Synthesis, optical properties and photovoltaic applications of hybrid rod-coil diblock copolymers with coordinatively attached CdSe nanocrystals

Article abstract of DOI:10.1039/c4ra04347b

The performance of hybrid solar cells based on conjugated polymers and nanostructured inorganic semiconductors is often limited by the poor interfacial interaction and the lack of controlled phase separation. Improvements are being made by building intima

Full text of DOI:10.1039/c4ra04347b

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Synthesis, optical properties and photovoltaic
applications of hybrid rod–coil diblock copolymers
with coordinatively attached CdSe nanocrystals†
Cite this: RSC Adv., 2014, 4, 35823
Shaohua Li,‡a Yong Li,‡a Clarissa A. Wisner,b Lu Jin,a Nicholas Leventisb
a
*
and Zhonghua Peng
The performance of hybrid solar cells based on conjugated polymers and nanostructured inorganic
semiconductors is often limited by the poor interfacial interaction and the lack of controlled phase
separation. Improvements are being made by building intimate contact between the two components
through coordinative linkages. In this contribution, three rod–coil diblock copolymers (DCPs) of the
modified poly(3-hexylthiophene)-polystyrene (P3HT-PS) type with different phosphorus-containing
functional groups for binding to inorganic nanoparticles are reported. Their corresponding P3HT-PS-
CdSe hybrid DCPs (HDCPs) were prepared by ligand-exchange with chemically prepared CdSe
nanocrystals. The three DCPs have different size disparity between the rod and coil blocks, where the
dominant block dictates their solid state aggregation behavior. As a result, the three DCPs show very
different fluorescence properties in the solid state. After binding with CdSe nanocrystals, nanocrystal
association appears to dominate the solid state aggregation in all three HDCPs, making them exhibit
comparable solid state optical properties. Solar cell devices of HDCPs showed high open circuit voltages
of 1.13–1.40 V and improved power conversion efficiencies (PCEs) over devices fabricated from the
corresponding DCPs without CdSe attachment. It is believed that the improvement of the PCE is
brought about by intimate contact between the P3HT and the CdSe components, which enhances the
initial charge separation from P3HT to the CdSe nanocrystals. The device performance is however
hampered by the low nanoparticle loading and the short P3HT block length, which are being addressed.
Received 9th May 2014
Accepted 8th August 2014
DOI: 10.1039/c4ra04347b
www.rsc.org/advances
CdSe,9,17,18 CdTe,19,20 CuInSe2,21 PbS,22 TiO2,23,24 and ZnO,25,26 etc.)
and of different nanostructures (nanoparticles,19 nanocrystals
(NCs),14 quantum dots, nanowires,16 nanorods,15,17,20 nanoporous
1 Introduction
Organic–inorganic hybrid solar cells (HSCs) have drawn
increasing attention in recent years as a promising photovoltaic
technology.1–5 HSCs composed of either bulk6–8 or ordered9–12
heterojunctions of conjugated polymers and inorganic semi-
conductor nanostructures have been demonstrated and their
device efficiencies have been steadily climbing.5,13 Poly(3-
hexylthiophene-2,5-diyl) (P3HT), as a benchmark photovoltaic
material, is one of the most extensively utilized conjugated
polymers in HSCs.5,6 Composites based on P3HT and inorganic
semiconductors of different compositions (such as Si,14 CdS,15,16
structures,27 etc.) have been studied in solar cell devices. In these
HSCs, P3HT oen plays the roles of photosensitizer, excitonic
electron donor and hole transporter, while the inorganic
components serve as electron acceptors and electron trans-
porters. It is envisioned that the high charge carrier mobility of
inorganic semiconductors may help lead to signicantly
enhanced photoinduced charge separation efficiencies due to
fast electron dissipation through the semiconductor network.5
HSCs may be fabricated by using simple physical mixtures of
P3HT and inorganic semiconductors. These composites,
however, without strong chemical interactions between the
organic and inorganic components, oen exhibit inefficient
interfacial charge separation.28 In addition, because of the
drastic structural and property differences between those two
components, macroscopic phase separation is almost inevi-
table, which eventually limits the performance and long-term
stability of the resulting devices.29 To improve the compati-
bility and to ensure intimate contact between the two compo-
nents as well as the structural stability of the blend lms,
researchers have developed some viable approaches such as
aDepartment of Chemistry, University of Missouri-Kansas City, Kansas City, Missouri
64110, USA. E-mail: pengz@umkc.edu; Fax: +1-816-235-2290; Tel: +1-816-235-2288
bDepartment of Chemistry, Missouri University of Science and Technology, Rolla,
Missouri 65409, USA
† Electronic supplementary information (ESI) available: The synthesis, absorption
and uorescence spectra of HDA/TOPO-capped CdSe NCs, the MALDI-TOF MS
results of the vinyl-terminated P3HT and the P3HT-MI, FT-IR of P3HT-PS-2 and
P3HT-PS-CdSe-2, EDX analysis of HDCPs, the calculation of the weight ratio of
the hybrids, TGA gures of DCPs and HDCPs, and the J–V curves of the solar
cells of HDCPs are included. See DOI: 10.1039/c4ra04347b
‡ These authors contributed equally to this work.
