Degradation Of Organic Solar Cells Research Articles

Understanding the metal-electrode-correlated degradation of organic solar cells under illumination/thermal stress

One of the main challenges limiting the commercialization of organic solar cells (OSCs) is the instability problem. Besides the active materials, the electrode especially metal electrode correlated degradation could mainly account for the efficiency decay of a device under illumination and/or thermal stress. However, this has often been neglected compared to the active layer. An in-depth and comprehensive understanding of the electrode-correlated degradation of OSCs is essential to conquer the instability challenge of OSCs. In this work, the detailed degradation of metal electrodes under light illumination and thermal stresses is systematically studied, by probing the morphological and chemical changes. According to the in-depth analyses, a degrading mechanism is proposed for the metal-electrode-correlated degradation of OSCs, which should be taken into account for further stability investigation and the industrial production of OSCs.

Manipulating the Alkyl Chains of Naphthodithiophene Imide-Based Polymers to Concurrently Boost the Efficiency and Stability of Organic Solar Cells.

Morphology instability holds the major responsibility for efficiency degradation of organic solar cells (OSCs). However, how to develop polymer donors simultaneously with high efficiency and excellent morphology stability remains challenging. Herein, we reported naphtho[2,1-b:3,4-b']dithiophene-5,6-imide (NDTI)-based new polymers PNDT1 and PNDT2. The alkyl chain engineering leads to high crystallinity, high hole mobility (>10-3 cm2 V-1 S-1), and nanofibrous film morphology, which enable PNDT2 to exhibit an efficiency of 18.13% and a remarkable FF value of 0.80. Moreover, the NDTIs have short π-π stacking and abundant short interactions, and their polymers exhibit superior morphological stability. Therefore, the PNDT2-based OSCs exhibit much better device stability than that of PNDT1, PAB-α, and benchmark polymers PM6 and D18. This work suggests the great importance of the large conjugated backbone of the monomer and alkyl chain engineering to develop high-performance and morphology-stable polymers for OSCs.

Characterization Tools to Probe Degradation Mechanisms in Organic and Perovskite Solar Cells

Organic solar cells (OSCs) and perovskite solar cells (PSCs) are promising due to their low cost and potential for renewable solar energy conversion. They are compatible with many substrates and varied deposition techniques, including solution processing. They can be coupled with other solar cell types in tandem and multijunction structures. Despite these great attributes and advancements in power conversion efficiencies over the years, they suffer from severe degradation, leading to low lifetime. In terms of research, their stability studies lag. One reason is the complexity of degradation studies and, sometimes, the lack of adequate tools to do an in‐depth probe. Another reason is the lack of comprehensive literature on metrologies’ appropriateness for this kind of study. Although there are reviews on stability and improvement in the efficiency of devices, they focus either on the degradation mechanisms or efforts to use specific tools. There is little on comprehensive characterization tools for their degradation studies. Herein, the experimental tools and techniques researchers use in general to probe degradation in OSCs and PSCs are studied. This review is intended as a starting point and a go‐to material for current and future researchers and (under‐)graduate students interested in stability studies.

Overcoming Moisture‐Induced Degradation in Organic Solar Cells

Unencapsulated organic solar cells are prone to severe performance losses in the presence of moisture. Accelerated damp heat (85 °C/85% RH) studies are presented and it is shown that the hygroscopic hole‐transporting PEDOT:PSS layer is the origin of device failure in the case of prototypical inverted solar cells. Complementary measurements unveil that under these conditions a decreased PEDOT:PSS work function along with areas of reduced electrical contact between active layer and hole‐transport layer are the main factors for device degradation rather than a chemical reaction of water with the active layer. Replacements for PEDOT:PSS are explored and it is found that tungsten oxide (WO3) or phosphomolybdic acid (PMA)—materials that can be processed from benign solvents at room temperature—yields comparable performance as PEDOT:PSS and enhances the resilience of solar cells under damp heat. The stability trend follows the order PEDOT:PSS << WO3 < PMA, with PEDOT:PSS‐based devices failing after few minutes, while PMA‐based devices remain nearly pristine over several hours. PMA is thus proposed as a robust, solution‐processable hole extraction layer that can act as a one to one replacement of PEDOT:PSS to achieve organic solar cells with significantly improved longevity.

