Favorable Mixing Thermodynamics in Ternary Polymer Blends for Realizing High Efficiency Plastic Solar Cells

Ternary blends with broad spectral absorption have the potential to increase charge generation in organic solar cells but feature additional complexity due to limited intermixing and electronic mismatch. Here, a model system comprising the polymers poly[5,5-bis(2-butyloctyl)-(2,2-bithiophene)-4,4-dicarboxylate-alt-5,5-2,2-bithiophene] (PDCBT) and PTB7-Th and PC70BM as an electron accepting unit is presented. The power conversion efficiency (PCE) of the ternary system clearly surpasses the performance of either of the binary systems. The photophysics is governed by a fast energy transfer process from PDCBT to PTB7-Th, followed by electron transfer at the PTB7-Th:fullerene interface. The morphological motif in the ternary blend is characterized by polymer fibers. Based on a combination of photophysical analysis, GIWAXS measurements and calculation of the intermolecular parameter, the latter indicating a very favorable molecular affinity between PDCBT and PTB7-Th, it is proposed that an efficient charge generation mechanism is possible because PTB7-Th predominantly orients around PDCBT filaments, allowing energy to be effectively relayed from PDCBT to PTB7-Th. Fullerene can be replaced by a nonfullerene acceptor without sacrifices in charge generation, achieving a PCE above 11%. These results support the idea that thermodynamic mixing and energetics of the polymer-polymer interface are critical design parameter for realizing highly efficient ternary solar cells with variable electron acceptors

Unraveling Complexity: A Strategy for the Characterization of Anisotropic Core Multishell Nanoparticles

Abstract In this work, a widely applicable routine to characterize the core, surface, stability, and optical properties of CdSe/CdS/ZnS core–shell–shell nanorods after multiple growth steps is established. First, size, shape, and shell thickness of the nanorods are characterized by transmission electron microscopy (TEM), analytical ultracentrifugation (AUC), and small angle X‐ray/neutron scattering (SAXS/SANS). In the next step, Fourier‐transform infrared (FT‐IR) spectroscopy, thermogravimetric analysis (TGA), and SANS measurements are applied to determine the surface species of nanorods. Then, the colloidal stability of the nanorods is investigated by UV–vis spectroscopy and dynamic light scattering (DLS) after different washing cycles. Finally, photoluminescence quantum yield (PLQY) of the nanorods during washing and sample storage is determined. With this highly complementary routine for particle characterization, the core, surface, stability, and optical properties of nanorods after multiple growth steps are resolved. The results demonstrate the importance of the developed toolbox to characterize such highly complex, anisotropic nanorods for a technical environment. This is of major importance for the handling of colloidal quantum materials and their quality control in industrial applications

Unraveling the Complex Nanomorphology of Ternary Organic Solar Cells with Multimodal Analytical Transmission Electron Microscopy

Elucidating the complex materials distribution in the active layers of ternary organicsolar cells is one of the greatest challenges in the eld of orga nic photov oltaics.Knowledge of the nanomorphology is key to understanding photophysical processes(e.g., charge separation, adjustment of the recombination mechanism, and suppres-sion of the radiationless and energetic losses) and thus improving the device perfor-mance. Herein, for the rst time, the successful discrimination and spatial mappingof the active layer components of a ternary organic solar cell are demonstrated usinganalytical transmission electron microscopy. The material distribution of all threeorganiccomponentsissuccessfullyvisualized by multimodal imaging using com-plementary electron energy loss signals. A complete picture of the morphologicalaspects can be gained by studying the lateral and cross-sectional morphology as wellas the morphology evolution as a function of the mixing ratio of the polymers. Finally,a correlation between the morphology, photophysical processes, and device perfor-mance of the ternary and t he reference binary syst em is achieved, explain ing thedifferences of the powe r conversion efciency between the two systems

Enhanced Cu and Cd sorption after soil aging of woodchip-derived biochar: What were the driving factors?

International audienceBiochar (BC) is increasingly tested as a soil amendment for immobilization of heavy metals (HMs) and other pollutants. In our study, an acidic soil amended with wood chip-derived BC showed strongly enhanced Cu and Cd sorption after 15 months of aging under greenhouse conditions. X-ray absorption near edge structure suggested formation of Cu(OH)2 and CuCO3 and upon aging increasingly Cu sorption to the BC organic phase (from 9.2% to 40.7%) as main binding mechanisms of Cu on the BCs. In contrast, Cd was predominantly bound as CdCO3 on the BCs even after 15 months (82.7%). We found indications by mid-infrared spectroscopy that the formation of organic functional groups plays a role for increased HM sorption on aged BCs. Yet, our data suggest that the accessibility of BC's pore network and reactive surfaces is likely to be the overriding factor responsible for aging-related changes in HM sorption capacity, rather than direct interactions of HMs with oxidized functional groups. We observed highly weathered BC surface structures with scanning electron microscopy along with strongly increased wettability of the BCs after 15 months of soil aging as indicated by a decrease of water contact angles (from 62.4° to 4.2°)

