• Energy Research
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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

Polymer conformation in solution is more important for R2R solar cell performance than the crystallinity of the final coated film.

Aqueous nanoparticle dispersions of a series of three low-band-gap polymers poly[4,8-bis(2-ethylhexyloxy)benzo(1,2-b:4,5-b')dithiophene-alt-5,6-bis(octyloxy)-4,7-di(thiophen-2-yl)(2,1,3-benzothiadiazole)-5,5'-diyl] (P1), poly[(4,4'-bis(2-ethylhexyl)dithieno[3,2-b:2',3'-d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl] (P2), and poly[2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (P3) were prepared using ultrasonic treatment of a chloroform solution of the polymer and [6,6]-phenyl-C(61)-butyric acid methyl ester ([60]PCBM) mixed with an aqueous solution of sodium dodecylsulphate (SDS). The size of the nanoparticles was established using small-angle X-ray scattering (SAXS) of the aqueous dispersions and by both atomic force microscopy (AFM) and using both grazing incidence SAXS (GISAXS) and grazing incidence wide-angle X-ray scattering (GIWAXS) in the solid state as coated films. The aqueous dispersions were dialyzed to remove excess detergent and concentrated to a solid content of approximately 60 mg mL(-1). The formation of films for solar cells using the aqueous dispersion required the addition of the nonionic detergent FSO-100 at a concentration of 5 mg mL(-1). This enabled slot-die coating of high quality films with a dry thickness of 126 ± 19, 500 ± 25, and 612 ± 22 nm P1, P2, and P3, respectively for polymer solar cells. Large area inverted polymer solar cells were thus prepared based on the aqueous inks. The power conversion efficiency (PCE) reached for each of the materials was 0.07, 0.55, and 0.15% for P1, P2, and P3, respectively. The devices were prepared using coating and printing of all layers including the metal back electrodes. All steps were carried out using roll-to-roll (R2R) slot-die and screen printing methods on flexible substrates. All five layers were processed using environmentally friendly methods and solvents. Two of the layers were processed entirely from water (the electron transport layer and the active layer).