Supporting Information

Optimization of Si/ZnO/PEDOT:PSS Tri-Layer Heterojunction

Photodetector by Piezo-Phototronic Effect Using Both Positive

and Negative Piezoelectric ChargesFangpei Li, Wenbo Peng,* Zijian Pan, and Yongning He*

Content:A. Fabrication procedures of n-Si/n-ZnO/p-PEDOT:PSS tri-layer HPDB. Fundamental characterization of n-Si/n-ZnO/p-PEDOT:PSS tri-layer HPDC. Experiment apparatus, measurement system, and calculation of strain and piezo-

potential distribution in ZnO NWsD. Complementary electrical performance of n-Si/n-ZnO/p-PEDOT:PSS tri-layer

HPD modulated by the strainE. Complementary photoresponse performance of n-Si/n-ZnO/p-PEDOT:PSS tri-

layer HPD modulated by the strainF. Theoretical calculation of strain-induced modulations on energy band diagrams of

n-Si/n-ZnO/p-PEDOT:PSS tri-layer HPD

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A. Fabrication procedures of n-Si/n-ZnO/p-PEDOT:PSS tri-layer HPD

The fabrication procedures of the n-Si/n-ZnO/p-PEDOT:PSS tri-layer HPD are as-

below:

(1) Prepare a polished n-type silicon wafer (boron doped, 5-10 Ω∙cm, <100>), ultrasonically

clean it for 5 min in acetone, ethanol, deionized water, consecutively, and dry it by the

high-purified N2.

(2) Deposit a very thin layer of ZnO seed by the radio frequency magnetron sputtering. Use

a 3-inch-diameter ZnO ceramic target. The sputtering power is set to 120 W at room

temperature and the argon to oxygen ratio is 1:1 under 1.0 Pa with time duration of 10

min.

(3) Ultrasonically clean the sample for 3 min in acetone, ethanol, deionized water,

consecutively, and dry it by the high-purified N2. The nutrient solution for

hydrothermally growing ZnO NWs consists of 25 mM zinc nitrate and 12.5 mM

hexamethylenetetramine per 100 mL deionized water. In order to get separated and long

ZnO NWs, 5.4 mL ammonium hydroxide (Alfa Aesar) per 100 mL is added into the

mixed solution. The container is then pre-heated at 95 °C for 1 h in a mechanical

convection oven after which the sample is carefully immersed in the solution with the

ZnO seed layer facing down. At the same constant temperature another 40 min heating is

required to grow ZnO NWs of proper lengths. After cooling down the whole system, the

sample is collected and repeatedly washed by the deionized water. Then it is dried by the

high-purified N2.

(4) Spin-coat a very thin layer of PEDOT:PSS film on top of the as-synthesized ZnO NWs

at 5000 rpm for 50 s, followed by a 5 min annealing at 120 °C, in order to obtain the

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well-formed ZnO/PEDOT:PSS heterojunction.

(5) Finally, ITO and Al are deposited as the top and the bottom electrodes by radio

frequency magnetron sputtering at room temperature, respectively. Copper wires are

connected to the as-fabricated electrodes by silver pastes. The effective area of the HPD

is 7 mm × 10 mm.

Figure S1. Schematic fabrication procedure flow diagram of the n-Si/n-ZnO/p-PEDOT:PSS

tri-layer HPD.

B. Fundamental characterization of n-Si/n-ZnO/p-PEDOT:PSS tri-layer HPD

The SEM images of the as-grown ZnO NWs are shown in Figures S2a and S2b,

indicating that the sputtered ZnO seed layer is about 100 nm thick, and the ZnO NWs are

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2.33 μm high. The top view SEM image of the ZnO/PEDOT:PSS heterojunction is given in

Figure S2c, showing that the PEDOT:PSS film is very thin with no obvious bumps or

hollows observed on the surface.

Eg is strongly dependent on the preparation methods and particle size, the energy band

gap change of the ZnO NWs is therefore investigated by UV-visible spectroscopy. The

corresponding absorption curve of ZnO NWs to 320 ~ 800 nm lights is plotted in Figure S2d.

ZnO NWs were grown on a piece of glass in the same way as described in Section A.

Tauc equation

(αhυ)2 = C (hυ - Eg)

where α is the absorption coefficient of the material, h is the Planck’s constant, υ is the

frequency of the light, C is a proportionality constant, and Eg is the band gap energy of the

material, gives a method to calculate the Eg of semiconductor.[3] Since α is proportional to

the value of absorption, the Tauc equation can be further written into

(Ahυ)2 = C’ (hυ - Eg)

where A is absorption and C’ is a proportionality constant. From the result in Figure

R1a, the (Ahυ)2 – hυ curve of ZnO NWs can be derived and is plotted in Figure S2e. By linear

fitting the linear part of the (Ahυ)2 – hυ curve and extrapolating the fitting line until it meets

the x-axis (red dash line), the value of Eg is obtained. It can be observed that the band gap

energy of ZnO NWs is about 3.09 eV.

