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DEVICE OPERATION 597<br />

1.4<br />

V OC<br />

[Volts]<br />

1.2<br />

1.0<br />

0.8<br />

c<br />

b<br />

a<br />

0.6<br />

0.4<br />

0 100 200 300<br />

T<br />

[K]<br />

Figure 13.18 Temperature dependence of V OC for the devices shown in Figure 13.16<br />

and (13.8) and assuming G ≪ J L /V OC , the open-circuit voltage becomes<br />

V OC = E g<br />

q − AkT<br />

q<br />

( )<br />

JOO<br />

ln<br />

J L<br />

(13.9)<br />

with the barrier height b = E g .<br />

The different recombination paths are effectively connected in parallel so that<br />

V OC will be controlled by the single dominant mechanism with the highest current.<br />

The values of b and A can be used to distinguish between recombination in the bulk<br />

absorber, in the space charge region of the Cu(InGa)Se 2 , or at the Cu(InGa)Se 2 /CdS<br />

interface [27, 169]. Each of the curves in Figure 13.16 can be fit to equation (13.7) with<br />

A = 1.5 ± 0.3. For a wide range of thin-film solar cells, it has been demonstrated that<br />

V OC (T → 0) = QE g and 1

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