Decoupling the metal layer of back contact solar cells – optical and electrical benefits

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7th International Conference on Silicon Photovoltaics, SiliconPV 2017 7th International Conference on Silicon Photovoltaics, SiliconPV 2017

Decoupling the metal layer of back contact solar cells – optical and Decoupling the metal layer of back contact solar cells – optical and electrical benefits The 15th International Symposium on District Heating and Cooling electrical benefits a a a b b Udo Römer , Zhongtian Liof Liaathe , Xueling Zhang a, Ning Song afeasibility a, Yang b, Yuan Shengzhaob, Assessing the using heat demand-outdoor b a Udo Römer , Ning Song ,Pierre Zhongtian Li , Yang Li , Xueling Zhang , Yuan Shengzhao , b, Alison Lennona temperature function forJ.J. aVerlinden long-term district Pierre Verlinden , Alison Lennonheat demand forecast a

The School of Photovoltaic and Renewable Energy Engineering, UNSW Sydney, NSW, 2052, Australia a The School of Photovoltaic and Renewable Energy Engineering, UNSW Sydney, NSW, 2052, Australia State Key Laboratory of PV Science andaTechnology, Trina a,b,c a Solar Limited, No. b2 Trina Road, Changzhou,c Jiangsu, 213031, China c b State Key Laboratory of PV Science and Technology, Trina Solar Limited, No. 2 Trina Road, Changzhou, Jiangsu, 213031, China b

I. Andrić

*, A. Pina , P. Ferrão , J. Fournier ., B. Lacarrière , O. Le Corre

a

IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal b Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France Abstract c Département Systèmes Énergétiques et Environnement - IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France

Abstract

We report on our work on optimising the optical and electrical properties of back contact solar cells. We use a spin coated We report on ourlayer workbetween on optimising the optical and electrical properties of back contact solar cells. use acell spinand coated Novolac polymer metal layer and solar cell. This helps to decouple the metal optically fromWe the solar also Novolac polymer layer of between metal layer solar cell. This helpscells to decouple metal optically fromemitter the solar cell and also enables the application symmetrical metaland finger widths on solar with thinthe BSF regions and large regions. While Abstract enables the application of symmetrical metal finger widths oncell solar cells with thin BSF regions and large emitterasregions. While the application of the polymer layer adds process steps to the flow it simplifies the metal contact separation, the metal can the application offrom the polymer adds process stepsthe the literature cell flow itassimplifies themost metal contact separation, asand theused metalascan beDistrict laser ablated the polymer without damaging solar cell. Further, can be inkjet patterned an heating networks arelayer commonly addressed into the onetheofpolymer the effective solutions for decreasing the be laser ablated from the polymer without damaging the solar cell. Further, the polymer be inkjet and usedthe as heat an etch mask forgas a wet chemical contact opening of the underlying dielectric passivation layer.can which greenhouse emissions from the building sector. These systems require high investments arepatterned returned through etch mask fortoa wet of the passivation sales. Due the chemical changed contact climate opening conditions andunderlying building dielectric renovation policies, layer. heat demand in the future could decrease, the investment return ©prolonging 2017 The Authors. Published byperiod. Elsevier Ltd. © 2017 2017 The The Authors. Published by Elsevier Ltd. © Authors. Published Ltd. Thereview main scope this paper isby to Elsevier assess the feasibility of using 2017 the heat demand – outdoorof Peer by conference committee of SiliconPV SiliconPV 2017 under responsibility oftemperature PSE AG. AG. function for heat demand Peer review by the theofscientific scientific conference committee of under responsibility PSE Peer review by the scientific conference committee of SiliconPV 2017was under responsibility of PSEThe AG.district is consisted of 665 forecast. The district of Alvalade, located in Lisbon (Portugal), used as a case study. Keywords: IBC solar cell; inkjetperiod structuring; metallisation; rear side optics;scenarios contact separation buildingsBJBC that solar vary cell; in both construction and typology. Three weather (low, medium, high) and three district Keywords: BJBC solar cell; IBC developed solar cell; inkjet structuring; metallisation; rear To side estimate optics; contact separation renovation scenarios were (shallow, intermediate, deep). the error, obtained heat demand values were compared with results from a dynamic heat demand model, previously developed and validated by the authors. results showed that when only weather change is considered, the margin of error could be acceptable for some applications 1.The Introduction 1.(the Introduction error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation scenarios, the error valuesolar increased up to 59.5% are (depending on the weather and renovation scenarios combination considered). The highest silicon cell efficiencies today realized with back junction back contact (BJBC) solar cells TheThe value ofthe slope coefficient increased on average within the reduction rangewith of 3.8% up to losses 8% back perdue decade, that corresponds to the highest silicon solar cell efficiencies arecells today back junction contact (BJBC) solar cells [1-4]. While main advantage of BJBC solar is realized the of optical to the absence of a front decrease in the of heating hours of 22-139h during heating (depending ondue thea to combination weather and [1-4]. While thenumber main ofofBJBC solar is the reduction of rear optical the absence of a front side metallisation, the advantage application both nandcells p-type regions onseason the sidelosses implies trade-off ofofgeometrical renovation scenarios considered). other function intercept increased 7.8-12.7% per decade (depending on in the side metallisation, application oftheboth n-hand, and p-type regions the rear side implies a trade-off of geometrical aspects: In order tothe keep losses On from electrical shading effects [5]onsmall, thefor BSF area should be minimized and coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and aspects: In order to keep losses from electrical shading effects [5] small, the BSF area should be minimized and in order keep the keep the metal resistance losses small, the metal cross sections for both polarities should be equally improve accuracy of resolve heat demand order keep keep to the metal resistance small,geometry the metalneeds cross to sections for both from polarities should be equally large [6].the Inthe order this estimations. issue,losses the metal be decoupled the doping geometry.

