Band Tail Research Articles

Photo-Excited Carrier Dynamics in Ammonothermal Mn-Compensated GaN Semiconductor.

We investigated the carrier dynamics of ammonothermal Mn-compensated gallium nitride (GaN:Mn) semiconductors by using sub-bandgap and above-bandgap photo-excitation in a photoluminescence analysis and pump-probe measurements. The contactless probing methods elucidated their versatility for the complex analysis of defects in GaN:Mn crystals. The impurities of Mn were found to show photoconductivity and absorption bands starting at the 700 nm wavelength threshold and a broad peak located at 800 nm. Here, we determined the impact of Mn-induced states and Mg acceptors on the relaxation rates of charge carriers in GaN:Mn based on a photoluminescence analysis and pump-probe measurements. The electrons in the conduction band tails were found to be responsible for both the photoconductivity and yellow luminescence decays. The slower red luminescence and pump-probe decays were dominated by Mg acceptors. After photo-excitation, the electrons and holes were quickly thermalized to the conduction band tails and Mg acceptors, respectively. The yellow photoluminescence decays exhibited a 1 ns decay time at low laser excitations, whereas, at the highest ones, it increased up to 7 ns due to the saturation of the nonradiative defects, resembling the photoconductivity lifetime dependence. The fast photo-carrier decay time observed in ammonothermal GaN:Mn is of critical importance in high-frequency and high-voltage device applications.

Preparation, characterization, and structural studies of methylcellulose/CuO hybrid materials

Flexible hybrid materials composites of methylcellulose (MC) and CuO are fabricated successfully with solution-casting preparation method as confirmed by the energy-dispersive X-ray spectroscopy (EDX), transmission electron microscope (TEM), and Fourier transform infrared methods. The EDX of MC/CuO recorded the peaks of C, O, and Cu with percentage ratios of 49.61%, 47.35%, and 3.04%, which verify the appropriate formation of the MC/CuO. The TEM image shows that the size of CuO is from 11 to 51 nm. The images from the SEM also show that the CuO nanoparticles are evenly distributed throughout the MC/CuO composite. The impact of CuO on MC transparency is also investigated via UV/vis spectroscopy. The optical parameters of the polymer and composites films were recorded. The bandgap energy of MC is reduced from 4.83 to 4.69, 4.33, and 4.2 eV when mixed with 1.5%, 3%, and 4.5% CuO, respectively, while the band tail is increased from 1.6 to 2.93, 3.66, and 4.15 eV. Carbon cluster numbers also rise from 50 for MC to 54, 63, and 67 for the addition of CuO concentrations of 1.5%, 3%, and 4.5%, respectively. The purpose of this work is fabrication of new nanocomposite materials with innovative flexible properties that may find application in high-performance optoelectronics.

Tuning the bandgap without compromising efficiency: ambient solution processing of Ge-alloyed (Ag,Cu)2Zn(Sn,Ge)(S,Se)4 kesterite thin-film solar cells

The photovoltaic sector currently experiences a growing demand for deeply-integrated technologies in agriculture, buildings, vehicles, among others. Kesterite compounds provide a versatile solution through their tunable bandgap via isovalent alloying. However, this should be done while preserving material opto-electronic quality and device performance, which is an unresolved challenge. Indeed, tuning the bandgap of kesterites must ensure low crystalline disorder to counteract the associated open-circuit voltage (Voc) losses when used in solar cells. To achieve this goal, this work introduces a simple molecular ink recipe realized in ambient air to process single-phase high-quality Ag,Gealloyed kesterite absorbers. By varying the Ge content from 0 to 100%, the absorber bandgap covers the range from 1.15 to 1.5 eV, in which the Urbach energy remains close to 20 meV and promisingly low Voc deficits are observed. In particular, the champion efficiency of 12.1% obtained with 40% Ge breaks the record for Ge-alloyed kesterites with a 1.2 eV bandgap by remarkably reaching 62% of the Shockley-Queisser limit for Voc. This study demonstrates a promising strategy to solve the issue of large band tailing and low opto-electronic quality in widebandgap kesterites while establishing a foundation for solution-processed kesterite solar cells with tunable bandgap and promising performance.

