Efficient CdSe Quantum Dot-Sensitized Solar Cells Prepared by an ...
Efficient CdSe Quantum Dot-Sensitized Solar Cells Prepared by an Improved Successive Ionic Layer Adsorption and Reaction (SILAR) Process HyoJoong Lee,*,# Mingkui Wang, Peter Chen, Daniel R. Gamelin,§ Shaik M. Zakeeruddin, Michael Grätzel* and Md. K. Nazeeruddin*
Laboratory for Photonics and Interfaces, Institute of Chemical Sciences and Engineering, School of Basic Sciences, Swiss Federal Institute of Technology, CH1015 Lausanne, Switzerland1
Corresponding author e-mail: [email protected], [email protected] and [email protected]; phone: +41-21-693-6124; fax: +41-21-693-4311. # Current address: Department of Chemistry, Pohang University of Science and Technology (POSTECH), Gyeongbuk, South Korea. § On sabbatical leave from the Department of Chemistry, University of Washington, Seattle, USA.
Supporting information:
Experimental details and supporting figures/tables
(1) Chemicals and electrolytes. Cadmium nitrate tetrahydrate (Fluka, ≥ 99.0 %), lead nitrate (Aldrich, 99.99 %), zinc nitrate hexahydrate (Aldrich, 98 %), SeO2 (Aldrich, 99.9+ %), TeO2 (Aldrich, 99+ %), titanium diisopropoxide bis(acetylacetonate) (Aldrich), and NaBH4 (Fluka, purum >97.0 %) were used as received. Ethanol was of HPLC grade. Na2Se (99.9 % pure, 60 mesh) and Na2Te (99.9 % pure, 60 mesh) were purchased from Cerac, Inc. The cobalt (II) complex, [Co(o-phen)3](TFSI)2, (o-phen=1,10-phenanthroline and TFSI = bis(trifluoromethanesulfonyl)imide) was synthesized according to literature procedures.1 The cobalt electrolytes were prepared at concentrations of 0.75 M Co2+ complex, 0.075 M Co3+ complex, and 0.2 M LiClO4
in acetonitrile/ethylene
carbonate (4:6/v:v). The Co3+ complex itself was synthesized and isolated as reported earlier.1d
(2) Preparaton of CdSe(Te), ZnSe, and PbSe QDs over TiO2 films from in situgenerated or commercially available selenide and telluride (a) Weigh metal nitrates in vials in order to make 30 mM Cd(NO3)2, 20 mM Pb(NO3)2, and 50 mM Zn(NO3)2 solution later and put those inside the glove bag. (b) 0.09986 g of SeO2 is dissolved in 30mL ethanol (30mM), and purged/stirred for about 2 minutes as like pictures below (Figure S1) and 0.068g of NaBH4 (60mM) added into the round-bottom flask (r. b. f) containing SeO2, and purged/stirred for about 1 hr (You will see a series of clear color changes like below; from deep red to transparent one!!). (c) Pure EtOH in large volume is also prepared and purged for 1 hr separately for making metal nitrate solutions and washing the electrodes used inside glove bag. (d) Move the r.b.f.s in (b) and (c) inside the glove bag, which then was sealed by zipper and evacuated by vacuum pump and purged with inert gas (Ar or N2). Evacuation/purging were repeated 5 times to make the inside atmosphere with a low oxygen level. (e) Inside the glove bag, prepare metal nitrate solution designated above in one beaker and selenide solution in another beaker. (f) The TiO2-modified
electrode was dipped into the metal2+ solution, pure ethanol (then dried), the Se2solution, and then pure ethanol (then dried) successively for 30 second for deposition each and longer time for washing and dried. Such an immersion cycle was repeated several times (CdSe: 1~6 times, ZnSe: 6 times and PbSe: 4 times). The electrode became darker as the number of SILAR cycles was increased.
For deposition of CdTe or ZnTe, all the process was the same except one step about telluride preparation; the amount of NaBH4 (0.6 M) used to reduce TeO2 (30 mM) was higher than in the case of SeO2 and purging/stirring was carried out for 2 hrs.(Be careful not to clog your needle while purging for 2 hrs, So, it is recommended to check the purging condition regularly). Then, somewhat grey or pale black solution will be seen from TeO2/NaBH4 after 2hr due to the precipitation of Te and limited solubility of Te2- while watching the transparent solution from SeO2/NaBH4 after about 1 hr. After moving the r.b.f. containg telluride and black precipitates inside the glove bag and doing evacuation/purging 5 times, you will see a pale pink solution [Figure S1 (h)] after all precipitates settled down. Then, the typical SILAR was done as in CdSe described above.
