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Cite this: DOI: 10.1039/c5cc01375e Received 14th February 2015, Accepted 12th March 2015
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An organic photovoltaic featuring graphene nanoribbons† Seung Joo Lee,a Jae-Yeon Kim,a Hyeong Pil Kim,a Dongcheon Kim,a Wilson Jose da Silva,*b Fabio Kurt Schneider,b Abd. Rashid bin Mohd Yusoffa and Jin Jang*a
DOI: 10.1039/c5cc01375e www.rsc.org/chemcomm
A combination of graphene nanoribbons (GNRs) and carbon nanotubes (CNTs) was deployed as a potential candidate to replace the commonly used hole transport material poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) in a high performance organic photovoltaic. A power conversion efficiency (PCE) of 7.6% has been obtained using inkjet printing to fabricate the photovoltaic along with the presence of C60-bis as an electron transporting material.
To date, huge efforts have been devoted toward developing highly efficient organic photovoltaic (OPV) devices and it is accepted that highly efficient devices basically depend on excellent active materials as well as device optimization. Very recently, a few excellent active materials have been reported with a power conversion efficiency (PCE) of 49%, including poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b0 ]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]:6,6-phenyl-C71-butyric acid methyl ester (PTB7:PC71BM).1 In fact, further enhancement can be anticipated through tuning the surface morphology,2 optical modulation,3 and interfacial modification (hole transport layer (HTL) or electron transport layer (ETL)).4 In order to select suitable hole and electron transport layers (HTLs or ETLs, respectively), there are a few criteria one must satisfy such as visible transparency, conductivity, carrier concentration, mobility, suitability to flexible electronics, work function, deposition temperatures, chemical stability and interfacial chemistry and surface states.5 Hence, recent studies on various new HTLs such as poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS),6 molybdenum oxide (MoO3),7 and conjugated polyelectrolytes (CPE-K)8 as well as ETLs such as copper hexadecafluorophthalocyanine (F16CuPc), cesium stearate (CsSt), and titanium a
Department of Information, Display, Kyung Hee University, Dongdaemun-ku, Seoul 130-171, Republic of Korea. E-mail: [email protected]; Fax: +82 2 961 0270; Tel: +82 2 961 0270 b Universidade Tecnologica Federal do Parana, GPGEI – Av. Sete de Setembro, 3165 – CEP 80230-901 – Curitiba, Parana, Brazil. E-mail: [email protected]; Fax: +55 41 331 04691; Tel: +55 41 331 04691 † Electronic supplementary information (ESI) available: HR-TEM image, dark current, EQE, FTIR, and UPS spectra. See DOI: 10.1039/c5cc01375e
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oxide (TiOx),9–11 poly[(9,9-bis((N-(4-sulfonate-1-butyl)-N,N-dimethylammonium)propyl)-2,7-fluorene)-alt-N-phenyl-4,4-diphenylamine] (PFNSO-TOA),12 poly[3-(6-trimethylammoniumhexyl)thiophene]: anionic surfactant sodium dodecylbenzenesulfonate (PTMAHT:SDS),13 amino-functionalized conjugated metallopolymer (PFEN-Hg),14 2-methoxyethanol (2-ME) and ethanolamine (EA) co-solvents,15 CdS/2,9-dimethyl-4,7-diphenyl-1,10-henanthroline (CdSBCP),16 bathocuproine (BCP)17 and additives based on PTB7:PC71BM such as tetrabromothiophene (Br-ADD)18 have brought a significant improvement in efficiency. These approaches to enhance the device performance have shed light on the future development of a highly efficient OPV. It is worth noting that PTB7:PC71BM has also responded to metallic nanoparticles, providing a huge improvement in the efficiency.19,20 For many years, graphene nanoribbons (GNRs) have been of interest to chemists, due to their theoretically predicted physical properties.21 GNRs can be thought of as planar analogues of CNTs, with band gaps depending upon the ribbon width. Therefore, producing GNRs with defined widths and edge structures constitutes a great challenge that many chemists and materials scientists have sought to tackle. GNRs display a finite band gap when their width is less than 10 nm. Unlike