UC Santa Barbara

Photovoltaic devices that directly convert sunlight to electrical power have the potential to generate terawatts of usable power if they can be produced economically at scale. Solution-processed bulk heterojunction organic photovoltaics (BHJ OPVs) are a particularly interesting photovoltaic technology primarily because they have demonstrated power conversion efficiencies (PCEs) exceeding 10% and can be solution-processed over large areas, indicating their potential as a cost-effective, scalable source of renewable energy. Whereas BHJ OPVs utilizing polymer donors and functionalized fullerene acceptors dominate the organic solar cell literature, BHJ OPVs have recently been fabricated using small molecule donors with PCEs on par with their polymeric counterparts. The work comprising this dissertation therefore consists of two separate but related goals: 1) understanding and controlling the crystallization of small molecule donor materials in BHJ blends with functionalized fullerene acceptors and 2) identifying the efficiency limiting processes of organic solar cells utilizing perylene diimide (a commercial dye) acceptor molecules and small molecule donor materials.

It is well established that the specifics of the bulk heterojunction morphology (phase separation, crystallinity, etc.) profoundly affect BHJ OPV device PCE. Controlling the BHJ morphology is thus of great importance. Using in-situ thermal annealing grazing incidence wide and small angle x-ray scattering it was discovered that the driving force for phase separation in BHJ OPV systems utilizing small molecule donor materials and functionalized fullerene acceptors is the crystallization of the small molecule donor material. Additionally, it was shown that this crystallization process, and therefore the development of the bulk heterojunction morphology and device performance, can be controlled by using commercially available nucleating agents designed for the clarifying of isotactic polypropylene.

Functionalized fullerene acceptors, ubiquitous in high efficiency BHJ OPVs, are produced via particularly solvent and energy intensive techniques and exhibit small extinction coefficients across the terrestrial solar spectrum. There then exists obvious benefits for using inexpensive, easily mass-produced fullerene alternatives that strongly absorb solar photons. Perylene diimides (PDIs) are a class of organic dye molecules with high electron affinity similar to fullerenes, large extinction coefficients across the terrestrial solar spectrum and relatively high electron mobilities, thus making them attractive for use as electron acceptors in organic solar cells. A BHJ OPV with a PCE of 3.1% was fabricated using a small molecule donor material and a PDI acceptor, making this one of the most efficient BHJ OPV devices utilizing a non-fullerene acceptor. Use of the solvent additive 1,8-diiodooctane (DIO) was shown to greatly improve the PCE of these devices. Using UV-Vis and transient absorption pump-probe experiments it was shown that use of DIO increases the structural order of both the donor and acceptor molecule, corresponding to a drastic increase in the efficiency with which excited states separate into free charge carriers and therefore largely explaining the drastic increase in solar cell figures of merit when using the solvent additive.