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Solution Processed Bismuth Sulfide Nanowire Array Core/Silver Sulfide Shell Solar Cells Yiming Cao, María Bernechea, Andrew Maclachlan, Valerio Zardetto, Mariadriana Creatore, Saif A. Haque, and Gerasimos Konstantatos Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b00783 • Publication Date (Web): 24 Apr 2015 Downloaded from http://pubs.acs.org on May 3, 2015

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Chemistry of Materials

Solution Processed Bismuth Sulfide Nanowire Array Core/Silver Sulfide Shell Solar Cells Yiming Cao†, María Bernechea†, Andrew Maclachlan‡, Valerio Zardetto§, Mariadriana Creatore§, Saif A. Haque‡, Gerasimos Konstantatos*† †

ICFO-The Institute of Photonic Sciences, Mediterranean technology Park, 08860 Castelldefels, Barcelona, Spain

‡

Centre for Plastic Electronics and Department of Chemistry, Imperial College London, South Kensington Campus, Exhibition Road, SW7 2AZ, U.K. §

Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, Eindhoven, The Netherlands

ABSTRACT: Low bandgap inorganic semiconductor nanowires have been served as building blocks in solution processed solar cells to improve their power conversion capacity and reduce fabrication cost. In this work, we first reported bismuth sulfide nanowire arrays grown from colloidal seeds on a transparent conductive substrate via mild aqueous chemistry, and demonstrated a novel core-shell nanowire architecture to enhance the photovoltaic performances of hybrid solar cells. We found that the bismuth sulfide nanowire core/silver sulfide shell structure reduces the interfacial charge recombination between the core and a hole transporter layer, and enables efficient charge separation in a type-II core-shell heterojunction. The bismuth sulfide nanowire core/silver sulfide shell combining with spiro-OMeTAD reached solar-toelectricity power conversion efficiency of 2.5 %, advancing the field of solution processed solar cells based on environmentally friendly metal chalcogenide semiconductors.

1. Introduction Solution processed solar cells have relied on inorganic semiconductor nanostructures, such as wires1 and colloids2, as building blocks with a view to enhancing the power conversion efficiency and reducing the fabrication cost. Inorganic semiconductor nanowires have demonstrated improved charge collection efficiencies and have been considered as a promising architecture for high efficiency solar cells1a,1b. So far, various low bandgap inorganic semiconductor nanowires, including CdX (X= S, Se, Te)3,1a,4-6, GaAs prepared via metal-organic chemical vapour deposition7, and silicon fabricated through metal ion-assisted aqueous electrolysis etching of crystalline wafer8 have been applied in efficient solution processed solar cells. The future large scale production of photovoltaic cells, however, requires inorganic semiconductors with advantageous properties: low toxicity, earth abundance, and facile fabrication. In nanowire-based solar cells, interfacial charge recombination also governs the yield of charge collection. Reducing the recombination has partly relied on the conception of all-inorganic nanowire core-shell structure presenting in Scheme 1a. Typically, as shown in Scheme 1b, an ultrathin alumina coating layer on ZnO nanowires via

atomic layer deposition (ALD) has been used as an insulating barrier in dye-sensitized solar cells to increase elec-

Scheme 1. (a) All-inorganic nanowire core-shell structure in solar cells to improve carrier transport and to reduce interfacial charge recombination. (b) An ultrathin alumina film coating on a wide bandgap semiconductor nanowires, such as ZnO, depresses the interfacial charge recombination, but retards electron injection dynamics in dye-sensitized solar cells9.

