The lateral photovoltaic effect in CdS-Cu2S heterojunction solar cell

The lateral photovoltaic effect has been observed in CdS-Cu₂S thin-film solar cells. The effect is more pronounced on the CdS side than on the Cu₂S side of the cells. On the CdS side, where the contacts were formed by soldering Cu wire by indium and then applying Ag paint, the photovoltage developed were found to increase as the point of illumination was moved towards the contact. The spectral response of photovoltage for coevaporated cells shows a peak at=0.5m (2.45 eV). But for topotaxial cells two peaks, one at=0.5m and the other at=0.65m (1.89eV) were observed. A band model has been proposed for the heat-treated optimized cells.     

Microcrystalline silicon and micromorph tandem solar cells

“Micromorph” tandem solar cells consisting of a microcrystalline silicon bottom cell and an amorphous silicon top cell are considered as one of the most promising new thin-film silicon solar-cell concepts. Their promise lies in the hope of simultaneously achieving high conversion efficiencies at relatively low manufacturing costs. The concept was introduced by IMT Neuchatel, based on the VHF-GD (very high frequency glow discharge) deposition method. The key element of the micromorph cell is the hydrogenated microcrystalline silicon bottom cell that opens new perspectives for low-temperature thin-film crystalline silicon technology. According to our present physical understanding microcrystalline silicon can be considered to be much more complex and very different from an ideal isotropic semiconductor. So far, stabilized efficiencies of about 12% (10.7% independently confirmed) could be obtained with micromorph solar cells. The scope of this paper is to emphasize two aspects: the first one is the complexity and the variety of microcrystalline silicon. The second aspect is to point out that the deposition parameter space is very large and mainly unexploited. Nevertheless, the results obtained are very encouraging and confirm that the micromorph concept has the potential to come close to the required performance criteria concerning price and efficiency. Received: 1 March 1999 / Accepted: 28 March 1999 / Published online: 1 July 1999     

Enhanced photovoltaic properties of polymer-fullerene bulk heterojunction solar cells by thermal annealing

The effect of a thermal annealing treatment on the performance of bulk heterojunction photovoltaic cells based on poly[2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene] (MEH-PPV) and fullerene (C₆₀) composites is investigated. Upon thermal annealing at 120 ^°C, short-circuit current and power conversion efficiency (η) are more than tripled, while a sharp rise by eight times in and η is found for the device annealed at 200 ^°C. It is concluded that the improved phase separation between MEH-PPV and C₆₀ leads to the enhancement of and η at 120 ^°C, while thermodynamic molecule arrangement at the higher temperature of ∼200 ^°C induces a significant increase in all photovoltaic parameters of composite devices except the open-circuit voltage .     

CuGaSe2 solar cells with 9.7% power conversion efficiency

Heterojunctions, such as ZnO/CdS/CuGaSe₂, were fabricated for photovoltaic applications. Optimization of device structures based on monocrystalline CuGaSe₂ led to the highest-to-date power conversion efficiencies for CuGaSe₂ solar cells. At room temperature under 100 mW/cm² AM1.5 illumination a maximum cell efficiency of 9.7% was achieved, given by an open-circuit voltage of 946 mV, a short circuit current density of 15.5 mA/cm², and a fill factor of 66.5%. Preparation and performance of the optimum device are described. Current voltage characteristics dependent on illumination intensity and temperature, spectral response and electron-beam-induced current measurements were performed to determine the device parameters as well as to analyse the current transport and loss mechanisms. Tunneling, assisted by defect levels in the CdS layer, seems to play a major role. High injection effects are observed at forward bias ofV > 0.5 V or an illumination level ofP > 10 mW/cm². Under such conditions, as well as at low temperatures, the non-zero series resistance comes into play. Effects of the shunt resistance, however, are negligible in all cases.     

Photoconductive response times of Si-on-sapphire damaged with Si28 ions

We present autocorrelation measurements showing the high-speed sampling and switching capabilities of SOS photoconducting elements which have been ion implanted with 60 and 160 keV Si²⁸ ions. An analysis of the circuit shows the intrinsic photoconductive decay to be very fast (3.5 ps) and that the measured response is primarily limited by the gap capacitance and the associated R-C time.     

