Chemical Bath Deposition of α-GaOOH with Tunable Morphology on Silicon Using the pH Adjustment.

The growth of GaOOH by chemical bath deposition has received great attention over the past years as a first step to form Ga2O3 with the α- or ß-phases by combining a wet chemical route with thermal annealing in air. By using gallium nitrate and sodium hydroxide in aqueous solution, we show that the structural morphology of GaOOH deposits is thoroughly tunable in terms of both dimensions, density, and nature by varying the initial pH value from acidic to basic conditions. In the low-pH region associated with a low supersaturation level and where Ga3+ ions represent the dominant Ga(III) species, GaOOH microrods with a low aspect ratio and low density prevail. In the intermediate-pH region associated with a high supersaturation level and where GaOH2+ ions represent the dominant Ga(III) species, GaOOH prismatic nanorods with a high aspect ratio and high density are preferentially formed. In the high-pH region where Ga(OH)4- complexes are predominantly formed, the growth of partially crystallized GaOOH thin films with a typical thickness of about 1 µm proceeds. These findings show the correlation between the characteristics of the chemical bath and the resulting structural morphology of GaOOH deposits. They further open great perspectives to grow GaOOH and hence Ga2O3-based materials on silicon with a dedicated structural morphology using chemical bath deposition for engineering devices in the fields of gas sensing, solar-blind UV-C photodetection, and power electronics.

Interplay Effects in the Co-Doping of ZnO Nanowires with Al and Ga Using Chemical Bath Deposition.

The simultaneous co-doping of ZnO nanowires grown by chemical bath deposition is of high interest for a large number of engineering devices, but the process conditions required and the resulting physicochemical processes are still largely unknown. Herein, we show that the simultaneous co-doping of ZnO nanowires with Al and Ga following the addition of Al(NO3)3 and Ga(NO3)3 in the chemical bath operates in a narrow range of conditions in the high-pH region, where the adsorption processes of respective Al(OH)4- and Ga(OH4)- complexes on the positively charged m-plane sidewalls are driven by attractive electrostatic forces. The structural morphology and properties of ZnO nanowires are significantly affected by the co-doping and mainly governed by the effect of Al(III) species. The incorporation processes of Al and Ga dopants are characterized by significant interplay effects, and the amount of incorporated Ga dopants into ZnO nanowires is found to be larger than the amount of incorporated Al dopants owing to energetic considerations. The Al and Ga dopants are located in the bulk of ZnO nanowires, but a part of Al and Ga lies on their surfaces, their incorporation processes in the bulk being enhanced by thermal annealing under oxygen atmosphere. Eventually, the Al and Ga dopants directly affect the incorporation of hydrogen-related defects, notably by annihilating the formation of VZn-nH defect complexes. These findings present an efficient strategy to proceed with the co-doping of ZnO nanowires grown by chemical bath deposition, opening perspectives to control their electronic structure properties with a higher precision.

Chemical Bath Deposition of ZnO Nanowires Using Copper Nitrate as an Additive for Compensating Doping.

The controlled incorporation of dopants like copper into ZnO nanowires (NWs) grown by chemical bath deposition (CBD) is still challenging despite its critical importance for the development of piezoelectric devices. In this context, the effects of the addition of copper nitrate during the CBD of ZnO NWs grown on Au seed layers are investigated in detail, where zinc nitrate and hexamethylenetetramine are used as standard chemical precursors and ammonia as an additive to tune the pH. By combining thermodynamic simulations with chemical and structural analyses, we show that copper oxide nanocrystals simultaneously form with ZnO NWs during the CBD process in the low-pH region associated with large supersaturation of Cu species. The Cu(II) and Zn(II) speciation diagrams reveal that both species show very similar behaviors, as they predominantly form either X2+ ions (with X = Cu or Zn) or X(NH3)42+ ion complexes, depending on the pH value. Owing to their similar ionic structures, Cu2+ and Cu(NH3)42+ ions preferentially formed in the low- and high-pH regions, respectively, are able to compete with the corresponding Zn2+ and Zn(NH3)42+ ions to adsorb on the c-plane top facets of ZnO NWs despite repulsive electrostatic interactions, yielding the significant incorporation of Cu. At the highest pH value, additional attractive electrostatic interactions between the Cu(NH3)42+ ion complexes and negatively charged c-plane top facets further enhance the incorporation of Cu into ZnO NWs. The present findings provide a deep insight into the physicochemical processes at work during the CBD of ZnO NWs following the addition of copper nitrate, as well as a detailed analysis of the incorporation mechanisms of Cu into ZnO NWs, which are considered beyond the only electrostatic forces usually driving the incorporation of dopants such as Al and Ga.

