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Colloidal nanocomposite of reduced graphene oxide and quantum dots for enhanced surface passivation in optoelectronic applications
Solar Energy Materials & Solar Cells 144 (2016) 181–186
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Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Colloidal nanocomposite of reduced graphene oxide and quantum dots for enhanced surface passivation in optoelectronic applications Anush Mnoyan a, Kyungmok Kim a, Jin Young Kim b, Duk Young Jeon a,n a Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305701, Republic of Korea b Fuel Cell Research Center, Korea Institute of Science and Technology (KIST), Hwarang-ro 14-gil 5, Seongbuk-gu, Seoul 136-791, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 1 May 2015 Received in revised form 30 August 2015 Accepted 1 September 2015
Colloidal graphene/PbS quantum dots (QDs) hybrid nanostructures are fabricated with chemical grafting in one-pot solution methods. In the hybrid nanocomposites, PbS QDs are decorated on the reduced graphene oxide (rGO) nanosheets (NSs). By employing X-ray photoelectron spectroscopy (XPS) analysis, it is shown that the rGO NSs are bonded to PbS nanocrystals through oxygen functional groups, leading to improved surface passivation and electrical conductivity in the hybrids. The results obtained by the recordings of time-resolved photoluminescence spectra, field effect mobility, and photovoltaic performance revealed that rGO grafted to PbS QD composite structures provided better charge transport by 16 times compared to PbS alone, which is attributed to suppressed charge recombination and improved interfacial charge transport processes. Thus, the developed hybrid photoactive film enhanced open circuit current (Jsc) and power conversion efficiency (PCE) by 12% and 14% correspondingly. & 2015 Published by Elsevier B.V.
Keywords: rGO/PbS nanocomposite QD-based solar cell QD passivation carrier transport
1. Introduction The formation of a colloidal QD composite with an appropriate host matrix can provide several advantages, such as the improved stability of QDs by the passivation effects of the matrix, and very efficient charge and energy transport between QDs with strong electronic interaction with the matrix [1,2]. In general, inorganic carbon matrices are known to have distinctive thermal and optical resistances compared with their organic or polymeric counterparts and could be good candidates for QD encapsulation [3–5]. Graphene-based composite materials with semiconductor QDs have shown to improve electrical and thermal conductivity [6–12]. The anchoring of semiconductor nanoparticles on graphene sheets potentially provides a new way to develop catalytic, magnetic, and optoelectronic materials. Encapsulation of semiconductor QDs inside multi-shell graphitic cages is usually achieved during carbon arc-discharge at high temperatures [13–15]. Similarly, interesting hybrid structures with carbon nanotubes [1,16] and graphitic oxide nanoplatelets [17] have been developed by incorporating semiconductor nanoparticles. Here, we demonstrate a facile method to prepare the composite structure of rGO NSs and PbS QDs hybrids via one-pot solution synthesis. Efforts have also been made to synthesize semiconductor n
Corresponding author. Tel.: þ 82 42 350 3337; fax: þ82 42 350 3310. E-mail address: [email protected] (D.Y. Jeon).
http://dx.doi.org/10.1016/j.solmat.2015.09.001 0927-0248/& 2015 Published by Elsevier B.V.
particles protected by rGO sheets and assemble them as thin films using layer-by-layer self-assembly technique. The ability of rGO sheets to react with cationic defect sites on the surface of QDs via oxygen containing functional groups has enabled us to develop chemical synthetic strategies and tailor their properties with chemical functionalization. Chemical grafting and ligand exchange were used to control the interface of rGO/PbS nanohybrids and the PbS QD interparticle distance, respectively. The effects of the controlled interface on the optical and electrical properties are presented here. We find that the surface functionalization of PbS QDs via the rGO grafting gives rise to efficient surface passivation and trap removal, significantly improving their charge transport and photovoltaic performance.
2. Experimental section 2.1. List of materials Lead oxide (99.999%), oleic acid (90%), 1-octadecene (90%), octane (anhydrous, Z99%), hexamethyldisilathiane (TMS, synthesis grade), 1,2-ethanedithiol (EDT, Z98%) were purchased from Sigma Aldrich for the synthesis of PbS QDs. Hexane and acetone used in QD purification were purchased from Merck. To prepare the reduced graphene oxide nanosheets the following list of materials were used such as natural graphite powder (Kanto Chemical, Z80%), potassium permanganate (Z99%), sulfuric acid (extra pure, 95%), phosphoric acid ( Z85%), hydrochloric acid
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(32%), hydrogen peroxide (35%), hydrazine monohydrate (98%), N, N-dimethylformamide (DMF, anhydrous, 99.8%). Zinc acetate dehydrate (Z98%), lithium hydroxide monohydrate ( Z98%) and absolute ethanol were used to prepare ZnO nanocrystal solution. To accomplish the device preparation gold (99.99%) and molybdenum (VI) oxide (metals basis, 99.9995%) is used as electrode materials. 