Light harvesting enhancement upon incorporating alloy structured CdSeXTe1-Xquantum dots in DPP:PC61BM bulk heterojunction solar cells

Hybrid solar cells based on the pDPP5T-2 electron donating polymer, [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) and cadmium selenide telluride (CdSeXTe1X) quantum dots (QDs) are fabricated and their photovoltaic performance and optoelectronic properties are investigated as a function of QD loading. The power conversion efficiency (PCE) of hybrid solar cells is improved up to 5.11% for the device containing 4 wt% of QDs which is mainly due to the enhancement in short circuit current density (Jsc) resulting from increased light harvesting. A full-fledged study is performed on the microstructure, charge transfer/transport and recombination mechanisms of our ternary hybrid solar cells by employing various advanced techniques. The transmission electron microscopy (TEM) results reveal the nonagglomerated and uniform distribution of the CdSeXTe1X QDs within the pDPP5T-2:PC61BM host matrix at low QD concentrations. Transient absorption spectroscopy (TAS) showed a slower charge carrier recombination rate due to the introduction of QDs into the photoactive layer. It can be attributed to the more efficient exciton dissociation in ternary systems. These findings are consistent with the photovoltaic properties of the device

Understanding and controlling the evolution of nanomorphology and crystallinity of organic bulk-heterojunction blends with solvent vapor annealing

Solvent vapor annealing (SVA) has been shown to significantly improve the device performance of organic bulk-heterojunction solar cells, yet the mechanisms linking nanomorphology, crystallinity of the active layer, and performance are still largely missing. Here, the mechanisms are tackled by correlating the evolution of nanomorphology, crystallinity, and performance with advanced transmission electron microscopy methods systematically. Model system of DRCN5T:PC71BM blends are SVA treated with four solvents differing in their donor and acceptor solubilities. The choice of solvent drastically influences the rate at which the maximum device efficiency establishes, though similar values can be achieved for all solvents. The donor solubility is identified as a key parameter that controls the kinetics of diffusion and crystallization of the blend molecules, resulting in an inverse relationship between optimal annealing time and donor solubility. For the highest efficiency, optimum domain size and single-crystalline nature of DRCN5T fibers are found to be crucial. Moreover, the π–π stacking orientation of the crystallites is directly revealed and related to the nanomorphology, providing insight into the charge carrier transport pathways. Finally, a qualitative model relating morphology, crystallinity, and device efficiency evolution during SVA is presented, which may be transferred to other light-harvesting blends.</p

Crystal-structure of active layers of small molecule organic photovoltaics before and after solvent vapor annealing

It is demonstrated by a detailed structural analysis that the crystallinity and the efficiency of small molecule based organic photovoltaics can be tuned by solvent vapor annealing (SVA). Blends made of the small molecule donor 2,2′-[(3,3′″,3″″,4′-tetraoctyl[2,2′:5′,2″:5″,2′″:5′″,2″″-quinquethiophene]-5,5″″-diyl)bis[(Z)-methylidyne(3-ethyl-4-oxo-5,2-thiazolidinediylidene)]]bis-propanedinitrile (DRCN5T) and the acceptor [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) were annealed using solvent vapors with either a high solubility for the donor (tetrahydrofuran), the acceptor (carbon disulfide) or both (chloroform). The samples were analyzed by grazing-incidence wide-angle X-ray scattering (GIWAXS), electron diffraction, X-ray pole figures, and time-of-flight secondary ion mass spectrometry (ToF-SIMS). A phase separation of DRCN5T and PC71BM is induced by SVA leading to a crystallization of DRCN5T and the formation of a DRCN5T enriched layer. The DRCN5T crystallites possess the two dimensional oblique crystal system with the lattice parameters a = 19.2 Å, c = 27.1 Å, and β = 111.1° for the chloroform case. No major differences in the crystal structure for the other solvent vapors were observed. However, the solvent choice strongly influences the size of the DRCN5T enriched layer. Missing periodicity in the [010]-direction leads to the extinction of all Bragg reflections with k ≠ 0. The annealed samples are randomly orientated with respect to the normal of the substrate (fiber texture)

Solution-Growth Liquid-Phase Epitaxy of CsPbBr 3 on NaCl by Optimizing the Substrate Dissolution

Since the earliest days of liquid-phase epitaxy (LPE) research, substrate stability has been a prerequisite for investigating nucleation and epitaxial growth. For flux- and melt-growth LPE, soluble substrates have always been viewed as problematic and difficult. However, solution-growth LPE transforms this disadvantage into a benefit by facilitating the creation of graded interfaces. In this study, we investigated the nucleation and growth of solution-grown LPE in the evaporation mode of soluble CsPbBr3 on freshly cleaved NaCl single-crystal substrates. Only two of the four main types of CsPbBr3 crystallization observed are epitaxial. The microscopic observation of composition gradients at the interface of CsPbBr3 grains and substrates reveals a promising opportunity to manipulate the lattice constant at the interface by simply controlling the temperature and concentration