Blocking the back reaction in quantum dot sensitized solar cells via surface modification with organic molecules

Abstract

Back reactions were suppressed effectively by double blocking barrier, organic molecules and ZnS on the photoanode of quantum dot (QDs) sensitized solar cells (QDSSCs), thereby achieving higher conversion efficiency. In this work, four different organic molecules were applied in QDSSCs, the efficiency increased two-fold from 2.21% to 4.25% when co-modifications with 4-tert-butyl pyridine (TBP) and ZnS were sequentially applied. The incident photon-to-current efficiency (IPCE) and parameters obtained from impedance spectroscopy (IS) such as recombination resistance (R_rec), chemical capacitance (C_μ), and electron lifetimes were consistent with the measured photovoltaic performance. We speculated that organic molecules mainly inhibit the charge recombination of the injected electrons in TiO₂ with electrolyte, because of its steric factor and the electron-donating property of the tert-butyl group. The selective site of modification has been tested for assessing the dominant mechanism underlying the improvement of solar cell characteristics.

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Introduction

Quantum dot sensitized solar cells (QDSSCs) as one of the most promising solar cells for third generation photovoltaics, have been attracting considerable attention in the past several years.^1–6 Sensitizer quantum dots (QDs) exhibit many advantages over organic small dyes, including higher extinction coefficients, a larger intrinsic dipole moment, multiple excitons and size-dependently tunable band gaps. These properties contribute to the feasibility of using QDs as materials for fabricating higher photovoltaic conversion efficiency sensitized solar cells. However, higher efficiencies of liquid-junction QDSSCs are reported to be about 8–9%,^7–12 which still lags much behind those of dye sensitized solar cells (DSSCs).^13,14

The major difference between QDSSCs and DSSCs involves a certain limitation of QDSSCs, in particular, relatively slower electron injection from QDs to porous oxide TiO₂ than that from dye to TiO₂.^15,16 Therefore, the main recombination process in DSSCs is the injected electrons in TiO₂ reacting with the oxidation state of the electrolyte; as for QDSSCs, a considerably more severe recombination pathway exists at the interface between the QDs and the electrolyte. Overall, the main back reactions limiting the photovoltaic conversion efficiency of QDSSCs are as follows: (1) the recombination of excited electrons in QDs with the electrolyte, which is denoted as BR-QE, and (2) the charger combination of injected electrons in TiO₂ with the electrolyte, denoted as BR-TE.¹⁷ To develop highly efficient solar cells, the electrons on the photoanode must preferentially transfer along the oriented channels, and the back reaction should be blocked. For this purpose, surface modifications have been adopted as an effective method for suppressing the recombination of electrons in DSSCs and QDSSCs.^18–20 For instance, TiO₂ has been coated with high band gap metal oxide layers, e.g., Al₂O₃, ZrO₂, MgO, and ZnO, which are suggested to act as barriers for interfacial electron transfer reactions.^21–23 In addition, the band alignment has been modified in DSSCs by using different additives in the electrolyte solution, such as the commonly used 4-tert-butyl pyridine (TBP).^24,25 The addition of TBP to the (I₃⁻/I⁻) electrolyte has been proven to decrease electron recombination. Likewise, surface treatments (ZnS or SiO₂) have been utilized in QDSSCs to passivate QD surface states and block back reactions.^17,26 Several studies have shown that ZnS nanometric barriers affect the passivation and blockage of the QD surface (blocking the BR-QE) rather than the TiO₂ films (blocking the BR-TE).^27,28 This phenomenon may be due to the relatively weak interaction between ZnS and TiO₂ films.²⁷ Moreover, several reports used organic species (including thiols, amines, and carboxylic acid) to passivate the surface states of QDs and mainly block the back reaction BR-QE.^28–34

In this paper, organic molecules that can effectively adsorb on the TiO₂ surface are introduced to further inhibit the back reaction in QDSSCs, and combine with ZnS nanometric barriers, thereby improving photovoltaic performance, as shown in Scheme 1. The blocking effects of organic molecules with different units such as 4-tert-butyl pyridine (TBP), 4-tert-butyl benzoic acid (TBBA), 4-methylpyridine (MP) or 4-cyanopyridine (CP) were investigated in this paper (shown in Scheme 1). Pyridine is proven to be prone to interact with TiO₂ via the formation of Ti–N bonds, and TBBA can react with TiO₂ via the anchoring group (–COOH).³⁵ In addition, the selective site of modification has been tested for assessing the dominant mechanism underlying the improvement of solar cell characteristics.

