Fast Recombination of Charge-Transfer State in Organic Photovoltaic Composite of P3HT and Semiconducting Carbon Nanotubes Is the Reason for Its Poor Photovoltaic Performance

Abstract

Although the photovoltaic performance of the composite of poly-3-hexylthiophene (P3HT) with semiconducting single-walled carbon nanotubes (s-SWCNT) is promising, the short-circuit current density jSC is much lower than that for typical polymer/fullerene composites. Out-of-phase electron spin echo (ESE) technique with laser excitation of the P3HT/s-SWCNT composite was used to clarify the origin of the poor photogeneration of free charges. The appearance of out-of-phase ESE signal is a solid proof that the charge-transfer state of P3HT+/s-SWCNT− is formed upon photoexcitation and the electron spins of P3HT+ and s-SWCNT− are correlated. No out-of-phase ESE signal was detected in the same experiment with pristine P3HT film. The out-of-phase ESE envelope modulation trace for P3HT/s-SWCNT composite was close to that for the polymer/fullerene photovoltaic composite PCDTBT/PC70BM, which implies a similar distance of initial charge separation in the range 2–4 nm. However, out-of-phase ESE signal decay with delay after laser flash increase for P3HT/s-SWCNT composite was much faster, with a characteristic time of 10 µs at 30 K. This points to the higher geminate recombination rate for the P3HT/s-SWCNT composite, which may be one of the reasons for the relatively poor photovoltaic performance of this system.

Structure of the Charge-Transfer State in PM6/Y6 and PM6/Y6:YT Composites Studied by Electron Spin Echo Technique

“…The emission/absorption spectral patterns are often observed in echo-detected EPR experiments on organic photovoltaic composites at low temperatures and small DAF values. This was observed for composites of donor polymer with different acceptors: fullerene [17]; nonfullerene small molecule [12]; and semiconducting carbon nanotubes [35]. Such a pattern is generated in the course of spin evolution of the CT state accompanying its thermalization in the constant magnetic field of the EPR spectrometer, during which the spins of the charges constituting the CT state acquire the opposite polarization [16,36,37].…”