Mixed Halide Perovskite Solar Cells. Consequence of Iodide

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Letter

Mixed Halide Perovskite Solar Cells. Consequence of Iodide Treatment on Phase Segregation Recovery R. Geetha Balakrishna, Steven M. Kobosko, and Prashant V. Kamat ACS Energy Lett., Just Accepted Manuscript • Publication Date (Web): 28 Aug 2018 Downloaded from http://pubs.acs.org on August 28, 2018

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ACS Energy Letters

Mixed Halide Perovskite Solar Cells. Consequence of Iodide Treatment on Phase Segregation Recovery

R. Geetha Balakrishna#, Steven M. Kobosko, and Prashant V. Kamat* Radiation Laboratory and Department of Chemistry and Biochemistry, Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556, United States

*Address correspondence to this author: [email protected] #

Permanent Address: Centre for Nano and Material Sciences, Jain University, Kanakapura,

Ramanagaram, Bangalore – 562112, India. E-mail: [email protected]

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2 Abstract Photoinduced halide ion segregation in mixed halide perovskites can introduce a detrimental effect on the photovoltaic performance of perovskite solar cells over extended light exposure. The photoconversion efficiency decreases from 5.5 to 2.3% when methyl ammonium lead mixed halide perovskite (MAPbBr1.5I1.5) solar cells are exposed to 25 minutes of continuous illumination with visible light. The formation of iodide rich and bromide rich regions create traps, which serve as bottlenecks to the flow of charge carriers. Iodide treatment overcomes some of these effects and stabilizes the solar cell performance. It also facilitates quicker recovery in the dark. The decrease in open-circuit voltage following the exposure of visible light was minimal in iodide treated solar cells. Transient absorption measurements provide insight into the influence of light exposure on charge carrier recombination processes in mixed halide perovskite films. The iodide treatment strategy presented here can aid in designing stable mixed halide perovskite solar cells and the development of multijunction solar cells.

TOC Graphic


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ACS Energy Letters

3 Mixed halide perovskites with ABX3 structure have been advocated as a new class of semiconductors with the ability to tune the bandgap by varying halide ratio.1-4 By changing the halide ion composition (Cl:Br and Br:I) it is possible to selectively harvest photons in the entire visible region. The ease of halide ion exchangeability offers the convenience to prepare gradient structures within the thin films and in nanocrystals.5-6 The tunable optical properties of these mixed halides make them attractive candidates to develop single junction7-9 and multijunction tandem 10-12 solar cells. Despite the attractive optical and electronic properties, mixed halide perovskites suffer from phase segregation under light irradiation.13-15 For example, upon exposure of MAPbBr1.5I1.5 films to visible light one observes formation of iodide and bromide rich regions. Previous studies have focused on probing the origin of such segregation effects and the dark recovery.14-16 Different models have been proposed to explain the mobility of halide ions resulting in phase segregation in mixed halide films.14, 17-19 In our previous study we have shown that halide ion defects play a major role in enriching Br and I species selectively within the area under light exposure.15, 17, 20-21 The effect of halide ion segregation and the mobility of halide ions under light irradiation or applied bias is still being actively investigated to understand its role in dictating the performance of the mixed halide perovskite solar cells.4, 9, 19, 22-26 A preliminary investigation conducted recently in our laboratory showed decrease in photovoltaic performance of MAPbBrxI3-x mixed halide perovskite solar cell upon extended exposure to visible light.22 The traps arising from the halide ion segregation were considered to be a factor in lowering the efficiency of mixed solar cells. Interestingly, when stored in the dark for several hours the photovoltaic performance recovered. It was also noted that the residual trap sites formed during light induced segregation seem to persist for several hours. One possibility to overcome the adverse effects of photoinduced segregation is to subject the mixed halide perovskite film to iodide treatment and remediate the halide ion defects that typically drive the photoinduced segregation. Spectroscopic studies have shown that mixed halide films treated with methyl ammonium iodide (MAI) accelerate the dark recovery and minimize light induced segregation effects.20 In addition, the iodide management has been shown to improve the performance of methyl ammonium lead iodide (MAPbI3) perovskite solar cells.27 It was suggested that the iodide treatment remediates the surface defects of MAPbI3 films. We have now undertaken a study to investigate the impact of iodide species addition on the halide ion segregation in mixed halide (MAPbBr1.5I1.5) film and its photovoltaic performance under extended light exposure.


