Process development of amorphous silicon/crystalline silicon solar cells

We have already investigated some crucial limiting process steps of the amorphous silicon (a-Si)/crystalline silicon (c-Si) solar cell technology and some specific characterization tools of the ultrathin amorphous material used in devices. In this work, we focus our attention particularlyon the technology of the ITO front contact fabrication, that also is used as an antireflective coating. It is pointed out that this layer acts as a barrier layer against the diffusion of metal during the annealing treatments of the front contact grid. The criteria of the selection of the metal to be used to obtain good performance of the grid and the deposition methods best suited to the purpose are shown. We were able to fabricate low temperature heterojunction solar cells based p-type Czochralski silicon, and a conversion efficiency of 14.7% on 3.8 cm² area was obtained without back surface field and texturization.     

Comparison of multicrystalline silicon surfaces after wet chemical etching and hydrogen plasma treatment: application to heterojunction solar cells

The modifications of the surface and subsurface properties of p-type multicrystalline silicon (mc-Si) after wet chemical etching and hydrogen plasma treatment were investigated. A simple heterojunction (HJ) solar cell structure consisting of front grids/ITO/(n)a-Si:H/(p)mc-Si/Al was used for investigating the conversion efficiency. It is found that the optimized wet chemical etching and cleaning processes as a last technological step before the deposition of the a-Si:H emitter are more favorable to HJ solar cells fabrication than the hydrogenation. Solar cells on p-type mc-Si were prepared without high-efficiency features (point contacts, back surface field). They exhibited efficiencies up to 13% for a cell area of 1 cm² and 12% for a cell area of 39 cm².     

Easy-to-fabricate 20% efficient large-area silicon solar cells

Recently, an innovative silicon solar cell structure has been developed at ISFH which is capable of achieving very high cell efficiencies on industrial-size wafers with a simple photolithography-free processing sequence. As the corresponding solar cells essentially rely on the application of obliquely evaporated contacts they are denoted as OECO cells. In this paper the successful up-scaling of the novel OECO process from 21% efficient 4 cm² laboratory devices to the fabrication of large-area (100 cm²) silicon solar cells is described, and independently confirmed total area efficiencies of 20% are reported for 10×10 cm² OECO-type solar cells fabricated on p-type float-zone silicon.     

High-efficiency OECO Czochralski-silicon solar cells for mass production

To improve the economy of photovoltaics, efficiencies of solar cells have to be drastically increased without using complex technologies. This work demonstrates that with the obliquely evaporated contact metal-insulator-semiconductor (MIS)-n⁺p solar cell structure recently developed at ISFH efficiencies exceeding 21% can be obtained using only four simple fabrication steps: (i) mechanical surface grooving, (ii) P-diffusion, (iii) oblique vacuum evaporation of Al, and (iv) plasma silicon nitride deposition. Cell design and processing sequences are outlined together with the importance of MIS contacts as both low-cost and high efficiency features. The custom-made pilot line equipment for mass production of 20% efficient 10×10 cm² Cz silicon solar cells including Ga doped wafers is described.     

A new generation of crystalline silicon solar cells: Simple processing and record efficiencies for industrial-size devices

In contrast to the general opinion that very high efficiencies can only be obtained using complex processing, with the novel technologically simple and environmentally sound obliquely evaporated contact (OECO) type solar cell efficiencies exceeding 21% could be obtained without applying masks or photolithography. Two different approaches of OECO cells using MIS contacts and exclusively Al as metallization are discussed: (i) with a diffused n⁺-emitter (MIS-n⁺p) and (ii) with an inversion layer emitter (MIS-IL). The most important results particularly for industrial production are efficiencies of 19% and 20% for simply to fabricate 10×10 cm² OECO cells on commercial CZ-Si and FZ-Si, respectively. These are the highest efficiencies ever reported for solar cells of industrial size.     

The brand‐new metallization patterning processes for plated solar cells

This paper discusses two brand‐new patterning methods for solar cell front metallization by using a layer of amorphous silicon (a‐Si) and the laser processed patterning process. These methods have the advantages of simplicity, rapidity, and low cost for the mass production of plated solar cells and also have the potential to overcome the shortcomings of the existing complex processes for Ni/Cu plated cells. In this paper, we reveal the processes and show the metallization performance. The patterning results were disclosed and had the line width of about 45 µm in our experiment. The specific contact resistivity (ρ_c) between plated Ni and silicon wafer exceeded the order of 10^?4 Ω cm². In addition, the patterning mechanisms are also proposed and discussed. Copyright © 2015 John Wiley & Sons, Ltd.     

Progress in monolithic series connection of wafer-based crystalline silicon solar cells by the novel ‘HighVo’ (High Voltage) cell concept

Progress in the development of the new HighVo cell concept for monolithic series connection of wafer-based crystalline silicon solar cells is presented. HighVo cells have been produced using standard low-cost silicon wafer technology without any photo lithographic masking step. The cells obtained with a total area of 21 cm² exhibited a voltage at the maximum power point V_MPP of 2.8 V and a conversion efficiency η of 11.4 %. To our knowledge this is the highest conversion efficiency reported so far for monolithically integrated non-thin-film silicon solar cells.     

