Improvement Of Minority Carrier Lifetime Research Articles

The role of boron related defects in limiting charge carrier lifetime in 4H–SiC epitaxial layers

One of the main challenges in realizing 4H–SiC (silicon carbide)-based bipolar devices is the improvement of minority carrier lifetime in as-grown epitaxial layers. Although Z1/2 has been identified as the dominant carrier lifetime limiting defect, we report on B-related centers being another dominant source of recombination and acting as lifetime limiting defects in 4H–SiC epitaxial layers. Combining time-resolved photoluminescence (TRPL) measurement in near band edge emission and 530 nm, deep level transient spectroscopy, and minority carrier transient spectroscopy (MCTS), it was found that B related deep levels in the lower half of the bandgap are responsible for killing the minority carriers in n-type, 4H–SiC epitaxial layers when the concentration of Z1/2 is already low. The impact of these centers on the charge carrier dynamics is investigated by correlating the MCTS results with temperature-dependent TRPL decay measurements. It is shown that the influence of shallow B acceptors on the minority carrier lifetime becomes neutralized at temperatures above ∼422 K. Instead, the deep B related acceptor level, known as the D-center, remains active until temperatures above ∼570 K. Moreover, a correlation between the deep level concentrations, minority carrier lifetimes, and growth parameters indicates that intentional nitrogen doping hinders the formation of deep B acceptor levels. Furthermore, tuning growth parameters, including growth temperature and C/Si ratio, is shown to be crucial for improving the minority carrier lifetime in as-grown 4H–SiC epitaxial layers.

Improved minority carrier lifetime in p-type GaN by suppressing the non-radiative recombination process

Time-resolved photoluminescence and capacitance-voltage measurement were performed on p-type GaN and InGaN films to study the minority carrier recombination mechanism. The minority carrier lifetime (τ PL) for p-GaN with a Mg concentration of 1.7 × 1019 cm−3 was 46 ps. The non-radiative recombination due to gallium vacancies (V Ga)-related defects is confirmed to dominate the minority carrier transport process. To suppress the formation of V Ga defects, the indium atoms were added into p-GaN. As a consequence, the V Ga-related non-radiative recombination centers were reduced from 8 × 1015 to 5 × 1014 cm−3 and a record long τ PL of 793 ps was obtained for p-In0.035Ga0.95N film.

Reduction of trapping and recombination in upgraded metallurgical grade silicon: Impact of phosphorous diffusion gettering

Upgraded metallurgical grade (UMG) silicon (Si) has raised interest as an alternative material for solar cells due to its low cost, low environmental impact and low CAPEX. Maximum cell efficiencies at the level of those obtained from high purity poly-Si have been reported. However, a higher defect density and the compensated doping character result in UMG-based cell efficiencies varying over wider ranges in frequency distribution charts. In this report we characterize mc-Si UMG samples with different defect densities, comparing them with mono-Si UMG and commercial high-performance mc-Si samples, analysing the impact of carrier trapping by means of photoconductance (PC) decay measurements, and its evolution after applying a phosphorous diffusion gettering (PDG) process. When analyzing the decay time constant of the PC measurements, slow (66.8+-14.3 ms) and fast (16.1+-3.5 ms) traps are found in mc-Si samples, while no evidence of trapping is found in mono-UMG samples. Slow traps are effectively removed after the PDG process, while fast traps do remain. The influence of dislocations clusters and the possible role of oxygen, as revealed by Fourier-transform infrared spectroscopy (FTIR) is discussed. Finally, the improvement in minority carrier lifetime due to the PDG treatment is reported for each sample type, reaching values up to 140 us in mc-Si samples with no slow traps neither interstitial oxygen FTIR-peaks

Improved minority carrier lifetime in p-type GaN segments prepared by vacancy-guided redistribution of Mg