ACS Energy Letters

4 Mixed Halide Perovskite Films Under Light Exposure. Methyl ammonium mixed halide films (MAPbBr1.5I1.5) with Br:I ratio of 1:1 are prepared by mixing the lead halide and methyl ammonium halide in DMF (dimethyl formamide). The precursor solution was spin coated on glass slides or TiO2 coated FTO

Absorbance

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

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1.0

electrodes and annealed at 65 °C (1 min) and

0.8

then 100 °C (2 min). Triiodide-treated films (referred to in this work as iodide treated)

0.6

were prepared using a modified procedure

0.4 0.2 0.0 400

reported earlier by Seok and coworkers.27

b

500

b'

Crystals of I2 were dissolved in propanol to

a a'

produce I3- species. This solution was

600

700

800

Wavelength (nm) Figure 1. The absorption spectra of MAPbBr1.5I1.5 films (a,a’) without and (b,b’) with iodide treatment. The spectra were recorded (a,b) before and (a’b’) after exposure of visible light for 30 minutes (simulated 1Sun condition).

introduced into the precursor solution of PbI2 and MAI before casting the films. This one step procedure to incorporate I3- in the precursor solution was satisfactory to obtain stable films and achieve reproducible performance of MAPBI3 cells (Figure S2). The details of the experimental procedures are given in the supporting information.

The absorption spectra of the MAPbBr1.5I1.5 films with and without iodide treatment are presented in Figure 1. The mixed halide films exhibit absorption below 700 nm with exciton peak appearing at 630 nm. The I3− treated films exhibited slightly higher absorbance than the untreated ones. It is interesting to note that the iodide treatment did not introduce any noticeable spectral shifts or changes in the overall spectral profile as both treated and untreated films exhibited excitonic peak around 630 nm. This ruled out any secondary exchange of halide ions in the film following the iodide treatment. Upon exposure to visible light (100 mW/cm2) we observed a decrease in the 630 nm band in both these films indicating the changes in the mixed halide composition. We also observed a small increase in the absorbance concurrently at longer wavelengths, which is an indication of the formation of I-rich regions. As discussed in our previous studies as well as that reported by others, these changes in the absorption arise from phase segregation (reaction 1).13-16


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ACS Energy Letters

5 nMAPbBrxI3-x+ hν → (n – 2m)MAPbBrxI3-x + mMAPbBrx-yI3-x+y+ mMAPbBrx+yI3-x-y

(1)

The kinetics of photoinduced segregation leading to the formation of iodide rich and bromide rich regions have been tracked by absorption and emission changes.15-16 The halide ion defects are considered to be the possible origin for such a selective movement of halide ions.17, 20 It is interesting to note that the halide ion segregation is reversible and original spectral features are retained after storing in the dark. The time of recovery in the dark depends on the exposure light intensity and the duration of exposure employed to induce halide ion segregation. In the following discussion we will focus on the impact of halide ion segregation on the photovoltaic performance of the solar cells employing mixed halide films. Effect of Extended Light Exposure on the Perovskite Solar Cells with Mixed Halide Composition. Perovskite solar cells were prepared by depositing MAPbBr1.5I1.5 films on FTO electrode coated with mesoscopic TiO2 film. Spiro-OMeTAD was deposited as a hole conducting layer. This step was followed by the deposition of Au layer. The schematic diagram of the perovskite solar cell (PSC) is shown in Figure 2 (left panel) and details of solar cell fabrication are included in the supporting information.

Figure 2: Left: The schematic illustration of cell design employed for evaluating photovoltaic performance. Center and Right: J-V characteristics of perovskite solar cell (PSC) with MAPbBr1.5I1.5 films under 1-Sun continuous illumination for 25 minutes. Cells were prepared with perovskite film (A) without and (B) with iodide treatment. Two sets of PSC prepared with mixed halide methyl ammonium lead perovskite (with and without iodide treatment) were subjected to 1-Sun light exposure under open circuit conditions and J-V characteristics were recorded periodically. Representative J-V curves recorded at different time exposures of PSC with and without iodide treatment are shown in Figure 2. The iodide treated PSC performs slightly better (PCE =6.8%) than the untreated (PCE=5.5%) PSC when recorded before the extended light exposure. The



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6 improvement achieved through iodide treatment is in agreement with the earlier reports on iodide management in perovskite solar cells.28 The remediation of the surface defects by the iodide treatment were showed in this study to improve the photovoltaic performance. It is evident in Figure 2 that both open-circuit voltage and short circuit current deteriorate with increased time of exposure. The effect is more obvious in untreated PSC. The power conversion efficiency of the untreated PSC drops from 5.5% to 0.26% within 25 minutes of light exposure. Both photovoltage and photocurrent decrease with increased time of irradiation. The shape of J-V curves change dramatically as we expose the PSC to continuous illumination. The change in the shape of J-V curve and corresponding decrease in fill factor, is likely to arise from the creation of deep traps as they impede the flow of charge carriers. On the other hand, the photovoltaic performance of iodide treated PSC appears to be less susceptible to such a decrease. The photovoltage of iodide treated PSC decreased from 0.88 V to 0.8 V following 25 minutes of light exposure. We see a drop in the power conversion efficiency from 6.77% to 2.33% during similar light exposure. In particular, the photovoltage remained steady in iodide treated PSC during illumination. Although not completely suppressed, the iodide treatment minimizes the impact of performance decrease. As shown earlier, halide ion defects play an important role in inducing phase segregation and remediation of such defects with iodide treatment helps to speed up the recovery. 20

Figure 3. External Quantum Efficiency (EQE) determined at different excitation wavelengths of mixed halide (MAPbBr1.5I1.5) perovskite solar cells (A) without and (B) with iodide treatment of perovskite film. The cells were exposed to 1-Sun light irradiation under open circuit conditions for 60 minutes and EQE were recorded periodically.


