Carrier recombination flux in bulk heterojunction polymer:fullerene solar cells: Effect of energy disorder on ideality factor

Energy disorder reduces the achievable open-circuit voltage in organic bulk-heterojunction solar cells. Here the effect of disorder on charge carrier recombination flux is numerically modeled. The recombination current follows an exponential dependence on voltage (Fermi level splitting) parameterized by β (inverse of the diode ideality factor), which reduces the power conversion efficiency through lower fill factors. β-Parameter approaches unity (Boltzmann approximation) at room temperature only in the case of weak disorder (σ ≈ 50 meV). For larger disorder values (σ ⩾ 100 meV) usually encountered in real devices, a huge reduction in β (open-circuit voltage, and fill factor) is predicted following a relationship as β ∝ ln σ−1.

► Energy disorder of electronic states and ideality factor are connected in BHJ devices.
► Boltzmann (ideal) statistics only valid for low disorder.
► Reduction in open-circuit voltage and fill factor occur as disorder increases.