2.2. Sol–gel synthesis of ZnO nanocrystals The fabrication of nanocrystalline (NC) ZnO was performed according to the method reported in literature [18] with certain modifications. For the first step, 0.1 M solution of organometallic Zn precursor was prepared by making a mixture of 2 mmol zinc acetate dehydrate with 20 ml of absolute ethanol in 50 ml round shape 1-neck flask. The flask was connected to the reverse condenser and placed into an oil bath to maintain the temperature at 80 °C under atmospheric pressure. The mixture was stirred with magnetic bar vigorously for 1 h until the transparent solution was obtained. Next, 2.8 mmol of lithium hydroxide monohydrate powder was added to the solution. Finally, the suspension was placed into an ultrasonic bath at room temperature to accelerate the release of OH ions, resulting in formation of stable solution of ZnO nanocrystals. The ZnO NCs were precipitated by adding hexane into the main solution with 1:3 volume ratio and centrifuged at 10,000 r.p.m. for 5 min. Then, the obtained ZnO nanocrystal precipitate was dispersed in anhydrous ethanol to obtain 80 mg/ml solution. The characteristics of ZnO NCs colloidal solution are presented in Supplementary material Figs. S1–S3. Finally, 0.20 μm pore size syringe filter was used to filtrate the NC solution before using it in PV cell structure. 2.3. Synthesis of reduced graphene oxide nanosheets The rGO NSs were initially synthesized by modified Hummer's method using natural graphite powder [19]. Obtained graphene oxide NSs after multiple purification, were exposed to hydrazineassisted reduction process to improve the conductivity by reducing the number of oxygen containing functional groups ( OH, H– C ¼O, O–C ¼O, and epoxy) on the surface [20]. The suspension of rGO NSs with a concentration of 1 mg/ml in DMF was used to fabricate rGO/PbS hybrid material. 2.4. Synthesis of rGO/PbS hybrid composite material The synthesis of rGO/PbS hybrid material was performed by the hot injection method that is based on previously reported synthesis route for PbS QDs [21] with certain modifications. For this purpose, preliminary prepared suspension of rGO NSs in DMF was injected into the reaction vessel as lead precursor was transformed to the lead oleate complex, then reaction mixture was degassed for 17 h in 95 °C. TMS solution in ODE, which was preliminary degassed for 12 h at 80 °C, was injected to the reaction mixture to burst the nucleation. The heating mantle was turned off as nucleation induced (solution color changed to dark brown), and the solution was cooled down naturally. After reducing the solution temperature to 38 °C the crystal growth process was inhibited by injection of 20 ml of distilled acetone. To purify as-prepared QD solution it was centrifuged three times in the mixture of toluene/ acetone (1:3 volume ratio) at the spin speed of 6000 r.p.m. for 5 min. After drying in a vacuum condition the obtained solid was dissolved in octane to obtain a solution with 60 mg/ml concentration and stored in N2 atmosphere in glove box for further use. A reference PbS QD solution was also prepared without adding graphene.
2.5. Material characterization The morphology and particle size of hybrid composite material was analyzed by transmission electron microscopy (JEOL, JEM3010 (300 kV)). The portion area of rGO covered by PbS QDs (PA, %) was estimated statistically among the same scaled TEM images of different 24 rGO/PbS samples. First we assumed that single QD projection area is matched to its covered area. As so, the integrated area of the QD covered rGO is calculated as a sum of projection area of all QDs ( ∑ QDA = n·πr 2, where n is the counted number of QDs, and r is the average radius of QD). Then this value is compared to the selected area of rGO NSs (rGOA) covered by those QDs as given in Eq. (1)
PA =
n · πr 2 ·100% rGOA
(1)
The X-ray diffraction analysis were carried out with RIGAKU, D/ MAX-2500 diffractometer (Cu Kα radiation). The device crosssectional view and the surface properties of all type of samples were by scanning electron microscope (Hitachi S-4800). The optical absorption features were investigated by Mecasys, Optizen POP, Korea, and the XPS analysis was carried out using Thermo VG Scientific, ESCALAB 200i spectrometer equipped with a microfocused monochromator X-ray source. The luminescence lifetime values were evaluated using FLP920 (Edinburgh Instruments) fluorescence decay analysis software. For low temperature measurement the liquid nitrogen cryostat was used in combination with diaphragm vacuum pump (Edwards, RH 10 9LW) to maintain 2 10 4 mbar vacuum condition. EPL-785, picosecond pulsed diode laser (785 nm) was used as a light source, and the signals were detected by NIR-PMT detector with the range of 300– 1400 nm. Samples were prepared on glass by drop casting in the case of OA-capped PbS and rGO–PbS films, or by layer-by-layer (LBL) deposition for EDT ligand exchanged ones. The fluorescent decay curve was fitted with triexponential decay fit. The goodness of fit was judged by χ2 (1 7 0.2) values. The average decay time (τavg) values of PbS QD and rGO/PbS composite films were analyzed using Eq. (2) [22]
τavg =
α1τ12 + α2 τ22 + α3 τ32 , α1τ1 + α2 τ 2 + α3 τ 3
(2)
where τ is the lifetime and α is the pre-exponential factor with subscripts 1, 2, and 3 representing various species. 2.6. Photovoltaic device fabrication ITO-coated glass substrates were processed to three step sonication for 30 min in methanol, acetone and isopropyl alcohol solvents. Then, freshly synthesized ZnO NCs solution was spin coated on top of ITO to make a film with 90 nm thickness, then annealed in open air at 260 °C for 30 min to improve film crystallinity. The next steps of device fabrication were performed in N2 filled glove box. On top of ITO/ZnO substrate the active layer was deposited by LBL spin coating of rGO/PbS hybrid material with EDT-assisted solid state ligand exchange procedure [23]. To perform the LBL deposition 4 drops of QD solution (60 mg/ml) for 20 s, 5 drops of EDT solution (1% vol. in acetonitrile), 9 drops of acetonitrile and 10 drops of toluene were spin coated for 5 s one by one at fixed spinning speed of 2500 r.p.m. Overall procedure results in formation of active layer with 280 nm thickness after 8 LBL cycle. Then, 10 nm of MoO3 layer and 50 nm of Au top electrode were deposited by thermal evaporator under high vacuum condition (10 7 Torr) with 0.5 Å/s and 1 Å/s deposition rate correspondingly.
