All-solid-state quantum-dot-sensitized solar cells with compact PbS quantum-dot thin films and TiO2 nanorod arrays

All-solid-state quantum-dot-sensitized solar cells with compact PbS quantum-dot thin films and TiO2 nanorod arrays

Ceramics International xxx (xxxx) xxx–xxx Contents lists available at ScienceDirect Ceramics International journal homepage: www.elsevier.com/locate...

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Ceramics International xxx (xxxx) xxx–xxx

Contents lists available at ScienceDirect

Ceramics International journal homepage: www.elsevier.com/locate/ceramint

All-solid-state quantum-dot-sensitized solar cells with compact PbS quantum-dot thin films and TiO2 nanorod arrays ⁎

Zhengguo Zhanga,b, Chengwu Shia, , Guannan Xiaoa, Kai Lva, Chengfeng Maa, Jiangyu Yuea a b

School of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei 230009, PR China School of Chemistry and Chemical Engineering, Beifang University of Nationalities, Yinchuan 750021, PR China

A R T I C L E I N F O

A BS T RAC T

Keywords: Compact PbS quantum-dot thin film TiO2 nanorod array 1,2-ethanedithiol All-solid-state quantum-dot-sensitized solar cell

To improve the electron injection efficiency from PbS quantum dots to TiO2 nanorods and prevent the direct contact of spiro-OMeTAD and TiO2 nanorods, a compact PbS quantum-dot thin film can be successfully obtained on TiO2 nanorod arrays 360 nm in length by repeated spin coating of Pb(Ac)2, Na2S and 1,2ethanedithiol solution in a step-by-step process. The corresponding solid-state quantum-dot-sensitized solar cells are fabricated using a novel structured FTO/compact PbS quantum-dot thin film sensitized TiO2 nanorod array/spiro-OMeTAD/Au that achieves a photoelectric conversion efficiency of 3.57% under AM 1.5 G illumination (100 mW cm−2), which represents a high value among all-solid-state PbS quantum-dot-sensitized TiO2 nanorod array solar cells.

1. Introduction PbS quantum dots (QDs) are considered a promising candidate material for quantum-dot-sensitized solar cells (QDSSCs) due to their unique properties, such as a large exciton Bohr radius (18 nm), high absorption coefficient, possibility of multiple exciton generation, low cost and band gap tunability [1–4]. PbS QDSSCs are typically composed of QD-sensitized metal oxide photoanodes, electrolytes and counter electrodes [5]. The preparation of QDs, the chemical composition of the electrolytes, and the microstructure of the metal oxide photoanodes strongly affect the photovoltaic performance of QDSSCs. Zhong et al. fabricated colloidal PbS/CdS QDSSCs using polysulfide liquid electrolytes and mesoporous TiO2 thin film (15 µm in thickness) and achieved a remarkable photoelectric conversion efficiency (PCE) of 7.19% [6]. Liu and Shen prepared colloidal PbS QDSSCs with liquid electrolytes containing a redox couple of I3-/I- and TiO2 nanotube arrays 4 µm in length and achieved a PCE of 3.41% [7]. Jia and Yi assembled PbS QDSSCs using polysulfide liquid electrolytes and TiO2 nanorod arrays 2 µm in length, and the corresponding PCE was 0.77% [8]. Seok et al. prepared colloidal PbS QDSSCs using poly-3-hexylthiophene (P3HT) as all-solid-state electrolytes and mesoporous TiO2 thin film (1 µm thickness) that produced a PCE of 2.9% [9]. Kim et al. fabricated colloidal PbS QDSSCs using P3HT and TiO2 nanorod arrays 1 µm in length and obtained a PCE of 3.9% [10]. Although individual colloidal PbS QDs have been widely investigated in QDSSCs, compact PbS QD thin film has been rarely applied for fabrication of QDSSCs.