Chemical Synthesis of ß-Ga2O3 Microrods on Silicon and Its Dependence on the Gallium Nitrate Concentration.

ß-Ga2O3 microrods have attracted increasing attention for their integration into solar blind/UV photodetectors and gas sensors. However, their synthesis using a low-temperature chemical route in aqueous solution is still under development, and the physicochemical processes at work have not yet been elucidated. Here, we develop a double-step process involving the growth of α-GaOOH microrods on silicon using chemical bath deposition and their further structural conversion to ß-Ga2O3 microrods by postdeposition thermal treatment. It is revealed that the concentration of gallium nitrate has a drastic effect on tuning the morphology, dimensions (i.e., diameter and length), and density of α-GaOOH microrods over a broad range, in turn governing the morphological properties of ß-Ga2O3 microrods. The physicochemical processes in aqueous solution are investigated by thermodynamic computations yielding speciation diagrams of Ga(III) species and theoretical solubility plots of GaOOH(s). In particular, the qualitative evolution of the morphological properties of α-GaOOH microrods with the concentration of gallium nitrate is found to be correlated with the supersaturation in the bath and discussed in light of the standard nucleation and growth theory. Interestingly, the structural conversion following the thermal treatment at 900 °C in air results in the formation of pure ß-Ga2O3 microrods without any residual minor phases and with tunable morphology and improved structural ordering. These findings reporting a double-step process for forming high-quality pure ß-Ga2O3 microrods on silicon open many perspectives for their integration onto a large number of substrates for solar blind/UV photodetection and gas sensing.

The Path of Gallium from Chemical Bath into ZnO Nanowires: Mechanisms of Formation and Incorporation.

ZnO nanowires grown by chemical bath deposition (CBD) are of high interest, but their doping with extrinsic elements including gallium in aqueous solution is still challenging despite its primary importance for transparent electrodes and electronics, and for mid-infrared plasmonics. We elucidate the formation mechanisms of ZnO nanowires by CBD using zinc nitrate and hexamethylenetetramine as standard chemical precursors, as well as gallium nitrate and ammonia as chemical additives. A complete growth diagram, revealing the effects of both the relative concentration of gallium nitrate and pH, is gained by combining a thorough experimental approach with thermodynamic computations yielding theoretical solubility plots as well as Zn(II) and Ga(III) speciation diagrams. The role of Ga(OH)4- complexes is specifically shown as capping agents on the m-plane sidewalls of ZnO nanowires, enhancing their development and hence decreasing their aspect ratio. Additionally, the gallium incorporation into ZnO nanowires is investigated in detail by chemical analyses and Raman scattering. They show the predominant formation of gallium substituting for zinc atoms (GaZn) in as-grown ZnO nanowires and their partial conversion into GaZn-VZn complexes after postdeposition annealing under oxygen atmosphere. The conversion is further related to a significant relaxation of the strain level in ZnO nanowires. These findings reporting the physicochemical processes at work during the formation of ZnO nanowires and the related gallium incorporation mechanisms offer a general strategy for their extrinsic doping and open the way for carefully controlling their physical properties as required for nanoscale device engineering.

