Subscriber access provided by UNIVERSITY OF CONNECTICUT
Energy, Environmental, and Catalysis Applications
Experimental and Theoretical Investigation of the Function of 4-tert-butyl pyridine for Interface Energy Level Adjustment in Efficient Solid-State Dye-Sensitized Solar Cells Lei Yang, Rebecka Lindblad, Erik Gabrielsson, Gerrit Boschloo, Håkan Rensmo, Licheng Sun, Anders Hagfeldt, Tomas Edvinsson, and Erik M. J. Johansson ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16877 • Publication Date (Web): 21 Mar 2018 Downloaded from http://pubs.acs.org on March 21, 2018
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 10 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Materials & Interfaces
Experimental and Theoretical Investigation of the Function of 4-tertbutyl pyridine for Interface Energy Level Adjustment in Efficient Solid-State Dye-Sensitized Solar Cells Lei Yang1,2, Rebecka Lindblad3, Erik Gabrielsson4, Gerrit Boschloo1, Håkan Rensmo3, Licheng Sun4, Anders Hagfeldt5, Tomas Edvinsson6, Erik M. J. Johansson1* 1
Physical Chemistry, Department of Chemistry - Ångström, Uppsala University, SE-751 20 Uppsala, Sweden.
2
School of Environmental and Municipal Engineering, Xi’an University of Architecture and Technology, Xi’an, Shaanxi Province, 710055, China.
3
Department of Physics and Astronomy, Uppsala University, SE-751 20 Uppsala, Sweden.
4
Organic Chemistry, School of Chemical Science and Engineering, Royal Institute of Technology, Stockholm, Sweden. 5
Laboratory of Photomolecular Sciences, Department of Chemistry and Chemical Engineering, Swiss Federal Institute of Technology, Station 6, CH-1015 Lausanne, Switzerland.
6
Inorganic Chemistry, Department of Chemistry - Ångström, Uppsala University, SE-751 20 Uppsala, Sweden.
KEYWORDS Keywords: mesoporous, TiO2, photovoltaic, dye, solar energy
ABSTRACT: 4-tert-butylpyridine (t-BP) is commonly used in solid state dye-sensitized solar cells (ssDSSCs) to increase the photovoltaic performance. In this report, the mechanism how t-BP functions as a favorable additive is investigated comprehensively. ssDSSCs were prepared with different concentration of t-BP and a clear increase in efficiency was observed up to a maximum concentration and for higher concentrations the efficiency thereafter decreases. The energy level alignment in the complete devices was measured using hard X-ray photoelectron spectroscopy (HAXPES). The results show that the energy levels of titanium dioxide are shifted further away from the energy levels of spiro-OMeTAD as the tBP concentration is increased. This explains the higher photovoltage obtained in the devices with higher t-BP concentration. In addition, the electron lifetime was measured for the devices and the electron lifetime was increased when adding t-BP which can be explained by the recombination blocking effect at the surface of TiO2. The results from the HAXPES measurements agree with those obtained from density functional theory (DFT) calculations and give an understanding of the mechanism for the improvement, which is an important step for the future development of solar cells including t-BP.
