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Fast Voltage Decay in Perovskite Solar Cells Caused by Depolarization of Perovskite Layer Qiong Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01033 • Publication Date (Web): 13 Feb 2018 Downloaded from http://pubs.acs.org on February 14, 2018

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The Journal of Physical Chemistry

Fast Voltage Decay in Perovskite Solar Cells Caused by Depolarization of Perovskite Layer Qiong Wang* Department of Chemistry, Michigan State University, 578 S Shaw Lane, East Lansing, USA, 48824.

ABSTRACT: The open-circuit voltage decay course in perovskite solar cells is studied in this work under the assistance of three techniques, i.e. electrochemical impedance spectroscopy (EIS), light intensitymodulated photovoltage spectroscopy (IMVS) and open-circuit voltage decay (OCVD). In contrast to what is known in dye-sensitized solar cells, where Voc decay is due to back transfer of electrons from TiO2 conduction band to redox species in the electrolyte, we find that in perovskite solar cells, half of the voltage drop is initially resulted from the fast depolarization of perovskite layer. In other words, we observe that the polarization of perovskite film contributes to about half of Voc in the device. The loss of the other half of Voc is found to be due to interfacial charge recombination.

Introduction Although the power conversion efficiency of the newly emerged lead halide perovskite solar cells has been increased up to over 20% using mesoporous-TiO2 based perovskite solar cells,1-3 the physical processes happened inside the perovskite solar cells are still under debate.4-7 One typical example is the origin of open-circuit voltage (Voc) in perovskite solar cells. It is generally found that photovoltage in a solar 1

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cell is governed by the difference in the Fermi level of electrons in the electron selective contact and the Fermi level of holes in the hole selective contact, or the “Fermi level splitting” induced by illumination.89

However, a Voc lager than the energy difference between the conduction band of electron transport ma-

terial (ETM) and the highest occupied molecular orbital (HOMO) of hole transport material (HTM) has been reported in the regular “n-i-p” structured CH3NH3PbBr3 or MAPbBr3 perovskite solar cells.10-11 Meanwhile, there are some reports found that Voc will be increased by down-shifting the HOMO level of the HTM layer.12 On the other hand, potential drop at the interface of TiO2/perovskite and across the perovskite layer has been observed under the measurement of Kelvin probe force microscopy (KPFM).13 Therefore, the Voc of perovskite solar cells can be influenced by the interfacial contacts but are not fully determined by the energy difference between the two selective contacts.14

To figure out the origin of Voc, we conducted this experiment by investigating the decay course of V oc when we switch on and off the lamp. We recorded the change in Voc as a function of time. During this process, the photovoltage decreases towards the dark equilibrium value of zero volt.15 The voltage decay in dye-sensitized solar cells (DSCs) has been examined by electrochemical impedance spectroscopy (EIS) measurement in comparison with OCVD measurement. For EIS measurement of DSCs, there are normally two semicircles observed in the Nyquist plot. The high-medium frequency semicircle represents the interface between TiO2 nanoparticles and electrolyte, and the low frequency semicircle originates from ion diffusion in the electrolyte.16-17 It has been reported that in DSCs, the electron characteristic time calculated from EIS measurement matches well with the characteristic time calculated from the OCVD measurement.18-19 Therefore, this confirms that in the OCVD measurement, the voltage loss is due to back transfer of electrons from TiO2 conduction band to the redox couple in electrolyte or hole transport layer.15, 18-20

2


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However, this may not be the case for perovskite solar cells. First, the interpretation of impedance spectroscopy response of perovskite solar cells is quite different from DSCs, although normally there are also two semicircles observed in perovskite solar cells. Based on the pioneering work conducted by GarciaBelmonte’s group,21-22 it is known that the first semicircle appeared in high frequency range is originated from the dielectric property of perovskite material and the second semicircle showed up in low frequency range is rooted from charge accumulation induced by illumination at TiO2 and perovskite interface. Secondly, it is found that the electron characteristic time of perovskite solar cells extracted from the second semicircle in EIS measurement is located in the range of 0.1 second (s) to 10 s. However, the “characteristic time” deviated from the OCVD measurement can go down to 100 μs.23 Thirdly, it is known from EIS and other techniques that when perovskite solar cells are at work, a geometric capacitance that is reversed to the film thickness of perovskite layer but not affected by bias, is formed inside the bulk perovskite layer due to the bulk polarization of the perovskite film.24-26 Previously, Luca et al.27 and Baumann et al.23 did some independent works using OCVD to investigate the photovoltage decay process. However, in their work, they were more interested in the slow decay process of Voc, or the “persistent photovoltage”. In addition, before Garcia-Belmonte’s pioneering work,2122

the understanding of two different time domains in perovskite solar cells is still not clear at that time.28-

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Herein, to elucidate a full picture of the voltage decay process, we use three techniques including EIS,

OCVD and intensity-modulated photovoltage spectroscopy (IMVS) and study two types of perovskite solar cells: planar perovskite solar cells and perovskite sensitized TiO2-mesoporous structured solar cells. It is revealed from our work that the fast depolarization of perovskite film contributes to the first half loss in Voc when the light is switched off, and then the other half of the Voc is vanished slowly due to the interfacial charge recombination. Results & Discussion In this work, MAPbI2.85Br0.15 perovskite is chosen as the light absorber in solar cells because it exhibits a much better moisture stability than MAPbI3, and at the same time, it shows similar optical band gap and 3



