Quaternary Cu2ZnSnS4 quantum dot-sensitized solar cells: Synthesis, passivation and ligand exchange

Journal of Power Sources 318 (2016) 35e40

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Quaternary Cu2ZnSnS4 quantum dot-sensitized solar cells: Synthesis, passivation and ligand exchange Bing Bai, Dongxing Kou*, Wenhui Zhou, Zhengji Zhou, Qingwen Tian, Yuena Meng, Sixin Wu** The Key Laboratory for Special Functional Materials of MOE, Collaborative Innovation Center of Nano Functional Materials and Applications, Henan University, Kaifeng 475004, PR China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Synthesize small-sized CZTS QDs by using 1-dodecanethiol as capping ligand.
The strong bonded nature cationDDT units are first exchanged by Cd-oleate.
A type-II core/shell structure is formed during the ligand exchange procedure.
The improvement of electron transport and recombination processes are achieved.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 21 February 2016 Received in revised form 31 March 2016 Accepted 2 April 2016

The quaternary Cu2ZnSnS4 (CZTS) QDs had been successfully introduced into quantum dot-sensitized solar cells (QDSC) via hydrolysis approach in our previous work [Green Chem. 2015, vol. 17, p. 4377], but the obtained cell efficiency was still limited by low open-circuit voltage and fill factor. Herein, we use 1-dodecanethiol (DDT) as capping ligand for fairly small-sized CZTS QDs synthesis to improve their intrinsic properties. Since this strong bonded capping ligand can not be replaced by 3-mercaptopropionic acid (MPA) directly, the nature cation (Cu, Zn or Sn)-DDT units of QDs are first exchanged by the preconjugated Cd-oleate via successive ionic layer adsorption and reaction (SILAR) procedure accompanied with the formation of a core/shell structure. The weak bonded oleic acid (OA) can be finally replaced by MPA and the constructed water soluble CZTS/CdSe QDSC achieves an impressive conversion efficiency of 4.70%. The electron transport and recombination dynamic processes are confirmed by intensitymodulated photocurrent spectroscopy (IMPS)/intensity-modulated photovoltage spectroscopy (IMVS) measurements. It is found that the removal of long alkyl chain is conducive to improve the electron transport process and the type-II core/shell structure is beneficial to accelerate electron transport and retard charge recombination. This effective ligand removal strategy is proved to be more convenient for the applying of quaternary QDs in QDSC and would boost a more powerful efficiency in the future work. © 2016 Elsevier B.V. All rights reserved.

Keywords: Cu2ZnSnS4 Cation exchange Core-shell Quantum dot-sensitized solar cells Charge transport

1. Introduction * Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (D. Kou), [email protected] (S. Wu). http://dx.doi.org/10.1016/j.jpowsour.2016.04.009 0378-7753/© 2016 Elsevier B.V. All rights reserved.

Over the past years, the quantum-dot-sensitized solar cell (QDSC) constitutes one of the most promising candidates for third-

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generation solar cells due to its high absorption coefficient, tunable band gap, generation of multiple electrons and solution processability [1e6]. The working principle of this device is based on the injection of electrons from photo-excited states of quantum dots (QDs) to the conduction band of TiO2 films followed by the reduction of the oxidized QDs with a charge mediator [7]. During these processes, QDs play a dominant role with regard to light harvesting, photoinduced charge injection and electron recombination processes [8,9]. Up to now, the highest record of photoelectric conversion efficiency for liquid junction QDSC over 9% has been obtained, but this value is still remarkably lower than their theoretical efficiency 44% [10e13]. New panchromatic QD sensitizers need to be developed for more efficient light harvesting and charge collection. Due to the “optical bowing” effect [14], quaternary QDs possess more excellent photoelectric properties than binary or ternary materials, such as better corrosion resistance, higher absorption coefficient and tunable band gap without changing particle size [15e19]. However, such promising materials have seldom been applied for QDSC in the previous reported work [20e22]. The key hindrance is that strong bonded capping ligand is needed for the synthesis of small-sized quaternary QDs, which is difficult to be replaced by bifunctional molecular linker due to the strong coordination capacity of thiol group [23e25]. In our last work, the earth-abundant Cu2ZnSnS4 (CZTS) QDs have been successfully introduced into QDSC via hydrolysis approach, but the cell efficiency is still limited by the intrinsic properties of CZTS QDs [24]. The fact that the existence of more defects in the multicomponent CZTS QDs can induce both internal charge recombination inside QDs and photoexcited electron recombination from TiO2 films to QDs, leading to relative poor cell performances [26]. Rigorous nucleation conditions and more efficient ligand removal strategy must be developed for the further application of CZTS QDs in QDSC. Herein, we use 1-dodecanethiol (DDT) as capping ligand for the synthesis of fairly small-sized CZTS QDs and successfully applied them in QDSC. To remove the strong bonded capping ligand, the nature cation (Cu, Zn or Sn)-DDT units at the surface of QDs are first exchanged by preconjugated Cd-oleate via successive ionic layer adsorption and reaction (SILAR) procedure and a type-II CZTS/CdSe core/shell structure is formed simultaneously [27]. After the weak bonded OA is replaced by 3-mercaptopropionic acid (MPA), the effect of band level alignment and ligand exchange on electron transport and recombination dynamic processes in CZTS QDSC are finally confirmed.

