One-pot synthesis of colloidal Cd_x:CuInS₂ quaternary quantum dots used as sensitizers in photovoltaic cells

Abstract

In this work, we present the synthesis of Cd_x:CuInS₂ quaternary quantum dots (q-QDs) using a one-pot non-injection approach of alloying CuInS₂ with Cd²⁺. Photoluminescence measurements showed that an increase in the Cd mole fraction in Cd_x:CuInS₂ q-QDs caused a systematic blue-shift in the QD emission wavelength. The as-prepared Cd_x:CuInS₂ q-QDs exhibited emissions in the range of 560–645 nm, and a maximum fluorescence quantum yield of 22%. Time-resolved photoluminescence measurements indicated that the average lifetime of Cd_x:CuInS₂ q-QDs became shorter compared to that of the CuInS₂ ternary QDs (t-QDs), clearly indicating that a certain amount of Cd²⁺ defects exist inside the CuInS₂ host. The photovoltaic performance of QD-sensitized solar cells (QDSSCs) was investigated by sandwiching a polysulfide electrolyte between Cd_x:CuInS₂ q-QDs photoanodes and Cu₂S photocathodes. A maximum energy conversion efficiency of 1.74% was obtained under AM1.5 G simulated solar light for the cell fabricated with Cd_x:CuInS₂ q-QDs (x = 1) as the sensitizer, which is about 70% and 35% better than the cells sensitized with pristine CuInS₂ t-QDs and Zn:CuInS₂ q-QDs, respectively. More interestingly, it was noted that J_SC systematically improved as the quantum yields of Cd_x:CuInS₂ q-QDs increased, resulting in an enhancement in power conversion efficiency. Furthermore, the power conversion efficiency of the solar cells co-sensitized with Cd:CuInS₂ and CdSe can be elevated further to an encouraging 2.86%.

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1. Introduction

In recent years, quantum-dot-sensitized solar cells (QDSSCs), as a derivative of dye-sensitized solar cells (DSSCs), have attracted considerable interest as a new type of photovoltaic technology. There are many distinctive benefits to adopting quantum dots (QDs) as a light-harvesting sensitizer. For example, QDs can provide improved light harvesting performance and matching of the sunlight spectral distribution, as their optical properties are easily tunable through controlling their size and shape. Moreover, QDs possess large absorption coefficients over a broad spectral range, are able to effect multiple electron–hole pair generation, can be solution processed, and can be easily manufactured potentially at low cost. Various semiconductor QDs sensitizers have been explored to fabricate QDSSCs, including CdS/CdSe, CdSe/CdS, CdSeS, CdTe/CdSe, Mn:CdS/CdSe, PbS, Ag₂S/CdS, and Cd_1−xMn_xSe.^1–9 Among metal chalcogenide materials, CuInS₂ and related chalcopyrite compounds draws particular interest because of their high optical absorption coefficient (on the order of 10⁵ cm⁻¹ in the visible wavelength range), superior stability under solar radiation, and direct band gaps of 1.0–1.5 eV (which overlaps well with the solar spectrum). Owing to these advantageous characteristics, numerous recent efforts have been devoted to the fabrication of CuInS₂-based QDSSCs.^10–16

To further improve the photovoltaic performance of CuInS₂-based QDSSCs, it is still of great interest to improve the material properties of CuInS₂ – for example, by reducing the presence of intrinsic defects or defect clusters. These defects act as recombination sites for photogenerated electrons and holes, which consequently influence the emission and photocurrent properties of the material. Most of the bare CuInS₂ ternary QDs (t-QDs) formed exhibit relatively low emission efficiencies because of intrinsic defect related emissions. In previous studies, it was reported that chalcopyrite Cu–In–S semiconductor compounds have abundant structural tolerance for a wide range of anion-to-cation off-stoichiometric variations, and exhibit plentiful intrinsic defects – such as copper and sulfur vacancies, and interstitial copper – that affect their physicochemical properties. Therefore, two main strategies have been developed to modulate the optical property of CuInS₂ t-QDs through controlling their constituent stoichiometries and internal structures. The first strategy is based on adjusting the initial feed ratio of [Cu]/[In] to obtain Cu-deficiency in CuInS₂ t-QDs (as emissions from this material mainly originate from large population donor–acceptor pair (DAP) recombination).^17–19 Another important strategy for tailoring the optical properties of CuInS₂ t-QDs is through composition modulation, by incorporating t-QDs with foreign metal ions. Some recent approaches have indicated that when incorporating processes are used to introduce various metal ions into the CuInS₂ lattice, the result is formation of quaternary QDs (q-QDs). However, these studies have so far been limited to select metal ions such as Zn²⁺ and Mn².^17,20–23 It is still important to develop ways of incorporating new metal ions into CuInS₂ to further understand the how the process affects the optoelectronic properties of the material. Referring to early studies, it has been reported that the presence of a CdS shell on the surface of CuInS₂ t-QDs could effectively eliminate the fast emission decay process and result in a uniform single-exponential decay compared with ZnS passivation.²⁴ However, to the best of our knowledge, there is no emission data of Cd_x:CuInS₂ q-QDs available yet in literature. Furthermore, there have been no previous investigations on the use of Cd_x:CuInS₂ q-QDs in the fabrication of solar cells.

