Photogenerated Space Charge Research Articles

Impedance analysis of the multilayered organic solar cells with and without hole buffer layer

We studied the effects of a buffer layer [molybdenum oxide (MoO3) and poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)] and the thickness of a p-type tetraphenyldibenzoperiflanthene (DBP) layer on the performance of multilayered organic solar cells in indium–tin oxide (ITO)/buffer/DBP/fullerene C60/bathocuproine (BCP)/Al multilayered photovoltaic devices. The insertion of a hole buffer layer reduced the resistance across the cell in forward biasing under illumination and in the dark, and the negative capacitance behaviors were observed at a voltage higher than VOC, probably owing to the enhancement of the double injection. On the other hand, a hole-blocking ITO/DBP interface reduced the hole injection and extraction, and DBP became similar to a dielectric layer in the dark, as determined from the capacitance–frequency measurement. The photogenerated carriers caused an increase in the capacitance C and the conductance G in the devices with and without the MoO3 buffer layers. Both C and G reached their local minima at a voltage close to VOC, and increased with a decrease in external voltage probably owing to the enhancement of charge extraction efficiency. At a lower voltage, however, C and G reached their peak values and then decreased again with a decrease in external voltage, probably owing to the reduction (ejection) in the number of photogenerated space charges in active layers. The increase in the thickness of the dielectric DBP layer resulted in a reduction in charge extraction efficiency, and an S-shaped curve was observed for the thick DBP cell. The peak voltages in C and G moved toward negative values with an increment in thickness, and this was discussed using the simplified rate equation model.

Fill factor related issues in hydrogenated amorphous Si solar cells

It is a fact that the fill factor (FF) decreases with decreasing μτ product, increasing illumination intensity, increasing absorber thickness, and long wavelength illumination in well-engineered hydrogenated amorphous silicon (a-Si:H) solar cells. The FF issues in the a-Si:H solar cell are not just a contact issue as in crystalline Si solar cell, but are more interesting and complicated issues that relate to the quality of materials and solar cell operating conditions. In past, a simple parameter of “collection length” or drift length, which is defined as μτE, was quite successful to explain the fact. In this article, we extensively study the collection length in terms of the internal electric field, which changes with the quality of material and operating condition. The Analysis of Microelectronic and Photonic Structures (AMPS) simulation shows that the decrease in fill factor is caused by photogenerated space charge trapped in the band-tail states rather than in defects when the defects are relatively small. This charge screens the applied field, reducing the internal field and the collection length, which reduce the FF. A voltage Vs from the photogenerated space charge was introduced to describe the behavior. This voltage varies with light intensity for large valance band-tail width and low hole mobility a-Si:H absorbers. The simulation also found that the space charge in mid-gap states is small compared with that in the tails and can be ignored under normal solar-cell operating conditions. Experimentally, the photocapacitance measurement was used as a means to probe the space charge. The decrease of FF with the increasing light intensity can be qualitatively explained by the light intensity dependence of photocapacitance, the declining electric field, and the decreasing collection length.

Optoelectronic conversion by polarization current, triggered by space charges at organic-based interfaces

We report that a highly efficient optoelectronic conversion can be achieved by photogenerated space charges, which usually damp photocurrent. Theoretical analysis of metal-organic insulator-organic semiconductor-metal photocells indicates the generation of a large transient current that is triggered by photogenerated space charges and governed by the dielectric properties of the insulator layer. We experimentally demonstrated this mechanism with model photocells, revealing that the quantum efficiency can be dramatically increased by increasing the dielectric constant of the insulator.

Optical control of orientational bistability in photorefractive liquid crystals

Materials with light-controlled properties are extremely attractive for information technologies and, among the wide range of approaches, that of photorefractive media is particularly relevant1,2. Photorefractivity is observed in photoconductors, where charge carriers photogenerated by non-uniform illumination redistribute over macroscopic distances, and where the resulting electric field modulates the refractive index. This effect has been used in a variety of devices for applications ranging from image treatment to beam steering and medical imaging. Compared with the more commonly studied inorganic crystals, photorefractive non-crystalline organic systems3,4,5,6 offer a more technologically appealing alternative, as these materials are mechanically flexible, can be easily modified chemically and are inexpensive. Here we show how a photogenerated space–charge field can be used to control the switching between the two stable orientational states of ferroelectric liquid crystals. The resulting refractive-index patterns are stable, easily erasable and rewritable. In contrast to more traditional organic photorefractive materials, where the refractive index is continuously modulated by the applied field, the liquid-crystalline device described here is a ‘binary’ system, in which the refractive index n switches between two values (Δn∼3×10−2 in this case, but Δn of the order of 10−1 is, in principle, possible).

Evidence of photogenerated space charges in the charge ordered state of Pr 0.63Ca 0.37MnO 3 single crystals

Time-resolved thermoelectric power measurements on bulk single crystals with composition close to charge ordered compound Pr 2/3Ca 1/3MnO 3 show a voltage transient V 2( t) for T< T co, which is superimposed to and faster as the quasistatic thermovoltage decay V 3( t) which is usually the only transient observed with manganite ceramic. From V 3( t) we derive the heat conductivity k and thermopower S in the temperature range 90 K< T<350 K. The extra transient is ascribed to the decay of an ambipolar charge distribution in trap states which appears only in the charged ordered regime of x=0.37. First data on intrinsic and/or extrinsic trap level distributions are given.

