Tandem Absorber Research Articles

Extremely high temperature stable nanometric scale multilayer spectrally selective absorber coating: Emissivity measurements at elevated temperatures and a comprehensive study on ageing mechanism

Spectrally selective W/WAlSiN/SiON/SiO2 solar absorber coatings were sputter-deposited on stainless steel and silicon substrates. The optimized as-deposited sample exhibits a high solar absorptance of 0.955 and a low thermal emissivity of 0.10 at 82 °C. The coating exhibits a very low reflectance of 1.2% in the wavelength range of 0.5–1.5 μm in the solar spectrum. The spectral emissivity measurements at various operating temperatures and with varying emergence angles were investigated. Also, the impact of emissivity on the photothermal efficiency at different temperatures of the developed solar absorber coating was calculated. The calculated optical properties of the as-deposited sample exhibited low thermal emissivity of 0.157 (at 500 °C) and a high heliothermal efficiency of 89.5%. Angular reflectance measured at room temperature illustrates an insignificant change in the hemispherical and near normal emissivities of the samples. In addition to these studies, the ageing tests of the as-deposited samples at various operating temperatures were studied in detail. The thermal ageing tests of the samples in air and vacuum environments indicated excellent thermal stability at elevated temperatures, i.e., in air at 400 °C for 500 h, at 450 °C for 175 h and at 500 °C for 100 h, whereas, in a vacuum it was stable at 700 °C for 200 h. The top anti-reflection layers and the fine nano-multilayers of WAlSiN (W2N and AlSiN) prevent the inward diffusion of oxygen and thereby improve the overall thermal stability of the tandem absorber.

Quantifying and Comparing Fundamental Loss Mechanisms to Enable Solar‐to‐Hydrogen Conversion Efficiencies above 20% Using Perovskite–Silicon Tandem Absorbers

Photovoltaic (PV)‐based solar hydrogen generation is a promising pathway for the scalable production of renewable fuels. Understanding the limitations of solar‐to‐hydrogen (STH) conversion efficiencies is critical to identify performance limits and conceptualize practical device designs. Herein, the losses in PV‐based solar hydrogen generation systems are quantified and the potential of loss‐mitigation techniques to improve the STH efficiency is assessed. The analysis shows that the two largest losses in an ideal system are current and voltage mismatches due to suboptimal system configurations and energy lost as heat in the PV component. A temperature‐dependent model is developed to evaluate the relative potential of two techniques to mitigate these losses: decoupling the PV system to remove current and voltage matching requirements and thermal integration to use the heat losses from PV to increase the electrolyte temperature and improve the reaction dynamics for water splitting. It is shown that optimal system configuration strategies provide more than three times the STH efficiency increase of thermal integration at high operating temperatures. Combining both techniques results in predicted STH efficiencies approaching 20% for low‐cost perovskite–silicon tandem‐based systems with earth‐abundant catalysts at realistic working temperatures.

Design and on-Sun Testing of Tandem III-V Photoelectrochemical Water-Splitting Systems

Photoelectrochemical (PEC) water-splitting systems utilize sunlight to directly split water to hydrogen and oxygen, providing a storable chemical fuel.1 While a variety of semiconductor material systems have demonstrated unassisted solar hydrogen production, III-V semiconductors have shown the highest reported solar-to-hydrogen (STH) conversion efficiencies and best device longevities.2–4 However, to date, work in the field has focused mostly on laboratory conditions, with few PEC systems having been tested outdoors using natural sunlight.In this work, we design III-V semiconductor systems with a molybdenum disulfide catalyst layer and demonstrate unassisted PEC hydrogen generation on-sun. First, a window and capping layer in conjunction with the MoS2 catalytic protection layer are shown to improve the photovoltage and durability of a single-junction pn-GaInP2 photocathode over a MoS2/pn-GaInP2 photocathode. Translating this top cell architecture to a tandem absorber device allows for more meaningful outdoor measurements and improved unassisted water-splitting performance. Specifically, we employ a tandem, lattice-matched GaInP2/GaAs (1.8/1.4 eV bandgaps) system that has been developed with high quality fabrication and yields>10% STH efficiency.2,3 An on-sun photoreactor platform was designed for PEC benchmarking and material testing that incorporates a 2-axis solar tracker and gas collection system.5 Insolation and environmental instrumentation at the co-located NREL Solar Radiation Research Laboratory provide a full record of the solar irradiance and environmental conditions, allowing for more accurate benchmarking of on-sun performance. PEC device characteristics were probed via current-voltage and chronoamperometry measurements under both natural and simulated sunlight illumination, while material structure is probed by XPS and SEM.The MoS2-protected tandem absorbers exhibit >10% STH efficiency under both laboratory and on-sun conditions. During a day of PEC testing under natural sunlight, >11 standard mL of hydrogen is generated with no applied bias. We will discuss challenges in catalyst and semiconductor stability and in cell design that limit PEC durability and hydrogen yield.

