Near infrared (NIR) photodetectors (PDs) refer to light responsive device that convert NIR optical signals into electric signals, which is central to modern science and technology due to their great application, such as image sensing, chemical and biological detection, remote control and so on. Currently, NIR PDs market mainly dominated by photodiodes based on crystalline inorganic semiconductors, such as silicon, Ge and InGaAs based PDs. However, these inorganic PDs require high driving voltage (> 40V) and some of them need to be operated at very low temperature, which substantially limits their application. Thus, it is of important development of room temperature operated solution-processed NIR PDs. In this thesis, we report our studies in the development of room temperature operated NIR PDs.
In Chapter 1, a brief introduction of PDs and its applications is given, which specially introduces the important of low cost and room temperature processed PDs compared with inorganic PDs.
Chapter 2 mainly presents a broaden overview of NIR PDs, which includes basic operation mechanism of PDs, device performance figure of merits of PDs and recent advancement in NIR PDs. Current challenges for fabrication of solution-processed near-NIR PDs are reviewed and the methods to circumvent existed problems are proposed.
In Chapter 3, we report the fabrication of broadband (PDs) with high responsivity and detectivity from the ultraviolet (UV) to the mid-infrared (MIR) region (400 nm-2500 nm) through a novel device structure. The hole-trap assisted photomultiplication (PM) effects
successfully circumvents the low coefficient drawback of PbSe CQDs in the NIR to MIR region. Then we applied a layer of water/alcohol soluble n-type polymer poly[(9,9-bis(3'-(N,N-dimethylamino)propyl)- 2,7-fluorene)-alt-5,5'-bis(2,2'-thiophene)-2,6-naphthalene- 1,4,5,8-tetracaboxylic-N,N'-di(2-ethylhexyl)imide] (PNDIT-F3N), which offers significant hole injection resistance for ensuring a low dark current. Under illumination, electrical conductivity of the polymer is dramatically increased due to the photo-induced self-doping process of the polymer, which reduces the electron transfer resistance and guarantees decent external quantum efficiency. Ultimately, we obtained detectivity of over ~1012 Jones in the visible and NIR region, and 4×1011 Jones (1 Jones=1 cm Hz1/2w-1) in the MIR region at small reverse bias (-1V). These results outperform those from the commercialized NIR and MIR PDs, which shows the PbSe CQDs has great potential as substituents for uncooled, inexpensive MIR PDs.
In Chapter 4, we report solution-processed broadband PDs derived from a bilayer CH3NH3PbI3/PbSe structure. In order to extend photo response of CH3NH3PbI3 in NIR region, PbSe quantum dots, which possesses a deep low LUMO (-4.76 eV), was developed on the top of CH3NH3PbI3. By formation the inter-absorption between HOMO of CH3NH3PbI3 (-5.4 eV) and LUMO of PbSe (-4.76), the incident photons lower than 2000 nm could be absorbed by CH3NH3PbI3 at the interface between CH3NH3PbI3 and PbSe. As a result, the responsivities of 400 mA/W and detectivities of 7 × 1011 Jones are obtained in 1350 nm from CH3NH3PbI3/PbSe bilayer structure. It is the highest reported performance at 1350 nm and at 2400 nm from CH3NH3PbI3 based PDs.
In Chapter 5, we present solution-processed polymer PDs with spectral response ranging from 350 nm to 2500 nm. The low bandgap donor-acceptor (D-A) conjugated copolymer contributes to the IR photo-response. Implementation of bulk heterojunction device structure with polypyrrole as an interfacial layer to match the energy level of low bandgap D-A conjugated copolymer and Ba/Al bi-cathode to compress dark current gave rise to a large photocurrent to dark current ratio, which resulted in detectivities greater than 1011 Jones (1 Jones =1 cmHz1/2W-1) over the wavelength ranging from 350 nm to 2500 nm. Thus, our finding of utilization of bulk heterojunction composite consisting of a D-A low bandgap polymer blended with a fullerene derivative provides a facile way to detect IR radiation, indicating that broadband polymer PDs are a promising photoelectronic technology in the future.
In Chapter 6 and Chapter 7, we briefly summarize our works and present some of novel ideas for realizing high performance broadband NIR PDs.