Nanoscale Microstructure and Chemistry of Cu2ZnSnS4/CdS Interface in Kesterite Cu2ZnSnS4 Solar Cells

Sulfurization with various atmosphere and postheat treatments has been reported for earth abundant kesterite Cu₂ZnSnS₄ (CZTS) preparation as cost‐effective material for next‐generation solar cells. A full understanding of the nanoscale microstructure and chemistry of CZTS/CdS interface obtained from these different fabrication routes is currently lacking, yet is critical to developing optimal processing routes for high‐performance kesterite solar cells. Here, the first detailed investigation of the interfacial microstructure and chemistry of CdS/Cu₂ZnSnS₄ heterojunctions is presented. For CZTS obtained from sulfurization in a sulfur‐only atmosphere where highly defective surfaces are present, air annealing followed by etching in the initial stage of chemical bath deposition (CBD) process can effectively eliminate interfacial defects and allow the epitaxial growth of CBD‐CdS, improving the minority lifetime, open circuit voltage (V_OC), and fill factor (FF) of the devices, while blocking Cd diffusion and deteriorating short circuit current (J_sc). For CZTS from sulfurization in a combined sulfur and SnS atmosphere where CBD‐CdS can directly epitaxially grow on CZTS and Cd‐diffusion is clearly observed, associated devices show the longest lifetime and the highest efficiency of 8.76%. Epitaxial growth of CdS and Cd diffusion into CZTS are found to be two crucial features minimizing interfacial recombination and achieving high‐efficiency devices. This will not only enhance the understanding of the device structure and physics of kesterite based solar cells, but also provide an effective way for designing other chalcogenide heterojunction solar cells.     

Suppressed Deep Traps and Bandgap Fluctuations in Cu2CdSnS4 Solar Cells with ≈8% Efficiency

The identification of performance‐limiting factors is a crucial step in the development of solar cell technologies. Cu₂ZnSn(S,Se)₄‐based solar cells have shown promising power conversion efficiencies in recent years, but their performance remains inferior compared to other thin‐film solar cells. Moreover, the fundamental material characteristics that contribute to this inferior performance are unclear. In this paper, the performance‐limiting role of deep‐trap‐level‐inducing 2Cu_Zn+Sn_Zn defect clusters is revealed by comparing the defect formation energies and optoelectronic characteristics of Cu₂ZnSnS₄ and Cu₂CdSnS₄. It is shown that these deleterious defect clusters can be suppressed by substituting Zn with Cd in a Cu‐poor compositional region. The substitution of Zn with Cd also significantly reduces the bandgap fluctuations, despite the similarity in the formation energy of the Cu_Zn+Zn_Cu and Cu_Cd+Cd_Cu antisites. Detailed investigation of the Cu₂CdSnS₄ series with varying Cu/[Cd+Sn] ratios highlights the importance of Cu‐poor composition, presumably via the presence of V_Cu, in improving the optoelectronic properties of the cation‐substituted absorber. Finally, a 7.96% efficient Cu₂CdSnS₄ solar cell is demonstrated, which shows the highest efficiency among fully cation‐substituted absorbers based on Cu₂ZnSnS₄.     

Kesterite Thin‐Film Solar Cells: Advances in Materials Modelling of Cu2ZnSnS4

Quaternary semiconducting materials based on the kesterite (A₂BCX₄) mineral structure are the most promising candidates to overtake the current generation of light‐absorbing materials for thin‐film solar cells. Cu₂ZnSnS₄ (CZTS), Cu₂ZnSnSe₄ (CZTSe) and their alloy Cu₂ZnSn(Se,S)₄ consist of abundant, low‐cost and non‐toxic elements, unlike current CdTe and Cu(In,Ga)Se₂ based technologies. Zinc‐blende related structures are formed by quaternary compounds, but the complexity associated with the multi‐component system introduces difficulties in material growth, characterization, and application. First‐principles electronic structure simulations, performed over the past five years, that address the structural, electronic, and defect properties of this family of compounds are reviewed. Initial predictions of the bandgaps and crystal structures have recently been verified experimentally. The calculations highlight the role of atomic disorder on the cation sub‐lattice, as well as phase separation of Cu₂ZnSnS₄ into ZnS and CuSnS₃, on the material performance for light‐to‐electricity conversion in photovoltaic devices. Finally, the current grand challenges for materials modeling of thin‐film solar cells are highlighted.     

