Introduction
Semitransparent solar cells are an ideal choice for building integrated photovoltaics, which can be used as electricity-generating facades, roofs, and windows [1,2]. Moreover, such devices can also be used in electric cars and tandem solar cells [3]. Organic–inorganic hybrid perovskite solar cells (PSCs) have generated wide interest since 2009 due to their unprecedented high efficiency and potential economic advantages as a low-cost photovoltaic technology [4–6]. There are many amazing optoelectronic properties of perovskite materials, such as a high carrier mobility, a long charge diffusion length, and a high absorption coefficient, which promise high power conversion efficiencies (PCEs) for PSCs [7] of more than 25.0% [8]. By regulating the thickness of the perovskite layers and/or using wide-bandgap perovskite materials and transparent electrodes, semitransparent perovskite solar cells (ST-PSCs) can easily be prepared for future potential applications due to their solution-compatible processing. Wide-bandgap perovskite materials, such as formamidinium lead bromide (FAPbBr3), with a bandgap of ~2.23 eV and absorption spectra in the range of 300–550 nm, are especially appropriate for ST-PSCs with low temperature (150°C) processability compared to their inorganic counterpart CsPbBr3. Currently, the highest performance of a FAPbBr3 solar cell has reached 10.6% using a spin-coating method for the device preparation [9].
With the goal of the abovementioned applications, it would be beneficial to prepare large-scale PSCs under ambient conditions to keep production costs as low as possible. The development of high-yielding printing methods is crucial for rapid and cost-effective production. However, most of the reported devices have been fabricated via spin coating with active device area of <1 cm2. Therefore, an alternative route for the large-scale output of perovskite film is preferred. In the past few years, large-scale printing processes have been reported, such as spray coating [10], soft-cover deposition [11], doctor-blade coating [12–14], and slot-die coating [15,16]. Due to the simplicity of doctor-blade coating, this method has successfully drawn the attention of researchers and has become one of the most widely used methods in laboratories. Meanwhile, blade coating is a printing method that is highly compatible with high-yielding roll-to-roll coating [17,18]. In contrast to spin coating, there is no doubt that doctor-blade coating is a cost-effective and simple process that has a great advantage in utilizing the rheological characteristics and drying kinetics of precursor solutions [19]. In 2015, blade coating was used in PSC fabrication for the first time achieving an ~15.0% PCE [18]; this efficiency then very quickly increased to 19.0% for small-area devices [14,20]. Due to the increased difficulty of film morphology control during blade coating [21], it is generally more difficult to obtain high-quality films with blade coating than with spin coating [5]. How to control the uniformity of bladed-coated films is the main obstacle to obtaining large-area devices with high performance.
In this study, we demonstrated a doctor-blading process to fabricate high-quality perovskite FAPbBr3 thin films taking advantage of the excellent solution processability of FAPbBr3 perovskite and its high-performance humidity stability [22]. We studied the effects of the addition of surfactants to the FAPbBr3 perovskite precursor for the doctor-blading process, which significantly enhanced the quality of the film on the substrate. For planar heterojunction PSCs with a regular classical architecture of fluorine-doped tin oxide-coated glass (FTO)\TiO2\FAPbBr3\Spiro-OMeTAD\Au, the PCEs of the devices prepared via blade coating were up to 7.29%. For the semitransparent perovskite devices, we selected silver nanowires (AgNWs) as the electrodes to replace the opaque gold metal. The PCEs of the wide-bandgap ST-PSCs fabricated via blade coating were up to 5.11%.
Experimental
Materials
Titanium chloride (TiCl4, 99.9%) was purchased from Shanghai HUSHI Laboratory Equipment Co., Ltd., China; formamidinium bromine (FABr, 99.99%) and lead bromide (PbBr2, 99.99%) were purchased from Xi’an p-OLED Photoelectric Technology Co., Ltd., China; Spiro-OMeTAD was purchased from Shenzhen Feiming Science and Technology Co., Ltd., China; and all of the solvents were purchased from Aladdin without further purification.
