Controlling Thin-film Morphology and Incorporating Novel Semiconducting Molecules toward High Performance Organic Optoelectronic Devices

Organic optoelectronic devices have been widely used in display, energy-storage, and consumer electronics. Insightful understanding on material properties, device architecture, and fabrication processes is inevitable to improve the performance of organic optoelectronic devices. My PhD research focuses on improving the performance of organic photovoltaics (OPV) and organic light-emitting diode (OLED) through the systematic processing and material design. The first part of the dissertation describes how to construct a highly conductive morphology of mixed donor:acceptor heterojunction. Organic vapor phase deposition (OVPD) was utilized to enhance crystallinity of C70 acceptor in the mixed tetraphenyldibenzoperiflanthen (DBP):C70 thin-film. Forming the face-center-cubic (fcc) structure of C70 facilitated charge extraction, thereby improving fill factor (FF) of the corresponding OPVs. The second part presents the study on the morphological stability and reliability of OPVs. The cathode buffer, bathophenanthroline (BCP), undergoes significant morphological degradation. This morphological degradation was successfully suppressed by making the underlying DBP:C70 layer rougher via the moving N2 carrier gas in OVPD. The open-circuit voltage (Voc) of the obtained heterojunction OPVs of DBP:C70 grown by OVPD experienced a negligible drop (< 3 % change) while the equivalent OPVs grown by VTE showed a significant decrease in Voc from 0.91±0.01 V to 0.74±0.01 after 1 Sun illumination for 250 h. The third part explains a more precise way to control the morphology of organic mixed layer. It was found that increase in the growth pressure of OVPD induced reorganization of molecules to form the equilibrium morphology. The morphology of the electron-filtering buffer layer of 3,5,3′,5′-tetra(m-pyrid-3-yl)phenyl[1,1′]biphenyl (BP4mPy):C60 was optimized to achieve the highest electron mobility by means of the control of the growth pressure. Consequently, the resulting OPVs with optimized BP4mPy:C60 buffer showed FF = 0.65±0.01 and a much higher PCE = 8.0±0.2 % compared to PCE = 6.6±0.2 % of the equivalent OPVs with the same composition buffer layer grown by VTE. The fourth part summarizes the effects of the inclusion of novel block-copolymers on the performance of the polymer bulk-heterojunction photovoltaic cells. The block-copolymers were composed of thiophene units with and without a dangling phenyl-C61-butyric acid methyl ester (PCBM) side chain. The added copolymer into the poly(3-hexylthiophene) (P3HT): PCBM active layer resulted in greatly improved thermal stability of P3HT:PCBM. Furthermore, electron conductivity also increased since the fullerene units of the copolymers contribute to the formation of a percolation pathway for electron transport. While PCE of conventional P3HT:PCBM bulk-heterojunction solar cells decreases significantly from 2.6±0.2 to 1.2±0.2% after 90-min of thermal annealing, the equivalent OPVs with the copolymer shows a much smaller decrease in PCE from 3.1±0.2% to 2.7±0.2%. The last section of this dissertation covers the design of phosphorescent OLED employing a metal-free purely organic phosphor. Owing to their much longer triplet lifetime in the millisecond regime compared to microseconds of organometallics, a more careful consideration should be given in the device design. The requirements for the host materials in metal-free purely organic phosphor OLEDs are identified to be a high triplet energy, suitable HOMO and LUMO energy levels, and large spectral overlap with the absorption of the phosphors. Systematic investigation on various host molecules, electron transporting molecules, and the layer thickness of each layer allows us to demonstrate an optimized phosphorescent OLED having an external quantum efficiency (EQE) of 2.5 % at 1 mA/cm2.PHDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/144195/1/bssong_1.pd

Random Copolymers Outperform Gradient and Block Copolymers in Stabilizing Organic Photovoltaics

