Kesterite Layers Research Articles

High efficiency titanium nitride bowtie nanoparticle and upconverter layer incorporated kesterite tandem solar cell

Thin-film photovoltaic (PV) technology offers an economically viable and flexible way to harness solar energy. Using a material that absorbs more solar light than silicon significantly reduces material consumption and costs. Solar cells based on the kesterite mineral structure and its alloys stand out from other thin-film solar cells due to their earth-abundance and non-toxicity. Incorporating plasmonic nanoparticles in the kesterite absorber layer opens a promising path of enhancing light trapping efficiency. Again, the main losses in solar cells result from the incomplete utilization of the solar spectrum. The otherwise unused sub-bandgap photons can be utilized by adding an upconverting layer on a solar cell. This paper demonstrates a theoretical design and performance analysis of a high-efficiency kesterite solar cell by incorporating titanium nitride (TiN) plasmonic bowtie nanoparticles (NPs) and an upconverter layer. We compared the optical characteristics of various shapes and sizes of nanoparticles to find the optimum solar cell efficiency. The 100 nm thick bowtie-shaped NP had the highest power conversion efficiency, η of 24.80% and had the open circuit voltage Voc=0.59V, and short circuit current density, Jsc = 45.43 mA/cm2. The absorbed power for bowtie-shaped TiN NP incorporation enhanced the average absorption from ∼61% to ∼88% for the kesterite layer. For 100 nm thickness, the absorption enhancement, g, was greater than 1 for the whole spectral range, and the maximum value was ∼1.2. Incorporating a yttrium aluminum garnet upconverter layer increased the solar cell efficiency to ∼27.44.%.

Effect of Synthesis Approach on Characteristics of CZTS Kesterite Layers for Photovoltaic Applications

Effect of Synthesis Approach on Characteristics of CZTS Kesterite Layers for Photovoltaic Applications

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.

Optimization of sulphurization temperature for the production of single-phase CZTS kesterite layers synthesized by electrodeposition

ABSTRACT Cu2ZnSnS4 films were grown onto FTO/glass substrate by electrodeposition. The influence of sulphurization temperature on CZTS properties was examined using XRD, Raman spectroscopy, SEM, optical transmittance and electrical resistivity measurements. The film sulphurized at 400°C exhibited CuS as the major phase mixed with CZTS and SnS2 phases. However, the films sulphurized at 450 and 500°C are mainly composed of the CZTS kesterite phase with SnS2 and CuS as secondary phases. Further sulphurization temperature increase up to 550°C led to the complete disappearance of CuS and SnS2 phases and the obtained film is a pure CZTS kesterite mono-phase. The Raman spectra exhibit a line centred at 334 cm−1; it is the most intense recorded in the spectra of the films sulphurized at 500 and 550°C. The film sulphurized at 550 °C, had an ideal band gap of 1.40 eV and electrical resistivity of (41.4 ± 5.5) Ω cm.

Influence of Zn excess on compositional, structural and vibrational properties of Cu2ZnSn0.5Ge0.5Se4 thin films and their effect on solar cell efficiency

The effect of Zn content on compositional, structural and vibrational properties of Cu2ZnSn1-xGexSe4 (CZTGSe, x ~ 0.5) thin films is studied. Kesterite layer is deposited by co-evaporation onto 5 × 5 cm2 Mo/SLG substrate followed by a thermal treatment at maximum temperature of 480 °C, obtaining areas with different composition and morphology which are due to the sample position in the co-evaporation system and to the non-uniform temperature distribution across the substrate. Kesterite layers with higher Zn amounts are characterized by lower Cu and Ge contents; however, a uniform Ge distribution through the absorber layer is detected in all cases. The excess Zn concentration leads to the formation of ZnSe secondary phase on the surface and in the bulk of the absorber as determined by Raman spectroscopy. When higher Ge content and no ZnSe are present in the absorber layer, a compact structure is formed with larger grain size of kesterite. This effect could explain the higher Voc of the solar cell. The Zn content does not affect the bandgap energy significantly (Eg near 1.3 eV), although the observed effect of Zn excess in CZTGSe results in a decreased device performance from 6.4 to 4.2%. This investigation reveals the importance of the control of the off-stoichiometric CZTGSe composition during the deposition process to enhance solar cells properties.

