Recombination processes in dye-sensitized solid-state solar cells with CuI as the hole collector

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Solar Energy Materials & Solar Cells 79 (2003) 249–255

Letter

Recombination processes in dye-sensitized solid-state solar cells with CuI as the hole collector V.P.S. Perera, K. Tennakone* Institute of Fundamental Studies, Hantana Road, Kandy, Sri Lanka Received 1 December 2002; received in revised form 30 March 2003

Abstract Construction of dye-sensitized solid-state solar cells requires identification of hole collectors and understanding of the dissipative processes that limit the energy conversion efficiency. One of the hole collectors fairly well studied and giving a reasonably high efficiency is CuI. In this note we show that stoichiometrically excess iodine molecules adsorbed at the CuI surface acts as hole trapping sites (located atB0.2 eV above the conduction band edge) that mediate recombination, affecting the performance of the cell. Fluorescence measurements reveal that exposure of CuI films to iodine vapor generates surface traps and as iodine diffuses to the bulk, surface trap density is greatly reduced. Methods by which the recombination originating from this effect may be circumvented are also discussed. r 2003 Elsevier B.V. All rights reserved. Keywords: Dye-sensitization; Solar cells; Photovoltaic cells; Carrier recombination; Hole trapping; Copper (I) iodide

Dye-sensitized solid-state photovoltaic cell is a heterojunction of the form N/D/P where N is a high surface area nanocrystalline n-type semiconductor film coated with a monolayer of the dye D and P is an hole conducting material which should be in contact with the dyed surface. Photoexcited dye molecules inject electrons and holes to the n- and p-type materials, respectively, generating a photovoltage and a photocurrent across the external circuit connecting n- and p-type regions [1–6]. The maximum open-circuit voltage (Voc ) approachable is the difference between the positions of the conduction band edge of the n-type material and the valence band edge of the p-type material. However the observed Voc is well below the above theoretical limit indicating the severity of recombination. Of the hole collectors *Corresponding author. Tel.: +94-8-232002; fax: +94-8-232131. E-mail address: [email protected] (K. Tennakone). 0927-0248/03/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0927-0248(03)00103-X

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Emission Intensity / Arb. Units

tested, the highest recorded efficiency has been obtained from CuI (p-type semiconductor of band gap 3.1 eV) with TiO2 as the n-type semiconductor and N3 dye [cis-dithiocyanato-bis(2,20 -bipyridyl-4,40 dicarboxylate)-ruthenium (II)] as the sensitizer [4]. In this note we present evidence to show that excess iodine molecules present in CuI diffusing to the CuI surface create surface hole traps that mediate recombination. CuI films were coated on glass plates heated toB150 C (0.5  3.0 cm2) by spraying a solution of CuI in acetonitrile until a filmB10 mm gets deposited. TiO2 films were deposited on conducting tin oxide glass plates (15 O sq1) using a solution of hydrolysed titanium isopropoxide by the method described in Ref. [3]. CuI was deposited on dye coated TiO2 films by spreading solution of CuI in acetonitrile over the surface, placing the plates on a hot plate heatedB150 C [3]. We found that a CuI film exposed to iodine vapor shows a strong fluorescence at 417 nm when the film is irradiated with light of wavelength loCuI band gap radiation (B400 nm). Fig. 1 shows the emission spectrum (excitation wavelength=350 nm) from a CuI film inserted to a cell (volume 9 cm3) filled with nitrogen saturated with iodine vapor. When the emission spectrum is measured the height of the emission peak gradually decreases with time (i.e., maximum occurs almost immediately after exposure of the film to iodine vapor corresponding to curve [a] of Fig. 1, 106 M solution of Rhodamine 6G was used as the calibration standard to estimate the fluorescence quantum yield). This simple experiment demonstrates that the fluorescence results from iodine adsorbed at the CuI surface and when iodine diffuses to the bulk the emission peak decreases. Samples of CuI always contain a stoichiometric excess of iodine [7] of which some naturally reach the surface. The emission peak of a CuI film unexposed to iodine (Fig. 1 curve (d)) originates for this reason. Again as expected, an enhancement of the emission peak was observed when the film is warmed. Obviously, heating increases the diffusion of iodine from bulk to the surface. In another experiment we irradiated a CuI film with

a

60.0

b c

40.0

d

20.0

0.0 380

400

420

440

460

480

λ / nm Fig. 1. Fluorescence spectrum of a CuI film: (a) immediately after exposure to iodine vapor; (b) 5 min after exposure; (c) 20 min after exposure; and (d) before exposure to iodine. (The fluorescence quantum yield at the peak position of the curve (a)B8%.)

