Award Date

8-1-2023

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

First Committee Member

Shubhra Bansal

Second Committee Member

Brendan O'Toole

Third Committee Member

Hui Zhao

Fourth Committee Member

Thomas Hartmann

Fifth Committee Member

Eunja Kim

Sixth Committee Member

Pradip Bhowmik

Number of Pages

184

Abstract

In recent years, perovskite photovoltaic technology has offered enormous viability and dimensionality in solar cell research. As a light-harvesting active layer, Perovskite generated remarkable development in device efficiency of 25.7% for the single-junction solar cell, and over 33% for the perovskite/silicon tandem solar cell. Also, perovskite-perovskite tandem solar cell (also called all-perovskite tandem solar cell) shows great potential in device performance and achieved a power conversion efficiency (PCE) of 26.4%. Transitioning photovoltaic technology from the laboratory to commercial products, high PCE, low cost, long lifetime, and low toxicity are some of the critical factors to consider during material selection. Pb-halide perovskites have been the most studied compositions in next-generation photovoltaics due to their excellent optoelectronic properties, such as the high PCE and ideal bandgap. However, the practical relevance of these materials is hindered as they offer multifarious disadvantages, including toxicity, high water solubility and bioavailability, and thermodynamic instability. Following the toxicity and instability issues present in the Pb-based perovskites, Pb-free perovskite compounds have been the mainstay of this thesis. Several alternative cations isoelectronic to Pb, preferablyfrom groups 14 and 15, have been explored by the perovskite research community, such as Sn2+, Ge2+, Sb3+, Pt2+, Ti2+, Bi+, Ag2+, etc. In project 1, a numerical analysis of several potential Pb-free perovskite light absorber materials has been performed by modeling the device structure using the solar cell capacitance simulator, a one-dimensional (SCAPS 1D) modeling tool, and optimizing the device characteristics to theoretically determine the optimum efficiency limits for the Pb-free compounds compared to the state-of-the-art Pb-based (FA, MA, Cs)Pb(I, Br)3 perovskite. To investigate the criteria for choosing an efficient composition and key strategies to boost their performances, 4 device parameters have been optimized, such as interface band alignments, midgap defect density, the absorption coefficient, and the thickness of the perovskite absorber.

Among the several alternative cations, the Sn-based perovskite absorbers are considered the best potential substitution for Pb, due to its isoelectronic configuration similar to Pb. In recent years, high-performance Sn-based HPSCs with PCE over 20% were made possible by introducing more than 50% of Sn in Pb-based perovskite, indicating a promising approach to alleviating the toxicity of lead in organic-inorganic metal halide perovskite. In project 2, an inorganic Cs(Sn,Pb)I3 perovskite with 60% Sn has been synthesized via solution processing and the effects of additive engineering on the improvement of its photovoltaic properties and stability have been investigated. The addition of the guanidinium thiocyanate (GuaSCN) additive lowers the bandgap significantly to 1.5 eV, enhances the absorption coefficient, and reduces the oxygen concentration, compared to the films with no additive. The highly oriented mixed phase of CsSnI3 and CsPbI3 has been observed in XRD, suggesting the formation of Cs(Sn, Pb)I3 perovskite. Also, it is observed that the Cs(Sn, Pb)I3 films with GuaSCN are thermally stable as they have retained the bandgap and crystal structure after being exposed to thermal annealing for 100 hours. This study suggests the addition of the GuaSCN additive is a potential route to enhance the optical properties and stability of perovskite films.

The oxidation tendency of Sn2+ to Sn4+ imbalances the charge neutrality of the CsSnI3. Recently Cs2SnI6 double perovskite has emerged as a light absorber, derived from the cubic 3D structure of CsSnI3, with a higher oxidation state of Sn4+ which accounts for the intrinsic resistance to oxidation. In project 3, SnF2-doped, p-type Cs2SnI6 perovskite has been synthesized via solution processing and the influence of several solvents and additives on its photovoltaic properties and stability has been analyzed. GBL as a solvent helps in the formation of low bandgap (1.3-1.5 eV) Cs2SnI6 perovskite phase with a small amount of unreacted CsI, which has been reduced with additives. SnF2-doped crystalline Cs2SnI6 films with pyrazine and ethylene diamine (EDA) additives show significantly suppressed CsI and Cs2SnI6 preferred orientation along the (222) crystal plane and have a minority carrier lifetime of over 6 ns, which is almost twice the lifetime in films with only SnF2. This EDA-induced Cs2SnI6 perovskite film has achieved a PCE of 5.18%.

Recently a Pt-based perovskite with a composition of Cs2PtI6 was developed in our previous work with a high PCE of 13.88%. However, the high cost of Pt is a hindrance to commercialization. In project 4, an experimental screening study to assess how partially replacing Pt in cesium platinum triiodide (CsPtI3) perovskite with different concentrations of earth-abundant and low-cost nickel (Ni) influences its crystallographic and optoelectronic properties has been undertaken. We report a partial substitution of PtI2 with NiCl2 in mixed PtI2-NiCl2-based films and this work will be conducted as the future work.

Keywords

Perovskite; solar cell materials; solution-processing; spectroscopy; stability; Thin film

Disciplines

Engineering | Engineering Science and Materials | Materials Science and Engineering | Oil, Gas, and Energy

File Format

pdf

File Size

5640 KB

Degree Grantor

University of Nevada, Las Vegas

Language

English

Rights

IN COPYRIGHT. For more information about this rights statement, please visit http://rightsstatements.org/vocab/InC/1.0/

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