Photoinduced Charge Carrier Generation in a Poly(3-hexylthiophene) and Methanofullerene Bulk Heterojunction Investigated by Time-Resolved Terahertz Spectroscopy

Abstract

We report on the ultrafast photoinduced charge separation processes in varying compositions of poly(3-hexylthiophene) (P3HT) blended with the electron acceptor [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM). Through the use of time-resolved terahertz spectroscopy, the time-and frequency-dependent complex photoconductivity is measured for samples with PCBM weight fractions (W PCBM ) of 0, 0.2, 0.5, and 0.8. By analysis of the frequency-dependent complex conductivity, both the charge carrier yield and the average charge carrier mobility have been determined analytically and indicate a short (<0.2 nm) carrier mean free path and a suppressed long-range transport that is characteristic of carrier localization. Studies on pure films of P3HT demonstrate that charge carrier generation is an intrinsic feature of the polymer that occurs on the time scale of the excitation light, and this is attributed to the dissociation of bound polaron pairs that reside on adjacent polymer chains due to interchain charge transfer. Both interchain and interfacial charge transfer contribute to the measured photoconductivity from the blended samples; interfacial charge transfer increases as a function of increasing PCBM. The addition of PCBM to the polymer films surprisingly does not dramatically increase the production of charge carriers within the first 2 ps. However, charge carriers in the 0.2 and 0.5 blended films survive to much longer times than those in the P3HT and 0.8 films.

Carrier dynamics in semiconductors studied with time-resolved terahertz spectroscopy

“…Apart from pristine polymers, films blended with fullerene derivatives [usually phenyl-C61-butyric acid methyl ester (PCBM)] have been investigated as well (Hendry, Schins et al, 2004;Ai et al, 2006;Cunningham and Hayden, 2008;Nemec et al, 2008;Parkinson et al, 2008;). The inclusion of PCBM is primarily intended for the application in organic solar cells; they provide a means to dissociate excitons after photoexcitation by acting as electron scavengers.…”

On the mechanism of charge transport in pentacene

“…These measurements were performed at 20 K using 400 nm pump pulses to photoinject charge carriers. 21 recently observed a terahertz resonance upon photoexcitation of the polymer P3HT. They showed how several Drude-like models cannot account for this observation and attributed the resonance to bound polaron pairs, having observed a marked dependence of the dynamics on charge density.…”

“…[5][6][7][19][20][21] By recording the modulation of the transmission of terahertz probe pulses after optically creating carriers in the sample, the technique allows for the determination of the photoconductivity spectrum in the range of 0.2-1.2 THz. The terahertz conductivity spectrum has been shown to contain detailed information on the nature of the charge transport.…”

Theoretical estimation of the rate of photoinduced charge transfer reactions in triphenylamine C60 donor–acceptor conjugate

“…It has been extensively studied experimentally and theoretically. [12,[15][16][17][18][19][20][21] For instance, rate constants (kct, subscript ct stands for charge transfer; it can be turned into subscripts cs and cr standing for charge separation and charge recombination, respectively) for photoinduced charge transfer (CT) reactions occurring in P3HT-PCBM reveal the potential of such materials in the construction of efficient DSSCs. Based on quantum chemistry methods, these kct have been demonstrated [22] to be tunable by a few parameters so that the power conversion efficiency may be increased; [23][24][25] which highlights, in fact, the importance of theoretical developments for the understanding and design of DSSCs with improved efficiency.…”