PhD Candidate, Princeton University
My PhD research centers on organic semiconductors and organic electronics, and focuses more specifically on the chemical doping of these materials. The purpose of chemical doping is to control electronic properties and conductivity of materials, both of which are key elements in the on-going development of high performance devices, such as organic light-emitting diodes (OLEDs) and organic solar cells.
My first major research achievement was the discovery of the impact of low concentrations of molecular dopants on the density of states of the organic semiconductor copper phthalocyanine (CuPc). The original intent was to reduce the detrimental effect of electronic trap states that are present in the energy gap of the material, by filling them with charges introduced by a controlled amounts of dopants. During this work, I realized that the incorporation of the dopants led to a significant broadening of the CuPc density of states, and I developed a sophisticated theoretical modeling of this behavior. This work not only helps researchers understand the physics of organic materials, but also provides a more specific approach to eliminate these ubiquitous gap states and enhance electronic transport in molecular semiconductors. This work was published in Chemistry of Materials, 28, 2677 (2016).
The second major piece of research was recently submitted and is under review in Nature Materials. It concerns the n-doping of, i.e. electron transfer to, organic molecular semiconductors with low electron affinity. These materials, which are key elements of OLEDs, are notably difficult to n-dope, however doping is required to lower device operating voltages. I demonstrated that certain organometallic dimer compounds could be photo-activated to reduce, i.e. transfer electrons to, more challenging materials. This work is potentially quite important and has already generated some interest from a couple of companies.
In addition, I also contributed to an extensive review entitled “Experimental Characterization of Interfaces of Relevance to Organic Electronics” that has been included as a chapter in a two volume Reference on Organic Electronics (2016). I am also a contributor to the paper “Pairing of near-ultraviolet solar cells with electrochromic windows for smart management of the solar spectrum” published in Nature Energy (2017) and the paper “Morphological tuning of the energetics in singlet fission organic solar cells” published in Advanced Functional Materials (2016).
Abstract: To make high-performance semiconductor devices, a good ohmic contact between the electrode and the semiconductor layer is required to inject the maximum current density across the contact. Achieving ohmic contacts requires electrodes with high and low work functions to inject holes and electrons respectively, where the work function is the minimum energy required to remove an electron from the Fermi level of the electrode to the vacuum level. However, it is challenging to produce electrically conducting films with sufficiently high or low work functions, especially for solution-processed semiconductor devices. Hole-doped polymer organic semiconductors are available in a limited work-function range, but hole-doped materials with ultrahigh work functions and, especially, electron-doped materials with low to ultralow work functions are not yet available. The key challenges are stabilizing the thin films against de-doping and suppressing dopant migration. Here we report a general strategy to overcome these limitations and achieve solution-processed doped films over a wide range of work functions (3.0-5.8 electronvolts), by charge-doping of conjugated polyelectrolytes and then internal ion-exchange to give self-compensated heavily doped polymers. Mobile carriers on the polymer backbone in these materials are compensated by covalently bonded counter-ions. Although our self-compensated doped polymers superficially resemble self-doped polymers, they are generated by separate charge-carrier doping and compensation steps, which enables the use of strong dopants to access extreme work functions. We demonstrate solution-processed ohmic contacts for high-performance organic light-emitting diodes, solar cells, photodiodes and transistors, including ohmic injection of both carrier types into polyfluorene-the benchmark wide-bandgap blue-light-emitting polymer organic semiconductor. We also show that metal electrodes can be transformed into highly efficient hole- and electron-injection contacts via the self-assembly of these doped polyelectrolytes. This consequently allows ambipolar field-effect transistors to be transformed into high-performance p- and n-channel transistors. Our strategy provides a method for producing ohmic contacts not only for organic semiconductors, but potentially for other advanced semiconductors as well, including perovskites, quantum dots, nanotubes and two-dimensional materials.
Pub.: 25 Nov '16, Pinned: 30 Jul '17
Abstract: Tail states in organic semiconductors have a significant influence on device performances by acting as traps in charge transport. We present a study of the controlled passivation of acceptor tail states in fullerene C(60) by the addition of electrons introduced by molecular n doping. Using ultralow doping, we are able to successively fill the traps with charges and examine the changes in conductivity, activation energy, mobility, and Fermi-level position. Passivation of the traps leads to an increase of the electron mobility in C(60) by more than 3 orders of magnitude, to reach 0.21 cm(2)/(V s).
Pub.: 12 Dec '12, Pinned: 30 Jul '17
Abstract: We investigate the distribution of valence and tail states in copper phthalocyanine (CuPc) upon the introduction of minute amounts of the p-dopant molybdenum tris[1,2-bis(trifluoromethyl)ethane-1,2-dithiolene] (Mo(tfd)3), using a combination of electron spectroscopy and carrier transport measurements. Density of gap states, conductivity, and hole-hopping activation energy are measured. We observe the progressive filling (and deactivation) of the deepest tail states by charges introduced by the dopants, as well as significant broadening of the CuPc density of states. Simulations relate this broadening to the electrostatic and structural disorder induced by the dopant in the CuPc matrix.
Pub.: 05 Apr '16, Pinned: 30 Jul '17
Abstract: Air-stable dimers of sandwich compounds including rhodocene and (pentamethylcyclopentadienyl)(arene)ruthenium and iron derivatives can be used for n-doping electron-transport materials with electron affinities as small as 2.8 eV. A p-i-n homojunction diode based on copper phthalocyanine and using rhodocene dimer as n-dopant shows a rectification ratio of greater than 10(6) at 4 V.
Pub.: 08 Nov '11, Pinned: 30 Jul '17