Growth and characterization of CuxAg1-xInSe2 thin films by pulsed laser deposition

We report on structural and optical measurements made on thin films of the quaternary compounds CuxAg1-xInSe2(x = 0; 0.3; 0.5; 0.7; 1.0). The films were prepared by pulsed laser deposition (PLD) of prereacted material onto glass substrates. The substrate temperature was about 450-480 degrees C. The beam of a Nd:YAG laser was directed onto a rotating target. The resulting films were characterized by XRD, SEM, and EDAX, The films were single phase, polycrystalline and stoichiometric within 4 %. The refractive index n and the absorption coefficient alpha of CuxAg1-xInSe2 thin films were obtained by measuring the transmittance (T) and reflectance (R) in the photon energy range from 0.4 to 2.5 mu m. The optical properties were determined from rigorous expression for the transmission and reflection in an air/film/glass substrate/air multilayer system. The films had high optical absorption about 10(4) - 10(5) cm(-1) and the band gaps of 0.99 eV (CuInSe2) and 1.25 eV (AgInSe2). The energy gaps observed in laser-deposited CuxAg1-xInSe2 thin films near and above the fundamental absorption edge exhibit a nonlinear composition dependence

Wide band-gap tuning Cu2ZnSn1-xGexS4 single crystals: optical and vibrational properties

The linear optical properties of Cu2ZnSn1-xGexS4 high quality single crystals with a wide range of Ge contents (x = 0.1, 0.3, 0.5, 0.7, 0.9 and 1) have been investigated in the ultraviolet and near infrared range using spectroscopic ellipsometry measurements. From the analysis of the complex dielectric function spectra it has been found that the bandgap E0 increases continuously from 1.49 eV to 2.25 eV with the Ge content. Furthermore, the evolution of the interband transitions E1A and E1B has been also determined. Raman scattering using three different excitation wavelengths and its analysis have been performed to confirm the absence of secondary phases in the samples, and to distinguish between stannite, wurtzite, wurzstannite and kesterite structures. Additionally, the analysis of the high resolution Raman spectra obtained in samples with different [Ge]/([Ge]+[Sn]) ratios allows describing a bimodal behavior of the dominant A modes. The understanding of the incorporation of Ge into the Cu2ZnSnS4 lattice is fundamental in order to develop efficient bandgap engineering of these compounds towards the fabrication of kesterite based solar cells with enhanced performanc

Wide band-gap tuning of Cu2ZnSn1-xGex(S;Se)4 solid solutions for photovoltaic multi-junction devices development: a study of their optical; structural and vibrational properties

2015 MRS Spring Meeting & Exhibit, San Francisco, California, April 6-10, 2015Cu2Zn(Sn,Ge)(S,Se)4 (CZTGSSe) solid solutions have a strong interest for photovoltaic (PV) applications. Cu2ZnSn(S,Se)4 (CZTSSe) kesterites have already been proposed as medium term alternative to more mature Cu(In,Ga)(S,Se)2 (CIGSSe) based technologies, that are already entering industrial production stage, avoiding the use of scarce elements as In. Addition of Ge gives the possibility to control the bandgap of the absorber in a wider region from 1 to 2.3 eV, expanding the range of values available by varying the anion compositions in CZTSSe. This is of paramount interest for the development of new multi-junction devices where photovoltaic conversion is optimized by improving the device efficiency in different spectral regions. Recently device prototypes based in these compounds have been reported, with efficiencies of 2.7% for Cu2Zn(Sn,Ge)Se4 [1] and 6.3% for Cu2Zn(Sn,Ge)S4 [2]. Further development of these technologies requires a much deeper knowledge on the fundamental properties of these complex compounds, which requires for the study of reference single crystals with high crystalline quality and well known composition. In this work we provide an in depth analysis of the fundamental optical and vibrational properties of the Cu2ZnSn1- xGex(S,Se)4 solid solutions. For this purpose Cu2Zn(Sn,Ge)S4 (CZTGS) and Cu2Zn(Sn,Ge)Se4 (CZTGSe) high quality single crystals, as confirmed by XRD measurements, have been grown by chemical vapour transport with different [Ge]/([Sn] + [Ge]) atomic ratios (x varying from 0 to 1) and have been fully characterized using optical, structural, and Raman scattering techniques. The optical characterization has been performed by variable angle spectroscopic ellipsometry in the full visible to near infrared range (0.7-4.7 eV) [3] in order to obtain the dielectric functions (¿1 and ¿2). Analysis of the dielectric response shows that by varying the composition indeed solid solutions with a wide band gap variation can be obtained ranging from 1.5 to 2.3 eV for the CZTGS, and a more moderate variation only from 1 to 1.3 eV for CZTGSe. Finally, vibrational properties were thoroughly studied by Raman spectroscopy in resonant and non-resonant conditions under different excitation wavelengths (325, 532, 633 and 785 nm). This has allowed to describe the evolution of the Raman modes as a function of the [Ge]/([Sn]+[Ge]) accurately. The results reveal one-mode and bi-mode behaviour of the main modes for the CZTGSe and CZTGS systems, respectively. This characterization provides us a deeper knowledge of the fundamental properties of the CZTGS and CZTGSe systems. Additionally, this work shows the way to evaluate the [Ge]/([Sn] + [Ge]) ratio of these compounds using only non-destructive optical methods.Peer Reviewe

Towards the growth of Cu2ZnSn1-xGexS4 thin films by a single-stage process : effect of substrate temperatura and composition

Cu2ZnSn1-xGexS4 (CZTGS) thin films prepared by flash evaporation of a Zn-rich Cu2ZnSn0.5Ge0.5S4 bulk compound in powder form, and a subsequent thermal annealing in S containing Ar atmosphere are studied. The effect of the substrate temperature during evaporation and the initial composition of the precursor powder on the growth mechanism and properties of the final CZTGS thin film are investigated. The microstructure of the films and elemental depth profiles depend strongly on the growth conditions used. Incorporation of Ge into the Cu2ZnSnS4 lattice is demonstrated by the shift of the relevant X-ray diffraction peaks and Raman vibrational modes towards higher diffraction angles and frequencies respectively. A Raman mode at around 348-351 cm-1 is identified as characteristic of CZTGS alloys for x = [Ge]/([Sn]+[Ge]) = 0.14-0.30. The supply of Ge enables the reduction of the Sn loss via a saccrifical Ge loss. This fact allows increasing the substrate temperature up to 350º C during the evaporation, forming a high quality kesterite material and therefore, reducing the deposition process to one single stag