Interfaces of high-efficiency kesterite Cu_2ZnSnS(e)_4 thin film solar cells

Cu2ZnSnS（e）4 （CZTS（e）） solar cells have attracted much attention due to the elemental abundance and the non- toxicity. However, the record efficiency of 12.6% for CuzZnSn（S,Se）4 （CZTSSe） solar cells is much lower than that of Cu（In,Ga）See （CIGS） solar cells. One crucial reason is the recombination at interfaces. In recent years, large amount inves- tigations have been done to analyze the interfacial problems and improve the interfacial properties via a variety of methods. This paper gives a review of progresses on interfaces of CZTS（e） solar cells, including： （i） the band alignment optimization at buffer/CZTS（e） interface, （ii） tailoring the thickness of MoS（e）2 interfacial layers between CZTS（e） absorber and Mo back contact, （iii） the passivation of rear interface, （iv） the passivation of front interface, and （v） the etching of secondary phases.

First-principles study on the alkali chalcogenide secondary compounds in Cu(In,Ga)Se_2 and Cu_2ZnSn(S,Se)_4 thin film solar cells

The beneficial effect of the alkali metals such as Na and K on the Cu（In.Ga）Se2 （CIGS） and Cu2ZnSn（S,Se）4 （CZTSSe） solar cells has been extensively investigated in the past two decades, however, in most of the studies the alkali metals were treated as dopants. Several recent studies have showed that the alkali metals may not only act as dopants but also form secondary phases in the absorber layer or on the surfaces of the films. Using the first-principles calculations, we screened out the most probable secondary phases of Na and K in CIGS and CZTSSe, and studied their electronic structures and optical properties. We found that all these alkali chalcogenide compounds have larger band gaps and lower VBM levels than CIGS and CZTSSe, because the existence of strong p-d coupling in CIS and CZTS pushes the valence band maximum （VBM） level up and reduces the band-gaps, while there is no such p-d coupling in these alkali chalcogenides. This band alignment repels the photo-generated holes from the secondary phases and prevents the electron-hole recombination. Moreover, the study on the optical properties of the secondary phases showed that the absorption coefficients of these alkali chalcogenides are much lower than those of CIGS and CZTSSe in the energy range of 0-3.4eV, which means that the alkali chalcogenides may not influence the absorption of solar light. Since the alkali metal dopants can passivate the grain boundaries and increase the hole carrier concentration, and meanwhile their related secondary phases have innocuous effect on the optical absorption and band alignment, we can understand why the alkali metal dopants can improve the CIGS and CZTSSe solar cell performance.

Photovoltaic properties of Cu_2O-based heterojunction solar cells using n-type oxide semiconductor nano thin films prepared by low damage magnetron sputtering method

We improved the photovoltaic properties of Cu_2O-based heterojunction solar cells using n-type oxide semiconductor thin films prepared by a sputtering apparatus with our newly developed multi-chamber system. We also obtained the highest efficiency(3.21%) in an AZO/p-Cu_2O heterojunction solar cell prepared with optimized pre-sputtering conditions using our newly developed multi-chamber sputtering system. This value achieves the same or higher characteristics than AZO/Cu_2O solar cells with a similar structure prepared by the pulse laser deposition method.

In-doping collaboratively controlling back interface and bulk defects to achieve efficient flexible CZTSSe solar cells

Focusing on the low open circuit voltage(V_(OC))and fill factor(FF)in flexible Cu_(2)ZnSn(S,Se)_(4)(CZTSSe)solar cells,indium(In)ions are introduced into the CZTSSe absorbers near Mo foils to modify the back interface and passivate deep level defects in CZTSSe bulk concurrently for improving the performance of flexible device.The results show that In doping effectively inhibits the formation of secondary phase(Cu(S,Se)_(2))and VSndefects.Further studies demonstrate that the barrier height at the back interface is decreased and the deep level defects(Cu_(Sn)defects)in CZTSSe bulk are passivated.Moreover,the carrier concentration is increased and the V_(OC) deficit(V_(OC,def))is decreased significantly due to In doping.Finally,the flexible CZTSSe solar cell with 10.01%power conversion efficiency(PCE)has been obtained.The synergistic strategy of interface modification and bulk defects passivation through In incorporation provides a new thought for the fabrication of efficient flexible kesterite-based solar cells.