To accelerate the development of GaN power devices, reproducible fabrication of p-type GaN (p-GaN) segments by ion-implantation (I/I) that enables selective-area doping is preferred. In this Letter, the results of time-resolved photoluminescence (PL) and spatially resolved cathodoluminescence (CL) measurements on p-GaN fabricated by I/I are presented. The p-GaN samples of a 220-nm-deep box-shaped Mg concentration profile ([Mg] = 3×1018 cm−3) were fabricated by sequential I/I of Mg and N followed by atmospheric-pressure post-implantation annealing (PIA). During PIA, Mg redistribution was guided by N-I/I-induced vacancies. The intensities of PL peaks originating from Mg acceptors were increased by increasing the PIA temperature (Ta), indicating progressive activation of Mg and simultaneous decrease in the concentration of nonradiative recombination centers (NRCs). As a result, a record-long room-temperature PL lifetime for the near-band edge emission of Ga-polar Mg-implanted p-GaN (1.4 ps) was obtained when Ta = 1300 °C. Because the species of major vacancy-type defects in the present sample was determined as vacancy clusters comprised of few Ga-vacancies (VGa) and N-vacancies (VN) such as (VGaVN)2–3, the NRC concentration is roughly estimated at around 1017 cm−3 provided that (VGaVN)2–3 are major NRCs with the electron-capture-coefficient of 7×10−6 cm3 s−1. Cross-sectional CL linescan images revealed certain activation of Mg from the surface to the leading edge of the box-shaped [Mg] profile, while NRCs remained at the depths right below the box-shaped profile. The results support advantages of using the vacancy-guided Mg redistribution technique for controlling the activated [Mg] profile and realizing p-type Ga-polar GaN:Mg segments by I/I.

Interface management of silicon-nanowire based hybrid solar cells through facile solution-processed oxidation

Surface recombination of photogenerated carriers turned to be critical issues against the realization of high-performance nanowire-based solar cells. To overcome it, the strategy toward passivating nanowire surfaces was essential where the improvement of minority carrier lifetime was identified in recent years; instead, the examinations of optical management as well as charge-transfer resistance at heterojunction were still limited. In this study, we herein investigated the facile, inexpensive and reliable HNO3-based chemical passivation that directly introduced SiOx coating on Si nanowires. By adjusting the oxidation durations, the cell performance exhibited the improved conversion efficiency with more than 39% of efficiency enhancement than reference cells. Impacts of light harvesting and charge separation characteristics on cell performances were in-depth envisioned.

