Present status and future of crystalline silicon solar cells in Japan

Crystalline silicon solar cells show promise for further improvement of cell efficiency and cost reduction by developing process technologies for large-area, thin and high-efficiency cells and manufacturing technologies for cells and modules with high yield and high productivity.In this paper, Japanese activities on crystalline Si wafers and solar cells are presented. Based on our research results from crystalline Si materials and solar cells, key issues for further development of crystalline Si materials and solar cells will be discussed together with recent progress in the field. According to the Japanese PV2030 road map, by the year 2030 we will have to realize efficiencies of 22% for module and 25% for cell technologies into industrial mass production, to reduce the wafer thickness to 50–100 μm, and to reduce electricity cost from 50 Japanese Yen/kWh to 7 Yen/kWh in order to increase the market size by another 100–1000 times.     

Interface effects on the carrier transport and photovoltaic properties of hydrogenated amorphous silicon/crystalline silicon solar cells

Current-voltage-temperature (I-V-T) characteristics evaluated near 150K and 300K were used to study the photovoltaic property variations in hydrogenated amorphous silicon (a-Si:H)/crystalline silicon (c-Si) solar cells. The possible carrier transport mechanisms in such devices were examined from the I-V-T data which indicated a significant influence of the amorphous /crystalline interface on the short-circuit current density (J_sc) and open-circuit voltage (V_oc) of the solar cells. Carrier transport near 300K for forward biases was by a multi-tunneling mechanism and became space charge limited with increasing bias. For devices having low J_sc and V_oc an additional region was seen in both forward and reverse biases, at low temperatures, where the current simply varied linearly with the applied bias. This characteristic manifested in both high and low temperatures region for devices with still lower photovoltaic properties, which has been reasoned to be due to a higher interface density. Passivating the c-Si surface with HF just prior to the amorphous layer deposition resulted in a large improvement in the properties. The most significant effect was on the J_sc which improved by an order of magnitude. The treatment also affected the lower temperature I-V-T data in that the current fell to very low levels. The spectral response of the treated solar cells showed enhanced blue/violet response compared with the unpassivated devices. The interface passivation plus reducing a-Si thickness has improved the solar cell efficiency from 0.39% to 9.5%.     

Amorphous silicon/p-type crystalline silicon heterojunction solar cells

We have investigated the photovoltaic (PV) characteristics of both glow discharge deposited hydrogenated amorphous silicon (a-Si:H) on crystalline silicon (c-Si) in a n⁺ a-Si:H/undoped a-Si:H/p c-Si type structure, and DC magnetron sputtered a-Si:H in a n-type a-Si:H/p c-Si type solar cell structure. It was found that the PV properties of the solar cells were influenced very strongly by the a-Si/c-Si interface. Properties of strongly interface limited devices were found to be independent of a-Si thickness and c-Si resistivity. A hydrofluoric acid passivation prior to RF glow discharge deposition of a-Si:H increases the short circuit current density from 2.57 to 25.00 mA/cm² under 1 sun conditions.DC magnetron sputtering of a-Si:H in a Ar/H₂ ambient was found to be a controlled way of depositing n type a-Si:H layers on c-Si for solar cells and also a tool to study the PV response with a-Si/c-Si interface variations. 300 Å a-Si sputtered onto 1–10 ω cm p-type c-Si resulted in 10.6% efficient solar cells, without an A/R coating, with an open circuit voltage of 0.55 V and a short circuit current density of 30 mA/cm² over a 0.3 cm² area. High frequency capacitance-voltage measurements indicate good junction characteristics with zero bias depletion width in c-Si of 0.65 μm. The properties of the devices have been investigated over a wide range of variables like substrate resistivity, a-Si thickness, and sputtering power. The processing has focused on identifying and studying the conditions that result in an improved a-Si/c-Si interface that leads to better PV properties.     

Advanced cost-effective crystalline silicon solar cell technologies

An overview is given concerning current industrial technologies, near future improvements and medium-term developments in the field of industrially viable crystalline silicon terrestrial solar cell fabrication (without concentration).     

