Wafer-scale detachable monocrystalline Germanium nanomembranes for the growth of III-V materials and substrate reuse

Germanium (Ge) is increasingly used as a substrate for high-performance optoelectronic, photovoltaic, and electronic devices. These devices are usually grown on thick and rigid Ge substrates manufactured by classical wafering techniques. Nanomembranes (NMs) provide an alternative to this approach while offering wafer-scale lateral dimensions, weight reduction, limitation of waste, and cost effectiveness. Herein, we introduce the Porous germanium Efficient Epitaxial LayEr Release (PEELER) process, which consists of the fabrication of wafer-scale detachable monocrystalline Ge NMs on porous Ge (PGe) and substrate reuse. We demonstrate monocrystalline Ge NMs with surface roughness below 1 nm on top of nanoengineered void layer enabling layer detachment. Furthermore, these Ge NMs exhibit compatibility with the growth of III-V materials. High-resolution transmission electron microscopy (HRTEM) characterization shows Ge NMs crystallinity and high-resolution X-ray diffraction (HRXRD) reciprocal space mapping endorses high-quality GaAs layers. Finally, we demonstrate the chemical reconditioning process of the Ge substrate, allowing its reuse, to produce multiple free-standing NMs from a single parent wafer. The PEELER process significantly reduces the consumption of Ge during the fabrication process which paves the way for a new generation of low-cost flexible optoelectronics devices.Comment: 17 pages and 6 figures along with 3 figures in supporting informatio

Photoconversion Optimization of Pulsed-Laser-Deposited p-CZTS/n-Si-Nanowires Heterojunction-Based Photovoltaic Devices

We report on the achievement of novel photovoltaic devices based on the pulsed laser deposition (PLD) of p-type Cu2ZnSnS4 (CZTS) layers onto n-type silicon nanowires (SiNWs). To optimize the photoconversion efficiency of these p-CZTS/n-SiNWs heterojunction devices, both the thickness of the CZTS films and the length of the SiNWs were independently varied in the (0.3–1.0 µm) and (1–6 µm) ranges, respectively. The kësterite CZTS films were directly deposited onto the SiNWs/Si substrates by means of a one-step PLD approach at a substrate temperature of 300 °C and without resorting to any post-sulfurization process. The systematic assessment of the PV performance of the ITO/p-CZTS/n-SiNWs/Al solar cells, as a function of both SiNWs’ length and CZTS film thickness, has led to the identification of the optimal device characteristics. Indeed, an unprecedented power conversion efficiency (PCE) as high as ~5.5%, a VOC of 400 mV, a JSC of 26.3 mA/cm2 and a FF of 51.8% were delivered by the devices formed by SiNWs having a length of 2.2 µm along with a CZTS film thickness of 540 nm. This PCE value is higher than the current record efficiency (of 5.2%) reported for pulsed-laser-deposited-CZTS (PLD-CZTS)-based solar cells with the classical SLG/Mo/CZTS/CdS/ZnO/ITO/Ag/MgF2 device architecture. The relative ease of depositing high-quality CZTS films by means of PLD (without resorting to any post deposition treatment) along with the gain from an extended CZTS/Si interface offered by the silicon nanowires make the approach developed here very promising for further integration of CZTS with the mature silicon nanostructuring technologies to develop novel optoelectronic devices

Substrate temperature optimization of pulsed-laser-deposited and in-situ Zn-supplemented-CZTS films and their integration into photovoltaic devices

The pulsed laser deposition (PLD) technique was used to deposit CZTS thin films onto SLG/Mo substrates via the KrF-laser ablation of a composite target consisting of Cu2ZnSnS4 pellet onto which Zn strips were purposely affixed. The effect of the substrate temperature (Tsub) of the PLD-CZTS films on their structure and properties was systematically studied over the 25–500 °C temperature range. The Zn content of the films was found to increase mainly when Tsub is raised from 300 to 500 °C. While both XRD and Raman analyses confirmed that the films consist of the kësterite-single-phase of which crystallinity improves when Tsub is increased (from RT up to 400 °C), the near resonant Raman (at 325 nm) revealed the presence of ZnS phase at high Tsub (> 400 °C). The optical energy band gap (Eg) of the PLD-CZTS films was consistently found to decrease from 1.9 to 1.4 eV when Tsub is increased from RT to 500 °C. Our results pointed out the Tsub = 400 °C as the optimal deposition temperature that meets at best the properties required for the PLD-CZTS films for PV application. The post-annealing (in presence of S and Sn vapors at 560 °C) of the PLD-CZTS films has improved further their crystallinity and led to the formation of some ZnS secondary phase at their surface. By appropriately integrating these post-annealed films into SLG/Mo/CZTS/CdS/ZnO/ITO photovoltaic devices, we were able to demonstrate their photoconversion ability with a PCE of 3.3 % (Voc = 512 mV, Jsc = 12.5 mA/cm2 and a FF = 51.5 %). The analysis of their EQE spectrum suggests that the effective carrier collection length in the CZTS absorption layer needs to be extended further to achieve higher photoconversion efficiencies.Peer ReviewedPostprint (author's final draft