Abstract
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 band offset in a π‐conjugated polymer–metal oxide hybrid donor–acceptor system in order to maximize the V~OC~. It is demonstrated that substitution of magnesium into a zinc oxide acceptor (ZnMgO) reduces the band offset and results in a substantial increase in the V~OC~ of poly(3‐hexylthiophene) (P3HT)–ZnMgO planar devices. The V~OC~ is seen to increase from 500 mV at x = 0 up to values 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‐conversion 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 maximizing the photovoltage and power‐conversion efficiency in such devices.