In the later stage of this study, we also found a commercial source for selenide and telluride (Na2Se and Na2Te, from Cerac Inc.). When the same SILAR process was performed using Na2Se (30 mM in EtOH) and Na2Te (20 mM in EtOH or acetonitrile), the same color changes were observed, but the best electrodes gave only about 80 % of the overall photovoltaic efficiencies of those prepared using the above-mentioned in-situ generation (Table S1). .
Figure S1. A series of pictures showing a round-bottom flask containing SeO2 in ethanol (a) and its reduction process by color changes after addition of NaBH4 (b-g) while purged with argon gas. (h) Telluride solution (pale pink) used for deposition of metal telluride in this experiment.
(3) Absorption measurements To check the optical properties of QD-sensitized electrodes which were sensitized with ZnSe6-, PbSe4-, CdSe5-, CdSe6- and CdSe5Te1-QDs by SILAR process, absorption spectra (over transparent TiO2 film, about 2 µm thick) were measured by using a Varian Cary 5 spectrophotometer.
Figure S2. Absorption spectrum of ZnSe6- and PbSe4-sensitized TiO2 film (number means the cycle times of SILAR process and reference was the bare TiO2 film).
(4) SEM and TEM measurements
The SEM images were performed in the CIME EPFL with FEI XL30 SFEG at ultra-high resolution mode. Electron beams with 5 Kev and spot size 3 were applied on the samples (bare-, CdSe6-, and CdSe5Te1-sensitized TiO2 films) to acquire the surface morphology. HR-TEM was carried out using a JEM-2200FS instrument, JEOL. The CdSe6sensitized TiO2 films were prepared by following the same procedures as in testing photovoltaic performances. Then, they were scratched off FTO glass as small as possible and dispersed in EtOH, from where a few drops were taken over TEM grid and dried for TEM measurements.
Figure S3. SEM image of (a) a bare TiO2 film, (b) the same film after 6 SILAR cycles for CdSe deposition and (c) for CdSe5Te1.
(5) Cell assembly and measurement of J-V curves and IPCE Photoelectrodes were prepared by following the optimal condition consisted of a TiO2 film with a triple layer structure.2 A compact blocking underlayer of spraypyrolyzed titanium dioxide (ca. 120 nm thick) was deposited onto a cleaned conducting glass substrate (NSG, F-doped SnO2, resistance 8 Ωsq-1). A solution of titanium diisopropoxide bis(acetylacetonate) in ethanol was sprayed 16 times over the conducting glass surface, which was maintained at 450 oC. The treated glass plates
were fired at 450 oC for 30 min. more to remove remaining organic traces. Successive depositions of about 2.3 µm thick transparent layer and about 5.8 µm thick 60 nm light-scattering layer by screen-printing, and final post-treatment with an aqueous solution of TiCl4 were then carried out according to typical procedures done in our laboratory for dye cells.3 Dye (Z907Na) sensitization followed the typical procedures, and QD’s derivatization of nanocrystalline oxide films was obtained by following the typical SILAR process with above-described cationic and anionic precursor solutions which are kept in different beakers inside the glove bag purged with inert gas (Ar or N2). Then, the QD-sensitized electrodes were assembled and sealed with a thin transparent hot-melt 25 µm thick Surlyn ring (DuPont) to the counter electrodes (Pt on FTO glass, chemical deposition of 0.05 M hexachloroplatinic acid in 2-propanol at 400 ℃ for 20 min). The electrolyte was injected into the interelectrode space from the counter electrode side through a predrilled hole, and then the hole was sealed with a Bynel sheet and a thin glass slide cover by heating. All the procedures in preparing electrodes and assembling those were the same as in our typical dye-sensitized cells,3 except one step of QDs attachment over TiO2 layers. For solid-state cell, the working electrode consisted of a TiO2 film with a double layer structure; A compact blocking underlayer of spray-pyrolyzed titanium dioxide (ca. 120 nm thick) onto a cleaned conducting glass substrate (F-doped SnO2, resistance 15 Ωsq-1), about 2 µm thick active layer (30 nm TiO2) by doctor-blading, and final post-treatment with an aqueous solution of TiCl4 were carried out according to typical procedures done in our laboratory for solid state dye-cells.4 The as-prepared electrodes were sensitized with CdSe5Te1 QDs by following the same SILAR process described above. The organic hole conductor [2, 2’, 7, 7’,- tetrakis(N, N-di-pmethoxyphenylamine)-9,
9’-spirobifluorene
(spiro-OMeTAD,
Merck)]
was
introduced into the mesopores of TiO2 film by spin coating 0.17 M chlorobenzene solution of spiro-OMeTAD with three additives; 13 mM Li salt (Li[CF3SO2]2N), 0.10 M tert-butylpyridine (tBP), and 0.30 mM antimony dopant (N[p-C6H4Br)3SbCl6Sb)
(The concentration values are given for the spiro-OMeTAD hole conductor solution). Finally, about 70 nm thick gold was deposited by a thermal evaporation as a counter electrode and to define the active area (~0.12 cm2). The irradiation source for the photocurrent-voltage (J-V) measurement is a 450 W xenon light source (Osram XBO 450, USA), which simulates the solar light. The incident light intensity was calibrated with a standard Si solar cell. The spectral output of the lamp matched precisely the standard global AM 1.5 solar spectrum in the region of 350-750 nm (mismatch < 2%) by the aid of a Schott K113 Tempax sunlight filter (Präzisions Glas & Optik GmbH, Germany). Various irradiance intensities from 0.01 to 1.0 sun can be provided with neutral wire mesh attenuators. The currentvoltage curves were obtained by measuring the photocurrent of the cells using a Keithley model 2400 digital source meter under an applied external potential scan. The measurement of incident photon-to-current conversion efficiency (IPCE) was performed by a similar data collecting system but under monochromatic light. IPCE was plotted as a function of excitation wavelength. The incident light from a 300 W xenon lamp (ILC Technology, USA) was focused through a Gemini-180 double monochromator (Jobin Yvon Ltd., UK) onto the cell under test.
