Heterojunction Contact Research Articles

Back Contact Heterojunction Solar Cells Patterned by Laser Ablation

Back contact heterojunction (IBC-HIT) solar cells is one of the most promising technology for the upcoming generations of high efficiency crystalline-Silicon (c-Si) based photovoltaic modules [1]. However, the industrialization of the IBC-HIT technology is actually constrained by the complexity of the back side cell processing, which usually involves costly and time consuming photolithography steps. CEA-INES is currently developing a method based only on laser ablation for the structuration of IBC-HIT solar cells [2]. Laser ablation is indeed a fast and low cost technique that also allows the patterning of the back side amorphous (a-Si:H) layers on large area IBC-HIT solar cells. However laser irradiation might induce some damage at the c-Si/a-Si:H interface thus limiting the final performance of the devices. In this work, we compare the results obtained with our laser patterning process for different stack configurations and laser ablation conditions (532nm and 355nm). We will also discuss about the criteria used for the choice of the different materials and the laser ablation conditions needed in order to successfully pattern both the emitter and BSF (Back Surface Field) regions of the cell. Our best cell efficiency achieved up to now is 20.55% on an area of 18.11 cm2.

Titanium dioxide/silicon hole-blocking selective contact to enable double-heterojunction crystalline silicon-based solar cell

In this work, we use an electron-selective titanium dioxide (TiO2) heterojunction contact to silicon to block minority carrier holes in the silicon from recombining at the cathode contact of a silicon-based photovoltaic device. We present four pieces of evidence demonstrating the beneficial effect of adding the TiO2 hole-blocking layer: reduced dark current, increased open circuit voltage (VOC), increased quantum efficiency at longer wavelengths, and increased stored minority carrier charge under forward bias. The importance of a low rate of recombination of minority carriers at the Si/TiO2 interface for effective blocking of minority carriers is quantitatively described. The anode is made of a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) heterojunction to silicon which forms a hole selective contact, so that the entire device is made at a maximum temperature of 100 °C, with no doping gradients or junctions in the silicon. A low rate of recombination of minority carriers at the Si/TiO2 interface is crucial for effective blocking of minority carriers. Such a pair of complementary carrier-selective heterojunctions offers a path towards high-efficiency silicon solar cells using relatively simple and near-room temperature fabrication techniques.

Graphene/MoS2 hybrid technology for large-scale two-dimensional electronics.

Two-dimensional (2D) materials have generated great interest in the past few years as a new toolbox for electronics. This family of materials includes, among others, metallic graphene, semiconducting transition metal dichalcogenides (such as MoS2), and insulating boron nitride. These materials and their heterostructures offer excellent mechanical flexibility, optical transparency, and favorable transport properties for realizing electronic, sensing, and optical systems on arbitrary surfaces. In this paper, we demonstrate a novel technology for constructing large-scale electronic systems based on graphene/molybdenum disulfide (MoS2) heterostructures grown by chemical vapor deposition. We have fabricated high-performance devices and circuits based on this heterostructure, where MoS2 is used as the transistor channel and graphene as contact electrodes and circuit interconnects. We provide a systematic comparison of the graphene/MoS2 heterojunction contact to more traditional MoS2-metal junctions, as well as a theoretical investigation, using density functional theory, of the origin of the Schottky barrier height. The tunability of the graphene work function with electrostatic doping significantly improves the ohmic contact to MoS2. These high-performance large-scale devices and circuits based on this 2D heterostructure pave the way for practical flexible transparent electronics.

