Charge Transfer and Strongly Coupled States in Organic Photovoltaics

Restricted (Penn State Only)
Author:
Brigeman, Alyssa Nicole
Graduate Program:
Electrical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
August 16, 2018
Committee Members:
  • Noel Christopher Giebink, Dissertation Advisor
  • Noel Christopher Giebink, Committee Chair
  • Thomas Nelson Jackson, Committee Member
  • Xingjie Ni, Committee Member
  • Enrique Daniel Gomez, Outside Member
Keywords:
  • organic semiconductors
  • organic photovoltaics
  • charge transfer states
  • polaritons
Abstract:
Organic semiconductors have attracted widespread interest over the past few decades due to their relative ease of processing, ability to be deposited on low-cost and flexible substrates, and the prospect of limitless tunability of their electrical and optical properties. A fundamental feature of organic semiconductors is the excitonic nature of molecular excited states supported by the weakly-bound, low dielectric constant solids, which makes organics scientifically interesting yet challenging with respect to incorporation into marketable optoelectronic devices. The first component of this thesis focuses on the implications of excitons on practical organic semiconductor devices, particularly organic photovoltaics (OPVs) and the complicated process of free charge generation enforced by excitons. The second part of this work concerns an investigation of how excitons respond to strong light-matter coupling in organic semiconductor microcavities, which has proven to be interesting for applications in organic optoelectronic devices including OPVs, organic light-emitting diodes (OLEDs) and lasers. OPVs currently lag behind their inorganic counterparts in overall performance, due in part to energy losses stemming from the excitonic nature of organic semiconductors which has inhibited widespread commercial viability. In order to overcome the current set of challenges associated with OPVs, a better understanding of the charge generation process and associated loss mechanisms is critical. Unlike inorganic semiconductors which directly generate free electrons and holes upon absorption of a photon, a photoabsorption event in an OPV device instead produces a tightly-bound electron-hole pair, called an exciton, which requires energetic assistance from interfacial charge transfer (CT) states to dissociate. Despite wide recognition of the importance of CT states to charge generation, recombination losses, and open-circuit voltage, characterization of these states is lacking due to stronger overlapping excitonic features in absorption and emission by composite molecules. A method to extract CT state absorption from the overlapping excitonic background by exploiting the natural alignment of CT states at a planar donor/acceptor (DA) heterojunction via polarized external quantum efficiency (EQE) measurements was developed in this thesis, which allows for an accurate calculation of critical OPV parameters such as CT state energy, reorganization energy, and energetic disorder-induced broadening. The polarized EQE analysis introduced in this work offers a technique to more comprehensively the understand nature and role of CT states and rigorously determine CT state parameters that directly affect the power conversion efficiency of OPVs. Another fundamental unknown regarding operational OPVs concerns the CT density of states (DOS) at the DA interface and its occupation under illumination. The CT DOS carries important implications that affect the performance of OPVs, such as added voltage losses and the potential for barrier-less charge separation, yet surprisingly little is known about its functional form, how carriers relax within it, and how the CT DOS relates to the joint density of free electron and hole states. By sensitively measuring weak emission from CT states in small molecule OPV systems in time with picosecond resolution, as a function of temperature down to 100 K, and with applied bias, the most complete picture to date of the CT DOS occupation and relaxation was realized in this thesis. Better understanding the relaxation processes that contribute to voltage losses is the first step towards engineering OPV devices with maximum power conversion efficiencies. The last part of this work investigates strong exciton-photon coupling in OPV-inspired microcavity devices resulting in exciton-polaritons. Polaritons are a result of the interaction between electronic excitations in a material (typically excitons) and the photon mode associated with a microcavity. Strong coupling of the exciton and photon occurs when the light-matter interaction far exceeds cavity damping or exciton dephasing, and is relatively easily observed from organic semiconductor microcavities due to the stability of excitons at room temperature in molecular solids. Ultrastrong coupling, a regime in which the interaction energy is a significant fraction of the bare exciton energy, is achieved from planar and bulk heterojunction devices. A thorough study of emission from the ultrastrongly-coupled cavities provides insight into the population of polariton states.