Lecture 6. Monocrystalline Solar Cells

Transcription

1 Lecture 6 Monocrystalline Solar Cells References: 1. Physics of Solar Cells. Jenny Nelson. Imperial College Press, Photovoltaic Materials, Series on Properties of Semiconductor Materials, Vol.1, Richard H. Bube, Imperial College Press, Handbook of Photovoltaic Science and Engineering. Antonio Luque, Steven Hegedus. Wiley, Photovoltaic Solar Energy Generation. Adolf Goetzberger, Volker U. Hoffmann. Springer, Wikipedia ( 1

2 Principles of Cell Design Efficient photovoltaic energy conversion requires efficient light absorption, efficient charge separation and efficient charge transport. For good optical absorption, (1) large optical depth, α(e)(xp+xn) for photon energy E>Eg; (2) small reflectivity of the surface, R(E). For good charge separation, (1) large built-in bias Vbi demanding high doping gradient across the junction; (2) slow charge recombination in the junction region; (3) junction located close to the surface for effective charge separation over a range of wavelengths. For efficient minority and majority carrier transport, (1) long minority carrier lifetimes (τn, τp) and diffusion lengths (Ln, Lp), small surface recombination velocities (Sn, Sp); (2) small series resistance Rs and large shunt resistance Rsh. The band gap should be close to the optimum for the intended solar spectrum, bs(e). Some of the demands are contradictory and a compromise must be found. 2

3 Efficiency/ % General Material/Design Features Ge Si InP GaAs Single band gap solar cell under AM Band Gap/ ev Monocrystalline, Multicrystalline: crystal grain size is comparable with or larger than the device thickness. Polycrystalline, Microcrystalline: the grain size is much smaller than device thickness. Grain boundary effects may dominate charge transport. The thickness should exceed the absorption length, for efficient light absorption. The junction should be shallow compared to both the diffusion length in the emitter and the absorption length, to avoid having a dead layer at the front of the cell. The emitter should be doped heavily to improve conductivity to the metallic contacts on the front of the cell. Heavy emitter doping also allows the base to be doped lightly, improving collection in the neutral base region without limiting VOC. Reflection of light should be minimized, and treated with an antireflection (AR) coating (refractive index between sc and air). 3

4 Silicon Material Properties SILICON Group IV element; Tetrahedral crystal structure (ambient); Indirect band gap semiconductor; Band gap is 1.1 ev (ambient); Refractive index is ~3.4 and natural reflectivity is ~40% over visible wavelength. The Czochralski method 4

5 Production of Single Crystal Silicon 5

6 Silicon Material Properties Doping Silicon can be readily doped n type by the addition of pentavalent phosphorus impurity atoms, p type by the addition of trivalent boron. Doping is needed in the emitter to reduce Rs and increase Vbi/VOC, while heavily doping may reduce VOC and induce recombination. Recombination Electron recombination in the lightly doped p region is dominant, as the heavily doped n region (emitter) is rather thin and photogeneration is negligible. The net recombination rate U U = U rad + U Aug + U SRH Different processes will have different temperature and doping dependence. In lightly doped p-silicon at room temperature, SRH dominates. In more heavily doped silicon or at higher temperature, Auger dominates. Carrier Transport Electron mobility in p-silicon is higher than the hole mobility in n-silicon doped to the same level. Carrier collection is more efficient in p-silicon than in an n- layer. So cells are designed as n-p cells with a thin n type emitter on top of a thick p type base. 6

7 Silicon Solar Cell Design A typical silicon solar cell is an n-p junction made in a wafer of p-silicon. The p-wafer forms the base of the cell and is thick ( µm) in order to absorb as much light as possible and is lightly doped (~10 16 cm -3 ) to improve diffusion length. EC Energy emitter n-type base p-type EF The n type emitter is created by dopant diffusion and is heavily doped (~10 19 cm -3 ) to reduce Rs. This layer should be thin to allow as much light as possible to pass through to the base. EV 0.3 μm doping density 300 μm Distance The dopant profile in the diffused n layer is not uniform and the junction not abrupt. The front surface is anti-reflection coated. Both front/back surfaces are contacted before encapsulation in a glass covering. n+ p distance 7

