Dual Role of Graphene Quantum Dots in Active Layer of Inverted Bulk Heterojunction Organic Photovoltaic Devices.

Graphene quantum dots (GQDs) have shown broad application prospects in the field of photovoltaic devices due to their unique quantum confinement and edge effects. Here, we prepared GQDs by a photon-Fenton reaction as reported in our previous work, which has great advantage in the preparation scale. The photoelectric properties of the inverted hybrid solar cells based on poly(3-hexylthiophene) (P3HT):(6,6)-phenyl-C61 butyric acid methylester (PCBM):GQDs and P3HT:GQDs with different contents of GQDs as the active layers are demonstrated, as well as their morphology and structure by atomic force microscopy images. Then, the different roles of GQDs played in the ternary (P3HT:PCBM:GQDs) and binary (P3HT:GQDs) hybrid solar cells are studied systematically. The results indicate that the GQDs provide an efficient excition separation interface and charge transport channel for the improvement of hybrid solar cells. The preliminary exploration and elaboration of the role of GQDs in hybrid solar cells will be beneficial to understand the interfacial procedure and improve device performance in the future.

Introduction

Graphene quantum dots (GQDs) have attracted a great deal of attention in photovoltaic applications due to a series of excellent properties such as high carrier transport mobility, large surface area, tunable band gaps, quantum confinement, and environmentally friendly nature.[1−8] Unremitting efforts have been made in the preparation of GQDs and their application as the electron-acceptor materials in the active layer of organic solar cells.[9−13] However, hybrid solar cells that completely replaced the original electron-acceptor materials with GQDs in the heterojunction active layer did not perform well. Worse, the performances of these hybrid solar cells are even much lower than those of the devices assembled with the original electron-acceptor material.[11,13] For example, Li et al. used GQDs prepared by the electrochemical approach in the aqueous phase system to completely replace the fullerence derivative (6,6)-phenyl-C61 butyric acid methylester (PCBM), which is the representative electron-acceptor material, and obtained a power conversion efficiency (PCE) of 1.28%.[11] Gupta et al. prepared GQDs functionalized with aniline (ANI), and the as-fabricated hybrid solar cells based on poly(3-hexylthiophene) (P3HT):ANI-GQDs presented a PCE of 1.14%.[13] Accordingly, the real role of GQDs in hybrid solar cells still needs to be studied thoroughly and systematically.

Ternary blend organic solar cells, which have the advantage of well absorbtion and utilization of solar irradiation using the three materials of donors/acceptor with complementary absorptions, have attracted great attention for improving the device performance. Besides, the simple fabrication process of the single-junction device and the potential synergistic interaction among three active materials make it more competitive.[14−18] Recently, ternary hybrid solar cells using GQDs as the second electron acceptor material were also fabricated by the community to obtain better performance. For example, Wang et al. achieved a 15% improvement of the PCE by introducing GQDs into the active layer (p-DTS(FBTTh2)2:PC71BM) of solar cells.[19] Dang et al. have fabricated ternary hybrid solar cells based on the active layer of PCDTBT:PC71BM:GQDs and achieved a 21% enhancement of the PCE compared to the device based on the active layer without GQDs.[20] Although the device performance of ternary hybrid solar cells can be significantly improved by employing GQDs as the second electron acceptors, the different effect of GQDs in the film morphology of the active layer between binary and ternary solar cells has not been compared and investigated comprehensively. It was well-known that the film morphology of the active layer would be strongly influenced by the number, type, and ratio of the electron-donor/acceptor materials. Thus, the film morphology of binary and ternary hybrid solar cells would be critically different and afford tremendous difference in device performance. These differences between the binary and ternary hybrid solar cells have been studied in many research studies;[21−23] however, to the best of our knowledge, only few studies have focused simultaneously on the difference between binary and ternary hybrid solar cells using GQDs as an electron acceptor.

