Recent Advances to Understand Morphology Stability of Organic Photovoltaics
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Organic photovoltaic devices are on the verge of commercialization with power conversion efficiencies exceeding 10 % in laboratory cells and above 8.5 % in modules. However, one of the main limitations hindering their mass scale production is the debatable inferior stability of organic photovoltaic devices in comparison to other technologies. Adequate donor/acceptor morphology of the active layer is required to provide carrier separation and transport to the electrodes. Unfortunately, the beneficial morphology for device performance is usually a kinetically frozen state which has not reached thermodynamic equilibrium. During the last 5 years, special efforts have been dedicated to isolate the effects related to morphology changes taking place within the active layer and compare to those affecting the interfaces with the external electrodes. The current review discusses some of the factors affecting the donor/acceptor morphology evolution as one of the major intrinsic degradation pathways. Special attention is paid to factors in the nano- and microscale domain. For example, phase segregation of the polymer and fullerene domains due to Ostwald ripening is a major factor in the microscale domain and is affected by the presence of additives, glass transition temperature of the polymers or use of crosslinkers in the active layer. Alternatively, the role of vertical segregation profile toward the external electrodes is key for device operation, being a clear case of nanoscale morphology evolution. For example, donor and acceptor molecules actually present at the external interfaces will determine the leakage current of the device, energy-level alignment, and interfacial recombination processes. Different techniques have been developed over the last few years to understand its relationship with the device efficiency. Of special interest are those techniques which enable in situ analysis being non-destructive as they can be used to study accelerated degradation experiments and some will be discussed here.
KeywordsOrganic photovoltaics Intrinsic degradation Morphology Thermal degradation Interface
Organic photovoltaic technology are on the verge of commercialization with power conversion efficiencies (PCE) exceeding 10 % in laboratory cells  and above 8.5 % in modules . A wide range of consumable applications may be reached, thanks to these PCE values and to the potentially low production costs and light weight. However, one of the main limitations hindering its mass scale production is the arguably low stability of organic photovoltaic. Stability requirements depend exclusively on the application where the organic photovoltaic (OPV) will be used. For example, requirements for futuristic applications like single-use/disposable electronics will be very mild with short degradation times in the range of months may be permissible. On the other hand, requirements for other applications like building integrated devices are highly demanding with a threshold of 10 % efficiency drop over a period of 20 years.
Significant efforts have been devoted to understand degradation processes of OPV and comprehensive reviews can be found in literatures [3, 4, 5, 6]. In general, degradation mechanisms can be divided into extrinsic and intrinsic factors. External agents such as water , oxygen , light [9, 10], or heat  that may trigger accelerated degradation pathways constitute extrinsic degradation sources. If these external factors are controlled, degradation kinetics of the devices can be reduced. For example, encapsulation/lamination of devices is the most common practice to create a physical barrier for water and oxygen permeation into the device. Similarly, the use of UV filters dramatically reduces the photodegradation of the active layer . Intrinsic degradation pathways include electrode diffusion, morphology evolution, or generation of charge transfer complexes between donor and acceptor molecules which act as source of photobleaching and recombination centers [12, 13].
Adequate donor/acceptor morphology of the active layer is required in the nanoscale to provide carrier separation and transport to the electrodes. Unfortunately, the beneficial morphology for device performance is usually a kinetically frozen state which has not reached thermodynamic equilibrium. For this reason, donor/acceptor morphology evolution during device operation is one of the major intrinsic degradation pathways and this topic will be discussed in detail in the present manuscript. Strictly speaking, degradation pathways due to light soaking and high temperature cannot be regarded as intrinsic processes. However, devices under operation conditions will always be under light and will suffer some degree of heating. Then, it is clear that these two factors can also be regarded as intrinsic factors. In general, light and heat induce morphology evolution of the active layer , interlayer and electrode diffusion , and electrode interaction with the organic materials .
The present manuscript aims at providing the latest developments to understand morphological degradation pathways present in most organic photovoltaics (OPVs). Recent advanced techniques to understand morphology evolution are presented. In addition, their use in some recent degradation studies is discussed.
