Organic Photodetectors in Analytical Applications

Eeshita Manna, Teng Xiao, Joseph Shinar, Ruth Shinar
2015 Electronics  
This review focuses on the utilization of organic photodetectors (OPDs) in optical analytical applications, highlighting examples of chemical and biological sensors and lab-on-a-chip spectrometers. The integration of OPDs with other organic optical sensor components, such as organic light emitting diode (OLED) excitation sources and thin organic sensing films, presents a step toward achieving compact, eventually disposable all-organic analytical devices. We discuss recent advances in developing
more » ... ances in developing and integrating OPDs for various applications as well as challenges faced in this area. Keywords: organic photodetectors; organic electronics in analytical applications; integrated sensors; lab-on-a-chip; spectrometer-on-a-chip layers include thermal evaporation (for π-conjugated organic small molecules), spin-coating (typically for polymers), spray-coating, screen printing, micro-printing, and roll-to-roll processing [16] [17] [18] [19] . Importantly, the optical and electronic properties of an organic material can be tuned to make it compatible with a specific application [20] . There are generally four stages of converting light into electric current in photovoltaic devices. An excited electron-hole pair (exciton) state is formed by photons absorbed by the active layer. The generated excitons diffuse to an interface where charge separation occurs. The separated charges travel to the corresponding electrodes, where they are collected. The efficiency of an OPD corresponds directly to the number of created charges that are collected at the electrodes and this number depends on the fraction of photons that are absorbed, the fraction of excitons that dissociate to electrons and holes, and the charge collection efficiency at the electrodes. The OPD performance is enhanced by optimizing any of these factors. The spectral response of detectors can be tuned mostly by material choice and by adjusting the thickness of the resonant cavity sandwiched between two metal electrodes, using an optical spacer at the anode [21] . For instance, PTB7 (polythieno [3,4-b]-thiophene-cobenzodithiophene)-based OPDs absorb more in a longer wavelength range (550-750 nm) than the well-known P3HT-based OPDs. In addition to the material choice, there are also several novel methods utilizing light trapping or plasmonic effects to maximize absorption in the active layer to enhance exciton formation [22] [23] [24] [25] . The exciton diffusion efficiency depends on where the excitons are formed and whether they can diffuse to the donor/acceptor (D/A) interface, where charge dissociation occurs. Since the exciton diffusion length is much smaller in organic materials than in their inorganic counterparts, bulk heterojunction (BHJ) structures are mostly used to ensure exciton formation very close to the D/A interface [26] . The charge collection highly depends on the carriers' mobility within the transporting layers. Reducing the density of deep traps, which act as recombination centers, whether originating from impurities or structural disorder, can improve carrier mobility. Indeed, carrier mobility was shown to increase with crystalline structure of the organic semiconductor formed during annealing [27] . Specifically, attributes that are important in characterizing OPDs include responsivity, external quantum efficiency (EQE)/gain, spectral response, dynamic range, response speed, response linearity, the noise equivalent power (NEP), detectivity, and stability. The ratio of the current or voltage output signal to the input power is defined as the responsivity, which improves with increasing EQE. A constant responsivity within a certain wavelength range or a linear responsivity is highly desired, so that the output signal can be predicted based on a given power input. In OPDs the EQE is typically less than 100%, hence they typically have no internal gain. High gain in OPDs was recently reported. In an OPD of the structure a ITO/poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)/fullerene (C60)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)/Al OPD [28] . The high gain was explained by a trapped hole-enhanced electron-injection process, where the photo-generated holes get trapped at the interface of the hole transport layer (PEDOT:PSS) and the active layer component (C60). The high density of trapped holes reduces the electron injection barrier via band bending at the interface, which leads to secondary electron injection from the hole transport material to the active layer. A buffer layer to strongly reduce the dark current and increase the detectivity was inserted between the PEDOT:PSS and the C60 layers, but it eliminated the gain.
doi:10.3390/electronics4030688 fatcat:o46ebuts6jcfflnj4yy3co7zku