Gate‐Controlled Energy Barrier at a Graphene/Molecular Semiconductor Junction

The formation of an energy‐barrier at a metal/molecular semiconductor junction is a universal phenomenon which limits the performance of many molecular semiconductor‐based electronic devices, from field‐effect transistors to light‐emitting diodes. In general, a specific metal/molecular semiconductor combination of materials leads to a fixed energy‐barrier. However, in this work, a graphene/C₆₀ vertical field‐effect transistor is presented in which control of the interfacial energy‐barrier is demonstrated, such that the junction switches from a highly rectifying diode at negative gate voltages to a highly conductive nonrectifying behavior at positive gate voltages and at room temperature. From the experimental data, an energy‐barrier modulation of up to 660 meV, a transconductance of up to five orders of magnitude, and a gate‐modulated photocurrent are extracted. The ability to tune the graphene/molecular semiconductor energy‐barrier provides a promising route toward novel, high performance molecular devices.     

Evaluation of decay photocurrent measurements in dye‐sensitized solar cells: Application to laser beam‐induced current technique

The use of the laser beam‐induced current (LBIC) technique in photovoltaic devices is widespread, but its use in photoelectrochemical cells, such as dye‐sensitized solar cells (DSSCs), is limited due to the configuration of these devices. The main reason is the very slow response time of DSSCs in the decay process, and therefore LBIC scans take too long to perform. We have designed a procedure published in the literature to correct the photocurrent values obtained by the LBIC technique, based on an algorithm that uses a decreasing mono‐exponential model. This work presents a study of the decay measurements in DSSCs using several functions, in order to improve the algorithm designed. It concludes that functions such as a decreasing bi‐exponential or Becquerel function generate better fits to experimental data. Copyright © 2010 John Wiley & Sons, Ltd.     

势垒层对黑硅光电探测器性能影响的研究

讨论势垒层Si3N4的引入对黑硅光电探测器光电性能的影响。探测器采用金属一半导体一金属（MSM）器件结构在黑硅层和电极间增加一层势垒层以提高肖特基势垒,从而减低器件的暗电流。实验表明,在相同的光照情况下,有势垒层的探测器暗电流比无势垒层的探测器暗电流至少低1个数量级,且势垒层厚度增加30nm其暗电流降低约1个数量级,而...     

Investigation of recombination loss in organic solar cells by simulating intensity-dependent current-voltage measurements

After numerically solving the Poisson and continuity equations that characterize the electrical behavior of semiconductor devices, we investigated the loss mechanism in bulk heterojunction organic solar cells (OSCs) by simulating intensity-dependent current-voltage curves. Comparison of simulation results with real experimental measurements reveals that monomolecular recombination is the primary loss mechanism in OSCs. This conclusion is consistent with direct observations that bimolecular recombination is significantly suppressed in bulk heterojunction OSCs. The determination of recombination mechanism is important to model and predict the operation of OSCs precisely and further optimize their performance.     

A large discrepancy between CPM and ESR defect densities in light-soaked a-Si:H

A large discrepancy between defect densities evaluated by constant photocurrent method (CPM) and electron spin resonance (ESR) is investigated for hydrogenated amorphous silicon (a-Si:H) with various thicknesses in the process of light soaking (LS). It is shown by a computer simulation that the inhomogeneous defect distribution across the film thickness caused by a rather small penetration depth of the white light in the thick film is one of the possible origins of the discrepancy. But various other causes should also bring about the discrepancy, and the problem is still a puzzling one.     

Photocurrent derivative spectra of ZnCdSe-ZnSe double multi-quantum wells

Photocurrent (PC) spectra of ZnCdSe-ZnSe double multi-quantum wells are measured at different temperature. Its corresponding photocurrent derivative (PCD) spectra are obtained by computing, and the PCD spectra have greatly enhanced the sensitivity of the relative weak PC signals. The polarization dependence of the PC spectra shows that the transitions observed in the PC spectra are heavy-hole related, and the transition energy coincide well with the results obtained by envelope function approximation including strain. The temperature dependence of the photocurrent curves indicates that the thermal activation is the dominant transport mechanism of the carriers in our samples. The concept of saturation temperature region is introduced to explain why the PC spectra have different temperature dependence in the samples with different structure parameters. It is found to be very useful in designing photovoltaic devices.     

Photocurrent near-field microscopy of Schottky barriers

We used a combination of internal photoemission and of near-field optical microscopy (SNOM) to study the lateral variations in solid interface properties such as energy barriers and electron–hole recombination. In particular we investigated the fully formed Pt–GaP, Au–GaAs, Au–SiN _x –GaAs and PtSi–Si Schottky barriers. Our approach enabled us to measure large lateral variations in the photocurrent with spatial resolution on the nanometric scale. Due to the ability of SNOM to supply parallel topographic information, we observed photocurrent variations from zone to zone that only correlated in a few cases with local variations in surface morphology. We assigned the uncorrelated fluctuations to local variations in the interface stoichiometry, the presence of interface states induced by the metallic overlayer and to defect states at the junction. Furthermore, by tuning the photon energy and applied bias we were able to measure the surface distribution of the diffusion length.     

GaN基MSM结构紫外探测器光响应特性

在求解一维电流连续性方程和传输方程的同时考虑表面态陷阱的作用,获得了GaN基MSM结构紫外探测器在稳态光照下的电流随电压变化的解析解,从而导出了其光响应特性主要参数,并解释了电流和响应度随偏压变化的原因和光增益现象.将该模型应用于具体器件,实验测得饱和临界偏压约6 V,稳态电流6×10-8A,响应率0.085 7 A/W,与理论计算较吻合.     

Opto-Mechanical and Electronic Design of a Tunnel-Trap Si Radiometer

A transmission-type light-trap silicon radiometer has been developed to hold the NIST spectral power and irradiance responsivity scales between 406 nm and 920 nm. The device is built from replaceable input apertures and tightly packed different-size silicon photodiodes. The photodiodes are positioned in a triangular shape tunnel such that beam clipping is entirely eliminated within an 8 field-of-view (FOV). A light trap is attached to the output of the radiometer to collect the transmitted radiation and to minimize the effect of ambient light. The photodiodes, selected for equal shunt resistance, are connected in parallel. The capacitance and the resultant shunt resistance of the device were measured and frequency compensations were applied in the feedback network of the photocurrent-to-voltage converter to optimize signal-, voltage-, and loop-gain characteristics. The trap radiometer can measure either dc or ac optical radiation with high sensitivity. The noise-equivalent-power of the optimized device is 47 fW in dc mode and 5.2 fW at 10 Hz chopping. The relative deviation from the cosine responsivity in irradiance mode was measured to be equal to or less than 0.02 % within 5° FOV and 0.05 % at 8° FOV. The trap-radiometer can transfer irradiance responsivities with uncertainties comparable to those of primary standard radiometers. Illuminance and irradiance meters, holding the SI units (candela, color- and radiance-temperature), will be calibrated directly against the transfer standard trap-radiometer to obtain improved accuracy in the base-units.