ZnO nanowires for solar cells: a comprehensive review.

As an abundant and non-toxic wide band gap semiconductor with a high electron mobility, ZnO in the form of nanowires (NWs) has emerged as an important electron transporting material in a vast number of nanostructured solar cells. ZnO NWs are grown by low-cost chemical deposition techniques and their integration into solar cells presents, in principle, significant advantages including efficient optical absorption through light trapping phenomena and enhanced charge carrier separation and collection. However, they also raise some significant issues related to the control of the interface properties and to the technological integration. The present review is intended to report a detailed analysis of the state-of-the-art of all types of nanostructured solar cells integrating ZnO NWs, including extremely thin absorber solar cells, quantum dot solar cells, dye-sensitized solar cells, organic and hybrid solar cells, as well as halide perovskite-based solar cells.

Formation mechanisms of ZnO nanowires on polycrystalline Au seed layers for piezoelectric applications.

ZnO nanowires are considered as attractive building blocks for piezoelectric devices, including nano-generators and stress/strain sensors. However, their integration requires the use of metallic seed layers, on top of which the formation mechanisms of ZnO nanowires by chemical bath deposition are still largely open. In order to tackle that issue, the nucleation and growth mechanisms of ZnO nanowires on top of Au seed layers with a thickness in the range of 5-100 nm are thoroughly investigated. We show that the ZnO nanowires present two different populations of nano-objects with a given morphology. The majority primary population is made of vertically aligned ZnO nanowires, which are heteroepitaxially formed on top of the Au (111) grains. The resulting epitaxial strain is found to be completely relieved at the Au/ZnO interface. In contrast, the minority secondary population is composed of ZnO nanowires with a significant mean tilt angle around 20° with respect to the normal to the substrate surface, which are presumably formed on the (211) facets of the Au (111) grains. The elongation of ZnO nanowires is further found to be limited by the surface reaction at the c-plane top facet in the investigated conditions. By implementing the selective area growth using electron beam lithography, the position of ZnO nanowires is controlled, but the two populations still co-exist in the ensemble. These findings provide an in-depth understanding of the formation mechanisms of ZnO nanowires on metallic seed layers, which should be taken into account for their more efficient integration into piezoelectric devices.

Well-ordered ZnO nanowires with controllable inclination on semipolar ZnO surfaces by chemical bath deposition.

Controlling the formation of ZnO nanowire (NW) arrays on a wide variety of substrates is crucial for their efficient integration into nanoscale devices. While their nucleation and growth by chemical bath deposition (CBD) have intensively been investigated on non-polar and polar c-plane ZnO surfaces, their formation on alternatively oriented ZnO surfaces has not been addressed yet. In this work, the standard CBD technique of ZnO is investigated on [Formula: see text] and [Formula: see text] semipolar ZnO single crystal surfaces. A uniform nanostructured layer consisting of tilted ZnO NWs is formed on the [Formula: see text] surface while elongated nanostructures are coalesced into a two-dimensional compact layer on the [Formula: see text] surface. By further combining the CBD with selective area growth (SAG) using electron beam-assisted lithography, highly tilted well-ordered ZnO NWs with high structural uniformity are grown on the [Formula: see text] patterned surface. The structural analysis reveals that ZnO NWs are homoepitaxially grown along the polar c-axis. The occurrence of quasi-transverse and -longitudinal optical phonon modes in Raman spectra is detected and their origin and position are explained in the framework of the Loudon's model. These results highlight the possibility to form ZnO NWs on original semipolar ZnO surfaces. It also opens the way for comprehensively understanding the nucleation and growth of ZnO NW arrays on poorly and highly textured polycrystalline ZnO seed layers composed of nanoparticles with a wide range of non-polar, semipolar, and polar plane orientations. Eventually, the possibility to tune both the inclination and dimensions of well-ordered ZnO NW arrays by using SAG on semipolar surfaces is noteworthy for photonic and optoelectronic nanoscale devices.

