DOI: 10.1002/chem.201400119
Full Paper
& Solar Cells
Performance Enhancement of Bulk Heterojunction Solar Cells with Direct Growth of CdS-Cluster-Decorated Graphene Nanosheets Kai Yuan,[a] Lie Chen,[a, b] Licheng Tan,[a] and Yiwang Chen*[a, b]
Abstract: Two-dimensional graphene–CdS (G–CdS) semiconductor hybrid nanosheets were synthesized in situ by graphene oxide (GO) quantum wells and a metal–xanthate precursor through a one-step growth process. Incorporation of G– CdS nanosheets into a photoactive film consisting of poly[4,8-bis-(2-ethyl-hexyl-thiophene-5-yl)-benzo[1,2-b:4,5-b]dithiophene-2,6-diyl]-alt-[2-(2-ethyl-hexanoyl)-thieno[3,4-b]thiophen-4,6-diyl] (PBDTTT-C-T) and [6,6]-phenyl C70 butyric acid methyl ester (PC70BM) effectively decreases the exciton lifetime to accelerate exciton dissociation. More importantly, the decreasing energy levels of PBDTTT-C-T, PC70BM, and G– CdS produces versatile heterojunction interfaces of PBDTTTC-T:PC70BM, PBDTTT-C-T:G–CdS, and PBDTTT-C-T:PC70BM:G– CdS; this offers multi-charge-transfer channels for more effi-
cient charge separation and transfer. The charge transfer in the blend film also depends on the G–CdS nanosheet loadings. In addition, G–CdS nanosheets improve light utilization and charge mobility in the photoactive layer. As a result, by incorporation of G–CdS nanosheets into the active layer, the power-conversion efficiency of inverted solar cells based on PBDTTT-C-T and PC71BM is improved from 6.0 % for a reference device without G–CdS nanosheets to 7.5 % for the device with 1.5wt % G–CdS nanosheets, due to the dramatically enhanced short-circuit current. Combined with the advantageous mechanical properties of the PBDTTT-CT:PC70BM:G–CdS active layer, the novel CdS-cluster-decorated graphene hybrid nanomaterials provide a promising approach to improve the device performance.
Introduction
rated charges of excitons before collected at the corresponding electrodes.[5, 6] To complement the OSCs performance, semiconductor nanocrystals with high charge mobility has been exploited as acceptors for OSCs.[7, 8] The absorption range and band gap of semiconductor nanocrystals can be precisely controlled by quantum-confinement effects.[9] The tunable energy levels of semiconductor nanocrystals favor charge separation and transfer between high-electron-affinity semiconductor nanocrystals and relatively low-ionization-potential conjugated polymers.[7] In spite of these conceptual advantages, OSCs based on conjugated polymers combined with semiconductor nanocrystals has so far exhibited poor performances. Because of the uncontrolled phase separation of semiconductor nanocrystals from conjugated polymer matrices, it is hard to form the desired, bicontinuous, interpenetrating, network nanomorphology. Moreover, the charge-carrier transport is allowed through inefficient random hopping among discrete semiconductor nanocrystalline structures.[10] 2D materials, such as quantum wells, offer particular capacity for efficient photogenerated charge-carrier collection. The 2D quantum-well morphology of graphene, possess excellent electron and hole mobility, electrochemical and thermal stability, solution processability, and good compatibility with conjugated polymers; this makes graphene a desirable candidate for photovoltaic and optoelectronic applications.[11–13] Efforts have been devoted to combine semiconductor nanocrystals with 2D nanostructures, because 2D nanostructures not only provide support for anchoring semiconductor nanocrystals, but also
Organic solar cells (OSCs) are promising materials for renewable and mobile energy generation, due to the low-costs, solution-synthesis, light-weight, and large-scales manufacturing characteristics.[1] Recently, the power-conversion efficiency (PCE) of OSCs has been significantly improved, up to 8–9 % for single-cell devices, by rational design of new, narrow-bandgap, conjugated polymers, optimization of device architectures, and controlling the active-layer morphology.[2] Nevertheless, the overall device performance of OSCs is still limited by various factors, such as extremely short length of the exciton diffusion (ca. 10 nm), inefficient charge separation of excitons and low charge-carrier mobility within the photoactive layer.[3, 4, 5] Amongst these, low electron and hole mobility is the crucial limitation for improvement of PCE. A low charge-carrier mobility leads to recombination or/and trapping of the sepa-
[a] K. Yuan, Prof. Dr. L. Chen, Dr. L. Tan, Prof. Dr. Y. Chen Institute of Polymers/Department of Chemistry Nanchang University, 999 Xuefu Avenue Nanchang 330031 (P.R. China) E-mail: [email protected] [b] Prof. Dr. L. Chen, Prof. Dr. Y. Chen Jiangxi Provincial Key Laboratory of New Energy Chemistry Nanchang University, 999 Xuefu Avenue Nanchang 330031 (P.R. China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201400119. Chem. Eur. J. 2014, 20, 1 – 10
These are not the final page numbers! ÞÞ
1
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
Full Paper act as acceptors to accelerate exciton dissociation and electron-hole transport.[13–16] However, graphene oxide (GO) or reduced graphene oxide (RGO) quantum wells and the semiconductor nanocrystalline structures, commonly used for these semiconductor nanocrystals/quantum-well hybrid materials, are synthesized separately.[13, 15, 17] In this work, we present in situ fabrication of 2D, graphene– CdS cluster (G–CdS), semiconductor nanocrystals/quantumwell hybrid nanosheets in one step by employing GO as quantum wells and metal–xanthate as CdS precursor. During the one-step growth process, the reduction of GO and the spontaneous deposition of CdS nanocrystals on the graphene surface occurred simultaneously. The tight contact between CdS and graphene in the G–CdS nanosheets without any adhesive media is favorable for charge injection, separation, and transport. The favorable energy alignment among PBDTTT-C-T (poly[4,8-bis-(2-ethyl-hexyl-thiophene-5-yl)-benzo[1,2-b:4,5-b]dithiophene-2,6-diyl]-alt-[2-(2-ethyl-hexanoyl)-thieno[3,4-b]thiophen-4,6-diyl]),[18] PC70BM ([6,6]-phenyl C70butyric acid methyl ester), and G–CdS produces versatile heterojunction interfaces in this film; this offers multi-charge-transfer channels for more efficient charge separation and transfer. Consequently, by introducing G–CdS nanosheets into the active layer, the PCE of inverted bulk heterojunction solar cells based on PBDTTT-C-T and PC70BM was improved from 6.0 % to 7.5 %. Furthermore, compared with the reference active layer without G–CdS nanosheets, the PBDTTT-C-T:PC70BM:G–CdS ternary film shows improved mechanical property, which is beneficial for practical applications.
Figure 1. (a) Schematic of the in situ synthesized 2D G–CdS nanosheets by the one-pot process, and (b) the device configuration of the PBDTTT-CT:PC70BM:G–CdS active layer fabricated in this work.
Results and Discussion Figure 1 a presents the simple process of in situ fabricated G– CdS nanosheets. Hydroxyl or carboxyl groups on the surface of GO first react with the CdS precursor, the metal–xanthate [Cd(S2COEt)2(C5H4N)2]. The grafted [Cd(S2COEt)2(C5H4N)2] further decomposes to form CdS by a solvothermal reduction process at 180 8C and GO is reduced at the same time. The decomposing process is initiated at the GO surface to produce a 2D G– CdS hybrid structure. G–CdS nanosheets are mixed with a PBDTTT-C-T:PC70BM solution to afford a photosensible layer deposited by spin coating. Figure 1 b displays the device configuration based on the PBDTTT-C-T:G-CdS:PC70BM blend. The nanoarchitecture of the G–CdS nanosheets can be discerned through microscopic characterization. Figure 2 a shows the typical transmission electron miscroscopy (TEM) image of GO, and the characteristic wrinkles, a property of a flexible, single-layer, 2D, graphene sheets structure, are observed. After solvothermal reduction, the CdS clusters were dispersed on the surface of the graphene sheets without evident phase separation, revealed by the large-scale scanning electron microscopy (SEM) image (Figure 2 b). The proper well-proportioned emission obtained from a laser scanning confocal microscopy image (Figure 2 c) also confirms the homogeneous distribution of the CdS clusters on the graphene sheets. To gain information on the internal microstructure of the G–CdS nanosheets, TEM was employed. Similar to the SEM observation, TEM re&
&
Chem. Eur. J. 2014, 20, 1 – 10
www.chemeurj.org
Figure 2. (a) TEM micrograph of GO. (b) SEM, (c) laser scanning confocal microscopy, and (d) TEM image of the G–CdS nanosheets. (e) High-magnification TEM image of the G–CdS nanosheets. The inset shows the EDX spectrum, which confirms the presence of CdS. (f) HRTEM images of the G–CdS nanosheets. The insets show a selected-area diffraction pattern of the G– CdS nanosheets (upper right) and a lattice fringes image of a CdS nanocrystal (bottom right).
2
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
Full Paper veals that all of the graphene sheets are tightly decorated with CdS clusters, and no free CdS cluster or pure graphene sheets are seen in the images of Figure 2 d and e. The strong interaction between CdS and graphene without any adhesive media and the uniform dispersion of CdS clusters on the graphene sheets would enable G–CdS nanosheets with highly efficient optoelectronic characteristic.[16] The sizes of the distributed CdS nanoparticles are range from about 5 to 7 nm as determined by TEM (Figure 2 e) and high-resolution TEM (HRTEM) images in Figure 2 f. The lattice spacing of 0.337, 0.208, and 0.176 nm is attributed to the (111), (220), and (311) lattice planes of CdS, respectively;[14] this agrees well with the X-ray diffraction (XRD) results of G–CdS (Figure S1 in the Supporting Information). In the XRD pattern, the three main scattering angles at 26.898, 43.968, and 52.058 are assigned to the diffractions of the (111), (220), and (311) planes of CdS (JCPDS 100454), respectively.[16] The three typical lattice planes can also been evidenced by three diffraction rings in the selected-area electron diffraction (SAED) profile (inset of Figure 2 f), indicating that these nanoparticles are polycrystalline with blende structure. Additionally, the energy dispersive X-ray spectrum (EDX, inset of Figure 2 e) and the well-defined diffraction ring with six spots (inset of Figure 2 f) provide the direct evidence for the formation of CdS cluster and the reduction of GO to afford graphene flakes. GO possesses a great quantity of oxygen-related functional groups with sp2 and sp3 hybridizations.[19] The sp3 hybridization forms through breaking the C=C bonds under oxidative conditions. Hydroxyl groups, carbonyl groups, and epoxy pairs can be formed by further oxidation.[20] During the solvothermal reduction process, the oxygen-containing functional groups can be gradually eliminated.[11] To investigate the changes to the functional groups, the formation of CdS, and the reduction degree of GO after the solvothermal reduction process, highresolution C1s X-ray photoelectron spectroscopy (XPS) was performed on the samples of GO, GO:CdS precursor (GO–Pro), and G–CdS (Figure 3). Figure 3 a shows the high-resolution C1s XPS spectrum of GO, and the fitted peaks correspond to carbon atoms in diverse functional groups. The peaks located at 284.8, 286.9, and in the approximate range 287.9–288.9 eV are associated to the sp2 C atoms, the C atoms in the epoxy/ hydroxyl groups (C O), and the carbonyl groups (C=O and O C=O), respectively.[21] The Cd2 + ion can interact with GO through electrophilic epoxy ring opening or coordination to oxygen-based functional groups.[22] Interaction between Cd2 + and GO also occur in the GO:CdS precursor solution. As shown in the C1s XPS spectrum of GO-Pro (Figure 3 b), the intensity of the peaks corresponding to C O and O C=O dramatically decrease, indicating that attractive interaction between the CdS precursor and the oxygen-based functional groups. In comparison, after the solvothermal reduction process, the peaks of C O, C=O, and O C=C almost disappear in the C1s XPS spectrum of G–CdS (Figure 3 c). The content of graphitic carbon and oxidized carbon in GO are 45.6 % and 54.4 %, respectively; in G-CdS nanosheets these numbers are changed to 67.3 and 32.7 %, respectively. The significantly decreased intensity of C O, C=O, and O C=C and the content changes of the comChem. Eur. J. 2014, 20, 1 – 10
www.chemeurj.org
These are not the final page numbers! ÞÞ
Figure 3. High-resolution C1s XPS of (a) GO (b) GO-Pro, and (c) G–CdS nanosheets.
