Soumya Dutta

In this report we address the open circuit voltage (Voc) variation with temperature (T) in details by using TCAD numerical simulation and by developing physics based model. The proposed model is in good agreement with the TCAD simulation and is able to explain the experimentally observed (Voc) variation with different temperature and intensity.

Whether organic semiconductors, in their pristine form, are inherently intrinsic or involuntarily doped is an open question that is yet to be addressed. In this work, the source of charge carriers within pristine organic semiconductors, processed in a controlled inert ambience, is investigated using organic metal–insulator–metal capacitor (OMISCAP) devices. Top-gate and bottom-gate architecture of OMISCAP devices based on various organic semiconductors along with different combinations of metal–semiconductor junctions are demonstrated. The impact of the metal–semiconductor junction and the semiconductor thickness on the capacitance–voltage (C–V) characteristics of MISCAP devices is studied to ensure the role of contact-induced charge injection into the semiconductor, contrary to involuntary doping as the source of charge. The charge injection from the metal to the semiconductor is found to be the predominant source of charge carriers inside a pristine organic semiconductor rather than inadvertent doping. The findings in this report are further substantiated using TCAD device simulation.

Electroabsorption (EA) measurements can be used to identify the type of excitons contributing to the absorption spectra of semiconductors. However, the shape of the EA spectrum may vary depending on the mode of measurement due to the optical interference effects. Analysis without considering these effects may lead to erroneous conclusions. In this work, we present EA measurements and analysis for reflection mode measurements considering optical interference effects. We compared the inferences with transmission mode measurements and discuss the limitations. We identified the nature of excitons associated with each transition in the absorption spectrum of poly[(2,5-bis(2-hexyldecyloxy)-phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c]-[1,2,5]-thiadiazole)] thin film from EA measurements. The bands at 1.89, 2.05, and 2.27 eV had a mixed nature consisting of charge transfer and Frenkel characteristics. Of these, the band at 2.05 eV showed the strongest charge transfer characteristic. From thickness dependent measurements, we showed that the interference effects increase with the thickness of the semiconductor layer. The nature of excitons, however, could still be deduced qualitatively from reflection mode EA measurements.

In an organic semiconductor optoelectronic device, the built-in field within the active layer is typically determined by the difference in contact potentials of the device. However, the presence of space charges and trap states contribute to the electric field within the thin film. Depending on the maximum applied forward voltage, the trap states can be charged, inducing hysteresis in the optoelectronic response of the system. In this work, we investigate the electric fields inside organic photovoltaic device structures, in the presence of traps, using electroabsorption (EA) spectroscopy. Comparing simulations with our experimental results, we explained the origin of hysteresis in the electroabsorption signal as a function of applied DC bias. We solved Poisson's equation to estimate the densities of trapped carriers in the active layers. The filled trap densities in poly(3-hexylthiophene-2,5-diyl) (P3HT) and poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c]-[1,2,5]thiadiazole)] (PPDT2FBT) were found to be ∼ 1 × 10 17 and ∼ 6 × 10 16 cm - 3, respectively. From the transient EA measurements, the estimated values of energies of the trap states with respect to the HOMO level were 0.82 and 0.76 eV in P3HT and 0.70 and 0.64 eV in PPDT2FBT, which indicated the presence of midgap traps in these organic semiconductor thin films. Such trap induced changes in the internal fields within the active layers, affect the mobility and carrier transport in the organic optoelectronic devices. The midgap traps lead to exciton quenching and also act as non-radiative recombination centers, resulting in reduction in luminescence efficiency of the active layers.

A tetramethoxyfluorene derivative (TMD) based small molecule (C53H54O4) has been used as a donor in a bulk-heterojunction donor-acceptor blend system to realize room temperature solution-processed ultraviolet (UV) photodetector. A systematic opto-electronic study to optimize the weight ratio of the blend of TMD and a common acceptor [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) is presented in this report. The fabricated detector is found to cover the most harmful UVA region while transmitting nearly the entire visible spectrum of light. The responsivity of 6 × 10−2 A/W, specific detectivity of 9.36 × 1011Jones and photosensitivity of 105, which are on par with the existing UV photodetectors reported in the literature, are obtained at the wavelength 394 nm under zero bias condition.