DonorâAcceptor Interfaces by Engineered Nanoparticles Assemblies

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Donor-Acceptor Interfaces by Engineered Nanoparticles Assemblies for Enhanced Efficiency in Plastic Planar Heterojunction Solar Cells Camillo Sartorio, Stefano Scaramuzza, Sebastiano Cataldo, Valeria Vetri, Michelangelo Scopelliti, Maurizio Leone, Vincenzo Amendola, and Bruno Pignataro J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07302 • Publication Date (Web): 08 Nov 2016 Downloaded from http://pubs.acs.org on November 10, 2016

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The Journal of Physical Chemistry

Donor-Acceptor Interfaces by Engineered Nanoparticles Assemblies for Enhanced Efficiency in Plastic Planar Heterojunction Solar Cells Camillo Sartorio,a Stefano Scaramuzza,b Sebastiano Cataldo,a Valeria Vetri,a, c Michelangelo Scopelliti,a,d Maurizio Leone,a,c Vincenzo Amendola,b,* and Bruno Pignataroa, c* a.

Università degli Studi di Palermo, Dipartimento di Fisica e Chimica, V.le delle Scienze, Parco d’Orleans II, 90128, Palermo, Italy

b.

Università degli Studi di Padova, Dipartimento di Scienze Chimiche, Via Marzolo 1, I-35131 Padova, Italy. c.

Aten Center, Università di Palermo, Ed. 18 V.le delle Scienze, Parco d'Orleans II 90128 Palermo, Italy d.

Consorzio Interuniversitario di Ricerca in Chimica dei Metalli nei Sistemi Biologici. (C.I.R.C.M.S.B.), Piazza Umberto I, n. 1 - 70121 Bari

ABSTRACT. Precisely positioning functionalized gold nanoparticles assemblies at planar donoracceptor interfaces results in fourteen fold enhancement of power conversion efficiency in

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P3HT/PCBM organic solar cells on plastic (ITO/PET) substrates. This result has been achieved by employing naphthalenethiol capped gold nanoparticles (NT-Au-NPs) produced by laser ablation in liquid and size varied in the 10 – 30 nm range. Upon surface functionalization with the aromatic thiol, these particles self-assemble in sub-micrometer aggregates which give increased light scattering. When these aggregates are deposited in the planar heterojunction between the donor and the acceptor systems, the localized scattering leads to a larger exciton formation just in the region of interest for charge transfer.

INTRODUCTION Organic solar cells (OSCs) are attracting significant interest because of their remarkable features such as low cost, ease of manufacture, mechanical flexibility and lightness suitable for portability.1-4 OSCs have been widely studied in the past few decades, showing promising power conversion efficiency (PCE).5-13 Devices based on poly(3-hexylthiophene) (P3HT) as donor (D) and soluble fullerene derivatives (PCBM) as acceptor (A) are among the most studied systems, which showed PCEs between 3-6% in bulk heterojunction (BHJ), whereas these values lower in the range of 0.1–3.1% in the planar heterojunction architectures mainly because of the loss of donor-acceptor interface area.14-25 The above BHJ efficiency values typically concern rigid substrates (e.g. glass) and it remarkably decreases when the devices are built up on plastic/flexible substrates (0.08-2.25%) because of the higher surface resistivity of the electrode on the substrate and the lower light transparency of plastic with respect to glass.26-29 No significant contribution has been given for the development of plastic planar heterojunction (PHJ) solar cells. This in spite of the remarkable advantages these devices may have in terms of processing and applications. Indeed, to use PHJ instead of BHJ solar cells makes easier the processing of large area plastic solar cells. With BHJs this may be difficult because the


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morphology control involves out-of-equilibrium phases, typically by annealing processes.20,

