05 mihailetchi isc konstanz

Bifacial n-type crystalline silicon solar cellsBifacial n-type crystalline silicon solar cells
bifiPV workshop, Konstanz, April 2012
Valentin D. Mihailetchi, A. Halm, A. Edler, G. Galbiati, L. J. Koduvelikulathu, R.
Roescu, C. Comparotto, R. Kopecek, K. Peter,
International Solar energy research Center (ISC), Konstanz (Germany)
1

OutlineOutline
Motivation
• why bifacial cells/modules?
• installation possibilities of bifacial modules
Bifacial n-type solar cells
• front and back contacted cells
• all back contacted cells (IBC)
Summary
• our best cell results
Preliminary tests of several small modules
• standard cell versus back contact cell
• indoor and outdoor tests
2

MotivationMotivation
Why bifacial cells & modules?
Advantages:
• increase power generation per m2
• enables owners to generate more kWh per rated KWp
A bifacial module generates electricity from the light reflected off surrounding
surfaces, and combines with power from the front face of the module.
• enables owners to generate more kWh per rated KWp
• …
Disadvantages:
• more expensive fabrication !?
• power output depends on installation site and conditions
• difficult to predict the gain due to bifacial contribution
• …
with bifacial (€/kWh) < without bifacial (€/kWh) ???
3

Installation possibilities of bifacial modules
Reflected
sunlight
Direct
sunlight
Slanted
Direct
sunlight
Horizontal
Direct
sunlight
Direct
sunlight
Vertical
sunlight
Reflected
sunlight
Reflected
sunlight
Reflected
sunlight
Balconies, Bus Shelters, Walkway Covers, Deck & Porch Coverings, Canopies, Carports,
Fences, Facades, Siding, Trellises, Tracking Systems, BIPV, Ground Mount, Roof Mount,
and more…!
Bifacial modules can be installed on:
4
… actually there are much fewer places where installation of bifacial modules is not beneficial !

Cell fabrication and module installation
Reflected
sunlight
Direct
sunlight
Slanted
Direct
sunlight
Horizontal
Direct
light
Reflected
light>
Cells (or modules) fabrication:
• can have asymmetric front/rear performance
• should be optimized based on front side performance
sunlight
Reflected
sunlight
light light
Preferred orientation:
• South for slanted
5

Direct
sunlight
Direct
sunlight
Vertical
Cell fabrication and module installation
Front
sunlight
Rear
sunlight=if
Reflected
sunlight
Reflected
sunlight
Cells (or modules) fabrication:
• could have symmetric front/rear performance
Preferred orientation:
• East – West
6

OutlineOutline
Motivation
Summary
7

Bifacial n-type solar cellsBifacial n-type solar cells
N-type cell process development at ISC-Konstanz
IBC solar cells (ZEBRA)
• n-type Cz substrate • n-type Cz substrate
Standard n-type solar cells
• n-type Cz substrate
• 156×156 mm2 area
• homogeneous diffusion
• front side textured, rear side flat
• industrial partner:
• n-type Cz substrate
• 156×156 mm2 area
• homogeneous diffusion
• front side textured, rear side flat or
textured
• industrial partner:
8

Standard n-type solar cells IBC solar cells (ZEBRA)
Saw damage etching, alkaline texturization, cleaning steps
BBr3 diffusion, p+ emitter
POCl3 diffusion, n+ BSF
SiO2/SiNx passivation & ARC
Screen printing and firing of p and n fingers, busbars
PECVD masking
Laser ablation
9

IBC solar cells (ZEBRA)Standard n-type solar cells
Both cell concepts are bifacial !!!
10

Best cell results (monofacial)
Cell type
area
[ cm2 ]
JSC
[ mA/cm2 ]
VOC
[ mV ]
FF
[ % ]
ηηηη
[ % ]
Pmax
[W]
Standard (both sides contacted) 241 38.8 643 77.6 19.4 4.66
ZEBRA (back contacted) 238.9 41.2 646 79.1 21.0 5.03
Highlights:
• 156×156 mm2 Cz substrates!
• pseudo-FF ≥ 82% → no indication of shunts!
• implied Voc ≥ 670 mV → good surface passivation!
What is the bifacial performance?
11

