Heterovalent Cation Substitutional and Interstitial Doping in Semiconductor Sensitizers for Quantum Dot Cosensitized Solar Cell

Abstract

Doped films of TiO₂/PbS/CdS have been prepared by successive ionic layer adsorption and reaction (SILAR) method. Bi- and Ag-doped-PbS quantum dot (QD) were produced by admixing Bi³⁺ or Ag⁺ during deposition and the existing forms of the doping element in PbS QD were analyzed. The results show that Bi³⁺ entered the cube space of PbS as donor yielding interstitial doping Bi-doped-PbS QD, while Ag⁺ replaced Pb²⁺ of PbS as acceptor yielding substitutional doping Ag-doped-PbS QD. The novel Bi-doped-PbS/CdS and Ag-doped-PbS/CdS quantum dot cosensitized solar cell (QDCSC) were fabricated and power conversion efficiency (PCE) of 2.4% and 2.2% was achieved, respectively, under full sun illumination.

1. Introduction

Growing attention has been paid to quantum dots for their remarkable characteristics of high absorption coefficient, tunable band gap, and multiple exciton generation (MEG) effect [1–4]. In theory, the PCE of QDSSCs could reach to 66% [5]; however, the actual PCE of QDSSC is not ideal at present. Introducing impurity into QD sensitizer can successfully improve the performance of QDSSC [6–9]. Using this method, CdSe QDs and Mg-doped CdSe QDs deposited on TiO₂ electrode displayed a broad spectral response [6] and Mn-doped CdS created electronic states in the midgap region of the CdS QD [7], while they did not give the existing form of doping elements. Lee group reported a PbS:Hg QD-sensitized solar cell with an unprecedentedly high of 30 mA/cm², owing to reinforcing Pb–S bond via incorporation of Hg²⁺ ion into the interstitial site of PbS lattice and broadening the absorption spectrum [8] and encouraging power conversion efficiency have been achieved on such devices.

For semiconductor nanocrystals (NCs), a single dopant which entered into NCs introduces impurity levels [10]. In the case of multiple impurities in a single dot, the nature of delocalization and interaction of the impurity charge carriers may be greatly modified relative to the bulk case [11]. The effects of doping on QD are akin to the influence of impurity on NCs, in virtue of QD which have a stronger confinement effect. In this report, Bi³⁺ and Ag⁺ were heavily doped into PbS QD and created impurity energy band in the band gap of PbS QD.

Herein, Bi- and Ag-doped-PbS quantum dot (QD) were produced by admixing Bi³⁺ or Ag⁺ with SILAR method and the existing forms of the doping element in PbS QD were analyzed. Optical measurements, Hall measurement, and XPS coupled with theoretical analysis showed that Bi³⁺ entered the cube space of PbS as donor yielding interstitial doping Bi-doped-PbS QD, while Ag⁺ replaced Pb²⁺ of PbS as acceptor yielding substitutional doping Ag-doped-PbS QD. The novel quantum dot cosensitized solar cell (QDCSC) with a photoanode of TiO₂-Bi-doped-PbS/CdS or TiO₂/Ag-doped-PbS/CdS, Cu₂S counter electrode, and sulfide/polysulfide electrolyte was fabricated and power conversion efficiency (PCE) of 2.4% and 2.2% was achieved, respectively, under full sun illumination, which were higher than undoped TiO₂/PbS/CdS. By Uv-Vis and UPS measurements, we noted that the changes in energy band of QD lead to the differences in the performance of the QDCSC. The research has a great importance on the development of QDSSC.

2. Materials and Methods

2.1. Sensitization of QD on Mesoporous TiO₂ Film

First, FTO-coated glass substrates were cleaned by ultrasonication in an alkaline, aqueous washing solution, rinsed with deionized water, ethanol, and acetone, and dried with nitrogen. The substrates were treated in an 0.04 M aqueous solution of TiCl₄ for 30 min at 70°C, rinsed with deionized water, and dried at 450°C for 30 min. After cooling to room temperature (25°C), the mesoporous TiO₂ layer composed of 20-nm-sized particles was deposited on substrates by using doctor-blade method and annealed at 450°C for 30 min. Successive ionic layer adsorption and reaction (SILAR) method was employed to sensitize TiO₂ mesoporous film with QD. Briefly, mesoporous TiO₂ coated electrode was first dipped in ethanol and deionized water (1 : 1) solution of 0.1 M Pb(NO₃)₂ for 1 min, followed by dipping in methanol solution of 0.1 M Na₂S for 1 min. Between each dipping, the electrode was thoroughly washed with ethanol or methanol and dried with nitrogen. These processes were defined as one cycle. Then, the TiO₂ substrate electrodes deposited with PbS were successively dipped in ethanol solution of 0.1 M Cd(NO₃)₂ for 5 min and methanol solution of 0.1 M Na₂S for another 5 min. Between each dipping, the electrode was thoroughly washed with ethanol or methanol and dried with nitrogen. For Bi-doped-PbS and Ag-doped-PbS QD, 5 mmol of BiCl₃ and 6.7 mmol AgNO₃ were added to Pb(NO₃)₂ cationic precursor solution, respectively. To obtain the optimum photovoltaic performance in each condition, several coating cycles were repeated (2 cycles for PbS, Bi-doped-PbS, and Ag-doped-PbS, 6 cycles for CdS of Ag-doped-PbS/CdS, and 7 cycles for CdS of Bi-doped-PbS/CdS).

