In situ growth of CuInS2 nanocrystals on nanoporous TiO2 film for constructing inorganic/organic heterojunction solar cells
© Chen et al.; licensee Springer. 2013
Received: 17 June 2013
Accepted: 8 August 2013
Published: 16 August 2013
Inorganic/organic heterojunction solar cells (HSCs) have attracted increasing attention as a cost-effective alternative to conventional solar cells. This work presents an HSC by in situ growth of CuInS2( CIS) layer as the photoabsorption material on nanoporous TiO2 film with the use of poly(3-hexylthiophene) (P3HT) as hole-transport material. The in situ growth of CIS nanocrystals has been realized by solvothermally treating nanoporous TiO2 film in ethanol solution containing InCl3 · 4H2O, CuSO4 · 5H2O, and thioacetamide with a constant concentration ratio of 1:1:2. InCl3 concentration plays a significant role in controlling the surface morphology of CIS layer. When InCl3 concentration is 0.1 M, there is a layer of CIS flower-shaped superstructures on TiO2 film, and CIS superstructures are in fact composed of ultrathin nanoplates as ‘petals’ with plenty of nanopores. In addition, the nanopores of TiO2 film are filled by CIS nanocrystals, as confirmed using scanning electron microscopy image and by energy dispersive spectroscopy line scan analysis. Subsequently, HSC with a structure of FTO/TiO2/CIS/P3HT/PEDOT:PSS/Au has been fabricated, and it yields a power conversion efficiency of 1.4%. Further improvement of the efficiency can be expected by the optimization of the morphology and thickness of CIS layer and the device structure.
The quest and demand for clean and economical energy sources have increased interest in the development of various solar cells, such as Si solar cells, Cu(In,Ga)(S,Se)2 film solar cells[3–6], organic solar cells, and dye-sensitized solar cells (DSSCs)[8–12]. Among these solar cells, DSSCs have been currently attracting widespread scientific and technological interest due to their low cost and high efficiency[8–12]. The typical working principle of DSSCs is based on ultrafast electron injection from a photoexcited dye into the conduction band of TiO2 and subsequent dye regeneration and hole transportation to the counter electrode. The power conversion efficiency of DSSCs with organic solvent-based electrolyte has been reported to exceed 11%[9, 13, 14]. However, DSSCs still suffer from some problems, such as high cost of Ru-based dyes, leakage and/or evaporation from organic solvent-based electrolyte.
For reducing the cost, the use of inorganic semiconductor nanocrystals instead of Ru-based dyes in DSSCs has attracted an enormous interest[15–18]. Semiconductor nanocrystals as the sensitizers have many fascinating advantages, such as high extinction coefficients, large intrinsic dipole moments, and the tuned bandgap. In particular, semiconductor quantum dots have capability of producing multiple electron/hole pairs with a single photon through the impact ionization effect. For depositing semiconductor nanocrystals on TiO2 films, two typical approaches have been developed. The first and most common route is the in situ synthesis of the nanocrystals on TiO2 film, for example, by chemical bath deposition or by successive ionic layer adsorption and reaction (SILAR). This method provides high surface coverage, but the lack of capping agents leads to a broad size distribution and a higher density of surface defects of nanocrystals, which deteriorates solar cell performance. The second route is the assembly of already-synthesized nanocrystals to TiO2 substrates by direct adsorption or linker-assisted adsorption. This ex situ approach could achieve better control over the sizes and electronic properties of nanocrystals but suffers from low surface coverage and poor electronic coupling. Up to now, many different semiconductor nanocrystals as the sensitizers have been investigated, including CdSe[17, 22, 25], CdS[21, 26], and PbS[27–29]. Unfortunately, these metal chalcogenide semiconductors are easily oxidized when exposed to light, and this unfavorable situation is even more detrimental when the metal sulfide is in contact with a liquid electrolyte containing sulfur. It is well known that the choice of semiconductors and the method of their deposition play a paramount role in affecting cell efficiency. Therefore, it is still necessary to develop new materials and deposition methods for improving DSSCs with semiconductors as the sensitizers.
