††thanks: present address: Institute for Materials Discovery, University
College London, Torrington Place, London, WC1E 7JE, UK††thanks: present
address: Faculty of Engineering and Science, University of Greenwich, Central
Avenue, Chatham Maritime, Kent, ME4 4TB, UK††thanks: present address:
Institute for Materials Discovery, University College London, Torrington
Place, London, WC1E 7JE, UK††thanks: Optical Networks Group, University
College London, London, WC1E 6BT, UK

# Graphene-perovskite fibre photodetectors

S. Akhavan  Cambridge Graphene Centre, University of Cambridge, JJ Thompson
Avenue, Cambridge CB3 0FA, UK  A. Taheri Najafabadi  Cambridge Graphene
Centre, University of Cambridge, JJ Thompson Avenue, Cambridge CB3 0FA, UK  S.
Mignuzzi  Cambridge Graphene Centre, University of Cambridge, JJ Thompson
Avenue, Cambridge CB3 0FA, UK  M. Abdi Jalebi  Cavendish Laboratory,
University of Cambridge, JJ Thompson Avenue, Cambridge CB3 0HE, UK  A. Ruocco
Cambridge Graphene Centre, University of Cambridge, JJ Thompson Avenue,
Cambridge CB3 0FA, UK  I. Paradisanos  Cambridge Graphene Centre, University
of Cambridge, JJ Thompson Avenue, Cambridge CB3 0FA, UK  O. Balci  Cambridge
Graphene Centre, University of Cambridge, JJ Thompson Avenue, Cambridge CB3
0FA, UK  Z. Andaji-Garmaroudi  Cavendish Laboratory, University of Cambridge,
JJ Thompson Avenue, Cambridge CB3 0HE, UK  I. Goykhman  Cambridge Graphene
Centre, University of Cambridge, JJ Thompson Avenue, Cambridge CB3 0FA, UK
Technion - Israel Institute of Technology, Haifa 3200003, Israel  L. G.
Occhipinti  Cambridge Graphene Centre, University of Cambridge, JJ Thompson
Avenue, Cambridge CB3 0FA, UK  E. Lidorikis  Department of Materials Science
and Engineering, University of Ioannina, Ioannina 45110, Greece  S. D. Stranks
Department of Chemical Engineering and Biotechnology, University of Cambridge,
Philippa Fawcett Drive, Cambridge CB3 0AS, UK  A. C. Ferrari  Cambridge
Graphene Centre, University of Cambridge, JJ Thompson Avenue, Cambridge CB3
0FA, UK

###### Abstract

The integration of optoelectronic devices, such as transistors and
photodetectors (PDs), into wearables and textiles is of great interest for
applications such as healthcare and physiological monitoring. These require
flexible/wearable systems adaptable to body motions, thus materials
conformable to non-planar surfaces, and able to maintain performance under
mechanical distortions. Here, we prepare fibre PDs combining rolled graphene
layers and photoactive perovskites. Conductive fibres ($\sim$500$\Omega$/cm)
are made by rolling single layer graphene (SLG) around silica fibres, followed
by deposition of a dielectric layer (Al2O3 and parylene C), another rolled SLG
as channel, and perovskite as photoactive component. The resulting gate-
tunable PDs have response time$\sim$5ms, with an external
responsivity$\sim$22kA/W at 488nm for 1V bias. The external responsivity is
two orders of magnitude higher and the response time one order of magnitude
faster than state-of-the-art wearable fibre based PDs. Under bending at 4mm
radius, up to$\sim$80% photocurrent is maintained. Washability tests
show$\sim$72% of initial photocurrent after 30 cycles, promising for wearable
applications.

##  I Introduction

Electronics on fibres and textiles is of interest for a range of applications,
including healthcare (e.g. monitoring heart rate)[1], wearable displays[2],
energy harvesting[3, 4] and storage[5]. The current vision for electronic-
textiles is for them to be an integral part of everyday outfits[6, 7], and
remain functional after washing[8]. One of the most common approaches to test
the washability of e-textiles was developed by the American Association of
Textile Chemists and Colorists (AATCC)[10]. One cycle in AATCC is equivalent
to 5 regular-machine (household) washing cycles[9], achieved by the addition
of steel or rubber balls to the washing container[9], with agitator speed
179$\pm$2spm and spin speed 645$\pm$15rpm[10]. Washability tests are performed
for at least 20 cycles[9], with each cycle being 45min at 40$\degree$C,
with$\sim$0.37% volume of detergent (AATCC standard reference detergent
124[10]), and 10 stainless steel balls (6mm) or load ballast (130$\pm$10g for
cotton) to mimic abrasion[10, 11]. The analysis of washability depends on the
intended application, the expected frequency of cleaning, and the total number
of cleaning cycles during the tested product’s lifetime. E.g., sports clothing
will be mainly washed after every use[12], while a jacket will be cleaned once
or twice a year[13, 9].

cccccccc Structure Properties

(Substrate)[Reference] Rext (A/W) Spectral range (nm) Rise/Fall time D∗
(Jones) Bending cycle Washing cycle

SLG/PVK (Fibre) [THIS WORK] 2.2$\times$104 @1V 488nm 480-870 5/35ms 1013 4mm
100 cycles AATCC 30 cycles [fill=lightgray,borders=left,right]10-1¡¿Flexible
PDs
CuZnS/TiO2 (Fibre) [28] 640 @3V 320-400nm 260-420 200/200ms -  600 200 cycles
\-
Carbon fibre/ZnO-CdS (Fibre) [31] 105 @2V 372nm 372-548 $>$1s -  -0.38%strain
- \-
P3HT/ZNO (Fibre) [36] 156$\mu$ @0V 365nm 365-880 40/40ms 0.74x109 1.96% strain
- \-
Carbon fibre/PVK (Fibre) [32] 0.563 @0V 800nm 350-1050 200/200ms 2.15x1013 900
60 cycles \-
PVK (PET) [121] 3.49 @3V 365nm 365-780 0.14/2.9s -  ND 120 cycles \-
PVK/PDPP3T (PET) [138] 154x10-3 @1V 835nm 365-835 - 8.8×1010 7mm 1000 cycles
\-
SLG/MoS2 (1L) (PET) [124] 45.5 @1V 642nm \- - -  14mm 30 cycles \-
InP (PET) [136] 0.12 @-0.3V 533nm 533-980 - -  38.1mm - \-
SLG/PVK (PA) [137] 115 @1V 515nm \- 5.3s 3x1012 12mm 3000 cycles \-
SLG/CsPbBr3 (Fibre(facet)) [29] 2$\times$104 @0.2V 400nm 400-520 24.2s
8.6x1010 NA NA [fill=lightgray,borders=left,right]8-1¡¿Rigid PDs
MoS2 (1L) (Fibre(facet)) [30] 0.6 @4V 400nm \- 7.1/3.5s - NA NA
PVK (Glass/ITO) [125] 27 @5V 830nm 400-83030/20$\mu$s 1013 NANA
SLG (Si/SiO2) [118] 6.1$\times$10-3 @0.4V 1150nm 300-6000 16GHz - NA NA
SLG/PVK (Si/SiO2) [112] 180 @0.1V 520nm 400-980 87/540ms 109 NA NA
SLG/PVK (Si/SiO2) [122] 109 @0.5V 598nm 350-1100 4.5/57.5s 1014 NA NA
SLG/QDs (Si/SiO2) [67] 107 @5V 532nm 532-1600 10/100ms 7x1013 NA NA
SLG/MoS2 (Si/SiO2) [123] 107 @1V 532nm 290-680 - - NA NA
Conductive microfluidized graphite-coated textile [37] NA NA NA NA  1800 150
cycles BS EN ISO 105 C06 A1S 10 cycles
[fill=lightgray,borders=left,right]3-1¡¿Flexible Non PDs
Polymer solar cells [38] NA NA NA NA  52% compression 20 cycles Submersed in
water 20 cycles
Textile-based field-effect transistors [71] NA NA NA NA  4mm - Tumble-washed
20 cycles

