Supplementary Material of
An Efficient Fast Response and High-Gain Solar-Blind Flexible
UV Photodetector Employing Hybrid Geometry
Amreen A. Hussain1, Arup R. Pal1* and Dinkar S. Patil2
1
Physical Sciences Division,
Institute of Advanced Study in Science and Technology, Guwahati, Assam, India
2
Laser and Plasma Technology Division,
Bhabha Atomic Research Center, Trombay, Mumbai, India
* Email: arpal@iasst.gov.in
1
Table 1: Comparison of the characteristic parameters for the present PPani-TiO2 hybrid UV
photodetector and other organic-inorganic based hybrid UV photodetectors
PD
UV light
(Ip-Id)/Id
(nm)
Rλ
τr
τd
(AW-1)
(s)
(s)
G
Ref.
ZnO: CuPc
350
3 × 104
2.9 × 106
2.4
3.0
1 × 107
1
ZnO: CuSCN
380
~ 102
0.0075
500 n
6.7 μ
0.04
2
ZnO: CNT
365
~ 1.25
-
< 0.5
< 0.5
-
3
TiO2: PFH
365
~ 103
0.0332
< 200 m
< 200 m
-
4
ZnO: PFH
365
~ 103
0.68
< 200 m
< 200 m
-
5
ZnO: PANI
350
46
-
47
58
-
6
TiO2: PFH
365
~ 103
0.0564
< 200 m
< 200 m
-
7
TiO2: PFH
365
~ 104
0.00692
< 200 m
< 200 m
-
8
TiO2: PPani
254
9.18 × 102
2.15 × 104
22.87 m
34.23 m
1.05 × 105
Present
work
All the measurements were carried out at room temperature in air. Photodetectors (PD),
Photosensitivity - ((Ip - Id) / Id) (Ip – Photocurrent, Id – Dark current), Responsivity (Rλ), Rise
time (τr), Decay time (τd), Photoconductive gain (G)
In the present work, we fabricated a flexible hybrid UV photodetector based on PPani-TiO2
nanocomposite by a single step plasma process. Our main effort in this work is to maintain a
balance between the photoconductive gain and response speed trade-off of the photodetectors.
We obtained a high photoconductive gain of the order of 105 and a fast response time of the
order of 10-3 s. Moreover, the preparation condition of the present hybrid nanocomposite
includes the use of non-toxic components and a completely solvent free plasma based process
that integrates the way for mass production of low cost and large area nanoscale hybrid UV
photodetectors.
2
S1: Electrochemical determination of the band alignment using cyclic voltammetry
Cyclic voltammetry is conducted with a potentiostat/galvanostat (Gamry Instruments, USA) for
electrochemical analysis which are carried out in a standard three electrode cell consisting of a
ITO working electrode, a platinum counter electrode and a Ag/AgCl reference electrode using a
sweep rate of 100 mV/s. PPani is studied in the film form, which are prepared by plasma
polymerization of aniline monomer on ITO electrode. The supporting electrolyte is KCl in
acetronitrile and ferrocene is used for calibration.
Efficient charge transfer from donor to acceptor component, effective charge transport and
charge injection into the electrodes are important parameters for design and optimization of
hybrid photodetectors. In this regard electrochemical data allow the estimation of the relative
position of Highest Occupied Molecular Orbital / Lowest Unoccupied Molecular Orbital
(HOMO/LUMO) levels of the investigated materials. The redox couple ferrocene/ferricenium
ion (Fc/Fc+) is used as external standard. The corresponding HOMO and LUMO levels are
𝑟𝑒𝑑
𝑜𝑥
𝑟𝑒𝑑
calculated using 𝐸1/2
and 𝐸𝑜𝑛𝑠𝑒𝑡
/ 𝐸𝑜𝑛𝑠𝑒𝑡
for experiments in PPani films.
