Thesis (M. S.)--Electrical and Computer Engineering, Georgia Institute of Technology, 2010. / Committee Chair: Cressler, John; Committee Member: Laskar, Joy; Committee Member: Papapolymerou, John. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Thesis (Ph. D.)--Ohio State University, 2007. / Title from first page of PDF file. Includes bibliographical references (p. 157-165).

Organic photovoltaic (OPV) is an exciting energy harvesting technique. Although its power conversion efficiency (PCE) now exceeds 10% in a research laboratory, the processing window of an OPV cell is still narrow. A fundamental understanding of the OPV materials is desired. This thesis presents the charge carrier transport properties and electronic interactions in the bulk heterojunction (BHJ) active layer of OPV cells. They were found to be well correlated with OPV device performances. Space-charge-limited current (SCLC) measurements and admittance spectroscopy (AS) were employed to study the charge transports, while photothermal deflection spectroscopy (PDS) was used to probe the trap densities inside the materials. Beneficial effects of a common solvent additive, 1,8-diiodooctance (DIO), on PTB7:PC71BM OPV cells have been investigated. With DIO present in the casting solution, the resulting BHJ films have much enhanced electron mobilities, whereas the impact on the hole mobility is negligible. The origin of increased electron mobility is the reduced average electron hopping distance for those films prepared with DIO solvent additive. A balance of hole-electron mobility by tuning the DIO concentration was demonstrated to be the way to optimize the OPV device performance. In light of carrier transport measurement results, a "polymer-rich" strategy with preserved device performance was demonstrated. After understanding the importance of balanced hole-electron mobility, the impact of donor-acceptor weight ratio on the performance of PTB7 : PC71BM based OPV cells was explored. Early stage electronic donor-acceptor interactions were revealed using ultra-low dosages of fullerenes. Before electron transport pathways percolate, the unconnected fullerene domains act as traps and hinder electron transport. From PDS, the trap density observed inside BHJ films was found to be anti-correlated with the fill factor of OPV devices. The origin of low FFs is mainly due to electron traps and localized states from fullerenes. Based on the observations, it is proposed that PC71BM tends to intercalate with PTB7 backbone instead of forming self-aggregates before the electron pathway percolation. Apart from investigating the fundamentals in OPV devices, a solution to improve its processing window was proposed in this thesis. Thermally stable polymer : fullerene OPV cells were fabricated by employing fluorenone-based solid additives. A charge transfer interaction between the additives and donor moiety of polymer formed a locked network which freezes the BHJ morphology under thermal stress. The most promising result retains 90% of the origin efficiency, upon thermal aging at 100 °C for more than 20 hours in PTB7:PC71BM solar cells. Besides fullerene-based OPV, all-polymer photovoltaic solar cells (all-PSCs) were also investigated. Two new difluorobenzene-naphthalene diimide based polymer electron acceptors, one random (P1) and one regioregular (P2) structure, were compared. P2 exhibited a much better molecular packing, a higher electron mobility and more balanced hole-electron mobilities in its composite film with polymer donor, PTB7-Th. An optimized PTB7-Th:P2 device can achieve a respectably high PCE over 5% for all-PSC devices. These all-PSCs should open a new avenue for next generation OPVs.

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Group III-nitride materials have attracted great attention for applications in high efficiency electronic and optoelectronics devices such as high electron mobility transistors, light emitting diodes, and laser diodes. On the other hand, group VI transition metal dichalcogenides (TMDs) in the form of MX2 has recently emerged as a novel atomic layered material system with excellent thermoelectric, electronic and optoelectronic properties. Also, the recent investigations reveal that the dissimilar heterojunctions formed by TMDs and III-nitrides provide the route for novel devices in the area of optoelectronic, electronics, and water splitting applications. In addition, integration of III-nitrides and TMDs will enable high density integrated optoelectronic circuits and the development of hybrid integration technologies. In this work, we have demonstrated kinetically controlled growth processes in plasma assisted molecular beam epitaxy (PAMBE) for the III-nitrides and their engineered heterostructures. Techniques such as Ga irradiation and nitrogen plasma exposure has been utilized to implement bulk GaN, InGaN and their heterostructures in PAMBE. For the growth of III-nitride based heterostructures, the in-situ surface stoichiometry monitoring (i-SSM) technique was developed and used for implementing stepped and compositionally graded InGaN-based multiple quantum wells (MQWs). Their optical and microstrain analysis in conjunction with theoretical studies confirmed improvement in the radiative recombination rate of the graded-MQWs as compared to that of stepped-MQWs, owing to the reduced strain in graded-MQWs. Our achievement also includes the realization of the p-type MoS2 by engineering pristine MoS2 layers in PAMBE. Mainly, Ga and nitrogen plasma irradiation on the pristine MoS2 in PAMBE has resulted in the realization of the p-type MoS2. Also, GaN epitaxial thin layers were deposited on MoS2/c-sapphire, WSe2/c-sapphire substrates by PAMBE to study the band discontinuity at GaN/TMDs heterointerface. The determination of band offset parameters for both GaN/MoS2 and GaN/WSe2 heterostructures revealed realization of type-II band alignment. Also, heterojunctions such as AlGaN/MoS2 is implemented to achieve type-I heterojunction. This work may open up a new avenue towards photonic quantum devices based on the integration of III-nitrides with 2D TMDs.

<p> Two methods for preparing semiconductor grade copper sulfide films, to be used in low cost thin film solar cells,have been investigated. The sulfurization method involves the controlled chemical conversion of copper films into the desired copper sulfide phase. The other method of evaporating Cu2S pellets is more adaptable for an all evaporated thin film solar cell. The copper sulfide films obtained by these methods were characterized using x-rays, cathodoluminescence, electrical and optical methods. The use of optical method in monitoring the stoichiometry of thin copper sulfide films has been illustrated. The photovoltaic properties of thin copper sulfide films obtained by these methods, were also investigated using Cu2S - Si heterojunctions. The behaviour of these junctions indicates that 900 Ȧ thick copper sulfide film is required for optimum photovoltaic conversion. This result may be of some importance in Cu2S - CdS solar cells in further reducing their thickness. Cu2S - Si heterojunctions can also be used to monitor the properties of copper sulfide, as silicon is a well characterized substrate.</p> / Thesis / Master of Science (MSc)

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