Analysis of Optical Potential and Electrical Loss between Upright Pyramids and Inverted Pyramids on Silicon Heterojunction Solar Cells

Surface passivation and optical design of silicon heterojunction solar cells

Silicon heterojunction (SHJ) solar cells are crystalline silicon wafer-based photovoltaic devices fabricated with thin-film deposition technology. The SHJ solar cells hold great potential for large-scale deployment for high conversion efficiencies with low-cost manufacturing. Recently Kaneka Corporation has fabricated an interdigitated-back-contact (IBC) SHJ solar cell with a certified 26.33% conversion efficiency in a large area (180.4 cm2), which is a world record for any 1-sun crystalline silicon wafer-based solar cell. The key feature of SHJ solar cells is the impressive highopen-circuit voltages (Voc) achieved by the excellent amorphous/crystalline silicon interface passivation. Generally, in SHJ solar cells, the boron doped hydrogenated amorphous silicon [(p)a-Si:H] serves as hole collector and the phosphorus doped hydrogenated amorphous silicon [(n) a-Si:H] functions as electron collector. In order to improve the lateral carrier transport of these layers, transparent conductive oxides (TCOs) are usually deposited on both sides of the solar cell. Therefore the parameters such as the heterointerface passivation quality, doping concentration and thickness of the a-Si:H doped layer, and work function of the transparent conductive oxide layer are the key factors that affect the performances of SHJ solar cells. Enormous research efforts have been devoted to studying the effects of the aforementioned influencing parameters on the photovoltaic characteristics of SHJ solar cells. Some research groups have addressed the physical mechanism behind the limitation of the solar cell efficiency. Owing to the insight into the physical mechanism some guidelines for optimally designing the high-performance solar cells in future are obtained. It seems therefore important to summarize the research efforts devoted to the physical mechanism and optimal design of SHJ solar cells.In the present review, we mainly discuss three important issues: 1) the amorphous/crystalline silicon interface passivation; 2) the Schottky barrier resulting from the work function mismatch between the (p)a-Si:H doped layer and the transparent conductive oxide layer; 3) the screening length that is required to efficiently shield the parasitic opposing band from bending originating from the work function mismatch between the (p)a-Si:H doped layer and the transparent conductive oxide layer. The numerical simulation and optimal design of SHJ solar cells are analyzed, and three strategies that may improve the solar cell performances are presented: 1) a hybrid SHJ solar cell structure with a rear heterojunction emitter and a phosphorus-diffused homojunction front surface field; 2) replacing the (p)a-Si:H doped layer by higher doping efficiency microcrystalline silicon alloys such as c-Si:H, c-SiOx:H or c-SiCx:H; 3) replacing the (p)a-Si:H doped layer by higher work function transition metal oxides such as MoOx, WOx or VOx. Finally, the research progress and future development of SHJ solar cells are also described.

Reducing indium consumption in transparent conductive oxide (TCO) layers is crucial for mass production of silicon heterojunction (SHJ) solar cells. In this contribution, optical simulation‐assisted design and optimization of SHJ solar cells featuring MoOx hole collectors with ultra‐thin TCO layers is performed. Firstly, bifacial SHJ solar cells with MoOx as the hole transport layer (HTL) and three types of n‐contact as electron transport layer (ETL) are fabricated with 50 nm thick ITO on both sides. It is found that bilayer (nc‐Si:H/a‐Si:H) and trilayer (nc‐SiOx:H/nc‐Si:H/a‐Si:H) as n‐contacts performed electronically and optically better than monolayer (a‐Si:H) in bifacial SHJ cells, respectively. Then, as suggested by optical simulations, the same stack of tungsten‐doped indium oxide (IWO) and optimized MgF2 layers are applied on both sides of front/back‐contacted SHJ solar cells. Devices endowed with 10 nm thick IWO and bilayer n‐contact exhibit a certified efficiency of 21.66% and 20.66% when measured from MoOx and n‐contact side, respectively. Specifically, when illuminating from the MoOx side, the short‐circuit current density and the fill factor remain well above 40 mA cm−2 and 77%, respectively. Compared to standard front/rear TCO thicknesses (75 nm/150 nm) deployed in monofacial SHJ solar cells, this represents over 90% TCO reduction. As for bifacial cells featuring 50 nm thick IWO layers, a champion device with a bilayer n‐contact as ETL is obtained, which exhibits certified conversion efficiency of 23.25% and 22.75% when characterized from the MoOx side and the n‐layer side, respectively, with a bifaciality factor of 0.98. In general, by utilizing a n‐type bilayer stack, bifaciality factor is above 0.96 and it can be further enhanced up to 0.99 by switching to a n‐type trilayer stack. Again, compared to the aforementioned standard front/rear TCO thicknesses, this translates to a TCO reduction of more than 67%.

