Breakthroughs in Photonics and Energy

Toward bulk heterojunction polymer solar cells with thermally stable active layer morphology

[+] Author Affiliations
Ilaria Cardinaletti, Jurgen Kesters, Sabine Bertho, Bert Conings, Jan D’Haen, Milos Nesladek

Hasselt University, Institute for Materials Research (IMO-IMOMEC), Wetenschapspark 1, Diepenbeek 3590, Belgium

Fortunato Piersimoni

Universität Potsdam, Institut für Physik und Astronomie, Potsdam-Golm 14476, Germany

Laurence Lutsen

IMEC vzw, Associated Lab IMOMEC, Diepenbeek 3590, Belgium

Bruno Van Mele, Guy Van Assche

Vrije Universiteit, Physical Chemistry and Polymer Science, Faculty of Engineering Sciences, Brussels 1050, Belgium

Koen Vandewal

Technische Universität, Institut für Angewandte Photophysik, Dresden 01069, Germany

Alberto Salleo

Stanford University, Department of Materials Science and Engineering, Stanford, California 94305

Dirk Vanderzande, Wouter Maes, Jean V. Manca

Hasselt University, Institute for Materials Research (IMO-IMOMEC), Wetenschapspark 1, Diepenbeek 3590, Belgium

IMEC vzw, Associated Lab IMOMEC, Diepenbeek 3590, Belgium

J. Photon. Energy. 4(1), 040997 (Jun 10, 2014). doi:10.1117/1.JPE.4.040997
History: Received March 1, 2014; Revised April 11, 2014; Accepted May 5, 2014
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Abstract.  When state-of-the-art bulk heterojunction organic solar cells with ideal morphology are exposed to prolonged storage or operation at elevated temperatures, a thermally induced disruption of the active layer blend can occur, in the form of a separation of donor and acceptor domains, leading to diminished photovoltaic performance. Toward the long-term use of organic solar cells in real-life conditions, an important challenge is, therefore, the development of devices with a thermally stable active layer morphology. Several routes are being explored, ranging from the use of high glass transition temperature, cross-linkable and/or side-chain functionalized donor and acceptor materials, to light-induced dimerization of the fullerene acceptor. A better fundamental understanding of the nature and underlying mechanisms of the phase separation and stabilization effects has been obtained through a variety of analytical, thermal analysis, and electro-optical techniques. Accelerated aging systems have been used to study the degradation kinetics of bulk heterojunction solar cells in situ at various temperatures to obtain aging models predicting solar cell lifetime. The following contribution gives an overview of the current insights regarding the intrinsic thermally induced aging effects and the proposed solutions, illustrated by examples of our own research groups.

Figures in this Article

In the last decades, organic bulk heterojunction (BHJ) solar cells have gradually evolved from a purely academic research topic to a promising emerging technology with world-wide R&D activities. With the first commercial applications on the market (e.g., the solar bags of Neuber1) and efficiencies reaching >10%,2 the lifetime of the devices (predicted to be several years3) becomes important. In order to test the durability of solar cells during operation, a variety of stress factors must be considered, such as humidity and oxidizing agents in ambient atmosphere, incident ultraviolet (UV), and visible radiation, and the high temperatures reached after prolonged exposure to sunlight. Since gathering fundamental knowledge on these degradation processes represents an important step toward the realization of more durable devices, various laboratories are investigating them and have agreed on testing protocols that allow for an easier comparison of results among different groups.4 Regarding the failure mechanisms in BHJ organic photovoltaics, several review papers can be found.57 Herein, we restrict ourselves to a brief overview of some of the most reported causes of losses in power conversion efficiencies (Table 1).

Table Grahic Jump Location
Table 1Extrinsic and intrinsic degradation mechanisms in organic solar cells.

Reversible degradation can occur due to oxygen doping of some metal oxides, widely employed as interfacial layers between the BHJ and the cathode.7 On the other hand, exposure to light accelerates the reversible p-doping of the polymer as it absorbs oxygen from the atmosphere, with a resulting decrease in short-circuit current density (Jsc).6,10 Alongside these effects, other mechanisms lead to irreversible degradation and fatal device failure. The ingress of water into the device can cause fast oxidation of the low-work function metal cathode (Yb, Ca, Al).8 Moreover, oxygen was found to promote the rupture of the π–conjugated backbone in the electron donor polymer within the active blend under UV illumination, with a subsequent drop in absorbance. This phenomenon is less effective in the presence of a fullerene derivative,6 such as the most widely used acceptor material, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM).

Some of these issues can be prevented by appropriate encapsulation14 and can, thus, be considered as extrinsic degradation mechanisms.15,16 Next to extrinsic failures, a variety of intrinsic failure mechanisms, linked directly to the properties of the materials used (as further defined in 16), may also occur. Table 1 provides a selection of the important extrinsic and intrinsic degradation phenomena observed in organic solar cells.

