Bhaskar Dudem

CH3NH3PbI3 planar perovskite solar cells with antireflection and self-cleaning function layers

We report CH3NH3PbI3 planar perovskite solar cells with multifunctional inverted micro-pyramidal structured (IMPS) polydimethylsiloxane (PDMS) antireflection (AR) layers for enhancing the device efficiency. These IMPS-PDMS films were fabricated via a facile and cost-effective soft lithography using micro-pyramidal structured silicon (Si) master molds formed by alkaline anisotropic wet-etching treatment of (100)-oriented monocrystalline Si substrates. The IMPS-PDMS laminated on the bare glass (i.e., IMPS-PDMS/glass) exhibited a higher solar weighted transmittance (TSW) value of ∼95.2% (or the lowest solar weighted reflectance (RSW) of ∼4.7%) than those of the bare glass and flat-PDMS/glass, i.e., TSW/RSW ∼ 90.7/9.1 and 91.5/8.2%, respectively. Additionally, it showed a much higher average haze ratio (HA) value of ∼93.1% compared to the bare glass and flat-PDMS/glass (i.e., HA ∼ 1.6 and 2.8%, respectively). By employing the IMPS-PDMS onto the outer surface of CH3NH3PbI3 planar perovskite solar cells as an AR layer, an improved short-circuit current density (Jsc) value of 21.25 mA cm−2 was obtained, as compared to the reference device and the device with flat-PDMS (i.e., Jsc = 20.57 and 20.87 mA cm−2, respectively), while showing the almost same Voc and FF values as those of the reference device. As a result, the power conversion efficiency was improved from 17.17 and 17.42% for the reference and flat-PDMS devices, respectively, to 17.74% for the IMPS-PDMS device. Also, the fluorooctyltrichlorosilane-treated IMPS-PDMS surface revealed a superhydrophobic behavior with a water contact angle of ∼150° which is useful for self-cleaning applications in outdoor environments.

A multifunctional hierarchical nano/micro-structured silicon surface with omnidirectional antireflection and superhydrophilicity via an anodic aluminum oxide etch mask

We fabricated hierarchical nano/micro (HNM) architectures on a silicon (Si) surface for efficient antireflection. To form the micropyramid (MP) structures on the Si surface, a potassium hydroxide-based wet etching process was carried out. Meanwhile, for the nanostructures (NS), an anodic aluminum oxide (AAO) film with nanopores as an etch mask was used, followed by an inductive coupled plasma (ICP) etching process. To obtain optimized structures with efficient antireflection, the etching was performed under different process conditions including RF power, ICP power, process pressure, gas flow rate, and etching time. The AAO/NS-Si largely reduced the reflectivity of bare Si over a wide wavelength range of 300–1100 nm, showing an average reflectance (Ravg) value of 3.4% (i.e., Ravg = 38% for the bare Si). The AAO/HNM-Si consisting of NS/MP arrays had a lower reflectance spectrum than 2% at wavelengths of 300–1050 nm, exhibiting an Ravg value of 1.5%. Superior antireflection characteristics were also observed in the wide light incident angle range of 20–70° at wavelengths of 300–1100 nm. For theoretical analysis, reflectance calculations were also performed by a rigorous coupled-wave analysis simulation, which indicated a similar trend to the experimental results. For surface wetting behavior, it revealed a superhydrophilic surface with water contact angles of < 5°.

Multifunctional polymers with biomimetic compound architectures via nanoporous AAO films for efficient solar energy harvesting in dye-sensitized solar cells

We report the considerable enhancement of the solar power conversion efficiency (PCE) in dye-sensitized solar cells (DSSCs) using polydimethylsiloxane (PDMS) patterned with a novel biomimetic compound architecture (CA) (i.e., hierarchical nanobumps/microcone arrays) with light-harvesting and self-cleaning functions as a protective cover-layer. The CA-PDMS is transferred from a nanoporous anodic alumina oxide mold by a facile and cost-effective soft imprint lithography via a microcone-patterned sapphire substrate. A lamination of CA-PDMS on the glass leads to increased total and diffuse transmittance properties (i.e., antireflection and light scattering effects), simultaneously, compared to bare glass over a wide wavelength range of 350–800 nm, exhibiting a much larger solar weighted total transmittance (TSW) value of ∼94% and an average haze ratio (HA) value of ∼49.7% (TSW ≈ 90.4% and HA ≈ 1.4% for bare glass). In addition, sand grains on the hydrophobic surface with a water contact angle of ∼134° are clearly washed by rolling down water droplets (i.e., self-cleaning effect). To simply demonstrate the device applicability, CA-PDMS is introduced onto an outer surface of the front glass substrate in a DSSC. The resulting DSSC with CA-PDMS exhibits a boosted PCE value of ∼8.24% mainly due to a strongly increased short-circuit current density (JSC) value of ∼18.11 mA cm−2 compared to the reference DSSC with bare glass (PCE ≈ 7.45% and JSC ≈ 16.37 mA cm−2) under AM1.5G illumination, indicating a large PCE enhancement percentage value of ∼10.7%.

