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*For correspondence. (e-mail: sunandan.baruah@dbuniversity.ac.in)

tion. We present here a study on quantum dot sensi- tized solar cells considering their construction and working, impact of incorporation of nanomaterials in solar cells and various structures for improving the performance of solar cells.

Keywords: Light harvesting, multiple exciton genera- tion, quantum dots, solar cells, tunable band gap.

ENERGY generation in the modern era always had a pro- blem in keeping pace with the increased demand of the ever burgeoning population. The world is changing; the standard of living of the people is resulting in rapid increase in the global consumption of energy (10 trillion kWh at present) and the US Energy Information Adminis- tration predicts global energy demand of 35 trillion kWh in 2035 (ref. 1). The amount of energy that is showered by the sun on the earth in an hour is enough to fulfil the global need of energy for a year2. The need of the hour, therefore, is some innovation that will provide elec- tricity using the abundantly available solar energy at minimal expenses to support the base of the pyramid population.

As a cost-effective alternative to silicon-based photo- voltaic systems, recently, quantum dot sensitized solar cells (QDSSCs) have gained considerable popularity.

QDSSCs, an evolution from dye sensitized solar cells (DSSCs) which were first reported by O’Regan and Grätzel in 1991, are considered to have great potential as the next generation of solar cells (SCs). Several efforts have been made to obtain an ideal organic dye as a sensi- tizer to absorb photons in the full visible spectra. It has been a challenge to obtain such an ideal organic dye.

Hence, narrow band-gap semiconductor quantum dots (QDs), such as CdSe, InAs, CdS and PbS became more popular as photosensitizers due to their versatile optical and electrical properties, such as higher stability towards oxygen and water, tuneable band gap depending on the

tion of various wavelengths of the visible spectrum of light can be achieved. Coupling QDs with semicon- ducting nanorods which have high surface area allows better tapping of sunlight as more photon-absorbing QDs can be coupled to the surface. The incorporation of nanomaterials improves photoenergy absorption owing to high available surface area. In this article, the impact of nanomaterials in QDSSCs and various possible struc- tures for improving the performance of QDSSCs are pre- sented.

Construction of QDSSCs

Figure 1 shows the typical construction schematic of QDSSC. It consists of a photoanode and a counter electrode separated by a redox couple5,6. The photoanode consists of a wide band gap, mesoporous semiconductor layer attached to conducting glass and QDs adsorbed onto the semiconductor layer. QDs work as sensitizer in which electron–hole pairs are created upon exposure to light.

Mesoporous structure of the semiconducting layer provides enhanced surface-to-volume ratio, which in turn facilitates enhancement in the adsorption of QDs onto it.

The redox couple scavenges the photogenerated holes and produces electrical equilibrium in the semiconducting layer. Sulphide/polysulphide redox couple is most widely used because of its higher open circuit voltage and better stability for photovoltaic operation7,8. Various additives have been explored with sulphide/polysulphide redox couple9. In fact, a new record of average power conver- sion efficiency of 12.3% of Zn–Cu–In–Se QD-based QDSSCs has been reported9, where 6 vol% tetraethyl or- thosilicate is used as an additive in polysulphide electro- lyte. CdS10,11, CdSe12,13, ZnSe14,15, PbS16,17, Ag2S18,19, CuInS220,21, CdTe22,23, InP24,25 and CdHgTe26,27 are mate- rials of choice for QDs to be used as sensitizer in QDSSC design. The most popularly used wide band gap semicon- ductor in QDSSCs is TiO2 (ref. 28). ZnO, SnO2 and Nb2O5 are also reported to be used as mesoporous semi- conducting layer in QDSSCs29–31.

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6

a c ti th e a ro a m C ic th F tr s 4 Q b c c s a u s S m e a g m a (T b h

REVIEW A

60

Figure 1 showing

Liu et al.32 and CdS/CdSe coating using ion (SILAR) hod, and ac efficiency of and efficiency

ods by CdS and CdSe QD method. Yu CdS/CdSe QD

conductor and hod and achi Fang et al.35

rode, CdSe Q emiconductor 4.81%. Radic QDSSCs usin band gap sem cy of 4.4%. Z ciency of 4.92 emiconductor al.38 reported used CdSe QD

emiconductor SILAR metho method to pre efficiency of 5 and TiO2.