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crystal structure to MAPbI3.30 Perovskite solar cells are prepared using TiO2 and spiro-OMeTAD as electron and hole selective layer, respectively. MAPbI2.85Br0.15 perovskite solar cells without and with TiO2 mesoporous layer are studied, and are referred as planar perovskite solar cells and mesoporous perovskite solar cells, and noted as “Planar_PV” and “Meso_PV” in the figures and tables, respectively. The photovoltaic performance of perovskite solar cells of these two configurations are given in supporting information. (Figure S1 and Table S1) It should be noted that for the photovoltaic measurement, a 400 nm UV filter was applied to cut off the UV light in the spectrum to avoid the photo-oxidization of perovskite film during the measurement in the ambient air. In addition, a large active area of 0.36 cm2 is used for all cells. A thin perovskite film of around 150 nm was adopted in this study because considering the charge carrier diffusion length of around 100 nm in polycrystalline perovskite film,31-32 we would expect an efficient charge transport through the bulk perovskite layer, which will help to simplify our analysis of the physical process occurred in perovskite solar cells. Histograms of the photovoltaic parameters for perovskite solar cells with a 150 nm perovskite layer and 600 nm perovskite layer with a small active area of 0.09 cm2 can be found in Figures S2-S3. Figure 1a displays the dependence of open-circuit voltage (Voc) on light intensity using 470 nm LED as light source. The light intensity of LED is controlled by the current goes through the LED driver. The diode ideality factor, m can be defined in the following expression:33 𝝏𝑽𝒐𝒄 𝝏𝐥𝐧𝑰𝟎

=

𝒎𝒌𝑩 𝑻 𝒒

(1)

where I0 is the photon flux, kBT is the thermal energy, kB is the Boltzmann constant, T is the temperature, q is the elemental charge. The diode ideality factor, m reflects how the photovoltaics deviate from an ideal solar cell. A big value of m means that more recombination is happened in the system. It can be seen from Figure 1a that mesoporous perovskite solar cells show a smaller diode ideality factor (4.56) than that of planar perovskite solar cells (5.84). This is not surprising because the mesoporous structure provides a more efficient charge separation path than the planar structure.34 The record of Voc decay as a function a

4


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time when the light is switched off is given in Figure S4. The photovoltage decay characteristic time, τOCVD can be deviated from Figure S4 based on the following expression:15 𝝉𝑶𝑪𝑽𝑫 = −

𝟐𝒌𝑩 𝑻 𝒅𝑽𝒐𝒄 −𝟏 ( 𝒅𝒕 ) 𝒒

(2)

The definition of kBT, kB, T and q are the same as in equation (1). The dependence of τOCVD on Voc is given in Figure 1b. It can be seen that mesoporous perovskite solar cells exhibit a longer “electron characteristic time” than planar perovskite solar cells.

Figure 1. a) The dependence of Voc on light intensity measured using 470 nm LED as light source. b) The photovoltage decay “characteristic time” calculated from OCVD measurement given in Figure S4.

5



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Figure 2. a) An illustration of Nyquist plot of planar MAPbI2.85Br0.15 perovskite solar cells measured at open-circuit at light intensity of 0.1 mWcm-2 (the Voc at this light intensity is about 0.1 V); b) the equivalent circuit used for fitting Nyquist plots; c) bulk capacitance, Cbulk, d) transport resistance, R1, e) dielectric relaxation time, τ1, f) chemical capacitance, Cμ, g) recombination resistance, R2, and h) recombination characteristic time of electrons, τ2 of planar perovskite cells as a function of Voc. In panels f) and g), m is fitted to be 5.74 and 5.54, respectively. To understand what’s happened during the Voc decay process, impedance spectroscopy measurement is conducted at open-circuit in a galvanostatic mode. The light intensity is adjusted from 0.01 mWcm-2 to 25 mWcm-2 using the neutral filters. Voc is also recorded at each light intensity. Figure 2a illustrates an example of the Nyquist plot of MAPbI2.85Br0.15 perovskite planar solar cells. It can be seen that two semicircles are presented in the Nyquist plot. According to Garcia-Belmonte’s work,22 an equivalent circuit 6


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given in Figure 2b is used to interpret the impedance response of perovskite solar cells. In Figure 2b, Rs presents the contact resistance; Cbulk refers to the bulk capacitance of perovskite film originated from the dielectric property of the material; R1 coupled with the bulk capacitance is determined by the transport resistance of the bulk perovskite film and is affected by contact resistance.35 τ1 is the dielectric relaxation time, which measures the time from the point the perovskite film gets polarized to the point it gets depolarized. Cμ is the chemical capacitance, reflecting the charge accumulation at the interface of TiO2/perovskite, which is also affected by another interface between spiro-OMeTAD/perovskite. R2 refers to the recombination resistance for back transfer of electrons from the conduction band of TiO2 to the valence band of perovskite. We realize that there are another two possible paths for recombination to occur, i.e. the recombination of electrons in the conduction band of TiO2 with holes in the highest occupied molecular orbital (HOMO) level of spiro-OMeTAD, and the recombination of electrons in the conduction band of perovskite with holes in the HOMO level of spiro-OMeTAD. But based on the time scales reported for the charge recombination dynamics of perovskite solar cells studied by transient absorption (TA) technique,36 where it is found that it takes 60 𝜇𝑠 and 0.37 𝜇𝑠 for electrons at the conduction band of TiO2 and perovskite respectively to recombine with holes in the HOMO level of spiro-OMeTAD, compared to 0.14 𝜇𝑠 for electrons at the conduction band of TiO2 to recombine with holes in the valence band of perovskite film, we think it is reasonable to believe that the dominant recombination process happens at the interface of TiO2 and perovskite layer. τ2 is the average electron characteristic time for all the electrons generated in the system, which measures how long electrons can survive before it gets recombined with holes in the device. The fitted results of the impedance data using Zview software together with the Nyquist plots can be found in the supporting information. (Figure S5-S6, Table S2-S3) Figure 2c-d show the bulk capacitance, Cbulk and transport resistance, R1 of the perovskite layer extracted from the first semicircle. It can be seen that the bulk capacitance is almost the same at different Voc. It further confirms that the bulk capacitance is not attributed by the charge carrier concentrations in the system. The decrease in transport 7



