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Boosting solar-cell efficiencywith quantum-dot-basednanotechnology

Seth Hubbard and Ryne Raffaelle

Inserting indium arsenide quantum dots into crystalline III-V

semiconductor-based photovoltaics results in both enhanced short-

circuit current and improved efficiency.

Every hour, the sun radiates more energy onto the Earths

surface than is consumed globally in one year. However, to best

harness this vast source of power, efficient and cost-effective so-

lar photovoltaic (PV) energy conversion is required. In partic-

ular, improving the efficiency of III-V-type semiconductor PV

devices is a goal for both the space and terrestrial research

communities. Current high-performance space technology relieson crystalline III-V materials to produce conversion efficiencies

greater than 31%.1 High power-conversion efficiencies directly

increase mass-specific power, thus lowering the costs associ-

ated with spacecraft deployment. Additionally, recent advances

have demonstrated that III-V-based concentrator PVs can pro-

duce high efficiencies (> 40%) and, therefore, reduce the cost of

power generation.2

Numerous approaches can increase both solar-cell efficiency

and/or mass-specific power, including lightweight substrates,

substrate removal, or bandgap engineering of multijunction

solar cells (MJSCs) using quantum dots (QDs).3, 4 Luque and

Marti5 also proposed a novel extension of the bandgap engi-

neering approach. It uses multiple QD superlattices to form an

optically isolated intermediate band (IB) within the bandgap of

a standard single-junction solar cell. Photons with energy below

the host bandgap are absorbed from the host valence to the inter-

mediate band, and from the intermediate to the host conduction

band. As these lower-energy photons are normally lost to trans-

mission in standard single-junction solar cells, the IB approach

may result in a high limiting efficiency. Recently, several exper-

iments have demonstrated the key operating principles of the

IB solar cell using both indium arsenide6, 7 (InAs) and gallium

antimonide8

QDs in a gallium arsenide (GaAs) host.

Figure 1. (a) Quantum-dot (QD)-enhanced solar-cell design concept.

(b) Current density-voltage curves for control and 520 layer enhanced

cells under one sun global air mass 1.5 (AM1.5g) light. These cells

did not have antireflective coating. InGaP: Indium gallium phosphide.

GaAs: Gallium arsenide.

The Rochester Institute of Technologys NanoPower Research

Laboratories (NPRL) have made significant advances in this area

by developing new nanomaterials and devices. We have engi-

neered III-V-type solar cells to take advantage of the extended

absorption

spectrum of lower-bandgap heterostructures (such as QDs) in-

serted into the current-limiting junction of an MJSC.4 The larger

absorption spectrum of the nanostructures enhances the over-

all short-circuit current and global efficiency of the cell. Mod-

els of an indium gallium phosphide (InGaP)/indium gallium

arsenide (InGaAs)/germanium (Ge) triple-junction solar cell, in

which QDs extend the middle junctions absorption spectrum,

indicate that we could raise the theoretical limiting efficiency to

47% under one sun illumination. These devices may also have

additional benefits, such as enhanced radiation tolerance and

temperature coefficients.9

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Figure 2. 520 layer QD-enhanced solar cells show a net increase in

external quantum efficiency (EQE) at IR wavelengths compared to the

baseline GaAs.

We demonstrated the feasibility of the QD tuning approach us-

ing strain-balanced InAs dots inserted in the intermediate (i)

region of a GaAs p-i-n solar cell.7,10 The solar-cell structure

(grown by organometallic vapor-phase epitaxy) is shown in

Figure 1(a). Figure 1(b) shows the illuminated one sun global

air mass 1.5 (AM1.5g) current density-voltage .JV / curves for

a control GaAs cell without quantum dots and for cells with

520 stacked layers of InAs QDs. We fabricated these cells us-

ing standard III-V processing technology and a concentrator

grid designed with 20% metallization coverage. The control re-

sults were equal or close to the standard values for GaAs solar

cells. Figure 1(a) clearly shows the enhancement in short-circuit

current using QDs. This is a direct result of photo-generated

current contributed by the nanoparticles. We expect the trend

to continue with addition of QDs, because a greater portion

of the sub-GaAs bandgap solar spectrum should be absorbed

with increasing number (i.e., volume). Additionally, the strain-balancing technique employed for QD growth led to higher ma-

terial quality in layers grown on top of the InAs nanoparticles.

