In my thesis, I report the results obtained during my three years PhD regarding the study of novel materials for light-energy conversion devices, whose preparation and processing are in agreement with a near-future large scale production. This work is divided into two topics: 1) The study of novel emitters for luminescent solar concentrators (LSCs); 2) The study of novel functional materials for bulk hetero junction solar cells (BHJ-SC) regarding the photoactive layers and interface engineering layers. In the first part, I studied the application of silicon Quantum Dots (Si-QDs) as novel chromophores in LSCs. The use of these semiconductor nanostructures overcomes the limitations in terms of industrial scaling which is characterizing the QDs so far used in LSC. Non-toxicity, low cost of raw material and ultra-abundancy make Si-QDs promising chromophores for large scale production of LSCs. Upon quantum confinement the optical properties of Si-QDs are significantly modified, resulting in a high Stokes Shift quantum efficiency. The device assembled by embedding the Si-QDs in a co-polymeric matrix exhibits a high optical quality and results in an almost ideal LSC in which the losses related to luminescence self-absorption and scattering, the two most important efficiency-limiting factors, are strongly reduced. Similar characteristics were demonstrated to be achieved also in flexible LSCs which still preserve the efficiency and the optical properties of standard flat LSC. In the second part I performed a comparative study about the photovoltaic performances in BHJ-SC of PBDTTPB, a polymer showing a good trade-off between PV performances and structural complexity, which makes it a promising candidate for a future industrial scale production. The comparison of devices PV characteristics was performed between two PBDTTPDs with different synthetic routes: one prepared via Stille reaction, a synthesis involving the use of high toxicity organo-tin compounds and requiring the pre-functionalization of the monomers, and a second one prepared via Direct (Hetero)Arylation Polymerization (DHAP), an alternative synthetic route, which avoids pre-functionalized monomers and the stoichiometric production of heavy-metal-containing waste. This study showed that the polymer synthetized via DHAP exhibits a power conversion efficiency comparable to that prepared via Stille. Moreover, through the use of optical, morphological characterizations and charge recombination analysis, I could highlight the relationship between the PBDTTPDs macromolecular properties, which are heavily dependent on their preparation, and their PV behavior in BHJ through the processing. In the last section, I studied a novel poly-fluorene-based (PFN) material as cathode interfacial material (CIM) for BHJ-SC with direct geometry, whose solution processing is totally compatible with roll to roll fabrication requirements. The PFN backbone was functionalized with two side groups: alkyl-ammonium bromide (NBr), whose high polarity allows high solubility in alcohol, a solvent orthogonal to organic solvent for active layer, and an effective work function modification and ethyl-phosphonate (EP) groups, which exhibit a selective interaction with aluminium, the most common metal for cathodes. The complete study through photoelectric, electric and morphologic characterization techniques highlighted an enhancement of charges extraction and a reduction of active layer/electrode interfacial recombinations due to the effective surface engineering upon insertion of a thin layer of PFO-NBr-EP. Consequently the BHJ-SC based on PTB7:PC71BM blends with CIM/Al as cathode showed an enhancement of the PV parameters compared to the pristine Al devices, resulting in 7.24% PCE. Finally, I discussed the effect of the side groups on the device stability, through a comparison with other PFN-based interlayers already used in literature as CIM for PTB7:PC71BM-based devices.
In questa tesi ho riportato i risultati ottenuti durante il mio dottorato sullo studio di nuovi materiali per dispositivi di conversione otpo-elettronica, la cui preparazione e processabilità li rende ottimi candidati per il loro scaling-up industriale. Questa tesi è divisa in due argomenti: 1) lo studio di emettori per conentratori solari luminescenti (LSC); 2) lo studio di materiali funzionali per celle solari a eterogiunzione (BHJ-SC) per l’applicazione come layer attivo e come interlayer. Nella prima parte, ho studiato l’applicazione di quantum dots basati sul silicio (Si-QDs) come emettitori in LSC. L’utilizzo di questi nanocristalli è in grado di superare le limitazioni in termini di scaling-up industriale che hanno caratterizzato i QDs fino ad oggi impiegati in LSC. La non tossicità, il basso costo di produzione e la grande disponibilità di materia prima rendono i Si-QDs ottimi candidati per una produzione di LSC su larga scala. L’opportuno confinamento quantico modifica fortemente le proprietà ottiche di questo materiale, conferendogli un elevato Stokes Shift e un’alta efficienza emissiva. Il LSC ottenuto dall’inserimento dei Si.QDs in una matrice copolimerica presenta un’elevata qualità ottica e proprietà vicine ad un LSC ideale, in cui le perdite per auto-assorbimento e scattering, i due più importanti fattori che ne limitano l’efficienza, sono fortemente ridotti. Ho dimostrato infine che le stesse caratteristiche si possono ottenere su LSC con matrice flessibile, preservandone l’efficienza e le proprietà ottiche. Nella seconda parte ho riportato uno studio comparativo sulle performance di BHJ-SC basate su PBDTTPD, un polimero che presenta un ottimo compromesso tra l efficienza di foto-conversione nei dispositivi e complessità molecolare, che lo rendono un promettente candidato per lo scaling-up industriale. Lo studio ha confrontato le efficienze di BHJ-SC basate su due tipologie di PBDTTPD: una preparata via Stille, una sintesi che richiede l’uso di stannilati e la pre-funzionalizzazione dei monomeri, e una seconda preparata vai etero arilazione diretta (DHAP), una via sintetica che evita l’uso di monomeri pre-funzionalizzati e la produzione di