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resistance as the Voc increases indicates the stronger polarization in the bulk perovskite layer as the light intensity goes up, which agree with the observed enhancement in dielectric constant in perovskite film as reported by Juarez-Perez et al.37 The time constant, τ1 can be calculated as R1Cbulk. Alternatively, the time constant can be calculated as the reverse of the angular frequency where the imaginary part of impedance reaches the maximum, i.e. τ1=(2π·fmax)-1. In most cases when the semicircles can be clearly identified, the time constant calculated from these two methods should match with each other. Figure 2e presents the dielectric relaxation time, τ1 as a function of Voc. It can be seen that dielectric relaxation time, τ1 exhibits similar trend as resistance, R1 against Voc because the geometric capacitance of perovskite layer, Cbulk is barely affected by the change in potential. From the second semicircle, chemical capacitance, Cμ and recombination resistance, R2 at TiO2/perovskite interface can be extracted. Figure 2f and 2g display the dependence of chemical capacitance, Cμ and recombination resistance, R2 on Voc, taken from the expressions:22, 33 At open-circuit: 𝝏𝐥𝐧𝑪𝝁 𝝏𝑽𝒐𝒄 𝝏𝐥𝐧𝑹𝟐 𝝏𝑽𝒐𝒄

𝒒

= 𝒎𝒌

(3)

𝑩𝑻

𝒒

= − 𝒎𝒌

𝑩𝑻

(4)

The definition of m, kBT, kB, T and q are the same as in equation (1). It should be noted that m is the same ideality factor as defined in equation (1). As a matter of fact, equations (3) and (4) are derived from equation (1) based on the dependence of charge capacitance or recombination resistance on light intensity. It can be seen that the chemical capacitance, Cμ varies exponentially with Voc with the slope of 1/(mkBT/q), where m is 5.74. Recombination resistance, R2 exhibits the opposite variation with Voc, presenting an inverse slope of -1/(mkBT/q), where m is about 5.54. Although the values of m extracted from equation (1) and equations (3) and (4) should be identically the same, we are happy to see that the values extracted from the dependence of Cμ on Voc and R2 on Voc are very close to the value of m (5.84) obtained from 8


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Figure 1a. Recombination characteristic time, τ2 is calculated from R2Cμ, and is displayed in Figure 2h. It measures the average characteristic time of electrons after photo-generation and before it gets recombined. It should be noted that the interpretation of the impedance data in our work is heavily influenced by Garcia-Belmonte’s work,21-22 but we further discussed the decay process of photovoltage in perovskite solar cells based on the knowledge we learnt from their works. The discussion of the impedance data of mesoprous perovskite solar cells (Figure S7) and the comparison between planar cells and mesoporous cells are given in the supporting information.

Figure 3. a) An illustration of light intensity-modulated photovoltage spectroscopy (IMVS) response of planar MAPbI2.85Br0.15 perovskite solar cells measured using 470 nm LED light source. At the light intensity of 4.84 mWcm-2, the Voc is around 0.86V. b) Two time constants, τ1 and τ2 extracted from EIS and IMVS measurements as a function of Voc. Another technique involved in this work is intensity-modulated photovoltage spectroscopy (IMVS). IMVS is measured at a constant light intensity with a small perturbation. Different from the EIS technique, where a small perturbation of current is applied to the system under galvanostatic mode and leads to a small fluctuation in the split Fermi levels of perovskite solar cells, in the IMVS measurement, the perturbation in the energetic levels in perovskite solar cells is provided by a small sinusoidal modulation of the 9



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illumination intensity. Similarly, the steady state Voc is determined by the light intensity for both EIS and IMVS. Figure 3a shows the complex plane of IMVS response of planar perovskite solar cells. It can be seen that two semicircles are appeared, similar to what is observed in impedance spectroscopy. Two time constants obtained as the reverse of the angular frequency at maximum of imaginary component of IMVS response are extracted from Figure 3a. In Figure 3b, two time constants obtained from IMVS measurement are compared with two time constants extracted from the impedance measurement. It can be seen that the time constants measured via two techniques match well with each other. As a result, it can be assigned that in the IMVS measurement, the first semicircle happened in high frequency represents dielectric property of the bulk perovskite layer, while the second semicircle appeared in small frequency presents the charge accumulation at the interface of TiO2 and perovskite layer.

