Kesterite Thin-film Solar Cells Research Articles

Lattice Dynamics of Cu2ZnSn(S x ,Se1-x )4 Kesterite Thin-Film Solar Cells Studied by Nuclear Inelastic Scattering.

Phonons play a crucial role in thermalization and non-radiative recombination losses in semiconductors, impacting the power conversion efficiency of solar cells. To shed light on the lattice dynamics in Cu2ZnSn(S x ,Se1-x )4 (CZTSSe) thin-film solar cells and validate the extensive number of theoretical studies, we determine the 119Sn-partial phonon density of states (Sn-PDOS) by nuclear inelastic X-ray scattering. CZTSSe-based devices, one with near-stoichiometric and two with off-stoichiometric compositions, are investigated, and the results are correlated with the corresponding power conversion efficiencies (PCEs) of 3.2, 7.6, and 10.6%, respectively. Compared to the near-stoichiometric cell, the main Sn-PDOS peak of the off-stoichiometric cells broadens and slightly shifts to higher energy; this effect is correlated with the type and concentration of the characteristic defects in the studied samples. Furthermore, the Sn-PDOS of the 10.6% device is also obtained under operando (maximum power point) and open-circuit conditions. A comparison of the Sn-PDOS before and after the operando measurements suggests structural changes, likely due to the formation of metastable defects. In agreement with the theoretical studies, the Sn-PDOS of the CZTSSe absorber shows additional peaks compared to CZTSe attributed to coupling of Sn to the vibrations of Se and S atoms. This work paves the way for a further understanding of the lattice dynamics and subsequent enhancement of the PCEs of thin-film solar cells as well as other applied materials and devices containing elements that are Mössbauer-active and hence suitable for nuclear inelastic scattering.

Efficient window layer modification enabling the remarkable FF and efficiency improvement of kesterite solar cell

As the core structure of kesterite thin-film solar cell (TFSC), the nature of N-type functional layer and its interface is a key issue impacting device performance, typically its influence on carrier recombination and electron charge transportation. Here, a modified window layer to tailor N-type bulk and its interface properties for efficiency carrier collection and device performance is incorporated by co-sputtering ZnO and SiO2. Research has found that the modified window layer not only has an excellent wide optical spectrum with high transmittance, enabling additional photon harvesting and short-wave absorption, but also modulates the energy band position for better band alignment to facilitate charge transportation, ensuring higher conductivity. Simultaneously, the significantly reduced carrier recombination and increased uniformity of the device further accelerate charge efficient collection and enhance filling factor (FF). Consequently, the modified window layer of kesterite TFSC enables a remarkable efficiency improvement from 8.4% to 12.0% with a boosted FF from 61% to 73%. These novel results provide the inherent physical origin for improving the photovoltaic performance of kesterite TFSC by appropriate modification of ZnO:SiO2 window layer and open up an effective and feasible strategy, which can also be applied to other optoelectronic devices.

Critical review on the controllable growth and post-annealing on the heterojunction of the kesterite solar cells

Abstract Kesterite-structured solar cells have drawn significant attention due to their low-cost and environmental friendly composition. Recently, a remarkable certified power conversion efficiency (PCE) of 14.9% has been achieved, indicating a broader prospect for kesterite solar cells. However, this PCE is still far below the theoretical efficiency and the PCE of predecessor Cu(In,Ga)Se2 solar cells, which have been commercialized successfully. The relatively low device efficiency primarily originates from the unfavourable bulk and heterojunction of kesterite solar cell. Therefore, the achievement of high PCE in kesterite solar cells heavily relies on high-quality absorber layers and appropriate heterojunction contact. In this review, we first summarize the recent studies on the controllable growth of kesterite thin film. Based on different fabrication methods, various endeavors in revealing the reaction mechanism and manipulating the growth pathway of kesterite thin films have been introduced. Subsequently, studies related to the optimization of heterojunction by post-annealing process are also summarized. This simple and convenient approach can effectively enhance the heterojunction contact and promote the carrier transportation. Finally, this article discusses the future development strategy and perspectives towards achieving enhanced PCE in kesterite thin film solar cells