Recent advances have led to conjugated polymer‐based photovoltaic devices with efficiencies rivaling amorphous silicon. Nevertheless, these devices become less efficient over time due to changes in active layer morphology, thereby hindering their commercialization. Copolymer additives are a promising approach toward stabilizing blend morphologies; however, little is known about the impact of copolymer sequence, composition, and concentration. Herein, the impact of these parameters is determined by synthesizing random, block, and gradient copolymers with a poly(3‐hexylthiophene) (P3HT) backbone and side‐chain fullerenes (phenyl‐C61‐butyric acid methyl ester (PC61BM)). These copolymers are evaluated as compatibilizers in photovoltaic devices with P3HT:PC61BM as the active layer. The random copolymer with 20 mol% fullerene side chains and at 8 wt% concentration in the blend gives the most stable morphologies. Devices containing the random copolymer also exhibit higher and more stable power conversion efficiencies than the control device. Combined, these studies point to the random copolymer as a promising new scaffold for stabilizing bulk heterojunction photovoltaics.Photovoltaic devices made from conjugated polymers now exhibit efficiencies rivaling amorphous silicon; however, the poor longevity of these devices continues to stymie their commercial impact. Copolymer additives represent a promising solution, yet little is known about how the copolymer sequence, composition, and concentration influence their compatibilizing abilities. Herein, random copolymer additives lead to higher efficiency and longer‐lasting photovoltaic devices.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/150505/1/adfm201900467.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/150505/2/adfm201900467_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/150505/3/adfm201900467-sup-0001-S1.pd

Nanoscale Control of Morphology in Fullerene-Based Electron-Conducting Buffers via Organic Vapor Phase Deposition

Small molecular weight organic thin film mixtures of the electron-conducting C₆₀ in a wide energy gap matrix, 3,5,3′,5′-tetra­(<i>m</i>-pyrid-3-yl)­phenyl­[1,1′]­biphenyl (BP4mPy) forms a high efficiency electron filtering buffer in organic photovoltaics (OPV). Electrons are conducted via percolating paths of C₆₀ whereas excitons are blocked by the BP4mPy. We find that the conductivity and exciton blocking efficiency of the blends are strongly dependent on film morphology that can be precisely controlled by the conditions used in the organic vapor phase deposition (OVPD). Specifically, we find that a background carrier gas pressure of 0.28 Torr leads to extended and highly conductive crystalline C₆₀ domains. Furthermore, the structure is strongly influenced by carrier gas pressure. Via a combination of morphological measurements and molecular dynamics simulations, we find that this dependence is due to kinetically induced structural annealing at the growth interface. The highest electron mobility of (6.1 ± 0.5) × 10^–3 (cm²/V·s) is obtained at 0.28 Torr, which is approximately 2 orders of magnitude higher than for amorphous C₆₀ films. The fill factors and power conversion efficiencies of vacuum deposited tetraphenyldibenzoperiflanthene (DBP):C₇₀ planar mixed heterojunction OPVs using an OVPD-grown buffer layer are (8.0 ± 0.2)% compared to (6.6 ± 0.2)% using amorphous buffers grown by vacuum thermal evaporation

Surprisingly High Conductivity and Efficient Exciton Blocking in Fullerene/Wide-Energy-Gap Small Molecule Mixtures

We find that mixtures of C₆₀ with the wide energy gap, small molecular weight semiconductor bathophenanthroline (BPhen) exhibit a combination of surprisingly high electron conductivity and efficient exciton blocking when employed as buffer layers in organic photovoltaic cells. Photoluminescence quenching measurements show that a 1:1 BPhen/C₆₀ mixed layer has an exciton blocking efficiency of 84 ± 5% compared to that of 100% for a neat BPhen layer. This high blocking efficiency is accompanied by a 100-fold increase in electron conductivity compared with neat BPhen. Transient photocurrent measurements show that charge transport through a neat BPhen buffer is dispersive, in contrast to nondispersive transport in the compound buffer. Interestingly, although the conductivity is high, there is no clearly defined insulating-to-conducting phase transition with increased insulating BPhen fraction. Thus, we infer that C₆₀ undergoes nanoscale (<10 nm domain size) phase segregation even at very high (>80%) BPhen fractions