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Ultrathin atomic layer deposited TiO2 for surface passivation of hydrothermally grown 1D TiO2 nanorod arrays for efficient solid state perovskite solar cells Sawanta S Mali, Chang Su Shim, Hui Kyung Park, Jaeyeong Heo, and Chang Kook Hong Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 09 Feb 2015 Downloaded from http://pubs.acs.org on February 11, 2015

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Chemistry of Materials

Ultrathin atomic layer deposited TiO2 for surface passivation of hydrothermally grown 1D TiO2 nanorod arrays for efficient solid state perovskite solar cells Sawanta S. Malia, Chang Su Shima, Hui Kyung Parkb, Jaeyeong Heob, Pramod S. Patila, Chang Kook Honga*

a

Polymer Energy Materials laboratory, School of Applied Chemical Engineering, Chonnam National University, Gwangju, 500-757 (South Korea) b

Department of Materials Science and Engineering and Optoelectronics Convergence Research Center, Chonnam National University, Gwangju 500-757 (South Korea).

Abstract: In the current work, we have studied the effect of the passivation of atomic layer deposited (ALD) ultrathin TiO2 on the hydrothermally grown one dimensional (1D) TiO2 nanorod (NR) arrays for solid state perovskite sensitized solar cells. Different thicknesses of ALD passivated TiO2 were deposited on the hydrothermally grown 1D TiO2 NR samples. The ALD TiO2 thickness was varied from 1nm to 5nm by varying growth cycles. Our controlled results revealed that, the 4nm thin layer passivated TiO2 nanorod sample shows power conversion efficiency (PCE) as high as 12.53% (without masking) for the CH3NH3PbI3 perovskite absorbing layer. Our results revealed that, the solar cell performance with different ALD passivation thickness strongly affect on the open circuit voltage (VOC) as well as current density (JSC). However, compared to high temperature processed standard device configurations based on TiCl4 treated mesoporous TiO2 (mp-TiO2) (~10%) and TiCl4 treated TiO2 nanorods (~9%) perovskite solar cells, our low temperature processed, pinhole free ALD passivated devices exhibit higher PCE. The 4nm passivated sample exhibits

ACS Paragon Plus Environment


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η=12.53±0.35%,

with

Jsc=19.23±0.53mAcm-2,

fill

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factor

(FF)=0.70±0.4,

and

VOC=0.931±0.01V. By controlling ultrathin thickness our champion cell for 4.8 nm ALD passivated TiO2 nanorods demonstrated 13.45 % PCE with JSC=19.78mAcm-2, VOC=0.945V and FF=0.72. These results further emphasize the hydrothermally grown 1D TiO2 and ALD passivated electron transporting layer (ETL) for efficient perovskite solar cell applications.


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1. Introduction Organometallic tri-halides perovskites, such as methylammonium lead iodide (CH3NH3PbI3) and its family are excellent light absorbing materials for solid state perovskite solar cells [1-5]. High diffusion length (>100nm) and proper band gap (Eg~ 1.5 eV) nearly equal to silicon and high absorption coefficient (4.3 x105 cm-1 at 360 nm and 1.5 x 104 cm-1 at 550 nm) [6,7] are key features of perovskite materials. Mesoporous TiO2 (mpTiO2) sensitized with organometallic halide perovskite sensitizers (CH3NH3PbI3) and its optimization is one of the major issues in perovskite sensitized solar cells [3, 4]. The nanoparticulate architecture also suffers from a large number of particles to particle grain boundaries that hamper the fast flow of electrons, which subsequently limits the efficiency. Therefore, recently, perovskite solar cells based on one dimensional (1D) nanostructures such as rutile 1D TiO2 and 1D ZnO have been investigated and demonstrated 9.4% and 11% power conversion efficiency (PCE), respectively [8,9]. The metal oxide photoelectrode or electron transporting layer (ETL), perovskite loading, hole transporting material (HTM) i.e. spiro-MeOTAD and a counter electrode (Au) are the key components of the perovskite solar cells. When the photon energy illuminates perovskite solar cells, an electron is excited from the valence band to the conduction band of the perovskite absorber, generating an exciton. The exciton separation is achieved by the injection of an electron into the conduction band (CB) of TiO2 and a hole into the HTM. The circuit will be completed by the transport of photo-generated electrons in TiO2 via the external circuit recombining with holes at the counter electrode. The photovoltage generated is equal to the difference between the quasiFermi level of electrons in TiO2 and redox potential of hole conductor. Therefore, the



