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Final Report Summary - ALHSOLAR (Advanced Light Harvesting for Organic based Solar Energy Conversion)

On the basis of the project proposal, the major objectives to be fulfilled were:
1. to study the optical and the electronic properties of low-bandgap materials - both polymers and inorganic nanoparticles as donor phase to be used in Bulk Hetero-Junction (BHJ) solar cells;
2. to study the optical and electronic properties of inorganic nanoparticles to be used as acceptor phase in BHJ solar cells;
3. to optimize the device structure in order to increase the fraction of absorbed photons from the incoming solar photon flux studying the light harvesting by Resonant Surface Plasmon coupling;
4. to optimize the light harvesting by Resonant Energy Transfer and/or new conceptual spectral shaping;
5. to increase general knowledge in the photophysics of different classes of materials to be used in solar energy conversion;
6. to collaborate with people and institutes known in the framework of previous Marie Curie RTN ”SolarNType” project.

The results of the project activities are the following:

• with reference to the points n. 1 and 4 of the list above:

– Photoluminescence and absorption experiments have been performed in order to extract first basic data parameters on materials; moreover the local emission properties down to the sub-micrometer scale have been studied by Confocal Laser Scanning Spectroscopy (CLSM). Pump&probe and photo induced absorption (PIA) techniques are mandatory in order to deeply understand optical and electronic processes in materials and/or blends; although Dr Lattante should has had access to the aforementioned experimental set-ups, unfortunately those set-ups are still under optimization behind schedule.
– no new low-band gap materials have been exploited with success in a working device: several thiophene-based modified compounds synthesized by collaborating researchers from ”Istituto per la Sintesi Organica e la Fotoreattività of Consiglio Nazionale delle Ricerche (ISOF-CNR)” have been tested; these compounds have shown poor photoactivity. Further job will be performed by collaborating chemists in order to obtain suitable molecular structures thanks to a local research grant obtained by Dr Lattante after the ALHSolar project ending;

• With reference to the points n. 2,3 and 5 of the above mentioned list:

– ZnO and TiO2 from aqueous nanoparticle dispersion have been successfully exploited as inorganic nanoparticles to be used in inverted bulk heterojunction solar cell devices as electron collecting/hole blocking layers. Other attempts to use inorganic nanoparticles as acceptor phase have given no reliable results. The explanation of this is still under investigation;
– different device configurations have been investigated: initial attempts to realize working solar cells with standard configuration fully in air - prototype transparent electrode/active blend/metal electrode - have prevented a reproducible and stable characterization of device parameter due to fast degradation in air. As the used instrumental set constrains to work in ambient condition (except for vacuum metal electrode deposition no inert atmosphere/glove box system is available), the inverted device configuration (namely transparent electrode/electron transport layer/active blend/hole extracting metal electrode) has been chosen in all the subsequent research activity; this choice, moreover, has allowed better performance of devices and will allow a low cost realization by printing techniques, currently in setting-up progress;
– a method for the characterization of the device parameters aging, like series and parallel resistance, has been developed, namely a simple fitting procedure to the analytical solution of the complete diode equation obtained by means of the LambertW function; this work has been published on Synthetic Metals 161, 949-952 (2010);
– silver nanostructured electrodes are currently under investigation: Dr Lattante will continue to try to realize optimal nanostructured metal layer by simple steps procedures using spin coated or casted layer of self nanostructured Poly(methyl methacrylate) (PMMA) or poly(bisphenol A)carbonate (PC) as well as other suitable polymers as matrix to obtain the replica structures on metal films in order to optimize the surface plasmonic resonance effects to be exploited in complete devices;
– new thiol-based molecular additives have been exploited in order to optimize the active layer morphology and hence the overall performance of devices; this work has been summarized in a paper ready for submission to Organic Electronics;
– the close correlation between morphology, emission properties and photocurrent generation has been studied by Confocal Laser Scanning Microscopy, simultaneously detecting the luminescence signal map and the photocurrent signal map. This activity allow a deep insight into the basic photophysics and electronic properties of the studied systems; a conference talk has been given (EOSAM 2012) on these results and two papers are ready to be submitted;
– the morphological behavior at interfaces of polymer-fullerene blends has been studied by Confocal Laser Scanning Microscopy in order to investigate possible different phase segregation due to the interaction between the organic molecules and the substrate nature. This activity is preliminary to an optimal device design (standard or inverted). Two conference talks have been given on these results (EOSAM 2012 and FUTURMAT 2) and a full paper has been submitted to Macromolecules.

• With reference to the point n. 6:

– the activity has been performed mainly in the laboratories of Dipartimento di Ingegneria dell’Innovazione of Università del Salento (DII), in strong collaboration with Consiglio Nazionale delle Ricerche Istituto per la Microelettronica e i Microsistemi (CNR-IMM) and research Institutes contacted from previous Marie Curie Fellowship, in particular the Institute for Organic Solar Cells (LIOS) in Linz, Austria, directed by Prof. N. S. Sariciftci, where part of the experimental activity has been carried out in the period January 11, 2010 - January 25, 2010 with the support received from the European Science Foundation (ESF) for the activity ORGANISOLAR - ’New Generation
of Organic based Photo-voltaic Devices’ - Grant Reference Number 3238. The research visit in LIOS by Dr Lattante has been very fruitful, in particular he has had the opportunity to train one master thesis student from the Università del Salento in organic solar cell realization and characterization and to allow him to experience a work activity in a international environment like LIOS, partly fulfilling the ”transfer of knowledge” requested by the Marie Curie ERG grant. Moreover an active collaboration is in progress with ”Dipartimento di Chimica - Università degli Studi di Bari ”Aldo Moro (Italy) thanks to local contacts established in LIOS by Dr Lattante and with CNR-ISOF in Bologna (Italy). More collaboration have been tempted with other institutes with which a previous collaboration was established in the framework of the SolarNTypeMarie Curie RTN project by submitting a joint project proposal in the framework of the MI.S.E.-ICE-CRUI-2010 (accordo quadro MISE-ICE-CRUI ANNO 2010) with title ”Realizzazione e ottimizzazione di celle solari organiche flessibili per applicazioni a coperture fotovoltaiche”: unfortunately the proposal has not been granted. Nevertheless two important research projects have been approved and granted, namely SMILE - Smart Innovation using Leds - Partenariati Regionali per l’Innovazione and ”Nanostrutture gerarchiche fotosintetiche per la produzione di energia” - PRIN 2010-2011, in which the expertise and the experience of Dr Lattante will be fully exploited.

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