Stable P3HT: amorphous non-fullerene solar cells with a high open-circuit voltage of 1 V and efficiency of 4%

Blade Coating Poly(3-hexylthiophene): The Importance of Molecular Weight on Thin-Film Microstructures

Poly(3-hexylthiophene) is one of the most prevalent and promising conjugated polymers for use in organic electronics. However, the deposition of this material in thin films is highly dependent on the process, such as blade coating versus spin coating and material properties such as molecular weight. Typically, large polymer dispersity makes it difficult to isolate the effect of molecular weight without considering a distribution. In this study, we characterize oligothiophenes of exactly 8, 11, and 14 repeat units, which were deposited into thin films by varying blade coating conditions and postdeposition annealing. From synchrotron-based grazing incidence wide-angle X-ray scattering (GIWAXS), scanning transmission X-ray microscopy (STXM) and near-edge X-ray absorption fine structure spectroscopy (NEXAFS), Raman microscopy, optical microscopy, and X-ray diffraction (XRD), it was suggested that higher molecular weight polymers exhibit a fast-forming crystalline polymorph (form-1) while low molecular weight polymers exhibit a slow forming polymorph (form-2) with large domain boundaries. As molecular weight is gradually increased, the polymorph formed transitions from form-1 and form-2, where 11 repeat unit oligomers display both polymorphs. We also found that processing conditions can increase the formation of the form-2 polymorph. We also report improved organic thin film transistor (OTFT) performance when form-1 is present. Overall, oligothiophene polymorph formation is highly dependent on the molecular weight and processing conditions, providing critical insight into the importance of polymer weight control in the development of thin-film electronics based on conjugated polymers.

Recent Advances of Film–Forming Kinetics in Organic Solar Cells

Solution–processed organic solar cells (OSC) have been explored widely due to their low cost and convenience, and impressive power conversion efficiencies (PCEs) which have surpassed 18%. In particular, the optimization of film morphology, including the phase separation structure and crystallinity degree of donor and acceptor domains, is crucially important to the improvement in PCE. Considering that the film morphology optimization of many blends can be achieved by regulating the film–forming process, it is necessary to take note of the employment of solvents and additives used during film processing, as well as the film–forming conditions. Herein, we summarize the recent investigations about thin films and expect to give some guidance for its prospective progress. The different film morphologies are discussed in detail to reveal the relationship between the morphology and device performance. Then, the principle of morphology regulating is concluded with. Finally, a future controlling of the film morphology and development is briefly outlined, which may provide some guidance for further optimizing the device performance.

Progress in Stability of Organic Solar Cells

The organic solar cell (OSC) is a promising emerging low-cost thin film photovoltaics technology. The power conversion efficiency (PCE) of OSCs has overpassed 16% for single junction and 17% for organic-organic tandem solar cells with the development of low bandgap organic materials synthesis and device processing technology. The main barrier of commercial use of OSCs is the poor stability of devices. Herein, the factors limiting the stability of OSCs are summarized. The limiting stability factors are oxygen, water, irradiation, heating, metastable morphology, diffusion of electrodes and buffer layers materials, and mechanical stress. The recent progress in strategies to increase the stability of OSCs is surveyed, such as material design, device engineering of active layers, employing inverted geometry, optimizing buffer layers, using stable electrodes and encapsulation materials. The International Summit on Organic Photovoltaic Stability guidelines are also discussed. The potential research strategies to achieve the required device stability and efficiency are highlighted, rendering possible pathways to facilitate the viable commercialization of OSCs.