graphene, GNRs demonstrate distinctive features in their electronic structure and optical properties, such as the opening of a finite band gap, which makes them interesting materials for carbon-based nanoelectronics.22–24 Therefore, in the context of this study, a general strategy is to design a novel HTL to make use of the unique features of GNRs at the molecular scale. We successfully fabricated high performance PTB7:PC71BM solar cells with GNRs as the hole transport layer using inkjet printing technology. We used inkjet printing technology to demonstrate the versatility of graphene in comparison with PEDOT:PSS. When GNRs were applied as the HTL, this optimized device exhibits an efficiency of over 5%. In fact, in the presence of multiple layers of GNRs, the efficiency is significantly decreased with a concomitant significant decrease in the photocurrent, open-circuit voltage as well as the fill factor (FF), nearly destroying the performance. This effect was tested in a regular structure, where the regular OPV geometry is set up, in which the photons enter through
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Fig. 1 (a) Chemical structures of the polymer (PTB7), fullerene (PCBM), carbon nanotubes (CNTs) and graphene nanoribbons (GNRs) used in the OPVs. (b) Energy band diagram of the studied device. The numbers below and above the bars in the histogram correspond to the energies of the highest occupied molecular orbital and the lowest unoccupied molecular orbital, respectively.
a transparent anode and must travel through the hole transport layer to reach a photoactive layer. Single and double stacked hole transport layers were then used to evaluate their compatibility. The ability to serve as a HTL is very useful for regular OPVs, as any suitable pair will result in much higher photovoltaic performance. Fig. 1 shows the chemical structures of PTB7, PCBM, CNTs, and GNRs used in this study as well as a schematic energy diagram of the PTB7:PC71BM bulk heterojunction (BHJ) solar cells with the GNR hole transport layer and the C60-bis selfassembled molecule (SAM) electron transport layer. All energy levels used in this study were taken from the literature.25–28 The control-cell structure, without the hole transport layer, consists of ITO-glass/PTB7:PC71BM/C60-bis SAM/Ag. The GNRs from the printing film are well attached to the surface of ITO. After printing the film, the GNR layer was dried at a low temperature (50 1C) for 30 min. Then, the photoactive layer is evenly spin coated on the GNR layer. Finally, the Ag cathode was deposited via thermal evaporation to complete the device. The detailed synthesis of the GNRs, the fabrication steps of the inkjet printing film, and the synthesis of the devices are described in the Experimental Section (ESI†). To explain the influence of the GNR layers on the energy level alignment, ultraviolet photoelectron spectroscopy (UPS) was employed. Fig. S1 (ESI†) shows
Fig. 2
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the UPS spectra for the PTB7:PC71BM, PTB7:PC71BM/PEDOT:PSS, PTB7:PC71BM/MoO3/GNRs and PTB7:PC71BM/CNTs/GNRs films. Fig. 1b illustrates the energy levels of all of the materials used in this work. Fig. 2a shows the Raman spectra of the chemical unzipping of multi-walled carbon nanotubes (MWCNTs) recorded using a Raman spectrometer (LabRAM HR) with the 633 nm line of an Ar laser. As shown in Fig. 2a (black triangles), the G and D bands, located at 1596 cm1 and 1320 cm1, respectively, are clearly observed. The presence of G and D bands demonstrates the reduction in size of the in-plane sp2-carbon domains due to oxidation. The G band corresponds to the formation of graphitized MWCNTs, while the latter originates from the finite size effect. The appearance of these bands shows that the acid treatment does not affect the structure of MWCNTs. It indicates that the highly oxidative unzipping process of MWCNTs breaks some bonds. In addition, the overtone of the D band is located at 2637 cm1 and is assigned as the D0 band.29 This is indeed in good agreement with the previously reported work by Ruoff et al.29 Fourier transform infrared spectroscopy (FTIR) was