Graphene Quantum Dots Heterojunction Solar Cells

Article pubs.acs.org/JPCC

Crystalline Si/Graphene Quantum Dots Heterojunction Solar Cells Peng Gao,‡ Ke Ding,‡ Yan Wang, Kaiqun Ruan, Senlin Diao, Qing Zhang, Baoquan Sun,* and Jiansheng Jie* Institute of Functional Nano & Soft Materials (FUNSOM) & Collaborative Innovation Center of Suzhou Nano Science and Technology, Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, Jiangsu 215123, People’s Republic of China S Supporting Information *

ABSTRACT: Graphene quantum dots (GQDs) possess extraordinary optical and electrical properties and show great potential in energy applications. Here, with combing of crystalline silicon (c-Si) and GQDs, a new type of solar cells based on the c-Si/GQDs heterojunction was developed. Thanks to the unique band structure of GQDs, photogenerated electron−hole pairs could be effectively separated at the junction interface. The GQDs also served as an electron blocking layer to further prevent the carrier recombination at the anode. These characteristics endow the heterojunction solar cells with much enhanced photovoltaic performance compared to the device counterparts without GQDs or with graphene oxide sheets. Eventually, an optimum power conversion efficiency of 6.63% was obtained by tuning the GQDs size and layer thickness. Our results demonstrate the great potential of the c-Si/GQDs heterojunctions in future low-cost and high-efficiency solar cells. performance of such kind of solar cells.13 Through performing appropriate surface passivation or introducing a polymer electron blocking layer, a high efficiency of over 10% was demonstrated recently,14,15 highlighting the great potential of the graphene/Si solar cells. In spite of the large progress, we note that graphene films only serve as transparent electrodes in the graphene/Si solar cells and there are no other effects for the device performance improvement. Also, the fabrication of largearea graphene films necessitate high-temperature chemical vapor deposition (CVD) growth and complicated transfer process.16,17 This will inevitably increase the cost of the devices. Recent studies demonstrated the marvelous properties of graphene quantum dots (GQDs) arising from the strong quantum confinement and edge effects.18 GQDs show continuous absorption in the UV−visible region because of the overlap of electronic absorption bands caused by closely spaced electronic energy levels and vibronic coupling.19 The extraordinarily long lifetimes of hot carriers in GQDs could potentially allow for efficient hot-carrier harvesting or multiexciton generation that could exceed the Shockley-Queisser limit in solar energy utilization.20 With their large abundance, nontoxicity, high mobilities, and tunable band gaps, GQDs could hold great promise for efficient photovoltaic devices as light absorbers. For instance, Qu and co-workers fabricated bulk heterojunction polymer solar cells based on an active layer of poly(3-hexylthiophene) (P3HT) and electrochemically synthesized GQDs.21 The addition of GQDs to P3HT

1. INTRODUCTION Harvesting electricity from solar light offers one of the most efficient approaches to meet the enormous demand for clean energy.1 Crystalline silicon (c-Si) p-n junction solar cells dominate the current photovoltaic (PVs) production due to their high efficiency as well as long lifetime.2 However, the high cost arising from the use of high-purity Si wafers and hightemperature fabrication process is a main problem faced by the current PV industry.3 As new types of low-cost solar cells, quantum dots (QDs), such as CdSe, CdTe, and PbS QDs, based solar cells have attracted much attention owing to the size-tuned optical response, efficient multiple carrier generation, and potential in exceeding the Shockley-Queisser limit.4,5 Nevertheless, the effort to develop high-performance QD solar cells is still hindered by the low carrier mobilities of QDs, which prevent the effective separation and transfer of photogenerated electron−hole pairs. As a result, a relatively low efficiency of 8.5% is achieved for the QDs-based solar cells so far.6 Graphene possesses extraordinary properties in terms of high optical transparency, large sheet conductivity, outstanding mechanical properties, and excellent physical/chemical stability.7,8 Therefore, there is a large and growing interest to explore its potential applications in energy-related fields, including solar cells,9 lithium ion batteries,10 and supercapacitors.11 Recently, by using graphene films as transparent conductive electrodes, graphene/Si heterojunction solar cells were intensively investigated.12 Graphene film can form Schottky junction with n-type Si; the existence of built-in electric field can facilitate the separation of photoinduced electron−hole pairs at graphene/Si interface. The high transparency and large electrical conductivity of graphene make for the high © 2014 American Chemical Society

