Hot carrier extraction from 2D semiconductor photoelectrodes Rachelle Austin1aYusef Farah1aThomas Sayer2aBrad M. Luther1 Andr es Montoya-Castillo2bAmber Krummel1cand Justin Sambur1 3d

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Hot carrier extraction from 2D semiconductor photoelectrodes

Rachelle Austin,1, a) Yusef Farah,1, a) Thomas Sayer,2, a) Brad M. Luther,1

Andr´es Montoya-Castillo,2, b) Amber Krummel,1, c) and Justin Sambur1, 3, d)

1)Department of Chemistry, Colorado State University; Fort Collins, CO, USA

2)Department of Chemistry, University of Colorado Boulder; Boulder, CO, USA

3)School of Advanced Materials Discovery, Colorado State University; Fort Collins, CO, USA

(Dated: 26 October 2022)

Hot carrier-based energy conversion systems could double the eﬃciency of conventional solar energy technology

or drive photochemical reactions that would not be possible using fully thermalized, “cool” carriers, but

current strategies require expensive multi-junction architectures. Using an unprecedented combination of

photoelectrochemical and in situ transient absorption spectroscopy measurements, we demonstrate ultrafast

(<50 fs) hot exciton and free carrier extraction under applied bias in a proof-of-concept photoelectrochemical

solar cell made from earth-abundant and potentially inexpensive monolayer (ML) MoS2. Our approach

facilitates ultrathin 7 ˚

A charge transport distances over 1 cm2areas by intimately coupling ML-MoS2to an

electron-selective solid contact and a hole-selective electrolyte contact. Our theoretical investigations of the

spatial distribution of exciton states suggest greater electronic coupling between hot exciton states located on

peripheral S atoms and neighboring contacts likely facilitates ultrafast charge transfer. Our work delineates

future 2D semiconductor design strategies for practical implementation in ultrathin photovoltaic and solar

fuels applications.

I. INTRODUCTION

All semiconductors absorb photon energies greater

than their bandgap, temporarily creating hot carriers

with excess energy. In a conventional solar cell material

like Si, 40% of the absorbed solar energy is lost as heat

because hot carriers rapidly cool in <100 fs.1This ultra-

fast hot-carrier cooling process prevents current solar cell

technology from reaching theoretical eﬃciency limits.2

A longstanding challenge in the ﬁeld is to develop ma-

terials and selective charge-extraction contacts that ef-

ﬁciently collect hot carriers before they cool.3The ﬁrst

experimental demonstration of hot-carrier extraction in-

volved the transfer of photogenerated hot carriers from

a bulk InP crystal to p-nitrobenzonitrile molecules in an

electrochemical cell.4Unfortunately, a signiﬁcant frac-

tion of hot-electrons thermalized in the InP bulk. Re-

cent eﬀorts in solid-state photovoltaics have focused on

enhancing hot carrier populations in the semiconductor

absorber and using charge carrier-selective contacts to

preferentially extract hot electron and hole populations

(e.g., In0.78Ga0.22As0.81P0.19 quantum well surrounded

by thick In0.8Ga0.2As0.435P0.565 barriers contacted by

n- and p-doped InP contact layers)5. Unfortunately,

such multi-layer structures require expensive materials

and growth methods, especially for the critical charge-

selective contacts.

Nanostructured materials possess unique photophysi-

cal and structural properties that could make hot-carrier-

based energy conversion systems both eﬃcient and inex-

pensive, but this remains a formidable challenge.6For

a)These authors contributed equally to this work

b)Andres.MontoyaCastillo@colorado.edu

c)Amber.Krummel@colostate.edu

d)Justin.Sambur@colostate.edu

example, while hot carrier extraction in graphene opto-

electronics is possible,7graphene-based photovoltaics ex-

hibit low quantum eﬃciency and small photovoltages.8,9

Plasmonic metal nanostructure solar energy conversion

systems suﬀer from low power conversion eﬃciency due

to fast hot carrier cooling.10 While hot carrier extrac-

tion at model interfaces made of solution-processed or-

ganic–inorganic lead halide perovskite11 and lead chalco-

genide nanocrystals12 has been demonstrated, electrical

measurements of hot-carrier eﬀects in a working solar en-

ergy conversion system are scant. Indeed, recent ultrafast

X-ray measurements of lead halide perovskites highlight

the need for concurrent electrical measurements because

interpretation and quantiﬁcation of hot electron and hole

temperatures can be diﬃcult using optical measurements

alone.13

In this work, we investigate the intriguing possibil-

ity of using inexpensive, earth-abundant, and potentially

scalable14 transition metal dichalcogenides (TMDs) such

as monolayer (ML) MoS2for hot carrier extraction us-

ing a proof-of-concept photoelectrochemical solar cell.

