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.
Absorption edge of perovskite (PVK) solar cells consisting of MAPbI3 is 800nm. According to our simulation, light harvesting in the area of near IR is also necessary for enhancing the efficiency more. We have already reported that mixed metal perovskite (MAPbSnI3) shows photo-conversion in IR region (1-6). The short circuit current (Jsc) was high, reaching to 30 mA/cm2 because of the wide range of light harvesting. However, the open-circuit voltage (Voc) was lower than 0.3 V and the estimated voltage loss was 0.6-0.7 V, which was much larger than that of MAPbI3 (0.4 V), suggesting the presence of high density charge recombination center. We found that Ti-O-Sn bonds formed at the interface between Tiania and MAPbSnI3 layer, create trap states, resulting in increasing charge recombination at the interfaces. The surface trap density and the trap depth was quantitatively monitored by thermally stimulated current methods. When the Ti-O-Sn linkage was formed at the interface between TiO2 and PVK, the trap density increased by one order of magnitude. The interface was analyzed by XPS and confirmed that Ti-O-Sn linkage was formed. After removing the Ti-O-Sn bond at the interface between TiO2 and MAPbSnI3, the efficiency drastically increased from 4.0 % to 13.8 % and the stability was improved. It was proved that interface architecture is quite important for enhancing the MAPbSnI3 solar cells.
References
1. S. Nakabayashi, et al., J. Photonics for Energy; 2015, 5, 057410. 2. Y. Ogomi, et al., J. Phys. Chem. Lett. 2014, 5, 1004-1011.
Cu(In,Ga)Se2 (CIGS) solar cells are leading candidates for low-cost and high-efficiency solar cells. A band gap energy
(Eg) of CIGS can be controlled from 1.0 eV (CuInSe2) to 1.7 eV (CuGaSe2). The Eg of CIGS can be adjusted to the
theoretically estimated optimum value of 1.4 eV. However, maximum efficiencies for CIGS solar cells were achieved at
Eg=1.1~1.2 eV. A higher-Ga addition degrades the electronic properties of CIGS films. Compared to CIGS,
Cu(In,Al)Se2 (CIAS) can be adjusted the same Eg by a small Al addition. We report on the fabrication of the CIAS film
on Mo/soda-lime glass (SLG) substrate by a three-stage evaporation process. The film composition was
Cu/(In+Al)=0.89, Se/Metal=0.99 and Al/(In+Al)=0.15. The Eg of the film was 1.15 eV from the quantum efficiency
measurement. The cross-sectional scanning electron microscope image of the film showed a grain size of approximately
1μm. The composition depth profile by secondary ion mass spectroscopy showed the V-shape distribution of Al in the
depth direction. The CIAS solar cell consisted of Al/ITO/ZnO/CdS/CIAS/Mo/SLG was fabricated. The active cell area
was 0.12 cm2. A current-voltage measurement under illumination (AM1.5, 100mW/cm2) at 25°C showed the area
efficiency of 13.1% without antireflection coating.
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