ponents indicate the elimination of oxygen-based functional groups caused by the solvothermal reduction process. In addition, the binding energy of S2p (Figure S2–4 in the Supporting Information) peaked at 161.8 eV; this demonstrates the formation of CdS.[23] The main findings of the GO–Pro interaction and the formation of the G–CdS hybrid nanosheets can be explained by density functional theory (DFT) calculations. Figure 4 presents the optimized binding constructions of the CdS precursor on the GO surface. The theoretical simulations reveal that the CdS precursor can covalently interact with the oxygen-based functional groups. Figure 4 a and b depict the adhesion of the CdS precursor on the GO functionalized by hydroxy ( OH) and carboxyl ( COOH) groups to form covalent Cd O C and Cd O C=O linkages, respectively. The corresponding bond lengths of the Cd O bond are calculated to be about 2.28 in the Cd O C unit and 2.46 in the Cd O C=O unit. The short bond length means that the main characteristic of the strong interaction between the CdS precursor and the OH, COOH groups on the GO surface is chemisorption. The molecular orbitals from DFT calculations (B3LYP/6-31G**) for GO–Pro with the covalent Cd O C and Cd O C=O linkages are shown in Figure 4 d–g. The DFT calculations also demonstrate that the pyridine moiety in the CdS precursor has strong affinity to the basal plane of graphene through p–p stacking to form the versatile noncovalent composites GO–Pro (Figure 4 c). The molecular orbitals are depicted in Figure S5 in the Supporting Information. The interaction energy of the CdS precursor and graphene is estimated to be 5.21 kcal mol 1 and the vertical distance between the pyridine moiety and the graphene surface is about 2.75 . The strong physisorption and improved interfacial electronic coupling by means of the formation of 3
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
Full Paper the G band in the Raman spectra results from reduction of GO.[16] Meanwhile, the increased intensity of the G/D ratio also verifies successful reduction of GO,[6] to give good graphitic crystallinity.[25] A shift of the position of the G band usually means changes to the charge-transfer and electrical properties of graphene.[26] In the G–CdS nanosheet hybrid structures between the CdS clusters and graphene, no intermittent layer was observed. Because of the well-matched energy levels, shown in Figure 6 a, barrier-free charge transmission is expected between the CdS clusters and graphene. Figure 6 a illustrates the energy-level scheme of various materials employed for fabrication of the solar-cell devices. The
Figure 4. DFT simulation of the electronic coupling. (a) Representative adsorption configuration of a CdS precursor on GO with OH functionalities and formation of a covalent Cd O C linkage, the corresponding molecular orbitals from DFT calculations (B3LYP/6-31G**) are presented in (d) and (e). (b) Adsorption configuration of a CdS precursor on GO with COOH functionalities with formation of a covalent Cd O C=O linkage, the corresponding molecular orbitals from DFT calculations (B3LYP/6-31G**) are presented in (f) and (g). (c) Adsorption of a CdS precursor on graphene through the strong affinity between pyridine moieties and graphene through p–p stacking to form versatile noncovalent composites. C atoms are gray, S atoms are yellow, N atoms are blue, Cd atoms are faint yellow, O atoms are red, and H atoms are white.
a covalent linkage between the CdS precursor and GO would facilitate spontaneously deposition of the CdS nanocrystals on the graphene surface during the solvothermal reduction process.[16, 24] These results also agree well with the SEM, TEM, and XPS measurements. The obtained G–CdS nanosheets were further examined by Raman spectroscopy. The Raman spectra of the GO and G–CdS nanosheets are shown in Figure 5. Two characteristic peaks located at around 1330 and 1580 cm 1 correspond to the D and G bands of graphene, respectively. The G band of G–CdS is slightly shifted to lower wavenumbers in comparison with that of GO, whereas the D band remains unchanged. The shift of
Figure 6. (a) Energy-level diagram of various materials used for fabricating solar cells. Excitation and charge-transfer pathways in: (b) PBDTTT-CT:PC70BM- and (c) PBDTTT-C-T:PC70BM:G–CdS-based solar cells.
energy levels of G–CdS and RGO were determined by ultraviolet photoelectron spectroscopy (UPS) measurements (Figure S6 in the Supporting Information). As depicted in Figure 6 b, for the reference device based on PBDTTT-C-T:PC70BM without G–CdS nanosheets, the excitons in the PBDTTT-C-T donor can diffuse to the PBDTTT-C-T:PC70BM interface and dissociate through electron migration to the lowest unoccupied molecular orbital (LUMO) of PC70BM. Whereas, for devices based on the PBDTTT-C-T:PC70BM:G–CdS ternary blend (Figure 6 c), in addition to the fullerene acceptor, G–CdS nanosheets also act as electronic acceptors due to the low LUMO level of 4.3 eV. The exciton dissociation can occur both at the PBDTTT-C-T:PC70BM interface and at PBDTTT-C-T:G–CdS interface. More interesting, the LUMO energy level of PC70BM (4.2 eV) is located between the LUMO of PBDTTT-C-T (3.25 eV) and the conduction band of G–CdS (4.3 eV). Therefore, the favorable energy alignment between the LUMO levels enable PC70BM to function as an energy-gradient intermediate, so that
Figure 5. Raman spectra of GO and G–CdS nanosheets.
&
&
Chem. Eur. J. 2014, 20, 1 – 10
www.chemeurj.org
4
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
Full Paper electrons can efficiently be transported to the cathode through this energetic downhill cascade pathway, as shown in Figure 6 c. The multi-charge-transfer channels not only significantly improve the charge-carrier separation capacity, but also remarkably enhance the collection and transportation efficiency of the photoinduced charge carriers. The charge transfer at the interface through multi-chargetransfer channels within the active materials was predominantly investigated by static photoluminescence (PL) and time-resolved photoluminescence (TRPL) spectroscopy measurements as shown in (Figure 7, and Figure S7 in the Supporting Information). PBDTTT-C-T shows a strong PL emission peak at 780 nm assigned to the radiative decay of excitons to the ground state (Figure 7 a). For the PL spectra of PBDTTT-C-T with various loadings of RGO (Figure S7 a in the Supporting In-
transfer at PBDTTT-C-T:PC70BM and PBDTTT-C-T:G–CdS. In addition, as the content of G–CdS increases, the emission intensity of PBDTTT-C-T decreases sharply (Figure S7 b in the Supporting Information). The PL quench demonstrates the efficient charge separation that occurs at PBDTTT-C-T:G–CdS. A further quenched emission can be detected in the PBDTTT-CT:PC70BM:G-CdS ternary blends, suggesting that versatile heterojunction interfaces offer multi-charge-transfer channels for more efficient charge transfer. TRPL spectroscopy was carried out to measure the PL lifetime of PBDTTT-C-T:PC70BM films with various G–CdS nanosheets loadings (0.5, 1.5, and 3.0 wt %, Figure 7 b). The lifetime of PBDTTT-C-T:PC70BM film was determined to be 728 ps, whereas all of G–CdS-modified PBDTTT-CT:PC70BM blends show reduced PL lifetimes of 536, 418 and 591 ps for 0.5, 1.5 and 3.0 wt % G–CdS nanosheets, respectively. The reduced PL lifetimes are related to the improved exciton dissociation rates. Among the PBDTTT-C-T:PC70BM:G-CdS ternary blends, the film with 1.5 wt % loading of G–CdS achieve the shortest lifetime, suggesting an appropriate loading of nanosheet can induce the most favorable charge transfer in the active layer. In addition to exciton dissociation, the effect of G–CdS nanosheets on charge-carrier mobility in solar-cell devices was also investigated by measurements of hole-only and electron-only devices by using Mott-Gurney space charge limited current (SCLC) model (Figure 8 and Table S1 in the Supporting Information). The devices based on PBDTTT-C-T:PC70BM with G–CdS nanosheets exhibit improved electron-transport and holetransport behaviors. The hole-mobility increases more than twofold from 1.63 10 3 cm2 V 1 s 1 (without G–CdS) to 3.65 10 3 cm2 V 1 s 1 (with 1.5 wt % G–CdS), the corresponding electron mobility increased from 2.21 10 3 to 4.66 10 3 cm2 V 1 s 1. The improvement in charge-carrier mobility caused by the G–CdS nanosheets originates from the versatile charge-transport pathways in the active layers supported by the barrier-free charge injection, which is guaranteed by the well-matched energy levels of PBDTTT-C-T, PC70BM, and G–CdS at their interfaces (Figure 6). Because the conductivity of the active layers will exert influence on the charge-carrier mobility, the conductivities of the active layers with and without G–CdS nanosheets were determined by conductive atomic force microscopy (CAFM). Figure 9 presents the CAFM current images with a scan area of 2 2 mm for PBDTTT-C-T:PC70BM films without and with 1.5 wt % G–CdS nanosheets. The mean current increases from 76.3 pA for the film without G–CdS to 167.6 pA for the film with 1.5 wt % G–CdS. The enhanced conductivity of PBDTTT-CT:PC70BM:G–CdS is consistent with the improved charge-carrier mobility, which would be favorable for the enhancement of Jsc. The UV/Vis absorption spectra of GO and G–CdS films are presented in Figure 10 a. The absorption band of GO is mainly located in the short-wavelength range. The absorption of the G–CdS nanosheet is extended into the longer-wavelength region; this represents a characteristic absorption of CdS clusters, from 400 to 600 nm. Furthermore, the absorption range of nanosheets can be tuned by the quantum-confinement effect.[27] Figure 10 b shows the UV/Vis absorption spectra of
Figure 7. (a) Static PL spectra and (b) TRPL spectra of PBDTTT-T-C and PBDTTT-C-T:PC70BM with different concentrations of G–CdS nanosheets. The lifetime of PBDTTT-C-T:PC70BM films with 0, 0.5, 1.5, and 3.0 wt % of G–CdS nanosheets are 728, 536, 418, and 591 ps, respectively.
formation), there is no noticeable PL quench with increasing RGO concentrations; this means that the direct charge transfer from PBDTTT-C-T to RGO is hampered by the remarkable energy barrier at the PBDTTT-C-T:RGO interface. On the contrary, after blended with PC70BM or G–CdS, the PL of PBDTTTC-T is significantly quenched, confirming the effective charge Chem. Eur. J. 2014, 20, 1 – 10
www.chemeurj.org
These are not the final page numbers! ÞÞ
5
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
Full Paper
Figure 8. J1/2–V characteristics of (a) hole-only and (b) electron-only devices based on PBDTTT-C-T:PC70BM with different concentrations of G–CdS nanosheets according to SCLC modeling and measured at ambient temperature. The insets represent the architecture of hole- and electron-only devices.
