Letter pubs.acs.org/NanoLett
Controllable Sequential Deposition of Planar CH3NH3PbI3 Perovskite Films via Adjustable Volume Expansion Taiyang Zhang,† Mengjin Yang,‡ Yixin Zhao,*,† and Kai Zhu*,‡ †
School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China Chemistry and Nanoscience Center, National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, United States
‡
S Supporting Information *
ABSTRACT: We demonstrate a facile morphology-controllable sequential deposition of planar CH3NH3PbI3 (MAPbI3) film by using a novel volumeexpansion-adjustable PbI2·xMAI (x: 0.1−0.3) precursor film to replace pure PbI2. The use of additive MAI during the first step of deposition leads to the reduced crystallinity of PbI2 and the pre-expansion of PbI2 into PbI2·xMAI with adjustable morphology, which result in about 10-fold faster formation of planar MAPbI3 film (without PbI2 residue) and thus minimize the negative impact of the solvent isopropanol on perovskites during the MAI intercalation/conversion step. The best efficiency obtained for a planar perovskite solar cell based on PbI2·0.15MAI is 17.22% (under one sun illumination), which is consistent with the stabilized maximum power output at an efficiency of 16.9%. KEYWORDS: Perovskite, solar cells, sequential deposition, volume expansion
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inhibit the complete conversion of PbI2 to MAPbI3 by blocking the MAI diffusion to the deeper layer of PbI2. This issue has been discussed in a recent study and has been associated with the low reproducibility of the performance of planar perovskite solar cells using two-step sequential solution deposition.15 In contrast, it normally is relatively easier to have a complete PbI2to-MAPbI3 conversion using a mesoporous scaffold (e.g., nanocrystalline TiO2 film); this is due to the small PbI2 grain size coupled with the porous structure to provide sufficient pathways for MAI diffusion, facilitating a rapid conversion process.8 To address the PbI2 residue issue for preparing planar perovskite films, a long intercalation/conversion time is required during the MAI-solution-dipping step. However, a long solution-dipping time is expected to cause significant damage to the MAPbI3 layer resulting from the possible back extraction (deintercalation) of MAI from MAPbI3.21 Other than the issue of incomplete conversion, the morphologies of most reported MAPbI3 films fabricated by sequential deposition are relatively rough and difficult to control. A recent study found that carefully adjusting the synthetic conditions to control the perovskite morphology is critical to the performance of perovskite cells.12 Although this study is based on mesoporous cell architecture, the conclusion should also apply to the planar perovskite solar cells. Thus, it is highly desired for future perovskite-based device applications and fundamental investigations to develop synthetic approaches via sequential
ead halide perovskite solar cells have become one of the most promising candidates for low-cost and high-efficiency solar cell technologies as evidenced by their unprecedented cell efficiency progress during the past several years.1−6 Highperformance perovskite solar cells have been demonstrated by multiple groups using different deposition approaches.7−18 Most of the reported high-efficiency perovskite solar cells are based on either a planar or planar/mesoporous hybrid perovskite architecture. One of the key requirements for achieving high-performance perovskite solar cells is to have a uniform, pinhole-free perovskite layer with controlled morphology and composition.19 The pinhole-free compact perovskite film was initially demonstrated by using vapor-phase deposition (coevaporation).9 Recent development in solution chemistry with various engineering and synthetic controls has also led to the fabrication of high-quality perovskite films by different groups.19 Among various solution deposition approaches, twostep sequential solution deposition has shown promise for fabricating high-efficiency CH3NH3PbI3 (or MAPbI3) perovskite solar cells.8 In the standard two-step sequential solution growth, the initially deposited PbI2 film undergoes an intercalation reaction with MAI in the isopropanol (IPA) solution to form MAPbI3. As a result, the density of the material decreases from about 6.16 g/cm3 for PbI2 to 4.29 g/ cm3 for MAPbI3,20 corresponding to about a two-fold volume expansion per formula.21 However, this volume expansion associated with the intercalation processes could present several issues for preparing a compact, planar MAPbI3 film. In preparation of planar perovskite films, space-expansion-induced formation of a tight MAPbI3 surface layer would significantly © XXXX American Chemical Society
Received: March 2, 2015 Revised: April 26, 2015
