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

Doped films of TiO 2 /PbS/CdS have been prepared by successive ionic layer adsorption and reaction (SILAR) method. Biand Agdoped-PbS quantum dot (QD) were produced by admixing Bi or Ag during deposition and the existing forms of the doping element in PbS QD were analyzed. The results show that Bi entered the cube space of PbS as donor yielding interstitial doping Bi-doped-PbS QD, while Ag replaced Pb of PbS as acceptor yielding substitutional doping Ag-doped-PbS QD. The novel Bidoped-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.


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][2][3][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][7][8][9].Using this method, CdSe QDs and Mg-doped CdSe QDs deposited on TiO 2 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  SC of 30 mA/cm 2 , owing to reinforcing Pb-S bond via incorporation of Hg 2+ 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, Bi 3+ 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 Bi 3+ 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 Bi 3+ entered the cube space of PbS as donor yielding interstitial doping Bi-doped-PbS QD, while Ag + replaced Pb 2+ of PbS as acceptor yielding substitutional doping Ag-doped-PbS QD.The novel quantum dot cosensitized solar cell (QDCSC) with a photoanode of TiO 2 -Bi-doped-PbS/CdS or TiO 2 /Ag-doped-PbS/CdS, Cu 2 S 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 TiO 2 /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.

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 ( , 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.The black, red, blue, and green curves in Figure 2(a) show X-ray powder diffraction spectra of TiO 2 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 TiO 2 , suggesting the PbS crystal growth on the TiO 2 mesoporous film.In addition, new diffraction peak does not appear with Bidoped-PbS and Ag-doped-PbS, which suggests that no new substance is formed after doping with cation Bi 3+ and Ag + .However, a slight broadening of (311) crystal surfaces of Bidoped-PbS is caused by the size change after doping cation Bi 3+ .According to Figure 2(b), Bi-doped-PbS/CdS and Agdoped-PbS/CdS cosensitized QDs have been successfully deposited on mesoporous TiO 2 films, respectively.
The PbS, Bi-doped-PbS, and Ag-doped-PbS QDs sensitized mesoporous TiO 2 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   mesoporous TiO 2 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.Semiconductor doping is the process that changes an intrinsic semiconductor to an extrinsic semiconductor.Hall measurement suggests that PbS, Bi-doped-PbS, and Agdoped-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.
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 Bidoped-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 TiO 2 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.
Optical band gap of the QDs was estimated from diffuse reflection absorption spectra using ℎ] = (ℎ] −   ) 2 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.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.International Journal of Photoenergy  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  SC .Besides, for the two systems,  OC 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  OC being degraded.Thus power conversion efficiency is slightly improved through doping Bi or Ag impurities into PbS QDs.The electrochemical impedance spectroscopy (EIS) is shown in Figure 7(d).The series resistances (  ) and the electronic transfer resistances of TiO 2 /QDs and electrolyte interface (  ) and photocathode and the electrolyte solution interface ( ct ) are summarized in Table 5 derived from the spectra by using the equivalent circuit [15].The   and   decrease after doping Bi 3+ and Ag + , indicating that the charge transfer resistance reduces in doped QDCSCs.However, the decreased  ct values increase the electronic transfer resistances.Thus, the PCE of doped QDCSCs is slightly improved.
Schematic structure of the PbS, Bi-doped-PbS, and Agdoped-PbS QDs is shown in Figure 8, according to results of optical measurements, XPS, Hall measurement, and reported articles [8,16].The radius of Bi 3+ is equal to the clearance radius of PbS cubes [16] which are shown in Table 6, so the Bi 3+ can enter into the cube clearance to form interstitial doping.We infer that the (Pb-S) bonds are reinforced by doping Bi 3+ , 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 Pb 2+ ; thus, Ag + can substitute for Pb 2+ 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.
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 TiO 2 , 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 TiO 2 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.

Conclusions
Heavily doped-PbS QDs were prepared by doping cations Bi 3+ and Ag + and the existence form of impurities is interstitial or substitutional doping.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.

Figure 8 :Figure 9 :
Figure 8: The ball-stick mode of quantum dot unit cells: (a) PbS and Bi-doped-PbS and (b) PbS and Ag-doped-PbS.

Table 1 :
Content of atoms in TiO 2 /Bi-doped-PbS and TiO 2 /Ag-doped-PbS solid film. Pb , b  Bi , c  Cd , d  S , and e  Ag : mass percent of the atom of Pb, Bi, Cd, S, and Ag. a 2.1.Sensitization of QD on Mesoporous TiO 2 Film.First, FTOcoated 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 TiCl 4 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 TiO 2 layer composed of 20-nmsized particles was deposited on substrates by using doctorblade method and annealed at 450 ∘ C for 30 min.Successive ionic layer adsorption and reaction (SILAR) method was employed to sensitize TiO 2 mesoporous film with QD.Briefly, mesoporous TiO 2 coated electrode was first dipped in ethanol

Table 2 :
Valence state parameters of elements in PbS, Bi-doped-PbS, and Ag-doped-PbS QDs.

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.

Table 4 :
Photovoltaic performances extracted from current-voltage measurements of different working electrodes.
a  sc : short-circuit photocurrent density, b  oc : open-current voltage, c FF: fill factor, and d PCE: power conversion efficiency.

Table 5 :
,   , and  ct values of undoped and doped QDCSCs.
a   : series resistances, b   : electronic transfer resistances of TiO 2 /QDs and electrolyte interface, and c  ct : electronic transfer resistances of photocathode and the electrolyte solution interface.
The cation Bi 3+ entered the cube space of PbS as donor yielding interstitial doping Bidoped-PbS QD, and Ag + replaced Pb 2+ of PbS as acceptor

Table 6 :
Size parameters of each material in the PbS lattice.