Effect of signal-modulated optical radiation on the characteristics of a Modfet

The effect of signal-modulated optical radiation on the characteristics of a GaAlAs/GaAs MODFET has been studied analytically. It is found that the offset voltage increases with modulation frequency and the effect of frequency is negligible above 5 MHz. The drain-source current decreases with increase in signal frequency at a constant radiation flux density, doping concentration and drain-source voltage. Studies on sheet concentration and transconductance also show that the signal frequency has a significant effect upto a certain modulation frequency (5 MHz) above which the effect of frequency is insignificant.     

On the thermodynamic limit of photovoltaic energy conversion

An infinite stack ofp—n junctions with smoothly varying bandgap from ∞ to 0 is considered. AnI —V characteristic is derived, which is more correct than the classical exponential characteristic. It is shown that open-circuit operation is a reversible process and leads to the Carnot efficiency, if one defines the efficiency in the way that is usual in the theory of thermodynamic engines. If instead one uses the definition of efficiency usual in photovoltaics, open-circuit mode gives rise to zero efficiency. Then operation at maximum efficiency equals operation at maximum power and is not reversible.     

Reliability studies on MIS solar cells

Research on Cr/oxide/p-type Si solar cells has produced a 12.2% efficiency on 2 cm² area. Reliability studies have been conducted to determine if degradation occurs during use in an extreme environment. Several cells with A/R coatings and encapsulation have been tested with degradation occurring in some cases after 2 years of use. One cell without an A/R coating was used for 1+1/2 years with degradation occurring only after a crack had appeared. Dark I–V curves, photovoltaic data, and Auger analysis show a quite stable situation to exist. A stable MIS solar cell depends on selection of the insulator, deposition rate of the metal, elimination of moisture, selection of the A/R coating, bonding techniques, and choice of encapsulant. This work points to an optimistic view towards the fabrication of stable terrestrial MIS solar cells. Research supported by the NSF-RANN Program with technical supervision by the Department of Energy.     

Effect of optical radiation in a photo-DOVATT

Avalanche photodetectors are very important solid-state detectors currently used in long-distance and wide-band optical communication systems, due to their faster speed of response compared to other solid-state photodiodes. Furthermore, it has been found that by using heterostructures one can improve both multiplication gain and quantum efficiency of such a device. DOVATT is a heterojunction impatt device in which there is one avalanche zone followed by two drift zones at different scattering limited velocities. The device is very useful for generation of high power in the X-band.The present paper examines the effect of optical radiation on such a device. Studies have been made on the frequency-response characteristics of the device. The results show that the device has the potentiality of becoming a powerful photodetector in optical communication systems.     

The performance of Hg1−x Zn x Te photodiodes

This paper considers the Hg_1–x Zn _x Te alloy system as a potential material for the fabrication of infrared photodiodes. The influence of different junction current components (diffusion, tunneling and depletion layer currents) on the R ₀ A product of n⁺-pHg_1–x Zn _x Te photodiodes is analysed. The upper theoretical limits of the R ₀ A product and detectivity are determined. Results of calculations are compared with experimental data reported by other authors and those measured in our laboratory. Preliminary results on related technology and the properties of Hg_1–x Zn _x Te prepared by the ion-etching technique are presented.     

Pb2CrO5 photovoltaic device for the detecting light-beam position

A light position detector operating through a photovoltaic effect of a Pb₂CrO₅ ceramic disk with a pair of Au planar electrodes is investigated. A fabrication technique and the basic characteristics of the photovoltaic device for position detection are described. A peak photovoltage is obtained around the edge of the electrode for an incident light beam. The incident light beam shape and the electrode pattern are important factors for obtaining a linear relation between the light-beam position and the output signal from the device. A device fabricated for detecting one-dimensional light position has a high position resolution of 0.5 m and a good linearity of ±2 m or less. A two-dimensional device can be fabricated in the same way as the one-dimensional device, except for the electrode pattern. A method for two-dimensional light position detection using a Pb₂CrO₅ photovoltaic cell is demonstrated for a green LED as a light source.     

