Low-cost solar cells based on large-area unconventional silicon

Low-cost approaches to solar cell manufacture require the use of inexpensive low-grade nonsingle crystalline silicon. Earlier experimental results indicate that conventional polysilicon, as it is used as ingot for the single crystal growing process, leads to solar cells of poor photovoltaic performance. These problems were overcome by utilizing unconventional nonsingle crystalline silicon, which is characterized by controlled size and structure of the individual grains. With modified processing, optimized in respect to the unique structure of the material, large-area solar cells could be realized under production scheme methods. Cells exhibiting dimensions up to 11 cm × 11 cm were fabricated, AM0 efficiencies of 8 percent could be achieved corresponding to AM1 values exceeding 10 percent. On test samples of 2 cm × 2 cm area AM0 efficiency Of 12.5 percent (AM1 value equivalent 14.0 percent) could be reached. The new material together with the optimized processes offer potentials for significant cost reduction by virtue of their being applicable to volume production and to automated fabrication techniques.     

Spectral response and energy output of concentrator multijunction solar cells

The spectral response of concentrator multijunction solar cells has been measured over a temperature range of 25–75°C. These data are combined with reference spectra representing the AM1·5 standard as well as annual spectral irradiance at representative geographical locations. The results suggest that higher performance in the field may be obtained if multijunction cells are designed for an effective air mass higher than AM1·5. Copyright © 2008 John Wiley & Sons, Ltd.     

Bifacial solar cells on large-area low-cost silicon wafers

A comprehensive design/technological study was conducted with the aim of obtaining high-efficiency BSF-type bifacial solar cells with reproducible parameters on large-area n-silicon and p-silicon wafers. The test devices were processed on 2-in. and 3-in. lower-grade single-crystal silicon wafers. Minimization of the cells' cost was one of the sought-after objectives. Various procedures leading to a substantial increase of both the emitter and the base contribution to the generated photocurrent are described and tested experimentally     

Simulating material inhomogeneities and defects in CIGS thin‐film solar cells

Thin‐film CIGS solar cells are simulated using a hybrid model consisting of a distributed form of the analytical one diode model paired with a numerical finite element model of the d.c. conduction in the front contact layers. Variations in material quality over the substrate surface, from measured J–V curves, are incorporated into the model and the effects of cell width and window layer thickness are evaluated for homogeneous and inhomogeneous material quality. Furthermore, the effects of discrete shunt defects of different sizes are modelled, and in different positions on the cell surface. The results from optimizing cell width and window layer thickness show that the effects of material inhomogeneities include a small shift of the optimal parameters together with a less pronounced maximum. As expected, the defect size is important to the shunt conductance parameter of the resulting J–V curves. The passivating effect of the highly resistive ZnO layer is confirmed. Copyright © 2009 John Wiley & Sons, Ltd.     

Spectral characteristics of GaAs solar cells grown by LPE

This paper describes the spectral characteristics of GaAs solar cells grown by low-temperature liquid phase epitaxy (LPE). It demonstrates improvements in blue response and peak internal quantum efficiencies of 100 percent for an optimized cell structure with isovalent In doped base and ultrathin (<100Å) heavily doped cap p⁺-GaAs layer on the photosensitive surface. The conversion efficiency obtained from the optimized cells under one-sun AM 1.5 conditions is 23.4 percent. Our results indicate that the low-cost LPE-grown films are suitable for high-efficiency solar cells.     

Structural inhomogeneities in polycrystalline silicon on glass solar cells and their effects on device characteristics

The effects of electrostatic fluctuations due to charged extended defects and strain‐induced bandgap fluctuations are examined in polycrystalline silicon on glass solar cells. The analysis is based on models previously applied to Cu(In,Ga)Se₂ solar cells, but with a new interpretation of the local ideality factor associated with electrostatic fluctuations. It is shown that electrostatic fluctuations become influential to the cell voltage properties as the absorber dopant concentration falls below a certain threshold (a few 10¹⁵ cm⁻³), and the degradations to the open circuit voltage and fill factor are expected to increase with further lowering of dopant density. It is equally plausible that the electrostatic fluctuations originate from charged dislocations or grain boundaries. Bandgap fluctuations on the other hand can be detrimental to the open circuit voltage of cells of any absorber dopant density. However, this voltage degrading effect is seen only in the cells deposited by electron‐beam evaporation, and not amongst those made by plasma enhanced chemical vapour deposition. Copyright © 2011 John Wiley & Sons, Ltd.     

Spectral mismatch correction for GaAs solar cells with varying junction depths

This study experimentally demonstrates that large errors 25%) in solar cell current measurements can occur even if a properly calibrated reference cell of the same material system is used to set the intensity of the light source. The spectral mismatch correction procedure allows the accurate (2%) measurement of the illuminated I-V characteristics. This is done with respect to standard test conditions defined by the temperature, intensity and reference spectrum. The results are independent of the light source spectral irradiance, or reference cell spectral response.     

