ABSTRACTThis article appraises the thermal, photo‐ and photo‐oxidative stability of poly(fullerene)‐alt‐[bismethylbenzene)]s (PFBMBs) prepared by the atom transfer radical addition polymerization (ATRAP) with particular attention paid to their use as additives in organic photovoltaic devices. PFBMBs are of interest due to their well‐defined structures based on alternating, main‐chain, fullerene‐methylene links. This work shows by way of a wide range of characterization techniques and a small library of PFBMBs with varying side chains, however, that PFBMBs are relatively unstable. Given that prior work has shown that other main‐chain fullerene polymers, such as poly(pyrrolidinofullerene)s, are inherently stable, we suggest a degradation mechanism specific to the fullerene‐methylene links of PFBMBs, which explains their unusual behavior. This work suggests that polymers based on fullerene each have their own specific stabilities and qualities and that PFBMBs might be of more use in purposes other than OPVs where in situ delivery of fullerene is required, for example, in medical applications. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 1434–1452

We have identified modifications of material distributions in inverted organic solar cell structures during accelerated aging processes that are potentially linked to the loss of the mechanical integrity of the devices. Non-encapsulated devices were treated with ISOS-D-3 ageing (the so called damp heat exposure at 85 degrees C/85% room humidity in the dark). After performing pull-off tests, the exposed surfaces were analyzed by X-ray photo-emission spectroscopy and atomic force microscopy. For fresh devices, the results revealed that mechanical failure occurs close to the PEDOT:PSS/P3HT:PCBM interface, in agreement with the literature. However, after a short exposure to damp heat, XPS investigations indicate an enrichment of PCBM at the PEDOT:PSS/P3HT:PCBM interface and a change in the PEDOT:PSS surface composition. Both phenomena may explain the rapid drop in device performances and the increased tendency towards delamination.

Organic photovoltaic devices (OPVs) are one of the most promising applications of organic semiconductors due to their compatibility with flexible plastic substrates resulting in lightweight, inexpensive, ergonomic and aesthetic products. While the electrical failure mechanisms in OPVs have been thoroughly investigated, little is known about their mechanical stability, which is as important and critical to ensure long term reliability. The characteristic thin films stresses of each layer present in organic solar cells, in combination with other possible fabrication, handling and operational stresses, provide the mechanical driving force for delamination of weak interfaces or even their decohesion, leading to a loss of device integrity and performance. In this study, we developed a technique to probe weak layers or interfaces in inverted polymer:PC60BM solar cells, establishing a new set-up for the so-called probe tack making it similar to a pull-off test. With this in hand, the adhesion of a variety of active layers towards a standard water-based poly(ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) formulation was compared against that of a new organic-solvent based PEDOT:PSS. The organic solvent based PEDOT:PSS formulation showed improved adhesion compared to the standard water-based formulation. Chemical characterization of the exposed surfaces indicated a difference in their composition, suggesting a stronger interaction of the organic-solvent based PEDOT:PSS with the active layer and resulting in a different fracture path. Indeed, all OPVs with water-based PEDOT:PSS showed a delamination localized at the active layer/PEDOT:PSS interface, while those with the organic-solvent formulation were not fractured at this interface but rather, in the active layer.

In this report, surface engineering is used to improve the air-stability of direct polymer solar cells by replacing the PEDOT:PSS hole transporting layer (HTL) by a grafted polymeric layer. For that purpose a poly(3-hexylthiophene) bearing a triethoxysilane function at the end of the chain (P3HT-Si) was synthesized and anchored to the indium-tin oxide (ITO) electrode. The different characterization techniques used (UV–visible and X-ray photoemission spectroscopy) showed that the applied temperature during grafting is a determinant parameter to tune the P3HT-Si layer thickness/density. For low temperature grafting, polymeric chains were laying close to the surface while higher grafting temperature led to polymeric chains pointing upwards. This organization of the polymeric chains in the grafted layer has a direct impact on photovoltaic properties of the devices fabricated with P3HT-Si as HTL. The use of this new self-assembled HTL creates a hole selective membrane which drastically reduces the leakage current. As a result, the power conversion efficiency of such devices was improved compared to devices with bare ITO. Additionally, by replacing PEDOT:PSS by the hydrophobic P3HT-Si, the water penetration in the device is impeded which significantly improved the shelf lifetime of devices.