Electrical and optical characterization of crystalline silicon/porous silicon heterojunctions

We have investigated the photovoltage and photocurrent spectra of crystalline silicon/porous silicon heterojunctions. The porous silicon layers were prepared using anodic etching of p-type crystalline silicon at a current density of 25 mA/cm². From the spectral dependence of the photovoltage and photocurrent, we suggest that the photovoltaic properties of the junction are dominated by absorption in crystalline silicon only. We have also studied the effect of increase in the thickness of porous silicon layers on these spectra. We find that the open-circuit voltage of the devices increases, but the short-circuit current decreases with an increase in the thickness of the porous silicon layers. We propose a qualitative explanation for this trend, based on the increase in the series and the shunt resistance of these devices. The effect of hydrogen passivation on the junction properties by exposing the devices to hydrogen plasma is also reported.     

The absorption factor of crystalline silicon PV cells: A numerical and experimental study

The absorption factor of a PV cell is defined as the fraction of incident solar irradiance that is absorbed by the cell. This absorption factor is one of the major parameters determining the cell temperature under operational conditions. Experimentally the absorption factor can be derived from reflection and transmission measurements. The spectral reflection and transmission factors were measured for a set of crystalline silicon (c-Si) samples with a gradually increasing complexity. The experimental results agree very well with the results from a 2D numerical model that was developed. It was found that the AM1.5 absorption factor of a typical encapsulated c-Si photovoltaic cell is as high as 90.5%. Insight was gained in the cell parameters that influence this absorption factor. The presence of texture at the front of the c-Si wafer of sufficient steepness is essential to achieve such a high absorption factor. Sub-bandgap solar irradiance is mainly absorbed in the very thin emitter by means of free-carrier absorption. By minimizing reflective losses over the entire solar spectrum, the AM1.5 absorption of c-Si cells can theoretically be increased to 93.0%. The effect on the annual yield of PV and PV/thermal systems is quantified.     

Phosphorus-doped, silver-based pastes for self-doping ohmic contacts for crystalline silicon solar cells

Undoped and phosphorus-doped Ag-based pastes were applied as circular contacts to the (1 1 1) surface of dendritic web n-type Si. Current–voltage characteristics of as-deposited contacts and contacts annealed at 780°C for 10 min, 950°C for 5 min, 1000°C for 10 min were measured and compared. Annealing above the Ag–Si eutectic temperature (835°C) yielded Si precipitation within the Ag matrix, resulting in increased current across the metal/semiconductor interface. The contact resistivity was significantly lower for P-doped (<0.04 Ω cm²) than for undoped (1.90 Ω cm²) Ag contacts, both of which were annealed at 1000°C. As supported by secondary ion mass spectrometry analyses, these results are attributed to an enhanced P doping level in the Si substrate after annealing the P-doped contacts. A p–n junction diode was demonstrated by alloying the Ag–P paste with p-type Si at 1000°C. The contact resistance was inferred from diode I–V data to be 0.013 Ω cm², a value which is comparable to the 0.010 Ω cm² target value for solar cell contacts.     

Laser structuring of crystalline silicon thin-film solar cells on opaque foreign substrates

Crystalline silicon thin-film solar cells were fabricated on graphite substrates. A laser ablation process was developed for edge isolation of the thin-film cells. The shunt resistance was comparable to otherwise identical cells isolated by plasma etching, while the reproducibility of the laser isolation process was higher. The solar cells were characterized by current-voltage and light beam induced current measurements (LBiC). No interference was detected along the ablated edges. Spatial variations of the minority carrier lifetime are attributed to the grain structure of the seeding layer obtained by the zone melting recrystallization (ZMR).     

Characterization of crystalline silicon grown by plasma-enhanced CVD for thin-film solar cells

High growth-rate Si epitaxy by plasma-enhanced chemical vapor deposition (PECVD) has been investigated for a thin-film solar cell application. A high growth rate of 50 μm/h was obtained at 1050°C with plasma which is 50% larger than that by the conventional CVD without plasma. The electrical properties are almost the same for epitaxial layers with and without plasma. For undoped n-type layers, the Hall mobility and carrier density were about 600 cm²/V s and low 10¹⁵ cm⁻³, respectively. The electron diffusion length in doped p-type layers was about 20 μm. These electrical properties for the layer with plasma, in spite of higher growth rate, are comparable or better than those without plasma.     

