Publication date: October 2020
Source: Materials Science and Engineering: R: Reports, Volume 142
Author(s): Yuqiang Liu, Yajuan Li, Yiliang Wu, Guangtao Yang, Luana Mazzarella, Paul Procel-Moya, Adele C. Tamboli, Klaus Weber, Mathieu Boccard, Olindo Isabella, Xinbo Yang, Baoquan Sun
Open Access
  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.
Molecular surgery on π‐conjugated small molecules at the atomic level to tune optical absorption in the second near‐infrared region is reported. With this technique, an absorption peak at 1060 nm is achieved and its nanoparticle shows a record high photothermal conversion efficiency of 77% under 1064 nm excitation, and high performance of in vivo photothermal theranostics is demonstrated.
Extensive recent progress has been made on the design and applications of organic photothermal agents for biomedical applications because of their excellent biocompatibility comparing with inorganic materials. One major hurdle for the further development and applications of organic photothermal agents is the rarity of high‐performance materials in the second near‐infrared (NIR‐II) window, which allows deep tissue penetration and causes minimized side effects. Up till now, there have been few reported NIR‐II‐active photothermal agents and their photothermal conversion efficiencies are relatively low. Herein, optical absorption of π‐conjugated small molecules from the first NIR window to the NIR‐II window is precisely regulated by molecular surgery of substituting an individual atom. With this technique, the first demonstration of a conjugated oligomer (IR‐SS) with an absorption peak beyond 1000 nm is presented, and its nanoparticle achieves a record‐high photothermal conversion efficiency of 77% under 1064 nm excitation. The nanoparticles show a good photoacoustic response, photothermal therapeutic efficacy, and biocompatibility in vitro and in vivo. This work develops a strategy to boost the light‐harvesting efficiency in the NIR‐II window for cancer theranostics, offering an important step forward in advancing the design and application of NIR‐II photothermal agents.
Surface doping of a conjugated polymer is drastically modulated by molecular orientation. Face‐on orientation presents more reaction sites for dopant/host interactions, leading to effective charge trap filling facilitated by efficient vertical transport down to conductive channel. Hole mobility increases by fivefold to 3 cm2 V−1 s−1 in the face‐on case, compared to a minimal change in the edge‐on case.
Molecular orientation plays a critical role in controlling carrier transport in organic semiconductors (OSCs). However, this aspect has not been explored for surface doping of OSC thin films. The challenge lies in lack of methods to precisely modulate relative molecular orientation between the dopant and the OSC host. Here, the impact of molecular orientation on dopant–host electronic interactions by large modulation of conjugated polymer orientation via solution coating is reported. Combining synchrotron‐radiation X‐ray measurements with spectroscopic and electrical characterizations, a quantitative correlation between doping‐enhanced charge carrier mobility and the Herman's orientation parameter is presented. This direct correlation can be attributed to enhanced charge‐transfer interactions at host/dopant interface with increasing face‐on orientation of the polymer. These results demonstrate that the surface doping effect can be fundamentally manipulated by controlling the molecular orientation of the OSC layer, enabling optimization of carrier transport.
The relatively large non‐radiative recombination voltage loss (ΔV non‐rad) is the main challenge for the development of organic solar cells (OSCs). ΔV non‐rad of OSCs can be effectively reduced to 0.16 V by adopting material combinations that deliver high E CT (the energy of charge‐transfer state) and low ΔE CT (energetic difference between singlet excited state and CT state), together with chlorination in donors.
Compared with inorganic or perovskite solar cells, the relatively large non‐radiative recombination voltage losses (ΔV non‐rad) in organic solar cells (OSCs) limit the improvement of the open‐circuit voltage (V oc). Herein, OSCs are fabricated by adopting two pairs of D–π–A polymers (PBT1‐C/PBT1‐C‐2Cl and PBDB‐T/PBDB‐T‐2Cl) as electron donors and a wide‐bandgap molecule BTA3 as the electron acceptor. In these blends, a charge‐transfer state energy (E CT) as high as 1.70–1.76 eV is achieved, leading to small energetic differences between the singlet excited states and charge‐transfer states (ΔE CT ≈ 0.1 eV). In addition, after introducing chlorine atoms into the π‐bridge or the side chain of benzodithiophene (BDT) unit, electroluminescence external quantum efficiencies as high as 1.9 × 10−3 and 1.0 × 10−3 are realized in OSCs based on PBTI‐C‐2Cl and PBDB‐T‐2Cl, respectively. Their corresponding ΔV non‐rad are 0.16 and 0.17 V, which are lower than those of OSCs based on the analog polymers without a chlorine atom (0.21 and 0.24 V for PBT1‐C and PBDB‐T, respectively), resulting in high V oc of 1.3 V. The ΔV non‐rad of 0.16 V and V oc of 1.3 V achieved in PBT1‐C‐2Cl:BTA3 OSCs are thought to represent the best values for solution‐processed OSCs reported in the literature so far.
