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Sub-Nanometer Conformal TiO2 Blocking Layer for High Efficiency Solid-State Perovskite Absorber Solar Cells Aravind Kumar Chandiran, Aswani Yella,* Matthew T Mayer, Peng Gao, Mohammad Khaja Nazeeruddin,* and Michael Grätzel* Solid-state dye-sensitized solar cells (ssDSC) are shown to be the attractive candidate for effective and economical conversion of solar photons to electricity.[1–6] Since the first successful proof of concept of ssDSC in 1998, the photovoltaic power conversion efficiencies (PCE) have progressively improved to a maximum of ca. 7%.[1,7,8] Recently, new class of absorbers based on organic-inorganic hybrid perovskite structures possessing high absorption coefficient have shown significant potential for the solution processable high efficient solar cells.[9–13] The absorbers based on CH3NH3PbI3 nanocrystals have replaced dyes in the conventional ssDSC and PCE of 15% have been achieved.[11,12,14–18] Under irradiance, an electron is excited from the valence band to the conduction band of the absorber, generating an exciton. The exciton separation is achieved by the injection of an electron into TiO2 and hole into the hole conductor. The circuit is completed by the transport of photogenerated electrons in TiO2 via the external circuit recombining with holes at the counter electrode, generating a photovoltage equal to the difference in the quasi-Fermi level of electrons in TiO2 and redox potential of hole conductor.[10,11,19] Electron recombination from the photoanode to the hole conductor within the device, is one of the major known loss factors decreasing the open-circuit potential of the device.[6,20–23] To block this parasitic back reaction, conventionally, three layers of passivation is applied: two TiO2 layers, a 50–100 nm spray pyrolysis layer and a TiCl4 treatment, on the transparent conducting glass (fluorine doped tin oxide, FTO) and one TiO2 overlayer, by TiCl4 treatment.[7] Other modes of passivation using physical or chemical vapor deposition methods were also proven to be effective in arresting the electron recombination, even though not effectively implemented for high efficiency devices.[24] This multistage passivation is mainly intended to eliminate the contact between FTO and hole conductor and also to passivate the surface defects on nanoparticulate TiO2 (np-TiO2) mesoporous films. The underlayer passivation, on other hand, generates pinholes upon thermal treatment during the device fabrication. To surpass this issue and complexity, we developed a one step, low temperature, sub-nanometer and pin Dr. A. K. Chandiran,[+] Dr. A. Yella,[+] Dr. M. T. Mayer, Dr. P. Gao, Dr. M. K. Nazeeruddin, Prof. M. Grätzel Laboratory of Photonics and Interfaces Swiss Federal Institute of Technology (EPFL) Station 6, CH 1015, Lausanne, Switzerland E-mail: [email protected]; [email protected]; [email protected] [+]Authors A.K.C. and A.Y. contributed equally to this work.
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hole free titanium dioxide overlayer by atomic layer deposition (ALD), which blocks the parasitic back reaction effectively for high efficient solid-state perovskite absorber solar cells. ALD is capable of depositing conformal pin-hole free oxide layers on high aspect ratio structures (length to diameter of the material or pore) as it exploits the chemical reaction of volatile, metal and oxidizing precursors at two separate stages.[25,26] No sintering is carried out after the deposition of blocking overlayer which otherwise can break the conformality and lead to the creation of pinholes. Different thicknesses (0–4 nm) of TiO2 overlayer (TiO2 OL) are deposited on the spin-coated mesoporous np-TiO2 films and their efficacy of photovoltaic power conversion is investigated. We found that a mere 2 nm ALD overlayer on the top of mesoporous film is sufficient to block the back reaction both from FTO and np-TiO2 leading to an efficiency of 11.5%. In addition, the deposition of ALD TiO2 helped us investigating the effect of pore diameter of the np-TiO2 film on the performance of the perovskite absorber solar cells. Complete experimental details of photoanode film preparation, ALD of titania, device fabrication and other characterization methods are given in the Supporting Information. Briefly, np-TiO2 screen printable paste (average particle size – 20 nm and average pore diameter – 23 nm) diluted in ethanol is spin-coated on a bare FTO conducting glass and sintered with a series of thermal steps up to 500 °C to burn out polymeric binders and ensure better electronic contact between particles.[27] The resulting mesoporous films are 300 nm thick and are used as such, as photoanodes, or used with 1–4 nm thick TiO2 overlayer deposited by ALD. A block diagram of the photoanode film is presented in Figure 1A. The CH3NH3PbI3 absorber is deposited by sequentially spin coating the PbI2 solution and then dipping into the methyl ammonium iodide solution for 5 seconds, preceded by an annealing step at 70 °C for 15 min.[14] The monolithic cell fabrication is completed by spin coating the spiro-OMetad hole conductor and evaporating a gold back contact. The current-voltage (J–V) characteristics of the devices with and without ALD TiO2 overlayer (OL) are investigated in dark and under AM1.5G solar irradiance (Figure 1B, Table 1). The reference cell with no underlayer or overlayer have shown a short-circuit photocurrent density (JSC), open-circuit voltage (VOC), fill factor (ff), respectively, of 17.44 mA/cm2, 836 mV and 0.49 leading to a PCE of 7.2%. In the case of the sequential deposition, the perovskite layer formed is conformal and this helps in avoiding the contact of the spiro-OMetad on to the bare FTO when no underlayer or overlayer is deposited. In the case where the perovskite layer is not conformal and the spiro-OMetad is in contact with the FTO, the cells exhibited the ohmic behavior.
