Pingqi Gao

Large-Area Nanosphere Self-Assembly by a Micro-Propulsive Injection Method for High Throughput Periodic Surface Nanotexturing

A high throughput surface texturing process for optical and optoelectric devices based on a large-area self-assembly of nanospheres via a low-cost micropropulsive injection (MPI) method is presented. The novel MPI process enables the formation of a well-organized monolayer of hexagonally arranged nanosphere arrays (NAs) with tunable periodicity directly on the water surface, which is then transferred onto the preset substrates. This process can readily reach a throughput of 3000 wafers/h, which is compatible with the high volume photovoltaic manufacturing, thereby presenting a highly versatile platform for the fabrication of periodic nanotexturing on device surfaces. Specifically, a double-sided grating texturing with top-sided nanopencils and bottom-sided inverted-nanopyramids is realized in a thin film of crystalline silicon (28 μm in thickness) using chemical etching on the mask of NAs to significantly enhance antireflection and light trapping, resulting in absorptions nearly approaching the Lambertian limit over a broad wavelength range of 375-1000 nm and even surpassing this limit beyond 1000 nm. In addition, it is demonstrated that the NAs can serve as templates for replicas of three-dimensional conformal amorphous silicon films with significantly enhanced light harvesting. The MPI induced self-assembly process may provide a universal and cost-effective solution for boosting light utilization, a problem of crucial importance for ultrathin solar cells.

Realization of 13.6% Efficiency on 20 μm Thick Si/Organic Hybrid Heterojunction Solar Cells via Advanced Nanotexturing and Surface Recombination Suppression

Hybrid silicon/polymer solar cells promise to be an economically feasible alternative energy solution for various applications if ultrathin flexible crystalline silicon (c-Si) substrates are used. However, utilization of ultrathin c-Si encounters problems in light harvesting and electronic losses at surfaces, which severely degrade the performance of solar cells. Here, we developed a metal-assisted chemical etching method to deliver front-side surface texturing of hierarchically bowl-like nanopores on 20 μm c-Si, enabling an omnidirectional light harvesting over the entire solar spectrum as well as an enlarged contact area with the polymer. In addition, a back surface field was introduced on the back side of the thin c-Si to minimize the series resistance losses as well as to suppress the surface recombination by the built high-low junction. Through these improvements, a power conversion efficiency (PCE) up to 13.6% was achieved under an air mass 1.5 G irradiation for silicon/organic hybrid solar cells with the c-Si thickness of only about 20 μm. This PCE is as high as the record currently reported in hybrid solar cells constructed from bulk c-Si, suggesting a design rule for efficient silicon/organic solar cells with thinner absorbers.

Categorization of resistive switching of metal-Pr0.7Ca0.3MnO3-metal devices

Resistive switching (RS) characteristics of a Pr0.7Ca0.3MnO3 (PCMO) film sandwiched between a Pt bottom electrode and top electrodes (TE) made of various metals are found to belong to two categories. Devices with TE made of Al, Ti, and Ta exhibit a large I-V hysteresis loop and bipolar RS, but those with TE made of Pt, Ag, Au, and Cu do not. Transmission electron microscopy reveals that a thin metal-oxide layer formed at the interface between the former group of TE and PCMO, but not for the latter group of TE. Analysis shows that the categorization depends on the Gibbs free energy of oxidation of the TEs with respect to that of PCMO.

Improvement of the SiOx Passivation Layer for High-Efficiency Si/PEDOT:PSS Heterojunction Solar Cells

Interfacial properties currently hinder the performance of Si/organic heterojunction solar cells for an alternative to high-efficiency and low-cost photovoltaics. Here, we present a simple and repeatable wet oxidation method for developing the surface passivation layer, SiOx, on the Si surface for the fabrication of high-efficiency Si/poly(3,4-ethylene-dioxythiophene):polystyrenesulfonate (PEDOT:PSS) heterojunction solar cells. The uniform and dense SiOx thin layer introduced by the oxidizing aqueous solution of H2O2 or HNO3 provided the better surface passivation and stronger wettability of the Si surface, compared to those in the native oxide case. These two types of progress helped create a lower defect density at the Si/PEDOT:PSS interface and thus a high-quality p-n junction with a lower interface recombination velocity. As a result, the HNO3-oxidized device displayed better performance with a power conversion efficiency (PCE) of 11%, representing a 28.96% enhancement from the PCE of 8.53% in the native oxide case. The effects on the performance of the Si/PEDOT:PSS hybrid solar cells of the wet oxidation treatment procedure, including the differences in surface roughness and wettability of the Si substrate, the quality and thickness of the SiOx, etc., were explored extensively. Such a simple and controllable oxidizing treatment could be an effective way to promote the interfacial properties that are an important cornerstone for more efficient Si/organic hybrid solar cells.