Polarity-Dependent Growth Rates of Selective Area Grown ZnO Nanorods by Chemical Bath Deposition.

Polarity is known to affect the growth and properties of ZnO single crystals and epitaxial films, but its effects are mostly unknown in ZnO nanorods. To leave polarity as the only varying parameter, ZnO nanorods are grown by chemical bath deposition under identical conditions and during the same run on O- and Zn-polar ZnO single crystals patterned by electron beam lithography with the same pattern consisting of 15 different domains. The resulting well-ordered O- and Zn-polar ZnO nanorod arrays with high structural uniformity are formed on all the domains. The comparison of their typical dimensions unambiguously reveals that Zn-polar ZnO nanorods have much higher growth rates than O-polar ZnO nanorods for all the hole diameter and period combinations. The distinct growth rates are explained in the framework of the surface reaction-/diffusive transport-limited elongation regime analysis, which yields a much larger surface reaction rate constant for Zn-polar ZnO nanorods. The origin of the difference is attributed to polarity-dependent dangling bond configurations at the top polar c-faces of ZnO nanorods, which may further be affected by polarity-dependent interactions with the ionic species in aqueous solution. These findings show the relevance of considering polarity as an important quantity in ZnO nanorods.

Effects of the pH on the Formation and Doping Mechanisms of ZnO Nanowires Using Aluminum Nitrate and Ammonia.

The elucidation of the fundamental processes in aqueous solution during the chemical bath deposition of ZnO nanowires (NWs) using zinc nitrate and hexamethylenetetramine is of great significance: however, their extrinsic doping by foreign elements for monitoring their optical and electrical properties is still challenging. By combining thermodynamic simulations yielding theoretical solubility plots and speciation diagrams with in situ pH measurements and structural, chemical, and optical analyses, we report an in-depth understanding of the pH effects on the formation and aluminum doping mechanisms of ZnO NWs. By the addition of aluminum nitrate with a given relative concentration for the doping and of ammonia over a broad range of concentrations, the pH is shown to strongly influence the shape, diameter, length, and doping magnitude of ZnO NWs. Tuning the dimensions of ZnO NWs by inhibition of their radial growth only proceeds over a specific pH range, where negatively charged Al(OH)4- complexes are predominantly formed and act as capping agents by electrostatically interacting with the positively charged m-plane sidewalls. These complexes further favor the aluminum incorporation and doping of ZnO NWs, which only operate over the same pH range following thermal annealing above 200 °C. These findings reporting a full chemical synthesis diagram reveal the significance of carefully selecting and following the pH to control the morphology of ZnO NWs as well as to achieve their thermally activated extrinsic doping, as required for many nanoscale engineering devices.

Quantitative and simultaneous analysis of the polarity of polycrystalline ZnO seed layers and related nanowires grown by wet chemical deposition.

The polarity in ZnO nanowires is an important issue since it strongly affects surface configuration and reactivity, nucleation and growth, electro-optical properties, and nanoscale-engineering device performances. However, measuring statistically the polarity of ZnO nanowire arrays grown by chemical bath deposition and elucidating its correlation with the polarity of the underneath polycrystalline ZnO seed layer grown by the sol-gel process represents a major difficulty. To address that issue, we combine resonant x-ray diffraction (XRD) at Zn K-edge using synchrotron radiation with piezoelectric force microscopy and polarity-sensitive chemical etching to statistically investigate the polarity of more than 107 nano-objects both on the macroscopic and local microscopic scales, respectively. By using high temperature annealing under an argon atmosphere, it is shown that the compact, highly c-axis oriented ZnO seed layer is more than 92% Zn-polar and that only a few small O-polar ZnO grains with an amount less than 8% are formed. Correlatively, the resulting ZnO nanowires are also found to be Zn-polar, indicating that their polarity is transferred from the c-axis oriented ZnO grains acting as nucleation sites in the seed layer. These findings pave the way for the development of new strategies to form unipolar ZnO nanowire arrays as a requirement for a number of nanoscale-engineering devices like piezoelectric nanogenerators. They also highlight the great advantage of resonant XRD as a macroscopic, non-destructive method to simultaneously and statistically measure the polarity of ZnO nanowire arrays and of the underneath ZnO seed layer.

