Donor design and modification strategies of metal-free sensitizers for highly-efficient n-type dye-sensitized solar cells

Abstract

Dye-sensitized solar cells (DSSCs) cannot be developed without the research on sensitizers. As the key of light harvesting and electron generation, thousands of sensitizers have been designed for the application in DSSC devices. Among them, organic sensitizers have drawn a lot of attention because of the flexible molecular design, easy synthesis and good photovoltaic performance. Recently, new record photovoltaic conversion efficiencies of 11.5% for DSSCs with iodide electrolyte and 14.3% for DSSCs with cobalt electrolyte and co-sensitization have been achieved with organic sensitizers. Here we focus on the donor design and modification of organic sensitizers. Several useful strategies and corresponding typical examples are presented.

Key words： donors; organic sensitizers; dye-sensitized solar cells (DSSCs)

Introduction

The world’s energy crisis has forced and accelerated the development of many renewable energy technologies, among which the solar cell is regard as one of the most promising technologies due to its clean and abundant energy resource. Dye-sensitized solar cells (DSSCs) have been intensively studied as one of the third-generation solar cells for its potential for roll-to-roll mass production, cost-efficient and easy fabrication procedures, colorful appearance and high solar-to-electricity conversion efficiency even under low light intensity [1–3]. Having been developed for more than two decades, DSSCs have already achieved the photovoltaic conversion efficiency (h) as high as 14.3% with an alkoxysilyl-anchor organic dye co-sensitizing with a carboxy-anchor organic dye in cooperate with cobalt complex-based redox electrolyte, which is tempting for industrial applications of DSSCs [4]. Since the first report of the DSSC in 1991 [5], the device structure and working principles of a typical DSSC have not changed so much. There are several important components: 1) the transparent conducting glass. For DSSCs, fluorine-doped SnO₂ (FTO) glass is the most commonly-used substrate due to its chemical inertness and high-temperature resistance; 2) the mesoporous semiconductor film, normally TiO₂; 3) the sensitizers that absorbed on the surface of mesoporous semiconductor layer. Sensitizers are crucial to the photovoltaic performance of DSSCs because they are the key of converting solar energy into electricity; 4) the redox electrolyte, which is very important for regenerating dyes and completing the electronic circuit; 5) the counter electrode. Normally for liquid-state DSSCs, the counter electrode is made by depositing catalyst of the redox couples (such as platinum or carbon materials) on the top of FTO glass. Figure 1 shows the device structure and working principle of a typical DSSC.

When the sunlight illuminates on a DSSC, the dyes (D) harvest photons and reach excited states (D*). The electrons are injected into the conductive band (CB) of TiO₂ due to the energy level difference between E_{(D+ /D*)} and E_CB, and flow into the external circuit, generating the photocurrent. Then the oxidized dyes are regenerated by the reductive species of redox couples. The oxidized species of redox couples will diffuse to the counter electrode to get electrons and complete the whole electronic circuit. The energy level diagram and electron transfer processes of DSSCs are demonstrated clearly in Fig. 2. The green arrows show the routes of electrons inside a working DSSC, representing the process of light harvesting and excitation of the dyes (Arrow 1), electron injection (Arrow 2), dye regeneration (Arrow 3), excited state decay (Arrow 4), charge recombination of electrons in the mesoporous semiconductor films with oxidized dyes (Arrow 5) or oxidized species in the electrolyte (Arrow 6). Process 4, 5 and 6 pose a negative effect on the solar-to-electricity conversion, which we shall minimize through device and material optimization.

Fig.1 A simple model for device and working principles of a typical DSSC

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Fig.2 Energy level diagram and electron transfer processes in a typical DSSC device (vs. the normal hydrogen electrode, NHE). The insert shows several redox potential examples of redox couples: I^-/I₃^-, iodide/triiodide, Ref. [6]; [Co(dmb)₃]^{3+ /2+}, cobalt(II/III) tris(4,4′-dimethyl-2,2′-bipyridine) complexes, [Co(dtb)₃]^{3+ /2+}, cobalt(II/III) tris(4,4′-ditert-butyl-2,2′-bipyridine) complexes, [Co(bpy)₃]^{3+ /2+}, cobalt (II/III) tris(2,2′-bipyridine) complexes, [Co(phen)₃]^{3+ /2+}, cobalt(II/III) tris(1,10-phenanthroline) complexes, Ref. [7]; [Co(Cl-phen)₃]^{3+ /2+}, cobalt(II/III) tris(5-chloro-1,10-phenanthroline) complexes, [Co(NO₂-phen)₃]^{3+ /2+}, cobalt(II/III) tris(5-nitro-1,10-phenanthroline) complexes, Ref. [8]; [Co(bpy-pz)₂]^{3+ /2+}, cobalt(II/III) bis[6-(1H-pyrazol-1-yl)-2,2′-bipyridine] complexes, Ref. [9

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As shown in Fig. 2, the open-circuit voltage (V_oc) is determined by the difference between the quasi Fermi energy level of TiO₂ and the redox potential of electrolyte. The formula is as follows [10]:

(1)V_{\mathrm{oc}}={\displaystyle \frac{E_{\mathrm{CB}}}{q}}+{\displaystyle \frac{kT}{q}}\mathrm{In}\left({\displaystyle \frac{n}{N_{\mathrm{CB}}}}\right)-{\displaystyle \frac{E_{\mathrm{redox}}}{q}},

where E_CB is the CB energy level of TiO₂, q is the unit electron charge, n is the number of electrons in the TiO₂ film, N_CB is the accessible density of states and E_redox is the redox potential of redox couples.

Another crucial parameter of the device photovoltaic performance is short-circuit current density (J_sc). It corresponds to the incident photon-to-current conversion efficiency (IPCE) of the DSSC, which can be expressed as the product of light harvesting efficiency (LHE), electron injection efficiency (h_inj), charge collection efficiency (h_col) and dye regeneration efficiency (h_reg) [11]:

(2)\mathrm{IPCE}(\lambda )=\mathrm{LHE}(\lambda )\times {\eta }_{\mathrm{inj}}\times {\eta }_{\mathrm{col}}\times {\eta }_{\mathrm{reg}}.

And the LHE at a certain wavelength is defined as [12]

(3)\mathrm{LHE}(\lambda )=\mathrm{1}-{\mathrm{10}}^{-A},

where A is the absorbance of dye-sensitized semiconductor film, which is related to the molar extinction coefficient (ϵ) and dye loading amount.

The photovoltaic conversion efficiency (PCE) of a DSSC can be obtained via the following equation [13]:

(4)\mathrm{PCE}={\displaystyle \frac{J_{\mathrm{sc}}\times V_{\mathrm{oc}}\times FF}{P_{\mathrm{in}}}},

where P_in is the incident light intensity, FF is the fill factor which is defined as the ratio of the maximum power (P_max=J_max×V_max) of the DSSC, and the product of J_sc and V_oc, that is

(5)FF={\displaystyle \frac{J_{\max }\times V_{\max }}{J_{\mathrm{sc}}\times V_{\mathrm{oc}}}}.

Therefore, for the aim of obtaining high photovoltaic conversion efficiency, high J_sc and V_oc are necessary. There are several general ways to improve V_oc: 1) using redox shuttles with more positive redox potentials. I^-/I₃^- is the most traditional redox couple used in DSSCs owing to their desirable kinetic properties and high carrier collection efficiencies [6]. However, the standard potential of I^-/I₃^- redox couple is 0.35 V vs. NHE, which limits the open-circuit voltage. Many alternative redox couples have been applied to DSSCs for the purpose of getting a higher V_oc [14]. Some examples and their redox potentials have been displayed in Fig. 2; 2) adding additives to shift the conduction band of TiO₂, such as tert-butylpyridine (TBP) [15]; 3) optimizing the device and materials to retard the charge recombination of electrons in the photoanode with oxidized dyes and oxidized species in redox electrolyte at the interfaces of TiO₂/dye/electrolyte and FTO/electrolyte as well as to reduce the dye aggregation which could cause the self-quenching effect [16–18].

For DSSCs with a given redox electrolyte, the V_oc will not change too much thus the enhancement of J_sc is very important. That is the one of the reason why researchers are trying so hard for searching a well-performed dye with broad spectral response, high molar extinction coefficient, matched energy levels and good stability [19,20]. For now, the high efficient sensitizers employed in DSSCs can be roughly categorized into three groups: ruthenium dyes, porphyrin dyes and metal-free organic dyes. All of them have already been reported with device PCEs above 11% [4,21–26]. Compared with ruthenium dyes and porphyrin dyes, metal-free organic dyes have many advantages such as great flexibility in molecular design for fine tuning the optical and electrochemical properties, high molar extinction coefficient as well as low-cost and easy synthesis [27]. In general, a metal-free organic dye is consist of donor (D), p-bridge (p) and acceptor (A) due to the good intramolecular charge transfer (ICT) property. When the dye absorbs a photon, the donor part gives an electron, which goes through the p-bridge to the acceptor for the push and pull effect [28]. Then the electron is injected into the TiO₂ film. Different donors, p-bridges and acceptors and their influence on properties and device performance have been studied by many research groups. Some common units of donors, p-bridges and acceptors are displayed in Fig. 3. Donor is an important part where the electrons are generated. In this review, we are going to focus on the donor designs and modifications of organic sensitizers for highly-efficient n-type DSSCs with iodide electrolyte and cobalt electrolyte.