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DOI: 10.1021/acs.nanolett.5b00843 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters solution deposition to produce a planar MAPbI3 film that has controlled morphology, no PbI2 residue, and minimum damage from deposition solution. Here, we demonstrate a facile morphology-controllable sequential deposition of planar MAPbI3 film by using a novel PbI2·xMAI (x: 0.1−0.3) precursor film. The addition of small amount of MAI to the standard PbI2 precursor during the first step of deposition leads to about 10-fold faster complete MAPbI3 formation without any PbI2 residue during the (second) MAI intercalation step, and much improved device performance with higher reproducibility when 0.1−0.2 MAI is used. The best cell efficiency obtained is 17.22% using the PbI2· 0.15MAI precursor with the maximum stable power output at about 16.9% under one-sun illumination. The morphologies of PbI2·xMAI precursor films and the corresponding MAPbI3 films can be adjusted by tuning the relative amount of MAI used during the first-step deposition. The effects of adding xMAI on the perovskite film morphology, device characteristics, and electrical/optical properties are discussed with respect to the standard two-step sequential solution deposition approach. Figure 1, panel a shows the ultraviolet−visible (UV−vis) absorption spectra evolution of the pure PbI2 film after dipping
sample (Figure 1b) still exhibits the characteristic PbI2 peaks along with the MAPbI3 peaks, which indicates incomplete conversion of PbI2 to MAPbI3. The intensity of the PbI2 diffraction peak is substantially reduced with longer dipping time up to 20 min. The PbI2 peak disappears completely with 30 min dipping. The typical scanning electron microscopy (SEM) images of planar PbI2 and MAPbI3 (20 min dipping/ conversion) films are compared in Figure 1, panels c and d, respectively. The morphology of this 20 min-dipping MAPbI3 film is similar to that of a film with shorter dipping time (6 min; Figure S1, Supporting Information). The planar PbI2 film appears porous with a coarse top surface, which may be caused by the shrinkage of the precursor film during the drying process.11,15,22 The resulting MAPbI3 film is also full of tiny pinholes, and its surface is covered with rough perovskite nanocrystals; this appearance is similar to that of the perovskite capping layer grown on a mesoporous substrate using two-step sequential deposition.8,12,13,23 The increased roughness of the MAPbI3 film compared to the initial PbI2 film is attributable to the space expansion during the MAI intercalation/reaction step. Taken together, the XRD and absorption results (Figure 1a,b) illustrate the difficulty in preparing planar MAPbI3 thin films from PbI2 using the two-step sequential solution deposition approach. This challenge has been discussed in detail elsewhere.24 The effect of varying MAI dipping time on the device characteristics is shown in Figure S2 with the corresponding photovoltaic parameters listed in Table S1. The cell efficiency increases significantly with longer dipping time, from 3.15% at 0.5 min to 6.11% at 2 min to 9.30% at 6 min, which is consistent with the more complete conversion of PbI2 to MAPbI3. However, when the dipping time is further increased to 20 min, the device efficiency drops to only 1.29% even though the MAPbI3 film with 20 min dipping has much reduced PbI2 with stronger light absorption than the MAPbI3 film with 6 min dipping. Thus, it is evident that simply lengthening the reaction time to complete the PbI2-to-MAPbI3 conversion presents a challenge for the typical two-step sequential solution processing. The long dipping/reaction duration could cause damage to the MAPbI3 layer resulting from the possible back extraction of MAI from MAPbI3,21 and thus it deteriorates the device performance. Therefore, it is critical to develop a strategy to speed up the conversion process (or minimize the MAI solution dipping time) for preparing planar MAPbI3 films. To address the challenges associated with the incomplete conversion of PbI2 and rough perovskite surface discussed above, we examined the effect of using a new precursor (mixture of PbI2 and xMAI with x varying from 0.1−0.3) during the first deposition step. The resulting precursor film is denoted as PbI2·xMAI. Even in the presence of MAI, the deposited PbI2·xMAI films still look yellowish, without any indication of the formation of perovskite MAPbI3. The UV−vis absorption spectra (Figure 2a) of the PbI2·xMAI precursor films exhibit an absorption onset near 510 nm, which is characteristic for PbI2. The sharp absorption edge associated with PbI2 becomes less clear with increasing MAI. When x = 0.3, the baseline of the absorption spectrum increases substantially, indicating an enhanced light-scattering effect. Figure 2, panel b compares the XRD patterns of PbI2·xMAI (x: 0−0.3) precursor films. In the absence of MAI, the PbI2 film displays a strong characteristic XRD peak at about 12.5°. The addition of xMAI into PbI2 during the first deposition step does not result in any detectable MAPbI3 diffraction peaks such as
Figure 1. (a) UV−vis absorption spectra and (b) XRD patterns of MAPbI3 films grown via the typical two-step sequential solution deposition using pure PbI2 precursor film with different MAI solutiondipping times. SEM images of the (c) typical spin-coated PbI2 film and (d) converted MAPbI3 film with 20 min dipping (reaction) time in MAI solution.
