Textured Front Research Articles

Perovskite/silicon tandem solar cells with bilayer interface passivation.

Two-terminal monolithic perovskite/silicon tandem solar cells demonstrate huge advantages in power conversion efficiency compared with their respective single-junction counterparts1,2. However, suppressing interfacial recombination at the wide-bandgap perovskite/electron transport layer interface, without compromising its superior charge transport performance, remains a substantial challenge for perovskite/silicon tandem cells3,4. By exploiting the nanoscale discretely distributed lithium fluoride ultrathin layer followed by an additional deposition of diammonium diiodide molecule, we have devised a bilayer-intertwined passivation strategy that combines efficient electron extraction with further suppression of non-radiative recombination. We constructed perovskite/silicon tandem devices on a double-textured Czochralski-based silicon heterojunction cell, which featured a mildly textured front surface and a heavily textured rear surface, leading to simultaneously enhanced photocurrent and uncompromised rear passivation. The resulting perovskite/silicon tandem achieved an independently certified stabilized power conversion efficiency of 33.89%, accompanied by an impressive fill factor of 83.0% and an open-circuit voltage of nearly 1.97 V. To the best of our knowledge, this represents the first reported certified efficiency of a two-junction tandem solar cell exceeding the single-junction Shockley-Queisser limit of 33.7%.

Design and Numerical Analysis of Thin Film Solar Cells Based on Cu₂ZnSn (SₓSe₁₋ₓ)₄ with SnO₂ TCO and ZnS Buffer

Due to their earth-abundance, direct and adjustable bandgap in the range of visible light, and lower fabrication cost on large areas, Cu2ZnSn(SxSe1-x)4 semiconductors, commonly known as kesterite, are gaining recognition as potential materials for affordable, environment‐friendly, and high‐efficiency thin‐film photovoltaics. In this work, we numerically investigated the performance parameters of a single-heterojunction solar cell using Cu2ZnSnS4 (CZTS) / Cu2ZnSnSe4 (CZTSe) absorber layer and ZnS buffer layer, with SnO2 transparent conducting oxide (TCO) under a range of operating conditions and cell parameters. Maximum efficiency of 20.68% was obtained for a 5.5 µm absorber layer with 0.01 µm ZnS buffer layer for Cu2ZnSnSe4 solar cell, and 22.86% was obtained for a 6 µm thick Cu2ZnSnS4 absorber layer with 0.03 µm ZnS layer at room temperature and standard irradiance. The increase of irradiance to thrice the standard value led to 1% and 5% increase in efficiency for CZTS and CZTSe cells, respectively. Efficiency increased by 39% and 6.6% when the front surface was textured at an angle of 90° for CZTS and CZTSe cells, respectively. The CZTS and CZTSe based solar cells with optimized thickness and 90° textured front surface with three times the standard irradiation, showed 25.59 % and 21.52 % efficiency with open circuit voltage 1045 mV and 579.4 mV and short circuit current 85.84 mA/cm2 and 140.7 mA/cm2, respectively. The proposed architecture in this study will help produce next-generation solar cells that are both economical and energy-efficient.

Screen printable fire through nickel contacts for silicon solar cells

Metallization of crystalline silicon (Si) solar cells is indispensable for reducing the cost while increasing the overall efficiency. Developing alternative materials to the most commonly used screen printed silver (Ag) contacts is a critical factor. Here, in this study, a nickel (Ni) metal paste consisting of Ni metal particles, glass frit and an organic vehicle is fabricated for screen printed Ni contacts. To prevent possible Schottky barrier formation, a graphene layer is placed on the front surface of the Si solar cell between Ni and Si forming a metal-2D-semiconductor structure ensuring the ohmic (or ohmic-like) contacts. It is demonstrated here that the graphene is transferred successfully onto the textured front surface of the cells and G/2D peaks are observed clearly, indicating the good quality of the graphene layer. Constituents of Ni metal paste, glass frit and organic vehicle, are fabricated in concordance with nickel metal powder and mixed to achieve optimal electrical output parameters. The findings suggest that the transition temperature of the glass frit is between 270 °C and 300 °C resulting in around 475 °C softening temperature, which gives excellent etching behavior to the paste. The average contact resistance of the screen printed Ni contacts governed by etching process is measured around 6.9 mΩ cm2. The SEM images of the contacts show a uniform distribution and sintering behavior similar to that of silver counterparts. The light current-voltage measurements read the open circuit voltage of 660 mV, the short circuit density of 39.11 mA/cm2 and the fill factor of 81.4% resulting in around 21% efficiency.

