Multicrystalline Silicon Solar Cells Exceeding 20% Efficiency
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PROGRESS IN PHOTOVOLTAICS: RESEARCH AND APPLICATIONS
Prog. Photovolt: Res. Appl. 2004; 12:553–558
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/pip.583
Research SHORT COMMUNICATION: ACCELERATED PUBLICATION
Multicrystalline Silicon Solar
Cells Exceeding 20% Efficiency
O. Schultz*,y , S. W. Glunz and G. P. Willeke
Fraunhofer Institute for Solar Energy Systems, Heidenhofstr. 2, D-79110 Freiburg, Germany
This paper presents the first conversion efficiency above 20% for a multicrystalline
silicon solar cell. The application of wet oxidation for rear surface passivation
significantly reduces the process temperature and therefore prevents the degradation
of minority-carrier lifetime. The excellent optical properties of the dielectrically pas-
sivated rear surface in combination with a plasma textured front surface result in a
superior light trapping and allow the use of substrates below 100 m thickness. A
simplified process scheme with laser-fired rear contacts leads to conversion efficien-
cies of 203% for multicrystalline and 212% for monocrystalline silicon solar cells
on small device areas (1 cm2). Copyright # 2004 John Wiley & Sons, Ltd.
key words: high-efficiency; multicrystalline silicon; crystalline silicon solar cells; silicon oxide
INTRODUCTION
T
he electronic quality of multicrystalline silicon (mc-Si) has improved significantly in recent years due
to a substantial reduction of impurity contamination and advanced gettering and bulk passivation tech-
niques. In consequence, the minority-carrier diffusion length easily exceeds wafer thickness, a fact
which is intensified by the general move to thinner substrates. This creates the need for a solar cell design with
excellent light confinement and high-quality surface passivation. Phosphorus diffused emitters with a sheet
resistance of around 100 /square contribute only little to recombination and therefore rear surface passivation
becomes increasingly important. Efficiencies of mc-Si solar cells exceeding 18% with aluminium back surface
field1 or silicon nitride2 have been reported, but the best result so far of 198% has been achieved by an oxide-
passivated rear surface with locally defined contacts.3 This superior performance of oxides, together with addi-
tional restrictions arising from the other techniques mentioned above (i.e., limited passivation quality of alu-
minium back surface field and shunting of the field effect for silicon nitride4) make thermal oxidation a
promising technology for the production of high-efficiency mc-Si solar cells. The major drawback so far has
been the significant reduction of minority-carrier lifetime in multicrystalline silicon during high-temperature
oxidation at more than 1000 C.5,6 This thermal degradation can be prevented by lowering the process tempera-
ture to 800 C during wet oxidation of the rear surface. The rear contact pattern has to consist of closely spaced
contacts since the grain boundaries in mc-Si decrease lateral conductance. The contacts were fired through the
oxide by a laser (LFC)7 and provide a local back surface field.8
*Correspondence to: O. Schultz, Fraunhofer Institute for Solar Energy Systems, Heidenhofstr. 2, D-79110 Freiburg, Germany.
y
E-mail: oliver.schultz@ise.fraunhofer.de
Received 28 July 2004
Copyright # 2004 John Wiley & Sons, Ltd. Revised 4 August 2004554 O. SCHULTZ, S. W. GLUNZ AND G. P. WILLEKE
EXPERIMENTAL
The major task when developing a high-efficiency process for multicrystalline silicon is the prevention of
carrier lifetime degradation which is caused by very high processing temperatures. This can for example be
achieved by the replacement of high-temperature processes such as rear surface boron diffusion by the laser-
fired contacts, the use of a photoresist mask for texturing instead of a masking oxide and the application of a wet
oxidation in steam ambience, where the oxide growth rate is dramatically enhanced and therefore the tempera-
ture may be lowered. Figure 1(a) shows the minority-carrier lifetime of consecutive wafers, which were taken
from a batch of multicrystalline silicon solar cells. The wafers were processed in a standard process sequence
developed for monocrystalline silicon. The oxide and all diffused layers were etched off and a well-passivating
silicon nitride (S ffi 10 cm/s) was deposited on both surfaces, hence the effective lifetime represents the bulk
lifetime. The measurements were performed with the QSSPC technique on a spot diameter of approximately
Figure 1. (a) In a standard high-efficiency process sequence with many high-temperature steps the bulk lifetime of
multicrystalline silicon is severely reduced. (b) In a process sequence with reduced temperature, which is adjusted to
multicrystalline silicon, the bulk lifetime can even be increased
Copyright # 2004 John Wiley & Sons, Ltd. Prog. Photovolt: Res. Appl. 2004; 12:553–558MULTICRYSTALLINE SILICON SOLAR CELLS EXCEEDING 20% EFFICIENCY 555
37 cm at an excess carrier density of n ¼ 1015cm3 to avoid trapping artefacts. The material used was
15 cm solar grade mc-Si produced by ScanWafer. High-temperature oxidations at 1050 C severely damage
the bulk lifetime and only the emitter diffusion is beneficial for the multicrystalline material. In contrast the new
process sequence optimised for multicrystalline silicon does not degrade the material quality (Figure 1b). The
carrier lifetime of consecutive wafers is even enhanced during processing. The reason for the superior carrier
lifetime after the modified process sequence is the lowering of the temperature from 1050 to 800 C during
oxidation. This has been investigated by consecutive wafers where the bulk lifetime after oxidation is severely
deteriorated after the dry oxidation at 1050 C whereas hardly any change could be observed for the wet oxida-
tion at 800 C (Figure 2).
