Photoactive Area Research Articles

Improved photocurrent by using n-doped 2,3,8,9,14,15-hexachloro-5,6,11,12,17,18-hexaazatrinaphthylene as optical spacer layer in p-i-n type organic solar cells

We introduce 2,3,8,9,14,15-hexachloro-5,6,11,12,17,18-hexaazatrinaphthylene (HATNA-Cl6) as n-dopable electron transport material (ETM) for small molecule organic solar cells. Because of its large optical energy gap of 2.7 eV and its well suited energy level positions, the material can be implemented as a semitransparent spacer layer between the reflecting metal contact and the photoactive C60 acceptor layer in p-i-n type solar cells. By varying the ETM thickness, it is possible to shift the position of the photoactive area with respect to the interference maximum of the optical field distribution. Applying n-HATNA-Cl6 instead of the parasitically absorbing reference ETM n-C60 results in a considerably improved photocurrent density and accordingly in a higher efficiency. At dETM=100 nm the power conversion efficiency is more than doubled as it increases from (100 nm n-C60) = 0.5% to (100 nm n-HATNA-Cl6) = 1.1%.

Molecular Design and Photovoltaic Performances of Organic Dyes Containing Triphenylamine for Dye-Sensitized Solar Cell

Organic dyes containing multi-acceptors/anchors in a chromophore were synthesized for use in a dye-sensitized solar cell. The photovoltaic properties of organic dyes composed of different acceptor in their chromophores were measured to identify the effects on the DSSC performance. The organic dye, 4 containing multi-cyanoacrylic acid as the electron acceptor showed a power conversion efficiency of 4.7% under AM 1.5 illumination (100 mW cm−2) in an photo active area of 0.24 cm2, short circuit current density of 12.9 mA cm−2, open circuit photo voltage of 0.62 V and a fill-factor of 60%. The retarded recombination kinetics from TiO2 electrode to electrolyte enhanced the electron life time of organic dye, 4 in the photoanode in DSSC that was well confirmed with the impedance analysis.

Effect of Conducting Ability of Electrolytes on the Photovoltaic Performance of Quasi-Solid State Dye-Sensitized Solar Cells

DSSCs based on quasi-solid type of electrolytes were prepared to investigate the influence of ionic mobility and recombination kinetics in electrolytes on DSSC performances. The improved ionic mobility of electrolyte in PMMA system resulted in the enhancement of Jsc value in DSSC which was well confirmed with the incident photon-to-current spectra and the impedance analysis. Furthermore, the retarded recombination rate from TiO2 electrode to electrolyte in PMMA system enhanced the electron life time of DSSC in the photoanode. DSSCs based on quasi-solid type of electrolytes composed of PMMA showed a power conversion efficiency of 3.36% underAM 1.5 illumination (100 mWcm−2) in an photo active area of 0.24 cm2, short circuit current density of 7.69 mAcm−2, open circuit photo voltage of 0.69 Vandafill-factor of 64%.

ITO-free flexible inverted organic solar cell modules with high fill factor prepared by slot die coating

We report on the fabrication of inverted ITO-free P3HT:PCBM solar cell modules on glass and PET foil as substrate where the organic functional layers are deposited with slot die coating, a reel to reel compatible coating technique. The active layers have been processed in ambient atmosphere, which will be of advantage in a future production and is especially remarkable as the metallic cathode is already deposited on the substrate at this stage of fabrication. The modules comprise two busses of 11 cell elements connected in series each. The series connection leads to an open circuit voltage of up to 6.88 V on glass substrate, which translates to 625 mV per cell element, a very competitive value for P3HT:PCBM based solar cells on glass. Although the designated area is as large as 41 cm 2 and the active area 26.4 cm 2, we obtain fill factors of up to 65% for these modules, which again is a typical value for small area laboratory cells. Remarkably the values for PET foil as substrate with an open circuit voltage of 6.5 V and a fill factor of 64% are very close to the results on glass and to our knowledge the highest fill factors for flexible organic solar cells, even if compared to small area devices. The short circuit current densities and therefore efficiencies are also comparable to small area devices, if only the photoactive area is accounted for. Therefore we have demonstrated that the scale up of organic solar cells can be achieved with a suited circuitry scheme.