Description of excitonic absorption using the Sommerfeld enhancement factor and band-fluctuations

Abstract One of the challenges of excitonic materials is the accurate determination of the exciton binding energy and bandgap from optical measurements. The difficulty arises from the overlap of the discrete and continuous excitonic absorption at the band edge. Many researches have modeled the shape of the absorption edge of such materials on the seminal formulation proposed by Elliott in 1957 (Phys. Rev. 108 1384–9) and its several modifications such as non-parabolic bands, magnetic potentials and electron–hole-polaron interactions. However, exciton binding energies obtained from optical absorption often vary strongly depending on the chosen ‘Elliott formula’. Here, we propose an alternative and rather simple approach, which has previously been successful in the determination of the optical bandgap of amorphous, direct and indirect semiconductors, based on the band-fluctuations (BFs) model. In this model, the fluctuations due to disorder, temperature or lattice vibrations give rise to the well known exponential shape of band tail states. The formulation results in an analytic equation for the fundamental absorption with 6 parameters only. To test it, the binding energy and optical bandgap of GaAs and the family of tri-halide perovskites ( MAPbX 3 ), X = Br , I , Cl , over a wide range of temperatures, are obtained by fitting the modified Elliott model. The results for the bandgap, linewidth and exciton binding energy are in good agreement with reports based on non-optical measurements. Moreover, due to the polar nature of perovskites, the retrieved binding energies can be compared with those computed with a model proposed by Kane (1978 Phys. Rev. B 18 6849). In the latter model, the exciton is surrounded by a cloud of virtual phonons interacting via the Frölich interaction. As a consequence, the upper bound for the binding energy of the exciton-polaron system can be estimated. These results are in good agreement with the optical parameters obtained with the proposed Elliott equation including BFs.

Revisiting Band Tails and Localized States in Amorphous Semiconductors

Amorphous semiconductors, such as hydrogenated amorphous silicon (a-Si:H) and amorphous indium gallium zinc oxide (a-IGZO) have been central to the development of thin film transistors (TFTs) for large-area electronic applications such as displays. This is a result of the exceptional material uniformity that can be achieved when depositing over very large areas (~10 m2). This means that all TFTs are essentially identical over an entire large display panel. However, there is a technological price to pay – the carrier mobility is generally less in amorphous semiconductors compared to their crystalline equivalent.The origin of this reduced carrier mobility is a result of a key difference in the density of states between crystalline semiconductors and their amorphous counterparts. The long-range order of a crystalline lattice means that all carrier states in the conduction and valence bands exist over long distances. These ‘extended states’ are associated with generally high mobility carriers. However, there is no long-range order in amorphous semiconductors, but only short-range order over one to two bond lengths associated with local atomic coordination requirements (and to some degree a medium-range order over a few bond lengths). The result is that the bottom of the conduction band and top of the valence band gain ‘tail’ states which are localized in nature.Not only do these localized states frequently dominate (or at least significantly affect) conduction in amorphous semiconductors and hence the switching characteristics of TFTs, but they also have a role to play in other important processes such as hysteresis and bias stress instability. But what are these localized band tail states, how should we think about them in the context of conduction and how do they relate to extended states?These questions are revisited from the starting point that there is a perturbation in the delocalized charge concentration with position in an amorphous semiconductor compared to its crystalline counterpart, and this can be quantified as a spatially varying excess charge concentration (where a deficit is just a negative excess). The rate of spatial variation is long relative to the rate of change in the electron wavefunction, and reflects the short- and medium-range order distances. Charge conservation means that that this excess charge concentration will have a mean of zero and a Gaussian probability distribution is assumed. A model using this as a starting point has been created which reproduces band tails and which provides new insights into their physics which are discussed.

