Enhancing conversion efficiency of inverted organic solar cells using Ag nanoparticles and long wavelength absorbing tin (II) phthalocyanine

Wide wavelength inverted organic solar cells (IOSCs) were fabricated and characterized by doping the active layer with long wavelength absorbing tin (II) phthalocyanine (SnPc). The hole-transporting layer (HTL) comprised Ag nanoparticles (NPs)-embedded poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). The short-circuit current density (J_sc) and power conversion efficiency (PCE) were considerably enhanced. It is attributed to that Ag NPs result in the enhancement of the scattering and reflection of light, leading to increase absorption efficiency in the active layer of IOSCs. Moreover, the SnPc-doped active layer exhibits a long wavelength absorption and prevents the active layer from degradation by PCBM clusters. The optimized IOSCs exhibited an open circuit voltage (V_oc), J_sc, fill factor (F.F.), and PCE of 0.5 V, 10.34 mA/cm², 45.33% and 2.33%, respectively, under simulated AM1.5G illumination at 100 mW/cm².     

Enhanced efficiency and stability of polymer solar cells with TiO2 nanoparticles buffer layer

TiO₂ sols synthesized with a facile solution-based method were used as a buffer layer between the active layer and the cathode Al in conventional structure polymer solar cells (PSCs). Using transmission electron microscopy (TEM), selected area electron diffraction (SAED), X-ray diffraction (XRD) and atomic force microscopy (AFM), the morphological and crystallographic properties of synthesized TiO₂ nanoparticles (TiO₂ NPs) as well as the buffer layer were studied in detail. It was observed that by increasing H₂O in the process of peptization both the crystallinity and particle size of TiO₂ NPs were enhanced, while the particles in sol showed a narrower size distribution conformed by dynamic light scattering. Inserting TiO₂ NPs as a buffer layer in conventional structure PSCs, both the power conversion efficiency (PCE) and stability were improved dramatically. PSCs based on the structure of ITO/PEDOT:PSS/P3HT:PCBM/TiO₂ NPs/Al showed the short-circuit current (J_sc) of 12.83 mA/cm² and the PCE of 4.24%, which were improved by 31% and 37%, respectively comparing with the reference devices without a TiO₂ buffer layer. The stability measurement showed that PSC devices with a TiO₂ NPs buffer layer could retain 80% of the original PCEs after exposed in air for 200 h, much better than the devices without such a buffer layer. The effect can be attributed to the protection by the buffer layer against oxygen and H₂O diffusion into the active layers. The observations indicate that TiO₂ NPs synthesized by facile solution-based method have great potential applications in PSCs, especially for large-area printed PSCs.     

Dispersion control of Ag nanoparticles in bulk-heterojunction for efficient organic photovoltaic devices

This research focuses on the effect of different capping agents on Ag nanoparticles (NPs), for the improved efficiency of organic photovoltaic cells. Ag NPs were produced by solution chemistry of the polyol process, and then successfully capped with oleylamine (OA), polyvinylpyrrolidone (PVP), or thiol terminated polystyrene (PS-SH), as proven by FT-IR spectra. These Ag NPs with different capping agents were finally embedded in the photoactive layer of poly(3-hexylthiophene):6,6-phenyl-C61 butyric acid methyl ester (P3HT:PCBM) bulk heterojunction solar cells. Because of the presence of a suitable capping agent that prevents aggregation, the dispersity of the Ag NPs in organic solvent was significantly improved, in the sequence of OA, PVP, and PS-SH. The photovoltaic cells exhibit increased performance from 3.11% to 3.49%, at an optimized blend ratio of Ag NPs (2.5 wt%) capped with PS-SH. This enhancement is mainly attributed to the improved short circuit current (increased from 8.49 mA/cm² to 9.29 mA/cm²) and extinction with effective light scattering, caused by improved dispersion of the Ag NPs in BHJ films, through reducing unwanted particle aggregation.     