Introduction The nano-structured dye-sensitized solar cell (DSC) has become a promising technology due to their cost-effective fabrication.1,2 Based on the development through decades a world record power conversion efficiency of 13% has been reached by Grätzel et al. for the electrolyte based DSCs.3,4 However, the architecture of these devices inevitably suffers from the leakage of electrolyte in terms of long-term stability and industrial applications. By replacing the conventional liquid electrolyte with a solid-state p-type semiconductor hole transport material (HTM) 2,2',7,7'-tetrakis(N,N-dimethoxyphenylamine)-9,9'spirobifluorene, termed as spiro-OMeTAD (Figure 1), all solid-state DSC (ssDSSC) was first demonstrated by Bach et al..5 To date, the highest certified power conversion efficiency of ssDSSC has been improved up to 6.08% while the lab record has reached 7.2% by introducing an
addition of a cobalt complex to the HTM that act as a ptype dopant.6,7 In spite of different device architectures, two crucial additives are commonly employed for interfacial modifications in DSC to obtain the appropriate device performance, namely 4-tert-butylpyridine (t-BP, Figure 1) and lithium bistrifluoromethylsulfonyl imide (Li-TFSI). It has been observed that regardless of DSC structures the devices barely function without the presence of these two interfacial additives. These additives are usually introduced into the device during the fabrication of the HTM layer by mixing the additives in the HTM solution. The additives therefore also play an important role on the formation of the HTM layer in the porous electrode. The function of Li-TFSI in DSC has been intensively studied. It has been shown that Li+ cations tend to transport towards the negatively charged TiO2 surface by
ACS Paragon Plus Environment
ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
the light induced potential-difference across the device under illumination. Consequently the adsorbed Li+ cations may compensate the injected free electrons in the conduction band (CB) of TiO2.8 Hence, it is commonly agreed that Li+ cations interact with the injected electrons screening the columbic interaction between injected electrons and the holes and therefore significantly inhibiting the charge carrier recombination reaction at interfaces.9-12 On the other hand, it was also suggested that LiTFSI is required as a catalyst to facilitate the photodoping of spiro-OMeTAD with oxygen, to improve the hole conductivity of HTM layer.13-16 Even though t-BP is an important additive for improving the ssDSSC performance, the operating mechanism of t-BP still remains poorly understood. It has been suggested that t-BP may have two significant roles for DSC devices: 1) similar to Li-TFSI, t-BP tends to adsorb onto the TiO2 surface and to protect the area uncovered by dye molecules and therefore to inhibit the free charge carrier recombination;17,18 2) the molecular dipole moment of tBP leads to a favorable upward shift of the conduction band of TiO2.19 In ssDSSC devices, the open circuit voltage, Voc, is determined by the energy level difference between the quasi-Fermi level in the mesoporous TiO2 electrode and the quasi-Fermi level in the HTM layer. Therefore, Voc can be improved by either increasing the conduction band electron density or shifting the conduction band edge further away from the HOMO (highest occupied molecular orbital) level of the HTM. Apart from the fact that the lifetime of injected free electrons was normally prolonged by introducing t-BP at the interfaces, the former function of t-BP was barely discussed in terms of coordination chemistry, such as coordination angel and conformation. In this study, the coordination reaction of t-BP onto the 101 surface of anatase TiO2 (the dominating surface on nanoparticles and the surface with highest photo-catalytic activity) is investigated with DFT (Density Functional Theory) calculation. As regard to the latter function of t-BP, it has not been studied in detail experimentally or theoretically how the band structure of dye-sensitized mesoporous TiO2 electrode is shifted by the interfacial dipole moment of t-BP. In literature, the interfacial capacitance has been measured to evaluate the energy level shift of flat-band potential Efb of anatase TiO2 in the presence of t-BP as interfacial modifier. However, in these studies, the results only interpret the influence of t-BP on the dense crystalline layer of TiO2 (also referred as blocking layer, which is not the active light absorbing layer) rather than the dyesensitized mesoporous layer (active light absorbing layer where the charge separation really occurs).9,17,18 In general, Efb is a rather accurate approximation of the electron quasi-Fermi level in n-type semiconductor, such as TiO2, indicating approximately where the conduction band edge in the energy diagram. However, the mesoporous TiO2 electrode in DSCs is composed of a number of interconnected nanoparticles with relatively low doping density (generally around 1017 cm-3) and considerably small diameters (normally around 20 nm).19 Unlike the conven-
Page 2 of 10
tional p-n junction solar cells, the band-bending is usually negligible in this type of nanoparticle-based network since the charge depletion region can be hardly established in such a small dimension.20 Hence, the energy shift of Efb determined by fitting the Mott-Schottky plot with electrochemical measurements of interfacial capacitance is no longer an applicable approximation for determining the energy level shift of the conduction band of dye-sensitized mesoporous TiO2 electrode arriving from the interfacial t-BP dipole moment. In this work, a systematic study how t-BP influences ssDSSC photovoltaic performance was carried out by altering the t-BP concentration gradually from 0 mM to 240 mM in the spiro-OMeTAD (HTM) solution, which was used to prepare efficient ssDSSCs sensitized by a custom-synthesized dye LEG4.8,21 In these devices, the photovoltaic performance, especially Voc and the electron lifetime τe, were improved significantly by modifying the t-BP concentration. Hard X-ray photoelectron spectroscopy (HAXPES) was performed on the same complete working devices as studied in I-V measurements to investigate how the energy levels of the materials are affected by t-BP. In order to simulate the functioning mechanism of t-BP, DFT calculations were also performed. Results and Discussion Organic chemistry allows for structural modification of organic sensitizers in order to control the electron energy levels. One of the most successful strategies to design the dyes is to build a D-π-A (donor-linker-acceptor) structure. Such a “push-pull” design facilitates the electron injection and minimizes charge recombination. According to this strategy, organic sensitizer LEG4 (Figure 1) was designed and modified based on the molecular structure of the dye D35 by introducing an extra conjugated π linker.21,22 Due to the more rigid and planar conjugated structure, the absorption spectrum of LEG4 is broadened with around 25 nm red-shift meanwhile the extinction coefficient is also increased by 18900 M-1×cm-1 compared to D35. These new features allow LEG4 dye to harvest more photons in the visible region of solar spectrum.21 O
O
N
N
t-BP Molecular Weight: 135.21
N
N
O
N
O O
O O
spiro-OMeTAD Molecular Weight: 1225.43
O O
O HO C
O CN S S N
O
LEG4 C71H84N2O6S2 Molecular Weight: 1125.57
O
Figure 1. The chemical structures of spiro-OMeTAD, t-BP and LEG4.
