Photovoltaic limitations of BODIPY:fullerene based bulk heterojunction solar cells

Synthetic Metals 226 (2017) 25–30

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Photovoltaic limitations of BODIPY:fullerene based bulk heterojunction solar cells Derya Barana , Sachetan Tuladharb , Solon P. Economopoulosc , Marios Neophytouc , Achilleas Savvac, Grigorios Itskosd , Andreas Othonose , Donal D.C. Bradleyb , Christoph J. Brabeca,f , Jenny Nelsonb , Stelios A. Choulisc,* a

Institute for Materials in Electronics and Energy Technology, Friedrich-Alexander University Erlangen-Nuremberg, Erlangen, D-91054, Germany Department of Physics and Centre for Plastic Electronics, The Blackett Laboratory, Imperial College London, London, SW7 2BZ, UK c Molecular Electronics and Photonics Research Unit, Department of Mechanical Engineering and Materials Science and Engineering, Cyprus University of Technology, Limassol, 3603, Cyprus d Experimental Condensed Matter Physics Laboratory, Department of Physics, University of Cyprus, P.O. Box 20537, 1678 Nicosia, Cyprus e Department of Physics, Research Center of Ultrafast Science, University of Cyprus, Nicosia 1678, Cyprus f Bavarian Center for Applied Energy Research (ZAE Bayern), Erlangen, D-91058, Germany b

A R T I C L E I N F O

Article history: Received 10 January 2017 Accepted 10 January 2017 Available online xxx Keywords: Organic solar cells BODIPY polymer donors Fractionation process Charge transport Collection Morphology

A B S T R A C T

The photovoltaic performance of blends of a series of 4,40 -Difluoro-4-bora-3a,4a-diaza-s-indacenes)based (BODIPY) conjugated polymers donors with fullerene electron acceptors is investigated. Despite the high Voc values observed, solar cell device yields relatively low power conversion efficiencies. Our study takes into account the materials’ structure-property relationship, light harvesting capabilities, charge transport, collection properties and morphological characteristics to elucidate factors affecting the photovoltaic performance in this class of polymers. We show that elimination of low molecular weight species and suitable electrodes for hole collection can be used to overcome some of the observed limitations on photovoltaic performance. © 2017 Elsevier B.V. All rights reserved.

1. Introduction During the last decade, bulk heterojunction solar cells are attracting great scientific interest because of the potential for low fabrication cost and light weight. One of the niche markets for commercializing this technology is the ease of deposition on flexible substrates. All these features offer new advantages and functionalities in comparison to state of the art inorganic photovoltaics. Power conversion efficiencies (PCE) as high as 12%, have been achieved [1–4] through optimization of device architecture and material engineering. While solar cell performance has been improved via thermal and vapor annealing processes, the use of additives [5] and interface optimization for charge carrier transportation and collection [6–9], conjugated low band gap (Eg < 2 eV) polymers with better solar harvesting and transport properties than existing donor materials, are expected to further improve PCEs [10,11]. To do so, there are two synthetic routes that still allow a functional energy level alignment with

* Corresponding author. E-mail address: [email protected] (S.A. Choulis). http://dx.doi.org/10.1016/j.synthmet.2017.01.006 0379-6779/© 2017 Elsevier B.V. All rights reserved.

electron acceptor’s LUMO level. Either, by lowering the electron donor’s LUMO level or raising its HOMO level, in respect to the vacuum level. The former may reduce the interfacial exciton dissociation ability of the system and the latter will increase polymer oxidation and degradation potential and reduce open circuit voltage (Voc) of the device. In this publication we describe a class of fluorescent 4,40 -Difluoro-4-bora-3a,4a-diaza-s-indacenes conjugated polymers (BODIPY) -a novel electron donor candidate for organic photovoltaics achieving high photovoltage values. BODIPY dyes have recently attracted considerable attention owed mainly to their interesting optical properties [12,13]. Despite being introduced a few years ago, the synthetic versatility offered by this class of molecules paved the way for a large number of small molecule light harvesting moieties for solar cells [14–16], donoracceptor hybrids, either with fullerenes [17,18] or with other molecular electron acceptors [19,20], which is a testament to their interest and potential. Polymers incorporating the BODIPY moiety, however, are still scarce and reports on solar cells with BODIPY -based polymers as the electron donor yield relatively low efficiencies [21,22], attributed perhaps to low charge carrier mobilities [23]. The present work aims to add to the family of

