Midgap Defect Research Articles

Effects of mid-gap defects and barrier interface reactions on tunneling behaviors of ZnO-i-Si heterojunctions

Low-leakage pin diodes based on ZnO-i-Si are realized by redox reaction of aluminum with the native oxide SiOx into AlOx and by proper selection of annealing conditions. The main sources of electric leakage was found to arise from charge carrier tunneling via mid-gap states in the semiconductors or lowered tunneling barriers. Less mid-gap states in n-ZnO and high tunneling barrier of the i-layer are key to lowering the leakage. Proper post-annealing of pin diodes effectively heal the mid-gap defects, while maintaining the integrity of the tunneling layers, thus lowering the leakage currents to reach a rectification ratio of 2400, surpassing most similarly benchmarked devices reported in literature. Excessive annealing causes some part of the i-layer to transform into to ZnAl2O4 and Al:ZnO. High Al-doping and lowered potential barrier provided by ZnAl2O4 are responsible for high leakage currents in devices so fabricated.

Ultrafast Carrier Dynamics of CdTe: Surface Effects

Cadmium telluride has been an important absorber material used in solar cells for decades given its near-optimum band gap and lower cost for device fabrication. However, the overall efficiency has been low compared to the theoretical limit. One major contributor to the problem of the relatively low open-circuit voltage is the high surface recombination of photogenerated carriers. In this contribution, time-resolved pump–probe reflectivity was used to study the carrier dynamics of CdTe(111) under the influence of selected surface conditions. It was found that surfaces after the processes of chemical etching and thermal oxidation for short durations exhibit bulk-like transient reflectivity and a clearly reduced surface recombination velocity, according to a model calculation. The comparative study indicates that the origins of carrier traps include the midgap defect states that are associated with the uncapped Te surface atoms, namely the Te antisites and Te interstitials, and subsurface damages resulting f...

Physical device modeling of CdTe ultrathin film solar cells

The device physics of CdS/CdTe Ultrathin Film solar cells (UTF-SCs) is modeled using Sah–Noyce–Shockley (SNS) theory for the generation/recombination in the Space Charge Region (SCR) of an asymmetric pn+ junction. This theory is based on Shockley–Read–Hall (SRH) recombination mechanism related to midgap defects in the band gap. The maximum effect of SRH is within SCR where the carriers are mainly collected by the electric field. Since the depletion width in a UTF extends to the back region, a UTF resembles an asymmetric pn+ junction, which is most commonly modelled by SNS theory. Several materials and electrical parameters dominate the device physics of a fully depleted CdTe layer with dCdTe⩽1μm. The uncompensated defect density, recombination lifetime, and CdTe thickness are parameters which effectively control the current–voltage characteristics and quantum efficiency of UTF devices. Simulation results indicate that any parameter that narrows the SCR of a UTF will reduce the open-circuit voltage whereas only thinning the CdTe layer lowers the short-circuit current density. The electrical resistivity and quantum efficiency are a measure of defect density/level and dCdTe or band bending at the junction, respectively. A practical takeaway of our simulation results is to measure the electrical resistivity of the CdTe layer before metallization of UTF solar cells. Band bending is a measure of junction quality. The latter parameters are usually ignored in device fabrication and modeling. However, any parameter related to defect density in UTF devices must be considered in the modeling since the front and back contacts are closer here and the degradation rate is rather probable. The degradation of UTF devices based on carrier collection mechanisms dominated by drift component is briefly discussed.

Lifetimes of photogenerated electrons on a GaAs surface affected by nanostructural defects

The recombination dynamics of photogenerated carriers on temporal and spatial scales of 100 fs and 100 nm, respectively, on an inhomogeneous GaAs surface with structural defects was investigated using time-resolved photoemission electron microscopy with femtosecond laser pulses. The observed photocarrier lifetimes (τ) ranged from subpicoseconds to picoseconds at isolated structural defects and were inversely proportional to the photoemission intensity at each defect. We concluded that τ corresponds to the carrier trapping time to midgap defect states and estimated the density of the midgap states at each defect on the basis of the relation between τ and the photoemission intensity.

