Pulsed laser deposition of a Bi₂S₃/CuInS₂/TiO₂ cascade structure for high photoelectrochemical performance

Abstract

The PLD technique is used for the direct fabrication of QD sensitized solar cells (QDSSCs) without any encapsulation and/or resorting to any surface treatment, ligand engineering and/or post-synthesis processing which might involve some toxic chemical regents harmful to the performance of solar cells. In this paper, co-sensitizers of Bi₂S₃ and CuInS₂ quantum dots (QDs) are deposited on TiO₂ nanorods via a physical deposition-based pulsed laser deposition (PLD) technique to fabricate the cascade structure of Bi₂S₃/CuInS₂/TiO₂. The performance of the QDSSCs with a cascade structure is optimized by adjusting the laser energy, and an energy conversion efficiency of 4.81% is achieved under one sun illumination (AM 1.5, 100 mW cm⁻²). Besides, the photovoltaic device exhibits high stability in air without any specific encapsulation. The improved performance is attributed to enhanced absorption in the longer wavelength region, quicker interfacial charge transfer and less chance of electron recombination with holes. Moreover, the direct atomic contact by the PLD technique and the cascade structure are also favorable factors for the enhanced photoelectrochemical performance of QDSSCs.

---

1. Introduction

Semiconductor quantum dots (QDs) have attracted significant interest from both fundamental and application viewpoints over the past few decades as sensitizers in quantum dot sensitized solar cells (QDSSCs) because of the high extinction coefficient, quantum confinement effect and impact ionization effect.^1–4 TiO₂ with a band gap of 3.26 eV is one of the most commonly used photoelectrode materials in QDSSCs because of its low-cost synthetic routes and higher stability than other semiconductors.^5,6 However, due to its large band gap, semiconductor QDs such as PbS,⁷ Bi₂S₃,⁸ AgInS₂,³⁵ CdS³⁶ and CuInS₂ (ref. 9) are incorporated with TiO₂ photoelectrodes as sensitizers to enlarge the photo-response range of the photoelectrode into the visible light region.

Recently, we have reported an improved energy conversion efficiency of 3.95% for CuInS₂ QDSSCs fabricated by pulsed laser deposition (PLD) technique.¹⁰ However, the CuInS₂/TiO₂ photoelectrode absorbs mainly in the wavelength region from 300 to 700 nm because of the wide band gap of TiO₂ and CuInS₂ (1.5 eV), indicating that a large percentage of sunlight with the spectrum ranging 700–2500 nm is still wasted. To further improve the energy conversion efficiency of CuInS₂ QDSSCs, it is essential to improve the optical absorption of the CuInS₂/TiO₂ photoelectrode in the wavelengths greater than 700 nm. As an alternative, utilizing different kinds of QDs with complementary absorption spectra in the visible spectrum as sensitizers to co-sensitize TiO₂ with CuInS₂ QDs (i.e. co-sensitization) is a typical strategy for this purpose. Wang et al. deposited the co-sensitizer of CdS and PbS nanoparticles on the TiO₂ nanotubes which significantly enhanced the visible light response, photocurrents and the photoelectrochemical properties.¹¹ Teng et al. fabricated the QDSSCs with TiO₂/CuInS₂-QDs/CdS photoanode by solvothermal method and achieved the high energy conversion efficiency of 4.2%.¹⁵ More recently, Rosa et al. obtained the optimal energy conversion efficiency of 2.52% with the configuration TiO₂/CdS/Bi₂S₃ via successive ionic layer adsorption and reaction method.¹² Furthermore, the cascade structure with two kinds of co-sensitizers has emerged to construct light energy harvesting assemblies. The cascade nanostructure can not only achieve extraordinary performance for that the outer shell serves as a potential barrier which confines the charge carriers in the region, but also can suppress the recombination of photo-excited electron–hole pairs and protect core material from photocorrosion.^13,14

