Large-area multicrystalline silicon solar cell fabrication using reactive ion etching (RIE)

Surface texturing of crystalline silicon wafer improves the conversion efficiency of solar cells by the enhancement in antireflection property and light trapping. Compared to antireflection coating, it is a more permanent and effective scheme. Wet texturing with the chemicals such as alkali (NaOH, KOH) or acid (HF, HNO₃, CH₃COOH) is too difficult for thinner wafer to apply due to a large amount of silicon loss. However, Plasma surface texturing using Reactive Ion Etching (RIE) can be effective in reducing the surface reflectance with low silicon loss. In this study, we have fabricated a large-area (156×156 mm) multicrystalline silicon (mc-Si) solar cell by mask less surface texturing using a SF₆/O₂ reactive ion etching. We have accomplished texturing with RIE by reducing silicon loss by almost half of that in wet texturing process. By optimizing the processing steps, we achieved conversion efficiency, open circuit voltage, short circuit current density, and fill factor as high as 16.1%, 619 mV, 33.5 mA/cm², and 77.7%, respectively. This study establishes that it is possible to fabricate the thin multicrystalline silicon solar cells of low cost and high efficiency using surface texturing by RIE.     

Large-size multi-crystalline silicon solar cells with honeycomb textured surface and point-contacted rear toward industrial production

In this paper, we present a multi-crystalline solar cell with hexagonally aligned hemispherical concaves, which is known as honeycomb textured structure, for an anti-reflecting structure. The emitter and the rear surface were passivated by silicon nitride, which is known as passivated emitter and rear (PERC) structure. The texture was fabricated by laser-patterning of silicon nitride film on a wafer and wet chemical etching of the wafer beneath the silicon nitride film through the patterned holes. This process succeeded in substituting the lithographic process usually used for fabricating honeycomb textured structure in small area. After the texturing process, solar cells were fabricated by utilizing conventional fabrication techniques, i.e. phosphorus diffusion in tube furnace, deposition of anti-reflection film and rear passivation film by chemical vapor deposition, front and rear electrodes formation by screen printing, and contact formation by furnace. By adding relatively small complicating process to conventional production process, conversion efficiency of 19.1% was achieved with mc-Si solar cells of over 200 cm² in size. The efficiency was independently confirmed by National Institute of Advanced Industrial Science and Technology (AIST).     

Effect of the front surface field on crystalline silicon solar cell efficiency

The present paper reports on a simulation study carried out to determine and optimize the effect of the high–low junction emitter (n⁺-n) on thin silicon solar cell performance. The optimum conditions for the thickness and doping level of the front surface layer with a Gaussian profile were optimized using analytical solutions for a one dimensional model that takes on the theory relevant for highly doped regions into account. The photovoltaic parameters of silicon solar cells with front surface field layer (n⁺-n-p structure) and those of the conventional one (n-p structure) are compared. The results indicate that the most important role played by the front surface field layer is to enhance the collection of light-generated free carriers, which improves the efficiency of the short wavelength quantum. This is achieved by a drastic reduction in the effective recombination at the emitter upper boundary, a property primarily responsible for the decrease in the emitter dark current density. The findings also indicate that the solar cell maximum efficiency increase by about 2.38% when the surface doping level of the n⁺-region and its thickness are equal to 2.10²⁰ cm^?3 and 0.07 μm, respectively.     

Damage-free reactive ion etch for high-efficiency large-area multi-crystalline silicon solar cells

Texturing of silicon (Si) wafer surface is a key to enhance light absorption and improve the solar cell performance. While alkaline texturing of single-crystalline Si (sc-Si) wafers was well established, no chemical solution has been successfully developed for multi-crystalline Si (mc-Si) wafers. Reactive-ion-etch (RIE) is a promising technique for effective texturing of both sc-Si and mc-Si wafers, regardless of crystallographic characteristics, and more suitable for thin wafers. However, due to the use of plasma source generated by high power, the wafer surface gets a physical damage during the processing, which requires an additional subsequent damage-removal wet processing. In this work, we developed a damage-free RIE texturing for mc-Si solar cells. An improved self-masking RIE texturing process, developed in this study, produced ∼0.7% absolute efficiency gain on 156×156 mm² mc-Si cells, where the gas ratio and the plasma power density were keys to mitigate the plasma-induced-damage during the RIE processing while maintaining decent surface reflectance. In the self-masking RIE texturing, a mixture of SF₆/Cl₂/O₂ gases was found to significantly affect the surface morphology uniformity and reflectance, where an optimal etch depth was found to be 200-400 nm. We achieved J_sc gain of ∼1.3 mA/cm² while maintaining decent FFs of ∼0.78 without a V_oc loss after optimization of firing conditions.     