Performance of double junction a-Si solar cells by using ZnO:Al films with different electrical and optical properties at the n/metal interface

High-quality ZnO:Al films have been prepared by using RF-magnetron-sputtering method with resistivity ranging from 10⁻¹ to 10⁻⁴ Ω cm and transmittance above 90% in visible region. We have fabricated small area (1 cm²) double junction (a-Si/a-Si) solar cells using ZnO/Al and ZnO/Ag as back contact. The conversion efficiency of double junction a-Si solar cell increases from 9.9% to 10.9% by using ZnO/Al back contact and to 11.4% by using ZnO/Ag as back contact. Effect of variation of thickness of i-layer on performance of the cell has also been studied.     

Ribbon growth on substrate (RGS) silicon solar cells with microwave-induced remote hydrogen plasma passivation and efficiencies exceeding 11%

For the first time efficiencies above 11% for solar cells (4 cm²) based on Bayer ribbon growth on substrate (RGS) crystalline silicon have been demonstrated including mechanical V-structuring of the front surface, aluminum-gettering, microwave-induced remote hydrogen plasma (MIRHP) passivation and PECVD SiN/SiO₂ double-layer antireflection coating. MIRHP alone resulted in absolute improvements in the open-circuit voltage of 27 mV, in the short-circuit current density of 2.8 mA cm⁻² and in the cell efficiency of 1.9% leading to an open-circuit voltage of 538 mV and an efficiency of 11.1%.     

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.     

Analysis of series resistance of crystalline silicon solar cell with two-layer front metallization based on light-induced plating

Improving the front metallization quality of silicon solar cells should be a key to enhance cell performance. In this work, we investigated a two-layer metallization scheme involving light-induced plating (LIP) and tried to quantify its impact on the series resistance of the front grid metals and FFs on finished cells. To estimate the effect of LIP processing on a printed and fired seed layer, individual components of series resistance were measured before and after LIP processing. Among them, grid resistance and contact resistance were closely observed because of their large contribution to series resistance. To optimize the plating on the seed metal grid, the grid resistance of the two-layer metal grid structure was calculated as a function of cross section area of the plated layer. Contact resistivity of the grid before and after LIP processing was analyzed to understand the contact resistance reduction, as well. As a result, the efficiency of solar cells with 80 μm seed metal grid width increased by 0.3% absolute compared with conventional solar cells of 120 μm metal grid width. The total area of electrodes in conventional cells was 1800 mm² and electrodes area of LIP processed solar cells was 1400 mm². The efficiency gain was due to reduction of shadowing loss from 7.7% to 6.0% without the increase of resistance due to two-layer front metallization.     

Double porous silicon layer on multi-crystalline Si for photovoltaic application

Double porous silicon (d-PS) layers formed by acid chemical etching on a top surface of n⁺/p multi-crystalline silicon solar cells were investigated with the aim to improve the performance of standard screen-printed silicon solar cells. First a macro-porous layer is formed on mc-Si. The role of this layer is texturization of surface. Next, the cells have been manufactured using standard technology based on screen-printing metallization. Finally, a second mezo-porous layer in n⁺ emitter of cell has been produced. The role of this PS layer is to serve as an antireflection coating. In this way, we have obtained d-PS layers on these solar cells. The paper present observation of d-PS microstructure with SEM as well as measurements of its effective reflectance at the level of 2.5% in the 400–1000 nm length wave range. The efficiency of the solar cells with this structure is about 12%.     

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.     

Microcrystalline silicon solar cells fabricated by VHF plasma CVD method

A series of systematic investigations on microcrystalline silicon (μc-Si:H) solar cells at high deposition rates has been studied. The effect of high deposition pressure and narrow cathode-substrate (CS) distance on the deposition rate and quality of microcrystalline silicon is discussed. The microcrystalline silicon solar cell is adopted as middle cell and bottom cell in a three-stacked junction solar cell. The characteristics of large area three-stacked junction solar cells, whose area is 801.6 cm² including grid electrode areas, are studied in various deposition rates from 1 to 3 nm/s of microcrystalline silicon. An initial efficiency of 13.1% is demonstrated in the three-stacked junction solar cell with microcrystalline silicon deposited at 3 nm/s.     

