Article pubs.acs.org/cm
Iodide-Passivated Colloidal PbS Nanocrystals Leading to Highly Efficient Polymer:Nanocrystal Hybrid Solar Cells Haipeng Lu, Jimmy Joy, Rachel L. Gaspar, Stephen E. Bradforth,* and Richard L. Brutchey* Department of Chemistry, University of Southern California, Los Angeles, California 90089-0744, United States S Supporting Information *
ABSTRACT: Current state-of-the-art hybrid polymer:lead chalcogenide nanocrystal solar cells require postdeposition, thin film chemical treatments to remove insulating organic ligands from the nanocrystal surface, which is a kinetically hindered process. This is compounded by the fact that it can be especially difficult to obtain colloidally stable suspensions of PbS nanocrystals ligand exchanged with small ligands, and many atomic ligands require dispersion in solvents that are incompatible with polymer solubility. Herein, we report a novel one-step colloidal ligand exchange process for PbS nanocrystals employing lead iodide (PbI2) or ammonium iodide (NH4I) as surface ligands along with n-butylamine that allow the ligand-exchanged nanocrystals to be suspended in solvents compatible with polymer dissolution. While ligand exchange is shown to be near quantitative for both iodide sources, when compared to NH4I-exchanged PbS nanocrystals, the PbI2-exchanged PbS nanocrystals not only exhibit better colloidal stability but also display superior photovoltaic performance. When the PbI2-passivated PbS nanocrystals are combined with the donor polymer poly[2,6-(4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′3′-d]silole)-alt-4,7-(2,1,3-benzothiadiazole)] (Si-PCPDTBT), the optimized hybrid solar cells give a broad spectral response into the NIR, leading to a power conversion efficiency (PCE) of 4.8% under AM 1.5G illumination. Time-resolved photoluminescence and transient absorption spectroscopies suggest that excitonic processes between the PbS nanocrystals and Si-PCPDTBT are more favorable when PbS nanocrystals are ligand exchanged with PbI2, leading to superior device performance.
1. INTRODUCTION Organic polymer:fullerene bulk heterojunction (BHJ) solar cells offer the opportunity for low-cost device fabrication as a result of the solution processability of both active components, in addition to having the added benefit of employing a thin absorber layer that translates to low materials costs.1,2 While organic photovoltaics have now achieved power conversion efficiencies in excess of ∼10%,3−5 optimization of the organic donor phase may be nearing its limit to further increase device performance. This has led to increased emphasis on exploring new acceptor types.6,7 Semiconductor nanocrystals have been explored as alternatives to the more well-established fullerene acceptors, and they possess several attributes that make them attractive in this regard: (i) size- and composition-tunable absorption from the visible to near-infrared (NIR), (ii) intrinsically higher electron mobilities, (iii) the potential for multiple exciton generation (MEG), and (iv) higher dielectric constants to help overcome the strong exciton binding energy of organic materials.8−10 To date, hybrid solar cells based on polymer:CdSe nanocrystal BHJs have been extensively explored and optimized with respect to ligand engineering,11−16 size and shape effects,17−20 charge transfer dynamics,21−24 and device architectures,25−28 leading to power conversion efficiencies (PCEs) of 4−5%. However, the best performing polymer:CdSe nanocrystal BHJs only absorb visible light out to wavelengths that are limited by the donor polymer, which prevent them © XXXX American Chemical Society
from harvesting light into the NIR part of the solar spectrum. For example, CdSe nanocrystals are intrinsically limited by the bulk band gap (Eg = 1.7 eV) to an absorption edge of ∼730 nm. Moreover, the dielectric constant of bulk CdSe (εr ∼ 629) is not significantly different from that of the organic polymer phase (εr ∼ 3−430), whereas lower band gap semiconductors generally possess high dielectric constants (e.g., εr ∼ 17 for PbS31). Because of these attributes, lead chalcogenide (PbE, where E = S, Se) nanocrystals are being increasingly investigated as electron acceptors in hybrid polymer:nanocrystal BHJ solar cells.32−35 Although the photovoltaic performance of hybrid polymer:PbE