ACS Paragon Plus Environment
Page 3 of 10
80
3.0
0mM t -BP 60mM t -BP 120mM t -BP 240mM t -BP
70
-0.011 -0.010
2.5
-0.009
2.0
40
1.5
30 1.0
Absorbance
50
20
LEG4/TiO2
10
0.5
Current Density / Acm
-2
60
IPCE (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Materials & Interfaces
-0.008 -0.007 -0.006 -0.005 -0.004
0mM t -BP 60mM t -BP 120mM t -BP 240mM t -BP
-0.003 -0.002 -0.001
0 400
500
600
700
0.0 800
0.000 0.0
0.1
0.2
Wavelength / nm
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Voltage / V
Figure 2. Absorption spectrum of the sensitizer LEG4 on a mesoporous TiO2 electrode (pink dotted trace) and IPCE spectra for ssDSSCs using four different t-BP concentration respectively; the cell with 0 mM t-BP (black trace with square), the cell with 60 mM t-BP (red trace with circle), the cell with 120 mM t-BP (blue trace with uptriangle), the cell with 240 mM t-BP (green trace with diamond).
As shown in Figure 2, the contribution to the photocurrent in the IPCE spectra is mainly attributed to the absorption of LEG4 in the spectral region from 400 nm to 680 nm. For comparison, the adopted concentration of spiro-OMeTAD and Li-TFSI were fixed as 150 mM and 20 mM respectively in all ssDSSC devices. As expected, the device without t-BP shows the lowest IPCE spectrum of only 35% at the maximum absorption wavelength of 470 nm. It can be clearly seen that the IPCE increases when the t-BP concentration is increased in the device. Especially, the IPCE of the device with 120 mM t-BP has been improved up to 76% by the factor more than 2 compared to the device without t-BP. Of particular interest is that most IPCE spectra exhibit similar shape and curvature by following the absorption spectrum of LEG4 expect for the one with the highest tBP concentration, where it appears a clear loss in photocurrent around the maximum absorption wavelength. The slight decrease in IPCE is probably due to the effect that the electron injection driving force, the Gibbs free energy ΔG0 between the energy level of excited state of the sensitizer and the conduction band of TiO2, becomes less sufficient when the interfacial concentration of adsorbed t-BP is tuned too high. The reason for this effect is further discussed in the latter sections.
Figure 3. I-V characteristics for ssDSSC devices: the cell with 0 mM t-BP (black trace with square), the cell with 60 mM tBP (red trace with circle), the cell with 120 mM t-BP (blue trace with uptriangle), the cell with 240 mM t-BP (green trace with diamond). Voc
Jsc
FF
η
(mV)
(mAcm-2)
(%)
(%)
0 mM
560 ± 28
5.5 ± 0.3
53 ± 3
1.66 ± 0.08
60 mM
915 ± 46
6.7 ± 0.3
68 ± 3
4.12 ± 0.21
120 mM
930 ± 47
10.0 ± 0.5
63 ± 3
5.85 ± 0.29
240 mM
950 ± 48
9.8 ± 0.5
62 ± 3
5.81 ± 0.29
ssDSSC
Table 1. Data of I-V characteristics for ssDSSC devices with different t-BP concentration. The data are averages for 3 different devices.
