FeS₂-sensitized ZnO/ZnS nanorod arrays for the photoanodes of quantum-dot-sensitized solar cells

Abstract

FeS₂-sensitized ZnO/ZnS nanorod arrays were fabricated and used as the photoanodes for quantum-dot-sensitized solar cells (QDSSCs). The cell performance of the ZnO/ZnS nanorod arrays after sensitization was better than that of ZnO-based nanorod arrays without sensitizing treatment. Pyrite FeS₂ was found to be an effective photosensitizer for QDSSCs. Various QDSSCs were assembled using different counter electrodes, such as Pt, FeS₂ nanorods and FeS₂ nanoparticles, and comparisons of cell performance as well as catalytic activity were made among them.

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1. Introduction

Quantum-dot-sensitized solar cells (QDSSCs) have attracted considerable attention and been regarded as promising alternatives for silicon-based photovoltaics in recent years, owing to their low cost, easy fabrication and potential application prospect.¹ Different from dye-sensitized solar cells (DSSCs), QDSSCs use semiconductor quantum dots (QDs) instead of organic dyes as photosensitizers.^2–5 Semiconductor QDs show some important advantages over dye molecules, such as tunable bandgap, high extinction coefficient, generation of multiple electron–hole pairs per photon.^6–8 So far, various narrow-bandgap QDs, such as CdS,^9,10 CdSe,^11,12 CdTe,^13,14 and PbS,¹⁵ have been explored and employed as efficient photosensitizers for QDSSCs.¹⁶ However, the disadvantages of element-toxicity and resource-rarity limit the long-term development of the QDs mentioned above.

Pyrite FeS₂, also as a member of narrow-bandgap semiconductors, has long been considered as a favorable photosensitization semiconductor because it has appropriate energy gap of 0.95 eV and high absorption coefficient (α > 6 × 10⁵ cm⁻¹ for hν > 1.3 eV) and, most importantly, the advantages of earth-abundance and non-toxicity.¹⁷ Song et al.¹⁸ synthesized FeS₂-sensitized TiO₂ nanotube arrays and used it as photoanode to study the photon-induced properties. The visible light response and the photodegradation performances of TiO₂ were greatly enhanced after FeS₂ sensitization. So far, to the best of our knowledge, research of FeS₂ QDs as photosensitizers for QDSSCs applications has not been reported yet.

ZnO, as an excellent candidate material for photoelectrodes, has been used in both QDSSCs and DSSCs due to its high electron mobility,^19,20 easy fabrication and abundant structures. However, the efficiency of ZnO-based QDSSCs is lower than that of TiO₂-based counterparts, likely due to high surface charge recombination in the case of ZnO.²¹ Introducing a barrier layer on ZnO surface has been regarded as an effective method to prevent charge recombination and improve performance of QDSSCs.^22,23 The conduction band edge of the barrier layer usually is more negative than that of ZnO so as to suppress surface charge recombination.²⁴

Based on above considerations, ZnS barrier layer and FeS₂ QD-sensitizer were assembled onto the surface of ZnO nanorods in sequence, taking advantages of the two materials in harvesting light and suppressing charge recombination in this work. The performance of the QDSSC was improved compared to the cells without sensitization. Various QDSSCs based on ZnO/ZnS/FeS₂ photoanodes were fabricated using different counter electrodes and comparisons were made among them.

2. Experimental

2.1 Preparation of photoanodes

ZnO, ZnO/ZnS and ZnO/ZnS/FeS₂ nanorod arrays were used as photoanodes. ZnO nanorod arrays were prepared on fluorine-doped tin oxide (FTO, F:SnO₂) glass substrate via seed assistant hydrothermal growth method.²⁵ ZnO/ZnS and ZnO/ZnS/FeS₂ nanorod arrays were fabricated via direct sulfurization and two-step solution immersion and subsequent sulfurization, respectively. The detailed preparation process can be seen in our previous research.²⁶

2.2 Preparation of counter electrodes

Pt, FeS₂ nanorod (simplified as FeS₂ NR) and FeS₂ nanoparticle (simplified as FeS₂ NP) films grown on FTO substrate were used as counter electrodes. Pt film was prepared by solvent-assisted drop-casting technique. H₂PtCl₆ isopropanol solution (5 mM) was spread onto FTO glass by several times and the resulting films were annealed at 500 °C for 30 min. FeS₂ nanorod array and FeS₂ nanoparticle films were synthesized according to our previous research.²⁷