nanocrystal BHJ solar cells is promising, much less is known about the effects of ligand engineering for the lead chalcogenides on device performance when compared to CdSe acceptors.36,37 All of the high-performing, state-of-the-art hybrid polymer:PbE nanocrystal BHJ devices to date mandate a postdeposition, thin film ligand treatment to remove insulating native ligands from the nanocrystal surface.32−37 Such ligand exchange processes are extremely critical to provide the interparticle electronic coupling that is required for efficient excitonic processes to occur (i.e., charge transfer, charge Received: January 15, 2016 Revised: February 25, 2016
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DOI: 10.1021/acs.chemmater.6b00185 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
(90%), 1,2-ethanedithiol (99%) and 1-octadecene (90%) were purchased from Sigma-Aldrich. PEDOT:PSS (Clevios PH 500, pH 1−2, percent 1−1.5) was purchased from Heraeus. Si-PCPDTBT (40,500 MW, 2.2 PDI) was purchased from 1-Material. All chemicals are used as received without further purification. Synthesis of PbS nanocrystals. The synthesis of Eg = 1.3 eV (first exciton peak ∼920 nm) PbS nanocrystals was based on the procedure developed by Hines and Scholes and is described in detail in the Supporting Information.48 The final suspension was made in 20 mL of toluene with a concentration of 50 mg mL−1. Ligand exchange of PbS nanocrystals. All ligand exchange processes are done in air under ambient conditions. Ligand exchange with lead iodide (PbI2). A 0.02 M solution of PbI2 in a mixture of DMF (12 mL) and methanol (6 mL) was prepared first. The suspension of as-prepared PbS nanocrystals (2 mL, 50 mg mL−1) was then added into the PbI2 solution and shaken for 1− 2 min, leading to an immediate precipitation of the PbS nanocrystals. After centrifugation, the PbS nanocrystals were isolated from the clear supernatant, followed by redispersing them in a mixture of 1,2dichlorobenzene (1 mL) and BA (0.2 mL). The ligand-exchanged PbS nanocrystals were then filtered through a 0.45 μm PTFE syringe filter for device fabrication. The PbI2-exchanged PbS nanocrystals are stable up to several months without any sign of agglomeration or surface etching. Ligand exchange with ammonium iodide (NH4I). First, a 0.2 M solution of NH4I in methanol (8 mL) was prepared. The suspension of as-prepared PbS nanocrystals (2 mL, 50 mg mL−1) was then added into the NH4I solution, leading to an immediate precipitation of the PbS nanocrystals. After centrifugation, the PbS nanocrystals were isolated from the clear supernatant, followed by redispersing them in a mixture of 1,2-dichlorobenzene (1 mL) and BA (0.4 mL). The ligandexchanged PbS nanocrystals were then filtered through a 0.45 μm PTFE syringe filter for device fabrication. The NH4I-exchanged PbS nanocrystals are not quite colloidally stable, and brown solids will precipitate within 48 h. Ligand exchange with n-butylamine (BA). A dispersion of asprepared PbS nanocrystals (2 mL, 50 mg mL−1) was first precipitated by 20 mL of acetone. After the supernatant was removed, BA (8 mL) was then added to redisperse the PbS nanocrystals. The solution was stored in the dark under a nitrogen atmosphere for 2 d. The PbS nanocrystals were then precipitated with excess acetone (30 mL) and redispersed in 1 mL of DCB. The ligand-exchanged PbS nanocrystals were then filtered through a 0.45 μm PTFE syringe filter for device fabrication. The BA-exchanged PbS nanocrystals are not quite colloidally stable, and brown solids will precipitate within 24 h. Characterization. UV−vis-NIR absorption spectra were acquired on a PerkinElmer Lamba 950 spectrophotometer equipped with a 150 mm integrating sphere, using a quartz cuvette for liquid samples or a borosilicate glass microscope slide substrate for films. Thermogravimetric analysis (TGA) measurements were made on a TA Instruments TGA Q50 instrument, using sample sizes between 5 and 15 mg in an alumina crucible under a flowing nitrogen atmosphere. TGA samples were prepared by drying the colloid under flowing nitrogen at 80 °C for up to 90 min, then lightly crushing with a spatula prior to analysis. FT-IR spectra were acquired from pressed pellets on a Bruker Vertex 80. Pressed pellets were made of dried nanocrystals (3 mg) and an internal standard, Prussian blue Fe4[Fe(CN)6]3, in a dry KBr matrix (100 mg) in order to gain semiquantitative information. 