GaAs solar cells grown on Si substrates for space use

GaAs single‐junction and InGaP/GaAs multi‐junction thin‐film solar cells fabricated on Si substrates have great potential for high‐efficiency, low‐cost, lightweight and large‐area space solar cells. Heteroepitaxy of GaAs thin films on Si substrates has been examined and high‐efficiency GaAs thin‐film solar cells with total‐area efficiencies of 18·3% at AM0 and 20·0% at AM 1·5 on Si substrates (GaAs‐on‐Si solar cells) have been fabricated. In addition, 1‐MeV electron irradiation damage to GaAs‐on‐Si cells has been studied. The GaAs‐on‐Si cells are found to show higher end‐of‐life efficiency than the conventional GaAs cells fabricated on GaAs substrates (GaAs‐ on‐GaAs cells) under high‐fluence 1‐MeV electron irradiation of more than 1 × 10¹⁵ cm⁻². The first space flight to make use of them has been carried out. Forty‐eight 2 × 2 cm GaAs‐on‐Si cells with an average AM0 total‐area efficiency of 16·9% have been evaluated in the Engineering Test Satellite No.6 (ETS‐VI). The GaAs‐on‐Si cells have been demonstrated to be more radiation‐resistant in space than GaAs‐on‐GaAs cells and 50, 100 and 200‐μm‐thick Si cells. These results show that the GaAs‐on‐Si single‐junction and InGaP/GaAs‐on‐Si multi‐junction cells have great potential for space applications. Copyright © 2001 John Wiley & Sons, Ltd.     

Zone-melting recrystallization of silicon thin films for solar cell application

Zone-melting recrystallization (ZMR) has been applied successfully to fabricate a thin-film silicon solar cell with high conversion efficiency that also has the potential to lower the material cost. It is found that seeding from an Si substrate during ZMR is not necessary for high-quality thin-film Si with a low defect density and the dominant (100) crystallographic orientation. This feature is very important because one can separate the thin-film Si from the substrate in order to obtain a flexible solar cell and the substrate can be recycled. Lowering the scanning speed of the upper movable carbon strip heater has proved to be most effective for high-quality crystal. In order to realize thin-film Si solar cells, a 60-μm thick Si active layer is deposited by chemical vapour deposition on recrystallized Si film. Pyramidal shape formation at the surface for light confinement by using (100) orientation and low-energy H⁺ ion irradiation for the passivation of crystal defects has been applied to the fabrication of thin-film Si solar cells and we achieved high conversion efficiencies of more than 14% for a 10 × 10 cm² cell and 16% for a 2 × 2 cm² cell.     

Cost trade between multijunction,gallium arsenide and silicon solar cells

Multijunction (MJ),¹ gallium arsenide (GaAs) and silicon (Si) solar cells have respective test efficiencies of approximately 24%, 18.5% and 14.8%. Multijunction and gallium arsenide solar cells weigh more than silicon solar cells and cost approximately five times as much per unit power at the cell level.² A trade is performed for the Tropical Rainfall Measuring Mission (TRMM) spacecraft to determine which of these cell types would have offered an overall performance and price advantage to the spacecraft. A trade is also performed for the multijunction cells under the assumption that they will cost over ten times that of silicon cells at the cell level. The trade shows that the TRMM project, less the cost of the instrument, ground systems and mission operations, would spend approximately $552 000 per kilogram to launch and support

1 The term ‘support’ means to provide the scientific equipment (‘science’) with environmental protection from the space environ ment, to keep it at acceptable temperatures and to send the data it produces to Earth in readable form, i.e. the services provided by the spacecraft.

science in the case of the spacecraft equipped with silicon solar cells. If these cells are exch anged for gallium arsenide solar cells, an additional 31 kg of science can be launched and serviced at a price of approximately $90 000 per kilogram. The 31 kg array weight reduction is shown to derive from the smaller area of the array and hence reductio ns in the weight of the array substrate and supporting structure. If the silicon solar cells are changed out for multijunction solar cells, an additional 45 kg of science above the silicon baseline can be launched and supported at a price of approximately $58 000 per kilogram. The trade shows that even if the multijunction cells are priced over ten times that of silicon cells, a price that is much higher than projected, the additional 45 kg of science are launched and serviced at $180 000 per kilogram. This is still much less than the original $552 000 per kilogram to launch and service the science. Data and qualitative factors are presented to show that these figures are subject to a great deal of uncertainty. Nonetheless, the benefit of the higher efficiency solar cells for TRMM is far greater than the uncertainties in the analysis. © This article is a US Government work and, as such, is in the public domain in the United States of America     

