Amorphous Silicon Cells Research Articles

Tandem Photovoltaic Cells with Amorphous Silicon Cells and Organic Photovoltaic Cells

ABSTRACTWe demonstrate series-connected tandem photovoltaic cells consisting of hydrogenated amorphous silicon (a-Si:H) solar cells and polymer-based organic photovoltaic (OPV) cells. One of the limiting factors of a-Si:H solar cells is their narrow absorption spectrum as compared with that of crystalline silicon solar cells. In order to overcome this limitation, we fabricated a hybrid tandem solar cell by employing a solution-processed OPV subcell based on a low bandgap semiconducting polymer onto the a-Si:H subcell. It was found that the interfacial property of the hole transporting intermediate layer between the subcells strongly affects the photovoltaic property of the tandem cells. By using MoO3 as an efficient hole transporting intermediate layer instead of the conventional conducting polymer, we obtained the power conversion efficiency of 1.84% and the open-circuit voltage (VOC) of 1.50 V which corresponds closely to the sum of the VOCs of the subcells.

Degradation of thin film nanocrystalline silicon solar cells with 1 MeV protons

Thin film hydrogenated nanocrystalline silicon (nc-Si:H) solar cells were deposited at a high growth rate of 5 nm/s by Very High Frequency Plasma Enhanced Chemical Vapor Deposition (VHF PECVD). A single deposition yielded 30 working cells out of 30 test cells. After characterization of J-V characteristics under AM1.5 conditions, spectral response and Fourier Transform Photocurrent Spectroscopy (FTPS) for defect density, the cells were subjected to a beam of 1 MeV protons, with different irradiation times for a series of cells, up to a maximum fluence of 1015 protons per cm2, to investigate the feasibility of using this type of solar cells in space. After degradation, the cells were characterized again. The highest fluence reduced the conversion efficiency of the solar cells by a factor of 10. These values are compared to a similar experiment in literature for thin film amorphous silicon cells. The spectral response shows that the quantum efficiency for low energy photons is drastically reduced, suggesting that the damage is mainly inflicted to the bulk of the absorber material. This is strongly supported by the FTPS results, which show a clear trend of increasing defect density with increasing fluence. The optical absorption coefficient at 0.8 eV increased by a factor of 20. After characterization, the cells were isochronally annealed, up to the deposition temperatures, to investigate if the original performance of the cell could be restored. The mid-gap absorption decreased by a factor of 5 after annealing.

Resistive interlayer for improved performance of thin film silicon solar cells on highly textured substrate

The deposition of thin-film silicon solar cells on highly textured substrates results in improved light trapping in the cell. However, the growth of silicon layers on rough substrates can often lead to undesired current drains, degrading performance and reliability of the cells. We show that the use of a silicon oxide interlayer between the active area and the back contact of the cell permits in such cases to improve the electrical properties. Relative increases of up to 7.5% of fill factor and of 6.8% of conversion efficiency are shown for amorphous silicon cells deposited on highly textured substrates, together with improved yield and low-illumination performance.

Texture ZnO Thin-Films and their Application as Front Electrode in Solar Cells

In this paper, three kinds of textured ZnO thin-films (the first kind has the textured structure with both columnar and polygon, the second posses pyramid-like textured structure only, and the third has the textured structure with both crater-like and pyramid-like), were prepared by three kinds of methods, and the application of these ZnO thin-films as a front electrode in solar cell was studied, respectively. In the first method with negative bias voltage and appropriate sputtering parameters, the textured structure with columnar and polygon on the surface of ZnO thin-film are both existence for the sample prepared by direct magnetron sputtering. Using as a front electrode in solar cell, the photoelectric conversion efficiency Eff of 7.00% was obtained. The second method is that by sputtering on the ZnO:Al self-supporting substrate, and the distribution of pyramid-like was gained. Moreover, the higher (8.25%) photoelectric conversion efficiency of solar cell was got. The last method is that by acid-etching the as-deposited ZnO thin-film which possesses mainly both columnar and polygon structure, and the textured ZnO thin-film with both crater-like and pyramid-like structure was obtained, and the photoelectric conversion efficiency of solar cell is 7.10% when using it as front electrode. These results show that the textured ZnO thin-film prepared on self-supporting substrate is more suitable for using as a front electrode in amorphous silicon cells.