Highly efficient CdS/CdSe-sensitized solar cells controlled by the structural properties of compact porous TiO2 photoelectrodes

At present, the photovoltaic performance of quantum dot-sensitized solar cells (QDSCs) is still much lower than conventional DSCs. Appropriate porous TiO(2) photoanodes for QDSCs need to be further investigated, and optimization of the nanoparticle-based photoanodes is highly desirable as well. In this article, the influence of the structural properties of various TiO(2) photoanodes on CdS/CdSe-sensitized solar cells have been systematically studied. Quantitative analyses of light-harvesting efficiency (LHE) and electron-transfer yield (Φ(ET)) for the QDSCs are investigated for the first time. It is revealed that the LHE increases in the long wavelength region with the addition of large size TiO(2) particles to the transparent film. In the meantime, the balance between the light scattering and surface area also needs to be controlled, which can significantly restrain the dark current of the device. A double-layer photoanodic structure can give 4.92% of light-to-electricity conversion efficiency with a photoactive area of 0.15 cm(2).

Molecular design of donor–acceptor-type cruciform dyes for efficient dyes-sensitized solar cells

Novel donor–acceptor (D–A) type cruciform dyes were synthesized for use in a dye-sensitized solar cell. The geometries and electronic structures of the dyes were designed by the results of density functional theory (DFT) calculations. Two phenothiazine units in these dyes act as electron donors, while two cyanoacrylic acid groups act as electron acceptors. The solubility of these compounds is increased because of the presence of the hexyl groups in the phenothiazine rings. The photovoltaic properties of 14D-25A and 12A-45D were measured to identify the effects of the dye having two electron donors and two electron acceptors on the DSSC performance. The 12A-45D dye showed a power conversion efficiency of 4.5% under AM 1.5 illumination (100 mW cm −2) in an photoactive area of 0.42 cm 2, short circuit current density of 10.8 mA cm −2, open circuit photovoltage of 0.65 V and a fill factor of 64%.

Lateral drift-field photodiode for low noise, high-speed, large photoactive-area CMOS imaging applications

In this work a theoretical concept and simulations are presented for a novel lateral drift-field photodetector pixel to be fabricated in a 0.35 μm CMOS process. The proposed pixel consists of a specially designed n-well with a non-uniform lateral doping profile that follows a square-root spatial dependence. “Buried" MOS capacitor-based collection-gate, a transfer-gate, and an n-type MOSFET source/drain n + floating-diffusion serve to realize a non-destructive readout. The pixel readout is performed using an in-pixel source-follower pixel buffer configuration followed by an output amplifier featuring correlated double-sampling. The concentration gradient formed in the n-well employs a single extra implantation step in the 0.35 μm CMOS process mentioned and requires only a single extra mask. It generates an electrostatic potential gradient, i.e. a lateral drift-field, in the photoactive area of the pixel which enables high charge transfer speed and low image-lag. According to the simulation results presented, charge transfer times of less than 3 ns are to be expected.

Metal Hole Arrays as Resonant Photo-Coupler for Charge Sensitive Infrared Phototransistors

Metallic photo-couplers utilizing surface plasmon polariton (SPP) excitation have been studied experimentally and theoretically for improving the quantum efficiency of charge sensitive infrared phototransistors (CSIP). Metallic hole arrays deposited on top of the photo-active area of CSIPs (wavelength of 14.7 ¿m) induce intensified near fields for the intersubband transition in a GaAs quantum well at 100 nm below the metal/substrate interface. Cross-hole arrays yield the highest efficiency of 7%, which is by a factor of about four higher than the previously achieved value with square patch arrays.

Temperature dependence of the performance of charge-sensitive infrared phototransistors

The performance of charge-sensitive infrared phototransistors (λ∼14.7 μm) is studied at temperatures of up to 30 K. The devices, with a 16×4 μm2 photoactive area, are fabricated in GaAs/AlGaAs double-quantum-well structure. An excellent specific detectivity D∗=9.6×1014 cm Hz1/2/W is derived in a T range of up to T=23 K. Experimental results are theoretically studied based on WKB approximation, in which photogenerated holes in the floating gate (FG) are recombined with thermal emission or thermally assisted tunneling from the outside of FG through the barriers. The model well reproduces the experimental results, including the vanishing of photosignal at 30 K under 280 fW incident radiation. The model is used to predict a temperature-dependent specific detectivity D∗ in ideal devices free from 1/f noise.

Toward a Systematic Understanding of Photodetectors Based on Individual Metal Oxide Nanowires

We present a set of criteria to optimize photodetectors based on n-type metal oxide nanowires and a comparison methodology capable of overcoming the present lack of systematic studies dealing with such devices. The response of photoconductors is enhanced following different fabrication strategies, such as diminishing the distance between the electrical contacts, increasing the width of the photoactive area, or improving the electrical mobility of the nanomaterials. The validity of the theoretical background is verified by experimental results obtained with devices based on ZnO nanowires. The performances of our devices show that the normalized gain of single ZnO nanowire-based photodetectors exceeds those of thin films.

