Epitaxial preparation of germanium cells for photovoltaic and thermophotovoltaic applications

Germanium is widely used in the advanced III-V photovoltaic technology based on arsenides and phosphides to realise triple junction (TJ) solar cells for space application and in thermophotovoltaic (TPV) devices. Nowadays, Ge cells are realised by diffusion and the cell conversion efficiency is partially limited by the broad doping profile thus obtained. Better performances could be achieved by means of homo-epitaxy of Ge on the Ge substrate, as improved thickness and doping profile control can be obtained with the epitaxial process. TJ cells, made of a InGaP/InGaAs/Ge monolithic array, have reached an efficiency value over 40% under concentration. The AM1.5 current density of the TJ cells are in the range of 15 mA/cm2 and the limiting subcell is the GaAs one: theoretical models suggest that the Ge subcell could produce a current density up to 40 mA/cm2, so that a large amount of the Ge potential is not fulfilled in the TJ cell. Epitaxial deposition of Ge would permit novel cell design (e.g. stacking 2 Ge cells) in order to obtain higher open circuit voltage. Ge cells are also employed in TPV devices to produce electricity from a heating source, thus fulfilling all the energetic content of a particular fuel and obtaining both heat and power from a single source. In this application, the advantages of Ge compared to the more common GaSb are a larger wafer size and a cheaper price. Ge epitaxial layers were deposited on both Ge and GaAs substrates by means of a home made Metal-Organic Vapor Phase (MOVPE) reactor using isobutylgermane (iBuGe). The samples were characterised by X-Ray Diffraction (XRD), Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM). Ohmic contacts were deposited on the samples in order to perform electrical characterization and to realise a simple p-n junction which showed photovoltaic effect. This work analyses the epitaxial growth of Germanium and the effect of the growth parameters (iBuGe partial pressure, temperature) on growth rate, surface morphology and material quality. In the temperature interval between 500 and 600?C a mass transport controlled regime was observed. Surface morphology showed a dependence on both the growth rate and on the substrate orientation: by using a low iBuGe partial pressure, a large density of holes were observed both by TEM and AFM. The holes almost disappeared by increasing the growth rate up to a limit of about 1mm/h, after which the surface roughness increases, degrading sample quality. XRD showed a nearly perfect crystallographic structure for the samples deposited on exaclty oriented (001) Ge substrates, while a larger diffraction peak was obtained for samples grown on (001) Ge substrated 6?off toward (110) direction. On the latter, a rougher and wave-like surface was observed by AFM while on exaclty oriented the surface was mirrorlike. n/p junctions were characterised by means of I-V and C-V techniques, and an optimal rectifying behaviour was obtained. The illuminated Ge n/p junctions reported a VOC of about 170 mV

Displacement Damage dose and DLTS Analyses on Triple and Single Junction solar cells irradiated with electrons and protons

Space solar cells radiation hardness is of fundamental importance in view of the future missions towards harsh radiation environment (like e.g. missions to Jupiter) and for the new spacecraft using electrical propulsion. In this paper we report the radiation data for triple junction (TJ) solar cells and related component cells. Triple junction solar cells, InGaP top cells and GaAs middle cells degrade after electron radiation as expected. With proton irradiation, a high spread in the remaining factors was observed, especially for the TJ and bottom cells. Very surprising was the germanium bottom junction that showed very high degradation after protons whereas it is quite stable against electrons. Radiation results have been analyzed by means of the Displacement Damage Dose method and DLTS spectroscopy.Comment: Abstract accepted for poster session at 2017 IEEE Nuclear and Space Radiation Effects Conference, July 17-21, New Orlean

Relazione sulle attivit? completate nel 4? semestre, periodo 20/09/2009-20/02/2010 del progetto 1

Secondo il piano temporale dettagliato del progetto gli obiettivi individuati nel semestre 20/08/2009-20/02/2010 sono: A.1.1: progetto preliminare di componenti termo-fotovoltaici A.4.1: progetto preliminare di caldaia termofotovoltaicaSecondo il piano temporale dettagliato del progetto gli obiettivi individuati nel semestre 20/08/2009-20/02/2010 sono: A.1.1: progetto preliminare di componenti termo-fotovoltaici A.4.1: progetto preliminare di caldaia termofotovoltaic

Stress-Induced Local Trap Levels in Au/n-GaAs Schottky Diodes With Embedded InAs Quantum Dots

Local trap levels in Au/n-GaAs Schottky diodes with embedded InAs quantum dots, generated after a long time of the device operation, have been investigated with low-frequency noise measurements performed in the temperature range of 77-298 K and at the forward current of 30 nA. Whereas the initial devices show a pure 1/f noise behavior, after a long time of operation, recombination noise was observed at frequencies above 100 Hz, in addition to the 1/f noise at lower frequencies. Analysis of the recombination noise data obtained on structures where different GaAs cap layer thicknesses have been removed by etching allowed us to determine the activation energy of the local traps and have a rough estimation of their spatial distribution

