Date of Award

12-17-2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Applied Science

First Advisor

Alexandru Biris

Abstract

Scientists and engineers are developing photovoltaic devices as promising clean energy tools. Organic-inorganic hybrid perovskites have attracted considerable attention as next-generation thin film solar cells due to their high efficiency, solution processability, easy processing, and low-cost fabrication. Moreover, perovskite exhibits outstanding optoelectronic properties. These unique properties have led to a rapid rise in their power conversion efficiency (PCE) as single-junction from 3.9% to certified 23.4% in just about 9 years. However, the stability of perovskite solar cell (PSC) -based devices is still an issue, therefore, it is important to increase both the efficiency and stability of PSC devices. Based on planar inverted architecture, we explored two strategies to improve the stability and performance of PSCs. First, we synthesized and optimized a conducting polymer based on LS-PANI-CSA as a hole transport layer (HTL) for pure MAPbI3 PSCs. The result was a highly stable, well-performing PSC device that reached 10.8% PCE, performing much better than PEDOT:PSS, a commonly used HTL. Second, we developed a new strategy to grow cesium formadinium methylammonium triple-cation (CsFAMA) lead mixed-halide perovskites with microscale grains and novel passivation grain boundaries by mixing HC(NH2)2I (FAI) and CH3NH3Br (MABr) in isopropanol (IPA) as a post-treatment step to cause controlled Ostwald ripening, leading to secondary grain growth. Compared to the conventional preparation of perovskite films with a free organic halide anti-solvent, this mixed-cation mixed-halide post-treatment created higher quality perovskite films in terms of electronic properties, film morphology, energy level alignment, and carrier recombination, with the bandgap adjusted at the optimal concentration. Treatment with the single cation and single halide presented by FAI and/or MABr in IPA treatment was also investigated, and the results showed that multiple bandgap perovskite structures were obtained, which could be beneficial for bandgap engineering. This method of perovskite preparation was tested in an inverted planar configuration with an HTL based on sulfonated poly(thiophene-3-[2-(2-methoxy-ethoxy) ethoxy]-2,5-diyl) (S-P3MEET). The performance of PSCs created with a stoichiometric recipe of Cs0.04(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3 was dramatically boosted from 13.80% with the organic halide-free antisolvent to 17.62% with the mixed-cation mixed-halide approach. In addition, the PSCs treated with the mixed-cation mixed-halide solution exhibited excellent reproducibility, with 79% fill factor with eliminated hysteresis. The CsFAMA photovoltaic device based on the single organic cation and single halide treatment approach showed power conversion efficiency of 15.82% and 15.90% for the FAI and MABr-treated films, respectively. Characterization of the new S-P3MEET hole transport layer found good optoelectronic and morphology properties for use in inverted architecture perovskite devices.

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