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2.7. Device characterization All device characterizations were performed in N2 filled glove box. Current density–voltage (J–V) data were measured using Keithley 2400 sourcemeter. The devices were illuminated through the glass substrate using a 150 W xenon lamp with an AM1.5G filter (LS-150-Xe, Abet Technologies). The light intensity was adjusted to 100 mW/cm2 using a Si reference cell (BS-520, Bunko Keiki). The effective surface area of the fabricated unit cells was controlled to be 0.06 cm2.
3. Results and discussion rGO/PbS composites were formed with chemical grafting of rGO during the colloidal synthesis of organic ligand (e.g. oleic acid, OA)-passivated PbS QDs. A key to the grafting process is to dissolve both QDs precursor materials and rGO NSs in compatible solvents, allowing them to interact with each other under conditions that preserve the stability and photophysical properties of both components. We have treated rGO with hydrazine to modify oxygenfunctional groups to be capable of forming chemical bonds with the surface sites of PbS QDs. Thus, chemical grafting of two suspensions of rGO and PbS results in the binding of PbS QDs to rGO NSs. The relative amount of rGO in the preparation of the hybrid structure was determined to be 0.15%. These composites remain suspended in solution, minimizing aggregation effects. In order to confirm the absence of unbound QDs in the purified rGO/PbS QD hybrid structure, isolation of the free QD in the supernatant was attempted by gentle centrifugation that selectively precipitated rGO/PbS.
Fig. 2. Typical XRD spectra of PbS QDs, rGO/PbS hybrid material and rGO NSs.
Fig. 1 shows transmission electron microscopy (TEM) images of the as-prepared rGO/PbS composite structure, where the homogeneous and dense population of PbS QDs with a diameter of ∼3.1 nm linked to rGO NSs are observed. A high resolution TEM image further shows the crystalline nature of PbS QDs and the rGO NSs attached to the surface of PbS QDs in the composite, indicating successful integration of rGO and PbS with intimate interfacial contact. It should be emphasized that PbS QDs in the composite were observed with the same structure and similar particle size/ distribution as those for the preparation of the PbS QDs without the presence of rGO. This result strongly indicates that the presence of rGO plays a good passivating and stabilizing role in the
Fig. 1. Characteristic TEM images of (a) PbS QDs and (b) rGO/PbS hybrid material. Obviously, PbS QDs have 3.1 nm average size (c), and rGO/PbS hybrid has 2D structure where PbS QDs are well distributed on rGO surface without any agglomeration (d).
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formation of PbS QDs in the hybrids. Fig. 2 shows X-ray diffraction (XRD) patterns of the as-synthesized rGO/PbS composite. The diffraction peaks of the QDs alone and the rGO/PbS composite are both ascribed to a well-crystallized PbS with cubic structure. The XRD pattern of rGO presents a broad peak at 2θ ¼24.2° (002) corresponding to 3.69 Å interlayer distance. The peak from rGO is hard to be recognized in the XRD pattern of the rGO/PbS composite sample due to the relatively small concentration of rGO in the composite, the overlapping of diffraction peaks, and the broad shape of rGO main peak. In our preparation of the composite, the concentration of anchored PbS QDs to rGO NSs was tuned by controlling the amount of rGO addition in the synthesis. In particular, we attempted to maximize the interfacial contact of rGO NSs around PbS QDs, which is expected to be beneficial for the efficient surface passivation of PbS QDs. However, it is essential for films to achieve a balance between the sufficient supply of rGO for ample QD interfacial contact while also limiting rGO's detrimental filtering in the photo-induced carrier transport due to a loss of colloidal stability and light absorption. The portion of covered area of rGO NSs by PbS QDs in our optimal conditions is estimated to be ∼60–70% (the histogram over 24 samples is demonstrated in Supplementary Fig. S4). To understand the bonding nature of the PbS QD-grafted rGO, XPS spectra of PbS QDs with and without rGO grafting were obtained. We focus on the XPS spectra of Pb 4f peaks to more precisely identify the bond formed during chemical grafting. Fig. 3a shows the Pb 4f peaks for non-grafted PbS samples where the Pb bonding energies are well matched to the value from the bulk PbS state as determined with peak deconvolution [24,25]. In comparison, XPS spectra of the grafted rGO/ PbS reveals a new intermediate bonding energy. This can be assigned to the Pb–O bond connecting the PbS QDs and rGO NSs matrix via O-containing polar groups and Pb-rich (Pb2þ sites) surface of nonstoichiometric PbS QDs [26] forming close contact after grafting [12,27].
Fig. 3. XPS spectra comparison: (a) Pb 4f spectra of PbS QDs and rGO/PbS hybrid material, and (b) O 1s spectra of rGO/PbS hybrid material and rGO NSs.
