Flexible thin-film InAs/GaAs quantum dot solar cellsKatsuaki Tanabe, Katsuyuki Watanabe, and Yasuhiko Arakawa Citation: Applied Physics Letters 100, 192102 (2012); doi: 10.1063/1.4712597 View online: http://dx.doi.org/10.1063/1.4712597 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/100/19?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Multi-stacked InAs/GaAs quantum dots grown with different growth modes for quantum dot solar cells Appl. Phys. Lett. 106, 222104 (2015); 10.1063/1.4922274 Enhancement of current collection in epitaxial lift-off InAs/GaAs quantum dot thin film solar cell and concentratedphotovoltaic study Appl. Phys. Lett. 105, 113904 (2014); 10.1063/1.4896114 Challenges to the concept of an intermediate band in InAs/GaAs quantum dot solar cells Appl. Phys. Lett. 103, 141113 (2013); 10.1063/1.4822322 InAs/GaAs quantum dot solar cell with an AlAs cap layer Appl. Phys. Lett. 102, 163907 (2013); 10.1063/1.4803459 Reducing carrier escape in the InAs/GaAs quantum dot intermediate band solar cell J. Appl. Phys. 108, 064513 (2010); 10.1063/1.3468520

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Flexible thin-film InAs/GaAs quantum dot solar cells

Katsuaki Tanabe,1,a) Katsuyuki Watanabe,1 and Yasuhiko Arakawa1,2

1Institute for Nano Quantum Information Electronics, University of Tokyo, 4-6-1 Komaba, Meguro,Tokyo 153-8505, Japan2Institute of Industrial Science, University of Tokyo, 4-6-1 Komaba, Meguro, Tokyo 153-8505, Japan

(Received 2 February 2012; accepted 23 April 2012; published online 8 May 2012)

Thin-film InAs/GaAs quantum dot (QD) solar cells on mechanically flexible plastic films are

fabricated. A 4.1-lm-thick compound semiconductor photovoltaic layer grown on a GaAs

substrate is transferred onto a plastic film through a low-temperature bonding technique. We also

fabricate thin-film InAs/GaAs quantum dot solar cells on Si substrates, as alternative low-cost,

lightweight, robust substrates. The open-circuit voltages of the thin-film cells on plastic and Si

substrates are equal to that of the as-grown bulk cell on a GaAs substrate, indicating that no

material degradation occurs during our bond-and-transfer process. VC 2012 American Institute ofPhysics. [http://dx.doi.org/10.1063/1.4712597]

Photovoltaic solar cells with semiconductor quantum

dots (QDs) can potentially realize ultrahigh-efficiency solar-

energy conversion in single p-n junction structures utilizing

intermediate-level energy bands.1 Among the wide range of

semiconductor materials for QD solar cells currently under

intensive study,2 those using III-V semiconductor compound

InAs/GaAs QDs (i.e., InAs QDs embedded in GaAs matri-

ces) have exhibited the highest efficiencies and robustness.3

However, expensive, heavy, thick, and solid GaAs wafers

have been used as the growth substrates for these QD cells,

hindering their commercialization. Solar-cell modules with

low-cost, lightweight, flexible support substrates are of par-

ticular interest for a number of practical applications.4 For

flexible-solar-cell production, the direct growth of photovol-

taic semiconductor thin films on plastic films or metallic foils

as substrates would be the most desirable approach. Crystal-

line semiconductor materials, however, cannot be grown on

noncrystalline substrates; and therefore, this direct growth

scheme would inevitably result in poor-performance amor-

phous cells. Instead, crystalline cells on such noncrystalline

substrates can be prepared by transferring photovoltaic semi-

conductor layers grown in advance on proper crystalline sub-

strates onto noncrystalline substrates.5–8

In this work, we fabricated thin-film InAs/GaAs QD so-

lar cells on flexible plastic film substrates by layer transfer.

A 4.1-lm-thick compound semiconductor photovoltaic layer

was grown on a GaAs substrate, and then transferred onto a

plastic film through a bonding technique. Our bonding

scheme is mediated by a metal–epoxy agent for the realiza-

tion of bonding at low temperatures (below 200 �C), enabling

the use of plastic materials as support substrates, as well as

preventing the degradation of the semiconductor photovol-

taic layers including QDs. We also fabricated thin-film InAs/

GaAs QD solar cells on Si substrates, as alternative inexpen-

sive, lightweight, robust substrates, using the same layer-

transfer scheme. Thus, we have demonstrated the validity of

this scheme for the formation of thin-film photovoltaics on

any kind of support plate or film with no degradation of the

semiconductor layers.

Figure 1 shows a schematic flow diagram of our fabrica-

tion process for thin-film InAs/GaAs QD solar cells on plas-

tic films. The InAs/GaAs QD solar-cell structure was a p-i-nGaAs/Al0.4Ga0.6As double heterostructure with a 440-nm-

thick i-GaAs layer embedding ten layers of self-assembled

InAs QDs. This cell structure was grown inversely as p-on-non a GaAs (100) substrate by molecular beam epitaxy with

an Al0.7Ga0.3As etch-stop layer immediately above the GaAs

substrate. This results in an n-on-p configuration in the final

transferred cell structure. Figure 2(a) shows an atomic force

microscope image of the as-grown InAs QDs, which are

seen to be uniformly sized, coalescence-free, high-density

(3.9� 1010 cm�2) QDs. The sizes of the InAs QDs were

observed as roughly 30 nm in diameter and 10 nm in height.

