Kesterite Solar Cells Research Articles

An in-depth investigation on the grain growth and the formation of secondary phases of ultrasonic-sprayed Cu2ZnSnS4 based thin films assisted by Na crystallization catalyst

Up to date, the absorber low grain growth and the secondary compound formation remain the main factors causing the poor performance of kesterite solar cells based on spray pyrolysis derived Cu2ZnSnS4 (pure sulfide CZTS = PS-CZTS) compared with other deposition techniques. As the sodium (Na) inclusion is known to play a crucial role in affecting the grain growth, we evaluated in this report a thorough investigation in depth of the effect of Na doping content on crystallinity and phase segregation. The PS-CZTS was deposited by ultrasonic spray pyrolysis while the Na incorporation was carried out by soaking the pre-heated sprayed films in a solution of an environmentally-friendly NaCl salt. The change in the investigated parameters was followed throughout the absorber by scanning electron microscopy, Raman spectroscopy with different excitation wavelengths, grazing incidence X-ray diffraction (GIXRD) with various incidence angles, energy dispersive X-ray spectroscopy (EDS), secondary ion mass spectrometry (SIMS) and UV–Vis spectrometry. Increasing Na content significantly improves the crystallinity and the compactness simultaneously, resulting in smoothest surfaces with grains up to 2 µm in size. The Na was uniformly diffused leading to similar grain growth throughout the films. Whatever the Na content, the deposited materials are mostly consisting of ketserite-type PS-CZTS, whereas the use of different Raman excitations reveal the presence of Cu2S, ZnS and ZnO as secondary compounds in trace forms. The depth profile analysis with GIXRD showed that no further phase segregation has occurred while the EDS and SIMS results showed uniform distribution of PS-CZTS′ constituents along the surface and within the deposited films. By combining Raman, GIXRD and EDS analysis, the secondary compounds were found to be mostly gathered in the upper absorber layer. The absorption coefficient and the band gap range around 104 cm−1 and 1.55 eV respectively, the optimal values required for photovoltaic energy conversion.The Na doping developed in this study has a promising potential to yield high quality sprayed absorbers and opens up access to opportunities in enhancing of sprayed PS-CZTS thin film solar cell performances.

The effect of Cu-Zn disorder on charge carrier mobility and lifetime in Cu2ZnSnSe4

Cu-Zn disorder is one possible origin for the limited efficiencies of kesterite solar cells and its impact on the band gap and band tails have been intensively studied. However, the effect on charge transport and recombination, which are key properties for solar cells, has not been investigated so far. Therefore, we probe the impact of the Cu-Zn order on charge carrier mobility and lifetime. To this end, we change the Cu-Zn order of a co-evaporated Cu2ZnSnSe4 thin film by sequential annealing and probe the impact by time-resolved terahertz spectroscopy. Aside from of the well-known band gap shift, we find no significant change in mobility and lifetime with Cu-Zn order. This finding indicates that Cu-Zn disorder is not limiting efficiencies of kesterite solar cells at their current status by means of charge carrier recombination and transport.

Improvement of efficiency in CZTSSe solar cell by using back surface field

A kesterite CZTS based solar cell is studied with back surface field. In order to achieve this, a p+ layer of SnS is added to the standard cell configuration. The resulting p+-p-n configuration of SnS/CZTS/CdS is studied using Solar Cell Capacitance Simulator (SCAPS). Low Voc is responsible for lower efficiencies of kesterite solar cells. The proposed p+-p-n structure increases the Voc and thus the efficiency. Simulation of CZTSSe and CZTS with back surface p+ layer has shown considerably higher efficiency than the highest reported value for kesterite cell of CZTSSe from 12.3% to 15.7%. A comparative summary is presented with literature reported performance and our simulated performance for kesterite cell.