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.

The impact of intrinsic amorphous silicon bilayers in amorphous silicon/crystalline silicon (a‐Si:H/c‐Si) heterojunction solar cells is investigated. Intrinsic a‐Si:H films with a wide range of film densities and hydrogen contents are prepared via a plasma‐enhanced chemical vapor deposition (PECVD) technique by modifying various process parameters. For silicon heterojunction (SHJ) solar cells with a‐Si:H films applied as single i‐layers, the resulting surface passivation at the a‐Si:H/c‐Si interface is poor. However, surface passivation is significantly improved by applying intrinsic bilayers, which are composed of a porous interfacial layer (≈2 nm) and an overlying dense layer (≈8 nm). The microstructure factor R* of the interfacial a‐Si:H layer, which is related to the SiH bond microstructure and determined by infrared absorption spectroscopy, closely correlates to the surface passivation capability of the bilayers. A variety of PECVD process parameters (temperature, pressure, or precursor gas species) can be utilized to grow an interfacial layer for good surface passivation, provided that its R* is controlled within a suitable range. This indicates that R* is a key universal parameter for optimizing i‐bilayers and realizing high‐efficiency SHJ solar cells.

Amorphous/crystalline silicon heterojunction solar cells with varying i-layer thickness

Herein, a comparison of industrial silicon heterojunction (SHJ) solar cells formed using p‐type (boron‐ or gallium‐doped) Czochralski‐grown silicon (Cz‐Si) wafers is provided. Standard n‐type SHJ solar cells are also fabricated as a reference. Boron‐doped SHJ solar cells are heavily susceptible to boron–oxygen light‐induced degradation (BO‐LID), with an open‐circuit voltage (VOC) reduction of 100 mV in some cells with starting VOC of >720 mV. While an advanced hydrogenation process (AHP) is sufficient to completely stabilize BO‐LID in some cells, resulting in stable VOC of 724 mV, the impact in reducing BO‐LID is variable. This suggests that an AHP alone may not be a reliable method of reducing BO‐LID in industrial SHJ solar cells. In contrast, SHJ solar cells formed using gallium‐doped wafers exhibit VOC > 730 mV and show no degradation during light‐soaking. Yet, the same AHP treatment for gallium‐doped SHJ cells results in a 0.4%abs increase in the conversion efficiency to 22.6% (VOC of 734 mV). The conversion efficiency of the gallium‐doped SHJ solar cells is still lower than the n‐type reference cells, which is largely due to a reduced fill factor (FF). Further work is required to overcome this FF limitation to facilitate high‐efficiency gallium‐doped SHJ solar cells.

Achieving low contact resistivity for the p-contact in silicon heterojunction (SHJ) solar cells is challenging when classic n-type transparent conductive oxides (TCOs), such as indium tin oxide (ITO), are used in the contact stack. Here, we report on SHJ solar cells with interdigitated back-contact (IBC) and a direct aluminum (Al) metallization applied to the p-contact. We find that carefully annealing an Al/a-Si:H(p) (p-type amorphous silicon) contact at moderate temperatures leads to a specific contact resistivity that is half as low as its silver (Ag)/ITO counterpart. This is explained by Al diffusing into a-Si:H(p) upon temperature treatment, forming a partially crystallized aluminum silicide layer. For a sufficiently high doping level in a-Si:H(p), this enables an efficient tunnel-recombination of holes from a-Si:H(p) to the Al contact. An estimate for this tunneling-dominated specific contact resistivity is calculated as a function of the interface doping density. Best fabricated IBC SHJ solar cells with Al p-contact yield a fill factor of 77.5% and a power conversion efficiency of 22.3%. The main differences to devices with an Ag/ITO/a-Si:H(p) contact stack are a decrease in open-circuit voltage by 14 mV and a slightly higher series resistance ( R s). While the first aspect can be ascribed to increased interface recombination, the second one is unexpected and requires further investigation. Interestingly, omitting an intermediate TCO does not lead to current losses in devices with Al contacts, which is further investigated by optical simulations. Finally, electrical equivalent circuit simulations are conducted to describe the electrical behavior of the investigated devices.

Silicon hetero-junction (SHJ) with the a-Si:H/c-Si structure is one of the candidates for high efficiency solar cells. Despite SHJ solar cells show more than 22% efficiency, SHJ solar cells are limited by optical losses on the front surface. To overcome these limitations, silicon hetero-junction back contact (HBC) solar cells are suggested. HBC solar cells combined the advantages of silicon hetero-junction (SHJ) and Interdigitated Back Contact (IBC) solar cells. In this study, HBC solar cell structures were designed by simulation methods and optimized fabrication processes for industrial application. Dimensions of back contact patterns were design by TCAD ATLAS software. Based on simulation results, p-type and n-type back contact patterns were fabricated.