On the origin of spectrally selective high solar absorptance of TiB2-based tandem absorber with double layer antireflection coatings

Despite significant efforts in developing tandem ceramic coatings for photo-thermal conversion applications, the underlying physics behind spectrally selective high absorptance and thermal stability of the ceramic absorbers remains to be addressed. In this perspective, TiB2/Ti(B,N)/SiON/SiO2 and Ti/TiB2/Ti(B,N)/SiON/SiO2 films were developed on stainless steel substrates (SS 304) using pulsed direct current and radio frequency magnetron sputtering. The addition of double layer antireflection coating not only increases the solar absorptance, but also the thermal stability of the tandem absorber. The amorphous coatings exhibit a high solar absorptance of 0.981 at room temperature and an acceptable thermal emissivity of 0.15 at 82 °C. Variable angle spectroscopic ellipsometry analysis reveals a gradient in the refractive indices of the individual layers from the substrate to the top surface. The coatings with and without titanium interlayer were found to be stable for 2 h in air up to 450 °C and 400 °C, respectively. Under vacuum, the coatings exhibited good thermal stability up to 250 h at 500 °C. We make an attempt to explain the high absorptance of the tandem absorber in terms of the composition and gradient in refractive indices across the stack.

Solar Water Splitting: Over 17% Efficiency Stand‐Alone Solar Water Splitting Enabled by Perovskite‐Silicon Tandem Absorbers (Adv. Energy Mater. 28/2020)

Solar Water Splitting: Over 17% Efficiency Stand‐Alone Solar Water Splitting Enabled by Perovskite‐Silicon Tandem Absorbers (Adv. Energy Mater. 28/2020)

Over 17% Efficiency Stand‐Alone Solar Water Splitting Enabled by Perovskite‐Silicon Tandem Absorbers

AbstractRealizing solar‐to‐hydrogen (STH) efficiencies close to 20% using low‐cost semiconductors remains a major step toward accomplishing the practical viability of photoelectrochemical (PEC) hydrogen generation technologies. Dual‐absorber tandem cells combining inexpensive semiconductors are a promising strategy to achieve high STH efficiencies at a reasonable cost. Here, a perovskite photovoltaic biased silicon (Si) photoelectrode is demonstrated for highly efficient stand‐alone solar water splitting. A p+nn+ ‐Si/Ti/Pt photocathode is shown to present a remarkable photon‐to‐current efficiency of 14.1% under biased condition and stability over three days under continuous illumination. Upon pairing with a semitransparent mixed perovskite solar cell of an appropriate bandgap with state‐of‐the‐art performance, an unprecedented 17.6% STH efficiency is achieved for self‐driven solar water splitting. Modeling and analysis of the dual‐absorber PEC system reveal that further work into replacing the noble‐metal catalyst materials with earth‐abundant elements and improvement of perovskite fill factor will pave the way for the realization of a low‐cost high‐efficiency PEC system.

Theoretical limits of multiple exciton generation and singlet fission tandem devices for solar water splitting.

Photoelectrochemical (PEC) water splitting is one of the most important approaches being investigated for solar fuel generation. In this study, we determine the maximum thermodynamic power conversion efficiencies (PCEs) of PEC water splitting two-bandgap tandem devices that produce multiple carriers per photon absorbed via Multiple Exciton Generation (MEG) or Singlet Fission (SF) and in the presence of solar concentration. Here, we employ a detailed balance thermodynamic analysis to determine the effects of top cell thickness, solar concentration, carrier multiplication, electrode overvoltage (VO), and water absorption on PEC power conversion efficiency for water splitting cells. We have found a maximum PEC power conversion efficiency of 62.9% in cells using two ideal tandem MEG absorbers with bandgaps of 0.3 and 1.2 eV at 1000-suns solar concentration and 0 overvoltage; the maximum PCE for two tandem SF absorbers under the same conditions is nearly the same at 59% with the same values for the absorption thresholds. A very interesting and important result was that, upon thinning the top cell, the range of viable bandgaps for both the top and bottom cells is extended by as much as 0.5-1 eV while still maintaining high maximum conversion efficiency (60-63%). The effects of imposing different solar concentrations from 1X to 1000X and having different tandem configurations of SF and MEG layers were also studied.