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D. Baran et al. / Synthetic Metals 226 (2017) 25–30

BODIPY-based polymers and more importantly aspires to elucidate on the factors that, at present, prevent higher power conversion efficiencies (PCE) and propose methods for PCE improvement from this class of materials. 2. Experimental Absorption measurements were carried out with a Perkin Elmer’s Lambda 1050 UV/vis/NIR spectrophotometer. Thin films of each polymer were doctor bladed on glass substrates in ambient conditions. In order to measure hole mobility, space charge limited current (SCLC) devices were fabricated from pristine polymer and polymer:fullerene blends. Unipolar charge injection was guaranteed by the addition of an extra PEDOT:PSS layer on top of the optically active film. A serial resistance (Rs) of 4 V is considered for the voltage correction; V = Vappl-Vbi-Vrs, where V is the effective voltage, Vappl is the applied voltage, Vrs is the voltage drop and Vbi is the built-in voltage. The hole mobility was calculated by fitting dark J-V curves to the SCLC model in which the current density is given by J = 9e0ermV2/8L3, where e0 and er represents the permittivity of the material, m is the mobility, and L is the thickness of the active layer. Bulk heterojunction devices were elaborated using polymers as electron donors and [6,6]-phenylC61-butyric acid methyl ester (PC61BM) (Solenne BV) as electron acceptor. The standard device structure was as follows: ITO/ PEDOT:PSS/polymer:PC61BM/Ca/Ag. Pre-patterned ITO glass substrates were cleaned by ultrasonic treatment with acetone and isopropyl alcohol successively and dried under a flow of dry nitrogen. A PEDOT:PSS (AL4083, H.C. Stark) solution was doctorbladed at 50  C onto clean substrates resulting in a thickness of 40 nm as determined with Dektak profilometer. PEDOT:PSS layer was annealed for 15 min at 140  C in a nitrogen filled glovebox. The active layers consisting of polymers and PC61BM in a total of 35 mg/ ml, were stirred at 65  C for 12 h before use and filtered with 0.45 ml PTFE filters. The active layers were doctor-bladed at 80  C from ortho-dicholorobenzene (o-DCB) solution onto the PEDOT: PSS layer. A bimetal top electrode consisting of Ca/Ag (15/85 nm) was thermally evaporated via a shadow mask onto the active layers with an electrode area of 0.104 cm2. PCE was calculated from J-V characteristics recorded with a Botest source measure unit. Illumination was provided by an Oriel Sol1A 94061 solar simulator with an intensity of 100 mW/cm2 where the light intensity was calibrated with a standard silicon photodiode. Atomic force microscopy (AFM) images for morphological analysis of the active layers were acquired with a Veeco Model D3100 in tapping mode. Height images are shown with 5  5 mm image size. The PBTBT polymer was fractionated using a good solvent/bad solvent approach. It was diluted in CHCl3 and precipitated through the gradual addition of MeOH in order to exclude the precipitation of all the product while promoting the precipitation of the relative larger MW fractions first.

Scheme 1. Chemical structures of examined BODIPY polymers.

