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Scientific Reports volume 16, Article number: 4613 (2026)
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III–V nanostructure-based solar cells (SCs) have emerged as promising contenders for next-generation photovoltaic technologies due to their inherent antireflective properties and the pronounced excitation of optical resonance modes. Nonetheless, the high surface to volume ratio inherent to nanowires (NWs) typically leads to significantly low effective minority carrier lifetimes. Although radial junction architectures were introduced to mitigate this limitation, their experimental efficiencies remain lower than those of axial counterparts, primarily due to challenges in achieving uniform core–shell doping while maintaining low interfacial defect densities. To mitigate these limitations, we propose a core–shell heterojunction NW-SC comprising an hourglass (HG)-shaped GaAs0.99Bi0.01 core, conformally coated with a PEDOT:PSS/CuI shell. The proposed structure was simulated using the finite-difference time-domain (FDTD) and CHARGE modules of Lumerical software to evaluate its optical and electrical performance. Optical simulation results utilizing the FDTD module demonstrate that incorporating CuI as a hole-selective layer enhances optical absorption characteristics in GaAs0.99Bi0.01 NWs, and an optimized shell thickness minimizes the core material usage without compromising ideal current density, thereby lowering material usage and eliminating the need for additional epitaxial shell growth. Our 3D DEVICE simulations demonstrate that the proposed heterojunction structure can achieve a power conversion efficiency (PCE) of (sim)29% when configured with a core p-type doping concentration of 5 (times) 1018 cm−3, a minority carrier lifetime ((uptau)p) of 40 ns, and a surface recombination velocity (SRV) of 105 cm/s at the CuI/GaAs0.99Bi0.01 interface. This validates its high-efficiency operation even under low carrier lifetime conditions. These findings indicate that the proposed design is a promising and potentially fabrication efficient alternative to conventional epitaxially grown core-shell NW-SC architectures.
III–V compound semiconductor-based nanowire (NW) solar cells (SCs) are considered promising candidates for next-generation high-performance photovoltaic technologies due to their direct bandgap, excellent light absorption, high carrier mobility, and geometry-induced optoelectronic advantages. Despite these benefits, large-scale commercialization of III–V NW SCs utilizing p–n homojunctions remains economically unfeasible, primarily due to their high fabrication costs and the complexity of conventional growth techniques when compared to planar SC technologies. Moreover, the inherently large surface-to-volume ratio in NW structures leads to elevated surface recombination rates, which adversely affect carrier lifetimes and device efficiency. Nevertheless, nanowire array structures offer unique opportunities for performance enhancement. In particular, vertically aligned radial p–n junction SCs collect carriers perpendicular to the direction of light absorption. In addition to the excellent light-trapping ability of the nanowire structure, the radial p–n junction provides an enhanced charge-collection mechanism due to the significantly reduced carrier transport distance, thereby improving overall device performance 1,2,3,4,5.
To enhance device performance and reduce active material consumption in nanostructured SCs, extensive research has been devoted to the development of core–shell NW architectures. These configurations enable better light trapping and carrier separation but present notable fabrication challenges. Specifically, the epitaxial growth of core–shell structures often results in undesirable dopant diffusion during shell formation, non-uniform shell growth, and limited control over doping concentrations in both the core and shell regions, often resulting in interfacial defects 6,7. These imperfections, particularly along the core sidewalls, further introduce defect states and impurity levels that degrade the photovoltaic performance of the device 8,9. Such interfacial imperfections, particularly at the p–n junction formed over defective cores, substantially impair the overall device efficiency 8.
To circumvent these issues, passivating charge-selective contacts have emerged as a viable alternative. These structures have gained considerable interest for their ability to mitigate interface-related losses and eliminate the need for complex doping procedures, thereby enhancing interface quality and overall device performance 10. In particular, core–shell heterostructures incorporating hole or electron selective layers have demonstrated notable improvements in device operation by simplifying fabrication while maintaining or improving charge carrier dynamics.