Figure 10. (a) UV/Vis absorption profiles of thin films of GO and G–CdS composites. The inset shows a photograph of GO and G–CdS solutions. (b) UV/ Vis absorption spectra of PBDTTT-C-T:PC70BM:G–CdS films with various amounts of G–CdS nanosheets.
Figure 9. (a, f) AFM topography, (b, g) PeakForce error, (c, h) Young’s modulus, (d, i) adhesion, (e, j) CAFM of PBDTTT-C-T:PC70BM without and with 1.5 wt % of G–CdS nanosheets, respectively. The scanned areas are 2 2 mm.
&
&
Chem. Eur. J. 2014, 20, 1 – 10
www.chemeurj.org
6
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
Full Paper PBDTTT-C-T:PC70BM:G–CdS films with various amounts of G– CdS nanosheets (0.5–3.0 wt %). From the figure we can clearly see that incorporation of G–CdS nanosheets practically enhances the light harvesting of the blends compared with the reference film, and the absorption of the nanosheets is concentration dependent with regard to G–CdS nanosheets. Among these blends, PBDTTT-C-T:PC70BM:G–CdS films with 1.5wt % G– CdS shows the strongest absorption in the range 600–800 nm; this is beneficial for improving device performance. The contribution of well-characterized G–CdS nanosheets to the performance of bulk heterojunction solar cells based on PBDTTT-C-T:PC70BM:G–CdS were investigated and compared with the performance of the PBDTTT-C-T:PC70BM- and PBDTTT-C-T:PC70BM:RGO-based cells. Figure 11 presented the current-density–voltage (J–V) characteristics of the devices under AM1.5G (100 mW cm 2) light intensity illumination. The device based on PBDTTT-C-T:PC70BM has a PCE of 6.0 %, with a short-circuit current (Jsc) of 13.49 mA cm 2, an open-circuit voltage (Voc) of 0.765 V, a fill factor (FF) of 58.0 %. However, with the addition of RGO to the active layer, the PCE of the solar cell based on PBDTTT-C-T:PC70BM:RGO decreases from 6.0 % (without RGO) to 4.8 % (RGO 3.0 wt %), as shown in Figure 11 a. The reduced PCE mainly results from decreased Jsc ; this implies that RGO produces undesirable pathways for the recombination of electron holes;[6] this in turn is correlated to the PL observation. Incorporation of G–CdS nanosheets into the PBDTTT-C-T:PC70BM blends dramatically improve the device performance, and the device performance strongly depends on the loading of G–CdS sheets. The best PCE of 7.5 % is achieved together with an average Jsc value of 16.82 0.3 mA cm 2, Voc of 0.765 0.003 V, and FF of 58.1 1 % in the device based on the PBDTTT-C-T:PC70BM:G–CdS ternary blend with a G–CdS concentration of 1.5wt % (Figure 11 b). Table 1 summarizes the average solar-cell parameters of PBDTTT-CT:PC70BM with different concentrations of RGO and G–CdS nanosheets. A comparison of the reference device to the G– CdS-modified ones shows that both devices have almost the same values of Voc and FF. As expected, the improved performance of PBDTTT-C-T:PC70BM:G–CdS-based devices can be explained by the increased Jsc, which is caused by enhanced charge-carrier separation, transportation, and collection, as well as better light utilization, as mentioned above. The increased Jsc is consistent with the incident photon-to-current efficiency (IPCE) of the fabricated devices (Figure 11 c) The PBDTTT-C-T:PC70BM:G–CdS-based devices exhibit a higher IPCE value (72 %) than the reference devices (65 %) throughout the wavelength scope of 350–750 nm. The nanomorphology of the active layer is of major importance in charge separation and transport processes.[4, 28] The nanomorphology of PBDTTT-C-T:PC70BM and PBDTTT-CT:PC70BM:G–CdS (1.5 wt %) were measured by tapping-mode Figure 11. Current–voltage (J–V) characteristics of solar cells based on PBDTTT-C-T:PC70BM with different concentrations of (a) RGO and (b) G–CdS nanosheets. (c) IPCE spectra and (d) dark current–voltage (J–V) characteristics of PBDTTT-C-T:PC70BM with different concentrations of G–CdS nanosheets.
Chem. Eur. J. 2014, 20, 1 – 10
www.chemeurj.org
These are not the final page numbers! ÞÞ
7
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
Full Paper Table 1. Average solar-cell parameters for PBDTTT-C-T:PC70BM with different concentrations of RGO and G–CdS nanosheets under AM1.5G solar illumination. Rsh=shunt resistance. Devices
Jsc [mA cm 2] Voc [V]
FF [%]
PCE [%] Rs [W cm2] Rsh [W cm2]
reference RGO 0.5 wt % RGO 1.5 wt % RGO 3.0 wt %
13.49 0.2 12.88 0.2 11.92 0.3 11.35 0.2
58.0 1 56.5 1 58.1 1 55.6 1
6.0 0.1 5.5 0.1 5.3 0.1 4.8 0.1
G–CdS 0.5 wt % 15.52 0.2 G–CdS 1.5 wt % 16.82 0.3 G–CdS 3.0 wt % 14.54 0.2
0.765 0.002 0.761 0.002 0.765 0.003 0.761 0.002
3.28 4.39 4.89 5.35
578.3 529.3 488.1 446.7
0.765 0.003 58.5 1 6.9 0.1 2.18 0.765 0.003 58.1 1 7.5 0.1 2.06 0.765 0.003 58.4 1 6.5 0.1 2.27
622.7 683.6 582.2
To directly fabricate G–CdS from GO, GO was dispersed in DMSO, first by sonication, then a metal–xanthate precursor [Cd(S2COEt)2(C5H4N)2] was added. The metal–xanthate precursor was synthesized according to a previously published procedure.[33] The mixture was vigorously stirred for 1 h at room temperature to ensure sufficient interaction between GO and the xanthate precursor. Next, the mixture was moved into an oil bath and stirred at 180 8C overnight under N2 atmosphere for the solvothermal reduction process. The xanthate precursor spontaneously and completely decomposes at 180 8C, releasing C2H4, COS, and H2S. At the same time, the C2H4, COS, and H2S also act as reductants for the reduction of GO. The obtained solution was repeatedly centrifuged and washed with acetone and alcohol to achieve G–CdS nanosheets. The G–CdS nanosheets were dispersed in o-dichlorobenzene for use. The RGO was obtained by the same solvothermal reduction process.
Preparation of ZnO precursor The ZnO-precursor solutions were prepared by a previously described method.[34] Typically, zinc acetate dihydrate (Zn(CH3COO)2·2H2O, 1 g, Sigma Aldrich) and ethanolamine (NH2CH2CH2OH, 0.28 g, Sigma Aldrich) were dissolved in 2-methoxyethanol (CH3OCH2CH2OH, 10 mL, Sigma Aldrich), and vigorously stirred in air at room temperature overnight for the hydrolysis reaction.
In summary, 2D G–CdS nanosheets were in situ synthesized by a one-step process by using GO as a source for quantum wells. During the one-pot process, the reduction of GO and spontaneous deposition of CdS nanocrystals on the graphene surface occurred simultaneously. A tight contact is formed between CdS and graphene in the G–CdS nanosheets without any adhesive media. Incorporation of the 2D G–CdS nanosheets into the PBDTTT-C-T:PC70BM active layer provided versatile heterojunction interfaces, which not only significantly improve the Chem. Eur. J. 2014, 20, 1 – 10
www.chemeurj.org
GO synthesis
G–CdS and RGO synthesis
Conclusion
&
Experimental Section
GO was synthesized by a modified Hummers’ method.[15, 16] In a typical preparation, graphite (purchased from Sigma Aldrich), NaNO3, and concentrated sulfuric acid were mixed in a flask immersed in an ice bath. An oxidizing agent, KMnO4, was then slowly added into the mixture solution, which was stirred at 35 8C. After 1 h, deionized water was added slowly. Next, H2O2 (30 %) was added into the flask to end the reaction. The suspension was then repeatedly centrifuged and washed with HCl solution (10 %) and water to remove residue.
AFM images (Figure 9), the root-mean-squared (RMS) roughness of PBDTTT-C-T:PC70BM is 1.8 nm, whereas that of the PBDTTT-C-T:PC70BM:G–CdS (1.5 wt %) increases to 2.6 nm. Typically, a rougher surface will enlarge the contact area between the interface of the active layer and the electrodes; this results in more efficient charge transport and reduced series resistance (Rs).[29] The favorable nanomorphology of PBDTTT-CT:PC70BM:G–CdS is reflected by the reduced Rs of the devices (Figure 11 d and Table 1). The lowest Rs of 2.06 W cm2 is obtained in the device based on 1.5 wt % G–CdS, which also has the highest efficiency and Jsc.[30] Although the mechanical properties of the active layer is important for practical applications of PSCs, information related to these properties is scarcely reported.[31, 32] In this work, the mechanical properties of PBDTTT-C-T:PC70BM and PBDTTT-CT:PC70BM:G–CdS (1.5 wt %) are examined by Young’s modulus and adhesion measurements (Figure 9). Young’s modulus measurements provide information about the mechanical properties of the sample within several tens of nanometers from the surface (penetration depth is determined by the propagation of strain induced by AFM probe indentation) and allow inference about the composition of the surface layer.[31] Adhesion influences the degree of deformation of the roughness of the structures at the contact. The Young’s modulus of PBDTTT-C-T:PC70BM and PBDTTT-C-T:PC70BM:G–CdS (1.5 wt %) were 11.3 and 2.1 GPa, respectively, and the corresponding adhesion were 0.9 and 3.6 nN. The reduced Young’s modulus and increased adhesion of the film with G–CdS nanosheets favor imitate contact between the active layer and the electrodes and provide good flexibility to meet the practical applications, especially on the flexible substrate.
&
charge-carrier separation capacity, but also remarkably enhance the collection and transportation efficiency of the photoinduced charge carriers. The PCE was successfully improved from 6.0 % to 7.5 % by this synergistic exciton separation and charge-transport effect. These results make a promising pathway for future optimization of OSCs performance.