Chemical structures of these polymers are schematically depicted in Scheme 1. As light harvesting is mainly attributed to the conjugated polymer donor in the polymer:fullerene based bulk heterojunction organic solar cells and is an important component for photovoltaic materials, optical absorbance was the first property to be investigated. Neat polymers were diluted in chlorobenzene (CB) and were doctor-bladed on glass substrates at 70  C. Samples were probed at different spots and similar spectra concerning line shape and strength were obtained, suggesting layer homogeneity. All polymers showed main absorption coefficient peaks in the region of 565–600 nm (Fig. 1) as expected for this family of dye-based molecules [25,26] and a higher energy absorption band between 300 and 450 nm, with PBE exhibiting the broader absorbance in the UV–vis region. Polymer PB shows weak absorption strength and a long wavelength tail probably attributed to light scattering enhancement. PBTT and PBE polymers display significantly high absorption coefficient in comparison to the other BODIPY polymers. The redox properties of BODIPY-based polymers were evaluated using electrochemistry. [24] The optical and electrochemical data as well as the molecular weight of the synthesized polymers are summarized in Table 1. In various reports, different BODIPY-based polymers are synthesized and tested as electron donors in polymer:fullerene solar cells, with comonomers ranging from ethynylene [27] to EDOT-analogues [28]. Despite the variety of comonomers decorating the end products, the aforementioned properties seem to

3. Results and discussion In order to have a solid point of reference, standard poly(3hexylthiophene-2,5-diyl) (P3HT):PC61BM devices, were manufactured along with every batch of BODIPY:fullerene devices. Different moieties were incorporated onto the polymeric BODIPY-containing backbone, creating 4 new polymers using vinylene (PBE), thiophene (PBT), dithiophene (PBTT) and thiophenebenzothiadiazole-thiophene (PBTBT) as comonomers. The polymers’ synthesis, computational modeling, structural and optical characterization studies are described in details elsewhere [24]. n this work the light harvesting ability, charge carrier mobility, morphology and photovoltaic device performance of films and devices based on the polymer materials were investigated.

Fig. 1. Optical absorption coefficient spectra in the UV–vis region of pristine BODIPY polymers in solid state. From top to bottom PBTT polymer (-&-), PBE polymer (-*-), PBT polymer (-^-), PBTBT polymer (-b-) and PB homopolymer (-"-). As a point of reference the P3HT polymer is shown (dashed line).

D. Baran et al. / Synthetic Metals 226 (2017) 25–30 Table 1 Summary of optical characteristics and LUMO energy levels of the BODIPY polymers. Polymers

Egopt (eV)

ELUMO (eV)

EHOMO (eV)

labsfilm (nm)

Mw (Da)

PB PBT PBTT PBE PBTBT

1.97 1.87 1.81 1.73 1.79

3.47 3.56 3.60 3.55 3.59

5.44 5.84 5.78 5.59 5.83

577 577 576 592 573

4200 3220 5640 2230 1400

remain similar around certain values, determined by the optical characteristics of the central dye building block, as observed in our materials. Some exceptions exist in the cases where the comonomer is also a dye molecule such as perylene [29]. At this point it is worth mentioning the versatility that the BODIPY moiety affords as a low bandgap polymer building block. The library of BODIPY dyes includes molecules that span the visible spectrum [30], in order to drastically shift the optical characteristics of the BODIPY moiety, intricate chemical modifications beyond simple side chain alterations are needed [31]. These particular structureproperty characteristics of dye-based solar absorbers, differentiate this class of polymers from non-dye building blocks where the properties of the end-polymer vary greatly from its respective building blocks’ based on factors such as the push-pull strategy for low band gap polymers [32,33]. On a different note the removal of the substituted meso-groups has been shown to dramatically alter optical properties, quenching the PL intensity compared to mesosubstituted BODIPYs [34]. A crucial parameter for organic photovoltaics, is the charge carrier mobility of the polymer donors both as pristine materials but more importantly in blends with fullerene derivatives [35]. Various types of charge transport measurements have been deployed in the literature [36,37] for organic semiconductors, but for this publication, space-charge-limited-current (SCLC) measurements were chosen since the active layer thickness and processing conditions for SCLC samples are identical to corresponding solar cell devices [38]. It has been reported that charge carrier’s mobility can be determined from dark J/V characteristics by utilizing the appropriate electrodes which suppress one of the two charge species [39–41]. In theory, as charged species have to travel within the bulk a relatively long way until they reach respective electrodes, hopping between the polymeric chains takes place. In this way SCLC may also provide information about interpolymer interactions. Herein, hole only devices consisting of