Figure 4. The characteristic time deviated from OCVD measurement in comparison with time constants calculated from EIS and IMVS measurements for a) planar and b) mesoporous MAPbI2.85Br0.15 perovskite solar cells. Figure 4a puts the characteristic time, τOCVD calculated from OCVD measurement and the time constants obtained from EIS and IMVS measurements in one figure. It can be seen that in the high potential range, the photovoltage decay characteristic time matches well with the time constant, τ2, but in the small potential range, the photovoltage decay characteristic time tend to come up with the time constant, τ1. As 10


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discussed above, τ2 represents the dielectric relaxation time and τ1 presents the electron characteristic time determined by recombination in the system. As a result, it can be interpreted from Figure 4a that during the voltage decay process after the lamp is switched off, initially the voltage loss is caused by the relaxation/depolarization of perovskite layer. In another word, when it is under illumination, the polarized perovskite film experiences some voltage drop. However, the direction of the voltage drop may be reverse to the direction of the photovoltage. Therefore, the decrease in potential in perovskite film somehow contributes to the difference in the Fermi level of electrons in TiO2 and the Fermi level of holes in spiroOMeTAD. Hence, when the light is removed, the part of photovoltage contributed by polarized perovskite film is disappeared in 1-100 μs, consequently, what is observed in the OCVD measurement is the very fast decay in Voc. After the voltage drops to a certain level, the electron recombination at the interface of TiO2 and perovskite layer and hole transport layer starts to contribute to the voltage loss. Similar phenomenon is observed in mesoporous perovskite solar cells as well, as illustrated in Figure 4b. The IMVS data of mesoporous perovskite solar cells can be found in Figure S8. Conclusions In summary, by employing three techniques, i.e. EIS, IMVS and OCVD to investigate the perovskite solar cells, we come to the conclusion that a voltage drop is happened in the polarized perovskite film when the perovskite solar cells are under illumination. The polarized voltage somehow contributes to the overall photovoltage of the device by enlarging the Fermi level splitting in the system. During the OCVD measurement, where the perovskite solar cells are kept at open-circuit condition while switching the light from on to off, the very fast voltage decay at the initial stage is caused by the very fast disappearance of polarized voltage in the perovskite film that is relaxed from polarized to neutral. The persistent photovoltage at low values is caused by the slow back transfer of electrons from TiO2 to perovskite layer or hole transport layer, which leads to further loss in potential to zero volt and reaches the dark equilibrium. ASSOCIATED CONTENT 11



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Supporting Information. Experimental details, photovoltaics parameters of perovskite solar cells, and EIS and IMVS date of mesoporous perovskite solar cells can be found in the supporting information. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author Dr. Wang, Qiong [email protected] Present Addresses †Helmholtz-Zentrum Berlin, Institute of Silicon Photovoltaics, Kekulestreet 5, 12489, Berlin ACKNOWLEDGMENT Q.W. conducted the experiment at Prof. Thomas W. Hamann’s group under the support of Michigan State University Strategic Partnership Grant (SPG). Q.W. thanks to the great assistance and instruction from Prof. Hamann during the postdoctoral research experience. REFERENCES 1. Chen, W., et al., Efficient and Stable Large-Area Perovskite Solar Cells with Inorganic Charge Extraction Layers. Science 2015, 350, 944-948. 2. Li, X.; Bi, D.; Yi, C.; Décoppet, J.-D.; Luo, J.; Zakeeruddin, S. M.; Hagfeldt, A.; Grätzel, M., A Vacuum Flash–Assisted Solution Process for High-Efficiency Large-Area Perovskite Solar Cells. Science 2016, 353, 58-62. 3. Yang, W. S.; Noh, J. H.; Jeon, N. J.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I., High-Performance Photovoltaic Perovskite Layers Fabricated through Intramolecular Exchange. Science 2015, 348, 12341237. 4. Belisle, R. A.; Nguyen, W. H.; Bowring, A. R.; Calado, P.; Li, X.; Irvine, S. J. C.; McGehee, M. D.; Barnes, P. R. F.; O'Regan, B. C., Interpretation of Inverted Photocurrent Transients in Organic Lead Halide Perovskite Solar Cells: Proof of the Field Screening by Mobile Ions and Determination of the Space Charge Layer Widths. Energy & Environmental Science 2017, 10, 192-204. 5. Calado, P.; Telford, A. M.; Bryant, D.; Li, X. E.; Nelson, J.; O'Regan, B. C.; Barnes, P. R. F., Evidence for Ion Migration in Hybrid Perovskite Solar Cells with Minimal Hysteresis. Nat Commun 2016, 7. 6. O’Regan, B. C.; Barnes, P. R. F.; Li, X.; Law, C.; Palomares, E.; Marin-Beloqui, J. M., Optoelectronic Studies of Methylammonium Lead Iodide Perovskite Solar Cells with Mesoporous Tio2: Separation of Electronic and Chemical Charge Storage, Understanding Two Recombination Lifetimes, and the Evolution of Band Offsets During J–V Hysteresis. Journal of the American Chemical Society 2015, 137, 5087-5099. 12


