Sol-gel CdS layer for TiO2 nanorod-based quantum dots-sensitized solar cells

The CdS layer was essential for CdSe quantum dot-sensitized solar cells (QDSSCs) as the seed layer and energy barrier. Here, a novel sol–gel method was employed to prepare the CdS interlayer (SG-CdS) for TiO2 nanorod-based QDSSCs. Due to the sufficient reaction of the Cd and S sources in the sol–gel solution, SG-CdS exhibited fewer impurities than CdS produced by commonly used chemical bath deposition (CBD-CdS). QDSSCs with SG-CdS exhibited an open-circuit voltage of 490 mV, a short-circuit current density of 14.12 mA cm−2, and a fill factor of 0.35. The power conversion efficiency of the QDSSCs with SG-CdS was 2.48%, which was higher than that of the QDSSCs with CBD-CdS (2.02%). Moreover, electrochemical impedance spectroscopy showed that the QDSSCs with SG-CdS yielded a charge recombination resistance of 99.92 Ω at a bias voltage of −0.5 V, demonstrating less charge recombination than the QDSSCs with CBD-CdS (82.16 Ω). Therefore, the performance of the CdSe QDSSCs could be improved by reducing the impurities in CdS. This study revealed the advantages of SG-CdS in replacing CBD-CdS as the interlayer for charge transport, as well as good applicability with nanorod photoanodes in QDSSCs.

Chemical bath deposition (CBD) severs as the most common CdS preparation method due to its simple and fast solution deposition process [22].Lee et al demonstrated that CdS formed by the CBD method (CBD-CdS) could improve the power conversion efficiency (PCE) of QDSSCs by more than two times compared with those without CBD-CdS [23].However, a mass of Cd precursor solution must be used to achieve the CdS in the CBD process, resulting in wasted resources and environmental pollution [24].The incomplete intermediates and complex reaction process of CBD reaction also aggravate the trap states in CdS, increasing the risk of charge recombination.Therefore, an environmentally friendly and controllable design strategy of high-quality CdS layers may improve the efficiency of QDSSCs.
In this work, we prepared a CdS interlayer using the sol-gel method (SG-CdS), which was assembled into TiO 2 nanorod-based QDSSCs.To reduce the impurities of the CdS interlayer, we used excess thiourea (easy thermolysis) as the sulfur precursor to adequately react with the cadmium precursor during heat treatment.The sol-gel method used a small amount of cadmium precursor solution during CdS synthesis, which was more environmentally friendly than the CBD method.Scanning electron microscope (SEM) and Raman spectroscopy were used to confirm the formation of CdS on the surface of the TiO 2 nanorods (NRs).The function of SG-CdS in regulating the charge dynamics of QDSSCs was studied using electrochemical impedance spectroscopy (EIS) measurements.We compared recombination resistance at the interface between the photoanode and electrolyte of the QDSSCs with SG-CdS and CBD-CdS to demonstrate the effect of SG-CdS in suppressing charge recombination.As a result, the PCE of the QDSSCs with SG-CdS reached 2.48%, which was higher than that of the QDSSCs with CBD-CdS (2.02%).By improving the quality of the CdS interlayer, better charge transport and reduced recombination could improve the performance of the NR-based QDSSCs.Furthermore, SG-CdS could be expanded to various solar energy conversion systems, such as solar cells, photocatalysis, and water splitting, which require high-quality CdS.

