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We conducted optical and electrical simulations with the goal of determining the optimal design for conjugated polymer-fullerene tandem solar cells using poly͓2,6-͑4,4-bis-͑2-ethylhexyl͒-4H-cyclopenta͓2,1-b;3,4-bЈ͔dithiophene͒-alt-4,7-͑2,1,3-benzothiadiazole͔͒ ͑PCPDTBT͒: ͓6,6͔-phenyl C 61 butyric acid methyl ester ͑PC 61 BM͒ as a bottom cell and poly͑3-hexylthiophene͒ ͑P3HT͒: ͓6,6͔-phenyl C 71 butyric acid methyl ester ͑PC 71 BM͒ as a top cell. The effects of photon density, absorption, balanced and unbalanced charge carrier transport, and bimolecular recombination in the two subcells were incorporated into the simulations. We found that the maximum energy conversion efficiency ͑͒ is 9% when charge carrier mobilities in both top and bottom cells are balanced. However, the efficiency drops significantly if the carrier mobilities are unbalanced in either the top or bottom cell. In addition, we found that unbalanced carrier mobilities in the top cell require a reduction in the thickness of the bottom cell whereas unbalanced bottom cell mobilities require an increase in the thickness of the bottom cell to compensate for the reduced current.

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We performed detailed studies of the effect of polarization on III‐nitride solar cells. Spontaneous and piezoelectric polarizations were assessed to determine their impacts upon the open circuit voltages (VOC) in p–i(InGaN)–n and multi‐quantum well (MQW) solar cells. We found that the spontaneous polarization in Ga‐polar p–i–n solar cells strongly modifies energy band structures and corresponding electric fields in a way that degrades VOC compared to non‐polar p–i–n structures. In contrast, we found that piezoelectric polarization in Ga‐polar MQW structures does not have a large influence on VOC compared to non‐polar MQW structures. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)