Author: Zhang, Ying
Title: Investigation of high-performance non-fullerene organic solar cells prepared with novel nanostructures
Advisors: Li, Gang (EIE)
Degree: Ph.D.
Year: 2022
Subject: Organic photovoltaic cells -- Design and construction
Hong Kong Polytechnic University -- Dissertations
Department: Department of Electronic and Information Engineering
Pages: xviii, 152 pages : color illustrations
Language: English
Abstract: Organic solar cells (OSCs) or organic photovoltaic (OPVs) have been put huge research efforts benefiting from their distinguished merits of cheapness, low weight and flexibility. Thanks to the device engineering and materials innovation, impressive progress in the last two decades has triggered power conversion efficiencies (PCEs) of OSCs exceeding 18%, which is approaching benchmark 20% PCE and shows great promise for real world applications. Recently, non-fullerene acceptors (NFAs) have dominated high-performance OSCs on account for their tunable chemical structures and thus electronic properties to by replacing fullerene counterparts. However, the performances of OSCs still fall behind that of globally leading crystalline silicon (c-Si) solar cells and more recent inorganic-organic perovskite solar cells. The main reason is the excitonic characteristics of OSC with larger binding energy and the energy loss of the state-of-the-art OSC systems are generally around ~0.5-0.6 eV. Traditionally, the dominant OSC active layer is typically constituted of electron donating (D) and electron accepting (A) materials that is blended to form bulk heterojunctions (BHJs). It is generally believed that BHJ structure of OSCs complicates the morphology regulation of D-A blend, e.g., delicately balance of domain size/purity, phase separation and crystallinity, which has its beauty but also hinders the further improvement of PCE for OSCs. Therefore, further improvement of OSCs performances is expected to be realized by simultaneously optimizing morphological properties of active layer and minimizing energy loss.
During my PhD study, my major research activities are i) the OSC active layer morphology manipulation towards high-performance non-fullerene acceptor based binary OSCs; ii) investigation of the underlying mechanisms behind the performance improvement, providing scientific insights into the inherent correlation interconnecting the device engineering - morphological structure - charge dynamics - photovoltaic performances.
The introduction - Chapter one consists of a brief background on the general PV techniques, a brief introduction of OSCs, a concise description of mainstream progress of OSC photoactive layer structure, and a general introduction of NFA based ternary OSCs, aiming at providing a concise while solid understanding platform of OSCs.
In chapter two, I report my first research project in OSC - reliable and consistent bandgap (Eg) determination in representative OSC systems, in particular the impacts on the OSC energy loss (Eloss) study. Since the innovation of effective NFAs in 2016 enabled broader absorption and thus high current density, researchers and scientists started to shift more efforts on the VOC loss issue in OPV community, which have long been considered to be the most critical factor that determines the further improvement of PCE.[1] Therefore, to achieve higher PCEs in OSCs, an in-depth cognition of the origins of Voc and its loss becomes inevitable. This work provided a rigid comparison by summarizing the different bandgaps calculation approaches used in Eloss determination, which exerted direct impact on Eloss and OSC research.
In chapter three, based on the state-of-the-art binary NFA-OSC, we reduced the Eloss and improved the Voc of OSCs via introducing the rational third component (ternary strategy). The chemical compatibility issue becomes the key point that has to be considered when producing multi-donor OSC systems.[2] Guided by this criterion, two wide bandgap (WBG) polymer donors are designed and synthesized that are chemically compatible to the host donor PM6, and to afford highly efficient ternary OSCs. In binary OSCs, we achieved very high VOC (0.95 V) with low Eloss of 0.55 eV when paired with acceptor IT-4F. More importantly, the novel polymer donors also demonstrated their validity in reducing Eloss in ternary OSCs, achieving an improved PCE of 13.4% with simultaneously enhanced FF and JSC.
In chapter four, to further explore the effectiveness of WBG polymer donor in improving the performances of OSCs, another novel WBG polymer donor possessing more low-lying HOMO level of ~-5.73 eV was synthesized and incorporated into high performance binary OSCs. As a result, we not only proved that this material is effective in improving VOC, but more interestingly, the negative energy offsets between PBT(E)BTz and IT-4F exert no negative effects on the charge dissociation. Morphologically, a low dose of PBT(E)BTz with high crystallinity shows enhanced molecular packing which benefits the charge transport and thus higher FF in ternary solar cells. The ternary works formulate the general rules of selecting third components with regards to assisting high efficiency OSCs.
In chapter five, my research went beyond traditional BHJ. Delicate tailoring of active layer morphology via introducing a novel structure - graded heterojunction (G-BHJ) strategy. This work is motivated by the fact that different from fullerenes possessing caged structure, the prevailing ITIC-like or Y-series NFAs have similar conjugated backbones, making the aggregation manipulation and phase separation more complex and challenging in "blend" BHJ solution. Therefore, developing a novel structure-property correlation is indispensable, and sequential deposition (SD) comes onto the map. In this chapter, we revealed the solvent selection strategies for optimizing morphology via sequential or layer-by-layer (LBL) deposition, aiming at effective G-BHJ morphology. We also reappraised clear evidence of graded BHJ structure with the well-defined vertical phase separation as well as enhanced crystallinity in G-BHJ OSCs, which was quantitatively characterized for the first time. Ultimately, G-BHJ OSCs delivered superior performances than that of traditional BHJ OSCs in both spin-coating and blade-coating processing techniques. Moreover, this strategy was further extended into thick-film OSCs (16.25%/14.37% PCE with 300 nm/500 nm active layer thickness). A novel structural morphology-performances relationship is thoroughly discussed, and we provide a feasible and effective way towards efficient, eco-friendly, and scalable solar cells.
Chapter six gives conclusion of my research work, and proposes the future research prospects that I believe important for me and other researchers should pursue for the OSC technology future.
Rights: All rights reserved
Access: open access

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