Another requirement is a very good passivation quality of the wet oxide on mc-Si. This parameter was
determined by measuring the effective minority carrier lifetime. For the calculation of the surface recombination
velocity S, the bulk lifetime has to be known, a parameter which could in principle be extracted from samples the
surfaces of which have been passivated by other techniques. However, a possible detrimental effect of the oxida-
tion process on bulk lifetime would lead to falsified results. Thus we followed a purely experimental approach. A
set of neighbouring mc-Si wafers with varying thickness and identical surfaces was fabricated. Consequently it
was possible to determine the bulk lifetime and the surface recombination velocity independently. Therefore
pregettered solar grade mc-Si wafers of 15 cm base resistivity were ground by a solely mechanical process,9
and subsequently we etched off some micrometres of surface damage in hot 40% KOH. After an RCA cleaning
procedure, the wafers were oxidised in steam ambience for 35 h, resulting in an oxide thickness of 105 nm on
oriented silicon. After a forming gas anneal for 60 min at 425 C, the effective minority carrier lifetime
was measured at an excess carrier density of n ¼ 1015 cm3 in order to avoid trapping effects. Since the wafer
thickness varied between 42 and 192 mm, the generation of free carriers within the sample is thickness-dependent
and was calculated by the raytracing program SUNRAYS.10 With the equation
1 1 2S
¼ þ
eff b W
and a plot of the inverse effective lifetime against the inverse wafer thickness as shown in Figure 3(a), the
evaluation of the data leads to an extracted bulk lifetime b ¼ 41 ms (15 ms) and a surface recombination velo-
city of S ¼ 353 cm/s (32 cm/s).
Stocks and Cuevas performed a very similar experiment11 and published values of S ¼ 125 cm/s and
b ¼ 280 ms. The deviation might arise from the different surface treatment (they used an acidic surface etchant
and thus had a smoother surface) and a different supplier of mc-Si.
In order to draw a comparison, the passivation quality of the wet oxide was measured on 1 cm FZ-Si after
performing different process steps (Figure 3b). Directly after oxidation the surface passivation is rather poor, but
Figure 2. Oxidation at 1050 C deteriorates bulk lifetime considerably whereas a processing temperature of 800 C is not
harmful to multicrystalline silicon
Copyright # 2004 John Wiley & Sons, Ltd. Prog. Photovolt: Res. Appl. 2004; 12:553–558556 O. SCHULTZ, S. W. GLUNZ AND G. P. WILLEKE
Figure 3. (a) Plot of inverse effective lifetime against inverse wafer thickness at n ¼ 1015 cm3. The surface recombination
velocity S can be determined by the slope whereas the bulk lifetime can be evaluated from the y-axis intercept. (b) Effective
lifetimes of a 250-mm-thick 1 cm FZ wafer after different processing steps. Under the assumption of a high bulk lifetime of
b ¼ 1800 ms, this yields values of S ¼ 2500 cm/s directly after oxidation, S ¼ 320 cm/s after a forming gas anneal and
S ¼ 65 cm/s after an aluminium anneal. The latter condition corresponds to the applied solar cell process
a simple annealing in forming gas at 425 C for 60 min improves the effective minority-carrier lifetime signifi-
cantly.
Since the bulk lifetime of this high-quality silicon is not affected by the oxidation, we can calculate the upper
limit for S at n ¼ 1015 cm3 with the assumption that b is close to the intrinsic limit of about 1800 ms. The
result is S ¼ 320 cm/s at n ¼ 1015 cm3. This is in very good agreement with the data obtained for mc-Si. For
the application as rear surface passivation, it is important to determine the final value after an aluminium anneal-
ing as it takes place in the LFC process. Therefore 1 mm of aluminium was evaporated on both sides, and after a
forming gas anneal for 25 min at 425 C, the aluminium was etched off and the lifetime measured again.
