IBC Solar Cells Research Articles

Analyzing the operational versatility of advanced IBC solar cells at different temperatures and also with variation in minority carrier lifetimes

Analyzing the operational versatility of advanced IBC solar cells at different temperatures and also with variation in minority carrier lifetimes

Plasma‐Enhanced Chemical‐Vapor‐Deposited SiOx(Ny)/n‐type Polysilicon‐on‐Oxide‐Passivating Contacts in Industrial Back‐Contact Si Solar Cells

In this article, different in situ grown plasma‐enhanced chemical vapor deposition (PECVD)‐grown interfacial oxides for n‐type polysilicon‐passivating contacts are investigated. Herein, SiOx(Ny)/n‐type amorphous silicon stacks created from either N2O plasma or O2 plasma are applied to POLy‐silicon on Oxide interdigitated back‐contact (POLO IBC) solar cells using the structured deposition process through a glass mask to create the IBC layout. The impact of plasma exposure time for interfacial oxide growth on solar cell efficiencies is experimentally determined. In the POLO IBC cell results, it is shown that the PECVD oxides SiOxNy and SiOx with optimized plasma exposure time give similar maximum efficiencies of 23.8% and 23.7%, respectively. In these data, the feasibility to deposit a high‐quality in situ PECVD interfacial SiOx(Ny) layers for surface passivation and current transport of passivated contacts at the same time is demonstrated. For the SiOx/n‐type polysilicon stack, it is found that both plasma exposure time for interfacial oxide growth and polysilicon anneal temperature variations can lead to similar optimum of solar cell efficiencies. The current open‐circuit voltage losses due to metallization for the solar cells are analyzed and a realistic efficiency of 25.22% is calculated to achieve optimized POLO IBC solar cells applying the synergistic efficiency gain analysis on Quokka3 simulations.

Evaluating the Practical Efficiency Limit of Silicon Heterojunction–Interdigitated Back Contact Solar Cells by Creating Digital Twins of Silicon Heterojunction Solar Cells with Amorphous Silicon and Nanocrystalline Silicon Hole Contact Layers

Improving power conversion efficiency (PCE) in photovoltaics has driven innovative approaches in solar cell design and technology. Silicon heterojunction (SHJ) solar cells exhibit advantages in PCE due to their effective passivating contact structures. SHJ–interdigitated back contact (SHJ–IBC) solar cells have the potential to surpass traditional SHJ cells, attributed to their advantage in short‐circuit current (JSC). Herein, Silvaco Atlas technology computer‐aided design is used to create digital twins of high‐efficiency SHJ solar cells with amorphous silicon and nanocrystalline silicon hole selective contact (HSC) layers. Using parameters from digital twins of SHJ solar cells, the practical efficiency limit of SHJ–IBC solar cells is assessed. The results show that SHJ–IBC cells can achieve potential efficiencies of 27.01% with amorphous HSC and 27.38% with nanocrystalline HSC. Further efficiency augmentation to 27.51% can be achieved by narrowing the gap from 80 to 20 μm. This study not only advances comprehension of SHJ–IBC solar cells but also provides insights into optimizing geometrical configurations for improved performance. The utilization of digital twins provides a valuable tool for predicting and evaluating the performance of SHJ–IBC solar cells, contributing substantively to the ongoing development of high‐efficiency photovoltaic technology.

Optically‐Boosted Planar IBC Solar Cells with Electrically‐Harmless Photonic Nanocoatings

AbstractAdvanced light management via front‐coated photonic nanostructures is a promising strategy to enhance photovoltaic (PV) efficiency through wave‐optical light‐trapping (LT) effects, avoiding the conventional texturing processes that induce the degradation of electrical performance due to increased carrier recombination. Titanium dioxide (TiO2) honeycomb arrays with different geometry are engineered through a highly‐scalable colloidal lithography method on flat crystalline silicon (c‐Si) wafers and tested on standard planar c‐Si interdigitated back‐contact solar cells (pIBCSCs). The photonic‐structured wafers achieve an optical photocurrent of 36.6 mA cm−2, mainly due to a broad anti‐reflection effect from the 693 nm thick nanostructured coatings. In contrast, the pIBCSC test devices reach 14% efficiency with 679 nm thick TiO2 nanostructures, corresponding to a ≈30% efficiency gain relative to uncoated pIBCSCs. In addition, several designed structures show unmatched angular acceptance enhancements in efficiency (up to 63% gain) and photocurrent density (up to 68% gain). The high‐performing (yet electrically harmless) LT scheme, here presented, entails an up‐and‐coming alternative to conventional texturing for c‐Si technological improvement that can be straightforwardly integrated into the established PV industry.