2.2. Solar Cell Fabrication

The polishing brass plate was cleaned by ultrasonication in an alkaline, rinsed with deionized water, ethanol, isopropanol, and acetone (volume ratio was 8 : 2), and dried with nitrogen. The clean brass plate was etched in hydrochloric acid (35.0~37.0%) at 80°C for 30 min and subsequently washed with deionized water and dried with nitrogen. Polysulfide electrolyte solution composed of 1 M Na₂S, 1 M S, and 0.1 M NaOH in deionized water was dropped on etched brass for 15–30 min and black colored Cu₂S formed on the brass foil. The QDs sensitized TiO₂ electrode and Cu₂S counter electrode were assembled using 60 mm thick sealing materials. Polysulfide electrolyte solution was injected into the gap between photoanode and counter electrode from the side using injector. The active area was 0.25 cm².

3. Results and Discussion

Field emission scanning electron microscope (FESEM, Hitachi S-4800) was used to characterize the morphology of the samples. As revealed by SEM images (Figures 1(a), 1(b), 1(c), and 1(d)), mesoporous films were deposited on FTO glass. By comparing the SEM images of TiO₂/Bi-doped-PbS (Figure 1(a)) and TiO₂/Bi-doped-PbS/CdS (Figure 1(b)), TiO₂/Ag-doped-PbS (Figure 1(c)), and TiO₂/Ag-doped-PbS/CdS (Figure 1(d)), the pore size distribution of mesoporous films decreased after the deposit of CdS QDs, which confirm that the cosensitized QDs formed. Energy dispersive X-ray spectrum (EDS) was collected to investigate the composition of the mesoporous films. According to the corresponding EDS spectra, the elements Bi and Ag exist in the mesoporous films. To further determine and analyse the elements in the mesoporous films, the inductively coupled plasma optical emission spectroscopy (ICP-OES) measurement was used. The results revealed that the elements Bi and Ag exist in the corresponding mesoporous films (Table 1), in agreement with the results obtained with EDS.

Table 1 

Content of atoms in TiO₂/Bi-doped-PbS and TiO₂/Ag-doped-PbS solid film.

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Figure 1 

SEM images of TiO₂/Bi-doped-PbS, TiO₂/Bi-doped-PbS/CdS, TiO₂/Ag-doped-PbS, and TiO₂/Ag-doped-PbS/CdS. Insets in images are corresponding EDS spectra.

The black, red, blue, and green curves in Figure 2(a) show X-ray powder diffraction spectra of TiO₂ mesoporous films deposited with the pure PbS, Bi-doped-PbS, and Ag-doped-PbS, respectively. For comparison, we find six additional diffraction peaks that do not originate from TiO₂, suggesting the PbS crystal growth on the TiO₂ mesoporous film. In addition, new diffraction peak does not appear with Bi-doped-PbS and Ag-doped-PbS, which suggests that no new substance is formed after doping with cation Bi³⁺ and Ag⁺. However, a slight broadening of (311) crystal surfaces of Bi-doped-PbS is caused by the size change after doping cation Bi³⁺. According to Figure 2(b), Bi-doped-PbS/CdS and Ag-doped-PbS/CdS cosensitized QDs have been successfully deposited on mesoporous TiO₂ films, respectively.

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Figure 2 

XRD patterns of TiO₂, Bi-doped-PbS, Ag-doped-PbS, Bi-doped-PbS/CdS, and Ag-doped-PbS/CdS.

The PbS, Bi-doped-PbS, and Ag-doped-PbS QDs sensitized mesoporous TiO₂ films were also characterized with a transmission electron microscope (TEM, FEI, Tecnai F20UT) operating at 200 KV, to investigate impact of doping on PbS QD. TEM images show very clear lattice fringes from the QDs (Figure 3). It is obviously that the size of Bi-doped-PbS (Figure 3(b)) QD is smaller than that of PbS QD (Figure 3(a)), while the size of Bi-doped-PbS QD (Figure 3(c)) is equal to that of PbS QD. This means that Bi impurity causes the lattice contraction of PbS and Ag impurity has no effect on the lattices of PbS, in agreement with the results obtained with XRD. X-ray photoelectron spectroscopy (XPS) analysis of PbS, Bi-doped-PbS, and Ag-doped-PbS QDs adsorbed on mesoporous TiO₂ surface was also performed (Figure 4). By analyzing XPS (Table 2), the valence state of elements S, Pb, Bi, and Ag is −2, +2, +3, and +1 (the relatively stable valence of Ag element is +1 [11]), respectively.