Improving device performance of sputtered CZTSe based solar cells by Manganese doping

Despite many efforts, Cu2ZnSnSe4 (CZTSe) based solar cells have remained relatively low efficiency compared to other thin film-based devices. One of the prevalent strategies used to improve cell performance in CZTSe is cation substitution. In this contribution, the effects of substituting Mn for Zn on CZTSe solar cell performance were systematically examined. Samples with various Mn content were grown by a two-stage method involving high temperature annealing of precursor layers comprising Cu, Zn, Sn, Mn and Se. All of the samples were found to have Cu-poor and (Zn + Mn)-rich or nearly stoichometric compositions. They crystalized in kesterite phase irrespective of the Mn content as confirmed by X-ray diffraction (XRD), Raman and X-ray photoelectron spectroscopy (XPS) measurements. The highest efficiency solar cell (5.56%) was obtained employing the 5% Mn doped CZTSe absorber. This film was found to have improved microstructure and reduced defect density. This is the first report on over 5% efficient kesterite thin film solar cell fabricated on a Cu2Mn0.05Zn0.95SnSe4absorber layer.

Comparative study on the effects of substitutional Ag and substitutional Ge on kesterite thin-film solar cells

Comparative study on the effects of substitutional Ag and substitutional Ge on kesterite thin-film solar cells

Surface recombination and peltier heat generation in Kesterite thin film solar cells due to defect increment under elevated heat

Thermal instability is the major barrier against the commercialization and upscaling of thin film solar cells. Heat generation in thin film solar cells occurs both at normal operation conditions and under stress conditions. In this paper, we developed a simulation model to investigate the heat generation in CZTSSe Kesterite thin film solar cells by coupling the optical, electrical, and thermal modules in the COMSOL simulation package. This strongly coupled model allows investigation of the two main heat generation factors in solar cell structure: Surface recombination heat and Peltier heat. These factors have been rarely analyzed in literature but can be significantly detrimental to the solar cell structure especially when the temperature of the cell rises over time under operation conditions. We extended our simulation to insert the defect growth at the Surface and bulk of the CZTSSe layer of the cell for elevated temperatures (400 °C. for 1000 h) and calculate the heat generation at CZTSSe/Mo(S,Se)2 and CZTSSe/CdS interfaces and at the deletion width of the junction where Surface recombination heat and Peltier heat generated from electrons and holes are generated. The simulation shows that Peltier heat generation from electrons is more dominant than Peltier heat from holes while the surface recombination heat is significantly more dominant compared to Peltier heat generation.

A critical review of solution-process engineering for kesterite thin-film solar cells: current strategies and prospects

The Cu2ZnSn(S,Se)4 (CZTSSe) material is considered a promising semiconductor material for commercial photovoltaic applications due to its high theoretical efficiency, high absorption coefficient, tunable direct bandgap, high element abundance, and low production cost.

Enhanced Carrier Collection in Cd/In-Based Dual Buffers in Kesterite Thin-Film Solar Cells from Nanoparticle Inks.

