Nano Res

1

Dithiol treatments enhancing the efficiency of hybrid

solar cells based on PTB7 and CdSe nanorods

Weining Luo1,2, Tonggang Jiu1 (), Chaoyang Kuang1, Bairu Li1, Fushen Lu2 (), and Junfeng Fang2 ()

Nano Res., Just Accepted Manuscript • DOI 10.1007/s12274-015-0810-2

http://www.thenanoresearch.com on May 4, 2015

© Tsinghua University Press 2015

Just Accepted

This is a “Just Accepted” manuscript, which has been examined by the peer-review process and has been

accepted for publication. A “Just Accepted” manuscript is published online shortly after its acceptance,

which is prior to technical editing and formatting and author proofing. Tsinghua University Press (TUP)

provides “Just Accepted” as an optional and free service which allows authors to make their results available

to the research community as soon as possible after acceptance. After a manuscript has been technically

edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP

article. Please note that technical editing may introduce minor changes to the manuscript text and/or

graphics which may affect the content, and all legal disclaimers that apply to the journal pertain. In no event

shall TUP be held responsible for errors or consequences arising from the use of any information contained

in these “Just Accepted” manuscripts. To cite this manuscript please use its Digital Object Identifier (DOI®),

which is identical for all formats of publication.

Nano Research

DOI 10.1007/s12274-015-0810-2

Dithiol Treatments Enhancing the Efficiency of Hybrid

Solar Cells Based on PTB7 and CdSe Nanorods

Weining Luo,ab Tonggang Jiu,a* Chaoyang Kuang, a Bairu

Li, a Fushen Lu,b* and Junfeng Fanga*

a Institute of New Energy Technology, Ningbo Institute of

Material Technology and Engineering (NIMTE), Chinese

Academy of Science (CAS), China.

b Department of Chemistry, College of Science, Shantou

University, China.

A new system of hybrid devices based on PTB7 and CdSe NRs was

thoroughly studied in this work. The power conversion efficiency

(PCE) of this system can be significantly enhanced by engineering the

PTB7/CdSe NRs interface with ethanedithiol (EDT) and

1,4-benzenedithiol (1,4-BDT) treatments and reached 2.58% and

2.79%, respectively, which were preferable to the pyridine treated

device (1.75%).

Dithiol treatments enhancing the efficiency of hybrid

solar cells based on PTB7 and CdSe nanorods

Weining Luoab, Tonggang Jiua ( ), Chaoyang Kuanga, Bairu Lia, Fushen Lub (), and Junfeng Fanga

( )

Received: day month year

Revised: day month year

Accepted: day month year

(automatically inserted by

the publisher)

© Tsinghua University Press

and Springer-Verlag Berlin

Heidelberg 2014

KEYWORDS

CdSe nanorods, PTB7,

ethanedithiol treatments,

1,4-BDT treatments,

hybrid solar cells

ABSTRACT

We have reported the fabrication of polymer/inorganic hybrid solar cells (HSCs)

based on CdSe nanorods (NRs) and the semiconducting polymer PTB7. The

power conversion efficiency (PCE) of hybrid solar cells can be significantly

enhanced by engineering the polymer/nanocrystal interface with ethanedithiol

(EDT) and 1,4-benzenedithiol (1,4-BDT) treatments and reached 2.58% and

2.79%, respectively. They were preferable to the pyridine coated nanorods

based device (1.75%). This improvement was ascribed to the thiols groups of

EDT and 1, 4-BDT which can tightly coordinate to the Cd ions to form

Cd-thialate on CdSe NRs surfaces thereby effectively passivating the NRs

surface and reducing the active layer defects. Therefore, the rate of exciton

generation and dissociation was enhanced and led to the improvement of the

device performance.

1. Introduction

Polymer/inorganic hybrid solar cells (HSCs) have

attracted increased attention for applications in

renewable energy because of their versatile and

synergistic optical and electronic properties of both

polymer and inorganic components [1-7].

Incorporating these nanocrystals (NCs) into

conjugated polymers matrix [8, 9] can supply the

additional advantage that light is absorbed by both

components and complement the visible light

absorption range of the polymers for solar cells

applications, which are insufficient for widely used

phenyl-C61-butyric-acid-methyl ester (PC61BM) in

organic solar cells [10]. Moreover, semiconductor

NCs (such as CdSe NCs) possess several attributes

that should make them attractive substitutes for

PC61BM, such as tunable band gaps and frontier

energy levels through both compositional control

and quantum confinement effect, high dielectric

constants to help overcome the strong exciton

binding energy of organic materials and high

electron mobilities [11-16].