2.3 Fabrication of solar cells

The photoanodes and counter electrodes were sandwiched together using 60 μm thick transparent Surlyn film (Meltonix 1170-25) and the internal space of the cells was filled with a electrolyte containing 1.0 mM 1,3-dimethylimidazolium iodide, 50 mM LiI, 30 mM I₂, 0.5 mM tert-butylpyridine, and 0.1 mM guanidinium thiocyanate in the mixed solvent of acetonitrile and valeronitrile (v/v, 85/15). The active area of the solar cells was 0.25 cm².

2.4 Characterization

The morphology of the sample was observed by field-emission scanning electron microscopy (FESEM, Hitachi SU-70). High-resolution transmission electron microscopy (HRTEM, JEM-2100) was applied to further investigate the microstructure and crystal structure of the products. Photocurrent density–voltage (J–V) characteristics of the solar cells were measured under AM 1.5 illumination at an intensity of 100 mW cm⁻². Electrochemical impedance spectroscopy (EIS) was performed in an electrochemical workstation (Parstat 2273, Princeton Applied Research) with an AC amplitude of 10 mV and a frequency range of 1 × 10⁵ to 0.01 Hz.

3. Results and discussion

Fig. 1a shows the as-synthesized ZnO/ZnS/FeS₂ nanorod arrays with diameters ranging from 80 nm to 100 nm. Rough surface was clearly observed for the nanorods, which was caused by the adhesion of FeS₂ nanoparticles onto the surface. Comparison of the bright-field and dark-field TEM images (Fig. 1b and c) of an individual nanorod further confirms the formation of FeS₂ nanoparticles around the nanorod surface. Various fringe spacings were observed from the HRTEM image of the nanorod edge (Fig. 1d). The lattice spacing measured for the single orientation plane in the upper right region is 0.26 nm, corresponding to the (0002) plane of hexagonal wurtzite structure ZnO (JCPDS36-1451). The plane adjacent to ZnO presented the same orientation and the lattice spacing was 0.31 nm, which was identified to be wurtzite structure ZnS (JCPDS36-1450). Fine crystallites with different orientations were observed around ZnS layer. The lattice spacings of the crystallites were 0.31 nm, which was indexed to be pyrite FeS₂ (JCPDS42-1340). The average size of the FeS₂ crystalline is around 10 nm. The results indicate the successful fabrication of ZnO/ZnS/FeS₂ nanorod arrays and the FeS₂ QD sensitization of ZnO/ZnS nanorods. Recently, various similar researches^28–31 on the successful fabrication and microstructure characterization of multi-component nanoarrays have been reported, which shows promising application in photovoltaics and water splitting.

Fig. 1 SEM, TEM, HRTEM images of the as-synthesized ZnO/ZnS/FeS₂ nanorods. (a) SEM image; (b) bright-field and (c) dark-field TEM images; (d) HRTEM image.

The photocurrent density–voltage (J–V) curves of the solar cells using ZnO, ZnO/ZnS and ZnO/ZnS/FeS₂ nanorods as photoelectrodes are shown in Fig. 2. The key performance parameters of the cells, such as open circuit potential (V_OC), short-circuit current density (J_SC), fill factor (FF), and the total energy conversion efficiency (η) are listed in Table 1. The FeS₂ QDSSC exhibits relatively better performance than the ZnO-based and ZnO/ZnS-based cells. The enhanced efficiency is mainly attributed to its broader light absorption range benefiting from the excellent light absorption of FeS₂. Besides, the high FF is probably due to the gradient energy level structure built in ZnO/ZnS/FeS₂ nanorod photoelectrode and the resulting rapid efficient electron injection.⁷ However, the FF and J_SC of the QDSSC are still low, which is why the efficiency of the QDSSC cannot compare with that of other ZnO-based DSSCs. The possible reasons are due to the insufficient QD loading, high defect states and large series resistance of the system, or improper utilization of counter electrode.⁷ By adjusting and improving process or employing other appropriate counter electrodes, it is possible to enhance the FF and J_SC, and a higher efficiency of the QDSSC is anticipated.