Received: December 24, 2013 Revised: February 18, 2014 Published: February 19, 2014 5164

dx.doi.org/10.1021/jp412591k | J. Phys. Chem. C 2014, 118, 5164−5171

The Journal of Physical Chemistry C

Article

and baked in air (Figure 1a). Thickness of the GQDs film was controlled by adjusting the amount of colloidal GQDs dropped

significantly improved the photovoltaic performance of the device. GQDs were used as sensitizers in dye-sensitized solar cells as well as the light-emitting layer in organic light-emitting diodes (OLED).19,22 GQDs/ZnO nanowires combined devices also exhibited a pronounced photovoltaic behavior with a large open-circuit voltage.23 Herein, we developed a new c-Si/GQDs heterojunction solar cell by using GQDs as the active layer. The unique band structure of GQDs allowed the efficient carrier separation at the interface and in the meantime served as an electron blocking layer for suppressing interface recombination. Owing to the size-tunable band gap, c-Si/GQDs heterojunction solar cells exhibited significant performance enhancement as compared to the device counterparts without GQDs. Eventually, optimum power conversion efficiency (PCE) of 6.63% was achieved by carefully controlling the GQDs size and layer thickness. Considering the simple and low-cost solution processed capability of the GQDs layer, it is expected that c-Si/GQDs heterojunction is a viable low-temperature technique for highefficiency silicon-based solar cells.

Figure 1. (a) Schematic illustration shows the procedure for the fabrication of c-Si/GQDs heterojunction solar cell. (b) Photograph of the GQDs aqueous solution taken under visible light. (c) TEM and high-resolution TEM images of GQDs with size distribution in the range of 2−6 nm. (d) Cross-sectional view SEM image of the c-Si/ GQDs heterojunction solar cell.

2. EXPERIMENTAL SECTION Preparation. GQDs were synthesized by Pan’s hydrothermal method.24 For cutting preoxidized graphene sheets into GQDs, graphene sheets were first obtained by thermal deoxidization of graphene oxide (GO) sheets, which were prepared using an improved Hummers’ method,25,26 in a tube furnace at 300 °C for 2 h with a heating rate of 5 °C/min in an Ar atmosphere. Graphene sheets (0.05 g) were then oxidized in concentrated H2SO4 (10 mL) and HNO3 (30 mL) mixed solution under mild ultrasonication (180 W, 59 kHz). The [H+] was estimated to be ∼21 M for the mixed acid. Different ultrasonication times of 14, 18, and 22 h could lead to the formation of GQDs with varied sizes in the following step. The mixture was then diluted with DI water (250 mL) to a [H+] value of ∼2.89 M and filtered through a 0.22 μm microporous membrane to remove the residual acids. Purified oxidized graphene sheets (0.2 g) were redispersed in DI water (40 mL) and then the pH value was tuned to 8 with NaOH. The suspension was transferred to a poly(tetrafluoroethylene) (Teflon) lined autoclave (50 mL) and heated at 200 °C for 10 h. After the mixture was cooled to room temperature, the resulting black suspension was filtered through a 0.22 μm microporous membrane and a brown filtered solution was separated. Then the colloidal solution was dialyzed in a dialysis bag (retained molecular weight: 3500 Da) overnight and GQDs with different sizes were obtained. Device Construction. To reduce the surface carrier recombination of planar silicon, surface methylation modification was performed through a two-step chlorination/alkylation method.27 First, the clean n-type (100) silicon wafer (resistivity 1−3 Ω·cm) was soaked in the HF (5%, aq.) for 10−15 min and immersed in PCl5 solution (dissolved in the chlorobenzene, 140 °C) for 2 h, and then the silicon wafer was transferred to the CH3MgCl/tetrahydrofuran (THF) (1:2, 90 °C) solution for 10−12 h. After passivation, hydrogen-terminated Si substrate (denoted as H−Si) changed into methyl-terminated Si (denoted as CH3−Si). On the other hand, to obtain the silicon oxide-terminated Si (denoted as SiOx−Si), the H−Si was directly exposed to air for 1.5 h to allow spontaneous oxidation of the Si. To construct the c-Si/GQDs solar cells, the colloidal GQDs were dropped onto the silicon wafer surface (1.5 × 1.5 cm2)