The Influence of Structure and Processing on the Behavior of TiO2 Protective Layers for Stabilization of n-Si/TiO2/Ni Photoanodes for Water Oxidation

Light absorbers with moderate band gaps (1-2 eV) are required for high-efficiency solar fuels devices, but most semiconducting photoanodes undergo photocorrosion or passivation in aqueous solution. Amorphous TiO2 deposited by atomic-layer deposition (ALD) onto various n-type semiconductors (Si, GaAs, GaP, and CdTe) and coated with thin films or islands of Ni produces efficient, stable photoanodes for water oxidation, with the TiO2 films protecting the underlying semiconductor from photocorrosion in pH = 14 KOH(aq). The links between the electronic properties of the TiO2 in these electrodes and the structure and energetic defect states of the material are not yet well-elucidated. We show herein that TiO2 films with a variety of crystal structures and midgap defect state distributions, deposited using both ALD and sputtering, form rectifying junctions with n-Si and are highly conductive toward photogenerated carriers in n-Si/TiO2/Ni photoanodes. Moreover, the photovoltage of these electrodes can be modified by annealing the TiO2 in reducing or oxidizing environments. All of the polycrystalline TiO2 films with compact grain boundaries investigated herein protected the n-Si photoanodes against photocorrosion in pH = 14 KOH(aq). Hence, in these devices, conduction through the TiO2 layer is neither specific to a particular amorphous or crystalline structure nor determined wholly by a particular extrinsic dopant impurity. The coupled structural and energetic properties of TiO2, and potentially other protective oxides, can therefore be controlled to yield optimized photoelectrode performance.

Ultrafast pre-breakdown dynamics in Al₂O₃SiO₂ reflector by femtosecond UV laser spectroscopy.

Ultrafast carrier dynamics in Al2O3/SiO2 high reflectors has been investigated by UV femtosecond laser. It is identified by laser spectroscopy that, the carrier dynamics contributed from the front few layers of Al2O3 play a dominating role in the initial laser-induced damage of the UV reflector. Time-resolved reflection decrease after the UV excitation is observed, and conduction electrons is found to relaxed to a mid-gap defect state locating about one photon below the conduction band . To interpret the laser induced carrier dynamics further, a theoretical model including electrons relaxation to a mid-gap state is built, and agrees very well with the experimental results.. To the best of our knowledge, this is the first study on the pre-damage dynamics in UV high reflector induced by femtosecond UV laser.

Modeling the effect of nanosecond laser conditioning on the femtosecond laser-induced damage of optical films.

The effect of nanosecond laser conditioning on the femtosecond laser-induced damage behaviors of Al2O3, HfO2, SiO2 single layers and Al2O3/SiO2 high reflectors (HR) are explored. During femtosecond laser damage test, negative effects on enhancing the femtosecond laser-induced damage threshold (LIDT) of optical films after the nanosecond laser conditioning is found, which is opposite to the LIDT improvement in the nanosecond range. To explain the mechanism after nanosecond laser conditioning, a theoretical model including multiphoton ionization (MPI), avalanche ionization (AI) and decays of electrons with one defect state is built to simulate the evolution of electron density in the conduction band. A permanent mid-gap defect state resulting from the process of laser conditioning is introduced in our model, which is found to contribute seed electrons to conduction band and hence accelerate the final breakdown. Both the experimental result and theoretical calculation agree very well with each other.

Towards printed perovskite solar cells with cuprous oxide hole transporting layers: a theoretical design

Solution-processed p-type metal oxide materials have shown great promise in improving the stability of perovskite-based solar cells and offering the feasibility for a low cost printing fabrication process. Herein, we performed a device modeling study on planar perovskite solar cells with cuprous oxide (Cu2O) hole transporting layers (HTLs) by using a solar cell simulation program, wxAMPS. The performance of a Cu2O/perovskite solar cell was correlated to the material properties of the Cu2O HTL, such as thickness, carrier mobility, mid-gap defect, and doping concentrations. The effect of interfacial defect densities on the solar cell performance was also investigated. Our simulation indicates that, with an optimized Cu2O HTL, high performance perovskite solar cells with efficiencies above 13% could be achieved, which shows the potential of using Cu2O as an alternative HTL over other inorganic materials, such as NiOx and MoOx. This study provides theoretical guidance for developing perovskite solar cells with inorganic hole transporting materials via a printing process.

