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Biwas Subedi PhD theis December 2020.pdf (4.5 MB)

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Sub-Gap Absorption and Performance Losses in Mixed Cation Perovskites for Solar Cells

Subedi, Biwas
http://rave.ohiolink.edu/etdc/view?acc_num=toledo16070863918356

Abstract Details

2020, Doctor of Philosophy, University of Toledo, Physics.
Organic-inorganic metal halide perovskite ABX3 (A: methylammonium-MA, formamidinium-FA, cesium-Cs; B: lead-Pb, tin-Sn; X: iodine-I, bromine-Br, chlorine-Cl) based photovoltaics (PV) is one the most promising candidates for generation of low-cost clean energy. Perovskite-based PV yield, commonly quantified as power conversion efficiency (PCE), largely depends upon the characteristics of component layers including that of perovskite absorber layer and the interfaces between the layers. This dissertation presents the wide range of studies performed on the morphology and optoelectronic properties of perovskite solar cell component layers using spectroscopic ellipsometry (SE) and photothermal deflection spectroscopy (PDS). The amount of defect induced sub-gap absorption in perovskite absorber layers as a function of perovskite composition and laboratory ambient exposure level are studied and their effect on the PCE of solar cells are discussed. In addition, this dissertation presents the study of electronic loss arising from defects in perovskite absorber layers and parasitic optical absorption loss in non-active component layers in several perovskite solar cells. Optical response ε in the terms of near infrared to ultraviolet complex dielectric function (ε = ε1 + iε2) spectra of varying Cs-to-FA ratios in solution processed FA1-xCsxPbI3 perovskite thin films are studied. Analysis of these ε spectra track changes in the positions of critical point (CP) energies, including bandgaps and above bandgap transitions, with varying Cs contents. Bandgap values are identified as the lowest energy CP, with additional sub-gap features attributed to the presence of defects. Absorption onset values, for which absorption coefficient (α) = 4000 cm1, are extracted, with FA0.8Cs0.2PbI3 showing the sharpest absorption edge and the least contribution to absorption from defects below the bandgap. External quantum efficiency (EQE) simulations of solar cells using ε spectra and layer thicknesses as input are compared to experimental results and corroborate that to the observed sub-gap absorption is due to defects as it does not contribute to EQE. The impact of mixed cation composition and atmospheric exposure of perovskite films on sub-gap absorption in films and performance of solar cells based on mixed Sn + Pb narrow-bandgap (FASnI3)x(MAPbI3)1-x perovskites are studied. These narrow-bandgap mixed Sn + Pb perovskites are used as the bottom sub-cell in high efficiency all-perovskite polycrystalline tandem solar cells. Structural and optical properties of (FASnI3)x(MAPbI3)1-x (0.3 ≤ x ≤ 0.8) perovskite thin film absorbers with bandgaps ranging from 1.25 (x = 0.6) to 1.34 eV (x = 0.3) are probed with and without atmospheric exposure. The Urbach energy, which quantifies the amount of sub-gap absorption, is tracked for pristine perovskite films as a function of composition, with x = 0.6 and 0.3 demonstrating the lowest and highest Urbach energies of 23 and 36 meV, respectively. Films with x = 0.5 and 0.6 compositions show less degradation upon atmospheric exposure than higher or lower Sn-content films having greater sub-gap absorption. The corresponding solar cells based on the x = 0.6 absorber show the highest device performance. Despite having a low Urbach energy, higher Sn-content solar cells show reduced device performances as the amount of degradation via oxidation is the most substantial. Optical and electronic losses detrimentally affect the PV PCE. Perovskite solar cells are not an exception to that behavior. To minimize these losses in solar cells, it is important to identify their sources. The optical and electronic losses arising from physically mixed interfacial layers between the adjacent component materials in highly efficient two terminal (2T) all-perovskite tandem, single junction wide-bandgap, and single junction narrow-bandgap perovskite-based solar cells are studied in this dissertation. Physically mixed interfacial layers as the sources of optical and electronic losses are identified from SE measurements and data analysis followed by comparisons of simulated and measured EQE spectra. Parasitic absorbance in the physically mixed regions between silver metal electrical contacts and electron transport layers (ETLs) near the back contact, and a physical mixture of commercial indium tin oxide and hole transport layers (HTL) near the front electrical contact lead to substantial optical loss. A lower-density void + perovskite nucleation layer formed during perovskite deposition at the interface between the perovskite absorber layer and the HTL causes electronic losses due to incomplete collection of photogenerated carriers likely originating from poor coverage and passivation of the initially nucleating grains.
Nikolas J. Podraza (Committee Chair)
Robert W. Collins (Committee Member)
Yanfa Yan (Committee Member)
Richard Irving (Committee Member)
Terry Bigioni (Committee Member)
186 p.
Materials Science; Optics; Physics
Perovskite; Films; Solar Cell; Sub-Gap Absorption; Defects; Optical and Electronic Loss

Recommended Citations

Citations

  • Subedi, B. (2020). Sub-Gap Absorption and Performance Losses in Mixed Cation Perovskites for Solar Cells [Doctoral dissertation, University of Toledo]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=toledo16070863918356

    APA Style (7th edition)

  • Subedi, Biwas. Sub-Gap Absorption and Performance Losses in Mixed Cation Perovskites for Solar Cells. 2020. University of Toledo, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=toledo16070863918356.

    MLA Style (8th edition)

  • Subedi, Biwas. "Sub-Gap Absorption and Performance Losses in Mixed Cation Perovskites for Solar Cells." Doctoral dissertation, University of Toledo, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=toledo16070863918356

    Chicago Manual of Style (17th edition)

toledo16070863918356
309
© 2020, some rights reserved.Creative Commons License Sub-Gap Absorption and Performance Losses in Mixed Cation Perovskites for Solar Cells by Biwas Subedi is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License. Based on a work at etd.ohiolink.edu.
This open access ETD is published by University of Toledo and OhioLINK.
Release 3.2.12