The promise of solar cells having high conversion efficiency and low production cost makes solar cells based on cadmium telluride (CdTe) an attractive contender for terrestrial solar power generation. The viability of CdTe for near-term low cost photovoltaics is demonstrated by the success of one thin film manufacturing company and the recent burst of CdTe-related R&D at over nine other commercial entities. CdTe is a direct band gap compound semiconductor with a nearly optimal match to the solar spectrum for terrestrial photovoltaic energy conversion. The direct band gap, EG = 1.5 eV, and high absorption coefficient, α > 5 x 105 cm-1, account for the high quantum yield from the ultraviolet to the CdTe band gap, λ ~ 825 nm. Short wavelength photons, with energy greater than EG, are absorbed near the CdTe surface, making CdTe an attractive absorber layer material in thin-film solar cells, with 99% photon absorption occurring within 2 µm of film thickness. Champion CdTe thin film solar cells have achieved conversion efficiencies exceeding 16%.
CdTe is a chemically stable compound which can be synthesized in thin film form by numerous techniques such as physical vapor deposition, vapor transport deposition, sputter deposition, electrodeposition, chemical bath deposition, sintering, and more. The CdTe conductivity type and doping concentration is controlled by the influence of intrinsic defects related to Cd-Te stoichiometry and extrinsic dopants. As a group II-VI compound with tetrahedral cation-anion bonding, CdTe can be alloyed in isostructural combinations with other group II and group VI elements, allowing variation of the band gap; for example, substituting Zn for Cd to form Cd1-xZnxTe or Se for Te to form CdTe1-ySey widens the band gap. Substituting Hg for Cd to form the alloy Hg1-xCd Te narrows the band gap, and has found wide application for infrared detectors.
CdTe Solar Cell R&D at the Institute of Energy Conversion
The Institute of Energy Conversion (IEC) has been involved in research and development of group II-VI semiconductor thin film photovoltaic devices since its founding in 1972. R&D in the 1970’s and early 1980’s was focused on CdS/Cu2S solar cells and methods for roll-to-roll coating onto flexible substrates. A program for the development of polycrystalline CdTe thin film solar cells was initiated in 1985 and continues to the present.
The IEC CdTe program has been a significant contributor to the thin film PV community for over 20 years, having contributed original R&D and IP, collaborated with other university and industrial groups, given support to industrial entities, trained researchers and maintaining active participation in the now defunct NREL CdTe R&D team, which provided extensive support to First Solar LLC and its predecessor Solar Cells, Inc as well as the now defunct BP Solar Apollo product. Highlights of IEC CdTe technical achievements over the years which provided the experience needed to offer support include:
- Development of low-temperature substrate configuration CdTe solar cells with evaporated CdTe films
- Development of transparent back contacts to CdTe based on ITO and ZnTe films
- Fabrication of the first monolithic polycrystalline thin film solar cell using CuInSe2 and CdTe
- Quantifying the effects of CdCl2 treatment on the chemical and optoelectronic properties of the CdS-CdTe couple
- Quantifying 3-dimensional CdS-CdTe interdiffusion in polycrystalline thin films using x-ray diffraction
- Refining the equilibrium T-x phase diagram for the CdTe-CdS pseudobinary system from 700°C to 350°C
- Quantifying role of CdTe oxidation during film growth and post-growth treatments in controlling film chemistry, interdiffusion and device operation
- Development of the chemical surface deposition method for CdS and Cd1-xZnxS window layer deposition
- Development of a vapor transport deposition process for II-VI compounds and alloys
- Development of safer vapor treatments of polycrystalline films using metal halide sources
- Demonstration of alloy absorber Cd1-xZnxTe solar cell with EG = 1.6 eV
- Determined reversible and non-reversible components of stress-induced degradation of CdTe/CdS solar cells
- Determined electrical operation of CdTe/CdS devices by using bifacial analysis of cells with transparent contacts, and development of high throughput processes for cell fabrication
- Evaluation of high throughput processing methods and incorporation of sub-micron CdTe absorber layers
Today IEC maintains a baseline cell fabrication process for fabricating CdTe/CdS solar cells onto commercially available tin-oxide coated glass. The best cells made at IEC on commercial glass have efficiency >13% and use IEC-patented processes for CdS and CdTe film deposition and vapor treatment of the semiconductors. With the primary goal of defining a pathway towards low-cost CdTe modules with 15% efficiency, R&D emphasis at the IEC continues on collaborative support of the CdTe industrial sector.
M. S. Angelo, B. E. McCandless, R. W. Birkmire, S. A. Rykov, J. G. Chen, “Contact wetting as a characterization technique for processing CdTe/CdS solar cells,” Progress in Photovoltaics: Research and Applications 15 (2): 93-112 (2007).
G. M. Hanket, B. E. McCandless, W. A. Buchanan, S. Fields, and R. W. Birkmire, “Design of a vapor transport deposition process for thin film materials,” Journal of Vacuum Science and Technology A 24(5) 1695-1701 (2006).
S. S. Hegedus and B. E. McCandless, “CdTe contacts for CdTe/CdS solar cells: effect of Cu thickness, surface preparation and recontacting on device performance and stability,” Solar Energy Materials and Solar Cells 88, 75-95 (2005)
B. E. McCandless and K. D. Dobson, “Processing options for CdTe thin film solar cells,” Solar energy 77, 839-856 (2004).