Annual available radiation for fixed and tracking collectors

Single crystal silicon solar cells are potential elements of large scale solar energy conversion systems. Current costs of these cells are too high at least in part because current production methods require single crystal wafers obtained by slicing cylindrical single crystal ingots. This paper reviews a U.S. research program aimed at reducing the cost of silicon cells by developing new methods of growing silicon ribbons and sheets from which high efficiency solar cells can be fabricated. The paper also describes novel techniques for lower cost processes for ingot growth and wafer slicing which are included in this research and development program.     

Development of low-cost silicon crystal growth techniques for terrestrial photovoltaic solar energy conversion

Single crystal silicon solar cells are potential elements of large scale solar energy conversion systems. Current costs of these cells are too high at least in part because current production methods require single crystal wafers obtained by slicing cylindrical single crystal ingots. This paper reviews a U.S. research program aimed at reducing the cost of silicon cells by developing new methods of growing silicon ribbons and sheets from which high efficiency solar cells can be fabricated. The paper also describes novel techniques for lower cost processes for ingot growth and wafer slicing which are included in this research and development program.     

Photocarrier drift measurements and solar cell models for amorphous silicon

The relationship between transport research and solar cell models for hydrogenated amorphous silicon is reviewed. It is argued that a complete program of steady-state photoconductivity, ambipolar diffusion length, and photoconductivity response time measurements is required to support modeling; the present knowledge of these measurements in electronic quality a-Si:H is summarized. A qualitative discrepancy between trap distributions used for steady-state transport models and estimated from transient photocurrent measurements is discussed.     

RESEARCH IN THE UNITED STATES ON HIGH-EFFICIENCY AMORPHOUS SILICON PHOTOVOLTAIC DEVICES

The purpose of this paper is to describe the research in the United States, especially the cofunded government/industry program, on amorphous silicon photovoltaic devices that has been done over the past 10 years, discuss progress achieved, describe the remaining problem areas and the future research program to overcome the remaining problems.     

多晶硅锭中硬质点分析及控制研究

以多晶硅锭中硬质点为研究对象,通过实验研究和数值模拟的方法,对多晶硅锭中硬质点进行形貌和成分分析,并提出改善控制方法。研究结果表明硅锭中部的硬质点较细小,主要由SiC组成;硅锭头部的硬质点较粗大,主要由SiC和Si₃N₄组成,还有少量O的存在。进一步研究发现多晶硅定向凝固铸锭炉的热场结构对于多晶硅锭硬质点形成有直接影响,通过改进热场结构,优化晶体生长界面,显著减少了铸锭中硬质点的数量。     

Identification of some key parameters limiting the performance of high-efficiency silicon solar cells

Several theoretical calculations and barriers to achieving high-efficiency silicon solar cells have been discussed in the past. Cell efficiencies between 44% and 17% have been estimated assuming different spectra of illumination and sets of cell parameters. Plausible reasons for such a large variation in the cell efficiency have also been discussed.This paper presents, for the first time, a detailed sensitivity analysis of key cell parameters on cell efficiency by incorporating advanced solar cell physics in a sophisticated numerical simulation program. It delineates the true physical barriers to obtaining a high-efficiency silicon solar cell. Specific parameters presently limiting cell efficiency are identified to be the minority carrier lifetime and the recombination velocities at the front and back surfaces. Practical cell efficiencies in the vicinity of 22% are estimated to be attainable by using good quality silicon crystal and substantially reducing surface recombination velocities.     

