Morphology modulation of organic photovoltaics with block copolymer additive based on rational design strategies

Highlights

• We demonstrate block copolymer-based stretchable solar cells on plastic foil substrates with good efficiency.

• We find that the block-copolymer-based solar cell can withstand tensile strain up to 37%.

• We conclude that the ternary blend systems are consistent with parallel-like solar cells and the compatibilizer.

Abstract

Organic photovoltaics are a promising alternative to silicon-based solar cells with benefits of low-cost production and large scalability. However, its performance is restricted by a non-equilibrium phase-separated morphology. Additive compositions of block copolymer P3HT-b-PFTBT are most likely to mix up and form donor and acceptor morphologies. The parallel bulk-heterojunction model was proposed to show the characteristic photovoltaic parameters and the effect of the parallel cascading heterojunction formation made up of isolated PCBM acceptor domains. We demonstrate block copolymer-based stretchable solar cells on plastic foil substrates, with good power conversion efficiency. To compare the efficiency and stretchability, organic photovoltaic devices were constructed using P3HT/PC₆₁BM, PTB7/PC₇₁BM and P3HT/P3HT-b-PFTBT/PCBM active layer combinations. We find that through rational design of the component ratio, the block-copolymer-based solar cell can withstand tensile strain up to 37%.

Introduction

Organic photovoltaics (OPVs) based on the conjugated polymers exhibit the shortest payback time among various photovoltaic technologies [[1], [2], [3], [4]]. Systematic materials design and processing optimization have made great progress in device performance, and the power conversion efficiencies (PCEs) of single-junction OPV devices have been continuously improved to 18% by adding novel absorbing materials which reduce the band gap voltage offset, along with high fill factor (FF) and external quantum efficiency (EQE) [3,4]. However, a key challenge in the further development of these devices is finding reliable strategies for controlling and directing the blend morphologies, which exert a significant impact on electronic properties, stability, mechanical robustness and overall functionality [[5], [6], [7], [8]]. A series of strategies have been proposed to control the film morphology and to optimize the device performance, including the use of processing additives, coating method, surface treatment, and others, as shown in details in recent reports. Materials and processing methods both have critical impacts on the morphology so that the device performance can be improved by a rational design of material composition and active layer coating process [4,[9], [10], [11]].

The use of block copolymer is an effective approach to control the donor and acceptor morphology since electron donor and acceptor materials form an interpenetrating network of nanoscale domains and create interconnected pathways for charge transport and large interfacial area for exciton dissociation into charge-separated states. However, the appropriate blending materials and the optimal ratio between different materials still requires experiments to figure out. For two of the primary challenges still limiting the marketability of these devices—comparing to the conventional silicon-based solar cell—are the relatively low efficiency of the device, as well as the long-term thermal instability of the active layer of devices [4,12]. To further enhance the device performance, a ternary blend structure was designed to tune the energy level of donor and acceptor and more importantly, to provide a cascading effect that aids in charge separation.

Polymer solar cells are known as plastic-based devices consisting of the carbon framework, which indicates they are promising candidates for the next generation of the flexible solar cell (FSC). But recent studies show that the mechanical properties are related to a wide range of values more than the predicted factor of molecular structure, although molecular structure plays an important role in the stretchability. Kaltenbrunner and colleagues have demonstrated that their flexible solar cell can survive quasi-linear compression below 70% on a pre-stretched elastomer, which may show a very good stretchability [13]. Lipomi and colleagues reported a stretchable organic solar cell fabricated on pre-stretched PDMS, which can attain a 27% tensile strain reversibly [14]. For the normal non-pre-stretched flexible substrate, they have achieved better tensile strain up to 20% by a new polymer material of DPPT-TT [15].

In this work, we investigate the morphologies formed by a ternary blend of conjugated polymer and all-conjugated block copolymer. The results indicate the effect of P3HT-b-PFTBT compatibilizer on the performance and electronic properties of photovoltaic devices and suggest that the development of block copolymer (BCP) is viable to tune optical and electronic properties, which are supported by the alloy model and parallel bulk-heterojunction model at different BCP composition. The step after designing appropriate photovoltaic material is to further implement the overall device properties and put forward its wide application in large-scale production. We study the effect of all-conjugated P3HT-b-PFTBT block copolymer (BCP) additives on the photovoltaic performance and stretchability of bulk heterojunction OPV active layers. By increasing the amount of P3HT-b-PFTBT additives while decreasing the amount of PC₆₁BM, the stretchability is greatly enhanced by this strategy. The stretchability-enhanced property could be associated with the formation of the fiber-like structure in the BCP-based polymer solar cell.