Band Tail Research Articles

Photoluminescence assessment of materials for solar cell absorbers

Absolute photoluminescence measurements present a tool to predict the quality of photovoltaic absorber materials before finishing the solar cells. Quasi Fermi level splitting predicts the maximal open circuit voltage. However, various methods to extract quasi Fermi level splitting are plagued by systematic errors in the range of 10-20 meV. It is important to differentiate between the radiative loss and the shift of the emission maximum. They are not the same and when using the emission maximum as the "radiative" band gap to extract the quasi Fermi level splitting from the radiative efficiency, the quasi Fermi level splitting is 10 to 40 meV too low for a typical broadening of the emission spectrum. However, radiative efficiency presents an ideal tool to compare different materials without determining the quasi Fermi level splitting. For comparison with the open circuit voltage, a fit of the high energy slope to generalised Planck's law gives more reliable results if the fitted temperature, i.e. the slope of the high energy part, is close to the actual measurement temperature. Generalised Planck's law also allows the extraction of a non-absolute absorptance spectrum, which enables a comparison between the emission maximum energy and the absorption edge. We discuss the errors and the indications when they are negligible and when not.

Optical characterization of RF sputtered copper oxide for thin film solar cell applications

The structural and optical properties of copper oxide thin film have been investigated using various characterization techniques. These thin films have potential applications in thin film solar cells that utilize the visible and UV regions of the solar spectrum. Copper oxide thin films are deposited using RF magnetron sputtering equipped with mass flow controllers. The samples are in-situ annealed post deposition at 300 °C in oxygen ambient to improve the structural properties. Such annealed sample exhibit narrow (002) and (020) CuO peaks and (111)Cu2O peak. X-ray diffraction results confirm the formation of polycrystalline CuO and mixed phases of CuO -Cu2O. A Lowest FWHM of 0.00079 rad has been observed in the XRD spectrum, indicative of excellent crystalline quality of copper oxide thin films. UV–Visible Spectroscopy measurement performed on the copper oxide samples, showed a wide range of absorption edge ranging from 3.9 eV to 3.75 eV for various deposition temperatures ranging from 50 °C to 150 °C. The annealing effect at high temperature for longer duration results in the formation of CuO phase as confirmed by absorption edge at 2.85 eV and the presence of (002) and (020) CuO XRD peaks. In-situ annealing also showed the onset of Urbach effect due to the transitions at tail states and band to tail states.

Stoichiometry modulates the optoelectronic functionality of zinc phosphide (Zn3-xP2+x).

Predictive synthesis-structure-property relationships are at the core of materials design for novel applications. In this regard, correlations between the compositional stoichiometry variations and functional properties are essential for enhancing the performance of devices based on these materials. In this work, we investigate the effect of stoichiometry variations and defects on the structural and optoelectronic properties of monocrystalline zinc phosphide (Zn3P2), a promising compound for photovoltaic applications. We use experimental methods, such as electron and X-ray diffraction and Raman spectroscopy, along with density functional theory calculations, to showcase the favorable creation of P interstitial defects over Zn vacancies in P-rich and Zn-poor compositional regions. Photoluminescence and absorption measurements show that these defects create additional energy levels at about 180 meV above the valence band. Furthermore, they lead to the narrowing of the bandgap, due to the creation of band tails in the region of around 10-20 meV above the valence and below the conduction band. The ability of zinc phosphide to form off-stoichiometric compounds provides a new promising opportunity for tunable functionality that benefits applications. In that regard, this study is crucial for the further development of zinc phosphide and its application in optoelectronic and photovoltaic devices, and should pave the way for defect engineering in this kind of material.

Irradiation of Cu(In, Ga)Se-=SUB=-2-=/SUB=- Thin Films by 10 MeV Electrons at 77 K: Effect on Photoluminescence Spectra

Thin films of Cu(In, Ga)Se2 on Mo/glass were irradiated by 10 MeV electrons at 77 K and examined by photoluminescence at 77 K before and after irradiation without warming the samples as well as after warming to 300 K. The photoluminescence spectra revealed a broad band constituting 3 merged peaks (P1, P2, P3) assigned to: band-to-band recombination (P1) and recombination of free electrons with holes localised at acceptors influenced by the valence band tail (P2, P3). Irradiation reduced the intensity of the peaks due to deep traps generated by electrons and anomalously reduced the degree of compensation of the material. Keywords: thin films, irradiation, photoluminescence, recombination, band-to-band.