Epitaxial germanium growth and electrical characterization

Low bandgap Ge homojunctions are normally used in photovoltaic multiple junction solar cells or thermo-photovoltaic cells and are usually realized by thermal diffusion, starting from an n-type or p-type substrate. However the diffusion process itself intrinsically precludes the possibility to obtain sharp junctions and to control the doping profile. A better thickness and doping control could be achieved by epitaxial deposition of Ge junctions, with the aim to obtain better photovoltaic efficiencies. Single junction epitaxial Ge cells with high efficiency could find use either in III-V multijunction high efficiency solar cells or in thermophotovoltaic devices coupled to a burner and to suitable selective emitters based on rare earth elements. The samples presented in this talk were deposited by means of Metal Organic Vapur Phase Epitaxy (MOVPE) on Ge and on GaAs using Iso-Butyl Germane (iBuGe) as organic precursor. Structures of different thickness were growth by varying the deposition temperature between 400 and 700?C on n and p-type Ge substrates and on GaAs substrate. Arsenic was used as both doping element and as surfactant: in the latter case it was found to improve the epitaxial quality at low temperature. Memory effect of AsH3 in the MOVPE reactor will be discussed. Nominally undoped Ge was found to be p-type, while n-type doping is obtained with the use of AsH3. n/p and p/n junctions were obtained by using a p or n substrate and depositing a n or p type layer, respectively. A completely epitaxial n/p junction on a p Ge substrate was also deposited. Vertical Ge/Ge mesa junctions were prepared on the above structures by using conventional photolithographic techniques. Ohmic contacts were obtained by evaporation, followed by a thermal annealing at 250 ?C for 60 seconds, of Au on the backside of the ptype substrate and by evaporation of Au dots, 400 mm in diameter, onto the n-type epilayer. 500 mm mesa structures, concentric to Au dots, were then prepared by chemical wet etching in a solution of H2O2 : H2O. I-V, C-V, DLTS and EBIC techniques were used for electrical characterization of the layers. Typical I-V characteristics of the mesa structure show rectification in the range of 104, reverse currents lower than 10-6 A at 1 V and ideality factor in the 1.008-1.010 range. The good rectifying properties indicate that the nominally undoped Ge layer is n-type. Capacitance-Voltage (C-V) measurements suggest that the doping of the epitaxial Ge layer is expected to be higher than that of the substrate

Progress on Low-Temperature Pulsed Electron Deposition of CuInGaSe2 Solar Cells

The quest for single-stage deposition of CuInGaSe2 (CIGS) is an open race to replace very effective but capital intensive thin film solar cell manufacturing processes like multiple-stage coevaporation or sputtering combined with high pressure selenisation treatments. In this paper the most recent achievements of Low Temperature Pulsed Electron Deposition (LTPED), a novel single stage deposition process by which CIGS can be deposited at 250 °C, are presented and discussed. We show that selenium loss during the film deposition is not a problem with LTPED as good crystalline films are formed very close to the melting temperature of selenium. The mechanism of formation of good ohmic contacts between CIGS and Mo in the absence of any MoSe2 transition layers is also illustrated, followed by a brief summary of the measured characteristics of test solar cells grown by LTPED. The 17% efficiency target achieved by lab-scale CIGS devices without bandgap modulation, antireflection coating or K-doping is considered to be a crucial milestone along the path to the industrial scale-up of LTPED. The paper ends with a brief review of the open scientific and technological issues related to the scale-up and the possible future applications of the new technology

Deviation from Regular Shape in the Early Stages of Formation of Strain-Driven 3D InGaAs/GaAs Micro/Nanotubes

Single-crystalline InGaAs/GaAs semiconductor micro/nanotubes have been obtained by the strain-driven self-rolling mechanism. This approach combines the advantages of bottom-up (epitaxial growth) and top-down (postgrowth processing) techniques, offering an exceptional opportunity to realize complex three-dimensional nanoarchitectures by using conventional photolithography and wet-etching processes. The method employed to obtain micro/nanotubes with selected orientation and length is described in detail. By means of high-resolution scanning electron microscopy characterization, we show a clear shape difference between single-wall and multiwalls tubes and we discuss it on the basis of strain release, taking into account also possible shape deformations induced during micro/nanotubes drying. We analyse the In-segregation profile in the nominal In0.20Ga0.80As/GaAs bilayer and we show its effect on the actual diameter of the tubes, concluding that a more accurate description of the structure should consider an In0.20Ga0.80As/In0.10Ga0.90As/GaAs trilayer. This work will be useful to set up reliable methodologies for the realization of strain-driven micro/nanotubes with controlled properties, necessary for their implementation in a large number of application fields