Organic photovoltaics (OPV) with bulk heterojunction photoactive layers have recently reached a certified power conversion efficiency of 14.9% for single junction solar-cells¹. These high efficiencies have been obtained using, most of the time, fluorinated polymers² as electron-donor material and non-fullerene acceptors (NFAs³) as electron-acceptor material and represent a significant step to prove the commercial interest of OPV.

Besides high efficiency, scaling up of OPV lab-scale devices into larger area modules free from scarce materials (e.g. indium) and fabricated without the use of halogenated solvents and/or harmful additives needs to be demonstrated to approach market-readiness. We recently published photovoltaic results obtained with an electron-donating fluorinated copolymer⁴ together with a fullerene derivative, demonstrating an efficiency of more than 10% on 12 mm² photovoltaic devices prepared from hot o-DCB solution and using an Indium-containing electrode. In the present study, non-halogenated solvents and harmless additives have been employed together in order to reach similar efficiencies on lab-scale and ITO-containing devices. Furthermore, we could successfully transfer the fabrication process for the realization of small scale ITO-free devices with 7.8% efficiency, in which the photoactive layer was deposited from an o-Xylene/p-Anisaldehyde mixture. The devices contained substrate-sided opaque metal electrodes and a metal grid electrode on top of the cell stack, similar to an architecture we have published before⁵. After a scale-up of the fluorinated-polymer synthesis, ITO-free OPV modules with an active area of 66 cm² and power conversion efficiency above 6% have been manufactured. By employing a shunt-proof opaque electrode, no voltage losses have been observed after monolithic serial interconnection of the 15 individual cells, which could have been caused e.g. by reduced parallel resistance due to local coating defects. Currently, further experiments with other non-halogenated solvents and additives to further increase the power conversion efficiency are performed.

References
1.    J. Yuan, et al. Joule (2019, in press).
2.    N. Leclerc, et al. Polymers 8, 11 (2016).
3.    P. Cheng, et al. Nat. Photonics 12, 131–142 (2018).
4.    O. A. Ibraikulov, et al. J. Mater. Chem. A 6, 12038–12045 (2018).
5.    S. B. Sapkota, et al. Sol. Energy Mater. Sol. Cells 130, 144–150 (2014).