Silicon Solar Cells with Front Hetero-contact and Aluminum Alloy 

Back Junction

Hao-Chih Yuan, Matthew R. Page, Eugene Iwaniczko, Yueqin Xu, Lorenzo Roybal, Qi Wang, Howard M. Branz, and Daniel L. Meier*

National Renewable Energy Laboratory • Golden, CO 80401 * Present address: Suniva Inc. • Atlanta, GA 30308

I. Objective: 

 To apply industrial back Al process in efficient n-wafer cells 

― with a-Si:H front surface passivation

 To evaluate the surface recombination velocity (SRV) of the a-Si:H passivated front surface with different surface preparation procedures

The information contained in this poster is subject to a government license.

Presented at the 33rd IEEE Photovoltaic Specialists Conference • San Diego, CA • May 11–16, 2008 • NREL/PO-520-43321

II. Experiment: 

n-Si (100) FZ, 

1.2 Ω-cm, 300 μm 

RCA clean

Evaporate 3-μm Al on backside 

900oC Al-Si alloy in tube furnace

Remove front oxide in DHF

Deposit front i/n-type a-Si:H

contact layers by HWCVD

Evaporate 93-nm ITO

on front

Evaporate front grid metals

through shadow mask

Dice cells into 1×1 cm2 area

Leave chemical

oxide intact

Re-grow 76-nm

thermal oxide

Spin SOG on

front surface

and curing

Measure Suns-Voc

Measure QE, LIV

Schematic cell cross section

. . . . . .

intrinsic a-Si:H

n-type a-Si:H

93-nm ITO

300-μm n-type Si (100) FZ,

resistivity= 1.2 Ω-cm

p+-Si (Al-Si alloy)

Al back contact

Ti/Pd/Ag/Pd front 

metal grids

10 nm

III. Results: Certified IV on present status 

Process flow

Port to

pump

Heater &

substrate

W or Ta filament 

at 1800~2000o

C

Gas inlet

Filament 

feedthrough

Shutter

Schematic of HWCVD system

Summary of Suns-Voc and IQE

Front surface 

preparation

Leave chemical

oxide intact

Re-grow 76-nm

thermal oxide

SOG on front

Suns-Voc (mV) 611 617 548

IQE at 700 nm (%) 77.7 83.4 ―

0

10

20

30

40

50

60

70

80

90

300 400 500 600 700 800 900 1000 1100 1200 1300

Wavelength (nm)

R, EQE, IQE (%)

PC1D simulated IQE

45

50

55

60

65

70

75

80

85

90

95

1 10 100 1000

Front SRV (cm/sec)

IQE at 700 nm (%)

Thermal oxide 

preparation

Chemical oxide 

preparation

L= 700 μm

L= 300 μm

 Thermal oxide is most effective for preserving the cleanliness

of the c-Si surface through the high-temperature Al-Si alloying.

Three surface preparation procedures were studied to evaluate the effectiveness of preserving 

the clean Si surface until a-Si:H deposition.

Simulation suggests finished cells have

 diffusion length better than 450 μm

 front SRV of thermal oxide prepared surface between 10-50 cm/s

 front SRV of chemical oxide prepared surface twice as high

 Voc= 604.7 mV

 Jsc

= 27.9 mA/cm2

IV. Summary: 

 An alternative cell structure that incorporates front a-Si:H heterocontact and Al-Si alloy back junction is demonstrated for the first time.

 Thermal oxide effectively preserves the cleanliness of the c-Si

through the high-temperature Al-Si alloying for a-Si:H passivation.

 Front SRV of the a-Si:H passivated surface prepared by

thermal oxide is between 10-50 cm/s and is close to that

on SHJ cell fabricated at low temperature.

 13.5% conversion efficiency is demonstrated on planar cell

with single-layer anti-reflection coating.

EQE

IQE

R

 FF= 79.9%

 η= 13.5%