Solar Cells Based on Cu(In,Ga,Zn)Se2 Thin Films
High Efficiency Solar Cells Based on Cu (In, Ga, Zn) Se2 Thin Films Prepared by Selenization of Co-Sputtered Metal Alloy Layers
Tech Area / Field
- MAT-SYN/Materials Synthesis and Processing/Materials
- NNE-SOL/Solar Energy/Non-Nuclear Energy
8 Project completed
Senior Project Manager
Zalouzhny A A
Institute of Solid State and Semiconductor Physics, Belarus, Minsk
- National Academy of Sciences of the Republic of Belarus / Institute of Electronics, Belarus, Minsk
- The University of Salford, UK, Salford\nUniversity of Strathclyde, UK, Glasgow\nUniversitat Leipzig, Germany, Leipzig\nTechnical University, Germany, Freiberg
Project summaryProject purpose:
The main objectives of the proposal are to establish and identify the conditions and basic mechanisms of CuInXGa1-XSe2 (CIGS) and (CuInSe2)X - (ZnSe2)1-X (CIZS) thin films growth under different technological parameters, to choose cadmium-free materials for buffer layer, to investigate their physical properties, and to develop an easy and efficient technological process for producing large-area thin films for high-efficiency solar cells.
Present status of the field:
This project is related to the field of non-nuclear sources of energy, renewable energy sources (RES) – solar energy. Energy is obviously an essential area of social and economic activity all over the world. Sunlight is the most widespread renewable energy source among all other forms of energy, therefore electricity produced by photovoltaic devices and plants may become the cheapest technique for large regions, especially in developing countries. Therefore, national and international institutions together with industry should give high priority to long-term programs, which aims are to research and develop the field of solar energy. In future every house could be provided with electricity for light and could production by that source.
Actually, the cells are based mainly on semiconductor poly or single crystals. That explains their high cost and causes limitation of areas of their application. However, the interest in this new energy has made possible some novel technologies development, and mainly thin film deposition techniques. It is inexpensive from the point of view of primary materials cost and involves simple deposition techniques. Application of these techniques allows to obtain large-area low-cost and efficient photovoltaic modules [T. Wada, Proceedings of the 11th international Conference on Ternary and Multinary Compounds (1997) 903; B. Dimmer, H.W. Schock, Prog. Photovolt. Res. Appl., 6 (1998) 193].
In recent 15 years, CuInxGa1-xSe2 semiconducting materials have attracted great attention as some of the most promising compounds for application as active absorber layers in thin film solar cells. The x value decrease from 1 to 0 results in progressive increase of the band gap from 1.02 eV to 1.68 eV - the optimum energy range for solar cell active absorber layers. These materials possess very large absorption coefficient in visible and near-infrared range (3×105 to 6×105 cm-1) which allows efficient and light-weight thin film solar cells to be fabricated (the thickness of formed structures may be about 3 to 5 micrometers). These properties give an opportunity to develop low-cost single and tandem thin film solar cells which are stable and possess high conversion efficiency. Recently, solar cells based on a CIGS absorber layer have been reported to exceed the efficiency of 17% for small area (0.5 cm2) and 11.5% for large area modules (30×30 cm2, Siemens Solar) [Dimmler B at. al. in Proceeding of the 2nd World Conf. on Photovoltaic Energy Convention, (1998) p. 419].
At the same time, development of such solar cells has not quitted out yet the stage of laboratory studies. Although many various techniques were used successfully to produce CIGS thin films, no fabrication process of high reliability, suitable for commercial fabrication of large area devices has not been demonstrated yet. The current position is ascribed mainly to the absence of a scalable, relatively simple technique for high-quality absorber films preparation. It is believed that higher efficiency is achieved for solar cells produced using the technique of co-evaporation of all constituent elements (Cu, In, Ga and Se) for CIGS film deposition. However, this method is difficult to scale-up to achieve a commercially viable process. A promising alternative technique involves deposition of the precursor metals at the first step, then reacting them to produce the Cu(In,Ga)Se2 films [Adurodia F. O., Carter M. I., Hill R. Solar Energy Materials and Solar Cells, Vol. 40, 1996, p. 359.]. This allows the metals to be deposited by more scalable deposition techniques. In addition, the important feature of this technique is precise fluxes control and well-defined metal alloy composition; moreover, it also shows a promise for large-area reproduction of film properties. Of important significance is also the choice of material and technique for buffer layer formation to this compound, which forms a p-n junction, since presently used CdS is dangerous for human health and ecology.
