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Highly Efficient Hybrid Photovoltaics Based on Hyperbranched Three-Dimensional TiO2 Electron Transporting Materials Khalid Mahmood, Bhabani Sankar Swain, and Aram Amassian* Solution-processed hybrid thin film photovoltaic technologies, such as perovskite solar cells (PSCs), dye sensitized solar cells (DSSCs), and colloidal quantum dot solar cells rely heavily on TiO2 as the electron transporting material (ETM), with organic solar cells now also making use of the material as electron transporting layer (ETL).[1–9] In the particular cases of DSSCs and mesostructured PSCs, the ETL is typically engineered to be mesoporous in order to promote a large surface area, good loading of the absorber and to provide a continuous pathway for electron transport and extraction with minimal recombination.[10–13] One-dimensional (1D) TiO2 nanostructured ETLs (nanotubes, nanorods, and nanowires) were recently shown to provide larger internal surface area than the nanoparticulate structure, promoting higher loading of absorber and distinct light-scattering effects.[14–21] 1D nanostructured ETLs have also been found to offer efficient charge separation and electron transport,[10,22] making them promising candidates for efficient mesoporous PSCs and DSSCs. However, practical efforts to fabricate PSCs and DSSCs based on 1D TiO2 (anatase) nanowire or nanotube arrays have not yielded performance improvements to date.[23–27] Substantial further developments are needed in the architecture of mesostructured ETMs in order to achieve important photovoltaic performance improvements in conditions compatible with scalable manufacturing. We take the view that threedimensional (3D) hyperbranched nanowire architectures with high surface area, low defect density, and a 3D interconnection network for electron extraction can be the basis of effective ETMs for mesostructured photovoltaics. Efforts have been deployed to develop 3D hierarchical branched nanowire architectures,[28–35] as these are of great interest to a wide range of applications including photocatalysis, energy storage, and photovoltaics.[36–38] Different methods of synthesizing 3D hyperbranched TiO2 nanowires have included vapor deposition,[29] pulsed-laser deposition,[39] and hydrothermal methods.[24]
Dr. K. Mahmood, Prof. A. Amassian Physical Sciences and Engineering Division, and Solar and Photovoltaic Engineering Research Center King Abdullah University of Science and Technology (KAUST) Thuwal 23955–6900, Saudi Arabia E-mail: [email protected] Dr. B. S. Swain School of Advanced Materials Engineering Kookmin University Seoul 136–702, South Korea
DOI: 10.1002/adma.201500336
Adv. Mater. 2015, DOI: 10.1002/adma.201500336
Seed-induced multistep and one-step hydrothermal methods have been successfully used by Lee et al.[31] and Wu et al.[15] to fabricate hyperbranched TiO2 nanostructures with improved light harvesting in DSSCs owing to their larger surface area. However, these synthesis methods have resulted in a low density of nanofibers,[15,31] and have suffered from slow carrier transport due to the presence of structural defects, including at grain boundaries between the nanofiber trunks and nanorod branches of the hyperbranched ETM. Electrospinning of metal oxide nanofibers has recently emerged as a potentially inexpensive, rapid, facile, and versatile route to growing 1D TiO2 nanomaterials on a variety of substrates,[40,41] and has been investigated as a 1D material in the context of DSSCs.[42–44] However, the combination of electrospun TiO2 fibers with hydrothermally grown TiO2 branches has not been investigated in the contexts of DSSCs or PSCs.[45] In this communication, we introduce a unique and scalable multistage electrospinning and hydrothermal route for the development of 3D hyperbranched anatase TiO2 nanorod– nanofiber arrays as electron transporting materials. The hyperbranched ETM with optimal electron transport and carrier lifetime leads to highly efficient mesostructured perovskite (CH3NH3PbI3) solar cells with an average power conversion efficiency (PCEavg) of 15.03% and a maximum power conversion efficiency (PCEmax) of 15.50%. Increasing the thickness of the hyperbranched ETM from 0.6 µm to ≈29 µm led to highly efficient DSSCs as well, with PCEmax = 11.22%. These remarkable performances were possible thanks to the development of 3D hyperbranched nanofiber–nanorod arrays made of high quality anatase TiO2 with few defects and capable of transporting electrons rapidly and over long distances, minimizing recombination losses. Light harvesting was also found to be significantly enhanced due to light scattering effects of the hyperbranched architecture, leading to significant performance boosts in DSSCs. This work demonstrates remarkable advantages in using hyperbranched ETMs for highly efficient, largearea, and low-cost hybrid photovoltaics. In Figure 1a, we show scanning electron micrographs (SEM) of nanofibers obtained by electrospinning of a first layer without subsequent hydrothermal synthesis of nanorod branches (E1H0). We have carefully selected the electrospinning conditions, such as the applied voltage, the solution viscosity, and flow rate, in order to obtain nanofibers assembled from small TiO2 nanoparticles (40–50 nm diameter) and exhibiting significant internal porosity within the nanofibers (see the Supporting Information). The formation of mesoporous nanofibers is also an important requirement to obtaining hierarchically mesostructured nanofiber–nanorod arrays during the subsequent
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Figure 1. Schematic representation of the multistep electrospinning and hydrothermal route for fabrication of up to four layers of hyperbranched nanofiber–nanorod arrays. Plan-view SEM images showing a) a TiO2 nanofiber ETM (E1H0) (inset shows a magnified view of nanofibers showing their highly porous nature) and b) a hyperbranched nanofiber–nanorod array ETM (E1H1). c,d) Cross-sectional SEM views of completed perovskite solar cells based on nanofiber and hyperbranched nanofiber–nanorod arrays, respectively. e–g) Plan-view SEM images showing 2-, 3-, and 4-layer thick hyperbranched ETMs, respectively, used to fabricate dye-sensitized solar cells.
hydrothermal synthesis step. In this respect, nanofibers with large diameter (120–190 nm) were prepared exhibiting excellent electrical contact, as well as higher internal porosity and surface area compared to smaller diameter fibers, but were also found to be less continuous (Figure S1a, Supporting Information). Processing conditions were also carefully adjusted to avoid forming larger diameter fibers with dense internal structure (Figure S1b, Supporting Information), as these tend to reduce absorber loading (see the Supporting Information). In Figure S1c, Supporting Information, we show a ≈600 nm-thick single-layer electrospun TiO2 nanofiber array after calcination at 400 °C for 1 h. The high internal porosity of these nanofibers is evident from the magnified SEM image shown in Figure S1d, Supporting Information. The electrospun TiO2 nanofibers were subjected to a hydrothermal cycle in order to fabricate the 3D hierarchical nanofiber–nanorod array ETM. In Figure 1b, we show SEM images of 3D hierarchical arrays (150–160 nm in diameter and 550–600 nm in length) obtained after a hydrothermal cycle of 1.5 h at 160 °C, followed by annealing at 300 °C for 30 min, as schematically illustrated in Figure 1 as layer 1b (E1H1). Larger area views of the hyperbranched network are presented in Figure S2a,b, Supporting Information, to highlight the ETM’s uniform coverage of the FTO-coated substrate.