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(c) Low band gap inorganic semiconductors coating on ZnO nanowire core1f both prohibit electrons within nanowires recombining with a hole transporter, and generate charges upon light absorption. (d) As compared with that in (c), the nanowire core-shell structure with binary low bandgap semiconductors and with type-II heterojunction is more advantageous: the nanowire core transports carriers and, unlike ZnO, generates charges upon harvesting low energy photons; the shell accepts photogenerated holes in the core to avoid electron-hole pair recombination, apart from injecting electrons and prohibiting interfacial charge recombination. tron lifetimes within nanowires and to improve opencircuit photovoltage (Voc)9. On the other hand, the alumina shell reduces electron injection dynamics more than retards interfacial charge recombination, deteriorating short-circuit photocurrent density (Jsc). However, low bandgap chalcogenide semiconductors wrapping on ZnO nanowires (Scheme 1c), by assembling chalcogenide nanoparticles1f or via successive ionic layer adsorption reaction (SILAR)10, have dual functions: preventing electrons within nanowires from recombining with a hole transporter and, advantageously, generating charges by absorbing low energy photons. We expected an all-inorganic nanowire core-shell structure with binary low bandgap semiconductors and with type-II heterojunction, as described in Scheme 1d, to be a promising building block in solution processed solar cells lying at the following factors: (i) the low bandgap nanowire core both transports carriers and, unlike ZnO, produces charges upon harvesting low energy photons; (ii) the photosensitive shell not only prohibits electrons within nanowire recombining with a hole transporter, but accepts photogenerated holes from the core, where electron-hole pair recombination is thereby inhibited. In this work, we first reported the synthesis of an environmentally friendly solution processed bismuth sulfide nanowire arrays and demonstrated the interface control by coating the nanowires with SILAR-deposited silver sulfide to improve the photovoltaic performances of hybrid solar cells. Our bismuth sulfide nanowire arrays, as distinct from reported one-dimensional bismuth sulfide1116 , are grown from colloidal seeds on a transparent conductive substrate in 55 °C aqueous chemical bath. The resulted bismuth sulfide nanowire arrays have favorable orientations and are capable of fabricating hybrid solar cells by infiltrating a hole transporter, such as 2,2̕,7,7̕tetrakis(N,N-di-p-methoxyphenylamine)-9,9̕spirobifluorene (spiro-OMeTAD) used in this work. We found that bismuth sulfide nanowire arrays combining with spiro-OMeTAD performed speedy interfacial charge recombination. To overcome this we herein introduced the nanowire array core-shell structure. The bismuth sulfide nanowire arrays coated by silver sulfide shell show a power conversion efficiency (PCE) of 2.5 % under simulated AM1.5G 100 mW cm−2 illumination conditions and

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external quantum efficiency (EQE) of 70 % across the visible and near infrared, advancing the inorganic-organic hybrid photovoltaics based on bismuth sulfide17 or silver sulfide18. We attributed this advancement mainly to the depression of interface charge recombination between the nanowire core and spiro-OMeTAD layer, and to efficient charge separations at nanowire core/silver sulfide shell type-II heterojunction. 2. Experimental section Synthesis of bismuth sulfide nanocrystalline seeds. The synthesis of bismuth sulfide nanocrystals using standard Schlenk technique was performed according to the reported method19. Briefly, 2.8 mmol of bismuth acetate (Bi(OAc)3) and 34 mmol of oleic acid were heated and degassed under vacuum at 100 °C overnight, and the reaction temperature was raised to 170 °C. When the reactions flask reached 170 °C, 0.9 mmol of hexamethyldisilathiane mixed with 5 ml of 1-octadecene was quickly injected to the flask, and the heating temperature lowered to 100 °C. After 2 h at 100 °C, the reaction was quenched by removing the heating mantle and by adding 10 mL of toluene and 20 mL of methanol. The oleate-capped bismuth sulfide nanocrystals were isolated after centrifugation. Purification of the nanocrystals was performed by successive dispersion and precipitation in toluene and methanol. Finally, the nanocrystals were dispersed in anhydrous toluene. Preparation of bismuth sulfide nanocrystalline seed layer. Bismuth sulfide nanocrystalline seeds were deposited on a TiO2-coated FTO substrate by dip-coating process. A thin layer of TiO2 was coated on the FTO substrates by hydrolysis of 40 mM of TiCl4 in aqueous solution at 70 °C for 50 min. To grasp the as-synthesized bismuth sulfide nanoparticles onto the substrate, the TiO2/FTO was immersed in 0.24 M of 1, 2-ethanedithiol (EDT) of acetonitrile solution for 40 s, rinsed with acetonitrile, and dried with N2 flow. The EDT-modified substrate was dipped in 2 mg/mL of bismuth sulfide nanoparticle of toluene solution for 40 s, rinsed with toluene, and dried with N2 flow. Growth of bismuth sulfide nanowire array. Bismuth sulfide nanowire arrays were grown from the seeds via mild aqueous chemistry. The aqueous bath solution was prepared by mixing 1 mL of 0.2 M Bi(NO3)3 solution and 6 mL of 0.1 M thiourea solution. The Bi(NO3)3 solution was prepared by dissolving 0.097 g of Bi(NO3)3·5H2O in 0.2 mL of concentrated nitric acid and then diluting the volume to 1.0 mL with deionized water. The as-prepared seeded substrate was immersed in the 55 °C aqueous bath solution. After growth, the solution was cooled down to room temperature, and the bismuth sulfide nanowire film was rinsed with deionized water and ethanol and dried with N2 flow. Fabrication of bismuth sulfide core/alumina shell structures by ALD. The alumina coating layer has been deposited by ALD technique using TMA (Al(CH3)3) as