Hence, it is a potential approach to improve the performance of QDSSCs by co-sensitization and the key issues lie in exploring suitable semiconductor QDs with suitable band gap to form cascade structure. Bismuth sulfide (Bi₂S₃), as a V–VI compound semiconductor with the direct band gap of 1.3 eV, is a promising candidate for applications in photovoltaic devices, thermoelectric devices, optoelectronic devices and IR spectroscopy.^16,17 It is also a good alternative to be co-sensitizer. However, the mostly reported methods to realize co-sensitization are always chemical solution ways: ex situ and in situ preparation. For the ex situ method, QDs are presynthesized and attached onto TiO₂ by employing ligand molecules. Although this method could capitalize on the advantage of the colloidal syntheses to control growth dynamics and particle size by using ligand molecules, the ligand molecules have some defects on the performance of QDSSCs for the reasons that it makes the QD attachment unstable, competes with the QDs in light absorption and creates transport barriers for photogenerated electrons. Moreover, the ligand exchange process is time-consuming and unrepeatable.^22,28–30 For the in situ preparation, such as successive ionic layer adsorption and reaction method and chemical bath deposition, QDs are anchored to TiO₂ directly without any ligands. However, the QD size is uncontrolled and the obtained QDs always show poor stability in air, which limits the long-term application of QDSSCs. Hence, a ligand-free and solution-free synthesis approach is needed to circumvent such limitations by enabling the in situ growth of QDs onto various nanostructures and thus their facile integration into high-performance and stable PV devices.

The PLD technique is a clean and low-cost method for preparing semiconductor and metal thin films on various substrates. It enables the in situ growth of QDs onto various nanostructures and could obtain high-quality stoichiometric thin films. In previous work,¹⁰ we have demonstrated the powerfulness and process latitude of the PLD technique to decorate the TiO₂ nanorods controllably with CuInS₂ QDs to achieve excellent photoelectrochemical performance. In this work, by capitalizing on the advantageous features of the PLD approach, we were further able to fabricate the QDSSCs with cascade structure of Bi₂S₃/CuInS₂/TiO₂ photoelectrode, which exhibited high energy conversion efficiency and excellent stability in air without any encapsulation. The size of Bi₂S₃ QDs can be adjusted by changing the laser energy and an optimal energy conversion efficiency of 4.81% was achieved under one sun illumination (AM 1.5, 100 mW cm⁻²). The surface morphology, structure characterization, optical property, and photoelectrochemical properties of the Bi₂S₃/CuInS₂/TiO₂ photoelectrode are studied in detail with the comparison to those of CuInS₂/TiO₂ photoelectrode.

2. Experimental section

Preparation of Bi₂S₃/CuInS₂/TiO₂ electrodes

TiO₂ nanorod arrays were prepared on fluorine-doped tin oxide (FTO) glass using a hydrothermal method that was described in detail elsewhere.¹⁸ The fabrication of CuInS₂/TiO₂ photoelectrode by PLD technique can be found in our previous work¹⁰ and the cascade structure was fabricated based on the CuInS₂/TiO₂ photoelectrode deposited at 350 mJ since the sample CIS(350)/TiO₂ obtained the highest energy conversion efficiency. The Bi₂S₃ QDs were deposited on the surface of CuInS₂/TiO₂ photoelectrode by PLD technique at room temperature. Prior to the deposition, the vacuum chamber was evacuated to a base pressure of 10⁻⁴ Pa. A KrF excimer laser with a wavelength of 248 nm was focused on the rotating Bi₂S₃ target with a distance of 6.5 cm from the substrate. All the films were deposited at the repetition frequency of 3 Hz, the deposition time of 10 min and certain laser energy. The sample deposited at the laser energy of n mJ was denoted as BS(n)/CIS(350)/TiO₂. After PLD deposition, the samples were annealed at 300 °C for 1 h in nitrogen atmosphere to improve their crystallinity. For comparison, the BS/TiO₂ photoelectrode was also fabricated by the PLD technique with the same parameters as above.

Fabrication of QDSSCs

Firstly, a ZnS passivation layer was deposited on the Bi₂S₃/CuInS₂/TiO₂ electrode with a modified SILAR method in the ref. 19. Typically, after 2 cycles of alternately dipping into 0.1 M Zn(OAc)₂ methanol solution and 0.1 M Na₂S solutions for 1 min per dip, the Bi₂S₃/CuInS₂/TiO₂ electrode was coated with ZnS. For the fabrication of QDSSCs, a 60 μm-thick Surlyn sealing material was placed between the obtained electrode and the Pt-coated counter electrode to fabricate a sandwich-type configuration solar cell. For the I–V characterization, the electrolyte was a polysulfide solution consisting of 0.5 M Na₂S, 2.0 M sulfur and 0.2 M KCl in a mixed solvent of methanol and water solution (3 : 7 by volume). For the other photoelectric properties, the electrolyte was composed of 0.05 M Na₂S and 0.95 M Na₂SO₃ aqueous solution. The active area of the solar cells was 0.5 cm².

