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

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

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

33 Scopus citations

Abstract

Using an analysis based on Marcus theory, we characterize losses in open-circuit voltage (VOC) 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. 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 NCT upon selenium substitution in donor polymers correlates with lower VOC for bulk heterojunction photovoltaic devices. The increase in NCT 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.

Original languageEnglish (US)
Pages (from-to)6583-6591
Number of pages9
JournalChemistry of Materials
Volume27
Issue number19
DOIs
StatePublished - Oct 13 2015

ASJC Scopus subject areas

  • Chemistry(all)
  • Chemical Engineering(all)
  • Materials Chemistry

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