Organic thermoelectrics: development of new dopants for N-type naphthalene dicarboximide copolymers

In the last years conducting polymers have found application into various electronic devices, including transistors, solar cells, OLEDs and, more recently thermoelectric generators. The thermoelectric effect is the direct conversion of thermal energy, triggered by thermal gradients, into electric energy and viceversa. A huge variety of conducting p-type polymers and related dopants have been deeply investigated, and impressive conductivity values have been recently achieved. However, both p-type and n-type semiconductors are required to implement thermoelectric devices. The uneven development of n-type polymers and dopants is still a key issue. Recently, new n-type polymers showing good electron mobility and stability to the environment have been developed. The first high performance n-type polymer was the poly([N,N’-bis(2-octyldodecyl) – 11-naphthalene-1,4,5,8 - bis(dicarboximide) - 2,6-diyl] - alt-5,5’(2,2’-bithiophene)) P(NDI2OD-T2). Using this class of polymers, we show that a tailored modification of the dopant chemical structure is highly effective in avoiding phase segregation and increasing doping ratio of n-type polymers. Specifically, we present a new series of N-alkyl 1H-benzimidazoles in order to study the doping of the P(NDI2OD-T2). Moreover, we do not limit the discussion to 1H benzimidazoles, but a new class of n-type dopants belonging to the class of tetrazafulvalenes are developed. Their thermal behavior and crystalline structure were analyzed by means of DSC and structural modeling with Avogadro, which revealed that the isopropyl-benzimidazole derivative show a greater tendency with respect to the other dopants to be solubilized and not to segregate (section 2.3). The different alkyl chains demonstrated a strong effect on the doping of P(NDI2OD-T2) (section 2.4), where longer alkyl chains resulte in higher electrical conductivity; effect that is highly enhanced with hindered isopropyl-substituents, which give the highest electrical conductivity so far reported for this polymer. AFM tomographies, differential scanning calorimetry and x-ray structural analysis, leads to the conclusion that increasing the length of the alkyl substituents the miscibility between dopants and polymer increase, increasing the electrical conductivity. So, the alkyl chain in 1H-benzimidazole could have a relevant role in the intercalation of small molecules as dopants in these conjugated polymers rather than affect the electron transfer from dopant to polymer. As a side project, the same n-type polymer is investigated in the pristine state while it is confined in nanofibers. Here, we show that electrospun P(NDI2OD-T2) nanofibers exhibit electron mobility strongly dependent on the solvent used for the process and, even more important, values higher than those obtained in thin films can be achieved. The role on the charge transport ability of polymer preaggregates in specific solvents, to form domains with long range order at the solid state, is investigated. Specifically, we highlight that chain orientation, which is driven by the high electric field applied in electrospinning, does not favor charge transport in the P(NDI2OD-T2). Finally, P(NDI2OD-T2) is electrospun with a p-type polymer (i.e. P3HT) to realize for the first time ambipolar nanofibers, which are highly desired in electronics for the development of logic circuits and inverters.

In questi ultimi anni i polimeri conduttori hanno trovato applicazioni in diversi dispositivi elettronnici, tra cui transistor, celle solari, OLED e, più recentemente generatori termoelettrici. Nei dispositivi termoelettrici sono richieste alte conducibilità elettriche, che nel caso di materiali organici vengono raggiunte drogando il materiale. Recentemente è stato sviluppato un nuovo polimero semiconduttore di tipo n che ha mostrato buone mobilità elettriche e stabilità ambientale, il poly([N,N’-bis(2-octyldodecyl) – 11-naphthalene-1,4,5,8 - bis(dicarboximide) - 2,6-diyl] - alt-5,5’(2,2’-bithiophene)) P(NDI2OD-T2). In questo lavoro di tesi abbiamo usato questo materiale drogandolo con dei droganti opportunamente modificati sui substituenti laterali. Mostreremo che la modificazione della struttura chimica dei droganti ha un effettivo effetto sulla diminuzione dell'effetto della segregazione di fase e sull'aumento della conducibilità del materiale. E' stato anche seguito un side project in cui lo stesso polimero è stato studiato nel suo stato pristino confinato nella forma di nanofibre. Abbiamo dimostrato che le nanofibre mostrano mobilità elettroniche molto diverse a seconda del solvente usato durante il processo di deposizione. Questo effetto è dovuto alla formazione di domini ordinati e la loro orientazione e il grado di cristallinità sono stati studiati tramite spettroscopia infrarossa in luce polarizzata e misure GIWAXS. Infine il polimero di tipo n è stato elettrofilato insieme a un polimero di tipo p per realizzare trasistor ambipolari a singola fibra. Ovvero dispositivi in grado di accumulare sia elettroni che buche, che sono molto richiesti nella realizzazione di circuiti logici.