Enhanced open-circuit voltage in organic photovoltaic cells with partially chlorinated zinc phthalocyanine

The open-circuit voltage (V _OC) in organic photovoltaic cells has been shown to depend on a number of parameters including the energy levels of the active materials, active layer structure, illumination intensity, and operating temperature. Here we report, a significant increase in V _OC from 0.43 to 0.63 V in zinc phthalocyanine (ZnPc)/C₆₀ planar heterojunction photovoltaic cells operated at room temperature under 1 sun AM1.5G solar illumination, when a home-synthesized and purified ZnPc source materials was used instead of a commercially obtained (and home purified) ZnPc source. While the two ZnPc source materials have nearly identical UV-Vis and IR absorption properties, the home-synthesized ZnPc contains chlorinated derivatives and has half of the electrical defect density (on the order of 10¹⁶ cm^-3) as compared to the commercial ZnPc. The improved V _OC in devices with the home-synthesized ZnPc is contributed from both a lower dark current and a higher magnitude of photocurrent. Additional experiments revealed that the different device characteristics are mostly associated with processes occurring at the ZnPc/C₆₀ interface, which we attribute to nongeminate recombination of charges built-up on either side of the interface.