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In graphene/semiconductor heterojunction, the statistic charge transfer between graphene and semiconductor leads to decreased junction barrier height and limits the Fermi level tuning effect in graphene, which greatly affects the final performance of the device. In this work, we have designed a sandwich diode for solar cells and photodetectors through inserting 2D hexagonal boron nitride (h-BN) into graphene/GaAs heterostructure to suppress the static charge transfer. The barrier height of graphene/GaAs heterojunction can be increased from 0.88 eV to 1.02 eV by inserting h-BN. Based on the enhanced Fermi level tuning effect with interface h-BN, through adopting photo-induced doping into the device, power conversion efficiency (PCE) of 10.18% has been achieved for graphene/h-BN/GaAs compared with 8.63% of graphene/GaAs structure. The performance of graphene/h-BN/GaAs based photodetector is also improved with on/off ratio increased by one magnitude compared with graphene/GaAs structure.
© 2016 Optical Society of America
1. Introduction
Graphene, a two-dimensional (2D) network of sp2-hybridized carbon atoms, holds great promise for next generation optoelectronic applications based on its unique electronic, optical, mechanical and thermal properties [1–4], including high intrinsic carrier mobility [5], broad absorption spectrum of light [6], high Young’s modulus [7] and extraordinary thermal conductivity [8]. Graphene is an excellent light-to-current converter with internal quantum efficiency close to 100% [9], however, the absorbance of one atomic layer graphene is too low to get high performance optoelectronic devices [6]. Various approaches have been employed to enhance the light absorbance in graphene [10–15], among which forming graphene based heterostructure with bulk semiconductors is a feasible way to obtain high performance devices [16–18]. The interface between graphene and the substrate semiconductor plays a key role in the electrical and optoelectronic performance [19,20]. For the interface engineering in graphene/semiconductor junctions, the barrier height between graphene and the semiconductor substrate is the most important physical parameter, which influences the current-voltage (I-V) characteristics to a large extent [21]. On the other hand, the devices based on graphene/semiconductor heterostructures have attracted numerous attentions [22–24] while charge transfer between graphene and semiconductor substrate has not been investigated systematically in the graphene/semiconductor optoelectronic devices, which is the key for improving the device performance.
The physical picture of the Schottky diode formed by graphene and bulk semiconductor is featured with the van der Waals type contact and the low density of states (DOS) near the Dirac point of graphene. When graphene touches with bulk semiconductor, charge transfer between graphene and bulk semiconductor occurs, which makes the Fermi level positions of the two sides closer and results in decreased barrier height, thus limits the optoelectronic performance of the Schottky diodes [25]. Moreover, because of the charge transfer, tuning the Fermi level of graphene for better performance is restricted [21,26]. GaAs is a good candidate for high performance optoelectronic applications [27–29], which has a direct band gap of 1.42 eV [30] and high electron mobility (8000cm2V−1s−1 at 300K [31] We have reported the power conversion efficiency (PCE) of 18.5% with graphene/GaAs heterostructure based on electrical gate tuning the Fermi level of graphene [32]. Further improvement requires more efficiently tuning the Fermi level of graphene. Herein, we introduce h-BN as an interlayer in graphene/GaAs Schottky junction to improve the performance of solar cells and photodetectors through suppressing the statistic charge transfer by forming graphene/h-BN/GaAs sandwich structure devices. The barrier height of the graphene/GaAs heterojunction can be increased from 0.88 eV to 1.02 eV by inserting h-BN. Moreover, based on the enhanced Fermi level tuning effect with interface h-BN, through adopting photo-induced doping into the device, PCE of 10.18% has been achieved for graphene/h-BN/GaAs compared with 8.63% of graphene/GaAs structure. The on/off ratio of the photodetector is increased by one magnitude with multilayer h-BN based on graphene/h-BN/GaAs sandwich structure. These results support that optoelectronic devices with enhanced performance can be achieved by designing the charge transfer properties between graphene and the semiconductor substrate.
