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Production process for industrial fabrication of low price amorphous-microcrystalline silicon solar cells

Final Report Summary - LPAMS (Production process for industrial fabrication of low price amorphous-microcrystalline silicon solar cells)

This project aimed at lower cost price per Wattpeak (Wp) for film-Si photovoltaics (PV) produced in a local production plant in Western Balkan Countries (WBCs) by a significant upgrade of the cell and module efficiency, while keeping the production costs per m2 almost constant.

The increase of cell efficiency would be achieved in several sequential steps:
1) improvement of TCO layers (better stability in hydrogen plasma, better transparency),
2) introduction of doped micro-crystalline Si layers as window layers (less optical losses),
3) introduction of intrinsic micro-crystalline Si layers as active layers (no light induced degradation, improved light absorption in part of the solar spectrum),
4) integration of previous steps leading to the introduction of a-Si/µc-Si tandem cell concept (enlarge the effective absorption spectrum).

The work programme of the project has been divided in four different work packages.

Work package 1- Preliminary activities:
1) Preparing industrial systems for experimental work: The system for deposition of silicon thin films in solar cells factory in split (SCS) was prepared. First, one typical vacuum chamber in the SCS factory was selected for experimental thin film deposition. The choice was done after performing the statistics of approximately 1 000 last runs. In next step, the selected chamber was adopted to enable optical emission spectroscopy measurements of plasma (OES).
2) TCO laboratory experiments: ZnO APCVD. Zinc oxide (ZnO) was cathodically deposited on a conductive glass substrate covered with SnO2 as cathode by a potentiostatic method. SnO2 films were prepared by spray pyrolisis using 0.1 M water solution of SnCl2 with complexes of NH4F. Also, a very simple apparatus was used for electrochemical deposition.
3) Depositions by MWPECVD: The depositions were performed in a single chamber microwave PECVD reactor, in which a substrate holder with a substrate area of 60×15 cm2 moves underneath a linear microwave plasma source. The depositions took place simultaneously on glass substrates and on silicon wafers to allow various characterisation techniques to be used on samples from the same batch.
4) Progress of the project in relation to the expected results: The expected results were: to have a RF-PECVD system ready for deposition of µc-Si layers, a selection of process for ZnO deposition by APCVD and feasibility shown for MWPECVD for industrial scale deposition of µc-Si layers.

Work package 2- Single layer deposition:
1) Growth of microcrystalline Si layers in industrial RF-PECVD system. The thin silicon films have been grown in a capacitatively coupled, parallel plate RF PECVD reactor. The gas mixture was silane highly diluted with hydrogen.
2) Deposition of intrinsic µc-Si by MW-PECVD: effect of argon. Intrinsic microcrystalline silicon films were deposited simultaneously on crystalline silicon wafers and on aluminosilicate glass in a single chamber MW-PECVD reactor, in which a substrate holder with a substrate area of 60×15 cm2 moved underneath a linear microwave source. A mixture of H2 and, if needed, Ar is injected in the vicinity of the microwave source, while silane (SiH4) diluted in H2 is injected closer to the substrate.
3) TCO development: capping layers. Several experiments were done regarding application of ZnO as protecting layer for the front TCO. On the SnOx layer produced in the SCS factory, co-workers from MANU-Skopje deposited by several methods a ZnO layer with a variety of thicknesses and doping levels. The prepared layers were exposed to the hydrogen plasma.
4) Optical analysis of reference TCO layers: At the Institute of Physics, Prague, optical, electrical and of scattering properties of several industrial and laboratory developed TCO's were analyzed and compared. For measurement of optical transparency, SnO2 or ZnO deposited on glass were polished to get rid of scattering effect.
5) Progress of the project in relation to the expected results. The expected results of this work package were: Applicable µc-Si p- and n- layers for a-Si/µc-Si tandem cells on glass, applicable top side TCO for a-Si/µc-Si tandem cells on glass, applicable µc-Si i-layers for industrial scale production of a-Si/µc-Si tandem cells on glass. All these results were achieved.

