Transient Photocapacitance Spectroscopy Research Articles

Transient photocapacitance spectroscopy of deep-levels in (001) β-Ga2O3

Defect levels in (001) β-Ga2O3 are investigated using transient photocapacitance (TPC) spectroscopy. For sub-band photon energies in the range of 1.13–3.10 eV, the TPC signal shows broad optical absorption at room temperature. Using the theoretical Pässler model, deep-level states at E T = 1.15 ± 0.07 eV (Trap 1) and E T = 1.69 ± 0.41 eV (Trap 2) below the conduction bands are demonstrated. The Franck–Condon energies ( D F C) of Trap 1 and Trap 2 are 0.26 ± 0.11 and 0.66 ± 0.55 eV, respectively. TPC measurements have been performed at temperatures ranging from 30 to 360 K. From 150 to 360 K, the TPC signal of Trap 1 decreases as the temperature increases. The decrease in the TPC signal of Trap 1 agrees with the thermal quenching model, and a thermal activation energy of 156 meV is estimated. Moreover, the effective phonon energy of β-Ga2O3 has been extracted. From 30 to 360 K, the effective phonon energy is in the range of 85–126 meV.

Transient photocapacitance spectroscopy on Au/TiO2 Schottky diodes with rolled-up nanomembrane electrodes

Rolled-up nanomembrane electrodes are used to prepare optically transparent Au/TiO2 Schottky diodes suitable for deep level transient photocapacitance spectroscopy. It is demonstrated that both the binding energy and the capture cross section of the oxygen vacancy can be extracted from the photocapacitance transients using a rate equation model. The values are consistent with those obtained from conventional deep level transient spectroscopy, taken from the same sample. Furthermore, information about the capture process can be extracted.

Study of defect properties and recombination mechanism in rubidium treated Cu(In, Ga)Se2 solar cells

Heavy alkali-metal treatment of Cu(In,Ga)Se2 (CIGSe) absorbers has been emerging as a key process for achieving over 23% high conversion efficiencies in CIGSe solar cells. Here, we investigate the effect of rubidium fluoride post-deposition treatment (RbF-PDT) on the electronic and carrier recombination properties of narrow bandgap (narrowgap) gap and wide bandgap (widegap) CIGSe solar cells using thermal admittance spectroscopy (TAS), transient photocapacitance spectroscopy (TPC), as well as time-resolved photoluminescence (TRPL). We find that the activation energy of the main capacitance step in TAS spectra of narrowgap and widegap CIGSe solar cells reduces after RbF-PDT. On the other hand, capacitance–voltage (C–V) and temperature-dependent current–voltage (IVT) measurements demonstrate that the built-in potential, as well as the activation energy Ea, increases upon RbF-PDT both for narrowgap and widegap samples, pointing to reduced interface recombination. TPC revealed an appreciable reduction of the optical response of bulk defects in the narrowgap and widegap CIGSe, suggesting improvement of bulk properties after RbF treatment. TRPL confirmed that RbF-PDT significantly reduces carrier recombination in the bulk of narrowgap and widegap CIGSe absorbers and at the surface, leading to extended carrier lifetimes. Analysis of open-circuit voltage (VOC) losses due to nonradiative recombination in the bulk of the CIGSe showed a strong correlation between enhanced carrier lifetime and improved VOC for narrow gap CIGSe cells. In contrast, although we observed a substantial decrease of VOC losses in widegap CIGSe bulk, the analysis indicated that the key to photovoltaic performance enhancement is improved interface quality.

Study of ion-implanted nitrogen related defects in diamond Schottky barrier diode by transient photocapacitance and photoluminescence spectroscopy

The study of nitrogen-vacancy (NV) centers in diamond are growing attractive in the application of quantum devices. Here, electrical control of NV charge state and defects induced by nitrogen ions implantation in diamond were investigated by transient photocapacitance (TPC) spectroscopy and photoluminescence (PL) spectroscopy. The experiments show that thresholds of 1.2 eV appeared in TPC spectra are probably due to the presence of excited defect energy levels related to vacancy or NV center. Alternatively, the 2.2 eV defect observed in the TPC spectrum is probably attributed to NV centers. The variation of TPC and PL spectra with different applied voltages suggests that bias voltages control the charge state of NV centers since their effect on the Fermi level shifting in the depletion region. Furthermore, the steady-state photocapacitance indicates that the 2.2 eV deep trap slows down the process of photocapacitance rise and fall, and these processes can be enhanced by a higher electrical field.

