Deep Defect Density Research Articles

Light soaking effect on defect states distribution of Hydrogenated amorphous silicon investigated By means of constant photocurrent technique

In the present paper we have investigated, by using the constant photocurrent method in dc-mode (dc-CPM), the effect of the light soaking (LS) on the deep defect density (Nd) and the slope of the Urbach tail (E0) of a slightly phosphorus-doped hydrogenated amorphous silicon (a-Si:H) film prepared by Plasma-Enhanced Chemical Vapour Deposition (PECVD). By applying the derivative method, we have converted the measured data into a density of states (DOS) distribution in the lower part of the energy gap. The evolution of the sub-band-gap absorption coefficient and the CPMdetermined density of gap-states distribution within the gap versus the illumination time leads to: (i) an increase in the deep defect absorption without any significant changes in the Urbach tail (exponential part), (ii) a presence of more charged than neutral defects as predicted by the defect pool model, and (iii) a saturation point of the degradation of both optical absorption coefficient and density of deep states of slightly P-doped sample measured by dc-CPM. The constant photocurrent technique in dc-mode as a spectroscopy method for the defect distribution determination is, therefore, most reliable to study the light soaking effect on the stability of hydrogenated amorphous silicon layers used in solar cells manufacturing.

Enhancement in the Efficiency of Sb2 Se3 Solar Cells by Triple Function of Lithium Hydroxide Modified at the Back Contact Interface.

The efficiency of antimony selenide (Sb2 Se3 ) solar cells is still limited by significant interface and deep-level defects, in addition to carrier recombination at the back contact surface. This paper investigates the use of lithium (Li) ions as dopant for Sb2 Se3 films, using lithium hydroxide (LiOH)as a dopant medium. Surprisingly, the LiOH solution not only reacts at the back surface of the Sb2 Se3 film but also penetrate inside the film along the (Sb4 Se6 )n molecular chain. First, the Li ions modify the grain boundary's carrier type and create an electric field between p-type grain interiors and n-type grain boundary. Second, a gradient band structure is formed along the vertical direction with the diffusion of Li ions. Third, carrier collection and transport are improved at the surface between Sb2 Se3 and the Au layer due to the reaction between the film and alkaline solution. Additionally, the diffusion of Li ions increases the crystallinity, orientation, surface evenness, and optical electricity. Ultimately, the efficiency of Sb2 Se3 solar cells is improved to 7.57% due to the enhanced carrier extraction, transport, and collection, as well as the reduction of carrier recombination and deep defect density. This efficiency is also a record for CdS/Sb2 Se3 solar cells fabricated by rapid thermal evaporation.

Quantitative analysis of defect states in InGaZnO within 2 eV below the conduction band via photo-induced current transient spectroscopy

This work investigates the function of the oxygen partial pressure in photo-induced current measurement of extended defect properties related to the distribution and quantity of defect states in electronic structures. The Fermi level was adjusted by applying a negative gate bias in the TFT structure, and the measurable range of activation energy was extended to < 2.0 eV. Calculations based on density functional theory are used to investigate the changes in defect characteristics and the role of defects at shallow and deep levels as a function of oxygen partial pressure. Device characteristics, such as mobility and threshold voltage shift under a negative gate bias, showed a linear correlation with the ratio of shallow level to deep level defect density. Shallow level and deep level defects are organically related, and both defects must be considered when understanding device characteristics.

Defects studies of BiI3-Polymer composites with carbon fillers to achieve better charge transportation for direct X-ray detectors