Work package 3: Binary layer deposition:
1) Fabrication of single junction test modules. From 14 panels received during LPAMS meeting in Split, 9 panels were investigated by FTPS and FTPS-QE. Disorder in the absorber material could be characterised by the Urbach edge.
2) Experimental deposition of structure glass/TCO/p(RF)/i crystal MW and glass/TCO/p(RF)-i(RF)-n(RF)-p(RF)-i crystal MW. The work done at ECN for this task could be divided in three parts: improving the quality of the µc-Si layer deposited in the batch reactor with MW-PECVD, optimising the deposition conditions with regard to value and homogeneity of microcrystalline volume fraction fc and depositions of µc-Si on glass/TCO substrates coated with p layers deposited with RF-PECVD at Solarne Celije, Split. To improve the layer quality and deposition rate for the µc-Si deposition, a series deposited at constant conditions was studied, with varying total flow rate. A transition from predominately µc-Si growth at lower flow rates to a mixed growth regime at higher flow rates was observed. These higher flow rates also caused a higher deposition rate. The main problem to deposit µc-Si on the Solarne Celije substrates was the size of the substrates in relation with the standard sample holders and heater of the MW batch reactor at ECN. The sample holder was designed to hold six 10×10 cm2 substrates, whereas the Solarne Celije substrates were 30×30 cm2. As the deposition width was larger than 10 cm, it should be possible to deposit on a 30 cm wide substrate in two runs, once on the far side and once on the near side of the substrate. Test runs on regular glass substrates (2-3 mm thick) showed an optically homogeneous deposition. However, depositions on substrates with a µc-Si p-layer resulted in broken substrates during the second deposition run.
3) Analysis of interface problem, possible blocking layers: From the data obtained it could be concluded that the interface between the two layers is rather sharp. At these conditions, no significant diffusion of fluorine atoms takes place.
4) Progress of the project in relation to the expected results: The expected deliverables of this work package were: Single junction µc-Si p/a-Si i/µc-Si n cells; efficiency > 4 %, single junction µc-Si p/µc-Si i/µc-Si n cells; efficiency > 4 % and test modules consisting of single junction microcrystalline Si devices. From these only the first was fulfilled.

Work package 4: Fabrication of integrated structures and design of upgraded plant:
1) Initial computer simulations for tandem cells: For the estimation of layers performances needed for proper function of tandem cell 1-D simulation was used.
2) Experimental fabrication of amorphous-crystalline tandems: In testing the concept of amorphous-crystalline cells, in Splits factory, in collaboration with Rudjer Boskovic Institute, number of tandem cells was deposited in order to test the interface problem and general concept of amorphous-microcrystalline tandem cell. The produced tandem cells, in the form of solar module, was tested in Petten during out-door exposure. When comparing the properties of second cells in a-Si/a-Si and nano-Si/a-Si cell the a-Si layer in first cell is better than in second cell (it has a lower defect density as characterized by the subgap defect-connected optical absorption). On the other hand, the whole tandem nano-Si/a-Si is much more stable than a-Si/a-Si, e.g. shows much less degradation.
3) Modeling of tandem cells with the help of optical model CELL at IPP:
4) Progress of the project in relation to the expected results: The expected deliverables of this workpackage were: Tandem cells with cell structure: glass/TCO/µc-Si p/a-Si i/µc-Si n /µc-Si p/µc-Si i/µc-Si n/TCO/metal, with efficiency > 8 %, test modules containing tandem cells, design for upgraded production line incorporating existing industrial production tools plus an industrial scale MW-PECVD system for growth of µc-i-Si layers, scientific publications and the final report.

The PECVD system developed by ECN and R&R was essential step in on-line production of thin film solar cells that is different than most of deposition system currently in use, particularly in SC Split. In on-line system, the substrate for solar cells was inserted in vacuum system piece by piece through load-lock chamber without disturbing atmosphere in main deposition chambers. The substrate moved trough different deposition chambers for sequential multilayer deposition. System was modular and could be easy extended for tandem or triple cell deposition. Also, this system needed low number of workers which lowers the final price of solar cell.

The procedure for deposition of nano-crystalline Si layer would be used in production of tandem solar cells in Solar cells factory. This cells had larger stabile efficiency (5.7 - 0.5%) than those in current production (˜ 4 %) while the production costs were only slightly higher. The most of increase in cost came from longer time of deposition which included more gas, electricity and labour cost. On the other side, since the tandem cells have larger efficiency on the same substrate (glass and TCO) and the same back contact and encapsulation costs, the price per unit power (price per Wp) decreased.

In summary when applying any of the two improvement that were mentioned before, the price for Wp in solar cells split should be at least two times lower, what was the main aim of the LPAMS project.