An analytical model for the edge-illuminated silicon solar cell under concentrated sunlight

An analytical model for the edge-illuminated p⁺nn⁺ solar cell is derived. The model employs the Fletcher boundary conditions for the p⁺n and nn⁺ space-charge regions and the ambipolar approach for the low region, the lightly-doped n-type base region. For high-level condition, the ambipolar approach yields complete information about the low region, including the ohmic drop, the Dember voltage, and the hole concentration profile.     

The open-circuit voltage of back-surface-field (BSF) p-n junction solar cells in concentrated sunlight

Simple analytical expressions for the open-circuit voltage of the n⁺?p?p⁺ and p⁺?n?n⁺ BSF solar cells, which are valid for both the low- and high-levels of optical illumination, are derived. Based on the principle of superposition the open-circuit voltage of both the n⁺?p?p⁺ and p⁺?n?n⁺ solar cells are expressed in terms of the short-circuit current and the known saturated dark current. Effects of the high-low junction doping, the energy-gap shrinkage, and the dimensions of the BSF solar cells on the open-circuit voltage are included. The numerical results of the derived expressions are found to be in good agreement with the exact numerical analysis of Fossum et al. The optimal design considerations based on the known characteristics of the open-circuit voltage are also discussed.     

Limiting efficiency of silicon solar cells under highly concentrated sunlight

Recent work has shown that the upper bound on the energy conversion efficiency of silicon cells under concentrated sunlight lies in the 36-37-percent range regardless of the concentration ratio. These bounds are reassessed at very high cencentration levels where loss of conductivity modulation and loss in carrier collection efficiency due to Auger effects become important. Previous work is shown to overestimate the efficiency bound at such levels as well as the cell thickness required to attain this bound.     

The limiting efficiency of silicon solar cells under concentrated sunlight

The intrinsic limits on the energy conversion efficiency of silicon solar cells when used under concentrated sunlight are calculated. It is shown that Auger recombination processes are even more important under concentrated sunlight than nonconcentrated sunlight. However, light trapping can be far more effective under concentrated light due to the better defined direction of incident light. As a result of these effects, the limiting efficiency lies in tile 36-37-percent range regardless of concentration ratio compared to the limiting value of 29.8 percent for a nonconcentrating cell with isotropic response.     

Validated front contact grid simulation for GaAs solar cells under concentrated sunlight

In a common approach, the electric behavior of a solar cell is modeled by dividing it into smaller sub‐circuits and solving the resulting network by a circuit simulator. In this paper detailed network simulations are presented for a GaAs single‐junction solar cell. All resistive losses and losses influencing the diode saturation currents, such as recombination in the depletion region or at the perimeter are taken into account. With this model the maximum power point of a solar cell can be calculated for one‐sun and for higher illumination intensities. The results were validated experimentally using suitable test structures. This includes solar cell devices with varying dimensions, grid finger spacing and lengths. An excellent agreement between theoretical and experimental results was obtained. The network simulation model allows determining the optimum size and concentration ratio at which a solar cell operates at its maximum efficiency. In the case of a GaAs single‐junction solar cell this global efficiency maximum was found for an area of 1 mm² and at a concentration ratio of 450 suns. Under these conditions the largest loss mechanisms are the finger shading with 36.1% and the emitter resistance losses with 21.5% of the total power losses. Copyright © 2010 John Wiley & Sons, Ltd.     

Performance of n+-p silicon solar cells in concentrated sunlight

Performance data for n⁺-p silicon solar cells operating at illuminations up to 90 suns (9 W/cm²) and temperatures up to 100°C are presented. Experimental results for 2-cm²cells with different base resistivities are compared to performances predicted by a numerical device analysis computer code. Excellent agreement between numerical simulation and experiment is observed. For the illumination levels considered, an optimum base resistivity of approximately 0.3 Ω. cm is predicted by the numerical analyses and verified experimentally. The 0.3-Ω. cm cells exhibit conversion efficiencies above 11.8 percent up to 90 suns with a peak efficiency of 14 percent at approximately 30 suns. Preliminary results for a large-area (15.2 cm²) circular cell design are also presented for illuminations up to 60 suns. A peak conversion efficiency of 13.5 percent is measured for this cell at ∼25 suns.     

Optimization of silicon solar cell design for use under concentrated sunlight

For efficient operation under concentrated sunlight, solar cells must be optimized in terms of the collection efficiency and series resistance. At low concentrations, collection efficiency is more important, however, series resistance becomes increasingly important at high intensities ultimately limiting the device output. At a given intensity a tradeoff between these properties results in optimum performance. The computations includes losses from recombination, series resistance, contact coverage, and reflection. There are nine design parameters that were optimized; junction depth, thickness of anti-reflecting coating, width of grid strips, width of contact strips, separation of grid strips, length of grid strips, wafer thickness, and doping concentration of both sides of the junction. An optimization computer program was used to find the optimum design by employing a sequential search technique. An example is given for the optimum design of a silicon solar cell operating under an incident intensity of 9 W/cm²with active and total area efficiencies of 12.8 and 11.6 percent. Active area efficiencies as high as 17.8 percent are possible but with lower total area efficiencies. The effect of each of the nine parameters on device performance is presented. The theory was confirmed by fabricating and testing experimental devices. When corrected for temperature, the theory accurately predicts device performance to intensities of 50 suns (5.2 W/cm²).     

