Charge Separation in Type-II Semiconductor Heterodimers - The

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Charge Separation in Type-II Semiconductor Heterodimers Toshiharu Teranishi* and Masanori Sakamoto Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan ABSTRACT: Highly efficient photoenergy conversion in semiconductor nanoparticle heterostructures requires the formation of epitaxial heterointerfaces and band alignment engineering. This requirement has led to attention being given to recent advances and prospects in the charge separation properties of type-II semiconductor heterodimers composed of chalcogenide−chalcogenide blends. Type-II semiconductor heterodimers with a staggered alignment of band edges at the heterointerface can be synthesized by seeded growth or ion exchange to promote the spatial charge separation between electrons and holes in different parts of the heterostructure. Special attention has been given to CdS−Cu2−xS (0 ≤ x ≤ 0.0625) and CdS−CdTe combinations where CdS is a commonly used n-type semiconductor and both Cu2−xS and CdTe are proper p-type semiconductors that are used as light absorbers in heterojunction solar cells.

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he 20th century saw the development of a way of life that is based on the consumption of fossil fuel energy and nuclear energy, and this is currently the driver of economic and social activity. These energy sources have provided us with not only positive but also negative contributions such as greenhouse gas evolution and nuclear waste disposal. The use of solar energy as an alternative inexhaustible energy source is inevitable if the environment is to be protected and if human activity is to be sustained. Radiant energy from the Sun totals 1.2 × 1034 J/year, and the energy supplied to the surface of the Earth totals 3.0 × 1024 J/year, which corresponds to 104 times the energy consumption of human beings. The effective use of solar energy, which is inexhaustible and has less regional deviation than fossil fuel energy does, is a scientific field that requires much attention in the 21st century. Photoenergy conversion into electricity with solar cells1−5 or into useful chemicals like hydrogen from water with photocatalysts6−11 is promising for future clean and sustainable energy. Long-lived charge separation is important in both cases. Heterostructured inorganic nanoparticles (NPs) in which two or more distinct inorganic materials are connected and exposed to an outer environment12−14 are promising because they provide new ways to manipulate electron and hole wave functions. Additionally, they possess superior molar absorption coefficients and superior durability.15,16 For highly efficient charge separation, an anisotropic heterostructure is required to realize vectorial carrier transfer before exciton recombination in the semiconductor phase, as shown in Figure 1. Important requirements for this purpose are the formation of epitaxial heterointerfaces and band alignment engineering together with high photovoltage. A combination of chemically synthesized heterodimers to realize charge separation includes chalcogenide− chalcogenide, chalcogenide−metal, and chalcogenide−oxide. In this Perspective, we thus focus on recent advances and prospects with regard to the charge separation properties of semiconductor−semiconductor heterodimers composed of chalcogenide−chalcogenide blends. Specifically, we focus on the CdS−Cu2−xS (0 ≤ x ≤ 0.0625) and CdS−CdTe © 2013 American Chemical Society

Figure 1. Schematic of vectorial charge transfer in heterostructured NPs.

combinations because CdS is a commonly used n-type semiconductor, and both Cu2−xS and CdTe are proper p-type semiconductors that can be used as light absorbers in heterojunction solar cells. Complex semiconductor heterostructures with other combinations are discussed elsewhere.17 Heterojunctions of Semiconductor−Semiconductor Heterodimers. Heterojunctions are junctions between two semiconductors with different energy band gaps. Semiconductor−semiconductor heterointerfaces can be classified into three types of heterojunctions, straddling alignment (type-I), staggered alignment (type-II), and broken alignment (type-III), as shown in Figure 2. Type-I and type-II alignments are useful for photoenergy conversion. Three properties of combined materials determine the heterojunction and carrier dynamics at a heterojunction, and these are the band gap, the electron affinity (lowest potential of the conduction band), and the work function (highest potential of the valence band). In NPs, quantum size effects, that is, the NP size-dependent band gap energy, Received: July 1, 2013 Accepted: August 8, 2013 Published: August 8, 2013 2867

dx.doi.org/10.1021/jz4013504 | J. Phys. Chem. Lett. 2013, 4, 2867−2873

The Journal of Physical Chemistry Letters

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Figure 2. Three kinds of band alignments at the semiconductor−semiconductor heterointerfaceL (a) type-I, (b) type-II, and (c) type-III alignments.

