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High-rate growth of hydrogenated amorphous and microcrystalline silicon for thin-film silicon solar cells using dynamic very-high frequency plasma-enhanced chemical vapor deposition

Thin-film silicon tandem solar cells based on a hydrogenated amorphous silicon (a-Si:H) top-cell and a hydrogenated microcrystalline silicon (μc-Si:H) bottom-cell are a promising photovoltaic technology as they use a combination of absorber materials that is ideally suited for the solar spectrum. Additionally, the involved materials are abundant and non-toxic which is important for the manufacturing and application on a large scale.

One of the most important factors for the application of photovoltaic technologies is the cost per watt. There are several ways to reduce this figure: increasing the efficiency of the solar cells, reducing the material consumption and increasing the throughput of the manufacturing equipment.

The use of very-high frequencies has been proven to be beneficial for the material quality at high deposition rates thus enabling a high throughput and high solar cell efficiencies. In the present work a scalable very-high frequency plasma-enhanced chemical vapor deposition (VHF-PECVD) technique for state-of-the-art solar cells is developed. Linear plasma sources are applied which facilitate the use of very-high frequencies on large areas without compromising on the homogeneity of the deposition process.

The linear plasma sources require a dynamic deposition process with the substrate passing by the electrodes in order to achieve a homogeneous deposition on large areas. State-of-the-art static radio-frequency (RF) PECVD processes are used as a reference in order to assess the potential of a dynamic VHF-PECVD technique for the growth of high-quality a-Si:H and μc-Si:H absorber layers at high rates.

In chapter 4 the influence of the deposition process of the μc-Si:H i-layer on the solar cell performance is studied for static deposition processes. It is shown that the correlation between the i-layer growth rate, its crystallinity and the solar cell performance is similar for VHF- and RF-PECVD processes despite the different electrode configurations, excitation frequencies and process regimes. It is found that solar cells incorporating i-layers grown statically using VHF-PECVD processes obtain a state-of-the-art efficiency close to 8 % for growth rates up to 1.4 nm/s compared to 0.53 nm/s for RF-PECVD processes.

The influence of dynamic deposition processes on the performance of μc-Si:H solar cells is studied. It is found that μc-Si:H solar cells incorporating dynamically grown i-layers obtain an efficiency of 7.3 % at a deposition rate of 0.95 nm/s. There is a small negative influence of the dynamic deposition process on the solar cell efficiency compared to static deposition processes which is related to the changing growth conditions the substrate encounters during a dynamic i-layer deposition process.

The changes in gas composition during a dynamic i-layer deposition process using the linear plasma sources are studied systematically using a static RF-PECVD regime and applying a time-dependent gas composition. The results show that the changes in the gas composition affect the solar cell performance if they exceed a critical level.

In chapter 5 dynamic VHF-PECVD processes for a-Si:H are developed in order to investigate the influence of the i-layer growth rate, process parameters and deposition technique on the solar performance and light-induced degradation. The results in this work indicate that a-Si:H solar cells incorporating i-layers grown dynamically by VHF-PECVD using linear plasma sources perform as good and better as solar cells with i-layers grown statically by RF-PECVD at the same deposition rate. State-of-the-art stabilized a-Si:H solar cell efficiencies of 7.6 % are obtained at a growth rate of 0.35 nm/s using dynamic VHF-PECVD processes.

It is found that the stabilized efficiency of the a-Si:H solar cells strongly decreases with the i-layer deposition rate. A simplified model is presented that is used to obtain an estimate for the deposition rate dependent efficiency of an a-Si:H/μc-Si:H tandem solar cell based on the photovoltaic parameters of the single-junction solar cells. The aim is to investigate the individual influences of the a-Si:H and μc-Si:H absorber layer deposition rates on the performance of the tandem solar cell.

The results show that a high deposition rate of the μc-Si:H absorber layer has a much higher potential for reducing the total deposition time of the absorber layers compared to high deposition rates for the a-Si:H absorber layer. Additionally, it is found that high deposition rates for a-Si:H have a strong negative impact on the tandem solar cell performance while the tandem solar cell efficiency remains almost constant for higher μc-Si:H deposition rates.

It is concluded that the deposition rate of the μc-Si:H absorber layer is key to reduce the total deposition time without compromising on the tandem solar cell performance. The developed VHF-PECVD technique using linear plasma sources is capable of meeting this criterion while promoting a path to scale the processes to large substrate areas.

Identiferoai:union.ndltd.org:DRESDEN/oai:qucosa.de:bsz:14-qucosa-131765
Date29 January 2014
CreatorsZimmermann, Thomas
ContributorsTechnische Universität Dresden, Fakultät Elektrotechnik und Informationstechnik, Prof. Dr. rer. nat. Johann Wolfgang Bartha, Prof. Dr. rer. nat. Johann Wolfgang Bartha, Prof. Dr. rer. nat. Uwe Rau
PublisherSaechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden
Source SetsHochschulschriftenserver (HSSS) der SLUB Dresden
LanguageEnglish
Detected LanguageEnglish
Typedoc-type:doctoralThesis
Formatapplication/pdf

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