As shown in Figure 3 and Table 1, in agreement with the IPCE spectra, the device without t-BP shows the lowest Jsc of 5.5 mAcm-2 while the one with 120 mM t-BP provides the highest Jsc of 10.0 mAcm-2 indicating that the Jsc follows the same trend as the IPCE. As expected, the Jsc of device with 240 mM t-BP declines slightly by 0.2 mAcm-2 compared to the device with 120 mM t-BP. On the other hand, the device without t-BP gives the lowest Voc of 560 mV while the one using 240 mM t-BP shows the highest Voc of 950 mV with the difference of 390 mV. Similar to the variation in Jsc, the improvement of Voc also appears significant when t-BP is applied into the device. However, Voc increases also when the t-BP concentration increases to 240 mM, whereas Jsc decreases at the highest concentration of t-BP. Due to the high boiling point (196 °C) for t-BP, itremains in the ssDSSC devices after physical vapor deposition of Ag contact in the high vacuum chamber during the last fabrication process. Due to the molecular structure of t-BP, the higher electronegativity of the pyridine ring (owing to the high electronegativity of nitrogen atom) and the electron-donating property of the tert-butyl group (due to the hyper-conjugation effect) collectively lead to an uneven distribution of charge density along the molecule. The electron density on the aromatic ring in
ACS Paragon Plus Environment
ACS Applied Materials & Interfaces the vicinity of the nitrogen atom is relatively higher than the electron density on the tert-butyl terminal, which results in a molecular dipole moment established along the molecule pointing from the nitrogen atom to the tertbutyl moiety. During device fabrication, t-BP molecules infiltrate through the mesoporous electrode along with HTM solution, the lone pair of electrons on the nitrogen atom may coordinate to the unoccupied 3d orbitals of a titanium atom on the TiO2 surface. This effect would result in that the adsorbed t-BP molecules protect the TiO2 surface from the carriers of holes (either oxidized sensitizers or HTM) after charge separation and therefore prohibit the recombination reactions between free charge carriers. Hence, the photocurrent is improved when t-BP is applied. It is well agreed that in ssDSSC devices Voc is determined by the energy level difference between the quasiFermi level in mesoporous TiO2 electrode (EF,n) and the quasi-Fermi level in HTM layer (EF,HTM) as shown in Equation 1.
= , − ,
(1)
Normally the EF,HTM of spiro-OMeTAD remains constant when adding t-BP since no chemical reaction between tBP and spiro-OMeTAD occurs. The EF,n of a n-type semiconductor material is determined by Equation 2, where ECB is the conduction band edge of the n-type semiconductor material, nCB denotes the electron density in the conduction band, NCB designates the density of states in the conduction band, kB is the Boltzmann constant and T is the absolute temperature. According to Equation 2, the EF,n of mesoporous TiO2 is shifted in the same direction with its conduction band.
, = + ln
(2)
Moreover, the arrangement of an interfacial dipole moment pointing away from the interface generally causes an upward energy level shift of the band structure of the semiconductor, and vice versa.23 When t-BP molecules are oriented with the nitrogen atom close to the TiO2 surface, the dipole moment of t-BP points away from the interface, this may lead to a favorable upward shift of the conduction band of mesoporous TiO2 further away from the HOMO of HTM. According to Equation 1 and 2, such as interfacial effect will eventually increase the Voc of ssDSSC. Band-bending has been observed to be negligible in TiO2 nanoparticles since the charge depletion region can be hardly established in such a small dimension. Thus, the approximation that the shift of Efb can be used to determine the energy level shift of bulk semiconductor material by fitting Mott-Schottky plot from electrochemical measurements of interfacial capacitance becomes no longer applicable for dye-sensitized mesoporous TiO2 electrode. Besides, the electrochemical measurements are normally carried out in a supporting electrolyte where the chemical environment would be totally different from that in a working ssDSSC device.