Wide bandgap heavy semiconductor materials capable of detecting X-rays at room-temperature without cryogenic cooling have great advantages that include portability and wide-area deployment with major applications include medical imaging, spectroscopy, and astrophysics. BiI3 being a wide band gap material, is explored as a prospective material for X-ray and gamma ray detection. However, thick samples of Bismuth tri-iodide (BiI3) are more prone to defects, which trap the photo-generated charge carriers thereby limiting the charge collection efficiency. Here, we report the defects and charge transportation properties of BiI3-polymer (Polystyrene and Polymethyl-methacrylate) composite pellets of thickness 1.5 mm, upon irradiation with X-rays at tube voltage 60 kV. The defects in the samples have been studied using photoluminescence and the lifetime calculation has been done using Time Resolved Photoluminescence Spectroscopy. In order enhance the transport properties, defect engineering in the samples have been done by embedding carbon based conductive fillers into BiI3-polymer composites using hot compression method leading to decrease in the deep defect density. The mobility-lifetime product (μτ) of the charge carriers in composite samples has been calculated to be 6.53 × 10−1 cm2/V, using modified Hetch equation which is enhanced from 10−4 cm2/V for pristine BiI3 and from 10−2 cm2/V for CdZnTe and perovskites-based detectors. Also, the charge collection has been achieved up to 90% at a bias of 0.8 V for BiI3-polystyrene/super carbon composite samples. The results indicate that defects have been mitigated in the stable BiI3-polymer/filler composite pellets, making them a promising material for room temperature X-ray detection and imaging applications.

Sb2 Se3 Thin-Film Solar Cells Exceeding 10% Power Conversion Efficiency Enabled by Injection Vapor Deposition Technology.

Binary Sb2 Se3 semiconductors are promising as the absorber materials in inorganic chalcogenide compound photovoltaics due to their attractive anisotropic optoelectronic properties. However, Sb2 Se3 solar cells suffer from complex and unconventional intrinsic defects due to the low symmetry of the quasi-1D crystal structure resulting in a considerable voltage deficit, which limits the ultimate power conversion efficiency (PCE). In this work, the creation of compact Sb2 Se3 films with strong [00l] orientation, high crystallinity, minimal deep level defect density, fewer trap states, and low non-radiative recombination loss by injection vapor deposition is reported. This deposition technique enables superior films compared with close-spaced sublimation and coevaporation technologies. The resulting Sb2 Se3 thin-film solar cells yield a PCE of 10.12%, owing to the suppressed carrier recombination and excellent carrier transport and extraction. This method thus opens a new and effective avenue for the fabrication of high-quality Sb2 Se3 and other high-quality chalcogenide semiconductors.

Numerical analysis of a novel HTL-free perovskite solar cell with gradient doping and a WS₂ interlayer

Lead Halide Perovskite Solar Cells have exhibited the highest efficiencies for Perovskite Solar Cells. As a result of its affordable processing costs, and stability, HTL-free PSCs have grown in popularity in recent years. A comprehensive numerical study of a novel HTL-free PSC, with a structure of FTO/ZnO/WS₂/CH₃NH₃PbI₃/Au, is reported in this article. Multiple factors that affect the performance of the solar cell have been thoroughly investigated like absorber thickness, doping density and deep level defect density. Also, the influence of the interface defects has been closely studied. Finally, the impact of a gradient doping concentration on the performance parameters has been analyzed. After optimization, we have obtained the final simulated device parameters of power conversion efficiency(PCE) of 26.25%, with an open-circuit voltage(Voc) of 1.20 ​V, a short-circuit current(Jsc) of 24.60 ​mA/cm2, and a Fill Factor(FF) of 89.04%, which is an improvement on the best values recorded in literature.

Hole injection improvement in quantum-dot light-emitting diodes using bi-layered hole injection layer of PEDOT:PSS and V2Ox

Quantum-dot light-emitting diodes (QD-LEDs) are a promising light source for next-generation displays. However, poor hole injection in QD-LEDs and the resulting poor charge balance are key bottlenecks in implementing them in actual displays. In this study, we introduce a bi-layered hole injection layer (HIL) using a transition-metal oxide layer with deep work-function via all-solution process, to improve the hole injection efficiency. For the purpose, vanadium oxide (V2Ox) layer was deposited on a typical HIL, poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) layer. The single-layered V2Ox HIL or PEDOT:PSS HIL has weakness such as a lower hole mobility and a higher density of deep defects, respectively. However, the bi-layered HIL alleviates the weakness of each single-layered HIL by compensating the weakness of the other layer. In addition, the step-wise energy level structure in the bilayer of the HIL device enhances the hole current density and improves the charge balance. As a result, the bi-layered-HIL device exhibits a 60% higher external quantum efficiency and more than twice higher maximum luminance than those of the single-HIL devices. The large improvement in the optoelectrical performance was theoretically analyzed.

