Reduction In Charge Carrier Recombination Research Articles

Effective Steady‐State Recombination Decay Times in Comparison to Time‐Resolved Photoluminescence Decay Times in Halide Perovskite Solar Cells

One of the key topics in perovskite solar cells is the reduction of charge carrier recombination, with the aim of increasing power conversion efficiency. The recombination lifetime is a commonly used tool, as it directly affects the current–voltage curve via the diffusion length. The lifetime is often estimated using time‐domain measurement methods such as time‐resolved photoluminescence. However, two obstacles emerge when applying the transiently measured decay times to the steady‐state theory. In general, the decay time depends on the charge carrier concentration, and it is often not clear under which conditions the transient measurement must be conducted to be comparable with the steady‐state performance of the device. Furthermore, diffusion and capacitive effects due to charge injection and extraction can influence transient techniques and cause the measured decay time to deviate from the sought‐after recombination lifetime. Voltage‐dependent steady‐state photoluminescence measurements can be used to estimate the internal voltage during device operation and allow the extraction of collection efficiencies and effective steady‐state decay times that are independent of transport and capacitive effects. Here, the differences between the steady‐state and transient decay times are identified and discussed, and the losses in the current–voltage curve caused by extraction issues are quantified.

Optimizing charge transport and band-offset in silicon heterojunction solar cells: impact of TiO2 contact deposition temperature

Carrier selective contacts are a primary requirement for fabricating silicon heterojunction solar cells (SHSCs). TiO2 is a prominent carrier selective contact in SHSCs owing to its excellent optoelectronic features such as suitable band offset, work function, and cost-effectiveness. Herein, we fabricated simple SHSCs in an Al/TiO2/p-Si/Ti/Au device configuration. Ultrathin 3 nm TiO2 layers were deposited onto a p-type silicon substrate using the atomic layer deposition method. The deposition temperature of TiO2 layers varied from 100 °C to 250 °C. X-ray photoelectron spectroscopic studies suggest that deposition temperature highly affects the chemical states of TiO2 and reduces the formation of defective state densities at the Fermi energy. The optical band gap values of TiO2 layers are also altered from 3.13 eV to 3.27 eV when the deposition temperature increases. The work function tuning from −5.13 eV to −4.83 eV has also been observed in TiO2 layers, suggesting the variation in Fermi level tuning, which arises due to changes in carrier concentrations at higher temperatures. Several device parameters, such as ideality factor, trap density, reverse saturation current density, barrier height, etc, have been quantified to comprehend the effects of deposition temperature on photovoltaic device performance. The results suggest that the deposition temperature significantly influences the charge transport and device performance. At an optimum temperature, a significant reduction in charge carrier recombination and trap state density has been observed, which helps to improve power conversion efficiency.

Lanthanum anchored NiCo2S4 nanocomposite and their structural, optical, and enhanced photodegradation of Indigo Carmine (InC) dye

The focus of this study is to explore the potential of lanthanum-anchored NiCO2S4 (L-NCS) as a photocatalyst for Indigo carmine (InC) dye under visible radiation. Different analytical techniques including XRD, SEM, TEM, and PL were used to confirm the uniform distribution of lanthanum on NiCO2S4. A study of the optical and photochemical properties of L-NCS revealed a reduction in charge carrier recombination and an increase in visible light sensitivity due to the introduction of soft metal dopants into NCS. The optimized L-NCS catalyst showed a significant increase in photodegradation rate, about 7.2 times higher than that of pure NCS when exposed to visible light. Furthermore, the reproducibility of L-NCS shows excellent stability as a photocatalyst for environmental remediation.

Controlling Charge Carrier Dynamics in Porphyrin Nanorings by Optically Active Templates.