Fortunately, hard X-ray photoelectron spectroscopy (HAXPES) provides a direct pathway to experimentally monitor the band structure shifts of the mesoporous TiO2 electrodes in the presence of t-BP in those complete working devices studied in the I-V measurements. HAXPES was performed to directly investigate how the energy level alignment in the photoactive interface is influenced when t-BP is applied. In general, photoelectron spectroscopy is a surface-sensitive technique where only the outermost molecular layers of a material can be measured. However, by using hard X-rays this technique becomes more bulk-sensitive and applicable for measuring buried interfaces as well, which is crucial for this study since the aim is to measure the change in energy level alignment between TiO2 and spiro-OMeTAD in complete working devices through a rather thick residual layer of spiro-OMeTAD covering the dye-sensitized TiO2 surface. Therefore, by using HAXPES, it becomes feasible to simultaneously measure the energy levels of both bulk TiO2 and spiro-OMeTAD for determining the changes in energy level alignment due to the effect of t-BP in the fully assembled ssDSSC devices. The energy level alignment of all components at the interface can be observed from both valence electronic levels and core electronic levels. In this study, the energy level alignment is monitored by following the core-level binding energies for the different components since the signals from core-levels are element-specific and well defined.23 This therefore allows to distinguish the energy levels of TiO2 (e.g., Ti2p) from those of spiro-OMeTAD (e.g., N1s or C1s) to compare the binding energy from different species. The N1s core-level was selected for energy calibration and the N1s peak was therefore set to the same binding energy for all samples. Then, the difference in energy level alignment can be readily traced by monitoring the change in binding energy for the Ti2p core level peaks. C1s
N1s
no 4-tbp
Intensity /arb. units
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 10
60 mM 4-tbp
240 mM 4-tbp
408
406 404 402 Bindning Energy /eV
400
no 4-tbp
Ti2p 3/2 no 4-tbp
60 mM 4-tbp 60 mM 4-tbp
240 mM 4-tbp 240 mM 4-tbp
294 292 290 288 286 284 282 Binding Energy /eV
466
464 462 Binding Energy /eV
460
Figure 4: Hard X-ray photoelectron spectroscopy (HAXPES) measurements of complete (working) ssDSSC devices with different t-BP concentrations. The left panel shows the N1s core-level spectra of different devices, the middle panel displays the C1s core-level spectra of different devices and the right panel displays the Ti2p3/2 core-level spectra of different devices.
As can be seen in Figure 4, the HAXPES spectra of N1s, C1s and Ti2p core-levels are displayed respectively for the ssDSSC devices with different concentration (0 mM, 60 mM, 240 mM) of t-BP.
ACS Paragon Plus Environment
Page 5 of 10 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Materials & Interfaces
The N1s core level of spiro-OMeTAD is used as binding energy reference, therefore, the N1s core-levels from different samples all show the same binding energy. The C1s core levels also have similar binding energy (around 288 eV) for all samples since the main contribution to the C1s spectra arises from spiro-OMeTAD. It is considered that contribution to the C1s spectra from surface contamination becomes negligible due to the bulk-sensitivity of HAXPES. In contrast, the Ti2p core-level spectra are clearly shifted for the different samples. An obvious spectral shift of 0.35 eV is observed for the sample with 60 mM t-BP compared to the sample without t-BP. In addition, an even larger shift of 0.7 eV is observed for the sample with 240 mM t-BP compared to the sample without t-BP. This distinct experimental result elucidates that the energy level matching between TiO2 and spiro-OMeTAD is changed markedly when t-BP is applied in ssDSSC devices. The Ti2p peak is shifted towards lower binding energy versus spiro-OMeTAD in the presence of t-BP and the dimension of the shift increases when the t-BP concentration increases. Since the electronic band structure of mesoporous TiO2 is not affected by the addition of t-BP, it can be inferred that the conduction band and valence band energy levels are both shifted in the same direction with the same amount as the Ti2p core-level. Thus, the ECB of mesoporous TiO2 is shifted upwards in comparison to the spiroOMeTAD energy levels due to the addition of t-BP. According to Equation 1 and 2, the EF,n of mesoporous TiO2 is also shifted upwards away from EF,HTM ultimately resulting in the improvement of Voc in ssDSSC devices. This explains why Voc increases as t-BP concentration increases, as observed in the I-V characteristics above. The results from HAXPES measurements therefore agree with the I-V characteristics of the ssDSSC devices. For instance, compared to the device without t-BP, the Voc of the device with 60 mM t-BP is improved by 0.335 V since the shift in energy level alignment of mesoporous TiO2 in the same device is 0.35 eV. From another perspective, the shift observed in the HAXPES data also proves that t-BP molecules tend to adsorb onto TiO2 surface since the shift of energy level alignment occurs only if the adsorption of tBP on TiO2 surface takes place. In summary, the results of HAXPES measurements are schematically illustrated in Figure 5.