Herein, binary (ITO/ZnO/P3HT:GQDs/MoO3/Ag) and ternary (ITO/ZnO/P3HT:PCBM:GQDs/MoO3/Ag) hybrid solar cells were fabricated to study the photovoltaic properties of the GQDs prepared via a photon-Fenton reaction as reported in our previous work.[24] The performances and the film morphology of their active layers were compared systematically. The different influence of GQDs on the performances of binary and ternary hybrid solar cells has been studied comprehensively.

Experimental Section

Preparation of the GQDs

The GQDs were prepared by a photon-Fenton reaction using the graphene oxide (GO) as the raw material; the method is described in detail in our previous work.[24] After the concentrated product of GQDs aqueous suspension was purified with the dialysis bag (retained the molecular weight: 1000 Da), the GQDs powder was obtained from the GQDs aqueous suspension using a freeze dryer, which utilizes the sublimation of the water directly under vacuum to maximize the preservation of the inherent properties and structure of the material.

Preparation of Zinc Oxide (ZnO) Precursor Solution

Zinc acetate (0.5 mmol) and monoethanolamine (0.5 mmol) were dissolved in 2-methoxy ethanol (10 mL) and stirred by a magnetic stirrer at 60 °C for 2 h, then a homogeneous and transparent sol was gained.[25] The different concentration (0.5, 1 M) precursor solution were prepared by keeping the molar ratio of monoethanolamine and zinc acetate at 1:1 (Figure S1). To improve the viscosity of the precursor solution, it was aged at room temperature for 12 h before spin-coating.

Fabrication of Organic Solar Cells

The solar cells were fabricated according to the inverted structure, and their typical structure is (ITO/ZnO/active layer/MoO3/Ag), in which the active layer could be (P3HT), (P3HT:PCBM), (P3HT:GQDs), or (P3HT:PCBM:GQDs). The indium tin oxide (ITO) coated glass substrates were cleaned with detergent, ultrapure water, acetone, and isopropyl alcohol in an ultrasonic bath for 30 min. Then, the ITO glass substrates were dried by a high-pressure nitrogen gun and pretreated at 100 °C for 8 h in an oven. Then, the ZnO precursor solution was deposited on the ITO glass substrates by spin-coating at 3000 rpm for 30 s, which were immediately annealed at 300 °C for 10 min and 350 °C for 20 min in an oven to form ZnO film.[25] For comparison, four kinds of dichlorobenzene (DCB) solutions were prepared and stirred at 30 °C for 10 h: DCB of P3HT (20 mg mL–1); P3HT (20 mg mL–1)/GQDs (the content is 0.25, 0.5, 1.0, 2.0, and 4.0 wt % relative to the mass of P3HT); P3HT (20 mg mL–1)/PCBM (16 mg mL–1); and P3HT (20 mg mL–1)/PCBM (16 mg mL–1)/GQDs (the content is 0.25, 0.5, 1.0, 2.0, and 4.0 wt % relative to the mass of P3HT:PCBM blend). The active layers were fabricated by spin-coating the DCB solutions on the ZnO films at 800 rpm for 40 s. The active layers were then put into a covered Petri dish and dried slowly for 30 min in air. Next, MoO3 (6 nm) and Ag (110 nm) layers were thermally evaporated onto the active layers at a rate of 0.01 and 0.5 nm s–1, respectively, under about 3 × 10–6 Torr. The effective area of the cells is 0.06 cm2, which is defined by the active layer and Ag electrode. All devices were annealed at 150 °C for 10 min after evaporation in a vacuum furnace.[26,27]

Characterization and Measurements

Atomic force microscopy (AFM) images were taken using a Multimode Nanoscope V scanning probe microscopy system (Bruker) in the tapping mode; the AFM cantilever tips were AN-NSC10 (ShnitCo., Russia) with a force constant of ∼37 N m–1. The aqueous suspension of GQDs was spin-coated on a freshly cleaved mica surface and dried in air overnight for AFM measurement. The UV–vis absorption and the photoluminescence (PL) spectra of the GQDs were measured with Shimadzu UV-2550 (Shimadzu, Japan) and a Cary Eclipse spectrofluorometer (Varian). The Fourier transform infrared (FTIR) spectra were recorded with an EQUINOX 55 FTIR spectrometer (Bruker, Germany). The samples for FTIR measurement were prepared by grinding the mixture of the dried GQDs and KBr powders and then compressing them into thin pellets. The current density–voltage (J–V) characteristics of the solar cells were measured in air using a Keithley 2401 source measurement unit under air mass 1.5 Global (AM 1.5G) irradiation with a solar simulator. Light intensity used in this study was 100 mW cm–2.

