- Email: [email protected]
GaN MQWs solar cells by coupling plasmonic with piezo-phototronic effect
Nano Energy 57 (2019) 300–306
Contents lists available at ScienceDirect
Nano Energy journal homepage: www.elsevier.com/locate/nanoen
Full paper
Enhanced photocurrent in InGaN/GaN MQWs solar cells by coupling plasmonic with piezo-phototronic effect
T
Chunyan Jianga,b,c, Yan Chend, Jiangman Suna,b, Liang Jinga,b, Mengmeng Liua,b, Ting Liua,b, ⁎ ⁎ ⁎ Yan Panc, Xiong Pua,b,f, Bei Mae, , Weiguo Hua,b,f, , Zhong Lin Wanga,b,f,g, a
CAS Center for Excellence in Nanoscience, Beijing Key Laboratory of Micro-nano Energy and Sensor, Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, China b School of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing 100049, China c State Key Laboratory of Advanced Power Transmission Technology, Global Energy Interconnection Research Institute Co. Ltd., Beijing 102209, China d Beijing Aerospace Automatic Control Institute, National Key Laboratory of Science and Technology on Aerospace Intelligence Control, Beijing 100854, China e Graduate School of Electrical and Electronic Engineering, Chiba University, 1-33 Yayoicho, Inage-ku, Chiba 263-8522, Japan f Center on Nanoenergy Research, School of Physical Science and Technology, Guangxi University, Nanning, 530004, China g School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245, USA
A R T I C LE I N FO
A B S T R A C T
Keywords: InGaN/GaN MQWs Solar cells Plasmonic Piezo-Phototronic effect
InGaN-based photovoltaics (PV) devices have attracted great attentions because of the excellent photoelectric performance over the past decades. The photocurrent of the InGaN/GaN MQWs solar cells could be further improved by coupling plasmonic with piezo-phototronic effect. Here, we fabricated an InGaN/GaN MQWs solar cells with Ag nanoparticles and the short circuit current of the solar cell was improved by 40% with a static external strain applied. The transmission spectrum and the electromagnetic field distributions of the nanoparticles array were simulated with a finite element analysis model. The physical mechanism for light absorption enhancement by the coupling effect in the quantum well structures was illuminated through a self-consistent numerical calculation with non-linear piezoelectricity polarization. This work demonstrated the coupling between the plasmonic and piezo-phototronic effect achieves significant increase in InGaN/GaN MQWs solar cell power conversion efficiency without any complicated process involved.
1. Introduction Photovoltaic (PV) cells have been under intensive researches because that the threat of energy shortage is becoming one of the great challenges around the world.[1–4]In the past decades, scientists and engineers have been engaged in finding the most efficient photovoltaic devices. III-nitride semiconductors (such as GaN, AlN and InN) have been utilized extensively in various optoelectronic devices, including light-emitting diodes and laser diodes. InGaN-based alloys have recently attracted considerable attention for photovoltaic applications, because of the high radiation resistance for space applications [5], high absorption coefficient (~105 cm−1)[6] and a tunable direct band gap that covers nearly full-solar spectrum (0.7–3.4 eV) which results in a very high theoretical conversion efficiency [7]. Actually, to improve the power conversion efficiency of the solar cell is the most important issue in this field [8]. The InGaN/GaN MQWs structures usually used as the active region of the solar cells.[9]Conductive indium-tin-oxide (ITO)
⁎
current spreading layer applied on the top p-type GaN layer [10], colloidal CdS quantum dots (QDs) combined with back side distributed Bragg reflectors (DBRs) [11], and plasmonic nanoparticles [12] have enhanced the performance of the InGaN/GaN Multiple quantum wells (MQWs) solar cells partly. Among various photovoltaic technologies, Surface Plasmon Polaritons (SPPs) that use of the scattering from metal nanoparticles has received tremendous interest in recent years.[1] SPPs are using metal particles localized the electromagnetic field surface waves propagating in the vicinity of the metal-dielectric interface and also induce electron-plasma resonance inside the material. This increases the local electromagnetic field and also provides an opportunity to engineer the optoelectronic device. Recently, surface plasmon has provided serials of opportunities, for example large enhancements of internal quantum efficiencies in InGaN-based light emitting diodes (LEDs) [13], constructing miniaturized optoelectronic circuits [14] and fabricating deep sub-wavelength patterns for next generation lithography.[15]Especially in third-generation solar cells,
Corresponding authors. E-mail addresses: [email protected] (B. Ma), [email protected] (W. Hu), [email protected] (Z.L. Wang).
https://doi.org/10.1016/j.nanoen.2018.12.036 Received 29 September 2018; Received in revised form 5 December 2018; Accepted 10 December 2018 Available online 11 December 2018 2211-2855/ © 2018 Elsevier Ltd. All rights reserved.
Nano Energy 57 (2019) 300–306
C. Jiang et al.
Fig. 1. (a) A schematic of the InGaN/GaN MQW solar cell structure with Ag nanoparticles. The magnified cross-sectional view of the multilayered structure shows the plasmonic nanostructures. (b) Atomic structure model and the corresponding energy band diagram of GaN/InGaN/GaN heterostructure. (c) X-ray diffraction measurement of the MQW structures. The inset is the enlarged 2theta/omega scans between the satellites −1 and 0.(d) The SEM image of Ag nanoparticles on p-GaN. The inset is the statistical size distribution of Ag nanoparticles by Gaussian fittings (solid lines), and the nanoparticles average diameter is 30 nm. (e) The experimental extinction spectra for Ag nanoparticles on p-GaN after annealing.
periods InGaN/GaN (3/13 nm) MQWs absorption layer, with about 25% indium composition, and finally an Mg-doped p-GaN layer. A schematic of the InGaN/GaN MQW structure with Ag nanoparticles is shown in Fig. 1a. Ag nanoparticles were thermally annealed on the surface of p-GaN, followed by 200 nm ITO layer. When specific wavelength of light incident on the surface of Ag nanoparticles, a strong resonant coupling between the SPPs and the MQW is expect to take place.[32] The atomic structure model and energy band profile of the GaN/InGaN/GaN heterostructure are shown in Fig. 1b. As the noncentral symmetric crystal structure in wurtzite InGaN and GaN multilayers, the spontaneous polarization charges created in the interfaces lead to the electron wave function and hole wave function separated in opposite directions in the wells.[33]The photocurrent in InGaN/GaN MQWs solar cells may be reduced by these spontaneous polarization charges.[34] Fig. 1c shows the triple axis mode X-ray diffraction spectrum 2theta/omega scan of the InGaN/GaN MQW structure, which give an approximate average In composition in the InGaN layers.[15] The distinct, periodic and high-order satellite peaks illustrates the high crystal quality of GaN films.[35] The inset in Fig. 1d is the enlarged spectrum between the satellites −1 and 0, corresponding to the number of the quantum well in the active region. The diameter of Ag nanoparticles on the p-GaN has been measured and analyzed, which is shown in Fig. 1d. From the SEM image and the Gaussian fittings, it can be seen that the main size of the particles is 30 nm. The optical behavior of Ag NPs was evaluated by the UV–Visible spectrophotometer. It is observed that the absorption peak of the Ag NPs is 442 nm, which is shown in Fig. 1e. The peak position of absorption spectrum may result from the SPPs resonance in the Ag NPs. The solar cell device structure was based on our previous work [34] and is shown schematically in Fig. 2a. The Ag nanoparticles are distributed in a rectangular region between each two metal finger electrodes. Fig. 2b shows the current density-voltage (J-V) curves with and without Ag NPs under AM1.5illumination yields. A solar simulator
the light absorption of the devices can be increased with the strong local field enhancement [16–18]. With the plasmonic resonance, an efficiency enhancement of 27% for InGaN-based solar cells [19], a 9.1% short circuit current enhancement for InGaN/GaN MQWs solar cells. [20]Moreover, a piezoelectric polarization has been demonstrated in the non-central symmetric crystal structure, it can be utilized to modulate the electrical states in piezoelectric semiconductors under mechanical deformation [21–26]. The piezo-phototronic effect, which is introduced as a three-way coupling among piezoelectric polarizations, semiconductor property and optical excitation, can effectively control/ tune the charge generation, transport, and recombination at an interface of the p-n junction, metal-semiconductor (M-S) Schottky contact and quantum wells [27–31]. Here, we present the design of InGaN/GaN MQWs solar cell coupling with Ag NPs and piezo-phototronic effect, where the enhanced photocurrent and conversion efficiency have been observed. The transmission spectrum of Ag nanoparticle has been systematically studied. Under the external strain of 0.152%, we have attained a 40% enhancement in the short circuit current and a 66% enhancement in the conversion efficiency as compared a solar cell without metallic nanostructures. Furthermore, a self-consistent numerical model combined with the nonlinear piezoelectric effect has been established to investigate the physical mechanism of the photocurrent response. This work is not only promising in developing high conversion efficiency InGaN/GaN MQW solar cells, but also providing light-harvesting scheme to variety optoelectronic devices. 2. Results and discussions The epitaxial layers of InGaN and GaN heterostructure were grown by Metal-organic Chemical Vapor Deposition (MOCVD) on c-plane patterned sapphire substrate. This epi-wafer consists of (from bottom to top) a 3 µm thick undoped GaN buffer, a 2 µm Si-doped n-GaN layer, 9 301