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a Xe lamp (1000 W/m2) for 5 min and the emission spectrum was examined at different intervals of time after switching off the Xe lamp (excitation wavelength=350 nm). Until about 20 min the height of the emission peak at 417 nm continued to increase (Fig. 2). This observation can be understood as follows. Light of wavelength lpCuI band gap radiation, photodecomposes CuI liberating iodine (i.e., CuI=Cu+I). Iodine produced in the bulk diffuses to the surface and the maximum surface concentration is reached after lapse of some time. Excess iodine in the bulk of CuI acts as an acceptor of electrons from the valence band to induce p-type conductivity [7,8]. Fig. 3 shows the time variation of the current through an 18 mm thick film of CuI deposited on a glass sheet (1  2 cm2) connected to a voltage

Emission Intensity / Arb. Units

50.0

d c b a

25.0

0.0 380

430

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λ / nm Fig. 2. CuI film exposed to UV light and the fluorescence spectrum measured at different times after switching off the light at time: (a) t ¼ 0; (b) t ¼ 5; (c) t ¼ 10; and (d) t ¼ 15 min. (The fluorescence quantum yield at the peak position of the curve (d)B6%.)

Current / mA

0.45

0.4

0.35

0.3

0

200

400

600

800

1000

Time / s

Fig. 3. Time development of the current through a CuI film connected to a voltage source (dotted arrow indicate the time of exposure to iodine).

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CB

3.1 eV T (surface trap) 0.2eV VB

Fig. 4. Schematic energy level diagram showing the band positions of CuI and the position of surface trap level induced by iodine.

source and exposed to iodine vapor. Current gradually increases until saturation is reached, owing to the diffusion iodine from the surface to the bulk. The subsequent slow decay of the current results from the loss iodine in the bulk through the surface. The cause of fluorescence in CuI films exposed to iodine vapor can be understood on basis of the following explanation. Adsorbed iodine create surface trapping sites B0.2 eV above the valence band edge of CuI. On excitation of the CuI film with a band gap photon, an electron is transferred from valence to the conduction band and the hole in the valence band gets trapped in one of the above sites. Subsequent recombination of the electron in the conduction band with the trapped hole causes emission at 417 nm (Fig. 4). The trapping of holes at CuI surface sites seems to be the major cause of recombination in the dye-sensitized solid-state cells that adopts CuI as the hole collector. A schematic diagram showing the band positions of TiO2, CuI and ground (S0) and excited (S) states of the dyes are shown in Fig. 5. Upon photoexcitation, dye molecules sandwiched between the TiO2 and CuI surfaces inject electrons to TiO2 and holes to CuI. As electrons and holes are confined to two different materials and transfer of conduction band electrons (valence band holes) in TiO2 (CuI) to CuI (TiO2) are energetically forbidden, bulk recombination is nonexistent. However a conduction band electron could recombine with a hole trapped at the site T (this involve electron tunneling, probability of which could be appreciable because the dye layer is only one molecule thick). Furthermore there would also be regions where TiO2 and CuI are in direct contact (because of the imperfect coverage of the dye on TiO2) that readily admit trap mediated recombination. Iodine also acts as the dopant that induces p-type conductivity to CuI [7] (i.e., iodine atoms in the bulk accepting electrons from the valence band to form I ). In order to collect the photocurrent, high conductivity of CuI is desirable (therefore excess iodine in the bulk). However

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Vacuum Energy (eV)

-1 CB -2

S*

-3 CB -4 -5 S -6

VB

-7 -8 VB TiO2

Dye

CuI

Fig. 5. Schematic energy level diagram showing the band positions of CuI, TiO2 and ground (S) and excited (S) levels of the dye.

