Systematic Process Development for Optimization of Manufacturable Organic Solar Cells
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Systematic Process Development for Optimization of Manufacturable Organic Solar Cells J. A. Weiss1, L. Zhu1, P. N. Kariuki2, B. Arfaei1, W. E. Jones, Jr.2, and P. Borgesen1 1 System Science and Industrial Engineering; 2Department of Chemistry Binghamton University, 4400 Vestal Pkwy E., Vestal, NY 13850 U.S.A. ABSTRACT The commercial viability of solar power will depend on a careful balance of reliability, efficiency, and overall cost. A systematic approach to the optimization of the latter two for the case of organic solar cells is outlined. This relies among other on the development of a detailed understanding of the charge generation process and the systematic application of analytical tools such as UV-vis, photoluminescence, lifetime measurements, and current-voltage (I-V) curves. INTRODUCTION Solar power has emerged as a strong candidate for the delivery of clean and renewable energy based on its abundance, predictability, availability, and alignment with peak power demand. However, a major barrier to widespread adoption is the high cost of both modules and installation1. Organic photovoltaics (OPV’s) address both of these cost drivers by employing low cost, solution-based materials compatible with roll-to-roll (R2R) manufacturing techniques and flexible substrates that help minimize panel weight and installation costs. Truly cost effective solar cells will be found at the intersection of low cost production, high module efficiency, and long module lifetime. A systematic research effort was initiated to address each of these categories. A major part of current OPV module costs is associated with the transparent ITO electrode2. Dedicated efforts aim to replace this with sputtered aluminum-doped zinc oxide (AZO) to alleviate anticipated indium supply limitations, or even eliminate the sputter deposition process by using poly (3,4-ethylenedioxythiophene) (PEDOT) deposited by vapor phase polymerization3. Below we present initial results on the pursuit of a low cost manufacturing process and high efficiency. For now layers are deposited by spin coating on pieces of flex while a separate effort is focused on the transition to R2R slot die coating. Design and Optimization Our cell is based on a poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bulk heterojunction with blocking layers of PEDOT:PSS and TiO2. Electrodes of ITO and Ag were selected based on their work functions and optical properties. This cell structure is shown in Figure 1.
Figure 1. OPV cell structure and energy level diagram. Approximate layer thicknesses are: 50 nm ITO, 1.5 μm TiO2, 375 nm P3HT:PCBM, 1.5 μm PEDOT:PSS, 40 μm Ag.
The cell efficiency is determined by the combination of four steps: absorption, exciton diffusion, charge transfer, and carrier collection4. Light absorbed in the P3HT creates an exciton which can only move a distance of about 10 nm before it decays5. If it encounters an interface with PCBM within its lifetime, charge transfer takes place with nearly 100% efficiency on time scales of a few h
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