Scholarly article on topic 'Laser Doped Screen-printed Back Contact Solar Cells Exceeding 21% Efficiency'

Laser Doped Screen-printed Back Contact Solar Cells Exceeding 21% Efficiency Academic research paper on "Materials engineering"

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{"laser doping" / IBC / "back contact" / screen-printing / "high efficency" / n-type}

Abstract of research paper on Materials engineering, author of scientific article — Morris Dahlinger, Kai Carstens, Jürgen R. Köhler, Renate Zapf-Gottwick, Jürgen H. Werner

Abstract We present the first laser doped screen-printed back contact silicon solar cells with an efficiency η = 21.4% avoiding any masking steps neither for doping, contact opening, nor metallization. We introduce a selective emitter doping without additional process steps. An optimized laser ablation process avoids damage to the wafer surface. Screen-printing of the base and emitter metallization at the same time, further simplifies our high efficiency solar cell process scheme.

Academic research paper on topic "Laser Doped Screen-printed Back Contact Solar Cells Exceeding 21% Efficiency"

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Energy Procedia 55 (2014) 410 - 415

4th International Conference on Silicon Photovoltaics, SiliconPV 2014

Laser doped screen-printed back contact solar cells exceeding 21%

efficiency

Morris Dahlinger, Kai Carstens, Jürgen R. Köhler, Renate Zapf-Gottwick,

and Jürgen H. Werner

Institute for photovoltaics, Pfaffenwaldring 47, 70569 Stuttgart, Germany

Abstract

We present the first laser doped screen-printed back contact silicon solar cells with an efficiency n = 21.4% avoiding any masking steps neither for doping, contact opening, nor metallization. We introduce a selective emitter doping without additional process steps. An optimized laser ablation process avoids damage to the wafer surface. Screen-printing of the base and emitter metallization at the same time, further simplifies our high efficiency solar cell process scheme.

© 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/3.0/).

Peer-review under responsibility of the scientific committee of the SiliconPV 2014 conference Keywords: laser doping; IBC; back contact; screen-printing; high efficency; n-type

1. Introduction

Low prices for photovoltaic cells and modules require low production cost at high power conversion efficiencies to compete at the market. Some approaches for industrial interdigitated back contact solar cells (IBC) are published [1-3], but still, mainly due to the higher production costs, the market share of IBC solar cells is quite low. We recently presented laser doped back contact solar cells metallized with evaporated and photo-lithographic structured contacts with n = 22.0% [4]. To further reduce the production cost and make the whole process more industrial relevant, we apply standard screen-printed contacts and reach n = 21.4%.

1876-6102 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

Peer-review under responsibility of the scientific committee of the SiliconPV 2014 conference doi:10.1016/j.egypro.2014.08.118

2. Experimental

2.1. Solar cell process

Our solar cell process starts with an one-sided textured Czochralski (Cz) grown n-type silicon wafer with a thickness t = 165 p.m. Hydrofluoric acid removes the native silicon dioxide SiO2 before the boron containing precursor layer is sputtered on the polished side of the wafer. Subsequently, a first laser doping step locally melts the silicon surface and the precursor layer for a few nanoseconds using a frequency doubled nanosecond pulsed Nd:YAG-laser with a wavelength X = 532 nm and a pulse duration tp > 50 ns [5,6]. The boron atoms diffuse from the precursor layer into the molten silicon. After the laser pulse terminates the liquid silicon recrystallizes epitaxially. The silicon is doped with boron thus forming the emitter. The high spatial resolution enables local doping with an accuracy of less than 30 pm without any masking. Wet chemistry removes the residual precursor layer, which is still present at un-irradiated areas on the wafer, and cleans the wafer prior to the following phosphorus diffusion. In our cell process an optimized POCl3 tube furnace diffusion forms a lightly doped front surface field (FSF) on both sides of the wafer accompanied by the growth of a phosphosilicate glass (PSG). Subsequently, the second laser doping step locally irradiates the rear side of the wafer between the already existing emitter areas. The highly doped back surface field (BSF) regions form utilizing the phosphorus contained in the PSG. Hydrofluoric acid removes the PSG and an acidic solution removes some phosphorus residuals. Before thermal oxidation, wet chemical cleaning is applied. The thermal oxidation acts as drive-in step for all diffused areas and a 20 nm thin silicon dioxide SiO2 layer passivates the surfaces. A silicon nitride layer SiNx deposited by plasma enhanced chemical vapor deposition PECVD on the front side serves as anti-reflective coating. A PECVD dielectric layer stack on the rear side improves the light trapping. Without any damage to the wafer surface, a laser ablation step locally opens circular areas through the dielectric layer to the base and the emitter. No special laser damage etching is necessary. Finally, standard screen-printing applies a silver paste as metallization for contacting the emitter and the base in only one printing, one drying and one fast firing step. Compared to standard IBC solar cell processes, this solar process avoids any masking for doping, contact opening, or metallization.