Table 1: Comparison of KPIs for PDs on rigid and flexible substrates. PA
stands for polyamide. NA means not applicable. ND means not defined. The last
three rows are neither PDs nor fibre-based structures. Ref.[37] reported 3.5
times higher resistance ($\sim$700Ω/cm) after 10 washing cycles. Ref.[37] used
10 home laundry washing cycles following a British Standard (BS EN ISO 105 C06
A1S). Ref.[38] reported$\sim$80% of the initial power conversion efficiency
after water immersion. Ref.[71] reported a$\sim$50% drop in drain current
after 20 washing cycles. Our PDs maintain up to$\sim$72% Iph after 30 AATCC
cycles

Bendability can be achieved by integrating rigid components (e.g. Si-
based[14]) within flexible systems[15], including textiles[16]. However,
device rigidity (i.e. intolerance to various forms of mechanical stress caused
by body motions[17]), and washability issues[18] are bottlenecks towards
commercialization[6]. Additionally, wafer-based manufacturing processes are
generally optimized for planar fabrication[19] and substrates[20].

Fibres are an ideal platform for wearable electronics due to their
deformability[6, 21] and ease of integration into clothes[6, 21]. They can be
used to cover various geometries to make functional surfaces for wearable
electronics[22]. To achieve this, fibre-based conductors, semiconductors and
insulators with controlled geometries and interfaces are required. These
components could be used to integrate electronics into a textile during
weaving[15], for applications such as light-harvesting[23, 24], light-
generation[25], and photodetection[26]. The integration of photodetectors
(PDs) into clothes would enable features such as photoplethysmography[17]
(i.e. analysing the skin’s optical absorption/reflectance to track changes in
blood vessels’ volume for cardiac pulse measurement[27]), and hazardous light
detection (for skin cancer prevention[28]).

A number of fibre-based PDs were reported for wearable applications[29, 30,
31, 32, 28]. These can be compared via a number of key performance indicators
(KPIs) including (i) external responsivity[33]:

$R_{ext}=\frac{\triangle{I_{ph}}hc}{q.P_{opt}.\lambda.A_{PD}/A_{0}}$ (1)

where Iph is the photocurrent (Iph=Ilight-Idark) the difference between
current under illumination and in dark, $h$ is the Planck’ constant, $c$ is
the speed of light, $q$ is the electron charge, $P_{opt}$ is the impinging
optical power, $\lambda$ is the wavelength of the incident laser, $A_{PD}$ and
$A_{0}$ are the PD area and the laser spot size, respectively. $A_{PD}/A_{0}$
is a scaling factor taking into account that only a fraction of optical power
impinges on the PD, (ii) response time ($\tau_{res}$), i.e. the lifetime of
the photogenerated charges in the PD light absorbing layer[34], (iii)
operating voltage[35, 34], (iv) Rext or Iph under mechanical distortion (e.g.
bending)[26], and (v) washability, defined as the ability to withstand
exposure to water and cleaning processes without being damaged or
malfunctioning[26].

State-of-the-art wearable fibre-based PDs have $\tau_{res}\sim$40ms[36] and
R${}_{ext}\sim$156$\mu$A/W[36] at 365nm. Under bending strain$\sim$1.96%,
Ref.[36] reported that Rext was enhanced$\sim$80% by the piezo-phototronic
effect (a three-way coupling of piezoelectric, semiconductor and photonic
properties[36]) compared to no strain. Refs.[32, 28] measured $\tau_{res}\sim
200ms$ and R${}_{ext}\sim$1.48A/W[32] at 800nm and$\sim$640A/W[28] under UV-A
(320-400nm) at 0.5 and 3V, respectively. Bending to 90$\degree$ up to 60
cycles[32] and 60$\degree$ for 200 cycles[28] resulted in$\sim$8% and$\sim$14%
decrease of Iph, respectively[28], see Table 1. However, Refs.[36, 32, 28] did
not discuss washability. Refs.[37, 38] addressed washability in wearable
e-textiles, but did not specifically focus on the washability of PD
electronics. Ref.[37] reported microfluidized graphite (20% $<$10nm thickness
after 20 cycles of microfluidization) based poly-cotton fabric[37], with a
polyurethane-based encapsulant to fabricate graphene-coated textile. They
measured 3.5 times higher resistance ($\sim$700$\Omega$/cm) after 10 washing
cycles[37]. Ref.[38] showed washable polymer solar cells made of donor-
acceptor polymer with quaterthiophene and
naphtho[1,2-$c$:5,6-$c^{{}^{\prime}}$]bis[1,2,5]thiadiazole (NTz) (PNTz4T) and
[6,6]-phenyl C71-butyric acid methyl ester (PC71BM). Ref.[38] demonstrated
that flexible photovoltaic modules encapsulated with$\sim$1$\mu$m parylene
and$\sim$500$\mu$m acrylic elastomers maintained$\sim$80% of the initial power
conversion efficiency (i.e. the percentage of solar energy shining converted
into usable electricity[35]) after being submersed in water for 100mins[38].
Ref.[30] prepared PDs on the end-face of an optical fibre using monolayer (1L)
MoS2 prepared by chemical vapor deposition (CVD). This was wet transferred and
dip coated onto the fibre facet (end-face)[30]. Ref.[30] reported
R${}_{ext}\sim$0.6A/W at 4V and 400nm, with rise time and fall times (i.e. the
time for the signal to rise from 10% to 90%[35] or fall from 90% to 10% of the
final value[35])$\sim$7.1s and$\sim$3.5s, respectively[30]. However, PD
fabrication on fibres’ facets is not practical for wearable applications,
since fibres’ facets can be covered and hindered during weaving[39], thus not
being able to receive light[26]. PDs built parallel to the longitudinal axis
of fibres are desired[26], so that the exposed portion receives light, and the
covered conductive terminals transmit the signals. Ref.[31] prepared PDs made
of ZnO-CdS double-shell nanowire arrays on carbon fibres[31], with
R${}_{ext}\sim 10^{5}$A/W at 372nm at 2V, $\tau_{res}\sim>$1s[31] with
operating wavelength from 372 to 548nm[31]. Ref.[32] reported fibre-shaped PDs
based on CH3NH3PbI3 perovskite(PVK)/TiO2/carbon fibres and Cu wires, achieving
$\tau_{res}\sim$200ms, with R${}_{ext}\sim$ 1.48A/W under 0.5V at 800nm and
0.563A/W at a 0V bias at 800nm. Ref.[28] demonstrated fibre-shaped CuZnS/TiO2
nanotubes PDs with Ti wires as inner electrodes and wrapped carbon nanotube
around the wire as the outer electrode[28], with R${}_{ext}\sim$640A/W under
UV-A (320-400nm) at 3V, $\tau_{res}\sim$200ms and operating
wavelength$\sim$260-420nm, limited by the absorption of TiO2 nanotubes[28].
However, none of these reported washability tests, see Table 1 for a summary.

