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Origin of Long Lifetime of Band-Edge Charge Carriers in Organic–Inorganic Lead Iodide Perovskites

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Origin of Long Lifetime of Band-Edge Charge Carriers in Organic–Inorganic Lead Iodide Perovskites Origin of long lifetime of band-edge charge carriers in organic–inorganic lead iodide perovskites Tianran Chena,1, Wei-Liang Chenb,1, Benjamin J. Foleyc, Jooseop Leed, Jacob P. C. Ruffd, J. Y. Peter Kod, Craig M. Browne, Leland W. Harrigere, Depei Zhanga, Changwon Parkf, Mina Yoonf, Yu-Ming Changb, Joshua J. Choic,2, and Seung-Hun Leea,2 aDepartment of Physics, University of Virginia, Charlottesville, VA 22904; bCenter for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan; cDepartment of Chemical Engineering, University of Virginia, Charlottesville, VA 22904; dCornell High Energy Synchrotron Source, Cornell University, Ithaca, NY 14853; eNIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD 20899; and fCenter for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37831 Edited by Peidong Yang, University of California, Berkeley, CA, and approved May 26, 2017 (received for review March 16, 2017) Long carrier lifetime is what makes hybrid organic–inorganic pe- Third, it was proposed that large polarons are formed by organic rovskites high-performance photovoltaic materials. Several micro- cations when they reorient themselves in response to the presence of scopic mechanisms behind the unusually long carrier lifetime have photoexcited electrons and holes (14–16). As a result, the screened been proposed, such as formation of large polarons, Rashba effect, carriers are protected from scattering by defects and phonons, ferroelectric domains, and photon recycling. Here, we show that leading to the prolonged lifetime. Testing these theories calls for the screening of band-edge charge carriers by rotation of organic experimental studies on microscopic correlations between the mo- cation molecules can be a major contribution to the prolonged tion of organic cations and the charge carrier lifetime. An experi- carrier lifetime. Our results reveal that the band-edge carrier life- mental challenge is how to directly correlate the relaxation of band- time increases when the system enters from a phase with lower edge carriers and intrinsic properties at the atomic level, without rotational entropy to another phase with higher entropy. These complications with extrinsic factors such as morphology and charge results imply that the recombination of the photoexcited electrons trap density across different samples. and holes is suppressed by the screening, leading to the formation Here, the intrinsic effects of organic molecules on the band-edge SCIENCES of polarons and thereby extending the lifetime. Thus, searching charge carrier lifetime were studied by probing band-edge time- APPLIED PHYSICAL for organic–inorganic perovskites with high rotational entropy resolved photoluminescence (TRPL) of HOIPs as a function of over a wide range of temperature may be a key to achieve supe- temperature, spanning different structural phases of each system with rior solar cell performance. different rotational entropy. Among HOIPs with various organic organic–inorganic hybrid perovskite | carrier lifetime | cations, we have selected two prototypes for this study: for- photoluminescence | polaron mamidinium lead iodide [HC(NH2)2PbI3]andmethylammonium lead iodide (CH3NH3PbI3). The primary reason of the choice is that + + – the HC(NH2)2 and CH3NH3 molecules have different geometries: he record efficiency of hybrid organic inorganic perovskite ∼ – – the 120° bent and the linear geometry of the C N bonds in HC T(HOIP)-based solar cells has reached above 22% (1 4), + + + (NH2)2 and CH3NH3 , respectively. As a consequence, CH3NH3 which is comparable to that of silicon solar cells. The most + dominant contribution to the high photovoltaic performance of has a strong electric dipole moment (6), whereas HC(NH2)2 has a HOIPs comes from their long carrier lifetimes (≥ 1 μs), which strong quadrupole moment that has not been considered before. translates to large carrier diffusion lengths despite their modest charge mobilities (5). Several microscopic mechanisms behind Significance the unusually long carrier lifetime have been proposed, such as formation of ferroelectric domains (6–9), Rashba effect (10–12), Hybrid organic–inorganic perovskites (HOIPs) are among the photon recycling (13), and large polarons (14–16). When the most promising materials for next-generation solar cells that HOIPs are replaced with all inorganic perovskites in the solar combine high efficiency and low cost. The record efficiency of cell architecture, the device can still function as a solar cell. This HOIP-based solar cells has reached above 22%, which is com- indicates that the photons excite