PbS Quantum Dot Solar Cells - The

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On the Role of Polymer in Hybrid Polymer/PbS Quantum Dot Solar Cells Rosanna Mastria, Aurora Rizzo, Carlo Giansante, Dario Ballarini, Lorenzo Dominici, Olle Inganäs, and Giuseppe Gigli J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b03761 • Publication Date (Web): 11 Jun 2015 Downloaded from http://pubs.acs.org on June 16, 2015

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The Journal of Physical Chemistry

On the Role of Polymer in Hybrid Polymer/PbS Quantum Dot Solar Cells Rosanna Mastria, 1,2 Aurora Rizzo,1* Carlo Giansante,1,3* Dario Ballarini,1 Lorenzo Dominici,1,3 Olle Inganäs,4 Giuseppe Gigli1,2 1

CNR NANOTEC - Istituto di Nanotecnologia, Polo di Nanotecnologia c/o Campus Ecotekne, via

Monteroni - 73100 Lecce, Italy 2

Dipartimento di Matematica e Fisica ‘E. De Giorgi’, Università del Salento, via per Arnesano,

73100 Lecce, Italy 3

Center for Biomolecular Nanotechnologies @UNILE, Istituto Italiano di Tecnologia, via Barsanti

1, 73010 Arnesano (LE), Italy 4

Biomolecular and organic electronics, Department of Physics, Chemistry and Biology (IFM),

Linköping University, SE-581 83 Linköping, Sweden

AUTHOR INFORMATION * Corresponding Authors: [email protected], +39-0832-298211 CNR NANOTEC Istituto di Nanotecnologia, Polo di Nanotecnologia c/o Campus Ecotekne, via Monteroni - 73100 Lecce, Italy; [email protected] +39-0832-298201 Center for Biomolecular Nanotechnologies @UNILE, Istituto Italiano di Tecnologia, via Barsanti 1, 73010 Arnesano (LE), Italy. 1

ACS Paragon Plus Environment


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ABSTRACT

Hybrid nanocomposites (HCs) obtained by blend solutions of conjugated polymers and colloidal semiconductor nanocrystals are among the most promising materials to be exploited in solutionprocessed photovoltaic applications. The comprehension of the operating principles of solar cells based on HCs thus represents a crucial step towards the rational engineering of high performing photovoltaic devices. Here we investigate the effect of conjugated polymers on hybrid solar cell performances by taking advantage from an optimized morphology of the HCs comprising lead sulfide quantum dots (PbS QDs). Uncommonly, we find that larger photocurrent densities are achieved by HCs incorporating wide-bandgap polymers. A combination of spectroscopic and electro-optical

measurements

suggests

that

wide-bandgap

polymers

promote

efficient

charge/exciton transfer processes and hinder the population of midgap states on PbS QDs. Our findings underline the key role of the polymer in HC-based solar cells in the activation/deactivation of charge transfer/loss pathways.

KEYWORDS hybrid solar cells; polymer/nanocrystal composites; nanocrystal surface chemistry; electroluminescence; nanocrystal midgap states. 2


