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

Outline Outline

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 surroundingsurfaces, 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 Coveri ngs, 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

Outline Outline






Summary





7

Bifacial n-type solar cells Bifacial 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 typearea

[ cm 2 ]

JSC

[ mA/cm 2 ]

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 1.20

102030405060708090

100

Monofacial optimized cell : front illumination rear illumination

Bifacial optimized cell : front illumination rear illumination

IQE

[%]

Wavelength [µm]

Cells with symmetric front & rearperformance can be fabricated !!

12

Outline Outline






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.


0.80.91.01.11.21.3

max

(1su

n, in

door

)

ZEBRA, transparent ZEBRA, black albedo module (Isc/Isc @1sun) Light intensity (suns)

bifacial versus monofacial module performance:

40

45

albe

do c

ontr

ibut

ion

(%)

morning & afternoon



07:0008:00

09:0010:00

11:0012:00

13:0014:00

15:0016:00

17:0018:00

0.00.10.20.30.40.50.60.70.8

Pm

ax/P

max

(1su

n, in

door

)

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 1.25

10

15

20

25

30

35

40

albe

do c

ontr

ibut

ion

(%)

Outdoor light intensity (suns)

noon

More sunlight is reflected to the back side in the morning and afternoon !


0.60.70.80.91.01.11.2

Zebra, transparent Zebra, black Linear fit

max

(1su

n)

bifacial versus monofacial module performance:

242832

net p

ower

gai

n (%

)

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 1.20.00.10.20.30.40.50.6

Standard, transparent Standard, black Linear fit

Pm

ax/P

max

(1su

n)

Outdoor light intensity (suns)0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2048

12162024

bifacial ZEBRA bifacial Standard albedo contribution (%)

net p

ower

gai

n (%

)

Outdoor light intensity (suns)

Standard n-type bifacial module performed better due to higher ηrear/ ηfront ratio!

One cell module: bifacial performance

282930



0.0 0.1 0.2 0.3 0.4 0.5202122232425262728

effic

ienc

y (%

)

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. Bothsolar cell concepts have a good bifacial characteristics.

SummarySummary

• we developed processes to fabricated n-type solar cells on 156×156mm2 (industry-standard) using exclusively process steps that arealready used in mass production for conventional cells.


• Outdoor measurements on small bifacial modules demonstrate at least12% gain in power (over a day) for a slanted installation for both cellconcepts. Cell efficiencies (combined front and back) over 23% abs canbe achieved on our IBC (ZEBRA) cells.

Thank you for your attention!

05 mihailetchi isc konstanz

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