Solution processable organic semiconductors and semiconducting single-walled carbon nanotubes (SWCNTs) are at the heart of the emerging field of printed electronics. Flexible electronic circuits, wearable biosensors, semi-transparent solar panels, edible electronics and neuromorphic devices are only a few examples of the innovative applications made possible by the unique optoelectronic and mechanical properties of carbon-based semiconductors. Still, boosting charge transport in solution processed carbon-based semiconductors is necessary to compete with the electrical performance offered by inorganic semiconductors. In order to design organic semiconductors with enhanced charge mobility and functionalities, it is crucial to understand relationships between electrical and structural properties from the molecular level up to the macro-scale. Likewise in solution processed random networks of semiconducting SWCNT, understanding the effects of the network composition on charge transport is important to establish efficient processing standards leading to high-performance devices. This thesis aims to shed light on the structure-property relationships in field-effect transistors based on thin films of three classes of printable semiconductors. The first study regards the effects of molecular encapsulation on the photophysical and charge transport properties of a high-mobility semiconducting polymer, PNDIT2, which is widely employed in n-type organic field effect transistors (OFETs) and as non-fullerene acceptor in organic photovoltaics. Encapsulating macrocycles are covalently bonded to the thiophene units, while the naphthalenediimide (NDI) moieties are free to establish intermolecular interactions. Molecular encapsulation emerges as a promising approach to enforce planarization of the polymer backbone and to avoid coiling of the polymer chains in solution. Furthermore, the encapsulating rings promote crystalline packing through an intermolecular-lock mechanism. Due to the presence of the macrocycles the π-π stacking distance between NDI units increases with respect to the naked polymer, which is reflected in lower charge carriers mobility. Still, charge transport is remarkably preserved in the encapsulated PNDIT2, as demonstrated by ideal OFETs characteristics. The second part of the thesis examines charge transport in field-effect transistors based on solution processed random networks of single-walled carbon nanotubes (SWCNTs). Monochiral (6,5) SWCNT networks and multichiral networks containing five different semiconducting species with different band gaps are investigated by charge modulation microscopy (CMM). CMM allows to map charge-induced optical features in the transistors channel with micrometric resolution. Hence, it is possible to visualize the distribution of the free carriers and identify correlations with local properties of the network, such as SWCNTs density and species composition. The CMM maps provide direct evidence that holes and electrons are transported preferentially through the same percolation paths. A dependence of the charge-modulated signal on the density of SWCNTs is observed for the monochiral network during the switching of the transistor. Moreover, it is found that the monochiral network presents less fragmented percolation pathways with respect to the multichiral network, which is in good agreement with the better performance of devices based on monochiral SWCNT. Finally, molecules based on a linear backbone of sp-hybridized carbon atoms, namely cumulenic sp-carbon wires, are introduced as a novel class of conjugated semiconductors for organic electronics. Solution-processed thin films of tetraphenyl[3]cumulene ([3]Ph) are deposited by a large-area meniscus-coating technique, and employed as active layers in OFETs. Optimized polycrystalline thin films of [3]Ph show ideal field-effect characteristics and hole mobility up to 0.1 cm2 V−1 s−1. Moreover, the devices display promising operational stability in air without specific encapsulation. A comprehensive spectroscopic and structural characterization, combined with DFT calculations, allow to correlate optical absorption features and charge transport properties with the thin film microstructure, thus providing guidelines for the molecular design of a whole new family of performant organic semiconductors with sp-hybridized backbone.
I semiconduttori organici processabili da soluzione e i nanotubi di carbonio sono materiali essenziali per il fiorente campo dell'elettronica stampata. Circuiti elettronici flessibili, biosensori indossabili, pannelli solari semi-trasparenti, elettronica commestibile e dispositivi neuromorfici sono solo alcuni esempi delle innovative applicazioni rese possibili dalle peculiari proprietà optoelettroniche e meccaniche dei semiconduttori a base di carbonio. Per sviluppare semiconduttori organici con mobilità di carica e funzionalità migliorate, è cruciale comprendere le relazioni tra le proprietà elettriche e strutturali dalla scala molecolare a quella macroscopica. Allo stesso modo, in reti disordinate di nanotubi di carbonio semiconduttori, è importante comprendere gli effetti delle proprietà della rete sul trasporto di carica al fine di stabilire processi produttivi che portino a dispositivi con alte prestazioni. Questa tesi mira a fare luce sulle relazioni tra struttura e proprietà di film sottili di tre classi di semiconduttori stampabili impiegati in transistor a effetto di campo. Il primo studio riguarda gli effetti dell'incapsulamento molecolare sulle proprietà fotofisiche e di trasporto di carica del PNDIT2, un polimero semiconduttore di tipo n ad alta mobilità ampiamente impiegato in transistor organici a effetto di campo (OFET) e in celle solari flessibili. L'incapsulamento molecolare emerge come un approccio promettente per promuovere la planarizzazione dello scheletro del polimero e per evitare l’aggregazione delle catene polimeriche in soluzione. Inoltre, gli anelli di incapsulamento facilitano l’impacchettamento cristallino del polimero in stato solido grazie ad un meccanismo ad incastro tra le diverse catene polimeriche. A causa dell’ingombro degli anelli di incapsulamento, la distanza di impaccamento tra le catene polimeriche aumenta rispetto al polimero pristino, il che si riflette in una inferiore mobilità dei portatori di carica. Tuttavia, è notevole che il trasporto di carica nel PNDIT2 incapsulato sia preservato, come dimostrato dalle caratteristiche ideali degli OFET. La seconda parte della tesi esamina il trasporto di carica nei transistor basati su reti disordinate di nanotubi di carbonio a parete singola (SWCNT) processati da soluzione. Vengono