Microscopic Analysis of Inherent Void Passivation ... - ACS Publications

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Microscopic Analysis of Inherent Void Passivation in Perovskite Solar Cells Gabseok Seo, Dongwook Lee, Sung Heo, Minsu Seol, Yonghui Lee, Ki-Hong Kim, Seong Heon Kim, Jooho Lee, Dongho Lee, Jaehan Lee, Dong Wook Kwak, Dongwha Lee, Hoon Young Cho, Jucheol Park, Tae Kyu Ahn, and Mohammad Khaja Nazeeruddin ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00484 • Publication Date (Web): 29 Jun 2017 Downloaded from http://pubs.acs.org on June 30, 2017

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Microscopic Analysis of Inherent Void Passivation in Perovskite Solar Cells Gabseok Seo,a† Dongwook Lee,b† Sung Heo,b* Minsu Seol,b Yonghui Lee,c Kihong Kim,b Seong Heon Kim,b Jooho lee,b Dongho Lee,b Jaehan Lee,b Dong Wook Kwak,d Dongwha Lee,d Hoon Young Cho,d Jucheol Park,e Tae Kyu Ahn,a* and Mohammad Khaja Nazeeruddinc* a

Department of Energy Science, Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon,

16419, Korea b

Samsung Advanced Institute of Technology, 130, Samsung-ro, Yeongtong-gu, Suwon, 16678,

Korea c

Group for Molecular Engineering of Functional Materials, EPFL Valais Wallis, CH-1951 Sion,

Switzerland d

e

Department of Physics, Dongguk University, Seoul, 04620, Korea

Business Support Department, Gumi Electrons & Information Technology Research Institute,

Gumi, 39171, Korea AUTHOR INFORMATION Corresponding Author

ACS Paragon Plus Environment

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*

E-mail: [email protected]

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ABSTRACT: The presence of voids in perovskite solar cells is influencing the efficiency due to accelerated charge recombination. The induced electric field near voids due to band bending attracts photo-generated electrons and holes toward the voids, leading to carriers’ recombination. However, if the surface of the voids coated by materials with higher band gap than that of a perovskite layer, the strong electric field induced near the voids on the opposite way prevents carriers from recombining. We identified voids in perovskite layer by using electron beam induced current (EBIC) technique and found the influence of field-assisted passivation by organic materials on the efficiency of the solar cell.

TOC

Perovskite solar cells, as an emerging photovoltaic technology, hold the potential to overcome current efficiency and performance limits of conventional Si-based,1,2 copper indium gallium (di)


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selenide (CIGS),3-5 dye-sensitized,6-7 and PbS quantum dot solar cells.8-11 The advantages of perovskite absorber are high absorption coefficient, low non-radiative carrier recombination rate, and high carrier transport length,12-28 which result in boosting their efficiency from 3.8% to 22.1%.20-21 Also, the solution processability endows perovskite solar cells with a simple process and low cost.12-19 However, it is very hard to control crystallinity and morphological properties of perovskite layer due to a significant volume increase by the addition of chlorobenzene, which instantaneously reduces the solubility of the perovskite in the DMSO/DMF solvent. This creates very high levels of supersaturation and nucleation, and the birth of new crystalline particles is favored.22-27 Although the study on solvents and additives has raised the efficiency further,24-29 investigation of improving surface coverage of perovskite layer, controlling crystal morphology, and realizing void-free perovskite layer is paramount for further improving the efficiency. One-pot solution process24 and sequential deposition12 are commonly used for preparing perovskite solar cells. Voids are created in the perovskite layer during the drying process, and their creation causes critical problems30-33 such as a recombination center for carriers. Also, the formation of voids physically weakens the adhesion force between the perovskite layer and FTO contact, leading to layer delamination. Even though many groups have tried to reduce voids in the perovskite layer, void creation is not entirely avoidable during sequential deposition of a perovskite layer, and the void passivation has attracted attention from researchers as an alternative to enhance the efficiency.34-36 The mechanism of curbing the voids in perovskite layers by passivation has not been understood yet, and no straight evidence for the mechanism is available. If a void is formed in a perovskite layer, the band structure of the perovskite layer is changed, i.e. the conduction band becomes concave down, and the valence band is convex up near voids.37 As a result, a strong electric field is generated near the voids (Einduced = -∇Φ), where


