Light-induced Metastability Research Articles

The Impact of P-Type Layer on Performance and Stability of Thin Film Silicon Solar Cells in Relation to Third Generation: A review Analysis

Thin-film Silicon (Si) Solar Cells (SCs) have emerged as a promising candidate for next-generation photovoltaic devices due to their potential for cost-effectiveness and versatility. However, achieving high efficiency and long-term stability remains a significant challenge in their development. The primary challenges with thin-film Si SCs have been the poor infrared light absorption of Hydrogenated Microcrystalline Silicon (c-Si: H) and the light-induced metastability of Hydrogenated Amorphous Silicon (a-Si: H). This study reviewed the influence of the p-type layer, a key component in thin-film Si SCs, on both performance and stability. Third generation SCs are optimized for high Power Conversion Efficiency (PCE) at low manufacturing costs. According to the review of the comparison analysis, Patel K. et al., (2021) obtained the highest PCE of 30.17% with the a-Si/MoTe2 material (Photocurrent Density Jsc = 32.41mA/cm2, An Open-Circuit Voltage Voc = 1.093V, and Fill Factor (FF) = 85.12%) of the state-of-the-art previous works. The findings presented here contribute valuable insights toward the development of efficient and durable SC technology, crucial for advancing the adoption of renewable energy sources in our ever-growing energy demand.

Light-Induced Transient Lattice Dynamics and Metastable Phase Transition in CH3NH3PbI3 Nanocrystals.

Methylammonium lead iodide (MAPbI3) perovskite nanocrystals (NCs) offer desirable optoelectronic properties with prospective utility in photovoltaics, lasers, and light-emitting diodes (LEDs). Structural rearrangements of MAPbI3 in response to photoexcitation, such as lattice distortions and phase transitions, are of particular interest, as these engender long carrier lifetime and bolster carrier diffusion. Here, we use variable temperature X-ray diffraction (XRD) and synchrotron-based transient X-ray diffraction (TRXRD) to investigate lattice response following ultrafast optical excitation. MAPbI3 NCs are found to slowly undergo a phase transition from the tetragonal to a pseudocubic phase over the course of 1 ns under 0.02-4.18 mJ/cm2 fluence photoexcitation, with apparent nonthermal lattice distortions attributed to polaron formation. Lattice recovery exceeds time scales expected for both carrier recombination and thermal dissipation, indicating meta-stability likely due to the proximal phase transition, with symmetry-breaking along equatorial and axial directions. These findings are relevant for fundamental understanding and applications of structure-function properties.

Current-voltage and capacitance study of light-induced metastabilities in CuZnSnSSe thin film solar cells

White, red and blue light-induced metastabilities in Cu2ZnSnS4(Se4) solar cells were investigated by temperature dependent current-voltage measurements, drive level capacitance profiling, impedance and thermal admittance spectroscopy. A set of devices were studied where white and blue light soaking at room temperature led to degradation of the device performance, while after red light soaking the solar cell efficiency did not change.We observed a significant effect of light soaking on capacitance data measured in both low and high-temperature ranges for these devices. In particular, the net doping concentration extracted from drive-level capacitance profiling substantially increased after light soaking treatments.Low and high-temperature capacitance steps observed in the reference capacitance-frequency spectra were assigned to Fermi level pinning and bulk defects, correspondingly. Light soaking with different-wavelength light led to a shift of both steps toward the high-frequency range, and hence a decrease in the thermal admittance activation energies.A low-frequency ‘inductive’ loop was detected in the impedance spectra after light soaking, regardless of wavelength. It was proposed that the appearance of the ‘inductive’ loop is due to the formation of a negative electric field at the highly defected CdS/Cu2ZnSnS4(Se4) hetero-interface. This result also leads us to conclude that such electric field is responsible for the metastable behaviour of these devices at room temperature, while the low temperature metastable changes might have a different origin.We also discuss the methodology for electrical characterization of the metastable solar cells in detail.

Thin-Film Optical Devices Based on Transparent Conducting Oxides: Physical Mechanisms and Applications

This paper provides a review of optical devices based on a wide band-gap transparent conducting oxide (TCO) while discussing related physical mechanisms and potential applications. Intentionally using a light-induced metastability mechanism of oxygen defects in TCOs, it is allowed to detect even visible lights, eluding to a persistent photoconductivity (PPC) as an optical memory action. So, this PPC phenomenon is naturally useful for TCO-based optical memory applications, e.g., optical synaptic transistors, as well as photo-sensors along with an electrical controllability of a recovery speed with gate pulse or bias. Besides the role of TCO channel layer in thin-film transistor structure, a defective gate insulator can be another approach for a memory operation with assistance for gate bias and illuminations. In this respect, TCOs can be promising materials for a low-cost transparent optoelectronic application.

