Si Heterojunction Solar Cells Research Articles

Interface properties study of black silicon solar cells with front surface a-Si:H/c-Si heterojunction

Abstract The exceptionally low reflectance of black silicon across a broad wavelength range makes it an intriguing surface texture for solar cell applications. Silicon heterojunction (SHJ) solar cells fabricated on black silicon (Si) formed by dry reactive ion etching (RIE) using inductive coupled plasma (ICP) on n-type Si are explored. The study is focused on the properties of the a-Si:H/c-Si interface, being a key issue for the photovoltaic performance of SHJ. Deep-level transient spectroscopy (DLTS) detected no radiation defect in Si after the etching. The surface of black Si was passivated with an intrinsic a-Si:H layer, followed by the deposition of p-type and n-type a-Si:H on the front and back sides, respectively. The SHJ solar cell photovoltaic performance based on black Si is strongly influenced by defect density at the a-Si:H/Si interface. Admittance spectroscopy and effective charge carrier lifetime measurements demonstrate that interface defect density decreases with the increase of (i)a-Si:H thickness. The value of 0.75 ms effective charge carrier lifetime was reached for single (i) a-Si:H layer passivation and 0.25 ms when (p)a-Si:H was deposited over the intrinsic a-Si:H layer. The measured open circuit voltage (VOC) values for the SHJ solar cells increase with the (i)a-Si:H layer thickness reaching 658 mV. However, the fill factor decreases with increasing (i) a-Si:H layer thickness, limiting the efficiency at the maximum value below 14% due to the thickness uniformity of the a-Si:H layer. The development of conformal growth of a-Si:H is a key issue for further improvement of black Si heterojunction solar cell photovoltaic performance. 

Formation of n-type polycrystalline silicon with controlled doping concentration by flash lamp annealing of catalytic CVD amorphous silicon films

We investigated the properties of polycrystalline silicon (poly-Si) films formed by the flash lamp annealing (FLA) of hydrogenated amorphous Si (a-Si:H) films. Phosphorus- (P-) doped a-Si:H films with a thickness of several micrometers deposited by catalytic chemical vapor deposition (Cat-CVD) on a silicon nitride- (SiN x -) coated textured glass substrate are crystallized by FLA. P doping was achieved by flowing phosphine (PH3) during Cat-CVD, and the P concentration was controlled within a range of 1016–1021 cm−3, which was measured by secondary ion mass spectrometry. We also fabricated simple back-contact Si heterojunction solar cells to estimate a potential of flash-lamp-crystallized poly-Si as an absorption layer. The current-density–voltage (J–V) characteristics of the cells measured under 1 Sun light irradiation show an open-circuit voltage (V OC) of >0.4 V.

Silicon heterojunction solar cells: Excellent candidate for low light illuminations

The current solar cells and modules are marketed according to the behaviour at standard test conditions (STC), however, these devices are more often operated at lower irradiance levels. The dependence on illumination stems mainly from the voltage at the open circuit and at the maximum power point. The latter might also be strongly influenced by serial resistance, but that is more an engineering problem not addressed here. More fundamentally, the voltage is determined by quasi-Fermi levels at the contacts. In the case of unconstrained conductivity between the absorber and electrode, the quasi-Fermi levels are flat, and determined by their splitting in the absorber. Our analysis shows that the modulation doping mechanism working in Si heterojunction solar cell between the doped amorphous Si layer with a higher bandgap and crystalline absorber with a lower bandgap can, for certain parameter settings, lead to strongly depleted contact layer that limits conduction. This is a prerequisite for decoupling the quasi-Fermi level between contact and absorber and offers the possibility for additional voltage increase. Based on this understanding, we simulate a heterojunction solar cell with a varying thickness and doping of amorphous silicon p-type contact. We demonstrate that for a certain combination of thinner or lower-doped contact, higher efficiency at low illumination can be achieved compared to the technological baseline. This is fully in line with the experimental findings. This analysis is crucial not only for using solar cells for indoor applications but also for designing photovoltaic modules optimized for low irradiance, potentially increasing the level of self-sufficiency of buildings.