Fig.3 A model for D-p-A structure and common groups for donor, p-bridge and acceptor part

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Strategies for donor design and modification in traditional DSSCs with iodide electrolyte

Use properly strong electron-donating group or planar donor to broad the spectral response

The electron-donating ability of a donor is a primary consideration since it is not only closely related to the light harvesting capacity but also the energy levels of the sensitizer, thus plays an important role in the generation of photocurrent and charge transfer kinetics at the interface of TiO₂/dye/electrolyte such as dye regeneration and recombination processes [29]. Many groups have investigated different donors and their relationships between device performances (Scheme 1). Our group compared N, N-dialkylaniline (Dye 1), triphenylamine (Dye 2) and indoline (Dye 3) as donor part with an isophorone-incorporated p-conjugation system. Results show that indoline as donor is more favorable than N, N-dialkylaniline and triphenylamine. Not only the maximum absorption wavelength was largely red-shifted but also the molar extinction coefficient was increased. Finally, the highest PCE of 7.41% was achieved with 3 under AM 1.5 illumination with a much higher J_sc of 18.63 mA/cm² than that of 1 (J_sc = 12.33 mA/cm²) and 2 (J_sc = 11.46 mA/cm²). DSSC based on 2 with triphenylamine as donor performed slightly worse than that of 1 with N, N-dimethylaniline. The J_sc of DSSC based on 2 was lower for the blue-shifted absorption peak and lower molar extinction coefficient. The sequence of electron donating ability of these three donor are as follows: indoline>N, N-dimethylaniline>triphenylamine, which is in accordance with their dye properties and device performance [30]. However, donor with stronger electron-donating ability does not always result in higher PCE. Xue’s group also studied N, N-dimethylaniline (Dye 4) and triphenylamine (Dye 5) in a system of cyclopentadithiophene (CPDT) as the p-bridge. Even though N, N-dimethylaniline as donor still offered a broader absorption range than its triphenylamine analog, the molar extinction coefficient of 4 is less than 5’s. With the co-adsorption of 1.5 mM chenodeoxycholic acid (CDCA), DSSC sensitized by 5 gave higher PCE of 6.3% with J_sc = 15.2 mA/cm² than that of 4 with PCE= 5.7% and J_sc = 13.7 mA/cm² [31]. The authors explained that N, N-dimethylaniline cannot provide enough steric hindrance to suppress charge recombination when CPDT was used as p-bridge, which is different with our system since the isophorone was specially chosen to provide steric hindrance. Therefore, the optical and electrochemical properties and device performance cannot be simply judged by the electron-donating capability because they are affected by every segment of the dye and also the condition of device fabrication.

Chang and Chow made a very careful investigation about the effect of donor changing. Comparing dye 6 and dye 10 with their corresponding triphenylamine anologues dye 7 and dye 11, they found that by replacing one phenyl with naphthalene group, the molar extinction coefficients increased largely. While for replacing one phenyl with thiophene, the outcomes were quite different. Thiophene has a lower oxidation potential than phenyl unit, which can cause the bathochromic shift of absorption peaks. However, the molar extinction coefficients dropped. What’s more, the strong electron-donating abilities of N-(naphthalen-1-yl)-N-phenylthiophen-2-amine and N, N-diphenylthiophen-2-amine pushed the highest occupied molecular orbital (HOMO) energy level so high (0.53 V vs. NHE for 10 and 0.58 V vs. NHE for 11) that the driving force of dye regeneration might not be enough anymore, resulting a low IPCE, which affects the power conversion efficiency directly. In the end, the highest PCE of 7.08% was obtained by DSSC based on 6 while 11 and 10 only provided PCEs of 3.75% and 3.74%, respectively. Therefore, the electron-donating ability of the donor is not the case that the stronger the better. Donor with too strong electron-donating capability will result in a large negative shift of HOMO energy level, which might cause the mismatch of energy levels and jeopardize the device performance [32].

Fig.4 Scheme 1 Molecular structures of dyes 1-15

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Bridged triphenylamine as a new donor has been introduced into organic sensitizers. The planar structure can improve the electron delocalization on the donor, which can lead to a large bathochromic shift of the absorption peak. Ko and his colleagues designed and synthesized dyes with bridged triphenylamine. Compared with its triphenylamine analog 12, 13 showed a red-shifted absorption band. The IPCE of 13 exceeded 80% over the wavelength region from 400 to 600 nm and reached 90% at 485 nm, providing J_sc of 15.2 mA/cm² which was higher than 13.0 mA/cm² for DSSC based on 12. In the end, DSSC based on 13 reached PCE of 7.87% in contrast to PCE of 6.00% obtained by DSSC based on 12 in the same conditions [33].

Another successful example is presented by Liu’s group and Grätzel’s group. They synthesized dye 15 with tert-butyl-substituted bridged triphenylamine as the donor and compared with its triphenylamine analog 14. Large enhancements on molar extinction coefficient and maximum absorption wavelength were observed, leading to big improvements of J_sc from 8.92 mA/cm² (14) to 15.37 mA/cm² (15) and PCE from 4.44% (14) to 7.51% (15) [34].

Expand donor part with additional electron-donating units to broaden the spectral response

One easy approach to increase the electron-donating ability of donor part is simply to expand the donor with additional electron-donating units (Scheme 2), thus forming a D-D-p-A configuration. Since our first report on starburst D-D-p-A sensitizer 16 exhibiting a good device performance of 6.02% in 2008 [35], many D-D-p-A sensitizers have been developed and applied to DSSCs by different groups. Wan and his colleagues compared effect of phenothiazine (17) and carbazole (18) as antenna in the starburst dyes. The results showed that phenothiazine as antenna had better contribution to improve the electron-donating ability than carbazole. The high molar extinction coefficient of 17 was consistent with the higher IPCE, leading to higher J_sc of 9.2 mA/cm² and PCE of 4.54% whereas J_sc of 7.3 mA/cm² and PCE of 3.26% for 18 in DSSCs [36]. By introducing one or two 4-(p-tolyl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indole as additional donor moiety onto the triphenylamine part of reference dye 19, 20 and 21 displayed red-shifted absorption peaks, increased molar extinction coefficients and positively shifted HOMO energy levels. Further investigation found out that the two indoline segments spreading out from triphenylamine can increase the possibility of interacting with the adjacent molecules and result in excited-state self-quenching. The IPCE curve of DSSC based on 21 without any coadsorbent was below 30%, which was magnificently enhanced to almost 70% after adding 5 mM<FootNote>

1) 1 mM= 1 mmol/L

</FootNote> CDCA. The DSSC devices with 5 mM CDCA based on 20 and 21 showed higher PCEs (6.38% for 20 and 5.99% for 21) and J_scs (11.33 mA/cm² for 20 and 11.15 mA/cm² for 21) than that of 19 (5.45%, 9.72 mA/cm²) [37].

Since heterocyclic groups containing electron-rich atoms such as sulfur or nitrogen also have good electron-donating properties, Zhang and his coworkers adopted 2-thienyl and 1-pyrazolyl and added them onto triphenylamine donor of dye 22, offering two new dyes 23 and 24, respectively. Calculated HOMO levels of 23 and 24 showed good electron distributions on the heterocyclic substituted triphenylamine donor parts. Compared to 22, the absorption peaks of 23 and 24 were red-shifted and higher molar extinction coefficients were also obtained, which were responsible for the ca. 31% enhancement of short-circuit current density. PCEs of 23- and 24- sensitized DSSCs were 5.21% and 4.92%, respectively, higher than PCE of 3.80% for 22 [38].

D-D-p-A can also be constructed in simple linear structure. For example, our group extended phenothiazine dyes by adding CH₃O- substituted triphenylamine (25), triphenylamine (26) and 1,1,2-triphenylethene (27). The maximum absorption wavelengths of these three dyes were almost the same but 26 had the highest molar extinction coefficient which benefited its light harvesting, thus gave the best J_sc of 10.84 mA/cm², resulting the highest PCE of 4.41% [39].

Fig.5 Scheme 2 Molecular structures of dyes 16-28

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Liu and his coworkers introduced an additional donor group into the indoline unit in the donor part to increase the electron-donating ability of the donor part (Scheme 3). 4-Methylphenyl, carbazole and fluorene were adopted to form three D-D-p-A dyes (28, 29 and 30, respectively). As expected, the absorption peaks of 29 and 30 were red-shifted by 32 and 13 nm compared with that of 28. So did their molar extinction coefficients increased. DSSC based on 29 with strong electron-donating carbazole unit as additional donor exhibited a broad IPCE spectrum stretching into near infrared (NIR) region. With the co-absorbance of 30 mM deoxycholic acid (DCA), DSSCs based on 29, 30 and 28 displayed the J_scs of 18.53, 15.29 and 11.63 mA/cm², respectively, which were in good accordance with their light-harvesting abilities. In the end, DSSCs based on 29, 30 and 28 yielded the PCEs of 8.49%, 6.84% and 5.08%, respectively, proving that constructing D-D-p-A sensitizers with suitable additional electron-donating units is an effective approach to increase light-harvesting capability and overall solar cell efficiency [40]. After optimization the p-bridge of 29 with thiophene (31) and furan (32), the device performance was improved further to 9.29% and 9.49%, respectively, for the increased photovoltage by removing the vinyl bond [41].

