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Charge carrier transport in bulk heterojunction that is central to the device performance of solar cells is sensitively dependent on the energy level alignment of acceptor and donor. However, the effect of energy level regulation induced by nickel ions on the primary photoexcited electron transfer and the performance of P3HT/TiO2 hybrid solar cells remains being poorly understood and rarely studied. Here we demonstrate that the introduction of the versatile nickel ions into TiO2 nanocrystals can significantly elevate the conduction and valence band energy levels of the acceptor, thus resulting in a remarkable reduction of energy level offset between the conduction band of acceptor and lowest unoccupied molecular orbital of donor. By applying transient photoluminescence and femtosecond transient absorption spectroscopies, we demonstrate that the electron transfer becomes more competitive after incorporating nickel ions. In particular, the electron transfer life time is shortened from 30.2 to 16.7 ps, i.e., more than 44% faster than pure TiO2 acceptor, thus leading to a notable increase of power conversion efficiency in organic/inorganic hybrid solar cells. This work underscores the promising virtue of engineering the reduction of ‘excess’ energy offset to accelerate electron transport and demonstrates the potential of nickel ions in applications of solar energy conversion and photon detectors.
© 2015 Optical Society of America
1. Introduction
Solar cells, as an abundant and sustainable energy resource, have attracted enormous interest from researchers to meet the clean-energy demand of the twenty-first century [1]. However, the low-cost solution-processed solar devices used for harvesting solar energy normally suffer from low power conversion efficiency (PCE) [2]. One of the effective and promising approaches to simultaneously fulfil the goals of low-cost and high efficiency is the development of organic/inorganic hybrid solar cells (HSCs) [3]. Such HSCs can take advantage of the beneficial properties of both types of materials, such as enormous donor-acceptor interfacial area of organic semiconductors and high electron mobility of inorganic semiconductors, etc [4]. The recently emerging poly(3-hexylthiophene) (P3HT) based HSCs, albeit still in infancy, are one of the promising candidates, which can offer much opportunity to harvest solar energy using a simplified, low-cost and flexible architecture [5]. However, at the current stage of development, the poor PCE of organic/inorganic HSC hinders the future industrial and commercial applications. Over the last five years, most of the reported PCE of organic/inorganic HSCs based on P3HT (without considering polymer solar cells based on P3HT) and metal oxide semiconductors (e.g., TiO2, ZnO) is around 1.5%, ranging from 1.0~3.0%, depending on the type of inorganic nanocrystals, their morphology, architecture and others [6–9].
Recently, it has been demonstrated that an effective strategy to enhance the efficiency is in virtue of the doping of foreign metal ions into inorganic semiconductors [10–13]. Among these, nickel-doped mesoporous titanium dioxide (TiO2) that is considered as a promising photocatalytic material has attracted considerable attention thus far owing to their intriguing electronic properties and diverse applications. Nickel-doped TiO2 nanocrystals have exhibited excellent characteristics such as Ni-doped effect, extending the photo response of TiO2 from ultraviolet to visible light, improving the anatase crystallinity, increasing the conductivity of the TiO2 electrodes and inhibiting recombination of photoexcited carriers, etc [14–18]. Nevertheless, the intrinsic driving force behind the excellent performance of solar cells by nickel ion dopant remains unknown. Especially, the role of the energy level control of nickel ions playing in solar cells has seldom been reported. Have a complete understanding of the complex interaction between TiO2 and P3HT will be beneficial to the design of highly efficient HSCs based on TiO2/P3HT bulk heterojunctions (BHJs) in future.
In this work, we investigate the energy level regulation of nickel-doped TiO2 acceptor for TiO2:Ni/P3HT hybrid bulk heterojunctions. Especially, the charge transfer dynamics at the interface of donor-acceptor within BHJs are studied by femtosecond transient adsorption spectrum (TAS) and the influences of energy level regulation of TiO2 acceptor induced by nickel ions on the charge transfer process are systematically addressed.
