It is known that organic solar cell (OSC) is a promising photovoltaic technology due to the intrinsic advantages of organic materials, including mechanical flexibility, environmental friendliness and compatibility with roll-to-roll printing. Due to recent progress in small molecular acceptors (SMAs), OSC field has moved on to non-fullerene stage and the power conversion efficiency has been achieved over 18% based on Y-series acceptors. Since the photovoltaic performance of OSCs is influenced significantly by the chemical structure of materials, chemical modifications are helpful to further explore the potential of materials, among which side-chain engineering is demonstrated as an effective strategy to manipulate material properties. In this thesis, I will mainly focus on device fabrica...[ Read more ]

It is known that organic solar cell (OSC) is a promising photovoltaic technology due to the intrinsic advantages of organic materials, including mechanical flexibility, environmental friendliness and compatibility with roll-to-roll printing. Due to recent progress in small molecular acceptors (SMAs), OSC field has moved on to non-fullerene stage and the power conversion efficiency has been achieved over 18% based on Y-series acceptors. Since the photovoltaic performance of OSCs is influenced significantly by the chemical structure of materials, chemical modifications are helpful to further explore the potential of materials, among which side-chain engineering is demonstrated as an effective strategy to manipulate material properties. In this thesis, I will mainly focus on device fabrication and material design toward high-performance OSCs based on non-fullerene acceptors with bulky flanking side-chains. The insights into the relationship between device performance and chemical structure will be provided.

The effect of bulky side chains on the flanking thienothiophene units of Y6 are investigated in Chapter II. The introduction of phenyl unit attached to the core improves the steric effect between the side chain and the ending group. Meanwhile, the linking phenyl can also provide additional conjugation effect. Several beneficial effects are provided by the bulkier PhC6 unit: upshifted energy levels resulting in higher V_oc and favorable film-morphology with reduced paracrystalline disorder leading to increased carrier mobilities and suppressed monomolecular recombination with J_sc and FF simultaneously enhanced. As a result, the PM6:BTP-PhC6-based devices yield a higher efficiency value of 16.7% than its counterpart of BTP-C6Ph (15.5%).

In Chapter III, the impact of different franking side chains on Y6 molecule are systematically studied. Even though the bulky thiophene and benzene units on the side chains introduce more steric hindrance and thus slightly reduce the crystallinity of the molecule, surprisingly, an interesting matching trend is observed that the less crystalline molecule appears to match with PTQ10 donor better, with the resulting PTQ10:BTP-PhC6 based devices yielding 17.1% efficiency. To investigate the ternary performance, the molecule of BTP-ThC8 possessing good miscibility with BTP-PhC6 and better molecular packing is incorporated into PTQ10:BTP-PhC6, which leads to extended photon response, enhanced charge transport and suppressed charge recombination. The ternary device achieves an outstanding PCE of 17.6%.

In Chapter IV, the influence of the orientation of side chains on the properties of NFAs and on the photovoltaic performance of corresponding OSCs is investigated. Three isomeric NFAs named o-BTP-PhC6, m-BTP-PhC6, and p-BTP-PhC6 are designed by changing the substitution positions and thus orientations of the side chains attached to the central core. The studies show that the optimal side-chain orientation can be achieved by the meta-positioned hexylphenyl group (of the m-BTP-PhC6 molecule), which introduces significant beneficial effects on optical absorption, intermolecular packing, and phase separation of the NFAs. By pairing a donor polymer PTQ10 with m-BTP-PhC6, device efficiencies of 17.7% can be yielded.

Chapter V is the summary of this thesis and provides the future prospects of organic solar cells.