| 1. |
- Barrigón, Enrique, et al.
(author)
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Degradation of Ge subcells by thermal load during the growth of multijunction solar cells
- 2018
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In: Progress in Photovoltaics: Research and Applications. - : Wiley. - 1062-7995. ; 26:2, s. 102-111
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Journal article (peer-reviewed)abstract
- Germanium solar cells are used as bottom subcells in many multijunction solar cell designs. The question remains whether the thermal load originated by the growth of the upper layers of the multijunction solar cell structure affects the Ge subcell performance. Here, we report and analyze the performance degradation of the Ge subcell due to such thermal load in lattice-matched GaInP/Ga(In)As/Ge triple-junction solar cells. Specifically, we have detected a quantum efficiency loss in the wavelength region corresponding to the emitter layer (which accounts for up to 20% loss in equivalent JSC) and up to 55 mV loss in VOC of the Ge subcell as compared with analogous devices grown as single-junction Ge solar cells on the same type of substrates. We prove experimentally that there is no direct correlation between the loss in VOC and the doping level of the base. Our simulations show that both the JSC and VOC losses are consistent with a degradation of the minority carrier properties at the emitter, in particular at the initial nanometers of the emitter next to the emitter/window heterointerface. In addition, we also rule out the gradual emitter profile shape as the origin of the degradation observed. Our findings underscore the potential to obtain higher efficiencies in Ge-based multijunction solar cells if strategies to mitigate the impact of the thermal load are taken into consideration.
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| 2. |
- Barrigón, Enrique, et al.
(author)
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Light current-voltage measurements of single, as-grown, nanowire solar cells standing vertically on a substrate
- 2020
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In: Nano Energy. - : Elsevier BV. - 2211-2855. ; 78
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Journal article (peer-reviewed)abstract
- Nanowire based solar cells hold promise for terrestrial and space photovoltaic applications. However, to speed-up and further continue with nanowire solar cell development, quick and reliable characterization tools capable of evaluating single nanowire performance of nanowires still standing on the substrate are necessary. Here, we present the use of a light emitting diode (LED) based setup, which combined with a nanoprobe system inside a scanning electron microscope, enables on-wafer, single, nanowire solar cell optoelectronic characterization. In particular, we study the I–V characteristics of single nanowire solar cells under in situ illumination and correlate the results with those of electron beam induced current measurements. Further, the LED setup enables the study of nanowire solar cell under varied incident power. We believe that this approach will enable rapid development of single and tandem nanowire based solar cells as well as other nanowire based optoelectronic devices.
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| 3. |
- Barrigon, Enrique, et al.
(author)
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Nanoprobe-Enabled Electron Beam Induced Current Measurements on III-V Nanowire-Based Solar Cells
- 2019
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In: 2019 IEEE 46th Photovoltaic Specialists Conference, PVSC 2019. - 0160-8371. - 9781728104942 ; , s. 2730-2733
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Conference paper (peer-reviewed)abstract
- Electron beam induced current (EBIC) is a well-established tool to, among others, locate and analyze p-n junctions, Schottky contacts or heterostructures in planar devices and is now becoming essential to study and optimize devices at the nanoscale, like III-V nanowire (NW) based solar cells. Here, we report on EBIC measurements on III-V single NW solar cells as well as on fully processed NW devices. This paper also highlights the importance of EBIC to optimize short circuit current density values of fully processed nanowire solar cells of 1 mm2.
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| 4. |
- Barrigón, Enrique, et al.