The interdigitated back contact solar cell: A silicon solar cell for use in concentrated sunlight

The theoretical and experimental performance of an interdigitated back contact solar cell is described. This type of cell is shown to have significant advantages over a conventional solar cell design when used at high concentration levels, namely, reduced internal series resistance, nonsaturating open-circuit voltage, and an absence of shadowing by front surface contacting fingers. The results of a computer study are presented showing the effects of bulk lifetime, surface recombination velocity, device thickness, contact dimensions, and illumination intensity on the conversion efficiency and general device operation. Experimental results are presented for solar illumination intensities up to 28 W/cm².     

An analytical model for optimizing the performance of graphene based silicon Schottky barrier solar cells

In this paper, a model taking into account the effects of carrier loss mechanisms has been developed. The model simulates the photovoltaic properties of the graphene/n-type silicon Schottky barrier solar cells (G/n-Si_SBSC), and it can reproduce the experimentally determined parameters of the G/n-Si_SBSC. To overcome the low efficiencies of G/n-Si_SBSC, their performances have been optimized by modifying the work function of graphene and Si properties, accounted for variation of its thickness and doping level. The obtained results show that the work function of graphene has the major impact on the device performance. Also, the temperature dependence of the G/n-Si_SBSC performance is investigated.     

Improved equivalent circuit and analytical model for amorphoussilicon solar cells and modules

An improved equivalent circuit for hydrogenated amorphous silicon (a-Si:H) solar cells and modules is presented. It is based on the classic combination of a diode with an exponential current-voltage characteristic, of a photocurrent source plus a new term representing additional recombination losses in the i-layer of the device. This model/equivalent circuit matches the I(V) curves of a-Si:H cells over an illumination range of six orders of magnitude. The model clearly separates effects related to the technology of the device (series and parallel resistance) and effects related to the physics of the p-i-n junction (recombination losses). It also allows an effective μτ product in the i-layer of the device to be determined, characterizing its state of degradation     

Thermophotovoltaic cells for conversion of thermal radiation and concentrated sunlight to electricity

The potential of the thermophotovoltaic conversion of thermal and solar energy to electricity using narrow-gap semiconductor photoconverters is shown. Liquid-phase epitaxy, metal-organic chemical vapor deposition, and Zn diffusion from the vapor phase are used to fabricate thermophotovoltaic converters based on GaSb and GaAs/Ge structures and characterized by increased values of both photocurrent and open-circuit voltage. This circumstance made it possible to obtain thermophotovoltaic cells that were based on the aforementioned structures and had efficiencies of 25% (GaSb) and 16% (GaAs/Ge) at a blackbody-radiation temperature of T=1473 K under the condition of 100% return of low-energy photons to the emitter.     

An improved circuit model for polymer solar cells

Authenticity of conventional circuit model, to interpret the characteristics of polymer solar cells (PSCs) is examined. Conventional circuit model is found to be quite limited, and various assumptions used there are not valid for PSCs. By understanding the nature of photovoltaic characteristics, through detailed investigations, we developed an improved circuit model, which explains correctly the behavior of PSCs under different environmental conditions. Investigations are carried out on the solar cells, made of the blend of regioregular poly(3‐hexylethiophene) (P3HT) and phenyl [6,6] C₆₁ butyric acid methyl ester (PCBM). The model is developed by treating both the dark and illuminated characteristics separately, even the characteristics were dealt with separately in reverse and forward biases. The formulated equivalent circuit model helps us in explaining many other important features, observed in the characteristics of PSCs. Copyright © 2013 John Wiley & Sons, Ltd.     

A comprehensive analytical model for metal-insulator-semiconductor(MIS) devices: a solar cell application

As an application of the authors previous model for MIS (metal-insulator-semiconductor) devices, a detailed model for MIS solar cells has been developed that covers a wide range of parameters, including surface states, silicon dioxide thickness, substrate doping, fixed oxide charges, substrate thickness, and metal work function. It also takes the nonequilibrium conditions into consideration. The effects of using the actual permittivity and barrier height of thin oxide are discussed     

An analytical model with flexible accuracy for deep submicron DCVSL cells

Differential cascoded voltage switch logic (DCVSL) cells are among the best candidates of circuit designers for a wide range of applications due to advantages such as low input capacitance, high switching speed, small area and noise-immunity; nevertheless, a proper model has not yet been developed to analyse them. This paper analyses deep submicron DCVSL cells based on a flexible accuracy-simplicity trade-off including the following key features: (1) the model is capable of producing closed-form expressions with an acceptable accuracy; (2) model equations can be solved numerically to offer higher accuracy; (3) the short-circuit currents occurring in high-low/low-high transitions are accounted in analysis and (4) the changes in the operating modes of transistors during transitions together with an efficient submicron I-V model, which incorporates the most important non-ideal short-channel effects, are considered. The accuracy of the proposed model is validated in IBM 0.13 µm CMOS technology through comparisons with the accurate physically based BSIM3 model. The maximum error caused by analytical solutions is below 10%, while this amount is below 7% for numerical solutions.     

