Basic Research on the Science and Technology ofHigh-efficiency Quantum Dot Superlattice Solar Cells
Principal Researcher: Yoshitaka Okada, Associate Professor
Research Center for Advanced Science and Technology, The University of Tokyo

I. Introduction
Recently, semiconductor quantum dots (QDs) incorporated in the active region of ap-i-n junction solar cell (SC) have attracted intense research as a possible means ofexploiting the below bandgap infra-red photons to generate additional photocurrents
beyond that corresponding to the band-to-band transition in a conventional single-junction
SC1,2). In QDSCs, QDs must be uniform in size and periodically spaced in all 3
dimensions, which lead to the formation of an intermediate band to achieve the predictedhigh energy conversion efficiencies.
The experimental demonstration of QDSC requires fabrication of a three-dimensional
QD superlattice placed in the active region. In our previous work3), we reported onInP-based QDSC with 30 stacked InAs/AlGaInAs QD layers on InP (311)B substratefabricated by strain compensation technique. Here the compressive strain due to QD layer
was compensated by a tensile strain induced by the spacer layer. Consequently, the size
homogeneity of QDs has improved to 12 ~ 13%. As our continuing effort, we applied thistechnique fabricate InAs/GaNAs QDSCs by the strain compensation technique4-6).
II. Experiments and Results
We fabricated 4 series of solar cells S1, S2, S3, and S4 by molecular beam epitaxy(MBE) with a radio frequency (RF) plasma as the nitrogen source on n+-GaAs (001)substrates7,8). The 4 samples were identical in the structure except for the i-layer region asshown in Fig. 1. The sample S1 and S2 were grown with 10 and 20 stacked pairs of anInAs QD layer with 2.0 monolayer (ML) thickness and a 20 nm-thick GaNAs strain
compensation layer (SCL), respectively. The sample S3 comprised of 20 pairs of 2.0ML
InAs QDs and 20 nm-thick GaAs spacer layers and hence the QD structure was strained.
Lastly, the sample S4 was a GaAs control cell with a 400 nm-thick i-GaAs layer used as areference. For electrical characterization, AuZn alloy was used for the top electrode andindium for the bottom, and a standard anti-reflection coating (ARC) was employed in 3 ×3mm2 sized SCs.

－ 205 －Fig. 1 Schematic layer structure of QDSC

Fig. 2 Symmetric (004) x-ray diffraction

with multi-stacks of InAs QDs embedded

patterns for samples with 20 stacked

by GaNAs strain compensation layers

layers of InAs QDs structure embedded(SCLs), or conventional GaAs spacers.

in (a) 20 nm-thick GaNAs SCLs (S2),and (b) 20 nm-thick GaAs spacer layers(S3), respectively.

First, we measured the symmetric scans around (004) reflection in a ω-2θ geometry inhigh resolution x-ray diffraction (HR-XRD) in order to characterize the effect of strain
compensation. Figure 2 show the XRD spectra for sample S2 and S3. The satellite peaks
originating from the periodic superlattice structure can be observed in each sample, andhence abrupt heterointerfaces are maintained. For S2, the compressive strain induced byInAs QD layers is compensated almost perfectly by 20 nm-thick GaNAs SCLs as shownin Fig. 2 (a), where the zeroth-order satellite peak indicated by an arrow in the figure
shows a near perfect lattice-match with GaAs substrate.
Figure 3 shows the cross-sectional scanning transmission electron microscope (STEM)
image measured for S2. A vertical alignment of lens-shaped QDs along the growth
direction is maintained without degrading the structure and size uniformity from one layer
to the next. Further, we observe no strain-induced defects or dislocations from the image,which would otherwise be generated if the build-up of lattice strain exceeds the criticallayer thickness during the stacking. The mean QD diameter, height, size dispersion indiameter, and in-plane density were 39.1 nm, 5.2 nm, 13.4%, and 3.6 × 1010/cm2,－ 206 －Fig. 3 Cross-sectional STEM image

Fig.4

IQE spectra measured for;of 20 stacked layers of InAs QDs(a) strain compensated InAs/GaNAs QDSC

embedded in 20 nm-thick GaNAs

with 10 QD stacks (S1),SCLs (S2).