Among the various III-V semiconductors, GaAs1-xBix is one of the promising materials for solar photovoltaic applications due to the large reduction in band gap (83-88 meV/Bi%)11,12 caused by incorporation of a small amount of bismuth (Bi) into gallium arsenide (GaAs). Additionally, GaAsBi shown improved temperature stability, high electron mobility13,14, and a suppression of nonradiative Auger recombination loss15,16. Based on previous studies of GaAs1-xBix, a Bi composition of x = 0.01 is selected in accordance with prior experimental and theoretical reports showing that low Bi incorporation (typically for (xle 1.4%)) produces a significant bandgap reduction in GaAs while maintaining minimal lattice strain and without substantially degrading electron transport properties17,18,19. Recent theoretical investigations of GaAsBi nanowire solar cells also demonstrate that compositions with x = 0.01 provide superior overall photovoltaic performance compared to higher Bi concentrations20.
In heterojunction solar cells (HSCs), transparent conductive oxide (TCO) layers such as indium tin oxide (ITO) and fluorine-doped tin oxide (FTO) are widely adopted as electrodes because of their exceptional electrical conductivity coupled with high optical transparency. However, there are several drawbacks associated with the commonly used ITO material like high manufacturing cost, brittleness, and long-term stability21,22. To address these limitations, various alternative transparent electrodes have been extensively investigated, including highly conductive polymers and graphene. In particular, the organic conductive polymer PEDOT:PSS (poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate)) has emerged as a very promising substitute for ITO. PEDOT:PSS is a heavily doped p-type conductive polymer that disperses in water and can be deposited as a transparent film via simple solution-coating techniques. It has several favourable properties like good electrical conductivity, high optical transparency in the visible range, mechanical flexibility, and a high work function, making it highly suitable for optoelectronic applications. Compared with brittle, indium-based TCOs (e.g., ITO) that suffer from scarcity of raw materials and lack of flexibility, PEDOT:PSS offers a low-cost, mechanically robust, and solution-processable alternative23,24,25,26,27,28.
Over the past few decades, various geometric variations of NWs such as cylindrical 29, nanocones 30, funnel-shaped NWs 31,32, elliptical 33, and hexagonal NWs 34 have been investigated as potential replacements for thin-film SCs. These studies have demonstrated that light-trapping performance can be effectively engineered by tailoring the size, geometry, and orientation of the NWs. However, an optimal NW design is crucial to ensures efficient light absorption and minimal surface reflection without substantially increasing the surface area.
In this study, we propose a heterojunction SC architecture employing the organic polymer PEDOT:PSS as a transparent conductive layer, deposited over a CuI-based hole-selective shell that encapsulates an n-type Hourglass (HG)-shaped GaAs(_{0.99})Bi(_{0.01}) NW core. This configuration aims to enable the fabrication of cost-effective SC structures. The proposed HG-shaped NW structure consists of a top inverted truncated cone and a bottom truncated cone, facilitating enhanced light absorption through the excitation of whispering-gallery-mode resonances, while simultaneously maintaining low surface reflectance and minimizing the overall surface area.
A three-dimensional (3D) finite-difference time-domain (FDTD) module using commercially available Ansys Lumerical’s suite (version 2025 R2; https://www.ansys.com/products/optics) was used to examine the optical response of the core–shell heterojunction device. Figure 1a illustrates the PEDOT:PSS/CuI/HG-GaAs0.99Bi0.01 NW hybrid heterojunction structure, consisting of a periodically arranged HG-shaped GaAs0.99Bi0.01 NW array (period P = 280 nm, height H = 2 µm) grown on a 0.2 µm-thick GaAs substrate, in line with previously fabricated designs 20,35,36. Each NW is conformally coated with a 10 nm CuI hole-selective contact (HSC) and a 40 nm PEDOT:PSS transparent conductive layer.
(a) 3-D schematic representation of the proposed heterojunction SC. (b) 2-D cross-sectional view of a core-shell unit cell comprising the PEDOT:PSS/CuI/HG-GaAs0.99Bi0.01 heterojunction structure, along with detailed geometrical parameters.