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surface modication on the inorganic component or the 13C and 31P NMR spectra were recorded in CDCl3 on a Varian
attachment of inorganic-binding ligands to the organic INOVA 400 MHz FTNMR spectrometer. All samples were refer-
component.24,30ꢀ33 Typically, a binding ligand is attached to the enced to the deuterated solvent for the 1H and 13C NMR
end of a conjugated oligomer or polymer which is then graed to measurements, while triphenylphosphine (TPP) was applied as
the surface of an inorganic nanostructure to form the organic– an internal standard for the 31P NMR measurements. Mass
inorganic hybrids.32,33 Improved compatibility between and measurements were carried out on a Voyager DE Pro (Perceptive
enhanced charge separation across the organic and inorganic Biosystems/ABI) MALDI-TOF mass spectrometer with dithranol
components have been demonstrated. Herein, we report the (1,8-dihydroxyanthrone) was used as the matrix. Gel permeation
detailed synthesis, optical property studies and photovoltaic chromatography (GPC) measurements were performed on a
performance evaluations of three hybrid rod–coil diblock copol- Tosoh Ecosec HLC-8320GPC system equipped with triple
ymers of modied P3HT-polystyrene (P3HT-PS) with coor- detectors (a differential refractometer,
a light scattering
dinatively attached CdSe nanocrystals in the coil block. Rod–coil detector, and a UV detector) and a styragel column. THF was
diblock copolymers (DCPs) are an important class of block poly- used as the mobile phase. The instrument was calibrated by the
mers with potentially hierarchical ordered structures at multiple use of ve polystyrene standards ranging from 8000 to 90 000 in
length scale.34ꢀ36 The rigid rod block, mostly an extended p- number-average molecular weights. FT-IR spectra were recorded
conjugated system, is usually a liquid-crystal-forming mesogen on a Shimadzu IRAffinity-1 FTIR spectrophotometer. Trans-
which plays an important role during microphase separation. mission electron microscopy (TEM) images were taken using a
Having semiconducting nanoparticles attached to the coil block FEI Tecnai F20 200 kV super twin lens TEM in standard mode.
helps preserve p-stacking of the rod block while at the same time Thermal gravimetric analyses were performed on Shimadzu TGA-
promotes microphase separation due to the extreme contrast in 50. UV/Vis absorption spectra were collected on a Hewlett-
architecture between the two blocks. Such hybrid diblock copol- Packard 8452A diode array spectrophotometer. Fluorescence
ymers are thus potentially attractive systems for HSCs.
spectra were measured with a Shimadzu RF-5301 PC spectro-
Fig. 1 shows the structures of the reported hybrid systems. uorophotometer. Fluorescence quantum yields for solutions
The rod block is P3HT while the coil block is PS which is were calculated with quinine sulfate in 1 N H2SO4 (4 z 0.58) as
chemically-modied with phosphorus-containing functional the standard. Cyclic voltammetry studies were performed with a
groups as pendants. It is envisioned that having multiple BAS Epsilon EC electrochemical station, using a Pt working
binding ligands in the PS block helps create a tight and strong electrode of 1.6 mm in diameter, a silver wire as the reference
binding between the DCP and the nanoparticles,37,38 which can electrode and a Pt wire as the counter electrode under argon
be conducive to efficient photoinduced charge transfer. Solar protection. A 0.1 M tetra-n-butylammonium hexauorophosphate
cells fabricated from these hybrids indeed showed improved solution in acetonitrile was used as supporting electrolyte. Cali-
performances over devices fabricated from the corresponding bration of the potential was carried out by a ferrocene/
DCPs without CdSe coordination and devices of covalently ferrocenium (Fc/Fc+) redox couple whose absolute energy was
bonded donor–acceptor DCPs.
assigned as ꢀ4.80 eV vs. vacuum. The highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO) energy levels were calculated by HOMO ¼ ꢀ(Eooxnset + 4.80)
(eV) and LUMO ¼ ꢀ(Eroendset + 4.80) (eV), respectively.
2 Experimental
2.1 Materials and characterization
All chemicals were purchased from Aldrich or Acros and were
used without further purication unless otherwise noted. H,
1
2.2 Synthesis of modied P3HT-PS DCPs
Diethyl (4-vinylbenzyl)phosphonate (M1).39 p-Vinylbenzyl
chloride (2.15 g, 14.1 mmol) was rst stirred with NaI (10.5 g, 70.4
mmol) in acetone solution (40 mL) overnight. Ether (30 mL) was
added to the reaction mixture, which was then washed three
times with deionized water (100 mL). The formed vinylbenzyl
iodide was collected by removing the organic solvents under
vacuum, which was then stirred with triethylphosphite (2.34 g,
14.1 mmol) at room temperature for 12 h. The side product
(iodoethane) was removed by rotary evaporation, and column
chromatography purication of the product on silica gel with
dichloromethane as the eluent afforded diethyl(4-vinylbenzyl)
1
phosphonate as pale yellow liquid (3.34 g, 93% yield). H NMR
(400 MHz, CDCl3): d/ppm ¼ 7.36 (d, J ¼ 8.0 Hz, 2H), 7.25 (d, J ¼
10.4 Hz, 2H), 6.69 (dd, J ¼ 10.8 Hz, 18.0 Hz, 1H), 5.73 (d, J ¼ 17.6
Hz, 1H), 5.22 (d, J ¼ 17.6 Hz, 1H), 4.11–3.92 (m, 4H), 3.14 (d, J ¼
21.6 Hz, 2H), 1.23 (t, J ¼ 7.2 Hz, 6H); 13C NMR (400 MHz, CDCl3):
d/ppm ¼ 135.7, 135.6, 130.8, 129.2, 125.8, 113.1, 61.4, 33.5, 32.2,
15.8; 31P NMR (400 MHz, CDCl3): d/ppm ¼ 21.2.