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

In 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.     

Cadmium Free Cu2ZnSnS4 Solar Cells with 9.7% Efficiency

Cu₂ZnSnS₄(CZTS) thin‐film solar cell absorbers with different bandgaps can be produced by parameter variation during thermal treatments. Here, the effects of varied annealing time in a sulfur atmosphere and an ordering treatment of the absorber are compared. Chemical changes in the surface due to ordering are examined, and a downshift of the valence band edge is observed. With the goal to obtain different band alignments, these CZTS absorbers are combined with Zn_1?xSn_xO_y (ZTO) or CdS buffer layers to produce complete devices. A high open circuit voltage of 809 mV is obtained for an ordered CZTS absorber with CdS buffer layer, while a 9.7% device is obtained utilizing a Cd free ZTO buffer layer. The best performing devices are produced with a very rapid 1 min sulfurization, resulting in very small grains.     

Strong Interplay between Sodium and Oxygen in Kesterite Absorbers: Complex Formation,Incorporation, and Tailoring Depth Distributions

Sodium and oxygen are prevalent impurities in kesterite solar cells. Both elements are known to strongly impact performance of the kesterite devices and can be connected to efficiency improvements seen after heat treatments. The sodium distribution in the kesterite absorber is commonly reported, whereas the oxygen distribution has received less attention. Here, a direct relationship between sodium and oxygen in kesterite absorbers is established using secondary ion mass spectrometry and explained by defect analyses within the density functional theory. The calculations reveal a binding energy of 0.76 eV between the substitutional defects Na_Cu and O_S in the nearest neighbor configuration, indicating an abundance of Na? O complexes in kesterite absorbers at relevant temperatures. Oxygen incorporation is studied by introducing isotopic ¹⁸O at different stages of the Cu₂ZnSnS₄/Mo/soda‐lime glass baseline processing. It is observed that oxygen from the Mo back contact and contaminations during the sulfurization are primary contributors to the oxygen distribution. Indeed, unintentional oxygen incorporation leads to immobilization of sodium. This results in a strong correlation between sodium and oxygen, in excellent agreement with the theoretical calculations. Consequently, oxygen availability should be monitored to optimize postdeposition heat treatments to control impurities in kesterite absorbers and ultimately, the solar cell efficiency.     

Solution‐Processed TiO2 Nanoparticles as the Window Layer for CuIn(S,Se)2 Devices

As a wide‐bandgap semiconductor, titanium dioxide (TiO₂) with a porous structure has proven useful in dye‐sensitized solar cells, but its application in low‐cost, high‐efficiency inorganic photovoltaic devices based on materials such as Cu(InGa)Se₂ or Cu₂ZnSnS₄ is limited. Here, a thin film made from solution‐processed TiO₂ nanocrystals is demonstrated as an alternative to intrinsic zinc oxide (i‐ZnO) as the window layer of CuInS_xSe_1?x solar cells. The as‐synthesized, well‐dispersed, 6 nm TiO₂ nanocrystals are assembled into thin films with controllable thicknesses of 40, 80, and 160 nm. The TiO₂ nanocrystal films with thicknesses of 40 and 80 nm exhibit conversion efficiencies (6.2% and 6.33%, respectively) that are comparable to that of a layer of the typical sputtered i‐ZnO (6.42%). The conversion efficiency of the devices with a TiO₂ thickness of 160 nm decreases to 2.2%, owing to the large series resistance. A 9‐hour reaction time leads to aggregated nanoparticles with a much‐lower efficiency (2%) than that of the well‐dispersed TiO₂ nanoparticles prepared using a 15‐hour reaction time. Under optimized conditions, the champion TiO₂ nanocrystal‐film‐based device shows even higher efficiency (9.2%) than a control device employing a typical i‐ZnO film (8.6%).     