2. Results and discussion
2.1. Characterizations of Ni-doped TiO2 nanocrystal
XPS spectra of Ni-doped TiO2 and bare TiO2 were carried out to confirm chemical status of the elements. Figure 1(a) shows the XPS spectrum survey of the two samples, in which Ti, O, and C elements are clearly revealed in each curve, with sharp photoelectron peaks at the binding energies of 530 eV (O1s), 459 eV (Ti2p), and 285 eV (C1s). Besides, the Ni 2p peaks are clearly revealed in XPS spectrum for Ni-doped TiO2. The XPS spectrum confined to the Ni window in Fig. 1(b) exhibits two peaks located at 855.1 and 873.3 eV for Ni-doped TiO2, corresponding to the 2p3/2 and 2p1/2 binding energies while no characteristic peak of Ni 2p is observed in the bare TiO2 as expected. Figure 1(c) shows the small energy window with high resolution for the O1s band: both of the XPS spectra exhibit two peaks at 530.1 and 532.2 eV, which are ascribed to Ti-O in TiO2 and the OH on the surface of TiO2. It is worth mentioning that the samples were prepared by hydrothermal treatment during which may introduce hydroxyl on the surface. Furthermore, the Ti2p peaks of the bare TiO2 located at 458.5 and 464.4 eV can be assigned to Ti4+ of TiO2 in Fig. 1(d). A slight shift of Ti 2p3/2 peak is observed after Ni doping: the presence of nickel ions causes the peak of Ti 2p to shift to a higher binding energy depending on their chemical environment. XPS results indicate that Ni is present in the form of oxidation state (Ni2O3) on the acceptor surface.
Fig. 1 The survey (a), Ni 2p (b), O 1s (c) and Ti 2p (d) XPS spectra of Ni-doped TiO2 and pure TiO2.
2.2. Energy level regulation of acceptor
In a bid to investigate the energy level control induced by the dopant, CV characteristics were performed. CV measurement is a dynamic electrochemical method where current-potential curves are recorded at a well-defined applied potential, and allows one to evaluate the valence band (VB) and conduction band (CB) of the acceptor. CV characteristics of bare TiO2, and TiO2:Ni nanocrystalline thin film electrodes are shown in Fig. 2(a). One pair of peaks locating at the oxidation and reduction potential appears, corresponding to the VB and the CB, respectively. The energy levels of CB and VB can be calculated using the following formula: CB (or VB) (eV) = −4.8-(E-E1/2) (eV), where E is a peak point of the redox potential [19], E1/2 is the ferrocene potential used as a standard, E1/2 vs Ag/Ag+ = 0.09 eV [20]. The frontier orbital and band gap of bare TiO2, and TiO2:Ni nanocrystalline thin film electrodes are listed in Table 1. The band gap of TiO2:Ni nanocrystalline thin film is 2.99 eV, which becomes narrower but still quite close to that of pure TiO2 of 3.03 eV. However, after incorporating Ni species, both the CB and VB energy levels of the Ni-doped TiO2, corresponding to -4.08 and-7.07 eV, respectively, have been elevated and becomes higher than those of pure TiO2 (i.e., -4.33 and −7.36eV, respectively). Note that the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) of donor P3HT are −3.0 and −5.1 eV, respectively [21]. After the incorporation of nickel species, a notable reduction of the CB-LUMO offset is observed, decreasing from 1.33 to 1.08 eV. As shown in Fig. 2(b), the energy levels of acceptor can be tuned and hence the CB-LUMO energy offset can be reduced. However, the energy offset is still greater than the Coulomb binding energy (typically 0.1−0.5 eV) and enables to break the Coulomb attraction [22] but not in an efficient way. Gong and his associates have demonstrated that an efficient charge transfer can still be observed from BHJ even though the energy offset between the donor polymer and the acceptor is only 0.12 eV [23]. In order to determine the absolute band edge positions with respect to vacuum level, we measured the flat-band potentials of the TiO2 nanocrystals with and without Ni doping. Figure 2(c) shows the Mott-Schottky plots of the two different films, that is, 1/C2 versus V, where the capacitance C was obtained from the impedance measurement and V is the applied potential. Positive slopes of the plots indicate that both TiO2 and TiO2:Ni are n-type semiconductors [24]. For the n-type semiconductor, the relationship between the capacitance (C) and the applied potential (V) has been clarified in literatures. As a result, the flat band potentials can be derived by Mott-Schottky plots as obtained by a linear extrapolation to C = 0, i.e., the intercept at the X axis. Then the CB values were calculated with Ag/Ag+ as reference. Besides, the band gap can be determined from the UV-vis absorption spectrum by the following relationship: Eg = 1240/λ eV. Finally, the VB values were obtained and listed in Table 1. As observed from Table 1, the CB, VB and band gap Eg obtained from the CV characteristics are in reasonable agreement with those determined from the UV-vis and Mott-Schottky method, indicating that the results obtained in this work are reliable. Now one will wonder what influence such change will exert on the primary dynamics of photoexcited electron at the interface of BHJs.
Fig. 2 (a) CV characteristics of bare TiO2 and TiO2:Ni nanocrystalline thin film; (b) The energy levels diagram of the semiconductor with/without Ni doping. (c) Mott–Schottky plots of the different films. Mott-Schottky measurements were carried out at the frequency of 1 kHz in the aqueous solution of 0.05 M Na2SO4; (d) UV–vis diffuse reflectance spectra of the films.
