Figure 1

Schematic of the devices used in the experiments. (a) Lateral device structure consisting of gold electrodes $200\times 800\times 0.1\mu \text{m}$ patterned on 330 nm silicon oxide on a silicon backgate or quartz (not shown). The electrodes are spaced by 1 or $2\mu \text{m}$. The NC film, typically 100–200 nm thick, was drop cast or spun onto the device and covers the entire surface of the device. (b) Layered device structure used in the photovoltaic study. 20-nm-thick PEDOT:PSS was deposited onto ITO covered glass. NC layers, each $\sim 25\text{nm}$ thick, are spun on top of the PEDOT:PSS. The top contact was a EGaIn which was set in a thin PDMS donut (3.5 mm outer diameter, 1.5 mm inner diameter). The EGaIn settles in the area defined by the PDMS donut and forms a conformal contact with the NC film. The EGaIn was contacted with a Pt wire and the ITO with a metal pin. Photoexcitation occurs through the ITO face of the device.Reuse & Permissions

Figure 2

(Color online) Photocurrent versus electric field with $32\text{mW}/{\text{cm}}^2$ 514 nm excitation for mixed films composed of CdTe and CdSe NCs. Each line represents the photocurrent measured in a film containing a different ratio by volume of CdTe to CdSe: (a) 100% CdSe, (b) 14% CdTe, (c) 20% CdTe, (d) 33% CdTe, (e) 50% CdTe, (f) 66% CdTe, (g) 80% CdTe, (h) 88% CdTe, and (i) 100% CdTe. The left inset displays the magnitude of the photocurrent at $7\times {10}^5\text{V}/\text{cm}$ as a function of volume percent of CdTe. The red lines and circles designate films in which the field dependence of the photocurrent resembles that of an all-CdSe NC film while the black lines designate those films in which it resembles the field dependence of the photocurrent through the all CdTe NC film. The right inset shows the photocurrent curves (a)–(c) plotted on a linear scale versus electric field, demonstrating the saturation of the 100% CdSe NC film and the loss of saturation with the addition of CdTe NCs.Reuse & Permissions

Figure 3

(a) Photocurrent versus electric field with $32\text{mW}/{\text{cm}}^2$ 514 nm excitation for a CdTe NC film (black line) and a CdSe/CdTe NB film (gray line). The magnitude of the photocurrent has not been scaled for either of the films but they are matched to have a similar absorbance value at 514 nm. (b) Room-temperature differential conductance $dI/dV$ plotted as a function of gate voltage for a CdSe/CdTe NB film held at $-90{\text{V}}_{\text{ds}}$. $V_{\text{g}}$ is applied for 5 s and followed by a recovery of 5 s at 0 $V_{\text{g}}$ before $V_{\text{g}}$ is stepped to the next value (Ref. 18).Reuse & Permissions

Figure 4

Room-temperature (a) dark and (b) photocurrent current-field curves as a function of exposure to air. The electric field is swept from 0 to $10\times {10}^5\text{V}/\text{cm}$ for each curve. Two curves are taken at each air exposure time to demonstrate that it is the air exposure and not sweeping of the electric field, which results in the enhancement of the current. The insets of graphs (a) and (b) are the same data scaled in current and plotted on a log plot to highlight the electric field dependence of the current. The current is measured in a cryostat under vacuum and then the sample is opened to air for the designated period of time.Reuse & Permissions

Figure 5

XPS spectra for a CdTe NC film before (gray line) and after (black line) exposure to air for 3 weeks. Characteristic $\text{Te}4d$, $\text{Te}3d$, and $\text{Cd}3d$ peaks are shown before (gray) and after (black) oxidation of the CdTe NC film. The unoxidized $\text{Te}4d$ doublet located at 40.73 and 42.2 eV is assigned to Te-Cd in accordance with bulk CdTe values (Ref. 21). In the $\text{Te}4d$ and $\text{Te}3d$ spectra of the oxidized film additional peaks appear shifted $\sim 3.5\text{eV}$ to higher binding energy. These are attributed to ${\text{TeO}}_2$ and ${\text{CdTeO}}_3$ surface states. The $\text{Cd}3d$ peak shows only a slight shift to higher energies, possibly due to Cd-O formation or oxidation of the ligands which are bonded to the Cd.Reuse & Permissions

Figure 6

(Color online) Energy-level diagrams for (a) device #1, (b) device #2, and (c) device #3. Energies are given relative to vacuum level and represent the values for each component prior to establishment of a uniform Fermi level across the device (band bending). For the ITO and EGaIn electrodes, the lines indicate the work functions of the metals. For the semiconductor materials, the bottom of the rectangle indicates the position of the valence band and the top of the rectangle indicates the position of the conduction band. The dotted line indicates the position of the proposed electron trapping state on the CdTe NC surface. All devices exhibit diode behavior and the arrows indicate the direction of forward bias in each device (see discussion).Reuse & Permissions

Figure 7

(Color online) (a) The absorption spectra of CdSe and CdTe NCs in solution. (b) The PCA spectra for device structure #1 is displayed at 0 V (black line), $+0.6\text{V}$ (green line, same as 0 V), and $-0.6\text{V}$ (red line). The absorbance of device #1 is given by the dotted line. (c) The PCA spectra for device #2 is displayed at 0 V (black line) and $+0.6\text{V}$ (gray line). The absorbance of device #2 is given by the dotted line. (d) The PCA spectra for device #3 is displayed at 0 V (black line) and $+0.6\text{V}$ (red line). The absorbance of device #3 is given by the dotted line.Reuse & Permissions

Figure 8

Energy-level diagram for a CdTe NC with an acceptor state 0.38 eV from the valence band denoted by $1{\text{S}}_{\text{h}}$. We ignore any intrinsic width of the valence and conduction bands. At room temperature this state can be populated with electrons (black circles) thermally excited from the valence band, leaving holes behind (white circles). As the temperature decreases, the number of electrons in the acceptor state (and thus the hole density) decreases.Reuse & Permissions