Introduction

Two-dimensional (2D) materials provide rich physics in designing of new optoelectronic devices1,2,3,4. Because of the low light absorbance of the atomic thin 2D materials5,6, the external semiconductor is usually incorporated to improve the performance of 2D material based devices7,8,9. Photodetectors based on monolayer graphene have been reported to show photo gain as high as ~108 and photo responsivity as high as ~107 A/W through the enhanced light absorption with covering semiconductor quantum dots on graphene8. Forming 2D materials/bulk materials heterostructure junctions is an alternative choice to obtain high performance optoelectronic devices as the bulk semiconductor can fully absorb incident light10,11. As the first discovered 2D material with many fascinating electrical and optical properties, graphene and its heterostructures have attracted much attention for solar cells worldwide12,13,14,15. Power conversion efficiency (PCE) of solar cells based on graphene/Si system has been improved from 1.65% to 15.6% since the first reported graphene/Si heterostructure solar cell in the year 201016,17. Recently, we have reported graphene/GaAs solar cell with PCE of 18.5%18. On the other hand, single layer 2D molybdenum disulfide (MoS2) is semiconductor with a direct band gap of 1.8 eV19. MoS2 with thickness less than 1 nm can absorb 5–10% incident light20. Also, MoS2 can be synthesized with large area by chemical vapor deposition (CVD) method21,22,23. Based on the abovementioned merits, the MoS2/bulk semiconductor system offers a new platform for optoelectronic device design. It has been reported that MoS2/Si heterostructure solar cell has an efficiency of 5.23% with the assistance of aluminum deposition on MoS26. However, much more work on MoS2/semiconductor heterostructure is highly desirable both for the fundamental research interest and the potential photovoltaic application. Among all the bulk semiconductors, GaAs has a suitable direct band gap of 1.42 eV and high electron mobility (8000 cm2V−1s−1 at 300 K)24, which makes itself one of the best candidates for high performance solar cells25,26.

Tunable Fermi level is one of the unique physical properties of 2D materials, which can be finely tuned by chemical doping or electrical gating27,28,29,30. Different from the conventional bulk p-n junctions, there is static charge transfer between 2D materials and bulk semiconductor, which could severely lower the Fermi level difference between bulk semiconductor and 2D material31 and lead to a decreased junction barrier height. The photovoltaic performance of the heterojunction is greatly influenced by the junction barrier height, which means suppressing the static charge transfer between 2D materials and semiconductor substrate are highly desirable. Herein, we introduce 2D hexagonal boron nitride (h-BN) into the MoS2/GaAs heterostructure to suppress the static charge transfer. More importantly, the inserted h-BN layer makes the tuning of Fermi level of MoS2 more effective, which greatly improves the performance of solar cells. Based on the interface band structure designing and Fermi level tuning of MoS2, 9.03% of PCE has been achieved.

Results

Physical design of the MoS2 based solar cell

The schematic electronic band structure of the independent MoS2 and GaAs is shown in Fig. 1a. The electron affinity (energy gap between vacuum level and the bottom level of conduction band EC-MS) of MoS2 (χMS) is 4.0 eV32 and the band gap of MoS2 is 1.8 eV. As the measured sheet resistance of MoS2 is in the range of 104–106 Ω/□, indicating the Fermi level of MoS2 (EF-MS) locates near the middle of the band gap. The electron affinity of GaAs (χGA) is 4.07 eV. The Fermi level of GaAs (EF-GA) used in this study locates around the bottom level of conduction band (EC-GA) because the n-type doping concentration is around 1018 cm−3. When MoS2 touches with GaAs, due to the Fermi level difference, some of majority electrons of GaAs inject into MoS2, shifting EF-MS by ΔEF-MS, as shown in Fig. 1b, which can be quantitatively expressed as:

Figure 1
figure 1

Schematic electronic band structure of independent MoS2 and GaAs (a), MoS2/GaAs Schottky junction (b), MoS2/h-BN/GaAs heterojunction (c) under equilibrium condition and the electronic band structure of MoS2/h-BN/GaAs heterojunction under illumination (d).