Highly efficient Cesium Titanium (IV) Bromide perovskite solar cell and its point defect investigation: A computational study

Lead-free, halide-based perovskites are drawing appreciable attention for solar cell applications due to the non-toxic nature and stable performance in ambient environment. In this study, we investigated different facets of a Cesium Titanium (IV) Bromide (Cs 2 TiBr 6 ) perovskite solar cell (PSC) with ZnO and MoO 3 as charge transport materials. Upon numerical optimization of perovskite layer thickness, its defect density , and the interface defect density, we propose a highly efficient, practically realizable PSC with a power conversion efficiency of 18.15%. Besides, we investigated different point defects in Cs 2 TiBr 6 and the effect of their electronic band positions on the PSC performance. It is found that for Cs 2 TiBr 6 based PSC applications, the material processing of Cs 2 TiBr 6 is a crucial aspect and the PSC may underperform for deep defect states (0.2 eV ~ 1.6 eV) with defect density over 10 15 cm −3 . In addition, we analyzed different back contact materials and found that carbon-based back contact can be a low-cost solution to gold for Cs 2 TiBr 6 PSCs. • A Cesium Titanium (IV) Bromide (Cs 2 TiBr 6 ) based perovskite solar cell with FTO/ZnO/Cs 2 TiBr 6 /MoO 3 /Au structure is proposed. • An optimum power conversion efficiency (PCE) of 18.15% is achieved. • Investigation on possible point defects in Cs 2 TiBr 6 and their electronic band position's effect on the PSC performance. • For deep defect state (0.2 eV ~ 1.6 eV) and defect density over 10 15 cm −3 , the PSC tends to underperform. • Carbon-based back contact can be a possible low cost replacement of gold for Cs 2 TiBr 6 PSCs.

Quantitative analysis of defect states in amorphous InGaZnO thin-film transistors using photoinduced current transient spectroscopy

The device and defect characteristics of amorphous indium–gallium–zinc oxide (In:Ga:Zn = 1:1:1 at.%) thin-film transistors (TFTs) as a function of the oxygen partial pressure were investigated. It was found that as the oxygen partial pressure increased, the field effect mobility decreased, the threshold voltage saw a positive shift, and this shift of threshold voltage increased under a negative gate bias stress. From our qualitative analysis of defect states below the conduction band, it was found that as the oxygen partial pressure increased, defect states in the shallow levels decreased, while defect states in the deep levels increased. A quantitative analysis of the defect states in the TFT structures was conducted using photoinduced current transient spectroscopy. It was found that as the oxygen partial pressure used during fabrication of the TFTs increased from 0% to 10% to 60%, the defect states in the shallow levels decreased from 2.74 × 1018 to 2.93 × 1017 to 3.55 × 1016 cm−3, while the defect states in the deep levels increased from non-availability to 1.86 × 1016 to 3.25 × 1016 cm−3. As the oxygen partial pressure increased, the decrease in shallow level defect density is strongly related to a decrease in carrier concentration; the increase in deep level defect density affects the mobility and causes device instability.

Lifetime, quasi-Fermi level splitting and doping concentration of Cu-rich CuInS2 absorbers

Cu(In,Ga)S2–based solar cells have been shown by Hiroi et al (Hiroi et al 2015 IEEE Journal of Photovoltaics 6 309–312) to achieve higher efficiencies with absorbers processed at high deposition temperatures. Additionally, it is known for CuInS2 cells that the main improvement from higher deposition temperatures is the reduction in the density of deep defects and increased quasi-Fermi level splitting. The increased quasi-Fermi level splitting could result from a reduction in the rate of recombination or from an increase in doping concentration. To investigate which effect is the dominant one, we perform time-resolved photoluminescence measurements and estimate the doping concentration from carrier lifetime and quasi-Fermi level splitting. We find no changes in the effective lifetime, which is in the range of 200 ps. The doping concentration increases from 1016 cm−3 to 1017 cm−3. Our study shows that the increase in quasi-Fermi level splitting with higher deposition temperatures is not due to reduction in non-radiative recombination but due to increased doping concentration.