Irradiation of Cu(In, Ga)Se-=SUB=-2-=/SUB=- Thin Films by 10 MeV Electrons at 77 K: Effect on Photoluminescence Spectra

Thin films of Cu(In,Ga)Se2 on Mo/glass were irradiated by 10 MeV electrons at 77 K and examined by photoluminescence at 77 K before and after irradiation without warming the samples as well as after warming to 300 K. The photoluminescence spectra revealed a broad band constituting 3 merged peaks (P1, P2, P3) assigned to: band-to-band recombination (P1) and recombination of free electrons with holes localised at acceptors influenced by the valence band tail (P2, P3). Irradiation reduced the intensity of the peaks due to deep traps generated by electrons and anomalously reduced the degree of compensation of the material. Keywords: thin films, irradiation, photoluminescence, recombination, band-to-band.

Voltage Loss Comparison in CdSe/CdTe Solar Cells and Polycrystalline CdSeTe Heterostructures

Cd(Se)Te solar cells have considerable headroom to increase voltage. Voltage losses occur due to incomplete absorption above bandgap <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E_g</i> and band tail absorption below <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E_g</i> ; such losses are quantified using radiative voltage. The largest voltage losses are attributed to nonradiative recombination, which is quantified via carrier lifetime and radiative efficiency. We compare radiative voltage, radiative efficiency, and carrier lifetime for Cu-doped CdSe/CdTe solar cells and for undoped polycrystalline CdSeTe heterostructures passivated with Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> . Using external quantum efficiency spectrum and a CdSeTe absorption spectrum obtained from absolute photoluminescence (PL), we show that the radiative voltage is greater than 1.1 V. Time-resolved PL experiments and modeling show that a major part of voltage losses can be attributed to recombination in the absorber bulk. The front interface recombination makes a larger impact within the first few nanoseconds after pulsed excitation, and the comparison of time-gated and time-integrated PL can be used to assess relative contributions of front interface and bulk recombination rates.

Showcasing the optical properties of monocrystalline zinc phosphide thin films as an earth-abundant photovoltaic absorber.

Zinc phosphide, Zn3P2, is a semiconductor with a high absorption coefficient in the spectral range relevant for single junction photovoltaic applications. It is made of elements abundant in the Earth's crust, opening up a pathway for large deployment of solar cell alternatives to the silicon market. Here we provide a thorough study of the optical properties of single crystalline Zn3P2 thin films grown on (100) InP by molecular beam epitaxy. The films are slightly phosphorus-rich as determined by Rutherford backscattering. We elucidate two main radiative recombination pathways: one transition at approximately 1.52 eV attributed to zone-center band-to-band electronic transitions; and a lower-energy transition observed at 1.3 eV to 1.4 eV attributed to a defect band or band tail related recombination mechanisms. We believe phosphorus interstitials are likely at the origin of this band.

The Signatures of Acid Concentration on the Optical Band Gap and Associated Band Tails of Chitosan from Shrimp for Application in Optoelectronic Devices

Over the recent years, the global increase of electronic wastes from electrical and electronic devices (e-wastes) has been on an alarming trend in quantity and toxicity and e-wastes are non-biodegradable resulting in its cumulative increase over time. Changes in technology and unrestricted regional movement of electrical devices have facilitated the generation of more e-wastes leading to high levels of air, soil and water pollution. To address these challenges, biodegradable organic components such as chitosan have been used to replace their inorganic counterparts for optoelectronic device applications. However, in-depth knowledge on how such materials can be used to tune the optical properties of their hybrid semiconductors is unrivaled. Thus, systematic studies of the interplay between the preparation methods and optical band gap and Urbach energy of such organic components are vital. This study has thus been dedicated to map out the effect of acid concentrations during chitosan extraction on the corresponding optical band gap and Urbach energy with a view to improving its applications in optoelectronic devices. The, 1.0 to 2.5 molar hydrochloric acid (HCl) was used for 12 hours at room temperature during demineralization and 2.0 molar sodium hydroxide (NaOH) during deprotonation processes. The absorbance spectrum of the samples was collected by UV-Vis spectrophotometer and band gap energies were analyzed by performing Tauc’s plot. This study revealed that the energy band gap of chitosan extracted from 1 M HCl, 1.5 M HCl, 2.0 M HCl and 2.5 M HCl were 3.72 eV, 3.50 eV, 3.45 eV and 3.36 eV respectively. Furthermore, the Urbach energy of chitosan extracted from 1 M HCl, 1.5 M HCl, 2.0 M HCl and 2.5 M HCl were 0.60496 eV, 0.5292 eV, 4724 eV and 0.2257 eV, respectively.

