Improved light-harvesting and suppressed charge recombination by introduction of a nanograss-like SnO₂ interlayer for efficient CdS quantum dot sensitized solar cells

Abstract

Quantum dot sensitized solar cell (QDSSC) performance is primarily limited by the recombination of charges at the interfaces of TiO₂/quantum dot (QD) sensitizer/electrolyte. Hence, blocking or suppressing the charge recombination is an essential requirement to elevate the QDSSC performance to the next level. To retard the charge recombination, herein, we propose the introduction of a SnO₂ nanograss (NG) interlayer on the surface of TiO₂ using the facile chemical bath deposition method. The SnO₂ NG interlayer not only inhibits the interfacial recombination processes in QDSSCs but also enhances the light-harvesting capability in generating more excitons. Hence, the TiO₂/SnO₂ NG/CdS QDSSCs can achieve the power conversion efficiency of 3.15%, which is superior to that of a TiO₂/CdS device (2.16%). Electrochemical impedance spectroscopy, open-circuit voltage decay and dark current analyses confirm that the recombination of charges at the photoanode/electrolyte interface is suppressed and the life time is improved by introducing the SnO₂ NG interlayer between the TiO₂ and CdS QD sensitizer.

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1. Introduction

Because of the increased environmental pollution and the limited fossil fuels, the demand for renewable and sustainable energy systems, such as solar energy, has increased over the last few decades. Among the different solar energy systems, third-generation solar cells such as quantum dot sensitized solar cells (QDSSCs) are gaining considerable interest owing to their advantages of inorganic structure, high theoretical efficiency, easy fabrication and low-cost.^1,2 Various metal chalcogenide-semiconductors such as CdS, CdSe, PbS, CdSe_xS_1−x, CuInS₂, CuInZnS, etc. have been investigated as promising sensitizers for the QDSSCs due to their tunable band gap, high absorption coefficient and multiple exciton generation (MEG).^3–5 A typical structure of the QDSSC is composed of a QD-sensitized TiO₂ photoelectrode, polysulfide electrolyte (S²⁻/S_n²⁻), and counter electrode, which leads to various hetero-interfaces.^6–8 Although QDs have unique characteristics, the QDSSCs achieved a low power conversion efficiency (PCE = 12%) due to the recombination of charges that occur at the photoanode/QDs/electrolyte interfaces.^9,10 Hence, considerable attempts have been carried out to suppress the charge recombination, such as the use of a passivation layer on QDs surface, the introduction of metal doping in QDs, and overcoating wide band gap (or insulating) materials over TiO₂ surface or QDs, which results in the enhanced QDSSCs performance.^11–13

Recently, Z. Ren et al. reported that the application of amorphous TiO₂ intermediate layer between the interface of QDs and ZnS/SiO₂ layer, effectively suppressed the charge recombination and improved the photovoltaic performance.¹³ Recently, Li et al. developed the effective integration of ultra-high density well-separated ZnO nanocrystals (NCs) homoepitaxially grown on the surface of an individual vertically oriented single-crystalline (SC) ZnO nanoneedle (NN).¹⁴ The fabricated monolithic ZnO NCs-on-NN SC hybrids may hold great promise for enhanced photovoltaic performance (e.g., electron mobility) of ZnO NC-sensitized solar cells. Very recently, Y. S. Lee et al. introduced the TiO₂/ZnS/CdS/ZnS cascade structure, in which the presence of ZnS intermediate layers are not only capable of retarding the charge recombination but also improve the light-harvesting efficiency, resulting in the enhanced power conversion efficiency (3.69%).¹⁵ Z. Du et al. studied the CdSe QDSSCs based on FTO/TiO₂ without and with TiCl₄ treatment.¹⁶ The QDSSCs based on FTO/TiO₂/TiCl₄/CdSe photoanode exhibited higher PCE (5.36%) than the FTO/TiO₂/CdSe (5.11%), which is mainly due to the TiCl₄ treatment that reduced the recombination of charges at the interface of TiO₂ and CdSe QDs. Very recently, Ozu et al. reported ZnO nanowire (NW) passivation by an ultrathin SnO₂ layer, reducing the density of the deep traps by the absorbed oxygen and hydroxyl groups on the ZnO NW surface.¹⁷ SnO₂ passivation not only reduced the surface recombination but also led to an upward shift of the Fermi energy, causing more efficient electron transfer to ZnO. Very recently, E. N. Jayaweera et al. developed the SnO₂/MgO films for the CdS/ZnS based QDSSCs (SnO₂/MgO/CdS/ZnS) to boost the solar cell performance by efficiently preventing the charge recombination at the interface of SnO₂/sensitizer.¹⁸ As a result, the SnO₂/MgO/CdS/ZnS QDSSC achieved a PCE of 1.33%. Therefore, the idea of deposition of wide band-gap semiconductor materials on the surface of semiconductor nanoparticles could effectively improve the QDSSC performance.

