Manuel Hinojosa - Academia.edu (original) (raw)

Papers by Manuel Hinojosa

Solar Energy, Dec 1, 2021

III-V compound semiconductors and SiGe alloys can be combined to develop multijunction solar cell... more III-V compound semiconductors and SiGe alloys can be combined to develop multijunction solar cells on Silicon substrates with optimum bandgap combinations. Current implementations of such devices have reached efficiencies over 20%, using thick-and thus costly-buffer layers which induce the appearance of cracks in large area 15 samples. As a strategy to mitigate these two issues (thick buffers and cracking), a GaAsP/SiGe tandem solar cell has been developed employing group IV reverse graded buffer layers grown on Ge/Si virtual substrates with a subsurface Silicon porous layer. Reverse buffer layers facilitate a reduction in the threading dislocation density with limited thicknesses but can also induce cracks. To minimise this, a porous silicon layer 20 has been incorporated close to the Ge/Si interface so that the ductility of this layer suppresses crack propagation. In terms of solar cell performance, this porous layer reduces the problem of cracks, not totally supressing them though. Accordingly, the low shunt resistance observed in previous designs has been increased thus improving solar cell efficiency, which is still notably behind designs using thicker forward graded buffer 25 layers. The first results of this new architecture are presented here.

Energies, Jul 14, 2023

This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY

Impact of annealing temperature, time and process gas on 1 eV GaNAsSb solar cells is assessed. In... more Impact of annealing temperature, time and process gas on 1 eV GaNAsSb solar cells is assessed. In situ and ex situ annealings are carried out in order to analyze their effect on the solar cell performance. Ex situ annealings on as grown samples using N2 as process gas increase the external quantum efficiency while using Eh as process gas degrades the solar cell performance. Best performance is obtained by in situ annealing in a molecular beam epitaxy reactor. External quantum efficiency increases in the short wavelength range (< 700 nm) and decreases in the long wavelength range (> 700 nm) after ex situ annealing using N2 as process gas on in situ annealed samples (in a molecular beam epitaxy reactor).

2018 Spanish Conference on Electron Devices (CDE)

This paper summarizes the state-of-the-art of the lattice matched GaInP/Ga(In)As/Ge triple-juncti... more This paper summarizes the state-of-the-art of the lattice matched GaInP/Ga(In)As/Ge triple-junction solar cell developed at the Solar Energy Institute of UPM (IES-UPM). Different research topics tackled over the last years about this structure are described. As result of this work, an efficiency of-40% at ~-415>i is presented.

2019 IEEE 46th Photovoltaic Specialists Conference (PVSC)

Zinc-diffusion can induce multiple failures in the electrical performance of a multijunction sola... more Zinc-diffusion can induce multiple failures in the electrical performance of a multijunction solar cell. In this work, we show an important Zn-diffusion from the AlGaInP back-surface-field layer to the emitter of the GaInP top cell of an inverted multijunction solar cell. Through the analysis of different doping profiles, we provide strong evidence that the diffusion mechanism is (1) triggered by the growth of the tunnel junction cathode and (2) involves point defects. We analyze the implications of Zn-diffusion on the bandgap, the rear-passivation and the minority carrier quality of the GaInP solar subcell by relating the electrical performance of different samples to its corresponding doping profile.

2017 IEEE 44th Photovoltaic Specialist Conference (PVSC), 2017

A GaInP/Ga(In)As/GaNAsSb/Ge 4-junction solar cell grown using combined MOVPE+MBE growth is used t... more A GaInP/Ga(In)As/GaNAsSb/Ge 4-junction solar cell grown using combined MOVPE+MBE growth is used to analyze the effects during the integration of the subcell components into the full 4J structure. In this preliminary study, the Ge subcell is observed to suffer about 15% Jsc drop and ~50 mV Voc loss at 1-sun, while the Voc of the GaNAsSb subcell drops by as much as ~ 140 mV. The degradation of the Ge and GaNAsSb subcells in the current-matched 4J structure can hinder its efficiency potential to a higher extent than in the GaInP/Ga(In)As/Ge 3J. Besides, high quality GaNAsSb and Ge subcells would still limit the current and require redesigning the top subcells to achieve optimum efficiencies.

Solar Energy, 2021

Please refer to published version for the most recent bibliographic citation information. If a pu... more Please refer to published version for the most recent bibliographic citation information. If a published version is known of, the repository item page linked to above, will contain details on accessing it.

2019 IEEE 46th Photovoltaic Specialists Conference (PVSC), 2019

A tandem GaAsP/SiGe solar cell has been developed employing group-IV reverse buffer layers grown ... more A tandem GaAsP/SiGe solar cell has been developed employing group-IV reverse buffer layers grown on silicon substrates with a subsurface porous layer. Reverse buffer layers facilitate a reduction in the threading dislocation density with limited thicknesses, but ease the appearance of cracks, as observed in previous designs grown on regular Si substrates. In this new design, a porous silicon layer has been incorporated close to the substrate surface. The ductility of this layer helps repress the propagation of cracks, diminishing the problems of low shunt resistance and thus improving solar cell performance. The first results of this new architecture are presented here.

