Photoconductivity Degradation Research Articles

Three Generations of Solar Cells

In photovoltaic system the major challenge is the cost reduction of the solar cell module to compete with those of conventional energy sources. Evolution of solar photovoltaic comprises of several generations through the last sixty years. The first generation solar cells were based on single crystal silicon and bulk polycrystalline Si wafers. The single crystal silicon solar cell has high material cost and the fabrication also requires very high energy. The second generation solar cells were based on thin film fabrication technology. Due to low temperature manufacturing process and less material requirement, remarkable cost reduction was achieved in these solar cells. Among all the thin film technologies amorphous silicon thin film solar cell is in most advanced stage of development and is commercially available. However, an inherent problem of light induced degradation in amorphous silicon hinders the higher efficiency in this kind of cell. The third generation silicon solar cells are based on nano-crystalline and nano-porous materials. Hydrogenated nanocrystalline silicon (nc-Si:H) is becoming a promising material as an absorber layer of solar cell due to its high stability with high Voc. It is also suggested that the cause of high stability and less degradation of certain nc-Si:H films may be due to the improvement of medium range order (MRO) of the films. During the last ten years, organic, polymer, dye sensitized and perovskites materials are also attract much attention of the photovoltaic researchers as the low budget next generation PV material worldwide. Although most important challenge for those organic solar cells in practical applications is the stability issue. In this work nc-Si:H films are successfully deposited at a high deposition rate using a high pressure and a high power by Radio Frequency Plasma Enhanced Chemical Vapor Deposition (RF PECVD) technique. The transmission electron microscopy (TEM) studies show the formations of distinct nano-sized grains in the amorphous tissue with sharp crystalline orientations. Light induced degradation of photoconductivity of nc-Si:H materials have been studied. Single junction solar cells and solar module were successfully fabricated using nanocrystalline silicon as absorber layer. The optimum cell is 7.1 % efficient initially. Improvement in efficiency can be achieved by optimizing the doped layer/interface and using Ag back contact.

The Effects of Nitrogen and Oxygen Atmosphere on the Photoconductivity of Trimethyl Phenyl Diamine Thin Films

Organic materials were previously used as insulators in electrical technology. These materials, however, are currently used as conductors once their photoconductivity is confirmed and studied. From the literature, it has shown that the photoconductivity of trimethyl phenyl diamine (TPD) increases in the air and decreased in the atmosphere of the vacuum. To the best of our knowledge, there is no detailed study of the effects of gas in the air that affect TPD photoconductivity. In this study we investigate the effects of nitrogen (N2) and oxygen (O2) gases on photoconductivity, degradation and residual decay of photoconductivity for thin film TPD. The results of the study show that in the atmosphere of O2, TPD produces about seven times higher photoconductivity compared to N2 conditions. It also shows that, N2 and O2 provide more effective response time during photoconductivity residual decay. Photoconductivity degradation occurs in all conditions and its recovery takes more than 65 hours.

Metastable light induced effects in pentacene

In this study, one of the environmental factor which could affect organic materials was analyzed. Pentacene thin film samples were fabricated and the stability of their electrical properties was monitored when the devices were exposed to different energy light sources. Steady-state photocurrent spectrum and transient behavior were investigated in order to explore the dispersive transport mechanism and the carrier trapping and recombination effects. Anomalous phenomena were revealed for effect of a persistent ultraviolet (UV) irradiation, in particular a photoconductivity degradation under light conditions and a dark recovery, whose evolutions were studied focusing on the metastable nature of defects. The results were reported in terms of a trap density model, which provides a description of the dynamics of light induced electrically active defects in an organic semiconductor.

Photoconductivity of amorphous As2S8 chalcogenide film under bandgap light irradiation

The photoconductivity of amorphous As2S8 chalcogenide film under the irradiation of bandgap light is investigated. In the temperature range 300–350 K, the dark conductivity and photoconductivity of the annealed As2S8 film increase with the temperature, and the dependence of the both on temperature shows that the conduction in As2S8 film is an activated process having single activation energy. Under the irradiation of bandgap light, the photocurrents of the annealed and illuminated As2S8 film increase with the irradiation intensity, and their difference indicates the existence of the light-soaked effect. Meanwhile, the photoconductivity degradation during the irradiation and the photocurrent decay after stopping the irradiation are observed. By adding the irradiation of the sub-bandgap light, the enhancement of photoinduced voltage occurs.

