12 herguth riegel ukn

bifi PV, Konstanz, 23.04.2012 A. Herguth, S. Riegel
Challenges occurring during the electrical
characterization of (bifacial) solar cells
A. Herguth, S. Riegel
Universität Konstanz
www.uni-konstanz.de/pv
bifi PV
Konstanz, 23.04.2012

Device
Classical Design Bifacial Design

Measurement Setup
Classical Design Bifacial Design
Massive brass chuck
• Electrical contact on whole area
• Thermal contact on whole area
• Local voltage sensing (anywhere)
Brass chuck with light absorbing,
(thermal) isolating layer on top
• Electrical contact only on busbars
• Thermal contact ?
• Choice of pins?

Defining the Challenges
3 types of problems challenges
• Optical
What about back side reflection?
• Thermal
What about uncontrolled heating?
• Electrical
How to contact the cell?

The Thermal Issue
Airtight
Insufficient underpressure or leakage
Estimation of the
thermal contact area
In between the fingers exists
often no thermal contact
Gap (air/vac) isolates well
Light absorbing layer isolates also
 Metalized area: ~8%
of course not to scale …

The Thermal Issue
screen opening
screen wire
schematic finger profile
Estimation of the
thermal contact area
In between the fingers exists
often no thermal contact
Gap (air/vac) isolates well
Light absorbing layer isolates also
 Metalized area: ~8%
Fingers feature few contact spots
 Thermal contact area: < 1%
A proper thermal coupling/control
is absolutely not self-evident

The Thermal Issue…
A rough calculation (of course too idealized…)
… or how is your voltage today ?
Si-wafer only Si wafer on brass chuck
Absorbed power Pabs 14 J/s
Absorbed power (constant light source):
Pabs = Irradiance * Area * (1 - Reflection)
= 100 mW/cm² * (12.5 cm)² * 90% = 14 W = 14 J/s

Electrical output Pel 0 .. 3 J/s (>11 J/s thermalized)
Electrical output:
Pel = 0 .. 20% * Pabs = 0 .. 3 J/s

Heat capacity C 5 J/K 500 J/K
Heat capacity:
C = cspec* Mass
cSi = 0.7 J/gK * 2.3 g/cm3 * 200 µm * (12.5 cm)² = 5 J/K
cBrass = 0.4 J/gK * 8.6 g/cm3 * 1 cm * (12.5 cm)² = 500 J/K

Initial heating rate dT/dt 3 K/s 0.03 K/s
Initial heating rate:
dT/dt = Pabs/C

Initial heating rate dT/dt 3 K/s 0.03 K/s
Voltage drop rate dVoc/dt - 6 mV/s - 0.06 mV/s
Voltage drop rate:
dVoc/dt = - 2 mV/K * (dT/dt)
Equilibrium temperature:
TSi ~ 70 - 80°C (convective cooling only)
TBrass ~ 25°C (with active cooling)
 Erroneous voltage measurement, if temperature is unknown

The Thermal Issue
PT 100 (PTC)
2 x 2 mm²
Measurement options
Large contact area-to-volume ratio
 Suited for (flat) surfaces
Higher heat capacity of element
 Requires good thermal coupling
Small contact area-to-volume ratio
 Suited for volumes (embedded)
Small heat capacity of element
 Less sensitive to thermal coupling

The Thermal Issue
Embedded in the chuck
 Assuming the cell’s temperature
Insufficient contact to cell
 Guessing the cell’s temperature
Good contact to cell
 Knowing the cell’s temperature

The Thermal Issue
How do we know we didn‘t hit a finger ?
- Metalize the element’s surface
- Measure the resistance between
elements surface and chuck
 Assures the correct placement
of the element between fingers
Ω

The Thermal Issue
Voc decays with temperature
K
mVoc
dT
dV
1.2..7.1−≈Measure Voc at or correct it to 25°C
…but…
What about the FF when temperature
increases during the measurement?

The Thermal Issue
K
mVoc
dT
dV
1.2..7.1−≈
sc
oc
oc jdT
dT
dV
V
p
dTFF
⋅




 ⋅+
≈ max
)(
j(V,dT)-characteristic features
a sharper bend and increased slope
Influence on FF may be estimated by
voltage reduction
by temperature
mpp
In this (exaggerated) example:
dT = 10 K from mpp to Voc

The Thermal Issue
Uncontrolled heating can lead
to an overestimation of FF
K
mVoc
dT
dV
1.2..7.1−≈
sc
oc
oc jdT
dT
dV
V
p
dTFF
⋅




 ⋅+
≈ max
)(
j(V,dT)-characteristic features
a sharper bend and increased slope
Influence on FF may be estimated by
Reminder:
A heating up to 3 K/s is possible
A ramp of ~100 mV/s yields dT = 3 K

• Current extraction with pins along busbars, not
via complete rear surface.
• FF determination depends on pin-to-pin
distance if V&I double pins are used.
• V-sensing pin placed in the middle between
two current extracting pins.
 Additional Rseries ignored, potential
along busbar appears homogenous.
• Voltage varies with distance between V-sensing
pin and current extracting pin.
• Voltage variation symmetrical between two
current extracting pins.
 Increased lateral inhomogeneity between
the fingers that influences FF measurement.
The Electrical Issue

In principle: Use of many pins recommended
…but…
the force of the pins may be problematic.
• Thin cells can break.
• The vacuum can be to weak.
Unfortunately…
the choice of spring stiffness is limited.

Contact pins disappear in the chuck
 Better placement and alignment
 Especially suited for thin bifacial cells
Vacuum self-regulated contact
pressure
 Less breakage of cells
 Better thermal contact

Summary
• Locally metalized rear side & absorber on chuck reduce thermal coupling
• Insufficient thermal coupling + inappropriate T measurement  incorrect VOC
 Steady state systems require a very accurate T measurement
 From a thermal point of view a flasher system is better suited for bifacial cells
• A temperature transient can lead to an overestimation of FF
• Usage of small pin-to-pin distance guarantees for reliable FF determination
• Vacuum self-regulated contact pressure allows for the use of user-defined number
of pins without cell breakage

12 herguth riegel ukn

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