Carrier Lifetime Analysis Research Articles

Oxygen vacancy enriched and Cu single-atom contained covalent organic frameworks: A competitive photocatalyst to promote hydrogen evolution under visible light

Single-atom loaded covalent organic frameworks (COFs) have been rapidly developed in the field of photocatalytic hydrogen production in recent years. Despite the insufficient active sites has been introduced, the high recombination rate of photogenerated carriers has hindered the progress of catalyst development. In this work, a novel approach is proposed to construct oxygen vacancies (OVs) on single atom Cu contained COF-TpPa, acting as an electron collector to facilitate the transfer and utilization of electrons. The Cu-OV-TpPa can provide a remarkable H2 production rate of 2.77 mmol g−1 h−1 under visible light, which is 2.6 times of Cu-P-TpPa (a H2 production rate of 1.02 mmol g−1 h−1). It achieves a high apparent quantum efficiency (AQE) of 20 % at 420 nm with no noble metal cocatalyst. A thorough analysis of carrier lifetime, recombination rate, electric conductivity and charge density reveals that the synergistic effect between OVs and single-atom Cu significantly enhances photogenerated carrier separation efficiency and reduces activation energy. This study presents a new strategy to design COF-based photocatalysts towards a competitive H2 production under visible light.

Double-diode model carrier lifetime-based internal recombination parameter analysis and efficiency prediction of crystalline Si solar cells

With the continuous and significant advancements in the performance of crystalline silicon (c-Si) solar cells, an effective method for a detailed analysis of the efficiency loss factors is increasingly important. Specifically, a detailed analysis of recombination parameter (J0) is crucial for devising strategies to approach the theoretical efficiency limit. In this study, we propose an analysis method that utilizes minority carrier lifetime based on the double-diode model and predicts the performance efficiency of c-Si solar cells. The analysis, conducted through the p-PERC structure, examines the contributions to J0 from different recombination areas. The advantages of this method include: (i) its relative simplicity and reliance on non-destructive analysis techniques; (ii) its capability to extract internal recombination parameters, such as J0,total emitter, J0,rear, and J02, from a fully assembled solar cell with metal contacts. Furthermore, this method serves as a diagnostic tool to identify the sequence in which parameters should be adjusted to enhance solar cell efficiency effectively and to pinpoint areas within the solar cell that significantly limit performance efficiency. Using this method, the diagnosis of the p-PERC cell revealed that the main cause of performance limitation was the emitter in the front region. The carrier lifetime analysis showed that for overcoming cell performance limitations, it is more effective to design process improvements in the following order: i) front emitter region (J0,em), ii) space charge region (J02), iii) rear surface region (J0r).

New Ternary Blend Strategy Based on a Vertically Self-Assembled Passivation Layer Enabling Efficient and Photostable Inverted Organic Solar Cells.

Herein, a new ternary strategy to fabricate efficient and photostable inverted organic photovoltaics (OPVs) is introduced by combining a bulk heterojunction (BHJ) blend and a fullerene self-assembled monolayer (C60 -SAM). Time-of-flight secondary-ion mass spectrometry - analysis reveals that the ternary blend is vertically phase separated with the C60 -SAM at the bottom and the BHJ on top. The average power conversion efficiency - of OPVs based on the ternary system is improved from 14.9% to 15.6% by C60 -SAM addition, mostly due to increased current density (Jsc ) and fill factor -. It is found that the C60 -SAM encourages the BHJ to make more face-on molecular orientation because grazing incidence wide-angle X-ray scattering - data show an increased face-on/edge-on orientation ratio in the ternary blend. Light-intensity dependent Jsc data and charge carrier lifetime analysis indicate suppressed bimolecular recombination and a longer charge carrier lifetime in the ternary system, resulting in the enhancement of OPV performance. Moreover, it is demonstrated that device photostability in the ternary blend is enhanced due to the vertically self-assembled C60 -SAM that successfully passivates the ZnO surface and protects BHJ layer from the UV-induced photocatalytic reactions of the ZnO. These results suggest a new perspective to improve both performance and photostability of OPVs using a facial ternary method.

