Highlights of mainstream solar cell efficiencies in 2021

Solar photovoltaic (PV) technology is expected to play a key role in global carbon neutrality. The global new installed solar PV capacity over the past 20 years has grown at a compound annual growth rate of approximately 40%, far outpacing other energy sources such as coal, oil, natural gas, and wind [1]. The strong momentum of the PV industry comes from its rapidly decreased levelized cost of electricity due to technological improvement and large-scale application. The lowest bid price for large-scale PV plants around the world has been reduced to 1.04 cents/kWh in 2021 [2], and both the US and Chinese governments have announced in 2021 that the PV power generation will become the biggest power source in 20–30 years with a proportion of approximately 40% of the total power generation. It is encouraging to see that much progress has been made every year in solar cells and this editorial highlights the certified power conversion efficiency (PCE) in 2021 of three mainstream (silicon, perovskite and organic) solar cells.

Silicon solar cells

Among the PV products, crystalline silicon (c-Si) solar cells have been the leader for 40 years and now have over 95% of the market share due to the advantages of mature industry, low manufacturing cost, and high material reliability. Si passivated emitter and rear cell (PERC) dominates the current PV market since its conception in 1989 [3]. The mass-production averaged PCE of p-Si PERCs approximated to 23.1% in 2021 with the world record PCE of 24.06% achieved in 2019. As the PERC is gradually approaching its PCE limit, the research hotspots in the c-Si community are focused on n-Si heterojunction (n-SHJ) and both n-Si and p-Si tunnel oxide passivated contact (n-TOPCon, p-TOPCon) solar cells. Si heterojunction (SHJ) solar cell has been maintaining the world’s highest efficiency in the field of c-Si solar cells due to its effective carrier selective contacts and heterojunction interface characteristics. The current world record PCE of single-junction c-Si solar cells is 26.7%, employing the SHJ structure with interdigitated back contact (IBC) electrodes [4]. Tunnel oxide passivated contact (TOPCon) solar cell, proposed in 2013 by using a passivated structure of ultrathin SiO_x and doped polycrystalline Si to achieve low recombination carrier selective contacts, has reached a certified PCE of 26.1% in combination with the IBC architecture in 2019 [5].

The efficiency difference of over 2.0% is the engine of the PV industry revolution from PERC to SHJ and TOPCon solar cells. Table 1 lists the achievements of the SHJ and TOPCon solar cells during 2021. The SHJ solar cell is based on the concept of hydrogenated amorphous silicon (a-Si:H)/c-Si heterojunction, which is prepared at a low temperature (<200°C). From Table 1, it can be found that the best certified efficiency of SHJ solar cells has reached 26.30% by Longi. As is known, the PCE of a solar cell is related to the open-circuit voltage (V_OC), short-circuit current density (J_SC), and fill factor (FF). The intrinsic a-Si:H (a-Si:H(i)) inserted between the c-Si and doped a-Si:H passivates the surface dangling bonds of c-Si effectively and provides the possibility of a higher V_OC for SHJ solar cells [6]. The listed V_OCs are higher than 746 mV, the best ones even more than 750 mV, which indicates their good passivation. To achieve a good passivation, the structure of a-Si:H(i) layer should be optimized. Actually, a bilayer of a-Si:H(i), i.e., an ultra-thin (0.5–1.0 nm) but porous buffer a-Si:H(i) with a high hydrogen content capped with a dense a-Si:H(i) layer, is beneficial to V_OC [7]. The champion SHJ solar cell in 2021 should have employed such an a-Si:H(i) bilayer.