The bulk MoS2|I–, I3–|Pt photoelectrochemical cell is

a stable >14%-eﬃcient solar cell15 that is limited by

a small 0.6 V photovoltage, which hot carrier collection

could potentially circumvent. ML TMDs are exciting

absorber materials because high energy photons gener-

ate hot excitons, often called C-excitons, with >100 ps

lifetimes.16 These long and tunable17 lifetimes exceed ul-

trafast photocurrent response in optoelectronic devices,18

which suggests hot carriers could be contributing to cur-

rent in the device. Optical signatures of hot carrier trans-

fer from MoS2to graphene16 and gold19 also suggest

hot carrier transfer could outpace cooling, but electrical

signatures of hot carrier transfer have remained elusive.

Electrical measurements of solid-state ML TMD devices

typically require edge-on contacts for charge extraction,

meaning charge carriers travel micron-long distances to

arXiv:2210.13588v1 [cond-mat.mtrl-sci] 24 Oct 2022

FIG. 1. Optoelectronic properties of the monolayer MoS2photoelectrochemical cell. (a) Cartoon illustration of the three-

electrode photoelectrochemical cell. The solid blue and rainbow arrows indicate pump and probe pulses for TA measurements.

Pt counter and Ag/AgI reference electrodes are omitted for clarity. (b) Absorbance spectra in 0.025 V increments from

0.000 V to 0.550 V. (c) EQE spectra versus applied potential from 0.35 V to 0.55 V. EQE(λ) = qi/I0(λ), where qis the

electronic charge (in units of C), iis the photocurrent (in units of A), and I0is the monochromatic light power (in units of s–1).

(d) Monochromatic i-Emeasurements for resonant A-, B-, and C-exciton excitation (i.e., 650 nm, 605 nm, and 405 nm,

respectively).

the charge-collecting interfaces. The long transport dis-

tances promote cooling and recombination, which likely

explains why hot carrier-induced currents have not been

reported in any 2D TMD-based solar energy conversion

device. The advantage of employing TMD absorbers in a

photoelectrochemical cell is that photogenerated charge

carriers need only travel across three atoms (0.7 nm) to

reach the electron-selective indium tin oxide (ITO) sub-

strate and hole-collecting redox electrolyte (Fig. 1a). The

long lifetime of C-excitons combined with short charge-

transfer distances and intimate charge carrier-selective

contacts thus raises the exciting possibility of extracting

hot carriers in the model ML-MoS2|I–, I3–|Pt solar cell.

Using an unprecedented combination of photoelectro-

chemical and in situ time-resolved spectroscopic tech-

niques, here we show hot carrier extraction outcompetes

exciton formation and relaxation in a working photoelec-

trochemical cell. We demonstrate that hot carriers can

be extracted to generate photocurrent before cooling to

the band-edge and develop a picture of the photocurrent

generation mechanism in ML-MoS2photoelectrodes.

II. OPTICAL AND PHOTOELECTROCHEMICAL

CHARACTERIZATION OF THE ML-MoS2

PHOTOELECTRODE

Here we test our hypothesis that one should be able

to preferentially extract hot carriers from the C-exciton

manifold in ML-MoS2using the electron- and hole-

selective contacts in a photoelectrochemical cell because

the three atom-thick transport distance minimizes the

electron and hole transport times relative to the cool-

ing times. Photoluminescence and Raman spectroscopy

conﬁrmed the chemical vapor deposition (CVD) growth

method produced ML-MoS2(Appendix 1, Fig. 4a,b). To

probe the hot-carrier generation, recombination, and ex-

traction processes, we constructed a transparent photo-

electrochemical cell that enables in situ ultrafast spec-

troscopy measurements to measure relative exciton pop-

ulations after photo-excitation (Fig. 1a and Appendix 2).

The ML-MoS2-coated ITO electrode serves as the work-

ing electrode in a three-electrode microﬂuidic electro-

chemical cell containing 1 M NaI electrolyte. Fig. 1a

schematically shows that, under illumination and ap-

plied positive potentials, photogenerated holes move to

the semiconductor/liquid interface and oxidize I–to I2

while photogenerated electrons transfer to the ITO sub-

strate, generating net anodic current ﬂow through the

cell.

