Abstract
This chapter aims to provide students/engineers/scientists in the field of photovoltaics with the basic information needed to understand the operating principles of screen‐printed front junction n‐type silicon solar cells. The relevant device fabrication processing is described, from texturing, diffusion, passivation and antireflection coating, to screen‐printed and fired‐through metallization as well as the technologies that are currently used for most industrially produced solar cells. A brief description of the characterisation approaches is given and discussed for an understanding and analysis of the loss mechanisms in a finished cell, including resistance loss, recombination loss, and optical loss. The application of advanced cell concepts and the improved technologies for further increasing cell efficiency, such as selectively doping structure and tunnel oxide passivated contact, are addressed for screen‐printed front junction n‐type silicon solar cells.
Keywords
- screen printed
- front emitter
- n‐type silicon solar cell
- recombination
- surface passivation
1. Introduction
Photovoltaics is the process of converting sunlight directly into electricity using solar cells. For the past few decades, the main research tendency in solar cells has been to develop cells which are both highly efficient and also cost‐effective. Because of the abundance and nontoxicity of silicon, the fabrication simplicity, and the vast amount of accumulated knowledge in processing developed in the integrated circuit industry, silicon wafer‐based solar cells dominate the very dynamic photovoltaic market. Silicon solar cells generate electricity via absorbing photons and generating electron–hole pairs, which are separated by a
Figure 1 shows a schematic of the basic structure for a typical screen‐printed front junction
2. Operating principles of a front junction n ‐type silicon solar cell
2.1. Energy‐band diagram
Figure 2 shows the schematic energy‐band diagram for the fundamental operating principles of a screen‐printed front junction
The
When the cell is illuminated, photons with energy greater than the silicon band gap energy are absorbed to excite electrons from the valence band to the conduction band, which generates an electron–hole pair (a hole refers to the missing electron in the valence band), as shown in Figure 3. The generated electrons and holes can diffuse within the solar cell until they reach the SCR, if they do not recombine. Then, the electric field at the
2.2. Solar cell output parameters
The
where
where
When the cell is illuminated, it is ideally modelled as
where
For an actual solar cell, Eq. (3) becomes
where
The resulting dark and illuminated
In Figure 4b, the maximum power point (
where
Finally, the cell energy‐conversion efficiency is defined as
where
2.3. Resistance loss
Actual silicon solar cells generally have a parasitic series resistance (
The
where
2.4. Recombination loss and saturation current density
The generated electron–hole pair can recombine if they are not efficiently separated and collected. There are typically three recombination mechanisms that can occur in parallel in silicon solar cells. First, radiative recombination is the process that electron makes a band‐to‐band transition while emitting a photon as light. Hence, it is the reverse of the light absorption. But it is often neglected for silicon solar cell, because silicon is an indirect‐band‐gap material and a phonon is required for this type of recombination. Second, Auger recombination refers to electrons and holes that recombine and use the excess energy to excite a free carrier. Then, this excited free carrier relaxes back to its original energy status by emitting phonons. This type of recombination is particularly effective in the heavily doped regions with doping concentration over 1017 cm-3, for instance, the
The carrier lifetime is typically used to define the time for recombination to occur after the electron–hole generation. Because the three recombination mechanisms occur in parallel, the silicon material bulk lifetime (
where
In addition, for crystalline silicon wafers, dangling bonds are present on the front and back surfaces, and introduce defect levels throughout the energy‐band gap. Surface recombination velocity (
where
For silicon solar cells, recombination after carrier generation not only reduces
In order to conduct a detailed analysis about the recombination contribution from each part of a finished screen‐printed
