Cu2ZnSnSe4 Thin Film Solar Cells Research Articles

Analyzing the synergistic effect of Ag and Ge co-incorporation on Cu2ZnSnSe4 thin-film solar cells

At present, the large number of inherent CuZn anti-site defects and harmful [2CuZn + SnZn] defect clusters in the CZTSe film layer limit the further progress of device efficiency. In this paper, Ag and Ge double cations were introduced into the CZTSe film layer, and (CuAg)2ZnSnGeSe4 (CAZTGSe) films were synthesized successfully by the sol-gel method to cut down the above defects and defect clusters to obtain high-efficiency devices. The influences of double cation substitution on CZTSe by partly replacing Cu with Ag and Sn with Ge were developed. The role of Ag, Ge, and Ag + Ge substitution was researched by X-ray diffraction, scanning electron microscopy, current density-voltage (J-V), and external quantum efficiency measurements. By incorporating 5% Ag and 20% Ge double cations into the CZTSe film, the device demonstrated the highest efficiency of 10.12%. In addition, the open circuit voltage (VOC) of 503.57 mV, the short circuit current density (JSC) of 31.36 mA/cm2, and the fill factor (FF) of 64.1% were obtained.

Efficient Cu2ZnSnSe4 thin film solar cells prepared from an ethanolamine-dimethyl formamide based sulfur/selenium-free solution

Most reported solution processed Cu2ZnSn(S,Se)4 thin film solar cells are fabricated from precursor solution with sulfur source. However, the use of sulfur-containing solution is undesirable from the viewpoint of environment and security due to the release of sulfur-containing harmful gases when drying precursors. Here, a novel sulfur/selenium-free solution method using ethanolamine-dimethyl formamide as solvent was developed for the preparation of Cu2ZnSnSe4 thin films. It was found that Cu-Sn alloy phases were formed in precursor during drying process. The formation and phase transition mechanisms of the Cu-Sn phases were illustrated. The aggregation level of the alloy phases and their distribution in the precursor depend on the drying time. Effects of different drying time on structural, morphological and photoelectric properties of the resulted Cu2ZnSnSe4 films were systematically investigated. The Cu2ZnSnSe4 absorbers exhibit a sandwiched structure or a four-layer structure, which is determined by the drying time of precursor, implying two different grain growth mechanisms. The devices with drying time of 6 min show larger grain size, longer carrier life time, higher external quantum efficiency and better average photovoltaic performance.

Investigation into complex defect properties of near-stoichiometric Cu2ZnSnSe4 thin film

In this work, a systematic investigation of the effects of point defects on the efficiency of a Cu2ZnSnSe4 (CZTSe) thin film solar cell was conducted using temperature-/power-dependent photoluminescence (PL) and time-resolved photo-luminescence (TRPL). The studied stoichiometric CZTSe absorber layer for a CZTSe solar cell with an efficiency of 4.4 % was prepared using the low-toxicity selenization process. The fitting to the Arrhenius plot and the stretched tail at the long-wavelength side of PL spectrum obtained at 10 K indicated a high concentration of zinc on copper (CuZn) point defects. The dispersive-photon-energy-dependent TRPL at 10 K exhibited non-exponential stretched PL decay at all photon energies and, in particular, revealed the highly fluctuating potential of CZTSe that caused carrier localization. This effect was observed to be caused by [2CuZn− + SnZn2+] defect clusters, which produce a significant conduction/valence band-edge shift in their vicinity in CZTSe and can cause shunt leakage and an increase in series resistance. This systematic investigation presents the experimental characterization of [2CuZn− + SnZn2+] defect clusters and provides perspective to help clarify the performance deterioration of CZTSe solar cells.

Defect passivation in Cu2ZnSnSe4 thin-film solar cells by novel sodium doping

Sodium (Na) alkali doping is one of the effective ways to improve Cu2ZnSnSe4 (CZTSe) device performance by passivating detrimental native defects caused by off-stoichiometric composition and Cu–Zn cationic disordering. Various methods to incorporate Na in CZTSe films have been considered to realize the ideal distribution of Na in CZTSe films. Na content, regardless of Na incorporation method, is significantly increased in bulk CZTSe. Interestingly, when a NaF precursor is supplied after CZTSe film growth, the surface Na content is slightly increased, resulting in improved device performance. For the first time, a sodium bifacial treatment was employed to obtain an optimal profile of Na in CZTSe films. Using this novel route for Na doping, the photovoltaic (PV) device exhibited a maximum efficiency of 6.02%, which was higher than that compared to CZTSe devices with single Na doping and CZTSe device with Na diffused from soda-lime glass. According to defect analyses, the improved device performance is associated with a reduction in the defect density from 1.1 × 1016 (without Na doing) to 5.7 × 1014 cm−3 (with optimal Na profile). These results indicate that the proposed bifacial treatment method is a desirable means to fabricate high-efficiency, flexible CZTSe solar cells.

