Performance Optimization of Organic Solar Cells by Simulation and Characterization

Compared with silicon based solar cells, organic solar cells (OSCs) are less expensive alternatives because the thin and flexible OSCs can be easily fabricated onto the substrate by roll to roll painting. The current power conversion efficiency (PCE) of OSCs is about 8%. For commercialization of OSCs, a reasonable PCE (10%), at which those devices could generate electricity at a comparable cost to that of silicon based solar cells, is required. This requirement is the major driving force of this Ph.D. thesis to optimize OSCs with high PCE.To optimize the performance of OSCs, guidance from theoretical and simulation studies will play a key role. After solving coupled Poisson and Continuity equations, we first developed a macroscopic simulation tool which can precisely describe the current-voltage (J-V) characteristics of organic solar cells under known conditions such as device physical dimension (device layer thickness), physical parameters (absorption, carrier mobility), carrier generation/recombination kinetics, and boundary conditions. With this macroscopic simulation tool, the loss mechanism in BHJ OSCs is first investigated by fitting the simulated intensity dependent current-voltage (J-V) curves to experimental measurements. It is found that monomolecular recombination is dominant. Then, we have used the simulation tool to investigate performance optimization regarding thickness optimization, lowering bandgap of conjugated polymer, and balancing carrier transport in OSCs.For the characterization, single-walled carbon nanotubes (SWCNTs) were introduced in OSCs to increase the carrier mobility. It is observed that the performance of the device increased with small amount of SWCNTs but decreased if large amount of SWCNTs were introduced. The effects of semiconducting and metallic SWCNTs were explored by J-V characterization. It is found that semiconducting SWCNTs benefit the transport of photoexcited carrier while metallic SWCNTs introduce severe bimolecular recombination. Moreover, Kelvin Probe Force Microscopy is utilized to locally investigate the electrical role of SWCNTs in OSCs. The observation indicates that SWCNTs work as donor materials to transport holes.The simulation and characterization studies not only provide fundamental understanding on the physics of OSCs but also offer a feasible way to further optimize their performance