Figure 1b shows the 2D cross-sectional view of the simulation design employed in the FDTD module for optical analysis. Since the optical absorption is largely insensitive to substrate thickness, a thinner substrate is selected to minimise computational expense. An incident plane wave, directed vertically from the top, is employed to illuminate the SC within a wavelength range of 300 to 1000 nm. The chosen upper bound corresponds to the bandgap wavelength of GaAs0.99Bi0.01. The wavelength-dependent reflectance (R(lambda )) and transmittance (T(lambda )) are captured by frequency-domain monitors located at the top and bottom of the computational cell, respectively. The absorptance (A(lambda )) is then obtained from2
Assuming a 100% internal quantum efficiency, the ideal short-circuit current density ((J_{text {sc, ideal}})) can be calculated as:37
Here, q denotes the charge of an electron, (I(lambda )) represents the photon flux density of the AM1.5G solar irradiance, and (A(lambda )) corresponds to the optical absorption spectrum. The optical simulation utilizes periodic boundary conditions (PBCs) along the x and y axes, facilitating the modeling of a single unit cell to represent the full structure, which significantly reduces simulation complexity and computational time. Perfectly matched layers (PMLs) are applied at the top and bottom boundaries in the z-direction. These layers suppress internal reflections and enhance the absorption of incoming electromagnetic radiation. A fine mesh size of (Delta)x=(Delta)y=(Delta)z=3 nm is used to achieve high numerical accuracy, ensuring the mesh dimension remains below 1/100 of the shortest simulated wavelength. Optical constants, including the refractive index (n) and extinction coefficient (k), for GaAs0.99Bi0.01, CuI, and PEDOT:PSS are obtained from established experimental studies reported in the literature 20,38,39. These parameters are then added to the material database within the Lumerical software framework. The key optical parameters extracted from FDTD simulations include the electric field distribution (E), ideal short-circuit current density (Jsc, ideal), and 3D optical generation rate (G). Nevertheless, simulation results may be affected by parasitic absorption due to frequency-resolved scattering beyond the absorption threshold, potentially introducing unphysical features in the results 40. Figure 2 illustrates the bandgap energies and band alignments associated with the materials employed in the proposed heterojunction device 41,42,43.
The structural design of the proposed heterojunction NW-SC is illustrated in Fig. 1, comprising a radial junction between the core GaAs0.99Bi0.01 NW, a surrounding CuI-based HSC, and a conformal coating of PEDOT:PSS. The electrical performance of this structure is evaluated using 3D CHARGE simulations, focusing on key photovoltaic parameters such as open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF), and power conversion efficiency (PCE). The 3D optical generation profiles calculated using FDTD are integrated into the finite-element mesh of the NW array within the charge solver. This integration enables a coupled solution of Poisson’s and carrier drift-diffusion equations, allowing for the extraction of the J–V characteristics and essential photovoltaic performance metrics. The electrical simulation is conducted by defining a small section of the periodic system as the charge solver region. A thick dielectric (SiO2) layer is placed on top of the 200 nm GaAs substrate to prevent electrical contribution from the substrate, while a 100 nm Al electrode at the base ensures efficient carrier collection. The simulation accounts for all relevant recombination processes, including radiative, Auger, and Shockley-Read-Hall (SRH) recombination. Furthermore, the Caughey-Thomas model is employed to capture doping-dependent carrier mobility. The physical parameters used for the device simulation are summarized in Table 1.
Band alignment diagram of the constituent layers in the proposed Heterojunction structure, illustrating the relative alignment of the LUMO, conduction band minimum, HOMO, and valence band maximum energy levels.
Optimizing the geometrical dimensions of nanostructure plays a key role in enhancing photovoltaic performance. In our optical analysis, we use Jsc,ideal obtained from FDTD simulation for geometrical optimization. Accordingly, the Jsc,ideal corresponding to the uncoated NW structure is adopted as the reference performance metric and is computed based on Eq.2. Based on previously reported studies on GaAs0.99Bi0.01 NW-SC architectures 41, a NW array with a pitch of 280 nm, height of 2 µm, and core radius of 70 nm was adopted. To optimize the performance of the proposed PEDOT:PSS/CuI/GaAs0.99Bi0.01 heterojunction NW-SC, the shell thickness was optimized for a fixed core radius. The total shell comprises of a fixed 10 nm CuI layer and an outer PEDOT:PSS coating, with the CuI thickness selected in accordance with earlier experimental work 39. The overall diameter of the NW was designed to satisfy 2 (times) (Rcore + Rshell) < P to prevent overlap between adjacent structures.