should be considered when creating heterointerfaces. In other words, for nanoscale semiconductor−semiconductor heterodimers, the type and size of the semiconductor material allows for control of the manner of confinement of the electron and hole wave functions in the NPs. The CdSe@ZnS core−shell NP is a well-known nanoscale heterostructure, and it has a straddling gap (type-I) offset.18−20 In this type-I NP, the largeband-gap ZnS shell passivates the surface of the fluorescent CdSe core, thereby increasing the quantum efficiency of luminescence. On the other hand, type-II NPs with a staggered alignment of band edges at the heterointerface can promote the spatial charge separation of electrons and holes in different parts of the heterostructure, which is useful for photovoltaic and photocatalytic applications. In type-II NPs, the synthesis should not produce a core−shell structure but rather an anisotropic dimer structure so that electrons and holes are accessible and are present in the distinct semiconductor phases.

The ion exchange reaction is superior in terms of the selective formation of a dimer structure with an epitaxial heterointerface. Several chemical methods such as seeded growth and ion exchange reactions have been developed for the synthesis of type-II semiconductor−semiconductor heterodimers. Several reviews are available on the fabrication of semiconductor heterostructures including semiconductor−semiconductor heterodimers that were produced using seeded growth15−17 and ion exchange.21 The ion exchange reaction is superior in terms of the selective formation of a dimer structure with an epitaxial heterointerface without changing the prototypical shape of the ionic semiconductor NPs. The complete and reversible cation exchange reactions of ionic semiconductor NPs were first reported by Alivisatos et al. in the reverse CdSe and Ag2Se system.22 Cation exchange in nanosized crystals is often complete within seconds, and the crystal shape is likely conserved because of the large surface-to-volume ratio and lower activation barriers for the diffusion of smaller cations compared to that of anions. For example, we used Cu7S4 nanodisks as good templates to synthesize CdS nanodisks.23 Cation exchange from Cu7S4 to CdS occurred upon the addition of Cd2+ and tri-n-butylphosphine, which binds strongly to the soft Cu+ cations.24−26 Figure 3 shows TEM images of various size Cu7S4 nanodisks before and after cation exchange. The shape of the Cu7S4 nanodisks was maintained after cation exchange, resulting in formation of monodisperse CdS nanodisks. A lattice expansion of 6% in terms of diameter was observed, which agrees well with the calculated value of 7%. The versatility of cation exchange with regard to accessing NPs with covalent lattices has been recently demonstrated by the cation exchange

Figure 3. TEM images of the Cu7S4 nanodisks (a−c) before and (d−f) after the cation-exchange reactions. The diameters are (a) 5.4 ± 0.5, (b) 9.6 ± 0.6, (c) 13.7 ± 0.8, (d) 5.9 ± 0.6, (e) 9.7 ± 0.7, and (f) 14.7 ± 0.9 nm. Reprinted with permission from ref 23. Copyright 2012, Wiley VCH.

of cadmium pnictide NPs with group 13 cations to yield III−V semiconductor NPs.27 These NPs have proven difficult to synthesize by the conventional direct synthesis. CdS−Cu2−xS Heterodimers. A type-II thin film solar cell consisting of Cu2S and CdS films was developed more than 50 years ago. The Cu2S film with a band gap of 1.21 eV absorbs most of the incident solar energy, and the photoexcited electrons are transferred into the conduction band of the CdS film. However, the diffusion of Cu+ ions into the CdS phase leads to a drastic decrease in the overall solar energy conversion efficiency. Despite this, recent progress in nanoscale inorganic 2868



