CAREER: Interfaces and Their Effect on Charge Transfer in Extremely Thin Absorber Solar Cells

Drexel University, Philadelphia PA

Investigators

Abstract

0846464 Baxter The research objective of this renewable energy related CAREER proposal is to investigate nano-structured interfaces, material properties, and their effect on electron charge transfer in extremely thin absorber (ETA) solar cells. ETA cells will be economically competitive with fossil fuels at their predicted efficiencies of 15% because they can be produced by low-cost solution methods. However, demonstrated ETA cell efficiencies are only 2.5%. The discrepancy between realized and predicted efficiency is due to lack of fundamental understanding of the role of interfaces and material properties on charge transfer processes within the cell. ETA cells employ a mesoporous n-type semiconductor coated at the interface with a thin absorber film, with pores filled by a p-type semiconductor to create an interpenetrating heterojunction. The operating principle of the ETA cell is that the large junction area presented by an n-type nanowire array allows thin absorber layers to be used, such that charge separation across the interfaces is faster than competing bulk recombination. Nanowires are also the ideal geometry to quickly transport the separated charges to opposite contacts before they can undergo interfacial recombination. To date, ETA cells have primarily been studied by making solar cells and measuring their I-V characteristics, with few fundamental studies of either the individual materials and interfaces or charge transport within the cell. The proposed approach will use a combination of spectroscopy and electron microscopy of individual materials and interfaces as well as steady-state and perturbation studies of ETA solar cells to gain fundamental insight into charge transfer mechanisms in the ZnO-nanowire/CdSe/CuSCN materials system. Techniques including impedance spectroscopy, intensity-modulated photocurrent spectroscopy, and time-resolved terahertz spectroscopy will be employed to (1) measure carrier lifetime and mobility in individual materials and thin film stacks, (2) measure characteristic times to compare charge separation vs bulk recombination and charge transport vs. interfacial recombination, (3) determine processes that limit cell performance, and (4) design interfaces and material properties to improve charge transfer and, hence, increase ETA cell efficiency. Intellectual Merit: The transformational, spectroscopy-driven approach described in this proposal will be applied to ETA cells for the first time in order to understand the bulk and interfacial phenomena that govern cell performance. This work will explore the nature of charge transfer across nano-structured semiconductor interfaces, specifically focusing on the role of architecture, defect structure, and electronic band structure. Solution methods of depositing extremely thin coatings of high quality and precise thickness will be developed. Ex situ characterization of materials and interfaces will be combined with measurements of ETA cells to identify the properties or processes that limit cell efficiencies. This detailed understanding, which cannot be achieved using conventional methods, will allow comparison of experimental observations with theoretical predictions and will aid in the design of materials, interfaces, and molecular architectures that enable higher energy conversion efficiencies. Broader Impact: If successful, the proposed renewable energy related work will provide the fundamental understanding of interfacial phenomena that is necessary to increase efficiencies of solar cells from 2.5% toward the 15% predicted by theory. Such an improvement could potentially transform ETA cell technology to be economically competitive with fossil fuels. Low-cost ETA cells would provide a source of clean, secure, sustainable energy that could shift the U.S. portfolio away from fossil fuels and toward energy independence. The design principles learned from the CdSe system can be applied to other absorbers such as CuInGaSe2 for even higher potential efficiency. Additionally, the enhanced understanding of charge transfer at nano-structured semiconductor heterojunctions will be useful for many other nano-structured systems where controlling interfaces is critical to achieving high performance. These applications include organic-inorganic solar cells, displays, electrochromics, and batteries, all of which are important for sustainable energy or energy efficiency. The novel experimental approach described in this proposal can be employed by other researchers to enable new advances in a broad range science and technology fields. The educational objectives of this proposal are to educate students through research activities and curriculum development and to expose people of diverse ages and backgrounds to general concepts in renewable energy. A "Solar Energy Seminar Series" will be established to engage the general Drexel community; and an "Energy and Sustainability Workshop" for 5th-6th grade students and their teachers will be initiated in partnership with the School District of Philadelphia. This workshop will naturally be directed toward underrepresented groups since the district is over 80% African-American and Hispanic and will involve 100 students and 25 teachers over 5 years. This project is jointly supported by the Interfacial Processes and Thermodynamics program and the Sustainable Energy program in the NSF CBET Division.