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We anticipate the 3D hyperbranched ETM (E1H1) should have several important benefits, such as large surface area and enhanced absorber loading, beneficial light scattering behavior and enhanced charge extraction capabilities as compared with nanofiber films (E1H0). Using these ETMs and adequately scaled up thicknesses, we have fabricated both PSC and DSSC devices, respectively. In Figure 1c,d, we show crosssectional SEM images of the completed PSC devices with the structure of glass/FTO/nanofibers (E1H0) OR hyperbranches (E1H1)/MAPbI3/spiro-OMeTAD/Ag, respectively, where FTO is fluorine-doped tin oxide, MAPbI3 is methyl ammonium lead iodide, and spiro-OMeTAD is 2,2′,7,7′-tetrakis(N,N-di-pmethoxyphenylamine)-9,9′-spirobifluorene. In Figure 2a,b, we present the current–voltage (J–V) characteristics and external quantum efficiency (EQE) of the PSC devices based on porous nanofiber networks (E1H0) and hyperbranched nanostructures (E1H1). PSC devices based on nanofibers produced an average – for 50 individual device parameters averaged between forward and reverse scan directions – short-circuit current density (Jsc) = 17.3 mA cm−2, open-circuit voltage (Voc) = 955 mV, and fill factor (FF) = 0.70, which results in PCEavg = 11.65% and PCEmax of 12.25%. By contrast, the hyperbranched nanostructure-based PSC devices exhibit on average a high Jsc of
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COMMUNICATION Figure 2. Comparison of a) J–V characteristics both in the forward and reverse directions at a scan rate of 10 mV s−1, b) EQE spectra, c) electron diffusion coefficient, and d) time constant for electron recombination as a function of Jsc of the perovskite devices based on the as-prepared nanofibers and hyperbranched arrays, respectively.
19.85 mA cm−2, a Voc of 972 mV, and a FF of 0.735, which lead to a PCEavg of 14.19% and PCEmax of 14.75% (see Table 1). The integrated JSC calculated from the EQE data are 17.2 mA cm−2 and 19.75 mA cm−2 for the devices based on nanofibers and hyperbranched nanostructures, respectively, in good agreement with the measured JSC. The devices exhibited a reasonably low hysteresis, as indicated by comparing the forward and reverse scan directions in Figure 2a. Histograms of the average PCE of forward and reverse scans for 50 individual devices using the optimized nanofiber and hyperbranched ETMs are presented in Figure S3, Supporting Information, showing good reproducibility with >80% of devices yielding PCE >11.6% and >14.1%, respectively. The plot of EQE versus wavelength for typical nanofiber- and hyperbranched-based PSC devices (Figure 2b)
shows that the PSC based on hyperbranched ETM has higher values over the entire spectrum (370–750 nm). PSC devices fabricated with thicker hyperbranched or nanofiber ETMs (850 nm and 1230 nm instead of 600 nm), exhibited lower Voc and Jsc (Figure S4 and Table S1, Supporting Information), indicating that the lower ETM thickness is more suitable for highly efficient PSC devices. The improved performance of the hyperbranched nanoarrays is believed to be due to better infiltration of the perovskite absorber and to enhanced electron transport characteristics. In Figure 2c,d we plot the electron diffusion coefficients and time constants for electron recombination as a function of Jsc. The highest values of diffusion coefficients and longest electron lifetimes are obtained in the case of hyperbranched arrays compared with nanofiber ones, suggesting the
Table 1. Summary of device parameters obtained for mesostructured perovskite solar cells based on two different ETMs, namely, a nanofiber (E1H0) ETM and a 3D hyperbranched nanofiber–nanorod array ETM (E1H1) under one Sun illumination (AM 1.5G, 100 mW cm−2). The results are the average of 50 devices. Device type
Scan mode
Hyperbranch-based PSC
Nanofiber-based PSC
Adv. Mater. 2015, DOI: 10.1002/adma.201500336
JSC [mA cm−2]
VOC [mV]
FF
PCEavg [%]
PCEmax [%]
Reverse
20.0
980
0.74
14.50 ± 0.20
15.20
Forward
19.70
965
0.73
13.87 ± 0.25
14.30
Average
19.85
972
0.735
14.19 ± 0.22
14.75 12.80
Reverse
17.50
965
0.71
11.99 ± 0.21
Forward
17.10
945
0.70
11.32 ± 0.30
11.70
Average
17.30
955
0.705
11.65 ± 0.26
12.25
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hyperbranched hierarchical structure contains a lower density of surface traps which can suppress electron recombination, while the interconnected network of the hyperbranch enhances electron lifetime. Similarly, a multistep approach consisting of thicker individual layers (≈6–8 µm) was used to prepare thicker ETMs suitable for DSSCs, including 2 (E2H2), 3 (E3H3), and 4 (E4H4) repetitions followed by calcination and annealing treatments. The corresponding plan-view SEM images of these structures are shown in Figure 1e–g, respectively, while a cross-sectional view of E4H4 is shown in Figure S5, Supporting Information. The total thickness of E4H4 is ≈29 µm, while the individual layer thicknesses were 6, 8, 7, and 8 µm, from bottom to top. In order to minimize lattice defects and improve the crystallinity of the ETM, each hydrothermal cycle was preceded by thermal treatment of the electrospun layer. X-ray diffraction (XRD) patterns of hyperbranched nanostructures before and after annealing (Figure S6a, Supporting Information) confirm that the materials possess good crystallinity, with a small but notable improvement observed after annealing. The diffraction peaks obtained for these nanostructures were indexed to the anatase phase of TiO2 (JCPDS 21–1272). No diffraction peaks associated with rutile or brookite TiO2 phases were observed, confirming the phase purity of the TiO2 nanoarrays. A comparison of XRD pattern of single-layer porous nanofibers and nanorods grown over the fiber surface is shown in Figure S6b, Supporting Information. Single-layer hyperbranched arrays
exhibit higher peak intensities compared to nanofiber-only films suggesting enhanced crystallinity. Importantly, the multilayer hyperbranch synthesis method presented herein is both rapid and efficient as it produces 4 layers of anatase TiO2 hyperbranched structures suitable for DSSCs in under 15 h. Conventional methods require more than 36 h[3] and 24 h[18] to produce 1D ultra-long anatase TiO2 nanowire arrays and rutile TiO2 nanorods, respectively. In Figure 3a,b, we present the typical J–V characteristics and incident photon-to-current conversion efficiency (IPCE) or EQE spectra of DSSC devices fabricated using hyperbranched TiO2 with different layer thicknesses as well as with standard mesoporous titania nanoparticles (TNPs). The averaged photovoltaic parameters of 40 independent DSSCs fabricated for each ETM thickness are summarized in Table 2. We observe an enhancement of the JSC and PCE by increasing the number of layers to four, as more light was harvested by the thicker device capable of more dye loading. The amount of dye loading increasing with the number of layers as shown in Table 2. The best dye uptake (300.8 nmol cm−2) was observed for the five layer assembly of hyperbranched TiO2 and was much larger than the dye-loading capacity achieved for a single layer of nanofibers (48.5 nmol cm−2). The largest surface area (≈175 m2 g−1) obtained was for 4 layers of anatase TiO2 hyperbranched structures; it was ≈4.3 times larger than that from a single-layer of nanofibers (≈40 m2 g−1). The increase in dyeloading enhances the Jsc values from 8.48 to 17.0 mA cm−2
Figure 3. a) J–V characteristics, b) IPCE or EQE spectra, c) electron diffusion coefficient, and d) electron recombination lifetimes for DSSCs prepared using the 3D hyperbranched array with different film thicknesses prepared by repetitive layering of nanofibers and hydrothermal growth of branches. Standard TiO2 nanoparticle-based DSSCs are also presented in a) and b) for comparison.