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electrons and holes are trapped, leading to carriers’ recombination. However, if the surface of voids passivated by the material with a higher bandgap than that of a perovskite, band bending occurs in the opposite direction. The conduction band becomes convex up, and the valence band is concave down near voids, where the induced high electric field prevents carriers from trapping near the voids and the carriers’ recombination from taking place. This is called “field assisted passivation.” In our previous work,38 we demonstrated that field-assisted passivation on the surface of voids in Cu(InGa)Se2 layer suppresses carriers’ recombination effectively. In this report, we present a direct evidence of field-assisted void passivation in the perovskite solar cells via the electron beam induced current (EBIC). The perovskite solar cell was fabricated by sequential deposition.12 As shown in Figure S1, the device comprising fluorine-doped tin oxide (FTO)/compact TiO2 (c-TiO2)/mesoporous TiO2 (mp-TiO2)/CH3NH3PbI3/Spiro-OMeTAD/Au shows good photovoltaic parameters in shortcircuit current density (JSC), open-circuit voltage (VOC) and fill factor (FF) with a high PCE of 17.65 %. Junction EBIC (JEBIC) measurements were performed to identify the location of the generated current with an FEI Sirion scanning electron microscope (SEM) and Point electronic EBIC system. A voltage of 2 kV and a fixed current of 388 pA were used to excite a constant amount of carriers. In EBIC,39 electron-hole pairs are created when an electron beam impinges upon a sample. The photo-created carriers diffuse into the p-n junction in the solar cell. The carriers are separated and thus collected by the built-in electric field at the p-n junction due to the depletion region as they arrive at the junction. In the depletion region which is also called spacecharge region (SCR), all the carriers are assumed to be collected in a short-circuit condition. On the other hand, electrons and holes generated in the quasi-neutral region (QNR) diffuse into the depletion edge and metal electrode, respectively.


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During the measurement, the interface between the perovskite and mp-TiO2/perovskite layers was depleted due to the high carrier/defect density at room temperature, while most of the perovskite layer was in a quasi-neutral condition. Most of the voids observed by SEM were placed in the QNR and at the interface between the perovskite layer and the metal contact. Since the electron beam in EBIC was very small, it was scanned across the sample. The current was measured and superimposed on a SEM image.


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Figure 1. SEM and EBIC images of the perovskite solar cell. (a) Cross-sectional SEM image of the perovskite solar cell. (b) EBIC image from the perovskite solar cell superimposed on a SEM image. (c) 3-dimensional EBIC image of the perovskite solar cell.

Figure 1(a) shows SEM cross-sectional image of a perovskite layer on a metal electrode. There are many voids in the perovskite layer. The voids are highlighted by a white dotted circle and white box, which are numbered from 1 to 6. The large void numbered 3 is enlarged in the topright corner. In Figure 1(b), EBIC signal is overlaid with SEM image. Since the created carriers located deep into the perovskite surface are easily recombined, the EBIC signal is so weak for the perovskite bulk area that the EBIC signal of the most area below the junction is dark in Figure 1(b). However, the region corresponding to the SCR has a distinctive high intensity, indicating that generated carriers can be separated and transferred to either side of the junction before recombination. The void regions numbered 1 to 5 look very bright, indicating that it has very high collection efficiency. But, the void region numbered 6 is not bright, showing that its collection efficiency is low. Figure 1(c) illustrates the EBIC signal 3-dimensionally. Among six voids in Figure 1(a), the void numbered 3 was investigated in-depth by EBIC. Four different line profiles of the EBIC results are shown in Figure 2(a). The void region numbered 3 for this measurement is enlarged in the upper-right corner of Figure 2(a). In Figure 2(b), there is a very bright area corresponding to the inside and peripheral of the voids. EBIC line profiles are displayed in Figure 2(c). Line 1 is the representative EBIC line profile of a normal perovskite layer without any voids. Lines 2 and 4 represent the charge collections around the void, while Line 3 is the charge collection through the void. The EBIC intensity through the void is very high (L3), and the peripheral of the void area has a higher intensity compared with the other


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QNR (L2, and L4), revealing that the generated carriers in this area are expeditiously separated and moved to the electrode despite that voids formed in the QNR can act as a recombination center with high probability. From the EBIC results, we could deduce that the created charge carriers in the voids migrate to each electrode without recombination due to the strong electric field induced near the voids, which prevents carriers’ recombination and makes electrons nearby the voids detour to the electrode.