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More importantly, the employment of plastic substrates allows for completely new applications including artificial retinas, photovoltaic sails and any other applications where weightlessness and tissue integration might be of interest. 30-33 In order to better develop plastic planar heterojunction devices for real applications, their PCE needs a considerable improvement that is limited by the above discussed issues of D/A interface loss and plastic substrate employment. Efforts have been mainly directed at the design and synthesis of new materials with higher air-stability, carrier mobility and strong absorption in a wide range of wavelengths of the solar spectrum.34-38 At the same time the ideation of new BHJ concepts or designs, and the development of improved electrodes were considered.39-43 However, the low carrier mobility in heterojunction thin films remains the major obstacle. In fact, the poor charge transport in the active layer is the principal cause of a competition between separation and recombination of the photogenerated carriers. In addition, OSCs suffer from inefficient light absorption due to the small thickness of active layer, which is a necessary condition for reducing the recombination of the charge carriers. Thus, a major obstacle to enhance OSCs efficiency is developing strategies to increase light harvesting without increasing film thickness.44-45 Noble metal nanoparticles (NM-NPs) are receiving a lot of interest for the useful optical and electrical properties. In particular, NPs of metals such as Au and Ag are recognized for the intense plasmon resonances in the visible region of the electromagnetic spectrum, which is exploitable for the generation of plasmon-enhanced absorption.46-48 In this way, the appropriate insertion of NM-NPs in BHJ could ensure a greater photon absorption and a consequent improved photogeneration of mobile carriers in the heterojunction films.49-54 Many studies have been carried out incorporating NM-NPs in different positions of the OSC, such as in the active


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layer, hole transport layer (HTL), electrode layer or at the interface between the abovementioned layers.55-60 Despite this, the real function of NM-NPs on the performance of OSCs is still not clear and their action mechanism is not yet well understood. Indeed, different groups have obtained opposite effects with the same materials. For example, Topp et al. reported decreased performance of OSCs when Au-NPs were introduced into BHJ layer, because of local shunts due to strong tendency to aggregation of the NPs.61 Contrariwise, Cheng et al. reported improved device performance with Au-NPs-embedded HTLs. They demonstrated that the lower contact resistance at the P3HT/HTL interface causes the improvement in the hole collection and the reduction of charge recombination.62 Wang et al. showed the positive effects of Au-NPs incorporated in BHJ films due to enhanced light absorption, by light scattering, and decreased series resistance.63 Alternatively, NPs placed in the heterojunction layer could ensure better performance of the active layer through the reduction of its resistance and increase of exciton dissociation. Indeed, it was demonstrated that plasmon resonance induces strong local electric field that can enhance the exciton dissociation in OSCs, contributing positively to power conversion.64-67 Paci et al. have shown also that Au-NPs in the active layer are able to prevent the photo-oxidation process occurring in organic materials. Furthermore, Au-NPs could stabilize the BHJ morphology which tends to evolve undesirably under working conditions, due to the low glass transition temperature of the polymeric component.68 Several approaches have been proposed to synthesize NM-NPs which can be easily incorporated in organic devices, e.g. high vacuum thermal evaporation, electron beam evaporation, electrodeposition and so on. 55, 69-71 Also, the functionalization of NM-NPs has been studied mainly to improve solubility and inhibit aggregation. It has been found that alkyl ligands hamper the orbitalic conjugation between Au-NPs and polythiophenes, constituting an insulating