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26. Yang, Y.; Yang, M. J.; Zhu, K.; Johnson, J. C.; Berry, J. J.; van de Lagemaat, J.; Beard, M. C., Large Polarization-Dependent Exciton Optical Stark Effect in Lead Iodide Perovskites. Nat Commun 2016, 7. 27. Bertoluzzi, L.; Sanchez, R. S.; Liu, L. F.; Lee, J. W.; Mas-Marza, E.; Han, H. W.; Park, N. G.; Mora-Sero, I.; Bisquert, J., Cooperative Kinetics of Depolarization in Ch3nh3pbi3 Perovskite Solar Cells. Energy & Environmental Science 2015, 8, 910-915. 28. Dualeh, A.; Moehl, T.; Tetreault, N.; Teuscher, J.; Gao, P.; Nazeeruddin, M. K.; Gratzel, M., Impedance Spectroscopic Analysis of Lead Iodide Perovskite-Sensitized Solid-State Solar Cells. Acs Nano 2014, 8, 362-373. 29. Pascoe, A. R.; Duffy, N. W.; Scully, A. D.; Huang, F.; Cheng, Y.-B., Insights into Planar Ch3nh3pbi3 Perovskite Solar Cells Using Impedance Spectroscopy. The Journal of Physical Chemistry C 2015, 119, 4444-4453. 30. Noh, J. H.; Im, S. H.; Heo, J. H.; Mandal, T. N.; Seok, S. I., Chemical Management for Colorful, Efficient, and Stable Inorganic-Organic Hybrid Nanostructured Solar Cells. Nano Lett 2013, 13, 17641769. 31. Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J., Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber. Science 2013, 342, 341-344. 32. Xing, G.; Mathews, N.; Sun, S.; Lim, S. S.; Lam, Y. M.; Grätzel, M.; Mhaisalkar, S.; Sum, T. C., Long-Range Balanced Electron- and Hole-Transport Lengths in Organic-Inorganic Ch3Nh3Pbi3. Science 2013, 342, 344-347. 33. Pockett, A.; Eperon, G. E.; Peltola, T.; Snaith, H. J.; Walker, A.; Peter, L. M.; Cameron, P. J., Characterization of Planar Lead Halide Perovskite Solar Cells by Impedance Spectroscopy, OpenCircuit Photovoltage Decay, and Intensity-Modulated Photovoltage/Photocurrent Spectroscopy. J Phys Chem C 2015, 119, 3456-3465. 34. Anaya, M.; Zhang, W.; Hames, B. C.; Li, Y.; Fabregat-Santiago, F.; Calvo, M. E.; Snaith, H. J.; Miguez, H.; Mora-Sero, I., Electron Injection and Scaffold Effects in Perovskite Solar Cells. Journal of Materials Chemistry C 2017, 5, 634-644. 35. Guerrero, A.; Garcia-Belmonte, G.; Mora-Sero, I.; Bisquert, J.; Kang, Y. S.; Jacobsson, T. J.; Correa-Baena, J. P.; Hagfeldt, A., Properties of Contact and Bulk Impedances in Hybrid Lead Halide Perovskite Solar Cells Including Inductive Loop Elements. J Phys Chem C 2016, 120, 8023-8032. 36. Shen, Q., et al., Charge Transfer and Recombination at the Metal Oxide/Ch3nh3pbcli2/SpiroOmetad Interfaces: Uncovering the Detailed Mechanism Behind High Efficiency Solar Cells. Phys Chem Chem Phys 2014, 16, 19984-19992. 37. Juarez-Perez, E. J.; Sanchez, R. S.; Badia, L.; Garcia-Belmonte, G.; Kang, Y. S.; Mora-Sero, I.; Bisquert, J., Photoinduced Giant Dielectric Constant in Lead Halide Perovskite Solar Cells. The Journal of Physical Chemistry Letters 2014, 5, 2390-2394.

14


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Table of Contents (TOC): Fast Voltage Decay in Perovskite Solar Cells Caused by Depolarization of Perovskite Layer

15



Page 16 of 29

Supporting information

Fast Voltage Decay in Perovskite Solar Cells Caused by Depolarization of Perovskite Layer Qiong Wang* Department of Chemistry, Michigan State University, 578 S Shaw Lane, East Lansing, USA, 48824. E-mail: [email protected]; [email protected]

Experimental section Materials 2,2’,7,7’-Tetrakis(N,N-di-p-methoxy-phenylamine)-9,9’-spirobifluorene (spiro-OMeTAD) used in this work was purchased from Lumtec company. Methylammonium iodide (MAI) used for the preparation of lead halide perovskite films was purchased from Dyesol company. Titania (TiO2) paste (Ti-Nanoxide T/SP) used from the preparation of TiO2 mesoporous layer was purchased from Solaronix Company. Fluorine-doped tin oxide (FTO) glass was pre-pattered and purchased from Xinyan Technology LTD. All other chemicals were purchased from Sigma-Aldrich. All chemicals were used as received. Device Fabrication Pre-patterned FTO glass was cleaned by ultrasonicating in the order of warm soap water, acetone, and isopropanol respectively for 10 mins before use. TiO2 compact layer (~ 30 nm) was spin-coated from a sol-gel solution prepared by diluting titannium isopropoxide (97%, Sigma Aldrich) in isopropanol, and then calcined at 450°C for 30 min. TiO2 mesoporous layer (~ 200 nm) was spin-coated from diluted commercial TiO2 paste (Ti-Nanoxide HT/SP, Solarnoix), and then annealed at 450°C for 30 min. MAPbI2.85Br0.15 perovskite precursor was prepared by adding 170 mg MAI, 415 mg PbI2, and 36.7 mg PbBr2 in 1 ml anhydrous DMF, and then kept stirring at 100°C for 10 min until a transparent yellowish solution was formed. Before use, the room temperature perovskite solution was filtered (0.45 μm filter, Sigma). The deposition of perovskite film adopted methods given in the reference. More specifically, MAPbI2.85Br0.15 perovskite films (~ 153 nm) was prepared by spin-coating the above solution at 6 krpm S1 Environment ACS Paragon Plus

