ARTICLE

High open circuit voltage organic solar cells based uponfullerene free bulk heterojunction active layersAla’a F. Eftaiha, Jon-Paul Sun, Arthur D. Hendsbee, Casper Macaulay, Ian G. Hill, and Gregory C. Welch

Abstract: We have recently reported on a small organic molecule containing a bithiophene core with end-capping phthalimideunits (PthTh2Pth) that exhibited a H-aggregation tendency in the solid state and high electron mobility in organic field effecttransistors. In this contribution, we have studied both the physical and electrical properties of poly(3-hexylthiophene) (P3HT)and PthTh2Pth thin films by measuring the optical absorption, Frontier molecular orbital energy levels, photoluminescencequenching, thermal properties, and photovoltaic response. Our results have provided a useful insight into the use of PthTh2Pthas an electron acceptor material for organic photovoltaic applications. In comparison with high-performance, fullerene-based,solution-processed bulk heterojunction solar cells reported in the literature, a relatively high open circuit voltage (�0.94 V) wasobtained for various donor–acceptor blend ratios. These results highlight the potential for PthTh2Pth to act as an alternative tofullerenes as acceptors in organic solar cell devices.

Key words: organic photovoltaics, plastic solar cells, bulk heterojunction, solution processing, nonfullerene acceptors, donor–acceptor �-conjugated small molecules, H-aggregation, open circuit voltage.

Résumé : Nous avons récemment mis en évidence l’existence d’une petite molécule organique contenant du bithiophène en soncœur et des coiffes terminales phtalimide (PthTh2Pth). Cette molécule montrait une tendance a produire des agrégats d’hy-drogène a l’état solide et présentait une mobilité électronique élevée dans des transistors organiques a effet de champ. Dans laprésente étude, nous nous sommes intéressés aux propriétés physiques et électriques du poly(3-hexylthiophène) (P3HT) et defines pellicules de PthTh2Pth en effectuant des mesures d’absorption optique, de niveaux d’énergie des orbitales moléculairesfrontières, de désactivation de photoluminescence, des propriétés thermiques et de réponse photovoltaïque. Nos résultats ontfourni un aperçu utile de l’utilisation de PthTh2Pth comme matériau accepteur d’électrons dans les applications photovoltaïquesorganiques. En comparaison avec les cellules solaires a hétérojonction volumique traitées dans une solution a haute perfor-mance de fullerène décrites dans la littérature, un voltage en circuit ouvert relativement élevé (�0,94 V) a été obtenu avec diversmélanges de proportions en donneurs et accepteurs différentes. Ces résultats mettent en évidence la capacité de PthTh2Pth aremplacer les fullerènes en agissant comme accepteurs dans les dispositifs équipés de cellules solaires. [Traduit par la Rédaction]

Mots-clés : photovoltaïque organique, cellules solaires en plastique, hétérojonction volumique, traitement en solution, accepteursnon fullerèniques, petites molécules donneur—accepteur �-conjuguées, agrégats d’hydrogène, voltage en circuit ouvert.

IntroductionSolution processable organic solar cells have attracted great

attention in the past few years due to their light weight, flexibil-ity, semitransparency, ease of processing, and low-cost produc-tion compared with current technologies. These features areneeded to enable integration of solar cells into a wide range ofproducts and applications.1–6 To date, the polymer/fullerene bulkheterojunction (BHJ) composite offers one of the most successfulorganic solar cell device architectures.7,8 Key to this architectureis a high polymer (donor)/fullerene (acceptor) interaction that al-lows for photogenerated excitons to be readily dissociated intofree charges at the polymer–fullerene interface, from which theresulting holes and electrons are transported to the correspond-ing electrodes via donor and acceptor phases, respectively.9,10

Over the past several years, tremendous progress has been madein improving the power conversion efficiency of polymer/fuller-ene BHJ solar cells through advanced polymer design,11 innovativeactive layer processing,12 and device/interface engineering.13,14