Preparation of high quality QDs is of crucial importance to the photovoltaic performance of QDSSC, because the presence of recombination sites for photogenerated electrons and holes, which includes surface states and internal defects, would reduce the carrier transport efficiency of the QD.²⁵ In this paper, we successfully demonstrate that highly fluorescent Cd_x:CuInS₂ q-QDs (where x = 0–1) can be prepared by introducing Cd²⁺ into CuInS₂ via a one-step route. With the incorporation of Cd²⁺ ions into the CuInS₂ host, we show that the emission colors of the obtained q-QDs can be tuned continuously from 560 to 645 nm, with a higher emission intensity than that of native CuInS₂ t-QDs. In addition to the superior optical properties, our research also reveal that Cd_x:CuInS₂ q-QDs can exhibit enhanced photovoltaic performance compared with pristine CuInS₂ t-QDs. In QDSSCs fabricated using Cd:CuInS₂/CdSe, we measured an efficiency of 2.86% under simulated one sun illumination (AM1.5 G, 100 mW cm⁻²), which was 1.7 times higher than that of native CuInS₂ QDSSCs.

2. Experimental section

2.1 Chemicals

Copper(I) iodide (99.999%), sodium borohydride (NaBH₄, 98.0%), and sulfur powder (S, 99.98%) was purchased from Sigma-Aldrich. Copper(II) nitrate hemipentahydrate (Cu(NO₃)₂, 98%), indium acetate (99.99%) selenium oxide (99.4%), and 3-mercaptopropionic acid (MPA, 99%) were purchased from Alfa-Aesar. 1-Dodecanethiol (98%), potassium chloride (99+%) and sodium sulfide nonhydrate (Na₂S, 98+%) were purchased from Acros Organics. Zinc acetate dehydrate (Zn(Ac)₂, 99.8%) and cadmium acetate (99%) were obtained from J. T. Baker. Cadmium nitrate (99%) was purchased from Hayashi Pure Chemical. All chemicals were used as received without any further purification.

2.2 Preparation of Cd_x:CuInS₂ q-QDs

All synthetic preparations were performed under an argon atmosphere using standard Schlenk techniques. Five Cd_x:CuInS₂ q-QDs with [Cd]/[In] compositions of 0/1, 0.25/1, 0.50/1, 0.75/1, and 1/1 were synthesized at a fixed [Cu]/[In] molar ratio of 0.25/1; for convenience, these samples are respectively denoted as CuInS₂, Cd_0.25:CuInS₂, Cd_0.50:CuInS₂, Cd_0.75:CuInS₂, Cd:CuInS₂. Here, the synthetic procedure for Cd:CuInS₂ q-QDs with a [Cd]/[Cu]/[In] molar ratio of 1/0.25/1 is described. Copper(I) iodide (0.2 mmol), indium acetate (0.8 mmol), and cadmium acetate (0.8 mmol) were mixed with 1-dodecanethiol (10 mL) in a 50 mL three-neck round-bottom flask. The mixture was heated to 120 °C under a flow of argon gas while under continuous stirring for 30 min. Then, the resultant solution was heated to 200 °C for the desired reaction time, before being allowed to cool to room temperature. An excess of methanol/acetone mixture (1 : 9, v/v) was next added to the solution. The solid precipitate formed was collected by centrifuging the suspension and decanting the supernatant liquid. The solid residue was redispersed in a small amount of dichloromethane. This methanol/acetone washing process was repeated 2–3 times to remove the by-products generated during the reactions.

2.3 Preparation of QDs-sensitized photoanodes

The double-layer TiO₂-film electrode was prepared using the procedure we previously described.^14,15 The bottom transparent layer and top scattering layer on the fluorine-doped tin oxide (FTO) glass plate were prepared respectively by using screen printed TiO₂ paste (Solaronix, Ti-Nanoxide T/SP) and scattering particles (Solaronix, Ti-Nanoxide R/SP). The obtained TiO₂-film electrode was gradually heated to 500 °C for 60 min, and maintained at the temperature for a further 30 min. Subsequently, the TiO₂-film electrode was dipped into 45 mL of acetonitrile containing MPA (2 M) for 24 h. Pre-treatment of the TiO₂-film surface with MPA modification can facilitate the adsorption of QDs. The TiO₂-film electrode with MPA modification was immersed into a hexane solution of QDs (3 mg mL⁻¹) for 24 h in the dark to obtain the Cd_x:CuInS₂ sensitized photoanodes. After the deposition was complete, the electrodes underwent post-treatment with a ZnS passivation layer. The photoanodes were dipped alternately into solutions of Zn(Ac)₂ in methanol (0.03 M) and Na₂S in water/methanol (7/3 vol%) mixture (0.03 M) for 1 min per dip. The successive ionic layer adsorption and reaction (SILAR) process was repeated twice. Finally, the TiO₂ photoanode was subjected to sintering treatment at 250 °C for 3 min.

For the preparation of Cd:CuInS₂/CdS co-sensitized photoanodes, the as-prepared Cd:CuInS₂ sensitized photoanodes was immersed into a 0.05 M Cd(NO₃)₂ methanol solution for 1 min. Successively, the photoanodes were dipped into a 0.05 M Na₂S in a 50 mL of water/methanol mixture (7/3 vol%) for 1 min. The SILAR process was repeated 2–4 times to obtain Cd:CuInS₂/CdS co-sensitized photoanodes. After deposition was complete, the electrodes underwent post-treatment with a ZnS passivation layer. The above photoanodes were dipping alternately into Zn(Ac)₂ (0.03 M) methanol solution and Na₂S (0.03 M) water/methanol mixture (7/3 vol%) solutions for 1 min per dip and the SILAR process was repeated twice. Subsequently, TiO₂ photoanode was subjected to sintering treatment at 250 °C for 3 min.

