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Submitted on 1 Jan 1982
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SIGNIFICANCE OF PASSIVATION TECHNIQUES ON SEMICRYSTALLINE SILICON FOR THE IMPROVEMENT OF SOLAR CELLS EFFICIENCY
L. Sardi, S. Pidatella, G. Figari
To cite this version:
L. Sardi, S. Pidatella, G. Figari. SIGNIFICANCE OF PASSIVATION TECHNIQUES ON SEMICRYSTALLINE SILICON FOR THE IMPROVEMENT OF SOLAR CELLS EFFICIENCY.
Journal de Physique Colloques, 1982, 43 (C1), pp.C1-125-C1-128. �10.1051/jphyscol:1982117�. �jpa-
00221773�
SIGNIFICANCE OF PASSIVATION TECHNIQUES ON SEMICRYSTALLINE SILICON FOR THE IMPROVEMENT OF SOLAR CELLS EFFICIENCY
L. Sardi, S. Pidatella and G. Figari
ANSALDO S.p. A. Genoa, University o f Cenoa, I t a l y
Rdsumd - La diffusion de llhydrogCne moldculaire et la diffusion du phosphore en 2 Btapes amdliorent considdrablement le rendement des cellules solaires se- micristallines. Nous prssentons une etude de ces techniques de passivation,
rbalisde, sur le silicium Wacker SILSO
;Les mesures de resistance distribude, OCVD, EBIC et de rendement donnent pour les 2 techniques les rdsultats suivants
:i) Un accroissement de la dur6e de vie des porteurs minoritaires ii) fin accroissement du rendement (10%).
L'analyse de bore montre qu'apr2s diffusion de l'hydrogene moldculaire, la valeur theorique de la rdsistance est approche'e, alors qu'avant le traitement, une rdsistance beaucoup plus grande Qtait trouvee.
Abstract - Molecular hydrogen diffusion and "two steps phosphorus" diffusion enhance considerably the efficiency of semicrystalline solar cells. We present in this paper the results of a systematic investiga-ion of bot) these passi- vation tbchniques performed on Wacker, SILSO material.
Spreading resistance, OCVD, EBIC and efficiency measurements were carried out and showed that on both the techniques
:i) a lifetime increase is observed
ii) an enhancement of the solar cells efficiency of about 10% on hydrogenated samples.
Boron analysis shows the raw material resistivity to be far from its theore- tical value. The expected value is approached as a results of hydrogen diffu- sion.
A preliminary discussion of these results is presented.
1. Introduction - Semicrystalline silicon is going to become the most econc mica1 and most widely used material for solar cell production.
Semicrystalline silicon solar cell efficiencies have not yet reached the values obtained from crystalline silicon cells. This is mainly due to high recombination near the grain boundaries.
Several grain boundary passivation techniques have been developed in order to improve semicrystalline solar cell efficiencies. Two of the most promising techniques are: "Two steps diffusion" and hydrogen (both atomic and molecular) diffusion. These techniques are investigated in this paper through an experi- mental approach.
O C M lifetime and efficiency measurements were performed on two-step diffused samples.
Spreading resistance, OCVD lifetime, EBIC and efficiency measurements were performed on hydrogen-diffused samples.
The results of this investigation lead to an improvement of the efficiency of about 10%.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982117
JOURNAL DE PHYSIQUE
Two step diffusion process - This process is performed by a low temperature (700-800OC) diffusion for several hours and a second diffusion at 9 W ° C for a few minutes. According to theory, the phosphorous diffusion coefficient at low temperature along the grain boundary is greater than in the bulk.
Therefore the enclosure of the grain is obtained in the first diffusion, and the junction is made in the second.
Tests were performed on 6 groups of 20 samples each. Starting material was Wacker Semicrystalline silicon slices. SILSO measuring 10x10 cm, of resisti- vity 3-5 - cm.
Each slice was cut into four samples of dimensions 5x5 cm. The four samples were then given different diffusion times at the same temperature, as follows:
a) single step diffusion at 950°C only b) 7W°C for 6 hours
c) 750°C for 9 hours d) 750°C for 16 hours
A
previous work (1) indicated that 750°C is the best temperature for the first diffusion step.
The results obtained are shown in Table 1.
These results indicate that 750°C/9 hours are best conditions for the first diffusion.
The possibile explanations for these results obtained:
a) the grain boundary enclosure by a junction decreases the minoritary carrier recombination and increases the short circuit current.
b)getter action of phosphorous takes place.
The two mechanism are indistinguishable through the measurements performed.
Hydrogen Diffusion Process - It is already established that both atomic and molecular hydrogen diffusion considerably enhance the efficiency of semicrystal line solar cells.
Molecular hydrogen diffusion at low temperature has been analyzed.
Starting material was Wacker SILSO 10x10 cm slices. These slices were cut into four samples measuring 5x5 cm. The four samples were then given different diffusion temperatures.
I0 Step process Efficiency Open circuit Short circuit
rZ voltage V OC current I SC
Reference .98 565 650
750°C/64 10.0 565 670
75OoC/94 11.5 565 730
750°C/164 11. 565 700
Table 1 - Average values obtained "two step phosphorus" diffused samples.
d) 700°C for lh
Spreading resistance, OCVD lifetime, efficiency and EBIC measurements were per- formed on the treated samples.
A
spreading resistance measurement was performed on the same sample following the same scansion line before and after the hydrogen diffusion process.
Differences of about 30% were observed.
This leads to the results showed in Table 2.
Table 2 - Resistivity (Acm)
before hydrogen after hydrogen H diffusion diffusion diffusion 2
2.65 1.59 T
=500°C; t
=lh
2.72 1.91 T
=600°C; t
=lh
2.29 1.56 T = 700°C; t
=lh
I
The measurements were performed on the same sample before and after hydrogen diffusion process.
Chemical analysis shows
4parts per milion of boron in the raw silicon.
This is equivalent to a resistivity of about l n c m , but measurements of the resi stivity yeld 3ncm.
After hydrogenation this resistivity value approaches the expected value.
OCVD lifetime measurements show a significant improvement of about
40%(Table
3)EBIC measurements performed near the grain bandariesshow that the variation of lifetime is due only to the enhancement near the grain boundary.
Solar cells have been realized with this material and show a significant impro- vement of efficiency (Table 4).
Table 3 - Lifetime
Reference 1
T
=W ° C ; t
=1 h 1.4 T
=600°C; t
=lh 1.42 T
=700°C; t
=lh 1.3
Lifetime measurements (normalized to reference sample) Reference- lifetime
:1.1 usec.
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