Hybrid Photovoltaic Thermal (PV/T) Air and Water Based Solar Collectors Suitable for Building Integrated Applications

Problem statement: Experiments have been conducted to investigate the effect of mass flow rates on the electrical, thermal and combined of photovoltaic thermal efficiencies of the hybrid collectors. Approach: Two photovoltaic thermal solar collectors were designed and fabricated. The first collector, known as spiral flow absorber collector, designed to generate hot water and electricity. The second collector, known as single pass rectangular tunnel absorber collector designed to generate hot air and electricity. Both absorber collectors were fixed underneath the flat plate single glazing sheet of polycrystalline silicon PV module. Water was used as a heat transfer medium in spiral flow absorber collector and air for the Single pass rectangular tunnel absorber collector respectively. Results: The experiment results showed that the single flow absorber collector generates combined PV/T efficiency of 64%, electrical efficiency of 11% and power maximum achieved at 25.35 W. Moreover, Single pass rectangular tunnel absorber collector generated combined PV/T efficiency of 55%, electrical efficiency of 10% and maximum power of 22.45 W. Conclusion/Recommendations: The best mass flow rate achieved for spiral flow absorber collector is 0.011 kg sec-1 at surface temperature of 55% and 0.0754 kg sec-1 at surface temperature of 39°C for single pass rectangular collector absorber. It was recommended for PV/T system to further improve its efficiency by optimizing the contact surfaces between the solar panel (photovoltaic module) and the tubes underneath and also recommended to use other type of photovoltaic cell such as amorphous silicon cell that posses the black mat surfaces property that will improve it thermal absorption.

Development of thin-film Si HYBRID solar module

The device current–voltage ( I– V) characteristics of thin-film silicon stacked tandem solar modules (HYBRID modules), consisting of a hydrogenated amorphous silicon (a-Si:H) cell and a thin-film crystalline silicon solar cell (μc-Si), have been investigated under various spectral irradiance distributions. The performance of the HYBRID module varied periodically in natural sunlight due to the current-limiting property of the HYBRID module and the environmental effects. The behavior based on the current-limiting property was demonstrated by the modelling of the I– V curves using the linear interpolation method for each component cell. The improvement of the performance for the HYBRID module in natural sunlight will also be discussed from the viewpoint of the device design of the component cells.

Presumptive technique of power generation of amorphous silicon cell including various factors

This paper describes the presumptive technique of power generation of amorphous silicon module considering various factors. The influences exerted by solar spectrum, spectral response, angle of incidence and efficiency deviation are evaluated. Then, various factors are formulated. The presumptive technique of power generation including these factors is examined. As a result, the proposal technique is able to presume with a high accuracy as compared with the conventional technique.

Thin film silicon n–i–p solar cells deposited by VHF PECVD at 100 °C substrate temperature

The applicability of the very high frequency (VHF) plasma-enhanced chemical vapor deposition (PECVD) technique to the fabrication of solar cells in an n–i–p configuration at 100 °C substrate temperature is being investigated. Amorphous and microcrystalline silicon cells are made with the absorber layers grown in conditions close to the amorphous-to-microcrystalline transition, which proved to give the best quality layers. It was observed that post-deposition annealing at 100 °C resulted in a relative increase of the efficiency of up to 50% for both amorphous and microcrystalline cells. For an amorphous solar cell deposited on stainless steel foil with a non-textured back reflector, an efficiency of 5.3% was achieved. A too rough substrate (textured back reflector), with an rms roughness higher than 80 nm, was found to give rise to shunting paths.

SSTE-4 Program Advanced Photovoltaic Cell Technologies: Ground Chamber Test Results

Plasma ground-testing results, obtained at the John H. Glenn Research Center National Plasma Interaction Facility, are presented for a number of thin-film photovoltaic cells. The cells represent a mix of promising new technologies identified by the Air Force Research Laboratory under the space science technology experiment (SSTE-4) Program. The current ground-testing efforts are aimed at characterizing both the performance and survivability of thin-film technologies in the harsh Low Earth Orbit space environment where they are intended to be flown. Measurements of parasitic currents and arc threshold voltages were performed <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in</i> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">situ</i> under strictly controlled charging conditions for both amorphous silicon (a-Si) and copper indium gallium selenide cells. Surface flashing on the large-area a-Si cells revealed that microdischarges in the dielectric surface do not appear to cause any apparent degradation to the cells. Catastrophic arc testing between adjacent cells resulted in no sustained arcs. Similar catastrophic arc tests between adjacent strings resulted in self-extinguished nonsustained arc extensions on time scales of approximately 100 mus in length. All cell efficiency measurements were performed in the Solar Cell Calibration Laboratory prior to plasma testing and then once again after the completion of the plasma tests.