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barrier for charge transport.61 Furthermore, aromatic ligands showed efficient bridge properties for electron transfer processes.72 In this paper, we specifically engineer donor-acceptor interfaces of organic thin film heterojunctions by using naphthalene-thiol capped Au-NPs with different size, in order to enhance the efficiency of the photovoltaic devices that have been here specifically developed on plastic ITO/PET substrates. To this purpose, we used a new synthetic procedure for obtaining thiol-capped size-selected Au-NPs larger than 5 nm. Differently from previous work, we also positioned the hybrid nanoparticles at the planar interface between the donor and acceptor systems by controlling the aggregation level. Then, we investigated the main physico-chemical features affecting the photovoltaic efficiency also by comparing different thin film structural conditions. EXPERIMENTAL SECTION Preparation of thiol-capped Au-NPs by laser ablation. The pristine gold nanoparticles (AuNPs) dispersion with concentration of 0.6 mg/mL was obtained by laser ablation synthesis in solution (LASiS), according to a well established protocol.73 In LASiS, the 1064 nm laser pulses (50 Hz, 30 J/cm2) of a Quantel Brilliant50 Nd-YAG laser were focused on a bulk Au 99.99 % plate dipped in a 10-4 M NaCl solution in distilled water.73-74 Three fractions containing particles with different average size were extracted from the pristine Au-NPs dispersion by a selective sedimentation based separation (SBS) protocol.75 In the selective SBS protocol, multiple 1.5 mL plastic centrifuge tubes were filled each with 1 mL of the Au-NPs dispersion as obtained by LASiS and centrifuged at 126 rcf, then the surnatant was separated from the sediment and centrifuged again at 600 rcf. The centrifugation time was set to 1 hour in both cases. The three Au-NPs fractions consisted of the sediment of the 126 rcf run (Au-NPs-3), the sediment of the


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600 rcf run (Au-NPs-2) and the surnatant of the 600 rcf run (Au-NPs-1). Nanoparticle conjugation was carried out in one step by addition of either 2-naphthalenethiol (NT, 99% pure, Sigma) or 1-dodecanethiol (DT, ≥98%, Sigma) pre-dissolved in ethanol (HPLC grade, Sigma) at a concentration of 0.5 mM. The Au-NPs dispersions in water were added dropwise to the thiol solution in ethanol with a proportion of 1:5 in volume. Then, the water:ethanol 1:5 mixtures were left at room temperature for 15 hours before starting the purification procedure. The purification procedure consisted in the removal of water and ethanol by rotavapor, followed by repeated addition of n-hexane (HPLC grade, Sigma), ultrasonication and complete removal of the solvent for 5 times. Finally, cleaned NT-Au-NPs were redispersed in ethanol by ultrasonication. Spectroscopic characterization. A Varian Cary 5 spectrometer and 2 mm optical path quartz cells were used for UV-visible optical absorption. Dynamic light scattering (DLS) measurements were performed with a Malvern Zetasizer Nano ZS. Light scattering analysis was performed with a Varian Cary Eclipse fluorescence spectrophotometer in 1 cm optical path fluorescence quartz cells, using the synchronous scan modality. Numerical calculations of light scattering in Au-NPs clusters were performed with the Discrete Dipole Approximation (DDA) method.76-77 We used a number of dipoles of 5 105 for each target, constituted by a 60 nm x 80 nm cluster of, respectively, 10 nm, 20 nm or 30 nm Au NPs. The average over 27 different orientations was considered for the computation of the scattering intensity of each cluster. We used the size corrected dielectric constant for Au, as previously reported.78 Raman measurements on Au-NPs were performed with a Renishaw inVia micro-Raman spectrometer equipped with a 50x objective and the 633 nm line of a He-Ne laser (output power of 2.6 mW). Samples were obtained by drop casting of the Au-NPs dispersions in ethanol on a soda lime glass substrate. Measurements on BHJ films were performed with a 20x objective at 633 nm (1.3 mW). Fourier-