Improved PERC Solar Cell Design by TCAD Simulation

In this work, we aim to identify the performance limiting factors and consequently improve the performance of PERC solar cells through extensive TCAD based device simulation and modelling. Initially, a simplified planar PERC solar cell structure is simulated in Silvaco (Athena/Atlas), where the device geometry is selected according to an experimentally fabricated cell with an efficiency of 17.86%. The J-V curves and solar cell parameters such as Jsc, FF, Voc and efficiency (η) of the simulated cell are then fitted to the experimental performance parameters by incorporating relevant models as suggested by the literature. These include: carriers’ generation-recombination, mobility, statistics and bandgap narrowing. A good agreement is obtained, where the average percentage difference between simulated and experimental performance parameters is 0.65%. The solar cell performance is then improved to 21.52% by optimising the anti-reflective coating stack composition and thickness, and adding surface texturing. This increase in efficiency is attributed to lower surface recombination and reduced reflection due to light trapping. In addition, a textured front surface enhances the path-length of light, causing it to undergo multiple internal reflections which further increases light trapping, thus increasing Jsc by 7.31 mA/cm2.

High haze Ga and Zr co-doped zinc oxide transparent electrodes for photovoltaic applications

Creating textured front electrodes to improve optical path length is promising to enhance the power conversion efficiency of solar cells. Deposition of transparent conductive oxides with textured surfaces usually requires additional processing steps, such as etching and coating nanoparticles. However, this makes the process complicated and inefficient. We demonstrate a one-step fabrication process to deposit high haze gallium and zirconium co-doped zinc oxides (GZO:Zr) prepared by atmospheric pressure plasma jets. GZO:Zr (2 at%) films achieve a low resistivity (7.88 × 10–4 Ω cm), a high haze (34.8%), and a great FoM (8.22 × 10−3 Ω−1). The haze factor increases from 7.19% to 34.8% (+384%) when 2% Zr is doped into GZO films. Such an enhancement of haze is attributed to the increased surface roughness and deterioration of crystallinity. AFM results show that roughness increases from 18.4 to 122 nm after 2% Zr is doped. SEM images show that spherical particles appeared on the film surface when Zr was doped into GZO films. Unlike conventional methods, our method produces hazy transparent electrodes in one step without changing any operational parameters and is suitable for industrial-scale mass production. Our findings pave the way for new applications of co-doping in transparent conductive oxides.

Toward high efficiency for long-term stable Cesium doped hybrid perovskite solar cells via effective light management strategy

Power conversion efficiency (PCE) and device stability are the two most important concerns for next generation perovskite solar cells (PSCs). Herein, we report an investigation for long-term stable perovskite Cs0.05(FA0.85MA0.15)0.95Pb(I0.85Br0.15)3 solar cells toward high efficiency via the light management strategy. To that end, we first experimentally determine complex optical constants of this kind of perovskite thin film by using variable angle spectroscopic ellipsometry measurements. By constructing the proper model, the calculated photocurrent density (Jsc) of 23.2 mA/cm2 of the planar PSC model is only slightly higher than the measured 23.0 mA/cm2 of our champion planar cell with excellent consistency, where the PCE is up to 20.5% with active area of 0.16 cm2, demonstrating a long-term stability capable of retaining 91% of initial PEC for 40 days. It is further found that when the front texture of nanocone array design is applied to the planar cell as a surface antireflection, the Jsc value of 24.8 mA/cm2 with the resulting PCE of 22.1% is projected from the present real planar PSC device with active area of 0.16 cm2.