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where Δn is the change of electron concentration in MoS2 which affected by the injected electrons from GaAs, ni is the intrinsic carrier concentration in MoS2, k is the Boltzmann constant and T is the absolute temperature. The barrier height (Φbarrier) of the MoS2/GaAs heterojunction can be presented as:

It is very clear that suppressing the static charge transfer during the formation of MoS2/GaAs heterojunction can result in higher Φbarrier. We propose a device by inserting 2D h-BN into MoS2/GaAs Schottky diode as the interface layer to suppress the static charge transfer. h-BN is one of 2D materials with a band gap of 5.9 eV and dielectric constant of 4.033. The electronic band alignment of MoS2/h-BN/GaAs heterojunction can be seen in Fig. 1c. As h-BN has a negative electron affinity34, the electron transfer from GaAs to MoS2 is suppressed during the formation of the MoS2/h-BN/GaAs heterojunction. As a result, ΔEF-MS is reduced and Φbarrier of the junction is lifted up. Under illumination, photo generated excess electrons and holes are collected by GaAs and MoS2, respectively. As shown in Fig. 1d, transport of holes from GaAs to MoS2 is almost unaffected after inserting the ultrathin 2D BN layer, which dominates the power conversion from light to electricity. In other words, the open circuit voltage (Voc) of the solar cell can be increased by the inserted h-BN while short circuit current density (Jsc) stays almost unchanged, thus, solar cell with a better performance can be expected.

Fig. 2 shows the schematic fabrication processes of MoS2/GaAs and MoS2/h-BN/GaAs Schottky junction based solar cells. After removal of the native oxide on the GaAs substrate, Au with a thickness of 60 nm was evaporated on the rear surface of GaAs forming ohmic contact. Then the front surface of GaAs was cleaned with dilute HCl aqueous solution. Front surface passivation is achieved with remote NH3 plasma treatment for 5 min with power of 120 Watt and frequency of 27.5 MHz. After the passivation treatment, h-BN and MoS2 in sequence or MoS2 alone is directly transferred onto the front surface of GaAs substrate, followed with the deposition of front Au contacts (60 nm) with mask. Inset in Fig. 2 shows the digital photographs of the typical MoS2/GaAs and MoS2/h-BN/GaAs heterojunction based solar cells, where the thin line shape of the active area can be seen, which is designed for efficiently current collection based on the high resistance of monolayer MoS2.

Figure 2
figure 2

Schematic fabrication processes of MoS2/GaAs and MoS2/h-BN/GaAs Schottky junction based solar cells.

The up-left inset shows the digital photographs of the corresponding devices. shows the digital photographs of the corresponding devices.

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Basic properties of the MoS2/h-BN/GaAs heterostructure solar cell

Fig. 3a shows the schematic cross section structure of the MoS2/h-BN/GaAs heterojunction solar cell, which is composed of rear Au contact, GaAs substrate, h-BN layer, MoS2 layer and the front Au contact from bottom to top. The active area of the device is defined with the opened window in the front Au contact, as shown in Fig. 3b. The width of the active area is 120 μm and the length is 5 mm, making the active area 0.6 mm2. As the thickness of the front Au contact is 60 nm, no light can be absorbed by the device in the Au shadowed area, which guarantees the precise active area. The high resolution transmission electron microscopy (HRTEM) image of the MoS2 is shown in Fig. 3c, which shows the six fold symmetry nature of the MoS2. The inset of the Fig. 3c shows the HRTEM image of MoS2 layer, which clearly indicates the CVD grown MoS2 is monolayer. The electron diffraction pattern can be seen in Supplementary Information Fig. S1, which also implies the monolayer nature of MoS2. Fig. 3d shows the absorption spectrum of the CVD grown MoS2, where three absorption peaks corresponding to 436 nm, 619 nm and 662 nm can be seen in the wavelength range of 350–800 nm. The peak of the absorbance locates at 436 nm is 8.9%, which is in agreement with the reported absorbance of the monolayer MoS220. The Raman spectrum of the MoS2 on Si/SiO2 substrate is shown in Fig. 3e. The Raman peaks corresponding to E12g and A1g modes of MoS2 locate at 384.3 cm−1 and 404.5 cm−1, respectively, indicating the grown MoS2 is monolayer35. Fig. 3f presents the Raman spectrum of h-BN, where the 1371 cm−1 peak indicates it is monolayer36. The digital photographs of transferred h-BN on Si/SiO2 substrate, optical microscopy image and atomic force microscopy image of MoS2 and optical microscopy image of h-BN can be seen in Supplementary Information Fig. S2, where can be seen that the homogeneity of CVD grown MoS2 and h-BN is good.