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Figure 1. (A) Block diagrams of mesoscopic photoanodes with thinner (left) and thicker (right) ALD TiO2 overlayer (red) deposited on the mesoporous nanoparticle TiO2 (light blue) films. The np-TiO2-ALD TiO2 film is infiltrated with CH3NH3PbI3 absorber (green). For thicker ALD TiO2 overlayers, the pore diameter is reduced and improper pore filling of the perovskite absorber is highlighted (right).(B) J–V characteristics of solar cells with and without ALD overlayers under 1 sun irradiance (100 mW/cm2 photon flux, solid lines) and under dark (dotted lines).
Figure S1 shows the J–V characteristics of the device where the perovskite is deposited by a conventional, one step process from a DMF solution of lead iodide and methyl ammonium iodide.[14] Since we find sequential deposition more efficient for the device performance, we followed this procedure for the rest of our study. With 1 nm ALD TiO2 OL, PCE is increased to ca.10% and further increase in the deposition, up to 3 nm, enhanced the conversion efficiency to 11.5%. The improvement comes mainly from the increase in VOC by around 145 mV and fill factor from 0.49 to 0.67. However, on increasing the thickness to 4 nm, JSC reduced dramatically leading to a drop in the PCE of the device. The incident photon-to-electron conversion efficiency (IPCE) spectrum of the champion device with 2 nm OL is presented in Figure S2. The integrated current under IPCE matches with the JSC measured under 1 sun illumination. The J–V curve of the reference cell measured in dark shows an onset of the dark current at ∼600 mV. On increasing the thickness, the onset shifts to over 750 mV, indicating a decline in the back flow of electrons from the conduction band of TiO2 and FTO glass to the hole conductor or perovskite absorber. It has to be noted that the ALD TiO2 OL is deposited both on the surface of np-TiO2 and FTO glass. So the apparent shift in the onset of the dark current and the corresponding increase in the open-circuit potential might have resulted cumulatively from the reduction in the electron recombination and the passivation of np-TiO2 surface traps.[22,23] Table 1. Photovoltaic parameters of the solid-state mesoscopic solar cells with and without ALD TiO2 overlayers. (JSC: short-circuit current density, VOC: open-circuit potential, ff: fill factor, PCE: photovoltaic power conversion efficiency.) Thickness of the overlayer
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Electrochemical impedance spectroscopic (EIS) analyses, in dark, were performed on these devices to further understand the blocking effect of electron back reaction.[28] Figure 2A presents the Nyquist plot measured at a voltage bias of 1V. The reference cell and the device with different overlayer thicknesses exhibit two distinct semi-circles. The first semi-circle, at high frequencies, is attributed to the charge transfer resistance at the gold back contact and the second semi-circle, at low frequencies, is attributed to the charge transfer resistance (Rrec, inversely proportional to electron recombination) between TiO2/FTO and the hole-conductor or perovskite absorber. In conventional dye-sensitized solar cells, the second semi-circle arises due to the electron back reaction from the conduction band of TiO2 or FTO to the redox mediator.[28–30] In the perovskite absorber solar cell, the dominant electron loss pathway is the recombination of electrons from the titanium dioxide film to the hole conductor.[31]This recombination lowers the steady state electron density in the TiO2 film affecting the open-circuit potential of the device. On increasing the thickness of the ALD TiO2 overlayer, the diameter of the second semi-circle is increasing, resulting from a decline in the electron back reaction. The EIS spectrum is fitted using transmission line model and Rrec is presented as a function of the applied voltage bias, in Figure 2B.[29] Two different observations can be made from the plot: one is, for all devices, Rrec decreases as a function of applied voltage. In other words recombination rate increases when the quasi Fermi level of electrons in TiO2 moves toward its conduction band and this trend is similar to the conventional ssDSC.[6,22,23] Second important observation is that with the increasing thickness of ALD TiO2 overlayer, Rrec increases over the range of measured voltages. This observation indicates the enhanced resistance to the parasitic electron back reaction and is consistent with dark current onset shown in Figure 1B. These results are in excellent agreement with the observed open-circuit potentials of the devices, with ALD surface passivation. In addition to the blocking effect, ALD TiO2 overlayer deposition helps in tuning the pore properties of the mesoporous np-TiO2 structure. The average pore diameter of the pristine film is 23 nm. For every 1 nm deposition of TiO2 overlayer by
