Homojunction Cells
5.3 Functionality and Losses of the Al-BSF Cell .1 Architecture of the Al-BSF Cell.1Architecture of the Al-BSF Cell
In the previous Sect. 5.1we developed the fabrication processes and the physical concepts leading to the standard solar cell: to the so called Al-BSF solar cell. The Al-BSF cell (Aluminium Back Surface Field cell) consists ofp-type silicon and has a cross-section, which is shown in Fig.5.8.
A homogeneous emitter (pn-junction) represented as an++layer23heavily doped with phosphorous and having a thickness of ~300 nm is located on the textured silicon surface. In addition, there is an antireflection layer of silicon nitride SiNx, with a thickness of about 80 nm, which greatly reduces the reflectance of the surface, so that as much light as possible is absorbed. During the deposition of the silicon nitride layer, it is enriched with hydrogen during the plasma process in order to passivate the surface of the cell. Hydrogen atoms are very small and diffuse easily through every material, especially at higher temperatures; and they do this also during the production of the standard solar cell. Hydrogen acts as a positive, negative or neutral atom and occupies open bonds of the crystal atoms. Electrons cannot recombine later
Fig. 5.8 Cross section of a Al-BSF homojunction solar cell withp-type wafer material
23n++means higher doped with phosphorous thann+.p++means higher doped with boron thanp+.
5 Crystalline Silicon Solar Cells: Homojunction Cells 117 at these defects—because they are already occupied by hydrogen—and their lifetime increases. On the reverse side, an aluminium layer with a thickness of ~30μm is applied by screen-printing. A subsequent firing process leads to contact formation via an Al–Si alloy. This forms an electric field (Back Surface Field, BSF).
5.3.2 Band Diagram
We begin with a description of Fig.5.9, so that we can apply it to the cell structure in Fig. 5.10. In Fig.5.9, the band diagram is shown for an undoped and a doped semiconductor [9], as well as the Fermi level.24 Under sunlight, the electrons flow in the direction of then-side and the holes in the direction of thep-side.
The bandgap energyEgis 1.12 eV for silicon. When irradiated by sunlight, car-riers are generated. The field in the space charge zone causes the electrons in the conduction band to flow to then-side, e.g. energetically downwards, and the holes in the valence band slide to flow to thep-side.
Fig. 5.9 The Fermi level for undoped silicon (top, left), forp-doped silicon (top, middle) and for n-doped silicon (top, right). In the undoped case the Fermi level is exactly in the centre of the bandgap. It is pushed down forp-doped silicon and it is pushed up forn-doped silicon. When one puts ap-doped silicon region and an-doped silicon region together in the same device, the various Fermi levels have to align themselves, to form a single level for the whole device (lower part of the figure). This results in a potential step for conduction and valence band
24The Fermi level is the energy where the probability is just 50% to occupy this level. For undoped semiconductors, the Fermi level is exactly in the middle of the bandgap. For doped semiconductors, the Fermi level is near the edge of the conduction band forn-type material, since many electrons are present (n-doping) or close to the valence band edge inp-doped semiconductors.
118 S. Leu and D. Sontag
Fig. 5.10 The Al-BSF solar cell: cross section (left) with the corresponding bandgap diagram;
the bandgap diagram (right) shows the increase in potential through the Aluminium Back Surface Field, which protects electrons from recombining at the back
In Fig.5.10, the cell and the corresponding band diagram are now shown. The band diagram shows a small potential mountain on the back of the cell, which is caused by the back surface field. This repulses the electrons on the back and prevents them from recombining. Charge carrier transport is visualized in Fig.5.11.
5.3.3 The Losses of the Al-BSF Solar Cell
In Fig.5.11the typical loss zones of a solar cell are marked; they are explained in more detail below.
The following description follows the 7 points in Fig.5.12. The losses listed below are based on their geometric origins within the cell.
➀ (a) The photon has too little energy or the light strikes the solar cell at an incorrect angle. It will be reflected at the surface or it may enter silicon but will not be absorbed, it may be reflected by the back surface or escape silicon
(b) Parasitic absorption by free carriers (FCA)
➁ (a) Recombination losses underneath the metal contact (b) Ohmic losses on the metal contact
(c) Shading losses due to the metal contacts (d) Recombination at the surface
5 Crystalline Silicon Solar Cells: Homojunction Cells 119
Fig. 5.11 Simplified charge carrier transport. Blue light with a short wavelength does not penetrate deep into the solar cell. Red light penetrates far into the solar cell. The electron-hole-pairs are now forced through different gradients within the space charge zone. The gradients are generated by chemical potentials or electrical potentials resulting in Drift and Diffusion. Situation A: The electrons are driven upwards and the holes downwards and reach the contacts. Situation B: Some electrons and holes recombine at impurities in the crystal (or at the surface). Situation C: The Al-BSF pushes electrons back and prevents them from recombining at the back
Fig. 5.12 Typical loss zones (1–7) of a standard solar cell
120 S. Leu and D. Sontag
➂ (a) Pinholes in the silicon nitride layer, which affect negatively the quality of the surface passivation and lead locally to increased recombination (b) Ohmic resistance in thepn-junction
➃ Auger recombination
➄ (a) Scattering absorbs light in impurities.
(b) Impurities in the silicon such as iron and defects in crystal lattice cause recombination.
➅ (a) Reduced charge carrier lifetimes due to a high content of impurities (oxygen, carbon, metals)
(b) Series resistance in silicon material (low doping)
(c) Shockley-Read-Hall recombination in the bulk material because of foreign atoms, dislocations, interstitial deposits of foreign atoms, precipitates by oxygen, vacancies
(d) photons with energy Eph = hν > Eg. will most likely release their excess energy by thermalisation (see Chap.3)
➆ Recombination on the back side in spite of passivation and back surface field.
The losses can be also classified according to their nature:
(a) Optical losses:➀,➁c,➄a,➅d
(b) Recombination losses:➁a,➁d,➂a,➃,➄b,➅a,➅c (c) Ohmic losses:➁b,➂b,➅b.
They can be split up as follows: recombination losses: 2.5%; Ohmic and optical losses 2.0% each. Overall, the losses amount to about 6–7%. At a thermodynamic efficiency limit of29.4% for silicon single junction solar cellswith sunlight without light concentration,25the maximum cell efficiency achievable in mass production is approximately ~23.5% (=29.4–6%).
The development of standard solar cell was focused on improving passivation and reducing losses on the front side. Sophisticated coating processes and optimiza-tion in phosphorus diffusion led to substantial improvements, reducing thereby the recombination losses on the front side of the solar cell to one third of the recom-bination losses on the back side. The development of the standard solar cell with the Al-BSF is a big step in cell manufacturing. Nevertheless, the cell shows limi-tations. The back contributes only moderately to light trapping, for light with long wavelengths. The reflectance of the back is about 60–70%,26 and not over 90%, as it should be. Also, the surface recombination rate of 200–600 cm s−1can be further improved. This leads us to the concept of the Passivated Emitter Rear Cell (PERC), whose motivation was to improve the back side, especially its passivation and light trapping properties. The standard Al-BSF solar cell achieves today, at best, 20% cell efficiency. So, there is still considerable room for improvement to reach the 23.5%
efficiency limit mentioned above.
25This is valid for all homojunction cells.
26Long-wave light penetrates the solar cell and can escape from the cell on the back. To prevent this, the light-trapping on the back can be improved so that the light on the back is reflected and directed back into the cell.
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