RF Transformer Coupled Multiplexing Circuits for APD PET Detectors

Avalanche-photodiodes (APDs) as photosensors in positron emission tomography (PET) detectors have been extensively investigated in this field. Compared with conventional photosensors such as the photomultiplier tubes (PMTs), most APDs have advantages of higher quantum efficiency (~70% for APD vs. ~20% for PMT), robust packaging and very low magnetic susceptibility. However, it usually has very low gain (~200 for APD vs. ~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> for PMT), and a smaller photoactive area (~ 5 mmtimes5 mm for APD vs. 10-52 mm diameter for PMT). The proposal described in this paper was based on a previous APD block detector design, in which each block consists of a 2times2 APD array reading out an 8times8 array of Lutetium Oxyorthosilicate (LSO) crystals. Each crystal is 2 mmtimes2 mmtimes20 mm. Due to the small block size, in order to build an APD PET system with similar axial field-of-view of a conventional PET scanner, substantially more APD detectors would be needed. Consequently, more electronics processing channels would be required. To simplify the detector electronics, we initiate a multiplexing concept based on RF transformers. This approach may reduce the signal-processing channels by a factor of 16 (from 64 channels to four). The circuits would work from both current and voltage sources, as opposed to resistor networks which map signals only from current sources. We built prototype printed-circuit-boards (PCBs) to evaluate different multiplexing schemes. The initial measurements demonstrate that the multiplexing circuits can be implemented in the detector electronics to reduce signal output channels, without increasing signal rise-time and degrading signal-to-noise ratio (SNR). The detector maintains an energy resolution of 19% and timing resolution of about 2 ns (block to single crystal). Moreover, the transformer can function as a single-ended (pseudo-differential) to true-differential converter; this would facilitate retaining signal integrity in transmission through long twisted-pair cables

CMOS integrated pixel array for low-level radiation detection using ‘time compression’ parametric amplification

In this work, theoretical modeling and simulations of a ‘time compression’ parametric amplification pixel array to be used in CMOS imaging for low-level or near IR radiation are presented. The implementation in the higher resistivity substrate process used for smart-power devices, namely the 30 V thin-film 1.0 μm SIMOX-CMOS based on Silicon-on-Insulator technology, enables isolated separation of photoactive and readout circuitry areas when integrated on the same chip. The method presented shows that very high readout current peaks can be generated which are much higher than the mean value of the photocurrent induced. Thus, the parametrically amplified readout signals can be detected by a low-noise peak-detector circuit. The 500×500 μm 2 area pixels with internal parametrically controllable amplification and increased pixel fill-in factor (>90%) present higher detection volume due to enlarged deep-depletion region depth caused by a low resistivity substrate. Moreover, the X– Y addressing and the inverse exponential dependence of the parametric amplification on the incoming radiant flux for each pixel provide separately controllable single-pixel internal amplification which improves the intra-scene high dynamic range demands.

Efficient organic solar cells by penetration of conjugated polymers into perylene pigments

We report here efficient air-stable p-n heterojunction organic solar cells with a structure consisting of an n-type insoluble perylene pigment penetrated by a p-type-conjugated polymer, where the interfacial area for photocurrent generation increases. The solar cells are easily produced by infiltrating a soluble-conjugated polymer intentionally into an opening among insoluble microcrystalline perylene layer under a saturated chloroform vapor. This approach can be regarded as an alternative convenient way to achieve bulk heterojunction solar cells. The cell performance is further enhanced by inserting an additional layer between the electrode and the photoactive layer to confine exciton in the photoactive layer. The overall attempt to improve the cell performance, so far, results in maximum quantum efficiency up to 45% under illumination of 485-nm monochromatic light and power conversion efficiency up to 1.9% under a simulated solar light (AM1.5) with a 100mWcm−2 intensity. The approach is promising to achieve practical efficiency because tuning the opening size can further widen the photoactive area.

