A series of wide-bandgap (WBG) copolymers with different alkyl side chains are synthesized. Among them, copolymer PBT1-EH with moderatly bulky side chains on the acceptor unit shows the best photovoltaic performance with power conversion efficiency over 10%. The results suggest that the alkyl side-chain engineering is an effective strategy to further tuning the optoelectronic properties of WBG copolymers.

A new donor (D)-acceptor (A) conjugate, benzodithiophene-rhodanine-[6,6]-phenyl-C 61 butyric acid methyl ester (BDTRh-PCBM) comprising three covalently linked blocks, one of p-type oligothiophene containing BDTRh moieties and two of n-type PCBM, is designed and synthesized. A single component organic solar cell (SCOSC) fabricated from BDTRh-PCBM exhibits the power conversion efficiency (PCE) of 2.44% and maximum external quantum efficiency of 46%, which are the highest among the reported efficiencies so far. The SCOSC device shows efficient charge transfer (CT, ≈300 fs) and smaller CT energy loss, resulting in the higher open-circuit voltage of 0.97 V, compared to the binary blend (BDTRh:PCBM). Because of the integration of the donor and acceptor in a single molecule, BDTRh-PCBM has a specific D-A arrangement with less energetic disorder and reorganization energy than blend systems. In addition, the SCOSC device shows excellent device and morphological stabilities, showing no degradation of PCE at 80 °C for 100 h. The SCOSC approach may suggest a great way to suppress the large phase segregation of donor and acceptor domains with better morphological stability compared to the blend device.

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.

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.

In this contribution, we report the working mechanisms of several processing additives for controlling the morphology of four all-polymer systems.

This study demonstrates high‐performance, ternary‐blend polymer solar cells by modifying a binary blend bulk heterojunction (PPDT2FBT:PC71BM) with the addition of a ternary component, PPDT2CNBT. PPDT2CNBT is designed to have complementary absorption and deeper frontier energy levels compared to PPDT2FBT, while being based on the same polymeric backbone. A power conversion efficiency of 9.46% is achieved via improvements in both short‐circuit current density (JSC) and open‐circuit voltage (VOC). Interestingly, the VOC increases with increasing the PPDT2CNBT content in ternary blends. In‐depth studies using ultraviolet photoelectron spectroscopy and transient absorption spectroscopy indicate that the two polymers are not electronically homogeneous and function as discrete light harvesting species. The structural similarity between PPDT2CNBT and PPDT2FBT allows the merits of a ternary system to be fully utilized to enhance both JSC and VOC without detriment to fill‐factor via minimized disruption of semi‐crystalline morphology of binary PPDT2FBT:PC71BM blend. Further, by careful analysis, charge carrier transport in this ternary blend is clearly verified to follow parallel‐like behavior.

Diphenyl ether acts as a predominant processing additive regardless of the polymer crystallinity in bulk-heterojunction polymer solar cells.