Enhanced photovoltaic performance with co-sensitization of porphyrin and an
organic dye in dye-sensitized solar cells
Chi-Ming Lan,
aHui-Ping Wu,
aTsung-Yu Pan,
aChia-Wei Chang,
aWei-Shan Chao,
bChien-Tien Chen,*
cChin-Li Wang,
dChing-Yao Lin*
dand Eric Wei-Guang Diau*
aReceived 12th January 2012, Accepted 6th February 2012 DOI: 10.1039/c2ee21104a
We designed a stepwise approach for co-sensitization of a zinc porphyrin sensitizer (LD12) with a spirally configured organic dye (CD5) for dye-sensitized solar cells. The co-sensitized LD12 + CD5 device showed significantly enhanced VOCandJSC relative to its individual single-dye sensitized devices. Upon optimization, the device made of the LD12 + CD5 system yieldedJSC/mA cm2¼
16.7,VOC/V¼ 0.74, FF ¼ 0.73 and h ¼ 9.0%; this performance is superior to that of either individual device made from LD12 (h ¼ 7.5%) and CD5 (h ¼ 5.7%) under the same conditions of fabrica-tion. To understand the effects of the potential shift and charge recombination on the cell performance, we measured charge-extraction (CE) and intensity-modulated photovoltage spectra (IMVS). Upon sensitization with each dye, the TiO2potentials are similar, but co-sensitization causes the potential to shift down (cathodic shift). Charge recombination was significantly retarded for the co-sensitized system relative to each individual dye-sensitized
system, to account for the enhancedVOCfor the former relative to the latter. A test of stability indicates a systematic trend between the LD12 + CD5 and LD12 devices; the performance of the co-sensi-tized device degraded only15% and remained stable during the period of 500–1000 h near 295 K.
Dye-sensitized solar cells (DSSC) attract much attention because of their great advantages—light weight, low cost and easy processing, with colorful and transparent features as next-generation photovol-taic devices.1 Photosensitizers such as ruthenium complexes,2 zinc porphyrin3 and a metal-free organic dye4 have been developed to
serve as efficient light harvesters for DSSC. The devices made of ruthenium complexes5 and a porphyrin sensitizer6 have attained
remarkable efficiencies,h ¼ 11.0–11.5%, of power conversion under one-sun illumination. For comparison, with a cobalt-based redox electrolyte, the efficiency of the device made from a push–pull zinc porphyrin (YD2-o-C8) attained 11.9%,7whereas a system
co-sensi-tized with an organic dye (Y123) boosted the cell performance toh ¼ 12.3%,7which is a new milestone in this research field, stimulating
investigation of the development of key materials to promote the device performance.
Co-sensitization is an effective approach to enhance the device performance through a combination of two or more dyes sensitized on semiconductor films together, extending the light-harvesting ability so as to increase the photocurrents of the devices. For example, the co-sensitization of two organic dyes (JK2 and SQ1) with
aDepartment of Applied Chemistry and Institute of Molecular Science,
National Chiao Tung University, Hsinchu 30010, Taiwan. E-mail: diau@ mail.nctu.edu.tw; Fax: +886 35723764; Tel: +886 35131524
bDepartment of Chemistry, National Taiwan Normal University, #88, Sec.
4, Ding-jou Road, Taipei 11677, Taiwan
cDepartment of Chemistry, National Tsing Hua University, Hsinchu 30013,
Taiwan. E-mail: [email protected]; Fax: +886-3-5711082; Tel: +886 35739240
dDepartment of Applied Chemistry, National Chi Nan University, Puli,
Nantou Hsien 54561, Taiwan. E-mail: [email protected]; Fax: +886 49-2917956; Tel: +886 49-2910960 ext. 4152
Broader context
Co-sensitization of two or more dyes with complementary absorption spectra on TiO2film is a well-known strategy to further enhance the light-harvesting ability of dye-sensitized solar cells (DSSC). There are two ways to make a co-sensitization: the cocktail approach makes the co-sensitization in a mixed dye solution with certain molar ratios of the two dyes, whereas the stepwise approach accomplishes adsorption of the two different dyes in a consecutive manner. Here we followed a stepwise approach to make the co-sensitization of porphyrin with an organic dye for which the co-sensitized device exhibited enhancements of photovoltaic perfor-mance not only in JSCbut also in VOC. We found that co-sensitization stabilizes the potential of TiO2but it also effectively retards the charge recombination to lead to the enhanced VOC, which is unprecedented for the porphyrin + organic dye co-sensitization system. Porphyrins are known to suffer from dye aggregation due to their planar structural nature. In the present study we have shown that co-sensitization of a zinc porphyrin with a well-designed organic dye does help in changing the aggregation morphology of the porphyrin adsorbed on TiO2film to further improve its device performance.
