Formation pathway of copper indium (di)selenide nanocrystals and solution deposition of copper indium (di)selenide films for photovoltaic applications
Abstract
The development of suitable colloidal nanocrystal inks are a key step in the development of low-cost solar cells since they enable the use of fast and inexpensive coating processes such as spray coating and roll coating to form a thin film photo-absorbing layer. Copper indium selenide (CISe) nanocrystals were synthesized from CuCl, InCl3 and elemental selenium via a simple batch reaction in a coordinating solvent. The use of these nanocrystals for solar cells has been demonstrated by fabricating devices, with the first solar cells having an efficiency of 2.8%. Insights into the formation pathway of these nanocrystals are a key step to gain a better control over their electronic properties. Upon investigations, it was found that the formation of CISe nanocrystals in oleylamine solvent (cis-1-amino-9-octadecene) is preceded by the formation of copper selenide (CuSe) and indium selenide (InSe). The reaction takes place via the chlorination of the hydrocarbon (-CH, -CH2 & -CH 3) groups in oleylamine. In support of the proposed formation pathway, CISe nanocrystals have been synthesized in solvents without any amine or alkene groups, e.g. octadecane. Also, it was found that the reaction between the binary selenides, CuSe and InSe, primarily resulted in the formation of the disordered sphalerite phase of CISe. However, the liquid-solid phase reaction between InCl3 and CuSe in the presence of Se increased the yield of the ordered chalcopyrite phase. The knowledge of the formation pathway, where CuSe (or CuS films deposited on a substrate) reacts with InCl3 and Se in oleylamine to form CISe, has been used to develop a chemical liquid deposition (CLD) technique which enables the deposition of CISe thin films directly on to a substrate from a solution of precursors. This technique enables the fabrication of solar cells on low-cost, easy-to-install, flexible polymeric substrates by circumventing the high temperature (500 Celsius) vapor-phase processing steps. The use of these CLD-based films for applications in solar cells has been demonstrated by fabricating Glass/Mo/CuInSe2/CdS/i-ZnO/ITO/Ag devices of 2% efficiency. Based on the advantages of the CLD technique, a solution-selenization technique was also developed which facilitates the reaction between any deposited film and selenium in the liquid phase at temperatures in the vicinity of 300 Celsius. CuInS2 films deposited on flexible molybdenum foil substrates were reacted with selenium in oleylamine to form CuIn(S,Se)2 thin films of the desired thickness. Our first solar cells processed from these films demonstrated an efficiency of 2.85%.
Degree
Ph.D.
Advisors
Hillhouse, Purdue University.
Subject Area
Alternative Energy|Chemical engineering|Materials science
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