Graphene and related materials (GRMs) are ideal for a variety of
optoelectronics applications[41], such as PDs[34], solar cells[42, 43, 44],
and modulators[45, 46]. Wafer-scale growth, transfer and fabrication of CVD
SLG have improved over the years[47, 48, 49], with carrier mobility, $\mu$,
reaching$\sim$18500$cm^{2}V^{-1}s^{-1}$ at room temperature on a rigid
substrate such as Si/SiO2[64]. CVD SLG without encapsulation on ethylene vinyl
acetate (EVA)/polyethylene terephthalate (PET) was reported with $\mu$ up
to$\sim$7000$cm^{2}V^{-1}s^{-1}$[50]. However, the absence of a gain mechanism
that can generate multiple charge carriers from one incident photon, resulted
in limited R${}_{ext}\sim$0.01A/W for SLG-only PDs[68].

PVKs are promising photoactive materials, due to their long($>1\mu$m[51])
charge-carrier diffusion lengths[51], strong ($\sim$10${}^{5}cm^{-1}$[52])
absorption coefficients[52], and bandgap tunability (from 1.1eV[53] to
2.5eV[53]) via chemical composition control[53]. PVKs could be applied for
coloured emitters[54], energy harvesting[55], and PDs[56, 57]. PVKs based PDs
have low R${}_{ext}\sim$0.4A/W in the UV–visible range (350-900nm)[58, 59],
mainly due to low $\mu\sim$40$cm^{2}V^{-1}s^{-1}$[60], since Rext is
proportional to $\mu$[33]:

$R_{ext}=\frac{\Delta{I}}{P_{opt}}\frac{\tau_{life}\mu V_{ds}}{L^{2}}$ (2)

where $\tau_{life}$ is the response time, Vds is the bias applied between
source and drain, and $L$ is the channel length. The term
$\frac{\tau_{life}\mu V_{ds}}{L^{2}}$ is called gain[33]. By increasing $\mu$,
the gain increases, which results in higher Rext. Consequently, the
combination of solution-processed PVK with SLG having $\mu$ well above PVK’s,
to enhance gain, is promising for light sensing and flexible applications.

Ref.[29] prepared PDs based on SLG and CsPbBr3 PVK nanocrystals at the end-
face of silica fibres with R${}_{ext}\sim 2\times 10^{4}$A/W under 400nm
illumination. SLG was transferred onto the facet, followed by self-assembly of
CsPbBr3 nanocrystals[29]. However, the operating wavelength
was$\sim$400-520nm, limited by the CsPbBr3 nanocrystals absorptions[29], with
$\tau_{res}\sim$24.2s, Table 1.

Here, we report a wet transfer approach to roll CVD SLG around silica fibres,
to be used as a gate for flexible PDs. This is followed by the deposition of
Al2O3 and parylene C as dielectrics. Another rolled SLG is used as channel
with $\mu\sim$1300$cm^{2}V^{-1}s^{-1}$ at RT, before depositing
$(Cs_{0.06}FA_{0.79}MA_{0.15})Pb(I_{0.85}Br_{0.15})_{3}$, where
MA=methylammonium, $CH_{3}NH^{3+}$ and FA=formamidinium, $CH_{3}(NH_{2})^{2+}$
PVK as photoactive layer. Our PVK does not degrade up to 85$\degree$C[69] and
is chemically stable (ions do not migrate to other layers (i.e. SLG)[70]. We
get R${}_{ext}\sim$22,000A/W at 488nm, with rise and fall times$\sim$5ms and
$\sim$35ms, respectively, at 1V, outperforming $R_{ext}$ of fibre-based PDs by
at least two orders of magnitude[28, 32, 30] and one order of magnitude in
$\tau_{res}$[28, 32, 29], Table 1. Bending and washing tests are also
performed. After 100 bending cycles at 4mm bending radius Iph is$\sim$80% of
the non-bent one. After 20 and 30 washing cycles according to the AATCC
standard[10] we get$\sim$6% and$\sim$28% degradation of Iph. The washability
of our fibre-based PDs goes beyond the typical 20 cycles reported for textile-
based field-effect transistors (FETs)[71]. Ref.[71] had a$\sim$50% drop in
drain current after 20 cycles[71]), while our PDs maintain up to 72% Iph after
30 cycles, compared to unwashed fibre-based PDs, making them promising for
wearable applications.

##  II Results and discussion

Figure 1: (a) Scheme of the transfer process for CVD SLG around fibres. (b)
Top view SEM images for NR=6. (c) Cross-section SEM image for NR=6. (d) The
current vs voltage of fibres as a function of NR.

Fig.1 plots a scheme of the transfer process of CVD SLG around fibres. CVD SLG
is prepared on a $35\mu{m}$ Cu foil as for Ref.[72]. The Cu foil is annealed
up to 1000∘C for 30mins under H2 at 20sccm. Then, a gas mixture of CH4 (5sccm)
and H2 (20sccm) is added to initiate growth at 1000∘C for 30mins. Finally, the
sample is cooled in vacuum (1mTorr) to RT. In order to transfer SLG,
poly(methyl methacrylate) (PMMA) dissolved in acetone is spin coated on SLG/Cu
at 4000rpm for 40s, followed by oxygen etching of SLG on the Cu backside using
an RIE-NanoEtch (3W, 30s)[73, 72]. Cu/SLG/PMMA is then transferred to an
aqueous ammonium persulfate (APS) solution ((NH4)2S2O8, 0.5 mol/L) for Cu
dissolution[73, 72]. The PMMA membrane and CVD SLG are then cleaned of APS
residuals by dipping in DI water for 20mins. This is followed by the rolling
process. First, the fibre is placed on a supporting film (e.g. poly(ethylene
terephthalate) (PET)) and the edge of the floating SLG/PMMA is fished onto the
fibre, while the rest of the SLG/PMMA stays on the supporting film. Then, the
fibre is rotated clockwise at$\sim$10rpm to roll SLG/PMMA. The PMMA layer is
then removed by immersing the coated fibre in acetone for 15mins, followed by
rinsing with isopropanol for 10mins and drying in N2. Fig.1b is the top view
of 6 SLG layers coated on fibre glass, taken by scanning electron microscopy
(SEM, Magellan 400L). Fig.1c shows the cross-section.