The alterations of carrier separation in kesterite solar cells

Cu2ZnSn(S,Se)S4 (CZTSSe) thin films have attracted attention as low-cost absorber materials for solar cells; however, further studies are required to develop flexible solar cells from this material and to achieve a high power conversion efficiency. Toward this objective, this work investigated eight types of precursors applied on flexible Mo foil substrates, some of which also contained a layer of NaF. Secondary phases, defects, and defect clusters were different in the various samples, and the surface electrical characteristics of the CZTSSe absorber layer varied accordingly. In contrast to those in the CIGS-based cells, defects and defect clusters generated in the CZTSSe absorber layer caused an upward band bending-like band structure to form at the grain boundaries (GBs), thereby forming an intra-grain (IG) current path. By improving carrier separation, a flexible CZTSSe thin-film solar cell was developed on a Mo foil substrate with a power conversion efficiency of 7.04%. Thus, the efficiency of CZTSSe thin-film solar cells could be increased through carrier separation measures that enabled the collection of holes toward the GBs and of electrons toward IGs.

Passivation and thickness control of highly efficient kesterite solar cells

Kesterite Cu2ZnSn(SxSe1−x)4 (CZTSSe) is an attractive photovoltaic absorber material because of its tunable bandgap, earth abundance, and low toxicity. However, efficiency and open circuit voltage remain significantly below theoretical limits. We recently showed that back-contact engineering with MoO3/Au on exfoliated vapor-deposited kesterite solar cells can improve device performance. Here, we demonstrate more promising results, which translate into high power conversion efficiencies of up to 12.2% for solution-deposited CZTSe with thicknesses as low as 1.1 μm. Time-resolved terahertz spectroscopy of exfoliated films showed significantly faster recombination at the back surface than at the front. When atomic layer deposited Al2O3 was used to passivate the exposed back surface of exfoliated films, front and back surfaces showed nearly identical recombination dynamics. After thermally depositing high work function MoO3 and reflective Au as the back contact on the Al2O3-passivated absorber, we obtained devices with efficiencies of up to 11.6%. Applying the same strategy of exfoliating working devices and engineering the back contact resulted in efficiencies of up to 12.2% for passivation with a 10 nm layer of Se instead of Al2O3. Further development of such passivation and back-contact engineering approaches may lead to higher efficiency devices with absorber thicknesses below 1 μm.

Comparing strategies for improving efficiencies in vacuum processed Cu2ZnSnSe4 solar cells

In this study, we detail a Cu2ZnSnSe4 based solar cell fabrication process based on the selenization of metallic precursor stacks with elemental Se. 9.4% efficient devices without antireflection coating have been obtained. First, reproducibility issues of the process are carefully shown and discussed. It is demonstrated that device performances are strongly impacted by the precise control of the precursor composition. Then, starting from this robust process, a review of existing strategies to improve kesterite efficiencies is conducted. A significant increase in efficiency (+1.4% absolute efficiency and +50 mV VOC) is obtained with absorber surface treatment and post-annealing, while no effect of Ge incorporation in the precursor stack is observed. This contradictory result to most of the recent publications raises the question of the universality of this strategy to improve kesterite solar cell performance. Finding a universal activation step to boost kesterite efficiencies and bring it to the market remains a crucial need for the community.

Fabrication of high band gap kesterite solar cell absorber materials for tandem applications

Fabrication of high band gap kesterite solar cell absorber materials for tandem applications

Sulfurization of co-evaporated Cu2ZnSnSe4 thin film solar cells: The role of Na

The sulfurization of co-evaporated Cu2ZnSnSe4 (CZTSe) thin films is studied. In this work, a relationship between the Na concentration and the further S incorporation in the CZTSe absorber layer is found. Na is added by a NaF precursor layer before CZTSe co-evaporation and/or by diffusion from the soda-lime glass substrate. Higher Na concentrations result in bigger grain sizes, higher S/(S + Se) atomic ratios and photovoltaic devices with higher VOC, as investigated by SEM, GIXRD, Raman and I-V measurements. Results using an alternative substrate as ceramic verify the importance of the control of the Na content, which alters the carrier concentrations and elements distribution with much higher S and Na concentrations at the surface in this particular case. By using this methodology, it is possible to tune the kesterite band gap energy by the creation of S-gradient through the absorber thin film. This may provide a new approach for the control of Cu2ZnSn(S,Se)4 growth and development of high efficiency kesterite solar cells. Efficiencies of 5.5% and 6.4% are obtained using ceramic and SLG substrates respectively (without antireflection coatings and without grids).