The performance of such solar cells has reached upwards of9%.15,16 While promising, there are several drawbacks associatedwith fullerene-based acceptor materials (the chemical structure ofthe most common soluble fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) is shown in Fig. 1A), which canlimit long-term stability and mass production. Key issues includepoor photochemical stability, weak visible light absorption coef-ficients, and high production costs.17–20 With respect to that lattertwo, weak light absorption in the visible region results in thedonor polymer being the primary material responsible for lightabsorption, while the known high cost of fullerene derivatives(vide infra)21 is an obstacle to commercializing low-cost solar cells.Thus, the use of cost-effective and highly absorbing organic ma-terials as acceptors may serve to both increase efficiency andlower overall solar cell cost.

Small molecular semiconductors with an electron rich–electrondeficient (donor–acceptor, D–A) type framework have started toattract considerable attention as alternatives to fullerene deriva-

Received 23 February 2014. Accepted 12 April 2014.

A.F. Eftaiha. Department of Chemistry, Dalhousie University, 6274 Coburg Road, Halifax, NS B3H 4R2, Canada; Department of Physics, DalhousieUniversity, 1459 Oxford Street, Halifax, NS B3H 4R2, Canada.J.-P. Sun and I.G. Hill. Department of Physics, Dalhousie University, 1459 Oxford Street, Halifax, NS B3H 4R2, Canada.A.D. Hendsbee, C. Macaulay, and G.C. Welch. Department of Chemistry, Dalhousie University, 6274 Coburg Road, Halifax, NS B3H 4R2, Canada.Corresponding authors: Ian G. Hill (e-mail: ian.hill@dal.ca) and Gregory C. Welch (e-mail: gregory.welch@dal.ca).This article is part of a Special Issue dedicated to Professor Barry Lever in recognition of his contributions to inorganic chemistry across Canada and beyond.

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Can. J. Chem. 92: 1–8 (2014) dx.doi.org/10.1139/cjc-2014-0099 Published at www.nrcresearchpress.com/cjc on 14 April 2014.

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tives for use in organic solar cells (two examples are shown inFig. 1B). Such compounds can combine a facile and elegant syn-thesis, excellent thermal and chemical stability, near ideal opticaland electrochemical characteristics, and most importantly desir-able film-forming properties.22,23,10 Several classes of solution-processed, small organic acceptors have been explored usingeither polymers24–30 or molecular donors31,32 to fabricate organicphotovoltaic (OPV) devices. Poly(3-hexylthiophene) (P3HT, Fig. 1C)is one of the most widely used donor materials due to its commercialavailability and solution processability. To date, the maximumpower conversion efficiency (PCE) reported for P3HT/nonfullereneacceptor devices is 2.9%,26 while it is approaching 4%29 for otherconjugated polymer/nonfullerene systems. Therefore, there is plentyof room for improvement in this field of research.

In addition to the guidelines reported elsewhere in the litera-ture to design stable acceptor molecules with high electronmobilities,10,33 the design strategies for solution processable mo-lecular acceptors are directed by other factors such as film-forming capability, donor–acceptor optical properties, and thematerials response to postdeposition treatments (i.e., thermalor solvent vapor annealing). It has been reported that the film-forming ability of a �-conjugated molecular scaffold was im-proved by incorporating alkyl-substituted end-cap units.34 Inaddition, Meredith and co-workers35 indicated that the comple-mentary absorption spectra of donor and acceptor materials max-imized the light harvesting ability of donor–acceptor compositesand enhanced the generated photocurrent. Moreover, there areseveral reports that indicated the performance of organic solarcells was improved by the application of post film-deposition anneal-ing treatments such as thermal or solvent–vapor annealing.36–38

Recently, our group has reported the optimized synthesis39 andthe opto-electronic characteristics40 of thiophene-core moleculeswith end-capping phthalimide units that bear different terminalalkyl chains. These compounds exhibited a H-aggregation ten-dency in the solid state and high field effect transistor electronmobility up to 0.2 cm2 V−1 s−1. In comparison with other investi-gated compounds, the relatively high solubility of 5,5=-(2-octyl)isoindoline-1,3-dione bithiophene (PthTh2Pth, Fig. 1D), strongvisible light absorption coefficients (�44 000 (mol/L)−1 cm−1 at424 nm in CHCl3), and low cost make it a potential candidate as anelectron-transporting material for solution processable organicsolar cell application.

