Triazine-Bridged Porphyrin Triad as Electron Donor for Solution

Feb 21, 2014 - Triazine-Bridged Porphyrin Triad as Electron Donor for Solution-Processed Bulk Hetero-Junction Organic Solar Cells. Ganesh D. ... *E-ma...
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Triazine-Bridged Porphyrin Triad as Electron Donor for SolutionProcessed Bulk Hetero-Junction Organic Solar Cells Ganesh D. Sharma,†,* Galateia E. Zervaki,§ Panagiotis A. Angaridis,§ Theophanis N. Kitsopoulos,‡ and Athanassios G. Coutsolelos*,§ †

R&D Centre for Engineering and Science, JEC Group of Colleges, Jaipur Engineering College Campus, Kukas, Jaipur, Raj 303101, India ‡ Department of Chemistry University of Crete and IESL-FORTH, P.O Box 1527, 71110 Heraklion, Crete, Greece § Department of Chemistry, Laboratory of Bioinorganic Chemistry, University of Crete, Voutes Campus, P.O. Box 2208, 71003 Heraklion, Crete, Greece ABSTRACT: In this report we describe the use of a novel porphyrin triad (PPT) consisting of two zinc-metalated porphyrin units and one free-base porphyrin unit covalently linked through their peripheral amino-phenyl groups to a central s-triazine unit, in combination with PC70BM ([6,6]-phenyl C70 butyric acid methyl ester), as electron donor and electron acceptors, respectively, for the fabrication of small-molecule based, solution-processed, bulk heterojunction (BHJ) organic solar cells. Photoluminescence studies of PPT:PC70BM blend films indicated that charge transfer is possible from PPT to PC70BM molecules. The solutionprocessed BHJ organic solar cell with the PPT:PC70BM blend in 1:1 weight ratio, processed from THF, was found to exhibit an overall power conversion efficiency (PCE) of 2.85%. When the BHJ active layer of PPT:PC70BM was processed from a 5% v/v mixture of 1-chloronaphathalene (CN) in THF, the PCE of the solar cell was increased up to 3.93%. This was attributed to the enhancement of the short circuit current Jsc of the solar cell, which was ascribed to a stronger and broader incident photon to current efficiency (IPCE) response and to the higher degree of crystallinity of the active layer of the latter solar cell. The different surface morphologies of the two differently processed active layers result in different electron transport kinetics, and, as shown by electrochemical impedance spectra (EIS) and relaxation time measurements, the device with the active layer with the higher degree of crystallinity results in faster charge transfer process and more efficient exciton dissociation at the PPT/PC70BM interface.

1. INTRODUCTION The need for reduction of the overall cost of solar power production directed a considerable amount of research toward the development of unconventional solar cells based on earth abundant materials. A very interesting and promising technology that resulted from these efforts is organic photovoltaic devices.1−4 These are based on low-cost, semiconducting organic materials and offer the advantage of lightweight and large-area solar cell devices, using roll-to-roll processing on flexible substrates.5 At present, the most efficient architecture of organic solar cell devices is based on a solutionprocessed bulk heterojunction (BHJ) active layer, which usually consists of a blend of a photoactive, conjugated polymer as electron donor, and functionalized fullerene molecules as electron acceptors,6−9 sandwiched between two electrodes. Upon absorption of photons by the electron donor, electron− hole pairs (excitons) are formed; these diffuse to the donor/ acceptor interface, separate into free holes and free electrons (as electrons fall from the donor conduction band to the acceptor conduction band), which are finally directed to the © 2014 American Chemical Society

corresponding electrodes, generating current. Solar cells of this type have been reported to result in power conversion efficiencies (PCEs) that exceed 10% at laboratory scale.10,11 Recently, small organic molecules have attracted considerable attention as photoactive electron donors alternative to organic polymers, owing to their potential advantages in terms of defined molecular structure, definite molecular weight, high purity, ease of purification, and good batch-to-batch reproducibility.12−28 A variety of small, organic molecules have been designed and utilized for this purpose, resulting in solar cells with PCE values in the range of 5−7%.29−32 A particular type of compounds that have been reported to result in highly efficient organic solar cell are π-conjugated molecules with a donor− acceptor (D-A) molecular structure.33−35 In these π-delocalized systems the inherent internal charge transfer that follows light absorption leads to very efficient electron−hole separation. Received: January 4, 2014 Revised: February 18, 2014 Published: February 21, 2014 5968