Towards 28 %-efficient Si single-junction solar cells with better passivating POLO junctions and photonic crystals

We conduct numerical device simulations to study to what extend poly-Si on oxide (POLO)2 IBC solar cells can be optimized. In particular, we evaluate the benefit of the concept of photonic crystals (PCs) for “standard” cell thicknesses compatible with industrial wafer handling. We find that for our current surface passivation quality, implementing PCs and decreasing the wafer thickness down to 15 μm would increase the efficiency by „only“ 1% absolute due to limiting surface recombination losses. We deduce a high c-Si/SiOx interface state density Dit of 2.9 × 1012 eV−1cm−2 by analyzing special two-terminal IV measurements on small pads that contact the intact interfacial oxide between pinholes with our MarcoPOLO model. Consequently, we improve the hydrogenation process of our POLO junctions by an Al2O3/SiNx/Al2O3 rear-side dielectric layer stack. For n-type POLO (p-type POLO) J0 is reduced from 4 (10) fA/cm2 down to 0.5 ± 0.3 (3.3 ± 0.7) fA/cm2. For this improved surface passivation, our numerical device simulations predict an efficiency potential of 29.1% (27.8%) for POLO2 IBC cells with (without) PCs for a standard thickness of 150 μm. This shows that the “practical limit” for Si solar cells with poly-Si on oxide-based passivating contact schemes is above 27%, and, in general, that the efficiency potential of Si single-junction cells is still far from being exhausted. The first implementation of the improved POLO junctions into cell precursors confirms the predicted improvement on the level of suns - implied open-circuit voltage curves.

Colored optic filters on c‐Si IBC solar cells for building integrated photovoltaic applications

AbstractBuilding Integrated Photovoltaic systems can produce a significant portion of the energy demand of urban areas. Despite their potential, they remain a niche technology that architects and project engineers still find esthetically limited. The dark blue or black color of standard photovoltaic panels is considered inappropriate for restoration projects of historic buildings and represents a major constraint on the development of new projects. This work will provide insight into how the use of optic filters can offer new pathways for architectural acceptance of photovoltaic panels. Optic filters selectively reflect or transmit light by interference and can be designed and fabricated using cost‐effective and industrially compatible processes. By using in‐house developed ray tracing software coupled with TCAD Sentaurus, more than 400 colors were obtained, and their impact on the opto‐electrical performance of interdigitated back‐contacted solar cells was studied. Results show a maximum efficiency loss of 1.6% absolute at the perpendicular incidence of light on the range of obtained colors when compared with a standard dark blue solar cell. Simulations for different angles of incidence showed that the current reduction on the standard device could be modeled using a cosine relationship. The colored cells, however, deviated significantly from this relationship. We propose that the angular behavior of any cell (colored or standard) could be simulated by modifying the effective irradiance with scaling factors equal to the ratios of the photogenerated current at any angle with respect to the value at normal incidence. We demonstrate that this approach accurately models the effect of the color filter and allows for an easy transition from a bare cell to an encapsulated one. Due to the spectral effect of the filter, we developed both a spectrally resolved optical model and a two‐dimensional finite volume transient thermal model. In case of the optical model, we demonstrate an accuracy in the prediction of the reflectance produced by the color with values of mean bias error (MBE) between 2.0% and 3.9%. As for the thermal model, it was validated by first analyzing a standard model under conditions of nominal operating cell temperature and then comparing its results with published scientific literature. Later, we compare its prediction against 2 weeks of measurements. In both cases the thermal model proves an adequate accuracy, yielding differences below 1.5°C with respect to other scientific works and an MBE value of 0.89°C as well as a root‐mean‐square error value of 2.10°C for the entire measurement period. With the validated models, we studied the effect of the encapsulation on the color perception. We present two options of color filters. The first one produces relatively low reflectance losses and presents relative annual direct current (DC) energy losses of up to 6.4% for Delft, in the Netherlands, and up to 5.9% for Alice Springs in Australia. However, this first option has very poor color brightness. The second studied filter produces highly saturated bright colors. Improving brightness can increase the annual DC relative losses up to 13.7% and 13.5% for Delft and Alice Springs, respectively. Overall, we demonstrate that colored filters based on multilayer optical stacks are a versatile option for coloring cells that allow a good compromise between esthetics and performance.

