12 herguth riegel ukn
Post on 20-Jan-2017
185 Views
Preview:
TRANSCRIPT
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
bifi PV, Konstanz, 23.04.2012 A. Herguth, S. Riegel
Device
Classical Design Bifacial Design
bifi PV, Konstanz, 23.04.2012 A. Herguth, S. Riegel
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?
bifi PV, Konstanz, 23.04.2012 A. Herguth, S. Riegel
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?
bifi PV, Konstanz, 23.04.2012 A. Herguth, S. Riegel
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?
bifi PV, Konstanz, 23.04.2012 A. Herguth, S. Riegel
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 …
bifi PV, Konstanz, 23.04.2012 A. Herguth, S. Riegel
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
bifi PV, Konstanz, 23.04.2012 A. Herguth, S. Riegel
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
bifi PV, Konstanz, 23.04.2012 A. Herguth, S. Riegel
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 Electrical output Pel 0 .. 3 J/s (>11 J/s thermalized)
Electrical output:
Pel = 0 .. 20% * Pabs = 0 .. 3 J/s
bifi PV, Konstanz, 23.04.2012 A. Herguth, S. Riegel
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 Electrical output Pel 0 .. 3 J/s (>11 J/s thermalized) 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
bifi PV, Konstanz, 23.04.2012 A. Herguth, S. Riegel
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 Electrical output Pel 0 .. 3 J/s (>11 J/s thermalized) Heat capacity C 5 J/K 500 J/K Initial heating rate dT/dt 3 K/s 0.03 K/s
Initial heating rate:
dT/dt = Pabs/C
bifi PV, Konstanz, 23.04.2012 A. Herguth, S. Riegel
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 Electrical output Pel 0 .. 3 J/s (>11 J/s thermalized) Heat capacity C 5 J/K 500 J/K 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
bifi PV, Konstanz, 23.04.2012 A. Herguth, S. Riegel
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
bifi PV, Konstanz, 23.04.2012 A. Herguth, S. Riegel
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
bifi PV, Konstanz, 23.04.2012 A. Herguth, S. Riegel
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
bifi PV, Konstanz, 23.04.2012 A. Herguth, S. Riegel
The Thermal Issue
Voc decays with temperature
KmVoc
dTdV 1.2..7.1−≈Measure Voc at or correct it to 25°C
…but…
What about the FF when temperature
increases during the measurement?
bifi PV, Konstanz, 23.04.2012 A. Herguth, S. Riegel
The Thermal Issue
Voc decays with temperature
KmVoc
dTdV 1.2..7.1−≈
scoc
oc jdTdTdVV
pdTFF⋅
⋅+
≈ 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
bifi PV, Konstanz, 23.04.2012 A. Herguth, S. Riegel
The Thermal Issue
Uncontrolled heating can lead
to an overestimation of FF
Voc decays with temperature
KmVoc
dTdV 1.2..7.1−≈
scoc
oc jdTdTdVV
pdTFF⋅
⋅+
≈ 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
bifi PV, Konstanz, 23.04.2012 A. Herguth, S. Riegel
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?
bifi PV, Konstanz, 23.04.2012 A. Herguth, S. Riegel
• 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
bifi PV, Konstanz, 23.04.2012 A. Herguth, S. Riegel
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.
The Electrical Issue
bifi PV, Konstanz, 23.04.2012 A. Herguth, S. Riegel
The Electrical Issue
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
bifi PV, Konstanz, 23.04.2012 A. Herguth, S. Riegel
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
top related