Tail States Research Articles

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.

Realization of grain growth and suppressed bulk defects for efficient solution-processed Cu2ZnSn (S, Se)4 solar cells via co-doping strategy

Environmentally benign and earth-abundant constituent’s kieserite Cu2ZnSn (S, Se)4 (Cotises) is considered as a hopeful photovoltaic material. Unfortunately, the severe tail state caused by various harmful deep defects in CZTSSe absorber prevents further development of its device efficiency. Here, we report firstly that the Co was incorporated into CZTSSe films to form the Cu2CoxZn1-xSn (S, Se)4 (CCZTSSe) films using a two-step annealing and solution-processing technique method. Through optimized the Co/(Co+Zn) ratio, the CCZTSSe film has excellent good surface morphology and single-phase composition. Appropriate amount of Co elements replacing Zn elements in CZTSSe films can effectively reduce the degree of Cu-Zn disorder, thus inhibiting [2CuZn+SnZn] defect clusters and CuZn defects. The detrimental deep defects located in the film were effectively passivated to reduce electrostatic potential fluctuations, resulting in relief of the trailing state. Therefore, a champion device having an efficiency of 7.50 % with a fill factor (FF) of 61.00 % was achieved from CCZTSSe with 3 % Co content. This work provides new insights into the impacts of Co doping in CZTSSe by a solution processed method and offers a deeper comprehension of the origin of the efficiency enhancement of CCZTSSe devices.

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.

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.

Cd‐Free High‐Bandgap Cu2ZnSnS4 Solar Cell with 10.7% Certified Efficiency Enabled by Engineering Sn‐Related Defects

AbstractCd‐free high‐bandgap Cu2ZnSnS4 (CZTS) solar cells are expected to offer green and low‐cost solutions to single‐junction and tandem photovoltaic markets. Despite attracting considerable attention in past years, reported certified efficiency has yet to exceed the 10% threshold. Sn‐related defects, especially those originating from Sn2+ oxidation states, lead to severe recombination loss and limit device performance. Here, the formation of these detrimental defects is suppressed by creating a more benign chemical environment during sulfurization annealing. With dominant Sn4+ states in the source material and the precursor, the oxidation state of Sn remains primarily in its native Sn4+ state during annealing and in the final film. Engineering the Sn‐related defects leads to shallower tail states in bulk CZTS and fewer interfacial defects. As a result, both radiative and non‐radiative recombination losses are alleviated, contributing to a certified 10.7% efficiency of the Cd‐free high‐bandgap CZTS solar cell. This strategy may advance various kesterite materials and other technologies with multivalent constituents.

Heavy Boron-Doped Silicon Tunneling Inter-layer Enables Efficient Silicon Heterojunction Solar Cells.

P-type hydrogenated nanocrystalline silicon (nc-Si:H) has been used as a hole-selective layer for efficient n-type crystalline silicon heterojunction (SHJ) solar cells. However, the presence of an additional valence band offset at the interface between intrinsic amorphous hydrogenated silicon and p-type nc-Si:H films will limit the hole carrier transportation. In this work, it has been found that when a heavily boron-doped silicon oxide layer deposited with high hydrogen dilution to silane (pB) was inserted into their interface, the fill factor of SHJ solar cells increases 3% absolutely because of the reduced valence band offset and the increased opportunity to provide a hopping tunnel assisted by the doping energy level and valence band tail states. Furthermore, the additional boron incorporation in intrinsic amorphous silicon adjacent to pB helps to enhance the built-in electric field, thus increasing the hole selectivity. By these means, the power conversion efficiency was improved from 23.9% to approximately 25%.

