Recently, Sn perovskite solar cell (Sn PVK PV) are attracting attention. However, the efficiency was still lower than that of Pb perovskite solar cells. Recently, the Sn PVK PVs with efficiency higher than 10% have been reported from several research groups. The crystal defects include the presence of Sn4+, Sn2+ defect, I- defect, the presence of Sn0, the interstitial I- and so on. In order to decrease these defect densities, we have proposed some processes such as addition of Ge2+ ion, introduction of ethylammonium cation into A site, and surface passivation of perovskite grain boundary with diaminoethane dilute solution. Our results on efficiency enhancement (13%) is explained by the conduction and valence band energy level against carrier trap depth. In addition, an inverted SnPb perovskite solar cells with 23.3% efficiency is discussed from the view point of optimization of energy alignment.
Certified efficiency of halogenated Pb-perovskite solar cells has reached 25.2 %. Because of the Pb usage restriction, researches on Pb free perovskite solar cells have been focused on. Halogenated Sn-perovskite is one of candidates for the Pb-free perovskite solar cells. In spite of the similarities of the electronic properties between Pb-perovskite and Sn-perovskite, the efficiency of the Sn-perovskite solar cell was still about 10%, which is far below that of Pb-perovskite solar cells. We report Ge ion doped Sn-perovskite solar cells (Pb free) with 13% efficiency by optimizing A site cations in ASnI3 composition and surface passivation.
Despite the high-efficiency of these lead-based perovskite solar cells, the problem associated from the toxic nature of lead has open a new research direction which focuses on lead-free perovskite materials. As an alternative, tin has been proposed to replace lead. The highest efficiency obtained with Sn only perovskite was 9 % which was based on 2D and 3D mixture of FASnI3. However, Sn-based perovskites are known to have low stability in air. The use of germanium-based perovskite in solar cell was first realized by Krishnamoorthy et. al. The measured solar cell performance was notably low, 0.11 % for CsGeI3 and 0.20 % for MAGeI3. A theoretical study exploring hybrid tin and germanium perovskite showed that it is possible to prepare a stable Sn-Ge perovskite material that absorbs the sunlight spectrum. In this study, a new type of SnGe mixed metal perovskite solar cells are reported with enhanced efficiency and stability. In this report, FA0.75MA0.25Sn1-xGexI3 (abbreviated as SnGe(x)-PVK) were used for the mixed metal SnGe perovskite. XRD spectra showed that the structure is perovskite.
The structure of Ge-doped Sn perovskite was also discussed from the view point of band gap, conduction and valence band level, XPS analysis, and the urbach energy. It can be concluded that most of the Ge atoms passivate the surface of the Sn perovskite (graded structure).For SnGe(0)-PVK device, the averageJsc was 17.61 mA/cm2, VOC was 0.46 V, FF was 0.41 and PCE of 3.31 %. Upon doping with 5 wt% of Ge, the JSC increased up to 19.80 mA/cm2, FF improved up to 0.55 with an overall efficiency of 4.48 %. Upon increasing the Ge content more than 10wt%, all the photovoltaic parameters decreased significantly which resulted in an efficiency as low as 0.80 % for SnGe(0.2)-PVK device. After optimization, 7.75% of SnGe(5)-PVK device is reported. Significant effect on Ge doping was seen in the enhancement of the stability. The stability in air has been improved significantly with the Ge doping, retaining 80 % of its original performance, remarkable stability enhancement, compared with 10 % retention for non-doped sample. This work provides a platform for further research on lead-free Sn-Ge based perovskite solar cells.
Dye-sensitized solar cells (DSSCs) are a type of organic solar cell often cited for their high efficiency and easy fabrication. Recent studies have shown that modification of the standard liquid electrolyte DSSC architecture by the changing one of the components or the addition of additives often results in the improvement in one of the photovoltaic parameters and hence the overall efficiency. Here we explore a dielectric liquid crystal material which is a known insulator but possesses a high degree of order and optical anisotropy. In the presence of an applied electric field, the equilibrium of positive and negative charges are displaced in opposite directions. In this work, different mixtures with different dielectric anisotropies ranging from negative, zero and positive are formulated. These mixtures are then used to prepare polymer dispersed liquid crystal (PDLC) electrolytes and subsequently DSSC devices based on these PDLC electrolytes are fabricated. The morphology of the PDLC is observed through polarizing optical microscopy (POM) and the electrical/photovoltaic characterizations are performed through current density-voltage (J-V) measurements and electrochemical impedance spectroscopy.
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