Ruilong Yang

High-efficiency CdTe thin-film solar cell with a mono-grained CdS window layer

High-crystalline-quality CdS thin films with a mono-grained layer of submicron grain size were successfully fabricated. CdS thin films prepared by chemical bath deposition were re-crystallized under different chemical atmospheres. The microstructure of the CdS films, the hetero-junction interface CdS/CdTe, and therefore the solar cell performance were critically dependent on the film process history of the window CdS layers. Heat treatment of a CdS precursor film coated with a CdCl2 layer and under a high CdCl2 vapor pressure reduced over-oxidation at the grain surface and promoted in-plane grain coalescence along the CdS/FTO (F-doped SnO2) interface. A high-crystalline-quality, mono-grain CdS layer ensured homogenous intermixing of CdS and CdTe at the junction interface. A short-circuit current as high as 25.1 mA cm−2 was obtained for a mono-grain-CdS/CdTe solar cell. The corresponding solar cell efficiency is 14.6%.

Cu-doped CdS and its application in CdTe thin film solar cell

Cu is widely used in the back contact formation of CdTethin filmsolar cells. However, Cu is easily to diffuse from the back contact into the CdTe absorber layer and even to the celljunction interface CdS/CdTe. This phenomenon is generally believed to be the main factor affecting the CdTesolar cell stability. In this study Cu was intentionally doped in CdSthin film to study its effect on the microstructural, optical and electrical properties of the CdS material. Upon Cudoping, the VCd− and the surface-state-related photoluminescence emissions were dramatically decreased/quenched. The presence of Cu atom hindered the recrystallization/coalescence of the nano-sized grains in the as-deposited CdSfilm during the air and the CdCl2 annealing. CdTethin filmsolar cell fabricated with Cu-doped CdS window layers demonstrated much decreased fill factor, which was induced by the increased space-charge recombination near the p-n junction and the worsened junction crystalline quality. Temperature dependent current-voltage curve measurement indicated that the dopedCu in the CdS window layer was not stable at both room and higher temperatures.

High efficiency CdTe solar cells with a through-thickness polycrystalline CdTe thin film

The absorber layer in a CdTe solar cell is made of a polycrystalline CdTe thin film with a thickness of several μm. It is highly desirable that the CdTe film could be made of through-thickness CdTe grains. In such a case, the photon-generated carriers could be transported to the positive and negative contacts without encountering any grain boundaries in CdTe, thus significantly reducing the carrier scattering/recombination occurring at the grain boundary. We propose and demonstrate that one of the most important roles for the presence of oxygen during the CdTe film growth is to in situ form a low-melting liquid mixture of oxides (CdTeOx, TeOx and CdOx), Te, and CdTe on the growth front of the CdTe grain surface. This liquid mixture, having a thickness of ∼50 nm, assists material transport from the vapor through the liquid to the liquid–solid interface, where Cd, Te, and Cd–Te molecules reacted to epitaxially deposit on the CdTe growth front. Assisted by a highly crystalline mono-grained CdS window layer template, CdTe preferential growth with through-thickness grains along the normal direction of the film has been achieved. CdTe solar cells fabricated with such CdTe absorbers demonstrate much enhanced external quantum efficiency, and cells with efficiencies as high as 14–15.2% have been fabricated.

Space-charge limited current in CdTe thin film solar cell

In this study, we demonstrate that space-charge limited current (SCLC) is an intrinsic current shunting leakage in CdTe thin film solar cells. The SCLC leakage channel, which is formed by contact between the front electrode, CdTe, and the back electrode, acts as a metal-semiconductor-metal (MSM) like transport path. The presence of SCLC leaking microchannels in CdTe leads to a band bending at the MSM structure, which enhances minority carrier recombination and thus decreases the minority carrier lifetime in CdTe thin film solar cells. SCLC was found to be a limiting factor both for the fill factor and the open-circuit voltage of CdTe thin film solar cells.In this study, we demonstrate that space-charge limited current (SCLC) is an intrinsic current shunting leakage in CdTe thin film solar cells. The SCLC leakage channel, which is formed by contact between the front electrode, CdTe, and the back electrode, acts as a metal-semiconductor-metal (MSM) like transport path. The presence of SCLC leaking microchannels in CdTe leads to a band bending at the MSM structure, which enhances minority carrier recombination and thus decreases the minority carrier lifetime in CdTe thin film solar cells. SCLC was found to be a limiting factor both for the fill factor and the open-circuit voltage of CdTe thin film solar cells.

High efficient thin film CdTe solar cells

CdTe thin film solar cell with an absorber layer as thin as 0.5 μm was fabricated. An efficiency of 7.9 % was obtained for a 1-μm-thick CdTe solar cell. The experimental results presented in this study demonstrated that 1-μm-thick absorber layer is thick enough to fabricate CdTe solar cell with a decent efficiency. Formation of mono-grain CdS layer with grain size in submicron was fabricated. Heat treatment of a CdS precursor layer coated with a CdCl2 layer and under a high CdCl2 vapor pressure reduced oxide formation at the grain surface and promoted grain coalescence. High crystalline quality mono-grain CdS layer ensured homogenous intermixing of CdS and CdTe at the CdS/CdTe interface. A saturated junction leakage current in the order of 10-10 A/cm2 and a short-circuit current as high as 25.1 cm/cm2 were obtained for a mono-grain-CdS/CdTe solar cell. The corresponding solar cell efficiency is 14.6%.