Trap Assessment and Mobility Simulation of Amorphous Indium Gallium Zinc Oxide Thin Film Transistors

Ultra-broadband photoconduction (UBPC) is used to measure the sub-gap trap density of amorphous indium gallium zinc oxide (a-IGZO) thin film transistors (TFTs) across the bandgap, from the valence band mobility edge to within 0.15 eV (~6kBT) of the conduction band mobility edge. Figure (a) is a plot of the total integrated trap density (Ntraps, green) and the sub-gap density of states (dNtraps/dE, blue) as measured by UBPC for traps located just below the conduction band mobility edge (E-EC = 0). Three Gaussian sub-gap states are observed with peak energies of 0.9, 0.7, and 0.4 eV below the conduction band mobility edge. Note that the width of the 0.4 eV peak (180 meV) is much larger than that of the other two deeper peaks (45 meV). All three of these peaks are likely associated with oxygen vacancies, but it is unclear why the 0.4 eV trap is so much broader than all the other deeper oxygen vacancy derived donor trap states.The black dotted curves included in Figure (b) are experimental EKV mobility (μEKV) versus overvoltage (VG-VON) plots obtained at three different temperatures (0 °, 20 °, and 100 °C). The red dashed curves are simulated mobility curves obtained assuming that only conduction band tail states act as electron traps to degrade the mobility. Since these red simulated curves grossly overestimate the experimental mobility, other traps must be operative in establishing the black experimental mobility curves. In fact, the inclusion of the 0.4 eV trap measured by UBPC in addition to the conduction band tail states in the simulation (blue curves) leads to excellent agreement between experimental and simulated mobility for all three temperatures (0 °, 20 °, and 100 °C). In summary, it is demonstrated how the mobility of a-IGZO can be simulated by including the experimentally observed DoS of a 0.4 eV centered Gaussian trap state.

High Broadband Optical Absorption and Bandstop Filter Characteristics of Pb/Nb2O5 Interfaces

AbstractIn this study, semitransparent lead films serve as substrates for depositing niobium pentoxide thin films, forming versatile electro‐optical devices. Using vacuum evaporation and ion sputtering techniques at ≈10−5 mbar, stacked layers of crystalline Pb and amorphous Nb2O5 are created. This process reduces free carrier absorption in Nb2O5 and forms Urbach tail states with a width of 0.91 eV. Pb/Nb2O5 thin films exhibit remarkable broadband absorption, exceeding 440% in the visible and 98% in the infrared. Moreover, Pb substrates induce a redshift in Nb2O5’s energy bandgap. Electrical analysis using impedance spectroscopy on Pb/Nb2O5/Ag structures reveals their series/parallel resonance and bandstop filter properties. Notably, the bandstop filters exhibit reflection coefficient minima at a notch frequency of 1.66 GHz, with a bandwidth of 280 MHz, return loss of 26 dB, and voltage standing wave ratio of 1.13. These findings underscore the device's potential for wide‐ranging electro‐optical applications across the electromagnetic spectrum.

Viable Strategy for Suppressing Antisite Defects and Band Tailing States in Solution-Processed Kesterite Solar Cells via IIIA Incorporation: Case of Aluminum

Viable Strategy for Suppressing Antisite Defects and Band Tailing States in Solution-Processed Kesterite Solar Cells via IIIA Incorporation: Case of Aluminum

Influence of low-temperature cap layer thickness on the structure and luminescence of InGaN/GaN multiple quantum wells

In order to effectively regulate the luminescence performance of MQW and enhance its optical quality, it is crucial to investigate the InGaN/GaN MQW's structure and luminescence properties. In this study the focus is how they are affected by low-temperature cap (LT-cap) layer's thickness which is grown above each InGaN well layer during growth process of InGaN/GaN multiple quantum well (MQW) samples in MOCVD system. This was achieved by analyzing high resolution X-ray diffraction (HRXRD) spectra, electroluminescence (EL) spectra, temperature-dependent photoluminescence (TDPL) spectra, and micro-area fluorescence imaging of these samples. The results show that changes in LT-cap layer's thickness even have no significant impact on some structural parameters of MQW, such as the thickness of the well layer, but have an influence on the In component of the well layer. Due to the existence of LT-cap layer, dissociation of InGaN can be effectually reduced. In addition, the augmentation of LT-cap layer's thickness will make polarization effect of the QW sample more remarkable, so that the blue shift of the EL peak with the augmentation of current injection increases. The change of LT-cap layer's thickness will also influence distribution of the tail states of the quantum wells, which leads to a different localization states for injected carriers. As LT-cap layer becomes getting thicker, the material's internal quantum efficiency (IQE) tends to decrease, which may result from an increase in non-radiative recombination centers.