Effect of an ambient environment on light-induced degradation of organic solar cells based on a benzodithiophene–quinoxaline copolymer in air

Degradation of polymer molecules is one of the main factors that reduces the lifetime of thin film organic solar cells.

Operando study of the influence of small molecule acceptors on the morphology induced device degradation of organic solar cells with different degrees of π–π stacking

A real time study of the active layer morphology evolution with different acceptors during the OSC operation under vacuum atmosphere.

Optimization and development of ITO-free plasmonic gold nanoparticles assisted inverted organic solar cells

We report an indium tin oxide (ITO) free inverted organic solar cell with an alternate aluminium doped zinc oxide (AZO) front cathode layer based on the consideration of blocking harmful ultraviolet (UV) rays, which is one of the prominent reasons for degradation of organic solar cells. To further enhance the power conversion efficiency of AZO/ZnO/PTB7:PC71BM/MoO3/Ag organic solar cell, plasmonic gold nanoflowers were embedded inside the active layer. The critical parameter of thickness of electron transport layer (ETL), hole transport layer (HTL) and active layer war first optimized through simulations using general purpose photovoltaic device model (GPVDM) software. The experimental fabrication of the desired geometry was then carried out based on optimized thickness. The power conversion efficiency (PCE) under UV exposure was found remarkably constant for AZO front electrode solar cell was as compared to conventional ITO based organic solar cells. This is attributed to the UV blocking property of AZO. The PCE was boosted from 6.19% to 7.01% on further use of plasmonic Au nanoparticles in the active layer of AZO/ZnO/PTB7:PC71BM/MoO3/Ag. This work provides a practical insight of developing an ITO free, plasmonic nanoparticles assisted inverted organic solar cell with enhanced efficiency and stability.

Systematic investigation on stability influence factors for organic solar cells

Degradation of organic solar cells has always hindered the commercialization of organic solar cells (OSCs). In this paper, we fabricate OSCs with a power conversion efficiency (PCE) of 16.20%. The impact of different layers on degradation is studied for the first time by performing accelerated aging test on each layer. The device experiencing accelerated aging process on ITO/ZnO barely decays, indicating the electron transport layer (ETL) ZnO hardly affects the device degradation. The device experiencing accelerated aging process on ITO/ZnO/active layer and the and the device experiencing accelerated aging process on ITO/ZnO/active layer/MoO3 decay fast, manifesting the big impact of active layer and MoO3 on degradation. During the accelerated aging process, the morphology of the active layer changes, hindering light absorption and charge transport. Besides, the work function of hole transport layer (HTL) MoO3 becomes shallower after accelerated aging, making the hole extraction less efficient. Transient absorption spectroscopy (TAS) is recorded to investigate exciton physics change during the accelerated aging process, proving that inefficient charge transfer (CT) state excitons formation and separation account for the PCE decay. The mechanism of the degradation of OSCs is discussed, laying the foundation of their commercialization.

Top‐Gate Field‐Effect Transistor as a Testbed for Evaluating the Photostability of Organic Photovoltaic Polymers

Light‐induced performance degradation in organic solar cells (OSCs) is a major impediment to their commercialization. As the photostability of OSCs strongly depends on the material's properties, the most effective solution for this concern is to develop a photostable material. However, the wide variety of causes of photo‐instability in a standard multilayered OSC structure complicates the evaluation of photostability of newly developed materials. To address this challenge, a top‐gate field‐effect transistor (FET) as a testbed for evaluating the photostability of OSC materials is proposed. This device test platform minimizes the internal and external origins of photo‐instability by employing a fluoropolymer gate dielectric. The photostability of an OSC material incorporated in this FET testbed can be evaluated by monitoring light‐induced mobility degradation. Two types of common donor polymers with similar chemical structures and crystallinity are employed as test materials, and their photostability is evaluated. The test results correspond to the photostability measurements conducted in the standard OSC structure, validating the proposed FET testbed. The proposed FET testbed enables rapid evaluation of the photostability of a newly developed OSC material, thereby providing timely feedback to material scientists. This boosts the development of photostable OSC materials.

