Effect Of Light Soaking Research Articles

Thermal Annealing Effect on the Conversion Efficiency of Si Solar Cells

We investigated the effect of thermal annealing on the performances of Si solar cells. The Si solar cells were fabricated on p-type microtextured Si wafers using a standard cell fabrication process. The fabricated cells were annealed at 250 °C for 60, 80, and 90 min using a halogen lamp heater in N2 ambient. The annealed sample with a time of 60 min showed enhanced optical properties of light absorption, quantum efficiency, and minority carrier lifetime but did not show enhanced cell conversion efficiency. However, while the cell annealed for 90 min showed different optical properties, it showed the highest conversion efficiency of 17.35% compared to the reference cell of 17.14%, indicating the total recovery of the light soaking effect. We further analyzed the hydrogen-related chemical bonding structures for the dopant activation throughout hydrogen diffusion and the electroluminescence by a radiative recombination of a p–n junction.

Three-Dimensional Observation of a Light-Soaked Photoreactant Layer in BTR:PCBM Solar Cells Treated with/without Solvent Vapor Annealing.

A key challenge to the commercialization of solution-processed solar cells is a proper understanding of the morphological variations during long periods, particularly under light-soaking conditions. Many research groups have competitively reported solvent vapor annealing (SVA)-treated small-molecule devices with efficiency rates exceeding 11%; however, their light-soaking effects have been rarely studied. Here, we investigate the morphological changes in the light-soaked devices with/without SVA treatments depending on the illumination time via three-dimensional observations. From the results, we found that the trends of morphological variations differ in the surface and bulk parts of the active film and that the difference is closely related to the device performance capabilities. Therefore, our research will enhance the underlying knowledge of the light-soaking effect on active morphologies over long term.

Silane-Capped ZnO Nanoparticles for Use as the Electron Transport Layer in Inverted Organic Solar Cells.

Zinc oxide (ZnO) nanoparticles are widely used as electron- transport layer (ETL) materials in organic solar cells and are considered to be the candidate with the most potential for ETLs in roll-to-roll (R2R)-printed photovoltaics. However, the tendency of the nanoparticles to aggregate reduces the stability of the metal oxide inks and creates many surface defects, which is a major barrier to its printing application. With the aim of improving the stability of metal oxide nanoparticle dispersions and suppressing the formation of surface defects, we prepared 3-aminopropyltrimethoxysilane (APTMS)-capped ZnO (ZnO@APTMS) nanoparticles through surface ligand exchange. The ZnO@APTMS nanoparticles exhibited excellent dispersibility in ethanol, an environmentally friendly solvent, and remained stable in air for at least one year without any aggregation. The capping of the ZnO nanoparticles with APTMS also reduced the number of surface-adsorbed oxygen defects, improved the charge transfer efficiency, and suppressed the light-soaking effect. The thickness of the ZnO@APTMS ETL could reach 100 nm without an obvious decrease in the performance. Large-area APTMS-modified ZnO films were successfully fabricated through roll-to-roll microgravure printing and exhibited good performance in flexible organic solar cells. This work demonstrated the distinct advantages of this ZnO@APTMS ETL as a potential buffer layer for printed organic electronics.

Eliminating light soaking effect of inverted polymer solar cells functionalized with a conjugated macroelectrolyte electron-collecting interlayer

To overcome the light soaking and hysteresis characteristics in indium tin oxides (ITO)/poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) based bulk-heterojunction polymer solar cells, a smart gate dielectrics nano-interfacial layer was incorporated by spin-coating an ultrathin polyetherimide (PEI) between ITO and PFN electron extraction layer. The ultrathin PEI micromolecule can effectively fill the surface defects of ITO and surmount electron transport barrier upon forming attached dipoles. The passivation surface decreased the total interfacial electron localized state and electron transfer barrier, which could re-modulate the interfacial property of buffer layer and facilitate electron collection. The additional insulating layer can influence the photoconductivity response and consequently restrain photocurrent hysteresis phenomena, leading to the improved power conversion efficiency. This finding would be helpful to achieve large-scale and low-cost polymer solar cells without light soaking effect.

