One-Pot Large-Scale Synthesis of Carbon Quantum Dots: Efficient

Apr 10, 2017 - External quantum efficiency (EQE) measurements and 2D excitation–emission topographical maps revealed that the C-dots down-shifted th...
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One-Pot Large-Scale Synthesis of Carbon Quantum Dots: Efficient Cathode Interlayers for Polymer Solar Cells Yuzhao Yang,§,† Xiaofeng Lin,§,† Wenlang Li,† Jiemei Ou,† Zhongke Yuan,† Fangyan Xie,† Wei Hong,† Dingshan Yu,† Yuguang Ma,‡ Zhenguo Chi,*,† and Xudong Chen*,† †

Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education of China, Guangdong Engineering Technology Research Center for High-performance Organic and Polymer Photoelectric Functional Films, Guangdong Provincial Key Laboratory for High Performance Resin-based Composites, School of Chemistry, Sun Yat-sen University, Guangzhou 510275, P. R. China ‡ Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, P. R. China S Supporting Information *

ABSTRACT: Cathode interlayers (CILs) with low-cost, lowtoxicity, and excellent cathode modification ability are necessary for the large-scale industrialization of polymer solar cells (PSCs). In this contribution, we demonstrated one-pot synthesized carbon quantum dots (C-dots) with high production to serve as efficient CIL for inverted PSCs. The C-dots were synthesized by a facile, economical microwave pyrolysis in a household microwave oven within 7 min. Ultraviolet photoelectron spectroscopy (UPS) studies showed that the C-dots possessed the ability to form a dipole at the interface, resulting in the decrease of the work function (WF) of cathode. External quantum efficiency (EQE) measurements and 2D excitation−emission topographical maps revealed that the C-dots down-shifted the high energy near-ultraviolet light to low energy visible light to generate more photocurrent. Remarkably improvement of power conversion efficiency (PCE) was attained by incorporation of C-dots as CIL. The PCE was boosted up from 4.14% to 8.13% with C-dots as CIL, which is one of the best efficiency for i-PSCs used carbon based materials as interlayers. These results demonstrated that C-dots can be a potential candidate for future low cost and large area PSCs producing. KEYWORDS: cathode interlayer, carbon quantum dots, inverted polymer solar cells, microwave pyrolysis, large-scale industrialization and TiO2)7,8 and organic molecular/polymer interfacial materials9,10 have gained remarkable successes as efficient CIL in PSCs. Recently, a range of carbon-based materials, such as graphene oxide (GO),11 graphene quantum dots (GQDs),12 and carbon nanotubes (CNTs),13 have been used as either the additive in the active layer or electron/hole extraction materials to improve the power conversion efficiency (PCE) of the PSCs. For example, Li et al. reported GO-type hole extraction layer (HEL) with suitable WF and good film quality in PSCs.14 Liu et al. reported the cesium-neutralized graphene oxide (GOCs) materials as the electron extraction layer, which modified the WF of the electrodes to match the LUMO level of the acceptor for PSCs.15 However, most of the carbon based materials were suffered from multisteps and time-consuming synthesis

1. INTRODUCTION Polymer solar cells (PSCs), which structurally consisted of organic semiconductor sandwiched between two electrodes, have attracted tremendous interests because of a variety of competitive advantages, such as low-cost solution processing and easy manufacturing of large-area and flexible devices, as compared with traditional Si-based solar cells.1 With rapid advances in synthesizing novel active materials,2 optimizing devices processing,3 controlling morphology evolution,4 interface engineering,5 much progress have been made in achieving PSCs with high performances. The cathode interlayer (CIL) materials also play a crucial role in determining the performance of PSCs since they can provide better energy level alignment, minimize the contact resistance between the photoactive layer and cathode and offer ohmic contacts at the interface for better charge extraction.6 Among a variety of solution-processable CIL materials, semiconducting transition metal oxides (TMO) (such as ZnO © 2017 American Chemical Society

Received: January 9, 2017 Accepted: April 10, 2017 Published: April 10, 2017 14953