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Remarkably, there has been a recent report of a small molecule based organic solar cell with a record breaking 12% cell efficiency,36 rendering the small molecule based BHJ organic solar cell devices strong competitors to their counterparts based on polymer donors. Given that in solution-processed BHJ organic solar cells donor molecules play a very critical role in absorbing solar radiation, they should exhibit a strong and wide light absorption profile. Additionally, they should have sufficient solubility in common organic solvents, ability to form strong intermolecular interactions in the solid state for efficient charge transfer, conduction band edge that is higher than that of the acceptor material, and exhibit high light- and air-stability. Porphyrin derivatives, owing to their very well established potential as light harvesting antennae for efficient energy and electron transfer processes in biological systems, appear to be very attractive as electron donors in organic solar cells.37,38 Due to their π-conjugated macrocyclic framework, in their absorption spectra they display an intense Soret band at 400−450 nm and moderate Q bands at 500−650 nm. Moreover, their electronic, spectroscopic, and physical properties can be appropriately tuned by synthetically modifying the substituents of the macrocyclic ring and/or the metal into the central cavity.39−41 Despite the successful employment of porphyrins as sensitizers in dye-sensitized solar cells42−47 (in which there have been reports of cells with PCE values higher than 12%48) and in vacuum processed organic solar cells,49−55 their utilization as donors in solution-processed BHJ organic solar cells has been rather limited.56,57 Recently, Matsuo and coworkers reported the synthesis of a meso-tetraethynyl substituted porphyrin derivative with terminal aromatic and aliphatic groups, and its use in a solution-processed BHJ organic solar cell, in combination with PC60BM ([6,6]-phenyl C61 butyric acid methyl ester) as electron acceptor, achieving PCE value of 2.5%.58 In addition, Huang et al. reported the use of a porphyrin compound containing two benzothiadiazole (BT) units, end-caped with 3-hexylthienyl, linked to opposite meso positions of the porphyrin core by ethynyl bridges, in combination with [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) as electron acceptor, resulting in a PCE value of 4.02%.59 Peng and co-workers, by employing a porphyrin with two diketopyrrolopyrrole units and ethynyl bridges at meso positions, along with PC60BM as electron acceptor, reported a BHJ organic solar cell with a PCE value of 3.71%, which was significantly improved to 4.78% when the BHJ active layer was processed with pyridine.60 Furthermore, we recently reported a porphyrin derivative with an ethynylpyridinyl group at a meso position, which was used in combination with PC70BM for the BHJ active layer of organic solar cells, achieving a PCE value of 2.54%.61 These results suggest that porphyrins are effective electron donors in small molecule based BHJ organic solar cells and that their performance could be significantly improved via suitable molecular design. Over the course of our studies on the investigation of novel porphyrin derivatives and porphyrin assemblies for applications in solar cells,62−65 we synthesized an unsymmetrical triazinebridged porphyrin triad (PPT), consisted of two zinc− metalated units and one free-base unit, which was used as sensitizer in dye-sensitized solar cells.66 Herein, we report the successful use of this triad with the 2D-π-A molecular architecture (Scheme 1) as electron donor, blended together with PC70BM (Scheme 2) as electron acceptor, for the photoactive layer in solution processed BHJ organic solar

Scheme 1. Chemical Structure of the Triazine-BridgedPorphyrin Triad PPT

Scheme 2. Chemical Structure of PC70BM

cells. By using a PPT:PC70BM blend cast in 1:1 weight ratio in tetrahydrofurane (THF), a solar cell with a PCE value of 2.85% was achieved. The cell efficiency was further improved up to 3.93% when the blend was processed from a solvent mixture of 5% v/v 1-chloronaphathalene (CN) in THF.

