Acc. Chem. Res. 2000, 33, 269-277
Molecular Photovoltaics
ANDERS HAGFELDT† AND
MICHAEL GRÄTZEL*,‡
Departm en t of Ph ysical Ch em istry, An gström Solar Cen ter,
Un iversity of Uppsala, S75121 Uppsala, Sw eden , an d
In situ te of Ph oton ics an d In terfaces, Sw iss Federal In stitu te of
Tech n ology, CH-1015 Lau san n e, Sw itzerlan d
Received June 24, 1999
ABSTRACT
Th e dye-sen sitized n an ocrystallin e in jection solar cell em p loys
tran sition m etal com plexes for spectral sen sitization of m esoporous
TiO 2 film s togeth er with su itable redox electrolytes or am orp h ou s
organ ic h ole con du ctors. Ligh t h arvestin g occu rs efficien tly over
th e wh ole visible an d n ear-IR ran ge du e to th e very large in tern al
su rface area of th e film s. Ju diciou s m olecu lar en gin eerin g allows
th e p h otoin du ced ch arge sep aration to occu r qu an titatively with in
a few fem tosecon ds. Th e certified overall p ower con version efficien cy of th e n ew solar cell for AM 1.5 solar radiation stan ds
p resen tly at 10.4%.
Scien tists h ave been in fatu ated with p h otoelectric m olecu les ever sin ce th e discovery of p h otograp h y m ore th an
100 years ago. Th e first p an ch rom atic film , able to ren der
th e im age of a scen e realistically in to black an d wh ite,
followed on th e work of Vogel in Berlin after 1873,1 in
wh ich h e associated dyes with silver h alide grain s. Th e
first sen sitization of a photoelectrode was reported shortly
th ereafter, u sin g a sim ilar ch em istry.2 However, th e clear
recogn ition of th e p arallelism between th e two p rocedu res, th e realization th at th e sam e dyes can fu n ction in
both ,3 an d th e verification th at th e op eratin g m ech an ism
is by in jection of electron s from p h otoexcited dye m olecu les in to th e con d u ction ban d of th e sem icon d u ctor
substrates 4 date to the 1960s. In subsequen t years th e idea
develop ed th at th e dye cou ld fu n ction m ost efficien tly if
ch em isorbed on th e su rface of a sem icon du ctor.5,6 Th e
con cep t em erged to u se disp ersed p articles to p rovide a
su fficien t in terface,7 an d th en p articu late p h otoelectrodes
were em p loyed.8 Titan iu m dioxide becam e th e sem icon du ctor of ch oice.9 Th e m aterial h as m an y advan tages: it
is ch eap , abon dan t, n on toxic, an d biocom p atible, an d is
widely u sed in h ealth care p rodu cts as well as in p ain ts.
Th e stan dard dye at th e tim e was tris(2,2′-bip yridyl, 4,4′-
Anders Hagfeldt w as born in Norrkoping, Sw eden, in 1964. He received a Ph.D.
(1993) in physical chemistry from the University of Uppsala and w orked as a
postdoctoral fellow w ith Professor M ichael Grätzel at EPFL in Lausanne,
Sw itzerland. At present he is an Associate Professor at the Department of
Physical Chemistry, Uppsala University, and Program Secretary of Angstrom Solar
Center. He is also a fellow of LEAD (Leadership for Environment and Development).
M ichael Grätzel is a professor at the Sw iss Federal Institute of Technology in
Lausanne, Switzerland, where he directs the Institute of Photonics and Interfaces.
His laboratory initiated studies in the domain of nanocrystalline semiconductors
and mesoporous oxide semiconductor films and developed the dye-sensitized
nanocrystalline solar cell. Prof. Grätzel, who is the author of over 400 publications,
has received a number of aw ards and honorary lectureships including an
honorary doctors degree from the University of Uppsala, Sweden. He is a member
of several editorial boards.
10.1021/ar980112j CCC: $19.00
Published on Web 02/23/2000
 2000 Am erican Chem ical Society
FIGURE 1. Scanning electron microscope pattern of a typical
nanocrystalline TiO2 film.
carboxylate)ru th en iu m (II), th e fun ction of th e carboxylate
bein g th e attach m en t by ch em isorp tion of th e ch rom op h ore on to th e oxide su bstrate.8,9 In 1991,10 th e first
dye-sen sitized n an ocrystallin e solar cell with a con version
yield of 7.1% was an n ou n ced, an d p resen tly th e certified
efficien cy is over 10%.