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Figure S2. SEM images on n-Si substrates: (a) cross-sectional view with an average height of

2.33 μm and (b) top view with a diameter of 40 nm of the as-synthesized ZnO NWs and (c)

top view of the ZnO/PEDOT:PSS heterojunction. (c) UV-visible (320 ~ 800 nm) absorption

curve and (d) the Tauc plot derived from the absorption curve of the ZnO NWs on glass

substrates.

C. Experiment apparatus, measurement system, and calculation of strain and piezo-

potential distribution in ZnO NWs

Figure S3a introduces how to apply an external strain onto the HPD: first connect the

device under test (DUT) to a piece of glass by a sheet of double-sided Kapton tape with a

thickness of 95 μm (consisting of 25 μm thick Kapton film and two 35 μm thick PMMA

layers), then fix the glass on an iron holder by Kapton tape as well. Another piece of glass is

pressed onto the surface of the device by a metal pole from a 3D mechanical stage of which

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the movement resolution is 10 μm.

In order to generate, measure and record the electric signals, a precision source/measure

unit (B2902A, Keysight) is used here. The source sweeping voltage is set from -2 to 4 V with

the ITO electrode defined as the positive whereas the Al electrode as the opposite. A

continuously variable optical filter is used to tune the incident optical power on the effective

area of the HPD. In terms of transient response, the bias is set to -2 V with the chopper

frequencies fixed at 10 and 5 Hz for 405 and 648 nm laser illuminations, respectively.

The commercial finite element analysis (FEA) software COMSOL Multiphysics is

utilized to conduct the theoretical simulation and calculation of the externally applied

compressive strains within the ZnO NWs. According to the schematic experiment apparatus

shown in Figure S3a and the device structure of the n-Si/n-ZnO/p-PEDOT:PSS tri-layer HPD

shown in Figure 1a, a three-dimensional (3D) model with exact the size of the practical HPD

is built. The cross-sectional view of the 3D model is shown in Figure S3b. All the materials’

parameters used in the simulation are listed below in Table S1. The nether surface of the

bottom PMMA is fixed, which means it has no displacements in any directions at all. Then a

downward displacement along the c-axis direction of the ZnO NW (i.e., the z-axis direction

in the coordinate) as a boundary condition is applied to the upper surface of the top glass to

describe the externally applied compressive displacements in the experiments. An electric

ground boundary condition is applied at infinite boundaries to serve as the reference for the

calculation of the piezo-potential distribution. Finally the piezoelectric constitutive equations

are solved using Solid Mechanics, Electrostatics and Piezoelectric Effect modules in

COMSOL. After the computation, the average strains and the piezo-potential distribution of

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the ZnO NWs under different compressive displacements can be derived and obtained by

post-processing.

Figure S3. (a) Schematic illustration of the experiment apparatus. (b) Cross-sectional view of

the 3D simulation model.

Table S1. Materials’ parameters used in the simulation.

Material εr

Density

(g/cm3)

Young’s modulus

(GPa)Poisson’s ratio

Glass 2.09 2.20 73.1 0.17

PMMA 3.0 1.19 3.0 0.40

Kapton 3.4 1.30 3.1 0.34

ITO 6.80 116 0.35

PEDOT:PSS 1.3 1.0 0.3

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Si 11.7 2.33 170 0.28

Al 2.70 70 0.35

D. Complementary electrical performance of n-Si/n-ZnO/p-PEDOT:PSS tri-layer HPD

modulated by the strain

The dark current at a constant bias of 4 V extracted from Figure 1e is plotted in Figure

S4a, indicating that as the compression increases the dark current rises. The linear fitting

straight line based on a selected range of (I, V) points from the linear part of the curve from

Figure 1e has an intersection with the x-axis, which is obtained and defined as Vth, the

forward turn-on voltage under a certain strain. Here, the Vth values of all strains are calculated

and plotted in Figure S4b, exhibiting a falling tendency as the external compressive strain

increases.

Figure S4. (a) Dark Current at a bias of 4 V as a function of the external compressive strain.

(b) Forward turn-on voltage under dark condition as a function of the external compressive

strain.

E. Complementary photoresponse performance of n-Si/n-ZnO/p-PEDOT:PSS tri-layer

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HPD modulated by the strain

Similar to Figure 2a and 2b, the I-V characteristics of the n-Si/n-ZnO/p-PEDOT:PSS tri-

layer HPD with different illuminant powers are measured and plotted in Figure S5 and S6 for

405 and 648 nm lasers, respectively.

Figure S5. I-V characteristics of the n-Si/n-ZnO/p-PEDOT:PSS tri-layer HPD under different

strain conditions with 405 nm laser illuminant power of (a) 0.31 (b) 1.10 (c) 3.96 (d) 14.24

mW.

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Figure S6. I-V characteristics of the n-Si/n-ZnO/p-PEDOT:PSS HPD tri-layer under different

strain conditions with 648 nm laser illuminant power of (a) 0.14 (b) 0.50 (c) 1.78 (d) 6.41

mW.

