Understanding electronic and optical properties of PbS QDs for improved photovoltaic performance
Author(s)Kim, Donghun, Ph. D. Massachusetts Institute of Technology
Massachusetts Institute of Technology. Department of Materials Science and Engineering.
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Photovoltaic (PV) solar cells that constitute semiconducting sunlight absorber and metallic electrical contacts convert solar energy to electricity. Even though silicon represents roughly 90% of installed solar PV capacity as the clear current leader among PV technology, another class of solid-state solar cells, referred to as quantum dot (QD) solar cells, have gained much attentions from both academia and industry with the ability to provide further substantial enhancement of PV efficiency, together with the low possible manufacturing/installation cost. The power conversion efficiencies (P.C.E.) of QD-PVs based on lead sulfide (PbS) have been enhanced dramatically in only several years: current leading groups are able to fabricate reliably QD-PVs with 7-10% P.C.E. owing to favorable optical properties of PbS QDs including facile tunability of bandgaps with the variation in dot sizes or shapes, wide spectral responses, and multiple exciton generation. To date, the efficiency advances of QD solar cells have been carried out almost exclusively through tremendous numbers of trial and error experiments. Examples include materials set variations, donor and acceptor layer thickness optimization, and device structure modification. The core of the work described in this thesis deals with the theoretical understanding and design of PbS QDs with the goal of achieving a deeper and more fundamental understanding of the wide range of material's properties at the atomic scale in these devices. To this end, we employ a technique of computational electronic structure calculation methods, namely density functional theory (DFT) calculations. In this thesis, we select and investigate, using DFT calculations, three important electronic or optical properties: 1) band-edge energy (Chapter 2), 2) trap states (Chapter 3), and 3) Stokes shift (Chapter 4), all of which can contribute to PV performance improvements only if appropriately tailored. It is worth emphasizing that ligands which are used during QD synthesis for prevention of QD agglomeration plays a key role in tuning each property of interest in this thesis. Our theoretical work of band-edge energy shifts presented in Chapter 2 identifies ligand-induced surface dipoles as a hitherto-underutilized means of control over the absolute energy levels in PVs, complementary to well known bandgap tuning. This work have guided our experimental collaborators to build up a device architecture with a novel interfacial band alignment where a surplus loss of current collection can be minimized, leading to "certified" efficiency of 8.6% in 2014. Improvements of JSC presented in Chapter 2 led us to pay much attention to another figure of merit, open-circuit voltage (VOC): maximum Voc of 0.5-0.6 (V) has been achieved in single-junction PVs using PbS QDs with the bandgap of 1.1-1.3 (eV). Such large deficit of Voc in QD-PVs is attributed to the following sources: (1) high density of mid-gap trap states, (2) large Stokes shift, each of which is investigated and elaborated on in Chapters 3 and 4. Based on the fundamental understanding on the origin of these properties obtained from DFT calculations, we together with our experimental collaborators are actively working to develop PbS QD films with improved properties and to incorporate them into PV devices for further performance enhancements. This thesis document is organized as follows: Chapter 1 introduces PbS QDs and PVs, Chapters 2,3, and 4 illustrates theoretical investigations of key electronic and optical properties of PbS QDs (i.e. band-edge energy, trap states, and Stokes shift, respectively) supported by relevant experimental results from collaborators for better understanding of the this thesis. Lastly Chapter 5 closes the thesis with brief summary of works and future impacts to PVs and other optoelectronic applications.
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2015.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (pages 131-139).
DepartmentMassachusetts Institute of Technology. Department of Materials Science and Engineering.
Massachusetts Institute of Technology
Materials Science and Engineering.