Reduction Of Surface Recombination Velocity Research Articles

A highly efficient n‐CdS/p‐Ag2S/p+‐SnS thin film solar cell: Design and simulation

AbstractSilver sulfide (Ag2S) chalcogenide compound can be a viable absorber in the applications of thin film solar cells owing to its optimum bandgap of 1.1 eV and high absorption coefficient. Herein, we propose a novel Ag2S‐based n‐CdS/p‐Ag2S/p+‐SnS double‐heterojunction solar cell. The numerical analysis of the device has been performed with SCAPS‐1D (Solar Cell Capacitance Simulator). In the case of single heterojunction, n‐CdS/p‐Ag2S manifests an efficiency of 19.75%, where VOC = 0.66 V, JSC = 36.99 mA/cm2 and FF = 81.50%. However, Ag2S‐based double‐heterojunction device with optimized structure provides the efficiency of 29.51% with VOC = 0.81 V, JSC = 42.81 mA/cm2 and FF =85.24%. The noteworthy augmentation of VOC and JSC in double‐heterojunction results from the reduction in surface recombination velocity and rise in built‐in voltage in the p‐Ag2S/p+‐SnS hetero‐interfaces that promote the higher efficiency of the device. These theoretical insights indicate a path for fabrication of an efficient Ag2S based thin film solar cell.

Passivation of the Buried Interface via Preferential Crystallization of 2D Perovskite on Metal Oxide Transport Layers.

The open-circuit voltage (Voc ) of perovskite solar cells is limited by non-radiative recombination at perovskite/carrier transport layer (CTL) interfaces. 2D perovskite post-treatments offer a means to passivate the top interface; whereas, accessing and passivating the buried interface underneath the perovskite film requires new material synthesis strategies. It is posited that perovskite ink containing species that bind strongly to substrates can spontaneously form a passivating layer with the bottom CTL. The concept using organic spacer cations with rich NH2 groups is implemented, where readily available hydrogens have large binding affinity to under-coordinated oxygens on the metal oxide substrate surface, inducing preferential crystallization of a thin 2D layer at the buried interface. The passivation effect of this 2D layer is examined using steady-state and time-resolved photoluminescence spectroscopy: the 2D interlayer suppresses non-radiative recombination at the buried perovskite/CTL interface, leading to a 72% reduction in surface recombination velocity. This strategy enables a 65mV increase in Voc for NiOx based p-i-n devices, and a 100mV increase in Voc for SnO2 -based n-i-p devices. Inverted solar cells with 20.1% power conversion efficiency (PCE) for 1.70eV and 22.9% PCE for 1.55eV bandgap perovskites are demonstrated.

GaAs surface passivation for InAs/GaAs quantum dot based nanophotonic devices

Several passivation techniques are developed and compared in terms of their ability to preserve the optical properties of close-to-surface InAs/GaAs quantum dots (QDs). In particular, the influence of N-passivation by hydrazine chemical treatment, N-passivation by hydrazine followed by atomic layer deposition (ALD) of AlO x and use of AlN x deposited by plasma-enhanced ALD are reported. The effectiveness of the passivation is benchmarked by measuring the emission linewidths and decay rates of photo-carriers for the near-surface QDs. All three passivation mechanisms resulted in reducing the oxidation of Ga and As atoms at the GaAs surface and consequently in enhancing the room-temperature photoluminescence (PL) intensity. However, long-term stability of the passivation effect is exhibited only by the hydrazine + AlO x process and more significantly by the AlN x method. Moreover, in contrast to the results obtained from hydrazine-based methods, the AlN x passivation strongly reduces the spectral diffusion of the QD exciton lines caused by charge fluctuations at the GaAs surface. The AlN x passivation is found to reduce the surface recombination velocity by three orders of magnitude (corresponding to an increase of room-temperature PL signal by ∼1030 times). The reduction of surface recombination velocity is demonstrated on surface-sensitive GaAs (100) and the passivating effect is stable for more than one year. This effective method of passivation, coupled with its stability in time, is extremely promising for practical device applications such as quantum light sources based on InAs/GaAs QDs positioned in small-volume photonic cavities and hence in the proximity of GaAs-air interface.

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.

Characterization of n-GaN / p-GaAs NP heterojunctions

Abstract n-GaN/p-GaAs heterojunctions were fabricated by plasma-assisted molecular beam epitaxy (PA-MBE). The fabrication was carried out by the growth of n-type cubic GaN on p-type GaAs(100) substrates, followed by Ohmic contact deposition using conventional e-beam and rapid thermal annealing. With the n-GaN/p-GaAs heterojunction we have obtained photovoltaic effect and demonstrated that this effect strongly depends of the GaN nucleating layer on the GaAs surface. Likewise, we found that the decrease of hexagonal-stable phase and planar defects inclusions in the cubic-metastable GaN decreases the leakage current, and therefore, increases the open circuit voltage (Voc). Furthermore, the increase in crystal quality also increases the External quantum efficiency (EQE) at longer wavelengths due to the reduction of surface recombination velocity at the GaN/GaAs interface. In addition, the heterojunctions were tested as radiation detectors, demonstrating that they could be a candidate for alpha particle detectors if the thickness of the absorber layer is increased.