Reversible increase of photocurrents in excimer laser-crystallized silicon solar cells

Excimer laser-crystallized silicon solar cells fabricated show a steady increment of the current densities with exposure to simulated sunlight, over a 30 min period. The current density of the amorphous silicon cell under identical conditions remains steady, with no significant change. The process was observed to be reversible upon cooling, and the performance increase is attributed to the energy barrier introduced by the enhanced bandgap of a nanocrystalline silicon middle layer, created as a result of the crystallization. It is suggested that the thermal energy due to prolonged illumination allows carriers to cross the barrier increasing output currents.

EXCIMER LASER CRYSTALLIZATION AND NANOSTRUCTURING OF AMORPHOUS SILICON FOR PHOTOVOLTAIC APPLICATIONS

Excimer laser crystallization of amorphous silicon has been extensively studied for electronic applications. Most of the early works has been on thin film transistor fabrication from laser crystallized silicon. However, in parallel, the applicability of the technique for photovoltaics has also been pursued. Direct crystallization of the absorber layer of a thin film amorphous silicon cell has proven unsuitable, due to poor device performance. The surface nanostructuring capability of the laser process, as a result of the crystallization appears to be of more scientific significance, and a number of applications have been reported. This review covers the established physics of excimer laser crystallization in the context of photovoltaics. It also expands on more recent applications of excimer laser nanostructuring of amorphous silicon, especially for photovoltaic applications. The outlook of the technique for photovoltaics is discussed with the use of reported successes, briefly discussing the fundamental improvements.

Comparison of photocurrent spectra measured by FTPS and CPM for amorphous silicon layers and solar cells

Fourier Transform Photocurrent Spectroscopy (FTPS) has been recently introduced as a fast and highly sensitive method for the evaluation of the optical absorption coefficient of photoconductive thin films such as microcrystalline silicon layers. This contribution represents the first study of FTPS utilization for amorphous silicon layers and cells. FTPS spectra are compared with results of Constant Photocurrent Method (CPM) and Dual Beam Photoconductivity (DBP) measured at different chopping frequencies. We will concentrate to highlight the appropriate measuring conditions and evaluation procedures for correct data interpretation. Moreover, we will present our novel approach for the interference free determination of absorption coefficients of thin films grown on transparent substrates, which is mainly important for very thin layers where broad interference fringes do not allow correct evaluation of parameters such as a slope of the Urbach tail and the defect density.

Modeling the effect of varying spectra on multi junction A-SI solar cells

The performance of multijunction amorphous silicon cells has been investigated for outdoor solar spectral radiation, using long term measurement for existing data at CREST, Loughborough University. It is a further study of the solar system, destined to analyze the outdoor performance of the amorphous silicon cells. The short circuit current for each subcells have been modeled and implemented into a computer program to calculate the mismatched short circuit current of the whole device.

Comparison of Photovoltaic Module Performance Measurements

Computer simulation tools used to predict the energy production of photovoltaic systems are needed in order to make informed economic decisions. These tools require input parameters that characterize module performance under various operational and environmental conditions. Depending upon the complexity of the simulation model, the required input parameters can vary from the limited information found on labels affixed to photovoltaic modules to an extensive set of parameters. The required input parameters are normally obtained indoors using a solar simulator or flash tester, or measured outdoors under natural sunlight. This paper compares measured performance parameters for three photovoltaic modules tested outdoors at the National Institute of Standards and Technology (NIST) and Sandia National Laboratories (SNL). Two of the three modules were custom fabricated using monocrystalline and silicon film cells. The third, a commercially available module, utilized triple-junction amorphous silicon cells. The resulting data allow a comparison to be made between performance parameters measured at two laboratories with differing geographical locations and apparatus. This paper describes the apparatus used to collect the experimental data, test procedures utilized, and resulting performance parameters for each of the three modules. Using a computer simulation model, the impact that differences in measured parameters have on predicted energy production is quantified. Data presented for each module includes power output at standard rating conditions and the influence of incident angle, air mass, and module temperature on each module’s electrical performance. Measurements from the two laboratories are in excellent agreement. The power at standard rating conditions is within 1% for all three modules. Although the magnitude of the individual temperature coefficients varied as much as 17% between the two laboratories, the impact on predicted performance at various temperature levels was minimal, less than 2%. The influence of air mass on the performance of the three modules measured at the laboratories was in excellent agreement. The largest difference in measured results between the two laboratories was noted in the response of the modules to incident angles that exceed 75deg.

