Thermal Admittance Spectroscopy Research Articles

Defect Modulation via SnX2 Additives in FASnI3 Perovskite Solar Cells.

Tin halide perovskites suffer from high defect densities compared with their lead counterparts. To decrease defect densities, SnF2 is commonly used as an additive in tin halide perovskites. Herein, we investigate how SnF2 compares to other SnX2 additives (X = F, Cl, Br) in terms of electronic and ionic defect properties in FASnI3. We find that FASnI3 films with SnF2 show the lowest Urbach energies (EU) of 19 meV and a decreased p-type character, as probed with ultraviolet photoemission spectroscopy. The activation energy of ion migration, as probed with thermal admittance spectroscopy, for FASnI3 with SnF2 is 1.33 eV, which is higher than with SnCl2 and SnBr2, which are 1.22 and 0.79 eV, respectively, resulting in less ion migration. Because of improved defect passivation, the champion power conversion efficiency of FASnI3 with SnF2 is 7.47% and only 1.84% and 1.20% with SnCl2 and SnBr2, respectively.

Investigations of defect levels in different sized HgTe nanocrystals based photovoltaic devices using thermal admittance spectroscopy

HgTe nanocrystals (NCs) have unique properties that make them suitable for optoelectronic devices in the mid-wave infrared (MWIR) and short-wave infrared regions. However, electrically active defects can trap charge carriers, reducing their mobility and diffusion length, which degrades the NCs’ optical and electrical properties. In this study, we used the thermal admittance spectroscopic (TAS) method to analyze defects in HgTe NC-based photovoltaic devices. The ITO/HgTe/Al device structure was used to study the defect levels in HgTe nanocrystals and the effect of these traps on transport properties. Using low-temperature I–V measurements, we calculated the trap activation energy as 0.14 eV, and the transport was found to occur mainly through these trap states. From the TAS measurements, the trap activation energy obtained was 0.14 eV, and the concentration of trap level was 3.16×1016cm−3eV−1. We have also fabricated ITO/TiO2/HgTe/Au and fluorine doped tin oxide/TiO2/HgTe/MoO3/Au devices and did TAS measurements to understand the effect of adding electron and hole extraction layer on the formation of defect levels. Intriguingly, a distinct reversal in the capacitance–frequency (C–F) behavior is observed at different temperatures, leading to positive slopes in the Arrhenius plot. This peculiar phenomenon is attributed to the size-dependent doping effects within the HgTe nanocrystals. The outcomes of this study shed light on the significance of understanding and quantifying electrically active defects in HgTe nanocrystals for the advancement of NC-based optoelectronic devices.

Effect of Hole Transport Materials and Their Dopants on the Stability and Recoverability of Perovskite Solar Cells on Very Thin Substrates after 7 MeV Proton Irradiation

AbstractThe drastic reduction in launch and manufacturing costs of space hardware has facilitated the emergence of "commercial" space. Radiation‐hard organometal halide perovskite solar cells (PSCs) with low‐cost and high‐efficiency potentials are promising for space applications.High‐efficiency PSCs are tested with different hole transport materials (HTMs) and dopants on 175µm sapphire substrates under 7MeV‐proton‐irradiation‐tests at accumulated fluences of 1011, 1012, and 1013 protons cm−2. While all cells retain >90% of their initial power conversion efficiencies (PCEs) after 1011 protons cm−2 irradiation, PSCs that have tris(pentafluorophenyl)borane (TPFB) as the HTM dopant and poly[bis(4‐phenyl)(2,5,6‐trimethylphenyl) amine (PTAA) or PTAA:C8BTBT (C8BTBT = 2,7‐Dioctyl[1]benzothieno[3,2‐b][1]benzothiophene) as the HTM are more tolerant to higher‐fluence radiation than their counterparts with the lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) dopant and the 2,2′,7,7′‐Tetrakis[N,N‐di(4‐methoxyphenyl)amino]‐9,9′‐spirobifluorene (Spiro‐OMeTAD) HTM. Radiation induces fluorine diffusion from the LiTFSI dopant toward the perovskite absorber (confirmed by depth‐resolved X‐ray photoelectron spectroscopy) introducing defects. Radiation‐induced defects in cells with the TPFB dopant instead are different and can be “annealed out” by thermal vacuum resulting in PCE recovery. This is the first report using thermal admittance spectroscopy and deep‐level transient spectroscopy for defect analyses on proton‐irradiated and thermal‐vacuum‐recovered PSCs. The insights generated are expected to contribute to efforts in developing low‐cost light‐weight solar cells for space applications.

