• Energy Research

Germanium is a low-cost alternative to the highly expensive III-V semiconductors commonly used on thermophotovoltaic (TPV) devices. Despite efficiencies up to 16.5 % having been theorized in previous works, no experimental efficiencies have been reported so far for germanium-based devices. In this work, we present the first experimental germanium TPV cell efficiency measured under high view factor and high temperature irradiance conditions. Efficiencies and electric power densities up to 11.2 % and 1.43 W/cm2 have been obtained using a graphite emitter heated at 1440ºC.

AbstractIn this work, a photovoltaic mini‐module combining interdigitated back‐contacted solar cells with black silicon in the front was implemented as a proof of concept. The module consists of nine solar cells connected in series with an active area of 86.5 cm2. Both the assembly and panel encapsulation were made using industrial back‐contact module technology. Noticeable photovoltaic efficiencies of 18.1% and 19.9% of the whole module and the best individual cell of the module, respectively, demonstrate that fragile nanostructures can withstand standard module fabrication stages. Open‐circuit voltage and fill factor values of 5.76 V and 81.6%, respectively, reveal that series interconnection between cells works as expected, confirming a good ohmic contact between cell busbars and the conductive backsheet. Additionally, the excellent quasi‐omnidirectional antireflection properties of black silicon surfaces prevail at module level, as it is corroborated by light incidence angle dependence measurements of the short‐circuit current parameter.

Germanium is a low cost alternative to the highly expensive III-V semiconductors commonly used on thermophotovoltaic (TPV) devices. Despite efficiencies up to 16.5 % have been theorized in previous works, no experimental efficiencies have been reported so far for germanium-based devices. In this work, we present the first TPV germanium efficiency measured in a high view factor calorimetry set up. Efficiencies and power densities up to 11.2 % and 1.43 W/cm2 have been obtained using a graphite emitter heated at 1544 ºC.

In this work, we study bifacial heterojunction silicon solar cells fabricated by hot-wire CVD on both p- and n-type crystalline silicon substrates. In these devices, hydrogenated microcrystalline silicon doped layers are combined with thin intrinsic amorphous silicon buffers to form the heterojunction emitter and the low-temperature back surface field contact. The maximum temperature achieved in the whole fabrication process is 200 °C. A comprehensive electrical characterization has been done, including current density-voltage characteristics and external quantum efficiency curves with illumination from both sides. These results evidence the feasibility of efficient bifacial heterojunction silicon solar cells fully processed at low temperature. Peer Reviewed

© 2022 Elsevier. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ Surface passivation is of paramount importance in photovoltaic devices based on high-quality crystalline absorbers such as crystalline germanium (c-Ge). In this work, we report on the surface passivation of p-type c-Ge by an amorphous silicon carbide/aluminium oxide film stack (a-SiCx/Al2O3). In particular, we focus on the impact of the thin (0, 1, 2, and 4 nm) a-SiCx interlayer which is used to provide chemical passivation while the Al2O3 is responsible for the field-effect passivation due to its high negative fixed charge density. The measured effective surface recombination velocity (Seff) values show that the introduction of the a-SiCx interlayer improves surface passivation, but the thinner the a-SiCx film the better. High frequency capacitance-voltage characterization leads to the determination of the fixed charge density (Qf) which indicates that the a-SiCx is shielding the negative charge located at the first nanometers of the Al2O3 film, as suggested by chemical characterization of the interface. As a consequence, the best result is Seff = 18 cm/s for the case of 1 nm of a-SiCx. This work has been supported by the Spanish government under projects PID2019–109215RB-C41 (SCALED), PID2020–116719RB-C41 (MATER ONE) and PID2020–115719RB-C21 (GETPV) funded by MCIN/AEI/10.13039/501100011033. The authors would like to thank the master student Guillem Ayats for his help in processing the samples, Dr. Alejandro Datas from Instituto de Energía Solar (IES) in Madrid for providing the c-Ge wafers and Dr. Rodrigo Fernández-Pacheco of the Laboratorio de Microscopias Avanzadas (LMA-INA) of Zaragoza for the EELS analysis. Peer Reviewed

Selective thermal emitters concentrate most of their spontaneous emission in a spectral band much narrower than a blackbody. When used in a thermophovoltaic energy conversion system, they become key elements defining both its overall system efficiency and output power. Selective emitters' radiation spectra must be designed to match their accompanying photocell's band gap and simultaneously, withstand high temperatures (above 1000 K) for long operation times. The advent of nanophotonics has allowed the engineering of very selective emitters and absorbers; however, thermal stability remains a challenge since nanostructures become unstable at temperatures much below the melting point of the used materials. In this paper we explore a hybrid 3D dielectric-metallic structure that combines the higher thermal stability of a monocrystalline 3D silicon scaffold with the optical properties of a thin platinum film conformally deposited on top. We show experimentally that these structures exhibit a selective emission spectrum suitable for TPV applications and that they are thermally stable at temperatures up to 1100 K. These structures are ideal in combination with HI-V semiconductors in the range E-g=0.4-0.55 eV such as InGaAsSb (E-g=0.5-0.6 eV) and InAsSbP (E-g=0.3-0.55 eV). (C) 2014 Elsevier B.V. All rights reserved. Peer Reviewed

Crystalline germanium (c-Ge) has historically been regarded as a cost-effective alternative to III-V semiconductors for thermophotovoltaic (TPV) device fabrication. However, Ge-based devices have not yet reported high efficiencies, partially due to the lack of an efficient back-surface reflector that turns back to the heat source out-band (sub-bandgap) thermal radiation. The difficulty of implementing back surface reflectors in Ge TPV cells is related to the simultaneous requirement of good back surface passivation, low electrical resistivity, and high out-band optical reflectivity. In this study, we demonstrate a highly reflective ohmic contact to p-type c-Ge (doping concentration of 2 × 1015 cm-3) made of an aSiCx(1 nm)/Al2O3 (50 nm)/aSiC (45 nm) stack that is laser processed using Nd:YVO4 laser emitting at 355 nm to create punctual p+ contacts (locally diffused Al regions). This stack is finally caped with a thick (1000 nm) Al layer that behaves as a metallic mirror and back electrode. As the laser processed area increases from 0.1 to 3 %, which is the typical range in the final devices, the surface recombination velocity increase from 10.5 to 60.0 cm/s, while the effective contact resistance reduces from 0.462 to 0.036 O cm2. Moreover, a sub-bandgap reflectance of 90–98 % is achieved. Simulations assuming ideal device configuration indicate that implementing these back contacts could potentially enable TPV cell conversion efficiencies comparable to the reported high-efficiency c-Ge TPV cells operating at similar illumination temperature. Objectius de Desenvolupament Sostenible::7 - Energia Assequible i No Contaminant