Muriel Firon

In situ energy dispersive x-ray reflectometry measurements on organic solar cells upon working

The change in the morphology of plastic solar cells was studied by means of time-resolved energy dispersive x-ray reflectivity (XRR). This unconventional application of the XRR technique allowed the follow up of in situ morphological evolution of an organic photovoltaic device upon working. The study consisted of three steps: A preliminary set of XRR measurements on various samples representing the intermediate stages of cell construction, which provided accurate data regarding the electronic densities of the different layers; the verification of the morphological stability of the device under ambient condition; a real-time collection of XRR patterns, both in the dark and during 15h in artificial light conditions which allowed the changes in the system morphology at the electrode-active layer interface to be monitored. In this way, a progressive thickening of this interface, responsible for a reduction in the performances of the device, was observed directly.

Controlling photoinduced degradation in plastic photovoltaic cells: A time-resolved energy dispersive x-ray reflectometry study

The electrode-active layer interface of organic photovoltaic cells, a critical point in the development of organic devices, was studied by the energy dispersive x-ray reflectivity (EDXR) technique applied in situ. An EDXR-based protocol allowing discrimination between the possible mechanisms that produce the aging process at the interface was established. The study detects photoinduced oxidation of the electrode at the buried interface, to which fading of the device performances could be attributed. This conclusion was further confirmed by results obtained on a new cell, of selectively modified architecture, whose performances turned out to be stable in time.

Conjugated alternating copolymer of dialkylquaterthiophene and fluorenone: synthesis, characterisation and photovoltaic properties

New π-conjugated alternating copolymers containing thienylene and fluorenone units, namely poly[(5,5‴-dioctyl-[2,2′;5′,2″;5″,2‴]quaterthiophene)-alt-(2,7-fluoren-9-one)] (PQTF8), as well as its analogue without fluorenone groups (PQT8), have been synthesized. Absorption studies carried out both in solution and in thin films indicate that the presence of fluorenone chromophores in PQTF8 leads to a significant extension of the absorption spectrum in the visible range as compared to PQT8. The redox properties of both polymers, in particular their LUMO and HOMO levels, have been characterized by cyclic voltammetry and have been found suitable for potential use of these systems in organic solar cells. Finally, these materials have been tested as donor components in bulk-heterojunction-type photovoltaic cells using PCBM as an electron acceptor. We demonstrate a strong effect of the polymers molecular weight on crucial cell parameters, such as the short-circuit current density Jsc, and therefore on the overall cell efficiency. This effect is particularly pronounced for PQTF8-based cells leading to power conversion efficiencies up to 1.5% for the highest molecular weights.

Impact of light on organic solar cells: evolution of the chemical structure, morphology, and photophysical properties of the active layer

Organic photovoltaic represents an emerging technology thanks to its ability to give flexible, light weight and large-area devices, with low production cost by simple solution process or printing technologies. But these devices are known to exhibit low resistance to the combined action of sunlight, oxygen and water. This paper is focused on the behaviour of the active layer of the devices under illumination in the presence and absence of oxygen. The monitoring of the evolution of the chemical structure of MDMO-PPV submitted to accelerated artificial ageing permitted the elucidation of the mechanisms by which the polymer degrades. Extrapolation of the data to natural ageing suggested that, if well protected from oxygen (encapsulation), MDMO-PPV:PCBM based active layer is photochemically stable for several years in use conditions. In addition the charge transfer between the two materials was observed to remain efficient under exposure. The study of P3HT:PCBM blends allowed to point out the Achilles heel of P3HT towards the impact of light. In addition, P3HT:PCBM blends were shown to be much more stable under illumination than MDMO:PCBM blends. Preliminary results devoted to the AFM monitoring of the morphological modifications of P3HT:PCBM blends under the impact of light are also reported.

Study of P3HT:PCBM bulk heterojunction solar cells: influence of components ratio and of the nature of electrodes on performances and lifetime

Among the class of conjugated polymers, polythiophenes and in particular 3-alkyl-substituted thiophenes seem to focus all the attention in the domain of photovoltaic conversion. At CEA, we are working on the optimization of bulk heterojunction solar cells made of poly-3-hexylthiophene (P3HT)and [6,6]-phenyl C61 butyric acid methylester (PCBM) blend. First we will describe the influence of the ratio of P3HT and PCBM blend on the efficiency of the resulting bulk heterojunction solar cells. Best cells based on 1:1 in weight ratio yield 3.6 % power conversion efficiency under air-mass 1.5, 100 mW/cm2 illumination. Then we will compare the efficiency and lifetime of different cells by changing the nature and thickness of cathode (Aluminum or Calcium/Silver). On the optimized cells, we have proceeded to ageing and accelerated lifetime measurements on devices with Ca/Ag cathode. It shows that the current densities decrease less than 3 % and that efficiency is still higher than 1.7 %, after 400 hours under AM 1.5, 100 mW/cm2 illuminations and at high temperature (60°C).