(author)
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Synthesis and Applications of III-V Nanowires
- 2019
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In: Chemical Reviews. - : American Chemical Society (ACS). - 0009-2665 .- 1520-6890. ; 119:15, s. 9170-9220
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Research review (peer-reviewed)abstract
- Low-dimensional semiconductor materials structures, where nanowires are needle-like one-dimensional examples, have developed into one of the most intensely studied fields of science and technology. The subarea described in this review is compound semiconductor nanowires, with the materials covered limited to III-V materials (like GaAs, InAs, GaP, InP,...) and III-nitride materials (GaN, InGaN, AlGaN,...). We review the way in which several innovative synthesis methods constitute the basis for the realization of highly controlled nanowires, and we combine this perspective with one of how the different families of nanowires can contribute to applications. One reason for the very intense research in this field is motivated by what they can offer to main-stream semiconductors, by which ultrahigh performing electronic (e.g., transistors) and photonic (e.g., photovoltaics, photodetectors or LEDs) technologies can be merged with silicon and CMOS. Other important aspects, also covered in the review, deals with synthesis methods that can lead to dramatic reduction of cost of fabrication and opportunities for up-scaling to mass production methods.
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| 5. |
- Barrigón, Enrique, et al.
(author)
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Unravelling processing issues of nanowire-based solar cell arrays by use of electron beam induced current measurements
- 2020
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In: Nano Energy. - : Elsevier BV. - 2211-2855. ; 71
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Journal article (peer-reviewed)abstract
- III-V vertical nanowire arrays have great potential for next generation photovoltaics. Development towards high performing nanowire solar cells, which consist of a parallel connection of millions of single nanowire solar cells, requires a fast characterization technique that establishes a link between device performance and device processing. In this work, we use electron beam induced current measurements to characterize fully processed InP nanowire array solar cells at the nanoscale. Non-functional areas on fully processed devices can be quickly identified and processing induced effects on device performance can be clearly distinguished from those arising from nanowire growth. We identify how limiting factors on device performance are related to the processing procedures and provide a path to improve device performance further. In this way, electron beam induced current measurements become an essential tool for nanowire solar cell efficiency optimization, providing fast and useful information at the nanoscale and thus enabling up-scaling of the technology.
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| 6. |
- Barrutia, Laura, et al.
(author)
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Effect of Ge autodoping during III-V MOVPE growth on Ge substrates
- 2017
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In: Journal of Crystal Growth. - : Elsevier BV. - 0022-0248. ; 475, s. 378-383
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Journal article (peer-reviewed)abstract
- During the MOVPE growth of III-V layers on Ge substrates, Ge atoms can be evaporated or etched from the back of the wafer and reach the growth surface, becoming incorporated into the epilayers. This is the so-called Ge autodoping effect, which we have studied through a set of growth experiments of GaInP and Ga(In)As layers lattice matched to Ge substrates, which have been characterized by Secondary Ion Mass Spectroscopy. The role of V/III ratio and growth rate on Ge autodoping has been studied and a MOVPE reactor pre-conditioning prior to the epitaxial growth of III-V semiconductor layers that mitigates this Ge autodoping has been identified. In addition, the use of 2-in. versus 4-in. Ge substrates has been compared and the use of a Si3N4 backside coating for the Ge substrates has been evaluated.
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| 7. |
- Barrutia, Laura, et al.
(author)
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Impact of the III-V/Ge nucleation routine on the performance of high efficiency multijunction solar cells
- 2020
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In: Solar Energy Materials and Solar Cells. - : Elsevier BV. - 0927-0248. ; 207
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Journal article (peer-reviewed)abstract
- This paper addresses the influence of III-V nucleation routines on Ge substrates for the growth of high efficiency multijunction solar cells. Three exemplary nucleation routines with differences in thickness and temperature were evaluated. The resulting open circuit voltage of triple-junction solar cells with these designs is significantly affected (up to 50 mV for the best optimization routine), whereas minimal differences in short circuit current are observed. Electroluminescence measurements show that both the Ge bottom cell and the Ga(In)As middle cell present a VOC gain of 25 mV each. This result indicates that the first stages of the growth not only affect the Ge subcell itself but also to subsequent subcells. This study highlights the impact of the nucleation routine design in the performance of high efficiency multijunction solar cell based on Ge substrates.
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| 8. |
- Chen, Yang, et al.
