Steering the Efficiency of Carbon Nanotube–Silicon Photovoltaic Cells by Acid Vapor Exposure: A Real-Time Spectroscopic Tracking

Abstract

Hybrid carbon nanotube-silicon (CNT-Si) junctions have been investigated by angle resolved photoemission spectroscopy (AR-XPS) with the aim to clarify the effects of a nonstoichiometric silicon oxide buried interface on the overall cell efficiency. A complex silicon oxide interface has been clearly identified and its origin and role in the heterojunction have been probed by exposing the cells to hydrofluoric (HF) and nitric (HNO3) acid. Real-time monitoring of the cell efficiencies during the steps following acid exposure (up to 1 week after etching) revealed a correlation between the thickness and chemical state of the oxide layer and the cell efficiencies. By matching the AR-XPS and Raman spectroscopy with the electrical response data it has been possible to discriminate the effects on the cell efficiency of the buried SiO(x) interface from those related to CNT acid doping. The overall cell behavior recorded for different thicknesses of the SiO(x) interface indicates that the buried oxide layer is likely acting as a passivating/inversion layer in a metal-insulator-semiconductor junction.

A cross-functional nanostructured platform based on carbon nanotube-Si hybrid junctions: where photon harvesting meets gas sensing

“…Indeed, the overall cell structure can be described as a FET, where the gate potential is represented by the built-in potential of the p-n heterojunction. It is important to recall that CNT-Si cells based on a p-type CNT bundle layer can be regarded as MIS944, as they can display a Si oxide layer at the interface between the CNT bundle layer and the n-Si. This justifies the choice to describe these junctions as a FET architecture.…”

“…As the present PV devices are usually described in a MIS scheme944, V OC (Equation 2) can be expressed6162 as:…”

“…In particular the CNT-Si junction prepared by using a layer of CNT bundles can be regarded as an open system: for example chemical interactions are known to affect also the CNT-Si interface and ultimately the cell properties through selective etching or optimal oxidation of the buried SiO x layer at the interface134445. Etching with HNO 3 or HF is known to selectively tailor the thickness and the chemical properties of the buried SiO x layer, allowing to explore its role in determining the PV cell efficiency.…”

Semiconducting Carbon Nanotubes: Properties, Characterization and Selected Applications

“…For understanding the operation mechanism, a combination of suitable spectroscopic tools has been used recently [39]: AR-XPS for a direct probe of the buried oxide interface, Raman spectroscopy to obtain information on CNT properties, and the acquisition of current density-voltage (J-V) characteristics for monitoring the behavior of the photovoltaic device after exposure to acid.…”

Improving Carbon Nanotube‐Silicon Solar Cells by Solution Processable Metal Chlorides

“…According to previous studies, introducing functional materials into the CNT‐Si solar cells has proved to be an effective strategy to improve the cell efficiency. To this end, a variety of materials, such as acids, metallocenes, Au nanodots, AuCl 3 , colloidal TiO 2, polymethylmethacrylate, MoO x , and CuCl 2 /Cu(OH) 2 , have been explored, whose functions generally fall into three categories, including 1) chemical doping; 2) antireflection; and 3) acting as functional layers. For example, it was found that those introduced materials could dope CNTs and modulate the work function of CNTs as well as the diode/junction characteristics (e.g., for HNO 3 and Au salt) or act as an antireflection layer to enhance the light absorption and external quantum efficiency (e.g., for TiO 2 and polymethylmethacrylate) .…”