Pvi20 Front Cover indd

Voltage losses 
The open-circuit voltage of a solar 
cell is determine d by internal cell 
recombination. The open-circuit voltage 
which would result if there were only bulk 
recombination (for a given wafer type) is 
considered to be the upper 
V
oc
limit of the 
investigated solar cell. There are then four 
main loss mechanisms for the open-circuit 
voltage of the solar cell: (1) front-surface 
voltage loss due to surface recombination 
at the non-contacted (passivated) regions 
of the cell; (2) front-surface voltage loss 
due to surface recombination at the 
contacted regions (metal contacts) of the 
cell; and (3, 4) rear-surface voltage losses 
due to contacted/non-contacted solar cell 
regions. The total recombination can be 
specified by a total recombination current 
Figure 5. Investigation of two different coating materials with refractive indices 
n
 = 1.9 and 
n
 = 2.7 for a silicon solar cell with 
pyramidal texture and encapsulated with a material with refractive index 
n
 = 1.5: (a) calculated WAR, and (b) calculated WAT.
(a)
(b)

---

52
w w w. p v - te ch . o rg
Cell 
Processing
J
0
given by the equation
J
0
= 
J
0b
+ 
J
0fp
+ 
J
0fc
+ 
J
0rp
+ 
J
0rc
(2)
where 
J
0
is the sum of the individual 
components stemming from the bulk and 
the front and rear non-contacted/contacted 
regions respectively (see Fig. 6). The 
relationship between 
J
0
and 
V
oc
is given by
(3)
Because the doping level and the bulk 
lifetime of the starting wafers are usually 
known (or the maximum bulk lifetime 
can be evaluated using the model of Kerr-
Cuevas [6,7]), the bulk recombination 
current 
J
0b
can be easily calculated [5–7]. 
This in turn provides an upper 
V
oc
limit that 
can be achieved with the particular wafers 
used. The surface recombination current 
J
0fp
of the passivated (non-contacted) regions 
of the solar cell can be extracted from 
photoconductance decay measurements 
performed on symmetrically passivated 
lifetime samples according to the Kane-
Swanson method [3]. The total surface 
recombination current associated with the 
front and rear metal contacts 
J
0fc
and 
J
0rc
can then be determined. This subsequently 
allows the open-circuit voltage to be 
calculated using Equation 3, and this value 
can then be compared with the measured 
open-circuit voltage. Alternatively, the 
individual components 
J
0fc
and 
J
0rc
can be 
measured by means of lifetime-calibrated 
photoluminescence spectroscopy. 
As an example, Fig. 7 shows the relative 
V
oc
loss of a typical industrial p-type Al-BSF 
solar cell. As can be seen, the main voltage 
losses, as expected, are due to the metallized 
regions of the solar cell (i.e. the full-area 
Al-BSF). This explains why the PV industry 
is moving towards passivated rear surfaces 
and reduced front-surface metallization.
“
The main voltage losses, 
as expected, are due to the 
metallized regions of the
solar cell.
”