Internet-of-thing (IoT) is an assembly of devices that collect and share data with other devices and communicate via the internet. This massive network of devices, generates and communicates data and is the key to the value in IoT, allowing access to raw information, gaining insight, and making an intelligent decisions. Today, there are billions of IoT devices such as sensors and actuators deployed. Many of these applications are easy to connect, but those tucked away in hard-to-access spots will need to harvest ambient energy. Therefore, the aim is to create devices that are self-report in real-time. Efforts are underway to install a self-powered unit in IoT devices that can generate sufficient power from environmental conditions such as light, vibration, and heat. In this review paper, we discuss the recent progress made in materials and device development in power- and, storage units, and power management relevant for IoT applications. This review paper will give a comprehensive overview for new researchers entering the field of IoT and a collection of challenges as well as perspectives for people already working in this field.

Molecular thin films of N,N′-di-1-naphthalenyl-N,N′-diphenyl [1,1′:4′,1″:4″,1‴-quaterphenyl]-4,4‴-diamine (4P-NPD) have been demonstrated to function as efficient exciton blocking layers in organic solar cell devices, leading to improved device performance by minimizing exciton losses and by providing hole extraction selectivity. However, the exact mechanisms have been debated, as ultrathin thicknesses of less than 1 nm are required to observe optimized device performance improvements. In this work, we conduct photoelectron spectroscopy to gain information about core levels, HOMO/LUMO levels, and work functions for the hole extraction side of an organic solar cell device consisting of the small molecule tetraphenyldibenzoperiflanthene (DBP) as an electron donor and 4P-NPD for exciton blocking/hole extraction, the latter being in contact with the hole transport layer MoO x . Using in situ deposition and characterization, we demonstrate that a negative HOMO energy offset increases with 4P-NPD thickness on the DBP donor layer, which cannot account for the improvement observed in device performance. Investigation of the 4P-NPD/MoO x interface, on the other hand, reveals shifts of the electronic levels in 4P-NPD and a band alignment that favors hole extraction while blocking for exciton/electron leakage. This appealing behavior is enhanced for ultrathin 4P-NPD films of less than 1 nm. Thus, the exciton blocking/hole extraction behavior of 4P-NPD interlayers in organic solar cell devices is confirmed and understood from the detailed energy level alignment across both interfaces, as extracted from the in situ photoelectron spectroscopy studies.

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