where
where
where
In Eq. (12),
where the intrinsic carrier concentration
Similar to
where
where
So, in order to obtain a low
2.5. Optical loss
Apart from the recombination that contributes to the
So, to reduce the optical loss in a finished cell, front gridline should be as narrow as possible to reduce metal shading while not sacrificing conductivity. Currently, the screen‐printed gridline in mass production typically demonstrates ∼60 μm width. The size of pyramids also needs to be as small as possible to reduce reflection at the front surface, and currently, typical size is in the range of 3–6 μm. Low doping levels in the diffused regions (
3. Cell fabrication process with screen‐printed metallization
In this section, the typical processes of fabricating screen‐printed front junction
3.1. Saw damage removal and texturing
After the silicon ingot is grown, wire sawing is typically used to slice silicon ingots into wafers with a resulting thickness of around 200 µm, and often in pseudo‐square shape (∼156 × 156 mm2) with total area of about 239–242 cm2 depending on the diameter of the original ingot. During this process, the sawing damages the entire surface of both sides of the silicon wafers, with the damage depth of approximately 10 µm. This saw‐induced damage has a very bad effect on the electronic quality of the wafer as they dramatically increase the surface recombination velocity, and hence have to be removed together with other contaminants prior to the next high‐temperature diffusion step. This etching of the saw damage normally occurs in heated potassium hydroxide (KOH) solution at ∼80°C for few minutes. This etching reaction takes place in three steps, including oxidation of silicon, formation of a solvable salt and dissolving of the salt in water, which is summarized in [15, 16]
In addition, this is a selective etching process as different crystallographic orientations have different etch rates, with the lowest etch rate for the <111> plane. In order to effectively reduce the reflection at the front surface, isopropyl alcohol (IPA) is normally added into KOH solution to form small pyramids with a square base randomly distributed over the <100> oriented silicon surface, as shown in Figure 9.
After texturing, the wafers are processed by a thorough cleaning to remove impurities present on the wafer surface that could diffuse into the wafer and cause carrier recombination. This cleaning typically consists of a rinsing in deionized (DI) wafer, a thorough etching in hydrochloric acid (HCl) to remove metal impurities from wafer surfaces, then another DI water rinsing, a short etching in hydrofluoric acid (HF) to etch off the native silicon dioxide (SiO2) and to form a hydrophobic surface feature, and a final DI water rinsing and then air drying [17]. The more aggressive and more expensive ‘RCA’ clean (‘SC‐1’ and ‘SC‐2’) is another standard set of wafer cleaning steps typically used in R&D labs [18].
3.2. Boron emitter formation
To form the
This reaction is often referred to as the deposition stage, as a very high concentration of boron forms in the very thin layer on the silicon surface. Next, the formed B2O3 reacts with the silicon atoms which can diffuse boron atoms into the silicon bulk to form the
which is often referred to as the diffusion stage. The formed SiO2/B2O3 stack on the silicon surface is the so‐called borosilicate glass (BSG) that needs to be removed to improve surface passivation quality. The resulting boron‐doped
Due to BBr3 diffusion being a double‐sided coating process, a mask on the rear side is needed to protect the rear surface where the
3.3. Formation of phosphorus‐doped back surface field
The most commonly used technique to form phosphorus‐doped
where the formed P2O5 acts as a phosphorus dopant source. This is often referred to a deposition stage, as a very high concentration of phosphorus forms in the very thin layer (only tens of nanometres) on the silicon surface. Then, in the same process, the furnace temperature is often slightly increased for the next drive‐in stage: in which the phosphorus atoms diffuse deeper into the silicon. Phosphorus atoms diffuse into the silicon substrate to create an
where the formed SiO2/P2O5 stack is the so‐called phospho‐silicate glass (PSG). The resulting doping profile depends on diffusion temperature, diffusion time and gas flow rates [25, 26].