8 Silicon Solar Cell Fabrication Typical Processing Sequence: TiO2, Ta2O5, Si3N4 8

9 Phosphorus Diffusion N2+POCl3 (a) A quartz furnace; and (b) a belt furnace for the diffusion of phosphorus 9

10 Screen Printing Process: 10

11 Optimization of Silicon Solar Cell Design Cumulative rate/ Incident flux nm light absorption recombination Cumulative rate/ Incident flux nm light absorption recombination Depth/μm Depth/μm For a silicon solar cell, Absorption of light close to the band gap is poor. Bulk recombination in p region is most important. Rear surface recombination is important. To improve the performance of cell it is necessary to maximize the absorption of red light, minimize recombination at the rear surface, and minimize series resistance. Bulk recombination is already as low as can be expected for good quality silicon. 11

12 Strategies to Enhance Absorption 1. Texturing of front surface: The front surface is textured to reduce reflectivity and increase the optical path length. Light trapping is improved by using inverted pyramids (improved total internal reflection of light). Mc-Si surface after acid etching 12

13 Strategies to Enhance Absorption 2. Optimization of contacts: Shading of the front surface by metal contacts reduces the surface area available to the incident light. Reduced contact area increases the available surface area but increases the resistance. A large contact area can be achieved without increasing the surface shading by embedding the contacts in the sc. Laser grooved buried grid solar cell process 13

14 Strategies to Reduce Surface Recombination 1. Back surface field (BSF): A more heavily doped layer is formed at the back surface of the p-base by alloying with Al or by diffusion. This introduces a p + -p junction and presents a potential barrier to the minority electron. The BSF reflects electrons and reduces the effective rear surface recombination. The extra p + -p junction also adds to Vbi, and may enhance VOC. EF e p p+ EC EV 2. Passivation of front surface with thin oxide coating: The high surface recombination velocity at a free surface tends to create a dead layer. Oxidizing the surface creates a thin layer of the wide band gap insulator, SiO2, preventing carriers from reaching the surface and hence reduces the effective surface recombination velocity. The Si/SiO2 interface is much less defective than a free Si surface. The microgrooved passivated emitter solar cell (PESC cell) of 1985, the first silicon cell to exceed 20% efficiency. 14

15 Strategies to Reduce Surface Recombination 3. Use of point contact at rear: Since the Si/M interface is more defective than Si/SiO2 interface, rear surface recombination can be reduced by contacting part of the rear p layer with metal, using point contacts. The rest of the surface can then be passivated with oxide. p p+ p contact p p+ p contact oxide layer In order to avoid problems with series resistance, the region of sc close to the point contacts is differentially doped p+. 15

16 Strategies to Reduce Series Resistance 1. Optimization of the n region doping: Reduced doping improves collection from the n region, giving a better response to the blue light. Increased n doping increases Vbi and reduces series resistance, while very high n doping may reduce VOC because of Auger recombination and band gap narrowing. 2. Differential doping of the area around the contacts: For point and grid contacts, the current density through the material close to the contacted area is high. Doping this volume heavily reduces the losses to series resistance. 3. Narrow but deep finger in front contact, as above: The high aspect ratio reduces surface area blocked by contacts without reducing finger cross sectional area, and the relatively high contact area between fingers and semiconductor reduces the current density at the contact. 16

17 Evolution of Silicon Solar Cell Design Space silicon cell design developed in the early 1960s, which became a standard design for over a decade. Chemically textured non-reflecting black cell (so called because of their almost zero reflectivity) in the early 1980s, and exhibited efficiencies of up to 17%. 17