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optimization of stoichiometric perovskite and its deposition, thickness of each layer are the key factors of the experimental process for achieving good efficiency [10]. Recently, few reports are available using Al2O3 [9], NiO [10], ZnO [11] and graphene/TiO2 [12] based perovskite solar cells. On the other hand, some reports are also available based on the rutile TiO2 nanorods (NR) [13, 6] and anatase nanotubes [14]. The 1D ordered TiO2 facilitates effectively the dye loading and thus improve electron collection efficiency [15-20]. These nanostructures will also provide low grain boundaries, effective charge separation and collection and much more surface area. Moreover, the inter-nanorod channel facilitates effective HTM filling, thus enhances the hole transporting efficiency. However, TiCl4 treatment is mandatory for both mp-TiO2 as well as TiO2 NR. The TiCl4 treatment or surface passivation is one of the key process for achieving fast electron transport, low recombination and excellent adsorption of perovskite onto surface of metal oxides. It also acts as a surface passivation for mp-TiO2 that reduces grain boundaries and surface traps. It is well known that, the surface passivation process may contribute to the better charge carrier separation and hindered recombination when the electrons excited from the perovskite absorber layer are injected into the conduction band of TiO2. This surface passivation layer can be achieved by chemical technique (Spray Pyrolysis technique (SPT) or TiCl4 solution method) as well as physical technique (Chemical Vapour Deposition (CVD) technique). However, these SPT or CVD passivation techniques generate pinholes onto surface of TiO2 nanorod due to thermal treatment which unlikely assists back reaction. Furthermore, the deposition of mp-TiO2 or TiCl4 treatment on to flexible substrate is not possible. Since, flexible transparent conducting oxide (TCO) substrates are not stable at high temperature (>200°C) and these process need >500°C


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temperature. Therefore, these high temperature processed passivation methods or TiCl4 treatments need to avoid for pinhole free passivation as well as flexible perovskite solar cells. On the other hand, atomic layer deposition (ALD) technique facilitates pin hole free deposition of ultrathin TiO2 on to TiO2 NRs at low temperature. Here, we have selected ALD technique for deposition of pinhole free TiO2 barrier layers (i.e. surface passivation) onto 1D TiO2 nanorods, since it ensures well-defined, stepwise and conformal growth technique [21, 22, 23]. This technique also provides exceptional control over nanoscale device composition at low temperature, which blocks the unwanted back reaction for perovskite solar cells. Therefore, fabrication of perovskite solar cells based on one-dimensional (1D) TiO2 nanostructures followed by pinhole free passivation by ALD technique will open a new approach for efficient perovskite solar cells. Recently, Q. Jiang et al. synthesized rutile TiO2 nanowires by hydrothermal route followed by annealing at 450°C and used for perovskite solar cell. The authors mentioned that the sample having 9000nm length exhibits 22mAcm-2 current density with 11.7% (FF=0.68) power conversion efficiency [24]. However, this process also suffers from high temperature TiCl4 treatment. To the best of our knowledge, there is no single report available based on ALD passivated TiO2 NR for perovskite solar cells. Here, we report a systematic study of bare and ALD coated TiO2 NR for enabling surface passivation, effective pore filling and controlling recombination rate. These nano-architectures enhance the charge generation and reduces recombination rate in perovskite solar cells due to pinhole free surface passivation by ALD, good pore filling by 1D nano-architecture, uniform distribution of perovskite materials and controlling back reactions. 2. Results and discussion



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Simple and cost effective solution process was aimed in this work for the synthesis of 1D TiO2 NR, CH3NH3PbI3 perovskite and TiO2 ALD passivation. Figure 1 represents the process involved in the synthesis of perovskite solar cells based on ALD TiO2@TiO2 NR by the hydrothermal process. Initially, we have prepared blocking TiO2 (Bl-TiO2) from 0.3M titanium diisopropoxide bis(acetylacetonate) Ti(acac)2OiPr2 solution spin coating onto pattern FTO followed by annealing. The vertically aligned TiO2 nanorods were grown from hydrolysis of TTIP in equal volume of HCl/water at 180°C directly onto FTO substrate [25]. The prepared vertically aligned 1D TiO2 NRs on FTO substrates were used for ALD TiO2 coating. The experimental details of hydrothermal synthesis of 1D TiO2 NR, TiO2 passivation layer by ALD, perovskite deposition by spin coating, HTM preparation and device fabrication are given in the Supporting Information S1 to S4. For the γ-butyrolactone (GBL) solvent evaporation and crystallization of CH3NH3PbI3, the spin coated samples were dried at 100°C for 5min and used for spiro-MeOTAD (HTM) deposition. The 1D TiO2 NR arrays were grown directly onto FTO substrates as per our hydrothermal method [26]. Figure 2(a) shows a typical top view field emission scanning electron micrographic (FESEM) image of hydrothermally grown TiO2 nanorods. The average nanorod diameters are found 80-160 nm and 1.8µm length. Notably, these nanorods are vertically aligned to the FTO substrate. The synthesized 1D TiO2 nanorods were further used for the ALD process followed by CH3NH3PbI3 deposition. The CH3NH3PbI3 solution was prepared in GBL solvent and spin cast at two steps 2500rpm and 3500rpm. Initially, we have optimized our CH3NH3PbI3 phase using grazing incidence X-ray diffraction (GIXRD) analysis (Figure 2(b)). The dried CH3NH3PbI3/TiO2/FTO sample shows ten prominent diffraction peaks (wine colored star “ ” symbols) of the CH3NH3PbI3 nanoparticles corresponding to (110), (112), (211),


