Conjugated polymer semiconductors with their solution processibility at room temperature make them a key component for light-weight, large-area, flexible electronics and optoelectronics. This dissertation focuses on a number of advanced polymer semiconductor electronic and optoelectronic devices.
Highly efficient polymer bulk heterojunction (BHJ) solar cells were demonstrated through process optimization including varying the thickness of the LiF cathode interlayer sandwiched between the photoactive layer and Al cathode along with changing the postproduction annealing temperature. While varying the thickness of the LiF interlayer between the photoactive layer and Al cathode and the postproduction annealing temperature, a power conversion efficiency up to 3.6% with a high fill factor (FF) of 66% and an open circuit voltage (Voc) of ~0.61V was obtained when the devices incorporated with a 0.6 nm thick LiF layer were postproduction annealed at 160 °C for 30 min.
Surface modifications to the anode were also investigated for enhanced efficiency of polymer solar cells. An improved efficiency for polymer BHJ solar cells was demonstrated, mainly due to the enhanced Jsc through the usage of a thin islanded plasma-modified Ag atop indium tin oxide (ITO) anodes. Under forward bias, an enhanced hole injection was recorded due to the interfacial energy step created between the ITO and the photoactive layer. An increased internal electric field increases at the anode interface may be assisting the extraction of photogenerated holes, leading to the increased Jsc without significant changes to Voc, FF, and the Rs in the solar cell with modified surface.
In order to improve the efficiency of organic solar cells, one approach could be to yield increased optical absorption and photocurrent generation in the photoactive layer over a broad range of visible wavelengths by inducing surface plasmons through careful control of metallic nanoparticle’s properties. Plasmon-enhanced polymer solar cells were demonstrated using unique self-assembled layers of silver nanoparticles. An enhanced optical absorption was observed that improved Jsc for polymer BHJ solar cells, mainly due to the localized surface plasmon-enhanced photogeneration through the usage of plasmon-active Ag nanospheres between the anode interfacial layer and the photoactive layer. In spite of the increased Jsc, significant Voc and FF losses were observed because of the surface recombination at the interface between the anode interfacial layer and the photoactive layer.
Conjugated polymer semiconductors and associated materials are very sensitive to intrusion of air or moisture and necessitate hermetic seals. A laser sealing approach was also investigated for air-stable operation of flexible organic solar cells. This work also aims to significantly reduce encapsulation times through rapid sealing via tailored laser welding, making immediate commercialization of organic solar cell technology highly probable. This project successfully demonstrated the feasibility of using a diode laser source to seal flexible polymer solar cells with anodes and cathodes. Throughout the study, moderate seal strengths were obtained for each electrode material combination. Active solar cells were encapsulated and their partial operation was verified.
Strong negative differential resistance (NDR) performance of polymer tunnel diodes exhibiting large and reproducible NDR with a peak-to-valley current as high as 53 at room temperature was demonstrated. Additionally, a basic logic circuit operation was demonstrated using a pair of these polymer tunnel diodes connected in series to form a monostable-bistable transition logic element latch. This result indicates that polymer tunnel diodes are potential candidates for many flexible, low-power logic and memory applications for organic devices by using low-cost and simple solution processing.
A major hurdle to progress foldable electronics is the development of new gate dielectrics for flexible all-polymer, high performance thin film transistors (TFT). Organic based flexible dielectric films with high-k for low voltage operation are desirable for future applications of organic TFTs, such as smartcards and radio frequency identification (RFID) tags, concurrently with flexible or conformable form factors. Polymeric bipolar thin films were investigated for their potential application as the gate dielectric in TFTs. A multilayer stack of polymeric thin films composed of alternating amine and carboxylic acid functional groups was deposited by pulsed plasma polymerization produced a composite structure having a relatively high-k and low leakage current density as obtained without postdeposition annealing. This high performance multilayer polymer film stack, deposited at ambient temperature was not subjected to further treatment of any kind, is very promising in terms of use as a flexible dielectric material.