Rational Design of Conjugated Polymers for Organic Solar Cells Based on Structure-Property Relationships

Conjugated polymers have a long history of exploration and use in organic solar cells, and over the last twenty-five years, marked increases in the solar cell efficiency have been achieved. With the remarkable advances in the efficiency of organic solar cells, the need to distill key structure-property relationships for semiconducting materials cannot be understated. The fundamental design criteria based on these structure-property relationships will help realize low-cost, scalable, and high efficiency materials. This dissertation details design strategies for both small molecule fused-ring electron acceptors and functionalized donor-acceptor copolymer electron donors. First, a series of fused-ring electron acceptors with structural differences will be investigated (such as extending the conjugated core, changing the solubilizing side chains, adding electron withdrawing groups, etc.) and key design criteria will be derived. This will lead to a model which outlines the key requirements in order for a small molecule fused-ring electron acceptor to exhibit high efficiency. Afterwards, new methodologies to prepare conjugated polymer donors will be explored in the context of structure-property relationships. A common polymer backbone will be functionalized with various modifications including fluorine substituents, nitrogen heteroatoms, and cyano substituents; and the location and number of these modifications will be explored. A complete investigation on the structure-property relationship for each functionalization will be undertaken, and the optical, electrochemical, morphological, and photovoltaic impact of each substituent detailed. Furthermore, the limits of these functionalizations will be investigated, and key design rationale will be developed. Finally, while many of these conjugated materials exhibit high performance, the cost and synthetic complexity of these materials is also too large. For that reason, a redesigned synthesis of a high-performance conjugated polymer which offers an 86% reduction in the materials cost will be highlighted. The insights gained from this dissertation can aid in the design on new donor and acceptor materials to obtain high efficiencies and make polymer-based organic solar cells competitive for industrial processing.Doctor of Philosoph

Emergent molecular traits of lettuce and tomato grown under wavelength-selective solar cells

The integration of semi-transparent organic solar cells (ST-OSCs) in greenhouses offers new agrivoltaic opportunities to meet the growing demands for sustainable food production. The tailored absorption/transmission spectra of ST-OSCs impacts the power generated as well as crop growth, development and responses to the biotic and abiotic environments. To characterize crop responses to ST-OSCs, we grew lettuce and tomato, traditional greenhouse crops, under three ST-OSC filters that create different light spectra. Lettuce yield and early tomato development are not negatively affected by the modified light environment. Our genomic analysis reveals that lettuce production exhibits beneficial traits involving nutrient content and nitrogen utilization while select ST-OSCs impact regulation of flowering initiation in tomato. These results suggest that ST-OSCs integrated into greenhouses are not only a promising technology for energy-neutral, sustainable and climate-change protected crop production, but can deliver benefits beyond energy considerations

Aggregation Controlled Charge Generation in Fullerene Based Bulk Heterojunction Polymer Solar Cells: Effect of Additive

Optimization of charge generation in polymer blends is crucial for the fabrication of highly efficient polymer solar cells. While the impacts of the polymer chemical structure, energy alignment, and interface on charge generation have been well studied, not much is known about the impact of polymer aggregation on charge generation. Here, we studied the impact of aggregation on charge generation using transient absorption spectroscopy, neutron scattering, and atomic force microscopy. Our measurements indicate that the 1,8-diiodooctane additive can change the aggregation behavior of poly(benzodithiophene-alt-dithienyl difluorobenzotriazole (PBnDT-FTAZ) and phenyl-C61-butyric acid methyl ester (PCBM)polymer blends and impact the charge generation process. Our observations show that the charge generation can be optimized by tuning the aggregation in polymer blends, which can be beneficial for the design of highly efficient fullerene-based organic photovoltaic devices

Measuring Temperature-Dependent Miscibility for Polymer Solar Cell Blends: An Easily Accessible Optical Method Reveals Complex Behavior

In bulk-heterojunction polymer solar cells (PSC), the molecular-level mixing between conjugated polymer donors and small-molecule acceptors plays a crucial role in obtaining a desirable morphology and good device stability. It has been recently shown that the thermodynamic limit of this mixing can be quantified by the liquidus miscibility, the composition of the small-molecule acceptor in amorphous phases in the presence of small-molecule crystals, and then converted to the Flory–Huggins interaction parameter χ. This conversion maps out the amorphous miscibility. Moreover, the quantitative relations between χ and the fill factor of PSC devices were established recently. However, the commonly used measurement of this liquidus miscibility, scanning transmission X-ray microscopy, is not easily and readily accessible. Here, we delineate a method based on common visible light microscopy and ultraviolet–visible absorption spectroscopy to replace the X-ray measurements. To demonstrate the feasibility of this technique and methodology, a variety of conjugated polymers (PffBT4T-C₉C₁₃, PDPP3T PBDT-TS1, PTB7-Th, and FTAZ) and their miscibility with fullerenes or nonfullerene small molecules (PC₇₁BM, PC₆₁BM, and EH-IDTBR) are characterized. The establishment of this methodology will pave the way to a wider use of the liquidus miscibility and the critical miscibility-function relations to optimize the device performance and obtain good stability in PSCs and other devices

Effect of Core Size on Performance of Fused-Ring Electron Acceptors

We report 4 fused-ring electron acceptors (FREAs) with the same end-groups and side-chains but different cores, whose sizes range from 5 to 11 fused rings. The core size has considerable effects on the electronic, optical, charge transport, morphological, and photovoltaic properties of the FREAs. Extending the core size leads to red-shift of absorption spectra, upshift of the energy levels, and enhancement of molecular packing and electron mobility. From 5 to 9 fused rings, the core size extension can simultaneously enhance open-circuit voltage (<i>V</i>_OC), short-circuit current density (<i>J</i>_SC), and fill factor (FF) of organic solar cells (OSCs). The best efficiency of the binary-blend devices increases from 5.6 to 11.7%, while the best efficiency of the ternary-blend devices increases from 6.3 to 12.6% as the acceptor core size extends