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overlayer (red) increases the loading significantly compared to the reference film (black) despite an expected reduction in the pore 1000 -50 no OL diameter and the surface area. This might 2 nm TiO -40 be due to the formation of thick perovskite 4 nm TiO -30 no OL overlayers on the surface of the mesoporous 100 1 nm TiO -20 2 nm TiO titanium dioxide film, as observed from 3 nm TiO -10 Figure S3. Increasing the thickness of the 4 nm TiO 1V 0 10 ALD layer to 2 nm reduces the loading com0 20 40 60 80 100 120 140 160 700 750 800 850 900 950 1000 pared to the absorbance of the reference cell Voltage (mV) Z'(Ω) ∆t and further deposition, lowers the perovskite 3.5 loading progressively. Even though a very 4 no OL dense overlayer of perovskite absorber is ∆I 3.0 1 nm TiO ∆I formed for the ALD surface modified films, 2 nm TiO 2.5 3 nm TiO 3 the overall decrease in the absorbance can be 4 nm TiO 2.0 due to the reduction in the perovskite formation within the pores of the titania film. This 2 1.5 no OL variation in the absorber loading affects the 1 nm 1.0 2 nm short-circuit density of the device. The ini1 3 nm 4 nm 0.5 tial small increase in JSC from 17.4 mA/cm2 (0 nm overlayer) to 18.1 mA/cm2 (1 nm 0.0 0 0 2 4 6 8 10 400 450 500 550 600 650 700 750 800 overlayer) could be attributed to an increase Time (s) Wavelength (nm) in the absorber loading. However, the photoanodes with 2–3 nm TiO2 overlayer exhibit Figure 2. (A) Nyquist plot of the reference cell (black) and cells containing 2 (blue) and 4 nm only a slight decrease in the JSC despite sig(green) ALD TiO2 at an applied voltage of 1V, (B) Charge transfer resistance (Rrec), estimated nificant reduction in the perovskite loading. from the Nyquist plot of electrochemical impedance spectra, for the electron recombination from TiO2 or transparent conducting glass to the perovskite absorber or hole conductor, is In spite of significant variation in the light plotted as a function applied voltage for devices with and without ALD TiO2 overlayer, (C) harvesting efficiency between the reference Absorption spectra of CH3NH3PbI3 on 300 nm mesoporous np-titania films containing difcell and 1–3 nm overlayer, the current denferent thicknesses of ALD TiO2 overlayer and, (D) Current dynamics measured for the solar sity remains between 17.4 and 18.1 mA/cm2. cells possessing different thicknesses of ALD TiO2 overlayer, at 0.64 sun and 1 sun illumination. These results convey a message that the absorber loading is not the only factor deciding JSC in the perovskite absorber solar cells. ALD, the pore diameter decreases by 2 nm. The deposition also reduces the available internal surface area as described in our To understand this further, current dynamics was investiprevious publications.[32,33] These variations in the pore diameter gated for the reference device and the devices with ALD OL by switching the light ON and OFF at different light intensities affects the ability of the perovskite absorber loading in the film (Figure 2D). At 1 sun illumination, after opening the shutter, or the infiltration of spiro-OMetad hole conductor. The cross secthe current was progressively increasing for the reference cell tional microscopic image of different photoanode films with and during the entire duration of illumination. For the devices with without ALD TiO2 overlayers are presented in Figure S3, Figure 1–3 nm OL, the current rises to a maximum in time Δt, folS4 and Figure S5. Two notable differences could be observed lowing which a steady state is reached. But after time Δt, the from the micrographs: (1) the formation of perovskite overlayer device with 4 nm OL TiO2 exhibits a current drop by ΔI4. The crystals on the surface of the mesoporous titanium dioxide, and (2) the infiltration of absorber inside the pores of the titania film. possible explanation for the constant rise with the reference With increasing thickness of ALD TiO2 overlayer from 0 nm to device indicates the excessive charge accumulation at the electronic defect states either within perovskite absorber or at the 4 nm, the packing density of absorber crystals increase, on the interfaces of TiO2/perovskite, which limits the transport of the surface of mesoporous titanium dioxide film. The magnified images presented in Figure S4 and Figure S5 show mesoporous charges to the selective contacts.[28] The electronic defects might titania films infiltrated with perovskite absorber (denoted by red have arisen from the growth of perovskite on the defective arrows). However, for the films containing ALD TiO2, the infilnp-TiO2 surface or unconverted PbI2 at TiO2/perovskite intertration of the absorber is less and they exhibit high porosity. The face. The ALD layers which are known for passivating the surpoor loading of absorber could be due the improper infiltration faces could have led to the enhanced crystal growth of absorber of PbI2 solution because of the reduction in the pore diameter. with fewer defects leading to observation of steady state current for 1–3 nm ALD passivated surfaces.