Zinc oxide gaining attent most sought advantage of u TiO2 has an i binding energ higher electr

ARTICLES

1. a, Complete the energy band

used TiO2 as e QDs as sens

successive i and chemica chieved a fi 2.05%. Seol y of 4.15%, w

shells to red Ds were utili et al.34 repo Ds as sensitize d Cu2S in cou ieved efficien using carbo QDs as sensiti r achieved F ch et al.36 pr g CdS/CdSe iconductor an Zhang et al.37 2% using larg r and CdS/C

FF of 57% a D as sensitiz

r with a buff od. Santra and pare QDSSC 5.42% when t

with a direc tion along w after for D using ZnO ov indirect band gy of 60 meV ron mobility

set-up of quant ds.

s wide band g sitizer with a ion layer ads al bath depo ill factor (F

et al.33 repo when they co duce the char ized as sens orted a QDS er, TiO2 as wi unter electrod ncy of 4% a n nanofibres izer and TiO2

FF of 60% a repared SILA as sensitizer nd got FF of

7 reported FF er sized TiO2

CdSe as sens and efficiency er and TiO2

fer layer of C d Kamat39 als

s, and achiev they used Mn ct band gap o with TiO2, wh DSSC photo

ver TiO2 is it gap of 3 eV) V (refs 41, 42 y (200 cm2/

tum dot sensitize

gap semicond pplication of sorption and sition (CBD) FF) of 41%

rted FF of 3 overed ZnO n rge recombin

itizer using SSC design u

ide band gap de using CBD and FF of 60 as counter

2 as wide band and efficienc AR method b and TiO2 as 46% and effi of 63% and

2 as wide band itizer. Hossa y of 5.21%.

as wide band CdS followin

o used the SI ved FF of 47%

n-doped CdS/

of 3.37 eV is hich has been oelectrodes40.

ts direct band ) and high ex 2). ZnO also h

V/s) than

ed solar cell. b,

ductor f SiO2

reac- ) me- and 8.3%

nano- nation

CBD using

sem- D me- 0.1%.

elec- d gap cy of based

wide icien- d effi- d gap ain et They d gap

g the ILAR

% and CdSe s also

n the The d gap xciton has a TiO2

(30 c (ref.

large nano shee struc elect gle c diffu grow face grow them mero struc effic TiO2

Imp Inco vide band with cient Am gene impa

Tun Band owin metr prop

CURRENT Single nanopart

cm2/V/s), wh 43). Anothe est collection orods/nanowir ets, nanoneed

cture of single tron hopping crystalline Zn usion of elec wn ZnO nano

area compa wn using sim m the most su

ous reports ar ctures in Q ciency of ZnO

2-based ones.

pact of nano orporation of s higher stabi d gap depend h single-photo

t and low cos mong these, b eration are ga act on photon

able band ga d gap of QD ng to quantu re scale48,49, perties of QD

T SCIENCE, VO ticle with an ads

hich makes it er advantage n of nanostru res, hierarch dles, nanotub

e crystalline Z and enhance nO nanorods p

ctrons throug orod arrays ca ared to volum mple hydroth uitable for low re also availab QDSSC45–47.

O-based DSS

omaterials i QDs in SCs ility towards ding on size, m

on absorption st.

band gap ten aining import n absorption a

ap

Ds can be tu um confinem

which leads Ds. This is a

OL. 115, NO. 4, 2 sorbed quantum

t a better cho of ZnO is t uctures ever hical nanostru es, etc. By c ZnO, it is pos

electron mob provide a dire gh them. Fu an provide a me. ZnO nan hermal meth w-cost QDSS ble on the use However, SCs is still l

n QDSSCs is advantage oxygen and w multiple exci n, larger ext ability and m tance as they and photocurr

uned by vary ment phenome to size-dep advantageous

25 AUGUST 20 dot (QD)

oice than TiO that it has th recorded lik uctures, nano controlling th ssible to reduc bility. The sin ect path for th urther, dense

very high su norods can b hods44, makin SC design. Nu e of ZnO nano the reporte lower than th

eous as it pro water, tuneab iton generatio tinction coeff multiple excito y have a dire rent generatio

ying their siz enon in nano endent optic

for controlle

18

O2

he ke

o- he ce n- he

ly ur-

be ng u- o- ed he

o- ble

on fi- on ect n.

ze o- cal

ed

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proved photoelectrochemical response and photoconver- sion efficiency have been reported by Kongkanand et al.52 by varying the size of CdSe QDs. So a combination of different sized ODs must improve the photon absorp- tion scenario, which will definitely improve the efficien- cy of SCs. Figure 2 is a schematic of photoanode which will absorb the entire visible spectrum; hence it can be termed as photoanode of rainbow SC.