Solar Energy Materials and Solar Cells, Dec 1, 2022

The growth of heavily doped tunnel junctions in inverted metamorphic multijunction solar cells in... more The growth of heavily doped tunnel junctions in inverted metamorphic multijunction solar cells induces a strong diffusion of Zn via a point-defects-assisted diffusion mechanism. The redistribution of Zn can compensate the n-type doping in the emitter of the GaInP top junction, degrading severely the conductivity of the whole solar cell and its conversion efficiency. This work evaluates different epitaxial growth strategies to achieve control on the [Zn] profile of an inverted metamorphic triple-junction structure, including: the reduction of the doping concentration in the tunnel junction to minimize the injection of point defects that trigger the diffusion mechanism; the use of different barrier layers to keep the injected point defects away from active layers and, finally, the minimization of Zn in the AlGaInP back-surface-field layer of the GaInP subcell. This last approach enables a high-conductivity multijunction solar cell device without redesigning the tunnel junction as well as a high electronic quality in the GaInP subcell, which shows a collection efficiency higher than 93% and an open-circuit-voltage offset of 410 mV at 1 sun irradiance. The characterization of final triple-junction devices, including quantum efficiency, electroluminescence, and light current-density-voltage curves at different irradiances, demonstrates a successful integration of all the subcell and tunnel junction components. This way, final solar cells with peak efficiencies exceeding 40 % at ~500 suns are demonstrated, despite using doping levels as low as 1•10 17 cm-3 in the AlGaInP:Zn back-surface-field of the GaInP subcell and using nonoptimized antireflective coatings.

Zenodo (CERN European Organization for Nuclear Research), Jun 4, 2022

The non-uniform irradiance of fiber lasers influences the performance of power-by-light systems. ... more The non-uniform irradiance of fiber lasers influences the performance of power-by-light systems. We analyze the nonuniformity in both single and triple junction power converters in addition to other parameters that could reduce the efficiency such as manufacturing tolerances or series resistance effect. We present an experimental efficiency of 67.0 ± 0.9 % in triple junction power converters for an input power of 2.8 W using a laser wavelength of 857 nm. We demonstrate thus a high efficiency for non-uniformity irradiance.

2017 Spanish Conference on Electron Devices (CDE), 2017

Metamorphic solar cells require high-quality compositional graded buffer (CGB) layers to perform ... more Metamorphic solar cells require high-quality compositional graded buffer (CGB) layers to perform as virtual substrate. Several technical growth issues arisen during the implementation of an ordered-GalnP CGB at IES-UPM MOVPE reactor are discussed in this work. These considerations focuses on achieving high-quality metamorphic buffers by increasing phosphine partial pressure and growth rates, which requires some reactor adjustments, and attaining composition control for different GalnP alloys. Improvements on the material quality are proved. It is also presented a 1 eV GalnAs metamorphic subcell and an inverted metamorphic triple junction solar cell using these CGB.

2019 European Space Power Conference (ESPC), 2019

Virtual substrates based on thin Ge layers on Si substrates by direct deposition have recently ac... more Virtual substrates based on thin Ge layers on Si substrates by direct deposition have recently achieved high quality. In this work, their application as low cost, removable substrates for the growth of high efficiency, lightweight and flexible multijunction solar cells for space applications is analyzed. Experimental Ge single-junction solar cells and GaInP/Ga(In)As/Ge triple-junction solar cells using the Ge/Si virtual substrate as an active bottom junction (being the Si inactive), are implemented using medium quality Ge/Si virtual substrates with a 5 urn Ge layer thickness. A lower quality in the Ge material, as compared to standard substrates, but enough carrier collection efficiency for a standard triple-junction, are shown. The expected formation of cracks during growth, due to the large thermal expansion coefficient mismatch with the Si substrate, is confirmed, and is found to be a major limiting factor for the performance of the solar cells. Strategies such as thinning the Ge + III-V structure and minimizing the thermal cycling during growth are discussed. Using an embedded porous Si layer to serve as buffer for the strain is being investigated. This porous layer could also serve as sacrificial layer for high throughput mechanical epitaxial lift-off in the manufacturing of lightweight and flexible multijunction cells. These embedded porous Si layers need to be engineered for optimum performance and compatibility with the Ge and III-V deposition processes.

