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Journal ArticleDOI

Determination of defect distributions from admittance measurements and application to Cu(In,Ga)Se2 based heterojunctions

15 Oct 1996-Journal of Applied Physics (American Institute of Physics)-Vol. 80, Iss: 8, pp 4411-4420
TL;DR: In this article, a method to deduce energy distributions of defects in the band gap of a semiconductor by measuring the complex admittance of a junction is proposed, which consists of calculating the derivative of the junction capacitance with respect to the angular frequency of the signal corrected by a factor taking into account the band bending and the drop of the ac signal.
Abstract: A method to deduce energy distributions of defects in the band gap of a semiconductor by measuring the complex admittance of a junction is proposed. It consists of calculating the derivative of the junction capacitance with respect to the angular frequency of the ac signal corrected by a factor taking into account the band bending and the drop of the ac signal over the space charge region of the junction. Numerical modeling demonstrates that defect distributions in energy can be reconstructed by this method with high accuracy. Defect distributions of polycrystalline Cu(In,Ga)Se2 thin films are determined by this method from temperature dependent admittance measurements on heterojunctions of Cu(In,Ga)Se2 with ZnO that are used as efficient thin film solar cells.
Citations
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Journal ArticleDOI
TL;DR: The trap states on the surface and grain boundaries of the perovskite materials are demonstrated to be the origin of photocurrent hysteresis and that the fullerene layers deposited onperovskites can effectively passivate these charge trap states and eliminate the notorious photocurrent Hysteresi.
Abstract: The large photocurrent hysteresis observed in many organometal trihalide perovskite solar cells has become a major hindrance impairing the ultimate performance and stability of these devices, while its origin was unknown. Here we demonstrate the trap states on the surface and grain boundaries of the perovskite materials to be the origin of photocurrent hysteresis and that the fullerene layers deposited on perovskites can effectively passivate these charge trap states and eliminate the notorious photocurrent hysteresis. Fullerenes deposited on the top of the perovskites reduce the trap density by two orders of magnitude and double the power conversion efficiency of CH(3)NH(3)PbI(3) solar cells. The elucidation of the origin of photocurrent hysteresis and its elimination by trap passivation in perovskite solar cells provides important directions for future enhancements to device efficiency.

2,440 citations

Journal ArticleDOI
TL;DR: Zheng et al. as discussed by the authors showed that quaternary ammonium halides can effectively passivate ionic defects in several different types of hybrid perovskite with their negative-and positive-charged components.
Abstract: The ionic defects at the surfaces and grain boundaries of organic–inorganic halide perovskite films are detrimental to both the efficiency and stability of perovskite solar cells. Here, we show that quaternary ammonium halides can effectively passivate ionic defects in several different types of hybrid perovskite with their negative- and positive-charged components. The efficient defect passivation reduces the charge trap density and elongates the carrier recombination lifetime, which is supported by density-function-theory calculation. The defect passivation reduces the open-circuit-voltage deficit of the p–i–n-structured device to 0.39 V, and boosts the efficiency to a certified value of 20.59 ± 0.45%. Moreover, the defect healing also significantly enhances the stability of films in ambient conditions. Our findings provide an avenue for defect passivation to further improve both the efficiency and stability of solar cells. Losses in solar cells can be caused by material defects in the bulk or at interfaces. Here, Zheng et al. use quaternary ammonium halides to passivate various perovskite absorbers and prepare solar cells with certified efficiency above 20%, suggesting that both anionic and cation defects are affected.

1,536 citations

Journal ArticleDOI
TL;DR: Solvent-annealing is found to be an effective method to increase the grain size and carrier diffusion lengths of trihalide perovskite materials.
Abstract: Solvent-annealing is found to be an effective method to increase the grain size and carrier diffusion lengths of trihalide perovskite materials. The carrier diffusion length of MAPbI3 is increased to over 1 μm. The efficiency remains above 14.5% when the MAPbI3 thickness changes from 250 nm to 1 μm, with the highest efficiency reaching 15.6%.

1,521 citations

Journal ArticleDOI
TL;DR: The density of midgap trap states in CQD solids is quantified and shown to be limited by electron-hole recombination due to these states, and a robust hybrid passivation scheme is developed that can passivate trap sites that are inaccessible to much larger organic ligands.
Abstract: Improved performance in a photovoltaic device made of colloidal quantum dots is achieved through a combination of passivation by halide anions and organic crosslinking.

1,183 citations

Journal ArticleDOI
TL;DR: The interpretation of the impedance parameters for determining the internal features of the device, concerning the carrier distribution, materials properties such as the density of states and/or doping of the semiconductors, and the match of energy levels for photoinduced charge generation and separation are emphasized.
Abstract: We review the application of impedance spectroscopy in dye-sensitized solar cells, quantum dot-sensitized solar cells and organic bulk heterojunction solar cells. We emphasize the interpretation of the impedance parameters for determining the internal features of the device, concerning the carrier distribution, materials properties such as the density of states and/or doping of the semiconductors, and the match of energy levels for photoinduced charge generation and separation. Another central task is the determination of recombination mechanisms from the measured resistances, and the factors governing the device performance by combined analysis of resistances as a function of voltage and current–voltage curves.

