Direct Observation of the Tunneling Phenomenon in Organometal

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Direct Observation of Tunneling Phenomenon in Organometal Halide Perovskite Solar Cells and its Influence on Hysteresis Tae Woong Kim, Myoung Kim, Ludmila Cojocaru, Satoshi Uchida, and Hiroshi Segawa ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b01701 • Publication Date (Web): 16 Oct 2018 Downloaded from http://pubs.acs.org on October 17, 2018

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ACS Energy Letters

Direct Observation of Tunneling Phenomenon in Organometal Halide Perovskite Solar Cells and its Influence on Hysteresis Tae Woong Kim*,†, Myoung Kim†, Ludmila Cojocaru‡, Satoshi Uchida*,§, and Hiroshi Segawa*,†, § †Department of General Systems Studies, Graduate School of Arts and Sciences, The University of Tokyo, Komaba 3-8-1, Meguro-ku, Tokyo 153-8902, Japan ‡Freiburg Center for Interactive Materials and Bioinspired Technologies, Laboratory for Photovoltaic Energy Conversion, Department of Sustainable Systems Engineering, University of Freiburg, Georges-Köhler-Allee 105, Freiburg 79110, Germany §Research Center for Advanced Science and Technology, The University of Tokyo, Komaba 4-6-1, Meguro-ku, Tokyo 153-8904, Japan AUTHOR INFORMATION Corresponding Author

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*E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected] Abstract Recently, organometal halide perovskites have shown the unprecedented success in solar cell application and attracted widespread attention as the next-generation photovoltaic materials. Regardless of the remarkable achievements, the hysteretic behavior of the organometal halide perovskite solar cells is not fully understood yet and its origins suggested are still in dispute, even though the hysteresis deteriorates stability of the perovskite solar cells and, thus, prevents their commercialization. Here we report the direct observation of the tunneling phenomenon by using nm-scale in situ current density-voltage characterization and suggest that the tunneling phenomenon can induce the hysteresis in the perovskite solar cells. We identified that the tunneling phenomenon was originated from the local-heavy doping caused by the electrostatic dipole at rough interface and that the hysteresis of the perovskite solar cells is possibly originated by piling the tunneling currents. This report shows a new possible origin of the hysteresis and the importance of the interface condition in the perovskite solar cells.

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Organometal halide perovskite solar cells (PSCs) have attracted great attention as the most promising materials for the energy-harvesting because of their unprecedented achievements in power conversion efficiency (PCE),1–7. Since the first trial of the PSCs using methylammonium lead iodide (MAPbI3, MA = CH3NH3),8 the certified PCE has reached to 23.3%.9 The outstanding results are contributed to the specific properties of the organometal halide perovskite, for instance, long diffusion length of charge carrier,10,11 large light absorption coefficient12,13 and low recombination rate,14 and the investigation of their fundamental properties such as self-organized superlattice15 is still ongoing. Following the initial stage of the PSCs replacing dye molecules with organometal halide perovskite,2,12 various fabrication methods for the PSCs have been developed and also accelerated the dramatic achievements.16-20 Despite of the intensive researches of the PSCs, characteristics of the PSCs are not fully disclosed. Among the unrevealed characteristics of the PSCs, a typical example is hysteretic behavior of the PSCs commonly showing higher PCE at reverse scan than forward scan during current density-voltage (J-V) characterization; reverse (forward) scan is conducted by decreasing (increasing) the scanning voltage from near open circuit voltage, Voc, (0 V) to near 0 V (Voc) under forward bias during J-V characterization. To understand the hysteresis of the PSCs, various researches have been conducted and several possible origins such as capacitive characteristic of the organometal halide perovskite,21,22 trapping and detrapping of charges at interfaces,23,24 ferroelectric polarization25,26 and band bending caused by ion migrations27-29 have been proposed. Recently, it is reported that the hysteresis of the PSCs hinders the stability of the PSCs and, thus, their commercialization30,31. In this respect, the clear understanding of the


