Organic solar cells have the potential to realise a significant cost reduction for photovoltaic (PV) energy conversion if improvements of the power efficiency and the lifetime can be achieved. There are still some crucial obstacles to be overcome before large-scale production of plastic solar cells can be considered. This large-scale production is the clear aim of industrial partners here involved. The feasibility of this approach will be demonstrated by the development of a new generation of organic PV having better efficiency (³ 5% on 1 cm2 glass substrate and ³ 4% on 1 cm2 flexible substrate), longer lifetime and a production cost far below those of competing technologies based on silicon.To reach MOLYCELL goals, the following points are addressed in parallel:
1. Design and synthesis of new materials to overcome the large mismatch between the absorption characteristics of currently available polymer materials and the solar spectrum and also to improve the relatively slow charge transport properties of these polymer materials.
Solar emission spectrum (black curve) and absorption spectrum of the polymers which gave at the moment the best results in “all-organic” solar cells: P3HT (blue curve) and MDMO-PPV (red curve).
2. Development of two device concepts to improve efficiencies: the “all-organic” solar cells concept and the nanocrystalline metal oxides /organic hybrid solar cells concept.
All-organic solar cells :
Devices are based on donor-acceptor bulk heterojunction built by blending of two organic materials serving as electron donor (hole semiconductor, low band gap polymers) and electron acceptor (n-type conductor, here soluble C60 derivative) in the form of an homogeneous blend and sandwiching the organic matrix between two electrodes. One of these electrodes is transparent and the other is usually an opaque metal electrode. In addition to the incorporation of polymers with improved light harvesting and charge transport properties, two concepts are developed to improve efficiencies:
i) an innovative junction concept based on the orientation of polar molecules and
ii) a multi-junction bulk donor-acceptor heterojunction concept.
Nanocrystalline metal oxides /organic hybrid solar cells:
Devices are based upon solid-state heterojunctions between nanocrystalline metal oxides and molecular/polymeric hole conductors. Two strategies are addressed for light absorption: the sensitization of the heterojunction with molecular dyes, employing transparent organic hole transport materials and the use of polymeric hole conductors with the additional functionality of visible light absorption.
| Expected end results:|
The project MOLYCELL was founded by European Commission for 30 months and its starting date was the 1st January 2004.
The objectives to be achieved at the end of the project with one or more devices concepts are:
· Certified 5 % solar to electric energy conversion efficiency under Standard Test conditions (AM1.5 simulated sunlight, 100 mW/cm2, 25°C) for a 1 cm2 cell on glass substrate.
· Certified 4 % solar to electric energy conversion efficiency under Standard Test conditions (AM1.5 simulated sunlight, 100 mW/cm2, 25°C) for a 1 cm2 cell on flexible substrate.
· Fabrication methodologies compatible with large-scale reel to reel production on flexible substrates.
· >3000 hours stable operation under indoor conditions defined in consultation with end-users, with roadmap for establishing stability required for outdoor operation.
· Fabrication from non-toxic materials.
· Materials and fabrication costs determined to be consistent with projected production costs < 1 €/Watt peak.
(NB: all these objectives will not necessarily be achieved with a single device)
| Work performed and main achievements in the 2nd year of the project:|
The achievement of these technical objectives requires significant scientific progresses. We are pleased to say that the project is proceeding according to the planned schedule, having met or exceeded all of the year 2 deliverables and milestones. This work has already led to 13 publications in international referred journals, with a further 23 publications submitted or in preparation. Central to achieving these successes has been our integration of state of the art knowledge and best practice from both the dye sensitized nanocrystalline and polymer/fullerene research communities. This has been achieved both through 6 monthly project meetings which have focused on project co-ordination and smaller workshops dedicated to specific workpackages which have focused on detailed scientific discussion and the exchange of technical expertise and best practice. The project is managed as a series of 6 linked workpackages (WP), summaries of the achievements of each workpackage are given below.
· WP1: Design, Synthesis and Basic Chemical Analysis of Novel Organic Hole Conductors
The main objectives of this workpackage are the design and the development of new soluble oligomers and polymers having a good match between their absorption spectrum and the solar emission spectrum and having a mobility of charge carriers as high as possible. Second generation of polymers are now available for characterization (in WP3) and test of device performances (in WP4) is actually under way to design and synthesize via a continuous feedback a third generation of optimized polymers. Thus, main achievement has been obtained with the overcome of the difficulties encountered during some of the synthesis as some better strategy has been now established. Requested materials have been delivered at the end of the year 2 and analyses are on going by the different partners of the Consortium.
· WP2: Metal Oxide Development
WP2 deals with the design, fabrication and optimisation of mesoporous, nanocrystalline metal oxide films suitable for glass and flexible substrates and incorporation into solid-state nanocrystalline metal oxide / organic hybrid solar cells. The efforts of the first year were continued for the investigation, development and evaluation of various low temperature fabrication processes for nc-TiO2 and dense TiO2 blocking layers. New remarkable cell geometry was built on flexible Ti foils with an inverted structure which shows highly promising initial results. Alternative methodologies for the fabrication of mesoporous, nanocrystalline metal oxide films on transparent conductive oxide plastic substrates have also been studied. Among these, evaluation of mesoporous films made by supramolecular templating has led to promising results and a novel approach has been developed in which the porous metal oxide layer is replaced by a blend of TiO2 nanorods with a conjugated polymer.