(Keynote) High Efficiency Heterojunction c-Si Based Solar Cell and Module

1. INTRODUCTION High-efficiency heterojunction (HJ) back-contact crystalline Si (c-Si) solar cells with record-breaking conversion efficiencies are reported. We have so far focused our attention on the HJ technology using amorphous Si (a-Si) passivation layers, and we reported the fabrication of a large-area both-side-contact HJ c-Si solar cell with a record-breaking cell conversion efficiency of 25.1% in 2015 [1]. Using our HJ technology, we have achieved a record-breaking cell conversion efficiency of 26.7%, and a module conversion efficiency of 24.5% in c-Si solar cells [2]. Recent improvements of perovskite solar cells and c-Si solar cells have opened a path to realizing conversion efficiencies of over 30% by using a tandem structure solar cell. Recently, a 29.15%-efficiency perovskite-silicon tandem solar cell was reported [3]. In this paper, we report our state-of-the-art technologies for high-efficiency HJ c-Si solar cells and present perspectives for further improvement of conversion efficiency using tandem solar cells with high-efficiency HJ c-Si and perovskite solar cells based on our recent results. 2. HJBC solar cell Heterojunction back-contact c-Si solar cells (HJBC solar cells) are fabricated using n-type CZ crystalline Si wafers. Figure 1 shows appearance and a typical schematic image of a cross section and of the HJBC solar cell. To improve the conversion efficiency of the HJBC solar cell, we have improved our HJ technology with high-quality a-Si and a low-resistance electrode technology for a back-contact structure. By combining and optimizing the technologies, we developed a HJBC solar cell with a conversion efficiency of 26.3% in 2016, which was a record-breaking efficiency for Si solar cells [5]. In our preliminary experiments of the development of passivation technology, we have confirmed that further improvement of minority carrier lifetime is possible, which reduces the extrinsic recombination loss in the conversion efficiency by an absolute value of ~0.7% from ~1.2%. Small reductions in series resistance loss and optical loss are also possible by a small optimization. Therefore, it is reasonable to expect that a conversion efficiency of 27.1% is achievable by using our state-of-the-art technology [5]. Targeting a conversion efficiency of over 27%, we have improved our HJBC technologies. Now, we reach a conversion efficiency of 26.7% [2]. Besides the cell, using high-efficiency HJBC solar cells and newly developed technologies to minimize the resistance loss of intercell connection in the module and to raise the collection efficiency of the light radiated into the module, we achieved a conversion efficiency of 24.5% efficiency in a module with a size of a building-integrated photovoltaics (BIPV) product [2]. 3. Perovskite/c-Si tandem solar cell One of the low-cost solar cell candidates with efficiencies higher than the theoretical limit of Si is a combination of a perovskite solar cell and a c-Si solar cell. An organic-inorganic hybrid perovskite solar cell has rapidly progressed, and its conversion efficiency has reached 25.2% [3]. A perovskite solar cell is highly attractive as a top cell of a multijunction solar cell owing to its wide- and tunable-bandgap absorber. A record efficiency of 29.15% is reported for a tandem device comprising a perovskite component cell on a Si solar cell [3]. We have also developed perovskite solar cells and examined their multijunction structures with two-terminal and three-terminal tandem as shown in Fig. 2. Our derailed results can be found in ref 6-8. We believe that the improvement of the processes and materials of perovskite solar cells will lead to conversion efficiencies of over 30%. Acknowledgments The authors would like to thank Dr. Rudi Santbergen of Delft University of Technology for performing optical simulations of perovskite/c-Si tandem solar cells. This work was supported in part by the New Energy and Industrial Technology Development Organization (NEDO) under the Ministry of Economy, Trade and Industry of Japan. Reference D.Adachi et al., Appl. Phys. Lett. 107, 23350 (2015).K. Yamamoto et al., Jpn. J. Appl. Phys 57, 08RB20 (2018).https://www.helmholtz-berlin.de/pubbin/news_seite?nid=21020;sprache=en;seitenid=73236M A Green et al., Prog Photovolt Res Appl. 28, 3 (2020).Y, Yoshikawa et al., Nat. Energy 2, 17032 (2017).R. Mishima et al. Appl. Phys. Express 10, 062301 (2017).R. Santbergen et al., Optics Express. 24, A1288 (2016).R. Santbergen et al., IEEE J. Photovolt. 9, 446 (2019). Figure 1

The effect of diffusion barrier on minority carrier lifetime improvement of seed assisted cast silicon ingot

A newly designed diffusion barrier (DB) was introduced to industrial silicon ingot casting with various seeding techniques, in order to reduce the height of bottom low lifetime region. The DB was fabricated by alternatively brushing silica and silicon nitride coatings with high purity on the inner bottom of the crucible. The effects of DB on mono-like (ML) and multicrystalline (assisted by chips and slabs) ingots were characterized and compared. The results showed that the DB had much stronger influence in mono-like ingot than in multicrystalline ingots. The related mechanism on ingot bottom lifetime improvement was discussed.

Understanding of Molecular Contribution of Quinhydrone/Methanol Organic Passivation for Improved Minority Carrier Lifetime on Nanostructured Silicon Surface

In this report, we present a study of the quinhydrone/methanol (QHY/MeOH) organic passivation technique for a silicon (Si) surface. The roles of p-benzoquinone (BQ) and hydroquinone (HQ), which make up QHY, in controlling the uniformity and coverage of the passivation layer as well as the minority carrier lifetime (τeff) of Si were investigated. The uniformity and coverage of the passivation layer after treatment with diverse mixture ratios of BQ and HQ in MeOH were studied with two different atomic force microscope (AFM) techniques, namely tunneling mode (TUNA) and high-resolution tapping mode AFM (HR-AFM). In addition, the τeff and surface potential voltages (SPV) of passivated surfaces were measured to clarify the relationship between the morphologies of the passivation layers and degrees of surface band bending. The molecular interactions between BQ and HQ in MeOH were also analyzed using Fourier-transform infrared spectroscopy (FT-IR). In our study, we successfully demonstrated the role of each molecule for effective Si surface passivation with BQ working as a passivation agent and HQ contributing as a proton (H+) donator to BQ for accelerating the passivation rate. However, our study also clearly revealed that HQ could also hinder the formation of a conformal passivation layer, which raises an issue for passivation over complex surface geometry, especially a nanostructured surface.