We show that the highly conductive polymer poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) can successfully be applied as a hole selective front contact in silicon heterojunction (SHJ) solar cells. In combination with a superior electron selective heterojunction back contact based on amorphous silicon (a-Si), mono-crystalline n-type silicon (c-Si) solar cells reach power conversion efficiencies up to 14.8% and high open-circuit voltages exceeding 660 mV. Since in the PEDOT:PSS/c-Si/a-Si solar cell the inferior hybrid junction is determining the electrical device performance we are capable of assessing the recombination velocity (vI) at the PEDOT:PSS/c-Si interface. An estimated vI of ~400 cm/s demonstrates, that while PEDOT:PSS shows an excellent selectivity on n-type c-Si, the passivation quality provided by the formation of a native oxide at the c-Si surface restricts the performance of the hybrid junction. Furthermore, by comparing the measured external quantum efficiency with optical simulations, we quantify the losses due to parasitic absorption of PEDOT:PSS and reflection of the device layer stack. By pointing out ways to better passivate the hybrid interface and to increase the photocurrent we discuss the full potential of PEDOT:PSS as a front contact in SHJ solar cells.

Electrical losses in silicon heterojunction (SHJ) solar cells are difficult to identify and to control as multiple material layers and several entangled physical phenomena are involved. In this context, this contribution aims to accurately investigate and characterize the electrical losses affecting the collection of photogenerated carriers in SHJ solar cells and to provide means to mitigate them. In particular, the material properties controlling the physical coupling between the n-type hydrogenated silicon and the transparent conductive oxide (TCO) layers are studied. To this aim, the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">top-down</i> and the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">bottom-up</i> approaches are introduced and applied to develop different material layers and to study the final device performance once these layers are integrated inside actual solar cells. First, the bulk and interface layer properties required for an efficient carrier transport within the n-type carrier selective passivating contacts of SHJ devices are decoupled by using multilayers which combine thin n-type hydrogenated amorphous and nanocrystalline silicon layers. The multilayer characteristics yielding efficient transport are investigated for two different TCOs, which are indium-tin-oxide (ITO) and aluminium-zinc-oxide (AZO). Second, the passivation quality is studied at various process steps and the contact resistivity is investigated. Finally, these multilayers are further optimized to obtain low series resistance and high final passivation once coupled with both TCOs. As major outcomes, a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$2\times 2$</tex-math></inline-formula> -cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> screen-printed SHJ solar cell with 82.33% fill factor and 24.24% efficiency was reached using AZO as rear TCO and the best certified <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$2\times 2$</tex-math></inline-formula> -cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> solar cell integrating ITO demonstrated a fill factor of 82.28% along with an efficiency of 24.21%.

Silicon heterojunction (SHJ) solar cells are attracting attention as high-efficiency Si solar cells. The basicSHJ structure is p-type amorphous Si (a-Si)/i-type (non-doped) a-Si/n-type mono-crystalline Si (c-Si). Wehave developed and actively evolved this SHJ solar cells from early 1990s, and introduced the moduleequipped with SHJ solar cells named as well-known “HIT”. The features of HIT are: (1) high efficiency,(2) good temperature characteristics, that is, a small output decrease even in the temperature environmentactually used, (3) easy application to double-sided power generation (bifacial module) using symmetricstructure. Although bifacial module has been receiving much attention in recent years, we launched it asearly as 2000.How was this excellent solar cell born? Originally, it was born in the process of developing thin-filmpolycrystalline Si (poly-Si) solar cells as the bottom cells of tandem-type solar cells with a-Si top cells.As an initial stage of the development, we used p-type a-Si as a window layer and n-type c-Si wafer as aphotovoltaic layer because the characteristics of c-Si was clearer than thin film poly-Si. The insertion of ilayerbetween p-type a-Si and n-type c-Si came from two ideas to improve the interface properties. Onewas a structural approach (control the Si bonding network between c-Si and a-Si with microcrystal bufferlayer), and the other was an attempt to suppress the mutual diffusion of impurities. For the former, it wasconfirmed that it was better to use a-Si instead of microcrystals to passivate the c-Si surface effectively,and for the latter, it was found that better characteristics could be obtained by surface passivation of ilayerwith few defects, instead of the effect of impurity separation. The surface passivation technologyusing amorphous semiconductor film was a new invention different from conventional passivation usinginsulating films such as SiOx and SiNx. With this new structure, a large improvement of about 30 mV inopen circuit voltage (Voc) was confirmed. Next we applied this approach to the back side, and again wefound that i-layer could greatly reduce interface defects. Thus, the basic structure of the symmetrical SHJsolar cell was determined. With this structure (Ag electrode/TCO/(p/i)-a-Si/n-c-Si/(i/n)-a-Si/TCO/Agelectrode), we achieved a conversion efficiency of 20% (cell size of 1 cm x 1 cm) in 1994, and after thatthe efforts for mass production started. To expand to a practical size, we newly developed the electrodeforming technology using low-temperature curing type silver paste, and successfully started the massproduction in 1997.In parallel, our R&D team continued to make various conversion efficiency improvements whichincluded optimizing a-Si deposition conditions, developing new TCO materials and deposition methods,and lowering the resistivity of silver paste. In addition, we succeeded in demonstrating in experiments forthe first time that the open-circuit voltage increased associated with the thinning of wafers that had beenpredicted in simulations. The excellent surface passivation capability of i-layer minimized losses whenthinning wafers and maintained the conversion efficiency, which was a very advantageous feature fromthe viewpoint of cost reduction. At the end of 2012 (announced in 2013), we achieved a cell conversionefficiency of 24.7% with a wafer thickness of 98 μm, demonstrating that both low cost and highefficiency can be obtained. Some of these technologies were introduced into mass production.In 2014, we applied SHJ technology the back-contact type solar cell, and achieved a conversionefficiency of 25.6% with both the high Isc due to elimination of front electrode and the high Voc due toexcellent surface passivation. This was a new world record at that time for non-concentrating siliconbasedsolar cells at the research level, the previous one having stood for 15 years.