Fig. 2 Photocurrent density–voltage characteristics (J–V) of solar cells using various photoelectrodes and common Pt counter electrodes. (a) ZnO nanorods; (b) ZnO/ZnS nanorods; (c) ZnO/ZnS/FeS₂ nanorods. The measurements were performed under the illumination of simulated sunlight (AM 1.5, 100 mW cm⁻²).

Table 1 Performance parameters of the cells using various photoanodes

Samples	VOC (mV)	JSC (mA cm^−2)	FF (%)	η (%)
ZnO/ZnS/FeS2	390	0.87	36.08	0.12
ZnO/ZnS	310	0.57	28.24	0.05
ZnO	250	0.26	30.02	0.03

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Though the photoanode plays an important role in the performance of QDSSCs, the cell performance is also related to the counter electrode materials.³² The effect of counter electrode on the FeS₂-QDSSCs performance is also explored. Besides serving as photon sensitizer for QDSSCs, pyrite FeS₂ has also been used as an efficient counter electrode material for DSSCs and has been regarded as a promising alternative to noble Pt counter electrode due to its excellent stability and catalytic activity for I₂ reduction.^33–36 A series of FeS₂-QDSSCs were assembled using Pt, FeS₂ NR and FeS₂ NP as counter electrodes, respectively. Fig. 3 presents the J–V curves of the cells, and their corresponding performance parameters are summarized in Table 2. Taking the Pt counter electrode as a reference, the QDSSCs based on FeS₂ NR and FeS₂ NP counter electrodes both exhibit worse performance in J_SC, FF and η. However, the V_OC (∼500 mV) of the two structured FeS₂ counter electrodes is higher than that of Pt counter electrode (380 mV). The QDSSCs with different counter electrodes show different plots, indicating that the counter electrode materials do have effect on the cell performance. Compared with related research on FeS₂ counter electrode in DSSCs/QDSSCs,^34,36,37 the cell performance in our case is low. The reason for the low performance might be due to the unsuitable electrolyte or low match degree between photoanode and counter electrode. The exact reason and improving procedures are ongoing.

Fig. 3 Photocurrent density–voltage (J–V) characteristics of ZnO/ZnS/FeS₂ QDSSCs using various counter electrodes. (a) Pt; (b) FeS₂ NR; (c) FeS₂ NP.

Table 2 Electrochemical and performance parameters of the QDSSCs using different counter electrodes

Samples	Rs (Ω cm^2)	Rct (Ω cm^2)	VOC (mV)	JSC (mA cm^−2)	FF (%)	η (%)
Pt	21.9	7.2	380	0.88	36.1	0.12
FeS2 NR	21.5	17.9	510	0.32	28.3	0.09
FeS2 NP	21.9	47.7	500	0.29	26.8	0.07

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EIS measurements were performed on symmetric sandwich-like devices (that is, counter electrode/electrolyte/counter electrode) constructed using two identical electrodes, which were separated by a resin spacer. Fig. 4a shows the schematic illustration of the symmetrical structure cells. The Nyquist plots of the various symmetrical cells using different counter electrodes are shown in Fig. 4b. All of the curves show a common feature: the left semicircle accounting for the charge transfer resistance (R_ct) at the electrode/electrolyte interface and the right semicircle reflecting the Nernst diffusion impedance (W) in the electrolyte (not studied in this work).^38,39 The onset of the first arc indicates the solution resistance (R_s). Parameters obtained by fitting the impedance data according to the equivalent circuit are listed in Table 2. The three cells show a similar R_s of ∼22 Ω cm². The R_ct shows a significant decrease from 47.7 Ω cm² for the FeS₂ NP counter electrode to 17.9 Ω cm² for the FeS₂ NR counter electrode, and further to 7.2 Ω cm² for the Pt counter electrode. Small R_ct implies excellent catalytic activity. The smallest R_ct of Pt counter electrode indicates the highest capability for transferring electrons to electrolyte. Thus, the J_SC of the Pt counter electrode-based QDSSC is also the highest among the three counter electrodes-based cells. Phase impurity and defect states have been commonly observed in FeS₂ system,⁴⁰ leading to poor conductivity and catalytic properties of FeS₂ counter electrode. As a result, the J_SC and efficiency of the two FeS₂ counter electrode-based cells are lower than that of Pt counter electrode. However, compared with FeS₂ NP counter electrode, FeS₂ NR counter electrode shows a much smaller R_ct. It is probably related to the unique one-dimensional nanorod array structure, which provides direct electron transport channel and facilitates rapid electron transfer.^41,42 Though FeS₂ has been regarded as a promising alternative to Pt counter electrode, in this work Pt is more suitable than FeS₂ to be the counter electrode for the FeS₂-QDSSC system.

