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Letter
Tailoring mixed halide, wide gap perovskites via multi-step conversion process Dowon Bae, Axel F. Palmstrom, Katherine Roelofs, Bastian Timo Mei, Ib Chorkendorff, Stacey F. Bent, and Peter C. K. Vesborg ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01246 • Publication Date (Web): 26 May 2016 Downloaded from http://pubs.acs.org on May 29, 2016
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Figure 1. (a) Schematic illustration of the architecture for the MAPbI3−XBrX-based perovskite device fabricated in this work, (b) diagram of the approximate energy levels of the device, and (c) procedure for preparing the perovskite solar cell device via multistep conversion process. Note that the conduction band (CB) and valence band (VB) positions of the layers are values reported in the literature.21,22 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 f1
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washing treatment, resulting in PbI2 with pores.19 Ying et al. have formed the additional nanosheet PbI2 layer by thermal evaporation to promote the perovskite conversion process.20 However, those approaches introduce additional process to the fabrication process. In this work, we demonstrate a facile way to control PbI2 thin-film configuration and morphology to promote MAPbI3−XBrX conversion by means of multistep spin-coating without any additional chemical or thermal post-treatment, which can increase the fabrication complexity. Especially for the tandem device application, this is particularly critical as any fabrication steps need to be compatible with the layers previously deposited in the tandem stack. The PbI2 precursor films are deposited on either planar or meso-porous structures using spin-coating with various spin-rate combinations in order to investigate its effect on perovskite conversion and, consequently, J−V performance. MABr is subsequently spincoated and the samples are annealed into a MAPbI3−XBrX perovskite. For the PbI2 precursor deposition step, we investigate the effects of spin-rate in a two-step deposition of PbI2. It is found that sequential deposition of PbI2 precursor with two different spin rate (2000 rpm followed by 6000 rpm) results in a dense PbI2 underlying layer with porous surface morphology which promotes effective MABr absorption and forms a sufficient thickness for light absorption. These properties enable the fabrication of solar cells with enhanced VOC and short-circuit current (JSC) as compared with perovskite solar cells from single-step deposited PbI2 precursor. The device architecture of the MAPbI3‑XBrX perovskite solar cell (FTO/TiO2/Perovskite/Spiro-MeOTAD:Li/Au) and its fabrication procedure are illustrated in Figure 1a, c, respectively. The perovskite layer and Spiro-MeOTAD:Li (hole transporting
material) were deposited by spin-coating in glovebox to prevent the reversion to PbI2 or forming of a hydrate product (e.g., MA4PbI6·2H2O).23 The spin-coating conditions of the PbI2 precursors were varied as follows in order to identify the effect on perovskite morphology 2000 rpm (15 s); 6000 rpm (30 s); 2000/6000 rpm (15/30 s); 6000/2000 rpm (30/15 s); and 2000/2000 rpm (15/15 s). After spin coating, the PbI2 films were heated at 90 °C (20 min) to drive off residual solvent. MABr (in isopropyl alcohol) was then spin-coated onto the PbI2 films followed by annealing at 90 °C (20 min) in order to form a perovskite layer via diffusion of the MABr into the PbI2 layer. Further details are also provided in the Supporting Information. The electronic band structure is shown in Figure 1b, where the band gap of the MAPbI3−XBrX was determined from a UV−vis measurement (Figure 2a). The excited state of the MAPbI3‑XBrX is higher than the TiO2 conduction band (CB), and the valence band (VB) level of Spiro-MeOTAD is located near the CB of Au; this alignment allows the injection of photogenerated electrons and holes toward the back (FTO) and front (Au) contact materials, respectively. Figure 2a shows the absorbance of MAPbI3−XBrX deposited on the planar FTO substrate as measured in transmission UV− vis spectroscopy. The optical band gap (Eg) of MAPbI3−XBrX was estimated to be ∼1.9 eV, which is slightly larger than that of previously reported value (1.8 eV) for the MAPbI2Br.16 This is likely attributed to the excessive halide exchange during the wetting time of MABr (∼1 min) prior to the spinning step,24 resulting in a higher proportion of Br. It was found that the Eg of the MAPbI3−XBrX layer is reproducible, as confirmed by additional UV−vis measurements (Figure S2). On the basis of correlations between bandgap and I:Br ratio found in the B