Thin-film poly-Si solar cells on glass substrate fabricated at low temperature

The performances of thin-film poly-Si solar cells with a thickness of less than 5 μm on a glass substrate have been investigated. The cell of glass/back reflector/n-i-p type Si/ITO is well characterized by the structure of naturally surface texture and enhanced absorption with a back reflector (STAR), where the active i-type poly-Si layer was fabricated by plasma chemical vapor deposition (CVD) at low temperature. The cell with a thickness of 2.0 μm demonstrated an intrinsic efficiency of 10.7% (aperture 10.1%), an open-circuit voltage of 0.539 V and a short-current density of 25.8 mA/cm² as independently confirmed by Japan Quality Assurance. No light-induced degradation is observed. The optical and transport properties of poly-Si cells are summarized. Received: 1 March 1999 / Accepted: 28 March 1999 / Published online: 7 July 1999     

High performance a-Si solar cells and new fabrication methods for a-Si solar cells

The super chamber, a separated UHV reaction-chamber system has been developed. A conversion efficiency of 11.7% was obtained for an a-Si solar cell using a high-quality i-layer deposited by the super chamber, and a p-layer fabricated by a photo-CVD method.As a new material, amorphous superlattice-structure films were fabricated by the photo-CVD method for the first time. Superlattice structure p-layer a-Si solar cells were fabricated, and a conversion efficiency of 10.5% was obtained.For the fabrication of integrated type a-Si solar cell modules, a laser pattering method was investigated. A thermal analysis of the multilayer structure was done. It was confirmed that selective scribing for a-Si, TCO and metal film is possible by controlling the laser power density. Recently developed a-Si solar power generation systems and a-Si solar cell roofing tiles are also described.     

High-resolution mapping of the energy conversion efficiency of solar cells and silicon photodiodes in photovoltaic mode

We demonstrate an optical technique to derive the two-dimensional energy conversion efficiency (η_CE), fill factor (FF) and external quantum efficiency (η_QE) distributions across the surface of photovoltaic devices. A compact, inexpensive optical-feedback laser diode microscope is constructed to acquire the confocal reflectance and efficiency maps enabling the observation of the local parametric behavior in silicon photodiodes in photovoltaic mode and single-junction solar cells. The η_CE and η_QE distributions are greatly influenced by local parasitic resistances that depend on laser irradiance. These parasitic resistances decrease the η_CE and η_QE values with distance from the contact electrode at high laser irradiance. The optical technique enables microscopic comparison of η_CE and η_QE within the pn-overlay region of the photodiode sample, revealing its optimization for photodetection rather than power generation. The technique also elucidates the decreasing local η_CE of the solar cell under intense irradiation.     

The electrical performance of thinn +-window layers ina-Si: H solar cells

The electrical properties ofn ⁺-window layers inp-i-n a-Si:H solar cells were characterised as a function ofn ⁺-layer thickness, , by measuring firstly the activation energyE _a of the dark conductivity and secondly the built-in potentialV _bi of the cells.E _a was found to increase with decreasing attaining values as high as 0.8 eV for 5nm; bulk values, e.g.E _a . 2eV in the amorphous andE _a<0.01 eV in the microcrystalline case, were only observed for >20nm and for >200nm, respectively. In contrast,V _bi did not depend on at all and was further found to be consistent with expectations based on the Fermi level positions in bulkn ⁺ andp ⁺-material. As a consequenceE _a in very thin films can no longer be considered as a measure of (E _C –E _F), the distance of the Fermi level from the conduction band edge. The apparent inconsistency inherent to theE _a and theV _bi results can be resolved by assuming that the deposition of then ⁺-material proceeds via the growth and coalescence of small islands.     

The influence of interband tunneling on leakage current in manganite/titanate heterojunction

The behavior of leakage current at reverse bias in p-La_0.9Sr_0.1MnO₃/n-SrNb_0.01Ti_0.99O₃ heterojunction has been theoretically studied by calculating interband tunneling current with various doping densities and temperatures. Our results reveal that the reduction of leakage current with decrease of doping density and increase of temperature originates from properties of interband tunneling.     