The vertical NW array can provide a longer light-trapping path through its multiple interactions of light within the space between NWs. All these advantages are retained by the HG-shaped NW array while providing additional advantages and enhanced performance, as described below. The HG-NW is a unique structure consisting of upper and lower asymmetric parts with no increase in surface area, which not only serves as a good antireflective layer but also greatly increases the light-trapping path induced by resonant modes. The HG-NW structure consists of a top tapered section made of a cone or truncated cone, and a bottom tapered section formed by an inverted truncated cone. Hence, to begin with, the cylindrical GaAsBi NW structure was geometrically refined into an HG-shaped configuration to enhance optical performance. Using FDTD simulations, we identified the optimal structural parameters as (R_text {top} = R_text {bott} = 70) nm and (R_text {mid} = 64) nm, which yielded maximum absorption. This optimised HG geometry effectively suppresses light reflection via two distinct mechanisms: the tapered upper region promotes multiple internal reflections of incident light 44, while the tapered lower section ensures impedance matching via an effective refractive index gradient, enhancing antireflection characteristics across broad spectral and angular domains 45.
The geometric fill factor (GFF) for the HG-NW array is evaluated based on the following formula:
Several studies have previously shown that achieving a Jsc above 32 mA/cm2 requires a nanowire array with optimized periodicity or a GFF greater than 0.17, to enhance light scattering, reduce transmission and reflection losses by increasing the effective optical path length 19,20,46,47. A GFF value of 0.18 is obtained for the present HG-NW configuration, which lies within this optimal regime and promotes efficient photon harvesting. As a result, the optimised hybrid heterojunction NW-SC exhibits a high Jsc of 38.3 mA/cm2.
To further optimize the structural parameters of the proposed heterojunction device, the total shell thickness was varied from 0 to 80 nm, and the optimal PEDOT:PSS thickness was identified as 40 nm based on the analysis of Jsc,ideal. Figure 3a illustrates a comparative analysis of absorption spectra for different NW configurations, including a bare vertically aligned GaAs0.99Bi0.01 NW (core radius = 70 nm), a geometrically optimized HG-GaAs0.99Bi0.01 NW, a HG-GaAs0.99Bi0.01 NW with a 40 nm PEDOT:PSS shell, and HG-GaAs0.99Bi0.01 NW with a composite shell structure of 50 nm (10 nm CuI + 40 nm PEDOT:PSS). The results demonstrate that the coated NW structure provides significantly better absorption characteristics than the uncoated structures. The average absorptance values shown in Fig. 3c confirm that the PEDOT:PSS/CuI-coated HG-NW structure exhibits the highest absorption (95.16 %), outperforming the individual PEDOT:PSS-coated, uncoated HG, and bare NWs configurations. This is consistent with the optical Jsc,ideal results illustrated in Fig. 3b, wherein the proposed PEDOT:PSS/CuI/HG-GaAs0.99Bi0.01 heterojunction NW structure attains a peak value of 38.3 mA/cm(^2), thereby validating its superior light-harvesting capability and aligning with the corresponding absorption characteristics.
(a) Optical absorption spectra of bare GaAs0.99Bi0.01 NW, HG-GaAs0.99Bi0.01 NW, PEDOT:PSS/HG – GaAs(_{0.99})Bi(_{0.01}) NW and PEDOT:PSS/CuI/HG-GaAs0.99Bi0.01 with (b) Simulated Jsc,ideal and (c) Average absorptance of the corresponding structures.
Reflectance of HG and bare cylindrical GaAs0.99Bi0.01 NW solar cells.
Figure 4 shows the reflectance of the bare and optimized NW devices over the wavelengths between 300-1000 nm. The optimized hourglass-shaped GaAs0.99Bi0.01 NW array exhibits much lower reflectance than the cylindrical NW array across the entire spectral region. This reduction arises from the combined effect of the tapered and inversely tapered structures in the hourglass geometry. The upper tapered region enhances optical confinement by promoting multiple internal reflections of the incident light within the nanowire, thereby increasing the optical path length and light-trapping efficiency and lower part of the HG-NW array forms a gradual refractive-index transition between air and the NW array, providing superior impedance matching and functioning as an effective broadband antireflective layer. The transmittance of structures is almost 0% in the investigated wavelength range due to a 200 nm GaAs substrate and the Al layer at the back side, resulting in A((uplambda)) = 1 – R((uplambda)), and this significant reduction in reflectance directly corresponds to enhanced absorption in the optimized hourglass NW device.
Photogeneration rate profile (in cm-3s-1) for (a) the bare GaAs0.99Bi0.01 NW and (b) the HG- GaAs0.99Bi0.01 NW core encapsulated by a composite shell structure comprising 10 nm CuI and 40 nm PEDOT:PSS.
