CdS/CdSe system of quantum dot cosensitized solar cells (QDCSCs) is one of the most attractive structures for high-efficiency due to its effect of level adjusting. However, the stepwise structure formed between levels of CdS and CdSe has a limitation for enhancing the efficiencies. Metal ions doping in quantum dots have emerged as a common way for changing the Fermi level, band gap, and conductance. Here we report an innovative concept for the rare earth materials La-doped of the CdS layer in the CdS/CdSe QDCSCs by means of the successive ionic layer adsorption and reaction (SILAR). Then we tested that La doped quantum dots can help more electrons accumulate in CdS film, which makes the Fermi level shift up and form a stepped structure. This method leads to enhanced absorption intensity, obviously increasing current density in CdS/CdSe QDCSCs. Our research is a new exploration for improving efficiencies of quantum dot sensitized solar cells.
Emerging new technology of metal ions doping in quantum dots (QDs) has the potential to improve photoelectric conversion efficiency (PCE) of solar cells for the change of Fermi level, band gap, and conductance [
CdS/CdSe system is one of the most popular structures in research field and one of the most efficient QDSCs systems due to the effect of adjusting level. For example, Santra and Kamat employed the CdS/CdSe structure and deposited Mn-doped-CdS/CdSe on mesoporous TiO2 film as the photoanode preparing the QDCSCs, which led to record the standard light intensity short circuit current of 20.7 mA/cm2 and PCE of 5.42% [
In our experiment, deposition process of CdS : La QDs on nanostructured TiO2 surface used the SILAR method. First, 0.1 M Cd(NO3)2·4H2O was dissolved in ethanol using ultrasonic bath for 45 min. Then, LaCl3 was added to the Cd(NO3)2 ethanol solution with corresponding concentration as cation source. Different doping ratios of La3+ : Cd2+ (1 : 10, 1 : 50, 1 : 100, 1 : 200, and undoped) were chosen to compare with each other. Na2S (0.1 M) was dissolved in methanol as anion source. The FTO glass electrode was precoated with transparent active TiO2 layers, which formed a wide band gap semiconductor TiO2 photoanode. Next, we performed the SILAR cycle procedure. The TiO2 photoanode was firstly dipped into metal cation solution for 5 min, rinsed with ethanol, and dried with N2 and then dipped into sulfide anion solution for 5 min, rinsed again with methanol, and dried with N2. Specifically, the anion source was prepared by dissolving Na2SO3 (0.12 M), Se (0.06 M), and a little permutite in deionized water, then refluxing in aqueous condition at 70°C for about 7 hours, and finally filtering out the unreacted Se and permutite [
Na2S (1 M) and S (1 M) were dissolved in deionized water as polysulfide electrolyte. Pt counter electrodes were used in our solar cells. Then the La-doped-CdS/CdSe cosensitized TiO2 photoanode, polysulfide electrolyte, and Pt counter electrode were assembled simply. The packaged solar cells should be put for about 40 min and avoid light processing. Then we can perform the testing.
We have investigated the effect of diverse doping density on properties of La-doped-CdS/CdSe QDCSCs. The specific experiment parameter and sample calibration used are shown in Table
Experiment parameters and sample calibration of different doping concentrations of La doped CdS.
Samples | Doping concentration (La : Cd) | Total SILAR cycles |
|
|
FF |
|
---|---|---|---|---|---|---|
M-0 | 0 | 4 + 4 | 3.88 | 0.42 | 0.34 | 0.54 |
M-10 | 1 : 10 | 4 + 4 | 5.20 | 0.43 | 0.29 | 0.66 |
M-50 | 1 : 50 | 4 + 4 | 5.80 | 0.43 | 0.27 | 0.69 |
M-100 | 1 : 100 | 4 + 4 | 5.79 | 0.42 | 0.26 | 0.66 |
M-200 | 1 : 200 | 4 + 4 | 5.65 | 0.40 | 0.28 | 0.64 |
Figure
The
We have investigated the effect of different SILAR cycles on properties of La-doped-CdS/CdSe QDCSCs based on the optimal doping ratio mentioned in preamble. In this experiment, the doping proportion was chosen as 1 : 50. Furthermore, detailed experiment parameters and sample calibration used are shown in Table
Experiment parameters and sample calibration of different SILAR cycles of La doped CdS.