Preparation of PBDTTT-C-T:G–CdS or RGO:PC70BM ternary solutions PBDTTT-C-T:G–CdS or RGO:PC70BM ternary solutions were prepared according to the following procedures: PBDTTT-C-T and PC70BM (1:1.5 w/w) were dissolved into o-dichlorobenzene in a glove box by stirring at 60 8C overnight, and then mixed with varying amounts of G–CdS or RGO (0, 0.5, 1.5, and 3.0 wt % dispersed in o-dichlorobenzene) to obtain 10 mg mL 1 (PBDTTT-C-T:o-dichlorobenzene) solutions.
8
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
Full Paper Device fabrication
[2] a) Y. Li, Acc. Chem. Res. 2012, 45, 723; b) M. T. Dang, L. Hirsch, G. Wantz, J. D. Wuest, Chem. Rev. 2013, 113, 3734; c) L. Yang, H. Zhou, S. C. Price, W. You, J. Am. Chem. Soc. 2012, 134, 5432; d) J. You, L. Dou, K. Yoshimura, T. Kato, K. Ohya, T. Moriarty, K. Emery, C. C. Chen, J. Gao, G. Li, Y. Yang, Nat. Commun. 2013, 4, 1446. [3] a) G. Dennler, M. C. Scharber, C. J. Brabec, Adv. Mater. 2009, 21, 1323; b) B. Kannan, K. Castelino, A. Majumdar, Nano Lett. 2003, 3, 1729. [4] L. Lu, T. Xu, W. Chen, J. M. Lee, Z. Luo, I. H. Jung, H. I. Park, S. O. Kim, L. Yu, Nano Lett. 2013, 13, 2365. [5] J. M. Lee, B.-H. Kwon, H. I. Park, H. Kim, M. G. Kim, J. S. Park, E. S. Kim, S. Yoo, D. Y. Jeon, S. O. Kim, Adv. Mater. 2013, 25, 2011. [6] G. H. Jun, S. H. Jin, B. Lee, B. H. Kim, W.-S. Chae, S. H. Hong, S. Jeon, Energy Environ. Sci. 2013, 6, 3000. [7] W. U. Huynh, J. J. Dittmer, A. P. Alivisatos, Science 2002, 295, 2425. [8] Y. F. Zhou, M. Eck, M. Kruger, Energy Environ. Sci. 2010, 3, 1851. [9] D. Wang, J. K. Baral, H. Zhao, B. A. Gonfa, V.-V. Truong, M. A. El Khakani, R. Izquierdo, D. Ma, Adv. Funct. Mater. 2011, 21, 4010. [10] Y. Xia, T. D. Nguyen, M. Yang, B. Lee, A. Santos, P. Podsiadlo, Z. Tang, S. C. Glotzer, N. A. Kotov, Nat. Nano 2011, 6, 580. [11] G. Eda, G. Fanchini, M. Chhowalla, Nat. Nano 2008, 3, 270. [12] V. C. Tung, M. J. Allen, Y. Yang, R. B. Kaner, Nat. Nano 2009, 4, 25; A. Iwan, A. Chuchmała, Prog. Polym. Sci. 2012, 37, 1805. [13] a) S. Guo, D. Bao, S. Upadhyayula, W. Wang, A. B. Guvenc, J. R. Kyle, H. Hosseinibay, K. N. Bozhilov, V. I. Vullev, C. S. Ozkan, M. Ozkan, Adv. Funct. Mater. 2013, 23, 5199; b) U. N. Maiti, W. J. Lee, J. M. Lee, Y. Oh, J. Y. Kim, J. E. Kim, J. Shim, T. H. Han, S. O. Kim, Adv. Mater. 2014, 26, 40. [14] Q. Li, B. Guo, J. Yu, J. Ran, B. Zhang, H. Yan, J. R. Gong, J. Am. Chem. Soc. 2011, 133, 10878. [15] I. V. Lightcap, P. V. Kamat, J. Am. Chem. Soc. 2012, 134, 7109. [16] A. Cao, Z. Liu, S. Chu, M. Wu, Z. Ye, Z. Cai, Y. Chang, S. Wang, Q. Gong, Y. Liu, Adv. Mater. 2010, 22, 103. [17] Z. Yin, Z. Zeng, J. Liu, Q. He, P. Chen, H. Zhang, Small 2013, 9, 727. [18] L. Huo, S. Zhang, X. Guo, F. Xu, Y. Li, J. Hou, Angew. Chem. 2011, 123, 9871; Angew. Chem. Int. Ed. 2011, 50, 9697. [19] J. K. Kim, M. J. Park, S. J. Kim, D. H. Wang, S. P. Cho, S. Bae, J. H. Park, B. H. Hong, ACS Nano 2013, 7, 7207. [20] J.-L. Li, K. N. Kudin, M. J. McAllister, R. K. Prud’homme, I. A. Aksay, R. Car, Phys. Rev. Lett. 2006, 96, 176101. [21] S. Stankovich, D. A. Dikin, R. D. Piner, K. A. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S. T. Nguyen, R. S. Ruoff, Carbon 2007, 45, 1558. [22] P. K. Narayanam, G. Singh, V. Divakar Botcha, D. S. Sutar, S. S. Talwar, R. S. Srinivasa, S. S. Major, Nanotechnology 2012, 23, 325605. [23] T. Rakshit, S. P. Mondal, I. Manna, S. K. Ray, ACS Appl. Mater. Interfaces 2012, 4, 6085. [24] M. Ding, D. C. Sorescu, A. Star, J. Am. Chem. Soc. 2013, 135, 9015. [25] Y. Su, S. Li, D. Wu, F. Zhang, H. Liang, P. Gao, C. Cheng, X. Feng, ACS Nano 2012, 6, 8349. [26] D. Wei, Y. Liu, Y. Wang, H. Zhang, L. Huang, G. Yu, Nano Lett. 2009, 9, 1752. [27] M. P. Hendricks, B. M. Cossairt, J. S. Owen, ACS Nano 2012, 6, 10054. [28] W. Chen, T. Xu, F. He, W. Wang, C. Wang, J. Strzalka, Y. Liu, J. Wen, D. J. Miller, J. Chen, K. Hong, L. Yu, S. B. Darling, Nano Lett. 2011, 11, 3707. [29] C. Li, Y. Chen, Y. Wang, Z. Iqbal, M. Chhowalla, S. Mitra, J. Mater. Chem. 2007, 17, 2406. [30] C. Renaud, S. J. Mougnier, E. Pavlopoulou, C. Brochon, G. Fleury, D. Deribew, G. Portale, E. Cloutet, S. Chambon, L. Vignau, G. Hadziioannou, Adv. Mater. 2012, 24, 2196. [31] M. P. Nikiforov, S. B. Darling, Prog. Anesth. Mech. Prog. Photovoltaics: Res. Appl 2013, 21, 1433. [32] S. Savagatrup, A. S. Makaram, D. J. Burke, D. J. Lipomi, Adv. Funct. Mater. 2014, 24, 1169. [33] H. C. Leventis, S. P. King, A. Sudlow, M. S. Hill, K. C. Molloy, S. A. Haque, Nano Lett. 2010, 10, 1253. [34] Y. Sun, J. H. Seo, C. J. Takacs, J. Seifter, A. J. Heeger, Adv. Mater. 2011, 23, 1679.
The desired geometric configuration of indium tin oxide (ITO)coated glass substrates were patterned by etching. The ITO glass substrates were cleaned with detergent, ultrasonicated in de-ionized water, acetone, and isopropyl alcohol for 20 min each. The cleaned ITO glass substrates were covered by spin coating with the ZnO-precursor solution. After annealing at 200 8C for 1 h, the photoactive layer of the PBDTTT-C-T:G–CdS or RGO:PC70BM ternary solutions were spin coated on the substrates at 1000 rpm for 2 min. Before spin coating the photoactive layer, 1,8-diiodooctane (DIO, 3 %, v/v), was added to the solutions. After the photoactive layer had dried in a glove box, an interfacial layer of MoO3 was thermal deposited. Finally, a 100 nm thick Ag top electrode was deposited to complete the device with an active area of 0.04 cm2 controlled by a shadow mask.
Characterization The nanoarchitectures and morphology of GO and G–CdS were characterized by environmental SEM (FEI, Quanta-200F), field emission TEM, HRTEM (JEOL, JEM-2100F), and laser scanning confocal microscopy (CarlZeiss LSM710). The TEM and HRTEM samples were prepared by drop casting solution onto carbon-coated copper grids. The optical characteristics of GO, G–CdS, and the photoactive layers were carried out by UV/Vis-NIR spectroscopy (Lambda 750, PerkinElmer). The charge transfer among the active materials was investigated with a fluorescence spectrometer (Hitachi F-7000) and TRPL (Edinburgh Instrument FLS920) measurements. The XPS spectra were performed with a Kratos AXIS Ultra XPS system. UPS spectra were collected with an AXIS-ULTRA DLD spectrometer (Kratos Analytical Ltd.). Raman spectroscopy data were performed with a LabRam-1B Raman microscope. XRD study of GO and G–CdS were recorded on a Bruker D8 Focus X-ray diffractometer with a copper target (l = 1.54 ). Photovoltaic characterization was performed under 100 mW cm 2 simulated AM 1.5G irradiation (Abet Solar Simulator Sun2000) and in the dark. The current–voltage characterization was recorded by using a Keithley 2400 Source Meter. IPCE was carried out under monochromatic illumination (Oriel Cornerstone 260 1/4 m monochromator equipped with Oriel 70613NS QTH lamp), the incident light was calibrated with a monocrystalline silicon diode. The topography of the active layer was investigated by AFM (Bruker, MultiMode 8) PeakForce Tapping module, the Young’s modulus and adhesion measurements were detected by AFM (PeakForce QNM), the peak currents were measured by CAFM by using peak force tapping tunneling AFM (PeakForce TUNA).
Acknowledgements This work was supported by the National Natural Science Foundation of China (51273088). Keywords: CdS · exciton dissociation nanosheets · polymer solar cells
·
graphene
·
[1] a) G. Li, R. Zhu, Y. Yang, Nature Photonics 2012, 6, 153; b) S. Gnes, H. Neugebauer, N. S. Sariciftci, Chem. Rev. 2007, 107, 1324; c) N. Espinosa, M. Hosel, D. Angmo, F. C. Krebs, Energ Environ. Sci. 2012, 5, 5117; d) J. M. Lee, J. S. Park, S. H. Lee, H. Kim, S. Yoo, S. O. Kim, Adv. Mater. 2011, 23, 629.
Chem. Eur. J. 2014, 20, 1 – 10
www.chemeurj.org
These are not the final page numbers! ÞÞ
Received: January 11, 2014 Published online on && &&, 0000
9
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
Full Paper
FULL PAPER & Solar Cells
Two-dimensional exciton dissociation centers were fabricated in situ. By incorporating these centers into active layers (see figure, ITO = indium tin oxide, GO = graphene oxide), exciton dissociation and separation was dramatically improved with remarkably enhanced collecting and transporting efficiency of photoinduced electrons and holes, and thus elevated device performance.