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ACS Energy Letters

7 We also recorded photocurrent action spectra of these two sets of PSC to further evaluate the deteriorating photovoltaic performance during continuous light exposure. The external quantum efficiency (EQE) spectra for the two sets of cells, as determined from the photocurrent at different wavelengths, are presented in Figure 3. It is evident that the EQE drops quickly upon exposure to light across all wavelengths in both cases. The drop in the photocurrent response, as indicated by the EQE measurement, is quick for PSC without iodide treatment, showing more than a 90% decrease in 30 minutes of light exposure whereas iodide treated solar cells exhibited about a 40% decrease in EQE performance. These results are in line with the decrease observed from the J-V characteristics in Figure 2. Dark Recovery. It is interesting to check whether the observed decrease in the photovoltaic performance arises from the permanent photodegradation of mixed halide perovskite film or from the reversible phenomenon of halide ion segregation. If the observed decrease in the photovoltaic performance (Figures 2 and 3) is reversible we would expect the PSCs to recover if stored in the dark. It is interesting to note that the photoinduced segregation reverses back to the original mixed halide composition if left in the dark for several hours. The dark recovery of the solar cell performance is slower than the recovery of the optical spectra. The remediation of the deep traps created during light exposure took longer time to wane. In the present experiments we exposed sets of three cells (in each case) to 25 minutes of light exposure and left them in the dark while recording J-V characteristics and recording power conversion efficiency periodically. The changes in Voc, Jsc, PCE and Fill Factor during illumination and dark recovery are presented in Figure 4. Both treated and untreated cells exhibited a drop in PCE during first 25 minutes upon exposure to light. The iodide treated PSC showed a smaller decrease in PCE than the untreated cells. It is interesting to note that iodide treated cells exhibited almost complete recovery within 24 hours while untreated cells recovered only about 60% during the same time period. The remediation effect arising from iodide is also evident in the open circuit voltage. The change in the voltage was very small (only about 80 mV) for iodide treated PSC during light exposure while it decreased by 0.5 V in untreated cells. The photocurrent behavior was also similar to that of open circuit voltage. However, the major impact seems to be in the fill factor, which dropped nearly 50% for both iodide treated and untreated PSC. These trends in Voc, Jsc and fill factor collectively contribute to the variation seen in PCE performance.



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Figure 4. Photovoltaic performance of (a) MAPbBr1.5I1.5 and (b) iodide treated MAPbBr1.5I1.5 solar cells during 25 minute of steady state light exposure (open-circuit condition) followed by dark recovery. The JV curves were recorded periodically to monitor (A) Open Circuit Voltage (B) Short circuit current, (C) Power Conversion Efficiency (PCE) and (D) Fill Factor. (Note the break in X- axis at 65 min.) A total of 3 cells were evaluated in each set of the experiment and average values are presented.

Transient Absorption Measurements. The photoinduced charge separation and charge recombination can be conveniently studied using transient absorption spectroscopy.29 We carried out these measurements by depositing MAPbBr1.5I1.5 films (with and without iodide treatment) on glass slides. The annealed films were transferred to a spectroscopic cell with provision for evacuation. All measurements with these mixed halide films were


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ACS Energy Letters

9 carried out in vacuum at room temperature. Upon excitation with 387 nm laser pulse (pulse width 150 fs) the MAPbBr1.5I1.5 films exhibit intense bleaching of the exciton band as they undergo charge separation. The recovery of the bleaching reflects the carrier recombination time, a crucial factor in dictating the overall photovoltaic performance. In our previous studies we employed this technique to establish bimolecular charge recombination process.30-31 Figure 5 shows transient absorption spectra of MAPbBr1.5I1.5 films with and without iodide treatment. The transient spectra also show the effect of 25 min light exposure on the excited state behavior of both these films. The transient absorption spectra of MAPbBr1.5I1.5 films (with and without iodide treatment) recorded before the photoirradiation show similar spectral features. The bleaching arising from the charge separation seen around 640 nm confirms that the iodide treatment does not introduce any spectral shift or additional features to the excited state characteristics. With increasing time the bleached absorption recovers as the charge carriers recombine. The recovery of bleaching is shown in Figure 6. It is interesting

Figure 5. Transient absorption spectra recorded following 387 nm laser pulse excitation of MAPbBr1.5I1.5 films: (A) and (B) without iodide treatment and (C) and (D) with iodide treatment. (A) and (C) were recorded prior to visible light exposure and (B) and (D) were recorded after exposing the films for 30 min. All measurements were conducted in an evacuated spectroscopic cell at room temperature.