Scheme 1 Blocking effect of the organic barrier in QDSSCs.

Results and discussion

TBP was initially investigated as an organic barrier layer to block the back reaction in QDSSCs. After TBP modification, TiO₂ sensitized with CdS/CdSe exhibits a significant increase in photovoltaic conversion efficiency (as seen Fig. 1a) compared with that of the control sample (TiO₂/QDs). Detailed parameters, such as the open-circuit voltage (V_oc), short-circuit photocurrent (J_sc), fill factor (FF), and efficiency (η) are summarized in Table 1. The obtained efficiency of 3.32% is comparable with that of the electrode with ZnS coating, suggesting that the TBP organic barrier layer could efficiently act as a blocking barrier that inhibits electron recombination and increases the photovoltaic conversion efficiency of the QDSSCs. We speculate that the electron-donating group (tert-butyl) in the organic barrier layer and, additionally, the large steric factor of the tert-butyl group may serve a dominant function. To verify this hypothesis, organic molecules with a small steric effect (MP) or electron-drawing (CP) organic molecules are investigated as organic barriers in QDSSCs. Fig. 1a and Table 1 show that in comparison with the TiO₂/QDs electrode, a relatively lower efficiency is observed for MP modification, whereas a much lower efficiency is observed for the CP modified electrode, suggesting that the large steric group and its electron-rich property could efficiently block the pathway of electron recombination, while an electron-drawing group is unfavorable for electron transfer to the external circuit. Moreover, TBBA molecules have the same group (tert-butyl) as TBP, but a different anchoring group (–COOH) is introduced into the QDSSCs. Therefore, the improved efficiency is achieved. We can deduce that the large steric group and its electron-rich property could efficiently inhibit electron recombination.

Fig. 1 (a) I–V curves of QDSSCs modified with different molecules, (b) incident photon-to-current efficiency (IPCE) of QDSSCs modified with TBP or ZnS.

Table 1 Photovoltaic parameters of the QDSSCs based on different surface organic molecule modifications tested under standard conditions (100 mW cm⁻² AM1.5)

Photoanode	Jsc/mA cm^−2	Voc/V	FF	η (%)
TiO2/QDs	9.70	0.52	0.40	2.21
TiO2/QDs/ZnS	13.32	0.59	0.39	3.01
TiO2/QDs/TBP	13.90	0.58	0.41	3.32
TiO2/QDs/MP	12.25	0.51	0.39	2.55
TiO2/QDs/TBBA	13.16	0.59	0.37	2.84
TiO2/QDs/CP	8.53	0.56	0.37	1.77
TiO2/QDs/TBP/ZnS	15.56	0.59	0.46	4.25
TiO2/QDs/MP/ZnS	13.90	0.58	0.42	3.40
TiO2/QDs/TBBA/ZnS	14.25	0.59	0.43	3.57
TiO2/QDs/ZnS/TBP	14.96	0.58	0.46	4.02

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When co-modification with TBP and ZnS is sequentially applied, as shown in Fig. 2a, the photovoltaic parameters are as follows: V_oc = 0.59 V, J_sc = 15.56 mA cm⁻², FF = 0.46, and η = 4.25%. An improved solar cell performance is evident in the co-modified sample, and a nearly two-fold increase efficiency is observed. Likewise, co-modification with MP/TBBP and ZnS can also improve the efficiency of the QDSSCs, but the effect is not as remarkable as that with TBP. For MP, the electron-donating effect of methyl is obviously weaker than that of tert-butyl. For TBBP, the relatively poor interaction with TiO₂ films may be a major reason.

Fig. 2 (a) I–V curves and (b) IPCE of QDSSCs co-modified with TBP, TBBA, MP, and ZnS.

As shown in Table 1, efficiency changes are mostly due to the variations observed in J_sc, whereas V_oc and FF remain nearly unchanged for all samples. Fig. 1b and 2b illustrate the incident photon-to-current efficiency (IPCE) for the QDSSC samples modified with different molecules, the results show a good correlation with the J–V curves. A higher enhancement in IPCE is obtained after TBP modification. On the other hand, the TBP and ZnS co-modified electrode shows the highest IPCE (about 70%) compared with the control electrode.