To obtain a detailed analysis of the enhanced optical response, we analyzed the optical generation rate profiles in the x-z plane cross-sections at wavelengths of 800 nm, 900 nm, and 1000 nm for the bare GaAs0.99Bi0.01 NW and the optimized HG GaAs0.99Bi0.01 NW under identical conditions, as shown in Fig. 5a–c. The color scales in all plots were normalized to the maximum optical generation rate of the HG structure. The HG shaped NW exhibits the highest optical generation rate at 800 nm, consistent with the enhanced absorption and reduced reflectance observed in the spectral analysis. In addition, at 900 nm and 1000 nm, the HG-NW shows distinctly stronger optical generation compared with the bare NW. This indicate that the HG geometry supports improved optical confinement, as the gradual diameter variation enables repeated internal reflections and strengthens the formation of resonant modes within the nanowire.
In addition to evaluating the absorption spectra and 2D optical generation rate profiles, the electric field (E-field) distributions for both uncoated GaAs0.99Bi0.01 NWs and PEDOT:PSS/CuI/HG-GaAs0.99Bi0.01 NW are analyzed across a wavelength range of 400–900 nm, as depicted in Fig. 6a–l, to investigate the enhancement in light absorption and photogeneration in the coated structure. For the wavelength range of (lambda = 400) to 600 nm (Fig. 6a–c), the E-field distribution in uncoated GaAs0.99Bi0.01 NW is largely confined to the spaces between adjacent NWs, resulting in significant light transmission rather than effective absorption within the GaAs0.99Bi0.01 NWs. In contrast, within the same wavelength range, the E-field in the PEDOT:PSS/CuI/HG-GaAs0.99Bi0.01 NW structure (Fig. 6g–i), is predominantly concentrated near the top region of the NW, with minimal field distribution extending towards the bottom along their length. This enhanced field confinement results in high absorption in the coated HG-NWs compared to their uncoated counterparts.
For (uplambda) = 700 nm, both PEDOT:PSS/CuI/HG-GaAs0.99Bi0.01 NW and the uncoated GaAs0.99Bi0.01 NW exhibit strong electric field confinement at the top regions of the NWs, resulting in enhanced absorption at this wavelength for both structures (Fig. 6d,j). At longer wavelengths ((uplambda) = 800 and 900 nm), the uncoated GaAs0.99Bi0.01 NWs exhibit E-field distribution along their entire axial length, which results in poor light confinement and consequently lower absorption (Fig. 6e,f). In contrast, the coated PEDOT:PSS/CuI/HG-GaAs0.99Bi0.01 NWs (Fig. 6k,l) confine the E-field predominantly at the top surface of the NW structure, leading to superior absorption at these wavelengths relative to the uncoated counterpart. These E-field profiles are in strong agreement with the corresponding absorption spectra, which reveal significant absorption enhancement starting from (uplambda) = 400 nm. However, a noticeable decline in absorption is observed beyond (uplambda) >700 nm, primarily due to increased reflection and transmission losses through the coated hybrid NW configuration at higher wavelengths.
Comparison of normalized electric field (E) distributions for the uncoated GaAs0.99Bi0.01 nanowire (top row) and the optimized PEDOT:PSS/CuI/HG-GaAs0.99Bi0.01 nanowire (bottom row) at wavelengths of (a,g) 400 nm, (b,h) 500 nm, (c,i) 600 nm, (d,j) 700 nm, (e,k) 800 nm, and (f,l) 900 nm.
To carry out electrical simulations for the geometrically optimized PEDOT:PSS/CuI/HG-GaAs0.99Bi0.01 heterojunction NW-SC, the GaAs0.99Bi0.01 core was doped n-type at a concentration of 5 (times) 1018 cm-3, while the GaAs substrate was heavily doped n-type at 5 (times) 1019 cm-3. The CuI shell, serving as the HSC, and the PEDOT:PSS layer, acting as the hole transport layer (HTL), were assigned fixed p-type doping concentrations of 1 (times) 1019 cm-3 and 1 (times) 1020 cm-3, respectively 38,39. A high doping level in the core is essential for achieving high Voc, which enhances overall device efficiency by reducing recombination losses. Conversely, insufficient doping can deplete carriers in the core, decreasing conductivity and increasing series resistance, ultimately lowering the FF. Therefore, maintaining a core doping concentration above 1 (times) 1018 cm-3 is essential for achieving optimal values of Voc and FF, thereby enhancing the overall efficiency of the device.