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metal source and water as oxidant. The deposition was carried out in the commercial Oxford instrument reactor, FlexAlTM. The substrate temperature was at 150 °C. Each cycle of ALD was performed by 40 ms of TMA dosing, 5 s of TMA purging, 250 ms of water dosing, and 20 s of water purging, yielding a thickness of 0.45 nm alumina coating layer. Fabrication of bismuth sulfide core/silver sulfide shell structures by SILAR. The core-shell structures were prepared by SILAR method. Briefly, the as-grown bismuth sulfide nanowire film was immersed in 0.1 M AgNO3 of ethanol solution for 20 s, rinsed with ethanol, and dried with N2 flow; then the film was dipped in 0.1 M Na2S·9H2O aqueous solution for 40 s, rinsed with deionized water, and dried with N2 flow. The above process was repeated to get desired structures. Device fabrication and characterization. Before spin coating spiro-OMeTAD, we annealed the core-shell films at 100 °C for 5 min in N2 glovebox and soaked the films in 0.24 M EDT of acetonitrile solution for 20 min. The spiroOMeTAD solution was prepared by dissolving 60 mg spiro-OMeTAD in 500 µL cholorobenzene via heating at 80 °C in the glovebox for 30 min; then 8.33 µL of lithium bistrifluoromethanesulfonimidate (LiTFSI) solution (85 mg/mL of LiTFSI in acetonitrile) and 24.4 µL 4tertbutylpyridine were added into the spiro-OMeTAD solution. The as-prepared spiro-OMeTAD solution was put on the bismuth sulfide nanowire or core-shell films and kept static for 30 sec before spinning at 2000 rpm for 30 sec. The spiro-OMeTAD infilltrated films were annealed at 90 °C for 15 min in air. A 10 nm-thick MoO3 followed by 150 nm-thick Ag was deposited on top of the films by a Kurt J. Lesker Nano 36 system at a base pressure lower than 5 × 10–6 mbar. Keithley 2400 source meter equipped with Newport 9600 records current-voltage characteristics of devices under simulated AM 1.5G illumination conditions; Keithley 2400 source meter equipped with New port Cornerstone 260 monochromator measures external quantum efficiency. Transient absorption spectroscopy (TAS) measurements. The TAS samples were prepared on the glass slides coated with a thin compact layer of TiO2 from thermal hydrolysis of TiCl4, which improves the adhesion of bismuth sulfide nanowire films. During TAS measurements, the samples were excited by a dye laser (Photon Technology International Inc. GL-301) pumped by a nitrogen laser (Photon Technology International Inc. GL3300). A 100 W quartz halogen lamp (Bentham, IL, 1) with a stabilizer power supply (Bentham, 605) was used as a probe light source. The probe light passing through the sample was detected by silicon photodiode (Hamamatsu Photonics, S1722-01), from which the signal was amplified before being passed through electronic band-pass filters (Costronics Electronics). The amplified signal was collected with a digital oscilloscope (Tekeronics, DPO3012), which was synchronized with a trigger signal from the