Samples | Doping concentration (La : Cd) | Total SILAR cycles |
|
|
FF |
|
---|---|---|---|---|---|---|
S-4 | 1 : 50 | 4 + 4 | 5.92 | 0.41 | 0.21 | 0.52 |
S-6 | 1 : 50 | 6 + 4 | 6.97 | 0.40 | 0.25 | 0.70 |
S-8 | 1 : 50 | 8 + 4 | 6.72 | 0.37 | 0.21 | 0.53 |
S-10 | 1 : 50 | 10 + 4 | 5.58 | 0.35 | 0.18 | 0.36 |
The
Compared with the aforementioned results of different doping concentrations of La-doped CdS, we can find that current density of this experiment enhances slightly, but the open-circuit voltage and fill factor have a greater reduction. As a result, it finally leads to a very small increase in cell efficiency. The voltage reduction is mainly because we have changed the TiO2 slurry and screen used in silk-screen printing in the latter experiments. It results in TiO2 films painted a little thinner than the previous films and a relatively poor uniformity. The fill factor reduction is related to fabrication of Pt counter electrode. Pt slutty painted too little and counter electrode put too long will affect the fill factor.
First, we performed the materials characterization of La-doped-CdS/CdSe photoanode. Figure
ICP-OES quantitative analysis of La-doped CdS. The doping ratio of La : Cd is 1 : 50 in precursor solution. And the real doping ratio of La : Cd is 1 : 400 which we measured in the table.
Sample | Cd ( |
La ( |
La : Cd molar ratio |
---|---|---|---|
La-CdS | 31.75 | 0.0982 | 1 : 400 |
SEM image of photoanode and corresponding EDS energy spectrum diagram: (a) SEM image and (b) EDS energy spectrum.
The TEM images of La-doped-CdS/CdSe films: (a) low magnification figure and (b) high magnification figure.
Figure
Figure
The XRD patterns of La-doped-CdS/CdSe films.
The effect of rare earth materials doping on the optoelectronic properties of the CdS/CdSe is studied by comparing the ultraviolet-visible absorption spectrum of La-doped-CdS(4)/CdSe(4) with that of CdS(4)/CdSe(4) photoanodes, as shown in Figure
The UV-visible absorption spectra of La-doped-CdS(4)/CdSe(4) and CdS(4)/CdSe(4).
In Figure
UV-Vis absorption spectroscopy of La-CdS/CdSe photoanode of different SILAR cycles.
To confirm that doped quantum dots can change the energy level of quantum dots, we have performed the ultraviolet photoelectron spectroscopy test to ascertain the top of valence band of quantum dots. Quantum dots band gap can be obtained by utilizing the absorption spectrum function:
Known from the tangent at photon energy axial intercept in Figure
The absorption spectrum around doped CdS calculated by using
The ultraviolet photoelectron spectroscopy around doped CdS.
The diagram of La doping to the adjustment of CdS energy level.
Figure
IPCE curves of La-doped-CdS(6)/CdSe(4) and CdS(8)/CdSe(4) QDCSCs.
In summary, the rare earth materials La-doped-CdS/CdSe QDCSCs were prepared for the first time using SILAR method. The dopants La in solar cells can help to increase the short circuit current. When La doping ratio was 1 : 50, the efficiency of La-doped-CdS(6)/CdSe(4) solar cells reached the maximum value (
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was partially supported by Key Project of Beijing Natural Science Foundation (3131001), Key Project of Natural Science Foundation of China (91233201 and 61376057), Key Project of Beijing Education Committee Science & Technology Plan (KZ201211232040), State 863 Plan of MOST of China (2011AA050527), Beijing National Laboratory for Molecular Sciences (BNLMS2012-21), State Key Laboratory of Solid State Microstructures of Nanjing University (M27019), State Key Laboratory for New Ceramic and Fine Processing of Tsinghua University (KF1210), Key Laboratory for Renewable Energy and Gas Hydrate of Chinese Academy of Sciences (y207ka1001), Beijing Key Laboratory for Sensors of BISTU (KF20141077207 and KF20141077208), and Beijing Key Laboratory for photoelectrical measurement of BISTU (GDKF2013005).