Understanding the dynamics of photogenerated charge carriers is essential for enhancing the performance of solar and optoelectronic devices. Using atomistic quantum dynamics simulations, we demonstrate that a short π-conjugated optically active template can be used to control hot carrier relaxation, charge carrier separation, and carrier recombination in light-harvesting porphyrin nanorings. Relaxation of hot holes is slowed by 60% with an optically active template compared to that with an analogous optically inactive template. Both systems exhibit subpicosecond electron transfer from the photoactive core to the templates. Notably, charge recombination is suppressed 6-fold by the optically active template. The atomistic time-domain simulations rationalize these effects by the extent of electron and hole localization, modification of the density of states, participation of distinct vibrational motions, and changes in quantum coherence. Extension of the hot carrier lifetime and reduction of charge carrier recombination, without hampering charge separation, demonstrate a strategy for enhancing efficiencies of energy materials with optically active templates.

Reduction of charge carrier recombination by Ce gradient doping and surface polarization for solar water splitting

Photoelectrochemical (PEC) water splitting is one of the most strategies for solar energy storage and utilization. Hematite as one of the promising candidates for PEC research, has a short hole diffusion length and short carrier lifetime, resulting in PEC conversion efficiency far below the theoretical value. In this work, a facile but efficient strategy was introduced for the growth of gradient Ce doped hematite nanorods, following with the sequent photoactivation process. The gradient Ce doping not only increased the conductivity of bulk material, but also importantly, constructed a built-in electric field by the addition of Ce element, which remarkably enhanced the charge separation and transfer from the bulk to the surface. Moreover, the surface polarization under illumination was applied for the reduction of overpotential and the enhancement of photocurrent density, taking a role of cocatalyst decoration. It reduced the oxygen vacancies concentration at the surface, which further reduced the surface recombination owing to the passivation from photoactivation. According to these, the resulting photoanode exhibited a photocurrent of 1.92 and 2.50 mA cm−2 at 1.23 and 1.50 VRHE, respectively, with onset potential of 0.64 VRHE under AM 1.5G illumination. This work opens a useful strategy of modifying the electronic structure and constructing the interface structure for improving PEC performance on hematite-based photoelectrodes for solar energy storage and utilization.

The comparison of ZnO/polyaniline nanocomposite under UV and visible radiations for decomposition of metronidazole: Degradation rate, mechanism and mineralization

ZnO/polyaniline (ZnO/PANI) nanocomposite was produced through aniline in-situ chemical polymerization method on ZnO nanostructure. Different techniques including FT-IR, FE-SEM, EDX, DRS and TGA were employed to identify the composition and structure of the nanocomposites. The results confirmed the good quality of produced ZnO/PANI nanocomposite so that, it could be activated under UV and visible radiations. Photocatalytic activity of the nanocomposites for the degradation of metronidazole (MNZ) under UV and visible radiations as a function of time, MNZ concentration, catalyst dose, solution pH and catalyst stability was evaluated for several stages of the process. The maximum MNZ degradation (97%) under UV and visible radiations was happened over 120 and 150min, respectively and 1.0mgL−1 ZnO/PANI nanocomposite, 10mgL−1 MNZ concentration at pH 7.0. In the optimal condition, kinetic studies, COD, TOC, AOS, reusability test and predicted degradation mechanism were considered in the present study. The photocatalytic activity of ZnO/PANI nanocomposite was higher than ZnO under UV and visible radiations. The constant degradation rate of MNZ by ZnO/PANI nanocomposite was 2.53×10−2min−1, which was almost 63 times higher than ZnO photocatalysts. Besides, it was confirmed the important role of hydroxyl radicals (OH) and superoxide anion radical (O2−) in MNZ degradation. The photocatalytic performance under UV and visible radiations was associated with the significant absorption of UV and visible light and reduction of charge carrier recombination. With regard to the findings, it can be concluded that ZnO/PANI nanocomposite is a auspicious procedure for the removal of MNZ under UV and visible radiations.

