No abstract

All-polymer solar cells have shown great potential as flexible and portable power generators. These devices should offer good mechanical endurance with high power-conversion efficiency for viability in commercial applications. In this work, we develop highly efficient and mechanically robust all-polymer solar cells that are based on the PBDTTTPD polymer donor and the P(NDI2HD-T) polymer acceptor. These systems exhibit high power-conversion efficiency of 6.64%. Also, the proposed all-polymer solar cells have even better performance than the control polymer-fullerene devices with phenyl-C61-butyric acid methyl ester (PCBM) as the electron acceptor (6.12%). More importantly, our all-polymer solar cells exhibit dramatically enhanced strength and flexibility compared with polymer/PCBM devices, with 60- and 470-fold improvements in elongation at break and toughness, respectively. The superior mechanical properties of all-polymer solar cells afford greater tolerance to severe deformations than conventional polymer-fullerene solar cells, making them much better candidates for applications in flexible and portable devices.

A new acceptor-donor-acceptor-structured nonfullerene acceptor ITCC (3,9-bis(4-(1,1-dicyanomethylene)-3-methylene-2-oxo-cyclopenta[b]thiophen)-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d':2,3-d']-s-indaceno[1,2-b:5,6-b']-dithiophene) is designed and synthesized via simple end-group modification. ITCC shows improved electron-transport properties and a high-lying lowest unoccupied molecular orbital level. A power conversion efficiency of 11.4% with an impressive V of over 1 V is recorded in photovoltaic devices, suggesting that ITCC has great potential for applications in tandem organic solar cells.

Additives are known to improve the performance of organic photovoltaic devices based on mixtures of a low bandgap polymer, poly[2,6‐(4,4‐bis(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b;3,4‐b′]‐dithiophene)‐alt‐4,7‐(2,1,3‐benzothiadiazole)] (PCPDTBT) and [6,6]‐phenyl C61‐butyric acid methyl ester (PCBM). The evolution of the morphology during the evaporation of the mixed solvent, which comprises additive and chlorobenzene (CB), is investigated by in‐situ grazing incidence X‐ray scattering, providing insight into the key role the additive plays in developing a multi‐length‐scale morphology. Provided the additive has a higher vapor pressure and a selective solubility for PCBM, as the host solvent (CB) evaporates, the mixture of the primary solvent and additive becomes less favorable for the PCPDTBT, while completely solubilizing the PCBM. During this process, the PCPDTBT first crystallizes into fibrils and then the PCBM, along with the remaining PCPDTBT, is deposited, forming a phase‐separated morphology comprising domains of pure, crystalline PCPDTBT fibrils and another domain that is a PCBM‐rich mixture with amorphous PCPDTBT. X‐ray/neutron scattering and diffraction methods, in combination with UV–vis absorption spectroscopy and transmission electron microscopy, are used to determine the crystallinity and phase separation of the resultant PCPDTBT/PCBM thin films processed with or without additives. Additional thermal annealing is carried out and found to change the packing of the PCPDTBT. The two factors, degree of crystallinity and degree of phase separation, control the multi‐length‐scale morphology of the thin films and significantly influence device performance.

The structure–property relationships of PTB7‐phenyl‐C61‐butyric acid methyl ester (PCBM)‐based organic photovoltaics are investigated. The morphology is investigated in an active layer setting where a multi‐length‐scale morphology is observed using a solvent additive‐assisted film processing. This multi‐length‐scale structure consists of a phase separated morphology with a characteristic length scale of ≈30 nm, which is critical for producing large currents in devices; a second length scale of ≈130 nm, arises from face‐on PTB7 crystalline aggregates. This latter morphological feature is also observed in films prepared without the use of an additive. By observing the structure formation in situ during solvent evaporation for blade coated thin films, the additive is found to promote the formation of ordered domains of the PTB7 at an earlier stage during the solvent evaporation, which is critical in the development of the final morphology. In studies on PTB7/PCBM bilayers, PCBM is found to diffuse into the PTB7 layer. However, the performance of devices prepared in this manner is low. This diffusion leads to a swelling of the PTB7 and a reduction in the crystallinity of the PTB7, reflecting the strong miscibility of PCBM with PTB7. The morphology resulting from the interdiffusion is single‐length‐scale with slightly large phase separation. This leads to devices with poor performance.