At the heart of the device is the m esop orous oxide layer
com p osed of a n etwork of TiO 2 n an op articles wh ich h ave
been sin tered togeth er to establish electron ic con du ction
(Figu re 1). Th e p revailin g m orp h ologies of th e an atase
n an op articles are squ are bip yram idal, p seu docu bic, an d
stablike. Accordin g to tran sm ission electron m icroscop y
m easu rem en ts,11 th e (101) face is th e m ost exp osed,
followed by (100) an d (001) orien tation s. Attach ed to th e
su rface of th e n an ocrystallin e film is a m on olayer of th e
ch arge-tran sfer dye. Ph otoexcitation of th e latter resu lts
in th e in jection of an electron in to th e con du ction ban d
of th e oxide (Figu re 2). Th e origin al state of th e dye is
su bsequ en tly restored by electron don ation from th e
electrolyte, u su ally an organ ic solven t con tain in g th e
iodide/ triioide redox system . Th e regen eration of th e
sen sitizer by iodide in tercep ts th e recap tu re of th e con du ction ban d electron by th e oxidized dye. Th e iodide is
regen erated, in tu rn , by redu ction of triiodide at th e
cou n ter electrode, th e circu it bein g com p leted th rou gh
th e extern al load. Th e voltage gen erated u n der illu m in ation correspon ds to the differen ce between the Ferm i level
of th e electron in th e solid an d th e redox p oten tial of th e
electrolyte. Overall, electric p ower is gen erated with ou t
p erm an en t ch em ical tran sform ation .
A solid-state version of th e cell is th e sen sitized
h eteroju n ction (Figu re 3). Here, th e electrolyte is rep laced
†
‡
Un iversity of Up p sala.
Swiss Federal In stitu te of Tech n ology.
VOL. 33, NO. 5, 2000 / ACCOUNTS OF CHEM ICAL RESEARCH
269
M olecular Photovoltaics Hagfeldt and Grätzel
FIGURE 2. Schematic representation of the principle of the
nanocrystalline injection cell to indicate the energy level in the
different phases. The cell voltage observed under illumination
corresponds to the difference in the quasi-Fermi level of the TiO2
under illumination and the electrochemical potential of the electrolyte. The latter is equal to the Nernst potential of the iodide/triiodide
redox couple used to mediate charge transfer betw een the
electrodes.
by a wide ban d gap in organ ic sem icon du ctor of p -typ e
p olarity, su ch as Cu I12 or Cu SCN,13 or a h ole-tran sm ittin g
solid, e.g., an am orp h ous organ ic arylam in e.14 Th e excited
dye in jects electron s in the n -type oxide, an d it is regen erated by h ole in jection in th e p -typ e m arterial.
Th is Accou n t focu ses on th e m olecu lar en gin eerin g of
efficien t h eterogen eou s electron -tran sfer sen sitizers an d
scru tin izes th e factors th at afford qu an titative ch arge
separation at the dye-sem iconductor interface. The m echan ism of electron p ercolation across th e n an ocrystallin e
oxide film is also dicu ssed. Fin ally, p ersp ectives for fu tu re
develop m en t are p resen ted.
FIGURE 3. Dye-sensitized heterojunction solar cell where the
electrolyte in Figure 1 is replaced by an organic hole conductor.
Energy levels of the different redox species and TiO2 vs NHE are
show n. Electron-transfer reactions are symbolized by black arrows.
(NCS)2, kn own as th e N3 dye, h as em erged as th e
paradigm of a heterogen eous charge-tran sfer sen sitizer for
m esop orou s solar cells. Discovered in 1993,15 its p erform an ce h as been u n m atch ed by h u n dreds of oth er com p lexes th at h ave been syn th esized an d tested sin ce th en .