performed to support the Raman data. As we can see from Fig. S2 (ESI†), the vibration intensities of oxygen-containing groups (e.g. CQO at 1730 cm1) can be seen in the GNRs spectrum. The lowest angle diffraction peak after the chemical unzipping shifted to 2y B 9.71, which corresponds to the (002) reflection plane from intercalated stacked graphite (Fig. 2b).30 It is also worth mentioning that the peak shifts are a consequence of the existence of oxygen-containing functional groups after the highly oxidative unzipping process, which in turn enables intercalation of solvent and water molecules, thereby further increasing the distance between the partially stacked graphene oxide nanoribbons (GONRs). The absence of the peak at 2y B 25.81, corresponding to the interwall separation of MWCNTs, evidences a high degree of unzipping. The transmission electron microscopy (TEM) images of the GNRs are shown in Fig. 3a and b. In principle, the sensitivity of GNR properties is based on the width of the nanoribbons as well as the crystallographic orientation of the edges. There are two distinct types of edges (depending on the cutting direction): (i) zigzag, and (ii) armchair. Son et al. reported that a GNR with zigzag edges is predicted to have a magnetically ordered edge state while an armchair GNR is calculated to have a band gap that scales with the width of the nanoribbons.31 Moreover, the cutting direction also gives a boundary at the edge and this indicates that the geometry of the edge establishes
(a) Raman spectra of pristine CNTs and pristine GNRs, and (b) X-ray diffraction patterns of pristine CNTs and pristine GNRs.
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Fig. 3 (a, b) TEM images of GNRs, (c) HR-TEM image of the GNRs, and (d, e) AFM images of the CNTs and inkjet printed GNR/CNT. A zoomed in view of c can be found in the ESI† (Fig. S3).
a significant difference in the p-electron structure at the edge. As shown in Fig. 3c, our high resolution TEM (HR-TEM) image illustrates zigzag chains, which can be obviously seen from the graphene network. This implies that the thickness of the GNRs is within an atomic scale.32 It is worth noting that disorder at the edges of nanoribbons could potentially suppress the edge specific properties.33–36 Hence, in order to have minimal disorder at the edges of the nanoribbons, one has to minimize the disorder at the nanoribbon edges. The surface morphology of the CNT layers is shown in a 3D image obtained by atomic-force microscopy (AFM; Fig. 3d). The image shows uniformly percolated structures and flat surfaces with a root-mean-square (rms) roughness value of 0.3 nm. In addition, Fig. 3e depicts the printed GNR/CNT layer. Fig. 4a demonstrates the current density–voltage (J–V) plots of the PTB7:PC71BM OPVs with and without HTLs, such as GNRs (1 layer), and poly(3,4-ethylenedioxylenethiophene)-polystyrene sulfonic acid (PEDOT:PSS) (30 nm). Throughout this study, we refer to the layers by giving the number of layers printed. The regular-device structure consists of ITO-glass/HTL/PTB7:PC71BM/C60-bis/Ag. The device with the GNRs demonstrated a sharply increased power conversion efficiency (PCE) of 5.88% with an improved short-circuit current density (JSC) of about 13.74 mA cm2 and a FF of 56.14%. The PCE of the control device without the hole transport layer (ITO-only) was 4% with a JSC of about 11.20 mA cm2 and FF of 51.05% (Table S1, ESI†). However, the GNR device has been outperformed the device with a commonly used hole transport material, PEDOT:PSS, with
