Lateral device techniques for characterizing organic bulk heterojunction photovoltaic materials
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This work is focused on developing novel techniques for characterizing organic bulk heterojunction (BHJ) materials for organic photovoltaic (OPV) applications. Polymer:fullerene BHJs are a promising class of photovoltaic materials, but an improved understanding of the charge transport processes and materials science of BHJs is required for them to effectively compete with other photovoltaic systems. Key parameters of BHJ systems that need to be evaluated include both electron and hole mobilities, the carrier concentrations, the recombination mechanism and the recombination coefficient. For these studies, poly(3-hexylthiophene) (P3HT):(6,6)-phenyl C₆₁-butyric acid methyl ester (PCBM) have been characterized due to its wide use among researchers. Traditional characterization techniques have focused on transient measurements in a vertical device configuration, but we demonstrate the use of lateral BHJ devices as materials diagnostic platforms. Lateral devices allow for direct access to the active layer for spatially resolved and environmental effect measurements. The devices are also measured under steady state operation, similar to a working OPV cell. Under these conditions, lateral BHJ devices exhibit space charge limited transport behavior. A detailed charge transport model is presented to describe the potential, electric field, and carrier concentration profiles of lateral BHJ devices, as well as the current versus voltage characteristics of different regions of the device. We are able to calculate the slower carrier mobility from photocurrent measurements of lateral devices and the carrier mobility ratio from the device potential profile, even in ambipolar BHJ systems. In situ potentiometry is used to construct detailed potential profiles of the device channel and calculate both carrier mobilities. The carrier concentration and recombination coefficient are calculated from lateral conductivity measurements, and we show that bimolecular recombination is the dominant mechanism in bulk P3HT:PCBM. A simplified in situ potentiometry and photocurrent measurement technique is presented to measure the time evolution of organic BHJ performance. Due to the open geometry of the lateral BHJ device, we are also able to monitor the change in key charge transport parameters, including the recombination mechanism, in response to environmental degradation, analyte exposure, and ambient temperature. We show increased geminate recombination in P3HT:PC₇₁BM after prolonged light exposure. Lateral BHJ device measurements offer a useful complement to measurements on vertical photovoltaic structures and provide a more complete and detailed picture of OPV materials.