Category Thin-Film Crystalline Silicon Solar Cells

Fitting experimental data

Experimental data for the injection level dependence of the surface recombination velocity at a Si/Si02 interface are shown in Figure 2.25 on p. 42. In this and the subse­quent paragraphs we apply the extended SRH recombination model to these experimen­tal data.

We assume energy-independent values for Dit, om and op9 since the interface parame­ters of exactly the cell under test were not measured. We fix the ratio on /op at 100 [188]. The thin-film cell has p-type doping with an acceptor concentration NA = 4xl016 cm-3. The fitted injection level dependence is also shown in Figure 2.25 on p. 42. The theoreti­cal curve describes the measurement to within the measurement uncertainty...

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Experimental data on capture cross-sections

Capture cross-sections are the second important ingredient for modeling SRH recom­bination. Unfortunately, an accurate measurement is difficult. Cross-sections published in the literature vary by many orders of magnitude. One reason for the scatter of data published in the literature is the high sensitivity of the interface properties on sample preparation. Different types of defects prevail, depending on the sample treatment, e. g. alneal or not. Different capture cross-sections are expected from different defects. Hence it is advisable to measure the cross-sections and the interface state density with exactly the same sample as is used for the determination of the surface recombination. The ex-

image467 image468

perimental data in Figure B...

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Experimental data on the interface state density

Figure B.10 shows experimental interface state densities measured on alnealed Si/Si02 interfaces. The data represented by diamonds were measured by Fiissel on an oxide fabricated in our lab and processed similarly to the 20.6%-efficient thin-film cell [4]. In this case the state distribution peaks at Et – Ev = 0.75 eV, indicating a superposi­tion of Ut and PH centers. The data for the triangles were measured for an oxide pre­pared at 1050°C with 2 vol% trichlorethylene (TCA). These data are from Ref. [176]. After cooling the samples in an Ar atmosphere, Al was evaporated and the sample was annealed at 450°C in forming gas. This so-called alneal procedure often generates an interface symmetrical to the midgap...

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Defects at the Si/Si02 interface

Our 20.6%-efficient thin-film Si cell has an Si/Si02/Al back surface reflector [113]. In order to elucidate the physical origin of the back surface recombination, we start with a short review of the nature of the defects at the Si/Si02 interface.

The energy levels of chemically bonded atoms split into a bonding and an anti­bonding state. The bonding state is lower in energy and neutral when occupied. The anti­bonding state is neutral when empty. The regular tetrahedral bonding in crystalline Si causes the energy gap, which is free of electronic states. Stretching weakens the bonds and thus reduces the energy difference between bonding and anti-bonding states.

Figure B.9 shows the interface state density de-convoluted according to the model of Fiissel et al. [400]...

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Calculation of the interface charge Qit

We now consider the charge Qit in the interface states. The interface state density

image460(B.32)

has a contribution DA from acceptor states and a contribution Dd from donor states. The interface charge

Подпись: (B.33)

image462
Подпись: (В.ЗО)

Є, =-] DA{E)UE)dE+ DD{E)fd{E)dE

depends on the occupation function defined by Eqs. (B.23) and (B.24) on page 218. Determination of charges QG and Qf

An expression for the functional dependence of the gate charge Qg of a metallized surface is given in Ref. [184]. If Qg is a corona charge on a non-metallized surface, it does not depend on %иг and is determined experimentally from an integration of the charging current over the charging time. The fixed charge Qf in the dielectric layer is also independent of *Р5МГ and is experimentally accessible by capacitance-voltage meas­urements...

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Extended Shockley-Read-Hall recombination

В. 3.1 Surface recombination

The discussion of SRH recombination at Si surfaces or interfaces on p. 217 neglected the presence of space charge regions. In Figure B.8 we consider the more general case of a silicon surface with band bending due to charges of a passivating dielectric film of thickness t. This film may or may not be metallized. The system has four charged layers as shown in the upper part of Figure B.8.

(i) The surface charge density QG (in C cm-2) on the outer surface of the di­electric layer. If the dielectric layer is metallized, this charge could be induced by the work function difference of the metal and the Si and/or by applying a gate voltage UG [184]. For a non-metallized dielectric the charge QG could be depos­ited with a corona discharge chamber (see Figure 2...

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Bulk defects

The model to describe the recombination via defect states at energy Et in the bandgap was developed by Shockley, Read [169], and Hall [170]. The defect states are assumed to be non-interacting gap states. The gap state captures holes and electrons with the capture cross-section ap and an alternately. The process is shown schematically in Figure 2.12a on p. 24. The thermally activated release of a captured electron into the conduction band or a captured hole into the valence band is also possible. In the stationary state the occupation of the defect states as well as the carrier concentration in the bands is con­stant (in time average). This condition leads to the Shockley-Read-Hall (SRH) recombi­nation rate

Подпись: (B.14)(np-njjG v,

R – ДГ —————————————— > Г n p th—

SRH ‘ {п + п,)<т„+{р + рІ)ар...

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Defect recombination

Dislocations, surfaces, interfaces, and grain boundaries, as well as point defects such as vacancies or impurities, cause deviations from the ideal periodicity of the semicon­ductor lattice. Electronic states that would not exist in an ideal and infinitely extended Si crystal are introduced. Electronic defect states in the bandgap facilitate carrier recombi­nation. The energy of the recombining electron-hole pair is transferred to the lattice. We call the defect recombination extrinsic since it is, at least in principle, avoidable. Grain boundaries, for example, may be avoided by growing single crystals. A strictly zero defect concentration is only achievable in small samples because all defects have a non­zero equilibrium concentration.

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