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COMPUTATION OF SERIES-CONNECTED DEVICE PERFORMANCE 381
is more complex. Again taking the GaInP/GaAs tandem as an example, recall that GaAs
subcell J SC depends not only on the GaAs band gap but also on the GaInP band gap,
because the GaInP subcell filters the light to the GaAs subcell. When the tandem-cell
temperature is raised, the bottom-subcell band gap decreases, tending to increase its J SC ;
at the same time, however, the top-subcell band gap also decreases, which decreases
the amount of light going to the bottom cell and thus minimizes the increase in the
bottom-subcell J SC with temperature.
The tandem J SC is limited by the least of the subcell J SC s. In general, these
subcell J SC s will not have identical temperature coefficients. For a tandem cell that is
nearly current matched, there will, therefore be a crossover temperature below which the
tandem J SC is limited by one subcell and above which the tandem J SC is limited by the
other subcell. Figure 9.11 illustrates this crossover for a modeled GaInP/GaAs tandem
16.0
(a) J SC
J SC
[mA/cm 2 ]
15.5
15.0
Top subcell
Bottom subcell
Tandem cell
dJ SC /dT
[µA/cm 2 K]
dFF/dT
[10 −4 /K]
11
10
9
8
7
−3.5
−4.0
−4.5
−5.0
−5.5
−0.05
(b) dJ SC /dT
(c) dFF/dT
dEff/dT
[%/K]
−0.06
(d) dEff/dT
300
320
340 360
Temperature
[K]
380
Figure 9.11 (a) Subcell and corresponding tandem-cell J SC s as a function of temperature for a
GaInP/GaAs tandem cell that is slightly top-subcell current limited at 300 K. (b) The corresponding
temperature derivatives dJ SC /dT . As the cell temperature is raised above ∼340 K, the cell crosses
over from top limited to bottom limited, and dJ SC /dT changes correspondingly. (c) Tandem-cell
fill factor temperature derivative dFF/dT . (d) Efficiency temperature coefficient dEff/dT