High‐Efficiency (LixCu1−x)2ZnSn(S,Se)4 Kesterite Solar Cells with Lithium Alloying

AbstractThe performance‐boosting effect of alkali treatments is well known for chalcogenide thin‐film solar cells based on Cu(In,Ga)Se2 (CIGS) and Cu2ZnSn(S,Se)4 (CZTSSe–kesterite) absorbers. In contrast to heavier alkali elements, lithium is expected to alloy with the kesterite phase leading to the solid solution (LixCu1−x)2ZnSn(S,Se)4 (LCZTSSe), which offers a way of tuning the semiconductor bandgap by changing the ratio Li/(Li+Cu). Here is presented an experimental series of solution‐processed LCZTSSe with lithium fraction Li/(Li+Cu) ranging from x = 0 to 0.12 in the selenized absorber as measured by means of inductively coupled plasma mass spectrometry. The proportional increase in lattice parameter a and bandgap from 1.05 to 1.18 eV confirms the lithium alloying in the kesterite phase. Increase in grain size is observed for x up to 0.07, whereas a higher lithium fraction leads to a porous absorber morphology due to an inhomogeneous distribution of Li‐containing compounds in the kesterite layer. An increase of the photoluminescence quantum yield is observed as the Li fraction increases in the absorber layer. A champion device exhibits a remarkable efficiency of 11.6% (12.2% active area) for x = 0.06, close to the world record value of 12.6% demonstrating the effectiveness of lithium alloying.

Complex Interplay between Absorber Composition and Alkali Doping in High‐Efficiency Kesterite Solar Cells

AbstractSodium treatment of kesterite layers is a widely used and efficient method to boost solar cell efficiency. However, first experiments employing other alkali elements cause confusion as reported results contradict each other. In this comprehensive investigation, the effects of absorber composition, alkali element, and concentration on optoelectronic properties and device performance are investigated. Experimental results show that in the row Li–Na–K–Rb–Cs the nominal Sn content should be reduced by more than 20% (relative) to achieve the highest conversion efficiency. The alkali concentration resulting in highest device efficiencies is lower by an order of magnitude for the heavy alkali elements (Rb, Cs) compared to the lighter ones (Li, Na, K). Utilization of a wide range of characterization techniques helps to unveil the complex interplay between absorber composition and alkali doping. A ranking of alkali for best device performances, when employing alkali treatment, resulted in the order of Li > Na > K > Rb > Cs based on the statistics of more than 700 individual cells. Finally, a champion device with 11.5% efficiency (12.3% active area) is achieved using a high Li concentration with an optimized Sn content.

Inkjet-printed CZTSSe absorbers and influence of sodium on device performance

Inkjet printing is a promising route for thin film deposition due to its low material wastage and potential of high throughput production. In this work, the inkjet printing technique is used for the deposition of Cu2ZnSn(SxSe1−x)4 (CZTSSe) thin films from Cu-Zn-Sn-S molecular inks. The key step for using the inkjet printing technique is to formulate a stable ink with suitable viscosity. The ink was prepared by dissolving Cu, Sn, and Zn metal salt precursors, thiourea and different amount of NaF (varied from 0M to 1.12M) in dimethyl sulfoxide. NaF was added to verify the influence of sodium (Na) in ink formation and CZTSSe device properties. It was found from the kesterite structure analysis that increasing NaF leads to segregation of a NaCl phase in the resulting CZTSSe films. Cross-sectional morphologies of the kesterite layer as revealed by scanning electron microscopy showed an enhancement from a multi-layered sandwich type to a double-layered structure by adding NaF. The photoelectron conversion efficiencies of solar cells, with a glass/Mo/CZTSSe/CdS/ZnO/AZO/Ni-Al configuration, increased from 1.7% to 6.4% with increasing NaF concentration from 0M to 0.144M and thereafter decreasing at higher NaF concentration (≥ 0.288M).

Structure, optical, electrical and electronic parameters of a new thin film composition for kesterite solar cell

The work is devoted to the development of a new thin film composition for kesterite solar cell with the base Cu2ZnSnS4 (CZTS) layer obtained by the combination of two low-cost liquid methods, namely electrodeposition and Successive Ionic Layer Adsorption and Reaction (SILAR). The p-CZTS absorber is fabricated by the creation of the main Cu2ZnSnS4 layer through sulfurization of a stack of the electrodeposited Cu/Sn/Zn metal films. The pores in this main Cu2ZnSnS4 layer were filled by SILAR-deposited nanocrystalline kesterite layer. For a creation of semiconductor heterojunction n-ZnS/p-Cu2ZnSnS4 we prepared ZnS thin films by SILAR method. The work shows the results of analysis of diode characteristics of the developed ZnS/Cu2ZnSnS4 heterostructures (series resistance Rs, shunt resistance Rsh, ideality factor nd and the saturation current densities Jo). Other investigated electrical and electronic parameters obtained from the current-voltage (I-U) and capacitance-voltage (C-U) characteristics are the electrical resistivities ρCZTS and ρZnS, the ZnS/Cu2ZnSnS4 rectifying barrier Φ, the height φ of symmetrical double Schottky barriers in the grain boundary regions in the polycrystalline kesterite layers, the width of the electron depletion regions ω, the concentration of the fully ionized donor impurity Nd in n-ZnS and acceptor impurity Na in p-Cu2ZnSnS4, the density NSS of the surface states in the kesterite layers. Structure and optical properties of the ZnS films as such as of the individual kesterite layers and of their combination have been also researched.

