In-situ Structural/Morphological study of polymer-based active materials for Organic Photovoltaic devices: bulk, surface and interface properties and aging effects

ABSTRACT

The main purpose of the present work is to investigate properties and issues limiting performance and lifetime of active materials used in organic solar cell, in particular Poly(3-hexylthiophene)] (P3HT) and [6,6]phenyl-C61-butyric acid methyl ester (PCBM) blends. The influence of structural and morphological properties on such aspects is investigated.

Indeed, organic Photovoltaic (OPV) devices have gained more and more interest in scientific and research community. Moreover, as material’s cost is lower compared to silicon cells and structural- mechanical material’s property are interesting for utility-scale applications, they are gaining interest also in the industrial field.

However their performances are still quite far from their inorganic counterpart.
Hence, a great effort is being done to improve cells performances and durability. In this perspective, while on one hand the research for new photoactive materials or devices structures can be of great importance, on the other hand the study of morphological and structural properties can be a key aspect which can unroll Organic Devices possibilities. Indeed, these properties have been shown to be strictly linked to cell performances, even if they still present uncovered issues. In particular, the possibility to perform non-invasive time resolved experiments examining not only the films bulk structure but also their surface and interfaces properties may play a crucial role.
This works deals, in particular, with P3HT:PCBM (Poly (3-Hexylthiophene):[6,6]-phenyl-C61 butyric acid methyl ester) blends, which is one of the most studied because of its promising PV properties.

The investigation of such multi-layered systems, which present thickness of each layer of the order of 100-200nm, implies that techniques able to detect nano and meso-scale structures must be used. Moreover when the experiments are carried out in-situ and in real time, as the case of this work, non perturbative techniques are required.

Here, structural and morphological properties where studied by means of an unconventional tool which allows to use jointly, in situ and in real-time, Energy Dispersive X-ray Reflectometry (EDXR) and Atomic Force Microscopy (AFM) techniques. Thus, non-destructive investigations, both in reciprocal and direct space could be performed. Also, Energy Dispersive X-ray (EDXR) Diffraction and complementary FTIR studies were carried on.

A first part of this work was devoted to the set-up the joint EDXR and AFM tecnique. Hence information both on the reciprocal (EDXR) space and on the direct space (AFM) are obtained, and the possibility to probe the properties of buried interfaces is gained. The technique was validated by performing a series of preliminary test experiments on a reference sample and enabled to carry on the subsequent work of examining the multilayered system composing the intermediate stages of OPV cells.

There’s no de-facto standard in OSC device structure, however two common configurations are ITO/PEDOT:PSS/P3HT:PCBM/Al.Electrode and ITO/P3HT:PCBM/Al.Electrode, the only difference being the buffer PEDOT:PSS (Poly(3,4-ethylenedioxythiophene) poly (styrenesulfonate)) layer. This buffer layer is often used for its smoothing and conduction-enhancing properties, but in working conditions it has shown to have some drawbacks, in particular when exposed to environmental humidity.

Hence a second phase of this work has been dedicated to the understanding of the structural and morphological processes which occur in the bulk of PEDOT:PSS and at its interface with ITO. These investigations, performed by time resolved EDXR and AFM experiments, used jointly in- situ, pointed out that this layer is subject to a water uptake/release when exposed to humidity and then to heat induced by solar irradiation. This process induces bulk morphological modifications which result to be irreversible, as confirmed by FTIR analysis, thus compromising its role as conduction enhancer layer, and indicating that a more stable polymer buffer layer and better encapsulation techniques are required.

The study then focused on the P3HT:PCBM blend active layer. A preliminary investigation on the role of PEDOT:PSS buffer layer and of the relative P3HT and PCBM weight ratios on the blend characteristics was carried on. The results show that in any case the photoactive layer results to be characterized by the morphological and structural parameters in the optimal range for application for organic solar devices.

Being aging one of the most consistent factor which limits performances, a systematic study of morphological and structural degradation of P3HT:PCBM active layer bends has been carried on up-on illumination, making use of the aforementioned joint AFM/EDXR technique.
In pristine samples a bulk aging effect was detected, and further Energy Dispersive X-ray Diffraction (EDXD) experiments allowed to relate this phenomenon to a secondary crystallization process of the P3HT counterpart. On the other hand, annealed samples active layer showed to have a good bulk stability, and only a slight increase of roughness limited to the buried interface was detected. In addition, FTIR spectroscopy studies strongly supported the hypothesis that inter- diffusion of ITO into the organic layer (in ITO/P3HT:PCBM sample structure) was most likely the cause of morphological degradation. Therefore, the joint application of the in situ X-ray and AFM techniques, together with FTIR ex situ analysis, provided a clear picture of the concomitant chemical physical process occurring in the organic film. Importantly, such study demonstrated, for the first time, that the present approach for in situ non-invasive investigations of organic systems for PV devices is able to discriminate among the bulk, interface and surface aging effects.

The detected structural modification was then studied in-situ during illumination by means of a time-resolved EDXD analysis, thus pointing out the kinetics of the rearrangement of the active layer molecules. Such rearrangement could be described in real-time evidencing two different processes (P3HT crystallization and PCBM clustering into larger domains) which together with FTIR and AFM analysis support the hypothesis of a phase separation of the two components of the blend, which is one of the most relevant issues restraining conduction in active layer blends.

With these studies, also, a further proof that annealing results to be an effective treatment to stabilize P3HT:PCBM bulk hetherojunction active layer is given.

However, for the development of a flexible OPV technology, plastic substrates are needed. These letter are characterized by rather low glass transition temperatures, so that the employ of high temperature thermal treatments should be avoided.
Thus, in order to explore different promising solutions for cell stability, avoiding thermal annealing, the investigation of P3HT:PCBM blend doped with Silver Nanoparticles (Ag-Nps) was carried on. These materials are of great interest as the inclusion of Nps leads to an impressive improvement of device power conversion efficiency, up to 250% with respect to reference devices based on un doped P3HT:PCBM blends. Indeed, the incorporation of metallic nanoparticles in the active layer is expected to enhance absorption due to a plasmon mediated effect, causing improved initial cell efficiency.

Importantly, such improved performances come together with an enhanced stability. Indeed, pristine Ag-Nps doped samples, studied during illumination with EDXR/AFM joint technique, show improved bulk properties, the only morphological modification which was detected consisted in a minor roughening process taking place at the interface with PEDOT:PSS (in ITO/PEDOT:PSS/P3HT:PCBM sample structure). Moreover, the blends structural properties remained stable over time, thus evidencing that the doping of the blend with Ag Nps gives rise to enhanced structural stability.

The latter investigations are very encouraging, if we consider that the blend was not annealed. In this perspective, our finding pave the way to a systematic study of organic PV layers doped with metallic nanostructures, which can be key strategy towards the development of efficient and durable Organic Solar Devices.