Recent advances of high-efficiency single crystalline silicon solar cells in processing technologies and substrate materials

Single crystalline silicon solar cells have demonstrated high-energy conversion efficiencies up to 24.7% in a laboratory environment. One of the recent trends in high-efficiency silicon solar cells is to fabricate these cells on different silicon substrates. Some silicon wafer suppliers are also involved in such development. Another recent trend is the increased production of high-efficiency silicon cells, some of them with low-cost structures. This paper will discuss the progress at the University of New South Wales, and these trends in other organisations.     

Latest results on semitransparent POWER silicon solar cells

The paper presents the latest results of the polycrystalline wafer engineering result (POWER) silicon solar cell research (G. Willeke, P. Fath, The POWER silicon solar cell, Proceedings of the 12th EPVSEC, Amsterdam, 1994, pp. 766–768). Mono – as well as bifacially active semitransparent silicon solar cells have been created by forming perpendicularly overlapping grooves on the front and the rear side of a silicon wafer resulting in a regular pattern of holes. The developed very simple manufacturing process is fully compatible with an industrial production and uses POCl₃-tube diffusion, PECVD silicon nitride as single ARC and screen-printing metallization. Maximum efficiencies of η=11.2% for monofacial POWER cells on 0.4 Ω cm Cz material with a transparency of 18.2% and η=12.9% for bifacial cells on 1 Ω cm Cz material with a transparency of 16% have been obtained. Results for multicrystalline (mc) semitransparent mono- and bifacially active silicon solar cells are also presented.     

Interconnection through vias for improved efficiency and easy module manufacturing of crystalline silicon solar cells

It has become an opportune to develop new module technologies because manufacturers are using larger wafers leading to problems with interconnection of cells. ECN has developed a new cell and module design for crystalline silicon solar cells called pin-up module (PUM) based on old patents (Jong, US Patent 3,903,428, 1975; Pack, US Patent 3,903,427, 1975). In this design a limited number of holes serve as vias for interconnection of the front-side metallisation to a foil at the rear side by using pins. In this way the busbars at the front side are eliminated, thus reducing shadow losses. Calculations show that for 100 cm² cells, the efficiency will be 0.4% absolute higher. For larger cells, the efficiency gain will be as high as 1%. The PUM concept gives the possibility to increase cell dimensions without reducing the output.     

High-efficiency low-cost integral screen-printing multicrystalline silicon solar cells

This paper describes how the efficiency and throughput of industrial screen-printed multi-Si solar cells can be increased far beyond the state-of-the-art production cells. Implementation of novel processes of isotropic texturing, shallow emitter or single diffusion selective emitter, combined with screen-printed metallization fired through a PECVD SiN_x ARC layer, have been described. Novel dedicated fabrication equipment for emitter diffusion and a PECVD SiN_x deposition system are developed and implemented thereby removing the processing bottlenecks linked to the diffusion and bulk passivation processes. Several types of back-contacted solar cells with improved visual appeal required for building integrated photovoltaic (BIPV) application have been developed.     

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.     

Substrates for thin crystalline silicon solar cells

Choice of substrate for thin crystalline silicon solar cells requires a compromise between cost and quality. There are three generic substrate types, namely a transparent substrate (such as glass), an opaque substrate (such as a ceramic or metal) and low-cost multicrystalline silicon. Glass has the advantage of eliminating absorption within the substrate. However, the larger effective diffusion length, the improved surface passivation and the increased process flexibility obtainable with an opaque substrate, particularly low-cost multicrystalline silicon, may considerably outweigh the modest optical benefits of a transparent substrate. In this paper it is shown that the advantage in effective diffusion length that is required of a cell grown on an opaque substrate in order to offset the light-trapping advantages of a glass substrate is about a factor of two.     