Investigation on the impact of hydrogen on the passivation of silicon surface states in clean and copper contaminated conditions

Hydrogen is identified as a useful technique to passivate defects within crystalline silicon. However, the effect of hydrogen passivation for a silicon surface is normally characterized as a reduction in surface recombination velocity (SRV), which is not enough to reflect the detailed changes of electronic properties, such as defect density, defect energy levels, and capture cross section, of silicon surface states. In this paper, we utilized the transient capacitance measurement to characterize the detailed electronic properties of silicon surface states before and after hydrogenation. The differences, in terms of the effects of hydrogenation on silicon surface states, either in copper contaminated conditions or clean conditions, are presented and discussed.

Deleterious electrostatic interaction in silicon passivation stack between thin ALD Al2O3 and its a-SiNX:H capping layer: numerical and experimental evidences

This study focuses on the electrostatic interaction between the ALD Al2O3 passivation layer and the PECVD a-SiNX:H capping layer, while reducing ALD Al2O3 thickness. The first one embeds negative fixed charges while the second is well known to possess a fixed charge density with an opposite polarity. We reduced the Al2O3 thickness from 20 to 2 nm while keeping constant the a-SiNX:H thickness. To increase the magnitude of the field effect passivation provided by Al2O3, we used an original light-induced field effect enhancement strategy. QSS-PC was used to quantify the initial passivation properties. We monitored the evolution of the passivation quality under low-intensity light-soaking until its stabilization. We report here a minority carrier lifetime enhancement up to 900% and surface recombination velocity reduction below 10 cm.s-1. The evidence of field effect compensation at c-Si surface due to a-SiNX:H positive charges has been supported by numerical simulations (SILVACO ATLAS). While keeping the parameters of the a-SiNX:H layer constant, the configuration of the Al2O3 layer (thickness and fixed charge density) were varied. These results allow us to conclude that in order to use thinner Al2O3 passivation layer (2 nm), ideal capping layer has to be free of positive fixed charges and to release enough hydrogen at desired temperature to ensure the chemical passivation.

A 14.7% Organic/Silicon Nanoholes Hybrid Solar Cell via Interfacial Engineering by Solution-Processed Inorganic Conformal Layer.

We demonstrated a high-performance Si-organic hybrid heterojunction solar cell utilizing low-temperature and liquid-phase-processed TiO2 as an interlayer between poly(3,4-ethylene dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and Si nanoholes to produce a conformal contact on the surface of the Si nanostructure. The hydrophilic TiO2/Si-nanohole surface enabled the PEDOT:PSS to flow into the spacing of the close-packed nanoholes. Scanning electron microscopy images were used to confirm the PEDOT:PSS nanohole filling induced by the TiO2. With forming gas annealing of the double-sided TiO2, high Voc (0.63 V) and Jsc (35.7 mA/cm2) values were obtained, yielding a high power conversion efficiency of 14.7%. The high Voc was attributed to the surface passivation of Si by annealed TiO2. The X-ray photoelectron spectroscopy investigation at the TiO2/Si interface indicates the TiOx signal decreased and the TiO2 and SiOx signals increased after annealing. The Si-O bonding found in the O 1s study appeared in the form of Si-O-Si bonding to serve surface passivation. The band alignment of the PEDOT:PSS/TiO2/n-Si hetero-interfaces was postulated and plotted. The Vbi in the system after annealing was assumed to be higher because of the reduction of bulk and surface states that yield high Voc. After annealing, the Vbi increased from 0.805 to 0.905 V. The reduction of surface recombination velocity proved the passivation ability of TiO2 after annealing. With proven surface passivation and conformal PEDOT:PSS/Si nanohole interfaces for enhanced contact, this Si-organic hybrid heterojunction solar cell with solution-processed TiO2 interlayers has excellent potential for application as a high-efficiency and low-cost Si solar cell.