Optimization of W/WAlSiN/SiON/SiO2 tandem absorber consisting of double layer anti-reflection coating with broadband absorption in the solar spectrum region

A novel spectrally selective tandem stack of W/WAlSiN/SiON/SiO2 was deposited on stainless steel 304 and silicon substrates using a four-cathode reactive unbalanced magnetron sputtering system. The coatings were deposited by sputtering of W, Al and Si targets in Ar, Ar + N2, Ar + N2 + O2, and Ar + O2 plasmas. The process parameters were optimized by studying the optical properties of the individual layers using UV-VIS-NIR spectrophotometer and Fourier transform infrared spectroscopy measurements. The high spectral selectivity of the tandem stack was achieved by varying the reactive gas flow rates of N2 and O2 and thickness of individual layers. In the tandem stack, W layer acts as an IR reflector, WAlSiN acts as the main absorber layer, SiON, and SiO2 layers act as anti-reflecting layers. The tandem stack was designed based on graded refractive indices of individual layers with a double layer anti-reflection coating. The tandem stack exhibits superior spectral selectivity with a high solar absorptance of 0.955 in the broadband solar spectrum region and low thermal emissivity of 0.10 in the infrared region. The coating was found to be thermally stable up to 600 °C in vacuum for 200 h under cycling heating conditions.

High temperature optically stable spectrally-selective Ti1-xAlxN-based multilayer coating for concentrated solar thermal applications

Spectrally-selective solar absorbing coatings based on the Ti1-xAlxN system were deposited using DC magnetron sputtering. Due to their refractory nature and very suitable optical properties, these were considered for high temperature solar thermal energy conversion. The composition of Ti1-xAlxN, (effectively, the Ti/Al ratio) was optimized to achieve a maximized solar absorptance. The optimum composition was then tested in a tandem absorber which included anti-reflective layers. A stainless steel substrate was used in order to simulate service in parabolic trough-based power plants that use stainless steel pipe to carry the heat-transfer fluid. High temperature annealing of the stack caused structural modifications but the solar absorptance of 92% was retained even after annealing at 900 °C.

Thin polymeric CuO film from EPD designed for low temperature photothermal absorbers

The creation of a tandem solar absorber based on highly UV–Vis–NIR absorbing nanofilm deposited onto a highly IR reflecting platinized silicon wafer is processed through electrophoretic deposition (EPD). The stabilization of a CuO colloidal aqueous suspension is first studied by adding polyethyleneimine (PEI) as cationic polymer acting both as charging agent and as electro-steric stabilizer. The colloidal stability as a function of the suspension pH is investigated prior to EPD, by Laser Doppler Velocimetry and atomic force microscopy. Tandem absorbers are obtained by varying different EPD parameters to control the thickness and the morphology of the film, in order to tune and optimize the final optical properties. The deposition thickness is then compared relative to the applied potential difference and the deposition time range. The morphology of the deposits and the thickness of the coatings are analysed by scanning electron microscopy (SEM). The density is obtained from energy-dispersive X-ray spectroscopy (EDX) using X-film software, total organic carbon (TOC) and Hamaker equation. CuO tandem absorbers are found to possess a high density with homogeneous and crack-free surfaces. Finally, absorptance (α) and emittance (ε) are calculated from the reflectance spectra of the UV–Vis–NIR and the Fourier Transform Infra-Red (FTIR) spectroscopy respectively. These latter values are combined to determine the efficiency (ƞ) of the tandem material.