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Figure 4. (a) High-resolution TEM image showing an example heterojunction after redispersion. The left inset shows the simulated CdS structure, providing evidence that the higher-contrast portion of the rod is CdS while the lower-contrast portion of the rod is the lower-symmetry Cu2S. The right inset shows the energy-filtered TEM image of redispersed nanorods, with the energy filter positioned at the Cu-M1 edge. The scale bars in the panel are 5 nm. (b) Schematic of parallel nanodiode or nanophotodiode arrays and the bulk energy band alignment of our material system. Selfassembled colloidal CdS nanorods are partially converted (top ends only) to Cu2S using our in-film cation exchange procedure. Reprinted with permission from ref 28. Copyright 2011, American Chemical Society.

w-CdS by the partial cation exchange of preformed hexagonalshaped Cu1.94S nanodisks (41.7 ± 3.3 nm in diameter and 15.9 ± 1.0 nm in thickness) with a Cd precursor.32 The carrier dynamics of the CdS−Cu1.94S nanodisks using ultrafast laser spectroscopy revealed that the carrier separation is based on donor−acceptor charge transfer and that ultrafast electron transfer takes place from the Cu1.94S phase to the CdS phase with a charge transfer time of 0.80 ps. Recently, we synthesized type-II CdS−Cu31S16 heterodimers (8.3 ± 1.2 nm long and 6.6 ± 0.9 nm wide) by the selective in situ seeded growth of the Cu31S16 phase on preformed hexagonal CdS NPs (Figure 5),33 as has been observed in the PdSx− Co9S8 system.34,35 These studies were carried out to investigate the charge separation behavior that depends on the copper vacancies of the Cu2−xS phase (x = 0.0625 for Cu31S16).

material synthesis has led to renewed interest in type-II semiconductor solar cells. Alivisatos et al. were the first to apply cation exchange to the fabrication of a dense array of perpendicularly aligned Cu2S− CdS heterodimer nanorods for high-performance photoelectric conversion devices, as shown in Figure 4.28 A controlled structure of wurtzite (w) CdS nanorods was first fabricated using self-assembly, and then, the cation exchange of Cd2+ ions with Cu+ ions in f ilm was carried out by soaking the aligned heterodimer nanorod film in a methanolic salt solution of [MeCN]4CuIPF6 in a dry inert atmosphere to produce an array of oriented CdS−Cu2S heterodimer nanorods. In a vertically aligned, close-packed superlattice of w-CdS nanorods on a substrate cation access is limited to only the top end of the individual nanorods, ensuring that exchange only takes place from one end of the nanorod and that all heterodimer nanorods are oriented in the same direction. Unfortunately, the photovoltage produced by this device is rather low (0.13 V) compared with its theoretical value (∼0.8 V)29 or its practical value (∼0.45 V).30 The low photovoltage could be the result of high recombination band alignment that differs from that observed in the bulk or insulating ligand layers. To overcome the poor performance of photoelectric conversion devices, the recombination of photogenerated electrons and holes needs to be suppressed. Because defects greatly accelerate recombination of the excitons in the light-absorbing phase (Cu2−xS); both the exciton lifetime and the spatial charge separation in CdS−Cu2−xS (x > 0) heterodimers are affected. The expected efficient separation of photogenerated carriers has been observed with charge separation times of approximately 0.5 ps in type-II semiconductor heterostructures. Cu2S NPs possess exciton lifetimes longer than 0.5 ps, although the nonstoichiometric Cu2−xS (x > 0) NPs that were transformed by Cu2S NP oxidation showed a decrease in the exciton lifetime. This decrease was correlated to photoluminescence (PL) quenching upon near-IR localized surface plasmon evolution.31 Therefore, an investigation into the effect of copper vacancies on the photoinduced carrier dynamics of the CdS−Cu2−xS (x > 0) heterodimers is important for the practical application of Cu2−xS-containing semiconductor−semiconductor heterodimers. In 2011, Han et al. successfully synthesized disk-shaped heterodimers consisting of monoclinic Cu1.94S (x = 0.06) and

Figure 5. (a) TEM image, (b) nanospot EDX results, (c) HRTEM image, and (d) schematic of the CdS−Cu31S16 heterodimer. Reprinted with permission from ref 33. Copyright 2013, The Royal Society of Chemistry. 2869