What is Killing Organic Photovoltaics: Light‐Induced Crosslinking as a General Degradation Pathway of Organic Conjugated Molecules

AbstractIn view of a rapid development and increase in efficiency of organic solar cells, reaching their long‐term operational stability represents now one of the main challenges to be addressed on the way toward commercialization of this photovoltaic technology. However, intrinsic degradation pathways occurring in organic solar cells under realistic operational conditions remain poorly understood. The light‐induced dimerization of the fullerene‐based acceptor materials discovered recently is considered to be one of the main causes for burn‐in degradation of organic solar cells. In this work, it is shown that not only the fullerene derivatives but also different types of conjugated polymers and small molecules undergo similar light‐induced crosslinking regardless of their chemical composition and structure. In the case of conjugated polymers, crosslinking of macromolecules leads to a rapid increase in their molecular weight and consequent loss of solubility, which can be revealed in a straightforward way by gel permeation chromatography analysis via a reduction/loss of signal and/or smaller retention times. Results of this work, thus, shift the paradigm of research in the field toward designing a new generation of organic absorbers with enhanced intrinsic photochemical stability in order to reach practically useful operation lifetimes required for successful commercialization of organic photovoltaics.

Energy Transfer to a Stable Donor Suppresses Degradation in Organic Solar Cells

AbstractDespite many advances toward improving the stability of organic photovoltaic devices, environmental degradation under ambient conditions remains a challenging obstacle for future application. Particularly conventional systems employing fullerene derivatives are prone to oxidize under illumination, limiting their applicability. Here, the environmental stability of the small molecule donor DRCN5T together with the fullerene acceptor PC70BM is reported. It is found that this system exhibits exceptional device stability, mainly due to almost constant short‐circuit current. By employing ultrafast femtosecond transient absorption spectroscopy, this remarkable stability is attributed to two separate mechanisms: 1) DRCN5T exhibits high intrinsic resistance toward external factors, showing no signs of deterioration. 2) The highly sensitive PC70BM is stabilized against degradation by the presence of DRCN5T through ultrafast, long‐range energy transfer to the donor, rapidly quenching the fullerene excited states which are otherwise precursors for chemical oxidation. It is proposed that this photoprotective mechanism be utilized to improve the device stability of other systems, including nonfullerene acceptors and ternary blends.

Underwater Organic Solar Cells via Selective Removal of Electron Acceptors near the Top Electrode

Electron acceptor degradation of organic solar cells is considered a main contributor to performance instability and a barrier for the commercialization of organic solar cells. Here, we selectively...

Aging and Degradation of Organic Solar Cells Using QUV Accelerated-Weathering Tester

In this paper, we have reported some investigation on the QUV aging of organic solar cells (OSCs). The use of QUV chamber leads to study the effect of cyclic aging on the electrical properties namely PCE, Jsc, FF and Voc under variation of UV irradiation, temperature, humidity and dark. Samples of commercial encapsulated cells supplied by Infinity OPV manufacturer were subjected to 45 cycles (1080 h) of aging. Each cycle of 24 h contains 8 h of illumination with UVA-340 lamp under 50 °C, followed by 4 h of spray (raining) and condensation (humidity generation) at 50 °C, and finished by 12 h of dark. The effect of three different UV irradiation doses (0.83 W/m2, 1.2 W/m2 and 1.3 W/m2) was studied. The degradation process of the OSCs under cyclic aging fits with the earlier finding in the literature and is occurred in two phases with deferent aging rates. The first phase is characterized by the fast decrease of overall electrical properties at the beginning of aging and is due the synergistic effects of rapid variation in the temperature, light and humidity. The second phase is more stable and all the curves reach some saturation. This stabilized phase is the consequence of no bleaching of the semiconductor layer over time. Time constants and constant of degradation were deduced from the first exponential and linear fits of the studied properties. Furthermore, it has been found that the OSCs lose more than 60% of their initial performances, and for some properties like PCE the decrease reaches 80%. This drastic drop of the OSCs performances, associated to the formation of bubbles, indicates that samples undergo very hard bulk degradation.

