Through careful selection of materials and rigorous engineering of device interfaces, organic photovoltaic devices have broken the 10% power conversion efficiency mark. However, these devices are still operating far below their thermodynamic potential and enhanced understanding of the excitonic processes and free carrier dynamics can bridge that gap. Amongst various loss factors, foremost is a substantial offset between the optical gap of the donor-acceptor materials and the open-circuit voltage of the device. This offset is a direct representation of the energy losses during carrier generation and recombination processes, and are linked to the charge transfer (CT) states of the device. Thus understanding its density of states (DOS) can give us valuable insights to free carrier generation and recombination mechanisms. In this work, we investigate the CT state DOS by applying various external stresses to the normal operating condition of the solar cell. We measure the effect of electric field, excess free carriers and dynamic disorder on the CT states; through voltage bias, background light intensity and temperature controlled external quantum efficiency (EQE) measurements. Analyzing the behavior of the CT states at these stressed conditions we intend to understand the nature of the CT state DOS and its role towards successful free carrier generation and non-radiative recombination processes.
In our voltage bias measurements, we apply an external DC voltage across the solar cell and measure the EQE. Changing the bias voltage, we can operate the solar cell anywhere between the open circuit and short circuit condition, which forces the device to go through different degrees of free carrier recombination. The EQE in Frenkel exciton absorbing region is enhanced at reverse bias and degraded at forward bias, as expected due to exciton polaron annihilation. However, the CT region is less affected by the applied bias voltage compared to the Frenkel region. This is possibly due to a shift in CT energy with increasing bias. In addition, the CT energy shifts are less visible in bulk heterojunction devices compared to planar heterojunctions, possibly due to relatively lower junction electric fields. To probe the role of dynamic disorder on the findings of the voltage bias, we performed temperature controlled EQE measurements. Data showed well-resolved red shifts in CT energy with decreasing temperature, consistent with the literature. But the relative change in EQE vs. temperature reveals that the CT region is also less affected by the decreasing temperature when compared to Frenkel region, similar to the forward voltage bias case. Ongoing EQE measurements under background light bias to selectively flood the CT region with excess excitons and measure how that affects the quantum efficiency, coupled with the electric field and temperature stresses if possible.