Engineering of Porphyrin Molecules for use as Effective Cathode

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Engineering of Porphyrin Molecules for use as Effective Cathode Interfacial Modifiers in Organic Solar Cells of Enhanced Efficiency and Stability Marinos Tountas, Apostolis Verykios, Ermioni Polydorou, Andreas Kaltzoglou, Anastasia Soultati, Nikolaos Balis, Panagiotis A. Angaridis, Michael Papadakis, Vasilis Nikolaou, Florian Auras, Leonidas C. Palilis, Dimitris Tsikritzis, Evangelos Evangelou, Spyros Gardelis, Matroni Koutsoureli, George Papaioannou, Ioannis D. Petsalakis, Stella Kennou, Dimitris Davazoglou, Panagiotis Argitis, Polycarpos Falaras, Athanassios G. Coutsolelos, and Maria Vasilopoulou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03061 • Publication Date (Web): 22 May 2018 Downloaded from http://pubs.acs.org on May 22, 2018

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Engineering of Porphyrin Molecules for use as Effective Cathode Interfacial Modifiers in Organic Solar Cells of Enhanced Efficiency and Stability

Marinos Tountas,†, ‡ Apostolis Verykios,†, § Ermioni Polydorou,† ,§ Andreas Kaltzoglou,† Anastasia Soultati,† Nikolaos Balis,† Panagiotis A. Angaridis,∥ Michael Papadakis,⊥ Vasilis Nikolaou,⊥ Florian Auras,# Leonidas C. Palilis,§ Dimitris Tsikritzis,∇ Evangelos K. Evangelou,¶ Spyros Gardelis,⌡ Matroni Koutsoureli,⌡ George Papaioannou,⌡ Ioannis D. Petsalakis,╝ Stella Kennou,∇ Dimitris Davazoglou,† Panagiotis Argitis,† Polycarpos Falaras,† Athanassios G. Coutsolelos,⊥, * Maria Vasilopoulou†,*

†

Institute of Nanoscience and Nanotechnology, National Center for Scientific Research

Demokritos, Agia Paraskevi, 15310 Athens, Greece ‡

School of Applied Mathematical and Physical Sciences, National Technical University of

Athens, Zografou Campus, 15780 Athens, Greece §

Department of Physics, University of Patras, 26504 Patras, Greece

∥Department

of Chemistry, Aristotle University of Thessaloniki, Thessaloniki 54124, Greece

⊥Department

of Chemistry, University of Crete, Laboratory of Bioinorganic Chemistry,

Voutes Campus, Heraklion 70013, Crete, Greece 1 ACS Paragon Plus Environment

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#

Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, United Kingdom

∇Department

of Chemical Engineering, University of Patras, 26504 Patras, Greece

¶

Department of Physics, University of Ioannina, 45110 Ioannina, Greece

⌡

Solid State Physics Section, Physics Department, National and Kapodistrian University of

Athens, Panepistimioupolis, 15784 Zografos, Athens, Greece ╝

Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, Vas.

Constantinou Avenue 48, 11635 Athens, Greece *email: [email protected] (M. Vasilopoulou), [email protected] (A. G. Coutsolelos).

Keywords: Porphyrin, TiO2, organic solar cells, triazine, anchoring groups, molecular dipole moment, electron transfer.

Abstract In the present work, we effectively modify the TiO2 electron transport layer of organic solar cells with an inverted architecture by using appropriately engineered porphyrin molecules. The results show that the optimized porphyrin modifier bearing two carboxylic acids as the anchoring groups and a triazine electron-withdrawing spacer significantly reduces the work function of TiO2 thereby reducing the electron extraction barrier. Moreover, the lower surface energy of the porphyrin-modified substrate results in better physical compatibility between the latter and the photoactive blend. Upon employing porphyrin-modified TiO2 electron 2 ACS Paragon Plus Environment


transport layers in PTB7:PC71BM-based organic solar cells we obtained an improved average power conversion efficiency up to 8.73%. Importantly, porphyrin modification significantly increased the lifetime of the devices which retained 80% of their initial efficiency after 500 h of storage in the dark. Due to its simplicity and efficacy, this approach should give tantalizing glimpses and generate an impact into the potential of porphyrins to facilitate electron transfer in organic solar cells and related devices.

