The European photovoltaics (PV) market still represents the predominant share of worldwide installations and electricity generated from PV is becoming increasingly competitive, with an average levelised cost of energy (LCOE) estimated to be between 0.10–0.16 €/kWh in 2011. This constant reduction of LCOE means that the European industry can only regain its competitiveness with (i) a concomitant reduction of production and investment costs (current net price level ~0.8–1.0 €/Wp today) in Europe in order to face the strong price competition of emerging countries (China and Taiwan), (ii) investment in novel “advanced” industrial processes allowing for high efficiencies and low-cost device production (iii) the development of high-end tools and processes which are more difficult to master and duplicate, securing a technology leadership. These conditions are necessary to ensure sustainable PV technology production in Europe and the construction of a robust European PV industry able to beat international competition. However, ultra-high-efficiency PV devices require manufacturing processes that are increasingly complex, which results in an increase in the related investment and fabrication costs. Given that the market still requires a reduction of the technology price, we are left with a paradox, and we must find ways to produce high-efficiency devices with competitive industrial processes.
1.2 HERCULES concept
The concept proposed by the HERCULES project is to develop innovative n-type monocrystalline c-Si device structures based on back-contact solar cells with alternative junction formation, as well as related structures including hybrid concepts (homo-heterojunction). These concepts are the most promising technologies to reach ultra-high efficiencies with industrially relevant processes. The HERCULES strategy is to transfer the developed processes to the industrial scale by considering all major cost drivers of the entire manufacturing process chain of modules. The concepts developed in HERCULES will demonstrate 25% efficiency at cell level and pave the way to ultra-high efficiencies for the next generation of c-Si based solar cells devices.
1.3 HERCULES objectives
The key objectives of the HERCULES project are:
- To develop ultra-high efficiency modules at the pilot scale (mean power conversion efficiency η > 21%)
- To reduce production/investment complexity and demonstrate costs down to 0.7 €/Wp at pilot scale
- To increase the durability of modules up to 35 years
- To demonstrate ultra-high efficiency solar cells: η > 25%
The combination of three main concepts is investigated in order to reach the project objectives:
- PERT (Passivated Emitter, Rear Totally diffused) cells They have the advantage of being compatible with most of the usual equipment currently used in a p-type production line. Only two additional steps are required to switch from a p-type to an n-type wafer based process. PERT cells can have a bifacial design, leading to up to 30% higher power output as compared to single sided cells.
- IBC (interdigitated back contact) cells If both electrical contacts are on the rear side of the cell, shading losses due the the front grid are absent and an independent front side optimisation is possible. Furthermore, IBC cells open new opportunities for module interconnection.
- HET (Heterojunction) cells In amorphous/crystalline silicon heterojunctions, excellent passivation of the wafer’s surfaces leads to solar cell Vocs exceeding 740mV. Furthermore, all process steps are carried out at temperatures not exceeding 250°C, which makes the HET concept very attractive especially for very thin (100 µm and below) substrates.
These technologies and their combination are evaluated throughout the whole process chain, starting from the wafer material and up to the module integration and testing stage.