Identifying and mapping the polytypes and orientation relationships in ZnO/CdSe core-shell nanowire arrays.

Identifying and mapping the crystalline phases and orientation relationships on the local scale in core-shell ZnO nanowire heterostructures are of primary importance to improve the interface quality, which governs the performances of the nanoscale devices. However, this represents a major difficulty, especially when the expected polytypes exhibit very similar properties as in the case of CdSe. In the present work, we address that issue in ZnO nanowire heterostructures involving a uniform and highly conformal CdSe shell grown by molecular beam epitaxy. It is shown by x-ray diffraction and Raman spectroscopy through the occurrence of the (101̄0) and (101̄1) diffraction peaks and of the [Formula: see text] mode at 34 cm-1, respectively, that the CdSe shell is mostly crystallized into the wurtzite phase. By using automated crystal phase and orientation mapping with precession (ASTAR) in a transmission electron microscope and thus by benefiting from highly precise electron diffraction patterns, the CdSe shell is found to crystallize also into the minority zinc blende phase. The wurtzite CdSe shell is epitaxially grown on the top of ZnO nanowires, and some specific orientation relationships are mapped and revealed when grown on their vertical sidewalls. Zinc blende CdSe domains are also formed exclusively in the center of wurtzite CdSe grains located on the vertical sidewalls; both wurtzite and zinc blende CdSe crystalline phases have a strong orientation relationship. These findings reveal that ASTAR is a powerful technique to elucidate the structural properties on the local scale and to gain a deeper insight into their crystalline phases and orientation relationships, which is highly promising for many types of semiconducting nanowire heterostructures.

Light absorption processes and optimization of ZnO/CdTe core-shell nanowire arrays for nanostructured solar cells.

The absorption processes of extremely thin absorber solar cells based on ZnO/CdTe core-shell nanowire (NW) arrays with square, hexagonal or triangular arrangements are investigated through systematic computations of the ideal short-circuit current density using three-dimensional rigorous coupled wave analysis. The geometrical dimensions are optimized for optically designing these solar cells: the optimal NW diameter, height and array period are of 200 ± 10 nm, 1-3 µm and 350-400 nm for the square arrangement with CdTe shell thickness of 40-60 nm. The effects of the CdTe shell thickness on the absorption of ZnO/CdTe NW arrays are revealed through the study of two optical key modes: the first one is confining the light into individual NWs, the second one is strongly interacting with the NW arrangement. It is also shown that the reflectivity of the substrate can improve Fabry-Perot resonances within the NWs: the ideal short-circuit current density is increased by 10% for the ZnO/fluorine-doped tin oxide (FTO)/ideal reflector as compared to the ZnO/FTO/glass substrate. Furthermore, the optimized square arrangement absorbs light more efficiently than both optimized hexagonal and triangular arrangements. Eventually, the enhancement factor of the ideal short-circuit current density is calculated as high as 1.72 with respect to planar layers, showing the high optical potentiality of ZnO/CdTe core-shell NW arrays.

Light trapping in ZnO nanowire arrays covered with an absorbing shell for solar cells.

The absorption properties of ZnO nanowire arrays covered with a semiconducting absorbing shell for extremely thin absorber solar cells are theoretically investigated by optical computations of the ideal short-circuit current density with three-dimensional rigorous coupled wave analysis. The effects of nanowire geometrical dimensions on the light trapping and absorption properties are reported through a comprehensive optical mode analysis. It is shown that the high absorptance of these heterostructures is driven by two different regimes originating from the combination of individual nanowire effects and nanowire arrangement effects. In the short wavelength regime, the absorptance is likely dominated by optical modes efficiently coupled with the incident light and interacting with the nearby nanowires (i.e. diffraction), induced by the period of core shell ZnO nanowire arrays. In contrast, in the long wavelength regime, the absorptance is governed by key optically guided modes, related to the diameter of individual core shell ZnO nanowires.