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(202), (220), (310), (312), (224), (411) and (330) at 14.04°, 19.93°, 23.52°, 24.55°, 28.42°, 30.99°, 31.93°, 40.34°, 42.49° and 43.25°, respectively, of the tetragonal crystal structure [2, 6]. All diffraction peaks that are consistent with the reported data confirm CH3NH3PbI3 with a tetragonal phase. The small peak originated at 12.61° is due to excess PbI2. The other diffraction peaks (110), (101), (111), (211) and (220) at 27.45°, 36.23°, 41.20°, 51.75° and 54.56° respectively

are originated from the rutile phase of TiO2 nanorods (blue colored tringle

“▲”symbols). The peaks indicated by green color (plus “+” symbol) are originated from FTO substrate. In order to check the variation of ALD passivation of the TiO2 nanorods, we have also recorded XRD patterns of bare TiO2 NRs, ALD passivated TiO2 NRs, CH3NH3PbI3/TiO2 NRs and CH3NH3PbI3/ALD TiO2@TiO2 NRs. However, no variation has been observed in TiO2 NR and CH3NH3PbI3 peaks before or after ALD passivation. This is due to amorphous nature of TiO2 layer deposited by ALD technique (Supporting information Figure S1 a and b). The perovskite solar cells fabricated in the present work are composed of bare, TiCl4 treated and ALD coated 1D TiO2 NR/CH3NH3PbI3/spiro-MeOTAD heterojunction, deposited on a FTO substrate and thermally evaporated gold electrodes. Figure 2 (c) shows the FESEM micrograph of CH3NH3PbI3 perovskite deposited onto ALD passivated TiO2 nanorods. The CH3NH3PbI3 nanoparticles have been crystallized onto TiO2 NR. This sample was further used for the spiro-MeOTAD deposition. From Figure 2(d), it is clear that, CH3NH3PbI3 as well as HTM material have been well penetrated between interspacing of the nanorods, thus enhancing the pore filling. Such type of architecture facilitates enhanced hole transport efficiency. For more detailed analysis, these samples were further characterized by high-resolution transmission electron microscopy (HRTEM). Figure 3 (a) shows TEM image of CH3NH3PbI3 decorated TiO2 nanorods. From Figure 3 (a), it can be clearly seen that a CH3NH3PbI3 perovskite nanoparticles



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are deposited onto TiO2 nanorods. In order to study more details, we have recorded HRTEM images of selected area. Figure 3 (b) shows the clear lattice fringes of the nanorod of the selected area. The deposited TiO2 NRs are single crystalline along their entire length. The interplanar spacing is obtained from the HRTEM lattice fringes along distance of d110=0.32nm between the adjacent lattice fringes perpendicular to the rod axis can be assigned to the rutile TiO2 (110). The lattice spacing of d001=0.29 nm along the longitudinal axis direction pertains to the d-spacing of rutile TiO2 (001) crystal planes. The synthesized nanorods are single crystalline in nature that was confirmed by their spot selective area electron diffraction (SAED) pattern (Inset Figure 3(b)) [23]. Figure 3 (c) shows a highly magnified HRTEM image of synthesized CH3NH3PbI3 nanoparticles onto ALD passivated TiO2 NR. From Figure 3 (c), it can be clearly seen that a perovskite sensitized layer composed of ~5-7nm CH3NH3PbI3 nanoparticles has been formed on the surface of the TiO2 NRs. Nearly 4nm amorphous TiO2 layer between TiO2 NR and CH3NH3PbI3 nanoparticles has been shown which is deposited by ALD technique. The synthesized CH3NH3PbI3 nanoparticles are highly crystalline in nature with ~5-7nm in diameter. These CH3NH3PbI3 nanoparticles are single phase highly crystalline in nature were confirmed by HRTEM analysis as shown in Figure 3 (d). The calculated lattice spacing along d110=0.270±0.01nm confirming the tetragonal phase. Moreover, these nanoparticles are single phase confirmed by FFT analysis (Inset Figure 3(d)). Figure 4 (a) shows a typical cross-sectional FESEM image of fabricated solid state perovskite solar cell device based on ALD passivated 1D TiO2 NR. Nearly 1.8µm long TiO2 NRs are well covered with crystalline CH3NH3PbI3 absorber nanoparticles. Moreover, the HTM layer has been well penetrated into interspacing. The thickness of Au contact is 80nm. Figure 4 (b) shows respective schematic diagram of the fabricated device. Efficient HTM filling between


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the spacing of NRs helps fast hole transport towards Au electrode. i.e., this architecture facilitates enhance efficiency in hole transport [27]. The current density-voltage (J–V) characteristics of the fabricated devices based on 1D TiO2 NR with and without ALD process are represented in Figure 4 (c). The J-V measurements were recorded under AM1.5G solar irradiance, and solar cell parameters are summarized in Table-1. All devices were measured without any electrode masking. Further details of the device fabrication and determination of device active areas are described in the supporting information, Figure S2. Here we have used a laser pattern FTO coated glass substrate for deposition of TiO2 NRs by hydrothermal process. The device area was calculated as per cross area between laser patterned FTO line (0.3cm) and gold (0.3cm) line pattern contact width (device area=0.3x0.3=0.09cm2) as shown in Figure S2. In the present experiments, we have deposited TiO2 layers only on FTO substrate as shown in the photographs of as synthesized 1D TiO2 NR sample (Supporting information, Figure S2). The solar cell parameters based on various photoelectrodes are summarized in Table-1. The bare TiO2 NR device shows short-circuit current density (JSC) 13.73mAcm-2, open-circuit voltage (VOC) 0.748V, fill factor (FF) 0.49 leading to PCE η=5.03% (Figure 4c). For comparison, we have treated these nanorods with 0.04M TiCl4 treated at 70°C followed by 500°C annealing. The TiCl4 treated TiO2 nanorod sample shows 7.98% conversion efficiency with JSC=16.16mAcm-2, FF=58% and VOC=0.852V (Figure S3). The ALD processed 1nm TiO2 passivated layer coated photoelectrode shows VOC=0.787V, JSC=12.85mAcm-2, FF=0.59 leading to PCE η=5.96%. However, the 2 mm thick layer coated samples show a drastic increment in VOC up to 0.833V. This increment may be attributed to increased electrical isolation between spiro-MeOTAD and FTO. If spiro-MeOTAD is directly in contact with FTO substrate, then cell exhibits ohmic behavior. Moreover, the JSC=14.25 mAcm-2, FF=0.60 and η=7.12% have



