Pvi20 Front Cover indd

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48
w w w. p v - te ch . o rg
Cell
Processing
Introduction
In the PV industry there is continual 
pressure to increase solar cell efficiency. 
However, it is actually not that important 
to know the the oretical maximum 
efficiency limit of a certain solar cell 
design; instead, it is more important 
to understand – and quantif y – the 
loss processes that currently limit cell 
efficiency. Consequently there is a need for 
a full bottom-up solar cell loss analysis that 
is based on high-precision measurements 
and quantifies the losses for the most 
relevant solar cell parameters, specifically 
short-circuit current (
I
sc
), open-circuit 
voltage (
V
oc
), fill factor (
FF
) and efficiency 
(
η
). In this paper, the work of Aberle et al. 
[1] is extended by further analyzing the 
losses limiting 
V
oc
and 
FF
. The results will 
be demonstrated using standard industrial 
aluminium-back-surface field (Al-BSF) 
silicon wafer solar cells from the R&D pilot 
line of the Solar Energy Research Institute 
of Singapore (SERIS).
“
It is more important to 
understand – and quantify – the 
loss processes that currently 
limit cell efficiency.
”
Standard high-precision 
measurements associated with 
the advanced loss analysis
The presented loss analysis is based on a 
set of high-precision measurements, i.e. 
secondary calibrated dark and light current–
voltage characteristics (
J-V
) and full-area 
illuminated spectral response (internal 
quantum efficiency IQE and external 
quantum efficiency EQE), and effective 
carrier-lifetime measurements by the 
photoconductance decay method. A detailed 
quantification of the 
I
sc
, 
V
oc
and 
FF
losses of 
the solar cell are provided, and thus the cell’s 
most severe efficiency losses can be analyzed.
First, the electrical properties of the 
solar cell are determined. From the light 
J-V
curve in Fig. 1(a), the standard solar 
cell parameters are derived, i.e. open-
circuit voltage 
V
oc
, short-circuit current 
density 
J
sc
, fill factor 
FF
, efficiency 
η
, and 
maximum power point voltage 
V
mpp 
and 
corresponding current 
J
mpp
. From the dark
 
J-V
curve in Fig. 1(b), the shunt resistance is 
determined by a linear fit in the –50mV to 
+50mV range. The series resistance under 
one-sun maximum power point conditions 
R
s,mpp
and the total recombination current 
(the effective saturation current density 
J
0
) of the solar cell are then determined 
from the light and dark 
J-V
measurements 
The cell doctor: A detailed ‘health check’ 
for industrial silicon wafer solar cells