Effects of the position of silver nanoprisms on the performance of organic solar cells

Silver nanoprisms （AgNPs） affect the performance of organic solar cells （OSCs） in different ways depending on their positions in the device. To investigate this issue, we incorporate AgNPs in different positions of OSCs and compare their performance. The power conversion efficiency （PCE） is improved by 23.60% to 3.98% when the AgNPs are in- corporated in front of the active layer. On the other hand, when AgNPs are incorporated in the back of the active layer, the short-circuit current density （Jsc） is improved by 17.44% to 10.84 mA/cm2. However, if AgNPs are incorporated in the active layer, both open-circuit voltage （Voc） and Jsc are decreased. We discuss the position effect on the device performance, clarify the absorption shadow and exciton recombination caused by AgNPs, and finally indicate that the optimal position ofplasmonic AgNPs is in front of the active layer.     

Room‐Temperature Metallic Fusion‐Induced Layer‐by‐Layer Assembly for Highly Flexible Electrode Applications

To fabricate flexible electrodes, conventional silver (Ag) nanomaterials have been deposited onto flexible substrates, but the formed electrodes display limited electrical conductivity due to residual bulky organic ligands, and thus postsintering processes are required to improve the electrical conductivity. Herein, an entirely different approach is introduced to produce highly flexible electrodes with bulk metal–like electrical conductivity: the room‐temperature metallic fusion of multilayered silver nanoparticles (NPs). Synthesized tetraoctylammonium thiosulfate (TOAS)‐stabilized Ag NPs are deposited onto flexible substrates by layer‐by‐layer assembly involving a perfect ligand‐exchange reaction between bulky TOAS ligands and small tris(2‐aminoethyl)amine linkers. The introduced small linkers substantially reduce the separation distance between neighboring Ag NPs. This shortened interparticle distance, combined with the low cohesive energy of Ag NPs, strongly induces metallic fusion between the close‐packed Ag NPs at room temperature without additional treatments, resulting in a high electrical conductivity of ≈1.60 × 10⁵ S cm^?1 (bulk Ag: ≈6.30 × 10⁵ S cm^?1). Furthermore, depositing the TOAS–Ag NPs onto cellulose papers through this approach can convert the insulating substrates into highly flexible and conductive papers that can be used as 3D current collectors for energy‐storage devices.     

Solution-Processed Polyfluorene:Naphthalenediimide–N-Doped TiO2 Hybrids for Ultraviolet Photodetector Applications

In this study, highly efficient ultraviolet (UV) photodetectors based on a solution-processed system are introduced. Nitrogen-doped TiO₂ nanoparticles are embedded in poly(9,9-dioctylfluorenyl-2,7-yleneethynylene) (PFE):N,N′-bis-n-butyl-1,4,5,8-naphthalenediimide (BNDI) (3:1 wt.%) blends. An UV- active layer of [(PFE:BNDI)(3:1):ammonium hydroxide-TiO₂] [9:1 wt.%] gave a photoresponsivity value of 545 ± 6.92 mA/W at ?4 V under 1 mW/cm² UV light at 365 nm, and this value was increased to 597 ± 9.22 mA/W on annealing the active layer at 60°C. The efficiencies obtained are strongly dependent on the nitrogen source nature, their donor–acceptor relationship, and the morphological interaction with the PFE:BNDI blend.     

Enhanced efficiency of organic photovoltaic cells with Sr2SiO4:Eu2+ and SrGa2S4:Eu2+ phosphors

We report the effect of yellow Sr₂SiO₄:Eu²⁺ and green SrGa₂S₄:Eu²⁺ phosphors on the efficiency of organic photovoltaic (OPV) cells. Each phosphor was coated on the back side of indium tin oxide (ITO)/glass substrates by spin coating with poly(methyl methacrylate) (PMMA). The maximum absorption wavelength of the active layer in the OPV cells was ～512 nm. The emission peaks of Sr₂SiO₄:Eu²⁺ and SrGa₂S₄:Eu²⁺ were maximized at 552 nm and 534 nm, respectively. The short circuit current density (J_sc) and power conversion efficiency (PCE) of the OPV cells with Sr₂SiO₄:Eu²⁺ (8.55 mA/cm² and 3.25%) and with SrGa₂S₄:Eu²⁺ (9.29 mA/cm² and 3.3%) were higher than those of the control device without phosphor (7.605 mA/cm² and 3.04%). We concluded that phosphor tuned the wavelength of the incident light to the absorption wavelength of the active layer, thus increasing the J_sc and PCE of the OPV cells.     

