Electric‐Field‐Driven Reversible Conversion Between Methylammonium Lead Triiodide Perovskites and Lead Iodide at Elevated Temperatures

DOI: 10.1002/aenm.201501803 device effi ciency. [ 34 ] On the other hand, migration of the iodine vacancies (V I • ) has not been yet experimentally observed. Theoretical calculations suggest that the V I • might be even more mobile than MA + ions, although the activation energy values for V I • migration calculated by different methods vary from 0.08 to 0.58 eV. [ 35,36 ] In view of the widely observed oxygen vacancies migration in many oxide perovskite materials, the assumption of V I • migration in MAPbI 3 materials is quite reasonable. In this contribution, we present a direct evidence for the macroscopic migration of V I • in MAPbI 3 perovskite fi lms at an elevated temperature of 330 K. Most interestingly, we report a reversible conversion between MAPbI 3 and lead iodide (PbI 2 ) phases under a small electric fi eld at the elevated temperature as a result of a set of solid-state chemical reactions, and show that the conversion of MAPbI 3 to PbI 2 is slower than the conversion of PbI 2 to MAPbI 3 at room temperature (RT). The MAPbI 3 lateral devices were fabricated on glass substrates to enable observation of ion migration process by an optical microscope or a charge-coupled device (CCD) camera. Figure 1 a shows the working area of a fresh MAPbI 3 device at 330 K in vacuum taken with the CCD camera. The cross-sectional structure of the device is illustrated in Figure 1 b. After applying a constant electrical fi eld of ≈3 V μm −1 for 20–60 s, a thread formed near the anode region, and then gradually moved toward the cathode along the applied electric fi eld direction. Figure 1 c,e show the snapshot optical images obtained during the migration of this thread under the applied electric fi eld. Sketches in Figure 1 b,d,f illustrate a change in the device crosssection due to the formation of the thread (red region) and its subsequent motion across the channel from the anode to the cathode. (The process of thread formation and its motion can be seen in Video S1, Supporting Information). The thread appears darker than the neighboring regions due to the scattering of light incident at an angle of 50°–60°. When observed with vertical incident light in refl ection mode, the thread (region B) has a similar color with the neighboring regions (Figure 1 g). The optical microscopy image in transmission mode in Figure 1 h shows that this thread has higher transparency than the neighboring regions. To determine composition of the formed thread, we conducted energy-dispersive X-ray spectroscopy (EDX) and X-ray diffraction analysis (XRD) measurements. The scanning electron microscope (SEM) ( Figure 2 a) and EDX images (Figure 2 c–h) show the distributions of gold (Au), I and lead (Pb) elements in a device poled by an applied electric fi eld of 3 V μm −1 for 1 min. The quantitative distribution of these elements along the Solution-processed solar cells based on organolead trihalide perovskite (OTP) materials are emerging as a new generation of photovoltaic devices due to their low cost and superior performance. [ 1–17 ] The power conversion effi ciency (PCE) of the perovskite solar cells increased dramatically from 3.8% to over 20% after only a few years of research. [ 1–7,9–16,18–22 ] The advances in low-cost, high-throughput processing methods, such as doctor-blading and spray coating, are also fast, which allowed fabrication of high-effi ciency large-scale devices. [ 23–28 ] One of the remaining issues is whether the OTP materials and devices have suffi cient stability that is needed for the commercialization of the OTP solar cells. [ 3,29,30 ] Among all factors that cause the instability of the hybrid perovskites, ion migration has been recently been identifi ed to be intrinsic to the hybrid perovskite polycrystalline fi lms and cannot be removed by device encapsulation. To solve the instability problem of the OTP-based devices, a deeper insight into the ion migration effect is necessary, since it might provide hints for the development of new materials with better stability. [ 30,31 ]