In this contribution, we focus on one particular intrinsic failure mechanism: the thermally induced disruption of the active layer blend (nano) morphology of polymer:fullerene BHJ organic solar cells (Fig. 1). During thermal annealing, the BHJ system gains energy, which allows it to order itself in a more thermodynamically stable way. This leads to a thermally induced separation of the polymer and fullerene phases. As a result, small PCBM crystals tend to group into larger crystalline domains, a phenomenon called Ostwald ripening.14,17 The formation of these PCBM-rich features leads to a decrease of the interfacial area between donor and acceptor and, thus, to a diminution of exciton dissociation and of the number of percolation paths to the electrodes.6 In this phase separation phenomenon, the glass transition temperature (Tg) of the blend plays an important role.18 Above Tg, the blend component molecules become mobile, which allows diffusion and clustering of the fullerene derivatives.

Graphic Jump LocationF1 :

Schematics of the evolution of fullerene aggregation in thermally annealed bulk heterojunction organic solar cells.

Better insights regarding the nature of the phase separation process have been obtained through a variety of characterization methods. In the next paragraph, we provide a short overview of the analytical, thermal analysis and electro-optical techniques used to investigate the morphology of BHJ active layers at their initial stage as well as during and after thermal stress. Subsequently, the effects of thermally induced morphological changes on the photovoltaic properties are described. Models to predict the impact of structural degradation are introduced, together with analyses of its impact on the durability of the devices. Finally, we introduce various routes proposed to improve the thermal stability of the active layer blend morphology, such as the use of high-Tg polymers, the addition of functionalized/thermo-cleavable side-chains, cross-linking, or the photo-induced dimerization of the fullerene component.

Investigation of the Active Layer Morphology

The stability of the active layer blend morphology is crucial for the long-term operation of the solar cells. Optimal intermixing requires that the distance between the domain interfaces of the polymer and fullerene materials does not exceed the exciton diffusion length, typically in the order of 10 nm.15 A variety of techniques have been employed to determine on which length scale the intermixing occurs. Optical microscopy can be used to study the presence of large structures in the active layer film. Information on their height is obtained via stylus profilometry or atomic force microscopy (AFM) for micro- and nanoscale features, respectively.19,20 AFM represents a valuable tool to analyze the film topography,21 and it also enables one to distinguish structures from the different components in the blend through peak force tapping22 and conductive-AFM.22,23 Scanning electron microscopy (SEM)24 and transmission electron microscopy (TEM)20 techniques are used to image thin layers of organic semiconductors as well. Electron tomography reconstructions of three-dimensional volumes,25 together with cross-sectional SEM and TEM,6 allow the investigation of the vertical disposition of donor-acceptor domains throughout a layer2,25 or throughout a cross-section of a device.21Figure 2 (top row) shows an example of the evolution of the morphology of a poly[2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV): PCBM (14 wt%) blend after various durations of thermal annealing at 110°C (above the Tg of the blend), as visualized by TEM. It is clearly observed that the initially finely dispersed phases rapidly evolve toward the formation of aggregated structures, identified as PCBM domains. Due to the thermal treatment, the PCBM molecules diffuse out of the originally intermixed photoactive layer and aggregate into large domains. This Ostwald ripening (Fig. 1) is observed for numerous polymer:fullerene blends.20,21,24

Graphic Jump LocationF2 :

Transmission electron microscopy (TEM) images of poly[2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV): [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) (14) (top) and high-Tg PPV: PCBM (bottom) blend films after various annealing times at 110°C. The selected-area electron diffraction (SAED) insets show the crystalline nature of the aggregates. Reprinted from 26, with permission from Elsevier.

Diffraction studies can also provide information on ordered structures in BHJ films. Selected-area electron diffraction (SAED) allows attributing the observed structures to specific blend components.20,27 X-ray diffraction measurements enable determination of the nature of the crystalline structures.23 In grazing-incidence geometry, information is obtained on the ordering and orientation of crystallites and crystalline planes in a thin film2,27 or on the stacking of the π-chains in largely amorphous materials.28 As an illustration of the use of diffraction techniques in the study of thermally induced Ostwald ripening in BHJ organic solar cells, the insets of Fig. 2 (top row) show the crystalline nature of the PCBM aggregates formed during thermal annealing, as identified via SAED.

Together with UV-visible absorption29,30 and Raman spectroscopy,29 these microscopy and diffraction techniques can be used to determine the initial morphology of BHJ active layers21 as well as the changes in the blend structures due to annealing treatments23,27 or aging of the film.6,14

Thermal Analysis

It is nowadays recognized that a physicochemical approach based on dedicated thermal analysis protocols can strongly contribute to an increased understanding of (nano) morphology development and stabilization in BHJ organic solar cell blends. Calorimetric techniques, such as differential scanning calorimetry (DSC),3135 modulated DSC (32), and especially rapid heat-cool calorimetry,18 allow characterization of both ordered and disordered material phases and determination of phase/state diagrams. The state diagram for the donor-acceptor system is of primordial importance for a systematic understanding of the phase-separation behavior and for designing appropriate thermal annealing and accelerated aging procedures (see also Sec. 3.1). In this respect, the crystallization kinetics of the crystallizable component(s) as a function of temperature in the temperature window of interest is indispensable.18 In the state diagram of poly(3-hexylthiophene) (P3HT):PCBM, for example, the glass transition of PCBM (ca. 140°C) and the role of mobility restrictions by (partial) vitrification was demonstrated.32 The results reported are in line with actual statements regarding the importance of disorder and the amorphous phase in the active layer.36 In addition, the state diagram also permits the detection of the presumed eutectic phase behavior, with a finely intermixed morphology of donor and acceptor constituents, which might lead to an efficient thermal procedure to predict/propose mixing conditions of new blends for optimum solar cell performance.31,3335

