with Plasma-Enhanced Chemical Vapour Deposition (PE-CVD)
6.1.2 Physical Properties of Amorphous Silicon Layers
1. General structure
Crystalline solids, such as monocrystalline silicon wafers, have a fully regular and periodic structure; they possess what is called bothshort- andlong-range order.
In such a crystalline silicon network (or crystallinematrixas it is also called), each silicon atom is bonded to four neighbouring silicon atoms. Figure6.3a schematically shows this situation. Amorphous solids, such as hydrogenated amorphous silicon (a-Si:H) do not have a fully regular and periodic structure, but rather a random or
“chaotic” structure: They only have short-range order (see Fig.6.3b).
In crystalline silicon, the bond angle—i.e., the angle between two adjacent bonds—is fixed at a value of 109°28and remains the same throughout the whole crystalline network (Fig.6.4). Thebond length, or distance between two neighbour-ing silicon atoms within such a network, is also fixed and remains constant throughout the whole network at a value of approximately 0.235 nm.
In amorphous silicon, both the bond angles and the bond lengths vary in a random fashion: there is a whole distribution of values. As an example: The bond angles in a-Si:H have a random distribution centred on 109°28and a standard deviation
142 A. Shah
a) Crystalline silicon b) Amorphous silicon
Fig. 6.3 Schematic representation ofaCrystalline silicon matrix andbamorphous silicon matrix
Fig. 6.4 Atomic model for a silicon atom within a crystalline silicon network, indicating the bond angle formed between two adjacent bonds. In amorphous silicon, this angle has a distribution of values. Reproduced from Shah [1] with permission of the EPFL Press
of 6°–9°. The “best”1a-Si:H has bond angles with a narrow distribution (standard deviation of 6°–7°).
Because of the random or “chaotic” structure of amorphous silicon, not all silicon atoms within the amorphous layer, find four other silicon atoms as “next neighbours”.
From time to time, there is a silicon atom, which only has three other silicon atoms as “next neighbours”. This specific silicon atom has, therefore, a “broken bond” or
“dangling bond” as it also called (see Fig. 6.5a). In amorphous silicon layers, as deposited by Plasma-Enhanced Chemical Vapour Deposition (PE-CVD) from silane (SiH4), a large proportion (over 99%) of the original dangling bonds are “passivated”
by hydrogen, during the deposition process: The “passivated” dangling bonds have a hydrogen atom sitting on them, as represented in Fig.6.5b. The “passivated” dangling bonds do not act as recombination centres and do not constitute gap states; they will therefore not be counted as dangling bonds in the following discussion. In some very rare cases, a silicon atom has only two other silicon atoms as “next neighbours”—they constitute a “SiH2-configuration” as represented in Fig.6.5c.
1«best» meaning «usable for the production of devices, such as solar cells and thin-film transistors».
6 Amorphous Silicon Solar Cells 143
Fig. 6.5 Model for silicon atom witha unpassivated dangling bond (acting as recombination centre or mid-gap state)bdangling bond “passivated” by a hydrogen atom (and no longer acting as a dangling bond)ctwo hydrogen atoms connected to it (SiH2-configuration). Reproduced from [1], with the kind permission of the EPFL Press
2. Light-induced degradation or Staebler-Wronski Effect (SWE)
Because of the random or “chaotic” structure of amorphous silicon layers, these layers change their properties, when exposed to light: Some of the dangling bonds passivated by a hydrogen atom (Fig. 6.5b), lose their hydrogen atom, under the influence of light—this was discovered as early as 1977 by Staebler and Wronski [5]. It was designated as the “Staebler-Wronski Effect (SWE)” Since then, there has been, right up to 2005, a huge research effort to find ways of suppressing the SWE, albeit without any real success [6,7]. Well, although, one does not know how to fully suppress the SWE, many things are known about the SWE:
(a) If the amorphous silicon layer contains many SiH2-configurations (Fig.6.5c), the SWE will be more pronounced. Such SiH2-configurations are created if the deposition is done too rapidly by increasing the “RF Power” in the PE-CVD deposition system (Fig.6.1).
(b) SWE is a reversible effect: By heating the amorphous layer, during a few hours at about 200 °C (so-called “annealing” process), the original state is restored.
(c) SWE is an “asymptotic effect”. If one continues exposing the layers to light over a very long period (typically many thousands of hours), one reaches what is called a “stabilized final state”—with roughly ten times more dangling bonds than in the beginning.
(d) SWE can be influenced by the deposition parameters (Fig.6.1): an increase in deposition temperature [8] or an addition of atomic hydrogen in the Reaction gases [9] will both lead to layers with a less pronounced SWE.