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The interfacial lattice plane was determined to be the w-CdS (100) plane. HRTEM revealed that the (110) planes of the w-CdS phases are aligned parallel to the (046) planes of the monoclinic Cu31S16 phases, both of which are perpendicular to the heterointerfaces. The excitonic Bohr radius for the muchstudied CdS is 3 nm, while that for Cu2−xS is calculated to be about 3−5 nm if the relative dielectric constant for copper sulfides is assumed to be about 10−15.36 Considering these excitonic Bohr radii, it is reasonable to speculate that the resulting CdS−Cu31S16 heterodimers, in which two distinct phases have a similar size, possess type-II band alignment. The CdS−Cu31S16 heterodimers were found to have an interband transition of the CdS and Cu31S16 phases at high energy (800 nm was assigned to the localized surface plasmon resonance of the vacancy-doped Cu31S16 NPs because the copper defects in Cu2S create free carriers (positive holes).31,37 Note that Cu1.94S or CdS−Cu1.94S nanodisks with the same crystal structure did not show a clear localized surface plasmon band.31 This infers that the copper vacancies are introduced more efficiently in the Cu31S16 phases because of the difference in the surfactant and/or the exposed facet.37 PL of the CdS−Cu31S16 heterodimers was almost completely quenched in the UV−vis region, probably indicating electron or energy transfer between CdS and Cu31S16, and this was also observed in CdS−Cu1.94S nanodisks.32 Transient absorption measurements of the Cu31S16 NPs and the CdS−Cu31S16 heterodimers were obtained using femtosecond laser flash photolysis to determine their exciton lifetimes. The exciton lifetime of Cu31S16 NPs (0.4 ± 0.1 ps) was significantly shorter than the previously reported value for Cu1.94S nanodisks and for Cu2S NPs. Photogenerated excitons in the Cu31S16 NPs are quickly consumed through an Augertype charge recombination (a vacancy-induced quick recombination of an electron and a hole takes place, followed by the energy transfer to a third carrier, an electron in the conduction band) and/or energy transfer to a localized surface plasmon.31 The exciton lifetime of the Cu31S16 phase in the CdS−Cu31S16 heterodimer was found to be similar to that of the Cu31S16 NPs (0.3 ± 0.1 ps). This indicates that the photogenerated excitons are predominantly consumed by vacancy-induced decay processes (i.e., Auger-type charge recombination and/or energy transfer to a localized surface plasmon) in the identical Cu31S16 phase rather than undergoing electron transfer to the CdS phases. Considering that electron transfer occurs from Cu1.94S to CdS in the CdS−Cu1.94S nanodisks, it is interesting and important that slightly more copper vacancies in the Cu31S16 phase of the heterodimer suppressed electron transfer from Cu31S16 to CdS. A theoretical calculation indicates that the band structure of Cu2−xS (x > 0) should not significantly change upon such a slight increase in vacancies.38 The only remarkable change should be an increase in the evolution of localized surface plasmon resonance in the Cu31S16 phase of the heterodimer compared with that of the Cu1.94S phase of the CdS−Cu1.94S nanodisk. Therefore, it was concluded that the dominant exciton quenching process in the CdS−Cu31S16 heterodimer is an ultrafast energy transfer to the localized surface plasmon rather than electron transfer to the CdS phase. Consequently, vacancies in the light-absorbing phase were found to determine the carrier dynamics of the semiconductor heterodimers. To improve the charge separation efficiency of the semiconductor heterodimers, it is thus important to use a narrow-band-gap semiconductor with fewer vacancies as light absorbers. The presence of localized surface plasmon resonance

in the NIR region derived from vacancies would give an indication of whether charge separation takes place.