Fabrication of high band gap kesterite solar cell absorber materials for tandem applications

Using the thermal annealing of evaporated metallic precursors in successive H2Se and H2S atmospheres, it was possible to reproducibly manufacture kesterite absorber material for solar cell applications with a sulfur content varying from 30% to 100%. Respective band gaps for these sulfur inclusions were measured at approximately 1.45 eV and 2.0 eV. A recipe was devised for which results could be reproduced within an error margin of ±5% and the influence of the H2S pressure during the post sulfurization was negligible on all measurable and observable parameters. The evolution of the S/Se ratio in the sample was observed to be linearly dependent on the annealing time. It was also observed that at very early stages of the post-sulfurization, both the original Cu2(Zn,Ge)Se4 (CZGSe) and a primary Cu2(Zn,Ge)(S,Se)4 (CZGSSe) phase with a sulfur inclusion of ~30% coexist in the sample. The (112) x-ray diffraction (XRD) reflection of the CZGSe phase progressively disappears in favor for the first mixed CZGSSe phase. Using grazing incidence-XRD, the S/Se ratio was shown to be inhomogeneous. Indeed, the XRD measurement of the top layers led to the calculation of higher sulfur inclusions than was the case when measuring the bulk material. Top-scanning electron microscopy (SEM) as well as cross-SEM measurements were taken in order to determine the impact of the sulfur inclusion on the crystal growth and the overall quality of the produced absorber layers. The obtained images revealed a reduction in crystal size and the appearance of numerous holes in the layer as the S/Se ratio is increased.

The Role of Hydrogen from ALD‐Al2O3 in Kesterite Cu2ZnSnS4 Solar Cells: Grain Surface Passivation

AbstractIn this research, a new route of surface passivation is reported by introducing hydrogen from the atomic layer deposited (ALD) Al2O3 layer into pure sulfide Cu2ZnSnS4 (CZTS) solar cells. Different amounts of hydrogen are incorporated into the Cu2ZnSnS4/CdS interface through controlling the thickness of the ALD‐Al2O3 layer. The device with three cycles of ALD‐Al2O3 yields the highest efficiency of 8.08% (without antireflection coating) with improved open‐circuit voltage of up to 70 mV. With closer examination on the passivation route of ALD‐Al2O3, it is revealed by the surface chemisty study that the Al2O3 can be etched away by ammonium hydroxide in the CdS buffer deposition process. Instead, the hydrogen is detected within a shallow depth from the CZTS surface, and makes a significant difference in the measured distribution of contact potential difference and device performance. This may be interpreted by the effect of hydrogen passivation of the CZTS surface by curing dangling bonds at the surface of CZTS grains. This work may provide a new direction of further improving the performance of kesterite solar cells.

Pre-annealing of metal stack precursors and its beneficial effect on kesterite absorber properties and device performance

A variety of approaches is being used to fabricate kesterite absorbers, such as sputtering, co-evaporation and solution based techniques. Annealing at high temperatures is a common processing step to stimulate elemental mixing and grain growth. This study investigates the effect of pre-annealing of metal stack precursors at 150 °C, 200 °C, 300 °C and 450 °C on kesterite absorber and solar cell properties. Structural, morphological and vibrational properties of the thin films were investigated. It was found that pre-annealing at 450 °C exhibited a structural mixture distinct from precursors annealed at lower temperatures. The absorber showed improved thickness uniformity, compositional surface homogeneity and absence of Sn-S phases. Lower temperatures resulted in Sn-S compounds in the absorber. In situ XRD is used to monitor phase transitions during pre-annealing. It is shown that implementing a pre-annealing step can improve the absorber properties and the device efficiency. The best performing solar cell had a 4.02% efficiency and was achieved by pre-annealing the precursor layer at 450 °C.

Role of electron-phonon coupling and thermal expansion on band gaps, carrier mobility, and interfacial offsets in kesterite thin-film solar cells

The efficiencies of solar cells based on kesterite Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) are limited by a low open-circuit voltage due to high rates of non-radiative electron-hole recombination. To probe the origin of this bottleneck, we calculate the band offset of CZTS(Se) with CdS, confirming a weak spike of 0.1 eV for CZTS/wurtzite-CdS and a strong spike of 0.4 eV for CZTSe/wurtzite-CdS. We also consider the effects of temperature on the band alignment, finding that increasing temperature significantly enhances the spike-type offset. We further resolve an outstanding discrepancy between the measured and calculated phonon frequencies for the kesterites, and use these to estimate the upper limit of electron and hole mobilities based on optic phonon Fröhlich scattering, which uncovers an intrinsic asymmetry with faster (minority carrier) electron mobility.

