2, Lund University, Lund, , Sweden
In an organic solar cell (OPV), the key photophysical process is free charge carrier generation. Ultrafast long-range charge separation have been proposed1, there is however no universal consensus about the role of the charge-neutral electron-hole pairs (excitons) and charge transfer (CT) state in this process2.
Here, we aim at getting more insight on the timescale and process leading to the formation of the charge carriers from thermalized excited states, by using ultrafast transient absorption spectroscopy with 20fs time resolution based on one efficient ternary D1-D2-A OPV system. The use of extremely short pulses enables us to measure the characteristic vibrational modes in the excited states of the system. The analysis of these modes allow to study how they evolved during charge generation, in addition to unraveling the exciton photophysics in this process.
The vibrational modes created by the pump are clearly visible from the TA data (fig.1, a). The electronic response shows that the ternary blend have significant oscillation signal instantly while binary blend short of distinct signal. Time-resolved photoluminescence data (fig.1, b,c) showing that exciton in the ternary blend dissociate almost totally in the initial 10ps, while the binary blend still emitted photons after 1ns. In combination with other time –resolved and steady state optical measurements, we can identify that compared to binary blend, the ternary blend show less charge transfer state and more charge carriers come from exciton directly. This shows that thermalized excited states is not hindering charge generation as the vibronic coupling accelerates ultrafast charge separation dynamics.
1. Gelinas S et al, Ultrafast long-range charge separation in organic semiconductor photovoltaic diodes. Science 2014, 343(6170): 512-515.
2. Lienau C et al, Coherent ultrafast charge transfer in an organic photovoltaic blend. Science 2014, 344(6187): 1001-1005.