Table 1. Frontier orbital and band gap of bare TiO2 and TiO2:Ni nanocrystalline thin films obtained from CVs and Mott-Schottky plots.
2.3. Photoexcited charge transfer dynamics
Photoluminescence (PL), coupled with an appropriate electron or hole extraction layer, enables one to evaluate the photoexcited charge carrier transport [25–27]. In a bid to investigate the role of Ni dopant playing in the photoexcited charge carrier transfer, we prepared two different types of BHJs: pure TiO2/P3HT and TiO2:Ni/P3HT blend films for comparison. Meanwhile, the pure P3HT film was also fabricated for reference. Under almost identical experimental conditions, the PL quantum yield of TiO2:Ni/P3HT decreased much more significantly than that of bare TiO2/P3HT in Fig. 3(a), indicating that an enhanced charge transfer from donor P3HT to acceptor indeed occurred after incorporating Ni species. The PL quenching is expected to originate from the photoexcited charge carrier extraction across the interface. High degrees of PL quenching suggests that a highly efficient carrier transport occurs at the interface of TiO2:Ni/P3HT BHJ. In order to further investigate the charge transfer, time-resolved PL measurements were performed. The PL decays for the three films are shown in Fig. 3(b). Compared to the neat P3HT film, both TiO2/P3HT and TiO2:Ni/P3HT films exhibit instantaneous response near zero delay, which is well interpreted as auto correlation of laser pulse. The decay of either the TiO2/P3HT or TiO2:Ni/P3HT BHJ is significantly faster than that of pure P3HT film with emission intensively quenched after approximately 300 ps, which can be attributed to the hot electron transfer. However, on the long time scale, the three films exhibit almost the same tails. Thus, the transient signal can be well fitted to a double-exponential decay with a hot electron transfer lifetime of τfast = 892, 85 and 75 ps for pure P3HT, TiO2/P3HT and TiO2:Ni/P3HT, respectively. The measured lifetime of τfast = 892ps for P3HT reasonably match other reported results [28–30]. However, the time resolution of these measurements (~60 ps) is not enough to resolve the fast quenching that originates from hot electron transfer at this stage. Therefore, femtosecond transient absorption spectroscopy (TAS) with a temporal resolution of ~120 fs and a high SNR (10−4) was applied to obtain the more details of the photoexcited charge carrier dynamics.
Fig. 3 (a) PL intensity of neat P3HT, TiO2/P3HT and TiO2:Ni/P3HT films under 580 nm excitation; (b)The PL decay of the neat P3HT, TiO2/P3HT and TiO2:Ni/P3HT films under 580 nm excitation.
In order to further confirm faster electron transfer at the interface, additional experiments were carried out. Transient absorption spectra of neat P3HT and TiO2:Ni/P3HT films were measured for comparison to demonstrate that electron transfer indeed occurred at the interface. As shown in Fig. 4, the TAS spectra bear one pronounced photoabsorption (PA) band. When probed at 650 nm, it is reasonable to attribute the positive PA band to the state-filling effects [25, 31, 32]. In this report, the variation of electron population in first excited state should dominate the contribution to PA transients. To confirm this, the TAS measurements of neat P3HT and TiO2:Ni/P3HT films were performed for comparison. As observed from Fig. 4, for both neat P3HT and TiO2:Ni/P3HT, the difference absorption (ΔOD) of the PA band reduces by the almost same degree from 0 to 1 ps, i.e., 52% and 51% for neat P3HT, TiO2:Ni/P3HT, respectively. This indicates that the fast decay that originates from the vibrational relaxations whose typical time scale is less than 1 ps and affected very little as expected. However, from 1 to 30 ps, the PA band decreases by 56% and 23% for TiO2:Ni/P3HT and neat P3HT, respectively. This remarkable difference suggests that there exists electron transfer from P3HT to TiO2:Ni within 1~30 ps, thus resulting in the quenching of PA signal. Although the origin of the enhanced charge transfer at the donor-acceptor and the relationship between LUMO-CB energy offset and charge transfer rate are still in doubt, nonetheless the effect of doping on the enhanced charge transfer at the interfaces has been solidly confirmed.
Fig. 4 Transient absorption spectra of neat P3HT film (a) and TiO2:Ni/P3HT blend film (b) measured at 0, 1, 30, 100, and 1000 ps (from top to bottom).