For the preparation of Cd:CuInS₂/CdSe sensitized photoanodes, the Cd:CuInS₂ sensitized photoanodes was immersed into a 0.05 M Cd(NO₃)₂ methanol solution for 1 min. Successively, the photoanodes were dipped into a selenium precursor for 1 min. The selenium precursor was prepared by heating a mixture of 9 × 10⁻⁴ mol SeO₂ in 25 mL ethanol and then reducing the mixture with NaBH₄ in ethanol. The SILAR process was repeated 3–5 times to obtain Cd:CuInS₂/CdSe sensitized photoanodes. After deposition was complete, the electrodes underwent post-treatment with a ZnS passivation layer. The above photoanodes were dipping alternately into Zn(Ac)₂ (0.03 M) methanol solution and Na₂S (0.03 M) water/methanol mixture (7/3 vol%) solutions for 1 min per dip and the SILAR process was repeated twice. Subsequently, TiO₂ photoanode was subjected to sintering treatment at 250 °C for 3 min.

2.4 Solar cell fabrication

The QDSSCs were assembled by sandwiching the above photoanode and a Cu₂S counter electrodes using 60 μm-thick hot-melt ionomer films (Surlyn) as the spacer. A polysulfide electrolyte of 1.8 M Na₂S, 2 M S, and 0.2 M KCl in methanol/water (3/7 vol%) was injected into the cells. The active area of each solar cell is 0.16 cm². To obtain Cu₂S counter electrodes, we employed a FTO glass coated with a thin film of Cu₂S, which was prepared by spin-coating of 200 μL (0.5 M) of Cu(NO₃)₂ methanol solution and 100 μL (0.5 M) of Na₂S aqueous solution at a speed of 1000 rpm for 30 s alternatively. This process was performed at room temperature and repeated for two cycles. The Cu₂S counter electrode was then annealed at 300 °C for 5 min.

2.5 Characterization

Transmission electron microscopy (TEM) images and high-resolution TEM (HRTEM) was used to record the images of each samples with a FEI Tecnai G2 F20 microscope (Philips, Holland) equipped with an X-ray energy dispersive spectrometer (EDS) operating at 200 kV. The X-ray diffraction (XRD) patterns were obtained by a Bruker D8 Discover diffractometer using Cu K_α1 line (λ = 1.54 Å). Inductively coupled plasma atomic emission spectroscopy (ICP-AES) was performed on a JY 2000-2 (Jobin Yvon Horiba). Absorption spectra were obtained on a JASCO V630 spectrophotometer, and photoluminescence spectra were recorded on a JASCO FP-6500 fluorescence-spectrometer. The photoluminescence excitation spectrum was obtained on a FLuoromax-4 spectrofluorometer (HORIBA Jobin Yvon, Inc) equipped with a 150 W xenon lamp as the excitation source. Time-resolved single photon counting was conducted with a PicoQuant PDL 200-B pulsed diode laser at a wavelength of 450 nm. Photoluminescence decay profiles were estimated using the FluoFit software (PicoQuant, Germany) to obtain lifetime parameters. The quality of the curve fitting was evaluated by reduced chi-square (χ²). The current–voltage measurements of the cell were tested using a computer-controlled Keithley 2400 source meter under simulated AM1.5 solar light (450 W xenon arc lamp, Oriel 6691), calibrated using a standard silicon reference cell. The incident photon-to-current conversion efficiency (IPCE) spectra measurements were performed using a system comprising a 300 W xenon lamp, a monochromator (Peccell Technologies, PEC-S20), a calibrated silicon photodiode, and a power meter. Photoluminescence quantum yields (Φ) were measured by comparison with a Rhodamine 6G (Φ_dye = 95% in methanol) and using data derived from the luminescence and the absorption spectra, as follows:where subscript QD and dye represented the as-synthesized QD and Rhodamine 6G respectively; a was optical density values at the excitation wavelength were set the same in the range of 0.02–0.05, I was the integrated area of emission spectra, ε was the refractive index of the solvent used. Cyclic voltammetry curves (scan rate = 10 mV s⁻¹) were performed on a Zennium electrochemical workstation in a three-electrode cell, adopting glassy carbon as the working electrode, a Pt wire as the auxiliary electrode, and a Ag/Ag⁺ as the reference electrode. A few drops of diluted QDs solution were placed onto the surface of the working electrode and then the solvent was removed to form a film. 0.1 M tetrabutylammonium hexafluorophosphate dissolved in acetonitrile was utilized as the supporting electrolyte. The three-electrode cell was bubbled with high-purity argon for 15 min before the scan. The energy levels were calculated according to the following equations

E_HOMO = −(E^ox + 4.71) eV (2)

E_LUMO = −(E^red + 4.71) eV (3)

whereby E^ox and E^red represent the onset oxidation and reduction potential value, respectively.