The light stability of microcrystalline silicon thin films deposited by VHF–PECVD method

Microcrystalline silicon thin film is deposited under different conditions by plasma enhanced chemical vapor deposition. The light stability with different crystallinity and grain size is studied, and the growth mechanism is analyzed using the scaling behavior of roughening surface evolution. Degradation of photoconductivity mainly depends on crystallinity and grain size, but fundamentally, on the growth mechanism. Materials with high crystallinity and large grain size are more stable under light soaking. With the increasing of deposition pressure and input power, growth process transfers to zero diffusion limit growth mechanism, and films deposited present less grain size and poor light stability.

Light induced degradation in nanocrystalline Si films and related solar cells: Role of crystalline fraction

Silicon thin films with different crystalline volume fractions have been deposited at different power and pressure conditions. Structural properties of the films have been investigated. The effects of crystalline volume fractions and grain sizes on the degradation of photoconductivity have been studied. Single-junction solar cells have been fabricated with protocrystalline and nanocrystalline Si as absorber layer. Protocrystalline silicon solar cells show less than 1% degradation upto 50 h of light soaking. Then the cells degrade upto 500 h and thereafter become steady. Nanocrystalline solar cells show degradation initially and become steady after 10 h of light soaking. Using protocrystalline silicon as absorber layer the solar cell efficiency degrades 9% before stabilization, whereas using nanocrystalline silicon as absorber layer ( X c∼65%) the solar cell efficiency degrades 2.9%. Stabilized efficiency of the second type of cell is better than that of the first cell, but initial efficiency is higher for the first cell ( η=7.1%).

Nanocrystalline silicon: A material for thin film solar cells with better stability

Nanocrystalline silicon thin films have been deposited under different conditions by plasma enhanced chemical vapor deposition. The variation of light induced degradation of photoconductivity with crystalline volume fraction, grain size, microstructural defect, bonded hydrogen content and defect density have been studied in detail. Degradation of photoconductivity mainly depends on crystallinity, grain size and defect density. Degradation of photoconductivity does not directly depend upon the bonded hydrogen content. Materials with high crystallinity and low degradation do not produce best quality solar cells but absorber layers with moderate crystallinity with slightly lower stability produce better quality solar cells with around 2.8% light induced degradation of efficiency.

Deposition of hydrogenated amorphous silicon (a-Si:H) films by hot-wire chemical vapor deposition (HW-CVD) method: Role of substrate temperature

Hydrogenated amorphous silicon (a-Si:H) thin films were deposited from pure silane (SiH 4) using hot-wire chemical vapor deposition (HW-CVD) method. We have investigated the effect of substrate temperature on the structural, optical and electrical properties of these films. Deposition rates up to 15 Å s −1 and photosensitivity ∼10 6 were achieved for device quality material. Raman spectroscopic analysis showed the increase of Rayleigh scattering in the films with increase in substrate temperature. The full width at half maximum of TO peak ( Γ TO) and deviation in bond angle (Δ θ) are found smaller than those obtained for P-CVD deposited a-Si:H films. The hydrogen content in the films was found <1 at% over the range of substrate temperature studied. However, the Tauc's optical band gap remains as high as 1.70 eV or much higher. The presence of microvoids in the films may be responsible for high value of band gap at low hydrogen content. A correlation between electrical and structural properties has been found. Finally, the photoconductivity degradation of optimized a-Si:H film under intense sunlight was also studied.

Semi-quantitative study on the Staebler–Wronski effect of hydrogenated amorphous silicon films prepared with HW-ECR-CVD system

The method of numerical simulation is used to fit the relationship between the photoconductivity in films and the illumination time. The generation and process rule of kinds of different charged defect states during illumination are revealed. It is found surprisingly that the initial photoconductivity determines directly the total account of photoconductivity degradation of sample.

Optical feedback system with integrated color sensor on LCD

TFT LCDs have the largest market share of all digital flat panel displays. A three color RGB LED LCD backlighting system is very attractive considering wide color gamut, tunable white point, high dimming ratio, long lifetime and environmental compatibility. But the high intensity LED has thermal and time-based dependencies. Color and white luminance levels are not stable over a wide range of temperature due to inherent long-term aging characteristics. In order to minimize color point and brightness differences over time, optical feedback control is a key technology for any LED backlighting system. In this paper, we present the feasibility of an optical color sensing feedback system for an LED backlight by integrating the amorphous silicon color sensor onto the LCD panel. To improve the photoconductivity degradation of amorphous silicon, a new LASER immersion treatment has been applied. The integrated color sensor optical feedback controlled LED backlighting system improved the color variation to less than 0.008 Δu′v′ (CIE1976) compared with 0.025 for an open loop system over the temperature range of 42–76 °C.