esaminati reti monochirali di SWCNT (6,5) e reti multichirali contenenti cinque specie semiconduttrici con diversa chiralità mediante microscopia a modulazione di carica (CMM). La CMM consente di mappare con risoluzione micrometrica le variazioni dello spettro di assorbimento del semiconduttore indotte dall’accumulazione di carica nel canale del transistor. Pertanto, è possibile visualizzare la distribuzione dei portatori liberi e identificare le correlazioni con le proprietà locali della rete, come la densità di SWCNT e la composizione. Le mappe CMM forniscono una prova diretta che gli elettroni e le lacune sono trasportati preferenzialmente lungo gli stessi percorsi di percolazione. Per la rete monochirale, si riscontra una dipendenza del segnale modulato dalla densità di SWCNT durante la fase di accensione del transistor. Inoltre, si scopre che la rete monochirale presenta percorsi di percolazione meno frammentati rispetto alla rete multichirale, il che è in buon accordo con le migliori prestazioni dei dispositivi basati su SWCNT monochirali. Infine, vengono introdotte molecole basate su una catena lineare di atomi di carbonio ibridati sp (fili di carbonio cumulenici) come una nuova classe di semiconduttori coniugati per l'elettronica organica. Le proprietà di trasporto di film sottili policristallini di tetrafenil[3]cumulene ([3]Ph) sono ottimizate mediante una tecnica di deposizione da soluzione a larga area e integrati in transistor a effetto campo organici. I dispositivi ottimizzati mostrano caratteristiche ideali di trasporto e mobilità fino a 0.1 cm2 V-1 s-1. Inoltre, i transistor dimostrano una promettente stabilità operativa in aria anche in assenza di una specifica incapsulazione. Una completa caratterizzazione spettroscopica e strutturale, combinata con calcoli DFT, consente di correlare le caratteristiche di assorbimento ottico e le proprietà di trasporto di carica con la microstruttura del film sottile, fornendo quindi linee guida per il design molecolare di una nuova famiglia di semiconduttori organici performanti basati su fili di carbonio.
Charge Transport in Printable Carbon-Based Semiconductors: from sp2-conjugated materials to sp-carbon wires
Pecorario, Stefano
2022/2023
Abstract
Solution processable organic semiconductors and semiconducting single-walled carbon nanotubes (SWCNTs) are at the heart of the emerging field of printed electronics. Flexible electronic circuits, wearable biosensors, semi-transparent solar panels, edible electronics and neuromorphic devices are only a few examples of the innovative applications made possible by the unique optoelectronic and mechanical properties of carbon-based semiconductors. Still, boosting charge transport in solution processed carbon-based semiconductors is necessary to compete with the electrical performance offered by inorganic semiconductors. In order to design organic semiconductors with enhanced charge mobility and functionalities, it is crucial to understand relationships between electrical and structural properties from the molecular level up to the macro-scale. Likewise in solution processed random networks of semiconducting SWCNT, understanding the effects of the network composition on charge transport is important to establish efficient processing standards leading to high-performance devices. This thesis aims to shed light on the structure-property relationships in field-effect transistors based on thin films of three classes of printable semiconductors. The first study regards the effects of molecular encapsulation on the photophysical and charge transport properties of a high-mobility semiconducting polymer, PNDIT2, which is widely employed in n-type organic field effect transistors (OFETs) and as non-fullerene acceptor in organic photovoltaics. Encapsulating macrocycles are covalently bonded to the thiophene units, while the naphthalenediimide (NDI) moieties are free to establish intermolecular interactions. Molecular encapsulation emerges as a promising approach to enforce planarization of the polymer backbone and to avoid coiling of the polymer chains in solution. Furthermore, the encapsulating rings promote crystalline packing through an intermolecular-lock mechanism. Due to the presence of the macrocycles the π-π stacking distance between NDI units increases with respect to the naked polymer, which is reflected in lower charge carriers mobility. Still, charge transport is remarkably preserved in the encapsulated PNDIT2, as demonstrated by ideal OFETs characteristics. The second part of the thesis examines charge transport in field-effect transistors based on solution processed random networks of single-walled carbon nanotubes (SWCNTs). Monochiral (6,5) SWCNT networks and multichiral networks containing five different semiconducting species with different band gaps are investigated by charge modulation microscopy (CMM). CMM allows to map charge-induced optical features in the transistors channel with micrometric resolution. Hence, it is possible to visualize the distribution of the free carriers and identify correlations with local properties of the network, such as SWCNTs density and species composition. The CMM maps provide direct evidence that holes and electrons are transported preferentially through the same percolation paths. A dependence of the charge-modulated signal on the density of SWCNTs is observed for the monochiral network during the switching of the transistor. Moreover, it is found that the monochiral network presents less fragmented percolation pathways with respect to the multichiral network, which is in good agreement with the better performance of devices based on monochiral SWCNT. Finally, molecules based on a linear backbone of sp-hybridized carbon atoms, namely cumulenic sp-carbon wires, are introduced as a novel class of conjugated semiconductors for organic electronics. Solution-processed thin films of tetraphenyl[3]cumulene ([3]Ph) are deposited by a large-area meniscus-coating technique, and employed as active layers in OFETs. Optimized polycrystalline thin films of [3]Ph show ideal field-effect characteristics and hole mobility up to 0.1 cm2 V−1 s−1. Moreover, the devices display promising operational stability in air without specific encapsulation. A comprehensive spectroscopic and structural characterization, combined with DFT calculations, allow to correlate optical absorption features and charge transport properties with the thin film microstructure, thus providing guidelines for the molecular design of a whole new family of performant organic semiconductors with sp-hybridized backbone.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/203172