Ambient annealing influence on surface passivation and stoichiometric analysis of molybdenum oxide layer for carrier selective contact solar cells

Effective carrier contact with excellent surface passivation is essential for archiving high efficiency silicon solar cells. In this study, optical, passivation and stoichiometric characteristics of MoOx thin films were analyzed by varying thickness, temperature and ambient annealing. The 10 nm thick MoOx films showed high optical transmittance (89.81%) in the visible near infra-red (NIR) wavelength (380–1100) nm region and wide optical bandgap of 3.25 eV. An improved minority carrier lifetime of 2.24 ms was recorded for O2 ambient at 130 °C due to less additional defects in the MoOx films. X-rays photoelectron spectroscopic analysis confirmed that O2 ambient annealing greatly influenced the MoOx stoichiometric and reduce the formation defective state densities at the Fermi energy initiating from the tail of an oxygen vacancy and inducted the defect band within the energy bandgap. Carrier selective contact (CSC) solar cells were fabricated by using 10 nm thick MoOx film as hole extraction layer exhibited the high performance as; open circuit voltage (Voc) = 708 mV, current density (Jsc) = 37.80 mA/cm2, fill factor (FF) = 74.95% and efficiency (η) = 20.06%. These results implies that the MoOx is an important material to use in future CSC solar cell applications with the concept of new advantages such as dopant free, non-toxic and low temperature fabrication process.

Analysis for efficiency potential of crystalline Si solar cells

Abstract

Effect of Cu2O hole transport layer and improved minority carrier life time on the efficiency enhancement of Cu2NiSnS4 based experimental solar cell

C u 2 NiSn S 4 is a non-toxic earth abundant material and a promising quaternary semiconductor compound. Due to its optimum direct band gap, it has been considered as a suitable absorber material for photovoltaic cells. It is a conspicuous and suitable class of material for the fabrication of low cost and high efficiency thin film devices. This paper presents numerical modeling for the efficiency enhancement of Cu2NiSnS4 based experimental photovoltaic cells. In this work, the experimental cell results were reproduced in the SCAPS software. These simulated results are validated and compared with the experimental reference cell. Cu2O as the hole transport layer is also proposed for further efficiency enhancement of the photovoltaic cell. After optimization of cell parameters, the power conversion efficiency of an optimized device is increased up to 4.60%. By applying the hole transport layer and analyzing the minority carrier life time, the conversion efficiency increases up to 10.35%. This work presents a novel concept in numerical modeling by analyzing the experimental solar cell, which will categorically offer new directions for the fabrication of high efficiency photovoltaic devices.

Combining Low-Temperature Gettering With Phosphorus Diffusion Gettering for Improved Multicrystalline Silicon

We have investigated low-temperature (≤500 °C) gettering in combination with phosphorus diffusion gettering with a view to improving poor quality multicrystalline silicon. Low-temperature gettering applied after standard phosphorus diffusion gettering is found to provide a >40% improvement in minority carrier lifetime in samples from the top and bottom of an ingot. The best results are achieved at 300 °C with very long annealing times (>24 h). Improvements in the lifetime do not correlate with changes in interstitial iron concentration. Experiments are performed to assess whether the presence of a phosphorus-diffused emitter affects low-temperature gettering, and results from sister samples show the low-temperature gettering behavior is not affected by the existence of an emitter. Further experiments show that low-temperature gettering prior to phosphorus diffusion results in a 20% higher lifetime after phosphorus diffusion. Low-temperature gettering can, therefore, enhance lifetime even when used in conjunction with standard phosphorus diffusion gettering.

