A multi-variant investigation of CdTe/CdS photovoltaic material

Abstract

Six matrices of experiments were designed and carried out to investigate various deposition parameters of CdS/CdTe solar cells deposited by MOCVD. The results of the matrices were analysed using a criterion to establish which growth parameters are significant and warrant further investigation. The most significant parameters were CdTe growth temperature and in situ arsenic
doping of the CdTe absorber layer. Characterisation of the device structures showed CdTe grain enlargement from I μm to -3μm at the higher deposition temperature of 390°C.
A series of CdS/CdTe de vices with different TDMAAs partial pressures were grown by MOCVD to investigate the incorporation of arsenic in the bulk. Characterisation of the device structures
using SIMS analysis showed arsenic concentrations ranging from Ix l0¹⁶ to lx10¹⁹ atoms cm^-3• A square law dependence of arsenic concentration on the TDMAAs vapour concentration was observed. A reaction mechanism for the decomposition of TDMAAs precursor via dimerisation is presented and discussed in terms of reaction kinetics. A model for the behaviour of arsenic in
polycrystalline CdTe material, based on only partial passivation of grain boundaries and saturation of the grain boundaries at arsenic concentrations greater than 2.5xl0¹⁸ atoms cm^·3 is outlined and discussed.
A LBIC apparatus was designed and constructed in house and used to characterise CdS/CdTe device structures. The LBIC response from the devices was found to generate a small AC signal superimposed on a large DC current. Results are presented on the effect of arsenic concentration in the CdTe bulk on the induced current and uniformity. A plot of the arsenic concentration versus LBIC response and de vice efficiency clearly goes through a maximum in terms of induced current and device efficiency at 2x 10¹⁸ atoms cm^·3. Each device scan shows microscopic variations in the induced current with new linear "furrow defects" features that are too large to correlate directly to the grain boundaries.
Attempts to engineer large grains in the CdS and CdTe layers by depositing the device structures onto Cd nuclei, Au nano-structured templates and patterned photo-resist were investigated. The Cd nuclei templates exhibit templating effects in CdTe but not uniformly across the substrate. Texturing the glass substrates increases the density of nuclei across the substrate area. The Au nano-structured templates also produce a templating effect and show evidence of a change in preferred orientation in CdTe from the (111) plane to the (3 11) plane.

Date of Award	May 2008
Original language	English
Awarding Institution	Bangor University
Sponsors	EPSRC
Supervisor	Stuart Irvine (Supervisor)