Measurements

The surface morphologies and microstructures of the samples were investigated by field emission scanning electron microscope (FESEM, JSM-6701F, SU8020, Japan), transmission electron microscope (TEM) and high-resolution transmission electron microscope (HRTEM, FEI Tecnai F30, acceleration voltage: 300 kV). The composition and crystal structure of samples were analyzed by X-ray diffraction (XRD, Rigaku D/Max-2400 X-ray diffractometer with Cu-Kα irradiation). A PHI5702 multifunctional X-ray photoelectron spectroscope (XPS, USA) with the Al-Kα irradiation (1486.6 eV) as the excitation source was used to determine the composition and chemical states of the elements in the samples. Spect-50 UV-vis spectrophotometer (Jena, Germany) was used to measure the optical absorption in the wavelength range of 300–1000 nm. The J–V characteristics were measured by a CELL-S500 (China) solar simulator at 100 mW cm⁻² (AM1.5 G). The other photoelectrical properties of the samples were recorded on the electrochemical workstation (CHI660d) with a standard three-electrode system consisting of a working electrode, a Pt wire as the counter electrode, and a saturated calomel electrode (SCE) as a reference electrode. The illumination source was a 300 W Xe lamp, and the sunlight was simulated in full spectra. The light intensity was calibrated by a standard crystalline silicon cells, equivalent to AM 1.5 G light at 100 mW cm⁻². The EIS plots were measured at an open circuit bias (−V_OC) under dark conditions in the frequency range from 0.01 Hz to 100 kHz. Mott–Schottky plots were measured in the dark at an AC frequency of 3.0 kHz. The incident photon to current conversion efficiency (IPCE) spectra of QDSSCs were measured with a Crown instrument over the wavelength range from 400 to 900 nm and with a Si photovoltaic cell as reference.

3. Results and discussion

Fig. 1a–e show the FESEM images of BS(n)/CIS(350)/TiO₂ deposited at the laser energy n of 200–350 mJ. By varying the laser energy, we can control the quantity and size of Bi₂S₃ QDs onto the surface of CuInS₂/TiO₂. There are still some porosities between the nanorods when applying lower laser energy (Fig. 1b and c). The Bi₂S₃ QDs are incorporated onto CuInS₂/TiO₂ directly and generated the firm, direct interfaces between QDs and CuInS₂/TiO₂ arrays due to the fact that in the PLD process sputtered plasma with high velocity can intensely collide with the molecules on the surface of CuInS₂/TiO₂, and therefore the obtained QDs can integrate closely with CuInS₂/TiO₂ without linker molecules. The quantity of QDs on nanorods and the size of QDs raise along with the increase of laser energy. Meanwhile, the porosities reduce and the nanorods become thicker compared with that deposited at lower laser energy (Fig. 1d). While further increasing the laser energy, the QDs integrate cauliflower-like nanoparticles and the top of nanorod becomes domed (Fig. 1e). Fig. 1f shows the XRD patterns of as-prepared samples and the curves from bottom to top represent the XRD patterns of FTO, TiO₂/FTO, CIS(350)/TiO₂/FTO, BS(200)/CIS(350)/TiO₂/FTO, BS(250)/CIS(350)/TiO₂/FTO, BS(300)/CIS(350)/TiO₂/FTO and BS(350)/CIS(350)/TiO₂/FTO respectively. From the pattern of FTO/TiO₂, TiO₂ nanorods can be classified as tetragonal rutile phase (JCPDS file no. 21-1276). Other characterizations about TiO₂ nanorods can be found in the ESI (Fig. S1, ESI†). In addition to the diffraction peaks of FTO substrate and TiO₂ film, the rest peaks well match the tetragonal CuInS₂ (JCPDS file no. 27-0159) and bismuthinite Bi₂S₃ (JCPDS file no. 06-0333). The diffraction peaks located at the diffraction angle of 27.9°, 46.2° and 54.9° can be ascribed to the crystal planes of (112), (204)/(220) and (312)/(116) of the CuInS₂ QDs. The fact that the diffraction peaks of CuInS₂ and FTO coincide with each other obviously widens the diffraction peak at 54.9°. Meanwhile, the diffraction peaks at 22.3°, 23.7°, 25.2°, 28.6° and 31.8° can be indexed to the crystal planes of (220), (101), (310), (230) and (221) of Bi₂S₃ QDs. Besides, the average sizes of Bi₂S₃ QDs estimated by the Scherrer equation are ∼8, ∼10, ∼11, and ∼14 nm, respectively (varying with the laser energy from 200 to 350 mJ). The increase of QDs size along with the increase of laser energy is mainly due to that the more Bi₂S₃ plasma would be generated from the target when higher laser energy was employed and the Bi₂S₃ plasma will continue to grow on CuInS₂/TiO₂ to form QDs with larger diameter. Additionally, the XPS measurement has also been applied to determine the composition and valence state of elements in BS(300)/CIS(350)/TiO₂. As shown in Fig. S2,† all the peaks correspond to Bi, Cu, In, S, Ti, O and C elements (in which the C element was used to calibration) can be clearly detected and their valence states are consistent with the corresponding standardized binding energies. Combining the XRD and XPS analysis, it can be substantiated that the Bi₂S₃ QDs are successfully deposited on the surface of CuInS₂/TiO₂ and the as-obtained BS(300)/CIS(350)/TiO₂ samples compose no obvious impurities.