K. Yuan, L. Chen, L. Tan, Y. Chen* && – && Performance Enhancement of Bulk Heterojunction Solar Cells with Direct Growth of CdS-Cluster-Decorated Graphene Nanosheets
&
&
Chem. Eur. J. 2014, 20, 1 – 10
www.chemeurj.org
10
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
Full Paper
& Solar Cells
Performance Enhancement of Bulk Heterojunction Solar Cells with Direct Growth of CdS-Cluster-Decorated Graphene Nanosheets Kai Yuan,[a] Lie Chen,[a, b] Licheng Tan,[a] and Yiwang Chen*[a, b]
Abstract: Two-dimensional graphene–CdS (G–CdS) semiconductor hybrid nanosheets were synthesized in situ by graphene oxide (GO) quantum wells and a metal–xanthate precursor through a one-step growth process. Incorporation of G– CdS nanosheets into a photoactive film consisting of poly[4,8-bis-(2-ethyl-hexyl-thiophene-5-yl)-benzo[1,2-b:4,5-b]dithiophene-2,6-diyl]-alt-[2-(2-ethyl-hexanoyl)-thieno[3,4-b]thiophen-4,6-diyl] (PBDTTT-C-T) and [6,6]-phenyl C70 butyric acid methyl ester (PC70BM) effectively decreases the exciton lifetime to accelerate exciton dissociation. More importantly, the decreasing energy levels of PBDTTT-C-T, PC70BM, and G– CdS produces versatile heterojunction interfaces of PBDTTTC-T:PC70BM, PBDTTT-C-T:G–CdS, and PBDTTT-C-T:PC70BM:G– CdS; this offers multi-charge-transfer channels for more effi-
cient charge separation and transfer. The charge transfer in the blend film also depends on the G–CdS nanosheet loadings. In addition, G–CdS nanosheets improve light utilization and charge mobility in the photoactive layer. As a result, by incorporation of G–CdS nanosheets into the active layer, the power-conversion efficiency of inverted solar cells based on PBDTTT-C-T and PC71BM is improved from 6.0 % for a reference device without G–CdS nanosheets to 7.5 % for the device with 1.5wt % G–CdS nanosheets, due to the dramatically enhanced short-circuit current. Combined with the advantageous mechanical properties of the PBDTTT-CT:PC70BM:G–CdS active layer, the novel CdS-cluster-decorated graphene hybrid nanomaterials provide a promising approach to improve the device performance.
Introduction
rated charges of excitons before collected at the corresponding electrodes.[5, 6] To complement the OSCs performance, semiconductor nanocrystals with high charge mobility has been exploited as acceptors for OSCs.[7, 8] The absorption range and band gap of semiconductor nanocrystals can be precisely controlled by quantum-confinement effects.[9] The tunable energy levels of semiconductor nanocrystals favor charge separation and transfer between high-electron-affinity semiconductor nanocrystals and relatively low-ionization-potential conjugated polymers.[7] In spite of these conceptual advantages, OSCs based on conjugated polymers combined with semiconductor nanocrystals has so far exhibited poor performances. Because of the uncontrolled phase separation of semiconductor nanocrystals from conjugated polymer matrices, it is hard to form the desired, bicontinuous, interpenetrating, network nanomorphology. Moreover, the charge-carrier transport is allowed through inefficient random hopping among discrete semiconductor nanocrystalline structures.[10] 2D materials, such as quantum wells, offer particular capacity for efficient photogenerated charge-carrier collection. The 2D quantum-well morphology of graphene, possess excellent electron and hole mobility, electrochemical and thermal stability, solution processability, and good compatibility with conjugated polymers; this makes graphene a desirable candidate for photovoltaic and optoelectronic applications.[11–13] Efforts have been devoted to combine semiconductor nanocrystals with 2D nanostructures, because 2D nanostructures not only provide support for anchoring semiconductor nanocrystals, but also
Organic solar cells (OSCs) are promising materials for renewable and mobile energy generation, due to the low-costs, solution-synthesis, light-weight, and large-scales manufacturing characteristics.[1] Recently, the power-conversion efficiency (PCE) of OSCs has been significantly improved, up to 8–9 % for single-cell devices, by rational design of new, narrow-bandgap, conjugated polymers, optimization of device architectures, and controlling the active-layer morphology.[2] Nevertheless, the overall device performance of OSCs is still limited by various factors, such as extremely short length of the exciton diffusion (ca. 10 nm), inefficient charge separation of excitons and low charge-carrier mobility within the photoactive layer.[3, 4, 5] Amongst these, low electron and hole mobility is the crucial limitation for improvement of PCE. A low charge-carrier mobility leads to recombination or/and trapping of the sepa-
[a] K. Yuan, Prof. Dr. L. Chen, Dr. L. Tan, Prof. Dr. Y. Chen Institute of Polymers/Department of Chemistry Nanchang University, 999 Xuefu Avenue Nanchang 330031 (P.R. China) E-mail: [email protected] [b] Prof. Dr. L. Chen, Prof. Dr. Y. Chen Jiangxi Provincial Key Laboratory of New Energy Chemistry Nanchang University, 999 Xuefu Avenue Nanchang 330031 (P.R. China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201400119. Chem. Eur. J. 2014, 20, 1 – 10
These are not the final page numbers! ÞÞ
1
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
Full Paper act as acceptors to accelerate exciton dissociation and electron-hole transport.[13–16] However, graphene oxide (GO) or reduced graphene oxide (RGO) quantum wells and the semiconductor nanocrystalline structures, commonly used for these semiconductor nanocrystals/quantum-well hybrid materials, are synthesized separately.[13, 15, 17] In this work, we present in situ fabrication of 2D, graphene– CdS cluster (G–CdS), semiconductor nanocrystals/quantumwell hybrid nanosheets in one step by employing GO as quantum wells and metal–xanthate as CdS precursor. During the one-step growth process, the reduction of GO and the spontaneous deposition of CdS nanocrystals on the graphene surface occurred simultaneously. The tight contact between CdS and graphene in the G–CdS nanosheets without any adhesive media is favorable for charge injection, separation, and transport. The favorable energy alignment among PBDTTT-C-T (poly[4,8-bis-(2-ethyl-hexyl-thiophene-5-yl)-benzo[1,2-b:4,5-b]dithiophene-2,6-diyl]-alt-[2-(2-ethyl-hexanoyl)-thieno[3,4-b]thiophen-4,6-diyl]),[18] PC70BM ([6,6]-phenyl C70butyric acid methyl ester), and G–CdS produces versatile heterojunction interfaces in this film; this offers multi-charge-transfer channels for more efficient charge separation and transfer. Consequently, by introducing G–CdS nanosheets into the active layer, the PCE of inverted bulk heterojunction solar cells based on PBDTTT-C-T and PC70BM was improved from 6.0 % to 7.5 %. Furthermore, compared with the reference active layer without G–CdS nanosheets, the PBDTTT-C-T:PC70BM:G–CdS ternary film shows improved mechanical property, which is beneficial for practical applications.
Figure 1. (a) Schematic of the in situ synthesized 2D G–CdS nanosheets by the one-pot process, and (b) the device configuration of the PBDTTT-CT:PC70BM:G–CdS active layer fabricated in this work.
Results and Discussion Figure 1 a presents the simple process of in situ fabricated G– CdS nanosheets. Hydroxyl or carboxyl groups on the surface of GO first react with the CdS precursor, the metal–xanthate [Cd(S2COEt)2(C5H4N)2]. The grafted [Cd(S2COEt)2(C5H4N)2] further decomposes to form CdS by a solvothermal reduction process at 180 8C and GO is reduced at the same time. The decomposing process is initiated at the GO surface to produce a 2D G– CdS hybrid structure. G–CdS nanosheets are mixed with a PBDTTT-C-T:PC70BM solution to afford a photosensible layer deposited by spin coating. Figure 1 b displays the device configuration based on the PBDTTT-C-T:G-CdS:PC70BM blend. The nanoarchitecture of the G–CdS nanosheets can be discerned through microscopic characterization. Figure 2 a shows the typical transmission electron miscroscopy (TEM) image of GO, and the characteristic wrinkles, a property of a flexible, single-layer, 2D, graphene sheets structure, are observed. After solvothermal reduction, the CdS clusters were dispersed on the surface of the graphene sheets without evident phase separation, revealed by the large-scale scanning electron microscopy (SEM) image (Figure 2 b). The proper well-proportioned emission obtained from a laser scanning confocal microscopy image (Figure 2 c) also confirms the homogeneous distribution of the CdS clusters on the graphene sheets. To gain information on the internal microstructure of the G–CdS nanosheets, TEM was employed. Similar to the SEM observation, TEM re&
&
Chem. Eur. J. 2014, 20, 1 – 10
www.chemeurj.org
Figure 2. (a) TEM micrograph of GO. (b) SEM, (c) laser scanning confocal microscopy, and (d) TEM image of the G–CdS nanosheets. (e) High-magnification TEM image of the G–CdS nanosheets. The inset shows the EDX spectrum, which confirms the presence of CdS. (f) HRTEM images of the G–CdS nanosheets. The insets show a selected-area diffraction pattern of the G– CdS nanosheets (upper right) and a lattice fringes image of a CdS nanocrystal (bottom right).
2
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
Full Paper veals that all of the graphene sheets are tightly decorated with CdS clusters, and no free CdS cluster or pure graphene sheets are seen in the images of Figure 2 d and e. The strong interaction between CdS and graphene without any adhesive media and the uniform dispersion of CdS clusters on the graphene sheets would enable G–CdS nanosheets with highly efficient optoelectronic characteristic.[16] The sizes of the distributed CdS nanoparticles are range from about 5 to 7 nm as determined by TEM (Figure 2 e) and high-resolution TEM (HRTEM) images in Figure 2 f. The lattice spacing of 0.337, 0.208, and 0.176 nm is attributed to the (111), (220), and (311) lattice planes of CdS, respectively;[14] this agrees well with the X-ray diffraction (XRD) results of G–CdS (Figure S1 in the Supporting Information). In the XRD pattern, the three main scattering angles at 26.898, 43.968, and 52.058 are assigned to the diffractions of the (111), (220), and (311) planes of CdS (JCPDS 100454), respectively.[16] The three typical lattice planes can also been evidenced by three diffraction rings in the selected-area electron diffraction (SAED) profile (inset of Figure 2 f), indicating that these nanoparticles are polycrystalline with blende structure. Additionally, the energy dispersive X-ray spectrum (EDX, inset of Figure 2 e) and the well-defined diffraction ring with six spots (inset of Figure 2 f) provide the direct evidence for the formation of CdS cluster and the reduction of GO to afford graphene flakes. GO possesses a great quantity of oxygen-related functional groups with sp2 and sp3 hybridizations.[19] The sp3 hybridization forms through breaking the C=C bonds under oxidative conditions. Hydroxyl groups, carbonyl groups, and epoxy pairs can be formed by further oxidation.[20] During the solvothermal reduction process, the oxygen-containing functional groups can be gradually eliminated.[11] To investigate the changes to the functional groups, the formation of CdS, and the reduction degree of GO after the solvothermal reduction process, highresolution C1s X-ray photoelectron spectroscopy (XPS) was performed on the samples of GO, GO:CdS precursor (GO–Pro), and G–CdS (Figure 3). Figure 3 a shows the high-resolution C1s XPS spectrum of GO, and the fitted peaks correspond to carbon atoms in diverse functional groups. The peaks located at 284.8, 286.9, and in the approximate range 287.9–288.9 eV are associated to the sp2 C atoms, the C atoms in the epoxy/ hydroxyl groups (C O), and the carbonyl groups (C=O and O C=O), respectively.[21] The Cd2 + ion can interact with GO through electrophilic epoxy ring opening or coordination to oxygen-based functional groups.[22] Interaction between Cd2 + and GO also occur in the GO:CdS precursor solution. As shown in the C1s XPS spectrum of GO-Pro (Figure 3 b), the intensity of the peaks corresponding to C O and O C=O dramatically decrease, indicating that attractive interaction between the CdS precursor and the oxygen-based functional groups. In comparison, after the solvothermal reduction process, the peaks of C O, C=O, and O C=C almost disappear in the C1s XPS spectrum of G–CdS (Figure 3 c). The content of graphitic carbon and oxidized carbon in GO are 45.6 % and 54.4 %, respectively; in G-CdS nanosheets these numbers are changed to 67.3 and 32.7 %, respectively. The significantly decreased intensity of C O, C=O, and O C=C and the content changes of the comChem. Eur. J. 2014, 20, 1 – 10
www.chemeurj.org
These are not the final page numbers! ÞÞ
Figure 3. High-resolution C1s XPS of (a) GO (b) GO-Pro, and (c) G–CdS nanosheets.