Fig.1d plots the conductivity (measured via a Keithley source meter at the two
ends of the channel layer) of different coated fibres depending on the number
of rolled SLG, NR, for a channel length$\sim$1mm. The linear relation between
current and voltage indicates Ohmic conductivity. Given that increasing NR
reduces sheet resistance[42] due to increased levels of conductive paths[74],
NR=6 is chosen as a gate electrode. NR=10 leads to 284$\Omega$/mm, however the
roughness caused by PMMA residuals between layers could result in a short
circuit when the dielectric layer is positioned between gate and channel
layer. Thus, we use NR=6 to fabricate conductive fibres with a uniform surface
coverage of SLG as for Figs.1b,c.

The presence and quality of SLG are further studied by Raman spectroscopy at
514.5nm using a Renishaw InVia with a 50$\times$ objective and $<$0.1mW power
on the sample. Fig.2 plots the Raman spectrum of the film as grown on Cu. The
2D peak is a single Lorentzian with FWHM(2D)$\sim$34cm-1, signature of
SLG[76]. The position of the G peak, Pos(G), is$\sim$1591cm-1, with
FWHM(G)$\sim$22cm-1. The 2D peak position, Pos(2D), is$\sim$2720cm-1, with
FWHM(2D)$\sim$34cm-1, while the 2D to G peak intensity and area ratios,
I(2D)/I(G) and A(2D)/A(G), are$\sim$4.4 and$\sim$6.7. No D peak is observed,
indicating negligible defects[77]. Fig.2 plots Raman spectra of CVD SLG for
NR=1, 2, 4, 6, 8, 10. The 2D line shape is independent of NR, suggesting no
interaction between transferred SLGs[77, 78], Fig.2.

Fig.3 plots Pos(G), FWHM(G), Pos(2D), FWHM(2D), I(2D)/I(G), A(2D)/A(G),
I(D)/I(G) as a function of NR. For each NR, 10 points are collected along the
fibre. I(2D)/I(G) changes from$\sim$2.4 for NR=1 to$\sim$1.7 for NR=10,
Fig.3a. A(2D)/A(G) changes from$\sim$4 for NR=1 to $\sim$2.9 for NR=10,
Fig.3b. The decrease in I(2D)/I(G) and A(2D)/A(G) indicates doping increases
from E${}_{F}\sim$225meV for NR=1 to$\sim$320meV for NR=10[80, 79]. This is
further supported by the decrease of Pos(G) as function of NR
from$\sim$1585.4cm-1 for NR=1 to 1584.3cm-1 for NR=10, Fig.3d. From I(D)/I(G),
Fig.3g, the defect density changes from$\sim$5.74$\times$1010cm-2 for NR=1 to
7.1$\times$1010cm-2 for NR=10[81, 82] for excitation energy 2.41eV and
E${}_{F}\sim$225meV and 320meV, respectively, Fig.3g. FWHM(2D,G) are broadened
as NR increases due to defects and inhomogeneities of rolled CVD graphene,
Figs.3c,e.

Figure 2: 514.5nm Raman spectra of CVD SLG on Cu and for NR=1, 2, 4, 6, 8, 10,
normalized to have the same I(G).

Figure 3: 514.5nm Raman spectra for NR=1, 2, 4, 6, 8, 10. For each NR, 10
points are collected along the fibre. (a-g) Average and standard deviations of
(a) I(2D)/I(G), (b) A(2D)/A(G), (c) FWHM(G), (d) Pos(G), (e) FWHM(2D), (f)
Pos(2D), (g) I(D)/I(G). (h) Pos(2D) as a function of Pos(G).

For NR=6, Pos(G)$\sim$1584cm-1, FWHM(G)$\sim$21cm-1, Pos(2D)$\sim$2694cm-1,
FWHM(2D)$\sim$38cm-1, I(2D)/I(G)$\sim$1.7 A(2D)/A(G)$\sim$2.9, indicating p
doping with E${}_{F}\sim$310meV by taking into account the dielectric
constant$\sim$3.9 of the silica fibre[83]. I(D)/I(G)$\sim$0.1 corresponds to a
defect density$\sim$4.9$\times$1010cm-2[81, 82]. Pos(G,2D) are affected by the
presence of strain. Biaxial strain can be differentiated from uniaxial from
the absence of G-peak splitting with increasing strain[84], however at low
($\lessapprox$0.5%) strain the splitting cannot be resolved[84, 85]. For
uniaxial(biaxial) strain, Pos(G) shifts by $\Delta$Pos(G)/$\Delta$
$\epsilon_{strain}\sim$23(60)cm-1/%[84, 85]. Pos(G) also depends on doping[79,
80]. However, Pos(2D) and Pos(G) are not correlated, Fig.3h. Thus, the spread
of Pos(G) and Pos(2D) is mostly due to doping. Table 2 summarizes the Raman
fit parameters and resulting EF, doping type, strain, defect density.

Figure 4: Fibre-based SLG/PVK PDs fabrication process. (a) Silica fibre
substrate. (b) NR=6 around the fibre as gate electrode. (c) Gate dielectric
layer composed of Al2O3 and parylene C. (d) Rolled NR=6 channel. (e) Spin-
coated PVK on rolled CVD SLG channel. (f) Assembled fibre-based SLG/PVK PD.

Figure 5: (a) Raman spectra of SLG/PVK on fibre (green) and SLG/PVK on Si/SiO2
(gray). (b) Backgrounds subtracted spectra from (a). (c) Raman spectra of
fibre (red), fibre/dielectric (Al2O3 and parylene) (blue),
fibre/dielectric/NR=6 (orange), fibre/dielectric/NR=6/PVK (green). The PL
background of PVK is subtracted for the green spectrum. All spectra for
514.5nm.