Sliver solar cells for concentrator PV systems with concentration ratio below 50

This paper links together two different yet complimentary technologies: concentrator photovoltaics (CPVs) and Sliver technology. Recent research and development and commercialisation efforts in concentrator technologies have centred on high‐concentration systems, encouraged by the availability of high‐efficiency, multi‐junction III‐V cells. In contrast, little attention has been paid to the potential of systems with low‐to‐medium levels of concentration. Arguably, this is due to the absence of any suitable, low‐cost concentrator cells, readily available at a commercial scale. Sliver technology is a candidate for the supply of commercial low‐cost cells suitable for systems with concentration ratios in the range of 5–50. This can be achieved via judicious choice of cell design parameters and with only minor changes to the fabrication process suitable for 1‐sun Sliver cells. Device modelling is used to show that Sliver cells are suitable for illumination intensities up to 5 W/cm², with unavoidable emitter resistance limiting performance for higher intensities. The best cells manufactured for operation at low and medium concentration had efficiencies of 18·8% at 9 suns (above 18·6% between 5 and 15 suns) and 18·4% at 37 suns (above 18·2% between 30 and 50 suns), respectively. Incorporation of sidewall texturing and SiN anti‐reflection coatings would yield efficiencies exceeding 20% for similar cells. Concentrator Sliver cells can be produced to almost any length and are perfectly bifacial, features which add significantly to their attractiveness to concentrator system designers. The availability of cheap concentrator Sliver cells could provide opportunities for new, low‐cost concentrator systems, which in turn has the potential to provide a pathway to low‐cost solar electricity. Copyright © 2009 John Wiley & Sons, Ltd.     

美国洁净煤技术项目中的IGCC

在美国的45个洁净煤技术示范项目中,有7个IGCC示范项目,其投资合计为3亿多美元,约占示范项目总投资(7亿多美元)的44％。这7个项目几乎应用了美国所有新一代的煤气化技术,包括:CE两段式粉煤气流床气化、Texaco水煤浆气流床气化、Destec两段式水煤浆气流床气化、KRW粉煤流化床气化、U-GAS粉煤流化床气化、BG/L固定床熔渣气化及COREX与炼铁结合的粉煤熔渣池气化,而且其中有些项目还部分采用了先进的高温煤气净化技术。     

Recent progress in MIS solar cells

Metal–insulator–semiconductor (MIS)-type solar cells have an inherent cost advantage compared to p-n junction solar cells. First-generation MIS–inversion layer (MIS–IL) solar cells, already successfully produced in an industrial pilot line, are restricted to efficiencies of 15–16%. With the second-generation MIS–IL silicon solar cells, based on drastically improved surface passivation by plasma-enhanced chemical vapour-deposited silicon nitride, simple technology can be combined with very high efficiencies. The novel inversion layer emitters have the potential to outperform conventional phosphorus-diffused emitters of Si solar cells. A 17.1% efficiency could already be achieved with the novel point-contacted ‘truncated pyramid’ MIS–IL cell. A new surface-grooved line-contact MIS–IL device presently under development using unconventional processing steps applicable for large-scale fabrication is discussed. By the mechanical grooving technique, contact widths down to 2 μm can be achieved homogeneously over large wafer areas. Bifacial sensitivity is included in most of the MIS-type solar cells. For a bifacial 98 cm² Czochralski (Cz) Si MIS-contacted p-n junction solar cell with a random pyramid surface texture and Al as grid metal, efficiencies of 16.5% for front and 13.8% for rear side illumination are reported. A 19.5% efficiency has been obtained with a mechanically grooved MIS n⁺p solar cell. The MIS-type silicon solar cells are able to significantly lower the costs for solar electricity due to the simple technology and the potential for efficiencies well above 20%.©1997 John Wiley & Sons, Ltd.     

