LETTER pubs.acs.org/JPCL
Illumination Intensity-Dependent Electronic Properties in Quantum Dot Sensitized Solar Cells Menny Shalom, Zion Tachan, Yaniv Bouhadana, Hannah-Noa Barad, and Arie Zaban* Chemistry Department, Bar Ilan University, Ramat Gan 52900, Israel
bS Supporting Information ABSTRACT: New results of photoelectrochemical solar cells that consist of quantum dots (QDs) deposited directly onto FTO glass identify chemical potential within the QD layer as the source for the observed photovoltage. Charge extraction and transient photovoltage measurements of this cell quantify the lifetime and density of the photogenerated electrons within the QDs layer. At open circuit voltage, the electron density approaches 1 1019/ cm3, which corresponds to one electron per dot. The electron lifetime varies from 10 ms at low photovoltage to 0.1 ms at open circuit. These results lead to new understanding of the photoelectrochemical mechanisms in quantum dot sensitized solar cell. Under illumination, the QD sensitizer layer can charge up to levels that alter the relative energetics within the cell thus affecting both the generation and recombination mechanisms. The new insight, identifying a conceptual difference between QD and dye-sensitized solar cells, opens new paths for improvement and optimization of QD sensitized solar cell. SECTION: Energy Conversion and Storage
T
he quantum dot sensitized solar cell (QDSSC) is considered to be a simple analogue of dye-sensitized solar cell14 (DSSC). The only apparent difference involves the replacement of the organometallic or organic dyes with QD sensitizers such as CdS,5,6 CdSe,711 PbS,1215 PbSe,16,17 and InP.18 Otherwise, the basic cell mechanisms, including charge separation by the sensitizer, charge transport in both the mesoporous electrode and the electrolyte, and the recombination paths seem to be similar to those of DSSC. The replacement of dye sensitizers by QDs is motivated by their absorption coefficient, which is higher than most dyes, and the size confinement that allows tailoring of their absorption spectrum.19,20 Moreover, the use of QDs opens new possibilities for third-generation solar cell configurations such as multiple carrier generation (MEG)12,17 and hot electron injection.21,22 However, despite their great potential, the conversion efficiency of QD sensitized solar cells has reached only ∼4%,23,24 a low value compared with the DSSC analogue.25The rather low efficiencies of QDSSC were attributed mainly to the charge separation and recombination processes at the TiO2/ QD/electrolyte junctions.1,3,23,2629 Recently, we reported on the use of a pure QDs photoactive electrode to form a tandem photoelectrochemical solar cell.30 The half cell arrangement of this QD solar cell (entitled QDFTO in the following) contained a QD layer deposited directly on fluorine-doped tin oxide (FTO) glass, polysulfide electrolyte (based on 1 M Na2S, 0.1 M S, and 0.1 M NaOH), and a Pt counter electrode, as shown in Figure 1. Under one sun illumination, QDFTO exhibit photovoltages up to 650 mV and r 2011 American Chemical Society
Figure 1. Schematic drawing of (a) dye-sensitized solar cell (DSSC), (b) quantum-dot-sensitized solar cell (QDSSC), and (c) solar cell consisting solely of quantum dot, which are deposited directly of FTO glass (QDFTO).
short circuit currents of ∼2 mA/cm2, which implies that QDs can build up chemical potential in the presence of the liquid electrolyte.31,32 This understanding opens a discussion on the mechanisms of QDSSCs that also utilize a QD layer as the photoactive component. Here we report on new measurements of QDFTO, which point toward conceptual differences between DSSC to QDSSC. The QDFTO configuration, in which a QD layer is deposited directly on FTO glass, enables direct photoelectrochemical study of the QDs without interference of the mesoporous TiO2 electrode. We show that solar cell based solely on QDs can generate Received: June 26, 2011 Accepted: July 20, 2011 Published: July 20, 2011 1998
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Figure 2. Energy band diagram showing the charge transfer processes in (a) DSSC: electron injection from the excited dye state to the TiO2CB while holes are removed by the electrolyte. The major path for recombination is from the TiO2CB to the electrolyte. (b) QDSSC: fast electron injection from the QD excited state directly to the TiO2CB or through the QD surface states (slower injection process) while holes are removed by the electrolyte. The main recombination paths are (1) from the TiO2CB to the electrolyte, (2) from the QD (CB or surface states) to the electrolyte, and (3) internal recombination within the QD. (c) QDFTO: photogenerated holes are removed by the electrolyte while the excited electrons diffuse within the QDs layer. The major recombination paths in QDFTO are (1) from the QD (CB or surface states) to the electrolyte and (2) internal recombination within the QD.