A Temporary Barrier Effect of the Alloy Layer During Selenization: Tailoring the Thickness of MoSe2 for Efficient Cu2ZnSnSe4 Solar Cells

The influence of a prealloying process on the formation of MoSe₂ and thus on the performance of Cu₂ZnSnSe₄ (CZTSe) solar cells is investigated using sputtering deposition and post‐annealing approaches. The dense alloy layer, which is made by a low‐temperature prealloying process, acts as a temporary Se diffusion barrier during a subsequent high‐temperature selenization process. The formation of thick interfacial MoSe₂ can be suppressed effectively by this temporary barrier, cooperating with subsequent quick formation of compact CZTSe layer. The thickness of interfacial MoSe₂ layer in CZTSe solar cells can be tailored by adjusting the preannealing process during selenization. As a consequence, the series resistance of CZTSe solar cells is reduced to a low level (≈0.6 Ω cm²), and the performance of CZTSe solar cells is improved significantly. A CZTSe solar cell with efficiency of 8.7% is fabricated.     

Significant Efficiency Enhancement in Ultrathin CZTS Solar Cells by Combining Al Plasmonic Nanostructures Array and Antireflective Coatings

In the few past years, the economic and eco-friendly Cu₂ZnSnS₄ (CZTS) solar cells have caught lots of attentions. However, due to rather poor efficiency, identifying deficiencies and making improvements is necessary. In the present study, the performance improvement of ultrathin CZTS solar cells was achieved through (1) incorporation of anti-reflective coating (ARC) on the surface of cell and (2) embedding Al plasmonic nanostructures with different radius, periods, and vertical positions in the absorber layer. Various thicknesses of CZTS absorber layer were simulated optically and electrically using FDTD and DEVICE solver of Lumerical software. The reference solar cell consists of a 1.5-nm-thick CZTS absorber and exhibit an efficiency of up to 5.67%, short-circuit current density (Jsc) of 18.48 mA cm⁻² and open circuit voltage of 0.58 V. Result showed a remarkable performance enhancement of the solar cell in spite of a very thin absorber layer. For a 500-μm-thick CZTS solar cell with the assistance of ARC and embedding Al plasmonic nanostructures, the efficiency is increased to 7.45% due to an increase in Jsc to 22.62 mA cm⁻² with an open circuit voltage of 0.62 V.

     

Near‐Infrared Light‐Induced Photocurrent from a (NaYF4:Yb‐Tm)/(Cu2O) Composite Thin Film

Utilization of photons of sub‐bandgap energy, mostly near‐infrared (NIR) photo­ns, is highly desirable for photovoltaic devices. We show that (NaYF₄:Yb‐Tm)/(Cu₂O) composite films formed by electrodeposition exhibit robust photoactive current generation under NIR light excitation. The composite films consist of homogeneous crystalline particles of the fluoride and Cu₂O in sub‐micrometer size. From spectroscopic characterization, it is found that the NaYF₄:Yb‐Tm layer in the composite film converts NIR radiation by up‐conversion into visible emission, which is efficiently absorbed by the covering semiconducting Cu₂O film, producing photoelectrons. Accordingly, the composite films exhibit highly photoactive current generation by means of a photoelectrochemical process driven by NIR irradiation. The methodology demonstrated here may have certain implications for the utilization of NIR radiation in solar cells, photocatalysts, and infrared photodetectors.     