3. Results and discussion

Previous work on enhancing the photoluminescence of CuInS₂ t-QDs had used highly off-stoichiometric effects to prepare the QDs. It is noted that preparing copper-deficient CuInS₂ with an initial [Cu]/[In] molar feed ratio of 0.25 : 1 could result in high-quality QDs with an enhanced emission intensity for better photoluminescence (thought to be attributed to an increased amount of copper vacancies that behave as acceptors in DAP recombination).^21,26,27 Thus, the synthetic procedure of Cd:CuInS₂ q-QDs with the precursor molar ratios of Cu/In of 0.25/1, and Cd/In of 1/1, was selected. Cd:CuInS₂ q-QDs were prepared through simultaneous thermolysis reaction of a mixture of copper(I) iodide, indium acetate, and cadmium acetate in 1-dodecanethiol. The 1-dodecanethiol solvent here simultaneously behaves as the sulfur source as well as the capping ligand for the as-prepared q-QDs because it can coordinate strongly to the metal cations and has the ability to balance the reactivity of the three cationic precursors. Fig. 1a and b reveal the temporal evolution of the absorbance and photoluminescence spectra of Cd:CuInS₂ q-QDs prepared at reaction temperature of 200 °C. Both the absorption and emission wavelengths of q-QDs exhibit a gradual shift in peak position to lower photon energy regions after longer reaction times. This phenomenon is mainly attributed to the effect of weaker quantum confinement with the increasing particle sizes.

Fig. 1 Temporal evolution of normalized (a) optical absorption and (b) photoluminescence spectra of Cd:CuInS₂ q-QDs synthesized at 200 °C. The (c) absorption and (d) photoluminescence spectra recorded for Cd_x:CuInS₂ q-QDs with different compositions: (1) CuInS₂, (2) Cd_0.25:CuInS₂, (3) Cd_0.50:CuInS₂, (4) Cd_0.75:CuInS₂, and (5) Cd:CuInS₂, obtained at 200 °C for 60 min.

To better understand the influence of the Cd²⁺ concentration on the optical properties of the QDs, we performed a study on the preparation of Cd_x:CuInS₂ q-QDs with varying feed ratios of Cd between 0 and 1 (other reaction conditions were unchanged). Fig. 1c and d shows the absorption and emission spectra of the Cd_x:CuInS₂ q-QDs prepared with constant amounts of copper(I) iodide and indium acetate (0.2 mmol and 0.8 mmol respectively) at various molar ratios of Cd to In (0/1, 0.25/1, 0.50/1, 0.75/1 and 1/1). The peak position in the photoluminescence spectra featured a gradual blue-shift (or higher band gap energy) with increasing Cd content. The optical blue-shift of the Cd_x:CuInS₂ q-QDs compared with the original CuInS₂ t-QDs can be regarded as the incorporation of Cd constituent into the host structure of CuInS₂. Because the band gap of bulk CdS is wider (E_g = 2.4 eV) than that of CuInS₂ (E_g = 1.5 eV), a blue-shift is expected during the alloying process. The alloy formation process is possible due to a small lattice mismatch of only 3.2% between CuInS₂ and CdS.²⁸ Moreover, the continuous shift of the peak position to shorter wavelengths suggest that the obtained products are Cd_x:CuInS₂ q-QDs and allow the phase separation or separated nucleation of CuInS₂ and CdS to be ruled out. This observation is in accordance with previous results where increases in the ZnS constituent in the Zn:CuInS₂ q-QDs causes a blue-shift in the resulting QD emission peak position (as the band gap of ZnS (E_g = 3.6 eV) is much larger than that of CuInS₂).^21,22,29

Fig. 2 summarizes a representative plot of the emission peak position and quantum yield versus reaction time during formation of Cd_x:CuInS₂ q-QDs as the feed ratio of Cd increases from 0 to 1. With longer reaction times and a higher Cd feed ratio, the photoluminescence emission peaks of all q-QDs exhibited a gradual red-shift. Additionally, it was found that quantum yields of Cd_x:CuInS₂ q-QDs were enhanced when the Cd content was increased. The quantum yields of all Cd_x:CuInS₂ q-QDs were superior to that of corresponding native CuInS₂ t-QDs (Φ = ∼2%). The results indicated that when the molar ratio of Cd/In is equal to 1, the maximal quantum yield of Cd:CuInS₂ q-QDs can reach 22% under optimal conditions. We attributed this observation to be due to the decrease in the amount of Cu vacancy defects when more Cd is incorporated into the CuInS₂ lattice, hence reducing the number of non-radiative recombination sites available.

Fig. 2 Plots of the (a) emission peak position and (b) quantum yield versus time obtained from Cd_x:CuInS₂ q-QDs with different Cd feed ratios: (1) CuInS₂, (2) Cd_0.25:CuInS₂, (3) Cd_0.50:CuInS₂, (4) Cd_0.75:CuInS₂, and (5) Cd:CuInS₂. The reaction temperature was 200 °C and the excitation wavelength was set to 430 nm.

Fig. 3 presents XRD patterns of purified Cd_x:CuInS₂ q-QDs of various compositions. The XRD pattern of the CuInS₂ t-QDs had characteristic features at 28.3°, 47.2°, and 54.1° that corresponded to the (112), (024), and (132) planes of chalcopyrite CuInS₂ (JCPDS Card no. 47-1372). When Cd²⁺ ions are added to change the composition of the QDs, the diffraction patterns of the QDs are gradually shifted to smaller scattering angles, suggesting the formation of Cd_x:CuInS₂ q-QDs.

