Influence of Ga back grading on voltage loss in low-temperature co-evaporated Cu(In,Ga)Se2 thin film solar cells

The performance of Cu(In,Ga)Se2 (CIGS) solar cells is limited by the presence of the highly recombinative CIGS/Mo interface. The recombination at the CIGS/Mo interface is influential for the open circuit voltage (VOC) in high quality CIGS absorbers with increased charge carriers diffusion length. A quantitative understanding of the role of the Ga back grading height (ΔGGI) in suppressing back interface recombination is needed. In this work, we take advantage of a low temperature process to modify the ΔGGI while keeping the composition in the notch and front regions almost unchanged. Improvement in both VOC deficit and time-resolved photoluminescence lifetime are observed with increasing ΔGGI. With a combination of back surface modification experiments and numerical simulations, we quantify a voltage loss in ungraded devices of approximately 100 mV solely from the back interface recombination. Nice agreement between simulation and experimental data is reached while constraining the values of possible diffusion lengths. Our results suggest that a ΔGGI of about 0.50 is required to effectively suppress the back interface recombination, highlighting the importance of grading control in high-performance CIGS solar cells and devices.Bundesamt für Energie, Grant/Award Number: SI/501614-01; Horizon 2020 Framework Programme, Grant/Award Number: EMPIR project HyMet; Swiss State Secretary for Education, Research and Innovation (SERI), Grant/Award Number: 17.00105 (EMPIR project HyMet

Controlled Li Alloying of CZTSSe Absorbers by Electrochemical Treatment

<p>Oral presentation at 2023 MRS Spring Meeting</p&gt

Hybrid Ionic Liquid/water-in-Salt Electrolytes Enable Stable Cycling of LTO/NMC811 Full Cells

Water-in-salt electrolytes have successfully expanded the electrochemical stability window of aqueous electrolytes to up to 3 V. While particularly the reductive stability of water-in-salt electrolytes can be further improved by simply increasing the salt concentration, this approach has its limitations as it leads to very viscous and hence poorly conducting solutions. An alternative strategy is the partial substitution of water by ionic liquids, which boosts the lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) solubility while maintaining adequate transport properties at very high salt concentrations.Here, we study such ternary electrolytes based on LiTFSI, water, and imidazolium-type ionic liquids in terms of their thermal, electrochemical, and transport properties. We find that the LiTFSI solubility increases from 21 mol kg−1 in water to up to 60 mol kg−1 in the presence of these ionic liquids. Deconvolution of the different contributions to the LiTFSI solubility shows that particularly the ionic liquid 1-ethyl-3-methylimidazolium trifluoromethanesulfonate acts as solubility enhancer.The increased reductive stability of these ternary electrolytes enables stable cycling of both TiO2 and Li4Ti5O12 anodes. We further show that the low water content of these electrolytes also allows stable cycling of LiNi0.8Mn0.1Co0.1O2 (NMC811) cathodes. For instance, a TiO2/NMC811 full cell based on such a hybrid electrolyte shows an energy density of 121 Wh kg−1 on the active material level, a Coulombic efficiency of >99.7% at C/2, and retains 80% of its initial capacity after 290 cycles at this rate. Owing to the 300 mV lower redox potential of Li4Ti5O12 compared to TiO2, Li4Ti5O12/NMC811 full cells reach energy densities of 141 and 150 Wh kg−1 at a rate of 1C and C/2, respectively. These cells still show Coulombic efficiency of 99.4% and 99.2%, respectively, and maintain 80% of their initial capacity after 230 and 154 cycles, respectively. Considering the large number of potential lithium salt–ionic liquid combinations, we anticipate further improvements in electrolyte properties and subsequently cell performance.<br /

Monolithically-stacked thin-film solid-state batteries

The power capability of Li-ion batteries has become increasingly limiting for the electrification of transport on land and in the air. The specific power of Li-ion batteries is restricted to a few thousand W/kg due to the required cathode thickness of a few tens of micrometers. We present a new design of monolithically-stacked thin-film cells that has the potential to increase the power ten-fold. We demonstrate an experimental proof-of-concept consisting of two monolithically stacked thin-film cells. Each cell consists of a silicon anode, a solid-oxide electrolyte, and a lithium cobalt oxide cathode. The battery can be cycled for more than 300 cycles between 6 and 8 V. Using a thermo-electric model, we predict that stacked thin-film batteries can achieve specific energies >250 Wh/kg at C-rates above 60, resulting in a specific power of tens of kW/kg needed for high-end mobile applications such as drones, robots, and eVTOLs