Progress and limitations of thin-film silicon solar cells

The major issues of thin-film silicon solar cells have been the light-induced metastability of hydrogenated amorphous silicon (a-Si:H) and the weak infrared light absorption of hydrogenated microcrystalline silicon (μc-Si:H). In order to overcome these challenges, we have developed a novel deposition process for a-Si:H and an improved light trapping scheme for μc-Si:H light absorbers, leading to four record independently-confirmed conversion efficiencies with single-junction and multijunction device structures. In this article, we review some technological progresses that led to the notable improvement in thin-film silicon solar cells, and address some limitations imposed by the absorber layer quality as well as the cell design.

Towards high performance Cd-free CZTSe solar cells with a ZnS(O,OH) buffer layer: the influence of thiourea concentration on chemical bath deposition

High-performance kesterite- (CZTSe-)based solar cell devices usually employ an absorber/buffer heterostructure using toxic CdS deposited by chemical bath deposition (CBD) as a buffer layer. This is due to the favourable spike-like conduction band alignment of the CdS buffer and CZTSe absorber. ZnS(O,OH) buffer layers provide a promising nontoxic alternative. Here, variation of the thiourea concentration in the CBD of ZnS(O,OH) buffer layers and its influence on device performances of pure selenide CZTSe heterostructure solar cells is presented. Furthermore, the influence of buffer layer deposition conditions on light-induced metastabilities is discussed. A ZnS(O,OH) buffer layer deposited with a high thiourea concentration leads to distorted illuminated J–V curves as expected for devices with unfavourably high spike-like conduction band alignment between the buffer and CZTSe absorber. By adjusting the thiourea concentration J–V curve distortions can be reduced. An optimized CBD process leads to a device efficiency of up to 6.5% after light soaking, which is comparable to the efficiency of a reference device that employs CdS as the buffer layer (6.9%).

Light-induced metastability in pure and hydrogenated amorphous silicon

Light soaking is found to increase the specific heat C and internal friction of pure (a-Si) and hydrogenated (a-Si:H) amorphous silicon. At the lowest temperatures, the increases in C and are consistent with an increased density of two-level systems (TLS). The light-induced increase in C persists to room temperature. Neither the sound velocity nor shear modulus change with light soaking indicating that the Debye specific heat is unchanged which suggests that light soaking creates localized vibrational modes in addition to TLS. The increase can be reversibly added and removed by light soaking and annealing, respectively, suggesting that it is related to the Staebler-Wronski effect (SWE), even in a-Si without H, and involves a reversible nanoscale structural rearrangement that is facilitated by, but does not require, H to occur.

Validated Method for Repeatable Power Measurement of CIGS Modules Exhibiting Light-Induced Metastabilities

We report on the validation of a stabilization procedure designed to minimize variations in repeated power measurements at standard test conditions caused by transient light-induced metastabilities in copper indium gallium diselenide (CIGS) modules. Such metastable effects frustrate the repeatable and accurate measurement of a module's performance in the electrical state to which it stabilizes under normal operation outdoors. The procedure studied here is based on a light exposure followed by forward electrical bias as the module cools to the measurement temperature. The procedure was tested in a lab-to-lab intercomparison involving five different labs. Results show that the procedure is effective in yielding repeatable measurements and that the variations due to metastabilities are of roughly the same magnitude as those associated with variations in illumination conditions between different flash simulators. We also find that temperature-corrected measurements made immediately upon completion of the light exposure are less repeatable than those made after the module has cooled to 25°C under bias.

Light soaking induced doping increase and sodium redistribution in Cu(In,Ga)Se2-based thin film solar cells

Light-induced metastabilities of Cu(In,Ga)Se2 (CIGS)-based thin film solar cells have been extensively researched for many years. It is commonly observed that junction capacitance and p-doping level in CIGS absorbers increase considerably after light soaking (LS). In this work, we focus on the LS behaviors of cells with different minority carrier lifetimes (τn). Experiments show that high efficiency cells with long τn lose open circuit voltage (Voc) and fill factor (FF) upon LS, whereas low efficiency cells with short τn lose less or even gain Voc and FF. The sodium content measured with glow discharge optical emission spectroscopy (GD-OES) increases in the region close to the CdS/CIGS interface with LS and may contribute to the observed LS behaviors. The change in electrical parameters is explained with simulations, which relate the Voc and FF changes to a reduced recombination rate in the space charge region due to the light-induced doping increase. The simulations also suggest that cells with higher n-doping in the CdS are less sensitive to changes in interface recombination rate and doping of CIGS, which agrees with the hypothesis that the CdS buffer deposition is important for the LS behavior.

