Vertical Nanochannels in Gallium Nitride for Hybrid Organic/Inorganic Photovoltaics

by Mark Schwab

Institution: Yale University
Year: 2015
Keywords: Chemical engineering; Nanotechnology; Materials science
Posted: 02/05/2017
Record ID: 2125874
Full text PDF: http://pqdtopen.proquest.com/#viewpdf?dispub=3663661


Hybrid organic/inorganic photovoltaics can overcome many traditional shortcomings of organic photovoltaics, including recombination due to short exciton diffusion length scales, incomplete or tortuous charge transport pathways, and low charge mobility. In this work, aligned pore arrays are electrochemically etched into GaN films, and the semiconducting polymer polyhexylthiophene (P3HT) is intruded into these porous films. This hybrid device uses the polymer as the photoactive phase, electron donor, and hole transport medium, and the GaN as the electron acceptor and electron transport medium. Not only does the nanoporous geometry result in ultrafast charge transfer between the P3HT and the GaN, but a nanoconfined geometry can also drastically enhance charge mobility in the polymer by orienting the polymer alignment such that the fast charge transport direction is oriented vertically. Optimal etching parameters are found in various etchants to produce an aligned morphology, and a method to remove the low-porosity overlayer via UV-assisted etching is described. Additionally, the first reported pore formation in GaN using a neutral etching solution is demonstrated, opening up the possibility of safe and environmentally-friendly etching of GaN, in contrast to traditional methods that use extremely toxic hydrofluoric acid. Multiple methods to introduce polymer into the pores are described, and it is shown that hot pressing can achieve favorable polymer alignment. Ultrafast charge transport is demonstrated between the confined polymer and the GaN template by time-resolved terahertz spectroscopy. This geometry of an aligned nanoporous template surrounding an organic semiconductor is proposed as a general and beneficial strategy to improve performance of organic solar cells.