Ultrafast Terahertz Photoconductivity of Photovoltaic Polymer-Fullerene Blends: A Comparative Study Correlated with Photovoltaic Device Performance.
Ultrafast photoinduced carrier dynamics in prototypical low band gap polymer:fullerene photovoltaic blend films PTB7:PC70BM and P3HT:PC70BM is investigated using ultrafast terahertz (THz) spectroscopy. The subpicosecond and few-picosecond decays of THz-probed photoconductivities for both compounds are observed, attributed to the rapid formation of polaron pairs by exciton-exciton annihilation and subsequent polaron pair annihilation, respectively. The transient THz photoconductivity spectra of PTB7:PC70BM are well described by the Drude-Smith (DS) model, directly yielding the important charge transport parameters such as charge carrier density, momentum scattering time, and effective localization. By comparison with P3HT:PC70BM, we find that in PTB7:PC70BM the mobile charge carrier photoconductivity is significantly enhanced by a factor of 1.8 and prevails for longer times after charge formation, due to both improved mobile charge carrier yield and lower charge localization. In PTB7:PC70BM, a strong dependency of electron momentum scattering time on electron density was found, well parametrized by the empirical Caughey-Thomas model. The difference in ultrafast photoconductivities of both P3HT:PC70BM and PTB7:PC70BM is found to correlate very well with the performance of photovoltaic devices based on those materials.
†Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany.
Zuanming
Z
Jin
†Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany.
Dominik
D
Gehrig
†Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany.
Clare
C
Dyer-Smith
‡NIST - National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, Maryland 20899, United States.
Edwin J
EJ
Heilweil
†Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany.
Frédéric
F
Laquai
†Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany.
Mischa
M
Bonn
†Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany.
Dmitry
D
Turchinovich
charge transfer; organic photovoltaics; photoconductivity; semiconducting polymers; terahertz spectroscopy