Chemical Bath Deposition of ZnO/ZnGa2O4 Core-Shell Nanowire Heterostructures Using Partial Chemical Conversion.

The development of innovative heterostructures made of ZnO nanowires is of great interest for enhancing the performances of many devices in the fields of optoelectronics, photovoltaics, and energy harvesting. We report an original fabrication process to form ZnO/ZnGa2O4 core-shell nanowire heterostructures in the framework of the wet chemistry techniques. The process involves the partial chemical conversion of ZnO nanowires grown via chemical bath deposition into ZnO/ZnGa2O4 core-shell nanowire heterostructures with a high interface quality following their immersion in an aqueous solution containing gallium nitrate heated at a low temperature. The double-step process describing the partial chemical conversion relies on successive dissolution and reaction mechanisms. The present finding offers the possibility to fabricate ZnO/ZnGa2O4 core-shell nanowire heterostructures at low temperatures and over a wide variety of substrates with a large surface area, which is attractive for nanostructured solar cells, deep-UV photodetectors, and piezoelectric devices.

Tunable Hydrogen-Related Defects in ZnO Nanowires Using Oxygen Plasma Treatment by Ion Energy Adjustment.

The chemical bath deposition (CBD) process enables the deposition of ZnO nanowires (NWs) on various substrates with customizable morphology. However, the hydrogen-rich CBD environment introduces numerous hydrogen-related defects, unintentionally doping the ZnO NWs and increasing their electrical conductivity. The oxygen-based plasma treatment can modify the nature and amount of these defects, potentially tailoring the ZnO NW properties for specific applications. This study examines the impact of the average ion energy on the formation of oxygen vacancies (VO) and hydrogen-related defects in ZnO NWs exposed to low-pressure oxygen plasma. Using X-ray photoelectron spectroscopy (XPS), 5 K cathodoluminescence (5K CL), and Raman spectroscopy, a comprehensive understanding of the effect of the oxygen ion energy on the formation of defects and defect complexes was established. A series of associative and dissociative reactions indicated that controlling plasma process parameters, particularly ion energy, is crucial. The XPS data suggested that increasing the ion energy could enhance Fermi level pinning by increasing the amount of VO and favoring the hydroxyl group adsorption, expanding the depletion region of charge carriers. The 5K CL and Raman spectroscopy further demonstrated the potential to adjust the ZnO NW physical properties by varying the oxygen ion energy, affecting various donor- and acceptor-type defect complexes. This study highlights the ability to tune the ZnO NW properties at low temperature by modifying plasma process parameters, offering new possibilities for a wide variety of nanoscale engineering devices fabricated on flexible and/or transparent substrates.

ZnO Nanowires: Growth, Properties, and Energy Applications.

As a biocompatible semiconductor composed of abundant elements, ZnO, in the form of nanowires, exhibits remarkable properties, mainly originating from its wurtzite structure and correlated with its high aspect ratio at nanoscale dimensions [...].

Implementing the Reactor Geometry in the Modeling of Chemical Bath Deposition of ZnO Nanowires.

The formation of nanowires by chemical bath deposition is of great interest for a wide variety of optoelectronic, piezoelectric, and sensing devices, from which the theoretical description of their elongation process has emerged as a critical issue. Despite its strong influence on the nanowire growth kinetics, reactor size has typically not been taken into account in the theoretical modeling developed so far. We report a new theoretical description of the axial growth rate of nanowires in dynamic conditions based on the solution of Fick's diffusion equations, implementing a sealed reactor of finite height as a varying parameter. The theoretical model is applied in various chemical bath deposition conditions in the case of the growth of ZnO nanowires, from which the influence of the reactor height is investigated and compared to experimental data. In particular, it is found that the use of reactor heights smaller than 2 cm significantly decreases the ZnO nanowires' axial growth rate in typical experimental conditions due to the faster depletion of reactants. The present approach is further used predictively, showing its high potential for the design of batch reactors for a wide variety of chemical precursors and semiconductor materials in applied research and industrial production.