On the other hand, for avoiding the sealing problem in DSSCs, many attempts have been made to substitute liquid electrolytes with quasi-solid electrolytes or solid-state hole transporting material (HTM). Similarly, when semiconductor nanocrystals are used as the sensitizers, some HTMs including poly(3-hexylthiophene) (P3HT) have been developed, resulting in the construction of all-solid-state inorganic/organic heterojunction solar cells (HSCs)[32–36], which possesses the advantages of both DSSCs and traditional organic solar cells. In particular, the efficiency of HSCs with the structure of TiO2/Sb2S3/P3HT has reached 5%, which is very close to the efficiencies reported for solid DSSCs using Ru-based molecular dyes. In addition, Sb2S3 nanocrystals are non-toxic compared with Cd/Pb-based semiconductors. These facts show the great potentiality of all-solid HSCs, which also encourages to further achieve other kind of robust, efficient, and cheap HSCs without toxic component.
Copper indium disulfide (CuInS2, abbreviated as CIS) has a small direct bandgap of 1.5 eV that matches well the solar spectrum, a large absorption coefficient (α = 5 × 105 cm−1), and low toxicity. It has been regarded to be a promising light-absorbing material for film solar cells. As semiconductor sensitizers in DSSCs, CIS nanocrystals have been prepared by different methods and then were coated/adsorbed on TiO2 film to construct DSSCs with liquid electrolyte[24, 37, 38]. In addition, the in situ growth of CIS on TiO2 film has also been realized, by electrodeposition, spin-coating/anneal, and SILAR method, to construct DSSCs with liquid electrolyte. However, there is little report on solvothermal growth of CIS nanocrystals on TiO2 film for the construction of all-solid HSCs. In this paper, we report a facile one-step solvothermal route for the in situ growth CIS nanocrystals on nanoporous TiO2 film. The effects of reagent concentration on the surface morphology of CIS have been investigated. The all-solid HSC with the structure of FTO/compact-TiO2 /nanoporous-TiO2/CIS/P3HT/PEDOT:PSS/Au is fabricated, and it exhibits a relatively high conversion efficiency of 1.4%.
All of the chemicals were commercially available and were used without further purification. Titanium butoxide, petroleum ether, TiCl4, CuSO4 · 5H2O, InCl3 · 4H2O, thioacetamide, ethanol, methanol, and 1,2-dichlorobenzene were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). TiO2 (P25) was obtained from Degussa. Transparent conductive glass (F:SnO2, FTO) was purchased from Wuhan Geao Instruments Science & Technology Co., Ltd (Wuhan, Hubei, China). P3HT was bought from Guanghe Electronic Materials Co., Ltd. (Henan, China). The poly(3-4-ethylenedioxythiophene) doped with poly(4-stylenesulfonate) (PEDOT:PSS) solution (solvent, H2O; weight percentage, 1.3%) was obtained from Aldrich (St. Louis, MO, USA).
Preparation of compact and nanoporous TiO2 film
A part of FTO glass was chemically etched away in order to prevent direct contact between the two electrodes. A compact (about 100-nm thick) TiO2 layer was first deposited onto the FTO glass as follow. FTO glass was dipped into the mixture of titanium butoxide and petroleum ether (2:98 V/V), taken out carefully, hydrolyzed in air for 30 min, and sintered in oven for 30 min at 450°C.
Then, nanoporous TiO2 films were prepared by the doctor-blading technique with TiO2 (P25) colloidal dispersion and subsequently sintered at 450°C for 30 min, according to a previous study. This film was soaked into a TiCl4 (20 mM in water) solution for 12 h. It was then washed with deionized water and ethanol, dried with air, and sintered again at 450°C for 30 min.
In situ solvothermal growth of CuInS2nanocrystals
CIS layer was in situ grown on nanoporous TiO2 films by a solvothermal process. In a typical process, thioacetamide (0.24 mmol, 0.02 M) was added into a 12 mL ethanol solution containing InCl3 · 4H2O (0.01 M) and CuSO4 · 5H2O (0.01 M) under magnetic stirring, until a clear solution was formed. The resulting solution was transferred into a Teflon-lined stainless steel autoclave with 30-mL capacity. Subsequently, FTO/compact-TiO2/nanoporous-TiO2 film as the substrate was vertically immersed into the solution. Lastly, the autoclave was kept in a fan-forced oven at 160°C for 12 h. After air-cooling to room temperature, CIS film on non-conductive glass side was scraped off, while CIS film on nanoporous TiO2 film side was washed with deionized water and absolute ethanol successively, and dried in air. For comparison, the effects of InCl3 · 4H2O concentrations (0.01, 0.03, 0.1 M) on the morphologies CIS layer were investigated. The concentration ratio of InCl3 · 4H2O, CuSO4 · 5H2O, and thioacetamide was maintained constant (1:1:2) for all the cases.
