A GIWAXS Investigation of the Small Molecule Donor X2

Organic photovoltaics (OPVs) are a promising technology for low-cost renewable energy in applications such as wearable solar cells and semitransparent photovoltaics because of their tunable bandgap and solution processability. The core of an OPV device is the bulk heterojunction (BHJ), which is composed of donor and acceptor molecules or polymers that form an interpenetrating network of domains. Photons absorbed in the BHJ form bound electron-hole pairs that diffuse to the donor-acceptor interface and split into free charges, where the charges then travel to their respective electrodes. In the ideal BHJ morphology, domains are 10 to 50 nm in size and exhibit high crystallinity with continuous pathways to the transport layers. The BHJ is typically prepared by depositing a solution of donor and acceptor material and relying on thermodynamic and kinetic processes for film formation. Due to a wide range of processing conditions available, and difficulty in maintaining exact parameters, reproducibility of OPV has remained a challenge. Furthermore, specialized characterization methods must be used to probe the BHJ morphology. Grazing incidence X-ray scattering (GIWAXS) is one such technique and is used to look at length scales ranging from single to tens of nanometers; ideal for investigating the molecular packing and crystallinity of donor and acceptor materials. One interesting donor material to investigate reproducibility and stability in OPVs is X2, a small molecule donor. X2 has demonstrated robust device efficiencies over wide ranges of donor-acceptor ratios and processing conditions. Determining the molecular packing of X2 may explain the reasons for its robustness, which could then be exploited for improved reproducibility and stability in other OPV systems. GIWAXS data on neat films of X2 has been collected over a range of temperatures and is currently being analyzed to determine its molecular packing structure for the first time. Preliminary results show that X2 forms a highly crystalline structure and that it remains stable over wide temperature ranges. The results from this data analysis are expected to help guide further rational design of reproducible and efficient OPV devices.