science.sciencemag.org/content/370/6517/698/suppl/DC1

Supplementary Materials for Large-area low-noise flexible organic photodiodes for detecting

faint visible light

Canek Fuentes-Hernandez*, Wen-Fang Chou, Talha M. Khan, Larissa Diniz, Julia Lukens, Felipe A. Larrain, Victor A. Rodriguez-Toro, Bernard Kippelen*

*Corresponding author. Email: canek@ece.gatech.edu (C.F.-H.); kippelen@gatech.edu (B.K.)

Published 6 November 2020, Science 370, 698 (2020) DOI: 10.1126/science.aba2624

This PDF file includes:

Materials and Methods Supplementary Text Figs. S1 to S14 Table S1 References

2

Materials and Methods

OPD fabrication

ITO preparation. Organic photodiodes (OPDs) were prepared as follows: 14”×14”×0.043” sheets

of polished soda lime float glass coated with tin-doped indium oxide (ITO) of sheet resistance 9-

15 Ω/□ (Colorado Concept Coatings LLC, 96041) were cleaved into 1”by 4”stripes. 0.5”Kapton®

tape was adhered onto half of the stripes lengthwise, as a mask for subsequent patterning. The ITO

strips were patterned using wet etching in a solution of 3:1 HCl : HNO3 v/v for 10 min at a bath

temperature of 80 °C. After etching, the patterned ITO stripes were rinsed with copious amounts

of distilled water and scrubbed with detergent (Alconox Liquinox). The patterned ITO stripes were

cut into 1” × 1” pieces after rinsing in distilled water and dried under a stream of nitrogen.

The patterned ITO substrates were then solvent cleaned in sequential ultrasonic baths (Branson

5510) of detergent in distilled water, distilled water, acetone, and isopropanol, and blown dry with

a stream of nitrogen. Baths lasted at least 30 min at a temperature of 65 °C.

PEIE coating of ITO. Polyethylenimine (PEIE), 80% ethoxylated solution (Sigma-Aldrich

423475, 35-40 wt. % in H2O) with an average Mw ~ 70,000 was diluted in 2-methoxyethanol

(Sigma-Aldrich 284467, anhydrous, 99.8%) to a concentration of 0.4 wt. % and magnetically

stirred overnight at 1,000 rounds per minute (RPM). In some devices, PEIE, 80% ethoxylated

solution (Sigma-Aldrich 306185, 05019JE, 37 wt. % in H2O) with an average Mw ~ 110,000 was

diluted in 2-methoxyethanol (Sigma-Aldrich 284467, anhydrous, 99.8%) to a concentration of 0.37

wt. % and magnetically stirred overnight at 1,000 RPM. No significant difference was observed in

OPD performance using either batch of PEIE.

PEIE was dispensed onto patterned ITO substrates through a 0.2 µm pore size

polytetrafluoroethylene (PTFE) filter and spin-coated at a spin speed of 5,000 RPM, with an

acceleration of 928 RPM/s for 60 s. PEIE-coated ITO substrates were thermally annealed on a hot

plate at 100 °C for 10 min. Using spectroscopic ellipsometry measurements on PEIE-coated Si-

wafers, we estimate a PEIE average layer thickness of 10 nm for either of the solutions described.

The PEIE-coated ITO substrates were then transferred into a nitrogen-containing glovebox for

further processing.

Photoactive layer (P3HT:ICBA) coating. A dry mixture of highly regioregular P3HT (Rieke

Metals, 4002-E, PTL 24-71) and indene-C60 bisadduct (ICBA, Luminescence Technology, S9030

or Nano-C, JC100722) in a 1:1 weight ratio was dissolved in 1,2-dichlorobenzene to a total

concentration of 80 mg/ml, 100 mg/ml or 120 mg/ml. All solutions were magnetically stirred

overnight at 500 RPM and 75 °C inside a nitrogen-filled glovebox, where the samples were kept

for the rest of the device processing. Then, P3HT:ICBA was spin coated onto a PEIE-coated ITO

substrate through a 0.2 µm pore size PTFE filter at 800 RPM, with acceleration of 10,000 RPM/s

for 30 s. The resulting wet films were then slowly dried in covered glass Petri dishes for at least 7

h. After this solvent annealing process, a portion of the P3HT:ICBA was wiped off from the slides

using chloroform or chlorobenzene in order to expose the underlying ITO electrode and allow

electrical contact to be made. The ITO/PEIE/P3HT:ICBA slides were then thermally annealed on

a hot plate at 150 °C for 10 min.

The P3HT:ICBA film thickness was estimated to be 500 nm for films spin coated from 80 mg/ml

solutions, 700 nm for films processed from 100 mg/ml solutions and 950 nm for films spin-coated

3

from 120 mg/ml solutions. Film thickness was estimated based on variable angle spectroscopic

ellipsometry measurements taken on films processed under identical conditions on glass

substrates.

Top electrode deposition. The ITO/PEIE/photoactive layer slides were then transferred through an

antechamber to an adjacent nitrogen-containing glovebox, which is integrated with a thermal

evaporator (SPECTROS, Kurt J. Lesker). Slides were mounted onto a sample holder and affixed

to a shadow mask with openings defining 5 finger electrodes for individual devices along with a

rectangular opening for a common electrode deposited onto the exposed ITO area. The vacuum

chamber was pumped down to a base pressure of 7×10-8 Torr. 10 nm of MoOx was deposited by

evaporating MoOx at a deposition rate between 0.1 Å/s (initial) and 1.4 Å/s (final). Following the

MoOx, 200 nm of Ag was thermally evaporated on the slides at a deposition rate between 0.4 and

1.5 Å/s. The overlap of the ITO and MoOx/Ag electrodes defined an active area of ca. 0.1 cm2.

