Minority Carrier Recombination Research Articles

Minority Carrier Recombination Properties of Crystalline Defect on Silicon Surface Induced by Plasma Enhanced Chemical Vapor Deposition

This research investigates the carrier recombination properties of a crystalline defect layer introduced by the plasma enhanced chemical vapor deposition (PECVD) process of amorphous hydrogenated silicon nitride (SiNx) passivation films. A direct PECVD technique was used for SiNx films deposition. A crystalline defect layer existed on the surface of the silicon substrate and is under the SiNx passivation film. The recombination lifetime in this defect layer was obtained by focusing on the thickness of the defect layer and the effective lifetime before and after the defect layer etching. After etching a few nanometer thickness, effective lifetime drastically increased. On the other hands, the carrier recombination center could be electrically inactivated by 600°C annealing after SiNx deposition. According to the depth profile of effective lifetime, it was clarified the high carrier recombination region were concentrated near the surface of silicon substrate.

원자층 증착법으로 형성된 Al2O3박막의 질소 도핑에 따른 실리콘 표면의 부동화 특성 연구

To improve the efficiency of the Si solar cell, high minority carrier life time is required. Therefore, the passivation technology is important to eliminate point defects on the silicon surface, causing the loss of minority carrier recombination. PECVD or post-annealing of thermally-grown SiO2 is commonly used to form the passivation layer, but a high-temperature process and low thermal stability is a critical factor of low minority carrier lifetime. In this study, atomic layer deposition was used to grow the Al2O3 passivation layer at low temperature process. Al2O3 was selected as a passivation layer which has a low surface recombination velocity because of the fixed charge density. For the high charge density, an improved minority carrier lifetime, and a low surface recombination, nitrogen was doped in the Al2O3 thin film and the improvement of passivation was studied.

Hydrogen passivation effect on the conversion efficiency of Si solar cells by low-energy proton implantation

We investigated the hydrogen passivation effect on the conversion efficiency of Si solar cells by including low-energy hydrogen ion implantation in the cell fabrication process. The solar cells were implanted with various proton energies and doses just before the co-firing process. The quantum efficiency specifically in the blue wavelengths of the solar cells increased with a low implantation dose and energy indicating improvement of surface passivation. However, quantum efficiency decreased when the implantation dose and energy were increased, indicating a high defect-assisted surface recombination of minority carriers. The cells implanted with a low dose and energy in the presence of SiNx showed interfacial passivation with enhanced photocurrent, minority carrier lifetime and short-circuit current density for improved conversion efficiency.

Investigation of long lifetimes in Cu(In,Ga)Se2 by time-resolved photoluminescence

The main objective of time-resolved photoluminescence (TRPL) is to characterize minority carrier recombination in semiconductors. However, trap states in the band gap can lead to artificially long decay times thus distorting the measured minority carrier lifetime. In this work, we propose to measure TRPL under elevated temperature and excitation in order to reduce minority carrier trapping. Taking three Cu(In,Ga)Se2 layers as examples, we show that the decay time decreases with increasing temperature—in accordance with simulations. Under increasing excitation, the decay time can become smaller due to trap saturation but also can become larger due to asymmetric hole and electron lifetimes. By comparison of simulation and experiment, we can find the energy, the density, and the electron capture cross-section of the trap which in the present example of Cu(In,Ga)Se2 films gives values of ∼200 meV, ∼1015 cm−3, and ∼10−13 cm2, respectively.

Theoretical study of time-resolved luminescence in semiconductors. III. Trap states in the band gap

In the third part of this series, we study the influence of trap states in the band gap of semiconductors on the time-resolved luminescence decay (TRL) after a pulsed excitation. The results based on simulations with Synopsys TCAD® and analytical approximations are given for p-doped Cu(In,Ga)Se2 as a working example. We show that a single trap can be mostly described by two parameters which are assigned to minority carrier capture and emission. We analyze their influence on the luminescence decay and study the difference between a single trap and an energetic Gaussian trap distribution. It is found that trap states artificially increase the TRL decay and obscure the recombination dynamics. Thus, there is a demand for experimental methods which can reveal the recombination of minority carriers in a TRL experiment without trapping effect. In this regard, a variation of the device temperature, the excitation frequency, the injection level, as well as a bias illumination may be promising approaches. We study these methods, discuss advantages and disadvantages, and show experimental TRL for prove of concept. At the end, we validate our approach of simulating only band-to-band radiative recombination although photoluminescence spectra often exhibit free-to-bound radiative recombination of charge carriers.