Photoconducting response on bending of individual ZnO nanowires

The bending effect of individual zinc oxide (ZnO) nanowires on photoconducting behavior has been investigated by an in situtransmission electron microscopy (TEM) method. By increasing the nanowire bending, the photocurrent of ZnO nanowire under ultraviolet illumination drops dramatically and the photoresponse time becomes much shorter. A possible mechanism has been proposed and discussed. The improved photo response performance by bending ZnO nanowires could be of significance for their optoelectronics and sensor applications.

Ambipolar to Unipolar Conversion in Graphene Field-Effect Transistors

Typical graphene field-effect transistors (GFETs) show ambipolar conduction that is unfavorable for some electronic applications. In this work, we report on the development of unipolar GFETs. We found that the titanium oxide situated on the graphene surface induced significant hole doping. The threshold voltage of the unipolar p-type GFET was tunable by varying the density of the attached titanium oxide through an etching process. An annealing process followed by silicon nitride passivation was found to convert the p-type GFETs to unipolar n-type GFETs. An air-stable complementary inverter integrated from the p- and n-GFETs was also successfully demonstrated. The simple fabrication processes are compatible with the conventional CMOS manufacturing technology.

In situ probing electrical response on bending of ZnO nanowires inside transmission electron microscope

In situ electrical transport measurements on individual bent ZnO nanowires have been performed inside a high-resolution transmission electron microscope, where the crystal structures of ZnO nanowires were simultaneously imaged. A series of consecutively recorded current-voltage (I-V) curves along with an increase in nanowire bending show the striking effect of bending on their electrical behavior. The bending-induced changes of resistivity, electron concentration, and carrier mobility of ZnO nanowires have been retrieved based on the experimental I-V data, which suggests the applications of ZnO nanowires as nanoelectromechanical sensors.

Silicon/Organic Hybrid Solar Cells with 16.2% Efficiency and Improved Stability by Formation of Conformal Heterojunction Coating and Moisture‐Resistant Capping Layer

Silicon/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) heterojunction solar cells with 16.2% efficiency and excellent stability are fabricated on pyramid-textured silicon substrates by applying a water-insoluble ester as capping layer. This shows that a conformal coating of PEDOT:PSS on textured silicon can greatly improve the junction quality with the main stability failure routes related to the moisture-induced poly(3,4-ethylenedioxythiophene) aggregations and the tunneling silicon oxide autothickening.

Efficient light trapping in low aspect-ratio honeycomb nanobowl surface texturing for crystalline silicon solar cell applications

We report a significant reflection reduction over a broadband light spectrum in crystalline silicon via introduction of low aspect-ratio honeycomb nanobowl front surface textures. A restructuration technique is developed to shape nanopores into nanobowls, enabling excellent impedance matching and efficient mode coupling. As a result, an overall reflection down to 2% in the spectrum range of 400–1 100 nm wavelength is achieved. In comparison to nanopores-structured light-trapping configurations, the nanobowls-textures have much smaller parasitic surface area, which mitigates the surface recombination losses. The texturing technique offers a promising approach to high efficiency c-Si thin-film solar cells.

High-Efficiency Silicon/Organic Heterojunction Solar Cells with Improved Junction Quality and Interface Passivation

Silicon/organic heterojunction solar cells (HSCs) based on conjugated polymers, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), and n-type silicon (n-Si) have attracted wide attention due to their potential advantages of high efficiency and low cost. However, the state-of-the-art efficiencies are still far from satisfactory due to the inferior junction quality. Here, facile treatments were applied by pretreating the n-Si wafer in tetramethylammonium hydroxide (TMAH) solution and using a capping copper iodide (CuI) layer on the PEDOT:PSS layer to achieve a high-quality Schottky junction. Detailed photoelectric characteristics indicated that the surface recombination was greatly suppressed after TMAH pretreatment, which increased the thickness of the interfacial oxide layer. Furthermore, the CuI capping layer induced a strong inversion layer near the n-Si surface, resulting in an excellent field effect passivation. With the collaborative improvements in the interface chemical and electrical passivation, a competitive open-circuit voltage of 0.656 V and a high fill factor of 78.1% were achieved, leading to a stable efficiency of over 14.3% for the planar n-Si/PEDOT:PSS HSCs. Our findings suggest promising strategies to further exploit the full voltage as well as efficiency potentials for Si/organic solar cells.