Highly reproducible c-Si texturing by metal-free TMAH etchant and monoTEX agent

Surface texturing of a silicon solar cell is critical to provide surface antireflection and light trapping. The common texturing method based on KOH as an etchant with isopropyl alcohol (IPA) as a wetting agent suffers two disadvantages: introducing metal contamination and low repeatability. To circumvent the limitation of the KOH-IPA method, we develop a new texturing regime by substituting TMAH for KOH, and a commercial surfactant RENA monoTEX for IPA. This TMAH-monoTEX method shows advantages of non-metal contamination, high reproducibility, short process time and small random pyramids. IBC solar cells fabricated with the TMAH-monoTEX texture achieved an efficiency of 25% with Jsc of 42.9 mA/cm2, Voc of 719 mV and FF of 81.1%.

EL and LBIC Characterization of Cut Edge Recombination in IBC Solar Cells

EL and LBIC Characterization of Cut Edge Recombination in IBC Solar Cells

Separating the two polarities of the POLO contacts of an 26.1%-efficient IBC solar cell

By applying an interdigitated back contacted solar cell concept with poly-Si on oxide passivating contacts an efficiency of 26.1% was achieved recently. In this paper the impact of the implemented initially intrinsic poly-Si region between p-type poly-Si and n-type poly-Si regions is investigated. Two recombination paths are identified: The recombination at the interface between the initially intrinsic poly-Si and the wafer as well as the recombination across the resulting p(i)n diode on the rear side which is aimed to be reduced by introducing an initially intrinsic region. By using test structures, it is demonstrated that the width of the initially intrinsic region ((i) poly-Si region) has a strong influence on the recombination current through the p(i)n diode and that this initially intrinsic region needs to be about 30 μm wide to sufficiently reduce the recombination across the p(i)n diode. Lateral and depth-resolved time of flight secondary ion mass spectrometry analysis shows that the high-temperature annealing step causes a strong lateral inter-diffusion of donor and acceptor atoms into the initially intrinsic region. This diffusion has a positive impact on the passivation quality at the c-Si/SiOx/i poly-Si interface and is thus essential for achieving an independently confirmed efficiency of 26.1% with 30 μm-wide initially intrinsic poly-Si regions.

High-efficiency Crystalline Silicon Solar Cells: A Review

Solar energy is a clean renewable energy resource that can be converted to electricity with photovoltaic (PV) technology without environmental damage. Solar energy can be transformed to electricity using a range of technologies, but crystalline silicon (c-Si)-based PV technology dominates in the PV market due to the high efficiency, long-term stability, reliability, and second most abundant (27%) material. Recently, c-Si solar cells achieved an outstanding efficiency of 26.7% through silicon heterojunction technology combined with an interdigitated back contact structure. Most industries and researchers are attempting to improve the efficiency further to reach the silicon limit. The dominant position of crystalline silicon solar cell in large-area electricity production and industrialization motivated us to write this review paper. This review paper covers the key factors that affect the efficiency, such as structure, process optimization, cost reduction strategies. In addition, some promising cell structures, such as PERC, IBC, HIT, and HBC solar cell, and their efficiencies are reported. Overall, this study provides a detailed idea to the new photovoltaic researchers regarding the solar cell structure, their efficiencies, and future potential of solar cells.