Organic Dye for Highly Efficient Solid‐State Dye‐Sensitized Solar Cells

The feasibility of solid-state dye-sensitized solar cells as a low-cost alternative to amorphous silicon cells is demonstrated. Such a cell (see Figure) with a record efficiency of over 4 % under simulated sunlight is reported, made possible by using a new organic metal-free indoline dye (see Figure) as the sensitizer with high absorption coefficient.

Efficiency improvement in solid-state-dye-sensitized photovoltaics with an amphiphilic Ruthenium-dye

We report a solid-state-dye-sensitized solar cell with an efficiency of 4% over the standard air mass 1.5 spectrum (100mW∕cm2). This was made possible by using an amphiphilic dye with hydrophobic spacers. We attribute the performance to the self-assembly of the dye to a dense layer on the TiO2 surface with its carboxylate groups as anchors and with its hydrophobic isolating chains as blocking layer between hole conductor and TiO2. In addition we studied the dependence of the thickness of the nanoporous TiO2 layer and the device performance. These results show the high potential for solid-state-dye-sensitized solar cells to compete with amorphous silicon cells as low-cost alternative.

Optical simulation of the role of reflecting interlayers in tandem micromorph silicon solar cells

The role of a reflecting interlayer in micromorph silicon thin-film solar cells is investigated from the optical point of view. Detailed optical modelling and simulation are used to study the effects of different interlayers on quantum efficiency and short-circuit current of the top, amorphous silicon, and bottom, microcrystalline silicon, solar cell. The role of refractive index of interlayers on quantum efficiency of the top and bottom cell is analysed. Critical issues, such as enhanced total reflection from the solar cell and decreased quantum efficiency of the bottom cell due to interlayer are studied. Besides the single interlayer concept, double and triple interlayer stacks are investigated and improvements in comparison to the single ZnO interlayer are demonstrated. Potential thickness reductions of the top amorphous silicon cell related to different interlayers are presented.

Numerical simulation of the defect density influence on the steady state response of asilicon-based p–i–n cell

This paper is concerned with the numerical simulation of the defect density influence onthe steady state response of a silicon-based p–i–n cell under reverse bias dark conditions.To show this effect, the numerical simulation is performed on a crystalline cell containing asingle discrete level of defects in the energy gap and in which the density of defects isvaried. Afterwards, we extend our model to a typical amorphous silicon cell by includingband tails and dangling bonds. The density of dangling bonds is calculated according to thedefect pool model. For both cases, a detailed description of the physical model and itsmathematical formulation is presented. By analysing the different variables which describethe electrical behaviour of the cell in the steady state such as the free carrierdistributions, the carrier lifetimes and the quasi-Fermi levels, we show how thedensity of defects changes the semiconductor regime from lifetime to relaxation.

Amphiphilic Dye for Solid-State Dye-Sensitized Solar Cells

ABSTRACTWe report a solid-state dye-sensitized solar cell with a record efficiency of 4% under simulated sunlight (AM1.5global, 100mW/cm2). This was made possible by using a new amphiphilic dye with hydrophobic spacers in combination with spiro-OMeTAD. We attribute the significant improvement in the device performance to the self-assembly of the dye to form a compact layer on the TiO2 surface and to the hydrophobic chains working as blocking layer between spiro-OMeTAD and TiO2 to reduce the back electron transfer. In addition, we studied the influence of nanoporous TiO2 film thickness on the performance of the device. These results demonstrate the high potential for solid state dye sensitized solar cells to compete with amorphous silicon cells as low cost alternative.

State-of-the-art mid-frequency sputtered ZnO films for thin film silicon solar cells and modules

This article reports on the use of ZnO films in silicon thin film p–i–n solar cells and modules. It summarizes the status in the final phase of a joint research project aiming at the development of high-quality ZnO/glass substrates feasible for an industrial solar module production. The samples were prepared by reactive mid-frequency (mf) sputtering on large area (60×100 cm 2) glass sheets using low-cost metallic Zn:Al targets. These ZnO films exhibit resistivities down to 2.6×10 −4 Ω cm and high optical transmittance. Variation of the sputter pressure leads to films with significantly differing etching behavior in diluted acids. ZnO layers prepared in the high pressure regime develop strongly textured light scattering surfaces after etching, which is necessary to obtain highly efficient solar cells. Initial efficiencies of small area amorphous silicon (a-Si:H) cells on texture-etched ZnO-films prepared by mf-sputtering on 60×100 cm 2 soda-lime glass (3 mm thick) range from 8 to 9% (highest efficiency 9.2%, i-layer thickness 350 nm). First 0.6 m 2 modules on ZnO prove the principal applicability of the films for an industrial manufacturing process.