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Table 2. Summary of device parameters obtained for DSSCs based on single and multilayered configurations of the 3D hyperbranched nanofiber– nanorod array ETMs under one Sun illumination (AM 1.5G, 100 mW cm−2). DSSCs based on a TiO2 nanoparticle ETM are also presented. DSSCs
JSC [mA cm−2]
VOC [mV]
FF
PCEavg [%]
PCEmax [%]
JSC from IPCE [mA cm−2]
Adsorbed dye [nmol cm−2]
Thickness [µm]
1 layer
8.48
840
0.64
4.56 ± 0.23
4.86
8.42
48.05
6
2 layer
13.46
840
0.65
7.35 ± 0.30
7.66
13.42
118.6
14
3 layers
15.50
830
0.66
8.50 ± 0.29
8.79
15.46
180.4
21
4 layers
17.0
829
0.68
9.58 ± 0.27
9.90
16.98
240.5
29
5 layers
15.9
800
0.66
8.39 ± 0.32
8.68
15.86
300.8
37
TNPs
10.9
800
0.63
5.49 ± 0.35
5.78
10.84
290.3
29
as the number of layers increases from 1 to 4 (equivalent to a film thickness increase from 6 to 29 µm). However, Jsc values decreased to 15.9 mA cm−2 upon further increase of the film thickness to 37 µm, possibly owing to the extended distance carriers must travel prior to extraction, or due to gradual fusing and cracking of hyperbranches (Figure S7, Supporting Information) which can reduce the light scattering ability of the ETM (see below) and lead to more recombination events within the cell (Figure S8, Supporting Information), resulting in decreased Voc and Jsc. In Figure 3b, we show the corresponding IPCE spectra for single-layered and multilayered TiO2 hyperbranched arrays revealing the same trend of Jsc values as described above. Clearly, the multilayered configuration with four layers exhibits an impressive photocurrent of 17.0 mA cm−2 and IPCE values as compared to the other layer structures over the whole spectral range of 400–800 nm, as well as in the wavelength range of 450 to 650 nm which corresponds to the region of maximum absorption of N719. For comparison, we have prepared a DSSC device with a TNPs photoanode of similar thickness (29 µm) to the four layer hyperbranched ETM (see Figure 3a,b). The Voc and Jsc are found to be 800 mV and 10.9 mA cm−2 (PCE = 5.49%), respectively. The performance of the TNP photoanode compares poorly to the hyperbranched TiO2 nanoarrays (829 mV and 17.0 mA cm−2, PCE = 9.85%). Part of the dramatic performance improvement of the hyperbranched structure as opposed to the TNPs film is attributed to enhanced light scattering by the former. The diffuse reflectance spectra of the single and multilayered TiO2 hyperbranched arrays are shown in Figure S9, Supporting Information. The reflectance spectra reveal the remarkable enhancement of light-scattering ability attributed to the hyperbranched photoanode architecture as compared with TNPs films, leading to improved light harvesting in the former. The diffuse scattering data with respect to the thickness of the hyperbranched ETM also reveal the reflectance peaks in case of four layers, possibly contributing to the remarkably high Jsc and PCE achieved for the DSSC devices fabricated at this particular thickness rather than at three or five layers. Further investigation of charge transport (Figure 3c) in DSSCs fabricated with different thicknesses of hyperbranched TiO2 revealed the diffusion coefficient increased steadily with thickness and peaked for the four layer configuration, subsequently decreasing in case of five layers, possibly due to an increase of the density of surface states (Figure S8, Supporting Information).[19,46] In comparison, a single-layer of nanofibers exhibited the lowest value of diffusion coefficient. The recombination
Adv. Mater. 2015, DOI: 10.1002/adma.201500336
lifetime (Figure 3d) associated to the hyperbranched ETM was also found to increase with thickness, peaking at four layers. The rapid charge transport associated with the multilayered hyperbranched TiO2 arrays can be attributed to the elimination of lattice defects and grain boundaries between the nanobranches as a result of successive layer depositions, including electrospinning, hydrothermal synthesis and calcination. It is believed the interconnected network created by the hyperbranches possess an inherent ability to carry charges over long distances within the photoanode, which can be credited with enhancing the charge collection efficiency in DSSCs. On the basis of these observations, we have performed additional thermal annealing of the single layer (600 nm) and multilayer (29 µm) hyperbranched ETMs at 300 °C for 30 min and fabricated PSC and DSSC devices, respectively. In Figure 4a, we have plotted the J–V characteristics of the best performing perovskite and DSSC devices. The corresponding EQE and IPCE spectra are plotted in Figure 4b. The DSSC device achieved a remarkable PCEavg of 10.85% with Jsc of 18.6 mA cm−2 (calculated JSC from EQE equals 18.57 mA cm−2), FF of 0.69, and Voc of 845 mV with PCEmax of 11.22%. The perovskite device also exhibited significantly enhanced efficiency, achieving PCEavg of 15.03% and PCEmax of 15.50% (as summarized in Table S2, Supporting Information). For the annealed samples the corresponding EQE spectra (Figure 4b) also show enhanced response over the whole spectral range for both PSC and DSSC devices. Comparison of the DSSC and PSC devices reveals that the latter exhibits far superior performance despite the use of a well-engineered ETL and the latter being much thinner in the case of the PSC device. The IPCE in the DSSC can reach values of up to 87%, but this occurs in a limited spectral range. By contrast, the PSCs achieved consistently high EQE. This is attributed to faster electron transport and reduced charge recombination in the PSC devices despite using the same ETL architecture. In fact, charge transport in the DSSCs appears to be “electron-limited” at short circuit, meaning that the transport of electrons through the hyperbranched TiO2 is still slower than the transport of holes through the hole-transporting medium.[47,48] We speculate this to be due to the ability of the PSC to transport both electrons and holes, whereas in the DSSC the ETL and electrolyte are independently responsible for these critical steps. In summary, hyperbranched TiO2 electron transporting materials based on a 3D hierarchical nanorod–nanofiber array have been developed using a viable multistage electrospinning and hydrothermal synthesis route. These ETMs not only
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Acknowledgements Part of this work was supported by Round 2 of the Collaborative Research Grant from the Office of Competitive Research Funds and by the Career Development SABIC Chair held by AA. Received: January 21, 2015 Revised: February 16, 2015 Published online:
Figure 4. a) J–V characteristics, b) EQE/IPCE spectra of best performing PSC and DSSC based on, respectively, 0.6 µm and 29 µm thick annealed 3D hyperbranched nanofiber–nanorod arrays.