Local aluminum-silicon contacts by layer selective laser ablation

We demonstrate damage free selective laser ablation of silicon nitride from a silicon nitride/amorphous silicon double layer. This approach allows local contact formation to passivated silicon. Thereby the remaining amorphous silicon dissolves in evaporated aluminum by annealing. This technique is especially useful for contacting thin emitters since it avoids any damage to the silicon substrate. We demonstrate a local contact resistivity of 0.8±0.3 mΩ cm² on a phosphorous diffused emitter with a peak doping density of 2×10²⁰ cm⁻³. Laser treated as well as non-treated areas show the same carrier lifetime of 2000 μs on 100 Ω cm mono-crystalline silicon, proving the selective ablation.     

The influence of porous silicon coating on silicon solar cells with different emitter thicknesses

The influence of the emitter thickness on the photovoltaic properties of monocrystalline silicon solar cells with porous silicon was investigated. The measurements were carried out on n⁺p silicon junction whose emitter depth was varied between 0.5 and 2.2 μm. A thin porous silicon layer (PSL), less than 100 nm, was formed on the n⁺ emitter. The electrical properties of the samples with PS were improved with decrease of the n⁺p junction depth. Our results demonstrate short-circuit current values of about 35–37 mA/cm² using n⁺ region with 0.5 μm depth. The observed increase of the short-circuit current for samples with PS and thin emitter could be explained not only by the reduction of the reflection loss and surface recombination but also by the additional photogenerated carriers within the PSL. This assumption was confirmed by numerical modeling. The spectral response measurements were performed at a wavelength range of 0.4–1.1 μm. The relative spectral response showed a significant increase in the quantum efficiency of shorter wavelengths of 400–500 nm as a result of the PS coating. The obtained results point out that it would be possible to prepare a solar cell with 19–20% efficiency by the proposed simple technology.     

Enhancement of photovoltaic properties of multicrystalline silicon solar cells by combination of buried metallic contacts and thin porous silicon

Photovoltaic properties of buried metallic contacts (BMCs) with and without application of a front porous silicon (PS) layer on multicrystalline silicon (mc-Si) solar cells were investigated. A Chemical Vapor Etching (CVE) method was used to perform front PS layer and BMCs of mc-Si solar cells. Good electrical performance for the mc-Si solar cells was observed after combination of BMCs and thin PS films. As a result the current-voltage (I-V) characteristics and the internal quantum efficiency (IQE) were improved, and the effective minority carrier diffusion length (Ln) increases from 75 to 110 μm after BMCs achievement. The reflectivity was reduced to 8% in the 450-950 nm wavelength range. This simple and low cost technology induces a 12% conversion efficiency (surface area = 3.2 cm²). The obtained results indicate that the BMCs improve charge carrier collection while the PS layer passivates the front surface.     

Monolithic series-interconnection for a thin film silicon solar cell

To raise the output voltage of silicon solar cells several solar cells on one wafer can be monolithically interconnected. A solar cell system consisting of 20 solar cells on a 2×2 cm² area has been produced on a 4” SOI-wafer with a 15 μm thick monocrystalline active layer. Under irradiation with an AM1.5G spectrum an open-circuit voltage of 7.5 V and current densities up to 17 mA/cm² for the system have been measured. An increase in performance is expected, when the doping and contact processing is better suited and a light trapping structure is realized for the solar cell system.     