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hysteresis in the PSCs is considered as a critical issue, however, the suggested origins of the hysteresis are still in dispute. In typical diodes showing rectifying behavior, current flow is an important component for device performance and affected by various factors. Interface condition is a factor influencing the current flow and, thus, roughness of interface is strongly concerned with the performance of the diodes because the current flow is changed by interface roughness and alters internal electric field.32-36 Although the roughness of interface is important for the current flow and the device performance, researches relevant to the interface condition inside the PSCs have not been actively considered. In this research, influence of the interface condition of the PSCs was investigated. Surprisingly, we directly observed tunneling phenomenon at rough hetero-interface including nanopyramids and identified that the hysteretic behavior of PSCs can be obtained by assembling additional currents induced by the tunneling phenomenon. For the investigation of the influence of the interface condition and the tunneling phenomenon, nm-scale in situ J-V characterization and electron beam-induced current (EBIC) measurement were conducted and two kinds of planar type conventional MAPbI3 PSCs containing rough and flat hetero-interfaces formed between MAPbI3 and TiO2 layers were fabricated (Figure S1). Nanoscale device characterization analysis system (2 point nano-probe, Hitachi NP6800) and Hitachi SU8020 were used for the nm-scale in situ J-V characterization and the EBIC measurement, respectively. Details of the fabrication of the MAPbI3 PSCs and the experiments are described in the Supporting Information. Firstly, we investigated influence of the rough hetero-interface on performance of the PSCs using the 2 point nano-probe. Figure 1a and b are a scanning electron microscope (SEM) image


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of the planar type conventional MAPbI3 PSC containing the rough MAPbI3/TiO2 interface and a result of the nm-scale in situ J-V characterization. In Figure 1a, we can observe spiro-OMeTAD, MAPbI3, TiO2 (red solid line), Fluorine doped tin oxide (FTO) and glass layers, and identify that

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Figure 1. (a) A SEM image showing a planar type MAPbI3 PSC including rough MAPbI3/TiO2 hetero-interface and needles for the nm-scale in situ J-V characterization. (b) Results of the nm-scale in situ J-V characterization conducted via the rough MAPbI3/TiO2 hetero-interface. A sharp current peak is observed only at forward scan (blue solid line). the rough hetero-interface (MAPbI3/TiO2) is composed of many nanopyramids. The two needles


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applying scanning voltage (forward bias) during the nm-scale in situ J-V characterization are also well observed. To introduce forward bias (scanning voltage) through the rough heterointerface, the upper and lower needles were contacted to MAPbI3 and FTO layers, respectively and then current for scans was applied from the upper needle (+) to the lower needle (-) (Figure 1a). Electron beam of SEM takes the role of photon irradiation during the nm-scale in situ J-V experiment, although energy supply by the electron beam is much lower than by solar source. Throughout this report, the applied scanning voltage and the detected current density of the nmscale in situ J-V characterization are non-absolute and normalized values, respectively, because of different current flow environments at each measurement depending on contact condition between the needles and layers of the PSC. The contact condition issue was confirmed by Hitachi High-Technologies Corporation. Figure 1b is the nm-scale in situ J-V characterization obtained at the rough hetero-interface. In Figure 1b, results of the forward (blue solid line) and reverse (red solid line) scans conducted across the rough MAPbI3/TiO2 interface are identified and their results are totally different. As is seen at Figure 1b, the forward scan shows a sharp current peak at near 4 V (non-absolute value), whereas the reverse scan demonstrates typical rectifying characteristics of a diode from 3 V (non-absolute value). After the sudden emergence of the sharp current peak, the current density of the forward scan returns to the typical current density of the reverse scan. Interestingly, the behavior of the forward scan showing the sudden sharp current peak and then returning to typical J-V characteristics of a diode is distinct feature of tunneling diodes. From the above results, it is identified that the rough hetero-interface in the PSCs induces tunneling phenomenon during forward scan and the unexpected tunneling may influence current flow of the inside PSCs. Figure S2 is results of


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additional tunneling phenomena measured at other area including the rough hetero-interface and Figure S3 is the graph showing the absolute values of the current density of Figure 1. Generally, current flow is altered by internal electric field of devices and the altered current flow influences on performance of the devices. The internal electric field is affected by distribution of charges (carriers) in devices and, therefore, accumulated charges change the internal electric field of the devices and energy band as well. Thus, the distribution of charges is an important factor for performance of devices. The carrier barrier in energy band of lightemitting diode (LED) caused by accumulation of charges at its hetero-interface forming electrostatic dipole and, thus, inducing band bending is a good example of deteriorated device performance by the accumulated charges and the changed internal electric field.37 In the case of a flat hetero-junction in a diode, when electrons are swept to direction of higher conduction band by scanning voltage, the electrons are blocked at and accumulated on the flat hetero-interface uniformly because of the difference in height of conduction bands. Contrast to the flat heterointerface, charges of a rough hetero-interface distribute non-uniformly on its rough interface and, therefore, change the internal electric field and energy band from the case of the flat heterointerface. The PSC incorporating a rough MAPbI3/TiO2 interface discussed at Figure 1 is the case of the diode including the rough hetero-interface. Figure 2 is schematics showing charge distribution (a, b and c) and energy band (d, e, and f) at the apex of a nanopyramid in the rough hetero-interface of a PSC. Figure 2a and d show the starting stage of forward scan in the PSC including a nanopyramid. Different from the case of the flat hetero-interface, the swept electrons (red solid ball) of Figure 2a and d do not uniformly accumulate at the hetero-interface but further move to the closest position to the + electrode because of geometric influence caused by existence of the nanopyramid (red dot arrows in Figure 2a). Consequently, the swept electrons


