Daniel Lamb

Atomic layer deposition of Ga-doped ZnO transparent conducting oxide substrates for CdTe-based photovoltaics

The atomic layer deposition of gallium doped zinc oxide films is investigated as a method of fabricating transparent conducting oxide substrates for cadmium telluride based photovoltaic cells. The growth parameters and properties of gallium-doped ZnO were established for a range of dopant concentrations. 1 at. % gallium-doped films exhibited the lowest electrical sheet resistances and were selected as substrates to deposit Cd1−xZnxS/CdTe photovoltaic cells. The average current density–voltage characteristics of 16 cells under AM1.5 illumination yielded a conversion efficiency of 10.8% and a fill-factor of 65%.

Combinatorial optimization of Al-doped ZnO films for thin-film photovoltaics

A rapid screening methodology for the development of transparent conducting oxides is presented. The methodology, based on a combination of spectrophotometry, ellipsometry and 4-point probe measurements, was used to map out the opto-electronic properties over a co-sputtered ZnO : Al2O3 film deposited from separate ceramic targets of ZnO and Al2O3. Clear distributions for the carrier density, ne, and mobility, μe, are determined as a function of wt%. Al2O3 content within the film. A minimum resistivity value of 7.6 × 10−4 Ω cm was achieved for a composition of 1.5 wt%. Al2O3.

SIMS analysis of intentional in situ arsenic doping in CdS/CdTe solar cells

A series of CdTe/CdS devices with different tris(dimethylamino)arsine (TDMAAs) partial pressures were grown by metal organic chemical vapour deposition (MOCVD) to investigate the incorporation of arsenic into the bulk. Characterization of the growth layers using secondary ion mass spectrometry (SIMS) showed arsenic concentrations ranging from 1 × 1016 to 1 × 1019 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 dimerization is presented and discussed in terms of reaction kinetics.

Comparative study of trap densities of states in CdTe∕CdS solar cells

Density of deep and shallow states has been investigated in three different kinds of CdTe∕CdS samples, two of which were grown by metal-organic chemical vapor deposition (MOCVD) and one by close-space sublimation (CSS) methods. The MOCVD samples were p doped by As and grown either with or without a ZnO buffer layer between the transparent conductor and CdS layers. Capacitance-voltage, admittance spectroscopy, and quantum efficiency measurements show pronounced effects of As doping and ZnO incorporation. It is found that A centers and vacancies of Cd, usually observed in CSS devices, are absent in the defect spectra of MOCVD samples.

CdCl2 treatment related diffusion phenomena in Cd1−xZnxS/CdTe solar cells

Utilisation of wide bandgap Cd1−x Zn xS alloys as an alternative to the CdS window layer is an attractive route to enhance the performance of CdTe thin film solar cells. For successful implementation, however, it is vital to control the composition and properties of Cd1−x Zn xS through device fabrication processes involving the relatively high-temperature CdTe deposition and CdCl2 activation steps. In this study, cross-sectional scanning transmission electron microscopy and depth profiling methods were employed to investigate chemical and structural changes in CdTe/Cd1−x Zn xS/CdS superstrate device structures deposited on an ITO/boro-aluminosilicate substrate. Comparison of three devices in different states of completion—fully processed (CdCl2 activated), annealed only (without CdCl2 activation), and a control (without CdCl2 activation or anneal)—revealed cation diffusion phenomena within the window layer, their effects closely coupled to the CdCl2 treatment. As a result, the initial Cd1−x Zn xS/CdS bilayer structure was observed to unify into a single Cd1−x Zn xS layer with an increased Cd/Zn atomic ratio; these changes defining the properties and performance of the Cd1−x Zn xS/CdTe device.

Characterization of MOCVD Thin-Film CdTe Photovoltaics on Space-Qualified Cover Glass

This paper details the AM0 conversion efficiency of a metal-organic chemical vapor phase deposition thin-film cadmium telluride (CdTe) solar cell deposited onto a cerium-doped cover glass (100 μm). An AM0 best cell conversion efficiency of 12.4% (0.25-cm2 contact area) is reported. An AM0 mean efficiency of 12.1% over eight cells demonstrated good spatial uniformity. Excellent adhesion of the cell structure to the cover glass was observed with an adhesive strength of 38 MPa being measured before cohesive failure of the test adhesive. The device structure on cover glass was also subject to severe thermal shock cycling of +80 °C to -196 °C, showing no signs of delamination and no deterioration of the photovoltaic (PV) performance.

Investigation into ultrathin CdTe solar cell Voc using SCAPS modelling

Abstract Ultrathin CdTe photovoltaic solar cells were produced by metal organic chemical vapour deposition in a single horizontally configured growth chamber. Solar cell activation was investigated by varying the duration of the CdCl2 layer deposition and 420°C thermal anneal to promote Cl diffusion into the CdTe. Thicker CdCl2 layers used in activation treatment resulted in a greater degree of sulphur interdiffusion, up to 2 at.-%, into the CdTe layer. The thicker CdCl2 activation layer was necessary to lower the reverse saturation current density for obtaining optimum experimental photovoltaic (PV) device performances. Modelling of the PV performances with equivalent solar cell structure for optimised devices using solar cell capacitance simulation software resulted in an overestimated open circuit voltage (Voc). The simulations showed that reduced acceptor states at the CdTe interface with the intermixed region resulted in the largest decrease in Voc when considering large back surface recombination velocities.