2. Experimental details
Graphene was grown on copper foil with chemical vapor deposition (CVD) technique using CH4 and H2 (CH4:H2 equals to 3:1) as the reaction gas sources at 1000 °C for 30min [33]. Monolayer h-BN was grown also on copper substrate with CVD technique using B3N3H6 as the precursor at 1000 °C for 30 min [34]. The GaAs substrate is heavily n-type doped with an electron concentration around 1 × 1018cm−3. Rear Au contact with a thickness of 60nm was thermally evaporated on back surface of GaAs. The active area (10mm × 1mm) of the devices was defined by the opened window in a plasma enhanced CVD deposited SiNx. This SiNx film with a thickness of 80nm was also used as the dielectric layer under metal/graphene contact. h-BN and graphene were transferred onto the GaAs substrate using polymethyl methacrylate (PMMA) as the supporting layer [35]. Multilayer h-BN was obtained by multiple transfers of monolayer h-BN. Prior to the transfer of h-BN and graphene layers, native oxide layer on GaAs was removed by dipping the GaAs wafer into 10% HCl aqueous solution for 5min. PMMA layer was removed in pure acetone for 20min after each transfer step, and finally Ag contact was evaporated onto graphene above SiNx coated area. For photo-induced doping, Si QDs solution in ethyl alcohol (10mg/mL) was spun on (1000rpm) the surface of graphene.
Graphene and h-BN were characterized by Raman spectroscopy (Renishaw inVia Reflex) with the excitation wavelength of 532nm. The photovoltaic properties were tested by Agilent B1500A system with a solar simulator under AM1.5G condition. The illumination intensity was calibrated with a standard Si solar cell. The photoresponse properties were measured with Coherent laser system and Tektronix TBS1052B digital oscilloscopes. The spot size of the lasers used in this study is 1 mm in diameter, and the laser illumination intensity was confirmed with a luminous intensity meter. Transient PL measurements were used to evaluate the charge separation behaviors at the graphene/GaAs interface with and without h-BN. The excitation light source (PicoHarp 300 system) was a 450 nm pulsed laser with 1 MHz repetition rate and 50 ps pulse duration with power of 10 μW. The diameter of the excitation laser spot was 10 μm. The PL signal with wavelength shorter than 1100nm was collected by a multimode optical fiber and recorded by a Horiba Jobin Yvon iHR550 spectrometer. All spectra were collected until the peak value reaching 5000 counts.
3. Results and discussion
The junction barrier height between graphene and semiconductor (Φbarrier) greatly influences the I-V characteristics of the graphene/semiconductor heterojunction. As graphene is atomic thin and the DOS near the Dirac point is quite low, the charge transfer from GaAs to graphene shifts the Fermi level of graphene and thus reduce the Φbarrier of the junction. Without considering the accurate interfacial states between graphene and GaAs, the Φbarrier under thermal equilibrium condition can be determined as a simplified equation compared with that in [25]:
where Φgraphene is the work function of graphene, χGaAs is the electron affinity of GaAs, Δg represents the Fermi level shift of graphene. Based on Eq. (1), by decreasing Δg with suppressed charge transfer, higher Φbarrier can be obtained. Here we propose a sandwich structure device by inserting 2D h-BN into graphene/GaAs Schottky diode to obtain high performance solar cell and photodetector devices.Figure 1(a) shows the electronic band structure of independent graphene and n-type GaAs. Graphene is p-type doped after wet transfer process [36], as confirmed by resistance of graphene reaches the peak value at a gate voltage of + 20V shown in Fig. 1(b). Thus Fermi level locates below Dirac point. The value of χGaAs is 4.07eV, and the work function of GaAs (ΦGaAs) is around 4.07eV according to the doping concentration of 1 × 1018cm−3. After forming the van der Waals contact, some amount of electrons in the conduction band of n-type GaAs transfers to graphene, which moves the EF-Gra upward by Δg and makes Φbarrier smaller than the barrier height based on the initial Fermi level difference [25], as shown in Fig. 1(c). Schematic structure and digital photograph of the graphene/h-BN/GaAs sandwich device is illustrated in Fig. 1(d), which consists of GaAs substrate, SiNx dielectric layer, 2D h-BN, 2D graphene and electrodes. The size of the GaAs substrate is typically 1 cm × 1 cm, and the active area of the device is 10 mm × 1 