A new texturing geometry for producing high efficiency solar cells with no antireflection coatings

Various schemes to trap weakly absorbed light into solar cells have been proposed. These schemes include texturing the cell, texturing the cover glass and geometric arrangements of the individual cells. The perpendicular slats geometry is considered to be the best cell texturing design for light trapping. In this paper a new cell surface texturing design is proposed which, without the use of anti-reflection coatings, can outperform the perpendicular slats geometry with a double layer anti-reflection coating by virtue of efficient internal light trapping and a decrease in the front surface reflectance. The single sided texture uses three perpeendicular planes on the front surface and a planar back surface. The three perpendicular planes provide a triple bounce for the incoming light and efficient confinement for light which has entered the cell. TEXTURE, a raytracing program for textured cells, was used to predict the performance of this new design. A quantitative comparison with other texturing schemes is also provided. It is shown that for a cell without an anti-reflection coating on the front and a 98% effective back surface reflector, the new design produces a maximum short circuit current density of 40.99 mA/cm² as compared to 41.46 mA/cm² and 35.16 mA/cm² for the perpendicular slats geometry and flat surfaces, respectively, with a conventional single layer AR coating on the front. Effects of different front surface reflection coefficients are examined to show that as the front reflectance is decreased by improved antireflection coatings, the importance of the triple bounce is reduced and most promising surface texturing schemes approach the same value of maximum current.     

Progress in emitter wrap-through solar cell fabrication on boron doped Czochralski-grown silicon

We report on RISE-EWT (Rear Interdigitated Single Evaporation-Emitter Wrap-Through) solar cells on full area (12.5×12.5 cm²) pseudo square boron doped Czochralski-grown silicon wafers. We investigate the main efficiency optimisation factors of these cells by investigating the dependence of RISE-EWT cell parameters on the base dopant concentration N_A. We furthermore detail the effects of large feature sizes in base and emitter regions at the rear of the solar cell and investigate these effects with particular attention to the edge regions. EWT solar cells typically exhibit rather low fill factors. However, our results show that the improved fill factors can be achieved by increasing N_A, which in return leads to optimised efficiency values. For our RISE-EWT solar cells made from boron doped Cz-Si wafers, this benefit is maintained even after light-induced degradation. Our investigation of edge area related effects shows the importance of proper cell design in these areas, leading to a further 2.8% absolute improvement in the fill factor. Combining increased base dopant concentration with optimised edge design, we achieve 19.0% efficiency on (12.5×12.5 cm²) boron doped Cz silicon wafers before light-induced degradation, resulting in 18.1% efficiency in the light-degraded state.     

Defect states on the surfaces of a P-type c-Si wafer and how they control the performance of a double heterojunction solar cell

“Heterojunction with intrinsic thin layer” solar cells combine the high efficiency of crystalline silicon (c-Si) cells, with the low cost of amorphous silicon technology. Here we use detailed numerical modeling and experiments to understand the influence, on the solar cell output parameters, of defects on the front and rear surfaces of the P-type c-Si wafer. Modeling indicates that the defects on the front surface of c-Si reduce the open-circuit voltage and fill factor, while those on the rear surface degrade mainly the short-circuit current density and fill factor, but only when their density exceeds 10¹² cm⁻².     

Surface texturing of sputtered ZnO:Al/Ag back reflectors for flexible silicon thin-film solar cells

Al-doped zinc oxide/silver (ZnO:Al/Ag) back reflectors for silicon thin-film solar cells with an n-i-p configuration were prepared on flexible stainless steel substrates by dc magnetron sputtering. The surface morphologies of the back reflectors were modified by changing the deposition temperature of the Ag films to improve the light-scattering properties on the back reflectors, resulting in the enhancement of the light-trapping effect in the solar cells. By elevating the deposition temperature from room temperature to 500 °C, the surface roughness of the Ag films increased from 6.62 to 46.64 nm. The films at 500 °C had coarse surface features with irregular grain size distributions between 200 and 900 nm, whereas the films produced at low temperatures below 100 °C had smooth surfaces consisting of small grains between 100 and 200 nm. Even after the 100-nm thick ZnO:Al films were deposited on the modified Ag surfaces, the surface microstructure of the ZnO:Al/Ag bilayers was similar to that of the Ag films. The surface roughness of bilayers increased from 7.12 to 39.30 nm with coarsening the Ag surfaces. Haze factor (a ratio of diffuse reflectance to total reflectance) of Ag films was enhanced remarkably from 59% to 74% in a wide wavelength range from 350 to 1100 nm with increasing the surface roughness of the Ag films from 6.62 nm to 46.64 nm. Enhancement in the haze factor was due to the increase of diffuse reflectance on the Ag films, because the total reflectance did not change much with increasing surface roughness of the Ag films. This increasing roughness indicated that the light scattering from the rough surface of the back reflectors improved. The enhanced light scattering from the back reflectors influenced the performance of the solar cells mainly in terms of the short-circuit current density (J_sc). Compared to the back reflectors with smooth surface features, leading to a J_sc value of 9.94 mA/cm², the back reflectors with large surface roughness improved the J_sc value of the solar cells to 13.36 mA/cm² without detrimental changes in the fill factor (FF) and open circuit voltage (V_oc); they eventually increased the conversion efficiency of the solar cells from 5.59% to 7.60%.     