scarti di reazione contenenti metalli pesanti. Questo studio ha dimostrato che le performance di questi polimeri in blend con PCBM in BHJ sono comparabili. Inoltre, tramite una caratterizzazione ottica, morfologica e lo studio della dinamica di ricombinazione dei portatori è stata messa in luce la relazione tra le caratteristiche macromolecolari dei polimeri, direttamente dipendenti dal tipo di sintesi, e il loro comportamento nei dispositivi in relazione al processing. Nell’ultima parte ho studiato un interlayer catodico basato su un derivato polifluorenico (PFN) in BHJ-SC, la cui deposizione da soluzione lo rende compatibile con le esigenze di fabbricazione roll to roll. Lo scheletro polifluorenico è stato funzionalizzato con due gruppi laterali alternati: un alchil-ammonio bromuro (NBr) il cui momento di dipolo garantisce una buona solubilità in alcool, solvente ortogonale a quello utilizzato per la deposizione dell’active layer,, e un alchil-etil-fosfonato (EP) che presenta un’interazione selettiva con l’alluminio, il metallo più comunemente usato come catodo. Lo studio completo tramite caratterizzazione elettrica e fotoelettrica ha messo in luce un miglioramento dell’estrazione di carica e una riduzione delle ricombinazioni interfacciali tra active layer e elettrodo grazie all’ingegnerizzazione superficiale data dal PFO-NBr-EP. Di conseguenza, le BHJ-SC basate su PTB7:PC71BM hanno mostrato un aumento di tutti i parametri fotovoltaici rispetto alla cella senza interlayer, migliorandone l’efficienza del 60%. Infine ho studiato il meccanismo di degradazione dei dispositivi, all’aria e in atmosfera controllata ,in relazione ai gruppi laterali presenti tramite uno analisi comparativa con altri interlayer commerciali.
(2018). Novel materials for solar energy conversion devices: Towards industrial scaling-up. (Tesi di dottorato, Università degli Studi di Milano-Bicocca, 2018).
Novel materials for solar energy conversion devices: Towards industrial scaling-up
CARULLI, FRANCESCO
2018
Abstract
In my thesis, I report the results obtained during my three years PhD regarding the study of novel materials for light-energy conversion devices, whose preparation and processing are in agreement with a near-future large scale production. This work is divided into two topics: 1) The study of novel emitters for luminescent solar concentrators (LSCs); 2) The study of novel functional materials for bulk hetero junction solar cells (BHJ-SC) regarding the photoactive layers and interface engineering layers. In the first part, I studied the application of silicon Quantum Dots (Si-QDs) as novel chromophores in LSCs. The use of these semiconductor nanostructures overcomes the limitations in terms of industrial scaling which is characterizing the QDs so far used in LSC. Non-toxicity, low cost of raw material and ultra-abundancy make Si-QDs promising chromophores for large scale production of LSCs. Upon quantum confinement the optical properties of Si-QDs are significantly modified, resulting in a high Stokes Shift quantum efficiency. The device assembled by embedding the Si-QDs in a co-polymeric matrix exhibits a high optical quality and results in an almost ideal LSC in which the losses related to luminescence self-absorption and scattering, the two most important efficiency-limiting factors, are strongly reduced. Similar characteristics were demonstrated to be achieved also in flexible LSCs which still preserve the efficiency and the optical properties of standard flat LSC. In the second part I performed a comparative study about the photovoltaic performances in BHJ-SC of PBDTTPB, a polymer showing a good trade-off between PV performances and structural complexity, which makes it a promising candidate for a future industrial scale production. The comparison of devices PV characteristics was performed between two PBDTTPDs with different synthetic routes: one prepared via Stille reaction, a synthesis involving the use of high toxicity organo-tin compounds and requiring the pre-functionalization of the monomers, and a second one prepared via Direct (Hetero)Arylation Polymerization (DHAP), an alternative synthetic route, which avoids pre-functionalized monomers and the stoichiometric production of heavy-metal-containing waste. This study showed that the polymer synthetized via DHAP exhibits a power conversion efficiency comparable to that prepared via Stille. Moreover, through the use of optical, morphological characterizations and charge recombination analysis, I could highlight the relationship between the PBDTTPDs macromolecular properties, which are heavily dependent on their preparation, and their PV behavior in BHJ through the processing. In the last section, I studied a novel poly-fluorene-based (PFN) material as cathode interfacial material (CIM) for BHJ-SC with direct geometry, whose solution processing is totally compatible with roll to roll fabrication requirements. The PFN backbone was functionalized with two side groups: alkyl-ammonium bromide (NBr), whose high polarity allows high solubility in alcohol, a solvent orthogonal to organic solvent for active layer, and an effective work function modification and ethyl-phosphonate (EP) groups, which exhibit a selective interaction with aluminium, the most common metal for cathodes. The complete study through photoelectric, electric and morphologic characterization techniques highlighted an enhancement of charges extraction and a reduction of active layer/electrode interfacial recombinations due to the effective surface engineering upon insertion of a thin layer of PFO-NBr-EP. Consequently the BHJ-SC based on PTB7:PC71BM blends with CIM/Al as cathode showed an enhancement of the PV parameters compared to the pristine Al devices, resulting in 7.24% PCE. Finally, I discussed the effect of the side groups on the device stability, through a comparison with other PFN-based interlayers already used in literature as CIM for PTB7:PC71BM-based devices.File | Dimensione | Formato | |
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Descrizione: tesi di dottorato
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