Tab.1 Summary of the achievements of SHJ and TOPCon solar cells in 2021

Solar cell type	PCE/%	Area/cm2	VOC/mV	JSC/(mA·cm−2)	FF/%	Test center	Report date	Description
n-SHJ	25.20	252.1	747.0	39.31	85.82	TüV Nord	Feb.	GS-Solar Back junction
n-SHJ	25.26	244.55	748.5	39.48	85.46	ISFH	Jun.	Longi Back junction
n-SHJ	25.23	274.5	746.2	40.00	84.64	ISFH	Jul.	Huasun/Maxwell Back junction
n-SHJ	25.54	274.5	746.0	40.23	85.08	ISFH	Sept.	SunDrive/Maxwell Back junction Cu plating
n-SHJ	25.82	274.5	750.4	40.20	85.57	ISFH	Oct.	Longi Back junction μc-SiOx:H a-Si:H bilayer
n-SHJ	26.30	274.3	750.2	40.49	86.59	ISFH	Oct.	Longi Back junction μc-SiOx:H a-Si:H bilayer
n-TOPCon	24.90	235.80	712.8	41.67	83.84	ISFH	Jan.	Jinko LPCVD
n-TOPCon	25.09	242.77	719.8	41.58	83.83	ISFH	Apr.	Longi LPCVD
p-TOPCon	26.00	4.00	732.3	42.05	84.30	Fraunhofer ISE	Apr.	Fraunhofer ISE PECVD
n-TOPCon	25.80	4.00	724.1	42.87	83.10	Fraunhofer ISE	Apr.	Fraunhofer ISE PECVD
n-TOPCon	25.25	267.40	–	–	–	NIM	May	Jinko LPCVD
n-TOPCon	25.21	242.97	721.6	41.64	83.90	ISFH	May	Longi LPCVD
n-TOPCon	25.23	247.57	721.4	41.75	83.78	ISFH	Jun.	Jinko LPCVD
p-TOPCon	25.02	–	–	–	–	ISFH	Jun.	Longi
p-TOPCon	25.19	–	–	–	–	ISFH	Jul.	Longi
n-TOPCon	25.40	330.15	–	–	–	NIM	Sept.	Jolywood PVD
n-TOPCon	25.41	235.79	719.1	42.24	83.66	ISFH	Nov.	Jinko LPCVD
n-TOPCon	25.53	3.99	700.7	43.04	84.64	FMI	Nov.	NIMTE-CAS PECVD

Notes: –: Not yet disclosed. TüV Nord: Technischer überwachungs Verein Nord, Germany; ISFH: Institut fur Solarenergieforschung Hameln, Germany; Fraunhofer ISE: The Fraunhofer Institute for Solar Energy Systems, Germany; NIM: National Institute of Metrology, China; FMI: Fujian Metrology Institute, China; NIMTE-CAS: Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences.

The efficiency of a SHJ solar cell is limited mainly by the J_SC losses in the amorphous layers [8]. The J_SC losses has been recognized as the optical losses, which include ① parasitic absorption in the short wavelength region (<500 nm) by the a-Si:H layers on the front side resulting in a reduction of J_SC by approximately 1.5 mA/cm², and ② refractive index (n, at 632 nm) mismatch among c-Si (n~3.8), a-Si:H (n~4.0), and the transparent conducting oxide (TCO) (n~3.8) at the front which may result in some optical reflection losses. To solve this issue, microcrystalline silicon (μc-Si:H) thin film has been utilized as the carrier-selective layers. The application of μc-Si:H film is helpful to improve J_SC because of the reduced parasitic absorption and less refractive index mismatch. The μc-Si:H film can reduce the parasitic absorption at a short wavelength region due to its higher optical band gap (E_g~2.0 eV) compared to the a-Si:H counterpart. The refractive index of μc-Si:H film (n~3.4) is lower than that of a-Si:H, although depending on the crystalline fraction and doping centration. Moreover, a higher optical band gap (E_g≅ 2.9 eV) and a lower refractive index (n≅ 2.8) can be achieved from the oxygen-alloyed μc-Si:H (or, μc-SiO_x:H) layer, which can significantly enhance the optical transparency and reduce the optical absorption at the front side of the SHJ solar cells. The J_SCs above 40 mA/cm² listed in Table 1 are beneficial to the utilization of μc-SiO_x:H window layers.

In addition to the improved performance of TCO layers and fine printing, the application of doped μc-Si:H or μc-SiO_x:H is also related, to a certain extent, to the achieved high FFs in SHJ solar cells in 2021 (nearly all over 85% with an extremely high value of 86.59% for the 26.30% champion one). The μc-Si:H or μc-SiO_x:H has a higher doping efficiency than a-Si:H, and the dark conductivity can be higher by more than two orders of magnitude than that of doped a-Si:H [8]. The higher doping efficiency also leads to lower series resistances and an adequate μc-Si:H (or μc-SiO_x:H)/TCO electronic contact. The carrier transport properties can be improved because of the better conductivity, lower series resistances, and lower contact resistivity with TCO, and hence a higher FF of the SHJ devices.