Table 2 

Valence state parameters of elements in PbS, Bi-doped-PbS, and Ag-doped-PbS QDs.

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Figure 3 

TEM images of (a) PbS, (b) Bi-doped-PbS, and (c) Ag-doped-PbS QDs.

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Figure 4 

XPS of (a) S 2s peaks, (b) Pb 4f peaks, (c) Bi 4f peaks, and (d) Ag 3d peaks for PbS, Bi-doped-PbS, and Ag-doped-PbS QDs adsorbed on mesoporous TiO₂ surface.

Semiconductor doping is the process that changes an intrinsic semiconductor to an extrinsic semiconductor. Hall measurement suggests that PbS, Bi-doped-PbS, and Ag-doped-PbS are p-type, n-type, and p-type semiconductor (Table 3), respectively. It demonstrates that Bi impurities “donate” their extra valence electrons to conduction band of PbS, providing excess electrons to PbS. Excess electrons increase the electron carrier concentration of PbS, forming impurity energy band which lies closer to the conduction band than the valence band. However, Ag impurities “accept” electrons from the valence band of PbS. Excess holes increase the hole carrier concentration of PbS, forming impurity energy band which lies closer to the valence conduction band than the conduction band.

Table 3 

Resistivity, Hall coefficient, conductivity type, carrier density, and Hall mobility of PbS:Hg thin film using Hall effect measurement system. The measurement conducted at room temperature under applied field was adjusted from 1 to 5 kG.

The diffuse reflection absorption spectra of QDs are shown in Figure 5(a). After doping impurities, a broad band appeared at near infrared region which is observed for Bi-doped-PbS and Ag-doped-PbS QD, which can be attributed to heavily doped Bi or Ag impurities that formed impurity band in the band gap of PbS. For Bi-doped-PbS based on mesoporous TiO₂ film, the optical absorption of Bi-doped-PbS was significantly enhanced in the visible light which is obvious, indicating that Bi-doped-PbS can generate more photogenerated electrons. In addition, Bi-doped-PbS QDs show the obvious blue shift in the absorption spectra due to the increase in the bulk energy band gap. In general, the shift of the onset absorption to lower wavelengths with decreasing particle size represents size quantization effects in these particles [12]. For Ag-doped-PbS, there is no obvious change expect for a broad band at near infrared region, suggesting that the size of PbS is unchanged after doping Ag impurities. These results are in agreement with the results obtained with XRD and TEM.

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Figure 5 

(a) Diffuse reflection absorption spectra, (b) the plot calculated by using equation from absorption spectra, and (c) UPS spectra of PbS, Bi-doped-PbS, and Ag-doped-PbS QDs.

Optical band gap of the QDs was estimated from diffuse reflection absorption spectra using equation [13]. As shown in Figure 5(b), the optical band gap of PbS, Bi-doped-PbS, and Ag-doped-PbS QD is 1.05 eV, 1.2 eV, and 1.07 eV, respectively. Ultraviolet photoelectron spectroscopy (UPS) can determine valence band maximum (VBM) [14]. Secondary cut-off is fitted to energy of He I light source (21.2 eV), where extrapolation of low energy region corresponds to potential energy of VBM from the vacuum level [8, 14] (Figure 5(b)). The position of conduction band minimum (CBM) is estimated based on VBM and optical band gap energy [8]. Heavily doped Bi-doped-PbS QD is n-type semiconductor, so the impurity band is near to CBM (Figure 6(a)), while the impurity band is near to VBM of heavily doped Ag-doped-PbS QD because it is p-type semiconductor. Band edge alignment is shown in Figure 6, where CBMs of the doped QDs move upward relative to pure QDs.

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Figure 6 

Band edge alignment diagram for (a) Bi-doped-PbS and (b) Ag-doped-PbS QDs.

