Date of Award
Doctor of Philosophy
Materials Science and Engineering
Materials Science and Engineering; Electrical Engineering
With concerns regarding climate change, pollution, and a limited supply of fossil fuels, photovoltaics are an attractive alternate energy source. Within the field of photovoltaics, thin film organic solar cells are alluring due to their potential low cost, mechanical flexibility, and ease of fabrication. However, there are many drawbacks that need to be overcome such as incomplete photon absorption, incomplete exciton dissociation, and carrier recombination. Three distinct projects addressing charge generation and collection in thin film photovoltaics are described.
The first details the use of microlens arrays (MLAs) as a nonintrusive method to increase photon absorption in organic solar cells. Laser holography and soft lithography were used to produce the MLAs on the glass side of an indium tin oxide substrate. In PTB7-based devices, we saw improvements in short circuit current (Jsc) of more than 10%, and achieved a high average power conversion efficiencies of 8.5%. Additionally, we used simulations utilizing the scattering matrix method to corroborate our experimental results. These simulations revealed that, for a given pitch of a MLA, a taller height typically yields more enhancement.
Second, the effects of using BaTiO3 nanoparticles as additives in polythiophene:fullerene solar cells are experimentally and theoretically investigated. BaTiO3 nanoparticles were chosen because of their high dielectric constant, which can increase exciton dissociation, and the potential for light scattering. To achieve stable suspensions for device fabrication, the nanoparticles were functionalized with organic ligands. Solar cells fabricated in air showed ~40% enhancement in the photocurrent primarily due to string-like aggregates of functionalized BaTiO3 particles, which increase light absorption without hindering charge collection. Solar cells fabricated in an inert atmosphere yielded overall more efficient devices, but the string-like aggregates were absent and enhancement in photocurrent was up to ~6%. Simulations with the excitonic drift-diffusion model demonstrate that a bare nanoparticle significantly increases exciton dissociation, whereas the functional group negates this effect. Simulations utilizing the scattering matrix method reveal that absorption enhancements caused by light scattering increase as the nanoparticles aggregate into string-like structures.
Lastly, a computational study investigating correlations between morphological features in two dimensional bulk heterojunctions and relevant photo physical processes is reported. A set of morphological descriptors were evaluated for a large set of morphologies utilizing a graph-based method. A morphology aware excitonic drift-diffusion model was used to compute current-voltage curves, fill factors, efficiencies, as well as spatial distributions of exciton generation, dissociation, and charge collection for each morphology. We find that the device efficiency primarily depends on the short circuit current, and has almost no dependence on the fill factor. Interestingly, we find that the fill factor is largely insensitive to many of the investigated descriptors. It is only weakly dependent on the contact area mismatch – the difference between the fraction of anode in direct contact with donor and the fraction of cathode in direct contact with acceptor. The fill factor is maximized when this quantity is nearly balanced. Since morphologies with a higher fraction of the electrodes in contact with the desirable material show higher short circuit current, we conclude that designing morphologies for a high short circuit current will necessarily lead to reasonably high fill factors.
Ryan Scott Gebhardt
Gebhardt, Ryan Scott, "Experimental and theoretical investigations of charge generation and transport in thin film photovoltaics" (2016). Graduate Theses and Dissertations. 15917.