Joule, 2021

Context & Scale The transmission of power using light instead of electricity-the so called power-... more Context & Scale The transmission of power using light instead of electricity-the so called power-by-light (PBL)has been around for some decades now. Despite this long history, it has been only in recent years when PBL systems have experienced enormous effervescence in situations where the direct use of electrical energy to power electronic equipment is either not possible or not recommendable. PBL systems are the smarter solution for monitoring and controlling processes under strict safety conditions in contexts with high fire or explosion risk (refineries, mines, fuel tanks in space and aircrafts), or that need galvanic insulation (high voltage power lines, lightingsafe monitoring, medical applications), etc. The core of a PBL system is the photovoltaic laser power converter (PVLPC) which transforms the laser light delivered through an optical fibre into electricity. Recently, a PVLPC has demonstrated the highest efficiency for any photovoltaic converter, i.e. 68.9% at a laser illumination of 858 nm. This review begins with a brief overview of the functionalities of PBL systems and the critical requirements imposed to PVLPCs. Afterwards, the history of PVLPCs is revisited, including the recent hatching of companies and technical conferences in the field. A detailed comparison amid the different typologies of PVLPCs is tackled in terms of their efficiency, delivered power, voltage, temperature effects and manufacturability, highlighting their advantages and disadvantages depending on the application. We also pinpoint the main aspects limiting the efficiency of PVLPCs and the ways to circumvent them. Advanced concepts such as photon recycling, high performance back reflectors, luminescent coupling, etc. and their potential to improve the PVLPC performance are discussed. Future PVLPCs must exhibit higher efficiencies and delivered power, robustness at rough environmental conditions and lower manufacturing cost. This review aims at showing the routes to achieve these goals. SUMMARY PhotoVoltaic Laser Power Converters (PVLPCs) currently exhibit the highest photovoltaic efficiency, i.e. 68.9% at a laser illumination of 858 nm. They are the core element of Power-by-Light (PBL) systems which basically made up of a power laser, an optical fibre and a PVLPC. PBL allows the safe transfer of power in situations where the direct use of electrical energy to power electronic equipment is either not possible or not recommendable. There is a huge market for PBL systems, for example in applications needing to monitor and control processes under strict safety conditions because of a high fire or explosion risk (refineries, mines, fuel tanks in space and aircrafts, etc.), the possible malfunction of sensitive electrical equipment in areas of high electromagnetic noise (nuclear plants), the need of galvanic insulation (high voltage power lines and lighting-safe monitoring), etc. The first PBL system was built in 1978 but it has been only recently when PBL systems are having an outburst with continuous efficiency improvements, creation of start-ups, big companies entering the business, and increasing number of scientific publications and specialized technical conferences. This review begins with an overview of the functionalities of PBL systems and the critical requirements imposed to PVLPCs. Afterwards, a brief outlook on the history of PVLPCs is presented. A detailed comparison among the different typologies of PVLPCs in terms of efficiency, delivered power, voltage, temperature effects and manufacturability is carried out, highlighting their advantages and disadvantages depending on the application. We also point out the main aspects limiting the efficiency of PVLPCs and possible ways to circumvent them. Finally, we discuss the perspectives of PVLPCs together with the possible routes to a steady deployment of PBL systems to serve a considerable number of applications in our daily life

Journal of Electronic Materials, 2020

The diffusion length of minority carriers in a p-doped InGaP layer is derived from the cathodolum... more The diffusion length of minority carriers in a p-doped InGaP layer is derived from the cathodoluminescence (CL) intensity profiles. Two procedures are used. First, the CL profile is recorded along a line crossing the intersection between a thin metallic mask and the semiconductor, a second approach consists of the measurement of the intensity profile around an intentional scratch on the surface of the sample. A longer diffusion length is measured when using the metallic mask as compared to the scratch. We discuss the role of nonradiative recombination centers in the reduction of the diffusion length around the scratch. The temperature dependence of the diffusion length is also measured, it is found to decrease with temperature.

Solar Energy Materials and Solar Cells, 2021

Reducing the formation of cracks during growth of GaInP/GaInAs/Ge 3-junction solar cells on Ge|Si... more Reducing the formation of cracks during growth of GaInP/GaInAs/Ge 3-junction solar cells on Ge|Si virtual substrates has been attempted by thinning the structure, namely the Ge bottom cell and the GaInAs middle cell. The theoretical analysis performed using realistic device parameters indicates that the GaInAs middle cell can be drastically thinned to 1000 nm while increasing its In content to 8% with an efficiency loss in the 3-junction cell below 3%. The experimental results show that the formation of macroscopic cracks is prevented in thinned GaInAs/Ge 2-junction and GaInP/GaInAs/Ge 3-junction cells. These prototype crack-free multijunction cells demonstrate the concept and were used to rule out any possible component integration issue. The performance metrics are limited by the high threading dislocation density over 2•10 7 cm-2 in the virtual substrates used, but an almost current matched, crack-free, thinned 3-junction solar cell is demonstrated, and the pathway towards solar cells with higher voltages identified.

2021 13th Spanish Conference on Electron Devices (CDE), 2021

Virtual Ge substrates fabricated by direct deposition of Ge on Si have become a pathway with high... more Virtual Ge substrates fabricated by direct deposition of Ge on Si have become a pathway with high potential to attain high-efficiency III-V multijunction solar cells on Si. We study the development of III-V triple junction solar cells using two types of Ge|Si virtual substrates. The first uses a thick (2-5 μm) Ge layer grown by CVD, which acts as the bottom Ge subcell. The second, grown by low-temperature RT-PECVD, has a thickness of a few tens of nanometres, with the Si substrate acting as Si bottom cell. We discuss the challenges related to each design (formation of cracks, parasitic absorption in the Ge layer, dislocations, ...), present the theoretical design and show the experimental results obtained. Finally, an advanced approach using embedded porous Si layers as buffer layers for crack mitigation is also presented.

