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.     

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.     

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.     

Self-aligned locally diffused emitter (SALDE) silicon solar cell

This paper presents, for the first time, a low-cost, high-throughput manufacturing approach for fabricating n-base dendritic web silicon solar cells with selectively doped emitters and self-aligned aluminum contacts using rapid thermal processing (RTP) and screen printing. The self-aligned locally diffused emitter (SALDE) structure is p⁺ nn⁺⁺ where aluminum is screen-printed on a boron-doped emitter and fired in a belt furnace to form a deep self-doped p⁺-layer and a self-aligned positive contact to the emitter according to the well-known aluminum-silicon (Al---Si) alloying process. The SALDE structure preserves the shallow emitter (20.2 μm) everywhere except directly beneath the emitter contact. There the junction depth is greater than 5 μm, as desired, in order to shield carriers in the bulk silicon from that part of the silicon surface covered by metal where the recombination rate is high. This structure is realized by using n-base (rather than p-base) substrates and by utilizing screen-printed aluminum (rather than silver) emitter contacts. Prototype dendritic web silicon (web) cells (25 cm² area) with efficiencies up to 13.2% have been produced.     

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.     

Fabrication of silicon nanowire arrays based solar cell with improved performance

We report fabrication of solar cell (n⁺-p-p⁺ structure) on black silicon substrates consisting of silicon nanowire (SiNW) arrays prepared by Ag induced wet chemical etching process in aqueous HF-AgNO₃ solution. SiNW arrays surface has low reflectivity (<5%) in the entire spectral range (400-1100 nm) of interest for solar cells. The solar cells were fabricated by conventional cell fabrication protocol. Performance of three types of cells, namely cell with SiNW over the entire front surface, cell with SiNW only in the active device area and control cell (on planar surface), has been compared. It was found that cell based on selectively grown shorter length SiNW arrays has the best cell performance.     

Polycrystalline silicon solar cells on low cost foreign substrates

The use of polycrystalline silicon layers on low-cost substrates is a promising approach for the fabrication of low-cost solar cells. Using low-carbon steel and graphite as substrates, solar cell structures have been deposited by the thermal decomposition of silane and appropriate dopants.Steel was selected as a substrate on the sole basis of its low cost. However, steel and silicon are not compatible in their properties, and an interlayer of a diffusion barrier, such as borosilicate, must be used to minimize the diffusion of iron from the substrate into the deposit. The deposited silicon on borosilicate/steel substrates is polycrystalline with a grain size of 1–5 μm, depending on deposition conditions. P-n junction solar cells were found to have low open-circuit voltages and poor current-voltage characteristics, and Schottky-barrier solar cells were found to show negligible photovoltages.Graphite is more compatible with silicon in properties than steel, and silicon deposited on graphite substrates shows considerably better microstructures. A number of solar cells, 2·5×2·5 cm in area, have been fabricated from n⁺-silicon/p-silicon/p⁺-silicon/graphite structures. The best cell to date had a V_oc of 0·35 V and an AMO efficiency of 1·5% (no antireflection coating). This type of solar cell is very promising because of the simplicity in fabrication.     

Fabrication of amorphous silicon p-i-n solar cells using ion shower doping technique

We have studied the fabrication of amorphous silicon (a-Si : H) p-i-n solar cells using an ion shower doped n⁺-layer. The p-i-n cells with ion-doped n⁺-layer exhibited open-circuit voltage of > 0.8 V, fill factor of > 0.62 and conversion efficiency of > 8.4% when the ion acceleration voltage was between 3 and 7 kV. The a-Si : H p-i-n solar cell fabricated under an optimized ion-doping condition exhibited an open-circuit voltage of 0.84 V, a fill factor of 0.66 and a conversion efficiency of 9.9% which was very similar to those of conventional a-Si : H p-i-n cells fabricated in the same deposition chamber. Therefore, ion shower doping technique can be applied to fabricate large area, high performance a-Si : H p-i-n solar cells.     

