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 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.     

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.     

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.     

Towards high-efficiency thin-film silicon solar cells with the “micromorph” concept

Tandem solar cells with a microcrystalline silicon bottom cell (1 eV gap) and an amorphous-silicon top cell (1.7 eV gap) have recently been introduced by the authors; they were designated as “micromorph” tandem cells. As of now, stabilised efficiencies of 11.2% have been achieved for micromorph tandem cells, whereas a 10.7% cell is confirmed by ISE Freiburg. Micromorph cells show a rather low relative temperature coefficient of 0.27%/K. Applying the grain-boundary trapping model so far developed for CVD polysilicon to hydrogenated microcrystalline silicon deposited by VHF plasma, an upper limit for the average defect density of around 2 × 10¹⁶/cm³ could be deduced; this fact suggests a rather effective hydrogen passivation of the grain-boundaries. First TEM investigations on μc-Si : H p-i-n cells support earlier findings of a pronounced columnar grain structure. Using Ar dilution, deposition rates of up to 9 Å/s for microcrystalline silicon could be achieved.     

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.     

Development of both-side junction silicon space solar cells

This paper reports the recent results of improving the radiation hardness of silicon solar cells, which is SHARP and NASDA's project since 1998 (Tonomura et al., Second World Conference on Photovoltaic Solar Energy, 1998, pp. 3511–3514). Newly developed 2×2 cm² Si solar cells with ultrathin substrates and both-side junction (BJ) structure showed 72.0 mW (13.3% efficiency) maximum output power at AM0, 28°C after 1 MeV electron irradiation up to 1×10¹⁵ e/cm² and the best cell showed 72.5 mW (13.4%) maximum output power. These solar cells have p–n junctions at both front and rear surfaces and showed less radiation degradation and better remaining factor than previous solar cells.     

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.     

Microcrystalline/micromorph silicon thin-film solar cells prepared by VHF-GD technique

Hydrogenated microcrystalline silicon prepared at low temperatures by the glow discharge technique is examined here with respect to its role as a new thin-film photovoltaic absorber material. XRD and TEM characterisations reveal that microcrystalline silicon is a semiconductor with a very complex morphology. Microcrystalline p–i–n cells with open-circuit voltages of up to 560–580 mV could be prepared. “Micromorph” tandem solar cells show under outdoor conditions higher short-circuit currents due to the enhanced blue spectra of real sun light and therefore higher efficiencies than under AM1.5 solar simulator conditions. Furthermore, a weak air mass dependence of the short-circuit current density could be observed for such micromorph tandem solar cells. By applying the monolithic series connection based on laser patterning a first micromorph mini-module (total area of 23.6 cm²) with 9% cell conversion efficiency could be fabricated.     

Experimental analysis of upconversion with both coherent monochromatic irradiation and broad spectrum illumination

Upconversion of sub-band-gap photons promises to increase solar cell efficiencies by making these photons useful. In this paper, we investigate the application of β-NaYF₄:20% Er³⁺ to silicon solar cells. We determine the external quantum efficiency of an upconverter silicon solar cell, both under monochromatic excitation and, for the first time in the context of silicon solar cells, under broad spectrum illumination as it is relevant for the application to harvest solar energy. The investigated upconverter silicon solar cell responds under broad spectrum illumination with an average upconversion efficiency of 1.07±0.13% in the spectral range from 1460 to 1600 nm. The resulting efficiency increase for the used solar cell with an overall efficiency of 16.7% is calculated to be 0.014% relative.     

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.     

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 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.     

New materials and deposition techniques for highly efficient silicon thin film solar cells

This paper reviews recent efforts to provide the scientific and technological basis for cost-effective and highly efficient thin film solar modules based on amorphous (a-Si:H) and microcrystalline (μc-Si:H) silicon. Textured ZnO:Al films prepared by sputtering and wet chemical etching were applied to design optimised light-trapping schemes. Necessary prerequisite was the detailed knowledge of the relationship between film growth, structural properties and surface morphology obtained after etching. High rate deposition using plasma enhanced chemical vapour deposition at 13.56 MHz plasma excitation frequency was developed for μc-Si:H solar cells yielding efficiencies of 8.1% and 7.5% at deposition rates of 5 and 9 Å/s, respectively. These μc-Si:H solar cells were successfully up-scaled to a substrate area of 30×30 cm² and applied in a-Si:H/μc-Si:H tandem cells showing initial test cell efficiencies up to 11.9%.     

Amorphous silicon solar cell modules fabricated with a single-chamber load-lock deposition system

Solar cell modules were fabricated in a single-chamber load-lock deposition system. This system routinely produces high efficiency solar cell modules on both small- and large-area substrates. The best solar cell efficiencies were 10.4% on 0.265 cm², 8.7% on 420 cm² and 8.1% on 1000 cm². The high throughput at one or two runs per hour makes the single-chamber load-lock deposition system a valuable research tool which permits rapid progress with great process control. Because of the large-area deposition capability, it can support pilot and manufacturing deposition systems.     

A new strategy for the fabrication of cost-effective silicon solar cells

Fabrication of solar cells with very high efficiencies currently requires extremely complex processing. In order to make photovoltaics an economical large scale source of energy, very high efficiencies have to be achieved by low-cost processing. The innovative approach for the cost-effective production of highly efficient silicon solar cells presented in this paper is characterised by only four simple and environmentally safe large-area fabrication steps. The basic processing sequence consists of: (i) mechanical surface grooving, (ii) simple diffusion or inversion, (iii) shallow angle metal evaporation, and (iv) plasma silicon nitride deposition. Cell design, fabrication techniques and processing sequences for metal-insulator-semiconductor contacted diffused n⁺-p junction (MIS-n⁺p) and MIS-inversion-layer (MIS-IL) silicon solar cells are outlined. The new simple approach turned out to be most successful, as demonstrated by mechanically grooved MIS-n⁺p silicon solar cells with efficiencies above 21% using exclusively aluminium as metallisation.     

Highly stable transparent and conducting gallium-doped zinc oxide thin films for photovoltaic applications

Transparent and highly conducting gallium zinc oxide (GZO) films were successfully deposited by RF sputtering at room temperature. A lowest resistivity of ∼2.8×10⁻⁴ Ω cm was achieved for a film thickness of 1100 nm (sheet resistance ∼2.5 Ω/□), with a Hall mobility of 18 cm²/V s and a carrier concentration of 1.3×10²¹ cm⁻³. The films are polycrystalline with a hexagonal structure having a strong crystallographic c-axis orientation. A linear dependence between the mobility and the crystallite size was obtained. The films are highly transparent (between 80% and 90% including the glass substrate) in the visible spectra with a refractive index of about 2, very similar to the value reported for the bulk material. These films were applied to single glass/TCO/pin hydrogenated amorphous silicon solar cells as front layer contact, leading to solar cells with efficiencies of about 9.52%. With the optimized deposition conditions, GZO films were also deposited on polymer (PEN) substrates and the obtained results are discussed.     

Very low light-reflection from the surface of incidence of a silicon solar cell

This paper reports very low light-reflection from the surface of incidence of a silicon solar cell. The measured hemispherical front surface light-reflectance of our epi thin film silicon solar cell is 1% over the optimum wavelength range 560 to 860 nm and below 2% in the range 440 to 960 nm. These reflectances are the lowest ever achieved for any silicon solar cell. The low reflection has resulted in a 7.9 mA/cm² higher current density and a 4.5% higher efficiency than those of our best thin film silicon solar cell prior to the optimisation described.     

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.