2. EXPERIMENTAL SECTION General Methods and Materials. All synthetic manipulations were carried out using standard Schlenk techniques under nitrogen atmosphere. 2,4,6-Trichloro-1,3,5-triazine (cyanuric chloride), diisopropylethylamine (DIPEA), Na2SO4, KOH, PC70BM, CN, and other chemicals and solvents were purchased from usual commercial sources and used as received, unless otherwise stated. Tetrahydrofuran (THF) was freshly distilled from Na/benzophenone. Synthesis of Porphyrin Triad PPT. PPT was prepared by adding to a THF solution of cyanuric chloride, in the presence of DIPEA, consecutively one equiv of 5-(4-carbomethoxyphenyl)-15-(4-aminophenyl)-10,20-bis(2,4,6-trimethylphenyl)-porphyrin)66 at low temperature, and excess of 5-(4-aminophenyl)10,15,20-triphenyl-porphyrin67 at elevated temperature (∼60 °C), followed by basic hydrolysis with KOH.66 Satisfactory analytical and spectroscopic characteristics were obtained. Photophysical Measurements. UV−vis absorption spectra were measured on a Shimadzu UV-1700 spectrophotometer using 10 mm path-length cuvettes. Photoluminescence spectra were measured on a JASCO FP-6500 fluorescence spectropho5969

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Figure 1. Normalized UV−vis absorption spectra of PPT in THF solution and thin film cast from THF solvent on quartz substrate.

Photovoltaic Measurements. The current−voltage characteristics of the BHJ organic solar cells were measured using a computer controlled Keithley 238 source meter in dark as well as under simulated AM1.5G illumination of 100 mW/cm2. A xenon light source coupled with optical filter was used to give the stimulated irradiance at the surface of the devices. The incident photon to current efficiency (IPCE) of the devices was measured illuminating the device through the light source and monochromator and the resulting current was measured using a Keithley electrometer under short circuit condition.

tometer equipped with a red sensitive WRE-343 photomultiplier tube. Electrochemistry Measurements. Cyclic voltammetry experiments were carried out at room temperature using an AutoLab PGSTAT20 potentiostat and appropriate routines available in the operating software (GPES, version 4.9). Measurements were carried out in freshly distilled and deoxygenated THF, at a rate of 100 mV/s with a solute concentration of 1.0 mM in the presence of tetrabutylammoniumhexafluorophosphate (0.1 M) as supporting electrolyte. A three-electrode cell setup was used with a platinum working electrode, a saturated calomel (SCE) reference electrode, and a platinum wire as counter electrode. X-ray Powder Diffraction (XRD) Measurements. XRD measurements were recorded on a Bruker D8 Advanced model diffractometer with Cu Kα radiation (λ = 1.542 Å) at a generator voltage of 40 kV. Solar Cell Fabrication. The BHJ organic solar cells were fabricated using the glass/ITO/PEDOT:PSS/PPT:PC70BM/Al device architecture. The indium tin oxide (ITO) patterned substrates were cleaned by ultrasonic treatment in aqueous detergent, deionized water, isopropyl alcohol, and acetone sequentially, and finally dried under ambient conditions. The anode consisted of glass substrates precoated with ITO, modified by spin coating with a PEDOT:PSS layer (60 nm) as hole transport and heated for 10 min at 100 °C. Mixtures of PPT with PC70BM with weight ratios of 1:05, 1:1, and 1:1.5 in THF were prepared and then spin-cast onto the PEDOT:PSS layer and dried overnight at ambient atmosphere. For the PPT:PC70BM blend processed from 5% v/v of 1-chloronaphathalene (CN) in THF solvent mixture only the 1:1 weight ratio mixture was used. The approximate thickness of the active layers was 90 nm. Finally, the aluminum (Al) top electrode was thermally deposited on the active layer at a vacuum of 10−5 Torr through a shadow mask of area of 0.20 cm2. All devices were fabricated and tested in ambient atmosphere without encapsulation. The hole-only and electron-only devices with ITO/PEODT:PSS/PPT:PC70BM/Au and Al/PPT:PC70BM/Al architectures were also fabricated in an analogous way, in order to measure the hole and electron mobility, respectively.

