Semitransparent organic solar cells (ST-OSCs) are of great interest for building-integrated photovoltaic applications and power-generating windows. So far, most efforts in this field have been devoted to the development of high-transparency electrodes. Much less attention has been devoted to optimizing the active layers, which generally consist of polymer donors and fullerene acceptors. Due to weak absorption of fullerene acceptors in the near-infrared (NIR) region, ST-OSCs exhibited relatively low power conversion efficiencies (PCEs); the best results reported in literature were 4-6% for single-junction devices and 7-8% for tandem devices.
In 2017, Zhan group proposed that an ideal active layer for a high-performance ST-OSC should consist of a narrow-bandgap donor and a narrow-bandgap nonfullerene acceptor, both of which exhibit strong NIR absorption but weak visible absorption. This kind of active layer can sufficiently utilize NIR solar irradiation to achieve high efficiency, while simultaneously maintain high visible transparency. They designed and synthesized a new NIR fused-ring electron acceptor, IHIC, with a narrow optical bandgap of 1.38 eV and absorption edge of 898 nm. The ST-OSCs based on blends of a narrow-bandgap polymer donor PTB7-Th and narrow bandgap IHIC acceptor exhibit a PCE of up to 9.77% with an average visible transmittance (AVT) of 36% (Adv. Mater. 2017, 29, 1701308, cited 51 times, ESI Hot Paper and ESI Highly Cited Paper).
Then, they designed and synthesized a fused tris(thienothiophene) unit with strong electron-donating and molecular packing properties, which is end-capped with strong electron-accepting FIC groups to afford a fused octacyclic electron acceptor (FOIC). FOIC exhibits optical bandgap of 1.32 eV and absorption edge of 942 nm, which red shifts 44 nm relative to IHIC. The as-cast ST-OSCs based on the PTB7-Th donor and FOIC acceptor without additional treatments show PCEs as high as 10.3% with an AVT of 37.4% (Adv. Mater. 2018, 30, 1705969).
In addition, they designed and synthesized three new fused-ring electron acceptors, F6IC, F8IC, and F10IC, and systematically investigated the effects of the core size on electronic properties, charge transport, film morphology, and photovoltaic properties. They found that core engineering appears to be a promising approach to maximize both short-circuit current density and open-circuit voltage at the same time (Adv. Mater. 2018, DOI: 10.1002/adma.201706571).
This research work was supported by the National Nature Science Foundation of China.
Photographs of Bo Ya Pogoda in Peking University taken with exactly the same camera settings on a sunny day: (a) without any filter; (b) with semitransparent device