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Delayed Luminescence Spectroscopy of Organic Photovoltaic Systems

Final Report Summary - DELUMOPV (Delayed Luminescence Spectroscopy of Organic Photovoltaic Systems)

The DELUMOPV project (Delayed Luminescence Spectroscopy of Organic Photovoltaic Systems, Grant Agreement number PIEF-GA-2011-299657) aimed to develop a novel spectroscopic characterization experimental technique that will provide a simple way to study the technologically important systems of organic photovoltaic (OPV) blend films and to screen the properties of their corresponding devices. OPV blend films are binary mixtures of electron-donor and electron-acceptor organic materials and currently their power generation efficiency has reached the level of 10%. One of the main reasons that keep the power-generation efficiency of these systems low is that after light absorption by the blends, several loss mechanisms operate that prevent the extraction of the photogenerated charges. This is mainly attributed to the unfavourable nanomorphology of these blends that instead of promoting extraction of the free charges, it favors free (non-geminate) charge recombination. It was suggested that non-geminated recombination event may be possible to be monitored by the detection of luminescence in the μs time-scale, originating from delayed exciplex (delplexl) charge transfer states activated via the recombination of the non-geminate charges. The main objectives of DELUMOPV are:

i) To confirm the detection of delplex species in the μs – ms timescale in the delayed luminescence spectra of OPV layers for a broad set of next generation electron-accepting materials.
ii) To study the strength of the delplex luminescence as a function of the OPV layer morphology in the systems under examination.
iii) To address the correlation of the delplex dynamics with the charge transport properties of the layer.

Given that one main disadvantage of the organic photovoltaic blend films today is their high price due to the expensive fullerene derivatives that are currently used as electron-acceptor materials, the identification of alternative electron acceptors is a top-priority that is not yet accomplished.

The DELUMOPV project set-out to explore the possibility of fabricating organic photovoltaic blend films based on low cost non-fullerene electron acceptor molecules and to develop a methodology for the tuning of the layer nanomorphology that can favor charge extraction and increase photocurrent generation, by experimentally studying the process of emissive non-geminate charge recombination based on the spectroscopic technique of delayed luminescence. In particular, delayed luminescence spectroscopy aims to correlate the strength of the delayed luminescence intensity of delplex states the μs time-scale with the photocurrent generation efficiency of OPV devices and with the charge transport capabilities and the microstructure of the OPV photoactive layers. The establishment of such a function-property-structure triangular correlation is expected to simplify the screening of the OPV material combination and to accelerate the realization of efficient OPV device products that can reach commercial availability.

Throughout the entire period of the DELUMOPV project, a state-of-the-art delayed luminescence spectroscopic rig was designed and set-up and many different material-combinations were studied and analyzed by using several non-fullerene based electron accepting materials and polymer electron-donor matrices. In parallel to this investigation, the delayed luminescence rig was utilized by PhD students supervised by the applicant for other spectroscopic investigations such as the study of the process of photon up-converted delayed luminescence in binary organic composite materials of donor/acceptor components, for the short-circuit transient photocurrent and open-circuit transient photovoltage optoelectronic characterization of non-fullerene based OPV devices and for the phosphorimetric characterization of solution-processed oxygen-barriers. The applicant secured additional research funding for travel expenses and consumables provided by The Royal Society in UK, in collaboration with the University of Nottingham, from the John S. Latsis Public Benefit Foundation and from SAES Getters (Lainate, Milan), an industrial partner of the Host institution. The latter case of external funding operated on the basis of a technology-transfer initiative of the applicant, in which the delayed luminescence rig was employed for the study of oxygen-barrier materials to be used as protective barriers for impeding oxygen permeation and prolonging the lifetime of organic electronic devices.
Concerting the findings of the DELUMOPV project, the detailed analysis of the many material combinations was performed by convoluting the results of several independent experimental characterization techniques with the main experimental body of data that was accumulated by the technique of delayed luminescence characterization. In particular, photocurrent generation was quantified based on external quantum efficiency (EQE) characterization measurements and was correlated to the observed photoluminescence quenching, J-V device characteristics were recorded either in the dark or under simulated solar illumination conditions (AM1.5G 1 Sun intensity), charge transport properties were studied on the basis of space-charge limited current (SCLC) characterization of unipolar devices, layer microstructure was studied by means of wide angle X-ray scattering (WAXS), surface topography was studied by atomic force microscopy imaging (AFM) and bulk morphology was studied by high-resolution scanning electron microscopy imaging (SEM) and X-ray photoelectron spectroscopy (XPS). For some of the studied systems, differential scanning calorimetry (DSC) measurements were performed in order to identify the critical temperatures that could be used for the accurate tuning of the photoactive layer microstructure by thermal annealing. During the project, interesting and unexpected results were collected on the observation of a previously unidentified memory effect in OPV polymeric composites, as seen by conductive AFM microscopy, in collaboration with the University of Ancona. It was also demonstrated that the use of a thin polymeric interlayer in OPV devices made of poly(indenofluorene)(PIF):PDI photoactive layers helps in the tuning of the PIF:PDI layer morphology so that optimized charge extraction is achieved. On the basis of the accumulated results high-efficiency organic solar cells were realized with a power-conversion efficiency of 3.7%.

By the completion of the project the analysis of all collected datasets have verified the initial postulate of the proposal; the strength of the delayed luminescence intensity of delplex emission in OPV photoactive layers positively correlates with the photocurrent generation efficiency of the corresponding OPV device. The decay dynamics of the delayed luminescence of the charge-transfer states in the μs time-scale can provide useful information concerning the charge transport capabilities of the photoactive layers under investigation and can predict the photocurrent generation efficiency. The nature of the excited state that is responsible for the generation of the delayed CT luminescence intensity was addressed by performing electric-field induced PL quenching experiments. The collected results suggested further that delayed CT luminescence in the μs-time scale is the result of non-geminate charge recombination in the OPV devices and some experimental indications were registered in favour of the notion that trapped charges result in power-law decays of the photocurrent transients in the μs-time scale.

EU holds a strong leadership record in the field of Organic Electronics and of OPV technology, and this research project aimed to contribute to the maintenance and the strengthening of this status. Ultimately the findings of DELUMOPV are expected to serve as a basis for a relative comparison between fullerene and non-fullerene based OPVs systems. Such a comparison will provide valuable feedback to the Materials Scientists and Chemists at EU and global level and it will enable the design of the next generation molecular structures that will combine the advantageous characteristics of both fullerenes and PDIs. The realization of cost-effective and efficient photovoltaic technologies is one of the most promising routes that can serve as an 'exit-crisis' solution and can benefit EU by creating new job opportunities in the research sector.