Characterization of LDMOS down to cryogenic temperatures and modeling with PSPHV

This article presents a detailed characterization and analysis of a 45 V LDMOS device from production technology across a wide temperature range from 33 to 385 K. For the first time, quasi-saturation behavior is consistently observed throughout the entire temperature range studied. Compared to prior published data, this device shows some notable differences, including a substantially higher saturation temperature of around 200 K for threshold voltage and subthreshold swing due to band tail and a typical low on-resistance down to 33 K, free of freezeout. To account for the observed temperature dependencies, we propose improved semi-empirical temperature scaling equations for the PSPHV model. We extend its applicable temperature range down to 33 K from the previous lower limit of 240 K. The enhancement models the temperature behaviors of key device parameters, including threshold voltage, subthreshold swing, mobility, velocity saturation, drift resistance, and quasi-saturation effects. These results provide new insights into the low-temperature behavior of LDMOS devices for cryogenic electronics applications.

Mitigating Band Tailing in Kesterite Solar Absorbers: Ab Initio Quantum Dynamics.

Open-circuit voltage deficits are limiting factors in kesterite solar cells. Addressing this issue by suppressing band tailing and nonradiative charge recombination is essential for enhancing the performance. We employ ab initio nonadiabatic molecular dynamics to elucidate the origin of band tailing and charge losses and propose a mitigation strategy. The simulations show that Cu-Zn disorder, associated with antisite defect clusters [CuZn+ZnCu], is a significant source of band tailing in kesterites, as evidenced by the much larger Urbach energy in disordered than ordered kesterites. Cu-Zn disorder gives rise to new sulfur-centered coordination polyhedra, increases structural inhomogeneity, changes electrostatic potential at sulfur centers, and shifts the S(3p) orbital energy. Differences in the S(3p)/Cu(3d) and S(3p)/Sn(5s) hybridization strengths and the S(3p) orbital energy shift reduce the band gap by 0.37 eV. Furthermore, Cu-Zn disorder enhances vibrational motion of sulfur anions and surrounding cations, increasing band gap fluctuations by 15 meV. The stronger electron-phonon interactions reduce charge carrier lifetimes and limit the kesterite solar cell efficiency. Partial substitution of Zn with Cd facilitates structural ordering and significantly suppresses band tailing, particularly in disordered systems. The improvement can be attributed to the larger atomic radius and mass of Cd, which weakens bonding around the anion, suppresses S-related vibrations within the covalent tetrahedra, and reduces nonadiabatic coupling, thereby increasing charge carrier lifetimes. The reported results establish the key influence of cation disorder on band tailing and reduced charge carrier lifetimes in kesterites and highlight cation disorder engineering as a strategy to achieve high-efficiency kesterite solar cells.

Extended Tauc-Lorentz Model for Amorphous Materials With Non-Exponential Band Tails

Extended Tauc-Lorentz Model for Amorphous Materials With Non-Exponential Band Tails

Temperature dependent radiative and non-radiative recombination lifetimes of luminescent amorphous silicon oxynitride systems

In our previous work, we deeply researched the absolute photoluminescence (PL) quantum yields of luminescent modulating a-SiNxOy films with various N/Si atom ratios under different measurement temperatures. In this work, we further systematically studied the temperature dependent kinetic processes of radiative and non-radiative recombinations in a-SiNxOy systems in the visible light range. First, we investigated the structure of a-SiNxOy films and obtained the concentrations of both trivalent Si and N-Si-O defects related dangling bonds through XPS, FTIR and EPR measurements. Then we further tested the transient fluorescence attenuation of a-SiNxOy films detected at different emission wavelengths. We found that the PL lifetimes of a-SiNxOy films vary with the change of N-Si-O defect state concentrations, which is different from the typical PL decay characteristics of band tail related a-SiNx films previously reported. By combining the resulting PL IQE values with the ns-PL lifetimes, we further intensively redetermined the radiative and non-radiative recombination lifetimes of a-SiNxOy systems. The related radiative recombination rates were obtained (kr∼108 s−1), which can be compared to the results in the direct band gap.