2.2. Laser ablation

For metallization we use a screen-printed silver paste that does not penetrate through our rear side dielectic layer, which is required for floating busbars. To locally define the contact areas to the base and the emitter we developed a laser ablation process utilizing a frequency tripled pulsed Nd:YAG-laser with a wavelength X = 355 nm and a pulse duration tp ~ 35 ns. The laser opens a circular area with a diameter dc = 55 pm in the dielectric layer during a single laser pulse. To ensure there is no laser damage to the wafer, we passivate float zone wafers with the same layer stack we use for our solar cells. Then we ablate a circular disc with a distance of 150 pm with varied laser pulse energy densities Habl1 = 0.9 Jcm2, Habl2 = 1.5 Jcm2, Habl3 = 1.8 Jcm2, and Habl4 = 2.0 Jcm2.

Fig. 1. QSSPC measured effective lifetime of passivated wafers before and after laser ablation with varied pulse energy density 0.9 Jcm2 ^ Habi ^ 2.0 Jcm2, after HF-dip, and after re-passivation with PECVD a-Si. For all pulse energy densities the lifetime is recovered after re-passivation. No laser damage is induced.

Figure 1 shows the effective lifetime Teff at an injection level of An = 1*1015 cm-3 measured by quasi-steady state photo conductance (QSSPC) before laser ablation, after laser ablation, after an additional HF-Dip, to make sure the circular patterns are opened, and re-passivation of the ablated areas with PECVD amorphous silicon a-Si [7]. In general, the effective lifetime recovers for all investigated laser pulse energy densities Habl after re-passivation. The additional hydrogen that is induced during the a-Si deposition additionally passivates the wafer, and the initial surface layers, hence the effective lifetime exceeds the initial level. For Habl > 1.5 J/cm2 the effective lifetime drops to Teff ~ 400 ps from initially Teff ~ 1130 ps after ablation. The ablated areas therefore can be considered as unpassivated and opened. For Habl1 = 0.9 J/cm2 the effective lifetime only decreases to Teff ~ 1070 ps. Suspecting the dielectric layer was not entirely ablated for Habl1 = 0.9 J/cm2, we apply an HF-dip of 4 min in 5 % HF-solution. An area of 1 cm2 on the same wafer is full area ablated with the same ablation parameters and checked if it is hydrophobic after the HF-dip, to ensure the ablated areas are completely opened. Leaving the samples at ambient air for three days reduces the hydrogen surface passivation left after the HF treatment. Still, the effective lifetime is Teff ~ 930 ps.

Figures 2a-c show optical microscope images of the locally opened rear dielectric layer of the solar cells with Habl = 0.9 J/cm2, 1.5 J/cm2, and 2.0 J/cm2 after the HF-dip. For Habl1 = 0.9 J/cm2 no optical change of the wafer surface appears. With increasing pulse energy density Habl the wafer surface visibly melts, but on cell level we do not observe any efficiency degradation using higher laser pulse energy densities.

2.3. Screen-printed metallization

As metallization we use an experimental silver based screen-printing paste which contacts n-type as well as p-type doped silicon. The paste does not penetrate through silicon oxide or silicon nitride and is fired at temperatures Tire ~ 550°C. Consequently only one screen-printing step and one drying step is required. To measure the sheet resistance dependent contact resistance pc, we adjust the sheet resistance of the n-type BSF and the p-type emitter by varying the laser pulse energy density H. As for the solar cells, thermal oxidation at 1000°C is applied, and the grown SiO2-layer is removed by HF-dip afterwards. After screen-printing, drying, and firing, we measure the contact resistance by the transfer length method [8].

Fig. 3 shows the sheet resistance dependent contact resistance on n-type and p-type doped regions. The contact resistance pc linearly increases with the sheet resistance Rsh for both n-type and p-type doping. For the n-type doping we reach low Rsh = 12 O/sq, utilizing the PSG as dopant source. Since the basic purpose of the BSF is to enable a low contact resistance to the solar cell base, we dope our solar cells BSF with Rsh,BSF = 12 O/sq and reach a contact resistance pc,BSF < 0.5 mO cm2. The contact resistance to p-type doping is overall lower at a certain sheet resistance compared to pc of the n-type doped region. At Rsh,HE ~ 115 O/sq, which is used as doping for the homogeneous emitters (HE), the contact resistance pc,HE ~ 4.0 mOcm2 which limits the solar cells fill factor with emitter contact areas Acont~ 0.8 %. As selective emitter doping applies a sheet resistance RshSE ~ 45 O/sq thus a contact resistance pc,SE ~ 1-4 mOcm2 is achieved.

