Supplementary Information for:

Enhanced NIR response of nano- and microstructured silicon/organic

hybrid photodetectors

Vedran Đerek, Eric Daniel Głowacki, Mykhailo Sytnyk, Wolfgang Heiss, Marijan

Marciuš, Mira Ristić, Mile Ivanda, and Niyazi Serdar Sariciftci

Materials

Boron-doped p-type silicon wafers with thickness of 500 µm and <100> orientation were

obtained from Topsil; R0= 5-10 Ω·cm. The wafers were one-side polished, with an

alkaline-etched backside.

A. General sample preparation

Silicon wafers were cut into 25×25 mm2 substrates. Substrates were pre-cleaned by

sonication in acetone, 2-propanol, 2% solution of chemical detergent Hellmanex III, and

DI water, followed by the RCA standard cleaning steps.1 Immediately prior to chemical

surface structuring, the chemical oxide formed by the RCA clean was removed from the

substrate by a 30 second dip in 5% HF solution. Next, the substrates were surface

structured by a number of well-established silicon structuring methods which will be

presented separately. Hierarchical Silicon surface structuring was accomplished by

combining the different structuring steps ordered by the dominant structure size. After the

structuring steps the prepared substrates were cut into 15×15 mm2 pieces and RCA

cleaned in order to remove the introduced organic impurities and metal contamination.1

Chemical oxide originating from the RCA cleaning process was removed from the

samples by a 30 second 5% HF dip. Immediately following the HF treatment, samples

were transferred to a vacuum chamber where an aluminum back contact was evaporated

through a shadow mask onto the alkaline-etched side of the wafer. After deposition of the

Al, the samples were annealed in N2 atmosphere at 450 C for 20 minutes to ensure an

Ohmic Al/p-Si contact. Next, samples were dipped into 5% HF for 30s, with Al contact

being protected from etching by adhesive polyimide tape. Then the sample was dipped

for 20s in RCA SC-2 solution heated at 70C to re-grow the oxide and remove residual

metal impurities. Next, a final 30s dip in 5% HF was conducted to generate a clean

hydride-terminated surface. The protective polyimide tape was removed and the samples

were immediately loaded into a vacuum chamber for evaporation of the Tyrian Purple

(TyP) layer. TyP is a natural indigoid pigment that has been reported to be an ambipolar

organic semiconductor with µh=µe=0.4 cm2/Vs.2,3 TyP was synthesized according to

known literature methods4 and purified thrice by temperature gradient sublimation using

a source temperature of 270C at a pressure of ~110-6 mbar. The vacuum evaporation

technique used was hot-wall epitaxy, allowing independent temperature control of the

substrates.5 At a pressure of ~110-7 mbar, the substrates were heated to 580 C for 10

minutes. The substrates were then cooled over 30 minutes to ~70C and then TyP was

evaporated at a rate of 0.15 Å/s, to a thickness of ~40 nm. Samples were then removed

from vacuum and a ~2500 nm-thick poly(methyl methacrylate) (PMMA) buffer layer

was coated from chlorobenzene solution along the edge of the sample to prevent later

shunting through the TyP layer to the p-Si beneath during application of measurement

contact pins to the top Al contact. After drying the PMMA at 70C for 10 minutes, the

samples were transferred to a vacuum chamber for thermal evaporation of 100 nm of Al

for top contacts. Top contacts were defined by a shadow mask to give an active device

area of 0.07 cm2.

B. Electrochemically-grown porous silicon (PS)

A 100 nm-thick layer of Aluminum was deposited on the alkaline etched side of the

substrates by thermal evaporation for back contact during the anodization. Substrates

were annealed at 450 °C in N2 atmosphere for 20 minutes in order to establish Ohmic

contacts between pSi/Al. The polished surface of the substrate was micro-structured by

anodization in the Standard Etch Cell as defined by Sailor.6 Back contact to the silicon

substrate was established by directly contacting aluminum foil with the aluminized back

of the substrate, while the counter electrode was a 15 cm long spirally bent platinum

wire. Anodization was conducted in galvanostatic mode using a Keithley 2401

Sourcemeter with a current density of 50 mA/cm2 for the duration of 5 minutes. The