Increasing the power conversion efficiency (PCE) of kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells has remained challenging over the past decade, in part due to open-circuit voltage (VOC)-limiting defect states at the absorber/buffer interface. Previously, we found that substituting the conventional CdS buffer layer with In2S3 in CZTSSe devices fabricated from nanoparticle inks produced an increase in the apparent doping density of the CZTSSe film and a higher built-in voltage arising from a more favorable energy-band alignment at the absorber/buffer interface. However, any associated gain in VOC was negated by the introduction of photoactive defects at the interface. This present study incorporates a hybrid Cd/In dual buffer in CZTSSe devices that demonstrate an average relative increase of 11.5% in PCE compared to CZTSSe devices with a standard CdS buffer. Current density-voltage analysis using a double-diode model revealed the presence of (i) a large recombination current in the quasi-neutral region (QNR) of the CZTSSe absorber in the standard CdS-based device, (ii) a large recombination current in the space-charge region (SCR) of the hybrid buffer CZTSSe-In2S3-CdS device, and (iii) reduced recombination currents in both the QNR and SCR of the CZTSSe-CdS-In2S3 device. This accounts for a notable 9.0% average increase in the short-circuit current density (JSC) observed in CZTSSe-CdS-In2S3 in comparison to the CdS-only CZTSSe solar cells. Energy-dispersive X-ray, secondary-ion mass spectroscopy, and grazing-incidence X-ray diffraction compositional analysis of the CZTSSe layer in the three types of kesterite solar cells suggest that there is diffusion of elemental In and Cd into the absorbers with a hybrid buffer. Enhanced Cd diffusion concomitant with a double postdeposition heat treatment of the hybrid buffer layers in the CZTSSe-CdS-In2S3 device increases carrier collection and extraction and boosts JSC. This is evidenced by electron-beam-induced current measurements, where higher current generation and collection near to the p-n junction is observed, accounting for the increase in JSC in this device. It is expected that optimization of the heat treatment of the hybrid buffer layers will lead to further improvements in the device performance.

Thermal degradation and defect growth in CZTSSe photovoltaic devices

Thermal degradation of Kesterite thin film solar cells (CdS/CZTSSe) has been studied using three different time-dependent models which formulate the defect growth kinetics within the depletion width of CZTSSe/CdS junction. Generation and annihilation of a shallow acceptor defect at 0.55 eV under the stress of heating at 400 °C for 1000 h have been modeled. Three cases were modeled for defect growth rate by relating defect generation to electrons and hole density increment linearly (n) or quadratically (np or n2). The non-radiative recombination has been modeled for the three models and enabled the calculation of heat generation across the depletion width. The heat generation was also mapped through a coupled optical-electrical-thermal model in COMSOL for defect growth models at elevated stressing time. Heat generation is then stronger for the n-model and rises to double times higher by increasing the stressing time to 1000 h.

Cadmium-Free Kesterite Thin-Film Solar Cells with High Efficiency Approaching 12.

Cadmium sulfide (CdS) buffer layer is commonly used in Kesterite Cu2 ZnSn(S,Se)4 (CZTSSe) thin film solar cells. However, the toxicity of Cadmium (Cd) and perilous waste, which is generated during the deposition process (chemical bath deposition), and the narrow bandgap (≈2.4eV) of CdS restrict its large-scale future application. Herein, the atomic layer deposition (ALD) method is proposed to deposit zinc-tin-oxide (ZTO) as a buffer layer in Ag-doped CZTSSe solar cells. It is found that the ZTO buffer layer improves the band alignment at the Ag-CZTSSe/ZTO heterojunction interface. The smaller contact potential difference of the ZTO facilitates the extraction of charge carriers and promotes carrier transport. The better p-n junction quality helps to improve the open-circuit voltage (VOC ) and fill factor (FF). Meanwhile, the wider bandgap of ZTO assists to transfer more photons to the CZTSSe absorber, and more photocarriers are generated thus improving short-circuit current density (Jsc). Ultimately, Ag-CZTSSe/ZTO device with 10nm thick ZTO layer and 5:1 (Zn:Sn) ratio, Sn/(Sn+Zn): 0.28 deliver a superior power conversion efficiency (PCE) of 11.8%. As far as it is known that 11.8% is the highest efficiency among Cd-free kesterite thin film solar cells.

Energy Band Alignment by Solution-Processed Aluminum Doping Strategy toward Record Efficiency in Pulsed Laser-Deposited Kesterite Thin-Film Solar Cell.

