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The emphasis of this project is the study of ZnTe:O as an intermediate band solar cell (IBSC) material. Intermediate band solar cells are an approach to provide higher energy conversion efficiency through increased absorption of photons with energy lower than the bandgap energy of the material, while maintaining a voltage that tracks the bandgap energy of the material. The increased absorption is accomplished via multi-photon transitions via the intermediate band electronic states. Oxygen doping in ZnTe, or ZnTeO alloys provide radiative optical transitions within the bandgap at energies that are highly favorable for efficient intermediate band solar energy conversion. In the first year of the project, the principle of intermediate band solar energy conversion in ZnTeO was demonstrated and reported. However, the efficiency remained low due to poor properties of the semiconductor junction.

In this second year, a primary emphasis was placed on improving a diode structure to provide a means of collecting carriers for ZnTeO active regions. The primary difficulty associated with ZnTe is that the material is intrinsically p-type due to the small electron affinity and native acceptors resulting from background Zn vacancies. While n-type ZnTe is problematic, this project utilized n-ZnSe/p-ZnTe heterojunction diodes. The n-ZnSe served as an emitter contact for the solar cell and wide bandgap window layer. The use of the n-ZnSe was found to significantly improve the I-V characteristics and fill factor of the devices. Measurements of the external quantum efficiency demonstrate a significant improvement in the conversion of photons above the ZnTe bandgap energy, and overall improvement in the efficiency. The improved diode characteristics will provide a means of more carefully characterizing intermediate band optical transitions in ZnTeO.

Similar to the n-ZnSe emitter approach, effort was devoted to develop an n-ZnS buffer layer. The ZnS layer provides an even higher bandgap energy “window layer” and hole blocking layer to potentially decrease leakage current and improve the open circuit voltage in a ZnS/ZnSe/ZnTeO/ZnTe device structure. Unlike the ZnSe and ZnTe(O) materials, a means of growing ZnS by MBE was not available. In this work, chemical bath deposition (CBD) was used to synthesize ZnSe. Initial diode structures demonstrated an incremental improvement over the ZnSe/ZnTe diode structures. More importantly, development of the ZnS CBD process resulted in a means of depositing a “Cd-free” buffer layer process to replace CdS thin films that are typically employed in thin film solar cell technologies including CIGS. A manuscript for journal submission is in preparation describing the synthesis and properties of ZnS thin films by CBD.

The PRF ND grant has been an invaluable resource for supporting both undergraduate and graduate student researchers in this project. The funds provided support for four graduate student researchers, where this project served as the primary topic for one doctoral thesis. In addition, an undergraduate student was directly working on this project over the course of summer 2011 to develop the CBD process. This undergraduate student was one of the top undergraduates in our department, plans to attend graduate school for doctoral study, and will likely be first author of a journal publication as a result of this project.

            The PRF ND grant has clearly provided the intended “New Direction” for the PI. After having no direct experience in the area of solar cells, the PI has now been recognized as a primary contributor to research on intermediate band solar cells. The PI has served as an invited organizer for multiple sessions related to intermediate band solar cells, including the Electronic Materials Conference, IEEE PVSC conference, and AVS annual meeting, and will be giving an invited talk on the topic of this project at the 2012 European MRS Spring Meeting. The outcomes of the work have also provided the intended function of “seed funding” for new research ventures. As a result of this effort, follow on funding has been obtained through the Department of Energy EFRC program. Additionally,  an international collaboration has been established through the National Science Foundation Materials World Network program with the Instituto de Energia Solar in Madrid, Spain to study intermediate band solar cell materials.