Modifications of the CZTSe/Mo Back-Contact Interface by Plasma Treatments

Molybdenum (Mo) is the most commonly used back-contact material for copper zinc tin selenide (CZTSe)-based thin-film solar cells. For most fabrication methods, an interfacial molybdenum diselenide (MoSe2) layer with an uncontrolled thickness is formed, ranging from a few tens of nm up to ≈1 μm. In order to improve the control of the back-contact interface in CZTSe solar cells, the formation of a MoSe2 layer with a homogeneous and defined thickness is necessary. In this study, we use plasma treatments on the as-grown Mo surface prior to the CZTSe absorber formation, which consists of the deposition of stacked metallic layers and the annealing in selenium (Se) atmosphere. The plasma treatments include the application of a pure argon (Ar) plasma and a mixed argon–nitrogen (Ar–N2) plasma. We observe a clear impact of the Ar plasma treatment on the MoSe2 thickness and interfacial morphology. With the Ar–N2 plasma treatment, a nitrided Mo surface can be obtained. Furthermore, we combine the Ar plasma treatment with the application of titanium nitride (TiN) as back-contact barrier and discuss the obtained results in terms of MoSe2 formation and solar cell performance, thus showing possible directions of back-contact engineering for CZTSe solar cells

Steep sulfur gradient in CZTSSe solar cells by H2_{2}S-assisted rapid surface sulfurization

Sulfur/selenium grading is a widely used optimization strategy in kesterite thin-film solar cells to obtain a bandgap-graded absorber material and to optimize optical and electrical properties of the solar-cell device. In this work, we present a novel approach to introduce a [S]/([S] + [Se]) grading for Cu$_{2}$ZnSn(S,Se)$_{4}$ solar cells. In contrast to commonly used methods with slow process dynamics, the presented approach aims to create a fast sulfurization reaction on the surface of pure selenide kesterite absorbers by using highly reactive H$_{2}$S gas and high sulfurization temperatures in a rapid flash-type process. With a combination of X-ray photoelectron spectroscopy, X-ray emission spectroscopy, Raman spectroscopy, and Raman-shallow angle cross sections spectroscopy, we gain depth-varied information on the [S]/([S] + [Se]) ratio and discuss the impact of different process parameter variations on the material and device properties. The results demonstrate the potential of the developed process to generate a steep gradient of sulfur that is confined mainly to the surface region of the absorber film

Steep Sulfur Gradient in CZTSSe Solar Cells by H2S-Assisted Rapid Surface Sulfurization

Sulfur/selenium grading is a widely used optimization strategy in kesterite thin-film solar cells to obtain a bandgap-graded absorber material and to optimize optical and electrical properties of the solar-cell device. In this work, we present a novel approach to introduce a [S]/([S] + [Se]) grading for Cu ZnSn(S,Se) solar cells. In contrast to commonly used methods with slow process dynamics, the presented approach aims to create a fast sulfurization reaction on the surface of pure selenide kesterite absorbers by using highly reactive H S gas and high sulfurization temperatures in a rapid flash-type process. With a combination of X-ray photoelectron spectroscopy, X-ray emission spectroscopy, Raman spectroscopy, and Raman-shallow angle cross sections spectroscopy, we gain depth-varied information on the [S]/([S] + [Se]) ratio and discuss the impact of different process parameter variations on the material and device properties. The results demonstrate the potential of the developed process to generate a steep gradient of sulfur that is confined mainly to the surface region of the absorber film. 2 4

Vapor-Phase Incorporation of Ge in CZTSe Absorbers for Improved Stability of High-Efficiency Kesterite Solar Cells

We report an approach to incorporate Ge into Cu2ZnSnSe4 using GeSe vapor during the selenization step of alloyed metallic precursors. The vapor incorporation slowly begins at T ≈ 480 °C and peaks at 530 °C, resulting in a Ge-based composition shift inside the previously formed kesterite layer. We initially observe the formation of a Ge-rich surface layer that merges into a homogeneous distribution of the incorporated element during the further dwelling stage of the annealing. This approach is very versatile and could be used in many similar fabrication processes for incorporating Ge into CZTSe-absorber layers. Because the vapor-based composition shift in the layer happens after the formation of the absorber film towards the end of the fabrication process, most process parameters and the precursor structure may not need any significant re-optimization. The careful integration of this step could help to reduce Sn-related deep defects and accompanying VOC losses. The best CZTGSe-power-conversion efficiency obtained in this series is 10.4 % (with EG = 1.22 eV, FF = 54%, JSC = 36 mA/cm2, VOC = 540 mV, VOCdef,SQ = 417 mV). These results demonstrate the potential of this approach for Ge incorporation into kesterite absorbers

Gasification of Crop Residue Briquettes in an Open Core Down-draft Gasifier

Gasification of crop residue briquettes was studied in an open core, throatless, down-draft gasifier of 1800 MJ.h-1 capacity. Briquettes of 25, 35 and 55 mm diameters were prepared using sugarcane baggase, cotton stalk and ground nut shell, separately, in a piston press briquetting machine. Series of experiments were conducted to study performance of the gasifier in terms of fuel consumption rate, calorific value of producer gas and gasification efficiency using briquettes of selected crop residue. The performance was compared with the performance data already available for babul wood. The gasifier was found working satisfactorily with the babul wood and briquettes of all the three crop residues of three sizes. The gasification efficiency was highest for groundnut shell briquettes, followed by babul wood, sugarcane baggase briquettes and cotton stalk briquettes. The study revealed that clinker formation was lower and calorific value of the gas higher for gasification of smaller sized biomass briquettes

