Si Heterojunction Solar Cells Research Articles

Double Heterojunction Crystalline Silicon Solar Cells: From Doped Silicon to Dopant-Free Passivating Contacts

Carrier-selective passivating contacts for effective electron and hole extraction are crucial to the attainment of high efficiency in crystalline silicon (Si) solar cells. In this comprehensive review, the principle of carrier extraction and recombination mechanisms in conventional industrial Si solar cells are discussed first. Passivating contacts based on (i) amorphous hydrogenated Si and (ii) polysilicon/silicon oxide are next reviewed, with emphasis on carrier selectivity mechanisms including contact layer band alignment with silicon, and localized carrier transport in ultrathin oxides. More recent developments in dopant-free passivating contacts deposited by lower-cost fabrication processes with lower thermal budget are then described. This third category of non-Si based electron- and hole-selective passivating contacts include transition metal oxides, alkali/alkali earth metal fluorides and organic conjugated polymers. The photovoltaic performance of asymmetric double heterojunction Si solar cells fabricated using these non-Si passivating contacts and their stability in damp heat conditions are discussed and compared with Si based passivating contacts.

Growth of MoS2 and metal back electrode-dependent performance of MoS2/Si heterojunction solar cells

In this work, MoS2 thin films were successfully synthesized by chemical vapor deposition (CVD) using MoO3 and sulfur powders as precursor sources. The effect of the MoO3 dosage and heating temperature on the quality of the synthesized MoS2 films are investigated and a detailed reaction mechanism is proposed to interpret the obtained experimental result. Furthermore, we have studied the influence of different metal back electrodes (including Ag and Au) on the photovoltaic performance of the fabricated MoS2/Si heterojunction solar cell. It is shown that, in comparison with the solar cell using Au as a metal back electrode, the solar cell using Ag as a metal back electrode has poor photovoltaic performance with the appearance of the S-type shape in the measured current density-voltage curve. Finally, numerical simulation based on AFORS-HET software has been carried out to investigate the photovoltaic performance of the [Formula: see text]-MoS2/[Formula: see text]-Si heterojunction solar cell under different work functions of the metal back electrode. The result of numerical simulation is consistent with our experiential result in the trend. This work is highly relevant to the development of transition metal dichalcogenides in the application of solar cells.

Energy yield analysis of different bifacial PV (photovoltaic) technologies: TOPCon, HJT, PERC in Hainan

The tunnel oxide passivated contacts (TOPCon) cell, and the Si heterojunction solar cell (HJT) based on Si cells with high-efficiency potential have recently been transferred from the laboratory to manufacturing technology. However, the harvested energy of advanced bifacial modules has not yet been proved in the field. Our project aims to present an experimental study on the energy yield performance of the TOPCon bifacial PV module compared to HJT and PERC bifacial PV modules in the southernmost of China, Hainan province. The experimental results have proven that the energy yield performance of HJT bifacial modules is 8.79% higher than that of the PERC modules, and the energy yield gain of the TOPCon modules is 5.55% higher compared to PERC modules.

Improved conversion efficiency of p-type BaSi2/n-type crystalline Si heterojunction solar cells by a low growth rate deposition of BaSi2

The effect of low growth rate deposition (LGD) of BaSi2 on the film quality and performance of silicon heterojunction solar cells was investigated. The total thickness of the BaSi2 layer decreased with increasing LGD duration (tLGD). Analysis using Raman spectroscopy indicated that an amorphous Si (a-Si) phase existed on the surface of the BaSi2 layer. The a-Si on the surface was converted into BaSi2 by post-annealing owing to the diffusion of Ba and Si atoms. X-ray diffraction analysis revealed that LGD improved the rate of a-axis orientation and crystallinity. Post-annealing was also observed to have significantly improved these structural properties. Furthermore, the solar cell performance was observed to be strongly dependent on tLGD, and the highest conversion efficiency of 10.62% was achieved by the p-BaSi2/n-c-Si heterojunction solar cells at a tLGD of 6 min. The improved structure and solar cell properties are attributed to improved atom rearrangement during LGD.