Improved Crystallization of Lead Halide Perovskite in Two-Step Growth Method by Polymer-Assisted "Slow-Release Effect".

Fast reaction between organic salt and lead iodide always leads to small perovskite crystallites and concentrated defects. Here, polyacrylic acid is blended with organic salt, so as to regulate the crystallization in a two-step growth method. It is observed that addition of polyacrylic acid retards aggregation and crystallization behavior of the organic salt, and slows down the reaction rate between organic salt and PbI2 , by which "slow-release effect" is defined. Such effect improves crystallization of perovskite. X-ray diffraction study shows that, after addition of 2mm polyacrylic acid, average crystallite size of perovskite increases from ≈40 to ≈90nm, meanwhile, grain size increases. Thermal admittance spectroscopy study shows that trap density is reduced by nearly one order (especially for deep energy levels). Due to the improved crystallization and reduced trap density, charge recombination is obviously reduced, while lifetime of charge carriers in perovskite film and devices are prolonged, according to time-resolved photoluminescence and transient photo-voltage decay curve tests, respectively. Accordingly, power conversion efficiency of the device is promoted from 19.96 (±0.41)% to 21.84 (±0.25)% (with a champion efficiency of 22.31%), and further elevated to 24.19% after surface modification by octylammonium iodide.

Impact of Na diffusion on Cu(In, Ga)Se2 solar cells: Unveiling the role of active defects using thermal admittance spectroscopy

In this work, Cu(In, Ga)Se2 (CIGS) based thin-film solar cells have been treated with the alkali element via incorporating Mo:Na layer into the device structure. A 600 nm thick Mo:Na layer behaved as a source for Na diffusion throughout the CIGS layer. The improvements have resulted in reduced depletion barrier width from ∼0.55 to 0.25 µm and increased p-type conductivity that improved the overall device performance from 7.06 to 12.06% for CIGS/Mo/Mo:Na in comparison with CIGS/Mo based solar cells. Thermal admittance spectroscopy has been analyzed to unveil the consequence of Na on shallow/deep-level defects and correlate with the CIGS/Mo junction properties for both the CIGS solar cells. The defect distribution at the CIGS/buffer interface has been altered, the activation energy reduced from 260 meV to 63 meV assigned to deep level defect and 22 meV to 12 meV assigned shallow defects (VCu acceptor) that can be assigned to in the CIGS layer.

Anti-solvent treatment time approach to high efficiency perovskite solar cells with temperature of coating environmental

The fabrication conditions of the light absorber layer (organic-inorganic halide perovskite) have a very large effect on the device performance in perovskite solar cells. In case of using the 1-step anti-solvent treatment (AST), their show very different characteristics depending on the AST time, amount, and temperature, etc. Here, we determined the optimal AST time for the synthesis of perovskite light absorber layer in relation to the turbidity point (TP) of the precursor solution during spin coating according to the temperature inside the glovebox. The TP was examined according to the fluid flow of the host solvent, and it depended on various conditions, such as the temperature and substrate. The measured TP was almost identical to the estimated TP, and both tended to decrease as the internal temperature increased. The AST time and the internal temperature of the glovebox significantly affected the crystallinity and device performance of the perovskite, and the best device performance of 18.9% for a methylammonium lead iodide (MAPbI 3 ) solar cell with n-i-p normal planar structure was achieved by applying an AST time of 9.5 s at 25 °C. Drive-level capacitance profiling and thermal admittance spectroscopy results confirmed that the optimal AST time of 9.5 s yielded a minimum trap density of 2.1 × 10 15 /cm 3 , which was related to the trap states of MAPbI 3 . • The investigation of optimal anti-solvent treatment time according to the temperature inside glove box for perovskite solar cells. • Analysis of defect density and state by drive level capacitance profile and admittance measurement for MAPbI 3 based perovskite solar cells. • Investigation of MAPbI 3 perovskite solar cells with n-i-p planar structure. • Study in relation to the Meyerhofer model, which is the fluid flow of the host solvent, and the antisolvent process for perovskite thin film layer.

