Improved Charge Collection in Highly Efficient CsPbBrI2 Solar Cells

Apr 13, 2017 - The meteoritic rise of solar cell efficiencies based on low- cost solution-processed metal halide perovskites. (MHPs) is raising an exc...
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Improved Charge Collection in Highly Efficient CsPbBrI2 Solar Cells with Light-Induced Dealloying J. Scott Niezgoda, Benjamin J. Foley, Alexander Z. Chen, and Joshua J. Choi* Department of Chemical Engineering, University of Virginia, Charlottesville, Virginia 22904, United States S Supporting Information *

ABSTRACT: Halide dealloying in CsPbBrI2 perovskite solar cells proves to be critical in achieving high performance. This dealloying occurs under steady illumination and is a reversible process, suggesting transitions between equilibrium states under dark and illuminated environments. Through decoupling charge collection in the electron- and hole-transporting layers, our photoluminescence, X-ray diffraction, and solar cell device characterization results suggest that the light-induced halide dealloying improves hole collection in solar cells, resulting in increased efficiency (9.2% ± 0.64 on average with a champion power conversion efficiency of 10.3%). Our results provide deeper insights on the impact of dealloying on perovskite solar cell performance and highlight the growing potential of all-inorganic perovskite solar cells.

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materials with compositions of CsPb(BrxI1−x)3 and CsPb(ClxBr1−x)3 for red- and blue-shifted photoemission, respectively. On the other hand, the implementation of Cs-based perovskites in thin-film photovoltaics has been less straightforward. CsPbI3 thin films, while compositionally stable to temperatures in excess of 450 °C, are not stable in the photovoltaic-active cubic phase at room temperature.14,24,25 Therefore, most of the initial studies on all-inorganic MHP solar cells have utilized CsPb(BrxI1−x)3 stoichiometries wherein the inclusion of bromide anions stabilizes the cubic phase. The CsPbBrI2 stoichiometry, in particular, has shown the most promising performance in solar cells.15,26 CsPbBrI2 thin films exhibit a band gap of ∼1.9 eV (630 nm) and are significantly more structurally stable in the cubic phase than their fully iodide counterparts. Recently, Snaith and co-workers obtained an average power conversion efficiency (PCE) of 6% with CsPbBrI2, TiO2-coated fluorine-doped tin oxide (FTO) electrodes, and spiro-OMeTAD hole transporting layers (HTLs), including a champion device PCE of 9.8%.15 In a similar report, the McGehee group developed a modified procedure that utilized an inverted architecture solar cell that achieved stabilized efficiencies of 6.5%.13 Interestingly, the CsPb(BrxI1−x)3 films described in this report were annealed at temperatures (65 °C) well below their purported phase transition to cubic. This suggests that the formation of a cubic phase in these Br-containing films can stably occur in

he meteoritic rise of solar cell efficiencies based on lowcost solution-processed metal halide perovskites (MHPs) is raising an exciting possibility of accelerating widespread usage of solar power.1−4 To realize this potential, the poor stability of MHP solar cells has been identified as a major challenge to be overcome.5−7 The majority of MHP solar cells to date have been based on hybrid organic−inorganic MHPs that typically consist of methylammonium (MA) or formamidinium (FA) molecules located in interstitial locations formed by a lead halide octahedra network. These organic moieties have non-negligible vapor pressures, and MHP thin films containing them have been shown to undergo thermal degradation at temperatures which can be commonly reached at geographic locations.6,8 Moreover, interaction between the organic moieties and water has been identified as a predominant degradation pathway.9−11 While recent efforts at encapsulating devices in polymeric coatings have proven quite effective at shielding water-sensitive MHP devices from the elements,12 this approach can concomitantly increase the cost of solar cells and the risk of device failure due to imperfect encapsulation. An alternative approach is to employ a composition of MHP that combines high efficiency with thermal and chemical stability. Recently, all-inorganic, Cs+ ionbased lead halide perovskites have been proposed as a means to overcome the thermal instability issues of organic cations in hybrid MHPs.13−16 Initial focus on Cs+-based perovskites (CsPbX3; X = I, Br, Cl) has been centered largely on their ability to form highly luminescent nanocrystals.17−23 These studies achieved band gap tuning largely through the formation of halide alloy © 2017 American Chemical Society

