A Grain Orientation-Independent Single-Step Saw Damage Gettering/Wet texturing Process for Efficient Silicon Solar Cells.

Improving electrical and optical properties is important in manufacturing high-efficiency solar cells. Previous studies focused on individual gettering and texturing methods to improve solar cell material quality and reduce reflection loss, respectively. This study presents a novel method called saw damage gettering with texturing that effectively combines both methods for multicrystalline silicon (mc-Si) wafers manufactured using the diamond wire sawing (DWS) method. Although mc-Si is not the Si material currently used in photovoltaic products, the applicability of this method using the mc-Si wafers as it contains all grain orientations is demonstrated. It utilizes saw damage sites on the wafer surfaces for gettering metal impurities during annealing. Additionally, it can crystallize amorphous silicon on wafer surfaces generated during the sawing process to allow conventional acid-based wet texturing. This texturing method and annealing for 10 min allow for the removal of metal impurities and effectively forms a textured DWS Si wafer. The results show that the open-circuit voltage (ΔVoc  = +29 mV), short-circuit current density (ΔJsc  = +2.5 mA cm-2 ), and efficiency (Δη = +2.1%) improved in the p-type passivated emitter and rear cells (p-PERC) manufactured using this novel method, as compared to those in the reference solar cells.

Novel Polymer-Based Organic/c-Si Monolithic Tandem Solar Cell: Enhanced Efficiency using Interlayer and Transparent Top Electrode Engineering.

Tandem solar cells which are electrically connected with various photoactive materials have the potential to solve the current challenges by exceeding the theoretically limited efficiency of single junction solar cells. Here the first monolithic organic/silicon tandem cell is reported based on a semitransparent polymer on a crystalline silicon (c-Si) substrate. Herein, experimental results are presented for four-terminal (4-T) and monolithic two-terminal (2-T) organic/c-Si tandem cells using organic cells with an inverted n-i-p structure and c-Si cells with an n-type TOPCon structure with detailed analysis. The best 4-T tandem cell efficiency is 15.22%, and 2-T results show that the top (organic) and bottom (c-Si) cells are electrically connected by an open-circuit voltage over 1.4 V. Further, a simulated efficiency of over 20% using the organic/c-Si tandem is achieved, implying the tandem efficiency can be enhanced through further improvement of electric and optical characteristics with the optimization.

Titanium Silicide: A Promising Candidate of Recombination Layer for Perovskite/Tunnel Oxide Passivated Contact Silicon Two-Terminal Tandem Solar Cells.

This study proposes a titanium silicide (TiSi2) recombination layer for perovskite/tunnel oxide passivated contact (TOPCon) 2-T tandem solar cells as an alternative to conventional transparent conductive oxide (TCO)-based recombination layers. TiSi2 was formed while TiO2 was made by oxidizing a Ti film deposited on the p+-Si layer. The reaction formation mechanism was proposed based on the diffusion theory supported by experimental results. The optical and electrical properties of the TiSi2 layer were optimized by controlling the initial Ti thicknesses (5-100 nm). With the initial Ti of 50 nm, the lowest reflectance and highly ohmic contact between the TiO2 and p+-Si layers with a contact resistivity of 161.48 mΩ·cm2 were obtained. In contrast, the TCO interlayer shows Schottky behavior with much higher contact resistivities. As the recombination layer of TiSi2 and the electron transport layer of TiO2 are formed simultaneously, the process steps become simpler. Finally, the MAPbI3/TOPCon tandem device yielded an efficiency of 16.23%, marking the first reported efficiency for a device including a silicide-based interlayer.

Titanium oxide nanomaterials as an electron-selective contact in silicon solar cells for photovoltaic devices.

To obtain high conversion efficiency, various carrier-selective contact structures are being applied to the silicon solar cell, and many related studies are being conducted. We conducted research on TiO2 to create an electron-selective contact structure that does not require a high-temperature process. Titanium metal was deposited using a thermal evaporator, and an additional oxidation process was conducted to form titanium oxide. The chemical compositions and phases of the titanium dioxide layers were analyzed by X-ray diffraction. The passivation effects of each titanium oxide layer were measured using the quasi-steady-state photoconductance. In this study, the layer properties were analyzed when TiO2 had a passivation effect on the silicon surface. The charge and interface defect densities of the layer were analyzed through CV measurements, and the passivation characteristics according to the TiO2 phase change were investigated. As a result, by applying optimized TiO2 layer thickness and annealing temperature conditions through the experiment for passivation to the cell-like structure, which is the structure before metal and electrode formation, an implied open-circuit voltage (iVoc) of 630 mV and an emitter saturation current density (J0) value of 60.4 fA/cm2 were confirmed.