Sub-cell characterization of two-terminal perovskite/silicon tandem solar cells

Although significant improvements in performance have been reported for monolithic perovskite/silicon tandem solar cells (PVSK/Si TSCs), the lack of simple electrical characterization methods impedes further development of monolithic TSCs. In this article, we report comprehensive electrical characterization methods that can accurately and easily analyze the sub-cells of two-terminal PVSK/Si TSCs. Using a highly efficient three-terminal PVSK/Si TSC conceived for the sub-cell analysis, fundamental electrical characterization methods, such as J - V characteristics, external quantum efficiency, impedance spectroscopy, and thermal admittance spectroscopy, are investigated to suggest a characterization platform for the sub-cell analysis of monolithic two-terminal PVSK/Si TSCs. • Sub-cells of monolithic perovskite/silicon tandem solar cells are characterized • Sub-cell-selective light bias enables simple nondestructive analysis • 3-T device is used to validate the suggested methods • J-V , EQE, EIS, and TAS of the sub-cells are investigated Park et al. report sub-cell characterization methods for monolithic perovskite/silicon tandem solar cells. By using sub-cell-selective light biases and highly efficient monolithic three-terminal perovskite/silicon tandem solar cells, the J-V characteristics, external quantum efficiency, impedance analysis, and thermal admittance spectroscopy of the sub-cells are analyzed.

Investigating the origin of the N1 signature in admittance spectroscopy of Cu(In,Ga)(S,Se)[formula omitted] thin-film solar cells by a variation of individual cell layers

Electronic defect levels in semiconductor devices are often investigated by thermal admittance spectroscopy, a method in which the device capacity is measured as a function of frequency and temperature. When applied to Cu(In,Ga)(S,Se)2-based thin film solar cells, these commonly show a characteristic signature which has an activation energy of roughly 0.1 eV and has been termed the “N1” signature. However, even though this feature has been observed for at least two decades, the origin of the N1 signature is still under quite some debate in the community. Explanations include a defect level in the absorber bulk or near the heterointerface, a transport barrier at the back contact or at the buffer/i-layer interface, as well as mobility freeze-out. In order to contribute to an answer on this issue, in this study we fabricated several Cu(In,Ga)(S,Se)2 devices in which the individual layers of the cell stack were systematically varied, so that between two variations only one layer was different. This approach allowed us to test different hypotheses on the origin of the N1 signature by measuring admittance spectroscopy on the variations and checking whether a change in a specific layer led to a change in the N1 signature. Additionally, we investigated the role of the N1 signature in non-radiative recombination losses. The results suggest that the N1 signature is caused by a transport barrier in the buffer/absorber region in the investigated devices.

Cation‐Diffusion‐Based Simultaneous Bulk and Surface Passivations for High Bandgap Inverted Perovskite Solar Cell Producing Record Fill Factor and Efficiency

AbstractHigh bandgap perovskite solar cells are integral to perovskite‐based multi‐junction tandem solar cells with efficiency potentials over 40%. However, at present, high bandgap perovskite devices underperform compared to their mid bandgap counterparts in terms of voltage outputs and fill factors resulting in lower than ideal efficiencies. Here, the low fill factor aspect of high bandgap perovskite is addressed by developing a cation‐diffusion‐based double‐sided interface passivation scheme that simultaneously provides bulk passivation for a 1.75 eV perovskite cell that is also compatible with a p‐i‐n cell architecture. The champion cell achieves a record fill factor of 86.5% and a power conversion efficiency of 20.2%. Results of ionic distribution profiling, Fourier transform infrared spectroscopy, and X‐ray diffraction crystallography reveal evidence of cation diffusion from the surface perovskite passivation layer into bulk. The diffused cations reduce Shockley–Read–Hall recombination in the perovskite bulk and at the surfaces with the latter being more dominant as confirmed by light‐intensity dependent and temperature‐dependent open‐circuit voltage measurements as well as thermal admittance spectroscopy. This concurrent bulk and surface passivation scheme renders record fill factor and efficiency in the double‐side passivated cells. This provides new insights for future passivation strategies based on ionic diffusion of functionalized materials.

Unraveling the Origin of Dark Current in Organic Bulk Heterojunction Photodiodes for Achieving High Near-Infrared Detectivity

