Minority and Majority Charge Carrier Mobility in Cu2ZnSnSe4 revealed by Terahertz Spectroscopy

The mobilities of electrons and holes determine the applicability of any semiconductor, but their individual measurement remains a major challenge. Here, we show that time resolved terahertz spectroscopy TRTS can distinguish the mobilities of minority and majority charge carriers independently of the doping type and without electrical contacts. To this end, we combine the well established determination of the sum of electron and hole mobilities from photo induced THz absorption spectra with mobility dependent ambipolar modeling of TRTS transients. The method is demonstrated on a polycrystalline Cu2ZnSnSe4 thin film and reveals a minority electron mobility of 128 cm2 V s and a majority hole carrier mobility of 7 cm2 V s in the vertical transport direction relevant for light emitting, photovoltaic and solar water splitting devices. Additionally, the TRTS analysis yields an effective bulk carrier lifetime of 4.4 ns, a surface recombination velocity of 6 104 cm s and a doping concentration of ca. 1016 cm 3, thus offering the potential for contactless screen novel optoelectronic material

Effect of Ag incorporation on structure and optoelectronic properties of Ag1 xCux 2 ZnSnSe4 solid solutions

The performance of Cu2ZnSnSe4 solar cells is presently limited by low values of open circuit voltage which are a consequence of strong band tailing and high level of nonradiative recombination. Recently, the partial substitution of Cu, Zn, and Sn by other elements has shown the potential to overcome this limitation. We explored the structural changes and the effect on the optoelectronic properties of the partial substitution of Cu with Ag in Cu2ZnSnSe4. This paper clari amp; 64257;es the crystal structure of Ag1 amp; 8722;xCux 2ZnSnSe4 solid solution series, deducing possible cationic point defects and paying special attention to the presence of Cu Zn disorder with a combination of neutron and x ray diffraction. The optoelectronic properties of the solid solution series are assessed using re amp; 64258;ection and quantitative photoluminescence spectroscopy, which allows us to estimate the fraction of nonradiative recombination, which would contribute to the open circuit voltage loss in devices. The results strongly suggest Ag incorporation as a promising route to eliminate Cu Zn disorder and to reduce nonradiative recombination losses in Cu2ZnSnSe4

The role of interparticle heterogeneities in the selenization pathway of Cu Zn Sn S nanoparticle thin films a real time study

Real time energy dispersive X ray diffraction EDXRD analysis has been utilized to observe the selenization of Cu Zn Sn S nanoparticle films coated from three nanoparticle populations Cu and Sn rich particles roughly 5 nm in size, Zn rich nanoparticles ranging from 10 to 20 nm in diameter, and a mixture of both types of nanoparticles roughly 1 1 by mass , which corresponds to a synthesis recipe yielding CZTSSe solar cells with reported total area efficiencies as high as 7.9 . The EDXRD studies presented herein show that the formation of copper selenide intermediates during the selenization of mixed particle films can be primarily attributed to the small, Cu and Sn rich particles. Moreover, the formation of these copper selenide phases represents the first stage of the CZTSSe grain growth mechanism. The large, Zn rich particles subsequently contribute their composition to form micrometer sized CZTSSe grains. These findings enable further development of a previously proposed selenization pathway to account for the roles of interparticle heterogeneities, which in turn provides a valuable guide for future optimization of processes to synthesize high quality CZTSSe absorber layer

Atomic Scale Structure of Ag,Cu 2 ZnSnSe4 and Cu2Zn Sn,Ge Se 4 Kesterite Thin Films

Kesterite based materials are being researched and developed as affordable, efficient, and mechanically flexible absorber materials for thin film photovoltaics. Both Ag,Cu 2ZnSnSe4 and Cu2Zn Sn,Ge Se4 based devices have shown great potential in overcoming some of the remaining challenges for further increasing the conversion efficiency of kesterite based solar cells. This study therefore investigates the long range crystallographic structure and the local atomic scale structure of technologically relevant thin films by means of grazing incidence X ray diffraction and low temperature X ray absorption spectroscopy. As expected, the unit cell dimensions change about an order of magnitude more than the element specific average bond lengths. In case of Cu2Zn Sn,Ge Se4, the thin film absorbers show a very similar behavior as Cu2Zn Sn,Ge Se4 powder samples previously studied. Small amounts of residual S in the thin films were taken into account in the analysis and the results imply a preferential formation of Sn S bonds instead of Ge S bonds. In Ag,Cu 2ZnSnSe4, the dependence of the Ag Se and Cu Se bond lengths on Ag Ag Cu might indicate an energetic advantage in the formation of certain local configuration

