Engineering CuInGaSSe 2 Surface Properties to Enhance Device Performance
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Engineering CuInGaSSe2 Surface Properties to Enhance Device Performance Shalini Menezes, Yan Li, Sharmila J. Menezes InterPhases Research, Thousand Oaks, California 91360, USA.
ABSTRACT The CuInGaSSe2/CdS heterostructure interface has a special effect on the performance of an important thin film photovoltaic device. The CdS buffer layer is essential to stabilize the performance of CuInGaSSe2 based devices. It adjusts the lattice mismatch at the absorber/window interface, repairs CuInGaSSe2 surface defects and protects it from air oxidation. Unfortunately, the CdS material has many environmental issues. This paper reports an alternate chemical approach to engineer the interface defects in CuInGaSSe2 and maximize its PV output. It describes a simple processing step to manipulate the defect density. This step could potentially reduce sensitivity to the ambience, widen the surface bandgap and replace the current hazardous processes used in state-of-the-art CuInGaSSe2 modules. Photocurrent and spectral response measurement in an electrolytic medium monitor the effects of surface modification, specific metal ions and time. The CuInGaSSe2 films respond easily to a number of external stimuli with either positive or negative changes in the electro-optic properties. Strong time dependence of the photocurrent suggests a dynamic equilibrium of point defects in the CuInGaSSe2 film. The results provide new insights into the effects of stoichiometry, deposition methods and oxide formation, on the defect chemistry. They also provide directions for reconfiguring the deep defects for enhanced device performance without the need for toxic etchants or buffer layers, and the environmental hazards associated with these steps. 1. INTRODUCTION Thin-film solar cells based on CuInSe2 alloys provide a viable path to affordable photovoltaic (PV) electricity generation in high-volume. To date, these solar cells offer the best combination of low cost & weight, high reliability and good performance for terrestrial and space power needs. Although these cells are now commercially available, many issues remain unresolved. For example, the energy conversion efficiency typically declines as cell size increases. The bandgap engineering of CuInSe2 with Ga to match the solar spectrum is ineffective with most deposition methods; the Ga tends to diffuse to the back surface of CuInGaSSe2 layer. Additional sulfurization steps are needed to increase the surface bandgap, which in turn increase the cost and complexity [1]. Also, the presence of Ga in CuInGaSSe2 film increases its sensitivity to the deposition parameters and its environment. Thus, a CdS buffer layer is indispensable to stabilize the performance of devices based on Ga-containing CuInSe2 alloys. The CdS can repair the surface defects in the CuInGaSSe2 film, adjust the lattice mismatch between CuInGaSSe2 and the transparent conducting oxide (TCO) window, and prevent the films from air oxidation. Unfortunately, the use and disposal of CdS pose many unfavorable repercussions to the manufactu
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