Fig. 4 EIS measurements on various symmetrical cells. (a) Schematic structure of symmetrical cell; (b) Nyquist plots of the symmetrical cells based on Pt, FeS₂ NR, FeS₂ NP counter electrodes. The lines express good results for corresponding EIS data and the inset shows the equivalent circuit.

4. Conclusions

In summary, this work demonstrated a FeS₂-sensitized ZnO/ZnS nanorod array photoanode for QDSSCs. The FeS₂-sensitized solar cell shows enhanced performance compared with the solar cells without sensitization treatment. The comparison of the FeS₂-QDSSCs using various counter electrodes indicates that counter electrode materials do have effect on the cell performance. Though the efficiency of the FeS₂-QDSSC cannot compare with other QDSSCs at present, we believe that the performance of the QDSSC can be further enhanced by improving processes. Our work provides an approach of using FeS₂ as efficient photosensitizer for QDSSCs.

Acknowledgements

We gratefully acknowledge the financial support provided by the National Science Foundation of China (No. 21301155), Zhejiang Provincial Natural Science Foundation of China (No. LR14B060002) and the National High Technology Research and Development Program (863) of China (No. 2011AA11A101).

Notes and references

1. Y. Lin, Y. Lin, Y. Meng, Y. Tu and X. Zhang, Opt. Commun., 2015, 346, 64–68.
2. J. H. Bang and P. V. Kamat, Adv. Funct. Mater., 2010, 20, 1970–1976.
3. V. González-Pedro, X. Xu, I. Mora-Sero and J. Bisquert, ACS Nano, 2010, 4, 5783–5790.
4. X.-Y. Yu, J.-Y. Liao, K.-Q. Qiu, D.-B. Kuang and C.-Y. Su, ACS Nano, 2011, 5, 9494–9500.
5. P. Lekha, A. Balakrishnan, K. Subramanian and S. V. Nair, Mater. Chem. Phys., 2013, 141, 216–222.
6. G. Zhu, L. Pan, T. Xu and Z. Sun, ACS Appl. Mater. Interfaces, 2011, 3, 3146–3151.
7. Y. L. Lee and Y. S. Lo, Adv. Funct. Mater., 2009, 19, 604–609.
8. J. Tian, R. Gao, Q. Zhang, S. Zhang, Y. Li, J. Lan, X. Qu and G. Cao, J. Phys. Chem. C, 2012, 116, 18655–18662.
9. J. Kim, H. Choi, C. Nahm, J. Moon, C. Kim, S. Nam, D.-R. Jung and B. Park, J. Power Sources, 2011, 196, 10526–10531.
10. M. Shalom, S. Ruhle, I. Hod, S. Yahav and A. Zaban, J. Am. Chem. Soc., 2009, 131, 9876–9877.
11. H. Lee, M. Wang, P. Chen, D. R. Gamelin, S. M. Zakeeruddin, M. Gratzel and M. K. Nazeeruddin, Nano Lett., 2009, 9, 4221–4227.
12. Z. Yang, C.-Y. Chen, C.-W. Liu and H.-T. Chang, Chem. Commun., 2010, 46, 5485–5487.
13. J. H. Bang and P. V. Kamat, ACS Nano, 2009, 3, 1467–1476.
14. G.-Y. Lan, Z. Yang, Y.-W. Lin, Z.-H. Lin, H.-Y. Liao and H.-T. Chang, J. Mater. Chem., 2009, 19, 2349–2355.
15. J. Jean, S. Chang, P. R. Brown, J. J. Cheng, P. H. Rekemeyer, M. G. Bawendi, S. Gradečak and V. Bulović, Adv. Mater., 2013, 25, 2790–2796.
16. H. Chen, L. Zhu, H. Liu and W. Li, Nanotechnology, 2012, 23, 075402.