The Investigation of a New NO<sub>2</sub> OTFT Sensor Based on Heterojunction F<sub>16</sub>CuPc/CuPc Thin Films

The bottom contact heterojunction organic thin film transistors (OTFTs) based on n-type hexadecafluorophthalocyaninatocopper (F16CuPc) and p-type copper phthalocyanine (CuPc) bilayer were developed by the vacuum evaporation, which were applied to detect nitrogen dioxide (NO2). The sensors with different thickness (5nm, 10nm, 15nm and 20nm) of CuPc were prepared to investigate the influence of CuPc film thickness on the properties of devices. The results showed that four parameters including the source-drain current (IDS), grid current (IGS), threshold voltage (VT) and carrier mobility (μ) changed in a few seconds when the sensors were exposed to the atmosphere of NO2. Further more, IDS and IGS presented extremely similar variation trend. So the grid current would be taken as a new parameter to reveal the response characteristic of OTFT gas sensor. By comparison, the device with 15nm CuPc thin film exhibited the optimum electronic and gas sensing properties.

Experimental and simulated analysis of front versus all-back-contact silicon heterojunction solar cells: effect of interface and doped a-Si:H layer defects

Front silicon heterojunction and interdigitated all-back-contact silicon heterojunction (IBC-SHJ) solar cells have the potential for high efficiency and low cost because of their good surface passivation, heterojunction contacts, and low temperature fabrication processes. The performance of both heterojunction device structures depends on the interface between the crystalline silicon (c-Si) and intrinsic amorphous silicon [(i)a-Si:H] layer, and the defects in doped a-Si:H emitter or base contact layers. In this paper, effective minority carrier lifetimes of c-Si using symmetric passivation structures were measured and analyzed using an extended Shockley–Read–Hall formalism to determine the input interface parameters needed for a successful 2D simulation of fabricated baseline solar cells. Subsequently, the performance of front silicon heterojunction and IBC-SHJ devices was simulated to determine the influence of defects at the (i)a-Si:H/c-Si interface and in the doped a-Si:H layers. For the baseline device parameters, the difference between the two device configurations is caused by the emitter/base contact gap recombination and the back surface geometry of IBC-SHJ solar cell. This work provides a guide to the optimization of both types of SHJ device performance, predicting an IBC-SHJ solar cell efficiency of 25% for realistic material parameters. Copyright © 2013 John Wiley & Sons, Ltd.

Silicon CMOS Ohmic Contact Technology for Contacting III-V Compound Materials

Silicon (Si)-encapsulated III-V compound (III-V) device layers enable Si-complementary metal-oxide semiconductor (CMOS) friendly ohmic contact formation to III-V compound devices, allowing for the ultimate seamless planar integration of III-V and Si CMOS devices in a common fabrication infrastructure. A method of making ohmic contacts to buried III-V films using silicide metallurgies has been established. NiSi/Si/III-V dual heterojunction contact structures are found to be optimal for integration. These structures allow contact resistivities to be controlled by Si/III-V interfaces and eliminate interactions between the buried III-V device and metal layers. Using a modified transmission line method (TLM) test structure fabricated using standard CMOS processing techniques, the specific contact resistivities of Si/GaAs and Si/InxGa1-xAs interfaces are extracted. The relationship between specific contact resistivity and heterojuntion barrier width is considered. Among the structures tested in this work, p-type Si/GaAs and n-type Si/InxGa1-xAs yielded the lowest contact resistivities. Using the p-type Si/GaAs interface, a GaAs/AlxGa1-xAs laser with NiSi top contact is demonstrated, confirming the feasibility of NiSi/Si/III-V contact structures for III-V devices.

Heterojunction bipolar transistors with hydrogenated amorphous silicon contacts on crystalline silicon

Heterojunction bipolar transistors with hydrogenated amorphous silicon (a-Si:H) contact layers on crystalline silicon (c-Si) substrates are reported. In particular, current gains exceeding 500 were achieved by heterojunction contacts further including embedded homo-junctions comprised of hydrogenated crystalline silicon (c-Si:H), having thicknesses much shorter than the diffusion length of minority carriers. The a-Si:H and c-Si:H layers were grown by plasma-enhanced chemical vapour deposition at temperatures close to 200°C.