In this work, a rutile TiO2 nanorod array was prepared via a hydrothermal method using an aqueous-grown solution containing 38 mM titanium isopropoxide and 6 M hydrochloric acid at 170 °C for 92 min. The compact PbS QD thin film was successfully obtained on the TiO2 nanorod array by repeated spin coating of Pb(Ac)2, Na2S and 1,2-ethanedithiol solution in a step-by-step process (spin-coatingassisted successive ionic layer absorption and reaction or spinSILAR). The influence of spin-SILAR cycle times on the deposition process of PbS QDs on the TiO2 nanorod array was investigated, and the photovoltaic performance of the corresponding solid-state compact PbS QD thin film sensitized TiO2 nanorod array solar cells was evaluated. 2. Experimental section 2.1. Preparation of the rutile TiO2 nanorod array The rutile TiO2 nanorod array was prepared via a hydrothermal method according to our previous report [11]. In brief, 20 mL of 37% hydrochloric acid was added to 20 mL of deionized water and sonicated for 5 min; then, 450 μL of titanium isopropoxide was added and further sonicated for 5 min. An aqueous-grown solution containing 38 mM titanium isopropoxide and 6 M hydrochloric acid was obtained. Subsequently, two pieces of FTO (2.0 cm×1.5 cm) with a TiO2 compact layer [12] of 60 nm in thickness were positioned in a tilted manner inside the Teflon liner with the active layer facing the wall. The above

Corresponding author. E-mail addresses: [email protected], [email protected] (C. Shi).

http://dx.doi.org/10.1016/j.ceramint.2017.05.022 Received 12 April 2017; Received in revised form 28 April 2017; Accepted 3 May 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Zhang, Z., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.05.022

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was completed at 4000 rpm for 30 s in a dry air-filled glove box with a relative humidity of 10%, and the active area of the solar cells was 0.09 cm2. The high-resolution XRD patterns of compact PbS QD thin films were recorded using Cu Kα radiation (λ=0.15418 nm, 40 kV, 40 mA, Rigaku, Japan) in the 2θ range of 28–32° at a scanning rate of 0.01°s−1. The surface and cross-sectional morphologies of the TiO2 nanorod array and compact PbS QD thin films were observed using field emission scanning electron microscopy (FE-SEM, Gemini SEM 500, Zeiss).

aqueous-grown solution was transferred to a Teflon liner containing two pieces of FTO. The properly sealed autoclave was placed inside an oven preheated to 170 °C, and the growth time was set to 92 min. After the autoclave was naturally cooled to room temperature in air, the two pieces of FTO with the TiO2 nanorod arrays were removed, rinsed with deionized water and ethanol, and annealed at 450 °C for 30 min in air prior to use. 2.2. Formation of compact PbS QD thin film on TiO2 nanorod arrays Compact PbS QD thin films can be formed on TiO2 nanorod arrays using spin-SILAR methods. An amount of 100 μL of fresh 5 mM Pb(Ac)2·3H2O solution in methanol/water (95/5, v/v) was dropped onto the TiO2 nanorod arrays and spin coated at 1500 rpm for 20 s. Subsequently, 100 μL of fresh 5 mM Na2S·9H2O solution in methanol/ water (95/5, v/v) was dropped and spin coated at 1500 rpm for 20 s, and 100 μL of 1% EDT/ethanol (1/99, v/v) solution was dropped and spin coated at 1500 rpm for 20 s. The above three spin-coating steps were denoted as one spin-SILAR cycle, and the cycle was repeated 20, 30 and 40 times. All processes were performed at ambient atmosphere.