Influence of n-type chemical doping layer on the performance of organic photovoltaic solar cells

We report the performance improvement of organic solar cell by addition of an n-type chemical doping layer in organic bulk heterojunction device. The power conversion efficiency (PCE) of P3HT and PCBM-71 based polymer solar cells increases by adding a mixture of TCNQ (7,7,8,8-tetracyanoquinodimethane) and LCV (Leucocrystal violet) between active layer and cathode electrode. The PCE of the cell increases by 14% compared to the control cell with Al-only cathode electrode. The device with an organic n-doped layer shows the J_SC of 8.88 mA/cm², V_OC of 0.51 V, FF of 60.1%, and thus the PCE of 2.72% under AM1.5 illumination of 100 mW/cm².     

Realization of high efficiency micromorph tandem silicon solar cells on glass and plastic substrates: Issues and potential

High conversion efficiency for (amorphous/microcrystalline) "micromorph" tandem solar cells requires both a dedicated light management, to keep the absorber layers as thin as possible, and optimized growth conditions of the microcrystalline silicon (μc-Si:H) material. Efficient light trapping is achieved here by use of textured front and back contacts as well as by implementing an intermediate reflecting layer (IRL) between the individual cells of the tandem. This paper discusses the latest developments of IRLs at IMT Neuchâtel: SiO_x based for micromorphs on glass and ZnO based IRLs for micromorphs on flexible substrates were successfully incorporated in micromorph tandem cells leading to high, matched, current above 13.8 mA/cm² for p-i-n tandems. In n-i-p configuration, asymmetric intermediate reflectors were employed to achieve currents of up to 12.5 mA/cm². On glass substrates, initial and stabilized efficiencies exceeding 13% and 11%, respectively, were thus obtained on 1 cm² cells, while on plastic foils with imprinted gratings, 11.2% initial and 9.8% stable efficiency could be reached. Recent progress on the development of effective front and back contacts will be described as well.     

Preparation of (n) a-Si: H/(p) c-Si heterojunction solar cells

Heterojunction solar cells have been manufactured by depositing n-type a-Si: H on p-type 1–2Ω cm CZ single crystalline silicon substrates. Although our cell structure is very simple - neither a BSF nor a surface texturing is used - a conversion efficiency of 13.1% has been achieved on an area of 1 cm². In this paper the technology is described and the dependence of the solar cell parameters on the properties of the n-type a-Si: H layer is discussed. It is shown that this cell type exhibits no degradation under light exposure.     

Large area 15.8% n-type mc-Si screen-printed solar cell with screen printed Al-alloyed emitter

A record efficiency of 15.8% (independently confirmed at Fraunhofer ISE calibration laboratory) is reported on large area (120 cm²) n-type mc-Si rear junction Si solar cell. Minor modifications to the industrial process for p-type, such as optimization of Al-alloyed screen-printed emitter and phosphorus front surface field, led to an improvement in cell properties. Large improvement in short-circuit current of the cell was possible by decreasing the cell thickness to 130 μm.     