On ly recen tly, a credible ch allen ger h as been fou n d with
th e black dye tri(cyan ato)-2,2′,2′′-terp yridyl-4,4′,4′′-tricarboxylate)ru th en iu m (II) th at exh ibits better n ear-IR p h otoresp on se th an N3.18
Molecular Engineering of Heterogeneous
Charge-Transfer Sensitizers
Th e ideal sen sitizer for a sin gle-ju n ction p h otovoltaic cell
sh ou ld absorb all ligh t below a th resh old wavelen gth of
abou t 900 n m . In addition , it sh ou ld be firm ly grafted to
th e sem icon du ctor oxide su rface an d in ject electron s to
th e con du ction ban d with a qu an tu m yield of u n ity. Its
redox p oten tial sh ou ld be su fficien tly h igh th at it can be
regen erated rapidly via electron don ation from the eletrolyte or a h ole con du ctor. Fin ally, it sh ou ld be stable
en ou gh to su stain at least 10 8 redox tu rn overs u n der
illu m in ation corresp on din g to abou t 20 years of exp osu re
to n atu ral ligh t. Th e best p h otovoltaic p erform an ce in
term s of both con version yield an d lon g-term stability h as
so far been ach ieved with p olyp yridyl com p lexes of
ruthen ium an d osm ium .15-18 Sen sitizers havin g the gen eral
stru ctu re ML2(X)2, wh ere L stan ds for 2,2′-bip yidyl-4,4′dicarboxylic acid, M for Ru or Os, an d X for h alide,
cyan ide, thiocyan ate, or water,15,17 are particularly prom isin g. In recen t years, th e ru th en iu m com p lex cis-Ru L2270 ACCOUNTS OF CHEM ICAL RESEARCH / VOL. 33, NO. 5, 2000
Configuration of Surface-Anchored N3 Sensitizer. The
crystal stru ctu re of N3 is sh own in Figu re 4. Ru th en iu m
adop ts a distorted octah edral geom etry in th e com p lex,
on e of th e carboxylate grou p s, viz. C(17)O(1)O(2), bein g
skewed u p to 30° from th e ideal orien tation wh ich is
cop lan ar to th e bip yridyl m oiety. Th e th iocyan ate grou p s
are bou n d th rou gh th e n itrogen , alth ou gh th e S-bon ded
isom er does ap p ear as an in term ediate sp ecies an d as a
fin al im purity durin g N3 syn thesis.19 Surface derivatization
of th e m esop orou s oxide film is n orm ally p erform ed by
dip p in g it in to a solu tion of N3 in a 50/ 50 (v/ v) solven t
m ixtu re of aceton itrile/ tert-bu tan ol. A m on olayer of sen sitizer is form ed sp on tan eou sly th rou gh attach m en t via
th e carboxylic acid an ch orin g grou p s. Th e adsorp tion
follows a Lan gm u ir isoth erm . Th e b in d in g con stan t is
M olecular Photovoltaics Hagfeldt and Grätzel
FIGURE 4. X-ray crystal structure of N3.
5 × 104 M -1, an d th e area occu p ied by on e N3 m olecu le
at th e an atase su rface at fu ll m on olayer coverage is 1.65
n m 2.
Th e in teraction between th e carboxylic grou p an d th e
TiO 2 is of fu n dam en tal im p ortan ce in determ in in g th e
geom etrical stru ctu re of th e adsorbed dye state an d
in flu en cin g th e electron ic cou p lin g with th e Ti(3d) con du ction ban d orbital m an ifold. Molecu lar dyn am ics calcu lation s h ave been u sed to m odel th e in teraction of th is
sen sitizer with th e (101) su rface p lan e of an atase.20 Th e
m ost likely con figuration supported by recen t IR an alysis 21
is sh own in Figu re 5. Th e dye is attach ed via two of its
fou r carboxylate grou p s. Th e carboxylate eith er bridges
two adjacen t rows of titan iu m ion s th rou gh biden tate
coordin ation or in teracts with su rface h ydroxyl grou p s
th rou gh h ydrogen bon ds. Of th e two rem ain in g carboxylate grou p s, on e is ion ized wh ile th e oth er rem ain s in
th e p roton ated state.
Model stu dies, u sin g th e L ligan d of N3 adsorbed on to
sin gle-crystal TiO 2 (110) ru tile, in vestigated by m ean s of
X-ray p hotoelectron spectroscopy, X-ray absorption spectroscop y, an d qu an tu m ch em ical calcu lation s,22 are also
in favor of the bridgin g biden tate con figuration . The ligan d
is orien ted at an an gle of abou t 40° with resp ect to th e
(001) crystallograp h ic direction . In terestin gly, from th e
calculation s it was foun d that, in addition to th e biden tate
carboxylate-titan ium lin kage described above, the m on oden tate ester bon d is also th erm odyn am ically stable.
However, th e form er bon din g is stron ger an d th u s wou ld
be th e p referred bin din g con figu ration for th e N3 dye.