Solar Energy Materials and Solar Cells, 2021

Abstract Qualification standards are on the base of the high endurance and resilience of space so... more Abstract Qualification standards are on the base of the high endurance and resilience of space solar cells. In these standards, such as the European ECSS-E-ST-20-08C or the American AIAA S-111A counterpart, life tests related with high temperature accelerated tests (among many others) are included. There are several issues that make it difficult to assess the multijunction solar cell life from temperature tests in these standards. For example, in the European standard, an activation energy of 0.7 eV determined many years ago for silicon devices is assumed. On the other hand, the American standard proposes temperature accelerated tests at the temperatures of 50 °C, 80 °C and 110 °C which are clearly low to really accelerate the life tests of solar cells. Accordingly, in this paper we present the resulting reliability figures (reliability function, failure probability and MTTF) derived of an innovative temperature ALT which allows the proper estimation of the activation energy of commercial lattice matched GaInP/Ga(In)As/Ge triple junction solar cells. The main conclusions are: a) an activation energy of 0.97 eV has been estimated. This value results in significantly higher lifetime values for the tested cells. b) the shape parameter obtained from Weibull failure density function, β, is 1.67; c) the tested solar cells perform as robust devices showing high reliability values in the temperature range of 80 °C–130 °C; d) for higher temperatures, especially beyond 150 °C, the reliability decays significantly; f) reliability functions and parameters can be assessed for any operation temperature and failure criterion.

2021 IEEE 48th Photovoltaic Specialists Conference (PVSC), 2021

Traditional front junction GaInP solar cells are radiation tolerant and can have good diffusion l... more Traditional front junction GaInP solar cells are radiation tolerant and can have good diffusion length, but have limited voltage and thus efficiency. Here, we investigate the impact of annealing on GaInP device performance. First, Zn-doped GaInP/AlGaInP double heterojunction structures are studied in order to investigate the impact of annealing in a simple structure. While standard anneals lower diffusion length and lifetime, annealing after injecting point-defects improves material quality, presumably by eliminating sources of non-radiative recombination. Atomic ordering is reduced by the anneal, which raises the device bandgap and requires consideration. These results can be used to improve front junction device performance while controlling the bandgap. Then, GaInP front-junction devices are created using anneals with and without point-defect injection. Voc and EQE trends follow the diffusion length and lifetime trends from the DH structures. Baseline devices have Woc of 0.484 V, raising to 0.504 V after a high temperature anneal. However, injecting point defects prior to the anneal results in a front junction device with Woc of 0.405 V and a diffusion length greater than 8 µm. Optimized front-junction GaInP devices with an ARC have ~20% efficiency without a rear reflector.

IEEE Journal of Photovoltaics, 2021

We investigate the dynamics of Zn diffusion in MOVPE-grown AlGaInP/GaInP systems by the compariso... more We investigate the dynamics of Zn diffusion in MOVPE-grown AlGaInP/GaInP systems by the comparison of different structures that emulate the back-surface field (BSF) and base layers of a GaInP subcell integrated into an inverted multijunction solar cell structure. Through the analysis of secondary ion mass spectroscopy (SIMS), electrochemical capacitance-voltage (ECV) and spectrally resolved cathodoluminescence (CL) measurements, we provide experimental evidence that 1) the Zn diffusion is enhanced by point defects injected during the growth of the tunnel junction cathode layer; 2) the intensity of the process is determined by the cathode doping level and it happens for different cathode materials; 3) the mobile Zn is positively charged and 4) the diffusion mechanism reduces the CuPt ordering in GaInP. We demonstrate that using barrier layers the diffusion of point defects can be mitigated, so that they do not reach Zn-doped layers, preventing its diffusion. Finally, the impact of Zn diffusion on solar cells with different Zn-profiles is evaluated by comparing the electrical I-V curves at different concentrations. The results rule out the introduction of internal barriers in the BSF but illustrate how Zn diffusion under typical growth condition can reach the emitter and dramatically affect the series resistance, among other effects.

Solar Energy Materials and Solar Cells, 2020

Tellurium allows attaining heavy n-type doping levels in GaAs, which is suited to achieve very lo... more Tellurium allows attaining heavy n-type doping levels in GaAs, which is suited to achieve very low contact resistivities in solar cells. Besides, it modifies the energy bandgap of MOVPE-grown GaInP by reducing the group-III sublattice ordering and presents a strong memory effect which induces residual n-type doping in subsequent layers, potentially detrimental to the performance of the solar cell. In this work, we present an inverted rearheterojunction GaInP solar cell that employs a thick Te-doped GaInP layer as absorber, with a doping profile obtained exclusively by controlling the memory effect of Te coming from the preceding growth of a heavily doped GaAs contact layer. In this way, GaInP is partially disordered with the use of no additional surfactant, leading to an increase in the solar cell bandgap of around 35 meV as compared to traditional samples doped with silicon. In the proof-of-concept experimental devices developed so far, the use of a rear-heterojunction configuration and the bandgap increase results in a global open-circuit voltage enhancement of 109 mV. The photocurrent decreases by 1.32 mA/cm 2 , mostly due to the bandgap blue-shift, with about 0.35 mA/cm 2 attributable to lower carrier collection efficiencies. These preliminary results are discussed by analyzing the I-V curve parameters and quantum efficiencies of a Te-doped rear-heterojunction, a Si-doped rear-heterojunction and a Si-doped front-junction solar cell. An additional advantage is that the emitter sheet resistance is reduced from 551 to 147 Ω/, which offers potential for higher efficiencies through lower front grid shadowing factors, as demonstrated with the concentrator measurements presented.

