~ ~ ~

ELSEVIER Solar Energy Materials and Solar Cells 38 (1995) 187-198

Solar Energy Materials and Solar Cells

Photosensitization of wide bandgap semiconductors with antenna molecules

Carlo Alberto Bignozzi a,*, Roberto Argazzi a, Jon. R. Schoonover b,

G.J. Meyer c, Franco Scandala a

Abstract

a Department of Chemistry, University of Ferrara, 44100 Ferrara, Italy b Los Alamos National Laboratory, Los Alamos, NM 87545, USA

c Department of Chemistry, fohns Hopkins University, Baltimore, MD 21218, USA

Polynuclear metal complexes, supporting efficient intramolecular energy transfer pro­cesses, can be used to increase the light harvesting efficiency of sensitized wide bandgap semiconductors. Experimental studies are discussed to emphasize: (i) how structural changes at the molecular level may affect the performances of photoelectrochemical cells based on antenna-sensitizer molecular assemblies, (ij) the availability of fast time-resolved resonance Raman and infrared spectroscopies for monitoring intercomponent energy transfer pro­cesses, and (iii) the possibility to design extended antenna units acting as molecular conduits for long-range energy transfer.

1. Introduction

The search for efficient solar energy conversion devices based on dye sensitiza­tion of wide bandgap semiconductors has recently experienced a sharp accelera­tion, due to the development of new nanocrystalline semiconductor electrodes of very high surface area [1,2]. Based on nanocrystalline TiOz electrodes, efficient photoelectrochemical cells for the conversion of visible light energy into electricity have been constructed [2,3]. The basic steps taking place in a sensitized semicon­ductor cell are shown in Fig. l.

The photovoltaic device consists of a molecular sensitizer chemically anchored to the semiconductor surface, a solution containing a relay electrolyte, and a metallic counterelectrode (Fig. la). The photosensitizer is first excited by light

• Corresponding author.

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188 C.A. Bignozzi et aL / Solar Energy Materials and Solar Cells 38 (J995) 187-198

, , , ,

;/ , , , , ,

, , , , , , , , ,

----EJ , , @] ---- Sens ReI , , ReI , , . , ,

, , , , , , , , , , , ,

a b

, , , , , ,

, , , " e ,

, , ,

----~M , , ,

ReI , , , ,

, , , , , , , , , , , ,

c d

, , , , , , , , , ,

, e" , , , e ,

, , ,

- ----8 Rei + Rel +

, , , , , , , , , , , ,

e f

Fig. 1. Schematic representation of the steps involved in a regenerative photoelectrochemical cell for light energy conversion based on dye sensitization of semiconductors.

absorption (in this example the excitation is a metal-to-ligand charge transfer) (Fig. Ib). The excited sensitizer then injects an electron into the conduction band of the semiconductor (Fig. Ic). The oxidized sensitizer is reduced by a relay molecule (Fig. Id), which then diffuses to discharge at the counterelectrode (Fig. Ie). As a result, a photopotential is generated between the two electrodes (Fig. 1f) in

CA. Bignozzi et al. / Solar Energy Materials and Solar Cells 38 (J995) 187-198 189

open-circuit conditions, and a corresponding photocurrent can be obtained upon closing the external circuit through an appropriate load.

A useful molecular sensitizer for applications in a regenerative photoelectro­chemical cell must, therefore, fulfil several important requirements, among which: (a) the ability to absorb firmly on the semiconductor; (b) high stability in both the oxidized and excited state; (c) efficient light absorption in the visible region; (d) a long-lived electronically excited state; (e) an excited-state redox potential negative enough for electron injection into the conduction band of the semiconductor; (f) a ground-state redox potential as positive as possible, compatible with (c) and (e); and (g) low kinetic barriers (small reorganizational energy) for excited- and ground-state redox processes, so as to minimize free energy losses in primary and secondary electron transfer steps.

A key parameter of cell performances is the incident photon to current efficiency, IPCE, which directly measures how efficiently incident photons are converted to electrons. The IPCE is the product of the quantum yield for charge injection (<P), the efficiency of collecting electrons in the external circuit (7]), and the fraction of radiant power absorbed by the material or the "light harvesting efficiency" (LHE),

IPCE = (LHE)( <P)( 7]).