Thermal-tolerant polymers with antireflective and hydrophobic grooved subwavelength grating surfaces for high-performance optics

We report thermal-tolerant polymer films integrated with antireflective and hydrophobic grooved subwavelength gratings (G-SWGs), which are fabricated by a soft imprint lithography from silicon molds with conical SWGs, for high-performance optics. For the poly-dimethylsiloxane (PDMS) polymer films with the G-SWGs at one- and both-side surfaces, their optical properties are investigated for different periods, exhibiting efficient broadband and wide-angle antireflection behavior with light scattering. To simply demonstrate their feasibility in optical and optoelectronic systems, the one-side G-SWGs PDMS film with a short period of 380 nm is laminated on indium tin oxide-coated glass (i.e., ITO glass) and polyethylene terephthalate (PET) substrates. This film effectively enhances the transmittance (or suppresses the reflectance) of both the bare ITO glass and PET substrates in wide ranges of wavelengths and incident angles. Additionally, it shows not only a relatively strong thermal durability at temperatures less than 180 °C, but also a hydrophobic surface with high water contact angle of around 140° (i.e., self-cleaning ability).

Triboelectric nanogenerators with gold-thin-film-coated conductive textile as floating electrode for scavenging wind energy

We report triboelectric nanogenerators (TENGs) composed of a flexible and cost-effective gold-coated conductive textile (CT) to convert wind energy into electricity. The Au-coated CT is employed because of its high surface roughness resulting from Au nanodots distributed on microsized fibers. Thus, the Au-coated CT with nano/microarchitecture plays an important role in enhancing the effective contact area as well as the output performance of the TENG. Moreover, the surface roughness of the Au-coated CT is controlled by adjusting the Au thermal deposition time or tailoring the diameter of the Au nanodots. At an applied wind speed of 10 m·s–1, a wind-based TENG (W-TENG) with dimensions of 75 mm × 12 mm × 25 mm produces an open-circuit voltage (VOC) of ∼39 V and a short-circuit current (ISC) of ∼3 μA by using the airflow-induced vibrations of an optimized Au-coated CT between two flat polydimethylsiloxane (PDMS) layers. To further specify the device performance, the electric output of the W-TENG is analyzed by varying several parameters such as the distance between the PDMS layer and Au-coated CT, applied wind speed, external load resistance, and surface roughness of the PDMS layers. Introducing an inverse micropyramid architecture on the PDMS layers further improves the output performance of the W-TENG, which exhibits the highest VOC (∼49 V) and ISC (∼5 μA) values at an applied wind speed of 6.8 m·s−1. Additionally, the reliability of the W-TENG is also tested by measuring its output current during long-term cyclic operation. Furthermore, the rectified output signals observed by the W-TENG device are used as a direct power source to light 45 green commercial light-emitting diodes connected in series and also to charge capacitors (100 and 4.7 μF). Finally, the output performance of the W-TENG device in an actual windy situation is also investigated.

Hierarchical structured polymers for light-absorption enhancement of silicon-based solar power systems

Light-absorption enhancement of silicon (Si)-based solar power systems (i.e., solar modules) is reported by employing a ultraviolet-curable polymer (i.e., NOA63) film integrated with a hierarchical structure (HS) consisting of nanonipples on micropyramids onto the PET cover as a protective antireflection (PAR) layer. The HS-patterns can be easily transferred from Si molds with the HS via poly-dimethylsiloxane stamps by soft lithography. The HS-NOA63 film increases the total and diffuse transmittances of the bare polyethylene terephthalate (PET), simultaneously, at wavelengths of 380–1100 nm, thus resulting in the reduced surface reflection (i.e., lower solar weighted reflectance (Rsw) of 6.1%) and the strong light scattering (i.e., much higher average haze ratio (Havg) of 90.7%) compared to the bare PET (i.e., Rsw = 11.7% and Havg = 6.9%). Consequently, the Si solar modules with the HS-NOA63 PAR layer of the PET cover exhibit a superior solar power generation in a wide incident angle range (0–70°) of solar irradiance, especially, showing a relative increment percentage of 6.3% in power conversion efficiency (PCE) at 0° (i.e., from PCE = 13.46 to 14.3%). Also, it has a superhydrophobic surface with a high water contact angle of 150°, which leads to a self-cleaning ability to protect Si solar modules from being polluted by water droplets and dust particles in outdoor applications.

