In the last 20 years, Organic Solar Cells have increasingly attracted the attention of researchers and of the nowaday’s insatiable energy market, thanks to their numerous advantages. For instance, they combine features like flexibility, lightness and semitransparency to ultrathin, rollable and bendable shapes. Moreover, their almost complete solution processability allows for roll-to-roll large-area fabrication with potential low-cost manufacturing and low environmental impact. On the downside OSCs show a modest power conversion efficiency and a wide range of stability issues related to the organic nature of the technology, that limit their commercialization. Consistent laboratory research efforts are being carried on worldwide, to overcome those issues, optimizing the materials employed, the device’s architecture, the fabrication process and the encapsulation. In this scenario, the interface between the organic electrodes and the active layer, is of crucial importance, and a dedicated interlayer is typically employed both at the anode and at the cathode. Their main task is to match the electronic levels of the electrodes with the transport levels of the organic materials, to facilitate a selective charge transport. For electron extraction, materials like titanium oxide (TiOx) and zinc oxide (ZnO) have been widely used. However, a reversible degradation effect, often referred to as “light-soaking issue” has been observed in such interlayers. The cells need to be activated through absorption of UV light. If this doesn’t happen, s-shaped J/V-characteristics with low fill factors (FF) and overall low power conversion efficiency (PCE) are found. The AM1.5 solar spectrum would in principle provide enough UV radiation to activate TiOx or ZnO, however, in outdoor applications, the light-soaking would consume roughly 1h every morning for the cells to be activated. In addition, to avoid UV-induced degradation, many active organic donor materials require to be protected with UV filters. Moreover, must be considered that OSCs are often employed in indoor applications, where UV light would also be missing. In that case, the lack of UV light would keep the cells inactivated and, therefore, not efficient. As a result, the development of charge extraction materials that do not rely on UV activation has been identified as a critical challenge to achieve highly efficient and long-term stable devices. In this work we explore a complete roll-to-roll fabrication of devices with a recently developed SnOx-based ETL, which is reported to be a good alternative to the conventional interlayers, since it doesn’t need to undergo the light soaking and at the same time it should improve the overall stability of the interface. We begin characterizing the behaviour of cells with pristine SnOx interlayers, from the point of view of the reversible degradation of light-soaking. Furthermore, we set the basis for a stability assessment, to observe the evolution of their performances during a longer period of time. The literature in support of short and long term stability of OSC from the interlayer’s point of view is still rather incomplete, and the understanding of the associated phenomena represents a fundamental step toward the optimization and commercialization of the organic photovoltaic technology (OPV).
Negli ultimi 20 anni le celle solari organiche hanno attratto sempre di più l’attenzione della ricerca e del mercato energetico grazie ai loro numerosi vantaggi. Esse combinano caratteristiche quali flessibilità, leggerezza e semitrasparenza, a forme super sottili e pieghevoli. Inoltre, i materiali alla base permettono una quasi completa processabilità da soluzione, che consente la fabbricazione in rotativa di dispositivi con aree grandi, potenzialmente a basso costo e basso impatto ambientale. Tra gli aspetti negativi, le OSC presentano efficienze di conversione al giorno d’oggi modeste, oltre a un ampio spettro di problemi legati alla stabilità derivanti dalla natura organica della tecnologia, che ne limitano la commercializzazione. Numerosi laboratori di ricerca in tutto il mondo stanno lavorando per superare tali problematiche, tramite l’ottimizzazione dei materiali, delle architetture e dei processi di fabbricazione e incapsulamento. In questo scenario, l’interfaccia tra gli elettrodi organici e il materiale attivo è di fondamentale importanza, e film dedicati (interlayers) vengono applicati in corrispondenza di anodo e catodo. La loro funzione principale è di accoppiare i livelli energetici degli elettrodi con i livelli di trasporto dei materiali organici, per facilitare il trasporto selettivo delle cariche. Per l’estrazione di elettroni, due tipici materiali utilizzati sono l’ossido di titanio (TiOx) e l’ossido di zinco (ZnO). Tuttavia, è stato dimostrato che tali materiali presentano un fenomeno di degradazione definito “light-soaking issue”. Le celle hanno bisogno di essere attivate tramite assorbimento di luce UV, e se questo non succede si originano curve caratteristiche con FF e PCE ridotti. Nonostante lo spettro solare AM1.5 sia in grado di provvedere la quantità di radiazione UV necessaria per