Colloidal Mn-doped ZnSe/CdS core/shell quantum dots (QDs) are synthesized for the first time and employed as a strategy to boost the power conversion efficiency of quantum dot sensitized solar cells. By using Mn-doping as a band gap engineering tool for core/shell QDs an effective improvement of absorption spectra could be obtained. The mid-states generated by a proper Mn content alleviate carrier separation and enhance the electron injection rate, thus facilitating electron transport to the TiO2 substrate. It is demonstrated that a device constructed with 0.25% Mn-doped ZnSe/CdS leads to an enhancement of the electron injection rate and power conversion efficiency by 4 times and 1.3, respectively.
Nanotechnology has led to huge progress in the use of semiconductor nanocrystals for applications in diverse areas like organic light emitting diodes [
Schematic illustration of charge transfer mechanism in Mn-doped ZnSe/CdS.
This offers charge carrier localization in two separate materials so that electrons and holes are confined in the shell and core, respectively. Moreover, the use of type II nanocrystals in solar cell applications leads to better power conversion efficiency compared to the corresponding nanocrystals made up entirely from the core or shell materials [
It has been shown that use of CdTe/CdSe core/shell nanocrystals prepared by the one-pot synthesis method without core seed purification could make structural and optical properties of nanocrystals comparable to the nanocrystals synthesized using purified core seed, which can give higher absorption and better crystallinity [
Here we present a colloidal synthesis of a novel type II Mn-doped ZnSe/CdS core/shell QD system as sensitizer with different Mn concentration (0–3%) and test its utilization in QD sensitized solar cells. This is the first time that Mn-doped ZnSe/CdS core/shell QDs are successfully synthesized by the method of hot injection and used as a strategy to boost solar cell efficiency. As explained in what is to follow it was found that a proper balance of Mn concentration could tailor the band gap and core/shell conduction band edge, causing a better electron transfer from QDs to the TiO2 photoelectrode, a broader absorption, and, consequently, higher solar cell efficiency.
Novel Mn-doped ZnSe/CdS QDs with different Mn concentration were synthesized by hot injection method. To investigate the effect of doping, all QDs were synthesized at the same conditions and with the same amount of ZnSe cores. To get precise amount of ZnSe core, they were synthesized in one batch and after washing by methanol and acetone, they were divided into several parts to be utilized as ZnSe core in ZnSe/CdS and Mn-doped ZnSe/CdS. To grow shell material onto ZnSe core, the mixture of S and (Mn-doped) Cd precursors was slowly injected into core solution at 240°C, followed by keeping 240°C for 20 min. Afterwards the reaction was cooled down to room temperature for purification and characterization. Transmission electron microscopy, operating in 100 KV, was employed to evaluate quality and size distribution of the nanocrystals. TEM images of ZnSe/CdS core/shell (Figure
TEM image of colloidal ZnSe/CdS core/shell.
In order to measure the actual Mn concentration incorporated into the nanocrystals, inductively coupled plasma atomic emission spectroscopy (ICP-AES, Thermo Scientific ICAP 6500) was employed. Nanocrystals were digested completely with nitric acid (67%, 0.1 mM) and diluted with DI water to obtain 10 mL of clear solution for ICP-AES measurement [
Summary of Cd and Mn concentration of nanocrystals.
Samples | (C) | (D) | (E) | (F) | (G) |
---|---|---|---|---|---|
Mn2576 (ppm) | 554 | 582 | 523 | 545 | 564 |
Mn2576 (ppm) | 0.7423 | 1.383 | 2.5000 | 2.2060 | 7.9190 |
Cd (mmol/L) | 4.9283 | 4.2878 | 4.6526 | 4.8483 | 5.0173 |
Mn (mmol/L) | 0.0135 | 0.0251 | 0.0455 | 0.0947 | 0.1441 |
Real Mn/Cd (%) | 0.2739 | 0.5853 | 0.9779 | 1.9532 | 2.8720 |
Expected Mn/Cd (%) | 0.25 | 0.5 | 1 | 2 | 3 |
After being successful in synthesizing the QDs, we prepared different photoanodes sensitized with these QDs: (A) ZnSe, (B) pure ZnSe/CdS and doped ZnSe/CdS with different concentration of Mn: (C) 0.25, (D) 0.5, (E) 1, (F) 2, and (G) 3%. Figure
Absorption of QDs-deposited photoelectrodes and photoluminescence of QDs: (A) ZnSe core, (B) pure ZnSe/CdS core/shell and Mn-doped ZnSe/CdS with concentration of (C) 0.25, (D) 0.5, (E) 1, (F) 2, and (G) 3%.
In order to calculate the band gap of the nanocrystals, the absorption coefficient (
Band gap values of nanocrystals derived from Figure
Samples | (A) | (B) | (C) | (D) | (E) | (F) | (G) |
---|---|---|---|---|---|---|---|
Optical band gap (eV) | 2.81 | 2.52 | 2.48 | 2.49 | 2.44 | 2.61 | 2.59 |
Absorption band edge (nm) | 437 | 488 | 495 | 493 | 504 | 471 | 474 |
Plot of
Therefore, the heterostructure band gap corresponds to the energy separation between the conduction band (CB) edge of the shell and the valence band (VB) edge of the core [
To examine the performance of the devices, they were tested under AM 1.5 G simulated solar irradiation with intensity of 100 mW·cm−2. Figure
Summary of device parameters.