Fig. 1 Hybrids of P3HT-polystyrene diblock copolymers with CdSe
nanoparticles.
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Dioctyl (4-vinylbenzyl)phosphine oxide (M2). The mixture
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P3HT-PS-n-CdSe. Detailed preparation of HDA/TOPO-
containing p-vinylbenzyl chloride (0.152 g, 1 mmol), dioctyl capped CdSe nanocrystals was described in the ESI.† The as-
phosphine oxide (0.220 g, 0.8 mmol), (TBA)2SO4 (29 mg), toluene prepared CdSe nanocrystals (100 mg) were stirred at 90 ꢁC in
(3 mL) and 30% NaOH (1.2 mL) was stirred for 24 h. The organic pyridine (10 mL) for 12 h. Aer cooled to room temperature,
phase was washed with water, and the product was precipitated hexane was added to the solution and pyridine-capped CdSe
by adding hexane. Recrystallization of the precipitates from nanocrystals were precipitated and collected by centrifuge. This
hexane afforded the title product as white solids (0.312 g, 91% process was repeated for two additional times. The resulting
1
yield). H NMR (400 MHz, CDCl3): d/ppm ¼ 7.36 (d, J ¼ 8.0 Hz, pyridine-capped CdSe nanocrystals were collected and dried
2H), 7.18 (d, J ¼ 10.4 Hz, 2H), 6.68 (dd, J ¼ 10.8 Hz, 17.6 Hz, 1H), with a stream of nitrogen gas. Pyridine-capped CdSe nano-
5.72 (d, J ¼ 17.6 Hz, 1H), 5.23 (d, J ¼ 10.8 Hz,1H), 3.10 (d, J ¼ 14.4 crystals (10 mg) were added to a solution of P3HT-PS-n (20 mg)
Hz, 2H), 1.6–1.5 (m, 8H), 1.4–1.2 (m, 20H), 0.87 (t, J ¼ 6.8 Hz, 6H); in THF (5 mL), and the suspension was subjected to ultra-
13C NMR (400 MHz, CDCl3): d/ppm ¼ 136.2, 131.8, 129.6, 129.0, sonication for 3 h. The mixture was cooled to room temperature
126.6, 113.4, 35.7, 32.2, 30.9, 28.8, 27.3, 26.7, 22.5, 21.4, 14.0; 31
P
and then passed through a 0.45 mm lter, where the undissolved
pyridine-capped CdSe nanocrystals were removed. Acetone was
added to the clear ltrate solution and the precipitated P3HT-
NMR (400 MHz, CDCl3): d/ppm ¼ 47.0.
Poly(3-hexylthiophene) macroinitiator (P3HT-MI).40
A
sample of 2-bromopropionyl bromide (1.5 mL, 12 mmol) was PS-n-CdSe nanocomposites were collected by centrifuge. The
added dropwise into a solution containing P3HT-OH (0.49 g, 0.02 precipitated nanocomposites were further washed with meth-
mmol), triethylamine (2 mL, 14.7 mmol) and anhydrous THF (20 anol and acetone for a few times.
mL) at room temperature under N2. Aer being stirred at 40 ꢁC
for 24 h, the solution was poured into methanol. The polymer
2.3 Photovoltaic devices fabrication and characterization
precipitates were collected by ltration and puried by Soxhlet
extraction with methanol and hexane. Yield: 96%. 1H NMR (400
Indium tin oxide (ITO) coated glass (Delta Technologies; sheet
resistance, 8–12 U per square) was used as substrates. Under the
MHz, CDCl3): d/ppm ¼ 6.96 (br), 4.35 (m), 3.11 (t, J ¼ 7.2 Hz), 2.78
protection of Magic tape, the ITO side of each substrate
was patterned by etching with aqua regia vapor. The patterned
(br), 1.83 (d, J ¼ 6.8 Hz), 1.68 (br), 1.5–1.3 (br), 0.89 (s).