Understanding defects and band tailing characteristics and their impact on the device performance of Cu2ZnSn(S,Se)4 solar cells

The Raman analysis of Cu2ZnSn(S,Se)4 thin films revealed a change in the relative defect concentration and device performance with composition.

Self-assembled CsPbBr3 quantum dots with wavelength-tunable photoluminescence for efficient active jamming.

CsPbBr3 perovskite quantum dots (QDs) show great potential in various applications due to their size-dependent and excellent optoelectronic properties. However, it is still challenging to synthesize size-tunable CsPbBr3 QDs with purple emission. Herein, CsPbBr3 nanospheres (NS) with purple emission (432 nm) and wavelength-tunable photoluminescence were synthesized using a two-step recrystallization method for the first time. A nanocube (NC) strategy resulting from CsPbBr3 nanosphere self-assembly via polar solvent-induced surface ligand mismatch was proposed. The self-assembly process endows the QDs with wavelength-tunable photoluminescence ranging from 432 to 518 nm. The significant reduction in defects during self-assembly was confirmed by transient optical spectroscopy measurements, photoluminescence quantum yields (PLQY), and the disappearance of tail bands in the long-wavelength region of the photoluminescence (PL) spectrum. This theory demonstrated that the decrease in high defect surfaces and increase in specific surface area were the reasons for the decline in defects. Most importantly, these QDs could be used for the active jamming of optical imaging systems based on charged-coupled devices (CCDs), including laser imaging radar and low light level (LLL) night vision systems. QDs significantly increase the mean square error (MSE) of the image, while the detection rate of the target by the artificial intelligence algorithm decreased by 95.17%. The wide wavelength tunable emission caused by structural changes makes it arduous for silicon-based detectors to avoid the interference of QDs by adding filters or by other means.

Determination of optical bandgap energy and optical characteristics of Cd30Se50S20 thin film at various thicknesses

At room temperature, the Cd30Se50S20 ternary thin films with thicknesses ranging from 100 to 400 nm were synthesized using a vacuum evaporation process in an adequate atmosphere of pure nitrogen. X-ray and scanning electron microscopy techniques were used to validate the amorphous nature of the investigated thin films. Changing the thickness of the pristine thin film had a significant effect on controlling the linear and nonlinear optical properties. The optical absorption coefficient was determined using the optical measurements (the transmittance, the reflectance spectra and the film’s thickness) in the range of (300–2500 nm) and the thickness of the thin layer. The optical parameters were calculated in the linear path: the band tail energy and the optical bandgap energy determined in this work by several methods, including the first derivative of the transmittance, reflectance, and optical absorption coefficient curves as a function of wavelength, as well as the second derivative of the transmittance curves. The optical bandgap energy was also computed as a function of photon energy using Tauc formula, the relaxation time, and the portions of the optical conductivity curves. It turns out that the bandgap energy and the tail energy were affected by the increase in thickness so that the first decreases and the other increases in a harmonious behavior. The linear optical constants (the extinction coefficient, the refractive index), the linear dispersion parameters (the single oscillator energy, the dispersion energy), and the linear dielectric parameters were all affected by the increase in thickness of the thin layer. The phase velocity, the group velocity, and Kirchhoff functions were determined, as well as the linear optical dispersion coefficient. The Boling formula, on the other hand, was used to compute the nonlinear refractive index. Finally, the spectral distribution of molar parameters was discussed.

Influence of sub-band gap density of states on the electrical performance of amorphous SiZnSnO thin film transistor

Fabrication, electrical characterizations and numerical simulations are performed on radio-frequency sputtered amorphous SiZnSnO thin film transistors (TFTs). Experimental transfer curves are compared with numerically simulated results to extract the nature of sub-band gap density of states (DOS) of the amorphous channel materials. Defect states arising due to disorder induced localization of electronic wave functions is considered in the numerical simulation process. The extracted DOS for conduction band tail is found to be roughly two order of magnitude lower than that of the a-Si:H. Moreover, disorder asymmetry is also observed between conduction and valence band tail states. In addition to band tails, Gaussian shallow donor-like states and deep level acceptor-like traps are also taken into consideration. The influence of these defect states on the electrical performance of SiZnSnO TFTs has been investigated in detail. Such studies are useful for fundamental understanding of device physics and further improvement of amorphous SiZnSnO based TFT performance and stability.

