Effect of the vertical composition gradient of active layer on the performance of bulk-heterojunction organic photovoltaic cell

A computational model is developed to investigate the effect of vertical composition gradient on the photovoltaic properties of organic photovoltaic cell based on poly (3-hexylthiophene) (P3HT) and [6,6]-phenyl C ₆₁-butyric acid methyl ester (PCBM). When the photovoltaic properties of model devices with systematic variation of vertical composition distribution are calculated by using this model, it is found that the efficiency increases almost linearly as the concentrations of P3HT and PCBM become richer near anode and cathode, respectively. The larger exciton generation near anode due to enrichment of P3HT near the anode contributes mainly to an increase of the device efficiency by compensating the lower hole mobility of P3HT with shorter pathway to the anode.