Shedding light on evolution of Raman line shape with probing laser power: Light-induced perturbation in electron-phonon coupling.

The laser power mediated changes in the Raman line shape have been considered in terms of interference between discrete phonon states ρ and the electronic continuum states ϰ contributed by Urbach tail states. The laser-induced effects are treated in terms of the increase in the surface temperature and thereby the scaling of electronic disorder, i.e., Urbach energy, which can further contribute to the electron-phonon interactions. Therefore, the visualization of this effect is attempted analytically as a perturbation term in the Hamiltonian, which clearly accounts for the observed changes with laser power. This has been investigated based on the experimental results of laser power dependent Raman spectra of bulk EuFeO3 and silicon nanowires, which are found to provide convincing interpretations.

Simulation and interpretation of zinc and nitrogen dopants induced defect emissions in monoclinic gallium oxide

Zinc (Zn) and nitrogen (N) are potential acceptor dopants for inducing visible emissions and p-type conductivity in monoclinic gallium oxide (β-Ga2O3), however, the poor understanding of these dopants and their recombination process severely limited the optoelectronic applications. Here, we investigate the zinc (Zn) and nitrogen (N) dopants-induced intraband states and the broadening of defect emission bands due to vibronic coupling of highly transparent β-Ga2O3 films through chemical and optical analyses, as well as density function theory. Incorporating Zn and N in β-Ga2O3 shifts the valence band edge towards the Fermi level and introduces band tail states above the valence band maximum. The emission band from pure β-Ga2O3 becomes significantly broad with the incorporation of Zn and N resulting in two additional emission bands: green luminescence (GL) and red luminescence (RL) along with the characteristic ultraviolet luminescence (UVL) and blue luminescence (BL) of pristine β-Ga2O3. Furthermore, the defect states responsible for these UVL, BL, GL, and RL emissions and their phonon coupling strengths are estimated by simulating the spectral line shape of these emission bands using the configuration coordinate model. The simulation results indicate that the Zn and N dopants-induced intraband states are responsible for the GL, and RL bands. These intraband states are acceptors, which provide p-type conductivity in β-Ga2O3 film.

Anion and Cation Co-Doping of NiO for Transparent Photovoltaics and Smart Window Applications

Materials engineering based on metal oxides for manipulating the solar spectrum and producing solar energy have been under intense investigation over the last years. In this work, we present NiO thin films double doped with niobium (Nb) and nitrogen (N) as cation and anion dopants (NiO:(Nb,N)) to be used as p-type layers in all oxide transparent solar cells. The films were grown by sputtering a composite Ni-Nb target on room-temperature substrates in plasma containing 50% Ar, 25% O2, and 25% N2gases. The existence of Nb and N dopants in the NiO structure was confirmed by the Energy Dispersive X-Ray and X-Ray Photoelectron Spectroscopy techniques. The nominally undoped NiO film, which was deposited by sputtering a Ni target and used as the reference film, was oxygen-rich, single-phase cubic NiO, having a visible transmittance of less than 20%. Upon double doping with Nb and N the visible transmittance of NiO:(Nb,N) film increased to 60%, which was further improved after thermal treatment to around 85%. The respective values of the direct band gap in the undoped and double-doped films were 3.28 eV and 3.73 eV just after deposition, and 3.67 eV and 3.76 eV after thermal treatment. The changes in the properties of the films such as structural disorder, direct and indirect energy band gaps, Urbach tail states, and resistivity were correlated with the incorporation of Nb and N in their structure. The thermally treated NiO:(Nb,N) film was used to form a diode with a spin-coated two-layer, mesoporous on top of a compact, TiO2 film. The NiO:(Nb,N)/TiO2heterojunction exhibited visible transparency of around 80%, showed rectifying characteristics and the diode’s parameters were deduced using the I-V method. The diode revealed photovoltaic behavior upon illumination with UV light exhibiting a short circuit current density of 0.2 mA/cm2 and open-circuit voltage of 500 mV. Improvements of the output characteristics of the NiO:(Nb,N)/TiO2 UV-photovoltaic by proper engineering of the individual layers and device processing procedures are addressed. Transparent NiO:(Nb,N) films can be potential candidates in all-oxide ultraviolet photovoltaics for tandem solar cells, smart windows, and other optoelectronic devices.