In situ Lithiated ALD Niobium Oxide for Improved Long Term Cycling of Layered Oxide Cathodes: A Thin-Film Model Study

Protective coatings applied to cathodes help to overcome interface stability issues and extend the cycle life of Li-ion batteries. However, within 3D cathode composites it is difficult to isolate the effect of the coating because of the additives and non-ideal interfaces. In this study we investigate niobium oxide (NbOx) as cathode coating in a thin-film model system, which provides simple access to the cathode-coating-electrolyte interface. The conformal NbOx coating was applied by atomic layer deposition (ALD) onto thin-film LiCoO2 cathodes. The cathode/coating stacks were annealed to lithiate the NbOx and ensure sufficient ionic conductivity. A range of different coating thicknesses were investigated to improve the electrochemical cycling with respect to the uncoated cathode. At a NbOx thickness of 30 nm, the cells retained 80% of the initial capacity after 493 cycles at 10 C, more than doubling the cycle life of the uncoated cathode film. At the same thickness, the coating also showed a positive impact on the rate performance of the cathode: 47% of the initial capacity was accessible even at ultrahigh charge-discharge rates of 100 C. Using impedance spectroscopy measurements, we found that the enhanced performance is due to suppressed interfacial resistance growth during cycling. Elemental analysis using TOF-SIMS and XPS further revealed a bulk and surface contribution of the NbOx coating. These results show that in situ lithiated ALD NbOx can significantly improve the performance of layered oxide cathodes by enhancing interfacial charge transfer and inhibiting surface degradation of the cathode, resulting in better rate performance and cycle life.</p

Monolithically-stacked thin-film solid-state batteries

Abstract The power capability of Li-ion batteries has become increasingly limiting for the electrification of transport on land and in the air. The specific power of Li-ion batteries is restricted to a few thousand W kg−1 due to the required cathode thickness of a few tens of micrometers. We present a design of monolithically-stacked thin-film cells that has the potential to increase the power ten-fold. We demonstrate an experimental proof-of-concept consisting of two monolithically stacked thin-film cells. Each cell consists of a silicon anode, a solid-oxide electrolyte, and a lithium cobalt oxide cathode. The battery can be cycled for more than 300 cycles between 6 and 8 V. Using a thermo-electric model, we predict that stacked thin-film batteries can achieve specific energies >250 Wh kg−1 at C-rates above 60, resulting in a specific power of tens of kW kg−1 needed for high-end applications such as drones, robots, and electric vertical take-off and landing aircrafts

Controlled Li Alloying by Postsynthesis Electrochemical Treatment of Cu&lt;sub&gt;2&lt;/sub&gt;ZnSn(S, Se)&lt;sub&gt;4&lt;/sub&gt; Absorbers for Solar Cells

Li-alloying of Cu2ZnSn(S, Se)4 (CZTSSe) absorbers is widely accepted for its beneficial influence on the performance of CZTSSe-based thin film solar cells. Given the degraded morphology characteristic of absorbers synthesized in the presence of excess Li concentrations, it is speculated that Li may be best incorporated into the absorber after synthesis. Here, we report an innovative method to add Li to synthesized CZTSSe by an electrochemical treatment using a liquid electrolyte. Our approach decouples Li addition from absorber synthesis, allowing one to possibly overcome morphology issues associated with high Li concentration. We show that Li is thereby transferred to the absorber and is incorporated into the crystal lattice. The resulting Li concentration in the absorber can be easily controlled by the treatment parameters. Using liquid electrolytes allows a straightforward disassembly of the lithiation setup and hence the fabrication of solar cells after electrochemical treatment. Electrochemically lithiated solar cells reached power conversion efficiencies of up to 9.0%. Further optimization of this innovative method is required to reduce expected interface issues resulting from the electrochemical treatment to demonstrate a gain in the power conversion efficiency of the CZTSSe solar cells. Finally, our results indicate strong lateral Li diffusion, which deserves further investigation. Moreover, the method could be transferred to other material systems, such as Cu(In, Ga)Se2 (CIGS), and adapted to treat layers with other alkali elements such as Na

Controlled Post-Synthesis Li-Alloying of CZTSSe Absorbers by Electrochemical Treatment for PV Applications

&lt;p&gt;Oral presentation at 13th European Kesterite+ Workshop&lt;/p&gt