Figure 2(b) shows a photoluminescence spectrum from the

as-grown inverted cell structure. The photoluminescence

measurement was conducted at room temperature using a

640-nm-wavelength continuous-wave semiconductor laser

diode as the excitation source. The pump laser beam irradi-

ated the sample surface with a spot size of �100 lm and a

power of �3 mW. The photoluminescence spectrum shown

in Figure 2(b) exhibits a peak associated with the ground-

state emission of the InAs QDs at 1.28 lm with a full-width

at half-maximum of 25.1 meV.

Next, the QD solar-cell structure was layer-transferred

onto a plastic film through wafer bonding and subsequent re-

moval of the GaAs growth substrate. We used a 130-lm-

thick polyimide film as the support substrate. A 30-nm-thick

AuGeNi alloy (80:10:10 wt. %) layer followed by a 150-nm-

thick Au layer were first deposited by electron-beam evapo-

ration onto the bonding surfaces of both the QD cell structure

and the plastic film. In this work, we adopted an epoxy agent

containing Ag nanoparticle clusters to enable electrical con-

tact as well as bonding at low temperatures, in order to avoid

degradation of the bonded materials, particularly the plastic

films. The diced cell wafer piece and plastic film were

brought into contact via the Ag–epoxy agent and annealed at

150–200 �C in ambient air for 1 h, with no pressure applied.

No significant difference in cell performance according to

bonding temperature was observed between 150 and 200 �C,

except for a slightly higher degree of buckling of the plastica)E-mail: tanabe@iis.u-tokyo.ac.jp.

0003-6951/2012/100(19)/192102/4/$30.00 VC 2012 American Institute of Physics100, 192102-1

APPLIED PHYSICS LETTERS 100, 192102 (2012)

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film at 200 �C, which was presumably due to the mismatch

of thermal expansion between the plastic and the metal.

After bonding, the GaAs substrate was removed at room

temperature by selective chemical etching with H3PO4/H2O2

(3:7 vol/vol) followed by 50% citric acid/H2O2 (4:1 vol/vol),

with the edges of the GaAs wafer coated with photoresist to

avoid undercutting of the cell structure. The H3PO4/H2O2

and citric acid/H2O2 solution compositions were chosen to

maximize the etching rate of GaAs and the etching selectiv-

ity between GaAs and AlGaAs, respectively.8 The

Al0.7Ga0.3As etch-stop layer was then removed with aqueous

hydrofluoric acid (HF) (20 vol. %) at room temperature. Fol-

lowing the bonding and layer transfer, the front surface of

the transferred photovoltaic thin film was partially metalized

with Au/AuGeNi. The nonmetalized part of the top nþ-GaAs

contact layer was removed by the citric acid/H2O2 solution,

and then an antireflection coating with MgF2/ZnS was

applied. The finished device was a 4.1-lm-thick thin-film

InAs/GaAs QD solar-cell structure bonded to a plastic film,

as shown in Figure 3. Using the same scheme, we also fabri-

cated QD thin-film cells on Si substrates, which are strong

candidates as alternative support substrates with lighter

weight, higher mechanical robustness, thermal stability, and

conductivity relative to GaAs substrates.

Figure 4 shows the light current–voltage (I–V) character-

istics of the highest-efficiency cells for each of the fabricated

thin-film InAs/GaAs QD solar cells on plastic films (active

area 0.0039 cm2) and Si substrates (0.059 cm2), as well as the

as-grown, bulk reference cells on GaAs substrates (0.21 cm2),

under AM1.5 G, 1-sun (100 mW cm�2) illumination. Signifi-

cantly, the thin film cells on plastic films can be easily cut

by household scissors into arbitrary shapes and sizes and

every separated piece exhibits photovoltaic operation, and we

intentionally cut the cells into smaller pieces to pick the best-

efficiency data for Figure 4. Note that the open-circuit vol-

tages (VOC)� 0.6 V are commonly seen for such 1.3-lm-band

QD solar cells,3 in contrast to �1.0 V for GaAs cells without

QD, according to the bandgap offset. Table I summarizes the

cell performance parameters including the energy-conversion

efficiency g, VOC, short-circuit current JSC, and fill factor FFfor the highest-efficiency cells as well as the highest VOC val-

ues observed for each of the three types of cells under

AM1.5G illumination (1 sun). The highest VOC values of the

transferred thin-film cells on plastic films and Si substrates are

quite similar to that of the bulk reference cell. This result indi-

cates that our bond-and-transfer process does not degrade the

quality of the cell material, since any generated crystal defects

that act as recombination centers would reduce VOC.9 In addi-

tion, the bonded interfaces have no significant carrier recom-

bination rate to reduce VOC.