(b) strain compensated InAs/GaNAs QDSC
with 20 QD stacks (S2),(c) InAs/GaAs QDSC with 20 stacks (S3),(d) GaAs p-i-n control cell (S4), and(e) PL spectrum at room temperature for S2.

respectively. As a result, the total density of QDs amounts to on the order of 1012/cm2,
which would be difficult to achieve by the conventional strained heteroepitaxy.
Figure 4 (a) ~ (d) show the internal quantum efficiencies (IQEs) of QDSCs, S1 throughS4, respectively. The IQE was determined from a direct measurement of the external QE
under a constant photon irradiation of 1016/cm2, then discounting for the shadowing loss
and residual reflection loss arising from ARC. We also measured photoluminescence (PL)for each sample at room temperature by using a standard lock-in technique with a cw 532
nm second harmonic generation (SHG) Nd:YVO4 laser as an excitation source andInGaAs photodetector. The PL spectrum obtained for S2 is shown in Fig. 4 (e), which isused to identify the photoabsorption edge for each layer. Firstly, a single PL emission
peak from the ground-state of InAs QDs is observed at around 1170 nm with a full-width－ 207 －half-maximum (FWHM) of 65.4 meV. In addition, a shoulder peak originating fromGaNAs spacer layer and InAs wetting layer (WL) is seen at around 980 nm for S1 and S2,and 880 nm for S3, respectively. Secondly, the absorption edge redshifts from 1150 nm
for S3 up to 1200 nm for S2 and this is because the quantum confinement is lower at theconduction band interface when GaNAs is used as the barrier instead of GaAs.
The filtered short-circuit current density (Isc) of sample S2 (strain compensated, 20stacks) above the GaAs bandedge of ~ 880 nm, which gives the contribution from QD
layer was calculated to be 2.47 mA/cm2 by taking the product of IQE and 1.5 air-masssolar spectrum. This value is twice that of S1 (strain compensated, 10 stacks), and 4 times
larger than that of S3 (strained, 20 stacks), respectively. Though the obtained value isnotably higher than the reported values for QDSCs, the maximum IQE from the QD
layers even for S2 is still limited to ~ 5 %, and thus a larger stack of QD layers is required
to obtain higher photoabsorption. Furthermore, the fact that degradation of QEs in theshorter wavelength region is not observed after increasing the QD layer from 10 to 20stacks suggests that the crystalline quality is maintained in our strain compensated QDSCs
as the contribution to the photocurrent generated by the p-GaAs emitter is not affected.
The dark I-V measurements show an improved diode factor of n = 1.42 for S2 compared
to n = 1.62 for S3 due to the reduction of recombination loss as result of strain
compensation. To this end, Isc improved to 21.1 mA/cm2 for S2 compared to 19.0 mA/cm2for S1, and 19.6 mA/cm2 for S3, respectively, and Isc of control cell S4 with 400 nm-thicki-GaAs layer was 19.7 mA/cm2 as a reference.
III. Summary
In summary, we have successfully fabricated and characterized InAs/GaNAs QDSCsby strain compensation technique on GaAs(001) substrates. Compensating for the strain
induced by QDs with a spacer layer that produces the opposite strain works well toachieve (1) improved size uniformity, (2) redshift of absorption edge, and (3) avoid
generation of defects and dislocations. The total QD density of our QDSC with 20 InAs
QD stacks is on the order of 1012/cm2, and Isc = 21.1 mA/cm2. The filtered Isc above GaAs
bandedge of 2.47 mA/cm2 is 4 times higher than that for a strained QDSC with theidentical cell structure.
I’d like to thank Dr. R. Oshima and the hard-working students in my laboratory atRCAST, The University of Tokyo. The JFE 21st Century Foundation is also gratefully
acknowledged for their support.

－ 208 －References
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