Fig. 1 FESEM images of (a) CIS(350)/TiO₂, (b) BS(200)/CIS(350)/TiO₂, (c) BS(250)/CIS(350)/TiO₂, (d) BS(300)/CIS(350)/TiO₂ and (e) BS(350)/CIS(350)/TiO₂. (f) XRD patterns of BS(n)/CIS(350)/TiO₂ electrodes.

Fig. 2a shows the TEM image of BS(300)/CIS(350)/TiO₂. It can be clearly seen that the QDs are uniformly and closely deposited on the entire TiO₂ nanorod. There are many single crystal diffraction spots with messy arrangement in the SAED pattern (inset in Fig. 2a) because of the quantity and density of QDs, which demonstrates the high crystallinity of nanorods and QDs. In the HRTEM image (Fig. 2b), the clearly distinguishable lattice fringes also confirm the high crystallinity of the nanorods and QDs. The distance between lattice fringes of 0.32 nm and 0.29 nm is corresponding to the (112) plane of tetragonal CuInS₂ (JCPDS file no. 27-0159) and the (221) plane of bismuthinite Bi₂S₃ (JCPDS file no. 06-0333), respectively. Besides, the lattice fringe distance of 0.35 nm is indexed to the (110) plane of rutile phase of tetragonal rutile TiO₂ (JCPDS file no. 21-1276). The FFT image for a single QD verifies the high crystallinity of QDs, which is consistent with the analysis in XRD patterns. EDS line scan profiles of BS(300)/CIS(350)/TiO₂ for Ti, O and Bi, Cu, In, S elementals are conducted and displayed in Fig. 2c and d to illustrate the composition and structure of QD-layer, which reveals the relative locations of the embedded Bi₂S₃ QDs and CuInS₂ QDs. There is an O-rich area in the middle of the samples (Fig. 2c) and the outer shell is the S-rich QD-layer (Fig. 2d). Moreover, the outermost shell of the QD-layer is Bi-rich and the inner layer is Cu-rich with a very clear line drawn in 25 nm and 165 nm, manifesting the cascade structure of BS(300)/CIS(350)/TiO₂ photoelectrode. Therefore, we can conclude that the Bi₂S₃ QDs and CuInS₂ QDs are uniformly deposited on the surface of TiO₂ nanorods to form the cascade structure.

Fig. 2 TEM (a) and HRTEM (b) images of BS(300)/CIS(350)/TiO₂. The insets in (a) and (b) show the SAED pattern of BS(300)/CIS(350)/TiO₂ and FFT image of a single QD. EDS line scan profiles of BS(300)/CIS(350)/TiO₂ for Ti, O (c) and Bi, Cu, In, S (d) elementals. The inset in (d) is the STEM image.