Theoretical analysis of trapping and recombination of photogenerated carriers in amorphous silicon solar cells

We have made theoretical studies on the trapping and recombination of photogenerated carriers in hydrogenated amorphous silicon (a-SiH) p-i-n solar cells. We discuss in detail the following points: 1) The limitations of the assumptions in the previous analysis. It has been clarified that the single-level Shockley-Read-Hall model for carrier recombination and the treatment of trap occupation in terms of quasi-Fermi levels are inadequate for exact analysis. 2) The superlinear dependence of carrier recombination rate on the free-carrier density which can explain the enhancement of photo-induced changes ina-SiH under high intensity light. 3) The estimation of capture cross section of the tail states ina-SiH. We show that the charged and neutral tail states have rather small capture cross sections of less than 10^–16 cm² and of less than 10^–19 cm², respectively. 4) The effect of the recombination of photogenerated (PG) carriers at the p/i and the n/i interfaces. We estimate the recombination velocityS of PG carriers at these interfaces to be about 10³ cm/s. It has been also clarified that the decrease inS is effective to improve the cell performance, especially the open circuit voltage.     

Effect of an Al-doped ZnO electron transport layer on the efficiency of inverted bulk heterojunction solar cells

Doping is a widely-implemented strategy for enhancing the inherent electrical properties of metal oxide charge transport layers in photovoltaic devices because higher conductivity of electron transport layer (ETL) can increment the photocurrent by reducing the series resistance. To improve the conductivity of ETL, in this study we doped the ZnO layer with aluminum (Al), then investigated the influence of AZO on the performance of inverted bulk heterojunction (BHJ) polymer solar cells based on poly [[4,8-bis [(2-ethylhexyl)oxy]benzo [1,2-b:4,5-b’]dithiophene-2,6-diyl]-[3-fluoro-2[(2-ethylhexyl)-carbonyl]-thieno-[3,4-b]thiophenediyl ]] (PTB7):[6,6]-phenyl C71 butyric acid methyl ester (PC₇₁BM). The measured conductivity of AZO was ~10⁻³ S/cm, which was two orders of magnitude higher than that of intrinsic ZnO (~10⁻⁵ S/cm). By decreasing the series resistance (R_s) in a device with an AZO layer, the short circuit current (J_sc) increased significantly from 15.663 mA/cm² to 17.040 mA/cm². As a result, the device with AZO exhibited an enhanced power conversion efficiency (PCE) of 8.984%.     

Theoretical analysis on the limitations of the open-circuit voltage of a hydrogenated amorphous silicon p-i-n solar cell

We have made theoretical studies on the limitation of the open-circuit voltageV _oc of a hydrogenated amorphous silicon (a-Si:H) p-i-n type solar cell. The effects of the tail states in the a-Si:H i layer and of the interface recombination are discussed in detail. The opencircuit voltage increases when the distribution of the tail states is sharp and/or the capture cross sections of these states are small. This is because the recombination rate of photogenerated carriers and/or the density of space charge due to trapped carriers in these states become low in these conditions. These effects of the tail states on the value ofV _oc become pronounced when the built-in potential of the p-i-n junction is high. The decrease in the effective recombination velocity of carriers at the p/i and n/i interfaces results in an increase ofV _oc. This increase becomes remarkable when the effects of the tail states on the value ofV _oc are small. Both the sharp distribution of tail states and the small value of the interface recombination velocity are necessary to increase considerably the value ofV _oc. We show the conditions of the material parameters necessary to obtain an open-circuit voltage of 1.0 V.     

The origin of reduced fill factors of emitter‐wrap‐through‐solar cells

Low fill factors generally limit the efficiency of emitter‐wrap‐through (EWT) solar cells. Until now, a conventional series resistance limitation along the laser‐drilled EWT vias has usually been assumed to be responsible for this effect. We demonstrate that the characteristic fill factor loss is caused by a crucial change in the diffusion currents inside the base, which are influenced by the conductivity along the laser‐drilled EWT vias. In addition, we show that the EWT via conductivity influences the fill factor loss caused by an iron contaminated base. This result affects the proposition that the EWT design is suitable for multicrystalline silicon in which interstitial iron is known to be the main contaminant. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)