SunPower Discovers the “Surface Polarization” Effect in
High Efficiency Solar Cells
Introduction
In August, 2005 SunPower announced the discovery of a new performance effect observed in
high efficiency silicon solar cells. This new effect, called “surface polarization,” creates the nondestructive and reversible accumulation of static charge on the surface of high-efficiency solar
cells such as the company’s all back contact A-300.
The high efficiency of SunPower’s A-300 solar cell is obtained in part by covering its front
surface with a proprietary coating which prevents the loss of the charge carriers generated by
sunlight. This layer performs much like a transistor that is turned off, preventing current flow. If
a large enough voltage is applied to the front of the cell, the “transistor” effectively turns on,
allowing charge carriers to recombine at the front surface. When this happens, surface
polarization reduces the output current of the cell. Also, like a transistor this effect can be fully
reversed and current returned to the original level.
How Surface Polarization Occurs
Surface polarization can occur when a module is put into operation at high positive voltages. If
the module is operated at a positive voltage with respect to the earth ground, then minute leakage
current will flow from the cells to ground. As a result, over time a negative charge is left on the
front surface of a cell. This negative charge turns on the surface transistor, attracting positive
charge carries to the front surface where they recombine with electrons and are lost. This
sequence of events is illustrated in Figure 1.
Figure 1: Cross-section of an A-300 solar cell embedded in a module with surface polarization.
1) Current leaks from cell through glass to frame and earth
ground when cell is at high (+) voltage to ground
2) Current leakage leaves negative
charge on ARC surface which acts like
the insulator of a MOS transistor
Glass
3) EVA acts like the gate
of a MOS transistor
EVA
----------------------------------------+
N-type Silicon
4) Negative surface charge attracts positively charged, EVA
light-generated holes to surface, where they recombine
with electrons in n-type silicon instead of being collected
at the positive junction
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Reversing Surface Polarization
When a module is operated at negative voltage with respect to ground, the surface polarization
reverses. In this case the performance of the module is not affected; in fact, it can improve
slightly because the front now repels light generated holes, guiding them to the back collecting
junction. Figure 2 shows the situation at negative voltage.
Figure 2: Reversing surface polarization by operating an A-300 solar cell at negative voltage
with respect to ground.
1) Current leaks from ground through glass to the cell surface
when cell is at negative voltage to ground
2) Current leakage leaves positive
charge on ARC surface which acts like
the insulator of a MOS transistor
Glass
3) EVA acts like the gate
of a MOS transistor
EVA
+++++++++++++++++++++++++++++++++++++
+
N-type Silicon
4) Positive surface charge repels positively charged,
light-generated holes from the surface, so they are
eventually collected at the positive junction
EVA
Figure 3 shows the current-voltage characteristic of a module that has undergone surface
polarization after operating in the field at a positive voltage of 160 V. The blue line labeled
“before voltage bias” shows that with surface polarization, the module output has declined to 140
W from its initial 200 W rating. After biasing the module at a minus 1000 V for one hour to
reverse the surface polarization, the negative surface charge has been bled off and the module
completely recovered, as illustrated by the red line labeled “after voltage bias.” The surface
polarization effect is completely reversible, with no lingering effect.
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Figure 3: Module performance during and after reversing surface polarization with voltage bias.
40
Current density (mA/cm2)
35
30
25
After voltage bias
20
Before voltage bias
15
10
Power (Watts)
202.7
140.0
After voltage bias
Before voltage bias
Isc (A)
5.334
4.864
Fill Factor Voc (volts)
79.2%
47.99
62.7%
45.91
5
0
0
100
200
300
400
500
600
700
Module voltage per cell (mV)
The polarization effect can be easily avoided by designing systems with proper grounding so that
modules only see negative voltage. All of SunPower’s system integration partners have
developed grounding procedures that prevent module operation at positive voltage. In Europe,
SunPower worked closely with its exclusive distribution partner, SunTechnics, to characterize
the polarization effect and to develop system configurations that prevent the effect. SunTechnics
also developed a range of system solutions and electronic devices to optimize SunPower system
performance and meet European security standards. For European tracker systems, SunPower
worked with Solon to characterize and develop system configurations to prevent the polarization
effect. In North America, SunPower modules are exclusively installed with SunPower positiveground inverters which eliminate surface polarization.
If a system is mistakenly installed with the incorrect grounding, modules will temporarily lose
power, but no lingering effect remains after surface polarization is reversed. Because the surface
polarization effect is completely reversible, once the grounding problem is corrected, modules
will quickly recover to their initial performance without any further intervention.
SunPower Systems Field Performance
SunPower has installed systems around the world that have been operating since early 2005.
These systems have confirmed that the use of proper system grounding can eliminate the surface
polarization effect. For example, two side-by-side arrays have been operating in Germany since
February, 2005 and have been continuously monitored in partnership with SunTechnics. An
array of ten 160 W conventional multi-crystalline modules compared with an array of eight 210
W SunPower modules allows for an accurate comparison. Recent data from SunTechnics for
these two arrays is plotted in Figure 4. The left bar at each date is the energy delivery of the
SunPower array and the right bar the energy delivery of the multi-crystalline array. The black
triangles give the ratio of kWh/kW rated for the two arrays. The SunPower array consistently
out-performs the multi-crystalline array by the amount displayed on the right axis scale. The
SunPower array delivers about 4% more energy delivery on a kWh/kW rated basis, due in part to
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the excellent low light performance and low temperature coefficient of efficiency of SunPower
modules.
SunPower Daily Energy Delivery Advantage
6/
1
6/
1
6/
1
6/
1
/0
5
Conventional Array
6/
1
6/
9
6/
7
6/
5
6/
3
6/
1
5/
3
5/
2
5/
2
5/
2
5/
2
5/
2
SunPower
9/
05
0%
7/
05
0
5/
05
2%
3/
05
2
1/
05
4%
/0
5
4
/0
5
6%
/0
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/0
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8%
0/
05
8
8/
05
10%
6/
05
10
4/
05
12%
2/
05
12
0/
05
Daily kWh Production
Figure 4: Energy delivery comparison of SunPower and conventional solar arrays in Bavaria.
SunPower Advantage
Source: SunTechnics SunReader
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