3. RESULTS AND DISCUSSION Synthesis, Photophysical, and Electrochemical Properties of Porphyrin Triad PPT. As shown in Scheme 1, PPT is a propeller-shaped, unsymmetrical porphyrin triad, which consists of two zinc−metalated porphyrin units and one freebase porphyrin unit (with a terminal carboxylic acid group) that are covalently linked through their peripheral amino-phenyl groups to the central s-triazine group. Therefore, the structure of PPT can be described as having the 2D-π-A molecular architecture. Its synthesis was achieved via temperaturedependent, stepwise amination reactions of cyanuric chloride with two different amino-phenyl porphyrins, in the presence of a weak base (DIPEA).66 The UV−vis absorption spectra of PPT in THF solution and in thin film form are depicted in Figure 1 (black line). In solution, PPT exhibits the characteristics absorptions of zinc− metalated porphyrin macrocycles, with an intense Soret band at 425 nm and Q bands in the region 550−630 nm. The absorption coefficients of these bands were found to be 5.6 × 105, 2 × 104, and 8.9 × 103 M−1 cm−1, respectively, indicating that the photophysical properties of the individual porphyrin units were not significantly affected after they are linked by the s-triazine unit, which is an indication of negligible interaction in their ground states. Analogous observations have also been made for other similar triazine-bridged porphyrin assemblies.68 The absorption spectrum of PPT thin film (Figure 1, red line) is similar but with broader bands than those of the solution spectrum, which may be attributed to porphyrin aggregation. The optical band gap estimated from the onset absorption edge 5970

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Figure 2. Iso-absorbing photoluminescence spectra of the PPT and PPT:PC70BM films on quartz substrate.

λonset of Q bands on thin film and the expression Eopt g = 1240/ λonset was calculated to be 1.94 eV. To get information about the ability of PPT to serve as electron donor in the BHJ active layer, in combination with PC70BM as electron acceptor, photoluminescence spectra of PPT, and PPT:PC70BM thin films spin-cast on quartz substrates were recorded with an excitation wavelength of 425 nm (maximum absorption peak of PPT). As shown in Figure 2, the PPT emission is almost completely quenched by PC70BM, indicating the appearance of electronic interactions, either in the form of energy or electron transfer, between the excited state of PPT and PC70BM. If the emission of donor in the blend is not completely quenched then the energy transfer takes place as in the case of light emitting diodes. Thus, it is suggested that electron transfer from the porphyrin triad (electron donor) to PC70BM (electron acceptor) can take place. Therefore, the blend of porphyrin triad PPT with PC70BM can be used as active layer in BHJ organic solar cell for efficient light harvesting of solar energy, facilitating exciton dissociation and charge transfer.69,70 The study of the redox behavior of porphyrin triad PPT by cyclic voltammetry measurements in THF revealed two reduction processes at E1red = −1.16 and E2red = −1.32 V vs SCE and one oxidation process at Eox1 = 0.92 V vs SCE, as shown at Figure 3.68 The highest-occupied molecular orbital (HOMO) and lowest-occupied molecular orbital (LUMO) energy levels of the porphyrin triad were estimated from the first oxidation potential (E1ox) and the first reduction potential (E1red), respectively, according to the expressions

Figure 3. Cyclic voltammogram (red line) and the square wave voltammogram (black line) of PPT in THF. The ferrocene/ ferrocenium (Fc/Fc+) redox couple wave (not shown) was observed at 0.59 V vs SCE.

relative amounts of the donor and acceptor materials used in the photoactive layer is of great importance for the observed photovoltaic performance, since there should be a balance between absorbance and charge transporting network of the active layer. When the acceptor content is too low, the electron transporting ability will be limited, while when the acceptor content is too high, the absorbance and hole transport ability in the active layer will be decreased. BHJ active layers with mixtures of PPT and PC70BM blended in THF in different weight ratios were tested, i.e. 1:05, 1:1 and 1:1.5. The optimum device performance was found for the 1:1 ratio. The current−voltage (J−V) characteristics, under stimulated illumination (AM1.5, 100 mW/cm2) of the BHJ organic solar cell with the PPT:PC70BM blend in 1:1 weight ratio processed from THF is depicted in Figure 4a (black line), while the corresponding photovoltaic parameters, i.e., short circuit current (Jsc), open circuit voltage (Voc), and fill factor (FF), are compiled in Table 1. The device showed an overall power conversion efficiency (PCE) value of 2.85%, with photovoltaic parameters Jsc = 6.45 mA/cm2, Voc = 0.96 V, and FF = 0.46.