exhibit large surface area, but also rapid charge transport and reduced recombination, which are credited with remarkable enhancement of photovoltaic efficiency both in perovskite and dye sensitized solar cells. Perovskite solar cells were fabricated by employing a submicron layer of the hyperbranched arrays to yield a high power conversion efficiency of 15.50%. DSSCs were fabricated based on a 29 µm thick multilayer hyperbranched array, yielding a PCEmax of 11.22%, the highest so far achieved using N719-sensitized hyperbranched hierarchical TiO2 photoelectrodes. These results demonstrate that hyperbranched nanofiber–nanorod ETMs have great potential in next generation mesostructured photovoltaics and may also benefit numerous other energy fields such as fuel cells, energy storage devices, photocatalysis, and artificial photosynthesis.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
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[1] X. Lan, S. Masala, E. H. Sargent, Nat. Mater. 2014, 13, 233. [2] J. T.-W. Wang, J. M. Ball, E. M. Barea, A. Abate, J. A. Alexander-Webber, J. Huang, M. Saliba, I. Mora-Sero, J. Bisquert, H. J. Snaith, R. J. Nicholas, Nano Lett. 2014, 14, 724. [3] W. Q. Wu, Y. F. Xu, C. Y. Su, D. B. Kuang, Energy Environ. Sci. 2014, 7, 644. [4] P. Docampo, J. M. Ball, M. Darwich, G. E. Eperon, H. J. Snaith, Nat. Commun. 2013, 4, 2761. [5] K. Mahmood, B. S. Swain, H. S. Jung, Nanoscale 2014, 6, 9127–9138. [6] K. Wojciechowski, M. Saliba, T. Leijtens, A. Abatea, H. J. Snaith, Energy Environ. Sci. 2014, 7, 1142. [7] B. J. Kim, D. H. Kim, Y.-Y. Lee, H.-W. Shin, G. S. Han, J. S. Hong, K. Mahmood, T. K. Ahn, Y.-C. Joo, K. S. Hong, N.-G. Park, S. Lee, H. S. Jung, Energy Environ. Sci. 2015, DOI: 10.1039/C4EE02441A. [8] H.-S. Kim, I. Mora-Sero, V. Gonzalez-Pedro, F. Fabregat-Santiago, E. J. Juarez-Perez, N.-G. Park, J. Bisquert, Nat. Commun. 2013, 4, 2242. [9] S. D. Stranks, G. E. Eperon, G. Grancini, C. Menelaou, M. J. P. Alcocer, T. Leijtens, L. M. Herz, A. Petrozza, H. J. Snaith, Science 2013, 342, 34. [10] K. Mahmood, B. S. Swain, A. R. Kirmani, A. Amassian, J. Mater. Chem. A 2015, DOI: 10.1039/C4TA04883K. [11] G. S. Han, H. S. Chung, B. J. Kim, D. H. Kim, J. W. Lee, B. S. Swain, K. Mahmood, J. S. Yoo, N. G. Park, J. H. Lee, H. S. Jung, J. Mater. Chem. A 2015, DOI: 10.1039/C4TA03684K. [12] K. Mahmood, S. B. Park, J. Mater. Chem. A 2013, 1, 4826. [13] K. Mahmood, H. W. Kang, S. B. Park, H. J. Sung, ACS Appl. Mater. Interfaces 2013, 5, 3075. [14] P. Roy, S. Berger, P. Schmuki, Angew. Chem., Int. Ed. 2011, 50, 2904. [15] W. Q. Wu, B. X. Lei, H. S. Rao, Y. F. Xu, Y. F. Wang, C. Y. Su, D. B. Kuang, Sci. Rep. 2013, 3, 1352. [16] M. Guo, K. Y. Xie, J. Lin, Z. H. Yong, C. T. Yip, L. M. Zhou, Y. Wang, H. T. Huang, Energy Environ. Sci. 2012, 5, 9881. [17] J. R. Jennings, A. Ghicov, L. M. Peter, P. Schmuki, A. B. Walker, J. Am. Chem. Soc. 2008, 130, 13364. [18] X. Feng, K. Zhu, A. J. Frank, C. A. Grimes, T. E. Mallouk, Angew. Chem. Int. Ed. 2012, 51, 2727. [19] X. Sheng, D. He, J. Yang, K. Zhu, X. Feng, Nano Lett. 2014, 14, 1848. [20] H.-S. Kim, J.-W. Lee, N. Yantara, P. P. Boix, S. A. Kulkarni, S. Mhaisalkar, M. Grätzel, N.-G. Park, Nano Lett. 2013, 13, 2412. [21] M. Yang, R. Guo, K. Kadel, Y. Liu, K. O’Shea, R. Bone, X. Wang, J. He, W. Li, J. Mater. Chem. A 2014, 2, 19616. [22] K. Mahmood, B. S. Swain, A. Amassian, Nanoscale 2014, 6, 14674. [23] J. Y. Liao, B. X. Lei, H. Y. Chen, D. B. Kuang, C. Y. Su, Energy Environ. Sci. 2012, 5, 5750. [24] W. Q. Wu, H. S. Rao, Y. F. Xu, Y. F. Wang, C. Y. Su, D. B. Kuang, Sci. Rep. 2013, 3, 1892. [25] D. Kuang, J. Brillet, P. Chen, M. Takata, S. Uchida, H. Miura, K. Sumioka, S. M. Zakeeruddin, M. Grätzel, ACS Nano 2008, 2, 1113. [26] D. Zhonga, B. Cai, X. Wang, Z. Yang, Y. Xing, S. Miao, W.-H. Zhang, C. Lia, Nano Energy 2015, 11, 409.
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[27] M. Yang, R. Guo, K. Kadel, Y. Liu, K. O’Shea, R. Bone, X. Wang, J. He, W. Li, J. Mater. Chem. A 2014, 2, 19616. [28] D. Wang, F. Qian, C. Yang, Z. H. Zhong, C. M. Lieber, Nano Lett. 2004, 4, 871. [29] J. Shi, Y. Hara, C. L. Sun, M. A. Anderson, X. D. Wang, Nano Lett. 2011, 11, 3413. [30] I. S. Cho, Z. B. Chen, A. J. Forman, D. R. Kim, P. M. Rao, T. F. Jaramillo, X. L. Zheng, Nano Lett. 2011, 11, 4978. [31] D. H. Lee, Y. Rho, F. I. Allen, A. M. Minor, S. H. Ko, C. P. Grigoropoulos, Nanoscale 2013, 5, 11147. [32] J. W. Shiu, C. M. Lan, Y. C. Chang, H. P. Wu, W. K. Huang, E. W. G. Diau, ACS Nano 2012, 6, 10862. [33] W. Q. Wu, H. L. Feng, H. S. Rao, Y. F. Xu, D. B. Kuang, C. Y. Su, Nat. Commun. 2014, 5, 3968. [34] W. Q. Wu, Y. F. Xu, H. S. Rao, C. Y. Su, D. B. Kuang, J. Am. Chem. Soc. 2014, 136, 6437. [35] Z. Zhu, J. Qiu, K. Yan, S. Yang, ACS Appl. Mater. Interfaces 2013, 5, 4000. [36] A. Kubacka, M. Fernández-García, G. Colón, Chem. Rev. 2012, 112, 1555. [37] S. N. Habisreutinger, L. Schmidt-Mende, J. K. Stolarczyk, Angew. Chem. Int. Ed. 2013, 52, 7372.
[38] N. Cai, S. J. Moon, L. Cevey-Ha, T. Moehl, R. Humphry-Baker, P. Wang, S. M. Zakeeruddin, M. Grätzel, Nano Lett. 2011, 11, 1452. [39] F. Sauvage, F. D. Fonzo, A. L. Bassi, C. S. Casari, S. Russo, G. Divitini, C. Ducati, C. E. Bottani, P. Comte, M. Grätzel, Nano Lett. 2010, 10, 2562. [40] D. Li, Y. Xia, Nano Lett. 2003, 3, 555. [41] D. Li, J. T. McCann, Y. Xia, M. Marquezz, J. Am. Ceram. Soc. 2006, 89, 1861. [42] K. Onozuka, B. Ding, Y. Tsuge, T. Naka, M. Yamazaki, S. Sugi, S. Ohno, M. Yoshikawa, S. Shiratori, Nanotechnology 2006, 17, 1026. [43] S. Yang, P. Zhu, A. S. Nair, S. Ramakrishna, J. Mater. Chem. 2011, 21, 6541. [44] M. Y. Song, D. K. Kim, K. J. Ihn, S. Jo, D. Y. Kim, Nanotechnology 2004, 15, 1861. [45] M. Shang, W. Wang, W. Yin, J. Ren, S. Sun, L. Zhang, Chem. Eur. J. 2010, 16, 11412. [46] K. Zhu, N. Kopidakis, N. R. Neale, J. van de Lagemaat, A. J. Frank, J. Phys. Chem. B 2006, 110, 25174. [47] T. Leijtens, J. Lim, J. Teuscher, T. Park, H. J. Snaith, Adv. Mater. 2013, 25, 3227. [48] H. J. Snaith, M. Grätzel, Adv. Mater. 2007, 19, 3643.