Multiple exciton generation

If a photon with energy greater than the bandgap (Eg) of a semiconductor is incident on it, then the energy in excess

Figure 2. Schematic structure of photoanode of a rainbow solar cell.

means of emission of phonons. The relaxation dynamics is largely affected by quantization effect, which can be produced in a semiconductor by dint of quantum con- finement62–64. When the size of the nanosemiconductor crystals is comparable to Bohr radius, the rate of impact ionization increases extensively and becomes comparable to the rate of cooling of the hot carriers. Schaller and Klimov57 carried out a detailed study on carrier multipli- cation in PbSe nanocrystals through impact ionization.

Figure 3a shows the generation of biexcitaton. In the Auger process, two excitons recombine and produce highly energetic single excitons (Figure 3b). Figure 3c shows the immediate consequences of high photon exci- tation (hω/Eg > 3); initially high-energy excitons form in nano semiconducting crystals and then some (nxx) of the excitons go through impact ionization and produce biex- citons while some (nx) simply relax to the band edge and remain as single excitons and with time biexcitons go through the Auger process and produce single excitons.

The change in population of exciton for low pump photon energies shows a step function with time while those with high energies exhibit an exponentially decaying function (Figure 3 d).

Structures of QDSSCs

Apart from the structure discussed earlier in the text, many structures of SCs have been evolved based on QD sensitizers. Among them tandem SCs, core-shell SCs and plasmonic SCs have been able to draw the attention of researchers.

Tandem solar cells

The tandem SCs simultaneously address two key prob- lems of SCs, viz. energy loss due to thermalization of hot charge carriers and sub-band gap transmission. It was reported that stacking multiple sub-cells in series can provide theoretical efficiency more than that of the Shockley–Queisser limitation65. With increase in the

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6

n m is c su o n th th u lo fo o su re n th d

REVIEW A

62

Figu b, au

number of su mum efficienc

s 68% (ref. 6 cal tandem SC

ub-cells stack on acceptor–d not absorbed he bottom su he sub-cells w unlike band g

oss. The inte for the recom one sub-cell w

ub-cell. This ecombination need to be sele

he acceptor o donor of the b

ARTICLES

ure 3. Demonst uger process; c, d

b-cells, effic cy possible w 6). Figure 4a C. It compris ked on top of donor compos in the top su ub-cell. So th will be differe gaps are used rmediate laye mbination of e

with that of is precisely w n layer. The m ected to ensu of the top sub bottom sub-ce

tration of multip dynamic carrier p

iency increas with infinite nu a shows the s ses two (or m f one another site. The part ub-cell will i he absorption ent as (Figure d to reduce t er needs to p electrons that holes comin why this layer materials for ure that the qu -cell is aligne ell (Figure 4c

ple excitation g population; d, ch

ses and the m umber of sub- structure of a more) indepen

r, which are b of the wavele mpinge furth n spectra for e 4b). Materia the thermaliz provide a plat t are coming ng from the r is also calle intermediate uasi-Fermi lev

ed with that o c) or vice-ver

generation in QD hange in populati

maxi- -cells

typi- ndent based ength her to both als of zation tform from other ed the layer vel of of the rsa. A

very to b layer p-typ hole abov inter hole If sub- circu

VT

In ge

VT

CURRENT DSSCs (adapted

ion with time.

y thin layer of e used as th r is decompo pe material, j s and the oth ve the bottom rface of the p s and electron f VOC1 and V cells 1 and 2 uit voltage of

Tandem OC1

V =V +

eneral,

Tandem OC1

V =V +

T SCIENCE, VO from ref. 57).

f Ag (ref. 67) he intermedia osed into two just under th her layer ma m sub-cell to t - and n-type ns will take p VOC2 are the respectively f the tandem S

OV2. +V

OV2 OC3

V V

+ +

OL. 115, NO. 4, 2 a, Impact ioniz

), or Au (ref.

ate layer. Th o layers, one he top sub-ce ade of n-type transport elec materials, rec place.

e open circu , then ideally SCs will be

3+ ⋅⋅⋅.

