Persistent Dopants and Phase Segregation in Organolead Mixed-Halide Perovskites

Organo lead mixed -ha l ide perovsk i t e s such as CH3NH3PbX3−aX′a (X, X′ = I, Br, Cl) are interesting semiconductors because of their low cost, high photovoltaic power conversion efficiencies, enhanced moisture stability, and band gap tunability. Using a combination of optical absorption spectroscopy, powder X-ray diffraction (XRD), and, for the first time, Pb solid state nuclear magnetic resonance (ssNMR), we probe the extent of alloying and phase segregation in these materials. Because Pb ssNMR chemical shifts are highly sensitive to local coordination and electronic structure, and vary linearly with halogen electronegativity and band gap, this technique can provide the true chemical speciation and composition of organolead mixed-halide perovskites. We specifically investigate samples made by three different preparative methods: solution phase synthesis, thermal annealing, and solid phase synthesis. Pb ssNMR reveals that nonstoichiometric dopants and semicrystalline phases are prevalent in samples made by solution phase synthesis. We show that these nanodomains are persistent after thermal annealing up to 200 °C. Further, a novel solid phase synthesis that starts from the parent, single-halide perovskites can suppress phase segregation but not the formation of dopants. Our observations are consistent with the presence of miscibility gaps and spontaneous spinodal decomposition of the mixed-halide perovskites at room temperature. This underscores how strongly different synthetic procedures impact the nanostructuring and composition of organolead halide perovskites. Better optoelectronic properties and improved device stability and performance may be achieved through careful manipulation of the different phases and nanodomains present in these materials. ■ INTRODUCTION Organolead halide perovskites (CH3NH3PbX3, X = I, Br, Cl) have emerged as promising semiconductors for photovoltaics due to their low cost, solution processability, and high power conversion efficiencies (>21−22%). Among their many interesting properties, organolead halide perovskites benefit from large absorption coefficients, low exciton binding energies, long exciton diffusion lengths, high dielectric constants, and intrinsic ferroelectric polarization. Organolead mixed-halide perovskites (CH3NH3PbX3−aX′a, X, X′ = I, Br, Cl) are of particular interest because they appear to further benefit from enhanced moisture stability, improved carrier relaxation time, and visible range tunability. Mixed-halide perovskites are thus useful in tandem solar cells, and because they also display intense photoluminescence, they have potential utility in lightemitting devices (LEDs). In spite of these advantages, questions surrounding the extent of alloying and phase segregation in mixed-halide perovskites remain. Films of CH3NH3PbI3−xClx cast from precursors that contain chloride exhibit improved film coverage, tunable morphologies, increased diffusion lengths, and reduced photocurrent hysteresis compared to CH3NH3PbI3 films prepared without chloride, even though no chloride is present by compositional analysis. Whether chloride is incorporated into the structure is uncertain, but it has been suggested that residual chloride collects at grain boundaries. In the ‘CH3NH3PbI3−aBra’ series, a recent computational study proposed that bromide-rich phases such as CH3NH3PbIBr2 and CH3NH3PbI0.5Br2.5 can be thermodynamically stable against phase segregation at room temperature; however, a miscibility gap between 30 and 60% Br is only overcome above 70 °C. Photoinduced phase segregation of organolead mixedhalide perovskites has also been observed. Structural issues aside, phase segregation in mixed-halide perovskites is intriguing because these materials are known to easily undergo anion exchange in solution as well as between gas and solid phases. In fact, the fast rate of diffusion and high overall mobility of halide ions throughout the crystalline perovskite lattice is likely responsible for photoinduced phase separation and other unusual perovskite properties such as giant dielectric constant and photocurrent hysteresis. More research is needed to understand whether (and which) mixed-halide perovskites form stable alloys, what other phases and impurities exist as phase-segregated domains, Received: May 9, 2016 Revised: July 24, 2016 Published: July 25, 2016 Article