1. Introduction In the field of organic solar cells (OSCs) substantial progress has been recently achieved, increasing the device efficiency beyond the milestone value of 10%.1 The significant advancement in OSCs performance is due to the development of novel small bandgap conjugated polymers and non-fullerene acceptors.2,3 The ability to fabricate high-efficiency OSCs needs however to be combined with a long lifetime. Inverted devices provide a facile and reliable strategy to improve OSC stability. Moreover, effective interface modification represents a key factor to further increase the efficiency and to boost the stability of OSCs, with the electron transport layer (ETL) to play a pivotal role in promoting the OSC operational characteristics.4 Zinc oxide (ZnO) is the most common metal oxide semiconductor-based ETL due to its adequate electron mobility, excellent optical transparency in the visible region, abundance, low cost and low toxicity.5,6 Titanium dioxide (TiO2), on the other hand, although it could enable lower recombination rates at surface trap states and faster electron injection than ZnO,7 it is not very often studied for its electron transporting functionality in OSCs due to its relatively higher work function (WF ~4.5 eV), lower electron mobility and higher temperature of post annealing compared to ZnO. To overcome the limitations arising from the relatively large WF, the presence of defect states and the incompatibility between the 3 ACS Paragon Plus Environment

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hydrophilic surface of TiO2 and the hydrophobic polymers used in OSCs,8 inserting an interfacial modifier at the metal oxide/organic interface has been actively pursued. An effective approach is the use of solution-processable organic modifiers, especially polyelectrolytes with various charged groups and self-assembled monolayers (SAMs), which optimize the effective WF of the metal oxide layer as well as the contact between the oxide and the photoactive film.9,10 The decoration of the metal oxide surface with organic quantum dots has also extended its effectiveness as electron transport material in OSCs.11,12 Compared to polymers and quantum dots, small organic molecules have numerous advantages, namely simple modification and purification, monodispersity and well-defined structure. Smallmolecule interfacial modifiers such as fullerene derivatives,13,14 perylene diimides,15 quinacridones,16

pyrene

sulfonates,17

perylene

bisimides,18

rhodamines,19

metal-

phthalocyanine derivatives,20,21 and triphenylamine-fluorene oligomers have also been actively explored.22 Metallated porphyrins represent a widely investigated class of macrocyclic coordination compounds with applications in multidisciplinary fields.23-26 They exhibit strong absorption in the visible spectral region and near-infrared,27,28 while ordered aggregates consisting of self-assembled porphyrin molecules may enable ultra fast energy and electron transfer due to delocalized excited states present in the aggregates as compared to the localized π-π∗ transitions within the monomer.29 Functionalized porphyrin compounds have been widely used as light harvesting elements in dye-sensitized solar cells (DSSCs),30,31 and OSCs,32 while they have also recently emerged as hole transport layers in perovskite solar cells (PSCs).33-35 Despite the remarkable electron transfer capability of porphyrin assemblies, their application as electron transport materials in photovoltaic devices is only limited to conventional OSCs previously reported by our group and others.36,37 In these two cases, appropriately oriented porphyrin self-assemblies (aggregates) were used to improve electron 4 ACS Paragon Plus Environment


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transport towards the metal cathode. However, possible benefits of using such molecules in OSCs of the more stable inverted architecture have not been explored thus far probably due to the strong absorption characteristics of poprhyrins in the visible. Here we report on the first use of a series of appropriately engineered zinc-metallated porphyrin molecules as cathode interfacial modifiers deposited as very thin films on TiO2 for efficiency enhancement in OSCs with an inverted architecture. We examine how changes in WF and surface properties (surface energy; morphology) of TiO2 upon porphyrin modification influence the performance of the fabricated OSCs. A power conversion efficiency (PCE) of 8.73% was obtained in poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)

carbonyl]

thieno[3,4-b]

thiophenediyl]]:[6,6]-phenyl-C71butyric acid methyl ester (PTB7:PC71BM)-based devices using the optimized porphyrin modifier, corresponding to a 34% improvement compared to the control device (6.52%). The improved device performance upon porphyrin modification is attributed to the synergistic effect of (i) the significant reduction of the WF of TiO2, which decreases the electron extraction barrier and increases the built-in voltage, (ii) the enhanced physical compatibility of the modified TiO2 (which becomes more hydrophobic and lowers its surface energy) with the organic layer. Moreover, porphyrin-modification enhanced the lifetime of non-encapsulated devices which preserved nearly 80% of their PCE after 500 h of storage in the dark.