Mechanism study of Zn/Sn ratio on the MoSe2 formation in Zn-rich Cu2ZnSnSe4 absorber layer

Controlling the thickness of the interfacial MoSe2 layer in Cu2ZnSnSe4 (CZTS) thin film solar cells can have a profound impact on device performance. The Zn/Sn ratio in the absorber layer of CZTS solar cells significantly affects the thickness of MoSe2 after selenization; however, the underlying mechanism remains unknown. In this study, we employed a fast-cooling selenization system to study the phase transformation and analyzed the back side of the CZTS absorber layer to characterize the mechanism by which Zn/Sn ratio affects the thickness of the MoSe2 absorber layer. Experiments revealed the formation of various metal-selenide phases at the bottom of the CZTS absorber layer during selenization. At temperatures below 400 °C, this mainly involves Sn- and CuSn-selenides; above 450 °C, this mainly involves Cu- and Zn- selenides. Since MoSe2 grows significantly above 450 °C, Cu- and Zn-selenides are the main reasons for reducing the thickness of MoSe2. The fact that MoSe2 growth generally occurs above 450 °C means that a higher Zn/Sn ratio leads to the formation of more Zn-selenides, such that there is less Se available for the formation of MoSe2, resulting in a thinner layer.

衬底温度对共蒸发法制备Cu2ZnSnSe4太阳电池的影响

衬底温度对共蒸发法制备Cu2ZnSnSe4太阳电池的影响

Efficiency enhancement of bifacial Cu2ZnSnSe4 thin-film solar cells on indium tin oxide glass substrates by suppressing In-Sn diffusion with Mo interlayer

We fabricate bifacial Cu2ZnSnSe4 (CZTSe) thin-film solar cells on indium tin oxide (ITO) substrates using a vacuum procedure. We find that, as the annealing temperature increases from 500 to 540 °C, the performance of the CZTSe solar cell rapidly degrades due to Sn–In diffusion. We implement Mo interlayers at the CZTSe absorber/ITO interface to suppress this Sn–In movement. During annealing, the selenized Mo interlayer with MoSe2 effectively acts as a barrier layer to reduce the In diffusion into the CZTSe absorber. The relationship between the Mo thickness and the CZTSe performance is addressed. Because the Mo interlayer is used, the conversion efficiency of the CZTSe solar cells greatly improves, from 0.40% to 5.21% under front illumination. Also, the effects of the bifacial conditions and Mo interlayer thickness on the performance of the CZTSe solar cells are investigated. Under bifacial illumination (front: 1.0 sun/rear: 0.3 sun), the conversion efficiency of the CZTSe solar cells increases by up to 10% compared with that under front illumination only, achieving a value of 5.71% for CZTSe/ITO.

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).

Control of Zn Content and Influence on Cu2ZnSnSe4 Thin-Film Solar Cells Fabricated by Coevaporation

Control of Zn Content and Influence on Cu₂ZnSnSe₄ Thin-Film Solar Cells Fabricated by Coevaporation

Improving the open‐circuit voltage of Cu2ZnSnSe4 thin film solar cells via interface passivation

AbstractThis study reports on substantial improvement of the open‐circuit voltage (Voc) of Cu2ZnSnSe4 (CZTSe) thin film solar cells by applying a passivation strategy to both the top and bottom interfaces of the CZTSe absorber, which involves insertion of a thin dielectric layer between the CZTSe and the surrounding layers. The study also presents in‐depth material characterizations using transmission electron microscopy, energy dispersive X‐ray spectroscopy, low‐temperature photoluminescence, and secondary ion mass spectrometry, to reveal the effects of the interface passivation. To passivate the bottom Mo/CZTSe interface, a dielectric layer with patterned local contacts of dimensions down to ~100 nm was prepared using nanosphere lithography. With this, the Voc, short‐circuit current, and fill factor (FF) were significantly enhanced due to reduction in carrier recombination at the bottom Mo/CZTSe interface. The top CZTSe/CdS interface was passivated by a thin dielectric layer which prevented inter‐diffusion of Cd and Cu at the top interface, thereby improving the junction quality. Application of the top passivation layers resulted in substantial improvement in Voc and FF, thereby achieving the Voc deficit of 0.542 V which is the record value among reported CZTSe solar cells. Copyright © 2017 John Wiley & Sons, Ltd.

