Vincent Barrioz

Grain and crystal texture properties of absorber layers in MOCVD-grown CdTe/CdS solar cells

The microstructure of 4?13 ?m thick CdTe absorber layers in CdTe/CdS/ITO/glass solar cell structures grown by metal-organic chemical vapour deposition (MOCVD) at 350 ?C has been studied. The crystalline texture, lattice parameter and grain size were measured as a function of thickness for the as-grown layers, and as a function of annealing temperature and time for annealing in both nitrogen (N2) and cadmium chloride (CdCl2) environments. The average grain sizes developed with thickness as r (?m) = 0.050x ? 0.10 (4 < x < 12 ?m), and this behaviour is contrasted with that for close-spaced sublimation material grown at 500 ?C. Annealing in both ambients promoted grain growth (with Rayleigh grain size distribution functions and Burke?Turnbull exponents being n = 7 at 440 ?C and ~4 at 400 ?C), a development of the grown-in preferred orientation from [1?1?1] to [2?1?1], and relief of the grown-in compressive stress. A growth mechanism by which development of the [2?1?1] preferred orientation may accompany grain growth is described. It is concluded that MOCVD growth at temperatures higher than 350 ?C used here will be required to produce the larger grain sizes required for photovoltaic applications.

Highly Arsenic Doped CdTe Layers for the Back Contacts of CdTe Solar Cells

In an effort to overcome the lack of a suitable metal as an ohmic back contact for CdTe solar cells, a study was carried out on the potential for using a highly arsenic (As) doped CdTe layer with metallization. The deposition of full CdTe/CdS devices, including the highly doped CdTe:As and the CdCl2 treatment, were carried out by metal organic chemical vapour deposition (MOCVD), in an all-in-one process with no etching being necessary. They were characterized and compared to control devices prepared using conventional bromine-methanol back contact etching. SIMS and C-V profiling results indicated that arsenic concentrations of up to 1.5 × 1019 at·cm-3 were incorporated in the CdTe. Current-voltage (J-V) characteristics showed strong improvements, particularly in the open-circuit voltage (Voc) and series resistance (Rs): With a 250 nm thick doped layer, the series resistance was reduced from 9.8 Ω·cm2 to 1.6 Ω·cm2 for a contact area of 0.25 cm2; the J-V curves displayed no rollover, while the Voc increased by up to 70 mV (~ 12 % rise). Preliminary XRD data show that there may be an As2Te3 layer at the CdTe surface which could be contributing to the low barrier height of this contact.

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.

Progress towards highly stable and lead-free perovskite solar cells

High-performance perovskite solar cells have attracted increased attention for photovoltaic applications and potentially replacing the predecessor generations. Nevertheless, the stability issues and the lead content has always been among the major concerns that barriers perovskite solar cells from commercialization. This review presents the discussion towards the inherent instability of perovskite solar cells and the development towards replacing lead with discussion towards their performance and challenges. The degradation of perovskite active layer would release toxic substance into the environment. The development towards low-toxic, lead-free and efficient perovskite solar cells is the key for a sustainable solar energy generation with the application of perovskite solar cells.

A comparison of in situ As doping with ex situ CdCl2 treatment of CdTe solar cells

A comparison has been made of MOCVD grown CdTe/CdS solar cells processed either by ex situ annealing with CdCl2 or doping with arsenic, in situ, together with various optional anneals. A materials comparison was made of both routes using Jsc measurements on arrays of gold contacts to the CdTe. The Jsc increased from around 1 mA cm-2 for undoped and unannealed layers to a range of 25-30 mA cm-2 for CdCl2 annealed layers. In situ arsenic doping resulted in Jsc values up to 18 mA cm-2. The annealing characteristics were very different for these films, compared with the CdCl2 annealed films, with annealing at 500°C dramatically reducing the Jsc. Only annealing under nitrogen at 400°C produced an improvement in Jsc and further evidence from SIMS analysis suggests that hydrogen passivation of the arsenic dopant may have a significant effect on the dopant activity.

In-situ Stress Monitoring with a Laser-Fibre System

A novel laser-fibre system is used for in-situ stress monitoring of thin film deposition on a glass substrate. The authors have shown that it could also be used to monitor the temperature of the substrate, as well as the thickness of the growing layer, using the thin film interference equation. Single layers of ZnS and YF 3 optical coatings have been evaporated onto borosilicate glass, doped with cerium oxide, in the environment of a high vacuum (HV) chamber. The results obtained show that the novel laser-fibre system used as a stress monitor can give insights into the causes of thin film residual stress in vacuum evaporation.

Reducing series resistance in Cu2ZnSn(S,Se)4 nanoparticle ink solar cells on flexible molybdenum foil substrates

Earth abundant Cu2ZnSnS4 nanoparticle inks were deposited on molybdenum foil substrates and subsequently converted to high quality thin film Cu2ZnSn(S,Se)4 photovoltaic absorbers. Integration of these absorbers within a thin film solar cell device structure yields a solar energy conversion efficiency which is comparable to identical devices processed on rigid glass substrates. Importantly, this is only achieved when a thin layer of molybdenum is first applied directly to the foil. The layer limits the formation of a thick Mo(S,Se)x layer resulting in a substantially reduced series resistance.

Chapter 2:Fundamentals of Thin Film PV Cells

This chapter sets out the science behind thin film photovoltaic (PV) materials, the fundamentals of the PV effect and requirements from a material point of view. It provides the basis for the reader to reach an understanding of the solar energy available for a PV solar cell to convert to electricity. The fundamental limitations of the absorbing material within a single junction solar cell, due to its characteristics and in particular its bandgap energy, are described. The material and device design opportunities to go beyond the limits of single junction solar cells are also introduced.

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.