2 Connection Between Performance Parameters and Morphology of the Active Layer
The first physical process that will limit the photocurrent is the optical properties of the active layer as it will ultimately determine the maximum photons that can be absorbed by the material. Without any doubt, the absorption is determined by the bulk properties of the active layer. For example, the optical density can be increased by increasing the thickness of the active layer or by creating an optical cavity [17, 18]. On the other hand, photobleaching is a commonly observed factor during degradation tests that gives rise to serious reduction in photocurrent. The relationship between optical properties of the active layer and photocurrent is probably the most intuitive and easy parameter to explain. However, from this point and due to the very complex nature of the donor and acceptor blend, efficiency will often be determined by a balance of different electrical processes: transport, recombination, and extraction of carriers.
Bulk morphology of the organic layer is undoubtedly one of the aspects most studied in OPV due to the high impact on device performance [19, 20]. Mixed donor/acceptor domains are required to provide optimum surface area to enable efficient charge separation . On the other hand, continuous phases are required to allow charge transport toward the electrodes . The combination of both factors will determine the final photocurrent of the device. Indeed, any reduction on the donor/acceptor surface area will proportionally decrease the photocurrent as it will decrease the photogeneration rate, for example, formation of fullerene aggregates in polymer:fullerene blends has been shown to consistently lead to a reduction in the observed J sc by increased geminate recombination [23, 24, 25]. On the other hand, enhanced crystallinity of the polymer phase has been correlated with high mobility  and improved device performance , leading to reduced charge recombination processes .
3 Effect of Morphology in the Nano- and Microscale
4 Morphological Stability
As discussed above, the electronic properties of organic photovoltaics are strongly influenced by various physical processes and one of the general aims in the field has been to correlate structure and function. Solar panels under operation conditions usually reach temperatures as high as 65–85 °C and thermal degradation is a key factor that needs to be controlled . Studies on highly efficient OPV devices show severe efficiency losses after a short operation time, being a morphological reorganization which is the main factor reducing the device efficiency . Different techniques have been developed to attempt to lock the nano-morphology and avoid generation of microscale domains under thermal stress as it will be discussed below. For example, high glass transition temperature polymers have been used with the aim to avoid fullerene diffusion at temperatures close to the glass transition temperature (T g) of polymer . It is important to note here that it is difficult to predict the thermal behavior of a bulk-heterojunction (BHJ) solar cell through the bare analysis of the T g of pristine materials due to several factors. For example, it is difficult to measure the T g of an amorphous polymer.
Regarding the morphological stability of different type of OPV devices by far the most studied systems regarding morphology stability by solution process are devices based on polymer donor and fullerene acceptor materials. However, it is important to highlight that efficiency of OPV containing non-fullerene acceptors, small-molecule donor material and all polymer solar cells have sharply progressed very recently. Morphological stability is likely to follow the same rules as those described for polymer:fullerene devices. However, examples on thermal degradation studies on these systems are rather scarce in the literature . To provide some guidelines on what expect for each system here we compare small molecule devices processed from high vacuum conditions with some solution-processed devices. Evaporated small molecules have shown superior thermal stability compared to polymer-based devices as they are actually processed at temperatures above 100 °C. In addition, these evaporated small molecules do not need long hydrocarbon side chains to confer solubility in solvents which typically would reduce the T g in the polymer analogues. Alternatively, devices based on solution process small molecules suffer from the need of these solubilizing groups which reduces the crystallinity and T g of the films. Therefore, it is expected that morphological stability to be lower for this last group and this may be the reason behind the lack of reported data for high efficiency devices.
4.1 Additives Effect
Importantly, small amounts of additive usually remain in the film and the effect on the device stability has not been much studied. In a recent study it has been shown that residual 1,8-diiodooctane (DIO) remains after vacuum electrode deposition and the presence of this additive shows a very negative impact on device stability . Indeed, devices containing DIO additive show improved initial device performance of 7.7 % as compared to devices not containing the additive 3.4 %. However, degradation decay using the protocol ISOS-D-1 (shelf life in air conditions) provided devices with similar efficiencies after 300 h, 3.5 versus 2.8 %. The authors claim that diffusion of O2 and H2O takes place differently depending on whether additive has been used or not. However, intrinsic morphology of degraded devices was not studied in detail.
