Charge Separation and Recombination Dynamics in Organic Photovoltaics using Low-Bandgap Conjugated Terpolymers

초록

In the development of bulk-heterojunction-type organic solar cells, low-bandgap polymers have recently attracted much interest with the aim of achieving better overlap with the solar spectrum and thus increasing the photoconversion efficiency. However, most common low-bandgap D-A copolymers do not exhibit broad absorption spectrum, but instead the absorption maximum is only red shifted, resulting in the decrease of the absorption in the visible region. As an alternative approach to circumvent this limitation, random terpolymers have been developed based on the copolymerization of one electron-rich unit and two different electron-deficient units (D-A-A) or two electron-rich units and one electron-deficient unit (D-D-A). The random terpolymers show broad absorption spectrum from visible to near-IR and the photoconversion efficiency of solar cells using those terpolymers dramatically changes depending on the composition of the repeat unit. While charge separation and recombination at the donor/acceptor interface are the most critical steps that govern the photoconversion efficiency, their dynamics and mechanism have never been explored for these novel low-bandgap terpolymers. In this work, we use femtosecond transient absorption (TA) spectroscopy to investigate the dynamics of charge separation and recombination in a bulk-heterojunction solar cell containing a random D-A-A terpolymer, PBDTT-DPP-TPD, and the one containing a D-A copolymer, PBDTT-TPD. From the TA measurement, we found that charge separation (CS) state is formed on sub-100 fs time scale for both D-A-A terpolymer and D-A copolymer solar cells. Subsequently, the CS state decays via a bimolecular process of charge loss between free electrons and holes in ~1 ps and geminate recombination in ~100 ps. Interestingly, we found that the fraction of the bimolecular charge loss and geminate recombination is apparently smaller at the D-A-A terpolymer heterojunction than at the D-A copolymer heterojunction.