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Cite this: Energy Environ. Sci., 2012, 5, 6460
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complementary spectral responses showed an enhanced photovoltaic performance relative to the devices fabricated with the individual dyes;8phthalocyanine dye (TT1) co-sensitized with various organic
dyes (JK2 or D2) exhibited an enhanced device performance through the improved light-harvesting efficiency of the cell;9co-sensitization of TiO2films with a black dye and organic dyes (D131 or Y1) resulted in a significantly enhanced photocurrent such that the device perfor-mance attainedh ¼ 11.0–11.4%.10For porphyrins, a device based on
co-sensitization of a TiO2film (thickness 2.4mm) with YD2 and an organic dye (D205) showed a short-circuit current density (JSC) increase by 20%,6 but for the champion system (YD2-o-C8 with Y123) the enhancement in JSCwas limited (17.3 vs. 17.7 mA cm2) because the gap between the Soret and the Q bands in the IPCE spectrum for the porphyrin-alone device was small.7 For most
co-sensitization systems above,6–9 the values of the open-circuit
voltage (VOC) are between those of the separate individual single-dye sensitized devices, which limits the enhancement of the overall device performance.
Here we report a promising DSSC system based on a zinc porphyrin bearing two ortho-substituted long alkoxyl chains (LD12)11 co-adsorbed with a spirally configured donor–acceptor
organic dye (CD5).12The syntheses and characterizations of these novel photosensitizers are published elsewhere;11,12the corresponding molecular structures appear in Chart 1. Both JSCand VOCof the co-sensitized devices were significantly enhanced relative to their corresponding single-dye sensitized devices, improving the overall performance for the LD12 + CD5 device by 20% and 58% with respect to LD12 and CD5 single-dye devices. To understand how the VOCenhancement arose, we measured charge-extraction (CE)13and
intensity-modulated photovoltage spectra (IMVS).14
Fig. 1a and b show the absorption spectra of LD12 and CD5 in THF solutions and on TiO2films, respectively. For LD12, the Soret band of the thin-film spectrum was significantly broadened relative to that in solution; the maximum shifted from 442 nm to 447 nm, a spectral feature similar to that reported for a push–pull porphyrin (LD16).15 Upon co-sensitization with CD5 on TiO
2 film, the maximum of the Soret band of LD12 shifted back to 443 nm. This observation indicates that aggregations of both H- (parallel orienta-tion) and J-type (tilted orientaorienta-tion) occur for LD12 adsorbed on TiO2, but co-sensitization with CD5 adjusts the arrangement of the porphyrin molecules toward a more nearly parallel configuration on the surface of TiO2. The absorption spectrum of CD5 complements that of LD12 for the co-sensitized film showing the panchromatic feature to render an optimal light-harvesting effect in the region of 350–650 nm.
The co-sensitization of LD12 with CD5 on a TiO2 film11 of thickness20 mm was achieved via a stepwise approach: the TiO2 electrode was immersed in the LD12 solution for 3 h and then
immersed in the CD5 solution for 2 h. Afterwards, the LD12 + CD5 co-sensitized film was assembled into a DSSC device of sandwich type with a Pt-coated counter electrode and filled with an appropriate
Chart 1 Molecular structures of LD12 and CD5.
Fig. 1 Absorption spectra of (a) LD12 and CD5 in THF and (b) LD12, CD5 and LD12 + CD5 sensitized on TiO2films (thickness 2mm). The
absorption coefficients of CD5 in (a) are magnified 10 times; the dashed gray line indicates the peak absorption (l ¼ 442 nm) of the Soret band of LD12 in solution.