As illustrated above, annealing at high temperatures promotes morphological rearrangement in BHJ blends because of the available thermal energy. The new phase distribution can have specific effects on the final photovoltaic performance of the devices, depending on the nature of the components. When a partially crystalline polymer (such as P3HT) is mixed with PCBM, an increase in hole mobility is reached by thermal annealing, as the latter promotes a strengthening of the crystalline character of the polymer.23 Concurrently, high temperatures induce PCBM aggregation. As a consequence, systems of this sort show an optimal annealing approach, which allows for high mobility in the crystalline polymer, but beyond which the formation of large PCBM domains causes a decrease in performance.23 On the other hand, when there is no ideal crystalline structure to be reached, as in the case of largely amorphous polymers, thermal annealing above the Tg of the blend leads to the formation of fullerene clusters only.17,37 In the following section, it will be shown that these thermally induced morphological changes influence the short-circuit current density, the fill factor (FF), and the open-circuit voltage (Voc), thus altering the final photovoltaic energy conversion efficiency.

Effect of Morphology Degradation on Jsc

The formation of aggregates of fullerene derivatives in BHJ films leads to a deterioration of the Jsc of the devices, partially due to a reduction of the interfacial area between the donor and acceptor domains, and partially due to the diminution of the photoactive area.38 For organic BHJ solar cells, it has been reported by several authors that the short-circuit current density (Jsc) decreases upon prolonged annealing and that the degradation occurs at a faster rate when higher annealing temperatures are applied.9,26 In blends of MDMO-PPV with PCBM, the degradation curves for Jsc have been fitted with Eq. (1) to reflect the Ostwald ripening occurring in the active layer during annealing17Display Formula

Jsc(t)=Jsc()+Jsc(0)exp(kdegt).(1)

In Eq. (1), t represents the annealing time and kdeg=Aexp(Ea/kBT) is a rate constant that characterizes how fast the degradation evolves, with Ea the activation energy in eV, kB the Boltzmann constant (8.62*105eVK1), and A, a constant that depends on the degradation mechanisms and the experimental conditions. Figure 3 shows the fitted curves (solid lines) and the measured decay in short circuit current at different annealing temperatures.

Graphic Jump LocationF3 :

Simulated curves matching well with the drop in short-circuit current (Isc) of MDMO-PPV: PCBM solar cells upon thermal annealing. Reproduced with permission from 17.

A similar approach was proposed to predict the lifetime of devices using the Lipshitz-Slyozov theory for spinodal decomposition, which predicts domain-size (R) evolution following Rtn, with n a material-dependent exponent,39 stressing the strong relationship between the employed polymer:fullerene system and its inherent rate of morphological degradation.

The link between the rearrangement of the active blend’s structure on the nanoscale and the drop in Jsc has also been investigated by Schaffer et al.38 through the acquisition of IV characteristics and in situ micro-focused grazing incidence small-angle x-ray scattering (μGISAXS) patterns of P3HT:PCBM devices during operation. They documented the change in P3HT domain size and density within the active layer blend matrix and introduced a model to quantitatively determine the Jsc of a device from the recorded morphology data by correlating the active interfacial area between the two species in the blend with the dimension of the structures, as measured with μGISAXS.

Effect of Morphological Degradation on Other Photovoltaic Parameters

Thermal annealing not only has an effect on Jsc, but it also influences the open-circuit voltage and FF. It was reported that the nanomorphology of the active layer plays a significant role in the determination of Voc40,41 due to the correlation between the latter and the energy (ECT) of the interfacial donor-acceptor charge transfer complex (CTC).42,43ECT can be determined through modeling the sub-bandgap photocurrent spectra,44 as measured by Fourier-transform photocurrent spectroscopy, an ultrasensitive technique that allows one to measure the external quantum efficiency over nine orders of magnitude.45 Given the reported linear relation between ECT and Voc, the variation of ECT during the aging process was studied to understand the origin of the drop in Voc upon morphological reorganization.41,43,46,47 Thermally annealed MDMO-PPV:PCBM devices showed a reduction of Voc and ECT, with the PCBM photocurrent peak at 1.74 eV becoming more prominent due to the formation of PCBM aggregates and the amount of interfacial CTCs decreasing. This indicates less interaction between the donor and acceptor and, thus, a more phase-separated BHJ blend morphology.35 A more detailed discussion on the effect of structural factors on Voc, ECT, and photovoltaic performance in organic solar cells is given in a recent paper by Vandewal et al.48

FF is also observed to change in relation to reorganizations of the materials at the nanoscale.23 The optimal morphology of the BHJ photoactive layer blends ensures good percolation paths from the exciton dissociation site to the collecting electrodes.49 When the mixing of donor and acceptor domains is not optimal any more, as in the case of thermally degraded devices, the number of percolation paths is reduced and the series resistance of the devices increases, which leads to a decrease of FF.49

Thermally induced structural changes in BHJ active layers at operating temperatures represent an intrinsic limit to the employability of certain material systems in outdoor conditions,15 in which temperatures >100°C can be reached (temperatures up to 175°C were measured for silicon modules in Arizona50). For this reason, we dedicate this section to some of the methods that have been proposed to improve the morphological stability of photoactive material blends.6