In this report, the potential utility of PthTh2Pth as an electronacceptor for solar cell application has been tested in a BHJ archi-tecture using P3HT as the donor component. The opto-electronic

properties, crystallinity data, and high open circuit voltage (Voc) val-ues obtained from conventional BHJ architectures suggest the poten-tial importance of PthTh2Pth for future photovoltaic applications.

Results and discussionPthTh2Pth provides a highly tunable framework that can be

used to produce a wide variety of compounds using low-cost build-ing blocks such as substituted thiophenes, 4-bromo phthalic an-hydride, and primary amines. Commercial sources of PCBM listthe retail price at $300–$3000 per gram depending on the level ofpurity and type of fullerene derivative,41 while a rough estimate ofthe cost of synthesis of 1 g of PthTh2Pth is approximately $30 pergram on a laboratory scale (cost analyses are shown in the Supple-mentary data, Scheme S1). Even in the case of 100% error in thecost analysis, and not taking into account the economy of scale, itis apparent that materials bearing the PthTh2Pth architecture aremuch less expensive than fullerene materials.42 In addition, thesematerials are made via direct (hetero) arylation reactions, whichfurther increases the economy of synthesis by removing the needfor attaching directing functional groups such as boronic acids ortoxic alkyl tin compounds.43,44 The final products are easily puri-fied to a high degree using standard chromatographic techniquesdue to their relatively high organic solvent solubility.

The ability of PthTh2Pth to form uniform thin films from solu-tion is of great importance for solar cell applications. P3HT–PthTh2Pth blend films spun from 20 mg mL−1 chloroform solutionsformed uniform films at various blend ratios (photographs of pureand blend films are shown in the Supplementary data, Fig. S1). Thismakes PthTh2Pth an attractive candidate for spin-coating or print-ing technologies amenable to large scale processing.

The light harvesting and photoluminescence (PL) quenchingability of PthTh2Pth blended with P3HT have been investigated toexplore its potential utility as an electron acceptor for photovol-taic application. Fig. 2A shows the UV-visible absorption spectrafor P3HT and PthTh2Pth as thin films spun from chloroform solu-tion. Both spectra were comparable with those reported elsewherein the literature.39,45,46 The absorption maximum of PthTh2Pth at380 nm was assigned to the �–�* transition. The absorption onsetat 514 nm corresponds to an optical gap of 2.4 eV. Using theionization energy obtained by ultraviolet photoelectron spectros-copy (UPS) measurements (6.1 eV), the LUMO energy was esti-mated to be –3.7 eV relative to the vacuum. The absorptionspectrum of the P3HT film showed two peaks at 520 and 550 nmand a shoulder at 600 nm. These bands were also attributed to

Fig. 1. Chemical structures of (A) [6,6]-phenyl-C61-butyric acid methyl ester, (B) examples of small organic acceptors used in organicphotovoltaics (OPVs),24,28 (C) poly(3-hexylthiophene) (P3HT), and (D) 5,5=-(2-octyl)isoindoline-1,3-dione bithiophene (PthTh2Pth).

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�–�* transitions.47 The absorption onset of the film was observedat 650 nm (1.9 eV). The highest occupied molecular orbital andlowest unoccupied molecular orbital (HOMO–LUMO) energy levelsof P3HT obtained using the UPS derived HOMO and the absorptiononset of the film were –4.7 and –2.8 eV, respectively.