Three homologous small molecule donors with hydrogen, fluorine, and chlorine substitution afford organic solar cells with efficiencies over 10% in combination with a common acceptor. The chlorinated derivative exhibits a more crystalline nanomorphology with relatively pure domains and provides more than 12% efficiency.
Compared to conjugated polymers, small‐molecule organic semiconductors present negligible batch‐to‐batch variations, but presently provide comparatively low power conversion efficiencies (PCEs) in small‐molecular organic solar cells (SM‐OSCs), mainly due to suboptimal nanomorphology. Achieving precise control of the nanomorphology remains challenging. Here, two new small‐molecular donors H13 and H14, created by fluorine and chlorine substitution of the original donor molecule H11, are presented that exhibit a similar or higher degree of crystallinity/aggregation and improved open‐circuit voltage with IDIC‐4F as acceptor. Due to kinetic and thermodynamic reasons, H13‐based blend films possess relatively unfavorable molecular packing and morphology. In contrast, annealed H14‐based blends exhibit favorable characteristics, i.e., the highest degree of aggregation with the smallest paracrystalline π–π distortions and a nanomorphology with relatively pure domains, all of which enable generating and collecting charges more efficiently. As a result, blends with H13 give a similar PCE (10.3%) as those made with H11 (10.4%), while annealed H14‐based SM‐OSCs have a significantly higher PCE (12.1%). Presently this represents the highest efficiency for SM‐OSCs using IDIC‐4F as acceptor. The results demonstrate that precise control of phase separation can be achieved by fine‐tuning the molecular structure and film formation conditions, improving PCE and providing guidance for morphology design.
In tandem organic photovoltaics, most ultraviolet–visible photons are absorbed by the front sub‐cell, so in the rear sub‐cell, excitons generated on large‐bandgap donors will be reduced significantly. This reduces the conductivity and limits the hole‐transporting property of the rear sub‐cell. An infrared‐absorbing polymer donor is introduced, which provides a second hole‐generation/transporting mechanism to minimize the aforementioned detrimental effects.
In tandem organic photovoltaics, the front subcell is based on large‐bandgap materials, whereas the case of the rear subcell is more complicated. The rear subcell is generally composed of a narrow‐bandgap acceptor for infrared absorption but a large‐bandgap donor to realize a high open‐circuit voltage. Unfortunately, most of the ultraviolet–visible part of the photons are absorbed by the front subcell; as a result, in the rear subcell, the number of excitons generated on large‐bandgap donors will be reduced significantly. This reduces the (photo) conductivity and finally limits the hole‐transporting property of the rear subcell. In this work, a simple and effective way is proposed to resolve this critical issue. To ensure sufficient photogenerated holes in the rear subcell, a small amount of an infrared‐absorbing polymer donor as a third component is introduced, which provides a second hole‐generation and transporting mechanism to minimize the aforementioned detrimental effects. Finally, the short‐circuit current density of the two‐terminal tandem organic photovoltaic is significantly enhanced from 10.3 to 11.7 mA cm−2 (while retaining the open‐circuit voltage and fill factor) to result in an enhanced power conversion efficiency of 15.1%.
A high‐performance all‐polymer solar cell (PCE of 12.06%) is achieved based on a novel polymer acceptor with a voltage loss of 0.52 eV, which is one of the smallest values reported for all‐polymer solar cells to date.
Although the field of all‐polymer solar cells (all‐PSCs) has seen rapid progress in device efficiencies during the past few years, there are limited choices of polymer acceptors that exhibit strong absorption in the near‐IR region and achieve high open‐circuit voltage (V OC) at the same time. In this paper, an all‐PSC device is demonstrated with a 12.06% efficiency based on a new polymer acceptor (named PT‐IDTTIC) that exhibits strong absorption (maximum absorption coefficient: 2.41 × 105 cm−1) and a narrow optical bandgap (1.49 eV). Compared to previously reported polymer acceptors such as those based on the indacenodithiophene (IDT) core, the indacenodithienothiophene (IDTT) core has further extended fused ring, providing the polymer with extended absorption into the near‐IR region and also increases the electron mobility of the polymer. By blending PT‐IDTTIC with the donor polymer, PM6, a high‐efficiency all‐PSC is achieved with a small voltage loss of 0.52 V, without sacrificing J SC and FF, which demonstrates the great potential of high‐performance all‐PSCs.