DOI: 10.1021/acsami.7b00282 ACS Appl. Mater. Interfaces 2017, 9, 14953−14959

Research Article

ACS Applied Materials & Interfaces

purchased from Sigma-Aldrich (United States). The polymer solar cells, using PTB7-Th as the electron donor and PC71BM as the acceptor, C-dots as the CIL, were fabricated using the following device structures: ITO/PTB7-Th:PC71BM/MoO3/Al (reference device) and ITO/C-dots/PTB7-Th:PC71BM/MoO3/Al (C-dots device). Indium−tin−oxide (ITO)-coated glasses were cleaned using the following steps. First, they were cleaned ultrasonically with detergent for about 15 min, and then followed with sonication in distilled water, ethanol, acetone, and isopropyl alcohol (IPA) sequentially for about 15 min, respectively. Finally, the glasses were dried by N2 prior to use. As for the C-dots devices, the C-dots (different concentrations, hydrosolvent) were spin-coated on the surface of the precleaned ITO-coated glass at 1000 rpm for 50 s. Then the residual water in C-dots layer was removed by thermal annealing at 100 °C for 10 min. Afterward, the blending solution of PTB7-Th:PC71BM (1:1.5 weight ratio) was prepared in the mixed-solvent of chlorobenzene (CB) and 1,8diiodoctane (DIO) (97:3, v/v). The final concentration of the mixture was 15 mg/mL. The solution was then spin-coated (∼180 nm) on top of the C-dots layer. To remove DIO, the samples were placed inside a vacuum glovebox for at least 3 h. Finally, the devices were accomplished by the deposition of 10 nm MoO3 and 80 nm Al on top of the active layer through a shadow mask in a chamber with the base pressure of 1 × 10−6 Torr (the active area was defined to be 0.09 cm2). As comparison, devices without C-dots CIL were fabricated as reference. 2.3. Characterization. The J−V characteristics of these devices were measured by using a Keithley 2400 source unit under 100 mW/ cm 2 simulated AM1.5G illumination. The external quantum efficiencies (EQE) of these devices were measured using the EQE evaluation system (PV measurements, Inc.). The transmission electron microscopy (TEM) images investigations of C-dots were carried out by a transmission electron microscope (JEM-2010HR). The atomic force microscopy (AFM) measurements were recorded by Bruker Multimode 8, yielding the tapping mode. The X-ray diffraction (XRD) measurement was recorded by an Advance X-ray diffractometer (Bruker D8). The UV−vis spectra were carried out by a UV/vis/NIR spectrometer (PerkinElmer-Lambda 750). The elemental analysis (EA) was measured by Elemental Analyzer (Elementar, Germany). The Fourier transform infrared (FTIR) spectra were collected by a FTIR Spectrophotometer (Bruker Tensor 27). X-ray photoelectron spectroscopy (XPS) characterizations were carried out on an ESCALAB 250Xi spectrometer made by Thermo Fisher Scientific, using Al Kα radiation (energy 1486.6 eV) in a vacuum of 2 × 10−9 mbar. The steady state photoluminescence (PL) spectra were taken using a FLS980 fluorescence spectrometer (Edinburgh, UK) with xenon lamp as the excitation sources. The transient photocurrent (TPC) measurement was carried out by applying 500 nm laser pulses in dark.