To assess which excitonic transitions contribute to cur-

rent ﬂow in the ML-MoS2photoelectrode, we simulta-

neously measured optical absorbance and photocurrent

signals under working photoelectrochemical conditions

(Appendix 3). Fig. 1b shows absorbance spectra of ML-

MoS2as a function of the applied potential (E, refer-

enced to the Ag/AgI electrode). All spectra feature three

peaks corresponding to the band-edge A- (1.99 eV) and

B- (2.01 eV) excitons, and the higher energy C-exciton

(2.98 eV). The A- and B-exciton peak intensities increase

FIG. 2. Center: Eigenspectrum of unsupported ML-MoS2

in vacuum calculated from BSE. The purple and green lines

represent the A-exciton ‘hydrogenic series’ around the K point

and the C-exciton states in the band nesting region between

K and Γ, respectively. The transparency of each energy level

is set to the oscillator strength of the transition, normalized

by the A-1s transition (labeled EA

ex), which is the strongest.

ex is the energy of the ﬁrst C-exciton state. The dotted blue

line represents the fundamental electronic band gap at 2.9 eV.

Top: The hole-averaged isosurface plots for the electron den-

sity in the aforementioned A-exciton (below, purple box) and

C-exciton (above, green box) states. Right: The absorption

spectra as calculated for the BSE states pictured (orange)

and the underlying single, independent particle states (blue).

The energy diﬀerence between peaks represents the A- and

C-exciton binding energies (EA

band EC

b).

and blue shift with increasing positive potential, while

the high energy C-exciton increases slightly with posi-

tive bias and does not shift with potential. The observed

potential-dependent absorbance changes (Fig. 1b) have

important consequences for interpreting the relative pop-

ulations of excitons gleaned from ultrafast TA measure-

ments, as will be discussed below.

Photocurrent measurements performed concurrently

with the absorbance measurements revealed at which po-

tentials the diﬀerent excitons dissociate and contribute

to current ﬂow in the cell. Figure 1c shows potential

dependent external quantum eﬃciency (EQE) spectra,

where EQE(λ) = qi/I0(λ) and iis the photocurrent,

qis the elementary charge, and I0is the light power.

At positive bias (e.g., E > 0.5 V), the EQE spectrum

mimics the absorbance spectrum (solid purple line in

Fig. 1c), indicating the applied potential generates a suf-

ﬁciently strong interfacial electric ﬁeld to eﬀectively dis-

sociate all three excitonic species. Close examination of

the potential-dependent spectra reveals subtle diﬀerences

between the photocurrent onset potential for the A-, B-

and C-excitons that could be further distinguished in

monochromatic i-Ecurve measurements (Fig. 1d). In-

terestingly, photocurrent generation onsets ﬁrst (i.e., less

positive potentials) for the C-exciton and the slope of

the C-exciton i-Ecurve is signiﬁcantly steeper than that

of the A/B-excitons. The lower onset potential means

that, under conditions of equivalent interfacial electric

ﬁeld strength, photo-excited C-excitons require less driv-

ing force to dissociate and contribute to current ﬂow in

the cell compared to the lower energy A/B-excitons.20

Since we did not observe photocurrent upon directly ex-

citing the A/B-excitons at the same applied potential as

that of the C-exciton (speciﬁcally E= 0.35 V in Fig. 1d),

our monochromatic i-Edata strongly suggests that C-

excitons are extracted before they cool to the band edge

and form low energy A/B-excitons.

III. NATURE OF EXCITONIC STATES AND THEIR

IMPLICATION FOR CHARGE TRANSFER

Why is C-exciton extraction more eﬃcient than low

energy A/B-exciton extraction in our ML-MoS2photo-

electrode? To shed light on this critical question, we

turned to a theoretical treatment of the spatial distribu-

tion of the A/B- versus C-excitons in ML-MoS2. The

goal of this theoretical investigation is to qualitatively

compare the magnitude of electronic coupling between

the diﬀerent exciton states in the charge donor (MoS2)

and the electron/hole acceptors (ITO substrate/iodide

anions), which ultimately allows us to rationalize why

charge transfer from the C-exciton is more eﬃcient than

A/B-excitons.

We ﬁrst analyzed the excitonic wavefunctions obtained

by solving the Bethe-Saltpeter equation (BSE) for un-

supported ML-MoS2in vacuum (see Appendix 4). This

approach accounts for excitonic eﬀects by explicitly treat-

ing electron-hole correlation. Figure 2-center shows the

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Hotcarrierextractionfrom2DsemiconductorphotoelectrodesRachelleAustin,1,a)YusefFarah,1,a)ThomasSayer,2,a)BradM.Luther,1AndresMontoya-Castillo,2,b)AmberKrummel,1,c)andJustinSambur1,3,d)1)DepartmentofChemistry,ColoradoStateUniversity;FortCollins,CO,USA2)DepartmentofChemistry,UniversityofColoradoBoulde...

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Hot carrier extraction from 2D semiconductor photoelectrodes Rachelle Austin1aYusef Farah1aThomas Sayer2aBrad M. Luther1 Andr es Montoya-Castillo2bAmber Krummel1cand Justin Sambur1 3d

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