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Highly Efficient Hybrid Photovoltaics Based on Hyperbranched Three-Dimensional TiO2 Electron Transporting Materials Khalid Mahmood, Bhabani Sankar Swain, and Aram Amassian* Solution-processed hybrid thin film photovoltaic technologies, such as perovskite solar cells (PSCs), dye sensitized solar cells (DSSCs), and colloidal quantum dot solar cells rely heavily on TiO2 as the electron transporting material (ETM), with organic solar cells now also making use of the material as electron transporting layer (ETL).[1–9] In the particular cases of DSSCs and mesostructured PSCs, the ETL is typically engineered to be mesoporous in order to promote a large surface area, good loading of the absorber and to provide a continuous pathway for electron transport and extraction with minimal recombination.[10–13] One-dimensional (1D) TiO2 nanostructured ETLs (nanotubes, nanorods, and nanowires) were recently shown to provide larger internal surface area than the nanoparticulate structure, promoting higher loading of absorber and distinct light-scattering effects.[14–21] 1D nanostructured ETLs have also been found to offer efficient charge separation and electron transport,[10,22] making them promising candidates for efficient mesoporous PSCs and DSSCs. However, practical efforts to fabricate PSCs and DSSCs based on 1D TiO2 (anatase) nanowire or nanotube arrays have not yielded performance improvements to date.[23–27] Substantial further developments are needed in the architecture of mesostructured ETMs in order to achieve important photovoltaic performance improvements in conditions compatible with scalable manufacturing. We take the view that threedimensional (3D) hyperbranched nanowire architectures with high surface area, low defect density, and a 3D interconnection network for electron extraction can be the basis of effective ETMs for mesostructured photovoltaics. Efforts have been deployed to develop 3D hierarchical branched nanowire architectures,[28–35] as these are of great interest to a wide range of applications including photocatalysis, energy storage, and photovoltaics.[36–38] Different methods of synthesizing 3D hyperbranched TiO2 nanowires have included vapor deposition,[29] pulsed-laser deposition,[39] and hydrothermal methods.[24]
Dr. K. Mahmood, Prof. A. Amassian Physical Sciences and Engineering Division, and Solar and Photovoltaic Engineering Research Center King Abdullah University of Science and Technology (KAUST) Thuwal 23955–6900, Saudi Arabia E-mail: [email protected] Dr. B. S. Swain School of Advanced Materials Engineering Kookmin University Seoul 136–702, South Korea
DOI: 10.1002/adma.201500336
Adv. Mater. 2015, DOI: 10.1002/adma.201500336
Seed-induced multistep and one-step hydrothermal methods have been successfully used by Lee et al.[31] and Wu et al.[15] to fabricate hyperbranched TiO2 nanostructures with improved light harvesting in DSSCs owing to their larger surface area. However, these synthesis methods have resulted in a low density of nanofibers,[15,31] and have suffered from slow carrier transport due to the presence of structural defects, including at grain boundaries between the nanofiber trunks and nanorod branches of the hyperbranched ETM. Electrospinning of metal oxide nanofibers has recently emerged as a potentially inexpensive, rapid, facile, and versatile route to growing 1D TiO2 nanomaterials on a variety of substrates,[40,41] and has been investigated as a 1D material in the context of DSSCs.[42–44] However, the combination of electrospun TiO2 fibers with hydrothermally grown TiO2 branches has not been investigated in the contexts of DSSCs or PSCs.[45] In this communication, we introduce a unique and scalable multistage electrospinning and hydrothermal route for the development of 3D hyperbranched anatase TiO2 nanorod– nanofiber arrays as electron transporting materials. The hyperbranched ETM with optimal electron transport and carrier lifetime leads to highly efficient mesostructured perovskite (CH3NH3PbI3) solar cells with an average power conversion efficiency (PCEavg) of 15.03% and a maximum power conversion efficiency (PCEmax) of 15.50%. Increasing the thickness of the hyperbranched ETM from 0.6 µm to ≈29 µm led to highly efficient DSSCs as well, with PCEmax = 11.22%. These remarkable performances were possible thanks to the development of 3D hyperbranched nanofiber–nanorod arrays made of high quality anatase TiO2 with few defects and capable of transporting electrons rapidly and over long distances, minimizing recombination losses. Light harvesting was also found to be significantly enhanced due to light scattering effects of the hyperbranched architecture, leading to significant performance boosts in DSSCs. This work demonstrates remarkable advantages in using hyperbranched ETMs for highly efficient, largearea, and low-cost hybrid photovoltaics. In Figure 1a, we show scanning electron micrographs (SEM) of nanofibers obtained by electrospinning of a first layer without subsequent hydrothermal synthesis of nanorod branches (E1H0). We have carefully selected the electrospinning conditions, such as the applied voltage, the solution viscosity, and flow rate, in order to obtain nanofibers assembled from small TiO2 nanoparticles (40–50 nm diameter) and exhibiting significant internal porosity within the nanofibers (see the Supporting Information). The formation of mesoporous nanofibers is also an important requirement to obtaining hierarchically mesostructured nanofiber–nanorod arrays during the subsequent
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Figure 1. Schematic representation of the multistep electrospinning and hydrothermal route for fabrication of up to four layers of hyperbranched nanofiber–nanorod arrays. Plan-view SEM images showing a) a TiO2 nanofiber ETM (E1H0) (inset shows a magnified view of nanofibers showing their highly porous nature) and b) a hyperbranched nanofiber–nanorod array ETM (E1H1). c,d) Cross-sectional SEM views of completed perovskite solar cells based on nanofiber and hyperbranched nanofiber–nanorod arrays, respectively. e–g) Plan-view SEM images showing 2-, 3-, and 4-layer thick hyperbranched ETMs, respectively, used to fabricate dye-sensitized solar cells.
hydrothermal synthesis step. In this respect, nanofibers with large diameter (120–190 nm) were prepared exhibiting excellent electrical contact, as well as higher internal porosity and surface area compared to smaller diameter fibers, but were also found to be less continuous (Figure S1a, Supporting Information). Processing conditions were also carefully adjusted to avoid forming larger diameter fibers with dense internal structure (Figure S1b, Supporting Information), as these tend to reduce absorber loading (see the Supporting Information). In Figure S1c, Supporting Information, we show a ≈600 nm-thick single-layer electrospun TiO2 nanofiber array after calcination at 400 °C for 1 h. The high internal porosity of these nanofibers is evident from the magnified SEM image shown in Figure S1d, Supporting Information. The electrospun TiO2 nanofibers were subjected to a hydrothermal cycle in order to fabricate the 3D hierarchical nanofiber–nanorod array ETM. In Figure 1b, we show SEM images of 3D hierarchical arrays (150–160 nm in diameter and 550–600 nm in length) obtained after a hydrothermal cycle of 1.5 h at 160 °C, followed by annealing at 300 °C for 30 min, as schematically illustrated in Figure 1 as layer 1b (E1H1). Larger area views of the hyperbranched network are presented in Figure S2a,b, Supporting Information, to highlight the ETM’s uniform coverage of the FTO-coated substrate.