procedures with dangerous strong oxidizing reagents (e.g., HNO3, KMnO4, or H2SO4),16,17 as summarized in Table S1. These drawbacks highly hindered their practical applications. Thus, developing novel carbon based CIL materials with safe reagents and simple synthesis processes is an important issue. Carbon quantum dots (C-dots) as a novel carbon-based material can be easily prepared in some circumstance. It has been reported in the previous works that C-dots can enhance the efficiency of PSCs. For example, Huang et al. use C-dots as a luminescent down-shifting layer to improve the performance of PSCs with the effective light conversion of near-ultraviolet and blue-violet portions of sunlight.18 Lin et al. used C-dots to modify ZnO or AZO layers to promote the device performance by lowering the WF and down-shifting the light.19 However, the application of C-dots using in PSCs has just started, and there is few report on using C-dots as CIL in the high-efficiency PSCs, as summarized in Table S2. Herein, we demonstrated facile one-pot synthesis of C-dots, which was first utilized as efficient CIL for inverted PSCs. The synthesis was carried out in a household microwave oven within 7 min, without strong oxidizing reagents. The resulted product can be well dispersed in water for direct use in device fabrication. Our synthesis method is well reproducible, and the resulted homogeneous C-dots solution can be massively produced with liter scale. Inverted PSCs with poly[4,8-bis(5(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′’]dithiopheneco-3-fluorothieno[3,4-b]thiophene-2-carboxylate] (PTB7-Th) and (6,6)-phenyl-C71-butyric acid methyl ester (PC71BM) as the bulk heterojunction (BHJ) layer and C-dots as CIL were fabricated. The results showed that remarkable PCE improvement from 4.14% to 8.13% with C-dots CIL was achieved (comparable to commonly used ZnO as CIL), which is one of the best efficiency for i-PSCs used carbon based materials as interlayers. This performance improvement lies in that the Cdots decrease the WF of cathode by forming an interfacial dipole at the interface, and down-shift the high energy nearultraviolet light to low energy visible light. As compared with ZnO, the C-dots dispersion is more environmentally friendly (water as solvent), stable (stored for more than half years), easy preparation (household microwave for 7 min). Furthermore, the PSCs with C-dots as the CIL exhibited outstanding reproducible performance by using an environmentally friendly solvent (water). Our achievements demonstrated that the Cdots are efficient and low cost CIL materials that can be a potential candidate for future low cost and large area PSCs producing.

3. RESULTS AND DISCUSSION The C-dots were prepared through an economical and facile microwave-assisted method within several minutes, which is depicted in the Experimental Section and Figure S1. The FTIR (Figure S2) and XPS (Figure S3) were used to identify the organic functional groups of bulk C-dots. The peaks at 1358, 1461, and 1631 cm−1 were attributed to the existence of C−N, N−H, and CO, respectively (Figure S2).20,21 In addition, the strong peak at 1114 cm−1 and broad peak at 3404 cm−1 correspond to the existence of C−O and −OH/N−H. The XPS spectrum (Figure S3a) shows three characteristic peaks of C 1s, N 1s and O 1s at 285.86, 399.87, and 532.77 eV, respectively. Figure S3b shows the C 1s spectrum of the C-dots. As can be seen, the C 1s spectrum can be divided into five peaks at 284.5, 284.8, 285.4, 286.2, and 287.2 eV by the Gaussian function, which are ascribed to the CC, C−C, C− N, C−O, and CO groups, respectively.22 The C−O peak could come from the raw material which is hydroxyl in the

2. EXPERIMENTAL SECTION 2.1. Synthesis of C-Dots. C-dots were synthesized by the microwave-assisted method procedures. Glucose (Sigma-Aldrich) and 4,7,10-trioxa-1,13-tridecane-diamino (TTDDA, Sigma-Aldrich) were used as received as the carbon precursor without further purification. Briefly, glucose and TTDDA with the mass ratio of 1:1 were mixed in distilled water (20% w/w). After several minutes of sonication, the mixture became homogeneous. The solution was subsequently heated with a power of 800 W in a household microwave (Galanz) for 7 min. Finally, a dark brown solution was obtained, indicating the reaction completed. After the aforementioned microwave reaction, the product was dialyzed (membrane molecular weight cutoff ∼3500) to obtain the ultimate aqueous solution of C-dots with the concentration of ∼20 mg/mL, which was freeze-dried for a yield of ∼30%. 2.2. Device Fabrication. PTB7-Th (PCE-10, Polymer: OSO100, Lot # YY8226) was purchased from 1-Material (Canada). PC71BM (purity >99%) and 1,8-diiodoctane (DIO) (purity >98%) were 14954