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10 to note that iodide treated films show slightly longer lifetime. This further ascertains the role of iodide treatment in trap remediation and extending carrier lifetime. This is in agreement with the observation that iodide treatment is beneficial for improving the performance of mixed halide MAPbI3 solar cells. The films exposed to 25 minutes of light exposure show broadened bands of bleaching. In particular, it is noticeable in untreated film. The full width at half maximum (FWHM) at 2 ps for the untreated film before light exposure was 47 nm, and it increased to 66 nm after 30 min light exposure. For the treated films the FWHM was 35 nm before light exposure and 50 nm after. The magnitude of bleaching is also lower for untreated film that was previously exposed to visible light. As seen in the absorption spectra (Figure 1) the exciton band at 640 nm decreases and absorption in the IR increases as halide ions segregate. These transient absorption spectral features of untreated film are in agreement with the expected trend from the segregation of halide ions following 25 min. of continuous light exposure. In the case of iodide treated film the light exposure seems to exhibit marginal effect on the transient spectral behavior. Another interesting feature is the transient bleach recovery of films exposed to 30 minutes of light exposure (Figure 6). The untreated film recovers faster, thus projecting a shorter lifetime of charge carriers. The traps introduced by halide ion segregation are likely Figure 6. Bleaching recovery recorded at 640 nm following 387 nm laser pulse excitation of MAPbBr1.5I1.5 films. Top panel: (a, a’) without iodide treatment and Bottom panel: (b, b’) with iodide treatment. Traces correspond to films (a, b) before and (a’, b’) after 30 min light exposure.

to influence the charge carrier recombination in these light exposed films. On the other hand, iodide treated film show relatively smaller change in the bleaching recovery time. Although the undesired effects are not


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ACS Energy Letters

11 completely alleviated, these results highlight the beneficial role of iodide treatment in overcoming some of the limitations of halide ion segregation. As

discussed

in

this

study,

the

photoinduced halide ion segregation is Scheme 1. Formation of iodide-rich phase and bromide-rich phase during white light irradiation of CH3PbBr1.5I1.5 film deposited on an FTO/TiO2 electrode. The arrows show charge separation and charge flow within the film. The iodide-rich phase CH3NH3PbI3 facilitates charge recombination, thus impeding the flow of charge carriers. The numbers indicated next to conduction band (CB) and valence band (VB) are the energy levels with respect to vacuum.

a

deterrent

to

achieving

steady

photovoltaic output. The iodide-rich domains formed during segregation is the lowest bandgap material and impede the flow of charge carriers. The charge carriers from nearby crystallites of mixed halides and bromide-rich phases accumulate at the iodide-rich

phase and undergo charge carrier recombination. These processes are illustrated in Scheme 1. Thus the formation of iodide-rich phase impedes the flow of charge carriers as evidenced from the marked decrease in fill factor and other cell parameters. It is interesting to note that the impact of segregation is noticeably suppressed when the films are treated with iodide. As shown in our previous study the presence of excess iodide species remediates halide defect sites and induces fast dark recovery.20 We expect similar defect remediation by triiodide species and thus be responsible for minimizing the impact of photoinduced segregation on the photovoltaic performance. However, slower recovery in fill factor suggests some of the deep traps still remain and thus prolong full recovery process. Although our success of stabilizing the performance of mixed halide perovskite solar cell was rather limited, the studies presented here show the importance of iodide treatment in minimizing the impact of halide ion segregation on the photovoltaic behavior. Further capping of mixed halide crystallites with PbSO4-Oleate can further reduce the impact suppress the migration of halide ions.32 Efforts are underway to introduce such additional measures to improve the photovoltaic performance of mixed halide perovskites.



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ASSOCIATED CONTENT Supporting Information. Experimental methods including solar cell design, photovoltaic measurements MAPbI3 cell performance, and spectroscopic measurements are included. AUTHOR INFORMATION Corresponding Author *Address correspondence to this author: [email protected] twitter: @kamatlabND Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT GB acknowledges Fulbright Foundation for the research fellowship. PVK and SK acknowledge the support of the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy, through award DE-FC02-04ER15533. SMK acknowledges the support of the Arthur J. Schmitt Leadership Fellowship and CEST-Bayer Fellowship. This is contribution number NDRL No. 51XX from the Notre Dame Radiation Laboratory.


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ACS Energy Letters

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Mixed Halide Perovskite Solar Cells. Consequence of Iodide

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