3. Results and discussion 3.1. Microstructure and crystal phase of the TiO2 nanorod array Fig. 1 shows the surface and cross-sectional SEM images, XRD pattern, and UV–Vis absorption spectrum of the TiO2 nanorod array on the TiO2 compact layer with a 60 nm thickness and FTO. According to Fig. 1(a, b), the TiO2 nanorod array had a length of 360 nm, diameter of 20 nm, and an areal density of 690 µm−2 and was obviously different from the TiO2 nanorod arrays with length/diameter of 2 µm/100 nm [8] and 1 µm/150 nm [10]. The short length of 360 nm is expected to be beneficial to infiltration of spiro-OMeTAD into the TiO2 nanorod array and can allow the holes to transport the solid-state electrolytes of spiro-OMeTAD. The high areal density of 690 µm−2 and small diameter of 20 nm ensure sufficient loading of PbS QDs on the TiO2 nanorod array. From Fig. 1(c), the weak peaks at 2θ=36.1° and 62.8° appeared corresponding to the spacing of the (101) and (002) planes of the tetragonal rutile phase (JCPDS: 71–0650), and a preferred orientation along the (101) plane was observed [11]. From Fig. 1(d), the absorption onset of the TiO2 nanorod array was 400 nm, and the corresponding optical band gap was 3.1 eV.

2.3. Solar cell fabrication and characterization All-solid-state compact PbS QD thin film sensitized TiO2 nanorod array solar cells were fabricated using a novel structure of FTO/ compact PbS QD thin film sensitized TiO2 nanorod array/spiroOMeTAD/Au and spiro-OMeTAD [13] was applied as all-solid-state electrolytes. The preparation of the TiO2 compact layer with a 60 nm thickness, a spiro-OMeTAD layer and an Au electrode; the characterization of the UV–Vis, UV–Vis–NIR, XRD patterns; and the photovoltaic performance measurements were the same as described in our previous reports [11,12]. Spin coating of the spiro-OMeTAD solution

Fig. 1. Surface (a) and cross-sectional (b) SEM images, XRD pattern (c) and UV–Vis absorption spectrum (d) of the TiO2 nanorod array.

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78–1057). The intensity of the diffraction peak increased with the increase in spin-SILAR cycle times, which could be related to the loading quantities of PbS QDs on the TiO2 nanorod arrays. It is worth noting that the full width at half maximum (FWHM) of the PbS QD diffraction peak in Fig. 2(b) did not increase with the increase in the spin-SILAR cycle times from 20 to 30 and 40, and the average crystal sizes of PbS QDs were all 6.5 nm, as estimated using the Scherrer equation from FWHM. Therefore, the average crystal size of PbS QDs remained unchanged with the increase in the spin-SILAR cycle times. The PbS QD average crystal size of 6.5 nm resulting from spin-SILAR methods was smaller than the PbS exciton Bohr radius of 18 nm but larger than that of the 3–5 nm material produced by hot injection methods [6,9,10]. The decrease in the average crystal size from 6.5 nm to 3–5 nm implies that the band gap of PbS QDs increased due to the quantum size effect, and the onsets of the optical absorption were blueshifted to reduce the light harvesting efficiency in the visible light region. The relative position of the conduction band minimum was upshifted with the decrease in the average crystal size, and the driving force of the electron injection from PbS QDs to TiO2 nanorods increased to improve the charge separation efficiency at the interface of PbS QD/TiO2 [14]. Fig. 3 shows the surface and cross-sectional SEM images of PbS QDs on TiO2 nanorod arrays with spin-SILAR cycle times of 20, 30 and 40. Compared with Fig. 1(a, b), it can be clearly observed that the TiO2 nanorod arrays were fully capped by PbS QDs along the TiO2 nanorods at 20 spin-SILAR cycle times. In other words, the compact PbS QD thin film was successfully deposited on TiO2 nanorod arrays. The microstructure of the TiO2 photoanode with the compact PbS QD thin film was obviously different from that with individual PbS QDs [8,9,15]. The application of the compact PbS QD thin film in QDSSCs should improve the electron injection efficiency from PbS QDs to TiO2 nanorods and prevent the direct contact of spiro-OMeTAD and TiO2 nanorods to suppress the recombination between the electrons of the TiO2 conduction band and the holes of the spiro-OMeTAD layer. Additionally, the diameter of TiO2 nanorods with the compact PbS QD thin film slightly increased from 33 nm to 39 nm and 44 nm with the increase in the spin-SILAR cycle times from 20 to 30 and 40, which indicated that the loading quantities of PbS QDs on the surface of TiO2 nanorods increased and the corresponding electron quantities injected into the TiO2 conduction band should increase. Unfortunately, loose and porous PbS QD covering layers appeared on the top of TiO2

Fig. 2. XRD patterns of PbS QDs on TiO2 nanorod arrays with spin-SILAR cycle times of 20, 30 and 40.

