Increase in internal quantum efficiency in small molecular oligothiophene: C60 mixed heterojunction solar cells by substrate heating

We present small molecule solar cells with alpha,omega-bis-(dicyanovinylene)-sexithiophene:C-60 mixed heterojunctions, reaching power conversion efficiencies of 4.9 +/- 0.2%. We use substrate heating during deposition of the mixed layer to achieve an optimized morphology and show that this significantly improves the internal quantum efficiencies (IQEs) to values approaching 70%. By optical modeling, we evaluate the amount of loss due to absorption in inactive layers and show that IQE of the active layer itself is about 80%. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3475766]