To further highlight the reason for the great change of photocurrent after modification, impedance spectroscopy for TBP modification is performed under dark conditions at a varying forward applied bias. The obtained IS spectra are characterized by the presence of two semiarcs in a Nyquist plot (data not shown). The high frequency arc is due to the resistance and capacitance at the platinum counter electrode, whereas the low frequency arc is related to the recombination resistance (R_rec) associated with electron recombination at the interface, combined with the chemical capacitance (C_μ) of the electrons in TiO₂.^36,37 As reported by Bisquert’s group, IS enables extraction of the voltage drop in the photoanode VF at each applied voltage by subtracting the effects of the series resistance and counter electrode on both R_rec and C_μ.^38,39 VF can obtained from IS measurements, taking into account the voltage drop at the series resistance VS (contacts, transport resistance, counterelectrode), as VF = V_appl − VS, where V_appl is the applied voltage. Fig. 3a and b represent the C_μ and R_rec of the analyzed samples as a function of VF in the photoanode. The chemical capacitance records the density of states in TiO₂. The C_μ of the modified electrode shifts to a lower potential at a high overpotential, thereby indicating the downward displacement of the TiO₂ conduction band (CB) after modification with TBP or/and ZnS. This displacement leads to an increase in J_sc due to a higher electron injection driving force. Meanwhile, a downward displacement of the TiO₂ CB is generally associated with a decrease in V_oc which is due to a reduction of splitting between the TiO₂ Fermi level and the redox position.^40,41 However, this trend did not appear, as shown in the results, possibly because of the reduction of electron recombination, which would increase V_oc as reported. The recombination resistance is considered to be a reliable parameter to evaluate the electron recombination in sensitized solar cells. The recombination resistance of QDSSCs evidently increases with TBP modification, as shown in Fig. 3b. In addition, when co-modification (TBP and ZnS) is applied in the cells, R_rec presents a significant increase at each potential.

Fig. 3 (a and c) Capacitance and (b and d) recombination resistance obtained from impedance spectroscopy (IS) as a function of VF for modified electrode.

The Bode plot of EIS is further investigated to support the abovementioned results (see Fig. 4). The characteristic frequency peak of the photoanode (peak at a lower frequency) shifts to a lower frequency after modification with TBP.³⁵ The shift of peak reveals a more rapid electron transport process, because the frequency is inversely related to the electron lifetime in TiO₂ films. This abovementioned finding suggests that the TBP organic barrier layer can efficiently further block the back reaction in QDSSCs, along with ZnS treatment, thereby resulting in high efficiency QDSSCs. In addition, the EIS results show that for the TBBA, MP and CP modifications, the recombination resistance of the QDSSCs obviously increased when TBBA and MP modifications were applied, as shown in Fig. 3d. The organic barrier layer is suggested to contain a larger steric unit or electron-donating groups that can block the back reaction in QDSSCs. However, the R_rec for CP modification exhibits opposite results, explaining why organic molecules containing small steric factors and electron-drawing groups cannot inhibit electron recombination.

Fig. 4 Bode plot obtained from impedance spectroscopy for the electrode modified with TBP, and/or ZnS.

It is worth noting that the efficiency and related IPCE are sensitive to the site of organic modification. The photovoltaic performance of QD/ZnS/TBP solar cells is close to that of QD/TBP/ZnS as shown in Table 1, suggesting that ZnS passivation could not completely block the back reaction in QDSSCs; the organic molecule TBP would further inhibit electron recombination. However, when the TiO₂ film is modified in the order of TBP/QDs/ZnS, the efficiency and IPCE of the solar cells do not show any improvement (Fig. 5a and b), while the efficiency and IPCE for the electrode modified in the order of QDs/TBP/ZnS are dramatically increased. There are two possible factors resulting in this phenomenon, as follows: (1) the decrease in the amount of QDs absorbed by TiO₂ film, and (2) this large steric group and relatively electron-rich unit in TBP inhibits the electron transfer from the excited state of QDs to the TiO₂ conduction band. As the UV results show (Fig. 5c), the absorbance intensity of the two modified electrodes are basically the same in the range of 400 nm to 600 nm. Thus, the position of TBP modification does not influence the QD amounts. Fig. 5d shows the fluorescence intensity of different samples. When the site of TBP modification is found before QD absorption, the fluorescence intensity significantly increases in comparison with the reference sample (TiO₂/QDs). This result suggests that a less number of excited electrons on the QDs transfer to the TiO₂ conduction band, thereby increasing the fluorescence intensity.^42–44 By contrast, when an organic modification was applied after QD absorption, the excited electrons on QDs could easily transfer to the TiO₂ conduction band, thereby decreasing the fluorescence intensity.