Identification of deep level defects in CdTe solar cells using transient photo-capacitance spectroscopy

The carrier lifetime in CdTe is strongly limited by the nonradiative recombination via defects. Here, deep level defects in CdTe thin-film solar cells are revealed by transient photo-capacitance (TPC) measurement. A broad defect band centered at 1.07 eV above the valance band is identified at 90 K. The defect signal is reduced with the insertion of the CdSe layer between the CdS/CdTe heterojunction. The TPC signals are rapidly quenched with increased temperature, which suggests that this deep level defect is highly possible to act as an effective recombination center. Based on the thermal quenching model, the activation energy (E a) of the defect is estimated to be ∼0.2 eV. With the configuration coordinate model, the temperature-dependent TPC signal and the corresponding electronic transition process can be well interpreted. All the observations strongly indicate that the introduction of Se atoms into CdTe is promising to suppress the formation of deep defects.

Investigation of Deep-level Defects in CuGaSe2 Thin-film Solar Cells by Transient Photo-capacitance Spectroscopy

Properties of deep-level defects in CuGaSe2 thin-film solar cells were investigated using transient photo-capacitance (TPC) spectroscopy. Two Gaussian-shaped deep-level defects centered at around 0.8 eV and 1.54 eV above the valence band were identified. The electronic structure of the two defects was illustrated by a configuration coordinate model to explain the thermal quenching effect in the two defects, which considered a large lattice distortion for the 0.8 eV defect while no distortion for the 1.54 eV defect.

Determination of deep-level defects in Cu2ZnSn(S,Se)4 thin-films using photocapacitance method

Deep-level defects were investigated in Cu2ZnSn(S,Se)4 and Cu2ZnSnS4 thin-films using transient photocapacitance (TPC) spectroscopy. A deep-defect, OH1 centered around 1.0 eV above the valance-band (EV) of Cu2ZnSnS4 has been identified at room temperature (RT). However, OH1-defect could be identified in Cu2ZnSn(S,Se)4 at low temperature only. Absence of OH1-defect in Cu2ZnSn(S,Se)4 at RT explains its better performance comparing to Cu2ZnSnS4 solar-cell. A comparative study of the TPC spectra of the Cu(In,Ga)Se2 solar-cells was performed. Low intensity of defect-signal together with lower broadening of exponential band-tail in the TPC spectra were attributed to superior performance of Cu(In,Ga)Se2 solar-cells comparing to Cu2ZnSn(S,Se) counterpart.

The effect of copper on the sub-bandgap density of states of CdTe solar cells

Two optical sub-bandgap transitions in CdTe thin-film solar cells have been identified using detailed transient photocapacitance and transient photocurrent spectroscopy measurements. A broad response centered at EV + 0.9 eV directly correlates with the quantity of Cu present in the absorber layer, while a second response at EV + 1.2 eV does not depend on Cu or Zn and may be an intrinsic defect. These results demonstrate the influence of Cu on the sub-bandgap density of states of CdTe, and they are critical to understanding, modeling, and improving its optoelectronic properties.

Study of optical and electrical assessments of the quaternary MgZnSnO system containing different Mg content

Magnesium zinc tin oxide (MZTO) quaternary systems were prepared using solution-processed method. Assessment of the Mg dopant content of the MZTO quaternary system was done to obtain a better performance of the films in the device applications. The combination of silicon and MZTO system exhibited rectifying behavior. It has been observed that the current in the reverse direction is increased by illumination of 30 mW/cm2. The capacitance-time characteristics were measured by transient photocapacitance spectroscopy. The composition of MZTO was studied by absorbance, transmittance and reflectance measurements. It has been seen that the band gap (~4.07 eV) of MZTO films remains almost constant with regard to the increasing Mg dopant content while the thin film of ZTO ternary system without Mg has band gap of 3.9 eV.

Optical response of deep defects as revealed by transient photocapacitance and photocurrent spectroscopy in CdTe/CdS solar cells

Transient photocapacitance (TPC) and photocurrent (TPI) spectroscopy were used to characterize defects in CdTe/CdS solar cells. A broad defect band is observed at an optical energy of 1.28eV above the valence band, and such a defect is not indicated by admittance spectroscopy and drive-level capacitance profiling. Temperature-dependent TPC measurements of the defect band follow a thermal quenching model in which the defect׳s energy, 0.22eV below the conduction band, is consistent with the optical response. We provide a theoretical basis for the use of a thermal quenching model in TPC.

