Minority Carrier Concentration Research Articles

(Invited) In Situ Photoelectron Spectroscopy Reveals the Chemical Nature of Semiconductor Surface States

The accessible photovoltage of semiconducting photoabsorbers is typically 0.5-1 V below the theoretically achievable values predicted by the Shockley-Queisser limit [1]. Although the reason for this is still not well understood, surface and interface states within the photoabsorbers energy band gap may play a crucial role as they generally induce Fermi level pinning [2]. Within the PEC community, two key elements have been identified for the maximization of the photovoltage in photoelectrodes for water splitting: (i) the passivation of surface defects which is needed to avoid Fermi level pinning, and (ii) the increase of the minority carrier concentration at the interface to improve contact selectivity and optimize carrier extraction [3,4].To understand the role of surface defects on Fermi level pinning, detailed information on the chemical nature and electronic properties of surface states is needed. Such information is usually obtained with UHV surface science techniques such as XPS, but the photoelectrode surface under UHV conditions has little, if any, relevance to the electrified surface when immersed in the electrolyte. Moreover, recent studies have shown that semiconductor surfaces are not static, but undergo extensive structural and chemical transformations during PEC device operation [5,6]. In this talk, we will show our recent development of state-of-the-art techniques to study the structure and dynamics of semiconductor/water interfaces under practically relevant conditions [7]. The chemical nature of the electronic states for selected semiconductors prepared at the Institute for Solar Fuels has been explored using synchrotron-based resonant, ambient pressure soft and hard X-ray photoelectron spectroscopy (AP-XPS and AP-HAXPES) [6,8-11], and with in situ/operando Raman and photoluminescence spectroscopy. In particular, we will show that it is possible to study photon-induced chemical changes at solid/liquid interfaces using AP-XPS and AP-HAXPES [6,8].We will conclude this contribution by discussing about future perspectives and technical implementations for multimodal in situ/operando investigations of photoelectrocatalytic processes.[1] M.T. Mayer. Curr. Opin. Electrochem. 2017, 2, 104.[2] A. J. Bard et al. J. Am. Chem. Soc. 1980, 102, 3671.[3] A.G. Scheuermann et al., Nat. Mater. 2016, 15, 99.[4] M. Schleuning et al., Sustainable Energy Fuels, 2022, 6, 3701.[5] F.M. Toma et al., Nat. Commun. 2016, 7, 12012.[6] M. Favaro et al., J. Phys. Chem. B 2018, 122, 801.[7] M. Favaro et al., Surf. Sci. 2021, 713, 121903.[8] M. Favaro et al., J. Phys. D: Appl. Phys. 2021, 54, 164001.[9] M. Favaro et al., J. Phys. Chem. C 2019, 123, 8347.[10] W. Wang et al., J. Am. Chem. Soc. 2022, 144, 17173.[11] P. Schnell et al., Sol. RRL 2023, 2201104.

Exploring the border traps near the valence band in the SiO2 - SiC system using above-band-gap optical excitation

Studying near-valence-band (EV) defects at the insulator–n-type 4H-SiC interface is challenging due to the low minority carrier concentration. Herein, we present a technique for characterizing the border traps near EV in an n-type 4H-SiC MOS capacitor by generating holes using above-band-gap optical excitation (OE). A rise in capacitance was observed under OE (due to hole capture by border traps), remaining stable long after the optical stimulus was removed, resulting in a persistent photocapacitance effect. We show the dynamics of the capture process and perform a quantification of the captured holes in samples with thermally grown oxide and different postoxidation annealing treatments. Published by the American Physical Society 2024

Fabrication of Ordered Macropore Arrays in n-Type Silicon Wafer by Anodic Etching Using Double-Tank Electrochemical Cell.

In this work, ordered macropore arrays in n-type silicon wafers were fabricated by anodic etching using a double-tank electrochemical cell. The effects of the wafer thickness, etching time and voltage on the quality of macropore arrays were investigated. Homogeneous macropore arrays could be achieved in 200 μm thick silicon wafers, but could not be obtained from 300 and 400 μm thick silicon wafers. Highly ordered macropore arrays with an aspect ratio of 19 were fabricated in 200 μm thick n-type silicon at 4.5 V. The etching current decreases in 200 μm thick silicon but increases in thicker silicon with an increase in time. It demonstrates that the minority carrier transportation capability from the illuminated surface to the reactive surface is different for silicon wafers with different thicknesses. The minority carrier concentration at the illuminated surface for stable macropore formation and the current under different etching voltages were calculated based on a hole transport model. The results show that appropriately decreasing wafer thickness and increasing voltage can help stable macropore array fabrication in the illumination-limited double-tank cell.

