Photoconductivity studies of treated CdSe quantum dot films exhibiting increased exciton ionization efficiency

We present measurements of photoconductivity in CdSe quantum dot films treated with a variety of reagents. While the photocurrent of untreated samples is highly voltage dependent at all voltages, after treatment the photocurrent is much larger, depends strongly on voltage at low voltage, displays a linear region above a voltage threshold, and finally saturates at high voltage. All regions of the current-voltage curves after treatment can be reproduced with a model that requires noninjecting contacts and a field dependent exciton ionization efficiency that saturates to unity. This model is shown to be consistent with the trends observed with different treatments. The changes in photocurrent with treatment are shown to be largely a consequence of increased quantum dot surface passivation and decreased quantum dot spacing, regardless of whether the molecules used for treatment are conjugated or able to cross-link the quantum dots.

Figure 1

Photocurrent at $77\mathrm{K}$ before (open squares, dashed line) and after (open circles, solid line) treatment with (a) butylamine (dried at room temperature overnight), (b) buytlamine (dried at $70°\mathrm{C}$ for $1\mathrm{h}$), (c) aniline, (d) tri-$n$-butylphosphine, (e) 1,6-diaminohexane, and (f) 1.4-phenylenediamine.

Figure 2

Photocurrent at $77\mathrm{K}$ before (open squares, dashed line) and after (open circles, solid line) treatment with sodium hydroxide.

Figure 3

Dark current at $77\mathrm{K}$ (a) before (open squares, dashed line) and (b) after treatment with butylamine and drying at $70°\mathrm{C}$ for $1\mathrm{h}$ (open circles, solid line).

Figure 4

(a) GAXS raw data for $\mathbf{a}$ reference, $\mathbf{b}$ 1,4-phenylenediamine, $\mathbf{c}$ aniline, $\mathbf{d}$ tri-$n$-butylphosphine, $\mathbf{e}$ butylamine (dried at room temperature overnight), $\mathbf{f}$ 1,6-diaminohexane, $\mathbf{g}$ butylamine (dried at $70°\mathrm{C}$ for $1\mathrm{h}$), $\mathbf{h}$ sodium hydroxide. Data are offset for clarity. (b) The pair distribution function $g\left(r\right)$ for the reference and treated samples. The solid line marks the center of the peak and the labels are the same as in (a).

Figure 5

Intensity dependence of the photocurrent for a sample treated with sodium hydroxide at $100\mathrm{V}$, after the photocurrent has saturated with voltage. Open circles represent data and the solid line represents a linear fit.

Figure 6

Photocurrent data (open circles) for a sample treated with butylamine and dried at $70°\mathrm{C}$ for $1\mathrm{h}$, plotted with reference data (solid line). The current and voltage values for the reference data are scaled to illustrate that the shape of the butylamine treated $i\text{−}V$ curve, at low voltages, is the same as the shape of the reference at all voltages.

Figure 7

(a) Fluorescence quenching data (filled squares) for a sample treated with 1,6-diaminohexane compared to the $i\text{−}V$ curve (open circles, solid line) for the same sample. The shaded region contains voltages at which the $i\text{−}V$ curve is linear and a saturated exciton ionization efficiency is predicted. (b) Time trace of the fluorescence intensity for the same sample as in (a) where the white region corresponds to $0\mathrm{V}$ and the shaded region corresponds to $+20\mathrm{V}$ (open squares, dashed line) or $+100\mathrm{V}$ (open circles, solid line). The curves corresponding to $+20\mathrm{V}$ and $+100\mathrm{V}$ are offset for clarity. A quantitative number for the percent fluorescence quenching in (a) and (b) was not obtainable due to high background fluorescence.

Figure 8

(a) Photocurrent before treatment at $77\mathrm{K}$ (open circles, solid line) and $300\mathrm{K}$ (open squares, dashed line). (b) Photocurrent after treatment with butylamine (oven-dried) at $77\mathrm{K}$ (open circles, solid line) and $300\mathrm{K}$ (open squares, dashed line).

Figure 9

Simulation of the current-voltage characteristics (solid line) for a photoconductor in a sandwich structure with two mobile carriers, noninjecting electrodes, and a field-dependent generation rate. For this simulation, the field characterizing the generation rate saturation is set to 0.7 times the field characterizing the current saturation. The ratio of $\mu \tau$ for the two carriers is set to 0.6. The simulation breaks down at saturation, at which point the current is given by the horizontal dashed line. For comparison, photocurrent data (open circles) for a sample treated with butylamine and dried at $70°\mathrm{C}$ are also presented. The current and voltage are scaled so that both are equal to one when the photocurrent saturates. The data are meant to be qualitatively compared to the simulation, since the simulation is not a result of a fit to the data.