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7.32% PCE. The obtained PCE for our PEDOT:PSS device is slightly low compared to the GNRs/CNTs device due to a lower FF value. The lower FF in the PEDOT:PSS-based device is because the RS is larger compared to that of the GNRs/CNTs-based device. To confirm the effectiveness of GNRs as a hole transport layer, we have designed another study employing bilayer hole transport materials using the same device geometry. The regularcell structure with a bilayer consists of ITO-glass/HTL/HTL/ PTB7:PC71BM/C60-bis/Ag. Because of the well-matched highest occupied molecular orbital (HOMO) level (4.95 eV) and lowest unoccupied molecular orbital (LUMO) level (2.6 eV) of CNT with the BHJ,37 hole transport and electron blocking are effective during the device operation. The PCE performance of the photovoltaic with GNRs/CNTs between the photoactive layer and ITO increased significantly to 7.6% with an enhanced JSC of 15.79 mA cm2 and a FF of 63.84% after the printing process (Table S2, ESI†). The enhanced JSC and PCE are possibly attributed to a reduced electronic charge barrier between the BHJ and the GNR interface. This device exhibits the best PCE value of 7.60% (47.95% increase) with JSC = 15.79 mA cm2 and FF = 64% when compared to the control device without any HTL. Similarly, in the case of the device with GNRs/molybdenum oxide (MoO3), the exhibited PCE value increased substantially to 6.11%, with an enhancement of B4 mA cm2, and an increase of over 15% compared to the control device. The device with the GNRs/ CNTs showed an increase of 29.15%, 24.47% and 13.6% over GNRs/MoO3, GNR and PEDOT:PSS, respectively. All photovoltaic parameters are tabulated in Table S1 (ESI†). Moreover, the device with the GNRs/CNTs HTL exhibits a reduced series resistance (RS) of 12.5 O cm2 compared to that of the device without HTL (where RS = 20 O cm2), RS = 18.35 O cm2 with GNRs/MoO3, RS = 13.75 O cm2 with GNRs, and RS = 13.03 O cm2 with PEDOT:PSS. The reduced RS is well correlated with the enhanced JSC. A high shunt resistance (RSH) was obtained with the device with GNRs/ CNTs compared to the device without the interlayer (see the J–V curves at the dark current shown in Fig. S4, ESI†). Therefore, the GNRs layer plays an important role in reducing RS and increasing RSH, both of which cause improvement in the PCE. The HOMO energy level of GNRs (B4.9 eV)38 is close to the LUMO level of the fullerene acceptor, thus resulting in efficient hole charge transport. The average PCE values and the deviation of the PCE of the devices
Fig. 4 (a) J–V characteristics of OPVs with different HTLs, (b) external quantum efficiency, and (c) J–V characteristics with different numbers of layers of GNRs.
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without interlayers, with GNRs/CNTs, GNRs/MoO3, GNRs, CNTs and PEDOT:PSS were calculated using more than fifty devices to demonstrate the reproducibility. Fig. 4b illustrates the EQE corresponding to the J–V characteristics of Fig. 4a. The calculated JSC values from these spectra are in agreement with the values obtained from Fig. 4a. To confirm the layer dependence on the GNRs of the BHJ solar cells, the layer-by-layer printing of GNRs (one-layer, twolayers, and three-layers) was performed by sequential printing onto the patterned ITO-glass substrate (Table S2, ESI†). The device with a single single layer of transferred GNRs/CNTs exhibits the best average PCE of 7.60% with open-circuit voltage (VOC) = 0.75 V, JSC = 15.79 mA cm2 and FF = 64%; while the two-layer GNRs/CNTs device and three-layer GNRs/CNTs device show a gradually decreasing PCE of 5.14% (VOC = 0.73 V, JSC = 12.42 mA cm2, FF = 57%), and 4.44% (VOC = 0.70 V, JSC = 11.20 mA cm2, FF = 56%), respectively, as shown in Fig. 4c and by the external quantum efficiency (EQE) in Fig. S5 (ESI†). Thus, the sequential printing has a negative effect on the PCE of the devices because of both the reduced JSC and VOC, which originate from unstable contact between the GNR layers. Furthermore, a thick layer of GNRs will lead to an increased RS in the device. Therefore, optimized thickness and good contact with the GNR layer are important factors required to obtain high-performing solar cells. In summary, we successfully fabricated high-performance PBT7:PC71BM BHJ solar cells comprised of a GNR hole transport layer using inkjet printing technology. The GNR HTL plays a vital role in improving the JSC and PCE in OPVs compared to OPVs without any interlayer. In addition, the device with a HTL consisting of GNRs/CNTs exhibits the highest PCE of 7.60%, which has outperformed the PEDOT:PSS only device. OPVs with printed GNRs serving as the HTL is an encouraging architecture for realizing highly efficient OPVs.