Another breakthrough in the 2021 SHJ research and development was the exploration of copper (Cu) plating. The screen-printing of low temperature silver paste is commonly employed because of the low temperature processing for SHJ solar cells. However, the low temperature silver paste utilized in the SHJ solar cell accounts significantly for about 30% of the total processing cost due to its large consumption. Reducing the use of silver paste is key for low-cost SHJ solar cells and Cu plating is of great interest and regarded as an ideal alternative electrode solution and industrially proven technology [9]. A 25.54% efficiency large-area SHJ solar cell featured by using Cu plating was achieved by SunDrive/Maxwell in 2021, indicating the possibility of replacing silver screen-printing by Cu plating for SHJ solar cell metallization. Nevertheless, the obstacles such as not cost-effective processes, complicated electroplating steps, long-term degradation and reliability are to be resolved before Cu plating can be industrially used [9].

It should be noted that these breakthroughs in 2021 were made in the certified PCEs with large-area solar cells, paving the way for rapid mass-production of advanced SHJ solar cells in the averaged PCE of 25%–26%. To pursuit a higher efficiency, the interest in industry begins to move to the heterojunction back contact (HBC) solar cell, a combination of SHJ structure with IBC electrodes, holding the present world record efficiency of 26.7% in c-Si solar cells [4]. The HBC solar cell is featured by no grids in the front side, which can improve J_SC distinctly, while the good passivation and thus high V_OC characteristics can be maintained because of the SHJ structure. Moreover, the HBC configuration can make it possible to replace the high-cost TCO (normally indium tin oxides) layer in the front side and apply the low-cost ZnO:Al (AZO) in the back side. Cheap materials such as Cu and Al can also be used as the grids since there is no shadow issue and the grids on the back side can be wide enough to satisfy the conductivity. The HBC solar cell may be a good choice for cutting the cost of the SHJ solar cell to the PERC solar cell level, while increasing the efficiency up to 27%–28%. For the further development of SHJ solar cells, the trend is clear to investigate the perovskite/SHJ tandem solar cell to achieve an efficiency of over 30%.

On the other hand, thanks to their compatibility with the PERC solar cell process, TOPCon solar cells are considered as the forthcoming technology to PERC counterparts. As can be seen in Table 1, the PCE of TOPCon solar cells was enhanced by over 25% in both laboratory and industry in 2021. The researchers from Fraunhofer ISE have successfully reached a high PCE of 26.0% in laboratory on the front and back contact solar cells with TOPCon structure on p-type silicon substrates by using plasma enhanced chemical vapor deposition (PECVD), while the champion PCE of 25.41% in industry has been achieved by Jinko on n-type silicon substrates due to the high-quality n-type polysilicon passivating contacts formed by low pressure chemical vapor deposition (LPCVD) [10]. It is worth noting that the silicon substrates which were used for fabricating the world record TOPCon solar cells are p-type wafers [5]. The p-TOPCon solar cells with the highest PCE treated tunneling oxide and polysilicon stack layers as emitters rather than collectors in mass-production, indicating a potential for the application of p-type Si/tunneling oxide/poly-Si(n) structure. Longi has attempted to improve the quality of p-type silicon substrates and succeeded in reaching 25.19% on p-type wafers. However, the PCE of p-TOPCon is greatly sensitive to the quality of p-type silicon substrates. That is the reason why the p-type silicon wafers cannot be available for fabricating well-performance TOPCon solar cells in mass-production.

Although the efficiency of n-type silicon-based TOPCon solar cells gradually approaches the world record PCE kept by SHJ solar cells, there are still several barriers expected to be overcome during mass-production. The first difficulty in mass-production is the uniformity of tunneling oxide and polysilicon stack layers, which determines the surface recombination and charge carrier transport quality on the rear side, especially for large-area solar cells. The polysilicon passivating contact property highly depends on the polysilicon layer deposition technique including LPCVD, PECVD, and physical vapor deposition (PVD). LPCVD polysilicon layers take advantage of the high uniformity and low process time but severely suffer from the overwrapping issue, which needs an additional wet chemical process to remove the polysilicon on the other side. PECVD and PVD polysilicon layers can reach the same passivation quality and carrier transport characteristics as the LPCVD one but are poor at the uniformity and process stability [11]. Therefore, the next step for industry is mainly to optimize any of the processes to achieve a low-cost and high-quality polysilicon layer.

