Evidences of photocurrent generation by hole–exciton interaction at organic semiconductor interfaces

The charge–exciton interaction at the donor/acceptor interface plays a significant role in the exciton dissociation processes, and thus influences the performance of organic solar cells. In this work, the evidences of photocurrent generation via hole–exciton interaction (HEI) at the organic semiconductor interface in organic solar cells, which is the counterpart of photocurrent generated by electron–exciton interaction, is demonstrated. A heterojunction, composed of copper phthalocyanine (CuPc) and fullerene (C₆₀), is used to provide free holes that interact with the excitons supplied by perfluorinated hexadecafluorophthalo-cyaninatozinc (F16ZnPc). The fact that photocurrent generation via HEI is well evidenced by: (1) a short circuit current of 0.38 mA cm⁻²; (2) the jump of an external quantum efficiency (EQE) around 800 nm after adding a bias light; (3) the EQE variations under bias light of different wavelengths and light intensities; and (4) the superlinear dependence of the photocurrent on the light intensity.     

States confined in the barriers of type-III HgTe/CdTe superlattices

We present a theoretical study of states quasi-localized in CdTe barriers of HgTe/CdTe superlattices. We show that the quasi-localization of both electrons and holes will lead to strong Coulomb interaction, and thus to the formation of excitons. It is further demonstrated that such quasi-localized states, including excitons, exhibit confinement effect similar to those of loclaized states in quantum wells.     

Efficient small molecule-based bulk heterojunction photovoltaic cells with reduced exciton quenching in fullerene

Most highly efficient small molecule-based bulk heterojunction (BHJ) photovoltaic cells contain a large concentration of fullerene in their blend active layers. However, the excitons generated in fullerene can seriously quench at the surface of the commonly used MoO₃ buffer layer, becoming a key limitation to the photovoltaic performance of these cells. In this study, we’ve investigated various anode buffer layers in the thermally evaporated tetraphenyldibenzoperiflanthene (DBP) and C₇₀-based BHJ cells with high C₇₀ concentration. It’s been found that obviously enhanced power conversion efficiency (PCE) of up to 6.26% can be obtained in DBP and C₇₀-based BHJ cells via simply replacing the MoO₃ buffer by poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS), which is also a commonly used anode buffer material in polymer-based BHJ cells. Photoluminescence spectra results have confirmed the suppression of exciton quenching at the anode interface by inserting this PEDOT: PSS buffer. Moreover, after adding a C₇₀ interlayer for better electron extraction and the further suppression of exciton quenching, the DBP and C₇₀-based M-i-n photovoltaic cells show a remarkable PCE of 7.04% under illumination with 100 mW/cm², AM 1.5G simulated solar light.     

Cathodoluminescence study of h– and c–GaN single crystals grown by a Na or K flux

Luminescence properties of hexagonal (h-) and cubic (c-) GaN freestanding single crystals were studied by means of cathodoluminescence spectroscopy. The h-GaN crystals of about 0.2–2 mm in dimension were synthesized at 750 °C by the reaction of Ga and N2 in a Na flux, while c-GaN crystals of about 0.3 mm or less in a K flux. The h-GaN showed rather strong band edge emission at room temperature compared with the crystal grown by using NaN3 as a nitrogen source. At 20 K, the band edge emission of h-GaN was split into four peaks. The main energy peak position was 3.478 eV, which was assigned as the A-free exciton emission. The energy position of the main peak of c-GaN was 3.268 eV. Assuming the binding energies of excitons in h- and c-GaN as 25 and 26 meV, respectively, the energy difference of bandgap between h-and c-GaN is estimated to be 209 meV. Since these crystals are free from strain from the substrates, the peak energies are reliable for the intrinsic GaN crystals. The full widths at half maximum of the emission of c-GaN were much broader than those of h-GaN, suggesting that the cubic phase is much defective compared with the hexagonal one.     

Effects of idling time between depositions of organic layers and metal electrode in organic photovoltaic device

Deposition of metal cathode in organic photovoltaic (OPV) devices is known to cause serious metal atom penetration into the active layers and as a remedy, exciton blocking layer (EBL) is implemented. It is found that the metal penetration through EBL can be controlled by optimizing the idling time between the depositions of EBL and cathode layers. Both electrical and optical data show that the resistance of BPhen layer (EBL) to metal (Mg:Ag) penetration increase with increasing idling time. Also, significant variation in power conversion efficiency (PCE) is observed in subphthalocyanine chloride (SubPc)/fullerene (C₆₀) based organic photovoltaic (OPV) cells by simply adding idling time as an additional control parameter. Device with optimized BPhen thickness and idling time resulted in a PCE of 2.8% whereas that of no idling time consideration has PCE limited at 2.3%. These observations are attributed to slow processes of self-organization in BPhen after their depositions.     

Triplet exciton diffusion and delayed interfacial charge separation in a Tio2/PdTPPC bilayer: Monte Carlo simulations

Nanosecond photoexcitation of a bilayer of smooth anatase TiO₂ coated with palladium tetrakis(4-carboxyphenyl)porphyrin (PdTPPC) results in a delayed, after-pulse growth of the conductivity over many microseconds as monitored by time-resolved microwave conductivity. This phenomenon is attributed to the slow diffusion of PdTPPC triplet excitons followed by electron injection into the TiO₂ conduction band. The temporal form and intensity dependence of this process have been simulated by Monte Carlo (MC) calculations of exciton diffusion and exciton–exciton annihilation. Good agreement between the experimental results and MC simulations are obtained using a triplet exciton diffusion coefficient D_E=8×10⁻¹¹ m²/s, exciton lifetime τ_E≥10 μs, effective triplet–triplet annihilation distance R_a=1.5 nm and interfacial electron injection efficiency φ_inj=0.44.     

MoS_2激子效应的压电光声研究

本文首先利用光声压电法,研究了MoS_2单晶在540-740nm波段的室温本征吸收边的光声吸收光谱,得到了在2.0eV附近的A、B两个强激子的吸收峰及杂质峰A,与以前报道的用透射光谱法测得的吸收谱及光电流谱的结果一致.     

势阱形状对电场下量子阱子带和激子能移的影响

在束缚态近似下,用级数解法求解电场下各种不同形状GaAs/GaAlAs量子阱中电子和空穴子带,进一步采用变分方法得到激子结合能.由此,我们首次得到电场下由抛物阱至方位阱子带和激子峰能移的变化图象.在考虑GaAs/GaAlAs量子阱形状影响的基础上,我们计算所得结果与实验吻合得很好.     

Chain helicity of a poly(phenylacetylene) with chiral centers between backbone and mesogenic groups on side chains

An optically active poly(phenylacetylene), poly{2-(R)-methyl-2-[4′-(tetradecyloxy)biphenyl-4-yloxy]propyl 4-ethynylbenzoate} (P1) is synthesized in a high yield with a high molecular weight (1.3 × 10⁶ g/mol) by [Rh(nbd)Cl]₂ catalyst. The chemical structure of the polymer is characterized by IR and NMR spectroscopies with satisfactory analysis data. The polymer is soluble in many common organic solvents. It shows circular dichroism (CD) band in the absorption region of the polyacetylene backbone, indicating that P1 is chiroptical owing to the formation of helical conformation with an excess screw sense. Combining the results of CD and light scattering experiments, we find that addition of poor solvent can induce the neighboring helical segments within a single polymer strand to become closer, resulting in exciton coupling prior to polymer precipitation. The exciton coupling can be kept in the solid state after solvent evaporation.