A technological evolution from bulk crystalline age to multilayers thin film age in solar photovoltaics

Recent advances in solar cell device technologies are surveyed, and a new trend underlying is predicted by a term “technological evolution from the bulk crystalline age to the multilayered thin film age”. In the paper, firstly, recent progress of thin film fabrication technologies for active materials of photovoltaic device are reviewed, and their significancies such as wide area, low temperature growth etc., are pointed out from currently developed live technologies. Secondly, some R & D efforts to develop the next generation type solar cells utilized by full use of multi-layers thin film growth technology are introduced together with some newly developed integrated process technology for the thin film solar cells. Then, some topics in the high cost performance multi-layers thin film solar cells are also introduced. In the final part of this paper, the current state of the art in the field of thin film solar cells and their industrialization are overviewed and the market expansion toward the 21st century is forecast, and discussed.     

Amorphous silicon solar cells

The perfectioning of the deposition techniques of amorphous silicon over large areas, in particular film homogeneity and the reproducibility of the electro-optical characteristics, has allowed a more accurate study of the most intriguing bane of this material: the degradation under sun-light illumination. Optical band-gap and film thickness engineering have enabled device efficiency to stabilize with only a 10–15% loss in the as-deposited device efficiency. More sophisticated computer simulations of the device have also strongly contributed to achieve the highest stable efficiencies in the case of multijunction devices. Novel use of nanocrystalline thin films offers new possibilities of high efficiency and stability. Short term goals of great economical impact can be achieved by the amorphous silicon/crystalline silicon heterojunction. A review is made of the most innovative achievements in amorphous silicon solar cell design and material engineering.     

Photovoltaic technology in Malaysia: past,present, and future plan

This article presents solar energy or specifically the solar photovoltaic (PV) development outlook in Malaysia. The paper first introduces the massive potential of solar energy in the country, the key players in the solar energy development and the early solar energy policies, and programmes in the country. The most important to the PV development is the Malaysia Building Integrated Photovoltaic initiative, which is presented in this paper followed by an explanation on the Feed-in Tariff recently introduced in the country to encourage new solar PV projects. The outlook for solar PV in Malaysia is optimistic and as the uptake of solar PV increases, the unit cost is coming down rapidly. Solar PV is expected to be the most competitive Renewable Energy (RE) source, with the potential to achieve grid parity for electrical power in the country in the near future, and surpassing all other REs combined by 2050.     

硅太阳电池稳步走向薄膜化

考察了硅太阳电池在光伏产业中所处的地位,分析了薄膜硅太阳电池的发展趋势。指出硅太阳电池在未来15a仍将保持优势地位,并继续沿着晶硅电池和薄膜硅电池两个方向发展。在此发展过程中,两个发展方向的主流很可能会汇合到一起,共同促使低成本、高效率、高可靠薄膜晶硅电池的诞生和产业化,从而继续保持硅太阳电池的优势地位。     

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.     

Photovoltaic devices based on electrochemical-chemical deposited CdS and poly3-octylthiophene thin films

Cadmium sulfide (CdS) is a well-known wide bandgap semiconductor for solar cell applications. In this work we report an electrochemical/chemical method to prepare CdS thin films on gold (Au)-coated glass substrates. 10 nm thick of titanium (Ti) film was first sputtered on glass surface to improve the adhesion between the subsequent sputtered Au film and the glass surface. Cadmium films were then electrochemically deposited on Au surface in an acidic solution with negative potential, and the obtained glass/Ti/Au/Cd samples were annealed in H₂S atmosphere to convert Cd into CdS. XRD pattern of H₂S-annealed Cd samples shows a hexagonal wurtzite phase in CdS with (0 0 2) as the preferential crystalline plane. Photovoltaic properties were clearly shown in hybrid heterojunctions of CdS and poly3-octylthiophene (P3OT) with Au as the top and the back metal contacts.     

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%.     