As for the potential origin of enhanced charge transfer, in our case, it could be ascribed to the reduced energy offset. On one hand, an appropriate energy offset between the LUMO of donor and conduction band (CB) edge of acceptor larger that is than the exciton binding energy (typically in the range of 0.1-0.5 eV) is necessary to break the Coulomb attraction and then that releases the electron and allows conduction to occur. Especially the electron acceptor used in this study, i.e., TiO2, have a quite large dielectric constant (εr≈80), which benefits the electron or hole to develop into freely mobile charge carriers. Therefore, it is believed that large energy offset is not requisite for TiO2-P3HT system. On the other hand, one efficient approach to facilitate charge transfer at the interface is by virtue of foreign ions to adjust energy level, which can reduce the energy offset and boost the charge transfer rate as evidenced by our results (Fig. 4 and Fig. 5). This is also in accordance with the Markus theory predicting “inverted region”, briefly speaking, in which reduced energy offset can lead to faster charge transfer. Although practical experiments are much more complicate than the theoretically idealized case, nevertheless at the current stage of development, the Markus theory can be reasonably confirmed by many experiments and provide some useful insights for researcher to explain the results.
Fig. 5 (a) Normalized femtosecond TAS of TiO2:Ni/P3HT (open circle) and TiO2/P3HT (square) films excited at 400 nm with an irradiance of 17 μJ·cm−2 and a probe wavelength of 650 nm. (b) The early time decays.
Figure 5 shows the TAS decays of the different BHJs based on pure TiO2/P3HT and TiO2:Ni/P3HT. Analysis of the transients would give detailed information about the photoexcited electron transfer rates.The decays of both the TiO2/P3HT and TiO2:Ni/P3HT films exhibit an instantaneous response at zero delay time and a long tail as shown in Fig. 5(a). The early time signals in Fig. 5(b) reveal an ultrafast decay within ~1 ps followed by a fast decay in the time domain 1~50 ps. Therefore, we decomposed these decays into four distinctive components: a scalar multiple of the laser pulse autocorrelation function that responses instantaneously to the applied laser field; a nonradiative decay of hot electron in P3HT after the high photo energy excitation (3.10 eV) with a vibronic relaxation time of τvib (within 1 ps); a fast electron transfer from donor to acceptor with a transfer time of τfast (tens of ps); the contribution of the recombination of the electron-hole pairs with a time constant τre (nanoseconds). The method of least-squares deconvolution (LSD) was employed to obtain the parameters by fitting the TA transients. Accordingly, the transient absorption data that was observed may be taken as a convolution of the background-free laser pulse autocorrelation function G(t) with the response function [33, 34]:
whereis a linear superposition of electronic contributions, which can be described with a superposition of the four contributions:where δ(t) is the purely electronic hyperpolarizability that responses instantaneously to the applied laser field. All these decays were fitted with a three-exponential function and the constants were listed in Table 2. It reveals that the hot electron transfer in TiO2:Ni/P3HT BHJ (16.7ps) becomes much faster than pure TiO2/P3HT BHJ (30.2 ps). These results agree well with the transient PL decays in Fig. 3. Moreover, the efficiency of the fast electron transfer can be estimated by using the ratio of the amplitudes B/(A + B + C), i.e., 12.1% for TiO2/P3HT and 14.8% for TiO2:Ni/P3HT. After the incorporation of Ni species, the CB-LUMO has decreased by 0.25 eV, i.e., from 1.33 down to 1.08 eV. Despite of the reduction of the energy level offset, it is still greater than the Coulomb binding energy (typically 0.1~0.5 eV) [22], thus enabling the initial electron transfer step to be energetically downhill. The reduction of the ‘excess’ energy offset enables the energetic electron transfer to the interface with less energy loss, which can leads to an improvement in charge transfer efficiency. Similar phenomena have been observed in organic solar cells with narrowed CB-LUMO energy offset and remarkable improvement in PCE [35, 36].Table 2. Lifetimes and the amplitudes fitted from TAS using the linear superposition of electronic contributions.