Structure, thermal stability and chromaticity investigation of TiB2 based high temperature solar selective absorbing coatings

Transition metal titanium diboride (TiB2) is a promising material for the solar selective absorbing coatings due to its favorable spectral selectivity and thermal stability. In this research, spectrally selective TiB2/Al2O3 tandem absorber is deposited on stainless steel (SS), which exhibits a high solar absorptance (αs = 0.93) and a low thermal emissivity (ε = 0.11). In this tandem absorber, the TiB2 acts as the main absorber layer and the Al2O3 acts as an antireflection layer. The tandem absorber coating is systematically characterized by ultra-high resolution scanning electron microscope (SEM), UV–vis-NIR spectrophotometer, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and Micro-Raman techniques. The thermal stability in vacuum of the SS/TiB2/Al2O3 tandem absorber coatings is investigated in the temperature range of 400–800 °C for 2 h and 100 h, respectively. SEM and Raman spectra indicate that the change of surface morphology and chemical composition for the coating result in the degradation of optical performance. With the different annealed temperature, the SS/TiB2/Al2O3 tandem absorber coatings exhibit different color, which is further investigated using chromaticity diagram.

High performance aperiodic metal-dielectric multilayer stacks for solar energy thermal conversion

A novel spectrally selective Cr/AlCrN/AlCrNO/AlCrN/AlCrNO/AlCrO multilayer coating was deposited on stainless steel substrates using cathodic arc ion plating. In this tandem absorber coating, Cr, AlCrN, AlCrNO and AlCrO act as the reflector layer, semi-absorbing layer, the main absorbing layer and the antireflection layer, respectively. The spectral properties of this multilayer coating during long-term annealing in air have been investigated with the aim of evaluating the thermal stability of this aperiodic metal-dielectric multilayer stacks. An average absorptance of 0.90 and emittance of 0.15 are achieved in this multilayer coatings fabricated with optimized parameters. More importantly, the coating exhibits an outstanding thermal stability with a selectivity of 0.94/0.11 even after annealed at 500 °C for 1000 h in air. The microstructure analysis indicate that the multilayer coating consists of aperiodic AlCrN and AlCrON layers in addition to the Cr and AlCrO layers. After the long-term annealing, small amounts of AlN, CrN and Cr2N nanocrystallites are observed to be homogeneously embedded in the AlCrN and AlCrON amorphous matrices. The nanoparticles in the AlCrN and AlCrON layers can effectively scatter the incident light into a broadband wavelength range, increasing the optical path length in the absorbing layers and thus resulting in a pronounced enhancement in the absorptivity. A handful of Cr2O3 and Al2O3 nanograins are observed to embedd in the amorphous AlCrO antireflection layer, which can efficitively reflect the solar infrared radiation and the thermal emittance from the substrate and thus result in pretty low infrared emissivity. The good thermal stability is attributed to the excellent thermal stability of the cermet-based amorphous matrices and the sluggish atomic diffusion in the nanoparticles, which could effectively slow down the inward diffusion of oxygen and avoid agglomeration of naonparticles. These results suggest that the AlCrON-based selective absorbing coating is a good candidate for photo-thermal conversion at high temperatures.

Scalable Triple Cation Mixed Halide Perovskite–BiVO4 Tandems for Bias‐Free Water Splitting

AbstractStrong interest exists in the development of organic–inorganic lead halide perovskite photovoltaics and of photoelectrochemical (PEC) tandem absorber systems for solar fuel production. However, their scalability and durability have long been limiting factors. In this work, it is revealed how both fields can be seamlessly merged together, to obtain scalable, bias‐free solar water splitting tandem devices. For this purpose, state‐of‐the‐art cesium formamidinium methylammonium (CsFAMA) triple cation mixed halide perovskite photovoltaic cells with a nickel oxide (NiOx) hole transport layer are employed to produce Field's metal‐epoxy encapsulated photocathodes. Their stability (up to 7 h), photocurrent density (–12.1 ± 0.3 mA cm−2 at 0 V versus reversible hydrogen electrode, RHE), and reproducibility enable a matching combination with robust BiVO4 photoanodes, resulting in 0.25 cm2 PEC tandems with an excellent stability of up to 20 h and a bias‐free solar‐to‐hydrogen efficiency of 0.35 ± 0.14%. The high reliability of the fabrication procedures allows scaling of the devices up to 10 cm2, with a slight decrease in bias‐free photocurrent density from 0.39 ± 0.15 to 0.23 ± 0.10 mA cm−2 due to an increasing series resistance. To characterize these devices, a versatile 3D‐printed PEC cell is also developed.

