Charge-carrier Generation, Transport, and Collection in Polymer-based Organic Solar Cells

Solar cells based on semiconducting, pi-conjugated polymers have attracted intense research interest in the past few years because of their potential to be inexpensive, lightweight, and flexible. However, their low power conversion efficiencies currently limit their viability for commercial production. We have studied both organic and hybrid organic-inorganic solar cells as a way to develop design strategies to improve performance.;Organic solar cell performance is quite sensitive to subtle variations in device processing conditions. We used transient photocurrent measurements to investigate the dependence of charge-carrier extraction rates on the morphology of regioregular poly(3-hexylthiophene) (P3HT)/fullerene blend films. We discovered that the addition of a fullerene capping layer on top of the P3HT/fullerene mixed layer consistently ensures balanced extraction of charge-carriers, and thus improves the device performance reproducibility.;Next, we used a series of thienyl-substituted fullerenes to control polymer/ - fullerene film microstructure. We tailored the side-groups of the fullerene to resemble the molecular structure of P3HT and showed that this alters the propensity of the fullerene to mix with P3HT in a film. Thus, we ensured that the average length scale of mixing coincides with the average P3HT exciton diffusion length. This leads to more exciton dissociation and more mobile charge-carrier generation.;Furthermore, we have explored the potential to enhance light absorption in the polymer/fullerene blend film using silver nanoparticles (Ag NPs). Efficient light absorption is a critical component of a solar cell's overall photoconversion mechanism. The addition of Ag NPs to organic solar cells can greatly improve absorption by plasmonic enhancement; however it is generally detrimental to the overall device performance. We found that the addition of Ag NPs to the active layer significantly enhances carrier mobility but decreases the total extracted carrier density. We conclude that the one needs to prevent the carriers from localizing on the metal NPs, so that the NPs only enhance absorption without altering the motion of the carriers through the device.;In addition, we also have examined why the distance over which a photo-excitation in P3HT can be harvested into free charge-carriers is another important parameter to consider when designing the solar cell device architecture. We showed that by carefully engineering the donor/acceptor interface with a self-assembled fullerene monolayer bound to a titania acceptor surface, we can capture P3HT excitons farther away from the donor/acceptor interface, thus increasing the total number of useful charge-carriers created in the active layer of the device.;Finally, we present a novel type of hybrid conjugated polymer/inorganic photovoltaic device based on an array of patterned P3HT-coated n-doped GaAs nanopillar heterojunctions. We designed the device to simultaneously exploit the high carrier mobility of the inorganic semiconductor and strong absorption coefficient of the polymer. Such a hybrid approach could effectively remove the carrier transport bottleneck suffered in conjugated polymers through the use of integrated III-V inorganic nanostructures.