A Simulated Study of Silicon Solar Cell Power Output as a Function of Minority-Carrier Recombination Lifetime and Substrate Thickness

A Simulated Study of Silicon Solar Cell Power Output as a Function of Minority-Carrier Recombination Lifetime and Substrate Thickness

Swift heavy ion induced capacitance and dielectric properties of Ni/n-GaAs Schottky diode

The in-situ capacitance and dielectric properties of 25 MeV C4+ ion irradiated Ni/n-GaAs Schottky barrier diode (SBD) were studied at 100 kHz in the fluence range 5 × 1010 – 5 × 1013 ions/cm2. The investigation shows reduction in capacitance and charge density with increase in ion fluence. Consequent changes were observed in other related parameters like conductance, dielectric constant, dielectric loss, loss tangent and electrical modulus. The results were interpreted in terms of generation of swift heavy ion induced acceptor trap states by electronic energy loss mechanism. Besides, the switch over characteristics of depletion to inversion regions in the CV plot reveals minority carrier recombination centers also. The dispersion and relaxation peaks observed in bias dependent dielectric plots were ascribed to the polarization and relaxation mechanism due to the interfacial trap states. The traps and recombination centers were found to alter the barrier characteristics of the fabricated SBD depending upon the ion fluence.

Tunnel oxide passivated carrier-selective contacts based on ultra-thin SiO2 layers

Carrier-selective contacts are one of the key enabling technologies to approach very high conversion efficiencies close to the theoretical limit of silicon solar cells. The tunnel oxide passivated contact (TOPCon) approach is an alternative to classical heterojunction solar cells enabling efficiencies up to 24.4%. The tunnel oxide is a core element of this contact as it has to reduce the minority carrier recombination but simultaneously must not hamper the majority carrier flow. This paper focuses on ozone-based oxidation techniques, which can potentially be cost effective and industrially feasible methods for the realization of ultra-thin tunnel oxide layers as an alternative to the oxidation in nitric acid (HNO3) reference process. All investigated oxides were applied to the electron-selective contact (n-TOPCon) on planar and textured surfaces. It will be shown that both ozone based oxidation techniques (UV/O3 photo-oxidation and wet-chemical oxidation in ozonized DI-H2O) enable high implied open circuit voltage (iVOC) values exceeding 720mV on planar and 710mV on textured surfaces, respectively. Further oxide properties as stoichiometry and layer thickness were analyzed by means of X-ray photoelectron spectroscopy (XPS), spectral ellipsometry (SE) and transmission electron microscopy (TEM). In compliance with earlier results it was found that a minimum oxide layer thickness (approximately 1.3nm) and a high amount of oxygen-rich sub oxide species are required to obtain a good surface passivation. Using such oxides, a wider range of temperatures can be used during the TOPCon annealing. Applying the ozone-based oxide layers to n-TOPCon solar cells resulted in a high VOC of up to 719mV and a peak efficiency of 24.9%. Similar results were obtained with the HNO3 reference process (VOC=716mV, η=24.8%).

Response of as grown dislocation structure to temperature and stress treatment in multi-crystalline silicon

Dislocations and dislocation networks in multi-crystalline (mc) silicon wafers for photovoltaic applications act as minority carrier recombination centers and thus limit the efficiency of solar cells. The literature shows results for a massive dislocation reduction by applying an annealing procedure in combination with and without an impurity gettering step. In this work different kinds of annealing experiments with mc silicon samples were carried out at 1200°C and 1365°C for 1h to 96h under an applied stress of up to 4.2MPa under pure inert or boron containing atmospheres. The results show clearly that, under the used process conditions, a dislocation reduction could not be observed via defect selective etching using different kinds of etchants. It was found, that it is essential to carefully select the etching solution as the electrical resistivity of the samples might change after a respective thermal treatment, such that the eventually still present dislocations will not be overseen.