2. Results and discussion 2.1 Design and synthesis of porphyrin modifiers. A primary prerequisite for the design and functionality of porphyrin molecules as TiO2 interfacial modifiers is that they have to bind 5 ACS Paragon Plus Environment

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strongly on TiO2. This is regularly achieved by incorporation of an anchoring group for the coordination of the target molecule to titanium atoms.38 Starting from the parent compound 5,10,15,20-tetraphenyl-porphyrin zinc (hereafter termed as TPPZn), which serves as the reference molecule, we designed 5-(4-carboxy-phenyl)-10,15,20-triphenyl-porphyrin zinc (termed as TPPCOOHZn) by directly introducing one carboxylic acid (-COOH) group into the molecule. Introducing two –COOH groups can further improve the porphyrin anchoring into TiO2. Furthermore, adding an electron-withdrawing spacer, which links the porphyrin ring and the carboxylic acid groups, such as a triazine moiety with high electron affinity,39 can enhance the intramolecular charge transfer properties through a polarizable π-electron density.40 Furthermore, the triazine is substituted at 3- and 5-positions by two glycines (NH2‐ CH2‐COOH), bearing the -COOH anchoring groups. The resulting molecule is 5-(4-{3,5[glycine]-triazinyl}-aminophenyl)-10,15,20-triphenyl-porphyrin

zinc

(termed

as

TPPtriazinegly2Zn). Since the covalent bonding of porphyrins to TiO2 strengthens their electronic coupling, it is anticipated that the latter porphyrin compound will not only exhibit a stronger binding capacity to TiO2 but will also induce a faster electron injection rate. The synthetic procedure for these compounds is given in the Supporting Information file (Figure S1 and relevant text).41 2.2 Theoretical calculations. The optimized ground state geometries and the corresponding dipole moments of the three porphyrins are shown in Figure 1a. Calculations were carried out using the G09 computational package. The CAM-B3LYP functional and the 6-31G(d,p) basis set were employed.42 The molecular dipole moment µMD of TPPZn is nearly zero (0.01 D). On the contrary, the substituted porphyrin TPPCOOHZn containing a carboxyl acid group exhibits a dipole moment of 2.61 D. Furthermore, TPPtriazinegly2Zn bearing two carboxylic acid and amino groups shows a higher molecular dipole moment of 4.99 D. We also approached the carboxylic acid groups of the glycine moieties with two TiO2 sites and we 6 ACS Paragon Plus Environment


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optimized the geometry of the system to see the effect of the attachemen to TiO2 on the calculated dipole moment. This attachement resulted in a further increase in molecular dipole moment of TPPtriazinegly2Zn to 9.19 D (Figure S2). The large magnitude of the molecular dipole moment of TPPtriazinegly2Zn is expected to have a high impact on the work function (WF) modification of TiO2.43 The frontier orbitals of the synthesized porphyrins were also calculated; the positions of the two highest occupied molecular orbitals (HOMOs) and the two lowest unoccupied molecular orbitals (LUMOs) of each porphyrin are given in Figure S3. The calculated LUMO levels of these compounds lie above the CB of TiO2 (around 4.0 eV) which suggests that they should be efficient to inject electrons to TiO2. Electron density plots of these orbitals are shown in Figure 1b. It is obvious that while HOMO-1 plots are localized exclusively on the porphyrin ring, HOMO plots have a significant contribution from the central Zn atom in accordance with the literature.44 However, zinc atom does not contribute to the lower lying states which can be expected since the lowest atomic excitation in Zn is at about 4.0 eV, while the low lying electronic states of i.e. TPPtriazinegly2Zn are calculated by TD-DFT at 2.32, 2.32, 3.25 and 3.28 eV, all involving porphyrin excitations. It should be further noticed

that the

undertaking of an extended theoretical examination in the future by attaching different porphyrins to a big TiO2 cluster is justified by the experimental findings of the current investigation. 2.3 Characterization of TiO2/porphyrin interfaces. The porphyrin-modification of TiO2 was probed by X-ray photoelectron spectroscopy (XPS) measurements. The expected elements (e.g C, N, O and Zn) were found in the wide XPS scans (not shown) of TiO2 covered with porphyrins deposited via spin coating from 2 mg mL-1 methanol solutions. The Zn 2p3/2 peaks are located in all cases at binding energies of about 1022.0 eV (Figure S4)