Thus, the interaction between PbS QD and rGO via Pb–O bonds may lead to the additional passivation of the Pb cationic defect sites on the QD surface. The presence of Pb–O bonding can be further confirmed by the O 1s XPS peak at 530.4 eV in the spectrum which indicates the presence of residual oxygen containing groups bonded with Pb atoms in rGO (Fig. 3b). It is posited that the C/O ratio for rGO in the composite is larger than that of the as-prepared rGO from the oxygen species involved in Pb–O chemical bonds anchored on the surface of rGO, which may result in a large electronic conductivity in the rGO sheets. This highlights the potential for rGO sheets to serve as the conductive channels between PbS QDs, improving the overall transport in the solid-state. We sought to study the posited benefits to charge transport by comparing PbS QDs with and without rGO NSs both optically and electrically. The rGO/PbS nanocomposites were first deposited on substrates with layer-by-layer deposition and a ligand exchange with ethanedithiol (EDT). The use of spin casting in the composite film preparation resulted in homogenous films without any deterioration (see Fig. 3 in Ref. [28]). We examined photoluminescence decay by time-correlated single photon counting (TCSPC) technique which is a good indirect probe of the photo-induced carrier transport process in the assemblies. Fig. 4 compares the time-resolved photoluminescence spectra recorded following 785 nm laser pulse excitation of PbS QDs with and without rGO-grafting in films. The decay time values were measured at the peak position of the photoluminescence spectrum (see Fig. 2 in Ref. [28]). Note that the absorption feature of PbS QDs before and after the addition of rGO remained
Fig. 4. Normalized transient PL decay features of (a) oleic acid capped PbS and rGO/ PbS films measured at 1120 nm emission wavelength at room temperature and (b) EDT treated PbS and rGO/PbS films measured at 1360 nm emission wavelength at 77 K conditions.
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Table 1 PL decay parameters detected at room temperature and at 77 K conditions. Material RT, 1120 nm
PbS-OA rGO/PbS-OA
77 K, 1360 nm
PbS-EDT rGO/PbS-EDT PbS-OA
τ1 (ns)
α1
τ2 (ns)
α2
τ3 (ns)
α3
Χ2
τavg (ns)
117 121
4898 4919
262 233
3508 4562
467 772
1105 626
1.034 0.916
273 361
10 56 1554
229 1750 298
85 219 2771
2201 1713 8954
558 230 –
3937 258 –
1.029 1.144 1.13
520 190 2749
unchanged, demonstrating that the integrity of the optical quality of the QDs in the composite is preserved (see Fig. 1 in Ref. [28]). The decay time for EDT-treated samples was measured at 77 K to enhance PL intensity. During the measurement, all samples were contained in vacuum to prevent oxidation. Fig. 4a shows the time-resolved photoluminescence spectra of the PbS QD film capped by oleic acid (emission maximum at 1120 nm). With rGO grafting, we observed an increase in the exciton lifetime. The significant longer-lived lifetime transient seen in the composite structure confirms the ability of rGO to efficiently passivate the surface of PbS QDs, reducing non-radiative recombination. However, after exchanging the dots with EDT in the solid-state, the rGO/PbS composite structures exhibited a much shorter exciton lifetime than PbS QDs alone (Fig. 4b). The average exciton lifetime data are presented in Table 1. Considering that the EDT-treatment improves the charge transport among neighboring QDs, the significant fast decay time is attributed to efficient charge transport processes within the PbS QDs. Therefore, we believe that rGO mainly contributes to the passivation of QD surface defects, consequently suppressing charge recombination and accelerating carrier transport processes in the assemblies. We then sought to identify QD optoelectronic devices that would uniquely benefit from the improved carrier transport. We first sought to measure field effect mobility. Fig. 5a shows a schematic of our field effect transistor (FET) device, and Fig. 5b and c shows the transfer characteristics where current (IDS) is plotted as a function of gate voltage (VG) with fixed source-drain voltage VDS ¼1 V. Both devices show typical p-type FET behavior demonstrating that holes are the majority carrier. The field effect mobility (μ), is calculated from the transfer characteristics, using Eq. (3)
ID =
WCμVG2 , 2L
(3)
where L is the channel length, W is the channel width, and C is the capacitance per unit area of the gate insulator. The channel length and channel width of our device are 102 μm and 104 μm, respectively. The field effect mobility of the rGO/PbS-based FET is calculated to be 16 times larger than that of PbS-based device: the mobility values for rGO/PbS and PbS structures are 6.23 10 6 cm2 V 1 s 1 and 3.91 10 7 cm2 V 1 s 1, respectively. Fig. 6 shows the current–voltage characteristics of rGO/PbS photovoltaic cells made with both EDT treated non-grafted, and grafted films as the photoactive layer. Table 2 shows a summary of the photovoltaic performance data for both samples. Our results show that improved short-circuit current density (Jsc) is revealed in the hybrid devices. The Jsc advantage could be attributed to the improved surface passivation in the photoactive film enabled by the chemical grafting of PbS QDs onto rGO NSs that provide more effective transport pathways for charge carries in the hybrid solar cells.
Fig. 5. (a) Schematic of bottom contact FET device. Transfer characteristics of (b) PbS QD and (c) rGO/PbS hybrid material based FET devices.
4. Conclusion In conclusion, we have presented a facile and simple solution method to synthesize hybrid rGO/PbS nanocomposites. The chemical grafting of PbS QDs onto rGO NSs results in not only the controllable dispersion of PbS QDs within the rGO matrix, but also efficient passivation of PbS QDs via enhancing the interfacial interactions and controlling nanomorphology between the organic and inorganic materials. The developed hybrid photoactive film enhanced charge transport efficiency by 16 times, and as a results,
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Fig. 6. Current density vs. voltage characteristics of PbS and rGO/PbS-based devices under 100 mW/cm2 AM1.5 illumination condition. Table 2 Summary of PV device parameters for PbS control and the best rGO/PbS-based devices. Material Vmax (V) Jmax (mA/cm2) Voc (V) Jsc (mA/cm2) FF (%) PCE (%) PbS rGO/PbS
0.34 0.32
10.47 12.7
0.51 0.50
17.2 19.3
41.39 42.11
3.56 4.06
Jsc and PCE by 12% and 14% correspondingly, which are essential for hybrid inorganic–organic solar cells.