Because the POCl3 diffusion is also a double‐sided coating process, a mask on the front side is needed to protect the
3.4. Surface passivation and antireflection coating
To obtain high cell performance, surface passivation plays an important role in reducing recombination in the finished cell. There are two fundamental mechanisms for surface passivation: (1) chemical passivation that the surface defect states are removed or reduced; (2) field‐effect passivation that a fixed‐charge dielectric is deposited on the surface to create an internal electrical field that repels or screens minority carriers inside the wafer from the defective surfaces. For field‐effect passivation, the positive‐fixed‐charge dielectrics (i.e., SiNx and SiO2) repel the positively charged holes inside the silicon wafer from the surfaces and are ideally suitable to passivate
In order to further reduce reflection losses at the textured front side, a layer of hydrogen‐rich silicon nitride (SiNx:H) is normally deposited by PECVD on top of the passivation layer (Al2O3 or SiO2) as an ARC. Since there is significant amount of H in this ARC layer, it can be released during the metal contact firing step at high temperature (∼800°C) and diffuse into the silicon wafer bulk region to passivate bulk defects, which reduces bulk recombination. The thickness of this ARC can be calculated by the quarter wavelength law [31]
where
where
3.5. Screen‐printed metallization
Screen‐printed metallization is very robust, simple and widely used for PV applications since its introduction about four decades ago [32]. Figure 10 shows the schematic of a screen‐printing process. The squeegee is moved with a proper pressure over the screen that consists of emulsion and mesh wires, which presses down the screen locally against the wafer surface and pushes the paste on the wafer surface through the well‐defined opened region (typically 40–60 µm wide openings). During the printing, wafer stays on the stage under vacuum condition. The printer settings, i.e., snap‐off distance, print pressure and print speed, are very critical parameters to obtain a good aspect ratio (height to width ratio) of screen‐printed gridlines.
The paste ingredients are also crucial to obtain a high aspect ratio, good conductivity and low contact resistance. For front junction
Figure 11 shows an example of firing‐temperature profile, including the firing‐temperature ramp up and ramp down. During this firing step in a conveyor belt furnace at a peak temperature of over 700°C, the metal contact is formed on both sides (
During the firing process, organic binders are burned out below 600°C, which typically occurs during the plateau stage as shown in Figure 11. In the higher‐temperature zones of the furnace, including the peak firing temperature, both the front and rear contacts are simultaneously formed by etching through the ARC and passivation layers, Ag particle sintering, and forming the ohmic contact [35]. In the meantime, the hydrogen of the SiNx:H ARC layer is released into the wafer to passivate electrical defects at interfaces and in the wafer bulk regions. The duration of the peak temperature often only lasts for a few seconds. Figure 12b shows the physical appearance of a finished cell with screen‐printed contacts, featuring 5 bus‐bars on both front and rear sides. The final gridline on the cell is typically ∼60 µm wide, and ∼20 µm high, as shown in optical microscope image of Figure 12a.
3.6. Characterization
After cell fabrication, the illuminated current density–voltage (
To obtain a better understanding of the loss mechanisms in a finished cell, detailed analysis is typically needed. For instance, a cell's internal and external quantum efficiency (
4. Advanced cell concepts and fabrication
4.1. Selective doping
Due to the recent improvements in material quality and surface passivation, current high‐efficiency silicon solar cells are often limited by the recombination at the metal/semiconductor contacts. A feasible solution to minimize contact recombination is a selectively doped structure, which allows decoupling of the metallized and non‐metallized areas of the doped regions. Figure 14 shows an example of a selective doping structure on the front emitter (
There are several selective doping technologies that have been developed over years in the field of photovoltaics. One of these promising technologies is based on ion implantation through a mask which only increases the dopant dose on the regions underneath the screen‐printed contacts. The advantages of this technology are eliminating the formation of PSG and/or BSG, and fewer process steps. Other selective doping technologies, such as the etch‐back process, laser‐doping, oxide mask process, etc., have been addressed by Hahn, et al. [37]. As a rule of thumb, in order to implement these selective doping technologies to an industrial production line, every extra process step should provide the enhancement in cell efficiency of 0.2–0.3% absolute considering the related extra manufacturing costs.
4.2. Tunnel oxide passivated contact
Another feasible solution to minimize contact recombination is to put a passivating material with offset bands between the metal and silicon, also known as a passivated contact. Introduction of a thin passivating interlayer between the high recombination regions and the silicon absorber mitigates their negative impact because they are not in direct contact with the absorber. This reduces total recombination or saturation current density (
A promising approach to achieving a carrier selective passivated contact involves an ultra‐thin (∼15 Å, see the TEM image in Figure 15b) tunnel oxide capped with phosphorus doped
5. Summary and outlook
The annual shipment and installation of PV cells and modules up to date are still dominated by the standard industrial solar cell fabrication process on
Although it is hard to exactly predict which process approach and cell architecture will be the most cost‐effective in the future, selective doping and tunnel oxide passivated contacts have become active areas of investigation for silicon solar cells, because they can produce higher cell efficiency due to reduced minority carrier recombination. Combining these two promising technologies into a screen‐printed front junction
Acknowledgments
The author would like to thank Professor Ajeet Rohatgi for his kind support at University Center of Excellence for Photovoltaics (UCEP) at the Georgia Institute of Technology. The author also thanks Dr. Adam Payne of Suniva Inc. for proof‐reading.