Cell type optimized for best: ηηηηfront (%) ηηηηrear / ηηηηfront
Standard monofacial 19.4 0.75-0.8
Standard bifacial* 18.0** 0.91***
* compared with monofacial cell, it has larger rear side metal finger pitch and higher BSF sheet resistance
** FF of ≤ 72% due to higer series resistance
*** ratio different than 1 because of a flat rear side (different Jsc values)
Optimization route for best bifacial performance
100
0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2
0
10
20
30
40
50
60
70
80
90
100
Monofacial optimized cell:
front illumination
rear illumination
Bifacial optimized cell:
front illumination
rear illumination
IQE[%]
Wavelength [µm]
Cells with symmetric front & rear
performance can be fabricated !!
12

OutlineOutline
Motivation
Summary
13

Preliminary tests of several small modulesPreliminary tests of several small modules
One cell module: for bifacial test
Back sheet variation: black, white, transparent
Indoor measurements (as reference): @ 1 sun and 25°C
bifiPV workshop, Konstanz, April 2012 14
module type back sheet ηηηηrear / ηηηηfront (module) ηηηηrear / ηηηηfront (cell)
Standard transparent 0.85 0.79
ZEBRA transparent 0.75 not measured
• front versus rear performance ratios

One cell module: for bifacial test
Back sheet variation: black, white, transparent
Indoor measurements (as reference): @ 1 sun and 25°C
• white versus black back sheet
module type back sheet gain in Isc
Standard white vs. black +0.80%
ZEBRA white vs. black +0.81%

One cell module: outdoor measurements
test condition and assumptions:
• ground reflectivity → ≈ 70% (300-1200nm) (white back sheet)
• orientation and inclination → South, 30°
• measurement interval → every 5 min. (10 sec. between each module scan)
• all modules assumed to have the same temperature during an IV scan
• front glass taped black outside cell area
• front glass taped black outside cell area
• one additional module was used to measured the back side light intensity
Photograph of the outdoor setup.

0.8
0.9
1.0
1.1
1.2
1.3
max(1sun,indoor)
ZEBRA, transparent
ZEBRA, black
albedo module (Isc/Isc @1sun)
Light intensity (suns)
bifacial versus monofacial module performance:
40
45
albedocontribution(%)
morning &
afternoon
07:00
08:00
09:00
10:00
11:00
12:00
13:00
14:00
15:00
16:00
17:00
18:00
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Pmax
/Pmax(1sun,indoor)
Time (HH:mm)
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2
5
10
15
20
25
30
35
40
albedocontribution(%)
Outdoor light intensity (suns)
noon
More sunlight is reflected to the back side in the morning and afternoon !

0.6
0.7
0.8
0.9
1.0
1.1
1.2
Zebra, transparent
Zebra, black
Linear fit
max(1sun)
bifacial versus monofacial module performance:
24
28
32
netpowergain(%)
morning &
afternoon
noon
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Standard, transparent
Standard, black
Linear fit
Pmax
/Pmax(1sun)
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2
0
4
8
12
16
20
24
bifacial ZEBRA
bifacial Standard
albedo contribution (%)
netpowergain(%)
Standard n-type bifacial module performed better due to higher ηrear/ ηfront ratio!

One cell module: bifacial performance
28
29
30
0.0 0.1 0.2 0.3 0.4 0.5
20
21
22
23
24
25
26
27
28
efficiency(%)
Rear side light intensity (suns)
estimated best cell efficiency
(combined front and rear)

• we achived 21% conversion efficiency on back-contacted (IBC)
cells and 19.4% on standard (both sided metalized) solar cells. Both
solar cell concepts have a good bifacial characteristics.
SummarySummary
• we developed processes to fabricated n-type solar cells on 156×156
mm2 (industry-standard) using exclusively process steps that are
already used in mass production for conventional cells.
• Outdoor measurements on small bifacial modules demonstrate at least
12% gain in power (over a day) for a slanted installation for both cell
concepts. Cell efficiencies (combined front and back) over 23%abs can
be achieved on our IBC (ZEBRA) cells.
Thank you for your attention!

05 mihailetchi isc konstanz

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