Although it is known that molecular interactions govern morphology formation and purity of mixed domains of conjugated polymer donors and small-molecule acceptors, and thus largely control the achievable performance of organic solar cells, quantifying interaction-function relations has remained elusive. Here, we first determine the temperature-dependent effective amorphous-amorphous interaction parameter, χ(T), by mapping out the phase diagram of a model amorphous polymer:fullerene material system. We then establish a quantitative 'constant-kink-saturation' relation between χ and the fill factor in organic solar cells that is verified in detail in a model system and delineated across numerous high- and low-performing materials systems, including fullerene and non-fullerene acceptors. Our experimental and computational data reveal that a high fill factor is obtained only when χ is large enough to lead to strong phase separation. Our work outlines a basis for using various miscibility tests and future simulation methods that will significantly reduce or eliminate trial-and-error approaches to material synthesis and device fabrication of functional semiconducting blends and organic blends in general.

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Enhancing the luminescence property without sacrificing the charge collection is one key to high-performance organic solar cells (OSCs), while limited by the severe non-radiative charge recombination. Here, we demonstrate efficient OSCs with high luminescence via the design and synthesis of an asymmetric non-fullerene acceptor, BO-5Cl. Blending BO-5Cl with the PM6 donor leads to a record-high electroluminescence external quantum efficiency of 0.1%, which results in a low non-radiative voltage loss of 0.178 eV and a power conversion efficiency (PCE) over 15%. Importantly, incorporating BO-5Cl as the third component into a widely-studied donor:acceptor (D:A) blend, PM6:BO-4Cl, allows device displaying a high certified PCE of 18.2%. Our joint experimental and theoretical studies unveil that more diverse D:A interfacial conformations formed by asymmetric acceptor induce optimized blend interfacial energetics, which contributes to the improved device performance via balancing charge generation and recombination.

To the Editor, Coronavirus disease 2019 (COVID-19), caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is a disease characterized by pneumonia. The main clinical presentations are fever, dry cough, and fatigue, but in addition to respiratory symptoms, a minority of patients may present only with muscle soreness, gastrointestinal symptoms, or dispiritedness in the early stages. 1 According to limited pathological autopsy results, in addition to lung involvement, the heart, liver, kidneys, spleen, hilar lymph nodes, bone marrow, and even brain tissues are also affected in patients with COVID-19. 2 Herein, we report a case of a patient diagnosed with COVID-19 who manifested with neurological symptoms.

of this distribution in the semiclassical approximation is given by [15] 2 D 2 B k T σ λ =(1)where λ denotes the reorganization energy related to the electron transfer process between the CT state and the ground state; k B , the Boltzmann constant; and T, the temperature. The static disorder arises from the amorphous nature of the active layers and the positional inequivalency (even in the absence of vibrational motions) of the donor and acceptor (macro)molecules, which results in a time-independent distribution of the CT states with a standard deviation σ S . If both static and dynamic contributions have Gaussian distributions, the standard deviation of the overall (total) disorder σ T can be expressed as [12] T 2 S 2 D 2 σ σ σ = +The knowledge of the dynamic and static disorder contributions in D-A systems is important for an accurate determination of exciton dissociation rates, nonradiative recombination rates, charge transport, and charge-transfer optical transitions. In the semiclassical approximation, accounting for the impact of static disorder on the nonradiative recombination rate (k nr ) results in Equation (3), which is similar to the classical Marcus formula [16] except that the 2term appearing in the latter is replaced with T 2 σ [14,17,18] 2 1 2 exp 2 nr el 2 T 2 CT a 2 T 2 k V E  π σ λ σ ( ) = − −        where V el denotes the electronic coupling between the CT states and the ground state and CT a E , the adiabatic (relaxed) CT energy. Equation (3) underlines that both dynamic and static disorders can have a major impact on the nonradiative recombination rates and consequently on open-circuit voltage losses. Therefore, there is increasing interest, triggered in particular by the rapid advance in the efficiency of organic solar cells, in better understanding the role of disorder on device performance. [9,12,13,[17][18][19][20][21][22][23][24] In the semiclassical approximation used to derive Equation (3), the CT absorption band is also characterized by a Gaussian distribution with a variance given by Equation (2); [13] the static Molecular dynamics simulations are combined with density functional theory calculations to evaluate the impact of static and dynamic disorders on the energy distribution of charge-transfer (CT) states at donor-acceptor heterojunctions, such as those found in the active layers of organic solar cells. It is shown that each of these two disorder components can be partitioned into contributions related to the energetic disorder of the transport states and to the disorder associated with the hole-electron electrostatic interaction energies. The methodology is applied to evaluate the energy distributions of the CT states in representative bulk heterojunctions based on poly-3-hexyl-thiophene and phenyl-C 61 -butyric-acid methyl ester. The results indicate that the torsional fluctuations of the polymer backbones are the main source of both static and dynamic disorders for the CT states as well as for the transport levels. The impact of static and dynamic disorders on radia...