Xue’s group also constructed a series of indoline based D-D-p-A sensitizers with different additional donors like dipropylfluorene (33), hexyloxybenzene (34), tert-butylbenzene (35), and hexapropyltruxene (36). 33 and 34 had better light harvesting ability compared to 35 and 36, contributing to their slightly higher J_sc. With the co-absorption with a small triphenylamine-based dye, the dye aggregation was further impeded. Increments in V_oc and J_sc led to the high PCEs of 33, 34, 35 and 36 as 8.18%, 7.06%, 7.36 and 8.08%, respectively [42].

However, this strategy is not always efficacious in every system. For example, our group showed two starburst dyes 37 and 38 with diphenylamine and carbazole as additional donor had almost the same device performance (PCE of 4.41% for dye 37 and PCE of 4.44% for dye 38) with their D-p-A triphenylamine analog 19 (PCE of 4.32%) with the co-adsorption of 5 mM CDCA. And the possibility of getting mismatched energy levels also becomes one drawback of this strategy. However, we found the dyes 37 and 38 exhibited much better long-term stability in DSSCs with quasi-solid-state electrolyte over 1200 h at full sunlight and at 50°C [43].

Fig.6 Scheme 3 Molecular structures of dyes 29-38

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Expand donor part into p-bridge to broaden the spectral response and accelerate ICT progress

Ko’s group investigated thoroughly on fluorene-substituted triarylamine dyes and their derivatives (Scheme 4). By changing the phenyl unit of 39 (PCE of 7.20%) that connected the donor and p-bridge into benzo[b]thiophene, benzo[b]furan and 4, 4-dimethyl-4H-indeno[1,2-b]thiophene, they got three new dyes 40 (PCE of 7.43%), 41 (PCE of 6.65%) and 42 (PCE of 8.2%), respectively [44–47]. Absorption spectra measured in ethanol solution showed that the absorption peak was largely red-shifted all the way from 436 to 480 nm (436 nm for 39; 456 nm for 40; 463 nm for 41 and 480 nm for 42). Unfortunately the HOMO level of 41 reached only 0.6 V vs. NHE, which might render a low dye regeneration efficiency and diminish the J_sc and PCE. Frontier molecular orbitals of 39, 40, 41 and 42 showed quite good planarity of phenyl, benzo[b]thiophene, benzo[b]furan and 4,4-dimethyl-4H-indeno[1,2-b]thiophene with thiophene unit. In 35, the dihedral angle between phenyl and thiophene was 20.5° [44]. Interesting finding was that the dihedral angle between the indenothiophene and the thienyl unit in 42 was only 1.6°. What’s more, the cyanoacrylic acid group is also almost coplanar with thiophene, presenting extremely good planarity from 4,4-dimethyl-4H-indeno[1,2-b]thiophene to the acceptor. The calculations also showed the HOMO of 42 was delocalized over the whole p-conjugated system from the fluorenylamino unit to the cyanoacrylic group and the lowest unoccupied molecular orbital (LUMO) was delocalized through the 4, 4-dimethyl-4H-indeno[1,2-b]thiophene to cyanoacrylic unit, indicating a good HOMO-LUMO coupling [47]. This shows that the 4,4-dimethyl-4H-indeno[1,2-b]thiophene moiety already became a part of p-bridge favors the ICT process. The highest PCE of 8.2% was achieved by DSSCs based on 42 under AM 1.5 illumination.

Fig.7 Scheme 4 Molecular structures of dyes 39-48

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Similar strategy was also applied to organic dyes 43 and 44. The absorption peak bathochromically shifted by 10 nm and the molar extinction coefficient increased from 73800 to 85000 M^-1·cm^-1 upon replacing the phenyl by benzo[b]thiophene. Time-dependent density functional theory (TD-DFT) calculations revealed that the dihedral angle between donor part and 3,6-dihexylthieno[3,2-b]thiophene-based p-bridge decrease from 51° (43) to 46.6° (44). The better planarity of molecular structure not only benefited the optical properties but also accelerated the ICT progress. The IPCE response of 44 was extended to 770 nm and highest IPCE of 93% was reached at 475 nm. The IPCEs were over 80% at the wavelength range of 400 to 640 nm. The broad IPCE spectra of DSSCs of 44 led to the high J_sc of 17.61 mA/cm². Excellent PCE of 9.1% was achieved by DSSC based on 44. In the same condition, high PCE of 8.0% was obtained by DSSC based on 43 with J_sc = 15.7 mA/cm² [48].

A further extension of this strategy is to use donor units with good electron-donating ability and hole transporting property as p-bridges and attach them directly to the acceptor part. Thus these units not only act as part of the donor but also the p-bridge. The push and pull effect is reinforced and the charge transfer and separation between the electron donor and acceptor in the molecule become more effective, resulting in a good IPCE and high J_sc. One of the good examples is presented by Sun’s group. They constructed three D-D-p-A dyes with different p-bridges such as thiophene (47), 3-hexylthiophene (46) and 3,4-ethyldioxythiophene (45) and compared them with their analog without the p-bridge (48). 45, 46 and 47 all exhibited higher molar extinction coefficients and more red-shifted absorption peaks than that of 48. However, the astonishing fact is that all of their IPCE were much lower than 48’s over the spectral region of 380-600 nm. DSSC based on 48 displayed extremely good IPCE over 90% from 450 to 575 nm. In contract, the IPCE of other three dyes are all below 80%, except the highest IPCE of 45 reaching 80%. Even though 45, 46 and 47 had much broader IPCE response ranges than 48, the highest J_sc of 14.90 mA/cm² and the highest PCE of 7.5% were obtained by DSSC based on 48, while PCEs of DSSCs based on 45, 46 and 47 ranged from 6.1 to 6.4%, with the J_scs ranging from 13.25 to 14.01 mA/cm² [49].

Fig.8 Scheme 5 Molecular structures of dyes 49-54

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We also observed the similar phenomenon (Scheme 5). By comparing two sensitizers based on triphenylamine substituted N-annulated perylene with (49) and without (50) thiophene as p-bridge, we found that the absorption peak of 50 was largely red-shifted by 40 nm compared to that of 49, which can be ascribed to the good co-planarity of triphenylamine and N-annulated perylene and better ICT process. The IPCEs of 50 were much higher than that of 49. As for the device performance, DSSC sensitized with 50 showed a high PCE of 8.28% with J_sc = 16.50 mA/cm², V_oc = 0.734 V and FF = 0.684 in contrast to the PCE of 4.90% obtained by DSSC based on 49 with J_sc = 10.75 mA/cm², V_oc = 0.655 V and FF = 0.700 [50].

Fused thiophene groups usually afford good charge transport properties and have been used as good donors for organic photovoltaic (OPV) and organic field-effect transistor (OFET) materials [51]. Zhou and his coworkers investigated the effect of the tetrathienoacene position within the sensitizers on its photovoltaic performance. Interestingly when the tetrathienoacene was attached directly to the triphenylamine donor, the dyes (51 and 53) demonstrated higher IPCE than their counterparts (52 and 54) with thiophene spacer between triphenylamine and tetrathienoacene. The results of femtosecond time-resolved photoluminescence (FTR-PL) quenching experiment on dye-sensitized TiO₂ films showed the sequence of electron injection efficiencies are 51>53>55>52, offering indirect evidence for the benefits of extending the donor part into the p-bridge since tetrathienoacene can be regarded as donor and p-bridge. In addition, dye 53 gave the best IPCE spectra reaching 85% and an excellent PCE of 10.1% in DSSC with iodine electrolyte with J_sc = 16.5 mA/cm², V_oc = 0.833 V and FF = 0.737 [52].

Add blocking groups on donor unit to resist dye aggregation and charge recombination

Research showed that organic dyes are prone to form aggregates on the semiconductor surface, which can cause self-quenching of the excited state and increase the chance of charge recombination at the interface [53]. Adding blocking groups like long alkyl or alkyloxy groups has been proved to be an efficient approach to resist dye aggregation and charge recombination (Scheme 6). For example, in the research of bridged triphenylamine as donor done by Ko’s group, the DFT/TD-DFT calculations of two 13 dye molecules adsorbed on adjacent Ti (IV) rows showed the formation of tight dye aggregates. They introduced C₆H₁₃O- and C₉H₁₉- onto the bridged triphenylamine to get dyes 55 and 56 respectively. Higher J_sc, V_oc and electron lifetime were all obtained in DSSCs based on 55 and 56 in the sequence of 56>55>13, proving that the long chains on a donor unit can efficiently retard dye aggregation and charge recombination at the interface of TiO₂/dye/electrolyte, prolonging the electron lifetime and improving the photovoltaic performance. As a result, a high PCE of 8.71% with V_oc = 0.75 V was achieved by DSSC based on 56 and PCE of 8.28% with V_oc = 0.73 V was reached by DSSC based on 55 [33].

