In-situ Characterization of Polymer-Fullerene Bilayer Stability
Lee J. Richter, Regis J. Kline, Hyun Wook Ro, Chad R. Snyder, Andrew A. Herzing, Sebastian Engmann
A consensus is emerging that every phase present in a bulk heterojunction organic photovoltaic (OPV) device is not material pure. In general, there will exist at least one region characterized by a mixture of donor and acceptor. Significant insights into this mixed phase have come from bilayer stability measurements, in which an initial sample consisting of material pure layers of donor and acceptor is thermally treated, resulting in swelling of one layer by the other. We present a comparative study of the stability of polymer/fullerene bilayers using two common OPV polymer donors poly(3-hexylthiophene), P3HT, and poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)], PCDTBT, and four fullerene acceptors phenyl-C61-butyric acid methyl ester, phenyl-C71-butyric acid methyl ester, PCBM-61 bis-adduct and Indene C60 bis-adduct. Using in-situ spectroscopic ellipsometry to characterize the quasi-steady state behavior of the films, we find that the polymer glass transition temperature is critical to the bilayer stability, with no significant changes occurring below Tg of the high Tg PCDTBT. Above the polymer Tg, we find the behavior is irreversible and most consistent with swelling of the polymer by the fullerene, constrained by tie chains in the polymer network and influenced by the glassy dynamics of the mixed region. The swelling varies significantly with the nature of the fullerene and the polymer. Across the 8 systems studied, there is no clear relationship between swelling and OPV device performance
, Kline, R.
, , H.
, Snyder, C.
, Herzing, A.
and Engmann, S.
In-situ Characterization of Polymer-Fullerene Bilayer Stability, Macromolecules, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=916304
(Accessed February 21, 2024)