Effect of forming gas annealing on the inversion response and minority carrier generation lifetime of n and p-In0.53Ga0.47As MOS capacitors

In0.53Ga0.47As MOS capacitors with a range of Al2O3 oxide thickness values were examined using multi-frequency capacitance–voltage and conductance–voltage characterization for n and p-type In0.53Ga0.47As epitaxial layers. The samples exhibited a minority carrier response consistent with surface inversion for both n and p-type In0.53Ga0.47As MOS structures. An equality between the peak magnitude of the frequency scaled conductance, G/ω, and the derivative of capacitance with frequency, −ωdC/dω in strong inversion allows estimation of accurate Cox values for the varying thickness MOS devices. Minority carrier generation lifetimes, τg, were extracted from the experimentally measured transition frequencies, ωm, through physics based a.c. simulations. It was observed that τg was reduced following a post-metallization 275°C forming gas anneal treatment over both n and p-In0.53Ga0.47As. This is indicative of an improvement in the In0.53Ga0.47As quality by means of hydrogen passivation of the mid-gap bulk defects responsible for the observed minority carrier response.

Fast exciton annihilation by capture of electrons or holes by defects via Auger scattering in monolayer metal dichalcogenides

The strong Coulomb interactions and the small exciton radii in two-dimensional metal dichalcogenides can result in very fast capture of electrons and holes of excitons by mid-gap defects from Auger processes. In the Auger processes considered here, an exciton is annihilated at a defect site with the capture of the electron (or the hole) by the defect and the hole (or the electron) is scattered to a high energy. In the case of excitons, the probability of finding an electron and a hole near each other is enhanced many folds compared to the case of free uncorrelated electrons and holes. Consequently, the rate of carrier capture by defects from Auger scattering for excitons in metal dichalcogenides can be 100-1000 times larger than for uncorrelated electrons and holes for carrier densities in the $10^{11}$-$10^{12}$ cm$^{-2}$ range. We calculate the capture times of electrons and holes by defects and show that the capture times can be in the sub-picosecond to a few picoseconds range. The capture rates exhibit linear as well as quadratic dependence on the exciton density. These fast time scales agree well with the recent experimental observations, and point to the importance of controlling defects in metal dichalcogenides for optoelectronic applications.

High internal quantum efficiency in AlGaN multiple quantum wells grown on bulk AlN substrates

The internal quantum efficiency (IQE) of Al0.55Ga0.45N/AlN and Al0.55Ga0.45N/Al0.85Ga0.15N UVC MQW structures was analyzed. The use of bulk AlN substrates enabled us to undoubtedly distinguish the effect of growth conditions, such as V/III ratio, on the optical quality of AlGaN based MQWs from the influence of dislocations. At a high V/III ratio, a record high IQE of ∼80% at a carrier density of 1018 cm−3 was achieved at ∼258 nm. The high IQE was correlated with the decrease of the non-radiative coefficient A and a reduction of midgap defect luminescence, all suggesting that, in addition to dislocations, point defects are another major factor that strongly influences optical quality of AlGaN MQW structures.

Synthesis and characterization of ZnS with controlled amount of S vacancies for photocatalytic H2 production under visible light.

Controlling amount of intrinsic S vacancies was achieved in ZnS spheres which were synthesized by a hydrothermal method using Zn and S powders in concentrated NaOH solution with NaBH4 added as reducing agent. These S vacancies efficiently extend absorption spectra of ZnS to visible region. Their photocatalytic activities for H2 production under visible light were evaluated by gas chromatograph, and the midgap states of ZnS introduced by S vacancies were examined by density functional calculations. Our study reveals that the concentration of S vacancies in the ZnS samples can be controlled by varying the amount of the reducing agent NaBH4 in the synthesis, and the prepared ZnS samples exhibit photocatalytic activity for H2 production under visible-light irradiation without loading noble metal. This photocatalytic activity of ZnS increases steadily with increasing the concentration of S vacancies until the latter reaches an optimum value. Our density functional calculations show that S vacancies generate midgap defect states in ZnS, which lead to visible-light absorption and responded.

