An overview of ohmic contacts on solar cells is presented The fundamentals of metal-semiconductor contacts are reviewed, including the Schottky approach, Fermi level pinning by surface states, and the mechanisms of thermionic emission, thermionic/field emission, and tunneling for current transport The concept of contact resistance is developed and contact resistance data for several different contact materials on both silicon and gallium arsenide over a range of doping densities are summarized Finally, the requirements imposed by solar cells on contact resistance are detailed

Solar cell contact resistance&#8212;A review

A theoretical analysis is given of the induced current profiles at grain boundaries in polycrystalline solar cells, as obtained by light or electron beam excitation. The area A and the variance σ2 of the contrast profile of a grain boundary are calculated for realistic generations as functions of the interface recombination velocity vs and the minority carrier diffusion length L. A graphical new procedure is proposed which allows the simultaneous determination of vs and L from the measured values of A and σ. The evaluation of an experimental electron beam induced current profile illustrates the applicability of the theory.

Theory of beam induced current characterization of grain boundaries in polycrystalline solar cells

A study of non-Darcian forced convection in an asymmetric heating sintered porous channel is carried out to investigate the feasibility of using this channel as a heat sink for high-performance forced air cooling in microelectronics. A volume-averaging technique is applied to obtain the macroscopic equations with the non-Darcian effects of no-slip boundary, flow inertia, and thermal dispersion. Local non-thermal-equilibrium is assumed between the solid and the fluid phases. The analysis reveals that the particle Reynolds number significantly affects the solid-to-fluid heat transfer coefficients. A wall function is introduced to model the transverse thermal dispersion process for the wall effect on the lateral mixing of fluid. The local heat transfer coefficient at the inlet is modeled by a modified impinging jet result, and the noninsulated thermal condition is considered at exit. The numerical results are found to be in good agreement with the experimental results in the ranges of 32 ≤ Red ≤ 428 and q = 0.8 ~ 3.2 W/cm2 for Pr = 0.71.

Investigation of Non-Darcian Forced Convection in an Asymmetrically Heated Sintered Porous Channel

Experiments have been conducted to study the heat transfer of a porous channel subjected to oscillating flow. The surface temperature distributions for both steady and oscillating flows were measured. The local and length-averaged Nusselt numbers were analyzed. The experimental results revealed that the surface temperature distribution for oscillating flow is more uniform than that for steady flow. Due to the reversing flow direction, there are two thermal entrance regions for oscillating flow. The length-averaged Nusselt number for oscillating flow is higher than that for steady flow. The length-averaged Nusselt number for both steady and oscillating flows increase linearly with a dimensionless grouping parameter k * /k f (D e /L) 1/2 Pe *1/2 . The porous channel heat sink subjected to oscillating flow can be considered as an effective method for cooling high-speed electronic devices

An Experimental Study of Heat Transfer of a Porous Channel Subjected to Oscillating Flow

A quantitative theory for evaluation of light‐beam‐induced profiles at grain boundaries is presented. Signals and deduced characteristic values, i.e., the maximum and the half‐width, are calculated in a practically important region of the semiconductor parameters. From this numerical simulation the influence of the various parameters on the signal properties is deduced. Light beam experiments are performed with high spatial resolution on semicrystalline solar cells. The light‐beam‐induced current can be calibrated to give the spatially resolved quantum yield. The experimental data are analyzed using the presented theory. Excellent agreement is obtained between experiment and theory as well as between light‐beam‐induced current analysis and previous electron‐induced experiments.

Light-beam-induced current characterization of grain boundaries

Lifetimes (τn), diffusion lengths (Ln) and consequently mobilities (μn) of minority carriers and also mobilities (μp) of majority carriers have been measured on small MESA diodes (1.5 × 1.5 mm) revealed on photocells realized on boron-doped cast polysilicon obtained by a fast variant Bridgmen method. The lifetime τn and the diffusion length Ln decrease drastically when the number of grains per diode NG increases. It has been deduced that the minority carrier mobility μn depends on the grain size even when this size is larger than 0.1 mm. This influence of the grain size is often screened by intragrain defects. This explains that generally the electronic properties are degraded at the bottom or at the periphery of the ingots where the dislocation density is high and at the top where the impurities are concentraled by the Bridgman process.