Advanced carrier lifetime analysis method of silicon solar cells for industrial applications

Over the last few years, the silicon photovoltaic industry has been searching for efficient technologies to reduce the cost of cost/Wpeak by improving their conversion efficiency. However, with the silicon solar cell efficiency approaching the theoretical limit, it has become important to increase the efficiency distribution of cells by managing the loss factors existing in the production process. Although some companies are managing the efficiency distribution through in-line analysis equipment, a preventive action for reducing process defects has not yet been accomplished. To prevent process defects in advance, big data management is required to correlate the process conditions and solar cell parameters. Hence, an analysis method that can simply evaluate the detailed properties of completed solar cells is a key technique in future PV industries. In this paper, we introduce an advanced injection-dependent carrier lifetime analysis logic that can subdivide detailed recombination factors by fitting the carrier lifetime graph obtained through Suns-Voc measurement with a theoretical graph. The industrial applicability of this analysis method was tested with 160 commercially manufactured solar cells, demonstrating the expected quantitative gains of low-efficiency solar cells. The results show that the proposed method helps in determining the process priority for improving the efficiency distribution and provides a research direction for increasing the absolute efficiency.

Photovoltaics literature survey (No. 181)

Photovoltaics literature survey (No. 181)

Analysis of carrier lifetime in a drift layer of 1.2-kV class 4H–SiC devices toward complete suppression of bipolar degradation

We analyzed the carrier lifetime in a drift layer of 1.2 kV-class SiC p-n diodes to suppress bipolar degradation. According to the device simulation results, the required carrier lifetime in the drift layer was estimated to be shorter than 10 ns with a current density of 300 A/cm2 if the threshold hole density for the expansion of the stacking fault at the interface of the drift layer and substrate was 2 × 1016 cm−3. Numerical analysis revealed that to ensure a carrier lifetime shorter than 10 ns, the 1/e2 lifetime obtained by microwave photoconductivity decay (μ-PCD) measurements should be shorter than 7.2 ns. Experimental μ-PCD measurements showed that 1/e2 lifetimes obtained from the as-received epiwafer were much longer than 7.2 ns, and even after H+ implantation, high-temperature annealing, or electron irradiation, 1/e2 lifetimes were still long. Therefore, carrier lifetime control in the drift layer is not sufficient to suppress bipolar degradation, and a combination of carrier lifetime control with other methods is necessary for the fabrication of 1.2 kV-class SiC p-n diodes for the complete suppression of bipolar degradation at a current capacity of 300 A/cm2.

The role of phase impurities and lattice defects on the electron dynamics and photochemistry of CuFeO2 solar photocathodes

CuFeO2 is a promising photocathode for H2 evolution and CO2 reduction reactions. To better understand the complex defect chemistry and role of impurity phases in this material and their effect on the photochemical performance, we employ visible light transient absorption spectroscopy and density functional theory (DFT) calculations to investigate the electron dynamics in electrochemically deposited Cu-Fe oxide thin films. Kinetic analysis of carrier lifetime shows a fast, sub-ps contribution to relaxation followed by persistence of a long-lived state to time delays greater than 2 ns. Increasing amplitude of the long-lived state is shown to correlate with the rate of fast initial relaxation, and this is explained in terms of a competition between charge carrier trapping and charge separation. Charge separation in CuFeO2 occurs via hole thermalization from O 2p to Cu 3d valence band states leading to segregation of electrons and holes across layers in the CuFeO2 lattice. Correlation between transient absorption measurements and DFT calculations suggest that Cu vacancies enhance photochemical performance by facilitating charge separation kinetics. In contrast, O interstitials are predicted to switch the relative positions of O 2p and Cu 3d valence band states, which would inhibit charge separation by inter-band hole thermalization. Finally, we find no evidence for electron injection from CuFeO2 to CuO suggesting that charge separation at this heterostructure interface does not play a role in the carrier lifetime or photochemical performance of the catalysts studied here.

Theoretical analysis of minority carrier lifetime and Cd-free buffer layers on the CZTS based solar cell performances

In this report, one dimensional simulation program (SCAPS-1D) has been used to study the crucial effect of minority carrier lifetime on Cu2ZnSnS4 based solar cells. The results have led to a strong correlation between the minority lifetime and the Cu2ZnSnS4 absorber thickness. Then, the modeling and investigation of Cu2ZnSnS4 based solar cells with graded Zn1-xMgxO and ZnO1-ySy buffer layers revealed that, these buffer layers could be an alternative to the CdS layer if the Mg and S compositions are suitably adjusted. In the case of Zn1-xMgxO buffer layer, the device with Zn0.75Mg0.25O shows the best efficiency and produces an improvement of 4.8%, whereas cells with ZnO1-ySy show an enhancement above 5% once the S composition is larger than 0.3. These performances have ascribed to the favorable band alignment at the buffer layers/Cu2ZnSnS4 interface, a strong electric field at the p-n junction and great transparency of these alternative buffer layers. The simulations have also showed a high potential barrier at the ZnO-intrinsic/alternative buffer layers interface which harms the electrons’ transport.

