The performance of organic solar cells is determined by the delicate, meticulously optimized bulk-heterojunction microstructure, which consists of finely mixed and relatively separated donor/acceptor regions. Here we demonstrate an abnormal strong burn-in degradation in highly efficient polymer solar cells caused by spinodal demixing of the donor and acceptor phases, which dramatically reduces charge generation and can be attributed to the inherently low miscibility of both materials. Even though the microstructure can be kinetically tuned for achieving high-performance, the inherently low miscibility of donor and acceptor leads to spontaneous phase separation in the solid state, even at room temperature and in the dark. A theoretical calculation of the molecular parameters and construction of the spinodal phase diagrams highlight molecular incompatibilities between the donor and acceptor as a dominant mechanism for burn-in degradation, which is to date the major short-time loss reducing the performance and stability of organic solar cells.

achieve the highest performance so far in combination with the commonly used methanofullerene derivatives like phenyl-C 61 -butyric acid methyl ester ([60]PCBM) or [6,6]-phenyl-C 71 -butyric acid methyl ester ([70]PCBM). [11] Functional fullerenes, on the one hand, can preserve the properties of pristine fullerenes, such as superior electron-transporting ability and high electron affinity. On the other hand, by properly attaching suitable addends, one can manipulate a wide variety of chemical, physical, or thermo dynamic properties, such as better solubility in organic solvents, higher lowest unoccupied molecular orbital (LUMO) level, and improved compatibility with organic donors. [12] These advantages endow functional fullerenes with extraordinary capability, acting as unique electron-accepting components in OPV devices. [2,13] Although PCBM is the most prominent acceptor in OPV devices, [5,14] the energy levels of PCBM should be further optimized in order to achieve high open circuit voltage (V OC ) in combination with the state-of-theart organic donors. [15] Fullerene derivatives with more suitable electronic levels are expected to reduce the bandgap to V OC loss and approach the theoretical efficiency limit. [13,16] PCBM bisadduct [17] and indene-C 60 bisadduct [18] made their glory debut for being well compatible with poly(3-hexylthiophene-2,5-diyl) (P3HT) and for achieving promising high V OC and PCE compared to the PCBM-based counterparts. However, resulting from constitutional isomer impurities and inefficient charge generation, fullerene bisadducts failed to produce similarly satisfying performance with the latest outstanding organic donors. [19] Another effective way of increasing the LUMO level of fullerene derivatives is to add an electron-donating group to the fullerene cage. [20] A series of newly developed functional fullerenes have shown higher photovoltaic performance than PCBM resulting from their high-lying LUMO levels. Itoh and co-workers carried out a systematic study on a series of fulleropyrrolidine derivatives, some of which possess higher V OC and efficiency than PCBM when blended with P3HT. [21] Troshin and co-workers reported that solar cells based on the novel pyrrolidinofullerene acceptors exhibited impressively high V OC and promising efficiency in combination with Poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) and P3HT compared to the PCBM-based references. [22] As the PCE of OPV devices was improved to over 11%, currently, more research efforts are required to address the long-term stability issue. Many factors may cause degradation Extensive research efforts around the world have been devoted to organic photovoltaics (OPV) over the last decade due to their unique advantages for commercial application, such as low cost, light weight, flexibility, and easy manufacture on large-scale. [1][2][3] By introducing the bulk heterojunction (BHJ) [4,5] structure into OPV devices, tremendous progress has been achieved to boost the powe...

a Solution-processed organic photovoltaic devices are advantageous due to their low-cost large area manufacturing techniques, such as slot-die coating, gravure printing and roll-to-roll coating. The final microstructure of a polymer:fullerene bulk-heterojunction (BHJ) film is a fine interplay between solution thermodynamics (e.g. solubility, miscibility. . .) and kinetics (e.g. solvent evaporation, polymer ordering, phase separation. . .) during the drying process. In order to design better performing organic photovoltaic devices, gaining knowledge over the drying properties of polymer:fullerene thin films is essential.A novel in situ thin film drying characterization chamber, equipped with white-light reflectometry, laser light scattering and photoluminescence, is presented in combination with grazing-incidence X-ray diffraction on two different polymer:fullerene bulk heterojunctions based on poly-(3-hexylthiophene-2,5-diyl) (P3HT) and polythieno [3,2b]thiophene-diketopyrrolopyrrole-co-thiophene (DPP-TT-T) polymers.With photoluminescence applied for the first time as an in situ method for such drying studies, these single-chamber measurements track the fine interplay between thermodynamics and kinetics of thin film drying and provide invaluable information on solution behavior and microstructure formation.