Alternatively, residual DIO additive has been successfully removed by using an orthogonal solvent which is able to remove the additive not affecting the organic layer. This methodology seems to be general as it has been reported for 5 different polymers and morphological stability is enhanced as well as the efficiency and reproducibility after spin-coating of inert solvents . The orthogonal solvents must follow the following requirements: 1-Must not dissolve de organic layer, 2-Must be fully miscible with the additive, and 3-Swelling of the organic layer should be avoided as this would modify the morphology. In this regard different alcoholic solvents have been studied for the benzodithiophenes (BDT)-based polymers and the use of methanol seems to be the best choice regarding the photovoltaic performance compared to isopropanol .
4.2 Use of Crosslinkers
Latest examples include the work by Durrant et al. where illumination with UV light during film processing is shown to improve device stability due to oligomerization of PCBM molecules . Stability test were carried out during 20 h only and during this time efficiencies compared to the control were already very similar, 4.8 and 4.4 %, respectively. This result is not really surprising as oligomerization of PCBM promoted by light is probably a reversible reaction following Diels–Alder chemistry. Alternatively, specially designed fullerenes with crosslinking units can be introduced to lock the morphology conformation of the bulk of the active layer  or the interface with the electron selective layer . In addition, the crosslinking unit may be added in the donor material . Finally, unselective and highly reactive units such as azide crosslinkers can be used to create chemical bonds with several chemical functionalities .
The use of crosslinkers to lock the morphology may be regarded as a final solution. However, this approach usually shows some practical limitations which are often difficult to overcome. For example, after the expected crosslinking chemical reaction some unreacted units may still be present in the active layer which could be negative for the device performance. Indeed, introduction of molecules which do not participate in the charge generation and transport usually show a negative impact on the initial device performance. See for example the high efficiency system described by McCulloch et al. in which it is observed an efficiency drop from 7.0 to 5.7 % during the curing process . Also important is that the final device performance after thermally aging at 85 °C during 130 h using the azide crosslinker cannot be considered as high efficiency any longer (4.1 %). Therefore, it appears that more work in this direction is needed to provide a definite solution.
4.3 Thermal Stability Probed by Capacitance–Temperature
5 Recent Techniques to Probe the Contact/Active Layer Interface
The link between morphology modification and performance has usually been discussed in terms of the bulk properties of the active layer , but very little attention has been paid to the evolving properties of the active layer/outer contact interfaces in the nanoscale domain. For example, a recent study has shown that migration of polymer/fullerene molecules toward the electrodes during thermal aging correlates with a loss in V oc for degraded devices .
In spite of this recent work degradation studies to specifically understand burn-in of the device performance are very scarce in the literature and further work is needed in this direction. For example, it is of utmost importance to understand if these “light-induced” traps are somehow related to the charge transfer complexes described above or if the superior stability is related with a better morphology stability, i.e., the different materials study may give different vertical segregation profile depending on the nature of the polymer.
Indeed, charge extraction can be modified during the lifetime of the device by a migration of either polymer  or fullerene  adhering to the top contact to generate a skin layer. The overall effect is the formation of barriers for selective extraction of carriers or hindered transport regions depending on the device architecture. In addition, selectivity of the contact may be lost increasing the leakage current which can ultimately limit the achievable photocurrent [35, 69, 70].
Unfortunately, direct observation of the active layer/contact interface is not trivial and latest developments have been focused to obtain physical techniques to offer information at the interfacial level. Hence, one of the issues that still require intensive work is the development of physical tools to probe the specific mechanisms behind active layer intrinsic evolution and its relationship to device performance . In this section, we will provide a range of new techniques which certainly need to be used in the future to further understand the effect of the external interfaces.