The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the as-prepared GQDs were evaluated by cyclic voltammetry (CV) measurement with a CHI 660C electrochemical workstation (Chen Hua Co., China) using a standard three-electrode system, which consists of a stick of platinum wire as the counter electrode, an Ag/AgCl electrode as the reference electrode, and a glassy carbon electrode coated with GQDs by drop-casting as the working electrode. Tetrabutylammonium hexafluorophosphate (TBAPF6, 0.1 M, Aldrich) in acetonitrile was used as an electrolyte for the CV measurement. The electrolyte solution was purged with high-purity argon gas for 20 min to remove oxygen before each measurement.

Results and Discussion

The GQDs were prepared from GO via a photo-Fenton reaction as reported in our previous work.[24] The AFM image of the as-prepared GQDs is shown in Figure a, with a height profile of GQDs along the line. The topographic heights have an average thickness of ∼0.8 nm, assuming a single atomic layer motif. The UV–vis absorption spectrum of GQDs shows a typical absorption band at 230 nm (Figure b), which is assigned to the π → π* transition of the aromatic sp2 domains. As shown in Figure c, the GQDs emitted a yellow-green fluorescence under UV light (302 nm). With the increase of the excitation wavelength beyond 340 nm, the emission wavelength of GQDs had a red shift caused by the wide size distribution of the GQDs. The emission spectra of the as-prepared GQDs show an excitation-dependent feature that is similar to those of GQDs prepared by the hydrothermal method.[28] Also, with the excitation wavelength of 340 nm, the PL emission has the highest intensity at around 450 nm, which is attributed to the conjugated carbon backbone of the GQDs.[29] As shown in the FTIR spectrum (Figure d), several significant FTIR peaks corresponding to oxygen functional groups are observed, such as the C=O stretching vibration peak at 1726 cm–1, the C–O (epoxy) stretching vibration peak at 1247 cm–1, the C–O (alkoxy) stretching peak at 1060 cm–1, and vibration and deformation peaks of O–H groups at 3429 and 1385 cm–1, respectively.

Figure 1

(a) AFM image of GQDs as well as height profile along the line. (b) UV–vis absorption of GQDs. (c) PL spectra of GQDs with different excitation wavelength at 300, 320, 340, 360, and 380 nm. The inset is a photo of GQD aqueous solution under UV irradiation (302 nm). (d) FTIR spectrum of GQDs.

As shown in Figure a, the inverted bulk heterojunction organic solar cells based on ITO/ZnO/active layer/MoO3/Ag were chosen to demonstrate the photoelectric application potential of GQDs in the active layer. The active layers of binary and ternary hybrid solar cells are P3HT:GQDs and P3HT:PCBM:GQDs blend films, respectively.

Figure 2

(a) Schematic illustration of the binary and ternary hybrid solar cells with P3HT:GQDs and P3HT:PCBM:GQDs as active layers, with the chemical structure of P3HT, GQDs, and PCBM. J–V characteristics of (b) binary and (c) ternary hybrid solar cells with different contents of GQDs under AM 1.5G 100 mW cm–2 illumination.