in a 10 mg/mL MAI IPA solution for various durations, from 0−30 min. The characteristic absorption shoulder near 750 nm for perovskite MAPbI3 increases slowly with the MAI dipping time up to 20 min. The absorption spectrum increases very little after 20 min dipping, which indicates that most PbI2 has been converted to MAPbI3. The baseline of the absorption spectra also increases simultaneously with longer dipping time, which reflects the stronger light-scattering effect associated with a rougher perovskite morphology. A recent study shows that the grain size of perovskites near the top surface of the perovskite layer increases significantly with increasing MAI soaking/dipping time.12 Although the MAPbI3 film grown with 6 min dipping has already exhibited strong MAPbI3 absorbance spectrum, the X-ray diffraction (XRD) pattern of the same B
DOI: 10.1021/acs.nanolett.5b00843 Nano Lett. XXXX, XXX, XXX−XXX
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In contrast to the slow (difficult) conversion of planar PbI2 film into MAPbI3 via the typical two-step sequential deposition, using the new PbI2·xMAI precursor films is found to convert to red−brown planar MAPbI3 film much more quickly when using the same sequential deposition processes. It takes about 1−3 min for all planar PbI2·xMAI (x: 0.1−0.3) films to convert completely into MAPbI3 without any PbI2 residue (Figure S3). This contrasts significantly to the 20−30 min required for the pure PbI2 film to have full conversion to MAPbI3 (Figure 1b). It is worth noting that the conversion time for making planar MAPbI3 film reported in literature varies over a wide range; for example, it changes from 10 min15 (complete conversion) to 45 min8 (incomplete conversion) depending on the precursor composition and processing conditions. Figure 2, panel c shows the UV−vis absorption spectra of the planar MAPbI3 films using various PbI2·xMAI (x: 0.1−0.3) precursor films. All of these films exhibit the same absorption shoulder near 750 nm, which is typical for MAPbI3. A noticeable increase of the baseline is observed for the PbI2·0.3MAI sample, which corresponds to a larger light-scattering effect as discussed in connection with Figure 2, panel a. By using the PbI2·0.15MAI precursor film as an example, Figure 2, panel d demonstrates the effect of MAI on the evolution of the absorption spectra of the precursor films. The rapid increase of absorbance near 750 nm indicates a quick conversion process from the PbI2· 0.15MAI precursor film to the final perovskite MAPbI3 film. Furthermore, there is no obvious increase of the baseline of the absorption spectra during the entire conversion process, which suggests that the MAPbI3 film sequentially deposited from PbI2·0.15MAI may have a similar surface roughness as the initial PbI2·0.15MAI film. Figure 3 panels a−d show the typical SEM images of top views of the PbI2·xMAI (x = 0.1, 0.15, 0.2, and 0.3) precursor films. These films have very different morphologies than the pure PbI2 film, and their morphologies vary with the amount of MAI used in the precursor. When x ≤ 0.2, introducing MAI into PbI2 leads to the formation of a smoother PbI2·xMAI film with fewer pinholes than the pure PbI2 film. However, the morphology of the PbI2·0.3MAI film becomes even coarser than that of pure PbI2 film, which is consistent with the higher absorption baseline of the PbI2·0.3MAI film (Figure 2a). Figure
Figure 2. (a) UV−vis absorption spectra and (b) XRD patterns of PbI2·xMAI precursor films before the second conversion step (inset shows the magnified view of XRD patterns from 5−10°); (c) UV−vis absorption spectra of MAPbI3 films prepared from PbI2·xMAI films after the second conversion step; (d) evolution of UV−vis absorption spectra using the PbI2·0.15MAI precursor film with different dipping times in the MAI solution.