References
- 1.
Hahn G, Joos S. State‐of‐the‐art industrial crystalline silicon solar cells. In: Semiconductors and Semimetals; G.P. Willeke and E.R. Weber, Eds, 1st ed. San Diego, CA, USA: Academic, 2014, vol. 90, pp. 1–62. - 2.
Macdonald D and Geerligs L J. Recombination activity of interstitial iron and other transition metal point defects in p‐and n‐type crystalline silicon. Applied Physics Letters. 2004; 85 (18):4061–4063. - 3.
Glunz SW, Rein S, Lee JY, Warta W. Minority carrier lifetime degradation in boron‐doped Czochralski silicon. Journal of Applied Physics. 2001; 90 (5):2397–2404. - 4.
Zhao J, Wang A, Altermatt P P, Green M, Rakotoniaina J P & Breitenstein O. High efficiency PERT cells on n‐type silicon substrates. In: 29th IEEE Photovoltaic Specialists Conference; May 2002; pp. 218–221. - 5.
Green M G. Solar cells: operating principles, technology, and system applications. Englewood Cliffs, NJ: Prentice‐Hall, Inc.; 1982. 288 p. - 6.
Shockley W, Queisser HJ. Detailed balance limit of efficiency of p–n junction solar cells. Journal of Applied Physics. 1961; 32 (3):510–519. - 7.
Meier DL, Good EA, Garcia RA, Bingham BL, Yamanaka S, Chandrasekaran V, Bucher C. Determining components of series resistance from measurements on a finished cell. In: Photovoltaic Energy Conversion, Conference Record of the 2006 IEEE 4th World Conference; IEEE; Wailohoa, HI, USA 2006; pp. 1315–1318. - 8.
Shockley W, Read WT, Jr. Statistics of the recombinations of holes and electrons. Physical Review. 1952; 87 (5):835. - 9.
Hall RN. Electron‐hole recombination in germanium. Physical Review. 1952; 87 (2):387. - 10.
Sinton RA, Cuevas A. Contactless determination of current–voltage characteristics and minority‐carrier lifetimes in semiconductors from quasi‐steady‐state photoconductance data. Applied Physics Letters. 1996; 69 (17):2510–2512. - 11.
Altermatt PP. Models for numerical device simulations of crystalline silicon solar cells—a review. Journal of Computational Electronics. 2011; 10 (3):314–330. - 12.
Feldmann F, Bivour M, Reichel C, Hermle M, Glunz SW. Passivated rear contacts for high‐efficiency n‐type Si solar cells providing high interface passivation quality and excellent transport characteristics. Solar Energy Materials and Solar Cells. 2014; 120 :270–274. - 13.
Misiakos K, Tsamakis D. Accurate measurements of the silicon intrinsic carrier density from 78 to 340 K. Journal of Applied Physics. 1993; 74 (5):3293–3297. - 14.
Richter A, Werner F, Cuevas A, Schmidt J, Glunz SW. Improved parameterization of Auger recombination in silicon. Energy Procedia. 2012; 27 :88–94. - 15.
Seidel H, Csepregi L, Heuberger A, Baumgärtel H. Anisotropic etching of crystalline silicon in alkaline solutions: I. Orientation dependence and behavior of passivation layers. Journal of the Electrochemical Society. 1990; 137 (11):3612–3626. - 16.
Neuhaus DH, Münzer A. Industrial silicon wafer solar cells. Advances in OptoElectronics. vol. 2007 (2007):1–15. - 17.
Kern W. The evolution of silicon wafer cleaning technology. Journal of the Electrochemical Society. 1990; 137 (6):1887–1892. - 18.
