Morphological investigation of P3HT/PCBM heterojunction and its effects on the performance of bilayer organic solar cells

Organic solar cells utilizing a P3HT/PCBM bilayer (BL) as their photoactive layer are fabricated using a sequential solution process. The film morphology and solar cell performance are investigated by changing the thickness of PCBM layer. The power conversion efficiency (PEC) of a BL solar cell strongly depends on the thickness of the PCBM layer; this value increases significantly after thermal annealing above the glass transition temperature of the P3HT. Based on the water contact angle and photoluminescence (PL) measurements, most of the PCBM was diffused into the amorphous P3HT region after thermal annealing. The external quantum efficiency spectrum reveals that the morphology of the P3HT in the BL solar cell is crystalline, even without thermal annealing, in contrast to the bulk heterojunction films. Moreover, thermal annealing improves charge collection efficiency by generating crystalline P3HT and PCBM domains. The transient photovoltage experiments suggest that the morphology of P3HT and PCBM in the BL solar cell reduces the charge recombination better than that in the BHJ solar cell. The BL solar cell exhibits a PEC of 3.54%, similar to that of the BHJ solar cell.