Motivated from the recent results of hindering the charge recombination at the TiO₂/QDs/electrolyte interfaces, herein, we propose the introduction of a novel SnO₂ nanograss (NG) interlayer on TiO₂ surface via a simple chemical bath deposition (CBD) route. The deposition of SnO₂ NG between the TiO₂ and CdS QDs suppresses charge recombination, resulting in enhanced QDSSC performance. Further, the photoanode of TiO₂/SnO₂ NG/CdS achieves improved light-harvesting and also provides efficient charge transport. The QDSSCs based on TiO₂/SnO₂ NG/CdS photoelectrode showed an impressive PCE of 3.15% under one sun illumination, which was much higher than that with TiO₂/CdS photoelectrode (2.16%). The achieved high efficiency by SnO₂ NG interlayer signifies an important approach to further elevate QDSSCs performance to the next level.

2. Experimental section

2.1. Materials

Tin chloride dihydrate (SnCl₂·2H₂O), hexamethylenetetramine (HMTA, C₆H₁₂N₄), cadmium acetate dihydrate (Cd(CH₃COO)₂·2H₂O), sodium sulfide (Na₂S), sulfur (S) and potassium chloride (KCl) were obtained from Sigma-Aldrich Co., South Korea and used as such without purification. TiO₂ paste (Ti-nanoxide HT/SP) was supplied by Solaronix.

2.2. Preparation of TiO₂/SnO₂ NG/CdS photoelectrode

Prior to the deposition, the fluorine-doped tin oxide (FTO, 13 Ω sq⁻²) glasses (1.3 × 1.5 cm²) were well washed with acetone, ethanol and deionized (DI) water for 10 min each. Mesoporous TiO₂ electrodes were prepared using doctor-blading a TiO₂ paste (20 nm diameter, Solaronix SA) onto FTO substrate and then the samples were maintained at 450 °C for 30 min. Then, SnO₂ NG was grown on the TiO₂ surface via a facile CBD reaction. In detail, 0.05 M SnCl₂·2H₂O and 0.15 M HMTA were mixed in 70 mL DI water under magnetic stirring for 30 min. The as-prepared TiO₂ samples were dipped horizontally into the SnO₂ growth solution and heated at 95 °C for 10 h. The HMTA in precursor solution not only serves as a surfactant by reducing the surface tension between the growth solution and substrate, but also it supports the growth of SnO₂ layer. After deposition, the SnO₂ NG loaded FTO/TiO₂ films were rinsed with DI water and ethanol, and then heated at 450 °C for 30 min. The as-fabricated electrodes are termed the TiO₂/SnO₂ NG.

Next, the CdS QDs were loaded on TiO₂/SnO₂ NG by a facile successive ionic layer adsorption and reaction (SILAR) method. Briefly, TiO₂/SnO₂ NG films were dipped in aqueous 0.1 M Cd(CH₃COO)₂·2H₂O solution for 5 min, rinsed with ethanol; then dipped in aqueous 0.1 M Na₂S solution for 5 min, and then once again rinsed with ethanol. Such a process is called one SILAR cycle and a totally 8 SILAR cycles were repeated. The as-obtained films are termed the TiO₂/SnO₂ NG/CdS. The schematic of the fabrication process of TiO₂/SnO₂ NG/CdS on FTO substrate is shown in Fig. 1.

Fig. 1 The schematic of the preparation process of TiO₂/SnO₂ NG/CdS on FTO substrate.

2.3. Counter electrode and QDSSC device fabrication

CuS counter electrodes were prepared on FTO substrates using CBD method following our previous report.¹⁹ Finally, the QDSSCs were sandwiched with as-fabricated TiO₂/SnO₂ NG/CdS photoelectrodes and CuS counter electrodes by a sealant (SX 1170-60, Solaronix) and polysulfide electrolyte containing of 1 M Na₂S, 2 M S and 0.1 M KCl in methanol and water solution (7 : 3).