18 Evolution of Silicon Solar Cell Design hν n+ n- n- p+ n+ oxide layer n contact p contact n contact Rear point contact solar cell which demonstrated 22% efficiency in 1988 (cell rear shown uppermost). The cell is made from lightly doped n type silicon with heavily doped n and p type regions close to contacts on the rear surface. The front surface is passivated and textured as usual. 18

19 Evolution of Silicon Solar Cell Design The passivated emitter and rear cell (PERC cell). The passivated emitter, rear locally diffused cell (PERL cell) which took efficiency above 24% in the early 1990s. 19

20 Evolution of Silicon Solar Cell Design The passivated emitter, rear totally diffused cell (PERT cell). the passivated emitter, rear floating junction cell (PERF cell) 20

21 Fabrication of Silicon Photovoltaic Module Exploded view of a standard silicon photovoltaic module. The different layers shown are laminated together under pressure at a temperature around o C where the transparent EVA (ethylene vinyl acetate) softens and binds the different layers together on cooling. 21

22 Future Directions in Silicon Cell Design The performance of silicon solar cells is now fairly close to the theoretical maximum of 29%. The main challenges are now in improving cell production techniques in order to mass-produce efficient cells more cheaply. An approach is the thin film microcrystalline silicon cell with the objective to reduce bulk recombination losses without losing absorption and effective light trapping. Silicon is not an ideal solar cell material because: Its band gap (1.1 ev) is smaller than the optimum (1.4 ev) for terrestrial solar energy conversion. It is indirect band gap material with small absorption coefficient. So a relatively thick layer of silicon is needed which increases the cost. The temperature dependence of efficiency makes silicon less suitable for application under concentrated light and in space. 22

23 III-V Semiconductor Material Properties III-V Semiconductor An alloy containing equal numbers of atoms from groups III and V; Zinc blende crystal structure (ambient); Band gap is controllable by replacing some of the group III atoms with another group III elements. Eg/eV at 300 K Nature of Energy Gap Density at 300K Dielectric Constant AlP 2.45 Indirect AlAs Indirect AlSb Indirect GaP Indirect GaAs Direct GaSb 0.75 Direct InP Direct InAs 0.76 Direct InSb 0.17 Direct

24 III-V Semiconductor Material Properties The best understood and most widely used III-V semiconductor is gallium arsenide (GaAs). It is also the most suitable for solar energy conversion. As a direct band gap semiconductor, its absorption coefficient is 10 times that of silicon, so only a few microns rather than hundreds of microns are needed for the active layer of the solar cell. GaAs solar cell has better temperature coefficient than silicon, and performs better under concentration and in space where temperature is normally high. InP has a suitable Eg for photovoltaic conversion and is particularly attractive for space applications because of its radiation hardness. Ternary alloys: AlxGa1-xAs, InxGa1-xP, InxGa1-xAs, etc. 24

25 III-V Semiconductor Material Properties Doping III-V sc can be doped by replacing one of the elements with one of different valence. n-gaas can be obtained by replacing some of the trivalent Ga with controlled amount of tetravalent Si/Sn. p-gaas can be prepared by replacing some of the Arsenic atoms with Carbon, or replacing some of Ga with Be. Recombination Radiative recombination is faster in GaAs than in Si and may dominate in very pure material. In practical materials, SRH recombination through defect states dominates. Hole lifetime is up to one order of magnitude shorter than that of electron in p-gaas doped to the same level. In p-n devices, SRH recombination in the space charge region dominates performance, and the dark current tends to vary with m=2 at low bias. At higher forward bias, m=1 as radiative recombination begins to dominate. Carrier Transport Electrons have a higher mobility than holes in GaAs. The diffusion length of minority carrier is a few microns in moderately doped GaAs. Ternary alloys such as n-alxga1-xas, minority carrier diffusion length is less than 0.1 µm. 25