Solar Energy, Dec 1, 2021

III-V compound semiconductors and SiGe alloys can be combined to develop multijunction solar cell... more III-V compound semiconductors and SiGe alloys can be combined to develop multijunction solar cells on Silicon substrates with optimum bandgap combinations. Current implementations of such devices have reached efficiencies over 20%, using thick-and thus costly-buffer layers which induce the appearance of cracks in large area 15 samples. As a strategy to mitigate these two issues (thick buffers and cracking), a GaAsP/SiGe tandem solar cell has been developed employing group IV reverse graded buffer layers grown on Ge/Si virtual substrates with a subsurface Silicon porous layer. Reverse buffer layers facilitate a reduction in the threading dislocation density with limited thicknesses but can also induce cracks. To minimise this, a porous silicon layer 20 has been incorporated close to the Ge/Si interface so that the ductility of this layer suppresses crack propagation. In terms of solar cell performance, this porous layer reduces the problem of cracks, not totally supressing them though. Accordingly, the low shunt resistance observed in previous designs has been increased thus improving solar cell efficiency, which is still notably behind designs using thicker forward graded buffer 25 layers. The first results of this new architecture are presented here.

Energies, Jul 14, 2023

This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY

Impact of annealing temperature, time and process gas on 1 eV GaNAsSb solar cells is assessed. In... more Impact of annealing temperature, time and process gas on 1 eV GaNAsSb solar cells is assessed. In situ and ex situ annealings are carried out in order to analyze their effect on the solar cell performance. Ex situ annealings on as grown samples using N2 as process gas increase the external quantum efficiency while using Eh as process gas degrades the solar cell performance. Best performance is obtained by in situ annealing in a molecular beam epitaxy reactor. External quantum efficiency increases in the short wavelength range (< 700 nm) and decreases in the long wavelength range (> 700 nm) after ex situ annealing using N2 as process gas on in situ annealed samples (in a molecular beam epitaxy reactor).

2018 Spanish Conference on Electron Devices (CDE)

This paper summarizes the state-of-the-art of the lattice matched GaInP/Ga(In)As/Ge triple-juncti... more This paper summarizes the state-of-the-art of the lattice matched GaInP/Ga(In)As/Ge triple-junction solar cell developed at the Solar Energy Institute of UPM (IES-UPM). Different research topics tackled over the last years about this structure are described. As result of this work, an efficiency of-40% at ~-415>i is presented.

2019 IEEE 46th Photovoltaic Specialists Conference (PVSC)

Zinc-diffusion can induce multiple failures in the electrical performance of a multijunction sola... more Zinc-diffusion can induce multiple failures in the electrical performance of a multijunction solar cell. In this work, we show an important Zn-diffusion from the AlGaInP back-surface-field layer to the emitter of the GaInP top cell of an inverted multijunction solar cell. Through the analysis of different doping profiles, we provide strong evidence that the diffusion mechanism is (1) triggered by the growth of the tunnel junction cathode and (2) involves point defects. We analyze the implications of Zn-diffusion on the bandgap, the rear-passivation and the minority carrier quality of the GaInP solar subcell by relating the electrical performance of different samples to its corresponding doping profile.

2017 IEEE 44th Photovoltaic Specialist Conference (PVSC), 2017

A GaInP/Ga(In)As/GaNAsSb/Ge 4-junction solar cell grown using combined MOVPE+MBE growth is used t... more A GaInP/Ga(In)As/GaNAsSb/Ge 4-junction solar cell grown using combined MOVPE+MBE growth is used to analyze the effects during the integration of the subcell components into the full 4J structure. In this preliminary study, the Ge subcell is observed to suffer about 15% Jsc drop and ~50 mV Voc loss at 1-sun, while the Voc of the GaNAsSb subcell drops by as much as ~ 140 mV. The degradation of the Ge and GaNAsSb subcells in the current-matched 4J structure can hinder its efficiency potential to a higher extent than in the GaInP/Ga(In)As/Ge 3J. Besides, high quality GaNAsSb and Ge subcells would still limit the current and require redesigning the top subcells to achieve optimum efficiencies.

Solar Energy, 2021

Please refer to published version for the most recent bibliographic citation information. If a pu... more Please refer to published version for the most recent bibliographic citation information. If a published version is known of, the repository item page linked to above, will contain details on accessing it.

2019 IEEE 46th Photovoltaic Specialists Conference (PVSC), 2019

A tandem GaAsP/SiGe solar cell has been developed employing group-IV reverse buffer layers grown ... more A tandem GaAsP/SiGe solar cell has been developed employing group-IV reverse buffer layers grown on silicon substrates with a subsurface porous layer. Reverse buffer layers facilitate a reduction in the threading dislocation density with limited thicknesses, but ease the appearance of cracks, as observed in previous designs grown on regular Si substrates. In this new design, a porous silicon layer has been incorporated close to the substrate surface. The ductility of this layer helps repress the propagation of cracks, diminishing the problems of low shunt resistance and thus improving solar cell performance. The first results of this new architecture are presented here.