Efficient planar organic solar cells with the high near-infrared response

Efficient planar organic solar cells extending the response into the near-infrared (NIR) were fabricated using the highly ordered Titanyl phthalocyanines (TiOPc) films as the donor layer. This type of films obtained through the weak epitaxy growth (WEG) method presents good continuity and integrity with the low density of grain boundaries. More importantly the films own a strong absorption in the NIR (750–950 nm) and a broad absorption spectrum from 550 to 950 nm. Meanwhile the high external quantum efficiency (EQE) is obtained in the NIR with the peak value over 38% and the EQE is over 18% in the entire response range, which could benefit from the long exciton diffusion length and the high carrier mobility of the highly ordered films. Thereby the fabricated planar solar cells achieve a high short-circuit current density (J_sc) of 9.26 mA cm^?2 and a power conversion efficiency (PCE) of 2.67%.     

High-power microwaves response characteristics of silicon and GaAs solar cells

The high-power microwave (HPM) effect heats solar cells, which is an important component of a satellite. This creates a serious reliability problem and affects the normal operation of a satellite. In this paper, the different HPM response characteristics of two kinds of solar cells are comparatively researched by simulation. The results show that there are similarities and differences in hot spot distribution and damage mechanisms between both kinds of solar cell, which are related to the amplitude of HPM. In addition, the duty cycle of repetition frequency contributes more to the temperature accumulation of the solar cells than the carrier frequency. These results will help future research of damage assessment technology, reliability enhancement and the selection of materials for solar cells.     

The collection probability and spectral response in isotype heterolayers of tandem solar cells

An analytical model for the position-dependent collection probability in uniformly doped one-dimensional layers with abrupt compositional and bandgap changes is presented. The collection probability is derived from coupled system of diffusion equations for low-level injection of photo-generated minority carriers in a stack of isotype heterolayers. Collection probability maintained continuity across isotype heterojunctions despite discontinuities of excess minority carrier concentration. An estimate for window and BSF passivation showed that the effective surface recombination velocity exponentially depended on the energygap difference, and linearly depended on increased doping, reduced mobility and increased thickness of sub-diffusion length passivation layers. Analytical expressions for the photo-generated current, and the internal quantum efficiency from each heterolayer were developed, and applied to the analysis of reported spectral response of a dual junction GaInP/GaAs tandem solar cell. Calculated internal quantum efficiencies closely matched reported experimental results, with the exception of sub-band absorption due to sub-bandgap deficiencies in the optical models and photon recycling. Calculated spectral response showed that upper AlInP₂ window, quasi-neutral emitter, and SCR layers dominated collection of photo-generated carriers in the top GaInP₂ cell, whereas, the base dominated collection of photo-generated carriers in the bottom GaAs cell. Results show that augmenting Hovel’s three layers (emitter, SCR, and base) analysis with the response from the top window layer should be sufficient to capture the spectral response of solar cells with thin passivation layers.     

Spectral response of photoconductivity in polycrystalline semiconductors

Photoconductivity in polycrystalline semiconductors can arise from the modulation, by the optical illumination, of the diffusion potentials at the grain boundaries, which in turn control the majority carrier current in these materials. The photogenerated carriers are captured by interface states at the grain boundaries through Shockley-Read-Hall processes, and under constant illumination this affects the steady-state charge at these boundaries so as to reduce the diffusion potentials from their dark values. The dependence of the diffusion potentials and of the photoconductivity upon position is calculated from the dependence of the optical absorption on wavelength. Finally, the average photoconductivity (expected from measurement) of polycrystalline semiconductor thin films is determined as a function of the intensity of the optical illumination, the grain size, the film thickness and the grain-boundary interface parameters.     

Effect of alkali treatment on the spectral response of silicon-nanowire solar cells

The effect of alkali treatment of Si nanowires (SiNWs) on the spectral response of solar cells was investigated using monochromatic incident photon-to-electron conversion efficiency spectroscopy. SiNWs were prepared on a substrate by metal-assisted etching and were then treated with NaOH/isopropanol. The results show that alkali treatment of SiNWs for 30 s obviously improved the cell conversion efficiency. This was attributed to enhancement of the red light response and a decrease in surface reflectivity from 6% to ~2%. However, SiNW alkali treatment led to poor blue light response, which is a major limiting factor for efficient SiNW solar cells. To improve the photovoltaic properties of SiNW cells, a near-complete response over the whole solar spectrum is essential.     