Economic feasibility of bifacial silicon solar cells

Bifacial solar cells and modules are a promising approach to increase the energy output of photovoltaic systems, and therefore decrease levelized cost of electricity (LCOE). This work discusses the bifacial silicon solar cell concepts PERT (passivated emitter, rear totally diffused) and BOSCO (both sides collecting and contacted) in terms of expected module cost and LCOE based on in‐depth numerical device simulation and advanced cost modelling. As references, Al‐BSF (aluminium back‐surface field) and PERC (passivated emitter and rear) cells with local rear‐side contacts are considered. In order to exploit their bifacial potential, PERT structures (representing cells with single‐sided emitter) are shown to require bulk diffusion lengths of more than three times the cell thickness. For the BOSCO concept (representing cells with double‐sided emitter), diffusion lengths of half the cell thickness are sufficient to leverage its bifacial potential. In terms of nominal LCOE, BOSCO cells are shown to be cost‐competitive under monofacial operation compared with an 18% efficient (≙ p_MPP = 18 mW/cm²) multicrystalline silicon (mc‐Si) Al‐BSF cell and a 19% mc‐Si PERC cell for maximum output power densities of p_MPP ≥ 17.3 mW/cm² and p_MPP ≥ 18.1 mW/cm², respectively. These values assume the use of $10/kg silicon feedstock for the BOSCO and $20/kg for the Al‐BSF and PERC cells. For the PERT cell, corresponding values are p_MPP ≥ 21.7 mW/cm² and p_MPP ≥ 22.7 mW/cm², respectively, assuming the current price offset (≈50%, at the time of October 2014) of n‐type Czochralski‐grown silicon (Cz‐Si) compared with mc‐Si wafers. The material price offset of n‐type to p‐type Cz‐Si wafers (≈15%, October 2014) currently accounts for approximately 1 mW/cm², which correlates to a conversion efficiency difference of 1%_abs for monofacial illumination with 1 sun. From p‐type mc‐Si to p‐type Cz‐Si (≈30% wafer price offset, October 2014), this offset is approximately 2.5 mW/cm² for a PERT cell. When utilizing bifacial operation, these required maximum output power densities can be transformed into required minimum rear‐side illumination intensities for arbitrary front‐side efficiencies η_front by means of the performed numerical simulations. For a BOSCO cell with η_front = 18%, minimum rear‐side illumination intensities of ≤ 0.02 suns are required to match a 19% PERC cell in terms of nominal LCOE. For an n‐type Cz‐Si PERT cell with η_front = 21%, corresponding values are ≤ 0.11 suns with 0.05 suns being the n‐type to p‐type material price offset. This work strongly motivates the use of bifacial concepts to generate lowest LCOE. Copyright © 2016 John Wiley & Sons, Ltd.     

太阳能电池及其设备

以太阳能电池为代表的光伏产业是朝阳产业,目前它处在历史上最大爆炸式的成长期。2007年全球光伏产业市场212亿美元,2012年增长至709亿美元,2007～2012年年均复合增长率27.31%。目前太阳能电池处于以单晶或多晶硅为衬底的第一代产品,正在向以硅体或非晶硅(隐瓷或玻璃)为衬底的薄膜太阳能电池(第二代产品)过渡。目前太阳能电池的原材料多晶硅处于供不应求,预计2010年可得到缓得。     