Historical Analysis of High-Efficiency, Large-Area Solar Cells: Toward Upscaling of Perovskite Solar Cells.

The status and problems of upscaling research on perovskite solar cells, which must be addressed for commercialization efforts to be successful, are investigated. An 804 cm2 perovskite solar module has been reported with 17.9% efficiency, which is significantly lower than the champion perovskite solar cell efficiency of 25.2% reported for a 0.09 cm2 aperture area. For the realization of upscaling high-quality perovskite solar cells, the upscaling and development history of conventional silicon, copper indium gallium sulfur/selenide and CdTe solar cells, which are already commercialized with modules of sizes up to ≈25 000 cm2 , are reviewed. GaAs, organic, dye-sensitized solar cells and perovskite/silicon tandem solar cells are also reviewed. The similarities of the operating mechanisms between the various solar cells and the origin of different development pathway are investigated, and the ideal upscaling direction of perovskite solar cells is subsequently proposed. It is believed that lessons learned from the historical analysis of various solar cells provide a fundamental diagnosis of relative and absolute development status of perovskite solar cells. The unique perspective proposed here can pave the way toward the upscaling of perovskite solar cells.

Layer-by-Layer Self-Assembly of Hollow Nitrogen-Doped Carbon Quantum Dots on Cationized Textured Crystalline Silicon Solar Cells for an Efficient Energy Down-Shift.

Enhancing the efficiency of the crystalline silicon solar cell (c-Si SC) by coating the energy shifting layer with quantum dots (QDs) is a recent approach to efficiently utilize the high-energy spectrum of light. Carbon QDs are an attractive candidate for such applications; however, a small Stokes shift and nonuniform coating due to high aggregation are the bottlenecks to fully utilize their potential. For this purpose, here, we propose a layer-by-layer self-assembled uniform coating of eco-friendly red-emissive hollow nitrogen-doped carbon QDs (NR-CQDs) as an efficient energy-down-shifting layer. A unique hollow and conjugated structure of NR-CQDs was designed to achieve a large Stokes shift (UV-excited red emission) with a quantum yield (QY) comparable to Cd/Pb QDs. A highly uniform coating of intrinsically negatively charged NR-CQDs on c-Si SCs was achieved by cationizing the c-Si SC by bovine serum albumin (BSA) under mildly acidic conditions. By an opposite-charge-assisted, self-assembled overlayer, the short-circuit current density (Jsc) and power-conversion efficiency were increased by 5.8%, which is attributed to the large Stokes shift (255 nm) and high QY. Blue-emissive undoped carbon QDs were synthesized for comparison with the proposed NR-CQDs to elucidate the significance of the novel proposed structure.

Correction: Role of polysilicon in poly-Si/SiO x passivating contacts for high-efficiency silicon solar cells.

[This corrects the article DOI: 10.1039/C9RA03560E.].

Perovskites fabricated on textured silicon surfaces for tandem solar cells.

The silicon surface texture significantly affects the current density and efficiency of perovskite/silicon tandem solar cells. However, only a few studies have explored fabricating perovskite on textured silicon and the effect of texture on perovskite films because of the limitations of solution processes. Here we produce conformal perovskite on textured silicon with a dry two-step conversion process that incorporates lead oxide sputtering and direct contact with methyl ammonium iodide. To separately analyze the influence of each texture structure on perovskite films, patterned texture, high-resolution photoluminescence (µ-PL), and light beam-induced current (µ-LBIC), 3D mapping is used. This work elucidates conformal perovskite on textured surfaces and shows the effects of textured silicon on the perovskite layers with high-resolution 3D mapping. This approach can potentially be applied to any type of layer on any type of substrate.

Role of polysilicon in poly-Si/SiO x passivating contacts for high-efficiency silicon solar cells.