Hydrogen generation from NaBH₄ hydrolysis using Co-B/AlPO₄ and Co-B/bentonite catalysts

<p>Aluminium phosphate and bentonite supported Co-B catalyst were synthesized via two step impregnation-reduction method for sodium borohydride hydrolysis. The synthesized catalysts were characterized by XRD, FTIR, XPS, FE-SEM, FE-TEM, BET, ICP-AES techniques and tested for NaBH₄ hydrolysis reaction. The results demonstrated that the synthesized supported Co-B catalysts greatly facilitate the NaBH₄ hydrolysis reaction. Highest hydrolysis rate observed for Co-B/AlPO₄ and Co-B/bentonite catalysts are 6.50 and 3.91 L min⁻¹ g⁻¹, respectively, with 2 wt% NaBH₄, 5 wt% NaOH solution at 30 °C. The hydrogen generation rate was found to increase with experimental temperature. Activation energy for the hydrolysis reaction was observed to be 37 and 40.2 kJ mol⁻¹ for Co-B/AlPO₄ and Co-B/bentonite catalysts, respectively.</p

Modification of mono-layer MoS2 through post-deposition treatment and oxidation for enhanced optoelectronic properties

Precise control of the optical and electrical properties of mono-layer (ML) thin MoS2 is crucial for future applications in functional devices. Depending on the synthesis route and the post-deposition annealing protocols, the number of sulfur vacancies in the material is different, which has a profound impact on the properties of the 2D layer. Here, we show that the sulfur vacancy-rich ML MoS2 films oxidize already at room temperature, which changes the photoluminescence (PL) yield, the MoS2–Al2O3 substrate interaction, and the structural integrity of the films. We used x-ray photoelectron spectroscopy to monitor the formation of MoO3 and possibly MoS3−xOx after exposure to air and to quantify the number of sulfur defects in the films. Atomic force microscopy measurements allow us to pinpoint the exact regions of oxidation and develop a dedicated low temperature heating procedure to remove oxidized species, leading to MoO3-free MoS2 films. AFM and Kelvin probe force microscopy show that the MoS2–Al2O3 substrate coupling is changed. The reduction in the MoS2–substrate coupling, combined with a preferential oxidation of sulfur vacancies, leads to a sevenfold increase in the PL intensity, and the ratio between trions and neutral excitons is changed. Our work highlights the importance of oxidized sulfur vacancies and provides useful methods to measure and manipulate their number in MoS2. Furthermore, changes in the MoS2–substrate interaction via sulfur vacancies and oxidation offer an elegant pathway to tune the optoelectronic properties of the two-dimensional films

Optoelectronic Properties of MoS2 in Proximity to Carrier Selective Metal Oxides

Transition metal dichalcogenides are an exciting class of new absorber materials for photovoltaic applications due to their unique optoelectronic properties in the single to few-layer regime. In recent years, these materials have been intensively studied, often utilizing conventional substrates such as sapphire and silicon dioxide on silicon. This study investigates the optical properties of molybdenum disulfide (MoS2) mono-, bi-, and multilayer films prepared by flake exfoliation and atomic layer deposition (ALD). These films are transferred to different photovoltaic relevant carrier-selective contacts, such as titanium oxide, titanium–titanium oxide, molybdenum oxide, and silicon-silicon dioxide reference substrates. Raman and photoluminescence (PL) spectra of single-crystalline exfoliated MoS2 flakes and ALD-grown MoS2 films on different substrates are compared in order to investigate the influence of the different contact materials on the corresponding optical transitions in MoS2. It is demonstrated that the different substrates influence the Raman and PL spectra of MoS2 layers due to doping and charge transfer effects, and similar effects are observed in both the exfoliated single-crystalline flakes and ALD-grown MoS2 layers

SiOxNy back-contact barriers for CZTSe thin-film solar cells.

The formation of molybdenum diselenide (MoSe2) is widely observed at the back-contact interface for copper zinc tin selenide (CZTSe) thin-film solar cells. Depending on individual selenium (Se) supply and thermal conditions for forming CZTSe absorbers on molybdenum (Mo) substrates, the thickness of MoSe2 can vary from a few hundreds of nanometers up to ≈ 1 μm, which is comparable to the commonly adopted thickness of 1 ~ 1.5 μm for CZTSe absorbers. In this study, for controlling the thickness of interfacial MoSe2, thin diffusion barrier layers of silicon oxynitride (SiOxNy) are deposited onto Mo layers prior to the growth of CZTSe absorbers in the fabrication process. As a result, a reduction in the thicknesses of MoSe2 layers is achieved. In terms of energy conversion efficiency (η), CZTSe solar cells grown on Mo/SiOxNy back contacts suffer a deterioration as the SiOxNy layers get thicker. CZTSe solar cells grown on Mo/SiOxNy/Mo back contacts preserve their efficiencies at ≈ 11% with thin 10 nm SiOxNy layers