mm. The Raman peak of h-BN corresponding to E2g phonon locates at 1371 cm−1, suggesting the monolayer nature of the CVD grown h-BN [37]. h-BN with different layer numbers are sandwiched between graphene and GaAs as the interlayer to disclose the dependence of the performance of optoelectronic devices on the thickness of interlayer h-BN. h-BN is a 2D dielectric material with a band gap of 5.9 eV and dielectric constant of 4.0 [38]. The electronic band alignment of graphene/h-BN/GaAs heterojunction can be seen in Fig. 1(e). As h-BN has a negative electron affinity [39], the electron transfer from GaAs to graphene is suppressed during the formation of the graphene/h-BN/GaAs heterojunction. As a result, Δg can be reduced and increased Φbarrier can be expected. Raman G peaks of graphene on SiO2(300nm)/Si substrate, in graphene/GaAs and graphene/h-BN/GaAs heterostructures are shown in Fig. 1(f). For the as-grown graphene on SiO2(300nm)/Si substrate, G peak position is 1595 cm−1, which is blue-shifted compared with 1580 cm−1 of undoped graphene [40] and corresponds to the Fermi level locating 0.24 eV below Dirac point [41]. When transferring graphene on GaAs substrate, electrons injected from GaAs moves up the Fermi level of graphene, resulting in red-shifting of Raman G peak position to 1583 cm−1 (0.02 eV from Dirac point). h-BN is designed in this study to suppress this charge transfer process. Compared with the case of graphene/GaAs structure, it is expected that the G peak of graphene should be blue-shifted for the graphene/h-BN/GaAs heterostructure. As shown in Fig. 1(f), Raman G peak positions of graphene in the graphene/h-BN/GaAs heterostructures match the theoretical expectation. Raman G peak locates at 1584 cm−1 (0.06 eV below Dirac point), 1585 cm−1 (0.09eV below Dirac point) and 1587 cm−1 (0.16eV below Dirac point) when the interlayer h-BN is 1 layer, 3 layers and 5 layers, respectively. These results demonstrate that the interlayer h-BN works well for suppressing the charge transfer as designed.
Fig. 1 (a) Electronic band structure of independent graphene and GaAs. (b) Dependence of resistance of graphene on gate voltages. Inset shows the structure for this measurement. (c) Electronic band structure of graphene/GaAs heterojunction. (d) Left: Schematic structure of the graphene/h-BN/GaAs sandwich device. Right: Digital photograph of one typical device. (e) Electronic band structure of graphene/h-BN/GaAs heterojunction. (f) Raman G peak of graphene on SiO2(300nm)/Si substrate, in graphene/GaAs and graphene/h-BN/GaAs heterostructure.
Figure 2(a) shows the typical dark current density-voltage (J-V) curves of the graphene/h-BN/GaAs devices with different layer numbers of h-BN, where all the devices show good rectifying behavior. Under forward bias, device without h-BN has the highest current density under the same voltages. For the devices with h-BN, the more layers of h-BN, the lower the current density. The threshold voltage values (defined as the voltage corresponding to the 1mA/cm2 of current density in this study) are increased by the added interlayer h-BN, which are 0.43V, 0.48V, 0.53V, 0.56V and 0.60V for the devices without h-BN and with 1, 3, 5 and 7 layers of h-BN, suggesting that Φbarrier of the junction has been increased by h-BN interlayers as expected. Figure 2(b) shows the typical J-V curves of the solar cells based on graphene/h-BN/GaAs sandwich heterostructures under illumination. Compared with the device without h-BN, open circuit voltage (Voc) values are increased, while short circuit current density (Jsc) values are decreased for the devices with h-BN. The thicker the interlayer h-BN film, the higher the values of Voc, while the trend is the opposite for Jsc of the devices. The PCE of the device without h-BN is 6.51%. PCE values of the graphene/h-BN/GaAs structure with 1, 3, 5 and 7 layers of h-BN are 6.83%, 6.92%, 7.10% and 6.38%, respectively. 5 layers of h-BN is the best to enhance the solar cell performance among all the tested conditions.
Fig. 2 (a) Dark J-V curves of the solar cells based on graphene/h-BN/GaAs sandwich heterostructure with different layers of h-BN. (b) J-V curves of the solar cells based on graphene/h-BN/GaAs sandwich heterostructure with different layers of h-BN under AM1.5G illumination. (c) Φbarrier and NIF of the devices based on graphene/h-BN/GaAs sandwich heterostructure. (d) Rs and Rshunt of the solar cells based on graphene/h-BN/GaAs sandwich heterostructure with different layers of h-BN.