2.4. Characterization and measurements

The crystalline phase, elemental analysis and morphology of the electrodes were investigated using X-ray diffraction (XRD, D/Max-2400), X-ray photon spectroscopy (XPS, VG Scientific ESCALAB 250) and scanning electron microscope (SEM, S-2400, Hitachi) with energy-dispersive X-ray spectroscopy (EDX, 15 kV) mapping characterizations, respectively. The optical absorption properties of the as-prepared electrodes were analyzed by UV-visible absorption spectra (OPTIZEN 3220UV). The photocurrent–voltage (J–V) measurements were conducted by an ABET Technologies (USA) solar simulator, which can exhibit an intensity of 100 mW cm⁻². Electrochemical impedance spectroscopy (EIS) was carried out by a SP-150 BioLogic station with a frequency range of 500 kHz to 0.1 Hz and AC amplitude of 10 mV.

3. Results and discussion

3.1. Structural and morphological characterization

Facile and cost-effective CBD method was used to grow the SnO₂ NG interlayer onto the TiO₂ surface. Initially, the crystalline structure of the TiO₂ and TiO₂/SnO₂ NG electrodes was investigated using XRD characterization. The XRD patterns of the as-prepared TiO₂ and TiO₂/SnO₂ NG are depicted in Fig. 2. As seen in Fig. 2, both the TiO₂ and TiO₂/SnO₂ NG samples exhibit peaks at 2θ = 25.1°, 37.5°, 47.8°, 53.9° and 62.3°, which are well-matched with the TiO₂ tetragonal anatase structure (JCPDS: 21-1272). After the deposition of SnO₂ over TiO₂ surface, new diffraction peaks are observed at 2θ = 26.7°, 33.6°, 51.9°, 54.9°, 61.5° and 65.8°, which are well-consistent with the SnO₂ tetragonal structure (JCPDS: 41-1445).²⁰

Fig. 2 XRD patterns of TiO₂ and TiO₂/SnO₂ NG on FTO substrate.

XPS characterization was performed to investigate the valence states of elements in TiO₂/SnO₂ NG electrode. The XPS survey spectra of TiO₂/SnO₂ NG sample is shown in Fig. 3a, which clearly shows the presence of only Ti, Sn, O and C elements. The presence of carbon element in the total survey scan spectrum is ascribed to the exposure of air sample to air. As depicted in Fig. 3b, the XPS spectrum of Ti 2p, exhibits the Ti 2p_3/2 and Ti 2p_1/2 peaks that are centered at 458.7 and 464.5 eV, respectively. These spectra are consistent with the presence of Ti⁴⁺ valence state. Due to the spin–orbit coupling of Ti 2p, the obtained peaks are separated by 5.8 eV.²¹ The Sn 3d spectrum presented in Fig. 3c exhibited two peaks at 486.8 eV and 495.3 eV which are indexed to Sn 3d_5/2 and Sn 3d_3/2, respectively, and agree with the Sn⁴⁺ state in SnO₂.²² As depicted in Fig. 3d, the spectrum of O 1s displays a peak that is centered at 530.9 eV, corresponding to O₂ in the TiO₂ and SnO₂ crystal lattices.^22,23 The XRD and XPS results confirmed the formation of SnO₂ NG on the TiO₂ surface.

Fig. 3 (a) XPS survey spectra and high-resolution XPS spectra of (b) Ti 2p, (c) Sn 3d and (d) O 1s in TiO₂/SnO₂ NG sample.

The surface morphologies of the as-prepared TiO₂ and TiO₂/SnO₂ NG electrodes were studied by SEM analysis, and the corresponding images are depicted in Fig. 3a and b for TiO₂ on FTO substrate, and in Fig. 4c and d for SnO₂ onto the TiO₂ surface (TiO₂/SnO₂) NG. As shown in Fig. 4a, the TiO₂ nanoparticles are uniformly distributed throughout the substrate. It can be clearly seen from Fig. 4b, that the TiO₂ exhibits spherical nanoparticle morphology with diameter in the range of ∼16 to ∼20 nm. CBD method was used to grow the SnO₂ nanostructures on the surface of TiO₂. It can be clearly observed in Fig. 4c, that the SnO₂ exhibits the nanograss (NG) morphology. The diameter of the SnO₂ NG was estimated to be in the range of ∼21 to ∼46 nm (Fig. 4d). The SEM images clearly reveal that the SnO₂ NG nanostructures were successfully grown on TiO₂ surface. In addition, SEM-EDX color mapping was carried out to examine the distribution of elements in the TiO₂/SnO₂ NG. The mapping images shown in Fig. S1† clearly revealed the homogeneous distribution of Ti, Sn and O elements in the as-prepared TiO₂/SnO₂ NG electrode.