Impact of spectral irradiance distribution and temperature on the outdoor performance of amorphous Si photovoltaic modules

Performance of photovoltaic (PV) modules is evaluated under the standard test condition, which rarely meets actual outdoor conditions. Environmental conditions greatly affect the output energy of PV modules. The impact of environmental factors, especially solar spectrum distribution and module temperature, on the outdoor performance of amorphous Si (a-Si) and multicrystalline Si (mc-Si) PV modules is characterized. The results show that the output energy of a-Si modules mainly depends on spectrum distribution and is higher under blue-rich spectrum. In contrast, the output energy of mc-Si module is sensitive to module temperature but not to spectrum distribution.     

Calculations of light trapping, responsivity, and internal quantum efficiency of In-doped silicon (n) structure

This research intends to investigate a mathematical model for In-doped silicon (n) structure, and calculate the absorbance of the simulated cell, then study the effect of impurity photovoltaic effect on the responsivity and internal quantum efficiency using the Shockley–Read–Hall model. It is found that the internal flux inside the simulated Lambertian cell could be enhanced as much as 25 times as a result of light trapping. Maximum responsivity and internal quantum efficiency of the simulated cell was obtained at a wavelength around 1 μm and 10¹⁷ cm⁻³ indium concentration. Near infrared response of the simulated cell is improved due to the enhancements of sub-band gap response by indium doping. To compare the mathematical model with the practical results, few samples of In-doped n-type silicon structures have been fabricated using the thermal vacuum resistive technique. Maximum responsivity and maximum internal quantum efficiency are obtained at wavelength 0.9–1 μm and 3.96×10¹⁷ cm⁻³ indium concentration. The results of the simulated and practical cells agree.     

Optoelectronic properties of doped layers for a-Si solar cells prepared using helium dilution

This paper is the first part of a work about the preparation and characterisation of doped layers for hydrogenated-amorphous-silicon (a-Si:H) thin film solar cells. An approach for RF-glow discharge deposition of a-Si consisting of dilution of silane (SiH₄) in helium and application of high RF-power densities, has been tested. In this first part the optimisation of n-type layers has been accomplished. The influence of preparation conditions on the optical and electrical properties of the films has systematically been studied. It has been found that the use of high RF-power densities and high dilution levels of SiH4 in He favour the doping efficiency and film quality when the substrate temperature is 300°C. As a result of these investigations, n-type layers with thicknesses between 250 and 360Å, an optical gap about 1.95 eV, a dark-conductivity of 0.1 (Ωcm)⁻¹ and an extended-state conductivity activation energy of 0.1 eV have been prepared. Such properties make them suitable for their use as n-type layers for a-Si:H thin-film solar cells.     

Amorphous solar cells, the micromorph concept and the role of VHF-GD deposition technique

During the last two decades, the Institute of Microtechnology (IMT) has contributed in two important fields to future thin-film silicon solar cell processing and design:

(1) In 1987, IMT introduced the so-called “very high frequency glow discharge (VHF-GD)” technique, a method that leads to a considerable enhancement in the deposition rate of amorphous and microcrystalline silicon layers. As a direct consequence of reduced plasma impedances at higher plasma excitation frequencies, silane dissociation is enhanced and the maximum energy of ions bombarding the growing surface is reduced. Due to softer ion bombardment on the growing surface, the VHF process also favours the formation of microcrystalline silicon. Based on these beneficial properties of VHF plasmas, for the growth of thin silicon films, plasma excitation frequencies f_exc in the range 30–300 MHz, i.e. clearly higher than the standard 13.56 MHz, are indeed scheduled to play an important role in future production equipment.

(2) In 1994, IMT pioneered a novel thin-film solar cell, the microcrystalline silicon solar cell. This new type of thin-film absorber material––a form of crystalline silicon––opens up the way for a new concept, the so-called “micromorph” tandem solar cell concept. This term stands for the combination of a microcrystalline silicon bottom cell and an amorphous silicon top cell. Thanks to the lower band gap and to the stability of microcrystalline silicon solar cells, a better use of the full solar spectrum is possible, leading, thereby, to higher efficiencies than those obtained with solar cells based solely on amorphous silicon.