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arrive and accumulate in front of built-in potential barrier (gray parallelogram in Figure 2d) at the apex of the nanopyramid [a group of solid balls at the apex (A in Figure 2a) in Figure 2a] by sidestepping the valley (V in Figure 2a) and the sloped planes of the nanopyramid, as shown at the red dot arrows in Figure 2a. Simultaneously with the electron accumulation, holes (blue solid

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Figure 2. Schematics showing charge distribution at stage of (a) beginning, (b) formation of the electrostatic dipole and (c) tunneling, and band diagram (d), (e) and (f) of the charge distribution (a), (b) and (c), respectively. ball) generated at the inside of the MAPbI3 layer by illumination (or electron beam irradiation) are attracted to the apex of the nanopyramid because of the electrostatic imbalance caused by the accumulated electrons at the apex of the nanopyramid (Figure 2a and d). As shown at Figure 2b and e, the accumulated electrons and attracted holes form electrostatic dipoles (a pair of blue and


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red squares) at the apex of the nanopyramid similar with the carrier barrier of LED and, thus, band bending occurs at the apex (slopes of conduction and valence bands in front of the gray parallelogram in Figure 2e). Interestingly, the electrostatic dipole induces additional internal electric field (yellow solid arrows) and the direction of the additional internal electric field is corresponding to the direction of the electric field of the forward scan (blue solid arrows). Therefore, the local electric field at the apex becomes stronger than before and other area by the additional internal electric field. As the result, the locally strengthened electric field at the apex attracts more electrons and, thus, more holes are captured near the apex. Consequently, the electrostatic dipole becomes stronger than before and the strengthened electrostatic dipole will reinforce the electric field at the apex again, i.e. chain reaction. Particularly, the aforementioned special properties of the organometal halide perovskites such as long diffusion length of carriers and large light absorption coefficient provide numerous carriers for the electrostatic dipoles and, therefore, enable the chain reaction. As the result, the chain reaction can circulate continuously and leads extreme accumulation of charges at the apex by forming strong electrostatic dipole (Figure 2c) and huge band bending storing many movable charges denoted as opaque red (electrons) and blue (holes) triangles at Figure 2f. The huge band bending at interface is able to match electron and hole band states between conduction band of the TiO2 and valence band of the MAPbI3 layers (red arrow in Figure 2f) and, interestingly, the matching of the band states is corresponding to the case of heavily doped junction (interface) showing tunneling phenomenon (Figure S4). Consequently, the movable charges can pass through the interface by tunneling the built-in potential barrier along the red arrow (matched band states), as shown at Figure 2f. According to literature,37 tunneling by the accumulated electrons is introduced as the way to overcome (pass through) the carrier barrier of LED formed by the electrostatic dipole. On the


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basis of the above discussion, it is confirmed that the nanopyramids composing the rough heterointerface are able to bring geometric heavy doping at their apex and that the tunneling phenomenon observed at Figure 1b is possibly acquired by the geometric heavy doping, if the

Figure 3. A result of EBIC measurement showing electron density map at apexes (black A), valleys (white V) and flat interface (blue F) on a cross-sectional SEM image of a PSC. electrostatic dipoles inducing the chain reaction of electron accumulation are successfully formed. As is discussed above, the electrostatic dipole triggers the chain reaction of charge accumulation and, thus, creates the heavy doping at the apex of the nanopyramids. Therefore, the tunneling phenomenon can be proved by verifying the formation of the electrostatic dipole at the apex of the nanopyramids and the formation of the dipole is verified by confirming existence and absence of electrons at apexes and valleys of the nanopyramids, respectively. In this respect, we