Fast light-induced metastability and thermal recovery for protocrystalline silicon multilayers proved by sub-bandgap absorption spectra

Light soaking and thermal annealing behaviors of an alternately hydrogen-diluted hydrogenated protocrystalline silicon (pc-Si:H) multilayer are investigated using a constant photocurrent measurement. The pc-Si:H multilayer includes isolated nano-sized silicon (nc-Si) grains embedded in a hydrogenated amorphous silicon (a-Si:H) matrix with a vertically regular distribution. A wide optical bandgap of the pc-Si:H multilayer is inspected mainly due to an non-uniform hydrogen distribution and quantum size effect. Compared to a conventional a-Si:H film, the pc-Si:H multilayer exhibits a superior light-induced metastability with a fast stabilization and low degradation. In addition, the pc-Si:H multilayer shows a reversible thermal recovery in a short annealing time.

Unique features of alternately hydrogen-diluted protocrystalline silicon multilayers as wide bandgap and highly stable absorbers of thin-film silicon solar cells

We investigate the unique structural and optical properties of an alternately H2-diluted protocrystalline silicon multilayers (pc-Si:H) compared to a hydrogenated amorphous silicon (a-Si:H) layer. Strong evidence for the isolated nano-sized silicon (nc-Si) grains embedded in the a-Si:H matrix is provided. The vertically regular distribution of the nc-Si grains is concluded as the main reason for the fast light-induced metastability with the low degradation for the fabricated pc-Si:H multilayer solar cell by localizing the photocreation of dangling bonds near the defective grain-boundary layers.

Light-induced Open-circuit Voltage Increase in Amorphous Silicon/Microcrystalline Silicon Tandem Solar Cells

ABSTRACTLight-induced metastability of amorphous/microcrystalline (micromorph) silicon tandem solar cell, in which the microcrystalline bottom cell was deposited in a single-chamber system, has been studied under a white light for more than 1000 hours. Two different light-induced metastable behaviors were observed. The first type was the conventional light-induced degradation, where the open-circuit voltage (Voc), fill factor (FF), and short-circuit current density (Jsc) were degraded, hence the efficiency was degraded as well. This phenomenon was observed mainly in the tandem cells with a bottom cell limited current mismatch. The second type was with a light-induced increase in Voc, which sometimes resulted in an increase in efficiency. The second type of light-induced metastability was observed in the tandem cells with a top cell limited current mismatch. The possible mechanisms for these phenomena are discussed.

Light-induced metastability in thin nanocrystalline silicon films

This paper examines the influence of light-induced metastability on conduction in thin nc-Si:H films. To investigate the role of surface effects, two sample types are considered: one with the surface intentionally oxidized to form an oxide cap layer and the other with etched oxide layer. Both the Staebler–Wronski effect (SWE) and persistent photo-current (PPC) have been observed, albeit at different phases of light soaking. For the nc-Si:H sample with a cap layer, we attribute the presence of SWE and PPC to defect generation and interface charge trapping, while, in the absence of the cap layer, these effects could be caused by unidentified photo-structural changes and defect generation.

Defects in AlSb: A density functional study

We carry out density functional calculations to study both intrinsic and extrinsic defects in AlSb. We focus on the carrier compensation and trapping properties of these defects, which are important to the radiation detection applications. We show that the Sb antisite $({\text{Sb}}_{\text{Al}})$ is a low-energy defect, with interesting property of light-induced metastability, similar to the As antisite in GaAs. ${\text{Sb}}_{\text{Al}}$ is effective in compensating holes induced by the residual carbon but is also a deep electron trap that reduces the carrier drifting length. We discuss the possibility of using hydrogenated isovalent N impurity in AlSb and GaAs to pin the Fermi level without causing efficient carrier trapping.

Highly and quickly stabilized p–i–n/p–i–n-type protocrystalline silicon multilayer tandem solar cells

We have developed p–i–n/p–i–n-type protocrystalline silicon (pc-Si:H) multilayer tandem solar cells. The purpose of this work is to make a thin film silicon solar cell with low degradation by combining the virtues of a pc-Si:H multilayer and tandem structure. The usefulness of the pc-Si:H multilayer as a low degradation top and bottom cell was confirmed when we achieved a low degradation ratio of 10.0%. Notably, this tandem cell stabilized rapidly, within 1 h. Nanocrystalline silicon (nc-Si) grains embedded in a pc-Si:H multilayer were detected with the aid of a planer transmission electron microscope. The isolated nc-Si grains may suppress the photocreation of dangling bonds due to non-radiative recombination in an a-Si:H matrix. Because of these embedded nc-Si grains, the pc-Si:H multilayer has a fast and high light-induced metastability.