Fig. 2 Optimized photovoltaic properties: (a) current–voltage charac-teristics and (b) the corresponding IPCE action spectra of devices made of LD12, CD5, and LD12 + CD5 with the same fabricated TiO2films
(17 + 5mm) under one-sun AM-1.5G irradiation.
electrolyte. Fig. 2a and b show the current–voltage characteristics and the corresponding IPCE action spectra, respectively, for a compar-ison of the photovoltaic performance of the LD12 + CD5 co-sensi-tized device with that of either individual dye-sensico-sensi-tized device; the corresponding photovoltaic parameters are summarized in Table 1. The results clearly show that, upon co-sensitization, the performance of the LD12 + CD5 device improved to yield JSC/mA cm2¼ 16.7, VOC/V¼ 0.74, FF ¼ 0.73 and h ¼ 9.0%; the latter is significantly greater thanh ¼ 7.5% for LD12 and h ¼ 5.7% for CD5 in the separate devices under the same conditions of fabrication.
For the co-sensitized system, the enhancement of JSC is under-standable from the IPCE spectra shown in Fig. 2b; the gap of the photoresponse of about 500 nm for the LD12 device was filled in the action spectrum of the LD12 + CD5 device by the contribution of CD5 that shows its maximum photoresponse in that spectral region. As a result, JSC of the co-sensitized device increased from 14.97 mA cm2to 16.74 mA cm2to contribute12% enhancement of the overall performance. VOCof the co-sensitized device increased from 0.711 V (LD12) and 0.689 V (CD5) to 0.736 V, which is unprecedented for reported co-sensitized porphyrin and organic-dye systems.6–9
To account for the enhanced VOC upon co-sensitization, we derived the shift of the conduction band of TiO2by measuring the charge densities (Ne) for the three devices at each VOCwith the CE method, and converted the Nevalues into the density of states (DOS) via DOS¼ Ne/VOC.16Fig. 3a displays plots of VOCvs. DOS for the three systems at six intensities of white light from a LED. The CE results indicate that the TiO2potentials of the LD12 and CD5 devices are similar to each other, but that of the LD12 + CD5 device exhibits a potential shift down (cathodic shift)25 mV with respect to the individual dye-sensitized devices. Co-sensitization of LD12 with CD5 on a TiO2 film has thus the effect to stabilize the TiO2potential, which we explain as follows. As shown in Fig. 1 the spectral broad-ening feature indicates that LD12 molecules aggregate in two geometries on the surface of TiO2; the blue spectral shift corresponds to the parallel (H-type) and the red shift to the tilted (J-type) orien-tations. Co-sensitization of LD12 with CD5 on TiO2would produce a blue shift of the Soret band, indicating that the LD12 molecules adjust to a more nearly parallel conformation so as to leave space for the CD5 co-sensitizer. Such a structural modification would allow an additive (e.g., Li+) to reach a new equilibrium state on the surface of
TiO2such that a lower potential is observed. The stabilization of the co-sensitized system in this case cannot account for the observed VOC enhancement for the LD12 + CD5 system, for which we performed IMVS measurements to obtain kinetic information about the charge recombination of the system.
We measured IMVS with a AC perturbation (modulation 10%) superimposed on the six CW bias light intensities, the same as in the
CE experiments, in the frequency range of 0.1–1000 Hz; the frequency of the minimum IMVS response corresponds to the inverse of the electron lifetime (sR) of the device at the open-circuit condition. Fig. 3b shows plots ofsRvs. DOS for the three systems at the six light intensities. The results indicate a systematic trend with the electron lifetimes (corresponding to the degree of charge recombination) showing the order LD12 + CD5 > LD12 > CD5, which is consistent with the variation of VOCshowing the same order. Our results are consistent with those of the black dye + Y1 co-sensitized system showing an increase of20 mV in VOC, which is also explicable according to the electron lifetimes increased upon co-sensitization.10b
For the TT1 + D2 co-sensitized system, these observed lifetimes in the co-sensitized device are between those of the D2 and the TT1 devices, correlating well with the trend of VOC.9bBased on the cobalt redox electrolyte, the decreased VOC of the YD2-o-C8 + Y123 device relative to that of the device containing only the YD2-o-C8 dye is also explicable by the smaller electron lifetimes for the former than for the latter.7In our case, we observed that V
OCbecame enhanced upon co-sensitization of LD12 with CD5; this result is due to the effect of a retarded charge recombination even though the potential of TiO2 shifted slightly down. We regard such a retardation as due to an effective combination between zinc porphyrin with long alkoxyl chains and the organic dye with bulky spiral cis-stilbene/fluorene hybrids, such that they formed a compact layer on the surface of TiO2 to impede the approach of triiodide anions in the electrolyte to the TiO2surface.