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mapped the distribution of electrons using EBIC system showing distribution of the electron density stimulated by electron beam on a cross-sectional SEM image of layers in a PSC. Figure 3 is the result of the EBIC measurement, and Au, spiro-OMeTAD, MAPbI3, TiO2 and FTO layers are well observed. The red signals of Figure 3 demonstrate the electron density map constructed by the EBIC measurement. The apex, valley of the rough MAPbI3/TiO2 interface and the flat MAPbI3/TiO2 interface are denoted as black A, white V and blue F, respectively. As is seen at Figure 3, it is clearly identified that the apexes (black A) are covered by strong red signals whereas the red signals are weak at the valleys (white V). Because charge accumulation at the apex in Figure 3 is just maintained by electrostatic dipole (without scanning voltage, forward bias), the force of binding carriers in Figure 3 is much weaker than in Figure 2 (with scanning voltage, forward bias). Therefore, the red signals are detected not only on apex but also near apex in Figure 3 because of the weak binding force. By checking the much stronger intensity of the red signals at near the apexes than the signal intensity of the MAPbI3 layer good supplier of carriers, the observed strong electron density at the apex is considered enough to prove the existence of the electrostatic dipoles. In general, the EBIC signal is relatively stronger at depletion region because photovoltaic effect actively occurs on the depletion region. However, the strong red signals observed at the apex are not originated by the depletion region issue, because intensity of the red signals at the flat interface (blue F) also including the depletion region is too weak to elucidate the strong signal intensity at the apex, as shown at Figure 3. On the basis of the above results, we confirmed that the electrostatic dipoles inducing the heavy doping exist at the apex of nanopyramids and, therefore, the tunneling phenomenon at the rough MAPbI3/TiO2 interface is possible. Figure S5 is an EBIC data reconfirming the above result.


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To finish the proof of tunneling phenomenon at the rough hetero-interface, it is essential to confirm that a flat hetero-interface not including the nanopyramids does not induce the tunneling phenomenon during the J-V characterization. To investigate whether the tunneling phenomenon

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Figure 4. SEM images of the nm-scale in situ J-V characterizations of a PSC incorporating flat (a) MAPbI3/TiO2 and (b) spiro-OMeTAD/MAPbI3 hetero-interfaces, and results of the nm-scale in situ J-V characterizations conducted via the flat (c) MAPbI3/TiO2 and (d) spiroOMeTAD/MAPbI3 hetero-interfaces. occurs at the flat hetero-interface, we conducted the nm-scale in situ J-V characterizations at flat hetero-interfaces composed of MAPbI3/TiO2 and spiro-OMeTAD/MAPbI3 layers. Figure 4a and b are SEM images showing cross-sectional PSCs including the flat hetero-interfaces. In Figure 4a and b, spiro-OMeTAD, MAPbI3, TiO2, FTO and glass layers are identified, and the flat hetero-interfaces between MAPbI3 and TiO2 layers (red arrow in Figure 4a), and between spiro-


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OMeTAD and MAPbI3 layers (blue arrow in Figure 4b) are also well observed. Figure 4c and d are the result of the nm-scale in situ J-V characterizations obtained via the flat hetero-interfaces of the MAPbI3/TiO2 and spiro-OMeTAD/MAPbI3 layers, respectively. As is seen at Figure 4c and d, the results show no tunneling current at the both forward (blue solid line) and reverse (red solid line) scans, and typical rectifying behavior of a diode. Based on the typical rectifying characteristics at the flat hetero-interface during the nm-scale in situ J-V characterization, it is verified that the tunneling phenomenon observed at Figure 1b is originated from the rough hetero-interface including the nanopyramids. As is seen at Figure 1b, the tunneling phenomenon is only observed at the forward scan. The unique situation is reasonable when the tunneling phenomenon is considered to be originated from the interface condition. In the view of the geometric tunneling, reverse scan is hard to form the strong electrostatic dipole inducing the huge band bending for the tunneling phenomenon because of its inactive chain reaction of charge accumulation. In the case of the forward scan, the chain reaction begins from the start of scan and is accelerated with increasing scanning voltage (forward bias). As a result, the accelerated chain reaction reinforces the electrostatic dipole and the strengthened electrostatic dipole leads the chain reaction to the second round of its circulation. Therefore, the scanning voltage and the electrostatic dipole of the forward scan reinforce each other, and the chain reaction continues until the tunneling occurs. Whereas, in the case of the reverse scan, no electrostatic dipole is generated at the beginning moment, because no built-in barrier exists at the beginning of the reverse scan. Although the electrostatic dipole of the reverse scan appears at the voltage less than Voc (the moment of changing direction of the current density), the chain reaction and the electrostatic dipoles of the reverse scan are insignificant to generate the huge band bending inducing the tunneling


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phenomenon because of the decreasing scanning voltage (forward bias) which decelerates the chain reaction and the belatedly constructed electrostatic dipoles. Therefore, the scanning voltage of the reverse scan is hard to reinforce the electrostatic dipole, and vice versa. For this reason, the

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Figure 5. (a) A result of nm-scale in situ J-V characterization showing typical hysteresis. The hysteresis is possibly formed by piling tunneling currents on the conventional current (colored triangles) caused by several nanopyramids of rough hetero-interface. (b) J-V characterizations comparing degree of hysteresis between rough (black lines) and flat (red lines) MAPbI3/TiO2 hetero-interfaces. tunneling phenomenon is not easily observed at the reverse scan during the J-V characterization.