The second restriction to the PCE improvement of n-TOPCon solar cells is the screen-printing metallization process, which moderately affects the series resistance and contact recombination [12]. Unlike PERC solar cells, the requirement of the silver paste for the rear passivating contacts is to partially etch the polysilicon layers and not to directly contact the silicon substrates. However, to avoid the parasitic absorption emerged from heavily-doped polysilicon layers, the thickness of polysilicon layers has to be kept under about 120 nm, which elevates the difficulty in controlling the etching depth during the firing process. A proper firing process that can realize a perfect passivating contact metallization without any passivation degradation assists in reaching a high level PCE of n-TOPCon solar cells. The optimization of the firing process requires suitable silver paste ingredients, firing temperature, and even uniformly firing equipment, to meet the need of large-area solar cells.

Besides the uniformity of polysilicon passivating contacts and screen-printing metallization process, the parasitic electrical and optical loss should be avoided to the greatest extent. According to the loss analysis of the certified n-TOPCon solar cells with PCE of 24.79% fabricated by Jinko in 2020 [13], about 1.19 mW/cm² power loss occurs from the front sides, including the electrical loss from weak passivation of metal contacts regions and lateral hole transport as well as the optical loss from front side finger shadings. In addition, about 0.38 mW/cm² power loss is contributed by the silicon substrates due to the bulk recombination and carrier transports while about 0.27 mW/cm² power loss attributes to rear side polysilicon passivating contacts. The feasible solution to dealing with the main loss is to introduce the selective emitter technology to front side metal contacts, descending the recombination at the metal contacts region and narrowing the finger size to enhance the effective optical absorption. However, the selective emitter techno-logy in TOPCon solar cells is still immature by the reason of the lack of a proper laser system. It can be predicted that a PCE improvement of about absolutely 1% could be realized in mass-production after the success in application of the reliable selective emitter technology.

Above all, a lot of work has still to be done to assist PCE of n-TOPCon solar cells in reaching a higher level, though an averaged PCE of 24% in mass-production have been realized by many PV companies. From the point of process in mass-production, the uniformity of polysilicon passivating contacts and metallization still require further optimization including the equipment and process recipes. The introduction of the selective emitter technology can also be remarkably helpful to improve the PCE of TOPCon solar cells only if proper lasers can be put into use. Anyway, n-type silicon-based TOPCon solar cells will gradually substitute p-Si PERC solar cells to dominate PV markets, and the PCE of n-TOPCon solar cells can be improved up to an averaged 26% in mass-production in 3–5 years.

Perovskite solar cells (PSCs)

Metal halide perovskites possess the general formula of ABX₃, where A is a monovalent inorganic or organic cation, B is a divalent metal cation, and X is a halide anion. These materials exhibit unique optoelectronic properties and a high photon-to-electron conversion efficiency. They can also be easily processed at a relatively low-cost. Therefore, they are ideal for PV applications such as solar cells and large-scale modules. Moreover, the broad range and high tunability of the bandgap of perovskites endow them suitable candidates as subcells in a multi-junction tandem architecture, which holds good potential on achieving a higher device efficiency by overcoming the Shockley-Queisser limit of 33.7% that is posed for single-junction solar cells.