25 AUGUST 20 zation;

68) is reporte he intermedia

layer made o ell to transpo e material, ju ctrons69. At th

combination o uit voltages o y the total ope

18

ed ate of ort ust he of of en

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T e c re th o g In C e

F ce

TiO2-based ph efficiency wa compared to

eported that i he maximum observed to b greater than t

n another stu Cu–ZnS was d enhance ligh

Figure 4. Tande ells; c, charge tra

hotoanode of as reported t single-layer in two- and t m power c be 3.2% and the three indi udy by Lee e deposited in t ht harvesting

em solar cell: a, ansport in tandem

the QDSSC.

o increase u r CdSeS. Th three-layered conversion e d 3.0% respe ividually laye et al.71, a pa tandem with g as well

structure; b, abs m SCs.

Power conve up to 1.97–2 he authors70

tandem QDS efficiencies ectively, whi

ered photoan assivation lay TiO2/CdS lay as suppress

sorption of its tw

ersion .81%

also SSCs,

were ch is nodes.

yer of yer to

the

wo sub-

surfa crea Vito both porte tion auth tion losse Cor Figu PbS

Figur ref. 7

ace of the ads se in cell eff oreti et al.72 al h CdS and Z ed to increase

compared to ors72 propose range and es with the el

e-shell solar ure 5 shows t quantum dot

re 5. Structure 5).

sorbed QDs i ficiency, but lso studied do ZnS in tandem

e by 600% w o the cells c ed that CdS ZnS layer r ectrolyte.

r cell the structure t is at the cor

e of core–shell

s reported to cell stability ouble layer pa m. Cell effic with double lay

containing on layer increas reduces the

of a core–sh re which is co

based QDSSC

have 350% in y is hampere

assivation wi ciency was r yer of passiv nly CdTe. Th ses the absorp recombinatio

hell SC, whe overed by Cd

(reproduced fro

n- d.

th re-

a- he

p- on

re dS

om

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6

s th c th s ra c su th s h s w h o ra c th Z k c s w re a b tw (C th 7

P F s a v h S tr

REVIEW A

64

hell. The ZnS he effect of b combine with hat may trap c

Even thoug olar spectrum apid cooling cooling of the ufficiently th he core and hell itself, w higher and lo low cooling which helps in hot hole in the of the core, ex ates a second core and CdS han what is o ZnS after dep keeping the co cells fixed at hell. A maxi was reported a

eported that alignment for by introducing

ween CdSe CdS)1 core/sh he maximum 76).

Plasmonic so For higher ab

tructure and a thinner stru vital paramete have an optica SC, which can rapping mech

ARTICLES

S coating give blocking elect h the electrol

carriers74. gh a typical S

m, most of th of the hot ca e hot carriers hick, the lowe higher energ which leads to ower-energy

of hot carrie n carrier mul e shell collide xciting it to cr d electron–hol shell and fou obtained with position. Selo ore size of Cd 1.65 nm and imum photoc at a shell thic a favourab better electro g a CdSexS1−

core and C hell QD-based m photoconver

olar cells bsorption, des

for higher ca ucture. Thickn

er to be optim ally thick but n be achieved hanism is in

Fig

es twofold ad trons73, which lyte, and coa SC absorbs a he absorbed arriers. In co s is slower. W er energy hol gy holes prim

o the electron holes. This ers in the cor

ltiplication. A es with a vale

ross the band le pair. Lai et und four times h simple PbS opal et al.76 c

dSe/CdS-base d varying the onversion eff ckness of ≈1.9 ble stepwise on transfer rat

−x interfacial CdS shell. C d QDSSCs ar rsion efficien

sign of SCs d arrier collecti

ness of the S mized. It is h

t physically t d by plasmoni ntroduced in

gure 6. Demon

dvantage of ha h possibly wi ating defect s

wide band o energy is lo re–shell struc When the sh es are confin marily stay in

nic decouplin is the reason re–shell struc A photo-gene ence band ele d gap, which g

t al.75 studied s higher effici QDs coated conducted a s ed core–shell thickness of ficiency of 3 96 nm. It was

electronic te can be achi

alloyed laye CdSe/(CdSexS re reported to ncy of 6.86%

demands a th on, the dema SCs is theref highly desirab

thin structure ic SCs where already desi

nstration of plasm

aving ill re- states of the st by cture, ell is ned to n the ng of n for cture, erated ectron gene- d PbS

iency with study solar f CdS .01%

s also band ieved er be- S1−x)5/

have (ref.

hicker and is fore a ble to e of a light igned

cells of li lead elect Th abso three 1.

path parti a pro one.

scatt back med ably That top o lowe tered place face cells tion both This ticle effic light over betw layer rear with of li tonic alon the p 2.

arou prob elect

CURRENT monic effect in S

s. Trapping ca ight at the s s to higher ab tron–hole pai he phenomen orption of ligh

e ways.