Role of ex-situ HfO2 passivation to improve device performance and suppress X-ray-induced degradation characteristics of in-situ Si3N4/AlN/GaN MIS-HEMTs

The role of ex-situ HfO2 passivation in in-situ Si3N4/AlN/GaN-based metal–insulator–semiconductor high-electron-mobility transistors (MIS-HEMTs) to improve the device performance and protect the device from severe degradation under high-energy X-ray irradiation has been investigated. Here, the root cause and mechanism for the degradation have been studied utilizing different material/device characterizations. X-ray photoelectron spectroscopy results suggest that several Si-N bonds in stoichiometric Si3N4 and Hf-O bonds in HfO2 passivation film can be broken under X-ray irradiation, where numerous charges might be trapped easily and deteriorate the interface quality. Furthermore, due to the generation of chemical vacancies under X-ray, a band tail near the edge of valence band maximum in Si3N4 and HfO2 films can be created. The obtained results suggest that MIS-HEMT fabricated with an atomic layer deposited (ALD)-HfO2 (MIS-HEMT B) has relatively greater performance compared to MIS-HEMT fabricated without ex-situ HfO2 (MIS-HEMT A) according to the higher maximum drain current (Idmax), peak transconductance (gmmax) and lower current collapse, denoting the effectiveness of ALD-HfO2 to passivate the surface traps. It is noticed that X-ray irradiation induces numerous negative charges in the dielectric layers and degrades the device performance. Interestingly, after X-ray irradiation, the degradation rates in Idmax, gmmax, and current collapse are significantly lower in MIS-HEMT B compared to MIS-HEMT A, suggesting the potential irradiation hardening mechanism of ALD-HfO2 to protect the MIS-HEMTs from unexpected degradations during X-ray irradiation. Finally, these results emphasize the importance of high-quality dielectric films in MIS-HEMTs to enhance the device performance and safeguard the device under high-energy irradiation, especially in the space environment.

Passivation of Deep‐Level Defects through Chemical Environment Regulation for Efficient CZTSSe Solar Cells Based on Active Selenium Adsorption–Desorption Process

AbstractInsufficient selenization and uneven distribution of elements caused by the poor diffusion and reaction activity of selenium clusters is one of the main issues limiting the efficiency of Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. Here, this work designs a simple and feasible strategy to improve the activity of selenium (Se) by implementing high‐temperature treatment on graphite boxes loaded with Se pellets. The rapid adsorption/desorption characteristics of graphite on active gaseous small‐molecule selenium have successfully introduced hyperactive Se4, Se3, and Se2 into the selenization process. The results indicate that the adsorbed non‐toxic gaseous active Se3 and Se4 can quickly and uniformly diffuse into the precursor film at low temperatures, thereby inducing nucleation and grain growth at both surface and back interface simultaneously, which inhibits the upward migration and aggregation of cations, especially Cu, and promotes the homogenization of elements. The overall relatively Cu‐poor chemical environment suppresses the formation of CuZn defects and [2CuZn+SnZn] defect clusters, and also promotes the generation of favorable VCu. The band tail states and non‐radiative recombination are then optimized. Finally, the CZTSSe solar cells achieve a power conversion efficiency (PCE) of 14.5%, with VOC/VOCSQ of 67% being one of the highest in the literature.

Electrical conduction and optical characteristics of Li2O and Bi2O3 Co-doped zinc-phosphate glassy nanocomposites: A critical observation of mixed ionic electronic effect

In the present report, the combined effects of the co-doping of metal oxide (Bi2O3) and Lithium oxide (Li2O) into the glassy matrix with chemical composition xLi2O-(0.45-x)Bi2O3-(0.15ZnO-0.40P2O5) (x = 0.05, 0.15, 025, and 0.35 mol%) are investigated thoroughly. The FESEM and X-ray diffraction data affirm the crystalline nature of the studied glassy composites. The obtained band gap energy values are declined from (3.68–3.17) eV, while the Urbach energy values are declined from (0.38–0.27) eV. Moreover, the mechanism responsible for AC conductivity has been analyzed using Almond–West formalism and Jonscher's universal power law. The observed rise in both AC and DC conductivity in the glass samples is attributed to the mixed ionic electronic effect. In the ionic diffusion model, an increased defect site concentration improves Li+ ion migration through percolation channels. The reduction in both electrostatic binding energy and strain energy lowers the Anderson-Stuart activation energy, facilitating Li+ ion migration and enhancing conductivity. Additionally, in the small polaron hopping model, an increase in the density of defect states suggests more defect sites that facilitate electronic hopping, creating deep localized states and transitions between localized and extended states. Replacing a Bismuth atom with a Lithium atom increases the number of non-bridging oxygen, enhancing the conduction band tail, reducing the energy needed for hopping conductivity, and increasing conductivity.