Recovery of minority carrier lifetime in low-cost multicrystalline silicon

Lifetime of minority carriers has been widely identified to be the key material parameter determining the conversion efficiency of pn-junction silicon solar cells. Impurities and defects in the silicon crystal lattice reduce the charge carrier lifetime and thus limit the performance of the solar cells. Removal of impurities by silicon material purification is often contradictory with low cost production of photovoltaic devices. In this paper, we present experimental results of an efficient gettering technique which can be applied to low cost processing of multicrystalline silicon solar cells without any additional process steps or compromises with optimal device design parameters. This technique is based on well-known phosphorous gettering. We have discovered that if the silicon wafers are kept in the furnace after the emitter diffusion at the 700°C, significant improvement in the lifetime will take place. At this temperature the properties of the pn-junction remain unaffected meanwhile many lifetime killers are still mobile. The time needed for this temperature program can be easily modified in order to respond to the material quality variations in substrates originating from different parts of multicrystalline ingot. Better control of lifetime can lead to higher degree of starting material utilization.     

Screen printed titanium oxide and PECVD silicon nitride as antireflection coating on silicon solar cells

Silicon nitride and titanium oxide coatings have been used to reduce the reflection losses from silicon solar cells. Both 100-mm-diameter circular and 100 × 100 mm pseudo-square single crystalline silicon solar cells have been used in the present studies. More than 27% enhancement in the short circuit current has been demonstrated in polished cells using screen printed titanium oxide antireflection coating. Solar cells made from textured silicon wafers were used for plasma enhanced CVD grown silicon nitride antireflection coating on them. In these cells more than 23% enhancement in short-circuit current has been observed after silicon nitride antireflection coating.     

Contribution of silicon substrates to the efficiencies of silicon thin layer solar cells

Although silicon solar cells based on layers less than 50 μm thick have become very popular, little attention has been paid to the role of the underlying silicon substrate. This treatment uses the device simulation program PC-1D and the ray tracing program SUNRAYS to examine the role of the substrate in contributing to the current and efficiency of textured and non-textured thin layer solar cells. For the case of a heavily doped silicon substrate, substrate contributions can be significant for cells with sufficiently thin base layers. For example, for the case of a silicon thin layer cell with a base layer thickness of 20 μm and a substrate doping of 6 × 10¹⁸ cm⁻³, the substrate contributes no more than 4% of the total short-circuit current. However, decreasing the base width to 5 μm results in an increase in this substrate contribution to 20%. Light trapping tends to alleviate the substrate contribution by increasing the effective path length in the base. Examination of the current components under forward bias reveals that for a thin layer cell with a high quality base and good front surface passivation, back diffusion of electrons into the substrate limits cell performance.     

Optical modelling of thin film solar cells with textured interfaces using the effective medium approximation

Simple methods for increasing the maximum achievable current density of amorphous silicon (a-Si:H) solar cells include bandgap and layer thickness optimisation, and light confinement strategies. The goal of the optical modelling work presented here has been to examine the nature and potential of these effects, in particular the optical enhancement resulting from the use of finely textured transparent conducting oxides. A computer program that combines coherent and incoherent optical theory has been used as a flexible tool for simulating the performance of any general thin film solar cell structure. An effective medium approximation has been used to model the optical effects of microroughness (texturing with correlation lengths smaller than the wavelength of light). This work suggests that effective interface grading due to microroughness does have a significant effect on the optical performance of a-Si:H solar cells, and that both enhancement and deterioration in the maximum achievable current density can be the outcome. Where both effective interface grading (microroughness) and larger scale texturing (macroroughness) are fully exploited, optical yields may be increased beyond their current level. This work emphasises the importance of characterising and controlling the interface morphology to optimise the short circuit current and maintain the open circuit voltage.     

R & D activities of crystalline silicon solar cells in Japan

Research and development of crystalline silicon solar cells in Japan have greatly advanced for the past 10 years. Fundamental research has been conducted on the recombination and passivation of minority carriers at Si/SiO₂ interfaces and in bulk regions including grain boundaries. Qualities of Si feedstock and substrates have been improved. A small-area cell efficiency using monocrystalline silicon substrates has reached 21 % and that for large-area, multicrystalline solar cells up to 17% by using low-cost cell fabrication processes. Such high efficiency values are realized by tenacious improvement of substrate quality and the development of new processes for fabricating solar cells.     