A careful choice of the donor-acceptor ratio51 and the casting solvent can result in not only more efficient,52 but also more durable organic solar cells.30 Depositing photoactive layers via different solvents was demonstrated to dictate the Tg and the ordering of the π-π stacking in amorphous poly[N-9’’-heptadecanyl-2,7-carbazole-alt-5,5-(4’,7’-di-2-thienyl-2’,1’,3’-benzothiadiazole)] (PCDTBT):[6,6]-phenyl-C71-butyric acid methyl ester (PC70BM) blends.37,53 This system, stable at outdoor solar cell operating temperatures, thanks to its high Tg (130°C),37 needs to be optimized through processing conditions52 or solvent annealing54 and has been determined to reach an extrapolated lifetime of seven years.3 Removing residual solvents by treating the layers with methanol also proved to reduce thermally induced fullerene aggregation.55 Additionally, a lower regioregularity and higher purity of the donor polymers, as well as the addition of block copolymer compatibilizers, has also been demonstrated to result in more stable films.5,5658

It was shown that employing polymers with a Tg higher than the temperature agreed on for standard testing [85°C (4)] helps to slow down PCBM clustering over the lifetime of the solar cells, due to the high rigidity of the polymer chains in said conditions.6,26,59 An example is shown in Fig. 2 (bottom row), where TEM analysis indicates that the employment of a high-Tg PPV polymer results in a more thermally stable blend as compared to standard MDMO-PPV. A similar comparison was reported by Vandenbergh et al. (poly[2-methoxy-5-(2’-phenylethoxy)-1,4-phenylene vinylene] versus MDMO-PPV).60 A high Tg also ensures stability of the electronic properties of (semicrystalline) polymers.53

A number of research groups have investigated the effect of polymer side-chain substitution on the blend stability.6165 Kesters et al.66 showed that the introduction of a small amount (5 to 15%) of functionalized side-chains in poly(3-alkylthiophene) (P3AT) copolymers results in a considerably increased blend stability upon prolonged thermal stress. Devices employing functionalized P3AT copolymers in blends with PCBM showed initial efficiencies comparable to regular P3HT:PCBM solar cells and drastically reduced fullerene aggregation at the ISOS-3 standard testing temperature of 85°C for a timescale of 700 h.4Figure 4 illustrates TEM images of functionalized P3AT copolymer:PCBM blends after different thermal annealing times (in comparison with standard P3HT:PCBM). SAED allowed identifying the formed clusters as PCBM crystals.

Graphic Jump LocationF4 :

TEM images and SAED patterns of polymer (P2 to P4): PCBM (11) blends degraded at high temperature. Reprinted with permission from 66.

The use of block copolymers containing both donor and acceptor domains has also been proposed as a strategy to improve photoactive layer stability,67 although their synthesis remains challenging.7 In these compounds, the donor and acceptor phases are covalently linked, allowing the material to be employed in single-component organic solar cells with stable morphology. Furthermore, rigid polymers can also be obtained by thermal cleaving of the side-chains, introduced to ensure good solubility but, in principle, not needed for optoelectronical functionality. This strategy has also been proven to lead to morphologically stable BHJ films.59

Another popular strategy uses cross-linking of the active materials, either via polymer or fullerene component, to freeze in the optimal morphology. This can, for instance, be achieved by adding cross-linkable groups to the polymer side-chains68 and curing the film with UV light. The employment of a cross-linked fullerene derivative, aligned in vertical nanorods, achieving a (quasi)ordered organic/organic nanostructure, allowed for an improved thermal stability, as well.69

Modifying the nature or the number of side-chains on the fullerene results in more amorphous compounds,55,70 which can also lead to an improved morphological stability. Liao et al. were able to exploit supramolecular interactions to impose a given order to the BHJ film and ensure structural firmness.71 They introduced a fullerene derivative, [6,6]-phenyl-C 61-butyric acid pentafluorophenyl ester (PCBPF), in which the methyl group of PCBM is substituted by a pentafluorophenyl ring, and then exploited the known interaction between C60 and the pentafluorophenyl moiety to produce solar cells with enhanced high-temperature stability.

Piersimoni et al.20 and Li et al.72 demonstrated the stabilizing effect of fullerene photodimerization on the active layer blend morphology. Light soaking of polymer:PCBM films before or during annealing above the Tg of the blend resulted in reduced fullerene aggregation, although the effect becomes less appreciable with increasing temperatures20 due to thermally induced dissociation of the dimers. These experiments were conducted using either PCBM or PC70BM, in combination with MDMO-PPV, P3HT, and PCDTBT as donor polymers (annealing the latter at higher temperature because of the elevated Tg of the polymer). Optical microscopy images, shown in Fig. 5, confirmed that thermal aging in the dark promotes the formation of clusters, attributed to the PCBM, while the same treatment under illumination does not significantly affect the morphology of the BHJ layers. While these images prove the stabilizing effect of this photoreaction on the blends’ morphology, its impact on the performance of devices resulted strongly system-dependent. Indeed, it can contribute to raising the Jsc in PCDTBT-based solar cells,72 but it was also reported by other authors to be responsible for performance degradation when combined with a bithiophene-co-thiazolothiazole push-pull copolymer.73

Graphic Jump LocationF5 :

Comparison between optical reflection microscopy images of as-cast films (top row), films annealed at 180°C (b), and 110°C [(e) and (h)] in the dark, and films annealed at the same temperatures under solar illumination. Reproduced with permission from 20.