ponents indicate the elimination of oxygen-based functional groups caused by the solvothermal reduction process. In addition, the binding energy of S2p (Figure S2–4 in the Supporting Information) peaked at 161.8 eV; this demonstrates the formation of CdS.[23] The main findings of the GO–Pro interaction and the formation of the G–CdS hybrid nanosheets can be explained by density functional theory (DFT) calculations. Figure 4 presents the optimized binding constructions of the CdS precursor on the GO surface. The theoretical simulations reveal that the CdS precursor can covalently interact with the oxygen-based functional groups. Figure 4 a and b depict the adhesion of the CdS precursor on the GO functionalized by hydroxy ( OH) and carboxyl ( COOH) groups to form covalent Cd O C and Cd O C=O linkages, respectively. The corresponding bond lengths of the Cd O bond are calculated to be about 2.28 in the Cd O C unit and 2.46 in the Cd O C=O unit. The short bond length means that the main characteristic of the strong interaction between the CdS precursor and the OH, COOH groups on the GO surface is chemisorption. The molecular orbitals from DFT calculations (B3LYP/6-31G**) for GO–Pro with the covalent Cd O C and Cd O C=O linkages are shown in Figure 4 d–g. The DFT calculations also demonstrate that the pyridine moiety in the CdS precursor has strong affinity to the basal plane of graphene through p–p stacking to form the versatile noncovalent composites GO–Pro (Figure 4 c). The molecular orbitals are depicted in Figure S5 in the Supporting Information. The interaction energy of the CdS precursor and graphene is estimated to be 5.21 kcal mol 1 and the vertical distance between the pyridine moiety and the graphene surface is about 2.75 . The strong physisorption and improved interfacial electronic coupling by means of the formation of 3
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
Full Paper the G band in the Raman spectra results from reduction of GO.[16] Meanwhile, the increased intensity of the G/D ratio also verifies successful reduction of GO,[6] to give good graphitic crystallinity.[25] A shift of the position of the G band usually means changes to the charge-transfer and electrical properties of graphene.[26] In the G–CdS nanosheet hybrid structures between the CdS clusters and graphene, no intermittent layer was observed. Because of the well-matched energy levels, shown in Figure 6 a, barrier-free charge transmission is expected between the CdS clusters and graphene. Figure 6 a illustrates the energy-level scheme of various materials employed for fabrication of the solar-cell devices. The
Figure 4. DFT simulation of the electronic coupling. (a) Representative adsorption configuration of a CdS precursor on GO with OH functionalities and formation of a covalent Cd O C linkage, the corresponding molecular orbitals from DFT calculations (B3LYP/6-31G**) are presented in (d) and (e). (b) Adsorption configuration of a CdS precursor on GO with COOH functionalities with formation of a covalent Cd O C=O linkage, the corresponding molecular orbitals from DFT calculations (B3LYP/6-31G**) are presented in (f) and (g). (c) Adsorption of a CdS precursor on graphene through the strong affinity between pyridine moieties and graphene through p–p stacking to form versatile noncovalent composites. C atoms are gray, S atoms are yellow, N atoms are blue, Cd atoms are faint yellow, O atoms are red, and H atoms are white.
a covalent linkage between the CdS precursor and GO would facilitate spontaneously deposition of the CdS nanocrystals on the graphene surface during the solvothermal reduction process.[16, 24] These results also agree well with the SEM, TEM, and XPS measurements. The obtained G–CdS nanosheets were further examined by Raman spectroscopy. The Raman spectra of the GO and G–CdS nanosheets are shown in Figure 5. Two characteristic peaks located at around 1330 and 1580 cm 1 correspond to the D and G bands of graphene, respectively. The G band of G–CdS is slightly shifted to lower wavenumbers in comparison with that of GO, whereas the D band remains unchanged. The shift of
Figure 6. (a) Energy-level diagram of various materials used for fabricating solar cells. Excitation and charge-transfer pathways in: (b) PBDTTT-CT:PC70BM- and (c) PBDTTT-C-T:PC70BM:G–CdS-based solar cells.
energy levels of G–CdS and RGO were determined by ultraviolet photoelectron spectroscopy (UPS) measurements (Figure S6 in the Supporting Information). As depicted in Figure 6 b, for the reference device based on PBDTTT-C-T:PC70BM without G–CdS nanosheets, the excitons in the PBDTTT-C-T donor can diffuse to the PBDTTT-C-T:PC70BM interface and dissociate through electron migration to the lowest unoccupied molecular orbital (LUMO) of PC70BM. Whereas, for devices based on the PBDTTT-C-T:PC70BM:G–CdS ternary blend (Figure 6 c), in addition to the fullerene acceptor, G–CdS nanosheets also act as electronic acceptors due to the low LUMO level of 4.3 eV. The exciton dissociation can occur both at the PBDTTT-C-T:PC70BM interface and at PBDTTT-C-T:G–CdS interface. More interesting, the LUMO energy level of PC70BM (4.2 eV) is located between the LUMO of PBDTTT-C-T (3.25 eV) and the conduction band of G–CdS (4.3 eV). Therefore, the favorable energy alignment between the LUMO levels enable PC70BM to function as an energy-gradient intermediate, so that
Figure 5. Raman spectra of GO and G–CdS nanosheets.
&
&
Chem. Eur. J. 2014, 20, 1 – 10
www.chemeurj.org
4
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
Full Paper electrons can efficiently be transported to the cathode through this energetic downhill cascade pathway, as shown in Figure 6 c. The multi-charge-transfer channels not only significantly improve the charge-carrier separation capacity, but also remarkably enhance the collection and transportation efficiency of the photoinduced charge carriers. The charge transfer at the interface through multi-chargetransfer channels within the active materials was predominantly investigated by static photoluminescence (PL) and time-resolved photoluminescence (TRPL) spectroscopy measurements as shown in (Figure 7, and Figure S7 in the Supporting Information). PBDTTT-C-T shows a strong PL emission peak at 780 nm assigned to the radiative decay of excitons to the ground state (Figure 7 a). For the PL spectra of PBDTTT-C-T with various loadings of RGO (Figure S7 a in the Supporting In-
transfer at PBDTTT-C-T:PC70BM and PBDTTT-C-T:G–CdS. In addition, as the content of G–CdS increases, the emission intensity of PBDTTT-C-T decreases sharply (Figure S7 b in the Supporting Information). The PL quench demonstrates the efficient charge separation that occurs at PBDTTT-C-T:G–CdS. A further quenched emission can be detected in the PBDTTT-CT:PC70BM:G-CdS ternary blends, suggesting that versatile heterojunction interfaces offer multi-charge-transfer channels for more efficient charge transfer. TRPL spectroscopy was carried out to measure the PL lifetime of PBDTTT-C-T:PC70BM films with various G–CdS nanosheets loadings (0.5, 1.5, and 3.0 wt %, Figure 7 b). The lifetime of PBDTTT-C-T:PC70BM film was determined to be 728 ps, whereas all of G–CdS-modified PBDTTT-CT:PC70BM blends show reduced PL lifetimes of 536, 418 and 591 ps for 0.5, 1.5 and 3.0 wt % G–CdS nanosheets, respectively. The reduced PL lifetimes are related to the improved exciton dissociation rates. Among the PBDTTT-C-T:PC70BM:G-CdS ternary blends, the film with 1.5 wt % loading of G–CdS achieve the shortest lifetime, suggesting an appropriate loading of nanosheet can induce the most favorable charge transfer in the active layer. In addition to exciton dissociation, the effect of G–CdS nanosheets on charge-carrier mobility in solar-cell devices was also investigated by measurements of hole-only and electron-only devices by using Mott-Gurney space charge limited current (SCLC) model (Figure 8 and Table S1 in the Supporting Information). The devices based on PBDTTT-C-T:PC70BM with G–CdS nanosheets exhibit improved electron-transport and holetransport behaviors. The hole-mobility increases more than twofold from 1.63 10 3 cm2 V 1 s 1 (without G–CdS) to 3.65 10 3 cm2 V 1 s 1 (with 1.5 wt % G–CdS), the corresponding electron mobility increased from 2.21 10 3 to 4.66 10 3 cm2 V 1 s 1. The improvement in charge-carrier mobility caused by the G–CdS nanosheets originates from the versatile charge-transport pathways in the active layers supported by the barrier-free charge injection, which is guaranteed by the well-matched energy levels of PBDTTT-C-T, PC70BM, and G–CdS at their interfaces (Figure 6). Because the conductivity of the active layers will exert influence on the charge-carrier mobility, the conductivities of the active layers with and without G–CdS nanosheets were determined by conductive atomic force microscopy (CAFM). Figure 9 presents the CAFM current images with a scan area of 2 2 mm for PBDTTT-C-T:PC70BM films without and with 1.5 wt % G–CdS nanosheets. The mean current increases from 76.3 pA for the film without G–CdS to 167.6 pA for the film with 1.5 wt % G–CdS. The enhanced conductivity of PBDTTT-CT:PC70BM:G–CdS is consistent with the improved charge-carrier mobility, which would be favorable for the enhancement of Jsc. The UV/Vis absorption spectra of GO and G–CdS films are presented in Figure 10 a. The absorption band of GO is mainly located in the short-wavelength range. The absorption of the G–CdS nanosheet is extended into the longer-wavelength region; this represents a characteristic absorption of CdS clusters, from 400 to 600 nm. Furthermore, the absorption range of nanosheets can be tuned by the quantum-confinement effect.[27] Figure 10 b shows the UV/Vis absorption spectra of
Figure 7. (a) Static PL spectra and (b) TRPL spectra of PBDTTT-T-C and PBDTTT-C-T:PC70BM with different concentrations of G–CdS nanosheets. The lifetime of PBDTTT-C-T:PC70BM films with 0, 0.5, 1.5, and 3.0 wt % of G–CdS nanosheets are 728, 536, 418, and 591 ps, respectively.
formation), there is no noticeable PL quench with increasing RGO concentrations; this means that the direct charge transfer from PBDTTT-C-T to RGO is hampered by the remarkable energy barrier at the PBDTTT-C-T:RGO interface. On the contrary, after blended with PC70BM or G–CdS, the PL of PBDTTTC-T is significantly quenched, confirming the effective charge Chem. Eur. J. 2014, 20, 1 – 10
www.chemeurj.org
These are not the final page numbers! ÞÞ
5
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
Full Paper
Figure 8. J1/2–V characteristics of (a) hole-only and (b) electron-only devices based on PBDTTT-C-T:PC70BM with different concentrations of G–CdS nanosheets according to SCLC modeling and measured at ambient temperature. The insets represent the architecture of hole- and electron-only devices.