Theoretically, the complementary absorption profiles and theenergetic offsets between the HOMO–LUMO and the LUMO–LUMOof P3HT and PthTh2Pth (Fig. 2B) make the use of PthTh2Pth as anelectron acceptor an appropriate choice when combined withP3HT for solar cell applications. As shown in Fig. 2A, the absorp-tion spectrum of neat PtTh2Pth film covered the high-energy re-gion of the visible spectrum where P3HT did not absorb. Thisenables the blend films to absorb between 300–650 nm, whichensures significant light absorption by P3HT–PthTh2Pth BHJ cells.The energy difference between the HOMO of the P3HT as a donorand the LUMO of the PthTh2Pth as an acceptor exhibits a highbuilt-in potential (VBI) of �1 V, which is of fundamental impor-tance because it provides the upper limit of the attainable Voc inBHJ OPV cells.48–50 It is important to note that we utilized theabsorption onset of PthTh2Pth to determine its LUMO energylevel, which neglects the exciton binding energy. Therefore, theactual built-in potential might be several hundred mV higher inenergy. In addition, the HOMO–HOMO and LUMO–LUMO offsetare larger than the exciton binding energy (0.3–0.5 eV), which isnecessary for efficient charge dissociation at the donor–acceptorinterface. This enables electrons from the donor excitons to betransferred from the donor LUMO to the acceptor LUMO, andholes from the acceptor excitons to be transferred from the acceptorHOMO to the donor HOMO, when both compounds are combinedfor OPV applications.10,18,38

For a given combination of donor and acceptor, it is importantto gain knowledge about the influence of blend ratios (i.e., thin-film composition) on optical and thermal properties as well ascrystallinity to provide a base for rational device fabrication.51 Theabsorption spectra of P3HT–PthTh2Pth blends spun from chlo-roform solutions (Fig. 2C) showed the characteristic peaks of theindividual components at various weight ratios, where the blendmade of 2:3 P3HT–PthTh2Pth indicated near normalized absorp-tion maxima. The UV-vis spectra of the blend films showed smallchanges in the absorbance of PthTh2Pth with changing concentra-tion, whereas it is strongly influenced by the concentration ofP3HT. This is likely attributed to the higher absorption coefficientof P3HT in thin films compared to PthTh2Pth. Unfortunately, theequilibrium solubility of PthTh2Pth in chloroform (15.2 mg mL−1)and low solution viscosity limit the ability of the small moleculeto form uniform thin films (with varying thicknesses), preventingthe accurate measurement of its absorption coefficient.

PL quenching provides valuable insight about the ability of thedonor–acceptor interface to dissociate excitons. To assess the PL

quenching of P3HT by PhtTh2Pth, the PL intensity of blend films ofvarious donor–acceptor ratios spun from 20 mg mL−1 solutionswere measured and compared with the intensity of neat P3HTfilms spun from solutions containing the same amount of poly-mer. It is worth mentioning that considering PhtTh2Pth is emis-sive in the range of 500–600 nm,40 we measured the PL spectralintensity of both neat P3HT and blend films excited at the absorp-tion maxima of PthTh2Pth, and noted that the emission spectrawere similar. The PL spectra of both neat and blend films of vari-ous donor–acceptor ratios are presented in Fig. 3A. The emissionpeaks of photoexcited P3HT were observed at 650 and 710 nm,which is consistent with the literature.52,53 The spectral intensityof the neat films increased with increasing polymer concentrationin the predeposition solution, but at approximately >16 mg mL−1

(the top two spectra in Fig. 3A), the intensity decreased due toP3HT self-quenching in thick films.54 Moreover, the relative inten-sity of the low energy peak was enhanced at lower polymer con-tent. This was explained by the tendency of self-absorption inthick films.55

The emission maxima of P3HT in the blend films was slightlyblueshifted from approximately 650 nm to 630 nm with increas-ing the acceptor concentration (see the arrows in Fig. 3A) Thisshift was ascribed to interruption in polymer aggregation.56–59