Spectroscopic and Microscopic Defect and Carrier-Lifetime Analysis in Cadmium Telluride

Using experiments and theory, we analyze “carrier-lifetime-killer” semiconductor bulk defects in CdTe. We can estimate the concentration of tellurium antisite (TeCd) recombination centers in undoped CdTe from the kinetic modeling of injection-dependent photoluminescence (PL) lifetimes. For the Cd-rich stoichiometry, the concentration of lifetime-limiting [TeCd] is ≥1013 cm–3. For the Te-rich stoichiometry, we find a smaller hole-capture rate constant than predicted by theory, which could be because of TeCd association with the Te interstitials. We also compare PL emission spectra, carrier lifetimes, and radiative efficiencies for Cu, P, and As doping and analyze the microscopic distribution of carrier lifetimes in As-doped single-crystal CdTe. In all the cases examined here, point defects are the dominant bulk recombination centers. Carrier lifetimes are the longest in undoped CdTe, but radiative efficiencies are the highest with group-V doping.

Doping Dependent Assessment of Accumulation Mode and Junctionless FET for 1T DRAM

This paper demonstrates the use of double-gate accumulation mode (AM) and junctionless (JL) transistors for dynamic memory applications at 85 °C. The doping dependent assessments of AM and JL devices include an analysis of storage volume, carrier lifetime, and depth of potential well to determine characteristics of Dynamic Random Access Memory (DRAM). This paper shows significant impact of carrier lifetime for channel doping $({N}_{\text{d}}) \le 10^{ {18}}$ cm−3 on Retention Time (RT), while the depth of potential well is more critical at higher doping (>1018 cm−3). RT of ~2.5 s at 85 °C and ~4.5 s at 27 °C is achieved for gate length ( ${L}_{\text{g}}$ ) of 400 nm with ${N}_{\text{d}} = 10^{{17}}$ cm−3 which reduces to ~90 ms at 85 °C for ${L}_{\text{g}} = 25$ nm. This paper discusses the storage volume ( ${L}_{\text{g}} \times \text {film}$ thickness for a fixed volume with width of $1~ {\mu }\text{m}$ ) optimization to attain maximum retention. Insights and guidelines, as a function of doping and device dimensions, are outlined for dynamic memory applications.

Identification of the limiting factors for high-temperature GaAs, GaInP, and AlGaInP solar cells from device and carrier lifetime analysis

We analyze the temperature-dependent dark saturation current density and open-circuit voltage (VOC) for GaAs, GaInP, and AlGaInP solar cells from 25 to 400 °C. As expected, the intrinsic carrier concentration, ni, dominates the temperature dependence of the dark currents. However, at 400 °C, we measure VOC that is ∼50 mV higher for the GaAs solar cell and ∼60–110 mV lower for the GaInP and AlGaInP solar cells compared to what would be expected from commonly used solar cell models that consider only the ni2 temperature dependence. To better understand these deviations, we measure the carrier lifetimes of p-type GaAs, GaInP, and AlGaInP double heterostructures (DHs) from 25 to 400 °C using time-resolved photoluminescence. Temperature-dependent minority carrier lifetimes are analyzed to determine the relative contributions of the radiative recombination, interface recombination, Shockley-Read-Hall recombination, and thermionic emission processes. We find that radiative recombination dominates for the GaAs DHs with the effective lifetime approximately doubling as the temperature is increased from 25 °C to 400 °C. In contrast, we find that thermionic emission dominates for the GaInP and AlGaInP DHs at elevated temperatures, leading to a 3–4× reduction in the effective lifetime and ∼40× increase in the surface recombination velocity as the temperature is increased from 25 °C to 400 °C. These observations suggest that optimization of the minority carrier confinement layers for the GaInP and AlGaInP solar cells could help to improve VOC and solar cell efficiency at elevated temperatures. We demonstrate VOC improvement at 200–400 °C in GaInP solar cells fabricated with modified AlGaInP window and back surface field layers.