Ultra‐Thin Poly‐Si Layers: Passivation Quality, Utilization of Charge Carriers Generated in the Poly‐Si and Application on Screen‐Printed Double‐Side Contacted Polycrystalline Si on Oxide Cells

Herein, the various measures to improve the efficiency of large‐area screen‐printed double‐side contacted polycrystalline Si on oxide (POLO)‐cells are experimentally demonstrated. The short‐circuit current density Jsc increases by 0.6 mA cm−2 upon reducing the thickness of poly‐Si from 25 to 10 nm due to the reduction of the parasitic absorption in the poly‐Si layer at the textured front side of the cell. Additionally, it is shown for the first time that the minority carriers generated by light absorbed in the poly‐Si layer can at least partially be transferred into the crystalline Si base. Remarkably high implied open‐circuit voltage Voc,impl values are achieved with n‐type cell precursors by introducing an hydrogenation step by AlxOy after reducing the poly‐Si thickness, and by an additional annealing step after sputtering of transparent conductive oxides (TCOs). All cell precursors show Voc,impl values of up to 740 mV independent of the poly‐Si thickness. A reduction in the open‐circuit voltage Voc is observed during back‐end processing to 728 mV as measured on the final cells. A certified cell energy conversion efficiency of 22.3% is reported.

Wavelength and angle resolved reflectance measurements of pyramidal textures for crystalline silicon photovoltaics

AbstractWavelength and angle resolved scattering (WARS) reflectance measurements are attractive to the photovoltaic (PV) industry as a means of characterizing the light‐trapping properties of a textured front surface. Moreover, at the PV module level, where a stack comprising encapsulants and glass is present, large angle scattering can promote total internal reflection at the interfaces and redirect light back towards the solar cell, thus increasing the photocurrent of the device. In this work, we present WARS measurements of a potassium hydroxide (KOH)etched random pyramid surface in the 6°–90° range and identify the main paths the photons experience through reflections from various facets of the pyramids. Our results, combined with ray‐tracing predictions, show that a reassessment of the morphology for simulation inputs is advised for a more comprehensive description of the experimental light paths due to a distribution of power across multiple scattering angles and a lower average pyramid base angle. In addition, we discuss the implications on the total amount of light trapped at the glass‐air interface and show that for a typical encapsulant refractive index of 1.5, approximately 14.5% of the scattered light is predicted to be trapped by the fabricated pyramidal texture. This is a significant increase over the 3.8% calculated to be trapped when assuming a dihedral base angle fixed to 54.74°.

Investigating the impact of layer properties on the performance of p-graphene/CH3NH3PbI3/n-cSi solar cell using numerical modelling

Abstract The use of multilayer p-graphene as hole transporting layer has been successfully shown to improve the performance of perovskite solar cell. The structure of p-graphene/CH3NH3PbI3/n-cSi is designed and simulated in AFORS-HET software. We optimized the parameters of single layer p-graphene and obtained power conversion efficiency (PCE) of 12.21% under an illumination of AM 1.5G. With an increase in the number of p-graphene layers, the PCE falls down to 10.01%. The optimization of active layer parameters increases the PCE up to 12.27%. Further optimization of n-cSi parameters lead to the highest PCE of 16.75%. A significant effect of the operating temperature on the solar cell performance is observed. The effect of textured front surface on the solar cell performance is also studied and a PCE of 17.90% is obtained for textured surface as compared to 16.75% for planar surface. Our studies indicated that p-graphene may act as an efficient hole transporting layer in perovskite solar cell.

Ray Tracing of Light Trapping Schemes in Thin Crystalline Silicon for Photovoltaics

Thin crystalline silicon (c-Si) suffers from poor light absorption which hinders generation of high photocurrent in photovoltaic (PV) devices. To overcome this issue, efficient light trapping (LT) schemes need to be incorporated into the thin c-Si absorber. This paper presents ray tracing of LT schemes in thin c-Si to enhance broadband light absorption within 300-1200 nm wavelength region. For the ray tracing, mono c-Si wafer with 100 μm thickness is investigated and solar spectrum (AM1.5G) at normal incidence is used. Front and rear pyramid textures, silicon nitride (SiNx) anti-reflective coating (ARC) and back surface reflector (BSR) are the LT schemes being studied in this work. With incremental LT schemes, optical properties of the thin c-Si are analyzed. From the absorption curve, maximum potential photocurrent density (Jmax) is calculated, assuming unity carrier collection. The c-Si reference (without LT) exhibits Jmax of 24.93 mA/cm2. With incorporation of incremental LT schemes into the thin c-Si, the Jmax increases, owing to enhanced light coupling and light scattering in the c-Si absorber. The Jmax up to 42.12 mA/cm2 is achieved when all the LT schemes are incorporated into the thin c-Si absorber. This represents 69% enhancement when compared to the Jmax of the c-Si reference.