[24,34] However, the drop Probably, this led to an accumulation of PbI2 on the surface of the ALD overlayer films leading to the formation of dense perof photocurrent ΔI4 with 4 nm overlayer could be attributed to ovskite overlayer. This might prevent the infiltration of the hole the poor pore filling of the perovskite absorber or spiro-OMetad conductor into the mesoporous film. hole conductor because of the reduction in the pore diameter Figure 2C shows the absorbance spectra of the perovskite (Figure 1(A), right). Under such inadequate pore filling circumabsorber loaded onto the reference np-TiO2 film and films stances, the hole generated in absorber closer to FTO, has to diffuse/migrate a longer distance within the perovskite layer to modified with the ALD overlayer. The deposition of 1 nm 2
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find the nearest hole injection layer. This enhances the parasitic charge recombination of h+ in the perovskite absorber with the photogenerated electrons leading to a decrease in the current, at short-circuit. This explanation is verified by measuring current transients separately, on a flat TiO2 film and on a mesoporous np-Al2O3 photoanode based device. In the latter cases, unlike mesoporous np-TiO2 photoanode, excitons have to diffuse/ migrate longer distances within the perovskite layer before getting injected into their respective contacts.[11] Figure S6 and Figure S7 shows the current dynamics of these devices where an initial increase in the short-circuit current in observed, but drops progressively with time because of poor charge collection. This observation is similar to the 4 nm TiO2 OL device. So, the drop in the JSC observed with 4 nm TiO2 OL arises cumulatively from the lower perovskite loading and improper pore filling of the spiro-OMetad. Thus, in this work, we have shown an alternate and efficient way to block the electron recombination by employing a subnanometer ALD TiO2 overlayer for high efficient solid state mesoscopic solar cells, overriding the conventional multistep passivation prevailing in the DSC community for more than a decade. Our observations show that a 2 nm TiO2 can block the electron back reaction effectively, both from FTO and TiO2 surface, leading to a photovoltaic power conversion efficiency of 11.5%. The study on current dynamics reveals the issue of charge collection while transporting the carriers with the absorber for longer lengths, stressing the necessity for effective pore filling of the perovskite absorber and hole conductors. The layers employed are ultrathin which lead to an overall reduction in the passivation layer film capacitance, displaying a potential for devices with higher open-circuit voltage. As ALD titania OL is deposited at a low temperature (120 °C) and no further heating was carried out, these blocking layers can be readily implemented for low temperature processable solar cells on flexible plastic substrates. Authors do not expect these layers to passivate the surface efficiently, when a thermal post-treatment is carried on ALD layers, as it might lead to thermal shrinkage leaving electronic pin holes behind.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The authors acknowledge the financial contribution from EU FP7 projects “ORION” (grant agreement number: NMP-229036) and “GLOBASOL” (grant agreement number: 309194). The authors are grateful for the financial support from the Balzan Foundation as a part of the 2009 Balzan prize awarded to Michael Grätzel. Received: December 24, 2013 Revised: March 5, 2014 Published online:
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[1] U. Bach, D. Lupo, P. Comte, J. E. Moser, F. Weissortel, J. Salbeck, H. Spreitzer, M. Grätzel, Nature 1998, 395, 583. [2] M. Grätzel, Inorg. Chem. 2005, 44, 6841. [3] B. E. Hardin, H. J. Snaith, M. D. McGehee, Nat. Phot. 2012, 6, 162. [4] H. J. Snaith, L. Schmidt-Mende, Adv. Mater. 2007, 19, 3187. [5] I. Chung, B. Lee, J. He, R. P. H. Chang, M. G. Kanatzidis, Nature 2012, 485, 486. [6] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pettersson, Chem. Rev. 2010, 110, 6595. [7] J. Burschka, A. Dualeh, F. Kessler, E. Baranoff, N.-L. Cevey-Ha, C. Yi, M. K. Nazeeruddin, M. Grätzel, J. Am. Chem. Soc. 2011, 133, 18042. [8] C. Xu, J. Wu, U. V Desai, D. Gao, Nano Lett. 2012, 12, 2420. [9] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc. 2009, 131, 6050. [10] H.-S. Kim, C.-R. Lee, J.-H. Im, K.-B. Lee, T. Moehl, A. Marchioro, S.-J. Moon, R. Humphry-Baker, J.-H. Yum, J. E. Moser, M. Grätzel, N.-G. Park, Sci. Rep. 2012, 2. [11] M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami, H. J. Snaith, Science 2012, 338, 643. [12] J. H. Heo, S. H. Im, J. H. Noh, T. N. Mandal, C.-S. Lim, J. A. Chang, Y. H. Lee, H. Kim, A. Sarkar, Md. K. Nazeeruddin, M. Grätzel, S. Il Seok, Nat Phot. 2013, 7, 486. [13] T. Miyasaka, J. Phys. Chem. Lett. 2011, 2, 262. [14] J. Burschka, N. Pellet, S.