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which is attributed to Zn-N bonding with Zn in the +2 oxidation state.45 The N1s spectrum of TPPZn (Figure 2a) exhibits a major nitrogen chemical state locating at 399.8 eV. Take note that the N1s peak has a FWHM of 2.46 eV which is rather large indicating that possibly there is other nitrogen at different chemical state. Therefore, the spectrum is fitted by using two peaks, one of which is at lower binding energy (BE) and is attributed to the Zn-N bonds and the other is at higher BE and can be attributed to protonated nitrogen, like in the metal free porphyrins.46 Probably, there is also a degree of demetallation of TTPZn at the first layers of the film close to the substrate.47 The N1s peak of TPPCOOHZn (Figure 2b) shows some interesting features. First, it is clear that in order to fit the N1s peak at least two peaks are needed. The main peak is located at 398.6 eV, at 1.2 eV lower BE compared to TPPZn. In addition, there is a peak at higher BEs at 401.1 eV which is not expected. The lower BE peak at 398.6 eV can be attributed to Zn-N bonds in good agreement with the literature for TPPZn multilayers.48,49 The peak at 401.1 eV is at BEs higher from that observed for the pyrrole component of free base porhyrins (400.1 eV). Thus, the peak at 401.1 eV is probably associated to protonated nitrogen atoms interacting with hydrogen bonds with other porhyrin molecules or the substrate.50,51 The N1s spectrum of TPPtriazinegly2Zn reveals two peaks of equal intensity at 398.6 eV and 400.4 eV. The higher energy peak probably belongs to nitrogen of the glycine moiety and the lower energy peak to the Zn-N bonds and the triazine nitrogen.52 In the molecule of TPPtriazinegly2Zn the glycine nitrogen is less than the Zn-N and the triazine nitrogens, and this fact should be reflected in the N1s spectrum, which is not the case. Therefore, as for TPPCOOHZn, the XPS data from the N1s peak of TPPtriazinegly2Zn suggest that we may have an intermolecular hydrogen bond between adjacent molecules.53 To further investigate possible intermolecular hydrogen bonding interactions in porphyrins, we next recorded their UV-Vis absorption spectra (Figure 2d-f). Absorption spectra of 8 ACS Paragon Plus Environment


porphyrins in 10-5 mol L-1 methanol solutions are quite similar. In particular, they consist of a Soret band appearring at ~420 nm attributed to the S0 to S2 transition and two weaker Q bands at 555 and 595 nm which are characteristic absorption bands of a zinc-containing porphyrin, attributed to S0 to S1 transition.54 In the very thin films deposited on TiO2 via spin coating the 2 mg mL-1 solutions (note that we were not able to determine the thickness of those layers with profilometer), the Soret band is broader and red shifted to ~430 nm in all cases, suggesting a tendency of porphyrin molecules to self-assemble into J-aggregates.55 The Q bands are also red-shifted as expected for J-aggregation. However, these bands are hardly distinct as the intensity of the spectra corresponding to porphyrin very thin films on TiO2 is extremely low. This indicates that is not the strong absorption of porphyrins but their exceptional electron transfer behavior the primary issue for their succesfull application in inverted OSCs as will be discussed below.To be able to unabigously detect the absorption bands of porphyrin films on TiO2 we also recorded the UV-Vis spectra of denser porphyrin layers formed via drop casting 4 mg mL-1 solutions on TiO2 (Figure S5). These spectra are of similar shape to those taken in thin films although of higher intensity while the red shift of the Soret band (that appears at ~430 nm) is more pronounced suggesting that these porphyrins are preferentially self-assembled into J-aggregates, when forming a solid film on TiO2. Notably, the bathochromic shift of absorption peaks seems to be slightly larger in the substituted porphyrins which indicates that intermolecular interactions could include carboxyl-Zn coordination. Interestingly, there is an increment in optical absorption of TPPCOOHZn and, especially, of TPPtriazinegly2Zn on TiO2 in the 460-520 nm region. This is probably an indication for the formation of charge transfer states upon binding of carboxylic acid on TiO2; such states are considered beneficial for electron injection.56 Furthermore, the assembly of porphyrin molecules in the solid state was also verified by the strong quenching in the short-wavelength peak of the photoluminescence spectra (Figure S6)