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for 30 s, while N2 purge was introduced at 5 s, followed by annealed at 100°C for 10 min on a hot plate. It is worth noting that the film thickness of TiO2 compact layer, TiO2 mesoporous layer, and MAPbI2.85Pb0.15 perovskite films are all measured using Atomic Force Microscopy (see more details in the Characterizations section). After the deposition of the perovskite layer in ambient atmosphere, all samples were transferred into a nitrogen-filled glove box, where spiro-OMeTAD layer was prepared. 1 ml spiro-OMeTAD solution was prepared by adding 79.65 mg spiro-MeTAD in anhydrous chlorobenzene (CBN) with 16.5 μl LiTFSI (Bis(trifluoromethane)sulfonimide lithium salt, 99.95%, Sigma) out of 520 mg/ml stock solution in acetonitrile, and 29.3 μl tBP (4-tert-Butylpyridine, 96%, Sigma). spiro-OMeTAD layer was prepared by spin-coating the above solution at 4 krpm for 30s. Then all samples were taken out of the glove box and kept inside a desiccator in the dark. The dark oxygen-soaking time is controlled to be around 20 hours. Finally, Au of around 80 nm was thermally deposited on top as the metal contact. Characterizations The film thickness of TiO2 compact layer, TiO2 mesoporous layer and MAPbI2.85Br0.15 perovskite layer was conducted on an Asylum MFP- 3D-Bio Atomic Force Microscopy (AFM). Surface morphology was characterized on the scanning electron microscopy (Hitachi S-4700 II FESEM). Photovoltaic measurements were performed with a potentiostation (Autolab PGSTAT 128N). Light source is provided by a xenon arc lamp. AM1.5 global filter was used to simulate 1 Sun at 100 mW/cm2. The light intensity was calibrated with a certified reference Si cell system (Oriel Reference Solar Cell & Meter). The active area of perovskite solar cells is 0.36 cm2, and a 400 nm UV filter to cut off < 400 nm wavelength in spectrum was applied during the JV measurement. Light intensity-modulated photovoltage spectroscopy (IMVS) was recorded using a 470 nm LED with an LED driver from Metrohm Autolab. The photon flux was calibrated using the photodiode integrated with the LED setup. The IMVS response of devices were measured by applying 10% of the DC LED driving current as the AC perturbation of the incident light intensity in the frequency ranging from 20kHz to 0.1Hz (10 frequencies per decade). Electrochemical impedance spectroscopy (EIS) measurements were performed at open-circuit at 1 Sun using an FRA2 integrated with the PGSTAT 128N. Each impedance measurement was recorded at frequency ranging from 1 MHz to 0.1 Hz in equally spaced logarithmic steps. Neutral intensity filters were used to adjust the incident light intensity from 100 mW/cm2 to 0.01 mW/cm2.

S2 Environment ACS Paragon Plus


Figure S1. The photovoltaic performance of champion MAPbI2.85Br0.15 perovskite solar cells with and without TiO2 mesoporous layer measured at AM1.5G (100mWcm-2). A 400 nm UV filter was applied to cut off the UV light. The active area of devices studied in this work is 0.36 cm2.

12 Current Density (mAcm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 29

10 8 6 4 2 0 0.0

Planar_PV-Re Planar_PV-Fw Meso_PV_Re Meso_PV_Fw 0.2

0.4

0.6

0.8

1.0

Voltage (V)

Table S1. The average photovoltaic parameters of six MAPbI2.85Br0.15 perovskite solar cells with and without TiO2 mesoporous layer measured at AM1.5G (100mWcm-2). A 400 nm UV filter was applied to cut off the UV light. The active area of devices studied in this work is 0.36 cm2. Samples

Efficiency (%)

Jsc (mAcm-2)

Voc (V)

FF (%)

Planar_PV_Re

5.51 ± 1.29

9.84 ± 1.12

1.00

55.6 ± 6.83

Planar_PV_Fw

3.29 ± 1.09

8.77 ± 1.39

1.00

37.1 ± 6.53

Meso_PV_Re

5.91 ± 0.57

10.03 ± 1.08

1.00 ± 0.002

59.0 ± 0.82

Meso_PV_Fw

4.74 ± 0.98

9.60 ± 1.22

1.00 ± 0.001

49.3 ± 5.28


Page 19 of 29

Figure S2. The histograms of the photovoltaic parameters, i.e. a) PCE, b) Jsc, c) Voc, and d) FF for mesoporous perovskite solar cells out of 10 devices measured under 1 sun (100 mWcm-2) without a 400 nm UV-filter with an active area of 0.09 cm2. Scan rate: 10 mV/s. Perovskite film thickness of 150 nm was adopted in the cells.

a)

b) 4.5 PCE

3.5 3.0

Count

4

Count

Jsc

4.0

5

3

2.5 2.0 1.5

2

1.0 1

0.5 0.0

0

2

3

4

5

6

7

8

9

9

10

10

11

12

13

14

15

Jsc (mA/cm2)

PCE (%)

c)

d) 9 8

5

Voc

7

FF

4

Count

6

Count

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5 4

3 2

3 2

1

1 0 0.5

0.6

0.7

0.8

0.9

1.0

1.1

0 0.4

0.5

0.6

0.7 FF

Voc (V)


0.8

0.9


Figure S3. The histograms of the photovoltaic parameters i.e. a) PCE, b) Jsc, c) Voc, and d) FF for mesoporous perovskite solar cells our of 24 cells with 600 nm perovskite film. Measured under 1 sun (100 mWcm-2) without a 400 nm UV-filter with an active area of 0.09 cm2. Scan rate: 100 mV/s. 10 PCE

14

Jsc

12 10

6

Count

Count

8

4

8 6 4

2

2 0

0 12

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

13

14

15

PCE (%)

16

17

18

19

20

21

Jsc (mAcm-2)

18 16

Voc

10

FF

14 8

12 10

Count

Count

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 29

8 6

6 4

4 2

2 0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 20

30

Voc (V)

40

50 FF (%)


60

70

80

Page 21 of 29

Figure S4. The record of Voc decay as a function of time after light is switched off.