(6) Determination of recombination rate constant and the electron diffusion length in the titania film by transient photovoltage and photocurrent decay measurements Transient photovoltage decay measurements were performed with a similar method as by O’Regan et al.5 A white light bias was generated from an array of diodes. Green light pulse diodes (0.05 s square pulse width, 100 ns rise and fall time) controlled by a fast solid-state switch were used as the perturbation source. The voltage dynamics were recorded on a PC-interfaced Keithley 2400 source meter with a 500 µs response time. The perturbation light source was set to a suitably low level in order for the voltage decay kinetics to be mono-exponential. This enabled the charge recombination rate constants to be obtained directly from the exponential decay rate
of the voltage decay. By varying the white light bias intensity, the recombination lifetime could be estimated under open circuit condition. As for the electron diffusion length measurement, the photovoltage and photocurrent decay dynamics were recorded under different conditions with a constant bias light intensity given by white LED (about 10% sun light intensity in this experiment).5 The voltage decay measurements were performed with zero current (open-circuit) but also over a range of fixed current, corresponding to ‘scanning’ the voltage perturbation at various positions on the photocurrent voltage curve. Small perturbation transient photocurrent measurements were performed in a similar manner to the open-circuit voltage decay measurement. However, the signal was recorded with the Keithley source meter in series with the solar cell and the oscilloscope, holding a potential difference across the device over a range of potentials between short-circuit and open-circuit. For the voltage decay measurements, no extra current is allowed to flow, following the light pulse, therefore the decay of the measured perturbation signal is entirely governed by the charge recombination within the cell. For the current decay measurements, while the charge is being collected the charges are also simultaneously recombining within the cell. Therefore the decay rate constant for the current signal (ksignal) is a combination of the decay rate constant for the transport out of the cell (ktrans) and the rate constant for the recombination in the cell (krec) as k singnal = ktrans + k rec . From the voltage decay measurements, we can estimate an electron lifetime (τre) and from the current decay measurements we can estimate an effective diffusion coefficient for electrons, De = d 2 2.77 × τ trans , where τtrans is the current collection lifetime, and d is the film thickness. From the current decay measurements, the lifetime of the current signal (τsignal) is a combination of the lifetime for the transport out of the cell (τtrans) and the recombination lifetime in the cell (τre). τtrans is calculated as 1 τ trans = 1 τ signal − 1 τ re . From the derived effective diffusion coefficients and charge
recombination lifetimes we can estimate the electron diffusion length in the titania film following this equation: Le = Deτ re .
(7) Electrochemical impedance spectroscopy measurements Electrochemical impedance spectra of QD-sensitized cells were measured using an Autolab Frequency Analyzer set-up which consists of an Autolab PGSTAT 30 (Eco Chemie B.V., Utrecht, The Netherlands) producing a small amplitude harmonic voltage, and a Frequency Response Analyzer module. The EIS experiments were performed at a constant temperature of 20 oC under dark conditions. The impedance spectra of the samples were obtained at various potentials (from -0.8 V to -0.2 V) at frequencies ranging from 0.05 Hz to ~200 kHz; the oscillation potential amplitudes having values of approximately 10 mV. In the EIS experiments, the photoanode (the QD-sensitized TiO2 electrode) was used as the working electrode, the Pt counter electrode (CE) being used simultaneously as both the auxiliary electrode and the reference electrode. EIS was employed to scrutinize the effect of QDs on the photovoltaic performance. Figure S4 compares the Nyquist plot of a nanocrystalline TiO2 film covered by CdSe6 and CdSe5Te1 QDs. The impedance data in Fig. S4 were measured at forward bias of -0.50 V under dark condition and 20 °C in presence of Co(o-phen)32+/3+ redox electrolyte. Under those conditions, we observed that the adsorption of CdSe5Te1 enlarges dramatically the radius of the semicircle (related to the interface recombination between the TiO2 conduction band electrons and Co(o-phen)33+ complex in the electrolyte) at low frequencies range (from 100 Hz to 0.05 Hz) in the Nyquist plot compared to that of the nanocrystalline TiO2 film sensitized with CdSe6. By fitting the EIS spectroscopy with the transmission line model, suggested by Bisquert et al,6 we can obtain information on the charge transfer, interfacial recombination, and charge collection yield in photovoltaic devices. A slight difference in electron diffusion coefficient (De: De = Rt C µ , Rt being electron diffusion resistance in the TiO 2 and C µ being chemical capacitance) of the devices with QDs
Figure S4. Impedance spectra (Nyquist plot) of the devices sensitized with CdSe6 and CdSe5Te1 at forward bias of -0.50 V under dark conditions at 20 oC. The solid line corresponds to derived values using the transmission line model.