the main (110) peak near 14°. This is consistent with the appearance of the yellow color and the corresponding absorption spectra. The intensity of the main PbI2 diffraction peak (∼12.5°) decreases substantially with increasing amount of MAI in the precursor. This observation suggests that a partial incorporation of MAI in the PbI2 film decreases the PbI2 crystallinity in the PbI2·xMAI films (compared to the pure PbI2 film) without forming the perovskite MAPbI3 phase. It is noteworthy that there are some unknown peaks formed below 10° when MAI is used in the precursor film. By comparing the XRD patterns of the PbI2·xMAI precursor films to those of the MAPbI3 and MAI (Figure S8), we can attribute these peaks below 10° to the formation of certain complexes, which become stronger for PbI2·0.3MAI. Similar observation of complex formation was discussed previously.11
Figure 3. Typical SEM images of (a−d) PbI2·xMAI (x = 0.1, 0.15, 0.2, and 0.3) precursor films and (e−h) MAPbI3 films prepared from their respective precursor films as indicated. C
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in the standard conversion process with pure PbI2, the expansion ratio of the final MAPbI3 film thickness (df) to the thickness of the initial PbI2·xMAI film (di) can be calculated with the expression df/di = m/[1 + (m − 1)x], where m is the expansion ratio. Best fits of the data to this express yield m = 2.02 ± 0.07; the best-fitted line is shown in Figure 4, panel b. The derivation of the expression for the expansion ratio is given in the Supporting Information (eqs S1−S3). It is interesting that the expansion is also about two, which suggests that the pre-expansion to form PbI2·xMAI and the standard expansion to form MAPbI3 have essentially the same volume expansion ratio. It is also interesting to note that the XRD intensity of the PbI2 peak is much reduced even when the PbI2 phase still constitutes the majority of the PbI2·xMAI precursor film (e.g., x = 0.15; Figure 2b). Thus, the addition of a small amount of MAI results in several features that favor the rapid conversion reaction during the second step. The features of using PbI2· xMAI matrix include the reduced PbI2 crystallinity, the preexpansion of PbI2 into PbI2·xMAI, and the controllable morphology by varying the amount of MAI in PbI2·xMAI. Moreover, using different amounts of MAI allows the control of the pre-expansion of the PbI2·xMAI precursor film, which gives rise to a distinctive morphology, as shown in Figure 3, and also affects the final photovoltaic performance of perovskite solar cells based on the planar MAPbI3 films, as discussed next. Figure 5 shows the typical photocurrent density−voltage (J− V) curves of planar MAPbI3 solar cells prepared using different
3, panels e−h show the typical SEM images of the MAPbI3 films prepared from PbI2·xMAI film after the second dipping/ conversion step. It is worth noting that the MAPbI3 films fabricated from PbI2·xMAI (x = 0.1−0.2) are nearly pinholefree and show a relatively uniform, smooth surface, especially for the 0.15MAI sample, which is significantly different from the relatively rough MAPbI3 films prepared from the pure PbI2 film. The grain sizes of the MAPbI3 films also show a dependence on the amount of MAI used in the precursor. When x = 0.1, the MAPbI3 film mainly consists of relatively small MAPbI3 nanocrystals with sizes less than 200 nm. When 0.15−0.2 MAI is used, the MAPbI3 films are composed of many relatively large (∼500 nm) crystals filled with ∼200 nm small crystals; such film morphology is comparable to that of the vapor-phase-grown perovskite films. This result is also consistent with the absence of an absorption baseline increase during the sequential deposition, as shown in Figure 2, panel d. When started with the relatively coarse PbI2·0.3MAI film, the MAPbI3 film looks similar to that prepared from the pure PbI2 film. These results clearly demonstrate that our new PbI2· xMAI-based two-step sequential deposition allows (to a certain degree) the control of the morphology of the MAPbI3 film. In the typical two-step sequential solution deposition of MAPbI3, the layer-structured PbI2 crystal is intercalated by MAI to form the MAPbI3, as shown in Figure 4, panel a. As
Figure 4. (a) Schematic illustration of the transformation from PbI2 to MAPbI3 in the typical two-step sequential deposition process. (b) Plot of the film thickness (or volume) expansion ratios of converting PbI2· xMAI to MAPbI3 films as a function of the relative amount xMAI used. The solid line is the best fit as discussed in the text.
