Bastian Siepchen

Effect of chlorine activation treatment on electron beam induced current signal distribution of cadmium telluride thin film solar cells

We have investigated CdTe thin film solar cells without activation treatment and with CdCl2 activation treatment at temperatures between 370 and 430 °C using a constant activation time of 25 min. For this purpose, CdS/CdTe layers were deposited by closed-space-sublimation on FTO coated float glass. The solar cells were characterized by measurements of the JV characteristics and quantum efficiencies. In addition, ion polished cross sections of the solar cells were prepared for high-resolution FE-SEM imaging of the microstructure and the simultaneous registration of electron beam induced current (EBIC) signal distribution. By measurement of the EBIC signal distribution, it can be shown that without activation treatment the CdTe grain boundaries itself and grain boundary near regions exhibit no EBIC signal, whereas centres of some singular grains already show a distinct EBIC signal. In contrast, after the chlorine activation treatment, the grain boundary near regions exhibit a significant higher EBIC signal ...

How to control the stoichiometry of cadmium telluride thin film for photovoltaic applications

In this study, Bubes growth model for a cadmium telluride (CdTe) polycrystalline thin film was re-examined with a view of avoiding his assumptions that neglect the vapor pressures of Cd and Te2 near the film. We proposed a new thermodynamic growth model based on the fact that there is an experimentally verified characteristic ratio (α) of equilibrium partial pressures PCd/2PTe2 that depends on the temperature T and CdTe stoichiometry. By writing PCd(0)=2α(0)PTe2(0) and PCd(h)=2α(h)PTe2(h), where α(0) is determined by source stoichiometry, we can solve the equations for α(h) and thereby determine the stoichiometry of the CdTe thin film grown under physical vapor deposition (PVD) conditions. Simulation was performed to predict the stoichiometry of the CdTe thin film as a function of source stoichiometry for various source-film temperature combinations. The results show that for a typical CdTe PVD process with Tsource>Tthin film: (1) stable deposition, without a non-stoichiometric composition shift, can be ...

CdTe solar cells: Nucleation, structure, and performance

CdTe solar cells are strongly influenced in their performance by the morphology of the absorber layer. Therefore, we have systematically investigated CdTe film formation deposited by close space sublimation. At low sample temperature the films tend to form compact layers with preferred (111) orientation which is lost at elevated temperatures above 420°C. For CdS layers without (0001) texture there is in addition a low temperature regime (350°C) with (111) texture loss. Within this temperature regime we have found a large concentration of plane defects related to a hexagonal CdTe layer sequence. After activation treatment the (111) texture is lost for all deposited layers leading to strong recrystallisation of the grains and to rough surfaces and the tendency of pin-hole formation. This effect is minimized for the 350°C deposited layers. As a consequence we have observed maxima in the conversion efficiencies for this temperature in addition to the high temperature regime.