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also been increased. The 3nm ALD processed sample shows VOC=0.865V, JSC=15.44 mAcm-2, FF=0.66 leading 8.82% conversion efficiency. The 4nm sample exhibits VOC=0.931V, JSC=19.23mAcm-2, FF=70% with PCE=12.53%. Our champion cell having 4.8nm ALD passivated thickness shows VOC=0.945V, JSC=19.78mAcm-2, FF=0.72 leading to 13.45% conversion efficiency. On the other hand, further increment in ALD thickness affects current density. Our 5nm passivated TiO2 NRs based device shows VOC=0.972V, JSC=16.81mAcm-2, FF=0.66 leading to 10.78% conversion efficiency. This improvement in VOC as a function of ALD thickness reveals that the backflow of electrons from the TiO2 conduction band (CB) to CH3NH3PbI3 and HTM is minimum. From the above observation, it is clear that, the 4 nm TiO2 layer deposited sample shows the highest average PCE of 12.53% and FF of 0.70. It is also observed that the 5 nm sample exhibits higher VOC values compared to other samples. The thicker TiO2 layer facilitates the low recombination rate and passivation of TiO2 NR surface traps leading to increasing in FF and VOC. However, in the present investigation, our 5 nm sample showed decrease in the JSC value that is the main cause for the low PCE. This 5nm thick over layer reduces the interspacing between adjacent nanorods. This reduced interspacing affects the penetration of CH3NH3PbI3/GBL and formation of crystalline CH3NH3PbI3 nanoparticles onto TiO2 NRs. Furthermore, due to thick amorphous nature of ALD TiO2 limits electron transport. Notably, these devices show quite stable efficiency. We have also checked a set of ten devices at each condition and the results are summarized in Figure S4. Moreover, we have also fabricated perovskite device based on mp-TiO2. Figure S5 (‘a’ and ‘b’) shows typical cross section and J-V plots of mp-TiO2 based perovskite solar cell under forward and reverse condition. The thickness of CH3NH3PbI3/mp-TiO2 composite is around ~300nm, ~250nm HTM, and 80nm gold contacts clearly demonstrated from the cross-sectional image. The mp-TiO2 sample shows


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average VOC=0.887V, JSC=19.64mAcm-2, FF=0.60 leading to PCE=9.58%. In order to check further details, we have recorded incident photon-to-electron conversion efficiency (IPCE) spectra of all samples. Figure 4(d) shows the IPCE spectra of Bare and ALD passivated TiO2 NRs devices. The photocurrent generation starts at ~775 nm, in agreement with the band gap of the CH3NH3PbI3 and reached maximum in the visible spectrum. The bare TiO2 NR sample shows 32% IPCE while ALD processed samples show much higher IPCE in the range of 80-92%. The 4nm sample shows highest 92% IPCE leading to 12.45% conversion efficiency. The calculated integrated current under IPCE matches well with the JSC values from J-V plots. It is well known that, the back reaction in DSSCs can be controlled by compact blocking layer. Therefore, the VOC of these devices increases with respect to ALD thickness from 0 nm (bare TiO2 nanorod sample) to 5 nm. On the other hand, increasing the thickness to >4nm, the interspacing may be filled by the overlay. This dramatic decrease in Jsc value is the main cause for the drop in efficiency. Moreover, the ALD overlay limits good penetration of CH3NH3PbI3/GBL solution and lower the formation of perovskite. Here we have used different ALD thickness for passivation of the TiO2 nanorods. Initially we have used 1nm ALD thickness, that means the interspacing of the two adjacent nanorods will be filled by 2nm (the spacing between two nanorods, 1nm coating for each nanorod). Similarly, for 2, 3 and 4 nm samples the inter spacing of the nanorod will be filled by 4, 6 and 8nm respectively. For these samples, they have enough spacing for proper penetration of CH3NH3PbI3/GBL solution and formation of crystalline CH3NH3PbI3 nanoparticles. However, for sample 5nm, the inter-nanorods spacing almost filled by ALD overlay shell, probably 10 to 11nm. This higher deposition reduces the interspacing between two adjacent nanorods will be directly affect ability of the perovskite loading. This data is also consistent with our J-V measurements. Based on the discussion above, we propose



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possible charge transport mechanisms within our perovskite solar cell consisting of 1D TiO2 NRs. The working principle of the perovskite solar cell is as shown in Figure 5. The CH3NH3PbI3/TiO2 onto FTO coated substrate acts as a working electrode. Here, CH3NH3PbI3 nanoparticles act as the absorber layer which sandwiched between an electron transport layer (ETL) i.e. TiO2 and hole transport layer (HTL) i.e. spiro-MeOTAD and gold contact act as a counter electrode.

