• Energy Research

To date, alternating co-polymers based on electron-rich and electron-poor units are the most attractive materials to control functionality of organic semiconductor layers in which ultrafast excited-state processes play a key role. We present a computational study of the photoinduced excited-state dynamics of the 4-(2-thienyl)-2,1,3-benzothiadiazole (BT-1T) molecule, which is a common building block in the backbone of $\pi$-conjugated polymers used for organic electronics. In contrast to homo-polymer materials, such as oligothiophene, BT-1T has two non-identical units, namely, thiophene and benzothiadiazole, making it attractive for intramolecular charge transfer studies. To gain a thorough understanding of the coupling of excited-state dynamics with nuclear motion, we consider a scenario based on femtosecond time-resolved x-ray absorption spectroscopy using an x-ray free-electron laser in combination with a synchronized ultraviolet femtosecond laser. Using Tully's fewest switches surface hopping approach in combination with excited-state calculations at the level of configuration interaction singles, we calculate the gas-phase x-ray absorption spectrum at the carbon and nitrogen $K$ edges as a function of time after excitation to the lowest electronically excited state. The results of our time-resolved calculations exhibit the charge transfer driven by non-Born-Oppenheimer physics from the benzothiadiazole to thiophene units during relaxation to the ground state. Furthermore, our $ab$ $initio$ molecular dynamics simulations indicate that the excited-state relaxation processes involve bond elongation in the benzothiadiazole unit as well as thiophene ring puckering at a time scale of 100 fs. We show that these dynamical trends can be identified from the time-dependent x-ray absorption spectrum. Structural dynamics 7(4), 044101 - (2020). doi:10.1063/4.0000016 Published by AIP Publishing LLC, Melville, NY

Due to their potential for solar energy harvesting and storage, molecular solar thermal energy storage (MOST) materials are receiving wide attention from both the research community and the public. MOST materials absorb photons and convert their energy to chemical energy, which is contained within the bonds of the MOST molecules. Depending on the molecular structure, these materials can store up to 1 MJ/kg, at ambient temperature and with storage times ranging from minutes to several years. This work is the first to thoroughly investigate the potential of MOST materials for the development of energy saving windows. To this end, the MOST molecules are integrated into thin, optically transparent films, which store solar energy during the daytime and release heat at a later point in time. A combined experimental and modeling approach is used to verify the system's basic functionality and identify key parameters. Multi-physics modeling and simulation were conducted to evaluate the interaction of MOST films with light, both monochromatic and the entire solar spectrum, as well as the corresponding dynamic energy storage. The model was experimentally verified by studying the optical response of thin MOST films containing norbornadiene derivatives as a functional system. We found that the MOST films act as excellent UV shield and can store up to 0.37 kWh/m2 for optimized MOST molecules. Further, this model allowed us to screen various material parameters and develop guidelines on how to optimize the performance of MOST window films.

The mark of 18% power conversion efficiency (PCE) was recently overcome by laboratory-scale organic solar cells (OSCs) thanks to the development of nonfullerene acceptors (NFAs). NFA-based solar cells show improved performance and stability compared with those of their fullerene-acceptor-based counterparts. However, only a few studies focus on scalable deposition techniques or roll-to-roll compatible processing, which is of paramount importance for the commercialization of the technology. Here, we report a simple and fast fabrication of slot-die-coated poly(3-hexylthiophene-2,5-diyl):(5Z,5'Z)-5,5'-{7,7'-(4,4,9,9-tetraoctyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithiophene-2,7-diyl)bis(benzo[c][1,2,5]thiadiazole-7,4-diyl)]bis(methanylylidene)}bis(3-ethyl-2-thioxothiazolidin-4-one) (P3HT:O-IDTBR) OSCs using a roll platform on flexible ITO-free substrates under ambient conditions. We show that the optical band gap of the active layer increases when an isopropanol-diluted poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) hole-transport layer is coated on top of it, changing the device properties. Optimization of the coating conditions leads to the achievement of up to 3.6% PCE for single cells of $1\phantom{\rule{0.1em}{0ex}}{\mathrm{cm}}^{2}$ fabricated under ambient conditions with flexographic printed $\mathrm{Ag}$ back electrodes, compared with solar cells with evaporated Ag (3.8% PCE), $\mathrm{Au}$ (2.1% PCE), or $\mathrm{Cu}$ (3.0% PCE) back contacts. OSCs with larger areas of $4\phantom{\rule{0.1em}{0ex}}{\mathrm{cm}}^{2}$ with 2.3% PCE are also fabricated, where the fast increase of the series resistance with the area is the main PCE-limiting factor. The efficiencies herein reported for NFAs obtained by roll processing show the excellent potential of the P3HT:O-IDTBR blend for large-scale fabrication.