Dark current mechanisms in amorphous selenium-based photoconductive detectors: an overview and re-examination

The transient and steady-state dark current behaviors in various amorphous selenium (a-Se) detectors are analyzed by developing mathematical models considering all possible mechanisms (e.g., carrier depletion, thermal generation, and carrier injection from electrodes) and charge carrier transport properties of a-Se. The theoretical models are validated by comparing them with recently published measured transient and steady-state dark currents in various a-Se detectors. The fittings of the models with the experimental results reveal various important material and device properties such as the optimum trap depth and concentrations in the blocking layers for faster stabilization and lower dark current, and the effective barrier height between the metal/a-Se contacts. The thermal generation current is significantly higher in avalanche detectors (at extremely high fields) than that in conventional detectors. The effective blocking layers for both holes and electrons are necessary for the minimum level of the dark current. The minimum dark current is determined by the thermal generation current. The thermal generation current can be lowered by reducing the mid gap defect states.

Optimizing Ga-profiles for highly efficient Cu(In, Ga)Se2 thin film solar cells in simple and complex defect models

Highly efficient Cu(In,Ga)(S,Se)2 photovoltaic thin film solar cells often have a compositional variation of Ga to In in the absorber layer, here described as a Ga-profile. In this work, we have studied the role of Ga-profiles in four different models based on input data from electrical and optical characterizations of an in-house state-of-the-art Cu(In,Ga)Se2 (CIGS) solar cell with power conversion efficiency above 19%. A simple defect model with mid-gap defects in the absorber layer was compared with models with Ga-dependent defect concentrations and amphoteric defects. In these models, optimized single-graded Ga-profiles have been compared with optimized double-graded Ga-profiles. It was found that the defect concentration for effective Shockley–Read–Hall recombination is low for high efficiency CIGS devices and that the doping concentration of the absorber layer, chosen according to the defect model, is paramount when optimizing Ga-profiles. For optimized single-graded Ga-profiles, the simulated power conversion efficiency (depending on the model) is 20.5–20.8%, and the equivalent double-graded Ga-profiles yield 20.6–21.4%, indicating that the bandgap engineering of the CIGS device structure can lead to improvements in efficiency. Apart from the effects of increased doping in the complex defect models, the results are similar when comparing the complex defect models to the simple defect models.

Defect density and dielectric constant in perovskite solar cells

We report on measurement of dielectric constant, mid-gap defect density, Urbach energy of tail states in CH3NH3PbIxCl1−x perovskite solar cells. Midgap defect densities were estimated by measuring capacitance vs. frequency at different temperatures and show two peaks, one at 0.66 eV below the conduction band and one at 0.24 eV below the conduction band. The attempt to escape frequency is in the range of 2 × 1011/s. Quantum efficiency data indicate a bandgap of 1.58 eV. Urbach energies of valence and conduction band are estimated to be ∼16 and ∼18 meV. Measurement of saturation capacitance indicates that the relative dielectric constant is ∼18.

Electron–Hole Pair Generation of the Visible-Light Plasmonic Photocatalyst Ag@AgCl: Enhanced Optical Transitions Involving Midgap Defect States of AgCl

We explored how the visible-light energy absorbed by noble-metal nanoparticles (NPs) is converted to electrons and holes in the semiconductor in a visible-light plasmonic photocatalyst by studying the representative system Ag@AgCl on the basis of density functional calculations and classical electrodynamics calculations. These calculations suggest that the energy transfer from the Ag NPs to the semiconductor AgCl requires the presence of midgap defect states in the semiconductor and that the surface plasmon resonance (SPR) of the Ag NPs strongly enhances the optical transitions of the semiconductor involving the defect states. We verified this suggestion experimentally by preparing Ag@AgCl samples possessing different degrees of bulk and surface defects and subsequently by carrying out photodegradation experiments using these samples.