Table 1. CurrentVoltage Characteristics of the Two CdSeBased Solar Cells: QDSSC and CdSe Deposited Directly on FTO (Simulated 1 sun, Cell Area = 1 cm2) cell type
Figure 3. IV characteristics of the QDSSC (black) and the QDFTO (red) under one sun illumination.
photovoltaic activity when immersed in polysulfide electrolyte. Advanced characterization, utilizing charge extraction and open circuit voltage (Voc) decay techniques, quantifies the charge accumulated in the QD layer and the rate at which it recombines with the surrounding electrolyte. The results provide new insight into a fundamental difference between DSSC and QDSSC, which is critical for further improvement of QD-sensitized solar cell. Figure 1 illustrates the configuration of three photoelectrochemical cells: DSSC (1a), QD sensitized solar cell (QDSSC, 1b), and QD solar cell (QDFTO, 1c). The important generation and recombination paths of these cells are presented in Figure 2. Charge separation in DSSCs involves dye excitation, ultrafast electron injection to the TiO2 substrate, followed by slower hole transfer to the electrolyte33,34 (Figure 2a, marked black). The charges separated into the TiO2 and the electrolyte can recombine at the interface between them (Figure 2a, marked red). Other recombination paths are often negligible. In a QDFTO, the dominant process is hole extraction from the excited QD layer by the electrolyte (Figure 2c, black). The remaining electrons accumulate within the film, mostly at surface states, generating the photovoltage response.31,35 Here electrons can react with holes at the QD electrolyte interface, recombining from either the CB or the surface states of the QDs (Figure 2c, red arrows). QDSSC operation involves charge separation and recombination mechanisms of the two former systems (Figure 2b). Part of QD excitations results in fast electron injection to the TiO2 substrate, followed by hole transfer to the electrolyte (similar to DSSC). The other mechanism starts with electron trapping at the QD surface and hole extraction by the electrolyte as in QDFTO, followed by slow electron transfer to the TiO2 (Figure 2b, black arrows). The second slow injection mechanism becomes
Jsc [mA/cm2]
Voc [mV]
FF (%)
η [%]
QDSSC
3.89
473
33
0.6
ODFTO
1.87
524
31
0.3
dominant in two cases: (i) when some of the QD sensitizers are not in direct contact with the TiO2, that is, QD multilayer sensitization, and (ii) for QDs that exhibit high concentration of surface states that are positioned energetically above the TiO2 CB.3 The properties of the surface states depend strongly on the QDs preparation procedure, mainly differentiating growth on the TiO2 surface from deposition of presynthesized particles.35 Similar to charge separation, recombination in QDSSCs involves mechanisms of both DSSC and QDFTO. Electrons injected into the TiO2 or those trapped at the QDs surface can react with holes prior or subsequent to their transfer to the electrolyte (Figure 2b, red arrows).35 Here also, the ratio between the DSSC-like recombination and that resembling QDFTO depends on the properties of the QD layer. QDs exhibiting high concentration of surface states and multi-QD-layer sensitization seem to enhance the portion of recombination that involves electrons that are accumulated in the sensitizer layer.3 In the following, we discuss quantitative measurements of charge accumulation and recombination in QDFTO, resulting in new insight into QDSSC mechanisms. Figure 3 and Table 1 summarize the performance of a basic QDSSC and a flat QDFTO, both utilizing CdSe-QDs as a sensitizer. The optical density of the QDSSC that consists of a mesoporous TiO2 electrode was higher compared with the flat FTO electrode-based QDs (Figure S1 of the Supporting Information). Consequently, whereas both cells exhibit similar photovoltages and fill factors, there is a significant difference in the photocurrents and thus in the overall efficiencies. We note that it is possible to achieve much higher performance of QDSSC by surface modification,6,10,26 optimization of the sensitizer parameters,36,37 and replacement of the Pt-based cathode.38,39 However, to simplify the analysis of the measurements, we used the basic cell in both cases. The photovoltaic performance of QDFTO indicates the ability of QDs to build up charge under illumination. The high Voc values indicate that hole removal from the QD by the redox couple is faster than back electron transfer to the electrolyte. 1999
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Figure 4. (a) Extracted charge and electrons density of QDSSC (black) and QDFTO (red) versus photovoltage, both normalized to 1 cm3. (b) Number of electrons per nanoparticle in the nanocrystalline TiO2 (black) and the QD layer (red) versus photovoltage.