When this device is illuminated by photons with enough energy, the

following processes take place. Here the light absorbing layer CH3NH3PbI3 absorbs the photon energy in the visible region to create an electron–hole pair. photonenergy CH 3 NH 3 PbI 3   → CH 3 NH 3 PbI 3 (h + + e − )

(1)

Due to band alignment of TiO2 and CH3NH3PbI3, the generated electrons

will be

transferred immediately to the conduction band (CB) of TiO2, while holes will be transferred through spiro-MeOTAD via hopping mechanism. Since, CB of CH3NH3PbI3 is higher than TiO2. tion CH 3 NH 3 PbI 3 (h + + e − ) + TiO2 transporta  → CH 3 NH 3 PbI 3 (h) + TiO2 (e)

(2)

The transferred electrons subsequently flow from FTO to an external circuit to produce electricity. Recently, Grätzel et al. reported bare mp-TiO2 passivated by the ALD technique with various thickness and concluded that 2nm overlayer sample shows excellent PCE 11.5% with FF=0.67 and VOC=0.919V [28]. These results are very interesting and different from our present results, which may be from rutile TiO2 phase and different experimental conditions. Interestingly, the HTM thickness has been drastically decreased. This HTM thickness decrease is a good indication for good pore filling of HTM [12]. The compactness of HTM layer


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facilitates excellent hole-extraction from perovskite active layer and transfer to counter electrode. Such morphology also provides high surface area which is not available in mp-TiO2 samples. As described above, the 1D TiO2 NR configuration promotes the enhancement in PCE. It is also beneficial for the improvement of electron lifetime, resulting in a reduced recombination rate. This architecture facilitates low interface resistance, leading to high current density and FF. From our experimental condition, this architecture enables excellent pore filling property of HTM layer as well as perovskite. It is well known that, the optimization of HTM thickness layer strongly affects FF as well as JSC values of the perovskite devices [29, 30]. Also, we have optimized the HTM film thickness and varied the HTM composition. Interestingly, it is observed that, the over layer thickness of HTM is lower compared to bare samples. Hence, our ALD passivated samples show higher FF leading to higher conversion efficiency compared to respective bare samples. The optimized device shows the highest PCE >13.45% with FF=0.72, which is higher than previous reports based on a similar configuration [11]. The extracted device parameters of the mp-TiO2, bare and TiCl4 treated TiO2 nanorods, and ALD passivated TiO2 nanorods with different thicknesses are shown in Figure 6. We observed that, 4±0.2nm ALD passivated TiO2 NRs samples show higher FF=70±4%, VOC=0.931±0.01V, JSC=19.23±0.53mAcm-2 with PCE 12.53±0.35% compared to the rest of samples. In order to study the interface properties and qualitative analysis of TiO2 NR based devices, we have recorded the solid state impedance spectroscopy (ssIS) in dark at 1V biasing potential [31, 32]. Figure 7 (a) presents the Nyquist plots of typical nanorod based devices with different ALD thickness. Detailed analysis and fitting were done using an equivalent circuit (Inset Figure 7 (b)). The traditional dye sensitized solar cell (DSSC) exhibits two semicircles at



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low and high frequency respectively. However, in the present devices, the high frequency semicircle is absent due to low thickness of photoelectrodes. Generally, >2 to 3µm thicker electrodes are needed to exhibits this feature [33]. The parameter R1 represents the resistance and Q1 the capacitance at the interface between TiO2 and CH3NH3PbI3. This lower frequency arc is attributed to a recombination resistance (R1=Rrec), in parallel with a chemical capacitance (Q1), related to the electron Fermi level in the TiO2 [33]. While R2 (Rrec) and Q2 represents the interface between the composite electrode (TiO2+CH3NH3PbI3) and spiro-MeOTAD. The Rs is the series resistance of the device due to composite electrode and electrode contacts. A series resistance Rs is also attributed to wires and FTO substrate. The main elements of this system that have been analyzed are electron transport resistance, (R1), and electron chemical capacitance (Q1) in TiO2, transport resistance in spiro-MeOTAD (R2), and calculated the recombination resistance,(Rrec), at the interface between the electron and hole transport materials. Controlling the recombination rate is an important factor in all types of solar cells. Recombination processes are present in all photovoltaic devices, and the feature observed in the low-frequency region in the Nyquist plot of perovskite solar cells. In order to verify this, we have calculated the recombination resistance (Rrec) using above equivalent circuit. Usually, the recombination rate is inversely proportional to the recombination resistance (Rrec). In terms of absolute values, it can be seen that; the most significant effect of selective contacts is observed for Rrec (Figure 7 (b)). Such type of behaviour is possible due to the following reason; usually interface recombination is severe than in the bulk region of the perovskite solar cell at low applied potential. In other words, at low applied potential the recombination from TiO2 to spiroMeOTAD is higher than exciton dissociation. It is well known that, the Rrec value strongly influences the VOC of the device; the Rrec is inversely proportional to electron recombination. The