As listed in Table 2, the year of 2021 has witnessed many encouraging achievements on pushing the PCE of single-junction lead halide-based PSCs [14,15]. A new certified record has been set in December by Ulsan National Institute of Science and Technology (UNIST) on a thin-film solar cell efficiency of 25.7%, a value that has exceeded the performance of many other thin-film solar cells and is one step closer to that of c-Si solar cells [16]. Compared to the parameters of the previous record efficiency of 25.5% set by the same institution [17], the differences on V_OC (from 1.189 V to 1.179 V) and J_SC (from 25.68 mA/cm² to 25.8 mA/cm²) are not distinct. However, there is a remarkable improvement on FF from 83% to 86.4%, which is close to the theoretical maximum FF for such a V_OC value and suggests a highly efficient charge extraction and transport inside the solar cell. Another record set for single-junction lead halide PSCs in 2021 is on minimodules. A certified PCE of 21.4% was achieved by Microquanta with a designated illumination area (da) of 19.32 cm² [15,18]. For the other record-holders of single-junction PSCs, the state-of-the-art quantum dot solar cell (QDSC) comes from UNIST with PCE reported in late 2020 as 18.1% [14]. Technical details for the fabrication process of this QDSC have not yet been released, except for the fact that perovskite with BHT (full name not disclosed) additive was used as the QD material. Meanwhile, for the certified record set on large-area (da over 1 cm²) PSCs, the strategy from the research group in Australian National University was revealed in January [19]. A nanopatterned electron transport layer was coupled with the polymer passivation layer to retain both high values of V_OC (1.18 V) and FF (84.4%), thereby leading to a high PCE of 22.6%[15].

Tab.2 Summary of the achievements of single-junction and multi-junction perovskite-based solar cells and submodules in 2021

Solar cell type	PCE/%	Area/cm2	VOC/V	JSC/(mA·cm−2)	FF/%	Test center	Report date	Description
Lead halide perovskite (thin-film)	25.7	0.096 (ap)	1.179	25.8	86.4	Newport	Dec.	UNIST
Lead halide perovskite (minimodule)	21.4	19.32 (da)	1.149	23.4	79.6	JET	Oct.	Microquanta
Sn-based perovskite	14.63	0.04 (da)	0.91	20.6	77.1	SIMIT	Jul.	ShanghaiTech University
Perovskite/perovskite tandem	26.4	0.049 (da)	2.048	16.54	77.9	JET	Feb.	Two-terminal, NJU
Perovskite/perovskite tandem (minimodule)	21.7	20.25 (da)	2.009	14.22	75.9	JET	Aug.	Two-terminal, NJU
Perovskite/silicon tandem	29.8	1.016 (ap)	Not yet disclosed			Fraunhofer ISE	Nov.	Two-terminal, HZB

Notes: ap: aperture area; da: designated illumination area. JET: Japan Electrical Safety and Environment Technology Laboratories; SIMIT: Shanghai Institute of Microsystem and Information Technology, China; Fraunhofer ISE: Fraunhofer Institute for Solar Energy Systems, Germany.

Although lead is currently the most prevailing choice as B-site cation, the associated toxicity issues and environmental concerns have impelled the development of PSCs based on lead-free compositions, such as replacing Pb²⁺ with cations like Sn²⁺, Ge²⁺, Cu²⁺, etc. Among these choices, tin exhibit similar outer electronic structure, ionic radius, and decent semiconductor characteristics to Pb. The use of Sn-based perovskite in PSCs has shown its good potential as a less-toxic alternative. The current record on certified efficiency of lead-free PSCs is 14.63% [20], which was set in July by researchers from ShanghaiTech University. From the perspective of regulating the growth of perovskite film, highly coordinated SnI₂·(DMSO)_x adduct was synthesized and used to prepare Sn-based films with a uniform crystal orientation and a homogeneous structure. The resulted tin-halide PSCs demonstrate an enhanced diffusion length, an improved stability and a record-high PCE.

Much exciting progress has also been made in perovskite-based multi-junction tandem solar cells and submodules. For the construction of two-terminal all-perovskite tandems, lead halide perovskite with the bandgap tuned to the range of 1.65 eV to 1.8 eV can be exploited as a wide-bandgap top cell. For the candidates of the narrow-bandgap bottom cell, the composition of mixed Sn-Pb perovskites can realize a bandgap as low as 1.2 eV, which is suitable for the tandem structure so long as the oxidation issue of Sn²⁺ is well addressed during fabrication. In 2021, one record on small-area device and one on minimodule were reported in February and August, respectively [14]. Both records were achieved from the same research group in Nanjing University (NJU). The confirmed PCE was 26.4% for an area of 0.049 cm², and 21.7% for a 20 cm² minimodule. Moreover, the up-to-date record on 1 cm² large-area PSCs is also hold by this group [21]. A PCE value of 24.2% was reported in late 2019, with the methods published in late 2020. The high efficiency of the tandem cell was established upon the high PCE (certified 20.7%) of the narrow-bandgap bottom cell. In detail, for this subcell of mixed Sn-Pb perovskites, surface-anchoring zwitterionic antioxidant was utilized to inhibit Sn²⁺ oxidation, passivate defects, and improve the homogeneity.