5.1 Scanning Transmission Electron Microscopy Spectral Imaging
The effect of the incorrect vertical segregation profile has been studied by STEMSI in shelf life degradation studies and evolution of the morphology in the bulk and the effect of vertical segregation profile have been separated. The experiment was carried out in conditions to probe shelf life degradation of OPV by excluding extrinsic factors (i.e., light, heat, water and oxygen). Of particular relevance to study intrinsic stability is a variation of the protocol ISOS-D-1 in which devices are not stressed keeping them in the dark under nitrogen atmosphere and ambient temperature . A study completed using chloroform as solvent provides a kinetically frozen morphology even after a thermal treatment. Storage of the device in the dark and in the glovebox enables a morphology rearrangement which doubles the efficiency over a period of 1 year . As can be observed in Fig. 10a evolution of morphology in aged samples shows that demixing and distribution of domains does not change in agreement with absorption measurements Fig. 10c. On the other hand, a PCBM-rich layer is developed at the ZnO contact (Fig. 10b) in agreement with fullerene cathode coverage calculated by C–V measurements, a technique which will be discussed below. Interestingly, improvement in device efficiency is observed due to a modification of the selectivity of the cathode contact. In particular, j sc nearly doubles by a reduction of the leakage current and decreasing the recombination of carriers. Importantly, by carrying out impedance spectroscopy measurements if an incorrect vertical segregation profile is observed a feature in the low-frequency region is observed which can be attributed to the effect of an undesirable contact resistance.
5.2 Impedance Spectroscopy
5.3 Capacitance–Voltage Analysis
5.4 External Quantum Efficiency
5.5 Further Techniques to Test Contact Evolution
Several comprehensive reviews of advance physical techniques have been published in the field of OPV to understand morphology configurations . Emerging technologies such as helium ion microscopy provides high sensitivity and low beam-induced damage of the samples but equipment is not widely available . Tip-enhanced Raman spectroscopy (TERS) is another promising technique for nanoscale chemical analysis of surfaces. TERS exploits the effect that Raman signals of probed molecules can be enhanced by many orders of magnitude . Very high resolution can be obtained since it only depends on the apex size and shape. By reduction of the diameter, chemical information with resolution in the range of 10 nm is achievable. This creates possibilities for various potential applications in biology and materials science fields, which are summarized in a recent review article . Applicability of TERS to polymer blends and BHJs was demonstrated. Yeo et al. reported a 20 nm resolution for compositional profiling of a polyisoprene and polystyrene blend . For a P3HT:PCBM blend chemical contrast at 10 nm resolution was achieved by Wang et al. . In that work, P3HT-rich, PCBM-rich, and mixed regions as identified by Raman signals were compared with photoluminescence and topographic mapping of the layer surface.
6 Conclusions and Further Work Needed
The present manuscript discusses some of the advances witnessed over the last 5 years to better understand morphological degradation of OPVs. A range of new techniques have been developed to separate effects related to morphology changes in the bulk of the active layer to those related to the interfaces with the external contacts. Overall, all factors contributing to an increased morphology stability at all levels (bulk and vertically) will help to increase the device stabilities. The use of polymers benefiting from a high transition temperature can definitely improve the thermal stability in terms of bulk morphology. Importantly, more work is urgently needed to stabilize the contact interfaces, i.e., the use of crosslinking molecules or SAM at the contacts may help to increase the stability toward modification of the leakage currents during device operation. Importantly, it is yet to be found that crosslinkers not only improve the morphological stability but also allow the achievement of high efficiencies. Efficiency of OPV containing non-fullerene acceptors, small-molecule donor material, and all polymer solar cells have sharply progressed very recently. Morphological stability is likely to follow the same rules as those described in the present manuscript, but further data are needed.
This work was partially supported by FP7 European collaborative project SUNFLOWER (FP7-ICT-2011-7-contract No. 287594), the Spanish Ministerio de Economía y Competitividad (project MAT2013-47192-C3-1-R), and Generalitat Valenciana (project ISIC/2012/008 Institute of Nanotechnologies for Clean Energies). A.G. would like to thank the Spanish Ministerio de Economía y Competitividad for a Ramón y Cajal Fellowship (RYC-2014-16809).
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