Critical role of H-aggregation for high-efficiency photoinduced charge generation in pristine pentamethine cyanine salts

The mechanism of photoinduced symmetry-breaking charge separation in solid cyanine salts at the base of organic photovoltaic and optoelectronic devices is still debated. Here, we employ femtosecond transient absorption spectroscopy (TAS) to monitor the charge transfer processes occurring in thin films of pristine pentamethine cyanine (Cy5). Oxidized dye species are observed in Cy5-hexafluorophosphate salts upon photoexcitation, resulting from electron transfer from monomer excited states to H-aggregates. The charge separation proceeds with a quantum yield of 86%, providing the first direct proof of high efficiency intrinsic charge generation in organic salt semiconductors. The impact of the size of weakly coordinating anions on charge separation and transport is studied using TAS alongside electroabsorption spectroscopy and time-of-flight techniques. The degree of H-aggregation decreases with increasing anion size, resulting in reduced charge transfer. However, there is little change in carrier mobility, as despite the interchromophore distance increasing, the decrease in energetic disorder helps to alleviate the trapping of charges by H-aggregates.

Dynamics of Electronic and Ionic Charges in Cyanine Organic Semiconductor Devices

Sandra Christina Jenatsch

Organic semiconductor materials have been widely applied in optoelectronic devices to replace their inorganic counterparts and explore new fields of applications. Due to their high extinction coefficients, chemical tunability and solubility, cyanine dyes are an interesting class of molecules to be employed in organic semiconductor devices. Their beneficial properties have been demonstrated in organic solar cells (OSC), photodetectors and recently in organic light-emitting electrochemical cells (OLEC). The focus of this thesis is to analyse different aspects regarding the dynamics of electronic and ionic charges in cyanine based devices. Models and conclusions are not restricted to cyanine based devices but can potentially be applied in various systems where combined ionic and electronic charges are present. In order to focus on electronic processes in OSCs, a combination of steady-state, transient and impedance analysis is used. It is resolved that trapped charges can be introduced at the dye/hole extracting layer interface by using specific solvents for the cyanine layer fabrication. These measurements also reveal the low hole mobility of cyanine films which in turn limits the possible upper layer thickness in OSCs. A frequently used way to enhance the conductivity of organic semiconductors is the addition of extra charges by chemical doping. This concept is employed for a near-infrared absorbing dye which is used as donor in a bilayer solar cell. The dependence of all figures of merit versus doping concentration and layer thickness is studied in detail. It is found that the performance of the optimised cell is not further improved by doping, but that it presents a good option if thicker layers are required, e.g. on rough substrates. Furthermore, the stability and degradation pathway of the p-doped species is analysed. This information is of general relevance as they also occur after charge transfer at the heterojunction interface of an OSC and during operation of OLECs. The dynamics of ionic charge carriers is separated into two contributions. In a first step, the steady-state diffusion profile of the counter ion within a cyanine/fullerene bilayer structure is scrutinised using different profiling techniques. An unexpected constant ion profile is found throughout the C60 layer. Simultaneously, the potential within the acceptor is measured by the Kelvin probe technique. A mechanism of combined electron and ion transfer from the C60 to the dye and vice versa is proposed which is in agreement with the experimental results. A prerequisite for OLEC operation is the mobility of ionic charge carriers. Their dynamics in cyanine salts under the effect of an external field is therefore studied in such devices. Several methods are developed and applied to characterise the dynamic p-i-n structure in detail, i.e. the position and width of the intrinsic region as well as the doping concentrations in p- and n-doped regions. The combination of all these methods helps to understand the transient behaviour of these cyanine devices but can also be adapted to other OLEC systems. A major drawback of cyanine dyes as emissive material is their low fluorescence quantum yield in solid films. To overcome this restriction and improve the OLEC efficiency, host-guest systems with various visibly emitting dyes are studied. The gain in fluorescence quantum yield in the film can directly be transferred to an increase in external quantum efficiency of the device.

EPFL2017

Exciton Dynamics in 2D Organic Assemblies for Realization of Next-generation Optoelectronic Devices

Surendra Babu Anantharaman

Supramolecular assemblies of cyanine dye molecules formed by weak van der Waals forces have gained immense attention for their distinct optical properties (high extinction coefficient, narrow absorption and photoluminescence line-width and small Stokes shift) compared to their monomer counterparts. Strong light-matter interaction in the J-aggregates causes coherent exciton migration and high color purity emission. This motivates to fabricate J-aggregate thin films for different opto-electronic devices such as solar cells, where J-aggregates serve as exciton transport channels, organic light emitting diodes (OLEDs) and polariton lasers. Exciton migration and photoluminescence quantum yield (PLQY) of the J-aggregates in thin films dictate the efficiency of energy harvesting and light emitting devices, respectively. The main focus of this thesis is the development of a new J-aggregate growth route which favors the formation of large, coherent domains of 2D J-aggregate thin films and to investigate the exciton dynamics. Furthermore, non-radiative decay channels suppressing the PLQY of J-aggregates in thin film and in solution were identified. The coherent crystalline domain size in thin films is typically limited to nanoscale length because dye molecules spontaneously self-assemble already in solution. This can be overcome by maintaining equilibrium between monomers and the critical nucleus of a J-aggregate in solution. This approach is shown here as a successful route to achieve large coherent domains on a functionalized substrate. The growth model was further confirmed with small-angle x-ray scattering (SAXS) studies. Using time-resolved photoluminescence (TRPL) spectroscopy, non-radiative decay channels limiting the exciton migration were identified. The results suggest that the increase in domain size and order is directly proportional to the increased radiative decay of the excitons. Furthermore, the PLQY of J-aggregates in water at room temperature is typically ~5%. Upon addition of alkylamine, the quantum yield was drastically improved while maintaining line-width and peak position. Using small-angle neutron scattering (SANS), a two-phase region was identified in the water-dye-alkylamine ternary phase diagram in which high PLQY J-aggregates form. From TRPL studies, an increase in radiative lifetime is in agreement with the increase in PLQY. As an application, ultra-narrowband photodetectors were explored owing to the narrow line-width of J-aggregate absorption. The concept proved advantageous over other strategies for narrowing the response width, with competing figure of merits. Realization of an inkjet printed prototype with higher frequency response compared to spin-coated devices highlights their potential for industrial applications. Excitonic channels using J-aggregate nanowires were formed by complexing J-aggregates with dendronized polymers. SAXS studies revealed a core-shell configuration of the DP-J-aggregate hybrid structure. A correlation between polymer conformation and J-aggregates allowed to assemble J-aggregate nanowires as thin films. Centrosymmetric cyanine dimers are known for their non-luminescent nature. Twisted dimer packing of dye molecules in a polymer matrix are confirmed from circular dichroism spectroscopy studies. This explains a strongly red-shifted photoluminescence with high quantum yield in the thin film.

EPFL2019

Exciton Dynamics in 2D Organic Assemblies for Realization of Next-generation Optoelectronic Devices

Surendra Babu Anantharaman

EPFL2019

Dynamics of Electronic and Ionic Charges in Cyanine Organic Semiconductor Devices

Sandra Christina Jenatsch

EPFL2017