Figure 3
figure 3

(a) Schematic structure of the MoS2/h-BN/GaAs heterostructure. (b) Optical micrograph of the MoS2/h-BN/GaAs solar cell. (c) HRTEM of the monolayer MoS2, inset is the TEM image of the edge of the MoS2 layer, where the bar represents 10 nm. (d) Absorption spectrum of the monolayer MoS2. Raman spectra of monolayer MoS2 (e) and monolayer h-BN (f) used in this study.

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Fig. 4a shows the dark current density-voltage (J-V) curves of the MoS2/GaAs and MoS2/h-BN/GaAs heterojunctions, both of which show good rectifying characteristics. It is noteworthy that in this study if no mentioned, GaAs substrate is n-type doped. We also test the J-V curve of MoS2/p-GaAs, which shows bad rectifying characteristics as presented in Supplementary Information Figure S3. The threshold voltage (the voltage needed to reach a current density of 2 mA/cm2 here) for the MoS2/GaAs heterojunction is 0.41 V, while the value for the MoS2/h-BN/GaAs hetrostructure is 0.52 V, suggesting that Φbarrier is increased by the interlayer h-BN. The value of Φbarrier can be deduced through fitting of dark J-V curves as expressed by:

Figure 4
figure 4

(a) Dark J-V curves of the MoS2/GaAs and MoS2/h-BN/GaAs heterojunctions. (b) J-V curves of the MoS2/GaAs and MoS2/h-BN/GaAs heterojunctions under AM1.5G illumination. (c) Linear fitting of dV/dLnI~I data for obtaining Rs of the devices. (d) Transient PL of the bare GaAs substrate, MoS2/GaAs and MoS2/h-BN/GaAs heterojunctions. (e) Fitting of the PL decay time constant in the fast decay range and (f) in the slow decay range.

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where K is the Boltzmann constant, NIF is the junction ideality factor and q is the value of electron charge. Based on thermionic-emission theory, saturation current density J0 can be described as:

where A* is the effective Richardson’s constant of n-type GaAs (8.16 A/k•cm2)37. Based on equations (3) and (4), the values of NIF for MoS2/GaAs and MoS2/h-BN/GaAs heterojunctions are 3.18 and 2.73 and the Φbarrier values are 0.71 eV and 0.78 eV, respectively. The lower NIF of the MoS2/h-BN/GaAs device compared with that of the MoS2/GaAs device indicates that the interface recombination rate is decreased with interlayer h-BN. Fig. 4b shows the J-V curves of the MoS2/GaAs and MoS2/h-BN/GaAs heterostructure solar cell under AM1.5G illumination. As Φbarrier is increased, with interlayer h-BN, Voc of the solar cell is increased from 0.51 V to 0.57 V, while the Jsc is slightly decreased from 20.6 mA/cm2 to 20.2 mA/cm2. With the values of fill factor (FF, FF = Pmax/(Voc × Jsc)) as 45.9% and 47.0%, the PCE values (PCE = Voc × Jsc × FF) for the MoS2/GaAs and MoS2/h-BN/GaAs solar cells are 4.82% and 5.42%, respectively. The series resistance (Rs) fitting results of the MoS2/GaAs and MoS2/h-BN/GaAs solar cells are shown in Fig. 4c, which demonstrates Rs is increased from 56.8 Ω for the MoS2/GaAs device to the value of 81.9 Ω for the MoS2/h-BN/GaAs device. FF is mainly influenced by NIF and Rs. Lower NIF leads to higher FF while higher Rs leads to lower FF. For the MoS2/h-BN/GaAs device, even with increased Rs, the decreased NIF increases the FF compared with the value of MoS2/GaAs device.

