Muons penetrate much further than X-rays, they do essentially zero damage, and they are provided for free by the cosmos.
Image credit: Science Source/Digital Globe.
Edited by Peidong Yang, University of California, Berkeley, CA, and approved May 26, 2017 (received for review March 16, 2017)
Significance
Hybrid organic–inorganic perovskites (HOIPs) are among the most promising materials for next-generation solar cells that combine high efficiency and low cost. The record efficiency of HOIP-based solar cells has reached above 22%, which is comparable to that of silicon solar cells. HOIP solar cells can be manufactured using simple solution processing methods that can be drastically cheaper than the current commercial solar cell technologies. Despite the progress so far, the microscopic mechanism for the high solar cell efficiency in HOIPs is yet to be understood. Our study shows that rotation of organic molecules in HOIPs extends the lifetime of photoexcited charge carriers, leading to the high efficiency. This insight can guide the progress toward improved solar cell performance.
Abstract
Long carrier lifetime is what makes hybrid organic–inorganic perovskites high-performance photovoltaic materials. Several microscopic mechanisms behind the unusually long carrier lifetime have been proposed, such as formation of large polarons, Rashba effect, ferroelectric domains, and photon recycling. Here, we show that the screening of band-edge charge carriers by rotation of organic cation molecules can be a major contribution to the prolonged carrier lifetime. Our results reveal that the band-edge carrier lifetime increases when the system enters from a phase with lower rotational entropy to another phase with higher entropy. These results imply that the recombination of the photoexcited electrons and holes is suppressed by the screening, leading to the formation of polarons and thereby extending the lifetime. Thus, searching for organic–inorganic perovskites with high rotational entropy over a wide range of temperature may be a key to achieve superior solar cell performance.
Footnotes
↵1T.C. and W.-L.C. contributed equally to this work.
- ↵2To whom correspondence may be addressed. Email: shlee{at}virginia.edu or jjc6z{at}virginia.edu.
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., 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 W.-L.C. analyzed data; and J.J.C. and S.-H.L. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1704421114/-/DCSupplemental.
Freely available online through the PNAS open access option.
Origin of long lifetime of charge carriers in HOIP
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