vacuum for 30 min. Subsequently, the solution was stayed at 230  C under argon flow for 1 h. After the flask cooling down to room temperature, the purified CZTS QDs were obtained by precipitation and centrifugation with the use of ethanol. 2.3. Synthesis of CZTS/CdSe QDs A chloroform solution containing 0.1 mmol purified CZTS QDs, 5 ml ODE and 2 ml OA was loaded into a flask. The mixture was heated at 170  C under argon flow for the deposition of CdSe. Cd precursor was obtained by dissolving CdO in OA and ODE solution, while the Se precursor was obtained by dissolving Se powder in TOP and ODE solution. Equimolar amount of Cd and Se precursor solutions (0.4 M) were injected into the reaction system alternately at 30 min intervals. This SILAR procedure was repeated for three times. The volume of the precursor stock solution added in each cycle should not exceed the amount needed for a whole monolayer (ML) of the CdSe shell to eliminate the formation of isolated CdSe QDs [29,30]. 2.4. Preparation of water-soluble CZTS/CdSe QDs The water-soluble CZTS/CdSe QDs were prepared by ligand exchange procedure [31]. Typically, 200 ml MPA was dissolved in 1 ml methanol, and the pH of the mixture was adjusted to 12 with NaOH deionized water solution. 5 ml CZTS QDs chloroform solution was added into the mixture. After stirring for 30 min, 10 ml deionized water was added. After centrifugation of the upper layer, the precipitate was dispersed in deionized water for next step. 2.5. Sensitization of TiO2 films and fabrication of solar cells The TiO2 mesoporous electrodes were immersed into QDs deionized water solution for 24 h and then rinsed with water and ethanol. The sensitized TiO2 electrodes were successively dipped into 0.1 M Zn(NO3)2 solution and 0.1 M Na2S solution for 3 times to prepare ZnS passivation layer. To enhance the stability of our devices, Cu2S counter electrodes were obtained by screen-printed Cu2S pastes onto FTO glass and sintered at 400  C for 30 min according to previous literature [32]. The QDSCs were constructed by sandwiching the photoanodes and counter electrodes using thermal adhesive films (Surlyn, Dupont). The polysulfide electrolyte solution consisting of 2.0 M Na2S, 2.0 M S, 0.2 M KCl and 5 ml deionized water, was injected by vacuum backfilling followed with sealing process. The active area of the device was 0.25 cm2.

2. Experimental section 2.6. Characterization 2.1. Chemicals Copper(II) acetylacetonate (Cu(acac)2, 98.5%), Tin(II) chloride dehydrate (SnCl2$2H2O, 98%), 3-mercaptopropionic acid (MPA, 99%), selenium powder (Se, 99þ%) and 1-octadencene (ODE, 90%) were purchased from Alfa Aesar. Zinc acetate (Zn(OAc)2, 99.99%), oleylamine (OAm, 80e90%), cadmium oxide (CdO, 99.99%), trioctylphosphine (TOP, 90%) and oleic acid (OA, AR) were purchased from Aldrich. 1-dodecanethiol (DDT, CP) was purchased from Shanghai Chemical Reagents Company. All the chemical reagents were used as received. 2.2. Synthesis of CZTS QDs The CZTS QDs were prepared according to the previous literature with further modified [28]. Typically, 0.5 mmol Cu(acac)2, 0.25 mmol Zn(OAc)2, 0.25 mmol SnCl2$2H2O, 12 ml OAm and 1 ml DDT were loaded into a flask. The mixture was kept at 130  C under