Synthesis of Cu_2ZnSnS_4 thin film from mixed solution of Cu_2SnS_3 nanoparticles and Zn ions

The Cu2ZnSnS4 thin film was prepared by a facile solution method without vacuum environment and toxic substance. The formation mechanism of the film was studied by transmission electron microscopy (TEM), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and Raman scattering measurements. Through cyclic voltammetry and photo-electricity tests, the electrocatalytic activity of the prepared film as the counter electrode of dye-sensitizedsolar cell was also studied. The results show that the mixed precursor solution mainly consists of Cu2SnS3 nanoparticles and Zn ions.After 550 °C annealing process on the precursor film prepared from the mixed solution, Cu2ZnSnS4 thin film is obtained. Besides, itis found that the prepared Cu2ZnSnS4 thin film has the electrocatalytic activity toward the redox reaction of I3?/I? and the dye-sensitized solar cell with the prepared Cu2ZnSnS4 thin film as the counter electrode achieves the efficiency of 1.09%.

Theoretical study on the kesterite solar cells based on Cu_2ZnSn(S,Se)_4 and related photovoltaic semiconductors

The kesterite thin film solar cells based on the quaternary Cu2ZnSnS4 and Cu2ZnSnSe4 and their alloys Cu2ZnSn（S,Se）4 have been considered as environment-friendly and non-toxic alternatives to the currently commercialized CdTe and Cu（In,Ga）Se2 thin film solar cells. From the theoretical point of view, we will review how the group I2-II-IV-VI4 quaternary compound semiconductors are derived from the binary CdTe and the ternary CuInSe2 or CuGaSe2 through the cation mutation, and how the crystal structure and electronic band structure evolve as the component elements change. The increased structural and chemical freedom in these quaternary semiconductors opens up new possibility for the tailoring of material properties and design of new light-absorber semiconductors. However, the increased freedom also makes the development of high-efficiency solar cells more challenging because much more intrinsic point defects, secondary phases, surfaces, and grain-boundaries can exist in the thin films and influence the photovoltaic performance in a way different from that in the conventional CdTe and Cu（In,Ga）Se2 solar cells. The experimental characterization of the properties of defects, secondary phase, and grain-boundaries is currently not very efficient and direct, especially for these quaternary compounds. First-principles calculations have been successfully used in the past decade for studying these properties. Here we will review the theoretical progress in the study of the mixed-cation and mixed-anion alloys of the group I2-II-IV- VI4 semiconductors, defects, alkaline dopants, and grain boundaries, which provided very important information for the optimization of the kesterite solar cell performance.

Cu_2ZnSnS_4 thin films prepared by sulfurization of ion beam sputtered precursor and their electrical and optical properties

Cu2ZnSnS4 (CZTS) thin films were successfully prepared by sulfurization of ion bean sputtered precursors on soda-lime glass substrate. The single phase of stannite-type structure CZTS films were obtained as revealed in EDS and XRD analysis when the ratios of the constituents of CZTS thin films are close to stoichiometric by optimizing the conditions of precursor preparation and sulfurization. A low sheet resistivity as about 0.156 Ω·cm and a high absorption coefficient as 1×104 cm-1 were achieved in this method by Hall effect measurements and UV-VIS spectrophotometer. The optical band-gap energy of the CZTS sample is about 1.51 eV, which is very close to the optimum value for a solar-cell absorber.

