We report effects of controlled humidity in ambient condition on grain boundary potential and charge transport within the grains of Pervoskite films prepared by sequential deposited technique. Grain boundary exhibited variation of their electronic properties with change in humidity level from sample kept inside glove box to 75% RH. X-ray diffraction (XRD) indicates the formation of PbI2 phase with increasing humidity level. The degradation of Pervoskite solar cell is mainly associated with the increase of PbI2 phase with increase in humidity level and hydration of the grain boundaries with the formation of hydrated phases. Spatial mapping of surface potential in the Perovskite film exhibits higher positive potential at grain boundaries compared to the surface of the grains. Grain boundary potential barrier were found to increase from ~35 meV to 80 meV for perovskite film exposed to 75% RH level compared to perovskite film kept inside glove box. Nanoscale current sensing measurement (Cs-AFM) shows that charge transport in perovskit solar cell strongly depends in humidity level. Performances of the solar cell was maximum for 25% humidity with 14.01 %. Transient measurement shows decrease in charge carrier life time and charge transport time with increase in humidity level. Our results show strong correlation between humidity level, electronic grain boundary properties and device performance.
Smooth, compact and defect free morphology of perovskite is highly desired for enhanced device performance. Several routes such as thermal annealing, use of solvent mixtures, growth under controlled humidity has been adopted to obtain crystalline, smooth and defect free perovskite film. Herein we showed direct use of water (H2O) as co-solvent in precursor solution and have optimized the water content required to obtain smooth and dense film. Varying concentration of water was used in precursor solution of CH3NH3I and PbI2 mixed in γ-butyrolactone (GBL) and dimethylsulfoxide (DMSO). Perovskite films were crystallized using toluene assisted solvent engineering method using GBL:DMSO:H2O as solvent mixture. The amount of water was varied from 1% to 25%, which resulted in change in film morphology and perovskite crystallinity. It was concluded that an appropriate amount of water is required to assist the crystallization process to obtain smooth pin-hole free morphology. The change in morphology led to improved fill factor in the device, with highest efficiency ~14%, which was significantly higher than devices made from perovskite film without adding water. We also showed that addition of up to 25% by volume of water does not significantly change the device performance.
KEYWORDS: Solar cells, Polymers, Annealing, Silver, Zinc oxide, Solar energy, Atomic force microscopy, 3D image processing, Photovoltaics, Electrical engineering
Solution processed tandem polymer solar cell has drawn a great deal of attention due its low cost, ease of production and capability of harvesting solar energy more efficiently. In solution processed tandem polymer solar cell, the most challenging part is the optimization of interfacial layer. In this work, we have investigated the robustness of PEDOT:PSS/AZO/PEIE interfacial layer to develop tandem polymer solar cell. While developing triple junction polymer solar cell, temperature of second interfacial layer has also a great impact on overall device performance. Here, the performance of tandem polymer solar cell was investigated on different temperature of interfacial layer.
Conjugated polymers are potential materials for photovoltaic applications due to their high absorption coefficient, mechanical flexibility, and solution-based processing for low-cost solar cells. A bulk heterojunction (BHJ) structure made of donor–acceptor composite can lead to high charge transfer and power conversion efficiency. Active layer morphology is a key factor for device performance. Film formation processes (e.g., spray-coating, spin-coating, and dip-coating), post-treatment (e.g., annealing and UV ozone treatment), and use of additives are typically used to engineer the morphology, which optimizes physical properties, such as molecular configuration, miscibility, lateral and vertical phase separation. We will review electronic donor–acceptor interactions in conjugated polymer composites, the effect of processing parameters and morphology on solar cell performance, and charge carrier transport in polymer solar cells. This review provides the basis for selection of different processing conditions for optimized nanomorphology of active layers and reduced bimolecular recombination to enhance open-circuit voltage, short-circuit current density, and fill factor of BHJ solar cells.
Recently, excellent solar cell device performances have been achieved with solution-processed small-molecule donor materials. Small molecules have well defined structures and thus allow better control of self-assembly in the solid state. However, the easy formation of H-type aggregates and lack of strong interactions between nanodomains could limit charge transport, device performance, and long-term stability. We have recently explored the synthesis of ring-protected small molecules (with rings surrounding the center of the molecules), studied the intermolecular interactions in solution and solid state, and conducted preliminary solar cell device fabrications. It has been found that the molecules behave very differently from conventional flat small molecules in both solution and solid states. Proton NMR study of solutions of different concentrations revealed the presence of strong intermolecular interactions as a result of absence or shortage of open-ended alkyl side chains; however, such strong interactions do not lead to precipitation of the molecules even at high concentrations. Excellent films are routinely obtained from the neat small molecules despite the much reduced number of solubilizing groups. The New findings strongly suggest that ring protection is an effective strategy to avoid Haggregation and maintain strong pi-pi interactions simultaneously. Such materials are expected to form head-tail selfassemblies that will open new possibilities for small molecule organic materials. Conceptually, thin films of such materials are potentially more isotropic in charge transport than conventional small molecule and polymer films, a property desirable for photovoltaics and some other optoelectronic applications.
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