Formation , location and beneficial role of PbI 2 in lead halide perovskite solar cells

Formation, location and beneficial role of PbI2 in lead halide perovskite solar cells Tian Du, Claire H. Burges, Jinhyun Kim, Jiaqi Zhang, James R. Durrant and Martyn A. McLachlan* Department of Materials and Centre for Plastic Electronics, Imperial College London, SW7 2BP Department of Chemistry and Centre for Plastic Electronics, Imperial College London, SW7 2AZ *martyn.mclachlan@imperial.ac.uk Abstract Here we report the investigation of controlled PbI2 secondary phase formation in CH3NH3PbI3 (MAPI) photovoltaics through post-deposition thermal annealing, highlighting the beneficial role of PbI2 on device performance. Using high-resolution transmission electron microscopy we show the location of PbI2 within the active layer and propose a nucleation and growth mechanism. We discover that during the post-deposition annealing


Introduction
Lead halide perovskite solar cells have created significant interest since they were first proposed a little more than 5-years ago, such that now they are being referred to the next big thing in the photovoltaics industry. 1 The methylammonium lead halide system has been most commonly reported i.e. CH 3 NH 3 PbX 3 (X=I, Br, Cl), although other mixed halide perovskites including CH 3 NH 3 PbI 3-x Cl x , 2 CH 3 NH 3 PbI 3-x Br x , 3 as well as formamidinium triiodide perovskite HC(NH) 2 PbI 3 have also been reported. 4 Current state-of-the-art power conversion efficiencies exceed 20% in these systems, 5,6 a remarkable achievement given 2 that fabrication is enabled through facile solution processing routes, which have remarkable cost saving implications compared with incumbent photovoltaic technologies. 7 Post-deposition thermal annealing of the perovskite active layers has emerged as a key step in performance enhancement and film stabilization in i) the numerous solution based processing routes implemented, these include: one-step spin-coating, 8 two-step sequential deposition, 9 and solvent-engineering spin-coating 10 and ii) the materials systems thus far developed i.e. CH 3 NH 3 PbI 3-x Cl x , 8 CH 3 NH 3 PbI 3 , 10 and HC(NH) 2 PbI 3 . 4 Unsurprisingly, a variety of annealing regimes have emerged with typical temperatures in the range 70 -150 °C, 48 and times from 10 minutes to > 1 hour, 911 in all cases the annealing step is cited as being essential for improving power conversion efficiency (PCE) and reducing current density-voltage (J-V) hysteresis. 12 Regardless of the annealing conditions cited it is apparent that this processing has a profound impact on film morphology, crystallinity and homogeneity of the perovskite layers resulting in a dramatic impact on measured device performance. 13 In parallel, it has been observed that excessive or secondary annealing can decompose the active layer, irreversibly forming species including PbI 2 . This is exemplified by post-annealing at elevated temperature, 150 °C, for a few minutes 14 or lower temperatures, ~ 100 °C, for extended periods 15 where the formation of a secondary PbI 2 phase is observed. To compound this issue thermally induced decomposition can be accelerated by the presence of illumination, 16 moisture, 17 and interfacial alkaline ligands, 18 and is known to occur even at 85 °C, the upper limit of standard thermal cycling tests in inert atmosphere. 19 These observations are made despite the reported decomposition temperature of CH 3 NH 3 PbI 3 being close to 300 °C, clearly degradation occurs below 100 °C, 20 perhaps at sites such as grain boundaries, 21 surfaces and interfaces. The generation of PbI 2 secondary phase should therefore be considered not only in post-3 deposition treatments but also during the operation under environmental and/or thermal stress.
Degradation of the perovskite absorber may therefore seem like something to be avoided, however the presence of PbI 2 can actually be beneficial to device performance. For example Chen et al have shown impressive PCE improvements, 0.66% to 12%, by thermally forming PbI 2, which is attributed to surface defect passivation. 22 Alternatively, PbI 2 can also be incorporated into perovskite absorbers by adding excess PbI 2 to the precursor solution, which has been proposed as a means of reducing J-V hysteresis by blocking ionic migration. 23 As a further example, unreacted PbI 2 remaining as a secondary phase during dip-coating, is reported to suppress charge carrier recombination at the perovskite/cathode interface thus enhancing device performance. 24 Herein, we report the investigation of controlled PbI 2 secondary phase formation in CH 3 NH 3 PbI 3 (MAPI) photovoltaics through post-deposition annealing, highlighting the beneficial role of PbI 2 and through the use of high resolution transmission electron microscopy propose a nucleation and growth mechanism for PbI 2 . In summary the MAPI films were prepared via a "toluene-dripping" method reported by Jeon et al 10 following which brief (20 s) anneal at 100 °C was carried out on which the color of films changed from pale yellow to a characteristic dark brown. We have investigated the influence of a subsequent thermal anneal between 100 -150 °C for 10 minutes. We discover that during the second annealing step PbI 2 forms mainly in the grain boundary regions of the MAPI films and that at certain temperatures the PbI 2 formed can be highly beneficial to device performance. Our analysis shows that the MAPI grain boundaries as susceptible areas that, under thermal loading, initiate the conversion of MAPI into PbI 2 .