We tested the stability (for a period of 1000 h) near 295 K for the three devices—co-sensitized LD12 + CD5, LD12 alone, and CD5 alone. Fig. 4a–d show the temporal variations of JSC, VOC, FF andh for the three systems. The results indicate that the LD12 + CD5
Table 1 Optimized photovoltaic parameters of DSSC fabricated with LD12, CD5, or LD12 + CD5 adsorbed on the TiO2 films of
thick-ness (15 + 5) mm under simulated AM-1.5G illumination (power 100 mW cm2) and active area 0.16 cm2
Dye JSC/mA cm2 VOC/V FF h (%)
LD12 14.97 0.711 0.705 7.5 CD5 11.12 0.689 0.744 5.7 LD12 + CD5 16.74 0.736 0.731 9.0
Fig. 3 (a) Open-circuit voltage (VOC) and (b) electron lifetimes (sR) as
a function of density of states (DOS) for the devices made of LD12 alone, CD5 alone and their co-sensitized combination.
device attained the best performance (h z 9%) during the period of 40–140 h; the efficiency then decreased to 8.1% at 300 h and further to 7.7% at 500 h. After 500 h, the performance of the co-sensitized device remained stable until the end of the test (1000 h). The degraded efficiency of the LD12 + CD5 device reflects mainly the decreased JSC, from 16.7 mA cm2at 40 h to 14.7 mA cm2at 500 h, whereas the VOCand the FF values remained approximately constant during the entire period. The degraded performance of the LD12 device is also due to the decreased JSC, but that of the CD5 device is mainly due to the decreased VOC. A decrease of JSC implies a possible desorption of the dye molecules from the TiO2surface for decreased light harvesting, whereas the decreased VOC might be due to an altered ionic equilibrium on the TiO2surface for a shift down of the potential or for an increased recombination of charge. As the trend of performance degradation of the co-sensitized device is similar to that of the LD12 device, we conclude that the degradation of15% of the cell performance in the initial 500 h results from desorption of some porphyrin molecules weakly bound to the TiO2surface.
In conclusion, we have designed a co-sensitized system with a zinc porphyrin enveloped with long alkoxyl chains (LD12) and a spirally structured push–pull organic dye (CD5); the co-sensitized device exhibits significant improvement not only of JSCbut also of VOC, yielding an overall efficiency of 9.0% greater than that of a device containing only the porphyrin (7.5%) or the organic dye (5.7%). JSCis enhanced because of the combined light-harvesting effect of the two
dyes that have complementary absorption spectra, and VOC is enhanced because the retarded charge recombination overwhelmed the shift down of the TiO2potential. In the test of stability, the performance of the co-sensitized device remained stable after 500 h with prior degradation by only15%. The present work can thus provide guidance, with appropriate molecular design, to seek more promising co-sensitized systems with the ability to harvest an increased portion of the solar spectrum and to enhance the photo-voltage. Work is in progress in this direction.