Improving the Power Conversion Efficiency of Organic Solar Cell by Blending with CdSe/ZnS Core–Shell Quantum Dots

We have blended poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) with CdSe/ZnS core–shell quantum dots (QDs) as the active layer to produce organic solar cells (OSC). The size of the CdSe/ZnS core–shell QDs was determined to be about 4 nm using transmission electron microscopy. The OSC were characterized by measuring the absorption spectra, current–voltage characteristics, and external quantum efficiency (EQE) spectra. The samples doped with 0.5 wt.% CdSe/ZnS core–shell QDs exhibited higher power conversion efficiency (PCE) than samples doped with other concentrations of QDs. The PCE of the OSC increases from 2.10% to 2.38% due to an increase of the short circuit current density (J _sc) from 6.00 mA/cm² to 6.25 mA/cm². The open circuit voltage (V _oc) was kept constant when comparing OSC that were undoped and doped with 0.5 wt.% CdSe/ZnS core–shell QDs. These CdSe/ZnS core–shell QDs can increase optical absorption as well as provide extra exciton dissociation and additional electric pathways in hybrid OSC.     

A Cross‐Linkable Donor Polymer as the Underlying Layer to Tune the Active Layer Morphology of Polymer Solar Cells

For polymer solar cells (PSCs) with conventional configuration, the vertical composition profile of donor:acceptor in active layer is detrimental for charge carrier transporting/collection and leads to decreased device performance. A cross‐linkable donor polymer as the underlying morphology‐inducing layer (MIL) to tune the vertical composition distribution of donor:acceptor in the active layer for improved PSC device performance is reported. With poly(thieno[3,4‐b]‐thiophene/benzodithiophene):[6,6]‐phenyl C₇₁‐butyric acid methyl ester (PTB7:PC₇₁BM) as the active layer, the MIL material, PTB7‐TV , is developed by attaching cross‐linkable vinyl groups to the side chain of PTB7. PSC device with PTB7‐TV layer exhibits a power conversion efficiency (PCE) of 8.55% and short‐circuit current density (J_SC) of 15.75 mA cm^?2, in comparison to PCE of 7.41% and J_SC of 13.73 mA cm^?2 of the controlled device. The enhanced device performance is ascribed to the much improved vertical composition profile and reduced phase separation domain size in the active layer. These results demonstrate that cross‐linked MIL is an effective strategy to improve photovoltaic performance of conventional PSC devices.     

Interfacial layer material derived from dialkylviologen and sol–gel chemistry for polymer solar cells

Sol–gel processible organosilicate material based on dialkylviologen (1,1-(bis-trimethoxysilane)-[4,4′]bipyridium dibromide (bis-trimethoxypropylsilane)-yl-viologen, PV-Si) was synthesized and used as an interfacial layer material for polymer solar cells based on poly(3-hexylthiophene): [6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT:PCBM). PV-Si is very good soluble in polar protic solvents because of two pyrinium bromide salts and PV-Si pre-polymer can be easily prepared by sol–gel chemistry under the mild acidic conditions. From the ultraviolet spectroscopy (UPS) study, the reduction of the work function of Al and ITO is observed by the formation of interface dipole, which is induced by the thin film of thermally cured PV-Si pre-polymer (cPV-Si) at 180 °C. The open circuit voltage (V_oc) of conventional type polymer solar cell (CPSC) with a structure of ITO/active layer (P3HT:PCBM)/cPV-Si(<5 nm)/Al is 0.58 V, which is higher than the CPSC without cPV-Si (0.55 V). This indicates that the favorable interface dipole is generated by the thin film of cPV-Si. Besides, the power conversion efficiency (PCE) of CPSC with cPV-Si reaches at 2.90%, which is higher than that of the device without cPV-Si (2.69%). Surprisingly, the PCE and the short circuit current (J_sc) of inverted type polymer solar cell (IPSC) with a structure of ITO/cPV-Si (<5 nm)/active layer/WO₃/Ag are 2.83% and ?9.19 mA/cm², respectively, which are higher than those of the device with ZnO (2.51% and ?8.63 mA/cm²) as an electron transporting/injecting layer. This is due to that the work function of ITO is also reduced by the formation of interface dipole. The IPSC with cPV-Si as an interfacial layer (IFL) shows very good rectification and a contact property as well. From the results, the thin layer of cPV-Si is potential material for an IFL for either CPSC or IPSC. Especially, ZnO can be replaced by cPV-Si because of their improved device performances and pretty low processing temperature.     