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.solmat.2015.09. 001.
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Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Colloidal nanocomposite of reduced graphene oxide and quantum dots for enhanced surface passivation in optoelectronic applications Anush Mnoyan a, Kyungmok Kim a, Jin Young Kim b, Duk Young Jeon a,n a Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305701, Republic of Korea b Fuel Cell Research Center, Korea Institute of Science and Technology (KIST), Hwarang-ro 14-gil 5, Seongbuk-gu, Seoul 136-791, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 1 May 2015 Received in revised form 30 August 2015 Accepted 1 September 2015
Colloidal graphene/PbS quantum dots (QDs) hybrid nanostructures are fabricated with chemical grafting in one-pot solution methods. In the hybrid nanocomposites, PbS QDs are decorated on the reduced graphene oxide (rGO) nanosheets (NSs). By employing X-ray photoelectron spectroscopy (XPS) analysis, it is shown that the rGO NSs are bonded to PbS nanocrystals through oxygen functional groups, leading to improved surface passivation and electrical conductivity in the hybrids. The results obtained by the recordings of time-resolved photoluminescence spectra, field effect mobility, and photovoltaic performance revealed that rGO grafted to PbS QD composite structures provided better charge transport by 16 times compared to PbS alone, which is attributed to suppressed charge recombination and improved interfacial charge transport processes. Thus, the developed hybrid photoactive film enhanced open circuit current (Jsc) and power conversion efficiency (PCE) by 12% and 14% correspondingly. & 2015 Published by Elsevier B.V.
Keywords: rGO/PbS nanocomposite QD-based solar cell QD passivation carrier transport
1. Introduction The formation of a colloidal QD composite with an appropriate host matrix can provide several advantages, such as the improved stability of QDs by the passivation effects of the matrix, and very efficient charge and energy transport between QDs with strong electronic interaction with the matrix [1,2]. In general, inorganic carbon matrices are known to have distinctive thermal and optical resistances compared with their organic or polymeric counterparts and could be good candidates for QD encapsulation [3–5]. Graphene-based composite materials with semiconductor QDs have shown to improve electrical and thermal conductivity [6–12]. The anchoring of semiconductor nanoparticles on graphene sheets potentially provides a new way to develop catalytic, magnetic, and optoelectronic materials. Encapsulation of semiconductor QDs inside multi-shell graphitic cages is usually achieved during carbon arc-discharge at high temperatures [13–15]. Similarly, interesting hybrid structures with carbon nanotubes [1,16] and graphitic oxide nanoplatelets [17] have been developed by incorporating semiconductor nanoparticles. Here, we demonstrate a facile method to prepare the composite structure of rGO NSs and PbS QDs hybrids via one-pot solution synthesis. Efforts have also been made to synthesize semiconductor n
Corresponding author. Tel.: þ 82 42 350 3337; fax: þ82 42 350 3310. E-mail address: [email protected] (D.Y. Jeon).
http://dx.doi.org/10.1016/j.solmat.2015.09.001 0927-0248/& 2015 Published by Elsevier B.V.
particles protected by rGO sheets and assemble them as thin films using layer-by-layer self-assembly technique. The ability of rGO sheets to react with cationic defect sites on the surface of QDs via oxygen containing functional groups has enabled us to develop chemical synthetic strategies and tailor their properties with chemical functionalization. Chemical grafting and ligand exchange were used to control the interface of rGO/PbS nanohybrids and the PbS QD interparticle distance, respectively. The effects of the controlled interface on the optical and electrical properties are presented here. We find that the surface functionalization of PbS QDs via the rGO grafting gives rise to efficient surface passivation and trap removal, significantly improving their charge transport and photovoltaic performance.
2. Experimental section 2.1. List of materials Lead oxide (99.999%), oleic acid (90%), 1-octadecene (90%), octane (anhydrous, Z99%), hexamethyldisilathiane (TMS, synthesis grade), 1,2-ethanedithiol (EDT, Z98%) were purchased from Sigma Aldrich for the synthesis of PbS QDs. Hexane and acetone used in QD purification were purchased from Merck. To prepare the reduced graphene oxide nanosheets the following list of materials were used such as natural graphite powder (Kanto Chemical, Z80%), potassium permanganate (Z99%), sulfuric acid (extra pure, 95%), phosphoric acid ( Z85%), hydrochloric acid
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(32%), hydrogen peroxide (35%), hydrazine monohydrate (98%), N, N-dimethylformamide (DMF, anhydrous, 99.8%). Zinc acetate dehydrate (Z98%), lithium hydroxide monohydrate ( Z98%) and absolute ethanol were used to prepare ZnO nanocrystal solution. To accomplish the device preparation gold (99.99%) and molybdenum (VI) oxide (metals basis, 99.9995%) is used as electrode materials. 