One possible strategy to increase the light harvesting efficiency is to use antenna sensitizer molecular assemblies. We have been interested for some time in the study of the photoinduced energy and electron transfer processes that take place within polynuclear metal complexes [4]. These are supramolecular [5] systems made up of a discrete number of metal complex units connected by suitable bridging ligands. In this article we will point out some of the ways in which the use of a supramolecular species (instead of a simple molecular species) as sensitizer can enhance the performance of photoelectrochemical cells for light energy conversion. We will present a few examples of the types of polynuclear complexes that can be designed for such purposes, and of some experimental techniques that must be applied to verify the complex sequence of elementary steps taking place in these systems.

2. Antenna-sensitizer molecular devices

With a conventional semiconductor electrode and a simple molecular sensitizer, light absorption is often quite inefficient even at monolayer coverage. On the other hand, multilayer adsorption does not help, as the inner layers tend to act as insulators with respect to the outer ones [6]. Although this type of limitation is now much less severe thanks to the introduction of nanostructured electrodes of exceptionally high surface area [2,3], the search for sensitizers with high intrinsic light harvesting efficiency is still of considerable interest in the field.

One possibility in this direction is to replace the sensitizer molecule at the semiconductor-solution interphase with some type of an antenna-sensitizer molecu­lar assembly [7] (e.g., the simple triad systems of Fig. 2).

190 CA. Bignozzi et al. / Solar Energy Materials and Solar Cells 38 (J995) 187-198

hv

hv

a

b

Fig. 2. Schematic representation of branched (a) and one dimensional (b) antenna-sensitizer molecular devices.

In principle, such a device can take advantage of efficient intercomponent energy transfer from a number of "antenna" chromophoric units to a specific chromophoric unit which behaves as an energy collector and a charge injection sensitizer. In this way, the light energy absorbed by all the chromophoric compo­nents can be used to effect charge injection, with an increase in the overall cross-section for light absorption compared to the case of a simple molecular sensitizer. As a further advantage, an appropriate selection of the spectral proper­ties of the antenna and sensitizer chromophores can lead to better matching between absorption spectrum and solar emission spectrum.

In a first attempt to demonstrate the feasibility of such an approach, the [NC-Ru(bpY)2-CN-Ru(bpy-4,4'(COO)2)2-NC-Ru(bpY)2-CNF- trinucIear complex (Fig. 3) has been developed [7].

Efficient energy funnelling from the peripheral chromophores to the central -Ru(bpy-4,4'(COO)2)2-unit in this trinucIear complex was demonstrated by con­ventional photophysical experiments [7] and in particular by time-resolved reso­nance Raman [8]. The transient Raman spectra of Ru(bpY)2(CN)2' Ru(bpy-4,4'(COO)2MCN)~- and of the trinucIear complex [NC-Ru(bpY)2-CN-Ru(bpy-

CA. Bignozzi et al. / Solar Energy Materials and Solar Cells 38 (J995) 187-198 191

a

hv ~ Ch

b

Ch'

+

Ch

hv' ~

hv ~

Fig. 3. Schematic structure (a) and behaviour as an antenna-sensitizer molecular device (b) of the trinuclear complex [NC-Ru(bpy )2-CN-Ru( 4,4' -(COO )2-bpy )2-NC-Ru(bpy )2-CNF -.

4,4'(COO)2)2-NC-Ru(bpY)2-CNF- measured at room temperature in acetonitrile are shown in Figs. 4a-4c. The frequencies of the Raman bands are listed in Table l.

These spectra were obtained under identical conditions using 354.7 nm laser pulses to both excite the sample and interrogate the excited state. The spectrum of Ru(bpy)zCCN)2 (Fig. 4B) shows enhancement of (bpy·-) modes as observed for Ru(bpy)~+· [8]. The transient Raman spectrum of the trinuclear complex (Fig. 4A) lacks the typical intense Raman bands for (bpy·-) at 745, 1212, 1285, 1424, and

192 CA. Bignozzi et al. / Solar Energy Materials and Solar Cells 38 (J995) 187-198

B

800 1000 1200 1400 1600 Wavenumber (em· I)

Fig. 4. Transient resonance Raman spectra of: (A) [NC-Ru(bpY)2-CN-Ru(4,4'-(COO)2-bpY)2-NC­Ru(bpY)z-CN]2- in CH 3CN, (B) Ru(bpy>z(CN)2 in CH3CN, and (C) [Ru(4,4'(COO)2-bpY)2(CN)2]2- in H 20 at pH = 7. The spectra were measured with 354.7 nm laser pulses for excitation and as a source for the Raman scattering following excitation. The spectra are result of 10 min. total integration time and the Raman band energies are listed in Table 1.