Effects of environmental and operational variability on the spectrally selective properties of W/WAlN/WAlON/Al2O3-based solar absorber coating

The stability of solar selective absorber coatings in hostile environments (e.g., humid condition, corrosive medium, longer exposure at elevated temperature) needs to be critically assessed to ensure durability of such coatings. In the present work, magnetron sputtered W/WAlN/WAlON/Al2O3-based absorber coating with a high absorptance (0.958) and low emittance (82 °C) has been tested in humid and corrosive environments. The selectivity (absorptance/emittance) of the coating did not change after keeping it in 95% humidity at 37 °C for 400 h. Corrosion study in 3.5% NaCl solution reveals that this novel multilayer coating has a better corrosion resistance than that of uncoated stainless steel (SS) substrate. The nanoindentation test on the coating indicates that it has a hardness of 9.6 ± 0.5 GPa. The performance of the coating did not degrade after heat treatment at 350 °C in air for 1000 h. Additionally, the activation energy for degradation has been determined to predict the stability of the coating at high temperature. Further, the normal spectral emissivity of the tandem solar absorber was measured in the full angular range from 200 to 500 °C. The results of emissivity measurements by varying observation angles are analysed using numerical integration to obtain the total hemispherical emissivity. In summary, W/WAlN/WAlON/Al2O3 stack is an attractive candidate absorber coating for photo-thermal conversion systems.

Zirconia nanoparticles embedded spinel selective absorber coating for high performance in open atmospheric condition

Spectrally selective absorber coatings which have high thermal stable at temperatures ≥ 500°C in open air atmosphere are more beneficial for all types of concentrated solar thermal power (CSP) applications. In order to achieve this, high crystalline zirconia nanoparticles are synthesized via Lyothermal process were embedded into a spinel matrix to form a novel composite solar selective absorber layer (Mn-Cu-Co-Ox-ZrO2). To enhance the absorption substantially, an ink-bottle type mesoporous MgF2 nanoparticles are synthesized via novel route have been deposited further. As a result, a novel tandem absorber system (Mn-Cu-Co-Ox-ZrO2/MgF2) has been developed with α/ε = 0.97/0.17 @500°C. The composite layer comprises of tetragonal phase Zirconia particles which were uniformly distributed in the spinel matrix responsible for high optical performance and high thermal stability. These properties are thoroughly investigated by micro X-ray diffraction (µXRD), X-ray photoelectron spectrometer (XPS) and Elemental mapping by Energy filtered transmission electron microscope (EF-TEM) technique. This novel composite absorber layer developed by a facile route exhibits superior stability up to 700°C ideally in the air makes a promising advancement for the cost-effective power generation by CSP systems.

Measurement of high temperature emissivity and photothermal conversion efficiency of TiAlC/TiAlCN/TiAlSiCN/TiAlSiCO/TiAlSiO spectrally selective coating

A spectrally selective TiAlC/TiAlCN/TiAlSiCN/TiAlSiCO/TiAlSiO coating was deposited on stainless steel substrate by unbalanced magnetron sputtering system. Each individual layer of the tandem absorber was optimized by varying the reactive gas flow rates (C2H2, N2 and O2) and target power densities (Ti, Al and Si). The optimized tandem absorber shows a solar absorptance of 0.960 and an emittance of 0.15 at 82°C, measured using solar spectrum reflectometer and emissometer, respectively. In order to study the optical properties of the deposited tandem absorber at high operating temperatures the reflectance spectra of the tandem absorber were measured at temperatures ranging from 80°C to 500°C by UV–Vis–NIR spectrophotometer and FTIR spectrometers. The reflectance spectra of the as-deposited sample and after high temperature reflectance measurements did not show any significant changes. The thermal emittance of the tandem absorber at high temperatures (80–500°C) was studied in detail. At the temperature of 200°C, 300°C, 400°C and 500°C the tandem absorber shows the emittance of 0.152–0.157, 0.181–0.19, 0.214–0.246 and 0.251–0.275, respectively with an absorptance of ~0.930. These results show the good selectivity of the tandem absorber even at high operating temperatures (e.g., 500°C) with a photothermal conversion efficiency of 88%, thus demonstrating that the tandem absorber is suitable for solar thermal power generation applications. Reflectance and roughness data of the absorber coating post annealing in air up to 600°C for 2h, carried out independently, corroborated the present results.