Light‐Induced Reversible Optical Properties of Hydrogenated Amorphous Silicon: A Promising Optically Programmable Photonic Material

Reversible optical properties (refractive index, n, and extinction coefficient, k) upon light soaking and annealing are predicted to originate from the metastable properties of hydrogenated amorphous silicon (a‐Si:H). Optically programmable photonic devices can be demonstrated when a‐Si:H is established as a potential programmable photonic material. Therefore, the effects of prolonged high intensity light soaking and annealing are investigated. A set of a‐Si:H films is deposited by inductively coupled plasma‐enhanced chemical vapor deposition (ICP‐PECVD) near the amorphous/nano‐crystalline phase transition that have significantly different hydrogen content and microstructures. Spectroscopic ellipsometry shows that the imaginary part of the pseudo dielectric function <ϵ2> values near the peak clearly decreases after light soaking and this decrease can be reversed after annealing. No loss of Si‐bonded hydrogen is observed after repeated cycles of light soaking and annealing and an inverse correlation between this reversibility and the hydrogen content is found. A change in the bulk optical properties is likely the main contributor to the observed metastable effect, suggesting a reversible refractive index change. Repeated cycles of reversible <ϵ2> behavior are demonstrated which together with the magnitude of reversibility (as high as 3.7% at 3.6 eV) illustrates the potential of a‐Si:H for future programmable photonic devices.

Dynamic study of the light soaking effect on perovskite solar cells by in-situ photoluminescence microscopy

Organic-inorganic halide perovskite solar cells (PSCs) have emerged as promising candidates for next generation solar cells due to the rapid increase in their power conversion efficiency. The instability of these cells under illumination, however, remains a major technical barrier for commercialization. In this work, by fabricating full perovskite cells (not test structures) that is compatible with in photoluminescence (PL), for the first time we have achieved in-situ monitoring of the localized charge carrier and ion dynamics in an operating perovskite solar cell under light soaking, using nanoscale resolved in-situ PL and time-resolved PL (tr-PL) microscopy. By analyzing the dynamic PL lifetime and intensity under different light soaking conditions and its correlation with the shape of the voltage current curve, we explain the different scenarios of ion migration and accumulation at the interface and in the bulk that result in different hysteresis behaviors. Our results suggest that mobile positive ions, predominantly iodide vacancies pre-accumulate near the spiro-MeOTAD/perovskite interface of as-fabricated devices, reducing charge-carrier separation and increasing recombination at that electrode. After light soaking for a short time at open circuit, these positive ions drift away from the interface under the altered electric field, improving device performance. After prolonged light soaking, however, negative ions, predominantly iodide interstitials drift to the spiro-MeOTAD/perovskite interface, significantly enhancing carrier recombination at that electrode. In contrast, light soaking had less effect at short-circuit because the electric field is invariant at short circuit. In addition to the light-soaked-induced ion movement under short circuit is by diffusion rather than by drift. This result in ionic redistribution and ion-recombination increases PL intensity uniformity across the device and resulting in relatively stable device performance. Our work reveals that the bias voltage during light soaking results in different dynamic processes, which can be either positive or negative. Analysis on the corresponding PL images revealed a difference in charge carrier and ion dynamics between grain interior and grain boundary during light soaking. The larger density of grain boundaries causes a faster ion migration rate in the regions with smaller grains. Therefore, it may be possible to reduce J-V hysteresis by producing larger grains. Our results provide novel insight into the effect of light soaking on ions and subsequent effect on carrier dynamics for better understanding of the operation of perovskite solar cells.

Light-soaking free organic photovoltaic devices with sol-gel deposited ZnO and AZO electron transport layers.

This paper investigates light-soaking effects in inverted organic photovoltaic (OPV) devices with zinc oxide (ZnO) and aluminum doped ZnO (AZO) electron transport layers (ETL), which is important for the development of low-cost and stable solar cells. The samples demonstrated high solar harvesting properties with power conversion efficiency up to 3.9%. Air-stability tests of up to 150 days were performed on devices with different Al doping levels. The devices maintained higher than 60% of the initial PCE after 50 days of open-air exposure. The light-soaking mechanism was investigated with experiments and simulations and shown to be eliminated when the Al fraction of the AZO is higher than 4%. The simulated band diagram of the OPV devices indicates that the low carrier density in the ZnO layer by virtue of depletion is the main reason of the light-soaking effect. Doping the ZnO layer as well as exposing the devices under UV irradiation will introduce additional free carriers into the ETL and reduce the width of the depletion region at both sides of the ETL.