The PL polymer quenching was assessed quantitatively by measur-ing the percentage of PL quenching, which was defined accordingto the following equation:

%PL quenching

� �1 �Intensity obtained for blend film

Intensity obtained for neat P3HT film� × 100%

Figure 3B shows the %PL quenching of blend films measured at650 and 710 nm. The PL quenching of neat P3HT films progres-sively increased with increasing concentration of PthTh2Pth. Theobserved trend was attributed to the photoinduced electron trans-fer from the polymer to the PthTh2Pth.35,60 An exception wasobserved at a 1:1 weight ratio. The origin of such an anomaly isunknown at this time, but is likely due to morphological reasons.Nonetheless, the fact that all concentrations of PthTh2Pth quenchthe emission of P3HT at a minimum of 40% suggests the potentialof PthTh2Pth to act as an acceptor material for OPV applications.

Postproduction thermal annealing is one of the most effectivemethods to improve the performance of organic solar cells. Con-sequently, exploring the thermal properties of both the donorand acceptor is important in optimizing the active layer morphol-ogy of donor–acceptor blends.22,61,62 Differential scanning calo-rimetry (DSC) was used to measure the melting and crystallizationtemperatures of the pure components and blends drop cast from

Fig. 2. (A) Normalized UV-vis absorbance spectra of P3HT and PthTh2Pth in thin films spun from chloroform solution. (B) Band structurediagram illustrating the HOMO and LUMO energies of P3HT and PthTh2Pth obtained from UPS and absorption onset of the thin film,respectively. (C) UV-vis absorbance spectra of P3HT–PthTh2Pth film blends spun from chloroform solution.

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chloroform. The first heating cycle of the thermograms (data areshown in the Supplementary data, Fig. S2) was different from thesubsequent heating curves, while the three cooling cycles wereconsistent. Therefore, we used the first cooling and the secondheating cycles to extract melting and crystallization tempera-tures. For P3HT, the melting and crystallization temperatures wereobserved at 216 °C and 184 °C, respectively, whereas PthTh2Pthmelted at 220 °C and crystallized at 206 °C. These values werecomparable with those reported by our group and others.39,63

Table 1 shows the melting and crystallization temperatures of thedrop-cast blends. In comparison with the endotherms of the purematerials, melting temperatures of the blends were lower thanthe two components. This suggested a simple eutectic phase be-havior displayed by the blends. The eutectic composition wasfound to exist at 40 wt% of P3HT, and the eutectic temperaturewas observed at �196 °C. Thermodynamically, the melting tem-perature depression implied intimately mixed components.64–66

Solidification of the molten eutectic blend would result in finelyintermixed solid structures. However, thermal annealing ofblends below or above the eutectic concentration would result incrystals made of the excess component and surrounded by a finematrix of both constituents.67–69

X-ray diffraction (XRD) was used to measure the crystallinity ofboth as-cast and thermally annealed films comprised of the purecomponent and their blends. The XRD patterns of the as-cast filmspresented in Fig. 4A showed no detectable peaks within the mea-sured range. It is worth mentioning that crystalline P3HT filmsshould have a diffraction peak at 2� of 5°.70 However, our XRDconfiguration enabled us to do the measurements starting from6°. The absence of diffraction from 2� = 6°–24° in as-cast filmsmight indicate either that the spun films did not produce diffrac-tion peaks above the instrument signal-to-noise threshold or anamorphous structure of the films. Pure and blend films were an-nealed below and above the eutectic temperature (at 150 and225 °C, respectively) using a hot plate. We noticed that the filmcolor turned from red to yellow upon heating at 225 °C, then uponcooling, the films returned to their original color. The UV-vis ab-sorption spectra of the thermally annealed films were almostidentical with the as-cast films (data are shown in the Supplemen-tary data, Fig. S3). The XRD spectra of the films heated at 150 °C for10 min did not show any diffraction peaks. However, two broaddiffraction peaks were recorded for PthTh2Pth film that washeated at 225 °C for 10 min (Fig. 4B). These peaks were displayed atangles of 8.2° and 13.7° and are in good agreement with the spec-trum of drop-cast PthTh2Pth films.40 Peak broadening may indi-cate either the formation of small crystal domains (<1 �m) or thepresence of a large density of lattice defects.71