Experimental
The LD12, CD5 and LD12 + CD5 co-sensitized devices were fabricated with a working electrode based on TiO2 nanoparticles (NP) and a Pt-coated counter electrode reported elsewhere.11,12For the working electrode, a paste composed of TiO2NP (particle sizez 25 nm) prepared with a sol–gel method for the transparent nano-crystalline layer was coated on a TiCl4-treated FTO glass substrate (TEC 7, Hartford, USA) to obtain the required thickness of the film with repetitive screen printing. To improve the performance of the device, we screen-printed an additional scattering layer (particle sizez 300 nm) on the transparent active layer. The TiO2electrode was immersed in a solution containing LD12 (0.15 mM) and CDCA (10 mM) in ethanol/toluene (volume ratio 4 : 1) at 25C for 3 h. Afterwards, this LD12-sensitized film was washed in absolute ethanol, dried in air, and immersed in a solution containing CD5 (0.15 mM) and CDCA (0.3 mM) in ethanol and kept at 25C for 2 h. The co-sensitized working electrode was assembled with the counter electrode into a cell of sandwich type and sealed with a hot-melt film (SX1170, thickness 60mm) at 90C. The electrolyte solution con-taining LiI (0.1 M), I2(0.05 M), PMII (1.0 M), and 4-tert-butylpyr-idine (0.5 M) in a mixture of acetonitrile and valeronitrile (volume ratio 85 : 15) was introduced into the space between the two elec-trodes, so completing the fabrication of these DSSC devices. The photovoltaic performance of the device was assessed through measurement of an I–V curve with a solar simulator (AM-1.5G, XES-502S, SAN-EI) calibrated with a standard silicon reference cell (Oriel PN 91150V, VLSI standards). The incident monochromatic efficiencies for conversion from photons to current (IPCE) spectra of the corresponding devices were measured with a system comprising a Xe lamp (PTi A-1010, 150 W), a monochromator (PTi, 1200 g mm1blazed at 500 nm), and a source meter (Keithley 2400,
computer-controlled). A standard Si photodiode (Hamamatsu S1337-1012BQ) served as a reference to calibrate the power density of the lamp at each wavelength.
The intensity-modulated photovoltage spectra (IMVS) were measured with the CIMPS equipment (Zahner) at an open-circuit condition based on a CW while light at six intensities (0.03–1.1 W cm2) controlled with a slave system (XPOT, Zahner) to obtain the photovoltaic response induced by the modulated light. The modu-lated light was driven with a 10% AC perturbation current super-imposed on a DC current in a frequency range from 0.1 to 1000 Hz. The measurements of charge extraction (CE) were performed with the same CIMPS system under the same bias light irradiations. For that experiment, the system was initially set to an open-circuit condition for 10 s for the photovoltage of the device to attain a steady state; the white light from the LED was then terminated while the device simultaneously switched to a short-circuit condition to extract the charges generated at that bias light intensity.
Fig. 4 A test of stability of devices over 1000 h showing the degradation of the photovoltaic performance for (a) short-circuit current density (JSC), (b) open-circuit voltage (VOC), (c) fill factor (FF) and (d) efficiency
(h) of power conversion for the devices made of LD12 alone, CD5 alone and their co-sensitized combination.
Acknowledgements
We thank Prof. Chien-Chon Chen for providing the Zahner equip-ment for the CE and IMPS measureequip-ments. National Science Council of Taiwan and Ministry of Education of Taiwan, under the ATU program, provided support for this project.
Notes and references
1 (a) M. Gr€atzel, Acc. Chem. Res., 2009, 42, 1788–1798; (b) A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo and H. Pettersson, Chem. Rev., 2010, 110, 6595–6663.
2 (a) M. K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker, E. M€uller, P. Liska, N. Vlachopoulos and M. Gr€atzel, J. Am. Chem. Soc., 1993, 115, 6382–6390; (b) N. Robertson, Angew. Chem., Int. Ed., 2006, 45, 2338–2345.
3 (a) H. Imahori, T. Umeyama and S. Ito, Acc. Chem. Res., 2009, 42, 1809–1818; (b) M. V. Martınez-Dıaz, G. de la Torre and T. Torres, Chem. Commun., 2010, 46, 7090–7108; (c) X.-F. Wang and H. Tamiaki, Energy Environ. Sci., 2010, 3, 94–106.
4 (a) G. Zhang, H. Bala, Y. Cheng, D. Shi, X. Lv, Q. Yu and P. Wang, Chem. Commun., 2009, 2198–2200; (b) Z. Ning and H. Tian, Chem. Commun., 2009, 5483–5495; (c) Z. Ning, Y. Fu and H. Tian, Energy Environ. Sci., 2010, 3, 1170–1181.
