Role of surface passivation and doping on the development of quantum dot solar cells

Abstract

The work in this thesis is focused on the use of advanced nanotechnology for the development of next-generation solar cells. Noble processes are developed to harness the unique optoelectronic properties of quantum dots to improve the performance of solar cells. This thesis explores the use of lead sulfide (PbS) quantum dots (QDs) as the main light-absorbing material for the development of QD solar cells. Near-infrared active and earth-abundant PbS QDs have emerged as a viable alternative to conventional materials (like Si solar cell, CIZS solar cell, etc.) due to their many distinct advantages, like solution-phase facile processability, bandgap tenability, and low material cost. Despite significant breakthroughs over the years, low performance has remained the major roadblock for the commercialization of QD solar cells. It has been understood that the primary reasons for the underperformance of QD solar cells are originated from not so high carrier mobility, low open-circuit voltage, and high charge recombination rate in QDs based solar cells. We posited that most of these drawbacks could effectively be mitigated by a comprehensive surface passivation strategy which would prohibit trap state formation and allow fast transport of photo carriers through the QD solids. In this thesis, by understanding the surface chemistry of PbS QDs in detail, we have strategically developed the surface passivation methods to improve the power conversion efficiency (PCE) of QD solar cells. A low-temperature processed TiO2 layer (acts as an electron extracting layer in solar cell device) has been demonstrated to make QD solar cells on flexible substrates. The solvent induced 2D matrix layer has been tracked on the surface of QD ink. The 2D matrix layer thickness on the QD surface has been engineered to improve the PCE of QD solar cell. A two-step hybrid (organic and inorganic) ligand passivation strategy has been developed for the first time to make high quality QDs ink. Further, the one-step hybrid passivation method has been realized to develop QD solar cells. Through these strategic developments, in this thesis work, the PCE of PbS QD solar cell has been improved from 3.7% to 10.6%. Lastly, based on the findings in this thesis work, possible future directions that could further improve the efficiency of QD solar cells are discussed.