2.4. Device performances
Nyquist plots of the HSCs in Fig. 6(a) illustrate the impedance characteristics of the photoanode with/without nickel doping. According to the equivalent circuit model in Fig. 6(a), the intercept on the real axis represents the series resistance (Rs), i.e., an impedance stemming from the contact resistance between PEDOT:PSS and the electrodes. Rct1 and Rct2 arise from the charge-transfer resistance at the interfaces [37]. Electrochemical impedance spectroscopy (EIS) curves of the two HSCs both exhibit two semicircles: the small semicircle can be attributed to the charge transfer resistance (Rct1) at the interface of the Pt and PEDOT:PSS while the large semicircle represents the charge transfer resistance (Rct2) at TiO2 (or Ni-doped TiO2)/PEDOT:PSS interface [38–40]. The EIS data were fitted numerically by the equivalence model [37], and the parameters from the best fit were summarized in Table 3.The Rct2 values with/without Ni doping are 11.19 and 15.09 Ω, respectively. Lower charge transfer resistance ensures that the photoexcited electrons have enough time to reach electrode before recombination. In a bid to explore the doping effect of nickel species on the final photovoltaic performance of HSCs, two types of BHJs were fabricated for comparison: TiO2/P3HT and Ni-doped TiO2/P3HT blend films. As observed from Fig. 6(c), the prepared TiO2:Ni acceptor layers clearly exhibits a porous structure, which favors the physical adsorption of P3HT by trapping the solution in the microporomerics. The thickness of BHJ layer including TiO2:Ni and P3HT was estimated to be about 500 nm as observed from Fig. 6(d). The intimate interface favors the efficient charge separation in BHJs, because the photoexcited charge carrier in P3HT cannot diffuse too long, typically ~12 nm [27]. The photocurrent-voltage (J-V) curves of the TiO2/P3HT and TiO2:Ni/P3HT HSCs under a simulated solar light irradiation of 100 mW cm−2 are shown in Fig. 6(b). Each type of the HSCs was tested for 5 times under identical experimental conditions, and the statistics of the device parameters with standard derivations were listed in Table 3. After Ni doping, the short-circuit current density JSC increases from 6.10 to 6.85 mA·cm−2. Meanwhile, the open-circuit voltage Voc increases from 0.65 to 0.70 V, thus leading to a notable PCE enhancement from 2.14% to 2.71%. Since the photocurrent is influenced by the efficiency of photoexcited charge carrier transfer and charge collection [22], which may give a hint to the increase of open-circuit current density JSC. After the incorporation of Ni species, both the electron transfer lifetime and the charge transfer resistance decrease. It is known that the Voc is partly governed by the energy offset between highest occupied molecular orbital (HOMO) of the donor and CB of the acceptor [41]. In this work, the CB energy level of acceptor has been elevated by 0.25 eV while the energy level of donor remains unchanged, which may provide some insight into the slightly increased Voc.
Fig. 6 (a) Nyquist plots of the HSCs and the inset gives the equivalent circuit; (b) Current–potential (J–V) curves for different BHJs: TiO2/P3HT (red) and TiO2:Ni/P3HT (blue); Cross-sectional SEM images of bare TiO2:Ni acceptor(c) Ni-doped TiO2/P3HT BHJ (d).
Table 3. Photovoltaic parameters of the HSCs and EIS parameters from the best fit.
3. Experiment
Nickel-doped TiO2 colloid was prepared by the procedure in a similar way as done in the previous works [42, 43]. A layer of acceptor film with a thickness of about 250 nm was prepared by coating the Ni-doped TiO2 colloid on FTO glass using spin-coating technique, followed by sintering in air at 450 °C for 30 min. Then the acceptor film is pretreated at 60 °C for 30 min. After that the acceptor film was soaked in a 0.15 M conjugated polymer P3HT methylbenzene solution for 12 hr to uptake P3HT molecules. Next, the PEDOT:PSS layer was spin-coated onto the BHJ. Finally, Pt electrodes were deposited on the top of the PEDOT:PSS layer by thermal evaporation under vacuum.
Transient photoluminescence was measured on a spectrometer (Bruker Optics 250IS/SM) with intensified charge coupled device detector (Andor, IStar740). The samples were excited by 120 fs laser pulses at 400 nm with a repetition rate of 10 Hz. The time resolution was determined to be ~60 ps. TAS measurements of the BHJs were performed by the mode-locked Ti:sapphire laser (Coherent Mira 900) in combination with a regenerative amplifier (Coherent Legend-F). The ultrafast light source with a temporal resolution of ~120 fs was generated by a mode-locked titanium-sapphire laser operating at 800 nm. The laser system was operated at a repetition rate of 10 Hz, and the corresponding time interval of each pulse is 0.1 s. Since the time interval was long enough for the sample to reach fully thermodynamic equilibrium before the next pulse arrived, thus the already fully relaxed sample can be excited by each pulse. Each data was obtained by averaging 100 individual measurements to improve the signal-to-noise ratio, and the typical detection sensitivity of the difference absorption (ΔOD) was better than 10−4.