on the substrate; 100, 200, 400, 600, and 800 μL led to a thickness of 30, 50, 80, 100, and 150 nm, respectively, with thickness error in the range of ±10 nm. Afterward, semitransparent gold top electrodes (14 nm) with area of 1 × 5 mm2 were deposited on the GQDs film using a shadow mask via e-beam evaporation. Indium−gallium (In−Ga) alloy was pasted onto the rear side of the Si substrate to form ohmic contact. Characterization. Scanning electron microscope (SEM) images were taken using FEI Quanta 200 FEG. Transmission electron microscope (TEM) observations were performed on a FEI Quanta G2 F20 electron microscopy operated at 200 kV. Absorption and PL spectra were recorded at room temperature on a Perkin-Elmer/Lambda 750 and a HORIBA Flworomax-4 fluorescence spectrophotometer, respectively. Ultraviolet photoelectron spectroscopy (UPS) spectra were collected in a Kratos AXIS UltraDLD ultrahigh vacuum surface analysis system. Newport 91160 solar simulator equipped with a 300 W xenon lamp and an air mass (AM) 1.5 filter was used to generate simulated AM 1.5G solar irradiation (100 mW·cm−2).

3. RESULTS AND DISCUSSION Fabrication of c-Si/GQDs heterojunction solar cells could be accomplished in air via a facile solution process. The colloidal GQDs are light yellow in the aqueous solution (Figure 1b), and the size in the range of 2−6 nm (Figure 1c). The plane spacing of 0.21 nm is consistent with the lattice spacing of (1120) inplane.28 The excellent water solubility of the GQDs comes from the epoxy, carboxyl, and other oxygenous functional groups on the edges.29 To fabricate the device, the colloidal GQDs were dropped onto the c-Si surface and baked in air until the solvent was fully evaporated, leaving behind a layer of GQDs on c-Si wafer. From the cross-sectional view SEM image of the device (Figure 1d and Supporting Information, Figure S1), it is seen that the GQDs layer is smooth with relatively uniform thickness. More importantly, the GQDs layer is compact without any visible cracks, avoiding the formation of short circuit channels between the top electrode and c-Si wafer. The electrical conductivity of the GQDs layer was further measured by depositing two Au electrodes on it (Figure S2). 5165



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Figure 2. J−V curves of c-Si/GQDs heterojunctions with different surface terminations of H−, SiOx−, and CH3− (a) in the dark and (b) under AM 1.5 G simulated solar light illumination, respectively. (c) J−V curves and (d) external quantum efficiency (EQE) spectra of CH3−Si/Au, CH3−Si/ GO, and CH3−Si/GQDs devices.