Fig. 4 SEM images of (a and b) TiO₂ and (c and d) TiO₂/SnO₂ NG on FTO substrate at different magnifications.

3.2. Optical properties

UV-vis absorption measurements were carried out to demonstrate the optical properties of the TiO₂, TiO₂/SnO₂ NG, TiO₂/CdS and TiO₂/SnO₂ NG/CdS samples and the results are plotted in Fig. 5 in the rage of 300 to 800 nm. The TiO₂ sample exhibits the absorption edge at around 378 nm. The growth of SnO₂ NG on TiO₂ surface increased the absorption intensity. In addition, the absorption onset of TiO₂/SnO₂ NG increased to 388 nm. The absorption onsets of the TiO₂/CdS and TiO₂/SnO₂ NG/CdS samples were measured to be 510 nm and 525 nm, respectively. The absorption onset of the TiO₂/SnO₂ NG/CdS electrode was increased and with higher absorbance compared to TiO₂/CdS. These observations can be attributed to the growth of SnO₂ NG passivation/barrier layer over TiO₂ surface. Also, the SnO₂ NG interlayer supports the nucleation and growth of CdS QDs, which results in the high rate absorption of TiO₂/SnO₂ NG/CdS electrode.

Fig. 5 UV-vis absorption spectra of as-prepared electrodes.

3.3. Photovoltaic performance and charge transfer of the QDSSCs

To evaluate the impact of the SnO₂ NG interlayer on the photoanodes, the photovoltaic behaviors of TiO₂/CdS and TiO₂/SnO₂ NG/CdS based QDSSCs have been investigated. The J–V profiles of the as-fabricated QDSSCs were measured under one sun illumination and the related photovoltaic considerations are depicted in Fig. 6a and Table 1. The TiO₂/CdS QDSSC exhibits J_SC of 6.64 mA cm⁻², V_OC of 0.614 V, and FF of 0.530, resulting in a PCE of 2.16%. When a SnO₂ NG layer was grown on TiO₂ surface (TiO₂/SnO₂ NG/CdS), the J_SC, V_OC and FF were considerably improved to 8.92 mA cm⁻², 0.619 V, and 0.570, respectively, resulting in the elevated PCE of 3.15%, which is much higher than the PCE of TiO₂/CdS (2.16%) photoelectrode. The enhanced V_OC and J_SC of the TiO₂/SnO₂ NG/CdS based QDSSCs are attributed to the reduced charge recombination and the fast charge transfer at the TiO₂/QDs/electrolyte interfaces. Hence, the PCE of QDSSC has improved from 2.16% to 3.15% by the growth of SnO₂ NG interlayer over the TiO₂ surface. Furthermore, incident photon to current conversion efficiency (IPCE) study was carried out to illustrate the light absorption and electron generation behaviors in the QDSSCs. Fig. 6b depicts the IPCE plots of the as-prepared QDSSCs. It is evident from the IPCE spectra that the introduction of SnO₂ NG interlayer enhanced the IPCE response from 66% to 75% and also enlarged the IPCE response edge from 548 nm to 568 nm. The TiO₂/SnO₂ NG/CdS based QDSSCs exhibits higher IPCE response than the TiO₂/CdS, which is in good agreement with the J_SC of the J–V plots.

Fig. 6 (a) J–V, (b) IPCE and (c) EIS profiles of QDSSCs based on TiO₂/CdS and TiO₂/SnO₂ NG/CdS photoelectrodes. (d) J–V profiles of QDSSCs under dark current.

