Optical modeling of finely textured amorphous silicon solar cells

It has long been known that the use of finely textured transparent conducting oxide layers substantially improves the performance of thin film amorphous silicon (a-Si:H) solar cells. Major efforts to understand the nature of this effect and to fully capture its potential have been made by researchers using advanced modeling techniques. In this work, modeling the oblique angle optical performance and use of an effective medium approximation to simulate microrough interfaces suggests that effective interface grading makes a significant contribution to optical enhancement.     

Characterization of PIII textured industrial multicrystalline silicon solar cells

Optimized textured structure is one of the most important elements for high efficiency multicrystalline silicon solar cells. In this paper, in order to incorporate low reflectance nanostructures into conventional industrial solar cells, structures with aspect ratios of about 1:1 and average reflectance of 8.0% have been generated using plasma immersion ion implantation. A sheet resistance of 56.9 Ω/sq has been obtained by adjusting the phosphorous diffusion conditions, while the thickness of the silicon nitride vary in 70–80 nm by extending the deposition time by 100 s. Under the conventional co-firing conditions, a solar cell with efficiency of 16.3% and short-circuit current density 34.23 mA/cm² has been fabricated.     

Concept and design of modular Fresnel lenses for concentration solar PV system

In this paper, we propose a new configuration of solar concentration optics utilizing modularly faceted Fresnel lenses to achieve a uniform intensity on the absorber plane with a moderate concentration ratio. The uniform illumination is obtained by the superposition of flux distributions resulted from modularly faceted Fresnel lenses. Based on the concept of modularly faceted Fresnel lenses, the cost effective 3-D concentration solar PV system is designed for future applications. Mathematical treatments for deriving the flux distribution and the concentration efficiency at the absorber plane are introduced. As an example, the distribution of the solar flux, at the cell position, is simulated using ray-trace technique for 9, 25, 49, 81, and 121 suns concentration systems. The irradiance distributions at the cell plane are estimated to be uniform within 20%, with a transmission efficiency larger than 70% for low and medium concentration ratios (less than 50 suns).     

Optical analysis of textured plastic substrates to be used in thin silicon solar cells

Light confinement strategies in thin-film silicon solar cells play a crucial role in the performance of the devices. In this work, the possible use of Ag-coated stamped polymers as reflectors to be used in n–i–p solar cells is studied. Different random roughnesses (nanometer and micrometer size) have been transferred on poly(methylmethacrylate) (PMMA) by hot embossing. Morphological and optical analyses of masters, stamped polymers and reflectors have been carried out evidencing a positive surface transference on the polymer and the viability of a further application in solar cells.     

Dependence of efficiency on slat angles of microgrooved solar cells

The short circuit current of a monocrystalline silicon solar cell can be enhanced further by suitably modifying the slat angle of its microgroove surface due to reduction in reflection coefficient and increase in optical trapping with decreasing slat angle. In this paper the dependence of I_sc, V_oc and η of a solar cell on the slat angle have been computed taking into account the variation of the reflection coefficient with the slat angle.It is observed that I_sc increases while V_oc decreases significantly with decreasing slat angles leading to a maximum efficiency of about 22% corresponding to a slat angle range lying between 30° and 45° without antireflection coating. However, the efficiency can be increased further to about 25% with AR coating.     

Performance of solar powered thermoacoustic engine at different tilted angles

In this study, the thermodynamic performance of a thermoacoustic engine charged with different working fluids were examined at different tilted angles ranging from ?90° to 90° with 45° angular interval. The results suggest that the influence of the tilted angle on the onset temperature of the engine depends on the viscidity of the working gas. The lower the viscidity is, the more obvious the influence is. The difference between the maximum and the minimum onset temperature of the engine charged with nitrogen could be as high as 52 °C, but the difference for system charged with helium is only about 1.5 °C. The tilted angle has little or no effect on the pressure oscillation amplitude, pressure ratio, resonance frequency and the relation of the temperature versus heat power. They are mainly affected by the properties of the working gas. Furthermore, the interactions of the oscillatory motion and the natural convection of the working gas within the thermoacoustic core were also examined. The properties are of importance for the thermoacoustic engine driven by two-axis solar collector, for the tilted angle of the engine varies with the sun position.     

