xGaxSe2 Solar Cells Research Articles

Systematic survey of suitable buffer and high resistive window layer materials in CuIn1−xGaxSe2 solar cells by numerical simulations

This work investigates the impact of opto-electronical buffer (b) and high resistive window layer (w) properties, i.e. band gap Eg(b,w) and electron affinity χe(b,w), on the device performance of chalcopyrite CuIn1−xGaxSe2 (CIGS) solar cells by numerical simulations with SCAPS. We established an initial device model based on an experimental device and its J–V, C–V, and EQE data at room temperature as well as its quantified depth profile for the [Ga]/([Ga]+[In]) ratio (GGI). The device features a non-uniform CIGS doping profile as well as a strongly doped CIGS surface layer. Based on our simulations that include various buffer layer materials, we argue that the most suitable buffer and window layer is Zn1−zMgzO. The potential gain in efficiency is up to 0.9% absolute which corresponds to a relative gain of 4.1%.

Influence of process parameters on the gallium composition of a CuIn1−xGaxSe2 solar cell on the efficiency of non-vacuum blade coating stacking

The distribution of an energy gap gradient can influence photovoltaic conversion, which in turn influences the performance of solar cells. On the other hand, controlling the energy gap gradient can influence the range of the absorption spectrum, and thus the overall solar photovoltaic conversion efficiency is affected. To control the energy gap of a CuIn1−xGaxSe2 (CIGS) solar cell, this study employs a non-vacuum stack coating technique. The chemical composition of the ink is regulated and a multilayer stack is coated to investigate (1) the distribution range of the energy gap and (2) the variability of conversion efficiency in multilayer stack and single-layer composition conditions, in order to determine the feasibility of using a coating stack design to change and control an energy gap. The effect of film composition on crystallinity is tested through high-temperature selenylation. Finally, a miniature solar module is prepared and the conversion efficiency is found to improve from 5.02% to 7.08%, and the feasibility of energy gap control is demonstrated.

Influence of laser wavelength on the laser induced breakdown spectroscopy measurement of thin CuIn 1− xGaxSe2 solar cell films

Laser induced breakdown spectroscopy (LIBS) measurement of thin CuInxGa1−xSe2 (CIGS) films (1.2–1.9μm) with varying Ga to In ratios was carried out using the fundamental (1064nm) and second harmonic (532nm) wavelength Nd:YAG lasers (τ=5ns, spot diameter=150μm, top-hat profile) in air. The concentration ratios of Ga to In, xGa≡Ga/(Ga+In), of the CIGS samples ranged from 0.027 to 0.74 for which the band gap varied nearly proportionally to xGa from 0.96 to 1.42. It was found that the LIBS signal of 1064nm (1.17eV) wavelength laser was significantly influenced by xGa, whereas that of the 532nm (2.34eV) laser was consistent for all values of xGa. The observed dependency of the LIBS signal intensity on the laser wavelength was attributed to the large difference of photon energy of the two wavelengths that changed the absorption of incident laser energy by the film. The 532nm wavelength was found to be advantageous for multi-shot analysis that enabled depth profile analysis of the thin CIGS films and for improving measurement precision by averaging the multi-shot LIBS spectra.

Electrodeposition of CuIn1−xGaxSe2 solar cells with a periodically-textured surface for efficient light collection

The photoresponse of CuIn1−xGaxSe2 (CIGS) solar cells is improved using a periodically-textured structure as an antireflection layer. The CIGS absorber layers were prepared by one-step electrodeposition from an aqueous solution containing 12mM CuSO4, 25mM In2(SO4)3, 28mM Ga2(SO4)3, and 25mM SeO2. The electrodeposited CIGS films exhibit the (112)-preferred orientation of the chalcopyrite structures and feature improved film stoichiometry after the selenization process. In addition, the lower bandgap value of 0.97eV is caused by the discrepancy of the reduction potentials for each constituent, resulting in insufficient Ga content in the deposited films. Using self-assembled silica nanoparticles as the etching mask, periodically-textured structures can be easily formed on an indium tin oxide (ITO)-coated soda lime glass to achieve a low average reflection (<10.5%) in a wide wavelength and incident angle range. With the periodic textured structures suppressing light reflections from the front surface, the photogenerated current in the semi-transparent CIGS solar cells made with transparent conducting electrodes is 1.82 times higher than they otherwise would be.