Results and Discussion
Typical X-ray diffraction patterns of the post-annealed MAPI films are shown in Figure 1a.
The films annealed at 100 °C and 110 °C show the presence of polycrystalline MAPI with the (110) diffraction peak clearly visible around 14.2 ° (2θ). As temperature is increased a (001) diffraction peak of PbI 2 appears around 12.5° (2θ), which increases with intensity as temperature is raised, at 150 °C the (101) and (003) peaks of PbI 2 are also visible. Based on these data we suggest that post-deposition annealing leads to the thermal decomposition of MAPI to CH 3 NH 3 I (MAI) and PbI 2 . The MAI is vaporized from the film due to its low sublimation temperature whilst the inorganic PbI 2 remains in the film resulting  by voids in the film. In Figure 2a, grains of PbI 2 (circled in red) can be seen at the edge of a MAPI grain. The PbI 2 secondary phase appears brighter than the MAPI in the dark-field images, due to its higher average atomic number (Z). Figure 2b shows a PbI 2 crystal (dashed circle) penetrating the MAPI layer. It can be seen that where there are PbI 2 crystals there are also neighboring voids within the film, which appear significantly darker in STEM images due to the reduction in amount of material present in that area. We propose that the voids form in the MAPI grains as a consequence of mass transfer into the grain boundary region that initiates at/near the TiO 2 interface. The areas marked "1-4" were probed using HRTEM with the lattice fringes are shown in Figure 2c-2f, respectively.
The presence of PbI 2 was confirmed by measuring these fringes that contained (101),

Conclusions
In conclusion, our work shows conclusively that grain boundaries are the preferred locations for PbI 2 nucleation and growth during post deposition thermal anneal of MAPI based perovskite photovoltaic devices. By combining morphological and microstructural analysis, using X-ray diffraction and high-resolution electron microscopy, with transient optical probes we identify the benefits to device performance characteristics enabled by controllably introducing PbI 2 at the grain boundaries. We ascribe our improvements to a combination of beneficial effects, namely i) PbI 2 increasing the shunt resistance of the active layer, ii) PbI 2 reducing ionic migration and, iii) PbI 2 assisting with hole injection into Spiro-OMeTAD. We observe that temperature is an easily accessed parameter for 11 controlling PbI 2 formation and identify an optimum temperature and processing duration.
Excessive thermal annealing results in the formation of too much PbI 2 that creates highly resistive devices. Thus by confirming the precise location of PbI 2 in MAPI based perovskite photovoltaics we present a logical explanation of changes in measured performance and optical characteristics and identify optimum processing conditions for producing good devices with low hysteresis.

Device measurement
Cyclic current density-voltage (J-V) characteristics were measured by applying external potential bias to the cell and recording the current with a Keithley 2400 source meter. The 13 cells were illuminated by an AM 1.5 xenon lamp solar simulator (Oriel Instruments). The intensity was adjusted to 1 sun by changing the working current, which was calibrated using a Si reference photodiode. All devices were stored in dark prior to measurement and were measured in a nitrogen-filled chamber. The voltage was scanned from 0V to 1.5V, back to -0.2 V and back to 0 V at a scan rate of 125 mV/s.

Film Characterisation
Scanning electron microscopy (SEM) images were obtained using a LEO Gemini 1525 field emission gun scanning electron microscope with a fixed operating voltage of 5 kV. Xray diffraction (XRD) patterns were obtained using a Bruker D2 diffractometer using a Cu