was observed and is illustrated in Figure S5a, indicating that the QDs have only a small influence on the electron transport in the TiO2 nanoparticles. At the same applied bias, device utilizing CdSe5Te1 has a consistently longer apparent recombination lifetime (τn: τ n = Rct Cµ , Rct being recombination resistance, see Figure S5b) than that found in device with CdSe6, more supporting evidence for a slower charge recombination occurring in the former relative to the latter. These values obtained from impedance measurements are consistent with the transient photovoltage decay measurements of the recombination rate constant as presented in Figure 3a. The electron diffusion length (L: L = d Rct Rt , d being the film thickness) is longer for device with CdSe5Te1 than that of device utilizing CdSe6 (Figure S2c), reflecting that the increase in Jsc is caused by a higher charge collection yield.
Figure
S5.
Derived
transport/recombination
parameters
from
impedance
measurements under dark conditions for two different devices indicated: (a) Effective electron diffusion coefficient (De), (b) apparent recombination lifetime (τn), and (c) effective electron diffusion length (L) as a function of the applied bias (U).
Table S1. Photovoltaic parameters of CdSe5Te1 QD-sensitized cell, prepared from Cd(NO)3 (30 mM in ethanol) and commercially available Na2Se (30 mM in ethanol) /Na2Te (20 mM in acetonitrile) (This data was obtained 7 days later after initial measurement).
9.2% sun 29.3% sun 50.5% sun 98.4% sun
Isc (mA/cm2) 0.83
Voc (V) 0.52
FF 0.76
Efficincy (%) 3.57
2.33
0.56
0.69
3.09
3.64
0.57
0.58
2.39
4.94
0.59
0.52
1.54
Table S2. Photovoltaic parameters of best CdSe5Te1 QD-sensitized cell at 100 W/m2, measured after some time elapsed. (The cell was stored in an office drawer between measurements)
fresh after 4 days after 7 days
Isc (mA/cm2) 0.63
Voc (V) 0.58
FF 0.79
Efficincy (%) 3.11
0.79
0.59
0.78
3.98
0.83
0.60
0.78
4.18
References 1. (a) Shklover, V.; Eremenko, I. L.; Berke, H.; Nesper, R.; Zakeeruddin, S. M.; Nazeeruddin, M. K.; Grätzel, M. Inorg. Chim. Acta 1994, 219, 11. (b) Dekorte, J. M.; Owens, G. D.; Margerum, D. W. Inog. Chem. 1979, 18, 15381. (c) Szalda, D. J.; Creutz, C.; Mahajan, D.; Sutin, N. Inog. Chem. 1983, 22, 2372. (d) Nusbaumer, H. Ph. D. thesis, EPFL, 2004. (e) Nusbaumer, H.; Zakeeruddin, S. M.; Moser, J.-E.; Grätzel, M. Chem. Eur. J. 2003, 9, 3756. 2. Lee, H. J.; Chen, P.; Moon, S.-J.; Frédéric, S.; Sivula, K.; Bessho, T.; Gamelin, D. R.;Comte, P.; Zakeeruddin, S. M.; Seok, S. I.; Grätzel, M.; Nazeeruddin, Md. K. Langmuir, 2009, 25, 7602. 3. Kuang, D.; Ito, S.; Wenger, B.; Clein, C.; Moser, J.-E.; Humphry-Baker, R.; Zakeeruddin, S. M.; Grätzel, M. J. Am. Chem. Soc. 2006, 128, 4146. 4. Schmidt-Mende, L.; Grätzel, M. Thin Solid Films 2006, 500, 296. 5. O’Regan, B.; Lenzmann, F. J. Phys. Chem. B 2004, 108, 4342. 6. a) Bisquert, J.; Cahen, D.; Hodes, G.; Rühle, S.; Zaban, A. J. Phys. Chem. B 2004, 108, 8106. b) Fabregat-Santiago, F.; Bisquert, J.; Cevey, L.; Chen, P.; Wang, M.; Zakeeruddin, S. M.; Grätzel, M. J. Am. Soc. Chem. 2009, 131, 558.