The annealed neat P3HT (Fig. 4B) film did not show any diffrac-tion peaks, possibly due to the presence of extremely small crys-talline domains that could not be detected using simple powderXRD techniques. No detectable peaks were observed for thermally

annealed blends containing more than 70 wt% P3HT. However,the peaks of the annealed films incorporating lower concentra-tions of the polymer resembled to a great extent the pattern of theannealed neat PthTh2Pth film (Fig. 4B). This implied that the for-mation of PthTh2Pth crystalline domains at high temperaturedominated the self-assembly and confirmed the DSC-based predic-tion that the solidification of hypoeutectic mixtures would inducethe crystallization of the excess component.

Conventional BHJ OPV architecture (Fig. 5A) was used to inves-tigate the performance of PthTh2Pth as an electron acceptor. ITO/PEDOT:PSS (indium tin oxide/poly(3,4-ethylenedioxythiophene:polystyrene sulfonate) was used as the anode and Ca/Al was usedas the cathode. Current–voltage curves of selected devices and theaverage device parameters (minimum of five devices) are pre-sented in Fig. 5B and Table 2, respectively. The postannealed de-vices (at 225 °C for 10 min) displayed higher short circuit current(Jsc) and Voc values in comparison with the as-cast ones.

As shown in Table 2, the Voc values obtained for devices 2–5were between the empirical Voc value estimated according toScharber et al.48 and the VBI (�0.7 and 1 V, respectively). Comparedwith highly efficient solution-processed polymer–fullerene andpolymer–molecular acceptor organic solar cells reported in theliterature (examples are shown Table 2), the high Voc attained byP3HT–PthTh2Pth OPV devices highlight their promising potentialfor OPV applications. The low Jsc (�1 mA cm–2 for devices 2 and 3)are indicative of nonoptimal active layer morphologies. This canbe explained by the high degree of crystallization, deduced fromthe XRD data and microscopy images (vide infra), that led to do-mains of larger size than the exciton diffusion length.

It has been reported that processing thin films from solventmixtures73–75 or the usage of solvent additives76,77 has a profoundeffect on the morphological characteristics, and thereby the op-toelectronic properties of the active layer. It is anticipated that theuse of a high boiling point, good solvent for one component, anda poor solvent for the other component or incorporating a smallamount of a high boiling-point additive into a host solvent mayinduce aggregation and enhance phase separation. Herein, theequally intense absorption maxima of both P3HT and PthTh2Pthobtained from the UV-visible absorption measurements of 2:3P3HT–PthTh2Pth blend film (Fig. 2C) suggested proceeding withthis blend ratio for further processing as the active layer.

As shown in Table 2, the active layer (device 5) spun from achlorobenzene/chloroform solvent mixture produced a lower Jscand a higher Voc compared to the other blends spun from chlo-roform solution (devices 1–4). Unfortunately, the incorporation ofdiiodooctane (DIO) into the solution (devices 6 and 7) reduced thephotovoltaic parameters of the blend devices.

To gain insight into the low photovoltaic performance, opticalmicroscopy was used to examine the morphology of neat andblend films at the micron-length scale. Optical images of as-castand thermally annealed films at 150 °C (images are shown in the

Fig. 3. (A) Photoluminescence emission spectra of neat P3HT (black) and blend films (red) with equivalent polymer content spun from chlo-roform solutions. (B) %PL quenching of blend films measured at 650 and 710 nm.