Fig. 3 XRD patterns of Cd_x:CuInS₂ q-QDs under different feed molar ratio of Cu/In/Cd: (1) CuInS₂, (2) Cd_0.25:CuInS₂, (3) Cd_0.50:CuInS₂, (4) Cd_0.75:CuInS₂, and (5) Cd:CuInS₂. The standard XRD patterns for CuInS₂ and CdS are shown below.

A low-resolution TEM image of Cd:CuInS₂ q-QDs is shown in Fig. 4a. Analysis of the high-resolution TEM images in Fig. 4b shows that Cd:CuInS₂ q-QDs display crystalline structures. EDS analysis (see Fig. S1 in the ESI†) confirmed that the Cd:CuInS₂ q-QDs have elemental compositions comprising Cd, Cu, In, and S. ICP-AES was used to quantify the Cd:CuInS₂ q-QDs (Cu : In : Cd = 0.27 : 1.22 : 0.91).

Fig. 4 (a) TEM and (b) high-resolution TEM images of the obtained Cd:CuInS₂ q-QDs. The white circles show the location of q-QDs.

Upon light irradiation, absorption of a photon with energy above the semiconductor band gap energy gives rise to the generation of an electron and a hole located, respectively, in the conduction and valence bands of the semiconductor. These photoelectrons are then consumed either by decay through the recombination of the electrons and holes to yield photoluminescence, or are separated by the photoelectric process to generate photocurrent. In terms of the transfer of photoelectrons, this process is dominated by the defect states in semiconductor nanocrystals because such defect states in the crystal structure could act as the recombination center for electrons and holes. A near-perfect or high quality semiconductor nanocrystal, which has a very low density of defects or trap states, would allow more radiative recombinations resulting in high photoluminescence quantum yields. In other words, QDs with high emission efficiency are thought to be correlated with low defect rates, which would benefit the process of photoelectron migration and deliver more photocurrent in a solar cell device. The correlation between the photocurrent of QDSSC and the emission intensity of QDs has been previously investigated. For example, Tian et al. reported that QDs with higher emission efficiency can yield more excitons (electrons and holes), which allowed more photoinduced electrons to be transferred to the electrodes.⁹ They indicated that the high emission intensity promote a high charge density in the solar cell. In our previous study, we found In-rich CuInS₂ QDs possessed higher emission efficiency than native CuInS₂ QDs due to the relative lower intrinsic defect states in In-rich CuInS₂ QDs.³⁰ We also found that In-rich CuInS₂ based QDSSC showed a photocurrent of 5.61 mA cm⁻², higher than the value obtained from CuInS₂ based QDSSC (1.79 mA cm⁻²).

Based on the above discussion, we adopted QDs with the highest QY as sensitizers in our QDSSC in our study. Different Cd_x:CuInS₂ q-QDs with the highest quantum yields were prepared with different Cd doping concentrations and reaction times while keeping all other experimental conditions the same. The synthetic conditions are listed in Table 1. Cyclic voltammetry was used to estimate the band edge position of Cd_x:CuInS₂ q-QDs (Fig. 5a). The corresponding HOMO and LUMO energy levels of these samples were shown in Fig. 5b. It can be seen that the conduction bands of the as-prepared q-QDs were positively shifted systemically with increasing x. Fig. S2† displays a comparison of the absorption and excitation spectra of the as-prepared sample. For the excitation spectra, the detection wavelength is set to the maximum of the photoluminescence position. The excitation spectrum of samples S1–S5 have features similar to those of the absorption spectrum, which indicated that the emission of the Cd_x:CuInS₂ q-QDs originates from the same recombination process in a nearly homogeneity sample. As a comparison, we also used Zn²⁺ precursors instead of Cd²⁺ precursors to synthesize Zn:CuInS₂ q-QDs with the precursor molar ratios of Cu/In of 0.25/1 and Zn/In of 1/1 following the same procedure as described for Cd:CuInS₂ q-QDs. TEM image, EDS spectrum, and XRD pattern of the Zn:CuInS₂ q-QDs are shown in Fig. S3.† Fig. 6a and b show absorption and photoluminescence spectra of Cd_x:CuInS₂ and Zn:CuInS₂ q-QDs. To gain detailed and profound insight into the optical features of the Cd_x:CuInS₂ and Zn:CuInS₂ q-QDs, we probed the temporal decay of each QD using 450 nm excitation from a pulsed laser (∼3 ns pulses, 20 Hz). As shown in Fig. 6c and d, the emission decay traces of all the samples can be well fitted using a double-exponential model with the reduced chi-square value χ² ≤ 1.1 described by eqn (4):

I(t) = α₁ e^−t/τ₁ + α₂ e^−t/τ₂ (4)

where I(t) is the photoluminescence intensity as a function of time (t) after the excitation pulse, α₁ and α₂ are the weighted coefficients, and τ₁ and τ₂ represent the fast and slow decay lifetime for the two exponential components.

Table 1 The experimental conditions of different QDs and their fluorescent properties and decay parameters

No.	Sample	α1 (%)	τ1 (ns)	α2 (%)	τ2 (ns)	〈τ〉 (ns)	Φ (%)	χ^2	Synthetic conditions
S1	CuInS2	29.4	39.2	70.6	313	232	3.4	1.00	200 °C, 80 min
S2	Cd0.25:CuInS2	26.9	57.5	73.1	361	280	7.1	1.07	200 °C, 80 min
S3	Cd0.50:CuInS2	26.6	58.1	73.4	377	292	12.3	1.04	200 °C, 80 min
S4	Cd0.75:CuInS2	20.1	68.2	79.9	381	318	17.4	1.01	200 °C, 100 min
S5	Cd:CuInS2	19.4	79.0	80.6	409	345	30.1	1.01	200 °C, 200 min
S6	Zn:CuInS2	31.0	63.4	69.0	341	255	9.73	1.01	200 °C, 360 min

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Fig. 5 (a) Cyclic voltammetry spectra and (b) HOMO and LUMO energy levels of samples S1–S5.