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ITO/PEDOT/polymer derivative/PEDOT/Ag were fabricated and characterized. The SCLC measurements were modeled by the following equation: b pffiffiffi 9 V2 0;89 pffi JSCLC ¼ e0 em 3 e L V 8 L

Where b is the field activation factor, L is speciment’s thickness, V applied voltage, eoe are defined as electrical permittivity and m is the hole zero field mobility [42]. The hole mobility values observed for the pristine BODIPY materials under study were in the range of 105–106 m2/V s. Accurate determination of mobility values cannot be reported since the deep HOMO level of the BODIPY polymers can create injection barrier issues which can affect SCLC mobility values. Taking into account the BODIPY-class of polymers that have been presented in the literature, the charge carrier mobility values seem to be related to other parameters besides low molecular weight. Analogous polymers with similar degrees of polymerization, as the ones studied here, exhibit carrier mobility values, orders of magnitude lower than the ones reported here [29], whereas BODIPY-based polymers with much higher degrees of polymerization [27] do not seem to be exempt from low-mobility issues [23]. A possible solution for this problem may be provided from increasing planarity of these polymers. It has already been shown that the optical absorption of the stand-alone dyes can be red-shifted through increased rigidity [31] but the mobility of the end polymer can also be affected through planarization. Initial attempts at planar BODIPY oligomers [43] yielded solubility issues, as the conjugation length increases, but improvements in this area will most likely address the issue of mediocre carrier mobility values and allow for the removal of a considerable loss mechanism in device efficiency of these polymers. Charge carrier mobility issues can be directly linked to morphology, as the latter can alleviate or enhance the performance bottleneck. Polymer morphology, in itself is of crucial importance in organic solar cells as it can directly influence exciton dissociation and ultimately determine the current obtained from the solar cell device. In order to gain a better understanding of the polymer:fullerene blend morphology, tapping mode AFM height and phase images were taken. Fig. 2 shows the topography of each polymer when blended with electron acceptor. PBTBT and PBT blend films look smoother than the rest with root mean squared (rms) roughness of 18 nm and 30 nm respectively. PB, PBE and PBTT blends, on the other hand, exhibit

Fig. 2. Tapping mode AFM (TOP) height and (BOTTOM) phase images of the optically active layer.

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D. Baran et al. / Synthetic Metals 226 (2017) 25–30

Fig. 3. Current density vs voltage representative characteristics of BODIPY:fullerene (PBTBT-red, PBE-blue, PBT-green and PBTT-purple) derivatives under 100 mW/cm2 illumination and in dark conditions in semi logarithmic (inset). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

high rms roughness values in the order of 200 nm suggesting unfavorable morphology as large domains are present throughout the surface. The highest observed phase segregation is found for PBE:PC61BM, a result that comes into agreement with photovoltaic performance exhibited below in Fig. 3. Hypothetically smoother layers should result in a better contact of the photoactive area and evaporated metal cathode but this is not the case. Both PBTBT and PBT:fullerene blend based OPV devices exhibit high Rs values. Solar cells were fabricated using the ITO/PEDOT:PSS/polymer: fullerene/Ca/Ag layer sequence with the optimum electron donor: acceptor ratio being 1:4. After thermal evaporation of the cathode, devices were measured under 1 sun 1.5 A.M. conditions and parameters are summarized in Fig. 4. Thermal annealing lacked the noticeable effect on device performance, evident on the P3HT: PC61BM system. Due to solubility problems but also because of the

