Category Thin-Film Crystalline Silicon Solar Cells

Voltage-biased

When studying injection level-dependent recombination processes, carrier injection by a bias voltage is an attractive alternative to carrier generation by bias illumination. The determination of the injection level is easier for a voltage bias since, in contrast to bias illumination, the complex optical properties of the cell do not need to be known.

Experimental set-up

Figure C.2 shows the circuit that we use to apply a time-constant bias voltage Ub [31]. The illuminated solar cell is represented by an ideal diode, a current source, and a series resistance Rs. The current source represents only the modulated fraction of the photogen­eration due to the chopped monochromatic light. The cell has a current-voltage curve I(U) in the dark...

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Quantum efficiency

С .1 Measurement

СЛЛ Light-biased

Quantum efficiency spectra are routinely measured under time-constant bias light in order to investigate the device under conditions similar to operation. We adjust the dis­tance from the lamp to the cell so as to yield a short-circuit current that is 50% to 100% of the short-circuit current that we measure with an AM1.5G spectrum at 1000 W nT2

[13] . The measurement set-up is shown in Figure C. l. Chopped monochromatic light is directed via a beam splitter onto the cell under test, towards and onto a monitor cell. Two lock-in amplifiers measure the modulation of currents AjceU and Ajmon, respectively. We record the ratio RceU = AjcM /Аjmon...

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Electrostatic potential in the space charge region

In a space charge region, the concentrations of electrons and holes varies by orders of magnitude. Since the electrons are majority carriers in the emitter and minority carriers in the base, the space charge region contains a “sweet plane” where the capture rate of electrons into the defect equals that of holes. For the one-dimensional case Choo gave an easy-to-handle analytical expression that evaluates the recombination in the space charge region of a monocrystalline asymmetric junction [347]. Rau and Werner introduced an approach that permits the analytical treatment of space charge region recombination in poly crystalline material [35]. Their trick is to assume an interface state density that is independent of energy...

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.4.2 Carrier concentration in the base

Fortunately, it is often possible to find approximate analytical solutions to the trans­port equations that give insight into the underlying physical effects. An important simpli­fication that is often required to enable analytical treatments is the assumption of low – level injection conditions. This assumption guarantees a recombination rate Urec = An/z that is linear in the excess minority carrier concentration An. If quasi-neutrality holds (as in the homogeneously doped base of a Si cell), the electric field – УФ does not need to be considered for the transport of the minority carriers. Then the minority carrier diffusion equation

r d2 An д2Ап An

Подпись:~дХГ dY2 dZ2

holds [79]. Неге X, Y, and Z denote the Cartesian coordinates.

B.4.2.1 Thin-film cells

Figure B...

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Transport in poly crystalline thin films

B.4.1 Transport equations

We restrict ourselves to steady-state situations. The electric field E = – УФ is the solution of the Poisson equation

– АФ =-^—{p + NB – n-NA) (B.40)

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where q is the elementary charge and e0xes is the dielectric constant of Si. The electron current density

jn = – qpnnVO + qDfi/n (B.41)

and the hole current density

jP = – ЯИрРУФ ~ ФРУР (B.42)

are caused by charge carrier drift in the electric field and by diffusion. We assume mo­bilities pn>p = q Dnp/kT that are independent of position and independent of the carrier concentrations. Both conditions are fulfilled only approximately. In thin-film materials the material quality is often spatially inhomogeneous...

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Grain boundary recombination

We model grain boundary recombination with the extended Shockley-Read-Hall model. The position of the various energy levels at a grain boundary in p-type Si is shown in Figure 2.32 on p. 49. Due to our assumption of flat quasi-Fermi levels, this model is limited to low interface recombination rates. An extension to high recombina­tion rates is outside the scope of this work. Numeric simulators, such as the programs PC1D [402] or DESIS [344], may be used to study high recombination rates. Here we have to keep in mind that our analytical simulations are only qualitatively correct for the case of large interface state densities.

Doping dependence

The doping concentration of the cell is easy to control during the growth of the thin – film absorber...

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Capture rates at the Si/SiNx interface

Figure B.13 shows the capture rates for electrons and holes at a silicon/silicon nitride interface. The simulation parameters are the same as those used for the simulation of the injection level-dependent surface recombination velocity of the 1.5 Q cm sample in Figure 2.26 on p. 43. At low injection levels the band bending is 4*^ = 0.1 V, similar to the oxidized sample. The cross-over region does not exist, however, because the holes have a larger capture cross-section than the electrons [187]. The surface recombination rate Usur is limited by the supply of electrons at all injection levels. The decrease in sur­face recombination velocity is solely due to the reduction of the surface potential when switching from low – to high-injection conditions. Consequently the reduction of the

CARRIE...

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Interpretation of the injection level dependence

As a result of the best fit, the carrier concentrations nsur and psur at the Si/Si02 inter­face and nw and pw at the edge of the space charge region are known. Figure B.12a shows these concentrations. The hole concentration is equal to the acceptor concentra­tion Na at low injection levels. We use the term injection level synonymously with the term excess carrier concentration Anw. The electron concentration is orders of magnitude smaller and increases linearly with the injection level tsnw. At high injections Anw » NA the electron and hole concentrations become equal.

The concentrations nsur and psur at the surface are different from nw and pw at the edge of the space charge region...

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