Fig.4 is a schematic drawing of our fibre-based PDs. The fabrication process
starts by wiping the silica fibre via acetone and isopropanol, Fig.4a. Then,
CVD SLG is rolled 6 times around the fibre (reaching a
conductivity$\sim$500$\Omega$/mm to form a gate electrode, Fig.4b). Then,
150nm Al2O3 and 200nm parylene C are deposited as gate-dielectrics, Fig.4c.
Al2O3 is commonly used as a gate dielectric in graphene field-effect
transistors[86] because of its dielectric constant ($k\sim$9)[87, 88],
compatibility with graphene[88] and breakdown voltage (up to 10 MV/cm)[87].
Parylene is also commonly employed as dielectric, pin-hole free[89, 90],
passivation layer[91] in flexible electronics[92, 93]. Al2O3 coating is
performed by atomic layer deposition (ALD, Cambridge Nanotech Savannah), to
achieve uniform coating and film thickness control (down to the nm range)[94].
We deposit a 150nm-thick Al2O3 dielectric layer at 150∘C at a base
pressure$\sim$0.5mbar using trimethylaluminum (TMA purity $>$98%, Strem
Chemicals 93-1360) as precursor and H2O as oxidant. Oxidant and precursors are
alternately introduced into the chamber in a 20sccm flow of N2 carrier gas. If
Al2O3 alone is used, we get dielectric failure because of crack formation, due
to the mechanical stress/strain involved in the fabrication process of our
PDs. 200nm parylene is applied as follows. A parylene dimer is vaporized at
80∘C, then, in a separate chamber, it is pyrolysed into monomers at 690∘C. The
fibre is held at room temperature (RT), so that the parylene polymerizes on
contact with the surface. The physical vapor deposition of parylene provides
coverage of all accessible surfaces[95]. This improves dielectric robustness
against deformations during manufacturing of fibre-based PDs, reducing the
risk of short circuits. The sole use of parylene C, however, creates
inconsistent dielectric properties: thin layers ($\sim$500nm) result in gate
leakage at low ($\sim$0.2MV/m) electric fields, while exceeding 500nm
compromises the channel modulation due to a decrease in gate dielectric
capacitance[96].

Figure 6: (a) SEM and (b) AFM image of PVK film on NR=6, inset image shows the roughness of spin-coated PVK film. (c-e) Optical microscopy images of final device. (d,e) Source-drain and gate electrodes, respectively, printed using Ag ink. s, d, g indicate source, drain, gate, respectively Sample |  Pos(G) cm-1 |  FWHM(G) cm-1 |  Pos(2D) cm-1 |  FWHM(2D) cm-1 | A(2D)/A(G) | I(2D)/I(G) | I(D)/I(G) |  EF meV |  Doping type |  Uniaxial strain% Biaxial strain% |  defect density $\times$1011cm-2
---|---|---|---|---|---|---|---|---|---|---|---
NR=6 |  1584 $\pm$1 |  21 $\pm$2 |  2694 $\pm$2 |  38 $\pm$3 |  2.9 $\pm$1.5 |  1.7 $\pm$0.9 |  0.10 $\pm$0.08 |  310 $\pm$110 | p |  0.40 $\pm$0.04 0.15 $\pm$0.02 |  0.49 $\pm$0.12
NR=6 |  1587 $\pm$1 |  17 $\pm$2 |  2686 $\pm$3 |  41 $\pm$3 |  4.7 $\pm$1 |  1.9 $\pm$0.3 |  0.49 $\pm$0.1 |  190 $\pm$20 | p |  -0.06 $\pm$0.06 -0.02 $\pm$0.02 |  1.82 $\pm$0.39
NR=6/PVK |  1583 $\pm$1 |  17 $\pm$2 |  2701 $\pm$3 |  43 $\pm$3 |  2.3 $\pm$0.6 |  0.9 $\pm$0.3 |  0.46 $\pm$0.1 |  380 $\pm$30 | p |  0.60 $\pm$0.03 0.20 $\pm$0.01 |  2.48 $\pm$0.62
Table 2: Raman fit parameters, resulting Fermi energy (EF), doping type (n or
p), strain, and defect density. The first row relates to rolled graphene
layers on top of the fibre (gate), while the last two rows correspond to fibre
with a dielectric layer (150nm Al2O3 and 200nm parylene C) (channel).

Fig.5 plots the Raman spectra of the fibre PD at different fabrication stages.
Raman spectra of fibre (red), dielectric layer (150nm Al2O3 and 200nm parylene
C) (blue), NR=6 on the dielectric layer (green), and coated PVK on NR=6
(orange). In the Raman spectrum of Al2O3 and parylene C, Fig.5c, the
peaks$\sim$1337cm-1, 1440cm-1, 1610cm-1, Fig.5c, are attributed to CH2 wagging
and twisting vibrations[97, 98], CH scissoring in CH2 and/or C-C skeletal in-
plane vibrations of aromatic rings[97, 98], and CH scissoring in CH2 and/or
C-C skeletal in-plane vibrations of aromatic rings[97, 98], respectively. For
NR=6, Pos(G)$\sim$1587cm-1, FWHM(G)$\sim$17cm-1, Pos(2D)$\sim$2686cm-1,
FWHM(2D)$\sim$41cm-1, I(2D)/I(G)$\sim$1.9 and A(2D)/A(G)$\sim$4.7.
Pos(2D)$\sim$2686cm-1 indicates that layers are p-doped[80]. From
A(2D)/A(G)$\sim$4.7 we estimate E${}_{F}\sim$190meV, corresponding to a
carrier concentration$\sim$2.2$\times$1012cm-2 by taking into account the
dielectric constant$\sim$3[99] of Parylene and the residual PMMA from transfer
of CVD SLG[80]. I(D)/I(G)$\sim$0.49 corresponds to a defect
density$\sim$1.82$\times$1011 cm-2[81, 82] for 2.41eV excitation and
E${}_{F}\sim$190meV. The doping level as calculated from A(2D)/A(G) should
correspond to Pos(G)$\sim$1586cm-1 for unstrained graphene[79]. However, in
our experiment Pos(G)$\sim$1587cm-1, which implies a contribution from
compressive uniaxial (biaxial) strain$\sim$0.06% (0.02%)[84]. Table 2
summarizes the Raman fit parameters and the resulting EF, doping, strain, and
defect density.

Source and drain electrodes are defined (channel length: 1000$\mu$m; diameter:
125$\mu$m) by inkjet printing Ag ink (Sigma-Aldrich, 736465)
(resistivity=11$\mu\Omega$cm). Ag ink is also printed on the gate, and the ink
is cured on a hot plate at 150∘C for 1h to remove the residual solvent
(triethylene glycol monomethyl ether). We use a Fujifilm Dimatix DMP-2800 with
a 21$\mu$m diameter nozzle. This approach removes the need for conventional
lithography[100], or shadow masks[101].

Then, a mixed-cation lead mixed-halide PVK[60] is prepared by dissolving PbI2
(1.2M), formamidinium iodide (1.11M), methylammonium bromide (0.21M) and PbBr2
(0.21M) in a mixture of anhydrous DMF:DMSO (4:1 volume ratio) followed by 5
vol% CsI stock solution (1.5M in DMSO). The final solution is diluted by
adding 50 vol% DMF:DMSO (4:1 volume ratio). The PVK solutions is spin-coated
(Fig.4e) on the surface-treated rolled SLG surface (10min under UV ozone
utilising UV-Ozone Cleaner UVC-1014, in order to facilitate the dispersion of
PVK, by making the surface hydrophilic[102]) using a two-step speed program at
2000 and 6000rpm for 10 and 30s, respectively, adding 150$\mu$l of
chlorobenzene 30s after the start of the spinning routine[103]. These
different speeds allow us to get more homogeneity with respect to one-step
spinning[103], and remove solvents during the PVK film formation[103]. The
samples are annealed at 100∘C for 1h to remove the solvents (Dimethylformamide
and Dimethyl sulfoxide)[103]. All processes to make and spin coat PVK are
performed in a N2-filled glove box to prevent degradation of PVK triggered by
H2O[104].