Experimental and Theoretical Investigation of Zr‐Doped CuO/Si Solar Cell

Copper oxide (CuO) is a nanostructured semiconductor material with the potential for solar energy conversion and can be suitable for solar cells when used as a thin film. Herein, nondoped and doped (doping ratios of 1%, 2%, and 3% zirconium [Zr]) CuO thin films on silicon (Si) with the spin‐coating technique are developed. Optical and topological characterizations of CuO thin films are examined by ultraviolet‐visible and X‐ray diffraction. The electrical properties of nondoped and Zr‐doped CuO/Si heterojunctions are investigated with experimental current–voltage measurements in the dark and under illuminated conditions. The electrical behavior of nondoped and Zr‐doped CuO/Si heterojunctions is obtained using the experimental J–V technique and computational Cheung–Cheung and Norde methods. A simulation based on nondoped and Zr‐doped CuO/n‐Si heterojunction solar cells using SCAPS‐1D is completed. Photovoltaic (PV) parameters of experimentally produced and theoretically calculated CuO and Zr‐doped CuO/Si heterojunction solar cells are compared. Accordingly, PV parameters of 1% Zr‐doped CuO/Si solar cells show the highest power conversion efficiency calculated as a function of interfacial defect density and hole carrier concentration.

High‐Throughput Indirect Gravure Printing Applied to Low‐Temperature Metallization of Silicon‐Based High‐Efficiency Solar Cells

Herein, this work is dedicated to a both exotic and promising printing technique in silicon (Si) photovoltaics (PV). Indirect gravure printing is investigated as a metallization technique focusing on low‐temperature applications, such as Si heterojunction (SHJ) and perovskite Si tandem solar cells. One advantage of this rotary printing technique is that its components are very durable. From graphical industry, it is known that gravure cylinders show almost no wearing during multiple printing. Additionally, rotary printing techniques offer great throughput. Therefore, indirect gravure printing can reduce costs significantly in PV production. In this work, short process cycle times of 0.9 s cell−1 for industrial wafers (edge length of 156.75 mm) are demonstrated. Using an appropriate production platform even enables cycle times of down to 0.5 s cell−1. Busbarless SHJ half cells with non‐optimized gravure‐printed front grids reach a mean photoconversion efficiency that is (1.7 ± 0.5) %abs lower compared to screen‐printed reference samples, however, cycle time is reduced. The metal contacts exhibit a mean shading width of (65 ± 16) μm and an average height of (5 ± 1) μm. Applied to SHJ solar cells’ rear, such metallization can replace screen‐printed contacts with 0.1%abs efficiency loss only, as device simulations reveal.

The impact of sputtering pressure on the TiO2/p-Si interface and its implications for hole-blocking and photodetection

One convenient approach to minimize the surface recombination loss in Si is to passivate the surface dangling bonds by depositing a thin TiO2 layer on the bulk Si. Besides, the p-Si/TiO2 interface can initiate hole-blocking and facilitate efficient electron transport through the TiO2 layer; thereby, promoting the extraction of electrons for efficient onward utilization. In this context, the current investigation involves studying how the Ti3+ donor states, produced by deliberately introducing O-vacancies controlled by changing the plasma pressure in the RF-magnetron sputtering chamber during the growth of the TiO2 thin films, can change the material's Fermi level and work function and modify the band bending and band offset as well as the fixed oxide charge density in the p-Si/TiO2 heterojunction interface. The TiO2/p-Si interface has been studied through capacitance-voltage (C-V) analysis to determine the interface parameters such as fixed oxide charge, the density of interface traps, and the dielectric constant of TiO2. The studied hole-blocking property of the interface has been correlated to the photoresponsivity of the metal-oxide-semiconductor (MOS) structure. The MOS device fabricated at a pressure of 30 mTorr displays a large saturation current (15.95 μA cm–2) when subjected to a positive gate bias, facilitated by the relatively low conduction band offset (ΔEC ∼0.20 eV), significant positive fixed oxide charge (+1.18 ×1011 cm–2) and the interface trap density (1.03 ×1012 cm–2), which enhanced the transport of minority carrier electrons from the p-Si to the metal, resulting in a good photo responsivity ∼0.158 AW–1 and photo gain of ∼37.5 under white light illumination. Optimum hole-blocking and surface passivation characteristics of the TiO2/p-Si interface together may facilitate fabricating improved heterojunction Si solar cells.