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Note that Figure 1b shows the higher current density at the forward scan because of the tunneling phenomenon than the case of the reverse scan and the tendency is corresponding to the hysteresis of the PSCs. From the correspondence, it is inferred that the higher current density of the forward scan than the reverse scan observed at the hysteresis of PSCs is possibly realized by piling the tunneling currents on the conventional current because the tunneling voltage and intensity of the tunneling phenomenon depends on the geometric factors of each nanopyramids such as their shape and sharpness. As shown at the SEM images (Figure 1a, 3 and S5), since each nanopyramids have different geometric characteristics actually, they are expected to induce tunneling currents having various tunneling voltages and intensities during J-V characterization. Figure 5a and S6 are nm-scale in situ J-V characterizations conducted via rough MAPbI3/TiO2 hetero-interface at different areas and show the typical hysteretic behaviors of the PSCs. Because the 2 point nano-probe used for the nm-scale in situ J-V characterization was executed at hundreds nm-scale area, the number of nanopyramids participated in the tunneling phenomena was ranged from one to several depending on situation. In this respect, the hysteretic behavior of Figure 5a and S6 showing not a sharp current peak but a broad current hill at forward scan is considered to be generated from not single nanopyramid but several nanopyramids. Colored triangles of Figure 5a show a possible combination of tunneling currents which form the increased current density at the forward scan. From the plural results of the nm-scale in situ J-V characterizations, we can identify that the tunneling currents caused by a number of nanopyramids can form the increase of current density at the forward scan, specific feature of the hysteresis in the PSCs, by piling themselves on the conventional current. So far, many reports have elucidated alleviation of hysteresis in PSCs by investigating the relation between mesoporous or organic layers and electron transporting layer (ETL) without consideration of the


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interface condition issue.21,38,39 However, their results used for the evidences of the alleviation of hysteresis were mostly obtained at the condition of flattened hetero-interface achieved by filling grooves of the rough hetero-interface with thick mesoporous layer or organic materials. Interestingly, when the rough hetero-interfaces are not fully filled which means still rough, the hysteretic behavior still appears despite of usage of the mesoporous or organic layers.21,38 Figure 5b is a comparison of hysteresis between rough (black lines) and flat (red lines) hetero-interfaces formed by MAPbI3/TiO2 layers. As expected, the flat hetero-interface shows lower hysteresis than the rough hetero-interface and, therefore, better hysteresis factor [(PCEReverse − PCEForward)/PCEReverse].40 The results are corresponding to previous report.41 More cases of the comparison between rough and flat hetero-interfaces are introduced at table S1 and 2 of supporting information. In addition, different experiment was conducted for reconfirmation of the above results by controlling roughness of hetero-interface with TiO2 mesoporous layer and the experiment also showed same hysteresis tendency (supporting information, Figure S7 and Table S3). All the J-V characterizations were acquired within 5 h after cell fabrication because cell performance including hysteresis of the PSCs fabricated on flat hetero-interface was abruptly decreased after 24 h. The origin of the sudden degradation in cell performance of the flat heterointerface PSCs seems drastic delamination of MAPbI3 from TiO2 ETL because of large difference in thermal expansion.41 In conclusion, we observed the tunneling phenomena at the rough hetero-interface composed of nanopyramids in PSCs during the nm-scale in situ J-V characterization. According to the results, it was identified that the tunneling phenomenon was caused by heavy doping at the apex of the nanopyramids and that the heavy doping is obtained through circulation of chain reaction which is begun and reinforced by the electrostatic dipole. Surprisingly, we confirmed


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that the tunneling phenomena can induce the hysteretic behavior of PSCs by piling their currents on the conventional current and, thus, suggest the tunneling phenomenon as a possible origin of the hysteresis in the PSCs. We believe that this report will show a new milestone for hysteresis research of the PSCs and serve as a momentum to shed light on the importance of the interface condition of the PSCs.


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Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXX Description of the solar cell fabrication, characterization methods, additional Experiments (J-V characterizations, EBIC and EDX results on a SEM images), schematics and reconfirmation experiment.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected] ACKNOWLEDGMENT This work is funded by the New Energy and Industrial Technology Development Organization (NEDO, Japan). We thank Hitachi HighTechnologies Corporation, for valuable supporting in the nanoscale device characterization analysis and EBIC experiments.


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Direct Observation of the Tunneling Phenomenon in Organometal

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