P3HT-PS-1,2,3. All three DCPs were prepared by the atom
ITO glass substrates were cleaned in an ultrasonic bath
transfer radical polymerization (ATRP).41,42 In a typical poly-
sequentially by hot detergent, water, deionized water,
merization process, CuBr, N,N,N0,N00,N00-pentamethyldiethyl-
toluene, acetone, and isopropyl alcohol, and then dried by
enetriamine (PMDETA, ligand), and styrene (M) together with
compressed air. Cleaned ITO substrates were treated with UV
ozone for 45 min before use. Highly conductive poly(3,4-
ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS;
Heraeus Precious Metals; Clevios P VP AI4083) thin layer was
the modied styrene monomer (M1 or M2) were added to dry p-
xylene. The reaction mixture was degassed by three freeze–
pump–thaw cycles, and then lled with N2 gas. A degassed
solution of P3HT-MI in p-xylene (1 mL) was then added, and the
spin-coated (4000 RPM, 30 s) onto the ITO substrates from an
reaction mixture was stirred at 110 ꢁC for 24 h under N2
aqueous solution. The PEDOT:PSS thin lms were dried at
atmosphere. For P3HT-PS-1, styrene and M1 were used as
120 ꢁC for 45 min on hotplate in air. The P3HT-PS-CdSe-1,
monomers with a molar ratio of 1 : 1. For P3HT-PS-2, and P3HT-
P3HT-PS-CdSe-2, and P3HT-PS-CdSe-3 solutions were prepared
PS-3, styrene and M2 were used as monomers with a molar ratio
in chloroform with concentration of 5 mg mLꢀ1. The above
of 1 : 1 and 4 : 1, respectively. The mole ratio of [M]total : [P3HT-
solutions were passed through a 0.45 mm lter and spin-coated
MI] : [CuBr] : [PMDETA] was set to be 100 : 1 : 1 : 6 for P3HT-PS-
on top of the PEDOT:PSS layer at 400 RPM for 30 s. The devices
1 and P3HT-PS-2, while 250 : 1 : 20 : 120 for P3HT-PS-3. Aer
were transferred into a glove box and thermally annealed
cooled to room temperature, a minute amount of THF was
added to the dark-red solution. The solution was then passed
composed of 45 nm thick Ca and 100 nm thick Al were depos-
through a short neutral Al2O3 column to remove the copper
catalyst. The eluent was poured into methanol and the resulting
precipitates were collected by centrifugation. The collected
polymers were redissolved in THF and reprecipitated from
there at 120 ꢁC for 10 min in dark. Subsequently, electrodes
ited on top in sequence by thermal evaporation under high
vacuum (<2 ꢂ 10ꢀ6 mb) through a shadow mask. The active area
of 0.14 cm2 of the devices was dened by the overlapped area of
the ITO and the deposited Ca/Al electrodes. Current–voltage
characteristics of the solar cells were measured using a Keithley
2400 Source Meter. Incident photon to current conversion effi-
ciency (IPCE) spectra were recorded using a Newport QE-PV-SI
methanol to give the corresponding diblock copolymers.
1
P3HT-PS-1 (23%). H NMR (400 MHz, CDCl3): d/ppm ¼ 7.02
(br), 6.97 (s), 6.50 (br), 3.96 (br), 3.04 (br), 2.79 (br), 1.84 (br),
1.69 (br), 1.4–1.1 (br), 0.90 (br). 31P NMR (400 MHz, CDCl3): d/
QE/IPCE measurement kit. The thickness of the thin lms
ppm ¼ 22.1.
1
was measured with a Tencor Alphastep 200 automatic step
proler.
P3HT-PS-2 (17%). H NMR (400 MHz, CDCl3): d/ppm ¼ 7.02
(br), 6.96 (s), 6.43 (br), 3.10 (br), 2.78–2.60 (br), 2.0–1.1 (br), 0.88
(br). 31P NMR (400 MHz, CDCl3): d/ppm ¼ 48.1.
1
P3HT-PS-3 (22%). H NMR (400 MHz, CDCl3): d/ppm ¼ 7.05
(br), 6.97 (s), 6.52 (br), 3.09 (br), 2.79 (br), 2.3–1.2 (br), 0.89 (br).
31P NMR (400 MHz, CDCl3): d/ppm ¼ 46.0.
3 Results and discussion
Scheme 1 shows the synthesis of the three hybrid DCPs. A P3HT
rod block end-functionalized with a-bromopropanoate (P3HT-MI),
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was synthesized following literature procedures40 and was used as
a macroinitiator to grow the coil block using atom-transfer
radical polymerization (ATRP).41,42 All three DCPs have the
same P3HT block length with an average molecular weight of
2338 (average number of repeating unit 14) and a polydispersity
of 1.03, measured by MALDI-TOF mass spectrometry. The P3HT
with shorter chain length was selected here as it has a higher
solubility in THF to ensure a better functionalization with the
end group and a narrow molecular weight distribution. To
realize a coil block which can bind to CdSe nanoparticles, two
styrene derivatives, one functionalized with a phosphonate
group (M1) and the other with a phosphine oxide (M2), both of
which are well known ligands for CdSe binding, were synthe-
sized as monomers for ATRP. M1 or M2 was copolymerized with
styrene in a 1 : 1 or 1 : 4 monomer ratio to ensure the incor-
poration of sufficient binding ligands in the coil block but
without steric congestion. Using M1 and M2 respectively as the
functionalized monomer, P3HT-PS-1 and P3HT-PS-2 were
synthesized under identical ATRP conditions, leading to coil
blocks with comparable sizes. P3HT-PS-3, on the other hand,
was synthesized using a much higher monomer-to-initiator
ratio, resulting in a DCP with a much longer coil block. All
three DCPs show excellent solubility in chloroform and THF,
and are moderately soluble in acetone.
Fig. 2 1H NMR spectra of P3HT-PS-1,2,3 in CDCl3.