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Cite this: DOI: 10.1039/c5cc01375e Received 14th February 2015, Accepted 12th March 2015
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An organic photovoltaic featuring graphene nanoribbons† Seung Joo Lee,a Jae-Yeon Kim,a Hyeong Pil Kim,a Dongcheon Kim,a Wilson Jose da Silva,*b Fabio Kurt Schneider,b Abd. Rashid bin Mohd Yusoffa and Jin Jang*a
DOI: 10.1039/c5cc01375e www.rsc.org/chemcomm
A combination of graphene nanoribbons (GNRs) and carbon nanotubes (CNTs) was deployed as a potential candidate to replace the commonly used hole transport material poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) in a high performance organic photovoltaic. A power conversion efficiency (PCE) of 7.6% has been obtained using inkjet printing to fabricate the photovoltaic along with the presence of C60-bis as an electron transporting material.
To date, huge efforts have been devoted toward developing highly efficient organic photovoltaic (OPV) devices and it is accepted that highly efficient devices basically depend on excellent active materials as well as device optimization. Very recently, a few excellent active materials have been reported with a power conversion efficiency (PCE) of 49%, including poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b0 ]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]:6,6-phenyl-C71-butyric acid methyl ester (PTB7:PC71BM).1 In fact, further enhancement can be anticipated through tuning the surface morphology,2 optical modulation,3 and interfacial modification (hole transport layer (HTL) or electron transport layer (ETL)).4 In order to select suitable hole and electron transport layers (HTLs or ETLs, respectively), there are a few criteria one must satisfy such as visible transparency, conductivity, carrier concentration, mobility, suitability to flexible electronics, work function, deposition temperatures, chemical stability and interfacial chemistry and surface states.5 Hence, recent studies on various new HTLs such as poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS),6 molybdenum oxide (MoO3),7 and conjugated polyelectrolytes (CPE-K)8 as well as ETLs such as copper hexadecafluorophthalocyanine (F16CuPc), cesium stearate (CsSt), and titanium a
Department of Information, Display, Kyung Hee University, Dongdaemun-ku, Seoul 130-171, Republic of Korea. E-mail: [email protected]; Fax: +82 2 961 0270; Tel: +82 2 961 0270 b Universidade Tecnologica Federal do Parana, GPGEI – Av. Sete de Setembro, 3165 – CEP 80230-901 – Curitiba, Parana, Brazil. E-mail: [email protected]; Fax: +55 41 331 04691; Tel: +55 41 331 04691 † Electronic supplementary information (ESI) available: HR-TEM image, dark current, EQE, FTIR, and UPS spectra. See DOI: 10.1039/c5cc01375e
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oxide (TiOx),9–11 poly[(9,9-bis((N-(4-sulfonate-1-butyl)-N,N-dimethylammonium)propyl)-2,7-fluorene)-alt-N-phenyl-4,4-diphenylamine] (PFNSO-TOA),12 poly[3-(6-trimethylammoniumhexyl)thiophene]: anionic surfactant sodium dodecylbenzenesulfonate (PTMAHT:SDS),13 amino-functionalized conjugated metallopolymer (PFEN-Hg),14 2-methoxyethanol (2-ME) and ethanolamine (EA) co-solvents,15 CdS/2,9-dimethyl-4,7-diphenyl-1,10-henanthroline (CdSBCP),16 bathocuproine (BCP)17 and additives based on PTB7:PC71BM such as tetrabromothiophene (Br-ADD)18 have brought a significant improvement in efficiency. These approaches to enhance the device performance have shed light on the future development of a highly efficient OPV. It is worth noting that PTB7:PC71BM has also responded to metallic nanoparticles, providing a huge improvement in the efficiency.19,20 For many years, graphene nanoribbons (GNRs) have been of interest to chemists, due to their theoretically predicted physical properties.21 GNRs can be thought of as planar analogues of CNTs, with band gaps depending upon the ribbon width. Therefore, producing GNRs with defined widths and edge structures constitutes a great challenge that many chemists and materials scientists have sought to tackle. GNRs display a finite band gap when their width is less than 10 nm. Unlike graphene, GNRs