Fig. 5 (a) I–V curves, (b) IPCE, (c) UV absorption, and (d) fluorescence spectroscopy for the TiO₂/QD electrode with TBP modification at different sites.

Conclusions

In conclusion, the TiO₂ films are modified using organic molecules to further block the back reaction in QDSSCs, combined with ZnS passivation of the QD surface, which results in an electron directional transfer to the external circuit, and a high conversion efficiency is achieved. An efficiency of 4.25% is obtained when co-modification with TBP and ZnS is sequentially applied, and such a increased efficiency is mainly due to the double blocking effect on QDSSCs. The results show that organic molecules containing large steric factors and electron-donating groups serve a positive function in blocking the back reaction in QDSSCs. The findings in the present work could pave the way for the assembly of high efficiency sensitized solar cells in the future.

Experimental

Electrode configuration

Mesoporous TiO₂ films were prepared by coating the TiO₂ paste, which was produced according to the literature on fluorine-doped tin oxide (FTO, ∼14 Ω square⁻¹, Nippon Sheet Glass, TCO-15) using the doctor-blade method. Prior to doctor blading, a solution of titanium isopropoxide (30 μL) in ethanol (10 mL) was spin-coated on FTO at 1000 rpm for 30 s. The surface area and thickness of the electrode were about 0.16 cm² and 8.5 μm, respectively. Chemical bath deposition (CBD) was used to assemble CdS and CdSe QDs in sequence onto the TiO₂ film, as described previously.⁴⁵ First, CdS QDs were deposited in the aqueous solution of the mixture of 20 mM CdCl₂, 66 mM NH₄Cl, 140 mM thiourea and 230 mM ammonia. After washing with pure water completely, the CdS-coated TiO₂ films were immersed into an aqueous solution with 80 mM CdSO₄, 90 mM N(CH₂COONa)₃ and 80 mM Na₂SeSO₃. All of the depositions were carried out at 10 °C in the dark. The deposition times for CdS and CdSe QDs were 30 min and 5.5 h, respectively. Surface passivation with ZnS was conducted twice by dipping into a 0.1 M Zn(CH₃COO)₂ and Na₂S aqueous solution for 1 min alternately. All of the depositions were carried out at 10 °C in the dark. The deposition times for CdS and CdSe QDs were 30 min and 5.5 h, respectively. For organic modification, TiO₂ or the sensitized electrodes were respectively immersed in a 0.6 M acetonitrile solution of TBP, MP, CP and a 0.6 M ethanol solution of TBBA for 24 h.

Solar cell configuration

QD sensitized TiO₂ electrodes with the corresponding surface treatment and Pt counter electrodes were assembled in a sandwich-type cell. The Pt counter electrodes were fabricated using a previously described method and via thermal decomposition of H₂PtCl₆ (30 mM in isopropanol) on an FTO glass at 400 °C for 30 min. Electrolytes composed of 1 M Na₂S and 1 M S in H₂O penetrated the cell.

Characterization

For photovoltaic testing, a solar light simulator (Oriel, 91192) was used to provide an illumination of 100 mW cm⁻² (AM 1.5). A digital source meter (2400 source meter, Keithley Instruments Inc., USA) was used to record current–voltage plots. The incident monochromatic photon-to-current conversion efficiency (IPCE) spectra were measured as a function of the wavelength from 350 nm to 800 nm, according to a monochromator (Crowntech M24-S, USA) equipped with a 150 W halogen lamp. The short photocurrent was measured with a Keithley electrometer (model 2000). Electrochemical impedance spectroscopy (EIS) measurements were carried out in dark conditions at a forward bias: 0–0.55 V, applying a 10 mV perturbation amplitude with a frequency ranging between 100 kHz and 50 MHz. Photoluminescence (PL) spectra were recorded at room temperature on a fluorescence spectrophotometer (Hitachi F-7000, Japan) using xenon lamps as the excitation source. All electrodes were excited at a wavelength of 400 nm.

Acknowledgements

The authors gratefully acknowledge the financial support from the Natural Science Foundation of China (No. 51202268), Shanxi Natural Science Foundation (No. 2013011013-4, and 2014021019-6).

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