Hole transport layer influencing the charge carrier dynamics during the degradation of organic solar cells

Here, we demonstrate the effect of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and molybdenum trioxide (MoO3) hole transport layers (HTLs) on degradation of the bulk-heterojunction organic solar cell (OSC) with the combination of two active layers—poly(3-hexylthiophene) and poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]. The study is performed for unencapsulated conventional structure OSCs exposed to the ambient dark condition. In the self-life test situation, it is found that PEDOT:PSS based devices show an initial higher short circuit current and subsequent faster degradation with time in comparison to the MoO3 based devices. The effects of these HTLs have been shown in terms of better charge extraction and a decrease in the shunt resistance as well as the bulk resistance of the active layer. The charge carrier generation probability evaluated from photocurrent data shows the dominant impact of active layer degradation rather than the oxidation of a top electrode under the ambient condition. This suggested mechanism is further supported by impedance spectroscopy as well as the evaluated transit time, global mobility, and exciton dissociation probability, establishing that the degradation does not much affect the transport property of the active material. Rather, it affects more the carrier generation rate. The low hole extraction barrier in PEDOT:PSS based devices show small transit time and high global mobility compared to MoO3. It is found that during the degradation process, the bulk resistance of the device significantly increases, which reduces the diffusion current in the device.

In-Operando Study of the Effects of Solvent Additives on the Stability of Organic Solar Cells Based on PTB7-Th:PC71BM

Degradation of organic solar cells is among the fundamental problems that hinder their successful breakthrough. We probe in-operando the degradation behavior of organic solar cells based on the high-efficiency low-bandgap benzodithiophene copolymer PTB7-Th and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). The influence of the solvent additives 1,8-diiodooctane and o-chlorobenzaldehyde on the degradation mechanism is studied with in situ grazing incidence small-angle X-ray scattering during operation. Changes in the IV characteristics are correlated for the first time in situ with changes in the morphology showing that the use of a solvent additive causes a change in device degradation.

Modeling of degradation in normal and inverted OSC devices

Degradation of organic solar cells (OSC) remains a problem which has prevented its commercial exploitation. While there are several experimental studies pointing towards different reasons behind the atmospheric degradation of these devices, yet a microscopic understanding is lacking. Here, we present a combined experimental and modeling study to understand the degradation in the electrical characteristics of OSC in normal and inverted architectures, further correlated with the interfacial degradation. Our investigations show that degradation of current density – voltage (J-V) characteristics of normal devices is extremely fast i.e. 50% in 9 h as against 21 days for inverted devices. Interestingly, degradation in normal devices is driven primarily by a decrease in short circuit current density (Jsc) without appearance of S-shape in J-V characteristics while degraded inverted devices exhibit S-shaped J-V characteristics. Modeling of J-V data along with time dependent capacitance-voltage (C-V) analysis suggests that the degradation in normal OSC devices can be modeled using degradation induced reduced effective area theory supporting the formation of Al2O3 pockets at the cathode-active layer interface which grow exponentially with storage time eventually forming a continuous layer. In contrast, inverted devices degrade much slowly, via deterioration of hole transporting MoO3 layer resulting in increased anode barrier height and reduced surface recombination velocity (Sp) at anode causing a S shaped J-V curve due to decreased carrier extraction rate. The study clearly highlights the differences between the way two differences architectures and elucidates the underlying reasons.

Reversing an S-kink effect caused by interface degradation in organic solar cells through gold ion implantation in the PEDOT:PSS layer

In this work, we performed a study on the recovery of the photovoltaic performance of an ITO/PEDOT:PSS/P3HT:PCBM/Ca/Al solar cell after the hole transport layer (PEDOT:PSS) had been degraded by contact with the environment. A device that was fully built in an inert environment exhibited a fill factor (FF) of 0.64, while the device whose hole transport layer was exposed to air presented a FF equal to 0.2. In addition, the J-V characteristic curve of the degraded device did not follow the photovoltaic pattern exhibiting the degenerate S shape. However, the elimination of the deleterious effect was achieved by bombarding gold ions on the contaminated surface of PEDOT:PSS by means of the Metal Plasma Immersion Ion Implantation technique. Due to the low energy of the ionic beam of gold, the implanted gold atoms were located at few nanometers off the surface, forming nanometric clusters, that is, gold nanoparticles. Most probably, the degradation of the J-V photovoltaic curve, represented by the S-kink effect, was caused by the appearance of a potential barrier at PEDOT:PSS/P3HT:PCBM interface, which was demolished by the gold nanoparticles that have work function close to HOMO of P3HT. This S-kink effect was also simulated by using an equivalent circuit model constituted by a two-diode circuit, one of which plays the role of the undesirable potential barrier formed at the PEDOT:PSS/P3HT:PCBM interface. Our analysis shows that deposition of gold nanoparticles next to the interface recovers the good hole injection condition from the PEDOT:PSS into the active layer, restoring the fill factor and the device efficiency.