Solar Energy Materials and Solar Cells, Dec 1, 2022

The growth of heavily doped tunnel junctions in inverted metamorphic multijunction solar cells in... more The growth of heavily doped tunnel junctions in inverted metamorphic multijunction solar cells induces a strong diffusion of Zn via a point-defects-assisted diffusion mechanism. The redistribution of Zn can compensate the n-type doping in the emitter of the GaInP top junction, degrading severely the conductivity of the whole solar cell and its conversion efficiency. This work evaluates different epitaxial growth strategies to achieve control on the [Zn] profile of an inverted metamorphic triple-junction structure, including: the reduction of the doping concentration in the tunnel junction to minimize the injection of point defects that trigger the diffusion mechanism; the use of different barrier layers to keep the injected point defects away from active layers and, finally, the minimization of Zn in the AlGaInP back-surface-field layer of the GaInP subcell. This last approach enables a high-conductivity multijunction solar cell device without redesigning the tunnel junction as well as a high electronic quality in the GaInP subcell, which shows a collection efficiency higher than 93% and an open-circuit-voltage offset of 410 mV at 1 sun irradiance. The characterization of final triple-junction devices, including quantum efficiency, electroluminescence, and light current-density-voltage curves at different irradiances, demonstrates a successful integration of all the subcell and tunnel junction components. This way, final solar cells with peak efficiencies exceeding 40 % at ~500 suns are demonstrated, despite using doping levels as low as 1•10 17 cm-3 in the AlGaInP:Zn back-surface-field of the GaInP subcell and using nonoptimized antireflective coatings.

Zenodo (CERN European Organization for Nuclear Research), Jun 4, 2022

The non-uniform irradiance of fiber lasers influences the performance of power-by-light systems. ... more The non-uniform irradiance of fiber lasers influences the performance of power-by-light systems. We analyze the nonuniformity in both single and triple junction power converters in addition to other parameters that could reduce the efficiency such as manufacturing tolerances or series resistance effect. We present an experimental efficiency of 67.0 ± 0.9 % in triple junction power converters for an input power of 2.8 W using a laser wavelength of 857 nm. We demonstrate thus a high efficiency for non-uniformity irradiance.

2017 Spanish Conference on Electron Devices (CDE), 2017

Metamorphic solar cells require high-quality compositional graded buffer (CGB) layers to perform ... more Metamorphic solar cells require high-quality compositional graded buffer (CGB) layers to perform as virtual substrate. Several technical growth issues arisen during the implementation of an ordered-GalnP CGB at IES-UPM MOVPE reactor are discussed in this work. These considerations focuses on achieving high-quality metamorphic buffers by increasing phosphine partial pressure and growth rates, which requires some reactor adjustments, and attaining composition control for different GalnP alloys. Improvements on the material quality are proved. It is also presented a 1 eV GalnAs metamorphic subcell and an inverted metamorphic triple junction solar cell using these CGB.

2019 European Space Power Conference (ESPC), 2019

Virtual substrates based on thin Ge layers on Si substrates by direct deposition have recently ac... more Virtual substrates based on thin Ge layers on Si substrates by direct deposition have recently achieved high quality. In this work, their application as low cost, removable substrates for the growth of high efficiency, lightweight and flexible multijunction solar cells for space applications is analyzed. Experimental Ge single-junction solar cells and GaInP/Ga(In)As/Ge triple-junction solar cells using the Ge/Si virtual substrate as an active bottom junction (being the Si inactive), are implemented using medium quality Ge/Si virtual substrates with a 5 urn Ge layer thickness. A lower quality in the Ge material, as compared to standard substrates, but enough carrier collection efficiency for a standard triple-junction, are shown. The expected formation of cracks during growth, due to the large thermal expansion coefficient mismatch with the Si substrate, is confirmed, and is found to be a major limiting factor for the performance of the solar cells. Strategies such as thinning the Ge + III-V structure and minimizing the thermal cycling during growth are discussed. Using an embedded porous Si layer to serve as buffer for the strain is being investigated. This porous layer could also serve as sacrificial layer for high throughput mechanical epitaxial lift-off in the manufacturing of lightweight and flexible multijunction cells. These embedded porous Si layers need to be engineered for optimum performance and compatibility with the Ge and III-V deposition processes.