The electrical properties mentioned above indicate the importance of the interface recombination condition for the MoS2/GaAs and MoS2/h-BN/GaAs devices. Here transient photoluminescence (PL) is employed to investigate the kinetics of the photo generated carriers near the interface. Fig. 4d shows the transient PL decay curves for bare GaAs substrate and the same MoS2/GaAs and MoS2/h-BN/GaAs heterostructure devices with PCE of 4.82% and 5.42% respectively. The decay curves show double channel dependent behavior, corresponding to a fast decay channel (in the range of 1 ns to 2 ns) and a slow decay channel (after 2 ns). The wavelength of the excitation laser is 450 nm and the absorption depth is close to the surface of GaAs (about 50 nm). The quick decay range in the first nanosecond is related to carrier kinetics at surface or interface and the subsequent slow decay range is dominated by the bulk recombination processes. PL decay time constants are deduced by exponentially fitting the PL intensity decay curves in the fast and slow decay ranges as shown in Fig. 4e,f, respectively. The fitted PL decay time constants in the fast decay range for bare GaAs, MoS2/GaAs and MoS2/h-BN/GaAs are 0.97 ns, 0.59 ns and 0.52 ns, respectively and the values in the slow decay range are 1.92 ns, 1.89 ns and 1.85 ns, respectively. For the bare GaAs, photo generated excess carriers recombine with emission of photons or phonons. In MoS2/GaAs heterostructure, besides the process mentioned above, parts of the excited holes in GaAs are separated by the heterojunction and collected by MoS2. The separated electrons and holes cannot participate in the radiation recombination process. Thus PL decay time constant is decreased. In the MoS2/h-BN/GaAs heterostructure, Φbarrier is increased, which leads to higher speed of carrier separation process and even shorter PL decay time constant. Recombination will take place when electrons produced in GaAs cross the interface. The recombination rate is influenced by the carriers crossing time, which corresponds to the PL decay time constant in the fast decay range. For the device with interlayer h-BN, crossing time is shortened, resulting in the lowered interface recombination rate and lower value of NIF. For the PL decay in the slow decay range, similar time constants imply that the PL decay process is dominated by the bulk recombination properties.

Enhance the performance of the MoS2/h-BN/GaAs heterostructure solar cell by chemical doping

The Fermi level and carrier concentration in MoS2 can be tuned by chemical doping. In this study, AuCl3 solution in nitromethane (1 mM) is used to doping 2D MoS2 to increase the PCE of the MoS2/h-BN/GaAs solar cell. The J-V curves in the dark and under AM1.5G illumination can be found in Fig. 5a. The threshold voltage increases from 0.45 V to 0.58 V for after doping of MoS2, indicating Φbarrier increases by AuCl3 doping of MoS2. The value of Φbarrier can be deduced through fitting of dark J-V curves based on equation (3) and (4). The obtained values are 0.79 eV and 0.85 eV for the undoped and doped MoS2/h-BN/GaAs devices, respectively. As Φbarrier increases, Voc of the MoS2/h-BN/GaAs heterostructure based solar cell increases from 0.56 V to 0.64 V. Meanwhile, Jsc is slightly increased from 20.6 mA/cm2 to 20.8 mA/cm2. The FF values are 46.6% and 53.7% and the PCE values are 5.38% and 7.15% for the undoped and doped devices, respectively. The increase of the FF after doping is related to the decrease of Rs, as shown in Fig. 5b, the value of Rs without doping of MoS2 is 56.0 Ω, while Rs is decreased to 45.9 Ω after doping. In addition, to explore the stability of the doped MoS2/h-BN/GaAs solar cell, we test the variation of the PCE values in 50 hours under AM1.5G illumination, as seen in Fig. 5c. The device was sealed by polymethyl methacrylate (PMMA) through spining-on-coating. The starting PCE value is 6.73%, while after illumination for 50 hrs, the PCE increases to 6.96%. Considering the light induced degradation of crystaslline silicon solar cell is usually happened in the first 24 hrs under AM1.5G illumination, it can be safely concluded the stability of MoS2/h-BN/GaAs solar cell under illumination is good with suitable encapsulation.