Fig.9 Scheme 6 Molecular structures of dyes 55-64

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Sun’s group and Hagfeldt’s group extended the triphenylamine donor with p-dimethylamine and/or p,o-butoxy groups substituted phenyl groups to investigate the relationship between dye structure and overall efficiency of DSSCs. They first synthesized unsymmetric neutral metal-free sensitizer 58 with both p-dimethylamine and p,o-butoxy groups substituted phenyl groups, expecting to combine the properties from its parent dyes, 57 (only p-dimethylamine groups) and 59 (only p,o-butoxy groups). Because phenyl with p-dimethylamine has better electron-donating ability, 57 showed the broadest absorption band while 59 had the narrowest band. However, breaking the symmetry of the molecule led to orbital rearrangements and rendered the LUMO energy level of 58 much higher than that of 57 and 58 as well as the low molar extinction coefficient, which was only about half of 57’s or 59’s molar extinction coefficient. As for the device performance, J_scs and V_ocs were both in the order of 59>58>57, which probably because p,o-butoxy groups had much better effect on retard charge recombination and dye aggregation than p-dimethylamine. In the end, DSSC based on 59 yielded the highest PCE of 6.00%, thus showing the importance of using blocking chains to retard the charge recombination and dye aggregation to improve the overall solar cell efficiency [54].

Later, they used two long alkyl chains to replace the methyl groups on the tetrahydroquinoline part of 60 to get a new NIR dye 61. Compared to 60, there is a big increase on the molar extinction coefficient of 61, reaching 88867 M^-1·cm^-1. Also the DSSC based on 61 displayed an extremely high IPCE of 93% at 660 nm. The absorption peaks of these two dyes on TiO₂ films were red-shifted compared to their corresponding absorption peaks in solutions, which was in favor of light harvesting. This probably was due to J-aggregation since the deprotonation of the dyes normally results in hypochromatic shift. Results also showed that the dye amount of 61 on the TiO₂ films was less than half of the amount of 60, indicating 60 had a serious dye aggregation problem. The electrochemical impedance spectroscopy (EIS) showed the longer electron lifetime in DSSC based on 61. As a result, DSSC based on 61 obtained PCE of 5.1% with J_sc = 13.35 mA/cm², V_oc = 0.519 V, FF = 0.73 while DSSCs based on 60 only gave PCE of 3.7%, J_sc = 11.76 mA/cm², V_oc = 0.464 V, FF = 0.674, proving the replacement with long alkyl chains can decrease the dye aggregation and charge recombination as well as improve the photovoltaic performance efficiently [55,56].

Our group synthesized a series of starburst truexene-based organic sensitizers (62, 63, and 64) with modifications of long hexyl chains on truexene part and methoxyl groups on diphenylamine segments. From 62 to 64, the absorption peaks in CH₂Cl₂ were red-shifted from 406 to 430 nm and HOMO energy levels were shifted toward negative. The IPCEs of 62 and 63 were close to 90% but the IPCE of 64 was lower. This could be due to the low dye regeneration efficiency for lacking of driving force since the HOMO level of 64 was pushed too close to the redox potential of iodide/triiodide electrolyte. However, this did not affect the improvement of V_oc too much. By lengthening the ethyl groups to hexyl groups, the V_oc of DSSCs increased from 0.689 to 0.731 V. Then the V_oc was further improved to 0.752 V via adding methoxy groups on diphenylamine part, higher than 0.728 V obtained by DSSC based on N719 in the same condition [57].

Construct non-planar donor with large steric hindrance to resist dye aggregation and charge recombination

To avoid the consideration of the length and positions of blocking chains, using non-planar donor with large steric hindrance to resist the day aggregation and charge recombination is also seems to be an available choice (Scheme 7). To prove this, Han’s group designed and synthesized two starburst organic dyes 65 and 66 with twisted and planar p-conjugation system. The absorption coefficient of 66 was higher than that of 65, which can be ascribed to the wider and more planar p-conjugation system of 66. They measured the maximum absorption wavelength of the dyes adsorbed on the surface of 2.3 mm-thick TiO₂ films with and without the co-adsorption of DCA and found that for 65, the maximum absorption wavelength was 441 nm no matter if DCA was adsorbed while for 66, the maximum absorption wavelength blue-shifted by 4 nm after co-adsorption with DCA, which can be attributed to the suppression of J-aggregates. In the case of no DCA, the IPCE maximum of 65 based device reached about 80%, much higher than that of 66, which is only about 50% despite of its high molar extinction coefficient. The 65-based DSSC offered a PCE of 5.35% with J_sc = 10.359 mA/cm², V_oc = 0.715 V, FF = 0.722 while DSSC based on 66 only gave PCE of 3.20%, J_sc = 6.866 mA/cm², V_oc = 0.687 V, FF = 0.678. After co-sensitized with 20 mM DCA, the J_sc and PCE of DSSC based on 65 were slightly decreased whereas the J_sc and PCE of DSSC based on 66 were increased to 7.481 mA/cm² and 3.75%, respectively. Therefore, they came up with a conclusion that the twisted dye molecular structure can also efficiently suppress the dye aggregation and charge recombination [58].

Tsai et. al. compared the shielding effect of twisted D-D-p-A (67) and its corresponding D-p-A with long alkyl chains (68) in a 1H-phenanthro[9,10-d]imidazole-based system. They found that incorporation of triphenylamine groups at 1H-phenanthro[9,10-d]imidazole not only can retard charge recombination of the electrons, but also benefit from the enhancement of light-harvesting ability. In the end, DSSC based on 67 yield a better PCE of 4.68% than 4.01% of DSSC based on 68 even though it had higher V_oc [59].

Xue’s group developed several truexene-based triphenylamine dyes. They found out that the large steric hindrance brought by truexene segments can effectively suppress dye aggregation and reach high V_oc. Taking 19 (PCE of 4.55%), 69 (PCE of 4.92%) and 70 (PCE of 5.23%) as examples, V_ocs of DSSCs based on 69 and 70 with no CDCA were 0.750 and 0.754 V, much higher than 0.690 V of 19-sensitized DSSC. According to the absorption spectra measured in CH₂Cl₂ solution with the same dye concentration of 1.0 × 10^-5 M, the bathochromic shifts of absorption peak and increase of molar extinction coefficients of 69 and 70 (65000 M^-1·cm^-1 at 486 nm for 69; 52000 M^-1·cm^-1 at 498 nm for 70) compared to 19 (28000 M^-1·cm^-1 at 474 nm) can be mainly attribute to the stronger electron-donating ability of truexene, which are favorable for light harvesting. However, the large steric hindrance also reduced dye loading amount. For instance, the dye loading amount of 19 (1.25 × 10^-7 mol/cm²) on the TiO₂ surface is about 1.5 times of that of 69 dye (0.80 × 10^-7 mol/cm²), rendering the similar IPCE of 19 and 69. Same case is also for 70, the dye loading amount of 19 (1.8 × 10^-7 mol/cm²) is as twice as that of 70 (0.91 × 10^-7 mol/cm²). Therefore, no obvious improvement of J_sc was observed in DSSCs of 19 (9.7 mA/cm²), 69 (9.8 mA/cm²) and 70 (10.2 mA/cm²), limiting the enhancement of PCE [60,61].

Chen and his colleagues worked on a series phenothiazine-based dyes (Scheme 8). They studied the difference of diphenylethylene (71), triphenylethylene (72), and tetraphenylethylene (73) substituted phenothiazine-based D-D-p-A dyes. The optical and electrochemical properties of these three dyes are almost the same, resulting in the similar IPCE spectra and J_scs. However, big increments in V_oc were observed with the increasing size of polyphenyl-substituted ethylene. High V_oc of 0.804 V was reached by 73-sensitized DSSC. The dark current and open-circuit voltage decay study suggested that more twisted donor structures of the dyes had an advantage in slowing down the electron recombination kinetics and lengthening the electron lifetime. Intensity modulated photovoltage spectroscopy (IMVS) and intensity modulated photocurrent spectroscopy (IMPS) revealed that the rigid tetraphenylethylene might pose a negative effect on the electron transport under increasing light intensity. With a twisted but a smaller steric hindrance effect on phenothiazine than tetraphenylethylene, triphenylethylene as the capping donor was more favorable in terms of charge transport and recombination suppression. With J_sc = 12.62 mA/cm², V_oc = 0.789 V and FF = 0.63, the highest PCE of 6.29% was obtained by DSSC based on 72 [62].

Fig.10 Scheme 7 Molecular structures of dyes 65-70

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Similarly, they compared the effect of phenothiazine and carbazole as the capping donor and p-bridge in four triphenylethylene-based sensitizers. Results showed comparing with planar carbazole, phenothiazine is not only a good choice for donor with strong electron-donating ability, but also favorable for getting high V_oc because its nonplanar butterfly conformation can effectively retard molecular aggregation and dye aggregation. The highest PCE of 6.55% was produced by DSSCs based on 77 employing phenothiazine as capping donors and p-bridge with J_sc = 12.18 mA/cm², V_oc = 0.826 V and FF = 0.65. DSSCs based on 74, 75 and 76 yielded the PCEs of 2.14%, 2.69% and 5.51%, respectively [63].

Fig.11 Scheme 8 Molecular structures of dyes 71-77

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Introduce electron-withdrawing groups to fine tune the optical and electrochemical properties and accelerate the ICT process

By systematical study, our group found that introducing a strong electron-withdrawing group onto the donor is a very efficient method to essentially facilitate the electron transfer from the donor to the acceptor and modulate the absorption spectra, energy levels, photovoltaic performance as well as the photostability of sensitizers [64,65]. Many kinds of electron-withdrawing groups have been applied to organic sensitizers using this strategy, such as benzothiadiazole [66–74], benzotriazole [75–78], benzoxadiazole [79], quinoxaline [80–84], pyrido[3,4-b]pyrazine [85–87], [1,2,5]thiodiazolo[3,4-c]pyridine [88–90], thieno[3,4-c]pyrrole-4,6-dione [91,92], diketopyrrolopyrrole [93–98], and isoindigo units [99–101]. Some examples have been showed in Fig. 4.