Exploiting optical anisotropy to increase the external quantum efficiency of flexible P3HT:PCBM blend solar cells at large incident angles

The external quantum efficiencies of P3HT:PCBM blend solar cells decrease significantly when they are bent or illuminated at large incident angles because of (i) optical anisotropy of the P3HT:PCBM films—primarily because a mismatch between the direction of the electric field of the incoming light and the orientation of the P3HT:PCBM blend nanocrystallites results in a significant reduction in the amount of TM-polarized light absorbed and (ii) interfacial reflection of multilayer structures - primarily because the outermost air-flexible substrate interface exhibits a distinct refractive index difference - at large incident angles. Textured moth-eye structures fabricated by nanoimprint lithography on the flexible substrates of organic solar cells reduce the degree of interfacial reflection at high incident angles; they should allow more TE-polarized light to absorb in the P3HT:PCBM films (active layers) of the organic solar cells.     

Reduced light reflection of textured multicrystalline silicon via NPD for solar cells applications

Texturing by negative potential dissolution (NPD) process of p-type multicrystalline silicon for solar cells application is reported. The effect of the negative potential, KOH concentration, and texturing time of cast multicrystalline silicon was studied. Rapid texturing of multicrystalline silicon was achieved in a time-frame of 2 min with the application of negative potential of −30 V and the use of optimal alkaline concentration of 32 wt%. While texturing process in these optimal NPD conditions results in a step-free morphology, necessary in solar cells contacts printing, light reflection was reduced to minimal values, as well.     

Data analysis on solar irradiance and performance characteristics of solar modules with a test facility of various tilted angles and directions

We examined and evaluated the output characteristics and the deterioration lowering factors of modules installed under various conditions (module directions and tilted angles) based on the data collected at the multi-position test facility, which was produced and installed in order to conduct evaluation using design parameters (correction coefficient). This report introduces example data and the results of our evaluation.     

Analysis of point-focused,non-imaging Fresnel lenses' concentration profile and manufacture parameters

The purpose of this study is to develop a mechanism of a curve-based, point-focused Fresnel lens concentrator system and use it to examine each spectral segment's distribution patterns on the lens' focal area. The mechanism incorporates optical geometry and ray tracing technique with the components of solar spectrum, refractive index information of lens materials, and the formulation for quantifying the concentrator systems' transmittance loss and prism-tip dispersion loss. In addition to the facet angles' role in refracting the incoming radiation, this research has addressed spectrum-filtering role of the side angle, the angle between the entry face and the side face of each facet on the lens. The theoretical aspect has been elaborated on the basis of the lens' design wavelength. A computerized model has been developed and the simulated outcome compared to the measured data from a previous research. The result of this study provides the information of illumination patterns under a circular lens, which will help to match up various spectrum distributions to their suitable solar applications.     

Incorporation and critical concentration of oxygen in a-Si:H solar cells

For different process conditions, series of hydrogenated amorphous silicon p-i-n solar cells with various oxygen concentrations in the intrinsic absorber layer were fabricated by plasma-enhanced chemical vapor deposition at 13.56 MHz using process gas mixtures of SiH₄ and H₂. Oxygen was introduced into the gas phase during the deposition process by a controllable leak in the chamber wall and the amount of oxygen supply is characterized by the oxygen base pressure p_b. It is found that for a certain deposition regime defined by silane and H₂ flows, deposition pressure and substrate temperature the oxygen incorporation follows an expected dependence on the ratio p_b/r_d with r_d the deposition rate. This relation is not valid for the comparison of different deposition regimes. A high hydrogen flow is found to reduce the oxygen incorporation strongly. The photovoltaic parameters of the solar cells were measured in the initial state as well as after 1000 h of light-soaking. The critical oxygen concentration (i.e. the upper limit of incorporated oxygen not leading to a decay of the solar cell performance) was determined for each regime in the initial and light-soaked state. For all deposition regimes, the results show no difference in these critical oxygen concentrations for the initial and light-soaked state. The critical oxygen concentration, is found to differ for the different process regimes and turns out to be the highest (approximately 1×10²⁰ cm⁻³) for the deposition regime with the highest hydrogen flow rate, which interestingly is the regime with the lowest oxygen incorporation at a given p_b/r_d ratio. This combination makes the regime of high hydrogen gas flow suitable for depositing high-efficiency solar cells at high base pressure.     