Approaches for III‐V/Si tandem solar cells and comparative studies on Si tandem solar cells

AbstractPhotovoltaic (PV)‐powered vehicle applications are very attractive for reducing CO2 emission and creation of new market. Development of Si tandem solar cells is very promising for high‐efficiency and low‐cost solar cells. This paper presents our approaches for III–V/Si 3‐junction tandem solar cells. In this paper, 24.9% efficiency Si heterojunction solar cells and mechanically stacked four‐terminal 35.8% InGaP/GaAs/Si three‐junction tandem solar cell are shown. Roadmap for realizing high‐efficacy three‐junction tandem solar cells of more than 40% is discussed in this paper. Because efficiencies of record cell of 36.1% with III–V/Si three‐junction tandem cells and 33.9% with perovskite/Si two‐junction cells are lower compared to 39.5% with III–V three‐junction solar cells, it is necessary to clarify and reduce several losses of Si tandem solar cells. This paper presents high efficiency potential of III–V/Si three‐junction and perovskite/Si two‐junction solar cells analyzed by using our analytical procedure and discusses about non‐radiative recombination, optical, and resistance losses in those Si tandem cells. Current status of various solar cell module efficiencies including our new record efficiency Si tandem solar cell module with an efficiency of 33.66% is also analyzed.

Enhancing performance of heterojunction silicon solar cells through atomic-layer-deposited MoOx hole contact and atomic-layer-deposited AlZnO layer

We present the performance enhancement of a heterojunction Si solar cell facilitated by incorporating an atomic-layer-deposited (ALD) MoOx hole contact layer and an ALD Al-doped ZnO (AZO) transparent conductive oxide (TCO). ALD MoOx thin films as a hole-selective contact layer instead of P+-doped amorphous Si in Si heterojunction solar cells, with subsequent growth of ALD AZO films grown on the MoOx layer preceding the indium-tin-oxide sputtering deposition. Integrating ALD MoOx hole-selective contact and ALD AZO TCO manifests an augmentation in the short-circuit current in heterojunction Si solar cells due to the heightened carrier transport and concurrent reduction in sputtering-induced damage. Consequently, the heterojunction Si solar cell employing the ALD MoOx film and ALD AZO layer yielded a 1.1 % increase in efficiency relative to the reference heterojunction solar cell.

HIT Solar Cell Modeling Using Graphene as a Transparent Conductive Layer Considering the Atacama Desert Solar Spectrum

The optical and geometrical properties of transparent conductive oxide (TCO) are crucial factors influencing the efficiency of a−Si:H/c−Si heterojunction (HIT) solar cells. Graphene is a potential candidate to be used as TCO due to its optical and electrical properties. Here, the effect of graphene as TCO is numerically analyzed by varying the number of graphene layers from one to ten. First, the optical properties are calculated based on the transmittance data, and then the HJT cell’s performance is simulated under the AM1.5 standard spectrum and the mean Atacama Desert solar spectral irradiance in Chile. In the modeling, the most relevant properties are calculated with the spectrum of the Atacama Desert. The most relevant values were obtained as follows: open circuit voltage Voc=721.4 mV, short circuit current Jsc=39.6 mA/cm2, fill factor FF=76.5%, and energy conversion efficiency Eff=21.6%. The maximum power of solar panels irradiated with the Atacama Desert spectrum exceeds the results obtained with the AM1.5 standard spectrum by 10%. When graphene is the transparent conducting oxide, quantum efficiency has a higher value in the ultraviolet range, which shows that it may be convenient to use graphene-based solar cells in places where ultraviolet intensity is high.

Efficiency enhancement of solar cell by using methylammonium lead tribromide/polymethyl methacrylate hybrid film as luminescent down shifting layer