Bright green organic light-emitting devices having a composite electron transport layer

A bright green organic light-emitting device employing a co-deposited Al-Alq₃ layer has been fabricated. The device structure is glass/indium tin oxide (ITO)/ N, N′-diphenyl-N, N′- (3-methylphenyl)-1, 1′-biphenyl-4, 4′-diamine (TPD)/tris(8-quinolinolato) aluminum (Alq₃)/ Al-Alq₃/Al. In this device, Al-Alq₃ is used as electron transport layer (ETL). The device shows an operation voltage of 6.1 V at 20 mA/cm². At optimal condition, the brightness of a device at 20 mA/cm² is 2195 cd/m² achieved a luminance efficiency of 5.64lm/W. The result proves that the composite Al-Alq₃ layer is suitable for the ETL of organic light-emitting devices (OLEDs).     

Insights into the influence of fluorination positions on polymer donor materials on photovoltaic performance

To explore the influence of fluoro substitution position and number on optical, electrochemical and photovoltaic properties, three novel donor-acceptor (D-A) alternative copolymers (PHF, PFH and PFF) were synthesized by Stille polycondensation of 2,3-diphenyl-5,8-di(thiophen-2-yl)quinoxaline (DTQx) acceptor unit and indacenodithiophene (IDT) donor unit. As films, PHF and PFF comprising two fluoro substituents on the lateral phenyl groups displayed a broad absorption ranging from 350 to 700 nm; whereas PFH containing two fluorine atoms on the polymer main chain exhibited a slightly narrower absorption ranging from 350 to 650 nm. In addition, fluoro substitution on the polymer main chain can lower the HOMO level of the resulted polymers. As expected, PFH and PFF possess deeper HOMO energy level than PHF. Polymer solar cells (PSCs) were fabricated with these three polymers as donor materials and PC₇₁BM as acceptor material. PHF based PSCs gave a power conversion efficiency (PCE) of 7.2% with a V_oc of 0.84 V, a J_sc of 12.46 mA/cm² and an FF of 0.69. And PFH based PSCs showed a PCE of 6.19% with a V_oc of 0.93 V, a J_sc of 9.57 mA/cm² and an FF 0.70. However, a PCE of only 2.9% with a V_oc of 0.92 V, a J_sc of 4.61 mA/cm² and an FF of 0.68 was obtained for PFF based PSCs. Transmission electron microscopy (TEM) and resonant soft X-ray scattering (R-SoXS) studies indicated that the introduction of four fluorine atoms at each repeating unit can spoil the morphology of active layer. These results highlight the importance of fluorination position and number to the performance of PSCs.     

高效硅基异质结太阳能电池的模拟

a-Si:H/c-Si异质结太阳能电池的基本参数,如层厚度、掺杂浓度、a-Si:H/c-Si界面缺陷、功函数等是影响载流子传输特性和电池效率的关键因素。在本文中,利用AFORS-HET程序,研究了这些参数与a-Si:H/c-Si电池的性能的关联性。最后,具有TCO/n-a-Si:H/i-a-Si:H/p-c-Si/p -a-Si:H/Ag结构的太阳能电池的最优化性能被获得,其光电转换效率为27.07%（VOC: 749 mV, JSC: 42.86 mA/cm2, FF: 84.33%）。深入地了解异质结电池的输运特性,对进一步提高电池的效率有很大的帮助,同时对实际太阳能电池的制造也能提供有益的指导。     

Photovoltaic response of Cu2O/In2S3 hetero-structure grown on Cu substrate

A thin layer of p-type Cu₂O was grown over flexible 30 μm thick copper substrates. Using Injection Chemical Vapor Deposition technique, n-type In₂S₃ thin films were grown over the Cu₂O layer. A p–n junction was thus realized. The Cu₂O/In₂S₃ hetero-structure showed photovoltaic behavior. A solar cell with the structure Cu/Cu₂O/In₂S₃/Ag could be fabricated. An acidic texturization sequence was developed which increased the photo-sensitivity of the In₂S₃ window layer. The Cu/Cu₂O/In₂S₃/Ag hetero-structure with the textured window layer had an open circuit voltage of 377 mV, short circuit current density of 0.118 mA/cm² and fill factor of 33.34%. It was found that the efficiency of the solar cell depended upon the photo-sensitivity of the In₂S₃ window layer. The work demonstrates the use of copper substrate for thin film solar cell fabrication.     