To quantify these, we performed observation charge extraction and transient photovoltage (TPV) measurements of the QDFTO. Charge extraction is an established technique that measures the charge density in the photoactive electrode as a function of steady-state open circuit voltage (Voc).4043 The technique utilizes white light of different intensities to realize a series of Voc values. When the light is turned off, the cell is shorted through a small resistor that enables monitoring of the discharge flow. Integration of the voltage drop across the resistor using the following equation reveals the total charge accumulated in the electrode 1 Q ¼ R
Z
t
V dt t0
where R is the circuit resistance in ohm unit (Ω), Q is the charge in coulombs (C), and V is the voltage in volts (V). Figure 4a plots the charge density and the total electron density (next) versus photovoltage for a QDFTO and a QDSSC. (Calculation parameters are provided in the Experimental Section.) In the case of QDFTO, the extracted charge relates to the QD layer only; the contribution of the FTO substrate was found to be negligible. In QDSSC, we attribute the measured charge to the mesoporous TiO2 based on the understanding that most of the electrons accumulated in the QD layer recombine with the electrolyte when illumination is turned off and due to the small volume of the QDs with respect to the mesoporous TiO2. For both the QD layer of the QDFTO and the TiO2 of the QDSSC, we find an exponential growth of charge density versus the photovoltage of the cell with a much shallower slope for the QD layer, which seems to result from a narrower distribution of density of states compared with the TiO2. Further insight is provided by a calculation of the number of electrons per nanocrystal as a function of the cell photovoltage (for calculation parameters, see the Experimental Section). Figure 4b shows that unlike the TiO2 that accumulates more that 100 electrons in a nanocrystal, the QD charging is limited to one electron per particle. This value, which seems to set an upper limit to the photovoltage of QDFTO, is probably the result of fast decay of excitons generated within charged QDs.44,45 The results presented in Figure 4 reveal a conceptual difference between DSSCs and QDSSCs. Unlike dye monolayers of DSSCs that promote ultrafast charge separation, QD sensitizers may be charged with significant electron densities depending on the thickness of the QD layer and their electronic properties. This understanding, which is based on quantitative measurements of the QDFTO analogue, remains at the qualitative level
because of interference of the QD layer and the mesoporous TiO2 with respect to the measured parameters. The new understanding opens a discussion regarding the relative energetics within QDSSCs. Similar to the nanocrystalline TiO2, which undergoes a potential shift upon buildup of chemical potential, we expect band movement of the charged QDs with respect to the redox potential. In addition, the TiO2-QD interface that is usually considered for dark conditions may also be altered under illumination because of the upward movement of the Fermi level in the two materials. In other words, our results suggest that the dark alignment of the energy levels within the cell can significantly change under illumination, including shifts at the TiO2QD interface. We leave these issues as open question until more information is available, noting that the photoinduced charging of the sensitizer in the triple TiO2-QD-electrolyte junction may open possibilities to improve the performance of QDSSCs. Further quantification of the QDSSC mechanisms is provided by TPV measurements. TPV is frequently used to reveal the electron lifetime, τ, in solar cells.42,46,47 The transients obtained can be fitted to an exponential decay providing the lifetime (τ) at the applied bias photovoltage V ¼ V0 þ Aeðt=τÞ where A is an exponential prefactor, t is the decay time, V0 is the initial voltage (at steady state for a given illumination intensity), and V is the actual photovoltage. For QDFTO, the major path of recombination involves the interface between the QDs and the electrolyte (Figure 2c), whereas QDSSC exhibits additional paths involving the electrons injected into the TiO2 (Figure 2b). TPV measurements of the two cells are shown in Figure 5, displaying, in both cases, a strong dependence of the electron lifetime on the photovoltage. Throughout the voltage scan, the lifetime of electrons accumulated in the QD layer is ∼10 times shorter compared with those injected into the TiO2. The operation of QDSSC involves the two recombination mechanisms presented in Figure 2: (i) loss of electrons that were transferred to the TiO2, the slower process that increases with the chemical potential of the TiO2, and (ii) loss of electrons at the QD-electrolyte interface prior to their transfer to the TiO2, a mechanism that influences the effective charge separation efficiency in QDSSCs. Moreover, Figure 5 shows that charge separation events that involve electron trapping on the QDs prior to transfer into the TiO2 become less efficient with increasing electron densities in the QD sensitizer. That is, the photoinduced charging of the QDs reduces the observed injection rate and increases the recombination of electrons from the QDs to the electrolyte. However, in the absence of a direct 2000
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Figure 5. Comprehension of the transient photovoltage (TPV) of QDSSC (black) and QDFTO (red) versus photovoltage.
measurement of the charge accumulated in the QD sensitizer of the QDSSC, the significance of this process cannot be evaluated. It is therefore left as an open question until more information will be available. In this work, we demonstrated a conceptual difference between DSSC and QDSSC. Although both of the cells have similar configurations, the use of QDs as sensitizers leads to new paths for charge transfer and recombination involving in the cell operation. Using solar cell based solely on quantum dot, we show that QDs have a photovoltaic response resulting in significant photocurrent and voltage. Charge extraction technique was utilized to measure, for the first time, the charge density and the number of electrons within the QDs under working conditions. TPV measurements reveal the lifetime of electrons within the QDs layer, which is more than 10 times shorter than in the TiO2. Consequently, we anticipate shifts of the relative energetic levels within the system and a decrease in the charge separation efficiency, both induced by the charging of the QD layer. The new understandings of the QDSSCs mechanism should enable their optimization toward highly efficient, low-cost photovoltaics.