Fig. 6 Absorption and emission spectra of (a) samples S1–S5 and (b) sample S6 synthesized at 200 °C. Time-resolved emission decay of (c) samples S1–S5 and (d) sample S6. Solid yellow curves represent the result of fitting the curves using a double-exponential function as described in the text.

The intensity-weighted average lifetime (〈τ〉) is determined by the expression:

〈τ〉 = [α₁(τ₁)² + α₂(τ₂)²]/[α₁τ₁) + α₂τ₂] (5)

The fitting results are summarized in Table 1 and a representative trace is plotted with its bi-exponential fit in Fig. 6c and d. The double-exponential behavior strongly suggests that the emission involves both a fast decay (τ₁) and a slow decay (τ₂) component. It is known that the relatively shorter decay component originates from surface trap state recombination (such as vacancies and dangling bonds), while the longer decay component is originate from DAP recombination (which are associated with internal defects that serve as donor or acceptor positions). We can see from Fig. 6c that as the content of Cd²⁺ increases, the emission decay rate of q-QDs becomes slower compared to that of the CuInS₂ t-QDs without the incorporation of Cd²⁺. For example, the average lifetime measured for the Cd:CuInS₂ q-QDs is estimated to be 345 ns, and for the CuInS₂ t-QDs, 232 ns. A possible explanation for the elongated lifetimes of the Cd_x:CuInS₂ q-QDs is that there are variations of the lattice strain with the changing composition of Cd_x:CuInS₂ q-QDs, which is associated with defects resulting from the lattice constant mismatch. This also indicates that a certain amount of Cd²⁺ defects exist inside the CuInS₂ host and supports the successful formation of the incorporation, which will create a large population of donor or acceptor defects in the q-QDs. A similar result was also found in the case of Zn:CuInS₂ q-QDs as shown in Fig. 6d. Table 1 revealed that the average lifetime of Zn:CuInS₂ q-QDs is longer (〈τ〉 = 255 nm) compare to CuInS₂ t-QDs without the incorporation of Zn²⁺.

To investigate the photovoltaic properties of the QDs, QDSSC were assembled by sandwiching the q-QDs-sensitized TiO₂ photoanode and a Cu₂S counter electrode together in the presence of a S²⁻/S_x²⁻ redox couple. The SEM image (Fig. 7) indicates that the film thickness of porous nanocrystalline is ∼3.8 μm thickness and the film thickness of light scattering layer is ∼1.4 μm. The conditions used to synthesize Cd_x:CuInS₂ q-QDs with high quantum yields are summarized in Table 1. As a comparison, we also used CuInS₂ t-QDs and Zn:CuInS₂ q-QDs as alternative sensitizers in the QDSSC. Fig. 8 illustrates the photocurrent–voltage (I–V) curves for QDSSCs with different types of QD sensitization under simulated one sun illumination (AM1.5 G, 100 mW cm⁻²). Cell performance parameters corresponding to Fig. 8, containing short-circuit current density (J_SC), open-circuit potential (V_OC), fill factor (FF), and power conversion efficiency (η), are listed in Table 2. It is interesting note in Tables 1 and 2 that J_SC was systematically improved with increasing the quantum yields of Cd_x:CuInS₂ q-QDs. Similar J_SC enhancement was also found in Zn:CuInS₂ q-QDs sensitized solar cells. It can be seen that J_SC increased in the order of Zn:CuInS₂ (4.57 mA cm⁻²) > Cd_0.25:CuInS₂ (4.33 mA cm⁻²) > CuInS₂ (3.58 mA cm⁻²) – the same order of increase as the quantum yields of the QDs. Moreover, it can be seen from Table 1 that the efficiency of all Cd_x:CuInS₂ sensitized solar cells are much superior compared with the CuInS₂ device. The highest efficiency obtained for a Cd:CuInS₂ sensitized solar cell was 1.73%, which is 70% greater than the native CuInS₂ QDSSC (η = 1.03%). This improvement may be attributed to the higher value of J_SC. These results may be understood based on the above discussion associating Cd:CuInS₂ q-QDs with higher quantum yields with having a lower defect density than native CuInS₂ t-QDs. The lower defect density reduces the amount of photogenerated electrons and holes being trapped in the QDs, and leads to a greater number of photoinduced electrons being injected into the TiO₂.

Fig. 7 SEM cross-section of a TiO₂ electrode composed of porous absorption layer and scattering layers.

Fig. 8 I–V characteristics of solar cells assembled with different photoanodes from cell C1 to C6.