Preparation of TiO2 films
The titanium dioxide (TiO2) films were prepared according to a previously reported procedure [23]. In brief, in an ice and water bath environment, 112 mL of TiCl4 was slowly dissolved in 388 mL of deionized water. The TiO2 precursor solution was stirred for 5 h at room temperature in air when the solution became pellucid. The TiO2 film was formed via chemical bath deposition. Cleaned FTO glass was covered by a 100-mL mixed solution of the TiO2 precursor solution and deionized water and placed in an oven at 70°C for 40 min; the FTO glass was then washed and the process repeated until the film was finally annealed at 450°C for 30 min. The resulting TiO2 electron transport layer was approximately 20-nm thick.
Device fabrication
First, the FTO glass was etched using a femtosecond laser to form the designed patterns. Then, ultrasonic cleaning was performed using deionized water and ethyl alcohol for 20 min. The mixed perovskite precursor solution was prepared by dissolving metal lead salts (PbBr2) and organic cations (FABr) into a mixture solvent of N,N-dimethylformamide (DMF)/dimethyl sulfoxide (DMSO) (4:1, by volume) forming a 0.7 mol/L solution of FAPbBr3. The formamidinium lead bromide (FAPbBr3) perovskite thin film was fabricated via blade coating on top of the electron transport layer under ambient conditions. During deposition, the substrate was heated and then a 10–20 mL perovskite solution was dropped onto the substrate followed by blading at a constant speed of 200 cm/min. The resulting films were sequentially annealed at 150°C for 20 min. The hole transport layer was spun on the perovskite films at 3000 r/min for 30 s using a chlorobenzene (CBZ) solution, which was prepared by dissolving 73 mg of Spiro-OMeTAD into 1 mL of CBZ doped by 30 mL of 4-tert-butylpyridine, 18 mL of 520 mg/mL lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI), and 29 mL of 300 mg/mL FK209. Finally, a 70-nm-thick Au electrode was deposited via thermal evaporation. For the ST-PSCs, the Au electrode was replaced by spray-prepared silver nanowires.
Characterization
Scanning electron microscopy (SEM, Phenom Pro) was used to characterize the morphologies of the different perovskite films. X-ray diffraction (XRD, Bruker AXS D8 Advance) was used to measure the XRD spectra of the different perovskite films. The optical properties of the perovskite films were measured using a UV/vis/NIR spectrophotometer (PerkinElmer Lambda 750s). Current density–voltage (J–V) curves were measured under AM 1.5 G one sun illumination (100 mW/cm2) using a solar simulator (Oriel 94023 A, 300 W) and a Keithley 2400 source meter. The external quantum efficiency (EQE) spectra were all calibrated using crystalline silicon batteries for EQE testing.
Results and discussion
Effect of surfactants
Figure 1(a) illustrates the perovskite film preparation process via blade printing of a perovskite precursor doped with surfactants. Figure 1(b) shows a large-area semitransparent perovskite film with an area of 5 cm × 5 cm produced via doctor-blading coating with a pinhole free, flat, and highly reflective surface. We demonstrate the planar photovoltaic device architecture of FTO\TiO2\ CH(NH2)2PbBr3\Spiro-OMeTAD\Au (Fig. 1(c)) using FAPbBr3 as an absorber layer, TiO2 as an electron transport layer, and Spiro-OMeTAD as a hole transport layer.
An island-like perovskite morphology often appears in scalable printing processes. This non-ideal morphology may be related to the solution flow kinetics during the drying process of the perovskite solution [6]. At the beginning of the drying stage during coating, the perovskite and solvent react and generate micron-scale particles; these particles then begin to move and crystallize to form perovskite islands [11]. Under the influence of such a large-scale migration, it is challenging to fabricate uniform, pinhole-free perovskite films over large areas.