Luminescent down-shifting (LDS) layer was introduced to tackle the issue of poor short-wavelength response of solar cell. Besides LDS effect, coating a LDS layer on the surface of a solar cell leads to a change in surface reflectance. LDS and antireflection (AR) effect are coupled and usually reported as a single effect. For those solar cell with optimized AR, it is difficult to enhance power conversion efficiency (PCE) with the LDS layer due to the variation of reflectance. In this work, we report an obvious PCE enhancement of two mainstream commercial silicon (Si) solar cells with the optimized AR by using methylammonium lead tribromide (MAPbBr3)/polymethyl methacrylate (PMMA) hybrid film as a LDS layer. The used MAPbBr3/PMMA nanoplatelets solutions for LDS layer have an absolute photoluminescent quantum yield of ∼85 % (excited by 350 nm) in air atmosphere for more than 30 days, laying a foundation for the realization of effective LDS effect. Theoretical analysis has separated the AR and LDS effect. It has demonstrated that AR effect has a positive influence on LDS effect, and PMMA plays the key role in the improvement of AR. Experimental studies further elucidated the LDS and AR effect provided by MAPbBr3/PMMA hybrid film. Thanks to this combined effect, the application of MAPbBr3/PMMA hybrid film as a LDS layer on Si heterojunction solar cell yields a PCE gain of 0.3 % absolute and that on passivated emitter and rear cell produces an absolute PCE enhancement of about 0.7 %.

Amorphous SnO2 as Earth‐Abundant Stable Transparent Conductive Oxide and Its Application to Si Heterojunction Solar Cells

Transparent conductive oxides that contain indium are widely used in various applications including solar cells. However, In is regarded as one of the critical and economically volatile elements, hindering its massive use in production. Herein, the possibility of using amorphous (a‐)SnO2 transparent conductive oxides (TCOs) instead of In2O3‐based TCOs in silicon heterojunction (SHJ) solar cells is explored. Reactive plasma deposition is utilized to fabricate a‐SnO2 thin films suitable for solar cells, demonstrating good electrical conductivity (>1 × 103 S cm−1) and high damp heat stability while maintaining high transparency in the visible and near‐infrared regions. Furthermore, the a‐SnO2 film exhibits a larger optical bandgap than a‐In2O3‐based TCOs. When the a‐SnO2 layer is applied to SHJ solar cells, it is found that the TCO layer shows almost no negative effect on fill factor, open‐circuit voltage, and short‐circuit current density compared to solar cells with indium tin oxide layers. In‐free rear‐junction SHJ solar cells with a‐SnO2 on both sides of the wafer show an efficiency of 22.2%, suggesting the potential of a‐SnO2 as a cost‐effective and sustainable substitute for conventional In2O3‐based TCOs used in solar cells and other applications.

Sub‐Stochiometric Nickel Oxide Hole‐Selective Contacts in Solar Cells: Comparison of Simulations and Experiments with Sputtered Films

Sub‐stochiometric nickel oxide (NiOx) films were investigated as a hole selective contact option in silicon (Si) heterojunction solar cells. Numerical simulations were carried out to evaluate the impacts of the NiOx electronic properties variations and the NiOx/Si interface defect density (Dit) on device performance. Simulation data suggest that the best performance is achievable for wide bandgaps (Eg) and corresponding high valence band edge (EVB) positions in the NiOx films. Overall, in simulations, the performance remains practically unchanged for the nickel vacancy concentrations [VNi] = 1017–1021 cm−3, assuming high EVB and low Dit. The experimental data measured using NiOx films prepared by radio‐frequency magnetron sputtering reveal that the increase in [VNi] lifts the conductivity, concurrently decreasing Eg and EVB. As a result, we concluded that the performance of the fabricated sputtered NiOx/Si heterojunction solar cell is limited by high Dit as well as narrow Eg and low EVB.

High‐performance perovskite/silicon heterojunction solar cells enabled by industrially compatible postannealing

AbstractIn recent years, developing dopant‐free carrier‐selective contacts, instead of heavily doped Si layer (either externally or internally), for crystalline silicon (c‐Si) solar cells have attracted considerable interests with the aim to reduce parasitic light absorption and fabrication cost. However, the stability still remains a big challenge for dopant‐free contacts, especially when thermal treatment is involved, which limits their industrial adoption. In this study, a perovskite material ZnTiO3 combining with an ultrathin (~1 nm) SiO2 film and Al layer is used as an electron‐selective contact, forming an isotype heterojunction with n‐type c‐Si. The perovskite/c‐Si heterojunction solar cells exhibit a performance‐enhanced effect by postmetallization annealing when the annealing temperature is 200–350°C. Thanks to the postannealing treatment, an impressive efficiency of 22.0% has been demonstrated, which is 3.5% in absolute value higher than that of the as‐fabricated solar cell. A detailed material and device characterization reveal that postannealing leads to the diffusion of Al into ZnTiO3 film, thus doping the film and reducing its work function. Besides, the coverage of SiO2 is also improved. Both these two factors contribute to the enhanced passivation effect and electron selectivity of the ZnTiO3‐based contact and hence improve the cell performance.

