Figure 34.6 shows a high efficient CdS/CdTe cell current voltage characteristic with Voc = 845 mV, jsc = 25.88 mA and a fill factor of 75.51 % for a total efficiency of 16.5 %. This cell was a typical CdS/CdTe cell, however deposited on conducting glass with a Cadmium stannate (CTO/ZTO)1 conducting/buffer layer.
In first approximation, the current-voltage characteristics follow the typical diode equation, shifted by the short circuit current, jL:
j = j0 exp-(eV/AkT) – jL (34.1)
1This is a double layer of highly conductive cadmium stannate, Cd2 SnO4 followed by a buffer layer ZTO (Cd2SnO3).
V-1J X Rs I or Voc/V
Fig. 34.7 Open circuit voltage as function of the temperature (a) (Phillips et al. 1996a),2 and of the logarithm of the shot circuit current (b) (Hegedus and McCandless 2005)
j0 = j00 exp-(Eg/AkT) (34.2)
if shunt and series resistances van be neglected in better cells. A is the diode ideality factor with is related to interface recombination and Eg is the band gap of the emitter (CdTe).
Voc = Eg/e – (AkT/e) ln(j00/jL). (34.3)
This relation is one criterion of an excellent cell, if Voc approaches the band gap of CdTe when extrapolated to T = 0 K (see Fig 34.7a). From Eq. (34.3) one also concludes the Voc increases with the logarithm of the short circuit current (shown for different base electrodes in Fig. 34.7b).
When including the series resistance with
Voc = V + jR (34.4)
one has an easily measurable diode ideality factor from the derivative:
dj/dV = (AkT/e)(j/jL) (34.5)
where, from Eqs. (34.4) and (34.5) one obtains the A factor +1.6 and the series resistance of 1.4 and 2.3 ^ cm2 for a cell with dry (evaporation of Cu and Te) and vet processing (etching) respectively (see Fig. 34.8). This reflects a more intimate contacting by evaporation compared to the etching (Hegedus and McCandless 2005).
Fig. 34.8 dV/dj as function of the reciprocal current for two CdS/CdTe solar cells with different back contact with sequential tellurium and copper layer as identified within the picture. All similarly produced cells show an ideality factor of 1.6. The curve shift shows the influence of the series resistance (Hegedus and McCandless 2005)