Results and discussion
SG-CdS was prepared on the surface of a TiO 2 NR film using the sol-gel method.A sol-gel precursor solution containing 0.4 M of thiourea and 0.2 M of Cd(NO 3 ) 2 was spin-coated on the TiO 2 NR film and heat-treated in an N 2 atmosphere at 350 °C for 3 min.The schematic of the SG-CdS synthesis process is shown in Scheme 1. Cd(NO 3 ) 2 could complete the reaction with excess thiourea in the heat-treated process to form high-quality CdS, and the remaining thiourea decomposed to reduce impurities.In addition, we employed TiO 2 NRs as the QDs absorption scaffold in the photoanodes, as they contained few grain boundaries, which was beneficial for photogenerated carrier transfer [25].A SEM was used to investigate the SG-CdS morphology on the TiO 2 NR surface.The TiO 2 NRs vertically grew on the fluorine-doped tin oxide (FTO) glass substrates and showed a regular rectangular structure (figures 1(a) and (b)), making them a suitable photoanode for QDSSCs.The length of the TiO 2 NRs was approximately 1.7 μm, and the diameter was in the range of 60-80 nm.After the deposition of SG-CdS, the TiO 2 NRs did not demonstrate any significant changes, indicating that SG-CdS was ultra-thin and uniform (figures 1(c) and (d)).As shown in figures 1(e) and (f), the introduction of CdSe QDs efficiently roughened the surface of the SG-CdS/TiO 2 NR film, confirming successful uniform deposition with sufficient coverage of CdSe QDs.These SEM images indicated that SG-CdS could be used as a seed layer to ensure the preferential growth of CdSe QDs on the TiO 2 NR surface.
The formation of SG-CdS was characterized using Raman spectroscopy, as shown in figure 2. For reference, we also fabricated a CdS using the common CBD method.And to enhance the intensity of the intense peaks, we increased the deposited thickness of the CdS.The Raman spectroscopy revealed the rutile nature of the asprepared TiO 2 NRs from the intense peaks located at 236, 447, and 612 cm -1 [26].After CdS deposition, a peak located at 303 cm -1 was present in the Raman spectra of the SG-CdS and CBD-CdS samples [27].This peak was assigned to the first-order longitudinal optical phonon mode (1LO) of CdS.The appearance of this peak indicated that the CdS layer was successfully synthesized through both the sol-gel and CBD methods.
As a critical photoanode characteristic, the light-harvesting ability was assessed using the UV-vis absorption spectra, as shown in figure 3. The SG-CdS and CBD-CdS films exhibited UV region absorption with an absorption edge of 400 nm and no apparent absorption in the visible region.Moreover, negligible changes were observed between the SG-CdS and CBD-CdS films, indicating that CdS acted as a seed layer and energy barrier,  1, according to the photocurrent density-voltage (J-V ) curves.The QDSSCs with SG-CdS exhibited a PCE of 2.48%, with V oc , J sc , and FF of 490 mV, 14.12 mA cm −2 , and 0.35, respectively.The QDSSCs with CBD-CdS acquired a V oc of 465 mV, J sc of 11.68 mA cm −2 , and FF of 0.37, yielding a PCE of 2.02%.Notably, SG-CdS increased the V oc and J sc of the QDSSCs when compared with CBD-CdS, resulting in a 23% increase in PCE.Because V oc and J sc were closely associated with the charge transport dynamics, we inferred that suppressed charge recombination might occur in the QDSSCs with SG-CdS.EIS was used to explore the influence of SG-CdS on the charge recombination processes in the QDSSCs.As shown in figure 5(a), the Nyquist curves of the QDSSCs with SG-CdS or CBD-CdS were recorded under dark conditions under a forward bias of -0.5 V, perturbation amplitude of 10 mV, and a frequency range of 10 kHz to 100 mHz.The test results were fitted using Z-view software, and the equivalent circuit is depicted in the inset of figure 5(a) [28].The Nyquist plots consisted of two semicircles, with R s being the series resistance, and the smaller semicircles at higher frequency corresponding to charge transfer resistance (R ct ) at the counter electrode and electrolyte interface [29].At a lower frequency, the larger semicircles represented charge recombination resistance (R rec ) at the photoanode and electrolyte interface.CPE 1 and CPE 2 are constant phase elements [30].R rec could be fitted as 99.92 or 82.16 Ω for the QDSSCs with SG-CdS or CBD-CdS.The larger R rec demonstrated suppressed charge recombination between the photoanode and electrolyte interface for the QDSSCs with SG-CdS.The above results indicated that SG-CdS with fewer impurities led to an increase in the V oc and J sc of the QDSSCs (figure 5   In summary, a CdS layer was synthesized on TiO 2 NRs using a convenient sol-gel method and successfully applied in QDSSCs.The SEM and UV-vis absorption spectra demonstrated the morphology and optical properties of SG-CdS.The formation of CdS was verified using Raman spectroscopy.SG-CdS with fewer impurities decreased charge recombination at the photoanode and electrolyte interface in the QDSSCs.The PCE of the QDSSCs with SG-CdS increased by 2.48%, which was higher than that of the QDSSCs with CBD-CdS (2.02%).A significant improvement of 23% in PCE was achieved with the SG-CdS QDSSCs compared with the CBD-CdS QDSSCs.EIS showed that R rec obtained in the QDSSCs with SG-CdS (99.92 Ω) was greater than that obtained in the QDSSCs with CBD-CdS (82.16 Ω) at a bias voltage of −0.5 V, and the improvement in device performance was attributed to an increase in the V oc and J sc .These results demonstrated that innovative CdS synthesis methods could enhance the PCE of QDSSCs.In addition, this work verified the potential of SG-CdS to replace CBD-CdS as the interlayer of various types of photovoltaic devices to improve performance.

Figure 1 .
Figure 1.Top-view and cross-sectional SEM images of (a and b) TiO 2 NR, (c and d) SG-CdS, and (e and f) CdSe/SG-CdS films.

Figure 5 .
Figure 5. (a) Nyquist curves of the EIS measurement measured under the dark condition at an applied forward bias of -0.5 V for the QDSSCs with SG-CdS or CBD-CdS (the inset displays the equivalent circuit used to fit the EIS data): the experimental curves are represented by symbols and the fitted curves with solid lines; (b) structure diagram of the QDSSCs with SG-CdS.

Table 1 .
Photovoltaic parameters of the QDSSCs with SG-CdS or CBD-CdS.