pump laser pulse from a photodiode (Thorlabs Inc., DET210). XRD, XPS, and UPS characterizations. The XRD measurements were performed with PANalytical X’Pert PRO MPD Alpha1 system coupled with Cu Kα, (λ = 1.5406 Å) at 45 KV/40 mA. XPS and UPS measurements were performed with a with a SPECS PHOIBOS 150 hemispherical analyzer (SPECS GmbH, Berlin, Germany) in ultrahigh vacuum conditions (10–10 mbar). XPS measurements were performed with a monochromatic Kα x-ray source (1486.74 eV) and UPS measurements with monochromatic HeI UV source (21.2 eV). 3. Results and discussion The bismuth sulfide nanowire arrays were synthesized, analogously to the growth of ZnO nanowires20,21, from bismuth sulfide nanocrystalline seeds on a transparent conductive substrate in a 55 °C aqueous bath solution, as schematically depicted in Scheme 2. A TiO2-coated fluorine-doped tin oxide (FTO) substrate (Figure 1a) was dipped into 2 mg/mL bismuth sulfide colloidal solution to produce a seeded substrate (Figure 1b). The seeds (Figure 1e) form crystalline nanowires (Figure 1f) in the aqueous bath by reacting bismuth ions, which are released from a bismuth-thiourea complex, with bisulfide, which is provided by decomposing thiourea14. Figure 1c shows that, at 120 min growth, the short nanowires randomly lay on the substrate. Oriented bismuth sulfide nanowire arrays were obtained by elongating the growth time: 330 min growth yields nanowires (Figure 1d) with favorable orientations and length of about 240 nm (Figure S1). For the solar cell studies, benefiting from low bandgap and high absorptivity of bismuth sulfide22, we focused on bismuth sulfide nanowires with length of ~150 nm, width of ~15 nm, and an average film thickness of 130 nm by 210 min growth showing in Figure 2a and 2b.

Scheme 2. Growth of bismuth sulfide nanowires from a seeded substrate via mild aqueous chemistry. X-ray diffraction studies performed on the bismuth sulfide nanowire arrays confirm their crystalline structure as presented in Figure S2 in the supplementary information. X-ray photoelectron spectroscopy (XPS) studies (Figure S3) show that the bismuth sulfide nanowire semiconductor is nonstoichiometric, with the ratio of Bi 4f to S2s content being 1:1.2, pointing to Bi rich nanowires. This is expected to yield a highly doped material. Indeed, ultraviolet photoelectron spectroscopy (UPS) characterization of the bismuth sulfide nanowires (Figure 3) manifests that the nanowires have a work function of 4.3 eV and an ionization energy of 5.6 eV. As the band gap of bismuth sul-



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fide nanowires is 1.3 eV, the calculated conduction band level is estimated to be 4.3 eV. The latter confirms that our bismuth sulfide nanowires are heavily doped due to Bi richness.

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enhanced FF of 0.54, yet reduced Jsc of 0.66 mA cm–2. Manifestly, the alumina shells are capable of reducing the interfacial charge recombination, which results in improved Voc; a thick alumina shell, however, retards the injection of photogenerated holes in bismuth sulfide into spiro-OMeTAD, deteriorating Jsc. Thereby, the coating layer, which not only prevents electrons within bismuth sulfide nanowires from recombining with spiro-OMeTAD, but also accepts photogenerated holes in the nanowire, is highly desired for designing efficient solution processed solar cells. In this regard, we coated the nanowires with silver sulfide via SILAR. We chose silver sulfide in view of its advantageous properties of bandgap of ~1.0 eV24, high absorptivity (104 cm–1)25, high photostability26,27, and, importantly, the energy level alignments with bismuth sulfide.

Figure 1. Scanning electron micrograph of (a) a thin layer of TiO2-coated FTO substrate, (b) bismuth sulfide colloidal seeds on the substrate, and bismuth sulfide nanowire at (c) 120 min and (d) 330 min growth. Transmission electron microscopy of (e) bismuth sulfide colloidal seeds and (f) bismuth sulfide nanowire at 210 min growth.