Figure 2. Cross-section EBIC line profiles near the void numbered 3 in Figure 1(a). (a) SEM image (b) EBIC image from the perovskite solar cell superimposed on a SEM image. (c) The EBIC line profiles.


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The requisite for field-assisted void passivation in the perovskite solar cell is the change of band structure, which could be realized by passivated surface of the voids with a higher bandgap than that of the perovskite layer. For example, the bandgap of HTM is in the order of ~3.33 eV which is confirmed in the Reflection Electron Energy Loss Spectroscopy (REELS) experiment (Supplementary information). If the surface of voids is coated with HTM, the band structure near voids is changed. To confirm the presence of HTM at the void surface, we employed a Transmission electron microscopy and Energy-dispersive spectrometer (EDS) analysis, of which results are displayed in Figure 3. Figure 3(a) illustrates voids in the perovskite layer. The size of voids is in the order of tens of nanometers. Figure 3(b) displays the EDS image of Figure 3(a). The elemental mapping in Figure 3(b) corroborates that the void encircled with a dashed red line is containing carbon atoms. Figure 3(c) illustrates the EELS spectrum of the void enclosed in Figure. 3(b). The bandgap is 3.5 eV, which is close to the bandgap (~3.33 eV) of HTM. The band structure of the perovskite solar cell was investigated with REELS and X-ray photoemission spectroscopy (XPS) in Figure S2, and the band alignment is illustrated in Figure S3. The effect of fields assisted passivation near voids can be schematically explained in Figure 4. The photo-generated electrons and holes move to the FTO and Au electrode, respectively (Figure 4(a)). However, the recombination path is changed if the unpassivated void is present in the perovskite layer shown in Figure 4(b). Some of the carriers recombine before they travel to each electrode, leading to the decrease of the efficiency. If the voids are passivated by HTM material, the conduction band becomes strongly convex up, and the valence band is also slightly convex up near voids (See the band diagram in Figure S3). As a result, the strongly induced electric field at the conduction band near the voids prevents electrons from trapping in the voids.


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Although the weakly induced electric field at the valence band might slightly attract holes inside the voids, electrons easily pass further to the FTO without recombination with holes due to the strong electric field at the conduction band (Figure 4(c)).

Figure 3. TEM image and EDS result. (a) TEM image (b) Elemental mapping around voids are overlaid with TEM image. (c) TEM-EELS spectrum of the void encircled in Figure 3(b)


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Figure 4. Schematic diagram of the recombination of carriers. (a) Carriers move to the electrodes without recombination (b) carriers will be recombined at the unpassivated void surface (c) carriers will bypass the passivated void.


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In conclusion, we investigated the presence of voids and field-assistant void passivation in the perovskite solar cell fabricated by sequential deposition. Although the voids have been considered of carriers’ recombination center and factor for decreasing the efficiency, they will prevent charge recombination if they are passivated with materials of which bandgap is larger than that of the perovskite layer. HTM materials near voids change the band structure, and as a result, a strong electric field is induced near the voids. Therefore, voids in the perovskite solar cells would improve the performance of the cells, if properly passivated with field assistance.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Details of fabrication, synthesis method, perovskite sample preparation, supporting analysis of EBIC, REELS, XPS spectra of TiO2, perovskite layer, HTM layers data, the schematic band diagram for the perovskite solar cell, and J-V characteristic data are described in Supporting Information. AUTHOR INFORMATION Corresponding Author *


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*E-mail: [email protected] Notes


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The authors declare no competing financial interest. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. †These authors contributed equally.

ACKNOWLEDGMENT Woo Jang Chun Special Project by Rural Development Association, Korea [Grant No. PJ009106022013] and the National Research Council of Science and Technology (NST) through Degree and Research Center (DRC) Program (2015) to G. Seo and T. K. Ahn

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