Laboratory for Photonics and Interfaces, Institute of Chemical Sciences and Engineering, School of Basic Sciences, Swiss Federal Institute of Technology, CH1015 Lausanne, Switzerland1
Corresponding author e-mail: [email protected], [email protected] and [email protected]; phone: +41-21-693-6124; fax: +41-21-693-4311. # Current address: Department of Chemistry, Pohang University of Science and Technology (POSTECH), Gyeongbuk, South Korea. § On sabbatical leave from the Department of Chemistry, University of Washington, Seattle, USA.
Supporting information:
Experimental details and supporting figures/tables
(1) Chemicals and electrolytes. Cadmium nitrate tetrahydrate (Fluka, ≥ 99.0 %), lead nitrate (Aldrich, 99.99 %), zinc nitrate hexahydrate (Aldrich, 98 %), SeO2 (Aldrich, 99.9+ %), TeO2 (Aldrich, 99+ %), titanium diisopropoxide bis(acetylacetonate) (Aldrich), and NaBH4 (Fluka, purum >97.0 %) were used as received. Ethanol was of HPLC grade. Na2Se (99.9 % pure, 60 mesh) and Na2Te (99.9 % pure, 60 mesh) were purchased from Cerac, Inc. The cobalt (II) complex, [Co(o-phen)3](TFSI)2, (o-phen=1,10-phenanthroline and TFSI = bis(trifluoromethanesulfonyl)imide) was synthesized according to literature procedures.1 The cobalt electrolytes were prepared at concentrations of 0.75 M Co2+ complex, 0.075 M Co3+ complex, and 0.2 M LiClO4
in acetonitrile/ethylene
carbonate (4:6/v:v). The Co3+ complex itself was synthesized and isolated as reported earlier.1d
(2) Preparaton of CdSe(Te), ZnSe, and PbSe QDs over TiO2 films from in situgenerated or commercially available selenide and telluride (a) Weigh metal nitrates in vials in order to make 30 mM Cd(NO3)2, 20 mM Pb(NO3)2, and 50 mM Zn(NO3)2 solution later and put those inside the glove bag. (b) 0.09986 g of SeO2 is dissolved in 30mL ethanol (30mM), and purged/stirred for about 2 minutes as like pictures below (Figure S1) and 0.068g of NaBH4 (60mM) added into the round-bottom flask (r. b. f) containing SeO2, and purged/stirred for about 1 hr (You will see a series of clear color changes like below; from deep red to transparent one!!). (c) Pure EtOH in large volume is also prepared and purged for 1 hr separately for making metal nitrate solutions and washing the electrodes used inside glove bag. (d) Move the r.b.f.s in (b) and (c) inside the glove bag, which then was sealed by zipper and evacuated by vacuum pump and purged with inert gas (Ar or N2). Evacuation/purging were repeated 5 times to make the inside atmosphere with a low oxygen level. (e) Inside the glove bag, prepare metal nitrate solution designated above in one beaker and selenide solution in another beaker. (f) The TiO2-modified
electrode was dipped into the metal2+ solution, pure ethanol (then dried), the Se2solution, and then pure ethanol (then dried) successively for 30 second for deposition each and longer time for washing and dried. Such an immersion cycle was repeated several times (CdSe: 1~6 times, ZnSe: 6 times and PbSe: 4 times). The electrode became darker as the number of SILAR cycles was increased.
For deposition of CdTe or ZnTe, all the process was the same except one step about telluride preparation; the amount of NaBH4 (0.6 M) used to reduce TeO2 (30 mM) was higher than in the case of SeO2 and purging/stirring was carried out for 2 hrs.(Be careful not to clog your needle while purging for 2 hrs, So, it is recommended to check the purging condition regularly). Then, somewhat grey or pale black solution will be seen from TeO2/NaBH4 after 2hr due to the precipitation of Te and limited solubility of Te2- while watching the transparent solution from SeO2/NaBH4 after about 1 hr. After moving the r.b.f. containg telluride and black precipitates inside the glove bag and doing evacuation/purging 5 times, you will see a pale pink solution [Figure S1 (h)] after all precipitates settled down. Then, the typical SILAR was done as in CdSe described above.
In the later stage of this study, we also found a commercial source for selenide and telluride (Na2Se and Na2Te, from Cerac Inc.). When the same SILAR process was performed using Na2Se (30 mM in EtOH) and Na2Te (20 mM in EtOH or acetonitrile), the same color changes were observed, but the best electrodes gave only about 80 % of the overall photovoltaic efficiencies of those prepared using the above-mentioned in-situ generation (Table S1). .