Overcoming the poor short wavelength spectral response of CdS/CdTe photovoltaic modules via luminescence down‐shifting: ray‐tracing simulations

The short‐wavelength response of cadmium sulfide/cadmium telluride (CdS/CdTe) photovoltaic (PV) modules can be improved by the application of a luminescent down‐shifting (LDS) layer to the PV module. The LDS layer contains a mixture of fluorescent organic dyes that are able to absorb short‐wavelength light of λ < 540 nm, for which the PV module exhibited low external quantum efficiency (EQE), and re‐emit it at a longer wavelength (λ > 540 nm), where the solar cell EQE is high. Ray‐tracing simulations indicate that a mixed LDS layer containing three dyes could lead to an increase in the short‐circuit current density from J_sc = 19.8 mA/cm² to J_sc = 22.9 mA/cm² for a CdS/CdTe PV module. This corresponds to an increase in conversion efficiency from 9.6% to 11.2%. This indicates that a relative increase in the performance of a production CdS/CdTe PV module of nearly 17% can be expected via the application of LDS layers, possibly without any making any alterations to the solar cell itself. Copyright © 2006 John Wiley & Sons, Ltd.     

Lending Triarylphosphine Oxide to Phenanthroline: a Facile Approach to High‐Performance Organic Small‐Molecule Cathode Interfacial Material for Organic Photovoltaics utilizing Air‐Stable Cathodes

Cathode interfacial material (CIM) is critical to improving the power conversion efficiency (PCE) and long‐term stability of an organic photovoltaic cell that utilizes a high work function cathode. In this contribution, a novel CIM is reported through an effective and yet simple combination of triarylphosphine oxide with a 1,10‐phenanthrolinyl unit. The resulting CIM possesses easy synthesis and purification, a high T _g of 116 °C and attractive electron‐transport properties. The characterization of photovoltaic devices involving Ag or Al cathodes shows that this thermally deposited interlayer can considerably improve the PCE, due largely to a simultaneous increase in V _oc and FF relative to the reference devices without a CIM. Notably, a PCE of 7.51% is obtained for the CIM/Ag device utilizing the active layer PTB7:PC₇₁BM, which far exceeds that of the reference Ag device and compares well to that of the Ca/Al device. The PCE is further increased to 8.56% for the CIM/Al device (with J _sc = 16.81 mA cm^?2, V _oc = 0.75 V, FF = 0.68). Ultraviolet photoemission spectroscopy studies reveal that this promising CIM can significantly lower the work function of the Ag metal as well as ITO and HOPG, and facilitate electron extraction in OPV devices.     

Molecular Engineering of Blue Fluorescent Molecules Based on Silicon End‐Capped Diphenylaminofluorene Derivatives for Efficient Organic Light‐Emitting Materials

Blue fluorescent materials based on silicone end‐capped 2‐diphenylaminofluorene derivatives are synthesized and characterized. These materials are doped into a 2‐methyl‐9,10‐di‐[2‐naphthyl]anthracene host as blue dopant materials in the emitting layer of organic light‐emitting diode devices bearing a structure of ITO/DNTPD (60 nm)/NPB (30 nm)/emitting layer (30 nm)/Alq₃ (20 nm)/LiF (1.0 nm)/Al (200 nm). All devices exhibit highly efficient blue electroluminescence with high external quantum efficiencies (3.47%–7.34% at 20 mA cm^?2). The best luminous efficiency of 11.2 cd A^?1 and highest quantum efficiency of 7.34% at 20 mA cm^?2 are obtained in a device with CIE coordinates (0.15, 0.25). A deep‐blue OLED with CIE coordinates (0.15, 0.14) exhibits a luminous efficiency of 3.70 cd A^?1 and quantum efficiency of 3.47% at 20 mA cm^?2.     