Energy Levels and Orbital Configuration. Th e in terfacial electron -tran sfer even ts will be stron gly affected by
th e electron ic stru ctu re of th e dye in th e adsorbed state
an d th e en ergy level m atch in g between its excited state
an d the con duction ban d of the sem icon ductor. Gen erally,
the optical tran sition of Ru com plexes has m etal-to-ligan d
ch arge-tran sfer (MLCT) ch aracter. Excitation of th e dye
in volves tran sfer of an electron from th e m etal to th e π*
orbital of th e ligan d. N3 h as two su ch tran sition s in th e
visible region . Th e absorp tion m axim a in eth an olic solu tion are located at 518 an d 380 n m , th e extin ction
coefficien ts bein g 1.33 × 104 an d 1.3 × 104 M -1 cm -1,
resp ectively. Th e com p lex em its at 750 n m , th e excitedstate lifetim e bein g 60 n s.15
Resu lts from an ab in itio calcu lation of a decarboxylated N3 com p lex are p resen ted in Figu re 6, sh owin g th e
highest occupied (HOMO) and lowest unoccupied (LUMO)
m olecular orbital surfaces.23 We n ote that the HOMO level
is actu ally sh ared by both th e Ru m etal an d th e NCS
ligan ds. Ph otoelectron sp ectroscop y h as been u sed to
stu dy th e den sity of states of th e N3 valen ce levels by
p h oton en ergy an d an gu lar-dep en den t sp ectra.24 Th ese
exp erim en ts con firm th at both Ru 4d an d atom ic orbitals
cen tered on th e -NCS grou p s, in p articu lar S3p wave
fu n ction s, con tribu te to th e fron tier orbitals of th e com plex. In a photovoltaic cell, the oxidized dye, after electron
in jection to th e con du ction ban d of th e oxide, sh ou ld
qu ickly be redu ced by a redox sp ecies in th e su rrou n din g
electrolyte. Th e observation th at th e ou term ost orbital
con tain s a su bstan tial am ou n t of S3p -ch aracter from th e
VOL. 33, NO. 5, 2000 / ACCOUNTS OF CHEM ICAL RESEARCH
271
M olecular Photovoltaics Hagfeldt and Grätzel
FIGURE 6. (a) Calculated HOM O and (b) LUM O molecular orbital
surfaces for Ru(bpy)2(NCS)2.
FIGURE 5. Anchoring of the N3 dye on the (101) surface of anatase.
-NCS grou p s m ay p lay an im p ortan t role in th is p rocess.
Th e -NCS grou p s p oin t in th e direction of th e electrolyte,
which m ay facilitate reduction by I-, m akin g it particularly
suitable for highly efficien t solar cells. The LUMO in Figure
272 ACCOUNTS OF CHEM ICAL RESEARCH / VOL. 33, NO. 5, 2000
6 is con cen trated on th e π* stru ctu re of th e ligan ds. Th u s,
th e absorp tion ban ds of Ru (bp y)2(NCS)2 can be assign ed
to a Ru NCS-bp y(π*) tran sition . Prelim in ary calcu lation s
of th e carboxylated N3 in dicate th at th e bp y rin gs sh are
th eir LUMO with th e COOH grou p s, en h an cin g electron ic
cou p lin g to th e TiO 2 con du ction ban d states.
Th e en ergetics at th e dye/ m etal oxide was sum m arized
in ou r p reviou s review article,25 in wh ich data were taken
from differen t optical an d electrochem ical m easurem en ts.
Recen tly, th e valen ce level sp ectra from u n sen sitized as
well as sen sitized n an ostru ctu red TiO 2 electrodes h ave
been directly derived from p h otoelectron sp ectroscop ic
m eth ods.22 Figu re 7 sh ows th e den sity of states in th e
valen ce region for an N3-derivatized n an ostru ctu red TiO 2
film togeth er with an en ergy level diagram . Th e HOMO
orbital of th e com p lex can clearly be distin gu ish ed above
M olecular Photovoltaics Hagfeldt and Grätzel
FIGURE 8. Schematic of the kinetics at the TiO2/N3/electrolyte
interface.
FIGURE 7. Valence band spectrum of a nanostructured TiO2 film
sensitized with N3.
th e valen ce ban d edge of th e sem icon du ctor. Its p osition
fits well with the en ergy requirem en ts for efficien t electron
in jection . Th e level of th e vibron ic state p rodu ced by 530n m excitation of th e dye is abou t 0.25 eV above th e
con du ction ban d edge. For th e 0-0 tran sition of N3 (∆E
) 1.65 eV15), the excited-state level m atches the lower edge
of th e con du ction ban d . Th is en ergy was estim ated by
u sin g a differen ce in th e valen ce ban d edge p osition an d
HOMO level position taken at the m axim um of the den sity
of states of 1.2 eV (with an u n certain ty of abou t 0.1 eV),
an absorp tion m axim u m for th e dye of 2.3 eV, an d a ban d
gap for an atase TiO 2 of 3.2 eV. Th is p ossibility of determ in in g th e p osition of th e ou term ost dye level in th e
sem icon du ctor ban d gap is of p articu lar in terest in
com p arin g differen t typ es of dyes in th e search for
ch rom op h ores h avin g op tim ized en ergy level m atch in g
between th e dye an d th e oxide an d th e dye an d th e redox
cou p le.
