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Organometal trihalide perovskites (OTPs) are arising as a new generation of low-cost active

materials for solar cells with efficiency rocketing from 3.5% to over 20% within only five years.

From “dye” in dye-sensitized solar cells (DSSCs) to “hole conductors” and “electron conductors”

in mesoscopic heterojunction solar cells, there has been a dramatic conceptual evolution on the

function of OTPs in photovoltaic devices. OTPs were originally used as dyes in Grätzel cells,

achieving a high efficiency above 15% which, however, did not manifest the excellent charge

transport properties of OTPs. An analogy of OTPs to traditional semiconductors was drawn after

the demonstration of highly efficient planar heterojunction structure OTP devices and the

observation of their excellent bipolar transport properties with a large diffusion length

exceeding 100 nm in CH₃NH₃PbI₃ (MAPbI₃) polycrystalline thin films. This review aims to

provide the most recent advances in the understanding of the origin of the high OTP device

efficiency. Specifically, we will focus on reviewing the progress in understanding

(1) the characterization of fantastic optoelectronic property of OTPs,

(2) the unusual defect physics that originate the optoelectronic property,

(3) morphology control of the perovskite film from fabrication process and film post-treatment,

(4) device interface and charge transport layers that dramatically impact device efficiency in the

OTP thin-film devices,

(5) photocurrent hysteresis,

(6) tandem solar cells and

(7) stability of the perovskite materials and solar cell devices.

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