Optimization of tunnel-junction IBC solar cells based on a series resistance model

This work presents the upscaling of the tunnel IBC technology on large area, Czochralski (Cz) n-type wafers. At the junction level, a self-aligned PECVD masking technology has been developed for the deposition of hydrogenated nano-crystalline silicon (nc-Si:H) layers on industrial 6-inch pseudo-square wafers. This damage free patterning technology allows state-of-the-art passivation with a minority carrier lifetime of 9 ms at an injection level of 1015 cm-3, thus enabling extremely long diffusion lengths up to several millimetres. The use of indium-free, cost effective aluminium-doped zinc oxide strongly reduces the materials bill of the tunnel-IBC technology while maintaining very low contact resistance for both the electron and the hole contacts. Remarkably, these tunnel-IBC devices demonstrated a conversion efficiency of 25% on large area (90.25 cm2) industrial wafer with a thickness of 155 μm. Series resistance analysis points out probable losses from the hole contact and the base. The limitation of the Transfer Length Method is discussed when used to extract the hole contact resistance.

Record-high solar-to-hydrogen conversion efficiency based on a monolithic all-silicon triple-junction IBC solar cell

We present a record-high solar-to-hydrogen conversion efficiency (STH) for monolithic all-silicon multi-junction solar devices. The device is based on an interdigitated back-contact silicon solar cell, in which the p- and n-regions are connected in a combination of series and parallel contacts, in order to triple the photovoltage compared to a single-junction cell. Our best device provides an open-circuit voltage of 1.99 V, larger than the water redox potential of 1.23 V plus overpotentials at the electrodes, as well as a short-circuit current density of 12.6 mA/cm2. Coupled to a sulphuric acid electrolysis system, with platinum and ruthenium dioxide electrodes, our device shows an STH of 14.5% at 1.63 V.

Efficient Low-Cost IBC Solar Cells with a Front Floating Emitter: Structure Optimization and Passivation Layer Study

In this paper, we investigate interdigitated back contact solar cells with the front floating emitter structure systematically by using simulated and experimental methods. By comparing the front floating emitter structure with the front surface field structure, it is found that the efficiency of solar cells with the front surface field structure quickly reduces with the increasing of back surface field width; while solar cells with the front floating emitter structure can have a wider front surface field width range with minimum impact on the cell efficiency. More importantly, solar cells with the front floating emitter structure have a larger fabrication process tolerance, especially for the back surface field width, emitter width, and the bulk resistivity, which means that the fabrication process flow can be simplified and the production cost can be reduced. Based on the above results, large area (156.75 mm × 156.75 mm) interdigitated back contact solar cells with the front floating emitter structure are fabricated by using the simplified process with only one masking step. SiOx:B is used as the passivation layer, which can lead to a higher open circuit voltage and lower surface saturation current density. Finally, an efficiency of 20.39% is achieved for the large area solar cells.

In situ simultaneous photovoltaic and structural evolution of perovskite solar cells during film formation

Simultaneous GI-WAXS diffraction patterns and JV measurement of IBC solar cells during in situ anneal.

Heterojunction IBC Solar Cells on Thin (< 50μm) Epitaxial Si Foils Produced from Kerfless Layer Transfer Process

Heterojunction IBC Solar Cells on Thin (< 50μm) Epitaxial Si Foils Produced from Kerfless Layer Transfer Process

Poly-Si(O)x passivating contacts for high-efficiency c-Si IBC solar cells

Highest conversion efficiency in crystalline silicon (c-Si) solar cells can be enabled by quenching minority carriers’ recombination at c-Si/contact interface owing to carrier-selective passivating contacts. With the semi-insulating poly-crystalline silicon (SIPOS, poly-Si) a very good passivation of c-Si surfaces was obtained. We have explored these passivating structures on IBC solar cells and obtained over 22% efficiency with over 23% within reach on the short term. We present in detail the passivation quality of p-type and n-type ion-implanted LPCVD poly-crystalline silicon (poly-Si) and its relation to the doping profile. Optimized poly-Si layers in the role of emitter and BSF showed excellent passivation (J0,emitter = 11.5 fA/cm2 and J0,BSF = 4.5 fA/cm2) and have been deployed in FSF-based IBC c-Si solar cells using a simple self-aligned patterning process. Applying an optimized passivation of FSF by PECVD a-Si:H/SiNx layer (J0,FSF = 6.5 fA/cm2) leads to a cell with efficiency of 22.1% (VOC = 709 mV, JSC = 40.7 mA/cm2, FF = 76.6%). Since over 83% FF has been reached with adjusted metallization technology on similar IBC structures, we believe 23% efficiency is within reach on the short term. Further improvement, especially at JSC level, is expected by deploying less absorbing carrier-selective passivating contacts based on poly-Si or wide bandgap poly-SiOx layers (J0 ~ 10 fA/cm2).