5913wileyonlinelibrary.com display at least a three-phase structure consisting of pure polymer regions, pure fullerene regions, and mixed polymerfullerene regions. [ 11,15,[20][21][22] Critical to the operation of a polymer-fullerene BHJ solar cell is the mixed phase, where the details of the polymer-fullerene packing are expected to play a major role in the processes of exciton dissociation, charge separation, and charge recombination. [ 15,23,24 ] Recently, Graham et al. reviewed some of the most effi cient D-A copolymers used in polymer-fullerene BHJ solar cells and investigated in detail the impact of the nature (branched or linear) and location [on the electron-rich, donor (D) moiety or the electron-poor, acceptor (A) moiety] of the alkyl side-chains along the polymer chains; in particular, they focused on polybenzo[1,2-b:4,5-b′]dithiophene-thieno[3,4c]pyrrole-4,6-dione (PBDTTPD). [ 15 ] These authors found that PBDTTPD with branched side-chains on the BDT (D) moiety and linear side-chains on the TPD (A) moiety shows the highest performance in BHJ, bilayer, and low-polymerconcentration devices; the PCE substantially decreases for PBDTTPD with linear side-chains on the D moiety and branched side-chains on the A moiety. [ 15 ] Furthermore, solid-state 2D heteronuclear correlation NMR analyses revealed that, in the case of the highest performing PBDTTPD derivative, fullerenes are in closer proximity to the A moieties compared to the lower performing PBDTTPD derivatives. This trend was also present in most of the other D-A copolymers reviewed in their work. Since similar quenching behaviors and similar nongeminate recombination dynamics were observed for all low-polymer-concentration devices, the authors attributed the differences in effi ciency to the dependence of exciton dissociation and/or geminate recombination on the polymerfullerene packing in the mixed region, i.e., whether the fullerenes are close to (interacting with) mainly the D moieties or A moieties of the PBDTTPD backbone. [ 15 ] Spectroscopic investigations by Laquai and co-workers on PBDTTPD-fullerene BHJ solar cells underline that, compared to PBDTTPD with only linear side-chains, a PBDTTPD polymer with branched side-chains on the D moieties and linear side-chains on the A moieties exhibits a lower rate of nongeminate recombination and a smaller fraction of geminate Polymer-fullerene packing in mixed regions of a bulk heterojunction solar cell is expected to play a major role in exciton-dissociation, charge-separation, and charge-recombination processes. Here, molecular dynamics simulations are combined with density functional theory calculations to examine the impact of nature and location of polymer side-chains on the polymerfullerene packing in mixed regions. The focus is on poly-benzo[1,2-b:4,5-b′] dithiophene-thieno[3,4-c]pyrrole-4,6-dione (PBDTTPD) as electron-donating material and [6,6]-phenyl-C 61 -butyric acid methyl ester (PC 61 BM) as electronaccepting material. Three polymer side-chain patterns are considered: i ) linear side-chains...

We outline a step-by-step protocol that incorporates a number of theoretical and computational methodologies to evaluate the structural and electronic properties of π-conjugated semiconducting materials in the condensed phase. Our focus is on methodologies appropriate for the characterization, at the molecular level, of the morphology in blend systems consisting of an electron donor and electron acceptor, of importance for understanding the performance properties of bulk-heterojunction organic solar cells. The protocol is formulated as an introductory manual for investigators who aim to study the bulk-heterojunction morphology in molecular details, thereby facilitating the development of structure–morphology–property relationships when used in tandem with experimental results.

Fused‐ring core nonfullerene acceptors (NFAs), designated “Y‐series,” have enabled high‐performance organic solar cells (OSCs) achieving over 18% power conversion efficiency (PCE). Since the introduction of these NFAs, much effort has been expended to understand the reasons for their exceptional performance. While several studies have identified key optoelectronic properties that govern high PCEs, little is known about the molecular level origins of large variations in performance, spanning from 5% to 18% PCE, for example, in the case of PM6:Y6 OSCs. Here, a combined solid‐state NMR, crystallography, and molecular modeling approach to elucidate the atomic‐scale interactions in Y6 crystals, thin films, and PM6:Y6 bulk heterojunction (BHJ) blends is introduced. It is shown that the Y6 morphologies in BHJ blends are not governed by the morphology in neat films or single crystals. Notably, PM6:Y6 blends processed from different solvents self‐assemble into different structures and morphologies, whereby the relative orientations of the sidechains and end groups of the Y6 molecules to their fused‐ring cores play a crucial role in determining the resulting morphology and overall performance of the solar cells. The molecular‐level understanding of BHJs enabled by this approach will guide the engineering of next‐generation NFAs for stable and efficient OSCs.