Effect of textured silicon pyramids size and chemical polishing on the performance of carrier-selective contact heterojunction solar cells

Role of textured crystalline silicon pyramids size and chemical polishing (CP) for isotropic etching of pyramid peaks/valleys are investigated from carrier-selective contact silicon heterojunction solar cell performance. With an increase of average pyramids size from 2 to 8 µm, the effective minority carrier lifetimes (τeff) are reduced from ∼126 to ∼65 µs with molybdenum oxide (MoOx) surface passivation layers. After the CP treatment, an increase in the τeff is observed for the same respective textures (from ∼154 to ∼99 µs) due to reduction of charge carrier recombination. The solar cell structure of Ag/ITO/MoOx/n-Si/LiFx/Al is fabricated at room temperature. With small pyramids (∼2 µm), the cell has shown the better power conversion efficiency of ∼14.53%, but, after the CP treatment not much efficiency variation is observed. Whereas; the CP treatment is beneficial for medium/large pyramids (∼5–8 µm) based cells, which has enhanced the open-circuit voltage (45–63 mV) after smoothing/rounding of sharp peak/valley surfaces. But, we have observed a reduction in photocurrent due to an increase of light reflection from smoothened pyramid surfaces. Quantum efficiency analysis has provided the better insight with the silicon surface morphology dependent energy conversion. The cells’ reverse saturation currents also have analyzed to understand the silicon surface passivation and MoOx/n-Si junction quality. The performance variation of cells is explained by considering low-barrier shunts at the pyramids’ peaks/valleys, which influence the junction built-in potential at the depletion region.

SiO2 surface passivation layers – a key technology for silicon solar cells

High-efficiency silicon solar cells strongly rely on an effective reduction of charge carrier recombination at their surfaces, i.e. surface passivation. Today's industrial silicon solar cells often utilize dielectric surface passivation layers such as SiNx and Al2O3. However, a passivation layer well-known from the microelectronic industry, SiO2, had and has a strong impact on silicon photovoltaics. It allowed to develop the first 20% efficient silicon solar cells in the past and currently experiences a renaissance as the interfacial oxide for silicon-based passivating contacts, thus enabling the next generation of silicon solar cells. This article reviews the properties of SiO2 passivation layers and their strong impact on the historical and current development of silicon solar cells.

High-Performance CH3NH3PbI3-Inverted Planar Perovskite Solar Cells with Fill Factor Over 83% via Excess Organic/Inorganic Halide.

The reduction of charge carrier recombination and intrinsic defect density in organic-inorganic halide perovskite absorber materials is a prerequisite to achieving high-performance perovskite solar cells with good efficiency and stability. Here, we fabricated inverted planar perovskite solar cells by incorporation of a small amount of excess organic/inorganic halide (methylammonium iodide (CH3NH3I; MAI), formamidinium iodide (CH(NH2)2I; FAI), and cesium iodide (CsI)) in CH3NH3PbI3 perovskite film. Larger crystalline grains and enhanced crystallinity in CH3NH3PbI3 perovskite films with excess organic/inorganic halide reduce the charge carrier recombination and defect density, leading to enhanced device efficiency (MAI+: 14.49 ± 0.30%, FAI+: 16.22 ± 0.38% and CsI+: 17.52 ± 0.56%) compared to the efficiency of a control MAPbI3 device (MAI: 12.63 ± 0.64%) and device stability. Especially, the incorporation of a small amount of excess CsI in MAPbI3 perovskite film leads to a highly reproducible fill factor of over 83%, increased open-circuit voltage (from 0.946 to 1.042 V), and short-circuit current density (from 18.43 to 20.89 mA/cm2).

Reduction of Charge-Carrier Recombination at ZnO-Polymer Blend Interfaces in PTB7-Based Bulk Heterojunction Solar Cells Using Regular Device Structure: Impact of ZnO Nanoparticle Size and Surfactant.

Cathode interfacial layers, also called electron extraction layers (EELs), based on zinc oxide (ZnO) have been studied in polymer-blend solar cells toward optimization of the opto-electric properties. Bulk heterojunction solar cells based on poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}) (PTB7) and [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM) were realized in regular structure with all-solution-processed interlayers. A pair of commercially available surfactants, ethanolamine (EA) and ethylene glycol (EG), were used to modify the surface of ZnO nanoparticles (NPs) in alcohol-based dispersion. The influence of ZnO particle size was also studied by preparing dispersions of two NP diameters (6 versus 11 nm). Here, we show that performance improvement can be obtained in polymer solar cells via the use of solution-processed ZnO EELs based on surface-modified nanoparticles. By the optimizing of the ZnO dispersion, surfactant ratio, and the resulting morphology of EELs, PTB7/PC70BM solar cells with a power-conversion efficiency of 8.2% could be obtained using small sized EG-modified ZnO NPs that allow the clear enhancement of the performance of solution-processed photovoltaic devices compared to state-of-the-art ZnO-based cathode layers.

