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Materials World Network:ZnTeO for Intermediate Band Solar Cells Jamie D. Phillips, University of Michigan Ann Arbor, DMR 1006154 Oxygen states in ZnTe result in highly efficient optical transitions approximately 0.5eV from the conduction band edge of ZnTe. These oxygen states may be used to provide intermediate optical transitions for efficient solar energy conversion by providing increased current for sub-bandgap photons while maintaining a voltage that tracks the bandgap of the host material.

Solar cells utilizing ZnTeO absorbers have been demonstrated, exhibiting enhanced response below the ZnTe bandedge, and not observed for reference cells with ZnTe absorbers. The subbandgap response further exhibits intermediate band solar energy conversion through the observation of photoresponse under subbandgap excitation at wavelengths corresponding to the intermediate spectral bands.

ECEVEO1.8eV0.5eV2.3eV

1Please utilize the PowerPoint Notes Page to provide details covering the aim of the project, the particular research results being highlighted, the significance of the results in terms of science and/or technological impact, and future plans. Include as much depth as space allows.

Aim of the Project:The goal of the project is to identify II-VI-O semiconductor materials for efficient intermediate band solar energy conversion, and to determine the limiting mechanisms for the energy conversion process. Semiconductor materials possessing intermediate electron states within the bandgap provide a path for the excitation of charge carriers to the conduction band using multiple photons at energy lower than the bandgap energy of the material. The intermediate electron states effectively provide multiple optical absorption bands in the material, which may result in more efficient conversion of solar energy as illustrated in the energy band diagram shown. Highly radiative states provided by oxygen incorporation in ZnTe appears to be an attractive approach for realizing efficient intermediate band solar energy conversion, with desirable energetic transitions. This project establishes an international collaboration between the University of Michigan (U-M) and the Instituto de Energia Solar at the Universidad Politcnica de Madrid (IES/UPM) to study intermediate band solar energy conversion in II-VI-O materials. The research combines expertise in II-VI-O materials and devices at U-M with expertise in intermediate band solar energy conversion and photovoltaics at IES/UPM. Research Results:In this collaboration, we have jointly conducted a series of experiments on the theory, materials, and solar cell devices based on ZnTeO. Firstly, samples grown at U-M were studied by photoreflectance at IES/UPM. While this technique was very insightful for prior experiments at IES/UPM on InGaAs/GaAs intermediate band solar cells, the technique has not provide a signal that is strong enough for detailed studies (alternative techniques are being investigated). Secondly, a model is being constructed jointly to determine fundamental limits for intermediate band solar energy conversion in the ZnTeO system. This model will incorporate material parameters known experimentally, and identify material parameters where experimental measurements are necessary. Heterojunction diodes of ZnTeO have been fabricated (U-M), and are being measured at IES/UPM, as shown in the photo. The diodes exhibit sub-bandgap spectral response as shown in the quantum efficiency spectrum, indicating the intermediate band process.Significance of the Results:Intermediate band solar energy conversion in ZnTeO is demonstrated and confirmed by the experts in IB solar cells (IES/UPM). This provides a solid foundation for further study of limiting mechanisms for the energy conversion process and improvement of the overall efficiency.

Future plans:Future plans include continued development of a theoretical model for ZnTeO IB solar energy conversion, development of improved junctions as a testbed for characterizing the energy conversion process, and the exploration of alternative II-VI-O materials such as ZnSeO.

Acronyms and symbols:DefineMaterials World Network: Intermediate Band Solar Energy ConversionJamie D. Phillips, University of Michigan Ann Arbor, DMR 1006154A strong collaborative effort has developed between U-M and IES/UPM taking advantage of materials and devices expertise at U-M and photovotlaics expertise at IES/UPM. In addition to contributing to the direct research activities, the complementary expertise is helping to establish new capabilities at each institutions (solar cell characterization at U-M, molecular beam epitaxy and pump-probe ultrafast optical characterization at IES/UPM). Top: Jamie Phillips, Antonio Luque, and Antonio Marti at a visit to a 50 MW solar installation in Puertollano, Spain (May 2011)Bottom: Antonio Marti and members of Jamie Phillips research group outside of the Lurie Nanofabrication Facility at the University of Michigan (Aug. 2011)

2The PI at Michigan (Jamie Phillips) visited IES/UPM in May 2011, while one of the PI s at IES/UPM visited U-M in Aug. 2011. Prof. Phillips gathered a great deal of information on photovoltaic systems through visits at the institute and several spin off companies and solar installations. In addition, details were learned regarding solar cell characterization that will help in the installation of a solar cell characterization system at U-M. Prof. Marti learned a great deal of information on materials growth by molecular beam epitaxy, where IES/UPM is in the process of acquiring and installing an MBE system (their first MBE system). Additionally, Prof. Marti learned details of ultrafast pump-probe optical spectroscopy at U-M, where the institute is planning to acquire a system in the near future. A postdoctoral researcher at IES/UPM is planning to spend 5 months at U-M in the coming year as a part of the collaboration.