Influence of thermal and solvent annealing on the morphology and photovoltaic performance of solution processed, D-A-D type small molecule-based bulk heterojunction solar cells

Four new solution-processable donor-acceptor-donor (D-A-D) structured organic small molecules (CSDPP9, CSDPP10, CSDPP11 and CSDPP12) with diketopyrrolopyrrole (DPP) as the central core unit and different electron donating groups as the terminal units, were designed and synthesized. These molecules employed as donors along with PC71BM as an electron acceptor in solution processed bulk heterojunction (BHJ) solar cells, exhibited a strong visible (450-650 nm) absorption band with suitable molecular orbital energy levels. OSCs based on these small molecules achieved moderate power conversion efficiency (PCE) in the range of 2.86-3.55% along with high open circuit voltage (V-oc) and the difference is correlated with the different electron donating units used in the DPP. The observed high open circuit voltage (V-oc) of the device is directly related to the HOMO energy level of the DPP. The BHJ devices, based on CSDPP11:PC71BM and CSDPP12:PC71BM subjected to solvent and thermal annealing showed improved PCEs of 5.47% and 4.88% respectively. The enhancement in the PCE has been attributed to the improved charge carrier mobility, crystallinity and light harvesting ability of the active layer.