Fig.12 Examples of electron-withdrawing groups used in organic sensitizers for DSSCs

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For instance, by incorporating benzothiadiazole with indoline donor (Scheme 9), dye 79 showed a 27-nm bathochromic shift of the absorption band compared to its reference D-p-A dye 78 in CH₂Cl₂. DSSC based on 79 yielded a higher PCE of 8.15% with J_sc = 16.91 mA/cm², V_oc = 0.672 V and FF = 0.717 while 5.97% for DSSC based on 78 with J_sc = 13.77 mA/cm², V_oc = 0.615 V and FF = 0.705. The higher J_sc can be attributed to the much higher and boarder IPCE spectra of DSSC based on 79 than that of 78. The results of stepped light-induced transient (SLIT) measurements and dipole moment calculations showed that the E_CB of TiO₂ can be upshifted by introducing benzothiadiazole in between the donor and p-bridge, thus improving V_oc. 79 also showed an excellent photostability, indicating the electron-withdrawing group can effectively stabilize indoline donor [73]. Different donors, triphenylamine (80), methoxyl substituted triphenylaime (81) and indoline (82) units, were also compared in benzothiadiazole-based system with CPDT as p-bridge. The sequence of electron-donating ability of the three donors is indoline>methoxyl substituted triphenylaime>triphenylamine. Interesting result of IPCE spectra was that even though the order of onset wavelength was in accordance with the electron-donating ability, the plateau of the IPCE spectrum of DSSC based on 81 was much lower than the others, only reaching 60%. Unfortunately this phenomenon was not explained in the paper. After all, DSSC based on 82 achieved the highest PCE of 10.08% with a high J_sc of 19.69 mA/cm² [74].

Introduction of electron-withdrawing groups also offers another approach for molecular modification. For example, 83 and 84 were designed and synthesized with methyl and octyl substituted benzotriazole groups, respectively. Results exhibited that the octyl chains posed a good effect on retarding the charge recombination and dye aggregation and increment of 0.1 V for V_oc was obtained under the same condition. PCEs of 6.74% and 8.02% were offered by DSSCs based on 83 and 84, respectively [75]. Similarly, 2,3-bis(4-methoxyphenyl)quinoxaline and 2,3-bis(4-(octyloxy)phenyl)quinoxaline were also adopted as electron-withdrawing group attaching to indoline donor, providing dye 85 and 86. The usage of long octyloxy chains successfully suppressed the charge recombination of electrons in TiO₂ conduction band to the electrolyte and dye aggregation, enhanced the electron lifetime and V_oc values. Without any co-adsorbent, DSSC based on 86 exhibited a high V_oc of 0.776 V and J_sc of 15.65 mA/cm², yielding a high overall DSSC efficiency of 8.50% under AM 1.5 illumination. PCE of 6.24% was given by DSSC based on 85 in the same condition with J_sc = 13.60 mA/cm² and V_oc of 0.685 V [80].

Fig.13 Scheme 9 Molecular structures of dyes 78-89

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Our group designed and synthesized a series of triphenylamine-based sensitizers with pyrido[3,4-b]pyrazine (87) and 2,3-bis(4-methoxyphenyl)pyrido[3,4-b]pyrazine (88 and 89) as electron-withdrawing groups. Although barely changing the optical and electrochemical properties of the sensitizer, methoxyphenyl substitutions on the pyrido[3,4-b]pyrazine unit produced immediate effects of preventing dye-aggregation thus enhancing the J_sc, V_oc and PCE from 7.10 to 12.11 mA/cm², from 0.570 to 0.671 V and from 3.11% (87) to 6.14% (88), respectively. Further improvement was done by introducing octoxyl group on the triphenylamine unit (89). The maximum absorption wavelength of 89 was red-shifted to 524 nm and molar extinction coefficient was increased, contributing to the high J_sc of 13.56 mA/cm². The V_oc was also enhanced to 0.691 V, offering PCE of 7.12% [85].

Diketopyrrolopyrrole (DPP) has a strong electron-withdrawing capability which is promising to extend the absorption of dyes into NIR region. But the large p-conjugated system of DPP can cause a strong p-stacked aggregation on TiO₂ (Scheme 10). As a solution for that, we attached the butyl and 2-ethyl-hexyl chains in the middle of the DPP unit and obtained two dyes 90 and 91. Both of them possessed broad spectral response and high molar extinction coefficients. Our results showed branched chains was better than straight chains in terms of reducing the dye aggregation and charge recombination for its bigger steric hindrance, resulting a high V_oc. As a result, DSSC based on 91 with indoline donor and 2-ethyl-hexyl substituted DPP unit achieved a high PCE of 7.43% with J_sc = 13.40 mA/cm², V_oc = 0.76 V, and FF = 0.73 while DSSC based on 90 with triphenylamine donor and butyl substituted DPP unit yielded the PCE of 5.18% with J_sc = 11.05 mA/cm², V_oc of 0.69 V, and FF of 0.68. 91 also exhibited a good long-term stability [95].

Fig.14 Scheme 10 Molecular structures of dyes 90-95

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Our group designed and synthesized a blue dye with the electron-withdrawing [1,2,5]thiadiazolo[3,4-c]pyridine unit attaching to the indoline donor. The absorption peak of 92 located at 593 nm with a molar extinction coefficient of 35800 M^-1·cm^-1. With 2 mM CDCA as coadsorbent, the DSSC based on 92 showed a J_sc of 13.3 mA/cm², V_oc of 0.631 V and FF of 0.76, corresponding to the PCE of 6.4% [89].

Kang and his colleagues synthesized three organic dyes (93, 94 and 95) with 5,6-difluoro-2,1,3-benzothiadiazole (DFBTD) as the electron-withdrawing group. Compared 93 with its 2,1,3-benzothiadiazole analog, they found two fluorine atoms on DFBTD unit can largely enhance the molar extinction coefficient but lead to a blue shift of the absorption band. By optimizing the p-bridge, strong absorption band with a red-shifted maximum absorption wavelength of 549 nm and a molar extinction coefficient of 55800 M^-1·cm^-1 was obtained by 94. Replacement of indoline with 4-(tert-butoxy)-N-(4-(tert-butoxy)phenyl)-N-phenylaniline led to a slightly blue shift of absorption peak and a decrease in molar extinction coefficient, resulting to a low J_sc. In the end, the highest PCE of 9.1% was yield by DSSC based on 94 with J_sc = 18.8 mA/cm², V_oc = 0.717 V, and FF = 0.673. In the same condition, PCEs of 7.4% and 6.6% were obtained by DSSCs based on 93 and 95, respectively [102].

Other donors

Hundreds of organic sensitizers have been developed but the basic donor units are much less. New donor units need to be developed, especially strong donors for panchromatic sensitizers. Several new donor have been investigated (Scheme 11). For example, Franco et. al. designed and synthesized four organic sensitizers based on phenyl (96 and 97) or tert-butyl (98, and 99) substituted H-pyran-4-ylidene as the donor for its proaromatic character. By adding hexyl chains on thiophene, the absorption peaks were successfully largely red-shifted by 24 and 33 nm for phenyl and tert-butyl substituted H-pyran-4-ylidene-based sensitizers, respectively, locating around 580 nm. However, the big drawback of this series was the low V_oc which were sacrificed for the broader spectrum response. The highest PCE of 5.37% was obtained by DSSC based on 99 with PCE of 6.9% for N719 based DSSC as reference [103].

Some novel fused aromatic hetercyclic groups with long alkyl chains also have been developed as the donor of sensitizers for DSSCs. For example, Hara’s group designed and synthesized two dyes based on 5,11-dioctylindolo[3,2-b]carbazole (100 and 101). Their absorption peaks were at about 500 nm and showed good IPCE. Under AM 1.5G irradiation, DSSC based on 100 gave a J_sc of 15.4 mA/cm² and V_oc of 0.71 V, yielding PCE of 7.3% while PCE of 6.7% for DSSC based on 101 with J_sc = 15.5 mA/cm² and V_oc = 0.70 V [104]. Paramasivam et. al. worked on benzocarbazole-based sensitizers since benzocarbazole has a good hole transporting property and thermal stability. Taking dyes 102 and 103 for example, they had high molar extinction coefficients of 50623 and 60782 M^-1·cm^-1 at 427 and 434 nm, respectively. Large bathochromic shifts (55 nm for 102 and 34 nm for 103) were observed after the dyes absorbed onto the surface of mesoporous TiO₂ layer, which can be ascribed to the J-aggregation. With an iodide electrolyte, DSSC based on 102 afforded a J_sc of 10.18 mA/cm² and V_oc of 0.733 V, yielding PCE of 5.74% under AM 1.5G illumination while 103 sensitized DSSC gave PCE of 4.63% under the same condition [105].