In this study, we focused on understanding the roles of a polysilicon (poly-Si) layer in poly-Si/SiO x /c-Si passivating contacts. Passivating contact formation conditions were varied by changing the doping method, annealing temperature and time, polysilicon layer thickness, and polysilicon doping concentration. Our observations indicated that the roles of polysilicon are contact, in-diffusion barrier action, field effect, gettering, and light absorption. Based on the observations, a iV OC of 741 mV was obtained. Finally, to increase J SC with high V OC, the polysilicon was etched after hydrogenation to reduce light absorption with high passivation quality. iV OC was not affected by etching; moreover, by etching the polysilicon from 300 nm to 60 nm, the cell efficiency increased from 20.48% to 20.59% with increasing J SC, constant V OC, and fill factor.

Pinhole-free TiO2/Ag(O)/ZnO configuration for flexible perovskite solar cells with ultralow optoelectrical loss.

Perovskite solar cells (PSCs) fabricated on transparent polymer substrates are considered a promising candidate as flexible solar cells that can emulate the advantages of organic solar cells, which exhibit considerable freedom in their device design thanks to their light weight and mechanically flexibility while achieving high photocurrent conversion efficiency, comparable to that of their conventional counterparts fabricated on rigid glasses. However, the full realization of highly efficient, flexible PSCs is largely prevented by technical difficulties in simultaneously attaining a transparent electrode with efficient charge transport to meet the specifications of PSCs. In this study, an effective strategy for resolving this technical issue has been devised by proposing a simple but highly effective technique to fabricate an efficient, multilayer TiO2/Ag(O)/ZnO (TAOZ) configuration. This configuration displays low losses in optical transmittance and electrical conductivity owing to its completely continuous, ultrathin metallic Ag(O) transparent electrode, and any notable current leakage is suppressed by its pinhole-free TiO2 electron transport layer. These features are a direct consequence of the rapid evolution of Ag(O) and TiO2 into ultrathin, completely continuous, pinhole-free layers owing to the dramatically improved wetting of metallic Ag(O) with a minimal dose of oxygen (ca. 3 at%) during sputtering. The TAOZ configuration exhibits an average transmittance of 88.5% in the spectral range of 400-800 nm and a sheet resistance of 8.4 Ω sq-1 while demonstrating superior mechanical flexibility to that of the conventional TiO2 on ITO configuration. The photocurrent conversion efficiency of flexible PSCs is significantly improved by up to 11.2% thanks to an optimum combination of optoelectrical performance and pinhole-free morphologies in the TAOZ configuration.

Impact of Buffer Layer Process and Na on Shunt Paths of Monolithic Series-connected CIGSSe Thin Film Solar Cells.

The illuminated current-voltage characteristics of Cu(In,Ga)(S,Se)2 (CIGSSe) thin film solar cells fabricated using two different buffer layer processes: chemical bath deposition (CBD) and atomic layer deposition (ALD) were investigated. The CIGSSe solar cell with the ALD buffer showed comparable conversion efficiency to the CIGSSe solar cell with CBD buffer but lower shunt resistance even though it showed lower point shunt defect density as measured in electroluminescence. The shunt paths were investigated in detail by capturing the high-resolution dark lock-in thermography images, resolving the shunt resistance contributions of the scribing patterns (P1, P3), and depth profiling of the constituent elements. It was found that the concentration of Na from the soda-lime glass substrate played a key role in controlling the shunt paths. In the ALD process, Na segregated at the surface of CIGSSe and contributed to the increase in the shunt current through P1 and P3, resulting in a reduction in the fill factor of the CIGSSe solar cells.

Structural evolution of tunneling oxide passivating contact upon thermal annealing.

We report on the structural evolution of tunneling oxide passivating contact (TOPCon) for high efficient solar cells upon thermal annealing. The evolution of doped hydrogenated amorphous silicon (a-Si:H) into polycrystalline-silicon (poly-Si) by thermal annealing was accompanied with significant structural changes. Annealing at 600 °C for one minute introduced an increase in the implied open circuit voltage (Voc) due to the hydrogen motion, but the implied Voc decreased again at 600 °C for five minutes. At annealing temperature above 800 °C, a-Si:H crystallized and formed poly-Si and thickness of tunneling oxide slightly decreased. The thickness of the interface tunneling oxide gradually decreased and the pinholes are formed through the tunneling oxide at a higher annealing temperature up to 1000 °C, which introduced the deteriorated carrier selectivity of the TOPCon structure. Our results indicate a correlation between the structural evolution of the TOPCon passivating contact and its passivation property at different stages of structural transition from the a-Si:H to the poly-Si as well as changes in the thickness profile of the tunneling oxide upon thermal annealing. Our result suggests that there is an optimum thickness of the tunneling oxide for passivating electron contact, in a range between 1.2 to 1.5 nm.