Figure 7 illustrates the J–V and P–V characteristics for both the uncoated radial p-n GaAs0.99Bi0.01 NW-SC and the coated PEDOT:PSS/CuI/HG-GaAs0.99Bi0.01 NW-SC, considering an interface surface recombination velocity (SRV) of 105 cm/s and a minority carrier lifetime ((uptau)p) of 40 ns. The results, summarized in Table 2, reveal that the hybrid heterojunction structure achieves a notable efficiency of 28.8%, significantly outperforming the 23.67% PCE obtained for the uncoated homojunction counterpart. Table 2 also summarizes the effect of minority carrier lifetime degradation on the electrical performance of the optimized PEDOT:PSS/CuI/HG-GaAs0.99Bi0.01 heterojunction NW-SC and clearly illustrates their superiority over conventional radial p–n homojunction GaAs0.99Bi0.01 NW-SCs. The radial p–n homojunction NW-SC achieves a Voc of 0.9312 V and a PCE of 21.9% at a minority carrier lifetime ((uptau)p) of 10 ns. As the value of (uptau)p decreases to 1 ns and 0.1 ns, the PCE drops to 18.88% and 15.7%, respectively, due to increased non-radiative recombination. In comparison, the heterojunction NW-SC incorporating CuI as the HSC exhibits a Voc of 0.93 V and a higher PCE of 28.3% under the same carrier lifetime ((uptau)p = 10 ns). For a minority carrier lifetime as low as 0.1 ns, the proposed heterojunction NW-SC incorporating CuI as the hole-selective contact yields a Voc of 0.8302 V and a PCE of 22.805%, as reported in Table 2.
(a) (J-V) and (b) (P-V) characteristics of the uncoated radial (p-n) doped GaAs(_{0.99})Bi(_{0.01}) NW and the PEDOT:PSS/CuI/HG-GaAs0.99Bi0.01 NW, simulated for a fixed (uptau)p = 40 ns and a SRV of 104 cm/s at the CuI/GaAs0.99Bi0.01 interface.
This improvement is indicative of suppressed non-radiative SRH recombination, which becomes more pronounced with decreasing carrier lifetime 48. Table 2 shows that Jsc is not significantly altered across individual cases with varying (uptau)p, since the series resistance remains unaffected due to the constant doping condition. The overall degradation of FF and PCE results primarily from Voc degradation due to the decrease in minority carrier lifetime. This reduction in Voc is attributed to less effective charge separation, which dissolves the generated exciton pairs, preventing the photo-generated carriers from reaching the electrodes. It is evident from the results that application of the HSC shell improves the electrical performance over the homojunction radial p–n doped GaAs0.99Bi0.01 NW-SC structure.
Our proposed structure, containing hole-selective contacts, exhibits a high Voc of 0.9897 V in comparison to the GaAs0.99Bi0.01-NW-based SCs due to the use of wide bandgap CuI as the HSC. Table 3 offers a comparative assessment between the performance of the proposed heterojunction NW-SC and various previously reported III-V NW-SC designs,encompassing both experimental and computational studies incorporating carrier-selective contacts (CSC).A comparative analysis with previously reported studies demonstrates that the proposed PEDOT:PSS/CuI/HG-GaAs0.99Bi0.01 structure, employing CuI as a HSC, holds substantial potential for achieving a high PCE of approximately 29%, even under a low minority carrier lifetime of 40 ns in the core.
