Temperature Dependence of Charge Carrier Generation in Organic Photovoltaics

The charge generation mechanism in organic photovoltaics is a fundamental yet heavily debated issue. All the generated charges recombine at the open-circuit voltage ($V_{\mathrm{OC}}$), so that investigation of recombined charges at $V_{\mathrm{OC}}$ provides a unique approach to understanding charge generation. At low temperatures, we observe a decrease of $V_{\mathrm{OC}}$, which is attributed to reduced charge separation. Comparison between benchmark polymer:fullerene and polymer:polymer blends highlights the critical role of charge delocalization in charge separation and emphasizes the importance of entropy in charge generation.

Figure 1

(a) Temperature-dependent $V_{\mathrm{OC}}$ for $\mathrm{P}3\mathrm{HT}:{\mathrm{PC}}_{60}\mathrm{BM}$, $\mathrm{P}3\mathrm{HT}:\mathrm{F}8\mathrm{TBT}$, and $\mathrm{PFB}:\mathrm{F}8\mathrm{BT}$ devices (chemical structures in the inset of Panel b). At low $T$, the $V_{\mathrm{OC}}\text{−}T$ plot deviates from the linear relation (dashed line), starting to saturate and even decrease. We attribute this sublinear $V_{\mathrm{OC}}\text{−}T$ relation to temperature-dependent charge separation. (b) Calculated temperature-dependent charge densities. The solid lines are guides for the eye. Panel (b) shares the same legend as Panel (a).

Figure 2

Temperature-dependent EQE values at a low intensity. The points are the raw data, and the solid lines are the fitted curves (see text for details).

Figure 3

Increasing electron-hole distance is required for charge separation with decreasing temperature. The inset shows the free energy dependence on the electron−hole distance at different temperatures. A negative value of $\mathrm{\Delta }G$ means that the entropy contribution cancels the Coulombic attraction, leading to spontaneous charge separation.

Figure 4

Temperature-dependent $V_{\mathrm{OC}}$ for $\mathrm{TQ}1:{\mathrm{PC}}_{70}\mathrm{BM}$ blends with different weight ratios. With decreasing fullerene concentration, the transition temperature, where $V_{\mathrm{OC}}$ starts to decrease, appears at higher $T$, indicating that charge separation becomes increasingly difficult with decreasing fullerene content.