2.2. Sol–gel synthesis of ZnO nanocrystals The fabrication of nanocrystalline (NC) ZnO was performed according to the method reported in literature [18] with certain modifications. For the first step, 0.1 M solution of organometallic Zn precursor was prepared by making a mixture of 2 mmol zinc acetate dehydrate with 20 ml of absolute ethanol in 50 ml round shape 1-neck flask. The flask was connected to the reverse condenser and placed into an oil bath to maintain the temperature at 80 °C under atmospheric pressure. The mixture was stirred with magnetic bar vigorously for 1 h until the transparent solution was obtained. Next, 2.8 mmol of lithium hydroxide monohydrate powder was added to the solution. Finally, the suspension was placed into an ultrasonic bath at room temperature to accelerate the release of OH ions, resulting in formation of stable solution of ZnO nanocrystals. The ZnO NCs were precipitated by adding hexane into the main solution with 1:3 volume ratio and centrifuged at 10,000 r.p.m. for 5 min. Then, the obtained ZnO nanocrystal precipitate was dispersed in anhydrous ethanol to obtain 80 mg/ml solution. The characteristics of ZnO NCs colloidal solution are presented in Supplementary material Figs. S1–S3. Finally, 0.20 μm pore size syringe filter was used to filtrate the NC solution before using it in PV cell structure. 2.3. Synthesis of reduced graphene oxide nanosheets The rGO NSs were initially synthesized by modified Hummer's method using natural graphite powder [19]. Obtained graphene oxide NSs after multiple purification, were exposed to hydrazineassisted reduction process to improve the conductivity by reducing the number of oxygen containing functional groups ( OH, H– C ¼O, O–C ¼O, and epoxy) on the surface [20]. The suspension of rGO NSs with a concentration of 1 mg/ml in DMF was used to fabricate rGO/PbS hybrid material. 2.4. Synthesis of rGO/PbS hybrid composite material The synthesis of rGO/PbS hybrid material was performed by the hot injection method that is based on previously reported synthesis route for PbS QDs [21] with certain modifications. For this purpose, preliminary prepared suspension of rGO NSs in DMF was injected into the reaction vessel as lead precursor was transformed to the lead oleate complex, then reaction mixture was degassed for 17 h in 95 °C. TMS solution in ODE, which was preliminary degassed for 12 h at 80 °C, was injected to the reaction mixture to burst the nucleation. The heating mantle was turned off as nucleation induced (solution color changed to dark brown), and the solution was cooled down naturally. After reducing the solution temperature to 38 °C the crystal growth process was inhibited by injection of 20 ml of distilled acetone. To purify as-prepared QD solution it was centrifuged three times in the mixture of toluene/ acetone (1:3 volume ratio) at the spin speed of 6000 r.p.m. for 5 min. After drying in a vacuum condition the obtained solid was dissolved in octane to obtain a solution with 60 mg/ml concentration and stored in N2 atmosphere in glove box for further use. A reference PbS QD solution was also prepared without adding graphene.
2.5. Material characterization The morphology and particle size of hybrid composite material was analyzed by transmission electron microscopy (JEOL, JEM3010 (300 kV)). The portion area of rGO covered by PbS QDs (PA, %) was estimated statistically among the same scaled TEM images of different 24 rGO/PbS samples. First we assumed that single QD projection area is matched to its covered area. As so, the integrated area of the QD covered rGO is calculated as a sum of projection area of all QDs ( ∑ QDA = n·πr 2, where n is the counted number of QDs, and r is the average radius of QD). Then this value is compared to the selected area of rGO NSs (rGOA) covered by those QDs as given in Eq. (1)
PA =
n · πr 2 ·100% rGOA
(1)
The X-ray diffraction analysis were carried out with RIGAKU, D/ MAX-2500 diffractometer (Cu Kα radiation). The device crosssectional view and the surface properties of all type of samples were by scanning electron microscope (Hitachi S-4800). The optical absorption features were investigated by Mecasys, Optizen POP, Korea, and the XPS analysis was carried out using Thermo VG Scientific, ESCALAB 200i spectrometer equipped with a microfocused monochromator X-ray source. The luminescence lifetime values were evaluated using FLP920 (Edinburgh Instruments) fluorescence decay analysis software. For low temperature measurement the liquid nitrogen cryostat was used in combination with diaphragm vacuum pump (Edwards, RH 10 9LW) to maintain 2 10 4 mbar vacuum condition. EPL-785, picosecond pulsed diode laser (785 nm) was used as a light source, and the signals were detected by NIR-PMT detector with the range of 300– 1400 nm. Samples were prepared on glass by drop casting in the case of OA-capped PbS and rGO–PbS films, or by layer-by-layer (LBL) deposition for EDT ligand exchanged ones. The fluorescent decay curve was fitted with triexponential decay fit. The goodness of fit was judged by χ2 (1 7 0.2) values. The average decay time (τavg) values of PbS QD and rGO/PbS composite films were analyzed using Eq. (2) [22]
τavg =
α1τ12 + α2 τ22 + α3 τ32 , α1τ1 + α2 τ 2 + α3 τ 3
(2)
where τ is the lifetime and α is the pre-exponential factor with subscripts 1, 2, and 3 representing various species. 2.6. Photovoltaic device fabrication ITO-coated glass substrates were processed to three step sonication for 30 min in methanol, acetone and isopropyl alcohol solvents. Then, freshly synthesized ZnO NCs solution was spin coated on top of ITO to make a film with 90 nm thickness, then annealed in open air at 260 °C for 30 min to improve film crystallinity. The next steps of device fabrication were performed in N2 filled glove box. On top of ITO/ZnO substrate the active layer was deposited by LBL spin coating of rGO/PbS hybrid material with EDT-assisted solid state ligand exchange procedure [23]. To perform the LBL deposition 4 drops of QD solution (60 mg/ml) for 20 s, 5 drops of EDT solution (1% vol. in acetonitrile), 9 drops of acetonitrile and 10 drops of toluene were spin coated for 5 s one by one at fixed spinning speed of 2500 r.p.m. Overall procedure results in formation of active layer with 280 nm thickness after 8 LBL cycle. Then, 10 nm of MoO3 layer and 50 nm of Au top electrode were deposited by thermal evaporator under high vacuum condition (10 7 Torr) with 0.5 Å/s and 1 Å/s deposition rate correspondingly.
