Preparation of Tree-Structured ZnO Films And Application In Quantum Dots Sensitized Solar Cells

This study reported the fabrication of tree-structured ZnO lms through hydrothermal method. Long ZnO nanowires (NWs) were rstly deposited on the transparent conductive glass substrates. ZnO braches were then grown on the surface of ZnO NWs. The morphology of the as-prepared tree-structured ZnO lms was affected by several factors, such as the concentration of Zn(CH 3 COO) 2 in the seed precursor solution, and the hydrothermal reaction time. ZnO branches were vertically grown on the ZnO NWs when the concentration of Zn(CH 3 COO) 2 was 0.04 M. Quantum dots (QDs)-sensitized solar cells were assembled using the CdS/CdSe QDs co-sensitized ZnO lms as the photoanodes. The value of Incident photon-to-current conversion e ﬃ ciency (IPCE) in the range of 550–650 nm was obviously enhanced with the growth of ZnO braches. The short-circuit current density (J SC ) was increased from 9.65 to 12.60 mA cm -2 when the growth time of ZnO branches was 6 h. The overall PCE increased from 3.21% to 5.19%.


Introduction
In the past few years, quantum-dot-sensitized solar cells (QDSCs) have attracted immense scienti c and technological interest because of their advantages, such as simple preparation process, costeffectiveness, and high power conversion e ciency (PCE) [1][2][3][4]. The PCE of QDSCs has been improved to be 14.4%, which is comparable with those of other solar cells [5,6]. There are three main parts in QDSCs, photoanode, electrolyte, and the counter electrode parts [7,8]. The photoanode is the key section of QDSCs which is responsible for the light capture and the generation of electrons [9][10][11]. The photoanodes have been prepared based on different oxide semiconductors with wide band gap. ZnO is an important environment-friendly, cost-effective n-type semiconductor material which has a high electron mobility (200-1000 cm 2 V − 2 s − 1 ) [12,13]. At the same time, the morphology of ZnO could be easily controlled using various methods. Various nanostructures of ZnO, such as nanorods (NRs), nanosheets (NSs), nanowires, nano owers, and nanoballs, have been successfully prepared and applied in solar cells [14][15][16].
ZnO nanorods were widely studied in QDSCs because the 1D crystalline nanostructured ZnO could provide a direct conduction pathway for the photogenerated electrons. However, the surface area of the ZnO NRs lm was much lower than the conventional nanoporous TiO 2 lms used in QDSCs. (SCRC, China). Ethanol and methanol (purities > 99.9%) were purchased from Aladdin Reagent Co.
(China). All of these materials were used as received without any further puri cation.

preparation of tree-structured ZnO lms
For the preparation of ZnO NWs, a thin ZnO seed layer were rstly deposited on glass substrates according to the former studies [20]. Long ZnO NWs were grown through hydrothermal reaction at 95 ℃ in a high pressure reactor. The primary precursors were zinc nitrate (((Zn(NO 3 ) 2 ), 4 mM) and hexamethylenetetramine (HMTA: C 6 H 12 N 4 , 4 mM) with polyethylenemine (PEI, (C 2 H 5 N) n ) as additive chemicals in the solution. The hydrothermal reaction time is controlled to be 24 h. The former hydrothermal reaction processes were repeated for the preparation of longer ZnO NWs. The detailed experiments were described in the former studies [20]. The prepared ZnO NWs were immersed in an ethanol solution containing Zn(CH 3 COO) 2 . Then, the ZnO NWs samples were hanged in deionized water solution which consisted of 0.05 mol/L Zn(NO 3 ) 2 and 0.04 mol/L C 6 H 12 N 4 under 90 ℃ to grow ZnO branches on the former prepared NWs. The growth time was controlled to be 1, 6, 12 and 24 h. The obtained tree-structured ZnO lms were dried under room temperature and sintered at 450 ℃ in mu e furnace.