Cation Substitution of Solution‐Processed Cu2ZnSnS4 Thin Film Solar Cell with over 9% Efficiency

To alleviate the limitations of pure sulfide Cu₂ZnSnS₄ (CZTS) thin film, such as band gaps adjustment, antisite defects, secondary phase and microstructure, Cadmium is introduced into CZTS thin film to replace Zn partially to form Cu₂Zn_1?xCd_xSnS₄ (CZCTS) thin film by low‐cost sol–gel method. It is demonstrated that the band gaps and crystal structure of CZCTS thin films are affected by the change in Zn/Cd ratio. In addition, the ZnS secondary phase can be decreased and the grain sizes can be improved to some degree by partial replacement of Zn with Cd in CZCTS thin film. The power conversion efficiency of CZTS solar cell device is enhanced significantly from 5.30% to 9.24% (active area efficiency 9.82%) with appropriate ratio of Zn/Cd. The variation of device parameter as a function of Zn/Cd ratio may be attributed to the change in electronic structure of the bulk CZCTS thin film (i.e., phase change from kesterite to stannite), which in turn affects the band alignment at the CZCTS/buffer interface and the charge separation at this interface.     

Oxygenated CdS Buffer Layers Enabling High Open‐Circuit Voltages in Earth‐Abundant Cu2BaSnS4 Thin‐Film Solar Cells

Earth‐abundant Cu₂BaSnS₄ (CBTS) thin films exhibit a wide bandgap of 2.04–2.07 eV, a high absorption coefficient > 10⁴ cm^?1, and a p‐type conductivity, suitable as a top‐cell absorber in tandem solar cell devices. In this work, sputtered oxygenated CdS (CdS:O) buffer layers are demonstrated to create a good p–n diode with CBTS and enable high open‐circuit voltages of 0.9–1.1 V by minimizing interface recombination. The best power conversion efficiency of 2.03% is reached under AM 1.5G illumination based on the configuration of fluorine‐doped SnO₂ (back contact)/CBTS/CdS:O/CdS/ZnO/aluminum‐doped ZnO (front contact).     

Influence of Thermal Annealing on PCDTBT:PCBM Composition Profiles

A variety of measurement techniques including photothermal deflection spectroscopy (PDS), auger electron spectroscopy (AES), (sub–bandgap) external quantum efficiency (EQE), and impedance spectroscopy are applied to poly[N‐900‐hepta‐decanyl‐2,7‐carbazole‐alt‐5,5‐(40,70‐di‐2‐thienyl‐20,10,30‐benzothiadiazole (PCDTBT)/[6,6]‐phenyl C₇₁ butyric acid methyl ester (PC₇₁BM) films and devices to probe the stability under thermal annealing. Upon annealing, solar cell performance is drastically decreased for temperatures higher than 140 °C. Detailed investigation indicate changes in polymer:fullerene interactions resulting in the formation of a polymer wetting layer upon annealing at temperatures higher than 140 °C. Upon device completion this wetting layer is located close to the metal electrode and therefore leads to an increase in recombination and a decrease in charge carrier extraction, providing an explanation for the reduced fill factor (FF) and power conversion efficiency (PCE).     

Chemical Consequences of Alkali Inhomogeneity in Cu2ZnSnS4 Thin‐Film Solar Cells

This study offers new insight into the role of Na in Cu₂ZnSnS₄ (CZTS) thin film solar cells by studying samples with a spatially varying alkali distribution. This is achieved by omitting a diffusion barrier between the soda‐lime glass substrate and the Mo back contact, where compositional variations of the glass inherently result in non‐uniform alkali distributions in the CZTS. By correlating light beam induced current (LBIC) maps with secondary ion mass spectrometry composition maps, it is shown that samples containing regions of higher Na concentration (“hot spots”) have corresponding LBIC hot spots on comparable length scales. Samples containing an alkali diffusion barrier have lower LBIC dispersion; thus, LBIC can be used to evaluate non‐uniformity in CZTS devices, where a common cause is Na inhomogeneity. Moreover, it is shown that the Na hot spots are strongly correlated with other compositional variations in the device, including increased Cu in‐diffusion with the underlying MoS₂ layer and decreased diffusion of Cd to the back contact. Neither of these effects are well understood in CZTS devices, and neither have previously been correlated with the presence or absence of Na.     