Flex-OPD fabrication

Polyethersulfone (PES) preparation. PES substrates (i-components) were cut into 1” x 1” pieces.

The protective film was peeled off and the substrates were shadow-masked for patterning, without

further cleaning, and then placed in a high-vacuum thermal evaporation system (SPECTROS, Kurt

J. Lesker) for the deposition of the bottom-electrode.

Bottom-electrode deposition. The vacuum chamber was pumped down to a base pressure of 5.0 x

10-7 Torr. A 10 nm-thick MoOx layer was deposited followed by an 11 nm-thick silver layer

deposited with a rate of 1.8 Å/s. PEIE, 80% ethoxylated solution (Sigma-Aldrich) had previously

been diluted in 2-methoxyethanol (Sigma-Aldrich) to a concentration of 0.37 wt.% and

magnetically stirred overnight. PEIE solution was then dispensed onto the patterned

PES/MoOx/Ag substrates through a 0.2 µm pore size polytetrafluoroethylene (PTFE) filter and

spin-coated at a spin speed of 5,000 RPM with acceleration of 928 RPM/s for 60 s, followed by

thermal annealing on a hot plate at 100 °C for 10 min. The PEIE-coated samples were then

transferred into a nitrogen-filled glovebox for photoactive layer deposition.

Photoactive layer (P3HT:ICBA) coating. A 100 mg/ml solution of highly regioregular P3HT

(Rieke Metals) and ICBA (Nano-C) in a 1:1 weight ratio mixed in 1,2-dichlorobenzene (Sigma-

Aldrich) was prepared and magnetically stirred overnight at 500 RPM at 70 °C in nitrogen-filled

glovebox. P3HT:ICBA was spin coated on the PES/MoO3x/Ag/PEIE substrates inside the

glovebox through 0.2 µm pore size PTFE filters at 800 RPM with acceleration of 10,000 RPM/s

for 30 s. The resulting wet films were solvent annealed in covered glass Petri dishes for at least 5

h and then thermally annealed at 150 °C for 10 min on a hot plate to remove any remaining solvent

and aid in the crystallization of P3HT in a N2-filled glovebox. A portion of the P3HT:ICBA films

were removed by chlorobenzene (Sigma-Aldrich) to expose bottom electrodes to allow electrical

contact.

Top electrode deposition. The samples were transferred to the thermal evaporation system

(SPECTROS, Kurt J. Lesker) for top electrode deposition. For the devices with an area of 0.1 cm2,

five openings defined five top electrodes, and a rectangular opening for a contact of the common

bottom electrode. For a ring-shaped Flex-OPD, with an area of 1.0 cm2, the top electrode was

deposited through a ring-shaped shadow mask, and a rectangular opening for a contact of the

bottom electrode. 10 nm of MoOx followed by 150 nm of Ag were thermally evaporated through

shadow masks at a pressure of < 5.0 x 10–7 Torr.

4

Electro-optic characterization

Photodiodes (PDs) are two-terminal photodetectors where a semiconductor layer transduces an

optical signal into an electrical one. A PD’s performance is quantified by two metrics: the

responsivity, ℜ(𝜙𝑜𝑝𝑡), defined by the ratio of the average steady-state photogenerated current

𝐼𝑝ℎ(𝜙𝑜𝑝𝑡) = 𝐼(𝜙𝑜𝑝𝑡) − 𝐼(𝜙𝑜𝑝𝑡 = 0), to the average incident optical power: 𝜙𝑜𝑝𝑡 (i.e. ℜ(𝜙𝑜𝑝𝑡) =

𝐼𝑝ℎ(𝜙𝑜𝑝𝑡) 𝜙𝑜𝑝𝑡⁄ ). And by the noise equivalent power, defined as the optical power: 𝜙𝑜𝑝𝑡,𝑁𝐸𝑃 ≡

𝑁𝐸𝑃, producing a signal-to-noise ratio, 𝑆𝑁𝑅 = 𝐼𝑝ℎ(𝜙𝑜𝑝𝑡)/𝐼𝑟𝑚𝑠, of one (i.e. 𝐼𝑝ℎ(𝜙𝑜𝑝𝑡,𝑁𝐸𝑃) =

𝐼𝑟𝑚𝑠). Here, time-independent quantities represent steady-state average values of their

corresponding physical variables.

To measure these physical quantities, the steady-state I-V characteristics were measured by

applying an electrical bias to a photodiode (PD) using the voltage source output of an electrometer

(Keithley 6517A or 6340). The electrometer, was programmed using a custom written LabView

program, to record a time dependent set of discrete current values I(tj) = ⟨i(t’)⟩T ; where the brackets

represent a temporal average, from tj-T to tj, performed over the time interval T=B-1; B is the

measurement bandwidth; and i(t) is the instantaneous current flowing through the PD. In our

measurements, the electrometer internal measurement bandwidth was 60 Hz and the effective

measurement bandwidth was estimated to be 1.5 Hz (Figure S3).