-J. Moon, R. Humphry-Baker, P. Gao, M. K. Nazeeruddin, M. Grätzel, Nature 2013, 499, 316. [15] A. Abrusci, S. D. Stranks, P. Docampo, H.-L. Yip, A. K.-Y. Jen, H. J. Snaith, Nano Lett. 2013, 13, 3124. [16] H. J. Snaith, J. Phys. Chem. Lett. 2013, 4, 3623. [17] N. J. Jeon, J. Lee, J. H. Noh, M. K. Nazeeruddin, M. Grätzel, S. Il Seok, J. Am. Chem. Soc. 2013, DOI 10.1021/ja410659k. [18] P. Qin, A. L. Domanski, A. K. Chandiran, R. Berger, H.-J. Butt, M. I. Dar, T. Moehl, N. Tetreault, P. Gao, S. Ahmad, M. K. Nazeeruddin, M. Grätzel, Nanoscale 2014. [19] D. Bi, S.-J. Moon, L. Haggman, G. Boschloo, L. Yang, E. M. J. Johansson, M. K. Nazeeruddin, M. Grätzel, A. Hagfeldt, RSC Adv. 2013. [20] D. Bi, L. Yang, G. Boschloo, A. Hagfeldt, E. M. J. Johansson, J. Phys. Chem. Lett. 2013, 4, 1532. [21] J. Nissfolk, K. Fredin, A. Hagfeldt, G. Boschloo, J. Phys. Chem. B 2006, 110, 17715. [22] L. M. Peter, Phys. Chem. Chem. Phys. 2007, 9, 2630. [23] B. C. O'Regan, J. R. Durrant, Acc. Chem. Res. 2009, 42, 1799. [24] T. C. Li, M. S. Góes, F. Fabregat-Santiago, J. Bisquert, P. R. Bueno, C. Prasittichai, J. T. Hupp, T. J. Marks, J. Phys. Chem. C 2009, 113, 18385. [25] S. M. George, Chem. Rev. 2009, 110, 111. [26] M. Leskelä, M. Ritala, Thin Solid Films 2002, 409, 138. [27] S. Ito, T. N. Murakami, P. Comte, P. Liska, C. Grätzel, M. K. Nazeeruddin, M. Grätzel, Thin Solid Films 2008, 516, 4613. [28] 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. [29] J. Bisquert, J. Phys. Chem. B 2001, 106, 325. [30] J. Bisquert, Phys. Chem. Chem. Phys. 2008, 10, 49. [31] Y. Zhao, K. Zhu, J. Phys. Chem. Lett. 2013, 2880. [32] A. K. Chandiran, A. Yella, M. Stefik, L.-P. Heiniger, P. Comte, M. K. Nazeeruddin, M. Grätzel, ACS Appl. Mater. Interfaces 2013, 5, 3487. [33] A. K. Chandiran, P. Comte, R. Humphry-Baker, F. Kessler, C. Yi, M. K. Nazeeruddin, M. Grätzel, Adv. Funct. Mater. 2013, 23, 2775. [34] H.-J. Son, X. Wang, C. Prasittichai, N. C. Jeong, T. Aaltonen, R. G. Gordon, J. T. Hupp, J. Am. Chem. Soc. 2012, 134, 9537.
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Sub-Nanometer Conformal TiO2 Blocking Layer for High Efficiency Solid-State Perovskite Absorber Solar Cells Aravind Kumar Chandiran, Aswani Yella,* Matthew T Mayer, Peng Gao, Mohammad Khaja Nazeeruddin,* and Michael Grätzel* Solid-state dye-sensitized solar cells (ssDSC) are shown to be the attractive candidate for effective and economical conversion of solar photons to electricity.[1–6] Since the first successful proof of concept of ssDSC in 1998, the photovoltaic power conversion efficiencies (PCE) have progressively improved to a maximum of ca. 7%.[1,7,8] Recently, new class of absorbers based on organic-inorganic hybrid perovskite structures possessing high absorption coefficient have shown significant potential for the solution processable high efficient solar cells.[9–13] The absorbers based on CH3NH3PbI3 nanocrystals have replaced dyes in the conventional ssDSC and PCE of 15% have been achieved.[11,12,14–18] Under irradiance, an electron is excited from the valence band to the conduction band of the absorber, generating an exciton. The exciton separation is achieved by the injection of an electron into TiO2 and hole into the hole conductor. The circuit is completed by the transport of photogenerated electrons in TiO2 via the external circuit recombining with holes at the counter electrode, generating a photovoltage equal to the difference in the quasi-Fermi level of electrons in TiO2 and redox potential of hole conductor.[10,11,19] Electron recombination from the photoanode to the hole conductor within the device, is one of the major known loss factors decreasing the open-circuit potential of the device.[6,20–23] To block this parasitic back reaction, conventionally, three layers of passivation is applied: two TiO2 layers, a 50–100 nm spray pyrolysis layer and a TiCl4 treatment, on the transparent conducting glass (fluorine doped tin oxide, FTO) and one TiO2 overlayer, by TiCl4 treatment.[7] Other modes of passivation using physical or chemical vapor deposition methods were also proven to be effective in arresting the electron recombination, even though not effectively implemented for high efficiency devices.[24] This multistage passivation is mainly intended to eliminate the contact between FTO and hole conductor and also to passivate the surface defects on nanoparticulate TiO2 (np-TiO2) mesoporous films. The underlayer passivation, on other hand, generates pinholes upon thermal treatment during the device fabrication. To surpass this issue and complexity, we developed a one step, low temperature, sub-nanometer and pin Dr. A. K. Chandiran,[+] Dr. A. Yella,[+] Dr. M. T. Mayer, Dr. P. Gao, Dr. M. K. Nazeeruddin, Prof. M. Grätzel Laboratory of Photonics and Interfaces Swiss Federal Institute of Technology (EPFL) Station 6, CH 1015, Lausanne, Switzerland E-mail: [email protected]; [email protected]; [email protected] [+]Authors A.K.C. and A.Y. contributed equally to this work.
