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.     

Survey of material options and issues for thin film silicon solar cells

The high production cost of thick high-efficiency crystalline silicon solar cells inhibits widespread application of photovoltaic devices whereas the most developed of thin film cell technologies, that based on amorphous silicon, suffers inherent instability and low efficiency. Crystalline thin-film silicon solar cells offer the potential for a long-term solution for low cost but high-efficiency modules for most applications. This paper reviews the progress in thin-film silicon solar cell development over the last two decades, including progress in thin-film crystal growth, device fabrication, novel cell design, new material development, light trapping and both bulk and surface passivation. Quite promising results have been obtained for both large-grain (>100 μm) polycrystalline silicon material and the recently developed microcrystalline silicon materials. A novel multijunction solar cell design provides a new approach to achieving high-efficiency solar cells from very modest quality and hence low-cost material. Light trapping is essential for high performance from thin-film silicon solar cells. This can be realized by incorporating an appropriate texture on the substrate surface. Both bulk and surface passivation is also important to ensure that the photogenerated carriers can be collected effectively within the thin-film device. © 1998 John Wiley & Sons, Ltd.     

Polycrystalline silicon thin-film solar cells: The future for photovoltaics?

Based on performance, material availability, consumer acceptance, life expectancy, environmental considerations and the potential for low cost, thin-film polycrystalline silicon solar cells are well placed to have a significant impact in the future. of key importance will be the achievement of performance targets, because module efficiencies of at least 15% are probably necessary in the long term for photovoltaics to have a significant impact in grid-connected applications. Strategies for achieving these performance levels with mediocre material quality and only moderate surface passivation and light trapping are presented. the challenges associated with the supporting substrate choice and layer depostion techniques and structures are discussed and the psesent practices reviewed. Important considerations include device performance, cost, throughput, device area and simplicity of fabrication and operation. Promising efficiencies in the vicinity of 15% have already been demonstrated using a number of different crystalline silicon layer-formation techniques. Novel device structures based on incorporation of narrow bandgap materials (Si/Ge alloys) or defect layers, quantum wells and the impurity photovoltaic effect are considered, with particular emphasis given to approaches that compensate for the current loss in thin-film cells. It appears increasingly likely that polycrystalline silicon thin-film solar cells will have an impact on the development of photovoltaics in the future and may in fact provide the means for the substantial cost reductions necessary for significant penetration into utility markets.     

Polycrystalline Silicon-Film™ Thin-film Solar Cells: Advanced Products

Thin-film polycrystalline silicon has the potential to achieve the cost reduction and performance improvement necessary for large-scale electricity markets. Reduced cost is achieved by capitalizing on the benefits of thin films grown on low-cost, large-area substrates. Improved efficiency is realized, in spite of reduced material quality, by incorporating enhanced optical absorption and back-surface passivation. The cornerstone of AstroPower's thin-film solar cell technology is the Silicon-Film™ process: a method for the manufacture of solar cell-quality, polycrystalline films of silicon on a variety of low-cost, supporting substrates. Three thin-film solar cell designs, based on this technology, are currently under development. This paper presents the key design features of these three products and briefly reviews the current status of the development of the key technologies that comprise the advanced thin-film solar cell products.     

Produktion von Dünnschichtsolarzellen

The high performance of CdTe thin-film solar cells made by fast and rugged manufacturing process indicates a significant promise for commercial production. ANTEC Solar GmbH has successfully created the financial basis for production on a scale of 10 MW_p per annum using close-spaced sublimation for the deposition of the semiconductor films. We present basic technical and business considerations comment the plant in construction.     

Recent progress in thin-film cadmium telluride solar cells

Cadmium telluride (CdTe) with a room-temperature bandgap energy of 1.45 eV has been shown to be the most promising low-cost, thin-film photovoltaic material for terrestrial applications. Significant progress has been made during the past several years, and thin-film CdTe solar cells of > 1 cm² area with conversion efficiencies higher than 12% have been prepared by several techniques. Thin-film CdTe photovoltaic modules with 10% efficiency have also been produced. They are of the heterojuntion configuration using a transparent conducting semiconductor (TCS) as the window and p-CdTe as the absorber. In this paper, the potential window materials for thin-film CdTe solar cells are discussed. Thus far, cadmium sulphide (CdS) with a bandgap energy of 2.42 eV at room temperature has been found to be best suited for efficient CdTe solar cells. the deposition techniques for p-CdTe films capable of producing efficient solar cells, including close-spaced sublimation (CSS), electrodeposition, screen printing and spraying, are briefly reviewed, and the characteristics of the resulting solar cells are discussed. It is concluded that the efficiency of thin-film CdTe solar cells can be increased to 18-19% in the near-term, leading to 15-16.5% efficient modules.     