A series of alternating oligothiophene (nT)–isoindigo (I) copolymers (PnTI) were synthesized to investigate the influence of the oligothiophene block length on the photovoltaic (PV) properties of PnTI:PCBM bulk-heterojunction blends. Our study indicates that the number of thiophene rings (n) in the repeating unit alters both polymer crystallinity and polymer–fullerene interfacial energetics, which results in a decreasing open-circuit voltage (Voc) of the solar cells with increasing n. The short-circuit current density (Jsc) of P1TI:PCBM devices is limited by the absence of a significant driving force for electron transfer. Instead, blends based on P5TI and P6TI feature large polymer domains, which limit charge generation and thus Jsc. The best PV performance with a power conversion efficiency of up to 6.9% was achieved with devices based on P3TI, where a combination of a favorable morphology and an optimal interfacial energy level offset ensures efficient exciton separation and charge generation. The structure–property relationship demonstrated in this work would be a valuable guideline for the design of high performance polymers with small energy losses during the charge generation process, allowing for the fabrication of efficient solar cells that combine a minimal loss in Voc with a high Jsc.

Organic Solar Cells (OSCs) have reached the highest efficiencies using lab-scale on active areas far below 0.1 cm^2. This tends to widen the so-called “lab-to-fab gap”, which is one of the most important challenges to make OSCs competitive. The most commonly used fabrication technique is spin-coating, which has poor compatibility with large-scale techniques and substantial material waste. Moreover, other techniques such as blade or slot-die coating are much more suitable for roll-to-roll manufacturing processes, which is one of the advantages the technology has compared, for example, to silicon solar cells. However, only a few studies report solar cells using these fabrication techniques. Additionally, for the environmentally friendly OSC scale-up, inks based on non-hazardous solvent systems are needed. In this work, slot-die coating has been chosen to coat the PM6:Y6 active layer, using o-xylene, a green solvent, without additives. The optimal coating parameters are defined through fine-tuning of the coating parameters, such as the drying temperature and solution concentration. Moreover, ternary devices with PCBM, and fully printed devices are also fabricated. Power conversion efficiencies of 6.26% and 7.16% are achieved for binary PM6:Y6 and ternary PM6:Y6:PCBM devices, respectively.

We demonstrate a non-destructive approach to provide structural properties on the grain level for the absorber layer of kesterite solar cells. Kesterite solar cells are notoriously difficult to characterize structurally due to the co-existence of several phases with very similar lattice parameters. Specifically, we present a comprehensive study of 597 grains in the absorber layer of a 1.64% efficient Cu2ZnSnS4 (CZTS) thin-film solar cell, from which 15 grains correspond to the secondary phase ZnS. By means of three dimensional X-ray diffraction (3DXRD), we obtained statistics for the phase, size, orientation, and strain tensors of the grains, as well as their twin relations. We observe an average tensile stress in the plane of the film of ~ 70 MPa and a compressive stress along the normal to the film of ~ 145 MPa. At the grain level, we derive a 3D stress tensor that deviates from the biaxial model usually assumed for thin films. 41% of the grains are twins. We calculate the frequency of the six types of $��$3 boundaries, revealing that 180�� rotations along axis <221> is the most frequent. This technique can be applied to polycrystalline thin film solar cells in general, where strain can influence the bandgap of the absorber layer material, and twin boundaries play a role in the charge transport mechanisms. 28 pages, 12 figures, 4 tables