By scatterin length and h icles scatter li olonged path

If the surro tering takes kward directio

ium is inhom into the m t is the case of the SC (Fi er dielectric c d light gets tr

e at an angle . This trappe s, leading to p

by the cells.

h thin and th s mechanism

s. Larger nan ciency, while t more in th rcome by pla ween the glas r. Moreover,

end help to hin the absorb ght either in c waveguide

g the lateral d path length an

Absorption e und the plasm bability of el tric field79. T

T SCIENCE, VO SCs.

an produce co urface of the bsorption and

rs.

non of plasm ht into the SC ng light into t hence absorpt ight at resona h of light com

ounding med place unifor ons. On the c mogeneous, th medium with when metal n igure 6a), wi constant) as rapped inside

larger than th ed light acts prolonged pat It also leads hick cells on is influenced noparticles res smaller ones he forward d acing the nan ss substrate a plasmonic n o excite pro bing layer. Th

surface plasm mode. In ei direction of S nd hence impr enhancement monic nanopar ectron excita Thus, an inten

OL. 115, NO. 4, 2

omparatively h e sensitizer,

hence greate mon resonance

Cs mainly by the SCs, ther tion. When pl ance frequenc mpared to the ium is homo rmly in both contrary, if th hen light is sc higher diele nanoparticles ith air (or any

surrounding the SCs, if s he critical ang as a wavegu th length and

to higher cur n index-match by the size o sult in increas s are preferred direction. Th

noparticles a and transpare nanoparticles opagating wa his can lead mon polariton ither cases, l SCs, which in roves absorba

due to intens rticle. It is k ation is propo nsified electri

25 AUGUST 20

higher intensi which in tur er generation o e increases th y the followin reby increasin lasmonic nano cy, they lead e non-scattere ogeneous, the h forward an he surroundin cattered prefe ectric constan s are placed o y medium wi

medium. Sca scattering tak gle of the inte uide inside th d hence absorp rrent density hed substrate of the nanopa se of scatterin d for scatterin is trade-off at the rear en

ent conductiv placed at th aveguide mod to propagatio n mode or pho light is guide n turn increas

ance77,78. sified near fie

known that th ortional to th ic field aroun

18

ity rn of he ng ng o- to ed en nd ng er-

nt.

on th at-

es er-

he p- in es.

ar- ng ng is nd ve he de on o- ed

es ld he he nd

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p e li a fi th b to o th b la n ro h tr th p b b p c p le e p w b th e su m a re p e

plasmonic nan excitation in th ight energy a also proportio field always r his energy in ble only if the

or is greater otherwise the

he plasmonic by Hägglund

argely influen nanoparticles,

oughness of t 3. Decay o hole pair in th

ron-hole pair he plasmon r particle work band are prope be transferred plasmonic nan crease in the s particles are

eads to enhan established th plasmonic dec when injectio between the m

he other han energy, many

ufficient ene metals have h above the Fer

everse curren particle, whic efficiency of S

hancement o

noparticles (F he surroundin as plasmon os onal to the el results in incr nto the surrou e absorption r than the in

incident ligh nanoparticle

and co-work nced by the s

metal–semi the metal and

f plasmon re he metal nano

in the metal resonance dec k function an erly aligned, d to the sem

noparticle ch short circuit

not electrica ncement in th hat based on cay, efficienc on is throug metal nanopar

nd, because y of the exci ergy to over high density rmi energy. H nt flows from

ch might con SCs.

of SCs via introd

Figure 6b) re ng semicondu scillation. Op lectric field. S

reased absorp unding semico

coefficient of nelastic plas ht energy wi es. This theory

kers80–82. Thi hape and size iconductor i presence of m esonance by e

oparticles. E nanoparticles cays (Figure nd semicond then the elect miconductor, harged. This

current of the lly isolated he performanc

charge injec ies up to 7%

h a Schottk rticle and sem

of the broa ited electrons rcome the S

of unoccup However, if b

the semicond ntribute towa

duction of Ag nan

esults in enha uctor and can tical absorpti So a high ele ption. Couplin onductor is p f the semicon mon decay ill be absorbe

y has been pr is phenomeno e of the plasm interface, su metal oxide la exciting elec Excitation of s takes place w