Fig.5a plots the Raman spectra of SLG/PVK on fibre (green) and Si/SiO2 (gray)
for 514.5nm excitation. These show three bands$\sim$52, 110, 249 $cm^{-1}$.
The PL background of PVK can be seen in Fig.5a and background subtracted
spectra are in Fig.5b. The bands$\sim$52 and 110$cm^{-1}$ are the main modes
of PVK[105] assigned to PbBr6 deformation[106, 107] and motion of the organic
cation[106, 107], respectively. The peak$\sim$249$cm^{-1}$ is assigned to
torsional mode of the methylammonium cation, due to PVK exposure to
moisture[108, 105]. Moisture could be introduced when the sample is removed
from the glove-box, prior to sealing with parylene. After the deposition of
PVK on NR=6, Fig.4e, the Raman spectrum of NR=6 has Pos(G)$\sim$1583cm-1,
FWHM(G)$\sim$17cm-1, Pos(2D)$\sim$2701cm-1, FWHM(2D)$\sim$43cm-1,
I(2D)/I(G)$\sim$0.9 and A(2D)/A(G)$\sim$2.3. Pos(2D)$\sim$2701cm-1 indicates
p-doping[80]. From A(2D)/A(G)$\sim$2.3 we estimate E${}_{F}\sim$380meV, which
corresponds to a carrier concentration$\sim$8.8$\times$1012cm-2, by taking
into account the dielectric constant of Parylene and residual PMMA[80].
I(D)/I(G)$\sim$0.46 corresponds to a defect density$\sim$2.5$\times$1011
cm-2[81, 82] for 2.41eV excitation and E${}_{F}\sim$380meV. EF as estimated
from A(2D)/A(G) should correspond to Pos(G)$\sim$1596cm-1 for unstrained
graphene[79]. However, in our experiment Pos(G)$\sim$1583cm-1, which implies
tensile uniaxial (biaxial) strain$\sim$0.6%(0.2%)[84].

Figure 7: (a) Absorbance of PVK and SLG/PVK. (b) PL of PVK and SLG/PVK for
514.5nm excitation. (c) TRPL spectra of PVK and SLG/PVK for 400nm excitation.
The arrow indicates a shorter lifetime.

Figure 8: (a) Ids as a function of Vds in the dark before (red) and after
(blue) deposition of PVK on NR=6. (b) Transconductance for Vds=0.5V for NR=6
in dark. (c) Illumination power-dependent transfer characteristics at 488nm.
(d) Photocurrent as a function of bias for different illumination power and
Vg=0. (e) Rext as a function of optical power density. (f) Temporal
photocurrent response for NR=6 under alternating dark (purple) and light
(yellow).

Fig.6a is a SEM image of the spin-coated PVK on NR=6. This shows uniform
coverage. An average roughness$\sim$20nm is measured with a Bruker Dimension
Icon atomic force microscope (AFM) in Fig.6b. Figs.6c-e are optical images of
the final device.

Fig.7a plots the absorbance from photo-thermal deflection spectroscopy (PDS)
of individual SLG and PVK films on quartz, compared with SLG/PVK. A
combination of a Light Support MKII 100 W Xe arc source and a CVI DK240
monochromator is used to produce a modulated monochromated light beam. The
probe beam comes from a Qioptiq 670nm fibre-coupled diode laser directed
parallel to the PVK film surface. PDS can measure the band-tails of PVK down
to$\sim$10-5absorbance[109]. Fig.7a shows that PVK absorption increases for
SLG/PVK compared to PVK. This indicates the presence of interface states that
alter the PVK bandgap[109]. The PL spectra of PVK and SLG/PVK are in Fig.7b.
Both show a PL peak$\sim$762nm arising from the PVK band gap[60]. The PL
intensity (integrated area under PL curve) of SLG/PVK is quenched$\sim$96%
compared to PVK. This can be assigned to charge carrier transfer between PVK
and SLG[110], though there is also increased non-radiative recombination due
to the increased sub-gap trap density, as shown by the PDS measurements in
Fig.7a. Time-resolved PL (TRPL) spectra are recorded at RT with a gated
intensified CCD camera (Andor iStar DH740 CCI-010) connected to an Andor
SR303i spectrometer with time resolution$\sim$0.1ns. A Ti:sapphire optical
amplifier (1kHz, 90fs pulse width) is used to generate narrow bandwidth
photoexcitation (FWHM=10nm at 400nm) via a noncollinear optical parametric
amplifier and a pulse fluence of 0.5$\mu$Jcm-2. We thus fit Fig.7c to
calculate average lifetimes as a means to compare the amount of charge
quenching, but we do not imply any physical meaning to this fit. We use the
relation[35]:

$f(t)=\sum_{i}A_{i}exp(-t/\tau_{i})+B$ (3)

where $A_{i}$ is decay amplitude, $\tau_{i}$ is decay time and B is constant.
The PL decay times for PVK are $\tau_{1}$=6.7ns (A1:0.802) and
$\tau_{2}$=32.9ns (A2:0.198) with average lifetime ($\Sigma
A_{i}\tau_{i)}$)/($\Sigma A_{i}$) of 11.8ns. The PL decay times for SLG/PVK
are $\tau_{1}$=2.0ns (A1:0.743) and $\tau_{2}$=10.7ns (A2:0.257) with average
lifetime$\sim$4.3ns, indicating charge transfer between SLG and PVK[111],
because of band energy alignment at the PVK/SLG interface.

We then characterize the photoresponse of the fibre-based PDs. The linear
dependence of current vs voltage in Fig.8a indicates Ohmic contact of CVD SLG
and Ag electrodes. By depositing PVK, the current increases in Figs.8a,b,
suggesting h transfer from PVK to SLG, in agreement with the Raman measurement
in Fig.5. A similar h transfer was previously reported for SLG/PVK
interfaces[112, 113]. The field-effect $\mu$ is calculated as[33]:

$\mu=\frac{\triangle I_{d}.L}{\triangle V_{g}.C_{ox}.V_{ds}.2\pi r}$ (4)

Figure 9: (a) Spectral noise power of fibre PDs. (b) Wavelength dependent Rext
for PDs and absorption of fibre PDs from the KK relations as a function of
excitation wavelength