A Facile and TGA Free Hydrothermal Synthesis of SnS Nanoparticles

In this paper, we employed a simple and cost-effective thioglycolic acid (TGA) free hydrothermal method, based on thiourea hydrolysis of stannous chloride dihydrate [SnCl2.2H2O] at 160[Formula: see text]C–190[Formula: see text]C for 6[Formula: see text]h, for the synthesis of SnS nanoparticles. The effect of hydrothermal autoclave reaction temperature on various properties of SnS nanoparticles have been examined at length using X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), field-emission scanning electron microscopy attached with EDAX (FE-SEM), transmission electron microscopy (TEM) and ultraviolet-visible (UV-Vis) spectroscopy. The results suggest that the crystallization of orthorhombic SnS nanoparticles, with size varying from 3[Formula: see text]nm to 5[Formula: see text]nm, formed at R[Formula: see text]C. Further, the formation of SnS phase was confirmed by an IR Sn-S characteristic bands around 2350[Formula: see text]cm[Formula: see text], 1041[Formula: see text]cm[Formula: see text] and 570[Formula: see text]cm[Formula: see text], and four distinguished Raman peaks at 95[Formula: see text]cm[Formula: see text], 160[Formula: see text]cm[Formula: see text], 189[Formula: see text]cm[Formula: see text] and 220[Formula: see text]cm[Formula: see text]. The mechanism for the formation of SnS nanoparticles have been proposed and discussed. The SnS nanoparticles have exhibited reaction temperature dependent morphological features like nanoflowers, nanoflakes, spherical nanoparticles and nanogranules. The absorbance studies indicated both strong direct and weak indirect allowed transitions for SnS nanoparticles and the associated band gaps were found to be 1.5[Formula: see text]eV and 1.19[Formula: see text]eV, respectively. The dual band gap combination of SnS would favor strong direct absorption of carriers and improved minority carrier life time due to indirect nature, which means the grown particles are suitable for ideal absorber material for solar cell applications.

Improvement of minority carrier lifetime and conversion efficiency by Na incorporation in Cu2ZnSnSe4 solar cells

The effect of Na incorporation in Cu2ZnSnSe4 (CZTSe) solar cells grown by the coevaporation method was investigated via photoluminescence (PL) and time-resolved PL (TRPL) measurements as well as photovoltaic properties. The TRPL decay curves showed a monotonic increase in CZTSe lifetime from 2 to 15 ns with increasing Na incorporation, which corresponds to the increase in the correction length estimated by quantum efficiency measurements. The TRPL decay curves included two decay components, fast and slow, which were discussed and concluded as originating from the recombination at the surface and bulk of CZTSe, respectively, which is also supported by TPRL measurements with various excitation wavelengths. The lifetime of CZTSe is limited by the surface-related nonradiative recombination compared to Cu(In,Ga)Se2 devices which are fabricated with the same device structure except for the absorber, and at present, it is concluded that the surface recombination of the CZTSe limits the cell performance. In addition to the above investigations, the relationship between the CZTSe bulk lifetime and carrier concentration is discussed; deep nonradiative recombination centers in the CZTSe bulk were found to decrease by one order of magnitude with Na incorporation. The Na incorporation primarily resulted in improvement in the short circuit current density and fill factor and not in the open circuit voltage, and the results are discussed. The best performing CZTSe solar cell with Na incorporation showed a conversion efficiency of 9.57%.

A novel passivation process of silicon nanowires by a low-cost PECVD technique for deposition of hydrogenated silicon nitride using SiH4 and N2 as precursor gases

In this work, a different SiNx passivation process of silicon nanowires has been opted for the deposition of a hydrogenated silicon nitride (SiNx:H) by a low-cost plasma enhanced chemical vapor deposition (PECVD) using silane ( SiH4 and nitrogen ( N2 as reactive gases. This study is focused on the effect of the gas flow ratio on chemical composition, morphological, optical and optoelectronic properties of silicon nanowires. The existence of Si-N and Si-H bonds was proven by the Fourier transmission infrared (FTIR) spectrum. Morphological structures were shown by scanning electron microscopy (SEM), and the roughness was investigated by atomic force microscopy (AFM). A low reflectivity less than 6% in the wavelength range 250-1200nm has been shown by UV-visible spectroscopy. Furthermore, the thickness and the refractive index of the passivation layer is determined by ellipsometry measurements. As a result, an improvement in minority carrier lifetime has been obtained by reducing surface recombination of silicon nanowires.