The Role of Sodium as a Surfactant and Suppressor of Non‐Radiative Recombination at Internal Surfaces in Cu2ZnSnS4

It is well‐known that sodium improves the performance of Cu₂ZnSnS₄ (CZTS) devices, yet the mechanism of the enhancement is still not fully understood. This work aims to present a unified account of the relationships between grain boundaries in CZTS, sodium content at these boundaries, non‐radiative recombination, and surfactant effects that produce large microstructural changes. Using temperature‐dependent photoluminescence measurements, it is demonstrated that samples containing dramatically different grain sizes display identical radiative and non‐radiative decay characteristics when sufficient sodium is present in the film. It is also shown that the sodium concentration needed to efficiently passivate non‐radiative defects is significantly less that the quantity needed to obtain micrometer‐sized CZTS grains. Finally, the high densities of donor‐acceptor pairs that are observed in CZTS films appear to reside within the grains themselves, rather than at grain boundaries.     

Targeting Ideal Dual‐Absorber Tandem Water Splitting Using Perovskite Photovoltaics and CuInxGa1‐xSe2 Photocathodes

Efficient sunlight‐driven water splitting devices can be achieved by pairing two absorbers of different optimized bandgaps in an optical tandem design. With tunable absorption ranges and cell voltages, organic–inorganic metal halide perovskite solar cells provide new opportunities for tailoring top absorbers for such devices. In this work, semitransparent perovskite solar cells are developed for use as the top cell in tandem with a smaller bandgap photocathode to enable panchromatic harvesting of the solar spectrum. A new CuIn_xGa_1‐xSe₂ multilayer photocathode is designed, exhibiting excellent performance for photoelectrochemical water reduction and representing a near‐ideal bottom absorber. When pairing it below a semitransparent CH₃NH₃PbBr₃‐based solar cell, a solar‐to‐hydrogen efficiency exceeding 6% is achieved, the highest value yet reported for a photovoltaic–photoelectrochemical device utilizing a single‐junction solar cell as the bias source under one sun illumination. The analysis shows that the efficiency can reach more than 20% through further optimization of the perovskite top absorber.     

The Importance of Fullerene Percolation in the Mixed Regions of Polymer–Fullerene Bulk Heterojunction Solar Cells

Most optimized donor‐acceptor (D‐A) polymer bulk heterojunction (BHJ) solar cells have active layers too thin to absorb greater than ～80% of incident photons with energies above the polymer's band gap. If the thickness of these devices could be increased without sacrificing internal quantum efficiency, the device power conversion efficiency (PCE) could be significantly enhanced. We examine the device characteristics of BHJ solar cells based on poly(di(2‐ethylhexyloxy)benzo[1,2‐b:4,5‐b′]dithiophene‐co‐octylthieno[3,4‐c]pyrrole‐4,6‐dione) (PBDTTPD) and [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) with 7.3% PCE and find that bimolecular recombination limits the active layer thickness of these devices. Thermal annealing does not mitigate these bimolecular recombination losses and drastically decreases the PCE of PBDTTPD BHJ solar cells. We characterize the morphology of these BHJs before and after thermal annealing and determine that thermal annealing drastically reduces the concentration of PCBM in the mixed regions, which consist of PCBM dispersed in the amorphous portions of PBDTTPD. Decreasing the concentration of PCBM may reduce the number of percolating electron transport pathways within these mixed regions and create morphological electron traps that enhance charge‐carrier recombination and limit device quantum efficiency. These findings suggest that (i) the concentration of PCBM in the mixed regions of polymer BHJs must be above the PCBM percolation threshold in order to attain high solar cell internal quantum efficiency, and (ii) novel processing techniques, which improve polymer hole mobility while maintaining PCBM percolation within the mixed regions, should be developed in order to limit bimolecular recombination losses in optically thick devices and maximize the PCE of polymer BHJ solar cells.     

