In this proposal, supported by the NSF CHE-DMR-DMS SOLAR Initiative, PIs describe their vision for new hybrid photovoltaic materials via nanoscale integration of organic and inorganic materials to exploit the unprecedented high extinction coefficient of conjugated polymers and high carrier mobility of the solid-state semiconductors. This study will address many fundamental questions pertinent to organic-inorganic hybrid solar cells that utilize nanoscale structures. Through unique world-class expertise in chemistry, materials, devices and mathematics, this project explores the effects of scale and structure as it applies to interface quality, material structure, energy transfer and electronic behavior. This work includes simultaneous study of polymer synthesis, polymer and nanostructure material as well as geometrical parameter optimization and spectroscopic study and device development. One of the compelling aspects of this work is the synthesis of polymers with complementary absorption and band offset to III-As and III-P inorganic semiconductors supported by mathematical calculations. In parallel, an in depth study will be conducted in the development of optimal nanostructure design for efficient light coupling, absorption and carrier extraction. A primary focus of this work will be on the roles of surface states, surface passivation and electronic coupling between organic semiconductors and inorganic nanostructures in hybrid solar cells. Intrinsic to PIs efforts will be start-of-the-art calculations on multiple conjugated polymer chains and surface passivation agents that will point the way toward enhancing carrier mobility and long-range energy transfer in these materials. Synthesis of new polymer materials to effectively pair with available III-V materials along with optimized nanostructure design will produce a new photovoltaic device fabric. NON-TECHNICAL SUMMARY: This program has been designed to include several targeted efforts to directly impact society through multidisciplinary, multi campus collaboration. At the core of this project is integrated research and education for students at all levels (K-12, undergraduate, graduate, postdoctorates). Through research involvement, summer workshops and campus exchange, this SOLAR program will generate highly skilled researchers and scientists. Direct outcomes of this project will be shared best practices in K-12 activities, a co-developed graduate course in solar cell development to emphasize organic synthesis/inorganic materials/modeling/analysis, URM student involvement in SOLAR research along with collectively strengthened industrial collaboration. In a more general sense, this research will provide fundamental understanding of the organic and inorganic material interfaces as well as electrical and optical properties of the hybrid solar cells. This study will broadly influence the basic design of future hybrid solar cells and their efficiency limits.

Project Report

Goals An inter-disciplinary and inter-institutional collaboration involving four research groups (Huffaker and Ratsch at UCLA, You at UNC Chapel Hill, and Zhu at Columbia) have explored their vision for new hybrid photovoltaic materials via nanoscale integration of organic and inorganic materials to exploit the unprecedented high extinction coefficient of conjugated polymers and high carrier mobility of the solid-state semiconductors. The goal of this joined effort is to investigate fundamental issues in hybrid solar cells comprised of semiconductor nanopillars embedded in organic polymer films. Intellectual Merit Specifically, through this collaboration, the You group (UNC Chapel Hill) achieved the following: 1. We systematically investigated GaAs/polymer hybrid solar cells in a simple planar junction, aiming to fundamentally understand the function of semiconducting polymers in GaAs/polymer-based heterojunction solar cells. We conclude that n-type GaAs/polymer planar heterojunctions are not a type II heterojunctions as originally assumed. Instead, n-type GaAs forms a Schottky barrier with its corresponding anode, while the semiconducting polymer of appropriate energy levels can function as hole transport layer and/or electron blocking layer. Additionally, we discovered that both GaAs surface passivation and thermal annealing can improve the performance of GaAs/polymer hybrid solar cells. 2. With a set of conjugated polymers having different energy levels and band gaps (P3HT, PNDT-DTffBT and PBnDT-DTffBT), we show that the photovoltaic behavior of GaAs/ultrathin polymer layer/PEDOT:PSS planar solar cells is noticeably affected by the surface orientation of n-GaAs, (111)B or (100). In all these hybrid solar cells, a Schottky barrier is formed between the GaAs and the anode, with these ultrathin polymer layers possibly serving as the electron blocking layer (EBL) and the hole transport layer (HTL). The lower density of surface states of GaAs(111)B helps reduce the surface recombination and results in a higher short circuit current for (111)B based hybrid solar cells than for (100) based ones. However, the higher valence band maximum (VBM) of (111)B than that of (100) could lead to increased recombination via blocking hole transport, if the highest occupied molecular orbital (HOMO) level of the polymer as HTL is lower than the VBM of GaAs. Considering all these effects, P3HT stands out as the best polymer in the studied set to pair with GaAs, with an efficiency of 4.2% achieved for the device based on P3HT/GaAs(111)B. Broader Impacts The broader impacts have been significant, including the following. 1. Research results: the experimental and theoretical outcomes of this work help understanding the role of surface energies and how to control polymer morphology on nano structures in hybrid solar cells. The nano pillars developed in this work can help other research areas such as nano transistors, sensors and detectors. This work also benefits other communities who try to understand carrier transport in hybrid structures. In addition, the elucidated design rationale for organic/inorganic hybrid solar cells helps others who want to venture into this field. 2. Education and training: this project has been a great opportunity to train graduate students and post doc fellows as it involves experts from different research fields. This project also helped students and post docs to develop research and communication skills and understand challenges involved in other fields and also how to work with researchers from different fields to achieve specific goals. 3. Publication and presentation: some of the results obtained in this work have been published in peer reviewed journals, and the participants (students, postdocs, and faculty members) have given research presentations in various venues. 4. Outreach to general public, and K-12: UNC team has participated in the UNC Science Expo (2013 and 2014) to show the importance of renewable energy. In addition, UNC team also participated in the Climate LEAP program and ran a few lab tours for high school students in the summer of 2012, 2013, and 2014.

National Science Foundation (NSF)
Division of Materials Research (DMR)
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Linda Sapochak
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University of North Carolina Chapel Hill
Chapel Hill
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