Dependence of electronic properties of polysilicon grain size and intragrain defects

Photocurrents, diffusion lengths (L) and g.b. recombination velocities (s) have been measured by optical scanning in N and P-type polysilicon. With type of grains, L is found to vary between 5 μm and 150 μm and s is always higher than 104 cm.s-1. The results on local measurements of diffusion lengths show that the vicinity of a g.b. is highly stressed. It seems that the g.b. potential barriers are higher in N-type than in P-type.

https://hal.archives-ouvertes.fr/jpa-00244979/document

Photocurrent and diffusion lengths at the vicinity of grain boundaries (g.b.) in N and P-type polysilicon. Evaluation of the g.b. recombination velocity)

2014 In order to passivate structural defects which degrade electronic properties, copper was diffused in p-type Wacker polysilicon at low temperature (~ 500 °C). Copper diffused samples exhibit homogeneous values of effective minority carrier diffusion lengths, which are practically as high as the higher values measured in the starting material. Light beam induced current scannings indicate that the grain responses are improved after copper diffusion while the grain boundary recombination activity is enhanced. Revue Phys. Appl. 18 (1983) 557-560 SEPTEMBRE 1983. : Classification Physics Abstracts 72.40

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Passivation of intragrain defects by copper diffusion in p-type polycrystalline silicon

The polysi 1 icon studied contains many c r i s t a l lographic defects (grainsboundaries, dislocations.. . ] which degrade the electronic properties. As these defects are randomly distr ibuted, the study of the i r detrimental ef fec ts needs the investigation of a large quantity of mesa diode N+P presenting approximately the same total length of grains boundaries L j in order to obtain the influence of the dislocations. The dislocations densit ies Ndis were evaluated by counting etch p i t s revealed by chemical etching. The evolution of [\\Idis were confirmed by X topography. The photocurrents Icc of the diodes and the diffusion length Ln decreased when Ndis increased. Icc and Ln decreased also markedly when the carbon concentration (determined by I.R. analysis) in the s i l icon varied from 3 . 1 0 ' ~ to 2.10~' cmV3. These results suggested that dislocations and excess carbon are correlated.

/pdf/correlation-entre-les-dislocations-les-concentrations-en-55pw09bh23.pdf

Correlation entre les dislocations, les concentrations en carbone et les longueurs de diffusion dans du silicium polycristallin c.g.e

Minority carrier properties (Ln, xn, ¼n) and photovoltaic parameters (Isc/ Voc) nave been measured in small MESA diodes revealed in N+P photocells cut out from different regions of polysilicon ingots (top, middle, bottom or periphery, centre). Their dependence on the physical properties (number of grains NG per diode, dislocations, impurities) has been analysed. It has been shown that the electronic and photovoltaic properties decrease strongly when the number of grains per diode NG increases. This influence of the grain size is often screened by intragrain defects. The dispersion of the results can be partially resolvedby taking into account the dislocation distribution. Local measurements of photocurrents Iph and diffusion lengths Ln performed with a 5 ¼m light spot confirm the influence of dislocations on the grain response.

G. Mathian

Papers

Dependence of electronic properties of polysilicon grain size and intragrain defects

Photocurrent and diffusion lengths at the vicinity of grain boundaries (g.b.) in N and P-type polysilicon. Evaluation of the g.b. recombination velocity)

Passivation of intragrain defects by copper diffusion in p-type polycrystalline silicon

Correlation entre les dislocations, les concentrations en carbone et les longueurs de diffusion dans du silicium polycristallin c.g.e

Grain Boundaries and Intragrain Defects Dependence of Local and Global Electronic and Photovoltaic Properties of CGE Polysilicon