High performance organic photovoltaic devices typically rely on type-II P/N junctions for assisting exciton dissociation. Heremans and co-workers recently reported a high efficiency device with a third organic layer which is spatially separated from the active P/N junction; but still contributes to the carrier generation by passing its energy to the P/N junction via a long-range exciton energy transfer mechanism. In this study the authors show that there is an additional mechanism contributing to the high efficiency. Some bipolar materials (e.g., subnaphthalocyanine chloride (SubNc) and subphthalocyanine chloride (SubPc)) are observed to generate free carriers much more effectively than typical organic semiconductors upon photoexcitation. Single-layer devices with SubNc or SubPc sandwiched between two electrodes can give power conversion efficiencies 30 times higher than those of reported single-layer devices. In addition, internal quantum efficiencies (IQEs) of bilayer devices with opposite stacking sequences (i.e., SubNc/SubPc vs SubPc/SubNc) are found to be the sum of IQEs of single layer devices. These results confirm that SubNc and SubPc can directly generate free carriers upon photoexcitation without assistance from a P/N junction. These allow them to be stacked onto each other with reversible sequence or simply stacking onto another P/N junction and contribute to the photocarrier generation.

The effects of gold nanoparticles (AuNPs) incorporated in the hole transporting layer (HTL) of poly[[4,8-bis[(2-ethylhexyl)oxy] benzo[1,2-b:4,5-b'] dithiophene-2, 6-diyl] [3-fluoro-2-[(2-ethylhexy)carbonyl]thieno[3,4-b]thiophened iyl]] (PTB7): [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) based solar cells are being systematically investigated in terms of the optical properties, electrical properties, and photovoltaic performance. The impacts of AuNPs on the optical response of the devices are modeled by finite-difference time-domain (FDTD) simulation. The size of the AuNPs used in this work is around 50-70 nm, so that 10-20 nm penetrated from the HTL into the active layer. We found that the power conversion efficiencies (PCEs) of the devices with AuNPs are significantly enhanced from 7.5%, for the control device, to 8.0%, 8.1%, and 8.2% for Au nanosphere-, nanorod-, and nanocube-incorporated devices, respectively. Among the photovoltaic parameters of the AuNP devices, the short circuit current density (JSC) exhibits the largest improvement, which can be attributed to the improved optical properties of the devices. On the basis of the calculation results, the scattering cross section for the samples in the presence of AuNPs can be enhanced by a factor of ∼10(10)-10(13) and Au nanocubes exhibit superior scattering cross section compared to the Au nanospheres and nanorods with the same linear dimension. From the experimental impedance spectroscopy results, we found that the addition of AuNPs had little effect on the electrical properties of the device. The device performance is also found to be sensitive to the concentration and morphology of the AuNPs.

CdO based transparent conducting oxide thin films have great potential applications in optoelectronic devices due its high mobility, low resistivity and high transparency over a wide spectral range. In this work, we report results of a comprehensive study of optical properties of CdO thin films doped with different donors (In, Ga, V, Ti) with carrier concentration in the range of 10 20 to >10 21 /cm 3 . Variable angle spectroscopic ellipsometry (SE) studies revealed that the complex dielectric function of CdO thin films drastically depends on the carrier concentration. Specifically, with increasing carrier concentration, (1) the net effect of Burstein-Moss shift and band gap renormalization gives rise to an increase in the optical band gap from 2.6 to 3.2 eV; (2) the free carrier absorption coefficient at wavelength of 1200 nm increases from 10 2 to 1×10 4 cm -1 ; (3) the refractive index decreases from 2.4 to 2.05 at 600 nm; (4) the high frequency dielectric constant reduces from 5.5 to 4.8. The SE results were analyzed with results from Hall measurements to obtain information on the electron effective mass and optical mobility of CdO thin films. The significantly higher effective mass of V and Ti doped CdO thin film is attributed to the modification of the conduction band due to anticrossing interaction between the localized d-levels of V and Ti atoms and the CdO conduction band extended states. The effective mass of In and Ga doped CdO increases with the electron concentration, consistent with the prediction from the nonparabolic conduction band model. We also found that the optical mobility is close to the Hall mobility when the 60 /Vs, while for materials with higher .

Electromodulation (EM) spectroscopy, a powerful technique to monitor the changes in polarizability p and dipole moment u of materials upon photo-excitation, can bring direct insight into the excitonic properties of materials. However, extracting Δp and Δu from the electromodulation spectrum relies on fitting with optical absorption of the materials where optical effect in different device geometries might introduce large variation in the extracted values. Here, we demonstrate a systematic electromodulation study with various fitting approaches in both commonly adopted reflection and transmission device architectures. Strikingly, we have found that the previously ascribed continuum state threshold from the deviation between the measured and fitting results is questionable. Such deviation is found to be caused by the overlooked optical interference and electrorefraction effect. A generalized electromodulation model is proposed to incorporate the two effects, and the extracted Δp and Δu have excellent consistency in both reflection and transmission modes in all organic film thicknesses.

In this work, we have synthesized CdGaO alloy thin films at room temperature over the entire composition range by radio frequency magnetron sputtering. We found that alloy films with high Ga contents of x > 0.3 are amorphous. Amorphous CdGaO alloys in the composition range of 0.3 < x < 0.5 exhibit a high electron mobility of 10-20 cm V s with a resistivity in the range of 10 to high 10 Ω cm range. The resistivity of the amorphous alloys can also be controlled over 5 orders of magnitude from 7 × 10 to 77 Ω cm by controlling the oxygen stoichiometry. Over the entire composition range, these crystalline and amorphous alloys have a large tunable intrinsic band gap range of 2.2-4.8 eV as well as a conduction band minimum range of 5.8-4.5 eV below the vacuum level. Our results suggest that amorphous CdGaO alloy films with 0.3 < x < 0.4 have favorable optoelectronic properties as transparent conductors on flexible and/or organic substrates, whereas the band edges and electrical conductivity of films with 0.3 < x < 0.7 can be manipulated for transparent thin-film transistors as well as electron transport layers.