Development of concentrator modules with dome‐shaped Fresnel lenses and triple‐junction concentrator cells

The status of the development of a new concentrator module in Japan is discussed based on three arguments, performance, reliability and cost. We have achieved a 26·6% peak uncorrected efficiency from a 7056 cm² 400 × module with 36 solar cells connected in series, measured in house. The peak uncorrected efficiencies of the same type of the module with 6 solar cells connected in series and 1176 cm² area measured by Fraunhofer ISE and NREL are reported as 27·4% and 24·8% respectively. The peak uncorrected efficiency for a 550× and 5445 cm² module with 20 solar cells connected in series was 28·9% in house. The temperature‐corrected efficiency of the 550 × module under optimal solar irradiation condition was 31·5 ± 1·7%. In terms of performance, the annual power generation is discussed based on a side‐by‐side evaluation against a 14% commercial multicrystalline silicon module. For reliability, some new degradation modes inherent to high concentration III‐V solar cell system are discussed and a 20‐year lifetime under concentrated flux exposure proven. The fail‐safe issues concerning the concentrated sunlight are also discussed. Moreover, the overall scenario for the reduction of material cost is discussed. Copyright © 2005 John Wiley & Sons, Ltd.     

Silicon solar cells

The key attributes for achieving high-efficiency crystalline silicon solar cells are identified and historical developments leading to their realization discussed. Despite the achievement of laboratory cells with performance approaching the theoretical limit, commercial cell designs need to evolve significantly to realize their potential. In particular, the development of cell structures and processes that facilitate entirely activated device volumes in conjunction with well-passivated metal contacts a nd front and rear surfaces is essential (and yet not overly challenging) to achieve commercial devices of 20% efficiency from solar-grade substrates. The inevitable trend towards thinner substrates will force manufacturers to evolve their designs in this direction or else suffer substantial performance loss. Eventually, a thin-film technology will likely dominate, with thin-film crystalline silicon cells being a serious candidate. Present commercial techniques and processes are in general unsuitable for t hin-film fabrication, with even greater importance placed on the achievement of devices with entirely activated volumes (diffusion lengths much greater than device thicknesses), well-passivated metal contacts and surfaces and the important inclusion of li ght trapping. The recent achievement of 21.5% efficiency on a thin crystalline silicon cell (less than 50 μm thick) adds credibility to the pursuit of crystalline silicon in thin films, with a key attribute of this laboratory cell being its extremely good light trapping that nullifies the long-term criticism of crystalline silicon regarding its poor absorption properties and correspondingly perceived inability to achieve high-performance thin-film devices. For low-cost, low-quality polycrystalline sil icon material, the parallel-multijunction cell structure may provide a mechanism for achieving entirely activated cell volumes with the potential to achieve reasonable efficiencies at low cost over the next decade.     

Thin-film solar cells: A unified analysis of their potential

The development and deployment of low-cost thin-film solar cells for the direct conversion of sunlight to electricity can be accelerated by the utilization of loss minimization and cost minimization methodologies. The solar cell is separated into its five constituent layers to provide a common basis for the development of these methodologies. Photovoltaic theory, materials science, and loss analysis are combined to develop the loss minimization methodology which can be used to systematically improve and optimize performance of any solar-cell material system. The techniques of the chemical process industry have been applied to achieve cost minimization. The loss-and cost-minimization methodologies have been combined into a generalized procedure for the accelerated development of all low-cost thin-film photovoltaic material systems.     