Defect structure regulation and thermoelectric transfer performance in n-type Bi<sub>2–<i>x</i></sub> Sb<sub><i>x</i></sub>Te<sub>3–<i>y</i></sub>Se<sub><i>y</i></sub>-based compounds

Bi<sub>2</sub>Te<sub>3</sub>-based compounds are thermoelectric materials with the best performance near room temperature. The existence of a large number of complex defects makes defect engineering a core stratagem for adjusting and improving the thermoelectric performance. Therefore, understanding and effectively controlling the existence form and concentration of defects is crucial for achieving high-thermoelectric performance in Bi<sub>2</sub>Te<sub>3</sub>-based alloy. Herein, a series of Cl doped n-type quaternary Bi<sub>2–<i>x</i></sub> Sb<sub><i>x</i></sub>Te<sub>3–<i>y</i></sub>Se<sub><i>y</i></sub> compounds is synthesized by the zone-melting method. The correlation between defect evolution process and thermoelectric performance is systematically investigated by first-principles calculation and experiments. Alloying Sb on Bi site and Se on Te site induce charged structural defects, leading to a significant change in the carrier concentration. For Bi<sub>2–<i>x</i></sub> Sb<sub><i>x</i></sub>Te<sub>2.994</sub>Cl<sub>0.006</sub> compounds, alloying Sb on Bi site reduces the formation energy of the <inline-formula><tex-math id="M6">\begin{document}${\mathrm{S}}{{\text{b}}_{{\mathrm{Te}}}}_{_2}$\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20240098_M6.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20240098_M6.png"/></alternatives></inline-formula> antisite defect, which generates the antisite defect <inline-formula><tex-math id="M7">\begin{document}${\mathrm{S}}{{\text{b}}_{{\mathrm{Te}}}}_{_2}$\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20240098_M7.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20240098_M7.png"/></alternatives></inline-formula> and accompanied with the increase of the minority carrier concentration from 2.09×10<sup>16</sup> to 3.99×10<sup>17</sup> cm<sup>–3</sup>. The increase of the minority carrier severely deteriorates the electrical transport properties. In contrast, alloying Se in the Bi<sub>1.8</sub>Sb<sub>0.2</sub>Te<sub>2.994–<i>y</i></sub>Se<sub><i>y</i></sub>Cl<sub>0.006</sub> compound significantly lowers the formation energy of the complex defect <inline-formula><tex-math id="M8">\begin{document}${\mathrm{S}}{{\mathrm{e}}_{{\mathrm{Te}}}}$\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20240098_M8.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20240098_M8.png"/></alternatives></inline-formula>+<inline-formula><tex-math id="M9">\begin{document}${\mathrm{S}}{{\mathrm{b}}_{{\mathrm{Bi}}}}$\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20240098_M9.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20240098_M9.png"/></alternatives></inline-formula>, which becomes more energetically favorable and suppresses the formation of the antisite defect <inline-formula><tex-math id="M10">\begin{document}${\mathrm{S}}{{\text{b}}_{{\mathrm{Te}}}}_{_2}$\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20240098_M10.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20240098_M10.png"/></alternatives></inline-formula>. As a result, the concentration of minority carriers decreases to 1.46×10<sup>16</sup> cm<sup>–3</sup>. This eliminates the deterioration effect of the minority carrier on the electrical transport properties of the material and greatly improves the power factor. A maximum power factor of 4.49 mW/(m·K<sup>2</sup>) is achieved for Bi<sub>1.8</sub>Sb<sub>0.2</sub>Te<sub>2.944</sub>Se<sub>0.05</sub>Cl<sub>0.006</sub> compound at room temperature. By reducing thermal conductivity through intensifying the phonon scattering via alloying Sb and Se, the maximum <i>ZT</i> value of 0.98 is attained for Bi<sub>1.8</sub>Sb<sub>0.2</sub>Te<sub>2.844</sub>Se<sub>0.15</sub>Cl<sub>0.006</sub> compound at room temperature. Our finding provides an important guidance for adjusting point defects, carrier concentrations, and thermoelectric performances in Bi<sub>2</sub>Te<sub>3</sub>-based compounds with complex compositions.