Since the seminal work on hybrid solar cells based

on CdSe NCs and MEH-PPV by Alivisatos and

co-workers [17] decades ago, advances in the areas

have increased the power conversion efficiency to

above 4%, which significantly lags behind that for

all-organic solar cells (maximum PCE values

exceeding 10%) [18-20]. A critical challenge for

Nano Research

DOI (automatically inserted by the publisher)

Address correspondence to Tonggang Jiu, jiutonggang@nimte.ac.cn; Fushen Lu, fslu@stu.edu.cn; Junfeng Fang, fangjf@nimte.ac.cn

Research Article

| www.editorialmanager.com/nare/default.asp

2 Nano Res.

polymer/inorganic HSCs lies in the complex hybrid

material interface which is important for the charge

carrier separation and recombination. Xue et al. [21]

report that ethanedithiol treatments result in removal

of previous surface ligands and formation of

Cd-thialate on CdSe NCs surfaces. This led to

reduced exciton and charge carrier recombination

and increased electron transport. In hybrid device

fabrication the tetrapods or NRs with large aspect

ratio has been verified more advantageous compared

to spherical NCs when blended with polymer due to

the existence of directional charge transport paths

[22-24].

Efficient polymer/inorganic HSCs containing CdSe

NCs have been fabricated in combination with P3HT

[25, 26], OC1C10-PPV [27] and PCPDTBT [28-31] as

the hole acceptor. For solar cell applications, novel

alternating copolymers have been developed

successfully by tuning the chemical structure of

conjugated backbones to harvest more solar photons

up to 700 nm [19, 28, 30]. Among them,

poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b’]dit

hiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl

]thieno[3,4-b]thiophenediyl]] (PTB7), widely used in

organic solar cells, exhibits a synergistic combination

of properties [32-35]. With a low band gap of 1.6 eV,

PTB7 shows efficient absorption around the region

with the highest photon flux of the solar spectrum

(about 700 nm) [36]. The rigid backbone of polymer

results in a good hole mobility, and the side chains

with the ester and benzodithiophene enable good

solubility in organic solution [37].

We reported here the fabrication of bulk

heterojunction solar cells based on composites of

PTB7 and CdSe NRs. Different methods had been

utilized to optimize the hybrid device performance.

After treating the hybrid films by EDT and 1,4-BDT,

the efficiency of the HSCs devices can increase by

47.4% and 59.4% to reach maximum PCE of 2.58%

and 2.79% under simulated AM 1.5 illumination,

respectively. The device performances were much

better than those based on the pyridine treatments

before optimization. The steady-state PL emission

spectra, changs of surface microstructure as well as

exciton generation rate and dissociation probability

were analyzed to investigate the mechanism of

efficent improvement.

2 Experimental

2.1 Materials and Characterization

Indium tin oxide (ITO) coated glass substrates were

purchased from Shenzhen Nan Bo Group, China.

Poly(3,4-ethylenedioxythiophene):polystyrene

sulfonic acid (PEDOT:PSS) (Clevious P VP AI 4083)

was purchased from H. C. Stark company. PTB7 were

purchased from 1-material Chemscitech.

Chlorobenzene and 1,8-diiodoctane were provided

by Sigma-Aldrich. n-Decylphosphonic acid (DPA)

was purchased from PolyCarbon Industries, Inc.

synthesis (Newburyport, Massachusetts).

Trioctylphosphine oxide (TOPO, 98%) and

trioctylphosphine (TOP, 90%) were purchased from

Shandong weitian finechemical Co., LTD. The other

reagents for NC synthesis, such as Cadmium oxide

were obtained from Aldrich. All chemicals were used

as received without further purification.

The J-V characterization was taken using a Keithley

2400 source measure unit under AM 1.5G simulated

solar light. The measurement of external quantum

efficiency (EQE) was performed using a IQE200TM

data acquisition system. An atomic force microscope

(AFM) was used to measure film roughness and

surface morphologies in tapping mode using a Veeco

dimension V atomic microscope at room temperature.

UV/vis spectra were collected with a Perkin Elmer

Lambda 950 scan UV/vis spectrophotometer over the

spectral range of 350-800 nm. The steady-state PL

spectra were taken on a FluoroMax-4 HORIBA Jobin

Yvon spectrofluorometer. X-ray powder diffraction

(XRD) analyses were performed on a D8 Advance

(Bruker AXS) with a Cu Kα (k= 1.54 Å) radiation

source. Cyclic voltammetry (CV) was carried out on a

CHI600D electrochemical workstation with a

platinum working electrode of 2 mm diameter and a

platinum wire counter electrode at a scan rate of 50

mV s-1 against a solution of 0.1 mol L-1

tetratutylammonium hexafluorophosphate (Bu4NPF6)

in anhydrous acetonitrile.

2.2 Preparation of the CdSe NRs solution and

Cosolutions of Pyridine-Coated CdSe NRs and

PTB7

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research

3 Nano Res.

CdSe NRs were synthesized according to the

procedure described in Ref. [22]. In brief, they were

prepared by injecting slowly (about 2 min) six times

0.2 mL of a 1.0 M TOPSe solution every 10 min at 255 oC into a solution containing 50 mg CdO, 150 mg

DPA, and 2.9 g TOPO, previously degassed for 0.5 h

at 300 oC. After a reaction time of 60 min, the NRs

were precipitated and washed with methanol. The

XRD of as-prepared CdSe NRs was obtained (Fig. S-1

in the Electronic Supplementary Material (ESM)).