A review of thin-film crystalline silicon for solar cell applications. Part 1: Native substrates

Approximately half the cost of a finished crystalline silicon solar module is due to the silicon itself. Combining this fact with a high-efficiency potential makes thin-film crystalline silicon solar cells a growing research area. This paper, written in two parts, aims to outline world-wide research on this topic. The subject has been divided into techniques which use native substrates and techniques which use foreign substrates. Light trapping, vapour- and liquid-phase deposition techniques, cell fabrication and some general considerations are also discussed with reference to thin-film cells.     

Influence of defects and band offsets on carrier transport mechanisms in amorphous silicon/crystalline silicon heterojunction solar cells

We have investigated the carrier transport mechanisms in undoped a-Si:H/p-type c-Si heterojunctions with and without a μc-Si buffer layer, as well as their effects on the photovoltaic properties of the junction. The conduction behavior of the junction is strongly affected by the defect state distribution and band offset at the hetero-interface. The recombination process involving the interface states on the thin film silicon (a-Si:H/μc-Si) side dominates at low forward bias (V<0.3 V), whereas multistep tunneling capture emission (MTCE) dominates in the higher bias region (0.3<V<0.55 V) until the conduction becomes space charge limited (V>0.55 V). The MTCE process seems to be more closely related to the bulk defects in the thin film silicon than the interface states. In addition, the position of a trapping level, where the tunneling process occurs, seems to be determined by the hole energy at the edge of the c-Si and the trap distribution in the thin film silicon. Despite the domination of MTCE in the indicated voltage range, the reduced band offset at the interface increases current levels by the enhanced diffusion and/or emission processes. The insertion of a 200 Å thick μc-Si buffer layer between the a-Si:H (700 Å)/c-Si increases the solar cell efficiency to 10%, without an antireflective coating, by improving both the carrier transport and the red response of the cell.     

A review of thin-film crystalline silicon for solar cell applications. Part 2: Foreign substrates

One of the most promising ways to reduce the cost of photovoltaics is thin-film crystalline silicon solar cells. This paper, together with part 1, reviews the current state of research in thin-film crystalline silicon solar cells. Deposition on silicon, novel techniques which use a high-quality, reusable silicon substrate and light trapping have been described in part 1 of this paper. This paper describes deposition on glass and ceramics and discusses cell designs for thin-film crystalline silicon solar cells.     

Optimization of fabrication process of high-efficiency and low-cost crystalline silicon solar cell for industrial applications

The process conditions for a high-efficiency and low cost crystalline silicon solar cell were optimized. Novel approaches such as wafer cleaning and saw -damage removal using 0.5 wt% of 2,4,6-trichloro-1,3,5-triazine, silicon surface texturing with optimized pyramid heights (∼5 μm), and a third step of drive-in after phosphosilicate glass (PSG) removal followed by oxide removal were investigated. A simple method of chemical etching adopted for edge isolation was optimized with edge etching of 5-10 μm, without any penetration of chemicals between the stacked wafers. The conversion efficiency, open-circuit voltage, short-circuit current, and fill factor of the cell fabricated with the optimized process were a maximum of 17.12%, 618.4 mV, 5.32 A, and 77% under AM1.5 conditions, respectively.     

Investigation of the damage as induced by 1.7 MeV protons in an amorphous/crystalline silicon heterojunction solar cell

Current–voltage under illumination and quantum yield characteristics of an amorphous silicon/crystalline silicon hetero solar cell have been measured before and after exposure to high-energy (1.7 MeV) protons. A comparison of the measured wavelength-dependent quantum yield with calculated values enabled to determine the effective electron diffusion length of the crystalline silicon, that dropped from a value of 434 μm before to a value of 4 μm after irradiation with 5×10¹² cm⁻² protons. Good agreement has been obtained between measured and simulated data using DIFFIN,¹ a finite-element simulation program for a-Si:H/c-Si heterojunction solar cells, enabling us to extract the depth profile of the recombination rate and the density of states distribution in the semiconductor layers before and after irradiation.