DOI: 10.1021/acsami.7b00282 ACS Appl. Mater. Interfaces 2017, 9, 14953−14959

Research Article

ACS Applied Materials & Interfaces glucose, as well as ether bond in the TTDDA. The CO peak indicates the existence of ketone groups in the C-dots, arising from the carbonization in glucose. The N 1s spectrum from 396 to 404 eV (Figure S3c) can be divided into three peaks. These peaks are ascribed to pyridinic-like N (−C3N/C−NC, 399.3 eV), pyrrolic-like N (C−N−C, 399.6 eV), and quaternary N (C−N−H, 401.2 eV),23 which manifested that the C-dots has been passivated by TTDDA successfully. The compositions of C-dots after the passivation via TTDDA were measured to be 53.29% C, 7.85% H, 7.76% N, and 31.30% O (see Table S3) by EA. The physical appearance of C-dots dispersion in deionized water is presented in Figure S4. The production of C-dots dispersion can be as high as liter scale in lab indicating that the C-dots can be massively produced. Besides, the C-dot dispersion can be stored for half a year without any apparent precipitation (Figure S5) because of the existence of −NH2 and −OH, which indicating that it was very stable and can be used as industrial product for PSCs fabrication. The TEM image of C-dots is shown in Figure 1a. It is presented that the C-dots

Figure 2. (a) Full-range XPS survey spectra of bare ITO and C-dots coated ITO. (b) High-resolution N 1s spectra of bare ITO and C-dots coated ITO.

N, pyrrolic-like N and amino groups on C-dots,26 as shown in Figure S3. It has been demonstrated that the amino groups possess the ability to form interfacial dipole to lower the cathode WF, as well as doped PCBM at interface to form a high conductive interlayer,27 thus improving electron extraction and transport. Furthermore, the presence of polar amino group on C-dots can endow C-dots with excellent solubility in water and other polar solvents, such as methanol. Thus, C-dots can be processed using these environmentally friendly solvents. Moreover, the C-dots are insoluble in organic solvents with low-polarity or nonpolarity, such as chlorobenzene (CB) and dichlorobenzene (DCB), which are the common solvents used for processing the photoactive layer of PSCs. The unique solubility of C-dots offers it the possibility to be used as cathode interlayer in inverted PSCs by using orthogonal solvent processing. Lowering the WF of the cathode is very important for a CIL. Because the low WF of cathode allows the formation of Ohmic contact at the interface to reduce the electron extraction barrier.28 Besides, the low cathode WF is benefit for large builtin potential across the PSC to enhance the exciton dissociation process.29 Both these effects are beneficial for achieving good device performance. Herein, ultraviolet photoelectron spectroscopy (UPS) was carried out to explore the WF change of ITO substrates before and after C-dots modification. As shown in Figure 3, the WF of bare ITO was −4.74 eV, consistent with

Figure 1. (a) TEM image of C-dots. The inset shows an individual Cdot. (b) AFM height image of C-dots (top) and the height profile of selected C-dots (bottom).

had a uniform lateral size of ∼7 nm. This is consistent with the height profile of the AFM image as shown in Figure 1b. The AFM images also showed that after the modification of C-dots, the surface roughness of ITO substrate was slightly reduced from 2.17 to 1.63 nm (as shown in Figure S6). The smoother surface could enhance the physical contact between the ITO/ C-dots layer and the PTB7-Th:PC71BM layer.9 The XRD pattern of C-dots (Figure S7) demonstrates a broad peak located at 22°, implying the amorphous nature of C-dots. In Figure S8a, the UV−vis absorption spectrum of C-dots demonstrates a typical absorption peak around 290 nm. The Tauc’s relation is used to calculated the energy band gap (Eg),24 αhν = A(hν − Eg)n, where α is the absorption coefficient, hν is the photon energy, Eg is the optical band gap, and n equals 1/2 or 2 for direct and indirect allowed transition, respectively. From the UV−vis absorption spectrum of C-dots and the Tauc’s relation, the calculated direct optical bandgap of the Cdots is ∼3.3 eV (Figure S8b). The XPS survey spectra of both ITO and C-dots coated ITO are shown in Figure 2a. Noted that there was a C 1s signal on bare ITO substrate at 285.84 eV. This was due to inevitable contamination when the bare ITO substrate was contact with air during the transport.25 However, the C-dots coated ITO substrate showed much higher C 1s intensity than bare ITO, indicating that the successful distribution of the C-dots on ITO substrate. As a label element, the N 1s spectrum of C-dots (Figure 2b) clearly showed the presence of N at the binding energy of 399.57 eV, which originated from the pyridinic-like