Proton Irradiation Effects on the Performance of III-V-Based, Unipolar Barrier Infrared Detectors

An examination of the collective results from recent experiments quantifying the performance degradation rates of III-V-based, unipolar barrier infrared detectors with various designs and materials, cutoff wavelengths and operating conditions due to 63 MeV proton irradiation is presented. Empirical relationships were established between the radiation damage factors for dark current density, lateral optical collection length, and quantum efficiency and the inverse product of the detectors' cutoff wavelength and operating temperature. Fitting the dark current density damage factor's empirical relationship reflected these detectors' tendency to remain diffusion-limited during irradiation, which was previously established using Arrhenius-analysis of the post-irradiation, temperature-dependent dark current measurements on each. Collectively, the results affirmed the performance degradation stemmed from a reduction of the minority carrier recombination lifetime via generation of additional defects by proton-induced displacement damage. For comparing detector's radiation-tolerance, the results indicated that damage factors alone were not ideal, but their empirical relationships would serve as heuristics in this role.

Titanium dioxide/silicon hole-blocking selective contact to enable double-heterojunction crystalline silicon-based solar cell

In this work, we use an electron-selective titanium dioxide (TiO2) heterojunction contact to silicon to block minority carrier holes in the silicon from recombining at the cathode contact of a silicon-based photovoltaic device. We present four pieces of evidence demonstrating the beneficial effect of adding the TiO2 hole-blocking layer: reduced dark current, increased open circuit voltage (VOC), increased quantum efficiency at longer wavelengths, and increased stored minority carrier charge under forward bias. The importance of a low rate of recombination of minority carriers at the Si/TiO2 interface for effective blocking of minority carriers is quantitatively described. The anode is made of a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) heterojunction to silicon which forms a hole selective contact, so that the entire device is made at a maximum temperature of 100 °C, with no doping gradients or junctions in the silicon. A low rate of recombination of minority carriers at the Si/TiO2 interface is crucial for effective blocking of minority carriers. Such a pair of complementary carrier-selective heterojunctions offers a path towards high-efficiency silicon solar cells using relatively simple and near-room temperature fabrication techniques.

New Architecture towards Ultrathin CdTe Solar Cells for High Conversion Efficiency

Solar Cell Capacitance Simulator in 1 Dimension (SCAPS-1D) is used to investigate the possibility of realizing ultrathin CdTe based solar cells with high and stable conversion efficiency. In the first step, we modified the conventional cell structure by substituting the CdS window layer with a CdS:O film having a wide band gap ranging from 2.42 to 3.17 eV. Thereafter, we simulated the quantum efficiency, as well as the parameters ofJ-Vcharacteristics, and showed how the thickness of CdS:O layer influences output parameters of Glass/SnO2/ZTO/CdS:O/CdTe1-xSx/CdTe/Ni reference cell. High conversion efficiency of 17.30% has been found using CdTe1-xSx(x=0.12) and CdTe layers of thickness 15 nm and 4 μm, respectively. Secondly, we introduced a BSR layer between the absorber layer and back metal contact, which led to Glass/SnO2/ZTO/CdS:O/CdTe1-xSx/CdTe/BSR/Ni configuration. We found that a few nanometers (about 5 nm) of CdTe1-xSxlayer is sufficient to obtain high conversion efficiency. For BSR layer, different materials with large band gap, such as ZnTe, Cu2Te, and p+-CdTe, have been used in order to reduce minority carrier recombination at the back contact. When ZnTe is used, high conversion efficiency of 21.65% and better stability are obtained, compared to other BSR.

Impact of tungsten contamination on the sensing margin of a CMOS image sensor cell

We investigated the impact of tungsten contamination on minority-carrier recombination lifetime, the photodiode dark/photo current and sensitivity of a pinned photodiode, and the sensing margin of a CMOS image sensor (CIS) cell. After an intentional tungsten contamination and followed by driving at 800 °C for 30 min, tungsten contaminant were located from the surface to p- and n-type regions of a photodiode. The tungsten contamination degraded minority-carrier recombination life-time and the dark current and sensitivity of a pinned photodiode; i.e., there was a good correlation between the minority recombination lifetime and the sensitivity of a photodiode. As a result, tungsten contamination directly degraded the sensing margin of a CIS cell photodiode; i.e., it decreased with increasing tungsten contaminant concentration.