Film cooling performance of a row of dual-fanned holes at various injection angles

Film cooling performance about a row of dual-fanned holes with injection angles of 30°, 60 ° and 90° were experimentally investigated at blowing ratios of 1.0 and 2.0. Dual-fanned hole is a novel shaped hole which has both inlet expansion and outlet expansion. A transient thermochromic liquid crystal technique was used to reveal the local values of film cooling effectiveness and heat transfer coefficient. The results show that injection angles have strong influence on the two dimensional distributions of film cooling effectiveness and heat transfer coefficient. For the small injection angle of 30 degree and small blowing ratio of 1.0, there is only a narrow spanwise region covered with film. The increase of injection angle and blowing ratio both leads to the enhanced spanwise film diffusion, but reduced local cooling ability far away from the hole. Injection angles have comprehensive influence on the averaged film cooling effectiveness for various x/d locations. As injection angles are 30 and 60 degree, two bands of high heat transfer coefficients are found in mixing region of the gas and coolant. As injection angle increases to 90 degree, the mixing leads to the enhanced heat transfer region near the film hole. The averaged heat transfer coefficient increases with the increase of injection angle.     

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.     

Technical note: Performance study of a linear Fresnel concentrating solar device

A linear Fresnel concentrating solar device was developed and its performance was studied. Various combinations of reflecting mirrors were tried to achieve the temperature at focus. A tubular absorber made from aluminium pipe (0.075 m diameter and 0.53 m long) filled with Hytherm-500 oil was used. The absorber tube was placed at focus of the collector. The quantum of heat absorbed by oil in absorber tube with 10, 15, and 20 numbers of mirrors was evaluated. Rise in oil temperature in absorber tube with different set of reflecting mirrors was compared. The efficiency of this solar device with different set of mirrors was also compared. It was observed that increase in peak oil temperature in absorber tube was not directly related with number of mirrors. The overall efficiency of the concentrator during initial 30 min of operation with 10, 15 and 20 number of mirrors was found to be 20.5, 17.6 and 16.8% respectively.     

Computer modelling of current matching in a-Si : H/a-Si : H tandem solar cells on textured TCO substrates

Computer modelling is used as a tool for optimising a-Si : H/a-Si : H tandem cells on textured substrate in order to achieve current matching between the top and bottom cell. To take light scattering at the textured interfaces of the cell into account, we developed a multirough-interface optical model which was used for calculating the absorption profiles in the tandem cells. In order to simulate multi-junction solar cell as a complete device we implemented a novel model for tunnel/recombination junction (TRJ), which combines the trap-assisted tunnelling and enhanced carrier transport in the high-field region of the TRJ.We investigated the influence of light scattering and thickness of the intrinsic layer of the bottom cell on the optimal ratio i2/i1 between the thicknesses of the bottom (i2) and top (i1) intrinsic layers in the current-matched cell. The simulation results show that increasing amount of scattering at the textured interfaces leads to a lower ratio i2/i1 in the current-matched cell. This ratio depends on the thickness of the intrinsic layer of the bottom cell. The simulation results demonstrate that a-Si : H/a-Si : H tandem cell with 300 nm thick intrinsic layer in the bottom cell exhibits higher efficiency than the cell with 500 nm thick bottom intrinsic layer.     

Luminescent spectral splitting: Efficient spatial division of solar spectrum at low concentration

The purpose of this study was to investigate the potential performance of a novel concept for dividing solar radiation into spectral components that separately illuminate photovoltaic (PV) cells of different band gaps using an optical design that (1) is simple, easily manufactured, and extensible to many spectral channels and (2) does not achieve high geometric concentration factors. The concept that we explore leverages the approach of stacked luminescent solar concentrators (LSCs) for dividing the solar spectrum using fluorophores that are tuned to different spectral bands. However, whereas multicolor LSCs must perform two functions using the same optical component—spectral division and concentration—we consider the performance of a similar design when only one demand—spectral division—is placed on it. We find that the optical quantum and power efficiencies can be quite high (QE>90%, PE>80%) compared to what one might intuitively expect. When we couple the light output to a simple detailed balance model of a solar cell using experimental performance parameters we find that solar-to-electric conversion could exceed 30% with four junctions, using existing PV materials. While this does not exceed what can be achieved by HCPV designs on multijunction epitaxially grown stacks, the concept presented here has the major advantage of being easily extensible to an arbitrarily large number of spectral channels. Because of this extensibility, the number of junctions in the system is limited only by the availability of PV cells with appropriate band gaps, so significantly higher system efficiencies should be accessible without major revision to the basic design presented here.     