P3HT-PS-1, using the integration ratio of well resolved signals 3
and c (4 : 1), one can calculate the average number of M1 unit in
the diblock copolymer to be 3 (the average number of P3HT
repeating unit is 14 based on MALDI-TOF measurements). With
P3HT and M1 compositions in the DCP known and using the
integration of the aromatic signals (either a/1/a or b/b or both),
one can calculate the number of styrene units in the DCP to be
5, giving P3HT-PS-1 an average molecular weight of 3640. For
the other two DCPs, well-resolved signal 2 was used as the
reference for the P3HT block. Based on the integration of
signals 3/e and 2, one can calculate the number of protons
corresponding to proton e, and thus the number of repeating
1
Fig. 2 shows the H NMR spectra of the three DCPs. While
the signals are broad and most are signicantly overlapped, one
can still identify certain signals which are characteristic to each
repeating unit. For example, signals marked as 1, 2 and 3 in
Fig. 2 can be attributed to the 3-hexylthiophene unit. The
phosphonate or phosphine oxide-functionalized styrene units
give distinct signals marked as a, b, c, d and e. The phenyl
protons (a, b) of styrene are overlapped with those of M1 or M2
(a & b). Based on the integration of the characteristic signals,
one can calculate the diblock copolymer compositions. For
Scheme 1 Synthesis of P3HT-PS DCPs and ligand exchange with pyridine-capped CdSe nanocrystals.
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unit M2. Once the P3HT and M2 compositions in the DCP are
known, the integration ratio of the aromatic signals versus that
of 2 can be used to calculate the number of styrene repeating
unit. Following these procedures, the number of M2 and styrene
repeating units in P3HT-PS-2 and P3HT-PS-3 are found to be 5
and 7, 12 and 51, respectively, from which their number-average
molecular weights are calculated to be 5038 and 12 355,
respectively. These numbers are rather consistent with the
molecular weights measured by Gel Permeation Chromatog-
raphy (GPC), which gave Mns of 4975, 5499, and 14 822 for
P3HT-PS-1, P3HT-PS-2 and P3HT-PS-3, respectively. The coil
block compositions in all three DCPs reasonably match the
monomer loading ratios with styrene only in slight excess,
indicating that M1 and M2 have similar reactivity to the styrene
monomer.
CdSe nanocrystals with average sizes around 5 nm were
synthesized using the well demonstrated hot-injection
method.43 The original hexadecylamine (HAD) and tri-
octylphosphine oxide (TOPO) capping ligands on the CdSe
nanocrystals were rst replaced with pyridine, which was then
exchanged with the ligands on the DCPs, as shown in Scheme 1.
Compared to the direct growth of nanocrystals in the DCP
matrix, this process is advantageous since nanocrystals are
synthesized separately, allowing much better control in their
size and size distribution. The ligand exchange was carried out
in THF where the pyridine-capped CdSe nanocrystals are not
soluble. Excess non-DCP bound nanocrystals were got rid of by
ltration. The CdSe-coordinated HDCPs were isolated by
precipitating from acetone where any DCPs without CdSe
binding remain soluble. The as-prepared hybrids are soluble in
chloroform and THF.
Fig. 3 FT-IR spectra of P3HT-PS-1,3 and P3HT-PS-CdSe-1,3.
The formation of DCP–CdSe hybrids was conrmed by FT-IR,
TEM and SEM-EDX analysis (see Fig. S7–S9 in ESI†). Fig. 3
shows the FT-IR spectra of P3HT-PS-1,3 before and aer CdSe
coordination. For all DCPs, the IR absorption at 2960–2860
cmꢀ1 (C–H stretching) and 1730–1600 cmꢀ1 (aromatic C]C
stretching) can be clearly observed. The characteristic P]O
stretching band appears at around 1250–1260 cmꢀ1, indicating
the existence of phosphonate or phosphine oxide groups. Aer
CdSe coordination, those P]O stretching bands shi to lower
wavenumbers, indicating O–Cd binding which weakens the
P]O bond.44
High-resolution TEM images of the three DCP–CdSe
hybrids were collected (Fig. 4). CdSe nanocrystals with distinct
crystalline lattice fringe can be clearly observed in all three
composite samples, and the average size of nanocrystals is
around 5 nm. The lattice fringe spacing of the CdSe NCs is
measured to be 0.334 nm and is consistent with the lattice
spacing of CdSe NCs with the Wurtzite structure.45 The inset in
Fig. 4 HRTEM images of (a) P3HT-PS-CdSe-1, (b) P3HT-PS-CdSe-2,
(c) and (d) P3HT-PS-CdSe-3. The inset in (d) displays the fast Fourier
transformation (FFT) pattern of the nanocrystals in P3HT-PS-CdSe-3.
The UV/Vis absorption spectra of DCPs before and aer CdSe
Fig. 4d shows the fast Fourier transformation (FFT) pattern of coordination are shown in Fig. 5. The spectra are dominated by
P3HT-PS-CdSe-3 hybrids, from which the crystallinity of the the absorption of the P3HT block with an absorption maximum
CdSe nanocrystals was conrmed. As can be seen from all around 438 nm. Aer CdSe coordination, this band is slightly
images, the CdSe nanocrystals are uniformly distributed red-shied, and a well separated weak absorption band at 570–
among all DCP substrates although some aggregation is noted. 580 nm is observed for all three hybrids as shown clearly in the
The aggregation may be due to the binding of one DCP to more inset of Fig. 5. This absorption is attributed to CdSe NCs. It is
than one nanocrystal since each coil block has multiple noted that the absorption maxima of the CdSe NCs in the hybrid
binding ligands.