Vacancies in the light-absorbing phase were found to determine the carrier dynamics of the semiconductor heterodimers. CdS−CdTe Heterodimers. CdTe is an ideal absorbing layer for solar cells because it has a direct band gap of 1.5 eV, which almost perfectly matches to the solar spectrum. Type-II CdS− CdTe-based solar cells that have a simple heterojunction design, such as p-type CdTe matched with n-type CdS, have achieved an efficiency of 16.5%.39 These types of solar cells have been used extensively and are still widely used. Precise volume control of both epitaxially attached phases may further improve efficiency. We recently investigated the formation and spatial charge separation efficiency of nanoscale CdS−CdTe heterodimers with epitaxial heterointerfaces in which the lightabsorbing CdTe phase usually possesses an exciton lifetime in the nanosecond time frame. As previously mentioned, the uniform heterodimer structure can be obtained in a facile manner by the partial transformation of NPs by ion exchange. In general, ionic NPs usually exhibit high ion exchange rates relative to their bulk form because of their larger surface-to-volume ratios. Compared with cation exchange, anion exchange, wherein the anions are exchanged in a cationic crystal while the cationic framework remains intact, requires high reaction temperatures and/or long reaction times because of the larger radii of the anions. This leads to the formation of poor-quality products, although the slow reaction can provide uniform heterostructures with the desired volume fractions by the selection of an adequate reaction temperature and time. As shown in Figure 6, type-II CdS−CdTe heterodimers were obtained by the anion exchange of ionic 10 nm w-CdS NPs with tri-n-octylphosphine telluride (TOPTe) as an intermediate during the formation of the completely exchanged product, CdTe NPs.40 The CdS−CdTe heterodimers were composed of thermodynamically stable w-CdS and zinc blende (zb) CdTe crystals, and the large strain at the heterointerface induced spontaneous phase segregation. The CdS− CdTe heterodimers had only one heterojunction because of the lack of inversion symmetry along the c axis of the w-CdS.41 Detailed STEM observations indicate that the w-CdS(001)/ zb-CdTe(−1−1−1) interface is more stable than the w-CdS(00−1)/zb-CdTe(111) interface. This leads to the formation of only one CdS−CdTe heterojunction in a single heterodimer. The preferential formation of the w-CdS(001)/ zb-CdTe(−1−1−1) interface could be explained by the surface free energy of the CdTe phase. Type-II band-edge alignment between CdS and CdTe allows the realization of spatial carrier separation in a heterodimer in which photoexcited electrons and holes are confined within the CdS and CdTe phases, respectively.42−45 To understand the photoexcited carrier dynamics in the CdS−CdTe heterodimers, femtosecond transient absorption spectroscopy measurements were carried out (Figure 7a). The pump pulse was tuned to 600 nm to photoexcite only the CdTe phases. The two bleaching peaks at 490 and 680 nm observed with a >2 ps delay were associated with the CdS and CdTe exciton transitions upon state filling, respectively. Because the 600 nm 2870



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CdTe phase to the CdS phase. Temporal changes in the transient absorption signals at the CdTe (680 nm) and CdS (490 nm) transitions within 3 ps after photoexcitation (Figure 7c) revealed an ultrafast decay for CdTe bleaching, which was accompanied by a subsequent increase in CdS bleaching. The relatively slow increase in CdS band-edge transition bleaching arises from electron transfer from the conduction band of CdTe to that of CdS. Because of the transient absorption signals resulting from the red shift of the CdS transition at very short delay times (0.2 ps), which could be because of the Stark effect,46,47 an electron-transfer time of ∼440 fs from the increase in CdS bleaching (Figure 7c) was evaluated. This electron-transfer time is approximately equal to the initial fast decay time of CdTe bleaching. From the ratio of the amplitude of the initial fast decay component to that of the slow decay component in the CdTe bleaching signal, the charge-transfer efficiency from CdTe to CdS was estimated to be 50%. This estimation gives the minimum value of the transfer efficiency because the state-filling-induced bleaching signal also includes a signal from the holes that remain in the CdTe valence band. On the other hand, the transient absorption spectra of the mixture of CdTe and CdS NPs showed bleaching of the CdTe bandedge transition but no bleaching of the CdS band-edge transition (Figure 7b). This clearly indicates that the CdS−CdTe heterointerface effectively produces a photoexcited spatial charge separation. The different polar characteristics of the w-CdS (001) and (00−1) surfaces leads to one-sided growth of the zb-CdTe phase, which can result in one-directional charge transfer. The CdS−CdTe heterodimers actually exhibit a spatial charge separation at the heterointerface, and this could be applied in the fields of photovoltaics and photocatalysis. The slow anion exchange reaction can be used to easily control the volume fractions of heterodimers by the selection of appropriate reaction temperatures and/or reaction times, which would allow for delicate band alignment engineering for efficient vectorial charge separation. Future Prospects for Charge Separation in Semiconductor Heterodimers. Group I−III−VI semiconductors such as copper indium selenide (CIS), copper gallium selenide (CGS), and their solid solutions like copper indium gallium selenide (CIGS) are candidates for use as nontoxic light-absorbing layers for solar cells. As previously mentioned, it should be noted that a certain amount of defects predominantly promotes

