A Study of SAM Modified ZnO in Hybrid Bilayer ZnO/P3HT Photovoltaic Devices

Abstract

Hybrid organic/inorganic solar cells such as ZnO/P3HT offer promise in increasing efficiency of organic-based devices. However there are many unresolved issues such as poor short-circuit current and open-circuit voltage that are hampering their widespread, commercial use. It is thought that surface trap states on ZnO are providing an open avenue for carrier recombination thus creating devices with poor current transport characteristics. Using self assembled monolayers (SAMs) may provide some key answers and solutions to this problem by passivating trap states. In the course of this work, benzoic acid, 4-aminobenzoic acid, 4-methoxybenzoic acid, phenylphosphonic acid, and 4-methoxyphenylphosphonic acid SAMs were studied in large part due to their commercial availability. It was found that the phenylphosphonic acids had a clear impact on decreasing dark current; therefore strongly suggesting that exciton recombination has been inhibited to some degree. These molecules also caused a decrease in efficiency by an order of magnitude as compared to a plain ZnO/P3HT bilayer cell (standard). There were pronounced negative effects on the other device parameters such as open circuit voltage and short circuit current. In the case of 4-methoxybenzoic acid and benzoic acid the effects are not so clear in that parts of the dark J-V curve indicate a decrease in dark current while other regions show an increase. Interestingly for the negative effect on efficiency and other device parameters was not as pronounced as the phenylphosphonic acids. In both cases it is hypothesized that because of their wide band gaps and poor energy level matching, they ultimately impact device performance negatively. In the future, use of simulations to determine optimal SAM molecular structures that can be synthesized in the lab or purchased commercially is suggested.