Ion Migration in Organometal Trihalide Perovskite and Its Impact on Photovoltaic Efficiency and Stability

Abstract

Organometal trihalide perovskites (OTPs) are emerging as very promising photovoltaic materials because the power conversion efficiency (PCE) of OTP solar cells quickly rises and now rivals with that of single crystal silicon solar cells after only five-years research. Their prospects to replace silicon photovoltaics to reduce the cost of renewable clean energy are boosted by the low-temperature solution processing as well as the very low-cost raw materials and relative insensitivity to defects. The flexibility, semitransparency, and vivid colors of perovskite solar cells are attractive for niche applications such as built-in photovoltaics and portable lightweight chargers. However, the low stability of current hybrid perovskite solar cells remains a serious issue to be solved before their broad application. Among all those factors that affect the stability of perovskite solar cells, ion migration in OTPs may be intrinsic and cannot be taken away by device encapsulation. The presence of ion migration has received broad attention after the report of photocurrent hysteresis in OTP based solar cells. As suggested by much direct and indirect experimental evidence, the ion migration is speculated to be the origin or an important contributing factor for many observed unusual phenomenon in OTP materials and devices, such as current-voltage hysteresis, switchable photovoltaic effect, giant dielectric constant, diminished transistor behavior at room temperature, photoinduced phase separation, photoinduced self-poling effect, and electrical-field driven reversible conversion between lead iodide (PbI2) and methylammonium lead triiodide (MAPbI3). Undoubtedly thorough insight into the ion-migration mechanism is highly desired for the development of OTP based devices to improve intrinsic stability in the dark and under illumination. In this Account, we critically review the recent progress in understanding the fundamental science on ion migration in OTP based solar cells. We look into both theoretical and experiment advances in answering these basic questions: Does ion migration occur and cause the photocurrent hysteresis in perovskite solar cells? What are the migrating ion species? How do ions migrate? How does ion migration impact the device efficiency and stability? How can ion migration be mitigated or eliminated? We also raise some questions that need to be understood and addressed in the future.

Capturing the Sun: A Review of the Challenges and Perspectives of Perovskite Solar Cells

“…[168,169] In detail, these ions originate from defect states during low-temperature fabrication and include vacancies (e.g., V MA , V Pb , and V I ) (i.e., Schottky defects), interstitial defects (e.g., MA i , Pb i , I i ), i.e., Frenkel defects, and anti-site substitutions (e.g., MA I , Pb I ), [167,170] as previously mentioned in Figure 7. Note that due to the high activation energy (explained in the following part), [167,171] migration of Pb 2+ ions can be excluded in this discussion.…”

“…The migration pathways of individual ions/defects are consistent with the ones proposed by Eames et al [171] Apart from hopping through point defects (Schottky and Frenkel defects) within the crystal lattice, [178] as shown in Figure 10f,g, a range of other possible ion migration channels can exist. Yuan et al suggest local lattice distortion, induced through charges or additional impurity atoms, lightinduced softening, and piezoelectric effects, as shown in Figure 10h-l. [168] Furthermore, grain boundaries (GB) that possess a large density of defects may play an even more important role, illustrated in Figure 10k. The influence of GBs has been demonstrated by different groups.…”

“…[198] The reaction between PCBM and I 3 trimers associated with the Pb I antisite defects leads to the creation of anionic fullerene derivatives. [168] In this respect, the iodide ions/defects are immobilized by combining with the PCBM molecules in the bulk of the perovskite, thus impeding their further motion towards the perovskite/TL interfaces.…”

Formation and Diffusion of Metal Impurities in Perovskite Solar Cell Material CH3NH3PbI3: Implications on Solar Cell Degradation and Choice of Electrode

“…The low defect formation energies in MAPbI 3 indicate that the energy cost for bond breaking and distortion is low, which further implies low defect diffusion barriers. Indeed, fast diffusion of native defects in MAPbI 3 14, 17, 18, 19, 20, 21, 22, 23, 24, 25 and the resulting phenomena (such as hysteresis in current–voltage curves,25, 26, 27, 28 giant dielectric constant,28, 29 switchable photovoltaic effect,17 photon‐induced phase separation,30 etc.) have been reported and extensively discussed in the literature.…”

Absence of ferroelectricity in methylammonium lead iodide perovskite

“…41 The migration of I ion at room temperature and under device operation conditions however has not been verified yet. 42 The experimental procedure applied here gives only an account of the total number of ions and the signs. Hence the hysteretic behavior can only be attributed the positive ions.…”