E HOMO = −(Eox + 4.71)eV E LUMO = −(Ered + 4.71)eV

and they were found to be −5.63 and −3.55 eV, respectively. Considering that the HOMO and LUMO energy levels of PC70BM are −6.2 and −3.95 eV, the differences between the HOMO and LUMO energy levels between PPT and PC70BM are 0.47 and 0.40 eV respectively, which indicates that there is sufficient driving force for efficient electron−hole dissociation.71 Photovoltaic Performance of the PPT:PC70BM Based BHJ Organic Solar Cells. In BHJ organic solar cells, the 5971

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electron donor for the fabrication of BHJ organic solar cells, resulting in a PCE value of 0.5%.72 The higher efficiency of the solar cell based on PPT presented herein could be attributed to the different electronic molecular architecture of PPT. Due to the presence of two zinc−metalated units and one free-base porphyrin unit attached to the triazine π-framework, PPT can be described as a 2D-π-A system which, upon light absorption, assists more efficient intramolecular charge transfer and electron−hole separation, leading to better photovoltaic parameters, than the totally symmetrical porphyrin triad. Nevertheless, the overall performance of the BHJ organic solar cell with the PPT:PC70BM active layer is considered to be poor, compared to the latest developments on polymer and small molecule based organic solar cell devices. Taking into account that the Voc value of 0.96 V of the organic solar cell presented herein appears to be high (which can be attributed to the low lying HOMO energy level of PPT with respect to the LUMO energy level of the acceptor molecules in the BHJ active layer), its poor performance can be attributed to the low Jsc, and FF values. These are directly related to the light harvesting efficiency of the BHJ active layer, film morphology, charge transport, and charge collection on the respective electrodes. In most BHJ organic solar cells, the Jsc value is determined by both electron and hole transport rates. Usually, the electron mobility is much higher than the hole mobility, resulting in an increase of the recombination loss of charge carriers (due to the unbalanced charge transport).73 Furthermore, the nanoscale morphology of the active layer can deteriorate the solar cell efficiency, if the electron acceptor domains grow beyond the length scale of exciton diffusion, thereby prohibiting efficient charge transfer. Previous reports on polymer and small molecule based BHJ organic solar have demonstrated that the morphology of the active layer can be significantly improved by appropriate treatment methodologies, which include thermal annealing,74,75 solvent annealing,76−79 and solvent additives.80−85 Particularly, the choice of solvent plays a very important role and greatly influences the overall PCE of the photovoltaic device. In an effort to improve the performance of the BHJ organic solar cell based on the PPT:PC70BM active layer presented in this report, we used the solvent additive method, and prepared the active layer from a 5% v/v mixture of CN in THF. The current− voltage (J−V) characteristics of the resulting device are shown in Figure 4a (red line), and the corresponding photovoltaic parameters are listed in Table 1. Remarkably, the PCE value of the solar cell was improved from 2.85 to 3.93%. The superior performance of the latter solar cell is attributed to its improved Jsc (= 8.06 mA/cm2) and FF (= 0.53) values. Particularly, the improved Jsc value is ascribed to the enhanced IPCE response of the solar cell. As shown in Figure 4b (red line), the device with the active layer processed from the CN/ THF solvent mixture exhibits a stronger and broader IPCE response, with respect to the device with the THF processed solvent mixture. In general, the Jsc value and the IPCE response of a solar cell are related by the equation

Figure 4. (a) Current−voltage (J−V) characteristics under illumination, and (b) IPCE spectra of the BHJ organic solar cells based on differently processed active layers of PPT:PC70BM.

Table 1. Photovoltaic Parameters of the BHJ Organic Solar Cells Based on Differently Processed Active Layers of PPT:PC70BM in 1:1 Weight Ratio active layer PPT:PC70BM cast from THF PPT:PC70BM cast from CN/ THF

Jsc (mA/cm2)

Voc (V)

FF

PCE (%)

6.45 8.06

0.96 0.92

0.46 0.53

2.85 3.93

The IPCE response of the solar cell device was estimated from the expression IPCE(λ) = 1240Jsc /λPin

where Jsc is the photocurrent under short circuit condition, λ is the wavelength of the incident monochromatic light, and Pin is the incident photon flux. As shown in Figure 4b (black line), the IPCE spectrum of the device closely resembles the UV−vis absorption spectrum of the BHJ active layer, which suggests that both PPT and PC70BM contribute to the photocurrent generation. It also indicates the formation of an efficient BHJ active layer and the presence of increased PPT/PC70BM interfaces. Recently, Luechai et al. reported a triazine-bridged, symmetrical porphyrin triad similar to PPT, consisting of three identical zinc-metalated porphyrin units, which was used as an

