Investigation on Photovoltaic Performance based on Matchstick-Like Cu2S–In2S3Heterostructure Nanocrystals and Polymer
© to the authors 2008
Received: 11 August 2008
Accepted: 3 October 2008
Published: 25 October 2008
In this paper, we synthesized a novel type II cuprous sulfide (Cu2S)–indium sulfide (In2S3) heterostructure nanocrystals with matchstick-like morphology in pure dodecanethiol. The photovoltaic properties of the heterostructure nanocrystals were investigated based on the blends of the nanocrystals and poly(2-methoxy-5-(2′-ethylhexoxy)-p-phenylenevinylene) (MEH-PPV). In comparison with the photovoltaic properties of the blends of Cu2S or In2S3nanocrystals alone and MEH-PPV, the power conversion efficiency of the hybrid device based on blend of Cu2S–In2S3and MEH-PPV is enhanced by ~3–5 times. This improvement is consistent with the improved exciton dissociation or separation and better charge transport abilities in type II heterostructure nanocrystals.
Since 1996, Greenham et al  reported the photovoltaic device based on inorganic nanocrystals and conjugated polymer; hybrid photovoltaic devices fabricated by incorporating inorganic nanocrystals (such as CdSe [2–4], CdS , CdSe x Te1−x , CuInS2, ZnO [8, 9], TiO2 [10, 11], PbS , and so on) into conjugated polymer matrixes have been extensively studied. This has been demonstrated that the performance of the hybrid photovoltaic devices could be enhanced by using the blends of these different-shaped inorganic nanocrystals and conjugated polymers [13, 14]. In these hybrid devices, the photo-induced charge transfer is favored between inorganic nanocrystals with high electron affinity and conjugated polymers with relatively low ionization potential. The neutral excitons in polymer and nanocrystals produced by photo-excitation are separated into free carriers at the nanocrystal/polymer interface and then are transported through their own pathways to the electrode, resulting in the generation of photocurrent and photo-voltage [15, 16].
Currently, a significant interest has been directed toward the design of semiconductor heterostructure nanocrystals for electroluminescence and photovoltaic applications [15, 16]. The semiconductor heterostructure nanocrystals, composed of at least two different types of materials with different band-gaps, can be generally classified into two types according to the electronic structures built up within the heterostructures. With respect to type I heterostructure, the mismatch between the energetic levels of each component is unfavorable for exciton dissociation, while type II heterostructure is in favor of charge separation upon photo-irradiation. Thus, type II heterostructure nanocrystals are believed to be useful for photovoltaic applications [15, 17]. There have been several reports on the synthesis of type II heterostructure nanocrystals, containing heavy metal such as cadmium or lead ions [15, 18, 19]. However, there are very few reports about the investigation of the photovoltaic properties on the type II heterostructure nanocrystals to date. Furthermore, taking the environmental problems into consideration, environmental friendly nanocrystals containing copper and indium should be more welcome for their applications in photovoltaic devices.
Cu2−x S is a p-type semiconductor possessing an x-dependent band-gap energy varying from ~1.2 eV for chalcocite (x = 0) to ~1.5 eV for digenite (x = 0.2), accompanied by a transformation from an indirect-gap semiconductor to a direct one, and it has high absorption coefficient of about 105 cm−1 (at 750 nm) . In contrast, In2S3 is an important n-type semiconducting material with a band-gap as narrow as 2.00–2.30 eV, which presents both direct and indirect conduction-to-valence transitions .
Herein, we report a new type II matchstick-like Cu2S–In2S3heterostructure nanocrystals, synthesized by successively pyrolizing copper (II) acetylacetonate (Cu(acac)2) and indium acetylacetonate (In(acac)3) in pure dodecanethiol, which is nontoxic and environmental friendly. Furthermore, we fabricated the hybrid photovoltaic devices using the blends of the Cu2S–In2S3nanocrystals and poly(2-methoxy-5-(2′-ethylhexoxy)-p-phenylenevinylene) (MEH-PPV). To study the photovoltaic performance of the Cu2S–In2S3/MEH-PPV films related to the built-in heterostructures, the photovoltaic performance of the single component spherical Cu2S nanocrystals and In2S3nanorods were also investigated.
