Silicon Heterojunction Solar Cells Research Articles

Observation of negative capacitance in silicon heterojunction solar cells: role of front contact in carrier depopulation

The origin of the low-frequency inductive loop in the Nyquist plot of the Ag/indium tin oxide (ITO)/p-a-Si:H/intrinsic hydrogenated amorphous silicon (i-a-Si:H)/c-Si/i-a-Si:H/n-a-Si:H/ITO/Al heterojunction (SHJ) solar cells and their effect on the device performance are investigated by adopting impedance spectroscopy under dark and light. The negative capacitance/low-frequency inductive loop originates from the depopulation of injected charge carriers due to a transport barrier at the p-a-Si:H/ITO interface. The p-a-Si:H hole-selective SHJ device with a low-frequency inductive loop also has shown an S-shape and associated performance degradation in the light current density–voltage characteristics due to the opposing field type transport barrier present at the p-a-Si:H/ITO interface, which was overcome after vacuum annealing at ∼200 °C. However, the NiO x -based hole-selective contact Ag/ITO/NiO x /i-a-Si:H/c-Si/i-a-Si:H/n-a-Si:H/ITO/Al SHJ cells have not shown any low-frequency inductive loop or corresponding S-shape and associated performance degradation due to the optimised contact (minimum resistance) between the NiOx and ITO layers.

Bifacial silicon heterojunction solar cells using transparent‐conductive‐oxide‐ and dopant‐free electron‐selective contacts

AbstractThe development of transparent electron‐selective contacts for dopant‐free carrier‐selective crystalline silicon (c‐Si) heterojunction (SHJ) solar cells plays an important role in achieving high short‐circuit current density (JSC) and consequently high photoelectric conversion efficiencies (PCEs). This becomes even more important when focusing on the development of bifacial solar cells. In this study, bifacial SHJ solar cells using a transparent‐conductive‐oxide‐free and dopant‐free electron‐selective passivating contacts are developed, showing a JSC bifaciality of up to 97%. Intrinsic ZnOX layer deposited by atomic layer deposition was used in this structure, which simultaneously provides negligible passivation loss after annealing and enables a low contact resistivity on the electron‐selective contact. With both side finger metal electrodes contact, this bifacial solar cell shows an efficiency of 21.2% under front‐side irradiation and 20.4% under rear‐side irradiation, resulting in an estimated output power density of 24.1 mW/cm2 when considering rear‐side irradiance of 0.15 sun.

The effect of the curing process on illuminated annealing of silicon heterojunction solar cells

The effect of the curing process on illuminated annealing of silicon heterojunction solar cells

Towards a cutting‐edge metallization process for silicon heterojunction solar cells with very low silver laydown

AbstractWithin this work, we investigate the potential to optimize the screen‐printed front side metallization of silicon heterojunction (SHJ) solar cells. Three iterative experiments are conducted to evaluate the impact of the utilized fine mesh screen configurations and grid layout adaption (finger pitch) for the front side metallization on silver laydown and electrical performance of the solar cells. With respect to the screen configuration, we compare the performance of a fine‐mesh knotless screen to a conventionally angled screen demonstrating an additional gain of Δη = +0.1%abs due to reduced shading losses. Additionally, a grid layout is improved by increasing the number of contact fingers from 120 to 156. Furthermore, the current possibility to push the fine‐line printing process for low‐temperature pastes to the limit is investigated by reducing the nominal finger width wn to 20, 18, and 15 μm. It is shown that even the smallest nominal width of wn = 15 μm can be printed with high quality, leading to an additional efficiency gain of Δη = +0.15%abs as well as a reduction of silver paste laydown by −5 mg. Finally, a batch of champion cells is fabricated by applying the findings of the previous experiments, which results in a maximum efficiency of ηmax = 23.2%. Compared to the reference group without optimization, this corresponds to a gain of Δη = +0.17%abs, which comes along with an additional decrease of the silver paste laydown by approximately −2 mg. This emphasizes the significance of consistent optimization of the screen‐printing process in terms of cell performance and resource utilization for SHJ solar cells.

