Gradient Al-doped ZnO multi-buffer layers: Effect on the photovoltaic
properties of organic solar cells
Xuan Yu
a,b,1
, Xiaoming Yu
a,b,1
, Jianjun Zhang
b,
n
, Hongjun Pan
a
a
Innovation Application Institute, Zhejiang Ocean University, Zhoushan, Zhejiang 316022, China
b
College of Electronic Information and Optical Engineering, Nankai University, Tianjin 300071, China
article info
Article history:
Received 6 August 2015
Received in revised form
2 September 2015
Accepted 4 September 2015
Available online 5 September 2015
Keywords:
Thin films
Semiconductors
Inverted organic solar cells
Al-doped ZnO
Multi-buffer layers
abstract
In this work, gradient Al-doped ZnO multi-buffer layers (MBLs) were used as the electron transport
layers (ETLs) of organic solar cells (OSCs) for the first time. Three kinds of MBLs were prepared using a
facile sol–gel process and deposited on a homo-buffer layer (ZnO). Compared to a device based on a
multi-ZnO buffer layer, devices with the gradient-doped MBL displayed enhanced power conversion
efficiency from 2.11% to 3.43%, which were caused by the efficient light transmission and charge carrier
transport, derived from the higher film qualities, denser and more uniform surface morphologies, and
improved conductivities of the gradient-doped MBLs. Results indicated that a device with ZnO/MBL
(0.5:1.0 at%(Al/Zn)) simultaneously enhanced the short-circuit current and fill factor. This study presents
an innovative and effective method for improving the quality of ETLs for efficient OSCs.
& 2015 Elsevier B.V. All rights reserved.
1. Introduction
Sol–gel derived ZnO thin-films have attracted much attention
on account of their good transparency in the visible spectral range,
easy production via a solution process, and potential applications
as light emitting diodes and photovoltaic devices. Recently, ZnO
has been placed on the indium tin oxide (ITO) electrodes and used
as an electron transport buffer layer (ETL) in organic solar cells
(OSCs) [1–3]. In order to improve device performance, the effects
of the amount of Al dopant [4], surface roughness [5], and the
nanostructure of Al-doped ZnO (AZO) [6] on the efficiency of OSCs
have been studied.
Annealing procedure is known to be critical for sol–gel AZO.
However, thermal expansion inevitably occurs in the ITO substrate
during the AZO annealing process. The crystal qualities and optical
properties of the AZO thin-films are affected by the mismatched
thermal expansion coefficients of ITO and AZO. Annealing also
increases the defect density on the surfaces of the AZO thin-films
[7–8]. Defects containing hydroxyl groups are known to act as
electron charge defect traps, which not only affect the electrical
properties at the interface of the AZO and organic layers [9] but
also act as charge trap sites or recombination centers [10],influ-
encing the photovoltaic performance of the organic devices.
Although the introduction of a homo-buffer layer and use of
gradient doping can improve AZO film quality [11–12], studies on
the influence of gradient doping and homo-buffer layer on the
properties of organic photovoltaic devices have not been per-
formed. In this work, a gradient AZO multi-buffer layer (MBL) was
used in organic devices as the ETL for the first time. Three kinds of
bistratal gradient AZO thin-films of 0.5/1.0 at%(Al/Zn), 1.0/1.5 at%
(Al/Zn), and 1.5/2.0 at%(Al/Zn) layers, which were based on an
intrinsic ZnO buffer layer, were used as the ETLs of inverted OSCs.
These results indicated that MBLs are promising tools for im-
proving the photovoltaic performance of OSCs.
2. Experiments
A sol–gel solution was prepared by dissolving zinc acetate di-
hydrate (Zn(CH
3
COO)
2
2H
2
O) and thanolamine (C
2
H
7
NO) in
ethanol. Aluminum nitrate (Al(NO
3
)
3
9H
2
O) was used as an Al
source [13] with Al-to-Zn ratios (Al/Zn) of 0.5, 1.0, 1.5, and 2.0 at%.
After 24 h, the solution was spin-coated onto clean ITO substrates.
Samples were then annealed at 300 °C for 30 min to obtain ZnO
thin-films. A ZnO/ZnO/ZnO reference layer, defined as A, was also
fabricated. To prepare multilayered gradient AZO, the samples
were spin-coated with different AZO solutions with concentra-
tions of 0.5/1.0 at%, 1.0/1.5 at%, and 1.5/2.0 at% on ZnO buffer layers,
and these layers were defined as B, C, and D, respectively.
An active layer consisting of a poly (3-hexylthiophene)(P3HT):
Contents lists available at ScienceDirect
journal homepage: www.elsevier.com/locate/matlet
Materials Letters
http://dx.doi.org/10.1016/j.matlet.2015.09.017
0167-577X/& 2015 Elsevier B.V. All rights reserved.
n
Corresponding author.
E-mail address: jjzhang@nankai.edu.cn (J. Zhang).
1
Contributed equally to this work.
Materials Letters 161 (2015) 624–627
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