Sol-Gel Titanium Dioxide Blocking Layers for Dye-Sensitized Solar Cells: Electrochemical Characterization

Efficient Perovskite Solar Cells Based on Tin Oxide Nanocrystals with Difunctional Modification

Tin oxide (SnO₂ ) nanocrystals-based electron transport layer (ETL) has been widely used in perovskite solar cells due to its high charge mobility and suitable energy band alignment with perovskite, but the high surface trap density of SnO₂ nanocrystals harms the electron transfer and collection within device. Here, an effective method to achieve a low trap density and high electron mobility ETL based on SnO₂ nanocrystals by devising a difunctional additive of potassium trifluoroacetate (KTFA) is proposed. KTFA is added to the SnO₂ nanocrystals solution, in which trifluoroacetate ions could effectively passivate the oxygen vacancies (O_V ) in SnO₂ nanocrystals through binding of TFA^- and Sn⁴⁺ , thus reducing the traps of SnO₂ nanocrystals to boost the electrons collection in the solar cell. Furthermore, the conduction band of SnO₂ nanocrystals is shifted up by surface modification to close to that of perovskite, which facilitates electrons transfer because of the decreased energy barrier between ETL and perovskite layer. Benefiting from the decreased trap density and energy barrier, the perovskite solar cells exhibit a power conversion efficiency of 21.73% with negligible hysteresis.

Understanding equivalent circuits in perovskite solar cells. Insights from drift-diffusion simulation

Perovskite solar cells (PSCs) have reached impressively high efficiencies in a short period of time; however, the optoelectronic properties of halide perovskites are surprisingly complex owing to the coupled ionic-electronic charge carrier dynamics. Electrical impedance spectroscopy (EIS) is a widely used characterization tool to elucidate the mechanisms and kinetics governing the performance of PSCs, as well as of many other semiconductor devices. In general, equivalent circuits are used to evaluate EIS results. Oftentimes these are justified via empirical constructions and the real physical meaning of the elements remains disputed. In this perspective, we use drift-diffusion numerical simulations of typical thin-film, planar PSCs to generate impedance spectra avoiding intrinsic experimental difficulties such as instability and low reproducibility. The ionic and electronic properties of the device, such as ion vacancy density, diffusion coefficients, recombination mechanism, etc., can be changed individually in the simulations, so their effects can be directly observed. We evaluate the resulting EIS spectra by comparing two commonly used equivalent circuits with series and parallel connections respectively, which result in two signals with significantly different time constants. Both circuits can fit the EIS spectra and by extracting the values of the elements of one of the circuits, the values of the elements of the other circuit can be unequivocally obtained. Consequently, both can be used to analyse the EIS of a PSC. However, the physical meaning of each element in each circuit could differ. EIS can produce a broad range of physical information. We analyse the physical interpretation of the elements of each circuit and how to correlate the elements of one circuit with the elements of the other in order to have a direct picture of the physical processes occurring in the device.

Role of Surface Coverage and Film Quality of the TiO2 Electron Selective Layer for Optimal Hole-Blocking Properties

Titanium dioxide (TiO2) is a commonly used electron selective layer in thin-film solar cells. The energy levels of TiO2 align well with those of most light-absorbing materials and facilitate extracting electrons while blocking the extraction of holes. In a device, this separates charge carriers and reduces recombination. In this study, we have evaluated the hole-blocking behavior of TiO2 compact layers using charge extraction by linearly increasing voltage in a metal-insulator-semiconductor structure (MIS-CELIV). This hole-blocking property was characterized as surface recombination velocity (S R) for holes at the interface between a semiconducting polymer and TiO2 layer. TiO2 layers of different thicknesses were prepared by sol-gel dip coating on two transparent conductive oxide substrates with different roughnesses. Surface coverage and film quality on both substrates were characterized using X-ray photoelectron spectroscopy and atomic force microscopy, along with its conductive imaging mode. Thicker TiO2 coatings provided better surface coverage, leading to reduced S R, unless the layers were otherwise defective. We found S R to be a more sensitive indicator of the overall film quality, as varying S R values were still observed among the films that looked similar in their characteristics via other methods.

Solution-processed two-dimensional materials for next-generation photovoltaics

In the ever-increasing energy demand scenario, the development of novel photovoltaic (PV) technologies is considered to be one of the key solutions to fulfil the energy request. In this context, graphene and related two-dimensional (2D) materials (GRMs), including nonlayered 2D materials and 2D perovskites, as well as their hybrid systems, are emerging as promising candidates to drive innovation in PV technologies. The mechanical, thermal, and optoelectronic properties of GRMs can be exploited in different active components of solar cells to design next-generation devices. These components include front (transparent) and back conductive electrodes, charge transporting layers, and interconnecting/recombination layers, as well as photoactive layers. The production and processing of GRMs in the liquid phase, coupled with the ability to "on-demand" tune their optoelectronic properties exploiting wet-chemical functionalization, enable their effective integration in advanced PV devices through scalable, reliable, and inexpensive printing/coating processes. Herein, we review the progresses in the use of solution-processed 2D materials in organic solar cells, dye-sensitized solar cells, perovskite solar cells, quantum dot solar cells, and organic-inorganic hybrid solar cells, as well as in tandem systems. We first provide a brief introduction on the properties of 2D materials and their production methods by solution-processing routes. Then, we discuss the functionality of 2D materials for electrodes, photoactive layer components/additives, charge transporting layers, and interconnecting layers through figures of merit, which allow the performance of solar cells to be determined and compared with the state-of-the-art values. We finally outline the roadmap for the further exploitation of solution-processed 2D materials to boost the performance of PV devices.