Defect termination on crystalline silicon surfaces by hydrogen for improvement in the passivation quality of catalytic chemical vapor-deposited SiNx and SiNx/P catalytic-doped layers

We investigate the role of hydrogen (H) in the improvement in the passivation quality of silicon nitride (SiNx) prepared by catalytic chemical vapor deposition (Cat-CVD) and Cat-CVD SiNx/phosphorus (P) Cat-doped layers on crystalline silicon (c-Si) by annealing. Both structures show promising passivation capabilities for c-Si with extremely low surface recombination velocity (SRV) on n-type c-Si. Defect termination by H is evaluated on the basis of defect density (Nd) determined by electron spin resonance (ESR) spectroscopy and interface state density (Dit) calculated by the Terman method. The two parameters are found to be drastically decreased by annealing after SiNx deposition. The calculated average Dit at midgap (Dit-average) is 2.2 × 1011 eV−1 cm−2 for the SiNx/P Cat-doped c-Si sample with a SRV of 2 cm/s, which is equivalent to 3.1 × 1011 eV−1 cm−2 for the SiNx/c-Si sample with a SRV of 5 cm/s after annealing. The results indicate that H atoms play a critical role in the reduction in Dit for SiNx/c-Si and SiNx/P Cat-doped c-Si, resulting in a drastic reduction in SRV by annealing.

Reduction of surface recombination velocity by rapid thermal annealing of p-Si passivated by catalytic-chemical vapor deposited alumina films

Excellent electrical-passivation of p-type Si (p-Si) in Si solar cells has been achieved by post-deposition rapid annealing of aluminum oxide (AlOx) films prepared by catalytic chemical vapor deposition (Cat-CVD) using trimethyl aluminum (TMA) and O2. Extremely small surface recombination velocity of below 0.1cm/s has been obtained at post-deposition annealing temperatures in the range of 350–400°C for an annealing time of 2min. The reduction of surface recombination velocity has been attributed to band bending induced by a fixed negative charge density of 5×1011charges/cm2 and an additional small interface trapping density of around 1010cm−2eV−1.

Optimal hydrogenated amorphous silicon/silicon nitride bilayer passivation of n-type crystalline silicon using response surface methodology

This study reports the highest quality surface passivation achieved with hydrogenated amorphous silicon and amorphous silicon nitride (SiNx) bilayer stack deposited using plasma enhanced chemical vapour deposition on 1–2 Ωcm n-type crystalline silicon. The SiNx deposition conditions were investigated using response surface methodology (RSM). Optimized deposition parameters obtained from the RSM study yielded a low surface recombination velocity (SRV) of 3.5 cm/s. Interface defect and charge densities, inferred using the interface dangling bond recombination model, revealed a strong influence of charge on the SRV reduction. The model predicts a lower SRV of 1.5 cm/s for the bilayer passivation scheme.

Surface passivation of Cu(In,Ga)Se2 using atomic layer deposited Al2O3

With Al2O3 passivation on the surface of Cu(In,Ga)Se2, the integrated photoluminescence intensity can achieve two orders of magnitude enhancement due to the reduction of surface recombination velocity. The photoluminescence intensity increases with increasing Al2O3 thickness from 5 nm to 50 nm. The capacitance-voltage measurement indicates negative fixed charges in the film. Based on the first principles calculations, the deposition of Al2O3 can only reduce about 35% of interface defect density as compared to the unpassivated Cu(In,Ga)Se2. Therefore, the passivation effect is mainly caused by field effect where the surface carrier concentration is reduced by Coulomb repulsion.

Low-temperature Phosphorus Doping To Silicon Using Phosphorus-related Radicals

ABSTRACTPhosphorus (P) doped ultra thin n+-layer is formed on crystalline silicon (c-Si) at low substrate temperatures of 80 – 350 °C using radicals generated by the catalytic reaction of phosphine (PH3) with a tungsten catalyzer heated at 1300 °C. The sheet carrier concentration obtained by Hall effect is in the range between 3×1012cm-2 and 8×1012cm-2. The distribution of P atoms obtained by secondary ion mass spectrometry (SIMS) indicates that P atoms locate within the depth of 4 nm from surface and the profile has almost the same distribution independent of any doping conditions such as substrate temperature or radical exposure time. The sheet carrier concentration is 1.15 – 2.12% of the amount of P atoms incorporated through the radical doping. The ratio of activated donors increases with substrate temperature during the radical doping, suggesting that P-related species bonded on the c-Si surface require thermal energy for their activation. Using the n+-layer formed by radical doping, the reduction of surface recombination velocity for n-type c-Si wafer is attempted. The effective minority carrier lifetime of the n-type c-Si sample covered with 6-nm-thick intrinsic amorphous Si (i-a-Si) layers on both side increases from 32 μs to1576 μs by the radical doping of P atoms to n-type c-Si surface, suggesting that the radical doping can be utilized for the formation of passivation layers on a-Si/ n-c-Si hetero-interface.