(Invited) In Situ Photoelectron Spectroscopy Reveals the Chemical Nature of Semiconductor Surface States

The accessible photovoltage of semiconducting photoabsorbers is typically 0.5-1 V below the theoretically achievable values predicted by the Shockley-Queisser limit [1]. Although the reason for this is still not well understood, surface and interface states within the photoabsorbers energy band gap may play a crucial role as they generally induce Fermi level pinning [2]. Within the PEC community, two key elements have been identified for the maximization of the photovoltage in photoelectrodes for water splitting: (i) the passivation of surface defects which is needed to avoid Fermi level pinning, and (ii) the increase of the minority carrier concentration at the interface to improve contact selectivity and optimize carrier extraction [3,4].To understand the role of surface defects on Fermi level pinning, detailed information on the chemical nature and electronic properties of surface states is needed. Such information is usually obtained with UHV surface science techniques such as XPS, but the photoelectrode surface under UHV conditions has little, if any, relevance to the electrified surface when immersed in the electrolyte. Moreover, recent studies have shown that semiconductor surfaces are not static, but undergo extensive structural and chemical transformations during PEC device operation [5,6]. In this talk, we will show our recent development of state-of-the-art techniques to study the structure and dynamics of semiconductor/water interfaces under practically relevant conditions [7]. The chemical nature of the electronic states for selected semiconductors prepared at the Institute for Solar Fuels has been explored using synchrotron-based resonant, ambient pressure soft and hard X-ray photoelectron spectroscopy (AP-XPS and AP-HAXPES) [6,8-11], and with in situ/operando Raman and photoluminescence spectroscopy. In particular, we will show that it is possible to study photon-induced chemical changes at solid/liquid interfaces using AP-XPS and AP-HAXPES [6,8].We will conclude this contribution by discussing about future perspectives and technical implementations for multimodal in situ/operando investigations of photoelectrocatalytic processes.[1] M.T. Mayer. Curr. Opin. Electrochem. 2017, 2, 104.[2] A. J. Bard et al. J. Am. Chem. Soc. 1980, 102, 3671.[3] A.G. Scheuermann et al., Nat. Mater. 2016, 15, 99.[4] M. Schleuning et al., Sustainable Energy Fuels, 2022, 6, 3701.[5] F.M. Toma et al., Nat. Commun. 2016, 7, 12012.[6] M. Favaro et al., J. Phys. Chem. B 2018, 122, 801.[7] M. Favaro et al., Surf. Sci. 2021, 713, 121903.[8] M. Favaro et al., J. Phys. D: Appl. Phys. 2021, 54, 164001.[9] M. Favaro et al., J. Phys. Chem. C 2019, 123, 8347.[10] W. Wang et al., J. Am. Chem. Soc. 2022, 144, 17173.[11] P. Schnell et al., Sol. RRL 2023, 2201104.

Synergy of Front-Surface Energy-Level Gradient and Lattice Anchoring Effect for Enhancing Perovskite Solar Cell Performance.

A front surface gradient of the absorber valence band can effectively reduce the open-circuit voltage (VOC) loss of perovskite solar cells by suppressing the minority carrier concentration near the front surface. However, the existing method is limited to the one-step fabrication process, resulting in underachieved photon harvesting and power conversion efficiency (PCE). To solve the problem, ZnCd-based alloy quantum dots (QDs) are utilized to create a valence-band-maximum gradient at the front surface of a two-step processed FAPbI3 absorber. This design significantly enhances VOC without requiring surface passivation. Furthermore, it is demonstrated that reducing the QD-perovskite lattice mismatch while maintaining QD's energy levels mitigates nonradiative recombination without compromising the front surface gradient effect. As a result, normal-structured perovskite solar cells achieve a VOC equivalent to 93% of the Schockley-Queisser limit and a PCE of 24.37%.