An all ambient, room temperature-processed solar cell from a bare silicon wafer.

Solar cells are a promising optoelectronic device for the simultaneous solution of energy resource and environmental problems. However, their high cost and slow, laborious production process so far severely hinder a sufficient widespread of clean, renewable photovoltaic energy as a major alternative electricity generator. This undesirable situation is mainly attributed to the fact that photovoltaic devices have been manufactured through a series of vacuum and high-temperature processes. Here we realize a poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS)/Si heterojunction solar cell fabricated only in ambient and room temperature conditions from a plain Si wafer, with an over 10% energy conversion efficiency. Our production scheme is based on our finding that PEDOT:PSS photovoltaic layers actively operate even on highly doped Si substrates, which substantially mitigates the condition requirements for electrode implementation. Our approach may pave the way for facile, low-cost, high-throughput solar cell fabrication, useful in various fields even including developing countries and educational sites.

Simulation of CdS/p-Si/p+-Si and ZnO/CdS/p-Si/p+-Si heterojunction solar cells

In this study, the antireflection effect of CdS and ZnO/CdS layers was investigated via computing and solar cell I-V graphs and efficiency of the solar cell is simulated using the SCAPS. The lowest average reflectance value for single layer CdS was calculated as 13.43 %, for ZnO/CdS double layer 9.46 %. The efficiency values of CdS/p-Si/p+-Si and ZnO/CdS/p-Si/p+-Si heterojunction solar cells were calculated by changing the CdS and ZnO thicknesses from 40 nm to 80 nm. The highest efficiency value for the CdS/p-Si/p+-Si structure was calculated as 20.30 %. The efficiency value for the ZnO/CdS/p-Si/p+-Si heterojunction solar cell was increased to 21.82 % with the effect of the ZnO layer.

Improved passivation performance of Al2O3 interlayer/MoOX thin films continuously grown via atomic layer deposition

Atomic-layer-deposited MoOx was investigated as a hole-selective layer in Si heterojunction solar cells. MoOx grown on Si through atomic layer deposition (ALD) produce numerous sub-oxidation states at the MoOx/Si interface. The passivation of interface states remains an important problem for hole-selective contact Si solar cells. We improved the passivation of interface defects in the ALD-MoOx/Si structure by introducing ultrathin ALD-Al2O3 at 443 K for continuous processing with ALD-MoOx. Consequently, the carrier lifetime and implied open-circuit voltage increased from 22 μs to 215 μs and 568 mV to 637 mV, respectively. The introduction of ultrathin ALD-Al2O3 significantly reduced the number of oxidation states that acted as defects. The best passivation was observed up to 907 μs and 676 mV after post-deposition annealing at 623 K. The ALD-MoOX film prepared by introducing ultrathin Al2O3 is a promising hole-selective contact layer.

Temperature and illumination dependence of silicon heterojunction solar cells with a wide range of wafer resistivities

AbstractRecently, the significant improvements in the surface and contact passivation of silicon (Si) solar cells as well as their bulk quality have shifted their operating point to higher injections. Hence, they are less dependent on wafer doping. This shift opens an opportunity of using high‐resistivity wafers for practical photovoltaic applications, introducing a promising approach to push the cell efficiency towards the intrinsic limit and to improve the module reliability by increasing the cell breakdown voltage. Therefore, insights into the performance of Si solar cells using high‐resistivity wafers at various operating temperatures are of significant interest. In this study, we investigate the temperature‐ and illumination‐dependent performance of Si heterojunction (SHJ) solar cells using a wide range of wafer resistivities (between 3 and 1000 Ω⋅cm). Although a reduction in the passivation quality of the passivating contacts is observed at elevated temperature, the impact on the temperature coefficient of the open‐circuit voltage (TCVoc)—the dominant contributor to the temperature coefficient (TC) of the cell efficiency—is very limited. Their TCVoc are still dominated by the temperature dependence of the effective intrinsic carrier concentration. Furthermore, we also find that the investigated cells are more sensitive to temperature variation at lower illumination intensities. It is noteworthy that the efficiency of the cells fabricated using high‐resistivity wafers is comparable to that of the reference cells at any given temperature, highlighting the potential of using high‐resistivity wafers for solar cells.