Organic infrared materials are attractive due to their high absorption coefficients and facile tuning of the absorption spectrum beyond that of silicon. Although bulk heterojunctions (BHJs) are widely used in organic photodetectors (OPDs), achieving a high signal-to-noise ratio in the near-infrared spectrum is often challenging due to the inherently large dark currents in low-bandgap polymers and their blending morphology effect. Herein, we investigate the origin of dark currents in near-infrared OPDs and reveal a strategy to improve the detectivity in terms of two aspects: (1) donor–acceptor blending morphology and (2) effects of carrier injection from outer electrodes. To do so, a series of random terpolymers were synthesized in a similar band structure and their blending morphology with the PC71BM acceptor was gradually controlled. As the phase separation was smaller, the dark current gradually reduced while the responsivity increased, leading to a higher detectivity. Complex charge-transporting paths formed under the fine percolating network account for the dark current reduction, while the increased heterojunction area facilitates the dissociation of the photogenerated excitons. From our thermal admittance spectroscopy, deeper sub-bandgap states were found under the smaller phase separation, further contributing to the dark current reduction. Second, the carrier injection effect was revealed by inserting a charge-blocking layer at the active layer/electrode interface. While manipulating both parameters could yield a high near-infrared detectivity up to 1012 Jones at −0.5 V, the blending morphology effect turns out to be more dominant over the carrier injection effect in suppressing the dark current and achieving higher detectivity in BHJ OPDs.

Multilayer Capacitances: How Selective Contacts Affect Capacitance Measurements of Perovskite Solar Cells

Capacitance measurements as a function of voltage, frequency, and temperature are a useful tool to gain a deeper insight into the electronic properties of semiconductor devices in general and of solar cells in particular. Techniques such as capacitance-voltage, Mott-Schottky analysis, or thermal-admittance spectroscopy measurements are frequently employed in perovskite solar cells to obtain relevant parameters of the perovskite absorber. However, state-of-the-art perovskite solar cells use thin electron- and hole-transport layers to improve the contact selectivity. These contacts are often quite resistive in nature, which implies that their resistance will significantly contribute to the total device impedance and thereby also affect the overall capacitance of the device, thus partly obscuring the capacitance signal from the perovskite absorber. Based on this premise, we develop a simple multilayer model that considers the perovskite solar cell as a series connection of the geometric capacitance of each layer in parallel with their voltage-dependent resistances. Analysis of this model yields fundamental limits to the resolution of spatial doping profiles and minimum values of doping and trap densities, built-in voltages, and activation energies. We observe that most of the experimental capacitance-voltage-frequency-temperature data, calculated doping and defect densities, and activation energies reported in the literature are within the derived cutoff values, indicating that the capacitance response of the perovskite solar cell is indeed strongly affected by the capacitance of its selective contacts.2 MoreReceived 21 December 2021Revised 16 February 2022Accepted 25 February 2022DOI:https://doi.org/10.1103/PRXEnergy.1.013003Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasCapacitanceImpedanceSolar energyPhysical SystemsPerovskitesSolar cellsTechniquesDielectric spectroscopyInterdisciplinary PhysicsCondensed Matter, Materials & Applied Physics

Mitigating deep-level defects through a self-healing process for highly efficient wide-bandgap inorganic CsPbI3−xBrx perovskite photovoltaics

Thermal admittance spectroscopy is used to explain the self-healing process in the wide bandgap inorganic CsPbI3−xBrx. It is revealed that the deep-level interstitial defects in the fresh film can be self-healed when stored in a low-humidity ambient condition.

Dominant acceptors in Li doped, magnetron deposited Cu2O films

Cu2O films deposited by reactive magnetron sputtering with varying Li concentrations have been investigated by a combination of temperature-dependent Hall effect measurement and thermal admittance spectroscopy. As measured by secondary ion mass spectrometry, Li concentrations up to 5 × 1020 Li/cm3 have been achieved. Li doping significantly alters the electrical properties of Cu2O and increases hole concentration at room temperature for higher Li concentrations. Moreover, the apparent activation energy for the dominant acceptors decreases from around 0.2 eV for undoped or lightly doped Cu2O down to as low as 0.05 eV for higher Li concentrations.

Defect Passivation via Additive Engineering to Improve Photodetection Performance in CsPbI2Br Perovskite Photodetectors.

Solution-processable all-inorganic lead halide perovskites are under intensive attention due to their potential applications in low-cost high-performance optoelectronic devices such as photodetectors. However, solution processing usually generates structural and chemical defects which are detrimental to the photodetection performance of photodetectors. Here, a polymer additive of polyethylene glycol (PEG) was employed to passivate the localized defects in CsPbI2Br films through the Lewis acid-base interaction. The interfacial defects were passivated efficiently by introducing a trace amount of a PEG additive with a concentration of 0.4 mg mL-1 into the CsPbI2Br precursor solution, as suggested by the significantly reduced trap density of state, which was revealed using thermal admittance spectroscopy. Fourier transform infrared spectrum characterization showed that rather than Cs+ or I-, a Lewis acid-base interaction was established between Pb2+ and PEG to passivate the defects in the CsPbI2Br perovskite, which leads to large suppression of noise current. Both specific detectivity and linear dynamic range improved from 4.1 × 109 Jones and 73 dB to 2.2 × 1011 Jones and 116 dB, respectively. Our work demonstrates the feasibility of employing an environmentally stable polymeric additive PEG to passivate defects for high photodetection performance in all-inorganic perovskite photodetectors.