Joule, 2021

Context & Scale The transmission of power using light instead of electricity-the so called power-... more Context & Scale The transmission of power using light instead of electricity-the so called power-by-light (PBL)has been around for some decades now. Despite this long history, it has been only in recent years when PBL systems have experienced enormous effervescence in situations where the direct use of electrical energy to power electronic equipment is either not possible or not recommendable. PBL systems are the smarter solution for monitoring and controlling processes under strict safety conditions in contexts with high fire or explosion risk (refineries, mines, fuel tanks in space and aircrafts), or that need galvanic insulation (high voltage power lines, lightingsafe monitoring, medical applications), etc. The core of a PBL system is the photovoltaic laser power converter (PVLPC) which transforms the laser light delivered through an optical fibre into electricity. Recently, a PVLPC has demonstrated the highest efficiency for any photovoltaic converter, i.e. 68.9% at a laser illumination of 858 nm. This review begins with a brief overview of the functionalities of PBL systems and the critical requirements imposed to PVLPCs. Afterwards, the history of PVLPCs is revisited, including the recent hatching of companies and technical conferences in the field. A detailed comparison amid the different typologies of PVLPCs is tackled in terms of their efficiency, delivered power, voltage, temperature effects and manufacturability, highlighting their advantages and disadvantages depending on the application. We also pinpoint the main aspects limiting the efficiency of PVLPCs and the ways to circumvent them. Advanced concepts such as photon recycling, high performance back reflectors, luminescent coupling, etc. and their potential to improve the PVLPC performance are discussed. Future PVLPCs must exhibit higher efficiencies and delivered power, robustness at rough environmental conditions and lower manufacturing cost. This review aims at showing the routes to achieve these goals. SUMMARY PhotoVoltaic Laser Power Converters (PVLPCs) currently exhibit the highest photovoltaic efficiency, i.e. 68.9% at a laser illumination of 858 nm. They are the core element of Power-by-Light (PBL) systems which basically made up of a power laser, an optical fibre and a PVLPC. PBL allows the safe transfer of power in situations where the direct use of electrical energy to power electronic equipment is either not possible or not recommendable. There is a huge market for PBL systems, for example in applications needing to monitor and control processes under strict safety conditions because of a high fire or explosion risk (refineries, mines, fuel tanks in space and aircrafts, etc.), the possible malfunction of sensitive electrical equipment in areas of high electromagnetic noise (nuclear plants), the need of galvanic insulation (high voltage power lines and lighting-safe monitoring), etc. The first PBL system was built in 1978 but it has been only recently when PBL systems are having an outburst with continuous efficiency improvements, creation of start-ups, big companies entering the business, and increasing number of scientific publications and specialized technical conferences. This review begins with an overview of the functionalities of PBL systems and the critical requirements imposed to PVLPCs. Afterwards, a brief outlook on the history of PVLPCs is presented. A detailed comparison among the different typologies of PVLPCs in terms of efficiency, delivered power, voltage, temperature effects and manufacturability is carried out, highlighting their advantages and disadvantages depending on the application. We also point out the main aspects limiting the efficiency of PVLPCs and possible ways to circumvent them. Finally, we discuss the perspectives of PVLPCs together with the possible routes to a steady deployment of PBL systems to serve a considerable number of applications in our daily life

Journal of Electronic Materials, 2020

The diffusion length of minority carriers in a p-doped InGaP layer is derived from the cathodolum... more The diffusion length of minority carriers in a p-doped InGaP layer is derived from the cathodoluminescence (CL) intensity profiles. Two procedures are used. First, the CL profile is recorded along a line crossing the intersection between a thin metallic mask and the semiconductor, a second approach consists of the measurement of the intensity profile around an intentional scratch on the surface of the sample. A longer diffusion length is measured when using the metallic mask as compared to the scratch. We discuss the role of nonradiative recombination centers in the reduction of the diffusion length around the scratch. The temperature dependence of the diffusion length is also measured, it is found to decrease with temperature.

Solar Energy Materials and Solar Cells, 2021

Reducing the formation of cracks during growth of GaInP/GaInAs/Ge 3-junction solar cells on Ge|Si... more Reducing the formation of cracks during growth of GaInP/GaInAs/Ge 3-junction solar cells on Ge|Si virtual substrates has been attempted by thinning the structure, namely the Ge bottom cell and the GaInAs middle cell. The theoretical analysis performed using realistic device parameters indicates that the GaInAs middle cell can be drastically thinned to 1000 nm while increasing its In content to 8% with an efficiency loss in the 3-junction cell below 3%. The experimental results show that the formation of macroscopic cracks is prevented in thinned GaInAs/Ge 2-junction and GaInP/GaInAs/Ge 3-junction cells. These prototype crack-free multijunction cells demonstrate the concept and were used to rule out any possible component integration issue. The performance metrics are limited by the high threading dislocation density over 2•10 7 cm-2 in the virtual substrates used, but an almost current matched, crack-free, thinned 3-junction solar cell is demonstrated, and the pathway towards solar cells with higher voltages identified.

2021 13th Spanish Conference on Electron Devices (CDE), 2021

Virtual Ge substrates fabricated by direct deposition of Ge on Si have become a pathway with high... more Virtual Ge substrates fabricated by direct deposition of Ge on Si have become a pathway with high potential to attain high-efficiency III-V multijunction solar cells on Si. We study the development of III-V triple junction solar cells using two types of Ge|Si virtual substrates. The first uses a thick (2-5 μm) Ge layer grown by CVD, which acts as the bottom Ge subcell. The second, grown by low-temperature RT-PECVD, has a thickness of a few tens of nanometres, with the Si substrate acting as Si bottom cell. We discuss the challenges related to each design (formation of cracks, parasitic absorption in the Ge layer, dislocations, ...), present the theoretical design and show the experimental results obtained. Finally, an advanced approach using embedded porous Si layers as buffer layers for crack mitigation is also presented.

