4,5-Ethylene-2,7-Carbazole-Based Medium-Bandgap Conjugated Polymers with Low-Lying HOMO Levels Toward Efficient Polymer Solar Cells with High Open-Circuit Voltage

Three medium-bandgap polymers based on a 4,5-ethylene-2,7-dithienyl carbazole as the electron-donating unit and different 5,6-dialkoxy-2,1,3-benzothiadiazoles as the electron-accepting units, are synthesized as polymer donors for photovoltaic applications. The three copolymers possess highest occupied molecular oribital (HOMO) levels around -5.47 eV and medium bandgaps of about 1.94 eV. The solar cells with polymer:[6,6]-phenyl C71-butyric acid methyl ester (PC71BM) = 1:4 as the active layer, show an especially high open-circuit voltage (V-oc) of 0.95 V and attain good power conversion efficiency up to 5.91%. The hole mobilities of the active layer films, measured by space-charge-limited current (SCLC), are up to 3.5 x 10(-4) cm(2) V-1 s(-1). Given the favorable medium bandgaps, low-lying HOMO levels, and good hole mobilities, these copolymers are promising candidates for the construction of a highly efficient front cell to harvest the shorter wavelength band of the solar radiation in a tandem solar cell with high V-oc.