Publisher's Note: “Enhancement of the conversion efficiency of thin film kesterite solar cell” [J. Renewable Sustainable Energy 10, 033501 (2018)

Publisher's Note: “Enhancement of the conversion efficiency of thin film kesterite solar cell” [J. Renewable Sustainable Energy <b>10</b>, 033501 (2018)

Enhancement of the conversion efficiency of thin film kesterite solar cell

C u 2 ZnSn S 4 ( CZTS ) is a non-toxic earth abundant material and a promising quaternary semiconductor compound of groups I−II−IV−VI having a kesterite symmetrical structure. Due to its optimum direct bandgap, it has been considered as a suitable material for absorber layers for photovoltaic cell applications. This paper presents the numerical simulation and modeling of CZTS based thin film kesterite photovoltaic cells using SCAP-1D software. The influence of device parameters such as the carrier concentration, thickness, densities of absorber, buffer and window layers, defect densities and the temperature effect on the performance of the ZnO/CdS/CZTS/Mo photovoltaic cell structure are analyzed. Defect densities are added to the absorber layer and the interface between the buffer layer and the absorber layer. This type of solar cell does not comprise any toxic material and can lead to non-toxic thin film photovoltaic cells with outstanding optical properties. In this work, promising optimized results had been achieved with a conversion efficiency of 23.72%, a fill factor of 82.54%, a short-circuit current (Jsc) of 44.87 mA/cm2, and an open circuit voltage (Voc) of 0.64V.

Analysis of Failure Modes in Kesterite Solar Cells

Intense research has been carried out in the past few years to improve efficiencies and understand limitations in kesterite-based solar cells. Despite notable efforts to determine and list the different failure modes affecting the photovoltaic properties of these devices, very few works have tried to quantify and classify the effects of these failure modes. In this study, an exhaustive literature review has first been conducted to determine the different causes leading to limited efficiencies in kesterite devices, with an additional focus on cadmium-free and critical raw material-free devices. Second, an original approach has been employed to quantify the impact of these failure modes on solar cells, based on the evaluation of feedback from 18 scientific experts working on kesterite technology. The result of this survey is analyzed, which allows us to determine what should be the research priority for the community to improve efficiencies and drive kesterite technology to the market.

Decoupling of optoelectronic properties from morphological changes in sodium treated kesterite thin film solar cells

Sodium is typically used during the synthesis of kesterite thin films to enhance the performance of solar cells. As sodium tends to affect grain growth and morphology, it is difficult to analyse solely the electronic effects of sodium as dopant. To decouple the structural and electronic effects from each other, two processes were designed in this work to successfully incorporate sodium into a vacuum-processed Cu2ZnSnSe4 absorber without changing the morphology. A thin layer of NaF is deposited before precursor deposition (Pre-NaF) or after absorber synthesis to undergo a post deposition treatment (NaF-PDT). While composition and distribution of matrix elements remain unchanged, the sodium concentration is increased upon sodium treatment up to 140 ppm as measured by inductively coupled plasma mass spectrometry. X-ray photoelectron spectroscopy showed that the surface composition was not altered. Within its detection limit, sodium was not present at the absorber surface. For a Pre-NaF sample measured with atom probe tomography a sodium concentration of 30 ppm was measured in a grain, suggesting that sodium might segregate at grain boundaries. The additional sodium content in the film leads to an increased acceptor concentration, which results in improved open-circuit voltage and fill factor.

