Halide heterogeneity affects local charge carrier dynamics in mixed

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Halide heterogeneity affects local charge carrier dynamics in mixed-ion lead perovskite thin films Sarah Wieghold, Jason Tresback, Juan-Pablo Correa-Baena, Noor Titan Putri Hartono, Shijing Sun, Zhe Liu, Mariya Layurova, Zachary A. VanOrman, Alexander S. Bieber, Janak Thapa, Barry Lai, Zhonghou Cai, Lea Nienhaus, and Tonio Buonassisi Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b00650 • Publication Date (Web): 29 Apr 2019 Downloaded from http://pubs.acs.org on May 2, 2019

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Chemistry of Materials

Halide heterogeneity affects local charge carrier dynamics in mixed-ion lead perovskite thin films Sarah Wieghold1, 3,*, Jason Tresback2, Juan-Pablo Correa-Baena1, Noor Titan Putri Hartono1, Shijing Sun1, Zhe Liu1, Mariya Layurova1, Zachary A. VanOrman3, Alexander S. Bieber3, Janak Thapa,1 Barry Lai4, Zhonghou Cai4, Lea Nienhaus3, Tonio Buonassisi1,* 1Massachusetts

2Center

Institute of Technology, Cambridge, MA 02139, USA

for Nanoscale Systems, Harvard University, Cambridge, MA 02139, USA

3Department

of Chemistry and Biochemistry, Florida State University, Tallahassee, FL 32306,

USA 4X-ray

Science Division, Advanced Photon Source, Argonne National Laboratory, Lemont, IL

60439, USA

Corresponding Author *SW: [email protected], TB: [email protected].

ACS Paragon Plus Environment

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ABSTRACT. The mechanisms and elemental composition that form the basis for the improved optical and electronic properties in mixed-ion perovskite solar cells are still not well understood compared to standard methylammonium lead triiodide perovskite devices. Here, we use synchrotron-based Xray fluorescence to map the composition of perovskite thin films. To get insights into the elemental distribution during film growth, we fabricate films with three different thicknesses. To create a link between the composition and the electronic properties, we perform Kelvin probe force microscopy and time-resolved photoluminescence spectroscopy. We find that the elemental composition is highly dependent on the film thickness, in particular, that the I/Pb ratio is altered for single grains based on the film thickness. The difference in the I/Pb ratio reveals to be the root cause for the underlying difference in film lifetime and defect density influencing charge carrier dynamics. Our results provide an in-depth analysis approach combining macro- and nanoscale techniques to shed light onto the fundamental processes which help to further engineer perovskite thin films and improve device efficiencies.


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INTRODUCTION. Despite the recent progress in perovskite solar cells (PSCs) reaching a power conversion efficiency (PCE) of 23.7%,1 the mechanisms and elemental compositions that form the basis for the improved optical and electronic behavior of these thin-films is still not well understood. Compositional engineering approaches to improve film quality and stability2, i.e. mixtures of methylammonium (MA) and formamidinium (FA),3–6 incorporation of Cs7–9 and/or Rb10,11 into the perovskite structure, have led to the remarkable progress and increase in device PCE.12 In particular, films consisting of four cations (Rb, Cs, MA, FA) showed a high open circuit voltage (VOC) of 1.24 V and an unprecedented stability under illumination for 500 h (conditions: 85 °C, continuous illumination).10 This was mainly attributed to a compositional film homogeneity to favor stability,13,14 by suppressing phase segregation into iodide- and bromide-rich domains.10,15 However, while these thin-film materials have been well characterized averaged over large areas when integrated in full device architectures, little is known about the local chemical composition and distribution of ions which influence the performance in these multi-element materials. Thus, mapping ion distribution on the microscale is of great importance to understand halide segregation and ion migration and to correlate it to performance relevant parameters. Over the last years, various techniques have been employed to gain insight into the elemental distribution and segregation. A helium ion microscopy coupled to a secondary ion mass spectrometer (HIM-SIMS) was used to map the elemental distribution and nanoscale segregation of

127I, 79Br,

and

12C

in

(FAPbI3)0.85(MAPbBr3)0.15 films.16 Iodide-rich domains were observed in addition to the thermodynamically stable domains composed of FAxMAyPbI3. This film inhomogeneity was attributed to be the root cause for the measured variation in the optical film properties. The distribution of lead and iodide in (FAPbI3)0.85(MAPbBr3)0.15 films and their impact on the local