The J–V curves of the binary hybrid solar cells with different contents of GQDs are shown in Figure b. The short-circuit current density (Jsc), open-circuit voltage (Voc), fill factor (FF), and power conversion efficiency (PCE) of the as-fabricated binary hybrid solar cells based on P3HT:GQDs active layers are summarized in Table . Typically, the solar cell based on P3HT:GQDs active layer with 1.0% GQDs showed a PCE of 0.25%, Voc of 0.56 V, Jsc of 1.22 mA cm–2, and FF of 36.89%. The PCE of the cells improve first and then drop when the weight ratio of GQDs/P3HT surpassed 1%. The maximum value of PCE is achieved in the cell in which 1% GQD was blended with P3HT. The Jsc increased first and then decreased with the increase of content of GQDs. The maximum value of Jsc was obtained from the cell that with 1% GQDs. Besides, the Voc of the hybrid solar cells is also strongly dependent on the content of GQDs. The trend of Voc and content of GQDs is similar to that of Jsc and PCE. Because the Voc of the organic solar cells is proportional to the offset between the HOMO of the donor and the LUMO of the acceptor, it increases from 0.45 to 0.56 V gradually with the increase in acceptor (GQDs) content from 0.25 to 1% for the binary hybrid solar cells. The decrease of the Voc from 0.56 to 0.45 V with the increase in content of GQDs from 2.0 to 4.0% is attributed to the addition of excess GQDs, which would result in much agglomeration and prevent donor (P3HT) and acceptor (GQDs) from mixing evenly. Besides, we also note that some similar change cases are displayed from the reported literature.[13] However, there is no significant change in the FF with the variation of content of GQDs. In the binary solar cell, the GQDs act as electron acceptors. Therefore, with the increase in content of GQDs from 0.25 to 1%, the Jsc and FF were improved due to the enhanced concentration of the excition dissociation centers in the active layer, which lead to the increase in PCE. However, with the ratio of GQDs surpassing 1%, the leakage current was increased because the agglomerated GQDs behaved as potential recombination sites, which induced to the drop of PCE.

Table 1

Performance Details (Voc, Jsc, FF, and PCE) of the Binary Solar Cells Based P3HT:GQDs Active Layers under Simulated AM 1.5G 100 mW cm–2 Illumination

contents of GQDs	Jsc (mA cm–2)	Voc (V)	FF (%)	PCE (%)
none GQDs	0.006	0.45	22.15	0.001
GQDs-0.25 wt %	0.55	0.51	37.85	0.11
GQDs-0.50 wt %	1.09	0.55	36.74	0.22
GQDs-1.0 wt %	1.22	0.56	36.89	0.25
GQDs-2.0 wt %	0.63	0.56	34.51	0.12
GQDs-4.0 wt %	0.32	0.45	35.72	0.05

To improve the performance of organic solar cells further, we have blended P3HT:PCBM:GQDs as the active layer to make ternary hybrid solar cells. The J–V curves of the as-assembled ternary hybrid with different contents GQDs are shown in Figure c, and the performance details of the solar cells were shown in Table . A PCE of 4.13% with a Voc of 0.60 V, Jsc of 12.31 mA cm–2, and FF of 55.77% is obtained from the hybrid solar cell based on P3HT:PCBM:GQDs active layer with 1.0% GQDs. Comparatively, the solar cell based on P3HT:PCBM active layer without GQDs displayed a PCE of 2.96%, Voc of 0.59 V, Jsc of 9.51 mA cm–2, and FF of 52.50%, which is consistent with PCE of the reported P3HT:PCBM-based inverted solar cells.[25,30−34] Similar to the aforementioned binary hybrid solar cells, the PCE and Jsc increased first and then decreased with the increase of content of GQDs. The maximum value of PCE was obtained from the cell with 1% content of GQDs. It was displayed that the Voc and FF of ternary hybrid solar cells changed slightly. The slight decrease of the open circuit voltage from 0.60 to 0.57 V with the content of GQDs increase from 1.0 to 4.0% is attributed to the addition of excess GQDs, which was consistent with the reported literatures.[30,31,35−37] The Jsc enhanced gradually with the increase of content of GQDs from 0.25 to 1% and then dropped rapidly with the content of GQDs rise to 4%. Thus, the variations of PCE in ternary hybrid solar cells are mainly attributed to the changes of their Jsc. The enhancement of PCE can be mainly attributed to the plenty excition dissociation interfaces and the improving charge transport pathway. However, the addition of excess GQDs may result in many agglomeration, which would prevent P3HT and PCBM from mixing evenly, thus greatly reducing the separation efficiency of excitons and the transport of charge.