One diode circuital model of light soaking phenomena in Dye-Sensitized Solar Cells

In this work, we report on the modelling of light soaking effect on Ruthenium-based Dye Sensitized Solar cells (DSSCs). Such a phenomenon can be detected when exposing the cells at increasing hours of illumination and produces a reversible performance increase. Starting from the results obtained through the electro-optical characterization of the cells, we applied a one-diode circuital-model. Our results show a good agreement between the experimental and the simulated data, with a mean square error in the order of 10−12 and a maximum error in current lower than 0.6%. Finally such results allowed us to infer some precise trends followed by the cells main electrical parameters and of their equivalent one-diode electrical model due to light soaking effect.

Irreversible light-soaking effect of perovskite solar cells caused by light-induced oxygen vacancies in titanium oxide

An irreversible light-soaking effect was disclosed in perovskite solar cells using TiO2 as an electron transporting layer. The power conversion efficiency of a fresh device was improved more by twice after light soaking for 15 min and then remained 70% even though the device was recovered in the dark for 4 days. The buried mechanism was explored by shedding light on the interaction between light and titanium dioxide. Oxygen vacancies in TiO2 were found to be increased by light-soaking, especially for wavelengths shorter than 400 nm. Such vacancies enhanced the N-type doping in the semiconductor, which not only increased the conductivity of the titania film but also accelerated the charge extraction rate between perovskite crystallites and titania, and finally contributed to upgraded power conversion efficiency.

Pinning Down the Anomalous Light Soaking Effect toward High-Performance and Fast-Response Perovskite Solar Cells: The Ion-Migration-Induced Charge Accumulation.

The light soaking effect (LSE) is widely known in perovskite solar cells (PVSCs), but its origin is still elusive. In this study, we show that in common with hysteresis, the LSE is owed to the ion migration in PVSCs. Driven by the photovoltage, the mobile ions in the perovskite materials (MA+/I-) migrate to the selective contacts, forming a boosted P-i-N junction resulting in enhanced charge separation. Besides, the mobile ions (MA+) can soften and suture the PCBM/perovskite interface and thus reduce the trap density, in keeping with a higher open-circuit voltage. Finally, almost LSE-free PVSCs can be prepared by using 0.1 wt % MAI-doped PCBM as the electron transport material, whereas overdoping (1 wt % MAI doping) makes the LSE even more pronounced due to excess mobile ions that need time to migrate to reach a new quasi-static state.

Investigation of the light-soaking effect in organic solar cells using dielectric permittivity and electric modulus approaches

We demonstrate the influence of a light-soaking treatment on current-voltage characteristics, external quantum efficiency and dielectric response of an encapsulated P3HT:PCBM bulk heterojunction inverted organic solar cell. Furthermore, we apply dielectric permittivity and electric modulus approaches to identify relaxation peaks observed in this device. We discuss previously reported explanations of the light-soaking effect on the basis of our experimental results, showing that the light-soaking effect originates from dipole formation at the ZnO/P3HT:PCBM interface. In addition, we find that metal oxide hole and electron selective layers prevent the P3HT:PCBM molecules' reorientations and, thus, enhance its thermal stability. This type of experimental observations are expected to provide new prospective on the fundamental aspect of elementary charge transfer modifications in organic solar cells.

Mechanism of Light-Soaking Effect in Inverted Polymer Solar Cells with Open-Circuit Voltage Increase.

In this study, we present novel insights into the light-soaking effect of inverted polymer solar cells (PSCs), where the open-circuit voltage (Voc) of the cells improves over time under light irradiation. The effect was investigated by electron spin resonance (ESR) studies of bare indium tin oxide (ITO) and piperazine derivative-modified ITO/regioregular poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C61 butyric acid methyl ester (PCBM) substrates. These results were combined with alternating current impedance spectroscopy (IS) measurements of inverted PSCs based on the above substrates. In ESR experiments with the substrates under white light irradiation, with a UV light component, many P3HT•+ radical cations were observed in the bare-ITO/P3HT:PCBM substrate. The number of radical cations was considerably suppressed in the ITO/P3HT:PCBM substrates with ITO modified by piperazine derivatives. This is because adsorbed oxygen molecules on the ITO acted as acceptor dopants for photoexcited P3HT, and the amount of adsorbed oxygen was decreased by modifying the ITO with piperazine derivatives. In IS measurements of the inverted PSCs under white light irradiation, a decrease in the electric capacitance (CPE2) of an electric double layer formed at the ITO/P3HT:PCBM interface was observed. A strong correlation was observed between the decrease of CPE2 and the increase of Voc. From these results, the light-soaking behavior was attributed to the removal of an electron injection barrier formed between ITO and PCBM, under white light irradiation.