Understanding the degradation mechanisms in organic photovoltaics is a major issue in order to develop stable organic semi-conductors and robust device architectures. We investigated the role of the architecture of organic solar cells (OSC) in the stability of the device under different aging conditions. The impact of the light, especially the UV part of the solar spectrum, on the evolution of the photovoltaic characteristics of the inverted OSC devices was investigated. The results show that UV light induces an important Voc loss for several active layers. More importantly, we show that the type of electron transport layer (ETL) induces different degradation mechanisms. For TiOx-based devices, the formation of an interface dipole was identified, resulting in a loss of the flat-band potential (Vfb). For ZnO-based devices, chemical modifications of the metal oxide and active layer at the interface were detected, resulting in a doping of the active layer. The stability of inverted OSC under thermal stress was also investigated and the role played by the hole transport layer (HTL) material and metal electrode was examined. A roughening phenomenon of the silver electrode was detected upon thermal ageing. For sufficiently thick silver layers, this has no impact of the device stability. However, for devices with thin silver electrode, this de-wetting phenomenon leads to a complete failure of the solar cell due to the formation of gaps in the electrode. This problem can be overcome with the addition of a thin metal-oxide layer on top of the electrode. Using XPS depth profile, we also demonstrated the presence of inter-layer diffusion when silver is used as an electrode material. This inter-diffusion is correlated to declines in the performance of the device when used in conjunction with MoO3 as an HTL. The stability under storage in air of OSC in direct architecture was also investigated and we developed a novel strategy to replace the PEDOT:PSS. A poly(3-hexylthiophene) bearing a triethoxysilane function at the end of the chain (P3HT-Si) was anchored to the indium-tin oxide (ITO) electrode. The use of this novel grafted HTL creates a hole selective membrane which drastically reduces the leakage current. As a result, the power conversion efficiency of OSC was improved compared to devices with bare ITO. By replacing PEDOT:PSS by the hydrophobic P3HT-Si, the water penetration in the device is impeded which significantly improved the shelf lifetime of devices. These different studies highlight the role of the architecture and, more specifically, of the ETL, HTL and electrode in the stability of OPV devices.

Understanding the degradation mechanisms in organic photovoltaics is crucial in order to develop stable organic semiconductors and robust device architectures. The rapid loss of efficiency, referred to as burn-in, is a major issue to be addressed. This study reports on the influence of the electron transport layer (ETLs) and UV light on the drop of open-circuit voltage (Voc) for P3HT:PC60BM-based devices. The results show that Voc loss is induced by the UV and, more importantly, that the ETL can amplify it, with TiOx yielding a stronger drop than ZnO. Using impedance spectroscopy (IS) and X-ray photoelectron spectroscopy (XPS), different degradation mechanisms were identified according to whether the ETL is TiOx or ZnO. For TiOx-based devices, the formation of an interface dipole was identified, resulting in a loss of the flat-band potential (Vfb) and, thus, of the Voc. For ZnO-based devices, chemical modifications of the metal oxide and active layer at the interface were detected, resulting in a doping of the active layer which impacts the Voc. This study highlights the role of the architecture and, more specifically, of the ETL in the severity of burn-in and degradation pathways.

A series of eight low band gap polymers based on benzodithiophene - diketopyrrolopyrrole (BDT-DPP) skeleton were designed with side chain variation on the benzodithiophene unit. The effect of different side chains, including -H, alkyl, alkoxy, and aryl moieties, over polymer stability is examined. Thin films were processed and submitted to photooxidative degradation. We experimentally confirm that alkoxy side chains are the most promising candidates for designing soluble and photo-resistant polymers. This conclusion is modulated by the polymer dispersity which also plays an important role: a low dispersity value being synonymous of higher photostability. The reasons why the side chain structure and dispersity value drive the polymer photostability are discussed. For the design of photo-resistant polymers and devices, side chain selection must go hand in hand with the control of the synthesis meaning a low dispersity.

X-ray Absorption Near Edge Structure (XANES) has been used to investigate the orientation of polymers in thin films. These polymers contain Sulphur and are examples of Low Band Gap (LBG) materials; consequently, they are of interest for use in Organic Photovoltaic (OPV) devices. The characterisation of such films and in particular the determination of molecular orientation is essential for assessing important properties in potential devices, such as the charge mobility, energy transfer or charge separation. XANES is particularly useful in this respect giving information about both the orientation and the electronic structure. The technique was used recently to empirically determine the orientation of polymers in thin films and the study of changes associated with different processing conditions (e.g. [12,13]). To obtain further insight into the near edge structure we use Density Functional Theory to calculate and fit to the experimental XANES spectrum and present the results for a few of the polymers studied. (C) 2016 Elsevier Ltd. All rights reserved.