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We anticipate the 3D hyperbranched ETM (E1H1) should have several important benefits, such as large surface area and enhanced absorber loading, beneficial light scattering behavior and enhanced charge extraction capabilities as compared with nanofiber films (E1H0). Using these ETMs and adequately scaled up thicknesses, we have fabricated both PSC and DSSC devices, respectively. In Figure 1c,d, we show crosssectional SEM images of the completed PSC devices with the structure of glass/FTO/nanofibers (E1H0) OR hyperbranches (E1H1)/MAPbI3/spiro-OMeTAD/Ag, respectively, where FTO is fluorine-doped tin oxide, MAPbI3 is methyl ammonium lead iodide, and spiro-OMeTAD is 2,2′,7,7′-tetrakis(N,N-di-pmethoxyphenylamine)-9,9′-spirobifluorene. In Figure 2a,b, we present the current–voltage (J–V) characteristics and external quantum efficiency (EQE) of the PSC devices based on porous nanofiber networks (E1H0) and hyperbranched nanostructures (E1H1). PSC devices based on nanofibers produced an average – for 50 individual device parameters averaged between forward and reverse scan directions – short-circuit current density (Jsc) = 17.3 mA cm−2, open-circuit voltage (Voc) = 955 mV, and fill factor (FF) = 0.70, which results in PCEavg = 11.65% and PCEmax of 12.25%. By contrast, the hyperbranched nanostructure-based PSC devices exhibit on average a high Jsc of
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COMMUNICATION Figure 2. Comparison of a) J–V characteristics both in the forward and reverse directions at a scan rate of 10 mV s−1, b) EQE spectra, c) electron diffusion coefficient, and d) time constant for electron recombination as a function of Jsc of the perovskite devices based on the as-prepared nanofibers and hyperbranched arrays, respectively.
19.85 mA cm−2, a Voc of 972 mV, and a FF of 0.735, which lead to a PCEavg of 14.19% and PCEmax of 14.75% (see Table 1). The integrated JSC calculated from the EQE data are 17.2 mA cm−2 and 19.75 mA cm−2 for the devices based on nanofibers and hyperbranched nanostructures, respectively, in good agreement with the measured JSC. The devices exhibited a reasonably low hysteresis, as indicated by comparing the forward and reverse scan directions in Figure 2a. Histograms of the average PCE of forward and reverse scans for 50 individual devices using the optimized nanofiber and hyperbranched ETMs are presented in Figure S3, Supporting Information, showing good reproducibility with >80% of devices yielding PCE >11.6% and >14.1%, respectively. The plot of EQE versus wavelength for typical nanofiber- and hyperbranched-based PSC devices (Figure 2b)
shows that the PSC based on hyperbranched ETM has higher values over the entire spectrum (370–750 nm). PSC devices fabricated with thicker hyperbranched or nanofiber ETMs (850 nm and 1230 nm instead of 600 nm), exhibited lower Voc and Jsc (Figure S4 and Table S1, Supporting Information), indicating that the lower ETM thickness is more suitable for highly efficient PSC devices. The improved performance of the hyperbranched nanoarrays is believed to be due to better infiltration of the perovskite absorber and to enhanced electron transport characteristics. In Figure 2c,d we plot the electron diffusion coefficients and time constants for electron recombination as a function of Jsc. The highest values of diffusion coefficients and longest electron lifetimes are obtained in the case of hyperbranched arrays compared with nanofiber ones, suggesting the
Table 1. Summary of device parameters obtained for mesostructured perovskite solar cells based on two different ETMs, namely, a nanofiber (E1H0) ETM and a 3D hyperbranched nanofiber–nanorod array ETM (E1H1) under one Sun illumination (AM 1.5G, 100 mW cm−2). The results are the average of 50 devices. Device type
Scan mode
Hyperbranch-based PSC
Nanofiber-based PSC
Adv. Mater. 2015, DOI: 10.1002/adma.201500336
JSC [mA cm−2]
VOC [mV]
FF
PCEavg [%]
PCEmax [%]
Reverse
20.0
980
0.74
14.50 ± 0.20
15.20
Forward
19.70
965
0.73
13.87 ± 0.25
14.30
Average
19.85
972
0.735
14.19 ± 0.22
14.75 12.80
Reverse
17.50
965
0.71
11.99 ± 0.21
Forward
17.10
945
0.70
11.32 ± 0.30
11.70
Average
17.30
955
0.705
11.65 ± 0.26
12.25
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hyperbranched hierarchical structure contains a lower density of surface traps which can suppress electron recombination, while the interconnected network of the hyperbranch enhances electron lifetime. Similarly, a multistep approach consisting of thicker individual layers (≈6–8 µm) was used to prepare thicker ETMs suitable for DSSCs, including 2 (E2H2), 3 (E3H3), and 4 (E4H4) repetitions followed by calcination and annealing treatments. The corresponding plan-view SEM images of these structures are shown in Figure 1e–g, respectively, while a cross-sectional view of E4H4 is shown in Figure S5, Supporting Information. The total thickness of E4H4 is ≈29 µm, while the individual layer thicknesses were 6, 8, 7, and 8 µm, from bottom to top. In order to minimize lattice defects and improve the crystallinity of the ETM, each hydrothermal cycle was preceded by thermal treatment of the electrospun layer. X-ray diffraction (XRD) patterns of hyperbranched nanostructures before and after annealing (Figure S6a, Supporting Information) confirm that the materials possess good crystallinity, with a small but notable improvement observed after annealing. The diffraction peaks obtained for these nanostructures were indexed to the anatase phase of TiO2 (JCPDS 21–1272). No diffraction peaks associated with rutile or brookite TiO2 phases were observed, confirming the phase purity of the TiO2 nanoarrays. A comparison of XRD pattern of single-layer porous nanofibers and nanorods grown over the fiber surface is shown in Figure S6b, Supporting Information. Single-layer hyperbranched arrays
exhibit higher peak intensities compared to nanofiber-only films suggesting enhanced crystallinity. Importantly, the multilayer hyperbranch synthesis method presented herein is both rapid and efficient as it produces 4 layers of anatase TiO2 hyperbranched structures suitable for DSSCs in under 15 h. Conventional methods require more than 36 h[3] and 24 h[18] to produce 1D ultra-long anatase TiO2 nanowire arrays and rutile TiO2 nanorods, respectively. In Figure 3a,b, we present the typical J–V characteristics and incident photon-to-current conversion efficiency (IPCE) or EQE spectra of DSSC devices fabricated using hyperbranched TiO2 with different layer thicknesses as well as with standard mesoporous titania nanoparticles (TNPs). The averaged photovoltaic parameters of 40 independent DSSCs fabricated for each ETM thickness are summarized in Table 2. We observe an enhancement of the JSC and PCE by increasing the number of layers to four, as more light was harvested by the thicker device capable of more dye loading. The amount of dye loading increasing with the number of layers as shown in Table 2. The best dye uptake (300.8 nmol cm−2) was observed for the five layer assembly of hyperbranched TiO2 and was much larger than the dye-loading capacity achieved for a single layer of nanofibers (48.5 nmol cm−2). The largest surface area (≈175 m2 g−1) obtained was for 4 layers of anatase TiO2 hyperbranched structures; it was ≈4.3 times larger than that from a single-layer of nanofibers (≈40 m2 g−1). The increase in dyeloading enhances the Jsc values from 8.48 to 17.0 mA cm−2
Figure 3. a) J–V characteristics, b) IPCE or EQE spectra, c) electron diffusion coefficient, and d) electron recombination lifetimes for DSSCs prepared using the 3D hyperbranched array with different film thicknesses prepared by repetitive layering of nanofibers and hydrothermal growth of branches. Standard TiO2 nanoparticle-based DSSCs are also presented in a) and b) for comparison.