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diffraction peaks consistent with TiO2 and FTO were also identified. In addition, as reported in previous work8 the PbI2 peak (2θ ≈ 12.7°) was also identified. This PbI2 peak may indicate incomplete conversion of precursor to the perovskite phase, or it could indicate partial decomposition of the perovskite during the annealing step at 90 °C. Figure 3 illustrates the J−V curves for solar cell devices with perovskite absorber prepared by various deposition conditions of PbI2 followed by the MABr diffusion step. Planar MAPbI3‑XBrX devices obtained from single-step deposited PbI2 precursors showed a VOC of 0.92 V for 2000 rpm deposition and a VOC of 0.94 V for 6000 rpm deposition, respectively. The series resistance (RS; 81.7 Ω cm2, 52.8 Ω cm2/2000, 6000 rpm) and JSC (2.10 mA cm−2, 1.87 mA cm−2/ 2000, 6000 rpm) both decreased with the higher spin rate for the PbI2 precursor deposition. The decrease of JSC of the device with increased spin-rate PbI2 is attributed to the decreased light absorption (see Figure S1), arising from reduced thickness of the deposited PbI2 at the higher spin-rate.26 The origin of RS in PV cells is typically attributed to the contact resistance of the top and/or back contact as well as the movement of current through the window and absorber layer of the solar cell (e.g., bulk resistance). Because process conditions for all other steps than the PbI2 spin-coating were kept the same, we ascribe the increased RS of the low-spin-rate samples to the hindered injection of the generated electrons into the TiO2 layer, which will be discussed later in this paper. Interesting behavior is observed in planar samples using the two-step PbI2 deposition (Figure 3b). A sequential deposition step for the PbI2 layer was conducted to provide sufficient thickness to absorb most of the light for high efficiency. The perovskite sample converted from PbI2 with a 2000/6000 rpm (2000 rpm followed by 6000 rpm step) combination exhibits a higher VOC of 1.0 V, a JSC of 3.02 mA cm−2 and an η of 2.04%, ascribed to the increase in absorber thickness, and fill factor (FF) of 68% with an RS of 41.7 Ω cm2. On the other hand, the cell with the PbI2 precursor deposited via a 6000/2000 rpm sequential process exhibits an η of 1.60% with JSC of 2.83 mA cm−2 and FF of 61.4% without any change in VOC; the VOC has almost the same level (0.94 V) as the perovskite sample with the single step (6000 rpm) PbI2 precursor (Figure 3a). Sequential deposition of low spin-rates (2000/2000 rpm) exhibits very poor device performance with low η (0.16%) and FF (31.7%) due to significantly increased RS (416.7 Ω cm2), even though such samples likely have the thickest absorber
Figure 2. (a) UV−vis absorbance spectrum and images of spin-coated wide-gap MAPbI3−XBrX (inset, left). Tauc plot converted from the absorbance to estimate its band gap also shown as an inset (upper right). (b) XRD patterns for MAPbI3−XBrX film, which was converted from the single step spun PbI2 on to compact TiO2 film followed by MABr diffusion step. Characteristic peaks indicate formation of the perovksite with high crystallinity. 135 136 137 138 139 140
literature,25 our perovskite layer is likely closer to a MAPbI3−XBrX with X ≈ 1.5. Figure 2b shows the X-ray diffraction pattern, which agrees well with previous reports8,16 and provides a confirmation that the sequential deposition process successfully produces crystalline perovskite. Together with the perovskite phases,
Figure 3. J−V curves of devices with the MAPbI3−XBrX converted from the single-step spun PbI2 precursor (a) and two-step spun on planar substrate (b) and two-step spun on mp-TiO2-coated substrate (c). Notation 2000 rpm corresponds to the sample with perovskite converted from the 2000 rpm (15 s) spun PbI2. 2000/6000 rpm, 2000 rpm (15 s) followed by 6000 rpm (30 s) spun PbI2, and vice versa. 2000/2000 rpm (black curve in b), repeated deposition of PbI2 with same condition (2000 rpm for 15 s). J−V performance was measured by reverse scans at a rate of 0.5 V/s under AM 1.5 G illumination. Detail information for the setup and J−V under forward bias for the best cell can be found in the Supporting Information. C