Improving the efficiency of titania aerogel-based photovoltaic electrodes by electrochemically grafting isopropyl moieties on the titania surface

We have fabricated thick, nanostructured composite films of titania (aerogel)–silica (aerogel) on transparent electrodes for use as composite photoelectrodes. Aerogels are excellent candidate materials for photovoltaic electrodes because the high specific surface area of titania aerogel (∼150m2/g) significantly amplifies photoactive area, while the continuous pore network permits rapid mass transport of diffusing electron donors through the film. The high surface-to-volume ratio of titania aerogel films can also be a cause for concern because the amplified surface character begets a large number of surface defects that can act as energetically deep traps and hinder percolative transport of electrons through the film. Surface hydroxyls (Ti–OH, titanols) act as charge traps, whereby electrons localize at the semiconductor surface and promote recombination of electrons with solution-phase acceptors, resulting in diminished measured photocurrents. Titania (aerogel)–silica (aerogel) composite films can be modified by electrochemically generated organic radical anions, which graft to the titanols at the electrode surface. This organic modification improves photocurrent yields under bandgap-excitation by a factor of >2 relative to unmodified films, and shifts the onset of photoanodic current negatively by ∼45mV. The improved photocurrent yields are likely due to more facile electronic transport through the modified mesoporous network, caused by changing the chemical nature of the surface species on the titania electrode. The negative shift of the onset potential for the flow of cathodic current is further evidence that the enhanced photocurrent arises from a change in electron mobility, as opposed to a lowering of the conduction band edge, which would improve photoelectron injection efficiency, but would involve a positive shift in the cathodic onset potential.

Improvement of Dark Current Using InP/InGaAsP Transition Layer in Large-Area InGaAs MSM Photodetectors

A large-area InGaAs metal-semiconductor-metal (MSM) photodetector with 1/spl times/1 mm/sup 2/ photoactive area for free-space optical communication applications has been designed, fabricated, and characterized. Interdigitated electrodes of 2-/spl mu/m widths and 15-/spl mu/m spacings are designed to maximize the responsivity, and enable MSM photodetectors to reach a maximum responsivity at 1.53-/spl mu/m wavelength. By employing a two-step InP/InGaAsP transition layer, the dark current density of 45 fA//spl mu/m/sup 2/ was achieved at 10-V bias and at room temperature. Dark current-bias voltage curves were measured as a function of temperature from 40 to 270 K to estimate the activation energy. A 3-dB bandwidth of 210 MHz was obtained at a 10-V bias, and the measured result was compared with the designed bandwidth.

Time-resolved fluorescence imaging of slab gels for lifetime base-calling in DNA sequencing applications.

A compact time-resolved near-IR fluorescence imager was constructed to obtain lifetime and intensity images of DNA sequencing slab gels. The scanner consisted of a microscope body with f/1.2 relay optics onto which was mounted a pulsed diode laser (repetition rate 80 MHz, lasing wavelength 680 nm, average power 5 mW), filtering optics, and a large photoactive area (diameter 500 microns) single-photon avalanche diode that was actively quenched to provide a large dynamic operating range. The time-resolved data were processed using electronics configured in a conventional time-correlated single-photon-counting format with all of the counting hardware situated on a PC card resident on the computer bus. The microscope head produced a timing response of 450 ps (fwhm) in a scanning mode, allowing the measurement of subnano-second lifetimes. The time-resolved microscope head was placed in an automated DNA sequencer and translated across a 21-cm-wide gel plate in approximately 6 s (scan rate 3.5 cm/s) with an accumulation time per pixel of 10 ms. The sampling frequency was 0.17 Hz (duty cycle 0.0017), sufficient to prevent signal aliasing during the electrophoresis separation. Software (written in Visual Basic) allowed acquisition of both the intensity image and lifetime analysis of DNA bands migrating through the gel in real time. Using a dual-labeling (IRD700 and Cy5.5 labeling dyes)/two-lane sequencing strategy, we successfully read 670 bases of a control M13mp18 ssDNA template using lifetime identification. Comparison of the reconstructed sequence with the known sequence of the phage indicated the number of miscalls was only 2, producing an error rate of approximately 0.3% (identification accuracy 99.7%). The lifetimes were calculated using maximum likelihood estimators and allowed on-line determinations with high precision, even when short integration times were used to construct the decay profiles. Comparison of the lifetime base calling to a single-dye/four-lane sequencing strategy indicated similar results in terms of miscalls, but reduced insertion and deletion errors using lifetime identification methods, improving the overall read accuracy.