Interdigitated‐back‐contacted silicon heterojunction solar cells featuring novel MoOx‐based contact stacks

AbstractThe fabrication process of interdigitated‐back‐contacted silicon heterojunction (IBC‐SHJ) solar cells has been significantly simplified with the development of the so‐called tunnel‐IBC architecture. This architecture utilizes a highly conductive (p)‐type nanocrystalline silicon (nc‐Si:H) layer deposited over the full substrate area comprising pre‐patterned (n)‐type nc‐Si:H fingers. In this context, the (p)‐type nc‐Si:H layer is referred to as blanket layer. As both electrodes are connected to the same blanket layer, the high lateral conductivity of (p)nc‐Si:H layer can potentially lead to relatively low shunt resistance in the device, thus limiting the performance of such solar cells. To overcome such limitation, we introduce a thin (<2 nm) full‐area molybdenum oxide (MoOx) layer as an alternative to the (p)nc‐Si:H blanket layer. We demonstrate that the use of such a thin MoOx minimizes the shunting losses thanks to its low lateral conductivity while preserving the simplified fabrication process. In this process, a novel (n)‐type nc‐Si:H/MoOx electron collection contact stack is implemented within the proposed solar cell architecture. We assess its transport mechanisms via electrical simulations showing that electron transport, unlike in the case of tunnel‐IBC, occurs in the conduction band fully. Moreover, the proposed contact stack is evaluated in terms of contact resistivity and integrated into a proof‐of‐concept front/back‐contacted (FBC) SHJ solar cells. Contact resistivity as low as 100 mΩcm2 is achieved, and fabricated FBC‐SHJ solar cells obtain a fill factor above 81.5% and open‐circuit voltage above 705 mV. Lastly, the IBC‐SHJ solar cells featuring the MoOx blanket layer are fabricated, exhibiting efficiencies up to 21.14% with high shunt resistances above 150 kΩcm2. Further optimizations in terms of layer properties and fabrication process are proposed to improve device performance and realize the efficiency potential of our novel IBC‐SHJ solar cell architecture.

Solution Processing Silicon Heterojunction Photocathode for Efficient and Stable Hydrogen Production.

Efficient and stable photocathodes are crucial for the development of photoelectrochemical (PEC) water-splitting devices. Silicon heterojunction (SHJ) solar cell is one of the most advanced photovoltaic cells. However, due to the instability of its outermost indium tin oxide (ITO) layers in the electrolyte, a protective layer needs to be introduced on its surface. Previously reported high-quality protective layers almost all involved the use of expensive thin film manufacturing techniques such as atomic layer deposition (ALD). In this work, for the first time, a new strategy is proposed of modifying SHJ-based photocathode with yttrium hydroxide (Y(OH)3) through two-step solution methods to simultaneously improve the stability and activity. The optimized SHJ photocathode exhibits a high applied bias photon-to-current efficiency (ABPE) of 8.4% under simulated 100mWcm-2 (1 Sun) with an AM 1.5G filter in 0.5m KOH. Furthermore, the obtained SHJ photocathode demonstrates excellent stability of at least 110h at 0.3V versus RHE. In this work, combining facile direct current magnetron sputtering with a solution treatment technique provides a novel design strategy, which lowers the threshold for preparing high-quality protective layer, and paves the way for developing economic, efficient, and stable SHJ-based PEC devices.

Structural optimization and growth of intrinsic hydrogenated amorphous silicon films by HWCVD

Although the silicon heterojunction (SHJ) solar cell is the crystalline silicon solar cell with highest conversion efficiency at present, its higher cost of production line has been a factor restricting its industrial development. The use of hot wire chemical vapor deposition (HWCVD) technology instead of the mainstream plasma enhanced chemical vapor deposition (PECVD) technology for the deposition of amorphous silicon films can effectively reduce the cost of equipment and processing, and has a bright future. In recent years, i-a-Si:H films grown by PECVD have obtained great improvement in passivation quality, whereas HWCVD technology has been neglected in this field. This has significantly limited the development and application of HWCVD technology in SHJ cell production. In this work, we exploited the differences in the films properties and microstructures deposited by various hot-wire temperatures and successfully developed a structure for the high-passivation-quality i-a-Si:H film grown by HWCVD, consisting of a buffer layer and a double-layer bulk stack. By introducing the buffer layer grown with the 1650 °C hot-wire temperature and pure silane, the effective minority carrier lifetime was improved from 2.3 ms to 7.5 ms, and a cell efficiency enhancement of 0.4%abs was obtained. By depositing the bulk layer sequentially with hot-wire temperatures of 1800 °C and 1900 °C, the passivation quality and the conductance were both improved. An effective minority carrier lifetime of 8 ms and a further cell efficiency enhancement of 0.15%abs were obtained. Finally, SHJ solar cell efficiency of 24.35% was obtained with a home-made HWCVD-based pilot SHJ cell line.