Electric field-assisted dissociation of singlet excitons in tris-(8-hydroxyquinolinato) aluminum (III)

Results based on the electric-field dependence of electro-modulated fluorescence and steady-state photoconduction in films of tris-(8-hydroxyquinolinato) aluminum (III) (Alq3) are consistent with a model where singlet excitons dissociate into electron–hole pairs through charge transfer state. Dissociation into free carriers or predominantly geminate recombination results from this intermediate state. The quantum efficiency of charge separation is in agreement with predictions based on a model using the three-dimensional Onsager theory of geminate recombination combined with volume (bimolecular) recombination of the photogenerated space charge. This model is used to account for the decrease in the electroluminescence quantum efficiency of Alq3-based organic light-emitting diodes at high electric fields.

Rapid collapse of Wannier–Stark localization caused by space charge electric field screening in short-period superlattices

A rapid optical absorption change is observed in a GaAs/AlAs short-period superlattice having Wannier–Stark localization. This phenomenon is clearly explained by a rapid collapse of Wannier–Stark localization due to electric field screening by photogenerated space charges. The screening causes a positive feedback loop between restoration of the blue-shifted wavelength of the absorption band-edge towards the red and an increase in optical absorption, which causes an additional field screening. The experimental bias voltage dependence of the intensity of photoluminescence and photocurrent under high optical excitation, agree well with a model applying Fowler–Nordheim tunneling at the heterointerface cladding layer. It is concluded that the space charges are stopped near the cladding layer and that the superlattice region is almost fully screened to near the flat-band bias condition.

Probe of Field Collapse in a-Si:H Solar Cells

AbstractWe study the effect of illumination intensity on solar cell performance in a-Si:H solar cells. We find that the fill factor strongly depends on light intensity. As we increase the illumination intensity from low levels to one sun we observe a decrease in fill factor of approximately 15% in as grown cells. We attribute this effect to electric field collapse inside the cell. We propose that photogenerated space charge (free and trapped charge) increases with light intensity and causes field collapse. We describe the origin of space charge and the associated capacitance - photocapacitance. We measure the photocapacitance as a barometer to probe the collapsed field. We obtain a good agreement between photocapacitance experiments and theory. We also explore the light intensity dependence of photocapacitance and explain the decrease of FF with the increasing light intensity.

Optical differentiator based on rapid collapse of Wannier–Stark localization due to space charge field screening

An optical differentiator operation is demonstrated using a GaAs/AlAs short-period superlattice near an optical absorption band edge. This operation is clearly explained by a rapid collapse of Wannier–Stark localization due to electric field screening by photogenerated space charges, because the screening causes a positive feedback loop between restoration of blueshifted wavelength of absorption band edge to the red side and an increase in optical absorption which causes an additional field screening.

Influence of space charges on the impulse response of InGaAs metal-semiconductor-metal photodetectors

The impulse response of interdigitated metal-semiconductor-metal photodetectors fabricated on an Fe-doped InGaAs absorbing layer and an Fe-doped InP barrier enhancement layer is investigated. For ultra-short pulse excitation of 150 fs at lambda =620 nm the photoresponse is found to be less than 13 ps FWHM for detectors with 1 mu m finger spacing. Above a certain level of illumination, the peak amplitude increases sublinearly and the relative contribution of the tail to the detector response is appreciably enhanced. The screening of the electric field by photo-generated space charges is responsible for this nonlinearity. For detectors with 5 mu m finger spacing illuminated with 1.3 mu m light pulses (FWHM=33 ps), space charge perturbation of the impulse response manifests itself by a decrease of the FWHM and an increase of the fall time with increasing illumination level. The practical consequences for the performance of MSM detectors in various applications are discussed. >

Role of Space Charge in a Memory-Type Organic Photoreceptor Coated on an Organic Polymer Electrode

Role of photo-generated space charges on the memory effect, observed in an iodine-doped conductive polymer / poly(N-vinylcarbazole) (PVK)-trinitrofluorenone (TNF) double-layered system, was investigated by means off voltage-current measurements. The results were consistent with the previous conclusion obtained by the xerographic technique. The memory effect was found to come from space charges formed at the electrode-photoreceptor interface containing iodine. The space charge formation induced apparent barrier height lowering against carrier injection and the magnitude of lowering was about 60 meV. It was also found that injected charges from polymer electrode per unit space charge was more than 2×103.

Modeling of thin-film solar cells: Nonuniform field

A model calculation for a thin film solar cell is presented to describe its photovoltaic behavior under the conditions that the photogenerated space charge causes significant distortion of the internal electric field. Expressions for the photocurrent, photocapacitance, and ambipolar-diffusion length are given.

Thermally stimulated depolarization of dipolar defects in MgO single crystals

For the first time, the TSD technique has been applied for the detection of dipolar defects in MgO single crystals. Three electrical depolarization bands at 200, 214, and 240 K were found and are ascribed to localized defect dipoles. The band at 214 K changed in height as long as 30 h after the crystals had been exposed to UV radiation of wavelengths between 250 and 300 nm. This indicates the formation of dipolar defects by solid state reactions between photogenerated space charge carriers and localized defect centers. The relations of the TSD bands to the V− and VAl centers are discussed. Another broad band was observed at 400 K which seems to arise from the depolarization of electronic space charge layers.