λ2

Jsc =

∫λ1

qIPCE(λ)Nph(λ) dλ

where Nph(λ) is the photon flux intensity at any wavelength (λ), q is the electron charge and λ1 and λ2 are the wavelengths that correspond to lower and upper limits of the IPCE spectrum, respectively. The Jsc values estimated from this expression are 6.23 and 7.94 mA/cm2 for devices with PPT:PC70BM active 5972

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reported method based on thermally annealed porphyrins,86−88 which, in general, had been proved very useful for BHJ organic solar cells.89−91 For efficient organic solar cells, the mobility of electrons and holes in the BHJ active layer is also of great importance, as these should be balanced in order to achieve efficient charge transport. The mobility of electrons and holes in the PPT:PC70BM blend films were estimated from electron-only and hole-only devices, respectively, prepared from THF and CN/THF processed films, by means of space charge limited current (SCLC) measurements.92−94 The current−voltage (J− V) characteristics of the hole-only and electron-only devices are shown in Figure 6a and 5b, respectively. The hole and electron

layer spin-cast from THF and CN/THF, respectively. These are consistent with the experimentally observed values. Another factor that also contributes to the increased PCE value of the latter solar cell is the higher degree of crystallinity of the PPT:PC70BM active layer blend cast from the CN/THF solvent mixture. In order to investigate the effect of solvent mixture on the morphology of the PPT:PC70BM active layer, the X-ray diffraction (XRD) patterns of the PPT:PC70BM blend films cast from THF and CN/THF were recorded (Figure 5b). In addition, the XRD patterns of pristine PPT and

Figure 5. X-ray diffraction (XRD) patterns of (a) pristine PPT and (b) PPT:PC70BM blended thin films deposited on quartz substrates.

PPT:PC70BM blend thin films cast from THF and CN/THF were also recorded for comparison reasons. The PC70BM thin film did not show any diffraction peak from 2θ = 4 to 20°, but for the pristine PPT thin film (Figure 5a), there is a strong diffraction peak at 2θ = 7.42° and a weak diffraction peak at 14.56° (not shown in Figure 5a). The intensity of both peaks is significantly increased in the case of the film with the CN additive (Figure 5a, red line), indicating that the crystallinity of the film is increased. For the PPT:PC70BM film cast from THF (Figure 5b, black line), the diffraction peak at 2θ = 7.42° that corresponds to PPT, becomes very weak, suggesting an effective mixing of PC70BM with PPT. When the blend is cast from the CN/THF mixture (Figure 5b, red line), the diffraction peak at 2θ = 7.42° becomes strong again. In addition, the peak width at half-maximum height is decreased in the case of the film with the CN/THF mixture. These observations suggest that the blend film cast from THF has low crystallinity, while in the case of the CN/THF mixture, the crystallinity and π-electron conjugation of PPT in the blend film increases. Apparently, due to the high boiling point of CN, the active layer dries more slowly, assisting in the formation of a better self-ordered structure of the blend. The improvement of crystallinity of the PPT:PC70BM active layer blend by means of solvent additives is a more effective method than an earlier

Figure 6. Current−voltage (J−V) characteristics in dark for (a) holeonly and (b) electron-only solar cell devices, using two differently processed PPT:PC70BM active layer blends.

mobility values in the active layers were estimated via fitting these curves, in which the current in the SCLC region (JSCLC) is expressed as