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and by the appearance of crystalline peaks at low angles in the x-ray diffraction (XRD) pattern of the dense layers (Figure S7). Based on the above observations, a possible intermolecular interaction mode of porphyrin molecules via the formation of J-aggregates in their solid films on TiO2 surface is illustrated in Figure 2g. Note that formation of Jaggregates may be beneficial to electron injection into the metal oxide substrate.55 The surface topography of the TiO2 substrate before and after porphyrin-modification was probed by atomic force microscopy (AFM) (Figure S8). The as-deposited TiO2 exhibits a nanocrystalline surface morphology with a root mean-square (RMS) roughness of 7.04 nm The porphyrin-modified layers exhibit similar morphology (as expected due to the very thin nature of porphyrin films) with a reduced RMS roughness of 5.56, 5.20 and 6.44 nm for TPPZn, TPPCOOHZn and TPPtriazinegly2Zn, respectively, which could allow better physical contact with the photoactive layer and reduced series resistance. In addition, the transmittance of an electrode and associated interfacial layers must be as high as possible so that all incident light can reach the photoactive layer. We verified that this criterion is met when TiO2 is modified with these very thin porphyrin layers, since the slight decrease in transmittance of porphyrin-modified TiO2 between 400 and 650 nm is not expected to have a significant impact on the light harvesting capability of the photoactive blend (Figure S9). Changes in wettability of porphyrin-modified TiO2 were also probed by measuring contact angles and calculating the surface energies (Figure S10 and Table S1) of the different substrates. The TiO2 substrate showed a low contact angle of 30° and a high surface energy of 65.7 mJ m-2 whereas increased contact angles of 42.1, 60.9 and 77.5° and decreased surface energies of 61.04, 52.63 and 48.09 mJ m-2 were calculated for the TPPZn, TPPCOOHZn and TPPtriazinegly2Zn-coated TiO2 substrate, respectively. The reduced surface energies of the porphyrin-modified metal oxide substrates might help to improve the nanomorphology of the blend.57 In particular, the reduced hydrophilicity of porphyrin10 ACS Paragon Plus Environment


modified TiO2 can improve the self-assembly of the organic film (which is generally highly hydrophobic) on the substrate. 2.4. Work function changes upon porphyrin modification of TiO2. The primary requirement of cathode modification layers is to reduce the work function (WF) of the electrode. Ultraviolet photoelectron spectroscopy (UPS) was used to investigate the surface electronic structure of as-deposited and porphyrin-modified TiO2 layers. Figure 3a, b and c displays the high binding energy cut-off (a), the full UPS spectra (b) and the near Fermi level region (c) of the UPS spectra. The large similarity between the UPS spectra of TPPZnmodified and as-deposited TiO2 may be attributed to poor surface coverage of the metal oxide with the specific porphyrin since the latter is nor functionalized with anchoring groups for effective anchoring onto the substrate. On the contrary, in the UPS spectra of TPPCOOHZn and TPPtriazinegly2Zn-modified TiO2 films, the appearance of new peaks attributed to the porphyrin molecules and the modification of the surface WF is evident. After deposition of a thin layer of TPPCOOHZn on TiO2, the WF reduces from the reference value of 4.5 to 4.2 eV. Moreover, the TPPtriazinegly2Zn modification of TiO2 results in a large decrease in the WF to 3.4 eV which is expected to be beneficial for electron extraction (Table 1). A WF reduction in the porphyrin-modified TiO2 surfaces was also confirmed by the decrease of about 0.07, 0.30 and 1.20 eV in the contact potential difference (CPD) of TPPZn, TPPCOOHZn and TPPtriazinegly2Zn-modified TiO2, respectively, as compared to the asdeposited metal oxide (Figure 3d). The small deviations between WF differences measured with these techniques (UPS, Kelvin probe) are probably attributed to the different size of the scanned surface area of the sample or to environmental conditions during measurements, namely high vacuum versus air.