Figure 3. (a) UPS secondary cutoff and (b) schematic energy levels of ITO substrates with or without C-dots modification.

commonly reported value (−4.6 to −4.8 eV).30 After modification with C-dots, the WF of ITO substrate was down-shifted to −3.95 eV. A ∼0.79 eV shift in the WF of ITO was achieved after introduction of the C-dots, suggesting that the interfacial dipole was formed to decrease the cathode WF. Importantly, the WF of C-dots modified ITO was very close to the lowest unoccupied molecular orbital (LUMO) level of PC71BM acceptor (∼4.0 eV),31 thus the Ohmic contact at the interface can be expected, which is benefit for electron extraction. Moreover, the reduced WF of ITO can increase 14955

DOI: 10.1021/acsami.7b00282 ACS Appl. Mater. Interfaces 2017, 9, 14953−14959

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reported ZnO as the cathode interlayer.33,34 Although the PEC of C-dots and ZnO as CILs is comparable, the dispersion of Cdots is much more stable than that of ZnO. As shown in Figure S5, after stored in room temperature for seven months, the ZnO dispersion turned to turbid, but the C-dots dispersion did not change apparently, and the corresponding J−V curves and devices data were summarized in Figure S11 and Table S5. From Figure S11 and Table S5, the PCE of 7-month C-dots as CIL (8.00%) is comparable with fresh C-dots as CIL (8.13%), but the PCE of 7-month ZnO as CIL (5.31%) is dramatically decreased as compared with fresh ZnO as CILs (8.37%). Moreover, the PCE dependence of ZnO and C-dots as CILs on the storage time is shown in Figure S12. As for C-dots as CIL, further increase the C-dots concentration to 2 mg/mL resulted in decreased PCE value. The averaged device characteristics of the inverted PSCs with C-dots as CILs of different concentrations were summarized in Figure S13. Apart from decreasing the WF of cathode, another attractive property of C-dots is that it can down-shift the high energy near-ultraviolet light to low energy visible light region.35,36 Figure 5a showed the 2-dimentional photoluminescence (2DPL) topographical map of C-dots. It can be seen that under the excitation of light with wavelength range of 360−520 nm, the C-dots just selectively absorbed the light ∼400 nm and emitted light ∼500 nm, demonstrating that C-dots can down-shift the higher-energy light of near-ultraviolet to low energy light of visible range. This feature is of significance since most active layer materials have low responsive intensity in the ultraviolet range but high response intensity in the visible range. The external quantum efficiency (EQE) spectra of inverted PSCs with 0.8 mg/mL C-dots modification were measured to analyze the enhancements of Jsc. The reference device without C-dots modification was also included. As demonstrated in Figure 5b, the EQE curve of C-dots modified device had apparent enhancement in the region of 350−750 nm as compared with the reference device because of the interfacial modification (improved exciton dissociation and electron extraction). Careful study the EQE curve of the device with the C-dots as interfacial modification layer reveal that the EQE curve is intensified in the wavelength range around 500 nm, with EQE enhancement larger than 1.15 in Figure 5c, this can be explained by the luminescent down-shifting effect of C-dots. After incorporating C-dots into a PSC, more incident sunlight can be effectively utilized and converted to the current due to effective light conversion of near-ultraviolet and sky-blue portions of sunlight, which is so-called the luminescent down-shift effect.18 The photocurrents of prepared devices (Figure S14a) are obtained from the data of Figure 5b. The curves of Figure S14b are obtained by integrating the curves of Figure S14a. The Jsc values calculated by integrating the EQE spectra from 300 to 800 nm are 12.70 mA·cm−2 (reference)

the built-in potential of the resulted PSCs. Consequently, the open voltage (Voc) will increase in the inverted PSC device. To explore the potential application of C-dots as cathode interlayer in PSCs, inverted PSCs with device structure of ITO/ (with or without) C-dots/active layer/MoO 3 /Al were fabricated (Figure 4a), and the corresponding schematic energy

Figure 4. (a) Inverted PSC device structure and (b) J−V curves of device with or without C-dots modification.