17. D. Y. Wang, Y. T. Jiang, C. C. Lin, S. S. Li, Y. T. Wang, C. C. Chen and C. W. Chen, Adv. Mater., 2012, 24, 3415–3420.
18. X. M. Song, J. M. Wu, L. Meng and M. Yan, J. Am. Ceram. Soc., 2010, 93, 2068–2073.
19. Q. Zhang and G. Cao, J.Mater. Chem., 2011, 21, 6769–6774.
20. Q. Zhang, T. P. Chou, B. Russo, S. A. Jenekhe and G. Cao, Angew. Chem., Int. Ed., 2008, 120, 2436–2440.
21. J. Tian, Q. Zhang, E. Uchaker, Z. Liang, R. Gao, X. Qu, S. Zhang and G. Cao, J. Mater. Chem. A, 2013, 1, 6770–6775.
22. K. Park, Q. Zhang, B. B. Garcia and G. Cao, J. Phys. Chem. C, 2011, 115, 4927–4934.
23. K. Park, Q. Zhang, B. B. Garcia, X. Zhou, Y. H. Jeong and G. Cao, Adv. Mater., 2010, 22, 2329–2332.
24. M. Law, L. E. Greene, A. Radenovic, T. Kuykendall, J. Liphardt and P. Yang, J. Phys. Chem. B, 2006, 110, 22652–22663.
25. M. Wang, C. Xing, K. Cao, L. Meng and J. Liu, J. Phys. Chem. Solids, 2014, 75, 808–817.
26. M. Wang, C. Chen, H. Qin, L. Zhang, Y. Fang, J. Liu and L. Meng, Adv. Mater. Interfaces, 2015, 2.
27. M. Wang, C. Xing, K. Cao, L. Zhang, J. Liu and L. Meng, J. Mater. Chem. A, 2014, 2, 9496–9505.
28. Z. Liu, K. Guo, J. Han, Y. Li, T. Cui, B. Wang, J. Ya and C. Zhou, Small, 2014, 10, 3153–3161.
29. Z. Liu, J. Han, K. Guo, X. Zhang and T. Hong, Chem. Commun., 2015, 51, 2597–2600.
30. K. Guo, Z. Liu, C. Zhou, J. Han, Y. Zhao, Z. Liu, Y. Li, T. Cui, B. Wang and J. Zhang, Appl. Catal., B, 2014, 154, 27–35.
31. J. Han, Z. Liu, K. Guo, J. Ya, Y. Zhao, X. Zhang, T. Hong and J. Liu, ACS Appl. Mater. Interfaces, 2014, 6, 17119–17125.
32. A. D. Mani, M. Deepa, N. Xanthopoulos and C. Subrahmanyam, Mater. Chem. Phys., 2014, 148, 395–402.
33. S. Shukla, N. H. Loc, P. P. Boix, T. M. Koh, R. R. Prabhakar, H. K. Mulmudi, J. Zhang, S. Chen, C. F. Ng and C. H. A. Huan, ACS Nano, 2014, 8, 10597–10605.
34. Y. C. Wang, D. Y. Wang, Y. T. Jiang, H. A. Chen, C. C. Chen, K. C. Ho, H. L. Chou and C. W. Chen, Angew. Chem., Int. Ed., 2013, 52, 6694–6698.
35. Z. Wei, Y. Qiu, H. Chen, K. Yan, Z. Zhu, Q. Kuang and S. Yang, J. Mater. Chem. A, 2014, 2, 5508–5515.
36. Q.-H. Huang, T. Ling, S.-Z. Qiao and X.-W. Du, J. Mater. Chem. A, 2013, 1, 11828–11833.
37. Y. Hu, Z. Zheng, H. Jia, Y. Tang and L. Zhang, J. Phys. Chem. C, 2008, 112, 13037–13042.
38. S. Das, P. Sudhagar, V. Verma, D. Song, E. Ito, S. Y. Lee, Y. S. Kang and W. Choi, Adv. Funct. Mater., 2011, 21, 3729–3736.
39. L. Kavan, J.-H. Yum, M. K. Nazeeruddin and M. Grätzel, ACS Nano, 2011, 5, 9171–9178.
40. C. Steinhagen, T. B. Harvey, C. J. Stolle, J. Harris and B. A. Korgel, J. Phys. Chem. Lett., 2012, 3, 2352–2356.
41. Q. Zhang, C. S. Dandeneau, X. Zhou and G. Cao, Adv. Mater., 2009, 21, 4087–4108.
42. K. Yu and J. Chen, Nanoscale Res. Lett., 2009, 4, 1–10.

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