Application of block copolymers in organic solar cells

After many years of development, block copolymers became one of the most important materials for organic solar cells. They were used in photovoltaic cells because of their self-assembly properties and controlled ordered nanoscale, which is ideal for electron-hole pair separation. However, block copolymers still faced several major challenges as organic solar materials, and this paper focused on their limitations and corresponding solutions in terms of material design, morphology control, and interface modification, as well as on the fundamental molecular engineering principles involved. In the paper’s conclusion, an perspective on the use of modern, advanced block copolymers in organic solar cells was provided.


Introduction
At present, the prospect of inorganic solar cells is somewhat limited by the extensive quantity of purification and post-processing required for silicon materials in the production [1].Compared to inorganic solar cells, organic solar cells' advantages lie in the inexpensive raw materials and high flexibility; they are also expected to be printed on a large scale on flexible substrates, thus offering greater scope for development.Block copolymers (BCPs) are special polymers obtained by joining two or even more polymer chain segments with different properties.By adjusting the structure of the BCP, the degree of polymerisation of each chain segment, the internal force between the chain segments, and the interaction between each chain segment and the environment, it can be self-assembled into a sizecontrolled (~10 nm) ordered structure after micro-phase separation, which can be applied to solar cells as electron transport or acceptor materials to compensate for the lack of easy separation of electron-hole pairs in the photoactive layer, effectively improving the photovoltaic conversion (PCE) of solar cells [2].However, the application of BCPs as organic solar cell materials is not yet very mature and there are many challenges to be faced, such as the limitation of charge transfer by conformational isomers, changes in the copolymer structure when assembled into thin films and the degradation of self-assembly performance due to glass transition or crystallisation when amorphous BCPs are made to incorporate functions such as carrier conduction or light absorption.Overall, these challenges can be categorised as material design, morphology control, interface modifications, photostability and substrate compatibility.Among these, material design, morphology control and interface modification are of considerable interest and major challenges in the area of molecular engineering.Therefore, this paper focuses on these three challenges and the prevailing solutions to provide a convenient and concise grasp of the application of BCPs in organic solar cells, which can have implications for research on organic solar cells.

Organic solar cell and molecular engineering principles
Solar cells are depended on the photovoltaic effect of the P-N junction, where photogenerated voltages occur on both sides of the P-N junction of a semiconductor under illumination conditions, thereby generating an electric current.The binding energy between free electron-hole pairs organic semiconductor materials (~0.5eV) exceeds the thermal energy at ambient temperature (~0.025eV), so the electron-hole pairs are bound by Coulomb forces at room temperature [3].In a solar cell, the one that provides electrons becomes the donor material and the one that accepts electrons is called the acceptor material.When photons with sufficient energy irradiate the photoactive layer, free electrons are excited from the donor material into the acceptor material, producing the separation of electron-hole pairs.As the electron-hole pair diffusion length in the material is only 5-10 nm, it tends to compound before diffusion reaches the electrode, causing a reduction of PCE [4].To overcome this problem, the following structures have emerged in organic solar cells in turn, like the single-layer organic solar cell shown in Figure 1(a) transformed into a donor-acceptor bilayer heterojunction structure, so that the separated electrons and holes diffuse in two different materials to reduce the chance of compounding, but the thickness of the material layer is still much greater than the electron-hole pair diffusion distance.As shown in Figure 1(b), in 1995 the bulk-phase heterojunction (BHJ) structure designed by the University of Cambridge and UCSB, where materials form the donor and acceptor are mixed so that the material interface distance is less than the diffusion distance, but the high instability of the morphology leads to local aggregation in the region, resulting in a lower PCE.The ordered nanostructure (~10 nm) formed by self-assembly based on BCPs is shown in Figure .1(c), which is also the theoretically optimal structure.The application of BCPs focuses on the use of self-assembly properties to obtain a series of ordered nanostructures, which can occur in melt and solution by controlling the structure of individual chain coil molecules [5].For BCPs, the thermodynamic force for driving self-assembly is related to the degree of polymerisation N, and an important thermodynamic parameter, the Flory-Huggins parameter χ.Negative values of χ indicate favourable mixing, with weakly biased polymers favouring disordered forms of each identity when χN<10, when the thermodynamic driving force is insufficient to overcome the entropic elasticity associated with demixing [6].Microphase segregation occurs when χN>10, and a series of ordered self-assembled structures are observed as the relative volume fraction f A of individual polymers varies.As shown in Figure 2, spheres, cylinders, gyroids, and lamellaes can be observed sequentially as f A increases.In addition, BCPs are often applied as templates in organic solar cells, where a BCP layer is synthesised and then sorted by annealing, one component is selectively removed (usually by etching), leaving a template with pores, followed by backfilling the other material using dip coating or electrochemical deposition, and finally degrading the remaining template [7].

