Effect of thionation on the performance of PNDIT2-based polymer solar cells

Kira Rundel, Young Hun Shin, Anthony S.R. Chesman, Amelia C.Y. Liu, Adam Welford, Lars Thomsen, Michael Sommer, Christopher R. McNeill

Research output: Contribution to journalArticleResearchpeer-review

Abstract

All-polymer solar cells have gained attention in recent years with a solar cell performance of over 11% power conversion efficiency (PCE) recently demonstrated. The n-type polymer PNDIT2, also known as N2200 or P(NDI2OD-T2), has been extensively used for both photovoltaic and field-effect transistor applications. When paired with donor materials that have appropriately aligned energy levels, PNDIT2 exhibited device efficiencies over 10% PCE, and organic field-effect transistors fabricated with PNDIT2 exhibited mobilities over 1 cm 2 /V s. Thionation of the naphthalene diimide (NDI) moiety, which is the substitution of imide oxygen with sulfur atoms, has been shown to improve the field-effect transistor performance of NDI-based small molecules. By applying this strategy to PNDIT2, we explored the effect that thionation, in a 2S-trans configuration, has on the performance of all-polymer solar cells fabricated with the donor polymer PTB7-Th. Solar cells were fabricated with the original polymer, PNDIT2, as a reference, and an optimized efficiency of 4.85% was achieved. As samples with 100% conversion to 2S-trans configuration could not be produced because of synthetic limitations, batches with increasing ratios of 1S to 2S-trans thionation (15:85, 7:93, and 5:95) were studied. Devices with thionated PNDIT2 exhibited a systematic lowering of photovoltaic parameters with increasing thionation, resulting in device efficiencies of just 0.84, 0.62, and 0.42% PCE, respectively. The lower performance of the thionated blends is attributed to poor π-πstacking order in the thionated PNDIT2 phase, resulting in lower electron mobilities and finer phase separation. Evidence in support of this conclusion is provided by grazing incidence wide-angle X-ray scattering, transmission electron microscopy, photoluminescence quenching, transient photocurrent analysis, and space-charge-limited current measurements.

Original languageEnglish
Pages (from-to)12062−12072
Number of pages11
JournalJournal of Physical Chemistry C
Volume123
Issue number19
DOIs
Publication statusPublished - 16 May 2019

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