DCPs are slightly red-shied compared to that of the original
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temperature to 600 ꢁC at a rate of 10 ꢁC minꢀ1. For all polymers,
weight loss occurred in the temperature range of 200–480 ꢁC. No
ꢁ
or negligible further weight loss was observed up to 600 C. At
600 ꢁC, the remaining weight percentage is 42, 38 and 36%, for
P3HT-PS-1, P3HT-PS-2, P3HT-PS-3, respectively. Their corre-
ꢁ
sponding HDCPs at 600 C have higher remaining weights of
62, 63, and 41%. Assuming CdSe is not losing weight in the
heating process, and the difference in remaining weight
percentage between the HDCPs and their corresponding DCPs
at 600 ꢁC is due to the coordinated CdSe NCs, one can calculate
the weight percentage of the NCs in the original HDCPs to be 34,
40, and 8%, respectively. While these values are higher than
those estimated based on UV/Vis absorption spectra, both
techniques indicate that P3HT-PS-CdSe-2 has the highest CdSe
NC loading.
The absorption spectra of the DCP lms before and aer
CdSe coordination have also been studied. As shown in Fig. 6,
all DCP lms show a broad absorption peak at 483–492 nm, over
40 nm red-shied compared to that of their solution spectra.
Such red shis are common for P3HT-containing DCPs due to
the P3HT inter-chain p-stacking in the thin solid lms.46 It is
noted that both the maximum absorption wavelength and the
absorption band edge of P3HT-PS-3 lm are clearly shorter than
those of the other two DCPs, indicating that the P3HT
Fig. 5 UV/Vis absorption spectra of P3HT-PS-1 (a), 2 (b) and 3 (c)
before (dotted) and after (solid) CdSe coordination.
TOPO-capped CdSe NCs (563 nm). The 10 to 15 nm redshi may
be due to the change in surface ligands (for P3HT-PS-1) or
ligand densities (for P3HT-PS-2 and P3HT-PS-3). Note that the
extent of redshi increases from P3HT-PS-1, to P3HT-PS-2 and
to P3HT-PS-3. Using a P3HT-MI solution and a TOPO-capped
CdSe NC solution with known concentrations as standards
and assuming that the total weight of surface ligands are
negligible compared to the weight of the core NCs, one can
calculate the DCP–CdSe weight ratio in the hybrids using
the absorbance at 444 nm and the maximum absorption
wavelength in the 560–580 nm range. Using this method, the
CdSe–DCP weight ratios are estimated to be 1 : 12, 1 : 8, and
1 : 11 for P3HT-PS-1,2,3 respectively (see ESI† for the detailed
calculation). While this estimate is very rough, it nonetheless
indicates that the nanocrystal contents in all three hybrids are
rather low.
The amount of NCs in the HDCPs was also estimated by the
thermogravimetric analysis (TGA) (see Fig. S10 in ESI†). All
Fig. 6 UV/Vis absorption spectra of thin films of DCPs before and after
DCPs and HDCPs were heated, under N2 protection, from room CdSe coordination.
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p-stacking in P3HT-PS-3 is not as strong as in the other two coordinated. It is interesting to note that the relative sizes of the
DCPs. In P3HT-PS-1 and P3HT-PS-2, the P3HT block is longer P3HT block and the PS block change from P3HT dominant
than the PS block and likely dictates the diblock copolymer (P3HT-PS-1) to comparable (P3HT-PS-2) to PS dominant (P3HT-
aggregation. In P3HT-PS-3, however, the PS block is about 4 PS-3). The different size disparity in the three DCPs likely gives
times as long as the P3HT block, and thus likely dominates the rise to their different emission changes in responding to CdSe
self-assemble of the diblock copolymer. In other words, the NC binding. It is noted that the lm emission spectra of the
arrangement of P3HT blocks in P3HT-PS-3 is dictated not so three HDCPs do not differ as much as those of the three DCPs
much by their own p–p stacking but by PS blocks, making P3HT do. Binding with CdSe NCs signicantly alters the interactions
stacking not as effective. Aer the coordination of CdSe NCs, among PS blocks. The ligand–cluster coordination is much
the absorption of all hybrid lms further red-shied to 496–511 stronger than other non-covalent interactions existed in the
nm. The maximum absorption wavelength of P3HT-PS-CdSe-3 HDCPs, such as p–p stacking and alkyl chain interdigitation. It
is again shorter than those of the other two HDCPs.
is thus reasonable to assume that it is the NC-binded PS block
The uorescence emission spectra of DCPs and HDCPs in dictates solid state aggregation for all three hybrid DCPs,
chloroform solutions and as solid lms have been studied. As regardless of their initial PS block sizes, making all three hybrid
shown in Fig. 7, when excited at 440 nm, all DCP and HDCP DCPs with comparable solid state optical properties.
solutions show nearly identical uorescence emission spectra
The HOMO/LUMO energy levels of the CdSe NCs, DCPs and
and comparable uorescence quantum yields (ꢃ0.065). The HDCPs were studied by cyclovoltammetry (CV) measurements.