Sensitization of tree-structured ZnO lms and QDSCs fabrication
CdS QDs were rstly deposited on the tree-structured ZnO lms through successive ion layer absorption and reaction technique (SILAR) method. The detailed processes were performed following the former studies [21]. For the deposition of CdSe QDs, the CdS sensitized tree-structured ZnO lms were then immersed in a mixed aqueous solution containing Na 2 SeSO 3 , CdSO 4 , and C 6 H 6 NNa 3 O 6 for ca. 4 h at room temperature. The CdS and CdSe QDs co-sensitized ZnO lms were rinsed with DI water and dry at room temperature in dark.
The CdS and CdSe QDs co-sensitized ZnO lms were used as photoanodes. PbS lms, which were prepared through the treatment of Pb sheets in a 2.0 M of Na 2 S, 0.5 M of S, and a 0.2 M KCl mixed solution for another 1 h at 70 ℃, were used as the counter electrodes. The electrolyte was polysul de aqueous solution containing 1 M of Na 2 S, 1 M of S, and 0.1 M of NaOH. The photoanodes and PbS counter electrodes were sandwiched to be a simple solar cell device.

Measurement and characterization
The morphology and structure of the tree-structured ZnO lms were characterized through eld-emission scanning electron microscopy (FE-SEM, JSM-7001F). The crystalline phase of the samples was examined using X-ray diffraction (XRD, DX-2700) with a monochromatic Cu K a irradiation (λ = 0.154145 nm).
The photovoltaic performance of QDSCs was measured using a Keithley 2440 source meter under AM 1.5G illumination from a Newport Oriel solar simulator with an intensity of 1 Sun. The incident light intensity was calibrated using a standard Si solar cell obtained from Newport Oriel. The active cell area of the assembled QDSCs was 0.25 cm 2 . Incident photon-to-current conversion e ciency (IPCE) spectra were recorded on an IPCE system especially designed for QDSCs (Crowntech. Inc.). To generate a monochromatic beam, a 150-W tungsten halogen lamp was used as the light source. A silicon solar cell was used as the standard during calibration. IPCE values were measured using a Keithley model 2400 source meter.

Results And Discussion
Regular ZnO NWs were mostly vertically grown on the FTO glass substrate. The length of the NWs could be regulated to be ca. 3.08, 6.27 and 11.3 μm, respectively (shown in Fig.1), with the extension of the hydrothermal reaction times. The hydrothermal reaction time was 24 h for one growth processes. So the growth time of the ZnO NWs were controlled to be 24 h, 48 h and 72 h. It also can be seen that there were almost no dividing lines in the whole ZnO NWs in Fig. 1(c) although the ZnO NWs were grown through three times of hydrothermal reaction. At the same time, there were large distance between the adjacent ZnO NWs, which provided convenience for the growth of ZnO branches.  Figure 2 shows the surface and cross section of the tree-structured ZnO lms after different hydrothermal treatment time. There were some short branches grown on the surface of the ZnO NWs when the hydrothermal treatment time was 1 h. The short branches were ca. 100 nm. The length of the branches was improved to be ca. 500 nm when the hydrothermal treatment time was 6 h. The length of the branches was further increased to be ca. 1.0 µm after 12 h hydrothermal treatment. The length of ZnO branches depended on the hydrothermal treatment time. It can also be seen from the cross section of these samples that the ZnO branches were almost grown surrounding the ZnO NWs. However, there were few ZnO branches at the end of ZnO NWs. This phenomenon might be related with the capillary effect during the immersion process of Zn seeds.
Though ZnO branches could grow at the surface of ZnO NWs, the ZnO branches were much messy. In this experiment, it was investigated about the effects of the concentration of Zn(CH 3 COO) 2 in the ZnO seed solution on the morphologies of the ZnO lms. The concentration of Zn(CH 3 COO) 2 was increased from 1.2 M to 2.4 M. The branches became more dense and chaotic (shown in Fig. 3 (a-d)). The branches were ca. 500 nm in length. The concentration of Zn(CH 3 COO) 2 was also decreased to 0.24 M and 0.04 M.
It can be seen from the SEM images ( Fig. S1 and Fig. 3