Kern W. Cleaning solutions based on hydrogen peroxide for use in silicon semiconductor technology. RCA Review. 1970; 31 :187–206. - 19.
Schiele Y, Fahr S, Joos S, Hahn G, Terheiden B. Study on boron emitter formation by BBr3 diffusion for n‐type Si solar cell applications. In: 28th EU PVSEC; Paris, France: 2013. - 20.
Werner S, Lohmuller E, Belledin U, Vlooswijk AHG, Naber RCG, Mack S, Wolf A. Optimization of BBr3 diffusion processes for n‐type silicon solar cells. In: 31st European PVSEC; 14–18 September; Hamburg, Germany. 2015. - 21.
Taniguchi K, Kurosawa K, Kashiwagi M. Oxidation enhanced diffusion of boron and phosphorus in (100) silicon. Journal of the Electrochemical Society. 1980; 127 (10):2243–2248. - 22.
Rohatgi A, Meier DL, McPherson B, Ok YW, Upadhyaya AD, Lai JH, Zimbardi F. High‐throughput ion‐implantation for low‐cost high‐efficiency silicon solar cells. Energy Procedia. 2012; 15 :10–19. - 23.
Hermle M, Benick J, Rüdiger M, Bateman N, Glunz SW. N‐type silicon solar cells with implanted emitter. In: 26th European Photovoltaic Solar Energy Conference; Hamburg, Germany; 2011; pp. 875–878. - 24.
Müller R, Benick J, Bateman N, Schön J, Reichel C, Richter A, Hermle M, Glunz SW. Evaluation of implantation annealing for highly‐doped selective boron emitters suitable for screen‐printed contacts. Solar Energy Materials and Solar Cells. 2014; 120 :431–435. - 25.
Peters S. Industrial diffusion of phosphorus n‐type emitters for standard wafer‐based silicon solar cells. Photovoltaics International. 2009; 3 :60–66. - 26.
Wolf A, Kimmerle A, Werner S, Maier S, Belledin U, Meier S, Biro D. Status and perspective of emitter formation by POCl3‐diffusion. In: 31st European PVSEC; 14–18 Sep; Hamburg, Germany. 2015. - 27.
Tao Y, Ok YW, Zimbardi F, Upadhyaya AD, Lai JH, Ning S, Upadhyaya VD, Rohatgi A. Fully ion‐implanted and screen‐printed 20.2% efficient front junction silicon cells on 239 cm n‐type CZ substrate. IEEE Journal of Photovoltaics. 2014; 4 (1):58–63. - 28.
Lanterne A, Le Perchec J, Gall S, Manuel S, Coig M, Tauzin A, Veschetti Y. Understanding of the annealing temperature impact on ion implanted bifacial n‐type solar cells to reach 20.3% efficiency. Progress in Photovoltaics: Research and Applications. 2015; 23 (11):1458–1465. - 29.
Hoex B, Schmidt J, Bock R, Altermatt PP, Van de Sanden MC, Kessels WM. Excellent passivation of highly doped p‐type Si surfaces by the negative‐charge‐dielectric Al2O3. Applied Physics Letters. 2007; 91 (11):112107. - 30.
Dingemans G, Kessels WMM. Recent progress in the development and understanding of silicon surface passivation by aluminium oxide for photovoltaics. In: 25th European PVSEC; Valencia, Spain. 2010. - 31.
Heavens OS. Optical properties of thin solid films. Courier Corporation; Dover, New York; 1991. - 32.
Ralph EL. Recent advancements in low cost solar cell processing. In: 11th IEEE PVSC; Scottsdale. 1975. pp. 315–316. - 33.
Ok YW, Upadhyaya AD, Zimbardi F, Tao Y, Cooper IB, Rohatgi A, Carroll AF, Suess T. Effect of Al content on the performance of Ag/Al screen printed N‐type Si solar cells. In: 39th IEEE Photovoltaic Specialists Conference (PVSC); June 16; IEEE; Tampa, FL, USA 2013. - 34.