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layer. Comparing the planar device performance with different spin-coating conditions for the PbI2 precursor thus reveals that VOC and RS of the device are significantly influenced by the spin-coating condition of the second PbI2 deposition. The effect of spin-rate combination on the device performance of the samples with mp-TiO2 substrate is even more obvious (Figure 3c). The 2000/6000 rpm PbI2 combination showed a significantly increased JSC of 6.15 mA cm−2 and η of 3.86%. The constant current under ambient conditions for the duration of the experiment using a sample from the same batch confirmed a stabilized efficiency of 3.56% (Figure S3). The photovoltaic parameters for 3−5 similar devices can be found in in Figure S4. This improvement over the planar devices is consistent with previous reports for perovskite solar cells with a mesoporous scaffold layer.15,17 The improvement of the mesoporous device compared to the planar device stems from the enhanced electron transport from the perovskite layer via the porous scaffold structure.1 But the observed VOC (>1.1 V) is not a significant increase considering the band gap of the deposited perovskite layer (∼1.9 eV). It is possible that this is caused by the halide segregation under illumination as suggested by Hoke et al.,25 but further study is beyond the scope of this work. We observed a hysteresis in the J−V curve of the perovskite cell with mp-TiO2 (Figure S5), which predominantly arises from the perovskite absorber.5 While the origins of the hysteresis behavior in perovskite solar cell is debated, one hypothesis for the hysteresis is the presence of mobile ionic species in the perovskite layer that creates unfavorable electronic contact under forward bias.5 Conversely, the mp-TiO2 structure appears detrimental for the cell with 6000/2000 rpm PbI2 precursor comparing to the cell with 2000/6000 rpm PbI2. It showed JSC of barely over 2 mA cm−2 with drastic decrease in FF (28.1%) due to a high RS (185.1 Ω cm2) of the device likely due to hindered electron injection toward FTO substrate from the pervoksite layer with 6000/ 2000 rpm PbI2 deposition. Figure 4 shows a comparison between the surface and crosssectional morphology of the 2000/6000 rpm and 6000/2000 rpm two-step deposited PbI2 layers versus the converted perovskite films created from these PbI2 deposition procedures. The top-view scanning electron microscope (SEM) images shown in Figure 4a highlight the substantial differences between the surface morphologies. The PbI2 layer deposited using the 2000/6000 rpm combination has crystalline facets with voids between them (Figure 4a, right), whereas PbI2 deposited using the 6000/2000 rpm combination (Figure 4e, right) showed relatively dense surface morphology without any obvious voids. Surface morphologies of those samples after the MABr diffusion step are quite similar (Figure 4a, e, left). The morphology of the perovskite layers appears to have formed by volume expansion due to the reaction with MABr. The crosssectional image of the perovskite converted from the 2000/ 6000 rpm PbI2 deposition (Figure 4b) depicts a uniformly converted thick perovskite throughout the device, while the one converted from the PbI2 with 6000/2000 rpm spin rate (Figure 4f) showed a thick, dense film on top of a relatively porous bottom layer. The cross-sectional Auger electron spectroscopy (AES) line-scan profile shows Pb and Br are evenly distributed along the depth of the perovskite converted from the 2000/ 6000 rpm PbI2 (Figure 4c), whereas there is a segregation of Pb and Br in the conversion of the 6000/2000 rpm PbI2 (Figure 4g). These results suggest that a complete conversion of PbI2 was not achieved in the 6000/2000 rpm case. Note that y-axis
Figure 4. Top-view and cross-sectional SEM images of the samples with PbI2 spun with (a, b) 2000/6000 rpm and (e, f) 6000/2000 rpm. Images on the left side correspond to the PbI2 precursors before MABr diffusion process; post-MABr samples are shown on the right side. (c, g) Cross-sectional AES linear-scan profiles for those samples. (d, h) Illustration of the the MABr diffusion process for the two spin speed deposition procedures.
