Band-offset engineering for enhanced open-circuit voltage in polymer-oxide hybrid solar cells

The power conversion efficiency of organic and hybrid solar cells is commonly reduced by a low open circuit voltage (V-OC). In these cases, the V-OC is significantly less than the energy of the lowest energy absorbed photon divided by the elementary charge q. The low photovoltage originates from characteristically large band offsets between the electron donor and acceptor species. Here a simple method is reported to systematically tune the hand offset in a pi-conjugated polymer metal oxide hybrid donor-acceptor system in order to maximise the V-OC. It is demonstrated that substitution of magnetisim into a zinc oxide acceptor (ZnMgO) reduces the band offset and results in a substantial increase in the V-OC of poly(3-hexylthiophene) (3HT) ZnMgO planar devices. The V-OC is seen to increase from 500 mV at x = 0 up to value in excess of 900 mV for x = 0.35. A concomitant increase in overall device efficiency is seen as x is increased from 0 to 0.25 with a maximum power-conversation efficiency of 0.5% obtained at x = 0.25, beyond which the efficiency decreases because of increased series resistance in the device. This work provides a new tool for understanding the role of the donor-acceptor band offset in hybrid photovoltaics and for maximising the photovoltage and power-conversion efficiency in such devices.