Data acquisition was controlled using the same LabView program, customized to allow manual

control over the data acquisition time (i.e., the total time during which the PD was held at a constant

bias V). In the dark or under illumination, I(tj) values were monitored until they reached their

steady-state value I, or until a sufficiently large number of points were acquired to enable a reliable

extrapolation by fitting the data to an exponential decay function (typically a biexponential

function). Mathematically, steady-state is defined as the condition when the average value of I(tj)

over N discrete points in time: 1

1

N

jj

j

jN I tI t

, becomes approximately invariant:

j j N rmsI t I t I ; (1)

where

2

1

1

2

j

j N

jj

rms j

j

I t

N I t

I I t

I t

. (2)

Averages typically ran between N = 10 to 50 points. If I(tj) fulfills eq. (1), then jI I t represents

the steady-state average current value. In practice, the condition in eq (1) is too strict given other

uncertainties associated with these measurements. Consequently, the condition in eq. (1) was often

relaxed to allow for variations between the temporal average of two adjacent sets to diverge by no

more than ca. 5%. After the current stabilized, the applied bias, or optical power, was changed,

and a new steady-state current recorded in time following the procedure previously described. The

process was repeated until the steady-state I-V values had been recorded for applied biases

5

typically varying between +1.5 and -1.5 V. Note that as stated in eq. (2), Irms represents the

standard deviation of the fluctuations around I (i.e. the electronic noise). The temperature

dependence of the dark current was recorded by continually monitoring the PD’s steady-state

current when mounted on a copper sample holder placed on top of a metal-stage coupled with a

thermoelectric controller (Oven Industries, Inc.). To ensure good thermal contact, portions of the

sample were coated with thermal grease prior to loading them to the sample holder. The bottom of

the sample holder was also coated with thermal grease before placing it on top of the thermoelectric

controller. The sample’s temperature was calibrated by correlating the thermoelectric controller

temperature readings (taken at the metal-stage surface) with those taken at the surface of a dummy

PD sample using an independent thermocouple connected to a digital thermometer (VWR

Traceable Expanded Range Thermometer).

Variable irradiance measurements of the photocurrent generated by the photodetectors were

conducted using illumination provided by a green LED (Super Bright LEDs, Inc., LD1-G) with a

525 nm band pass filter having a full-width half maximum (FWHM) of 10 nm. The optical power

over the sample was varied by changing the bias voltage onto the LED and by placing neutral

density filters to ensure that the LED was always operated within its linear region. The optical

power was measured using a calibrated silicon photodetector (OPHIR, PD300R-UV-SH-ROHS)

and the current was recorded by an electrometer using the same procedure previously described.

Under illumination, for a given optical power (i.e. bias voltage value to the LED) the open circuit

voltage (VOC) was measured by manually varying the applied voltage (with a 0.5 mV resolution)

onto the PD until J was minimized. Due to the voltage resolution of the electrometer’s source, at

higher optical powers we linearly interpolated VOC values between two points having J values of

different sign.

The spectral responsivity: ℜ(𝜆) = 𝐼𝑝ℎ (∫ 𝜙𝑜𝑝𝑡(𝜆)𝑑𝜆𝜆+Δ𝜆

𝜆−Δ𝜆)

−1, was acquired by using illumination

provided by either a xenon arc-lamp source (CVI Products) or a laser-driven light source

(Energetiq EQ-99X) coupled to a monochromator (CVI Spectral Products, CM110), supplying

illumination with a spectrally narrow profile characterized by ca. 10 nm FWHM = 2Δ𝜆. In order

to avoid the presence of higher order harmonics generated by the monochromator, high-

wavelength-pass colored glass filters (Newport, FSR-GG400 in the 400 - 700 nm range, FSR-

RG610 in the 700 - 1100 nm range) were placed within the beam path. The steady-state current

generated by the PDs at each wavelength was measured using an electrometer as previously

described. Custom written LabView programs interfaced with the monochromator and

electrometer to remotely set wavelength and bias, while measuring current.

The frequency response of the photodetectors was measured by biasing the PD using the output of

a voltage divider biased with a battery. The current generated by the photodetectors was passed

through a load resistor and the voltage across the resistor was passed through a low noise voltage

preamplifier (Stanford Research Systems, SR560) operating with a typical dc gain of 50× without

frequency filtering. The amplified voltage transient was then recorded using either a lock-in

amplifier (Stanford Research Systems, SR830) or an oscilloscope (Agilent Infiniium 54815). As

an illumination source, a green LED (Super Bright LEDs, Inc., LD1-G) was biased using a square

input wave with frequency in the range from 1 Hz to 105 Hz. The PD’s transient voltage amplitude

at each frequency was normalized to the amplitude measured at 1 Hz and then inserted into the

equation: 20 × log(normalized voltage amplitude) to estimate the -3 dB normalized responsivity

cut off frequency.

6

PPG Measurements

A surface-mounted LED emitting at 635 nm controlled by a DC power supply (Agilent E3647A)

and placed at a distance of 5 mm from the center of a PD or at the middle of the ring-shaped Flex-

OPD. PPG signals in reflection mode where acquired in the index finger. The PPG signal generated

by a PD was measured as the time-dependent voltage across a 1 MΩ resistor and this voltage

amplified 20× with a low-noise amplifier (Stanford Research Systems SR560) using a 0.3-10Hz

bandpass filter. The voltage output was read and recorded with an oscilloscope (Agilent 54815A).

The electrical power consumption by the LED was estimated as the value of the product: P = IV.