The morphology was characterized by scanning electronic microscope (SEM) equipped with an Energy Dispersive X-Ray Spectroscopy (EDX, Nano SEM 45050/EDX). Transition electron microscopy (TEM) images were obtained by using JSM-2100. The crystal structure of CZTS, CdSe and CZTS/CdSe QDs were taken on a powder X-ray diffraction (XRD, Bruker D8 Advance, Cu Ka radiation, l ¼ 1.5406 nm). FTIR spectra were performed with a Nicolet 360 FTIR instrument using KBr pellets. UVevisible transmission spectra were achieved by using PE-Lambda35. J-V characteristics of QDSC were detected with a Keithley 2420 digital source meter (Keithley, USA) under a 450 W xenon lamp (Oriel Sol3A Solar Simulator 94043, Newport Stratford Inc., USA). Intensity-modulated photocurrent spectroscopy (IMPS)/intensity-modulated photovoltage spectroscopy (IMVS) measurements were performed on IM6ex workstation using light-emitting diodes (l ¼ 610 nm) driven by Expot (Germany, Zahner). The amplitude of the modulated component was 10% or less of dc component. The frequency range

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was 3 kHz to 0.1 Hz. 3. Results and discussion Generally, the main route for immobilization of presynthesized QDs onto the TiO2 electrodes is achieved by the replacement of weak bonded oleylamine (OAm) or oleic acid (OA) with bifunctional molecular linker MPA via ligand exchange [33,34]. However, the fairly small-sized quaternary CZTS QDs require the strong coordination capacity of capping ligand, such as DDT, which is hard to be displaced by MPA (Fig. 1a) [10]. Supposing the cation exchange method, we can replace the nature cation (Cu, Zn or Sn)-DDT units at the surface of QDs by the preconjugated Cd-oleate. During this process, the DDT-capped QDs are converted into the OA-capped QDs. Aiming to passivate the surface of CZTS QDs, equimolar amount of Cd and Se precursor solutions are alternate injected via SILAR procedure to form a CdSe shell. Finally, the weak bonded OA can be easily replaced by MPA and the water soluble CZTS/CdSe core/shell QDs are obtained (Fig. 1b). The overall reactions during these procedures can be described as CZTS-(Cu/Zn/Sn-DDT) þ Cd2þ-OA / CZTS-(Cd-OA) þ Cuþ/Zn2þ/ Sn2þ-DDT (1) Cd2þ þ Se2 / CdSe

(2)

CZTS/CdSe-OA þ MPA / CZTS/CdSe-MPA þ OA

(3)

where Equations (1)e(3) is corresponding to the cation unit exchange, formation of shell via SILAR and ligand exchange of MPA, respectively. The small sized CZTS QDs are prepared by using DDT as capping ligand and the EDX quantitative analysis of as-synthesized QDs is shown in Fig. S1. The atomic percentages of Cu, Zn, Sn and S listed in Table S1 are 24.11%, 13.52%, 11.50% and 50.87%, which are close to the 2:1:1:4 stoichiometric ratios of CZTS. The XRD patterns of the CZTS core QDs are shown in Fig. S2, where the diffraction peaks appear at 27, 28 , 30 , 39 , 47, 51 and 56 , consistent with (100), (002), (101), (102), (110), (103) and (112), matching well with previous literature and exhibiting the feature of pure wurzite-phase structure [28]. Furthermore, Raman spectroscopy is also employed to determine the purity of CZTS QDs because XRD is unable to accurately distinguish the binary and ternary impurities. The characteristic vibrational peak mainly locates at 333 cm1, and no other impurity is detected, such as Cu3SnS4 (320 cm1), ZnS (351 cm1) and Cu2-xS (269 cm1) [24,28]. Both XRD and Raman results indicate that the pure wurzite-phase CZTS QDs are obtained. Fig. 2 shows the wide-field TEM and HRTEM images of as-