Fabrication and optical characterization of hybrid antireflective structures with zinc oxide nanorods/micro pyramidal silicon for photovoltaic applications

We investigated the hybrid antireflective (AR) structures with zinc oxide nanorods (ZnO NRs) grown on micro pyramidal silicon (MP-Si) structures. The MP-Si structures were fabricated by a simple and cost-effective anisotropic wet chemical etching technique under different ratios between potassium hydroxide (KOH) and isopropyl alcohol (IPA). For the MP-Si structures etched at 10:9 vol% of KOH and IPA, the relatively low average reflectance (Ravg) value of 14.5% was obtained in the wavelength range of 300-1100 nm. Its solar weighted reflectance (SWR) value was also estimated to be 12.6% in the wavelength range of 400-1100 nm. The ZnO NRs were hydrothermally grown on the MP-Si structures at various solution concentrations. For the ZnO NRs (25 mM)/MP-Si, the Ravg and SWR values were further decreased to 3.6% and 3.8%, respectively. The omnidirectional AR behavior was also observed in the wide incident angle range of 20-70°. The hybrid ZnO NRs/MP-Si AR structures revealed a superhydrophilic surface with water contact angles of < 5 o.

Improved light harvesting efficiency of semitransparent organic solar cells enabled by broadband/omnidirectional subwavelength antireflective architectures

We report organic solar cells (OSCs) with subwavelength architectured polydimethylsiloxane (SWA-PDMS) as an antireflective (AR) layer on a glass substrate for not only enhancing the efficiency but also increasing the transparency of the devices. These subwavelength architectures (SWAs) on PDMS layers are fabricated using a soft imprint lithography technique via an anodic aluminum oxide mold. The effect of optical characteristics of SWA-PDMS with respect to the period and diameter of SWAs, along with the theoretical analysis using rigorous coupled-wave analysis simulation, is investigated. Consequently, the SWA-PDMS/glass with a period and diameter of 125 nm and 80 nm, respectively, is obtained as the optimal sample, exhibiting the highest average transmittance (Tavg) of ∼95.2%, which is much higher compared to that of bare glass (Tavg ∼92.08%). By employing the optimal SWA-PDMS on the glass surface of opaque and semitransparent OSCs as an AR layer, their power conversion efficiency is improved from 8.67 to 10.59% and 7.07 to 8.52%, respectively. Additionally, the average visible transmittance (AVT) of the semitransparent OSC is increased from 23.0 to 26.2% with significantly improved color coordinates as the AR layer is employed. Also, the OSCs with SWA-PDMS having a relatively high hydrophobic nature exhibit a stable performance in the ambient environment.

Enhancing the output performance of hybrid nanogenerators based on Al-doped BaTiO3 composite films: a self-powered utility system for portable electronics

Enhancing the output performance of nanogenerators using composite films consisting of a piezoelectric material embedded into polymers has gained much attention over the last few years. Such composite films can provide a high surface charge density and dielectric permittivity, which can further efficiently enhance the performance of nanogenerators. We, for the first time, employed aluminum (Al)-doped barium titanate (BaTiO3; ABTO) particles to enhance the performance of nanogenerators. These ABTO particles were synthesized via a solid-state technique, and the effect of Al dopant concentration on their crystallinity and ferroelectric properties was systematically investigated. However, the BTO particles with 2% Al dopant concentration exhibited a high remnant polarization and piezoelectric coefficient, and they were further employed to efficiently enhance the output performance of the hybrid piezo/triboelectric nanogenerators. For this, these ABTO particles were first mixed with polydimethylsiloxane (PDMS) to prepare a composite film. Next, the ABTO/PDMS composite film was employed as a piezoelectric material and triboelectric material of the hybrid nanogenerator (HNG) and exhibited a high output performance owing to their synergetic effects. In addition, the influence of the surface roughness of the composite film on the performance of the HNG was also investigated and optimized. Consequently, the HNG device with the rough surface ABTO/PDMS composite film exhibited maximal open-circuit voltage, short-circuit current, and power density values of ∼945 V, ∼59.8 μA, and ∼42.4 W m−2, respectively. For practical device application, the stable and high electrical power generated from the HNG device was employed to light several light-emitting diodes and power portable electronic devices.

Biomimetic nano/micro double-textured silicon with outstanding antireflective and super-hydrophilic surfaces for high optical performance

We report the fabrication of nano/micro double-textured silicon (NMDT-Si) and its structural, optical, and surface wetting properties. The micro-pyramidal textured (MPTs) are formed on the Si surface by a simple potassium hydroxide-based wet etching process. On the other hand, for pillar-arrayed nano-textures (NTs), the thermally-dewetted gold (Au) nanoparticles are employed on the surface of the MPT-Si as an etch mask and the inductively coupled plasma etching is followed. The optical reflectance of the NMDT-Si is strongly dependent on the period and height of NTs on the surface of the MPT-Si, which can be controlled by the Au film thickness and etching time, respectively. Compared with the planar nano-textured Si, the NMDT-Si shows superior antireflection (or higher light absorption) and light-scattered propagation behaviors, which are verified from a finite-difference time-domain simulation, over wide ranges of wavelengths (350–1100 nm) and incident angles (0–70°), resulting in the average reflectance of ∼2.1% and the solar weighted absorption of ∼98.5% at normal incidence, respectively. In addition, it has a super-hydrophilic surface with water contact angles of <5°.