Grain boundaries are not the source of Urbach tails in Cu(In,Ga)Se2 absorbers

The presence of Urbach tails in Cu(In,Ga)Se2 (CIGSe) absorbers has been identified as a limiting factor for the performance of the CIGSe solar cells. The tail states contribute to both radiative and non-radiative recombination processes, ultimately leading to a reduction in the open-circuit voltage and, consequently, decreasing the overall efficiency of CIGSe devices. Urbach tails result from structural and thermal disorders. The Urbach tails can be characterized by the Urbach energy, which is associated with the magnitude of the tail states. Within polycrystalline CIGSe absorbers, grain boundaries can be considered as structural disorder and, therefore, can potentially contribute to the Urbach tails. In fact, it has been proposed that the band bending at grain boundaries contribute significantly to the tail states. This study focuses on examining the correlation between Urbach tails and the band bending at the grain boundaries. The Urbach energies of the CIGSe samples are extracted from photoluminescence (PL) measurements, which reveal that the introduction of Sodium (Na) into the material can lead to a reduction in the Urbach energy, and an even further decrease can be achieved through the RbF post-deposition treatment. The band bending at the grain boundaries is investigated by Kelvin probe force microscopy measurements. A thorough statistical analysis of more than 340 grain boundaries does not show any correlation between Urbach tails and grain boundaries. We measure small band bending values at the grain boundaries, in the range of the thermal energy (26 meV at room temperature). Furthermore, our intensity dependent PL measurements indicate that Urbach tails are, at least in part, a result of electrostatic potential fluctuations. This supports the model that the introduction of alkali elements mainly decreases the magnitude of electrostatic potential fluctuations, resulting in a subsequent reduction in the Urbach energy.

Characteristics of Fully-Depleted Poly-Si Thin Film Transistors Operated in Above-Threshold Region with Low Drain Bias

Transfer and output characteristics of fully-depleted polycrystalline silicon thin film transistors, including both tail and deep acceptor-like trap states in bulk, in the above-threshold region with low drain bias are presented under low or high state density in the situation without or with interface charge, respectively. The characteristics are calculated by a simple surface-potential-based drain current model in the strong inversion region with valid bias condition explained, and 2D-device simulation. The above-threshold region is found to be divided into Regions I and II, with Vsi , indicating the channel beginning to be completely strongly-inverted and large currents, and explication of deviations between the model and simulation in Region I. The large-traps effect on the range of Region I and Vth in the high state density situation is discovered.

Efficient, Color-Stable, Pure-Blue Light-Emitting Diodes Based on Aromatic Ligand-Engineered Perovskite Nanoplatelets.

Perovskite nanoplatelets (NPLs) show great potential for high-color-purity light-emitting diodes (LEDs) due to their narrow line width and high exciton binding energy. However, the performance of perovskite NPL LEDs lags far behind perovskite quantum dot-/film-based LEDs, owing to their material instability and poor carrier transport. Here, we achieved efficient and stable pure blue-emitting CsPbBr3 NPLs with outstanding optical and electrical properties by using an aromatic ligand, 4-bromothiophene-2-carboxaldehyde (BTC). The BTC ligands with thiophene groups can guide two-dimensional growth and inhibit out-of-plane ripening of CsPbBr3 NPLs, which, meanwhile, increases their structural stability via strongly interacting with PbBr64- octahedra. Moreover, aromatic structures with delocalized π-bonds facilitate charge transport, diminish band tail states, and suppress Auger processes in CsPbBr3 NPLs. Consequently, the LEDs demonstrate efficient and color-stable blue emissions at 465 nm with a narrow emission line width of 17 nm and a maximum external quantum efficiency (EQE) of 5.4%, representing the state-of-the-art CsPbBr3 NPL LEDs.

Excited-State Dynamics of MAPbBr3: Coexistence of Excitons and Free Charge Carriers at Ultrafast Times.