Modification of the Highly Conductive PEDOT:PSS Layer for Use in Silver Nanogrid Electrodes for Flexible Inverted Polymer Solar Cells.

Silver nanogrid based flexible transparent electrode is recognized as the most promising alternative to ITO electrode for organic electronics, owing to its low production cost and excellent flexibility. Typically, a highly conductive thin film coating layer, such as highly conductive PEDOT:PSS (HC-PEDOT:PSS) is usually deposited onto the Ag-grid electrode to smooth the surface and to minimize the sheet resistance. In this paper, we found that inverted flexible polymer solar cells with structure of Ag-grid/HC-PEDOT:PSS/ZnO/photoactive layer/MoO3/Al generally exhibits strong S-shaped J-V curves, which could be eliminated by light-soaking treatment. Kelvin probe force microscope (KPFM) measurement proved that a large work function (WF) difference (0.70 eV) between HC-PEDOT:PSS and ZnO is the main reason for the formation of S-shape. White light soaking of the Ag-grid/HC-PEDOT:PSS gradually decreased the WF of HC-PEDOT:PSS from 5.10 to 4.60 eV, leading to a reduced WF difference between HC-PEDOT:PSS and ZnO from 0.70 to 0.38 eV. Such a WF difference decrease was believed to be the working mechanism for the light-soaking effect in this flexible device. Based on this finding, the HC-PEDOT:PSS solution was then modified by doping with polyethylenimine (PEI) and aqueous ammonia. The modified PEDOT:PSS film is characteristic of adjusting WF through varying PEI doping concentrations. By using such a modified PEDOT:PSS layer, light-soaking-free flexible inverted polymer solar cell with a power conversion efficiency of 6.58% was achieved for PTB7-Th:PC71BM cells. The current work provides a useful guideline for interfacial modification for Ag-grid based flexible electrode.

Seasonal performance comparison of three grid connected photovoltaic systems based on different technologies operating under the same conditions

Three grid connected photovoltaic systems based on different PV technologies (poly-Si, a-Si and CdTe), operating under the same conditions were analysed to determine their degradation and the variation in seasonal performance. In addition to the intrinsically different degradation of each technology through indoor tests at standard test condition (STC), a deeper analysis of the variables affecting the fluctuations of the performance was performed. The performance seasonal fluctuations resulting from the solar spectrum and module temperature was studied during a single year (December 2014 to December 2015). A notable solar air mass spectrum dependence (−11.22%/AM) of the generated power but scarcely detectable instantaneous temperature dependence was observed in the a-Si array. In contrast, the poly-Si array suffered the highest power decline with an elevating temperature (−0.56%/°C). The CdTe array exhibited a tempered interaction with the solar spectrum (−4%/AM) and temperature (−0.25%/°C). Finally, the significant maximum power difference between two months (April and September) due to the thermal annealing and light soaking effects in the a-Si array is confirmed, where the quantified variation of these two effects were 5.9% and −5.7%, respectively.

Electro-optical characterization of ruthenium-based dye sensitized solar cells: A study of light soaking, ageing and temperature effects

In this work, we report on the electro-optical characterization of Dye Sensitized Solar Cells (DSSCs) which use ruthenium complexes as sensitizers. In particular, several kinds of measurements have been performed to study the electrical performances and the efficiency levels of the cells at different operating conditions. In detail, the measurements have been conducted at varying temperatures and hours of light soaking. Our results show that the short circuit current density JSC, the open circuit voltage VOC and the conversion efficiency η increase with the hours of light soaking, while they decrease with temperature. A thorough investigation on the light soaking process at the device level has been carried out, giving a wealth of experimental data which can be very useful to better single out and quantify the underlying physical phenomena and the effects on the cell parameters, which are still controversial among the scientific community. Finally, the ageing study demonstrates that the cell performance rapidly deteriorates in the first few months of exposure to light. All the obtained results are of great importance for a better understanding of the light soaking phenomenon and for improving the fabrication process of the DSSCs investigated during this study.