Improvement of selenium analysis during laser-induced breakdown spectroscopy measurement of CuIn1−xGaxSe2 solar cell films by self-absorption corrected normalization

The analysis of Se emission lines (196.089 nm and 203.984 nm) of CuIn1−xGaxSe2 (CIGS) thin solar cell films by laser-induced breakdown spectroscopy (LIBS) is reported. With CIGS thin films fabricated by co-evaporation or sputtering methods, LIBS measurement was carried out using a nanosecond laser (λ = 532 nm, τ = 5 ns, top-hat profile) and a CCD spectrometer (gate width = 1.05 ms, gate delay = 0.2 μs) in a flowing Ar gas environment (20 L min−1). From the comparison of calibration curves, it was found that the correlation between the Se/Cu LIBS intensity ratio and the Se/Cu concentration ratio could be improved significantly by applying the self-absorption correction procedure introduced in this study, demonstrating the significance of self-absorption in those Se emission lines. It was shown that normalization alone could reduce the relative standard deviation but was insufficient for accurate LIBS analysis of Se in CIGS thin films.

On microscopic compositional and electrostatic properties of grain boundaries in polycrystalline CuIn1−xGaxSe2

We report on the microscopic characteristics of polycrystalline CuIn1−xGaxSe2 thin films probed with Auger electron spectroscopy, cathodoluminescence spectroscopy, secondary ion mass spectrometry, and work function measurements. Confirming theory, we find a substantial reduction in Cu content from grain interior to boundary and a p-type potential barrier that acts to reduce hole recombination. Such compositional and electrostatic variations between grain boundaries and grain interiors in CuIn1−xGaxSe2 solar cell absorber layers may improve the overall photovoltaic efficiency. The high degree of intergranular inhomogeneity emphasizes the importance of detailed grain-by-grain analysis. These results show that careful specimen preparation and ultrahigh vacuum conditions, coupled with nanoscale instrumental resolution, are pivotal for such analysis.

Performance comparison of hybrid sputtering/evaporation CuIn1−xGaxSe2 solar cells with different transparent conducting oxide window layers

In this work we report the results of a study where solar cells were prepared with CuIn1−xGax Se2 (CIGS) as the absorber layer deposited by a two-stage hybrid sputtering/evaporation method and finished with two different transparent conducting oxides (TCO): i-ZnO/ZnO:Al or indium tin oxide (ITO). The solar cell structure is soda lime glass/Mo/CIGS/CdS/TCO. The ZnO and the ITO window layers were deposited by RF-magnetron and DC-magnetron sputtering at room temperature respectively. The deposition of these two TCOs was optimized to obtain similar parameters. In both cases the control samples showed sheet resistivities at around 50Ω/□ and transmission coefficients around 85% in the visible spectral range for layers 300nm thick. The I–V characteristics of finished solar cells under simulated solar spectrum illumination have been measured and the cell parameters analyzed.

Secondary barriers in CdS–CuIn1−xGaxSe2 solar cells

Previous work on CdS–CuInSe2 (CIS) solar cells, which reported distortions of their current-voltage (J–V) curves under red illumination, is expanded in this work to include CdS–CuIn1−xGaxSe2 cells with variable Ga and CIS cells with variable CdS thickness. Different amounts of J–V distortion were observed in these cells under red light. The details are in good agreement with predictions of a photodiode model, in which a secondary barrier caused by the positive conduction-band discontinuity (spike) at the buffer–absorber interface is responsible for the current limitation. The illumination of the cell with high-energy photons lowers the barrier due to buffer photoconductivity, and thus removes the J–V distortion.

Effect of band offset on the open circuit voltage of heterojunction CuIn1−xGaxSe2 solar cells

The reasons behind why the theoretically estimated open circuit voltage (Voc) of CuIn1−xGaxSe2 solar cells with large x values has not been realized are discussed. Typically, the reduction in Voc is estimated only on the basis of the conduction-band offset between the absorber and the window material. The importance of the electron affinity difference between the window and the transparent electrode must also be taken into account. Based upon both of these factors, a material selection guideline is reported for the window and the transparent electrode layers suitable for high-x CuIn1−xGaxSe2 absorber-based solar cells.