Abstract
The surface orientation of inorganic semiconductors affects the density of surface states, energy levels, and other intrinsic properties of the semiconductor, all of which would have an impact on the performance of related polymer/inorganic hybrid solar cells. With a set of conjugated polymers having different energy levels and band gaps (P3HT, PNDT-DTffBT, and PBnDT-DTffBT), we show that the photovoltaic behavior of GaAs/ultrathin polymer layer/PEDOT:PSS planar solar cells is noticeably affected by the surface orientation of n-GaAs, (111)B or (100). In all of these hybrid solar cells, a Schottky barrier is formed between the GaAs and the anode, with these ultrathin polymer layers serving as the electron blocking layer (EBL) and the hole transport layer (HTL). The lower density of surface states of GaAs(111)B helps reduce the surface recombination and results in a higher short circuit current for (111)B based hybrid solar cells than for (100) based ones. However, the higher valence band maximum (VBM) of (111)B compared to that of (100) could lead to increased recombination from blocked hole transport, if the highest occupied molecular orbital (HOMO) level of the HTL polymer is lower than the VBM of GaAs. Considering all these effects, P3HT stands out as the best polymer to pair with GaAs in the studied set, with an efficiency of 4.2% achieved for the device based on P3HT/GaAs(111)B.
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