We initially attempted to make solar cells by infiltrating bismuth sulfide nanowires with a hole transport layer, spiro-OMeTAD. The EQE spectrum of device presenting in Figure 4a shows an onset at around 950 nm as indicated by the bandgap of 1.3 eV, and reaches 6 % in the visible region. J-V characteristics of the device under simulated AM1.5G 100 mW cm−2 illumination conditions, as shown in Figure 4b, yield a poor PCE of 0.02 % with a Jsc of 0.46 mA cm−2, a Voc of 0.13 V, and a fill factor (FF) of 0.39. To supress interfacial charge recombination, as a potential recombination pathway in this structure, an ultrathin ALD-alumina interfacial layer has been typically employed as a recombination barrier9,23. In this respect, we deposited ALD-alumina film on the surface of bismuth sulfide nanowires. The ALD-alumina film homogeneously coating on the nanowires has a thickness of 0.45 nm for each cycle. With a 0.45 nm-thick shell, Jsc enhances to 0.93 mA cm–2, Voc improves to 0.20 V, FF increases to 0.44, and PCE approaches 0.08%; with 0.9 nm, PCE remains to be 0.08 %, by further improved Voc of 0.22 V,

Figure 2. Scanning electron micrograph of (a,b) bismuth sulfide nanowires at 210 min growth and bismuth sulfide nanowire core/silver sulfide shell structure fabricated from SILAR silver sulfide with cycles of 4 (c,d), 16 (e,f), and 32 (g,h). Scar bar: 500 nm.


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5

Intensity (10 counts)

(a)

sulfide coated nanowire film with merely 4 cycles of SILAR (Figure 2c,d) forms around 10 nm-thick homogeneous shell (Figure S4). Remarkably, the shell merges at nearby nanowires, forming an interconnected nanostructure. By increasing cycles from 16 (Figure 2e,f) to 32 (Figure 2g,h), silver sulfide successively grows and inevitably fuses, producing interconnected thick shells. The resulted novel core-shell structures are capable of infiltrating spiro-OMeTAD for efficient charge separation and inhibiting charge recombination between the nanowire core and hole transporter in hybrid solar cells (Figure S5).

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Bismuth sulfide nanowire coated by silver sulfide with n cycles of SILAR n=0 n=4 n=16 n=32

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5.61 Figure 3. (a) UPS of bismuth sulfide nanowires with and without SILAR silver sulfide. All the samples were applied with voltage of –3V during measurements to obtain the secondary electrons cutoff. Detailed presentations of UPS with high binding energies of secondary electron cutoff (b) and low binding energies of the onset (c) regions, which are obtained by the intercept of dashed yellow lines. (d) Energy level diagrams of bismuth sulfide nanowire arrays and the nanowire array core/silver sulfide shell structures deprived form UPS measurements.

Silver sulfide coating on the nanowires via SILAR evolves as a novel core-shell nanostructure. In comparison with the uncoated nanowire film (Figure 2a,b), silver

The work functions and ionization energies of coreshell structures can be mapped from UPS measurements28 showing in Figure 3a. The work function was calculated from the difference between the energy of the incident beam (He I, 21.2 eV) and the secondary electron cutoff energy (Figure 3b) as well as applied bias (−3V). The ionization energy was deduced by subtracting the energy of the incident beam with the difference in energy between the secondary electron cutoff and onset showing in Figure 3c. The conduction band level can then be inferred by taking the ionization energy from the UPS measurements together with the bandgap of the material. The silver sulfide shells with different cycles of SILAR slightly differ in ionization energies (5.19−5.28 eV), conduction band energies (4.19−4.28 eV), and work functions (4.54−4.60 eV), as displayed in Figure 3d. The work functions of shells are at least 250 meV downward shift with respect to that of uncoated bismuth sulfide nanowires. However, under equilibrium, the work functions of the core and shell align to create a built-in potential across the heterojunction forming a favorable energy offset to produce photogenerated electrons towards the bismuth sulfide nanowire cores and photogenerated holes towards the silver sulfide shells. Such type-II heterojunction can afford efficient charge separation at the core-shell interface. The evolution of EQE spectra of devices with different cycles of SILAR silver sulfide is displayed in Figure 4a. The EQE of device with 4 cycles rises at about 1200 nm, gradually improves to 25 % at 800 nm, and to over 60 % at 600 nm. Along with increasing cycles of SILAR, the EQE enhances in the near infrared region and reaches 50 % at 1000 nm with 32 cycles; while in the visible region, the peak values of EQE vary within 65-70 %, insensitive to the thickness of shells. From 400 to 600 nm wavelength, the silver sulfide shells boost EQE of devices by a factor of six higher than that of device based on uncoated bismuth sulfide nanowires; however, the shells enhance the absorbance of core-shell films merely by a factor of less than two (Figure S6). In this respect, we attributed the boosted EQE of devices with silver sulfide shells to efficient charge separation at the core-shell and shell/spiro-OMeTAD interface as evidenced by the following transient absorption spectroscopy measurements. J-V characteristics of the solar cells with silver sulfide shells were measured under simulated AM 1.5G 100 mW cm–2 intensity illumination conditions showing in Figure