Blistering-free carbon-doped polysilicon (n+) passivating contact with high surface passivation properties prepared by industrial tube PECVD

Tube plasma-enhanced chemical vapor deposition (PECVD) has garnered attention as a promising large-scale production tool for in-situ doped polysilicon due to its high capacity and low equipment cost. In this work, we explore the use of carbon (C)-doped a-Si:H (n+), deposited by tube PECVD, for tunnel oxide passivated contact (TOPCon) solar cells. By introducing carbon atoms into polysilicon, blistering-free polysilicon films with excellent passivation properties can be obtained. The optimized sample with C-doped polysilicon (n+) exhibits remarkable surface passivation with a single-sided saturation current density (J0,s) of 1.25 fA/cm2, an implied open-circuit voltage (iVoc) of 751 mV, and a corresponding effective lifetime exceeding 18 ms at a minority carrier concentration of 1 × 1015 cm−3. UV Raman measurements reveal that C doping suppresses the crystallization of polysilicon, leading to a significant stress in the film. Despite excessive C doping, the contact resistivity extracted from the TLM method remains below 13.6 mΩ cm2. As a result, a champion conversion efficiency of 23.64 % was achieved in a small-sized proof-of-concept n-type TOPCon solar cell. This demonstrates the feasibility of fabricating C-doped polysilicon (n+) prepared by tube PECVD for high-efficiency and low-cost TOPCon solar cells.

Understanding the temperature sensitivity of the photovoltaic parameters of perovskite solar cells

Perovskite solar cells (PSCs) have promised high-efficiency and low-cost solar-to-electrical conversion that now go outdoors for practical applications; however, the elevated outdoor temperature remarkably affects the photovoltaic efficiency. To date, there has been little work about understanding the temperature sensitivity of PSCs. Here, we build an analytical model to understand the temperature sensitivity and the limiting factors of temperature coefficient (TC) in PSCs. Our model is based on diffusion-drift numerical method by involving the temperature-dependent bandgap and band edge of perovskite layers, which quantitively describe the effect of many physical factors, including carrier recombination channels, trap density, band alignment and band variation of transport layers. It was found that the built-in electrical field and minority carrier concentration at interface are closely corrected with temperature, that is responsible for the temperature sensitivity of device performance. We show that this model can be used to explain the disparate temperature sensitivity of a series of reported real PSCs, and also gives potential pathways for the design and fabrication of temperature-insensitive photovoltaics towards practical applications.

Kinetics of charge carriers in bilateral macroporous silicon

The kinetics of charge carriers in bilateral macroporous silicon with macroporous layers of equal thicknesses is calculated by the finite difference method. A diffusion equation for a monocrystalline substrate and macroporous layers is solved. The boundary conditions are defined at the boundaries between the monocrystalline substrate and the macroporous silicon layers on both sides. Stationary distribution of excess charge carriers in the bilateral macroporous silicon with the macroporous layers of equal thicknesses calculated by the finite difference method is set as the initial condition. Under stationary conditions, excess charge carriers are generated by light with the wavelengths of 0.95 µm and 1.05 µm. It is shown that at the counting times much longer than the relaxation time, all the distributions of the concentration of excess minority carriers generated by light with any wavelength approach the same distribution with exponentially decreasing value.

Simulation study on the influence of metal contact and MOS interface trap states on the electrical characteristics of SiC IGBT

A 13 kV class n-channel 4H–SiC trench gate insulated gate bipolar transistor (IGBT) structure is designed based on Silvaco TCAD device simulator tool. The influence of metal/SiC and SiC MOS interface trap states on the static and dynamic characteristics of SiC IGBT devices are systematically studied. It is found that the electrical properties of SiC IGBTs are insensitive to the donor or acceptor traps at the interface of metal/SiC and the donor traps at the SiC/SiO2 interface. However, the acceptor traps at the SiC/SiO2 interface affect the electrical properties of SiC IGBTs greatly. When the acceptor trap density at the SiC/SiO2 interface (Dita) is up to the level of 1012 cm−2·eV−1, the turn-on voltage drop (VCEON) is increased gradually with the augmentation of Dita. The breakdown voltage (VBR) of the device is increased slightly. Further study of the device’s turn-off characteristics shows that the turn-off loss (EOFF) is decreased with the increase in Dita but a large tail current is formed. The mechanism may be that when the electrons are injected from the N+ source region into the N− drift region, the SiC/SiO2 interfacial acceptor traps at the side wall of trench gate capture a large number of electrons. As a result, the minority carrier concentration is decreased, and the conductance modulation effect on the N− drift region is weakened.