S decreased further to 65 cm/s, a value which is low enough as not to limit solar cell performance anymore. This
experiment has not been carried out with the thin mc-Si wafers, but the results are expected to be transferable.
CELL STRUCTURE
For further investigations of the passivation quality, the maintenance of high bulk lifetime and the full applic-
ability in a simple high-efficiency process, we prepared solar cells with the LFC7 structure (Figure 4). The rear
passivation is provided by a wet oxide which simultaneously serves as diffusion barrier during emitter forma-
tion. The front surface is textured by a masked plasma process12 and the single-step phosphorus emitter of about
120 /square is contacted with a photolithographically defined metal grid of Ti/Pd/Ag which has been thick-
ened by electroplating. Different front designs have been applied with the metallisation fraction varying
between 3 and 5%. This corresponds to 11–13 fingers of about 15–20 mm width for a 1 cm2 cell. All metallisa-
tion, including the busbar, is within the active cell area. Front surface passivation was achieved by a thin thermal
oxide of about 10 nm grown in a dry ambient at 840 C for 40 min and reflection is minimised by a double-layer
anti-reflection coating (TiO2/MgF2).
SOLAR CELL RESULTS
Results of solar cells on FZ-Si and high-quality mc-Si are shown in Table I. The size of the cells is 1 cm2
aperture area. Excellent values of 197% conversion efficiency for 17 cm material of ScanWafer and
199% (218-mm-thick wafer) and 203% (99-mm-thick wafer) for 06 cm material produced by Kawasaki
Steel are achieved and the latter represent the best values published for mc-Si so far.13 The distribution of cell
Copyright # 2004 John Wiley & Sons, Ltd. Prog. Photovolt: Res. Appl. 2004; 12:553–558MULTICRYSTALLINE SILICON SOLAR CELLS EXCEEDING 20% EFFICIENCY 557
Figure 4. Schematic drawing of the solar cell design with plasma textured front surface and wet oxide/LFC rear
Table I. Results of solar cells made from FZ and mc-Si measured under standard testing conditions (25 C, 1000 W/m2,
AM 15 global). The size of the cells is 1 cm2 aperture area
Material W (mm) Voc (mV) Jsc (mA/cm2) Fill factor (%) Efficiency (%)
FZ, 1 cm 330 660 398 807 212
mc-Si, 17 cm 218 649 385 788 197*
mc-Si, 06 cm 218 653 388 786 199*
mc-Si, 06 cm 99 664 377 809 203**
*Independently measured at Fraunhofer ISE CalLab.
**Independently measured at NREL.
Figure 5. Distribution of cell efficiencies of 60 cells from two wafers of the same brick made by Kawasaki Steel
efficiency of 60 cells from two 99-mm-thick wafers of the same brick are shown in Figure 5. The mean efficiency
of 181% is among the highest obtained so far for mc-Si, 16 cells exceed 19% and 4 cells the 20% efficiency
limit. The process was especially designed for mc-Si and consists of a limited number of high-temperature
processes only. Furthermore only two photolithographic steps (texturing and front metallisation) have been
applied, but the cell structure’s efficiency limit is beyond 21% as can be seen from the cell results on FZ mate-
rial. This means that the cell efficiency for mc-Si is still mainly determined by bulk lifetime which makes the
cell design a perfect tool for further investigations on multicrystalline silicon for photovoltaic applications.
CONCLUSION
This paper presents a relatively simple high-efficiency process adjusted to the low temperatures which are
required by multicrystalline silicon. However, the cell structure still proves able to manufacture multicrystalline
silicon solar cells in the 20% range. The best cell result of 203% efficiency is the highest value for
Copyright # 2004 John Wiley & Sons, Ltd. Prog. Photovolt: Res. Appl. 2004; 12:553–558558 O. SCHULTZ, S. W. GLUNZ AND G. P. WILLEKE
multicrystalline silicon reported so far and was obtained with an only 99-mm-thick wafer. A high average effi-
ciency of 181% for 60 cells was achieved, which shows the potential of solar cell efficiencies that may be
reached with this new process on large area wafers.
Acknowledgements
The authors would like to thank all members of the Fraunhofer ISE Solar Cell Department for their contribu-
tions to this work and Dr Saito of SOGA and Mr Hjemaas of ScanWafer for supplying the multicrystalline sili-
con material.
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