Thermal Admittance Research Articles

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.

Thermal and acoustic performance of cement fibreboard and bamboo buildings

The present study evaluates the thermal and acoustic performance of building constructed from aerated concrete-cement fibreboards and bamboo composite. The thermal, humidity, ambient, and indoor temperature parameters are analyzed for a 24-hours cycle from April to May and December to January. The average temperature plot shows the maximum indoor temperature does not exceed 28-32 º C for April to May and 12-16 ºC for December to January for both the buildings. Thermal admittance is observed as 7.3 W/m2 K and 12 W/m2 K for cement fibreboard and bamboo composite buildings respectively. Outdoor to indoor noise reduction (OINR) values at frequencies ranging from 80 Hz to 4000 Hz are noted and analysed. The maximum noise isolation provided by cement fibreboard and bamboo composite walls are found to be ~40 dBA and 35 dBA at 500 Hz. With proper ventilation and avoidance of direct sunlight in the room, a comfortable atmosphere can be achieved for these buildings.

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.

Heat storage efficiency and effective thermal output: Indicators of thermal response and output of radiant heating and cooling systems

An alternative indicator of thermal response of radiant heating and cooling systems called heat storage efficiency (HSE) has been tested. The heat storage efficiency was compared with established indicators represented by the time constant τ63, response time τ95, and thermal energy stored (TES). The comparison was performed for three wall cooling systems with various combinations of pipe location, configuration of material layers, and materials of the thermal core. Considering the whole response curve (HSE) instead of focusing on one specific point on the curve (τ63, τ95) allowed comparing the thermal response of complex systems by a single value. It also permitted predicting thermal response consistently regardless of the system and core material. Thermal energy stored predicted the thermal response of certain systems, but it may not be suitable for comparing the thermal response of radiant systems with different thermal admittance. Besides, a novel indicator of thermal output called effective thermal output (ETO) has been proposed. The effective thermal output considers both the steady-state thermal performance represented by the nominal thermal output and dynamic thermal performance represented by the heat storage efficiency. For thermally active building systems (TABS), effective thermal output was up to 57% lower than the nominal thermal output.

Where Are White Roofs More Effective in Cooling the Surface?

AbstractBy reflecting more sunlight, white (cool) roofs experience lower surface temperatures and produce less heating of the surrounding air. The effectiveness of cool roofs, quantified by the surface temperature difference between cool and regular roofs (ΔTs), is known to vary spatially. A common perception is that ΔTs is controlled by solar radiation that reaches the roof surface. Here we use an Earth System Model and a surface energy balance model to show that the spatial variability of ΔTs, when normalized by the albedo difference between cool and regular roofs Δα, is also controlled by an energy distribution factor that encodes the efficiencies of surface energy balance components in dissipating heat. Our results suggest that painting the roof white is more effective when the roof has less water‐holding capacity and smaller thermal admittance and is located in places with more solar radiation, less precipitation, and lower wind speed.

Water-Covered Roof Versus Inverted Flat Roof on the Mediterranean Coast: A Comparative Study of Thermal and Energy Behavior

Reservoir, or water-collecting roofs present greater thermal inertia than inverted flat roofs due to the mass of water they contain. This feature gives them better thermal performance and leads to greater stability in the indoor air temperature Ti and the wall surface temperatures. In the summer, they can dampen the effect of solar radiation and regulate external thermal loads thanks to their greater effusivity and thermal capacity. This research compares the thermal behavior of the roofs of two buildings located in Alicante on the Spanish Mediterranean coast: a loft flat in the city center and a water-covered roof in the Museum of the University of Alicante (MUA). Values for effusivity, diffusivity, thermal capacity, decrement factor, time lag and internal, as well as external thermal admittance were obtained. After monitoring both roofs during 2014, behavior simulations were performed in Design Builder using 6 different scenarios reflecting different combinations in both buildings of water-covered, inverted and conventional roofs and marble or terrazzo paving. The water-covered roof led to a higher decrement factor and time lag, as well as to a reduction of annual energy demands between 8.86% and 9.03%.

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.

