N AN O E X P R E S S Open Access
Performance-improved thin-film a-Si:H/μc-Si:H
tandem solar cells by two-dimensionally
nanopatterning photoactive layer
Cheng Zhang, Xiaofeng Li
*
, Aixue Shang, Yaohui Zhan, Zhenhai Yang and Shaolong Wu
Abstract
Tandem solar cells consisting of amorphous and microcrystalline silicon junctions with the top junction
nanopatterned as a two-dimensional photonic crystal are studied. Broadband light trapping, detailed electron/hole
transport, and photocurrent matching modulation are considered. It is found that the absorptances of both junctions
can be significantly increased by properly engineering the duty cycles and pitches of the photonic crystal; however, the
photocurrent enhancement is always unevenly distributed in the junctions, leading to a relatively high photocurrent
mismatch. Further considering an optimized intermediate layer and device resistances, the optimally matched
photocurrent approximately 12.74 mA/cm
2
is achieved with a light-conversion efficiency predicted to be 12.67%,
exhibiting an enhancement of over 27.72% compared to conventional planar configuration.
Keywords: Tandem solar cells; Photonic crystal; Photocurrent matching
Background
A common goal for photovoltaic (PV) design is to find ef-
fective ways to manage photons and excitons for high
conversion efficiency by for example reducing cell reflec-
tion loss, improving light absorption of photoactive layers,
and increasing charge collection [1]. The rapid progress of
PV science has witnessed a lot of advanced light-trapping
scenarios and technologies, such as impedance-matched
coating [2], moth's eye structures [3], optical antennas [4],
and photonic crystals [5]. Recent interests also focus on
the applications of plasmonics in photovoltaics [6], e.g., by
core-shell metallic nanowire design [7] or metallic gratings
[8]. However, the strong p aras itic absorption brings a
big c hallenge to strictly balance the (negative) para sitic
absorption loss and (positive) photocurrent gain of plas-
monic solar cells (SCs) [9]. Therefore, conventional dielec-
tric light -trapping structures are still attracting intensive
research/application interests. Among these designs, pho-
tonic crystals are usually employed as an effective way to
guide and confine the solar incidence, e.g., two-dimen
sional (2D) backside oxide grating [10] and low- or high-
dimensional photonic structures [11,12].
The above designs are mainly dedicated to single-
junction SCs. The strong demand f or high photocon-
version efficiency requires a more efficient use of the
broadband solar incidence, leading to the generations of
tandem and multi-junction cells. One important direction
is the silicon-based tandem thin-film SCs (TFSCs), which
are realized by introducing a layer of hydrogenated micro-
crystalline silicon (μc-Si:H) into conventional amorphous
silicon (a- Si:H) SCs [13]. Compared to single-junction cells,
a well-designed tandem solar cell has to be the combin-
ation of properly designed light trapping, efficient carrier
transportation with low carrier loss, and perfectly matched
photocurrent. Unlike the ordinary random texture or
nanopattern in transparent conductive oxide (TCO), we re-
cently proposed an a- Si:H/μc -Si:H tandem cell by nanopat-
terning the a- Si:H layer into one-dimensional (1D) grating.
It is found that the realistic output photocurrent density
(J
sc
) after current matching treatment can be greatly im-
proved arising from a broadband absorption enhancement,
which is stable against the changes of light polarization
and injection direction [14].
* Correspondence: xfli@suda.edu.cn
Institute of Modern Optical Technologies & Collaborative Innovation Center
of Suzhou Nano Science and Technology, Key Lab of Advanced Optical
Manufacturing Technologies of Jiangsu Province & Key Lab of Modern
Optical Technologies of Education Ministry of China, Soochow University,
Suzhou 215006, China
© 2014 Zhang et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons
Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction
in any medium, provided the original work is properly cited.
Zhang et al. Nanoscale Research Letters 2014, 9:73
http://www.nanoscalereslett.com/content/9/1/73