High Efficiency Large Area Back Contact Concentrator Solar Cells with a Multilevel Interconnection

Very high efficiencies have been demonstrated under concentration with silicon solar cells having interdigitated contacts on the backside. However, only laboratory cells of small dimension have reached very high efficiencies. The need for developing a multilevel metallization technology for back contact concentrator solar cells of large area is demonstrated. The particular features required for such a multilevel interconnection are studied and a process using anodic oxidation of aluminum is presented. Back contact silicon solar cells of 0.64 cm² have been processed in this technology resulting in 26.2% efficiencies at 10W/cm² (100 suns AM1.5, 25.5 ^°C). the highest efficiency reported to date for a solar cell of this area. The one-sun efficiency of this cell is 21.7% (AMI.5, 25.2^°C). We propose also a new design for the metallization of back contact cells which allows an increase in the size of the cell without increasing the series resistance.     

The infrared processing in multicrystalline silicon solar cell low-cost technology

New directions in photovoltaics depend very often on financial possibilities and new equipment. In this paper, we present the modification of a standard screen-printing technology by using an infrared (IR) furnace for forming a n⁺/p structure with phosphorus-doped silica paste on 100 cm² multicrystalline silicon wafers. The solar cells were fabricated on 300 μm thick 1 Ω cm p-type multicrystalline Bayer silicon. The average results for 100 cm² multicrystalline silicon solar cells are: I_sc=2589 mA, V_oc=599 mV, FF=0.74, E_ff=11.5%. The cross-sections of the contacts metallized in the IR furnace, as determined by scanning electron microscopy, and the phosphorus profile measured by an electrochemical profiler are shown. IR processing offers many advantages, such as a small overall thermal budget, low power and time consumption, in terms of a cost-effective technology for the continuous preparation of solar cells.     

SOLAR CELLS WITH 15.6% EFFICIENCY ON MULTICRYSTALLINE SILICON,USING IMPURITY GETTERING,BACK SURFACE FIELD AND EMITTER PASSIVATION

Three features have been combined to raise the efficiency of solar cells made on industrial multicrystalline silicon wafers: 1) reduction of bulk recombination by a special gettering process, 2) reduction of back recombination by using a p/p ⁺ junction, 3) reduction of front recombination by emitter back-etching and passivation.

A conversion efficiency of 15.6% has been achieved on 2 × 2 cm² solar cells. Spectral response measurements are used to identify the role of each processing parameter.     

Development of an emitter for industrial silicon solar cells using the doped oxide solid source diffusion technique

The aim of this paper is to demonstrate for the first time the feasibility of fabricating large-area screen-printed monocrystalline silicon solar cells using the Doped Oxide Solid Source (DOSS) diffusion technique. This process was applied to form the n⁺p emitter junction from highly doped sources prepared in a POCl₃ ambient. The diffusions were performed under a pure nitrogen flow in the temperature range 900–1050°C. In this investigation attention was devoted to the influence of the source doping level on the fill factor. The solar cells were fabricated on industrial quality 4-inch Cz wafers using a simple processing sequence incorporating screen-printed contacts and a TiO₂ antireflection coating deposited by spin-on. Fill factors as high as 79% were obtained. The potential benefit of retaining for passivation purposes the thin residual oxide grown during phosphorus diffusion was evaluated. These first experiments led to a cell efficiency close to 10%.     

High-efficiency μc-Si/c-Si heterojunction solar cells

P-type microcrystalline silicon (μc-Si (p)) on n-type crystalline silicon (c-Si(n)) heterojunction solar cells is investigated. Thin boron-doped μc-Si layers are deposited by plasma-enhanced chemical vapor deposition on CZ-Si and the V_oc of μc-Si/c-Si heterojunction solar cells is higher than that produced by a conventional thermal diffusion process. Under the appropriate conditions, the structure of thin μc-Si films on (1 0 0), (1 1 0), and (1 1 1) CZ-Si is ordered, so high V_oc of 0.579 V is achieved for 2×2 cm² μc-Si/multi-crystalline silicon (mc-Si) solar cells. The epitaxial-like growth is important in the fabrication of high-efficiency μc-Si/mc-Si heterojunction solar cells.     