Figure 10. (a) UV/Vis absorption profiles of thin films of GO and G–CdS composites. The inset shows a photograph of GO and G–CdS solutions. (b) UV/ Vis absorption spectra of PBDTTT-C-T:PC70BM:G–CdS films with various amounts of G–CdS nanosheets.
Figure 9. (a, f) AFM topography, (b, g) PeakForce error, (c, h) Young’s modulus, (d, i) adhesion, (e, j) CAFM of PBDTTT-C-T:PC70BM without and with 1.5 wt % of G–CdS nanosheets, respectively. The scanned areas are 2 2 mm.
&
&
Chem. Eur. J. 2014, 20, 1 – 10
www.chemeurj.org
6
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
Full Paper PBDTTT-C-T:PC70BM:G–CdS films with various amounts of G– CdS nanosheets (0.5–3.0 wt %). From the figure we can clearly see that incorporation of G–CdS nanosheets practically enhances the light harvesting of the blends compared with the reference film, and the absorption of the nanosheets is concentration dependent with regard to G–CdS nanosheets. Among these blends, PBDTTT-C-T:PC70BM:G–CdS films with 1.5wt % G– CdS shows the strongest absorption in the range 600–800 nm; this is beneficial for improving device performance. The contribution of well-characterized G–CdS nanosheets to the performance of bulk heterojunction solar cells based on PBDTTT-C-T:PC70BM:G–CdS were investigated and compared with the performance of the PBDTTT-C-T:PC70BM- and PBDTTT-C-T:PC70BM:RGO-based cells. Figure 11 presented the current-density–voltage (J–V) characteristics of the devices under AM1.5G (100 mW cm 2) light intensity illumination. The device based on PBDTTT-C-T:PC70BM has a PCE of 6.0 %, with a short-circuit current (Jsc) of 13.49 mA cm 2, an open-circuit voltage (Voc) of 0.765 V, a fill factor (FF) of 58.0 %. However, with the addition of RGO to the active layer, the PCE of the solar cell based on PBDTTT-C-T:PC70BM:RGO decreases from 6.0 % (without RGO) to 4.8 % (RGO 3.0 wt %), as shown in Figure 11 a. The reduced PCE mainly results from decreased Jsc ; this implies that RGO produces undesirable pathways for the recombination of electron holes;[6] this in turn is correlated to the PL observation. Incorporation of G–CdS nanosheets into the PBDTTT-C-T:PC70BM blends dramatically improve the device performance, and the device performance strongly depends on the loading of G–CdS sheets. The best PCE of 7.5 % is achieved together with an average Jsc value of 16.82 0.3 mA cm 2, Voc of 0.765 0.003 V, and FF of 58.1 1 % in the device based on the PBDTTT-C-T:PC70BM:G–CdS ternary blend with a G–CdS concentration of 1.5wt % (Figure 11 b). Table 1 summarizes the average solar-cell parameters of PBDTTT-CT:PC70BM with different concentrations of RGO and G–CdS nanosheets. A comparison of the reference device to the G– CdS-modified ones shows that both devices have almost the same values of Voc and FF. As expected, the improved performance of PBDTTT-C-T:PC70BM:G–CdS-based devices can be explained by the increased Jsc, which is caused by enhanced charge-carrier separation, transportation, and collection, as well as better light utilization, as mentioned above. The increased Jsc is consistent with the incident photon-to-current efficiency (IPCE) of the fabricated devices (Figure 11 c) The PBDTTT-C-T:PC70BM:G–CdS-based devices exhibit a higher IPCE value (72 %) than the reference devices (65 %) throughout the wavelength scope of 350–750 nm. The nanomorphology of the active layer is of major importance in charge separation and transport processes.[4, 28] The nanomorphology of PBDTTT-C-T:PC70BM and PBDTTT-CT:PC70BM:G–CdS (1.5 wt %) were measured by tapping-mode Figure 11. Current–voltage (J–V) characteristics of solar cells based on PBDTTT-C-T:PC70BM with different concentrations of (a) RGO and (b) G–CdS nanosheets. (c) IPCE spectra and (d) dark current–voltage (J–V) characteristics of PBDTTT-C-T:PC70BM with different concentrations of G–CdS nanosheets.
Chem. Eur. J. 2014, 20, 1 – 10
www.chemeurj.org
These are not the final page numbers! ÞÞ
7
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
Full Paper Table 1. Average solar-cell parameters for PBDTTT-C-T:PC70BM with different concentrations of RGO and G–CdS nanosheets under AM1.5G solar illumination. Rsh=shunt resistance. Devices
Jsc [mA cm 2] Voc [V]
FF [%]
PCE [%] Rs [W cm2] Rsh [W cm2]
reference RGO 0.5 wt % RGO 1.5 wt % RGO 3.0 wt %
13.49 0.2 12.88 0.2 11.92 0.3 11.35 0.2
58.0 1 56.5 1 58.1 1 55.6 1
6.0 0.1 5.5 0.1 5.3 0.1 4.8 0.1
G–CdS 0.5 wt % 15.52 0.2 G–CdS 1.5 wt % 16.82 0.3 G–CdS 3.0 wt % 14.54 0.2
0.765 0.002 0.761 0.002 0.765 0.003 0.761 0.002
3.28 4.39 4.89 5.35
578.3 529.3 488.1 446.7
0.765 0.003 58.5 1 6.9 0.1 2.18 0.765 0.003 58.1 1 7.5 0.1 2.06 0.765 0.003 58.4 1 6.5 0.1 2.27
622.7 683.6 582.2
To directly fabricate G–CdS from GO, GO was dispersed in DMSO, first by sonication, then a metal–xanthate precursor [Cd(S2COEt)2(C5H4N)2] was added. The metal–xanthate precursor was synthesized according to a previously published procedure.[33] The mixture was vigorously stirred for 1 h at room temperature to ensure sufficient interaction between GO and the xanthate precursor. Next, the mixture was moved into an oil bath and stirred at 180 8C overnight under N2 atmosphere for the solvothermal reduction process. The xanthate precursor spontaneously and completely decomposes at 180 8C, releasing C2H4, COS, and H2S. At the same time, the C2H4, COS, and H2S also act as reductants for the reduction of GO. The obtained solution was repeatedly centrifuged and washed with acetone and alcohol to achieve G–CdS nanosheets. The G–CdS nanosheets were dispersed in o-dichlorobenzene for use. The RGO was obtained by the same solvothermal reduction process.
Preparation of ZnO precursor The ZnO-precursor solutions were prepared by a previously described method.[34] Typically, zinc acetate dihydrate (Zn(CH3COO)2·2H2O, 1 g, Sigma Aldrich) and ethanolamine (NH2CH2CH2OH, 0.28 g, Sigma Aldrich) were dissolved in 2-methoxyethanol (CH3OCH2CH2OH, 10 mL, Sigma Aldrich), and vigorously stirred in air at room temperature overnight for the hydrolysis reaction.
In summary, 2D G–CdS nanosheets were in situ synthesized by a one-step process by using GO as a source for quantum wells. During the one-pot process, the reduction of GO and spontaneous deposition of CdS nanocrystals on the graphene surface occurred simultaneously. A tight contact is formed between CdS and graphene in the G–CdS nanosheets without any adhesive media. Incorporation of the 2D G–CdS nanosheets into the PBDTTT-C-T:PC70BM active layer provided versatile heterojunction interfaces, which not only significantly improve the Chem. Eur. J. 2014, 20, 1 – 10
www.chemeurj.org
GO synthesis
G–CdS and RGO synthesis
Conclusion
&
Experimental Section
GO was synthesized by a modified Hummers’ method.[15, 16] In a typical preparation, graphite (purchased from Sigma Aldrich), NaNO3, and concentrated sulfuric acid were mixed in a flask immersed in an ice bath. An oxidizing agent, KMnO4, was then slowly added into the mixture solution, which was stirred at 35 8C. After 1 h, deionized water was added slowly. Next, H2O2 (30 %) was added into the flask to end the reaction. The suspension was then repeatedly centrifuged and washed with HCl solution (10 %) and water to remove residue.
AFM images (Figure 9), the root-mean-squared (RMS) roughness of PBDTTT-C-T:PC70BM is 1.8 nm, whereas that of the PBDTTT-C-T:PC70BM:G–CdS (1.5 wt %) increases to 2.6 nm. Typically, a rougher surface will enlarge the contact area between the interface of the active layer and the electrodes; this results in more efficient charge transport and reduced series resistance (Rs).[29] The favorable nanomorphology of PBDTTT-CT:PC70BM:G–CdS is reflected by the reduced Rs of the devices (Figure 11 d and Table 1). The lowest Rs of 2.06 W cm2 is obtained in the device based on 1.5 wt % G–CdS, which also has the highest efficiency and Jsc.[30] Although the mechanical properties of the active layer is important for practical applications of PSCs, information related to these properties is scarcely reported.[31, 32] In this work, the mechanical properties of PBDTTT-C-T:PC70BM and PBDTTT-CT:PC70BM:G–CdS (1.5 wt %) are examined by Young’s modulus and adhesion measurements (Figure 9). Young’s modulus measurements provide information about the mechanical properties of the sample within several tens of nanometers from the surface (penetration depth is determined by the propagation of strain induced by AFM probe indentation) and allow inference about the composition of the surface layer.[31] Adhesion influences the degree of deformation of the roughness of the structures at the contact. The Young’s modulus of PBDTTT-C-T:PC70BM and PBDTTT-C-T:PC70BM:G–CdS (1.5 wt %) were 11.3 and 2.1 GPa, respectively, and the corresponding adhesion were 0.9 and 3.6 nN. The reduced Young’s modulus and increased adhesion of the film with G–CdS nanosheets favor imitate contact between the active layer and the electrodes and provide good flexibility to meet the practical applications, especially on the flexible substrate.
&
charge-carrier separation capacity, but also remarkably enhance the collection and transportation efficiency of the photoinduced charge carriers. The PCE was successfully improved from 6.0 % to 7.5 % by this synergistic exciton separation and charge-transport effect. These results make a promising pathway for future optimization of OSCs performance.
