The Impact of Thermal Annealing to the Efficiency and Stability of Organic Solar Cells based on PCDTBT:PC71BM

Bulk heterojunction organic solar cells have been fabricated based on PCDTBT:PC71BM blend in three different procedures involving thermal annealing treatment. The first solar cell has been fabricated by spin coating PEDOT:PSS solution onto ITO-coated glass substrate in order to form 40 nm buffer layer and followed by the formation of 70 nm PCDTBT: PC71BM active layer. Without undergoing any treatment, an aluminium layer of about 100 nm thick has been formed on top of this active layer by means of thermal evaporation in vacuum condition. For the second solar cell fabrication, the active layer has been thermally treated at 70 degrees C for 30 min before the deposition of Al top contact. And for the third solar cell, the fabrication has been done exactly as previous but having additional thermal treatment at 70 degrees C for 30 min after the deposition of Al top contact. All the devices have been characterized by I-V measurement in dark and under light illumination in ambient condition. The first solar cell has shown the highest efficiency up to 9.3%, followed by the second solar cells that achieved 4.8% efficiency and the third solar cell with 2.3% efficiency. However, it is observed that the stability of first solar cells is drastically dropped in a day indicates that the solar cell had experienced a serious instability. The second solar cell shows an improve stability by having less reduces in it efficiency. While the third solar cell showing even better stability than the second one by giving an enhanced stability. It is observed that, without thermal annealing, highest efficiency could be achieved but has poor stability. While by having twice thermal annealing, the solar cell will have lower efficiency but better stability. (C) 2015 The Authors. Published by Elsevier Ltd.