Preparation and Characterization of Nickel Oxide Photocathodes Sensitized with Colloidal Cadmium Selenide Quantum Dots

Abstract

Quantum dot sensitized solar cells (QDSCs) are receiving a lot of attention as promising third generation solar cells, being virtually all of them based on sensitized photoanodes. Finding efficient QD-sensitized photocathodes would pave the way toward the implementation of tandem QDSCs. In this context, NiO photocathodes have been sensitized with colloidal CdSe quantum dots directly attached to the semiconductor oxide surface. The emission spectra indicate effective hole injection from the excited state of the quantum dots to the valence band of the NiO. A maximum incident current to photon conversion efficiency of 17% at 420 nm has been achieved. For the sake of comparison, other ways to prepare and anchor the QDs have been tested. Sensitization routes based on presynthesized colloidal quantum dots show better results than in situ growth techniques such as successive ionic layer adsorption and reaction. Electrochemical impedance measurements have identified transport resistance in NiO as one of the limiting factors in the performance of the system under study. Interestingly, surface treatments based on the deposition of very thin films of either SiO 2 or Al 2 O 3 can diminish recombination at the NiO/CdSe/electrolyte interface. This work also identifies a number of possible routes for the improvement of this kind of electrodes, unveiling their potential use in tandem quantum dot solar cells.

Drastic difference between hole and electron injection through the gradient shell of CdxSeyZn1−xS1−y quantum dots

“…The overall collection efficiency of the photo-generated charges by the MO depends both on the surface passivation and the hole injection process. 20 In our previous work, we confirmed the existence of hole injection from Cd x Se y Zn 1−x S 1−y gradient CSQDs to NiO in the CSQD/NiO system. 22 Here, we aim at obtaining further insight into the dependence of the hole injection dynamics on the gradient shell thickness.…”

“…Here, the holes are injected from the valence band of the QDs to the p-type MO acceptor because the process is energetically favorable, while the electrons can be scavenged by the redox couples. 20,21 Hole injection from neat colloidal CdSe QDs to p-type MO was reported with a low incident photon-to-current conversion efficiency (17%) due to the strong competition between hole injection and surface hole trapping. 20 However, our preliminary results have shown that the core-shell structure can passivate such surface traps and consequently enhance the hole injection efficiency.…”

“…20,21 Hole injection from neat colloidal CdSe QDs to p-type MO was reported with a low incident photon-to-current conversion efficiency (17%) due to the strong competition between hole injection and surface hole trapping. 20 However, our preliminary results have shown that the core-shell structure can passivate such surface traps and consequently enhance the hole injection efficiency. 22 In this work, we systematically study the hole injection from gradient CSQDs (Cd x Se y Zn 1−x S 1−y ) to NiO mesoporous film by time-resolved photoluminescence (TRPL) and transient absorption (TA) spectroscopies.…”

Developments in and prospects for photocathodic and tandem dye-sensitized solar cells

“…The majority of reports into pQDSC have, however, used a polysulfide redox mediator, S 2-/Sx 2-, although (Co(II)/(III)) based electrolytes have also been successfully employed as well [120]. Furthermore, dense…”

“…CdS and CdSe have been the most popular choices of inorganic sensitizer [119,120,[122][123][124], including a dual sensitizer CdSe + CdS system, which forms an energy cascade [121].…”