Preparation of PBDTTT-C-T:G–CdS or RGO:PC70BM ternary solutions PBDTTT-C-T:G–CdS or RGO:PC70BM ternary solutions were prepared according to the following procedures: PBDTTT-C-T and PC70BM (1:1.5 w/w) were dissolved into o-dichlorobenzene in a glove box by stirring at 60 8C overnight, and then mixed with varying amounts of G–CdS or RGO (0, 0.5, 1.5, and 3.0 wt % dispersed in o-dichlorobenzene) to obtain 10 mg mL 1 (PBDTTT-C-T:o-dichlorobenzene) solutions.
8
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
Full Paper Device fabrication
[2] a) Y. Li, Acc. Chem. Res. 2012, 45, 723; b) M. T. Dang, L. Hirsch, G. Wantz, J. D. Wuest, Chem. Rev. 2013, 113, 3734; c) L. Yang, H. Zhou, S. C. Price, W. You, J. Am. Chem. Soc. 2012, 134, 5432; d) J. You, L. Dou, K. Yoshimura, T. Kato, K. Ohya, T. Moriarty, K. Emery, C. C. Chen, J. Gao, G. Li, Y. Yang, Nat. Commun. 2013, 4, 1446. [3] a) G. Dennler, M. C. Scharber, C. J. Brabec, Adv. Mater. 2009, 21, 1323; b) B. Kannan, K. Castelino, A. Majumdar, Nano Lett. 2003, 3, 1729. [4] L. Lu, T. Xu, W. Chen, J. M. Lee, Z. Luo, I. H. Jung, H. I. Park, S. O. Kim, L. Yu, Nano Lett. 2013, 13, 2365. [5] J. M. Lee, B.-H. Kwon, H. I. Park, H. Kim, M. G. Kim, J. S. Park, E. S. Kim, S. Yoo, D. Y. Jeon, S. O. Kim, Adv. Mater. 2013, 25, 2011. [6] G. H. Jun, S. H. Jin, B. Lee, B. H. Kim, W.-S. Chae, S. H. Hong, S. Jeon, Energy Environ. Sci. 2013, 6, 3000. [7] W. U. Huynh, J. J. Dittmer, A. P. Alivisatos, Science 2002, 295, 2425. [8] Y. F. Zhou, M. Eck, M. Kruger, Energy Environ. Sci. 2010, 3, 1851. [9] D. Wang, J. K. Baral, H. Zhao, B. A. Gonfa, V.-V. Truong, M. A. El Khakani, R. Izquierdo, D. Ma, Adv. Funct. Mater. 2011, 21, 4010. [10] Y. Xia, T. D. Nguyen, M. Yang, B. Lee, A. Santos, P. Podsiadlo, Z. Tang, S. C. Glotzer, N. A. Kotov, Nat. Nano 2011, 6, 580. [11] G. Eda, G. Fanchini, M. Chhowalla, Nat. Nano 2008, 3, 270. [12] V. C. Tung, M. J. Allen, Y. Yang, R. B. Kaner, Nat. Nano 2009, 4, 25; A. Iwan, A. Chuchmała, Prog. Polym. Sci. 2012, 37, 1805. [13] a) S. Guo, D. Bao, S. Upadhyayula, W. Wang, A. B. Guvenc, J. R. Kyle, H. Hosseinibay, K. N. Bozhilov, V. I. Vullev, C. S. Ozkan, M. Ozkan, Adv. Funct. Mater. 2013, 23, 5199; b) U. N. Maiti, W. J. Lee, J. M. Lee, Y. Oh, J. Y. Kim, J. E. Kim, J. Shim, T. H. Han, S. O. Kim, Adv. Mater. 2014, 26, 40. [14] Q. Li, B. Guo, J. Yu, J. Ran, B. Zhang, H. Yan, J. R. Gong, J. Am. Chem. Soc. 2011, 133, 10878. [15] I. V. Lightcap, P. V. Kamat, J. Am. Chem. Soc. 2012, 134, 7109. [16] A. Cao, Z. Liu, S. Chu, M. Wu, Z. Ye, Z. Cai, Y. Chang, S. Wang, Q. Gong, Y. Liu, Adv. Mater. 2010, 22, 103. [17] Z. Yin, Z. Zeng, J. Liu, Q. He, P. Chen, H. Zhang, Small 2013, 9, 727. [18] L. Huo, S. Zhang, X. Guo, F. Xu, Y. Li, J. Hou, Angew. Chem. 2011, 123, 9871; Angew. Chem. Int. Ed. 2011, 50, 9697. [19] J. K. Kim, M. J. Park, S. J. Kim, D. H. Wang, S. P. Cho, S. Bae, J. H. Park, B. H. Hong, ACS Nano 2013, 7, 7207. [20] J.-L. Li, K. N. Kudin, M. J. McAllister, R. K. Prud’homme, I. A. Aksay, R. Car, Phys. Rev. Lett. 2006, 96, 176101. [21] S. Stankovich, D. A. Dikin, R. D. Piner, K. A. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S. T. Nguyen, R. S. Ruoff, Carbon 2007, 45, 1558. [22] P. K. Narayanam, G. Singh, V. Divakar Botcha, D. S. Sutar, S. S. Talwar, R. S. Srinivasa, S. S. Major, Nanotechnology 2012, 23, 325605. [23] T. Rakshit, S. P. Mondal, I. Manna, S. K. Ray, ACS Appl. Mater. Interfaces 2012, 4, 6085. [24] M. Ding, D. C. Sorescu, A. Star, J. Am. Chem. Soc. 2013, 135, 9015. [25] Y. Su, S. Li, D. Wu, F. Zhang, H. Liang, P. Gao, C. Cheng, X. Feng, ACS Nano 2012, 6, 8349. [26] D. Wei, Y. Liu, Y. Wang, H. Zhang, L. Huang, G. Yu, Nano Lett. 2009, 9, 1752. [27] M. P. Hendricks, B. M. Cossairt, J. S. Owen, ACS Nano 2012, 6, 10054. [28] W. Chen, T. Xu, F. He, W. Wang, C. Wang, J. Strzalka, Y. Liu, J. Wen, D. J. Miller, J. Chen, K. Hong, L. Yu, S. B. Darling, Nano Lett. 2011, 11, 3707. [29] C. Li, Y. Chen, Y. Wang, Z. Iqbal, M. Chhowalla, S. Mitra, J. Mater. Chem. 2007, 17, 2406. [30] C. Renaud, S. J. Mougnier, E. Pavlopoulou, C. Brochon, G. Fleury, D. Deribew, G. Portale, E. Cloutet, S. Chambon, L. Vignau, G. Hadziioannou, Adv. Mater. 2012, 24, 2196. [31] M. P. Nikiforov, S. B. Darling, Prog. Anesth. Mech. Prog. Photovoltaics: Res. Appl 2013, 21, 1433. [32] S. Savagatrup, A. S. Makaram, D. J. Burke, D. J. Lipomi, Adv. Funct. Mater. 2014, 24, 1169. [33] H. C. Leventis, S. P. King, A. Sudlow, M. S. Hill, K. C. Molloy, S. A. Haque, Nano Lett. 2010, 10, 1253. [34] Y. Sun, J. H. Seo, C. J. Takacs, J. Seifter, A. J. Heeger, Adv. Mater. 2011, 23, 1679.
The desired geometric configuration of indium tin oxide (ITO)coated glass substrates were patterned by etching. The ITO glass substrates were cleaned with detergent, ultrasonicated in de-ionized water, acetone, and isopropyl alcohol for 20 min each. The cleaned ITO glass substrates were covered by spin coating with the ZnO-precursor solution. After annealing at 200 8C for 1 h, the photoactive layer of the PBDTTT-C-T:G–CdS or RGO:PC70BM ternary solutions were spin coated on the substrates at 1000 rpm for 2 min. Before spin coating the photoactive layer, 1,8-diiodooctane (DIO, 3 %, v/v), was added to the solutions. After the photoactive layer had dried in a glove box, an interfacial layer of MoO3 was thermal deposited. Finally, a 100 nm thick Ag top electrode was deposited to complete the device with an active area of 0.04 cm2 controlled by a shadow mask.
Characterization The nanoarchitectures and morphology of GO and G–CdS were characterized by environmental SEM (FEI, Quanta-200F), field emission TEM, HRTEM (JEOL, JEM-2100F), and laser scanning confocal microscopy (CarlZeiss LSM710). The TEM and HRTEM samples were prepared by drop casting solution onto carbon-coated copper grids. The optical characteristics of GO, G–CdS, and the photoactive layers were carried out by UV/Vis-NIR spectroscopy (Lambda 750, PerkinElmer). The charge transfer among the active materials was investigated with a fluorescence spectrometer (Hitachi F-7000) and TRPL (Edinburgh Instrument FLS920) measurements. The XPS spectra were performed with a Kratos AXIS Ultra XPS system. UPS spectra were collected with an AXIS-ULTRA DLD spectrometer (Kratos Analytical Ltd.). Raman spectroscopy data were performed with a LabRam-1B Raman microscope. XRD study of GO and G–CdS were recorded on a Bruker D8 Focus X-ray diffractometer with a copper target (l = 1.54 ). Photovoltaic characterization was performed under 100 mW cm 2 simulated AM 1.5G irradiation (Abet Solar Simulator Sun2000) and in the dark. The current–voltage characterization was recorded by using a Keithley 2400 Source Meter. IPCE was carried out under monochromatic illumination (Oriel Cornerstone 260 1/4 m monochromator equipped with Oriel 70613NS QTH lamp), the incident light was calibrated with a monocrystalline silicon diode. The topography of the active layer was investigated by AFM (Bruker, MultiMode 8) PeakForce Tapping module, the Young’s modulus and adhesion measurements were detected by AFM (PeakForce QNM), the peak currents were measured by CAFM by using peak force tapping tunneling AFM (PeakForce TUNA).
Acknowledgements This work was supported by the National Natural Science Foundation of China (51273088). Keywords: CdS · exciton dissociation nanosheets · polymer solar cells
·
graphene
·
[1] a) G. Li, R. Zhu, Y. Yang, Nature Photonics 2012, 6, 153; b) S. Gnes, H. Neugebauer, N. S. Sariciftci, Chem. Rev. 2007, 107, 1324; c) N. Espinosa, M. Hosel, D. Angmo, F. C. Krebs, Energ Environ. Sci. 2012, 5, 5117; d) J. M. Lee, J. S. Park, S. H. Lee, H. Kim, S. Yoo, S. O. Kim, Adv. Mater. 2011, 23, 629.
Chem. Eur. J. 2014, 20, 1 – 10
www.chemeurj.org
These are not the final page numbers! ÞÞ
Received: January 11, 2014 Published online on && &&, 0000
9
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
Full Paper
FULL PAPER & Solar Cells
Two-dimensional exciton dissociation centers were fabricated in situ. By incorporating these centers into active layers (see figure, ITO = indium tin oxide, GO = graphene oxide), exciton dissociation and separation was dramatically improved with remarkably enhanced collecting and transporting efficiency of photoinduced electrons and holes, and thus elevated device performance.
K. Yuan, L. Chen, L. Tan, Y. Chen* && – && Performance Enhancement of Bulk Heterojunction Solar Cells with Direct Growth of CdS-Cluster-Decorated Graphene Nanosheets
&
&
Chem. Eur. J. 2014, 20, 1 – 10
www.chemeurj.org
10
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
