Density Of Deep Defect States Research Articles

Low-Cost Antimony Selenosulfide with Tunable Bandgap for Highly Efficient Solar Cells.

About 10% efficient antimony selenosulfide (Sb2 (S,Se)3 ) solar cell isrealized by using selenourea as a hydrothermal raw material to prepare absorber layers. However, tailoring the bandgap of hydrothermal-based Sb2 (S,Se)3 film to the ideal bandgap (1.3-1.4eV) using the selenourea for optimal efficiency is still a challenge. Moreover, the expensive selenourea dramatically increases the fabricating cost. Here, a straightforward one-step hydrothermal method is developed to prepare high-quality Sb2 (S,Se)3 films using a novel precursor sodium selenosulfate as the selenium source. By tuning the Se/(Se+S) ratio in the hydrothermal precursor solution, a series of high-quality Sb2 (S,Se)3 films with reduced density of deep defect states and tunable bandgap from 1.31 to 1.71eV is successfully prepared. Consequently, the best efficiency of 10.05% with a high current density of 26.01 mA cm-2 is achieved in 1.35eV Sb2 (S,Se)3 solar cells. Compared with the traditional method using selenourea, the production cost for the Sb2 (S,Se)3 devicesis reduced by over 80%. In addition, the device exhibits outstanding stability, maintaining more than 93% of the initial power conversion efficiency after 30 days of exposure in the atmosphere without encapsulation. The present work definitely paves a facile and effective way to develop low-cost and high-efficiency chalcogenide-based photovoltaic devices.

Ultrasonically sprayed-on perovskite solar cells-effects of organic cation on defect formation of CH3NH3PbI3 films

Methylammonium lead iodide (CH3NH3PbI3) based perovskite having low degrees of the disorder is of great interest for optoelectronic and photovoltaic applications. In this work, a layer of CH3NH3PbI3 was successfully prepared using an ultrasonically sprayed-nebulous method. Changes in structural and optical properties alongside with photo-induced charge separation and transportation behavior were systematically studied. The surface photovoltage spectra reveal a significant reduction of the density of deep defect states as the organic content was increased. It was observed that the measured values of Urbach energies decrease from 33.36 to 28.24 meV as the amount of organic content was increased to an optimum value. The best perovskite solar cells obtained using the sprayed-on approach exhibited a Jsc of 16.54 mA/cm2, a Voc of 0.99 V, and a FF of 62.4, resulting in an overall PCE of 10.09%.

Device Characteristics of Band gap Tailored 10.04% Efficient CZTSSe Solar Cells Sprayed from Water-Based Solution.

A diverse S/(S + Se) ratio of Cu2ZnSn(Sx,Se1-x)4 (CZTSSe) thin-film solar cells is derived by the water-based spray pyrolysis approach. By fine-tuning the S/(S + Se) ratio, base CZTSSe device efficiency has significantly improved from 7.02 to 10.04% by minimizing the Voc deficit up to 616 mV and increasing fill factor (FF) from 56.42 to 62.38%. As the S/(S + Se) ratio was increased from 0 to 0.4, surface compactness was observed to be improved with slightly decreased grain size, which increased shunt resistance and resultantly increased FF. However, when S-alloying was more than S/(S + Se) = 0.4, grain size decreased too much and had a detrimental effect on device performance. To deeply understand the role effect of the S/(S + Se) ratio, detailed spectroscopic analysis is performed with admittance spectroscopy, temperature-dependent current-voltage characteristic (J-V-T), time-resolved photoluminescence, and Raman depth profiling. Experimental results revealed that the different power conversion efficiency limiting factors were developed with various S/(S + Se) ratios. High density of deep defect states generated with the S/(S + Se) ≥ 40% content and larger conduction band offset observed with red kink were formed in the Se/(S + Se) ≥ 80% content. Hence, in order to get the high-efficient CZTSSe solar cell, fine tuning of the S/(S + Se) ratio is necessary.

Preparation and Characterization of a-SiC:H Absorber Layer for Semi-transparent Solar Cells

This paper reports on device-quality silicon-carbon alloy (a-SiC:H) application as an absorber material in semi-transparent solar cells. Films with an optical bandgap ranging from 2 to 2.3 eV were prepared by plasma enhanced chemical vapour deposition (PECVD). The n-i-p structures with undoped SiC:H layers deposited under the same experimental conditions were also fabricated and characterized. The optimized devices showed forward current-voltage characteristics with a diode ideality factor in the range from 1.4 to 1.8, and an open circuit voltage up to 0.92 V. The density of deep defect states in a SiC:H was estimated from the transient current measurements and correlated with the optical bandgap.