Both the VHF-GD deposition technique and the “micromorph” tandem solar cell concept are considered to be essential for future thin-film PV modules, as they bear the potential for combining high-efficiency devices with low-cost manufacturing processes.     

Applications of carbon materials in photovoltaic solar cells

Carbon-based photovoltaic cells (PVCs) have attracted a great deal of interest for both scientific fundamentals and potential applications. In this paper, applications of various carbon materials in PVCs, especially in silicon-based solar cells, organic solar cells and dye-sensitized solar cells, are reviewed. The roles carbon materials played in the PVCs are discussed. Further research on solar cells comprised solely of carbon is prospected.     

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.     

太阳能电池研究的新进展

简要回顾了第一代晶体硅和第二代薄膜太阳能电池的发展状况,并介绍了基于薄膜技术的第三代高性能太阳能电池的基本原理和发展趋势。     

Australian educational and research opportunities arising through rapid growth in the photovoltaic industry

In recent years, the photovoltaic (PV) industry has been growing rapidly at the rate of 30–40% per annum. As a result of this rapid growth, new opportunities through collaborative research with industry and in the educational area have arisen. To address these needs, the Australian government, through the Australian Research Council (ARC), has established a Key Centre for Teaching and Research in Photovoltaic Engineering at the University of New South Wales (UNSW). This is one of only eight such centres established Australia-wide across all disciplines. The primary new initiative of this Key Centre is to establish the world's first undergraduate engineering degree in photovoltaics and solar energy, commencing in March 2000.     

A new perspective on the characterization of materials for a-Si:H solar cells

A study has been carried out on a-Si:H solar cell materials fabricated under a wide range of deposition conditions in different laboratories. The results on both thin films and corresponding Schottky barrier cell structures demonstrate that analysis and characterization based solely on the neutral dangling bonds are clearly inadequate. Contributions of charged defects to the properties of a-Si:H, their effect on light-induced changes are identified together with the limitations of methods commonly used to characterize the solar cell properties and stability of a-Si:H materials. Self-consistent fitting of a wide range of results on films and Schottky barrier cell structures is obtained with a gap state distribution in which charged defects are included.     

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.     

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.     

Phase engineering of a-Si:H solar cells for optimized performance

Until recently, the advances in hydrogenated amorphous silicon (a-Si:H) solar cell performance and stability have been achieved materials prepared with hydrogen dilution following primarily empirical approaches. This paper discusses the recently obtained insights into the growth, microstructure and nature of these materials. Such protocrystalline Si:H materials are more ordered than the a-Si:H obtained without dilution and evolve with thickness from an amorphous phase into first a mixed amorphous–microcrystalline and subsequently into a single microcrystalline phase. The development of deposition phase diagrams, characterize their microstructural evolution during growth which can be used to guide the fabrication of solar cell structures in a controlled way. Examples are presented and discussed of their application in solar cell fabrication to obtain a fundamental understanding of the properties of the phase transitions as well as the systematic optimization of cell performance.     

The influence of porous silicon on junction formation in silicon solar cells

In this work the results of a structural investigation by SEM of porous silicon (PS) before and after diffusion processes are reported. The formation of PS n⁺/p structures were carried out on PS p/p silicon wafers with two methods: from POCl₃ in a conventional furnace and from a phosphorous doped paste in an infrared furnace. Sheet resistance was found to be a strong function of PS structure. Further details on sheet resistance distribution are reported. The electrical contacts in prepared solar cells were obtained by screen printing process, with a Du Ponte photovoltaic silver paste for front contacts and home-prepared silver with 3% aluminium paste for the back ones. Metallization was done in the infrared furnace. Solar cell current–voltage characteristics were measured under an AM 1.5 global spectrum sun simulator. The average results for multi-crystalline silicon solar cells without antireflection coating are: I_sc=720 (mA), V_oc=560 (mV), FF=69%, Eff=10.6% (area 25 cm²).