LED-backlight feedback control system with integrated amorphous-silicon color sensor on an LCD panel

— Thin-film-transistor liquid-crystal displays (TFT-LCDs) have the largest market share of all digital flat-panel displays. An LCD backlighting system employing a three-color red-green-blue light-emitting diode (RGB-LED) array is very attractive, considering its wide color gamut, tunable white point, high dimming ratio, long lifetime, and environmental compatibility. But the high-intensity LED has problems with thermal stability and degradation of brightness over time. Color and white luminance levels are not stable over a wide range of temperature due to inherent long-term aging characteristics. In order to minimize color point and brightness differences over time, optical feedback control is the key technology for any LED-backlight system. In this paper, the feasibility of an optical color-sensing feedback system for an LED backlight by integrating the amorphous-silicon (a-Si) color sensor onto the LCD panel will be presented. To minimize the photoconductivity degradation of a-Si, a new laser exposure treatment has been applied. The integrated color-sensor optical-feedback-controlled LED-backlight system minimized the color variation to less than 0.008 Δu'v' (CIE1976) compared to 0.025 for an open-loop system over the temperature range of 42–76°C.

40.1: Distinguished Contributed Paper: Integrated Amorphous Silicon Color Sensor on LCD Panel for LED Backlight Feedback Control System

In this paper, we present the feasibility of an optical color sensing feedback system for an LED backlight by integrating the amorphous silicon color sensor onto the LCD panel. To improve the well-known photoconductivity degradation of amorphous silicon, a new LASER immersion treatment has been applied. The integrated color sensor optical feedback controlled LED backlighting system improved the color variation to less than 0.008 Δu‘v’ (CIE1976) compared with 0.025 for an open loop system over the temperature range of 42°C to 76°C.

Fingerprints of two distinct defects causing light-induced photoconductivity degradation in hydrogenated amorphous silicon

We find distinct experimental fingerprints of two metastable defects created during illumination of hydrogenated amorphous silicon. The well-studied threefold-coordinated silicon dangling bond defect has an anneal activation energy near 1.1 eV and dominates annealing experiments above about 110 °C. The second defect created by illumination is the “primary recombination” (pr) center, which causes most of the light-induced photoconductivity decrease and dominates annealing experiments below about 110 °C. Because the pr centers are created in linear proportion to the dangling bond defects, they are difficult to distinguish during degradation. However, we observe clear fingerprints of the pr center during their low T annealing: (1) an anneal activation energy of 0.85 eV; (2) a sharp increase in photoconductivity; and (3) a surprising increase in hν⩽1.1 eV optical absorption.

New experiments on the relationship between light-induced defects and photoconductivity degradation

ABSTRACTWe report experimental results that help settle apparent inconsistencies in earlier work on photoconductivity and light-induced defects in hydrogenated amorphous silicon (a-Si:H) and point toward a new understanding of this subject. After observing that light-induced photoconductivity degradation anneals out at much lower T than the light-induced increase in deep defect density, Han and Fritzsche[1] suggested that two kinds of defects are created during illumination of a-Si:H. In this view, one kind of defect degrades the photoconductivity and the other increases defect sub-bandgap optical absorption. However, the light-induced degradation model of Stutzmann et al.[2] assumes that photoconductivity is inversely proportional to the dangling-bond defect density. We observe two kinds of defects that are distinguished by their annealing activation energies, but because their densities remain in strictlinear proportionduring their creation, the two kinds of defects cannot be completely independent.In our measurements of photoconductivity and defect absorption (constant photocurrent method) during 25°C light soaking and during a series of isochronal anneals between 25 &lt; T &lt; 190°C, we find that the absorption measured with E ≤1.1 eV, first increases during annealing, then exhibits the usual absorption decrease found for deeper defects. The maximum in this absorption at E ≤1.1eV occurs simultaneously with a transition from fast to slow recovery of photoconductivity. The absorption for E ≤1.1eV showstwodistinct annealing activation energies: the signal rises with about 0.87 eV and falls with about 1.15 eV. The 0.87 eV activation energy roughly equals the activation energy for the dominant, fast, recovery of photoconductivity. The 1.15 eV activation energy roughly equals the single activation energy for annealing of the light-induced dangling bond absorption.

水素化アモルファスシリコンの光劣化と光誘起構造変化

Reversible photo-induced change has been observed in the polarization dependence of transverse electroabsorption signal in hydrogenated amorphous silicon (a-Si: H), which is indicative of structural change occurred in the whole material. The structural change in a-Si: H saturates in a few minutes upon light exposure, which is a short time in comparison with the saturation time of photoconductivity degradation and more than 10 hours' annealing is necessary for restoring to initial state. Accounting for the results of various investigations, photo-induced structural change may occur through the following processes: (1) The structural disorder increases with light exposure and is observed as a PEA. In particular regions, the local network distortion becomes significant and causes surround network to distort. The strained region enlarges by creating metastable strained bonds. (2) Bond breaking is caused by the collision of such a strained region with another such region. Since the region of intersection will be the most highly strained region. (3) The produced dangling bond pair move through such a bond switching process to reduce strain energy. (4) The PEA ratio saturates when the strained regions cover almost the whole volume. However the density and internal stress will not saturate until they reach a averaged state of distortion distributed throughout the material. Therefore the time to reach the averaged state is considered to depend on the light exposure history. Because the size and distribution of strained region may depend on their consecutive growth time. The breaking process continues with collisions of the strained region in smaller and smaller un-affected parts of the material. On the other hand, in annealing process, the structural restoration begins with the most highly strained regions which is located to near the dangling bond defects.