Nano Energy 57 (2019) 300–306
C. Jiang et al.
Fig. 2. Performances of the plasmonic nanoparticles enhanced InGaN/GaN MQWs solar cell. (a) A schematic diagram of the InGaN/GaN MQW solar cell with Ag nanoparticles. (b) J-V characteristics of InGaN/GaN MQWs solar cell with and without Ag nanoparticles. (c) Room temperature J-V characteristics of the solar cell as the light intensity increases from dark to 1.5 AM (d) P-V characteristics of the solar cell as the light intensity increases from dark to 1.5 AM (e-f) The illumination intensity dependence of the open-circuit voltage (VOC), the short-circuit current density (JSC), the conversion efficiency (η) and the fill factor (FF).
explain the photocurrent enhancement by incorporating metal nanoparticles with solar cells.[36]To illustrate the characteristics of the InGaN/GaN MQW solar cell with Ag NPs, the dependency of the current density and the power density on the illumination intensity (P) is demonstrated in Fig. 2c and d. The open-circuit voltage (VOC) is insensitive to variation of the illumination intensity. This is because that the addition of Ag NPs does not change the bandgap of the MQWs structure. The current density and the power density of the solar cell increase uniformly as the illumination intensity increases. Specifically, the VOC, JSC, η and fill factor (FF) under different illumination intensities are extracted and plotted in Fig. 2e, f. The short-circuit current increases almost linearity as the illumination intensities ranging from 0
(Model PT-SUN2S, Pharos Technology) provided the AM1.5 illumination, and had been calibrated with an amorphous silicon reference solar cell. It can be clearly seen that the rough surfaces covered with Ag nanoparticles lead to an enhanced short-circuit current (JSC). This is due to the Ag nanoparticles can not only enhance the light absorption in the quantum well region, but also can couple incident light into guided modes that propagate through the MQWs region, thus improving the current density of solar cells. The short-circuit current increased from 0.93 mA/cm2 to 1.03 mA/cm2, thereby the photoelectric conversion efficiency (η) increased from 0.76% to 0.98%, increased by 28%, which is close to previous reports.[19]The near-field concentration and light scattering are two main mechanisms which have been suggested to 302
Nano Energy 57 (2019) 300–306
C. Jiang et al.
Fig. 3. (a) The schematic illustration of the fringing electric field excited by the localized surface plasmonic resonance. (b) Diagrams illustrating the mechanism for enhanced SPPs resonant excitation. (c) Transmission spectrum for the Ag NPs configuration with ITO superstrate, and the inset is the Ez distribution at direct transmission (390 nm) and SP mode (445 nm).
diagram of our modified Raman system with a jig unit for stress analysis. The fixed plate and the clamp hold the device, the stress controller apply stress from the back at the same time. Structural mechanical model was established which is shown in the red dashed box, and arrays of solar cells were prepared on a sample. Raman shifts of E2 h mode and A1(LO) mode of InGaN/GaN MQWs under various external strains are shown in Fig. 4b. As the external stress increases, the Raman spectrum moves to the shortwave numbers, thus the internal residual compressive stress decreases. The value marked on the curve is the strain calculated according to the movement of the peak position of Raman E2 h. Due to the limitations of the sapphire substrate, the maximum strain that can be added to the devices is 0.152% in our experiment. Fig. 4c-d show J–V characteristics and P–V characteristics of the InGaN/GaN MQW solar cell with Ag nanoparticles under 100 mW/cm2 illuminations at different external applied strains. The gray dashed lines correspond to devices without Ag nanoparticles. With increasing external strains, J–V and P–V curves remarkably moves upper. The corresponding short-circuits current and open-circuit voltage at external strains is summarized in Fig. 4e. The short-circuits current increases from 1.03 to 1.37 mA/cm2 with increasing strain, and the open-circuit voltage is basically stable. Fig. 4f is the conversion efficiency and the fill factor of the InGaN/GaN solar cell as a function of the strains. The efficiency of the solar cell enhanced from 0.98% to 1.25%, while the fill factor is basically stable. Compared to InGaN/GaN solar cell without Ag plasmonic nanoparticles, the enhancement of short-circuit current and conversion efficiency is up to 40% and 66%, respectively. With the single plasmonic resonance, previous works reported 9.1% short circuit current enhancement and 27% efficiency enhancement in InGaN based solar cells [10,19]. This new coupling method is effective, simple and recoverable in developing high conversion efficiency photovoltaic. To further explain the physical mechanism of the plasmonic and piezo-phototronic coupling effects on InGaN/GaN multi quantum well solar cells, a self-consistent numerical calculation of Schrödinger equation and Poisson equation has been established. The schematic diagram of InGaN/GaN MQW structure with/without Ag NPs are shown in Fig. 5a1 and a2, the corresponding carrier mobility mechanism are shown in Fig. 5a2 and b2. For the structure with Ag NPs, the effective absorption cross-section increased and the optical reflection loss reduced.[37] In addition, more lattice relaxation under the compressive residual stress leads to an increase of photo-generated carriers, and the photoelectric current increases. In the calculation, the impact of the non-linear piezoelectricity on quantum wells has been considered. [38–40] Both first and second order piezoelectric coefficients have been considered. The total polarization in the multi-quantum well regions is given by the quadratic expression:
to AM1.5 illumination, corresponding to the increase of photo-generated carriers. The VOC slightly increases from 1.46 V to 1.6 V as the intensity of the light increases from 25 mW/cm2 to 100 mW/cm2, and the efficiency enhances from 0.75% to 0.98%. This also indicates that SPP enhance the absorption and the carrier collection of the solar cell under different light intensities. In addition, the FF is 0.58 at 25 mW/ cm2 and 0.59 at 100 mW/cm2，respectively, which is basically constant. In order to study the physical principles of the enhancement effect of SPP on electromagnetic fields, the schematic illustration of the electric field excited by the surface plasmon resonance is shown in Fig. 3a. The free electrons of metal particles can be driven to oscillate under the electromagnetic wave incidence and the amplitude reaches the maximum when the frequency of the incident electromagnetic wave is consistent with the resonance frequency of the metal nanoparticles. The metal nanoparticles size, shape and the dielectric parameters of the surrounding environment determine the resonant wavelength. Combined with the quasi-static approximation, the polarizability can be expressed as:
P = 4πa3
ε − εm ε +2εm
(1)
Where a is the diameter of the metal nanoparticles, ε and εm are the dielectric constant of the surrounding environment and the nanoparticles, respectively. As is expressed in Eq. (1), the resonant oscillation reaches maximum when ε + 2εm → 0 . In Fig. 1, the Ag nanoparticles are distributed between ITO layer and p-GaN layer, and Fig. 3b is the diffraction schematic diagram of light incident on the Ag NPs structure. A beam of light is directed toward the surface of the Ag nanoparticles, and multistage diffraction occurs simultaneously. Thus, when the momentum of transmitted component of diffracted or reflected light matches with SPP momentum, SPP excitation reaches the maximum. In addition, the field distributions and the transmission spectrum were obtained with a finite element analysis model established by COMSOL Multiphysics 5.0. In the model, the RF module was used, and the diameter of the particles is 30 nm, the refractive index of the ITO and p-GaN is 2 and 2.25, respectively. It was observed that the Ag NPs exhibited a dip at 445 nm as the wavelength of incident light range from 380 nm and 600 nm, which is shown in Fig. 3c. The inset is the Ez distribution at 390 nm and 445 nm, corresponding to the full transmission and SPP excitation respectively. This simulated transmission spectrum keeps a perfect consistent with the experimental data shown in Fig. 1e. The previous works [33,34] have studied the strain distribution of the sample under different stress. Raman microscopy system is one of the most available analyze internal strain methods for InGaN/GaN MQWs structures as the incident light can be accurately focused on the sample through a microscope objective lens. Fig. 4a is the schematic
PTot = Psp + Plm + Ppz + e33 ε⊥+2e31 ε∥ + e311 ε∥2 + e333 ε⊥2 + e133 ε∥ ε⊥ 303
(2)
