Colloidal quantum dots (CQDs) are promising candidates for fabricating large-scale, low-cost, flexible, and lightweight photovoltaic solar cells. However, their power conversion efficiency is still insufficient for commercial applications, partly and significantly, due to the not-well-understood carrier transport mechanisms and the lack of effective characterization techniques. Addressing these issues, carrier transport kinetics in CQD systems were studied to develop high-frequency dynamic testing and/or large-area quantitative imaging techniques: photocarrier radiometry (PCR), and homodyne (HoLIC) and heterodyne (HeLIC) lock-in carrierographies.
Based on the discrete carrier hopping transport in CQDs, various carrier drift-diffusion current-voltage (J-V) analytical models and new concepts including the imbalanced carrier mobilities, reversed Schottky barrier, and double-diode model were developed to quantitatively interpret carrier transport and J-V characteristics in CQD solar cells. The further quantitative study of carrier mobility, CQD bandgap energy, phonon-assisted carrier transport, and open-circuit voltage deficit revealed CQD solar cell efficiency optimization strategies. Applying these energy transport mechanisms, for the first time, an analytical PCR signal generation model for CQD systems was developed from a novel trap-state-mediated carrier hopping transport theory. Therefore, multiple carrier transport parameters including carrier hopping lifetime, diffusivity, and diffusion length were extracted to investigate carrier transport dependencies on temperature, quantum dot size, surface-passivation ligands, and carrier hopping activation energies. As an imaging extension of PCR, using a heterodyne method to overcome the limitations of camera frame rate and exposure time of even the state-of-the-art InGaAs cameras, the first camera-based HeLIC theoretical model for ultrahigh-frequency (up to 270 kHz) imaging of CQD solar cells was achieved. Therefore, quantitative imaging of carrier lifetime, diffusivity, and diffusion and drift lengths of CQD solar cells was accomplished to explore the influences of carrier transport and contact/CQD interface on CQD solar cells. Also, low-frequency HoLIC large-area imaging evaluated the sample homogeneity and quality, reflecting preliminary carrier lifetime distribution.
The combination of the novel carrier discrete hopping transport mechanism, J-V models, PCR, and the lock-in carrierography techniques (HoLIC and HeLIC) shows great potential for quantitative carrier transport study of CQD solar cells and for fast, all-optical, contactless, large-area, and nondestructive characterization of commercial photovoltaic materials and devices.Ph.D.2018-06-19 00:00:0
