Abstract
Thermally induced degradation of photovoltaic performance in bulk-heterojunction (BHJ) polymer solar cells as a result of changes either in the active layer morphology or at interfaces during operation at elevated temperature is a common phenomenon. In this work, we have studied the thermal stability of a high performance polymer:fullerene BHJ PSCcomprising a conjugated polymer poly{[4,8-bis-(2-ethyl-hexyl-thiophene-5-yl)-benzo[1,2-b:4,5-b0]dithiophene-2,6-diyl]-alt-[2-(20-ethyl-hexanoyl)-thieno[3,4-b]thiophen-4,6-diyl]} (PBDTTT-CT) and a fullerene derivative [6,6]-phenyl-C70-butyric acid methyl ester photoactive layer within a conventional device architecture of glass/ITO/PEDOT:PSS/active layer/Mg/Al. By varying the temperature exposure conditions, the degradation path has been identified as an interfacial change in the device rather than a bulk effect. Furthermore, charge carrier dynamics studied by open circuit corrected charge carrier extraction has shown that post-annealed devices suffer from charge extraction due to the development of interfacial changes as compared to the non-treated devices in both pristine and with 1,8-Diiodooctane added scenarios.