3.2. Influence of spin-SILAR cycle times on the deposition process of PbS QDs on TiO2 nanorod arrays Fig. 2 presents the XRD patterns of PbS QDs on TiO2 nanorod arrays with spin-SILAR cycle times of 20, 30 and 40. From Fig. 2(a), the diffraction peaks at 2θ=30.01° all appeared in the three samples, corresponding to the spacing of (200) plane of the cubic PbS (JCPDS:

Fig. 3. Surface and cross-sectional SEM images of PbS QDs on TiO2 nanorod arrays with spin-SILAR cycle times of (a, d) 20, (b, e) 30, and (c, f) 40.

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solid-state PbS QD-sensitized TiO2 nanorod array solar cells, and the corresponding photocurrent-photovoltage characteristics are shown in Fig. 5. With the increase in spin-SILAR cycle times from 20 to 30 and 40, the open-circuit voltage (Voc) and fill factor (FF) gradually decreased from 0.55 V and 0.62–0.54 V and 0.52, 0.49 V and 0.49. This observation should be ascribed to the appearance of loose and porous PbS QD covering layers observed from the SEM results. The short-circuit current density (Jsc) increased from 10.43 mA cm−2 to 12.38 mA cm−2 and 12.55 mA cm−2, and this result was in accordance with the loading quantities of PbS QDs on the TiO2 nanorod array from the results of the SEM and UV–Vis–NIR analyses. Therefore, the allsolid-state compact PbS QD thin film sensitized TiO2 nanorod array solar cells exhibited the highest PCE of 3.57% at a spin-SILAR cycle time of 20, which represents a high value among all solid-state PbS QDsensitized TiO2 nanorod array solar cells. We speculate that the PCE would be further increased if the PbS QDs with an average crystal size of 6.5 nm produced using spin-SILAR methods were replaced by colloidal PbS QDs with an average crystal size of 3–5 nm produced using hot injection methods, and this work is ongoing.

Fig. 4. UV–Vis–NIR absorption spectra of PbS QDs on TiO2 nanorod arrays with spinSILAR cycle times of 20, 30 and 40.

Table 1 Photovoltaic performance parameters of all-solid-state PbS QD-sensitized TiO2 nanorod array solar cells. Spin-SILARcycle times

Voc (V)

Jsc (mA cm−2)

FF

PCE (%)

