OPTIMISATION OF N+ DIFFUSION AND CONTACT SIZE OF IBC SOLAR CELLS

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1 OPTIMISATION OF N+ DIFFUSION AND CONTACT SIZE OF IBC SOLAR CELLS Kean Chern Fong 1, Kho Teng 1, Keith R. McIntosh 2, Andrew W. Blakers 1, Evan Franklin 1, Ngwe Zin 1, Andreas Fell 1. 1 Australian National University, Australia. 2 PV Lighthouse, Australia. Corresponding author: Kean Chern Fong, The Australian National University, School of Engineering, North Road, Acton, ACT 0200, Australia. Ph: , kean.fong@anu.edu.au ABSTRACT: The Australian National University (ANU) recently fabricated a 24.6% ± 0.5% efficiency IBC solar cell on n-type Cz-Si. This paper discusses the optimisation of the diffusions and contact openings of that solar cell, which combined detailed characterisation with 3D device modelling. Given the many competing effects in an IBC cell structure, the optimal diffusion profiles and geometry are very dependent on the device fabrication technology, particularly on the surface passivations and metalisation techniques. The crucial device parameters are discussed and the method of experimentally measuring the surface passivation quality, emitter recombination, contact recombination and specific contact resistivity are presented. With these experimental values and 3D device modelling, efficiency contour plots are generated for a pertinent range of phosphorus sheet resistance and contact fraction. Keywords: Characterisation, back contact, simulation 1 INTRODUCTION ANU fabricated a 24.6% efficiency IBC solar cell on n-type Cz-Si material. The chamption cell is 4cm 2, with a thickness of 190um. The front surface is undiffused and pasivated using PECVD SiN. The rear diffusions are photolitographically patterned, and have rear SiO 2 /LPCVD SiN stack passivation [1]. A diagram of the cell design is as presented in Figure 1. Sheet resistance of diffusion In this work, Quokka, a freeware 2D/3D quasi neutral simulation tool is utilized [5] for the 3D simulations. Quokka treats diffusion regions as a boundary condition, and assumes quasi-neutrality in the bulk. Spatial effects such as current crowding, internal series resistance, and loss due to laterally diffusing carriers are natively resolved. 2 EXPERIMENTAL DETAILS Figure 1: IBC device structure with localised N+ BSF The work presented in this paper is part of the optimisation study behind the n-type IBC solar cells fabricated at ANU. Optimal design of the diffused regions require consideration of several competing effects. Specific contact resistivity between n-type silicon and aluminium is often significant [2], necessitating formation of n+ diffusion to provide good ohmic contact. A diffused region introduces additional recombinative losses and optical loss via free carrier absorption [3] but at the same time reduces the amount of surface recombination at unpassivated regions such as aluminium contact surface [4]. A selective heavily doped diffusion beneath the metal contacts can potentially minimise contact resistance and recombination losses at the cost of an increase in process complexity. The differences between selective base diffusion versus large area diffusion is investigated. The key parameters necessary for finding the optimal device geometry and diffusion are identified: Passivated diffusion recombination of n+ region (measured by recombination current pre-factor, J 0n+ ) Contact area recombination Specific contact resistivity for a range of diffusions 2.1 Measurement of diffusion profiles Measurement of the diffusion profile is performed via electrochemical capacitance voltage(ecv) profiling. As published in [6], uncertainty of ECV etching area size causes error in the measurement, thus requiring etch area correction. In this work, the etch area was used as a floating parameter, varied to match the measured sheet resistance (using 4-point-probe) to the calculated sheet resistance. Once the accurate profile of the diffusion is obtained, the samples are etched in TMAH etchant and removed at different etch periods to produce different surface doping concentrations. The surface doping concentration of each sample is determined by first measuring the sheet resistance using 4-point-probe measurement, and calculating the etch depth based on the pre-etch doping profile. Figure 2 compares the calculated and measured diffusion profiles for different samples, and demonstrates how this method accurately determines the doping. The ability to determine surface doping concentration is particularly important since it is a crucial parameter for resolving the surface passivation quality and determining specific contact resistance. 2.2 J 0 measurement After preparation and determination of the diffusion profiles and surface concentration, the phosphorus glass and oxide are etched off in HF etch, and subjected to standard RCA cleaning before passivated with various passivation schemes. The J 0n+ of each sample is then measured using photoconductance decay (PCD) method. To determine contact recombination, samples were coated with approximately 5nm of Aluminium and J 0n+ was measured using PCD method. The idea behind using

