Ultrafast vibrational spectroscopy of charge-carrier dynamics in organic photovoltaic materials

Ultrafast vibrational spectroscopy is used to examine the dynamics of interfacial electron transfer, free-carrier formation, and bimolecular charge recombination and trapping in an organic photovoltaic material. The carbonyl (CO) stretch of the functionalized fullerene, PCBM, is probed as a local vibrational reporter of the dynamics in a blend with a conjugated polymer, CN-MEH-PPV. Ultrafast interfacial electron transfer from CN-MEH-PPV to PCBM occurs on time scales ranging from less than 100 fs to 1 ps. PCBM molecules at interfaces with the polymer have carbonyl vibrations that are higher in frequency compared to the ensemble. The frequency variation results in part from a vibrational Stark shift arising from an interfacial dipole formed by spontaneous charge transfer from the polymer to PCBM. The Stark shift provides a means to observe directly the formation of free carriers through the spectral evolution of the carbonyl stretch. Free carrier formation occurs surprisingly quickly on the 1-10 ps time scale, suggesting that the charges experience a smaller effective Coulombic binding energy than expected. The interfacial dipole decreases the Coulombic binding energy because the negative pole of the dipole repels electrons at the PCBM domain interface. Following free-carrier formation, electrons diffuse within the material and become trapped on the microsecond time scale resulting in the formation of a distinct peak in the vibrational spectra. The time scale of charge trapping corresponds to the carrier lifetime of similar PPV-based polymer blends that have been reported in the literature on the basis of transient photocurrent measurements.