Understanding efficiency improvement in organic photovoltaics with molecular modifiers

Abstract

Molecular dipole modification of metal oxides has become popular to improve the performance of organic photovoltaic devices through charge transport level matching to the bulk heterojunction species. Properly tuning the work function of a device interlayer can increase charge collection from the active layer, ultimately raising the efficiency of the device. Here a novel type of molecule is introduced for modulating the work function of charge transport layers in organic photovoltaics: the conjugated phosphonic acid. Due to its longer and double-bonded linkage, as well as its multi-dentate attachment, this type of molecule is shown to shift the work functions of ZnO and ITO through ranges of 2 eV. The vast dipolar aromatic groups possible in conjugated phosphonic acids allow for either increasing or decreasing the work function of the substrate incrementally. This facilitates the energy matching with the Fermi level of a photovoltaic material necessary to achieve maximum efficiency of that solar cell. The effectiveness of conjugated phosphonic acids is also demonstrated in an operational organic bulk heterojunction solar cell. A self-assembled monolayer of conjugated phosphonic acid on the electron transport layer of an inverted device is shown to significantly increase the power conversion efficiency of that cell, even compared to its non-conjugated counterpart. The improvement to device performance was largely due to an increase in the short circuit current, with minor boosts to the open circuit voltage and fill factor. Direct measurements of potential distributions inside phosphonic acid modified and unmodified cells are also given. Beneficial modification of the electron transport layer interface is shown to extend the electric field within the active layer. The electric field is thought to aid in carrier separation and extraction from the bulk heterojunction, which correlates with improved short circuit current. Additionally, the technique for measuring potential distributions within operational solar cells, cross-sectional scanning Kelvin probe microscopy (X-SKPM), is tested in ambient and inert conditions. X-SKPM of organic photovoltaics in air reveals oxygen p-doping of the bulk heterojunction, as well as sensitivity to surface contamination of oxide interlayers, while inert conditions facilitate a reliable measurement of the potential distribution in organic photovoltaics.