High quality sustainable Cu2ZnSnSe4 (CZTSe) absorber layers in highly efficient CZTSe solar cells

An evaporation-processed Cu2ZnSnSe4 thin film solar cell with a conversion efficiency of 7.18% was facilely fabricated in an environmentally benign selenium atmosphere.

Improving Open-Circuit Voltage in Cu2ZnSnSe4 Thin Film Solar Cells Via Interface Passivation

The earth-abundant Cu2ZnSnSe4 (CZTS) thin film solar cells have demonstrated promising results with the record power conversion efficiency of 12.6%. However, they suffer from the high open-circuit voltage (Voc) deficit that hinders further improvement in the device performance. Ongoing studies regarding the origins behind this critical issue suggest that it stems from the fluctuation of band structures and non-radiative recombination centers in CZTS solar cells. Here, we focused on passivating the interfaces of CZTS solar cells – both a front CdS/CZTS interface and a back CZTS/Mo interface – to reduce the interfacial recombination that may greatly contribute to a large Voc deficit of CZTS solar cells. By applying a thin dielectric on top of the CZTS and a nano-patterned dielectric layer between the CZTS and the Mo, Voc is significantly enhanced; the Voc-deficit value is comparable to that of the world record CZTS solar cell. Structural and electrical properties of the CZTS devices with passivation layers were studied using scanning electron microscopy, transmission electron microscopy, energy dispersive x-ray spectroscopy, J-V and external quantum efficiency measurement, low-temperature photoluminescence, and time-resolved photoluminescence analysis. The effects of interface passivation in correlation with the device performance will be presented as well.

High efficiency bifacial Cu2ZnSnSe4 thin-film solar cells on transparent conducting oxide glass substrates

In this work, transparent conducting oxides (TCOs) have been employed as a back contact instead of Mo on Cu2ZnSnSe4 (CZTSe) thin-film solar cells in order to examine the feasibility of bifacial Cu2ZnSn(S,Se)4 (CZTSSe) solar cells based on a vacuum process. It is found that the interfacial reaction between flourine doped tin oxide (FTO) or indium tin oxide (ITO) and the CZTSe precursor is at odds with the conventional CZTSe/Mo reaction. While there is no interfacial reaction on CZTSe/FTO, indium in CZTSe/ITO was significantly diffused into the CZTSe layers; consequently, a SnO2 layer was formed on the ITO substrate. Under bifacial illumination, we achieved a power efficiency of 6.05% and 4.31% for CZTSe/FTO and CZTSe/ITO, respectively.

Defect characterization in co-evaporated Cu2ZnSnSe4 thin film solar cell

Cu2ZnSnSe4 (CZTSe) solar cell fabricated by selenization of co-evaporated Cu–Zn–Sn–Se precursor thin film exhibited 6.2% conversion efficiency. Defects in the solar cell were investigated by temperature-dependent capacitance-voltage and admittance spectroscopy. Capacitance-voltage measurements showed that the p-type conductivity of CZTSe absorber layer was mainly attributed to two acceptor levels with activation energy of 0.02 eV and 0.11 eV. The admittance spectra exhibited capacitance steps that might have originated from a defect level with activation energy of 0.16 eV. The effects of these defects on the solar cell were discussed.

Vitreous enamel as sodium source for efficient kesterite solar cells on commercial ceramic tiles

The range of applications for kesterite photovoltaic devices can be widely broadened by the use of alternatives to the commonly used soda-lime glass (SLG) as substrate. This work explores the possibility to use commercial ceramic tiles as ecological substrates for Cu2ZnSnSe4 (CZTSe) thin film solar cells in view of an application in building integrated photovoltaics (BIPV). As a novel approach to control the Na-incorporation into the kesterite absorber, the substrates are coated with enamels containing different concentrations of Na2O (from 2 to 20% wt.). This way, the enamel becomes not only a smoothing element but also a source of Na during the absorber growth. The impact that an increased Na2O content has on the Mo back contact deposited on these enameled ceramic substrates is investigated. 3D optical profilometry shows surfaces with undulations, pinholes and cracks directly related to the Na2O content of the enamels. High quality kesterite (Cu2ZnSnSe4) absorbers with good crystallinity were produced on these substrates by sequential precursor sputtering and selenization. The good performance of first solar cell devices with a maximum efficiency of 7.5% gives a solid proof-of-concept for ceramic solar tiles based on efficient kesterite absorbers.