Energy yield analysis of textured perovskite silicon tandem solar cells and modules.

Perovskite silicon tandem solar cells combine potentially low production costs with the ability to surpass the efficiency limit of silicon single junction solar cells. Optical modeling and optimization are crucial to achieve this ambitious goal in the near future. The optimization should seek to maximize the energy yield based on realistic environmental conditions. This work analyzes the energy yield of perovskite silicon tandem solar cells and modules based on realistic experimental data, with a special focus on the investigation of surface textures at the front and rear side of the solar cell and its implication for reflection as well as parasitic absorption properties. The investigation reveals a 7.3%rel higher energy yield for an encapsulated tandem cell with a textured front side compared with an encapsulated high efficiency single junction solar cell with 24.3% harvesting efficiency for irradiance data of the year 2014 in Freiburg/Germany.

Large-area MACE Si nano-inverted-pyramids for PERC solar cell application

Silicon (Si) nanostructures are regarded as a competitive candidate for the front texture of future high-efficiency Si solar cells owing to their excellent light-trapping properties. In this paper, we present nano-inverted-pyramids (NIPs) textures on the surface of Cz Si wafer with an area of 156 × 156 mm2 fabricated by employing the metal-assisted chemical etching (MACE) technique. The optical properties of the surface are investigated in detail before and after the application of a SiOx/SiNx antireflection coating. A passivated emitter and rear contact (PERC) solar cell with NIPs (NIPs-PERC) is analyzed at the cell as well module level using simulation. The simulation results indicate that the NIPs-PERC solar cell possesses a higher efficiency (η) of 1.4% relatively than that of the traditional PERC solar cell, benefiting from a significant better reflection performance in both the short as well as long wavelength range. Furthermore, simulations show that a 60-cell module with NIPs-PERC solar cells can yield a peak power of 310 W which is 8 W higher than a traditional PERC module. The novel NIPs-PERC solar cell shows high potential for mass production and opens a broad way for the application of NIPs textures to other high-performance solar cells.

Phosphorous-Doped Silicon Carbide as Front-Side Full-Area Passivating Contact for Double-Side Contacted c-Si Solar Cells

We present an electron selective passivating contact based on a tunneling SiO $_x$ capped with a phosphorous doped silicon carbide and prepared with a high-temperature thermal anneal. We investigate in detail the effects of the preparation conditions of the SiC $_x$ (n) (i.e., gas flow precursor and annealing temperature) on the interface recombination rate, dopant in-diffusion, and optical properties using test structures and solar cells. On test structures, our investigation reveals that the samples annealed at temperatures of 800–850 °C exhibit an increased surface passivation toward higher gas flow ratio ( r = CH4/(SiH4 + CH4)). On textured and planar samples, we obtained best implied open-circuit voltages ( i-V OC) of 737 and 746 mV, respectively, with corresponding dark saturation current densities ( J0 ) of ∼8 and ∼4 fA/cm2. The SiC $_x$ (n) layers with different r values were applied on the textured front side of p-type c-Si solar cells in combination with a boron-doped SiC $_x$ (p) as rear hole selective passivating contact. Our cell results show a tradeoff between V OC and short-circuit current density ( J SC) dictated by the C-content in the front-side SiC $_x$ (n). On p-type wafers, best V OC = 706 mV, FF = 80.2%, and J SC = 38.0 mA/cm2 with a final conversion efficiency of 21.5% are demonstrated for 2 × 2 cm2 screen-printed cells, with a simple and patterning-free process based on plasma depositions and one annealing step 800 °C T < 850 °C for the formation of both passivating contacts.