High-efficiency ultra-thin Cu2ZnSnS4 solar cells by double-pressure sputtering with spark plasma sintered quaternary target

In recent years, Cu2ZnSnS4 (CZTS) semiconductor materials have received intensive attention in the field of thin-film solar cells owing to its non-toxic and low-cost elements. In this work, double-pressure sputtering technology is applied to obtain highly efficient and ultra-thin (~450 nm) pure Cu2ZnSnS4 (CZTS) solar cell. Using mixed materials with sulfides and copper powder as a quaternary target via spark plasma sintering (SPS) method and adopting double-layer sputtering (high + low pressure), a highly adhesive and large-grained CZTS thin film is achieved. As a result, the damage to the surface of Mo contact is decreased so that the reflectivity of incident light can be improved. Moreover, the composition of CZTS film was more uniform and the secondary phase separation at the Mo interface was reduced. Therefore, the interface defect state and deep level defect density in corresponding device with double-pressure is reduced and the ratio of depletion thickness to absorption layer thickness can reached to 0.58, which promoted the collection of photogenerated carriers. Finally, an efficiency of 9.3% for ultra-thin (~450 nm) CZTS film solar cell is obtained.

An antibonding valence band maximum enables defect-tolerant and stable GeSe photovoltaics

In lead–halide perovskites, antibonding states at the valence band maximum (VBM)—the result of Pb 6s-I 5p coupling—enable defect-tolerant properties; however, questions surrounding stability, and a reliance on lead, remain challenges for perovskite solar cells. Here, we report that binary GeSe has a perovskite-like antibonding VBM arising from Ge 4s-Se 4p coupling; and that it exhibits similarly shallow bulk defects combined with high stability. We find that the deep defect density in bulk GeSe is ~1012 cm−3. We devise therefore a surface passivation strategy, and find that the resulting GeSe solar cells achieve a certified power conversion efficiency of 5.2%, 3.7 times higher than the best previously-reported GeSe photovoltaics. Unencapsulated devices show no efficiency loss after 12 months of storage in ambient conditions; 1100 hours under maximum power point tracking; a total ultraviolet irradiation dosage of 15 kWh m−2; and 60 thermal cycles from −40 to 85 °C.

Efficient Passivation Strategy on Sn Related Defects for High Performance All‐Inorganic CsSnI3 Perovskite Solar Cells

AbstractDespite remarkable progress in hybrid perovskite solar cells (PSCs), the concern of toxic lead ions remains a major hurdle in the path towards PSC's commercialization; tin (Sn)‐based PSCs outperform the reported Pb‐free perovskites in terms of photovoltaic performance. However, it is of a particularly great challenge to develop effective passivation strategies to suppress Sn(II) induced defect densities and oxidation for attaining high‐performance all‐inorganic CsSnI3 PSCs. Herein, a facile yet effective thioamides passivation strategy to modulate defect state density at surfaces and grain boundaries in CsSnI3 perovskites is reported. The thiosemicarbazide (TSC) with SCN functional groups can make strong coordination interaction with charge defects, leading to enhanced electron cloud density around defects and increased vacancy formation energies. Importantly, the surface passivation can reduce the deep level trap state defect density originated from undercoordinated Sn2+ ion and Sn2+ oxidation, significantly restraining nonradiative recombination and elongating the carrier lifetime of TSC treated CsSnI3 PSCs. The surface passivated all‐inorganic CsSnI3 PSCs based on an inverted configuration delivers a champion power conversion efficiency (PCE) of 8.20%, with a prolonged lifetime over 90% of initial PCE, after 500 h of continuous illumination. The present strategy sheds light on surface defect passivation for achieving highly efficient all‐inorganic lead‐free Sn‐based PSCs.