Quasi-3-dimensional simulations and experimental validation of surface leakage currents in high operating temperature type-II superlattice infrared detectors

The surface leakage in InAs/GaSb type-II superlattice (T2SL) is studied experimentally and theoretically for photodiodes with small sizes down to 10 × 10 μm2. The dependence of dark current density on mesa size is studied at 110 and 200 K, and surface leakage is shown to impact both generation–recombination (GR) and diffusion dark current mechanisms. A quasi-3-dimensional model to simulate the fabrication process using surface traps on the pixel's sidewall is presented and is used to accurately represent the dark current of large and small pixels with surface leakage in the different temperature regimes. The simulations confirmed that the surface leakage current has a GR and diffusion component at low and high temperature, respectively. Finally, the surface leakage current has been correlated with the change in minority carrier concentration at the surface due to the presence of donor traps.

Deep Learning Extraction of the Temperature-Dependent Parameters of Bulk Defects.

Bulk defects in silicon solar cells are a key contributor to loss of efficiency. To detect and identify those defects, temperature- and injection-dependent lifetime spectroscopy is usually used, and the defect parameters are traditionally extracted using fitting methods to the Shockley-Read-Hall recombination statistics. In this study, we propose a deep learning-based extraction technique that is based on an alternative representation of the lifetime curves: lifetime mapping in the temperature and minority carrier concentration space. The deep learning approach successfully predicts all the defect parameters while addressing one of the main limitations of the traditional approach of locating the defect in the energy spectrum, which usually outputs two possible solutions. Furthermore, the approach is applied to temperature-dependent defect parameters where the traditional approach is not applicable, achieving satisfying levels of prediction of the defect parameters. Image representation and deep learning have the potential to bolster solar cell characterization techniques by extracting more insights from the characterization data.

(Digital Presentation) Calculation of the Energy Band Diagram and Estimation of Electronic Transport Parameters of Metastable Amorphous Ge2Sb2Te5

Phase change memory (PCM) is a high speed, high density non-volatile resistive memory technology that utilizes different phases (crystalline and amorphous) of phase change materials such as Ge2Sb2Te5 (GST) to store information [1]. Here, the material undergoes two types of reversible switching phenomena: (i) Ovonic Threshold Switching (OTS), which causes the amorphous phase of the material to switch from a highly resistive state to conductive state with application of high electric fields, resulting in current flow and (ii) Ovonic Memory Switching (OMS), which is due to the change of the phase of the material between amorphous and crystalline phases induced by heating [2]. Even though PCM entered high volume manufacturing, the electronic properties of the phase change materials are still not well understood [3,4]. In this work, we construct the energy band diagram of amorphous GST as a function of temperature using a temperature dependent model of effective activation energy of conduction in metastable amorphous GST [5], which we obtained from high-speed experiments on GST line-cells [6]. Assuming the bandgap energy to linearly decrease with temperature [7,8] and p-type conduction (based on positive Seebeck coefficients measured in a wide temperature range [9]) , we determine a temperature dependent Fermi energy level from 0K to melting temperature, Tmelt ~ 858 K. Liquid GST is expected to become metallic (bandgap collapsing to 0) at ~ 894K, based on the experimental results. We also estimate the carrier concentration at Tmelt utilizing the latent heat of fusion (126 kJ/kg) [10] to be pmelt = 1.47 x 1022 cm-3. Using the melt resistivity measured on GST thin film, we calculate the carrier mobility at melting point as µmelt ~0.187 cm2/V-s, close to the previously reported value of 0.15 cm2/V-s based on crystalline state mobility and a density of states calculation [2]. Assuming a weak temperature dependence of the mobility [5], we obtain the carrier concentration of ~3.37× 1017 cm-3 at room temperature which lies within the ~1017-1018 cm-3 range estimated in a former study [3]. Finally, we calculate conduction activation energy of as-deposited amorphous GST from temperature dependent Seebeck coefficient measured simultaneously with the resistance [9]. The activation energy varies as a parabolic function of temperature where it starts from 0 eV at 0 K, reaches a peak of ~0.257 eV near glass transition temperature (~400 K) with the room temperature value of ~0.24 eV and becomes 0 eV again at ~810 K. We also utilize the Seebeck coefficient measurements along with the band edges and Fermi energy level information from the energy band diagram to calculate the ratio of minority carrier concentration to the total carrier concentration as a function of temperature; this is useful to predict the temperature beyond which bipolar conduction becomes significant.Acknowledgment: Analysis is performed with the support of US National Science Foundation (NSF) award ECCS 1711626. The experimental data used for this analysis were collected with the support of US NSF DMR-1710468 on devices fabricated with the support of US Department of Energy Office of Basic Energy Sciences.