are replaced by GO, which has sheetlike morphology (Figure S3), VOC of the CH3−Si/GO device is remarkably improved to 0.44 V as compared to that of the CH3−Si/Au device. Nevertheless, the JSC of 14.71 mA/cm2 and resulting PCE of 3.99% are still much lower than that of the device based on GQDs. Moreover, the external quantum efficiency (EQE) values of these three devices were detected, as shown in Figure 2d. It is clear that EQE values of these devices have the same tendency with their photovoltaic characteristics. EQE values of the GQDs-based device is ∼65% at 550 nm, in contrast to the values of 32% and 44% for CH3−Si/Au and CH3−Si/GO devices, respectively. Table 1 summarizes the performance details of the solar cells studied in this work. For comparison, we calculated the JSC values from EQE spectra according to the ASTM G173-03 reference spectra derived from SMARTS v. 2.9.2. It is seen that they match well with the measured values. To further understand the underlying physical mechanism of the device performance enhancement by using GQDs, Figure 3a,b illustrates the energy band diagrams of the devices before and after inserting GQDs between the Au electrode and CH3− Si substrate. When the Au electrode directly contacts the CH3− Si substrate, a built-in electric field is generated at the interface, and the separation of photogenerated electron−hole pairs by the built-in electric field leads to the generation of photocurrent (solid arrows in Figure 3a). From Figure 3a, the difference in the Fermi energy level of Au and Si is close to 1 eV, which in principle means a large VOC of near 1 V for this kind of device. However, the real Schottky barrier is much lower than this value and thus impedes the improvement of VOC. The is because of the following: (i) The thin silicide layer formed by interface diffusion can result in the decrease of Schottky barrier.31 (ii) The Schottky barrier is further decreased because of the effect of image force.32 (iii) The barrier that impedes the recombination of electrons at the anode is relatively small (dashed arrow in Figure 3a), resulting in a large saturation current density (J0) and consequently small VOC.33 For these reasons, VOC of the c-Si/Au Schottky junction solar cells is only

We note that the device shows a large hysteresis because of the absorption of oxygen or water molecules in air.30 According to the I−V curve in air (Figure S2), the conductivity of the GQDs in air could be estimated to be ∼1.8 × 10−3 S/cm. Unpassivated Si with a large number of surface dangling bonds always leads to a large surface carrier recombination. In this study, c-Si/GQDs heterojunction solar cells with different Si surface passivation of H−, SiOx−, and CH3− terminations were investigated, as shown in Figure 2a. The thickness of the GQDs layer is fixed at 80 nm, and the GQDs’ size is 2−6 nm, for all the devices. It is noted that a pronounced rectifying behavior is observed from the current density versus voltage (J−V) characteristic curves of the devices in the dark, indicating the formation of a heterojunction between GQDs and c-Si. However, the diode performance of CH3−Si/GQDs is much better than that of the counterparts based on H−Si and SiOx− Si; the diode ideality factor (n) of CH3−Si based device is deduced to be 1.68, while the values are 2.90 and 3.28 for H− Si- and SiOx−Si-based devices, respectively, indicating a lower interface recombination in the CH3−Si device. Because of the effective surface passivation, the CH3−Si device also exhibits a much superior photovoltaic performance upon light illumination (Figure 2b). It exhibits a short circuit current density (JSC) of 23.38 mA/cm2, open circuit voltage (VOC) of 0.51 V, and fill factor (FF) of 0.55, yielding an efficiency of 6.63% under AM 1.5 G illumination, in contrast to the lower efficiencies of 2.24% and 2.92% for H−Si and SiOx−Si devices, respectively. To verify the essential roles of GQDs in the heterojunction solar cells, control experiments were conducted by removing the GQDs layer from the device, or replacing the GQDs layer with GO layer, as shown in Figure 2c. Without the use of the GQDs layer, CH3−Si/Au can also form a Schottky-type solar cell due to their large work function difference. However, although the CH3−Si/Au device shows pronounced photovoltaic behavior, its performance is much worse than the CH3− Si/GQDs device; JSC and VOC are 10.77 mA/cm2 and 0.31 V, yielding a low PCE of 2.26%. On the other hand, if the GQDs 5166