Optoelectrical analysis of TCO+Silicon oxide double layers at the front and rear side of silicon heterojunction solar cells

Silicon Heterojunction has become a promising technology to substitute passivated emitter and rear contact (PERC) solar cells in pursuance of lower levelized cost of electricity through high efficiency devices. While high open circuit voltages and fill factors are reached, current loss related to the front and rear contacts, such as the transparent conductive oxide (TCO) layers is still a limiting factor to come closer to the efficiency limit of silicon based solar cells. Furthermore, reducing indium consumption for the TCO has become mandatory to push silicon heterojunction technology towards a terawatt scale production due to material scarcity and costs. To address these issues dielectric layers, such as silicon dioxide or nitride cappings are implemented to reduce TCO thicknesses both diminishing parasitic absorption and material consumption. However, reducing the TCO thickness comes in cost of resistive losses. Furthermore, the TCO properties do vary with thickness and neighboring layer configuration altering the optimization frame of the device. In this paper we present a detailed analysis to quantify the optoelectrical losses trade-off associated to the TCO thickness reduction in such layer stacks. Through the analysis we show and explain why experimental bifacial cells with 20 nm front and rear TCO perform at a similar level to reference cells with 75 nm under front and rear illumination reaching efficiency close to 24% at 92% bifaciality. We present as well a simple interconnection method via screen printing metallization to implement a thin TCO/silicon dioxide/silver reflector enhancing current density from 39.6 to 40.4 mA/cm 2 without compromising resistive losses resulting in a 0.2% absolute solar cell efficiency increase from a bifacial design (23.5–23.7%). Finally, following this approach we present a certified champion cell with an efficiency of 24.6%. • Analysis of TCO variations on electron and hole contact for Si heterojunction solar cells by simulations and experiments. • Both sides 20 nm TCO + SiO 2 layer stack SHJ solar cells with 23.9% efficiency. • Champion solar cell with rear 20 nm TCO + SiO 2 layer stack and silver rear reflector with 24.6% efficiency.

Integration of Si Heterojunction Solar Cells with III-V Solar Cells by the Pd Nanoparticle Array-Mediated "Smart Stack" Approach.

This paper describes the way to fabricate two-terminal tandem solar cells using Si heterojunction (SHJ) bottom cells and GaAs-relevant III-V top cells by "smart stack", an approach enabling the series connection of dissimilar solar cells through Pd nanoparticle (NP) arrays. It was suggested that placing the Pd NP arrays directly on typical SHJ cells results in poor tandem performance because of the insufficient electrical contacts and/or deteriorated passivation quality of the SHJ cells. Therefore, hydrogenated nanocrystalline Si (nc-Si:H) layers were introduced between Pd NPs and SHJ cells to improve the electrical contacts and preserve the passivation quality. Such nc-Si:H-capped SHJ cells were integrated with InGaP/AlGaAs double-junction cells, and a certified efficiency of 27.4% (under AM 1.5 G) was achieved. In addition, this paper addresses detailed analyses of the 27.4% cell. It was revealed that the cell had a relatively large gap at the smart stack interface, which limited the short-circuit current density (thereby the efficiency) of the cell. Therefore, higher efficiency would be expected by reducing the interfacial gap distance, which is governed by the height of the Pd NPs.

High conductivity PEDOT:PSS thin films affording improved open circuit voltages in PEDOT:PSS-Si heterojunction solar cells

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)-Si heterojunction solar cells (HSCs) have the characteristics of low cost and simple preparation process, which is expected to realize the manufacture of high-performance solar cells. Among them, The modification of PEDOT:PSS has become the most important and feasible method to improve the photoelectric conversion efficiency (PCE) of PEDOT:PSS-Si HSCs. Here, we design a novel hybrid nanostructure of Co9S8 nanoparticles uniformly loaded onto the surface of multi-walled carbon nanotubes (Co9S8@N,P-MWCNT) as a dopant to improve the performance of PEDOT:PSS. The analysis of experimental data reveals that the dopant could effectively suppress the recombination losses of PEDOT:PSS-Si interface, which is conducive to the improvement of open circuit voltage (Voc). The HSCs reaching a PCE of 13.44%, while achieving a high Voc of 645 mV.

NiOx/MoOx bilayer as an efficient hole-selective contact in crystalline silicon solar cells

