High‐performance inorganic–organic lead halide perovskite solar cells (PSCs) are often fabricated with a liquid additive such as dimethyl sulfoxide (DMSO), which retards crystallization and reduces roughness and pinholes in the perovskite layers. However, DMSO can be trapped during perovskite film formation and induce voids and undesired reaction byproducts upon later processing steps. Here, it is shown that the amount of residual DMSO can be reduced in as‐spin‐coated films significantly through use of preheated substrates, or a so‐called hot‐casting method. Hot casting increases the perovskite film thickness given the same concentration of solutions, which allows for reducing the perovskite solution concentration. By reducing the amount of DMSO in proportion to the concentration of perovskite precursors and using hot casting, it is possible to fabricate perovskite layers with improved perovskite–substrate interfaces by suppressing the formation of byproducts, which increase trap density and accelerate degradation of the perovskite layers. The best‐performing PSCs exhibit a power conversion efficiency (PCE) of 23.4% (23.0% stabilized efficiency) under simulated solar illumination. Furthermore, encapsulated devices show considerably reduced post‐burn‐in decay, retaining 75% and 90% of their initial and post‐burn‐in efficiencies after 3000 h of operation with maximum power point tracking (MPPT) under high power of ultraviolet (UV)‐containing continuous light exposure.

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In an effort to gain a comprehensive picture for the interfacial states in bulk heterojunction solar cells, we provide a combined experimental-theoretical analysis of the energetics and dynamics of low-lying electronic charge-transfer (CT) states in donor:acceptor blends with large frontier orbital energy offset. By varying the blend composition and temperature, we unravel the static and dynamic contributions to the disordered density of states (DOS) of the CT states manifold, and assess their recombination to the ground state. Namely, we find that static disorder (conformational and electrostatic) shapes the CT DOS, and that fast non-radiative recombination crops the low-energy tail of the distribution probed by external quantum efficiency (EQE) measurements (thereby largely contributing to voltage losses). Our results then question the standard practice of extracting microscopic parameters such as exciton energy and energetic disorder from EQE.

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To understand why organic solar cells have a non-radiative energy loss of 0.2-0.4 eV, [7,8] a significantly larger value compared to state-of-the-art inorganic counterparts, [9,10] a fundamental understanding of all non-radiative loss pathways is crucial.Sources of non-radiative energy loss in OPVs are still not well understood, [11] but recent studies suggest an intrinsic origin controlled by molecular properties. [8] However, these studies mostly consider non-radiative recombination at the donoracceptor (D-A) interface, where charge transfer (CT) excitons (either generated directly by optical excitation or via injected charge) recombine at a rate exponentially dependent on the CT state energy (E CT ), [8,11,12] prescribed by the "energy-gap law." [13] Experimentally, a linear increase in non-radiative energy loss is observed as E CT is reduced. [8,12] Theoretical modeling [12] established design rules for using stiff molecules with strongly absorbing CT states [4] to minimize such losses at the D-A interface.Despite the success of this model in capturing the general trend, for a given E CT , large variations are observed in the estimated non-radiative energy loss values. [4] These variations are attributed to recombination away from the D-A interface (i.e., in the bulk or at the contacts), surface recombination, and effects of the energetic disorder, not considered explicitly in the model. [4,11] Moreover, recently it has been shown that the "energy-gap law" fails for non-fullerene acceptor (NFA) based solar cells, [7] which has been explained through hybridization of CT and local exciton states. In addition, Gillett et al. [14] suggested back-electron transfer from CT-triplet states to NFA-triplet states as a non-radiative recombination pathway for NFA solar cells. These latest studies highlight the importance of identifying fundamental device processes beyond the "energy-gap law" that could add to and perhaps unify our understanding of non-radiative energy losses in both fullerenes-and NFA-based OPVs.One key characteristic of organic semiconductors is high binding energy excitons (0.2-0.5 eV) owing to the small dielectric constant and confinement in localized molecules. [15] This excitonic nature of organic semiconductors produces unique non-radiative loss processes like exciton-exciton annihilation