Introduction of interface layers can maximize the performance of certain organic solar cells. We demonstrate that high efficiency non-fullerene acceptor based solar cells can be further improved with the insertion of PC70BM as an interlayer between the electron transport layer and the active layer. The combination of ZnO and PC70BM layers between a cathode and a bulk heterojunction active layer appears to serve as a better selective contact by reducing charge transport barrier and recombination. The enhanced short-circuit current density (JSC) is characterized by a low series-resistance (<2 Ω cm2), improved charge collection efficiency, and power conversion efficiency. These features are reflected in impedance spectroscopy and electrical noise measurements and provide a route for large-area organic solar cells.

Rapid degradation of bulk heterojunction (BHJ) organic solar cells employing non-fullerene acceptor molecules, especially during the initial period of operation (“burn-in”), poses a major challenge. Among the prominent factors causing rapid deterioration of device performance, such as light and thermal stress, UV-induced degradation is shown to significantly reduce the lifetime. It is observed that UV-exposed devices exhibit the sharpest decrease in the performance parameters during the burn-in period. Systematic monitoring of solar cell parameters under accelerated aging conditions reveals that the bulk and interfacial charge transport barriers are substantially affected by UV. These trends were observed even in the absence of residual solvent additive DIO (1,8-diiodooctane) in the devices, indicating the intrinsic tendency of the BHJ system to become modified.

A steady surge in device efficiencies of organic solar cells (OSCs) along with improvement in associated features, such as stability and facile processing methods, is expected to provide a realistic, feasible commercial option. The introduction of high performing donor and acceptor molecules along with tailored buffer layers has provided the impetus for the resurgence of this field. Further options of ternary and tandem architectures of these OSC systems should push this technology to competitive levels. A major hurdle, which is expected when these devices are evaluated for long-term performance in all weather conditions, is the level of degradation. We examine and address these stability-limiting factors in this perspective article. Modifications in microstructure/morphology and interfaces with time and energy levels defining the molecules form some of the critical intrinsic degradation pathways. Various strategies that have been used to limit the associated pathways of degradation of the active layer will be discussed. One such strategy is electric field-assisted thermal annealing treatment, which concomitantly also brings in a favorable vertical phase segregated active layer morphology. We also emphasize the utility of photocurrent noise measurements to monitor the level of degradation and possibly forecast the trajectory of long-term performance of OSCs.

To prepare coatings with enhanced self-cleaning properties, organic− inorganic hybrid materials were constructed that are composed of a polymer (poly(vinylidene fluoride)) with embedded particles (TiO 2 ) coated with a perfluorinated (C−H bond free) phthalocyanine photosensitizer and also containing the volatile salt ammonium hydrogen carbonate. Coatings prepared from this material were thermolyzed at 80 °C to volatilize the salt. The resulting coatings exhibit enhanced hydrophobicity as demonstrated by contact angles exceeding 150°, a property imparted by the surface roughness generated by gases and vapors released upon the thermal decomposition of the volatile salt. The surface roughening does not hinder the photogeneration of singlet oxygen, as evidenced by the photocatalyzed decomposition of a model dye, methyl orange, present in solutions placed in contact with the coatings. These new materials, which exhibit self-cleaning properties, could thus be useful for repelling pollutants through enhanced hydrophobicity and the lasting, catalytic degradation of contaminants by reactive oxygen species produced from light and air.

The utility of electric fields during the bulk heterojunction (BHJ) film drying process for tuning the morphology and stability of the device is demonstrated. An external electric-field-assisted annealing (EFTA) treatment is used to engineer the stability of amorphous donor polymer-based BHJs without compromising device performance. Residual additive in the device post fabrication is a major source of degradation. Thermal annealing of an active layer effectively removes residual additive, which in case of amorphous polymer donor-based BHJs, however, leads to unfavorable changes in the morphology. The detrimental effect of thermal annealing in amorphous donor polymer-based solar cells is mitigated by the presence of an electric field during the drying stage. The complete removal of additive is ensured by this treatment procedure and leads to improved packing and a rigid morphology. The structural stability is reflected in the performance parameters monitored over the long term and electrical noise measurements. The magnitude and polarity of the applied electric field are observed to control the vertical distribution of donor and acceptor components.

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