Samples | (A) | (B) | (C) | (D) | (E) | (F) | (G) |
---|---|---|---|---|---|---|---|
|
0.365 | 0.470 | 0.580 | 0.575 | 0.545 | 0.535 | 0.508 |
|
0.484 | 2.235 | 4.011 | 3.753 | 3.634 | 3.109 | 2.594 |
ff | 0.852 | 0.537 | 0.549 | 0.553 | 0.595 | 0.524 | 0.513 |
|
0.15 | 0.565 | 1.276 | 1.194 | 1.179 | 0.871 | 0.674 |
Current-voltage characteristics of the QDSSCs device sensitized with various nanocrystals: (A) ZnSe core, (B) pure ZnSe/CdS core/shell and Mn-doped ZnSe/CdS with concentration of (C) 0.25, (D) 0.5, (E) 1, (F) 2, and (G) 3%.
The ZnSe/CdS core/shell shows much higher current than the ZnSe core which may be attributed to special carrier extraction and extension of the light-absorption range in type II heterostructures. Compared to previous work in which the CdS shell of the investigated ZnSe/CdS QDs was synthesized by deposition of Cd and S separately [
A device sensitized with low Mn-doped ZnSe/CdS (0.25%) shows a dramatic increase in all device parameters with about 10 times and 3 times higher efficiency compared to ZnSe and ZnSe/CdS, respectively. Since all samples are coated with 2 cycles of a ZnS SILAR layer, which blocks the electron recombination with the electrolyte [
The best values of device parameters are found for the lowest Mn concentration, possibly due to lower carrier recombination induced by new states generated by proper concentrations of Mn. It should also be mentioned that, like Mn-doped GaAs [
The Incident Photon to Current Efficiency (IPCE) of all devices was measured to evaluate the photocurrent response to incident light (shown in Figure
The IPCE spectra of devices based on (A) ZnSe, (B) pure ZnSe/CdS and Mn-doped ZnSe/CdS with Mn concentration of (C) 0.25, (D) 0.5, (E) 1, (F) 2, and (G) 3%.
The power conversion efficiency is comparatively low for devices sensitized by ZnSe due to limited light absorption but increases in ZnSe/CdS. This is related to the broader absorption and the special carrier separation in this type II heterostructure. The observed rather dramatic increase in IPCE spectra of Mn-doped ZnSe/CdS compared to undoped ZnSe/CdS may be due to a more efficient electron collection and electron injection efficiency. It is worthwhile to stress that band structure and CB manipulation via band gap engineering (Figures
The excited states dynamics of QDs were further investigated by time-resolved fluorescence lifetime measurements (experimental details in the supporting information in Supplementary Material available online at
Time-resolved fluorescence intensity decay curves of QDs onto (a) insulator and (b) TiO2 film; QDs: (A) ZnSe, (B) pure ZnSe/CdS and Mn-doped ZnSe/CdS QDs with Mn concentration of (C) 0.25, (D) 0.5, (E) 1, (F) 2, and (G) 3%.
It is found that the average fluorescence lifetime of ZnSe/CdS QDs (49 ns) is much longer than that of ZnSe (2 ns) when they are attached to the insulator. This is due to special charge separation in type II nanostructures that can decrease their wave function overlap and delay radiative recombination, resulting in a long fluorescence lifetime [
To get more understanding of the charge transport in type II ZnSe/CdS QDs with different Mn concentration, a physical model like Scheme
Novel colloidal Mn-doped ZnSe/CdS core/shell QDs with various Mn concentrations were successfully synthesized and applied to sensitized solar cells. It was demonstrated that QDs with proper Mn-doping could cause an increase in the absorption spectra and red shift in the absorption band edge and in the photoluminescence emission peak. The mid-states generated by Mn can facilitate electron transfer from the QDs to the TiO2 substrate. With superior light absorption, better carrier separation, and efficient electron injection rate a power conversion efficiency of 1.27% is presented, which is about 2 and 3 times larger than those of core and undoped QDs sensitized solar cells, respectively. The present work suggests that band structure manipulation in type II core/shell nanostructure offers an effective way to improve light harvesting and control of charge transfer via efficient charge separation in sensitized solar cells and that Mn-doping opens a new window to increase device efficiency. There is a multitude of lines of research to embark upon for future improvement of QDs sensitized solar cell of the kind proposed here. One such line of research concerns surface passivation. In fact, the organic ligands used can act as dangling bonds on the QDs surface and trap carriers, which calls for surface passivation of the QDs as a way to improve device performance, for instance, by using hydrophilic materials or ions. These and other measures will be pursued in our future studies.