26 Overview of GaAs Solar Cells As the diffusion length of minority carrier in either doping GaAs is greater than the absorption depth, cell can be prepared either as p-n or n-p designs. In either case, the emitter should be as thin as possible without increasing series resistance too much. For p-n design, a 0.5 µm emitter doped to cm -3 is typical. For the n-p design, the emitter can be as thin as 0.2 µm because of the higher n type conductivity. In practice, p + -n designs seem to perform better than n + -p designs. the base is much shorter than in silicon cells, typically 2-4 µm which is comparable with the diffusion length. 26

27 Optimization of GaAs Solar Cell Design 800 nm light 500 nm light Cumulative rate/ Incident flux recombination absorption Cumulative rate/ Incident flux recombination absorption Depth/μm Depth/μm For a GaAs solar cell, Absorption of light is good at all wavelengths. Front surface recombination is important for long wavelengths Bulk recombination is unimportant relative to junction and surface. Rear surface recombination is negligible because of the high absorption. To optimize GaAs cell design it is necessary to minimize front surface recombination, minimize junction recombination, minimize series resistance, and minimize substrate cost. 27

28 Strategies to Reduce Front Surface Recombination Because of the high absorption coefficient, contribution of the emitter to the photocurrent is not negligible and that surface and bulk recombination in this region are important. EC p + -AlGaAs p-gaas 1. Front surface field (FSF): A heavily doped EV n-gaas layer is introduced by diffusion, as the BSF in silicon cells. 2. Window layers: Front surface recombination can be reduced by introducing a front surface window of a higher band gap material to reflect electron away from the surface. Similar to BSF, the higher band gap of window layer presents a potential barrier to electrons generated in the p region. The window layer is transparent to most visible light, but the interface with the bulk GaAs is much less defective. (Buried homojunction) 3. Heterojunctions: Fabricate the whole emitter from a wider gap material than the base, producing a p-n heterojunction. So the emitter still absorbs blue light but recombination is suppressed. In addition, the wider gap emitter may increase VOC. However, it is likely to introduce defect and assist recombination. EF 28

29 Strategies to Optimize GaAs Solar Cells 4. Graded emitters: Grade the composition of the p layer, from GaAs near to the junction, to a high band gap alloy at the front surface. The electric field introduced by the compositional gradient assists electron migration to the junction. (AlxGa1-xAs of varying Al fraction.) EC p + -AlGaAs EV p-alxga1-xas n-gaas EF Strategies to reduce series resistance: It is particularly important for GaAs concentrator cells. For a cell under 100 suns, the sheet resistance should be less than 10-3 Ohm-cm 2, while the front surface contact pattern should be designed to minimize shading without enhancing series resistance. Strategies to reduce substrate cost: Substrate should have the same lattice constant as the cell material in order not to introduce crystal defects at the rear surface. GaAs is prohibitively expensive to be the substrate. Ge can be used but is rather rare. One approach is to use polycrystalline GaAs as cell material, in which case lattice matching with a substrate is less important. 20% efficient poly-gaas cells have been obtained. 29

30 III-V Material Preparation and Cell Fabrication MBE MOVPE III-Vs are grown by a number of epitaxial techniques such as liquid phase epitaxy (LBE), molecular beam epitaxy (MBE), metal-organic chemical vapor deposition (MOCVD) and metal-organic vapor phase epitaxy (MOVPE). These techniques allow minute control of the composition and layer thickness, while very costly. 30

31 GaAs Solar Cell Development In the development of p + -AlGaAs/p-GaAs/n-GaAs heteroface junction plays a dominant role. Reflection is minimized using anti-reflection coat. Texturing is not suitable because of the larger size of light trapping structures compared to the device thickness, and sensitivity of front surface. The record for highest efficiency is 25.1% for a single junction cell without concentration, and 29.2% for a single junction with concentration. One of the advantages of GaAs solar cell is its insensitivity to an increase in temperature - a monotonic decrease in efficiency of 0.033% per degree, and it is suitable for concentrator and space application. 31

32 Summary 32