with that of the bismuth sulfide nanowires without spiroOMeTAD (Figure S7), has stronger positive signals at 600 and 1600 nm, indicating the polaron absorption of spiroOMeTAD+, which is produced by spiro-OMeTAD accepting photogenerated holes from bismuth sulfide. The TAS of bismuth sulfide/silver sulfide (for example, 16 cycles)/spiro-OMeTAD shows an even stronger polaron absorption of spiro-OMeTAD+, implying more efficient charge separation at silver sulfide/spiro-OMeTAD interface; besides, a bleach signal appears at around 900 nm, which is attributed to the depopulation of ground states of silver sulfide. (a)

µ∆OD

4b. The device with 4 cycles of SILAR silver sulfide achieves a PCE of 1.6 % yielded by a Jsc of 11.33 mA cm–2, a Voc of 0.31 V, and a FF of 0.45, highlighting the advantage of silver sulfide shells as compared with ultrathin ALDalumina coating layers. The enhanced Voc benefits from the reduced charge recombination by the shell. As the cycle increases to 16, the Jsc, Voc, and FF improve to 15.52 mA cm–2, 0.34 V, and 0.47, respectively, yielding a PCE of 2.5 %. Even with a shell thickness of over 200 nm with 32 cycles, the device has a PCE of 2.2 %, with a Jsc of 16.00 mA cm–2, a Voc of 0.33 V, and a FF of 0.41. The slightly decreased Voc and FF are probably due to increased charge recombination in the bulk silver sulfide. The photovoltaic parameters of devices with cycles of SILAR from 4 to 32 by a step of four are listed in Table S1, manifesting the best device with 16 cycles. This best device, after aging for one month in air, kept over 90 % of its initial efficiency.

3

2.5

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bismuth sulfide/spiro-OMeTAD bismuth sulfide/silver sulfide/spiro-OMeTAD

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60 0

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1400

180 120

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800

40 20

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80

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0.1

0.2

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0.4

V (V)

Figure 4. (a) EQE spectra of devices with 0, 4, 16, and 32 cycles of SILAR silver sulfide coating on bismuth sulfide nanowire arrays. (b) J–V characteristics of devices with 0, 4, 16, and 32 cycles of SILAR silver sulfide coating on bismuth sulfide nanowire arrays. The devices were measured under sim–2 ulated AM 1.5G 100 mW cm illumination conditions.

To shed light on the underlying principles we studied the charge kinetic mechanisms at the interfaces formed in those structures: bismuth sulfide nanowire/spiroOMeTAD and bismuth sulfide nanowire/silver sulfide/spiro-OMeTAD. Upon excitation at 510 nm, which is absorbed by the bismuth sulfide and silver sulfide, the absorbance of photoinduced charges was probed after 10 µs over a range of wavelengths. As presented in Figure 5a, the TAS of bismuth sulfide/spiro-OMeTAD, as compared

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t (s) Figure 5. (a) Transient absorption spectroscopy of bismuth sulfide/spiro-OMeTAD and bismuth sulfide/silver sulfide (16 cycles)/spiro-OMeTAD. Smooth lines serve as guide to the eye. (b) Kinetic decay traces of bismuth sulfide/spiroOMeTAD and bimuth sulfide/silver sulfide (4, 16, and 32 cycles)/spiro-OMeTAD. The traces were probed at 1600 nm after 1 µs once at 510 nm excitation. All samples were corrected for the number of photons absorbed.