· WP3: Advanced Characterization and Modelling
The objective of WP3 is to create a detailed understanding of the fundamental properties and behaviour of the novel materials developed in WP1 and WP2 and to check their mutual compatibility and suitability for improved solar cells (increased energy conversion efficiency). For that, quantitative models of device function are developed and validated by a range of experimental data, leading to:
a) Identification of parameters limiting device performances,
b) Identification of specific design improvements,
c) Prediction of optimum device efficiencies achievable with each device concept.
Experimental testing of prototype models were conducted upon as many device concepts as possible achieving the year 2 milestone for this workpackage. Following Deliverable D10, a model of device function with projected optimised device efficiencies has been developed.
· WP4: All-Organic Device Development
The objectives of WP4 are the realization of one or several all-organic test devices. Based on the donor-acceptor bulk heterojunction concept, 2 innovative principles are explored in parallel and low band gap polymers issued from WP1 are tested. The 2 innovative principles explored are one based on a junction induced by the orientation of polar molecules, and one based on multijunction bulk donor-acceptor heterojunction concept. Proofs of concept studies for the innovative devices are now under progress. First 2-terminal multijunction solar cells, in particular, were shown with near doubling of the open-circuit voltage as compared to the single junction device.
A prototype device with a certified efficiency of 4% on 1 cm2 glass substrate has been realised, thereby fulfilling Deliverable D7. An efficiency of 3% on 10 cm2 flexible substrate has also been demonstrated, thus fulfilling Deliverable D8.
Current / voltage characteristics of a 3% all organic device on flexible substrate.
Stripe module: active area = 930 mm2; 22.5 mW (80 mW/cm2, AM1.5); h =3.03%
· WP5: Metal Oxide / Organic Hybrid Device Development
The objectives of this workpackage are the fabrication and the characterization of solid-state metal oxide / organic solar cells on glass and flexible substrates, focusing on two device concepts:
- Dye sensitized solid-state metal oxide / organic heterojunctions employing an optically transparent organic hole conductor.
- Solid state metal oxide / organic heterojunctions in which the organic material serves the functions of both hole transport and light absorption.
A prototype device with an efficiency of 4% on glass substrate has been realised, thereby fulfilling Deliverable D7. In parallel with research on all organic flexible devices, research is on going on metal oxide / organic hybrid devices on flexible substrate. Efficient devices employing liquid electrolytes have already been achieved, work on devices employing organic hole conductors is in progress and will be reported in 6 months.
· WP6: Device Evaluation / Cost Assessment
Objectives of this workpackage are:
- Development of processes and materials for the production of PV modules on the basis of organic absorbers with efficiencies of >5 % (glass substrates) and >4% on flexible substrates and module cost perspectives < 1 €/Wp.
- Specifications of relevant accelerated ageing procedures for solar cells to be representative for in- and outdoor behaviours.
- Comparative evaluation of the different concepts explored within the project:
§ Solid-state metal oxide / organic heterojunctions
§ Dye sensitized metal oxide / organic heterojunctions
§ All-organic solar cells
and comparison against other benchmark technologies in terms of performance, stability, productivity and cost projections.
An initial evaluation of device processing and stability for metal oxide / organic and all organic devices was carried out, thereby fulfilling the Deliverable D6. A list of the critical stress factors and of the characterisation tools for analysis of aged cell/modules has been established meeting one of the mid-term milestones of this workpackage. A definition of the specifications requested for a 4% flexible solar cell (5% on glass substrate) has also been established meeting the second mid-term milestones of this workpackage.
· WP7: Project Management
The objectives of this workpackage are to structure, manage and monitor project progress and also to guarantee full use of the dissemination and exploitation opportunities. A project website has been set up where a complete description of the project may be found: consortium, research activities, publications…(http://www-molycell.cea.fr/). Moreover, in order to allow the broad dissemination of the knowledge built up during the project and the spreading of these new technologies, the Consortium will organize the ECHOS’06 conference (European Conference on Hybrid and Organic Solar Cells) in Paris at the close of the project.
| Expected impact:|
Photovoltaics is one of the most promising sustainable energy sources for the mid- and long-term future. In order to reach the very ambitious European goal of 3-4 GWp installed PV capacity in 2010, significant cost reduction to < 1 €/Wp in 2010 is mandatory.
In the long-term there is thus a need for alternative cost-effective PV technologies that do not depend on silicon feedstock. This will assure continuation of high PV growth rates beyond 2010. After 2010 the organic PV should be able to penetrate this classic PV market in a significant fraction as long as costs, efficiency and long-term stability targets can be met.
Hence, the success of organic solar cells will allow an entire new European business segment in the areas of (a) materials fabrication and supply, (b) equipment manufacturing, (c) solar cell manufacturing and (d) product integration. The potential of this project is stimulation of a new prosperous primary and secondary industry for new type of photovoltaics in Europe. In particular new markets are envisaged for polymer and nanoparticles suppliers that are traditionally not in the photovoltaics business. This is also true for industry involved in flat-glass, display, battery or electrical products.
| Consortium and funding:|
Commissariat à l’Energie Atomique
Laboratoire Cellules et Composants
F-91191 Gif-sur Yvette Cedex - France
Dr Carole Sentein
Tel.:33 (0)1 69 08 52 34
Companies: Konarka Austria (AU), Konarka Technologies (CH);
Universities: LIOS (AU); ICL (UK), EPFL (CH), JHIPC (CZ), Vilnius (LT), Ege (TR);
Research institutes: CEA (FR), ECN (NL), Fraunhofer-ISE (DE), IMEC (BE).
Start date: 1 January 2004
Duration: 30 months
EC funding: € 2 500 000