Short-Term Environmental Effects and Their Influence on Spatial Homogeneity of Organic Solar Cell Functionality.

In this study, we focus on the induced degradation and spatial inhomogeneity of organic photovoltaic devices under different environmental conditions, uncoupled from the influence of any auxiliary hole-transport (HT) layer. During testing of the corresponding devices comprising the standard photoactive layer of poly(3-hexylthiophene) as donor, blended with phenyl-C61-butyric acid methyl ester as acceptor, a comparison was made between the nonencapsulated devices upon exposure to argon in the dark, dry air in the dark, dry air with illumination, and humid air in the dark. The impact on the active layer's photophysics is discussed, along with the device physics in terms of integral solar cell performance and spatially resolved photocurrent distribution with point-to-point analysis of the diode characteristics to determine the origin of the observed integrated organic photovoltaic device behavior. The results show that even without the widely used hygroscopic HT layer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), humidity is still a major factor in the short-term environmental degradation of organic solar cells with this architecture, and not only oxygen or light, as is often reported. Different from previous reports where water-induced device degradation was spatially homogeneous and formation of Al2O3 islands was only seen for oxygen permeation through pinholes in aluminum, we observed insulating islands merely after humidity exposure in the present study. Further, we demonstrated with laser beam induced current mapping and point-to-point diode analysis that the water-induced performance losses are a result of the exposed device area comprising regions with entirely unaltered high output and intact diode behavior and those with severe degradation showing detrimentally lowered output and voltage-independent charge blocking, which is essentially insulating behavior. It is suggested that this is caused by transport of water through pinholes to the organic/metal interface, where they form insulating oxide or hydroxide islands, while the organic active layer stays unharmed.

Degradation Analysis of Encapsulated and Nonencapsulated TiO2/PTB7:PC70BM/V2O5 Solar Cells under Ambient Conditions via Impedance Spectroscopy.

Inverted organic cells are promising devices for sustainable and low-cost future electric generation. In this work, we present the degradation mechanisms studied in ITO/TiO2/PTB7:PC70BM/V2O5/Ag inverted organic solar cells (iOSCs) by impedance spectroscopy (IS). Measurements were performed on encapsulated (controlled environment) and nonencapsulated (ambient condition) cells following their temporal evolution under AM1.5 illumination for several voltage biases. From the impedance spectra, analyzed in terms of resistive/capacitive equivalent circuits, we were able to identify that the most sensitive layers inside of the device are contact layers. According with presented, IS technique is useful for determining the materials that have more influence on the degradation of organic solar cells. We demonstrate that IS is a powerful technique to identify the limiting mechanisms and to establish the limiting materials inside of the iOSCs.

Operational Stability and Degradation of Organic Solar Cells

Recently, Organic solar cells (OSC) have been increasingly utilized all over the world. The changes made in the organic components of the organic solar cells enable them to exhibit good features such as mechanical flexibility, lightness and high power generation efficiency even under lower light intensities. However, operational stability is an important parameter for organic solar cells. Despite the aforementioned advantages of organic solar cells, degradation in operational environments limits their use in harsh conditions. Studies have shown that the organic layer and the cathode layer of the OSCs are degraded by external factors, and this adversely affects the operational stability and productivity of OSCs considerably. The overall efficiency of an organic solar cell is defined as a function of life cycle and efficiency of energy generation. Therefore, the shorter the life cycle becomes, the lesser the overall efficiency of OSCs gets. Recent studies are focused on improving the operational stability and power generation efficiencies of OSCs by reducing the effects degradation induced by external factors, such as climatic conditions and thermal fatigue. The purpose of this study is to assess how organic solar cells work, how they degrade from external factors, such as water and water vapor, and how these parameters affect the operational stability as well as the efficiency of the organic solar cells.