Study on the process of hydrogen-doped indium oxide for silicon heterojunction solar cell mass production

In this work, we conducted a comparative study on water vapor (H2O) process and hydrogen gas (H2) process to prepare the IMO:H films based on industrialized DC magnetron sputtering system. To dynamically control the residual H2O pressure, we installed a set of polycold coil and kept it turned on during the sputtering process. The film characteristics of Hall mobility, carrier density, sheet resistance, and transparency was investigated. The results revealed that the silicon heterojunction (SHJ) solar cell with H2 process had a higher Hall mobility and greater transparency in the 400–600 nm wavelength range resulting in a lower contact resistivity and a slight increase current density compared to the H2O process. The median efficiency (Eff) achieved with the H2 process improved by 0.09% absolute to 25.43%, mainly because of the increased Jsc and FF. And the IMO:H films with H2 process had a large basal spacing with (400) and (222) crystal planes resulting in maintaining a more stable and reliable structure at temperatures below 200 °C. After the optimization of TCO layer process, we manufactured the industrial-grade SHJ cells with max efficiency (Eff), Voc, Jsc, and fill factor (FF) values as high as 26.27%, 753 mV, 41 mA/cm2, and 86%, respectively. The IMO:H films with H2 process can thus be considered more suitable for large-scale production.

Synergetic Effect of Gas Compositions on Material Properties of n‐Type Nanocrystalline Silicon Oxide Prepared by Plasma‐Enhanced Chemical Vapor Deposition

Hydrogenated nanocrystalline silicon oxide (nc‐SiOx:H) has drawn extensive attention during the past years for the utilization of silicon heterojunction (SHJ) solar cells. This material is demonstrated to be the key to the performance improvement of SHJ solar cells. Herein, the influence of the process gas compositions on the optical, electronic, chemical, and structural properties of n‐type nc‐SiOx:H films at the thickness of 38 nm is systematically investigated. The synergetic effect of CO2 and PH3 gas flow fraction ( and ) and the synergetic effect of CO2 and SiH4 gas flow fraction ( and ) on the growth of nc‐SiOx:H (n) films are demonstrated. Creatively combining the Fourier‐transform infrared, UV‐Raman scattering spectroscopy, spectroscopy ellipsometry, and photo‐thermal deflection spectroscopy measurement, the mechanism behind the synergetic effect of the gas compositions on the material properties is analyzed.

Low‐Cost Metallization Based on Ag/Cu Fingers for Exceeding 25% Efficiency in Industrial Silicon Heterojunction Solar Cells

Although Ag‐coated Cu technology is considered to be an effective approach to mitigate the metallization expenses associated with silicon heterojunction (SHJ) solar cells, there is limited reporting on the photovoltaic performance and reliability of devices employing Ag/Cu electrodes. Herein, industrial SHJ solar cells were successfully fabricated, yielding an average efficiency of 25.18% with bifacial Ag/Cu fingers, which caused a 0.13% decline in efficiency but saved 46% in Ag consumption when compared to traditional Ag fingers. Performance degradation is demonstrated to originate essentially from the front rather than rear Ag/Cu fingers, as revealed by a comprehensive analysis of resistances enhancement and optical losses. Notably, the Ag/Cu fingers exhibited 1.4% lower printed qualification rate and similar high‐temperature stabilities in contrast to Ag fingers. In the results, valuable insights were provided for further optimization of low‐cost and high‐efficiency SHJ solar cells based on Ag/Cu electrodes.

Highly Efficient and Highly Flexible Thin Crystalline Silicon Heterojunction Solar Cells Based on Dopant-Free Carrier-Selective Contacts Fabricated with Simple Processes

Highly Efficient and Highly Flexible Thin Crystalline Silicon Heterojunction Solar Cells Based on Dopant-Free Carrier-Selective Contacts Fabricated with Simple Processes

Optimization of a Solution-Processed TiOx/(n)c-Si Electron-Selective Interface by Pre- and Postdeposition Treatments.