Photodegradation in a-Si:H Prepared by Hot-Wire CVD as a Function of Substrate and Filament Temperatures

ABSTRACTWe have studied light-soaking effects, such as photoconductivity (PC) degradation kinetics, the changes of conductivity activation energy, Ea, and the defect density of states (DOS) in a-Si:H films deposited by hot-wire CVD. Films were deposited in a substrate temperature range from 280 to 440 °C for filament temperatures of 1900 and 2100 °C. We find that (a) the photodegradation kinetics does not follow the stretched exponential rule for all of the samples; (b) the Fermi level position moves up after light-soaking for most samples; and (c) the metastable defect DOS deduced from sub-band gap absorption is not consistent with that deduced from the electron mobility-lifetime product. The results are discussed according to the possible mechanism in which charged defects exist in hot-wire a-Si:H films.a

Properties of a-SiO:H films prepared by RF glow discharge

Hydrogenated amorphous silicon oxygen alloy (a-SiO:H) films have been prepared by RF plasma enhanced chemical vapour deposition from (SiH 4+CO 2+H 2) gas mixture. Films have been characterized, in detail, by electrical, optical as well as structural studies. The effect of the oxygen incorporation into the Si-network was studied by controlling various deposition parameters e.g., CO 2 to SiH 4 flow ratio, H 2 dilution of the plasma, total flow rate of the reacting gases, RF power applied to the electrodes, working gas pressure in the plasma chamber and the substrate temperature. Optical gap of the films increased due to the incorporation of O, and a lowering in photoconductivity with optical gap widening was monitored. Increasing polyhydrogenation at higher O-content resulted in a rise in defect density. O-incorporation into the Si-network increased the light-induced degradation in photoconductivity.

Radiation-induced two-step degradation of Si photoconductors and space solar cells

Since two-step degradation, i.e., gradual degradation at low fluence and steep at higher fluence, was found in Si space solar cells, extensive studies have been conducted to clarify the mechanism. Recent studies indicate that the steep degradation results from a bulk process. Either decrease in carrier mobility or carrier exhaustion has been proposed for the mechanism. Independently of the device degradation, we observed a similar two-step degradation in photoconductivity of bulk Si, and have investigated the relevant phenomena. Taking advantage of the bulk simplicity, the steep degradation mechanism has been experimentally evaluated. Consequently, a possibility of mobility change is excluded. A rapid decrease in photocarrier lifetime is evident, though the carrier exhaustion simultaneously occurs. The drop of lifetime is explained in terms of Fermi-level-dependent recombination cross-section of deep centers. As the Fermi-level jump is induced by the carrier exhaustion, the cross-section may steeply increase. The origin of two-step degradation in the solar cells is also discussed, based on the bulk degradation.

Light-induced recombination centers in hydrogenated amorphous silicon–sulfur alloys

Light-induced degradation in photoconductivity, photoluminescence, electron spin resonance (ESR), and subgap optical absorption has been performed on a sample of hydrogenated amorphous silicon–sulfur (a-SiSx:H) with x∼0.02. After light soaking of this sample, there are no changes, within errors of measurement, in (1) the power exponent, γ, for the intensity dependence of the photoconductivity, (2) the intensity of the defect luminescence near 0.8 eV, and (3) the dark ESR signal. However, the deep defect density, as inferred from subgap optical absorption, increases with light soaking.

Staebler-Wronski effect as a function of the Fermi level position and structure of nondoped, amorphous, hydrated silicon

The photoconductivity degradation rates γ (σph∼t−γ) of nondoped, amorphous, hydrated silicon films deposited at Ts=300–400 °C and subjected to illumination for 5 h at 300 K (light source 100 mW/cm2, λ<0.9 µm) were investigated. It was shown that the degradation rate γ depends on the preillumination position of the Fermi level ɛc−ɛF and often is not directly related to the hydrogen content in the film. It was found that there are correlations between the value of γ and the bonds in the silicon-hydrogen subsystem [isolated SiH and SiH2 complexes, clusters (SiH)n, and chains (SiH2)n].