levels are demonstrated in Figure S9. From Figure S9, the WF of C-dots modified ITO is very close to the LUMO level of PC71BM. Notes that here Al instead of Ag was chosen as the top electrode because the device performances with these two metal electrodes are comparable (Table S4), while Al is much cheaper than Ag, which is more suitable for future low cost and large area PSCs producing. The J−V curves and devices data were summarized in Figure 4b and Table 1. As can be seen that the performance of the reference device with bare ITO as the cathode was quite inferior with Voc of 0.66 V, short circuit current density (Jsc) of 13.07 mA·cm−2, fill factor (FF) of 48% and PCE of 4.14%. This can be easily understood since electron and hole might combine at the cathode if no proper cathode interlayer was used, so the Voc obtained was low, which resulted in poor device performance. However, with the modification of 0.5 mg/mL C-dots, the device performance was dramatically improved, with Voc of 0.77 V, Jsc of 14.56 mA·cm−2, FF of 65%, and PCE of 7.27%. The increased Voc and FF can be attributed to the enhanced electron extraction and hole-blocking by introducing C-dots as cathode interlayer. This speculation is verified by the series resistance (Rs) and shunt resistance (Rsh) values (summarized in Table 1) that extracted from J−V curves. As for the device without C-dots modification, the Rs and Rsh was 281 and 5542 Ω·cm2, respectively. After the modification of 0.5 mg/mL C-dots, the Rs was drastically reduced to 96 Ω·cm2 while the Rsh increased to 6593 Ω·cm2. Besides, the low WF of the C-dots modified cathode results in the enhanced built-in potential across the whole device, which improved exciton dissociation and suppressed charge recombination.32 When the concentration of the C-dots increased from 0.5 to 0.8 mg/mL (∼10 nm, Figure S10), the maximum PCE value of 8.13% was achieved. It is comparable to the device with commonly

Table 1. Summary of Electrical Parameters for the Inverted PSCs with C-Dots as CILs of Different Concentrations

a

CILC‑dots (mg/mL)

Voc (V)

Jsc (mA·cm−2)

FF (%)

0.0 0.5 0.7 0.8 1.0 2.0

0.66 0.77 0.78 0.77 0.78 0.78

13.07 14.56 15.20 15.35 13.86 13.95

0.48 0.65 0.66 0.68 0.66 0.64

PCEmax (PCEavea) (%) 4.14 7.27 7.79 8.13 7.13 7.00

(4.09) (7.18) (7.63) (8.06) (6.99) (6.81)

Rs (Ω·cm2)

Rsh (Ω·cm2)

281 96 73 65 90 115

5542 6593 9075 12351 10156 7171

The statistic data are obtained over 5 devices. 14956

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Figure 5. (a) 2-Dimensional excitation−emission topographical maps of C-dots. (b) EQE of reference device and C-dots device. (c) EQE enhancement of C-dots device compared to reference device.

Figure 6. J1/2−V characteristics of (a) electron-only devices (ITO/ZnO/w/o or with C-dots/PTB7-Th:PC71BM/Ca/Al) and (b) hole-only devices (ITO/MoO3/ w/o or with C-dots/PTB7-Th:PC71BM/MoO3/Ag). (c) Charge transient time for reference sample and C-dots sample by transient photocurrent measurement.