Nano Energy 57 (2019) 300–306
C. Jiang et al.
Fig. 4. Strain analysis by Raman scattering and enhanced device performances in InGaN/GaN MQWs Solar Cells by coupling plasmonic with piezo-phototronic effect. (a) The optical path of the Raman system integrated with a jig unit.(b) Raman shift of E2 hmode and A1(LO) mode with various external strain.(c-d) J−V and P − V characteristics of the InGaN/GaN MQW solar cell with Ag nanoparticles under 100 mW/cm2 illumination at different external strains. The gray dashed lines correspond to the solar cell without Ag nanoparticles.(e-f) External strain dependence of VOC, JSC, η and FF.
where Psp is the spontaneous polarization, Plm is lattice-mismatchinduced piezoelectric polarizations, Ppz is piezotronic-induced polarization, eij is the linear piezoelectric coefficient, and eijk is the second-order piezoelectric coefficient. The ε∥ and ε⊥ are strain in the growth plane and in the orthogonal direction, respectively. The detailed calculation parameters of wurtzite InN and GaN are listed in Table 1. Fig. 5c is the calculated band profile as GaN/InGaN/GaN heterostructure. As the external strains applied, the band profile of the MQWs becomes a flatter band condition. Such a modified band causes the electron and hole wave functions move toward each other, and provide
Table 1 Value list of physical parameters for wurtzite GaN and InN used in this work. Parameters(C/m2)
GaN
InN
Refs
Psp e31 e33 e311 e333 e133
0.029 −0.49 0.73 6.185 −8.090 1.543
−0.032 −0.57 0.97 5.151 −6.680 1.280
[41] [41] [41] [39] [39] [39]
304
Nano Energy 57 (2019) 300–306
C. Jiang et al.
Fig. 5. The schematic diagram of the InGaN/GaN MQW structure and the mechanism are shown in (a1) and (a2) respectively. (b1) and (b2) are the structure and the mechanism of the InGaN/GaN MQW solar cell after applying Ag NPs and a compressive stress on the device, which shows the modulation effect of the plasma coupling with piezo-phototronic effect. (c) Calculated valence band and conduction band of the GaN/InGaN/GaN heterostructure with (dashed line) and without strain (solid line). (d) Corresponding electron and hole wave function distribution with (dashed) and without strain (solid line).
4. Methods
greater absorption enhancement in the quantum well region.
4.1. Fabrication of InGaN/GaN MQW solar cells 3. Conclusions The samples were first cleaned in HCl:4H2O solution and then thoroughly rinsed with deionized water. Under 450 W power, SiH4, Ar and O2 flow rates were 130.5 sccm, 126 sccm and 13 sccm, and the reaction chamber temperature is 80 °C, a 1500 nm SiO2 layer was deposited on the surface of p-GaN by PECVD. For mesa isolation, the 2 × 2 mm2 square array pattern is transferred to the sample through the combination of photolithography and inductively coupled plasma reactive ion etching (ICP-RIE) system. The flow rates of the reaction gas are Cl2 (30 sccm), BCl3 (15 sccm), Ar (5 sccm) for GaN etching, and CHF3 (20 sccm), CF4 (40 sccm), Ar (10 sccm) for SiO2 etching. After etching, the sample was soaked in hydrogen fluoride (HF) solution for 40 min so that the SiO2 for mask could be removed. N-type GaN was exposed about 1 µm deep by ICP, and then Ti/Al/Ti/Au (30 nm/ 120 nm/45 nm/55 nm) n-contact was evaporated and annealing at 850 °C for 30 s. Silver particles were deposited on the surface of p-type GaN, and detailed synthesizing processes of the Ag nanoparticles array
In summary, we have demonstrated the InGaN/GaN MQWs solar cells with an enhanced photocurrent by coupling plasmonic with piezophototronic effect. A 40% enhancement in the short circuit current and a maximum 66% enhancement in the conversion efficiency have been obtained under 0.152% external strain compared with that without Ag plasmonic nanoparticles. In addition, the calculated results of our selfconsistent model have showed agreement with experimental values. The effective absorption cross-section increased and the optical reflection loss reduced by Ag plasmonic, while a significant reduction of the total piezoelectric field is found through the piezo-phototronic effect. The combination of Ag nanoparticles and the external mechanical strain provide a viable, promising path toward high conversion efficiency solar cells, which gives us a new idea to design novel photovoltaic devices to alleviate the current global energy crisis.
305
Nano Energy 57 (2019) 300–306
C. Jiang et al.
could be found in the Section 4.2. Conductive indium-tin-oxide (ITO) material, which serves as current spreading layer for InGaN/GaN MQW solar cells, has been deposited with RF magnetron sputtering. The final Ni/Au (30/150 nm) n-contact electrodes were formed by magnetron sputtering using lithography and a lift-off technique, and each grid width is 40 µm with a center-to-center spacing of 400 µm, as is shown in Fig. 2a.