To suppress the solution flow to form more uniform perovskite films, we added amphiphilic surfactants to the perovskite precursor solution. Several types of surfactants were investigated to assist in the perovskite blade coating. The concentrations of the different surfactants added to the precursors were the same (0.2 mg/mL); these surfactants included l-α-phosphatidylcholine (LP), polyoxyethylene sorbitan monooleate (Tween), sodium laurylsulfonate (SLS), and hexadecyl trimethyl ammonium bromide (CTAB). Figure 2 shows SEM images of perovskite films with different surfactants. The control sample without a surfactant shows a discontinuous morphology, with huge perovskite islands of approximately 10–50 µm observed in the perovskite film. This causes exposure of the TiO2 film and the formation of an island-like perovskite film (Figs. 2(a) and 2(f)), which is not suitable for device fabrication. When a small amount of surfactant was added, the interface state of the solution system changes significantly. This is because the solution–air interface transforms into a hydrocarbon–air interface, which reduces the surface tension. From the images, we can see that all of the surfactants suppress the island structure to varying degrees. As opposed to the other samples, on the micron scale, the films formed with the addition of CTAB were uniform and pinhole free. Conversely, the films with the addition of LP/SLS/Tween presented a few holes in the surface of the grain, which may affect the optoelectronic properties of the films (Figs. 2(b)–2(e)). From the optical microscopy observations, we determined that the CTAB samples had the best morphology (Figs. 2(g)–2(j)). The color distribution of the optical microscopy images with CTAB was uniform compared to the films with LP/SLS. The dark areas at the edges of the grains were significantly reduced, implying that the surface of the film became smoother.
Fig.2 SEM images of the FAPbBr3 films fabricated via doctor blading (a) without surfactants and with (b) LP; (c) SLS; (d) CTAB; and (e) Tween surfactants. Optical microscopy images of the FAPbBr3 films fabricated via doctor blading (f) without surfactants and with (g) LP; (h) SLS; (i) CTAB; and (j) Tween surfactants |
We fabricated solar devices with films using the addition of different surfactants. Figure 3(a) illustrates the J–V characteristics of different devices prepared via blade coating with different surfactants added to the perovskite precursor solution. The values of the statistical PCE, open circuit voltage (Voc), short-circuit current density (Jsc), and fill factor (FF) for the different samples are shown in Figs. 3(c)–3(f). We can see that the improvement in the device performance primarily arises from the increase in Voc when the surfactant is added; the detailed photovoltaic results are presented in Table 1. The best device, prepared with CTAB, exhibits a PCE of 6.90% with a Voc of 1.159 V, a Jsc of 8.00 mA/cm2, and an FF of 0.74. In contrast to devices prepared with other surfactants, all the photovoltaic parameters of the CTAB-doped devices improved, especially FF and Voc. We see that the integrated Jsc values in the EQE spectra of the devices based on other surfactants were lower than those in the devices based on CTAB. In addition, the highest EQE value, approaching 80%, was obtained for a device based on CTAB doping.
Fig.3 (a) J–V curves of PSCs with different surfactants; (b) EQE spectra of the best devices prepared using different surfactants; and the effects of the addition of different surfactants on the statistical J–V parameters of the doctor-blading-printed devices: (c) Jsc; (d) Voc; (e) FF; and (f) PCE |
Tab.1 Photovoltaic parameters of PSCs prepared using different surfactants |
| surfactant | Voc/V | Jsc/(mA·cm−2) | FF | PCE/% |
|---|---|---|---|---|
| CTAB | 1.159 | 8.00 | 0.74 | 6.90 |
| SLS | 0.961 | 7.17 | 0.66 | 4.58 |
| Tween | 0.964 | 7.92 | 0.68 | 5.19 |
| LP | 1.024 | 7.97 | 0.66 | 5.41 |
Optimization of coating temperature
As opposed to other perovskite materials, FAPbBr3 has an exceptionally superior phase stability at elevated temperatures compared to its iodine counterpart. Pure FAPbBr3 becomes tetragonal at approximately 150 K and then cubic at temperatures greater than 275 K [24–26]. This indicates that, during the preparation of a perovskite film, the temperature change causes limited stability issues for the phase of the FAPbBr3 perovskite. The annealing process plays a decisive role in the crystallization process of perovskite films and enables better grain sizes and fewer defects [27]. Accordingly, we studied the effect of the annealing temperature on the growth of the film and the performance of the FAPbBr3 devices.