typical narrow emission of CdSe NCs (see Fig. S4 in ESI†) was Films of the TOPO/HDA-capped CdSe NCs, DCPs and HDCPs
not discernable in the emission spectra of the hybrids, most were prepared by drop-casting their solutions onto a Pt-disc
likely overshadowed by the broad emission of P3HT segments. working electrode. Aer drying under the ow of Argon, the
The lack of uorescence quenching indicates that the energy/ CV measurements of all samples were run under identical
electron transfer from the photoexcited P3HT chromophores conditions. As shown in Fig. 8, the cyclic voltammogram of pure
to the CdSe NCs is inefficient in dilute solutions. In solid state, CdSe NCs shows a clear reversible reduction wave with a half
the three DCPs show very different emission spectra. While cell potential of ꢀ1.13 eV (vs. Fc/Fc+), from which the LUMO
lms of P3HT-PS-1 and P3HT-PS-2 both gave an emission energy level of CdSe NCs can be estimated to be ꢃꢀ3.67 eV.
maxima at 636 nm, the maximum emission wavelength of
P3HT-PS-3 lm is only 576 nm, less than 10 nm red-shied over
its solution emission spectrum. As explained earlier, the p-
stacking of P3HT in P3HT-PS-3 is not the driving force for the
DCP self-assembly but rather the interaction among the much
longer PS blocks, which accounts for its much less red-shied
emissions. The binding of CdSe NCs to the PS block resulted
in starkly different changes in their lms' uorescence emis-
sions. While the binding of CdSe NCs leads to negligible change
in the lm emission spectrum of P3HT-PS-2, the emission
spectra of P3HT-PS-1 are blue shied by 21 nm while those of
P3HT-PS-3 are red-shied by 34 nm aer CdSe NCs were
Fig.
8 Cyclic voltammograms of P3HT-PS-1 (black), P3HT-PS-2
Fig. 7 Photoluminescence emission spectra of DCPs before and after (green) and P3HT-PS-3 (red) before (dotted line) and after (solid line)
CdSe coordination in dilution solutions (a) or as thin solid films (b).
CdSe coordination.
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There is no oxidation wave observed during the anodic scan. All
DCPs and HDCPs show a similar reduction wave onsetting
around ꢀ2.3 eV and a semireversible oxidation wave onsetting
at 0.3 eV (vs. Fc/Fc+), both of which are attributed to the P3HT
block. Careful comparison shows that redox processes in
HDCPs are all less reversible than those in DCPs. At a lower
negative potential, all three HDCPs show a small but clear
reduction hump with a peak potential around ꢀ1.1 eV. This
hump is missing in the voltammograms of the three DCPs and
can thus be attributed to the CdSe NCs. The observation of the
reduction process of CdSe NCs in the HDCPs not only conrms
the successful incorporation of the NCs in the HDCPs, but also
indicates that those incorporated NCs are susceptible to elec-
tron transfers. Based on the redox onset potentials, one can
calculate the HOMO and LUMO levels of the P3HT block to be
ꢀ5.10 eV and ꢀ2.51 eV, respectively. The driving forces for
potential charge transfer between the P3HT moiety and the
CdSe NCs can be estimated by the Rehm–Weller equation,46
DG ¼ e[Eox(D) ꢀ Ered(A)] ꢀ Eg ꢀ C
(1)
where DG is the free energy change (in eV) associated with the
photoinduced charge transfer process, Eox(D) and Ered(A) are the
oxidation potential of the donor (P3HT) and the reduction
potential of the acceptor (CdSe NCs), respectively, Eg is the
bandgap of the donor or acceptor, and C is the Coulomb term
which is ca. 0.06 eV in acetonitrile.46 The free energy change
associated with the electron transfer process from the excited
P3HT moiety to the LUMO of the CdSe NCs is calculated to be
ꢀ1.22 eV, while the free energy change associated with the hole
transfer process from the excited CdSe NCs to the HOMO of the
P3HT moiety is calculated to be ꢀ0.72 eV. The large negative
free energy changes indicate that the charge transfer at the
P3HT/CdSe NC interface is thermodynamically feasible.
Fig. 9 (a) Device architecture. (b) Current density–voltage (J–V)
curves (empty: dark; filled: illuminated) of the solar cells of glass/ITO/
PEDOT:PSS/P3HT-PS-2 or P3HT-PS-CdSe-2/Ca/Al. The illuminated
curves were measured under AM 1.5 G 1-sun (100 mW cmꢀ2) illumi-
nation. For clear identification of VOCs, the inset in (b) shows the illu-
minated J–V curves of the devices with the J-axis plotted in log-scale.
Table 1 Parameters of the photovoltaic cells under AM 1.5 G 1 Sun
(100 mW cmꢀ2) illumination
To evaluate the photovoltaic properties of the HDCPs,
regular photovoltaic cells with the conguration of glass/ITO/
PEDOT:PSS/P3HT-PS-CdSe/Ca/Al as illustrated in Fig. 9a were
fabricated. In general, the solar cells were fabricated by spin-
coating the HDCP solutions on top of ITO/PEDOT:PSS, fol-
lowed by the deposition of Ca/Al electrodes. Fig. 9b shows the J–
V curves of the solar cells of P3HT-PS-2 and P3HT-PS-CdSe-2,
while Table 1 summarizes their J–V characteristics, including
open circuit voltage (VOC), short circuit current density (JSC), ll
factor (FF), and power conversion efficiency (PCE).
With lm thickness of 80 ꢄ 5, 140 ꢄ 5 and 110 ꢄ 5 nm,
respectively, P3HT-PS-CdSe-1–3 show rather high VOCs but low
JSCs and poor ll factors. Estimated from the illuminated J–V
curves, the series resistance of these solar cells is 1.35 kU cmꢀ2
or greater which is large and likely contributed to the poor FF.