光浴对CH3NH3PbI3薄膜光致发光量子效率的影响

光浴对CH₃NH₃PbI₃薄膜光致发光量子效率的影响

Efficient and Highly Air Stable Planar Inverted Perovskite Solar Cells with Reduced Graphene Oxide Doped PCBM Electron Transporting Layer

Reduced graphene oxide (rGO) is added in the [6,6]‐Phenyl‐C61‐butyric acid methyl ester (PCBM) electron transport layer (ETL) of planar inverted perovskite solar cells (PSCs), resulting in a power conversion efficiency (PCE) improvement of ≈12%, with a hysteresis‐free PCE of 14.5%, compared to 12.9% for the pristine PCBM based device. The universality of the method is demonstrated in PSCs based on CH3NH3PbI3−xClx and CH3NH3PbI3 perovskites, deposited through one step and two step spin coating process, respectively. After a comprehensive spectroscopic characterization of both devices, it is clear that the introduction of rGO in PCBM ETL results in an important increase of the ETL conductivity, together with reduced series resistance and surface roughness. As a result, a significant photoluminescence quenching of such perovskite/ETL is observed, confirming the increased measured short circuit current density. Transient absorption measurements reveal that in the rGO‐based device, the relaxation process of the excited electrons is significantly faster compared to the reference, which implies that the charge injection rate is significantly faster for the first. Furthermore, the light soaking effect is significantly reduced. Finally, aging measurements reveal that the rGO stabilizes the ELT/perovskite interface, which results in the stabilization of perovskite crystal structure after prolonged illumination.

The Effect of the Microstructure on Trap‐Assisted Recombination and Light Soaking Phenomenon in Hybrid Perovskite Solar Cells

Despite the rich experience gained in controlling the microstructure of perovskite films over the past several years, little is known about how the microstructure affects the device properties of perovskite solar cells (HPSCs). In this work, the effects of the perovskite film microstructure on the charge recombination and light‐soaking phenomenon in mixed halide HPSCs are investigated. Devices with noncompact perovskite morphology show a severe light soaking effect, with the power conversion efficiency (PCE) improved from 3.7% to 11.6% after light soaking. Devices with compact perovskite morphology show a negligible light soaking effect, with PCE slightly increased from 11.4% to 11.9% after light soaking. From device investigations, photoluminescence, and impedance spectroscopy measurements, it is demonstrated that interface electron traps at the grain boundaries as well as at the crystal surface dominate the light soaking effect. Severe trap‐assisted recombination takes place in HPSCs using noncompact films, while it is effectively eliminated in devices with compact films. Moreover, how the grain size of the perovskite film affects the light soaking phenomenon is investigated. In the case of compact perovskite films, the size of the grains has a limited effect on the light soaking. In these compact films, grains are fused and trap states are effectively reduced.

Metastability of a-SiOx:H thin films for c-Si surface passivation

The adoption of a-SiOx:H films obtained by PECVD in heterojunction solar cells is a key to further increase their efficiency, because of its transparency in the UV with respect to the commonly used a-Si:H. At the same time this layer must guarantee high surface passivation of the c-Si to be suitable in high efficiency solar cell manufacturing. On the other hand the application of amorphous materials like a-Si:H and SiNx on the cell frontside expose them to the mostly energetic part of the sun spectrum, leading to a metastability of their passivation properties. Moreover as for amorphous silicon, thermal annealing procedures are considered as valuable steps to enhance and stabilize thin film properties, when performed at opportune temperature. In this work we explored the reliability of a-SiOx:H thin film layers surface passivation on c-Si substrates under UV exposition, in combination with thermal annealing steps. Both p- and n-type doped c-Si substrates were considered. To understand the effect of UV light soaking we monitored the minority carriers lifetime and SiH and SiO bonding, by FTIR spectra, after different exposure times to light coming from a deuterium lamp, filtered to UV-A region, and focused on the sample to obtain a power density of 50μW/cm2. We found a certain lifetime decrease after UV light soaking in both p- and n-type c-Si passivated wafers according to a a-SiOx:H/c-Si/a-SiOx:H structure. The role of a thermal annealing, which usually enhances the as-deposited SiOx passivation properties, was furthermore considered. In particular we monitored the UV light soaking effect on c-Si wafers after a-SiOx:H coating by PECVD and after a thermal annealing treatment at 300°C for 30min, having selected these conditions on the basis of the study of the effect due to different temperatures and durations. We correlated the lifetime evolution and the metastability effect of thermal annealing to the a-SiOx:H/c-Si interface considering the evolution of hydrogen in the film revealed by FTIR spectra, and we developed a model for the effect of both treatments on the SiH bonding and the metastability shown in the lifetime of a-SiOx:H/c-Si/a-SiOx:H structure. We found that, after UV exposure, thermal annealing steps can be used as a tool for the c-Si passivation recovery and enhancement.