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Hybrid nanocomposites (HCs) based on conjugated polymers and colloidal inorganic semiconductor nanocrystals represent a promising class of active materials for photovoltaic devices.1 Such HCs are processed at low temperatures from solution phase and could potentially combine the mechanical flexibility, low specific weight, and large absorption cross sections peculiar of conjugated polymers2 with the small effective masses of charge carriers and good photochemical stability of colloidal nanocrystals.3 Among other colloidal nanocrystalline materials, lead sulfide quantum dots (PbS QDs) have attracted considerable attention in the photovoltaic field due to their tunable and extended absorption spectrum in the near infrared spectral region,4 possibility of multiple carrier generation,5 hot-carrier extraction,6 and prospective air-stability.7 Despite the recent developments and the potential advantages, the highest photovoltaic performances reached with such HCs8 still lag behind those obtained with their all-organic9 or all-inorganic counterparts.7,10 One of the most pressing issues concerns the chemical and photophysical complexity of the HC systems, which poses some peculiar challenges for the accurate comparison of solar cell performances and for the evaluation of the working mechanisms. As a consequence of the inherent differences between the organic and inorganic components, considerable efforts have been devoted to control the miscibility and thus phase separation between the two species, while preventing the undesired aggregation of nanocrystals. In addition, the undoubted advantage of absorbing an extended portion of solar energy from both polymers and nanocrystals complicates the identification of each component contribution to the photo-induced charge carrier generation processes in such hybrid systems. Generally, the photo-induced generation of free charges in HCbased solar cells may occur (i) at the polymer/nanocrystal interface and/or (ii) near the nanocrystal/metal contact in analogy with (i) bulk-heterojunction solar cells and (ii) Schottky diodes, respectively. Specifically, (i) in the case of hybrid heterojunction solar cells sunlight can be absorbed either via polymers or nanocrystals and fast exciton dissociation may occur at the interface of the two materials.11 The resulting free charges have to be transported to the electrodes to collect 3



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photocurrent. In order to prevent recombination of free electron-holes, large interpenetrated phases of polymers and QDs are preferred. However, small phases are needed to increase the number of excitons that can reach the hybrid interface to dissociate within the diffusion length. The optimum morphology to design high-efficiency devices is reached when charge separation and transport are balanced. In addition, (ii) the high loading of PbS QDs in the HCs may result in percolation pathways between the QDs and both electrodes that are suspended in the polymer matrices leading to field assisted separation of excitons and free charges occuring near the QD/metal contact interfaces, similarly to Schottky diode cells.12 This mechanism may include the polymer contribution to photocurrent generation via energy transfer, rather than charge transfer, to the PbS QDs. Still in this hypothesis, HC morphology has a preeminent influence in determining the device performances by maximizing the energy transfer process and charge extraction at the electrodes. To the end of exerting control on HC morphology, we have recently shown that chemical species at the QD surface, the ligands, can be exploited to mediate non-covalent bonding interactions with conjugated polymers, and their side chains particularly, leading to a drastic impact on HC morphology during formation from blend solutions.13 Indeed, the solution-phase replacement of the electrically insulating oleate ligands coming from the synthetic procedure with short arenethiolate molecules permits to achieve polymer/QD phase segregation in interconnected nanometer size scale domains.14 The arenethiolate-capped PbS QDs, prepared by surface modification of the assynthesized PbS QDs with p-methylbenzenethiolate, display good long-term colloidal stability and retain their peculiar cubic close-packing self-assembly in HCs comprising polymers with aliphatic side chains, thus leading to ideal nanometer scale phase separation and comparable morphologies in several HCs.15 In this study, we thus take advantage of such an optimized and stable HC morphology to pursue the description and explanation of conjugated polymer contribution to photocurrent generation mechanisms in HC-based solar cells. Notably, our findings suggest

4


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unexpected guidelines for polymer selection, which are obtained by comparing the opto-electronic properties and photovoltaic behavior of HCs comprising five organic semiconductor polymers.

EXPERIMENTAL METHODS PbS QD synthesis. All manipulations were performed by using air-free techniques. The synthesis of colloidal PbS QDs was carried out following a modified reported procedure.4 In a typical synthesis, 2 mmol of PbO, 4 mmol of oleic acid and 10 g of 1-octadecene were mixed and degassed via repeated vacuum-nitrogen cycles. Subsequently, the mixture was heated to allow dissolution of PbO (~100°C) until the solution became optically transparent and colorless indicating the formation of lead(II)-oleate complex. Then the solution was cooled at 80°C and switched again under vacuum to remove water formed upon lead(II)-oleate complex formation. At this point the solution was heated and kept under nitrogen flow at 110°C and then a solution of bis(trimethylsylil)sulfide (210 µL) in 2 mL of TOP was swiftly injected, corresponding to a Pb/S molar ratio equal to 2. The heating mantle was immediately removed and the resulting colloidal solution was left to cool to room temperature. The obtained QDs were purified by using a nonsolvent precipitation procedure, carried out by adding to the reaction product excess acetone, centrifuging at 4000 rpm and dissolving in toluene. The purification step was repeated two times using methanol as nonsolvent and finally the solution was filtered through a 0.2 µm polytetrafluoroethylene (PTFE) filter. Solution-based ligand exchange procedure. To produce colloidally stable arenethiolate-capped PbS