Fig.8c plots Ids vs Vg when the PD is illuminated at 488nm for normal
incidence with a collimated laser (Multi-Channel Source-Thorlabs) with
P=44$\mu$W to 2nW for V${}_{ds}\sim$0.5V. -2$<V_{g}<$+10 is used as a safe
range to avoid dielectric breakdown. The gate-source current is kept$<$1nA to
avoid leakage. Due to the ultrafast (sub-100fs) charge recombination in
SLG[115], the photogenerated carriers do not contribute to the photocurrent. A
gate-dependent photocurrent is not observed in our channel, Fig.8c. This could
be explained by the screening of the electric field by the SLG layers
underneath[116]. Under illumination, light is absorbed by PVK and part of the
photogenerated h are transferred from the PVK valence band into the lower
energy states in p-doped SLG[112, 113], leaving behind the uncompensated
charge of photogenerated e[112, 113]. The latter are trapped in PVK and act as
an additional negative gate voltage, applied to the SLG channel, altering the
electric field at the SLG/PVK junction[112, 113]. The photocurrent is[33]:

$\triangle{I_{ph}}=I_{light}-I_{dark}$ (5)

where $I_{light}$ is the output current of the device under the incident beam,
and $I_{dark}$ is the current measured in dark. The photocurrent of the fibre-
based SLG/PVK PDs is in Fig.8d, for P ranging from 980$\mu$W to 750pW, and Vds
from -1 to 1V. The corresponding Rext as a function of power density is in
Fig.8e. For APD and A${}_{0}\sim$0.18$\mu$m2 and 0.785$\mu$m2, respectively,
we get R${}_{ext}\sim$22,000A/W for P=750pW and Vds=1V. For V${}_{ds}>$1, the
free carriers drift velocity $\nu_{d}$ in SLG increases lineally until
saturation by scattering with optical phonons[117]. Therefore, all
measurements are done at V${}_{ds}\leq$1V to keep the device operation in the
linear (Ohmic) regime. A P rise leads to higher photocurrents until saturation
(44$\mu$W and 980$\mu$W), due to the increased recombination rate of photo
excited carriers[113]. This Rext is seven orders of magnitude higher than
the$\sim$mA/W reported for SLG on flat and rigid substrates (e.g. Si/SiO2)[68,
118, 119, 120], one order of magnitude higher than reported for PVK on
Si/SiO2[57], and four orders of magnitudes higher than pristine
PVK$\sim$3A/W[121] on flexible PET substrates. Our Rext is two orders of
magnitude higher than PVK/SLG hybrid PDs on Si/SiO2[112], but lower than the
highest R${}_{ext}\sim$109A/W PDs based on PVK/organic-
semiconductor(poly-(3,4-ethylenedioxythiophene):poly(styrenesulfonate))[122]
and lower than hybrid SLG/QDs[67] and SLG/MoS2 PDs[123], both with
R${}_{ext}\sim$107A/W on Si/SiO2, see Table 1. However, our Rext is the
highest amongst fibre-based PDs for wearable applications, table I.

The temporal photocurrent response is measured in Fig.8f via an digital
oscilloscope (MSO9404A). This gives a rise time$\sim$5ms and a fall
time$\sim$35ms, faster than the$\sim$200ms reported for carbon fibre/PVK
PDs[32] and$\sim$40ms for fibre based P3HT/ZnO PDs[36], and comparable to PDs
made of mechanically exfoliated SLG and QDs on Si/SiO2[67].

The specific detectivity (D∗) ($(cm.Hz^{1/2}/W)$ or Jones) relates the
performance of PDs in terms of Rext to the PD photoactive area, allowing the
comparison of PDs with different active areas[33]:

$D^{*}=\frac{(AB)^{1/2}}{NEP}$ (6)

where A is the photoactive area (projected incident light on fibre), B is the
electrical bandwidth (Hz) and NEP($W/Hz^{1/2}$) is the noise equivalent power
(the power that gives a signal-to-noise ratio of one in a 1Hz output
bandwidth) defined as[126]:

$NEP=\frac{i_{n}}{R_{ext}}$ (7)

where ${i_{n}}$ is the dark noise current. The fibre PD is DC biased with a
source meter (Keithley 2612B). A chopper is used in the path of the incident
beam to modulate the light[126]. The noise (A/$\sqrt{Hz}$) is characterized in
the time domain collecting the trace on a digital oscilloscope, with
subsequent Fourier transform to analyze the data in the spectral domain.

Figure 10: (a)Iph,bend/Iph,nobend for NR=6 as a function of Rb. (b)
Iph,bend/Iph,nobend for NR=6 as a function of bending cycles for Rb=4mm. (c)
Iph,wash/Iph,nowash for NR=6 as a function of AATCC washing cycles.

Fig.9a plots the spectral noise density as a function of frequency $f$. The
measured noise in the dark shows a 1/$f$ dependence. Thus, the noise density
(i.e. noise power per unit of bandwidth (dbm.Hz-1/2))[33] is inversely
proportional to $f$, due to charge traps and defects[33]. Considering the
measurement at 4Hz,$\sim$10 times less than the cut off $f$, defined as the
$f$ at which the detector Rext decreases by 3dB[33], we get
NEP$\sim$2.1$\times$10-11W/Hz1/2, D*$\sim$2$\times$109Jones for NR=6. The
noise current in the shot noise limit (due to generation and recombination of
e-h pairs and resistive current paths in PDs[6]), can be expressed as
$i_{n}=(2qI_{dark})^{1/2}$[126, 127], where the combined dark current
($I_{dark}$) is the sum of unamplified graphene current ($I_{dark(SLG)}$) and
amplified injection current ($I_{dark(PVK)}$) from PVK, caused by the thermal
excitations of charge carriers in the dark. $I_{dark(PVK)}$, which is due to
thermal excitations of charge carriers in the dark, is much smaller than
$I_{dark(SLG)}\sim$3.75mA. Therefore, following Ref.[127], the shot noise
limited noise current can be expressed as $i_{n}=(2qI_{dark(SLG)})^{1/2}$. D*
in the shot noise limit can be written as[126]:

$D^{*}=\frac{R_{ext}(AB)^{1/2}}{i_{n}}$ (8)

We get D${}^{*}\sim$1013 Jones for NR=6. Our D∗ is comparable with that of
state-of-the-art fiber based PDs[32], where D${}^{*}\sim 2.15\times 10^{13}$
Jones, but with R${}_{ext}\sim$562.9mA/W at 800nm and
$\tau_{res}\sim$200ms[32], see Table 1.

Fig.9b plots Rext as a function of wavelength from 490 up 870nm. This follows
the absorption of SLG/PVK films, Fig.7a, suggesting PVK is the main light
absorbing layer in which photoexcited charges are supplied[56]. To get a
better understanding of the spectral response of PDs versus wavelength,
optical simulations are performed by extracting the PVK index from Ref.[128].
We use the PVK refractive index in transfer matrix calculations to estimate
the absorption of SLG/PVK on our fibre structure. The imaginary and the real
part of the PVK average index is found by applying the Kramers-Kronig (KK)
relation, $n_{PVK}(w)$ = 1 + 2$\pi^{-1}$ $\mathcal{P}$
$\int_{0}^{\infty}\,w^{{}^{\prime}}\,K_{PVK}(w^{{}^{\prime}})/(w^{{}^{\prime}2}-w^{2})\,dw^{{}^{\prime}}$,
where $\mathcal{P}$ denotes the principal value of the integral and $w$ is the
angular $f$. The SLG refractive index is modelled by the Kubo conductance[129]
at RT and EF=380meV. The spectral response of the fibre PDs in Fig.9b follows
the optical absorption of SLG/PVK on fibre in Fig.9b, i.e. drop of Rext and
absorption with increasing wavelength.