TiO2
Dye
Spiro-OMeTAD
CB
Figure 5: Schematic illustration of the variation in energy level alignment among different components in the ssDSSC devices and the effect of t-BP measured with HAXPES. Upon addition of t-BP, the energy level matching between TiO2 and spiro-OMeTAD is altered and the TiO2 energy levels (Ti2p) are observed to change in binding energy versus the spiro-OMeTAD energy levels (N1s and C1s).
However, it is worth noting that the amount of increase in Voc measured in I-V characteristics does not equal to the amount of energy level shift measured in HAXPES between the two devices with 60 mM or 240 mM t-BP concentration. The Voc of the device with 240 mM t-BP is only improved by 0.035 V compared to the device with 60 mM t-BP while the energy level shift between these two samples is 0.35 eV as 10 times great as the difference in Voc. This is probably due to that the electron density of TiO2 conduction band nCB no longer increases as the electron injection driving force becomes less and less sufficient when t-BP concentration reaches higher and higher level. Marcus theory suggests that the kinetics of an electron transfer reaction is determined by the driving force (the difference in Gibbs free energy ΔG0) and the electronic coupling between the two transition states. The Gibbs free energy ΔG0 between the LUMO energy level of LEG4 (-2.98 eV, the excited state of sensitizer)21 and the energy level of conduction band edge of mesoporous TiO2 (ECB = 4.0 eV)24 is estimated to around 1 eV in the absence of tBP at the TiO2 interface, which provides a highly sufficient electron injection driving force. However, the ΔG0 decreases when 60 mM t-BP is applied due to the 0.35 eV upward shift in energy level alignment of mesoporous TiO2 electrode. Similarly, the ΔG0 further decreases when the t-BP concentration increases to 240 mM due to the larger upward shift of 0.7 eV in energy level alignment of mesoporous TiO2 electrode. Thus, the electron injection driving force becomes less sufficient and the electron density of TiO2 conduction band decreases. According to Equation 1 and 2, the decrease of nCB partially cancels out the increase of ECB eventually leads to a slower increase of EF,n and Voc. This elucidates that the slight photocurrent loss of the device with 240 mM t-BP compared to the device with 120 mM t-BP observed in IPCE and I-V characteristics also affects the photovoltage. Moreover, it is observed that the recombination seems to increase rapidly when t-BP concentration is too high, which also may be related to the energy difference between the conduction band of TiO2 and spiro-OMeTAD.
HOMO VB
t-BP addition
N1s Ti2p Binding Energy
ACS Paragon Plus Environment
ACS Applied Materials & Interfaces
0.01
0mM t -BP 60mM t -BP 120mM t -BP 240mM t -BP Electron Lifetime / s
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1E-3
10
Light Intensity / mWcm
Page 6 of 10
energy was a partial negative nitrogen group binding to a Ti4+ acting as a Lewis acidic site and stabilized by sideway hydrogen bonds towards oxygen on the 101 surface (Figure 7). Analyzing the density of states, the energy levels of both valence band (VB) edge and conduction band (CB) edge of the TiO2 active layer were shifted upwards to the vacuum level by 0.36 eV with the attachment and relaxation of the system with t-BP. The DFT calculation therefore suggests that the improvement of Voc in ssDSSC devices is due to the favorable shift of TiO2 conduction band by t-BP coordinating with the acidic sites localized around the Ti-atoms on the surface.
100 -2
Figure 6. Electron lifetime in correlation with light intensity for the ssDSSC devices: the cell with 0 mM t-BP (black trace with square), the cell with 60 mM t-BP (red trace with circle), the cell with 120 mM t-BP (blue trace with uptriangle), the cell with 240 mM t-BP (green trace with diamond).