The project is based on the results of detailed studies, which have been done at the Leipzig University, University of Strathclyde, and Belarus Institutions. Thin film samples, tested at the Leipzig University, have shown promising structural, composition, physical properties and good adhesion to substrate [Gremenok V.F., Schmitz W., Bodnar I.V., Bente K., Doering Th., Kommichau G, Eivler A., Riede V., Proc. 12th Int. Conf. on Ternary and Multinary Compounds, (1999); Zaretskaya E.P., Gremenok V.F., Zalesski V.B. Ivanov V.A., Victorov I.V., Kovalevski V.I., Ermakov O.V., Leonova T.P., Tech. Phys. Let., v.26, 2000]. The yield of good-quality films and their parameters are reproducible and predictable. However, the improvement of semiconductor material parameters and device performance is related to better understanding of some fundamental properties of CIGS-type polycrystalline layers. In the same way, device performance is related strongly to the interface properties; so, the better understanding of their fundamental physics is necessary.
Impact of the Proposed Project on the Progress in the Field:
The principal novelty of given proposal is the usage of co-sputtering for Cu-In-Ga and Cu-In-Zn alloys formation from specially designed Cu-In-Ga(Zn) targets. For gaseous H2Se is highly toxic, the selenization processing will be accomplished under low-vacuum conditions in partially closed system at moderate temperatures. This technique is simple and has a potential to fabricate large-area high-quality films. We shall concentrate on CIGS materials mainly, but we are going to study new objects based on the (CuInSe2)X - (ZnSe2)1-X thin films with energy gap from 1.02 eV (CuInSe2) to 2.67 eV (ZnSe2) which has attracted attention in last years as new materials for solar cells.
The potential of this technique for solar cells production is highly promising due to application of sputtering deposition technique at ambient temperature (no special substrate heating is required) and relatively simple and safe selenization processing. Scaling-up of the process should be relatively straight-forward as compared to the co-evaporation techniques. It has the potential of in-line integration into a commercial manufacturing processes using any deposition techniques to produce elemental precursors and develop a new industrial process for low-cost solar cells production. This technology could be used to fabricate high-efficiency solar cells and may be successfully extended to manufacture large-area photovoltaic modules.
Institute of Solid State and Semiconductor Physics, Minsk, Republic of Belarus.
Institute of Electronics, Minsk, Republic of Belarus.
The Institute of Solid State and Semiconductor Physics has a long tradition in preparation and investigation of semiconductor materials for opto-electronic and photovoltaic applications for commercial and weapon purposes. They have professional skills in the techniques for guidance and control of missiles from launching to impact includes optical guidance. In recent years, the main topics of study have been fabrication and characterization of crystals and thin films of chalcopyrite semiconductors. Therefore, the topic of the proposed joint research project fits very well the scientific direction of the Institute. The backgrounds of the Project are highly qualified staff, high-quality materials and consumables together with a slight and inexpensive upgrade of some components of the equipment set. It will provide the high yield of thin films and devices producing.
The Institute of Electronics is one of the leading institutions in the field of semiconductor devices development in the Republic of Belarus. Specialists of the Institute possess a long-term experience in producing opto-electronic detectors based on semiconductor structures, thin dielectric and semiconductor films for commercial and weapon utilization. They have large professional skills in the branch of producing optoelectronic components for optical guidance, present and terminal guidance, homing guidance, and some other applications. The Institute has a complete technology equipment set for fabrication thin films mentioned above in accordance with the Proposal aims: photo-lithography processing; metal and resistive layers deposition techniques; plasma-chemical etching, etc. The backgrounds of the Project are qualified and skilled personnel, as well as high-quality consumables for fabrication of thin films and semiconductor devices.