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Rrec values decreases with applied potential because of Fermi level of electrons in TiO2 moves toward its conduction band. In other words, this recombination from conduction band of TiO2 to spiro-MeOTAD, lowers the steady state electron density in the TiO2 film affecting the open circuit potential of the perovskite solar cell. Therefore, for higher Rrec values, the device shows low recombination rate with higher open circuit potential [34]. From the above discussion, it is clear that, the 4nm passivated TiO2 NR sample shows lower recombination with higher VOC. Moreover, the lower values of Rrec at low applied potential reveal lower FF and shunt resistance. In the present case, our bare TiO2 NR based perovskite solar cell shows lower Rrec at low applied potential compared to other devices. Therefore, the bare TiO2 NR device shows lower FF and conversion efficiency compared to the rest of all devices. Based on above results, we have fabricated the controlled ALD thickness nearly 45nm coating to enhance further performance of the TiO2 NR based perovskite solar cells. Figure 8 (a) shows the J-V curve of champion cell recorded for 4.8nm thin ALD TiO2 layer. The highest PCE of 13.45% has been achieved due to high VOC=0.972V and higher current density 19.78mAcm-2 for the 4.8nm thin ALD passivated nanorods. It is also observed that, the FF=72%, which is much higher than the rest of passivated samples. This enhancement is mainly due to the lower back reaction and lower recombination rate. In order to confirm this, we have also recorded IPCE data with integrated JSC values. The calculated integrated Jsc value (19.23mAcm-2) based on IPCE curve is very much consistent with the Jsc values obtained from a JV curves recorded under 100mWcm-2 solar simulator as shown in Figure 8 (b). Please note that the observed and calculated Jsc values show negligible mismatch. In the present case, the improvement in PCE is mainly ascribed to higher VOC and FF parameters. The 4nm and 4.8nm



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passivated samples show negligible improvement in current density (~0.55mAcm-2) however; the FF and VOC have been increased dramatically leading to overall increase in PCE. In order to check the hysteresis behavior of fabricated devices, the J-V measurements were carried out with a 40ms scanning delay. For the reverse (from the VOC to the short-circuit current (ISC)) and forward (that is, from Isc to Voc) modes have been used. Initially, we have checked, mp-TiO2 based perovskite solar cells for comparison. Figure S5 (a) shows typical cross sectional micrograph of mp-TiO2 based perovskite solar cells. The compact layer of HTM has been deposited onto mp-TiO2/CH3NH3PbI3 composite. Figure S5 (b) shows the J-V curves recorded under forward and reverse bias conditions. Table 2 summarizes the extracted Jsc, Voc and FF parameters. The mp-TiO2 based perovskite solar cell exhibits PCE=7.46% and 9.93% for forward and reverse scan respectively. The forward scan of mp-TiO2 based perovskite solar cell exhibits VOC=0.899V, JSC=20.24mAcm-2, FF=41% with η=7.46%. The reverse scan exhibits drastic enhancement up to 9.93% (VOC=0.887V, JSC=19.64, FF=57%). The FF is main cause of increment in PCE of mp-TiO2 based perovskite solar cells. From the hysteresis study, it is clear that, the mp-TiO2 based perovskite solar cells show little hysteresis. Further, we have also studied the effect of forward and reverse scan for 4.8nm passivated TiO2 based perovskite solar cells as shown Figure S6. The Jsc, Voc and FF values obtained from the J-V curve of the reverse scan were 19.78mAcm-2, 0.945V and 0.72, respectively, yielding a PCE of 13.45%. On the other hand, the corresponding values obtained in forward scan were 20.10mAcm-2, 0.952V and 0.42, respectively, showing 8.03% conversion efficiency. The reverse scan showed better FF as compared to the one in forward scan resulting in overall better PCE. Figure S7 shows the hysteresis study of different configuration CH3NH3PbI3 sensitized perovskite solar cells. All devices show hysteresis behavior in their forward and reverse scan. This high hysteresis


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tolerance harms the commercial application [35]. Actually, perovskite solar cell is suffering from hysteresis issue. The reverse sweep always shows the higher power conversion efficiency (PCE). On the other hand, when such type of solar cell will be used in commercial application, then there is possibility in decrement in PCE. Such type of behavior is possibly due to defect-assisted trapping and capacitive behavior of perovskite material. If perovskite solar cell illuminates under steady state, then it should show steady voltage and current density for an extended period of time for practical application. Unfortunately, due to hysteresis effect, the efficiency of perovskite solar cell decreases. This issue can be solved by optimizing the electron transport later (ETL), fast sweep speed during J-V measurements or optimization of mesoporous scaffold thickness [36,37]. These studies are undergoing in different research laboratories. However, this problem is not yet solved completely. This high hysteresis tolerance can harm the commercial application. Therefore, further study is needed in order to avoid the hysteresis effects tolerance. Typically, reliable solar cell J-V measurements should exhibit coincident curves for both the forward and reverse scans. The effect of nanorod length, diameter, different nanostructures and deposition time may be helpful for avoiding this hysteresis issue. This study, based on different biasing, scan sweep speed, controlling effect of HTM and perovskite loading time with concentration is currently underway in our laboratory. 3. Conclusions In summary, we have demonstrated a low-temperature synthesis approach for 1D TiO2 and good ultrathin passivation by hydrothermal and ALD techniques, respectively. The TiO2 ALD thickness shell has been varied from 1nm to 5nm and controlled the back flow reaction in perovskite solar cells. Our results show that, the passivation layer strongly affects the VOC, FF as well as JSC of the solar cells. The increment in ALD thickness enhances the VOC as well as FF.



