We report the growth and characterization of record‐efficiency ZnO/CdS/CuInGaSe2 thin‐film solar cells. Conversion efficiencies exceeding 19% have been achieved for the first time, and this result indicates that the 20% goal is within reach. Details of the experimental procedures are provided, and material and device characterization data are presented. Published in 2003 by John Wiley & Sons, Ltd.
Nitrogen-doped films of ZnO grown by two methods, metalorganic chemical vapor deposition (MOCVD) and reactive sputtering, were studied with x-ray and ultraviolet photoelectron spectroscopy (XPS and UPS). Systematic differences in the N chemical states were observed between films grown by sputtering and MOCVD: only two N chemical states were observed in films grown by reactive sputtering, whereas four N chemical states were observed in MOCVD films. To aid in the assignment of the N chemical states, photoemission data from the polycrystalline films were compared with data taken on N2+-implanted Zn metal and N2+-implanted ZnO. High-resolution core level spectra of the N1s region indicated that nitrogen can occupy at least four different chemical environments in ZnO; these include the NO acceptor, the double donor (N2)O, and two carbon–nitrogen species. Valence band spectra indicate that the Fermi energy of all films studied was near the conduction band minimum, implying that the films remained n-type after nitrogen doping. Analysis of the relative amounts of acceptors and donors identified by XPS in the sputter-grown films provides clues as to why only a small percentage of incorporated nitrogen is found to contribute to carriers, and points toward possible paths to higher quality ZnO:N films.
The role of hydrogen in nitrogen-doped ZnO thin films was studied by Fourier transform infrared (FTIR) absorption and modeled by first-principles calculations to understand the difficulty of doping ZnO p-type with nitrogen. Nitrogen-doped ZnO films were fabricated by low-pressure metal-organic chemical vapor deposition (MOCVD). High levels of nitrogen incorporation were observed, but the acceptor concentrations remained low. Theoretical analysis suggests there is a high probability that NO− and H+ charged defects combine to form the neutral defect complexes, thereby compensating the nitrogen-related acceptors. Calculated values of the vibrational frequencies of the related infrared modes agree well with the measured spectra. Thus, we believe the difficulty of achieving p-type doping in MOCVD-grown ZnO films is due, at least partially, to inadvertent passivation by hydrogen.
Our effort towards the attainment of high performance devices has yielded several devices with total-area conversion efficiencies above 16%, the highest measuring 16.8% under standard reporting conditions (ASTM E892-87, Global 1000 W/m'). The first attempts to translate this development to larger areas resulted in an efficiency of 12.5% for a 1 6.8-cm2 monolithically interconnected submodule test structure, and 15.3% for a 4.85-cm' single cell. Achievement of a 17.2% device efficiency fabricated for operation under concentration (22-sun) is also reported. All high efficiency devices reported here are made from graded bandgap absorbers. Bandgap grading is achieved by compositional Ga/(ln+Ga) profiling as a function of depth. The fabrication schemes to achieve the graded absorbers, the window materials and contacting will be described.
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