Prolonged storage or operation of polymer:fullerene BHJ organic solar cells can result in athermally induced disruption of the optimal photoactive layer morphology via phase separation and Ostwald ripening of the fullerene domains, leading to a significant reduction of the photovoltaic performance. Improving the morphological stability of the photoactive blends upon thermal stress is, therefore, an important challenge for future commercialization of these devices. We briefly reviewed the degradation mechanisms resulting in performance failure, as well as the main techniques employed to characterize the thermally induced changes in the active layer (nano)morphology. The most obvious indication for BHJ structural damage is the formation of large crystalline domains of the fullerene derivative, with a consequent reduction of the interfacial area between the donor and acceptor domains, and a consequent negative effect on the photovoltaic parameters. Insights on the underlying mechanisms provide the foundation to model the effect of this structural reorganization on the photovoltaic parameters, allowing the prediction of device lifetimes and degradation rates.

Toward more intrinsically stable BHJ organic solar cells, several roads have been proposed and are still under investigation, regarding the choice of materials, the exploitation of observed phenomena, and the modification of compounds. The reviewed solutions will undoubtedly continue to find their way toward the development of stable organic photovoltaic devices. On the other hand, dedicated thermal analysis of the blends, as well as accurate investigation of the morphological changes in degraded photoactive layers, can be regarded as valuable tools in the direction of intrinsically stable organic solar cells.

The authors would like to acknowledge the Interreg-project ORGANEXT and the Fund for Scientific Research, Flanders (Belgium) (FWO) for the financial support.

