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Nonequilibrium site distribution governs charge-transfer electroluminescence at disordered organic heterointerfaces
摘要： The interface between electron-donating (D) and electron-accepting (A) materials in organic photovoltaic (OPV) devices is commonly probed by charge-transfer (CT) electroluminescence (EL) measurements to estimate the CT energy, which critically relates to device open-circuit voltage. It is generally assumed that during CT-EL injected charges recombine at close-to-equilibrium energies in their respective density of states (DOS). Here, we explicitly quantify that CT-EL instead originates from higher-energy DOS site distributions significantly above DOS equilibrium energies. To demonstrate this, we have developed a quantitative and experimentally calibrated model for CT-EL at organic D/A heterointerfaces, which simultaneously accounts for the charge transport physics in an energetically disordered DOS and the Franck–Condon broadening. The 0–0 CT-EL transition lineshape is numerically calculated using measured energetic disorder values as input to 3-dimensional kinetic Monte Carlo simulations. We account for vibrational CT-EL overtones by selectively measuring the dominant vibrational phonon-mode energy governing CT luminescence at the D/A interface using fluorescence line-narrowing spectroscopy. Our model numerically reproduces the measured CT-EL spectra and their bias dependence and reveals the higher-lying manifold of DOS sites responsible for CT-EL. Lowest-energy CT states are situated ～180 to 570 meV below the 0–0 CT-EL transition, enabling photogenerated carrier thermalization to these low-lying DOS sites when the OPV device is operated as a solar cell rather than as a light-emitting diode. Nonequilibrium site distribution rationalizes the experimentally observed weak current-density dependence of CT-EL and poses fundamental questions on reciprocity relations relating light emission to photovoltaic action and regarding minimal attainable photovoltaic energy conversion losses in OPV devices.
关键词： organic electronics,Franck–Condon vibronic progression,energetic disorder,electroluminescence at organic interfaces,3D kinetic Monte Carlo model
更新于2025-09-12 10:27:22
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Energetic Disorder and Activation Energy in Efficient Ternary Organic Solar Cells with Nonfullerene Acceptor Eh‐IDTBR as the Third Component
摘要： Solution processed ternary organic solar cells (OSCs) contain a third component in the active layer besides the donor/acceptor materials. Two main avenues are considered to fabricate ternary OSCs: (i) to improve the short circuit current density by the selected third component that can broaden and/or enhance the absorption of the host films; (ii) to increase the fill factor by adding materials with diverse crystallinity to tune the film morphology. However, little work has been reported for the improvement of open circuit voltage (VOC), energetic disorder, charge transfer state energy (ECT), and activation energy in ternary OSCs. Herein, we used ternary OSCs with active layer composed of PCE10: F8IC: Eh-IDTBR as model to examine these parameters besides the morphology. We found in the ternary device that the additional Eh-IDTBR improved the crystallinity of the acceptor phase in the ternary mixture; the VOC was 58 mV higher than that of the reference caused by the reduced energetic disorder; due to the good miscibility of Eh-IDTBR with both PCE10 and F8IC, only 50 meV in ECT was observed; zero field activation energy was lower than that for the reference. Our findings provide an alternative way to understand the complex ternary structural-electrical properties correlations.
关键词： ternary organic solar cells,charge transfer state energy,crystallinity phase,energetic disorder,activation energy
更新于2025-09-11 14:15:04