Extremely Low Dark Current, High Responsivity, All‐Polymer Photodetectors with Spectral Response from 300 nm to 1000 nm

Abstract

COMMUNICATIONbetter organic photodetector performance could be achieved if a low dark current density and a high EQE can be reached at the same time.In this paper, we present the fabrication and characterization of an all polymer photodetector sensing from 300 nm to 1000 nm with a calculated peak detectivity greater than 1.0 × 10 13 Jones in the NIR region, on the basis of the shot noise limit. The device also shows a respectable EQE of 27.7% at a wavelength of 850 nm under −0.5 V bias. Importantly, the dark current density is as extremely low as 0.64 nA cm −2 , which should be the best result among all wide spectrum response organic photodetectors reported so far. It can be seen that introducing a thin cross-linkable hole transporting/electron blocking layer greatly reduces the dark current density, while simultaneously preserving the photoresponse.A low-bandgap diketopyrrolopyrrole (DPP)-based polymer poly(diketopyrrolopyrrole-terthiophene) (PDPP3T), as shown in Figure 1 a, is utilized as the donor material, which has proven to be a promising candidate for organic fi eld-effect transistors (OFETs) and organic solar cells. [21][22][23][24] This polymer possesses a narrow-bandgap of 1.3 eV and nearly balanced hole and electron mobilities of 0.04 and 0.01 cm 2 V −1 s −1 . [ 21 ] As shown, it is able to cover the UV/visible/NIR spectral region and outputs a high photoresponse when incorporated with a (6,6)-phenyl-C71-butyric acid methyl ester (PC 71 BM) acceptor.Poly[ N , N ′-bis(4-butylphenyl)-N , N ′-bis(phenyl)-benzidine] (poly-TPD) is an effi cient hole transport material with a hole mobility of about 2.0 × 10 −3 cm 2 V −1 s −1 and has been widely used in polymer/quantum dot light emitting diodes as hole transporting layer due to its resistance to nonpolar organic solvents, such as toluene and p-xylene. [25][26][27] As reported, poly-TPD is also cross-linkable under 254 nm UV light exposure, [ 28 ] which is favorable to multilayer solution processing even with polar solvent, such as chloroform (CF) and o-dichlorobenzene ( o-DCB). In our devices, the cross-linked polymer is used as a buffer layer to collect the photogenerated holes and block the electron injection from the indium tin oxide (ITO) anode to the active layer under reverse bias.To verify the resistance of cross-linked poly-TPD fi lm to polar solvent, the absorption measurements were carried out for the poly-TPD fi lms deposited on fused silica substrates, both with and without UV irradiation. Prior to testing, the fi lms were washed by spin coating with o-DCB solvent and then dried on a hotplate. As shown in Figure 2 , after washing, the absorption remains as high as 92% for the UV light treated fi lm, while the value for the untreated fi lm is only 9.7%. The results suggest that poly-TPD fi lm gets cross-linked and shows excellent polar solvent resistivity after UV light exposure.

Long-Term Stable Organic Photodetectors with Ultra Low Dark Currents for High Detectivity Applications

“…The photodetector is biased at −2 V and exhibits a rise time t r (time for which device response rises from 10 to 90%) and a fall time t f (time for which sensor response decreases from 90 to 10%) of 7.7 and 10.9 μs respectively. The rise and fall times are one order of magnitude faster than in the case of fully-inkjet-printed organic photodetectors with the well-known P3HT:PC 61 BM blend10, and one order of magnitude slower than for organic photodetectors with PDPP3T:PC 71 BM blend having the metallic electrode evaporated under high vacuum25. The relatively long response of devices indicates the slow extraction process of photogenerated charge carriers, solution-processed electrodes (here PEDOT:PSS) being less conductive than metal electrodes deposited under high vacuum (Ag, Au, Al).…”

“…2c. Our results imply that the key element is to carefully select the materials with regard to their energetic levels, and additional interlayers such as MoO 3 35, poly-TPD25, or PVK36 are not always required. Reducing the number of layers makes the fabrication process easier, faster and cheaper.…”

“…The main problem comes from the definition of J min.lin . It is reported as the dark current density J D at a given reverse bias1131620253738, or as the theoretical noise current density J N , discussed further15. One can also find J min.lin defined as the dark current density at “0 V” bias which might correspond to the noise current of the experimental setup8911.…”

“…One can also find J min.lin defined as the dark current density at “0 V” bias which might correspond to the noise current of the experimental setup8911. The constant B is reported to be either 2018911131620253738 or 1015353940. From such different and not clear ways of determining LDR, it is evident that comparing OPDs can lead to ambiguous conclusions since the values can differ from 50 to 240 dB, depending on the method and the photodetector.…”

“…Where it is possible, the value is given at −2 V reverse bias. Plain numbers indicate references3891011121314151617181920212223242526.…”