The photooxidation of a polymer blend film used in efficient solar cells based on poly[(4,40-bis(2-ethylhexyl)dithieno[3,2-b:20,30- d]silole)-2,6-diylalt-(2,1,3-benzothiadiazole)-4,7-diyl], (Si-PCPDTBT) and [6,6]-phenyl-C71-butyric acid methyl (PC70BM) has been investigated. A set of experiments from complementary techniques was developed to monitor the modifications during ageing that occur not only in the bulk but also at the surface. The surface analyses were performed by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), and the bulk analyses by UV–visible spectroscopy and infrared (IR) spectroscopy. The results reveal that the silicon bridge atom is the first target of degradation. We have identified the existence of a photooxidation profile within the 300nm thick film. Such a heterogeneous degradation was confirmed by time-of-flight secondary ion mass spectrometry (TOF-SIMS) depth profiling.

An approach to improve the efficiency of donor-acceptor based bulk heterojunction (BHJ) organic solar cells is the use of low band gap (LBG) polymers as donor materials. The properties of these materials in BHJ solar cells are strongly influenced by their morphology and ability for self-organization in thin-films. We studied for two related polymer pairs the influence of the introduction of additional (hexyl-) thiophene moieties on both the electronic structure and ability for self-organization in thin films using Ultraviolet photoelectron spectroscopy (UPS) and Near-Edge X-Ray Absorption Fine Structure spectroscopy (NEXAFS). In addition, the influence of mixing with PCBM and post-processing annealing on the molecular orientation is investigated. We find that the introduction of additional (hexyl-) thiophene moieties in the polymer structure does not affect significantly the electronic structure of the polymers in thin films, but has a strong impact on their self-organization properties and consequently affects their behavior in devices.

The impact of polymer side-chains on encapsulated OPV device stability is studied systematically in a series of low bandgap polymers.

This word was devoted to the stability in various conditions of materials used in the active layer of organic solar cells. The main goals of this work were first to provide deeper understanding about complex mechanisms occurring in the active layer and second to investigate interfacial degradation pathways involving the active layer. A first part was dedicated to the photo and thermal stability of the polymer blend materials which constitute the active layer of the solar cells. A second section focused on the role of the third component of the active layer which can be undesired residual additives coming from the processing or the desired insertion of a stabilizer additive. A third part concerned the delamination issue which takes place at the active layer / hole transporting layer interface. Finally, a last section was devoted to the energy level alignment between the C60 molecules and various electron transporting substrates. The photo and thermal stability of the active layer / electron transporting layer interface was also studied in this section.

We have investigated the impact of residual additives such as diiodooctane (DIO) and octanedithiol (ODT) on the photostability of state of the art P3HT:PCBM active layers. A series of active layers prepared with and without additives as well as neat additives were submitted to light irradiation in ambient air and analyzed by UV-vis and IR spectroscopy. We show not only that residues are sensitive to the combined action of light and oxygen but also that their presence can dramatically impact the polymer blend stability. DIO molecules are highly sensitive to light and can directly saturate the polymer conjugated backbone or be trapped by the fullerene moieties. ODT molecules can be photooxidized and may accelerate the intrinsic photooxidation of the active layer. Another important result is that the additives impact is directly linked to the presence of a top layer above the active layer. The confinement makes that additives react within the active layer, and thus accelerate its photodegradation, rather than decomposing in the gas phase (irradiation without top layer). Thus, a light-soaking step before top layer deposition could allow a clean removing of additives without affecting the optimized morphology and polymer blend stability. This process would be easily adaptable to industrial scale production.