demonstrate distinctive features in their electronic structure and optical properties, such as the opening of a finite band gap, which makes them interesting materials for carbon-based nanoelectronics.22–24 Therefore, in the context of this study, a general strategy is to design a novel HTL to make use of the unique features of GNRs at the molecular scale. We successfully fabricated high performance PTB7:PC71BM solar cells with GNRs as the hole transport layer using inkjet printing technology. We used inkjet printing technology to demonstrate the versatility of graphene in comparison with PEDOT:PSS. When GNRs were applied as the HTL, this optimized device exhibits an efficiency of over 5%. In fact, in the presence of multiple layers of GNRs, the efficiency is significantly decreased with a concomitant significant decrease in the photocurrent, open-circuit voltage as well as the fill factor (FF), nearly destroying the performance. This effect was tested in a regular structure, where the regular OPV geometry is set up, in which the photons enter through
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Fig. 1 (a) Chemical structures of the polymer (PTB7), fullerene (PCBM), carbon nanotubes (CNTs) and graphene nanoribbons (GNRs) used in the OPVs. (b) Energy band diagram of the studied device. The numbers below and above the bars in the histogram correspond to the energies of the highest occupied molecular orbital and the lowest unoccupied molecular orbital, respectively.
a transparent anode and must travel through the hole transport layer to reach a photoactive layer. Single and double stacked hole transport layers were then used to evaluate their compatibility. The ability to serve as a HTL is very useful for regular OPVs, as any suitable pair will result in much higher photovoltaic performance. Fig. 1 shows the chemical structures of PTB7, PCBM, CNTs, and GNRs used in this study as well as a schematic energy diagram of the PTB7:PC71BM bulk heterojunction (BHJ) solar cells with the GNR hole transport layer and the C60-bis selfassembled molecule (SAM) electron transport layer. All energy levels used in this study were taken from the literature.25–28 The control-cell structure, without the hole transport layer, consists of ITO-glass/PTB7:PC71BM/C60-bis SAM/Ag. The GNRs from the printing film are well attached to the surface of ITO. After printing the film, the GNR layer was dried at a low temperature (50 1C) for 30 min. Then, the photoactive layer is evenly spin coated on the GNR layer. Finally, the Ag cathode was deposited via thermal evaporation to complete the device. The detailed synthesis of the GNRs, the fabrication steps of the inkjet printing film, and the synthesis of the devices are described in the Experimental Section (ESI†). To explain the influence of the GNR layers on the energy level alignment, ultraviolet photoelectron spectroscopy (UPS) was employed. Fig. S1 (ESI†) shows
Fig. 2
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the UPS spectra for the PTB7:PC71BM, PTB7:PC71BM/PEDOT:PSS, PTB7:PC71BM/MoO3/GNRs and PTB7:PC71BM/CNTs/GNRs films. Fig. 1b illustrates the energy levels of all of the materials used in this work. Fig. 2a shows the Raman spectra of the chemical unzipping of multi-walled carbon nanotubes (MWCNTs) recorded using a Raman spectrometer (LabRAM HR) with the 633 nm line of an Ar laser. As shown in Fig. 2a (black triangles), the G and D bands, located at 1596 cm1 and 1320 cm1, respectively, are clearly observed. The presence of G and D bands demonstrates the reduction in size of the in-plane sp2-carbon domains due to oxidation. The G band corresponds to the formation of graphitized MWCNTs, while the latter originates from the finite size effect. The appearance of these bands shows that the acid treatment does not affect the structure of MWCNTs. It indicates that the highly oxidative unzipping process of MWCNTs breaks some bonds. In addition, the overtone of the D band is located at 2637 cm1 and is assigned as the D0 band.29 This is indeed in good agreement with the previously reported work by Ruoff et al.29 Fourier transform infrared spectroscopy (FTIR) was performed to support