Zinc stearate, gray selenium, cadmium oxide, sulfide, and manganese nitrate tetrahydrate were bought from Aldrich Company as precursors for Zn, Se, Cd, S, and Mn sources, respectively. The targeted heteronanocrystals are fabricated by a two-step synthesis composed of the fabrication of ZnSe core nanoparticle followed by a deposition of pure or Mn-doped CdS shell. Synthesis of both ZnSe core and pure or Mn-doped ZnSe/CdS core/shell under different Mn concentrations is based on previously published procedures [
Firstly, in order to synthesize ZnSe core nanocrystals from organic solution, specific amounts of selenium 0.0118 g and 0.4 mL of trioctylphosphine (TOP) were placed in a one-necked flask while stirring to make selenium dissolved in TOP. The reaction was conducted under nitrogen atmosphere. When the mixture became clear, the solution was kept in a clean syringe to be used in the next step. 0.0950 g zinc stearate, 0.1874 g stearic acid, and 2 mL of octadecane (ODA) were mixed together in a 25 mL three-necked flask. The mixture was slowly heated to 120°C (about 2°C/min) while stirring and pumping to remove additional elements from solution. The mixture was then heated to 240°C under nitrogen flow to make zinc dissolved in ODE where the solution appeared colorless and clear. Then a selenium stock solution prepared in the last step was swiftly injected into the reaction flask. The solution temperature was controlled and monitored to be kept at about 280°C. After 20 min, the reaction solution was cooled down to 60°C and 5 mL of chloroform was added to the solution to allow the quantum dots to be dissolved and suspended. The product of this step was ZnSe core, which contains byproducts and free ligands. To purify, it was washed 4 times by acetone and methanol. Typically, 5 mL of chloroform, 10 mL of acetone, and 2 mL of methanol were slowly added to the QDs solution followed by centrifugation for 3 min at 12400 rpm. The upper colorless layer was removed and QDs precipitated on the bottom were dissolved in chloroform for the next washing. Monodisperse ZnSe core produced in this step was sealed and kept in a cool and dark area to be used for shell deposition.
For preparing the CdS shell, three steps were considered. First, totally 0.0050 g of sulfur was dissolved in 3 mL of ODE while pumping and heating slowly to 80°C and then cooled to room temperature. Second, totally 0.0190 g of cadmium oxide (in case of Mn-doping, a proper amount of Mn was added to Cd precursor) was mixed in 0.4 mL of oleic acid (OA) and 3.5 mL of ODE in a 25 mL flask while stirring and heating slowly to 100°C. After degassing process, the temperature was increased to 280°C to make Cd dissolved completely in the solution. When the solution became clear, it was cooled down to 60°C. Third, mix these two precursor solutions (S and Cd) together as a source for shell growth. The final mixture was slowly injected (3 mL/h) into the reaction vessel, which contained 375 g ODA and ZnSe core dispersed in 2 mL of ODE at 240°C. After 20 min, the reaction was terminated and cooled down to room temperature for purification by chloroform and acetone.
FTO (fluorine tin oxide) conductive glasses (Sigma Aldrich, sheet resistance of 7 Ω/sq) were used as photoelectrode substrates. FTO glasses were cut to targeted size (1 × 2 cm) and sequentially washed by soap, KOH dissolved in 2-propanol, acetone, ethanol, and DI water via sonication for 30 min for each washing step, followed by immersion in 40 mM TiCl4 solution at 80°C for 40 minutes to give a TiO2 blocking layer. Mesoporous TiO2 layers were prepared by deposition of three transparent layers and one scattering layer with commercial TiO2 pastes (Ti-Nanoxide T/SP with 20 nm and Ti-Nanoxide R/SP with > 100 nm particle size for transparent and scattering layer, resp.) by screen-printing technique concluding by heating at 70°C for each layer deposition. The samples were postannealed at 500°C for 30 min to make the layers porous by removing the organic part at high temperature.
QDs dissolved in chloroform were deposited drop by drop on TiO2 films and left to be dried in the air. Excess QDs not adsorbed on TiO2 were washed by chloroform. To passivate QDs, ZnS block layers were deposited by two cycles of SILAR method. Each cycle contains dipping the samples into sulfur solution (Na2S dissolved in DI) for 1 min followed by washing with DI water and zinc solution (zinc nitrate hydrate dissolved in DI) for 1 min followed by washing with DI water.
Cu2S counter electrode was prepared by dipping brass sheet (Sigma Aldrich, resistivity of 1.673
Both photoelectrode and Cu2S counter electrode prepared in the last steps were stocked together by cell spacer while the electrodes were heating at 80°C. The
Time-correlated single-photon (TCSPC) measurements were performed on a spectrofluorometer with a TCSPC option (FluoroMax3, Horiba Jobin Yvon). A NanoLED (Horiba Jobin Yvon) emitting at 488 nm with a repetition rate of 1 MHz and pulse width of 1,4 ns was used as an excitation. Measurements were stopped when 3000 photon counts were collected in the peak channel using 2048 channels with 0,4 ns/channel. The instrument response function was recorded using a 2% Ludox (Sigma Aldrich) solution. To avoid reabsorption and reemission effects and also not to saturate the detectors, the sample concentration was kept strictly below 5
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors acknowledge Jing Huang for helpful advice. This work was supported by a grant from the Swedish Science Research Council (Contract G21-2012-3347).