6c). If the n ductor condu

tron generated thus leaving can cause a e SC, if the n from it, and ce of SCs83,84

tion generate can be achie ky barrier fo miconductor85 ad distributio s might not Scottky barrie pied energy s barrier is sm ductor to the m ards lowering

noparticles (repr

anced store ion is ectric ng of possi-

nduc- time;

ed in roved on is monic urface ayer.

tron–

elec- when nano- uction

d can g the

n in- nano- thus . It is ed by ved85 ormed

5. On on of

have er as states

all, a metal g the

Fi nopa gani and for p ratin and ficie meta exter 2.1 i the c To h al.86 more as pl MoO shor impr Re phot base InGa semi ptha such been high

Con Inco signi surfa gap up

roduced from ref

igure 7a show articles in ab

c SC with A found that bi predominant s ng a buffer la the metal nan ency of the S al nanoparticl

rnal quantum if optimum n cell and 30%

have greater suggested in e than 20 nm lasmonic nan O3/P3HT:PC6

t circuit curr roved up to 25 esearchers h tocurrent in d ed on CdSe88

aN/GaN93, In iconductors97 locyanine, a h as DSSCs99 n explored as hest quality fa

nclusion orporation of n

ificantly imp ace-to-volum

and multiple innumerable

f. 86).

ws the impact sorptance. Fl Ag nanopartic

gger size of m scattering. Th ayer of PED noparticles is SCs, compar les on ITO ( m efficiency w

nanoparticles

% increase in absorption ncorporation m. Aneesh et nomaterial in

61BM/Al, and rent density w

5%.

have demon different pho

8, CdS89 InP/

nGaAs/GaAs8

,98 such as and hybrid

9,100. Al, Au plasmonic na actor101.

nanostructure prove light a

e ratio offere exciton gene possibilitie

t of incorpora leetham et al cles as plasm metal nanopa hey also foun DOT:PSS bet s better in the red to direct (Figure 7b).

was enhanced size was inc photocurrent enhancement of Ag nanop al.87 used Au orgarnic SCs d achieved 16

while convers nstrated plas otovoltaics, in

/InGaAsP90,

4, c-Si95, a-S polythiophen organic–inor and Ag nano anomaterial, a

ed materials in absorption o ed by them. T eration in QD es in high-e

ation of Ag n l.86 studied o monic materi rticles is bett d that incorpo tween the IT e context to e adsorption o The maximu

in the scale o orporated wi

was achieve t, Fleetham partices of siz u nanoparticle s made of ITO 6% increase

sion efficienc smon-enhance ncluding tho

PbS91 GaAs9 i:H96, organ ne and copp rganic device oparticles hav and Ag has th

n QDSSCs ca wing to larg Tuneable ban Ds have opene efficiency S

a- or- ial ter o- TO

ef- of um of th ed.

et ze es O/

in cy ed

92se , nic er es ve he

an ge nd ed SC

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REVIEW ARTICLES

CURRENT SCIENCE, VOL. 115, NO. 4, 25 AUGUST 2018 666

design. Nanostructured TiO2 and ZnO are widely used as semiconducting material for the transport layer in QDSSCs. Though the efficiency obtained with TiO2 is better than ZnO at present, the latter is gaining popularity as it has the advantage of higher electron mobility (200 cm2/V/s) than the former (30 cm2/V/s). Many struc- tures of SCs have evolved based on QD sensitizers. Ener- gy loss due to thermalization of hot charge carriers and sub-bandgap transmission can be addressed using tandem structures. A tandem structure of TiO2/CdS/Cu–ZnS is reported to have an improved efficiency of 3.35%, which is 82% higher than TiO2/CdS-based QDSSCs71. It is re- ported that cell efficiency increases by 600% with double layer of passivation compared to cells containing only CdTe. In core–shell structure, cooling of the hot carriers is slower, which helps in carrier multiplication and hence improves efficiency. Lai et al.75 studied PbS core and CdS shell, and found four times higher efficiency than that obtained with simple PbS QDs coated with ZnS after deposition. CdSe/(CdSexS1−x)5/(CdS)1 core/shell QD- based QDSSCs are reported to have a maximum photo- conversion efficiency of 6.86% (ref. 76). An optically thick but physically thin structure of a SC is highly desir- able. It can be achieved by means of plasmonic SCs, where light trapping mechanism is introduced in already designed cells. This review will be helpful to SC enthu- siasts working on novel designs.

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Received 13 June 2017; revised accepted 6 June 2018 doi: 10.18520/cs/v115/i4/659-668

References

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