UV-vis absorption spectra of TiO₂ nanorod and BS(n)/CIS(350)/TiO₂ photoelectrodes are presented in Fig. 3a. The absorption edge of plain TiO₂ nanorod arrays is only 400 nm, but it is enlarged to 900 nm when the CuInS₂ QDs and Bi₂S₃ QDs are deposited on the surface of TiO₂ nanorod arrays respectively, which indicates that the CuInS₂ QDs and Bi₂S₃ QDs can improve the light absorption ability of TiO₂ nanorods effectively. Moreover, the value of optical absorption increases and the absorption edge moves to the long wavelength as the laser energy increases. The band gap of BS(300) QDs and CIS(350) QDs is estimated by the plots of (αhv)² against photon energy (hv) to be ∼1.45 and ∼1.64 (Fig. S3, ESI†), which have been enlarged compared with that of bulk phase and therefore indicates the presence of quantum size effect. Fig. 3b shows the J–V characteristics of QDSSCs employing BS(n)/CIS(350)/TiO₂ photoelectrodes with an active area of 0.5 cm² under AM 1.5 illumination of 100 mW cm⁻² and the corresponding photoelectrochemical parameters are listed in Table 1. The value of J_SC increases along with the increase of laser energy, which is mainly due to the enhancement of photo-absorption as shown in Fig. 3a. The value of V_OC exhibits a tendency of increasing firstly and then decreasing as the laser energy increases, which may be due to the fact that V_OC is related to the difference between the Fermi level in the semiconductor under illumination and the Nernst potential of the redox couple in the electrolyte.²⁰ Moreover, the FF exhibits an irregularity and is sensitive to the series resistance (R_S) which is also in an irregular distribution.²¹ The QDSSC employing BS(300)/CIS(350)/TiO₂ electrode obtains the maximum energy conversion efficiency (η) of 4.81%, which is significantly enhanced compared with that of QDSSC employing plain TiO₂ nanorods, revealing the superiority of this cascade structure. This high performance is due to the fact that the increasing amount and size of Bi₂S₃ QDs deposited on CIS(350)/TiO₂ photoelectrodes along with the increase of laser energy will enlarge the light absorption and provide more channels for electron transportation, which can enhance the collected photocurrent and increase the co-sensitization effect. As a result, the η is enhanced. However, the quantity of QDs on nanorods and the size of QD continue to raise when the laser energy exceeds certain value, which could enhance the absorbance and photocurrent density. Nevertheless, the increase in the quantity and size of QDs would make the aggregation of QDs into large nanoparticles, causing the quenching of the quantum confinement effect due to the overlapping electron wave functions and increasing the recombination loss of the photogenerated electrons inside the interconnecting QD structures.^22–24 Therefore, the η of QDSSCs employing BS(350)/CIS(350)/TiO₂ decreases. In addition, the fabricated QDSSCs show high reproducibility and the related J–V characteristics and photovoltaic parameters based on five cell devices in parallel are present in the Fig. S5 and Table S1† respectively, which also highlights the high reproducibility of PLD technique used for construction of cell devices.

Fig. 3 UV-vis absorption spectra (a) and J–V characteristics (b) of QDSSCs employing BS(n)/CIS(350)/TiO₂ photoelectrodes.

Table 1 Photovoltaic parameters of QDSSCs assembled with BS(n)/CIS(350)/TiO₂ photoelectrodes

Sample	VOC (V)	JSC (mA cm^−2)	FF	η (%)
TiO2	0.31	3.84	0.54	0.64
CIS(350)/TiO2	0.45	16.82	0.52	3.95
BS(300)/TiO2	0.46	15.84	0.45	3.31
BS(200)/CIS(350)/TiO2	0.47	17.63	0.49	4.05
BS(250)/CIS(350)/TiO2	0.48	18.77	0.49	4.40
BS(300)/CIS(350)/TiO2	0.49	19.62	0.50	4.81
BS(350)/CIS(350)/TiO2	0.46	20.57	0.48	4.54

---

Besides the high energy conversion efficiency for QDSSCs employing BS(n)/CIS(350)/TiO₂ photoelectrodes, it also exhibits excellent stability under ambient air. Fig. 4a shows the time stability of normalized η of QDSSC assembled with BS(300)/CIS(350)/TiO₂ photoelectrode (the fabricated QDSSC was tested once a week and placed under the ambient air in normal times). There is a slight relative decrease of ≈10% after 3 weeks. Then it is in a flat state and decrease ≈21% after 8 weeks in the ambient air with no specific encapsulation, which confirms a good capability to resist photocorrosion and the superiority of PLD QDSSCs with cascade structure compared with that chemically synthesized, of which the η decreases from 20% to more than 70% in the few days following their fabrication if no encapsulation is used.^10,25–27 The highlights of the excellent stability is believed to be owing to that in the PLD process, the sputtered plasma with high velocity can intensely collide with the molecules on the surface of CuInS₂/TiO₂, and therefore the obtained QDs can integrate closely with CuInS₂/TiO₂ without any ligand molecules which might make the QD attachment unstable, compete with the QDs in light absorption, and create transport barriers for photogenerated electrons.^22,28–30

Fig. 4 Time stability, under ambient air, of the normalized energy conversion efficiency (η) of QDSSC assembled with BS(300)/CIS(350)/TiO₂ photoelectrode (a). Incident photon to current conversion efficiency (IPCE) spectra (b), Nyquist plots (c) and Bode phase plots (d) of QDSSCs employing BS(n)/CIS(350)/TiO₂ photoelectrodes.

