Increasing the lifetime of polymer based organic solar cells is still a major challenge. Here, the photostability of bulk heterojunction solar cells based on the polymer poly[4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′,3′-d]silole)-2,6-diyl-alt -[2,5-bis(3-tetradecylthiophen-2-yl)thiazole[5,4-d]thiazole)-1,8-diyl] (PDTSTzTz) and the fullerene [6,6]-phenyl-C 61 -butyric acid methyl ester (PC 60 BM) under inert atmosphere is investigated. Correlation of electrical measurements on complete devices and UV-vis absorption measurements as well as highperformance liquid chromatography (HPLC) analysis on the active materials reveals that photodimerization of PC 60 BM is responsible for the observed degradation. Simulation of the electrical device parameters shows that this dimerization results in a signifi cant reduction of the charge carrier mobility. Both the dimerization and the associated device performance loss turn out to be reversible upon annealing. BisPC 60 BM, the bis-substituted analog of PC 60 BM, is shown to be resistant towards light exposure, which in turn enables the manufacture of photostable PDTSTzTz:bisPC 60 BM solar cells.

We have studied the utilization of PCDTBT, an alternating poly(2,7-carbazole) derivative, in organic bulk heterojunction solar cells. The effect of polymer molecular weight, PCDTBT:[60]PCBM ratio, and active layer thickness on the device performance is reported. The best performance was obtained when the number-average molecular weights (M n ) are around 20 kDa with a polydispersity index around 2.2. Both PCDTBT:[60]PCBM ratio and active layer thickness affect the light absorption and the charge transport properties. By optimizing these two parameters, power conversion efficiency (PCE) up to 4.35% was reached under calibrated AM1.5G illumination of 100 mW cm À2 . When blended with [70]PCBM, PCDTBT exhibited a PCE up to 4.6%.

A push-pull copolymer is presented which can be used in bulk heterojunction (BHJ) solar cells with active layers greater than 200 nm and fill factors above 60%. The efficiencies of most BHJ solar cells are limited by the fact that they have active layers which are between 60 and 110 nm. While this thickness regime enables peak quantum efficiencies (EQE) of 60%–70%, the ability to fabricate thicker devices would increase average EQE values and thus device efficiencies. Discovery of materials which can maintain high performance at large thicknesses will enable higher performance in BHJ hero cells and increase the commercial viability of this technology.

We combined experimental and computational approaches to tune the thickness of the films in poly(N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT)-based organic solar cells to maximize the solar absorption by the active layer. High power-conversion efficiencies of 5.2% and 5.7% were obtained on PCDTBT-based solar cells when using [6,6]-phenyl C61-butyric acid methyl ester (PC60BM) and [6,6]-phenyl C71-butyric acid methyl ester (PC70BM) as the electron acceptor, respectively. The cells are designed to have an active area of 1.0 cm2, which is among the largest organic solar cells in the literature, while maintaining a low series resistance of 5 Ω cm2.

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