Optical band-gap and associated Urbach energy tails in defected AlN thin films grown by ion beam sputter deposition: Effect of assisted ion energy

AIN thin films were grown by ion beam sputter deposition in reactive assistance of N+/N2+ ions on Si (100) substrates. During deposition, assisted ion energy (EA) was varied as 90 eV and 120 eV with a post deposition exposure to N2 plasma. The resultant films were characterized by grazing incidence x-ray diffraction (GIXRD) for their structure and atomic force microscopy (AFM) for their root mean square (rms) surface roughness (δ). UV-VIS spectrophotometry was carried out to explore their optical band-gap with associated Urbach (EU) and weak absorption tail (EWAT) energies. Our results show that, AlN thin film grown with 90 eV reactive ion assistance possesses larger optical band gap (Eg) of 5.3 eV associated with comparatively narrower band tails when compared to those AlN thin film samples which were grown for 120 eV reactive ion assistance. These optical band-gaps are further correlated with corresponding Urbach energy tails which can be used as a measure of disorder in microstructure of the film. Also, appropriate optimization of energy tails substantiates the possibility of band gap engineering as per the requirement of different thin film devices. Copyright © 2017 VBRI Press.

Photoluminescence spectroscopy of excitonic emission in CsPbCl3 perovskite single crystals

We analyzed low-temperature photoluminescence (PL) and the band edge excitonic emission in bulk halide perovskite CsPbCl3 single crystals. Strong near band edge excitonic emission is observed at 420 nm and a series of excitonic states was observed. Temperature-dependent PL measurements on cesium lead chloride over a wide temperature range from 13 K to 295 K revealed significant electron phonon coupling with a Huang-Rhys parameter ranging from 1.34 to 3.38. A primary experimental finding is that the near band edge PL emission shows existence of multiple sharp features at low temperatures that are attributable to free excitons, bound excitons, and their phonon replicas. The PL also shows evidence of band tailing effects due to disorder. We investigated the recombination mechanism of photogenerated charge carriers by means of time-resolved PL (TR-PL) spectroscopy at low temperatures and various excitation powers and observed competing mono- and bimolecular recombination processes. A dynamic model is proposed to describe the TR-PL. We observed both long-lived and short-lived recombination where the short-lived time decay is attributed to free exciton recombination, whereas the long-lived decay to bound exciton recombination involving traps due to intrinsic disorder.

Optical and A.C. Electrical Properties for Polypyrrole and Polypyrrole/Graphene (ppy/gn) Nanocomposites

In this work, chemical oxidation was used to polymerize conjugated polymer "Polypyrrole" at room temperature Graphene nanoparticles were added by in situ-polymerization to get (PPY-GN) nano. Optical and Electrical properties were studied for the nanocomposites. optical properties of the nanocomposites were studied by UV-Vis spectroscopy at wavelength range (200 -800 nm). The result showed optical absorption spectra were normally determined and the result showed that the maximum absorbance wave length at 280nm and 590nm. The optical energy gap has been evaluated by direct transition and the value has decreased from (2.1 eV for pure PPy) to (1.3 eV for 5 %wt. of GN). The optical constants such as the band tail width ΔE was evaluated, the value of ΔE for pure PPy was (0.0949eV) while for 5 wt. % of GN it was (0.5156 eV), It has been observed that the Urbach tail for pure PPy was smaller than that for PPy/GN nanocomposites and it increase as GN concentration increases. The A.C electrical conductivity at range of frequency (103Hz-106Hz) was increased by increasing the frequency and GN concentration about four order of magnitude. The s value was about (0.653-0.962) which means that the mechanism of conductivity is correlated hopping mechanism (C. H. P.). The dielectric constant and dielectric lose were determined and found to decrease with increasing frequency.

An Empirical Modeling of Gate Voltage-Dependent Behaviors of Amorphous Oxide Semiconductor Thin-Film Transistors including Consideration of Contact Resistance and Disorder Effects at Room Temperature.

In this paper, we present an empirical modeling procedure to capture gate bias dependency of amorphous oxide semiconductor (AOS) thin-film transistors (TFTs) while considering contact resistance and disorder effects at room temperature. From the measured transfer characteristics of a pair of TFTs where the channel layer is an amorphous In-Ga-Zn-O (IGZO) AOS, the gate voltage-dependent contact resistance is retrieved with a respective expression derived from the current–voltage relation, which follows a power law as a function of a gate voltage. This additionally allows the accurate extraction of intrinsic channel conductance, in which a disorder effect in the IGZO channel layer is embedded. From the intrinsic channel conductance, the characteristic energy of the band tail states, which represents the degree of channel disorder, can be deduced using the proposed modeling. Finally, the obtained results are also useful for development of an accurate compact TFT model, for which a gate bias-dependent contact resistance and disorder effects are essential.