During realization of this project the following science results should be obtained:
- development of reproducible technology for fabrication of CIGS and CIZS thin films by selenization of co-sputtered metal alloy layers,
- choice of cadmium-free materials and development of technology of forming buffer layers,
- choice of materials for front and back ohmic contacts,
- improvement of overall quantum efficiency of solar cells and spectral response to sunlight under AM 1,5 conditions,
- working out the technologies of high efficiency and reproducible solar cells manufacturing with high homogeneity over a large sensitive area up to 50-100 cm2.
Project Results Application:
Commercial importance of the Proposal is that chalcopyrite-based solar cells are cheaper and possess a number of above-mentioned advantages in comparison with solar cells based on monocrystalline and another thin film materials for world wide applications. So, they could find a wide area of application as a reliable and economical source of electricity for small scattered application places far from urban centers. This Proposal reflects the logical link between scientific studies and high technology production facilities. It should be noted that this Project is very important for high-tech industry in the Republic of Belarus to introduce them into the international market with products of competitive quality and technology methods.
Meeting ISTC Goals and Objectives:
· Providing weapons scientists and engineers of the Institute of Solid State and Semiconductor Physics, and the Institute of Electronics, Minsk, Republic of Belarus (there quota of participation in the project is more than 67%) opportunities to redirect their talents to peaceful activities;
· Promoting integration of scientists of the Belarus Institutions involved in the Project into the international scientific community.
· Supporting basic and applied research and developing technologies for peaceful purposes, notably in fields of environmental protection and energy production. The scientific objective will be the preparation and modification of thin film solar cells containing non-toxic compounds by means of using cheap techniques. At the end of the investigation of the subject, these cells are expected to have good efficiency.
· Contributing to the solution of national and international technical problems (other than those mentioned above). An important objective of this joint project is scientific exchange between different participants. Therefore a part of requested financial support is related to maintenance grants for researches coming from CIS states.
· Reinforcing the transition to market-based economies responsive to civil needs.
This project will contribute sustainable development, introducing a renewable, environmentally clean energy sources and enforce long term research of the involved EU human potential and safeguard the research potential of the involved Belarus Institutes.
Here are the aspects that motivated authors for Proposal realization: Scientific and social motivation is that the Proposal involves qualified former weapon scientists and modern equipment in civil commercial and scientific activity. Participation in the Proposal will give the involved specialists work and intensive communications in defense projects and it also promotes a long-term objective to intensify the cooperation between CIS and Western Europe scientists.
Scope of Activities:
The Project presents an applied research in the field of materials synthesis and solar energy. The duration of the Project is 24 months. Total Project efforts are 400 man×months, including 271 man×months for weapon scientists and engineers.
Foreign Collaborators Role:
Proposal Participants will work together with specialists of Leipzig University, Institut fur Experimentelle Physik, Bergakademie TU Freiberg, and University of Strathclyde for the next tasks:
· information and test structures exchange in the course of project implementation;
· investigation of physical properties of thin films and test solar cells using modern high-price equipment in Germany and Great Britain would be extremely fruitful for the current Proposal. Such utilization of the equipment should provide good economical effect for the Proposal;
· general coordination of the Proposal with the specialists of Leipzig University, Institut fur Experimentelle Physik, and University of Strathclyde and recommendations formulation for further applications;
· providing comments to technical reports (quarterly, annual, final, etc.) submitted by project participants to the ISTC, conduction of joint seminars and workshops;
· assistance and materials supply for participants of the project.
Technical Approach and Methodology:
The basic methodology and technical approaches of the Proposal include:
- development and modernizing experimental facilities for co-sputtering deposition and selenization processing of large-area CIGS(CIZS)-based thin films,
- fundamental studies of physical and chemical mechanisms of CIGS (CIZS) thin films formation using proposed technique of alloys co-sputtering and subsequent selenization processing,
- investigation of electrical, optical and photoelectric properties of thin films formed,
- investigation and development of cadmium-free processing for buffer layer formation,
- development of ohmic contacts formation technique and design for large-area solar cells,
- development of experimental process for fabrication of large-area solar cells,
- fabrication and testing of experimental samples of CIGS and CIZS thin-film solar cells.