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2.7. Device characterization All device characterizations were performed in N2 filled glove box. Current density–voltage (J–V) data were measured using Keithley 2400 sourcemeter. The devices were illuminated through the glass substrate using a 150 W xenon lamp with an AM1.5G filter (LS-150-Xe, Abet Technologies). The light intensity was adjusted to 100 mW/cm2 using a Si reference cell (BS-520, Bunko Keiki). The effective surface area of the fabricated unit cells was controlled to be 0.06 cm2.
3. Results and discussion rGO/PbS composites were formed with chemical grafting of rGO during the colloidal synthesis of organic ligand (e.g. oleic acid, OA)-passivated PbS QDs. A key to the grafting process is to dissolve both QDs precursor materials and rGO NSs in compatible solvents, allowing them to interact with each other under conditions that preserve the stability and photophysical properties of both components. We have treated rGO with hydrazine to modify oxygenfunctional groups to be capable of forming chemical bonds with the surface sites of PbS QDs. Thus, chemical grafting of two suspensions of rGO and PbS results in the binding of PbS QDs to rGO NSs. The relative amount of rGO in the preparation of the hybrid structure was determined to be 0.15%. These composites remain suspended in solution, minimizing aggregation effects. In order to confirm the absence of unbound QDs in the purified rGO/PbS QD hybrid structure, isolation of the free QD in the supernatant was attempted by gentle centrifugation that selectively precipitated rGO/PbS.
Fig. 2. Typical XRD spectra of PbS QDs, rGO/PbS hybrid material and rGO NSs.
Fig. 1 shows transmission electron microscopy (TEM) images of the as-prepared rGO/PbS composite structure, where the homogeneous and dense population of PbS QDs with a diameter of ∼3.1 nm linked to rGO NSs are observed. A high resolution TEM image further shows the crystalline nature of PbS QDs and the rGO NSs attached to the surface of PbS QDs in the composite, indicating successful integration of rGO and PbS with intimate interfacial contact. It should be emphasized that PbS QDs in the composite were observed with the same structure and similar particle size/ distribution as those for the preparation of the PbS QDs without the presence of rGO. This result strongly indicates that the presence of rGO plays a good passivating and stabilizing role in the
Fig. 1. Characteristic TEM images of (a) PbS QDs and (b) rGO/PbS hybrid material. Obviously, PbS QDs have 3.1 nm average size (c), and rGO/PbS hybrid has 2D structure where PbS QDs are well distributed on rGO surface without any agglomeration (d).
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formation of PbS QDs in the hybrids. Fig. 2 shows X-ray diffraction (XRD) patterns of the as-synthesized rGO/PbS composite. The diffraction peaks of the QDs alone and the rGO/PbS composite are both ascribed to a well-crystallized PbS with cubic structure. The XRD pattern of rGO presents a broad peak at 2θ ¼24.2° (002) corresponding to 3.69 Å interlayer distance. The peak from rGO is hard to be recognized in the XRD pattern of the rGO/PbS composite sample due to the relatively small concentration of rGO in the composite, the overlapping of diffraction peaks, and the broad shape of rGO main peak. In our preparation of the composite, the concentration of anchored PbS QDs to rGO NSs was tuned by controlling the amount of rGO addition in the synthesis. In particular, we attempted to maximize the interfacial contact of rGO NSs around PbS QDs, which is expected to be beneficial for the efficient surface passivation of PbS QDs. However, it is essential for films to achieve a balance between the sufficient supply of rGO for ample QD interfacial contact while also limiting rGO's detrimental filtering in the photo-induced carrier transport due to a loss of colloidal stability and light absorption. The portion of covered area of rGO NSs by PbS QDs in our optimal conditions is estimated to be ∼60–70% (the histogram over 24 samples is demonstrated in Supplementary Fig. S4). To understand the bonding nature of the PbS QD-grafted rGO, XPS spectra of PbS QDs with and without rGO grafting were obtained. We focus on the XPS spectra of Pb 4f peaks to more precisely identify the bond formed during chemical grafting. Fig. 3a shows the Pb 4f peaks for non-grafted PbS samples where the Pb bonding energies are well matched to the value from the bulk PbS state as determined with peak deconvolution [24,25]. In comparison, XPS spectra of the grafted rGO/ PbS reveals a new intermediate bonding energy. This can be assigned to the Pb–O bond connecting the PbS QDs and rGO NSs matrix via O-containing polar groups and Pb-rich (Pb2þ sites) surface of nonstoichiometric PbS QDs [26] forming close contact after grafting [12,27].
Fig. 3. XPS spectra comparison: (a) Pb 4f spectra of PbS QDs and rGO/PbS hybrid material, and (b) O 1s spectra of rGO/PbS hybrid material and rGO NSs.