Formation of porous silicon for large-area silicon solar cells: A new method

Luminescent porous silicon (PS) was prepared for the first time using a spraying set-up, which can diffuse in a homogeneous manner HF solutions, on textured or untextured (1 0 0) oriented monocrystalline silicon substrate. This new method allows us to apply PS onto the front-side surface of silicon solar cells, by supplying very fine HF drops. The front side of N⁺/P monocrystalline silicon solar cells may be treated for long periods without altering the front grid metallic contact. The monocrystalline silicon solar cells (N⁺/P, 78.5 cm²) which has undergone the HF-spraying were made with a very simple and low-cost method, allowing front-side Al contamination. A poor but expected 7.5% conversion efficiency was obtained under AM1 illumination. It was shown that under optimised HF concentration, HF-spraying time and flow HF-spraying rate, Al contamination favours the formation of a thin and homogeneous hydrogen-rich PS layer. It was found that under optimised HF-spraying conditions, the hydrogen-rich PS layer decreases the surface reflectivity up to 3% (i.e., increase light absorption), improves the short circuit current (I_sc), and the fill factor (FF) (i.e., decreases the series resistance), allowing to reach a 12.5% conversion efficiency. The dramatic improvement of the latter is discussed throughout the influence of HF concentration and spraying time on the I–V characteristics and on solar cells parameters. Despite the fact that the thin surfae PS layer acts as a good anti-reflection coating (ARC), it improves the spectral response of the cells, especially in the blue-side of the solar spectrum, where absorption becomes greater, owing to surface band gap widening and conversion of a part of UV and blue light into longer wavelengths (that are more suitable for conversion in a Si cell) throughout quantum confinement into the PS layer.     

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%.     

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.     

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.     

Aluminium BSF in silicon solar cells

The purpose of this work is to develop a back surface field (BSF) for industrial crystalline silicon solar cells and thin-film solar cells applications. Screen-printed and sputtered BSFs have been realised on structures which already have a n⁺p back junction due to the diffusion of the phosphorus in both faces of the wafer during solar cell emitter elaboration. Rapid thermal annealing temperatures from 700°C to 1000°C have been used. Thickness of the BSF has been measured by SIMS and confronted to the theoretical expected value and simulations. Electrical and optical measurements have been done in order to characterise the BSF. For 250 μm thick industrial solar cells, 6% relative increase in photocurrent has been reached.     

Experimental and theoretical radiation damage studies on crystalline silicon solar cells

An experimental facility was developed to asses in situ the degradation of crystalline silicon solar cells, fabricated by the Solar Energy Group of the National Atomic Energy Commission (CNEA), by measuring the current–voltage characteristic curve. The cells were irradiated with 10 MeV protons and fluences between 10⁸ and 10¹³ p/cm², using an external beam of the linear tandem accelerator TANDAR, at CAC-CNEA. Furthermore, theoretical simulations were performed to establish the relation between the variation of the electrical parameters and the degradation of the lifetime of minority carriers in the base. The damage constant for 10 MeV proton irradiated silicon solar cells of n⁺–p–p⁺ structure and 1 Ω cm base resistivity was determined. Finally, a proposal of a new model of radiation damage for silicon solar cells is discussed.     

Microcrystalline and micromorph device improvements through combined plasma and material characterization techniques

Hydrogenated microcrystalline silicon (μc-Si:H) growth by very high frequency plasma-enhanced chemical vapor deposition (VHF-PECVD) is studied in an industrial-type parallel plate KAI reactor. Combined plasma and material characterization techniques allow to assess critical deposition parameters for the fabrication of high quality material. A relation between low intrinsic stress of the deposited i-layer and better performing solar cell devices is identified. Significant solar cell device improvements were achieved based on these findings: high open circuit voltages above 520 mV and fill factors above 74% were obtained for 1 μm thick μc-Si:H single junction cells and a 1.2 cm² micromorph device with 12.3% initial (V_oc=1.33 V, FF=72.4%, J_sc=12.8 mA cm⁻²) and above 10.0% stabilized efficiencies.