We have applied an optical splitting system in order to achieve very high conversion efficiency for a full spectrum multi-junction solar cell. This system consists of multiple solar cells with different band gap optically coupled via an “optical splitter.” An optical splitter is a multi-layered beam splitter with very high reflection in the shorter-wave-length range and very high transmission in the longer-wave-length range. By splitting the incident solar spectrum and distributing it to each solar cell, the solar energy can be managed more efficiently. We have fabricated optical splitters and used them with a wide-gap amorphous silicon (a-Si) solar cell or a CH3NH3PbI3 perovskite solar cell as top cells, combined with mono-crystalline silicon heterojunction (HJ) solar cells as bottom cells. We have achieved with a 550 nm cutoff splitter an active area conversion efficiency of over 25% using a-Si and HJ solar cells and 28% using perovskite and HJ solar cells.

Silicon heterojunction (SHJ) solar cells have become one of the mainstream solar cells in the current photovoltaic market due to their high efficiency. Still, concerns about their long-term reliability are seen as a potential issue restricting further marketisation. In particular, the sensitivity of silicon heterojunction solar cells to high temperatures and moisture is a concern. Sodium (Na) in combination with humidity is widely considered one of the causes of degradation in silicon heterojunction solar cells. Yet, a comprehensive understanding of the mechanisms behind Na-induced decay remains lacking. This study will investigate humidity-induced degradation of industrial SHJ solar cells at elevated temperatures using various sodium-containing salts [sodium bicarbonate (NaHCO3), sodium chloride (NaCl), and sodium nitrate (NaNO3)] to improve our fundamental understanding of Na-induced degradation. We will show that SHJ solar cells exposed to NaHCO3 and NaCl show a significant reduction in efficiency, while solar cells exposed to NaNO3 show minimal degradation. Further analysis indicates that NaHCO3 may interact with the transparent conductive oxide (TCO) layer, leading to a reduction in surface passivation and a deterioration of the metal-TCO interface. NaCl primarily affects the Ag contact, resulting in a reduction of the adhesion of the screen-printed contact. Moreover, the TCO composition, particularly the oxygen content, influences its chemical tolerance. These results show that Na-related degradation is more complicated than initially thought. The chemistry is strongly influenced by the negative ions, as well as the composition of TCO and metal paste. These factors are determined by the bill of materials and the contaminants introduced during cell/module fabrication and operation of the SHJ module. The findings of this paper may lead to the development of new accelerated testing protocols for SHJ technology to ascertain long-term reliability in the field.

P-type silicon heterojunction (SHJ) solar cells with a-SiC:H(n) emitters were studied by numerical computer simulation in this paper. The influence of interface states, conduction band offset, and front contact on the performance of a-SiC:H(n)/c-Si(p) SHJ solar cells was investigated systematically. It is shown that the open circuit voltage () and fill factor (FF) are very sensitive to these parameters. In addition, by analyzing equilibrium energy band diagram and electric field distribution, the influence mechanisms that interface states, conduction band offset, and front contact impact on the carrier transport, interface recombination and cell performance were studied in detail. Finally, the optimum parameters for the a-SiC:H(n)/c-Si(p) SHJ solar cells were provided. By employing these optimum parameters, the efficiency of SHJ solar cell based on p-type c-Si was significantly improved.