Enhance the Performance of CZTSSe Solar Cells Through Inhibiting the Cu2+, Tu, and (─COOH) Association Reaction.

The Sol-gel precursor solution reaction mechanism has a significant impact on the Cu2ZnSn(S, Se)4 (CZTSSe) solar cells. It is discovered that in the Cu2ZnSnS4 (CZTS) precursor solution (CZTS-PS) in the preparation, there is an association reaction among Cu2+, thiourea (Tu), and carboxyl (-COOH), which is an important reason for the undesirable CZTSSe solar cells. The strong association reaction generates excessive Cu2+ ions, forming the CuxSe secondary phase on the surface of the CZTSSe absorber. The secondary phase causes a short circuit and deterioration of gadget performance. Following a 6-h aging period for the CZTS-PS, the average photoelectric conversion efficiency (PCE) of the device is enhanced to 8.02%, and there is also an improvement in device uniformity, as evidenced by a decrease in the standard deviation to less than 1. To inhibit the association reaction and eliminate the aging time phenomenon, a strategy is developed using hydrochloric acid to regulate the CZTS-PS environment. This strategy shifts the REDOX reaction in Cu2++Sn2+ toward the formation of Cu1++Sn4+, leading to a decrease in the defect concentrations of VSn(-/0) and CuSn(-/0), which increases the carrier concentration and reduces the impact of band tailing. The average power conversion efficiency (PCE) of the devices improved from 7.45% to 9.26%, the PCE of the best-performing CZTSSe solar cells increased from 9.25% to 9.83%, and the consistency among the devices is further enhanced, as indicated by a reduction in the standard deviation from 0.98 to 0.44. Ultimately, the device performance of the Cu2++Sn2+-DMF system improved by 11.01% (without the MgF2 layer) after optimization. This study serves as a reference for regulating the environment of the CZTS-PS to further enhance the CZTSSe devices' performance, and the photoelectric conversion efficiency is improved by ≈30%.

Fabrication, surface characterization and optical behavior of flexible PVA/Nd2O3 polymer nanocomposites materials for optoelectronics applications

The nanocomposite PVA/Nd2O3 with novel properties, that composed of Neodymium (III) oxide (Nd2O3) and polyvinyl alcohol (PVA), was synthesized by the casting preparation method for applied as flexible films for optoelectronic. The techniques of the FTIR, XRD, SEM and EDX showed that the PVA/Nd2O3 were successfully synthesized. The successful inclusion of Nd2O3 into PVA is indicated by a reduction in PVA/Nd2O3 peak intensities as observed by the FTIR and XRD. Additionally, the UV/Vis technique was used to record the optical behavior at wavelengths between 200 and 1100 nm. The measurements of carbon clusters, band gap, absorption edge, and band tail were determined. The band gap decreases from 5.01 eV to 4.60 eV and the band tail changes from 1.8 eV to 2.85 eV when the Nd2O3 is raised from 5 % to 20 %. In addition, the absorption edge reduced from 5.06 eV for PVA to 4.92, 4.81, 4.72, and 4.49 eV, accordingly, when 5 %, 10 %, 15 %, and 20 % of Nd2O3 are introduced in the composite. The result showed the composites made with PVA and Nd2O3 have excellent optical characteristics, especially in the UV–visible spectrum, where they are highly reflective and absorbent. The study emphasizes the fabricated composites enhanced optical transparency and absorption properties, which make it a good materials for uses in photonic devices, sensors, and optical instruments.