Considering the tunneling probability, J-V curve of the graphene/h-BN/GaAs heterostructure can be expresses as below:
where J0 is the saturation current density, m* is the effective mass of the tunneling carrier, d is the thickness of interface h-BN layer, A* is the effective Richardson’s constant of GaAs, T is the absolute temperature, NIF is the ideality factor, h and K represent Plank constant and the Boltzmann constant, respectively. The effective Richardson’s constant of Schottky junction is influenced by the interface conditions. And the contact between graphene and GaAs is different from the contact formed with metal and GaAs. Thus the effective Richardson’s constant and Φbarrier are obtained based on Arrhenius plotting of Eq. (3) at temperatures from 300 K to 350 K. The obtained A* for graphene/GaAs heterojunction is 12.1 A/(k2•cm2), and the values for the graphene/h-BN/GaAs heterojunctions with 1, 3, 5 and 7 layers of h-BN are 15.8, 17.4, 18.6 and 20.1 A/(k2•cm2), respectively. The obtained values of Φbarrier for the devices with different layer numbers of h-BN are shown in Fig. 2(c), where Φbarrier increases with increasing of the layer numbers of h-BN. The output voltage of Schottky diode based solar cell is proportional to the logarithm of the ratio of light current (IL) to dark current (ID). ID is exponential to the negative value of Φbarrier, which means higher Φbarrier leads to much lower ID. Thus, with the increased Φbarrier and slightly decreased Jsc, Voc of the solar cell with interlayer h-BN is increased. On the other hand, sandwiched h-BN interlayers increase the value of NIF, and more layers of h-BN lead to higher value of NIF, as shown in Fig. 2(c). NIF is mainly influenced by the recombination condition in junction region, which suggesting that the extra h-BN layers in the graphene/GaAs interface enhance the interface recombination rate, caused by the extra recombination centers introduced during the transfer processes. Series resistance (Rs) of the graphene/h-BN/GaAs solar cells can be extracted as the slope of linear fitting of dV/dLnI as a function of I [28], and shunt resistance (Rshunt) can be extracted as the reciprocal value of first-order derivation of the I-V curve under illumination at zero voltage point [42]. The obtained values are shown in Fig. 2(d), where Rs and Rshunt both show increasing trend with the increase of the h-BN layers. FF is influenced by NIF, Rs and Rshunt together [43]. Higher NIF and Rs lead to lower FF, while Rshunt show little effect on FF, thus FF values of the device with interlayer h-BN are decreased. The effect of interlayer h-BN in graphene/h-BN/GaAs solar cells can be clearly pictured as that extra h-BN layer with suitable thickness increases the PCE values with increased Voc as the Φbarrier increases, even though the values of Jsc and FF are slightly decreased related to the increased NIF and Rs. When the h-BN layer in the graphene/GaAs interface is too thick, low Jsc and FF leads to decreased PCE of the solar cell.By suppressing the charge transfer between graphene and GaAs, doping of graphene can more effectively improve the device performance based on the enhanced Fermi level tuning effect. Si QD has been chosen as a representative photo-induced dopant by simply spinning Si QDs onto graphene surface [44]. Here solar cell devices without h-BN and with 5 layers of h-BN are used to compare the effect of photo-induced doping. Figure 3(a) shows the J-V curves of the devices with photo-induced doping, corresponding to the J-V curves of the devices without h-BN and with 5 layers of h-BN shown in Fig. 2(b). For the device without h-BN, Jsc is increased by 28% and Voc is increased from 0.56V to 0.57V, together with FF slightly increased from 60.5% to 61.6%, thus the final PCE is 8.63%, increased by 32.6% relatively compared with that of the same device without photo-induced doping. For the device with 5 layers h-BN, Jsc is increased by 35%, Voc is increased from 0.66V to 0.69V, and FF is improved from 58.8% to 59.7%, leading to the final PCE increased from 7.10% to 10.18% by 43.3% relatively. Figure 3(b) shows the electronic band structure and charge transfer process of the Si QDs doped graphene/h-BN/GaAs solar cell. The valence and conduction bands of both Si QDs (EC-Si and EV-Si) and GaAs (EC-GaAs and EV-GaAs) bend upward near graphene layer to maintain Fermi level balance. Under illumination, excess electrons and holes generated in GaAs are collected by GaAs and graphene respectively. Based on the electronic band alignment, transport of excess holes from GaAs to graphene is almost unaffected after inserting the 2D h-BN layer, which dominates the power conversion from light to electricity. Photo generated excess holes in Si QDs inject into graphene while electrons are trapped in the QDs. The injected holes and the negative charged Si QDs on graphene surface both move the Fermi level of graphene downward, leading to higher barrier height and higher Voc.