Energy 41 (2017) 654–664. [19] J.-Y. Wang, F.-J. Tsai, J.-J. Huang, C.-Y. Chen, N. Li, Y.-W. Kiang, C.C. Yang, Opt. Express 18 (2010) 2682–2694. [20] J.-P. Shim, S.-B. Choi, D.-J. Kong, D.-J. Seo, H.-j. Kim, D.-S. Lee, Opt. Express 24 (2016) A1176–A1187. [21] Z.L. Wang, Nano Today 5 (2010) 540–552. [22] Z.L. Wang, W. Wu, Natl. Sci. Rev. 1 (2013) 62–90. [23] Z.L. Wang, Adv. Mater. 24 (2012) 4632–4646. [24] Y. Zhang, Z.L. Wang, Adv. Mater. 24 (2012) 4712–4718. [25] T. Liu, C. Jiang, X. Huang, C. Du, Z. Zhao, L. Jing, X. Li, S. Han, J. Sun, X. Pu, J. Zhai, W. Hu, Nano Energy 39 (2017) 53–59. [26] C. Jiang, T. Liu, C. Du, X. Huang, M. Liu, Z. Zhao, L. Li, X. Pu, J. Zhai, W. Hu, Z.L. Wang, Nanotechnology (2017). [27] Y. Zhang, Y. Yang, Z.L. Wang, Energy Environ. Sci. 5 (2012) 6850. [28] C. Pan, L. Dong, G. Zhu, S. Niu, R. Yu, Q. Yang, Y. Liu, Z.L. Wang, Nat. Photonics 7 (2013) 752–758. [29] P. Lin, L. Zhu, D. Li, L. Xu, Z.L. Wang, Nanoscale 10 (2018) 14472–14479. [30] C. Du, L. Jing, C. Jiang, T. Liu, X. Pu, J. Sun, D. Li, W. Hu, Mater. Horiz. (2017). [31] D.Q. Zheng, Z. Zhao, R. Huang, J. Nie, L. Li, Y. Zhang, Nano Energy 32 (2017) 448–453. [32] C.Y. Cho, M.K. Kwon, S.J. Lee, S.H. Han, J.W. Kang, S.E. Kang, D.Y. Lee, S.J. Park, Nanotechnology 21 (2010) 205201. [33] X. Huang, C. Jiang, C. Du, L. Jing, M. Liu, W. Hu, Z.L. Wang, ACS Nano 10 (2016) 11420–11427. [34] C. Jiang, L. Jing, X. Huang, M. Liu, C. Du, T. Liu, X. Pu, W. Hu, Z.L. Wang, ACS Nano (2017). [35] M.E. Vickers, M.J. Kappers, T.M. Smeeton, E.J. Thrush, J.S. Barnard, C.J. Humphreys, J. Appl. Phys. 94 (2003) 1565–1574. [36] K. Catchpole, A. Polman, Opt. Express 16 (2008) 21793–21800. [37] H.A. Atwater, A. Polman, Nat. Mater. 9 (2010) 205–213. [38] J. Pal, M.A. Migliorato, C.K. Li, Y.R. Wu, B.G. Crutchley, I.P. Marko, S.J. Sweeney, J. Appl. Phys. 114 (2013) 073104. [39] J. Pal, G. Tse, V. Haxha, M.A. Migliorato, S. Tomić, Phys. Rev. B 84 (2011). [40] J. Pal, G. Tse, V. Haxha, M.A. Migliorato, S. Tomić, Opt. Quantum Electron. 44 (2011) 195–203. [41] F. Bernardini, V. Fiorentini, D. Vanderbilt, Phys. Rev. B 56 (1997) R10024–R10027.
4.2. Fabrication of Ag nanoparticles array with thermal annealing Before the ITO layer was deposited on p-GaN as a current spreading layer, 5 nm Ag was deposited into the mesa region at a speed of 0.5 Å/s by a Denton Vacuum / Explore 14 electron beam evaporator system at a reacting pressure of around 1.5 × 10−5 Torr. Following the deposition, the photoresist masks were lifted off by dissolved in acetone solution. Then the sample was loaded into the RTP-1200 rapid thermal annealing system, and the atmosphere in the furnace was displaced with nitrogen. The device was annealed at 300 °C for 5 min, the heater current was turned off and the device was left to natural cool in the nitrogen condition to room temperature. Finally, the Ag nanoparticles were nested in the surface of GaN, and the morphology of the nanoparticles was shown in Fig. 1d. With the annealed nanoparticles, the substrate with Ag nanoparticles could serve as surface plasmon to modulate photon scattering and trapping. Acknowledgments Chunyan Jiang, Yan Chen and Jiangman Sun contributed equally to this work. The authors thank for the support from National Natural Science Foundation of China (Grant nos. 51432005, 61574018, and 51603013, 61704008), National Key Research and Development Program of China (2016YFA0202703), “Hundred Talents Program” of the Chinese Academy of Science, the Youth Innovation Promotion Association of Chinese Academy of Science, and the National Grid Science & Technology Project, China (Study of High Voltage SiC Power Device Terminal Technology Based on Dose Modulation Method, No. 5455GB180001).
Dr. Chunyan Jiang received a PhD. degree of Condensed Matter Physics from University of Chinese Academy of Sciences in July 2018.She is interested in wide bandgap semiconductors and is now focused on piezotronic devices based on the third generation wide bandgap semiconductor materials.
Competing financial interests Professor Weiguo Hu received his PhD degree from Institute of Semiconductors, Chinese Academy of Sciences in 2007. His current research interests include semiconductor material epitaxy, optoelectronics /electronic device development and simulation, and new energy devices. So far, he has published more than 60 papers in academic journals such as Science Advances, Advanced Materials, ACS Nano, and Nano Energy.
The authors declare no competing financial interest. References [1] J. Wu, S.C. Mangham, V.R. Reddy, M.O. Manasreh, B.D. Weaver, Sol. Energy Mater. Sol. Cells 102 (2012) 44–49. [2] J.P. Shim, S.R. Jeon, Y.K. Jeong, D.S. Lee, IEEE Electron Device Lett. 31 (2010) 1140–1142. [3] L. Zhou, Y. Tan, J. Wang, W. Xu, Y. Yuan, W. Cai, S. Zhu, J. Zhu, Nat. Photonics 10 (2016) 393–398. [4] K.Q. Peng, S.T. Lee, Adv. Mater. 23 (2011) 198–215. [5] Y. Nanishi, Y. Saito, T. Yamaguchi, Jpn. J. Appl. Phys. 42 (2003) 2549–2559. [6] A. David, M.J. Grundmann, Appl. Phys. Lett. 97 (2010) 033501. [7] R. Dahal, B. Pantha, J. Li, J.Y. Lin, H.X. Jiang, Appl. Phys. Lett. 94 (2009) 063505. [8] X. Li, Z. Lv, H. Zhu, Adv. Mater. 27 (2015) 6549–6574. [9] A.G. Bhuiyan, K. Sugita, A. Hashimoto, A. Yamamoto, IEEE J. Photovolt. 2 (2012) 276–293. [10] J. Bai, M. Athanasiou, T. Wang, Phys. Status Solidi (C.) 13 (2016) 297–300. [11] A. Freundlich, Y.-L. Tsai, C.-C. Lin, H.-V. Han, H.-C. Chen, K.-J. Chen, W.-C. Lai, J.K. Sheu, F.-I. Lai, P. Yu, H.-C. Kuo, J.-F. Guillemoles, 8620 (2013) 86201M. [12] I.M. Pryce, D.D. Koleske, A.J. Fischer, H.A. Atwater, Appl. Phys. Lett. 96 (2010) 153501. [13] J. Vuckovic, M. Loncar, A. Scherer, IEEE J. Quantum Electron. 36 (2000) 1131–1144. [14] A. Dadgar, C. Hums, A. Diez, J. Bläsing, A. Krost, J. Cryst. Growth 297 (2006) 279–282. [15] K.S. Ramaiah, Y.K. Su, S.J. Chang, B. Kerr, H.P. Liu, I.G. Chen, Appl. Phys. Lett. 84 (2004) 3307–3309. [16] D. Thrithamarassery Gangadharan, Z. Xu, Y. Liu, R. Izquierdo, D. Ma, Nanophotonics 6 (2017). [17] U. Palanchoke, H. Kurz, R. Noriega, S. Arabi, V. Jovanov, P. Magnus, H. Aftab, A. Salleo, H. Stiebig, D. Knipp, Nano Energy 6 (2014) 167–172. [18] N. Fu, Z.Y. Bao, Y.-L. Zhang, G. Zhang, S. Ke, P. Lin, J. Dai, H. Huang, D.Y. Lei, Nano
Zhong Lin (ZL) Wang received his Ph.D. from Arizona State University in physics. He now is the Hightower Chair in Materials Science and Engineering, Regents’ Professor, Engineering Distinguished Professor and Director, Center for Nanostructure Characterization, at Georgia Tech. Dr. Wang has made original and innovative contributions to the synthesis, discovery, characterization and understanding of fundamental physical properties of oxide nanobelts and nanowires, as well as applications of nanowires in energy sciences, electronics, optoelectronics and biological science. His discovery and breakthroughs in developing nanogenerators established the principle and technological road map for harvesting mechanical energy from environment and biological systems for powering personal electronics. His research on self-powered nanosystems has inspired the worldwide effort in academia and industry for studying energy for micro-nano-systems, which is now a distinct disciplinary in energy research and future sensor networks. He coined and pioneered the field of piezotronics and piezophototronics by introducing piezoelectric potential gated charge transport process in fabricating new electronic and optoelectronic devices.