Received: March 23, 2017 Accepted: April 13, 2017 Published: April 13, 2017 1043

DOI: 10.1021/acsenergylett.7b00258 ACS Energy Lett. 2017, 2, 1043−1049

Letter

http://pubs.acs.org/journal/aelccp

Letter

ACS Energy Letters

Figure 1. (a) Device schematic, showing architecture of champion device (ITO/c-TiO2/CsPbBrI2/spiro/Ag), absorbance spectrum of CsPbBrI2 thin film with an initial onset at 1.97 eV (628 nm), and SEM image of CsPbBrI2 thin film on ITO/TiO2 electrode (scale bar, 2 μm). (b) J−V characterization of our champion device with the architecture shown in panel a. (c) Histogram of power conversion efficiency values for 100 devices, with statistics for photovoltaic parameters listed in the inset table.

dealloying of halides is one of the main contributions to the high solar cell efficiency. Our results reveal deeper insights into the solar cell performance in alloyed perovskites and provide a path forward in further improving the device performance of MHP solar cells. Solutions of CsPbBrI2 in dimethylformamide (DMF) were spun cast on indium tin oxide (ITO) covered with compactTiO2 (ITO/c-TiO2) electrodes to obtain smooth and glossy, coffee-colored films which were annealed at 345 °C for 10 min. The resulting high-quality CsPbBrI2 thin films exhibit a first absorption feature at 630 nm (∼1.95 eV) and are composed of relatively large 0.5−2 μm diameter grains (Figure 1a). As can be seen in the inset image, pinhole formation in these films was present, but this was not a focus of this contribution because of the high reproducibility of the devices. Furthermore, X-ray diffraction patterns agree well with previous reports and can be found in the Supporting Information (Figure S2). On top of the CsPbBrI2 layer, spiro:OMeTAD and silver layers were deposited as hole-transporting layer and anode, respectively. These devices resulted in an average PCE of 9.22 ± 0.64 (n = 100) with open-circuit voltage (Voc) of 1.08 V ± 0.02, shortcircuit current density (Jsc) of 12.96 ± 1.00, and fill factor (FF) of 0.66 ± 0.4. Under extremely slow scanning conditions, these devices retain roughly 85% of their original PCE value and suffer from a ∼10% relative PCE loss due to hysteresis (Figure S3). We found roughness of our CsPbBrI2 layer to be suppressed in comparison to films prepared on top of FTO/ c-TiO2 electrodes,15 which we attribute to the inherent roughness of FTO compared to ITO. Detailed scanning electron microscopy (SEM) image comparison of several types of TiO2 electrodes on ITO and FTO can be found in the Figure S4. This improved surface uniformity on ITO/c-TiO2 electrodes and the relatively large grains in CsPbBrI2 lead to the high and consistent VOC values.