Fig. 3. Contact resistance pc of our screen-printed metallization depending on type of doping and sheet resistance Rsh. For the BSF we achieve pc,BSF ^ 0.5 mO cm2, for the selective emitter (SE) pc,SE ^ 1.4 mO cm2, and for the homogeneous emitter (HE) p cHE Ss 4.0 mO cm2.

Laser doping forms the p+ emitter, using a sputtered boron precursor as dopant source. The thickness of the boron precursor dB and the laser pulse energy density H tailor the emitter profile [9]. The boron layer acts as a finite dopant source, and its thickness limits the surface doping concentration after laser doping. The pulse energy density H adjusts the emitter depth zE. A high pulse energy density leads to a longer melt duration, to a deeper molten silicon layer and to a deeper emitter depth zE. Since the diffusion constant of boron atoms in liquid state is several orders of magnitude higher than in crystalline silicon, doping is only achieved in the molten part of the wafer. Therefore, we can co-optimize the emitter sheet resistance Rsh and emitter saturation current density J0e very elegantly. With these benefits of the laser doping process, we introduce a selective boron emitter doping (SE) by locally increasing the pulse energy density H in the areas where the emitter will be contacted. A selective emitter doping can be applied by laser beam shaping or overlaying two independently triggered laser beams without adding a single process step.

The here presented interdigitated back contacted solar cells have an active area of A = 2x2 cm2, with the busbars located outside the active area. The pitch p of the repeating n-type and p-type doping structure on the rear side is p = 1 mm. The width of the emitter is we = 800 ^m and the width of the back surface field is wBSF = 180 ^m. The contacts are locally opened as point contacts with a diameter dc = 55 ^m. Sixteen cells are manufactured at the same time on a standard 6 inch Czochralski CZ n-type wafers with a resistivity p = 8 Ocm. The FSF has a sheet resistance Rsh,FSF ~ 550 O/sq, the BSF Rsh, bsf ~ 12 O/sq, the emitter Rsh,HE ~ 115 O/sq, and the selective emitter

Rsh,sE ~ 45 Q/sq. The high versatility of our laser processes enables individual processing of each of the 16 cells on a wafer, allowing an easy tailoring of the cell parameters.

Table 1 shows the averaged results of current/voltage I/V-measurements of three identically processed cells with and three cells without selective emitter. The selective emitter increases the mean open circuit voltage Voc by AVoc = 3 mV. The increased emitter depth and doping shields the highly recombination active contacts electrically, hence increasing the Voc. Besides the contact recombination, the emitter passivation is not optimized yet, thus recombination at the emitter surface limits the open circuit voltage to Voc ^ 654 mV. The low front surface recombination combined with low front side reflection and good light trapping on the excellent 8 Qcm bulk material enable an excellent short circuit current density Jsc ~ 42 mA/cm2. Also the selective doping helps to keep the high Jsc level, since we found, that the metallization of the emitter decreases Jsc depending on doping, contact area, and firing temperature. Due to the reduced emitter contact resistance, the mean fill factor increases by AFF = 2.7 %. The FF ~ 77.3 % is partly limited by the low total contact area Ac = 1.6 % of the active cell area, hence the accruing contact resistance contributed to the series resistance. The mean efficiency increases by An ~ 0.9 %abs to n ~ 21.2 % with selective emitter doping. The best solar cell achieved an efficiency n ~ 21.4 %.

Table 1. Mean values of open circuit voltage Voc, short circuit current density Jsc, fill factor FF, and efficiency n of three cells each with and without selective emitter doping (in-house measurement).

Voc Jsc FF n

[mV] [mA/cm2] [%] [%]

with SE 654 ±1 42.0 ±0.1 77.3 ±0.9 21.2 ±0.2

without 651 ±1 41.8 ±0.1 74.6 ±0.8 20.3 ±0.2

3. Conclusion and Outlook

We fabricated laser doped screen-printed back contact silicon solar cells with an maximum efficiency n = 21.4% without any masking steps. The versatility of our laser processes enables local doping and ablation with an accuracy below 30pm. The laser ablation process does not induce damage to the wafer and thus avoids any laser damage removal. Screen-printing metallization forms the base and the emitter contacts in one step. The introduction of a selective emitter doping gains An ~ 0.9 %abs. Optimizing the rear side passivation and metallization conditions will further boost the solar cells efficiency.

Acknowledgements

The authors thank L. Beisel, H. Moldenhauer, M. Saueressig, and B. Winter for technological support. We thank the company DuPond for providing the metallization paste. This work was financially supported by the German Federal Ministry for Environment, Nature Conservation and Nuclear Safety (BMU) under project no. 327519.

References

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