Fig. 3 (a) J-V curves of solar cells based on graphene/GaAs and graphene/h-BN/GaAs heterostructure with Si QDs introduced photo-induced doping. (b) Electronic band structure of the graphene/h-BN/GaAs solar cell with Si QDs covering on graphene under illumination.
For further confirming the improvement of the device by inserting h-BN interlayer, graphene/h-BN/GaAs heterostructures without h-BN and with 5 layers of h-BN have been utilized as photodiode-type photodetectors with the same structure shown in Fig. 1(d), where only two electrodes, i.e. the top electrode touching graphene and bottom electrode forming ohmic contact with GaAs have been used in the test. The photoresponse properties are measured under illumination of monochromatic laser with wavelength of 325 nm and 635 nm. The typical photoresponse under 325 nm laser illumination is shown in Fig. 4(a), where the laser power is set from 2 μW to 10 μW. All the devices are reverse biased at −0.5 V during the measurements. The laser induced photocurrent increases linearly as the laser power increasing. The similar photoresponse under 635 nm laser illumination is not shown here. Figure 4(b) shows one normalized cycle of photoresponse to 325nm laser of the devices without h-BN and with 5 layers of h-BN, where identical photoresponse behaviors for the two devices can be found, suggesting the interlayer h-BN doesn’t influence the response speed of the photodetection properties. The response time (time interval for the response to rise from 10% to 90% of its peak value) is 67μs, and the recovery time (time interval for the response to decay from 90 to 10% of its peak value) is 813μs. The values of on/off ratio under 325 nm and 635 nm laser illumination, which is deduced from the ratio of IL and ID, i.e. IL/ID, are shown in Fig. 4(c). The values of on/off ratios for the device with 5 layers of h-BN are about one magnitude higher than those of the device without h-BN. The increase of on/off ratio corresponds to the decreased ID of the devices. The responsivity of the photodetector is defined as the following equation:
where PLaser is the incident laser power. The obtained values of responsivity are shown in Fig. 4(d). Under 325 nm laser illumination, responsivity values are in the range of 6 A/W to 21 A/W. The responsivities of the device with interlayer h-BN decreases slightly as the laser power decreases, while the responsivity of the device without h-BN decreases drastically when the laser power is low. The low responsivity to low laser power of the device without h-BN is attributed to the high reverse dark current. The condition under 635 nm laser illumination is similar, low laser power responsivity of the device with 5 layers of h-BN is much better than that of the device without h-BN, except that the responsivity values are in the range of 0.2 A/W to 6 A/W. From another point of view, the high dark current mainly affects the output voltage of the graphene/GaAs heterostructure devices as shown in the performance of solar cells in the former section, while it shows little effect on the photo-generated current under illumination. The low responsivity to weak light of the photodetector device without h-BN arises from the low IL under illumination with low intensity of incident laser and ID staying constant with the same reverse bias based on Eq. (4).Fig. 4 (a) Photoresponse under 325nm laser illumination condition of the photodetectors based on graphene/h-BN/GaAs sandwich heterostructure with different thickness of h-BN 325nm laser. (b) One normalized cycle of photoresponse of the device without h-BN and with 5 layer h-BN under 325 nm laser illumination. (c) on/off ratio values of the photodetectors for the device without h-BN and with 5 layer h-BN under 325 nm and 635 nm laser illumination. (d) Responsivity values of the photodetectors for the device without h-BN and with 5 layer h-BN under 325nm and 635nm laser illumination.