large [6]. In order to resolve issue, the metalhave geometry to be electrical decoupledinsulation from theproperties doping geometry. Unfortunately, commonly usedthis passivation layers shown needs insufficient on large © 2017 The Authors. Published bypassivation Elsevier Ltd.layers have shown insufficient electrical insulation properties on large Unfortunately, commonly used area to permit reliable overlapping of BSF metal fingers over emitter regions [6-8]. Possible solutions include the Peer-review under responsibility of theof Scientific Committee of over The 15th International Symposium on District Heating and the area permit overlapping BSF metal fingers emitter regionsinsulation [6-8]. Possible solutions include use oftotwo levelreliable metallisation schemes [9, 10], or the deposition of a reliable layer between the solar cell Cooling. use of two level metallisation schemes [9, 10], or the deposition of a reliable insulation layer between the solar cell 1876-6102 2017demand; The Authors. Published bychange Elsevier Ltd. Keywords:©Heat Forecast; Climate 1876-6102 The Authors. Published by Elsevier Ltd. Peer review©by2017 the scientific conference committee of SiliconPV 2017 under responsibility of PSE AG. Peer review by the scientific conference committee of SiliconPV 2017 under responsibility of PSE AG.

1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling.

1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer review by the scientific conference committee of SiliconPV 2017 under responsibility of PSE AG. 10.1016/j.egypro.2017.09.290

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and the metal layer [11]. In [11] we have shown that Novolac polymer layers applied between a Si wafer and evaporated metal provide an electrical resistance of 1010 Ωcm2. In this work we focus on further benefits of using this layer: In Section 3 we show that the application of the polymer layer can also enhance the internal reflection at the solar cells’ rear surface, thus increasing the short circuit current density of the cells, especially for contact metals such as Ni or Ti which strongly absorb in the IR spectral region. These results also present an update for the results in [11] as recent improvements in our ray tracer have highlighted an error in the previously reported results which is corrected in this paper. In Section 4 we further show that the polymer can also be used to help with the contact separation of rear contact solar cells. As the metal layer in these cells is not deposited directly on a ~ 200 nm thin dielectric layer, but on a ~3 μm thick polymer layer that is highly absorbing in the UV wavelength region, we are able to demonstrate a damage free UV laser ablation process for the scribing of the contact separation line between the emitter and BSF metal. 2. Experimental procedure The application of the polymer was as reported in [11]. A Novolac polymer layer was spin-coated on the rear surface of passivated solar cells or test structures and soft baked at 110 °C. A functional ink was then inkjet-printed at the point locations for the contact openings, the deposited ink preventing the polymer layer from crosslinking during the following UV exposure step and the subsequent baking at 110 °C. The non-crosslinked regions were then dissolved by immersing the samples in tetramethyl ammonium hydroxide (TMAH). After hard baking the remaining resist at 160 °C, it was HF resistant enabling it to act as an etch mask for the formation of contact openings in the underlying dielectric layer. After the formation of the contact openings there are a number of different metallisation possibilities. In this study, we evaporated Ti layers, which were capped with evaporated Ag as a seed layer system, for a Cu plating step. Before the plating step, the thin seed layer (in our experiments currently ~400 nm) was ablated with a pico-second UV laser for the contact separation of the BSF and emitter fingers. Copper was then plated following the seed layer geometry. 3. Optical benefits We used the angular matrix framework [12, 13] to simulate the optics of our solar cells. Fig. 1 shows the optical losses of a solar cell with different seed layer metals on the rear surface. The simulated cell structure comprised a 180 μm thick silicon wafer with a pyramid texture on the front surface, capped with a SiNx layer. The flat rear surface was capped with a 200 nm thick SiNx layer followed by the metal layer. It can be seen that even for a highly reflective Ag seed layer on a 200 nm thick SiNx layer, 0.29 mA/cm2 are absorbed at the rear surface of the cell.



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Rear metal type Fig. 1. Simulated current losses in solar cell structures comprising a 180 μm thick c-Si wafer with a 70 nm thick SiNx layer on a textured front surface and a 200 nm thick SiNx layer on a planar rear surface metallised with different 3 μm thick metal layers. Shown are the losses due to reflection R (including also light leaving the cell at the front side after internal reflections), absorption in the front dielectric layer Af, absorption at the rear layers Ar and due to transmission thorough the rear layers T.