306
Contents lists available at ScienceDirect
Nano Energy journal homepage: www.elsevier.com/locate/nanoen
Full paper
Enhanced photocurrent in InGaN/GaN MQWs solar cells by coupling plasmonic with piezo-phototronic effect
T
Chunyan Jianga,b,c, Yan Chend, Jiangman Suna,b, Liang Jinga,b, Mengmeng Liua,b, Ting Liua,b, ⁎ ⁎ ⁎ Yan Panc, Xiong Pua,b,f, Bei Mae, , Weiguo Hua,b,f, , Zhong Lin Wanga,b,f,g, a
CAS Center for Excellence in Nanoscience, Beijing Key Laboratory of Micro-nano Energy and Sensor, Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, China b School of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing 100049, China c State Key Laboratory of Advanced Power Transmission Technology, Global Energy Interconnection Research Institute Co. Ltd., Beijing 102209, China d Beijing Aerospace Automatic Control Institute, National Key Laboratory of Science and Technology on Aerospace Intelligence Control, Beijing 100854, China e Graduate School of Electrical and Electronic Engineering, Chiba University, 1-33 Yayoicho, Inage-ku, Chiba 263-8522, Japan f Center on Nanoenergy Research, School of Physical Science and Technology, Guangxi University, Nanning, 530004, China g School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245, USA
A R T I C LE I N FO
A B S T R A C T
Keywords: InGaN/GaN MQWs Solar cells Plasmonic Piezo-Phototronic effect
InGaN-based photovoltaics (PV) devices have attracted great attentions because of the excellent photoelectric performance over the past decades. The photocurrent of the InGaN/GaN MQWs solar cells could be further improved by coupling plasmonic with piezo-phototronic effect. Here, we fabricated an InGaN/GaN MQWs solar cells with Ag nanoparticles and the short circuit current of the solar cell was improved by 40% with a static external strain applied. The transmission spectrum and the electromagnetic field distributions of the nanoparticles array were simulated with a finite element analysis model. The physical mechanism for light absorption enhancement by the coupling effect in the quantum well structures was illuminated through a self-consistent numerical calculation with non-linear piezoelectricity polarization. This work demonstrated the coupling between the plasmonic and piezo-phototronic effect achieves significant increase in InGaN/GaN MQWs solar cell power conversion efficiency without any complicated process involved.
1. Introduction Photovoltaic (PV) cells have been under intensive researches because that the threat of energy shortage is becoming one of the great challenges around the world.[1–4]In the past decades, scientists and engineers have been engaged in finding the most efficient photovoltaic devices. III-nitride semiconductors (such as GaN, AlN and InN) have been utilized extensively in various optoelectronic devices, including light-emitting diodes and laser diodes. InGaN-based alloys have recently attracted considerable attention for photovoltaic applications, because of the high radiation resistance for space applications [5], high absorption coefficient (~105 cm−1)[6] and a tunable direct band gap that covers nearly full-solar spectrum (0.7–3.4 eV) which results in a very high theoretical conversion efficiency [7]. Actually, to improve the power conversion efficiency of the solar cell is the most important issue in this field [8]. The InGaN/GaN MQWs structures usually used as the active region of the solar cells.[9]Conductive indium-tin-oxide (ITO)
⁎
current spreading layer applied on the top p-type GaN layer [10], colloidal CdS quantum dots (QDs) combined with back side distributed Bragg reflectors (DBRs) [11], and plasmonic nanoparticles [12] have enhanced the performance of the InGaN/GaN Multiple quantum wells (MQWs) solar cells partly. Among various photovoltaic technologies, Surface Plasmon Polaritons (SPPs) that use of the scattering from metal nanoparticles has received tremendous interest in recent years.[1] SPPs are using metal particles localized the electromagnetic field surface waves propagating in the vicinity of the metal-dielectric interface and also induce electron-plasma resonance inside the material. This increases the local electromagnetic field and also provides an opportunity to engineer the optoelectronic device. Recently, surface plasmon has provided serials of opportunities, for example large enhancements of internal quantum efficiencies in InGaN-based light emitting diodes (LEDs) [13], constructing miniaturized optoelectronic circuits [14] and fabricating deep sub-wavelength patterns for next generation lithography.[15]Especially in third-generation solar cells,
Corresponding authors. E-mail addresses: [email protected] (B. Ma), [email protected] (W. Hu), [email protected] (Z.L. Wang).
https://doi.org/10.1016/j.nanoen.2018.12.036 Received 29 September 2018; Received in revised form 5 December 2018; Accepted 10 December 2018 Available online 11 December 2018 2211-2855/ © 2018 Elsevier Ltd. All rights reserved.
Nano Energy 57 (2019) 300–306
C. Jiang et al.
Fig. 1. (a) A schematic of the InGaN/GaN MQW solar cell structure with Ag nanoparticles. The magnified cross-sectional view of the multilayered structure shows the plasmonic nanostructures. (b) Atomic structure model and the corresponding energy band diagram of GaN/InGaN/GaN heterostructure. (c) X-ray diffraction measurement of the MQW structures. The inset is the enlarged 2theta/omega scans between the satellites −1 and 0.(d) The SEM image of Ag nanoparticles on p-GaN. The inset is the statistical size distribution of Ag nanoparticles by Gaussian fittings (solid lines), and the nanoparticles average diameter is 30 nm. (e) The experimental extinction spectra for Ag nanoparticles on p-GaN after annealing.
periods InGaN/GaN (3/13 nm) MQWs absorption layer, with about 25% indium composition, and finally an Mg-doped p-GaN layer. A schematic of the InGaN/GaN MQW structure with Ag nanoparticles is shown in Fig. 1a. Ag nanoparticles were thermally annealed on the surface of p-GaN, followed by 200 nm ITO layer. When specific wavelength of light incident on the surface of Ag nanoparticles, a strong resonant coupling between the SPPs and the MQW is expect to take place.[32] The atomic structure model and energy band profile of the GaN/InGaN/GaN heterostructure are shown in Fig. 1b. As the noncentral symmetric crystal structure in wurtzite InGaN and GaN multilayers, the spontaneous polarization charges created in the interfaces lead to the electron wave function and hole wave function separated in opposite directions in the wells.[33]The photocurrent in InGaN/GaN MQWs solar cells may be reduced by these spontaneous polarization charges.[34] Fig. 1c shows the triple axis mode X-ray diffraction spectrum 2theta/omega scan of the InGaN/GaN MQW structure, which give an approximate average In composition in the InGaN layers.[15] The distinct, periodic and high-order satellite peaks illustrates the high crystal quality of GaN films.[35] The inset in Fig. 1d is the enlarged spectrum between the satellites −1 and 0, corresponding to the number of the quantum well in the active region. The diameter of Ag nanoparticles on the p-GaN has been measured and analyzed, which is shown in Fig. 1d. From the SEM image and the Gaussian fittings, it can be seen that the main size of the particles is 30 nm. The optical behavior of Ag NPs was evaluated by the UV–Visible spectrophotometer. It is observed that the absorption peak of the Ag NPs is 442 nm, which is shown in Fig. 1e. The peak position of absorption spectrum may result from the SPPs resonance in the Ag NPs. The solar cell device structure was based on our previous work [34] and is shown schematically in Fig. 2a. The Ag nanoparticles are distributed in a rectangular region between each two metal finger electrodes. Fig. 2b shows the current density-voltage (J-V) curves with and without Ag NPs under AM1.5illumination yields. A solar simulator
the light absorption of the devices can be increased with the strong local field enhancement [16–18]. With the plasmonic resonance, an efficiency enhancement of 27% for InGaN-based solar cells [19], a 9.1% short circuit current enhancement for InGaN/GaN MQWs solar cells. [20]Moreover, a piezoelectric polarization has been demonstrated in the non-central symmetric crystal structure, it can be utilized to modulate the electrical states in piezoelectric semiconductors under mechanical deformation [21–26]. The piezo-phototronic effect, which is introduced as a three-way coupling among piezoelectric polarizations, semiconductor property and optical excitation, can effectively control/ tune the charge generation, transport, and recombination at an interface of the p-n junction, metal-semiconductor (M-S) Schottky contact and quantum wells [27–31]. Here, we present the design of InGaN/GaN MQWs solar cell coupling with Ag NPs and piezo-phototronic effect, where the enhanced photocurrent and conversion efficiency have been observed. The transmission spectrum of Ag nanoparticle has been systematically studied. Under the external strain of 0.152%, we have attained a 40% enhancement in the short circuit current and a 66% enhancement in the conversion efficiency as compared a solar cell without metallic nanostructures. Furthermore, a self-consistent numerical model combined with the nonlinear piezoelectric effect has been established to investigate the physical mechanism of the photocurrent response. This work is not only promising in developing high conversion efficiency InGaN/GaN MQW solar cells, but also providing light-harvesting scheme to variety optoelectronic devices. 2. Results and discussions The epitaxial layers of InGaN and GaN heterostructure were grown by Metal-organic Chemical Vapor Deposition (MOCVD) on c-plane patterned sapphire substrate. This epi-wafer consists of (from bottom to top) a 3 µm thick undoped GaN buffer, a 2 µm Si-doped n-GaN layer, 9 301