Influence of Charge Transport Layers on Capacitance Measured in Halide Perovskite Solar Cells

Summary Capacitance-based techniques have been used to measure the electrical properties of halide perovskite solar cells (PSCs) such as defect activation energy and density, carrier concentration, and dielectric constant, which provide key information for evaluating the device performance. Here, we show that capacitance-based techniques cannot be used to reliably analyze the properties of defects in the perovskite layer or at its interface, because the high-frequency capacitance signature is due to the response of charge carriers in the hole-transport layer (HTL). For HTL-free PSCs, high-frequency capacitance can be considered as the geometric capacitance for analyzing the dielectric constant of the perovskite layer because there is no trapping and de-trapping of charge carriers in the perovskite layer. We further find that the low-frequency capacitance signature can be used to calculate the activation energy of the ionic conductivity of the perovskite layer, but the overlapping effects with charge transport materials must be avoided.

Threshold Switching in Single Metal‐Oxide Nanobelt Devices Emulating an Artificial Nociceptor

AbstractElectronic devices that can simulate the dynamics of neurotransmission in the human body are of great interest for the development of artificial intelligence in modern information technology. An artificial nociceptor realized by a single metal‐oxide nanobelt device with a unique capacitive‐coupled threshold switching behavior is demonstrated. Via thermal admittance spectroscopy and temperature‐dependent sweeping study, the properties of the nanobelt devices are determined by Schottky emission at low bias and by defect‐assisted quantum tunneling at high bias subject to a threshold voltage. The low activation energy associated with dynamic electron trapping gives rise to a voltage‐dependent volatile threshold switching behavior. This threshold switching behavior allows the emulation of several characteristic features of a nociceptor, a critical type of sensory neuron in the human body, including “threshold,” “relaxation,” “no adaptation,” “allodynia,” and “hyperalgesia” behaviors, essential for the realization of electronic sensory receptors that detect noxious stimuli and signal rapid warning to the central nervous system. One‐dimensional metal oxide nanobelt devices of this type yield multifunctional nociceptor performance that is fundamental for applications in artificial intelligence systems, representing a key step in the realization of neural integrated devices via a bottom‐up approach.

Mechanically tuning spin-orbit coupling effects in organic-inorganic hybrid perovskites

Organic-inorganic hybrid perovskites have potential applications in flexible electronics based on solution-processing polycrystalline thin-films. This article reports an emerging phenomenon: mechanically tunable spin-orbit coupling (SOC) in flexible perovskite solar cells under elastic bending. Polarization-dependent photocurrent studies show that mechanical bending increases the orbit-orbit interaction, shown as an enhanced SOC, and consequently boosting the intersystem crossing to convert optically generated bright states (which are allowed to recombine) into dark states (which are forbidden to recombine) in flexible perovskite solar cells [PET/ITO/PEDOT:PSS/MAPbI3-xClx/PCBM/PEI/Ag]. Simultaneously, the photocurrent is increased from 15.39 mA/cm2 to 22.0 mA/cm2 by 43% upon such elastic bending with the curvature radius of 4.2 mm. It is further found that introducing mechanical stress leads to both grain boundary interaction and grain deformation shown as the decreased defects at grain boundaries through thermal admittance spectroscopy and the elastic strain verified by X-ray diffraction measurement. The capacitance-frequency characteristics indicate that applying this mechanical stress causes an increase on the bulk polarization by introducing grain boundary interaction and grain deformation. This provides necessary condition to realize mechanically tunable SOC effects in perovskites via electric-magnetic coupling. Essentially, mechanically tunable SOC effects present new mechanisms to control the optoelectronic properties in flexible perovskite electronic devices.