the Raman data. As we can see from Fig. S2 (ESI†), the vibration intensities of oxygen-containing groups (e.g. CQO at 1730 cm1) can be seen in the GNRs spectrum. The lowest angle diffraction peak after the chemical unzipping shifted to 2y B 9.71, which corresponds to the (002) reflection plane from intercalated stacked graphite (Fig. 2b).30 It is also worth mentioning that the peak shifts are a consequence of the existence of oxygen-containing functional groups after the highly oxidative unzipping process, which in turn enables intercalation of solvent and water molecules, thereby further increasing the distance between the partially stacked graphene oxide nanoribbons (GONRs). The absence of the peak at 2y B 25.81, corresponding to the interwall separation of MWCNTs, evidences a high degree of unzipping. The transmission electron microscopy (TEM) images of the GNRs are shown in Fig. 3a and b. In principle, the sensitivity of GNR properties is based on the width of the nanoribbons as well as the crystallographic orientation of the edges. There are two distinct types of edges (depending on the cutting direction): (i) zigzag, and (ii) armchair. Son et al. reported that a GNR with zigzag edges is predicted to have a magnetically ordered edge state while an armchair GNR is calculated to have a band gap that scales with the width of the nanoribbons.31 Moreover, the cutting direction also gives a boundary at the edge and this indicates that the geometry of the edge establishes
(a) Raman spectra of pristine CNTs and pristine GNRs, and (b) X-ray diffraction patterns of pristine CNTs and pristine GNRs.
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Fig. 3 (a, b) TEM images of GNRs, (c) HR-TEM image of the GNRs, and (d, e) AFM images of the CNTs and inkjet printed GNR/CNT. A zoomed in view of c can be found in the ESI† (Fig. S3).
a significant difference in the p-electron structure at the edge. As shown in Fig. 3c, our high resolution TEM (HR-TEM) image illustrates zigzag chains, which can be obviously seen from the graphene network. This implies that the thickness of the GNRs is within an atomic scale.32 It is worth noting that disorder at the edges of nanoribbons could potentially suppress the edge specific properties.33–36 Hence, in order to have minimal disorder at the edges of the nanoribbons, one has to minimize the disorder at the nanoribbon edges. The surface morphology of the CNT layers is shown in a 3D image obtained by atomic-force microscopy (AFM; Fig. 3d). The image shows uniformly percolated structures and flat surfaces with a root-mean-square (rms) roughness value of 0.3 nm. In addition, Fig. 3e depicts the printed GNR/CNT layer. Fig. 4a demonstrates the current density–voltage (J–V) plots of the PTB7:PC71BM OPVs with and without HTLs, such as GNRs (1 layer), and poly(3,4-ethylenedioxylenethiophene)-polystyrene sulfonic acid (PEDOT:PSS) (30 nm). Throughout this study, we refer to the layers by giving the number of layers printed. The regular-device structure consists of ITO-glass/HTL/PTB7:PC71BM/C60-bis/Ag. The device with the GNRs demonstrated a sharply increased power conversion efficiency (PCE) of 5.88% with an improved short-circuit current density (JSC) of about 13.74 mA cm2 and a FF of 56.14%. The PCE of the control device without the hole transport layer (ITO-only) was 4% with a JSC of about 11.20 mA cm2 and FF of 51.05% (Table S1, ESI†). However, the GNR device has been outperformed the device with a commonly used hole transport material, PEDOT:PSS, with
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7.32% PCE. The obtained PCE for our PEDOT:PSS device is slightly low compared to the GNRs/CNTs device due to a lower FF value. The lower FF in the PEDOT:PSS-based device is because the RS is larger compared to that of the GNRs/CNTs-based device. To confirm the effectiveness of GNRs as a hole transport layer, we have designed another study employing bilayer hole transport materials using the same device geometry. The regularcell structure with a bilayer consists of ITO-glass/HTL/HTL/ PTB7:PC71BM/C60-bis/Ag. Because of the well-matched highest occupied molecular orbital (HOMO) level (4.95 eV) and