Investigation of various surface passivation schemes for silicon solar cells

In this work, we have investigated three different surface passivation technologies: classical thermal oxidation (CTO), rapid thermal oxidation (RTO) and silicon nitride by plasma enhanced chemical vapor deposition (PECVD). Eight different passivation properties including SiO₂/SiN_x stacks on phosphorus diffused (100 and 40 Ω/Sq) and non-diffused 1 Ω cm FZ silicon were compared. Both types of SiO₂ layers, CTO and RTO, yield a higher effective lifetime on the emitter surface than on the non-diffused surface. For the SiN_x layers the situation is reverted. On the other hand, with SiO₂/SiN_x stacks high lifetimes are obtained not only non-diffused surface but also on the diffused surface. Thus, we have chosen the RTO/SiN_x stack layers as front and rear surface passivation in solar cells, which passivate relatively good on the surface and has very low-weighted reflection. On planar cells passivated with RTO/SiN_x a very high V_oc of 675.6 mV and a J_sc of 35.1 mA/cm² was achieved. Compared to a planar cell using CTO the efficiency of RTO/SiN_x cell is 0.8% higher (4.5% relative). It can be concluded that the RTO/SiN_x layers are the optimal passivation for the front and rear surface. On the other hand, for textured cells, the J_sc and FF of RTO/SiN_x cells are lower than those of CTO cells. The main reasons of these J_sc and FF losses were also discussed systematically.     

Numerical and experimental research of the characteristics of concentration solar cells

The development of automatic tracking solar concentrator photovoltaic systems is currently attracting growing interest. High concentration photovoltaic systems (HCPVs) combining triple-junction InGaP/lnGaAs/Ge solar cells with a concentrator provide high conversion efficiencies. The mathematical model for triple-junction solar cells, having a higher efficiency and superior temperature characteristics, was established based on the one-diode equivalent circuit cell model. A paraboloidal concentrator with a secondary optic system and a concentration ratio in the range of 100X–150X along with a sun tracking system was developed in this study. The GaInP/GalnAs/Ge triple-junction solar cell, produced by AZUR SPACE Solar Power, was also used in this study. The solar cells produced by Shanghai Solar Youth Energy (SY) and Shenzhen Yinshengsheng Technology Co. Ltd. (YXS) were used as comparison samples in a further comparative study at different concentration ratios (200X–1000X). A detailed analysis on the factors that influence the electrical output characteristics of the InGaP/lnGaAs/Ge solar cell was conducted with a dish-style concentrating photovoltaic system. The results show that the short-circuit current (I_sc) and the open-circuit voltage (V_oc) of multi-junction solar cells increases with the increasing concentration ratio, while the cell efficiency (η_c) of the solar cells increases first and then decreases with increasing concentration ratio. With increasing solar cell temperature, I_sc increases, while V_oc and η_c decrease. A comparison of the experimental and simulation results indicate that the maximum root mean square error is less than 10%, which provides a certain theoretical basis for the study of the characteristics of triple-junction solar cell that can be applied in the analysis and discussion regarding the influence of the relevant parameters on the performance of high concentration photovoltaic systems.     

The effect of a concentration graded cathode for organic solar cells

We present the effects of a concentration graded Li:Al cathode when it is made by one-step evaporation method using single alloy sources on the performance of organic solar cells. The concentration profile of the Li:Al cathode and related interface energy levels were investigated by means of secondary ion mass spectroscopy and ultraviolet photoelectron spectroscopy, in comparison with those of a common Al cathode. The results indicate that interfacial lithium accumulation introduces a cascade decrease of the work function (WF) of the cathode. The WF graded cathode applied to bulk heterojunction solar cells resulted in increased short circuit current and power conversion efficiency. Furthermore, the Li:Al cathode avoids the formation of interface Al-C complex, which may cause disruption of electron transport.