Fem tosecond Electron Injection. On e of th e m ost
astou n din g fin din gs over th e p ast few years con cern s th e
rate of electron in jection from th e excited N3 dye in th e
TiO 2 con du ction ban d. Alth ou gh assign m en ts of tran sien t
sp ectra are still u n der debate,26-29 it is n ow gen erally
accep ted th at th is in terfacial redox reaction is on e of th e
fastest ch em ical p rocesses kn own to date occu rrin g in th e
fem tosecon d tim e regim e. In a recen t elegan t exp erim en t,30 m id-IR sp ectroscop y was u sed to p robe directly
th e bu ildu p of electron con cen tration in side th e sem icon du ctor. Carefu l exam in ation at differen t wavelen gth s
an d tim e scales yield ed a d ou ble exp on en tial with rise
tim es of 50 ( 25 fs (>84%) an d 1.7 ( 0.5 p s (<16%). Th e
slower com p on en t was very sen sitive to th e sam p le
con dition , an d th e exact origin is still u n kn own . A ch allen ge for fu tu re research on th ese m olecu lar p h otovoltaic
system s is to explain the reason s for the ultrafast in jection .
It ap p ears also th at th e an ch orin g carboxylic grou p s do
n ot h ave to be con ju gated to th e π electron system of th e
chrom ophore for efficien t electron tran sfer 31 to take place.
Th ere is a n eed for detailed th eoretical m odelin g of th e
kin etics. Fu rth er exp erim en tal stu dies will focu s on th e
participation of hot vibron ic states in the electron in jection
which m an ifests itself by wavelen gth-depen den t quan tum
yields.32,33
The Recapture of the Injected Electron and Its Interception. Th e kin etics of back-electron -tran sfer kin etics
from the con duction ban d to the oxidized sen sitizer follow
a m u ltiexp on en tial tim e law, occu rrin g on a m icrosecon d
to m illisecon d tim e scale. Th e reason s su ggested for th e
relatively slow rate of th e back-reaction tran sfer reaction
are (i) weak electron ic cou p lin g between th e electron in
th e solid an d th e Ru (III) cen ter of th e oxidized N3, (ii)
trap p in g of th e in jected electron , an d (iii) th e kin etic
im p edim en t du e to th e in verted Marcu s region .34 Du rran t
an d co-workers h ave p resen ted som e n ew data on th is
issu e, sh owin g th e reaction to be stron gly dep en den t on
ap p lied p oten tial.35 Th is m ay be of relevan ce for th e
p erform an ce of th e cell an d sh ou ld be con sidered in th e
m odelin g of th e electrical p erform an ce togeth er with th e
reaction between I3- an d con duction ban d electron s.36 The
latter p rocess h as been fou n d to be secon d order with
resp ect to th e I3- con cen tration with a tim e con stan t of
abou t 10 m s at 1 su n .37
Th e in tercep tion of th e oxidized dye by th e electron
don or in the electrolyte, i.e., iodide, is crucial for obtain in g
good collection yields an d h igh cycle life of th e sen sitizer.
For N3, tim e-resolved laser exp erim en ts h ave sh own th e
in tercep tion to take p lace with in abou t 10 n s u n der th e
con dition s ap p lied in th e solar cell. Th e N3/ N3+ cou p le
sh ows reversible beh avior in differen t organ ic solven ts,
the stan dard redox poten tial in aceton itrile bein g E° ) 0.83
V vs SCE.38 Th e lower lim it of 1 s can be derived for th e
lifetim e of th e oxidized dye from cyclic voltam m etry. Th is
m ean s th at th e in tercep tion is 10 8 tim es faster th an th e
in trin sic lifetim e of th e oxidized sen sitizer, exp lain in g th e
fact th at N3 can su stain 100 m illion tu rn overs in con tin u ou s solar cell op eration with ou t loss of p erform an ce. Lack
of adequ ate con dition s for rap id regen eration of th e dye
leads to dye degradation . A su m m ary of th e p ossible
electron -tran sfer p ath ways at th e dye/ sem icon du ctor
in terface togeth er with observed kin etic data is sh own in
Figu re 8.