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hole free titanium dioxide overlayer by atomic layer deposition (ALD), which blocks the parasitic back reaction effectively for high efficient solid-state perovskite absorber solar cells. ALD is capable of depositing conformal pin-hole free oxide layers on high aspect ratio structures (length to diameter of the material or pore) as it exploits the chemical reaction of volatile, metal and oxidizing precursors at two separate stages.[25,26] No sintering is carried out after the deposition of blocking overlayer which otherwise can break the conformality and lead to the creation of pinholes. Different thicknesses (0–4 nm) of TiO2 overlayer (TiO2 OL) are deposited on the spin-coated mesoporous np-TiO2 films and their efficacy of photovoltaic power conversion is investigated. We found that a mere 2 nm ALD overlayer on the top of mesoporous film is sufficient to block the back reaction both from FTO and np-TiO2 leading to an efficiency of 11.5%. In addition, the deposition of ALD TiO2 helped us investigating the effect of pore diameter of the np-TiO2 film on the performance of the perovskite absorber solar cells. Complete experimental details of photoanode film preparation, ALD of titania, device fabrication and other characterization methods are given in the Supporting Information. Briefly, np-TiO2 screen printable paste (average particle size – 20 nm and average pore diameter – 23 nm) diluted in ethanol is spin-coated on a bare FTO conducting glass and sintered with a series of thermal steps up to 500 °C to burn out polymeric binders and ensure better electronic contact between particles.[27] The resulting mesoporous films are 300 nm thick and are used as such, as photoanodes, or used with 1–4 nm thick TiO2 overlayer deposited by ALD. A block diagram of the photoanode film is presented in Figure 1A. The CH3NH3PbI3 absorber is deposited by sequentially spin coating the PbI2 solution and then dipping into the methyl ammonium iodide solution for 5 seconds, preceded by an annealing step at 70 °C for 15 min.[14] The monolithic cell fabrication is completed by spin coating the spiro-OMetad hole conductor and evaporating a gold back contact. The current-voltage (J–V) characteristics of the devices with and without ALD TiO2 overlayer (OL) are investigated in dark and under AM1.5G solar irradiance (Figure 1B, Table 1). The reference cell with no underlayer or overlayer have shown a short-circuit photocurrent density (JSC), open-circuit voltage (VOC), fill factor (ff), respectively, of 17.44 mA/cm2, 836 mV and 0.49 leading to a PCE of 7.2%. In the case of the sequential deposition, the perovskite layer formed is conformal and this helps in avoiding the contact of the spiro-OMetad on to the bare FTO when no underlayer or overlayer is deposited. In the case where the perovskite layer is not conformal and the spiro-OMetad is in contact with the FTO, the cells exhibited the ohmic behavior.
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Figure 1. (A) Block diagrams of mesoscopic photoanodes with thinner (left) and thicker (right) ALD TiO2 overlayer (red) deposited on the mesoporous nanoparticle TiO2 (light blue) films. The np-TiO2-ALD TiO2 film is infiltrated with CH3NH3PbI3 absorber (green). For thicker ALD TiO2 overlayers, the pore diameter is reduced and improper pore filling of the perovskite absorber is highlighted (right).(B) J–V characteristics of solar cells with and without ALD overlayers under 1 sun irradiance (100 mW/cm2 photon flux, solid lines) and under dark (dotted lines).
Figure S1 shows the J–V characteristics of the device where the perovskite is deposited by a conventional, one step process from a DMF solution of lead iodide and methyl ammonium iodide.[14] Since we find sequential deposition more efficient for the device performance, we followed this procedure for the rest of our study. With 1 nm ALD TiO2 OL, PCE is increased to ca.10% and further increase in the deposition, up to 3 nm, enhanced the conversion efficiency to 11.5%. The improvement comes mainly from the increase in VOC by around 145 mV and fill factor from 0.49 to 0.67. However, on increasing the thickness to 4 nm, JSC reduced dramatically leading to a drop in the PCE of the device. The incident photon-to-electron conversion efficiency (IPCE) spectrum of the champion device with 2 nm OL is presented in Figure S2. The integrated current under IPCE matches with the JSC measured under 1 sun illumination. The J–V curve of the reference cell measured in dark shows an onset of the dark current at ∼600 mV. On increasing the thickness, the onset shifts to over 750 mV, indicating a decline in the back flow of electrons from the conduction band of TiO2 and FTO glass to the hole conductor or perovskite absorber. It has to be noted that the ALD TiO2 OL is deposited both on the surface of np-TiO2 and FTO glass. So the apparent shift in the onset of the dark current and the corresponding increase in the open-circuit potential might have resulted cumulatively from the reduction in the electron recombination and the passivation of np-TiO2 surface traps.[22,23] Table 1. Photovoltaic parameters of the solid-state mesoscopic solar cells with and without ALD TiO2 overlayers. (JSC: short-circuit current density, VOC: open-circuit potential, ff: fill factor, PCE: photovoltaic power conversion efficiency.) Thickness of the overlayer
2
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17.44
836.6
0.49
7.2
1
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919.9
0.59
9.9
2
17.64
969.4
0.67
11.5
3
17.92
980.8
0.64
11.2
4
14.44
981.7
0.64
9.1