Mechanistic Investigation into the Light-Soaking Effect Observed in Inverted Polymer Solar Cells Using Indium Tin Oxide Electrodes Modified By Piperazine Derivatives

The well-controlled electrical double layer in polymer solar cells, which is formed between active layer and electrode, promotes both increase in built-in electric field and decrease in carrier injection barrier, resulting in effective carrier collection on the electrode. Previously, in ITO/P3HT:PCBM/PEDOT:PSS/Au inverted solar cells, we observed the improvement of power conversion efficiency according to decrease in ionization potential of ITO by modifying with amine compounds. However, the light-soaking effect which the open-circuit voltage (Voc) gradually increased with white light irradiation containing UV component was observed. In this work, we have investigated the mechanism of light-soaking effect by combining time-dependences of photovoltaic properties and electric capacity of the inverted polymer solar cells. To prepare the amine-modified ITO electrodes, we used ethanol as a solvent for piperazine, N’(2-aminoethyl)piperazine (NAP) and 1,4-bis(3-aminopropyl)piperazine (BAP). The dilute solutions containing the amines were spin-coated on the ITO substrates. The substrate was subsequently dried at 120 °C for 10 min on a hotplate. Additionally, to remove any extra amines from the ITO, the same operation was performed using a solvent instead of the amine solution. Their ionization potentials (I P) were measured by a photoelectron spectroscopy in air. To fabricate the inverted polymer solar cells, a P3HT:PCBM blend solution and a PEDOT:PSS dispersion solution were spin-coated on the modified ITO substrates sequentially. The Au rear electrode was vacuum-deposited on the PEDOT:PSS layer. Finally, the device was mechanically protected by laminating it with a film at 150°C for 5 min under a reduced pressure. The time-dependence of the current-voltage (I-V) curves of these solar cells was measured under AM 1.5G-100 mW cm-2 simulated sunlight condition. Using ac impedance spectroscopic measurement, both time-dependences of resistance and electric capacity (CPE) of these solar cells were obtained in the dark and under simulated sunlight irradiation. The cell fabrications and the measurements were carried out under an ambient atmosphere. The IP values of BAP-modified ITO electrodes decreased with increasing the concentration of the BAP solutions. The minimum IP was 4.39 eV, decreasing by ca.0.3 eV compared to that of bare ITO. In addition, the power conversion efficiency of the inverted cells increased by using the BAP-modified ITOs. This improvement is perhaps because of an efficient drift migration of photo-produced charge carriers by increase in the built-in potential. However, when white light was irradiated to the cells, the light-soaking effect was observed. The time taken to arrive at a constant Voc decreased as the IP of the amine-modified ITO decreased. That is, the light-soaking effect was small when the amine-modified ITO which had smaller IP was used. In addition, the CPE in an electric double layer formed in the ITO/active layer interface was estimated by ac impedance spectroscopy. This CPE gradually decreased by continuous white light irradiation. Furthermore, the degree of change of the CPE was small when the amine-modified ITO with smaller IP was used. The good correlation between both time-dependences of VOC and CPE was observed. The decrease in CPE and the increase in VOC were caused by decrease in IP . That is, the CPE corresponded to an electron injection barrier in the ITO/active layer interface, and the electron injection barrier became small according to decrease in IP . In summary, the improvement of VOC was caused by decreasing the electron injection barrier because of decrease in IP .