Fig. 2. The wide-field TEM (a) and HRTEM (b) images of as-synthesized CZTS/CdSe QDs. Inset are the relative conduction band levels of CZTS/CdSe core/shell and TiO2 films.

prepared OA-capped CZTS/CdSe QDs. The SILAR method could control a precise thickness on the surface of QDs and the size of the obtained CZTS/CdSe QDs is about 8 nm. The HRTEM image clearly presents the formation of a uniform shell surrounding the inner core (Fig. S3, about 6.5 nm), and the thickness of this layer is calculated to be 1.5 nm. According to previous literature, the valence band (VB) of the CZTS core is located within the band gap of the expitaxial CdSe shell and a type-II core/shell structure is formed [18,27]. In the CZTS/CdSe QDs, hole is confined to the core and electron is primarily located in the shell, facilitating electron injection from QDs to TiO2 films because of the enhanced electron density at the surface of CZTS/CdSe QDs [12,27]. Meanwhile, the charge recombination process between the separated hole and electron in QDs will be also suppressed due to the shell acts as a tunneling barrier for the hole localized in the core [27]. After the attachment of CZTS/CdSe QDs onto TiO2 films, the conduction band (CB) levels of the three materials decrease in the order CZTS (3.45 eV)>CdSe (3.8 eV)>TiO2 (4.2 eV) as previous literature reported [18]. This resulting cascade structure as shown in Fig. 2b would give a higher driving force for the injection of excited electron and further support a higher photocurrent in the corresponding QDSC. The cation exchange happens when the new cation enter into the parent crystal as the original cation diffuse out of the crystal, which is mainly due to the enhanced surface access (especially concentration, pH and temperature). The high surface area of CZTS QDs, excess Cd2þ and relative high temperature used here can also reduce this reaction activation barrier and allow the reversible reaction direction to be changed with little effort [35]. Therefore, we can anticipate that the DDT-capped CZTS QDs could be converted into the MPA-capped CZTS/CdSe QDs via cation exchange, SILAR

Fig. 1. (a) The traditional ligand exchange approach and (b) the cation-ligand units exchange approach.

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Fig. 3. FTIR spectra of OAm and DDT-capped CZTS QDs, OA-capped CZTS/CdSe QDs and MPA-capped CZTS/CdSe QDs.

and ligand exchange processes according to Equations (1)e(3). Fig. 3 shows the FTIR spectra of the prepared oil soluble CZTS QDs, oil soluble CZTS/CdSe QDs and water soluble CZTS/CdSe QDs. From the characteristic absorption peaks of eNH2 (1640 cm1, OAm) and eC-S (621 cm1, DDT), we can infer that the as-synthesized CZTS QDs are capped with OAm and DDT simultaneously [36,37]. Because of the ionization, the absorption peak of -S-H (2566 cm1, DDT) as shown in Fig. S4 disappears here. After SILAR treatment, the symmetric (1421 cm1) and asymmetric (1532 cm1) stretching vibrations of -COO- group present [38] and the characteristic absorption peaks of eNH2 and eCeS disappear, indicating OAm and DDT are effectively removed by OA (Equations (1) and (2)). This new capping ligand with weak coordination capacity can be easily replaced by MPA (Equation (3)) and further supports the effective immobilization of CZTS QDs on TiO2 films [33]. The weaker intensity of eCeH stretch peak of MPA-capped CZTS/CdSe QDs after ligand exchange indicates the removal of OA. A well light harvesting ability will pave the way for high photocurrent in the resulting QDSC and Fig. 4a illustrates the UVevis absorption spectra of CdSe, CZTS and CZTS/CdSe QDs dispersions. It can be found that the light harvesting range of CZTS QDs dispersion covers the whole visible spectrum and extends to NIR region as far as 900 nm. Meanwhile, the core-shell dispersion exhibits a higher absorption up to 600 nm and an obvious blue shift of absorption edge than plain CZTS QDs, due to the introduction of