Using transient photovoltage decay measurements, it is possible to estimate how fast injected electrons recombine with the carriers of holes (either oxidized sensitizer or HTM), from which the electron lifetime (τe) can be determined. As shown in Figure 6, the electron lifetime is remarkably prolonged upon addition of t-BP and generally increases as the t-BP concentration increases. However, the device with 240 mM t-BP exhibits shorter τe compared to the device with 120 mM t-BP. This may be due to the higher recombination driving force (the ΔG0 between ECB of TiO2 and HOMO level of either LEG4 or spiroOMeTAD is enlarged). As one of the most important and necessary interfacial additives, the t-BP concentration was optimized according to the discussion above. It was found ssDSSC devices with 120 mM t-BP concentration provides the best photovoltaic performance with the power conversion efficiency of 5.9 %, Voc of 925 mV, Jsc of 9.6 mAcm-2 and FF of 0.67. In order to quantify the theoretical shift of energy levels when t-BP attaches to the mesoporous TiO2 electrode, DFT calculations were performed on the most dominant and active surface of the anatase TiO2 nanoparticles (the 101 surface). The influence of t-BP as additive was calculated by first geometry optimizing a 4 × 3 TiO2 crystal unit cell with the 101 surface exposed on the top layer. The slab contained 18 layers of Ti and O with a termination of an oxygen rich plane (Figure 7) and the system was fully geometry optimized with respective to both coordinates and cell parameters using the PBE functional in Crystal09 and (6x6x6) k-points in the Brillouin zone. The system was then transferred to a cluster calculation in Gaussian09 where the local Ti and O atoms at the surface around the nitrogen together with the attached t-BP were allowed to relax and freezing the remainder of the 4 × 3 unit cell of anatase. 70 atoms were free to geometrically relax around the nitrogen binding. The cluster DFT calculations were performed with the B3LYP functional on a 6311G(d,p) level and the cluster was geometry relaxed with and without t-BP. The binding coordination with lowest
Figure 7. The 18 layer TiO2 slab with the 101 surface and the locally relaxed t-BP on top (left inset). An energy level diagram of the calculated CB edge energy level in TiO2 (-3.99eV) and the relative shifts when t-BP is attached (right inset).
Although the shift obtained in the DFT calculations (0.36 eV) is in excellent agreement with the shift measured with HAXPES for the low coverage limit (0.35 eV for 60 mM t-BP), the calculated situation corresponds to an idealized situation without dye molecules or an outer spiroOMeTAD layer. We also performed cluster calculations of the isolated species showing an molecular dipole moment of 3.02 D for t-BP on a B3LYP/6-311G (d,p) level. The surface dipole contribution ΔΦdipole of t-BP with deviating angle can be estimated by using the Helmholtz equation ΔΦdipole=q·nd·μ·cos(θ)/εrεo where q is the elementary charge, nd is the packing density of molecular dipoles, εr is the relative permittivity, ε0 is the permittivity of vacuum, and θ is the angular deviation from a perpendicular configuration. A 20 or 30 degree tilting would then only give factors of 0.94 and 0.87 lower projected dipoles indicating that the result is not so sensitive to the local tilting but would instead be more dependent on the packing density of molecular dipoles (nd) when going beyond the diluted limit. Conclusion In summary, the mechanism how t-BP as additive influences ssDSSC photovoltaic performance was systematically investigated on samples with altered t-BP concentration. The results from the HAXPES measurements highly agree with the solar cell characterization measurements, which provides a direct experimental evidence showing that the observed improvements in photo-voltage upon
ACS Paragon Plus Environment
Page 7 of 10 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Materials & Interfaces