Kesterite-based Cu2ZnSnS4 (CZTS) thin-film photovoltaics involve a serious interfacial dilemma, leading to severe recombination of carriers and insufficient band alignment at the CZTS/CdS heterojunction. Herein, an interface modification scheme by aluminum doping is introduced for CZTS/CdS via a spin coating method combined with heat treatment. The thermal annealing of the kesterite/CdS junction drives the migration of doped Al from CdS to the absorber, achieving an effective ion substitution and interface passivation. This condition greatly reduces interface recombination and improves device fill factor and current density. The JSC and FF of the champion device increased from 18.01 to 22.33 mA cm-2 and 60.24 to 64.06%, respectively, owing to the optimized band alignment and remarkably enhanced charge carrier generation, separation, and transport. Consequently, a photoelectric conversion efficiency (PCE) of 8.65% was achieved, representing the highest efficiency in CZTS thin-film solar cells fabricated by pulsed laser deposition (PLD) to date. This work proposed a facile strategy for interfacial engineering treatment, opening a valuable avenue to overcome the efficiency bottleneck of CZTS thin-film solar cells.

Ge-alloyed kesterite thin-film solar cells: previous investigations and current status – a comprehensive review

Ge alloying in kesterite thin films enables to mitigate electronic defect and disorder, enhance morphology as well as realize bandgap grading, all contributing to higher performance of complete solar cells via resolved Voc and fill factor deficits.

The effect of heat generation via interface recombination in CZTSSe kesterite thin film solar cells: A simulation study

Heat generation via Interface recombination in CZTSSe/CdS kesterite thin film solar cells has been investigated using a simulation model by coupling the thermal, electrical, and optical modules. Three interface defects (CuSn, CuZn, SnZn) located at different energy levels were considered. The defect closer to the midgap (CuSn at 0.55 eV) generate the highest heat power and increasing the defect density from 1014 cm−3 to 1018 cm−3 raised the interface heat generation from 60 W/cm2 to 80 W/cm2. This is nearly half of heat generation rate for CuZn defect located near the valence band. Hsurf raised from 60 W/cm2 to 74 W/cm2 by increasing the surface recombination velocity (S). This detailed simulation analysis recognizes the significant role of interface recombination and its contribution to heat generation and thermal stability in solar cells and demonstrates the importance of interface quality and interface engineering strategies in such devices.

Analysis of opto-electrical, trap complexity and defect density in Cu–Zn–Ge–S/Se kesterite thin-film solar cell

Kesterite photovoltaic have received a lot of attention for its cheap cost and high efficiency. But, their performance efficiency is less than 12%. Its performances are restricted by the formation of defects that reduce short-circuit current by creating deep traps which undergo recombination. An approach, replacing Sn with Ge as Cu–Zn–Ge–S/Se (CZGS)/(CZGSe) kesterite solar cell is investigated to reduce defect development. In this article, various loss mechanisms that impact on the performance of CZGS/Se solar cells are investigated and compared. The impact of electrical, optical and various loss strategies such as radiative and non radiative recombination is investigated in depth. Additionally, trap and defect state densities are also propagated to the absorber layer. In the presence of defect states, the efficiency of the kesterite solar cell is significantly damaged and degraded about 11%.

The Degradation of Cu2ZnSn(S,Se)4 Kesterite Thin Film Solar Cells Induced by Proton Radiation (Adv. Mater. Interfaces 26/2022)

Proton-Irradiated CZTSSe Solar Cells In article number 2201049, Yun Zhao, Yan Li, Xiuxun Han, and co-workers report the degradation of Cu2ZnSn(S,Se)4 kesterite thin film solar cells by 200 keV proton radiation. The radiation hardness, impact factors and loss mechanism are presented. It is expected to be advantageous for improving the performance of the kesterite solar cells.

The Degradation of Cu2ZnSn(S,Se)4 Kesterite Thin Film Solar Cells Induced by Proton Radiation

AbstractAiming at the current bottleneck of Cu2ZnSn(S,Se)4 (CZTSSe) thin film solar cells, an experimental analysis of the performance and loss mechanism of CZTSSe thin film solar cells upon proton radiation exposure is presented. In order to deduce the important cell parameters, the environment of the cells is simulated by subjecting the devices to 200 keV proton irradiation with fluences from 1 × 1010 to 1 × 1017 cm‐2. It is found that the damage constants are sensitive to the elemental diffusion at heterojunction and Na redistribution in device. The incidence of the proton can lead to bandgap, tails fluctuations, and Urbach energy changes in the cells those directly impact defects and structural disorder in CZTSSe material thereby the device performance. The time‐resolved photoluminescence results show that the carrier lifetime is also very sensitive to the distribution of interface elements, especially Na, Cd, and S. These findings may provide new ideas for solving the VOC deficit of CZTSSe solar cells. From this work, it can be also suggested that the CZTSSe thin film solar cells are robust to proton irradiation and have the potential for future space applications.