2 a very thin layer is that the additional conductivity of the layer is very small and does not saturate the PCD coil, while producing a silicon-metal interface similar to a device contact surface. Results of the measurements for SiO 2 /LPCVD SiN passivated, Oxide passivated, PECVD SiN passivated, metal deposited surface are presented in Figure 3. Quokka treats diffusion at the surface as a recombinative boundary condition, thus making it convenient to directly use experimentally determined J 0 for the simulation with little loss of accuracy, provided the absence of injection dependent effects. Ther Auger recombination model and the mobility model used are the Richter model[8] and Klassen mobility[9] model Figure 2: Doping profile of samples after etch-back. Comparison between measured profiles and profiles calculated from sheet resistance show good agreement. 2.3 Contact Resistivity Measurement Evaporated aluminum is used as the metal contact for the fabricated cells. The specific contact resistivity is measured using Transmission Line method (TLM) [7]. Samples are etched in hydrofluoric acid immediately prior to metal evaporation to minimize formation of native oxide. The TLM structures are isolated via dicing saw before measured using a 4 point probe measurement. Results of the contact resistivity test are presented in Figure 4. Specific Contact resistivity (Ohm-cm) 1E-3 1E-4 1E-5 1E-6 N+ Contact P+ Contact 1E Sheet Resistivity (Ohms/sq) Figure 4: Contact resistivity as measured by TLM for aluminium-silicon contact. Two design structures are investigated in this paper; having a localised base diffusion versus having a sheet base diffusion. The IBC unit cell as generated from Quokka for the design examined in this study is presented in Figure 5. The device unit cell geometry is 250um long, 35um wide, and have a thickness of 190um, emulating the actual devices which were fabricated. The front surface is set to 5fA/cm 2 and rear undiffused surface was set to 20fA/cm 2 as is measured from experimental results. 0 N+ metal contact J 0N+ (fa/cm2) 10 PECVD SiN SiO 2 /LPCVD SiN Wet SiO 2 Dry SiO 2 Figure 5: 3D device mesh as generated by Quokka, for sheet N+ diffusion and localised N+ diffusion design Sheet Resistivity (Ohms/sq) Figure 3: Reverse saturation current density of n diffusion for various passivation techniques. 3 3D MODELLING USING QUOKKA To incorporate all the effects in calculating the optimal configuration, Quokka was utilized. 4 RESULTS & DISCUSSION A matrix of variables as collected from Figure 3 and 4 are inputs to the simulation tool, which runs a series of individual simulations to generate IV curves for each parameter set. The contour plot of N+ sheet resistivity versus contact dot size for sheet N+ diffusion is presented in Figure 6, and localised N+ diffusion is in Figure 7. The resulting contour plot for efficiency indicates that a common optimal dot contact size for both

3 investigated cell design, which is having a contact area 0.25% to 0.75% of total rear area. Having a large area sheet diffusion makes the device sensitive to base diffusion recombination. Thus the diffusion needs to be lightly doped to minimize diffusion recombination loss. N+ diffusions heavier than 40 ohms/sq causes losses to diffusion recombination while gaining little in reducing contact resistivity. The optimal for the sheet N+ diffusion is between Ohms/sq (Figure 6). In the case of a localised N+ diffusion surrounding the N+ contact region as illustrated in Figure 5 (right), the total diffused area is relatively small at only 2% of total rear area and therefore the effect of diffused region recombination becomes less significant. The optimal diffusion range is found to be quite large, from Ohms/sq (Figure 7). In both cases, having a combination of large contact area with light diffusions is undesirable due to increase in contact recombination losses. pitch), showing very little performance difference when the optimal diffusions and geometry is used for both designs. This of course depends heavily upon the passivation quality difference between the diffused phosphorus region and undiffused n-type base surfaces. However, free carrier absorption is not taken into account in these simulations, there is potentially more advantage in having less diffused regions. There is also more potential for efficiency improvement for localized N+ design. But requires a surface passivation with J 0 < 20fA/cm 2 on undiffused n- type surfaces. Having a localised base diffusion relaxes diffusion process control, having a broader optimal configuration of diffusion sheet resistivity and contact area coverage, but comes at the cost of the need for aligned contact openings. The champion cell at ANU utilizes a localized N+ contact diffusion, with 30 Ohm/sq diffusions with a contact area of 0.2% of the rear surface, which is very close to the optimal as shown by simulation. N+ Sheet Resistivity(Ω/sq) Contact recombination loss Diffusion recombination loss ACKNOWLEDGEMENT This work was performed under contract with the Solar Energy Research Institute of Singapore (SERIS), and supported by the joint research project between Trina Solar and SERIS. SERIS is sponsored by the National University of Singapore (NUS) and Singapore s National Research Foundation (NRF) through the Singapore Economic Development Board (EDB). This work is supported by the National High-Tech R&D Program (863 program) of the Ministry of Science and Technology of the P.R. China under project number 2012AA % N+ Contact Area Figure 6: Contour plot of Efficiency for a sheet N+ diffusion IBC design. N+ Sheet Resistivity (Ω/sq) Contact recombination loss % N+ Contact Area Figure 7: Contour plot of Efficiency for a localised N+ diffusion IBC design. 5 SUMMARY Two different base diffusion designs were investigated for a high efficiency IBC design (500um REFERENCES [1] A. W. Blakers, K. C. Fong, E. Franklin, T.C. Kho, K. R. McIntosh, Y. Wan, D. Wang and N.S. Zin "24.6% Efficient Back Contact Cell with Oxide - Nitride Passivation.", accepted to 23rd Photovoltaic Scienece & Engineering Conference, [2] D. K. Schroder and D. L. Meier, "Solar cell contact resistance; A review," Electron Devices, IEEE Transactions on, vol. 31, pp , [3] J. Isenberg and W. Warta, "Free carrier absorption in heavily doped silicon layers," Applied Physics Letters, vol. 84, pp , [4] A. Cuevas, P. A. Basore, G. Giroult-Matlakowski, and C. Dubois, "Surface recombination velocity of highly doped n-type silicon," Journal of Applied Physics, vol. 80, pp , [5] A. Fell, "A Free and Fast Three-Dimensional/Two- Dimensional Solar Cell Simulator Featuring Conductive Boundary and Quasi-Neutrality Approximations," Electron Devices, IEEE Transactions on, vol. 60, pp , [6] R. P. Bock, P. Altermatt, and J. Schmidt, "Accurate extraction of doping profiles from electrochemical capacitance voltage measurement.," in 23th EU- PVSEC, Valencia, Spain, 2008, pp [7] G. K. Reeves and H. B. Harrison, "Obtaining the specific contact resistance from transmission line model measurements," Electron Device Letters, IEEE, vol. 3, pp , 1982.

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