Cu(In,Ga)Se2 Cells Research Articles

Light-enhanced reverse breakdown in Cu(In,Ga)Se2 solar cells

Partial shading of solar modules can subject shaded cells to significant reverse bias, often large enough to force them into electrical breakdown, possibly resulting in irreversible damage. Therefore, better understanding of reverse current–voltage characteristics might lead to improvements in the design of solar modules. The focus of this study is the breakdown behavior of Cu(In,Ga)Se2 (CIGS) cells in darkness and under illumination. Two series of CIGS cells were investigated, with CdS and Zn–Sn–O buffer layers of varying thickness. Electrical breakdown was found to be highly dependent on the buffer layer. Under blue illumination a remarkable decrease in breakdown voltage was observed for both buffer types. Metastable defects in the buffer/CIGS interface region are tentatively proposed as the source of this effect and tunnelling is suggested as the main mechanism responsible for breakdowns.

Optical properties of Cd1−xZnxS thin films for CuInGaSe2 solar cell application

Abstract The photovoltaic Cd1−xZnxS thin films, fabricated by chemical bath deposition, were successfully used as n-type buffer layer in CuInGaSe2 (CIGS) solar cells. Comprehensive optical properties of the Cd1−xZnxS thin films were measured and modeled by spectroscopic ellipsometry (SE), which is proven to be an excellent and non-destructive technique to determine optical properties of thin films. The optical band gap of Cd1−xZnxS thin films can be tuned from 2.43 eV to 3.25 eV by controlling the Zn content (x) and deposition conditions. The wider-band-gap Cd1−xZnxS film was found to be favorable to improve the quantum efficiency in the wavelength range of 450–550 nm, resulting in an increase of short-circuits current for solar cells. From the characterization of quantum efficiency (QE) and current–voltage curve (J-V) of CIGS cells, the Cd1−xZnxS films (x = 0.32, 0.45) were demonstrated to significantly enhance the photovoltaic performance of CIGS solar cell. The highest efficiency (10.5%) of CIGS solar cell was obtained using a dense and homogenous Cd0.68Zn0.32S thin film as the buffer layer.

Development of new buffer layers for Cu(In,Ga)Se2 solar cells

Abstract Recent progress in the field of Cu(In,Ga)Se2 (CIGS) thin film solar cell technology is briefly reviewed. New wide-bandgap In x (OOH,S) y and ZnS x (OH) y O z buffers for CIGS solar cells have been developed. Advances have been made in the film deposition by the growth process optimization that allows the control of film properties at the micro- and nanolevels. To improve the CIGS cell junction characteristics, we have provided the integration of the developed Cd-free films with a very thin CdS film. Transmittances of the developed buffers were greatly increased compared to the standard CdS. In x (OOH,S) y buffer has been applied to low-bandgap CIGS devices which have shown poor photovoltaic properties. The experimental results obtained suggest that low efficiency can be explained by unfavorable conduction band alignment at the In x (OOH,S) y /CIGS heterojunction. The application of a wide-gap Cu(In,Ga)(Se,S)2 absorber for device fabrication yields the conversion efficiency of 12.55 %. As a result, the In x (OOH,S) y buffer is promising for wide-bandgap Cu(In,Ga)(Se,S)2 solar cells, however, its exploration for low-bandgap CIGS devices will not allow a high conversion efficiency. The role played by interdiffusion at the double-buffer/CIGS heterojunction and its impact on the electronic structure and device performance has also been discussed.

Investigation of rapid thermal annealing on Cu(In,Ga)Se2 films and solar cells

Rapid thermal annealing (RTA), with fast ramp rate, was performed on several Cu(In,Ga)Se2 (CIGS) films and solar cells under various peak annealing temperatures and holding times. Characterizations were made on CIGS films and cells before and after RTA treatments to study effects of RTA on the CIGS film properties and cell performance. In addition, AMPS-1D device simulation program was used to study effects of defect density on the cell performance by fitting the experimental data of RTA-treated CIGS cells. The results show that RTA treatments under optimal annealing condition can provide significant improvements in the electrical properties of CIGS films and cell performance while preserving the film composition and microstructure morphology.

Band-gap grading in Cu(In,Ga)Se2 solar cells

The quaternary system Cu(In,Ga)Se2 (CIGS) allows the band gap of the semiconductor to be adjusted over a range of 1.04–1.67 eV. Using a non-uniform Ga/In ratio throughout the film thickness, additional fields can be built into p-type CIGS-based solar cells, and some researchers have asserted that these fields can enhance performance. The experimental evidence that grading improves device performance, however, has not been compelling, mostly because the addition of Ga itself improves device performance and hence a consistent separation of the grading benefit has not always been achieved. Numerical modeling tools are used in this contribution to show that (1) there can be a beneficial effect of grading, (2) in standard thickness CIGS cells the benefit is smaller than commonly believed, (3) there is also the strong possibility of reduced rather than of increased device performance, and (4) thin-absorber cells derive more substantial benefit.

Rapid Thermal Annealing on Cu(In,Ga)Se2 Films and Solar Cells

AbstractRapid thermal annealing (RTA), with fast ramp up and down rates, was performed on several Cu(In,Ga)Se2 (CIGS) films and solar cells under various peak annealing temperatures and holding times. The XRD, SEM, Hall- effect, photo J-V, and quantum efficiency (Q-E) measurements were made on CIGS films and cells before and after RTA treatments to study the effects of RTA on the CIGS film properties and cell performance. The results show that RTA treatments under optimal annealing condition can provide significant improvements in the electrical properties (resistivity, carrier concentration, and mobility) of CIGS films and cell performance while preserving the film composition and microstructure morphology.

A stacked chalcopyrite thin‐film tandem solar cell with 1.2 V open‐circuit voltage

AbstractCuGaSe2 (CGS) thin films were prepared on tin‐doped indium oxide (ITO) coated soda‐lime glass substrates by thermal co‐evaporation to fabricate transparent solar cells. The films consisted of columnar grains with a diameter of approximately 1 μm. Some deterioration of the transparency of the ITO was observed after deposition of the CGS film. The CGS solar cells were electrically connected in series with Cu(In,Ga)Se2 (CIGS) solar cells and mechanically stacked on the CIGS cells to construct tandem cells. The tandem solar cell with the CGS cell as the top cell showed an efficiency of 7.4% and an open‐circuit voltage of 1.18 V (AM 1.5, total area). Copyright © 2003 John Wiley & Sons, Ltd.