Partial oxidation of the absorber layer reduces charge carrier recombination in antimony sulfide solar cells.

We investigate the effect of a post heat treatment of the absorber layer in air for antimony sulfide (Sb2S3) sensitized solar cells. Phenomenologically, exposing the Sb2S3 surface of sensitised solar cells to air at elevated temperatures is known to improve device performance. Here, we have investigated the detailed origins of this improvement. To this end, samples were annealed in air for different time periods and the build-up of an antimony oxide layer was monitored by XPS. A very short heat treatment resulted in an increase in power conversion efficiency from η = 1.4% to η = 2.4%, while longer annealing decreased the device performance. This improvement was linked to a reduction in charge carrier recombination at the interface of Sb2S3 with the organic hole conductor, arising from the oxide barrier layer, as demonstrated by intensity modulated photovoltage spectroscopy (IMVS).

Correlation between photoelectrochemical behaviour and photoelectrocatalytic activity and scaling-up of P25-TiO2 electrodes

The use of TiO2 electrodes may solve the two main drawbacks of photocatalytic processes: i) the necessity of recovering the catalyst and ii) the low quantum yield in the use of the radiation. This work focuses on the correlation between the photoelectrochemical properties of TiO2 electrodes and their activity for the photoelectrocatalytic oxidation of methanol. Particulate TiO2 electrodes prepared by deposition of P25-TiO2 nanoparticles on titanium (TiO2/Ti) or conductive glass support (TiO2/ITO) seem to be effective for charge carrier transference on TiO2 surface favouring the formation of OH radicals and consequently, the oxidation of molecules. In contrast, thermal TiO2 electrodes prepared by annealing of titanium (Ti) present better properties for charge carrier separation as a consequence of the application of a potential bias. Despite reducing charge carrier recombination by applying an electric potential bias, the activity of thermal electrodes remains lower than that of P25-particulate electrodes. TiO2 structure of P25-particulate electrodes does not completely allow developing a potential gradient. However, their adequate TiO2 layer characteristics for charge carrier transfer lead to a reduction in charge carrier recombination making up for the lack of charge carrier separation when applying an electric potential bias. TiO2/Ti showed the highest values of activity. Therefore, the combination of the suitable TiO2 surface properties for charge carrier transfer with an adequate conductive support seems to increase the properties of the electrode for allowing charge carrier separation. The scaling-up calculations for a TiO2/ITO electrode do lead to good estimations of the activity and photocurrent of larger electrodes since this photoanode made from ITO as conductive support does not seem to be significantly affected by the applied potential bias.

Prelimenary Study on the Photovoltaic and Impedance Characteristics of Dye Sensitized Solar Cell (DSSC) using Polymer Gel Electrolyte

Dye sensitized solar cells (DSSC) have been fabricated by using polymer gel electrolyte consisting of ionic liquids in a composite polymers of siloxane and ethylene oxide groups. The structure of those DSSCs are ITO/Ti:ZnO/TiO2/Ru-dye/gel electrolyte/Pt/ITO, with active area of about 0,5 cm2. At this preliminary work, the structure parameters and fabrication procedures had not been optimized for achieving the best performance, but it had been carefully controlled to be same for all fabricated DSSCs. Those DSSCs shows the solar cell performance similar to that observed in DSSCs fabricated in the same cell structure but using conventional electrolyte solution. They exhibit better open voltage and filling factor, which is suggested to the reduction of charge carrier recombination probably taking place at the double layer region formed at the electrode interfaces. This is consistent with their impedance spectrum which is dominated by a Warburg-like characteristics indicating a diffusion limited migration process. It seems then the cell performances is improved by the reduction of charge recombination loss but restricted by diffusion limited ion migration process.