Supplementary Text

Current transients and determination of steady-state values

The upper panels of Figures S1A and S1C shows Idark(tj) for S1133 SiPDs and P3HT:ICBA PDs,

respectively, at 0 V. Black lines in those Figures represent biexponential fits from which Idark

values can be derived. The lower panel of Figure S1A and S1C displays the residuals obtained

throught biexponential fits; demonstrating that this model describes current transients away from

the initial current spike, which is attributed to a displacement current. The rms current fluctuation

derived from these residual values represents the combined noise of the experimental conditions

and the intrinsic noise generated by the device itself. Minimum Idark,rms values of 5 fA have been

measured in our current setup. Data in Figures S1A and S1C provide examples of extremely

slow dynamics which are not typically observed in devices that have been illuminated and/or

forward biased. However, it helps to illustrate the time range upon which current transients may

be stabilizing and the importance of tracking down the current dynamics in order to obtain reliable

measurement of the steady-state current characteristics of a device. The dynamics of these current

transients have been found to depend upon the PD history of light exposure, magnitude and polarity

of applied electrical bias (e.g. slower dynamics are always found under reverse bias and ca. 0 V),

electrical bias step used to sweep the voltage (e.g. slower dynamics are associated with larger

changes of voltage between points in a voltage sweep), as well as the temperature at which the

data is acquired (faster transient observed at higher temperatures). Under illumination, the current

transients are typically fast, as shown in Figure 3 of the main text, except at very low optical

powers. These observations are consistent with the presence of trapping sites in the device.

Figures S1B and S1D show a typical example of the temporal evolution of the dark current in

P3HT:ICBA PD and Si PDs. For voltage values ca. 0 V, a biexponential fit was typically used to

determine the steady-state current values.

Estimation of electronic noise in PDs

To calculate the different current noise contributions, we used the following equations:

7

2

1

2

1

2

,

2

1/22 2 2

, , ,

,

4 /

rms thermal thermal B p

r

rms rms thermal rms shot noise rms other

f

thermalf

f

ms shot noise shot noise

shot Diode shot Light

shot

shot Inje

noisef

ction shot GR

I I

I S B k TB

I I

df R

I S B

S B S B

S

S

df

B

S

S B

0 02 exp

sh

i

ot

d

Light

PD PD ph PB DJ qV n

S B

q A J A Jk AT B

Where, 2 1f f B is the measurement bandwidth; and

2

, rms otherI represents all other statistically

independent contributions tormsI . These contributions may include but may not be limited to pink

noise: 2

1

2

,1/ 1/ ( ) f

rms f ff

I df S f where is a constant typically close to 1 and 1/ ( ) fS f I

where δ is generally between 1 and 2. The shot noise expression for the diode:

shot Diode shot Injection shot GRS S S , can also be written as , 0, )2 2[ ( ] shot Diode SM da k Dr PD PS J V JA ATq

and represents a thermodynamically consistent expression (refs. 20-21 in the main manuscript)

including the common term:,2 ( , )SM dark PDqJ V T A , but in addition, the shot noise generated from

statistically independent recombination and generation currents which are each proportional to J0

due to the principle of detailed balance in thermodynamic equilibrium. Therefore, at V = 0 V and

in the dark, 04shot Diode PDS qJ A . Note that since

, ( , ) ( , )SM dark darkJ V T J V T the expression:

4shot Diode dark PDS qJ A yields at least one evident non-physical result in that

( 0) 0 ( 0) 0dark shot DiodeJ V S V . Also note that our expression for shot noiseS

may not

completely describe all the shot-noise contributions in a real device, particularly at higher reverse

voltage values.

8

Fig. S1.

Current transients. A) Current transient and residuals measured on a SiPD in the dark. B) Temporal

evolution of voltage and current of a SiPD during a voltage sweep. A) Current transient and

residuals measured on an OPD in the dark. B) Temporal evolution of voltage and current of an

OPD during a voltage sweep.

-800

-600

-400

-200

0

rms current = 26 fA

Steady-state dark current: I = -14 fA

Average dark current: I(t)

Applied voltage: V = 0 V

I(t i)

(fA

)

OPD

0 1 2 3 4 5

-50

0

50

Time (h)

I(t i)-

I(t)

(fA

)

10-2

10-1

100

0 20 40 60 80 100 12010

-1410

-1210

-1010

-810

-610

-410

-2

V (

V)

OPD

|I(t

j)| (

A)

Time (min)

Dark

A B

-300

-200

-100

0

100

200

I(t j)

(fA

) Si PD (S1133)

Steady-state dark current: I = 107 fA

Average dark current: I(t)

Applied voltage: V = 0 V

0.25 0.50 0.75 1.00

-100-50

050

100

I(t j)-

I(t)

(fA

)

Time (h)

rms current = 25 fA

0.0

0.2

0.40.6

0.8

1.0

0 2 4 6 8 10 12 14 16 1810

-1410

-1210

-1010

-810

-610

-410

-2

V (

V)

Si PD (S1133)

|I(t

j)| (

A)

Time (min)

Dark

C D

9

Fig. S2.

Signal to noise ratio, responsivity and electronic noise current. A) Current transient and

photocurrent transient in a SiPD. B) Optical power dependence of signal-to-noise ratio of a SiPD.

C) Optical dependence of photocurrent of a SiPD. D) Optical power dependence of the

responsivity of a SiPD. E) Repeated measurements of the optical power dependence of the

responsivity of two SiPDs. F) Chart box plots of the electronic noise current of three SiPDs

measured N times. G) Current transient and photocurrent transient of an OPD. E) Repeated

measurements of the optical power dependence of the responsivity of two OPDs. F) Chart box

plots of the electronic noise current of six OPDs measured N times.