Enhancement of silicon solar cell performance by introducing selected defects in the [formula omitted] passivation layer

Deep level defects usually have harmful effect on solar cells. In this work it is shown; however, that correct incorporation of selected defects in the silicon oxide region of a silicon solar cell improves its efficiency. This is demonstrated by numerical simulation of n-type silicon-based solar cell including deep level defects in the silicon dioxide (SiO2) passivation layer. The defect density was varied to study its effect on the solar cell performance. The selected defects assist the majority carrier’s transport through their energy levels that are echoing with the band edge state, and repulse the minority carrier, therefore reducing recombination. It was found that well-defined deep defect density and a minimum thickness of the SiO2 passivation layer are required for high efficiency Si-based solar cells. The defects must be of a high density ( ∼1017cm-3), and energetically situated above the conduction band minimum (CBM) of the adjacent Si layer. With these considerations, the conversion efficiency attained 26 %. Furthermore, in the case of very thin passivation layer, the current through defects did not have any effect on the solar cell performance since the tunneling current goes directly through this tin layer.

Impact of Cation Multiplicity on Halide Perovskite Defect Densities and Solar Cell Voltages

Metal-halide perovskites feature very low deep-defect densities, thereby enabling high operating voltages at the solar cell level. Here, by precise extraction of their absorption spectra, we find t...

Enhancing the open circuit voltage of the SnS based heterojunction solar cell using NiO HTL

In this article, the numerical investigations on a novel (ITO/CeO2/SnS/NiO/Mo) heterostructure of the SnS-based solar cell have been presented using SCAPS-1D simulation software. The main objective of the study was the exploration of the effect of NiO hole transport layer (HTL) on the performance of the proposed SnS-based heterojunction solar cell. The open circuit voltage of the proposed cell has been enhanced up to 345 mV after using NiO HTL which indicates a significant reduction of surface recombination velocity at SnS/electrode interface. Around 12.78% efficiency enhancement was observed after using NiO HTL in the SnS-based heterojunction solar cell, and an efficiency of 25.1% could be obtained from the proposed heterojunction solar cell. It was found that the cell performance depended on the carrier concentration and thickness of the CeO2 electron transport layer (ETL), SnS absorber layer, and NiO HTL, respectively. The effects of deep level defect density of the SnS absorber layer and interface defect density of NiO/SnS and CeO2/SnS on the cell performance ware also studied. These findings reveal that the NiO could be a promising HTL for the fabrication of low cost, high-efficiency SnS-based heterojunction solar cell.

Sputtered and selenized Sb2Se3 thin-film solar cells with open-circuit voltage exceeding 500 mV

Antimony selenide (Sb 2 Se 3 ) is a potential absorber material for environment-friendly and cost-efficient photovoltaics and has achieved considerable progress in recent years. However, the severe open-circuit voltage ( V oc ) deficit ascribed to the interface and/or bulk defect states has become the main obstacle for further efficiency improvement. In this work, Sb 2 Se 3 absorber layer was prepared by an effective combination reaction involving sputtered and selenized Sb precursor thin films. The self-assembled growth of Sb 2 Se 3 thin films with large crystal grains, benign preferential orientation, and accurate chemical composition were successfully fulfilled under an appropriate thickness of Sb precursor and an optimized selenization scenario. Substrate structured Sb 2 Se 3 thin-film solar cells, a champion device with a power-conversion efficiency of 6.84%, were fabricated. This device is comparable to state-of-the-art ones and represents the highest efficiency of sputtered Sb 2 Se 3 solar cells. Importantly, the high V oc of 504 mV is closely related to the reduced deep level defect density for the Sb 2 Se 3 absorber layer, the passivated interfacial defects for Sb 2 Se 3 /CdS heterojunction interface, and the additional heterojunction heat treatment-induced Cd and S inter-diffusion. This significantly improved V oc demonstrates remarkable potential to broaden its scope of applications for Sb 2 Se 3 solar cells. • An effective combination reaction of sputtered and selenized Sb precursors for Sb 2 Se 3 thin film solar cells. • The self-assembled growth of Sb 2 Se 3 thin films with large crystal grains, benign preferential orientation and accurate chemical composition. • A highly interesting V oc record of 504 mV obtained due to the reduced deep level defect density, the passivated interfacial defects and heterojunction heat treatment-induced Cd and S inter-diffusion. • PCE of 6.84% represents the highest efficiency of sputtered Sb 2 Se 3 solar cells.