Defect study of molecular beam epitaxy grown undoped GaInNAsSb thin film using junction-capacitance spectroscopy

Defects in undoped GaInNAsSb thin film (i-GaInNAsSb) were investigated by junction-capacitance technique using admittance and transient photocapacitance (TPC) spectroscopy. An electron trap D2 was identified at 0.34 eV below the conduction band (EC) of i-GaInNAsSb using admittance spectroscopy. Optical transition of valance band (EV) electrons to a localized state OH1 (EV + 0.75 eV) was manifested in negative TPC signal. Combined activation energy of OH1 and D2 defect corresponds to the band-gap of i-GaInNAsSb, suggesting that OH1/D2 acts as an efficient recombination center. TPC signal at ∼1.59 eV above EV was attributed to the nitrogen-induced localized state in GaInNAsSb.

Photocapacitance study of MBE grown GaInNAsSb thin film solar cells

GaInNAsSb based solar cell structure has been grown using atomic hydrogen assisted molecular beam epitaxy (H-MBE). Transient photocapacitance (TPC) spectroscopy was applied to investigate the optically active defects in the Si-doped GaInNAsSb layer (n-GaInNAsSb) of the structure. In addition to the interband transitions, a sub-band-gap optical transition was also determined using TPC spectroscopy. This sub-band-gap transition corresponds to an electron trap (OE1) at 0.78eV below the conduction band (EC) edge of n-GaInNAsSb material. Thermally active defects were probed by the electrical measurement using admittance spectroscopy. Thus, a defect profile in the entire band gap of the n-GaInNAsSb film was obtained.

Electronically active defects in the Cu2ZnSn(Se,S)4 alloys as revealed by transient photocapacitance spectroscopy

Transient photocapacitance (TPC) spectra were obtained on a series of Cu2ZnSn(Se,S)4 absorber devices with varying Se:S ratios, providing bandgaps (Eg) between 1 eV and 1.5 eV. Efficiencies varied between 8.3% and 9.3% for devices with Eg ≤ 1.2 eV and were near 6.5% for devices with Eg ≥ 1.4 eV. The TPC spectra revealed a band-tail region with Urbach energies at or below 18 meV for the first group, but in the 25-30 meV range for the higher band-gap samples. A deeper defect band centered near 0.8 eV was also observed in most samples. We identified a correlation between the Urbach energies and the voltage deficit in these devices.

The electronic structure of Cu(In 1 − x Ga x)Se 2 alloyed with silver

We have examined the electronic properties of (Ag 1 − x Cu x)(In 1 − y Ga y)Se 2 (ACIGS) alloys over a wide range of compositions to assess whether such alloys might allow one to achieve larger values of V OC at larger band gaps compared to the Cu(In 1 − y Ga y)Se 2 (CIGS) alloys. Our studies employed junction capacitance techniques such as drive level capacitance profiling (DLCP) and transient photocapacitance (TPC) spectroscopy, as well as temperature dependent J–V measurements. The TPC spectra revealed not only that the band gap did indeed increase as the Ag-fraction was increased, but also that the bandtailing (or Urbach energies) in all ACIGS samples were substantially smaller than for CIGS samples of corresponding band gaps. This indicates that the Ag alloying somehow reduces the degree of disorder present. The DLCP measurements indicated very low free carrier densities, on the order of 10 14 cm − 3 , as well as evidence of defects located at the CdS/ACIGS junction. Temperature-dependent I–V measurements revealed a distinct “kink” in the V OC vs T characteristics, suggesting a transition from an interface-trap limited regime to a bulk-limited regime. At temperatures below 250 K, the V OC increased by up to 0.1 V as the sample was light soaked. This suggests that the interface traps limiting the V OC can be passivated by exposure to light.

Insights and challenges toward understanding the electronic properties of hydrogenated nanocrystalline silicon

Hydrogenated nanocrystalline silicon (nc-Si:H) is a complex mixed-phase material containing regions of silicon nanocrystallites interspersed with amorphous silicon. It is an important material in efforts to advance the production of more economical multijunction thin-film silicon-based photovoltaic technologies. We have applied the junction capacitance methods of transient photocapacitance spectroscopy and drive-level capacitance profiling to understand its fundamental electronic properties. We compare results in both the annealed and light-soaked states for nc-Si:H samples having a wide range of amorphous-to-crystallite volume fractions. Significant differences between samples with lower and higher amorphous fractions are observed and we propose a tentative microscopic model that may account for these differences.