Nano Energy 57 (2019) 300–306
C. Jiang et al.
Fig. 2. Performances of the plasmonic nanoparticles enhanced InGaN/GaN MQWs solar cell. (a) A schematic diagram of the InGaN/GaN MQW solar cell with Ag nanoparticles. (b) J-V characteristics of InGaN/GaN MQWs solar cell with and without Ag nanoparticles. (c) Room temperature J-V characteristics of the solar cell as the light intensity increases from dark to 1.5 AM (d) P-V characteristics of the solar cell as the light intensity increases from dark to 1.5 AM (e-f) The illumination intensity dependence of the open-circuit voltage (VOC), the short-circuit current density (JSC), the conversion efficiency (η) and the fill factor (FF).
explain the photocurrent enhancement by incorporating metal nanoparticles with solar cells.[36]To illustrate the characteristics of the InGaN/GaN MQW solar cell with Ag NPs, the dependency of the current density and the power density on the illumination intensity (P) is demonstrated in Fig. 2c and d. The open-circuit voltage (VOC) is insensitive to variation of the illumination intensity. This is because that the addition of Ag NPs does not change the bandgap of the MQWs structure. The current density and the power density of the solar cell increase uniformly as the illumination intensity increases. Specifically, the VOC, JSC, η and fill factor (FF) under different illumination intensities are extracted and plotted in Fig. 2e, f. The short-circuit current increases almost linearity as the illumination intensities ranging from 0
(Model PT-SUN2S, Pharos Technology) provided the AM1.5 illumination, and had been calibrated with an amorphous silicon reference solar cell. It can be clearly seen that the rough surfaces covered with Ag nanoparticles lead to an enhanced short-circuit current (JSC). This is due to the Ag nanoparticles can not only enhance the light absorption in the quantum well region, but also can couple incident light into guided modes that propagate through the MQWs region, thus improving the current density of solar cells. The short-circuit current increased from 0.93 mA/cm2 to 1.03 mA/cm2, thereby the photoelectric conversion efficiency (η) increased from 0.76% to 0.98%, increased by 28%, which is close to previous reports.[19]The near-field concentration and light scattering are two main mechanisms which have been suggested to 302
Nano Energy 57 (2019) 300–306
C. Jiang et al.
Fig. 3. (a) The schematic illustration of the fringing electric field excited by the localized surface plasmonic resonance. (b) Diagrams illustrating the mechanism for enhanced SPPs resonant excitation. (c) Transmission spectrum for the Ag NPs configuration with ITO superstrate, and the inset is the Ez distribution at direct transmission (390 nm) and SP mode (445 nm).
diagram of our modified Raman system with a jig unit for stress analysis. The fixed plate and the clamp hold the device, the stress controller apply stress from the back at the same time. Structural mechanical model was established which is shown in the red dashed box, and arrays of solar cells were prepared on a sample. Raman shifts of E2 h mode and A1(LO) mode of InGaN/GaN MQWs under various external strains are shown in Fig. 4b. As the external stress increases, the Raman spectrum moves to the shortwave numbers, thus the internal residual compressive stress decreases. The value marked on the curve is the strain calculated according to the movement of the peak position of Raman E2 h. Due to the limitations of the sapphire substrate, the maximum strain that can be added to the devices is 0.152% in our experiment. Fig. 4c-d show J–V characteristics and P–V characteristics of the InGaN/GaN MQW solar cell with Ag nanoparticles under 100 mW/cm2 illuminations at different external applied strains. The gray dashed lines correspond to devices without Ag nanoparticles. With increasing external strains, J–V and P–V curves remarkably moves upper. The corresponding short-circuits current and open-circuit voltage at external strains is summarized in Fig. 4e. The short-circuits current increases from 1.03 to 1.37 mA/cm2 with increasing strain, and the open-circuit voltage is basically stable. Fig. 4f is the conversion efficiency and the fill factor of the InGaN/GaN solar cell as a function of the strains. The efficiency of the solar cell enhanced from 0.98% to 1.25%, while the fill factor is basically stable. Compared to InGaN/GaN solar cell without Ag plasmonic nanoparticles, the enhancement of short-circuit current and conversion efficiency is up to 40% and 66%, respectively. With the single plasmonic resonance, previous works reported 9.1% short circuit current enhancement and 27% efficiency enhancement in InGaN based solar cells [10,19]. This new coupling method is effective, simple and recoverable in developing high conversion efficiency photovoltaic. To further explain the physical mechanism of the plasmonic and piezo-phototronic coupling effects on InGaN/GaN multi quantum well solar cells, a self-consistent numerical calculation of Schrödinger equation and Poisson equation has been established. The schematic diagram of InGaN/GaN MQW structure with/without Ag NPs are shown in Fig. 5a1 and a2, the corresponding carrier mobility mechanism are shown in Fig. 5a2 and b2. For the structure with Ag NPs, the effective absorption cross-section increased and the optical reflection loss reduced.[37] In addition, more lattice relaxation under the compressive residual stress leads to an increase of photo-generated carriers, and the photoelectric current increases. In the calculation, the impact of the non-linear piezoelectricity on quantum wells has been considered. [38–40] Both first and second order piezoelectric coefficients have been considered. The total polarization in the multi-quantum well regions is given by the quadratic expression:
to AM1.5 illumination, corresponding to the increase of photo-generated carriers. The VOC slightly increases from 1.46 V to 1.6 V as the intensity of the light increases from 25 mW/cm2 to 100 mW/cm2, and the efficiency enhances from 0.75% to 0.98%. This also indicates that SPP enhance the absorption and the carrier collection of the solar cell under different light intensities. In addition, the FF is 0.58 at 25 mW/ cm2 and 0.59 at 100 mW/cm2，respectively, which is basically constant. In order to study the physical principles of the enhancement effect of SPP on electromagnetic fields, the schematic illustration of the electric field excited by the surface plasmon resonance is shown in Fig. 3a. The free electrons of metal particles can be driven to oscillate under the electromagnetic wave incidence and the amplitude reaches the maximum when the frequency of the incident electromagnetic wave is consistent with the resonance frequency of the metal nanoparticles. The metal nanoparticles size, shape and the dielectric parameters of the surrounding environment determine the resonant wavelength. Combined with the quasi-static approximation, the polarizability can be expressed as:
P = 4πa3
ε − εm ε +2εm
(1)
Where a is the diameter of the metal nanoparticles, ε and εm are the dielectric constant of the surrounding environment and the nanoparticles, respectively. As is expressed in Eq. (1), the resonant oscillation reaches maximum when ε + 2εm → 0 . In Fig. 1, the Ag nanoparticles are distributed between ITO layer and p-GaN layer, and Fig. 3b is the diffraction schematic diagram of light incident on the Ag NPs structure. A beam of light is directed toward the surface of the Ag nanoparticles, and multistage diffraction occurs simultaneously. Thus, when the momentum of transmitted component of diffracted or reflected light matches with SPP momentum, SPP excitation reaches the maximum. In addition, the field distributions and the transmission spectrum were obtained with a finite element analysis model established by COMSOL Multiphysics 5.0. In the model, the RF module was used, and the diameter of the particles is 30 nm, the refractive index of the ITO and p-GaN is 2 and 2.25, respectively. It was observed that the Ag NPs exhibited a dip at 445 nm as the wavelength of incident light range from 380 nm and 600 nm, which is shown in Fig. 3c. The inset is the Ez distribution at 390 nm and 445 nm, corresponding to the full transmission and SPP excitation respectively. This simulated transmission spectrum keeps a perfect consistent with the experimental data shown in Fig. 1e. The previous works [33,34] have studied the strain distribution of the sample under different stress. Raman microscopy system is one of the most available analyze internal strain methods for InGaN/GaN MQWs structures as the incident light can be accurately focused on the sample through a microscope objective lens. Fig. 4a is the schematic
PTot = Psp + Plm + Ppz + e33 ε⊥+2e31 ε∥ + e311 ε∥2 + e333 ε⊥2 + e133 ε∥ ε⊥ 303
(2)