Pyridine-treated NRs were obtained by dispersing

DPA/TOPO coated NRs in neat pyridine. The

solution was stirred and heated to 100 oC for 10 min.

Hexane was added to the cooled solution to

precipitate the pyridine-treated NRs. The solution

was centrifuged, decanted and redispersed in a

CB:pyridine mixture (9:1, v:v). Then a solution of

pyridine-NRs was blended with a solution of PTB7 in

CB.

2.3 Photoluminescence (PL) measurement

The concentration of as-prepared CdSe NRs in CB

and pyridine mixture was 13 mg/mL. And the

concentration of PTB7 in CB was 0.5 mg/mL. Both

solutions were pumped at 467 nm. At first, in the PL

measurement the volume of PTB7 solution was 2 mL,

and then we added 9, 29, 39, 49 and 69 μL of CdSe

mixture step by step into the PTB7 solution and

measured the PL quenching respectively.

2.4 Device Fabrication

The hybrid solar cells studied in our work have a

structure of glass/ITO/PEDOT:PSS/PTB7:CdSe/Ca/Al.

The ITO coated glass substrates were pre-cleaned by

ultrasonic treatment with a sequence of detergent,

deionized water, acetone and isopropanol of 30 min

in an ultrasonic bath and then dried with a nitrogen

stream. The ITO substrates were then subjected to

UV−ozone treatment for 20 min. PEDOT:PSS was

spin-coated onto ITO substrates at 4000 rpm for 60 s;

then, the films were baked at 140 oC for 10 min on a

hot plate in air. Then the substrates were transferred

to a glove-box for spin coating of a PTB7/CdSe NRs

(1:5, w/w) active layer with a thickness of about 90

nm. The post-deposition treatments process was

carried out by soaking the as-prepared films in a

solution of 10 mM benzenedithiol or 1% EDT in

acetonitrile for 30 s, then rinsed with pure acetonitrile.

Finally, devices were completed by thermally

depositing 20 nm Ca on the active layer as a cathode

buffer layer and 100 nm of Al cathode in vacuum

under a base pressure of about 1 ×10−6 Pa. A shadow

mask was used during thermal evaporation to define

the active area of 0.09 cm2.

3 Results and discussion

Figure 1(a) Optical absorption spectra for spin-coated films of CdSe NRs (red, triangles), PTB7 (green, circles), and

PTB7/CdSe NRs blend (1/5, w/w) (black, squares) from chlorobenzene/pyridine mixture (9/1, v/v). (b) TEM micrograph of as-prepared CdSe NRs. (c) Molecular structure of PTB7.

Figure 1(a) presented the optical absorption spectra

of spin-coated films of the pyridine-treated CdSe

NRs, PTB7 and PTB7/CdSe (1:5, w/w) blend. Fig. 1(b)

showed the TEM micrograph of as-prepared CdSe

NRs. The diameters of CdSe NRs were 4.12 ± 0.94 nm,

and the lengths were 38.00 ± 8.15 nm (Fig. S-2 in the

ESM). The molecular structure of PTB7 was depicted

in Fig. 1(c). The film of pyridine-treated CdSe NRs

showed its first weak exciton absorption peak at ~650

nm and a strong absorbance in the shorter

wavelength region between 350 and 500 nm,

complementing the inferior absorption of the

polymers in this region. The PTB7 film exhibited a

very broad absorption spectrum in the near-infrared

and showed a distinct absorption band at 680 nm.

The optical absorption spectrum of the PTB7/NRs

composite was a simply superposition of the

respective absorption spectra of PTB7 and CdSe NRs,

indicating that there was no electronic interaction

between the PTB7 polymer and the CdSe NRs in the

ground state. In a composite film having ~17 wt% of

PTB7, contributing most of the absorption of the

blend, and the CdSe NRs contribution to the

| www.editorialmanager.com/nare/default.asp

4 Nano Res.

absorption spectrum of the film can be clearly seen as

a broad absorption range at wavelengths below 650

nm.

Figure 2 Photoluminescence (PL) emission spectra of PTB7 and PTB7/CdSe NRs mixture (both solutions pumped at 467 nm)

Steady-state PL measurements were conducted to

study the excited-state charge-transfer event in the

blend films. As shown in Fig. 2, PTB7/CdSe NRs

solution containing different amount of CdSe NRs

showed various levels of PL quenching compared to

the pure PTB7 solution. A reduction in the spectral

intensity of the composites relative to the pure PTB7

sample was observed, and the reduction of PL

intensity was increased with the increased

concentration of CdSe NRs. At a concentration of 67

wt% NRs in the mixture, more than 90% of the PL

from the PTB7 polymer was quenched. This

reduction of PL intensity was due to photogenerated

charge transfer between the PTB7 and CdSe NRs,

because the photogenerated exciton dissociated

before luminescence occurred [38, 39]. Once the

photogenerated excitons were dissociated, the

probability of excitons recombination should be

significantly suppressed, which was a precondition

for achieving highly efficient photovoltaic devices.

The composition of hybrid blend has proven to be

critical to the PCE of hybrid solar cells. Therefore, we

investigated the influence of the PTB7/CdSe NRs

weight ratio on the solar cell performance (Fig. S-3 in

the ESM). The devices were fabricated with an

optimized weight ratio of 1:5 for PTB7/CdSe NRs.