Figure 5
figure 5

(a) J-V curves in the dark and under AM1.5G illumination of undoped and doped MoS2/h-BN/GaAs solar cells. (b) Rs fitting of the MoS2/h-BN/GaAs solar cell device with and without AuCl3 doping. (c) Performance stability of the MoS2/h-BN/GaAs solar cell under AM1.5G illumination.

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Further improvement of the MoS2/h-BN/GaAs heterostructure solar cell by electrical gating

As a atomic thin 2D semoconductor, the Fermi level of MoS2 can be finely tuned with gating effect38. Here we employ PEO based ion polymer as the top gate electrode39 and the schematic structure of the field effect MoS2/h-BN/GaAs solar cell is shown in Fig. 6a. Ion gate is directly covered on the surface of AuCl3 doped MoS2. Negative voltage is applied on the ion gate and the rear Au contact is connected to the ground. Fig. 6b shows the J-V curves of the field effect solar cell under AM1.5G illumination. When gate voltage (Vgate) equals to −0.5 V, Voc of the solar cell is increased from 0.64 V to 0.72 V. Meanwhile, Jsc is slightly increased from 20.2 mA/cm2 to 20.7 mA/cm2, which might be attributed to the enhanced efficiency of charge seperation with gating. FF is increased from 53.1% to 54.9% and PCE is improved from 6.87% to 8.27%. when Vgate equals to −1.0 V, the obtained values of Voc, Jsc and FF are 0.76 V, 21.1 mA/cm2 and 56.3%, respectively. And the final PCE is 9.03%. Fig. 6c shows the dark J-V curves with different Vgate. The threshold voltage is increased when the negative Vgate increases. By fitting of the dark J-V curves shown in Fig. 6c, values of Φbarrier can be obtained and shown in Fig. 6d, where the values of Voc corresponding to different Vgate are also shown. Fig. 6d discloses that the increased Voc is mainly attributed to the improved Φbarrier under gating effect. The Φbarrier values are 0.85 eV, 0.91 eV and 0.95 eV corresponding to the Vgate values of 0 V, −0.5 V and −1.0 V, respectively. Vgate higher than −1.0 V causes gate leakage current in this experiment, causing the dark J-V curves away from the (0, 0) point and the J-V curves under illumination unreliable. Thus no further J-V curves are shown here for Vgate higher than −1.0 V. The effect of electrical gating on the MoS2/GaAs solar cell is shown in Supplementary Information Fig. S4. The PCE values are 4.71%, 5.81% and 6.15% with the Voc values as 0.50 V, 0.55 V and 0.57 V corresponding to the Vgate values of 0 V, −0.5 V and −1.0 V, respectively. The Φbarrier values correspondingly are 0.67 eV, 0.71 eV and 0.74 eV. The change of Φbarrier is 0.07 eV, suggesting the Fermi level of MoS2 is shifted by 0.07 eV with the gate voltage of −1.0 V. For the MoS2/h-BN/GaAs device, Fermi level of MoS2 can be tuned by 0.10 eV as Φbarrier is increased by 0.10 eV, 43% higher than that of the MoS2/GaAs device. In the 2D material/semiconductor heterostructure, change of the Fermi level of the 2D material can be suppressed by the charge transfer from GaAs substrate. In the MoS2/GaAs device, downshifting the Fermi level of MoS2 is inhibited by the electron transfer from GaAs to MoS2. By intersting h-BN to suppress the electron transfer, Fermi level tuning of MoS2 by the electrical gating can be more effective. From this point of view, interface h-BN can not only increase the initial Φbarrier of the MoS2/h-BN/GaAs heterostructure, but also guarantees more effectively Fermi level tuning of MoS2.