20 30 40

0.55 0.54 0.49

10.43 12.38 12.55

0.62 0.52 0.49

3.57 3.47 3.01

4. Conclusions The compact PbS QD thin film was successfully deposited on TiO2 nanorod arrays at a spin-SILAR cycle time of 20. The influence of the spin-SILAR cycle times on the deposition process of PbS QDs was investigated, and the results revealed that the average crystal size of PbS QDs remained unchanged with the increase in the spin-SILAR cycle times. The all-solid-state PbS QD-sensitized TiO2 nanorod array solar cells exhibited PCE values of 3.57%, 3.47% and 3.01% at spinSILAR cycle times of 20, 30 and 40, respectively. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (51272061, 51472071). References [1] G.-H. Kim, F.P. García, de Arquer, Y.J. Yoon, X. Lan, M. Liu, O. Voznyy, L.K. Jagadamma, A.S. Abbas, Z. Yang, F. Fan, A.H. Ip, P. Kanjanaboos, S. Hoogland, A. Amassian, J.Y. Kim, E.H. Sargent, High-efficiency colloidal quantum dot photovoltaics via robust self-assembled monolayers, Nano Let. 11 (2015) 7691–7696. [2] X. Lan, O. Voznyy, A. Kiani, F.P. García de Arquer, A.S. Abbas, G.-H. Kim, M. Liu, Z. Yang, G. Walters, J. Xu, M. Yuan, Z. Ning, F. Fan, P. Kanjanaboos, I. Kramer, D. Zhitomirsky, P. Lee, A. Perelgut, S. Hoogland, E.H. Sargent, Passivation using molecular halides increases quantum dot solar cell performance, Adv. Mater. 28 (2016) 299–304. [3] R. Zhou, J. Xu, F. Huang, F. Jia, L. Wan, H. Niu, X. Mao, J. Xu, G. Cao, A novel anion-exchange strategy for constructing high performance PbS quantum dotsensitized solar cells, Nano Energy 30 (2016) 559–569. [4] H. Jin, S. Choi, S.-H. Lim, S.-W. Rhee, H.J. Lee, S. Kim, Layer-by-layer-assembled quantum dot multilayer sensitizers: how the number of layers affects the photovoltaic properties of one-dimensional ZnO nanowire electrodes, Chemphyschem 15 (2014) 69–75. [5] D. Sharma, R. Jha, S. Kumar, Quantum dot sensitized solar cell: recent advances and future perspectives in photoanode, Sol. Energy Mater. Sol. Cells 155 (2016) 294–322. [6] S. Jiao, J. Wang, Q. Shen, Y. Li, X. Zhong, Surface engineering of PbS quantum dot sensitized solar cells with a conversion efficiency exceeding 7%, J. Mater. Chem. A 4 (2016) 7214–7221. [7] L. Tao, Y. Xiong, H. Liu, W. Shen, High performance PbS quantum dot sensitized solar cells via electric field assisted in situ chemical deposition on modulated TiO2 nanotube arrays, Nanoscale 6 (2014) 931–938. [8] L. Yu, J. Jia, G. Yi, M. Han, Photoelectrochemical properties of PbS quantum dot sensitized TiO2 nanorods photoelectrodes, RSC Adv. 6 (2016) 33279–33286. [9] S.H. Im, H. Kim, S.W. Kim, S.-W. Kim, S.I. Seok, All solid state multiply layered PbS colloidal quantum-dot-sensitized photovoltaic cells, Energy Environ. Sci. 4 (2011) 4181–4186. [10] S. Kim, J.H. Heo, J.H. Noh, S.-W. Kim, S.H. Im, S.I. Seok, PbS colloidal quantumdot-sensitized inorganic-organic hybrid solar cells with radial-directional charge transport, Chemphyschem 15 (2014) 1024–1027. [11] G. Xiao, C. Shi, Z. Zhang, N. Li, L. Li, Short-length and high-density TiO2 nanorod arrays for the efficient charge separation interface in perovskite solar cells, J. Solid

Fig. 5. Photocurrent-photovoltage characteristics of all-solid-state PbS QD-sensitized TiO2 nanorod array solar cells with spin-SILAR cycle times of 20, 30 and 40.

nanorod arrays when the spin-SILAR cycle times reached 30 and 40, and their thicknesses increased with the increase in the spin-SILAR cycle times. The loose and porous PbS QD covering layer might hinder the infiltration of spiro-OMeTAD and hamper the contact between spiro-OMeTAD and the compact PbS QD thin film on TiO2 nanorod arrays. Fig. 4 displays the UV–Vis–NIR absorption spectra of the PbS QDs on TiO2 nanorod arrays, and their absorption onsets were 600 nm, 700 nm and 750 nm for spin-SILAR cycle times of 20, 30 and 40, respectively. The red shift of the absorption onsets with the increase in the spin-SILAR cycle times was due to the increase in the PbS QD loading quantities, which was in agreement with the results from SEM, to suppress the reflection and transmission of the PbS QDs on the TiO2 nanorod arrays [16]. 3.3. Photovoltaic performance of all-solid-state PbS QD-sensitized TiO2 nanorod array solar cells Table 1 lists the photovoltaic performance parameters of the all4

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