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


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?


Measurement Setup


Massive brass chuck







• Choice of pins?


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





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





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





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





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


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

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?


The Thermal Issue


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


The Thermal Issue

Uncontrolled heating can lead

to an overestimation of FF


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


Measurement Setup


Massive brass chuck







• Choice of pins?


• 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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