Temperature dependent electrical characterization of thin film Cu2ZnSnSe4 solar cells

Impedance spectroscopy (IS) and current–voltage characteristics measurements were applied to study properties of a Cu2ZnSnSe4 (CZTSe) thin film solar cell. IS measurements were done in the frequency range 20 Hz to 10 MHz. The measurement temperature was varied from 10 K to 325 K with a step ▵T = 5 K. Temperature dependence of Voc revealed an activation energy of 962 meV, which is in the vicinity of the band gap energy of CZTSe and hence the dominating recombination mechanism in this solar cell is bulk recombination. Different temperature ranges, where electrical properties change, were found. Interface states at grain boundaries with different properties were revealed to play an important role in impedance measurements. These states can be described by introducing a constant phase element in the equivalent circuit.

Impact of Cu-rich growth on the Cu2ZnSnSe4 surface morphology and related solar cells behavior**Project supported by the Specialized Research Fund for the PhD Program of Higher Education (No. 20120031110039).

In order to study the influence of Cu-rich growth on the performance of the Cu2ZnSnSe4 (CZTSe) thin film solar cells, a multi-stage co-evaporation process is applied. The CZTSe films are grown at a lower substrate temperature to reduce the existence time of Cux Sey at the first period caused by the volatility of SnSex. This study examines the surface morphology and device performance in Cu-rich growth and close-to-stoichiometric growth. Although the grain size of Cu-rich growth film increases a little, the difference was not dramatic as the results of CIGS reported previously. A model based on the grain boundary migration theory is proposed to explain the experimental results. The mechanisms of Cu-rich growth between CZTSe and CIGS might be different.

Improvement of the Open Circuit Voltage of CZTSe Thin-Film Solar Cells by Surface Sulfurization Using SnS**Supported by the Specialized Research Fund for the Doctoral Program of Higher Education under Grant No 20120031110039.

The objective of this study is to find an effective method to improve Voc without Jsc loss for Cu2ZnSnSe4 (CZTSe) thin film solar cells, which have been fabricated by the one step co-evaporation technique. Surface sulfurization of CZTSe thin films is carried out by using one technique that does not utilize toxic H2S gas; a sequential evaporation of SnS after CZTSe deposition and the annealing of CZTSe thin films in selenium vapor. A Cu2ZnSn(S,Se)4 (CZTSSe) thin layer is grown on the surface of the CZTSe thin film after the annealing. The conversion efficiency of the completed device is improved due to the enhancement of Voc, which could be attributed to the formation of a hole-recombination barrier at the surface or the passivation of the surface and grain boundary by S incorporation.

Ge-incorporated Cu2ZnSnSe4 thin-film solar cells with efficiency greater than 10%

Ge-incorporated Cu2ZnSnSe4 (CZTGSe) thin films were prepared, and their materials and device properties were examined. The CZTGSe thin films were prepared using a two-step process comprising co-evaporation of each element and a subsequent annealing step. Ge atoms were successfully incorporated into the Cu2ZnSnSe4 thin films, and the band-gap (Eg) of CZTGSe was controlled via the full Ge/(Sn+Ge) ratio range of 0–1. In addition, the annealing environment containing GeSe2 led to CZTGSe thin films with flat surfaces, dense morphologies, and large grains comparable to their thickness. The highest efficiency achieved with the fabricated CZTGSe solar cells was 10.03%, with an open circuit voltage (VOC) of 0.54V. The CZTGSe thin-film solar cells exhibited an improved VOC deficit (Eg/q−VOC, q: electron charge) of 0.647V, which is comparable to that of high-efficiency Cu2ZnSn(SxSe1−x)4 solar cells.

Facile and Low-Cost Sodium-Doping Method for High-Efficiency Cu2ZnSnSe4 Thin Film Solar Cells

We present a simple and low-cost sodium-doping method for Cu2ZnSnSe4 (CZTSe) thin film solar cells. In this method, a piece of soda-lime glass (SLG) is served as the sodium source and is placed on top of the CZTSe precursor thin film during selenization. It was observed that the grain growth and the hole-carrier concentration can be significantly improved by the diffusion of sodium from the top SLG. Through this approach, high-quality CZTSe absorber layer is obtained after the selenization, and the photoelectric conversion efficiencies (PCE) of 7.51% and 6.09% are achieved for CZTSe thin film solar cells deposited on a Mo-coated SLG substrate and a Mo-coated quartz substrate, respectively. The difference in PCE on SLG and quartz substrate revealed that Na diffusion from the bottom SLG substrate and the top SLG was most effective for the high-performance of CZTSe solar cell devices.