Dynamics of Charge Separation and Recombination in Organic Solar Cells Employing Low-bandgap Conjugated Terpolymers

초록

Organic solar cells employing low-bandgap polymers have recently attracted much interest with the aim of achieving broad absorption spectrum and thus high power conversion efficiency (PCE). In fact, however, the absorption spectrum of common low-bandgap donor-acceptor (D-A) copolymers is not broad but 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 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, leading to dramatically increased PCE of solar cells based on those polymers. However, in a certain composition of D and A repeat units in terpolymers, terpolymer-based solar cells exhibit even lower PCE than copolymer-based solar cells. While charge separation and recombination at the donor/acceptor interface are the most critical processes that govern the PCE, their dynamics and mechanism in these novel terpolymers have been rarely explored. To understand the molecular origin of the dependence of PCE on terpolymer composition, we applied femtosecond transient absorption (TA) spectroscopy to a bulk- heterojunction solar cell employing random D-A-A terpolymers (PBDTT-DPP-TPD) and the one employing D-A copolymers (PBDTT-TPD). The former terpolymer-based solar cell exhibited a lower PCE value than the copolymer-based solar cell, which is contrary to the expectation.