of the AES profiles are relative ratios which correspond to the ratio of the element being investigated to the sum of the scanned elements. This behavior suggests the 2000 and 6000 rpm PbI2 spin-coating processes result in two different film morphologies: At 2000 rpm, the PbI2 layer is dense and impermeable to MABr diffusion (Figure 4h), whereas at 6000 rpm, the PbI2 film is sufficiently porous to be permeated by MABr, with which underlying PbI2 material can be effectively converted into perovskite (Figure 4d). The J−V and AES results above indicate that residual PbI2 in the perovskite film diminishes device performance. Further, the comparison study of the sample with different spin-rate combination indicates that the PbI2 precursor should be formed to have a porous morphology in order to promote the efficient absorption of MABr solution, and to consequently lead to the successful conversion to perovskite. From the UV−Vis results (Figure S1), it is clear that the perovskite layer converted from the two-step deposited PbI2 precursor with a rate of 2000 rpm (i.e., 2000/2000 rpm), which is likely attributed to the residual PbI2 layer, showed reduced light absorption (in turn reducing JSC) in the perovskite film in comparison with that of the sample with two-step 6000 rpm D
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PbI2 deposition (i.e., 6000/6000 rpm). In addition, excessive residual PbI2 may also lead to high charge accumulation inside the absorber layer and consequently decrease VOC.27 This explains why the perovskite PV cells with a low rpm-spun PbI2 top layer resulted in a relatively low level of VOC (Figure 3 and Figure S2). As shown in the band diagram (Figure 5), the CB
simply by the spin-coating process, minimizing the complexity of the perovskite device fabrication.
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ASSOCIATED CONTENT
S Supporting Information *
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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b01246. Experimental method, UV−vis absorption spectra, additional cross-sectional AES spectra, and box plot of J−V parameters for parallel devices (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Present Address
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B.M. is currently at Faculty of Science and Technology, 325 University of Twente, 7500 AE Enschede, The Netherlands 326 Notes
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The authors declare no competing financial interest. Figure 5. Approximate band energy diagram of the device with perovskite which was formed from the 6000/2000 rpm spun PbI2 precursor. Presence of PbI2 in the incompletely converted perovskite would build an energy barrier because of the relatively higher conduction band of the PbI2, which may lead to the unfavorable electron injection from the MAPbI3−XBrX (X ≈ 1.5) perovskite.
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ACKNOWLEDGMENTS The authors acknowledges the support of the Danish Council for Independent Research (DFF-4005-00463), and the support by the SUNCAT research visiting grant (4069-00006A) from the Danish Agency for Science, Technology and Innovation. The authors also would like to thank the U.S. Department of Energy through the Bay Area Photovoltaic Consortium under award No. DE-EE0004946 and the Precourt Institute for Energy. Part of this work, e.g. AES, UV−vis and SEM analysis, was performed at the Stanford Nano Shared Facilities (SNSF).
for the PbI2 interlayer is about 0.2 eV above the CB of the perovskite layer, accounting for the PbI2 band gap and its valence band (VB) (Eg = 2.3 eV; VB = −5.7 eV).28 Because PbI2 tends to be weakly p-type or intrinsic semiconductor29 the PbI2 barrier peak at the CB increases after the Fermi level equilibration with the perovskite, and thus injection of photogenerated electrons will be hindered, as suggested by Y. H. Lee et al.,30 and RS is increased. This explains why the perovskite cell with low-rate spun PbI2 on top showed significantly increased RS and poor FF (Figure 3b, c). However, several recent papers claim a PbI2 excess can be beneficial. Q. Chen et al.31 claim in their recent report that excess PbI2 lead to the electron recombination passivation spots at grain boundaries leading to enhanced device performance. This inconsistency is likely due to the sufficiently thick PbI2 layer case, as shown in Figure 4g, h, having an energy barrier may blocks charged carrier injection from the perovskite into the FTO cathode. In summary, we have developed a sequential deposition procedure with a multistep PbI2 precursor spin-coating process to promote the perovskite conversion process. A two-step deposited PbI2 precursor with a low-rate spun bottom layer followed by high-rate spun top layer shows the following advantages: (i) formation of a relatively thick perovskite layer for improved light absorption; (ii) formation of a porous surface morphology to allow for efficient MABr permeation and full conversion of the PbI2 precursor. Conversely, if the PbI2 top layer is deposited at a low spin-rate the unfavorable band alignment resulting from unconverted PbI2 obstructs the electron injection from the perovskite. The demonstrated device performance obviously lags behind performance of the state-of-the-art device with similar architecture.16,32 However, these results highlight how the morphology and the crosssectional structure of the PbI2 precursor layer can be controlled
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