Table 2

Performance Details (Voc, Jsc, FF and PCE) of the Solar Cells Based P3HT:PCBM:GQDs Active Layers under Simulated AM 1.5G 100 mW cm–2 Illumination

contents of GQDs	Jsc (mA cm–2)	Voc (V)	FF (%)	PCE (%)
none GQDs	9.51	0.59	52.50	2.96
GQDs-0.25 wt %	10.47	0.59	52.44	3.22
GQDs-0.50 wt %	11.51	0.59	52.68	3.56
GQDs-1.0 wt %	12.31	0.60	55.77	4.13
GQDs-2.0 wt %	10.50	0.59	52.04	3.23
GQDs-4.0 wt %	8.90	0.57	51.51	2.62

It is worth noting that for the ternary hybrid solar cells (P3HT:PCBM:GQDs), the maximum increment of Jsc is 2.8 mA cm–2 from the control group (P3HT:PCBM) of 9.51 mA cm–2 to the maximum value of 12.31 mA cm–2. However, for the binary hybrid solar cells (P3HT:GQDs), the maximum increment of Jsc is only 1.214 mA cm–2 from the control group (P3HT) of 0.006 mA cm–2 to maximum value of 1.22 mA cm–2. The obtained Jsc of the hybrid solar cells based on the P3HT:PCBM:GQD active layer is almost 30% higher than that based on the P3HT:PCBM active layer and 10 times higher than that based on the P3HT:GQD active layer. Similarly, the obtained PCE of the hybrid solar cells based on the P3HT:PCBM:GQD active layer is almost 41% higher than that based on the P3HT:PCBM active layer and 17 times higher than that based on the P3HT:GQDs active layer. After the addition of PCBM and GQDs, both the current density and efficiency of the solar cell have been significantly improved, which suggests the synergistic interaction between GQDs and PCBM. Besides, the opening circuit voltage and the filling factor of the hybrid solar cells based on the P3HT:PCBM:GQD active layer are just slightly higher than those of the P3HT:PCBM-based solar cell, which indicates that the efficiency of the P3HT:PCBM:GQD-based hybrid solar cell can be further improved by process optimization. The increments suggest that the roles of GQDs in ternary hybrid solar cells were more than that of a sole electron acceptor as in binary solar cells. As PCBM has a fullerene structure as shown in Figure a, the GQDs would be absorbed on the surface of PCBM by the van der Waals forces or π–π interactions between them, as they both have a benzene ring structure, which would construct more efficient exciton dissociation interfaces and be beneficial to the charge transport.[31,38]

Since the photocurrent is mostly related to the photoinduced charge carrier generation and transport, the morphology of active layer is a critical factor for performance of solar cells. To characterize the morphological variation depending on the content of GQDs, AFM was used to measure the morphology and structure of active layer. Figure a–f show the AFM phase images of P3HT film and the P3HT:GQDs active layers prepared by spin-coating the DCB solution of P3HT with different contents of GQDs. The fibrous phase and the yellow dot phase in images most probably are P3HT and GQDs.[29,39,40] With the increase of content of GQDs, the yellow dot phase become more and more obviously. When the content of GQDs reached 1%, almost every fiber was covered by yellow dots except sporadic aggregates of GQDs, as shown in Figure d. However, with the increase of content of GQDs, the aggregates of GQDs dominate the active layer, as shown in Figure e,f, which means excessive GQDs addition might cause agglomerates that are unfavorable for the separation and transmission photon-generated carriers. High quality hybrid blend film obtained from suitable content of GQDs, which can promote to reduce series resistance effectively, is quite important for the device performance. The morphology of the interpenetrating donor–acceptor networks has been well constructed with the optimizing of content of GQDs accompanying with nanoscale phase separation, which results in a large interfacial area for efficient charge generation. Therefore, the maximum value of PCE was obtained from the binary cell with 1% content of GQDs.