10-12

10-10

10-8

10-6

10-4

0.1

0.2

0.3

0.4

Each line represents data from

a different experimental run on

2 different SiPDs

(A

/W)

Optical Power (W)

0.275

N = 14

-300

-200

-100

0

1200 1400 1600 1800 2000-300

-200

-100

0

100

I(t j)

(fA

)

Idark

(t)

101

102

103

Irms

= 33 ± 5 fA

NEP = 111 fW V = 0 V

I(t j)-

I da

rk(t

) (f

A)

Time (s)

101

102

103

Op

tica

l P

ow

er

(fW

)

10-12

10-10

10-8

10-6

0.1

0.2

0.3

0.4

(

A/W

)

Optical Power (W)

0.275

-600

-500

-400

-300

100 150 200 250 300-200-150-100-50

050

Irms

= 36 ± 2 fA

NEP = 211 fW

I(t j)

(fA

)

101

102

Idark

(t)

V = 0 V

I(t j)-

I dark(t

) (f

A)

Time (s)

V = 0 V10

1

102

Optical P

ow

er

(fW

)

D

G

10-12

10-10

10-8

10-6

10-4

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Each line represents data from

a different experimental run on

2 different OPDs

(

A/W

)

Optical Power (W)

0.268

N = 13

0 100 200 300 400 500 600 7000

1

2

3

4

5

NEP = 211 fW

SN

R

Optical Power (W)

E10-1410-1310-1210-1110-1010-910-810-710-610-510-4

10-15

10-14

10-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

I ph (

A)

Optical Power (W)

Irms = 36 fA

NEPex = 131 fW

A B C

F

H I

10

100

N=7N=11

SiPD#3SiPD#2

I rms(f

A)

SiPD#1

N=26

10

100

N=10N=6 #6 #5OPD:

N=5 #4

I rms(f

A)

N=6 N=20 #1 #2 #3

N=57

10

Fig. S3.

Characterization of a 976 MΩ resistor. A) Current vs. voltage characteristics. B) The upper panel

shows a box chart of Irms values associated with V > 97 mV yielding I values > 100 pA. The lower

panel shows the same for V < 97 mV yielding I values < 100 pA. In the insets of both graphs, the

theoretical value expected for the thermal noise of this 976 MΩ resistor was calculated as: 1

, 4 rms thermal B pI k TB R . In the upper panel, an effective B value of 11 Hz yields an ,rms thermalI value

of 13.5 fA which is in good agreement with the measured median value of 13.6 fA. In the lower

panel, an effective B value of 1.5 Hz yields an ,rms thermalI value of 5.0 fA which is in good agreement

with the measured median value of 5.1 fA. It should be noted that the effective bandwith values

derived here are the result of the combined effects between the internal bandwidth of the

electrometer and the communication delays associated with the LabView program used to control

the measurements. The change of B values at I > 100 pA is expected and due to the use of the

current autorange setting on the Keithley code.

-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3-4x10-10

-3x10-10

-2x10-10

-1x10-10

0

1x10-10

2x10-10

3x10-10

4x10-10

R2 = 0.9998

Slope = 1.025x10-9 A/V

I (A

)

V (V)

R = 976 MW

12.813.013.213.413.613.814.014.214.4

25%~75%

Min~Max

Median Line

13.6 fA

Irms,thermal(B = 11±0.4 Hz) = 13.5±0.3 fA

for I > 100 pA

N = 5

3.5

4.0

4.5

5.0

5.5

6.0

976 MW

I rm

s (

fA)

Irms,thermal(B = 1.5±0.3 Hz) = 5.0±0.5 fA

for I < 100 pA

5.1 fA

N = 10

A B

11

Fig. S4.

Performance parameters of champion OPDs. (A) Current transient and photocurrent transient

during champion run in OPD#3. (B) Current transient and photocurrent transient during champion

run in OPD#6, (C) Comparison between extrapolated specific detectivity calculated for Si PDs

(S1133 and S1133-01) and P3HT:ICBA OPDs using median Irms and values as well as for

champion OPDs having an NEP of 63 fW.

-3200

-3000

-2800

40 60 80 100 120 140

-400

-300

-200

-100

0

Idark

(t)

I(t j)

(fA

)

102

103

Irms

= 10 ± 2 fA

NEP = 71 fW

I(t j)-

I dark(t

) (f

A)

Time (s)

OPD#6

102

103

Optical P

ow

er

(fW

)

A B

-1000-800-600-400-200

0

300 350 400 450 500 550 600 650

-800-600-400-200

0200

Idark

(t)

I(t j)

(fA

)

101

102

Irms = 106 ± 20 fA

NEP = 63 fW V = 0 VI(t j)-

I dark(t

) (f

A)

Time (s)

101

102

OPD#3

Optical P

ow

er

(fW

)

C

400 600 800 1000 1200

1011

1012

1013

De

x*

(cm

Hz

1/2

W-1

)

l (nm)

B = 1.5 Hz

D* extrapolated from

median irms & (525 nm)

values

S1133-01

S1133

OPD (P3HT:ICBA)

D* for champion OPD

NEP = 63 fW

OPD (P3HT:ICBA)

12

Fig. S5.

Simulation of ( )darkJ V characteristics of SiPDs using Prince’s equivalent circuit model (P-model).

(A) Current density characteristics of SiPDs having different shunt resistance values. (B) Semi

logarithmic plot of the current density characteristics of SiPDs having different shunt resistance

values under reverse bias. (C) Current density characteristics of SiPDs having different reverse

saturation current values. (D) Semi logarithmic plot of the current density characteristics of SiPDs

having different reverse saturation current values under reverse bias.

