4.5 Passivation layers
Passivation coatings aim to prevent processes that reduce the conversion efficiency of a photovoltaic cell.
The first process is charge recombination on the electrodes. Recombination processes occur in all parts of the cell, including the electrodes, although they are most intense in the active layer. Charges should be transferred from the electrodes to the external circuit, recombination reduces the amount of charge transferred and therefore the conversion efficiency. This is prevented by producing a tight, thin and corrosion resistant passivation layer through a chemical reaction. The second process prevented by passivation is electrode oxidation. Oxidation causes a change (deterioration) in the performance of the photovoltaic cells.
In the case of photovoltaic cells, passivation layers are placed on the front or back surface of the cell. However, the most common systems are those with a passivation layer between the silicon layer and the metal electrode. Simplified schemes of cells with a passivation layer are shown on Fig. 1.
An example of the passivation process developed by SoLayTec, InPassion is shown in the video "SoLayTec InPassion ALD for Al2O3". It shows the deposition of aluminium oxide films \( Al_{2}O_{3} \) using the atomic layer deposition (ALD) technique.
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An example of silicon passivation is silicon oxidation. On the silicon dioxide obtained as a result of oxidation ( \( SiO_{2} \)) aluminium is then applied. The layers are further processed in 400 \( _{}^{o}\textrm{C} \). Unfortunately, the high-temperature processing itself has a negative effect on the electrical properties of silicon [1]. Alternative passivation layers can be silicon nitride layers.
Cells with a bottom passivation layer have been manufactured and mass marketed. The most common cell types are 1:
- PERC (passivated emitter and rear cell),
- PERL (passivated emitter, rear locally diffused),
- PERT (passivated emitter, rear totally diffused) [2].
The term "passivated emitter" used in the name is understood in this case to mean the top layer of the cell. In all cases, the emitter layer is subjected to a passivation process. Additionally, the surface between semiconductor and metal electrode is passivated. PERC cells have a passivation layer which performs the reflection function. Photons reflected from the passivation layer reach the absorber, increasing the number of generated charge carriers. The principle of the operation of PERL cells is similar, except that the lower passivation layer is dispersed (composed of separated elements). In a PERT cell, the front and back surfaces of the monocrystalline cell are passivated. The back electrode is locally dispersed at the metal contacts only, to minimise recombination while keeping good electrical contact [3]. Cells with passivation layers as standard are becoming increasingly popular. Based on forecasts from ITRPV (International Technology Roadmap for Photovoltaic), PERC technologies will obtain \( 55\% \) panel market share by 2027 [4].
Przypis
1. The cells are described in the chapters: 6.2 PERL cells i 6.7 PERC and PERT type PV cells.Bibliography
1. A. G. Aberle: Surface passivation of crystalline silicon solar cells: a review, Progress in Photovoltaics 2000, Vol. 8, Iss. 5, pp. 473-487.2. J. Zhao, A. Wang, M. A. Green: High-efficiency PERL and PERT silicon solar cells on FZ and MCZ substrates, Solar Energy Materials and Solar Cells 2001, Vol. 65, Iss. 1-4, pp. 429-435, dostęp:14.12.2020
3. B. Szymański: Moduły fotowoltaiczne PERC – Co je wyróżnia i dlaczego zdobywają rynek?, GLOBEnergia : : Odnawialne Źródła i Poszanowanie Energii 2017/6.
4. M. Fisher: ITRPV 11th edition, October 2020, key findings - maturity report update, International Technology Roadmap for Photovoltaic (ITRPV).