Surface defect ordered Cu_(2)ZnSn(S,Se)_(4) solar cells with efficiency over 12% via manipulating local substitution

The environmentally friendly Cu_(2)ZnSn(S,Se)_(4)(CZTSSe) compounds are promising direct bandgap materials for application in thin film solar cells, but the spontaneous surface defects disordering would lead to large open-circuit voltage deficit(V_(oc,deficit)) and significantly limit kesterite photovoltaics performance,primarily arising from the generated more recombination centers and insufficient p to n conversion at p-n junction. Herein, we establish a surface defects ordering structure in CZTSSe system via local substitution of Cu by Ag to suppress disordered Cu_(Zn) defects and generate benign n-type Zn_(Ag) donors. Taking advantage of the decreased annealing temperature of Ag F post deposition treatment(PDT), the high concentration of Ag incorporated into surface absorber facilitates the formation of surface ordered defect environment similar to that of efficient CIGS PV. The manipulation of highly doped surface structure could effectively reduce recombination centers, increase depletion region width and enlarge the band bending near p-n junction. As a result, the Ag F-PDT device finally achieves maximum efficiency of 12.34% with enhanced V_(oc) of 0.496 V. These results offer a new solution route in surface defects and energy-level engineering, and open the way to build up high quality p-n junction for future development of kesterite technology.

Cu2ZnSn（S,Se）4 thin film solar cells fabricated with benign solvents

Cu2ZnSn（S,Se）4 （CZTSSe） is considered as the promising absorbing layer materials for solar cells due to its earth-abundant constituents and excellent semiconductor properties. Through solution-processing, such as various printing methods, the fabrication of high perfor- mance CZTSSe solar cell could be applied to mass production with extremely low manufacturing cost and high yield speed. To better fulfill this goal, environmentalfriendly inks/solutions are optimum for further reducing the capital investment on instrument, personnel and environmental safety. In this review, we summarized the recent development of CZTSSe thin films solar cells fabricated with benign solvents, such as water and ethanol. The disperse system can be classified to the true solution （consisting of molecules） and the colloidal suspension （consisting ofnanoparticles）.Three strategies for stabilization （i.e., physical method, chemical capping and self- stabilization） are proposed to prepare homogeneous and stable colloidal nanoinks. The one-pot self-stabilization method stands as an optimum route for preparing benign inks for its low impurity involvement and simple procedure. As-prepared CZTSSe inks would be deposited onto substrates to form thin films through spin-coating, spraying, electrodeposition or successive ionic layer adsorption and reaction （SILAR） method, followed by annealing in a chalcogen （S- or Se-containing） atmosphere to fabricate absorber. The efficiency of CZTSSe solar cell fabricated with benign solvents can also be enhanced by constituent adjustments, doping, surface treatments and blocking layers modifications, etc., and the deeper research will promise it a comparable performance to the non- benign CZTSSe systems.

Low-temperature phase transformation of CZTS thin films

The low temperature phase transformation in the Cu_2ZnSnS_4（CZTS） films was investigated by laser annealing and low temperature thermal annealing.The Raman measurements show that a-high-power laser annealing could cause a red shift of the Raman scattering peaks of the kesterite（KS） structure and promotes the formation of the partially disordered kesterite（PD-KS） structure in the CZTS films,and the low-temperature thermal annealing only shifts the Raman scattering peak of KS phase by several wavenumber to low frequency and the broads Raman peaks in the low frequency region.Moreover,the above two processes were reversible.The Raman analyses of the CZTS samples prepared under different process show that the PD-KS structure tends to be found at low temperatures and low sulfur vapor pressures.Our results reveal that the control of the phase structure in CZTS films is feasible by adjusting the preparation process of the films.

Lithium-assisted synergistic engineering of charge transport both in GBs and GI for Ag-substituted Cu2ZnSn(S,Se)4 solar cells

Although silver(Ag) substitution offers several benefits in eliminating bulk defects and facilitating interface type inversion for Cu2ZnSn(S,Se)4(CZTSSe) photovoltaic(PV) technology, its further development is still hindered by the fairly low electrical conductivity due to the significant decrease of acceptors amount.In this work, a versatile Li–Ag co-doping strategy is demonstrated to mitigate the poor electrical conductivity arising from Ag through direct incorporating Li via postdeposition treatment(PDT) on top of the Ag-substituted CZTSSe absorber. Depth characterizations demonstrate that Li incorporation increases ptype carrier concentration, improves the carrier collection within the bulk, reduces the defects energy level as well as inverts the electric field polarity at grain boundaries(GBs) for Ag-substituted CZTSSe system. Benefiting from this lithium-assisted complex engineering of electrical performance both in grain interior(GI) and GBs, the power conversion efficiency(PCE) is finally increased from 9.21% to 10.29%. This systematic study represents an effective way to overcome the challenges encountered in Ag substitution,and these findings support a new aspect that the synergistic effects of double cation dopant will further pave the way for the development of high efficiency kesterite PV technology.