Enhanced nanocrystalline silicon solar cell with a photonic crystal back-reflector

Nanocrystalline silicon solar cells were enhanced with a photonic crystal back-reflector. Rigorous scattering matrix simulations were used to optimize a photonic crystal back-reflector consisting of a triangular lattice of nano-holes, with a pitch near 800 nm. The photonic crystal back-reflector with a pitch of 800 nm was fabricated on the crystalline silicon substrate by photolithography and reactive-ion etching, and coated with silver and zinc oxide. Nanocrystalline silicon solar cells were grown on the patterned substrates. We observed ～7% enhancement of the absorption and photo-generated current relative to a Ag/ZnO substrate, with an enhancement ratio of 1.5 near the band edge. Significant enhancement occurred in photon absorption at near infrared wavelengths greater than 700 nm, due to diffraction resonances of the incoming light.     

Convective Heat Transfer Enhancement for Electronic Device Applications Using Patterned MWCNTs Structures

This article reports on the heat transfer characteristics of columnar, vertically aligned, multiwall carbon nanotubes grown on a patterned Si surface. In the first part, we describe the procedure for patterning the silicon (Si) surface and the growth of multiwall carbon nanotubes (MWCNTs) on these patterned surfaces. The diameter of MWCNTs grown by chemical vapor deposition technique was in the range of 30–80 nm. In the second part, structures mimicking macroscopic finned heat sinks are used for enhancing forced convective heat transfer on a silicon substrate. Convective heat transfer coefficient has been experimentally measured for silicon substrates with and without MWCNT-based fins on it. The configuration with MWCNTs based fins shows an enhancement in convective heat transfer of 40% and 20%, as maximum and average value, respectively, compared to the bare silicon. Experiments have been carried out in a wind tunnel with air as coolant in fully turbulent regime. These encouraging results and the possibility of growing structures directly on silicon can be regarded as a first step.     

How front side plasmonic nanostructures enhance solar cell efficiency

Dielectric roughness on the front surface enhances significantly solar cell efficiency by light trapping in the absorbing layer. However in plasmonic assisted thin silicon solar cells we show, by a detailed analysis of the various mechanisms, that front-surface plasmonic structures enhance the efficiency by a different mechanism—namely the effective broadband forward scattering into the silicon, while trapping (path length enhancement) is relatively small for these structures. The plasmonic local field enhancement contribution is even smaller in this configuration. Based on our study we optimized this “anti-reflection mechanism” by tuning the plasmonic structure such that the spectral location of the more efficiently scattering dipole and quadrupole resonances fit correctly the visible and NIR sun spectrum.     

Enhancement of on-chip combustion via nanoporous silicon microchannels

Due to its high energy density and MEMS compatible fabrication methods, on-chip porous silicon shows considerable promise as an energetic material. Rapid combustion events have been demonstrated with flame propagation speeds eclipsing 3 km/s, but much is still unknown about the controlling parameters of porous silicon combustion. Recent studies show that implementation of microstructure within a nanoporous silicon film greatly increases reaction rate of a relatively slow burning system. The present work utilizes porous silicon microchannels to enhance an already rapidly-reacting system. Reactions in channeled porous silicon regions of this system propagated at speeds up to 1.2 km/s faster than similar neat porous silicon films. The fastest propagation speed was 3660 m/s, the highest reported flame speed for comparable nanoenergetic systems to date. We provide evidence that the enhancement of flame propagation rates by channeled porous silicon is mechanistically different from the convectively controlled burning of neat porous silicon. This evidence suggests the presence of acoustically aided reactions for porous silicon channel combustion where the channels more readily ignite compared to neat porous silicon. We predict this allows for propagation of the reaction by intense sound waves within the porous medium.     