Front and rear contact Si solar cells combining high and low thermal budget Si passivating contacts

In this work we develop a rear emitter silicon solar cell integrating carrier-selective passivating contacts (CSPCs) with different thermal budget in the same device. The solar cell consists of a B-doped poly-Si/SiOx hole collector and an i/n hydrogenated amorphous silicon (a-Si:H) stack acting as electron collector placed on the planar rear and textured front side, respectively. We investigate the passivation properties of both CSPCs on symmetric structures by optimizing the interdependency among annealing temperature, time and environment. The optimized B-doped poly-Si/SiOx reaches a saturation current density of ~10 fA/cm2 on n-type wafers and an implied open circuit voltage (iVOC) of 716 mV. Furthermore, the i/n a-Si:H stack shows an effective carrier lifetime above 4 ms and iVOC of ~705 mV for cell-relevant layers thickness. After a post-deposition annealing in H2, lifetime is above 10 ms and iVOC = 708 mV. Finally, we optimize the optoelectronic properties of indium-based transparent conductive oxide (Indium Tin Oxide ITO and hydrogenated indium oxide IO:H) to reduce parasitic absorption with a gain in short circuit current density of 0.23 mA/cm2. In conclusion, the optimized layer stacks are implemented at device level obtaining a device with VOC = 704 mV, fill factor of 73.8%, a short circuit current of 39.7 mA/cm2 and 21.0% aperture-area conversion efficiency.

Light-Trapping Front Textures for Solar Cells from Tailored Mixtures of Nanospheres: A Numerical Study

Light-Trapping Front Textures for Solar Cells from Tailored Mixtures of Nanospheres: A Numerical Study

Bifunctional Hybrid a-SiO x(Mo) Layer for Hole-Selective and Interface Passivation of Highly Efficient MoO x/a-SiO x(Mo)/n-Si Heterojunction Photovoltaic Device.

The promising n-Si-based solar cell is constructed for the purpose of realizing hole- and electron-selective passivating contact, using a textured front indium tin oxide/MoO x structure and a planar rear a-SiO x/poly(Si(n+)) structure severally. The simple MoO x/n-Si heterojunction device obtains an efficiency of 16.7%. It is found that the accompanying ternary hybrid SiO x(Mo) interlayer (3.5-4.0 nm) is formed at the MoO x/n-Si boundary zone without preoxidation and is of amorphous structure, which is determined by a high-resolution transmission electron microscope with energy-dispersive X-ray spectroscopy mapping. The creation of lower-oxidation states in MoO x film indicates that the gradient distribution of SiO x with Mo element occurs within the interlayer, acting as a passivation of silicon substrate, which is revealed by X-ray photoelectron spectroscopy with depth etching. Specifically, calculations by density functional theory manifest that there are two half-filled levels (localized states) and three unoccupied levels (extended states) relating to Mo component in the ternary hybrid a-SiO x(Mo) interlayer, which play the roles of defect-assisted tunneling and direct tunneling for photogenerated holes, respectively. The transport process of photogenerated holes in the MoO x/n-Si heterojunction device is well-described by the tunnel-recombination model. Meanwhile, the a-SiO x/poly(Si(n+)) has been assembled on the rear of the device for direct tunneling of photoinduced electrons and blocking photoinduced holes.

Blue photon management by inhouse grown ZnO:Al cathode for enhanced photostability in polymer solar cells

We report the improvement in photostability of P3HT:PC60BM based bulk heterojunction solar cells deposited on Al-doped ZnO as a cathode layer replacing ITO as regularly used TCO in cells with N-I-P configuration. We experimentally and theoretically demonstrate that use of thicker ZnO:Al as cathode can successfully cut down the rate of photodegradation in short circuit current by ~40% and open circuit voltage by ~30% compared to the control device made on ITO based cathode. This effective reduction in photodegradation is understood to be coming from the absorption of ultraviolet and blue photon in the cathode layer itself. The loss in short circuit current due to the loss of blue photon in EQE is compensated by higher FF (lower series resistance) due to thicker ZnO:Al layer resulting in final device efficiency almost uncompromised with added benefit of reduced photo degradation. The experimental results are supported with optical simulations which show more absorption in the short wavelength region for the thicker ZnO films, compared to ITO films, deposited on glass substrates. This work also proposes using ZnO:Al cathode as a template for random textured front surface to potentially increase short circuit current by increase in photon absorption in active layer matrix by light scattering techniques. Our results provide an inexpensive pathway for improving the stability of organic photovoltaics without compromising the device performance.