In light of the more-mature development of PV technologies based on silicon and Cu(In, Ga)(S, Se)₂ (CIGS), pairing perovskite-based subcells with these narrow-bandgap solar cells has also shown a great potential on the performance of tandem devices. The bandgap is as narrow as 1.1 eV for silicon and approximately 0.98 eV for copper indium selenide. The highest PCE of perovskite-based tandem solar cells is achieved by the structure of perovskite/silicon [22,23]. A record value of 29.8% on a 1 cm² area was reported in November from Helmholtz-Zentrum Berlin (HZB) [16,22]. Detailed parameters for this PCE update have not yet been disclosed. For the architecture of perovskite/CIGS tandem, the current record value for a 1 cm² solar cell is 24.2%, which was reported from the same institution in early 2020 [15,16,24].

In all, with collaborated research efforts from academia and industry, many milestones have been achieved in 2021 on the performance of perovskite-based solar cells and submodules, including both single-junction devices and multi-junction perovskite tandems. For better development and commercialization of PSCs, there are several crucial aspects that need to be addressed, such as the improvement on device stability, the reduction of the gap between the solar cell and the module, and the considerations of cost and environmental impact. Nevertheless, it is believed that these challenges will be tackled in the near future, and perovskite-based solar cells and modules will have a promising prospect in the PV field.

Organic solar cells (OSCs)

OSCs based on bulk heterojunction (BHJ) blends have attracted much attention as a renewable energy technology due to the advantage of low-cost, flexibility, and semitransparency. Currently, the non-fullerene acceptor (NFA) based OSCs afford the highest efficient of about 20%. In the photoactive layer, the crystallization of both donor and acceptor leads to a complex interpenetrating crystalline network with a refined phase separation to balance the exciton dissociation and charge transport, giving rise to a high FF. The extended absorption from NFA toward the near infrared wavelength significantly enhances the light harvest, contributing to the rapid increase in J_SC. The low energy loss guarantees a high V_OC. These characteristics in combination enhance the device PCE, showing a potential for the commercial application in the near future.

Important progresses in 2021 have been listed in Table 3. For single-junction OSCs, the chemical and physical modification based on the Y6 acceptor blends induces a high device efficiency. At the beginning of 2021, the researchers from Shanghai Jiao Tong University (SJTU) and University of Massachusetts (UMass) proposed a quaternary physical blending concept, using PM6/PM7 as the dual donors and Y6/PC₇₁BM as the dual acceptors to construct double cascading charge transport pathways. The better energetics regulated by PC₇₁BM molecules and well-defined multi-length-scale morphology induced by PM6/PM7-Y6 crystallization yielded a recorded PCE of 17.4% for a device area of 0.032 cm², with simultaneous improved V_OC, J_SC, and FF [25]. It is the highest efficiency in the Best Research-Cell Efficiency Chart of National Renewable Energy Laboratory (NREL) that has been reported in publication in 2021. An efficiency of 17.5% for single-junction OSCs was obtained by City University of Hong Kong (City U HK) and University of Washington (UW), but detailed parameters for this PCE update have not yet been disclosed. In May, the research group in Beihang University (BUAA) reported a new acceptor named L8-BO by introducing branched alkyl chains to the thiophene β-position of Y6 molecules, which is an interesting spot that the pi-pi stacking occurs nearby. Such design helps to build an optimized, multi-length-scale morphology where high carrier generation, low charge recombination, and balanced transport are achieved. The single-junction device based on L8-BO showed a high PCE of 17.9% with a remarkably high FF of 81.0%, certificated by National Institute of Metrology (NIM) [26]. A certificated PCE of 18.2% for single-junction OSCs from SJTU and BUAA was declared in the Best Research-Cell Efficiency Chart of NREL and included in Solar Cell Efficiency Tables (Version 57–59) [15,27,28]. This record is still the best result in NREL Best Research-Cell Efficiency Chart. The device showed a high V_OC approaching 0.90 V and a J_SC of 25.72 mA/cm². In August, the researchers from Institute of Chemistry, Chinese Academy of Sciences (IC-CAS), reported a new designed large-bandgap donor PBQx-F and low-bandgap NFA eC9-2Cl. Cooperated with an NFA named F-BTA3 as the third component to construct the ternary OSC, a PCE of 18.7% was obtained with an area of 0.062 cm², certificated by NIM, which is the highest efficiency of single-junction OSCs reported in publication [29]. On the other hand, all polymer solar cells (APSCs) have attracted much attention in 2021, owing to the good stability and mechanical stability of the devices. The research group from Wuhan University (WHU) developed the PYT polymeric acceptor family. The ternary APSCs based on PM6:PYT:PY2F-T showed a PCE of 16.9% certificated by NIM, which is the highest efficiency of APSCs. Meanwhile, the device showed an impressive long-term light stability, that after 2000 h of illumination, 90.5% of its initial PCE maintained [30,31].

