Guillaume Zoppi

Inorganic photovoltaic cells

The inorganic semiconductor materials used to make photovoltaic cells include crystalline, multicrystalline, amorphous, and microcrystalline Si, the III-V compounds and alloys, CdTe, and the chalcopyrite compound, copper indium gallium diselenide (CIGS). We show the structure of the different devices that have been developed, discuss the main methods of manufacture, and review the achievements of the different technologies.

Optical properties of high quality Cu2ZnSnSe4 thin films

Cu2ZnSnSe4 thin films, fabricated on bare or molybdenum coated glass substrates by magnetron sputtering and selenisation, were studied by a range of techniques. Photoluminescence spectra reveal an excitonic peak and two phonon replicas of a donor-acceptor pair (DAP) recombination. Its acceptor and donor ionisation energies are 27 and 7 meV, respectively. This demonstrates that high-quality Cu2ZnSnSe4 thin films can be fabricated. An experimental value for the longitudinal optical phonon energy of 28 meV was estimated. The band gap energy of 1.01 eV at room temperature was determined using optical absorption spectra.

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.

Characterization of CuInSe2 material and devices: comparison of thermal and electrochemically prepared absorber layers

CuInSe2-based absorber layers for photovoltaic devices have been fabricated using two different scalable processes, electrodeposition and sputtering, both followed by thermal annealing. The structural properties of the absorber layers were studied by SEM, XRD and MiniSIMS. Sputtered absorber layers exhibit larger grain sizes than electrodeposited layers, but both types of film consist of randomly orientated crystallites. Electrodeposited layers appear to have a uniform composition with evidence of a MoSe2 layer at the back contact, whilst sputtered layers show no evidence for a MoSe2 layer. The external quantum efficiency spectrum of films and completed devices was measured, and the band gap and broadening parameters were obtained using electroreflectance spectroscopy. A device based on electrodeposited CuInSe2 achieved an AM 1.5 efficiency of 6.6%, whilst a device based on sputtered CuInSe2 had an efficiency of 8.3%. Impedance measurements were used to calculate doping densities of 2 ? 1016 and 4 ? 1015?cm?3 for the electrodeposited and sputtered devices, respectively.

Deposition and characterization of copper chalcopyrite based solar cells using electrochemical techniques

Cu(In,Ga)Se2 films were electrodeposited on molybdenum substrates from a single pH buffered bath and annealed in a reducing selenium atmosphere. The opto-electronic properties of the films were characterized using a potentiostatically- controlled three electrode setup and an electrolyte contact. Pulsed illumination was used to determine the carrier type and the speed of photoresponse. Chopped monochromatic illumination was used to measure photocurrent spectra. The electrodeposited copper chalcopyrite films were compared with films prepared by sputtering and spraying techniques.

Characterisation of Thin Films CuIn1-xAlxSe2 Prepared by Selenisation of Magnetron Sputtered Metallic Precursors

Thin films of CuIn1-xAlxSe2 have been produced by the selenisation of magnetron sputtered Cu/In/Al precursor layers using elemental selenium and the chemical and physical properties of the layers have been determined for different conditions of synthesis. For optimum conditions of synthesis it was found possible to produce single phase films with the chalcopyrite structure. These films were pinhole free, had good adhesion and were conformal to the substrate. The films had uniform depth profiles as determined using the MiniSIMS. The layers were highly photoactive, indicating that they have the potential to be used to fabricate thin film photovoltaic solar cell devices.

Influence of reaction conditions on the properties of solution-processed Cu2ZnSnS4 nanocrystals

Cu2ZnSnS4 nanocrystals were fabricated by hot injection of sulphur into a solution of metallic precursors. By careful control of the reaction conditions it was possible to control the elemental composition of the nanocrystals such that they are suitable for earth abundant photovoltaic absorbers. When the reaction temperature increased from 195 °C to 240 °C the energy band gap of the nanocrystals decreased from 1.65 eV to 1.39 eV. This variation is explained by the identification of a mixed wurtzite–kesterite phase at lower reaction temperatures and secondary phase Cu2SnS3 at higher temperatures. Moreover, the existence of wurtzite structure depends critically on the reaction cooling rate. The reaction time was also found to have a strong effect on the nanocrystals which became increasingly copper poor and zinc rich as the reaction evolved. As the reaction time increase from 15 min to 60 min, the energy band gap increased from 1.42 eV to 1.84 eV. This variation is discussed in terms of the sample doping. The results demonstrate the importance of optimizing the reaction conditions to produce high quality Cu2ZnSnS4 nanocrystals.

Thin film resistive materials: past, present and future

This paper explores the key developments in thin film resistive materials for use in the fabrication of discrete precision resistors. Firstly an introduction to the preparation of thin films and their fundamental properties is given with respect to well established systems such as NiCr, TaN and CrSiO. The effect of doping these systems in both solid and gaseous forms to further refine their structural and electrical properties is then discussed before the performance of more recent materials systems such as CuAlMo and MmAgCuN are reviewed. In addition to performance of the materials themselves, the effect of varying processing parameters such as deposition pressure and temperature and subsequent annealing environment, as well as laser trimming energy and geometry are also studied. It is shown how these parameters can be systematically controlled to produce films of the required properties for varying applications such as high precision, long term stability and high power pulse performance.

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.

A Study of Arsenic Dopant Concentration and Activity as a Function of Growth Conditions in Polycrystalline MOCVD-Grown CdTe

P-type CdTe can be produced via acceptor doping with As. However, as with other II/VI materials, the dopant behaviour is not simple, as there is the potential for compensating species to be formed from intrinsic defects and dopant-defect complexes. A further complication is introduced by the presence of grain boundaries in polycrystalline material. This study demonstrates that dopant concentration is a function of VI/II ratio in the growth ambient, and that resistivity is minimised for a dopant concentration of 18 at.cm -3 . Grain size is also affected by the VI/II ratio, increasing slightly as the growth ambient becomes more Te-rich.