Figure S1. A series of pictures showing a round-bottom flask containing SeO2 in ethanol (a) and its reduction process by color changes after addition of NaBH4 (b-g) while purged with argon gas. (h) Telluride solution (pale pink) used for deposition of metal telluride in this experiment.
(3) Absorption measurements To check the optical properties of QD-sensitized electrodes which were sensitized with ZnSe6-, PbSe4-, CdSe5-, CdSe6- and CdSe5Te1-QDs by SILAR process, absorption spectra (over transparent TiO2 film, about 2 µm thick) were measured by using a Varian Cary 5 spectrophotometer.
Figure S2. Absorption spectrum of ZnSe6- and PbSe4-sensitized TiO2 film (number means the cycle times of SILAR process and reference was the bare TiO2 film).
(4) SEM and TEM measurements
The SEM images were performed in the CIME EPFL with FEI XL30 SFEG at ultra-high resolution mode. Electron beams with 5 Kev and spot size 3 were applied on the samples (bare-, CdSe6-, and CdSe5Te1-sensitized TiO2 films) to acquire the surface morphology. HR-TEM was carried out using a JEM-2200FS instrument, JEOL. The CdSe6sensitized TiO2 films were prepared by following the same procedures as in testing photovoltaic performances. Then, they were scratched off FTO glass as small as possible and dispersed in EtOH, from where a few drops were taken over TEM grid and dried for TEM measurements.
Figure S3. SEM image of (a) a bare TiO2 film, (b) the same film after 6 SILAR cycles for CdSe deposition and (c) for CdSe5Te1.
(5) Cell assembly and measurement of J-V curves and IPCE Photoelectrodes were prepared by following the optimal condition consisted of a TiO2 film with a triple layer structure.2 A compact blocking underlayer of spraypyrolyzed titanium dioxide (ca. 120 nm thick) was deposited onto a cleaned conducting glass substrate (NSG, F-doped SnO2, resistance 8 Ωsq-1). A solution of titanium diisopropoxide bis(acetylacetonate) in ethanol was sprayed 16 times over the conducting glass surface, which was maintained at 450 oC. The treated glass plates
were fired at 450 oC for 30 min. more to remove remaining organic traces. Successive depositions of about 2.3 µm thick transparent layer and about 5.8 µm thick 60 nm light-scattering layer by screen-printing, and final post-treatment with an aqueous solution of TiCl4 were then carried out according to typical procedures done in our laboratory for dye cells.3 Dye (Z907Na) sensitization followed the typical procedures, and QD’s derivatization of nanocrystalline oxide films was obtained by following the typical SILAR process with above-described cationic and anionic precursor solutions which are kept in different beakers inside the glove bag purged with inert gas (Ar or N2). Then, the QD-sensitized electrodes were assembled and sealed with a thin transparent hot-melt 25 µm thick Surlyn ring (DuPont) to the counter electrodes (Pt on FTO glass, chemical deposition of 0.05 M hexachloroplatinic acid in 2-propanol at 400 ℃ for 20 min). The electrolyte was injected into the interelectrode space from the counter electrode side through a predrilled hole, and then the hole was sealed with a Bynel sheet and a thin glass slide cover by heating. All the procedures in preparing electrodes and assembling those were the same as in our typical dye-sensitized cells,3 except one step of QDs attachment over TiO2 layers. For solid-state cell, the working electrode consisted of a TiO2 film with a double layer structure; A compact blocking underlayer of spray-pyrolyzed titanium dioxide (ca. 120 nm thick) onto a cleaned conducting glass substrate (F-doped SnO2, resistance 15 Ωsq-1), about 2 µm thick active layer (30 nm TiO2) by doctor-blading, and final post-treatment with an aqueous solution of TiCl4 were carried out according to typical procedures done in our laboratory for solid state dye-cells.4 The as-prepared electrodes were sensitized with CdSe5Te1 QDs by following the same SILAR process described above. The organic hole conductor [2, 2’, 7, 7’,- tetrakis(N, N-di-pmethoxyphenylamine)-9,
9’-spirobifluorene
(spiro-OMeTAD,
Merck)]
was
introduced into the mesopores of TiO2 film by spin coating 0.17 M chlorobenzene solution of spiro-OMeTAD with three additives; 13 mM Li salt (Li[CF3SO2]2N), 0.10 M tert-butylpyridine (tBP), and 0.30 mM antimony dopant (N[p-C6H4Br)3SbCl6Sb)
(The concentration values are given for the spiro-OMeTAD hole conductor solution). Finally, about 70 nm thick gold was deposited by a thermal evaporation as a counter electrode and to define the active area (~0.12 cm2). The irradiation source for the photocurrent-voltage (J-V) measurement is a 450 W xenon light source (Osram XBO 450, USA), which simulates the solar light. The incident light intensity was calibrated with a standard Si solar cell. The spectral output of the lamp matched precisely the standard global AM 1.5 solar spectrum in the region of 350-750 nm (mismatch < 2%) by the aid of a Schott K113 Tempax sunlight filter (Präzisions Glas & Optik GmbH, Germany). Various irradiance intensities from 0.01 to 1.0 sun can be provided with neutral wire mesh attenuators. The currentvoltage curves were obtained by measuring the photocurrent of the cells using a Keithley model 2400 digital source meter under an applied external potential scan. The measurement of incident photon-to-current conversion efficiency (IPCE) was performed by a similar data collecting system but under monochromatic light. IPCE was plotted as a function of excitation wavelength. The incident light from a 300 W xenon lamp (ILC Technology, USA) was focused through a Gemini-180 double monochromator (Jobin Yvon Ltd., UK) onto the cell under test.
