Charge transport and photocurrent generation in PPAT:ZnO bulk heterojunction photovoltaic devices

In this paper, the blended systems composed of organic polymer and ZnO nano-particles were investigated through their optical and photo-electrochemical measurements for their potential applications in photovoltaic devices. The electrical and photoelectrical properties of the devices fabricated with the blend of poly (phenyl azo methane thiophene) (PPAT) and ZnO nano-particles sandwiched between ITO/PEDOT:PSS and metal electrode have been reported. Dependence of the performance this bulk heterojunction photovoltaic device on their composition has been investigated with respect to charge transport. We have observed that the electron mobility gradually increases on increasing ZnO concentration up to 60 wt.% and subsequently saturates to the bulk values. Similarly, the hole mobility in PPAT phase also shows an identical behavior and saturates beyond 50 wt.% ZnO, a value which is more than two orders of magnitude higher that that of the pure PPAT. The experimental electron and hole mobilities were used to study the photocurrent generation in PPAT:ZnO bulk heterojunction photovoltaic devices and discussed in detail. It is found that the device with 55 wt.% of ZnO in the blend shows maximum value of power conversion efficiency about 1.9%. The effect of controlled thermal annealing on the charge transport and generation of photocurrent in bulk heterojunction photovoltaic device made from blend film of PPAT:ZnO (55 wt.%) has also been studied with respect to the charge transport. It is concluded that the electron mobility dominates the charge transport of the device after thermal annealing. The hole mobility in PPAT phase of the blend is dramatically affected by thermal annealing. Additionally, upon thermal annealing the absorption spectrum of PPAT:ZnO blend under go a strong red shift, improving the spectral light harvesting, which results in an increase in the rate of charge carrier generation. The experimental results indicates that the most important factor leading to a strong enhancement of the efficiency, compared with non-annealed devices, is the increase in hole mobility in the PPAT phase of the blend and more light absorption by the blend.