L. Kreinin, N. Bordin, N. Eisenberg

bSolar, 21 Havaad Haleumi Street, Jerusalem 91160, Israel

P. Grabitz, S. Hasenauer, D. Obhof, G. Wahl, G. Wein, D. Zimmerling

bSolar GmbH,Theresienstraße 2, 74072 Heilbronn, Germany

1. INTRODUCTION. Parameter requirements and their implementation

2. EXPERIMENTAL� SR as a tool of recombination parameters � SR as a tool of recombination parameters

evaluation� Accuracy of SR determination 3. RESULTS AND DISCUSSION� Main results of industrial fabrication of bifacial cells� Limiting factors and improvements possibilities4. CONCLUSIONS

Electro-physical parameters:

�High bulk diffusion length L >>d (solar cell thickness)

Low effective back surface recombination, S�Low effective back surface recombination, Seff

�High potential of back high-low barrier

Optical parameters:

�Effective light trapping in photoactive wavelength region

�Light rejection in non photoactive wavelength region

� Use Si with high starting lifetime τ� Retain τ during cell fabrication� High doping at high-low junction formation� Proper pyramid texturized front with retained flat

back� Minimal contact shielding

Bulk L and S eff determination based on back IQE:Using the approximate equations, assuming zero

thickness of the back doped layer

�QE data were corrected by calculation and verified experimentally

�Chuck reflection effect ∆Jsc /Jsc evaluated by integration over sun spectrum of EQE measured using golden and black chucks is ~ 0.03 %

�Bifacial over mono facial cell IQE improvement is due to :- better light trapping- lower back surface recombination

� Short wavelength response of industrially fabricated bifacial cells is high similar to the response of mono facial cells� High photo response in long wavelength region results in short circuit current increase ∆Isc of ~0.4 mA/cm2short circuit current increase ∆Isc of ~0.4 mA/cm2

�Substitution of alloyed Al on back by B BSF leads to increase of Internal cell back reflection Rin b from ~ 0.6 to 0.76±0.05 �High recombination parameters of the cell (bulk lifetime and effective back surface recombination) contribute significantly to high Isc of the bifacial cell

Light trapping and recombination improvementsRin b = 60 % Rin b = 76 ± 5 %

Al solubility in Si ~3.1018 cm-3 B solubility in Si > 1020 cm-3

Seff = 400 ÷ 1000 cm/s Seff = 35÷ 110 cm/s

There is high response in long wavelength despite different type of back treatment – from flat to slightly textured

� Fabrication processing retains bulk lifetime τb in the range 0.2 – 1 ms� BSF provides effective surface recombination Seff in the range 35 – 110 cm/s� Back recombination limits solar cell open circuit voltage to 640 – 660 mV� Back to front short circuit current ratio (Isc b/Isc f) = 74 ÷ 79 %� Back antireflective coating optimization should result in (Isc b/Isc f) = 84 %

ExpectedAverageRangeParameter

3837.737.5-38Isc, mA/cm2

635621616-628Voc, mV

7978.578-79FF, %

19.118.318.0-18.7Eff (front)

807574-79Isc b/Isc f, %

EQUIVALENT EFFICIENCY is determined as EFFICIENCY OF MONOFACIAL CELL needed for generating same energy as bifacial cell under the given conditions

Equivalent efficiency, %Back contribution (energy gain), %

Front efficiency, %

20.3510 (field installation with low albedo)

18.5

22.220(field or roof installation with intermediate albedo)

18.5

23.125 (field or roof installation with high albedo)

18.5

�n+-p-p+ is promising structure for industrially produced terrestrial bifacial cells� Pilot batches demonstrate retained bulk lifetime τb and provide effective back surface recombination in the range 35 – 110 cm/s�Low Seff in combination with effective light trapping contribute to increased long wavelength response, full photo generated current increased long wavelength response, full photo generated current (by ~ 0.4 mA/cm2) and efficiency (by ~0.5 % abs).�Back to front short circuit current ratio is in the range 74 – 79 %�Equivalent efficiency as a quality criteria of bifacial cell achieved 22 - 23 % in the most typical applications.�Improvement cell efficiency above ~19 % is expected after tuning existing industrial fabrication processes