Synthesis of Nanocrystals
Cu2S nanocrystals, Cu2S–In2S3 heterostructure nanocrystals, and In2S3 nanorods were synthesized in our laboratory, and more details of the synthesis and characterizations are published elsewhere . All the synthetic processes were carried out under the protection of nitrogen gas. The synthesis of spherical Cu2S nanocrystals was accomplished by directly decomposing the dodecanethiol solution of Cu(acac)2 at the temperature of 200 °C. The matchstick-like Cu2S–In2S3 heterostructure nanocrystals were prepared as follows: the organometallic Cu(acac)2 was firstly pyrolyzed in pure dodecanethiol at 200 °C for several minutes, and three portions of dodecanethiol solutions containing In(acac)3 were intermittently injected into the Cu(acac)2 dodecanethiol solution without stopping the heating process. The In2S3 nanorods were prepared by chemically detaching the Cu2S segment from the matchstick-like Cu2S–In2S3 heterostructure nanocrystals by introducing 1,10-phenanthroline into the reaction system, and the reaction was allowed to take place at room temperature under magnetic stirring. The purification procedures of the samples were carried out by adding appropriate amount of ethanol into the samples and centrifuging at 4000 rpm for 10 min. After that, the precipitates were collected and washed twice with chloroform to remove precursor and surfactant residuals. Finally, the samples were re-dissolved into chloroform for TEM characterization.
Dimensions and morphologies of spherical Cu2S nanocrystals, matchstick-like Cu2S–In2S3heterostructure nanocrystals, and In2S3nanorods were characterized by transmission electron microscopy (TEM), and were recorded with a JEM-100CXII electron microscope operating at an accelerating voltage of 100 kV. TEM samples were prepared by dropping a dilute solution of the samples in chloroform on carbon-coated copper grids and then allowing the solvent to evaporate. Mean diameters, lengths, and widths were determined by counting at least 300 particles per sample for statistical purposes. Powder X-ray diffraction (XRD) were obtained with a Regaku D/Max-2500 diffractometer equipped with a Cu Kα1 radiation (λ = 1.54056 Å). The current density–voltage (J–V) characteristics of the photovoltaic devices were measured using a Keithley 2410 source measure unit both in dark and under illumination at 500 nm. Monochromatic illumination was produced by the output of a xenon lamp dispersed by a monochromator in SPEX Fluorolog-3 spectrophotometer.
Results and Discussions
Summary of device parameters for the photovoltaic devices using pristine MEH-PPV, MEH-PPV:Cu2S, MEH-PPV:In2S3, and MEH-PPV:Cu2S–In2S3as active layers under illumination
In summary, we studied the photovoltaic properties of the hybrid devices based on the blends of MEH-PPV and the type II Cu2S–In2S3heterostructure nanocrystals, which are environmental friendly and nontoxic. As compared to the hybrid device using single-composition Cu2S or In2S3, the power conversion efficiency of the Cu2S–In2S3:MEH-PPV device showed an obvious improvement, which could be attributed to higher exciton dissociation probability and more efficient charge transportation in type II heterostructure nanocrystals. This work may supply a new environmental friendly and type II heterostructure nanocrystals to design candidate materials for hybrid photovoltaic devices.
The authors gratefully acknowledge the financial support of National Key Project of Basic Research of China (No. 2003CB314707), National Natural Science Foundation of China (Nos. 10434030 and 90401006), Key Project of Chinese Ministry of Education (No. 105041). The author (Aiwei Tang) is also grateful to the Doctor Innovation Foundation of Beijing Jiaotong University (No. 48023).
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