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.510

-1410

-1310

-1210

-1110

-1010

-910

-810

-710

-610

-510

-410

-310

-210

-110

0

Rp (GW)

(Rs=0)

786

78.6

7.86

0.786

0.0786

|Jd

ark| (A

/cm

2)

V (V)

J0= 5 pA/cm

2

nid= 1.05

Rs = 35.7 W

APD

= 0.07 cm2

T = 297.15 K

-1 -0.1 -0.01 -1E-310

-14

10-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

J0= 5 pA/cm

2

Rp (GW)

(Rs=0)

786

78.6

7.86

0.786

0.0786

|Jd

ark| (A

/cm

2)

V (V)-0.001

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.510

-1410

-1310

-1210

-1110

-1010

-910

-810

-710

-610

-510

-410

-310

-210

-110

0

J0

0.050 pA/cm2

0.5 pA/cm2

5 pA/cm2

50 pA/cm2

500 pA/cm2

5 nA/cm2

nid= 1.05

Rp = 786 GW

Rs = 35.7 W

APD

= 0.7 cm2

T = 297.15 K

|Jd

ark| (A

/cm

2)

V (V)

-1 -0.1 -0.01 -1E-310

-14

10-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

Rp = 786 GW

J0

0.050 pA/cm2

0.5 pA/cm2

5 pA/cm2

50 pA/cm2

500 pA/cm2

5 nA/cm2

|Jd

ark| (A

/cm

2)

V (V)-0.001

A B

C D

13

Fig. S6.

Comparison of steady-state dark current density characteristics of SiPDs and OPDs under reverse

bias and simulated values using the P-model and S-model as discussed in the text.

14

Fig. S7.

Steady-state median dark current and 25-75% percentile range of OPD devices (N = 8 devices

fabricated in two different batches).

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.510

-14

10-12

10-10

10-8

10-6

10-4

10-2

297 ± 2 K

Range

Median

|Jdark| (A

/cm

2)

V (V)

15

Fig. S8.

Temperature and optical power-dependent studies of P3HT:ICBA PDs. (A) Comparison of

measured and modeled steady state average dark current density vs. voltage characteristics. (B)

Comparison of modeled and measured J-V characteristics under various illumination intensities.

C) Comparison of the measured and modeled open circuit voltage vs. short-circuit current density

at various temperatures. (D) Ideality factor and shunt resistance vs. temperature. (E) Reverse

saturation current density vs. inverse of the product between the thermal energy and ideality factor.

Solid lines represent a fit to an Arrhenius model. (F) Estimated charge-transfer energy value

derived from a Gaussian decomposition of the so-called reduced external quantum efficiency (i.e.

the product of the photon energy times the external quantum efficiency) measured in a

P3HT:ICBA (500 nm) PD. The red line corresponds to the charge-transfer band. Note that each

point is proportional to a measured steady-state photogenerated current in response to light with a

narrow spectral bandwidth (typically 10 nm FWHM); demonstrating that the ECT indeed represents

the transport bandgap of the material.

-2 -1 0 110-1310-1210-1110-1010-910-810-710-610-510-410-310-210-1

Exp., P Mod., T

287 K

297 K

307 K

318 K

328 K

|Jd

ark

| (A

/cm

2)

V (V)

23 24 25 26 2710-13

10-12

10-11

10-10

10-9

J0 (

A/c

m2)

[nid(T)kBT]-1 (eV-1)

J00= 650 ± 100 A/cm2

ECT = 1.31 ± 0.05 eV

1.50

1.51

1.52

1.53

273 293 313 333

101

102

103

nid

Rp (

GW

)

T (K)10-12 10-10 10-8 10-6 10-4 10-2

0.0

0.2

0.4

0.6

0.8

525 nm

Exp. P-Mod. T(K)

287

297

307

318

328

VO

C(V

)

Jsc (A/cm2)

1.2 1.4 1.6 1.8 2.0

10-6

10-5

10-4

10-3

10-2

10-1

100

h

E

QE

(eV

)

h(eV)

ECT

= 1.37 0.08 eV

l = 0.2 eV

f = 0.2 meV2

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.010-1310-1210-1110-1010-910-810-710-610-510-410-310-210-1

foptIopt:

|J| (A

/cm

2)

V (V)

100 nW (910 nW/cm2)

12 nW (110 nW/cm2)

430 pW (3.9 nW/cm2)

34 pW (309 pW/cm2)

Wavelegnth: 525 nm

Temperature: ca. 297 K

Symbols: Experiment

Lines: P-model

Dark

A B

C D

E F

16

Fig. S9.

Photocurrent dynamics and field dependent responsivity of P3HT:ICBA PDs. A) photocurrent

transients measured as a voltage across a 50 Ω load resistor biased at voltage values in the range

from 0 V to -16 V (32 V/µm). B) Normalized photocurrent transients shown in A) to demonstrate

that the shape of the transient response does not change substantially. C) Spectral responsivity

values measured at 0 V and at -1.5 V suggesting a week dependence of the responsivity with

respect to the electric field. D) Applied bias-dependent response time in 200 nm-thick OPDs. It

should be noted that the capacitance of these OPDs was measured to be ca. 2 nF at 20 Hz. Hence,

an RC-limited response time (2.2RC) is estimated to have a value of 0.22 µs; much faster than the

35 µs derived from these transients. Furthermore, considering that charge mobility values on

P3HT:ICBA are estimated on the order of 1-5×10-4 cm2V-1s-1 (30) using an average electric filed

approximation, we can estimate drift response times (d2(µ|V|)-1) ca. 5-25 µs at a bias field of -1 V.

However, at -16 V, the drift response time could be expected to drop to ca. 0.3-1.5 µs, which is

not observed experimentally. Consequently, it appears that the response time is dominated by the

diffusion of charge carriers, which at -1 V can be estimated to have a response time value ca. 24.5

µs (since the response times add in quadrature’s). Note that the little field dependence of the

0.0012 0.0014 0.0016 0.0018 0.0020

-0.12

-0.10

-0.08

-0.06

-0.04

-0.02

0.00

0.02

0.04

0 V

-1 V

-2 V

-4 V

-8 V

-16 V

Am

plif

ied

cu

rre

nt

tra

nsie

nt

(a.u

.)

