Effect of illumination on the space charge limited current in organic bulk heterojunction diodes

Current–voltage (J–V) characteristics of organic bulk heterojunction diodes based on an interpenetrating network of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C₆₁-butyric acid methyl esters (PCBM) have been studied in the dark and under halogen lamp illumination. The diodes contained 1:1 and 1:0.6 weight ratios of P3HT and PCBM. For both diodes the currents measured in dark (J _d , commonly known as the dark current) in forward bias are found to agree with the space charge limited current (SCLC). The illuminated current consists of a current due to applied voltage (J _da ) and the light generated current (J _L ). J _da  extracted from the illuminated current agrees well with Shockley’s diffusion and recombination current. This observation shows that illumination changes the SCLC into Shockley’s diffusion and recombination current. The forward current under illumination has been observed to be greater than the dark current, which is contrary to the photo–voltaic (PV) theory. This result is well explained by the change of SCLC into Shockley’s diffusion and recombination current. Former address of S.C. Jain: IMEC, Kapeldreef 75, 3001 Leuven, Belgium.     

Electronic behaviour of thin-film CdTe solar cells

New support is given for one of the controversial models about the electronic consequences of the CdCl₂ treatment of a thin-film CdTe solar cell: the assumption that deep acceptor states are introduced in the bulk of the CdTe layer as a result of the CdCl₂ treatment. A detailed study of the doping profile using capacitance–voltage (C-V) measurements is performed as a first step. The above assumption is numerically simulated with our simulation programme SCAPS. In this way, anomalous features of the C-V measurements are fully explained, and further correspondence between calculated and measurable quantities is found. Received: 1 March 1999 / Accepted: 28 March 1999 / Published online: 1 July 1999     

Photovoltaic Devices from Multi-Armed CdS Nanorods and Conjugated Polymer Composites

We demonstrate the preparation of composite photovoltaic devices by using the blends of multi-armed CdS nanorods with conjugated polymer, poly[2-methoxy-5-（2＇-ethylhexyloxy）-1,4-phenylenevinylene] （MEH-PPV）. Multi-armed CdS nanorods are prepared by thermolysing single precursor cadmium ethylxanthate [Cd（exan）2] in pure hexadecylamine solution under ambient conditions. The photoluminescence of MEH-PPV can be effectively quenched in the composites at high CdS nanocrystal （nc-CdS）//MEH-PPV ratios. Post-treatment of the multi-armed CdS nanorods by refluxing in pyridine significantly increases the performance of the composite photovoltaic devices. Power conversion efficiency is obtained to be 0.17% under AM 1.5 illumination for this composite device.     

Determination of diffusion length of minority carriers in epitaxial layers by photocurrent spectroscopy

We have determined the minority carrier diffusion length in n-type and p-type GaAs epitaxial layers grown on GaAs substrates as well as on intentionally misoriented Si substrates by photocurrent spectroscopy. It is found that for heteroepitaxial GaAs-on-Si, the minority carrier diffusion length is limited by the amount of dislocation density irrespective of the doping level. The value of dislocation density obtained from diffusion length measurements agrees well with that obtained from the double-crystal x-ray diffraction measurements.     

Structure due to indirect exciton in photocurrent spectrum of homoepitaxial diamond film

Photocurrent spectrum in homoepitaxal diamond film formed by chemical vapor deposition has been measured in the photon energy range 5-6 eV of the vicinity of the indirect band gap. It has been seen that the line shape of the spectrum in the photon energy range lower than 5.6 eV agrees with the fundamental absorption edge spectrum in natural diamond. Structures in the spectrum have been explained from the optical transition due to indirect excitons assisted by TO phonons and indirect band-to-band transition.     

Photoconductance of a one-dimensional quantum dot

The ac-transport properties of a one-dimensional quantum dot with non-Fermi liquid correlations are investigated. It is found that the linear photoconductance is drastically influenced by the interaction. While for weak interaction it shows peak-like resonances, in the strong interaction regime it assumes a step-like behavior. In both cases the photo-transport provides precise informations about the quantized plasmon modes in the dot. Temperature and voltage dependences of the sideband peaks are treated in detail. Characteristic Luttinger liquid power laws are found. Received 23 October 2001     

Transparent,flexible and low‐resistive precision fabric electrode for organic solar cells

We report the use of conducting precision fabrics as transparent and flexible electrode for organic semiconductor‐based thin film devices. Precision fabrics have well‐defined mesh openings, excellent flexibility and are fabricated by high‐throughput roll‐to‐roll manufacturing. Optimized fabrics reached light transmittance over 95% throughout the visible and near infrared spectra. A significant part of the transmitted light is scattered, which is particularly advantageous for solar cell applications. Surface resistivity is as low as ～3 Ohms/square, which decreases Ohmic losses when scaling up to large area devices. We demonstrate that solar cells fabricated onto these electrodes show very similar characteristics to those prepared on ITO. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Dynamics of Incoherent Photovoltaic Spatial Solitons

Propagation properties of bright and dark incoherent beams are numerically studied in photovoltaic-photorefractive crystal by using coherent density approach for the first time. Numerical simulations not only exhibit that bright incoherent photovoltaic quasi-soliton, grey-like incoherent photovoltaic soliton, incoherent soliton doublet and triplet can be established under proper conditions, but also display that the spatial coherence properties of these incoherent beams can be significantly affected during propagation by the photovoltaic field.     

Depth Dependence of Tetragonal Distortion of a ZnO/Mg0.1Zn0.9O/ZnO Heterostructure Studied by Rutherford Backscattering/Channeling

Rutherford backscattering and channeling are used to characterize the structure of a ZnO/Mg_0.1Zn_0.9O/ZnO heterostructure grown on a sapphire (0001) substrate by rf plasma-assisted molecular beam epitaxy. The results show that the Mg_0.1Zn_0.9O layer has the same hexagonal wurtzite structure as the underlying ZnO layer, and the heterostructure has a good crystalline quality with χ_min =5%, which is the ratio of the backscattering yields of aligned and random spectra in the near-surface region. Using the channeling angular scan around an off-normal <12^-13> axis in the {101^-0} plane of both ZnO and MgZnO layer, the tetragonal distortion e_T, which is caused by the elastic strain in the epilayer, is determined. The depth dependence of e_T is obtained by using this technique. It can clearly be seen that the elastic strain rapidly decreases with the increase in thickness of the ZnO film in the early growth stage and becomes slightly larger in the region of the Mg_0.1Zn_0.9O layer.     

Dependence of terahertz radiation on gap sizes of biased multi-energy arsenic-ion-implanted and semi-insulating GaAs antennas

We investigate the characteristics of terahertz radiation pulses using biased multi-energy arsenic-ion-implanted and semi-insulating GaAs photoconductive antennas with different gap sizes in terahertz time-domain spectroscopy. At a specific fluence excitation, with increasing antenna gap size, the absolute values of the (peak) normalized terahertz waveform minimum (valley), as well as the bandwidth, reveal an increasing trend for multi-energy arsenic-ion-implanted GaAs antennas and a decreasing trend for semi-insulating GaAs antennas. We find that the largest reachable bias fields applied to arsenic-ion-implanted GaAs antennas are higher than those applied to semi-insulating GaAs antennas. On the basis of pump fluence dependences of peak terahertz amplitude, we deduce that multi-energy arsenic-ion-implanted GaAs antennas have the ability to acquire higher THz power at even higher pump fluence in comparison with semi-insulating GaAs antennas.