and 14.64 mA·cm−2 (C-dots), respectively (Figure S14), which are consistent with Jsc obtained from J−V curve (less than 5% error). To further study the charge transport property of the devices with or without C-dots modification, the electron-only (ITO/ ZnO/w/o or with C-dots/PTB7-Th:PC71BM/Ca/Al) and hole-only (ITO/MoO 3 / w/o or with C-dots/PTB7Th:PC71BM/MoO3/Ag) devices were fabricated to study the electron mobility (μe) and hole mobility (μh), respectively. The J−V curves shown in Figure S15 were fitted by using space charge limited current (SCLC) method. The μ value is calculated from equation, JSCLC = (9/8)ε0εrμ(V2/d3),37 where JSCLC stands for the current density, ε0 is the permittivity of free space (8.85 × 10−12C·V−1·s−1), εr is the relative dielectric constant of the material, V is the effective voltage, d is the thickness of the active layer. The electron mobility and hole mobility can be calculated from the slope of the J1/2−V curves (Figure 6a and Figure 6b). The calculated μe for reference device and the C-dots device are 1.5 × 10−4 cm2/(v·s) and 3.2 × 10−4 cm2/(v·s), respectively. On the other hand, based on the hole-only devices, the calculated μh for reference device and the C-dots device are 1.9 × 10−3 cm2/(v·s) and 5.5 × 10−4 cm2/(v·s), respectively. The increase of μe value and the decrease of μh value suggest that the electron-transport ability of the resulted device is enhanced while the hole-transport ability is retarded by using C-dots as CILs, which directly contributes to the FF enhancement.38 To further investigate how the charge collection process was affected by C-dots, transient photocurrent (TPC) measurement was used to measure the average charge transit time across the devices after charge generation. As shown in Figure 6c, the C-dots device has a charge transient time of 0.16 μs, which is much shorter than the reference device without C-dots as CIL (0.24 μs), suggesting that C-dots as CILs is beneficial for the charge collection process.39

4. CONCLUSIONS In conclusion, we have demonstrated one-step synthesized Cdots serving as efficient CIL for inverted PSCs. The reaction was simply carried out in a household microwave oven within 7 min using water as solvent. Moreover, the production of C-dots solution can be as high as liter scale in lab and the C-dot solution can be stored for more than half a year without any apparent precipitation, which means a potential material for large-scale production. Mechanism study showed that the Cdots can not only decrease the WF of ITO by the interfacial dipole effect, but also down-shift the high energy nearultraviolet light to low energy visible light by the luminescent down-shifting effect. As a result, an optimized inverted polymer solar cell using C-dots as the CIL and the PTB7-Th:PC71BM as the photoactive layer achieved a maximum PCE of 8.13%, outperforming the reference devices without C-dots (4.14%). This work represents a major advance toward a low cost and low cytotoxicity CIL of carbon-based materials in highperformance PSCs.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b00282. Recent reports about graphene-based materials used as hole extraction layers (HEL) or electron extraction layers (EEL) in polymer solar cells; summary of all reports about PSCs using C-dots, a synthetic route of C-dots using glucose and TTDDA by microwave; the FTIR spectrum, the XPS spectrum, the EA results, the physical appearance, the XRD spectrum, the UV−vis spectrum, and the band gap plot of the C-dots; the physical appearance of fresh C-dots, 7-month C-dots, fresh ZnO and 7-month C-dots; AFM images of ITO and ITO/C14957

DOI: 10.1021/acsami.7b00282 ACS Appl. Mater. Interfaces 2017, 9, 14953−14959

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ACS Applied Materials & Interfaces

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dots; schematic energy band diagram of PSCs; summary of J−V parameters for the inverted PSCs with Al and Ag as the top electrodes; cross-section SEM images of Glass/ITO/PTB7-Th:PC71BM and Glass/ITO/C-dots (ca. 10 nm)/PTB7:PC71BM; J−V curves and corresponding parameters of device with fresh C-dots, 7month C-dots, fresh ZnO and 7-month C-dots as CILs; plot of normalized PCE for PSCs with C-dots or ZnO as CILs as a function of time; photovoltaic properties of the inverted PSCs with C-dots as CILs of different concentrations; photocurrent and total solar photocurrent of reference device and C-dots device; J−V curves of the electron-only devices (ITO/ZnO/w/o or with C-dots/PTB7-Th:PC71BM/Ca/Al) and the holeonly devices (ITO/MoO3/w/o or with C-dots/PTB7Th:PC71BM/MoO3/Ag) (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail addresses: [email protected]. *E-mail: [email protected]. ORCID

Xiaofeng Lin: 0000-0003-4802-0812 Dingshan Yu: 0000-0002-2913-2432 Xudong Chen: 0000-0001-9499-5421 Author Contributions §

Y.Y. and X.L. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the Natural Science Foundations of China (Grant No. 51233008, No. 51503228) and the Natural Science Foundations of Guangdong Province of China (2014A030311035).

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