Defect states and charge trapping characteristics of HfO2 films for high performance nonvolatile memory applications

In this work, we present significant charge trapping memory effects of the metal-hafnium oxide-SiO2-Si (MHOS) structure. The devices based on 800 °C annealed HfO2 film exhibit a large memory window of ∼5.1 V under ±10 V sweeping voltages and excellent charge retention properties with only small charge loss of ∼2.6% after more than 104 s retention. The outstanding memory characteristics are attributed to the high density of deep defect states in HfO2 films. We investigated the defect states in the HfO2 films by photoluminescence and photoluminescence excitation measurements and found that the defect states distributed in deep energy levels ranging from 1.1 eV to 2.9 eV below the conduction band. Our work provides further insights for the charge trapping mechanisms of the HfO2 based MHOS devices.

Influence of iron on the performance of CIGS thin-film solar cells

In the last years steel foil has become an attractive candidate for use as a flexible substrate material for Cu(Inx,Ga1−x)Se2 thin-film solar cells (CIGS). It is stable at the high temperatures present during CIGS processing and is also commercially available. However, one disadvantage is the diffusion of substrate elements into the CIGS layer, which are detrimental for the performance of solar cells. We found a direct correlation between the iron (Fe) content in the CIGS layer and corresponding solar cell parameters. A deep defect related to Fe impurities could be detected by admittance spectroscopy measurements. The solar cell parameters could be well fitted by simulation with recombination at an acceptor like deep defect in the bulk of the CIGS layer. The simulated density of deep defect states correlates nicely with the Fe concentration in the CIGS layer. From this we conclude that Fe replaces an In (or Ga) site in the CIGS lattice and creates an FeIn2+ (or FeGa2+) deep acceptor state in the bulk of CIGS layers, which is detrimental already at a low concentration in the sub ppm range. The simulations enabled us to estimate the maximum Fe concentration in CIGS layers which is tolerable without disturbing the performance of high-efficiency CIGS solar cells.

Optoelectronic properties of a‐Si1–xCx:H films grown in hydrogen diluted silane‐methane plasma

AbstractThis work reports on the optoelectronic properties and device application of hydrogenated amorphous silicon carbide (a‐Si1–xCx:H) films grown by plasma‐enhanced chemical vapour deposition (PECVD). The films with an optical bandgap ranging from about 1.8 to 2.0 eV were deposited in hydrogen diluted silane‐methane plasma by varying the radio frequency power. Several n–i–p structures with an intrinsic a‐Si1–xCx:H layer of different optical gaps were also fabricated. The optimized devices exhibited a diode ideality factor of 1.4‐1.8, and a leakage current of 190‐470 pA/cm2 at –5 V. The density of deep defect states in a‐Si1–xCx:H was estimated from the transient dark current measurements and correlated with the optical bandgap and carbon content. Urbach energies for the valence band tail were also determined by analyzing the spectral response within sub‐bandgap energy range. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of light soaking and annealing on the stability of hydrogenated amorphous silicon films deposited using pure and highly helium diluted silane

We present a comparative study of the effects of the light soaking and annealing on the density of deep defect states, the width of valence-band tail (Urbach tail) and the refractive index of two different types of undoped hydrogenated amorphous silicon (a-Si:H) films. The samples were prepared by radio-frequency plasma enhanced chemical vapour deposition (PECVD) from pure silane at low rates (0.7–2 Å s −1), known as ‘standard’ samples, and highly helium diluted silane at high rates (11–13 Å s −1), at two substrate temperatures equal to 250 and 350 °C for each type of films. The results obtained for the as grown samples show that increasing the substrate temperature from 250 to 350 °C leads to an increase in the refractive index for all films, suggesting an increase in their compactness. Moreover, the density of defects and Urbach tail measured for the helium diluted samples also decrease when increasing the deposition temperature, as a result of the elimination of the polyhydride Si–H 2 and (Si–H 2) n complexes present in the films grown at 250 °C. We obtain values comparable to those measured for optimised standard films. Light soaking induces a decrease in the static refractive index in all films however, the helium diluted samples deposited at 350 °C still exhibit a much higher compactness than the other ones. On the other hand, although the density of deep defect states increases upon illumination for all samples, without any changes either in the Urbach tail or in the related-hydrogen microstructure, the helium diluted films show comparable stability to that observed for the optimised standard ones deposited at low rates. The analysis of the data obtained for the annealed samples at 200 °C indicate that the native defects in the as grown films are of different nature from those created by illumination.

