Thin films of CdTel-xSx with bulk atomic compositions, x=-[S]/([S]+[Te]), ranging from 0 to 0.45 were deposited by vacuum co-evaporation of CdTe and CdS with substrate temperatures of 200 and 250'C. X-ray diffraction analysis revealed that films with x < 0.3 were predominately single phase having the zincblende structure. Films with 0.35 < x < 0.45 contained the wurtzite modification. Lattice parameter determination indicated that each phase exists with compositions well within the miscibility gap shown on published equilibrium phase diagrams. The variation of the optical band gap with x was determined by measuring transmission and reflection of the films. Heat treatment at 415'C in the presence of CdC1 2 caused the films to segregate into two phases consistent with the phase diagram. If the CdC1 2 is assumed to only promote the phase segregation process, then the compositions of the two phases after heat treatment may be taken as measurements of the solubility limits of S in CdTe and Te in CdS respectively. The solubility limit of S in CdTe was thus determined to be 5.8% at 415'C which is the temperature used for the common CdC1 2 treatment of CdTe-based solar cells. An analysis of CdTe/CdS solar cell device structures shows that the atomic composition of alloys created by interdiffusion are consistent with these solubility limits.

This publication was reproduced from the best available camera-ready copy submiited t.y the subconuactor and received no editoriai review at NREL NOTICE This report was prepared as an account of work sponsored by an agency of the United States government. Neither the United States goverment nor any agency thereof, nor any of their employees. makes any warranty, express or implied, or assumes any legal liabiiii or responsibility for the accurxy, completeness. or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily consiiute or imply its endorsement, recommendation, or tavoring by the United States government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or any agency thereo". Available to DOE and DOE contractors from:Cffice of Scientific and Technical Information (OSTI) P SUMMARYDuring the third phase of the subcontract, IEC researchers have continued to provide the thin film PV community with greater depth of understanding and insight into a wide variety of issues including: the deposition and characterization of CuInl-,GaxSe2, a-Si, CdTe, CdS, and TCO thin films; the relationships between film and device properties; and the processing and analysis of thin film PV devices. This has been achieved through the systematic investigation of all aspects of film and device production and through the analysis and quantification of the reaction chemistries involved in thin fdm deposition. This methodology has led to controlled fabrications of 15% efficient CuInl,GaxSe2 solar cells over a wide range of Ga compositions, improved process control of the fabrication of 10% efficient a-Si solar cells, and reliable and generally applicable procedures for both contacting and doping CdTe films. Additional accomplishments are listed below. Cu(InGa)Se, Multisource EvaporationCu(InGa)Se, films have been deposited by elemental evaporation with Ga composition ranging from 0.25 < x < 0.80. The films are deposited with the Ga uniformly distributed from the Mo back contact to the front surface. This allows the effects of increasing Ga to be characterized without differences in the device operation due to gradients in the electrical and optical properties of the Cu(InGa)Se,.The solar cells fabricated from these uniform films have 15% efficiency for x < 0.5 or Eg < 1.3 eV. V, increases over the entire range of Ga content, up to 820 mV, but the device efficiency declines with high Ga content due primarily to a drop in fill factor and short circuit current. Analysis of current-voltage and quantum efficiency results show that the main cause of this drop off is a voltage dependent current collection. Finally, preliminary results show that the fill factor can be improved by grading the bandgap of the Cu(InGa)Se,,...

The prospects for widespread commercial success of CdTe‐based photovoltaics are discussed and the key technical issues to be resolved are identified. Device efficiencies achieved using thin CdS on small‐area cells must he adapted to manufacturing, potential consumers must he convinced of module stability and environmental concerns must he dealt with effectively. All these issues are being actively addressed and there do not appear to be any insurmountable obstacles to the commercialization of CdTe photovoltaic technology.

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