QDs,

the

as-synthesized

QDs

were

treated

with

a

slight

excess

of

p-

methylbenzenethiol/triethylamine following an already published protocol.12 In a typical ligand exchange procedure, 300 equivalents of p-methylbenzenethiol/triethylamine were added slowly to a 1 mM PbS QDs solution in dichlorobenzene. Then the mixture was purified by adding hexane and methanol, centrifuging and dissolving in dichlorobenzene. The purification step was repeated and

5



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finally the precipitate dissolved in a mixture of dichlorobenzene:chloroform 3:2 vol/vol to obtain a 1 mM arenethiolate-capped PbS QDs solution. Morphological characterization. Low-resolution transmission electron microscope images were recorded with a Jeol Jem 1011 microscope operated at an accelerating voltage of 100 kV. Samples for analysis were prepared by spin-coating the blend solutions on glass/PEDOT:PSS substrates, then floated off the substrate onto the surface of a water bath, and transferred to carbon-coated Cu grids. Atomic force microscope imaging was carried out in air using a Park Scanning Probe Microscope (PSIA) operating in non-contact mode to reduce tip induced surface degradation and sample damages. The image acquisition was performed in air at room temperature Device fabrication and characterization. Patterned ITO-coated glass substrates (Visiontek) were cleaned using TL1 solution (H2O/NH4OH/H2O2 5:1:1) and covered with a 40 nm thick layer of poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS, Clevios P 4083). Concentrated PbS QD solutions (90 mg/ml in dichlorobenzene/chloroform 3:2) were blended with each polymer solution (13 mg/ml in dichlorobenzene) at a 9:1 (or 16:1) weight ratio and spin-casted at 1500 rpm for 60 sec yielding ~ 100 nm thick nanocomposite films. These films were treated with 1% vol/vol 1,3 benzenedithiol solution in acetonitrile solution and spin-coated at 1500 rpm for 30 sec and then rinsed with mere acetonitrile. An annealing step at 110 °C for 30 min was then performed. The thin films were kept under vacuum overnight. LiF (0.6 nm) and Al (130 nm) layers were deposited on top of the nanocomposites by thermal evaporation. The device area (4 mm2) was determined by the overlap of the ITO and Al electrodes and accurately measured using an optical microscope. The current–voltage characteristics were determined using an Air Mass 1.5 global (AM 1.5 G) solar simulator (Spectra Physics Oriel150W) with an irradiation intensity of 100 mW cm-2 and recorded with a Keithley 2400 source meter. The spectral response of the short-circuit current was measured by using a power source (Newport 300W xenon lamp, 66920) with a monochromator (Newport Cornerstone 260) and a multimeter (Keithley 2001). 6


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Spectroscopic measurements. The optical absorption spectra of the nanocomposites and their components were recorded with Varian Cary 5000 UV-Vis-NIR spectrophotometers. Steady-state PL and PIA spectra and time-resolved PL decays were recorded using homemade setups. In PIA measurements, the probe light was generated by a 250 W tungsten-halogen lamp (Thermo Oriel), and passing through a monochromator before being focused on the sample. The light transmitted through the sample was focused onto a second monochromator and detected by a Si photodiode connected to a current amplifier (Femto DLPCA200). In steady-state PL and PIA measurements, pump excitation is provided by a continuous wave laser (Spectra Physics Cyan 488 nm) with excitation power density kept as low as 50 mWcm-2 and modulation frequency of 380 Hz. The excitation light for time-resolved PL measurements was provided by a Mai Tai Spectra Physics pulsed laser (

PbS Quantum Dot Solar Cells - The

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