Tab.3 Summary of the achievements of organic solar cells with single and multiple junctions in 2021

Solar cell type	PCE/%	Area/cm2	VOC/V	JSC/(mA·cm−2)	FF/%	Test center	Report date	Description
Organic (thin film)	17.4	0.032	0.862	25.83	78.0	NREL	Jan.	SJTU/UMass
Organic (thin film)	17.5	–	–	–	–	NREL	–	City U HK/UW
Organic (thin film)	18.2	0.032	0.896	25.72	78.9	NREL	–	SJTU/BUAA
Organic (thin film)	17.9	0.032	0.871	25.38	81.0	NIM	May	BUAA/SJTU
Organic (thin film)	18.7	0.062	0.878	26.78	79.4	NIM	Aug.	IC-CAS
All-polymer (thin film)	16.9	0.051	0.894	24.76	76.5	NIM	Apr.	WHU
Organic (tandem)	20.0	0.022	2.019	12.97	76.3	NIM	Dec.	IC-CAS

Notes: – Not yet disclosed.

Much exciting progresses have also been made in multi-junction tandem solar cells recently. The tandem cell strategy is confirmed to be an effective way to improve the PCE, where the series-connected subcells can significantly reduce the electrical loss, and by manipulating the band structure of each subcell, a wide and efficient absorption could be achieved in this stacking mode, overcoming the thickness constrain of single-junction OSCs due to the low mobilities of organic semiconductors. The last breakthrough came from Nankai University in 2018 [32]. PTB7-th:O6T-4F:PC₇₁BM was used as the front cell while PBDB-T:F-M as the rear cell, with a thickness of 200 and 120 nm, respectively. The optimized two-terminal tandem cells gave a PCE of 17.29% with a V_OC of 1.640 V, a J_SC of 14.38 mA/cm² and an FF of 73.33%, certificated by National Center of Supervision and Inspection on Solar Photovoltaic Products Quality (CPVT). Recently, the researchers from IC-CAS reported an interconnecting layer, e-TiO_1.76/PEDOT:PSS, which showed a neat interface, a high conductivity, suitable energy levels, and a low Schottky barrier to match the subcells in the tandem device. PBDB-TF:BTP-eC9 was used as the front cell, PBDB-TF:GS-IOS as the rear cell and e-TiO_1.76/PEDOT:PSS as the interconnecting layer to construct the tandem solar cell, where a PCE of 20.0% certificated by NIM is the first OSCs with an efficiency of over 20% [33].

To conclude, 2021 is a prospectus year for the field of OSCs. Significant progresses have been made in all aspects, including increased efficiency in both single and multiple junction OSCs and device stability, and even large-scale module fabrication. For the better development and commercialization, the stability and costs of the OSCs need to be further emphasized. Meanwhile, it is advocated here that green solvents be used while striving for a higher efficiency to avoid the threat of toxic solvents to human beings and environment. Last but not least, more comprehensive evaluation criteria need to be established to guide the material and technology development and suit more demanding challenges for application.

Acknowledgments

This work was supported by the Major State Basic Research Development Program of China (Nos. 2018YFB1500501 and 2020YFB1505502) and the National Natural Science Foundation of China (Grant Nos. 11834011, 11974242, 22025505, 51973110, and 21734009).

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