Even though significant breakthroughs with over 17% power conversion efficiencies (PCEs) in polymer:non-fullerene acceptor (NFA) bulk heterojunction organic solar cells (OSCs) have been achieved, not many studies have focused on acquiring a comprehensive understanding of the underlying mechanisms governing these systems. This is because it can be challenging to delineate device photophysics in polymer:NFA blends comprehensively, and even more complicated to trace the origins of the differences in device photophysics to the subtle differences in energetics and morphology. Here, a systematic study of a series of polymer:NFA blends was conducted to unify and correlate the cumulative effects of i) voltage losses ii) charge generation efficiencies, iii) nongeminate recombination and extraction dynamics, and iv) nuanced morphological differences with device performances. Most importantly, a deconvolution of the major loss processes in polymer:NFA blends and their connections to the complex BHJ morphology and energetics were established. An extension to advanced morphological techniques, such as solid-state NMR (for atomic level insights on the local ordering and donor:acceptor π-π interactions) and resonant soft x-ray scattering (for donor and acceptor interfacial area and domain spacings), provided detailed insights on how efficient charge generation, transport, and extraction processes can outweigh increased voltage losses to yield high PCEs.

simple picture, be considered as an electron on the more and a hole on the less electronegative chromophore. The key step in the photogeneration of charges in organic solar cell consists in the separation of this electron-hole pair against their mutual coulomb attraction. In a medium with a dielectric constant of 3.5, the Coulomb energy of opposite point charges with mutual separation of 1 nm is 410 meV. At room temperature, the associated Boltzmann factor for dissociation, expwould thus be about 10 −7 . On other hand, in an efficient solar cell composed of appropriate electron donor and acceptor materials the internal quantum efficiency can be close to 100%, [1] and is only weakly temperature dependent. [2] For example, Gao and co-workers report activation energies for charge separation that are as low as 25 meV for P3HT:PCBM when the film is disordered, and that decrease to 9 meV when the film is better ordered after an annealing step. [2a] Similarly, Kurpiers and co-workers report 23 meV for PCPDTBT:PCBM, i.e., also in the range of thermal energy at room temperature. [2b] How can both facts, i.e., the expected high binding energy of a localized electron-hole pair and the efficient, only weaklyThe high efficiencies reported for organic solar cells and an almost negligible thermal activation measured for the photogeneration of charge carriers have called into question whether photoinduced interfacial charge transfer states are bound by a significant coulomb attraction, and how this can be reconciled with very low activation energies. Here, this question is addressed in a combined experimental and theoretical approach. The interfacial binding energy of a charge-transfer state in a blend of MeLPPP:PCBM is determined by using energy resolved electrochemical impedance spectroscopy and is found to be about 0.5 eV. Temperature-dependent photocurrent measurements on the same films, however, give an activation energy that is about one order of magnitude lower. Using analytical calculations and Monte Carlo simulation the authors illustrate how i) interfacial energetics and ii) transport topology reduce the activation energy required to separate the interfacial electron-hole pair, with about equal contributions from both effects. The activation energy, however, is not reduced by entropy, although entropy increases the overall photodissociation yield.

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.

We introduce an energy resolved electrochemical impedance spectroscopy method to map the electronic density of states (DOS) in organic semiconductor materials. The method consists in measurement of the charge transfer resistance of a semiconductor/electrolyte interface at a frequency where the redox reactions determine the real component of the impedance. The charge transfer resistance value provides direct information about the electronic DOS at the energy given by the electrochemical potential of the electrolyte, which can be adjusted using an external voltage. A simple theory for experimental data evaluation is proposed, along with an explanation of the corresponding experimental conditions. The method allows mapping over unprecedentedly wide energy and DOS ranges. Also, important DOS parameters can be determined directly from the raw experimental data without the lengthy analysis required in other techniques. The potential of the proposed method is illustrated by tracing weak bond defect states induced by ultraviolet treatment above the highest occupied molecular orbital in a prototypical r-conjugated polymer, poly[methyl(phenyl)silylene]. The results agree well with those of our previous DOS reconstruction by post-transient space-charge-limited-current spectroscopy, which was, however, limited to a narrow energy range. In addition, good agreement of the DOS values measured on two common p-conjugated organic polymer semiconductors, polyphenylene vinylene and poly(3-hexylthiophene), with the rather rare previously published data demonstrate the accuracy of the proposed method. Determination of the electronic structures of organic semiconductors has major relevance for studies of charge/ energy transport and recombination phenomena in organic electronics. However, weak molecular coupling and disordered structures often preclude application of the spectroscopic methods used for inorganic semiconductors. Electrochemical spectroscopic methods tend to fill this gap. Electrochemical impedance spectroscopy (EIS) has been known for decades and has served many purposes, from studies of electrochemical reaction mechanisms to investigations of passive surfaces.2 EIS development has also been positively influenced by activities related to clarification of solid-electrolyte processes over the last 40 to 50 years.3 A cumulative paper describing progress in the examination of various nanostructured and organic materials by EIS was published by Bisquert et al. 4 Additionally, several other electrochemical methods exist for study of electronic structures in organic semiconductors, based on direct determination of the density of states (DOS) at the Fermi energy. 5,6 Electrochemical cyclic voltammetry (CV) of conducting polymers and molecular solid films has also been interpreted in terms of the electronic DOS. 7 The CV of organic films is generally characterized by a broad non-Nernstian signal, which is interpreted as an indication of the underlying Gaussian DOS that is common in disordered organic materials. Recently, CV anal...

Although the density of states (DOS) distribution of charge transporting states in an organic semiconductor is vital for device operation, its experimental assessment is not at all straightforward. In this work, the technique of energy resolved–electrochemical impedance spectroscopy (ER‐EIS) is employed to determine the DOS distributions of valence (highest occupied molecular orbital (HOMO)) as well as electron (lowest unoccupied molecular orbital (LUMO)) states in several organic semiconductors in the form of neat and blended films. In all cases, the core of the inferred DOS distributions are Gaussians that sometimes carry low energy tails. A comparison of the HOMO and LUMO DOS of P3HT inferred from ER‐EIS and photoemission (PE) or inverse PE (IPE) spectroscopy indicates that the PE/IPE spectra are by a factor of 2–3 broader than the ER‐EIS spectra, implying that they overestimate the width of the distributions. A comparison of neat films of MeLPPP and SF‐PDI2 or PC(61)BM with corresponding blends reveals an increased width of the DOS in the blends. The results demonstrate that this technique does not only allow mapping the DOS distributions over five orders of magnitude and over a wide energy window of 7 eV, but can also delineate changes that occur upon blending.