Statistical analysis of recombination properties of the boron-oxygen defect in p-type Czochralski silicon

This paper presents the application of lifetime spectroscopy to the study of carrier-induced degradation ascribed to the boron-oxygen (BO) defect. Specifically, a large data set of p-type silicon samples is used to investigate two important aspects of carrier lifetime analysis: ①the methods used to extract the recombination lifetime associated with the defect and ② the underlying assumption that carrier injection does not affect lifetime components unrelated to the defect. The results demonstrate that the capture cross section ratio associated with the donor level of the BO defect (k1) vary widely depending on the specific method used to extract the defect-specific recombination lifetime. For the data set studied here, it was also found that illumination used to form the defect caused minor, but statistically significant changes in the surface passivation used. This violation of the fundamental assumption could be accounted for by applying appropriate curve fitting methods, resulting in an improved estimate of k1 (11.90±0.45) for the fully formed BO defect when modeled using the donor level alone. Illumination also appeared to cause a minor, apparently injectionindependent change in lifetime that could not be attributed to the donor level of the BO defect alone and is likely related to the acceptor level of the BO defect. While specific to the BO defect, this study has implications for the use of lifetime spectroscopy to study other carrier induced defects. Finally, we demonstrate the use of a unit-less regression goodness-of-fit metric for lifetime data that is easy to interpret and accounts for repeatability error.

Injection-dependent Carrier Lifetime Analysis of Recombination Due to Boron-Oxygen Complexes in Wafers Passivated with Different Dielectrics by QSSPL and QSSPC

This paper compares the use of quasi-steady state photoluminescence (QSSPL) and photoconductance (QSSPC) measurements for injection-dependent carrier lifetime analysis of the state of boron-oxygen (B-O) complexes in boron-doped Czochralski wafers passivated with different dielectrics. Use of QSSPL measurements enabled effective carrier lifetime measurements over a larger range of injection levels than possible with QSSPC, potentially increasing the accuracy of estimates of the capture cross-section ratio of the deep-level B-O Shockley-Read Hall (SRH) defect. Although the capture cross-section ratios estimated using QSSPL (9.7 ± 1.7) and QSSPC (9.7 ± 1.9) for wafers symmetrically-passivated with silicon nitride and subsequently rapidly fired were very similar and close to the value of 9.3 reported by Rein et al. for the B-O defect, significant differences in the ratio and larger variances in the measurements within a group resulted for wafers with silicon nitride that was not annealed and wafers which were passivated with the thermal oxide. It is suggested the low-injection QSSPL measurements may introduce inaccuracies arising from the conduction of excited carriers via the diffused emitter away from the measurement area. This may suggest that there is little advantage in using QSSPL over the more commonly-used QSSPC measurements for these SRH analyses. Additionally, the study confirmed that stable regeneration of the effective carrier lifetime was only achieved when wafers were passivated with silicon nitride and underwent a rapid thermal anneal after deposition. If wafers were not rapidly thermally-annealed after silicon nitride deposition, then the recovery of effective lifetime observed with regeneration was not stable during a second light soak. Furthermore, the effective carrier lifetime of wafers passivated with a thermal oxide continued to decrease with light soaking, regeneration and a second light soaking with the appearance of “rings” in the PL images of oxide-passivated wafers as they underwent light soaking and regeneration highlighting the possible role of oxide precipitates in reactions occurring with light soaking and regeneration.

Study on efficiency droop in InGaN/GaN light-emitting diodes based on differential carrier lifetime analysis

Efficiency droop is currently one of the most popular research problems for GaN-based light-emitting diodes (LEDs). In this work, a differential carrier lifetime measurement system is optimized to accurately determine carrier lifetimes (τ) of blue and green LEDs under different injection current (I). By fitting the τ-I curves and the efficiency droop curves of the LEDs according to the ABC carrier rate equation model, the impact of Auger recombination and carrier leakage on efficiency droop can be characterized simultaneously. For the samples used in this work, it is found that the experimental τ-I curves cannot be described by Auger recombination alone. Instead, satisfactory fitting results are obtained by taking both carrier leakage and carriers delocalization into account, which implies carrier leakage plays a more significant role in efficiency droop at high injection level.

Impact of the transparent conductive oxide work function on injection-dependent a-Si:H/c-Si band bending and solar cell parameters

An analysis of the contact formation between degenerated n-type transparent conductive oxide (TCO) and p-type amorphous silicon (a-Si:H) as it is used for front side contacts in high efficiency a-Si:H/crystalline silicon (c-Si) heterojunction solar cells is presented. It is shown that the deposition of a TCO on a (p)a-Si:H emitter layer causes a reduction of charge carrier lifetime in low injection levels which leads to a lowering of the implied fill factor. Simulation based analysis of charge carrier lifetime and direct measurements by surface photovoltage reveals that TCO deposition induces a change of the c-Si band bending. The magnitude of this change depends on the (p)a-Si:H doping level. Both observations are explained by the impact of the TCO/a-Si:H work function difference on the c-Si band bending. Based on numerical simulations, the reduced injection-dependent band bending is identified as the reason for the reduced fill factor of final solar cells.