1549 em -1. Bands due to ground state bpy vibrations are evident in the spectrum at 1561 and 1605 ern -1. Clear evidence that energy transfer has occurred to the inner Ru(bpy-4,4'(COO)2)2-unit is shown by the presence of bands at 1113, 1289 and 1446 em - 1 for ([bpy-4,4' (COO -)2]--) which also appear in the transient spectrum of Ru(bpy-4,4'(COO)2MCN)i- (Fig.4C).

The presence of the carboxylate groups at the central unit contributes to the stabilization of the 3MLCf states with respect to those localized on the lateral units, as shown by the lower energy of the emission of Ru(bpy-4,4'(COO)2MCN)i­versus Ru(bpy)iCN)2 [9]. An additional contribution to the driving force for intramolecular energy transfer is due to the electronic asymmetry of the bridge. This is shown by the room temperature transient Raman spectra of [NC­Ru(phen)2-CN-Ru(bpY)2-CN]+ and [NC-Ru(phen)2-NC-Ru(bpY)2-CN]+ in ace­tonitrile [8]. The spectrum of the former is essentially that of (bpy·-) in Ru(bpy)~+·, with the only features associated with the phen ligand arising from enhancement of ground state signals. This data is consistent with complete energy transfer to the lowest excited state localized on the Ru(bpy) moiety. The transient Raman spectrum of [NC-Ru(phen)2-NC-Ru(bpY)2-CN]+ is dominated by ground state bpy

CA. Bignozzi et al. / Solar Energy Materials and Solar Cells 38 (]995) 187-198 193

Table 1 Raman bands in cm- 1 from the transient (pulsed, 354.7 nm) and ground-state (cw, 363.8) Raman spectra of [«NCXbpY)2RuIl(CN))2Rull(dcb)2F (trimer) in CH 3CN, [RuIl (bpY)2(CN)2] in CH 3CN, and [Rull(dcb)Z<CN)214

- in H 20 at pH = 7 a

Trimer [Rull(bpy}z(CN)2 J [Rull(dcb)Z<CN)2J4 Origin

Pulsed Cw Pulsed Cw Pulsed Cw 354.7 nm 363.8 nm 354.7 nm 363.8 nm 354.7 nm 363.8 nm

1012 bpy • 1020 1017 deb •

1030 1027 1028 bpy+dcb 1039 1040 deb • 1113 1112 deb' 1177 1174 1176 bpy 1211 1213 deb •

1214 bpy . 1255 deb

1263 1263 bpy 1274 1276 1276 bpy

1285 bpy • 1289 1290 deb' 1315 1313 1317 1315 deb' +bpy

1373 1373 dcb 1427 bpy'

1430 1430 deb 1446 1444 deb'

1450 bpy 1488 1488 1490 1489 deb' +bpy

1499 bpy • 1542 1545 deb

1548 bpy • 1549 1552 deb' 1561 1560 1561 bpy 1605 1606 1605 bpy

1618 1618 deb

a The labels deb and bpy signify the origin of ground-state resonance Raman bands from these ligands, while an asterisk signifies an excited-state band.

modes which gain enhancement from ground state '1T-'1T' absorption near 320 nm. Bands assignable to (phen'-) are observed at 1315, 1432, 1455, and 1515 cm- 1

consistent with energy transfer to the Ru(phen) moiety. Although excited state energy transfer processes are very efficient in this class

of compounds, charge injection into the conduction band of the semiconductor could also be promoted by 3 MLCf states localized on the lateral units of the trinuclear complex [NC-Ru(bpy )2-CN-Ru(bpy-4,4' (COO)2 )2-NC-Ru(bpY)2-CN]Z -. This electron transfer process, however, is expected to be less efficient than that originating from the central unit due to direct electronic coupling to the semicon­ductor through the carboxylic acid groups. This expectation could be validated if the relative rates of charge injection into the semiconductor surface by the excited trinuclear complex and energy transfer between the lateral and central unit were