Electron Percolation through the Mesoporous
Oxide Film
Wh en th e dye-sen sitized m esop orou s solar cell was first
p resen ted, p erh ap s th e m ost p u zzlin g p h en om en on was
th e h igh ly efficien t ch arge tran sp ort th rou gh th e n an ocrystallin e TiO 2 layer. Th e m esop orou s electrodes are very
VOL. 33, NO. 5, 2000 / ACCOUNTS OF CHEM ICAL RESEARCH
273
M olecular Photovoltaics Hagfeldt and Grätzel
m u ch differen t com p ared to th eir com p act an alogu es
becau se (i) th e in h eren t con du ctivity of th e film is very
low, (ii) th e sm all size of th e in dividu al colloidal p articles
does n ot su p p ort a bu ilt-in electrical field, an d (iii) th e
oxide p articles an d th e electrolyte-con tain in g p ores form
in terp en etratin g n etworks wh ose p h ase bou n daries p rodu ce a ju n ction of h u ge con tact area. Th ese film s m ay be
viewed as an en sem ble of in dividu al p articles th rou gh
wh ich electron s can p ercolate by h op p in g from on e
crystallite to th e n ext, rath er th an regardin g th em as
p erforated com p act electrodes,25 su ggestin g a bottom -u p
ap p roach to ration alizin g th e tran sp ort p h en om en a.
Ch arge tran sp ort in m esop orous system s is un der keen
debate today,39-43 an d on ly th e m ost im p ortan t qu estion s
can be addressed h ere. How are th e electron s m ovin g? Is
th eir flow th rou gh th e n an op article film to th e collector
electrode m erely diffu se or is it driven by an electric field?
Is th e electric cu rren t sp ace ch arge con trolled? A first
attem p t to m odel carrier tran sp ort in n an ocrystallin e TiO2
film s suggested diffusion to be the operative m echan ism .39
However, it tu rn ed ou t to be erron eou s to describe th e
electron m otion by a fixed valu e for th e diffu sion coefficien t. Th e tran sp ort is com p lex as it in volves trap p in g
an d detrap p in g of electron s. Th e trap s h ave differen t
dep th s, leadin g to a distribu tion of trap p in g an d detrap p in g tim es. Wh ich typ e of trap th e electron visits du rin g
its ran dom walk th rou gh th e oxide film dep en ds on its
qu asi-Ferm i level u n der illu m in ation , i.e., on th e ligh t
in ten sity.40 At low ligh t levels, deep trap s p articip ate in
th e electron m otion , an d a slow tran sp ort, with a corresp on din gly low diffu sion coefficien t D(e-) is exp ected.
In creasin g the light in ten sity m ean s that deeper trap states
are filled un der steady-state con dition s. Th e tran sp ort will
be faster sin ce it in volves on ly sh allow trap s, resu ltin g in
a h igh er valu e for D(e-). Th e cen tral im p ortan ce of trap
states in th ese system s h as recen tly been discu ssed by
Nelson ,43 wh o ap p lied a disp ersive tran sp ort m odel based
on th e con tin u ou s-tim e ran dom -walk th eory of Sch er an d
Mon troll.44
An other curren tly debated issue con cern s space charge
con trol of th e p h otocu rren t. It is gen erally assu m ed th at
th e n egative ch arge of th e m ovin g electron is efficien tly
screen ed by cation s in th e electrical dou ble layer su rrou n din g th e sem icon du ctor n an op articles, m akin g it
m ove with its im age ch arge as an essen tially n eu tral
sp ecies. However, th ere is eviden ce th at th e ch arge
com p en sation on th e electrolyte side of th e ju n ction can
lag beh in d th e electron m ovem en t, n otably in ion -p aired
organ ic electrolytes wh en h igh p h otocu rren ts are drawn .
Th u s, in p h otocu rren t tran sien t m easu rem en ts it was
observed th at th e p h otocu rren t resp on se tim es becam e
lon ger with decreasin g electrolyte con cen tration .45 Also,
th e calcu lated valu e of th e effective diffu sion coefficien t,
1.5 × 10 -5 cm 2/ s, is several orders of m agn itu de sm aller
th an th at in th e bu lk crystallin e m aterial an d strikin gly
sim ilar to th e diffu sion con stan t of ion s in th e solu tion .