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respectively, based on the UPS measurements (Figure S4). On the other hand, as a rough estimation, the optical band gaps of GQDs could be estimated to be ca. 3.8, 3.7, and 3.5 eV for 2−6, 5−10, and 8−16 nm GQDs from the UV−vis absorption spectra (Figure S5). We note that the band gaps of the GQDs are comparable with the values obtained in previous reports.24,38 Considering that our GQDs were synthesized with the same method, which means they should have the same surface functional groups, it is reasonable to assume that the variation in their band gaps arises from the change in sizes. As a result, the lowest unoccupied molecular orbital (LUMO) of 2− 6, 5−10, and 8−16 nm GQDs could be estimated to be ca. 2.15, 1.98, and 2 eV (Figure S6), respectively. Upon irradiation, electron−hole pairs generated in Si would diffuse to the CH3− Si/GQDs interface and then be separated by the strong built-in electric field of the heterojunction. Electrons in the conduction band of Si were preferentially collected by the In/Ga electrode (cathode), while injection of electrons from Si to Au anode was prevented by the GQDs layer because of the large EC-LUMO offset. On the other hand, holes could inject into the HOMO level of GQDs and then collected by the Au anode (Figure 3b). Although the EV-HOMO offset is much smaller than the ECLUMO offset, from Figure 3b, it is still evident that an unfavorable energy barrier may present for hole transport. However, based on the device measurements, it seems that the existence of the energy barrier does not make significant influence on the device performance. We note that the conductivity of GQDs layer in air is much higher than that measured in vacuum (Figures S2 and S7). The surface absorption of oxygen molecules and moisture is responsible for the high conductivity of GQDs in air.41 Therefore, the energy barrier for hole transportation from Si to GQDs in air might be not as large as that shown in Figure 3b. It is likely that the holes generated in Si have sufficient energy to overcome the barrier or transport from the Si to GQDs layer via tunneling effect. On the basis of the above discussion, the GQDs layer can not only act as the hole transport layer but also serve as an electron blocking layer for reducing the carrier recombination at the anode. This should lead to a lower saturation current density and hence a larger VOC for the device.15,42 Moreover, the insertion of GQDs can also avoid the direct contact of Au with Si, suppressing the formation of thin silicide layer. The efficient carrier separation and transportation at the GQDs/Si interface are suggested to be responsible for the high efficiency of the heterojunction solar cells. On the other hand, although the GQDs show strong light absorption in the short-wavelength range, the contribution of light absorption to the improvement of JSC is not so evident from the EQE spectra in Figure 2d. The changes in photocurrent for the GQDs/Si heterojunction are mirrored by an increase in EQE in the spectral region corresponding to light absorption in Si. This result could be attributed to the weak sunlight intensity in the shortwavelength range; below 400 nm, the maximum obtainable photocurrent is only ∼1.4 mA/cm2 with 100% sunlight absorption and 100% efficient carrier collection. For comparison, GO instead of GQDs was also utilized to form Si/GO heterojunction solar cells (Figure 2c). It is known that GO has a relatively large work function of 4.9 eV, but its electrical conductivity is very poor.43,44 From Figure 2c, it is obvious that the device shows inferior performance in terms of lower JSC and VOC as compared to the GQDs/Si device. Figure 3c depicts the dark J−V characteristic curves of the solar cells. It is noted that J0 for the Si/Au device is the highest among these devices, while

Table 1. Performance Details of the Solar Cells Studied in This Work devices structures

VOC (V)

JSC (mA cm−2)

H−Si/ GQDs SiOx−Si/ GQDs CH3−Si/ GQDs CH3−Si/ Au CH3−Si/ GO CH3−Si/ GQDs 30 nm layer 50 nm layer 80 nm layer 100 nm layer 150 nm layer 2−6 nm size 5−10 nm size 8−16 nm size

0.44

JSC (mA cm−2) calculated from EQE

FF

PCE (%)