Fig.15 Scheme 11 Molecular structures of dyes 96-103

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Wu et. al. introduced cyclic thiourea/urea groups into triphenylamine donor (Scheme 12). They considered that the cyclic thiourea/urea group containing sulfur/oxygen and nitrogen atoms with lone pair electrons may pose a positive effect on the electron-donating ability and delocalizing the positive charges and the introduction of long alkyl chains can retard the charge recombination and improve the open circuit voltage. Dye 105 showed slightly weaker light harvesting capability than its thiourea analog 106. 106 also did not have much more advantages compared to 104 that had no p-bridge. After modification of p-bridge with two thiophene units, 107 exhibited the red-shifted absorption peak at 443 nm in CH₂Cl₂ solution and IPCE over 70% in the wavelength of 400 -600 nm in DSSCs, affording the highest J_sc of 14.8 mA/cm². With V_oc of 0.749 V and FF of 0.659, DSSC based on 107 yielded the highest PCE of 7.42%. The others gave PCEs of 4.94%, 4.73% and 5.33% for 104, 105 and 106 respectively [106].

Grätzel’s group reported a series of novel organic sensitizers with ullazine as donor. Ullazine is a planar p-system with an aromatic 14 p-electron annulene resonance structure. They connected the acceptor at the 4-, 5-, or 6-positions of the ullazine core and found that only acceptor at 5-position gave strong and red-shifted ICT band which was suitable for the sensitizers of DSSCs. With the thiophene p-bridge connecting to the 6-position of ullazine, the main absorption peak of 112 hypochromatically shifted by 189 nm and the shoulder peak at 540 nm almost disappeared, only giving a low molar extinction of 1600 M^-1·cm^-1. DSSC based on 112 also gave poor PCE of 1.7%. On the contrary, among those dyes with acceptor at the 5-position of ullazine core, dyes 108, 109 and 110 showed a broad absorption range with the maximum absorption wavelength of 582, 598 and 598, respectively. The one with only decyl substituted ullazine core as donor also afford an ICT band at 531 nm. Broad IPCE spectra of 108, 109, 110 and 111 sensitized DSSCs were observed with the onset wavelength exceeding 700 nm. The highest PCE of 8.4% was achieved by DSSC based on 108 with J_sc = 15.4 mA/cm², V_oc = 0.730 V and FF = 0.75 in conjunction with iodide/triiodide redox electrolyte under AM 1.5G illumination. In the same condition, 109-, 110- and 111-sensitized solar cells provided PCEs of 6.7%, 6.7% and 5.2%, respectively [107].

Fig.16 Scheme 12 Molecular structures of dyes 104-112

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In a nut shell, all these donor design and modification strategies of metal-free sensitizers aim at enhancing the photocurrent and photovoltage thus yielding a high overall solar cell efficiency. Recently, Joly and his coworkers developed several triphenylamine based sensitizers which achieved the new record PCE exceeding 10% with iodine electrolyte. Here we take four of them as examples since they used the strategies such as adding blocking chains, extending the donor part to p-bridge and connecting electron-withdrawing group with donor (Scheme 13). We can see compared to 113, the absorption peak red-shifted by 6 and 14 nm by adding the hexyl and hexyloxy groups, respectively. While bathochromic shift of 38 nm was shown via extending the donor by the fusion between the triphenylamine and the thiophene unit. In conjugation with iodine electrolyte consisting of 0.5 M 1-butyl-3-methyl-imidazolium iodide (BMII), 0.1 M LiI, 0.05 M I₂ and 0.5 M tert-butyl-pyridine in acetonitrile, all of the four dyes exhibited high IPCE over 80% in DSSC, especially IPCE of DSSCs based on 113 and 116 were close to 100%, offering high J_sc of 16.76-18.82 mA/cm². The V_ocs were almost the same for devices based on these dyes. After device optimization, DSSCs based on 113, 114, 115 and 116 yielded excellent PCEs of 10.20%, 9.67%, 10.11% and 9.69%, respectively [108].

Fig.17 Scheme 13 Molecular structures of dyes 113-116

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Strategies for donor design and modification in DSSCs with cobalt electrolyte

It has been pointed out that the large potential loss in DSSC system is one of the reasons that limit its PCE due to the over-potentials required by electron injection and dye regeneration processes [109]. The standard potential of I^-/I₃^- redox couple of 0.35 V vs. NHE results in a high overpotential for the dye-regeneration reaction and a big loss in potential, diminishing the V_oc and PCE, let alone its competitive light absorption and strong corrosiveness toward most of metals. Therefore, many alternative redox electrolytes have been developed [14,110–114]. Among them, cobalt complexes attracted most of researchers for its tunable redox potential, negligible light absorption and remarkable performance in DSSCs [114–117]. However, the thickness of TiO₂ films is limited because of the mass transport and charge recombination problems due to the larger size, heavier mass and lower diffusion coefficient of cobalt complexes [118] Therefore, organic sensitizers with high molar extinction coefficients are desirable to DSSCs with cobalt electrolyte.

In 2010, Hagfeldt’s group first reported DSSCs with cobalt electrolyte surpassing the PCE of their counterpart with iodide electrolyte using 59 with bulky triphenylamine donor and long alkyloxy chains. Four different cobalt complex-based redox couples: cobalt(III/II) tris(2,2′-bipyridine) ([Co(bpy)₃]^{3+ /2+}), cobalt(III/II) tris(4,4′-dimethyl-2,2′-bipyridine) ([Co(dmb)₃]^{3+ /2+}), cobalt(III/II) tris(4,4′-ditert-butyl-2,2′-bipyridine) ([Co(dtb)₃]^{3+ /2+}), and cobalt(III/II) tris(1,10-phenanthroline) ([Co(phen)₃]^{3+ /2+}) were compared in DSSCs based on 57 and 59. They found that recombination can be retarded more effectively by introducing insulating alkyl/alkyloxy chains on the dye rather than on the cobalt redox couples. The blocking chains on the ligands of cobalt complexes cannot only slow down the diffusion of the redox couples but also positively shift the redox potential, imperilling the J_sc, V_oc and thus PCE of the device. In the end, DSSCs based on 59 with o,p-dibutoxyphenyl substituted triphenylamine donor achieved the highest PCE of 6.7% with a high V_oc of 0.92 V and J_sc of 10.7 mA/cm² under AM1.5G illumination using 0.5 M Co(bpy)₃(PF₆)₂, 0.1 M Co(bpy)₃(PF₆)₃, 0.5 M TBP, and 0.1 M LiClO₄ in acetonitrile as redox electrolyte whereas PCE of 5.5% by its counterpart with iodine electrolyte in the same condition [7].

Fig.18 Scheme 14 Molecular structures of dyes 117-123

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Specified investigation was carried out in our group and Grätzel’s group on the donor size and their influence on DSSC performance with cobalt electrolyte and iodine electrolyte (Scheme 14). We constructed three organic sensitizers with different indoline donor size and electron-withdrawing quinoxaline. The sequence of the donor size is 117<118<119. Through the comparison in DSSCs with cobalt and iodide electrolyte with platinum counter electrode, we found the three dyes showed opposite trend (PCE: 117>118>119 in iodide electrolyte in contrast with 119>118>117 in cobalt electrolyte), indicating the electron recombination process was efficiently impeded because of the shielding effect of bulky donor with long alkyloxy chains in cobalt electrolyte whereas retarded the dye regeneration in iodide electrolyte. Since platinum is not a good catalyst for cobalt redox couples, graphene nanoplatelets (GNP) was used as counter electrode to further improve the performance of DSSCs with cobalt electrolyte. The device performance was the same and 119 sensitized DSSC obtained PCE of 9.60%. Further optimization was carried out using Au+ GNP as the counter electrode and 10.65% PCE was achieved with 0.22 M Co(bpy)₃[B(CN)₄]₂, 0.06 M Co(bpy)₃[B(CN)₄]₃, 0.1 M LiClO₄, and 0.5 M TBP in acetonitrile as redox electrolyte [119].

Therefore, the long blocking chains are necessary for good performed DSSCs employing cobalt electrolyte. The strategies of 2.2-2.7 are also valid for developing organic sensitizers for DSSCs with cobalt electrolyte. For example, truexene-based organic sensitizers are suitable for DSSC with cobalt electrolyte for its strong electron-donating ability and large steric hindrance with long alkyl chains. Xue’s group works on this types of sensitizers by combining with triphenylamine unit to construct high performance D-D-p-A dyes, such as 120 and 121. DSSC based on 120 showed PCE of 7.2% with J_sc = 12.0 mA/cm² and V_oc of 0.830 V in conjugation with [Co(phen)₃]^{3+ /2+} -based electrolyte. V_oc of 0.900 V, J_sc of 11.9 mA/cm², and PCE of 7.6%, were produced by DSSCs based on 121 with 0.25 M [Co(II)(phen)₃](PF₆)₂, 0.05 M [Co(III)(phen)₃](PF₆)₃, 0.8 M TBP and 0.1 M LiTFSI (TFSI= bis(trifluoromethanesulfonyl)imide) in acetonitrile as redox electrolyte [120,121]. Later, they introduced diphenylamine (122) and hexyloxy subsitituted diphenylamine (123). Interestingly, the photovoltaic performance of DSSC based on 123 was inferior to that of 122 although it had a stronger light harvesting ability. Lacking of driving force for dye regeneration might be responsible for the lower J_sc of DSSC based on 123. IMVS measurements also showed that the electron lifetime of the 123 sensitized DSCs was shorter than that of the 122, resulting the lower V_oc. DSSC based on 122 exhibited J_sc of 14.32 mA/cm², V_oc of 0.907 V and FF of 0.68, corresponding to a high PCE of 8.83% with [Co(phen)₃]^{3+ /2+}-based electrolyte whereas PCE of 7.81% for DSSC with 123 [122].

