With rapid advances in the development of new conjugated polymers, non-fullerene acceptors, the power conversion efficiency (PCE) of OPVs has been increased over 14%. However, a major drawback for the commercialization of OPVs is their long-term stability under continuous operation. Especially, OPVs suffer from a rapid decrease in PCE during initial device operation, which is known as the “burn-in loss”. It is considered that the origin of the burn-in loss is mainly related with the instability of the BHJ morphology and/or interface rather than the photooxidation of the photoactive layer. We find that the photoactive layer prepared by a sequential solution deposition is more stable than that prepared by blend solution deposition. We also find that the burn-in loss is closely related with stability of photoactive layer / electron transporting layer interface.
Colorful semitransparent organic photovoltaic cells (OPVs) are increasing in demand due to their application in building-integrated photovoltaics. In general, the colors in OPVs have been determined by the absorption properties of the active material, requiring one to use different active materials to achieve distinct colors. However, such a strategy presents challenges in fabrication, costs and implementation as different processes are needed to produce differently colored OPVs. Moreover, the photovoltaic performance cannot stay uniform from different active materials in colored OPVs. In this work, we present a simple solution to such problems by incorporating transmissive Fabry-Perot-type color filters (CFs) as the electrode in an OPV employing one type of active material. From this modification, we achieve widely tunable colors covering the whole visible range with high spectral purity, peak transmission efficiencies surpassing 25% at the expense of charge generation that is only a fraction of that of an opaque OPV, and uniformity in device performance regardless of transmitted color. Because the CF is spatially removed from the charge generation and transport pathway, the optical characteristics are largely decoupled from the electronic characteristics. This provides one the freedom to explore creative designs without having to consider the electronic properties and uniformity in performance. Furthermore, integration of CFs into OPVs ensures the transmitted colors to be spectrally pure, bidirectional, and of high saturation represented by sub-100nm resonance widths, providing optical characteristics suitable for use in colorful power generating windows.
We fabricated Si nanopillar (NP) arrays using e-beam lithography and coated them with poly(3-hexylthiophene-2,5-diyl) (P3HT) organic semiconductor layers. Optical reflection spectra showed that Mie resonance significantly increased the scattering cross-sections of the NPs and strongly concentrated incident light in the NPs. Such concentrated light should produce numerous charge carriers and affect the subsequent drift/diffusion of the carriers. Surface photovoltage (SPV), defined as the difference of the surface potential in dark and under light, could reveal the formation and separation of the photo-generated carriers. Especially, Kelvin probe force microscopy technique allowed us to obtain real space SPV maps with nanoscopic spatial resolution. The SPV values at the NP tops were much larger than those at the flat regions around the NPs. This study would provide us insights into improving performance of organic/inorganic hybrid nanostructure-based devices.
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