1H NMR spectra were collected on a Varian 500 spectrometer (500 MHz in 1H) with chemical shifts represented in units of ppm. All spectra are normalized relative to the residual benzene solvent peak at 7.16 ppm for the purpose of enabling semiquantitative comparison between different samples. NMR sample preparation and analyses were conducted according to previously published procedures for nanocrystal ligand analysis.49 Briefly, samples were prepared by digesting dried nanocrystal solids (∼50 mg) in half-concentrated aqua regia (7 mL), and then the organics were extracted using d6-benzene (2 mL). The solvent was then dried with MgSO4 and filtered before 1H NMR analysis. 32 scans were taken for each sample, and the data are presented as averages of those scans. XPS spectra were obtained using
separation). Common bidentate ligands (e.g., 1,2-ethanedithiol (EDT), 3-mercaptoproprionic acid) and atomic halide ligands (e.g., iodide) have been applied via thin film ligand exchange;36 however, thin film ligand exchange has several inherent drawbacks: (i) ligand exchange can be impeded by slow and/ or incomplete solid-state ligand diffusion through the BHJ film, (ii) reduction of the film volume through exchange of small ligands for large ones can lead to film cracking, and (iii) the dipwashing process used for thin film ligand exchange can lead to a low ligand exchange efficiency (yield) and may also be incompatible with large scale solution processing.38 With this in mind, a quantitative colloidal ligand exchange prior to BHJ deposition would be beneficial, but it still remains a challenge to generate a stable suspension of colloidal lead chalcogenide nanocrystals exchanged with small organic ligands because of their tendency to agglomerate,37 etch, and/or oxidize after ligand exchange.39 For example, simple alkylamines (e.g., n-butylamine or BA) typically require 48−72 h to achieve incomplete ligand exchange, and lead to surface etching and poor colloidal stability for PbS nanocrystals in air.39 Colloidal ligand exchange of PbS nanocrystals with arenethiolate ligands still required a postdeposition, thin film ligand treatment with 3-mercaptopropionic acid to achieve PCEs of 3% in hybrid poly(3-hexylthiophene) (P3HT):PbS nanocrystal BHJ solar cells.37 On the other hand, atomic halide ligands have proven successful in achieving high efficiency colloidal quantum dot solar cells,40−42 and in addition to allowing for efficient interparticle coupling, have been shown to effectively passivate surface trap states in PbS nanocrystals.43 While there are several successful examples of colloidal ligand exchange of lead chalcogenide nanocrystals with atomic halide ligands,44−47 these procedures require the ligand-exchanged nanocrystals to be dispersed in polar solvents that are orthogonal to the solubility of semiconducting polymers. Therefore, we sought to exploit the beneficial attributes of halide passivated PbS nanocrystals that are arrived at through a colloidal ligand exchange, but are dispersible in solvents that are compatible with the donor polymers. Herein, we report a simple colloidal ligand exchange process for PbS nanocrystals using iodide ligands (either NH4I or PbI2) that allows them to be dispersed in 1,2-dichlorobenzene. This facilitates, for the first time, a one-step deposition of hybrid poly[2,6-(4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′3′-d]silole)-alt4,7-(2,1,3-benzothiadiazole)] (Si-PCPDTBT):PbS nanocrystal BHJs to generate photovoltaic devices with halide-passivated nanocrystal acceptors. The optimized hybrid solar cells exhibit a broad spectral response into the NIR, leading to a PCE up to 4.8% under AM 1.5G illumination for the PbI2-exchanged PbS nanocrystal acceptors. This device performance is among the best for hybrid polymer:nanocrystal solar cells, and is the highest for a one-step BHJ deposition performed without any postdeposition ligand exchanges. Results from ultrafast transient absorption (TA) and time-resolved photoluminescence (PL) spectroscopies indicate more efficient charge transfer and slower recombination of the charge separated state in hybrid films with PbI2 ligand treatment when compared to hybrid films with NH4I surface ligands.