’ EXPERIMENTAL SECTION CdSe QDSSC was fabricated using Commercial TiO2 paste (TiNanoxide D, Solaronix, Switzerland) by the doctor blade technique on FTO and sintered at 450 °C for 30 min. The mesoporous film thickness was 2.2 μm, measured by a profilemeter (Surftest SV-500). A seeding layer of CdS was deposited by the SILAR method prior to the CdSe sensitization. Therefore, the TiO2 electrodes were dipped into 0.1 M Cd(ClO4)2 for 1 min, washed with deionized water, immersed into 0.1 M Na2S aqueous solution, and washed again. After seeding, chemical bath deposition (CBD) was used to sensitize the electrode with CdSe QDs following the procedure by Gorer and Hodes.48 Sodium selenosulphate (Na2SeSO3) solution (80 mM) was prepared by dissolving Se powder in a 200 mM Na2SO3 solution (solution A). CdSO4 (80 mM) and trisodiumsalt of nitrilotriacetic acid (N(CH2COONa)3) (120 mM) were mixed in a volume ratio 1:1 (solution B) before solutions A and B were mixed in a volume ratio 1:2. The mesoporous TiO2 electrodes were immersed into the final solution for 24 h at 10 °C and kept in the dark. For the QD solar cell, a seeding layer of CdS was utilized on FTO by CBD technique. For CdS QD deposition, FTO electrodes were immersed in a mixture of 2.35 mL of 0.5 M CdSO4 and 2.65 mL of 0.7 M potassium nitrilotriacetate (K3NTA) at pH 8.5 adjusted by 10% KOH. This solution was mixed with 4.25 mL of 0.4 M thiourea and then diluted with 7.55 mL of distilled water.
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Finally, the pH was readjusted to pH 11 using again 10% KOH. After the electrode was immersed in the solution, it was heated up to 70 °C for 2 h, resulting in 70 nm of CdS QDs on FTO. A predeposited CdS QD thin layer serves as a substrate for the growth of CdSe QDs in the CBD method mentioned before. The QD film thickness was ∼200 nm, measured by cross section made by focus ion beam (FIB) instrument. Photocurrentvoltage characteristics were performed with an Eco-Chemie potentiostat. A solar simulator class A (new port) calibrated to 100 mW/cm2 (AM 1.5 spectrum) served as a light source. The illuminated area of the cell was set to 1 cm2 using an aperture. Na2S (1 M), 0.1 M sulfur, and 0.1 M KOH solution served as the electrolyte. A sputtered Pt-coated FTO glass was used as a counter-electrode. For TPV measurements, devices were directly illuminated to reach steady-state conditions. Then, a small pulse of light, which provoked up to 10 mV increase in the voltage, was applied. The excess of charges recombine at approximately the same rate setting the equilibrium, which determines the Voc observed. For charge extraction measurements, the solar cell connected to a data acquisition under opencircuit conditions (1 GΩ). Then, the device was illuminated with white light to a desired Voc; at this point, equilibrium between charge formation, due to the illumination with light, and charge recombination was reached. Then, the light was turned off simultaneously to short circuiting the cell through a 10 Ω resistor so that the total charge flow can be derived from integration of the current measurement throw the cell. The number of extract electrons was calculated by dividing the charge density (normalized to cubic centimeters) by an electron charge. For the calculation of electrons per particle, we used a spherical shape of both QDs and TiO2 assuming 8 and 15 nm diameter, respectively. The thickness of both electrodes was determined by a high-resolution scanning electron microscopy (HRSEM) cross-section. Next, we calculated the number of particles (QDs or TiO2) within the electrode assuming porosity of 0.5. By knowing the number of particles and the total charge, we can extract the number of electrons per particle by the equation Q 1:6e19 number of particles where Q is the total charge and 1.6e19 is the charge of an electron, both in coulombs.
’ ASSOCIATED CONTENT
bS
Supporting Information. Absorption spectra of CdSe on conductive substrate and on TiO2. HRTEM image of CdSe QDs on mesoporous TiO2 and TPV row data are presented. This material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We thank the Israel Ministry of Science, Tshtiyot Program, for the financial support. M.S. thanks the “Converging Technologies” program. 2001
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