Despite the theoretical advantages of radial configurations, most experimental demonstrations of high-efficiency III–V NW-SCs have employed axial junction architectures due to interface defects and doping challenges in epitaxially grown core–shell structures. Hence, we discuss the practical significance and applicability of our proposed NW heterojunction structure as an alternative to epitaxially grown core–shell structures. To realize the proposed heterojunction NW-SC, fabrication begins with the formation of a SiO(_2) mask of approximately 200 nm thickness on an n-type GaAs substrate. This is achieved using electron beam lithography followed by wet chemical etching to define structural holes. These nanoholes act as etch openings that define the periodic arrangement of GaAs(_{0.99})Bi(_{0.01}) nanowires. The HG geometry is then realized via a top-down process: a single-step inductively coupled plasma reactive-ion etching (ICP-RIE) of the underlying material through the SiO(_2) mask using chlorine-based chemistries (e.g., Cl(_2)/Ar or Cl(_2)/N(_2)), producing a top tapered section and a bottom inverted tapered section 52. A two-step strategy is adopted for the uniform deposition of the outer CuI shell, involving the atomic layer deposition (ALD) of CuO followed by its conversion into CuI through exposure to hydrogen iodide (HI) vapor at room temperature 53. The final step involves depositing the organic transparent conductive layer PEDOT:PSS onto the CuI-coated GaAs(_{0.99})Bi(_{0.01}) nanowires. Previous experimental studies have shown that PEDOT:PSS can form shell-like coatings on NW arrays when its wettability is enhanced with solvent additives. The addition of ethanol or isopropyl alcohol to PEDOT:PSS solutions has been demonstrated to enable coating of nanowire sidewalls through spin coating or solution filling, resulting in uniform polymer shells on nanowire structures. These reports confirm the feasibility of achieving conformal PEDOT:PSS coverage on nanowires using low-temperature solution processing 54,55,56,57. Finally, an important advantage of the PEDOT:PSS/CuI/HG-GaAs(_{0.99})Bi(_{0.01}) configuration is that it avoids the requirement for high-temperature epitaxial core-shell junction formation. Instead, the GaAsBi nanowire is the only epitaxially grown component, while both CuI and PEDOT:PSS are deposited at low temperatures, thereby mitigating dopant diffusion and minimizing interface degradation during fabrication.
In conclusion, a comprehensive optoelectronic investigation of the PEDOT:PSS/CuI/HG-GaAs0.99Bi0.01-based radial p–n heterojunction NW-SC was carried out using the FDTD and DEVICE modules within the Lumerical simulation framework. The proposed heterojunction structure, incorporating CuI as a hole-selective contact (HSC) and an outer PEDOT:PSS shell, significantly outperforms the optical performance of the uncoated NW structure in the spectral range of 300–1000 nm. The 3D FDTD simulations reveal that the use of an optimized PEDOT:PSS/CuI shell of 50 nm thickness, composed of 10 nm CuI and 40 nm PEDOT:PSS conformally coated on the HG-shaped GaAs0.99Bi0.01 NW, leads to a 14.5% increase in Jsc, culminating in a peak value of 38.3 mA/cm2. The 2D optical generation profile and E-field distribution plots computed over a spectral range of 400–900 nm clearly demonstrate enhanced field confinement and photo-generation rate within the proposed radial heterojunction structure, validating its superior light-trapping and generation capabilities. Subsequently, detailed 3D DEVICE simulations were conducted to examine the impact of core doping and minority carrier lifetime on the PV performance of both uncoated and coated HG-GaAs0.99Bi0.01 NW structures. It was observed that for a core n-type doping concentration of 5 (times) 1018 cm-3, a minority carrier lifetime ((uptau)p) of 40 ns, and a low surface recombination velocity (SRV) of 105 cm/s at the CuI/GaAs0.99Bi0.01 interface, the proposed heterojunction NW-SC achieves a high PCE of approximately 29%. Comparative analysis with previously reported experimental and theoretical studies on III–V heterojunction NW-SC designs confirms the superior optoelectronic performance of the proposed device, emphasizing its advantages in reducing recombination losses and improving carrier collection. Hence, the proposed radial p–n heterojunction NW-SC structure demonstrates significant potential for achieving high efficiency. It also suggests a possible pathway toward lowering fabrication costs and complexities associated with conventional epitaxially grown homo and heterojunction core-shell NW structures. Finally, the proposed device structure is experimentally realizable using the fabrication steps discussed.
The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.
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The author(s) declare that no financial support was received for the research, authorship, or publication of this article.
Department of Electronics Engineering, Indian Institute of Technology (Indian School of Mines, Dhanbad, Jharkhand, 826004, India
Manisha Rautela, Sumit Sagar & Jitendra Kumar
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M.R. and S.S: Conceptualization, Methodology, Analysis, Results, Writing- original draft, J.K.: Validation, Resources, Supervision. All authors reviewed the manuscript.
Correspondence to Sumit Sagar.
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Rautela, M., Sagar, S. & Kumar, J. Hourglass-shaped GaAs0.99Bi0.01 nanowire solar cells with CuI-PEDOT:PSS double hole transport layers for enhanced photovoltaic performance. Sci Rep 16, 4613 (2026). https://doi.org/10.1038/s41598-025-34717-6
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