1,046 citations

References
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Journal ArticleDOI
TL;DR: Deep Level Transformer Spectroscopy (DLTS) as discussed by the authors is a high-frequency capacitance transient thermal scanning method useful for observing a wide variety of traps in semiconductors, which can display the spectrum of traps as positive and negative peaks on a flat baseline as a function of temperature.
Abstract: A new technique, deep‐level transient spectroscopy (DLTS), is introduced. This is a high‐frequency capacitance transient thermal scanning method useful for observing a wide variety of traps in semiconductors. The technique is capable of displaying the spectrum of traps in a crystal as positive and negative peaks on a flat baseline as a function of temperature. It is sensitive, rapid, and easy to analyze. The sign of the peak indicates whether the trap is near the conduction or valence band, the height of the peak is proportional to the trap concentration, and the position, in temperature, of the peak is uniquely determined by the thermal emission properties of the trap. In addition, one can measure the activation energy, concentration profile, and electron‐ and hole‐capture cross sections for each trap. The technique is presented with a simple theoretical analysis for the case of exponential capacitance transients. Various traps in GaAs are used as examples to illustrate certain features of the DLTS technique. Finally, a critical comparison is made with other recent capacitance techniques.

3,210 citations

Journal ArticleDOI
TL;DR: Drift mobility and conductivity measurements were made between 290 and 85 K on amorphous silicon specimens prepared by glow-discharge decomposition of silane as mentioned in this paper, and the results suggest that excess electrons drift in the extended states with a mobility of about 10
Abstract: Drift mobility and conductivity measurements were made between 290 and 85\ifmmode^\circ\else\textdegree\fi{}K on amorphous silicon specimens prepared by glow-discharge decomposition of silane. The results suggest that excess electrons drift in the extended states with a mobility of about 10 ${\mathrm{cm}}^{2}$ ${\mathrm{sec}}^{\ensuremath{-}1}$ ${\mathrm{V}}^{\ensuremath{-}1}$. At lower temperatures, phonon-assisted hopping occurs through localized states occupying a range of 0.2 eV below the extended states. Conductivity results also suggest hopping transport near the Fermi energy.

418 citations

Journal ArticleDOI
TL;DR: In this paper, an exact (i.e., to arbitrary accuracy) solution for the complex admittance of Schottky-barrier diodes as a function of temperature provided a spectroscopy of deep trapping levels.
Abstract: Measurements of the complex admittance of Schottky‐barrier diodes as a function of temperature provide a spectroscopy of deep trapping levels. The measurement conditions are usually close to thermal equilibrium, thus assuring the validity of equilibrium occupation probabilities. An exact (i.e., to arbitrary accuracy) solution for the junction admittance is given. The problem is reduced to solution of a simple initial‐value problem, the final integration being carried out by computer. It is shown that the dispersion in capacitance due to slow trapping levels can lead to serious errors in estimates of junction doping concentrations and barrier height determinations. Examples are given for p‐type ZnTe where Shockley‐Read‐Hall (SRH) centers are dominant and for n‐type CdTe and Cd1−xZnTe where double‐acceptor centers are dominant. For junctions formed on CdTe : Ga it is shown that one of the steps in capacitance observed in a thermally stimulated capacitance survey is actually a high‐frequency response step an...

395 citations

Journal ArticleDOI
TL;DR: In this paper, a generalized model for the electronic behavior of deep traps in a p −n junction depletion region is developed for the measurement of junction capacitance transients and photocapacitance.
Abstract: A generalized model is developed for the electronic behavior of deep traps in a p‐n‐junction depletion region. The depletion region is shown to consist of two parts: (i) a space‐charge region which is totally depleted of free carriers and (ii) a transition region which is only partially depleted. The influence of this junction structure on free‐carrier profiling measurements is considered in detail for donor and acceptor traps with both homogeneous and inhomogeneous spatial distributions. Experimental observations of deep‐trap distributions produced by proton bombardment of n‐type silicon are analyzed within the framework of the model. Implications of the model in the measurement of junction capacitance transients and photocapacitance are considered in Appendices A‐C.

303 citations

Journal ArticleDOI
TL;DR: In this paper, the phase shift between a sinusoidally modulated excitation light and its inducing photocurrent was analyzed to determine an energetic distribution of localized states, and a new method was proposed to determine the localized states.
Abstract: In order to determine an energetic distribution of localized states, a new method is proposed. The method is based on the analysis of the phase shift between a sinusoidally modulated excitation light and its inducing photocurrent. The theoretical relation between the phase shift and its relevant localized states is derived assuming a trap‐limited band conduction of unipolar photocarriers. The energetic profile of the localized states can be calculated from a modulation frequency dependence of the phase shift. The method has been applied to CdS crystal and the validity has been confirmed. It is suggested that the method is useful for amorphous semiconductors in which the localized states are distributed quasi‐continuously in the band gap.

167 citations