Th e dyn am ics an d m olecu lar descrip tion of th e screen in g
p rocess are issu es of great in terest for fu tu re stu dies. We
n ote th at m ass tran sp ort in th e electrolyte in m esop orou s
274 ACCOUNTS OF CHEM ICAL RESEARCH / VOL. 33, NO. 5, 2000
FIGURE 9. Spectral response curve of the photocurrent for the
DYSC sensitized by N3 and the black dye. The incident photon to
current conversion efficiency is plotted as a function of wavelength.
system s h as been m odeled 46 an d a descrip tion of a
cou p led electron ic-ion ic m otion p resen ted,36 albeit by
em p loyin g a con stan t valu e for th e electron diffu sion
coefficien t.
Th u s, alth ou gh a lot of data an d ideas con cern in g th e
ch arge tran sp ort p rocess in m esop orou s film s h ave been
p resen ted, th e p ictu re is far from bein g com p lete. Wh ere
are th e electron s (su rface or bu lk)? How m an y electron s
are th ere p er p article? Sch lich th örl et al.37 h ave calcu lated
th e steady-state carrier con cen tration in fu ll su n ligh t to
corresp on d to abou t on e electron p er TiO 2 p article.
However, u sin g th is valu e togeth er with a diffu sion
coefficien t of 1.5 × 10 -5 cm 2/ s, on e obtain s a resistan ce
for th e illu m in ated n an ocrystallin e film wh ich is at least
1000 tim es higher than the experim en tally observed value.
Th is discrep an cy m ay be exp lain ed by th e recen t su ggestion th at th e p h otodop in g of th e an atase p articles resu lts
in a Mott tran sition , stron gly in creasin g th eir con du ctivity.47 Clearly, th e cen tral qu estion wh ich rem ain s to be
an swered is h ow, in th e dye-sen sitized liqu id-ju n ction
solar cell, an in itially very p oorly con du ctin g n etwork of
an atase n an op articles can attain th e excellen t p h otocu rren t-voltage ch aracteristics, i.e., fill factor of 0.7 at a
cu rren t den sity of 20.5 m A/ cm 2, p resen ted below.
Photovoltaic Performance
Figu re 9 com p ares th e sp ectral resp on se of th e p h otocu rren t observed with th e N3 an d th e black dye sen sitizer.
Th e in ciden t p h oton to cu rren t con version efficien cy
(IPCE) of the solar cell is plotted as a fun ction of excitation
M olecular Photovoltaics Hagfeldt and Grätzel
will be u sed to in crease th e extin ction coefficien t su bstan tially in th e 700-900 n m region . Th e goal is to obtain
a p h otovoltaic cell h avin g op tical featu res sim ilar to th ose
of GaAs. A n early vertical rise of th e p h otocu rren t close
to th e 920 n m absorp tion th resh old wou ld in crease th e
sh ort-circu it p h otocu rren t from cu rren tly 20.5 to abou t
28 m A/ cm 2. Th is cou ld raise th e overall efficien cy to over
15%.
Future Outlook
FIGURE 10. Photocurrent-voltage characteristic of a nanocrystalline photoelectrochemical cell sensitized w ith the panchromatic
“ black” dye. The results plotted were obained at the NREL calibration
laboratory.
wavelen gth . Both ch rom op h ores sh ow very h igh IPCE
valu es in th e visible ran ge. After correction for losses du e
to ligh t reflection an d absorp tion by th e con du ctin g glass,
th e con version of p h oton s to electric cu rren t is p ractically
qu an titative in th e p lateau region of th e cu rves. However,
th e resp on se of th e black dye exten ds 100 n m farth er in to
th e in frared th an th at of N3. Th e p h otocu rren t on set is
close to 920 n m , i.e., n ear th e op tim al th resh old for sin gle
ju n ction con verters. From th ere on , th e IPCE rises gradu ally u n til at 700 n m it reach es a p lateau of ca. 80%. If on e
accou n ts for reflection an d absorp tion losses in th e
con du ctin g glass, th e con version of in ciden t p h oton s to
electric cu rren ts is p ractically qu an titative over th e wh ole
visible dom ain . From th e overlap in tegral of th e cu rves
in Figu re 9 with th e solar em ission sp ectru m , on e p redicts
th e sh ort-circu it p h otocu rren ts of th e N3 an d black dyesen sitized cells to be 16.5 an d 20.5 m A/ cm 2, resp ectively,
in agreem en t with exp erim en tal observation s. Th e overall
efficien cy (ηglobal) of the photovoltaic cell is calculated from
th e in tegral p h otocu rren t den sity (i p h ), th e op en -circu it
p h otovoltage (Voc ), th e fill factor of th e cell (ff), an d th e