Developing a vacuum-free and low-temperature deposition technique for dopant-free carrier-selective materials without sacrificing their performance can reduce the fabrication cost and CO2 footprint of silicon heterojunction (SHJ) solar cells. In this contribution, to activate the full capacity of the solution-processed TiOx as an electron-selective passivation contact, the effects of various pre- and postdeposition treatments on the passivation quality and contact resistivity are investigated simultaneously. It is demonstrated that the electrical properties of a thin TiOx layer spin-coated on an n-type silicon substrate can be remarkably improved through tailor-made pre- and postdeposition treatments. A notable low surface recombination velocity (SRV) of 6.54 cm/s and a high implied open-circuit voltage (iVoc) of 706 mV are achieved. In addition, by inserting a 1 nm LiFx buffer layer between TiOx and Al metal contact, a low contact resistivity (ρc) of 15.4 mΩ·cm2 is extracted at the n-Si/SiOx/TiOx heterojunction. Our results bring the solution-processed TiOx electrical properties to a level on par with those of state-of-the-art pure TiOx layers deposited by other techniques. Chemical and electrical characterizations elucidate that the improved electrical properties of the investigated Si/SiOx/TiOx heterojunction are mediated by the concomitant involvement of chemical and field-effect passivation.

LIFT metallization as an alternative to screen-printing for silicon heterojunction solar cells

The electrical characteristics of solar cells are significantly influenced by the metallization process, making it a crucial step. Screen printing is the standard metallization technique, but there is an increasing interest in the development of methods that allow more versatility, higher process control, and a more efficient use of the expensive metallic pastes used. We focus here on the comparison between the standard screen-printing method, and Light-Induced Forward-Transfer (LIFT), a non-contact, very precise technique, able to transfer volumes down to picolitres. The high flexibility, using free-form designs that do not depend on any mask or physical support, and the efficient use of the metallic paste with almost no waste, are other characteristics that point out LIFT as a very promising alternative. In this paper we include the electric characterization of contacts, and solar silicon heterojunction (SHJ) cells metalized with both techniques. The results show a slightly better efficiency for the screen-printed cells, but good series resistance and fill factor values imply that LIFT is a promising alternative for device metallization.

Light-activated surface passivation for more efficient silicon heterojunction solar cells: Origin, physics and stability

Silicon heterojunction (HJT) solar cells have world-leading efficiencies due to outstanding surface passivation. Yet, maintaining their performance during the lifetime of a photovoltaic module requires excellent quality and stability of the surface regions. It is well known that HJT solar cells can show an increase or reduction in performance under illumination, and this instability has been related to changes in the surface regions. This work investigates the stability of surface passivation in HJT solar cells by modelling the injection-dependent minority carrier lifetime of a range of symmetrically a-Si passivated silicon wafers. Fixed charges and defects at the interface are varied in the model to find the best fit to the injection-dependent lifetime before and after a high-intensity illumination treatment. The results indicate that the laser process induces an increase in field effect passivation at the surface, which is then reduced upon storage in the dark. The results show that lifetime spectroscopy is a useful tool to investigate the nature of a-Si passivation degradation.

Theoretical limiting‐efficiency assessment on advanced crystalline silicon solar cells with Auger ideality factor and wafer thickness modifications

AbstractWith the improvement of surface passivation, bulk recombination is becoming an indispensable and decisive factor to assess the theoretical limiting efficiency () of crystalline silicon (c‐Si) solar cells. In simultaneous consideration of surface and bulk recombination, a modified model of evaluation is developed. Surface recombination is directly depicted with contact selectivity while bulk recombination is revised on the aspects of ideality factor and wafer thickness. The of the double‐side silicon heterojunction (SHJ) and double‐side tunneling‐oxide passivating contact (TOPCon) solar cells are numerically simulated using the new model as 28.99% and 29.19%, respectively. However, the of single‐side TOPCon solar cells, the more practicable scenario, is only 27.79%. Besides, the of the double‐side SHJ solar cells would exceed the double‐side TOPCon solar cells if the recombination parameter of the non‐contacted area is higher than 0.6 fA/cm2, instead of perfect passivation. Our results are instructive in accurately assessing efficiency potential and accordingly optimizing design strategies of c‐Si solar cells.