small crystallites near room temperature, as also shown with CsPbI3 nanoparticles that maintain the cubic phase at room temperature because of nanostructuring.27 At present, record efficiencies for CsPbBrI2 MHP solar cells have reached 10% for solution-processed thin films28 and ∼11.5% for vacuumdeposited thin films,29 showing the promising potential of allinorganic perovskite solar cells. At this point, it must be emphasized that, unlike most solidstate semiconductors composed of covalently bound transition metal atoms, the atoms in MHPs are comparatively highly mobile.30−34 Huang and co-workers have recently outlined a process in MHP PVs (MAPbI3 in particular) that they term “light-induced self-poling”.35 In their findings, they attribute an increase in PCE during light soaking to the photovoltage-driven migration of space charges (i.e., I− anions, positively charged vacancies or ions) during photoexcitation. Findings from the Huang group are but a piece of the ever-emerging consensus that there exist highly mobile ions in MHP thin films and that they are susceptible to reorganization under irradiation and voltage bias.30,31,33,36−42 This is of particular relevance to CsPb(BrxI1−x)3 because they have been shown to exhibit lightinduced dealloying.13 An important question yet to be answered is, What is the impact of halide dealloying on solar cell performance? This is one of the main questions we address in this Letter. Herein, we present findings on CsPbBrI2-based solar cells with an average PCE of 9.22% ± 0.64 and a champion PCE of 10.34%. These are among the highest average and champion PCE based on inorganic MHP thin-film solar cells to date. Importantly, we find that the light-induced dealloying of CsPbBrI2 was crucial in achieving high efficiencies. By combining illumination-dependent X-ray diffraction (XRD) and photoluminescence (PL) measurements on systematically chosen heterojunction combinations, we conclude that improvement of hole collection from the device due to 1044

DOI: 10.1021/acsenergylett.7b00258 ACS Energy Lett. 2017, 2, 1043−1049

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Furthermore, it can be inferred from the plot of dark current density (Jdark) pre- and postlight soaking (Figure 2c) that the majority of FF enhancement stems from a lowering of series resistance (Rs) rather than improving shunt resistance (Rsh). Meanwhile, the open-circuit voltage remains relatively constant and short-circuit current stabilizes after roughly 5 min, well before the device reaches maximum PCE. More specifically, the as-prepared CsPbBrI2 device had a VOC of 0.93 V, JSC of 1.94 mA/cm2, and a FF of only 19.45% immediately upon illumination (Figure 2b). After 1 min of constant illumination (“light soaking”), the VOC, JSC, and FF increased to 1.04 V, 4.61 mA/cm2, and 19.93%, respectively, and after 20 min, 1.10 V, 12.19 mA/cm2, and 68.69%. We suspect the slight increase in VOC to be partly due to the bolstering of photocurrent density, as VOC scales logarithmically with the photocurrent density according to the Shockley equation. To better understand the nature of the light-induced change in solar cell performance, we performed XRD and PL measurements as a function of illumination. Figure 3a shows

Interestingly, our devices exhibit extreme device performance evolution under continuous illumination. Devices previously stored in the dark perform extremely poorly (Figure 2), but as

Figure 2. (a) Comparison of photovoltaic performance for CsPbBrI2 (red) and MAPbI3 (black) devices with ITO/TiO2 anodes and spiro/Ag cathodes. Each crosshair indicates one PCE measurement; dark regions represent times during which devices are not illuminated. CsPbBrI2-based devices exhibit a pronounced power conversion efficiency (PCE) curing with light soaking that is reversible once irradiation is removed (as shown in the timedependent drop in performance after dark sessions). (b) Effect of light-soaking time on J−V curve for device shown in panel a. Curves transition from a low fill factor (FF), S-shape state prior to soaking to a more ideal diode curve with a FF approaching 70% after 20 min of irradiation. Note that VOC is nearly constant throughout and that JSC stabilizes around ∼5 min of light soaking, indicating that the gradual increase in PCE is due to improvements in FF. (c) Comparison of dark current for a single CsPbBrI2-based device before and after light soaking. Lack of slope in reverse bias (positive V) indicates little shunting in either state, and the 2 orders of magnitude difference in current extraction in forward bias illustrates the enhanced charge transport properties after light soaking.