4. Conclusion
The effect of energy level control of acceptor by Ni doping on the charge transfer dynamic and photovoltaic performance was revealed in this work. After Ni doping, the CB and VB energy levels change from −4.33 to −4.08 eV and −7.36 to −7.07 eV, respectively. It was demonstrated that a remarkable acceleration in photoexcited electron transfer, i.e., from 30.2 down to 16.7 ps, is induced by the reduction of excess CB-LUMO offset. As a result, the photovoltaic parameters including Jsc and Voc have been significantly enhanced after the incorporation of nickel ions, thus leading to a notable PCE enhancement from 2.14 to 2.71%. This work presents an efficient way to obtain higher electron transport efficiency with better energy level alignment in acceptor-donor system by doping. We expect that with further construction that contain demanding materials, such as more reactive ions, the photoexcited carriers could be extracted even faster to achieve a much higher efficiency.
Acknowledgments
The authors gratefully acknowledge the financial support of the Natural Science Foundation of China (61366003), the science and technology project of the education department of Jiangxi Province, China (GJJ13474, GJJ14533).
References and links
1. R. M. Swanson, “Applied physics. photovoltaics power up,” Science 324(5929), 891–892 (2009). [CrossRef] [PubMed]
2. M. Graetzel, R. A. Janssen, D. B. Mitzi, and E. H. Sargent, “Materials interface engineering for solution-processed photovoltaics,” Nature 488(7411), 304–312 (2012). [CrossRef] [PubMed]
3. S. Avasthi, S. Lee, Y. L. Loo, and J. C. Sturm, “Role of majority and minority carrier barriers silicon/organic hybrid heterojunction solar cells,” Adv. Mater. 23(48), 5762–5766 (2011). [CrossRef] [PubMed]
4. J. M. Lee, B. H. Kwon, H. I. Park, H. Kim, M. G. Kim, J. S. Park, E. S. Kim, S. Yoo, D. Y. Jeon, and S. O. Kim, “Exciton dissociation and charge-transport enhancement in organic solar cells with quantum-dot/N-doped CNT hybrid nanomaterials,” Adv. Mater. 25(14), 2011–2017 (2013). [CrossRef] [PubMed]
5. Y. Lei, H. Jia, W. He, Y. Zhang, L. Mi, H. Hou, G. Zhu, and Z. Zheng, “Hybrid solar cells with outstanding short-circuit currents based on a room temperature soft-chemical strategy: the case of P3HT:Ag2S,” J. Am. Chem. Soc. 134(42), 17392–17395 (2012). [CrossRef] [PubMed]
6. S. Shoaee, J. Briscoe, J. R. Durrant, and S. Dunn, “Acoustic enhancement of polymer/ZnO nanorod photovoltaic device performance,” Adv. Mater. 26(2), 263–268 (2014). [CrossRef] [PubMed]
7. J. Lee, S. Mubeen, G. Hernandez-Sosa, Y. Sun, F. M. Toma, G. D. Stucky, and M. Moskovits, “High-efficiency panchromatic hybrid Schottky solar cells,” Adv. Mater. 25(2), 256–260 (2013). [CrossRef] [PubMed]
8. Y. Zhou, M. Eck, and M. Krüger, “Bulk-heterojunction hybrid solar cells based on colloidal nanocrystals and conjugated polymers,” Energy Environ. Sci. 3(12), 1851–1864 (2010). [CrossRef]
9. F. Gao, S. Ren, and J. Wang, “The renaissance of hybrid solar cells: progresses, challenges, and perspectives,” Energy Environ. Sci. 6(7), 2020–2040 (2013). [CrossRef]
10. N. Massihi, M. R. Mohammadi, A. M. Bakhshayesh, and M. Abdi-Jalebi, “Controlling electron injection and electron transport of dye-sensitized solar cells aided by incorporating CNTs into a Cr-doped TiO2 photoanode,” Electrochim. Acta 111, 921–929 (2013). [CrossRef]
11. A. E. Shalan and M. M. Rashad, “Incorporation of Mn2+ and Co2+ to TiO2 nanoparticles and the performance of dye-sensitized solar cells,” Appl. Surf. Sci. 283, 975–981 (2013). [CrossRef]
12. Y. Xie, N. Huang, S. You, Y. Liu, B. Sebo, L. Liang, X. Fang, W. Liu, S. Guo, and X.-Z. Zhao, “Improved performance of dye-sensitized solar cells by trace amount Cr-doped TiO2 photoelectrodes,” J. Power Sources 224, 168–173 (2013). [CrossRef]