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Ray  B., Alam  M. A., “A compact physical model for morphology induced intrinsic degradation of organic bulk heterojunction solar cell,” Appl. Phys. Lett.. 99, (3 ), 033303  (2011), CrossRef. 0003-6951 
Piersimoni  F. et al., “Influence of fullerene ordering on the energy of the charge-transfer state and open-circuit voltage in polymer: fullerene solar cells,” J. Phys. Chem. C. 115, (21 ), 10873 –10880 (2011), CrossRef. 1932-7447 
Vandewal  K. et al., “The relation between open-circuit voltage and the onset of photocurrent generation by charge-transfer absorption in polymer: fullerene bulk heterojunction solar cells,” Adv. Funct. Mater.. 18, (14 ), 2064 –2070 (2008), CrossRef. 1616-3028 
Tvingstedt  K. et al., “Electroluminescence from charge transfer states in polymer solar cells,” J. Am. Chem. Soc.. 131, (33 ), 11819 –11824 (2009), CrossRef. 0002-7863 
Hallermann  M., Haneder  S., Da Como  E., “Charge-transfer states in conjugated polymer/fullerene blends: below-gap weakly bound excitons for polymer photovoltaics,” Appl. Phys. Lett.. 93, (5 ), 053307  (2008), CrossRef. 0003-6951 
Vandewal  K. et al., “Relating the open-circuit voltage to interface molecular properties of donor: acceptor bulk heterojunction solar cells,” Phys. Rev. B. 81, (12 ), 12520  (2010), CrossRef. 0163-1829 
Vandewal  K. et al., “Fourier-transform photocurrent spectroscopy for a fast and highly sensitive spectral characterization of organic and hybrid solar cells,” Thin Solid Films. 516, (20 ), 7135 –7138 (2008), CrossRef. 0040-6090 
Goris  L. et al., “Observation of the subgap optical absorption in polymer-fullerene blend solar cells,” Appl. Phys. Lett.. 88, (5 ), 052113  (2006), CrossRef. 0003-6951 
Rand  B. P., Burk  D. P., Forrest  S. R., “Offset energies at organic semiconductor heterojunctions and their influence on the open-circuit voltage of thin-film solar cells,” Phys. Rev. B. 75, (11 ), 115327  (2007), CrossRef. 0163-1829 
Vandewal  K., Himmelberger  S., Salleo  A., “Structural factors that affect the performance of organic bulk heterojunction solar cells,” Macromolecules. 46, (16 ), 6379 –6387 (2013), CrossRef. 0024-9297 
Ray  B., Alam  M. A., “Random vs regularized OPV: limits of performance gain of organic bulk heterojunction solar cells by morphology engineering,” Sol. Energy Mater. Sol. Cells. 99, , 204 –212 (2012), CrossRef. 0927-0248 
Oh  J., TamizhMani  G., “Temperature testing and analysis of PV modules per UL 1703 and IEC 61730 standards,” in  35th IEEE Photovoltaics Specialists Conf. , pp. 000984 –000988 (2010).
Bertho  S. et al., “Improved thermal stability of bulk heterojunctions based on side-chain functionalized poly(3-alkylthiophene) copolymers and PCBM,” Sol. Energy Mater. Sol. Cells. 110, , 69 –76 (2013), CrossRef. 0927-0248 
Alem  S. et al., “Effect of mixed solvents on PCDTBT: PC70BM based solar cells,” Org. Electron.. 12, (11 ), 1788 –1793 (2011), CrossRef. 1566-1199 
Cho  S. et al., “A thermally stable semiconducting polymer,” Adv. Mater.. 22, (11 ), 1253 –1257 (2010), CrossRef. 0935-9648 
Gholamkhass  B., Servati  P., “Solvent-vapor induced morphology reconstruction for efficient PCDTBT based polymer solar cells,” Org. Electron.. 14, (9 ), 2278 –2283 (2013), CrossRef. 1566-1199 
Wang  T., Pearson  A. J., Lidzey  D. G., “Correlating molecular morphology with optoelectronic function in solar cells based on low band-gap copolymer: fullerene blends,” J. Mater. Chem. C. 1, (44 ), 7266 –7293 (2013), CrossRef. 0959-9428 
Mateker  W. R. et al., “Improving the long-term stability of PBDTTPD polymer solar cells through material purification aimed at removing organic impurities,” Energy Environ. Sci.. 6, (8 ), 2529 –2537 (2013), CrossRef. 1754-5692 
Sivula  K. et al., “Amphiphilic diblock copolymer compatibilizers and their effect on the morphology and performance of polythiophene: fullerene solar cells,” Adv. Mater.. 18, (2 ), 206 –210 (2006), CrossRef. 0935-9648 
Ebadian  S. et al., “Effects of annealing and degradation on regioregular polythiophene-based bulk heterojunction organic photovoltaic devices,” Sol. Energy Mater. Sol. Cells. 94, (12 ), 2258 –2264 (2010), CrossRef. 0927-0248 
Bundgaard  E. et al., “Advanced functional polymers for increasing the stability of organic photovoltaics,” Macromol. Chem. Phys.. 214, (14 ), 1546 –1558 (2013), CrossRef. 1022-1352 
Vandenbergh  J. et al., “Thermal stability of poly[2-methoxy-5-(2′-phenylethoxy)-1,4-phenylenevinylene] (MPE-PPV): fullerene bulk heterojunction solar cells,” Macromolecules. 44, (21 ), 8470 –8478 (2011), CrossRef. 0024-9297 
Lei  T., Wang  J.-Y., Pei  J., “Roles of flexible chains in organic semiconducting materials,” Chem. Mater.. 26, (1 ), 594 –603 (2014), CrossRef. 0897-4756 
Campo  B. J. et al., “Ester-functionalized poly(3-alkylthiophene) copolymers: synthesis, physicochemical characterization and performance in bulk heterojunction organic solar cells,” Org. Electron.. 14, (2 ), 523 –534 (2013), CrossRef. 1566-1199 
Tanenbaum  D. M. et al., “The ISOS-3 inter-laboratory collaboration focused on the stability of a variety of organic photovoltaic devices,” RCS Adv.. 2, (3 ), 882 –893 (2012), CrossRef. 2046-2069 
Andreasen  B. et al., “TOF-SIMS investigation of degradation pathways occurring in a variety of organic photovoltaic devices—the ISOS-3 inter-laboratory collaboration,” Phys. Chem. Chem. Phys.. 14, (33 ), 11780 –11799 (2012), CrossRef. 1463-9076 
Teran-Escobar  G. et al., “On the stability of a variety of organic photovoltaic devices by IPCE and in situ IPCE analyses—the ISOS-3 inter-laboratory collaboration,” Phys. Chem. Chem. Phys.. 14, (33 ), 11824 –11845 (2012), CrossRef. 1463-9076 
Kesters  J. et al., “Enhanced intrinsic stability of the bulk heterojunction active layer blend of polymer solar cells by varying the polymer side chain pattern,” Org. Electron.. 15, (2 ), 549 –562 (2014), CrossRef. 1566-1199 
Boudreault  P. R. T., Najari  A., Leclerc  M., “Processable low-bandgap polymers for photovoltaic applications,” Chem. Mater.. 23, (3 ), 456 –469 (2011), CrossRef. 0897-4756 
Carlé  J. E. et al., “Comparative studies of photochemical cross-linking methods for stabilizing,” J. Mater. Chem.. 22, (46 ), 24417  (2012), CrossRef. 0959-9428 
Chang  C.-Y. et al., “Enhanced performance and stability of a polymer solar cell by incorporation of vertically aligned, cross-linked fullerene nanorods,” Angew. Chem.. 50, (40 ), 9386 –9390 (2011), CrossRef. 0044-8249 
Kim  S.-O. et al., “Thermally stable organic bulk heterojunction photovoltaic cells,” Sol. Energy Mater. Sol. Cells. 95, (2 ), 432 –439 (2011), CrossRef. 0927-0248 
Liao  M.-H. et al., “Morphological stabilization by supramolecular perfluorophenyl-C 60 interactions leading to efficient and thermally stable organic photovoltaics,” Adv. Funct. Mater.. 24, (10 ), 1418 –1429 (2013), CrossRef. 1616-3028 
Li  Z. et al., “Performance enhancement of fullerene-based solar cells by light processing,” Nat. Commun.. 4, , 2227  (2013), CrossRef.
Distler  A. et al., “The effect of PCBM dimerization on the performance of bulk heterojunction solar cells,” Adv. Energy Mater.. 4, (4 ), 1300693  (2014), CrossRef. 1614-6840 

Biographies of the authors are not available.