Fabrication of microcrystalline silicon solar cells on a SnO2 coated substrate using seed layer insertion

We fabricated hydrogenated microcrystalline silicon (μc-Si:H) solar cells on SnO₂ coated glass using a seed layer insertion technique. Since rich hydrogen atoms from the μc-Si:H deposition process degrade the SnO₂ layer, we applied p-type hydrogenated amorphous silicon (p-a-Si:H) as a window layer. To grow the μc-Si:H layer on the p-a-Si:H window layer, we developed a seed layer insertion method. We inserted the seed layer between the p-a-Si:H layer and intrinsic bulk μc-Si:H. This seed layer consists of a thin hydrogen diluted silicon buffer layer and a naturally hydrogen profiled layer. We compared the characteristics of solar cells with and without the seed layer. When the seed layer was not applied, the fabricated cell showed the characteristics of a-Si:H solar cell whose spectral response was in a range of 400-800 nm. Using the seed layer, we achieved a μc-Si:H solar cell with performance of V_oc=0.535 V, J_sc=16.0 mA/cm², FF=0.667, and conversion efficiency=5.7% without any back reflector. The spectral response was in the range of 400-1100 nm. Also, the fabricated device has little substrate dependence, because a-Si:H has weaker substrate selectivity than μc-Si:H.     

Nanoimprint patterning for tunable light trapping in large-area silicon solar cells

We demonstrate the flexibility of UV nanoimprint lithography for effective light trapping in p-i-n a-Si:H/μc-Si:H tandem solar cells. A textured polymeric layer covered with pyramidal transparent conductive oxide structures is shown as an ideal system to promote front light scattering and thus enhanced photocurrent. The double structure incorporated into micromorph tandem thin film silicon solar cells is systematically investigated in order to find a relationship between interface morphology, optical properties and photovoltaic characteristics. To prevent the formation of defects during cell growth, a controllable smoothing of the imprinted texture is developed. Modules grown on polymer structures smoothed via multi-replication show excellent performance reaching a photocurrent of 12.6 mA/cm² and an efficiency of 12.8%.     

High-efficiency PERL and PERT silicon solar cells on FZ and MCZ substrates

High-efficiency PERL (passivated emitter, rear locally diffused) and PERT (passivated emitter, rear totally diffused) silicon solar cells have been fabricated on FZ and MCZ (magnetically confined Czochralski) substrates at the University of New South Wales. One of the PERL cells on FZ substrates demonstrated 24.7% efficiency at Sandia National Laboratories under the standard global AM1.5 spectrum (100 mW/cm²) at 25°C. Another PERT cell on a MCZ substrate, supplied by SEH, Japan, demonstrated 24.5% efficiency at Sandia under the same test conditions. Both these efficiencies are the highest ever reported for FZ and MCZ silicon cells, respectively. The cells made on MCZ substrates also showed stable cell performance.     

Spray pyrolytically deposited nanoporous Ti doped hematite thin films for efficient photoelectrochemical splitting of water

Nanoporous hematite (α-Fe₂O₃) thin films doped with Ti⁴⁺ deposited by spray-pyrolysis were successfully used in photoelectrochemical splitting of water for solar hydrogen production. X-ray diffraction, field emission scanning electron microscopy, UV–visible absorption and photoelectrochemical studies have been performed on the undoped and Ti⁴⁺ doped hematite thin films. Morphology of α-Fe₂O₃ thin films was observed to be nanoporous, with increased porosity (pore size ∼12 to 20 nm) on increasing doping concentration. A significant decrease in the bandgap energy from 1.95 to 1.27 eV was found due to doping. α-Fe₂O₃ film doped with 0.02 M Ti⁴⁺ ions exhibited best solar to hydrogen conversion efficiency (photoconversion efficiency) of 1.38% at 0.5 V/SCE. Highest photocurrent densities of 0.34 mA/cm² at zero bias and 1.98 mA/cm² at 0.5 V/SCE were obtained by incorporating 0.02 M Ti⁴⁺ in α-Fe₂O₃, which are significantly larger than earlier reported values. Donor density (30.8 × 10²⁰ cm⁻³) and flatband potential (−1.01 V/SCE) obtained were also maximum for this sample. Hydrogen collected in 1 hr at Pt electrode with the best photoelectrode was 2.44 mL with 150 mW/cm² visible light source.