Effect of localized states on the reciprocity between quantum efficiency and electroluminescence in Cu(In,Ga)Se2 and Si thin-film solar cells

We present the electroluminescence and quantum efficiency of three different types of thin-film solar cells based on absorbers made from Cu(In,Ga)Se2 as well as from μc-Si:H and a-Si:H. Simulations of our experimental results show that the main contribution to the electroluminescence spectrum originates from band-to-tail transitions in the case of Cu(In,Ga)Se2, from tail-to-tail transitions in μc-Si:H, and from tail-to-tail as well as from band-to-midgap-defect transitions in a-Si:H. By comparing the electroluminescence with the quantum efficiency, we analyze the effect of localized states on the optoelectronic reciprocity relation in the three material systems. The relatively steep band–tail density of states in Cu(In,Ga)Se2 is compatible with the reciprocity relation at room temperature while the shallower band–tail density of states in μc-Si:H and the deep mid-gap defect states in a-Si:H lead to substantial deviations from the reciprocity.

Passivation Properties of Subnanometer Thin Interfacial Silicon Oxide Films

Subnanometer thin silicon oxide films, applied as interlayer between crystalline silicon absorbers and functional layers, have demonstrated to improve interface passivation properties. Here we compare the interface defect density as well as the fixed charge of simple native air oxides to wet-chemical oxides on silicon substrates of different doping type, with different crystal orientations and after different chemical pre-treatment processes. We show that optimized wet-chemical pre-treatment and wet-chemical oxidation leads to strong improvement in terms of interface defect density as compared to simple oxidation in air. Furthermore we show that such subnanometer thin layers can contain large positive fixed charges of up to 1011 cm-2. Due to excellently low defect densities for thin layers <0.5nm, tunneling transport and thus application for solar cells with passivated contacts should be possible. While on p-type substrates such layers feature a high positive charge and would thus support an emitter band bending, on n-type substrates the charge is smaller and can even be negative due to electrons trapped in mid gap defects.

Luminescence and solar cell from ligand-free colloidal AgInS2 nanocrystals

Ligand-free AgInS2 nanocrystals display significant defect-related sub-band gap photo-absorption in addition to the excitonic absorption. The post-synthesis annealing of the nanocrystal dispersion at 150 °C reduces the intrinsic defect density, and also increases the nanocrystal size (5.8 nm) thereby reducing surface related defects. The AgInS2 nanocrystals exhibit a broad emission spectra, with a large Stokes shift compared to the excitonic absorption, which is attributed to two radiative electron–hole recombination processes: the first one involves the localized defect-state and the delocalized valence/conduction band with a lifetime of ~20 ns, and the second one involves two localized donor and acceptor states with a lifetime >250 ns. The faster decay dominates the higher energy end of the emission spectrum. Quantum dot sensitized solar cells using these ligand-free AgInS2 nanocrystals exhibit a maximum energy conversion efficiency of 0.8% and an open circuit voltage of 0.45 V. The device performance is better compared to previous AgInS2 nanocrystal based solar cells, due to both the use of the ligand-free nanocrystals that improve the charge transport and the post-synthesis annealing which partially removes the mid-gap defect states.

Impact of varied sulfur incorporation on the device performance of sequentially processed Cu(In,Ga)(Se,S)2 thin film solar cells

In order to improve the performance of chalcopyrite Cu(In1 − x,Gax)(Se1 − ySy)2 solar cells, the implementation of a bandgap widening at the absorber/buffer interface via an increase of the [S]/[S + Se] ratio is investigated. In this work we examine industrially processed samples, which were fabricated via the deposition-reaction process with varied H2S pressure during rapid thermal processing (RTP). Precursors which were exposed to a crucial amount of sulfur during the RTP step resulted in samples with significantly improved device performance. The increase of the open circuit voltage by about 150 mV cannot sufficiently be explained by bandgap widening. Observation of a strongly reduced saturation current density and ideality factor in intensively sulfurized samples suggest subdued recombination via mid-gap defect states located in the space charge region. This hypothesis is supported by results of deep-level transient spectroscopy measurements, which show that in both samples a mid-gap minority defect is present albeit its concentration is about one magnitude larger in sulfur-poor samples. These results confirm that sulfur passivates recombination centers in the depletion layer and hence significantly increases the open circuit voltage and the overall device performance of the photovoltaic devices.