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Table 2. Summary of device parameters obtained for DSSCs based on single and multilayered configurations of the 3D hyperbranched nanofiber– nanorod array ETMs under one Sun illumination (AM 1.5G, 100 mW cm−2). DSSCs based on a TiO2 nanoparticle ETM are also presented. DSSCs
JSC [mA cm−2]
VOC [mV]
FF
PCEavg [%]
PCEmax [%]
JSC from IPCE [mA cm−2]
Adsorbed dye [nmol cm−2]
Thickness [µm]
1 layer
8.48
840
0.64
4.56 ± 0.23
4.86
8.42
48.05
6
2 layer
13.46
840
0.65
7.35 ± 0.30
7.66
13.42
118.6
14
3 layers
15.50
830
0.66
8.50 ± 0.29
8.79
15.46
180.4
21
4 layers
17.0
829
0.68
9.58 ± 0.27
9.90
16.98
240.5
29
5 layers
15.9
800
0.66
8.39 ± 0.32
8.68
15.86
300.8
37
TNPs
10.9
800
0.63
5.49 ± 0.35
5.78
10.84
290.3
29
as the number of layers increases from 1 to 4 (equivalent to a film thickness increase from 6 to 29 µm). However, Jsc values decreased to 15.9 mA cm−2 upon further increase of the film thickness to 37 µm, possibly owing to the extended distance carriers must travel prior to extraction, or due to gradual fusing and cracking of hyperbranches (Figure S7, Supporting Information) which can reduce the light scattering ability of the ETM (see below) and lead to more recombination events within the cell (Figure S8, Supporting Information), resulting in decreased Voc and Jsc. In Figure 3b, we show the corresponding IPCE spectra for single-layered and multilayered TiO2 hyperbranched arrays revealing the same trend of Jsc values as described above. Clearly, the multilayered configuration with four layers exhibits an impressive photocurrent of 17.0 mA cm−2 and IPCE values as compared to the other layer structures over the whole spectral range of 400–800 nm, as well as in the wavelength range of 450 to 650 nm which corresponds to the region of maximum absorption of N719. For comparison, we have prepared a DSSC device with a TNPs photoanode of similar thickness (29 µm) to the four layer hyperbranched ETM (see Figure 3a,b). The Voc and Jsc are found to be 800 mV and 10.9 mA cm−2 (PCE = 5.49%), respectively. The performance of the TNP photoanode compares poorly to the hyperbranched TiO2 nanoarrays (829 mV and 17.0 mA cm−2, PCE = 9.85%). Part of the dramatic performance improvement of the hyperbranched structure as opposed to the TNPs film is attributed to enhanced light scattering by the former. The diffuse reflectance spectra of the single and multilayered TiO2 hyperbranched arrays are shown in Figure S9, Supporting Information. The reflectance spectra reveal the remarkable enhancement of light-scattering ability attributed to the hyperbranched photoanode architecture as compared with TNPs films, leading to improved light harvesting in the former. The diffuse scattering data with respect to the thickness of the hyperbranched ETM also reveal the reflectance peaks in case of four layers, possibly contributing to the remarkably high Jsc and PCE achieved for the DSSC devices fabricated at this particular thickness rather than at three or five layers. Further investigation of charge transport (Figure 3c) in DSSCs fabricated with different thicknesses of hyperbranched TiO2 revealed the diffusion coefficient increased steadily with thickness and peaked for the four layer configuration, subsequently decreasing in case of five layers, possibly due to an increase of the density of surface states (Figure S8, Supporting Information).[19,46] In comparison, a single-layer of nanofibers exhibited the lowest value of diffusion coefficient. The recombination
Adv. Mater. 2015, DOI: 10.1002/adma.201500336
lifetime (Figure 3d) associated to the hyperbranched ETM was also found to increase with thickness, peaking at four layers. The rapid charge transport associated with the multilayered hyperbranched TiO2 arrays can be attributed to the elimination of lattice defects and grain boundaries between the nanobranches as a result of successive layer depositions, including electrospinning, hydrothermal synthesis and calcination. It is believed the interconnected network created by the hyperbranches possess an inherent ability to carry charges over long distances within the photoanode, which can be credited with enhancing the charge collection efficiency in DSSCs. On the basis of these observations, we have performed additional thermal annealing of the single layer (600 nm) and multilayer (29 µm) hyperbranched ETMs at 300 °C for 30 min and fabricated PSC and DSSC devices, respectively. In Figure 4a, we have plotted the J–V characteristics of the best performing perovskite and DSSC devices. The corresponding EQE and IPCE spectra are plotted in Figure 4b. The DSSC device achieved a remarkable PCEavg of 10.85% with Jsc of 18.6 mA cm−2 (calculated JSC from EQE equals 18.57 mA cm−2), FF of 0.69, and Voc of 845 mV with PCEmax of 11.22%. The perovskite device also exhibited significantly enhanced efficiency, achieving PCEavg of 15.03% and PCEmax of 15.50% (as summarized in Table S2, Supporting Information). For the annealed samples the corresponding EQE spectra (Figure 4b) also show enhanced response over the whole spectral range for both PSC and DSSC devices. Comparison of the DSSC and PSC devices reveals that the latter exhibits far superior performance despite the use of a well-engineered ETL and the latter being much thinner in the case of the PSC device. The IPCE in the DSSC can reach values of up to 87%, but this occurs in a limited spectral range. By contrast, the PSCs achieved consistently high EQE. This is attributed to faster electron transport and reduced charge recombination in the PSC devices despite using the same ETL architecture. In fact, charge transport in the DSSCs appears to be “electron-limited” at short circuit, meaning that the transport of electrons through the hyperbranched TiO2 is still slower than the transport of holes through the hole-transporting medium.[47,48] We speculate this to be due to the ability of the PSC to transport both electrons and holes, whereas in the DSSC the ETL and electrolyte are independently responsible for these critical steps. In summary, hyperbranched TiO2 electron transporting materials based on a 3D hierarchical nanorod–nanofiber array have been developed using a viable multistage electrospinning and hydrothermal synthesis route. These ETMs not only
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Acknowledgements Part of this work was supported by Round 2 of the Collaborative Research Grant from the Office of Competitive Research Funds and by the Career Development SABIC Chair held by AA. Received: January 21, 2015 Revised: February 16, 2015 Published online:
Figure 4. a) J–V characteristics, b) EQE/IPCE spectra of best performing PSC and DSSC based on, respectively, 0.6 µm and 29 µm thick annealed 3D hyperbranched nanofiber–nanorod arrays.