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Such significant WF reduction upon the binding of TPPtriazinegly2Zn onto TiO2 arises from: (i) the component of the porphyrin molecular dipole moment (µMD) perpendicular to metal oxide and pointing away from the oxide’s surface and (ii) the interfacial dipole moment (µID) formed upon interfacial electron transfer from porphyrin to TiO2 upon binding of –COOH on the latter. As a result, a large net dipole is formed (Figure 3e) leading to a substantial vacuum level shift towards Fermi level and to a large decrease in surface WF of TiO2 upon TPPtriazinegly2Zn-modification.58,59 2.5 Organic solar cells performance. The effect of porphyrin-modification of TiO2 on OSCs was next investigated by fabricating devices with an inverted architecture illustrated in Figure 4a along with a sketch of the chemical structures of the organic semiconductors used. The corresponding energy level diagram of OSCs using the TPPtriazinegly2Zn modifier is shown in Figure 4b. It is observed that, upon porphyrin coverage, the effective WF of the porphyrinmodified TiO2 layer is significantly reduced to 3.4 eV. It was shown previously that a WF reduction of the metal oxide up to ~3.0 eV is expected to be beneficial for the device performance.60 This occurs because the initially large electron extraction barrier due to the band offset between the metal oxide’s CB and the PC71BM’s LUMO is expected to decrease. The formation of a large net dipole described above results in the high reduction of the WF of TPPtriazinegly2Zn-TiO2 to 3.4 eV. Despite that this WF value is lower than the LUMO of PC71BM, the direction of the dipole significantly enhances the device built-in voltage and sweeps electrons towards the cathode. In addition, according to the Integer Charge Transfer (ICT) model for weakly interacting metal oxide/organic interfaces, the low WF value of the modified metal oxide allows the formation of an ohmic contact with the fullerene acceptor with alignment (pinning) of the Fermi level of the metal oxide with the negative integer charge transfer state (EICT-) of PC71BM.61 Consequently, the substrate’s WF reduction upon TPPtriazinegly2Zn-coating could allow the alignment of energy levels at the metal 12 ACS Paragon Plus Environment


oxide/fullerene interface thus reducing the electron extraction barrier.62 However, this hypothesis needs further investigation which is out of the scope of this work. Figure 4c depicts the J-V characteristics of the PTB7:PC71BM-based devices under simulated A.M. 1.5 illumination (100 mW/cm2). Table 1 summarizes the performance characteristics of the devices. After coating the surface of TiO2 with a TPPZn layer the device showed a modest performance enhancement which became more evident upon coating with TPPCOOHZn. Remarkably, significantly enhanced Jsc, Voc, FF and therefore PCE values were obtained upon TPPtriazinegly2Zn-modification of TiO2. In particular, Jsc increased from 14.82 mA cm-2 to 17.11 mA cm-2, Voc from 0.71 V to 0.75 V, FF from 0.62 to 0.68 and PCE from 6.52% to 8.73% upon TPPtriazinegly2Zn-coverage of TiO2 which corresponds to a 34% enhancement compared to the control device. The dark J-V curves are presented in Figure 4d. The decrease in the series resistance, Rs, and the increase in the shunt resistance, Rsh, (Table 1) indicates the improved quality of the cathode contact of the porphyrin-modified devices which is related to enhanced Jsc and FF. Moreover, the suppression of the reverse saturation current could also contribute, in part, to the increase in Voc upon porphyrin modification.63,64 The enhancement in Jsc is also related to the increase in the external quantum efficiency (EQE) of the devices upon porphyrin modification (Figure 4e). The similarity between the shapes of EQE spectra indicates that porphyrins only act as a surface modifier without harvesting any light in our device configuration.64 The calculated short-circuit photocurrents, JscEQE (Table 1), are consistent with the Jsc value obtained from J-V curves taken under device illumination. The enhancement in Jsc might also, in part, originate from improved nanomorphology of the photoactive blend arising from better physical compatibility between the porphyrin-modified TiO2 and the organic layer as the result of different surface wetting characteristics of TiO2 layer before and after modification.65


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2.6 Additional device characterization. To unambiguously demonstrate that in porphyrin modified OSCs enhanced electron extraction and reduced recombination losses are achieved, (net) photocurrent density Jph versus effective voltage Veff characteristic curves were plotted and shown in Figure 5a. Note that the net Jph=JL-JD (JL and JD correspond to current densities under illumination and in the dark), while Veff =Vo-Vappl, where Vo is the voltage at net Jph=0 and Vappl corresponds to the applied voltage. As seen in Figure 5a, Jph is higher and reaches saturation (Jsat) at lower Veff for the porphyrin-modified devices compared to the reference one. This suggests that incorporation of the porphyrin layer (especially of TPPtriazinegly2Zn) facilitates exciton dissociation into free carriers and enhances the maximum exciton generation rate (limited only by the absorbed light intensity), thereby enhancing the net Jph of the modified OSCs. Moreover, the saturation of the net Jph at Veff>0.5 eV indicates that all photogenerated excitons are efficiently dissociated and free carriers are collected without significant recombination losses. For Veff f

Engineering of Porphyrin Molecules for use as Effective Cathode

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