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However, the JSC values decreases after 5nm thick over layer of ALD TiO2. We have demonstrated fine-tuned 4.8nm deposited ultrathin TiO2 layer can facilitate low recombination rate with high VOC=0.945V leading to good FF=0.72 and 13.45% conversion efficiency under standard conditions (AM 1.5 G radiation, 100mW cm-2). The proposed process is beneficial for the improvement of VOC as well as JSC. These passivated 1D TiO2 NRs are not only applicable to the boosting of light absorption, but also free from high temperature processed TiCl4 treatment. 4. Acknowledgments This work was supported by the Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2009–0094055). 5. Associated Content Supporting Information: Method for synthesis of TiO2 nanorods and its passivation by ALD, Preparation of Methylammonium lead iodide (CH3NH3PbI3), XRD data analysis of CH3NH3PbI3, device fabrication steps with determination of area, hysteresis study and stability of photovoltaic performance. This information is available free of charge via the Internet at http://pubs.acs.org/." 6. References 6. References [1]

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Figure captions Figure 1 Schematic representation of process involved in synthesis of perovskite solar cells: Single-step hydrothermal process followed by atomic layer deposited (ALD) passivation of 1D TiO2 NR. For the CH3NH3PbI3 deposition, CH3NH3I and PbI2 were dissolved in GBL solvent and spin-coated on the ALD TiO2@TiO2 NR followed by heating on a hot plate. Figure 2 Surface and crystallographic study. (a) FESEM micrograph of as posited TiO2 nanorods (b) GIXRD pattern of CH3NH3PbI3 deposited on TiO2 nanorods (c) CH3NH3PbI3 deposited TiO2 nanorods (d) spiro-MeOTAD deposition onto CH3NH3PbI3/TiO2 nanorods. Figure 3 TEM and HRTEM images of TiO2 nanorods coated with CH3NH3PbI3 (a) TEM image of CH3NH3PbI3 covered TiO2 nanorod. (b) HRTEM image of TiO2 nanorods. Inset shows SAED pattern (c) Highly magnified HRTEM image of the CH3NH3PbI3 and TiO2. All other ALD layers and TiO2 nanorods were mentioned by respective colors. (d) FFT live SAED pattern of the selected area of CH3NH3PbI3 material. HRTEM image of CH3NH3PbI3/TiO2 nanorod. Inset shows a live FFT pattern of selected area. Figure 4 Morphological and Photovoltaic properties. (a) Cross-sectional FESEM image of optimized ALD 4.8nm TiO2@TiO2 NR sample. (b) Schematic representation of respective device explaining pore filling mechanism. (c) J-V characteristics of fabricated devices with different ALD TiO2 thicknesses on TiO2 NR measured under 100mWcm-2 illumination. (d) Respective IPCE spectra recorded in the 300-900nm wavelength range. Figure 5 Schematic representation of energy level diagram of 1D TiO2 nanorod based perovskite solar cells. Figure 6 Perovskite solar cell device characteristics comprising mp-TiO2, bare TiO2, TiCl4 treated nanorods; ALD passivated TiO2 nanorods. (a) Variation of VOC and FF (b) variation of current density and efficiency with respect to the different TiO2 photoelectrodes. Figure 7 Impedance Spectroscopy (a) Nyquist plots of the perovskite solar cells based on hydrothermally grown TiO2 NR with respect to ALD coated TiO2 of different thickness. Inset shows magnified Nyquist plots (b) Charge transfer resistance calculated from the Nyquist plot of impedance spectra. Inset shows the equivalent circuit used for analysis. Figure 8 Current density–voltage curve (J-V) and incident photon-to-electron conversion efficiency (IPCE) data for the champion cell. (a) Current density–voltage curve of ALD TiO2@TiO2 nanorods passivated perovskite solar cell. Device configuration: FTO/1D TiO2 NR/ALD TiO2/CH3NH3PbI3/spiro-MeOTAD/Au. The J-V characteristics measured under AM 1.5G condition with the input solar power Pin of 100mWcm−2. (a) Incident photon-to-electron conversion efficiency (IPCE) spectrum (black circles and line) and integrated photocurrent density Jsc (blue dotted line) for the champion 4.8nm ALD passivated TiO2 nanorod based


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CH3NH3PbI3 perovskite solar cell. Integrated Jsc was calculated to be 19.23 mAcm−2. The IPCE data was collected under the constant energy DC mode with delay time 10ms under 50µWcm-2 light intensity. Table 1 Solar cell parameters of the solid-state perovskite solar cells based on various photoelectrodes. All measurements were carried out at room temperature. Table 2 Hysteresis solar cell studies of mp-TiO2 and 4.8 nm ALD @1D TiO2 nanorods based perovskite solar cells.



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Figure 1 Schematic representation of process involved in synthesis of perovskite solar cells: Single-step hydrothermal process followed by atomic layer deposited (ALD) passivation of 1D TiO2 NR. For the CH3NH3PbI3 deposition, CH3NH3I and PbI2 were dissolved in GBL solvent and spin-coated on the ALD TiO2@TiO2 NR followed by heating on a hot plate.


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Figure 2 Surface and crystallographic study. (a) FESEM micrograph of as posited TiO2 nanorods (b) GIXRD pattern of CH3NH3PbI3 deposited on TiO2 nanorods (c) CH3NH3PbI3 deposited on TiO2 nanorods (d) spiro-MeOTAD deposition onto CH3NH3PbI3/TiO2 nanorods.