Assessment of crystal quality and unit cell orientation in epitaxial Cu₂ZnSnSe₄ layers using polarized Raman scattering.

We use polarization-resolved Raman spectroscopy to assess the crystal quality of epitaxial kesterite layers. It is demonstrated for the example of epitaxial Cu₂ZnSnSe₄ layers on GaAs(001) that "standing" and "lying" kesterite unit cell orientations (c'-axis parallel / perpendicular to the growth direction) can be distinguished by the application of Raman tensor analysis. From the appearance of characteristic intensity oscillations when the sample is rotated one can distinguish polycrystalline and epitaxial layers. The method can be transferred to kesterite layers oriented in any crystal direction and can shed light on the growth of such layers in general.

Thermal treatments and characterization of CZTS thin films deposited using nanoparticle ink

The structural, optical, and optoelectronic properties of copper zinc tin sulfide (CZTS) films, deposited by a nonvacuum nanoparticle based approach were studied as a function of different annealing temperatures. The CZTS films for photovoltaic applications were deposited using the doctor blading method using an ink prepared with nanoparticles synthesized using the solvothermal method. Deposited films were annealed at different temperatures in N2–S atmosphere. The films were characterized using different tools such as X-ray diffraction, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDXS), ultraviolet–visible spectroscopy, Raman spectroscopy, and photoconductivity. The results showed that the kesterite phase was formed in the temperature range between 400 and 550 °C. At temperatures beyond 500 °C, many peaks of binary and ternary phases were detected, probably because of the decomposition of the kesterite layer. The cross section SEM images showed that the film is compact; however, there are isolated voids. The EDXS estimated chemical composition was found to vary with annealing temperature; the nearly stoichiometric film was obtained when annealed at 450 °C. The optical band gap of the stoichiometric film was 1.6 eV, and it showed photoconductivity.

Recent trends in direct solution coating of kesterite absorber layers in solar cells

This review describes a specific group of non-vacuum methods for depositing kesterite solar cell absorbers, which we refer to as direct solution coating (DSC). These methods work by coating a layer of precursor-containing solution on the surface of a substrate followed by appropriate thermal processing to yield the desired kesterite Cu2ZnSn(S,Se)4 phase. General aspects, challenges and future prospects of the DSC are discussed in detail. Two research highlights are presented in which kesterite layers are processed from solutions of metal salts in simple non-toxic organic solvents to yield up to 7.5% efficient solar cells.

Formula omitted] solar cell absorbers spin-coated from amine-containing ether solutions

A non-vacuum, solution-based method to deposit thin layers of Cu2ZnSnSe4 (CZTSe) as absorber layer for kesterite-based thin film solar cells was investigated. The concept is based on multiple spin-coated layers of ether solutions containing Cu, Zn, and Sn salts as precursors and monoethanolamine (MEA) as stabilizer, which are subsequently converted into CZTSe layers by annealing in selenium atmosphere. The influence of initial metal ratios and the number of spin-coated layers on the composition and homogeneity of the CZTSe layers was studied. Both zinc and, to a smaller extent, tin contents tend to decrease in selenized CZTSe layers, because of the partial evaporation of their volatile compounds during the selenization process. Using optimized metal ratios in the precursor solution, single phase kesterite layers of 1–2μm thickness were obtained. Increasing the number of spin-coating cycles caused the formation of “bubbles” between the CZTSe and the Mo layer, as a result of the compressive stress originating from the volume expansion during the selenization step. SEM micrographs and EDX compositional measurements of cross-sections also revealed a thin carbon-rich layer at the Mo back contact/CZTSe interface, which nevertheless provided a sufficient electrical contact between the metal contact and the absorber. The optical energy gap of the kesterite absorber determined from a quantum efficiency measurement is about 0.9eV. Solar cells with efficiencies of up to 2.76%, with Voc of 381mV, Jsc of 15.8mA/cm2 and fill factor of 42.1% were achieved for copper poor and zinc rich CZTSe absorbers.