40% efficient sunlight to electricity conversion

Increasing sunlight conversion efficiency is a key driver for on‐going solar electricity cost reduction. For photovoltaic conversion, the approach most successful in increasing conversion efficiency is to split sunlight into spectral bands and direct each band to a dedicated solar cell of an appropriate energy bandgap to convert this band efficiently. In this work, we demonstrate conversion of sunlight to electricity in a solar collector with an efficiency value above 40% for the first time, using a small 287‐cm² aperture area test stand, notably equipped with commercial concentrator solar cells. We use optical band‐pass filtering to capture energy that is normally wasted by commercial GaInP/GaInAs/Ge triple junction cells and convert this normally wasted energy using a separate Si cell with higher efficiency than physically possible in the original device. The 287‐cm² aperture area sunlight‐concentrating converter demonstrating this independently confirmed efficiency is a prototype for a large photovoltaic power tower system, where sunlight is reflected from a field of sun‐tracking heliostats to a dense photovoltaic array mounted on a central tower. In such systems, improved efficiency not only reduces costs by increasing energy output for a given investment in heliostats and towers but also reduces unwanted heat generation at the central tower. Copyright © 2015 John Wiley & Sons, Ltd.     

Evaluating the economic viability of CdTe/CIS and CIGS/CIS tandem photovoltaic modules

We analyze the potential cost competitiveness of two frameless, glass–glass thin‐film tandem photovoltaic module structures, cadmium telluride (CdTe)/CuInSe₂ (CIS) and CuIn_0.3Ga_0.7Se₂ (CIGS)/CIS, based on the demonstrated cost of manufacturing the respective component cell technologies in high volume. To consider multiple economic scenarios, we base the CdTe/CIS module efficiency on the current industrial production of CdTe modules, while for CIGS/CIS, we use an aspirational estimate for CIGS efficiency. We focus on four‐terminal mechanically stacked structures, thus avoiding the need to achieve current matching between the two cells. The top cell in such a tandem must have a transparent back contact, which has not been successfully implemented to date. However, for the purpose of understanding the economic viability of both tandems, we assume that this can be implemented at a cost similar to that of sputtered indium tin oxide. The cost of both tandem module structures was found to be nearly identical on an equal‐area basis and approximately $30/m² higher than the single‐junction alternatives. Both tandem modules are about 4% (absolute) more efficient than a module by using the top‐cell material alone. We find that these tandem modules might reduce total system cost by as much as 11% in applications having a high area‐related balance‐of‐system cost, such as area‐constrained residential systems; however, the relative advantage of tandems decreases in the cases where balance‐of‐system costs are lower, such as in commercial and utility scale systems. Copyright © 2017 John Wiley & Sons, Ltd.     

Perspectives of crystalline Si thin film solar cells: a new era of thin monocrystalline Si films?

A large number of competing approaches are currently being investigated around the world to develop crystalline silicon thin film solar cells on foreign substrates. These approaches can be broadly classified according to the crystalline state of the Si films employed: (i) thin film solar cells based on nano‐ or microcrystalline Si‐films; (ii) cells fabricated from large‐grained polycrystalline Si and (iii) recent approaches utilizing the transfer of monocrystalline Si films. The paper discusses prospects and limitations of these approaches and describes device results based on the transfer of quasi‐monocrystalline Si films. Using Si absorber films epitaxially grown on quasi‐monocrystalline Si, we achieve a conversion efficiency of 13˙6% for a 4 cm² sized thin film solar cell on glass. In contrast to the limited performance of polycrystalline Si thin film solar cells imposed by the presence of grain boundaries, transfer approaches are expected to result in thin film solar cell efficiencies in the range of 15 – 18% depending on process maturity and complexity. The transfer of monocrystalline Si films therefore opens a new avenue to an efficient and competitive Si‐based thin film technology. Copyright © 2000 John Wiley & Sons, Ltd     

All‐Solution‐Processed Random Si Nanopyramids for Excellent Light Trapping in Ultrathin Solar Cells

Si nanopyramids have been suggested as one of the most promising Si nanostructures to realize high‐efficient ultrathin solar cells or photodetectors due to their low surface area enhancement and outstanding ability to enhance light absorption. However, the present techniques to fabricate Si nanopyramids are either complex or expensive. In parallel, disordered nanostructures are believed to be extremely effective to realize broadband light trapping for solar cells. Here, a simple and cost‐effective method is presented to form random Si nanopyramids based on an all‐solution process, the mechanism behind which is the successful transfer of the generation site of bubbles from Si surface to the introduced Ag nanoparticles so that OH^? can react with the entire Si surface to naturally form random and dense Si nucleus. For optical performance, it is experimentally demonstrated that the random Si nanopyramid textured ultrathin crystalline Si (c‐Si) can achieve light trapping approaching the Lambertian limit. Importantly, it is revealed, by numerical calculations, that random Si nanopyramids outperform periodic ones on broadband light absorption due to more excited optical resonance modes. The finding provides a new opportunity to improve the performance of ultrathin c‐Si solar cells with a simpler process and lower cost.     

