Li-Hong Zhao

Surface-Directed Spinodal Decomposition in Poly[3-hexylthiophene] and C61-Butyric Acid Methyl Ester Blends

Demixed blends of poly[3-hexylthiophene] (P3HT) and C₆₁-butyric acid methyl ester (PCBM) are widely used in photovoltaic diodes (PV) and show excellent quantum efficiency and charge collection properties. We find the empirically optimized literature process conditions give rise to demixing during solvent (chlorobenzene) evaporation by spinodal decomposition. Ultraviolet photoemission spectroscopy (UPS) and X-ray photoemission spectroscopy (XPS) results are consistent with the formation of 1-2 nm thick surface layers on both interfaces, which trigger the formation of surface-directed waves emanating from both film surfaces. This observation is evidence that spinodal demixing (leading to a bicontinuous phase morphology) precedes the crystallization of the two components. We propose a model for the interplay of demixing and crystallization which explains the broadly similar PV performance for devices made with the bottom electrodes either as hole or electron collector. The process regime of temporal separation of demixing and crystallization is attractive because it provides a way to control the morphology and thereby the efficiency of PV devices.

Improved photoinduced charge carriers separation in organic-inorganic hybrid photovoltaic devices

We demonstrate enhanced performance of a hybrid photovoltaic device, where poly[3-hexylthiophene] (P3HT) is used as active material and a solution-processed thin flat film of ZnO modified by a self-assembled monolayer (SAM) of phenyl-C61-butyric acid (PCBA) is used as electron extracting electrode. Ultraviolet photoemission spectroscopy measurements reveal an increase in the substrate work function from 3.6 to 4.1 eV upon PCBA SAM deposition due to an interfacial dipole pointing away from the ZnO. External quantum efficiency (EQE) of the SAM modified devices reached 9%, greatly improved over the 3% EQE of the unmodified devices. This corresponds to full charge separation of all photoexcitations generated in the P3HT within an exciton diffusion range from the interface.

High internal quantum efficiency in fullerene solar cells based on crosslinked polymer donor networks

The power conversion efficiency of organic photovoltaic cells depends crucially on the morphology of their donor–acceptor heterostructure. Although tremendous progress has been made to develop new materials that better cover the solar spectrum, this heterostructure is still formed by a primitive spontaneous demixing that is rather sensitive to processing and hence difficult to realize consistently over large areas. Here we report that the desired interpenetrating heterostructure with built-in phase contiguity can be fabricated by acceptor doping into a lightly crosslinked polymer donor network. The resultant nanotemplated network is highly reproducible and resilient to phase coarsening. For the regioregular poly(3-hexylthiophene):phenyl-C61-butyrate methyl ester donor–acceptor model system, we obtained 20% improvement in power conversion efficiency over conventional demixed biblend devices. We reached very high internal quantum efficiencies of up to 0.9 electron per photon at zero bias, over an unprecedentedly wide composition space. Detailed analysis of the power conversion, power absorbed and internal quantum efficiency landscapes reveals the separate contributions of optical interference and donor–acceptor morphology effects.

Fabrication of negative index materials using dielectric and metallic composite route

In this work the authors proposed a dielectric and metallic composite route to design a negative index material. It was shown that a high-permittivity dielectric geometry could excite a magnetic dipolar resonance mode. Its incorporation with metallic wire resonators would generate the double negative resonance behavior, which gave the negative index. The homogenization ability of extending the composite combination into three dimensional structures was discussed.

Solvent effects and multiple aggregate states in high-mobility organic field-effect transistors based on poly(bithiophene-alt-thienothiophene)

Franck–Condon absorption analysis reveals the existence of several aggregate states in poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) thin films which impact their recrystallization and the attainable field-effect mobility (μFET). Poor solvents (toluene and mixed-xylenes) lock in both disordered and well-ordered states that cannot be annealed away even in the liquid crystalline phase. This reduces μFET and increases mobility activation energies compared with films from good solvents (chlorobenzene and o-dichlorobenzene). Despite its poor solubility characteristics, PBTTT can be ink-jet printed in dilute chlorobenzene, and devices can be operated unencapsulated in ambient, in the dark (>105cycles over several days) with only a moderate mobility loss.

Effective work functions for the evaporated metal/organic semiconductor contacts from in-situ diode flatband potential measurements

The diode built-in potentials (Vbi) of several polymer organic semiconductor (OSC) thin films [(2,5-dialkoxy-substituted poly(p-phenylenevinylene), poly(9,9-dialkylfluorene), poly(9,9-dialkylfluorene-alt-phenylene(N-phenyl)iminophenylene), and poly(9,9-dialkylfluorene-alt-benzothiadiazole)] sandwiched between p-doped poly(3,4-ethylenedioxythiophene) (PEDT:PSSH) and evaporated metal contacts have been measured by bias-dependent electromodulated absorption (EA) spectroscopy of the Stark-shifted π–π* band. From these values and the vacuum-level offsets at the PEDT:PSSH contacts evaluated by sub-gap EA spectroscopy, the following effective work functions for the buried evaporated metal contacts have been obtained: Al 3.4 ± 0.1, Ag 3.7 ± 0.1, Au 4.4 ± 0.1, and Ca 2.4 ± 0.1 eV. These work functions are smaller than those of the “clean” metal surfaces by up to 0.8 eV, and are substantially independent of the OSC in the absence of charge transfer.

Polarization effects on energy-level alignment at the interfaces of polymer organic semiconductor films

The thickness-dependent evolutions of the Fermi level EF and ionization potential Ip of ultrathin films of regioregular poly(3-hexylthiophene) and poly[2,5-bis(3-tetradecylthiophene-2-yl)thieno[3,2-b]thiophene], deposited as edge-on π-stacked lamellae on gold and on p-doped poly(3,4-ethylenedioxythiophene) electrodes, have been measured by ultraviolet photoemission spectroscopy. The Ip increases by 0.3 eV on going from a monolayer film to a few-nanometer-thick film. Correspondingly, the EF pinning depth increases from 0.2 eV to 0.5 eV. This valence “band bending” which occurs for a constant vacuum level can be quantitatively modeled by microelectrostatic self-consistent polarization field calculations that incorporate both substrate and organic semiconductor film effects. The EF pinning to the first monolayer is relatively shallow.