Material design
As mentioned above, organic solar cell structures consist of donor and acceptor materials, and BCPs are mainly used as donor materials.However, the design of donor materials can significantly affect the solubility and photostability of BCPs, and it is very challenging to design the chemical structure and side chains of BCPs to control their solubility and optical properties.In the past decades, there has been almost no breakthrough in acceptor materials and the PCE enhancement of organic solar cells has been largely dependent on the successful development of donor materials [9].In this regard, Huaxing Zhou and other scholars stated that for acceptor materials, due to fullerenes' inability to dissolve completely in most organic solven, the current mainstream approach uses the fullerene derivative [6,6]-phenyl-C 61butyric acid methyl ester (PC 61 BM); the self-assembled ordered structure of BCP and the excellent optical characteristics of conjugated polymers are used to control the electron donor capacity for donor materials, which has a substantial impact on the conjugated polymer's HOMO level and band gap.In order to achieve a band gap of 1.0 eV and a more planar configuration of the conjugated backbone, which promotes the delocalization of π-electrons along the conjugated backbone and results in a smaller band gap, copolymers of 3,4-aminothiophene and 3,4-nitrothiophene are usually used.This ensures the light absorption capability of the material while preventing steric hindrance from causing nearby repeating units to deviate from the intended coplanarity.Principles to address material design challenges also include optimising charge transport, PCE and stability of conjugated polymers by rationalising intramolecular substituents, constructing intramolecular bonding networks, and introducing strong absorbing groups [10].For example, Ji-Hoon Kim and other scholars discovered that octyl side chains on TT units produce the greatest level of molecular backbone coplanarity, the best molecular packing, and optimized hole mobility for copolymers using both the alkoxy or alkylth-ienyl-substituted BDT motifs [11].

Morphology control
Controlling the morphology of materials is arguably the most significant yet challenging task for the application of BCPs to organic solar cells.As previously mentioned, it is known that to avoid the recombination of electron-hole pairs with very short diffusion distances, suitable BCP self-assembly scales are chosen and thermally annealed to order them.However, the most favourable morphology typically exists in a kinetically frozen state rather than reaching thermodynamic equilibrium.Furthermore, pure phases are thermodynamically more stable than mixed phases, so the morphology of donor and acceptor molecules migrating can be unstable, leading to "degeneracy" under environmental factors [12].The formation of large fullerene crystals by Ostwald ripening is the main cause of the morphological change, while the formation of large-scale PCBM aggregates by attaching fullerenes to diblock copolymers and simulating environmental thermal degradation factors did not occur after 80 hours of treatment at 140°C.In addition, compatibilisers can be incorporated into the system to enhance the morphological stability of the BCPs.For example, in the PM6:Y6f non-fullerene system, Bin Li and other scholars designed a PM6-b-PTY6 BCP as a compatibiliser to be added to the blend, and the PCE increased to 16.48% and maintained an initial efficiency of 0.81 after 550 hours of burn-in testing under environmental and thermal stress, demonstrating that compatibilisers can optimise carrier dynamic processes and stabilise the BCPs' morphology [13].

Interface modification
For many solar cells where the cathode and anode materials are BCPs and inorganic quantum dot hybrids, the problem of inorganic and organic immiscibility can lead to the inability of charge collection at the corresponding electrodes, which is a major challenge for the application of BCPs to organic photovoltages (OPVs).In response, the mainstream approach is to use interface modification materials to lower the electrode performance and the energy level barrier for charge transfer at the interface to further improve device performance [14].For example, in the cathode-anode interface modification between organic and inorganic electrodes, Yueqin Shi and other scholars incorporated a BCP with epoxy-containing side chains into the P3HT: PC 61 BM active layer because of the lower surface energy of this oxygen-containing side chain, which migrated to the surface to form an interface during the spincoating process of the active layer, and this interface was able to improve the Ohmic contact; then combined with organic dyes and liquid crystal small molecules to modify the ZnO array to form a core/shell or core/double-shell array as a covering for transporting electrons.The modified layer is beneficial to passivate the ZnO array's surface defects and optimise the device energy level, ultimately increasing the PCE of the device to 8.0% [15].

Conclusion
This paper offers a concise introduction to the principles of organic solar cells and the molecular engineering of BCPs for OPV applications and the current promising prospects.It also identifies the main current limitations and focuses on three main challenges in terms of material design, morphology modulation and interface modification to present and discuss a summary of the best prevailing solutions.These include the combination of suitable conjugated polymers with BCPs to enhance light absorption and carrier transport while maintaining self-assembly performance, the attachment of fullerenes to BCPs in blends and the addition of compatibilisers to maintain BCP morphology and thermal stability; and the addition of interfacial modifiers to BCPs to improve carrier transport and device efficiency.It is reassuring to know that the nowadays state-of-the-art BCPs application, like the PBDB-T derivative in OPVs has come into being, with the main advantages of good donor-acceptor material compatibility and high morphological stability [16].Finally, it is hoped that the application of BCPs in OPVs will become increasingly more mature in the future, overcoming the mainstream challenges, and exploiting the prospect of low cost, high flexibility, and large-scale printing for the benefit of mankind in the commercial segment as soon as possible.

Figure. 1
Figure. 1 Three representations of organic solar cells structures.(a) A model of the photoactive layer of a bilayer organic solar cell, in which the material boundary length is much greater than exciton diffusion distance.(b) The bulk-heterojunction (BHJ) organic solar cell whose material boundary distance is less than exciton diffusion distance yet has an unstable morphology.(c) The BCP-based organic solar cell has an ideal nanoscale structure.Reprinted with permission from John Wiley and Sons [8].

Figure. 2
Figure. 2 Schematics of the most prevalent nanostructures embraced by coil-coil BCPs.S = sphere, C = cylinder, G = gyroid and L = lamellae.The red block denotes polymer B, while the blue block denotes polymer A (with volume fraction f A ). Reprinted with permission from John Wiley and Sons[8].