Review
Improved efficiency of InAs/GaAs quantum dots solar cells by Si-doping
Xiaoguang Yang
a
, Kefan Wang
a
, Yongxian Gu
a
, Haiqiao Ni
b
, Xiaodong Wang
c
, Tao Yang
a,
n
,
Zhanguo Wang
a
a
Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China
b
The State Key Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China
c
Engineering Research Center for Semiconductor Integrated Technology, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China
article info
Article history:
Received 3 July 2012
Received in revised form
1 February 2013
Accepted 4 February 2013
Keywords:
Solar cells
Intermediate-band
Quantum dots
Si-doping
abstract
This paper reports on significantly improved efficiency of InAs/GaAs quantum dot (QD) solar cells by
directly doping Si into InAs QDs during the QD growth. The devices which contain five stacked QDs in
their i-regions were grown using molecular beam epitaxy. It is shown that using appropriate Si-doing,
the open-circuit voltage of the device can be increased to 0.84 V. This is dramatically higher than the
value of 0.67 V obtained in undoped device using the same structure. Moreover, the efficiency of
corresponding device is improved from 11.3% to 17.0%. This improvement in efficiency is attributed to
greatly reduced energy loss in the devices that results from the reduction of the defect density in the
stacked InAs/GaAs QD layers due to the doping.
& 2013 Elsevier B.V. All rights reserved.
Contents
1. Introduction ......................................................................................................144
2. Experiment.......................................................................................................145
3. Results and discussion ..............................................................................................145
4. Conclusions ......................................................................................................146
Acknowledgments .....................................................................................................146
References . . .........................................................................................................147
1. Introduction
As novel photovoltaic devices, intermediate-band solar cells
(IBSCs) have attracted considerable attention because of their
very high efficiency limit of 63.1% under the Shockley and
Queisser model, which far exceeds the 40.7% limiting value of
ordinary single junction solar cells [1]. Because of their discrete
density of states, fabricating quantum dots (QDs) inside a semi-
conductor has proved to be an effective method for realizing
an independent IB between conduction band and valence band of
the host material, and InAs/GaAs QDs are a suitable material
system [2–4]. Previous studies on InAs/GaAs QD solar cells have
improved the available photocurrent compared to GaAs solar cells
using the same structure, however, there were severe drops in
open-circuit voltage (V
OC
) at the same time due to defects in the
QD layers that offset the benefit to devices’ efficiency introduced
by QDs [5–7]. These defects arise from the strain that accumulates
during the growth of the multilayers of InAs/GaAs QDs. The strain
results from the lattice mismatch between InAs and GaAs [8].
How to reduce the accumulated strain and related defect forma-
tion in the stacked QD layers has become a big challenge to
realizing high efficiency QD solar cells.
The use of strain compensation has been considered to be a
promising way of improving the material quality and the effi-
ciency of devices, but the practical effect of using this method is
limited because it changes the QD bandgap [9,10]. Recently, there
have been reports that Si-doping during the growth of InAs QDs
can lead to an enhancement of the photoluminescence intensity
from the QDs [11,12]. In this letter, the effect of the Si-doping on
the morphology and optical properties of InAs/GaAs QDs grown
Contents lists available at SciVerse ScienceDirect
journal h omepage: www.elsevier.com/locate/solmat
Solar Energy Materials & Solar Cells
0927-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.solmat.2013.02.005
n
Corresponding author. Tel./fax: þ 86 10 8230 4529.
E-mail address: tyang@semi.ac.cn (T. Yang).
Solar Energy Materials & Solar Cells 113 (2013) 144–147