The estimations are done with the empirical relation:
𝐸𝐻𝑂𝑀𝑂/𝐿𝑈𝑀𝑂 = [−𝑒𝑥𝑝 (𝐸𝑜𝑛𝑠𝑒𝑡(𝑣𝑠 𝐴𝑔/𝐴𝑔𝐶𝑙) − 𝐸𝑜𝑛𝑠𝑒𝑡(𝐹𝑐/𝐹𝑐+𝑣𝑠 𝐴𝑔/𝐴𝑔𝐶𝑙 ) )] − 4.8𝑒𝑉
including standard ferrocene value of -4.8 eV with respect to vacuum level [9]. The HOMO level
as calculated from CV and optical band gap based on tauc’s plot [10] from UV-Vis spectra of
PPani-TiO2 are -5.92 eV and 3.51 eV, respectively. The LUMO level of PPani is deduced to be 2.41 eV. Here, the optical band gap is used to calculate LUMO level. However, the LUMO level
can also be calculated from the reduction onset potential in CV measurements. It is noted that the
optical band gap is different from its energy band gap [11]. As the molecular simulation of the
basic unit of the polymer does not consider the effect of π conjugation of the macromolecule
chains, the simulated energy levels of the basic unit, in particular the LUMO level usually shows
some deviation from the experimental value. The optical band gap Eg can be determined from the
optical absorption coefficient (α) calculated as a function of incident photon energy (hʋ) and is
expressed by the tauc’s relation:
3
α hʋ = B (hʋ - Eg)m
where, B is an energy independent constant. The fundamental absorption edge in PPani is formed
by direct allowed transition where m = ½. The index ‘m’ is a constant that determines the type
of optical transition [12]. The value of ‘m’ can also be determined by the procedure described by
Yakuphanoglu et al [13]. The optical band-gap is obtained from the extrapolation of the linear
part of the plot (αhʋ)2 versus hʋ at (αhʋ)2 equals zero.
The HOMO and LUMO levels of TiO2 have the values -7.34 eV and -4.23 eV, respectively. The
relative position of donor LUMO and acceptor LUMO is crucial for the efficient charge transfer.
The HOMO/LUMO level of PPani is higher than the HOMO/LUMO of TiO2. At these
conditions it is energetically favorable for the photoexcited PPani to transfer electron to TiO2.
Therefore, PPani can be used as the donor and TiO2 as the acceptor to be used in the fabrication
of hybrid photodetectors.
References
[1] Q. Chen, H. Ding, Y. Wu, M. Sui, W. Lu, B. Wang, W. Su, Z. Cui, L. Chen, Nanoscale, 5,
4162 (2013)
[2] S. M. Hatch, J. Briscoe, S. Dunn, Adv. Mater. 25, 867 (2013)
[3] B. J. G. Ok, S. H. Tawfick, K. A. Juggernauth, K. Sun, Y. Zhang, A. J. Hart, Adv. Funct.
Mater. 20, 2470 (2010)
[4] Y. Han, C. Fan, G. Wu, H. -Z. Chen, M. Wang, J. Phys. Chem. C. 115, 13438 (2011)
[5] H. -G. Li, G. Wu, M. -M. Shi, L. -G. Yang, H. -Z. Chen, M. Wang, Appl. Phys. Lett. 93,
153309 (2008)
[6] S. Mridha, D. Basak, Appl. Phys. Lett. 92, 142111 (2008)
[7] Y. Han, G. Wu, H. Li, M. Wang, H. Chen, Nanotechnology, 21, 185708 (2010)
[8] Y. Han, G. Wu, M. Wang, H. Chen, Polymer, 51, 3736 (2010)
4
[9] M. Al-Ibrahim, H. -K. Roth, M. Schroedner, A. Konkin, U. Zhokhavets, G. Gobsch, P.
Scharff, S. Sensfuss, Organic Electronics 6, 65 (2005)
[10] J. Tauc (ed), Amorphous and liquid semiconductors. Plenum, London (1974)
[11] S. M. Sze, K. K. Ng, Physics of Semiconductor Devices (3rd ed), John Wiley & Sons
(2006)
[12] F. Yakuphanoglu, B. F. Şenkal, Polym. Adv. Technol. 19, 1876 (2008)
[13] F. Yakuphanoglu, E. Basaran, B. F. Şenkal, E. Sezer, J. Phys. Chem. B. 110, 16908 (2006)
5