Photocurrent Generation in Polymer-Fullerene Bulk Heterojunctions

The photocurrent in conjugated polymer-fullerene blends is dominated by the dissociation efficiency of bound electron-hole pairs at the donor-acceptor interface. A model based on Onsager's theory of geminate charge recombination explains the observed field and temperature dependence of the photocurrent in $\mathrm{P}\mathrm{P}\mathrm{V}:\mathrm{P}\mathrm{C}\mathrm{B}\mathrm{M}$ blends. At room temperature only 60% of the generated bound electron-hole pairs are dissociated and contribute to the short-circuit current, which is a major loss mechanism in photovoltaic devices based on this material system.

Figure 1

Room temperature $J_{\mathrm{p}\mathrm{h}}V$ characteristics of an ${\mathrm{O}\mathrm{C}}_1{\mathrm{C}}_{10}\mathrm{\text{−}}\mathrm{P}\mathrm{P}\mathrm{V}:\mathrm{P}\mathrm{C}\mathrm{B}\mathrm{M}$ $20:80$ device as a function of effective applied voltage ($V_0\mathrm{\text{−}}V$) (open circles), for a device with thickness of 120 nm. The solid line represents the calculated photocurrent from Eq. (1) using $G=1.56\times {10}^{27}{\mathrm{m}}^{-3}{\mathrm{s}}^{-1}$, whereas, the dotted line represents the drift current calculated from $J_{\mathrm{p}\mathrm{h}}=eGL$ using the same $G$. The inset shows the $J_L$, $J_D$, and $J_{\mathrm{p}\mathrm{h}}$ as a function of bias, whereas the arrows indicate the open-circuit voltage ($V_{\mathrm{O}\mathrm{C}}$) and the compensation voltage ($V_0$), respectively.

Figure 2

Temperature dependence of the photocurrent (symbols) versus effective applied voltage ($V_0\mathrm{\text{−}}V$) for a device of 100 nm thickness. The solid line represents the calculated photocurrent ($J_{\mathrm{p}\mathrm{h}}=eGL$) using field-dependent generation rate $G(T,E)$. The inset shows the spatial distribution $F(x)$ for $a=1.3\mathrm{n}\mathrm{m}$.

Figure 3

Normalized experimental photocurrent ($J_{\mathrm{p}\mathrm{h}}$) and normalized calculated generation rate [$G(T,E)$] from Fig. 2, as a function of $1/T$, taken in the linear regime at 0.01 V (solid symbols) and at short circuit (open symbols). The activation energies are written on the figure.

Figure 4

Experimental photocurrent ($J_{\mathrm{p}\mathrm{h}}$) as a function of effective applied voltage ($V_0\mathrm{\text{−}}V$) of ${\mathrm{O}\mathrm{C}}_1{\mathrm{C}}_{10}\mathrm{\text{−}}\mathrm{P}\mathrm{P}\mathrm{V}:\mathrm{P}\mathrm{C}\mathrm{B}\mathrm{M}$ $20:80$ device (symbols), at 295 and 250 K. The solid line represents the numerical calculation including diffusion, field dependent of generation rate $G(T,E)$, and recombination, for a device with a thickness of 120 nm. The dotted line represents calculated $J_{\mathrm{p}\mathrm{h}}$ from Eq. (1) using $G(T,E)$, at 295 K.