High-Performance Multi-Component Organic Solar Cells: Morphology and Device Performance
Restricted (Penn State Only)
- Author:
- Ayhan, Ismail Alperen
- Graduate Program:
- Chemical Engineering (PHD)
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- July 13, 2021
- Committee Members:
- Enrique Gomez, Chair & Dissertation Advisor
Michael Hickner, Major Field Member
Seong Kim, Professor in Charge/Director of Graduate Studies
Noel Giebink, Outside Unit Member
Joan Redwing, Outside Field Member
Christian Pester, Major Field Member - Keywords:
- Organic Solar Cells
Binary Blends
Ternary Blends
Organic Semiconductors
Fullerene
Nonfullerene
Morphology
Crystallinity
EF-TEM
GIWAXS - Abstract:
- Renewable energies significantly gained in importance in the last decade. Due to worldwide ecological problems and global warming, much effort is needed to increase the awareness and protect earth from the impacts with respect to the climate changes. Particularly, the excessive use of fossil fuels force countries to figure out some alternative applications for the energy supply. Other than hydro and wind-based applications, solar energy becomes the most important renewable technology to reduce the carbon dioxide emission and worldwide photovoltaic systems have recently been installed by gradually rising. Since only China and India are building several coal powers every week for energy needs based-on their population1, it is very important to develop efficient and low-cost solar energy systems to attract attention for especially newly develop countries other than develop ones. Organic solar cells (OSCs) are one of the most promising solar energy systems due to some unique advantages, such as low-cost, lightweight, flexible, semitransparent, large area compatibility, easy fabrication (roll to roll), easy processing (from solution), and energy level tunability, which enables new fields of applications. In contrast to inorganic solar cells, high temperature fabrication techniques are not needed to prepare organic photovoltaics. In solution processed OSCs, the organic semiconductor materials are used in active layer to absorb light and convert it to electrical energy. The active layers containing donor and accepter components where the optimized morphology is achieved through an interpenetrating phase-separated donor-acceptor network that is called as bulk heterojunction. The efficiency of bulk heterojunction OSCs is strongly correlated with the morphology of their active layers, which requires an optimized distribution of the donor and acceptor domains. A large interface between donor and acceptor domains are needed to obtain efficient exciton dissociation at interface. Also, percolation pathways are required to transport the charges to the corresponding electrodes. The nanomorphology depends on tendency of the active layer components to crystallization and phase-separation. These morphological properties can be optimized by altering the process parameters, such as selection of materials and solvents, composition, thickness, and thermal treatments. Recently, OSCs has been remarkably enhanced with power conversion efficiency exceeding 16% by virtue of new materials development and device optimizations.2–4 It is commonly known that the limited absorption window of organic semiconductors causes low photon-harvesting ability from active layer. Multi-component organic solar cells, which consist of one donor and two acceptors, are a promising strategy for broadening the light absorption spectrum and enhancing the power conversion efficiency (PCE) of the organic photovoltaic devices, due to the diverse small molecular materials with different optical band gaps and good compatibility. In this dissertation, the binary and ternary blend systems were studied through optimizing processing parameters and the effect of these parameters on the structural, morphological, electrical and photovoltaic properties were investigated by performing UV-vis absorption, EQE measurement, Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS) and energy-filtered transmission electron microscopy (EFTEM). In the first part of dissertation, it is found that optimized blends share a universal fibril-like structure, where the polymer donor appears to dictate fibril size. In the second part of section, the effect of third component in ternary blend OSCs is discussed where the optimum morphology is achieved and if the ternary blend systems also have similar morphology such as fibril-like structure. As expected, the ternary blend system showed significant enhancement of device performance due to broadening of the absorption spectrum and the fill factor. We attribute that the high morphology compatibility of the ternary system can benefit to optimized electron/hole mobility and diminished recombination.