© The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

Citation

Ilaria Cardinaletti ; Jurgen Kesters ; Sabine Bertho ; Bert Conings ; Fortunato Piersimoni, et al.
"Toward bulk heterojunction polymer solar cells with thermally stable active layer morphology", J. Photon. Energy. 4(1), 040997 (Jun 10, 2014). ; http://dx.doi.org/10.1117/1.JPE.4.040997


Figures

Graphic Jump LocationF1 :

Schematics of the evolution of fullerene aggregation in thermally annealed bulk heterojunction organic solar cells.

Graphic Jump LocationF2 :

Transmission electron microscopy (TEM) images of poly[2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV): [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) (14) (top) and high-Tg PPV: PCBM (bottom) blend films after various annealing times at 110°C. The selected-area electron diffraction (SAED) insets show the crystalline nature of the aggregates. Reprinted from 26, with permission from Elsevier.

Graphic Jump LocationF3 :

Simulated curves matching well with the drop in short-circuit current (Isc) of MDMO-PPV: PCBM solar cells upon thermal annealing. Reproduced with permission from 17.

Graphic Jump LocationF4 :

TEM images and SAED patterns of polymer (P2 to P4): PCBM (11) blends degraded at high temperature. Reprinted with permission from 66.

Graphic Jump LocationF5 :

Comparison between optical reflection microscopy images of as-cast films (top row), films annealed at 180°C (b), and 110°C [(e) and (h)] in the dark, and films annealed at the same temperatures under solar illumination. Reproduced with permission from 20.

Tables

Table Grahic Jump Location
Table 1Extrinsic and intrinsic degradation mechanisms in organic solar cells.