attivare gli ossidi, in applicazioni outdoor servirebbe 1h ogni mattina per attivare le celle. Inoltre, per evitare la degradazione indotta dall’UV, molti materiali attivi organici richiedono l’uso di filtri UV. Per di più, bisogna considerare che le OSC sono spesso impiegate in applicazioni da indoor, in cui la luce UV sarebbe mancante. In tal caso le celle rimarrebbero inattive, e dunque inefficienti. Come risultato, lo sviluppo di materiali per l’estrazione di carica che non dipendono dall’attivazione UV è stato identificato come un obiettivo fondamentale per ottenere dispositivi ad alta efficienza e stabili sul lungo periodo. In questo lavoro viene esplorata una fabbricazione roll-to-roll di dispositivi, che impiegano un layer di trasporto di recente sviluppo basato su ossido di stagno (SnOx). Esso è riportato in letteratura come una buona alternativa agli interlayer convenzionali, dal momento che non necessita di effettuare il light-soaking e allo stesso tempo dovrebbe aumentare la stabilità generale dell’interfaccia. Inizialmente il comportamento di celle con interlayers a base di SnOx viene caratterizzato rispetto ai più comuni interlayers, dal punto di vista del fenomeno del light-soaking. Inoltre, vengono gettate le basi per uno studio di stabilità a lungo termine, per osservare l’evoluzione delle loro performances nel tempo. La letteratura a supporto della stabilità a corto e lungo termine di OSC, dal punto di vista dell’interlayer, è ancora molto incompleta, e la comprensione dei fenomeni associati rappresenta uno step fondamentale verso l’ottimizzazione e commercializzazione della tecnologia fotovoltaica a base organica.
Improved reliability of organic solar cells with novel SnOx electron transport layers
Iacomini, Matteo
2019/2020
Abstract
In the last 20 years, Organic Solar Cells have increasingly attracted the attention of researchers and of the nowaday’s insatiable energy market, thanks to their numerous advantages. For instance, they combine features like flexibility, lightness and semitransparency to ultrathin, rollable and bendable shapes. Moreover, their almost complete solution processability allows for roll-to-roll large-area fabrication with potential low-cost manufacturing and low environmental impact. On the downside OSCs show a modest power conversion efficiency and a wide range of stability issues related to the organic nature of the technology, that limit their commercialization. Consistent laboratory research efforts are being carried on worldwide, to overcome those issues, optimizing the materials employed, the device’s architecture, the fabrication process and the encapsulation. In this scenario, the interface between the organic electrodes and the active layer, is of crucial importance, and a dedicated interlayer is typically employed both at the anode and at the cathode. Their main task is to match the electronic levels of the electrodes with the transport levels of the organic materials, to facilitate a selective charge transport. For electron extraction, materials like titanium oxide (TiOx) and zinc oxide (ZnO) have been widely used. However, a reversible degradation effect, often referred to as “light-soaking issue” has been observed in such interlayers. The cells need to be activated through absorption of UV light. If this doesn’t happen, s-shaped J/V-characteristics with low fill factors (FF) and overall low power conversion efficiency (PCE) are found. The AM1.5 solar spectrum would in principle provide enough UV radiation to activate TiOx or ZnO, however, in outdoor applications, the light-soaking would consume roughly 1h every morning for the cells to be activated. In addition, to avoid UV-induced degradation, many active organic donor materials require to be protected with UV filters. Moreover, must be considered that OSCs are often employed in indoor applications, where UV light would also be missing. In that case, the lack of UV light would keep the cells inactivated and, therefore, not efficient. As a result, the development of charge extraction materials that do not rely on UV activation has been identified as a critical challenge to achieve highly efficient and long-term stable devices. In this work we explore a complete roll-to-roll fabrication of devices with a recently developed SnOx-based ETL, which is reported to be a good alternative to the conventional interlayers, since it doesn’t need to undergo the light soaking and at the same time it should improve the overall stability of the interface. We begin characterizing the behaviour of cells with pristine SnOx interlayers, from the point of view of the reversible degradation of light-soaking. Furthermore, we set the basis for a stability assessment, to observe the evolution of their performances during a longer period of time. The literature in support of short and long term stability of OSC from the interlayer’s point of view is still rather incomplete, and the understanding of the associated phenomena represents a fundamental step toward the optimization and commercialization of the organic photovoltaic technology (OPV).| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/173806