A review of recent advances in transparent p-type Cu2O-based thin film transistors

One of the crucial challenges that face the wide-spread implementation of flexible and transparent electronics is the lack of high performance p-type semiconductor material. Cu₂O in thin-film form is a potentially attractive material for such applications because of its native p-type semi-conductivity, transparency, abundant availability, non-toxic nature, and low production cost. This review summarizes recent research on using copper oxide Cu₂O thin films to produce p-type transparent thin-film transistors (TFTs) and complementary metal–oxide–semiconductor (CMOS) devices. After a short introduction about the main advantages of Cu₂O semiconductor material, different methods for depositing and growing Cu₂O thin films are discussed. The hi-tech development, along with the associated obstacles, of the Cu₂O-based thin-film transistors is reviewed, with special emphasis on those made of sputtered Cu₂O films. Finally, the bilayer scheme as one of the most exciting and promising technique for both TFTs and CMOS devices will be considered.     

Modelling of GaAs solar cells under wide angle cones of homogeneous light

A new theoretical model is presented for the performance of solar cells when light in the shape of a cone impinges on them. This is the case for concentration applications. The model is applied to 1000 sun concentrator p/n heteroface GaAs solar cells for the general purpose of a cone angle of between 0° and 90°. A multidimensional optimization of the whole GaAs solar cell structure (antireflecting coatings + ohmic contacts + semiconductor structure) is carried out. It is the first time that the whole structure has been simultaneously optimized under a maximum efficiency criterion. This is thanks to the dual consideration of the opto-electrical characteristics of the window layer. An assessment of GaAs solar cells working inside optical concentrators is also derived. Copyright © 1999 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.     

Correlative microscopy analyses of thin-film solar cells at multiple scales

In the present work, a brief overview is given on how to apply transmission (TEM) as well as scanning electron microscopy (SEM) and their related techniques (electron diffraction, energy-dispersive X-ray spectrometry, electron energy-loss spectroscopy, electron holography; electron backscatter diffraction, electron-beam-induced current, cathodoluminescence) for the analysis of interfaces between individual layers or extended structural defects in a thin-film stack. All examples given in the present work were recorded on Cu(In, Ga)Se₂ thin film solar cells, however, the shown experimental approaches may be used on any similar thin-film semiconductor device. A particular aspect is the application of various techniques on the same identical specimen area, in order to enhance the insight into structural, compositional, and electrical properties. For (aberration-corrected) TEM, the spatial resolutions of such measurements can be as low as on the subnanometer scale. However, when dealing with semiconductor devices, it is often necessary to characterize electrical and optoelectronic properties at larger scales, of few 10 nm up to even mm, for which SEM is more appropriate. At the same time, these larger scales provide also enhanced statistics of the analysis. In the present review, it is also outlined how to apply SEM techniques in combination with scanning-probe and optical microscopy, on the same identical positions. Altogether, a multiscale toolbox is provided for the thorough analysis of structure-property relationships in thin-film solar cells using correlative microscopy approaches.     

Fabrication and Characterization of GaAs Solar Cells on Copper Substrates

This letter presents performance comparison between a GaAs/mirror/copper thin-film solar cell and a conventional GaAs solar cell with a thick GaAs substrate. The GaAs thin-film solar cell was fabricated by transferring a GaAs solar cell onto a AuGe/Au mirror-coated copper substrate. With the aid of the excellent copper conductor, the thin-film solar cell exhibits significant improvement in both open-circuit voltage and short-circuit current density. The improved current–voltage ($I$– $V$) performance of the thin-film solar cell originates from the following two factors: reduced reverse saturation current by good heat dissipation of copper and enhanced light absorption by the highly reflective AuGe/Au mirror. The role of the mirror can further be verified in the measurement of external quantum efficiency (EQE) response where the thin-film solar cell exhibits a larger EQE response in the wavelength range of 700–900 nm than the conventional GaAs solar cell with the same active absorbing thickness.      

Improving charge injection in organic thin-film transistors with thiol-based self-assembled monolayers

The aim of this work is to improve charge injection by interposing an appropriately oriented dipole layer between contact and semiconductor in organic thin-film transistors (OTFTs). OTFTs are fabricated with pentacene semiconductor and gold source and drain contacts. The contacts are modified with self-assembled monolayers (SAMs) made of alkane or fluorinated alkane thiols. Ultraviolet photoelectron spectroscopy (UPS) shows a respective decrease and increase of the work function of the electrodes. Consistent with these results, we observe an increase and a decrease, respectively, of the contact resistance of the OTFTs, and a further decrease when shortening the length of the fluorinated molecule.     