the device is irradiated with air mass 1.5 (AM1.5) light, the PCE gradually increases up to a point at which it reaches a maximum (generally after 45−60 min). Crucially, this effect is not interface-dependent. Regardless of the choice of various charge-transporting layers, CsPbBrI2 solar cells exhibit dramatic performance enhancement as a result of light soaking. To illustrate this, we fabricated devices with reverse polarity in the form of ITO/NiOx/CsPbBrI2/PCBM/Al that exhibit similar behavior with light soaking (Figure S5), indicating that this effect is apparently inherent to the CsPbBrI2 material itself. Figure 2a compares the time-resolved performance of a CsPbBrI2 device that highlights the vast difference in the power production as a function of time under illumination. Once the light source is removed (indicated by the gray regions), the device reverts back to a “dark state” with extremely low PCE. This change is reversible and timedependent, with longer periods of nonillumination causing more dramatic drops in efficiency.In contrast, the methylammonium lead iodide (MAPbI3) solar cell control device, with the same charge-transporting layers and electrodes, shows none of these effects on the same time scale. The enhancement in PCE for the CsPbBrI2 devices closely follows the curing of the fill factor as the devices remain illuminated (Figure 2b).

Figure 3. (a) Photoluminescence (PL) spectra of a CsPbBrI2 film on glass. Dealloying into I- and Br-rich regions is evident both in the ∼35 nm redshift of the initial peak (emission from increasingly I-rich phase) and the emergence of higher-energy peak near 520 nm (emission from formation of Br-rich regions). (b) X-ray diffraction (XRD) spectra of ITO/TiO2/CsPbBrI2/spiro device sealed with Kapton film. The shift to lower 2θ after light soaking is indicative of the formation of large I-rich regions in our PV devices during illumination.

PL spectra of CsPbBrI2 thin films as a function of white light soaking time. Note that the film was never exposed to ambient air to avoid any complexities arising from changes in PL through interaction with water and oxygen; the layer was formed in an inert glovebox on a glass substrate and covered with a glass coverslip which was sealed around the edges with epoxy. Initial PL maximum is centered at 650 nm, which signifies a ∼20 nm Stoke’s shift from the first absorption feature at ∼630 nm and indicates band edge recombination of the CsPbBrI2 alloy. Upon light soaking, the main PL peak redshifts 1045

DOI: 10.1021/acsenergylett.7b00258 ACS Energy Lett. 2017, 2, 1043−1049

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We hypothesized that the drastically improved FF and Jsc in CsPbBrI2 solar cells under light soaking is due to improved charge transport and collection in dealloyed perovskite thin films. From the literature, we highlight the work of Herz and co-workers that investigated the effects on electronic properties stemming from Br substitution for I in FAPb(BrxI1−x)3.45 They found an order-of-magnitude decrease in charge mobility with a minimum at roughly Br/I ratio of 0.33 compared to the pure monohalide phases. In addition, there is a precedent in the literature that suggests CsPb(BrxI1−x)3 alloys in the range of roughly 0.3 ≤ x ≤ 0.5 exhibit poor charge transport (“mobility gap”).45,46 Valence band maxima in lead halide perovskites are known to be largely composed of frontier p-orbitals of the halides,47 suggesting that the partial Br/I alloying will most heavily affect the conduction of photoexcited holes. Furthermore, the S-shaped J−V curves with extremely low FF from our CsPbBrI2 solar cells prior to light soaking can be interpreted as, among other things, an imbalance of electron and hole mobilities which causes charge accumulation at an interface.48−50 To investigate this possibility, we monitored the PL spectra of two different samples as a function of light soaking. One of the samples was CsPbBrI2 film interfaced with an ITO/c-TiO2 electron-acceptor layer and the other was CsPbBrI2 film interfaced with a spiro:OMeTAD hole-acceptor layer (Figure 4). The samples were illuminated from the perovskite side of the films such that the majority of excitation light absorption occurred primarily within the CsPbBrI2. If the mobility of a given charge carrier is changed during the light-soaking process,