To understand the photogeneration process of the photovoltaic devices, the incident photon to current conversion efficiency (IPCE) spectra of QDSSCs employing BS(n)/CIS(350)/TiO₂ photoelectrodes measured from the I_SC monitored at different excitation wavelengths are presented in Fig. 4b. The IPCE spectra show the similar trend with the UV-vis absorption spectra in Fig. 3a and J_SC in J–V characteristics in Fig. 3b. Compared with pristine TiO₂ nanorods photoelectrode, the BS(n)/CIS(350)/TiO₂ photoelectrodes have a significant increase to 54–67% around 390–800 nm, indicating the strong injection ability of photoexcited electrons in the cascade structure. Moreover, the BS(300)/CIS(350)/TiO₂ photoelectrodes have the highest value of 67% at 550 nm compared with other photoelectrodes, which manifests that the BS(300)/CIS(350)/TiO₂ photoelectrode possesses stronger injection ability of photoexcited electrons and affords sufficient light absorption without compromising electron transport and collection. However, the BS(350)/CIS(350)/TiO₂ photoelectrode suffers from the losses due to the increased carrier recombination with holes in the electrolyte and/or back-electron transfer to the semiconductor QDs and obtains the lower IPCE eventually.

In QDSSCs, the value of η is determined by the competition between the transport of electrons through the photoanode and the recombination of electrons with the oxidation state of electrolyte on the electrode–electrolyte interface.^37–40 In order to further study the interfacial charge transfer and recombination processes of QDSSCs based on the BS(n)/CIS(350)/TiO₂ photoelectrodes, the EIS spectra are measured and presented in Fig. 4c and d. All the Nyquist plots of BS(n)/CIS(350)/TiO₂ films display the typical semicircle arc in the measured frequency region, and the curve radius of BS(300)/CIS(350)/TiO₂ photoelectrodes is the smallest comparing with that of other samples (Fig. 4c). The smaller the radius is, the lower the charge transfer impedance at the electrode–electrolyte interface is, which demonstrates that the BS(300)/CIS(350)/TiO₂ photoelectrode possesses the lowest charge transfer impedance and could accelerate the interfacial charge transfer. So it can be concluded that the excellent photoelectrochemical performance of QDSSCs based on BS(300)/CIS(350)/TiO₂ photoelectrode is owing to the quicker interfacial charge transfer and electron injection process. As shown in the Bode phase plots (Fig. 4d), the characteristic peak frequency for the electron transport process of QDSSCs based on BS(300)/CIS(350)/TiO₂ photoelectrode shifts to the minimum value comparing with other photoelectrodes, which indicates that BS(300)/CIS(350)/TiO₂ photoelectrode possesses the longest electron lifetime since it can be determined by the equation: τ = 1/(2πf_p), where τ is electron lifetime and f_p is peak frequency.³¹ The QDSSCs based on BS(350)/CIS(350)/TiO₂ photoelectrode shows a shorter electron lifetime on account of the recombination of photogenerated electrons as mentioned in Fig. 3b. Therefore, with the small charge transfer resistance and long electron lifetime, QDSSCs assembled with BS(300)/CIS(350)/TiO₂ photoelectrode could achieve the best performance due to the quick interfacial charge transfer and few chance of carrier recombination with holes.

