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

Abstract

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 CH 3 NH 3 PbI 3 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.

Importance of Electric-Field-Independent Mobilities in Thick-Film Organic Solar Cells

“…We also investigated the energetic disorder in the active layers using photothermal deflection spectroscopy (PDS). − PDS is commonly used to characterize the degree of energetic disorder in disordered materials and organic semiconductors by analyzing the absorption tail with a characteristic Urbach energy ( E u ) below the band gap. , A smaller E u represents a smaller energetic disorder. As shown in Figure S7, the fitted values of E u of PM6:N3:PC 71 BM and PTQ10:N3:PC 71 BM blends are 26 ± 0.4 meV and 28 ± 0.5 meV, respectively, suggesting that the PTQ10 blend has a lower energetic disorder than the PM6 blend. , These results are consistent with the results from Zhang et al and Ng et al showing that a low E u value in the active layer results in a higher charge carrier mobility and device FF. , The results from the PDS measurements are also consistent with the findings in the field-dependent mobility measurements and recombination from the light-dependent analysis. A higher E u value stipulates the existence of deeper traps that can act as charge recombination centers, impacting the charge carrier mobility.…”

“…As shown in Figure S7, the fitted values of E u of PM6:N3:PC 71 BM and PTQ10:N3:PC 71 BM blends are 26 ± 0.4 meV and 28 ± 0.5 meV, respectively, suggesting that the PTQ10 blend has a lower energetic disorder than the PM6 blend. 53,56 These results are consistent with the results from Zhang et al and Ng et al showing that a low E u value in the active layer results in a higher charge carrier mobility and device FF. 57,58 The results from the PDS measurements are also consistent with the findings in the field-dependent mobility measurements and recombination from the light-dependent analysis.…”

Electronic Disorder Dominates the Charge-Carrier Dynamics in Two-Dimensional/Three-Dimensional Organic–Inorganic Perovskite Heterostructure

“…Recent investigations indicate that the defects in the actual materials can lead to structural disorder and give rise to electronic disorder. , Here, the electronic disorder is defined as the fluctuation of average band gap energy and is usually quantitatively characterized by the Urbach energy ( E U ) in experiments. , A low E U indicates that lattice structural disorder and defect density are very low. It is evident that the reduction of effective charge-carrier mobility and power PCE is strongly correlated to the Urbach energy (see Table S1).…”

“…It is evident that the reduction of effective charge-carrier mobility and power PCE is strongly correlated to the Urbach energy (see Table S1). ,− In addition, the Urbach energy also governs many important parameters such as quasi-Fermi-level splitting, absolute photoluminescence intensity, and sub-band gap absorption . The Urbach energy has been widely applied to characterize the defect density at the surface of semiconductor materials , and the effect of doping on the perovskite nanocrystal quality .…”

A Random Terpolymer Donor with Similar Monomers Enables 18.28% Efficiency Binary Organic Solar Cells with Well Polymer Batch Reproducibility

“…These results are consistent with the tendency obtained from photo-CELIV measurements. Compared to other devices, the D18–20%Cl:Y6-based devices possess higher and more balanced hole/electron mobilities, which can well explain the highest J SC and FF values in corresponding OSCs. , In addition, the charge transport properties intimate relationships with bulk resistance ( R b ) of the devices, which can be estimated by performing electrochemical impedance spectroscopy (EIS) measurements (Figure f). The fitted R b values are 12.9, 11.5, 13.6, and 15.2 Ω for D18:Y6, D18–20%Cl:Y6, D18–40%Cl:Y6, and D18-Cl:Y6-based devices, respectively.…”