1.0

Planar_PV Meso_PV

0.8 1.0

0.6

Planar_PV Meso_PV

0.9 Voc (V)

Voc (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.4

0.8 0.7 0.6 0.5 0.4 6.3

0.2

6.4

6.5 Time (s)

6.6

6.7

0.0 0

5

10 15 Time (s)

20


25


Figure S5. Nyquist plots of planar MAPbI2.85Br0.15 perovskite solar cells measured at open-circuit condition at light intensities of 0.01 mWcm-2 (dark), 0.1 mWcm-2 (red) and 1 mWcm-2 (blue). Insets are at light intensities of 10 mWcm-2 (pink), and 25 mWcm-2 (purple).

1.0x10

0.01mWcm-2 0.1mWcm-2 1mWcm-2

750

-Z'' (ohm)

1.5x104

-Z'' (ohm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 29

10 mWcm-2 25 mWcm-2

500

250

4 0

0

500

1000

1500

Z' (ohm)

5.0x103

0.0 0

1x104

2x104 Z' (ohm)


3x104

Page 23 of 29

Figure S6. Nyquist plots of mesporous MAPbI2.85Br0.15 perovskite solar cells measured at open-circuit condition at light intensities of 0.01 mWcm-2 (dark), 0.1 mWcm-2 (red) and 1 mWcm-2 (blue). Insets are at light intensities of 10 mWcm-2 (pink).

2.0x104

200

0.01 mWcm-2 0.1 mWcm-2 1 mWcm-2

10 mWcm-2

150

-Z'' (Ohm)

1.5x104

100

50

-Z'' (Ohm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0 100

4

200

300

400

500

Z (Ohm)

1.0x10

5.0x103

0.0 0

1x104

2x104

3x104

Z' (Ohm)


4x104

5x104


Page 24 of 29

Table S2. The fitted results of impedance response of planar perovskite solar cells measured at light intensities of 0.01 mWcm-2, 0.1 mWcm-2, 1 mWcm-2, 10 mWcm-2 and 25 mWcm-2. Light in-

Chi-Sqr

Sum-Sqr

Rs

tensity

Rs

Rs

Cbulk

Cbulk

Cbulk

R1

R1

Rct1

(Error)

(Error%)

(±)

(Error)

(Error%)

(±)

(Error)

(Error%)

(±)

-2

(mWcm ) 0.01

0.0288

2.709

23.88

0.634

2.656

1.04E-07

3.73E-09

3.578

10643

494.8

4.649

0.1

0.0263

2.474

24.07

0.688

2.860

1.08E-07

3.75E-09

3.452

9687

415.7

4.291

1

0.0235

2.208

23.85

0.738

3.093

1.06E-07

3.24E-09

3.048

3656

124.3

3.401

10

0.0291

2.383

23.76

0.689

2.901

9.79E-08

2.88E-09

2.946

429.7

11.06

2.574

25

0.0672

6.320

24.09

0.743

3.085

1.19E-07

4.99E-09

4.187

164.8

3.971

2.410

Continued with Table S2. Light intensity

CPE2-T

CPE2-T

CPE2-T

CPE2-P

CPE2-P

CPE2-P

R2

R2

R2

(mWcm-2)

(±)

(Error)

(Error%)

(±)

(Error)

(Error%)

(±)

(Error)

(Error%)

0.01

3.54E-06

6.72E-07

18.96

0.878

4.00E-02

4.550

12525

736.86

5.883

0.1

4.57E-06

5.89E-07

12.91

0.835

2.94E-02

3.520

20186

962.67

4.769

1

2.53E-05

2.78E-06

10.96

0.717

3.12E-02

4.347

11208

716.25

6.391

10

1.39E-04

3.07E-05

22.06

0.834

6.37E-02

7.549

948.2

126.65

13.357

25

1.32E-03

4.09E-04

31.00

0.896

0.116

12.980

125.7

16.332

12.993

In planar cells, the chemical capacitance at interface between perovskite layer and TiO 2 compact layer is fitted using a Constant Phase Element (CPE) rather than a parallel capacitor (C). This is because there will be large deviation when we use a capacitor to fit this interface. We think the deviation is due to the roughness of the perovskite film, which leads to a non-smooth interface.


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Table S3. The fitted results of impedance response of mesoporous perovskite solar cells measured at light intensities of 0.01 mWcm-2, 0.1 mWcm-2, 1 mWcm-2 and 10 mWcm-2. Light in-

Chi-Sqr

Sum-Sqr

Rs

tensity

Rs

Rs

CPE1-T

CPE1-T

CPE1-T

CPE1-P

CPE1-P

CPE1-P

(Error)

(Error%)

(±)

(Error)

(Error%)

(±)

(Error)

(Error%)

(±)

-2

(mWcm ) 0.01

0.0292

2.456

61.90

2.685

4.338

1.73E-07

1.44E-08

8.339

0.892

8.45E-03

0.948

0.1

0.0320

2.688

58.50

2.922

4.995

1.96E-07

1.86E-08

9.502

0.883

9.61E-03

1.089

1

0.0236

1.980

57.47

3.176

5.526

6.55E-07

6.68E-08

10.195

0.795

1.05E-02

1.318

10

0.0321

2.696

89.68

1.857

2.071

3.03E-07

5.61E-08

18.537

0.945

1.80E-02

1.903

Continued with Table S3. Light inten-

R1

R1

R1

C2

C2

C2

R2

R2

R2

(±)