2. EXPERIMENTAL SECTION Materials. Lead oxide (PbO, 99.99%), lead iodide (PbI2, 99.9985%), ammonium iodide (NH4I, 99.999%), and n-butylamine (99%) were purchased from Alfa Aesar. Bis(trimethylsilyl) sulfide ((TMS)2S, > 95.0%) was purchased from TCI America. Oleic acid B
DOI: 10.1021/acs.chemmater.6b00185 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials a Kratos Axis Ultra X-ray photoelectron spectrometer with an analyzer lens in hybrid mode. High resolution scans were performed using a monochromatic aluminum anode with an operating current of 5 mA and voltage of 10 kV using a step size of 0.1 eV, a pass energy of 20 eV, and a pressure range between 1 and 3 × 10−8 Torr. The binding energies for all spectra were referenced to the C 1s core level at 284.6 eV. Inductively coupled plasma optical emission spectroscopy (ICPOES) was performed on each nanocrystal sample for analysis of lead and sulfur using a Thermo Scientific Icap 7000 series ICP-OES. Film thicknesses were determined using a J. A. Woollam variable angle spectroscopic ellipsometer equipped with a 150 W Xe arc lamp. TEM images of BHJ films supported on copper (Ted Pella, Inc.) were obtained on a JEOL JEM-2100F microscope at an operating voltage of 200 kV, equipped with a Gatan Orius CCD camera. Photoluminescence lifetime studies. For time correlated single photon counting (TCSPC) measurements, the samples were spun cast onto borosilicate glass in a glovebox with optical densities between 0.1−0.2 at the excitation wavelength of 500 nm. To avoid any oxidative damage, the film also had an additional glass window placed on the top surface and the outer edges were sealed with epoxy under a nitrogen atmosphere. Lifetime measurements were carried out using the output of a Coherent RegA 9050 regenerative amplifier operating at 250 kHz. The amplified 800 nm pulse was then used to pump an optical parametric amplifier, Coherent OPA 9450, which produced the 500 nm excitation beam used in the experiment. The lifetime data were measured using pump pulses of power 0.5, 0.67, and 1.7 mW and with a spot size of 250 μm at the sample, resulting in excitation fluences of 4.0, 5.4, and 14 μJ cm−2 for the neat Si-PCPDTBT and hybrid Si-PCPDTBT:nanocrystal BHJ films with NH4I- and PbI2exchanged PbS acceptors, respectively. PL lifetimes were measured by detecting the emission at 700 nm for the neat polymer and hybrid films. The film samples were not moved for the duration of the experiment and remained static with respect to the excitation beam. TCSPC measurements were performed using a R3809U-50 Hamamatsu PMT with a B&H SPC-630 module (time resolution of 22 ps). The monochromator grating was blazed at 600 nm with 1200 g mm−1 and a slit width of 1.2 mm was used, giving a spectral bandpass of 4.17 nm. The lifetime measurements were limited by the response of the detector (22 ps), which was longer than the pulse width of the excitation beam from the OPA (