Modulating the growth of chemically deposited ZnO nanowires and the formation of nitrogen- and hydrogen-related defects using pH adjustment.

ZnO nanowires (NWs) grown by chemical bath deposition (CBD) have received great interest for nanoscale engineering devices, but their formation in aqueous solution containing many impurities needs to be carefully addressed. In particular, the pH of the CBD solution and its effect on the formation mechanisms of ZnO NWs and of nitrogen- and hydrogen-related defects in their center are still unexplored. By adjusting its value in a low- and high-pH region, we show the latent evolution of the morphological and optical properties of ZnO NWs, as well as the modulated incorporation of nitrogen- and hydrogen-related defects in their center using Raman and cathodoluminescence spectroscopy. The increase in pH is related to the increase in the oxygen chemical potential (µ O), for which the formation energy of hydrogen in bond-centered sites (HBC) and VZn-NO-H defect complexes is found to be unchanged, whereas the formation energy of zinc vacancy (VZn) and zinc vacancy-hydrogen (VZn-nH) complexes steadily decreases as shown from density-functional theory calculations. Revealing that these VZn-related defects are energetically favorable to form as µ O is increased, ZnO NWs grown in the high-pH region are found to exhibit a higher density of VZn-nH defect complexes than ZnO NWs grown in the low-pH region. Annealing at 450 °C under an oxygen atmosphere helps tuning the optical properties of ZnO NWs by reducing the density of HBC and VZn-related defects, while activating the formation of VZn-NO-H defect complexes. These findings show the influence of pH on the nature of Zn(ii) species, the electrostatic interactions between these species and ZnO NW surfaces, and the formation energy of the involved defects. They emphasize the crucial role of the pH of the CBD solution and open new possibilities for simultaneously engineering the morphology of ZnO NWs and the formation of nitrogen- and hydrogen-related defects.

Optimization of the Sb2S3 Shell Thickness in ZnO Nanowire-Based Extremely Thin Absorber Solar Cells.

Extremely thin absorber (ETA) solar cells made of ZnO/TiO2/Sb2S3 core-shell nanowire heterostructures, using P3HT as the hole-transporting material (HTM), are of high interest to surpass solar cell efficiencies of their planar counterpart at lower material cost. However, no dimensional optimization has been addressed in detail, as it raises material and technological critical issues. In this study, the thickness of the Sb2S3 shell grown by chemical spray pyrolysis is tuned from a couple of nanometers to several tens of nanometers, while switching from a partially to a fully crystallized shell. The Sb2S3 shell is highly pure, and the unwanted Sb2O3 phase was not formed. The low end of the thickness is limited by challenges in the crystallization of the Sb2S3 shell, as it is amorphous at nanoscale dimensions, resulting in the low optical absorption of visible photons. In contrast, the high end of the thickness is limited by the increased density of defects in the bulk of the Sb2S3 shell, degrading charge carrier dynamics, and by the incomplete immersion of the P3HT in the structure, resulting in the poor hole collection. The best ETA solar cell with a short-circuit current density of 12.1 mA/cm2, an open-circuit voltage of 502 mV, and a photovoltaic conversion efficiency of 2.83% is obtained for an intermediate thickness of the Sb2S3 shell. These findings highlight that the incorporation of both the absorber shell and HTM in the core-shell heterostructures relies on the spacing between individual nanowires. They further elaborate the intricate nature of the dimensional optimization of an ETA cell, as it requires a fine-balanced holistic approach to correlate all the dimensions of all the components in the heterostructures.