(6) Determination of recombination rate constant and the electron diffusion length in the titania film by transient photovoltage and photocurrent decay measurements Transient photovoltage decay measurements were performed with a similar method as by O’Regan et al.5 A white light bias was generated from an array of diodes. Green light pulse diodes (0.05 s square pulse width, 100 ns rise and fall time) controlled by a fast solid-state switch were used as the perturbation source. The voltage dynamics were recorded on a PC-interfaced Keithley 2400 source meter with a 500 µs response time. The perturbation light source was set to a suitably low level in order for the voltage decay kinetics to be mono-exponential. This enabled the charge recombination rate constants to be obtained directly from the exponential decay rate
of the voltage decay. By varying the white light bias intensity, the recombination lifetime could be estimated under open circuit condition. As for the electron diffusion length measurement, the photovoltage and photocurrent decay dynamics were recorded under different conditions with a constant bias light intensity given by white LED (about 10% sun light intensity in this experiment).5 The voltage decay measurements were performed with zero current (open-circuit) but also over a range of fixed current, corresponding to ‘scanning’ the voltage perturbation at various positions on the photocurrent voltage curve. Small perturbation transient photocurrent measurements were performed in a similar manner to the open-circuit voltage decay measurement. However, the signal was recorded with the Keithley source meter in series with the solar cell and the oscilloscope, holding a potential difference across the device over a range of potentials between short-circuit and open-circuit. For the voltage decay measurements, no extra current is allowed to flow, following the light pulse, therefore the decay of the measured perturbation signal is entirely governed by the charge recombination within the cell. For the current decay measurements, while the charge is being collected the charges are also simultaneously recombining within the cell. Therefore the decay rate constant for the current signal (ksignal) is a combination of the decay rate constant for the transport out of the cell (ktrans) and the rate constant for the recombination in the cell (krec) as k singnal = ktrans + k rec . From the voltage decay measurements, we can estimate an electron lifetime (τre) and from the current decay measurements we can estimate an effective diffusion coefficient for electrons, De = d 2 2.77 × τ trans , where τtrans is the current collection lifetime, and d is the film thickness. From the current decay measurements, the lifetime of the current signal (τsignal) is a combination of the lifetime for the transport out of the cell (τtrans) and the recombination lifetime in the cell (τre). τtrans is calculated as 1 τ trans = 1 τ signal − 1 τ re . From the derived effective diffusion coefficients and charge
recombination lifetimes we can estimate the electron diffusion length in the titania film following this equation: Le = Deτ re .
(7) Electrochemical impedance spectroscopy measurements Electrochemical impedance spectra of QD-sensitized cells were measured using an Autolab Frequency Analyzer set-up which consists of an Autolab PGSTAT 30 (Eco Chemie B.V., Utrecht, The Netherlands) producing a small amplitude harmonic voltage, and a Frequency Response Analyzer module. The EIS experiments were performed at a constant temperature of 20 oC under dark conditions. The impedance spectra of the samples were obtained at various potentials (from -0.8 V to -0.2 V) at frequencies ranging from 0.05 Hz to ~200 kHz; the oscillation potential amplitudes having values of approximately 10 mV. In the EIS experiments, the photoanode (the QD-sensitized TiO2 electrode) was used as the working electrode, the Pt counter electrode (CE) being used simultaneously as both the auxiliary electrode and the reference electrode. EIS was employed to scrutinize the effect of QDs on the photovoltaic performance. Figure S4 compares the Nyquist plot of a nanocrystalline TiO2 film covered by CdSe6 and CdSe5Te1 QDs. The impedance data in Fig. S4 were measured at forward bias of -0.50 V under dark condition and 20 °C in presence of Co(o-phen)32+/3+ redox electrolyte. Under those conditions, we observed that the adsorption of CdSe5Te1 enlarges dramatically the radius of the semicircle (related to the interface recombination between the TiO2 conduction band electrons and Co(o-phen)33+ complex in the electrolyte) at low frequencies range (from 100 Hz to 0.05 Hz) in the Nyquist plot compared to that of the nanocrystalline TiO2 film sensitized with CdSe6. By fitting the EIS spectroscopy with the transmission line model, suggested by Bisquert et al,6 we can obtain information on the charge transfer, interfacial recombination, and charge collection yield in photovoltaic devices. A slight difference in electron diffusion coefficient (De: De = Rt C µ , Rt being electron diffusion resistance in the TiO 2 and C µ being chemical capacitance) of the devices with QDs
Figure S4. Impedance spectra (Nyquist plot) of the devices sensitized with CdSe6 and CdSe5Te1 at forward bias of -0.50 V under dark conditions at 20 oC. The solid line corresponds to derived values using the transmission line model.