electrolyte consisted of a mixture of 2 M HF + 0.25 M of tetrabutylammonium

perchlorate + 2.4 M H2O in acetonitrile.7,8 After the anodization, the electrolyte was

removed with a pasteur micropipette and the sample was rinsed twice with ethanol. Pores

in the freshly prepared porous silicon were chemically widened by direct application of

an oxidizing solution (4:1:1 DMSO:Ethanol:48% HF) into the anodization cell for 10

minutes.6 After the pore widening, the solution was emptied, and the substrate was rinsed

twice with ethanol and removed from the anodization cell. The aluminum back contact

was etched away in an aluminum etchant solution (H3PO4:HNO3:CH3COOH:H2O =

73% : 3.1% : 3:1% : 20.8%), after which the substrates were thoroughly rinsed with DI

water and RCA SC-2 cleaned to remove the residual metal contamination introduced by

the aluminum etching.

C. Metal-assisted chemical etching of silicon (Si MACE)

A roughened silicon surface consisting of conical pores of size 10-100 nm was prepared

by metal-assisted chemical etching of silicon.9 Silver nanoparticles were deposited on the

substrate surface by a 5 minute dip in a mixture of NH4F:AgNO3:H2O = 2% : 0.2% :

97.8%.10 Silicon surface was etched by placing a silver nanoparticle coated substrate in a

mixture of 1:1 = 31% H2O2: 5% HF for 15 minutes. Silver nanoparticles were dissolved

from the substrate by a 5 minute dip in 35% nitric acid, followed by a thorough DI water

rinse and a RCA SC-2 cleaning step in order to eliminate residual silver ion

contamination from the substrate.

D. Silicon micropyramids (µ-pyramid Si)

Micropyramids were formed on <100> silicon substrates by anisotropic etching in a

water solution of 2% KOH and 10% 2-propanol.11,12 Etchant was heated to 90 °C in a

beaker topped by a reflux condenser on a stirrer/hot plate. A PTFE coated stir bar was

placed in the beaker, and the etchant was stirred at 1000 rpm. Substrates were positioned

vertically in a PTFE sample holder and placed in the beaker for 30 minutes. After the µ-

pyramids were formed the substrates were thoroughly rinsed with DI water and RCA SC-

2 cleaned to remove the residual metal ion contamination introduced by the KOH.

E. Characterization and measurement techniques

Diode devices were contacted using either pogo pin contacts or wires bonded to the

contacts with silver paste. Electrical IV measurements were conducted using a computer

controlled Keithley 2401 Sourcementer. IV measurements were taken in dark or under

the diode laser illumination, while JSC values were measured by keeping the device at

short circuit during illumination in order to measure the steady-state current. Photocurrent

output was measured for 2-3 minutes for each sample. Two IR sources were used: For IV

measurements a diode laser emitting at 1.48 µm, giving an irradiance of ~200 mW/cm 2;

for spectral responsivity a broadband light source coupled through a grating

monochromator was used. Temporal response was not measured for these devices due to

the large device area (~ 0.1 cm2), which from previous studies is known to give a

bandwidth of about 5 MHz due to the large RC time-constant.13 SEM images of

structured silicon and final devices were taken by FE-SEM Jeol 7000 and Zeiss 1540XB

CrossBeam FE-SEM/FIB.

FIG. S1. J-V characteristics of Al/p-Si/TyP/Al in dark and under illumination with a 1.48

µm laser diode (200 mW/cm2) for hierarchically structured substrates. Dark J-V curves

are shown with dotted lines, while solid lines represent J-V characteristics under

illumination. a) Comparison of PS/µ-pyramid Si with planar devices; b) µ-pyramid

Si/PS/Si MACE versus planar; c) µ-pyramid Si/Si MACE diodes versus planar.

FIG. S2. Comparison of spectral responsivity of Al/µ-pyramid Si/TyP/Al diodes biased

at -1V (purple line) with literature responsivity data for black silicon and InGaAs

FIG. S3. (a),(b) Reflectance values for the silicon samples without and with TyP,

respectively; (c),(d) Transmittance of Si samples, without and with TyP.

FIG. S4. (a) SEM of a silicon µ-pyramid sample (b) 3D model of the µ-pyramid

structured surface based on the same SEM. Surface area of the micro-pyramid covered

sample was calculated to be 1.55 times that of the planar sample of the same size.

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