12.27

0.42

2.24

0.42

12.52

0.56

2.92

0.51

23.38

0.55

6.63

0.31

10.77

10.5

0.68

2.26

0.45

14.71

14.4

0.60

3.99

0.51

23.38

22.7

0.56

6.63

0.48 0.47 0.48 0.48

20.49 22.50 25.17 20.20

21.4 24.5 19.8

0.47 0.48 0.48 0.57

4.60 5.06 5.78 5.54

0.48

18.03

0.63

5.46

0.50

21.03

20.7

0.59

6.22

0.47

23.88

22.6

0.53

5.95

0.44

25.98

24.4

0.49

5.58

Figure 3. Energy band diagrams of the (a) CH3−Si/Au Schottky junction device and (b) CH3−Si/GQDs heterojunction device. EC and EV represent the conduction band and valence band of silicon, respectively. EF is the Fermi level of silicon. (c) J−V curves of the CH3−Si/Au, CH3−Si/GO, and CH3−Si/GQDs solar cells in the dark.

0.31 V, which is in accordance with previous reports.34,35 As an alternative, heterojunction solar cells were studied in this work by inserting a layer of GQDs between Au and Si. Because of the quantum size effect, the GQDs show a large band gap, which is much different than the graphene that has zero band gap.7,36,37 However, it is noteworthy that the band gaps of GQDs are still not well-defined and in hot debate. Both surface functional groups and size can play important roles in determining the band gaps of GQDs.38−40 In this work, the highest occupied molecular orbital (HOMO) of 2−6, 5−10, and 8−16 nm GQDs could be estimated to be ca. 5.95, 5.68, and 5.50 eV, 5167



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Figure 4. (a) J−V curves and (b) EQE spectra of CH3−Si/GQDs heterojunction devices with different GQDs layer thickness. (c) Optical transmittance and absorption spectra of GQDs layers with different thickness. Quartz wafer was used as the substrate. (d) Cross-sectional view SEM images of the devices with different GQDs layer thickness.

Figure 5. (a) J−V curves and (b) EQE spectra of CH3−Si/GQDs heterojunction solar cells with different GQDs size. (c), (d), and (e) show the TEM images of as-prepared GQDs and the corresponding size distributions with Gaussian fits.

it decreases by introducing GO in the device. The CH3−Si/ GQDs device exhibits the lowest J0 due to the large junction barrier. The reduction in the dark current is clear evidence for reduced electron leakage and the suppression of electron combination at the anode,33 which is responsible for the large JSC and VOC of the CH3−Si/GQDs heterojunction solar cells.

Figure 4a plots the photovoltaic characteristics of the CH3− Si/GQDs devices with different GQDs thickness and the corresponding cross-sectional view SEM images of these devices are shown in Figure 4d. It is noted that the device performance increases when the GQDs layer thickness increases from 30 to 80 nm. However, further increasing the thickness over 80 nm deteriorates the device performance. EQE 5168



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Figure 6. Variations of (a) VOC and JSC, and (b) FF and PCE of the CH3−Si/GQDs heterojunction solar cell over time.

transportation and consequently results in the decrease of JSC. Therefore, a more efficient interface passivation method is much desired to further improve the durability of the c-Si/ GQDs solar cells.