Time (s)0 250 500 750

0

1

0 V

-1 V

-2 V

-4 V

-8 V

-16 V No

rma

lize

d P

ho

tocu

rre

nt

t (ms)

Rload = 50 WA

400 500 600 700 8000.00

0.05

0.10

0.15

0.20

0.25

0.30

P3HT:ICBA PD:

at 0 V

at -1.5 V

(

A/W

)

l (nm)

B

C

-2.0 -1.5 -1.0 -0.5 0.0 50.08

9

10

11

12

Re

sp

on

se

tim

e (m

s)

Applied bias (V)

OPD with 200 nm-tick photoactive layer

D

17

normalized photocurrent transient would also suggest that drift does not play a dominant role in

limiting the response time.

However, it should be noted that in thick OPDs with spectral responsivity characteristics as those

shown in Figure S12A, Dibb et.al. (24) found that the electric field at short circuit is not

homogenously distributed across the thickness of the device. Dibb et al. identified two distinct

regions, a first space-charge region (ca. 220 nm-thick in their OPDs) at the front of the device (here

close to the ITO/PEIE electrode) and a second neutral region at the back of the device (here close

to the MoOx/Ag electrode). In the space-charge region, charge transport is dominated by drift,

leading to very efficient collection of photogenerated carriers. In the neutral region at the back,

transport is dominated by diffusion, leading to a very poor collection efficiency. Consequently, a

plausible hypothesis is that the response time in these OPDs is limited by the time it takes for

charge carriers photogenerated in the neutral region to diffuse into the space-charge region. Hence,

a reduction of the device thickness to a value comparable to the thickness of the space-charge

region, should contribute significantly to reduce the response time. Indeed, preliminary

experiments on 200 nm-thick OPDs as the ones shown in Figure S10, reveal response time ca. 10

µs but still field-independent, suggesting that in our devices the space charge region may be thinner

than 200 nm. As discussed and revealed by these preliminary experiments, the origin of the

response time in these OPDs can be quite complex and is currently under investigation.

18

Fig. S10.

Comparison of measured and model steady-state dark current density of OPDs having different

shunt resistance values. A) OPDs with Rp values of 50 MΩ and 2.2 GΩ correspond to devices

having a photoactive layer thickness ca. 200 nm. Variation of Rp values are attributed to device-

to-device variations typically observed in OPDs with thin photoactive layers. OPDs with Rp values

ca. 200 GΩ have a photoactive layer thickness ca. 500 nm. OPDs with Rp values ca. 400 GΩ have

a photoactive layer thickness ca. 700 nm. The significant decrease of the dark current observed

for OPDs with photoactive layer thickness values beyond 500 nm, and corresponding increase in

shunt resistance value, reflects a drastic reduction of defects leading to shunts in a film. B)

Comparison of Irms values measured and estimated from Rp (thermal) and J0 (shot) values derived

from P-model fits to the experimental data. Note that differences in measured Irms values are within

device-to-device variations observed, as reported in the main text. It is believed that the weak

dependence of Irms with respect to Rp values is not statistically significant and thus appears

uncorrelated with respect to the predicted magnitude of the thermal noise in the diode. These data

also show that the measured Irms is dominated by noise contributions which are not related to shot

noise and thermal noise, presumably pink noise.

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.510-1410-1310-1210-1110-1010-910-810-710-610-510-410-310-210-1

108 109 1010 1011

10-15

10-14

10-13

Rp = 50 MW

|J

| (A

)

V (V)

Rp = 2.2 GW

Rp = 400 GW

Rp = 200 GW Measured

Other

Thermal

Shot

I rm

s (

A)

Rp (W)

A B

19

Fig. S11.

Photodetector characteristics of P3HT:PCBM PDs. A) Steady-state dark current density and fits

to the P-model. Inset shows parameters derived from this fit. B) Measured and calculated values

of 𝐼𝑟𝑚𝑠, 𝐼𝑟𝑚𝑠,𝑡ℎ𝑒𝑟𝑚𝑎𝑙, 𝐼𝑟𝑚𝑠,𝑠ℎ𝑜𝑡 and 𝐼𝑟𝑚𝑠,𝑜𝑡ℎ𝑒𝑟. C) Spectral responsivity of OPD devices. D) (upper

panel) Chart box of 𝐼𝑟𝑚𝑠distribution of P3HT:PCBM PDs and (lower panel) measured noise

equivalent power values and corresponding to measured specific detectivity values

400 500 600 700 800 900 10000.00

0.05

0.10

0.15

0.20

0.25

(A/W

)

l (nm)

A B

C D

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.510-1210-1110-1010-910-810-710-610-510-410-310-210-1

P-model, S-model

|J| (A

/cm

2)

V (V)

J0 = 35 pA/cm2

nid= 1.32

Rp = 5.5 GW

Rs = 225 W

T = 298 K

Glass

PEIE

ITOPEIE

MoOx

Ag

P3HT:PCBM

(960 nm)

10

100I r

ms(f

A)

OPD

(P3HT:PCBM)

A = 0.12 cm2

0

10

20

30

40

50 D* (cm×Hz1/2×W-1)

N = 3

N = 17

NE

P (

pW

)

B = 1.5 Hz

8.810-10 3.510-10

1.110-10

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.410-16

10-15

10-14

10-13

10-12

10-11

10-10

10-9

10-8

Exp.Irms

Irms,thermal

Irms,shot

Irms,other

Irms,total

I rm

s (

A)

V (V)

OPD (P3HT:PCBM) B = 1.5 Hz

20

Fig. S12.