Table 1 Photovoltaic parameters of the as-prepared QDSSCs based on TiO₂/CdS and TiO₂/SnO₂ NG/CdS photoelectrodes under one sun illumination

Cell	VOC (V)	JSC (mA cm^−2)	FF	PCE%	Rct (Ω)	Cμ (μm)	τe (ms)
TiO2/CdS	0.614	6.64	0.530	2.16	8.78	1358	11.92
TiO2/SnO2 NG/CdS	0.619	8.92	0.570	3.15	27.94	2183	60.99

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In order to demonstrate the impact of SnO₂ NG interlayer on the photoelectrode films, EIS measurement was conducted under forward bias (V_OC) and illumination. The Nyquist plots of the TiO₂/CdS and TiO₂/SnO₂ NG/CdS based QDSSCs are depicted in Fig. 6c. The Nyquist plots of both devices exhibit the two semicircles, in which the first (small) semicircle is obtained in the high-frequency region and represents the charge transfer resistance (R_CE), the chemical capacitance (C_CE) at the interface of the counter electrode/electrolyte, and the series resistance (R_S) which is obtained in the high-frequency region where the phase is zero. The recombination resistance (R_ct) at the TiO₂/QD/electrolyte interface and the chemical capacitance (C_μ) indexed to the bigger semicircle which is obtained at mid/low frequency.^24,25 Based on the equivalent circuit model (inset of Fig. 6c) and using Z-view software, the Nyquist plots were fitted to obtain the R_ct values and the fitting results are summarized in Table 1. There is no considerable change in the R_CE, which is due to the usage of similar electrolyte and counter electrode in the fabrication of QDSSCs. However, R_ct of the TiO₂/SnO₂ NG/CdS based QDSSC (27.94 Ω cm²) is much higher than that of the TiO₂/CdS (8.78 Ω cm²). The higher R_ct of the TiO₂/SnO₂ NG/CdS based QDSSC demonstrates that the growth of SnO₂ NG over the TiO₂ surface successfully suppresses the electron recombination at the TiO₂/QDs/electrolyte interfaces.²⁴ Hence, the growth of SnO₂ NG interlayer hinders the interfacial charge recombination and enhances the photovoltaic performance (J_SC and FF). Moreover, QDSSC based on TiO₂/SnO₂ NG/CdS photoelectrode achieves higher C_μ (2183 μF) than that of the TiO₂/CdS (1358 μF) system. The higher C_μ of the TiO₂/SnO₂ NG/CdS system demonstrates the improved collection of photo-excited electrons into the conduction band of photoanode, which is mainly due to the hindered recombination at the TiO₂/QDs/electrolyte interfaces. Further, the electron life time (τ_e) of the QDSSCs can be obtained using the following equation,²⁶

τ_e = R_ct × C_μ (1)

Interestingly, the τ_e of the TiO₂/CdS with SnO₂ NG interlayer (60.99 ms) is much higher than that of the SnO₂ NG-free device (11.92 ms), which reveals the higher charge collection efficiency of the TiO₂/SnO₂ NG/CdS system.

Under the dark condition, the J–V plots of the TiO₂/CdS and TiO₂/SnO₂ NG/CdS based QDSSCs were obtained and are shown in Fig. 6d. The electron recombination occurs at the interfaces of TiO₂/QDs and TiO₂/electrolyte, and this is the source of dark current.²⁷ Hence, a dark current study is a useful indicator of charge recombination. With the introduction of SnO₂ NG interlayer, the TiO₂/SnO₂ NG/CdS device delivers the reduced dark current, which is lower compared with that of the TiO₂/CdS device. The reduction of the dark current arose from an enhancement in electron transport with decreasing the internal resistance. As a result, the electron recombination was effectively reduced by the introduction of the SnO₂ NG interlayer, and this contributed to the enhanced current and decreased electron loss.

Furthermore, to investigate the impact of SnO₂ NG interlayer on the performance of QDSSCs, EIS tests were conducted at various bias applied voltages under dark condition in the 500 kHz to 100 mHz frequency range. The obtained recombination resistance (R_rec) and chemical capacitance (C_μ) from the corresponding EIS tests are depicted in Fig. 7a and b, respectively. The R_rec and C_μ reveal that the charge recombination process occurs at the TiO₂/QDs/electrolyte interfaces. A higher R_rec represents the low recombination rate and grater C_μ values denote the Fermi level upward shift, yielding the enhanced V_OC.¹⁶ It can be seen from Fig. 6a that the R_rec values of both devices decrease with the increment of the forward bias voltage due to the increased Fermi level of TiO₂ at the forward bias. Also, TiO₂/SnO₂ NG/CdS QDSSCs exhibits higher R_rec values than the TiO₂/CdS device under identical bias voltages. The recombination rate is inversely proportional R_rec.¹³ Moreover, the higher C_μ values of the TiO₂/SnO₂ NG/CdS device denote the upward shift of the Fermi level of TiO₂, resulting in large V_OC. These EIS studies reveal the reduced recombination rate for the TiO₂/SnO₂ NG/CdS device, which is favorable for the improved J_SC and FF.