These ratios were higher than those reported for

traditional polymer/CdSe NRs devices, where

generally weight ratios of 1:9 were necessary to

achieve optimal performances [25]. For PTB7-PC71BM

based devices, 1,8-diiodooctane (DIO) was used as

processing additives to optimize the active layer film

and it improved PCE values in the resulting solar

cells [40, 41]. We also explored the impact of DIO on

the PTB7/CdSe NRs HSCs (Fig. S-4 in the ESM).

However, DIO had a negative effect on its PCE

enhancements. The reason was that DIO cannot

selectively disperse CdSe NRs aggregates and vail

their intercalation into PTB7 domains. Without DIO

additives, the AFM image of blend film showed a

root-mean-square (RMS) value of 11.9 nm as shown

in Fig. S-5(a) in the ESM. On the other hand, the RMS

value increased to 54.6 nm with DIO additives (Fig.

S-5(b) in the ESM). So it increased the roughness of

PTB7/CdSe NRs blend film and led to poor efficiency

of the devices.

Figure 3 (a) Device structure of the HSCs. (b) Band diagram of the PTB7/CdSe NRs with different treatments. (c) J−V

characteristics of PTB7/CdSe NRs HSCs with different treatments at polymer/NRs interface under simulated 1 sun AM 1.5 solar illumination. (d) Corresponding dark currents of the devices.

Table 1 Summary of photovoltaic performance of PTB7/CdSe

NRs HSCs with different treatments at polymer/NRs interface. bThe average PCE values were calculated from 7 devices.

Treatments Voc (V)

Jsc (mA/c

m2)

FF (%)

PCE (%) best

(average)b

Rs (Ωcm2)

Pyridin-e 0.713 5.04 48.78 1.75 (1.68) 25.8

EDT 0.767 5.93 56.63 2.58 (2.45) 16.2

1,4-BDT 0.737 6.39 59.34 2.79 (2.46) 14.7

Figure 3(a) exhibited the architecture of HSCs

which was ITO/PEDOT:PSS/PTB7:CdSe NRs/Ca/Al.

The J–V characteristics of the devices with different

treatments under simulated 1 sun AM 1.5 solar

illumination were depicted in Fig. 3(c), and the

extracted parameters were summarized in Table 1.

Compared to the pyridine treatments, the results

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research

5 Nano Res.

after EDT treatments presented an increment in the Jsc from 5.04 to 5.93 mA/cm2 and FF from 0.488 to

0.566 and the increment of 1,4-BDT in the Jsc from

5.04 to 6.39 mA/cm2 and FF from 0.488 to 0.593.

Consequently, the EDT treatment at the PTB7/CdSe

NRs interface led to high HSC performance with a

PCE of 2.58%, which was 47% higher than that for

devices with pyridine treatment. The 1,4-BDT

treatment resulted in the highest HSC performance

with a PCE of 2.79%, which was 59% higher than that

for devices with pyridine treatments. We also fitted

the corresponding dark-current density

characteristics based on these treatments (see Fig.

3(d)). Fig. 3(b) exhibited the band diagram of

PTB7/CdSe NRs with different treatments calculated

by cyclic voltammograms (CV) (Fig. S-6 in the ESM).

When treated with different chemicals, HOMO and

LUMO of PTB7 and CdSe films changed a little. The

band offset still supported the donor-acceptor

relationship between PTB7 and CdSe NRs. The

distribution maps of device parameters were

presented in Fig. S-7 in the ESM. The distribution

ranges of device parameters were very narrow,

indicating that the errors of the device parameters

were relatively small. As summarized in Table 1, the

specific series resistance Rs decreased from 25.8 to

16.2, and further to 14.7 Ωcm2 as the active layer

treated with pyridine, EDT to 1,4-BDT. This

reduction in series resistance indicated better

morphology of blend film in the devices, which

could enhance the electron transport and block the

holes and reduce the recombination of elections and

holes in the film [42].

Figure 4 External quantum efficiency (EQE) and corresponding integral current of the devices.

The external quantum efficiency (EQE) of these

devices was shown in Fig. 4. The devices showed a

broad photoresponse range from λ = 300 to 800 nm.

Compared with pyridine treatments, the EQE was

drastically improved in the entire spectrum range

when the blend films were treated with 1,4-BDT. And

the EQE of EDT treatments showed apparent

improvement in the range between 450 and 700 nm,

corresponding to the absorption of the polymer.

Although there was only ~17 wt% PTB7 in the active

layer of HSCs, the low band gap polymer contributed

strongly to the light absorption due to its high

absorption coefficient and its ability to harvest the

near-infrared photons. The integrated currents, from

the EQE spectrum, were also displayed. From the integrated current, the values of Jsc from external

quantum efficiency test were close to that from the

IQE200TM data acquisition system, indicating that

the values of Jsc were more trusting.

Figure 5 AFM images of the surface morphology of (a) pyridine, (b) EDT and (c) 1,4-BDT treatments (scale bar: 5 μm).