Correlations of Ionic Migration and Deep-Level Traps Leads to Surface Defect Formation in Perovskite Solar Cells

Correlations between ion migration, charge carrier, and bulk deep-level defect in MAPbI3 perovskites are systematically invesitgated through voltage-dependent thermal admittance spectroscopy and in situ scanning probe microscopy. We show that iodine interstitials construct stable, deep-level bulk defects in the polycrystalline MAPbI3 with an activation energy (EA) level of 0.495 eV above the valence band maximum. Experimentally, EA varied with depletion region distribution and excess electron from charge transport layers, which can be modulated through external bias. Under forward bias, mobile bulk defects migrate and transform into surface defect states, introducing additional nonradiative recombination pathways. Overall, this study directly reveals electronic correlations between defect ions and charge carriers, pointing out the importance of iodine defect chemistry in surface recombination and degradation in long-term device stabilities.

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.

Decoupling the effects of defects on efficiency and stability through phosphonates in stable halide perovskite solar cells

Summary Understanding defects is of paramount importance for the development of stable halide perovskite solar cells (PSCs). However, isolating their distinctive effects on device efficiency and stability is currently a challenge. We report that adding the organic molecule 3-phosphonopropionic acid (H3pp) to the halide perovskite results in unchanged overall optoelectronic performance while having a tremendous effect on device stability. We obtained PSCs with ∼21% efficiency that retain ∼100% of the initial efficiency after 1,000 h at the maximum power point under simulated AM1.5G illumination. The strong interaction between the perovskite and the H3pp molecule through two types of hydrogen bonds (H…I and O…H) leads to shallow point defect passivation that has a significant effect on device stability but not on the non-radiative recombination and device efficiency. We expect that our work will have important implications for the current understanding and advancement of operational PSCs.

Investigation of Defect‐Tolerant Perovskite Solar Cells with Long‐Term Stability via Controlling the Self‐Doping Effect

AbstractAlthough there have been significant advances in the stability of perovskite solar cells through encapsulation techniques to remove extrinsic degradation factors, such as moisture and oxygen, irreversible photo‐degradation originating from intrinsic defects is still challenging and remains elusive. Herein, the photo‐aging mechanism due to intrinsic defects is investigated in nitrogen‐filled conditions, excluding extrinsic degradation factors. Devices with similar power conversion efficiencies (PCE) of 21%, but with different Fermi levels in the perovskite films, via controlling the self‐doping effect, have been investigated. Opto‐electronic investigations and depth profiles of the elemental constituents show that after photo‐aging, strain relaxation in the perovskite lattice and a Fermi level shift towards conduction band edge are observed, implying the formation of new defect states in Pb‐rich devices. Furthermore, thermal admittance spectroscopy measurement of the devices suggests that the formation of the deep‐traps in the perovskite leads to irreversible degradation. Thin‐film solar cells that are relatively Pb‐deficient (FA‐rich) exhibit improved long‐term stability, retaining over 90% of their initial PCE during 500 h of continuous 1‐Sun illumination. This study suggests passivation of the Pb‐I related antisite defects near the grain boundaries and the interface is crucial for the fabrication of solar cells with enhanced long‐term stability.

Identification of LiNi and VNi acceptor levels in doped nickel oxide

Nickel oxide, in particular in its doped, semiconducting form, is an important component of several optoelectronic devices. Doped NiO is commonly achieved either by incorporation of lithium, which readily occupies Ni sites substitutionally, producing the LiNi acceptor, or by supplying reactive oxygen species during NiO film deposition, which leads to the formation of Ni vacancies (VNi). However, the energetic position of these acceptors in the NiO bandgap has not been experimentally determined until today. In this work, we close this knowledge gap by studying rectifying n++p heterojunctions consisting of NiO thin films grown on top of fluorine-doped tin oxide. These structures show sufficient rectification to perform electric characterization by defect spectroscopic techniques, specifically capacitance–voltage and thermal admittance spectroscopy. Using these methods, the (0/−) charge transition levels are determined to be 190 meV and 409 meV above the valence band edge for the LiNi and the VNi acceptor, respectively.

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.