Mechanical characterizations are crucial to emulate real-life applications of
wearable PDs[26]. These include washing in a tumble washing machine[6, 130]
and deformations against body motions (such as bending[6]). We thus test our
fibre-based PDs according to AATCC[10] for washing. For mechanical
deformations, we perform bending cycles, starting from a radius of curvature
of 55mm where the PDs show almost no performance change, to 1mm where we
observe PD degradation.

Fig.10b is the schematic of a three-point bending setup (Deben Microtest) we
use to characterize the effect of bending on the PD photocurrent. The
photocurrent is first measured for no bending (Iph,nobend), and then under
bending (Iph,bend). We use the ratio of (Iph,bend) over (Iph,nobend) to
monitor the effect of mechanical deformations. The bending radius $(R_{b})$ is
calculated as[131]:

$R_{b}=\frac{[H^{2}+(L/2)^{2}]}{2H}$ (9)

where $H$ is the height at the chord midpoint of two ends of the arc, and $L$
is the chord of circumference connecting two ends of the arc.

Fig. 10 plots average and standard deviations (error bars) of
Iph,bend/Iph,nobend as a function of Rb and bending cycles, as well as
Iph,wash/Iph,nowash as a function of washing cycles. Iph,bend/Iph,nobend shows
up to$\sim$20% variation for bending radius as low as 4mm (Rb=4mm), Fig.10a.
Fig.10 suggests that variations primarily occur during the initial cycles.
Refs.[132, 133, 134] reported that the electrical behaviour of SLG on flexible
substrates (e.g. PET, PDMS) stabilizes after several deformations. Ref.[135]
reported that the morphology of SLG wrinkles undergoes gradual modification
with increasing bending, leading to a change of contact area. We attribute the
slight increase of Iph,bend/Iph,nobend up to R${}_{b}\sim$4mm to changes in
contact area of rolled CVD SLG on PVK. Upon further bending (R${}_{b}<$4mm),
Iph,bend drops to$\sim$10% of Iph,nobend, due to failure of various
components, such as contact of Ag electrodes with graphene and crack formation
in the fibre PD. Despite pushing our PDs to much lower Rb (4mm) compared to
InP-based semiconductor flexible PDs (R${}_{b}>$38.1mm)[136], we achieve five
orders of magnitude higher Rext (22,000A/W here vs 0.12A/W at 533nm[136]),
Table 1. Ref.[137] reported hybrid CVD SLG and PVK on Polyimide with
Rb=12mm[137], one order of magnitude lower Rext=115A/W at 515 nm[137] compared
to ours. Ref.[138] fabricated PVK/Conjugated-Polymer composite PDs on PET with
comparable Rb (R${}_{b}=$4mm), yet, the latter has six orders of magnitude
lower R${}_{ext}\sim$154mA/W at 835nm[138], see Table 1.

We then test our devices for a fixed Rb=4mm (as we observe$\sim$20% variation
in Iph,bend/Iph,nobend upto Rb=4mm in Fig.10a) as a function of bending
cycles, Fig.10b. After 100 bending cycles (Iph,bend)/Iph,nobend changes$<$20%,
Fig.10b. In similar conditions, Ref.[32] reported 6 fold lower
R${}_{ext}\sim$560mA/W at 800nm[32] for 60 less cycles, see Table 1.

We then test the PDs in a tumble washing machine (SKYLINE rotate washing
color) at 40$\degree$C up to 30 washing cycles, each programmed for 45min,
total volume$\sim$0.37% (Persil detergent powder), and with ten 6mm stainless
steel balls[10]. Two encapsulation approaches are used to protect the PDs
during washing. The fibres are conformally coated with$\sim$1$\mu$m parylene
C, since this acts as a barrier to moisture[139] and gases[139]. The PDs are
then further coated with PDMS for additional waterproofing and resistance
against tension/compression introduced during washing. Part A and B of the
commercial Si-based elastomer Polycraft T15 are mixed 1:1 using a speed mixer
(DAC 150 FVZ-K) at 2000rpm for 30s. The resulting solution is then spread on
the fibre and cured at 80∘C for 30min to expedite the process and ensure
crosslinking of polymer chains[140]. After encapsulation, the devices are
tested at 488nm at 1V. They have R${}_{ext}\sim$22,000A/W, with no degradation
after encapsulation. The photocurrent of fibre-based PDs is first measured for
no washing step (Iph,nowash). The PDs are then placed in a tumble washing
machine and washed. The PDs are then drip dried in a ventilated oven at 60∘C
(Genlab Air), then measured at various washing cycles (Iph,wash) from 1 to 30
cycles, Fig.10c. After 20 cycles, the PDs show$\sim$6% degradation in
Iph,wash/Iph,nowash, Fig.10c. After 30 washing cycles, Iph,wash/Iph,nowash
drops$\sim$28%, due to degradation of electrodes caused by stresses during
washing. Ref.[71] reported$\sim$50% drop in drain current after 10 and 20
washing cycles, however, our fibre PDs maintained up to$\sim$72% of the Iph
after 30 washing cycles.

##  III Conclusions

We reported fibre-based SLG/PVK PDs with $R_{ext}$ up to$\sim$22,000A/W at 1V
operating voltage at 488nm. The PDs have a broad spectral responsivity between
488 and 870nm, with rise and fall times$\sim$5ms and$\sim$35ms. The devices
were subjected to mechanical bending tests (100 cycles at R${}_{b}\sim$4mm)
with Iph,bend/Iph,nobend changes$<$10% and washing tests (AATCC standard) with
Iph,wash/I${}_{ph,nowash}\sim$94% and$\sim$72% at 20 and 30 cycles,
respectively. The combination of high ($\sim$22KA/W) responsivity, response
time, flexibility, washability, and low ($<$1V) operating voltage make our PDs
attractive for wearable applications.

##  IV Acknowledgments

We acknowledge funding from EU Graphene Flagship, ERC Grants Hetero2D, GIPT,
EU Grants CHARM, Graph-X, GreenCAP, EPSRC Grants EP/K01711X/1, EP/K017144/1,
EP/N010345/1, and EP/L016087/1, EP/X015742/1, EP/V000055/1, the Royal Society
and Tata Group (UF150033).For open access, we applied a Creative Commons
Attribution (CC BY) license to any Author Accepted Manuscript version arising
from this submission.

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