Dopant‐Free Partial Rear Contacts Enabling 23% Silicon Solar Cells

Over the past five years, there has been a significant increase in both the intensity of research and the performance of crystalline silicon devices which utilize metal compounds to form carrier‐selective heterocontacts. Such heterocontacts are less fundamentally limited and have the potential for lower costs compared to the current industry dominating heavily doped, directly metalized contacts. A low temperature (≤230 °C), TiO_x/LiF_x/Al electron heterocontact is presented here, which achieves mΩcm² scale contact resistivities ρ_c on lowly doped n‐type substrates. As an extreme demonstration of the potential of this heterocontact, it is trialed in a newly developed, high efficiency n‐type solar cell architecture as a partial rear contact (PRC). Despite only contacting ≈1% of the rear surface area, an efficiency of greater than 23% is achieved, setting a new benchmark for n‐type solar cells featuring undoped PRCs and confirming the unusually low ρ_c of the TiO_x/LiF_x/Al contact. Finally, in contrast to previous versions of the n‐type undoped PRC cell, the performance of this cell is maintained after annealing at 350–400 °C, suggesting its compatibility with conventional surface passivation activation and sintering steps.     

Quantum‐Dot Sensitized Solar Cells Employing Hierarchical Cu2S Microspheres Wrapped by Reduced Graphene Oxide Nanosheets as Effective Counter Electrodes

Hierarchical Cu₂S microspheres wrapped by reduced graphene oxide (RGO) nanosheets are prepared via a one‐step solvothermal process. The amount of graphene oxide used in the synthesis process has a remarkable effect on the features of Cu₂S microspheres. Compared to Pt and Cu₂S electrodes, RGO‐Cu₂S electrodes show better electrocatalytic activity, greater stability, lower charge‐transfer resistance, and higher exchange current density. As expected, RGO‐Cu₂S electrodes exhibit superior performance when functioning as counter electrodes in CdS/CdSe quantum dot‐sensitized solar cells (QDSSCs) using a polysulfide electrolyte. A power conversion efficiency up to 3.85% is achieved for the QDSSC employing an optimized RGO‐Cu₂S counter electrode, which is higher than those of the QDSSCs featuring Pt (2.14%) and Cu₂S (3.39%) counter electrodes.     

Atomic‐Scale Observation of Oxygen Substitution and Its Correlation with Hole‐Transport Barriers in Cu2ZnSnSe4 Thin‐Film Solar Cells

Kesterite‐type Cu₂ZnSn(S,Se)₄ has been extensively studied over the past several years, with researchers searching for promising candidates for indium‐ and gallium‐free inexpensive absorbers in high‐efficiency thin‐film solar cells. Many notable experimental and theoretical studies have dealt with the effects of intrinsic point defects, Cu/Zn/Sn nonstoichiometry, and cation impurities on cell performance. However, there have been few systematic investigations elucidating the distribution of oxygen at an atomic scale and the correlation between oxygen substitution and charge transport despite unavoidable incorporation of oxygen from the ambient atmosphere during thin‐film fabrication. Using energy‐dispersive X‐ray spectroscopy, scanning transmission electron microscopy, and electron energy‐loss spectroscopy, the presence of nanoscale layers is directly demonstrated in which oxygen is substantially substituted for Se, near grain boundaries in polycrystalline Cu₂ZnSnSe₄ films. Density‐functional theory calculations also show that oxygen substitution remarkably lowers the valence band maximum and subsequently widens the overall bandgap. Consequently, anion modification by oxygen can make a major contribution to the formation of a robust barrier blocking the holes from bulk grains into grain boundaries, thereby efficiently attaining electron?hole separation. The findings provide crucial insights into achieving better energy conversion efficiency in kesterite‐based thin‐film solar cells through optimum control of oxidation during the fabrication process.