Characterizing the effects of silver alloying in chalcopyrite CIGS with junction capacitance methods

AbstractA variety of junction capacitance-based characterization methods were used to investigate alloys of Ag into Cu(In1-xGax)Se2 photovoltaic solar cells over a broad range of compositions. Alloys show encouraging trends of increasing VOC with increasing Ag content, opening the possibility of wide-gap cells for use in tandem device applications. Drive level capacitance profiling (DLCP) has shown very low free carrier concentrations for all Ag-alloyed devices, in some cases less than 1014 cm−3, which is roughly an order of magnitude lower than that of CIGS devices. Transient photocapacitance spectroscopy has revealed very steep Urbach edges, with energies between 10 meV and 20 meV, in the Ag-alloyed samples. This is in general lower than the Urbach edges measured for standard CIGS samples and suggests a significantly lower degree of structural disorder.

The influence of Na on metastable defect kinetics in CIGS materials

The electronic properties of matched pairs of Cu(In x Ga 1 − x )Se 2 (CIGS) solar cells, with and without normal sodium levels, were studied by junction capacitance methods including admittance spectroscopy, drive level capacitance profiling (DLCP) and transient photocapacitance spectroscopy (TPC). The capacitance profiling measurements revealed a large deep defect density in the vicinity of the barrier interface that was likely responsible for the lower performance of the reduced Na samples. The metastable properties of CIGS solar cells were also examined, and these revealed marked differences between the two types of samples. These results directly address the predictions of theoretical microscopic models that have been proposed to account for metastable effects in CIGS.

Temperature dependence of photocapacitance spectrum of CIGS thin-film solar cell

Deep levels in Cu(In 1 − x ,Ga x )Se 2 (CIGS) are studied by transient photocapacitance (TPC) spectroscopy by varying the Ga concentration, x, from 0.38 to 0.7. The TPC spectra of CIGS thin-film solar cells at 140 K exhibited a defect level with an optical transition energy of about 0.8 eV. The spectrum shape in the sub-bandgap region is independent of the Ga concentration. Therefore, the optical transition energy to the defect level is almost constant with about 0.8 eV from the valence band. The TPC signals for defect level are quenched by increasing temperature. The activation energy of thermal quenching is estimated to be about 0.3 eV. The thermal and optical activation processes are explained using configuration coordinate diagram.

Electronic properties of amorphous zinc tin oxide films by junction capacitance methods

Amorphous zinc tin oxides have recently shown sufficiently high mobilities that fully transparent electronic devices with these materials are now considered promising. We report the first detailed characterization of the electronic properties of these materials using the methods of admittance spectroscopy, drive-level capacitance profiling (DLCP), and transient photocapacitance (TPC) spectroscopy. We have examined a series of 1–3μm thick zinc tin oxide films (at a composition of 1:1 ZnO:SnO2) incorporated into metal–insulator–semiconductor (MIS) devices. These were annealed at 400, 500, and 600°C. The DLCP measurement is largely insensitive to interface defects and thus provides a good measurement of the free carrier and defect densities within the bulk region of these films. For the best samples (annealed at 600°C), our results indicate a free carrier density of 5×1014cm−3 plus a deep defect density 1.5×1015cm−3. The TPC spectra disclose optical transitions between defect levels and the conduction and/or valence bands. They reveal two primary features in these materials: A dominant gaussian shaped deep defect band with an optical threshold near 1.6eV relative to the conduction band, and an exponential band-tail with a characteristic (Urbach) energy near 120meV. The Urbach energy indicates the degree of disorder in semiconducting materials, and is directly correlated to the anneal temperature for this series of samples.

Optical absorption spectra of a-Si:H MIS structure measured by transient photocapacitance spectroscopy

The transient photocapacitance spectroscopy (TPC) was applied to measure the optical absorption spectra of hydrogenated amorphous silicon (a-Si:H) in metal-insulator–semiconductor (MIS) structure. The measured spectra show the features of optical transitions from both D 0 and D − defect bands to conduction band. The stability of measurement was checked against the change of probe frequency. The availability of TPC in detecting the defect density changes was also tested by checking the well-known defect density increase in a-Si:H after light illumination.