Electrical properties and evaluation of band tail states in Mg doped p-type multilayer hBN

A series of Mg doped p-type multi-layered hBN films were prepared by atmospheric pressure chemical vapor deposition. Temperature dependent conductivity measurements were performed from 0.1 Hz to 10 MHz to analyze the characteristics of tail states close to the valence band edge. Jonscher's power law (Aωs) is successfully applied to understand charge carrier transport through these states. In this work, exponent S increases from 0.6 → 0.8, 0.8 → 0.995, and 1.4 → 1.6 for samples B (precursor temperature, 750 °C), D (850 °C), and E (900 °C), indicating that non-overlapping small polaron tunneling dominates to 548 K. Polaron binding energies of 0.2–0.40 eV and tunneling distances <4.9 Å are calculated, confirming transport through localized states. The density of states near the Fermi level N(EF) was extracted from a fit to the AC conductivity data, yielding values of 1015 and 1017eV−1cm−3 as the precursor temperature increases. Singular Mg acceptor levels of 74, 30, and 17 meV are identified for each sample. A hole concentration from 6.5 × 1017 to 1 × 1018 cm−3 and carrier mobility from 18 to 25 cm2/V s is measured at 300 K. From RC fitting, carrier recombination lifetimes of 1.2, 0.4, and 0.35 μs are determined. Fermi's golden rule is used to determine an optical joint density of states of 1.1 × 1021 eV−1 cm−3 at a band edge. Overall, we show that AC conductivity is an effective method to evaluate midgap states in 2D (two-dimensional) materials at EF and p-type hBN possesses sufficient electrical properties to be integrated into a wide range of semiconductor applications.

Role of Ag Addition on the Microscopic Material Properties of (Ag,Cu)(In,Ga)Se2 Absorbers and Their Effects on Losses in the Open‐Circuit Voltage of Corresponding Devices

ABSTRACTAg alloying of Cu(In,Ga)Se2 (CIGSe) absorbers in thin‐film solar cells leads to improved crystallization of these absorber layers at lower substrate temperatures than for Ag‐free CIGSe thin films as well as to enhanced cation interdiffusion, resulting in reduced Ga/In gradients. However, the role of Ag in the microscopic structure–property relationships in the (Ag,Cu)(In,Ga)Se2 thin‐film solar cells as well as a correlation between the various microscopic properties of the polycrystalline ACIGSe absorber and open‐circuit voltage of the corresponding solar cell device has not been reported earlier. In the present work, we study the effect of Ag addition by analyzing the differences in the various bulk, grain‐boundary, optoelectronic, emission, and absorption‐edge properties of ACIGSe absorbers with that of a reference CIGSe absorber. By comparing thin‐film solar cells with similar band‐gap energies ranging from about 1.1 to about 1.2 eV, we were able to correlate the differences in their absorber material properties with the differences in the device performance of the corresponding solar cells. Various microscopic origins of open‐circuit voltage losses were identified, such as strong Ga/In gradients and local compositional variations within individual grains of ACIGSe layers, which are linked to absorption‐edge broadening, lateral fluctuations in luminescence‐energy distribution, and band tailing, thus contributing to radiative VOC losses. A correlation established between the effective electron lifetime, average grain size, and lifetime at the grain boundaries indicates that enhanced nonradiative recombination at grain boundaries is a major contributor to the overall VOC deficit in ACIGSe solar cells. Although the alloying with Ag has been effective in increasing the grain size and the effective electron lifetime, still, the Ga/In gradients and the grain‐boundary recombination in the ACIGSe absorbers must be reduced further to improve the solar‐cell performance.

Design and Simulation of a High-Performance Tunneling Field Effect Transistor-Based Biosensor Using a Heterojunction Electron-Hole Bilayer

Abstract This study introduces a novel dielectrically-modulated heterojunction electron-hole bilayer tunnel field-effect transistor (DM-HEHBTFET) for bio-sensing applications. The device features a Ga0.85Sb0.15As/Ga0.8In0.2As heterojunction and a p-type pocket in the channel, achieving a remarkably low threshold voltage (VT) of 20 mV, an average subthreshold slope (SS) of 5.7 mV/dec, and a leakage current (IOFF) as low as 5 × 10⁻¹¹ A/μm. The staggered bandgap in the heterostructures enhances electric field control, enabling lower gate voltage operation. Furthermore, the strategically positioned nanogap cavities in non-overlapping regions of the top and bottom gates effectively mitigate gate control issues over the channel, ensuring improved device performance. A modified design, the modified DM-HEHBTFET, is also proposed, featuring source and drain regions engineered with Ga0.85Sb0.15As/Ga0.8In0.2As heterojunctions. This design mitigates leakage current and improves the average subthreshold slope (SS). For biomolecules with a dielectric constant of 12, the modified biosensor exhibits a drain current sensitivity (Scurrent) of 2.6e4, average SS=2.7 mV/dec, and IOFF=1e-12 A/μm. The device's performance is assessed by examining steric hindrance and band tailing effects. The modified biosensor outperforms recent DM-TFET biosensors, making it a promising candidate for low-power, high-switching speed bio-sensing.