Regimes of current transport mechanisms in CdS/CdTe solar cells

Forward bias recombination (current transport) mechanisms have been evaluated for thin film solar cells and correlated to the in-gap trap levels present. Here CdTe/CdS devices were chosen as an archetypal example of a modern thin film solar cell, and a set of devices with a range of design variables was used in order to reveal the full range of behaviours that may operate to limit current transport. Experimental current–voltage–temperature datasets were compared to mathematical models of transport, and the in-gap traps were evaluated by thermal admittance spectroscopy. The current transport mechanisms operating are presented on a temperature–voltage diagram. Three regimes were identified: at ‘intermediate’ voltages, the behaviour was temperature dependent. From 300 K down to 240 K, thermally activated Shockley Read Hall recombination mediated by a 0.38 eV trap (VCd) dominated the transport. Between 200 and 240 K the transport was thermally activated but below 200 K the mechanism became dominated by tunnel assisted interface recombination. At ‘low’ voltages (and for all devices at all voltages when measured at T < 200 K) band to band recombination is via multi-step tunnelling through in-gap states. At high voltage, the forward current is dominated by the well-known limiting effect of the back Schottky contact to the CdTe which is in reverse bias. The current transport behaviour is also correlated with the n-CdS thickness and CdCl2 processing conditions, both of which are critical to device performance.

Utilization of Temperature-Sweeping Capacitive Techniques to Evaluate Band Gap Defect Densities in Photovoltaic Perovskites.

Capacitive techniques, routinely used for solar cell parameter extraction, probe the voltage-modulation of the depletion layer capacitance isothermally as well as under varying temperature. In addition, defect states within the semiconductor band gap respond to such stimuli. Although extensively used, capacitive methods have found difficulties when applied to elucidating bulk defect bands in photovoltaic perovskites. This is because perovskite solar cells (PSCs) actually exhibit some intriguing capacitive features hardly connected to electronic defect dynamics. The commonly reported excess capacitance observed at low frequencies is originated by outer interface mechanisms and has a direct repercussion on the evaluation of band gap defect levels. Starting by updating previous observations on Mott-Schottky analysis in PSCs, it is discussed how the thermal admittance spectroscopy and the deep level transient spectroscopy characterization techniques present spectra with overlapping or even "fake" peaks caused by the mobile ion-related, interfacial excess capacitance. These capacitive techniques, when used uncritically, may be misleading and produce wrong outcomes.

Electrical Properties of Vertical p‐NiO/n‐Ga2O3 and p‐ZnCo2O4/n‐Ga2O3 pn‐Heterodiodes

Vertical β‐Ga2O3‐based pn‐heterostructures are investigated by current–voltage measurements and thermal admittance spectroscopy. The β‐Ga2O3 thin films are grown by pulsed laser deposition (PLD) on (00.1)ZnO:Ga/(11.0)Al2O3 substrates at 670 °C. Two different p‐type oxides are used to fabricate pn‐heterodiodes which are investigated with respect to rectification, temperature stability, and breakdown behavior. For that, p‐NiO and p‐ZnCo2O4 are grown by PLD at room temperature on top of a β‐Ga2O3 thin film, respectively. The rectification ratio at room temperature is about nine orders of magnitude, the ideality factor is about 2 and the on‐resistance is below 1 Ω cm2. Both diode types did not show degradation for temperatures up to 100 °C. Thermal admittance spectroscopy revealed defects in β‐Ga2O3 with thermal activation energy Ea of about 200 meV for both diode types. Additionally, a level with Ea ≈ 240 meV is detected for the NiO/Ga2O3 diodes only. In general, both diode types are suited to realize pn‐heterodiodes with high rectification, but NiO is especially interesting for defect characterization by deep‐level optical spectroscopy due to its transparency up to about 3.7 eV.

Dynamic thermal performance of conventional and alternative building wall envelopes

The current study investigates the thermal performance parameters of the building wall envelopes. The two wall systems, homogenous and composite wall envelope configurations were considered in this study by adopting conventional and alternative building materials. The study is based on theoretical investigation in evaluating the thermal performance parameters of the building envelope by adopting admittance method and finite difference methods. The effects of various wall configurations on time lag and decrement factor, interior surface instantaneous thermal load and the influence of exterior surface heat transfer coefficient on the time lag and decrement factors have been studied. Even dynamic thermal performance parameters such as thermal admittance, decrement factor, surface factor and its time leads and heat capacitance, effect of thermal conductivity and optimum thickness for maximum energy storage also studied. The study results reveal the impact of various building material thermal properties and its configurations as envelope on the thermal performance parameters.