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Electrochemical impedance spectroscopic (EIS) analyses, in dark, were performed on these devices to further understand the blocking effect of electron back reaction.[28] Figure 2A presents the Nyquist plot measured at a voltage bias of 1V. The reference cell and the device with different overlayer thicknesses exhibit two distinct semi-circles. The first semi-circle, at high frequencies, is attributed to the charge transfer resistance at the gold back contact and the second semi-circle, at low frequencies, is attributed to the charge transfer resistance (Rrec, inversely proportional to electron recombination) between TiO2/FTO and the hole-conductor or perovskite absorber. In conventional dye-sensitized solar cells, the second semi-circle arises due to the electron back reaction from the conduction band of TiO2 or FTO to the redox mediator.[28–30] In the perovskite absorber solar cell, the dominant electron loss pathway is the recombination of electrons from the titanium dioxide film to the hole conductor.[31]This recombination lowers the steady state electron density in the TiO2 film affecting the open-circuit potential of the device. On increasing the thickness of the ALD TiO2 overlayer, the diameter of the second semi-circle is increasing, resulting from a decline in the electron back reaction. The EIS spectrum is fitted using transmission line model and Rrec is presented as a function of the applied voltage bias, in Figure 2B.[29] Two different observations can be made from the plot: one is, for all devices, Rrec decreases as a function of applied voltage. In other words recombination rate increases when the quasi Fermi level of electrons in TiO2 moves toward its conduction band and this trend is similar to the conventional ssDSC.[6,22,23] Second important observation is that with the increasing thickness of ALD TiO2 overlayer, Rrec increases over the range of measured voltages. This observation indicates the enhanced resistance to the parasitic electron back reaction and is consistent with dark current onset shown in Figure 1B. These results are in excellent agreement with the observed open-circuit potentials of the devices, with ALD surface passivation. In addition to the blocking effect, ALD TiO2 overlayer deposition helps in tuning the pore properties of the mesoporous np-TiO2 structure. The average pore diameter of the pristine film is 23 nm. For every 1 nm deposition of TiO2 overlayer by
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Charge transfer Resistance (Ω)
overlayer (red) increases the loading significantly compared to the reference film (black) despite an expected reduction in the pore 1000 -50 no OL diameter and the surface area. This might 2 nm TiO -40 be due to the formation of thick perovskite 4 nm TiO -30 no OL overlayers on the surface of the mesoporous 100 1 nm TiO -20 2 nm TiO titanium dioxide film, as observed from 3 nm TiO -10 Figure S3. Increasing the thickness of the 4 nm TiO 1V 0 10 ALD layer to 2 nm reduces the loading com0 20 40 60 80 100 120 140 160 700 750 800 850 900 950 1000 pared to the absorbance of the reference cell Voltage (mV) Z'(Ω) ∆t and further deposition, lowers the perovskite 3.5 loading progressively. Even though a very 4 no OL dense overlayer of perovskite absorber is ∆I 3.0 1 nm TiO ∆I formed for the ALD surface modified films, 2 nm TiO 2.5 3 nm TiO 3 the overall decrease in the absorbance can be 4 nm TiO 2.0 due to the reduction in the perovskite formation within the pores of the titania film. This 2 1.5 no OL variation in the absorber loading affects the 1 nm 1.0 2 nm short-circuit density of the device. The ini1 3 nm 4 nm 0.5 tial small increase in JSC from 17.4 mA/cm2 (0 nm overlayer) to 18.1 mA/cm2 (1 nm 0.0 0 0 2 4 6 8 10 400 450 500 550 600 650 700 750 800 overlayer) could be attributed to an increase Time (s) Wavelength (nm) in the absorber loading. However, the photoanodes with 2–3 nm TiO2 overlayer exhibit Figure 2. (A) Nyquist plot of the reference cell (black) and cells containing 2 (blue) and 4 nm only a slight decrease in the JSC despite sig(green) ALD TiO2 at an applied voltage of 1V, (B) Charge transfer resistance (Rrec), estimated nificant reduction in the perovskite loading. from the Nyquist plot of electrochemical impedance spectra, for the electron recombination from TiO2 or transparent conducting glass to the perovskite absorber or hole conductor, is In spite of significant variation in the light plotted as a function applied voltage for devices with and without ALD TiO2 overlayer, (C) harvesting efficiency between the reference Absorption spectra of CH3NH3PbI3 on 300 nm mesoporous np-titania films containing difcell and 1–3 nm overlayer, the current denferent thicknesses of ALD TiO2 overlayer and, (D) Current dynamics measured for the solar sity remains between 17.4 and 18.1 mA/cm2. cells possessing different thicknesses of ALD TiO2 overlayer, at 0.64 sun and 1 sun illumination. These results convey a message that the absorber loading is not the only factor deciding JSC in the perovskite absorber solar cells. ALD, the pore diameter decreases by 2 nm. The deposition also reduces the available internal surface area as described in our To understand this further, current dynamics was investiprevious publications.