Enhancing photovoltaic response of organic solar cells using a crystalline molecular template

We demonstrate that a crystalline pentacene molecular templating layer considerably changes the morphology of the subsequently deposited lead phthalocyanine (PbPc) layer, resulting in an improved crystallinity at the early stages of growth of the PbPc film and a higher content of the triclinic phase. For bilayer PbPc (20 nm)/C₆₀ (40 nm) organic solar cells with or without the pentacene templating layer, the use of the pentacene templating layer leads to a 48% enhancement in the short-circuit current without noticeably affecting the solar cell open-circuit voltage or fill factor. A copper or zinc phthalocyanine molecular templating layer also leads to enhanced photovoltaic response from the PbPc/C₆₀ cells, though less significant than the pentacene template. The improved device performance originates from stronger absorption by the triclinic PbPc phase in the near infrared and the enhanced internal quantum efficiency over the entire spectrum where PbPc absorbs.     

Spectral response measurements of monolithic GaInP/Ga(In)As/Ge triple‐junction solar cells: Measurement artifacts and their explanation

Procedures for measuring the spectral response of multi‐junction cells in general require variation of the bias spectrum and voltage biasing. It is shown that a refined procedure including optimization of bias spectrum and voltage is necessary to minimize a measurement artifact, which appears if the subcell under test has non‐ideal properties, such as a low shunt resistance or a low reverse breakdown voltage. This measurement artifact is often observed on measuring the spectral response of the Ge bottom cell of GaInP/Ga(In)As/Ge triple‐junction cells. The main aspect of the measurement artifact is that the response of another subcell is simultaneously measured, while at the same time the signal of the Ge subcell is too low. Additionally, the shape of the spectral response curve is influenced under certain measurement conditions. In this paper the measurement artifact is thoroughly discussed by measurement results and simulation. Based on this analysis, a detailed procedure for the spectral response measurement of multi‐junction cells is developed, specially designed to minimize such measurement artifacts. Copyright © 2003 John Wiley & Sons, Ltd.     

Spectral and photoelectric parameters of high-voltage multi-junction solar batteries

The test samples of silicon high-voltage multi-junction solar batteries (SHVMJSB) are investigated experimentally. It is shown that the efficiency of the transformation of sunlight by silicon high-voltage multi-junction solar batteries without special antireflection coatings is not high and equal to 8%. It is due to the fact that the recombination rate of the minor charge carries is high on the damaged layer surface. The surface refinement and the antireflection coating make it possible to increase the efficiency of the transformation of sunlight by the silicon high-voltage multi-junction solar batteries of up to 13%.     

Thin-film solar cells

The R&D status of cells and modules based on hydrogenated amorphous silicon (a-Si:H) and those based on CdTe and CuInSe₂ is reviewed. The stability of a-Si:H solar cells is still a major concern. Improvements have been achieved on an empirical basis by application of multijunction structures, optimization of interfaces, etc. Stabilized efficiencies of close to 10% have been reported. In parallel, the introduction of the ‘defect-pool model’ led to remarkable progress in understanding; it follows that a-SiGe:H instead of a-Si:H should be used for the i-layer (absorber). Improved cell engineering concepts, however, such as enhancement of the built-in electric field via reduction of the i-layer thickness and/or folded structures, are believed to be more promising. Polycrystalline thin-film cells based on CdTe and CuInSe₂ are not affected by inherent degradation mechanisms. the specific properties of these materials demand heterojunctions, and particular problems arise due to the polycrystallinity of the films and to the lattice mismatch and mismatch of the electronic band structures of the materials involved. These are discussed in conjunction with measures currently applied for optimizing solar cell performance. Both cell types exhibit eficiencies in the range 16-17%. Estimations of production costs and energy payback times of thin-film photovoltaic modules are reviewed (even below 1 US$ W_p⁻¹ and as low as 4 months, respectively) and environmental concerns, especially for Cd-containing cells, are summarized.     

Amorphous-silicon solar cells

The status of a-Si solar cell technology is reviewed. This review includes a discussion of the types of solar cell structure that are being used in commercial products. An overview of the development efforts under way involving new materials, such as alloys and microcrystalline films, and their impact on device performance is given. The status of stability in a-Si solar cells and projections for costs for large-scale manufacturing facilities are reviewed. The development of markets for a-Si photovoltaics is also discussed     

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.     

Fiber-like solar cells

The human habit of wearing fiber materials and interwoven fabric can be dated back to the prehistoric time.In recent years, efforts have been devoted to make flexible energy devices, e.g., solar cells, into fiber shape, further expanding the concept of fiber from cloth materials to modern onbody electronic devices.The evolution of device shape has not only made changes to the sandwich-like device structure,but also provided more alternatives to classical electrode materials like transparent conductive oxide.In 2008, a flexible dye-sensitized solar cell (DSSC) was invented by Zou et al.[1] via twisting two electrodes into one fiber-like device, as a breakthrough to the coaxial-type or planar-type device structure.The entire cell was a fine wire with diameters of ～200μm.The working electrode was stainless steel fiber coated with a layer of dye-sensitized TiO2, and the counter electrode was Pt wire.Positive charges from the photoanode can be transferred to counter electrode by either liquid electrolyte or allsolid hole-transport materials[1-3].