The main distinction between hot blading and conventional post-annealing is the presence of the solvent concentration in the film. During the hot-blading process, the annealing process is carried out simultaneously with solvent evaporation, which could yield large crystalline grains [28]. The SEM images in Fig. 4 demonstrate that the perovskite thin films produced with hot blading (150°C) were uniform, pinhole-free, and covered the entire substrate. When the annealing temperature was below 150°C (130°C/140°C), the perovskite grain size was approximately 20–30 µm, which was smaller than the grain size that annealing at a temperature of 150°C. The images also indicate that the grain boundary was reduced compared to the condition at 150°C. As the blading temperature increased, the grain size stopped increasing and many pinholes appeared at the surface. This may be because temperatures that are too high accelerate the decomposition of the perovskite films and the removal of the residue solvent from the film.
Figure 5(a) shows the UV–vis absorption spectrum of different perovskite films prepared via blade coating at different blading temperatures. We see that the preparation temperature does not change the bandgap of the perovskite material. Meanwhile, the curves indicate that the different temperatures had an obvious influence on the absorption properties of the films. The sample with a 150°C blading temperature represented the highest absorption of the samples. This may be related to the obvious increase in the grain size for the perovskite films at this temperature.
XRD was used to characterize the crystal structure of the doctor-bladed perovskite films at the various blading temperatures. The perovskite phases were very similar except for their crystallinity, as seen from the XRD patterns in Fig. 5(b). The XRD spectra of the thin films fabricated at different blading temperatures (130°C–180°C) all showed their strongest diffraction peaks at 14.83°, 21.10°, and 29.87°, representing the (001), (011), and (002) crystal planes, respectively, of perovskite films [9,29] with typical cubic crystal structures. When the blading temperature was less than 150°C, in the cases of 130°C and 140°C, the XRD spectra presented an obvious diffraction peak at 12.25°, corresponding to diffraction from PbBr2 crystals [16]. Associating with the SEM images, we attributed the diffraction peak in the perovskite structure to the incomplete reaction of FABr and PbBr2. As the blading temperature increased to 150°C, the diffraction peak at 12.25° disappeared. Meanwhile, the diffraction peaks at 14.83°, 21.10°, and 29.87° were further enhanced. This result shows that the crystallinity of a thin film can be improved at higher temperatures. When the temperature was further increased, there was a slight decrease in the XRD intensity. The reduction in the peak intensity may be caused by the higher blading temperature, which promotes the evaporation of FABr and the incomplete reaction, in turn, results in the deterioration of the FAPbBr3 film quality [6]. Therefore, a very high temperature does not present positive effects for the preparation of films. Taking the blading process into account, it is necessary to prepare high-quality FAPbBr3 films at a suitable temperature.
The specific photovoltaic parameters of the different devices prepared at different temperatures are presented in Table 2. The average PCEs were 4.57% and 4.90% for samples processed at 130°C and 140°C, respectively. The PCEs were dramatically increased to 7.29% when the temperature increased to 150°C, with a Voc of 1.175 V, a Jsc of 8.22 mA/cm2, and an FF of 0.75. Compared to the PSCs prepared at 130°C and 140°C, all the performance parameters were significantly improved. The J–V curves and EQE spectra of the PSCs are shown in Figs. 5(c) and 5(d), respectively. Further increasing the temperature to 160°C, 170°C, and 180°C, the PCEs decreased distinctly, primarily due to decreases in Jsc and Voc. As discussed above, a high temperature results in the evaporation of FABr, which can deteriorate the FAPbBr3 phase and Jsc and Voc accordingly. Therefore, we postulate that the optimum annealing temperature is 150°C.