Fig. 7 EIS (a) recombination resistance (R_rec) and (b) chemical capacitance (C_μ) at various forward bias voltages (V).

The excited electrons life time was examined using the open-circuit voltage (V_OC) decay studies with time. Initially, the QDSSCs were irradiated with one sun illumination to a steady voltage, then the illumination was turned off and the V_OC decay data was obtained. Fig. 8a depicts the V_OC decay profiles of the TiO₂/CdS and TiO₂/SnO₂ NG/CdS based QDSSCs. The V_OC decay plots clearly exhibit the continued monitoring of V_OC values under illumination and approach to decay after switching off the illumination. It is evident from the V_OC decay plots that the TiO₂/SnO₂ NG/CdS based QDSSCs exhibit a slower voltage decay rate than the TiO₂/CdS photoanode. This behavior is mainly attributed to the SnO₂ NG interlayer which efficiently suppresses the charge recombination at the TiO₂/QDs/electrolyte interfaces and also promotes the efficient electron transfer. Further, the electron life time (τ_e) can be estimated using the following equation:²⁸

where, k_B, T and e have their usual meanings. Fig. 8b displays the plot of electron life time (τ_e, in log sign) as a function of V_OC.²⁹ The TiO₂/SnO₂ NG/CdS based QDSSCs deliver longer τ_e values than that of TiO₂/CdS, implying a suppressed recombination of the photo-generated electrons leading to the efficient charge transfer, which agrees well with the EIS analysis.

Fig. 8 (a) V_OC decay profiles of TiO₂/CdS and TiO₂/SnO₂ NG/CdS QDSSCs. (b) Electron life time (in log-linear notation) obtained from V_OC decay measurements.

Fig. 9 depicts the impact of the SnO₂ NG interlayer on the charge transfer mechanism in QDSSCs. As shown in Fig. 9a, upon illumination, CdS captures the photons and produces the electron–hole pairs, named excitons. Then the electrons transfer into TiO₂ conduction band, while the holes are reduced by polysulfide electrolyte. Simultaneously, the possibility of charge recombination also takes place at the interfaces of TiO₂/CdS QDs/electrolyte, which results in poor photovoltaic performance (Fig. 9a). Hence, SnO₂ NG interlayer was introduced between the TiO₂ and CdS QDs (TiO₂/SnO₂ NG/CdS) to prevent the charge recombination (Fig. 9b). The introduction of SnO₂ NG interlayer on TiO₂ surface retards the back injection of the electron from TiO₂ to QDs and also prevents the injection of excited electron from TiO₂ to redox couple, resulting in the high photovoltaic performance. Therefore, introducing SnO₂ NG interlayer over the TiO₂ surface achieves the outstanding properties, such as wide solar light-harvesting ability, enhanced charge transfer and suppressed charge recombination at the TiO₂/QDs/electrolyte interfaces.

Fig. 9 Schematic demonstration of the charge transfer pathways in (a) TiO₂/CdS and (b) TiO₂/SnO₂ NG/CdS based QDSSCs.

4. Conclusions

In summary, the introduction of SnO₂ NG layer between the TiO₂ and CdS QDs achieved the higher light-harvesting ability, improved charge transport and suppressed charge recombination at the TiO₂/QDs/electrolyte interfaces. Evident from the J–V, EIS, IPCE, V_OC decay and dark current measurements indicate that the photovoltaic properties of the TiO₂/CdS QDSSCs were drastically enhanced after the deposition of SnO₂ NG layer due to the improved light absorption, fast charge transfer, reduced charge recombination and prolonged electron life time. Hence, the introduction of SnO₂ NG interlayer could be a favorable and effective approach to considerably improve the performance of QDSSCs. As a result, TiO₂/SnO₂ NG/CdS based QDSSCs achieved the PCE (3.15%), V_OC (0.619 V), J_SC (8.92 mA cm⁻²), and FF (0.570), respectively, which are superior to TiO₂/CdS device (PCE = 2.16%, V_OC = 0.614 V, J_SC = 6.64 mA cm⁻², and FF = 0.530).

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by UAEU Program for Advanced Research (UPAR) under grant no. 31S312. Also, this work was supported by BK 21 PLUS, Creative Human Resource Development Program for IT Convergence, Pusan National University, Busan, South Korea.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ra08234d

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