Localization and other properties of electronic states in a semiconductor revealed by their response to surface doping and band-bending potential

Using photoelectron spectroscopy (PES), we study InAs(001) and InAs(110) surfaces, doped with Te atoms to induce strong band-bending phenomena and a formation of two-dimensional electron gases (2DEGs). The band structure of the 2DEGs as well as the coexisting 3D bulk electronic structure are investigated. We discuss several schemes to estimate the band-bending potentials at surfaces by PES methods, appropriate in different situations, e.g. in the presence of a significant band tailing. We use the Schroedinger–Poisson scheme as well as the Thomas–Fermi method to model the band-bending potential profiles and the 2DEG band energies. For the InAs crystal we find that the valence bands investigated with PES closely follow the band-bending potential which evidences that valence electronic states are localized to within few nm. In contrast, the conduction band is entirely insensitive to the band-bending potential, evidencing that the extent of electronic states within this band is at least 103nm.

Insights into the Heat‐Assisted Intensive Light‐Soaking Effect on Silicon Heterojunction Solar Cells

Heat‐assisted intensive light soaking has been proposed as an effective posttreatment to further enhance the performance of silicon heterojunction (SHJ) solar cells. In the current study, it is aimed to distinguish the effects of heat and illumination on different (doped and undoped) layers of the SHJ contact stack. It is discovered that both elevated temperature and illumination are necessary to significantly reduce interface recombination when working effectively together. The synergistic effect on passivation displays a thermal activation energy of approximately 0.5 eV. This is likely due to the photogenerated electron/hole pairs in the c–Si wafer, where nearly all of the incident light is absorbed. By distinguishing between the effects of light and heat effects on the conductivity of p‐ and n‐type doped hydrogenated amorphous silicon (a–Si:H) layers, it is demonstrated that only heat is accountable for the observed rise in conductivity. According to numerical device simulations, the significant contribution to the open‐circuit voltage enhancement arises from the reduced density of defect states at the c–Si/intrinsic a–Si:H interface. In addition, the evolution of the fill factor is highly dependent on changes in interface defect density and the band tail state density of p‐type a–Si:H.

Over 10% efficient Cu2ZnSn(S, Se)4 Thin-Film cells prepared by aluminium doping

Cu2ZnSn(S,Se)4(CZTSSe) solar cell is of excellent development tendency because of lower pollution and outstanding power conversion efficiency(PCE). Yet, current PCE of CZTSSe is limited because of a high open-circuit voltage deficit, primarily put down to severe band tailing and carrier recombination. In this paper, absorber films were obtained by solution method and the influences of different aluminum doping concentrations on the properties of CZTSSe thin film cell were studied. Analysis about X-ray diffraction (XRD) and Raman spectroscopy confirms which Al has incorporated into CZTSSe thin film lattice and replaced Sn. Hall effect measurements reveal the improvement of carrier concentration on thin film absorber layer attribute to the formation of AlSn acceptor defects. Furthermore, X-ray photoelectron spectroscopy (XPS) reveals that Al enter into the CZTSSe lattice and Al is in the appropriate state of + 3 valence without generating additional harmful defects. More important, the generation of deep energy level defects and defect clusters associated with Sn were suppressed by replacing Sn with Al. Finally, the photovoltaic performance of different Al doped CZTSSe cells were characterized by conducting J-V and external quantum efficiency (EQE) tests. Remarkably, the CZTSSe solar cell with PCE as high as 10.59 % is obtained under a doping level of 6 % Al, which has an open circuit voltage of 522 mv. Compared with reference CZTSSe solar cells with an efficiency of 7.29 %, which indicates with improvement of 45 % in efficiency. Consequently, Al doping process is considered as a prospective tactic to elevate the PCE of CZTSSe solar cells.