JSCLC = 5973

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where εo and εr are the permittivity of free space and relative dielectric constant of active layer, respectively, μ is the charge carrier mobility, i.e., electron and hole mobility for electrononly and hole-only devices, respectively, and d is the thickness of the active layer. The hole mobility values for the THF and the CN/THF cast films were found to be 2.45 × 10−6 and 4.5 × 10−5 cm2/(V s), respectively. The increased hole mobility for the latter film compared to the former one, may be attributed to the higher degree of crystallinity of the PPT:PC70BM active layer, as revealed by the corresponding XRD film patterns. On the contrary, the electron mobility was found to be slightly decreased for the CN/THF cast film, i.e., 2.8 × 10−4 vs 3.14 × 10−4 cm2/(V s) for the THF cast film. For an efficient BHJ organic solar cell, the electron mobility/hole mobility ratio should be as low as possible, so that the charge transport should be balanced (unity, for an ideal organic solar cell). For the THF and the CN/THF cast films, these ratios were found to be 114 and 7, respectively. The higher charge balance in the BHJ active layer processed from the CN/THF solvent mixture leads to a more efficient charge transport, which might be an additional reason for the enhanced Jsc and PCE values of the device. The two organic solar cell devices with the differently processed active layers were also studied by electrochemical impedance spectroscopy (EIS), which is a very powerful tool for getting information about the interface charge transport processes in solar cells.95−97 The Nyquist plots of the EI spectra of the two devices, in dark at frequencies from 1 Hz to 100 kHz, are presented in Figure 7a. Both devices exhibit one single semicircle in the imaginary (Z″) vs real part (Z′) of the complex impedance plane, indicating that there is only one type of interface charge transport process. The semicircles for the two devices have different diameters, which suggest that the solvent used for the processing of the active layer affects the charge transport process. The equivalent circuit used to fit the EIS data is shown in the inset of Figure 7a. In this circuit, Rp is associated with the interface charge transport process, defined as charge transfer resistance, while Rs represents the Ohmic resistance of the cells, which includes the electrodes and bulk resistance in the active layer, and CPE stands for the constant phase element. The Rp value for the device cast from CN/THF was found to be lower (105 Ohm cm2) than that for the device cast from THF (154 Ohm cm2), which suggests a more efficient charge transport process. Considering that the CN/ THF processed active layer has a higher degree of crystallinity than that for the THF cast film, it can be concluded that the electron transfer process is faster in the former active layer, resulting in a higher Jsc value. The frequency dependence of the imaginary part (Z″) of EI of the two cells is shown in Figure 7b. This type of plot is generally used to get information about relaxation time (τ) of charge carriers in solar cells devices. As shown in Figure 7b, the characteristic frequency centered at the maximum height of the peak (f max) is shifted to the higher frequency region for the device with the CN/THF processed active layer. Considering that f max value is related to the relaxation time τ with the relation

τ=

Figure 7. (a) Nyquist plots and (b) frequency dependent on the imaginary part of electrochemical impedance (EI) of the BHJ organic solar cells based on differently processed active layers of PPT:PC70BM. Insert in panel a is the equivalent circuit employed in fitting impedance curves.

more efficient exciton-dissociation at the PPT/PC70BM interface and a faster charge transport processes, which leads to the enhancement of the Jsc and PCE values of the cell. The efficiency of the organic solar cell device based on the PPT:PC70BM active layer processed from the CN/THF mixture presented herein is considered to be moderate, compared to the latest developments in the organic solar cells. However, the use of novel porphyrin derivatives and assemblies as molecular electron donors in organic solar cells is very promising for achieving high PCE values, and it is an area that we are currently working on.

4. CONCLUSIONS In summary, in this report we present the results of the use of an unsymmetrical, triazine-bridged porphyrin triad with the 2Dπ-A molecular architecture (PPT) as electron donor, together with PC70BM as electron acceptor, for the active layer of solution processed BHJ organic solar cells. Photophysical measurements of PPT and PC70BM films suggest that PPT can effectively harvest photons and transfer electrons to PC70BM molecules, resulting in a photovoltaic effect. The solar cell based on a PPT:PC70BM BHJ active layer in 1:1 weight ratio, processed from THF, displayed a PCE value of 2.85%. In order to improve the efficiency of this organic solar cell, the PPT: PC70BM BHJ active layer was processed from a solvent mixture of 5% of CN in THF. The PCE value of the resulting solar cell was improved up to 3.93%, which was attributed to

1 2πfmax

the τ values for the devices with the active layers processed from THF and CN/THF were found to be 0.33 and 0.14 ms, respectively. The lower τ value of the latter device suggests a 5974

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the enhancement of its Jsc and FF values. This is a result of the enhanced IPCE response and the higher degree of crystallinity of the active layer, which leads to more balanced charge transport, and enhanced hole mobility in the BHJ active layer. Furthermore, as it is shown by EIS data, the device based on the active layer processed from CN/THF shows more efficient exciton dissociation at the PPT/PC70BM interface, and more efficient charge transport.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the European Commission (FP7REGPOT-2008-1, Project BIOSOLENUTI No. 229927) is greatly acknowledged. This research has been also cofinanced by the European Union (European Social Fund−ESF) and Greek national funds through the Operational Program “Education and Lifelong Learning” of the National Strategic Reference Framework (NSRF)-Research Funding Program: Heraklitos II and THALIS-UOA-MIS 377252. Finally a Special Research account of the University of Crete is also acknowledged.

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REFERENCES

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