lowest unoccupied molecular orbital (LUMO) level (2.6 eV) of CNT with the BHJ,37 hole transport and electron blocking are effective during the device operation. The PCE performance of the photovoltaic with GNRs/CNTs between the photoactive layer and ITO increased significantly to 7.6% with an enhanced JSC of 15.79 mA cm2 and a FF of 63.84% after the printing process (Table S2, ESI†). The enhanced JSC and PCE are possibly attributed to a reduced electronic charge barrier between the BHJ and the GNR interface. This device exhibits the best PCE value of 7.60% (47.95% increase) with JSC = 15.79 mA cm2 and FF = 64% when compared to the control device without any HTL. Similarly, in the case of the device with GNRs/molybdenum oxide (MoO3), the exhibited PCE value increased substantially to 6.11%, with an enhancement of B4 mA cm2, and an increase of over 15% compared to the control device. The device with the GNRs/ CNTs showed an increase of 29.15%, 24.47% and 13.6% over GNRs/MoO3, GNR and PEDOT:PSS, respectively. All photovoltaic parameters are tabulated in Table S1 (ESI†). Moreover, the device with the GNRs/CNTs HTL exhibits a reduced series resistance (RS) of 12.5 O cm2 compared to that of the device without HTL (where RS = 20 O cm2), RS = 18.35 O cm2 with GNRs/MoO3, RS = 13.75 O cm2 with GNRs, and RS = 13.03 O cm2 with PEDOT:PSS. The reduced RS is well correlated with the enhanced JSC. A high shunt resistance (RSH) was obtained with the device with GNRs/ CNTs compared to the device without the interlayer (see the J–V curves at the dark current shown in Fig. S4, ESI†). Therefore, the GNRs layer plays an important role in reducing RS and increasing RSH, both of which cause improvement in the PCE. The HOMO energy level of GNRs (B4.9 eV)38 is close to the LUMO level of the fullerene acceptor, thus resulting in efficient hole charge transport. The average PCE values and the deviation of the PCE of the devices
Fig. 4 (a) J–V characteristics of OPVs with different HTLs, (b) external quantum efficiency, and (c) J–V characteristics with different numbers of layers of GNRs.
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without interlayers, with GNRs/CNTs, GNRs/MoO3, GNRs, CNTs and PEDOT:PSS were calculated using more than fifty devices to demonstrate the reproducibility. Fig. 4b illustrates the EQE corresponding to the J–V characteristics of Fig. 4a. The calculated JSC values from these spectra are in agreement with the values obtained from Fig. 4a. To confirm the layer dependence on the GNRs of the BHJ solar cells, the layer-by-layer printing of GNRs (one-layer, twolayers, and three-layers) was performed by sequential printing onto the patterned ITO-glass substrate (Table S2, ESI†). The device with a single single layer of transferred GNRs/CNTs exhibits the best average PCE of 7.60% with open-circuit voltage (VOC) = 0.75 V, JSC = 15.79 mA cm2 and FF = 64%; while the two-layer GNRs/CNTs device and three-layer GNRs/CNTs device show a gradually decreasing PCE of 5.14% (VOC = 0.73 V, JSC = 12.42 mA cm2, FF = 57%), and 4.44% (VOC = 0.70 V, JSC = 11.20 mA cm2, FF = 56%), respectively, as shown in Fig. 4c and by the external quantum efficiency (EQE) in Fig. S5 (ESI†). Thus, the sequential printing has a negative effect on the PCE of the devices because of both the reduced JSC and VOC, which originate from unstable contact between the GNR layers. Furthermore, a thick layer of GNRs will lead to an increased RS in the device. Therefore, optimized thickness and good contact with the GNR layer are important factors required to obtain high-performing solar cells. In summary, we successfully fabricated high-performance PBT7:PC71BM BHJ solar cells comprised of a GNR hole transport layer using inkjet printing technology. The GNR HTL plays a vital role in improving the JSC and PCE in OPVs compared to OPVs without any interlayer. In addition, the device with a HTL consisting of GNRs/CNTs exhibits the highest PCE of 7.60%, which has outperformed the PEDOT:PSS only device. OPVs with printed GNRs serving as the HTL is an encouraging architecture for realizing highly efficient OPVs.
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