Abstract
Organic solar cells (OSCs) have received wide research attention for their unique properties such as semi-transparency, light weight, low cost, flexibility, and easy to fabricate. The development of bulk heterojunction (BHJ) OSCs has been surging ahead accompanying with the rapid development of multifarious non-fullerene acceptors (NFAs). At present, the well-known NFAs such as ITIC and Y6 series, have replaced fullerene derivatives in fabricating highly efficient BHJ OSCs due to their simple synthesis and purification, tunable spectral absorption, and modulable frontier energy levels in comparison with fullerene acceptors, NFAs based OSCs have achieved a very promising power conversion efficiency (PCE) of approaching 19% recently.N-heteroarene based NFAs, as one of the main kinds of NFAs, exhibited great potential in fabricating high-performance OSCs for their unique features, such as higher energy levels facilitated by the strong electron donating nitrogen atom, red-shifted absorption, and an extra reaction site for side chain modification. Chapter 1 demonstrates the superiorities of these N-heteroarene based NFAs in fabricating high performance OSCs. Hence, Chapter 2 provides a comprehensive overview of N-heteroarene based NFAs. The development of N-heteroarene based NFAs and their construction are reviewed and discussed in this chapter. Subsequently, Chapter 3 presents the essential methodologies and analysis techniques employed in this thesis to characterize NFAs and the devices.
First of all, thiophene[3,2-b] pyrrole (TP) building block was introduced to construct two fused-ring conjugated NFAs, named as PTBTP-2F and PTBTP-4F (Chapter 4). When blending with donor PBDB-T, the PTBTP 4F fabricated devices afforded a PCE of 12.33% with a 𝑉OC of 0.86 V, a 𝐽SC of 20.74 mA cm-2 and a FF of 69.02%.
Core engineering has been demonstrated effective to achieve high performance NFAs, as molecular conjugation and intramolecular charge transfer can be readily realized through introducing or removing the aromatic rings on the backbone of NFAs. Additionally, the asymmetric strategy was adopted for molecule modification, which could precisely regulate the intrinsic optoelectronic properties of NFAs. Therefore, asymmetric molecular design strategy was employed to optimize PTBTP 4F and improve the photovoltaic performance (Chapter 5). TP block and thiophene[3,2-b] thiophene (TT) block were utilized to construct asymmetric acceptors, PTBTT-4F/4Cl. Ascribed to the great miscibility between PBDB-TF and PTBTT-4F and thus exquisite phase separation, an outstanding FF of 76.73% was achieved. Eventually, the PBDB-TF: PTBTT-4F based devices afforded a maximum PCE of 14.49%.
To further optimize the PTBTT backbone, we divided the PTBTT backbone into three segments and reconstructed the TPBTT backbone (Chapter 6). After the isomerization of TP block, the ICT properties improved a lot, owing to the inner placement of the pyrrole ring.. By alternating the position of pyrrole ring, the isomerized TPBTT backbone obtained improved molecular stacking, leading to higher crystallinity and amended energy disorder. Ascribed to improved charge mobility and suppressed 𝐸Loss, an impressive PCE of 15.72% was achieved in TPBTT-4F based devices, along with a reduced non-radiative energy loss (Δ𝐸nonrad) of 0.276 eV and a significant improvement in 𝐽SC.
In conclusion, this thesis proposes a commercial organic building block, TP block. Based on TP block, PTBTP series were created with suppressed 𝐸Loss . Asymmetric design strategy was adopted in PTBTP backbone modification, and constructed PTBTT series with improved device performance. Eventually, TPBTT series were obtained through isomerization, which further achieved more ordered molecular stacking and an improved PCE.
| Date of Award | Jul 2024 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Hainam Do (Supervisor), Yong Ren (Supervisor) & Fei Chen (Supervisor) |