electrons and holes out of the parable to that of silicon solar cells. HOIP solar cells can be inorganic metal halide atoms, which is consistent with the density manufactured using simple solution processing methods that functional theory (DFT) calculations that the corner interstitial can be drastically cheaper than the current commercial solar cations, whether organic or inorganic, do not directly contribute cell technologies. Despite the progress so far, the microscopic to the band-edge states (17). However, the efficiency of the mechanism for the high solar cell efficiency in HOIPs is yet to be purely inorganic perovskites is currently at ∼11% (18–20), which understood. Our study shows that rotation of organic mole- is far below 22% of HOIP-based solar cells. This suggests that cules in HOIPs extends the lifetime of photoexcited charge the presence of organic cation may be the key for achieving high carriers, leading to the high efficiency. This insight can guide solar cell efficiency. It is, however, yet to be understood how the the progress toward improved solar cell performance. organic cations enhance the efficiency. Author contributions: S.-H.L. designed research; T.C., W.-L.C., B.J.F., J.L., J.P.C.R., J.Y.P.K., Among the aforementioned microscopic mechanisms, three are C.M.B., L.W.H., D.Z., C.P., M.Y., Y.-M.C., J.J.C., and S.-H.L. performed research; T.C. and based on the role of organic cations. First, in the ferroelectric W.-L.C. analyzed data; and J.J.C. and S.-H.L. wrote the paper. domain theory, nanoscale ferroelectric domains are formed due to The authors declare no conflict of interest. alignment of organic cations (6–9). Such domains can spatially This article is a PNAS Direct Submission. separate the photoexcited electron and holes and thereby reduce Freely available online through the PNAS open access option. – their recombination. Second, in the Rashba effect theory (10 12), 1T.C. and W.-L.C. contributed equally to this work. the spin and orbit degrees of freedom of the inorganic atoms are 2To whom correspondence may be addressed. Email: [email protected] or jjc6z@virginia. coupled with the electric field generated by the organic cations. edu. This results in the electronic band splitting for different spins and This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. leads to an effectively indirect band gap and the prolonged lifetime. 1073/pnas.1704421114/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1704421114 PNAS | July 18, 2017 | vol. 114 | no. 29 | 7519–7524 Downloaded by guest on October 1, 2021 Furthermore, when caged in the environment of lead iodides, the molecules with different geometries will exhibit different character- istics of rotational motions and drive the HOIPs into different crystal structures. Upon cooling, CH3NH3PbI3 undergoes successive cubic- to-tetragonal-to-orthorhombic transitions (21, 22). On the other hand, HC(NH2)2PbI3 undergoes different structural-phase transi- tions depending on the cooling process (23); if cooled from room temperature, hexagonal structures are selected, whereas, if cooled from the high-temperature cubic phase, the hexagonal structures are entirely avoided. More importantly, the cubic phases of CH3NH3PbI3 and HC(NH2)2PbI3 have different molecular rota- + tional entropy; in CH3NH3PbI3 the CH3NH3 cation has preferen- + tial orientations (21, 22), whereas in HC(NH2)2PbI3 the HC(NH2)2 cation has nonpreferential orientations (23), leading to a much higher rotational entropy for HC(NH2)2PbI3 than for CH3NH3PbI3. To study the intrinsic relation between the charge carrier lifetime and the rotational entropy of the molecule in HOIPs, we have sys- tematically examined the charge carrier relaxation dynamics, crystal structures, and electronic band structures of the two HOIPs as a function of temperature spanning their structural phases, using time- averaged and time-resolved photoluminescence, neutron and X-ray scattering, and DFT analysis. We stress that, if any intrinsic corre- lation between the charge carrier relaxation and atomic properties of the system is present, it will manifest itself in the temperature dependences. Our results revealed that the band-edge carrier lifetimes in both HC(NH2)2PbI3 and CH3NH3PbI3 increase when the sys- tems enter from a structural phase with lower rotational entropy Fig. 1. Time-averaged photoluminescence (PL) spectra of HC(NH2)2PbI3. to another phase with higher entropy. For instance, the lifetime (A and B) HC(NH2)2PbI3 was first cooled from the room temperature Hex IT phase to the base temperature. Then, upon heating to 440 K (HEAT1), PL in HC(NH2)2PbI3 dramatically changes from ∼30 to ∼300 ns as, upon heating, the system enters into its cubic phase with high measurements were taken at 19 different temperatures. A shows a color rotational entropy. We stress that the
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