Thermal diffusion processes in bulk heterojunction formation for poly-3-hexylthiophene/C60 single heterojunction photovoltaics

We have fabricated bulk heterojunction (BHJ) photovoltaic (PV) devices by thermal annealing of poly-3-hexylthiophene (P3HT)/C-60 single heterojunction (HJ) PV devices at near the melting point of P3HT. The BHJ PV devices exhibited an increased efficiency of 12 times compared with single HJs. We found that the annealing of HJ devices produces an interpenetrated network of interfaces between the P3HT and C-60 layers. This plays a major role in carrier separation and mobility enhancement. Also the formation of crystalline C-60 domains, concurrent with polymer crystallinity, contributes to an increase in the overall external conversion efficiency. Surprisingly, the heterojunction morphology, as inferred through device performance, strongly depends on the thermal gradient across the film. (c) 2006 American Institute of Physics.