Low Cost Solution-Based Solar Cells
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There has been extensive research on reducing the cost of fabricating solar cells. Recently, due to the high fabrication cost of Silicon (Si) based solar cells, solution-based solar cells are receiving significant attention for low cost and mass production fabrication, and various materials have been investigated to obtain high quality film and high performance solar cells. This dissertation deals with two solution-based solar cell fabrication methods; the successive ionic layer adsorption and reaction (SILAR) process and a spray-based deposition method. In the first case, quantum dots, made from cadmium sulfide (CdS) and cadmium selenide (CdSe) with high absorption coefficients, were created on a Titanium dioxide (TiO 2 ) surface by the SILAR process. Those QDs were then used for absorbing light by using a single layer of CdS and also a cascaded double layer (CdS/CdSe) structure. The second approach to reduce the cost is to deposit the earth abundant materials using a spray-based method. Compounds containing Copper (Cu), Zinc (An), Tin (Sn) and Sulfur (S) were used as absorber layer materials, and a spray-based method was employed to deposit these absorber precursors on a heated substrate. The first solar cell structure investigated in this thesis is a quantum dot sensitized solar cell. CdS and CdS/CdSe quantum dot sensitized solar cells (QDSSC) fabricated using the SILAR process were investigated and a rate-equation model of trap induced power conversion efficiency (PCE) limits was developed and used to explain the experimental results. The highest power conversion efficiency (PCE) in the cascade structure was obtained with a CdS:CdSe 7:7 cycle ratio. This cycle ratio resulted in 2.55% PCE, 0.55V open circuit voltage (V oc ) and a short circuit current density (J sc ) of 10.5 mA/cm 2 with 44.1% fill factor (FF) under AM1.5G 1-sun illumination. However, excess cycles of CdSe beyond 7:7 decreases the device performance. The current loss when exceeding the optimum condition (7/7) is attributed to a trap induced space charge field which impedes the carrier extraction from the absorber layer to the TiO 2 . Increases in recombination due to dislocation generation when the critical thickness of the deposited layer exceeds the length for pseudomorphic growth. The simulation results (based on a phenomenological mode) are consistent with an increase of dislocations and corresponding increases in recombination rate. Taken together these effects impede the charge transfer at the interface between TiO 2 and the QDs. The second investigation in this thesis focuses on the development of solar cells using earth abundant materials deposited with a spraying technique. All layers are sprayed on a fluorine doped tin oxide (FTO) substrate at different temperatures. The solar cell structure that we used in this thesis is FTO/d-TiO 2 /In 2 S 3 /C 2 ZTS 4 /Au. A d-TiO 2 is a compactly deposited TiO 2 layer about 40nm of thickness by spray method. The spraying temperature for the In 2 S 3 buffer layer and the C 2 ZTS 4 absorber layer were systematically investigated. Devices fabricated under different spraying temperatures were investigated and characterized. The optimum temperature for the In 2 S 3 buffer layer and the C 2 ZTS 4 absorber layer were 360°C and 380°C, respectively. The C 2 ZTS 4 layer sprayed at low temperature (340°C) results in low quality crystallinity with secondary phases (ZnS and CuxS) and poor adhesion. The absorber layer sprayed at high temperature showed higher crystalline quality but the entire device performance was degraded due to poor fill factor (<25%). Moreover, the sheet resistance of the C 2 ZTS 4 layer was drastically increased at high temperature spraying. The best cell showed a PCE of 4.4%, V OC of 410mV, J SC of 30.4mA/cm 2 and FF of 35.3% under simulated AM 1.5G illumination condition. The future direction of this work is discussed within the context of the significant research underway globally to develop cost-effective photovoltaics.