to 685 nm after 20 min, corresponding to an alloy composition of x ≈ 0.2 for CsPb(BrxI1−x)3,13 and decreases in intensity concomitantly with the emergence of a higher-energy peak near ∼520 nm that corresponds to PL from the formation of Br-rich regions. This illumination-induced dealloying process has been reported previously in MA-, FA-, and Cs-based mixed halide perovskites, including the emergence and columinescence from two distinct stoichiometric states.13,43 Figure 3b shows further evidence of dealloying in CsPbBrI2 through a shift in XRD peak position as a result of light soaking of a sealed device (ITO/TiO2/CsPbBrI2/SPIRO/Kapton). Kapton film was used to seal the sample from ambient moisture while being X-ray transparent to ensure measurement mimicked the N2 environment under which our solar cells are tested without potential interference from external factors as much as possible. The Bragg peaks at 2θ = 14.58° and 29.52° indicate that the film is fairly well-oriented in the cubic ⟨100⟩ direction with respect to the substrate, which we find to occur on both ITO-TiO2 and glass substrates (Figure S5). The shift of peaks to lower angles (Δθ ≈ 0.1°) after light soaking indicates increased average lattice constant in the CsPbBrI2 film, which would result from the formation of bulk I-rich domains during a dealloying process. Light-induced halide phase segregation in XRD patterns has likewise been shown in a prior report concerning mixed-halide MHPs (MAPb(BrxI1−x)3).43 For x ≠ 0.6, the authors observed shifts in the (200) peak position after white light illumination without the appearance of distinctive peaks associated with Br-rich domains, consistent with our observations in this work.43 This indicates that the Br-rich domains are likely to be either nanostructured, forming thin layers on grain boundaries for example, or highly disordered. The driving force and precise mechanisms through which the dealloying occurs in MHPs are still unknown and remain areas of ongoing inquiry in the research field. Previous studies found that, for 0.1 < x < 1.0, the MAPb(BrxI1−x)3 alloy follows Vegard’s law.43 The shift in 2θ after light soaking for the CsPb(BrxI1−x)3 (x = 0.33) thin film in Figure 3b corresponds to a 0.41 Å increase in lattice constant, which, assuming replacement of MA+ with Cs+ does not cause the alloy system to deviate from Vegard’s law, corresponds to a shift from x = 0.33 to 0.20 (Supporting Information). Interestingly, the PL maximum of our samples before and after white light soaking in Figure 3a correlate extremely well with past reports of PL data as a function of x in CsPb(BrxI1−x)3 at x = 0.33 and 0.20, respectively.13 Our observations so far beg the question, How does dealloying improve the solar cell efficiency? The two major differences between our results with CsPbBrI2 and those previously reported with various other MHPs concerning the effects of light soaking on photovoltaic performance (PCE curing) are (1) the majority of the device performance resulting from FF improvement with nearly constant Voc and (2) the slow rate at which these effects take place in our samples. For example, even after 5 min of light soaking on the as-prepared device in Figure 2b, the PCE value was less than half the value it would reach after 20 min (4.36 and 9.18%, respectively). In contrast, the previously reported effects of light soaking of MHP solar cells occur within seconds and characteristically result in changes in Voc.32,35,44 On the basis of these results, we do not think that the increased efficiency in CsPbBrI2 solar cells can result solely from improved band bending due to lightinduced self-poling.

Figure 4. Photoluminescence (PL) spectra of ITO/TiO2/CsPbBrI2 (a) and CsPbBrI2/spiro (b) films on glass substrates. Illumination entered in the direction of the bare perovskite surface. Perovskite layers presented are of equal thickness. Unlike the ITO/TiO2/ CsPbBrI2 layer (which also emits with ∼6× less initial PL intensity, as labeled at the peak maxima for the initial curves), CsPbBrI2/ spiro emission mimics photovoltaic device performance during light soaking. Emission from CsPbBrI2/spiro decreases during continuous illumination and begins to regenerate when placed in the dark. The inset shows that, without CsPbBrI2, the glass/spiro sample does not show any change in PL intensity. 1046

DOI: 10.1021/acsenergylett.7b00258 ACS Energy Lett. 2017, 2, 1043−1049

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layer. Taken together, our results bolster the idea that inorganic halide perovskites have potential to result in high-efficiency solar cells.

