Degree Type

Dissertation

Date of Award

2020

Degree Name

Doctor of Philosophy

Department

Electrical and Computer Engineering

Major

Electrical Engineering( Microelectronicsand Photonics)

First Advisor

Vikram Dalal

Abstract

Perovskite solar cell has been attracting tremendous attention in the photovoltaic device research area for the past decade. Its material's optoelectonic properties make the perovskite a perfect candidate for the solar cell device. However, the main obstacles preventing the commercial deployment of the perovskite solar cells are its stability in air and at elevated temperatures. In this dissertation, we reported strategies to solve both stability issues by utilizing cesium based mixed halide perovskite material.

In the first part of this work, we introduced a layer-by-layer vacuum deposition method to fabricate the perovskite thin films, which allows us precise control over the perovskite crystal growth process and also other parameters of the fabrication procedures. CsPbI2Br inorganic perovskite solar cells were fabricated with the device structure of FTO/In:CdS/perovskite/P3HT/Au. After careful optimization of all the processing parameters, a high efficiency solar cell device with 1.16 V open-circuit voltage, 11.6 mA/cm2 short-circuit current, and 11.7% PCE was achieved. Histogram of 27 devices has shown that this layer-by-layer vacuum deposition technique makes perovskite solar cell fabrication process more controllable with great reproducibility. More importantly, superb thermal stability of the vacuum evaporated CsPbI2Br perovskite was reported here. The perovskite material's stability at high temperature was validated by preparing CsPbI2Br film on glass slides and annealing at 300 C for 24 hours. The XRD data of before and after annealing showed no degradation of crystal peaks at all. And the CsPbI2Br solar cell device's thermal stability was confirmed with 200C 72 hours thermal stress test. Device's J-V performance exhibited no change at all.

Fundamental material properties of CsPbI2Br perovskite were carefully measured. Bandgap was revealed by quantum efficiency measurement with the Tauc plot as 1.87 eV. Urbach energy was measured by subgap qe as 21 meV. CFT measurement indicated that deep defect states in the material has Gaussian distribution with peak value of 7E15 cm-3eV-1 located at 0.52 eV below the conduction band.

To address the air stability issue for perovskite, a whole bromide CsPbBr3 perovskite material was studied. With a two-step post annealing process after film deposition together with other careful optimization process, the CsPbBr3 perovskite solar cell with the device structure of FTO/compact TiO2/perovskite/P3HT/Au achieved an ultra-high open-circuit voltage of 1.62 V. Our champion cell has the Voc of 1.62V, Jsc of 5.3 mA/cm2 and 6.02% efficiency. As a wide bandgap thin film solar cell being the top cell for tandem structure, lower voltage loss and hence lower efficiency loss is the key to improve tandem cell's efficiency. Most importantly, the air stability of the CsPbBr3 perovskite solar cell has been demonstrated by exposing the device in ambient air without moisture control for 25 days and the cell has maintained 100% performance.

Further, fundamental material properties of CsPbBr3 were characterized. Capacitance-Voltage measurement revealed the built-in voltage of the solar cell as 1.61 V and the doping concentration of the perovskite material as 2.37E15 cm-3. Tauc plot from QE measurement showed the bandgap of CsPbBr3 is 2.31 eV. And the Urbach energy was measurement subgap QE as 23 meV.

Finally, we discussed the low current problem of CsPbBr3 devices using the QE integrated current as the confirmation. QE set up in our lab and the modification to it for more precise measurement of current density were introduced at the end. Several ideas about how to further improve the solar cell current density and hence efficiency were proposed.

DOI

https://doi.org/10.31274/etd-20210609-211

Copyright Owner

Junhao Zhu

Language

en

File Format

application/pdf

File Size

123 pages

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