# Crystallinity and Molecular Packing of Small Molecules in Bulk-Heterojunction Organic Solar Cells
###### Abstract  
Crystallinity has played a major role in organic solar cells (OSCs). In small molecule (SM) bulk-heterojunction (BHJ) OSCs, the crystallinity and crystalline packing of SM donors have been shown to have a dramatic impact on the formation of an optimum microstructure leading to high-power conversion efficiency (PCE). Herein we describe how crystallinity differs from polymers to SMs, and how the packing habits of SMs (particularly donors) in active layers of BHJ devices can be described as following two different main modes: a single crystal-like and a liquid crystal-like packing type. This notion is reviewed from a chronological perspective, emphasising milestone donor structures and studies focusing on the crystallinity in SM-BHJ OSCs. This review intends to demonstrate that a shift towards a liquid crystalline-like packing can be identified throughout the history of SM-BHJ, and that this shift can be associated with an increase in overall PCE.  
organic solar cell; photovoltaics; small molecule; bulk heterojunction; crystallinity; crystalline packing +
Footnote †: journal: Applied sciences  
reason behind it lies in the fact that the morphology (or microstructure) of the photoactive layer of OSCs is the key determinant of the overall device's characteristics [4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14]. Phase segregation into domains of donors and acceptors and crystallinity are essential features. The latter properties are challenging to predict, considering the sole chemical structure of the electron donor (D) and electron acceptor (A) components. In the case of small-molecule (SM) bulk-heterojunction (BHJ) OSCs in particular, the crystalline properties of the D and A molecules are critical to the device function since the formation of segregated D/A domains relies on the formation of crystalline domains of pure donor and pure acceptor. The kinetics and thermodynamics of the crystallisation process are likely the primary physical parameter involved in the genesis of the microstructure [15; 16; 17; 18; 19]. The recent progress in the field, with record PCEs over 14% using non-fullrerene acceptors (NFA) [20], demonstrates the importance of understanding the evolution of the crystallinity of the active layer from a fundamental perspective.  
Therefore, herein we describe the evolution of the crystalline properties of active layers in SM-BHJ OSCs from a historical perspective. We will first briefly introduce the concept of crystallinity in the context of polymers, liquid crystals and small molecules, and then review the methods currently used to characterise the crystallinity of SM-BHJ active layers. Next, the crystalline characteristics of the main classes of donor/acceptors blends will be reviewed, particularly focusing on the different modes of molecular packing in crystalline films of OSC active layers. We show that, throughout the evolution of the SM-BHJ field, SMs shifted from a single-crystalline-like to liquid-crystalline-like packing habit in active layers of OSCs. We believe that such a shift is directly linked with the trend in the design of SMs (D or A) consisting--in a broad sense--of adding a large proportion of alkyl chains to the conjugated core (far beyond the necessary amount for solubility). This trend is likely to have significantly contributed to the increase in PCEs in SM-BHJ OSCs, as we will show herein.