The kinetic traces of bismuth sulfide/spiro-OMeTAD and bismuth sulfide/silver sulfide (4, 16, and 32 cycles)/spiro-OMeTAD were probed at 1600 nm after 1 µs upon excitation at 510 nm, as displayed in Figure 5b. In comparison with that of bismuth sulfide/spiro-OMeTAD, the kinetic trace of bismuth sulfide/silver sulfide (4 cycles)/spiro-OMeTAD shows a slightly enhanced initial amplitude of ΔOD and half time (τ), which imply an efficient charge separation and sluggish charge recombination at silver sulfide/spiro-OMeTAD interface. This scenario, however, inadequately explains the boosted EQE spectrum of device with silver sulfide shells. We thereby attributed the improvement of EQE mainly to efficient charge separation at type-II bismuth sulfide/silver sulfide


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interface, where photogenerated holes in the nanowires are accepted by the shell to avoid electron-hole pair recombination. The kinetics traces of core-shell films with 16 or 32 cycles of SILAR silver sulfide shows an even higher initial ΔOD and a larger τ, probably due to the favorable change of shell morphology. It is interesting to note that films with 16 and 32 cycles of SILAR silver sulfide show negative signals starting at 100 µs, bottoming at 1 ms, and slowly recovering to zero after 100 ms, which could be related to the bleach of ground states of silver sulfide. 4. Conclusions In summary, we reported for the first time the synthesis of bismuth sulfide nanowire arrays from colloidal seeds on transparent conductive substrates via mild aqueous chemistry and employed this new form of bismuth sulfide to develop environmentally friendly panchromatic solar cells. We showed that the control of interface charge transfer plays a crucial role in device performance. The coating of silver sulfide onto the bismuth sulfide nanowires enables suppression of interface recombination between the nanowire core and the hole transporter layer, and simultaneously offers a type-II heterojunction that results in efficient charge separation between bismuth sulfide and silver sulfide. Our work, apart from introducing a novel bismuth sulfide nanowire array, provides important design guidelines by the introduction of coreshell nanowire architecture towards even higher device performance. At the same time, this methodology for growing bismuth sulphide nanowires may also find applications in the field of thermoelectrics.

ASSOCIATED CONTENT

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AUTHOR INFORMATION Corresponding Author

(6) Kang, Y.; Park, N.; Kim, D. Appl. Phys. Lett. 2005, 86, 113101.

*E-mail: [email protected]

SUPPORTING INFORMATION AVAILABLE The SEM, XRD, and XPS of bismuth sulfide, TEM of bismuth sulfide core/silver sulfide shell structure, FIB-SEM of devices, absorbance rational number of bismuth sulfide core/silver sulfide shell to uncoated bismuth sulfide, tables of photovoltaic parameters of devices with different cycles of SILAR, and TAS of bismuth sulfide. This information is available free of charge via the Internet at http: //pubs.acs.org/.

(7) Ren, S.; Zhao, N.; Crawford, S. C.; Tambe, M.; Bulović, V.; Gradečak, S. Nano Lett. 2011, 11, 408–413. (8) Shen, X.; Sun, B.; Liu, D.; Lee, S. J. Am. Chem. Soc. 2011, 133, 19408–19415. (9) Law, M.; Greene, L. E.; Radenovic, A.; Kuykendall, T.; Liphardt, J.; Yang, P. J. Phys. Chem. B. 2006, 110, 22652–22663. (10) Kim, H.; Jeong, H.; An, T. K.; Park, C. E.; Yong, K. ACS Appl. Mater. Interfaces 2013, 5, 268–275.

ACKNOWLEDGMENT We acknowledged Dr. Silke L. Diedenhofen for FIB-SEM measurement. The research leading to these results has received funding from Fundació Privada Cellex, European Commission's Seventh Framework Programme for Research under contract PIRG06-GA-2009-256355 and European Community's Seventh Framework program (FP7ENERGY.2012.10.2.1) under grant agreement 308997.

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Table of Contents (TOC)


Silver

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