Transient photoluminescence (PL) measurements are used to investigate the kinetics of charge separation at the graphene/h-BN/GaAs interface. PL transient kinetics of bare GaAs substrate, graphene/GaAs junction and graphene/h-BN/GaAs junction with 5 layers of h-BN are investigated. As shown in Fig. 5(a), the PL decay lines suggest that the PL decay depends on double channels [45], including a fast decay channel and a slow decay one. PL decay time constants can be deduced by fitting the PL intensity decay curves based on the equation as below:
where I0 is the starting PL intensity, It is the PL intensity at time t, τ is the time constant. It is noteworthy that the time point when the PL intensity reaches 5000 counts is set as t = 0 for the fitting, which is also the starting time point for PL decay. The fitted curves in the fast and slow decay ranges are shown in Fig. 5(b) and 5(c), respectively. The fitted PL decay time constants in the fast decay range for bare GaAs, graphene/GaAs junction, graphene/h-BN/GaAs junction are 0.92 ns, 0.82 ns, and 0.58 ns respectively, and the values in the slow decay range are 1.96 ns, 1.82 ns and 1.37 ns, respectively. The excitation laser with wavelength of 450 nm is used in the PL measurements, where the absorption depth is close to the surface of GaAs (about 50 nm). The quick decay range in the first few nanoseconds is related to the surface recombination or the charge separation by the heterojunction, and the subsequent slow decay range is influenced both by the bulk recombination and surface recombination or charge separation [46]. For bare GaAs, photo generated excess carriers go through band-to-band recombination and defects mediated recombination processes. For the junction samples, besides the recombination processes, some photo generated carriers are separated by the built-in barrier, thus the PL decay time constants are decreased. The schematic diagram of carrier separation processes in graphene/h-BN/GaAs heterojunction is shown in Fig. 5(d). Parts of the excited electrons in GaAs are separated by the heterojunction and collected by graphene. The increased Φbarrier for the device with h-BN leads to faster separation of excess carriers, which results in the shortened PL decay time constant in the fast decay range. Recombination will take place when holes generated in GaAs cross the interface. The recombination rate is influenced by the carriers crossing time and the total number of recombination centers. It is reasonable to assume that compared with the graphene/GaAs device, the extra h-BN layers in graphene/h-BN/GaAs device introduce more defects centers into the interface. Although the carrier crossing time in the graphene/h-BN/GaAs device can be decreased with the increased Φbarrier, the recombination is still increased as a result of extra recombination center introduced by h-BN transfer process, which is in agreement with the increased NIF for the device with interface h-BN layers. The slow decay range is both influenced by the properties of bulk recombination and charge separation. For different devices, bulk recombination conditions are expected to be identical as using the same substrates. While as mentioned above, the enhanced separation of the excess carriers leads to the slightly decreased time constant in the slow decay range for the heterojunctions with interlayer h-BN. The transient PL measurements further clarify the influence of interlayer h-BN in graphene/h-BN/GaAs heterostructures, and the results are in agreement with the electrical and optoelectronic behaviors mentioned above.Fig. 5 (a) Measured transient PL in bare GaAs, graphene/GaAs and graphene/h-BN/GaAs heterostructures. (b) Fitting of the PL decay time constant in the fast decay range. (c) Fitting of the PL decay time constant in the slow decay range. (d) Schematic diagram of carrier separation and recombination processes in graphene/h-BN/GaAs heterojunction after excited by laser source.
4. Conclusion
2D materials and their heterostructures offer great platforms for obtaining optoelectronic devices with enhanced functionality. In the graphene/GaAs system, the electron transfer from GaAs to graphene makes the Fermi level difference smaller and thus, decreases the Voc of the solar cell and also the on/off ratio of the photodetector. In this work, h-BN is sandwiched between graphene and GaAs to suppress the static charge transfer process, meanwhile not to affect the collection process of excess holes during the operation of the devices under illumination. The Voc of graphene/GaAs solar cell is enhanced from 0.56V to 0.66V with interface h-BN layer, and PCE is increased from 6.51% to 7.10% correspondingly. Moreover, photo-induced doping is more effective in the device with 5 layers of h-BN compared with that without interlayer h-BN. With photo-induced doping, PCE of 10.18% has been achieved for graphene/h-BN/GaAs compared with 8.63% of graphene/GaAs structure. The performance of graphene/h-BN/GaAs based photodetector is also greatly improved with on/off ratio increased by one magnitude compared with graphene/GaAs devices without h-BN. The main mechanism for the enhancement of the photo-detecting performance is that the interface h-BN reduces the dark current, not increases the photo generated current. Further experiments are needed in the future to optimize the transfer process and to obtain high quality graphene/h-BN/GaAs interface for obtaining solar cell and photodetector devices with even higher performance.
Acknowledgments
S. S. Lin and X. Q. Li thank the support from the National Natural Science Foundation of China (NSFC) (No. 51202216 and No. 51502264), Special Foundation of Young Professor of Zhejiang University (No. 2013QNA5007) and China Postdoctoral Science Foundation (2013M540491).
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