5 (a) C.-Y. Chen, M. Wang, J.-Y. Li, N. Pootrakulchote, L. Alibabaei, C. Ngoc-Ie, J.-D. Decoppet, J.-H. Tsai, C. Gr€atzel, C.-G. Wu, S. M. Zakeeruddin and M. Gr€atzel, ACS Nano, 2009, 3, 3103–3109; (b) Y. Cao, Y. Bai, Q. Yu, Y. Cheng, S. Liu, D. Shi, F. Gao and P. Wang, J. Phys. Chem. C, 2009, 113, 6290–6297; (c) Q. Yu, Y. Wang, Z. Yi, N. Zu, J. Zhang, M. Zhang and P. Wang, ACS Nano, 2010, 4, 6032–6038.
6 T. Bessho, S. M. Zakeeruddin, C.-Y. Yeh, E. W.-G. Diau and M. Gr€atzel, Angew. Chem., Int. Ed., 2010, 49, 6646–6649.
7 A. Yella, H.-W. Lee, H. N. Tsao, C. Yi, A. K. Chandiran, M. K. Kazeeruddin, E. W.-G. Diau, C.-Y. Yeh, S. M. Zakeeruddin and M. Gr€atzel, Science, 2011, 334, 629–634.
8 (a) D. Kuang, P. Walter, F. N€uesch, S. Kim, J. J. Ko, P. Comte, S. M. Zakeeruddin, M. K. Nazeeruddin and M. Gr€atzel, Langmuir, 2007, 23, 10906–10909; (b) H. Choi, S. Kim, S. O. Kang, J. J. Ko, M. S. Kang, J. N. Clifford, A. Forneli, E. Palomares, M. K. Nazeeruddin and M. Gr€atzel, Angew. Chem., Int. Ed., 2008, 47, 8259–8263.
9 (a) J.-J. Cid, J.-H. Yum, S.-R. Jang, M. K. Nazeeruddin, E. Martinez-Ferrero, E. Palomares, J. J. Ko, M. Gr€atzel and T. Torres, Angew. Chem., Int. Ed., 2007, 46, 8358–8362; (b) J. N. Clifford, A. Forneli, H. Chen, T. Torres, S. Tan and E. Palomares, J. Mater. Chem., 2011, 21, 1693–1696.
10 (a) R. Y. Ogura, S. Nakane, M. Morooka, M. Orihashi, Y. Suzuki and K. Noda, Appl. Phys. Lett., 2009, 94, 073308; (b) L. Han, A. Islam, H. Chen, C. Malapaka, S. Zhang, X. Yang, M. Yanagida and B. Chiranjeevi, Energy Environ. Sci., 2012, DOI: 10.1039/ c2ee03418b, advance article.
11 Y.-C. Chang, C.-L. Wang, T.-Y. Pan, S.-H. Hong, C.-M. Lan, H.-H. Kuo, C.-F. Lo, H.-Y. Hsu, C.-Y. Lin and E. W.-G. Diau, Chem. Commun., 2011, 47, 8910–8912.
12 W.-S. Chao, C.-T. Chen, W.-K. Huang, C.-M. Lan and E. W.-G. Diau, Chem. Commun., in revision (CC-COM-11-2011-017079.R1).
13 N. W. Duffy, L. M. Peter, R. M. G. Rajapakse and K. G. U. Wijayantha, Electrochem. Commun., 2000, 2, 658–662. 14 G. Schlichtho1rl, S. Y. Huang, J. Sprague and A. J. Frank, J. Phys.
Chem. B, 1997, 101, 8141–8155.
15 C.-L. Wang, C.-M. Lan, S.-H. Hong, Y.-F. Wang, T.-Y. Pan, C.-W. Chang, H.-H. Kuo, M.-Y. Kuo, E. W.-G. Diau and C.-Y. Lin, Energy Environ. Sci., 2012, DOI: 10.1039/c2ee03308a, in press.
16 F. Gao, Y. Wang, D. Shi, J. Zhang, M. Wang, X. Jing, R. Humphry-Baker, P. Wang, S. M. Zakeeruddin and M. Gr€atzel, J. Am. Chem. Soc., 2008, 130, 10720–10728.