in ten sity of th e in ciden t solar ligh t (Is ) 1000 W/ m 2):
ηglobal ) i p h Voc (ff)/ Is
At th is stage th e con firm ed efficien cy obtain ed with th e
black dye is 10.4% u n der stan dard air m ass 1.5 rep ortin g
con dition s, as con firm ed by th e PV calibration laboratory
of the National Energy Research Laboratory (NREL, Golden
CO). A typ ical I-V cu rve is sh own in Figu re 10. Fu rth er
develop m en t will con cen trate on th e en h an cem en t of th e
p h otoresp on se in th e n ear-IR region . Ju diciou s m olecu lar
en gin eerin g of th e black dye stru ctu re, e.g., th e su bstitu tion of th e carboxylic acid by p-carboxylp h en yl grou p s,
Masterin g th e in terface is th e first ch allen ge for fu tu re
developm en t of m olecular photovoltaics. The high con tact
area of th e ju n ction n an ocrystallin e solar cells ren ders
m an datory th e grasp an d m olecu lar con trol of su rface
effects for fu tu re im p rovem en t of cell p erform an ce. Syn th etic efforts will focu s on th e design of sen sitizers, su ch
as m olecu lar dyads,48 en h an cin g th e ch arge sep aration at
the oxide-solution in terface. The structural features of the
dye sh ou ld m atch th e requ irem en ts for m olecu lar rectification . In an alogy to th e p h otofield effect in tran sistors,
th e gate for u n idirection al electron flow from th e redox
electrolyte th rou gh th e ju n ction an d in to th e con du ction
ban d of th e oxide is op en ed by th e p h otoexcitation of th e
sen sitizer. Th e reverse ch arge flow, i.e., recap tu re of th e
electron by th e electrolyte, sh ou ld be im p aired by ju diciou s design of th e sen sitizer: wh ile con du ctin g electron s
in the excited state, it should be an in sulator in the groun d
state form in g a m olecu lar blockin g layer at th e h eteroju n ction between th e sem icon du ctor an d th e electrolyte
or th e solid h ole con du ctor. Molecu lar gatin g wou ld
ben efit greatly th e efficien cy of th e p h otocell. Even a
relatively sm all in crem en t, of e.g. 200 m V, in th e op en circu it voltage wou ld allow th e overall solar con version
yield to be raised from cu rren tly 10.4 to n early 15%.
Pan ch rom atic sen sitization exten din g th rou gh ou t th e
visible an d n ear-IR region s is an oth er ch allen ge for th e
fu tu re. Dye cocktails h ave already been ap p lied to m esop orou s TiO 2 film s in th e form of m ixtu res of p orp h yrin s
an d p h th alocyan in es.48 Th e resu lt was en cou ragin g in asm u ch as th e op tical effects of th e two sen sitizers were
fou n d to be additive an d th ere was n o destru ctive in terferen ce of th e dyes. Th ere are m an y op tion s for dyes th at
cou ld be com bin ed to im p rove th e p h otocu rren t of th e
device, op en in g up a fertile field for furth er in vestigation s.
An advan tage of dye-sen sitized solar cells is th at th ey
can be u sed to p rodu ce directly h igh -en ergy ch em icals
from su n ligh t. Su ch “p h otosyn th etic” devices can overcom e th e p rin cip al p roblem of all p h otovoltaic cells, i.e.,
th e lack of cap acity for en ergy storage. Th e “Holy Grail”
of all p h otocon version p rocesses is th e sp littin g of water
in to h ydrogen an d oxygen by su n ligh t, an d th ere is n o
dou bt th at th is will be on e of th e p rim ary targets of fu tu re
research .
M.G. expresses his gratitu de to co-w orkers w ho have con tribu ted
to the progress in the research on dye-sen sitized solar cells. Su pport
of th is w ork by th e Sw iss Nation al Scien ce Fou n dation as w ell as
th e Sw iss Nation al En ergy Office is gratefu lly ack n ow ledged.
Th an k s are also du e to th e in du strial licen sees of EPFL, in
VOL. 33, NO. 5, 2000 / ACCOUNTS OF CHEM ICAL RESEARCH
275
M olecular Photovoltaics Hagfeldt and Grätzel
particu lar th e In stitu te of Applied Ph otovoltaics in Gelsen kirch en ,
Germ an y, for th eir collaboration an d fin an cial con tribu tion . A.H.
th an k s th e Fou n dation for strategic en viron m en tal research
(MISTRA) an d th e Sw edish n ation al en ergy adm in istration for
fin an cial su pport an d th e research ers w ith in th e con sortiu m of
m esoporou s system s at Uppsala Un iversity for valu able com m en ts
an d assistan ce in draw in g som e of th e figu res.
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