one would expect the extent of charge injection into a respective charge transport layer (and therefore the extent of PL quenching from both CsPbBrI2 and charge transport layers) to vary accordingly. As can be seen in Figure 4a, the PL intensity of the I-rich peak (∼660 nm) of the CsPbBrI2 film on ITO/c-TiO2 fluctuates in a manner over the course of 20 min of light soaking that does not mimic the bulk device performance. On the other hand, with the CsPbBrI2/ spiro:OMeTAD sample, the additive PL of the spiro:OMeTAD and CsPbBrI2 films decreases steadily with light soaking time, mirroring the PV device behavior under J−V characterization (Figure 4b). Furthermore, as the CsPbBrI2/spiro:OMeTAD film sits in the dark after being illuminated (black curve), the PL signal begins to regenerate to its initial state, again mimicking the bulk device performance characteristics very well. The decreased PL intensity from the CsPbBrI2/ spiro:OMeTAD film is indicative of augmented hole transport and injection from CsPbBrI2 to spiro:OMeTAD. After the dealloying, holes in CsPbBrI2 become more mobile and a higher fraction of them get injected into spiro:OMeTAD. This quenches overall PL due to suppression of bimolecular recombination in CsPbBrI2 and higher probability of threebody Auger-like relaxation in spiro:OMeTAD. No such trend of changing PL intensity was observed in a control sample with spiro:OMeTAD-only on glass (inset, Figure 4). In closing, our results present high-performance CsPbBrI2 perovskite solar cells with average and champion device PCEs of 9.22 ± 0.64% and 10.34%, respectively. These high efficiencies result from light-induced dealloying of CsPbBrI2 that improves the collection of holes. The exact reason for and environment in which halide segregation occurs in MHPs are areas of ongoing inquiry. What does seem certain is that a particular impetus for its occurrence is the crystalline quality and stability of the starting material.51 More specfically, while many mono-A-cation MHP systems (of the form APb(BrxI(1−x))3, A = MA, FA, Cs) have been shown to undergo halide segregation, the mixed-cation lead mixed-halide system CsyFA(1−y)Pb(BrxI(1−x))3, with 0.10 < y < 0.30, has been shown to simultaneously exhibit high photostability to halide dealloying and more than 1 order of magnitude higher THz mobility than FA-only films within the traditional “instability gap” (0.3 < x < 0.5). These highly photostable alloys have proven effective and compositionally stable to extended light exposure, forming devices at above 17% efficiency.52 In the pursuit of fully inorganic perovskites for PV applications, alloying inorganic cations in the A-site to engineer the performance and stability will be an interesting future direction. Nonetheless, our findings in this Letter show that light-induced halide segregation can be harnessed to move an alloyed MHP out of its “mobility gap” and lead to bolstered PV performance. Furthermore, it seems that the trade-off of lower maximum PCE for increased stability of the cubic phase as Br content rises in mixed halide Cs-based perovskites may be alleviated by the formation of I-rich regions under working conditions. Indeed, JSC values in this and similar reports concerning these solar cells tend to closely approach the limit for CsPbBrI2 (16.3 mA/cm2)15,29,53 with AM1.5 illumination, suggesting that significant light absorption may be occurring in lower band gap dealloyed I-rich regions that allow for greater current production. In addition, because of the propensity of perovskite material to form type I band alignment at halide−alloy interfaces,54 it is likely that a significant portion of excited charge in Br-rich regions favorably relax into the I-rich active

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.7b00258. Experimental description, image of a solar cell device, XRD patterns, device hysteresis, SEM images, solar cell J−V curves, and a discussion of XRD analysis (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-amil: [email protected]. ORCID

Joshua J. Choi: 0000-0002-9013-6926 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by an Early Career Faculty Award grant from NASA’s Space Technology Research Grants Program (NNX15AU43G).

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REFERENCES

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DOI: 10.1021/acsenergylett.7b00258 ACS Energy Lett. 2017, 2, 1043−1049

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DOI: 10.1021/acsenergylett.7b00258 ACS Energy Lett. 2017, 2, 1043−1049