Inactivation of microorganisms in a flow-through photoreactor with an immobilized TiO2 layer

A laboratory flow-through photoreactor with an immobilized layer of TiO2 (total volume of the liquid 5000 cm 3, photoactive area 60 cm long and 30 cm wide; irradiation source UV lamps Eversun, Osram, light intensity from 0.9 to 6.2×10−9 Einstein cm−2 s −1) was tested for the inactivation of Escherichia coli (strain DH5α) and bacteriophage λNM1149. The kinetics of the deactivation were approximately first order and the initial reaction rate depended on the light intensity. At maximum intensity, the rate constants of the bacteria and viruses inactivation were 2.3×10−4 and 7.2×10−4 s−1, respectively. Bacterial inactivation was also accomplished with solar excitation. © 1999 Society of Chemical Industry

On the structural and photochemical studies of In 2O 3-admixed nanostructured TiO 2 with regard to hydrogen production through photoelectrolysis

A new photoelectrode system (for photoelectrolysis of water) employing nanostructured TiO 2 has been synthesized. In addition to using a bare TiO 2(ns) electrode, a system with an improved electrode obtained through admixing In 2O 3 into TiO 2(ns) has been synthesized and studied. The TiO 2(ns) photoelectrode has been prepared by the hydrolysis of Ti[OCH(CH 3) 2] 4 followed by deposition of thin film by a spin-on method. It has been shown that the TiO 2(ns) photoelectrode-bearing cell exhibits higher photoelectrochemical output. Possible causes for this are described and discussed. Another improvement in the TiO 2(ns) photoelectrode carried out in the present work comprised admixing with In 2O 3. The In 2O 3-admixed TiO 2(ns) exhibited a much higher photocurrent of 14mA cm −2. The hydrogen gas evolution under photoelectrolysis employing a TiO 2(ns) photoelectrode was found to be 61h −1 over a 1 m 2 electrode area. The TiO 2(ns)-In 2O 3 electrode exhibited a higher hydrogen gas evolution rate of 10.51 h −1 over a 1 m 2 electrode area. Evidence and arguments are put forward to show that, whereas for the bare TiO 2(ns) electrode the improvement in the performance of this photoelectrode compared with its original form was due to the higher quantum yield resulting from the nanostructured microstructural feature, for the TiO 2(ns)-In 2O 3 photoanode the improvement is due to improved spectral response, decrease of energy gap and availability of increased photoactive surface area.

Self-calibration as a tool for high-accuracy determination of silicon solar cell internal quantum efficiency

Light nonuniformity, uncertainty in the illuminated photoactive area, and relative, but not absolute radiometric data for the reference detector, can be the reasons for the inaccuracy or impossibility of solar cell spectral response and quantum efficiency determination. The use of a self-calibration principle permits minimization of the errors caused by the above factors. This principle consists of quite precise calculation of the internal quantum efficiency Q( λ m) of the test cell at λ m≈0.8 μm, where the cell response is weakly dependent on emitter and base parameters. Experimentally determined short- and long-wavelength internal quantum efficiencies, Q(0.4) and Q(0.95), respectively, based on relative radiometric data for a reference detector, are used as starting data for the Q( λ m) calculation. The ratio of the calculated to measured Q( λ m) values gives the correction factor for shifting the experimental quantum efficiency curve. Computer modeling supports the assumption that uniform deviation of measured Q( λ) can be precisely corrected by calculation. Analysis of the accuracy of the self-calibration method demonstrates very small uncertainties in the corrections of quantum efficiency measurements, attainable for many practical situations. Confirmation of correctness of the proposed method is shown by analysis of the results of spectral response measurements of several solar cells.

Construction of the energy diagram of an organic semiconductor film on SnO 2:F by surface photovoltage spectroscopy

The work functions of solid layers of the hole conductors diketopyrrolopyrrole (DPP) and dithioketopyrrolopyrrole (DTPP) on SnO 2:F were measured by the Kelvin probe technique. From the dependence of the work function on the layer thickness, the width of the space charge layer at the interface and the total voltage drop over the organic layer were deduced. The presence of two photoactive areas, one at the air/D(T)PP interface and one at the D(T)PP/SnO 2:F interface, was demonstrated by measuring the surface photovoltage (SPV) spectra under front and back side illumination, and their dependence on the layer thickness. Calculation of the contributions of each of these areas to the SPV permitted the construction of the energy diagram of the DTPP/SnO 2:F junction. The spectral dependence of the SPV for D(T)PP layers is weak. For thick layers of D(T)PP, the SPV spectrum under front side illumination follows the absorption spectrum, while the one under back side illumination is different in shape. For thinner layers the SPV spectrum is a superposition of the front side signal and back side signal, modulated by an internal filter effect of the organic layer itself.