exhibit large surface area, but also rapid charge transport and reduced recombination, which are credited with remarkable enhancement of photovoltaic efficiency both in perovskite and dye sensitized solar cells. Perovskite solar cells were fabricated by employing a submicron layer of the hyperbranched arrays to yield a high power conversion efficiency of 15.50%. DSSCs were fabricated based on a 29 µm thick multilayer hyperbranched array, yielding a PCEmax of 11.22%, the highest so far achieved using N719-sensitized hyperbranched hierarchical TiO2 photoelectrodes. These results demonstrate that hyperbranched nanofiber–nanorod ETMs have great potential in next generation mesostructured photovoltaics and may also benefit numerous other energy fields such as fuel cells, energy storage devices, photocatalysis, and artificial photosynthesis.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
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[1] X. Lan, S. Masala, E. H. Sargent, Nat. Mater. 2014, 13, 233. [2] J. T.-W. Wang, J. M. Ball, E. M. Barea, A. Abate, J. A. Alexander-Webber, J. Huang, M. Saliba, I. Mora-Sero, J. Bisquert, H. J. Snaith, R. J. Nicholas, Nano Lett. 2014, 14, 724. [3] W. Q. Wu, Y. F. Xu, C. Y. Su, D. B. Kuang, Energy Environ. Sci. 2014, 7, 644. [4] P. Docampo, J. M. Ball, M. Darwich, G. E. Eperon, H. J. Snaith, Nat. Commun. 2013, 4, 2761. [5] K. Mahmood, B. S. Swain, H. S. Jung, Nanoscale 2014, 6, 9127–9138. [6] K. Wojciechowski, M. Saliba, T. Leijtens, A. Abatea, H. J. Snaith, Energy Environ. Sci. 2014, 7, 1142. [7] B. J. Kim, D. H. Kim, Y.-Y. Lee, H.-W. Shin, G. S. Han, J. S. Hong, K. Mahmood, T. K. Ahn, Y.-C. Joo, K. S. Hong, N.-G. Park, S. Lee, H. S. Jung, Energy Environ. Sci. 2015, DOI: 10.1039/C4EE02441A. [8] H.-S. Kim, I. Mora-Sero, V. Gonzalez-Pedro, F. Fabregat-Santiago, E. J. Juarez-Perez, N.-G. Park, J. Bisquert, Nat. Commun. 2013, 4, 2242. [9] S. D. Stranks, G. E. Eperon, G. Grancini, C. Menelaou, M. J. P. Alcocer, T. Leijtens, L. M. Herz, A. Petrozza, H. J. Snaith, Science 2013, 342, 34. [10] K. Mahmood, B. S. Swain, A. R. Kirmani, A. Amassian, J. Mater. Chem. A 2015, DOI: 10.1039/C4TA04883K. [11] G. S. Han, H. S. Chung, B. J. Kim, D. H. Kim, J. W. Lee, B. S. Swain, K. Mahmood, J. S. Yoo, N. G. Park, J. H. Lee, H. S. Jung, J. Mater. Chem. A 2015, DOI: 10.1039/C4TA03684K. [12] K. Mahmood, S. B. Park, J. Mater. Chem. A 2013, 1, 4826. [13] K. Mahmood, H. W. Kang, S. B. Park, H. J. Sung, ACS Appl. Mater. Interfaces 2013, 5, 3075. [14] P. Roy, S. Berger, P. Schmuki, Angew. Chem., Int. Ed. 2011, 50, 2904. [15] W. Q. Wu, B. X. Lei, H. S. Rao, Y. F. Xu, Y. F. Wang, C. Y. Su, D. B. Kuang, Sci. Rep. 2013, 3, 1352. [16] M. Guo, K. Y. Xie, J. Lin, Z. H. Yong, C. T. Yip, L. M. Zhou, Y. Wang, H. T. Huang, Energy Environ. Sci. 2012, 5, 9881. [17] J. R. Jennings, A. Ghicov, L. M. Peter, P. Schmuki, A. B. Walker, J. Am. Chem. Soc. 2008, 130, 13364. [18] X. Feng, K. Zhu, A. J. Frank, C. A. Grimes, T. E. Mallouk, Angew. Chem. Int. Ed. 2012, 51, 2727. [19] X. Sheng, D. He, J. Yang, K. Zhu, X. Feng, Nano Lett. 2014, 14, 1848. [20] H.-S. Kim, J.-W. Lee, N. Yantara, P. P. Boix, S. A. Kulkarni, S. Mhaisalkar, M. Grätzel, N.-G. Park, Nano Lett. 2013, 13, 2412. [21] M. Yang, R. Guo, K. Kadel, Y. Liu, K. O’Shea, R. Bone, X. Wang, J. He, W. Li, J. Mater. Chem. A 2014, 2, 19616. [22] K. Mahmood, B. S. Swain, A. Amassian, Nanoscale 2014, 6, 14674. [23] J. Y. Liao, B. X. Lei, H. Y. Chen, D. B. Kuang, C. Y. Su, Energy Environ. Sci. 2012, 5, 5750. [24] W. Q. Wu, H. S. Rao, Y. F. Xu, Y. F. Wang, C. Y. Su, D. B. Kuang, Sci. Rep. 2013, 3, 1892. [25] D. Kuang, J. Brillet, P. Chen, M. Takata, S. Uchida, H. Miura, K. Sumioka, S. M. Zakeeruddin, M. Grätzel, ACS Nano 2008, 2, 1113. [26] D. Zhonga, B. Cai, X. Wang, Z. Yang, Y. Xing, S. Miao, W.-H. Zhang, C. Lia, Nano Energy 2015, 11, 409.
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[27] M. Yang, R. Guo, K. Kadel, Y. Liu, K. O’Shea, R. Bone, X. Wang, J. He, W. Li, J. Mater. Chem. A 2014, 2, 19616. [28] D. Wang, F. Qian, C. Yang, Z. H. Zhong, C. M. Lieber, Nano Lett. 2004, 4, 871. [29] J. Shi, Y. Hara, C. L. Sun, M. A. Anderson, X. D. Wang, Nano Lett. 2011, 11, 3413. [30] I. S. Cho, Z. B. Chen, A. J. Forman, D. R. Kim, P. M. Rao, T. F. Jaramillo, X. L. Zheng, Nano Lett. 2011, 11, 4978. [31] D. H. Lee, Y. Rho, F. I. Allen, A. M. Minor, S. H. Ko, C. P. Grigoropoulos, Nanoscale 2013, 5, 11147. [32] J. W. Shiu, C. M. Lan, Y. C. Chang, H. P. Wu, W. K. Huang, E. W. G. Diau, ACS Nano 2012, 6, 10862. [33] W. Q. Wu, H. L. Feng, H. S. Rao, Y. F. Xu, D. B. Kuang, C. Y. Su, Nat. Commun. 2014, 5, 3968. [34] W. Q. Wu, Y. F. Xu, H. S. Rao, C. Y. Su, D. B. Kuang, J. Am. Chem. Soc. 2014, 136, 6437. [35] Z. Zhu, J. Qiu, K. Yan, S. Yang, ACS Appl. Mater. Interfaces 2013, 5, 4000. [36] A. Kubacka, M. Fernández-García, G. Colón, Chem. Rev. 2012, 112, 1555. [37] S. N. Habisreutinger, L. Schmidt-Mende, J. K. Stolarczyk, Angew. Chem. Int. Ed. 2013, 52, 7372.
[38] N. Cai, S. J. Moon, L. Cevey-Ha, T. Moehl, R. Humphry-Baker, P. Wang, S. M. Zakeeruddin, M. Grätzel, Nano Lett. 2011, 11, 1452. [39] F. Sauvage, F. D. Fonzo, A. L. Bassi, C. S. Casari, S. Russo, G. Divitini, C. Ducati, C. E. Bottani, P. Comte, M. Grätzel, Nano Lett. 2010, 10, 2562. [40] D. Li, Y. Xia, Nano Lett. 2003, 3, 555. [41] D. Li, J. T. McCann, Y. Xia, M. Marquezz, J. Am. Ceram. Soc. 2006, 89, 1861. [42] K. Onozuka, B. Ding, Y. Tsuge, T. Naka, M. Yamazaki, S. Sugi, S. Ohno, M. Yoshikawa, S. Shiratori, Nanotechnology 2006, 17, 1026. [43] S. Yang, P. Zhu, A. S. Nair, S. Ramakrishna, J. Mater. Chem. 2011, 21, 6541. [44] M. Y. Song, D. K. Kim, K. J. Ihn, S. Jo, D. Y. Kim, Nanotechnology 2004, 15, 1861. [45] M. Shang, W. Wang, W. Yin, J. Ren, S. Sun, L. Zhang, Chem. Eur. J. 2010, 16, 11412. [46] K. Zhu, N. Kopidakis, N. R. Neale, J. van de Lagemaat, A. J. Frank, J. Phys. Chem. B 2006, 110, 25174. [47] T. Leijtens, J. Lim, J. Teuscher, T. Park, H. J. Snaith, Adv. Mater. 2013, 25, 3227. [48] H. J. Snaith, M. Grätzel, Adv. Mater. 2007, 19, 3643.
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