Silicon nanodisk array design for effective light trapping in ultrathin c-Si

The use of ultrathin c-Si (crystalline silicon) wafers thinner than 20 μm for solar cells is a very promising approach to realize dramatic reduction in cell cost. However, the ultrathin c-Si requires highly effective light trapping to compensate optical absorption reduction. Conventional texturing in micron scale is hardly applicable to the ultrathin c-Si wafers; thus, nano scale texturing is demanded. In general, nanotexturing is inevitably accompanied by surface area enlargements, which must be minimized in order to suppress surface recombination of minority carriers. In this study, we demonstrate using optical simulations that periodic c-Si nanodisk arrays of short heights less than 200 nm and optimal periods are very useful in terms of light trapping in the ultrathin c-Si wafers while low surface area enlargements are maintained. Double side texturing with the nanodisk arrays leads to over 90% of the Lambertian absorption limit while the surface area enlargement is kept below 1.5.

Effect of nanohole structure on pyramid textured surface on photo-voltaic performance of silicon solar cell

We investigated how the nanohole structure on a {111} pyramid textured surface affected the photovoltaic performance of silicon solar cells by varying nanohole depth. During the effective minority carrier recombination lifetime, the surface reflectance decreased with increasing nanohole depth, showing a trade-off relationship. The power conversion efficiency (PCE) of silicon solar cells with the {111} pyramid textured surface peaked at a specific nanohole depth, i.e., 0.43% PCE enhancement (3.32% enhancement relative to a reference cell) was obtained at the nanohole depth of 94.8 nm.

Nanocrystalline diamond on Si solar cells for direct photoelectrochemical water splitting

This article presents a study of a boron‐doped nanocrystalline diamond (B‐NCD) and crystalline silicon solar cell (c‐Si SC) heterostructure designed for direct photoelectrochemical water splitting application. A system is proposed which generates voltages sufficient for water‐splitting via light absorption by solar cells. The rear side (photo‐anode) of the c‐Si solar cell was shielded with the B‐NCD layer intended to serve as a protective coating to eliminate corrosion of the Si and operate as a p‐type conductive electrode suited for water decomposition. Information on the efficiency of the c‐Si/B‐NCD heterostructure and the distribution of the electric potential has been gained through investigation of minority carrier recombination mapping. The results of the B‐NCD layer characterization by Raman spectroscopy, AFM, SEM and optical reflection measurements are presented. It is demonstrated that inclusion of a boron (p+) diffusion layer into a c‐Si solar cell structure prevents lifetime (LT) degradation caused by the B‐NCD deposition process. In addition it is shown that the B‐NCD deposition temperature has a substantial effect on the properties of c‐Si solar cell.

Efficient carrier-selective p- and n-contacts for Si solar cells

The successful realization of carrier-selective contacts is a crucial prerequisite to approaching the theoretical efficiency limit of silicon solar cells. The herein proposed carrier-selective contacts resemble the polysilicon contacts, including a tunnel oxide, but make use of a wide bandgap semi-crystalline Si layer thereby forming a heterostructure with the crystalline silicon substrate. It will be shown that these tunnel oxide passivated contacts enable a low interface recombination, e.g. the electron contact achieved implied Voc (iVoc) of about 720mV. On the other hand, the interface passivation quality provided by the hole contact was significantly lower (iVoc=680mV) and it seems that its design is more challenging. The critical aspects which might explain the lower performance are addressed in this paper. On solar cell level good carrier selectivity was demonstrated which means that the passivated contacts effectively reduced minority carrier recombination and were transparent for majority carriers as well. The latter manifests itself in an external Voc of 694mV and a FF of 81%. Raman spectroscopy revealed that the Si layer can partially contain crystalline Si phases and it will be demonstrated that this significantly improved the cell׳s blue response. In combination with the higher thermal stability these carrier-selective contacts could be a very appealing option for next generation high-efficiency silicon solar cells once the limitations for the p-contact are overcome.