Nano Energy 57 (2019) 300–306
C. Jiang et al.
Fig. 4. Strain analysis by Raman scattering and enhanced device performances in InGaN/GaN MQWs Solar Cells by coupling plasmonic with piezo-phototronic effect. (a) The optical path of the Raman system integrated with a jig unit.(b) Raman shift of E2 hmode and A1(LO) mode with various external strain.(c-d) J−V and P − V characteristics of the InGaN/GaN MQW solar cell with Ag nanoparticles under 100 mW/cm2 illumination at different external strains. The gray dashed lines correspond to the solar cell without Ag nanoparticles.(e-f) External strain dependence of VOC, JSC, η and FF.
where Psp is the spontaneous polarization, Plm is lattice-mismatchinduced piezoelectric polarizations, Ppz is piezotronic-induced polarization, eij is the linear piezoelectric coefficient, and eijk is the second-order piezoelectric coefficient. The ε∥ and ε⊥ are strain in the growth plane and in the orthogonal direction, respectively. The detailed calculation parameters of wurtzite InN and GaN are listed in Table 1. Fig. 5c is the calculated band profile as GaN/InGaN/GaN heterostructure. As the external strains applied, the band profile of the MQWs becomes a flatter band condition. Such a modified band causes the electron and hole wave functions move toward each other, and provide
Table 1 Value list of physical parameters for wurtzite GaN and InN used in this work. Parameters(C/m2)
GaN
InN
Refs
Psp e31 e33 e311 e333 e133
0.029 −0.49 0.73 6.185 −8.090 1.543
−0.032 −0.57 0.97 5.151 −6.680 1.280
[41] [41] [41] [39] [39] [39]
304
Nano Energy 57 (2019) 300–306
C. Jiang et al.
Fig. 5. The schematic diagram of the InGaN/GaN MQW structure and the mechanism are shown in (a1) and (a2) respectively. (b1) and (b2) are the structure and the mechanism of the InGaN/GaN MQW solar cell after applying Ag NPs and a compressive stress on the device, which shows the modulation effect of the plasma coupling with piezo-phototronic effect. (c) Calculated valence band and conduction band of the GaN/InGaN/GaN heterostructure with (dashed line) and without strain (solid line). (d) Corresponding electron and hole wave function distribution with (dashed) and without strain (solid line).
4. Methods
greater absorption enhancement in the quantum well region.
4.1. Fabrication of InGaN/GaN MQW solar cells 3. Conclusions The samples were first cleaned in HCl:4H2O solution and then thoroughly rinsed with deionized water. Under 450 W power, SiH4, Ar and O2 flow rates were 130.5 sccm, 126 sccm and 13 sccm, and the reaction chamber temperature is 80 °C, a 1500 nm SiO2 layer was deposited on the surface of p-GaN by PECVD. For mesa isolation, the 2 × 2 mm2 square array pattern is transferred to the sample through the combination of photolithography and inductively coupled plasma reactive ion etching (ICP-RIE) system. The flow rates of the reaction gas are Cl2 (30 sccm), BCl3 (15 sccm), Ar (5 sccm) for GaN etching, and CHF3 (20 sccm), CF4 (40 sccm), Ar (10 sccm) for SiO2 etching. After etching, the sample was soaked in hydrogen fluoride (HF) solution for 40 min so that the SiO2 for mask could be removed. N-type GaN was exposed about 1 µm deep by ICP, and then Ti/Al/Ti/Au (30 nm/ 120 nm/45 nm/55 nm) n-contact was evaporated and annealing at 850 °C for 30 s. Silver particles were deposited on the surface of p-type GaN, and detailed synthesizing processes of the Ag nanoparticles array
In summary, we have demonstrated the InGaN/GaN MQWs solar cells with an enhanced photocurrent by coupling plasmonic with piezophototronic effect. A 40% enhancement in the short circuit current and a maximum 66% enhancement in the conversion efficiency have been obtained under 0.152% external strain compared with that without Ag plasmonic nanoparticles. In addition, the calculated results of our selfconsistent model have showed agreement with experimental values. The effective absorption cross-section increased and the optical reflection loss reduced by Ag plasmonic, while a significant reduction of the total piezoelectric field is found through the piezo-phototronic effect. The combination of Ag nanoparticles and the external mechanical strain provide a viable, promising path toward high conversion efficiency solar cells, which gives us a new idea to design novel photovoltaic devices to alleviate the current global energy crisis.
305
Nano Energy 57 (2019) 300–306
C. Jiang et al.
could be found in the Section 4.2. Conductive indium-tin-oxide (ITO) material, which serves as current spreading layer for InGaN/GaN MQW solar cells, has been deposited with RF magnetron sputtering. The final Ni/Au (30/150 nm) n-contact electrodes were formed by magnetron sputtering using lithography and a lift-off technique, and each grid width is 40 µm with a center-to-center spacing of 400 µm, as is shown in Fig. 2a.