Bulk heterojunction (BHJ) photovoltaic devices made of PCDTBT (poly[ N ‐9′‐hepta‐decanyl‐2,7‐carbazole‐ alt ‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)]) and PC 70 BM ([6,6]‐phenyl‐C 70 ‐butyric acid methyl ester) are among the most efficient and stable devices studied so far. However, during a short regime called “burn‐in”, a significant decrease of power conversion efficiency was observed. A study of the photochemical mechanisms involved in the PCDTBT:PCBM active layer exposed to light in encapsulated systems is presented. It is found that the photochemical reactions resulting from the absorption of light by PCDTBT involve crosslinking between the 2,7 carbazole unit of PCDTBT and the fullerene unit of PCBM. Those reactions stabilize the BHJ by avoiding the formation of microsized PCBM crystals known to cause failure of BHJ solar cells. Using classical electron paramagnetic resonance spectroscopy (EPR) (without illumination), paramagnetic defects along the polymer chains have been detected. The kinetics of defects intensity show a burn‐in trend. The evolution of their relaxation times upon aging is in good agreement with a structural change (crosslinking) of the BHJ observed from the nanomechanical properties. Finally, light‐induced electron paramagnetic resonance (LEPR) measurements performed on aged samples revealed that electron transfer is not significantly affected upon aging, confirming thus the stabilization of the BHJ in solar cell operating conditions.

The molecular orientation of selected low band gap polymers for organic photovoltaics (Si-PCPDTBT, PCPDTBT, and PCPDTBBT) and the influence of PCBM on the orientation is studied using Near-Edge X-Ray Absorption Fine Structure spectroscopy (NEXAFS). The results are compared to the state of the art material P3HT. A clear angular dependence in the S-K NEXAFS spectra points to a preferred orientation of the π-conjugated system and thus to a high degree of ordering for most materials. Generally, the degree of ordering decreases after blending the polymer with the fullerene PCBM. In the case of P3HT and PCPDTBT, the degree of ordering could be improved considerably due to the annealing of the film. The annealing temperatures depend however on the considered polymer.

The preservation of organic polymer solar cell (PSC) performances over time is of significant concern for their commercial development. A prime PSC degradation pathway is due to inherently photo-unstable conjugated polymers in the photo-active layer which bear continuous illumination in the presence of atmospheric oxygen that diffuses across the encapsulation layers to the whole device. This paper reports on the unexpected photostability of a low band gap polymer, namely poly[(benzo[1,2-b:4,5-b′]dithiophene)-alt-(thieno[3,4-c]pyrrole-4,6-dione)] (PBDTTPD), designed for efficient bulk heterojunction PSCs. An approach based on joint computational and spectroscopic studies is implemented to explain the unexpectedly high resistance of PBDTTPD towards photo-oxidation. It is shown that alkoxy side-chains on benzo[1,2-b;3,4-b]dithiophene (BDT) subunits mitigate the photodegradation of the whole polymer. Furthermore, PBDTTPD favours well-organized structures which inhibit the propagation of the chain oxidation process. Last but not least, results suggest that PBDTTPD is a self-protecting polymer. The first main highlight of this study is that the structure-photostability relationship of conjugated polymers can be dependent on both the macromolecular structure and the morphology of the polymer deposits. The second highlight is that the choice of solubilizing side-chains is a critical factor in the design of stable conjugated polymers for efficient PSCs.

This predictive study demonstrates that the introduction of aromaticoxy-alkyl links surprisingly makes materials more resistant to photo-oxidative degradation by reducing hydrogen abstraction. This revelation makes it possible, for the first time, to design a toolbox of substituents for soluble, photostable conjugated materials.

In organic photovoltaic (OPV), the photoactive layer is the main part, where the energy conversion process takes place. In our work we focus on Si-PCPDTBT:PC70BM (1:2) photoactive blend layers, where Si-PCPDTBT and PC70BM are the poly[2,6-(4,4-bis(2-ethylhexyl)dithieno[3,2-b:2,3-d]silole)-aft-4,7-(2,1,3 benzothiadiazole)] and [6,6]-phenyl-C-71-butyric acid methyl ester, respectively. The mixture of these materials was deposited on the ITO/glass substrate and investigated using different analytical methods such as secondary ion mass spectrometry (SIMS), atomic force microscopy (AFM) and scanning electron microscopy (SEM) before and after the degradation. The different degree degradation of layers measured as % of UV absorbance loss was performed under the illumination at ambient air using a solar simulator (AM 1.5, Xenon arc lamp). SIMS depth profiles reveal a diffusion of atmospheric oxygen through the photoactive layer leading to the photooxidation of organic materials as unveiled from the distribution of selected ions fragments (SO-, NO-, SO2-, NO2- and CO-). AFM and SEM images of the non-degraded sample show a relatively smooth surface. SEM investigations of the degraded layers reveal a homogenous formation of dispersed fibril-like nano-objects at the surface. The formation of such nano-objects may explain the increase of the layer surface oxidation and volatilization during the long-term degradation. (C) 2014 Elsevier B.V. All rights reserved.