The transferred cells have larger photocurrents than the

bulk reference cell, as shown in Figure 4 and Table I. First of

all, note that this result is not due to light absorption by the

QDs because our transferred cells and bulk reference cells

are from the identical grown QD solar-cell wafer. The photo-

current difference is mainly because the bulk reference cell

FIG. 1. Schematic flow diagram of the

fabrication process for the InAs/GaAs

QD solar cells on plastic films.

FIG. 2. (a) Atomic force microscope image and (b) room-temperature pho-

toluminescence spectrum of the InAs QDs grown in the InAs/GaAs QD

solar-cell structure.

192102-2 Tanabe, Watanabe, and Arakawa Appl. Phys. Lett. 100, 192102 (2012)

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has an improperly deep p-n junction due to its inverted

growth and consequently exhibits inefficient carrier collec-

tion. Other potential reasons for the larger photocurrents of

the transferred thin-film cells are the efficient carrier collec-

tion in the thin-film photovoltaic layer structure thinner than

the minority-carrier diffusion length and the enhanced opti-

cal path length because of the metallic back reflectors imple-

mented on the support substrates. In other words, the

metallic layer implemented between the semiconductor pho-

tovoltaic thin film and the support substrate has dual func-

tions, acting as the bottom electrode of the solar-cell

structure and providing back-reflection and scattering at the

GaAs/metal interfaces at the bottom of the photovoltaic thin

film to increase the path length of the incoming photons to

be absorbed. However, this light-trapping effect by the back

reflector may have a minor contribution for the larger photo-

currents for our relatively thick cell design in this work, but

would play more significant roles for thinner cells.7

The plastic film used in this work has a weight density of

1.4 g cm�3, in contrast to the density of 5.4 g cm�3 for GaAs.

Accounting for the difference in thickness of the substrates

used (130 lm and 450 lm for the plastic film and GaAs sub-

strates, respectively), we reduced the cell weight to less than

1/10 by transferring the photovoltaic thin film (which was

much thinner and, therefore, had negligible weight relative to

the semiconductor and plastic substrates) from the GaAs

growth substrate onto the plastic support film. In principle,

we could reduce the cell weight even further by using thinner,

commercially available plastic films. For the thin-film cell on

a Si substrate, the Si density of 2.3 g cm�3 provided a weight

of less than half that of the bulk cell on a GaAs substrate.

Our fabrication of thin-film QD solar cells on both plas-

tic films and Si substrates is a strong demonstration of the va-

lidity of our bond-and-transfer scheme for the formation of

thin-film photovoltaics on any kind of support plate or film,

with no material degradation from the as-grown semiconduc-

tor layers. In this work, we adopted an etch-back method to

detach the GaAs growth substrate to simplify the fabrication

process. Alternatively, the incorporation of an epitaxial lift-

off5,10 or ion-cutting7,11 technique would enable the reuse of

the GaAs substrates to reduce the production costs.

In summary, we have fabricated thin-film InAs/GaAs QD

solar cells on mechanically flexible plastic films by using a

bond-and-transfer technique as an approach for the production

of versatile QD solar cells. The use of a metal–epoxy media-

ting agent enabled low-temperature bonding and layer transfer

onto plastic support substrates. We have also fabricated

FIG. 4. Light I–V characteristics of the highest-efficiency cells for each of

the bulk reference cells and the thin-film cells on plastic films and Si sub-

strates under AM1.5G, 1-sun illumination.

TABLE I. Solar-cell performance parameters of the highest-efficiency cells

and the highest VOC values for each type of cell under AM1.5G, 1-sun

illumination.

Bulk reference Thin film on plastic Thin film on Si substrate

Highest g 4.69% 10.5% 8.45%

VOC 0.65 V 0.64 V 0.57 V

JSC 12.3 mA cm�2 23.5 mA cm�2 24.5 mA cm�2

FF 0.58 0.70 0.61

Highest VOC 0.675 V 0.673 V 0.664 V

FIG. 3. (a) Cross-sectional schematic diagram, (b) scanning electron micro-

scope image, and (c) photographs of the fabricated thin-film InAs/GaAs QD

solar cells on plastic films. Inset of (b) is a cross-sectional tunneling electron

microscope image of the InAs QD layers.

192102-3 Tanabe, Watanabe, and Arakawa Appl. Phys. Lett. 100, 192102 (2012)

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thin-film InAs/GaAs QD cells on Si substrates. No material

degradation was observed through our bond-and-transfer pro-

cess, as verified by VOC measurements. The bond-and-transfer

process, without material degradation, thus provides a path-

way for the production of lightweight, mechanically flexible,

low-cost, and highly efficient QD solar cells based on ultrathin

single-crystalline III-V semiconductors.

This work was supported by the Ministry of Education,

Culture, Sports, Science and Technology (MEXT), Japan,

through the Project for Developing Innovation Systems and

by the Japan Society for the Promotion of Science (JSPS)

through Grants-in-Aid for Young Scientists (B) 23760303.

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