Economic competitiveness of III–V on silicon tandem one‐sun photovoltaic solar modules in favorable future scenarios

Tandem modules combining a III–V top cell with a Si bottom cell offer the potential to increase the solar energy conversion efficiency of one‐sun photovoltaic modules beyond 25%, while fully utilizing the global investment that has been made in Si photovoltaics manufacturing. At present, the cost of III–V cells is far too high for this approach to be competitive for one‐sun terrestrial power applications. We investigated the system‐level economic benefits of both GaAs/Si and InGaP/Si tandem modules in favorable future scenarios where the cost of III–V cells is substantially reduced, perhaps to less than the cost of Si cells. We found, somewhat unexpectedly, that these tandems can reduce installed system cost only when the area‐related balance‐of‐system cost is high, such as for area‐constrained residential rooftop systems in the USA. When area‐related balance‐of‐system cost is lower, such as for utility‐scale systems, the tandem module offers no benefit. This is because a system using tandem modules is more expensive than one using single‐junction Si modules when III–V cells are expensive, and a system using tandem modules is more expensive than one using single‐junction III–V modules when III–V cells are inexpensive. Copyright © 2016 John Wiley & Sons, Ltd.     

Crystalline silicon photovoltaics: the hurdle for thin films

The dominant photovoltaic (PV) technology today is crystalline silicon, used in 85% of the terrestrial modules shipped in 1996. Thin-film PV technologies promise to allow significant reductions in the cost per watt of electricity generated by PV modules. However, thin films must meet or exceed the standards for performance, reliability, and cost set by crystalline silicon in order to successfully penetrate the market. This paper reports the results of a detailed economic analysis done for a 25 MW year⁻¹ multi-crystalline silicon production facility, including crystal growth, water slicing, solar cell fabrication and module assembly. The module manufacturing cost is projected to be $1·78 W⁻¹. The sensitivity of module cost to polysilicon cost and cell efficiency were determined. This analysis provides a near-term (1998–2000) cost/performance benchmark against which thin-film technologies can be compared. © 1997 John Wiley & Sons, Ltd.     

Thin Al2O3 passivated boron emitter of n‐type bifacial c‐Si solar cells with industrial process

We have presented thin Al₂O₃ (~4 nm) with SiN_x:H capped (~75 nm) films to effectively passivate the boron‐doped p⁺ emitter surfaces of the n‐type bifacial c‐Si solar cells with BBr₃ diffusion emitter and phosphorus ion‐implanted back surface field. The thin Al₂O₃ capped with SiN_x:H structure not only possesses the excellent field effect and chemical passivation, but also establishes a simple cell structure fully compatible with the existing production lines and processes for the low‐cost n‐type bifacial c‐Si solar cell industrialization. We have successfully achieved the large area (238.95 cm²) high efficiency of 20.89% (front) and 18.45% (rear) n‐type bifacial c‐Si solar cells by optimizing the peak sintering temperature and fine finger double printing technology. We have further shown that the conversion efficiency of the n‐type bifacial c‐Si solar cells can be improved to be over 21.3% by taking a reasonable high emitter sheet resistance. Copyright © 2017 John Wiley & Sons, Ltd.     