Influence of the feedstock purity on the solar cell efficiency

There is a strong need for low-cost silicon for solar cells and accordingly, there is an on-going debate about what are the acceptable contamination levels within the purified silicon feedstock to specify the material as solar grade silicon. Impurities in silicon can drastically modify the material properties relevant for solar cells. Transition metal impurities such as iron, copper and chromium are known to enhance the net recombination of minority carriers within the bulk material or negatively influence the emitter region and thus are detrimental for the cell efficiency. The knowledge about the level of metals and dopants, which are present in solar cell material, is of utmost interest for the use of alternative feedstock materials such as upgraded metallurgical silicon (UMG-Si) feedstock and for the improvement of actual industrial materials for solar cell production. Whereas most previous studies used artificial high contamination levels to investigate their influence on the solar cells efficiency, this work presents data using recent industrial feedstock material. For the first time, sensitive and direct chemical analyses of silicon before and after crystallisation were performed and correlated with final solar cell results.

Design of Extremely Proximity Gettering Using Hydrogen Ion Implantation for Si CMOS Image-Sensor

As the pixel size in CMOS image-sensor has scaled down less than 1 μm, photo sensitivity which is a ratio of photo and dark current has been important. To improve photo sensitivity in CMOS image-sensor, the achievement of lower dark current is essential. Leakage current is influenced by crystallographic, trapped charge at the SiO2/Si interface, structural defects, and especially metallic contaminant impurities. Also, these contaminant impurities degrade the minority-carrier recombination lifetime. Thus, we investigated the dependency of the properties of CMOS image-sensor on metallic contamination. Standard aqueous solutions of metallic ion were dropped on the surface of silicon wafers and the wafers were spin dry. They were heated to drive the metallic ion contaminants into the silicon bulk. After that, we fabricated 4-Tr CMOS image-sensor cells. In silicon bulk wafer, the minority-carrier recombination lifetime decreased with increasing a metallic contaminant concentration. These contaminations result in degradation of dark current, sensitivity of photodiode and sensing margin of CMOS image-sensor. These results indicate that metallic contaminants are critical for increasing dark current because metallic contaminants remain inside the photo-diode region, ~3 μm. In our study, we applied a proximity relaxation-type gettering method. We formed the nano-cavities close to the depletion region using hydrogen ion implantation, as shown in Fig 1. These cavities getter metallic impurities on the cavities wall. Sensing margin of CMOS image-sensor implanted by hydrogen ion was dramatically improved, as shown in Fig 2. In addition, although metallic contaminant concentration increased, the device performances of CMOS image-sensor were constant compare with that of bulk wafer which were not implanted (Fig. 3). These results indicates that proximity gettering by using hydrogen ion implantation was effective in metal gettering for image-sensor. We investigated the correlation of metallic contaminant and sensing margin of CMOS image-sensor. Finally, we will suggest an extremely proximity gettering method for removing metallic contaminants of CMOS image-sensor.* This work was financially supported by the Brain Korea 21 Plus Program in 2013 and SiWEDS (Silicon Wafer Engineering and Defect Science).

Parametric simulation of the back-surface field effect in the silicon solar cell

Recombination of minority carriers in the solar cell is a major contributing factor in the loss of quantum efficiency and cell power. While the surface recombination is dealt with by depositing a passivation layer of SiO2 or SiNx, the bulk recombination is minimized by use of nearly defect-free monocrystalline substrate. In addition, the back-surface field (BSF) effect has been very useful in aiding the separation of free electrons and holes in the bulk. In this study, the key BSF parameters and their effect on the performance of a typical p-type front-lit Si solar cell are investigated by use of Medici, a 2-dimensional device simulator. Of the parameters, the doping concentration of the BSF layer is found to be most significant. That is, for a p-type substrate of 1 × 1014 cm−3 acceptor concentration, the optimum doping concentration of the BSF layer is 1 × 1018 cm−3 or more, and the maximum cell power can be increased by 24%, i.e., 25.4 mW cm−2 vs. 20.5 mW cm−2, by using a BSF layer with optimum doping. With regards to the BSF layer thickness, the impact is less. That is, the maximum cell power is about 11% higher at 100 μm than at 5 μm, which translates to an increase of 1.2% μm−1. In practice, therefore, it would be better to rely on the control of the doping concentration than the thickness in maximizing the BSF effect in real Si solar cells.