Solar Energy Materials and Solar Cells, 2021

Abstract Qualification standards are on the base of the high endurance and resilience of space so... more Abstract Qualification standards are on the base of the high endurance and resilience of space solar cells. In these standards, such as the European ECSS-E-ST-20-08C or the American AIAA S-111A counterpart, life tests related with high temperature accelerated tests (among many others) are included. There are several issues that make it difficult to assess the multijunction solar cell life from temperature tests in these standards. For example, in the European standard, an activation energy of 0.7 eV determined many years ago for silicon devices is assumed. On the other hand, the American standard proposes temperature accelerated tests at the temperatures of 50 °C, 80 °C and 110 °C which are clearly low to really accelerate the life tests of solar cells. Accordingly, in this paper we present the resulting reliability figures (reliability function, failure probability and MTTF) derived of an innovative temperature ALT which allows the proper estimation of the activation energy of commercial lattice matched GaInP/Ga(In)As/Ge triple junction solar cells. The main conclusions are: a) an activation energy of 0.97 eV has been estimated. This value results in significantly higher lifetime values for the tested cells. b) the shape parameter obtained from Weibull failure density function, β, is 1.67; c) the tested solar cells perform as robust devices showing high reliability values in the temperature range of 80 °C–130 °C; d) for higher temperatures, especially beyond 150 °C, the reliability decays significantly; f) reliability functions and parameters can be assessed for any operation temperature and failure criterion.

2021 IEEE 48th Photovoltaic Specialists Conference (PVSC), 2021

Traditional front junction GaInP solar cells are radiation tolerant and can have good diffusion l... more Traditional front junction GaInP solar cells are radiation tolerant and can have good diffusion length, but have limited voltage and thus efficiency. Here, we investigate the impact of annealing on GaInP device performance. First, Zn-doped GaInP/AlGaInP double heterojunction structures are studied in order to investigate the impact of annealing in a simple structure. While standard anneals lower diffusion length and lifetime, annealing after injecting point-defects improves material quality, presumably by eliminating sources of non-radiative recombination. Atomic ordering is reduced by the anneal, which raises the device bandgap and requires consideration. These results can be used to improve front junction device performance while controlling the bandgap. Then, GaInP front-junction devices are created using anneals with and without point-defect injection. Voc and EQE trends follow the diffusion length and lifetime trends from the DH structures. Baseline devices have Woc of 0.484 V, raising to 0.504 V after a high temperature anneal. However, injecting point defects prior to the anneal results in a front junction device with Woc of 0.405 V and a diffusion length greater than 8 µm. Optimized front-junction GaInP devices with an ARC have ~20% efficiency without a rear reflector.

IEEE Journal of Photovoltaics, 2021

We investigate the dynamics of Zn diffusion in MOVPE-grown AlGaInP/GaInP systems by the compariso... more We investigate the dynamics of Zn diffusion in MOVPE-grown AlGaInP/GaInP systems by the comparison of different structures that emulate the back-surface field (BSF) and base layers of a GaInP subcell integrated into an inverted multijunction solar cell structure. Through the analysis of secondary ion mass spectroscopy (SIMS), electrochemical capacitance-voltage (ECV) and spectrally resolved cathodoluminescence (CL) measurements, we provide experimental evidence that 1) the Zn diffusion is enhanced by point defects injected during the growth of the tunnel junction cathode layer; 2) the intensity of the process is determined by the cathode doping level and it happens for different cathode materials; 3) the mobile Zn is positively charged and 4) the diffusion mechanism reduces the CuPt ordering in GaInP. We demonstrate that using barrier layers the diffusion of point defects can be mitigated, so that they do not reach Zn-doped layers, preventing its diffusion. Finally, the impact of Zn diffusion on solar cells with different Zn-profiles is evaluated by comparing the electrical I-V curves at different concentrations. The results rule out the introduction of internal barriers in the BSF but illustrate how Zn diffusion under typical growth condition can reach the emitter and dramatically affect the series resistance, among other effects.

Solar Energy Materials and Solar Cells, 2020

Tellurium allows attaining heavy n-type doping levels in GaAs, which is suited to achieve very lo... more Tellurium allows attaining heavy n-type doping levels in GaAs, which is suited to achieve very low contact resistivities in solar cells. Besides, it modifies the energy bandgap of MOVPE-grown GaInP by reducing the group-III sublattice ordering and presents a strong memory effect which induces residual n-type doping in subsequent layers, potentially detrimental to the performance of the solar cell. In this work, we present an inverted rearheterojunction GaInP solar cell that employs a thick Te-doped GaInP layer as absorber, with a doping profile obtained exclusively by controlling the memory effect of Te coming from the preceding growth of a heavily doped GaAs contact layer. In this way, GaInP is partially disordered with the use of no additional surfactant, leading to an increase in the solar cell bandgap of around 35 meV as compared to traditional samples doped with silicon. In the proof-of-concept experimental devices developed so far, the use of a rear-heterojunction configuration and the bandgap increase results in a global open-circuit voltage enhancement of 109 mV. The photocurrent decreases by 1.32 mA/cm 2 , mostly due to the bandgap blue-shift, with about 0.35 mA/cm 2 attributable to lower carrier collection efficiencies. These preliminary results are discussed by analyzing the I-V curve parameters and quantum efficiencies of a Te-doped rear-heterojunction, a Si-doped rear-heterojunction and a Si-doped front-junction solar cell. An additional advantage is that the emitter sheet resistance is reduced from 551 to 147 Ω/, which offers potential for higher efficiencies through lower front grid shadowing factors, as demonstrated with the concentrator measurements presented.