Gamma distribution excess minority carrier concentration model for solar cell performance modeling

Concentrated photovoltaic (PV) technology represents a growing market in the field of terrestrial solar energy production. As the demand for renewable energy technologies increases, further importance is placed on the modeling, design, and simulation of these systems. Given the cultural shift toward energy awareness and conservation, several concentrated PV systems have been installed across the world. This research presents a new model for carrier concentration within a solar cell. The goal of this innovation is to facilitate the determination of the steady-state operating temperature as a function of the concentration factor for the optical part of the concentrated PV system, to calculate the optimum concentration that maximizes power output and efficiency. This model will be shown to produce a more realistic estimate of the current through a solar cell, which will enable further research into dynamic thermal modeling.

Distribution of excess charge carriers in bilateral macroporous silicon with the same thickness of porous layers

In this work, to calculate the distribution of the excess minority carrier concentration in bilateral macroporous silicon, the solution of the diffusion equation for stationary conditions is used, which is written for a monocrystalline substrate and macroporous layers. The solution to the diffusion equation is supplemented by boundary conditions at the interface between macroporous layers and a monocrystalline substrate and at the boundaries of a bilateral macroporous silicon sample. The dependence of the distribution of the excess minority carrier concentration in bilateral macroporous silicon with the same thickness of porous layers on the depth of macropores, the thickness of the sample of bilateral macroporous silicon, and the bulk lifetime of minority charge carriers is calculated. It is shown that the distribution function of the excess minority carrier concentration in bilateral macroporous silicon exhibits two maxima. The maxima are located in the frontal macroporous layer, near the surface of the sample, and in a monocrystalline substrate, near the interface, the frontal macroporous layer - monocrystalline substrate.

Effect of Diode Field on Time-Resolved Photoluminescence of CdTe-Based Solar Cells

Time-resolved photoluminescence (TRPL) measurements with externally applied voltage bias were performed on a variety of thin-film CdTe-based solar cells. These measurements revealed a substantial variation in luminescence decays with the application of external voltage bias in many of the samples. Experimental results were replicated with a MATLAB-based modeling tool. Decay variations are attributed to the changing strength of internal p-n junction electric fields as bias is modulated; this exercise, therefore, illustrates whether or not field effects influence a particular TRPL measurement and how. It is found via modeling that the field influence on TRPL decays is dependent on the cell's bulk minority carrier lifetime, mobility, and carrier concentration, as well as the laser excitation power during a measurement. While nearly insignificant in poorer quality legacy cells, the field effects are substantial in state-of-the-art CdTe cells with high lifetime, mobility, and doping, complicating determination of bulk lifetime. It is found that forward bias is effective in eliminating junction fields. It is also shown that fields can be effectively suppressed by increasing excitation power, but doing so is imprecise and can also introduce additional errors unrelated to fields. It is therefore concluded that lifetime is most accurately measured in completed cells under forward bias at low excitation intensity.

The Influence of Minority Carrier Density on Degradation and Regeneration Kinetics in Multicrystalline Silicon Wafers

The rate at which silicon solar cells and wafers degrade and regenerate when subjected to light and elevated temperatures (LeTID) is known to depend on the minority carrier concentration. In this article, we utilize spatial differences in the rate of degradation and regeneration in multicrystalline silicon wafers to estimate the dependence of these rates on minority carrier concentration. We apply the relation obtained from spatial investigations in a model describing temporal LeTID defect evolution. With the updated model in which the evolving minority charge carrier density during degradation and regeneration is considered, we obtain a drastically improved fit between measured and modeled data. The activation energy for the regeneration process determined from our proposed model (0.56±0.12eV at 1 sun) is lower than most values reported in the literature but correspond perfectly to the activation energy reported by Graf et al. by whom evolving carrier injection is accounted for experimentally. Hence, our results underpin the importance of taking the evolution of charge carrier injection into account in the evaluation of LeTID kinetics. Finally, we use our updated model to obtain spatial maps of degradation and regeneration parameters. Clear correlations are shown between LeTID kinetics parameters and local processing conditions.