Fabrication of all-solid HSC
The P3HT solution (10 mg/mL in 1,2-dichlorobenzene) was spin-coated onto TiO2/CIS with 3,000 rpm for 60 s. Then, in order to improve the contact between P3HT and gold, a PEDOT:PSS solution diluted with two volumes of methanol was introduced onto TiO2/CIS/P3HT layer by spin-coating at 2,000 rpm for 30 s. In order to form a hybrid heterojunction, the TiO2/CIS/P3HT/PEDOT:PSS layer was then annealed at 90°C for 30 min in a vacuum oven. Gold layer as the back contact was prepared by magnetron sputtering with a metal mask, giving an active area of 16 mm2 for each device. The resulting HSC has a structure of FTO/compact-TiO2/nanoporous-TiO2/CIS/P3HT/PEDOT:PSS/Au.
Characterization and photoelectrical measurements
The sizes and morphologies of the sample were investigated by field emission scanning electron microscopy (FE-SEM; S-4800, Hitachi, Chiyoda-ku, Japan). During SEM measurement, energy dispersive spectroscopy (EDS; Quantax 400, Bruker AXS, Inc., Madison, WI, USA) line scan was also performed to locate and determine the distribution of different layer in the composite film. The X-ray diffraction (XRD; D/max-g B, Rigaku, Shibuya-ku, Japan) measurement was carried out using a Cu Kα radiation source (λ = 1.5418 Å). An ultraviolet/visible (UV-vis) spectrophotometer (U-3010 spectrophotometer, Hitachi, Chiyoda-ku, Japan) was used to carry out the optical measurements. The photocurrent density/voltage curves of HSC was measured under illumination (100 mW cm−2) using a computerized Keithley Model 2400 Source Meter unit (Keithley Instruments Inc., Cleveland, OH, USA) and a 300-W xenon lamp (Newport 69911, Newport-Oriel Instruments, Stratford, CT, USA) serving as the light source.
Results and discussion
The fourth step was to in turn deposit P3HT and PEDOT:PSS layer on FTO/compact-TiO2/nanoporous-TiO2/CIS film by the spin-coating process (Figure 1 (step D)). After the coating of P3HT, the photoabsorption of the film increases obviously in the range of 400 to 700 nm, as shown in Figure 8 (line B), since P3HT solution exhibits a wide and strong absorption with peak at about 445 nm. This fact also indicates the efficient deposition of P3HT in/on TiO2/CIS film. It should be noted that there are plenty of macro-pores among superstructures, nanopores inside CIS flower-shaped superstructures, and nanopores in TiO2 film due to the insufficient filling. The hierarchical combination of smaller nanopores and larger macro-pores can be considered as transport paths. It can be expected that P3HT solution can easily enter the deep layer of FTO/compact-TiO2/nanoporous-TiO2/CIS film through the transport paths, when they are coated onto its surface during the spin-coating process. This should lead to better effects of wetting and pore-filling and thus better interfacial contact among P3HT, CIS, and TiO2, probably resulting in more efficient separation of photoinduced electron/hole pairs and thus higher photocurrent.
In summary, an in situ growth of CIS nanocrystals has been demonstrated by solvothermally treating nanoporous TiO2 film in ethanol solution containing InCl3 · 4H2O, CuSO4 · 5H2O, and thioacetamide with a constant concentration ratio of 1:1:2. When InCl3 concentration is 0.1 M, there is a CIS layer on the top of TiO2 film, and the pores of TiO2 film have been filled by CIS nanoparticles. An HSC with the structure of FTO/TiO2/CIS/P3HT/PEDOT:PSS/Au has been fabricated, and it yields a power conversion efficiency of 1.4%. Further improvement can be expected by optimizing CIS layer and the cell structure.
This work was financially supported by the National Natural Science Foundation of China (grant nos. 21107013, 21171035, and 51272299), Specialized Research Fund for the Doctoral Program of Higher Education (grant no. 20110075120012), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, projects of the Shanghai Committee of Science and Technology (grant nos. 10JC1400100, 13JC1400300), Innovation Program of Shanghai Municipal Education Commission (grant no. 13ZZ053), the Fundamental Research Funds for the Central Universities, the Shanghai Leading Academic Discipline Project (grant no. B603), and the Program of Introducing Talents of Discipline to Universities (grant no. 111-2-04).
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