Designing effective carrier-selective contact is a prerequisite for high-efficiency crystalline silicon ( c -Si) solar cells. Compared to doped silicon thin films, wide-band-gap transition metal oxides (TMOs) feature low parasitic absorption, but their carrier selectivity and passivation being poor leads to a mediocre cell efficiency. Herein, we introduce a NiO x /MoO x bilayer as an efficient hole-selective contact in c -Si solar cells. A power conversion efficiency (PCE) of 21.31% is achieved using NiO x /MoO x bilayer, outperforming cells with a single layer of NiO x or MoO x . Upon depositing NiO x on MoO x , interfacial reactions modify the stoichiometry and defect chemistry in both oxides, leading to a band alignment beneficial for hole selectivity. By inserting a SiO x tunneling layer on c -Si surface to further suppress recombination, we achieve a PCE of 21.60% (fill factor 83.34%). Our work highlights a promising approach to improve the performance of dopant-free c -Si solar cells by employing cost-effective TMOs as hole-selective contact. A NiO x /MoO x bilayer as an efficient hole-selective contact in p -Si solar cells Interfacial reaction of NiO x /MoO x modifies doping thus band alignment p -Si solar cells with NiO x/ MoO x bilayer show efficiency of 21.31% Inserting an ultra-thin SiO x tunneling layer boosts to an efficiency of 21.60% Li et al. report a NiO x /MoO x bilayer as an efficient hole-selective contact in p -Si heterojunction solar cells, delivering an efficiency of 21.31%. Inserting an additional ultra-thin SiO x tunneling layer further boosts open-circuit voltage and fill factor, resulting in an efficiency of 21.60%. This work provides a design strategy to push forward the development of c -Si solar cells with transition metal oxides as hole-selective contact.

Study on the improvement of the open-circuit voltage of NiOx/Si heterojunction solar cell

Heterojunction solar cell based on nickel oxide ( NiO x ) as hole selective layer and n-type silicon as an active absorption layer is a promising high-efficiency solar cell. Unfortunately, this type of solar cells suffers from a low open circuit voltage ( V O C ) which limits any significant progress in its performance. Therefore, the main scope of this study is focused on V O C improvement. Discrete traps at the NiO x /Si interface are taken into account in the simulation to describe the real situation. The simulation was performed by SILVACO-Atlas using the interfacial traps NiO x /Si concentrations and hole mobility as tools to obtain agreement with measurement reported by Hsu et al. in [Thin Solid Films. 573 (2014) 159–163]. These parameters were found to be the cause of the low V O C value. V O C and efficiency improvements were achieved by optimizing the NiO x thickness and insertion a hydrogenated amorphous Si (a-Si:H) thin layer with optimal properties (electron affinity, bandgap and thickness)as a buffer layer between NiO x and Si. V O C increased from 423 to 906.16 mV and a good promising efficiency of η = 12.73 % is obtained in comparison to the initial efficiency (4.50%). • NiO x /Si heterojunction solar cell suffers from a low open circuit voltage. • This study focuses on finding the reason of low open circuit voltage. • A good agreement with the measured values was achieved. • An amorphous hydrogenated silicon buffer layer is inserted and optimized.

Analysis of temperature coefficients and their effect on efficiency of solar cell modules for photovoltaics-powered vehicles

Development of vehicles that are powered by photovoltaics (PV) is desirable, and is crucial for reduction in CO2 emissions from the transport sector to realize a decarbonized society. Our investigations show that the majority of the passenger cars that cruise only with solar energy can be realized by installing a high-efficiency PV module. Although the Toyota Prius demonstration car, which is equipped with a 860 W rated-output power PV module, has shown a 36.6 km d−1 PV-powered driving range at solar irradiance of 6.2 kWh m−2 d−1, practical driving ranges of PV-powered vehicles are shown to be shorter than the estimated values due to some losses of solar cell modules, such as temperature rise under sunny conditions. In this paper, we conduct a systematic analysis of the effects of these losses on the PV-powered driving range in order to obtain guidelines for the development of highly efficient solar cell modules for vehicle integrated applications. The analytical results show that the III–V compound solar cell modules have more suitable properties compared to other cells because of their higher potential conversion efficiencies of 37% with a smaller temperature coefficient of −0.19% °C−1 compared to −0.29% °C−1 for Si back contact solar cell modules and −0.26% °C−1 for Si heterojunction solar cell modules. Our theoretical calculations that take these losses into account suggest that installing the III–V-based triple-junction solar cell modules provides a potential PV-powered driving range of 30 km d−1 on average, and more than 50 km d−1 on a sunny day under the irradiation conditions in Japan.

Fabrication of heterojunction crystalline Si solar cells with BaSi2 thin films prepared by a two-step evaporation method

We have fabricated p-type BaSi2 (p-BaSi2)/n-type crystalline Si (n-c-Si) heterojunction solar cells using thermal evaporation. To control the defect around the heterointerface, samples were fabricated by two methods using different current profiles during evaporation. The one-step method was designed to avoid supplying Ba-rich vapor, and the two-step method was designed to intentionally introduce Ba-rich vapor at the first step. Deep-level transient spectroscopy measurements revealed that defect densities in the n-c-Si side were almost the same for both samples. Open-circuit voltage (V oc) was successfully improved from 319 to 463 mV by the two-step method. This may be due to the increase of carrier density in p-BaSi2 prepared using a two-step method. As a result of optimization, the conversion efficiency of 6.23% was achieved by the heterojunction solar cells.