[32,33] These variations in the pore diameter gated for the reference device and the devices with ALD OL by switching the light ON and OFF at different light intensities affects the ability of the perovskite absorber loading in the film (Figure 2D). At 1 sun illumination, after opening the shutter, or the infiltration of spiro-OMetad hole conductor. The cross secthe current was progressively increasing for the reference cell tional microscopic image of different photoanode films with and during the entire duration of illumination. For the devices with without ALD TiO2 overlayers are presented in Figure S3, Figure 1–3 nm OL, the current rises to a maximum in time Δt, folS4 and Figure S5. Two notable differences could be observed lowing which a steady state is reached. But after time Δt, the from the micrographs: (1) the formation of perovskite overlayer device with 4 nm OL TiO2 exhibits a current drop by ΔI4. The crystals on the surface of the mesoporous titanium dioxide, and (2) the infiltration of absorber inside the pores of the titania film. possible explanation for the constant rise with the reference With increasing thickness of ALD TiO2 overlayer from 0 nm to device indicates the excessive charge accumulation at the electronic defect states either within perovskite absorber or at the 4 nm, the packing density of absorber crystals increase, on the interfaces of TiO2/perovskite, which limits the transport of the surface of mesoporous titanium dioxide film. The magnified images presented in Figure S4 and Figure S5 show mesoporous charges to the selective contacts.[28] The electronic defects might titania films infiltrated with perovskite absorber (denoted by red have arisen from the growth of perovskite on the defective arrows). However, for the films containing ALD TiO2, the infilnp-TiO2 surface or unconverted PbI2 at TiO2/perovskite intertration of the absorber is less and they exhibit high porosity. The face. The ALD layers which are known for passivating the surpoor loading of absorber could be due the improper infiltration faces could have led to the enhanced crystal growth of absorber of PbI2 solution because of the reduction in the pore diameter. with fewer defects leading to observation of steady state current for 1–3 nm ALD passivated surfaces.[24,34] However, the drop Probably, this led to an accumulation of PbI2 on the surface of the ALD overlayer films leading to the formation of dense perof photocurrent ΔI4 with 4 nm overlayer could be attributed to ovskite overlayer. This might prevent the infiltration of the hole the poor pore filling of the perovskite absorber or spiro-OMetad conductor into the mesoporous film. hole conductor because of the reduction in the pore diameter Figure 2C shows the absorbance spectra of the perovskite (Figure 1(A), right). Under such inadequate pore filling circumabsorber loaded onto the reference np-TiO2 film and films stances, the hole generated in absorber closer to FTO, has to diffuse/migrate a longer distance within the perovskite layer to modified with the ALD overlayer. The deposition of 1 nm 2
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2 2 2 2
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find the nearest hole injection layer. This enhances the parasitic charge recombination of h+ in the perovskite absorber with the photogenerated electrons leading to a decrease in the current, at short-circuit. This explanation is verified by measuring current transients separately, on a flat TiO2 film and on a mesoporous np-Al2O3 photoanode based device. In the latter cases, unlike mesoporous np-TiO2 photoanode, excitons have to diffuse/ migrate longer distances within the perovskite layer before getting injected into their respective contacts.[11] Figure S6 and Figure S7 shows the current dynamics of these devices where an initial increase in the short-circuit current in observed, but drops progressively with time because of poor charge collection. This observation is similar to the 4 nm TiO2 OL device. So, the drop in the JSC observed with 4 nm TiO2 OL arises cumulatively from the lower perovskite loading and improper pore filling of the spiro-OMetad. Thus, in this work, we have shown an alternate and efficient way to block the electron recombination by employing a subnanometer ALD TiO2 overlayer for high efficient solid state mesoscopic solar cells, overriding the conventional multistep passivation prevailing in the DSC community for more than a decade. Our observations show that a 2 nm TiO2 can block the electron back reaction effectively, both from FTO and TiO2 surface, leading to a photovoltaic power conversion efficiency of 11.5%. The study on current dynamics reveals the issue of charge collection while transporting the carriers with the absorber for longer lengths, stressing the necessity for effective pore filling of the perovskite absorber and hole conductors. The layers employed are ultrathin which lead to an overall reduction in the passivation layer film capacitance, displaying a potential for devices with higher open-circuit voltage. As ALD titania OL is deposited at a low temperature (120 °C) and no further heating was carried out, these blocking layers can be readily implemented for low temperature processable solar cells on flexible plastic substrates. Authors do not expect these layers to passivate the surface efficiently, when a thermal post-treatment is carried on ALD layers, as it might lead to thermal shrinkage leaving electronic pin holes behind.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The authors acknowledge the financial contribution from EU FP7 projects “ORION” (grant agreement number: NMP-229036) and “GLOBASOL” (grant agreement number: 309194). The authors are grateful for the financial support from the Balzan Foundation as a part of the 2009 Balzan prize awarded to Michael Grätzel. Received: December 24, 2013 Revised: March 5, 2014 Published online:
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Adv. Mater. 2014, DOI: 10.1002/adma.201306271
