Electrochemical atomic layer deposition (E-ALD) of photovoltaic (PV) materials
Perdue, Brian Robert
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This dissertation investigates the layer-by-layer deposition of CdS and CdTe, two materials used to form photovoltaics (PV) by electrochemical atomic layer deposition (E-ALD). Like atomic layer deposition (ALD) E-ALD achieves compound growth by alternating the deposition of atomic layers of each component element using surface limited reactions, however, unlike ALD, E-ALD does this in the condensed phase. The electrochemical surface limited reactions used for compound formation using E-ALD are known as underpotential deposition (UPD). In E-ALD, compounds are formed by the alternating the UPD of one element onto another element. E-ALD allows for the optimization of solution composition, pH, and deposition potential for each element used in the E-ALD cycle. In this report, 100 cycle deposits of CdTe were grown using E-ALD, which were then analyzed using electron probe microanalysis (EPMA), X-ray diffraction (XRD), spectroscopic ellipsometry (SE) and photoelectrochemistry (PEC), to ascertain film quality. Results indicate that the E-ALD grown CdTe was essentially stoichiometric, crystalline, has the proper bandgap, optical constants, has an external quantum efficiency of 10%, and is p-type, without the need of a post deposition anneal step. CdS was grown by three different condensed phase deposition techniques, chemical bath deposition (CBD), successive ionic layer absorption and reaction (SILAR), and E-ALD. Deposits were then analyzed with the same thin film characterization techniques. Analysis indicates that the deposited CdS films were stoichiometric. E-ALD and SILAR produce crystalline films, which were n-type and had the proper bandgap and optical constants. PEC results showed that E-ALD of CdS was the most photosensitive while CBD was the least, prior to a post deposition anneal. CdS and CdTe are two II-VI compound semiconductors that form a PV ideally suited for terrestrial solar absorption. PV devices using these materials can be fabricated either using the superstrate configuration and the substrate configuration. In this study, PV devices fabricated in the superstrate configuration exhibited excessive stress between the layers and delaminated from the substrate. PV devices made in the substrate configuration produced current densities comparable to modern PV devices with 1/10th the material, however, they appeared to suffer from shunts.