1 Introduction
Organic solar cells (OSCs) have developed rapidly due to low cost, light weight, semitransparency, solution processability and so on, which have become one of the most dynamic research frontiers in the field of new materials and new energy.[1-9] At present, the power conversion efficiency (PCE) of OSCs has exceeded 20%.[10] In order to improve the PCE, researchers have made numerous efforts in optimization of material design, device structure and morphology of the active layer, etc.[1, 11-16]
The energy conversion process of OSCs mainly includes the following steps: photon absorption, diffusion of excitons, charge transfer state (CT state) separation, transport of charge and collection.[17-23] Through in-depth understanding of the working principle of OSCs, it is obvious that the film morphology of the active layer formed by the mixed donor and acceptor materials plays a decisive role in the effective separation of excitons and carrier transport.[24-26] To ensure the PCE of OSCs, the ideal morphology should have the following characteristics: (1) The bicontinuous phase separation structure: both donor and acceptor materials form a continuous pathway to ensure that carriers can be efficiently transmitted to the corresponding electrode and collected.[27-29] Simultaneously, it is necessary to ensure that the excitons diffuse to the donor/acceptor (D/A) interface within the lifetime by maintaining domain sizes of 10-20 nm.[30-34] (2) High degree of crystallinity: a significant level of crystallinity in the active layer molecules is advantageous for charge transfer as it allows carriers to move along the primary chain and interchain via the π-π stacking of molecules.[35-42] (3) Simultaneous face-on orientation: the face-on orientation enhances the vertical carrier transport within the active layer.[43-45] Furthermore, the same molecular orientation at the interface between the donor/acceptor promotes the separation of charge transfer states.[46-53] Therefore, the morphology of the active layer significantly influences the photophysical process and subsequently the PCE. The morphology is seriously affected by crystallization dynamics in the fabrication process of active layer, which mainly happens in the film-forming and post annealing process.[25]
Film-forming kinetics refer to the physical laws and dynamic processes that organic semiconductor materials follow during the formation of a film. In the initial stage of film-forming, the molecular clusters begin to nucleate, and then grow and form aggregates. Because of the influences of van der Waals interactions, hydrogen bonds, and π-π stacking interactions, these aggregates constantly change their size, shape, and orientation. As the concentration of the solution and the number of aggregates increase further, they are separated from the solution, forming liquid-liquid (L-L) or solid-liquid (S-L) separation. In solid-liquid phase separation, the clusters are deposited on the solid surface, forming a continuous film. In L-L separation, the clusters form discrete droplets that self-assemble into continuous films, usually under surface tension. Subsequently, the crystal continuously adsorbs organic molecules on the crystal surface, the volume increases with the increase of time, and eventually tends to mature. Since Heeger A.J. et al. invented OSCs with bulk heterostructure in 1995[54], researchers have carried out a lot of work to optimize the morphology of the active layer, and developed strategies such as solution state and post-annealing treatment to control the active layer morphology. During the solution treatment process, the morphology is affected by many factors, including solvent selection, temperature, deposition method, and the use of additives.[55] Therefore, understanding and optimizing these processes is critical in developing high-performance OSCs.
In addition, post-treatment processes, such as thermal annealing (TA) and solvent vapor annealing (SVA), can also optimize the morphology of the active layer. TA is an effective means to adjust the morphology of active layers in blends. Since Friend et al. first found that TA can improve the performance of P3HT:EP-PTC blends, TA has officially entered the field of regulation of active layer in OSCs.[56] The main principle of TA is that the polymer is heated above the glass transition temperature (T g) to promote the movement of polymer chain segments, so as to achieve the ordered aggregates of polymer molecules. At the same time, the molecules are diffused and crystallized during the annealing process, and finally form the interpenetrating network structure.[57, 58] For SVA, the blend film is placed in solvent vapor. After the solvent molecules penetrate into the active layer, the polymer chains are induced to reassemble into an ordered structure.[59, 60] For example, Yang et al. made the P3HT:PCBM blend film dry slowly through dichlorobenzene solvent vapor, so that the blend film self-assembled to form an ordered structure and good phase separation morphology, which is conducive to the balance of electron and hole mobility.[61]
This review focuses on the effect of crystallization dynamics on the morphology of OSCs and reveals its potential mechanisms of how it influencing the morphology. Firstly, the characterization methods and kinetic principles of the thin films are briefly described. Subsequently, the common strategies for the morphology optimization of OSCs active layers, including adjusting the film-forming process and the post-treatment process are summarized, and the typical examples are cited to understand how these treatments affect crystallization dynamics process to achieve the desired morphology. Finally, the future development of OSCs was prospected, which may provide some guidance for achieving high PCE of OSCs.