Large-area organic solar cells

Min Li; Jilin Wang; Liming Ding; Xiaoyan Du

doi:10.1088/1674-4926/43/6/060201

Organic solar cells (OSCs) have made significant progress due to the fast advances in nonfullerene acceptors (NFAs) since 2015^[1-7]. The power conversion efficiency (PCE) for small-area single-junction OSCs is around 19% with an active area <0.1 cm ^2[8-11]. Scalability is a key factor in developing this technology. When scaling lab cells to large-area modules, the device performance might drop. Brabec et al. proposed a stage–gate process for OSCs from R&D effort to commercialization, which includes materials development, processing, prototyping, pilot process and upscaling^[12]. Lab-to-fab transfer involves processing environment (glovebox or ambient air), coating technique (spin coating or scalable techniques), device size (<1, 1–200, or >200 cm²), device type (cells or modules), top electrode (evaporated or printed) and solvent (halogenated or green). To visualize the upscaling losses, representative high-efficiency lab cells with small active areas and those single cells and modules with active area over 1 cm² reported from 2018 to 2021 are summarized in Fig. 1(a) and Table 1. In addition, the records in NREL’s Champion Module Efficiencies Chart are also included for comparison^[13]. The PCE usually declines with the increase of the active area of the devices.

Figure 1.(Color online) (a) The PCEs for lab cells with small areas and for devices and modules with active area over 1 cm² (2018–2021); the records in NREL’s Champion Module Efficiencies Chart are included for comparison. (b) The chemical structures for the active materials. (c) Schematic for the device structure and J–V curves for 18 cm² module with a PCE of 14.45% (certified 13.98%). Reproduced with permission^[14], Copyright 2020, Elsevier. (d) 25.21 cm² module; J–V curve changes with number of subcells. Reproduced with permission^[15], Copyright 2021, Wiley. (e) Schematic for the blade-coating process and J–V curves for modules with different active layers. Reproduced with permission^[16], Copyright 2021, Springer Nature.

The representative high-performance solar modules with active area over 1 cm² are presented in Fig. 1. The chemical structures for the photoactive materials are shown in Fig. 1(b). In 2020, Huang et al. designed a nonfullerene acceptor named DTY6, and PM6:DTY6 module (area 18 cm²) gave a PCE of 14.45% (certified 13.98%) (Fig. 1(c))^[14]. Very recently, Zhou et al. found that the long side chain of BTP-eC9 inhibited excessive aggregation when processed with chlorobenzene (CB), and PM6:BTP-eC9 module offered a PCE of 14.07% with a wide processing window (area 25 cm²) (Fig. 1(d))^[15]. Liet al. reported a Y6 derivative named BTO, and PM6:Y6:BTO:PC₇₁BM module gave a PCE of 14.26% (area 36 cm²), which was realized without thermal annealing and with non-halogenated processing solvent. It is the record for OSC modules with an active area exceeding 20 cm² (Fig. 1(e))^[16].

The efficiency loss for solar modules relates to the change of processing method and increase of area^[17]. Spin-coating is a common method for making lab cells, but not suitable for making large-area modules^[18]. Slot-die coating is one of the most practical coating techniques for solar modules. In 2020, Wei et al. used slot-die coating to make PTB7-Th:CO_i8DFIC:PC₇₁BM ternary devices, achieving PCEs of 12.16% and 10.09% for 1 and 25 cm² flexible devices, respectively (Fig. 2(a))^[19]. Environmental-friendly manufacturing requires non-toxic solvents, also leading to PCE loss due to limited solubility of the active materials^[20]. Side-chain engineering is an effective approach to optimize active-layer morphology and attain high-performance organic photovoltaic (OPV) modules.

Figure 2.(Color online) (a) Schematic for spin coating and slot-die coating (top) and J–V curves for single cells and modules with different active area (bottom). Reproduced with permission^[19], Copyright 2020, Wiley. (b) Photos and J–V curves for 26 cm² module (PCE 12.6%) and 204 cm² module (PCE 11.7%) (top) certified by Fraunhofer ISE. Reproduced with permission^[12], Copyright 2020, Wiley. PCE changes with active cell width. Reproduced with permission^[21], Copyright 2020, Wiley. (c) Schematic for the device structure; J–V curves change with the number of subcells (module area is up to 54 cm²). Reproduced with permission^[23], Copyright 2021, Wiley.

The geometrical fill factor (GFF), defined as the ratio of active area and module area, needs to be considered when evaluating the upscaling loss in PCE. In 2020, Egelhaaf et al. optimized laser patterning parameters and the number of cells in the module for high GFF with minimum PCE loss. The certified PCEs were 12.6% for 26 cm² module and 11.7% for 204 cm² module. The GFF was over 95% (Fig. 2(b))^[21]. Before this report, the record efficiency 8.7% for OPV module (802 cm²) was demonstrated by Toshiba.

For large area and low cost, all functional layers should be processed via solution coating. PEDOT:PSS is commonly used with metal grid as electrode, but its transmittance is weak in the spectral region over 600 nm, which is of great importance to the absorption of the active layer^[22]. Recently, Zhou et al. made a solution-processed composite electrode AgNWs:PEI-Zn and it presented low roughness, high transmittance and good thermal stability. A 54 cm² flexible module with a 13.2% PCE was obtained (Fig. 2(c))^[23]. However, the top MoO_x/Ag layer was still thermally evaporated. In 2019, evaporation-free flexible OSC modules offered a PCE of 5.25% on an active area of 80 cm^2[24]. In 2021, Egelhaaf et al. developed a printable silver-nanoparticle (AgNP) film as top electrode and achieved a similar performance as the evaporated ones. The maximum PCE for 4 cm² modules with AgNP electrode was 7%^[25]. Great efforts are still needed to realize all-solution processed efficient OPV modules.

In short, large-area OSCs have made inspiring advances with the development of novel materials and processing techniques. Efforts are needed to improve the scalability of OSCs and achieve large-area, low-cost, environmental-friendly and all-layer-printed modules.

X. Du thanks National Natural Science Foundation of China (52103222), Natural Science Foundation of Shandong Province (ZR2021QA009), Taishan Scholar Foundation of Shandong Province (tsqn202103016) and Qilu Young Scholar Program of Shandong University. L. Ding thanks the National Key Research and Development Program of China (2017YFA0206600), the National Natural Science Foundation of China (51922032 and 21961160720), and the open research fund of Songshan Lake Materials Laboratory (2021SLABFK02) for financial support.

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