Dithienopyrrole-benzodithiophene based donor materials for small molecular BHJSCs: Impact of side chain and annealing treatment on their photovoltaic properties

Two small molecular organic materials denoted as ICT1 and ICT2 with A-D₁-D₂-D₁-A architecture have been synthesized and their thermal, photo-physical, electrochemical and photovoltaic properties are explored. Synthesized materials have n-butylrhodanine acceptor (A), dithienopyrrole (DTP) (D₁) and benzodithiophene (BDT) (D₂) (Alkoxy BDT and alkylthiophene BDT, respectively for ICT1 and ICT2) donor moieties. Both the materials have good solubility (up to 25 mg/mL) in most common organic solvents and have excellent thermal stability with the decomposition temperature (T_d) as 348 and 382 °C, respectively for ICT1 and ICT2. Both ICT1 and ICT2 have broad and intense visible region absorption (molar excitation coefficient is 1.71 × 10⁵ and 1.65 × 10⁵ mol⁻¹ cm⁻¹, respectively for ICT1 and ICT2) and have suitable HOMO and LUMO energy levels for PC₇₁BM acceptor. Bulk heterojunction solar cells with ITO/PEDOT:PSS/blend/Al structure are fabricated using these materials. The BHJSCs fabricated by spin cast of ICT1:PC₇₁BM and ICT2:PC₇₁BM (1:2 wt ratio) blend from chloroform showed power conversion efficiency (PCE) of 2.77% (J_sc = 6.84 mA/cm², V_oc = 0.92 V and FF = 0.44) and 3.27% (J_sc = 7.26 mA/cm², V_oc = 0.96 V and FF = 0.47), respectively. Annealing the active layer significantly improved the PCE of these BHJSCs to 5.12% (J_sc = 10.15 mA/cm², V_oc = 0.87 V and FF = 0.58) and 5.90% (J_sc = 10.68 mA/cm², V_oc = 0.92 V and FF = 0.60), respectively for ICT1 and ICT2 donors. The enhancement in the PCE is due to higher light harvesting ability of the active layer, better nanoscale morphology for efficient and balanced charge transport and effective exciton dissociation at the donor-acceptor interface.     

Highly Reversible Zn Anode Enabled by Controllable Formation of Nucleation Sites for Zn‐Based Batteries

Aqueous Zn batteries have drawn tremendous attention for their several advantages. However, the challenges of Zn anodes such as the corrosion and ZnO densification have compromised their application in rechargeable Zn‐based batteries. In this paper, a straightforward strategy is employed to facilitate the uniform Zn stripping/plating of the Zn anode through using a ZrO₂ coating layer, which contributes to the controllable nucleation sites for Zn²⁺ and fast Zn²⁺ transportation through the favorable Maxwell–Wagner polarization. As a result, the low polarization (24 mV at 0.25 mA cm^?2), high Coulombic efficiency (99.36% at 20 mA cm^?2), and long cycle life (over 3800 h at 0.25 mA cm^?2) can be obtained for the ZrO₂‐coated Zn anode. It is believed that the ZrO₂ coating layer can also act as an inert physical barrier to decrease the contact of the anode and electrolyte, thus reducing both the Zn corrosion and formation of ZnO densification, and then improve the reversibility of Zn anode. The results demonstrated in this work provide an appealing strategy for the future development of rechargeable Zn‐based batteries.     

Enhanced carrier mobility and photon-harvesting property by introducing Au nano-particles in bulk heterojunction photovoltaic cells

In this study, polymer solar cells (PSCs) doped with Au nanoparticles (Au NPs) were successfully fabricated to maximize the photon-harvesting properties on the photoactive layer. In addition, a conductivity-enhanced hybrid buffer layer was introduced to improve the photon absorption properties and effectively separate the generated charges by adding Au NPs and dimethylsulfoxide (DMSO) to the PH 500 as a buffer layer. The PSC performance was optimized with a 88% improvement over the conventional PSCs (photoactive area: 225 mm², power conversion efficiency (PCE): 3.2%) by the introduction to the buffer layer of Au NPs and DMSO at 10 wt% and 1.0 wt%, respectively, and with 15 wt% Au NP doping in the photoactive layer. The internal resistance was decreased due to the increased photocurrent caused by the localized surface plasmon resonance (LSPR) effect of the Au NPs in the photoactive layer and by the improvement of carrier mobility induced by the DMSO doping of the buffer layer. As a result, the series resistance (R_S) deceased from 42.3 to 19.7 Ω cm² while the shunt resistance (R_SH) increased from 339 to 487 Ω cm².