13. T. Hori, T. Shibata, V. Kittichungchit, H. Moritou, J. Sakai, H. Kubo, A. Fujii, and M. Ozaki, “MoO3 buffer layer effect on photovoltaic properties of interpenetrating heterojunction type organic solar cells,” Thin Solid Films 518(2), 522–525 (2009). [CrossRef]
14. M. Yang, L. Huo, L. Pei, K. Pan, and Y. Gan, “Enhanced photoelectrochemical performance and charge transfer properties in self-organized NiOx-doped TiO2 nanotubes,” Electrochim. Acta 125, 288–293 (2014). [CrossRef]
15. H. Melhem, P. Simon, J. Wang, C. Di Bin, B. Ratier, Y. Leconte, N. Herlin-Boime, M. Makowska-Janusik, A. Kassiba, and J. Bouclé, “Direct photocurrent generation from nitrogen doped TiO2 electrodes in solid-state dye-sensitized solar cells: towards optically-active metal oxides for photovoltaic applications,” Sol. Energy Mater. Sol. Cells 117, 624–631 (2013). [CrossRef]
16. M. Sun, Z. Chen, and J. Yu, “Highly efficient visible light induced photoelectrochemical anticorrosion for 304 SS by Ni-doped TiO2,” Electrochim. Acta 109, 13–19 (2013). [CrossRef]
17. J. Tian, H. Deng, L. Sun, H. Kong, P. Yang, and J. Chu, “Influence of Ni doping on phase transformation and optical properties of TiO2 films deposited on quartz substrates by sol-gel process,” Appl. Surf. Sci. 258(11), 4893–4897 (2012). [CrossRef]
18. L. Sun, J. Zhai, H. Li, Y. Zhao, H. Yang, and H. Yu, “Study of homologous elements: Fe, Co, and Ni dopant effects on the photoreactivity of TiO2 nanosheets,” Chem. Cat. Chem. 6(1), 339–347 (2014). [CrossRef]
19. I. Mora-Sero, L. Bertoluzzi, V. Gonzalez-Pedro, S. Gimenez, F. Fabregat-Santiago, K. W. Kemp, E. H. Sargent, and J. Bisquert, “Selective contacts drive charge extraction in quantum dot solids via asymmetry in carrier transfer kinetics,” Nat. Commun. 4, 2272 (2013). [PubMed]
20. Y. C. Li, H. Z. Zhong, R. Li, Y. Zhou, C. H. Yang, and Y. F. Li, “High-yield fabrication and electrochemical characterization of tetrapodal CdSe, CdTe, and CdSexTe1–x nanocrystals,” Adv. Funct. Mater. 16(13), 1705–1716 (2006). [CrossRef]
21. Y. X. Liu, M. A. Summers, C. Edder, J. M. J. Frechet, and M. D. McGehee, “Using resonance energy transfer to improve exciton harvesting in organic-inorganic hybrid photovoltaic cells,” Adv. Mater. 17(24), 2960–2964 (2005). [CrossRef]
22. T. M. Clarke and J. R. Durrant, “Charge photogeneration in organic solar cells,” Chem. Rev. 110(11), 6736–6767 (2010). [CrossRef] [PubMed]
23. X. Gong, M. Tong, F. G. Brunetti, J. Seo, Y. Sun, D. Moses, F. Wudl, and A. J. Heeger, “Bulk heterojunction solar cells with large open-circuit voltage: electron transfer with small donor-acceptor energy offset,” Adv. Mater. 23(20), 2272–2277 (2011). [CrossRef] [PubMed]
24. L. Liao, Q. Zhang, Z. Su, Z. Zhao, Y. Wang, Y. Li, X. Lu, D. Wei, G. Feng, Q. Yu, X. Cai, J. Zhao, Z. Ren, H. Fang, F. Robles-Hernandez, S. Baldelli, and J. Bao, “Efficient solar water-splitting using a nanocrystalline CoO photocatalyst,” Nat. Nanotechnol. 9(1), 69–73 (2013). [CrossRef] [PubMed]
25. S. D. Stranks, G. E. Eperon, G. Grancini, C. Menelaou, M. J. Alcocer, T. Leijtens, L. M. Herz, A. Petrozza, and H. J. Snaith, “Electron-hole diffusion lengths exceeding 1 micrometer in an organometal trihalide perovskite absorber,” Science 342(6156), 341–344 (2013). [CrossRef] [PubMed]
26. G. Xing, N. Mathews, S. Sun, S. S. Lim, Y. M. Lam, M. Grätzel, S. Mhaisalkar, and T. C. Sum, “Long-range balanced electron- and hole-transport lengths in organic-inorganic CH3NH3PbI3.,” Science 342(6156), 344–347 (2013). [CrossRef] [PubMed]