Lead oxide-modified TiO2 photocatalyst: tuning light absorption and charge carrier separation by lead oxidation state

Modification of TiO2 with metal oxide nanoclusters such as FeOx, NiOx has been shown to be a promising approach to the design of new photocatalysts with visible light absorption and improved electron–hole separation. To study further the factors that determine the photocatalytic properties of structures of this type, we present in this paper a first principles density functional theory (DFT) investigation of TiO2 rutile(110) and anatase(001) modified with PbO and PbO2 nanoclusters, with Pb2+ and Pb4+ oxidation states. This allows us to unravel the effect of the Pb oxidation state on the photocatalytic properties of PbOx-modified TiO2. The nanoclusters adsorb strongly at all TiO2 surfaces, creating new Pb–O and Ti–O interfacial bonds. Modification with PbO and PbO2 nanoclusters introduces new states in the original band gap of rutile and anatase. However the oxidation state of Pb has a dramatic impact on the nature of the modifications of the band edges of TiO2 and on the electron–hole separation mechanism. PbO nanocluster modification leads to an upwards shift of the valence band which reduces the band gap and upon photoexcitation results in hole localisation on the PbO nanocluster and electron localisation on the surface. By contrast, for PbO2 nanocluster modification the hole will be localised on the TiO2 surface and the electron on the nanocluster, thus giving rise to two different band gap reduction and electron–hole separation mechanisms. We find no crystal structure sensitivity, with both rutile and anatase surfaces showing similar properties upon modification with PbOx. In summary the photocatalytic properties of heterostructures of TiO2 with oxide nanoclusters can be tuned by oxidation state of the modifying metal oxide, with the possibility of a reduced band gap causing visible light activation and a reduction in charge carrier recombination.

Effect of counter electrode, thickness and sintering temperature of TiO 2 electrode and TBP addition in electrolyte on photovoltaic performance of dye sensitized solar cell using pyronine G (PYR) dye

Dye sensitized solar cells (DSSCs) assembled with nano-crystalline TiO 2 adsorbed with pyronine G (PYR) dye as photoanode and polar solvent-treated poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) coating on a conductive glass (fluorine-doped thin oxide, FTO), as a counter electrode were studied. It was found that the DSSC using carbon black (0.2 wt%) modified DMSO-PEDOT:PSS conductive coating as a counter electrode resulted highest power conversion efficiency. This is attributed to the role of DMSO in enhancing the conductivity of a PEDOT:PSS film and carbon black plays a vital role in increasing the catalytic activity of the film due to its larger surface area. The solar cell performance was studied as a function of sintering temperature and thickness of TiO 2 layer, dye sensitization time of TiO 2 electrode and electrolyte composition. The DSSCs consist of TiO 2 electrode sintered at a temperature of 450 °C shows higher photocurrent attributed to the formation of higher crystalline film. This enhancement in the interconnection between particles results a swift diffusion of electrolyte and good transportation of electrons in the film. All types of DSSC possess the same dependence of performance on the photoanode thickness, i.e. the efficiency increases with the anode thickness to a maximum value, and then decreases slightly when the thickness of photoanode increases further. The effect of pyridine derivative addition in electrolyte on the performance of photovoltaic parameters of DSSCs has also been investigated. A reduced interface defect density and a resulting reduction of charge carrier recombination were found to be the main mechanism for an increased open circuit voltage of DSSCs with TBP addition in the electrolyte. The negative shift of conduction band edge of TiO 2 after the addition of TBP in electrolyte was also to be a factor in improving the open circuit voltage. The over all power conversion efficiency of the DSSCs with TBP in the electrolyte is about 5.24%.

On the origin of increased open circuit voltage of dye-sensitized solar cells using 4-tert-butyl pyridine as additive to the electrolyte

A combination of electrical measurements and surface analysis techniques was used to investigate the electronic structure of the TiO2/electrolyte interface and its change upon addition of 4-tert-butyl pyridine to the electrolyte. A reduced interface defect density and a resulting reduction of charge carrier recombination were found to be the main mechanism for an increased open circuit voltage of dye-sensitized solar cells with 4-tert-butyl pyridine in the electrolyte. The reduction of interface defect states was traced back to specific binding of 4-tert-butyl pyridine at defect sites on the TiO2 surface.