Extracting Bulk-like Semiconductor Parameters from the Characterization of Colloidal Quantum Dot Film Photoconductors

Abstract

A simple photoconductor structure based on a HgTe colloidal quantum dot (CQD) film on a field-effect transistor substrate is analyzed to provide quantitative information on the mobility, Fermi energy, and carrier number. Then, from the photocurrent with a calibrated light source and the optical constants of the materials, the carrier lifetime and recombination mechanism can be determined. The study confirms that geminate recombination is the dominant lifetime mechanism for these midinfrared HgTe CQD films instead of trapping or Auger recombination. This work is a step toward answering the general question whether the individual properties of CQDs and their distribution in a film can be related to the simpler macroscopic optical and electrical properties of a homogeneous semiconductor.

Intraband Transition of HgTe Nanocrystals for Long-Wave Infrared Detection at 12 μm

“…The data are analyzed further to provide an insight into the performance limitations . The IQE of 3 × 10 –3 is assigned to the small ratio of the drift length to channel length and to the charge carrier generation efficiency, η, with Assuming unity efficiency of carrier generation, the drift length is then only 30 nm, such that the carriers barely move by a few quantum dots at low temperature.…”

“…The data are analyzed further to provide an insight into the performance limitations. 42 The IQE of 3 × 10 −3 is assigned to the small ratio of the drift length to channel length and to the charge carrier generation efficiency, η, with ( )…”

Room-Temperature 15% Efficient Mid-Infrared HgTe Colloidal Quantum Dot Photodiodes

“…Mobilities were calculated using the equation for a MOSFET in the linear regime, as given by , where μ is the mobility, V G is the gate voltage, I SD is the source-drain current across the dot film, L is the electrode gap, w is the channel width, V SD is the source-drain bias, and C̅ is the capacitance per unit area of the gate dielectric. At higher temperatures where there is hysteresis in the transfer curves, the values obtained from the slopes in both the forward and reverse scans are given …”

“…It is ∼160 ns at room temperature, much longer than the exciton lifetime or the hopping time. 24 Figure 1c shows that the diffusion length peaks at about 215 nm at 200 K and is 180 nm at 295 K. Since the mobility is not strongly temperature dependent between these temperatures, the decrease in diffusion length is attributed to a decreased carrier lifetime, caused by a greater concentration of thermal carriers. The small decrease of L D would not explain a 20-fold decrease in quantum efficiency reported in previous PV devices.…”

“…Device characterization is similar to that reported in the literature. 13,14,21,24 The photovoltaic devices are placed under vacuum (<30 mTorr) in a closed-cycle cryostat with a CaF 2 window for measurement and are optionally surrounded by a cold copper shield. The photoconductor devices are placed in a smaller liquid nitrogen cooled cryostat.…”