This absorbed fraction can be reduced by either further increasing the rear dielectric layer thickness or introducing layers with a lower refractive index between SiNx and metal. Fig. 2 shows that the former would require SiNx thicknesses of more than 600 nm in order to increase the photogenerated current density above the one for a 200 nm thick layer. In contrast, the deposition of an only 100 nm thick dielectric layer with a lower refractive index than SiNx can considerably increase the generated current.

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Fig. 2. Influence of the rear dielectric layer on the simulated photogenerated current density. The blue line shows the influence of the rear SiNx layer thickness (shown on the lower X-axis), the red line shows the impact of a 100 nm thick dielectric layer on top of a 200 nm thick rear SiNx layer (refractive index shown on the upper X-axis).

In our experiments we combined both methods, by depositing a ~ 3 μm thick Novolac polymer layer with a refractive index of ~ 1.6 (after curing) on top of the rear SiNx layer. Fig. 3 shows the influence of the addition of such a layer for different polymer thicknesses. As the polymer layer used in our experiments showed some infrared absorption, the application on Ag or Cu metallised solar cells only yields an increase in the current density for polymer thicknesses below 1 μm. For stronger absorbing metals like Ni or Ti the addition of the polymer layer can reduce the current loss by 0.35 mA/cm2 and 0.39 mA/cm2.



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4. Laser contact separation One further beneficial feature of the incorporation of the Novolac polymer layer between metal and solar cell is the possibility to laser-ablate the metal without damaging the solar cell. In our study we used a pico-second UV laser for the ablation. As the Novolac polymer is highly absorbing in the UV region the laser energy doesn’t penetrate to the solar cell and therefore the electrical passivation is not impacted. Fig. 4 (a) shows a photoluminescence image of a metallised test structure. In the marked area, metal has been laser ablated with a contact separation pattern as indicated in the drawing in Fig. 4 (b). No difference can be observed between the contact separated and non-contact separated regions, showing that the laser ablation step didn’t introduce damage to the solar cell. The inset in Fig. 4 (b) depicts an SEM cross-section image at a contact separation line after 5 μm copper plating, showing that only some part of the polymer layer was ablated in the process. It can also be seen that the separation line was wide enough to prevent merging of the fingers during copper plating. As the cross-section only shows a small section of the laser scribed lines, we further used multimeter measurements between adjacent metal fingers to ensure that no shunts have been formed.

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Fig. 4. (a) Photoluminescence image of a metallised test structure exhibiting a square laser contact separated region. (b) Schematic drawing of the contact separation layout, showing the emitter regions in red, the BSF regions in green and the laser scribed regions in blue. The inset in b) shows a SEM image at the cross-section of the final structure after plating, prepared by focused ion beam milling.

5. Conclusion The process presented makes possible the optical and electrical decoupling of the rear metallisation of a back contacted solar cell. This allows the independent optimization of the doping and the metal contact geometry. Further benefits arising from this metallization strategy include the possibility of using highly IR absorbing metals like Ti that provide a low contact resistivity to both, n- and p-type regions (we measured 0.24 mΩcm2 and 0.26 mΩcm2, respectively) without altering the optical properties of the solar cell too much. Further, the contact openings in the rear dielectric layer can be made by wet chemical etching, thus avoiding possible laser damage on the solar cell surface. Presence of the polymer interlayer also makes possible a simple laser ablation process featuring damage free metal contact separation. While our main motivation of using the polymer layer was the electrical decoupling of solar cell and metallisation geometry in order to optimize our BJBC solar cells, further aspects like the damage free contact opening process and especially the optical benefits of the polymer layer would also be beneficial for both side contacted solar cell architectures, like PERC or PERT cells. Acknowledgements The authors would like to thank Derwin Lau for many helpful hints and tips regarding the sample preparation and Wei Zhang for FIB measurements. Further, the authors acknowledge support from the Australian Government through the Australian Renewable Energy Agency (ARENA) and this work was performed in part at the OptoFab node of the Australian National Fabrication Facility. Responsibility for the views, information or advice expressed herein is not accepted by the Australian Government. The authors also acknowledge support from the scientific and technological achievement transformation of Changzhou City under the Project Number of CC20150005. References [1] Yoshikawa, K., Kawasaki, H., Yoshida, W., Irie, T., Konishi, K., Nakano, K., Uto, T., Adachi, D., Kanematsu, M., Uzu, H., Yamamoto, K. Silicon heterojunction solar cell with interdigitated back contacts for a photoconversion efficiency over 26%, Nature Energy, 2017, 2, p. 17032 [2] Masuko, K., Shigematsu, M., Hashiguchi, T., Fujishima, D., Kai, M., Yoshimura, N., Yamaguchi, T., Ichihashi, Y., Mishima, T., Matsubara, N., Yamanishi, T., Takahama, T., Taguchi, M., Maruyama, E., Okamoto, S. Achievement of More Than 25.6% Conversion Efficiency With Crystalline Silicon Heterojunction Solar Cell, IEEE Journal of Photovoltaics, 2014, 4, (6), p. 1433-1435



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