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Figure S7. Specific detectivity as a function of the external compressive strain under (a) 405

nm and (b) 648 nm laser illuminations, with corresponding ∆D*/D*0 values plotted in (c) and

(d), respectively.

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Figure S8. Transient I-t characteristics of the HPD under strain free condition in (a) 405 nm and (b) 648 nm laser illuminations at -2 V, with the extracted tr, tf as a function of the external compressive strain plotted in (c) and (d), respectively. Three cycles of transient response under different externally applied compressive strains are plotted in (e) for 405 and (f) 648 nm laser illuminations, respectively.

F. Theoretical calculation of strain-induced modulations on energy band diagrams of

n-Si/n-ZnO/p-PEDOT:PSS tri-layer HPD

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Commercial FEA software COMSOL Multiphysics is utilized to conduct the theoretical

simulation and derive the energy band diagrams of the n-Si/n-ZnO/p-PEDOT:PSS tri-layer

HPD under different compressive strains. For simplification, a one-dimensional (1D) model

is used here to investigate the effect of externally applied compressive strains on the energy

band diagrams of the n-Si/n-ZnO/p-PEDOT:PSS tri-layer heterojunction. The semiconductor

parameters of Si, ZnO and PEDOT:PSS used in the simulation are listed in Table S2. The

“Analytic Doping Model” built inside the Semiconductor module of COMSOL is applied to

describe the uniform doping and depletion regions of n-Si, n-ZnO and p-PEDOT:PSS,

respectively. The doping concentration of n-Si, n-ZnO and p-PEDOT:PSS are adopted as 5 ×

1015 cm-3, 1 ×1017 cm-3 and 1 ×1019 cm-3. Two Ohmic contacts are then applied to the ends of

n-Si and p-PEDOT:PSS to describe the externally applied bias. Next, a surface charge density

boundary condition is applied to the interfaces of the n-Si/n-ZnO/p-PEDOT:PSS tri-layer

heterojunction to introduce the piezoelectric polarization for the investigation of effects of

externally applied compressive strains on the energy band diagrams. Finally, the conventional

drift-diffusion approach is solved and computed using partial differential equations available

internally within the Semiconductor module in COMSOL. After the computation, the energy

band diagram of the n-Si/n-ZnO/p-PEDOT:PSS HPD under different compressive strains at -

2 V are derived and obtained by post-processing.

The piezoelectric polarization of different compressive strains used in the simulation can

be obtained from the piezoelectric polarization P, which can be retrieved from the

piezoelectric constitutive equation, as shown below:[1,2]

Pi = eijkεjk

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where the eijk is the piezoelectric coefficients matrix, εjk is the externally applied strain tensor,

and i, j, k = 1, 2, 3 are the corresponding coordinates along x, y, z directions, respectively.

As in the experiments compressive strains are applied to the devices along the c-axis of

the ZnO NWs, the stress matrix can be assumed to have only one component, σ33, which is

along the c-axis of the ZnO NWs. Thus, the strains induced by the stress can be expressed by:

where s11, s12, s13, s33, s44, and s66 belong to the compliance matrix S6×6 and can be calculated by

the elasticity matrix C6×6, ε11, ε22, and ε33 are strains along the a-, b-, and c-axis of the ZnO

NWs, respectively. Therefore, the ε11, ε22 can be calculated from ε33. Then, the piezoelectric

polarization along the c-axis of the ZnO NWs P3 can be calculated, which equals to the

piezoelectric polarization produced by the externally applied compressive strains at the

heterojunction interfaces of the n-Si/n-ZnO and the n-ZnO/p-PEDOT:PSS heterojunctions.

Table S2. Semiconductor parameters of Si, ZnO and PEDOT:PSS used in the simulation.

Material Band gap Electron affinity

Si 1.12[eV] 4.05[eV]

ZnO 3.37[eV] 4.50[eV]

PEDOT:PSS 1.70[eV] 3.50[eV]

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Figure S9. Theoretical calculation of externally applied compressive strain induced

modulations on energy band diagrams of the n-Si/n-ZnO/p-PEDOT:PSS tri-layer HPD. (a)

Full graphic of the calculated energy band diagram of the n-Si/n-ZnO/p-PEDOT:PSS tri-layer

HPD via FEA method for different compressive strains. Zoom-in graphics of (b) the enlarged

electron accumulation region in n-ZnO and the narrowing depletion region in n-Si, (c) the

hole “potential well” in n-Si that is becoming shallower and narrower, (d) the transformation

of the hole depletion region to a hole accumulation region in p-PEDOT:PSS, and (e) the

expansion of the electron depletion region in n-ZnO.

References: [1] W. Peng, X. Wang, R. Yu, Y. Dai, H. Zou, A. C. Wang, Y. He, Z. L. Wang, Adv. Mater. 29 (2017) 1606698. [2] Y. Liu, S. Niu, Q. Yang, B. D. B. Klein, Y. S. Zhou, Z. L. Wang, Adv. Mater. 26 (2014) 7209-7216. [3] J. Tauc, R. Grigorovici, A. Vancu, Phys. Status Solidi 15 (1966) 627-637.

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