A multi-port thermal coupling model for multi-chip power modules suitable for circuit simulators

This paper investigates two compact thermal model representations for multi-chip power modules, namely the thermal impedance matrix model and the thermal admittance matrix model. The latter can shape a multi-port thermal network without controlled temperature sources, and can be readily implemented in circuit simulators. The mutual transformation between the two models and their relationship to parameters in the multi-port network are revealed. In addition, practical tips for thermal model parameter extractions based on temperature measurements and curve-fitting are discussed. The multi-port thermal model is verified by simulations and experimental results. It confirms that more accurate temperature estimation can be achieved compared with the thermal model without the thermal coupling effect.

Impact of Insulation Type and Thickness on the Dynamic Thermal Characteristics of an External Wall Structure

The dynamic thermal characteristics of external wall structures are directly related to indoor thermal comfort and energy savings in buildings; they are also complicated and worth investigating. Thermal insulation in external wall structures has become a popular topic of investigation in the domain of building energy efficiency. This study aims to find the impact of insulation type and thickness on the dynamic thermal characteristics of external wall structures using a homogeneous multi-layer building external wall structure and three types of insulation materials that are widely used in Japan. The impact of insulation type and thickness on seven thermal characteristics of external walls, including thermal transmittance, decrement factor or amplitude attenuation, time lag, thermal admittance, time lead for thermal admittance, surface factor, and thermal capacity, was evaluated by numerical methods in this study. It was shown that insulation type and thickness would have a significant effect on thermal transmittance, decrement factor and time lag, but yield no significant change in thermal admittance, time lead for thermal admittance, surface factor, and the thermal capacity of external wall structures.

Alkali treatments of Cu(In,Ga)Se2 thin‐film absorbers and their impact on transport barriers

AbstractWe study the impact of different alkali post‐deposition treatments by thermal admittance spectroscopy and temperature‐dependent current‐voltage (IVT) characteristics of high‐efficiency Cu(In,Ga)Se2 thin‐film solar cells fabricated from low‐temperature and high‐temperature co‐evaporated absorbers. Capacitance steps observed by admittance spectroscopy for all samples agree with the widely observed N1 signature and show a clear correlation to a transport barrier evident from IVT characteristics measured in the dark, indicating that defects are likely not responsible for these capacitance steps. Activation energies extracted from capacitance spectra and IVT characteristics vary considerably between different samples but show no concise correlation to the alkali species used in the post‐deposition treatments. Numerical device simulations show that the transport barrier in our devices might be related to conduction band offsets in the absorber/buffer/window stack.

Understanding Thermal Admittance Spectroscopy in Low-Mobility Semiconductors

Thermal admittance spectroscopy (TAS) is frequently used to analyze the properties of trap states in semiconductor devices. We perform detailed simulations in combination with experiments to understand the effect of low carrier mobility on the analysis of trap states by TAS. We show that the apparent characteristic peak in the differential capacitance spectra is strongly dominated by the dielectric relaxation (DR) peak caused by low carrier mobilities for the case of shallow traps and low trap densities. The model for the DR dominated case is successfully applied to interpret the experimental results from poly(3-hexylthiophene-2,5-diyl) (P3HT) based diodes. In contrast, for deep states with high density of states, we are able to properly estimate the energetic position, but the low carrier mobility affects the correct determination of the attempt-to-escape frequency as well as the capture cross section. Our results reveal that low carrier mobilities cause inherent obstacles in accurately determining the t...

Energy Levels of Defects Created in Silicon Supersaturated with Transition Metals

Intermediate-band semiconductors have attracted much attention for use in silicon-based solar cells and infrared detectors. In this work, n-Si substrates have been implanted with very high doses (1013 cm−2 and 1014 cm−2) of vanadium, which gives rise to a supersaturated layer inside the semiconductor. However, the Mott limit was not exceeded. The energy levels created in the supersaturated silicon were studied in detail by means of thermal admittance spectroscopy. We found a single deep center at energy near EC − 200 meV. This value agrees with one of the levels found for vanadium in silicon. The capture cross-section values of the deep levels were also calculated, and we found a relationship between the capture cross-section and the energy position of the deep levels which follows the Meyer–Neldel rule. This process usually appears in processes involving multiple excitations. The Meyer–Neldel energy values agree with those previously obtained for silicon supersaturated with titanium and for silicon contaminated with iron.