Unraveling Urbach Tail Effects in High-Performance Organic Photovoltaics : Dynamic vs Static Disorder

The origin of Urbach energy (E_U) in organic semiconductors and its effect on photovoltaic properties remain a topic of intense interest. In this letter, we demonstrate quantitative information on the E_U value in emerging Y-series molecules by an in-depth analysis of the line shape of the temperature-dependent quantum efficiency spectra. We found that the static disorder (E_U(0)), which is dominated by the conformational uniformity in Y-series acceptors, contributes 10-25 meV to the total Urbach energy. Particularly, this static contribution in organic solar cells (OSCs) is much higher than those (E_U(0) ≈ 3-6 meV) in inorganic/hybrid counterparts, such as CH₃NH₃PbI₃ perovskite, crystalline silicon (c-Si), gallium nitride (GaN), indium phosphide (InP), and gallium arsenide (GaAs). More importantly, we establish clear correlations between the static disorder and photovoltaic performance and open-circuit voltage loss. These results suggest that suppressing the static disorder via rational molecular design is clearly a path for achieving higher performance.