Organic photovoltaics are a promising candidate for indoor applications. Recent progresses in optimization of indoor photovoltaic materials and devices, and the key strategies to optimize the indoor photovoltaic characteristics will be discussed.

Edged-selectively fluorine (F) functionalized graphene nanoplatelets (EFGnPs-F) with a p-i-n structure of perovskite solar cells achieved 82% stability relative to initial performance over 30 days of air exposure without encapsulation. The enhanced stability stems from F-substitution on EFGnPs; fluorocarbons such as polytetrafluoroethylene are well-known for their superhydrophobic properties and being impervious to chemical degradation. These hydrophobic moieties tightly protect perovskite layers from air degradation. To directly compare the effect of similar hydrophilic graphene layers, edge-selectively hydrogen functionalized graphene nanoplatelet (EFGnPs-H) treated devices were tested under the same conditions. Like the pristine MAPbI perovskite devices, EFGnPs-H treated devices were completely degraded after 10 days. The hydrophobic properties of EFGnPs-F were characterized by contact angle measurement. The test results showed great water repellency compared to pristine perovskite films or EFGnPs-H coated films. This resulted in highly air-stable p-i-n perovskite solar cells.

Four different kinds of photovoltaic polymers were synthesized by controlling the intrachain noncovalent coulomb interactions through the incorporation of alkoxy-or alkylthio substituted phenylene, 4,7-di(furan-2-yl)benzothiadiazole and 4,7-di(thiophen-2-yl)benzothiadiazole as a building block. Fine-modulation of the interplay of dipole-dipole, H-bond and chalcogen-chalcogen interactions (O•••S, O•••H, S•••S, S•••F, etc.) along the polymeric backbone influenced the chain planarity, interchain organization, film morphology, electrical, and photovoltaic properties significantly. By replacing the alkoxy substituents with alkylthio groups, the torsional angle increased (136~168°) due to the absence of an O•••S attractive coulomb interaction (and/or increased S•••S steric hindrance), enhancing the amorphous nature with hindered interchain packing. The alkoxy-substituted polymers exhibited nanofibrillar structures, showing strong interlamellar scattering peaks up to (300) with tight face-on π-π stacking in grazing incidence X-ray scattering. The measured carrier mobility of the alkoxy-containing polymers was 1~2 orders of magnitude higher than that of the alkylthio-containing polymers. The incident-light-intensity-dependent photovoltaic characteristics clearly suggested efficient charge generation/extraction with less charge recombination for the alkoxy-containing semi-crystalline polymers. The resulting photovoltaic energy conversion efficiency of the PPDT2FBT, PPDF2FBT, PPsDF2FBT and PPsDT2FBT blended devices with PC70BM was measured to be 8.28, 5.63, 5.12, and 0.55%, respectively. This study suggests an important molecular design guideline for the further optimization of photovoltaic polymers and devices by fine-controlling the interplay of the weak noncovalent coulomb interactions.

Solution-processed bilayer organic solar cells (OSCs) with high performance are demonstrated for nonfullerene small molecular acceptors (NFAs). Unlike fullerene acceptors, NFAs show significant spectral overlap between their absorption and the photoluminescence (PL) of a polymer donor, which makes the design of an efficient exciton-harvesting bilayer heterojunction possible. Efficient exciton diffusion in the organic bilayer heterojunction is realized by long-range energy transfer between a polymer donor and NFAs. We observed efficient exciton diffusion from the polymer/ NFA bilayer heterojunctions via thickness-dependent PL quenching and timeresolved PL measurements. Despite the strongly reduced donor−acceptor interface area, a substantial density of charge-transfer states is observed for the polymer/NFA bilayer heterojunctions by electroluminescence measurements. Overall, polymer/NFA bilayer heterojunction OSCs demonstrate a power conversion efficiency of 9%−10%, which is comparable to the photovoltaic performance of bulk heterojunction OSCs, with the additional advantage of simplified microstructure formation.