To understand the nature of the EDT and 1,4-BDT

treatments on the hybrid materials and its impact on

PV device performance, we have carried out

morphology characterization on the hybrid materials.

Fig. 5 showed the morphology of the PTB7/CdSe NRs

hybrid films before and after EDT or 1,4-BDT

treatments measured by atomic force microscopy

(AFM). Before EDT or 1,4-BDT treatments, the image

showed a RMS roughness value of 10.0 nm (Fig. 5(a)),

on the other hand, the RMS roughness values were

8.43 nm with EDT treatment and 7.87 nm with

1,4-BDT, respectively (as shown in Fig. 5(b) and (c)).

The decreased RMS roughness value gave a better

surface morphology due to the reduction of active

layer defects which indicated improved hybrid

interface and phase separation. Both EDT and

1,4-BDT had thiols groups (-SH), which had stronger

coordination ability to metal surface than pyridine

group (-NH) did. Moreover, 1,4-BDT, as short

conjugated-dithiol molecule, which can more tightly

coordinate to the Cd ions to form Cd-thialate on

| www.editorialmanager.com/nare/default.asp

6 Nano Res.

CdSe NRs surfaces thereby effectively passivating the

NRs surface and reducing the surface defects [18, 21].

The aromatic rings in the BDT also had an affinity

with polymers which were favorable for charge

transfer. Namely the exciton dissociation process was

facilitated and the exciton recombination rate was

inhibited, which was critical for achieving high

efficient photovoltaic device. The XRD patterns of

PTB7 film, CdSe film and PTB7/CdSe hybrid films

with different treatments were shown in the Fig. S-8

in the ESM. From the XRD patterns, the peaks of

hybrid film were a simply superposition of the

respective peaks of PTB7 and CdSe NRs. So the peaks

of CdSe NRs remained unchanged when treated with

EDT and 1,4-BDT. And the crystal structure of PTB7

didn't change after treatment by EDT and 1,4-BDT.

Figure 6 Photocurrent density (Jph) versus effective voltage (Veff)

characteristics of the devices based on different treatments.

To further investigate the influence of the EDT and

1,4-BDT treatments on the exciton generation and

dissociation, the maximum exciton generation rate

(Gmax) and exciton dissociation probability (P(E,T)) of

the device with the best performance of different

treatments were calculated based on the theory

reported [43-45]. Fig. 6 depicted the dependence of

the photocurrent density (Jph) on the effective voltage

(Veff) about these three HSCs. Here, Jph is described by

Jph= JL-JD, where JL and JD are the current densities

under illumination and in the dark, respectively. Veff

is described by Veff = V0-Va, where V0 is the voltage

when Jph equals zero and Va is the applied voltage. In

general, the saturation current density (Jsat) is

independent of the bias and temperature excepting

for being limited by the number of the absorbed photons, and it correlates with the value of Gmax,

which is mainly governed by light absorption and is

given by Jsat = qGmaxL, where q is the electronic charge

and L is the thickness of active layer (90 nm). With

the enhancement of Veff, the Jph increased sharply in

the lower Veff range and gradually saturated in the

higher Veff range. Meanwhile, the Jsat reached earlier

when the devices were treated by EDT and 1,4-BDT.

The values of Gmax for the devices with pyridine, EDT

and 1,4-BDT were 3.12 × 1027 m-3 s-1 (Jsat = 2.98

mA/cm2), 4.58 × 1027 m-3 s-1 (Jsat = 6.56 mA/cm2) and

4.82 × 1027 m-3 s-1 (Jsat = 6.96 mA/cm2), respectively.

EDT and 1,4-BDT treatments improved the Gmax.

Since the value of Gmax implied the maximum amount

of the absorbed photons, the enhanced Gmax indicated

that EDT and 1,4-BDT treatments improved the light

absorption. The P(E,T) could be obtained from the

ratio of Jph/Jsat. P(E,T) values under the short-circuit

condition increased from 65.4% for the device with

pyridine treatments to 89.6% and 91.4% for the

devices with EDT and 1,4-BDT treatments, respectively. The increased P(E,T) meant the

reduction of the exciton recombination rate, which

matched with the AFM results discussed above.

Therefore, the enhanced values of maximum exciton

generation and exciton dissociation rate led to the

improvement of the device performance.

4 Conclusions

In summary, a new system of hybrid devices based

on PTB7/CdSe NRs was reported. The effects of

treatments on the charge transport properties of

CdSe NRs, the surface topography on the blend films

and consequently the photovoltaic performance were

systematically investigated. High performance

hybrid solar cells based on PTB7/CdSe NRs were

demonstrated with the highest efficiency of 2.79%

and 2.58% when 1,4-BDT and EDT were used,

respectively. The 1,4-BDT and EDT treatments

resulted in removal of charged surface pyridine and

formation of Cd-thialate on CdSe NRs surfaces. This

effect was ascribed to the passivation of surface

defects on CdSe NRs, which led to reduced exciton

and charge carrier recombination and increased

electron transport. Further improvement in the

efficiency of such as adjusting the radio of

PTB7/CdSe NRs and adding additive were also

studied, and finally we draw the optimal ratio of 1:5

but the DIO additive had a negative effect on its PCE

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research

7 Nano Res.

enhancements. We anticipate that the findings will

stimulate further research to achieve more efficient

charge collection and devices performance.

Acknowledgements

This work was supported by the Ningbo City

Natural Science Foundation of China (2014A610037),

National Natural Science Foundation of China

(No.51202264 and 61474125), and Zhejiang Provincial

Natural Science Foundation of China(LR14E030002).

The work was also supported by Hundred Talent

Program of Chinese Academy of Sciences.

Electronic Supplementary Material: Supplementary

material (J−V characteristics of optimized devices

made from different blend ratios of PTB7/CdSe NRs,

J−V characteristics and AFM of PTB7/CdSe NRs

HSCs processed with and without DIO additives, the

device parameters distribution map of PTB7:PC71BM

devices, Cyclic voltammograms of PTB7 and CdSe

using different treatments and XRD patterns of

as-synthesized CdSe NRs) is available in the online

version of this article at

http://dx.doi.org/10.1007/s12274-***-****-*

(automatically inserted by the publisher).

References

[1] Snaith, H. J.; Whiting, G. L.; Sun, B.; Greenham, N. C.; Huck, W. T. S.; Friend, R. H. Self-organization of nanocrystals in polymer brushes. Application in heterojunction photovoltaic diodes. Nano Lett. 2005, 5,

1653-1657.

[2] Bansal, N.; Reynolds, L. X.; MacLachlan, A.; Lutz, T.;

Ashraf, R. S.; Zhang, W.; Nielsen, C. B.; McCulloch, I.;

Rebois, D. G.; Kirchartz, T.; Hill, M. S.; Molloy, K. C.; Nelson, J.; Haque, S. A. Influence of crystallinity and energetics on charge separation in polymer-inorganic

nanocomposite films for solar cells. Sci. Rep. 2013, 3, 1531.

[3] Dixit, S. K.; Madan, S.; Kaur, A.; Madhwal, D.; Singh, I.; Bhatnagar, P. K.; Mathur, P. C.; Bhatia, C. S. Enhancement of efficiency of a conducting polymer P3HT:CdSe/ZnS quantum dots hybrid solar cell by adding single walled

carbon nanotube for transporting photogenerated electrons. J. Renew. Sust. Energy 2013, 5, 033107.

[4] Böhm, M. L.; Kist, R. J. P.; Morgenstern, F. S. F.; Ehrler, B.; Zarra, S.; Kumar, A.; Vaynzof, Y.; Greenham, N. C. The Influence of nanocrystal aggregates on photovoltaic performance in nanocrystal-polymer bulk heterojunction solar cells. Adv. Energy Mater. 2014, 4, 1400139.

[5] Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Hybrid nanorod-polymer solar cells. Science 2002, 295, 2425-2427.

[6] Chang, J.; Waclawik, E. R. Colloidal semiconductor

nanocrystals: controlled synthesis and surface chemistry in organic media. RSC Adv. 2014, 4, 23505-23527.

[7] Greaney, M. J.; Brutchey, R. L. Ligand engineering in hybrid polymer:nanocrystal solar cells. Mater. Today 2015, 18, 31-38.

[8] Li, Y.; Liu, T.; Liu, H.; Tian, M. Z.; Li, Y. Self-assembly of intramolecular charge-transfer compounds into functional molecular systems. Acc. Chem. Res. 2014, 47, 1186-1198.

[9] Guo, Y.; Xu, L.; Liu, H.; Li, Y.; Che, C. M.; Li, Y.

Self-assembly of functional molecules into 1D crystalline nanostructures. Adv. Mater. 2015, 27, 985-1013.

[10] Jeltsch, K. F.; Schädel, M.; Bonekamp, J. B.; Niyamakom, P.; Rauscher, F.; Lademann, H. W. A.; Dumsch, I.; Allard, S.; Scherf, U.; Meerholz, K. Efficiency enhanced hybrid solar cells using a blend of quantum dots and nanorods. Adv. Funct. Mater. 2012, 22, 397-404.

[11] Robel, I.; Subramanian, V.; Kuno, M.; Kamat, P. V.

Quantum dot solar cells. harvesting light energy with CdSe

nanocrystals molecularly linked to mesoscopic TiO2 films. J. Am. Chem. Soc. 2006, 128, 2385-2393.

[12] Bang, J. H.; Kamat, P. V. Quantum dot sensitized solar cells. A tale of two semiconductor nanocrystals: CdSe and CdTe. ACS Nano 2009, 3, 1467-1476.

[13] Farrow, B.; Kamat, P. V. CdSe quantum dot sensitized solar

cells. Shuttling electrons through stacked carbon nanocups. J. Am. Chem. Soc. 2009, 131, 11124-11131.

[14] Hungría, A. B.; Juárez, B. H.; Klinke, C.; Weller, H.; Midgley, P. A. 3-D characterization of CdSe nanoparticles attached to carbon nanotubes. Nano Res. 2008, 1, 89-97.

[15] Fasoli, A.; Colli, A.; Martelli, F.; Pisana, S.; Tan, P. H.;

Ferrari, A. C. Photoluminescence of CdSe nanowires grown

with and without metal catalyst. Nano Res. 2011, 4, 343-359.

[16] Gao, B.; Lin, Y.; Wei, S.; Zeng, J.; Liao, Y.; Chen, L.; Goldfeld, D.; Wang, X.; Luo, Y.; Dong, Z.; Hou, J. Charge transfer and retention in directly coupled Au-CdSe nanohybrids. Nano Res. 2011, 5, 88-98.

[17] Greenham, N. C.; Peng, X. G.; Alivisatos, A. P. Charge separation and transport in

conjugated-polymer/semiconductor-nanocrystal composites studied by photoluminescence quenching and photoconductivity. Phys. Rev. B 1996, 54, 17628.

[18] Fu, W.; Wang, L.; Zhang, Y.; Ma, R.; Zuo, L.; Mai, J.; Lau,

T. K.; Du, S.; Lu, X.; Shi, M.; Li, H.; Chen, H. Improving

polymer/nanocrystal hybrid solar cell performance via tuning ligand orientation at CdSe quantum dot surface. ACS Appl. Mater. Interfaces 2014, 6, 19154-19160.

[19] You, J.; Dou, L.; Yoshimura, K.; Kato, T.; Ohya, K.; Moriarty, T.; Emery, K.; Chen, C. C.; Gao, J.; Li, G.; Yang,

Y. A polymer tandem solar cell with 10.6% power conversion efficiency. Nat. commun. 2013, 4, 1446.

[20] Chen, J. D.; Cui, C.; Li, Y. Q.; Zhou, L.; Ou, Q. D.; Li, C.;

Li, Y.; Tang, J. X. Single-junction polymer solar cells exceeding 10% power conversion efficiency. Adv. Mater. 2015, 27, 1035-1041.

| www.editorialmanager.com/nare/default.asp

8 Nano Res.

[21] Zhou, R.; Stalder, R.; Xie, D.; Cao, W.; Zheng, Y.; Yang, Y.; Plaisant, M.; Holloway, P. H.; Schanze, K. S.; Reynolds, J. R.; Xue, J. Enhancing the efficiency of solution-processed

polymer:colloidal nanocrystal hybrid photovoltaic cells using ethanedithiol treatment. ACS Nano 2013, 7, 4846-4854.

[22] Liao, H. C.; Chen, S. Y.; Liu, D. M. In-situ growing CdS single-crystal nanorods via P3HT polymer as a soft

template for enhancing photovoltaic performance. Macromolecules 2009, 42, 6558-6563.

[23] Sun, B.; Greenham, N. C. Improved efficiency of photovoltaics based on CdSe nanorods and poly(3-hexylthiophene) nanofibers. Phys. Chem. Chem. Phys. 2006, 8, 3557-3560.

[24] Kuo, C. Y.; Su, M. S.; Chen, G. Y.; Ku, C. S.; Lee, H. Y.; Wei, K. H. Annealing treatment improves the morphology

and performance of photovoltaic devices prepared from thieno[3,4-c]pyrrole-4,6-dione-based donor/acceptor conjugated polymers and CdSe nanostructures. Energy Environ. Sci. 2011, 4, 2316-2322.

[25] Jiu, T.; Reiss, P.; Guillerez, S.; Bettignies, R. D.; Bailly, S.;

Chandezon, F. Hybrid solar cells based on blends of CdSe

nanorods and poly(3-alkylthiophene) nanofibers. IEEE J. Sel. Top. Quant. 2010, 16, 1619-1626.

[26] Dixit, S. K.; Madan, S.; Madhwal, D.; Kumar, J.; Singh, I.; Bhatia, C. S.; Bhatnagar, P. K.; Mathur, P. C. Bulk heterojunction formation with induced concentration

gradient from a bilayer structure of P3HT:CdSe/ZnS quantum dots using inter-diffusion process for developing high efficiency solar cell. Org. Electron. 2012, 13, 710-714.

[27] Sun, B.; Snaith, H. J.; Dhoot, A. S.; Westenhoff, S.; Greenham, N. C. Vertically segregated hybrid blends for photovoltaic devices with improved efficiency. J. Appl. Phys. 2005, 97, 014914.

[28] Dayal, S.; Kopidakis, N.; Olson, D. C.; Ginley, D. S.;

Rumbles, G. Photovoltaic devices with a low band gap polymer and CdSe nanostructures exceeding 3% efficiency. Nano Lett. 2010, 10, 239-242.

[29] Albero, J.; Zhou, Y.; Eck, M.; Rauscher, F.; Niyamakom, P.; Dumsch, I.; Allard, S.; Scherf, U.; Krüger, M.; Palomares,

E. Photo-induced charge recombination kinetics in low

bandgap PCPDTBT polymer:CdSe quantum dot bulk heterojunction solar cells. Chem. Sci. 2011, 2, 2396-2401.