presence of internal deficient interpenetrating networks, it was impossible to obtain assessable signal out of devices using PB polymer. The device based on the thienyl thiophene derivative (PBTT) exhibited optimum device performance with the highest photocurrent values. This is attributed to improved light absorption, to its higher light harvesting ability as indicated by the absorption spectra of Fig. 1. The polymer PBE, that also showed high absorption coefficient, demonstrated significantly lower Voc and PCE values. This contradiction can be explained when taking into account the large domains which were formed in PBE-based photovoltaic devices (please see Fig. 2); interconnecting anode and cathode, resulting into shunts for the fabricated solar cell devices. Therefore, unfavorable morphology and recombination losses (and probably hindered exciton dissociation ability) are suggested as limiting factors. This comes in accordance with AFM images shown in Fig. 2 where non-optimum phase morphology was observed. High photovoltage values are observed for the other polymers in compliance with the cyclic voltammetry values and HOMO-LUMO energy difference. With the possible exception of PBTBT and PBT blends, the morphological data indicate large aggregate formation that can justify the limited Fill Factor (FF) observed in the J/V characteristics data. For these polymers (PBTBT and PBT) ohmic contributions between metal contact and the active layer result in high serial resistance (Rs), which limits FF values. Pristine polymer carrier mobility values are lower than those that theoretical predictions would assume as ideal [35]. Analyzing the photovoltaic values and superimposing the data onto similar BODIPY:fullerene systems appearing in the literature, it is evident that the open circuit voltage values (1 V) obtained are on par with the state of the art electron donors. Desirable, ionization potential values for the BODIPY polymers examined, confirm the experimental results obtained relevant to Voc values. The other crucial parameters for organic solar cells power conversion efficiency are Jsc and FF values that are linked to light harvesting, recombination, charge transport and collection. The light harvesting ability of the polymers examined can be considered satisfying but leaves room for improvement, although a trade-off between low energy bandgap

Fig. 4. Summary of the device performance for normal device architecture non fractionated BODIPY polymer:fullerene blends.

D. Baran et al. / Synthetic Metals 226 (2017) 25–30

6

Fractionated Non-fractionated

6

Current Density / mA/cm2

Current Density mA/cm

2

10 8 4 2 0 -2 -4 -6 -8 -10 -0.4

-0.2

0.0

0.2

0.4

0.6

29

0.8

1.0

1.2

fractionated_neat fractionated_annealed

4 2 0 -2 -4 -6 -0.2

0.0

Voltage / V

0.2

0.4 0.6 0.8 Voltage / V

1.0

1.2

Fig. 5. Normal device architecture Current density vs voltage representative characteristics of fractionated and non fractionated PBTBT: PC61BM derivatives under 100 mW/cm2 illumination.

Fig. 6. Inverted device architecture current density vs voltage representative characteristics of fractionated (neat and annealed) PBTBT: PC61BM derivatives under 100 mW/cm2 illumination.