Thermally Controlled Charge-Carrier Transitions in Disordered PbSbTe Chalcogenides.

Binary and ternary chalcogenides have recently attracted much attention due to their wide range of applications including phase-change memory materials, topological insulators, photonic switches, and thermoelectrics. These applications require a precise control of the number and mobility of charge carriers. Here, an unexpected charge-carrier transition in ternary compounds from the PbTe-Sb2 Te3 pseudo-binary line is reported. Upon thermal annealing, sputtered thin films of PbSb2 Te4 and Pb2 Sb2 Te5 undergo a transition in the temperature coefficient of resistance and in the type of the majority charge carriers from n-type to p-type. These transitions are observed upon increasing structural order within one crystallographic phase. To account for this striking observation, it is proposed that the Fermi energy shifts from the tail of the conduction band to the valence band because different levels of overall structural disorder lead to different predominant types of native point defects. This view is confirmed by an extensive computational study, revealing a transition from excess cations and SbPb for high levels of disorder to PbSb prevailing for low disorder. The findings will help fine-tune transport properties in certain chalcogenides via proper thermal treatment, with potential benefits for memories, thermoelectric material optimization, and neuromorphic devices.

Back Shallow Ge Gradient Enhanced Carrier Separation for CZTSe Solar Cells through a Coselenization Process.

Given the prominent success of the Ga gradient in CuIn1-xGaxSe2 (CIGSe) solar cells, Ge gradient implementation is a promising way to boost Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. However, Ge-graded CZTSSe solar cells only possess a low efficiency of 9.2%, far from that of Ge-incorporated CZTSSe without a gradient (12.3%). Herein, we demonstrated a shallow Ge gradient CZTSe solar cell with an improved efficiency over 10%. The Ge gradient was achieved through a GeSe2-Se coselenization process, where GeSe2 acts as a low-temperature fluxing agent to assist crystallization and induce Ge transport toward the back interface. The relieved band tails and improved junction quality, leading to a better carrier separation, were found to take a primary responsibility for device improvement. These results highlight a remarkable breakthrough for Ge-graded CZTSe solar cells and offer a promising way to develop Ge-involved solar cells.

Optical and Electrical Transport Evaluations of n-Type Iron Pyrite Single Crystals.

Iron pyrite [cubic FeS2 (cFeS2)] is considered as an earth-abundant and low-cost thin-film photovoltaic material. However, the conversion efficiency of cFeS2-based solar cells remains below 3%. To elucidate this limitation, we evaluate the optical and electrical characteristics of cFeS2 single crystals that are grown using the flux method, thus providing us an understanding of the electron transport behavior of cFeS2 single crystals. The oxide layer on the surface of cFeS2, which can possibly have an influence on the electrical characteristics of cFeS2, is removed prior to characterization via optical spectroscopy and electrical transport measurement. The optical property of cFeS2 was found to have both indirect and direct transitions. We also observed the presence of a band tail below the conduction band. The obtained electrical transport behavior indicates that cFeS2 bulk exhibits a high defect density and a disordered phase, thus leading to the hopping conduction mechanism. Our results will pave the way for the development of photovoltaic applications with iron pyrite.

Effect of copper doping on the optical and dielectric parameters of glycine zinc sulfate (GZS) single crystals

Glycine zinc sulfate (GZS) single crystals pure and doped with copper were grown by the slow evaporation of aqueous solutions at constant temperature (307 K). Cu/Zn molar ratios were 1%, 2% and 3% in the growth solutions. The optical reflectance was measured for pure and doped crystals (GZS:Cu) and it was used to calculate several optical and dielectric parameters. The direct optical band gap energy was found to be 5.44 eV for pure GZS crystal and it decreased as Cu/Zn molar ratio increased. The band tailing was studied for GZS:Cu crystals by using Urbach relation. The normal dispersion was investigated for GZS:Cu crystals by using both Wemplee–Di-Domenico and Cauchy-Sellmaier models. The real and imaginary parts of the complex dielectric constant of GZS:Cu crystals were calculated. Urbach parameters, normal dispersion parameters and dielectric parameters of GZS:Cu crystals were calculated in the present work for the first time. It was found that doping GZS crystal, with copper, affects the values of its optical and dielectric parameters and improves its electrical conduction.