(Error)

(Error%)

(±)

(Error)

(Error%)

(±)

(Error)

(Error%)

0.01

21401

804.2

3.758

2.55E-06

5.05E-07

19.79

12209

969.13

7.938

0.1

19573

821.4

4.197

2.12E-06

3.19E-07

15.03

17622

1091.7

6.195

1

8404

212.7

2.531

7.19E-05

8.25E-06

11.48

8785

626.6

7.133

10

342.9

5.027

1.466

7.26E-03

2.62E-03

36.04

55.32

9.818

17.75

sity (mWcm-2)

The fitting of the data for mesoporous perovskite solar cells at low light intensities, i.e., 0.01 mWcm-2 and 0.1 mWcm-2 is difficult because the two semicircles are not clearly separated compared to the data collected at light intensities of 1 mWcm-2 and 10 mWcm-2. We managed to fit the data by starting from the data measured at 10 mWcm-2. Then we used the same model to fit the data measured at 1 mWcm-2. Then we moved to data measured at 0.01 mWcm-2 and 0.1 mWcm-2. It should be noted that for the bulk capacitance of the perovskite film, we used a CPE to represent this physical process because in mesoporous perovskite solar cells, TiO2 is found to be sensitized by the perovskite material. As a result, different from a compact perovskite film formed in a planar cell, in a mesoporous cell, perovskite material is penetrated into TiO2 mesoporous layer and formed a sensitized structure with a highly increased interface area. In addition, we found that a capacitor is good enough to represent the chemical capacitance at the TiO2/perovskite layer interface. When we used a CPE to fit the second semi-circle, we found that the CPE-P was above 0.95 and very close to 1. Actually, the capacitance ACS Paragon S10 Plus Environment


can be calculated from a CPE using the expression, 𝐶 = 𝑅

1−𝑛 𝑛

Page 26 of 29

1

∗ 𝑄 𝑛 (where n is CPE-P, Q is CPE-T, R is

the recombination resistance coupled with CPE), so when CPE-P equals to 1, it is actually a capacitor. However, it is hard for us to explain why a capacitor is suitable for a mesoporous interface.

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Figure S7. MAPbI2.85Br0.15 perovskite solar cells with TiO2 mesoporous layer: a) bulk capacitance, Cbulk, b) resistance, R1, c) chemical capacitance, Cμ, d) recombination resistance, R2, e) dielectric relaxation time, τ1 and f) recombination lifetime of electrons, τ2 as a function of Voc. In panels c) and d), the average value of m is calculated to be 4.48 and 6.68 for Cμ and R2, respectively. The standard deviation is shown in the error bar.

Comparisons of the extracted data from impedance measurement for mesoporous perovskite solar cells (Figure S7) and planar perovskite solar cells (Figure 2):



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Firstly, for parameters related to the bulk property of perovskite film, i.e. Cbulk, R1 and 𝜏1, we can see that both planar and mesoporous cells share a similar trend for the dependence on Voc, and they also share a similar value range. The slight difference in Cbulk between planar and mesoporous cells could be caused by the influence of TiO2 mesoporous structure because TiO2 nanoparticles have a different dielectric constant from that of perovskite material. Secondly, the most significant difference is coming from the second semicircle appeared at low frequency range, which corresponds to the interface of TiO2/perovskite. When Voc is around 0.9 V, the chemical capacitance of the mesoporous cells shows nearly five times higher than that of planar cells. However, the recombination resistance for the mesoporous cells is only half of the value for planar cells. As a result, the electron lifetime, 𝜏2 calculated from 𝐶𝜇 𝑅2 locates in the same range for both planar cells (~ 0.16 s) and mesoporous cells (~ 0.4s). At other light intensities, 𝜏2 is also found to be in the same range for these two types of cells, with the mesoporous cells showing a slightly longer electron lifetime than the planar cells at higher light intensity. This explains the relatively better performance that we got with mesoporous cells. The higher chemical capacitance in mesoporous perovskite solar cells could be caused by the enhanced interfacial area due to the presence of TiO2 nanoparticles so that electrons have more chances to be transferred to TiO2 films. However, due to the not superior electron conductivity of TiO2 film, electrons are accumulated at the interface before it can be efficiently collected at the FTO contact. As a result, we observe a decrease in recombination resistance, indicating more recombination is happening at a higher light intensity meanwhile more charge carriers are generated at a higher light intensity.


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Figure S8. a) An illustration of light intensity-modulated photovoltage spectroscopy (IMVS) response of planar MAPbI2.85Br0.15 perovskite solar cells measured using 470 nm LED light source. Note: 50 mA is the current goes through the LED driver. It corresponds to around 4.84 mWcm-2, and the Voc is around 0.86V. b) two time constants, τ1 and τ2 extracted from EIS and IMVS measurements as a function of Voc.

2

100 10-1 10-2

1,2 (S)

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1

10-3 10-4 10-5

Meso-IMVS(1) Meso-EIS(1) Meso-IMVS(2) Meso-EIS(2)

10

-6

0.0

0.2

0.4

0.6

0.8

Voc (V)


1.0

Fast Voltage Decay in Perovskite Solar Cells ... - ACS Publications

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