and energy level alignment with the electron acceptor and the collection electrodes exist. As exhibited in the state of the art, BODIPY polymer:fullerene solar cell by Frechet and coworkers [23], an improvement on the optical bandgap, compared to the polymers in this study, can lead to a deterioration of Voc values. Despite the very high open circuit voltage values obtained for the BODIPY: PC61BM material system (1 V), Jsc and FF values ranging from 0.3 to 4 mA/cm2 and 24–34% respectively, are much lower compared to our reference P3HT: PC61BM based OPVs (Voc of 0.57 V, FF of 45.1 % and Jsc of 11.55 mA/cm2). Ultimately, PBTT exhibits the best normal device architecture solar cell device performance with Jsc of 4 mA/cm2, Voc of around 1 V, FF of 34 % and photon to electron conversion efficiency of 1.2%. Un-optimized morphology, carrier collection property and low carrier mobility affect Jsc and FF, which in turn result in the moderate PCE values measured. The morphology problems might be addressed, through chemical modification of alkyl chains or by improving diffusion of PC61BM in the polymer matrix [44]. In order to improve carrier selectivity, electrode modification is needed, to eliminate energetic barriers between semiconductor and contacts. Carrier mobility, on the other hand, can be improved by the elimination of low molecular weight species that can cause transport traps. The above strategies where implemented to the PBTBT: PC61BM material system as it particularly under performs (as shown in Fig. 4) in terms of power conversion efficiency (Jsc of 2.3 mA/cm2, Voc of 0.9 V, FF of 23% resulting to PCE of 0.3%). Fractionated process to eliminate low molecular weight species (impurities) was used for the PBTBT: PC61BM material system in normal device architecture OPVs. As can be seen from Fig. 5 Jsc have significantly increased (Jsc of 6.43 mA/cm2) in fractionated PBTBT:PC61BM OPVs indicating that transport is one of the limited factors affecting OPV performance. The PCE for normal device architecture fractionated PBTBT:PC61BM OPVs increase from 0.3% to 1.34%. Inverted structure OPVs based ITO/Polyethylenimine ethoxylated (PEIE)/PBTBT: PC61BM/MoO3/Ag were also used to replace the PEDOT:PSS hole selective contact with the deeper work function alternative MoO3. The current density vs voltage representative characteristics of inverted PBTBT: PC61BM with and without annealing under 100 mW/cm2 illumination are shown in Fig. 6. The annealed (100  C for 10 min) inverted PBTBT: PC61BM have Jsc of 4,27 mA/cm2, Voc of 0.93 V, FF of 30% and PCE of 1.2%. The improvement on the FF for the inverted PBTBT: PC61BM solar cells provides indications that anode selectivity is also one of the key

parameters limiting photovoltaic performance of BODIPY:fullerene based solar cells. 4. Conclusion In summary, we reported a comparative evaluation of five BODIPY-based electron donors for bulk heterojunction solar cells. Different moieties inserted on the main polymeric chain resulted in distinctive variations concerning energy levels, hole mobility, film morphology and overall device performance. Relatively high absorption coefficient, compared to established light harvesting polymers and photo-voltage values delivered (over 1 V) make these polymers attractive alternatives for PV applications. Despite the fact that the molecular weights of the polymers can be improved, once all the data have been factored-in, a preliminary comparison with other BODIPY-based polymers that have been reported shines a favorable light on these materials. A device performance-oriented characterization of the BODIPY polymers reported, allows for a spherical assessment of these conjugated polymers as electron donors and deep insight on potential factors relating to low PCE values. Un-optimized morphology, carrier collection property and low carrier mobility affect Jsc and FF, which in turn result in the moderate PCE values measured. We have shown that a fractionation process to eliminate low molecular weight species (impurities) and suitable choice of electrodes to reduce energetic barriers for hole carrier collection can be used to overcome some of the photovoltaic performance limitations. Acknowledgements This work was co-funded by the European Regional Development Fund and the Republic of Cyprus through the Research Promotion Foundation (Strategic Infrastructure Project NEA FPODOMH/STPATH/0308/06.) References [1] Z. He, C. Zhong, X. Huang, W.-Y. Wong, H. Wu, L. Chen, S. Su, Y. Cao, Simultaneous enhancement of open-circuit voltage, short-circuit current density, and fill factor in polymer solar cells, Adv. Mater. 23 (2011) 4636. [2] C.E. Small, S. Chen, J. Subbiah, C.M. Amb, S.-W. Tsang, T.-H. Lai, J.R. Reynolds, F. So, High-efficiency inverted dithienogermole-thienopyrrolodione-based polymer solar cells, Nat. Photon. 6 (2012) 115. [3] L. Dou, J. You, J. Yang, C.-C. Chen, Y. He, S. Murase, T. Moriarty, K. Emery, G. Li, Y. Yang, Tandem polymer solar cells featuring a spectrally matched low-bandgap polymer, Nat. Photon. 6 (2012) 180.

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