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Supplementary data, Fig. S4) were featureless. The films annealedat 225 °C that incorporated more than 40 wt% PthTh2Pth revealedthe emergence of micron-size domains. This is consistent with theXRD measurements that indicated the tendency of PthTh2Pth tocrystallize. The formation of spherical domains at various blendratios is explained by the tendency of the film components tominimize line tension between different coexisting phases. Theappearance of other domain shapes such as leaflike structures andelongated interwoven strands (Fig. 6) might be explained by theunequal distribution of P3HT across the film. Molecular-level or-ganization of blend films was further explored using atomic forcemicroscopy (AFM). Fig. 6 shows a good agreement between thefilm structure obtained by optical microscopy and AFM. The thick-

ness of the leaflike structure ranged from 80 to 100 �m, and thediameter of the circular structure and strands ranged from 25 to40 �m and 10 to 20 �m, respectively. While comprehensive filmstructure mapping is beyond the scope of this study, the largedomain sizes compared to exciton diffusion length (<20 nm) ex-plained the low PCE obtained for various blend ratios. This sug-gests using different processing techniques of P3HT–PthTh2Pthcomposites to reduce the domain size to the nanoscale, whichwould be useful for OPV applications.

In conclusion, we have presented an in-depth investigation ofthe optical, thermal, and crystal properties of neat and blendfilms comprising P3HT and PthTh2Pth to explore the potentialutility of PthTh2Pth as an electron acceptor for photovoltaic

Table 1. Melting and crystallization temperatures of P3HT, PthTh2Pth, and their blends obtained from the DSC measurements.

Blend composition (P3HT:PthTh2Pth) P3HT 9:1 4:1 7:3 3:2 1:1 2:3 3:7 1:4 1:9 PhtTh2Pht

Melting temperature (°C) 216 211 211, 196 200 201 204 205 207 208 207 220Crystallization temperature (°C) 184 178 175 180 180 190 193 195 198 194 206

Fig. 4. X-ray diffraction patterns of (A) as-cast and (B) thermally annealed films spun from 20 mg mL−1 chloroform solution.

Fig. 5. (A) Conventional BHJ device architecture. (B) Current–voltage curves of various P3HT–PthTh2Pth based devices.

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applications. The complementary optical absorption spectra ofP3HT and PthTh2Pth in thin films, the appropriate LUMO–LUMOlevel offset, and the polymer PL quenching measurements indi-cated the ability of blend films to harvest a relatively broad spec-trum of the solar energy and the role of PthTh2Pth to inducecharge transfer from the photoexcited polymer. These are desir-able properties for electron acceptor materials. Moreover, the

crystallinity of thermally annealed blend films suggested somesignificant potential to develop high-performance OPV materials.The high Voc obtained from the conventional BHJ device architec-ture, made of different blend ratios, which is limited by the en-ergy offset between the HOMO of donor and the LUMO of theacceptor, indicated the potential value of PthTh2Pth to act as anacceptor material. Optical microscopy and AFM measurements of

Table 2. Average photovoltaic parameters of P3HT–PthTh2Pth BHJ OPV devices; the bottom two rows show theparameters of highly efficient solution-processed OPV devices reported in the literature.

Donor–acceptor composite Voc (V) Jsc (mA cm–2) Fill factor (%) PCEav (%) PCEmax (%)