Sub-Pico-Second Hole Generation Lifetime in Thin Film IGZO Sputtered and Annealed on P-Silicon Substrate

Indium-gallium-zinc-oxide (IGZO) sputtered and annealed in the absence of oxygen shows large oxygen vacancy concentration leading to n-type doping of the thin film. However, because of low intrinsic carrier concentration minority carrier (hole) concentration remains weak. Thus, measurement of thermal hole generation lifetime in unipolar IGZO is challenging. In this work, utilizing the charge transient of an appropriately biased depleted HfO2/IGZO thin film MOS capacitor fabricated on a p-silicon substrate, we extracted hole thermal generation lifetime of IGZO. The measured lifetime was found between 0.62–0.064 ps whence IGZO film received 400 °C to 700 °C anneal treatments followed by its direct sputtering on p-silicon substrate. Further, a morphology analysis of the IGZO was presented. Difference in the lifetime which occurred due to amorphous to the nano-crystalline transition of the IGZO thin film with increased anneal temperature was furthermore validated through XRD, SEM and AFM measurements. Hole capture cross-sections as we obtained from surface generation velocities at the IGZO/HfO2 interface and energy distributions of interface state density were found in agreement, which indirectly demonstrates the accuracy of the measured hole generation lifetime employing the transient capacitance extrapolation, since under the steady state condition hole generation through the interface states of IGZO/HfO2 tends to balance its bulk (IGZO) counterpart

Comments on “A New Derivation of the Law of the Junctions”

Contribution: This brief comment highlights some crucial assumptions behind the “law of the junction” that are overlooked by the above paper and argues that the proposed derivation is not actually a “new” derivation at all. Background: The “law of the junction” is one of the most significant and useful results within the field of solid-state devices. The above paper is likely to confuse readers, particularly those who are undergraduate electrical engineering students studying semiconductor device physics for the first time. This is especially so because of the abstract nature of the underlying quantum mechanics framework and solid-state physics models (subjects which the typical student at that level lacks a substantial background in) as well as the plethora of tedious equations in the curriculum. Research Questions: What core physical concepts are essential to a fundamental yet intuitive understanding of the law of the junction? Methodology: Several key features of how semiconductor junctions behave under bias are explained. References to well-known textbooks are provided where appropriate. Findings: The above paper’s primary mistake is its assertion that its derivation does not rely on the assumption of thermal equilibrium. However, the law of the junction is equivalent to a calculation of depletion-edge minority carrier concentrations using Maxwell–Boltzmann statistics—a distribution which only holds under thermal equilibrium conditions. More rigorously, in a nondegenerate semiconductor, Fermi–Dirac statistics (which governs electrons) reduces to Boltzmann statistics only when the electrochemical potential is spatially uniform, a condition equivalent to having no net flow of thermal energy—the very definition of thermal equilibrium.

Carrier Transmission Mechanism-Based Analysis of Front Surface Field Effects on Simplified Industrially Feasible Interdigitated Back Contact Solar Cells

Interdigitated back contact (IBC) n-type silicon solar cells with a different front surface layer doping concentration were fabricated and studied and the influence of the front surface doping level was analyzed via simulation (PC1D). The IBC cells were processed by industrially feasible technologies including laser ablation and screen printing; photolithography was not used. A maximum efficiency of up to 20.88% was achieved at an optimal front surface field (FSF) peak doping concentration of 4.8 × 1019 cm−3 with a sheet resistance of approximately 95 Ω/square, corresponding to Jsc = 40.05 mA/cm2, Voc = 671 mV and a fill factor of 77.70%. The effects of the front surface doping level were studied in detail by analyzing parameters related to carrier transmission mechanisms such as minority carrier concentration, minority carrier lifetime and the saturation current density of the FSF (J0e). The influence of the front surface recombination velocity (FSRV) on the performance of IBC solar cells with different FSF layer doping concentrations was also investigated and was verified by examining the variation in the minority carrier density as a function of the distance from the front surface. In particular, the impact of the FSF doping concentration on the Jsc of the IBC cells was clarified by considering carrier transmission mechanisms and the charge-collection probability. The trends revealed in the simulations agreed with the corresponding experimental data obtained from the fabricated IBC solar cells. This study not only verifies that the presented simulation is a reasonable and reliable guide for choosing the optimal front surface doping concentration in industrial IBC solar cells but also provides a deeper physical understanding of the impact that front surface layer doping has on the IBC solar cell performance considering carrier transmission mechanisms and the charge-collection probability.