Effect of CZTSe BSF and minority carrier life time on the efficiency enhancement of CZTS kesterite solar cell

Cu2ZnSnS4(CZTS) is non-toxic earth abundant material and a promising quaternary semiconductor compound of group I-II-IV-VI having kesterite symmetrical structure. Due to its optimum direct band gap, it has been considered suitable material for absorber layer for photovoltaic solar cell applications. This paper presents numerical modeling of experimentally designed CZTS solar cell by applying CZTSe as back surface field (BSF) layer. In this work we reproduced results for experimentally designed CZTS solar cell in SCAPS software. The simulated results are validated and compared with real experimental results. After optimization of cell parameters conversion efficiency of an optimized device is increased up to 6.42% (with CZTS thickness of 0.8 μm). With further optimization and applying CZTSe BSF layer conversion efficiency increased up to 12.92%. Under 1.5 AM illumination condition from the proposed cell promising results had been achieved with short circuit current (Jsc) 27.41 mA/cm2, open circuit voltage (Voc) 744.4 mV, fill factor (FF) 63.35% and corresponding to a total area conversion efficiency of 12.92%.

Numerical modeling baseline for high efficiency (Cu2FeSnS4) CFTS based thin film kesterite solar cell

Cu2FeSnS4(CFTS) is auspicious nontoxic and earth abundant semiconductor compound having kesterite symmetrical structure. It is an attractive and suitable material for the fabrication of low cost, high efficiency and sustainable thin film photovoltaic cell. CFTS based kesterite photovoltaic cell device modeling was performed in this work. The influence of device parameters such as the thickness, acceptor and donor carrier concentration densities of absorber and electron transport layer (ETL), effect of back contact metal work function and the temperature effect on the performance of CFTS based kesterite photovoltaic cell is analyzed by using one dimensional solar cell capacitance simulator (SCAPS) software. In this work, promising optimized results had been achieved with the conversion efficiency of 19.97%, fill factor (FF) 85.94%, short-circuit current (Jsc) 23.37 mA/cm 2 and open circuit voltage (Voc) 0.995 V. The above results will give imperative baselines and feasible directions for the fabrication of higher efficiency CFTS based photovoltaic cell.

Role of intermediate metallic sub-layers in improving the efficiency of kesterite solar cells: concept and optimization

In this work, versatile CdS/Cu2ZnSnS4(CZTS) solar cell designs based on intermediate metallic sub-layers (Au, Ti, and Ag) engineering are proposed for enhancing light-scattering behavior and reducing recombination losses. The idea behind this work is to generate optical confinement regions in the CZTS absorber layer to achieve an improved absorption and appropriate antireflection effects. Moreover, the ultra-thin metal at the CZTS/Mo interface can be helpful for reducing the series resistance, where it behaves like a blocking layer for the Sulfur diffusion. We further combine the proposed designs with Particle Swarm Optimization (PSO)-based approach to achieve broadband absorption and boost the conversion efficiency. It is found that the optimized design with Ti sub-layer improves the CZTS solar cell properties, where it yields 31% improvement in short-circuit current and 60% in the power efficiency over the conventional one. Therefore, the optimized designs provide the opportunity for bridging the gap between improving the optical behavior and reducing the recombination losses.

Cation Substitution in Earth-Abundant Kesterite Photovoltaic Materials.

As a promising candidate for low‐cost and environmentally friendly thin‐film photovoltaics, the emerging kesterite‐based Cu2ZnSn(S,Se)4 (CZTSSe) solar cells have experienced rapid advances over the past decade. However, the record efficiency of CZTSSe solar cells (12.6%) is still significantly lower than those of its predecessors Cu(In,Ga)Se2 (CIGS) and CdTe thin‐film solar cells. This record has remained for several years. The main obstacle for this stagnation is unanimously attributed to the large open‐circuit voltage (V OC) deficit. In addition to cation disordering and the associated band tailing, unpassivated interface defects and undesirable energy band alignment are two other culprits that account for the large V OC deficit in kesterite solar cells. To capture the great potential of kesterite solar cells as prospective earth‐abundant photovoltaic technology, current research focuses on cation substitution for CZTSSe‐based materials. The aim here is to examine recent efforts to overcome the V OC limit of kesterite solar cells by cation substitution and to further illuminate several emerging prospective strategies, including: i) suppressing the cation disordering by distant isoelectronic cation substitution, ii) optimizing the junction band alignment and constructing a graded bandgap in absorber, and iii) engineering the interface defects and enhancing the junction band bending.