Extraction of individual components of series resistance using TCAD simulations meeting the requirements of 2- and 3-dimensional carrier flow of IBC solar cells

The series resistance (RS) of a solar cell, that influences fill factor and thus efficiency, is usually decomposed into individual components for development of optimization strategies. In this contribution we introduce a method to extract Rs of each part of the solar cell from TCAD simulation using free energy loss analysis. This method is particularly relevant for modern cell concepts where only TCAD simulation describes correctly high injection phenomena in lowly doped wafers, and where 2- or 3-dimensional carrier flow occurs (PERC, PERT, IBC) invalidating the classical analytical models for Rs description. We compare the lumped RS value extracted from the FELA to the RS extracted from the Multiple Light Intensity Method (MLIM) for IBC solar cells with various geometries and wafer doping levels, with and without a front surface field, and show that it is quite accurate even when high injection phenomena induce a slight underestimation of RS.

Low Cost Semi Automated Assembly Unit for Small Size Back Contact Modules and Low Cost Interconnection Approach

We present our low cost assembly unit to manufacture back contact solar modules based on the conductive backsheet (CBS) approach. This in house developed apparatus was built to assemble test modules containing one up to four 6 inch back contact solar cells. The system is a retrofit of a commercially available CNC system which is equipped with a cell grabber and a manual dispensing system (by Nordson). The total cost of the setup was roughly 4000 € excluding the dispenser unit. Using this equipment we assembled several small size modules containing one and four Zebra cells, which are low cost 6 inch IBC solar cells developed at ISC Konstanz [1,2]. The contact between copper backsheet and back contact cell of the one cell modules we present here is formed by low temperature solder paste (LTSP). First cell to module (CTM) loss evaluations and reliability results suggest that this material could be a viable alternative to electrically conductive adhesive (ECA) which is currently the most commonly used material for this purpose.

Universal Passivation for p++ and n++ Areas on IBC Solar Cells

We present a universal passivation for both, phosphorus and boron doped surfaces of interdigitated back contact solar cells. The amorphous silicon layer shows excellent passivation on boron emitter and phosphorus back surface field as long as the deposition temperature is below Tdep = 200°C. The amorphous silicon enables saturation current densities Jo,em = 46 fA/cm2 for laser doped p++ boron emitters with sheet resistance Rsh,em = 90Ω/sq as well as Jo,bsf = 73 fA/cm2 for laser doped n++ phosphorus back surface fields with Rsh,bsf°=°36°Ω/sq. Integration of the amorphous silicon low temperature passivation into our laser processed IBC solar cells yields a mean efficiency η = 22.8%. The open circuit voltage gain ΔVoc amounts to ΔVoc = 5mV compared to cells with thermal silicon dioxide rear passivation. The best solar cells achieve a confirmed efficiency η = 23.24%.

Front-floating Emitter Voltage Mapping of IBC Mercury Cells

Standard characterization techniques such as LBIC are difficult to apply to IBC solar cells with a front floating emitter (FFE). This is because the cells need to be under bias illumination, and this is often impossible in commercial LBIC measurement tools. To the best of our knowledge, LBIC measurements on FFE IBC cells have not been published so far. In this work we present a experimental method to spatially characterize the FFE of IBC cells and gain insights on electrical shading losses, without using LBIC. This method makes use of the commercially available CoreScan device, mapping the FFE voltage relative to the shorted back-contacts over the cell area. Using this technique we were able to resolve busbar, pads and finger features of both polarities with a scan of the front side. The scan shows that BSF features have higher voltage than emitter features, thus giving a direct evidence of the FFE pumping effect. If coupled to circuit simulation, FFE voltage maps can be the base for estimating electrical shading losses in IBC cells. The FFE voltage maps also give information on the homogeneity of the pumping effect across the wafer, e.g. affected by diffusion non-uniformity, and at least qualitatively its effect on the cell performance.