Effect of Time and Angle Etching in the Performance Heterojunction Si Solar Cells

Effect of Time and Angle Etching in the Performance Heterojunction Si Solar Cells

Impact of chemically grown silicon oxide interlayers on the hydrogen distribution at hydrogenated amorphous silicon/crystalline silicon heterointerfaces

We studied the impact of oxidizing pre-treatments (OPT) and post deposition annealing (PDA) on the passivation performance and the hydrogen distribution near the interface between crystalline silicon (c-Si) and hydrogenated amorphous silicon (a-Si:H), the critical functional region in Si heterojunction solar cells. The OPT prior to deposition of the a-Si:H layer consists of immersing the c-Si substrates into hydrogen peroxide solutions, which forms a silicon oxide interlayer. Spectroscopic ellipsometry (SE) indicates that slightly thicker a-Si:H layers result from OPT. The refractive index and the extinction coefficient are increased by inserting the oxide interlayers, suggesting that less deficient and denser a-Si:H layers can be formed. Under optimum conditions, OPT leads to at least 2-fold improvement of the effective photo-generated carrier lifetime. PDA at 200 °C further improves the passivation performance of samples with an interlayer. Hydrogen profiling with nuclear reaction analysis clarifies that higher hydrogen concentrations are present around the heterointerfaces of samples with an interlayer and that these hydrogen concentrations are maintained after PDA. Our results suggest that the oxide interlayer can suppress hydrogen desorption in the initial growth stage of high-quality a-Si:H layers and during subsequent PDA, resulting in excellent passivation performance.

From Femtoseconds to Gigaseconds: The SolDeg Platform for the Performance Degradation Analysis of Silicon Heterojunction Solar Cells.

Heterojunction Si solar cells exhibit notable performance degradation. We modeled this degradation by electronic defects getting generated by thermal activation across energy barriers over time. To analyze the physics of this degradation, we developed the SolDeg platform to simulate the dynamics of electronic defect generation. First, femtosecond molecular dynamics simulations were performed to create a-Si/c-Si stacks, using the machine learning-based Gaussian approximation potential. Second, we created shocked clusters by a cluster blaster method. Third, the shocked clusters were analyzed to identify which of them supported electronic defects. Fourth, the distribution of energy barriers that control the generation of these electronic defects was determined. Fifth, an accelerated Monte Carlo method was developed to simulate the thermally activated time-dependent defect generation across the barriers. Our main conclusions are as follows. (1) The degradation of a-Si/c-Si heterojunction solar cells via defect generation is controlled by a broad distribution of energy barriers. (2) We developed the SolDeg platform to track the microscopic dynamics of defect generation across this wide barrier distribution and determined the time-dependent defect density N(t) from femtoseconds to gigaseconds, over 24 orders of magnitude in time. (3) We have shown that a stretched exponential analytical form can successfully describe the defect generation N(t) over at least 10 orders of magnitude in time. (4) We found that in relative terms, Voc degrades at a rate of 0.2%/year over the first year, slowing with advancing time. (5) We developed the time correspondence curve to calibrate and validate the accelerated testing of solar cells. We found a compellingly simple scaling relationship between accelerated and normal times tnormal ∝ taccelT(accel)/T(normal). (6) We also carried out experimental studies of defect generation in a-Si:H/c-Si stacks. We found a relatively high degradation rate at early times that slowed considerably at longer time scales.

Intrinsic Amorphous Silicon Bilayers for Effective Surface Passivation in Silicon Heterojunction Solar Cells: A Comparative Study of Interfacial Layers

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.

Identification of embedded nanotwins at c-Si/a-Si:H interface limiting the performance of high-efficiency silicon heterojunction solar cells

The interface of high-quality crystalline silicon/hydrogenated amorphous silicon (c-Si/a-Si:H) is indispensable for achieving the ideal conversion efficiency of Si heterojunction solar cells. Therefore, it is extremely desirable to characterize and control the interface at the atomic scale. Here, we employ spherical aberration-corrected transmission electron microscopy to investigate the atomic structure of the c-Si/a-Si:H interface in high-efficiency Si heterojunction solar cells. Their structural evolution during in situ annealing is visualized at the atomic scale. High-density embedded nanotwins, detrimental to the device performance, are identified in the thin epitaxial layer between c-Si and a-Si:H. The nucleation and formation of these nanotwins are revealed via ex situ and in situ high-resolution transmission electron microscopy. Si heterojunction solar cells with low-density nanotwins are fabricated by introducing an ultra-thin intrinsic a-Si:H buffer layer and show better performance, indicating that the strategy to restrain embedded nanotwins can further enhance the conversion efficiency of Si heterojunction solar cells. Silicon heterojunction solar cells are expected to increase their market share in the near future. Qu et al. identify an embedded nanotwin structure at the crystalline silicon/hydrogenated amorphous silicon interface of silicon heterojunction cells that limits the device performance and devise an approach to suppress its formation.