Table 2 Photovoltaic parameters obtained from the I–V curves using various photoanodes from cell C1 to C6

Cell	Photoanode	JSC (mA cm^−2)	VOC (V)	FF	η (%)
C1	CuInS2	3.58	0.56	51.8	1.03
C2	Cd0.25:CuInS2	4.33	0.56	59.3	1.44
C3	Cd0.50:CuInS2	4.75	0.57	59.1	1.59
C4	Cd0.75:CuInS2	5.41	0.55	53.9	1.62
C5	Cd:CuInS2	5.48	0.57	56.1	1.74
C6	Zn:CuInS2	4.57	0.54	53.1	1.30

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To further improve cell performance, we used the SILAR approach to deposit CdS and CdSe onto the surface of Cd:CuInS₂ q-QDs, as tailoring alternative sensitizers onto the surface of the QDs could provide a complementary light harvest capacity and result in more energetically favorable interfacial electron transfers. The SILAR approach consisted of the successive surface adsorption of oppositely charged ions to allow nucleation and in situ growth of QDs onto the TiO₂ film. This approach provides precise control over the deposition of QDs and high coverage of QDs on the electrode surface. The effect of SILAR cycles on the cell performance was carefully evaluated, and the optimum cycles was found to be three and four cycles for CdS and CdSe respectively. From Fig. 9 and Table 3, the highest efficiency, 2.86%, was obtained with the Cd:CuInS₂/CdSe(4) sensitized cell, higher than the 2.40% efficiency achieved with the Cd:CuInS₂/CdS(3) sensitized cell, and much higher than 1.74% efficiency in achieved in the Cd:CuInS₂ sensitized cell without the CdSe treatment (Table 3). For all samples, the FF and V_OC values were not found to differ significantly. However, solar cells sensitized with the Cd:CuInS₂/CdSe(4) exhibited J_SC values of 9.73 mA cm⁻², which are 78% higher than the value obtained with the Cd:CuInS₂ QDs (5.48 mA cm⁻²) sensitized cell. The improvement of J_SC in Cd:CuInS₂/CdSe based QDDSCs could be attributed to the expanded absorption spectrum coverage, as Cd:CuInS₂ was co-sensitized with the higher band gap CdSe (1.7 eV in bulk). In addition, it is also possible that favorable realignment of the Fermi level at the TiO₂/Cd:CuInS₂/CdSe interfaces could have improved the electron injection from CdSe QDs to the TiO₂ photoanode compared to the co-sensitization of Cd:CuInS₂ with CdS. Similar phenomena have recently been observed elsewhere: CuInS₂/CdSe QDSSCs have been shown to exhibit an enhanced J_SC as compared with CuInS₂ and CuInS₂/CdS QDSSCs.¹⁵ The IPCE spectra for the Cd:CuInS₂/CdS(3) and Cd:CuInS₂/CdSe(4) sensitized cell are shown in Fig. S4.† The maximum IPCE values obtained at 520 nm is 38% for Cd:CuInS₂/CdS(3) based QDSSC. When CdSe was used as a co-sensitizer on Cd:CuInS₂, the IPCE curve of the Cd:CuInS₂/CdSe(4) based QDSSC cover almost the entire visible spectrum from 350 to 700 nm with a maximum value of 68% at 500 nm.

Fig. 9 I–V characteristics of solar cells assembled with different photoanodes from cell C7 to C12.

Table 3 Photovoltaic parameters obtained from the I–V curves using various photoanodes from cell C7 to C12

Cell	Photoanode	JSC (mA cm^−2)	VOC (V)	FF	η (%)
C7	Cd:CuInS2/CdS(2)	6.53	0.56	53.7	1.95
C8	Cd:CuInS2/CdS(3)	8.10	0.53	55.9	2.40
C9	Cd:CuInS2/CdS(4)	7.74	0.54	55.9	2.33
C10	Cd:CuInS2/CdSe(3)	8.89	0.58	51.5	2.66
C11	Cd:CuInS2/CdSe(4)	9.73	0.56	53.0	2.86
C12	Cd:CuInS2/CdSe(5)	8.99	0.53	45.2	2.13

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4. Conclusions

In conclusion, a facile method was developed to prepare Cd_x:CuInS₂ q-QDs. The synthesis could be accomplished in a one-pot reaction by mixing the cationic and anionic precursors in 1-dodecanethiol solvent without any complicated precursor preparation. This facile non-injection method is the first reported procedure for the synthesis of Cd_x:CuInS₂ q-QDs that could be performed in one vessel. The procedure has the advantages of being easy to handle, giving high synthetic reproducibility, and being easily scalable. The obtained Cd_x:CuInS₂ q-QDs showed blue-shifted emission wavelengths dependent on the Cd feed ratio. The highest quantum yield measured in the Cd:CuInS₂ q-QDs was ∼22%, higher than that of native CuInS₂ t-QDs (∼2%). We also showed that the photovoltaic performance of QDSSCs produced using of Cd:CuInS₂ q-QDs display an efficiency enhancement of about 35% and 70% respectively over solar cells produced using Zn:CuInS₂ q-QDs and pristine CuInS₂ t-QDs. Moreover, it is interesting note that J_SC was systematically improved along with an increase in the emission intensity of Cd_x:CuInS₂ q-QDs. Through the use of CdSe co-sensitizer, Cd:CuInS₂ q-QDs-based solar cells showed very promising photovoltaic performance, with J_SC = 9.73 mA cm⁻², V_OC = 0.56 V, and FF = 53%, to give an efficiency of 2.86%.

Acknowledgements

The authors thank the Ministry of Science and Technology of the Republic of China for financially supporting this research under Contract no. 102-2628-M-011-001-MY3.