Thus, the interaction between PbS QD and rGO via Pb–O bonds may lead to the additional passivation of the Pb cationic defect sites on the QD surface. The presence of Pb–O bonding can be further confirmed by the O 1s XPS peak at 530.4 eV in the spectrum which indicates the presence of residual oxygen containing groups bonded with Pb atoms in rGO (Fig. 3b). It is posited that the C/O ratio for rGO in the composite is larger than that of the as-prepared rGO from the oxygen species involved in Pb–O chemical bonds anchored on the surface of rGO, which may result in a large electronic conductivity in the rGO sheets. This highlights the potential for rGO sheets to serve as the conductive channels between PbS QDs, improving the overall transport in the solid-state. We sought to study the posited benefits to charge transport by comparing PbS QDs with and without rGO NSs both optically and electrically. The rGO/PbS nanocomposites were first deposited on substrates with layer-by-layer deposition and a ligand exchange with ethanedithiol (EDT). The use of spin casting in the composite film preparation resulted in homogenous films without any deterioration (see Fig. 3 in Ref. [28]). We examined photoluminescence decay by time-correlated single photon counting (TCSPC) technique which is a good indirect probe of the photo-induced carrier transport process in the assemblies. Fig. 4 compares the time-resolved photoluminescence spectra recorded following 785 nm laser pulse excitation of PbS QDs with and without rGO-grafting in films. The decay time values were measured at the peak position of the photoluminescence spectrum (see Fig. 2 in Ref. [28]). Note that the absorption feature of PbS QDs before and after the addition of rGO remained
Fig. 4. Normalized transient PL decay features of (a) oleic acid capped PbS and rGO/ PbS films measured at 1120 nm emission wavelength at room temperature and (b) EDT treated PbS and rGO/PbS films measured at 1360 nm emission wavelength at 77 K conditions.
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Table 1 PL decay parameters detected at room temperature and at 77 K conditions. Material RT, 1120 nm
PbS-OA rGO/PbS-OA
77 K, 1360 nm
PbS-EDT rGO/PbS-EDT PbS-OA
τ1 (ns)
α1
τ2 (ns)
α2
τ3 (ns)
α3
Χ2
τavg (ns)
117 121
4898 4919
262 233
3508 4562
467 772
1105 626
1.034 0.916
273 361
10 56 1554
229 1750 298
85 219 2771
2201 1713 8954
558 230 –
3937 258 –
1.029 1.144 1.13
520 190 2749
unchanged, demonstrating that the integrity of the optical quality of the QDs in the composite is preserved (see Fig. 1 in Ref. [28]). The decay time for EDT-treated samples was measured at 77 K to enhance PL intensity. During the measurement, all samples were contained in vacuum to prevent oxidation. Fig. 4a shows the time-resolved photoluminescence spectra of the PbS QD film capped by oleic acid (emission maximum at 1120 nm). With rGO grafting, we observed an increase in the exciton lifetime. The significant longer-lived lifetime transient seen in the composite structure confirms the ability of rGO to efficiently passivate the surface of PbS QDs, reducing non-radiative recombination. However, after exchanging the dots with EDT in the solid-state, the rGO/PbS composite structures exhibited a much shorter exciton lifetime than PbS QDs alone (Fig. 4b). The average exciton lifetime data are presented in Table 1. Considering that the EDT-treatment improves the charge transport among neighboring QDs, the significant fast decay time is attributed to efficient charge transport processes within the PbS QDs. Therefore, we believe that rGO mainly contributes to the passivation of QD surface defects, consequently suppressing charge recombination and accelerating carrier transport processes in the assemblies. We then sought to identify QD optoelectronic devices that would uniquely benefit from the improved carrier transport. We first sought to measure field effect mobility. Fig. 5a shows a schematic of our field effect transistor (FET) device, and Fig. 5b and c shows the transfer characteristics where current (IDS) is plotted as a function of gate voltage (VG) with fixed source-drain voltage VDS ¼1 V. Both devices show typical p-type FET behavior demonstrating that holes are the majority carrier. The field effect mobility (μ), is calculated from the transfer characteristics, using Eq. (3)
ID =
WCμVG2 , 2L
(3)
where L is the channel length, W is the channel width, and C is the capacitance per unit area of the gate insulator. The channel length and channel width of our device are 102 μm and 104 μm, respectively. The field effect mobility of the rGO/PbS-based FET is calculated to be 16 times larger than that of PbS-based device: the mobility values for rGO/PbS and PbS structures are 6.23 10 6 cm2 V 1 s 1 and 3.91 10 7 cm2 V 1 s 1, respectively. Fig. 6 shows the current–voltage characteristics of rGO/PbS photovoltaic cells made with both EDT treated non-grafted, and grafted films as the photoactive layer. Table 2 shows a summary of the photovoltaic performance data for both samples. Our results show that improved short-circuit current density (Jsc) is revealed in the hybrid devices. The Jsc advantage could be attributed to the improved surface passivation in the photoactive film enabled by the chemical grafting of PbS QDs onto rGO NSs that provide more effective transport pathways for charge carries in the hybrid solar cells.
Fig. 5. (a) Schematic of bottom contact FET device. Transfer characteristics of (b) PbS QD and (c) rGO/PbS hybrid material based FET devices.
4. Conclusion In conclusion, we have presented a facile and simple solution method to synthesize hybrid rGO/PbS nanocomposites. The chemical grafting of PbS QDs onto rGO NSs results in not only the controllable dispersion of PbS QDs within the rGO matrix, but also efficient passivation of PbS QDs via enhancing the interfacial interactions and controlling nanomorphology between the organic and inorganic materials. The developed hybrid photoactive film enhanced charge transport efficiency by 16 times, and as a results,
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Fig. 6. Current density vs. voltage characteristics of PbS and rGO/PbS-based devices under 100 mW/cm2 AM1.5 illumination condition. Table 2 Summary of PV device parameters for PbS control and the best rGO/PbS-based devices. Material Vmax (V) Jmax (mA/cm2) Voc (V) Jsc (mA/cm2) FF (%) PCE (%) PbS rGO/PbS
0.34 0.32
10.47 12.7
0.51 0.50
17.2 19.3
41.39 42.11
3.56 4.06
Jsc and PCE by 12% and 14% correspondingly, which are essential for hybrid inorganic–organic solar cells.
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.solmat.2015.09. 001.
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