Figure 3

AFM phase images of the P3HT:GQDs active layers with different contents of GQDs: (a) 0%, (b) 0.25%, (c) 0.5%, (d) 1%, (e) 2%, and (f) 4%.

Similarly, the film morphologies of active layers based on P3HT:PCBM and P3HT:PCBM:GQDs with different contents of GQDs were compared by the AFM phase images as shown in Figure . The image of the P3HT:PCBM film as shown in Figure a reveals that a relatively homogeneous phase of P3HT and PCBM has been obtained and interwoven network structure of P3HT fibers could be observed. With the increase of content of GQDs from 0 to 1% (Figure a–d), the interwoven network structure becomes more and more obvious. Homogeneous film morphology of P3HT, PCBM, and GQDs with well-interwoven network structure and an obvious microphase separation has been obtained when the content of GQDs reached 1% (Figure d). However, with the further increase of content of GQDs from 1 to 4% (Figure d–f), the network structure of films decreases and the uniformity of morphology becomes worse. Even some agglomerates could be observed from the AFM image, as shown in Figure f, which means that addition of excess GQDs is also unfavorable for the nanoscale phase separation of the active layer film of the ternary hybrid solar cell. The uniform morphology results in the enhancement of the exciton migration to the donor–acceptor interface, leading to a decrease in the resistance and a corresponding increase in the performance of the solar cells.[27,41,42] Thus, the maximum value of PCE was obtained from the cell with 1% GQDs.

Figure 4

AFM phase images of P3HT:PCBM:GQDs film active layers with different contents of GQDs: (a) 0%, (b) 0.25%, (c) 0.5%, (d) 1%, (e) 2%, and (f) 4%.

The HOMO and LUMO energy levels of the as-synthesized GQDs were calculated according to the reported equations[13] as followswhere Eox is the onset of the oxidation potential and Ered is the onset of the reduction potential. The electron energy levels of GQDs were studied by cyclic voltammetry as shown in Figure S2. The HOMO and LUMO energy levels of the GQDs were calculated to be −3.5 and −5.5 eV, respectively. As shown in Figure a,b, the electron energy level of the GQDs matches well with the electron energy level of the ZnO, PCBM, P3HT, and MoO3. The unique band structure of as-prepared GQDs in our work narrows the energy barrier between the HOMO and LUMO between P3HT and PCBM, which affords the dissociation of excitons, as displayed in Figure b. The processes of exciton dissociation and the as-separated electrons transport are promoted synergistically with the introduction of the suitable amount of GQDs. In addition, the existence of GQDs also contributes to the enhancement of the absorption of P3HT:PCBM in previous literatures.[12] All these beneficial factors afford the improved performance of ZnO/P3HT:PCBM:GQDs/MoO3 hybrid solar cells.

Figure 5

Energy band diagram of (a) P3HT:GQDs binary and (b) P3HT:PCBM:GQDs ternary hybrid solar cells.

Conclusions

In summary, GQDs prepared via a photo-Fenton reaction were introduced to the active layer to fabricate the binary (ITO/ZnO/P3HT:GQDs/MoO3/Ag) and ternary (ITO/ZnO/P3HT:PCBM:GQDs/MoO3/Ag) hybrid solar cells. A maximum PCE value of 0.25% was obtained for a binary hybrid solar cell, while that of 4.13% was obtained for a ternary hybrid solar cell. The photoelectric properties and intrinsic roles of the GQDs were studied systematically and highlighted by comparing the performances of hybrid devices and film morphologies of the corresponding active layers. The maximum PCE value of ternary hybrid solar cell (P3HT:PCBM:GQDs) was improved by 40% compared with that of the control group (P3HT:PCBM), which was contributed by the enhancement of exciton separation and charge transport processes with the help of GQDs as an electron acceptor and charge transport channel. The synergistic interaction between GQDs and PCBM helps donor/acceptor to mix evenly and obtain a uniform film morphology of the active layer. The preliminary exploration and elaboration of the role of GQDs in hybrid solar cells will be beneficial to understand the interfacial procedure and improve the device performance in the future.