Heterojunction bandgap engineered photodetector based on type-II InAs/GaSb superlattice for single color and bicolor infrared detection

We report a heterostructure bandgap engineered strained layer superlattice photodetector design for performance improvement for longwave infrared (LWIR) detection. At 77 K, the dark current density of the reported device was at least two orders of magnitude lower than that of the conventional PIN superlattice photodiode. We have obtained a shot-noise limited detectivity of 8.7 × 10 10 cm Hz 1/2 W −1 ( λ c = 10.8 μm), responsivity of 1.8 A/W, 23% QE at 250 mV of applied reverse bias. A three contact heterojunction bandgap engineered dual color detector was demonstrated with simultaneous detection of midwave and longwave infrared radiation. The design showed midwave responsivity of 0.93 A/W and detectivity of 1.0 × 10 11 cm Hz 1/2 W −1 (at 4 μm) at 77 K for V b = −70 mV. Longwave responsivity and detectivity of 1.5 A/W and 2.42 × 10 10 cm Hz 1/2 W −1 (at 10 μm) were observed at −100 mV of applied bias at 77 K.

Optimization of interdigitated back contact silicon heterojunction solar cells: tailoring hetero‐interface band structures while maintaining surface passivation

AbstractInterdigitated back contact silicon heterojunction (IBC‐SHJ) solar cells have the potential for high open circuit voltage (VOC) due to the surface passivation and heterojunction contacts, and high short circuit current density (JSC) due to all back contact design. Intrinsic amorphous silicon (a‐Si:H) buffer layer at the rear surface improve the surface passivation hence VOC and JSC, but degrade fill factor (FF) from an “S” shape J–V curve. Two‐dimensional (2D) simulation using “Sentaurus device” demonstrates that the low FF is related to the valence band offset (energy barrier) at the hetero‐interface. Three approaches to the buffer layer are suggested to improve the FF: (1) reduced thickness, (2) increased conductivity, and/or (3) reduced band gap. Experimental IBC‐SHJ solar cells with reduced buffer thickness (<5 nm) and increased conductivity with low boron doping significantly improves FF, consistent with simulation. However, this has only marginal effect on efficiency since JSC and VOC also decrease due to poor surface passivation. A narrow band gap a‐Si:H buffer layer improves cell efficiency to 13.5% with unoptimized passivation quality. These results demonstrate that tailoring the hetero‐interface band structure is critical for achieving high FF. Simulations predicts that efficiences >23% are possible on planar devices with optimized pitch dimensions and achievable surface passivation, and 26% with light trapping. This work provides criterion to design IBC‐SHJ solar cell structures and optimize cell performance. Copyright © 2010 John Wiley & Sons, Ltd.

2D simulations of interdigitated back contact heterojunction solar cells based on n‐type crystalline silicon

AbstractThe combination of amorphous silicon/crystalline silicon heterojunction (a‐Si:H/c‐Si) concept with interdigitated back contact leads to a very promising structure for high efficiency solar cell. In this work, we study the interdigitated back contact silicon heterojunction (IBC‐SiHJ) structure by two‐dimensional numerical simulations and we focus on IBC‐SiHJ solar cells based on n‐type c‐Si. Assuming low surface recombination velocity (10 cm/s), we study the effect of the following parameters: bulk lifetime and doping concentration of c‐Si, density of defects at the a‐Si:H/c‐Si interface. The influence of these parameters has been tested by generating the current‐voltage (I‐V) curves. Results indicate that with a high crystalline substrate quality and low recombining a‐Si:H/c‐Si interfaces efficiencies approaching 25% can be reached. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electroluminescence in nanoporousTiO2 solid-state heterojunctions

We report visible electroluminescence at room temperature in nanoporousTiO2 films using solid-state heterojunction contacts. Electrons are injected froman n-type polymer layer, while hole injection occurs from a conductiveSnO2 substrate. As the polymer material penetrates into theTiO2 nanopore structure a highly non-planar interface with a greatly enlarged area is created.This type of interface establishes very short current paths to the recombination regionof the device and allows high carrier fluxes from the electron-injecting contact.The electroluminescence onset occurs at 7 V applied bias, and involves a broademission band covering most of the visible region with a peak centred at 600 nm.