High Efficiency CIGS Solar Cells by Bulk Defect Passivation through Ag Substituting Strategy.

Cu(In,Ga)Se2 (CIGS) is considered a promising photovoltaics material due to its excellent properties and high efficiency. However, the complicated deep defects (such as InCu or GaCu) in the CIGS layer hamper the development of polycrystalline CIGS solar cells. Numerous efforts have been employed to passivate these defects which distributed in the grain boundary and the CIGS/CdS interface. In this work, we implemented an effective Ag substituting approach to passivate bulk defects in CIGS absorber. The composition and phase characterizations revealed that Ag was successfully incorporated in the CIGS lattice. The substituting of Ag could boost the crystallization without obviously changing the band gap. The C-V and EIS results demonstrated that the device showed enlarged Wd and beneficial carrier transport dynamics after Ag incorporation. The DLTS result revealed that the deep InCu defect density was dramatically decreased after Ag substituting for Cu. A champion Ag-substituted CIGS device exhibited a remarkable efficiency of 15.82%, with improved VOC of 630 mV, JSC of 34.44 mA/cm2, and FF of 72.90%. Comparing with the efficiency of an unsubstituted CIGS device (12.18%), a Ag-substituted CIGS device exhibited 30% enhancement.

Perceiving of Defect Tolerance in Perovskite Absorber Layer for Efficient Perovskite Solar Cell

Controlling the defect in the perovskite absorber layer is a very crucial issue for developing highly efficient and stable perovskite solar cells (PSCs) as it exhibits the existence of unavoidable defects even after the careful fabrication process. In this study, the presence of defects in the perovskite layer has been evaluated through the analysis of its structural and optical properties. Then the investigations on the impact of defect density on perovskite absorber layer and its associated solar cell parameters have been carried out by numerical simulation utilizing SCAPS-1D software. Besides the defect density, the thickness of the absorber layer has also been varied to find optimum values of cell parameters. It has been found that when the thickness of absorber and shallow defect density is increased from 200 nm to 800 nm and 1 × 10 13 cm -3 to 1 × 10 18 cm -3 respectively, power conversion efficiency (PCE) is varied from 26.7% to 0.90%. However, when the thickness and deep defect density are raised from 200 nm to 800 nm and 1 × 10 13 cm -3 to 1 × 10 16 cm -3 , respectively, the PCE is varied from 19.3% to 6.15%. It is revealed that optimum absorber thickness is 550 nm and the tolerances of shallow level and deep level defect density are 1 × 10 17 cm -3 and 1 × 10 15 cm -3 , respectively.

Study on the role of growth time on structural, morphological and optical properties of un-capped and L-cyst.-capped ZnO nanorods grown on a GZO seeded thin film layer from an aqueous solution

The influence of the deposition time (ranging from 30 to 300 min) and the role of l-cysteine on the structural and optical properties of the un-capped and L-cyst.-capped ZnO nanorods (ZNRs) grown using the chemical bath deposition method was discussed. X-ray diffraction patterns showed that the estimated crystallite size increased with the deposition time up to 240 min for both capped and un-capped samples, while the calculated strain was found to decrease to a minimum for the 240 min deposition time. The Fourier transform infra-red spectroscopy of the as-grown ZNRs films showed various absorption bands in the range of 250–4000 cm-1 indicating the presence of different functional groups. Scanning electron microscopy analysis illustrated that the aspect ratios of the L-cyst.-capped ZNRs increased six-fold as the growth duration varied from 30 to 240 min, but decreased slightly at longer growth time. The root mean square roughness increased continuously from 10.9 to 36.3 nm as the growth time was raised from 30 to 240 min for the un-capped ZNRs. The samples deposited at 240 min exhibited the highest photoluminescence emission intensity ratios suggesting a low density of deep level defects. An inverse relation between the optical band gaps and growth time was observed. The highly luminescent cyst.-capped sample, with well-aligned nanorods and highest aspect ratio and good crystallization grown at 240 min growth time can be used as a possible photoanode component of dye-sensitized solar cells.