Synthesis, theoretical and experimental characterisation of thin film Cu2Sn1-xGexS3 ternary alloys (x = 0 to 1): Homogeneous intermixing of Sn and Ge

Cu2Sn1-xGexS3 is a p-type semiconductor alloy currently investigated for use as an absorber layer in thin film solar cells. The aim of this study is to investigate the properties of this alloy in thin film form in order to establish relationships between group IV composition and structural, vibrational and opto-electronic properties. Seven single phase Cu2Sn1-xGexS3 films are prepared from x = 0 to 1, showing a uniform distribution of Ge and Sn laterally and in depth. The films all show a monoclinic crystal structure. The lattice parameters are extracted using Le Bail refinement and show a linear decrease with increasing Ge content. Using density-functional theory with hybrid functionals, we calculate the Raman active phonon frequencies of Cu2SnS3 and Cu2GeS3. For the alloyed compounds, we use a virtual atom approximation. The shift of the main Raman peak from x = 0 to x = 1 can be explained as being half due to the change in atomic masses and half being due to the different bond strength. The bandgaps of the alloys are extracted from photoluminescence measurements and increase linearly from about 0.90 to 1.56 eV with increasing Ge. The net acceptor density of all films is around 1018 cm−3. These analyses have established that the alloy forms a solid solution over the entire composition range meaning that intentional band gap grading should be possible for future absorber layers. The linear variation of the unit cell parameters and the band gap with group IV content allows composition determination by scattering or optical measurements. Further research is required to reduce the doping density by two orders of magnitude in order to improve the current collection within a solar cell device structure

Synthesis, theoretical and experimental characterisation of thin film Cu2Sn1-xGexS3 ternary alloys (x=0 to 1): Homogeneous intermixing of k for Sn and Ge

Cu2Sn1-xGexS3 is a p-type semiconductor alloy currently investigated for use as an absorber layer in thin film solar cells. The aim of this study is to investigate the properties of this alloy in thin film form in order to establish relationships between group IV composition and structural, vibrational and opto-electronic properties. Seven single phase Cu2Sn1-xGexS3 films are prepared from x = 0 to 1, showing a uniform distribution of Ge and Sn laterally and in depth. The films all show a monoclinic crystal structure. The lattice parameters are extracted using Le Bail refinement and show a linear decrease with increasing Ge content. Using density-functional theory with hybrid functionals, we calculate the Raman active phonon frequencies of Cu2SnS3 and Cu2GeS3. For the alloyed compounds, we use a virtual atom approximation. The shift of the main Raman peak from x = 0 to x =1 can be explained as being half due to the change in atomic masses and half being due to the different bond strength. The bandgaps of the alloys are extracted from photoluminescence measurements and increase linearly from about 0.90 to 1.56 eV with increasing Ge. The net acceptor density of all films is around 10(18)cm(-3). These analyses have established that the alloy forms a solid solution over the entire composition range meaning that intentional band gap grading should be possible for future absorber layers. The linear variation of the unit cell parameters and the band gap with group IV content allows composition determination by scattering or optical measurements. Further research is required to reduce the doping density by two orders of magnitude in order to improve the current collection within a solar cell device structure. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.We acknowledge support from the National Research Fund,Luxembourg, through the EATSS project under the grant agreementC13/MS/5898466 and through project C12/MS/3987081/TSDSN(L.W. and H.P.C.M.). The Laboratory for Photovoltaics and the Lab-oratory of Energy Materials at the University of Luxembourg arefully acknowledged for help and discussion, especially Dr. MicheleMelchiorre and Daniel Siopa for performing the majority of theWDX measurements. The working group of Structure and Dy-namics of Energy Materials at the HZB is acknowledged for accessand help with the measurements in the X-Ray CoreLab. Dr Inma-culada Pera Alonso is acknowledged for her complementary adviceon the Le Bail refinement

Strong Exciton-Coherent Phonon Coupling in Single-Layer MoS<inf>2</inf>

© 2019 The Author(s) 2019 OSA. We use broadband optical pump-probe spectroscopy to study coherent optical phonons in lL-MoS2. We detect a strong coupling with the A': mode, which is enhanced around the C-exciton peak. Ab-initio calculations of the phonon-induced band structure variation fully confirm this result

Strongly coupled coherent phonons in single-layer MoS2_2

We present a transient absorption setup combining broadband detection over the visible-UV range with high temporal resolution ($\sim$20fs) which is ideally suited to trigger and detect vibrational coherences in different classes of materials. We generate and detect coherent phonons (CPs) in single layer (1L) MoS$_2$, as a representative semiconducting 1L-transition metal dichalcogenide (TMD), where the confined dynamical interaction between excitons and phonons is unexplored. The coherent oscillatory motion of the out-of-plane $A'_{1}$ phonons, triggered by the ultrashort laser pulses, dynamically modulates the excitonic resonances on a timescale of few tens fs. We observe an enhancement by almost two orders of magnitude of the CP amplitude when detected in resonance with the C exciton peak, combined with a resonant enhancement of CP generation efficiency. Ab initio calculations of the change in 1L-MoS$_2$ band structure induced by the $A'_{1}$ phonon displacement confirm a strong coupling with the C exciton. The resonant behavior of the CP amplitude follows the same spectral profile of the calculated Raman susceptibility tensor. This demonstrates that CP excitation in 1L-MoS$_2$ can be described as a Raman-like scattering process. These results explain the CP generation process in 1L-TMDs, paving the way for coherent all-optical control of excitons in layered materials in the THz frequency range

Strong exciton-coherent phonon coupling in single-layer mos2

© 2020 OSA - The Optical Society. All rights reserved. Broadband transient absorption with sub-20fs temporal resolution, supported by abinitio calculations, quantitatively provides the strength of exciton-coherent phonon coupling in 1LMoS2, showing a resonant profile around the C exciton