27. P. E. Shaw, A. Ruseckas, and I. D. W. Samuel, “Exciton diffusion measurements in poly(3-hexylthiophene),” Adv. Mater. 20(18), 3516–3520 (2008). [CrossRef]
28. S. Cook, A. Furube, and R. Katoh, “Analysis of the excited states of regioregular polythiophene P3HT,” Energy Environ. Sci. 1(2), 294–299 (2008). [CrossRef]
29. B. Ferreira, P. F. da Silva, J. S. Seixas de Melo, J. Pina, and A. Maçanita, “Excited-state dynamics and self-organization of poly(3-hexylthiophene) (P3HT) in solution and thin films,” J. Phys. Chem. B 116(8), 2347–2355 (2012). [CrossRef] [PubMed]
30. B. Kraabel, D. Moses, and A. J. Heeger, “Direct observation of the intersystem crossing in poly(3‐octylthiophene),” J. Chem. Phys. 103(12), 5102–5108 (1995). [CrossRef]
31. R. A. Marsh, J. M. Hodgkiss, S. Albert-Seifried, and R. H. Friend, “Effect of annealing on P3HT:PCBM charge transfer and nanoscale morphology probed by ultrafast spectroscopy,” Nano Lett. 10(3), 923–930 (2010). [CrossRef] [PubMed]
32. H. Ohkita and S. Ito, “Transient absorption spectroscopy of polymer-based thin-film solar cells,” Polymer (Guildf.) 52(20), 4397–4417 (2011). [CrossRef]
33. D. McMorrow, W. T. Lotshaw, and G. A. Kenney-Wallace, “Femtosecond optical Kerr studies on the origin of the nonlinear responses in simple liquids,” IEEE J. Quantum Electron. 24(2), 443–454 (1988). [CrossRef]
34. D. McMorrow and W. T. Lotshaw, “Intermolecular dynamics in acetonitrile probed with femtosecond Fourier-transform Raman spectroscopy,” J. Phys. Chem. 95(25), 10395–10406 (1991). [CrossRef]
35. J. J. M. Halls, J. Cornil, D. A. dos Santos, R. Silbey, D. H. Hwang, A. B. Holmes, J. L. Brédas, and R. H. Friend, “Charge- and energy-transfer processes at polymer/polymer interfaces: a joint experimental and theoretical study,” Phys. Rev. B 60(8), 5721–5727 (1999). [CrossRef]
36. C. J. Brabec, C. Winder, N. S. Sariciftci, J. C. Hummelen, A. Dhanabalan, P. A. van Hal, and R. A. J. Janssen, “A low-bandgap semiconducting polymer for photovoltaic devices and infrared emitting diodes,” Adv. Funct. Mater. 12(10), 709–712 (2002). [CrossRef]
37. B. Sebo, N. Huang, Y. Liu, Q. Tai, L. Liang, H. Hu, S. Xu, and X.-Z. Zhao, “Dye-sensitized solar cells enhanced by optical absorption, mediated by TiO2 nanofibers and plasmonics Ag nanoparticles,” Electrochim. Acta 112, 458–464 (2013). [CrossRef]
38. P. Xu, Q. Tang, H. Chen, and B. He, “Insights of close contact between polyaniline and FTO substrate for enhanced photovoltaic performances of dye-sensitized solar cells,” Electrochim. Acta 125, 163–169 (2014). [CrossRef]
39. H. Cai, Q. Tang, B. He, M. Wang, S. Yuan, and H. Chen, “Self-assembly of graphene oxide/polyaniline multilayer counter electrodes for efficient dye-sensitized solar cells,” Electrochim. Acta 121, 136–142 (2014). [CrossRef]
40. Q. Li, Q. Tang, N. Du, Y. Qin, J. Xiao, B. He, H. Chen, and L. Chu, “Employment of ionic liquid-imbibed polymer gel electrolyte for efficient quasi-solid-state dye-sensitized solar cells,” J. Power Sources 248, 816–821 (2014). [CrossRef]
41. R. Kroon, M. Lenes, J. C. Hummelen, P. W. M. Blom, and B. de Boer, “Small bandgap polymers for organic solar cells (polymer material development in the last 5 Years),” Pol. Rev. 48(3), 531–582 (2008). [CrossRef]
42. X. Jin, W. Sun, Z. Chen, T. Wei, C. Chen, X. He, Y. Yuan, Y. Li, and Q. Li, “Exciton generation/dissociation/charge-transfer enhancement in inorganic/organic hybrid solar cells by robust single nanocrystalline LnPxOy (Ln = Eu, Y) doping,” ACS Appl. Mater. Interfaces 6(11), 8771–8781 (2014). [CrossRef] [PubMed]
43. J. Wu, S. Hao, J. Lin, M. Huang, Y. Huang, Z. Lan, and P. Li, “Crystal morphology of anatase titania nanocrystals used in dye-sensitized solar cells,” Cryst. Growth Des. 8(1), 247–252 (2008). [CrossRef]