was observed and is illustrated in Figure S5a, indicating that the QDs have only a small influence on the electron transport in the TiO2 nanoparticles. At the same applied bias, device utilizing CdSe5Te1 has a consistently longer apparent recombination lifetime (τn: τ n = Rct Cµ , Rct being recombination resistance, see Figure S5b) than that found in device with CdSe6, more supporting evidence for a slower charge recombination occurring in the former relative to the latter. These values obtained from impedance measurements are consistent with the transient photovoltage decay measurements of the recombination rate constant as presented in Figure 3a. The electron diffusion length (L: L = d Rct Rt , d being the film thickness) is longer for device with CdSe5Te1 than that of device utilizing CdSe6 (Figure S2c), reflecting that the increase in Jsc is caused by a higher charge collection yield.
Figure
S5.
Derived
transport/recombination
parameters
from
impedance
measurements under dark conditions for two different devices indicated: (a) Effective electron diffusion coefficient (De), (b) apparent recombination lifetime (τn), and (c) effective electron diffusion length (L) as a function of the applied bias (U).
Table S1. Photovoltaic parameters of CdSe5Te1 QD-sensitized cell, prepared from Cd(NO)3 (30 mM in ethanol) and commercially available Na2Se (30 mM in ethanol) /Na2Te (20 mM in acetonitrile) (This data was obtained 7 days later after initial measurement).
9.2% sun 29.3% sun 50.5% sun 98.4% sun
Isc (mA/cm2) 0.83
Voc (V) 0.52
FF 0.76
Efficincy (%) 3.57
2.33
0.56
0.69
3.09
3.64
0.57
0.58
2.39
4.94
0.59
0.52
1.54
Table S2. Photovoltaic parameters of best CdSe5Te1 QD-sensitized cell at 100 W/m2, measured after some time elapsed. (The cell was stored in an office drawer between measurements)
fresh after 4 days after 7 days
Isc (mA/cm2) 0.63
Voc (V) 0.58
FF 0.79
Efficincy (%) 3.11
0.79
0.59
0.78
3.98
0.83
0.60
0.78
4.18
References 1. (a) Shklover, V.; Eremenko, I. L.; Berke, H.; Nesper, R.; Zakeeruddin, S. M.; Nazeeruddin, M. K.; Grätzel, M. Inorg. Chim. Acta 1994, 219, 11. (b) Dekorte, J. M.; Owens, G. D.; Margerum, D. W. Inog. Chem. 1979, 18, 15381. (c) Szalda, D. J.; Creutz, C.; Mahajan, D.; Sutin, N. Inog. Chem. 1983, 22, 2372. (d) Nusbaumer, H. Ph. D. thesis, EPFL, 2004. (e) Nusbaumer, H.; Zakeeruddin, S. M.; Moser, J.-E.; Grätzel, M. Chem. Eur. J. 2003, 9, 3756. 2. Lee, H. J.; Chen, P.; Moon, S.-J.; Frédéric, S.; Sivula, K.; Bessho, T.; Gamelin, D. R.;Comte, P.; Zakeeruddin, S. M.; Seok, S. I.; Grätzel, M.; Nazeeruddin, Md. K. Langmuir, 2009, 25, 7602. 3. Kuang, D.; Ito, S.; Wenger, B.; Clein, C.; Moser, J.-E.; Humphry-Baker, R.; Zakeeruddin, S. M.; Grätzel, M. J. Am. Chem. Soc. 2006, 128, 4146. 4. Schmidt-Mende, L.; Grätzel, M. Thin Solid Films 2006, 500, 296. 5. O’Regan, B.; Lenzmann, F. J. Phys. Chem. B 2004, 108, 4342. 6. a) Bisquert, J.; Cahen, D.; Hodes, G.; Rühle, S.; Zaban, A. J. Phys. Chem. B 2004, 108, 8106. b) Fabregat-Santiago, F.; Bisquert, J.; Cevey, L.; Chen, P.; Wang, M.; Zakeeruddin, S. M.; Grätzel, M. J. Am. Soc. Chem. 2009, 131, 558.