Figure 6. (a) HAADF-STEM and (b) BF-STEM images of CdS− CdTe heterodimers. (c) Atomic-resolution HAADF-STEM image of a single CdS−CdTe heterodimer. (d) Z-contrast profile of anion/cation pair columns in the blue rectangular region in (c). (e,f) Magnified images of the yellow and red rectangular regions for (e) w-CdS and (f) zb-CdTe phases in (c), respectively. Scale bars = 1 nm. (g) Illustration of the w-CdS/zb-CdTe heterointerface. Reprinted with permission from ref 40. Copyright 2011, American Chemical Society.

pump pulse only excites electrons in the CdTe phases, state filling in CdS should be induced by electron injection from the

Figure 7. (a,b) Transient absorption spectra of (a) CdS−CdTe heterodimers and (b) a mixture of w-CdS and zb-CdTe NPs (λpump = 600 nm). (c) Kinetics of CdTe and CdS state-filling signals in (a) during the first 3 ps after photoexcitation. Reprinted with permission from ref 40. Copyright 2011, American Chemical Society. 2871



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for Scientific Research (A) (T.T., No. 23245028) and a Grantin-Aid for Young Scientists (B) (M.S., No. 23710121).

A slow anion exchange reaction can easily be used to control the volume fractions of heterodimers by selecting appropriate reaction temperatures and/or reaction times.

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the recombination of photogenerated excitons before spatial charge separation in semiconductor heterodimers. First, the optimal combination and volume ratio of the two phases of interest need to be determined to obtain the highest charge separation efficiency by transient absorption measurements. Second, the semiconductor heterodimers need to be aligned in the same direction between the electrodes for photovoltaic applications. The self-assembly of rod-shaped large w-CdS NPs48 in the [001] direction by use of their permanent dipoles or templates such as anodic aluminum oxide followed by ion exchange from a specific side of the CdS NPs is one approach. Another promising candidate is the heterostructure with the combination of semiconductor and metal, where the metal phase-sensitized electron transfer to the conduction band of the semiconductor phase takes place,49−52 as observed in dyesensitized solar cells. In both cases, reducing the interface resistance between the nanostructure and the electrode is the biggest challenge for the efficient extraction of photogenerated carriers from the heterodimers, and a solution might be found in electronic interactions between organic π and inorganic orbitals.53−55

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +81-774-38-3120. Fax: +81-774-38-3120. Notes

The authors declare no competing financial interest. Biographies Toshiharu Teranishi received his Ph.D. from The University of Tokyo under the direction of Prof. N. Toshima in 1994. He spent 7.5 years at the Japan Advanced Institute of Science and Technology as an Assistant Professor and as an Associate Professor. He became a Full Professor at the University of Tsukuba in 2004 and moved to Kyoto University in 2011. Current research interests include precise structural control of inorganic nanomaterials and structure-specific functions for high-performance devices and photoenergy conversion. Website: http://www.scl.kyoto-u.ac.jp/~teranisi/index_E.html Masanori Sakamoto received his Ph.D. from Osaka University under the direction of Prof. T. Majima in 2005. He spent 4 years at Osaka University, The Institute of Scientific and Industrial Research as an Assistant Professor. In 2009, he joined the research group of Prof. T. Teranishi at the University of Tsukuba and moved to Kyoto University in 2012. Current research interests include the formation of suprastructures composed of organic molecules and inorganic nanoparticles, functional composite materials for high-performance devices, and photoenergy conversion.

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ACKNOWLEDGMENTS We thank Dr. Furube (National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan) for his advice. This work was supported by a KAKENHI Grant-in-Aid 2872



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