Open-Circuit Voltage Losses in Selenium-Substituted Organic Photovoltaic Devices from Increased Density of Charge-Transfer States

Journal Article · Sat Sep 12 00:00:00 EDT 2015 · Chemistry of Materials

DOI:https://doi.org/10.1021/acs.chemmater.5b02133· OSTI ID:1215764

Sulas, Dana B. ^[1]; Yao, Kai ^[1]; Intemann, Jeremy J. ^[1]; Williams, Spencer T. ^[1]; Li, Chang-Zhi ^[1]; Chueh, Chu-Chen ^[1]; Richards, Jeffrey J. ^[1]; Xi, Yuyin ^[1]; Pozzo, Lilo D. ^[1]; Schlenker, Cody W. ^[1]; Jen, Alex K. -Y. ^[1]; Ginger, David S. ^[1]

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1. Department of Chemistry, ‡Department of Materials Science and Engineering, and #Department of Chemical Engineering, University of Washington, Seattle, Washington 98195, United States

Using an analysis based on Marcus theory, we characterize losses in open-circuit voltage (V_OC) due to changes in charge-transfer state energy, electronic coupling, and spatial density of charge-transfer states in a series of polymer/fullerene solar cells. Here, we use a series of indacenodithiophene polymers and their selenium-substituted analogs as electron donor materials and fullerenes as the acceptors. By combining device measurements and spectroscopic studies (including subgap photocurrent, electroluminescence, and, importantly, time-resolved photoluminescence of the charge-transfer state) we are able to isolate the values for electronic coupling and the density of charge-transfer states (NCT), rather than the more commonly measured product of these values. We find values for NCT that are surprisingly large (~4.5 × 10²¹–6.2 × 10²² cm^-3), and we find that a significant increase in N_CT upon selenium substitution in donor polymers correlates with lower VOC for bulk heterojunction photovoltaic devices. The increase in N_CT upon selenium substitution is also consistent with nanoscale morphological characterization. Using transmission electron microscopy, selected area electron diffraction, and grazing incidence wide-angle X-ray scattering, we find evidence of more intermixed polymer and fullerene domains in the selenophene blends, which have higher densities of polymer/fullerene interfacial charge-transfer states. Our results provide an important step toward understanding the spatial nature of charge-transfer states and their effect on the open-circuit voltage of polymer/fullerene solar cells

• Research Organization: Univ. of Washington, Seattle, WA (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES); National Science Foundation (NSF)
• Grant/Contract Number: SC0005153
• OSTI ID: 1215764
• Alternate ID(s): OSTI ID: 1436293
• Journal Information: Chemistry of Materials, Journal Name: Chemistry of Materials Vol. 27 Journal Issue: 19; ISSN 0897-4756
• Publisher: American Chemical Society
• Country of Publication: United States
• Language: English

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37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY