734

MRS BULLETIN

•

VOLUME 35

•

OCTOBER 2010

•

www.mrs.org/bulletin

734

MRS BULLETIN

•

VOLUME 35

•

OCTOBER 2010

•

www.mrs.org/bulletin

NEWS & ANALYSIS RESEARCH/RESEARCHERS

neighboring Tb

resulting in a reduction

in luminescence.

In addition, the emitting color varied

from bluish-green to greenish-yellow

with increasing Tb

content. Excited

at 330 nm, the Commission Interna -

tional de l’Eclairage (CIE) chromaticity

coordinates shifted from (0.25, 0.33) to

(0.40, 0.52) as the value of y increased

from 0.1 to 0.6 in the Mg(Y

3.8-y

0.2

)

materials.

Melissa A. Harrison

Quantum efﬁciency improved

in bulk heterojunction PVs

lthough bulk heterojunction photo-

voltaics (BHJs) are attractive as

low-cost solar cells due to their ease of

processability, devices incorporating low-

bandgap quantum dots (QDs) fall short

of the performance achieved by analo-

gous blends of polymers and CdSe QDs.

Previously, D.S. Ginger, S.A. Jenekhe,

and co-researchers at the University of

Washington, Seattle, hypothesized that

the poor performance by BHJs composed

of PbS QDs blended with common con-

jugated polymers is due to insufcient

photoinduced charge transfer at the

organic–inorganic interface. Recently,

Ginger and co-researchers showed that

BHJs made from PbS QDs and new

donor–acceptor polymers exhibit efcien-

cies two orders of magnitude greater than

those observed for blends of PbS with

conventional host polymers.

As reported in the July 14th issue of

Nano Letters (DOI: 10.1021/nl1013663;

p. 2635), Ginger and co-researchers

select ed three polymers (PDTPQx,

PDTPPPz, and PDTPBT; see gure) to

blend with PbS QDs because their ion-

ization potentials show that their highest

occupied molecular orbitals lie within the

PbS bandgap. Photoinduced absorp -

tion (PIA) spectroscopy shows that in the

range of 0.8 eV to 2.2 eV BHJ blends

of PDTPQx and PbS QDs exhibit new

sub-bandgap absorptions by the PDTPQx

and a bleach of the polymer’s bandgap

transition following photoexcitation,

while the PDTPBT blend shows very

weak PIA signal, and the PDTPPPz blend

shows no detectable PIA signals. Because

neat PDTPQx shows no PIA signal, the

researchers attribute the blend’s PIA

spectrum to long-lived polarons on the

PDTPQx polymer chains created by

photoinduced electron transfer from the

polymer to the PbS QDs, leading them

to predict PDTPQx/PbS to perform sig-

nicantly better in BHJ photo diodes than

PbS QDs blended with either of the other

two polymers considered. The researchers

verified this prediction by showing

that the quantum efciencies exhibited by

the PDTPQx/PbS blends are two orders

of magnitude higher than those exhibited

by blends with the other two polymers.

Under simulated AM 1.5 illumination,

the researchers estimated a power conver-

sion efciency (PCE) of about 0.55%,

which is modest in comparison to polymer-

fullerene BHJ cells but is signicantly

higher than BHJ devices made from pre-

vious polymer blends with low-bandgap

QDs. The researchers plan to improve

performance by increasing the PDTPQx

molecular weight to facilitate thicker

lms, and by controlling the shape of

the QDs.

The researchers said, “We anticipate

that the viability of new organic host

materials when blended with PbS should

reinvigorate the study of solution-

processable bulk-heterojunction exci tonic

solar cells made with a range of low-

bandgap nanoparticles and should

facilitate their use in both hybrid photo -

voltaics and photodetectors with bandgaps

tailored via quantum connement.”

Steven Trohalaki

Energy Focus

Normal modes and density of

states achieved in disordered

colloidal solids

olloidal suspensions have been used

as model systems in experimental

research on the fundamentals of statis -

tical mechanics. In colloidal systems,

a crystal or amorphous structural glass

can be produced using traditional hard-

sphere particles such as silica sphere.

However, the perfect crystals produced

with these conventional particles show

spatial homogeneous uctuations. These

uctuations are measured using optical

microscopy to observe individual particle

motion within the interior of the system.

Recently, D. Kaya, N.L. Green, C.E.

Maloney, and M.F. Islam of Carnegie

Mellon University developed an approach

to measure the correlation in particle dis-

placement using strongly disordered col-

loidal crystals composed of deformable

microgel colloidal particles to determine

the normal modes and the density of states

(DOS). Normal modes and the DOS of a

Structures of the polymers used in blends

with PbS quantum dots:

(a) PDTPQx: poly(2,3-didecyl-quinoxaline-5,

8-diyl-alt-N-octyldithieno[3,2-b:2´,3´-d]pyrrole);

(b) PDTPPPz: poly(2,3-didecyl-pyrido[3,4-b]

pyrazine-5, 8-diyl-alt-N-dodecyl -dithieno[3,

2-b:2´,3´-d]pyrrole); and

2-yl)-alt-N-dodecyldithieno [ 3,2-b:2´,3´-d]pyrrole).