Trap-Assisted Recombination via Integer Charge Transfer States in Organic Bulk Heterojunction Photovoltaics

Qinye Bao, Oskar Sandberg, Daniel Dagnelund, Simon Sandén, Slawomir Braun, Harri Aarnio, Xianjie Liu, Weimin Chen, Ronald Österbacka, Mats Fahlman

    Research output: Contribution to journalArticleScientificpeer-review

    54 Citations (Scopus)

    Abstract

    Organic photovoltaics are under intense development and significant focus has been placed on tuning the donor ionization potential and acceptor electron affinity to optimize open circuit voltage. Here, it is shown that for a series of regioregular-poly(3-hexylthiophene): fullerene bulk heterojunction (BHJ) organic photovoltaic devices with pinned electrodes, integer charge transfer states present in the dark and created as a consequence of Fermi level equilibrium at BHJ have a profound effect on open circuit voltage. The integer charge transfer state formation causes vacuum level misalignment that yields a roughly constant effective donor ionization potential to acceptor electron affinity energy difference at the donor-acceptor interface, even though there is a large variation in electron affinity for the fullerene series. The large variation in open circuit voltage for the corresponding device series instead is found to be a consequence of trap-assisted recombination via integer charge transfer states. Based on the results, novel design rules for optimizing open circuit voltage and performance of organic bulk heterojunction solar cells are proposed.
    Original languageUndefined/Unknown
    Pages (from-to)6309–6316
    Number of pages8
    JournalAdvanced Functional Materials
    Volume24
    Issue number40
    DOIs
    Publication statusPublished - 2014
    MoE publication typeA1 Journal article-refereed

    Keywords

    • solar cells
    • surface states
    • Trapping
    • Energy efficiency

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