Figure 5. Typical J−V curves of planar MAPbI3 solar cells prepared from PbI2·xMAI precursor films.
discussed above, the significant phase transformation normally leads to about a factor of two volume expansion per formula during the second conversion step.12,21 To help understand the effect of adding MAI (during the first step of deposition) on the MAPbI3 film formation, the thickness (or volume) expansion ratios of converting PbI2·xMAI to MAPbI3 films were examined by using a surface profiler. Figure 4, panel b shows the result of the expansion ratios for the films prepared with different amounts (x) of MAI from 0−0.3. The ratio continuously decreases with increasing amount of MAI, which could be accounted for by the following hypothesis. For the PbI2·xMAI films, the added MAI is likely incorporated into the PbI2 matrix and has partially pre-expanded the volume of the PbI2 matrix. The degree of this pre-expansion depends on the amount of MAI used during the first step of deposition. This preexpansion decreases the final film expansion ratio with increasing MAI amount. Assuming the pre-expansion ratio per formula of reactant MAI is the same as the expansion ratio
PbI2·xMAI (x: 0−0.3) under simulated one-sun illumination. The details of photovoltaic parameters of all these devices including the statistical analysis (mean values and standard deviations) for each type of device is given in Table S2 in the Support Information. These PbI2·xMAI films were dipped in the MAI solution for 2 min to form the MAPbI3 layers, which were adjusted to about 250 nm thick. In the absence of MAI (i.e., using the standard PbI2 precursor), the device shows a short-circuit photocurrent density (Jsc) of 8.85 mA/cm2, opencircuit voltage (Voc) of 0.969 V, fill factor (FF) of 0.712, and overall conversion efficiency (η) of 6.11%. The cell efficiency increases significantly when 0.1−0.2 MAI along with PbI2 is used during the first-step deposition, especially when 0.15 MAI is used. A typical efficiency for the 0.15 MAI-based device is increased to 15.62% with a Jsc of 19.89 mA/cm2, Voc of 1.065, and FF of 0.738. The performance improvement is largely determined by the increased Jsc value associated with the enhanced PbI2 conversion process and less perovskite damage D
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support by the U.S. Department of Energy/National Renewable Energy Laboratory’s Laboratory Directed Research and Development (LDRD) program under Contract No. DEAC36-08GO28308.
resulting from the minimized interaction with the IPA solvent. The Voc and FF values for the devices based on 0.1−0.2 MAI are also significantly larger than those of the cells based on pure PbI2 (Table S2). However, when 0.3 MAI is used, all device parameters are reduced, with a typical Jsc of 13.87 mA/cm2, Voc of 0.967 V, and FF of 0.668, yielding an efficiency of 8.98%. This is presumably due to the coarser morphology of MAPbI3 when 0.3 MAI is used because there is no PbI2 residue found in the perovskite film. The significant drop in cell performance of the 0.3 MAI-based device is consistent with its much reduced recombination resistance (or faster recombination) compared to the 0.1−0.2 MAI-based cells (Figure S4). Using 0.15 MAI, the best cell efficiency obtained is 17.22% (Figure S5a) with its Jsc value consistent with the external quantum efficiency (EQE) spectrum (inset of Figure S5a). The maximum power output of this device stabilizes at an efficiency of about 16.9% (Figure S5b), which is in good agreement with the value obtained from the J−V measurement. In summary, we report the use of a new composition precursor PbI2·xMAI consisting of mixed PbI2 and partial MAI (molar ratio 1:x, where x varies from 0.1−0.3) to replace the pure PbI2 used in the two-step sequential solution deposition of MAPbI3. In comparison to the standard two-step approach using pure PbI2, the use of additive MAI during the first step of deposition leads to about 10-fold faster MAPbI3 formation without any PbI2 residue during the (second) MAI intercalation step and much improved device performance when 0.1−0.2 MAI is used. The morphology of the MAPbI3 film depends on the relative amount of MAI used in the PbI2·xMAI precursor films (during the first-step deposition), and it is generally smoother when 0.1−0.2 MAI is used. The addition of a small amount of MAI is found to lead to the reduced crystallinity of PbI2 and the pre-expansion of PbI2 into PbI2·xMAI with adjustable morphology, which favor the complete conversion reaction within a short period of time and thus minimize the negative impact of the IPA solvent on the integrity of the MAPbI3 film during the (second) dipping/conversion step. Our results suggest that this novel first-step precursor (PbI2·xMAI) is promising for facile preparation of high-quality planar MAPbI3 film with controllable morphologies via two-step sequential solution deposition to fabricate high-performance perovskite solar cells.
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ASSOCIATED CONTENT
S Supporting Information *
Experimental method, SEM, XRD, solar cell parameters, impedance measurement, and best device characteristics. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b00843.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Author Contributions
T.Z. and M.Y. contributed equally. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Y.Z. and T.Z. are thankful for the support of the NSFC (Grants 51372151 and 21303103). K.Z. and M.Y. acknowledge the E
DOI: 10.1021/acs.nanolett.5b00843 Nano Lett. XXXX, XXX, XXX−XXX