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Figure 3 TEM and HRTEM images of TiO2 nanorods coated with CH3NH3PbI3 (a) TEM image of CH3NH3PbI3 covered TiO2 nanorod. (b) HRTEM image of TiO2 nanorods. Inset shows SAED pattern (c) Highly magnified HRTEM image of the CH3NH3PbI3 and TiO2. All other ALD layers and TiO2 nanorods were mentioned by respective colours. (d) FFT live SAED pattern of the selected area of CH3NH3PbI3 material. HRTEM image of CH3NH3PbI3 nanoparticles. Inset shows a live FFT pattern of selected area.


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Figure 4 Morphological and Photovoltaic properties. (a) Cross-sectional FESEM image of optimized ALD 4.8nm TiO2@TiO2 NR sample. (b) Schematic representation of respective device explaining pore filling mechanism. (c) J-V characteristics of fabricated devices with different ALD TiO2 thicknesses on TiO2 NR measured under 100mWcm-2 illumination. (d) Respective IPCE spectra recorded in the 300-900nm wavelength range.



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Figure 5 Schematic representation of energy level diagram of 1D TiO2 nanorod based perovskite solar cells.


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Figure 6 Perovskite solar cell device characteristics comprising mp-TiO2, bare TiO2, TiCl4 treated nanorods; ALD passivated TiO2 nanorods. (a) Variation of VOC and FF (b) variation of current density and efficiency with respect to the different TiO2 photoelectrodes.



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Figure 7 Impedance Spectroscopy (a) Nyquist plots of the perovskite solar cells based on hydrothermally grown TiO2 NR with respect to ALD coated TiO2 of different thickness. Figure 7 (a) inset shows magnified Nyquist plots of 2, 3 and 4nm ALD coated TiO2 samples (b) Charge transfer resistance calculated from the Nyquist plot of impedance spectra. Figure 7 (b) inset shows the equivalent circuit used for analysis.


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Figure 8 Current density–voltage curve (J-V) and incident photon-to-electron conversion efficiency (IPCE) data for the champion cell. (a) Current density–voltage curve of ALD TiO2@TiO2 nanorods passivated perovskite solar cell. Device configuration: FTO/1D TiO2 NR/ALD TiO2/CH3NH3PbI3/spiro-MeOTAD/Au. The J-V characteristics measured under AM 1.5G condition with the input solar power Pin of 100mWcm−2. (a) Incident photon-to-electron conversion efficiency (IPCE) spectrum (black circles and line) and integrated photocurrent density Jsc (blue dotted line) for the champion 4.8nm ALD passivated TiO2 nanorod based CH3NH3PbI3 perovskite solar cell. Integrated Jsc was calculated to be 19.23 mAcm−2. The IPCE data was collected under the constant energy DC mode with delay time 10ms under 50µWcm-2 light intensity.



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Table 1 Solar cell parameters of the solid-state perovskite solar cells based on various photoelectrodes. All measurements were carried out at room temperature.

Sample

Bare TiO2 NR TiO2 NR TiCl4 treated 1nm* 2nm* 3nm* 4nm* champion cell 4.8nm 5nm * mp-TiO2 TiCl4 treated

VOC (V)

JSC (mAcm-2)

FF (%)

η (% %)

0.748±0.01 0.852±0.01 0.787±0.01 0.833±0.01 0.865±0.01 0.931±0.01 0.945±0.01 0.972±0.01 0.887±0.01

13.73±0.73 16.16±0.32 12.85±0.56 14.25±0.83 15.44±0.38 19.23±0.53 19.78±0.67 16.81±0.54 19.64±0.63

49±3 58±4 59±3 60±2 66±2 70±4 72±4 66±3 60±3

5.03±0.35 7.98±0.49 5.96±0.42 7.12±0.33 8.82±0.65 12.53±0.35 13.45±0.35 10.78±0.35 9.58±0.43

* The determination of the TiO2 passivation layer thickness by ALD directly on the TiO2 NR surface is difficult; however we have measured thickness of sample on Si wafer which was kept simultaneously with TiO2 nanorod sample in ALD chamber. Therefore, there is possibility of error in thickness measurements because the growth rate on TiO2 NRs and Si wafer is not the same. (Several samples were deposited at each growth conditions and the above mentioned values correspond to the average values. Please consider error ±0.1nm for all ALD thickness samples)

Table 2 Hysteresis solar cell studies of mp-TiO2 and 4.8 nm ALD @1D TiO2 nanorods based perovskite solar cells.

Sample

mp-TiO2

4.8 nm 1D TiO2 nanorods

Scan

VOC

JSC

FF

η

direction

(V)

(mAcm-2)

(%)

(%)

Forward

0.899

20.24

41

7.46

Reverse

0.887

19.64

57

9.93

Forward

0.952

20.10

42

8.03

Reverse

0.945

19.78

72

13.45


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For Table of Contents Only

TOC synopsis: Atomic layer deposited (ALD) ultrathin TiO2 layer for passivation of hydrothermally grown TiO2 nanorods. The champion cell having 4.8nm thin passivation layer exhibits 13.45% power conversion efficiency.


Ultrathin Atomic Layer Deposited TiO2 for Surface ... - ACS Publications

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