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fluorescent characteristics were investigated using photoluminescence (PL) and fluorescence lifetime imaging microscopy (FLIM).17 Here, PbI2 deficient regions showed a different PL signature and the very fast PL component could be attributed to higher defect trapping. Halide migration was also mapped by synchrotron-based X-ray fluorescence (XRF).18 An enhancement in PL in CH3NH3PbBr3 films was correlated to Br- rich regions. Further, the distribution of the cations Cs+ and Rb+ in the mixed perovskites have been investigated by photoelectron spectroscopy (PES) with a synchrotron radiation source in the hard X-ray region19 and by synchrotron-XRF.11 It was observed that Rb+ in high concentrations segregates into large clusters (no Cs present), while the addition of CsI into the RbI mixed-perovskite films resulted in a lower density of Rb clusters. By using XRF/X-ray-beam induced current (XBIC), it was shown that the Rb clusters suppress charge collection. In this work, we investigate the elemental distribution of perovskite films containing Cs, Rb, MA, FA, Pb, I and Br by means of synchrotron-based XRF mapping, and their effect on the electronic properties by Kelvin probe force microscopy (KPFM) and time-resolved photoluminescence (TRPL) spectroscopy. Perovskite thin films with different thicknesses (prepared with an excess of PbI2,

i.e.

over-stoichiometric)

were

fabricated

alongside

full-size

devices

(FTO/SnO2/Rb0.05Cs0.05(FA0.83MA0.17)0.90Pb(I0.83Br0.17)3/Spiro/Au)20–22 to get insight into the elemental distribution during film growth. First, we study the impact of film thickness on device performance. Synchrotron-based XRF imaging is used to map the elemental distribution of the three different film thicknesses to reveal the difference in grain composition, and in particular the I/Pb ratio. To link the observed variation in grain composition to electronic features, we perform KPFM and TRPL to obtain the spatially distributed contact potential difference (CPD) and trap state density.


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RESULTS AND DISCUSSION. Influence of film thickness on device performance. Characterizing the composition of perovskite thin films has been identified as an important parameter to correlate device performance to elemental distribution, morphology and electronic properties.11 To study the impact of film thickness on the device properties, we prepare three perovskite thin film thicknesses composed of Rb0.05Cs0.05(FA0.83MA0.17)0.90Pb(I0.83Br0.17)3 by changing the molar precursor concentration.23,24 As a result, perovskite film thicknesses of 40, 150, and 380 nm are obtained (see cross-sectional SEM and AFM images in the Supporting Information Figure S1-S2). To study the bulk crystal structure, we perform grazing incidence X-ray diffraction (GIXRD). Figure S3 shows the GIXRD pattern of the three film thicknesses confirming the cubic perovskite phase for all three film thicknesses. Emission spectra of the three films can be found in Ref. [24]. To understand how the parameter, namely film thickness, changes the optoelectronic properties, we prepared PSCs composed of a stack of FTO/SnO2/mixed perovskite/Spiro-OMeTAD/gold as presented in the schematic in Figure 1a. Details on the preparation of each layer can be found in the experimental section in the Supporting Information. Figure 1b shows the absorption spectra as a function of film thickness of the half-devices. The optical density increases with increasing film thickness and all films exhibit broadband absorption with an absorption onset around 780 nm.24 The device characteristics as a function of film thickness under standard conditions (with AM1.5G illumination at 25 °C) are shown in Figure 1c-g. The J-V curves of the highest performing device for all film thicknesses are plotted in Figure 1c (gray: 40 nm, green: 150 nm, and blue 380 nm; backward scan direction) with the statistics of at least 4 devices for open circuit voltage (VOC),


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short circuit current (JSC), fill factor (FF) and PCE in Figure 1d-g. As can be seen, the lowest VOC and JSC are obtained for the thinnest films, whereas a similar VOC is obtained for the 150 nm and 380 nm films. The improvement in JSC can be explained by an increase in the optical density of the films, in particular, by an increase in light absorption in the range from 400 to 650 nm.25,26 The loss in VOC for the thinnest film can be attributed to a low shunt resistance promoting recombination losses at the interface. By increasing the film thickness, the recombination resistance increases accordingly, resulting in an enhanced VOC. Our results are also is in line with previous work, where only slight variations in VOC were measured for films thicker than >100 nm.25,27–29 In addition, it is suspected that the root cause of voltage loss in these devices is mainly due to interfacial recombination via trap states.30,31 Figure 1g shows the PCE for the three different film thicknesses. A similar PCE is obtained for the 380 and 150 nm-based devices, whereas a PCE 2.8 is seen between the grains (yellow), ii) a higher grain-to-grain variation is seen for the two thinnest films compared to the 380 nm film, and iii) a higher average ratio is extracted for the 380 nm film of approx. 2.86, whereas an average I/Pb ratio of 2.41 and 2.36 is extracted for the 150 and 40 nm films, respectively. The latter observation is also visualized in a box plot in Figure 3e. When we compare our data to the theoretical value of 2.55 for the I/Pb ratio for the bulk stoichiometry,34 a lower value (

Halide heterogeneity affects local charge carrier dynamics in mixed

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