To validate the superiority of the cascade structure, the contrastive photoelectrochemical properties have been measured and shown in Fig. 5. The J–V characteristics (Fig. 5a) show that the QDSSC with cascade structure achieves the highest η while the Bi₂S₃ QD-based QDSSC obtains the lowest. This may be due to that the cascade structure could harvest more visible light and the photoexcited electrons gain stronger injection ability and higher transport speed. Benefiting from the cascade structure, its absorbance is the highest and the absorption range can be enlarged to about 900 nm (Fig. S4†), indicating Bi₂S₃ is a good co-sensitizer and complementary and enhancement effects in light harvest present in the cascade structure. The IPCE spectra (Fig. 5b) confirm the strong injection ability of photoexcited electrons in the cascade structure as it obtains the IPCE of 67%, higher than CIS(350)/TiO₂ of 44% and BS(300)/TiO₂ of 36%. Moreover, the Mott–Schottky plots (Fig. 5c) are measured to evaluate the electron-transfer properties in the semiconductor interface. The slope of Mott–Schottky plot denotes the charge carrier density N_D as it can be calculated by the equation: N_D = (2/e₀εε₀)[d(1/C²)/dV]⁻¹, where e₀ is the elementary electron charge, ε is the permittivity of the semiconductor electrode, ε₀ is the permittivity of vacuum and d(1/C²)/dV is the slope of Mott–Schottky plot.³² Hence, the smaller slope of Mott–Schottky plot means higher charge carrier density in the semiconductor interface. Comparing the three photoelectrodes, the enormous difference between the slopes of the three plots indicates a higher carrier concentration in the heterojunction and more efficient charge transfer for the cascade structure. Meanwhile, the expected upwards shift of the Fermi level caused by the increased charge carrier density can lead to a significantly larger degree of band bending at the surface of the heterostructure, which could promote the charge separation at the interface of the heterostructure and electrolyte.^32–34 Meanwhile, the EIS Nyquist plots (Fig. 5d) also verify the enhanced charge separation and transport process in the cascade structure as it gets a smaller radius. This also indicates that the chance for recombination of electrons and holes is greatly reduced. The phenomenon that the radius of cascade structure is larger than that of CIS(350)/TiO₂ can be ascribed to the larger resistance of Bi₂S₃ itself than that of CuInS₂. In summary, the enlarged photoresponse range, quick interfacial charge transfer and few chance of electrons recombination with holes ensures the high photoelectrochemical performance of QDSSCs based on the cascade structure.

Fig. 5 Contrastive photoelectrochemical properties of QDSSCs assembled with different photoelectrodes: (a) J–V characteristics, (b) IPCE spectra, (c) Mott–Schottky plots and (d) EIS Nyquist plots.

4. Conclusion

In this paper, we demonstrate a physical, powerful and straightforward method of PLD technique to construct the cascade structure of BS(n)/CIS(350)/TiO₂ photoelectrode. The photoelectrochemical performance of QDSSCs assembled with BS(n)/CIS(350)/TiO₂ photoelectrode can be optimized by adjusting the laser energy and an optimal energy conversion efficiency of 4.81% is achieved by the QDSSC assembled with BS(300)/CIS(350)/TiO₂ photoelectrode under one sun illumination (AM 1.5, 100 mW cm⁻²). The photoelectric investigation reveals that the improved performance is attributed to the enhanced absorption in the longer wavelength region, quick interfacial charge transfer and few chance of carrier recombination with holes in the QDSSCs. Moreover, the QDSSCs with cascade structure fabricated by PLD technique exhibit high stability in the ambient air without any encapsulation. Thus, the PLD technique could be further applied for the assembly of QDs where ligand exchange is difficult and could possibly lead to reduced fabrication cost and improved device performance.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 51471181, 51562033).