Recombination lifetime in InGaN/GaN based light emitting diodes at low current densities by differential carrier lifetime analysis

AbstractCarrier lifetimes of the recombination processes in light emitting diodes (LEDs) depend strongly on the operating point. We measure the carrier lifetimes of InGaN/GaN LEDs at low current densities by applying the differential carrier lifetime analysis. In the measurement setup the LED is driven with constant current modulated with small amplitude alternating current (AC) and the carrier lifetime is obtained from the phase difference between input AC and light output.In addition to the optical method, we also extract the lifetimes directly from the capacitance and differential resistance of the LED obtained using an impedance analyzer and compare the results to lifetimes obtained by the optical setup. The carrier lifetimes were found to increase from hundreds of nanoseconds to microseconds when the current density was decreased from tens of A/cm2 to tens of mA/cm2. Both measurement methods resulted in similar lifetimes (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Dislocation Density Reduction During Impurity Gettering in Multicrystalline Silicon

Isothermal annealing above 1250 °C has been reported to reduce the dislocation density in multicrystalline silicon (mc-Si), presumably by pairwise dislocation annihilation. However, this high-temperature process may also cause significant impurity contamination, canceling out the positive effect of dislocation density reduction on cell performance. Here, efforts are made to annihilate dislocations in mc-Si in temperatures as low as 820 °C, with the assistance of an additional driving force to stimulate dislocation motion. A reduction of more than 60% in dislocation density is observed for mc-Si containing intermediate concentrations of certain metallic species after P gettering at 820 °C. While the precise mechanism remains in discussion, available evidence suggests that the net unidirectional flux of impurities in the presence of a gettering layer may cause dislocation motion, leading to dislocation density reduction. Analysis of minority carrier lifetime as a function of dislocation density suggests that lifetime improvements after P diffusion in these samples can be attributed to the combined effects of dislocation density reduction and impurity concentration reduction. These findings suggest there may be mechanisms to reduce dislocation densities at standard solar cell processing temperatures.

In situ analysis of carrier lifetime and barrier capacitance variations in silicon during 1.5 MeV protons implantation

Results of the in situ measurements of the recombination lifetime and of barrier capacitance variations in Si substrates and pin diodes, respectively, during 1.5 MeV protons implantation are presented. Carrier recombination lifetime has been measured by employing microwave probed photoconductivity method, while parameters of the barrier capacitance changes have been extracted by transient technique of barrier capacitance charging current measurements using linearly increasing voltage pulses. Sub-linear decrease of carrier lifetime as a function of fluence has been revealed and peculiarities of such characteristic are explained in terms of formation of two layered structure within implanted Si material. Carrier recombination processes determine the increase of dielectric relaxation time within electrically neutral region (ENR) of a diode base. Carrier capture/emission processes within space charge (SC) and transition layer (between ENR and SC) regions lead to increase of generation/recombination currents in the irradiated diode.

Submicron resolution carrier lifetime analysis in silicon with Fano resonances

AbstractDefect rich regions in multicrystalline silicon are investigated by Raman spectroscopy at high and low injection levels. By analyzing the Fano type asymmetry and the spectral position of the first order Raman peak crucial properties such as recombination lifetime, doping density and stress can be extracted simultaneously. Due to the small wavelength of the excitation laser the spatial resolution of these measurements is significantly below 1 µm, which gives new insight into the impact of defects on the carrier recombination lifetime. The results are evaluated by comparing them to micro‐photoluminescence and synchrotron X‐ray fluorescence measurements. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Determination of the bulk lifetime of bare multicrystalline silicon wafers

AbstractThe determination of the bulk lifetime of bare multicrystalline silicon wafers without the need of surface passivation is a desirable goal. The implementation of an in‐line carrier lifetime analysis is only of benefit if the measurements can be done on bare unprocessed wafers and if the measured effective lifetime is clearly related to the bulk lifetime of the wafer. In this work, we present a detailed experimental study demonstrating the relationship between the effective carrier lifetime of unpassivated wafers and their bulk carrier lifetime. Numerical modelling is used to describe this relationship for different surface conditions taking into account the impact of a saw damage layers with poor electronic quality. Our results show that a prediction of the bulk lifetime from measurements on bare wafers is possible. Based on these results we suggest a simple procedure to implement the analysis for in‐line inspection. Copyright © 2010 John Wiley & Sons, Ltd.