Figure 7(a) shows diffuse reflection absorption spectra of QDs cosensitized mesoporous TiO₂ films. Photocurrent density-voltage (-) curves of the QDCSCs based on TiO₂/PbS(2)/CdS(7), TiO₂/Bi-doped-PbS(2)/CdS(7), TiO₂/PbS(2)/CdS(6), and TiO₂/Ag-doped-PbS(2)/CdS(6) were presented in Figure 7(b). We show the solar cell performance parameters in Table 4. By comparing with the corresponding undoped system, of QDCSC is essentially increased after doping Bi or Ag impurities into PbS QDs. The of the cell can be influenced by the value of monochromatic incident photon-to-electron conversion efficiency (IPCE) and spectral response range. Doping of Bi or Ag impurities into PbS QDs leads to improvement of the value of IPCE and broadening of spectral response range, according to IPCE spectra (Figure 7(c)) resulting in the increase of . Besides, for the two systems, is degraded as photocurrent increases, and this may be because of the decreased shunt resistance. The fill factors are degraded which may be due to the being degraded. Thus power conversion efficiency is slightly improved through doping Bi or Ag impurities into PbS QDs.

Table 4 

Photovoltaic performances extracted from current-voltage measurements of different working electrodes.

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Figure 7 

(a) Diffuse reflection absorption spectra of photoanode, (b) - curves, (c) IPCE spectra, and (d) EIS spectra of TiO₂/PbS(2)/CdS(7), TiO₂/Bi-doped-PbS(2)/CdS(7), TiO₂/PbS(2)/CdS(6), and TiO₂/Ag-doped-PbS(2)/CdS QDCSCs.

The electrochemical impedance spectroscopy (EIS) is shown in Figure 7(d). The series resistances () and the electronic transfer resistances of TiO₂/QDs and electrolyte interface () and photocathode and the electrolyte solution interface () are summarized in Table 5 derived from the spectra by using the equivalent circuit [15]. The and decrease after doping Bi³⁺ and Ag⁺, indicating that the charge transfer resistance reduces in doped QDCSCs. However, the decreased values increase the electronic transfer resistances. Thus, the PCE of doped QDCSCs is slightly improved.

Table 5 

, , and values of undoped and doped QDCSCs.

Schematic structure of the PbS, Bi-doped-PbS, and Ag-doped-PbS QDs is shown in Figure 8, according to results of optical measurements, XPS, Hall measurement, and reported articles [8, 16]. The radius of Bi³⁺ is equal to the clearance radius of PbS cubes [16] which are shown in Table 6, so the Bi³⁺ can enter into the cube clearance to form interstitial doping. We infer that the (Pb–S) bonds are reinforced by doping Bi³⁺, which contribute not only to a decrease of the lattice size but to the increase of the optical band gap of PbS QD. The radius of Ag⁺ is a little smaller than the clearance radius of PbS cubes and is equal to the radius of Pb²⁺; thus, Ag⁺ can substitute for Pb²⁺ to form replacement doping, leading to lattice contraction. However, the breaking of (Pb–S) bonds makes lattice expansion. These two kinds of function cause a little lattice distortion and the size of Ag-doped-PbS is unchanged.

Table 6 

Size parameters of each material in the PbS lattice.

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Figure 8 

The ball-stick mode of quantum dot unit cells: (a) PbS and Bi-doped-PbS and (b) PbS and Ag-doped-PbS.

Based on optical measurements, UPS, and Hall measurement, we inferred the internal mechanism of QDCSCs and doped QDCSCS to explain the changes of performance for QDCSCs after doping. From Figure 9, the significant difference in doped QDCSCs is that there is impurity band in doped-PbS QDs. The PbS and CdS QDs can both absorb light, leading to the separation of electron-hole. For Bi-doped-PbS QDCSSC, the photogenerated electrons generated from impurity band can be transferred to the CBM of TiO₂, which means that Bi is donor impurity contributing electrons. For Ag-doped-PbS, the photogenerated electrons generated from CdS not only can be transferred to the CBM of TiO₂ but also to the Ag impurity band possibly, which means that Ag is acceptor impurity contributing holes. This means that there is higher probability recombination of electrons in impurity band leading to the decrease of Rc for doped-PbS QDCSC.

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Figure 9 

Schematic illustrations for internal mechanism of (a) TiO₂/PbS/CdS and TiO₂/Bi-doped-PbS/CdS and (b) TiO₂/PbS/CdS and TiO₂/Ag-doped-PbS/CdS QDCSCs.

4. Conclusions

Heavily doped-PbS QDs were prepared by doping cations Bi³⁺ and Ag⁺ and the existence form of impurities is interstitial or substitutional doping. The cation Bi³⁺ entered the cube space of PbS as donor yielding interstitial doping Bi-doped-PbS QD, and Ag⁺ replaced Pb²⁺ of PbS as acceptor yielding substitutional doping Ag-doped-PbS QD. The novel Bi-doped-PbS/CdS and Ag-doped-PbS/CdS quantum dot cosensitized solar cell (QDCSC) were assembled and PCE of 2.4% and 2.2% was obtained, respectively, under full sun illumination. The analysis of the changes of energy level of the cell has important influence upon understanding the effect of doping in theory and the development of QDSSC.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

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 PR 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).