Energy 41 (2017) 654–664. [19] J.-Y. Wang, F.-J. Tsai, J.-J. Huang, C.-Y. Chen, N. Li, Y.-W. Kiang, C.C. Yang, Opt. Express 18 (2010) 2682–2694. [20] J.-P. Shim, S.-B. Choi, D.-J. Kong, D.-J. Seo, H.-j. Kim, D.-S. Lee, Opt. Express 24 (2016) A1176–A1187. [21] Z.L. Wang, Nano Today 5 (2010) 540–552. [22] Z.L. Wang, W. Wu, Natl. Sci. Rev. 1 (2013) 62–90. [23] Z.L. Wang, Adv. Mater. 24 (2012) 4632–4646. [24] Y. Zhang, Z.L. Wang, Adv. Mater. 24 (2012) 4712–4718. [25] T. Liu, C. Jiang, X. Huang, C. Du, Z. Zhao, L. Jing, X. Li, S. Han, J. Sun, X. Pu, J. Zhai, W. Hu, Nano Energy 39 (2017) 53–59. [26] C. Jiang, T. Liu, C. Du, X. Huang, M. Liu, Z. Zhao, L. Li, X. Pu, J. Zhai, W. Hu, Z.L. Wang, Nanotechnology (2017). [27] Y. Zhang, Y. Yang, Z.L. Wang, Energy Environ. Sci. 5 (2012) 6850. [28] C. Pan, L. Dong, G. Zhu, S. Niu, R. Yu, Q. Yang, Y. Liu, Z.L. Wang, Nat. Photonics 7 (2013) 752–758. [29] P. Lin, L. Zhu, D. Li, L. Xu, Z.L. Wang, Nanoscale 10 (2018) 14472–14479. [30] C. Du, L. Jing, C. Jiang, T. Liu, X. Pu, J. Sun, D. Li, W. Hu, Mater. Horiz. (2017). [31] D.Q. Zheng, Z. Zhao, R. Huang, J. Nie, L. Li, Y. Zhang, Nano Energy 32 (2017) 448–453. [32] C.Y. Cho, M.K. Kwon, S.J. Lee, S.H. Han, J.W. Kang, S.E. Kang, D.Y. Lee, S.J. Park, Nanotechnology 21 (2010) 205201. [33] X. Huang, C. Jiang, C. Du, L. Jing, M. Liu, W. Hu, Z.L. Wang, ACS Nano 10 (2016) 11420–11427. [34] C. Jiang, L. Jing, X. Huang, M. Liu, C. Du, T. Liu, X. Pu, W. Hu, Z.L. Wang, ACS Nano (2017). [35] M.E. Vickers, M.J. Kappers, T.M. Smeeton, E.J. Thrush, J.S. Barnard, C.J. Humphreys, J. Appl. Phys. 94 (2003) 1565–1574. [36] K. Catchpole, A. Polman, Opt. Express 16 (2008) 21793–21800. [37] H.A. Atwater, A. Polman, Nat. Mater. 9 (2010) 205–213. [38] J. Pal, M.A. Migliorato, C.K. Li, Y.R. Wu, B.G. Crutchley, I.P. Marko, S.J. Sweeney, J. Appl. Phys. 114 (2013) 073104. [39] J. Pal, G. Tse, V. Haxha, M.A. Migliorato, S. Tomić, Phys. Rev. B 84 (2011). [40] J. Pal, G. Tse, V. Haxha, M.A. Migliorato, S. Tomić, Opt. Quantum Electron. 44 (2011) 195–203. [41] F. Bernardini, V. Fiorentini, D. Vanderbilt, Phys. Rev. B 56 (1997) R10024–R10027.
4.2. Fabrication of Ag nanoparticles array with thermal annealing Before the ITO layer was deposited on p-GaN as a current spreading layer, 5 nm Ag was deposited into the mesa region at a speed of 0.5 Å/s by a Denton Vacuum / Explore 14 electron beam evaporator system at a reacting pressure of around 1.5 × 10−5 Torr. Following the deposition, the photoresist masks were lifted off by dissolved in acetone solution. Then the sample was loaded into the RTP-1200 rapid thermal annealing system, and the atmosphere in the furnace was displaced with nitrogen. The device was annealed at 300 °C for 5 min, the heater current was turned off and the device was left to natural cool in the nitrogen condition to room temperature. Finally, the Ag nanoparticles were nested in the surface of GaN, and the morphology of the nanoparticles was shown in Fig. 1d. With the annealed nanoparticles, the substrate with Ag nanoparticles could serve as surface plasmon to modulate photon scattering and trapping. Acknowledgments Chunyan Jiang, Yan Chen and Jiangman Sun contributed equally to this work. The authors thank for the support from National Natural Science Foundation of China (Grant nos. 51432005, 61574018, and 51603013, 61704008), National Key Research and Development Program of China (2016YFA0202703), “Hundred Talents Program” of the Chinese Academy of Science, the Youth Innovation Promotion Association of Chinese Academy of Science, and the National Grid Science & Technology Project, China (Study of High Voltage SiC Power Device Terminal Technology Based on Dose Modulation Method, No. 5455GB180001).
Dr. Chunyan Jiang received a PhD. degree of Condensed Matter Physics from University of Chinese Academy of Sciences in July 2018.She is interested in wide bandgap semiconductors and is now focused on piezotronic devices based on the third generation wide bandgap semiconductor materials.
Competing financial interests Professor Weiguo Hu received his PhD degree from Institute of Semiconductors, Chinese Academy of Sciences in 2007. His current research interests include semiconductor material epitaxy, optoelectronics /electronic device development and simulation, and new energy devices. So far, he has published more than 60 papers in academic journals such as Science Advances, Advanced Materials, ACS Nano, and Nano Energy.
The authors declare no competing financial interest. References [1] J. Wu, S.C. Mangham, V.R. Reddy, M.O. Manasreh, B.D. Weaver, Sol. Energy Mater. Sol. Cells 102 (2012) 44–49. [2] J.P. Shim, S.R. Jeon, Y.K. Jeong, D.S. Lee, IEEE Electron Device Lett. 31 (2010) 1140–1142. [3] L. Zhou, Y. Tan, J. Wang, W. Xu, Y. Yuan, W. Cai, S. Zhu, J. Zhu, Nat. Photonics 10 (2016) 393–398. [4] K.Q. Peng, S.T. Lee, Adv. Mater. 23 (2011) 198–215. [5] Y. Nanishi, Y. Saito, T. Yamaguchi, Jpn. J. Appl. Phys. 42 (2003) 2549–2559. [6] A. David, M.J. Grundmann, Appl. Phys. Lett. 97 (2010) 033501. [7] R. Dahal, B. Pantha, J. Li, J.Y. Lin, H.X. Jiang, Appl. Phys. Lett. 94 (2009) 063505. [8] X. Li, Z. Lv, H. Zhu, Adv. Mater. 27 (2015) 6549–6574. [9] A.G. Bhuiyan, K. Sugita, A. Hashimoto, A. Yamamoto, IEEE J. Photovolt. 2 (2012) 276–293. [10] J. Bai, M. Athanasiou, T. Wang, Phys. Status Solidi (C.) 13 (2016) 297–300. [11] A. Freundlich, Y.-L. Tsai, C.-C. Lin, H.-V. Han, H.-C. Chen, K.-J. Chen, W.-C. Lai, J.K. Sheu, F.-I. Lai, P. Yu, H.-C. Kuo, J.-F. Guillemoles, 8620 (2013) 86201M. [12] I.M. Pryce, D.D. Koleske, A.J. Fischer, H.A. Atwater, Appl. Phys. Lett. 96 (2010) 153501. [13] J. Vuckovic, M. Loncar, A. Scherer, IEEE J. Quantum Electron. 36 (2000) 1131–1144. [14] A. Dadgar, C. Hums, A. Diez, J. Bläsing, A. Krost, J. Cryst. Growth 297 (2006) 279–282. [15] K.S. Ramaiah, Y.K. Su, S.J. Chang, B. Kerr, H.P. Liu, I.G. Chen, Appl. Phys. Lett. 84 (2004) 3307–3309. [16] D. Thrithamarassery Gangadharan, Z. Xu, Y. Liu, R. Izquierdo, D. Ma, Nanophotonics 6 (2017). [17] U. Palanchoke, H. Kurz, R. Noriega, S. Arabi, V. Jovanov, P. Magnus, H. Aftab, A. Salleo, H. Stiebig, D. Knipp, Nano Energy 6 (2014) 167–172. [18] N. Fu, Z.Y. Bao, Y.-L. Zhang, G. Zhang, S. Ke, P. Lin, J. Dai, H. Huang, D.Y. Lei, Nano
Zhong Lin (ZL) Wang received his Ph.D. from Arizona State University in physics. He now is the Hightower Chair in Materials Science and Engineering, Regents’ Professor, Engineering Distinguished Professor and Director, Center for Nanostructure Characterization, at Georgia Tech. Dr. Wang has made original and innovative contributions to the synthesis, discovery, characterization and understanding of fundamental physical properties of oxide nanobelts and nanowires, as well as applications of nanowires in energy sciences, electronics, optoelectronics and biological science. His discovery and breakthroughs in developing nanogenerators established the principle and technological road map for harvesting mechanical energy from environment and biological systems for powering personal electronics. His research on self-powered nanosystems has inspired the worldwide effort in academia and industry for studying energy for micro-nano-systems, which is now a distinct disciplinary in energy research and future sensor networks. He coined and pioneered the field of piezotronics and piezophototronics by introducing piezoelectric potential gated charge transport process in fabricating new electronic and optoelectronic devices.
306