Figure 6
figure 6

(a) Schematic structure of the field effect MoS2/h-BN/GaAs solar cell. (b) J-V curves of the field effect MoS2/h-BN/GaAs heterojunctions under AM1.5G illumination and different Vgate. (c) Dark J-V curves of the field effect MoS2/h-BN/GaAs heterojunctions under different Vgate. (d) Values of Φbarrier and Voc correspond to different Vgate.

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Discussion

MoS2/GaAs heterostructure based solar cell is investigated. Different from the traditional p-n junctions and metal/semiconductor Schottky junctions, the charge transfer between MoS2 and the GaAs substrate can greatly influence the position of Fermi level in the 2D material, which leads to a much lower barrier height than the ideal value originated from the Fermi level difference. The barrier height is a key factor for the electrical properties of electronic and optoelectronic devices. Thus, suppressing or preventing the charge transfer during the formation of the 2D material based heterojunctions is highly desired to achieve high performance devices. Herein, we demonstrated the performance of MoS2/GaAs based heterostructure solar cell is improved by inserting interlayer h-BN. The inserted h-BN layer can suppress the electron injection from n-type GaAs into MoS2 during the junction formation, while does not affect the hole separation and collection processes according to the electronic band structure of h-BN. Thus, PCE is increased from 4.82% to 5.42% after inserting the BN layer as higher barrier height and Voc can be achieved. Furthermore, by employing chemical doping and electrical gating into the solar cell device, PCE of 9.03% is achieved, which is the highest among all the reported monolayer transition-metal dichalcogenide-based solar cells. This physical picture and technique could be extended into other 2D materials/semiconductor heterostructure based electronic and optoelectronic devices.

Methods

Monolayer h-BN was grown on copper substrate with B3N3H6 as the precursor at 1000 oC for 30 min40. Single layer MoS2 film was grown on Si/SiO2 substrate in a quartz tube with CVD method41. MoO3 powder and sulfur powder (99.9%, both bought from Aladdin) was used as the precursor. Growth temperature was set at 650 oC. 60 nm Au was thermally evaporated on back surface of GaAs to form rear contact. GaAs substrate was cleaned by dipping the samples into 10%wt HCl solution for 5 min followed with DI water rinse. Surface passivation of GaAs was realized by remote NH3 plasma treatment for 5 min with 120 Watt 27.5 MHz RF generator. h-BN was transferred onto the GaAs substrate using PMMA as the sacrificing layer. After PMMA spun-on, MoS2 on Si/SiO2 substrate was immersed into deionized water to lift-off the PMMA-MoS2 films. After transferring, PMMA was removed by immersing the samples into acetone for 20 min.

MoS2 and h-BN were characterized by Raman spectroscopy (Renishaw inVia Reflex) with the excitation wavelength of 532 nm. The microstructure of MoS2 was examined by HRTEM (Tecnai F-20 operating at 200 KV). Atomic force Microscopy (AFM) characterization was performed using Veeco dimension 3100 system. The MoS2/h-BN/GaAs solar devices were tested by Agilent B1500A system with a solar simulator under AM1.5G condition. It is noteworthy that the illumination intensity was calibrated with a standard Si solar cell. Transient PL measurements were used to evaluate the charge recombination and separation behaviors at the interfaces of MoS2/h-BN/GaAs heterojunction. 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 1100 nm 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.

Additional Information

How to cite this article: Lin, S. et al. Interface designed MoS2/GaAs heterostructure solar cell with sandwich stacked hexagonal boron nitride. Sci. Rep. 5, 15103; doi: 10.1038/srep15103 (2015).