Nanofabrication processes for innovative nanohole-based solar cells

Abstract

The ability to form ordered nanostructures at the wafer level with low cost methodologies has represented a challenge in the last decade in many research fields spanning from nanoelectronics to photovoltaics (PVs). For the latter application the nanostructures have demonstrated interesting capabilities for exploiting the quantum effects in terms of efficient visible light absorption. To fabricate ordered nanostructures many solutions have been proposed but they provide feature densities lower than 10 9 cm À2 or present high fabrication costs. We propose a wafer level and low-cost Lithography based on block CoPolymers self-assembling (LCP), which allows the formation of nanofeatures controlled down to 10 nm and density higher than 5 Â 10 10 cm À2. We propose to use this technique to form radial junctions in nanoholes for solar cells. The approach is similar to that of the nanowires, i.e., it decouples the optical path of the visible photons from the electrical path of the carriers, but since the one-dimensional (1D) structures are embedded inside the bulk of the wafer the structure is more robust and allows easier implementation. To form the junction inside the nanoholes a novel strategy based on the deposition of monolayers of dopant-containing molecules is proposed. This technique allows to obtain shallow and controlled junction depths with peak carrier concentrations of about 10 19 cm À3 for both n-and p-type doping.

A comprehensive study on the physicochemical and electrical properties of Si doped with the molecular doping method

“…Another method recently proposed in literature for the doping of semiconductors is the molecular doping (MD) process [10][11][12][13][14][15]. It is based on the formation of selfassembled layers of dopant-containing molecules on the surface to be doped.…”

“…In all these mentioned approaches, the deposition process is followed by a high temperature annealing step when the molecule decomposes and the dopant atoms diffuse from the surface into the substrate lattice. The substrate material frequently used as a target is Si [15][16][17][18][19][20][21], but InAs and InGaAs have been successfully doped too [12,22], together with oxidized silicon, alumina, and mica to study the molecule anchoring mechanisms [23,24]. The dopant atoms typically injected by the MD technique are phosphorus, boron, and sulfur.…”

Applications of Monolayer‐Functionalized H‐Terminated Silicon Surfaces: A Review

“…Electrical characterization showed an increase in the fill factor from 10 to 35% for the MLD solar cells compared to planar control samples treated equally, although the open‐circuit voltage decreased from 0.42 to 0.27 V. The solar‐cell performance can be increased even further when using 3D micropillars/nanowires with a radial p–n junction, since the larger surface‐to‐volume ratio and larger junction area enhance the efficiency. The use of MLD to create radial junctions was also reported in solar cells based on nanohole arrays, where the doping was also found to be conformal in the vertical direction.…”

Monolayer functionalization of silicon micro and nanowires : towards solar-to-fuel and sensing devices

“…Electrical characterization showed an increase in the fill factor from 10 to 35% for the MLD solar cells compared to planar control samples treated equally, although the open-circuit voltage decreased from 0.42 to 0.27 V. The solar cell performance can be increased even further when using 3D micropillars/nanowires with a radial p-n junction, since the larger surface-to-volume ratio and larger junction area enhance the efficiency. The use of MLD to create radial junctions was also reported in solar cells based on nanohole arrays, 99,100 where the doping was also found to be conformal in the vertical direction.…”

“…6,13 In the case of Si (111) surfaces, immersion in a 40% aqueous ammonium fluoride (NH 4 F) solution results in Si monohydride sites. For Si (100), an aqueous 1% hydrogen fluoride (HF) solution is usually used for Si-H formation, which predominantly results in Si dihydride sites, but also monohydride and trihydride sites are formed because of the different crystal lattice compared to Si(111). 5,6 2 After Si-H formation (Scheme 2.1a), different routes have been reported for monolayer formation (Scheme 2.1b-i).…”