[30] Zhou, R.; Zheng, Y.; Qian, L.; Yang, Y.; Holloway, P. H.; Xue, J. Solution-processed, nanostructured hybrid solar cells with broad spectral sensitivity and stability. Nanoscale 2012, 4, 3507-3514.

[31] Couderc, E.; Greaney, M. J.; Brutchey, R. L.; Bradforth, S. E. Direct spectroscopic evidence of ultrafast electron transfer from a low band gap polymer to CdSe quantum dots in hybrid photovoltaic thin films. J. Am. Chem. Soc. 2013, 135, 18418-18426.

[32] He, Z.; Zhong, C.; Su, S.; Xu, M.; Wu, H.; Cao, Y.

Enhanced power-conversion efficiency in polymer solar

cells using an inverted device structure. Nat. Photonics 2012, 6, 593-597.

[33] Liu, C.; Wang, K.; Hu, X.; Yang, Y.; Hsu, C. H.; Zhang, W.; Xiao, S.; Gong, X.; Cao, Y. Molecular Weight Effect on the

Efficiency of Polymer Solar Cells. ACS Appl. Mater. Interfaces 2013, 5, 12163-12167.

[34] Tan, W. Y.; Wang, R.; Li, M.; Liu, G.; Chen, P.; Li, X. C.; Lu, S. M.; Zhu, H. L.; Peng, Q. M.; Zhu, X. H.; Chen, W.;

Choy, W. C. H.; Li, F.; Peng, J.; Cao, Y. Lending triarylphosphine oxide to phenanthroline: a facile approach to high-performance organic small-molecule cathode interfacial material for organic photovoltaics utilizing

air-stable cathodes. Adv. Funct. Mater. 2014, 24, 6540-6547.

[35] Zhang, K.; Zhong, C.; Liu, S.; Mu, C.; Li, Z.; Yan, H.; Huang, F.; Cao, Y. Highly efficient inverted polymer solar cells based on a cross-linkable water-/alcohol-soluble

conjugated polymer interlayer. ACS Appl. Mater. Interfaces 2014, 6, 10429-10435.

[36] He, Z.; Zhong, C.; Huang, X.; Wong, W. Y.; Wu, H.; Chen,

L.; Su, S.; Cao, Y. Simultaneous enhancement of open-circuit voltage, short-circuit current density, and fill factor in polymer solar cells. Adv. Mater. 2011, 23, 4636-4643.

[37] Liang, Y.; Xu, Z.; Xia, J.; Tsai, S. T.; Wu, Y.; Li, G.; Ray, C.;

Yu, L. For the bright future-bulk heterojunction polymer

solar cells with power conversion efficiency of 7.4%. Adv. Mater. 2010, 22, E135-138.

[38] Wang, P.; Abrusci, A.; Wong, H. M. P.; Svensson, M.; Andersson, M. R.; Greenham, N. C. Photoinduced charge transfer and efficient solar energy conversion in a blend of

a red polyfluorene copolymer with CdSe nanoparticles. Nano Lett. 2006, 6, 1789-1793.

[39] Fu, W.; Wang, L.; Ling, J.; Li, H.; Shi, M.; Xue, J.; Chen, H. Highly efficient hybrid solar cells with tunable dipole at the donor-acceptor interface. Nanoscale 2014, 6, 10545-10550.

[40] Lou, S. J.; Szarko, J. M.; Xu, T.; Yu, L.; Marks, T. J.; Chen, L. X. Effects of additives on the morphology of solution

phase aggregates formed by active layer components of

high-efficiency organic solar cells. J. Am. Chem. Soc. 2011, 133, 20661-20663.

[41] Ochiai, S.; Imamura, S.; Kannappan, S.; Palanisamy, K.; Shin, P.-K. Characteristics and the effect of additives on the nanomorphology of PTB7/PC71BM composite films. Curr. Appl. Phys. 2013, 13, S58-63.

[42] Sun, C.; Wu, Y.; Zhang, W.; Jiang, N.; Jiu, T.; Fang, J.

Improving efficiency by hybrid TiO(2) nanorods with 1,10-phenanthroline as a cathode buffer layer for inverted organic solar cells. ACS Appl. Mater. Interfaces 2014, 6, 739-744.

[43] Kyaw, A. K. K.; Wang, D. H.; Wynands, D.; Zhang, J.;

Nguyen, T.-Q.; Bazan, G. C.; Heeger, A. J. Improved light harvesting and improved efficiency by insertion of an optical spacer (ZnO) in solution-processed small-molecule solar cells. Nano Lett. 2013, 13, 3796-3801.

[44] Lu, L.; Luo, Z.; Xu, T.; Yu, L. Cooperative plasmonic effect of Ag and Au nanoparticles on enhancing performance of polymer solar cells. Nano Lett. 2013, 13, 59-64.

[45] Li, P.; Jiu, T.; Tang, G.; Wang, G.; Li, J.; Li, X.; Fang, J.

Solvents induced ZnO nanoparticles aggregation associated with their interfacial effect on organic solar cells. ACS Appl. Mater. Interfaces 2014, 6, 18172-18179.

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research

Nano Res.