Metastability in hydrogenated nanocrystalline silicon solar cells

Light-induced metastability in hydrogenated nanocrystalline silicon (nc-Si:H) single-junction solar cells was studied systematically. First, we observed no light-induced degradation when the photon energy was lower than the band gap of the amorphous phase; degradation occurred when the energy was higher than the band gap in the amorphous phase. The light-induced degradation could be annealed away at an elevated temperature. We concluded that the light-induced defect generation occurred mainly in the amorphous phase. Second, forward current injection did not degrade the nc-Si:H cell performance. However, a reverse bias during light soaking enhanced the degradation. Third, the nc-Si:H cells made with an optimized hydrogen dilution profile showed minimal degradation although these cells had a high amorphous volume fraction. This indicated that the amorphous volume fraction was not the only factor determining the degradation. Other factors also played important roles in the nc-Si:H stability.

Light- and bias-induced metastabilities in Cu(In,Ga)Se2 based solar cells caused by the (VSe-VCu) vacancy complex

We investigate theoretically light- and bias-induced metastabilities in Cu(In,Ga)Se2 (CIGS) based solar cells, suggesting the Se–Cu divacancy complex (VSe-VCu) as the source of this hitherto puzzling phenomena. Due to its amphoteric nature, the (VSe-VCu) complex is able to convert by persistent carrier capture or emission from a shallow donor into a shallow acceptor configuration, and vice versa, thereby changing in a metastable fashion the local net acceptor density inside the CIGS absorber of the solar cell, e.g., a CdS/CIGS heterojunction. In order to establish a comprehensive picture of metastability caused by the (VSe-VCu) complex, we determine defect formation energies from first-principles calculations, employ numerical simulations of equilibrium defect thermodynamics, and develop a model for the transition dynamics after creation of a metastable nonequilibrium state. We find that the (VSe-VCu) complex can account for the light-induced metastabilities, i.e., the “red” and “blue” illumination effects, as well as for the reverse-bias effect. Thus, our (VSe-VCu) model implies that the different metastabilities observed in CIGS share a common origin. A defect state in the band gap caused by (VSe-VCu) in the acceptor configuration creates a potentially detrimental recombination center and may contribute to the saturation of the open circuit voltage in larger-gap Cu(In,Ga)Se2 alloys with higher Ga content. Therefore, the presence of metastable defects should be regarded as a concern for solar cell performance.

Material structure and metastability of hydrogenated nanocrystalline silicon solar cells

We find that the volume fraction of amorphous component in hydrogenated nanocrystalline silicon intrinsic layers is not necessarily the determining factor for the light-induced metastability ofn-i-p solar cells. Small grains and/or intermediate range order may play an important role in improving the stability. The distribution of nanocrystallites along the growth direction is also important. Based on the findings, we have optimized the hydrogen dilution profiling for controlling the structural evolution and have reduced the light-induced degradation of solar cells. As a result, we have achieved initial and stable active-area efficiencies of 14.1% and 13.2%, respectively, using ana-Si:H/nc-Si:H/nc-Si:H triple-junction structure.

The nature of the defect states above midgap and their role in the light-induced metastability of a-Si:H

Defect states of a-Si:H above midgap, with which the electrons interact through trapping and thermal release and make an important contribution to the imaginary (Y) term of the modulated photocurrent, are determined. Analysis of the ‘transition’ region between the low- and high-frequency regimes enables the densities and capture coefficients of defect states to be probed. A reasonable fit to the data is obtained by assuming three defect states, and it is suggested that the defects with the higher capture probability are hydrogen-related centers that are removed during the initial stage of light degradation.

Inclusion of nanosized silicon grains in hydrogenated protocrystalline silicon multilayers and its relation to stability

Photoluminescence and Fourier transform infrared spectroscopy measured at room temperature produce strong evidence that nanosized silicon (nc-Si) grains embedded in hydrogenated protocrystalline silicon (i-pc-Si:H) multilayers. Thus, we propose the structure of the i-pc-Si:H multilayer possessing isolated nc-Si grains and their wrapping layers with a high hydrogen concentration embedded in highly hydrogen-diluted sublayers. The isolated nc-Si grains may act as radiative recombination centers of photoexcited carriers, and hence suppress the photocreation of dangling bonds caused by the nonradiative recombination in amorphous silicon matrix. Because of the repeatedly layered structure, the i-pc-Si:H multilayers have a fast light-induced metastability with a low degradation.