Advanced light trapping scheme in decoupled front and rear textured thin-film silicon solar cells

We present the study of an advanced light trapping scheme applied to thin-film silicon-based solar cells, overcoming the broadband Green absorption limit, that is the generalized case of the 4n2 classical absorption limit for all wavelengths. This result is achieved by the 3-dimensional optical modelling of a fully functional thin-film hydrogenated nano-crystalline silicon (nc-Si:H) solar cell endowed with decoupled front and back textures. Our results stem from rigorously characterized optical properties of state-of-the-art materials, optimized geometric nano-features on the front and rear surfaces of the solar cell, and thickness optimization of the front transparent oxide. The simulated improvements derive from a gain in light absorption, especially in the near-infrared part of the spectrum close to the band gap of nc-Si:H. In this wavelength region, the material is weakly absorbing, whereas we now find significant absorptance peaks that can only be explained by the concurrent excitation of guided resonances by front and rear textures. This insight indicates the need to modify the temporal coupled-mode theory, which fails to predict the absorption enhancement achieved in this work, extending its validity to the case of decoupled front/back texturing. Our approach results in substantially high photocurrent density (>36 mA/cm2), creating a platform suitable for high efficiency single and multi-junction thin-film solar cells based either on typical silicon alloys or on the novel and promising barium (di)silicide (BaSi2) absorber. In the latter case, using the same advanced light trapping employed for nc-Si:H, we demonstrate a very high implied photocurrent density of 41.1 mA/cm2, for a device endowed with 2-μm thick absorber.

Impact of Standard Cleaning on Electrical and Optical Properties of Phosphorus-Doped Black Silicon

Black silicon (b-Si) has been estimated to considerably grow its market share as a front texture of high-efficiency silicon solar cells. In addition to excellent optical properties, high-efficiency cell process requires extreme cleanliness of the bulk material, and thus cleaning of b-Si surfaces is often a critical process step. While standard clean (SC) 1 solution efficiently removes possible contamination from wafer surfaces, we show here that it may cause challenges in b-Si solar cells. First, the silicon etch rate in SC1 solution is shown to depend on the phosphorous concentration and as high rate as ∼1.4 nm/min is observed on planar emitter surfaces. When extending the study to b-Si, which has much larger surface area in contact with the cleaning solution, even higher volumetric Si consumption occurs. This is observed in significant changes in emitter doping profiles, for instance, a 10 and 30-min cleaning increases the sheet resistance from 47 to 57 Ω/□ and 127 Ω/□, respectively. Furthermore, the SC1 solution alters substantially the nanostructure morphology, which impacts the optics by nearly doubling and more than tripling the surface reflectance after a 30 and 60-min immersion, respectively. Thus, uncontrolled cleaning times may impair both the electrical and optical properties of b-Si solar cells.

Efficiency increase of a-Si:H solar cells with optimized front and back contact textures

This work investigated the effects of replacing the substrate of a typical n-i-p-p+ amorphous silicon solar cell by a textured and reflective one. To develop different front and back contact textures for n-i-p-p+ solar cells different triangular profile with different values of texture period (p) and height (h) were parameterized and investigated. All calculations have been carried out by using a realistic 2D model based on TCAD Sentaurus. The results of simulation shown that the designed back textured structures lead to high scattering and diffraction of light, and front textured lead to less reflection which helps to increase the potential solar cell external quantum efficiency (EQE). For it, we have obtained an enhancement of the EQE in the short wavelength part of the spectrum for triangular periods p = 200 nm with height of 75 nm, whereas the EQE in the red and infrared part of the spectrum is increased for triangular periods p = 500 with height of 75 nm. Accordingly, the best values of the short circuit current JSC = 15.73 mA/cm2 and the conversion efficiency Eff = 11.23% were obtained with the optimized values of p = 500 nm and h = 75 nm for the front and back contact textures. Finally, the conversion efficiency Eff of the simulated structure with front and back contact textures compared to that of an experimental standard structure without front and back contact textures was increased from 7.44% to 11.23%.