(author)
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Semiconductor nanowire array for transparent photovoltaic applications
- 2021
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In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 118:19
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Journal article (peer-reviewed)abstract
- The surface area of a building that could potentially be used for Building Integrated Photovoltaics would increase dramatically with the availability of transparent solar cells that could replace windows. The challenge is to capture energy from outside the visible region (UV or IR) while simultaneously allowing a high-quality observation of the outside world and transmitting sufficient light in the visible region to satisfactorily illuminate the interior of the building. In this paper, we show both computationally and experimentally that InP nanowire arrays can have good transparency in the visible region and high absorption in the near-infrared region. We show experimentally that we can achieve mean transparencies in the visible region of 65% and the radiative limit of more than 10% based on measured absorption and calculated emission. Our results demonstrate that nanowire arrays hold promise as a method to achieve transparent solar cells, which would fulfill the requirements to function as windows. In addition, we show that by optical design and by designing the geometry of nanowire arrays, solar cells can be achieved that absorb/transmit at wavelengths that are not decided by the bandgap of the material and that can be tailored to specific requirements such as colorful windows.
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| 9. |
- Cordoba, Cristina, et al.
(author)
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Three-Dimensional Imaging of Beam-Induced Biasing of InP/GaInP Tunnel Diodes
- 2019
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In: Nano Letters. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; , s. 3490-3497
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Journal article (peer-reviewed)abstract
- Electron holographic tomography was used to obtain three-dimensional reconstructions of the morphology and electrostatic potential gradient of axial GaInP/InP nanowire tunnel diodes. Crystal growth was carried out in two opposite directions: GaInP–Zn/InP–S and InP–Sn/GaInP–Zn, using Zn as the p-type dopant in the GaInP but with changes to the n-type dopant (S or Sn) in the InP. Secondary electron and electron beam-induced current images obtained using scanning electron microscopy indicated the presence of p–n junctions in both cases and current–voltage characteristics measured via lithographic contacts showed the negative differential resistance, characteristic of band-to-band tunneling, for both diodes. Electron holographic tomography measurements confirmed a short depletion width in both cases (21 ± 3 nm) but different built-in potentials, Vbi, of 1.0 V for the p-type (Zn) to n-type (S) transition, and 0.4 V for both were lower than the expected 1.5 V for these junctions if degenerately doped. Charging induced by the electron beam was evident in phase images which showed nonlinearity in the surrounding vacuum, most severe in the case of the nanowire grounded at the p-type Au contact. We attribute their lower Vbi to asymmetric secondary electron emission, beam-induced current biasing, and poor grounding contacts.
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| 10. |
- Dagyte, Vilgaile, et al.
(author)
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Time-resolved photoluminescence characterization of GaAs nanowire arrays on native substrate
- 2017
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In: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 28:50
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Journal article (peer-reviewed)abstract
- Time-resolved photoluminescence (TRPL) measurements of nanowires (NWs) are often carried out on broken-off NWs in order to avoid the ensemble effects as well as substrate contribution. However, the development of NW-array solar cells could benefit from non-destructive optical characterization to allow faster feedback and further device processing. With this work, we show that different NW array and substrate spectral behaviors with delay time and excitation power can be used to determine which part of the sample dominates the detected spectrum. Here, we evaluate TRPL characterization of dense periodic as-grown GaAs NW arrays on a p-type GaAs substrate, including a sample with uncapped GaAs NWs and several samples passivated with AlGaAs radial shell of varied composition and thickness. We observe a strong spectral overlap of substrate and NW signals and find that the NWs can absorb part of the substrate luminescence signal, thus resulting in a modified substrate signal. The level of absorption depends on the NW-array geometry, making a deconvolution of the NW signal very difficult. By studying TRPL of substrate-only and as-grown NWs at 770 and 400 nm excitation wavelengths, we find a difference in spectral behavior with delay time and excitation power that can be used to assess whether the signal is dominated by the NWs. We find that the NW signal dominates with 400 nm excitation wavelength, where we observe two different types of excitation power dependence for the NWs capped with high and low Al composition shells. Finally, from the excitation power dependence of the peak TRPL signal, we extract an estimate of background carrier concentration in the NWs.
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