Consist of the bulky donor of 59, CPDT as p-bridge and cyanoacetic acid as acceptor and anchor (Scheme 15), 124 has exhibited a good device performance in DSSC with cobalt electrolyte, offering PCE of 8.8% in the first report [123]. Later, with the usage of porous poly(3,4-ethylenedioxythiophene) (PEDOT) counter electrodes, DSSCs with 124 achieved PCE of 10.3% with [Co(bpy)₃]^{3+ /2+} electrolyte [124]. Many sensitizers based on 124 has been synthesized and applied to DSSC with cobalt electrolyte. For example, 125 was proposed as an analog of 124 with fluorene-substituted triarylamine donor attached with hexyloxy chains. It showed a stronger electron-donating property and slightly better device performance of 10.3% with J_sc = 16.2 mA/cm² and V_oc of 0.840 V whereas PCE of 9.8% for DSSCs based on 124 in the same condition [125]. Later, two analogs of 124 and 125 by replacing the CPDT unit with the dithieno[3,2-b:2′,3′-d]pyrrole p-bridge as DTP were also applied successfully to DSSCs with cobalt electrolyte, yielding PCE of 8.86% (126) and 8.72% (127), respectively [126].

Fig.19 Scheme 15 Molecular structures of dyes 124-130

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Our group compared the differences between triphenylamine- and indoline-based bulky donors in two dyes with electron-withdrawing 2,3-diphenylpyrido[3,4-b]pyrazine group. The absorption peak of 129 was red-shifted by 16 nm compared to 130 due to the stronger electron-donating property of indoline. With the assistant of density functional theory calculations, we found the biphenyl branch at the cis-position of N-phenylindoline and the indoline core had very limited electronic contribution to the donor, which probably acted as an insulating blocking group to inhibit the dye aggregation and charge recombination at the interface of TiO₂/dye/electrolyte. The measurement results also showed the charge recombination in DSSCs based on 129 was much less than that of 130, probably because of the bigger donor size and the shielding effect of the donor of 129. In DSSCs with [Co(bpy)₃]^{3+ /2+} redox electrolyte, 129 presented a higher performance of 8.57% with J_sc = 16.08 mA/cm² and V_oc = 0.802 V while 130 gave PCE of 7.74%, J_sc = 15.35 mA/cm² and V_oc = 0.790 V under standard AM 1.5 G simulated sunlight [127]. Another bulky triphenylamine sensitizer 128 was investigated with [Co(bpy)₃]^{3+ /2+} redox electrolyte containing stacked graphene platelet nanofibers (SGNF). With 0.22 M [Co(II)(bpy)₃](TFSI)₂, 0.06 M [Co(III)(bpy)₃](TFSI)₃, 0.1 M LiClO₄, 0.5 M TBP and 0.2 mg/mL SGNF in acetonitrile as electrolyte, DSSC based on 128 obtained the highest PCE of 9.81% with J_sc = 16.75 mA/cm² and V_oc = 0.830 V [128].

Grätzel’s group reported four blue-colored diketopyrrolopyrrole (DPP)-based sensitizers 131, 132, 133 and 134 with 4-(hexyloxy)-N-(4-(hexyloxy)phenyl)-N-phenylaniline, 4-(p-tolyl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indole,4-(2',4'-bis(hexyloxy)-[1,1'-biphenyl]-4-yl)-1,2,3,3a,4,8b-hexa-hydrocyclopenta[b]indole and 4-(4-(2,2-bis(2',4'-bis(hexyloxy)-[1,1'-biphenyl]-4-yl)vinyl)phenyl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indole as donor (Scheme 16). Indoline based sensitizer 132 (57100 M^-1·cm^-1 at 596 nm), 133 (62400 M^-1·cm^-1 at 600 nm) and 134 (69000 M^-1·cm^-1 at 602 nm) had more advantages in light-harvesting ability then triphenylamine-based sensitizer 131 (55700 M^-1·cm^-1 at 587 nm). With the expansion of indoline donor, the molar extinction coefficient increased and absorption peak slowly red-shifted. High IPCE was obtained at the range of 400-700 nm. 132 got the lowest V_oc for lacking of shielding protection from the charge recombination at the interface of TiO₂/dye/electrolyte. With the corporation of cobalt electrolyte, DSSCs based on 131, 132, 133 and 134 yielded high PCE of 8.97%, 8.23%, 9.81% and 10.1%, respectively [129].

Fig.20 Scheme 16 Molecular structures of dyes 131-139

Full size|PPT slide

Recently, Wang’s group extended the p-conjugation system of N-annulated indenoperylene donor of 135 fixed the adjacent phenyl group with cyclopentadiene. The novel dye 136 possessed a red-shifted absorption band and much higher molar extinction coefficient than 135 contributing to a better light harvesting capability. Without any coadsorbent, DSSC based on 136 achieved a record PCE of 12.5% with high J_sc = 17.03 mA/cm², V_oc of 0.956 V and FF of 0.770 while PCE of 10.6% for DSSC based on 135 with J_sc = 15.81 mA/cm², V_oc of 0.897 V and FF of 0.744 under AM1.5G sunlight. The cobalt electrolyte employed was consist of 0.25 M Co(phen)₃(TFSI)₂, 0.05 M Co(phen)₃(TFSI)₃, 0.7 M TBP, and 0.05 M LiTFSI in acetonitrile [130].

Alkoxysilyl as the anchor group exhibited a strong bonding with TiO₂ and has been shown higher electron transfer efficiency, photovoltage and better stability than its counterpart of carboxy group. Carbozole dye 137 with alkoxysilyl anchor group has been demonstrated brilliant PCE of 12.49% in DSSC with J_sc = 15.57 mA/cm², V_oc = 1.036 V and FF = 0.775 using a cobalt(III/II)tris(5-chloro-1,10-phenanthroline)-based redox electrolyte containing 0.25 M [Co(Cl-phen)₃]²⁺(PF₆^-)₂, 0.035 M [Co(Cl-phen)₃]³⁺(PF₆^-)₃, 0.07 M LiClO₄, 0.02 M NaClO₄, 0.03 M tetrabutylammonium hexafluorophosphate (TBAPF), 0.01 M tetrabutylphosphonium hexafluorophosphate (TBPPF), 0.01M 1-hexyl-3-methylimidazolium hexafluorophosphate (HMImPF), 0.30 M TBP, 0.10 M 4-trimethylsilylpyridine (TMSP), 0.10 M 4-methylpyridine (MP) in acetonitrile. Recently, by co-sensitization with coumarin dye 138, the maximum IPCE value of DSSC based on 137 reached 88%, offering a higher J_sc of 16.0 mA/cm², thus a high PCE of 12.81% has been made under the same condition [131]. Later they obtained a new record PCE of 14.3% with J_sc = 18.27 mA/cm², V_oc = 1.014 V and FF = 0.771 employing 139 as a co-sensitizer and [Co(phen)₃]^3+/2+ redox electrolyte in cooperated with Au+ GNP counter electrodes [4].

Conclusion

In this review, we have summarized several useful strategies for donor design and modification of metal-free sensitizers for DSSCs and typical examples are presented (Tables 1 and 2). Basically, those strategies are aimed at improving the short-circuit current density and open-circuit voltage: adapting donors or adding additional donors for strong enough electron-donating ability, planar structure and adding electron-withdrawing groups for facilitating intramolecular charge transfer process, using blocking chains and twisted molecular structure for decreasing dye aggregation and charge recombination. Recently, new record photovoltaic conversion efficiencies of 10.2% for DSSCs with iodide electrolyte and 14.3% for DSSCs with cobalt electrolyte and co-sensitization have been made. New challenges for donor may focus on the new design of panchromatic sensitizers and mechanics research.

Tab.1 Optical properties of mentioned dyes and their device performance with iodide/triiodide electrolyte