(1) 1:1 P3HT–PthTh2Ptha,b 0.61 (±0.002) 0.29 (±0.01) 33 0.06 (±0.01) 0.06(1) 1:1 P3HT–PthTh2Ptha,c 0.66 (±0.03) 0.58 (±0.03) 19 0.07 (±0.01) 0.09(2) 2:3 P3HT–PthTh2Ptha,b 0.55 (±0.03) 0.56 (±0.04) 34 0.10 (±0.03) 0.11(2) 2:3 P3HT–PthTh2Ptha,c 0.73 (±0.02) 1.11 (±0.08) 25 0.20 (±0.03) 0.23(3) 3:7 P3HT–PthTh2Ptha,b 0.66 (±0.01) 0.51 (±0.02) 31 0.11 (±0.005) 0.11(3) 3:7 P3HT–PthTh2Ptha,c 0.83 (±0.01) 1.00 (±0.05) 24 0.20 (±0.01) 0.21(4) 1:4 P3HT–PthTh2Ptha,b 0.89 (±0.01) 0.37 (±0.03) 26 0.08 (±0.01) 0.09(4) 1:4 P3HT–PthTh2Ptha,c 0.81 (±0.01) 0.53 (±0.04) 25 0.11 (±0.01) 0.12(5) 2:3 P3HT–PthTh2Pthb,d 0.85 (±0.01) 0.29 (±0.04) 28 0.07 (±0.01) 0.09(5) 2:3 P3HT–PthTh2Pthc,d 0.94 (±0.02) 0.44 (±0.05) 23 0.09 (±0.01) 0.11(6) 2:3 P3HT–PthTh2Pthb,e 0.71 (±0.01) 0.06 (±0.001) 23 0.01 (±0.001) 0.01(6) 2:3 P3HT–PthTh2Pthc,e 0.52 (±0.03) 0.13 (±0.03) 20 0.01 (±0.004) 0.02(7) 2:3 P3HT–PthTh2Pthb,f 0.56 (±0.07) 0.02 (±0.005) 21 0.003 (±0.001) 0.004(7) 2:3 P3HT–PthTh2Pthc,f 0.37 (±0.04) 0.11 (±0.02) 26 0.01 (±0.003) 0.011:1 P3HT–PCBMg 0.61 (±0.01) 7.93 (±0.31) 64 3.07 (±0.14) 3.24P3HT–PCBM72 0.62 10.1 66 4.10

aThe active layer was spun from CHCl3 solution.bAs-cast device.cThe device was annealed at 225 °C for 10 min.dThe active layer was spun from chlorobenzene (CB)–CHCl3 solution.eThe active layer was spun from CHCl3 solution using 2% diiodooctane (DIO) additive.fThe active layer was spun from CHCl3 solution using 4% DIO additive.gThe device was fabricated according to Nam et al.72 and used as a reference.

Fig. 6. Optical images (94 �m × 79 �m) and AFM height mode images (90 �m × 90 �m) of thermally annealed blend films at 225 °C of (A) and(B) 2:3 P3HT–PthTh2Pth, (C) and (D) 3:7 P3HT–PthTh2Pth, and (E) and (F) 1:4 P3HT–PthTh2Pth, respectively. The root mean square valuesobtained for images (B), (D), and € are 25, 52, and 76 nm, respectively.

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thermally annealed films indicated the formation of micron-sizedomains, which are considered the main reason for the low PCE ofthe photovoltaic devices. This suggests a further processing opti-mization of the blend composites to enhance the photovoltaicparameters of the BHJ devices.

Supplementary dataSupplementary data are available with the article through the

journal Web site at http://nrcresearchpress.com/doi/suppl/10.1139/cjc-2014-0099.

AcknowledgementsA.F.E. is grateful for the receipt of a Natural Sciences and Engi-

neering Research Council of Canada (NSERC) Collaborative Re-search and Training Experience Program (CREATE) DalhousieResearch in Energy, Advanced Materials and Sustainability (DREAM,http://dreams.irm.dal.ca/) fellowship. I.G.H. acknowledges the NSERCDiscovery Program for financial support. G.C.W. acknowledges theNSERC Discovery Program, the Canada Foundation for Innovation(CFI) Leaders Opportunity Fund, and the Canada Research ChairsProgram for financial support. Dr. Jessica Topple is acknowledgedfor assisting with the AFM measurements. Dr. Jeff Dahn andDr. Mark Obrovac are acknowledged for providing access to DSCand XRD instrumentations.

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