The role of dangling bond states in the picosecond recovery of photoinduced absorption in a-Si:H

We have investigated the influence of deep defect states on the picosecond decay of photoinduced absorption in a-Si:H using femtosecond laser-pulses. For device-quality a-Si:H material with a low density of deep defect states, N d ≃10 16 cm -3 , the recovery of photoinduced absorption is controlled by an intrinsic bimolecular recombination process

Distribution of occupied near-surface band-gap states in a-Si:H.

We investigate the distribution of occupied band-gap states in undoped, B-doped, and P-doped a-Si:H within the first \ensuremath{\sim}100 A\r{} of the surface using total-yield photoelectron spectroscopy in combination with the Kelvin probe. In clean, undoped a-Si:H the occupied density of states extracted from the measured yield spectrum consists of a linear valence-band edge, an exponential valence-band tail decreasing into the gap, and a broad band of deep defect states superposed on this tail. The deep-defect density of undoped a-Si:H measured by total yield is 2 orders of magnitude larger than that measured by photothermal deflection spectroscopy (PDS) on simultaneously prepared 1-\ensuremath{\mu}m-thick samples, from which we infer the existence of an excess density of deep defect states near the surface corresponding to an equivalent surface-state density of 3\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$.The addition of diborane (phosphine) to the glow-discharge plasma decreases (increases) the density of excess occupied near-surface defect states in the gap and shifts the Fermi energy towards the valence- (conduction-) band edge. A 0.5-eV increase in the work function over that in undoped a-Si:H accompanies the \ensuremath{\sim}98% reduction of these excess occupied near-surface deep defect states by ${10}^{\mathrm{\ensuremath{-}}5}$ B doping, from which we infer an (0.4--0.5)-eV downward band bending at the clean, undoped a-Si:H surface. The inverse logarithmic slope of the exponential intrinsic valence-band tail observed in B-doped a-Si:H is 45 meV, in excellent agreement with that inferred from dispersive-transport, electron-spin-resonance (ESR), and traveling-wave data. Dividing the occupied density of states of ${10}^{\mathrm{\ensuremath{-}}3}$ P-doped a-Si:H by the Fermi-Dirac distribution function, we observe an exponential conduction-band tail extending over more than 3 orders of magnitude in the density of states. Its inverse logarithmic slope of \ensuremath{\sim}35 meV agrees with that inferred from ESR and traveling-wave data on P-doped a-Si:H, but is slightly larger than the slope inferred from dispersive-transport measurements on undoped a-Si:H. This discrepancy arises from the presence of a large concentration of P donors which affects the deeper tail-state distribution observed in heavily-P-doped a-Si:H. The average density of occupied states at the Fermi energy ${E}_{F}$ for the 30 doped samples we have studied is 8(\ifmmode\pm\else\textpm\fi{}2)\ifmmode\times\else\texttimes\fi{}${10}^{15}$ states/eV ${\mathrm{cm}}^{3}$, in good agreement with the thermodynamic minimum density of defect states permitted in undoped a-Si:H. This implies that the Fermi level lies at a minimum in the a-Si:H density of states for all doping levels.From this result we infer that the position of the ${D}^{+}$ defect level in B-doped a-Si:H lies more than 0.5 eV above the ${D}^{\mathrm{\ensuremath{-}}}$ level in P-doped a-Si:H; an arrangement in conflict with a fixed energy distribution of deep defect states and with the generally accepted positive value of the defect correlation energy in a-Si:H. We resolve this seeming discrepancy by postulating that the amorphous network responds to changes in ${E}_{F}$ by changing its defect structure so as to minimize the total energy of the system. This postulate leads to a variable energy distribution of deep defect states that depends only on the position of ${E}_{F}$ and the defect-formation energies; intimate pairing of dopants and defects, which has been suggested to account for this discrepancy, is not required. The relation between the surface and bulk distributions of localized gap states in a-Si:H is discussed.