References

1. A. Yella, H.-W. Lee, H. N. Tsao, C. Yi, A. K. Chandiran, M. Nazeeruddin, E. W.-G. Diau, C.-Y. Yeh, S. M. Zakeeruddin and M. Grätzel, Science, 2011, 334, 629–634.
2. P. K. Snatra and P. V. Kamat, J. Am. Chem. Soc., 2012, 134, 2508–2511.
3. M. C. Beard, A. G. Midgett, M. C. Hanna, J. M. Luther, B. K. Hughes and A. J. Nozik, Nano Lett., 2010, 10, 3019–3027.
4. S. Guenes and N. S. Sariciftci, Inorg. Chim. Acta, 2008, 361, 581–588.
5. B. Liu and E. S. Aydil, J. Am. Chem. Soc., 2009, 131, 3985–3990.
6. T. Hong, Z. Liu, W. Yan, J. Liu and X. Zhang, Phys. Chem. Chem. Phys., 2015, 17, 30993–30998.
7. R. Wang, L. Wan, H. Niu, Q. Ma, S. Miao and J. Xu, J. Sol-Gel Sci. Technol., 2013, 67, 458–463.
8. L. M. Peter, K. G. U. Wijayantha, D. J. Riley and J. P. Waggett, J. Phys. Chem. B, 2003, 107, 8378–8381.
9. C. Chen, G. Ali, S. H. Yoo, J. M. Kum and S. O. Cho, J. Mater. Chem., 2011, 21, 16430–16435.
10. M. Han, W. Chen, H. Guo, L. Yu, B. Li and J. Jia, J. Power Sources, 2016, 318, 121–127.
11. Y. Zhu, R. Wang, W. Zhang, H. Ge and L. Li, Appl. Surf. Sci., 2014, 315, 149–153.
12. D. Esparza, I. Zarazúa, T. López-Luke, R. Carriles, A. Torres-Castro and E. D. L. Rosa, Electrochim. Acta, 2015, 180, 486–492.
13. S. Deka, A. Quarta, M. G. Lupo, A. Falqui, S. Boninelli, C. Giannini, G. Morello, M. De Giorgi, G. Lanzani and C. Spinella, J. Am. Chem. Soc., 2009, 131, 2948–2958.
14. P. Reiss, M. Protiere and L. Li, Small, 2009, 5, 154–168.
15. T. L. Li, Y. L. Lee and H. Teng, Energy Environ. Sci., 2012, 5, 5315–5324.
16. R. He, X. Qian, J. Yin and Z. Zhu, J. Cryst. Growth, 2003, 252, 505–510.
17. Y. Xi, C. Hu, X. Zhang, Y. Zhang and Z. L. Wang, Solid State Commun., 2009, 149, 1894–1896.
18. M. Han, J. Jia, L. Yu and G. Yi, RSC Adv., 2015, 5, 51493–51500.
19. L. J. Diguna, Q. Shen, J. Kobayashi and T. Toyoda, Appl. Phys. Lett., 2007, 91, 023116–023119.
20. B. O'Regan and M. Gratzel, Nature, 1991, 353, 737–740.
21. F. Fabregat-Santiago, J. Bisquert, E. Palomares, L. Otero, D. Kuang, S. M. Zakeeruddin and M. Grätzel, J. Phys. Chem. C, 2007, 111, 6550–6560.
22. Q. Dai, J. Chen, L. Lu, J. Tang and W. Wang, Nano Lett., 2012, 12, 4187–4193.
23. W. R. Algar and U. J. Krull, ChemPhysChem, 2007, 8, 561–568.
24. T.-L. Li, Y.-L. Lee and H. Teng, J. Mater. Chem., 2011, 21, 5089–5098.
25. J. Han, Z. Liu, B. Yadian, Y. Huang, K. Guo, Z. Liu, B. Wang, Y. Li and T. Cui, J. Power Sources, 2014, 268, 388–396.
26. Y. Wan, M. Han, L. Yu, G. Yi and J. Jia, CrystEngComm, 2016, 18, 1577–1584.
27. R. Suarez, P. K. Nair and P. V. Kamat, Langmuir, 1998, 14, 3236–3241.
28. J. Seo, S. J. Kim, W. J. Kim, R. Singh, M. Samoc, A. N. Cartwright and P. N. Prasad, Nanotechnology, 2009, 20, 095202–095208.
29. J. Tang, X. Wang, L. Brzozowski, D. A. R. Barkhouse, R. Debnath, L. Levina and E. H. Sargent, Adv. Mater., 2010, 22, 1398–1402.
30. S. H. Im, H.-J. Kim, S. Kim, S.-W. Kim and S. I. Seok, Org. Electron., 2012, 13, 2352–2357.
31. T. Hoshikawa, T. Ikebe, R. Kikuchi and K. Eguchi, Electrochim. Acta, 2006, 51, 5286–5294.
32. H. Cui, W. Zhao, C. Yang, H. Yin, T. Lin, Y. Shan, Y. Xie, H. Gu and F. Huang, J. Mater. Chem. A, 2014, 2, 8612–8616.
33. C. Yang, Z. Wang, T. Lin, H. Yin, X. Lu, D. Wan, T. Xu, C. Zheng, J. Lin, F. Huang, X. Xie and M. Jiang, J. Am. Chem. Soc., 2013, 135, 17831–17838.
34. G. Wang, H. Wang, Y. Ling, Y. Tang, X. Yang, R. C. Fitzmorris, C. Wang, J. Z. Zhang and Y. Li, Nano Lett., 2011, 11, 3026–3033.
35. 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.
36. Y. Xie, G. Ali, S. H. Yoo and S. O. Cho, ACS Appl. Mater. Interfaces, 2010, 2, 2910–2914.
37. A. J. Frank, N. Kopidakis and J. v. d. Lagemaat, Coord. Chem. Rev., 2004, 248, 1165–1179.
38. M. Grätzel, Inorg. Chem., 2005, 44, 6841–6851.
39. Y. Zhu, X. Meng, H. Cui, S. Jia, J. Dong, J. Zheng, J. Zhao, Z. Wang, L. Li, L. Zhang and Z. Zhu, ACS Appl. Mater. Interfaces, 2014, 6, 13833–13840.
40. G. Schlichthörl, S. Y. Huang, J. Sprague and A. J. Frank, J. Phys. Chem. B, 1997, 101, 8141–8155.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra14901d

---