High temperature decomposition of Cu 2 BaSnS 4 with Sn loss reveals newly identified compound Cu2Ba3Sn2S8

The earth abundant quaternary compound Cu2BaSnS4 is being currently studied as a candidate for photovoltaics and as a photocathode for water splitting. However, the chemical stability of this phase during synthesis is unclear. The synthesis of other quaternary tin sulphur based absorbers e.g., Cu2ZnSnS4 involves an annealing step at high temperature under sulphur gas atmosphere, which can lead to decomposition into secondary phases involving Sn loss from the sample. As the presence of secondary phases can be detrimental for device performance, it is crucial to identify secondary phase chemical, structural and optoelectronic properties. Here we used a combination of in situ EDXRD XRF and TEM to identify a decomposition pathway for Cu2BaSnS4. Our study reveals that, while Cu2BaSnS4 remains stable at high sulphur partial pressure, the material decomposes at high temperatures into Cu4BaS3 and the hitherto unknown compound Cu2Ba3Sn2S8 if the synthesis is performed under low partial pressure of sulphur. The presence of Cu4BaS3 in devices could be harmful due to its high conductivity and relatively lower band gap compared to Cu2BaSnS4. The analysis of powder diffraction data reveals that the newly identified compound Cu2Ba3Sn2S8 crystallizes in the cubic system space group I 43d with a lattice parameter of a 14.53 1 . A yellow powder of Cu2Ba3Sn2S8 has been synthesized, exhibiting an absorption onset at 2.19 e

Metastable defect response in CZTSSe from admittance spectroscopy

Admittance spectroscopy is a useful tool used to study defects in semiconductor materials. However, metastable defect responses in non ideal semiconductors can greatly impact the measure ment and therefore the interpretation of results. Here, admittance spectroscopy was performed on Cu2ZnSn S,Se 4 where metastable defect response is illustrated due to the trapping of injected car riers into a deep defect state. To investigate the metastable response, admittance measurements were performed under electrically and optically relaxed conditions in comparison to a device fol lowing a low level carrier injection pretreatment. The relaxed measurement demonstrates a single capacitance signature while two capacitance signatures are observed for the device measured fol lowing carrier injection. The deeper level signature, typically reported for kesterites, is activated by charge trapping following carrier injection. Both signatures are attributed to bulk level defects. The significant metastable response observed on kesterites due to charge trapping obscures accurate interpretation of defect levels from admittance spectroscopy and indicates that great care must be taken when performing and interpreting this measurement on non ideal devices

Synergistic Effects of Double Cation Substitution in Solution Processed CZTS Solar Cells with over 10 Efficiency

The performance of many emerging compound semiconductors for thin film solar cells is considerably lower than the Shockley Queisser limit, and one of the main reasons for this is the presence of various deleterious defects. A partial or complete substitution of the cations presents a viable strategy to alter the characteristics of the detrimental defects and defect clusters. Particularly, it is hypothesized that double cation substitution could be a feasible strategy to mitigate the negative effects of different types of defects. In this study, the effects of double cation substitution on pure sulfide Cu2ZnSnS4 CZTS by partially substituting Cu with Ag, and Zn with Cd are explored. A 10.1 total area power conversion efficiency 10.8 activearea efficiency is achieved. The role of Cd, Ag, and Cd Ag substitution is probed using temperature dependent photoluminescence, time resolved photoluminescence, current voltage IV , and external quantum efficiency EQE measurements. It is found that Cd improves the photovoltaic performance by altering the defect characteristics of acceptor states near the valence band, and Ag reduces nonradiative bulk recombination. It is believed that the double cation substitution approach can also be extended to other emerging photovoltaic materials, where defects are the main culprits for low performanc