Methylammonium lead tribromide perovskite (MAPbBr3) is an important material, for example, for light-emitting applications and tandem solar cells. The relevant photophysical properties are governed by a plethora of phenomena resulting from the complex and relatively poorly understood interplay of excitons and free charge carriers in the excited state. In this study, we combine transient spectroscopies in the visible and terahertz range to investigate the presence and evolution of excitons and free charge carriers at ultrafast times upon excitation at various photon energies and densities. For above- and resonant band-gap excitation, we find that free charges and excitons coexist and that both are mainly promptly generated within our 50-100 fs experimental time resolution. However, the exciton-to-free charge ratio increases upon decreasing the phonon energy toward resonant band gap excitation. The free charge signatures dominate the transient absorption response for above-band-gap excitation and low excitation densities, masking the excitonic features. With resonant band gap excitation and low excitation densities, we find that although the exciton density increases, free charges remain. We show evidence that the excitons localize into shallow trap and/or Urbach tail states to form localized excitons (within tens of picoseconds) that subsequently get detrapped. Using high excitation densities, we demonstrate that many-body interactions become pronounced and effects such as the Moss-Burstein shift, band gap renormalization, excitonic repulsion, and the formation of Mahan excitons are evident. The coexistence of excitons and free charges that we demonstrate here for photoexcited MAPbBr3 at ultrafast time scales confirms the high potential of the material for both light-emitting diode and tandem solar cell applications.

High quality a-InGaZnO and a-ZrAlO deposited at 375 °C by spray pyrolysis for low voltage operation TFTs

We report the high-performance, bottom-gate a-InGaZnO (IGZO) TFT with high-k ZrAlO (ZAO) gate insulator by spray pyrolysis (SP). The a-IGZO/ZAO TFT exhibits the average saturation mobility of 23.12 cm2 V−1 s−1, subthreshold swing of 121 mV dec-1, ION/OFFratio of ∼108 with no hysteresis and low off-state current of ∼10-19A µm−1. In addition, the TFT exhibits good interface quality with less trap density (∼9x1010 cm−2) and conduction band tail state (∼4.3x1018 cm−3) extracted from TCAD fitting. The high-performance is attributed to the formation of uniform and dense a-IGZO and ZAO thin films with low defect density. Therefore, the SP a-IGZO/ZAO TFTs have great potential to be employed in low-cost, low driving voltage TFTs for display application.

High‐Efficiency Single‐Photon Upconversion Photoluminescence of Perovskite Quantum Dots via Intragap State Regulation

AbstractLead halide perovskite quantum dots (PQDs) have shown remarkable single‐photon upconversion photoluminescence (SPUC‐PL) bringing them a brilliant perspective as the active media for biological imaging, optiag, and upconversion photovoltaics. However, the mechanism of SPUC in PQDs is still in dispute, and the specific strategy for optimizing PQDs toward a higher SPUC performance has yet to be established, which impedes the application of PQDs‐based SPUC‐PL in practice. In this work it is revealed that the PL quantum yield is declined by surface trap states, while the SPUC efficiency is promoted by Urbach tail states. These two types of intragap states can be chemically manipulated by engineering the surface ligand and the growth kinetics of perovskite nanocrystals, bringing on the improvement of light emission efficiency and the promotion of subgap low‐energy photon absorption, respectively. As a proof of concept, utilizing the synergistic effect of the proposed chemical manipulation method, the SPUC‐PL efficiency of PQDs is boosted by more than 40%, which gives rise to a spectral window of optical refrigeration gain as broad as ≈130 meV.

Numerical Simulation, Electrostatic and Physical Compact Modeling of C8-BTBT-C8 Organic Thin Film Transistor

This paper presents numerical simulation and compact modeling of 2,7-Dioctyl {1} benzothieno {3,2-b}{1} benzothiophene (C8-BTBT-C8) organic semiconductor-based TFT. It shows the entire modeling process flow of this organic semiconductor (OSC) and tests the device realization using a ring oscillator. The paper comprises OSC characterisation, band-gap modeling, electrostatic modeling, and capacitance modeling. The TCAD model consists of the Hopping and Pool Frenkel premise and characterizes the Density of State (DOS) for traps in deep and tail states. The findings from this research provide valuable information for improving OTFT models, enhancing their predictive accuracy, and advancing the understanding of organic semiconductor device behavior. The electrostatics demonstrate device structure dependency