194 CA. Bignozzi et al. / Solar Energy Materials and Solar Cells 38 (J995) 187-198

known. While the intramolecular rate is currently unknown, an estimation of the charge injection rate of about 6 X 109 S-1 has been obtained by Willig et al. [10] by monitoring the initial fast decay (170 ps) of the luminescence of the dye molecule adsorbed on Ti02. Time resolved infrared measurements in the picosecond time domain represent a valuable mean of gaining insight into the rate of intramolecu­lar energy transfer. We have already applied this powerful technique to the study of fast intercomponent energy transfer process in the binuclear complex [(CO)3Re(phen)-NC-Ru(bpY)2-CN]+ [11]. Because of varying degrees of "IT-back bonding, different CO stretching frequencies are observed for the ground state, the Re-based MLCT excited state and the Ru-based MLCT excited state. By monitoring of the CO stretching frequencies after excitation of the binuclear complex at 300 nm (involving substantial initial Re-based MLCT excitation), the energy transfer process can be time-resolved, yielding rate constants in the 5-10 ps range for disappearance of the Re-based MLCT state and appearance of the Ru-based MLCT state [11].

The idea of antenna systems for sensitization of semiconductors can be ex­tended towards larger systems. Branched Ru(II) polypyridine "arborols" of high nuclearity (presently up to 22 units) are already known [12,13]. On the other hand, one-dimensional systems that could stack perpendicularly to the surface (with the various antenna units working in series rather than in parallel) would be interest­ing in order to increase the ratio of chromophoric components to occupied surface area. Of course, the maximum size (in terms of number of components) of a one-dimensional antenna system depends critically on the driving-force demand of the single energy transfer steps involved. Interesting candidates for this role are the chain-like polychromophoric systems of general formula:

[(COhReI(phen) - CN - [RuII(bpYh - CNln

- RuII(bpyh - CNt+ 1)+.

Due to the cis-configuration at Ru(II), the metal-metal chain in this type of complexes is not linear, but presumably coiled in a more or less extended conformation. The supramolecular design is such that: (i) the highest-energy MLCT state is located on the Re-containing fragment; (iO a set of almost isoenergetic MLCT states is present in the intermediate Ru-containing units; (iii) the lowest energy MLCT state is located on the terminal Ru-containing fragment. General evidence for efficient energy transfer, in the expected direction, along such chains comes from trends in the photophysical parameters along the series and from solvent effect studies [14]. Direct evidence by time-resolved resonance Raman spectroscopy can also be obtained for the trinuclear member of the series [14]. The available evidence for vectorial energy transfer along such chains repre­sents good prospects for the use of extended one-dimensional antenna systems in the sensitization of semiconductors [15].

In the design of the antenna-sensitizer device a critical role is played by the sensitizer unit which directly interacts with the surface of the semiconductor. This unit should, in fact, meet all requisites of a mononuclear sensitizer, allowing a

CA. Bignozzi et al. / Solar Energy Materials and Solar Cells 38 (J995) 187-198 195

stable linkage with the antenna's unit and promoting an electron transfer process to the semiconductor. The sensitizer should therefore, have 3MLCf excited states higher in energy than the conduction band edge, but lower in energy than 3MLCf excited states localized on the different antenna moieties. Recently, a series of cis-Ru(bpy-5,5'COOH2)Z<X)2 (X = CI-, CN-, NCS -) complexes have been syn­thesized [16], and their properties compared to those of the analogous cis-Ru(bpy-4,4'COOH 2}z(X)2 species [3]. The change of the carboxylic function position from 4,4' to 5,5' causes a lowering of the energy of the 7r * accepting orbitals. The absorption spectra of the Ru(bpy-5,5'COOH 2)Z<X)2 species, which are dominated in the visible by d7r-7r * (bpy-5,5'COOH) transitions, are thus red shifted with respect to those of the corresponding Ru(bpy-4,4'COOH 2)Z<X)2 complexes. The performance of these molecules in photoregenerative cells reveal an enhanced response at longer wavelengths for Ru(bpy-5,5'COOH 2)Z<NCS)2 compared to Ru(bpy-4,4'COOH 2}z(NCS)2' which is, however, offset by a lower photocurrent efficiency at shorter wavelengths. This result is rationalized on the basis of the lower molar extinction coefficient shown by the 5,5' dyes in the visible and a lower quantum yield for charge injection [16].