Enhanced compactness and element distribution uniformity of Cu2ZnSnS4 thin film by increasing precursor S content

Cu2ZnSnS4 thin films were prepared by cosputtering with Cu(or Cu2S),ZnS and SnS2 targets in this study.S amount in the precursor of Cu2ZnSnS4 thin film was verified by using Cu or Cu2S target.The effect of S amount in the precursor on the microstructure and element distribution of Cu2ZnSnS4 thin film was discussed.It was found that S content is sufficient in the precursor thin film using Cu2 S instead of Cu target.The microstructure,composition homogeneity,and secondary phase formation of the Cu2ZnSnS4 thin film are seriously affected by S amount in the precursor thin film.Namely,sufficient S can improve the crystallization and orientation of the precursor thin film and enhance the compactness as well as composition homogeneity of the Cu2ZnSnS4 thin film after sulfurization.Moreover,the secondary phase formation in Cu2ZnSnS4 thin film can be greatly inhibited by increasing S content in the precursor thin film.

Na-doping-induced modification of the Cu_(2)ZnSn(S,Se)_(4)/CdS heterojunction towards efficient solar cells

It is very important to understand why a small amount of alkali metal doping in Cu_(2)ZnSn(S,Se)_(4)(CZTSSe)solar cells can improve the conversion efficiency.In this work,Na-doped CZTSSe is prepared by a simple solution method,and then the effects on the surface properties of the absorber layer,the buffer layer growth,and the modifications of the solar cell performance induced by the Na doping are studied.The surface of the absorber layer is more Cu-depletion and less roughness due to the Na doping.In addition,the contact angle of the surface increases because of Na doping.As a consequence,the thickness of the CdS buffer layer is significantly reduced and the optical losses in the CdS buffer layer are decreased.The difference of quasi-Fermi levels(EFn-EFp) increases with a small amount of Na doping in the CZTSSe solar cell,so that open circuit voltage(VOC) increased significantly.This work offers new insights into the effects of Na doping on CZTSSe via a solution-based approach and provides a deeper understanding of the origin of the efficiency improvement of Na-doped CZTSSe thin film solar cells.

Precisely tuning Ge substitution for efficient solution-processed Cu_2ZnSn(S,Se)_4 solar cells

The kesterite Cu2ZnSn（S,Se）4 （CZTSSe） solar cells have yielded a prospective conversion efficiency among all thin- film photovoltaic technology. However, its further development is still hindered by the lower open-circuit voltage （Voc）, and the non-ideal bandgap of the absorber is an important factor affecting this issue. The substitution of Sn with Ge provides a unique ability to engineer the bandgap of the absorber film. Herein, a simple precursor solution approach was successfully developed to fabricate Cu2Zn（SnyGel_y）（SxSe~ x）4 （CZTGSSe） solar cells. By precisely adjusting the Ge content in a small range, the Voc and Jsc are enhanced simultaneously. Benefitting from the optimized bandgap and the maintained spike structure and light absorption, the 10% Ge/（Ge＋Sn） content device with a bandgap of approximately 1.1 eV yields the highest efficiency of 9.36%. This further indicates that a precisely controlled Ge content could further improve the cell performance for efficient CZTGSSe solar cells.