(e) SWE is closely linked to the presence and behaviour of hydrogen2within the amorphous layer. In this context there is an optimal value for the hydrogen content of the amorphous silicon layer: This value is around 10 atomic % of hydrogen, meaning that there is 1 hydrogen atom for 10 silicon atoms. At this value, hydrogen is “helpful”, because it mainly passivates dangling bonds. If
2Interestingly, hydrogen also plays a major role in the instability of perovskite solar cells.
144 A. Shah the hydrogen content is higher, then the “excess” hydrogen atoms are no more linked to silicon atoms, but wander about freely within the amorphous layer, creating havoc and ultimately leading to an increase in SWE.
(f) The inclusion of impurities like oxygen within the amorphous layer, leads to an increased SWE.3
3. Density of states within the bandgap of amorphous silicon layers
Because of the random or “chaotic” structure of amorphous silicon, amorphous silicon does not have a real bandgap, like crystalline silicon. In comparison with crystalline silicon, a-Si:H layers have:
(a) An “equivalent bandgap”, which is filled with gap states, acting (partly) as recombination centres. This is shown schematically in Fig.6.6.
(b) A higher “equivalent bandgap”, higher than the “real bandgap” of crystalline silicon, i.e. approximately 1.75 eV instead of 1.12 eV
Figure6.6indicates the density of statesN(E) within the “equivalent bandgap” (called in the Figure “mobility gap”) of typical amorphous silicon layers. The “midgap states” act as recombination centres. Their density is increased by the SWE. (Fig.6.7).
4. Optical properties of amorphous silicon layers
The optical properties of a-Si:H layers are very different from those of c-Si. From a practical point of view, the main differences are:
(1) Absorption starts at a higher value of photon energy for a-Si:H→i.e. at a shorter wavelengthλof light: atλ≈700 nm for a-Si:H as compared toλ≈1100 nm for c-Si
3There is a long story behind this simple statement—when work started at IMT Neuchâtel, after 1985, on amorphous silicon layers and solar cells, there was the “justified hope” that by suppressing (reducing) the oxygen content within the layers we were depositing, we would be able to totally remove the SWE effect. We therefore invested heavily in so-called “gas purifiers”, so as to be able to produce a-Si:H layers with very low oxygen content. To our misfortune it turned out that these layers had just the same intensity of SWE as layers containing more oxygen. It was only above a certain threshold that oxygen led to an enhancement of SWE. However, when we started to deposit microcrystalline silicon layers, with our equipment, the gas purifiers were absolutely decisive: We at IMT Neuchâtel became the first laboratory able to produce microcrystalline silicon solar cells with conversion efficiencieswell above 5%. Thanks to this discovery, the Author of this chapter received in 2007 the Becquerel Prize.More importantly the Author of this Chapter would wish to communicate to the Readers the following lessons he learnt:
1. Never underestimate instability effects. They generally turn out to be unavoidable. no matter what tricks one tries to do. This should be remembered in the context of the present hype regardingperovskitesolar cells. These cells are at present unstable, and may remain so, no matter what tricks researchers try to do.
2. Research often leads to results you are totally unable to predict—our layers with low oxygen content were of no use for amorphous silicon solar cells, but they were a decisive asset, when we started our work on microcrystalline solar cells. The present heavy research investment in perovskite solar cells will probably also lead to some quite unexpected results.
6 Amorphous Silicon Solar Cells 145
Fig. 6.6 Diagram showing schematically the density of electronic “gap states”N(E), which are a result of the amorphous nature of the semiconductor. The density of statesN(E) is represented logarithmically and as ordinate (on the vertical axis); the energyEof the corresponding electronic state is represented as abscissa (on the horizontal axis). The “gap states” are situated in the mobility gap, i.e. in the energy range between the valence band edgeEVand the conduction band edge EC. The “midgap states” result from the dangling bonds and constitute recombination centres. The bandtails are also characteristic of the amorphous nature of the layers. Adapted (with modifications) from [1]
Fig. 6.7 Sketch indicating the difficulty of doing research on amorphous silicon.Courtesy
Dji-Illustrations, Neuchâtel
146 A. Shah (2) Amorphous silicon layers have a much stronger absorption than crystalline
silicon layers (see Chap. 3, Sect.3.2.2)
Because of (2) it became possible to fabricate amorphous silicon solar cells, which had to be kept very thin, in order not to suffer unduly from the SWE.
Because of (1) these amorphous cells have a spectral response that is different from the spectral response of crystalline silicon cells (see Chap. 3, Sect.3.6), rendering amorphous cells particularly suitable for use with indoor lighting.