References

1. M. A. Hossain, J. R. Jennings, Z. Y. Koh and Q. Wang, ACS Nano, 2011, 5, 3172–3181.
2. K. Yu, X. Lin, G. Lu, Z. Wen, C. Yuan and J. Chen, RSC Adv., 2012, 2, 7843–7848.
3. P. K. Santra and P. V. Kamat, J. Am. Chem. Soc., 2012, 134, 2508–2511.
4. Z. Pan, H. Zhang, K. Cheng, Y. Hou, J. Hua and X. Zhong, ACS Nano, 2012, 6, 3982–3991.
5. J. Wang, I. Mora-Seró, Z. Pan, K. Zhao, H. Zhang, Y. Feng, G. Yang, X. Zhong and J. Bisquert, J. Am. Chem. Soc., 2013, 135, 15913–15922.
6. P. K. Santra and P. V. Kamat, J. Am. Chem. Soc., 2013, 135, 877–885.
7. P.-J. Wu, J.-W. Yu, H.-J. Chao and J.-Y. Chang, Chem. Mater., 2014, 26, 3485–3494.
8. A. N. Jumabekov, T. D. Siegler, N. Cordes, D. D. Medina, D. Bohm, P. Garbus, S. Meroni, L. M. Peter and T. Bein, J. Phys. Chem. C, 2014, 118, 25853–25862.
9. J. Tian, L. Lv, C. Fei, Y. Wang, X. Liu and G. Cao, J. Mater. Chem. A, 2014, 2, 19653–19659.
10. J.-Y. Chang, L.-F. Su, C.-H. Li, C.-C. Chang and J.-M. Lin, Chem. Commun., 2012, 48, 4848–4850.
11. H. McDaniel, N. Fuke, N. S. Makarov, J. M. Pietryga and V. I. Klimov, Nat. Commun., 2013, 4, 2887.
12. H. Mcdaniel, N. Fuke, M. Pietryga and V. I. Klimov, J. Phys. Chem. Lett., 2013, 4, 355–361.
13. J. Luo, H. Wei, Q. Huang, X. Hu, H. Zhao, R. Yu, D. Li, Y. Luo and Q. Meng, Chem. Commun., 2013, 49, 3881–3883.
14. C.-C. Chang, J.-K. Chen, C.-P. Chen, C.-H. Yang and J.-Y. Chang, ACS Appl. Mater. Interfaces, 2013, 5, 11296–11306.
15. J.-Y. Chang, J.-M. Lin, L.-F. Su and C.-F. Chang, ACS Appl. Mater. Interfaces, 2013, 5, 8740–8752.
16. Z. Pan, I. Mora-Seró, Q. Shen, H. Zhang, Y. Li, K. Zhao, J. Wang, X. Zhong and J. Bisquert, J. Am. Chem. Soc., 2014, 136, 9203–9210.
17. W. Kato, M. Uehara, K. Nose, T. Omata, S. Otsuka-Yao-Matsuo, M. Miyazaki and H. Maeda, Chem. Mater., 2006, 18, 3330–3335.
18. M. Uehara, K. Watanabe, Y. Tajiri, H. Nakamura and H. Maeda, J. Chem. Phys., 2008, 129, 134709.
19. Y.-K. Kim, S.-H. Ahn, K. Chung, Y.-S. Cho and C.-J. Choi, J. Mater. Chem., 2012, 22, 1516–1520.
20. J. Zhang, R. Xie and W. Yang, Chem. Mater., 2011, 23, 3357–3361.
21. L. D. Trizio, M. Prato, A. Genovese, A. Casu, M. Povia, R. Simonutti, M. J. P. Alcocer, C. D'Andrea, F. Tassone and L. Manna, Chem. Mater., 2012, 24, 2400–2406.
22. W.-D. Xiang, H.-L. Yang, X.-J. Liang, J.-S. Zhong, J. Wang, L. Luo and C.-P. Xie, J. Mater. Chem. C, 2013, 1, 2014–2020.
23. Q. Liu, R. Deng, X. Ji and D. Pan, Nanotechnology, 2012, 23, 255706.
24. L. Li, A. Pandey, D. J. Werder, B. P. Khanal, J. M. Pietryga and V. I. Klimov, J. Am. Chem. Soc., 2011, 133, 1176–1179.
25. S. Hachiya, Y. Onishi, Q. Shen and T. Toyoda, J. Appl. Phys., 2011, 110, 054319.
26. B. Chen, H. Zhong, W. Zhang, Z. Tan, Y. Li, C. Yu, T. Zhai, Y. Bando, S. Yang and B. Zou, Adv. Funct. Mater., 2012, 22, 2081–2088.
27. E. S. Speranskaya, N. V. Beloglazova, S. Abé, T. Aubert, P. F. Smet, D. Poelman, I. Y. Goryacheva, S. D. Saeger and Z. Hens, Langmuir, 2014, 30, 7567–7575.
28. G.-C. Park, H.-D. Chung, C.-D. Kim, H.-R. Park, W.-J. Jeong, J.-U. Kim, H.-B. Gu and K.-S. Lee, Sol. Energy Mater. Sol. Cells, 1997, 49, 365–374.
29. W. Zhang and X. Zhang, Inorg. Chem., 2011, 50, 4065–4072.
30. J.-Y. Chang, S. C. Chang, S. H. Tzing and C. H. Li, ACS Appl. Mater. Interfaces, 2014, 6, 22272–22281.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra04275e

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