Enhancing Hole Density and Suppressing Recombination Centers through Illumination in Kesterite Thin Film Solar Cells.

Enhancing carrier density and increasing carrier lifetime are critical for the good performance of thin film solar cells. We apply illumination during the growth of kesterite Cu2ZnSnS4 (CZTS) to enhance hole density and suppress defects of nonradiative electron-hole recombination centers simultaneously. To examine the effect of the injected carriers generated by illumination, we first extend the scheme of detailed balance equations relating free carriers and defects beyond thermal equilibrium conditions by developing an extended Fermi level (EF') to characterize a homogeneous semiconductor with non-equilibrium carriers. On the basis of this scheme, we find that illumination can promote the formation of carrier-providing defects and suppress the formation of carrier-compensating defects. Then, we demonstrate that applying proper illumination during the growth of CZTS will help achieve a higher hole density and simultaneously suppress the formation of the SnZn antisite significantly, which are beneficial for the performance of CZTS solar cells.

Recovery Mechanisms in Aged Kesterite Solar Cells.

For successful long-term deployment and operation of kesterites Cu2ZnSn(SxSe1–x)4 (CZTSSe) as light-absorber materials for photovoltaics, device stability and recovery in kesterite solar cells are investigated. A low-temperature heat treatment is applied to overcome the poor charge extraction that developed in the natural aging process. It is suggested that defect states at aged CZTSSe/CdS heterojunctions were reduced, while apparent doping density in the CZTSSe absorber increased due to Cd/Zn interdiffusion at the heterojunction during the annealing process. In situ annealing experiments in a transmission electron microscope were used to investigate the elemental diffusion at the CZTSSe/CdS heterojunction. This study reveals the critical role of heat treatment to enhance the absorber/Mo back contact, improve the quality of the absorber/buffer heterojunction, and recover the device performance in aged kesterite thin-film solar cells.

Multi-modal characterization of kesterite thin-film solar cells: experimental results and numerical interpretation

We report a multi-modal study of the electrical, chemical and structural properties of a kesterite thin-film solar cell by combining the spatially-resolved X-ray beam induced current and fluorescence imaging techniques for the evaluation of a fully functional device on a cross-section. The data allowed the correlation of the chemical composition, defects at interfaces and inhomogeneous deposition of the layers with the local charge-collection efficiency of the device. We support our observations with Monte Carlo simulations of high-energy X-ray interactions with the semiconductor device, and finite-volume modeling of the charge-collection efficiency.

Large-Grain Spanning Monolayer Cu2 ZnSnSe4 Thin-Film Solar Cells Grown from Metal Precursor.

The persistent double layer structure whereby two layers with different properties form at the front and rear of absorbers is a critical challenge in the field of kesterite thin-film solar cells, which imposes additional nonradiative recombination in the quasi-neutral region and potential limitation to the transport of hole carriers. Herein, an effective model for growing monolayer CZTSe thin-films based on metal precursors with large grains spanning the whole film is developed. Voids and fine grain layer are avoided successfully by suppressing the formation of a Sn-rich liquid metal phase near Mo back contact during alloying, while grain coarsening is greatly promoted by enhancing mass transfer during grain growth. The desired morphology exhibits several encouraging features, including significantly reduced recombination in the quasi-neutral region that contributes to the large increase of short-circuit current, and a quasi-Ohmic back contact which is a prerequisite for high fill factor. Though this growth mode may introduce more interfacial defects which require further modification, the strategies demonstrated remove a primary obstacle toward higher efficiency kesterite solar cells, and can be applicable to morphology control with other emerging chalcogenide thin films.