Devices fabricated from P3HT-PS-CdSe-2 give the highest PCE of
0.17%. Devices fabricated from the corresponding P3HT-PS
DCPs (without CdSe attachment) gave PCEs of 0.01%, indi-
cating that the attached CdSe NCs do help improving the device
performance. The IPCE spectra of the devices, shown in
Fig. 10b, reasonably match the absorption spectra of the cor-
responding thin lms (Fig. 10a) especially in the visible range,
conrming the photosensitivity contribution of the DCPs. There
JSC
Active material
VOC (V)
(mA cmꢀ2
)
FF
PCE (%)
P3HT-PS-CdSe-1
P3HT-PS-CdSe-2
P3HT-PS-CdSe-3
P3HT-PS-2
1.40
1.39
1.13
0.37
0.73
0.217
1.042
0.119
0.083
2.27
0.125
0.118
0.158
0.324
0.229
0.038
0.170
0.021
0.010
0.379
P3HT : CdSe (1 : 8)
is a slight mismatch in the UV region between the IPCE spectra
and the lm absorption spectra, presumably due to the fact that
the insulating PS block absorbs in the UV region as well
(Fig. 10a inset) but produces no photocurrent. Only absorptions
from CdSe and P3HT are expected to generate photocurrent.
Although the coordinatively binded CdSe nanocrystals help
improve the solar cell performance, the performance of these
HDCPs is clearly not on par with those of the best P3HT:CdSe
composites. There are a number of reasons for the HDCP's not
so appealing performance. First of all, the incorporated CdSe
NCs in the HDCPs is rather low. Among the three HDCPs, P3HT-
PS-CdSe-2 has the highest CdSe loading, which may be the
reason why it showed the best performance. However, its NC
loading is still only 40%. For P3HT:CdSe blends, the best
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particular differ signicantly, reecting different extent of
interchain P3HT stacking in the solid state due to the different
size disparity of the rod and coil blocks in the three DCPs.
Binding with CdSe nanocrystals was achieved by ligand-
exchange and was conrmed by FT-IR, TEM, EDX and TGA
measurements. The resulting HDCPs showed very different
optical properties from their corresponding DCPs, particularly
in the solid state. The energy levels of HDCPs were studied by
CV measurements and shown to be suitably aligned for PV
devices. The HDCPs showed improved solar cell performance
over their corresponding DCPs, conrming that the coor-
dinatively binded CdSe nanocrystals help improve the initial
photoinduced charge separation. The CdSe loading in the
HDCPs is however very limited, presumably due to the low
density of binding ligands in the coil block. As a result, the CdSe
nanocrystals in the HDCPs are mostly isolated and not forming
connected networks, which led to poor ll factors and lower
than expected device efficiencies. The short P3HT block length
also limited the device performance. Nevertheless, the bene-
cial effect of CdSe coordination is clearly demonstrated. When
longer P3HT block is used and higher CdSe loading is achieved,
signicant improvement in solar cell performance is expected.
The higher CdSe loading may be realized by increasing the
density of binding ligands in the coil block or directly attaching
binding ligands as side chain pendants to the P3HT rod block.
Efforts along these lines are in progress.
Fig. 10 (a) Absorption spectra of thermally annealed (at 120 ꢁC for 10
min under nitrogen atmosphere) HDCP thin films fabricated on
PEDOT:PSS/ITO substrates. The inset in (a) shows the absorption of
the reference film of PEDOT:PSS/ITO glass (magenta) and the one with
a very thin PS layer coated on it (royal). (b) IPCE spectra of the solar
cells of glass/ITO/PEDOT:PSS/P3HT-PS-CdSe/Ca/Al.
performing devices typically have a P3HT to CdSe weight ratio
around 1 : 8.47 In other words, the weight percentage of CdSe in
the blends is over 85%. The low NC content likely prevented the
aggregation of NCs to form continuous electron transporting
networks, which is supported by the TEM studies where NCs are
shown to be isolated. The lack of morphologies supporting
bicontinuous charge transporting pathways may also account
for the poor ll factors. Another reason for the poor perfor-
mance may be due to the short P3HT block length.6 The P3HT in
the HDCPs has an average molecular weight of only about 2300
while the best performing P3HT:CdSe blends has P3HTs with
molecular weights over 100 000.5 Regioregular P3HT with large
molecular weight can self-organize and align polymer chains to
form semicrystalline lamellar morphologies which possess
highly ordered packing and alignment, high hole mobility, and
strong interchain and interlayer interactions.12 However, for all
the three HDCPs in the present work, no long range order of
P3HT packing was observed from their HRTEM images (Fig. 4).
Indeed, a bulk heterojunction device using the short P3HT in
the P3HT : CdSe (1 : 8) blends showed a PCE of only 0.38%,
signicantly lower than similar BHJ devices fabricated from
high molecular weight P3HT : CdSe blends.47 The fact that the
P3HT-PS-CdSe-2, albeit with much lower CdSe loading, showed
PCEs not that much lower than that of the corresponding
P3HT : CdSe blend indicates again that coordination of NCs
with the DCP backbone improves device performance. To
increase CdSe NC loading in the HDCPs, PS blocks with higher
density of binding ligands may be needed. Using a P3HT block
with high enough molecular weights and a PS block with higher
binding capacity to NCs, the resulting HDCPs are expected to
show better performance.
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