Fabrication of flexible CdTe solar modules with monolithic cell interconnection

CdTe solar cells and modules have been manufactured on polyimide (PI) substrates. Aluminum doped zinc oxide (ZnO:Al) was used as a transparent conductive oxide (TCO) front contact, while a thin high resistive transparent layer of intrinsic zinc oxide (i-ZnO) was used between the front contact and the CdS layer. The CdS and CdTe layers were evaporated onto the ZnO:Al/i-ZnO coated PI films in a high vacuum evaporation system followed by a CdCl₂ activation treatment and a Cu–Au electrical back contact deposition. In some cases prior to the cell deposition, the PI film was coated with MgF₂ on the light facing side and the effects on the optical and electrical properties of TCO and solar cells were investigated. The limitations on current density of solar cells due to optical losses in the PI substrate were estimated and compared to the experimentally achieved values. Flexible CdTe solar cells of highest efficiencies of 12.4% and 12.7% were achieved with and without anti-reflection MgF₂ coating, respectively.Laser scribing was used for patterning of layers and monolithically interconnected flexible solar modules exhibiting 8.0% total area efficiency on 31.9 cm² were developed by interconnection of 11 solar cells in series.     

Sliver solar cells: A new thin-crystalline silicon photovoltaic technology

A new technique for producing thin single-crystal silicon solar cells has been developed. The new technology allows for large decreases in silicon usage by a factor of 12 (including kerf losses) compared to conventional crystalline silicon wafer technologies. The new Sliver^® cell process uses a micromachining technique to form 60 μm-thick solar cells, fully processed while they are still supported by the silicon substrate at the edge of the wafer. The Sliver^® solar cells are capable of excellent performance due to their thickness and unique cell design with demonstrated efficiencies over 19.3% and open-circuit voltages of 683 mV. In addition, the cells are bifacial (accepts light from either sides) and very flexible. Several prototype modules have been fabricated using a new design approach that introduces a diffuse reflector to the rear of a bi-glass module. To save expensive silicon material, a significant gap is kept between cells. The light striking between cells is scattered from the rear reflector and is directed onto the rear surface of the bifacial Sliver^® cells. Module efficiency of 13% (AM1.5, 25C) has been demonstrated with a module presenting a 50% solar-cell coverage fraction, and 18.3% with a 100% Sliver^® cell coverage fraction.     

Analysis of thin silicon solar cells for high efficiency

A comprehensive theoretical analysis taking into account the contribution from both the emitter and base regions having finite surface recombination velocity has been developed for computing short-circuit current, open-circuit voltage, and efficiency of thin AR coated thin silicon solar cells with textured front surface. The dependence of efficiency on the front surface and back surface recombination velocities and on the cell parameters have been investigated in details for varying cell thickness considering the effects of bandgap narrowing and Auger recombination in the material. It is shown that efficiency exceeding 24% can be attained with silicon solar cells having thickness as low as 25 μm provided both front and back surfaces are well passivated (S < 10³cm/s) and the doping concentration in the base and emitter are in the range of 5 × 10¹⁶ to 10¹⁷cm⁻³ and 10¹⁸ to 5 × 10¹⁸cm⁻³, respectively. It is also shown that an efficiency of about 23% can be obtained for thin cells of 25 μm thickness with a much inferior quality materials having diffusion length of about 40 μm.     

Bulk and contact resistance in P3HT:PCBM heterojunction solar cells

In this paper, the series resistance of poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) bulk heterojunction (BHJ) organic solar cells (OSC) has been studied. The series resistance of thermal annealed and un-annealed devices with different active layer thicknesses was measured. The series resistance of the organic solar cells consists of the bulk resistance of the active layer itself and the specific contact resistance between the active layer and the electrode. The bulk resistance and contact resistance were extracted from the measured series resistance using the vertical transmission line model (TLM) method. By fabricating solar cell devices with different active layer thicknesses, a relationship of the series resistance with thickness was established from which bulk and contact resistances were derived. We have also found that thermal annealing helps reduce both contact resistance and bulk resistance significantly; the contact resistance dropped by a factor of 2, while the bulk resistance decreased by a factor of 8. Results have shown that for an annealed P3HT:PCBM device that has an active layer thickness of 85 nm (optimum thickness for high efficiency), 17% of the total series resistance was due to the contact resistance, and bulk resistance contributed the rest 83%. The bulk resistance value for thermal annealed organic solar cell device with an active area of 0.1 cm² was found to be 150 Ω, and the measured specific contact resistance was 3.1 Ω cm². The measured bulk and contact resistance values are much higher as compared to the high efficiency silicon solar cells. Bulk resistance and contact resistance need to be further decreased in order to achieve higher organic solar cell efficiency.     

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.     

A hybrid amorphous silicon photovoltaic and thermal solar collector

A hybrid amorphous silicon (a-Si) photovoltaic and thermal solar collector was developed and its performance tested. The solar cells, deposited on glass panels and having an average efficiency of 4% and a total area of 0.9 m², were bonded to the fin and tube aluminum heat-exchange plate using simple technology. This hybrid unit performed well as a thermal solar collector, heating water up to 65°C, while the electric characteristics of the photovoltaic modules showed little change. In addition to saving space this integral unit substantially reduces the balance-of-system cost of the photovoltaic generator. The transmission of light through various layers of an a-Si cell was measured and, in order to improve the thermal efficiency, a novel transparent type of a-Si cell was developed and tested in the hybrid unit. The results obtained show that it is possible to construct simple and cheap hybrid systems having good photovoltaic as well as thermal efficiencies.