Optical Transmittance Enhancement of Flexible Copper Film Electrodes with a Wetting Layer for Organic Solar Cells.

The development of highly efficient flexible transparent electrodes (FTEs) supported on polymer substrates is of great importance to the realization of portable and bendable photovoltaic devices. Highly conductive, low-cost Cu has attracted attention as a promising alternative for replacing expensive indium tin oxide (ITO) and Ag. However, highly efficient, Cu-based FTEs are currently unavailable because of the absence of an efficient means of attaining an atomically thin, completely continuous Cu film that simultaneously exhibits enhanced optical transmittance and electrical conductivity. Here, strong two-dimensional (2D) epitaxy of Cu on ZnO is reported by applying an atomically thin (around 1 nm) oxygen-doped Cu wetting layer. Analyses of transmission electron microscopy images and X-ray diffraction patterns, combined with first-principles density functional theory calculations, reveal that the reduction in the surface and interface free energies of the wetting layers with a trace amount (1-2 atom %) of oxygen are largely responsible for the two-dimensional epitaxial growth of the Cu on ZnO. The ultrathin 2D Cu layer, embedded between ZnO films, exhibits a highly desirable optical transmittance of over 85% in a wavelength range of 400-800 nm and a sheet resistance of 11 Ω sq-1. The validity of this innovative approach is verified with a Cu-based FTE that contributes to the light-to-electron conversion efficiency of a flexible organic solar cell that incorporates the transparent electrode (7.7%), which far surpasses that of a solar cell with conventional ITO (6.4%).

Relationship between ion migration and interfacial degradation of CH3NH3PbI3 perovskite solar cells under thermal conditions.

Organic-inorganic hybrid perovskite solar cells (PSCs) have been extensively studied because of their outstanding performance: a power conversion efficiency exceeding 22% has been achieved. The most commonly used PSCs consist of CH3NH3PbI3 (MAPbI3) with a hole-selective contact, such as 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spiro-bifluorene (spiro-OMeTAD), for collecting holes. From the perspective of long-term operation of solar cells, the cell performance and constituent layers (MAPbI3, spiro-OMeTAD, etc.) may be influenced by external conditions like temperature, light, etc. Herein, we report the effects of temperature on spiro-OMeTAD and the interface between MAPbI3 and spiro-OMeTAD in a solar cell. It was confirmed that, at high temperatures (85 °C), I- and CH3NH3+ (MA+) diffused into the spiro-OMeTAD layer in the form of CH3NH3I (MAI). The diffused I- ions prevented oxidation of spiro-OMeTAD, thereby degrading the electrical properties of spiro-OMeTAD. Since ion diffusion can occur during outdoor operation, the structural design of PSCs must be considered to achieve long-term stability.

Investigation of Thermally Induced Degradation in CH3NH3PbI3 Perovskite Solar Cells using In-situ Synchrotron Radiation Analysis.

In this study, we employ a combination of various in-situ surface analysis techniques to investigate the thermally induced degradation processes in MAPbI3 perovskite solar cells (PeSCs) as a function of temperature under air-free conditions (no moisture and oxygen). Through a comprehensive approach that combines in-situ grazing-incidence wide-angle X-ray diffraction (GIWAXD) and high-resolution X-ray photoelectron spectroscopy (HR-XPS) measurements, we confirm that the surface structure of MAPbI3 perovskite film changes to an intermediate phase and decomposes to CH3I, NH3, and PbI2 after both a short (20 min) exposure to heat stress at 100 °C and a long exposure (>1 hour) at 80 °C. Moreover, we observe clearly the changes in the orientation of CH3NH3+ organic cations with respect to the substrate in the intermediate phase, which might be linked directly to the thermal degradation processes in MAPbI3 perovskites. These results provide important progress towards improved understanding of the thermal degradation mechanisms in perovskite materials and will facilitate improvements in the design and fabrication of perovskite solar cells with better thermal stability.