A study on the improvement of amorphous silicon/crystalline silicon heterojunction solar cells by microwave processing

A study on the improvement of amorphous silicon/crystalline silicon heterojunction solar cells by microwave processing

Impact of Texture Height and Ozone-Based Rounding on Silicon Heterojunction Solar Cells Performance

In this study, the effect of ozone-based rounding was investigated for three random-pyramid texture height ranges for Silicon Heterojunction (SHJ) at the cell and module levels. In a first experiment, 5 ozone-based rounding process times (0, 2, 4, 6, 8 and 10 minutes) were applied to two texture-height ranges (Small and Medium) to observe variations in the reflection response of the wafers. In a second experiment, 3 ozone-based rounding treatment times (0, 2 and 6 minutes) were applied to three texture-height ranges (Small, Medium and Large) and to observe the results both on the cell and on the module levels. The first experiment has shown that ozone-based rounding increases reflection and has a more pronounced effect for smaller than larger sized texture-heights. An increase in the minimum wavelength of reflection is seen after the Transparent Conductive Oxide (TCO) deposition, indicating that shorter deposition times can be used. The results of the second experiment show that, even though the Short-Circuit Current (Jsc) decreases for the three textures with rounding time, it is compensated by an increase in the pseudo-Fill-Factor (pFF). A maximum is seen at the cell level for 2 minutes of rounding for the small and large texture sizes, but which is leveled at the module. In conclusion, these results indicate that 2 minutes of ozone-based rounding can be beneficial for the cell’s final efficiency as the improvement in passivation still compensates the optical losses resulting in a lower Jsc.

Demonstration of Excellent Crystalline Silicon Surface Passivation (S < 1.27 cm s−1) by High‐Rate DC‐Sputtered Hydrogenated Amorphous Silicon

The surface passivation quality of intrinsic hydrogenated amorphous silicon (i‐a‐Si:H) layers deposited by constant DC power‐facing target sputtering (FTS) technology is investigated. An n‐type crystalline silicon wafer passivated with a 42 nm‐thick i‐a‐Si:H shows an effective carrier lifetime of 11 ms, which corresponds to a low upper limit effective surface recombination velocity Seff of 1.27 cm s−1. An ultrathin 5 nm‐thick i‐a‐Si:H film achieves an implied open‐circuit voltage (iVoc) of 730 mV at a high deposition rate of 31 nm min−1. It is found that constant DC‐FTS does not cause sputtering damage under high deposition rate conditions. This work demonstrates that i‐a‐Si:H deposited by constant DC‐FTS can achieve the same passivation quality as i‐a‐Si:H deposited by plasma‐enhanced chemical vapor deposition or catalytic chemical vapor deposition, paving the way for the fabrication of silicon heterojunction solar cells without the use of explosive and toxic gases.

Practical Jsc Limits for SHJ Devices: Insights From Modelling

Modern industrial silicon heterojunction (SHJ) solar cells are increasingly limited by the short-circuit current density (Jsc) and there is a strong interest in understanding how much novel approaches such as window layers, novel transparent conductive oxides (TCOs) and anti-reflection coatings (ARCs) could improve the Jsc of SHJ solar cells. In this work, the practical Jsc limits of SHJ solar cells are determined using a carefully calibrated ray-tracing model, validated using empirical data from in-house solar cells as well as recently published high-efficiency front-and-back contacted (FAB) SHJ solar cells. The model is then further refined to obtain a detailed Jsc loss breakdown of the latest record efficiency FAB SHJ solar cells, for which there are no published Jsc loss breakdowns. Notable advances made in these advanced solar cells with regards to window layers, TCOs and ARCs at the cell level are analysed. Based on the magnitude of impact on the solar cell Jsc, the most critical factors for achieving high-Jsc SHJ solar cells are identified and ranked. Allowing for additional improvements and combining the best approaches identified, an estimate of the practical upper limit of Jsc for FAB SHJ solar cells is determined to be 41.81 mA/cm2. This work serves as a useful reference for the current state of play for Jsc improvements in SHJ solar cells, and highlights practical pathways and issues for improving commercial SHJ solar cells.

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.