Carbohydrate-attached fullerene derivative for selective localization in ordered carbohydrate-block-poly(3-hexylthiophene) nanodomains

Highlights

• 10 μm Thick films made from Carbohydrate-b-semi-conducting Block Copolymer System.

• A carbohydrate-attached fullerene for photovoltaic applications.

• Fullerene derivatives were selectively localized in the phase-separated nanodomains.

• A well-defined sub-10 nm scale donor/acceptor lamellar structure was achieved.

Abstract

A carbohydrate-based fullerene derivative (AcMal₇-C₆₁) is designed, synthesized and applied to a lamellar-forming high-χ block copolymer system, poly(3-hexylthiophene)-block-peracetylated maltoheptaose (P3HT-b-AcMal₇), to actualize an ordered donor/acceptor (D/A) network. A well-defined D/A lamellar structure of the P3HT-b-AcMal₇:AcMal₇-C₆₁ blend with sub-10 nm domain features is achieved upon thermal annealing. The AcMal₇-C₆₁ molecules are localized in the phase-separated AcMal₇ nanodomains without causing the formation of fullerene crystals while maintaining the lamellar morphology up to 1:0.5 (D:A) blending ratio. The cross-sectional TEM observation and GISAXS measurement reveals that the P3HT-b-AcMal₇ tends to spontaneously organize into lamellar structures oriented perpendicular to the film surface at the air/film interface while the domain orientation at the bottom interface depends on the nature of the substrate.

Introduction

Organic photovoltaics (OPVs) have been intensively studied in the past two decades in response to social demand for renewable energy technologies that can fulfill the needs for low-cost, light weight, and flexible energy devices (Dou et al., 2013). With extensive efforts on the device engineering and the material design, the power conversion efficiencies (PCEs) of single junction donor-acceptor (D/A) bulk-heterojunction (BHJ) solar cells have remarkably improved since the first demonstration of the BHJ concept in 1995 (Yu, Gao, Hummelen, Wudl, & Heeger, 1995).

Block copolymers have received significant attention in the area of organic optoelectronics because of their ability to self-organize into thermodynamically stable periodic nanostructures (Lee & Gomez, 2015; Segalman, McCulloch, Kirmayer, & Urban, 2009). Regioregular poly(3-hexylthiophene), P3HT, is one of the most employed p-type semiconducting polymers in the block copolymer systems. However, strong π-π interactions of the P3HT often prevents the microphase separation, eventually results in a randomly-oriented nanofibril structure (Boudouris, Frisbie, & Hillmyer, 2008; Iovu et al., 2007; Liu, Sheina, Kowalewski, & McCullough, 2002). Recently, we have reported the synthesis and morphological characterization of a carbohydrate-based block copolymer, poly(3-hexylthiophene)-block-peracetylated maltoheptaose (P3HT-b-AcMal₇). We revealed that, via the thermal annealing of the copolymer thin film, the P3HT-b-AcMal₇ can self-organize into a periodic lamellar structure having a sub-10 nm scale d-spacing. This periodicity is one of the smallest domain sizes ever reported in block copolymers containing crystalline π-conjugated polymer blocks (Sakai-Otsuka et al., 2017). This distinct morphology could be attained by the huge chemical and steric disparity (high χ) between the synthetic P3HT block and the naturally-derived carbohydrate block. Furthermore, an enhanced thermal stability of the P3HT crystalline structure was observed when the P3HT segments were confined in the phase-separated P3HT-b-AcMal₇ lamellae (Sakai-Otsuka et al., 2020).

While our previous works have highlighted the great potential of the P3HT-b-AcMal₇ as a system with an ideal morphology for OPV securing a high stability of the P3HT crystalline structure, the two following questions are yet to be answered: (i) how the semiconducting domains are organized in the film and (ii) how the ordered D/A nanostructures can be achieved by taking advantage of self-organization of the P3HT-b-AcMal₇. In this study, by addressing these questions, we aim at establishing a morphological control method best suited for our original P3HT-b-AcMal₇ system whilst providing the fine-tuned ideal D/A lamellar structures.

Controlling the morphology of photoactive layers remains a crucial challenge for performance enhancement of the OPVs. A perpendicularly-oriented periodic lamellar structure, in which electron donor and acceptor materials are alternately interpenetrating on the 10-nm length scale, has been recognized as an ideal geometry. This well-defined structure ensures (i) enlarged interface contact, (ii) optimal lateral domain size for exciton dissociation, (iii) the shortest charge transportation pathway in the vertical direction, and (iv) reduced charge trap sites (Coakley & McGehee, 2004; Shah & Ganesan, 2010). Although our previous study demonstrated the formation of the lamellar structures in the film, it was unclear how the semiconducting domains are organized with respect to the film surface.

Another issue is to achieve ordered D/A nanostructures by taking advantage of self-organization of the P3HT-b-AcMal₇. For instance, in case of a blend of P3HT and [6,6]-phenyl C₆₁ butyric acid methyl ester (PC₆₁BM), small PC₆₁BM molecules are easily diffused over the photoactive layer due to the lack of physical or chemical connections to the P3HT matrix. This instability causes inhomogeneous macrophase segregations of the blends, undesired crystallization or aggregation of PC₆₁BM, and eventually leads to the deterioration of device performance with long-term device operations or post-annealing process. Several strategies have been implemented to localize fullerene-based n-type acceptors in a specific area of phase-separated block copolymer systems. Synthesizing D-A block copolymers consisting of a semiconducting donor polymer and another polymer bearing covalently attached acceptor compounds is one of the promising approaches (Miyanishi, Zhang, Tajima, & Hashimoto, 2010; Miyanishi, Zhang, Hashimoto, & Tajima, 2012). Alternatively, non-covalent chemical interaction between acceptor materials and polymer blocks has also been exploited to immobilize the acceptors (Lin et al., 2012; Sary et al., 2010). Inspired by the latter approach, in this study, we designed a new fullerene derivative bearing AcMal₇ segment (AcMal₇-C₆₁) to immobilize the acceptor compounds in the AcMal₇ channel. We hypothesize that the AcMal₇-C₆₁ would be excluded from P3HT domains during a post-annealing process and predominately located within the phase-separated AcMal₇ domain owing to a miscibility between the AcMal₇-C₆₁ and AcMal₇ segments. Simultaneously, the AcMal₇-C₆₁ is not expected to cause any microphase separation or crystallization of fullerenes in the AcMal₇ matrix because of a steric constraint of AcMal₇ moiety.

Herein, we present the synthesis of carbohydrate-based fullerene derivative AcMal₇-C₆₁ and the reciprocal self-organization of the P3HT-b-AcMal₇:AcMal₇-C₆₁ blends. The chemical structure of the AcMal₇-C₆₁ was fully characterized by NMR, FT-IR, and mass spectroscopy. The validity of the approach was confirmed by investigating the morphology of the P3HT-b-AcMal₇:AcMal₇-C₆₁ blends and crystalline structure of P3HT domains using transmission electron microscopy (TEM) and Wide-angle X-ray scattering (WAXS) analysis. In addition, an internal orientation of the self-assembled P3HT-b-AcMal₇ lamellar was investigated to gain further insight into the thin film’s internal structure. TEM and grazing incidence small-angle X-ray scattering (GISAXS) measurements revealed that the P3HT-b-AcMal₇ tends to organize into perpendicularly-oriented lamellar structures relative to the film surface at the air/film boundary.