A steady‐state method to probe bimolecular recombination in organic solar cells is presented. The technique is applicable to thin‐film solar cells at any temperature and does not require a separate measurement setup other than conventional solar‐cell testing equipment. The key element in our method is the derivation of a simple analytical expression that directly gives access to the recombination strength.

Characterizing the density of states (DOS) width accurately is critical in understanding the charge-transport properties of organic semiconducting materials as broader DOS distributions lead to an inferior transport. From a morphological standpoint, the relative densities of ordered and disordered regions are known to affect charge-transport properties in films; however, a comparison between molecular structures showing quantifiable ordered and disordered regions at an atomic level and its impact on DOS widths and charge-transport properties has yet to be made. In this work, for the first time, the DOS distribution widths of two model conjugated polymer systems are characterized using three different techniques. A quantitative correlation between energetic disorder from band-bending measurements and charge transport is established, providing direct experimental evidence that chargecarrier mobility in disordered materials is compromised due to the relaxation of carriers into the tail states of the DOS. Distinction and quantification of ordered and disordered regions of thin films at an atomic level is achieved using solid-state NMR spectroscopy. An ability to compare solid-state film morphologies of organic semiconducting polymers to energetic disorder, and in turn charge transport, can provide useful guidelines for applications of organic conjugated polymers in pertinent devices.

The efficiency roll‐off in organic light‐emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) is attributed to either singlet–triplet or triplet–triplet annihilation (TTA) as well as triplet–charge annihilation. The origin of the efficiency roll‐off on a TADF OLED consisting of a host‐less single‐layer emitter is studied. Varying the charge‐carrier concentration at constant exciton density or the exciton density at constant charge‐carrier density with temperature unambiguously shows that the dominant contribution to the roll‐off originates from TTA. Using an analytical model, also the TTA rate constant can be obtained. These results show that single‐layer TADF OLEDs are suited not only to determine the roll‐off mechanism, but also to provide its rate constant directly from OLED efficiency measurements.

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