3D impurity profiles of doped/intrinsic amorphous-silicon layers composing textured silicon heterojunction solar cells detected by atom probe tomography

Laser-assisted atom probe tomography was used to identify the impurity distribution in Si heterojunction (SHJ) solar cells composed of thin doped/intrinsic amorphous Si layers on the textured surface of a crystalline Si wafer. A site-specific lift-out technique involving a focused ion beam enabled the selection of a ∼2 × 2 μm2 area on an arbitrary pyramidal surface. The distributions of B, P and C in the amorphous Si layers introduced by p-type (trimethyl-borane or diborane) or n-type (phosphine) dopant gases were investigated. Standard guidelines for the assessment of the H content in amorphous Si of SHJ solar cells were provided.

On the application of hole‐selective MoOx as full‐area rear contact for industrial scale p‐type c‐Si solar cells

AbstractWe present the feasibility of integrating substoichiometric molybdenum oxide (MoOx) as hole‐selective rear contact into the production sequence of industrial scale p‐type crystalline silicon (c‐Si) solar cells. Thin films of MoOx are deposited directly on p‐type c‐Si by thermal evaporation at room temperature. It is found that Ag/MoOx/p‐type c‐Si rear contact structure exhibits low contact resistivity and modest surface recombination current density. The attained peak efficiency (η) of the fabricated solar cells is 17.65% with Voc of 626 mV, Jsc of 36.8 mA/cm2, and fill factor (FF) of 76.63%. Next, a complete loss analysis of a MoOx/p‐type Si heterojunction solar cell is carried out for the first time by using Quokka simulation software that employs characteristics of different layers which constitute the fabricated solar cell. Based on this loss analysis, the dominant loss mechanisms are defined and a roadmap to attain the desired highest possible efficiency from industrial scale p‐type c‐Si solar cells with full‐area MoOx hole‐collecting rear contact is explored.

A simulation approach for device structure and thickness optimization of silicon heterojunction solar cells featuring TiO2 as carrier-selective contact

Transition metal oxide (TMO) based thin film contacts have gained attention in heterojunction solar cells because of their low fabrication cost and high-efficiency potential. In this work, a simulation study of heterojunction Si solar cells with TiO 2 as a carrier selective layer was carried out with the aid of AFORS-HET software. We have investigated the effects of variations in thicknesses of the front-side carrier selective layer (TiO 2 ), c-Si absorber, and back-side a-Si layer on the photovoltaic performance of TiO 2 /c-Si(p)/a-Si(p) heterojunction solar cells. The simulation results show that the device structure with 1 nm TiO 2 , 300 µm Si and 1 nm a-Si has the highest efficiency of 24.27%. The structure with 100 µm c-Si thickness also shows a high efficiency of 23.2%. It indicates that carrier-selective contacts can provide excellent performance even with thin Si and are highly suitable for reducing the material cost of Si photovoltaics.

Solution-Processed Titanium Oxide for Rear Contact Improvement in Heterojunction Solar Cells

In this work, we demonstrated a heterojunction Si solar cell utilizing chemically grown titanium oxide (TiOx) as an electron-selective contact layer at its rear surface. With TiOx, the rear surface was passivated to reduce carrier recombination. The reverse saturation current, which is an indicator of carrier recombination, exhibited a 4.4-fold reduction after placing a TiOx layer on the rear surface. With reduced recombination, the open-circuit voltage increased from 433 mV to 600 mV and consequently, the power conversion efficiency (PCE) increased from 9.57 to 14.70%. By X-ray photoemission spectroscopy, the surface passivation was attributed to a silicon oxide interfacial layer formed during the chemical growth process. This passivation results in a 625 cm/s surface recombination velocity for the TiOx-passivated Si surface, which is 2.4 times lower than the sample without TiOx, ensuring the carriers pass through the rear contact without extensive recombination. According to these results, the band alignment for the heterojunction solar cell with and without a TiOx rear contact layer was plotted, the reduced interfacial recombination and the electron and hole blocking structure are the main reasons for the observed efficiency enhancement.