Ballif C, Huljić DM, Willeke G, Hessler‐Wyser A. Silver thick‐film contacts on highly doped n‐type silicon emitters: structural and electronic properties of the interface. Applied Physics Letters. 2003; 82 (12):1878–1880. - 35.
Schubert G, Fischer B, Fath P. Formation and nature of Ag thick film front contacts on crystalline silicon solar cells. In: Proceedings of the Photovoltaic in Europe Conference; October 7; Rome, Italy; 2002; Vol. 343. - 36.
Institute for Solar Energy Research in Hamelin (ISFH). Ion implanted, co‐annealed and fully screen‐printed bifacial n‐type PERT solar cells with efficiencies of 21% and bifaciality factors exceeding 97% [Internet]. September 4, 2015. Available from: http://www.isfh.de - 37.
Hahn G. Status of Selective Emitter Technology. In: 25th European Photovoltaic Solar Energy Conference and Exhibition/5th World Conference on Photovoltaic Energy Conversion; 6–10 September; Valencia, Spain; 2010; pp. 1091–1096. - 38.
Taguchi M, Yano A, Tohoda S, Matsuyama K, Nakamura Y, Nishiwaki T, Fujita K, Maruyama E. 24.7% record efficiency HIT solar cell on thin silicon wafer. IEEE Journal of Photovoltaics. 2014; 4 (1):96–99. - 39.
Masuko K, Shigematsu M, Hashiguchi T, Fujishima D, Kai M, Yoshimura N, Yamaguchi T, Ichihashi Y, Mishima T, Matsubara N, Yamanishi T. Achievement of more than 25% conversion efficiency with crystalline silicon heterojunction solar cell. IEEE Journal of Photovoltaics. 2014; 4 (6):1433–1435. - 40.
Feldmann F, Bivour M, Reichel C, Hermle M, Glunz SW. Passivated rear contacts for high‐efficiency n‐type Si solar cells providing high interface passivation quality and excellent transport characteristics. Solar Energy Materials and Solar Cells. 2014; 120 :270–274. - 41.
Lee WC, Hu C. Modeling CMOS tunneling currents through ultrathin gate oxide due to conduction‐and valence‐band electron and hole tunneling. IEEE Transactions on Electron Devices. 2001; 48 (7):1366–1373. - 42.
Tao Y, Upadhyaya V, Jones K, Rohatgi A. Tunnel oxide passivated rear contact for large area n‐type front junction silicon solar cells providing excellent carrier selectivity. AIMS Materials Science. 2016; 3 (1):180–189. - 43.
Tao Y, Chang EL, Upadhyaya A, Roundaville B, Ok YW, Madani K, Chen CW, Tate K, Upadhyaya V, Zimbardi F, Keane J, Rohatgi A. 730 mV implied Voc enabled by tunnel oxide passivated contact with PECVD grown and crystallized n+ polycrystalline Si. In: Photovoltaic Specialist Conference (PVSC), IEEE 42nd; June 14; New Orleans; 2015. - 44.
Tao Y, Upadhyaya V, Huang Y, Chen C, Jones K, Rohatgi A. Carrier selective tunnel oxide passivated contact enabling 21.4% efficient large‐area n‐type silicon solar cells. In: 43rd IEEE Photovoltaic Specialists Conference; June 5; Portland, Oregon; 2016. - 45.
ITRPV. International Technology Roadmap for Photovltaic Results 2014_Rev.1 [Internet]. July 2015. Available from: http://www.itrpv.net/Reports/Downloads/2015/ - 46.
Hermle M, Feldmann F, Eisenlohr J, Benick J, Richter A, Lee B, Stradins P, Rohatgi A, Glunz SW. Approaching efficiencies above 25% with both sides‐contacted silicon solar cells. In: Photovoltaic Specialist Conference (PVSC), 2015 IEEE 42nd; June 14; pp. 1–3. - 47.
Glunz S, Feldmann F, Richter A, Bivour M, Reichel C, Benick J, Hermle M. The irresistible charm of a simple current flow pattern approaching 25% with a solar cell featuring a full‐area back contact. In: Proc. 31h Europ. Photovolt. Sol. Energy Conf., Hamburg, Germany, 2015 Sep 14.