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.