dye	lmaxa) /nm	ϵa) /(M-1·cm-1)	lmaxb) /nm	Jsc /(mA·cm-2)	Voc/V	FF	PCE/%	Ref.
1	464	32700		12.33	0.642	0.64	5.08	[30]
2	450	26900		11.46	0.643	0.66	4.93	[30]
3	497	37600		18.63	0.634	0.63	7.41	[30]
4	521	34000		13.7	0.606	0.69	5.7	[31]
5	491	36000		15.2	0.605	0.68	6.3	[31]
6	422	37700		16.81	0.74	0.57	7.08	[32]
7	427	29000		15.36	0.69	0.50	5.25	[32]
8	461	31300		14.28	0.71	0.60	6.12	[32]
9	461	27100		16.26	0.66	0.58	6.17	[32]
10	480	22200		11.88	0.58	0.54	3.74	[32]
11	468	22500		10.89	0.58	0.60	3.75	[32]
12	438	29420		13.0	0.66	0.71	6.00	[33]
13	455	20369		15.2	0.72	0.72	7.87	[33]
14	511	27900	459	8.92	0.630	0.79	4.44	[34]
15	558	42800	492	15.37	0.651	0.75	7.51	[34]
16	480	25000	505	13.8	0.632	0.69	6.02	[35]
17	422	29367	416	9.2	0.625	0.79	4.54	[36]
18	424	13834	424	7.3	0.603	0.74	3.26	[36]
19	442	13700	429	9.72	0.787	0.71	5.45	[37]
	415	20300	490	8.83	0.736	0.66	4.32	[43]
	474	28000	422	9.7	0.690	0.68	4.55	[60]
20	458	19800	440	11.33	0.792	0.71	6.38	[37]
21	479	21800	465	11.15	0.778	0.69	5.99	[37]
22	410	19400		8.88	0.764	0.560	3.80	[38]
23	425	27100		11.61	0.766	0.586	5.21	[38]
24	440	28400		11.71	0.709	0.592	4.92	[38]
25	460	24000	476	9.43	0.584	0.69	3.78	[39]
26	450	31000	475	10.84	0.592	0.69	4.41	[39]
27	449	23000	480	7.39	0.505	0.66	2.48	[39]
28	491	22300	471	11.63	0.639	0.68	5.08	[40]
29	523	27900	491	18.53	0.649	0.71	8.49	[40]
30	504	27200	490	15.29	0.627	0.72	6.84	[40]
31	523	58200		16.58	0.756	0.741	9.29	[41]
32	522	57100		16.28	0.779	0.748	9.49	[41]
33	492	33000	444	16.1	0.770	0.66	8.18	[42]
34	495	24000	423	14.8	0.723	0.66	7.06	[42]
35	465	21000	427	15.0	0.743	0.66	7.36	[42]
36	500	45000	448	15.8	0.775	0.66	8.08	[42]
37	447	27000	509	8.90	0.710	0.70	4.41	[43]
38	411	24300	483	8.45	0.753	0.70	4.44	[43]
39	436	30000		12.20	0.764	0.77	7.20	[44]
40	456	16000		15.33	0.74	0.66	7.43	[45]
41	463	25300		14.39	0.70	0.66	6.65	[46]
42	480	55000		13.84	0.790	0.75	8.2	[47]
43	480	73800		15.7	0.690	0.74	8.0	[48]
44	490	85000		17.61	0.710	0.72	9.1	[48]
45	550	31000	513	14.01	0.704	0.65	6.4	[49]
46	538	31000	487	13.37	0.714	0.66	6.3	[49]
47	542	38000	485	13.25	0.696	0.66	6.1	[49]
48	517	28000	468	14.90	0.738	0.69	7.5	[49]
49	472	26400	478	10.75	0.655	0.700	4.90	[50]
50	512	30100	484	16.50	0.734	0.684	8.28	[50]
51	498			7.66	0.946	0.658	4.76	[52]
52	482			10.1	0.893	0.681	6.15	[52]
53	490			16.5	0.833	0.737	10.1	[52]
54	513			11.8	0.832	0.703	6.91	[52]
55	444	20289		16.3	0.73	0.70	8.28	[33]
56	463	12614		16.8	0.75	0.70	8.71	[33]
57	482	70200	456	12.00	0.67	0.60	4.83	[54]
58	459	37200	446	12.50	0.71	0.59	5.24	[54]
59	445	70100	444	12.96	0.75	0.61	6.00	[54]
60	610	66111	632	11.76	0.464	0.674	3.7	[55]
61	615	88867	650	13.35	0.519	0.73	5.1	[56]
62	406	27500	422	7.75	0.689	0.73	3.90	[57]
63	420	24900	425	7.89	0.731	0.74	4.27	[57]
64	430	41300	426	6.86	0.752	0.70	3.61	[57]
65	485	21600	441	10.359	0.715	0.722	5.35	[58]
66	468	34300	454	6.866	0.687	0.678	3.20	[58]
67	426	29000		12.21	0.65	0.59	4.68	[59]
68	413	21200		9.42	0.69	0.60	4.01	[59]
69	486	65000	443	9.8	0.750	0.67	4.92	[61]
70	498	52000	466	10.2	0.754	0.68	5.23	[60]
71	471	16000		11.82	0.759	0.65	5.84	[62]
72	474	20000		12.62	0.789	0.63	6.29	[62]
73	474	20000		11.41	0.804	0.63	5.76	[62]
74	412	16000		4.55	0.682	0.69	2.14	[63]
75	412	21000		5.27	0.711	0.72	2.69	[63]
76	462	13000		10.76	0.793	0.64	5.51	[63]
77	466	14000		12.18	0.826	0.65	6.55	[63]
78	518	22900		13.77	0.615	0.705	5.97	[73]
79	545	23600		16.91	0.672	0.717	8.15	[73]
80	536	37300	514	16.23	0.692	0.716	8.04	[74]
81	546	41000	529	12.32	0.699	0.727	6.27	[74]
82	551	43000	533	19.69	0.700	0.731	10.08	[74]
83	495	17200	428	13.39	0.68	0.74	6.74	[75]
84	496	19200	438	13.18	0.78	0.78	8.02	[75]
85	521	18700	508	13.60	0.685	0.67	6.24	[80]
86	523	21900	522	15.65	0.776	0.70	8.50	[80]
87	500	16700	479	7.10	0.570	0.76	3.11	[85]
88	497	16800	482	12.11	0.671	0.76	6.14	[85]
89	524	23300	516	13.56	0.691	0.76	7.12	[85]
90	514	41000		11.05	0.69	0.68	5.18	[95]
91	526	46000		13.40	0.76	0.73	7.43	[95]
92	593	33700	558	13.3	0.631	0.76	6.4	[89]
93	538	24100		17.1	0.642	0.675	7.4	[102]
94	549	55800		18.8	0.717	0.673	9.1	[102]
95	540	40300		12.7	0.730	0.712	6.6	[102]
96	556	33899		9.35	0.545	0.685	3.49	[103]
97	580	28840		12.32	0.595	0.708	5.19	[103]
98	584	23700		10.78	0.645	0.715	4.97	[103]
99	551	36399		12.10	0.610	0.728	5.37	[103]
100	492	36000	429	15.4	0.71	0.67	7.3	[104]
101	501	29900	437	15.5	0.70	0.62	6.7	[104]
102	427	50623	483	10.18	0.733	0.769	5.74	[105]
103	434	60782	470	7.89	0.767	0.765	4.63	[105]
104	423	39950		9.9	0.770	0.650	4.94	[106]
105	428	29680		9.3	0.739	0.689	4.73	[106]
106	426	33530		9.9	0.780	0.690	5.33	[106]
107	443	40690		14.8	0.749	0.659	7.29	[106]
108	582	28000		15.4	0.730	0.75	8.4	[107]
109	598	33000		11.5	0.807	0.72	6.7	[107]
110	598	24000		13.3	0.716	0.70	6.7	[107]
111	531	12000		11.0	0.672	0.70	5.2	[107]
112	540	1600		3.7	0.553	0.78	1.7	[107]
113	484	27600	471	18.26	0.76	0.74	10.20	[108]
114	490	23000	484	16.76	0.76	0.76	9.67	[108]
115	498	19900	495	17.81	0.76	0.75	10.11	[108]
116	533	25900	553	18.82	0.71	0.72	9.69	[108]

Notes: a)---Absorption maximum wavelength and molar extinction coefficient in an organic solution. b)--- Absorption maximum wavelength on TiO2 film

Tab.2 Optical properties of mentioned dyes and their device performance with cobalt electrolyte

dye	lmaxa) /nm	ϵa) /(M-1·cm-1)	lmaxb) /nm	Jsc /(mA·cm-2)	Voc/V	FF	PCE/%	Ref.
59	445	70100	444	10.7	0.92	0.68	6.7	[7]
117	526	17300	487	14.83	0.767	0.666	7.57	[119]
118	522	21400	502	15.58	0.797	0.712	8.84	[119]
119	534	27400	508	15.71 16.25	0.882 0.890	0.693 0.737	9.60 10.65	[119]
120	498	64000		12.0	0.830	0.72	7.2	[120]
121	500	81000		11.9	0.900	0.71	7.6	[121]
122	531	59200		14.32	0.907	0.68	8.83	[122]
123	543	69100		12.79	0.885	0.69	7.81	[122]
124	542	50500	441	14.6	0.855	0.70	8.8	[123]
				15.9	0.910	0.71	10.3	[124]
				14.1	0.876	0.78	9.8	[125]
125	548	47500	468	16.2	0.840	0.76	10.3	[125]
126	526	57700		13.4	0.901	0.74	8.86	[126]
127	541	52600		14.1	0.811	0.77	8.72	[127]
128	548	35528	539	16.75	0.830	0.706	9.81	[127]
129	557	26300	540	16.08	0.802	0.66	8.57	[127]
130	541	25500	532	15.35	0.790	0.64	7.74	[128]
131	587	55700		15.6	0.743	0.78	8.97	[129]
132	596	57100		15.2	0.716	0.76	8.23	[129]
133	600	62400		17.6	0.745	0.75	9.81	[129]
134	602	69000		17.9	0.761	0.74	10.1	[129]
135	512		507	15.81	0.897	0.744	10.6	[130]
136				17.03	0.956	0.770	12.5	[130]
137	498	43200		15.57	1.036	0.775	12.49	[131]
				15.99	1.034	0.774	12.81	[131]
				18.27	1.014	0.771	14.3	[4]

Notes: a)---Absorption maximum wavelength and molar extinction coefficient in an organic solution. b)--- Absorption maximum wavelength on TiO2 film

Acknowledgements

For financial support of this research, we thank the Science Fund for Creative Research Groups (21421004), the National Basic Research Program of China (973 Program) (No. 2013CB733700), and the National Natural Science Foundation of China (Grant Nos. 21172073, 21372082, 21572062 and 91233207). J.-L. Hua appreciates Prof. H. Tian very much for his helpful discussion and valuable comments.

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