Improvement of silicon solar cell efficiency by ion beam sputtered deposition of AlO_xN_y thin films

Negative charge material, AlOxNy, has been fabricated to passivate the surface of p-type silicon. The fabrication of AlOxNy was possible by using ion beam sputtering deposition to deposit AlN thin film on the surface of a p-type silicon wafer and following annealing in oxygen ambient. Capacitance-voltage analysis shows the fixed charge density has increased from 10(11) cm(-2) to 2.26×10(12) cm(-2) after annealing. The solar cell efficiency increased from 15.9% to 17.3%, which is also equivalent to the reduction of surface recombination velocity from 1×10(5) to 32 cm/s.

Drastic reduction in surface recombination velocity of crystalline silicon by surface treatment using catalytically-generated radicals

A method of reducing surface recombination velocities (SRVs) of crystalline silicon (c-Si) wafers is presented for the case when c-Si surfaces are passivated by amorphous Si (a-Si) thin films. It is demonstrated that the surface treatment of c-Si wafers prior to a-Si deposition, using hydrogen atoms generated by catalytic reaction of hydrogen gas with heated catalyzer, lowers SRV effectively, and that SRV is drastically reduced to less than 3 cm/s when a small amount of doping impurity is added during atomic hydrogen treatment.

Surface-recombination-free InGaAs/InP HBTs and the base contact recombination

Surface-recombination-free InGaAs/InP HBTs with graded base have been demonstrated. The HBTs were passivated by ammonium sulfide. The current gain of the nonself-aligned HBTs was independent of the emitter periphery, indicating that the surface recombination was removed by the passivation. For the self-aligned HBTs, the current gain was still dependent on the emitter periphery after the passivation due to the base contact recombination. A surface leakage channel has been identified to result in a significant increase in the base contact recombination. The passivation has two effects: one is the surface recombination velocity reduction and the other is the surface leakage channel elimination.

Surface recombination velocity reduction in type-II InAs∕GaSb superlattice photodiodes due to ammonium sulfide passivation

The surface recombination velocity (SRV) of minority electrons in a type-II InAs∕GaSb superlattice photodiode is quantitatively investigated using the electron beam induced current technique and its value used to evaluate the effects of two different passivation methods. Before passivation, the SRV was determined to be (5.0±0.2)×104cm∕s. The SRVs of two samples passivated at room temperature are compared with that of the unpassivated sample. One passivation method, using a neutralized (NH4)2S solution for 60min, reduces the SRV by a factor of 2. The other passivation method, using 4% (NH4)2S solution for 30min, reduces the SRV by more than one order of magnitude.

Effects of sub‐gap irradiation on the time‐of‐flight current waveforms of high resistivity CdTe

AbstractEffects of IR irradiation on TOF current waveforms of a CdTe:Cl crystal with Pt electrodes on its (111) faces have been investigated. An increase of TOF current and a change in the evolution of DC photocurrent versus applied voltage by IR irradiation are observed. By taking into account an effect of surface recombination, satisfactory fits to theory are obtained for the charge collection efficiency and the DC photocurrent versus applied voltage. These results show that the observed effects are mainly explained by a reduction of surface recombination velocity rather than by an increase of bulk carrier lifetime. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Passivation Effect of Plasma Chemical Vapor Deposited SiNx on Single-Crystalline Silicon Thin-Film Solar Cells

Reduction of surface recombination velocity is one of the most important issues for high-efficiency single crystalline silicon thin-film solar cells. Surface passivation effects of silicon nitride (SiNx) films deposited by direct plasma chemical vapor deposition were analyzed focusing on the polarity of fixed charges and solar cell performance. For the solar cell, epitaxial single crystalline silicon layers with a typical thickness of 10 µm were grown by atmospheric pressure chemical vapor deposition. The solar cell passivated by SiNx film with positive fixed charges showed improved internal quantum efficiency in the wavelength range of 320–500 nm, which was attributed to the effect of an induced front surface field of an n++/n+ junction as holes. A very small surface recombination velocity of 4.6 ×103 cm/s and high cell performances (η=11%, Jsc=26.3 mA/cm2, Voc=609 mV) were realized.

Surface photovoltagespectroscopy of pulsed laser deposited undoped ZnSe/n+GaAs

We report surface photovoltage (SPV) spectra of ZnSe thin filmsdeposited on n+GaAs substrates in the wavelength rangeof 400-800 nm. In the above bandgap region of ZnSe(below 450 nm), we find that the major contribution to SPVcomes from trapping and re-emission from the slow states atthe ZnSe surface and ZnSe/GaAs interface. The effect of interference of light on the SPV spectra, has been analysed forsubbandgap wavelength excitation of ZnSe (470-800 nm). Inspite of the presence of a large number of subbandgap statesin ZnSe, the major contribution to SPV in this wavelength rangecomes from the substrate. The difference in the magnitudes ofthe SPV between the bare n+GaAs and the ZnSe/n+GaAs is due to the reduction of surface recombination velocity(SRV) of the minority carriers in n+GaAs.