Recent progress in defect engineering for kesterite solar cells

Kesterite Cu_(2)ZnSn(S,Se)_(4)(CZTSSe)thin film solar cells have been regarded as one of the most promising thin film photovoltaic technologies,offering a low-cost and environmentally friendly solar energy option.Although remarkable advances have been achieved in kesterite solar cells,the performance gap relative to mature thin film photovoltaic technologies such as CIGSe and Cd Te remains large.Significant open-circuit voltage(V_(OC))deficit has been recognized as the main limiting factor to performance improvement,with undesirable intrinsic defects being a key culprit contributing to the low V_(OC).To realize the promise inherent in kesterite CZTS to become an earth-abundant alternative to existing thin film photovoltaic technologies with comparable performance,significant research effort has been invested to tackle the challenging defect issues.In this review,recent progress and achievements relevant to engineering improvements to the defect properties of the semiconductor have been examined and summarized.Promising strategies include:(i)manipulating the synthesis process to obtain a desirable reaction pathway and chemical environment;(ii)introducing cation substitution to increase the ionic size difference and supress the related band tailing deep-level defects;(iii)applying post deposition treatment(PDT)with alkaline elements to passivate the detrimental defects.These advances obtained from work on kesterite solar cells may lead to future high performance from this material and may be further extended to other earth-abundant chalcogenide photovoltaic technologies.

Enhancing back interfacial contact by in-situ prepared MoO_3 thin layer for Cu_2ZnSnS_xSe_(4–x) solar cells

In-situ prepared MoO3 thin layer has been introduced to suppress the formation of too thick Mo(S,Se)2layer in Cu2ZnSnSxSe4–x(CZTSSe) solar cells. This MoO3 layer effectively improves the back interfacial contact between CZTSSe absorber layer and Mo substrate without poisoning the carrier transport. Up to 10.58% power conversion efficiency has been achieved.

Suppression of charge recombination by application of CueZnSnS4-graphene counter electrode to thin dye-sensitized solar cells

This paper proposes a new mechanism to explain the performance of thin dye-sensitized solar cells （DSSC）. Near-stoichiometric flower-like Cu2ZnSnS4 （CZTS） microspheres with a high specific surface area was fabri- cated for use as the photocathode in a DSSC. To improve the extraction and transfer of electrons, graphene was added to the CZTS. A DSSC with a 10-gin TiO2 pho- toanode layer exhibited a slightly degraded efficiency with a CZTS-graphene photocathode, relative to a Pt counter electrode （CE）. Nevertheless, when the thickness of the TiO2 photoanode was reduced to 2 lam, the efficiency of a DSSC with a CZTS-graphene photocathode was greater than that of a Pt-DSSC. It is speculated that, unlike the Pt CE, a CZTS-graphene photocathode not only collects electrons from an external circuit and catalyzes the reduction of the triiodide ions in the electrolyte, but also utilizes unabsorbed photons to produce photo-excited electrons and suppresses charge recombination, thus enhancing the performance of the cell. The use of narrowband gap p-type semiconductors as photocathodes offers a new means of fabricating thin dye-sensitized solar cells and effectively improving the cell performance.

In Situ Controllable Growth of Cu_2SnS_3 Film as Low-Cost Counter Electrodes for Dye-Sensitized Solar Cells

Morphology-controllable Cu2SnS3 thin films on solvothermal process and used in dye-sensitized solar cells as Mo-glass were prepared via a facile in situ one-step counter electrodes. The effects of different solvents on the morphology of films were investigated. DSC based on the porous net-like Cu2SnS3 thin film as counter electrodes showed a power conversion efficiency of 2.30%, which was improved to 3.35% after annealing.

Influence of selenium evaporation temperature on the structure of Cu_2ZnSnSe_4 thin film deposited by a co-evaporation process

Cu2ZnSnSe4 （CZTSe） thin film solar cells have been fabricated using a one-step co-evaporation technique. The structural properties of polycrystalline CZTSe films deposited at different selenium evaporation temperatures （TSe） have been investigated using X-ray diffraction spectra, scanning electron microscopy, and atomic force microscopy. A relationship between TSe and the secondary phases deposited in the initial stage is established to explain the experimental observations. The Se flux is not necessarily increased too much to reduce Sn loss and the consumption of Se during fabrication could also be reduced. The best solar cell, with an efficiency of 2.32%, was obtained at a medium Tse of 230 ℃ （active area 0.34 cm2）.

Impact of Cu-rich growth on the Cu2ZnSnSe4 surface morphology and related solar cells behavior

Inorderto study the influence ofCu-rich growth on the performance ofthe 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.