Nanocrystalline silicon solar cells from excimer laser crystallization of amorphous silicon

Excimer laser crystallized nanocrystalline silicon layers were used to fabricate solar cells. The laser crystallized layers are characterized with Raman spectroscopy for structural investigations and with atomic force microscopy to study surface modifications upon crystallization. The current–voltage characteristics of the devices completed with aluminium back contacts were investigated with air mass 1.5 G solar simulations. The resulting nanocrystalline solar cells show inferior performance compared with amorphous silicon devices. The degradation of open circuit voltages and short circuit currents with the increase of crystallization energy density is explained to be due to thermal modification of the p-type/intrinsic layer interface and band gap enhancement of fine-grained nanocrystalline layers of the devices along with increased surface and grain boundary recombination of carriers.     

Microwave heating characteristics of graphite based powder mixtures

The choice of material for a crucible is essential in hybrid microwave heating and graphite is one of the strong microwave absorbers which can sustain high temperatures. Experiments were carried out to investigate the microwave heating behavior of graphite combined with other materials to enhance efficiency. Mixtures comprising of graphite and magnetite or graphite and silicon carbide were found to have better microwave heating properties than pure components. The enhancement was likely due to microplasma generated by graphite in a microwave environment. Microwave heating of graphite–silicon carbide mixtures provided the best results in terms of high heating rates without arcing. The optimal combination of graphite–silicon carbide was identified and can be used as a starting material for making efficient crucibles and susceptors.     

Design, development and performance monitoring of a photovoltaic-thermal (PVT) air collector

The photovoltaic-thermal (PVT) systems allow the enhancement of the energy performance of photovoltaics, by removing thermal energy and subsequently decreasing the operating temperature of the cells. The possibility of the utilization of heat for climatization makes them attractive for the building integration. In order to diffuse this kind of solar systems it is necessary to translate the basic concepts into efficient and functional technological components and associated performance should be evaluated in a reliable manner. This paper presents the experimental and theoretical results of a research and development program carried out at the Politecnico di Milano on the design, development and performance monitoring of a hybrid PVT air collector. One of the main products of the research consists of a simulation model for performance prediction of the system. This R&D program led to the development of the TIS (tetto integrale solarizzato, i.e. integrated solar roof), an innovative technological system for building integration of hybrid PVT air collectors. The successful commercial application of the TIS in a research center building is also shown as a case study.     

P/A1 co-gettering effectiveness in various polycrystalline silicon

The various polycrystalline silicon materials (cast ingots, ribbons) which are commercially available for the solar cells manufacturing differ very much among themselves due to the different growth processes. The resulting microstructure and impurity content will influence differently the material characteristics during thermal treatments inherent to the device manufacturing. As the gettering efficiency depends on the kind of polycrystalline material, the variations observed in the optimal gettering conditions or passivation will be discussed. In this paper, we compare the performances of various types of polycrystalline silicon upon classical and rapid thermal-process-induced co-diffusion of phosphorus and aluminium. We show that a large bulk minority carrier diffusion length enhancement occurs in the case of co-diffusion when compared to the separate diffusion of phosphorus and aluminium.     

Effect of surface condition on boiling heat transfer from silicon chip with submicron-scale roughness

Boiling heat transfer on treated silicon surfaces was studied. Experiments were conducted to investigate the effects of submicron-scale roughness on the boiling heat transfer at a subcooled condition in FC-72 at the ambient pressure. Two-type of treated silicon surfaces were prepared for boiling surfaces using anodisation with HF (hydrofluoric acid) based electrolyte and DMF (dimethylforamide) based one. The back side of the treated surface was glued to the back side of the other silicon chip on which thin film heaters and thin film temperature sensors were fabricated using conventional MUPs processes with doped polysilicon. The treated chips with submicron-scale roughness which provide many possible nucleation sites showed considerable enhancement in the nucleate boiling heat transfer coefficients compared to the untreated silicon surface. Further, the critical heat flux (CHF) of the treated surfaces increase linearly to the increase in the effective area for boiling.