However, the Voc values of the prepared devices compared to the bandgap and reported studies are still very low. This may be because the coverage of the perovskite film fabricated via doctor blading still contains trance amounts of pinholes, which degrade Voc [30,31]. The solvent extraction due to the heat treatment is not as efficient as the antisolvent route in spin coating. The energy band alignment may also need to be adjusted to allow efficient carrier transportation at the interface.
Tab.2 Photovoltaic parameters of blade-coated PSCs at different temperatures |
| Temperature/°C | Voc/V | Jsc/(mA·cm−2) | FF | PCE/% |
|---|---|---|---|---|
| 130 | 1.052 | 8.02 | 0.54 | 4.57 |
| 140 | 1.022 | 7.97 | 0.60 | 4.90 |
| 150 | 1.175 | 8.22 | 0.75 | 7.29 |
| 160 | 1.139 | 8.08 | 0.76 | 6.98 |
| 170 | 1.118 | 8.06 | 0.73 | 6.56 |
| 180 | 1.033 | 8.02 | 0.77 | 6.39 |
Preparation of semitransparent devices
Based on the above optimization of the blade coating of perovskite thin films, we fabricated ST-PSCs with silver nanowires; such devices are convenient and cost-effective for the preparation of semitransparent electrodes [32]. The structure of the perovskite semitransparent device was FTO\TiO2\FAPbBr3\Spiro-OMeTAD\AgNWs. The transmittance spectra are shown in Fig. 6(a); the average transmittance (AVT) (400–800 nm) of the FTO glass with a perovskite thin film was 50.83%. When a layer of Spiro-OMeTAD was deposited on top of the perovskite thin film via spin coating, the average transmittance (400–800 nm) was 51.18%. A Spiro-OMeTAD coating on a perovskite film can decrease the interface roughness [33], which enhances the transmittance of the device. We fabricated the transparent electrode by spray coating different volumes of AgNWs solutions using our reported method [2]. The average transmittance of ST-PSCs with 50 and 100 mL of AgNWs were 36.97% and 25.42%, respectively. Figure 6(b) shows the J–V characteristics of the ST-PSCs. When 50 mL of AgNWs was applied via spray coating, the average PCEs of the ST-PSCs were 4.61%. When the AgNWs volume increased to 100 mL, the average PCEs were clearly enhanced to 5.11%. The average photovoltaic parameters of the ST-PSCs are shown in Table 3. With increasing AgNWs, Jsc decreased slightly with enhanced Voc. The ST-PSCs devices with and without AgNWs exhibit high transparency, as shown in Fig. 6(c).
Fig.6 (a) UV–vis transmittance spectra of samples with different stackings of the functional layers (PSK: perovskite); (b) J–V curves of the best devices prepared with different volumes of AgNWs; and (c) optical morphologies of devices without (left) and with (right) the transparent electrode |
Tab.3 Photovoltaic parameters of PSCs prepared with different volumes of AgNWs |
| volume/mL | Voc/V | Jsc/(mA·cm−2) | FF | PCE/% |
|---|---|---|---|---|
| 50 | 1.116 | 7.67 | 0.54 | 4.61 |
| 100 | 1.052 | 8.14 | 0.60 | 5.11 |
Conclusions
Uniform, pinhole-free FAPbBr3 perovskite films were fabricated by adding surfactants to the precursor solution to improve the crystallization of the films via doctor-blade coating. In contrast to the precursor solution without an added surfactant, the addition of a surfactant effectively inhibited the production of defects and the resulting films had higher charge transfer efficiency and better uniformity. Therefore, during the large-area preparation process of perovskite, the surfactant CTAB can effectively improve the photoelectronic properties of FAPbBr3 devices. PSCs prepared using this method exhibited on average a Jsc of 8.22 mA/cm2, a Voc of 1.175 V, and a PCE of 7.29%. The resulting semitransparent devices had a PCE of 5.11% and an AVT of 25.42%. This study therefore developed a doctor-blading process to fabricate wide-bandgap FAPbBr3 films with the aim of producing high efficiency and semitransparent FAPbBr3 solar cells.