Comparison of the A) responsivity, steady-state dark current density and electronic noise of OPDs

with B) “inverted” and C) “conventional” geometries. The use of thick photoactive layers in an

inverted device geometry (i.e. electron collection at the bottom and hole collection at the top

electrode) is enabled by effective hole-transport (diffusion) of photogenerated carriers through

P3HT. Note that comparable OPDs with a conventional geometry show reduced EQE values and,

as recently described by Armin et al. (10), spectral narrowing towards the band edge. Differences

in responsivity with respect to the device geometry have been attributed to the existence of two

distinct regions in the device, one where transport is dominated by diffusion and another, narrower,

dominated by drift due to band-bending close to the contact. While a detailed description is beyond

the scope of this contribution, the interested reader is directed to work reported by Dibb, et.al. (24)

21

Fig. S13.

Comparison of dark current density and electronic noise of OPDs and Flex-OPDs. (A) Comparison

of the steady-state current density characteristics of small and large area OPDs on glass. (B) Box

charts representing the distribution of electronic noise values measured in two large-area OPDs on

glass. (C) Comparison of the transmittance of PET and MoOx/Ag/PET from which we infer that

around 30% of losses in the responsivity between OPDs and Flex-OPDs are due to

absorption/reflection losses caused by the metallic electrode. (D) Comparison of the steady-state

current density characteristics in small and large area Flex-OPDs on PES. (E) Box charts

representing the distribution of electronic noise values measured on two large-area Flex-OPDs on

PES.

22

Fig. S14.

Comparison of PPG experiments using SiPDs and OPDs. (A) Progression of PPG signals for

increasingly smaller LED driving voltages using small area S1133 SiPDs and OPDs. (B)

Progression of PPG signals for increasingly smaller driving voltages using a small-area S1133

SiPDs and a large-area ring-shaped Flex-OPD.

0.00.20.40.60.81.0

0.00.20.40.60.81.0

-1 0 1

0.00.20.40.60.81.0

-1 0 1

VLED

= 5 V

VLED

= 6 V

VLED

= 7 V

Norm

aliz

ed P

hoto

curr

ent

OPD (0.11 cm2)

Time (s)

S1133 SiPD (0.07 cm2)

0.0

0.5

1.0

0.0

0.5

1.0

0 1 2 3 4

0.0

0.5

1.0

VLED

=6 V; ILED

= 1.62 mA; PLED

= 9.7 mW

VLED

=5 V; ILED

= 49.3 mA; PLED

= 247 mW

Norm

aliz

ed P

PG

sig

na

l

VLED

=4.9 V; ILED

= 23.2 mA; PLED

= 114 mW

Time (s)

0 1 2 3 4

VLED

=4.7 V; ILED

= 5.46 mA; PLED

= 26 mW

Time (s)

Ring-Shaped Flex OPD (1 cm2)S1133 SiPD (0.07 cm2)A B

23

Table S1. Summary of performance parameters of devices in this work

P

D

Ph

oto

act

ive

layer

I rm

s (fA

) B

= 1

.5 H

z I r

ms

,wh

ite

-no

ise (fA

) B

= 1

.5 H

z;V

= 0

V

(A

/W)

ϕo

pt >

10

0 p

W/c

m2 a

t

λ= 5

25

nm

NE

P (

fW)

B =

1.5

Hz,

λ= 5

25

nm

D

* (×

10

12

cm

Hz

1/2 W

-1)

B =

1.5

Hz,

λ= 5

25

nm

Typ

e

Are

a

Mate

rial

Th

ickn

ess

Me

dia

n

(Q1

; Q

3)

N

Ca

lcu

late

d

Me

dia

n

(SD

)

Extr

ap

ola

ted

from

me

dia

n

I rm

s &

Extr

ap

ola

ted

w

hit

e-n

ois

e

lim

ited

Me

as

ure

d

Me

dia

n

(Q1

; Q

3)

N

Extr

ap

ola

ted

from

me

dia

n

I rm

s &

Extr

ap

ola

ted

w

hit

e-n

ois

e

lim

ited

Me

as

ure

d

Me

dia

n

(Q1 ;

Q3)

N

S1133

0.0

7 c

m2

Si

24

(18

; 3

7)

44

0.6

0.2

75

(0.0

07

) 87

7

174

(121

; 2

12)

5

3.8

40

×B

-1/2

1.9

(1.8

; 3

.1)

5

OP

D

0.1

1 c

m2

P3H

T:I

CB

A

500

nm

37

(24

; 7

4)

104

0.5

0

.268

(0.0

09

) 136

2

230

(91 ;

316)

8

3.0

15

8×B

-1/2

1.8

(1.3

; 4

.7)

8

OP

D

0.9

0 c

m2

P3H

T:I

CB

A

500

nm

115

–

0.2

0

564

–

–

2.0

–

–

Fle

x- O

PD

0.1

0 c

m2

P3H

T:I

CB

A

500

nm

73

–

0.1

0

707

–

–

0.5

–

–

Fle

x- O

PD

1.0

0 c

m2

P3H

T:I

CB

A

500

nm

112

–

0.1

0

1120

–

–

1.1

–

–

OP

D

0.1

2 c

m2

P3H

T:P

CB

M

780

nm

30

(21

; 6

2)

17

2.8

0

.12

250

25

12

200

(4 8

00

; 37 0

00)

3

1.6

1

7×B

-1/2

0.0

35

(0.0

11 ;

0.0

88

)

8

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