spectra in Figure 4b confirm the tendency of the photovoltaic characteristics. The above results verify the core role of GQDs in improving the device performance of the CH3−Si/GQDs solar cells, which can promote the carrier separation by forming heterojunction with CH3−Si and suppress the recombination at the interface. As for the thickness-dependent performance, GQDs with an adequate thickness is necessary to ensure the full coverage on the Si substrate. The thick GQDs layer also leads to a higher light absorption, as shown in Figure 4c. Nevertheless, at a large GQDs thickness (100 nm), the holes separated at the junction interface cannot effectively transfer through the GQDs layer because of the relatively poor electrical condctivity of GQDs. The strong light absorption in thick GQDs layer will then aggravate the carrier recombination in GQDs layer. As a novel zero dimensional (0D) quantum dot, GQDs possess strong quantum confinement and size effects. As a result, the band gap and photoluminescence of GQDs also show size-dependent properties.40 By adopting an optimum layer thickness of ∼80 nm, the impact of the GQDs’ size on the photovoltaic performance was studied, as shown in Figure 5a,b. The size of the GQDs is controlled by varying the ultrasonic time during synthesis. Figure 5c−e shows the TEM images of the GQDs, which have relatively narrow size distributions in the range of 2−6, 5−10, and 8−16 nm, respectively. Gaussian fits were used to obtain the size distributions. Interestingly, from the J−V characteristics in Figure 5a, it is seen that VOC of the device increases with the decrease of GQDs’ size, while JSC shows the opposite tendency. EQE spectra in Figure 5b also confirm the decrease of JSC with decreasing GQDs’ size. It is known that the GQDs with smaller size have larger band gap.19 Therefore, the heterojunction barrier will increase with the size decreases, resulting in the enhancement of VOC. However, because of the decrease in the HOMO level, the barrier for hole transportation is increased. This factor results in the decrease of JSC. From the above discussion, it is clear that there is a tradeoff between VOC and JSC. The optimum GQDs’ size is deduced to be 2−6 nm on the basis of the experimental results. The use of GQDs as an active layer offers the device with excellent device stability. Figure 6a,b depicts the variation of the main parameters of the CH3−Si/GQDs solar cell over time by storing it in a N2-filled glovebox without any especial encapsulation. Significantly, the device can work even after storage for half a year (180 days); the JSC decreases from 28.71 to 22.35 mA/cm2, while the VOC and FF are nearly unchanged, leading to a degradation of device efficiency from 6.35% to 5.15%. Considering the high stability of GQDs, the decrease of JSC can be attributed to the gradual invalidation of surface CH3− termination over time. The oxidation of the Si surface might be another reason, which increases the barrier for carrier

4. CONCLUSIONS In summary, we have demonstrated that GQDs can form an excellent heterojunction with crystalline silicon for highly efficient solar cell application. The large junction gap between GQDs and n-Si allows the efficient separation of photogenerated electron−hole pairs at the junction interface, while the low GQDs’ LUMO level with respect to the silicon’s conduction band can ensure a low interface recombination. As a result, both the open-circuit voltage and short-circuit current of the c-Si/GQDs heterojunction solar cells have been substantially improved as compared to those of the device counterparts without GQDs. Preliminary results reveal a PCE of 6.63% for the c-Si/GQDs heterojunction solar cells. Significantly, the c-Si/GQDs solar cells exhibit relatively good stability and can retain high efficiency after storing for half a year. Although the efficiencies of c-Si/GQDs solar cells are still not as high as the state-of-the-art c-Si/graphene film solar cells, there is a large room for the device performance improvement by the means such as improving the GQDs’ mobilities, replacing Au top electrode with a more transparent electrode and so on. It is expected that the c-Si/GQDs heterojunctions demonstrated in this work will have important applications in future high-efficiency, low-cost solar cells.

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ASSOCIATED CONTENT

S Supporting Information *

Performance details of the devices studied in this work. Crosssectional view in the SEM image of the GQDs layer on a large scale. I−V measurement of the GQDs layer in air. TEM images of GO fragment. UPS spectra of the GQDs with different size distribution. PL spectra at different excitation wavelengths for 2−6, 5−10, and 8−16 nm GQDs. UV−vis absorption spectra of GQDs aqueous solution with different sizes. HOMO and LUMO levels of GQDs with different size distributions. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*Jiansheng Jie: Telephone: +86-512-65881265. E-mail: jsjie@ suda.edu.cn. *Baoquan Sun: Telephone: +86-512-65880951. E-mail: [email protected]. Author Contributions ‡

P.G. and K.D. contributed equally.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (Nos. 2012CB932400, 2013CB933500), the Major Research Plan of the National Natural Science Foundation of China (Nos. 91233110, 91027021), the National Natural Science Foundation of China (Nos. 51172151, 51173124, 50903059), and the Natural Science Foundation of Jiangsu Province (BK20131162).

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