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In organic bulk heterojunction solar cells, the donor/acceptor interfacialenergy offset ($\Delta E$) is found to provide the driving force for efficientcharge separation which gives rise to high short circuit current density($J_\mathrm{sc}$), but a high $\Delta E$ inevitably undermines the open circuitvoltage ($V_\mathrm{oc}$). In this paper, employing the device model method wecalculated the steady state current density-voltage ($J-V$) and the$J_\mathrm{sc}-\Delta E$ curves under two different charge separationmechanisms to investigate the optimum driving force required for achievingsizable $V_\mathrm{oc}$ and $J_\mathrm{sc}$ simultaneously. Under the Marcuscharge transfer mechanism, with the increased $\Delta E$ the Jsc increasesrapidly for $\Delta E\leq 0.2$ eV, and then maintains a nearly constant valuebefore decreasing at the Marcus inverted region, which is due to theaccumulation of undissociated excitons within their lifetime and is beneficialfor obtaining a sizable $J_\mathrm{sc}$ under a $\Delta E$ much smaller thanthe reorganization energy $\lambda$. For the coherent charge transfer mechanismin which the driving force act as the energy window of accessible chargeseparated states, with two typical types of density of states for the chargetransfer excitons, it is shown that the highest $J_\mathrm{sc}$ can also beachieved under a small $\Delta E$ of 0.2\,eV if the high-lying delocalizedstates are harvested in high proportion. This work demonstrates the existenceof the optimum driving force of 0.2\,eV and provides some guidelines forengineering the interfacial energetics to achieve the high balanced$J_\mathrm{sc}$ and $V_\mathrm{oc}$.

Achieving balanced open circuit voltage and short circuit current by  tuning the interfacial energetics in organic bulk heterojunction solar cells:  A drift-diffusion simulation