The goal of this project is to demonstrate proof-of-concept of the transformative idea of self-powered photo-electrochromic system based on 1D and 2D ZnO nanostructures on flexible substrates for significant potential payoffs. The power conversion efficiency may be below 3%, but the important metric of open circuit voltage to drive the electrochromic material is computed to be acceptably high (~2.25-2.75 V). Initially, UV photovoltaic Schottky and p-n junction devices, based on ZnO NWs and thin films, will be fabricated and characterized. Finally, monolithic integration of ZnO NW photovoltaic cells and electrocrhromic layers will be demonstrated using a flexible polymer as the substrate, to achieve self-powered smart windows. The intellectual merit of this work lies in the interdisciplinary and integrated approach, the success of which will be transformative in a number of areas.
Broader Impact:
The overall promise of this project is in creating a breakthrough for developing self-powered nanostructure based electrochrmoic devices on flexible substrates. In addition, future engineers will gain hands-on, problem-solving skills in multiple disciplines including nanosciences, while taking necessary courses in advanced characterization, solid state devices, nanofabrication, and photoelectrochromic technologies at ASU.
The US Department of Energy estimates that advanced dynamic window technologies, or smart windows, in buildings could save as much as 1.5 quadrillion BTUs of energy, or more than $10 billion in cost each year. A reduced energy demand for buildings will lead to lower energy needs currently dependent on foreign sources, as well as decrease the amount of greenhouse gas emissions. A number of mitigation ideas have been proposed including the integration of electrochromic stacks on existing windows to automatically detect peak sunlight and provide the necessary shading. Voltage-tunable light transmission characteristics of switching, inorganic electrochromic devices require a high drive voltage (>2 V), and for autonomous or smart systems, a simple architecture of the power supply module is also imperative. Therefore, transparent and single metal-semiconductor junction devices that convert ultraviolet (UV) photon energy into high open circuit voltage are highly desirable. The central theme of this exploratory, 2-year NSF EAGER project was to process and evaluate transparent photovoltaic devices that may eventually power an electrochromic stack of a smart window. Consequently, the experimental objectives were to process materials (nanowires and thin films), fabricate device test structures, characterize materials and devices, and simulate the photovoltaic response of visibly transparent UV solar cells based on metal- zinc oxide (ZnO) and/or metal-ZnO/ZnS Schottky-barrier devices including hybrid inorganic-organic structures. Initially, the demonstration of the photovoltaic effect in sputtered ZnO thin films, under the illumination of UV radiation was accomplished. The fabrication and evaluation of Schottky devices based on variety of contact metals on ZnO was carried out, and it was determined that silver was the most promising. Some preliminary information of defects was gained by capacitance transient measurements, and through the incorporation of a periodic grid pattern, the short circuit current and the power conversion efficiency was increased. Moreover, hybrid ZnO-organic devices were tested, with the latter being multiple grades of spun-on PEDOT:PSS films. The potential of ZnO as a UV photovoltaic absorber was also verified by simulation. Based on reported material parameters for the device configurations, an analytical theoretical model was tested. The results of the idealized, analytical model were compared with experimental data, and output from a more realistic solar simulation software package. Concurrently, zinc sulfide (ZnS), synthesized from the conversion of sputtered and solution grown ZnO, was also evaluated as an absorber material in multilayer stack of P-doped Si/ZnS/ZnO heterojunction. A Schottky solar cell, fabricated via the vacuum route, exhibited a high open-circuit voltage of ~1.35 V. Therefore, extensive effort was devoted to low temperature, low cost solution growth of ZnO thin films followed by partial conversion to ZnS. An initial result of high conductance of solution-derived ZnO reflects the potential for the development of low cost, Schottky-based Si/ZnS/ZnO heterojunction solar cells. The research team has indeed been able to meet a number of the milestones outlined in the original proposal. In particular, it was found that through simulation of and experimental efforts in fabricating ZnO and ZnO-ZnS-based Schottky UV cells, the potential to fulfill the necessary power requirements for electrochromic devices is high. In the hybrid inorganic and organic device, a large open circuit voltage was also demonstrated. The synergistic approach also gave a couple of PhD students, as well as a REU student the opportunities to both independently and collaboratively work on the synthesis of materials, device fabrication, characterization and testing, and simulation and testing. Consequently, the experiences gave them a greater insight into semiconductor processes, electrical and optical effects, and photovoltaic responses from both an experimental and theoretical points of view.