N-type diamane: An effective emitter layer in crystalline silicon heterojunction solar cell

In the presented work, cell's parameter of heterostructure silicon solar cell modelled as: ITO (front contact)/n-Dn/p-cSi/Ag under the illumination of monochromatic light at standard spectrum AM-1.5 G, has been optimized by using AFORS-HET v 2.5 software. We have used n-type diamane as an emitter layer and the nature of diamane has been considered 3D in spite of 2D as well as electronic nature of diamane is considered isotropic. To ensure that Schottky junction has been formed at interface, the electric contacts have been made along the c-axis so that maximum charge carriers get collected. To obtain the high efficiency, various parameters of n-type diamane as well as p-type c-Si layers have been optimized and the maximum efficiency of 16.84% has been obtained for single layer n-diamane at 100 µm thick silicon wafer. We also investigated the spectral response and dependency of temperature on the performance of exclusively designed solar cell and we obtained the best efficiency 16.84% at 300 K temperature. In order to check the performance on commercially available silicon we have optimized the same solar cell by considering the parameters of commercially available p-type crystalline silicon layer and maximum efficiency 10.41% was achieved. After getting the maximum efficiency 16.84% we further carried out the simulation by optimizing the layer numbers of n-diamane and found decrement in the efficiency up to 15.3% which indicates that, efficiency slightly decreases as layer number increases. We have demonstrated that n-type diamane might be used as an effective emitter layer in crystalline Si heterojunction solar cell.

Large Area and High‐Efficiency MXene–Silicon Solar Cells by Organic Enhanced Dispersity and Work Function

2D transition metal carbon/nitrides (MXenes) have lately attracted increased attention in photovoltaic (PV) because of their good optical and electrical properties. MXenes–silicon solar cells as a novel PV device are considered to be a low‐cost strategy for boosting Si‐based device development. Nevertheless, power conversion efficiency (PCE) and large area preparation are limited in their future applications. Here, the solubilization of Ti2CTx (T = O, F, OH) MXene multilayer nanosheets is reported and thus the ability of scaled coating of Ti2CTx MXene thin films using an organic Nafion polymer solution is enhanced. It is demonstrated that an effect of termination by Nafion to Ti2CTx MXene groups (MXene:Nafion) dramatically enhanced the work function of the Ti2CTx MXene from 3.96 to 5.17 eV. Due to the excellent passivation ability of the organic Nafion species, the MXene:Nafion increases the effective carrier lifetime of silicon wafers to 2.52 ms compared with unpassivated control (0.03 ms). These results finally contribute to the high PCE of 14.21% of MXene:Nafion–Si solar cell with a large area of 7.29 cm2. This work proposes a large area MXene:Nafion–Si heterojunction solar cell with high performance and low cost, which points to the great promise of such a device for future application.

Over 16% efficiency organic/nanostructured Si heterojunction solar cells with a p-doped organic small molecule layer

Organic/nanostructured Si hybrid solar cells have achieved high power conversion efficiency (PCE) and short-circuit current density ( J SC ), due to the excellent light harvesting ability of nanostructured Si. However, the separation of carriers is mainly affected by the low work function of PH1000-type PEDOT:PSS, resulting in poor open circuit voltage ( V OC ). In this work, an efficient and stable MoO 3 -doped copper phthalocyanine-3,4′,4″,4‴-tetra-sulfonated acid tetra sodium salt (TS-CuPc) organic small molecule film was introduced between the heterojunction interfaces. The photogenerated carrier separation is promoted by the enhanced built-in potential ( V bi ) owing to the high work function of TS-CuPc:MoO 3 , which also suppresses the carrier recombination at the surface of nanostructured Si. As a result, for PEDOT:PSS/nanostructured Si photovoltaic devices, the PCE was greatly improved from 14.15% to 16.09%. The excellent charge separation properties and interface passivation effect enable efficient devices to demonstrate the vast potential of this new type of interlayer in photovoltaic applications. • The p-type doping increases the work function of TS-CuPc films. • The interlayer substantially improves charge separation at the Si/organic interface. • The device efficiency is as high as 16.09%, and V OC and FF are improved overall.