iodine that diffuses to the surface promote recombination. Accurate control or estimation of the stoichiometrically excess iodine in CuI is difficult because, light and exposure to air liberate iodine. When a dye-sensitized solid-state solar cell is made from freshly prepared CuI (in the presence of a reducing agent, e.g., SO2 to remove excess iodine) Voc reaches 600–650 mV. However, during operation of the cell Voc decreases gradually followed by an increase in Isc : We believe that this effect originates at least partly (the other factor leading to unstability seems to be the loosening of the contact between CuI and the dyed surface) from release of iodine to the surface of CuI. Cutting off UV light and placing the cell in an oxygen free environment reduces the rate of change of Voc and Isc : Again we also noticed that exposure of the CuI surface of a cell to iodine vapor, immediately reduces the Voc to about 400 mV. In earlier reports we have shown that incorporation of small quantities of low melting point thiocyantes into the CuI coating solution greatly improves the performance of the cell [4,9]. Cells prepared by this method remain more stable and

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especially the rapid decay of Voc is suppressed. As explained in these reports thiocyantes acts as CuI crystal growth inhibitors enabling filling of the pores of the dye coated TiO2 film. Now it has become clear that the thiocyanate has another favorable effect. We found that CuI films prepared from CuI solutions containing a thiocyanate (e.g., few mili-moles of triethylamine thiocyanate per mole of CuI) did not show iodine induced fluorescence. However, such films showed a feeble fluorescence at 437 nm (Fig. 4). The position and the height of this peak was unaltered by exposure to iodine vapor and height depended only on the amount of thiocyante introduced (reaching a saturation when [CNS]/[CuI]B103). CNS is strongly adsorbed on CuI surface and quenching of the iodine induced fluorescence by thiocyante, indicates that thiocyante effectively prevents iodine surface adsorption on CuI and formation of iodine induced hole traps. Thiocynate adsorption also introduces a traps at 2.8 eV above the valence band edge, however a more feeble fluorescence originating from thiocyanate suggest that the hole trapping cross-section at these sites is smaller than trap centers resulting from iodine adsorption. Consequently the cell performance improves in the presence of thiocyanate. CuSCN which is also used for construction of dye-sensitized solidstate solar cells which does not show fluorescence similar to CuI when exposed to UV light and there is no indication that stoichiometrically excess SCN (which induces p-type conductivity in CuSCN) creates surface traps in CuSCN. This seems to be consistent with observation that dye-sensitized solid-state solar cells based on CuSCN have more stable open-circuit voltages. In conclusion the results presented above shows that although a stoichiometric excess iodine is necessary for the induction of p-type conductivity in the hole collector CuI, iodine adsorbed at the CuI surface has the detrimental effect of trapping holes and mediating recombination and iodine in the bulk gets diffused to the surface. Adsorption of thiocyantes passivates the CuI surface against formation iodine induced surface traps improving the cell performance. Sites where SCN is adsorbed also seem to trap holes with a relatively smaller trapping cross-section. As N/D/P-type cell exposes an interface more than one order of magnitude larger than geometrical projection, recombination mediated by trapping centers located on the surface becomes an important dissipative process. The superior performance of the standard liquid junction dye-sensitized solar cells indicates that recombination is much less in this device. Presumably wave functions of electronic states in the solid side (i.e., TiO2) overlaps less strongly with acceptor states on the liquid side of the interface. The exact physical reason for this remains unclear.

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[4] G.R.R.A. Kumara, A. Kono, K. Sahiratsuchi, J. Tsukuhara, K. Tennakone, Chem. Mater. 14 (2002) 954. [5] B. O’Regan, D.T. Schwartz, Chem. Mater. 7 (1995) 1349. [6] B. O’Regan, D.T. Schwartz, Chem. Mater. 10 (1998) 1501. [7] J.B. Ayers, W.H. Waggoner, J. Inorg. Nucl. Chem. 13 (1971) 721. [8] K. Tennakone, G.R.R.A. Kumara, I.R.M. Kottegoda, V.P.S. Perera, G.M.L.P. Aponsu, K.G.U. Wijayantha, Sol. Energy Mater. Sol. Cells 55 (1998) 283. [9] G.R.R.A. Kumara, S. Kaneko, M. Okuya, K. Tennakone, Langmuir 18 (2002) 10493.