ZnO nanoparticles (NPs) have great potential for their use in, e.g., thin film solar cells due to their electro-optical properties adjustable on the nanoscale. Therefore, the production of well-defined NPs is of major interest. For a targeted production process, the knowledge of the stabilization layer of the NPs during and after their formation is of particular importance. For the study of the stabilizer layer of ZnO NPs prepared in a wet chemical synthesis from zinc acetate, only ex situ studies have been performed so far. An acetate layer bound to the surface of the dried NPs was found; however, an in situ study which addresses the stabilizing layer surrounding the NPs in a native dispersion was missing. By the combination of small angle scattering with neutrons and X-rays (SANS and SAXS) for the same sample, we are now able to observe the acetate shell in situ for the first time. In addition, the changes of this shell could be followed during the ripening process for different temperatures. With increasing size of the ZnO core (d(core)) the surrounding shell (d(shell)) becomes larger, and the acetate concentration within the shell is reduced. For all samples, the shell thickness was found to be larger than the maximum extension of an acetate molecule with acetate concentrations within the shell below 50 vol %. Thus, there is not a monolayer of acetate molecules that covers the NPs but rather a swollen shell of acetate ions. This shell is assumed to hinder the growth of the NPs to larger macrostructures. In addition, we found that the partition coefficient μ between acetate in the shell surrounding the NPs and the total amount of acetate in the solution is about 10% which is in good agreement with ex situ data determined by thermogravimetric analysis.

The complex intermixing morphology is critical for the performance of the nanostructured polymer:fullerene bulk heterojunction (BHJ) solar cells. Here, time resolved in situ grazing incidence X‐ray diffraction and grazing incidence small angle X‐ray scattering are used to track the structure formation of BHJ thin films formed from the donor polymer poly(2,5‐bis(3‐hexadecylthiophen‐2‐yl)thieno[3,2‐b]thiophene) with different fullerene derivative acceptors. The formation of stable bimolecular crystals through the intercalation of fullerene molecules between the side chains of polymer crystallites is investigated. Such systems exhibit more efficient exciton dissociation but lower photo‐conductance and faster decay of charges. On the basis of the experimental observations, intercalation obviously takes place before or with the formation of the crystalline polymer domains. It results in more stable structures whose volume remains constant upon further drying. Three distinct periods of drying are observed and the formation of unidimensional fullerene channels along the π‐stacking direction of polymer crystallites is confirmed.

aOne of the most employed morphology optimization methods to design better performing organic photovoltaic devices is ink formulation engineering with additives. In this work, by employing a suboptimal host solvent mixture and 1,8-diiodooctane (DIO) as a very optimal solvent for both components in poly-thieno [3,2b]thiophene-diketopyrrolopyrrole-co-thiophene (DPP-TT-T)-based bulkheterojunctions (BHJ), an alternative, previously unknown mechanism of additive behavior on BHJ microstructure formation is presented. In situ characterization methods involving grazing incidence X-ray diffraction, white-light reflectometry, laser light scattering and photoluminescence during film drying reveal that the microstructure formation under the influence of DIO is led towards thermodynamic equilibrium during host solvent drying, and the kinetics of morphology formation (i.e. polymer crystallization, fullerene aggregation.) are controlled dominantly by the additive during its evaporation. Ex situ X-ray-based characterization methods, such as scanning transmission X-ray microspectroscopy (STXM) and resonant soft X-ray scattering (R-SoXS), additionally reveal that the microstructure of dried films favors smaller domain sizes with purer domains, smaller fullerene aggregates, bimodal polymer crystallization relative to the substrate and more face-on molecular orientation relative to the donor/acceptor interface, which at the end lead to better performing devices with power conversion efficiencies ranging from 1.25% to 4.68%.

High efficiency polymer:fullerene photovoltaic device layers self-assemble with hierarchical features from ångströms to 100’s of nanometers. The feature size, shape, composition, orientation, and order all contribute to device efficiency and are simultaneously difficult to study due to poor contrast between carbon based materials. This study seeks to increase device efficiency and simplify morphology measurements by replacing the typical fullerene acceptor with endohedral fullerene Lu3N@PC80BEH. The metal atoms give excellent scattering contrast for electron beam and x-ray experiments. Additionally, Lu3N@PC80BEH has a lower electron affinity than standard fullerenes, which can raise the open circuit voltage of photovoltaic devices. Electron microscopy techniques are used to produce a detailed account of morphology evolution in mixtures of Lu3N@PC80BEH with the record breaking donor polymer, PTB7 and coated using solvent mixtures. We demonstrate that common solvent additives like 1,8-diiodooctane or chloronapthalene do not improve the morphology of endohedral fullerene devices as expected. The poor device performance is attributed to the lack of mutual miscibility between this particular polymer:fullerene combination and to co-crystallization of Lu3N@PC80BEH with 1,8-diiodooctane. This negative result explains why solvent additives mixtures are not necessarily a morphology cure-all.

Abstract