The influence of electronic transport across interface junction between Si substrate and the root of ZnO micro-prism on field emission performance

ZnO micro-prisms are prepared on the p-type and n-type Si substrates, separately. The I–V curves analysed by AFM show that the interface junctions between the ZnO micro-prisms and the p-type substrate and between the ZnO micro-prisms and the n-type Si substrate exhibit p–n junction behaviour and ohmic contact behaviour, respectively. The formation of the p–n heterojunction and ohmic contact is ascribed to the intrinsic n-type conduction of ZnO material. Better field emission performance (lower onset voltage and larger emission current) is observed from an individual ZnO micro-prism grown on the n-type Si substrate. It is suggested that the n-Si/n-ZnO interfacial ohmic contact benefits the electron emission; while the p-Si/n-ZnO interface heterojunction deteriorates the electron emission.

Emitter series resistance effect of multiple heterojunction contacts for Pnp heterojunction bipolar transistors

For InP-based Pnp heterojunction bipolar transistors (HBTs), a set of epitaxial layers, frequently incorporating quaternaries, is used to make a low resistance electrical contact to the emitter layer. We describe an analytical approach to investigate the nonlinear effects of the multiple heterojunction interfaces on the emitter series resistance and emitter junction current–voltage characteristics of the device. The simulation results show that heterojunction interfaces can contribute a substantial portion (up to 20%) to the total emitter series resistance, especially at high levels of emitter current.

Low parasitic resistance contacts for scaled ULSI devices

Analysis of the components of parasitic series resistance in ULSI devices shows that interfacial contact resistivities less than 10−7 Ω cm2 will be required for sub 100-nm ULSI devices in order to stay on the historical performance trend. With dimensional scaling, the series resistance–width product decreases because channel lengths are scaled, while it increases in contacts because the contact length is decreased. Unless the contact resistivity is also reduced, the contact resistance ultimately becomes higher than the channel resistance, and no performance advantage will be obtained by making the device smaller. The challenge in meeting the contacting requirements in the 1997 National Technology Roadmap for Semiconductors is especially difficult in light of the desire to simultaneously contact both n+ and p+ junctions with a single material and given the trend towards lower processing temperatures, in which the equilibrium dopant electrical activity is lower. Several techniques, such as dielectric capping during junction annealing, are effective in reducing contact resistivity by maximizing interfacial dopant concentrations and minimizing contact barrier heights. Higher saturated drive currents, due to lowered parasitic series resistance, are observed in deep submicron devices made using silicides as diffusion sources (SADS); this technique eliminates the interfacial dopant segregation that is associated with conventional silicidation. The use of elevated source drains (ESD) also allows the use of thicker silicides while minimizing the consumption-induced increase in contact resistivity that normally accompanies silicidation; as a result, ESD devices give higher drive currents. The recrystallization of amorphous layers has been observed to result in non-equilibrium dopant activation which can be many times the equilibrium value. Finally, the use of heterojunction contacts using Si–Ge in the context of elevated source/drain devices presents another way to achieve lower contact resistance.

Portable P-I-N CdZnte Gamma-Ray Spectroscopy System

AbstractA portable Gamma-Ray Spectroscopy system has been designed and tested for identification of nuclear materials in the field. A P-I-N CdZnTe detector was developed to minimize the leakage current and to improve energy resolution. The detector has a sensitive volume of 200 mm3 and has been constructed with novel heterojunction contacts. Thermoelectric cooling is used to reduce leakage currents in the detector element and the input thermal noise to the FET. Risetime discrimination is used to offset the negative effects of incomplete charge collection. The detector probe is connected to a single unit comprised of detector electronics, an MCA and a portable computer, all operated from battery power. Results from testing with laboratory radioisotopic sources are presented.