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transformed infra-red (FTIR) measurements were collected with a Perkin-Elmer 1720X FTIR spectrometer. Au-NPs dispersions in ethanol were drop casted on a KBr window and the solvent dried with a nitrogen flow prior to the analysis. XPS spectra were recorded with a PHI 5000 VersaProbe II scanning XPS Microprobe (TM) using monochromatic Al-Kα radiation (hν = 1486.6 eV) from an X-ray source operating at 200 µm spot size, 50 W power and 15 kV acceleration voltage. The high-resolution XPS spectra were collected with the hemispherical analyzer (128 channels) at the pass energy of 93.900 eV for the carbon and 23.500 eV for Sulphur and Gold, the energy step size of 0.100 eV (S) and 0.050 eV (C, Au); the photoelectron take off angle with respect to the surface was 45 °. The Shirley background subtraction and the peak fitting with Gaussian–Lorentzian and Doniach-Sunjic shaped profiles were performed for the high-resolution XPS spectra analysis using the Multipak software version 9.6.0.15 (ULVACPHI). Morphological characterization. Transmission electron microscopy (TEM) was carried out with a FEI Tecnai G2 electron microscope operating at 100 kV and equipped with a TVIPS CCD camera. Au-NPs dispersions in ethanol were deposed by drop casting on carbon coated copper grids. Average Au-NPs size and relative standard deviation were calculated on more than 350 nanoparticles for each sample by the ImageJ software. Samples for AFM analysis were prepared by spin-coating (Laurell WS-400A-6TFM; 500 rpm for 5”, 1500 rpm for 15”, 500 rpm for 5”) 0.5 ml of the NP solutions onto the surface of freshly cleaved mica 5 mm size chips (Ted Pella Inc.). Once the solvent was dried, samples were imaged immediately. AFM imaging was performed by a Multimode/Nanoscope V (Bruker, Germany). Images were acquired by using commercially available etched-silicon probes (RTESP type; Bruker, Germany) and collecting 512 × 512 points per image by maintaining the scan rate at about 1 line/s. Size measurements


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were carried out by randomly sampling the NP structures on at least three sample regions with an area of about 5 µm2. Each size distribution histogram has been obtained by collecting measurements from tens of elements. Particles dimensions were measured by using the NanosScope software, then gathered inside single data sets and statistically elaborated by Origin 9.0 program. Preparation of PHJs by Langmuir Schaefer. LS films were deposited using a KSV Minitrough apparatus. Ultrapure Millipore filtered water with resistivity greater than 18.2 MΩ cm was used as subphase at a temperature of 25 °C. A solution of 0.1 mg mL-1 of P3HT (regioregular 91-94%, electronic grade, average Mn 50 000 – 70 000, Rieke Metals, Inc.) in chloroform was randomly spread over the aqueous surface by a microsyringe. After solvent evaporation (about 10 min) the floating films were linearly compressed by means of two mobile barriers at a rate of 5 mm/min. The ultrathin films (three consecutive layers), at a 20 mN m-1 surface pressure, were transferred on indium tin oxide coated poly(ethylene terephthalate) (ITO/PET) (Aldrich, surface resistivity 60 Ω/sq) square substrates (about 1 cm2), by the LS deposition technique, which consists in approaching the solid substrate parallel to the surface subphase. In the same way, to obtain a PHJ with the desired thickness, three consecutive layers of PCBM (MW 911, >99.5, Ossila) (0.2 mg mL-1 in chloroform) have been deposited onto thin films of P3HT previously deposited. For the realization of the three components heterojunction, a LS film of Au-NPs, from ethanol solution, has been deposited between P3HT and PCBM layers. In the same way, PHJ of P3HT and poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT, MW 21,500 with polydispersity 2.9, American Dye Source) was prepared by starting from a solution of 0.1 mg mL-1 of F8BT in chlorobenzene.


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Device fabrication by Spin Coating. Devices based on the ITO/PEDOT:PSS/P3HT/AuNPs/PCBM/Al structure were fabricated by SC technique on ITO coated PET substrates. The substrates were cleaned sequentially by sonication in methanol, acetone and isopropanol for 20 min each, followed by cleaning in a UV-ozone cleaner (Procleaner Plus, Bioforce) for 60 minutes. The hole-transport material poly (ethylenedioxythiophene) : poly(styrenesulfonate) (PEDOT:PSS, Aldrich, 1.3 wt % dispersion in H2O, conductive grade) was spin-coated onto the substrates at 4000 rpm and dried at 100 °C for 10 min on a hotplate. After this, the substrates were transferred in a gloveboxes system (MBraun, Germany) filled with N2 (

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