Environmental life cycle assessment of roof‐integrated flexible amorphous silicon/nanocrystalline silicon solar cell laminate

This paper presents an environmental life cycle assessment of a roof‐integrated flexible solar cell laminate with tandem solar cells composed of amorphous silicon/nanocrystalline silicon (a‐Si/nc‐Si). The a‐Si/nc‐Si cells are considered to have 10% conversion efficiency. Their expected service life is 20 years. The production scale considered is 100 MW_p per year. A comparison of the a‐Si/nc‐Si photovoltaic (PV) system with the roof‐mounted multicrystalline silicon (multi‐Si) PV system is also presented. For both PV systems, application in the Netherlands with an annual insolation of 1000 kWh/m² is considered. We found that the overall damage scores of the a‐Si/nc‐Si PV system and the multi‐Si PV system are 0.012 and 0.010 Ecopoints/kWh, respectively. For both PV systems, the impacts due to climate change, human toxicity, particulate matter formation, and fossil resources depletion together contribute to 96% of the overall damage scores. Each of both PV systems has a cumulative primary energy demand of 1.4 MJ/kWh. The cumulative primary energy demand of the a‐Si/nc‐Si PV system has an uncertainty of up to 41%. For the a‐Si/nc‐Si PV system, an energy payback time of 2.3 years is derived. The construction for roof integration, the silicon deposition, and etching are found to be the largest contributors to the primary energy demand of the a‐Si/nc‐Si PV system, whereas encapsulation and the construction for roof integration are the largest contributors to its impact on climate change. Copyright © 2012 John Wiley & Sons, Ltd.     

Proposed cut in US congressional solar energy budget

The proposed US congressional budget for solar energy is to be trimmed from $91.6m in 2001 to $89.4 m in 2002, with a request to cut it further to $87.6m for 2003. The strategic object for research and development in US solar technologies is to reduce the price paid for a photovoltaic system by an end-user (including operating and maintenance costs) from an average value of $6.25 per watt in 2000 to $4.50 per watt by 2006, which is equivalent to a reduction from $0.25 to $0.18 per kWh.     

The performance of PV‐t systems for residential application in Bangkok

This paper focused on the performance of photovoltaic‐thermal (PVT) systems working in Bangkok for residential applications. The PVT system is one which produces both electricity and low temperature heat at the same time. This paper investigated the performance of PVT systems that use different types of commercial solar PV panels. The characteristics of the PV panels were used as input parameters in the simulation. Each system comprises 2 m² of PVT collector area. Water draw patterns are those with a typical consumption of medium size houses in Bangkok, and the measured monthly average city water temperature of Bangkok has been used to estimate the energy output. The results show that the optimum water flow rate is 20 kg/h for all types of PVT collectors and the effect of water flow can improve the cell efficiency of PV cells. Moreover, the total energy output from the PVT collectors, which had glass covers is very significantly higher than those without one. The c‐Si PVT panel gave the best performance with the highest rate of primary energy reduction. The payback time of each system is 6.4, 11.8, and 13.4 years for a‐Si, mc‐Si, and c‐Si types of PVT system, respectively. This investigation concludes that from the viewpoint of system performance, c‐Si PVT is the most promising type than whereas from the viewpoint of economy, a‐Si PVT has the fastest payback time. Copyright © 2012 John Wiley & Sons, Ltd.     

Photovoltaic Cells and Modules towards Terawatt Era

Progresses in photovoltaic technologies over the past years are evident from the lower costs, the rising efficiency, to the great improvements in system reliability and yield. Cumulative installed power yearly growths were on an average more than 40% in the period from 2007 to 2016 and in 2016, the global cumulative photovoltaic power installed has reached 320 GWp. The level 0.5 TWp could be reached before 2020. The production processes in the solar industry still have great potential for optimization both wafer based and thin film technologies. Trends following from the present technology levels are discussed, also taking into account other parts of photovoltaic systems that influence the cost of electrical energy produced. Present developments in the three generations of photovoltaic modules are discussed along with the criteria for the selection of appropriate photovoltaic module manufacturing technologies. The wafer based crystalline silicon (c-silicon) technologies have the role of workhorse of present photovoltaic power generation, representing more than 90% of total module production. Further technology improvements have to be implemented without significantly increasing costs per unit, despite the necessarily more complex manufacturing processes involved. The tandem of c-silicon and thin film cells is very promising. Durability may be a limiting factor of this technology due to the dependence of the produced electricity cost on the module service time.     

半导体材料的光伏应用现状

世界光伏市场已成为高速增长的市场。介绍了各类太阳电池近年来所取得的进展和产业化现状及发展趋势。