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Schaffer  C. J. et al., “A direct evidence of morphological degradation on a nanometer scale in polymer solar cells,” Adv. Mater.. 25, (46 ), 6760 –6764 (2013), CrossRef. 0935-9648 
Ray  B., Alam  M. A., “A compact physical model for morphology induced intrinsic degradation of organic bulk heterojunction solar cell,” Appl. Phys. Lett.. 99, (3 ), 033303  (2011), CrossRef. 0003-6951 
Piersimoni  F. et al., “Influence of fullerene ordering on the energy of the charge-transfer state and open-circuit voltage in polymer: fullerene solar cells,” J. Phys. Chem. C. 115, (21 ), 10873 –10880 (2011), CrossRef. 1932-7447 
Vandewal  K. et al., “The relation between open-circuit voltage and the onset of photocurrent generation by charge-transfer absorption in polymer: fullerene bulk heterojunction solar cells,” Adv. Funct. Mater.. 18, (14 ), 2064 –2070 (2008), CrossRef. 1616-3028 
Tvingstedt  K. et al., “Electroluminescence from charge transfer states in polymer solar cells,” J. Am. Chem. Soc.. 131, (33 ), 11819 –11824 (2009), CrossRef. 0002-7863 
Hallermann  M., Haneder  S., Da Como  E., “Charge-transfer states in conjugated polymer/fullerene blends: below-gap weakly bound excitons for polymer photovoltaics,” Appl. Phys. Lett.. 93, (5 ), 053307  (2008), CrossRef. 0003-6951 
Vandewal  K. et al., “Relating the open-circuit voltage to interface molecular properties of donor: acceptor bulk heterojunction solar cells,” Phys. Rev. B. 81, (12 ), 12520  (2010), CrossRef. 0163-1829 
Vandewal  K. et al., “Fourier-transform photocurrent spectroscopy for a fast and highly sensitive spectral characterization of organic and hybrid solar cells,” Thin Solid Films. 516, (20 ), 7135 –7138 (2008), CrossRef. 0040-6090 
Goris  L. et al., “Observation of the subgap optical absorption in polymer-fullerene blend solar cells,” Appl. Phys. Lett.. 88, (5 ), 052113  (2006), CrossRef. 0003-6951 
Rand  B. P., Burk  D. P., Forrest  S. R., “Offset energies at organic semiconductor heterojunctions and their influence on the open-circuit voltage of thin-film solar cells,” Phys. Rev. B. 75, (11 ), 115327  (2007), CrossRef. 0163-1829 
Vandewal  K., Himmelberger  S., Salleo  A., “Structural factors that affect the performance of organic bulk heterojunction solar cells,” Macromolecules. 46, (16 ), 6379 –6387 (2013), CrossRef. 0024-9297 
Ray  B., Alam  M. A., “Random vs regularized OPV: limits of performance gain of organic bulk heterojunction solar cells by morphology engineering,” Sol. Energy Mater. Sol. Cells. 99, , 204 –212 (2012), CrossRef. 0927-0248 
Oh  J., TamizhMani  G., “Temperature testing and analysis of PV modules per UL 1703 and IEC 61730 standards,” in  35th IEEE Photovoltaics Specialists Conf. , pp. 000984 –000988 (2010).
Bertho  S. et al., “Improved thermal stability of bulk heterojunctions based on side-chain functionalized poly(3-alkylthiophene) copolymers and PCBM,” Sol. Energy Mater. Sol. Cells. 110, , 69 –76 (2013), CrossRef. 0927-0248 
Alem  S. et al., “Effect of mixed solvents on PCDTBT: PC70BM based solar cells,” Org. Electron.. 12, (11 ), 1788 –1793 (2011), CrossRef. 1566-1199 
Cho  S. et al., “A thermally stable semiconducting polymer,” Adv. Mater.. 22, (11 ), 1253 –1257 (2010), CrossRef. 0935-9648 
Gholamkhass  B., Servati  P., “Solvent-vapor induced morphology reconstruction for efficient PCDTBT based polymer solar cells,” Org. Electron.. 14, (9 ), 2278 –2283 (2013), CrossRef. 1566-1199 
Wang  T., Pearson  A. J., Lidzey  D. G., “Correlating molecular morphology with optoelectronic function in solar cells based on low band-gap copolymer: fullerene blends,” J. Mater. Chem. C. 1, (44 ), 7266 –7293 (2013), CrossRef. 0959-9428 
Mateker  W. R. et al., “Improving the long-term stability of PBDTTPD polymer solar cells through material purification aimed at removing organic impurities,” Energy Environ. Sci.. 6, (8 ), 2529 –2537 (2013), CrossRef. 1754-5692 
Sivula  K. et al., “Amphiphilic diblock copolymer compatibilizers and their effect on the morphology and performance of polythiophene: fullerene solar cells,” Adv. Mater.. 18, (2 ), 206 –210 (2006), CrossRef. 0935-9648 
Ebadian  S. et al., “Effects of annealing and degradation on regioregular polythiophene-based bulk heterojunction organic photovoltaic devices,” Sol. Energy Mater. Sol. Cells. 94, (12 ), 2258 –2264 (2010), CrossRef. 0927-0248 
Bundgaard  E. et al., “Advanced functional polymers for increasing the stability of organic photovoltaics,” Macromol. Chem. Phys.. 214, (14 ), 1546 –1558 (2013), CrossRef. 1022-1352 
Vandenbergh  J. et al., “Thermal stability of poly[2-methoxy-5-(2′-phenylethoxy)-1,4-phenylenevinylene] (MPE-PPV): fullerene bulk heterojunction solar cells,” Macromolecules. 44, (21 ), 8470 –8478 (2011), CrossRef. 0024-9297 
Lei  T., Wang  J.-Y., Pei  J., “Roles of flexible chains in organic semiconducting materials,” Chem. Mater.. 26, (1 ), 594 –603 (2014), CrossRef. 0897-4756 
Campo  B. J. et al., “Ester-functionalized poly(3-alkylthiophene) copolymers: synthesis, physicochemical characterization and performance in bulk heterojunction organic solar cells,” Org. Electron.. 14, (2 ), 523 –534 (2013), CrossRef. 1566-1199 
Tanenbaum  D. M. et al., “The ISOS-3 inter-laboratory collaboration focused on the stability of a variety of organic photovoltaic devices,” RCS Adv.. 2, (3 ), 882 –893 (2012), CrossRef. 2046-2069 
Andreasen  B. et al., “TOF-SIMS investigation of degradation pathways occurring in a variety of organic photovoltaic devices—the ISOS-3 inter-laboratory collaboration,” Phys. Chem. Chem. Phys.. 14, (33 ), 11780 –11799 (2012), CrossRef. 1463-9076 
Teran-Escobar  G. et al., “On the stability of a variety of organic photovoltaic devices by IPCE and in situ IPCE analyses—the ISOS-3 inter-laboratory collaboration,” Phys. Chem. Chem. Phys.. 14, (33 ), 11824 –11845 (2012), CrossRef. 1463-9076 
Kesters  J. et al., “Enhanced intrinsic stability of the bulk heterojunction active layer blend of polymer solar cells by varying the polymer side chain pattern,” Org. Electron.. 15, (2 ), 549 –562 (2014), CrossRef. 1566-1199 
Boudreault  P. R. T., Najari  A., Leclerc  M., “Processable low-bandgap polymers for photovoltaic applications,” Chem. Mater.. 23, (3 ), 456 –469 (2011), CrossRef. 0897-4756 
Carlé  J. E. et al., “Comparative studies of photochemical cross-linking methods for stabilizing,” J. Mater. Chem.. 22, (46 ), 24417  (2012), CrossRef. 0959-9428 
Chang  C.-Y. et al., “Enhanced performance and stability of a polymer solar cell by incorporation of vertically aligned, cross-linked fullerene nanorods,” Angew. Chem.. 50, (40 ), 9386 –9390 (2011), CrossRef. 0044-8249 
Kim  S.-O. et al., “Thermally stable organic bulk heterojunction photovoltaic cells,” Sol. Energy Mater. Sol. Cells. 95, (2 ), 432 –439 (2011), CrossRef. 0927-0248 
Liao  M.-H. et al., “Morphological stabilization by supramolecular perfluorophenyl-C 60 interactions leading to efficient and thermally stable organic photovoltaics,” Adv. Funct. Mater.. 24, (10 ), 1418 –1429 (2013), CrossRef. 1616-3028 
Li  Z. et al., “Performance enhancement of fullerene-based solar cells by light processing,” Nat. Commun.. 4, , 2227  (2013), CrossRef.
Distler  A. et al., “The effect of PCBM dimerization on the performance of bulk heterojunction solar cells,” Adv. Energy Mater.. 4, (4 ), 1300693  (2014), CrossRef. 1614-6840 

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