Examination of 1-D Position Sensitive Detector Performance Through Analysis of Front Contact Heterojunction

AbstractThe influence of different TCOs (SnO2 and ITO) on the photoelectrical properties of 1 -D position sensitive detectors based on p-i-n structures was studied. A strong cross-contamination in the p-layer and contamination in the i-layer reduce the quality of the device. Numerical analysis of TCO/p-i-n structure also revealed a strong increase in defect states at the p-layer surface which can be attributed to the reduction of TCO. ITO seems to be less appropriate for a front TCO, although the spectral response of the p-i-n structure under reverse bias is not significantly affected by the conditions at the TCO/p heterojunction.

Organic-on-organic semiconductor heterojunctions: building block for the next generation of optoelectronic devices

A novel class of optoelectronic devices utilizing thin films of stable crystalline organic semiconductors layered onto inorganic semiconductor substrates is described. The electrical properties of these devices are determined by the energy barrier at the heterojunction contact between the organic and inorganic materials, and in many ways are similar to those of ideal diffused-junction inorganic semiconductor devices. The organic materials can be layered onto semiconductor substrates without inducing large strains in either material, hence allowing a wide range of material combinations with a similarly broad range of optoelectronic functions to be realized. As examples, high-bandwidth photodetectors and field-effect transistors made using organic/inorganic semiconductor heterojunctions are discussed. Modification of the optical and electronic properties of the organic films by irradiation with energetic electron and ion beams is considered.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Highly conductive p-type microcrystalline SiC:H prepared by ECR plasma CVD

Highly conductive p-type microcrystalline SiC:H films have been prepared by ECR (electron cyclotron resonance) plasma CVD. The material with an optical energy gap of 2.25 eV exhibits a dark conductivity as high as 10 S cm -1 which is more than seven orders of magnitude higher than that of amorphous SiC:H prepared by conventional RF plasma CVD. The optimal energy gap can be controlled in the range from 2.0 to 2.8 eV, while retaining excellent conductivity. Utilizing this material as a wide-gap heterojunction contact in amorphous silicon solar cell, a conversion efficiency of 12.0% has been obtained with a large open circuit voltage.

Thermoelectric Effects in the Point Contact of Ce Intermetallic Compounds

The current-voltage characteristics of point contact heterojunctions between Mo and the intermetallic compounds CeB 6 , CeAl 2 , CeCu 2 Si 2 and CeCu 6 were investigated at liquid He temperatures. The dynamic resistance of these compounds shows a dip near zero bias voltage and large asymmetric bias voltage dependence. These behaviours are explained in terms of thermoelectric effects and resistivity due to local heating near the contact.

High responsivity HgCdTe heterojunction photoconductor

We present an experimental and theoretical study of n-type Hg1−xCdxTe photoconductors in which a large band-gap alloy was grown on top of a smaller band-gap active region and contacts were made to the larger gap material. The larger band-gap material causes an energy barrier to holes which decreases the rate at which they reach the high recombination region of the metal-semiconductor interface. As a result, this heterojunction contact greatly reduces the effects of carrier sweepout on device performance and leads to much higher detector responsivities. Experimental results in a symmetric device with a cutoff wavelength of 7.8 μm at 77 K show responsivities in excess of 106 V/W and detectivities close to the background limited value and nonsaturation of responsivity with bias voltage. In an asymmetric device, in which only one heterojunction contact was used, an order of magnitude increase in responsivity was observed when the heterojunction contact was biased to attract minority carriers, compared with the opposite bias polarity. A theoretical model of the heterojunction contact photoconductor is presented. Calculated results are in good agreement with experimental results. The results of the calculation suggest that the optimum compositional difference Δx of the two layers should be Δx∼0.04, and that the thickness of the large band-gap region should be 2–3 μm.