wide band gap shell material [39,40]. The coverage of CZTS QDs throughout the mesoporous TiO2 films can be verified by EDX elemental mapping accompanied with cross-sectional SEM [12]. As seen in Fig. 4b, The uniform distribution of Cu demonstrates a uniform coverage of QDs throughout the 10 mm thick TiO2 films. Atomic percentages from elemental analysis are found to be 18.46% and 3.41% for Ti and Cu, respectively. That is to say, the molecule ratio of TiO2 and CZTS is close to 11/1, which is in the same level as the data presented before [12]. The uniform coverage of CZTS QDs with such high Cu/Ti ratio further support that the particle size and ligand exchange techniques used in this work facilitate the penetration and adsorption of QDs throughout the mesoporous film electrode. The effect of the prepared core/shell structure and the subsequent ligand exchange on electron transport and recombination dynamics in our constructed QDSC were further estimated by IMPS/ IMVS measurements. The electron transport time td is associated with the electron transport process from the injection sites to FTO substrate and the recombination time tn is mostly determined by the back reaction between electrons in TiO2 films and S2 x in electrolyte. Both the constants can be calculated by the expression td ¼ 1/2pfImps and tn ¼ 1/2pfImvs, where fImps and fImvs is the minimum frequency of IMPS/IMVS imaginary component, respectively [12]. As shown in Fig. 5a, the IMPS results clearly show that the td of OA-capped CZTS/CdSe QDSC is shorter than that of DDTcapped CZTS QDSC at varied light intensities. This observation is related to the larger charge generation in the CZTS/CdSe device, which brings forward an enhanced electron concentration in TiO2 films [12]. In Fig. 5b, the type-II core/shell structure of the OAcapped CZTS/CdSe QDSC retards the back reaction of electrons and contributes a longer tn than that of DDT-capped CZTS QDSC [12]. After ligand exchange treatment, both td and tn of the MPAcapped CZTS/CdSe QDSC is remarkable shorter than those of the oil-soluble QDSCs, due to the removal of long hydrocarbon chains. The subtle balance between transport and recombination of the photoinduced electrons can be well characterized by collection efficiency ƞc, as expressed by the relation ƞc ¼ 1td/tn, and a larger ƞc could further support a higher photocurrent [11]. It can be found in Fig. 5c that both the core/shell structure and the ligand exchange treatments are beneficial to the increase of ƞc. The J-V characteristics of CZTS QDSCs with different treatments are presented in Fig. 6. An impressive enhancement in short-circuit current (Jsc), open-circuit voltage (Voc) and fill factor (FF) can be found for MPA-capped CZTS/CdSe QDSC due to the largest charge generation and the improvement of charge collection efficiency as discussed above. It is meaningful to note that this efficiency is much higher than our previously reported value [24], suggesting that a more effective ligand removal or surface passivation strategy could

Fig. 4. (a) UVevis absorption spectra of CdSe, core/shell and plain CZTS QDs dispersions and (b) Cross-section SEM image of CZTS quantum dots sensitized TiO2 films. Inset is the corresponding elemental mappings of Ti and Cu.

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Fig. 6. J-V characteristics of CZTS QDSCs with different treatment. The units of Voc, Jsc, FF and ƞ are V, mA/cm2, % and %, respectively.

4. Conclusions In conclusion, we applied the fairly small-sized quaternary CZTS QDs into QDSC by removing the strong bonded capping ligand DDT via SILAR procedure, where the nature cations (Cu, Zn or Sn)-DDT unit at the surface of QDs were exchanged by the preconjugated Cd-oleate. During these processes, a type-II CZTS/CdSe core/shell was formed, efficiently promoting charge separation and retarding charge recombination. After the weak bonded capping ligand OA is replaced by MPA, the constructed water soluble CZTS/CdSe core/ shell QDSC finally achieved an impressive conversion efficiency of 4.70% due to the removal of long hydrocarbon chains. Further strategies focus on more efficient QDs synthesis, ligand removal and surface passivation need to be developed for the future application of quaternary CZTS QDSC. Acknowledgements This project is supported by the National Natural Science Foundation of China (21203053, 21271064 and 61306016), the Joint Talent Cultivation Funds of NSFC-HN (U1204214), the Program for Changjiang Scholars and Innovative Research Team in University (PCS IRT1126) of Henan University. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2016.04.009. References [1] [2] [3] [4]

Fig. 5. Dependence of (a) td, (b) tn and (c) ƞc on light intensities for CZTS QDSCs with different treatment.

remarkably improve cell performances.

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