Optical properties of TiAlC/TiAlCN/TiAlSiCN/TiAlSiCO/TiAlSiO tandem absorber coatings by phase-modulated spectroscopic ellipsometry

TiAlC, TiAlCN, TiAlSiCN, TiAlSiCO, and TiAlSiO layers of thicknesses ~2.2 μm, 755, 491, 393, and 431 nm, respectively, were deposited on stainless steel, silicon, and glass substrates to study their refractive indices and extinction coefficients using the phase-modulated spectroscopic ellipsometry in the wavelength range of 300–1200 nm. Absorption coefficient of each layer was calculated from the extinction coefficient of the layer. The results indicate that the first three layers (i.e., TiAlC, TiAlCN, and TiAlSiCN) are absorbing in nature, while TiAlSiCO and TiAlSiO act as intermediate and antireflection layers. Subsequently, a tandem absorber of TiAlC/TiAlCN/TiAlSiCN/TiAlSiCO/TiAlSiO with layer thicknesses of 62, 20, 18, 16, and 27 nm, respectively, was deposited on stainless steel substrates to fabricate a spectrally selective coating with absorptance of 0.961 and emittance of 0.15 at 82 °C. The obtained refractive indices and extinction coefficients of the tandem absorber were used to simulate the reflectance of the deposited tandem absorber using SCOUT software. Simulated reflectance data of the tandem absorber showed a good agreement with the experimental data measured by UV–Vis–NIR and FTIR spectrophotometry. The angular dependence of the selective properties of the tandem absorber was studied by measuring the reflectance spectra of the tandem absorber at different incident angles.

Optical characterization of TiAlNx/TiAlNy/Al2O3 tandem solar selective absorber coatings

Solar selective absorber coatings, consisting in a TiAlNx / TiAlNy tandem absorber with low (metallic-like) and high (semiconductor-like) nitrogen content, and an Al2O3 antireflective coating, were deposited on stainless steel. A full optical characterization was carried out on these coatings. Reflectance spectra were measured by spectrophotometry in the UV to mid-IR range (0.25−25µm), allowing for the calculation of total solar absorptance and thermal emittance. An absorptance of 0.93 and an emittance of 0.22 at 550°C were obtained for the best coating. The temperature dependence of reflectance was investigated in the infrared region and results demonstrated the validity of the classical room temperature measurement approximation to calculate thermal emittance at high temperature for these coatings. The total hemispherical emittance of the coatings was calculated from hemispherical directional reflectance spectra measured at different angles, showing that the near normal hemispherical emittance is a good approximation of the total hemispherical emittance. The total solar absorptance was also directly measured under natural solar irradiation using a unique solar facility.

Spectrally selective coatings obtained from electrophoretic deposition of CuO nanoparticles

Cathodic electrophoretic deposition (EPD) of CuO nanoparticles is investigated for the formation of CuO tandem photothermal absorber starting from CuO-isopropanol suspension. Prior to the deposition step, the stabilization of the suspension in request is achieved via the addition of Mg(NO3)2 acting as a stabilizer but, also as a binder for the final coating. The colloidal dispersion stability is studied as a function of the concentration of Mg(NO3)2 added to the suspension by dynamic light scattering coupled with laser doppler velocimetry in order to determine the size and the charge of particles respectively. EPD parameters, such as the electric field and deposition time, have been investigated in order to control the thickness and the morphology in order to select the most promising coatings. The evolution of the thickness, obtained from scanning electron microscopy images, as a function of deposition time for different applied electric field is in agreement with a linear Hamaker law type. This is also used here as a first attempt to extract the effective density of the coating. Finally, the conversion efficiency of the tandem material is calculated from the reflectance spectra of the UV–vis-NIR and the Fourier transform InfraRed spectroscopy in order to link the EPD parameters to the spectral selectivity. The optimal coating was formed with an applied electric field of 50V·cm−1 for a deposition time of 30min yield the highest efficiency at 0.8.

Light management of tandem solar cells on nanostructured substrates

We report the use of nanostructured substrates as a simple approach to improve the performance of tandem micromorph silicon solar cells. In the proposed approach, nanostructured substrates are produced using a low-cost, self-assembled growth process. The use of a nanostructured substrate coated with a thick transparent conductive oxide electrode layer enables the conformal deposition of the tandem solar cell absorber layers while allowing the solar cell to exhibit a modified surface morphology caused by the underlying nanostructured morphology. Using this nanostructured substrate approach, we demonstrated a 78% relative enhancement in the conversion efficiency of a tandem micromorph silicon cell on a nanostructured substrate compared to a standard tandem micromorph cell deposited onto a flat substrate.