A study of the conversion efficiency limit of p+-i-n+silicon solar cells in concentrated sunlight

The conversion efficiency limit of p⁺-i-n⁺silicon solar cells in concentrated sunlight is explored with numerical simulations of an idealized p⁺-i-n⁺cell having field-induced junctions. Conversion efficiencies greater than 30 percent are calculated for this cell operating in sunlight concentrated 1000 times. The relative importance of bulk and surface recombination in limiting the cell conversion efficiency is illustrated for operation in 1 to 1000 suns. For surface recombination velocities below 100 cm/s, it is shown that bulk recombination losses limit the cell performance rather than recombination losses occurring in the p⁺or n⁺regions. The results show that Auger recombination in the bulk region will limit ultimately the cell conversion efficiency.     

一种VOD系统的分析模型

对全连网络的分布式视频点播系统进行了性能分析，提出了一种VOD系统的分析模型，可以利用这一模型计算视频请球发生拥塞的概率以及网络的带宽要求，同时还可以得到在通信和存储费用两者之间的一个折衷方案。     

Limiting factors of photon-to-current conversion in polymer/nanocrystal bilayer hybrid solar cells: An analytical quantum efficiency model study

This paper introduces an analytical external quantum efficiency (EQE) model of planar hybrid solar cells (HSCs) based on photon-to-current conversion processes and uses this to investigate the factors that limit the maximum EQE (EQE_m) of devices; i.e., the photon absorption coefficient α, exciton diffusion coefficient D_z, exciton lifetime τ_z, exciton dissociation rate k_dis, electron diffusion coefficient D_e, electron lifetime τ_e, nanocrystals thickness d, and thickness of the polymer l. Our simulations indicate that relying solely on modifying k_dis, D_e, or τ_e cannot achieve a breakthrough increase in the EQE_m of planar HSCs. However, increasing α, D_z, or τ_z could potentially lead to a large EQE_m (30–100%), especially in the context of high k_dis values. Moreover, the calculation results indicate that although both D_z and τ_z contribute to the exciton diffusion length (L_z) via the equation L_z² = D_zτ_z, the EQE_m has an asymmetric dependence on these variables. With a small k_dis (i.e., <10⁴ cm/s), an increase in D_z results in an initial increase and then decrease in EQE_m, resulting in a peak value that increases with increasing k_dis. When k_dis is sufficiently large (∼10⁵ cm/s), the EQE_m becomes saturated after the initial increase. Thus, although an increase in D_z can adversely affect device performance when the k_dis is lower than 10⁴ cm/s, increasing τ_z always improves device performance, regardless of large k_dis becomes. This behavior can be attributed to the detrimental effect of excitons accumulating at the D/A interface, and can be used to optimize the material design and device engineering of planar HSCs and related solar cells for maximum photon-to-current conversion performance. In addition, we also demonstrate that the model can fit to the experimental data.     

An analytical model for jitter in IP networks

Traditionally, IP network planning and design is mostly based on the average delay or loss constraints which can often be easily calculated. Jitter, on the other hand, is much more difficult to evaluate, but it is particularly important to manage the QoS of real-time and interactive services such as VoIP and streaming video. In this paper, we present simple formulas for the jitter of Poisson traffic in a single queue that can be quickly calculated . It takes into account the packets delay correlation and also the correlation of tandem queues that have a significant impact on the end-to-end jitter. We then extend them to the end-to-end jitter of a tagged stream based on a tandem queueing network. The results given by the model are then compared with event-driven simulations. We find that they are very accurate for Poisson traffic over a wide range of traffic loads and more importantly that they yield conservative values for the jitter so that they can be used in network design procedures. We also find some very counter-intuitive results. We show that jitter actually decreases with increasing load and the total jitter on a path depends on the position of congested links on that path. We finally point out some consequences of these results for network design procedures.     

An analytical dishing and step height reduction model for chemical mechanical planarization (CMP)

An analytical model for dishing and step height reduction in chemical mechanical planarization (CMP) is presented. The model is based on the assumption that at the feature scale, high areas on the wafer experience higher pressure than low areas. A Prestonian material removal model is assumed. The model delineates how dishing and step height reduction depend on slurry properties (selectivity and Preston's constants), pad characteristics (stiffness and bending ability), polishing conditions (pressure, relative velocity and overpolishing) and wafer surface geometry (linewidth, pitch and pattern density). Model predictions are in good agreement with existing experimental observations. The present model facilitates understanding of the CMP process at the feature scale. Based on the proposed model, design avenues for decreasing dishing and increasing the speed of step height reduction may be explored through modification of appropriate parameters for slurry, pad and polishing conditions. The proposed model may also be used as a design tool for pattern layout to optimize the performance of the CMP process.