Evaporated Te on CdTe: A vacuum-compatible approach to making back contact to CdTe solar cell devices

A commonly used process for forming low-resistance contacts to thin-film p-type CdTe involves the formation of a Te layer by etching the CdTe film in a concentrated mixture of nitric and phosphoric acids. The authors compare evaporated Te back contacts with ‘control’ back contacts formed by the usual etching process, and demonstrate that evaporating Te onto a CdTe thin film is a viable process for forming a low-resistance contact. The best efficiency achieved for a CdTe solar cell made with an evaporated Te back contact is 12.1%, whereas the efficiency of the device made with the control back contact was 11.9%. The evaporation process offers numerous advantages over acid etching, most notably vacuum compatibility amenable to large-scale production of CdTe solar cell modules.     

Scalability and pilot operation in solar cells of CuInSe2 and their alloys

The compound Cu(Ga,In)Se₂ (CIGS), and related compounds, have demonstrated their high potential for high-efficiency thin-film solar cells up to levels approaching 18%. It is expected that this quality can be further improved by optimizing process conditions and combining the CIGS with other group I, III and VI elements. Other material combinations are under development. Several companies and research institutes are developing CIGS-based technology with the aim of low cost/high volume production. The key process is a scalable technology to fabricate highest quality CIGS films on a large area with high throughput and process yield. The Centre for Solar Energy and Hydrogen Research (ZSW) and the University of Stuttgart (IPE) are working together on CIS technology. On the module basis ZSW is negotiating with private companies to commercialize module technology. These activities are compared with others worldwide. With the aim of developing relevant high-volume fabrication technologies, all laboratory deposition techniques that have proven highest device performance are applied also on the module level to prevent physical and chemical effects that could limit device performance. All film deposition techniques are developed for high-vacuum in-line fabrication on a large area except for the buffer layer of CdS, and monolithic integration is realized by patterning steps. Modules are prepared on substrate areas of 10×10 up to 30×30 cm². Actual results of modules of these sizes are 14% and 10%, respectively. Estimations of fabrication costs with increasing fabrication volume show that it is possible to produce CIGS modules at costs well below US$1 W⁻¹_p. © 1998 John Wiley & Sons, Ltd.     

Theoretical characteristics of a solar cell with a specific thin-film design

Expressions for the net generation rate of electron-hole pairs under illumination and the ideal Current-voltage (J-V) characteristics of a specific type of thin-film solar cell are derived. The thin-film configuration consists of an n- on p-type semiconductor with a rough upper surface (which is approximated by a distribution of smooth microscopic surfaces or facets randomly oriented) and a diffusive, perfectly reflecting lower surface. This configuration leads to a significant enhancement of the radiation flux in the film and theoretically to a higher collection probability and conversion efficiency compared to the conventional solar cell based on a much thicker crystalline film. TheJ-Vcharacteristics are finally expressed in terms of a set of double integrals in a form suitable for numerical analysis.     

Alternative contact designs for thin epitaxial silicon solar cells

Two major opportunities for increasing the performance of crystalline silicon solar cells involve reducing their thickness and reducing the losses associated with their front metallic grid contacts. Front grid contacted thin epitaxial silicon solar cells based on the growth of crystalline silicon films on a substrate or superstrate have been reported for many years, as have wafer‐based solar cells with alternative contact approaches. Integrating these two concepts into a single device presents an opportunity for simultaneously reducing two major loss mechanisms associated with crystalline silicon solar cells. The opportunities that exist and challenges that must be overcome in order to realize such a device are described in this paper. The design space is defined and explored by considering a wide range of possible approaches. A specific approach was chosen and used to design, grow, and fabricate a proof‐of‐concept thin epitaxial silicon solar cell with an embedded semiconductor grid as an alternative to a conventional front metallic grid. The work presented here has resulted in a thin epitaxial silicon solar cell with a 7·8% designated area conversion efficiency, well isolated contacts, negligible series resistive power loss, and less than 1% shading of the designated area. Copyright © 1999 John Wiley & Sons, Ltd.