Electric-Field-Induced Degradation of Methylammonium Lead Iodide Perovskite Solar Cells.

Perovskite solar cells have great potential for high efficiency generation but are subject to the impact of external environmental conditions such as humidity, UV and sun light, temperature, and electric fields. The long-term stability of perovskite solar cells is an important issue for their commercialization. Various studies on the stability of perovskite solar cells are currently being performed; however, the stability related to electric fields is rarely discussed. Here the electrical stability of perovskite solar cells is studied. Ion migration is confirmed using the temperature-dependent dark current decay. Changes in the power conversion efficiency according to the amount of the external bias are measured in the dark, and a significant drop is observed only at an applied voltage greater than 0.8 V. We demonstrate that perovskite solar cells are stable under an electric field up to the operating voltage.

Electrocatalytic activity of NiO on silicon nanowires with a carbon shell and its application in dye-sensitized solar cell counter electrodes.

To improve the catalytic activity of a material, it is critical to maximize the effective surface area by directly contacting the electrolyte. Nanowires are a promising building block for catalysts in electrochemical applications because of their large surface area. Nickel oxide (NiO) decoration was achieved by drop-casting a nickel-dissolved solution onto vertically aligned silicon nanowire arrays with a carbon shell (SiNW/C). Based on the hybridization of the NiO and silicon nanowire arrays with a carbon shell this study aimed to achieve a synergic effect for the catalytic activity performance. This study demonstrated that the resulting nanomaterial exhibits excellent electrocatalytic activity and performs well as a counter electrode for dye-sensitized solar cells (DSSCs). The compositions of the materials were examined using X-ray diffraction, X-ray photoelectron spectroscopy, and energy dispersive spectroscopy. Their micro- and nano-structures were investigated using scanning electron microscopy and transmission electron microscopy. The electrochemical activity toward I(-)/I3(-) was examined using cyclic voltammetry and electrochemical impedance spectroscopy. The obtained peak power conversion efficiency of the DSSC based on the NiO@SiNW/C counter electrode was 9.49%, which was greater than that of the DSSC based on the Pt counter electrode.

Effects of Current-injection Firing with Ag Paste in a Boron Emitter.

A high contact resistance for screen-printed contacts was observed when a conventional Ag paste was used on a boron emitter. The results of this study suggest that electron injection during firing is one of the processes that contribute to a lower contact resistance. Larger quantities of Ag precipitates formed upon electron injection into the boron emitter, which was confirmed by observing Ag crystallite or dendrite structures on the boron and by measuring the contact resistance between the boron emitter and the Ag bulk. The electron-injected sample had approximately 10000 times lower contact resistance than an untreated sample. The contact resistance of the electron-injected sample was 0.021 mΩ âˆ™ cm(2) under optimal conditions, which is lower than that of conventional p-type silicon solar cells. Thus, electron injection can effectively lower contact resistance when using Ag paste in n-type silicon solar cells. During the cooling in the firing process, dissolved Ag ions in the glass layer are formed as dendrites or crystallites/particles. The dendrites are formed earlier than others via electrochemical migration under electron injection conditions. Then, crystallites and particles are formed via a silicon etching reaction. Thus, Ag ions that are not formed as dendrites will form as crystallites or particles.

UV Degradation and Recovery of Perovskite Solar Cells.

Although the power conversion efficiency of perovskite solar cells has increased from 3.81% to 22.1% in just 7 years, they still suffer from stability issues, as they degrade upon exposure to moisture, UV light, heat, and bias voltage. We herein examined the degradation of perovskite solar cells in the presence of UV light alone. The cells were exposed to 365 nm UV light for over 1,000 h under inert gas at <0.5 ppm humidity without encapsulation. 1-sun illumination after UV degradation resulted in recovery of the fill factor and power conversion efficiency. Furthermore, during exposure to consecutive UV light, the diminished short circuit current density (Jsc) and EQE continuously restored. 1-sun light soaking induced recovery is considered to be caused by resolving of stacked charges and defect state neutralization. The Jsc and EQE bounce-back phenomenon is attributed to the beneficial effects of PbI2 which is generated by the decomposition of perovskite material.