HIGH EFFICIENCY MULTIPLE EXCITON GENERATION ENABLED QUANTUM DOT SOLAR CELLS

Abstract

Quantum dots (QDs), as sensitizer material, are a promising solution-processable low-cost candidate for solar cell research because of their many advantages, such as high absorption coefficient, tunable bandgap, and potential for multiple exciton generation (MEG). Their use in quantum dot sensitized solar cells (QDSSC) is being extensively studied as a prospective band engineered material for increasing power conversion efficiency. A typical QDSSC consists of multiple QD layers sandwiched between the electron transport layer (ETL) & hole transport layer (HTL), and ohmic metal contacts at both ends. In QDSSCs, only QDs absorb most of the irradiance and are responsible for the photo-generation process. One of the least studied application is MEG enabled solar cells. This is a phenomenon through which photons with energy sufficiently greater than semiconductor band gap energy can be employed in the generation of more than one exciton, and hence the total number of excitons generated is higher than that of the total number of absorbed photons, resulting in an increase in quantum efficiency. This improves the photocurrent and, in turn, enhances power conversion efficiency (PCE). This dissertation comprehends the carrier dynamics in a MEG empowered QDSSC. In this work, charge transport in quantum dot (QD) thin films is modeled by considering trap-state assisted tunneling among the QD thin films. Core-shell type quantum dots are considered here. A typical core-shell QD is always surrounded by a thin oxide layer or an organic ligand layer. The shell has a finite probability of having trap states. The trap states at the surface of quantum dots (at middle of energy barrier among the quantum dots) have been considered. Multiple parameters associated with traps like density, position, and capture/emission rates which can significantly affect tunneling, have been considered in the model. The model is then extended to the quantum dot sensitized solar cells, and the effects of tunneling rate variation in accordance with the trap-states at the QD-QD interfaces and/or in the shell of QDs is analyzed. In the proposed model, multiple silicon QD layers as active material are considered, which are sandwiched between ZnO as electron transport layer and a MoO3 as hole transport layer. The model couples drift-diffusion framework for charge transport in ETL and HTL, and a system of rate equations for carrier dynamics among the QDs and their interfaces with ETL and HTL. Our analysis shows that the variations in trap-state concentration significantly affect the charge extraction and recombination scenario in the active layer of QDSSC, consequently producing a direct impact on device characteristics. Further, we have derived an expression for the generation rate to include critical MEG parameters, namely MEG threshold and MEG efficiency, for a MEG enabled quantum dot solar cell. Next, the characteristics of a three layer quantum dot sensitized solar cell at varied MEG threshold, efficiency, and band gap of the active layer have been examined by numerical analysis. The model used for analysis includes various physical phenomena at interfaces, inside transport layers, and in QDs, including quantum effects. Charge transport in QDs is considered to be governed by trap assisted tunneling. It is found that we need a nanomaterial with such a property so that the MEGth must be below 3.5Eg and have a band gap of around 0.94 eV to have enhanced PCE due to MEG at the device level. We further demonstrate that the PCE is limited by the recombination rate at lower band gaps and effective solar flux at higher band gaps beyond the maximum efficiency point. The proposed model can be employed to improve the device’s structure by using the computed value to foresee the quantum dot’s size and composition since the QD’s band gap energy directly relates to these two.