Organic passivation of silicon through multifunctional polymeric interfaces

In this work, we demonstrate a solvent-free, low temperature (<25–170°C) approach using chemical vapor deposition (CVD) grafting and polymerization processes to passivate the unpaired electrons or “dangling” bonds on the surface of silicon. The multifunctional (dielectric and electronically conducting) passivating layers described here achieved several orders of magnitude improvement in minority carrier lifetime (>2ms) compared to bare silicon (~30μs), and remained stable in air for over 200h. These values approach that of SiNx films deposited at significantly higher temperatures. The polymer passivation processes and materials are shown to significantly reduce surface recombination rates: chemically by reducing density of surface states and electrostatically by altering band bending at the silicon interface. Passivation quality also improved on grafting using aliphatic monomers compared to aromatic ones suggesting a reduction in steric effects in the former, helping us posit design rules for polymer based surface passivation of silicon. Finally, the ability to use an electrically conducting polymer for passivation creates a direct interface between traditional silicon microelectronics and organic electronics. Beyond microelectronics and photovoltaics, these processes can enable the fabrication of hybrid or multifunctional devices, such as biosensors and light emitting diodes.

Effect of Sn doping on improvement of minority carrier lifetime of Fe contaminated p-type multi-crystalline Si ingot

The influence of Sn doping on improvement of minority carrier lifetime (MCL) of Fe contaminated directionally solidified p-type multi-crystalline Si ingots is studied. The macrostructure and resistivity distribution of the Si ingots indicate that no significant difference exists with and without Sn doping. The average MCL increase by 26.2%, 31.8%, 8.1% with 20ppmw, 40ppmw, 60ppmw Sn doping, respectively. The MCL was improved evidently due to the reduction of formation of interstitial Fe, FeB. The doping of Sn promotes formation of vacancies, which also contributes to passivation of interstitial Fe and FeB. A calculation of constitutional supercooling is carried out, which shows that Fe have great influence on the solidification interface stability, and Sn have little influence on the interface stability.

Hydrogenation Defect Passivation for Improved Minority Carrier Lifetime in Midwavelength Ga-Free InAs/InAsSb Superlattices

Two hydrogenation techniques were used to passivate defects in InAs/InAsSb superlattices: UV-photon assisted hydrogenation with and without DC bias enhancement. The effects of the hydrogenation on the minority carrier lifetime were studied using photoluminescence and optical modulation response. An increase of the minority carrier lifetime from 1.8 μs to 3.3 μs with hydrogenation using both methods was observed; however, the processing time shortened from 24 h to 90 min when using the DC bias enhancement. The largest increase in carrier lifetime corresponded to a deuterium density of 9 × 1014 atoms/cm2, as measured by nuclear reaction analysis.

Low Temperature Metalorganic Chemical Vapor Deposition of Semiconductor Thin Films for Surface Passivation of Photovoltaic Devices

ABSTRACT Three II-VI wide bandgap compound semiconductors have been investigated for surface passivation of various photovoltaic devices. First part of this work focuses on the surface passivation of HgCdTe IR detectors using CdTe. A new metalorganic chemical vapor deposition (MOCVD) process has been developed that involves depositing CdTe films at much lower temperature (&amp;lt; 175°C) than the conventional processes used till now. Deposition rate as high as 420nm/h was obtained using this novel experimental setup. Favorable conformal coverage on high aspect ratio HgCdTe devices along with a significant minority carrier lifetime improvement was obtained. Another II-VI semiconductor, namely, CdS was investigated as a surface passivant for HgCdTe IR detectors. It was deposited by MOCVD as well as atomic layer deposition (ALD) and was studied for optimal conformal coverage on high aspect ratio structures. Surface passivation of p-type Si wafer has also been demonstrated using p-ZnTe grown by MOCVD, for possible application in solar cells. Preliminary work showed a remarkable improvement in the minority carrier lifetime of Si light absorbing layer after passivation with a thin layer of ZnTe.

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.