Pulsed Laser Fabrication of TiO2 Buffer Layers for Dye Sensitized Solar Cells

We report on the fabrication of dye-sensitized solar cells with a TiO₂ buffer layer between the transparent conductive oxide substrate and the mesoporous TiO₂ film, in order to improve the photovoltaic conversion efficiency of the device. The buffer layer was fabricated by pulsed laser deposition whereas the mesoporous film by the doctor blade method, using TiO₂ paste obtained by the sol–gel technique. The buffer layer was deposited in either oxygen (10 Pa and 50 Pa) or argon (10 Pa and 50 Pa) onto transparent conducting oxide glass kept at room temperature. The cross-section scanning electron microscopy image showed differences in layer morphology and thickness, depending on the deposition conditions. Transmission electron microscopy studies of the TiO₂ buffer layers indicated that films consisted of grains with typical diameters of 10 nm to 30 nm. We found that the photovoltaic conversion efficiencies, determined under standard air mass 1.5 global (AM 1.5G) conditions, of the solar cells with a buffer layer are more than two times larger than those of the standard cells. The best performance was reached for buffer layers deposited at 10 Pa O₂. We discuss the processes that take place in the device and emphasize the role of the brush-like buffer layer in the performance increase.

n-Type phosphorus-doped nanocrystalline diamond: electrochemical and in situ Raman spectroelectrochemical study

Electrochemical and in situ Raman spectroelectrochemical characterization of n-type phosphorus-doped nanocrystalline diamond (P-NCD) is carried out.

Mesenchymal stem cell growth behavior on micro/nano hierarchical surfaces of titanium substrates

Surface topography of an orthopedic implant plays an essential role in the regulation of bone formation with surrounding bone tissue. To investigate the effects of surface topography of titanium (Ti) substrates on cellular behavior of mesenchymal stem cells (MSCs), a series of micro/nano hierarchical structures were fabricated onto micro-structured titanium (Micro-Ti) substrates via a sol-gel method with spin-coat technique. Scanning electron microscopy (SEM), surface profiler, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and water contact angle measurement were employed to certify the successful fabrication of micro/nano hierarchical structures with the presence of various nano-sized TiO2 grains (20 nm, 40 nm and 80 nm, respectively) onto micro-structured surfaces. The formation mechanism of the micro/nano hierarchical structures was proposed. Moreover, the effects of those hierarchical structures on the growth behavior of MSCs were evaluated both on cellular and molecular levels in vitro. The results confirmed that micro/nano hierarchical structures with large grains (80 nm) greatly promoted the proliferation and differentiation of MSCs comparing with other small grains (20 nm and 40 nm). The study provides an alternative for the fabrication of hierarchically structured Ti implants for potential orthopedic application.

Role of temperature in the recombination reaction on dye-sensitized solar cells

The DSC recombination kinetics was studied up to 100 °C without electrolyte leakage or external contamination using a unique laser assisted glass sealing process. In addition to temperature having a strong influence on the recombination reaction rate, the energy of the exited electrons was found to be critical for determining the recombination activation energy.

Visible-light sensitization of boron-doped nanocrystalline diamond through non-covalent surface modification

The boron-doped diamond electrode is efficiently sensitized with a light-harvesting molecule using a facile and versatile synthetic method.

Strong Photocurrent Amplification in Perovskite Solar Cells with a Porous TiO2 Blocking Layer under Reverse Bias

We investigate two different types of TiO2 blocking layer (BL) deposition techniques commonly used in solid-state methylammonium lead triiodide perovskite (MaPbI3)-based solar cells. Although these BLs lead to similar photovoltaic device performance, their structure and blocking capability is actually very different. In one case, the "blocking" layer is porous, allowing an intimate contact of the perovskite with the fluorine-doped tin-dioxide (FTO)-covered glass substrate serving as transparent electron collector. This interface between the perovskite and the FTO shows rectifying behavior. Reverse biasing of such a solar cell allows the determination of the valence-band position of the MaPbI3 and the theoretical maximum attainable photovoltage. We show that under reverse bias strong photocurrent amplification is observed, permitting the cell to work as a high-gain photodetector at low voltage. Without BL, the solar-cell performance decreased, but the photocurrent amplification increased. At 1 V reverse bias, the photocurrent amplification is above a factor of 10 for AM 1.5 solar light and over 100 for lower light intensities.