This has enabled short-circuit current enhancement and minimal

open-circuit voltage degradation.7

Measuring the cells external quantum efficiency (EQE) pro-

vided a spectroscopic means to study how QDs affect the de-

vice current-voltage characteristics. EQE measures the number

of electrons or holes collected as photocurrent per incident pho-

ton at a particular wavelength. Figure 2 shows the EQE for the

baseline and 520 QD samples. All three QD samples show an

increased response at wavelengths greater than the GaAs band

Figure 3. Direct air mass 1.5 (AM1.5d) efficiency and short-circuit

current (JSC ) versus concentration for the baseline and QD cells. The

enhanced cells show improved efficiency at higher concentration.

edge. This indicates that a portion of the short-circuit current of

these cells is generated by QD-related absorption processes. Ad-

ditionally, increasing the number of layers raises the EQE at all

QD-related transitions. At 909nm (1.36eV) this amounted to in-

creases from 2.5 to 9.2% by increasing the stacking from five to

20 (see Figure 3). This was clear evidence that raising the vol-ume of the quantum-dot absorbers successively increases short-

circuit current.

To assess the spectral enhancement of the QDs at higher con-

centration, we observed the control and 20-layer cells under

high-intensity illumination.11 The trends seen at one sun con-

tinued, with the enhanced cells showing improved short-circuit

currents (see Figure 3). All cells, including the baseline, show a

slight superlinear trend in current, as has been observed in GaAs

materials because of high-level injection effects. The 20-layer

QD cell generated the largest short-circuit current (7.53A/cm2)

at 440suns. This represents an 11% increase compared to the

control-cell current at the same concentration. The open-circuit

voltage for all cells rose logarithmically with concentration.

We calculated the efficiency for each type of cell under di-

rect air mass 1.5 (AM1.5d) conditions (see Figure 3). Efficiency

peaked near 400suns, consistent with our concentrator-grid de-

sign. The QD-enhanced cell showed almost 18% power effi-

ciency at 400suns. This represented a 1% absolute efficiency

improvement compared to the control (a 6% relative improve-

ment). Under high-illumination intensity, the reduced

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(longer-wavelength) effective bandgap of the cell, as well as in-

creased optical and thermal extraction of QD-generated carri-

ers, leads to direct improvement in cell efficiency. This boostwas due to higher short-circuit current combined with minimal

open-circuit voltage loss.

Nanomaterial-based energy technologies have great promise

for both terrestrial and extraterrestrial applications. Our exper-

imental demonstration of current and efficiency enhancement

in QD-based solar cells represents a milestone to breaking effi-

ciency limits through using nanomaterials. Our next steps aim

to increase the absorption cross section of the quantum dots by

both increasing the number of QD layers and investigating new

multilayer device designs. In addition, we are continuing our ef-

forts to understand and model radiation and temperature effectsin these solar cells.

We wish to acknowledge the hard work and dedication of the NanoPV

group at NPRL. Specific contributions to this work were submitted

by David Forbes, Cory Cress, John Anderson, and graduate students

Chris Bailey and Stephen Polly. We also acknowledge support for our

work from the US Department of Energy (DE-FG36-08GO18012) and

the US Government.

Author Information

Seth Hubbard

NPRL

Rochester Institute of Technology (RIT)

Rochester, NY

Seth Hubbard is assistant professor of physics at RIT and the

photovoltaics group leader at the NPRL. He currently leads a

team of graduate students and research staff working on the

epitaxial growth, fabrication, and characterization of nanostruc-

tured solar photovoltaic devices.

Ryne RaffaelleNational Center for Photovoltaics

National Renewable Energy Laboratory

Golden, CO

Ryne Raffaelle is currently the director of the National Center for

Photovoltaics. He was formerly the founder and director of the

NPRL. His research interests include energy storage and conver-

sion systems, nanomaterials, and carbon nanotubes.

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2010 SPIE