Andrew Clayton

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%.

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.

Chemical analysis of Cd1−xZnxS/CdTe solar cells by plasma profiling TOFMS

Abstract Thin film CdTe photovoltaic (PV) devices and reference layers obtained by the atmospheric pressure metalorganic vapour deposition (AP-MOCVD) method have been studied for their chemical structure using plasma profiling time-of-flight-mass spectroscopy (PP-TOFMS, also called glow discharge TOFMS). Different levels of arsenic (As) dopant in CdTe films were measured by PP-TOFMS and compared to results obtained from a more conventional depth profiling method (secondary ion mass spectrometry or SIMS). This comparison showed that PP-TOFMS has the sufficient sensitivity towards detection of the As dopant in CdTe and hence is suited as a rapid, low vacuum tool in controlling the large scale production of CdTe PV materials.

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.

A new approach to thin-film SnS PV using MOCVD

SnS thin films were deposited by an inline metal organic chemical vapour deposition process using tetramethyltin and diethyldisulfide as precursors. A N2/H2 carrier was used with pre-mixing of the precursors before overhead injection into the deposition chamber. NSG AB soda lime glass was used as the substrate with area of 50 × 50 mm2. The resulting SnS films had calculated band gaps between 1.3 and 1.5 eV. Scanning electron microscopy showed relatively large grains ranging from 0.5 to 1 μm across for a SnS film sample deposited at 556–558 °C. X-ray diffraction confirmed the films to be SnS, but with small concentrations of impure phases such as Sn2S3. Post-growth annealing treatment in a N2 atmosphere at 435 °C using SnCl2/MeOH solution at different molar concentrations only showed changes to the film at 0.05 M. The 0.05 M SnCl2/MeOH treatment was aggressive with blistering and etching occurring.

One-step growth of thin film SnS with large grains using MOCVD

Abstract Thin film tin sulphide (SnS) films were produced with grain sizes greater than 1 μm using a one-step metal organic chemical vapour deposition process. Tin–doped indium oxide (ITO) was used as the substrate, having a similar work function to molybdenum typically used as the back contact, but with potential use of its transparency for bifacial illumination. Tetraethyltin and ditertiarybutylsulphide were used as precursors with process temperatures 430–470 °C to promote film growth with large grains. The film stoichiometry was controlled by varying the precursor partial pressure ratios and characterised with energy dispersive X-ray spectroscopy to optimise the SnS composition. X-ray diffraction and Raman spectroscopy were used to determine the phases that were present in the film and revealed that small amounts of ottemannite Sn2S3 was present when SnS was deposited on to the ITO using optimised growth parameters. Interaction at the SnS/ITO interface to form Sn2S3 was deduced to have resulted for all growth conditions.

Accessing the quantum palette: quantum-dot spectral conversion towards the BIPV application of thin-film micro-modules

To demonstrate the potential for building integrated photovoltaics (BIPV) incorporation of thin-film photovoltaics, commercially available quantum dots (QDs) have been deposited, as part of a poly(methyl methacrylate) (PMMA) composite film, on a cadmium telluride (CdTe) micro-module. This resulted in an increase in photocurrent generation through the luminescent down-shifting (LDS) process. The optical properties of these films were characterized through UV–vis spectroscopy. The impact of the film on the micro-module was studied through current–voltage (I–V) and external quantum efficiency measurements. Further layers were added to the initial single-layer LDS film, however no additional improvement to the micro-module were observed. Additionally, a range of emission wavelengths have been explored. The majority of these films, when tested on a CdTe device, were shown to improve the photocurrent generation whilst also visually displaying the vivid colour palette provided by quantum confined materials. The future feasibility of using QD based LDS films for large scale BIPV-based power generation has also been discussed.

A combinatorial approach to the optimisation of Cd (1−x) Zn x S layers for CdTe solar cells

A combinatorial methodology has been adopted to determine the optimum composition of a Cd(1-x)ZnxS window layer for CdTe solar cells. The methodology generated a large, self consistent dataset which permitted an unambiguous relationship between x, conversion efficiency and related cell parameters to be determined. An optimum composition of x = 0.57 was shown to maximise cell efficiency. Analysis of J - V curves, measured over 72 separate cells show that both short circuit current, JSC, and fill factor, FF, values increase with respect to x over the range 0.1-0.57. EQE measurements show that further increases in JSC value are limited by the band gap of the highly resistive transparent (HRT) ZnO layer. The methodology demonstrates a rapid route, compared to conventional experiments, to the further optimisation of CdTe solar cells.

Chapter 5:Thin Film Cadmium Telluride Solar Cells

This chapter discusses a number of deposition techniques used to produce polycrystalline CdTe solar cells, including progress of photovoltaic (PV) performances in recent years. Focus is on the CdTe absorber and the effects from impurities, which are dependent on the process conditions used, but also due to self-compensating nature of the material itself influencing the acceptor levels in the layer. Impurities can introduce deep donor/acceptor levels that act as traps for both majority and minority carriers. This leads to greater recombination and reduced carrier lifetimes, causing a loss in the level of generated photocurrent and overall performance of the PV cell. Impurities are typically concentrated at the CdTe grain boundaries, making grain size an important parameter for defect density control. Post-growth treatment using CdCl2 and annealing improves PV performances in several ways: grain re-crystallisation and growth; inter-diffusion at the CdS–CdTe interface removing defects related to the lattice mismatch between the two layers; and passivation of deep acceptor states through complex formation with the ClTe+ shallow donor. High p-type doping is necessary for the formation of a back contact with good ohmic properties without a Schottky barrier restricting conduction of majority carriers. Stable back contacts are also required, with the Sb2Te3–Mo system possibly offering the best solution. Finally MOCVD is presented as a prospective technique for large-scale industrial production of CdTe solar modules, with discussion of the beneficial impact in reducing CdTe absorber thickness and the processing challenges associated with it.