30

--TNN4,4' a 25 ----- TNN5,5'

E 20 ~o

~ 15 , , , ,

'" ,

0 10 x to

5 -... _-

0

~ 60 W o Q: 40

20

-o-TNN4,4'

0". _. 0 __ TNN5,S'

',~~::-" .. ---... ~

..... ~~. '0..'::-. o~_

.....::.:. °

b 80

0~~4~OO~~--~50~0--~---6~OO--~--~70~0--~--~800

Wavelength (nm)

Fig. 5. Absorption spectra (a) and corrected photoaction spectra (b) of [NC-Ru(bpY)2-CN-Ru(4,4'­(COOXCOOH)-bpY)2-NC-Ru(bpY)2-CNl (TNN4,4', solid) and of [NC-Ru(bPY)2-CN-Ru(5,5'­(COOXCOOH)-bpY)2-NC-Ru(bpY)2-CNl (TNN5,5', dash). The photoaction spectra were measured in a thin layer cell containing a solution of 0,03 M 12 and 0.3 M LI in acetonitrile.

196 CA. Bignozzi et al. / Solar Energy Materials and Solar Cells 38 (1995) 187-198

Despite the poor performances observed in photoregenerative cells containing the 5,5' sensitizers, these complexes offer the possibility to verify possible improve­ments to the sensitization process which may arise from the antenna effect. With this goal, the two triads:

Ne - Ru(bpy)z - CN - Ru(bpy - 5,5'(eOOH)(eOO-) - Ne - Ru(bpy)z - eN, (A)

Ru(bpy - 5,5'(eOOH)(COO-)(CN) - eN - Ru(bpy)z - CN - Ru(bpy)z - eN, (B)

corresponding to the schematic structures reported in Figs. 2a and 2b, respectively, were synthesized. Both species contain the sensitizer with carboxylate function in 5,5' position, but different coordinations of CN ligands. In (A) two bridging cyanides are nitrogen bonded to the sensitizer unit, while in (B) one bridging and one terminal eN are carbon bonded to the sensitizer.

The absorption spectrum of the green complex (A) in acetonitrile is compared in Fig. 5a with that of the brown isomer,

Ne - Ru(bpy)z - CN - Ru(bpy - 4,4'(eOOH)(eOO-) - NC - Ru(bpy)z - eN. (C)

The band at 600 nm for (A) is due to a MLCf transition localized on the central unit [7]. The red shift of this band with respect to the analogous band of (e) is due to the lower energy of the LUMO of the 5,5' carboxylated complex. The photoac­tion spectra of (A) and (e), measured in acetonitrile and corrected for the

~ w ()

100r-------------------------------------~ __ B

~ ~~,".'·;COOH' • .,"."'~

'\.?- "'\

80

60

c.. 40 \ ~ 20

"­--:::::'-1=1 O~--~----~---L----~--~----~--~~~

400 500 600 700

Wavelength (nm)

Fig. 6. Corrected photoaction spectra of the trinuclear Ru(bpy-S,S'(COOHXCOO'XCN)-CN-Ru(bpy)z­CN-Ru(bpy)z-CN, (8) and of the mononuclear Ru[S,S'(COOH)-bpy]iCN)z' Experimental conditions as in Fig. S.

CA. Bignozzi et al. / Solar Energy Materials and Solar Cells 38 (1995) 187-198 197

absorption of the conductive glass (Fig. Sb) closely match the corresponding absorption spectra supporting the antenna effect. The lower IPCE values observed for (A), at comparable surface coverage with (C), are consistent with the lower absorptivity in the visible (Fig. Sa). The behaviour of the trinuclear complex (B) reveals that structural changes cause a considerable increase of IPCE (Fig. 6).

As illustrated in Fig. 6, the use of antenna-sensitizer complexes could be of some relevance for applications requiring thin Ti02 layers.

3. Conclusions

A "supramolecular approach" can be applied to the design of photosensitizers for wide band-gap semiconductors. Positive aspects are: (i) the cross-section for light absorption and the action spectrum can be improved by using antenna effects, (ii) as shown by the experimental studies discussed systems based on transition metal complexes are extremely versatile in terms of synthetic control and tunability of photophysical and redox properties, (iii) a variety of specific fast photophysical methods (luminescence techniques, visiblejUV laser flash spectroscopy, time-re­solved resonance Raman and infrared spectroscopies) are available for monitoring intercomponent energy transfer processes, (iv) a fair degree of kinetic control on such processes by molecular design can be achieved. The above features are of key importance in the development of practical devices based on supramolecular sensitization.

Acknowledgements

The work reported here relies on a number of collaborations. The authors are particularly indebted to T.J. Meyer (University of North Carolina, Chapel Hill) and R.B. Dyer (Los Alamos NL) for the equipment for the time-resolved resonance Raman and infrared spectroscopy.

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