addition of t-BP is due to the molecular dipole moment of t-BP resulting in a beneficial upward shift of the conduction band of dye-sensitized mesoporous TiO2 electrode. Moreover, the adsorbed t-BP molecules on TiO2 surface efficiently inhibit charge recombination leading to a positive effect on photovoltaic performance. In addition, DFT calculations were performed to visualize the adsorption of t-BP on TiO2 surface. In agreement with the HAXPES results, the surface dipole changes the work-function of the surface in the favorable direction. Hence, this study unveils the functioning mechanism of t-BP as an interfacial additive in ssDSSC devices with solid experimental evidence and predictive theoretical insight. We think that this mechanism is rather general, and provides possibilities for future improvements of hybrid organic/inorganic solar cells. Experimental Section Device Fabrication: All chemicals were purchased from Sigma-aldrich unless specified. ssDSSC devices were prepared on FTO-coated glass substrate (purchased from Pilkington, 15 Ω/□, 2.3 mm thick, high transparency). Firstly, a dense TiO2 blocking layer was deposited onto the pre-cleaned FTO substrate heated on hotplate at 450 °C by spray pyrolysis using an air brush from above in a distance of 5 cm. The thickness of this layer was controlled by using 10 spray cycles. The precursor solution used for spray pyrolysis contains 0.2 M Ti-isopropoxide and 2 M acetylacetone in isopropanol. Mesoporous TiO2 films were prepared on the top of blocking layer by spincoating a colloidal TiO2 paste (Dyesol DSL 18NR-T) containing nanoparticles with 20 nm average diameter. The paste was diluted with terpineol (46.2% in weight ratio). A spin-coating rate of 2400 rpm during 30 s was used to obtain a mesoporous film with thickness around 2.2 μm, as measured with DekTak profilometer and SEM. After sintering the TiO2 film on hotplate at 500 °C for 30 min, the film was cooled down to room temperature and then immersed into 20 mM aqueous TiCl4 solution at 70 °C for 30 min. Afterwards the film was rinsed with deionized water and again annealed on hotplate at 500 °C for 30 min. Then the film was dye-sensitized in a solution of 0.5 mM LEG4 (Dyenamo) in MeCN for 18 h. The hole transport material (HTM) solution was prepared in an Argon glovebox before spin-coating comprising 150 mM spiro-OMeTAD (Lumtech) (213 mg/mL), 20 mM LiN(CF3SO2)2 and 4-tert-butylpyridine in chlorobenzene, as regard to 4-tert-butylpyridine four different concentrations (0 mM, 60 mM, 120 mM, 240 mM) were employed for each sample. The HTM solution was then applied to the dye-sensitized TiO2 films by leaving the solution to penetrate into the films for 60 s followed by spin-coating at a rate of 2000 rpm for 30 s. To complete the solar cell an Ag (Sigma-aldrich; ≥ 99.99% trace metals basis) contact with thickness of 200 nm was deposited on the top of spiro-OMeTAD residual layer by physical vapor deposition in a vacuum chamber (Leica EM MED020). The base pressure of the vacuum chamber was around 10-5 mbar.
All measurements were performed on freshly made devices within one week after device fabrication. UV-Vis Spectroscopy: An HR-2000 Ocean Optics fiber optics spectrophotometer was used for measuring UVVisible absorption spectra of the dye-sensitized TiO2 films.. The background absorption of conducting glass was subtracted from the absorption spectra. Incident Photon to Current Conversion Efficiency (IPCE): The IPCE spectra were measured using a computercontrolled setup including a monochromator (Spectral Products CM110), a xenon lamp (Spectral Products ASBXE-175) and a potentiostat (EG&G PAR 273). The measurements were intensity calibrated with a certified silicon solar cell (Fraunhofer ISE) prior to measurements. All ssDSSC devices were illuminated from the glass side with an aperture area of 0.16 cm2 (0.4 × 0.4 cm2). Photocurrent Density - Voltage Measurement (I-V Measurement): For measuring current-voltage characteristics, a 300 W collimated xenon lamp (Newport) was used. The lamp was calibrated with the light intensity to 1000 Wm-2 at 1.5 AM Global condition by comparing to a certified silicon solar cell (Fraunhofer ISE). A computer controlled by a digital sourcemeter (Keithley Model 2400) was used to record the electrical data, with the scan direction from open-circuit to short-circuit at a scan rate of 50 mV/s. The prepared ssDSSC devices were masked during the measurement with an aperture area of 0.20 cm2 (0.4 × 0.5 cm2) exposed under illumination. Photovoltage Decay and Light Intensity Dependence Measurements: A custom-designed “toolbox setup” was used to measure the transient photovoltage decay as a function of light intensity. A white LED (Luxeon Star 1W) was used as light source to provide the base light intensity. A 16-bit resolution digital acquisition board (National Instruments) in combination with a current amplifier (Stanford Research Systems RS570) and a customdesigned electromagnetic switching system was used to measure the transient voltage and current response of the devices. By superimposing the base light with a small square wave modulation (
