Date of Award

2-22-2016

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Applied Science

First Advisor

Alexandru Biris

Abstract

Significant attention has been given during the last few years to overcoming technological and material barriers to the development of organic photovoltaic devices (OPVs) with cost efficiency comparable to that of inorganic photovoltaics (PVs). If OPVs are to become commercially viable, major improvements are necessary. In this work, polyaniline (PANI) protonated with various levels of camphorsulfonic acid (CSA) and dissolved in m-cresol has been used as a hole-transport layer (HTL) in organic bulk-heterojunction solar cells. Polyaniline was first synthesized using oxidative polymerization based on the MacDiarmid method. The polymerization parameters, such as polymerization temperature and polymerization pH control agent, were investigated to produce the best possible hole transport material. The effect of the protonation level on the hole transport properties was also studied. PANI with various protonation levels was inserted between P3HT:PCBM and an ITO glass transparent electrode to explore the effects of varying the protonation level to optimize the hole-transport properties. The three protonation concentrations (in molar ratios) of PANI used in this work were as follows: zero-protonated (PANI:CSA, 1:0), half-protonated (PANI:CSA, 1:1), and fully protonated (PANI:CSA, 1:2) thin films. Current-voltage measurements under AM 1.5 conditions revealed that a conversion efficiency of 1.3% was achieved when half-protonated PANI was used as the HTL. Several analytical methods were utilized for characterizing PANI to understand the effects of the protonation level on the electrical, optoelectronic, and structural characteristics, and their correlation with final device properties. The stability and lifetime of HP-PANI-based devices were investigated in parallel with PEDOT:PSS-based devices. The study was conducted over a period of approximately 600 days. Interestingly, PANI-based devices exhibited great durability compared to PEDOT:PSS-based devices. Tuning the work function of polyaniline by controlling the concentration level of camphorsulfonic acid as a protonic acid dopant and m-cresol as a solvent was demonstrated in this study. Optical, thermal, structural, and electronic properties, along with surface topography and elemental analysis of protonated polyaniline, were studied in detail to investigate the effect of camphorsulfonic acid on the work function of polyaniline. The results showed that an increase in camphorsulfonic acid content induces a gradual transformation in the polyaniline structure from an emeraldine base to an emeraldine salt phase, which is associated with an increase in electrical conductivity and an improvement in crystallinity. X-ray photoelectron spectroscopy (XPS) was used to evaluate the work function. The results showed that increasing the camphorsulfonic acid content from quarter-protonated to fully-protonated leads to an increase in the work function of polyaniline from 4.42±0.14 eV to 4.78±0.13 eV. The gradual increase in the work function of PANI led to the conclusion that PANI can be utilized as an electron transport interlayer (when it is quarter-protonated) and a hole transport interlayer (when it is fully protonated). Stacking the two PANI interlayers together introduces a recombination layer that can be applied in tandem architecture. The results of tandem architecture based on the wide bandgap subcell PCDTBT:PC70BM and the narrow bandgap subcell PCPDTBT:PC70BM with quarter-protonated/full-protonated PANI interlayer system showed an extension in the external quantum efficiency spectrum over a broader spectrum. It was found that the acidic nature of HTL in organic solar cells can greatly affect the cells' stability and lifetime. Producing a non-acidic HTL is necessary to produce a durable organic solar cell. An acid-free polyaniline:graphene-oxide (PANI:GO) nanocomposite was fabricated and applied as a hole transport layer in two different types of organic solar cells. Various GO concentrations were used in the composite to optimize the photovoltaic performance of the cells. The composite with GO of 7.3 % exhibited the best performance.

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