The Application Of Low Dimensional Inorganic Nanomaterials In Organic Photovoltaic And Photodetecting Devices

Organic electronics has been the focus of a growing body of investigation in the fieldsof physics and chemistry for more than60years. Organic semiconductors are expected tobe a potential alternative to the inorganic counterparts, due to their peculiar advantages,such as low cost, light weight, large-area fabrication, and so on. And the significant researchprogresses of organic semiconductors provide a feasible way to fabricate organicoptoelectronics devices, such as organic light-emitting diodes (OLEDs), organicphotovoltaic devices (OPVs), and organic photodetectors (OPDs). Especially, OLED hasbeen successfully applied for commercial use in display and interior illumination today.However, the performance of OPV and OPD has not been good enough yet for practicalapplication. Thus, there is a long way to go for the research of OPV and OPD. Conductingpolymer based optoelectronics devices, which belong to the category of organic electronicsdevices, are widely investigated in recent years due to their comparatively higherperformance and solution process. In this work, we investigated and identified the influenceof low dimensional inorganic nanomaterials on the performance of polymer solar cells(PSCs) and organic/inorganic hybrid ultraviolet photodetectors (UVPDs).Polymer solar cells:Even though the low-bandgap conjugated polymer donor materials emerge in anendless stream now, poly(3-hexylthiophene)(P3HT) is still one of the most popular donormaterials due to its high electron mobility and good absorption at the wavelength band of450-600nm. For the purpose of fundamentally enlarging the power conversion efficiency of polymer solar cells based on P3HT:PCBM bend, an further enhancement in the open-circuitvoltage of the devices is needed by breaking the intrinsic limitation of materials, besides theimprovement in the utilization of incident light.Firstly, NaYF4nanoparticles (NPs) was prepared by a facile solvothermal approachusing NaF and YCl3·xH2O as precursors, and polyvinylpyrrolidone (PVP) as a surfactant.The X-ray diffraction peaks of NaYF4sample are in good agreement with the data ofcubic-phase NaYF4nanocrystals. From the image of SEM and TEM, it can be estimatedthat the as-prepared NaYF4NPs have an average size of30-45nm, and a thin layer of PVPcapping on the surface of NPs can be observed clearly, which makes the NPs well dispersedin the blend of P3HT:PCBM in o-dichlorobenzene.Secondly, the NaYF4NPs were blended into the P3HT:PCBM solution with differentweight ratios, and the polymer bulk heterojunction (BHJ) solar cells were fabricated with astructure of indium tin oxide (ITO)/nano-crystal titanium dioxide(nc-TiO2)/P3HT:PCBM:NaYF4NPs/tungsten oxide (WO3)/silver (Ag). Comparing with theundoped devices, the best performance was achieved for the device with0.45wt%NPs,which exhibited a power conversion efficiency (PCE) of3.48%with short-circuit currentdensity (Jsc) of9.63mA/cm2, open-circuit voltage (Voc) of0.62V, and fill factor (FF) of58.3%under AM1.5G illumination with an intensity of100mW/cm2.A control experiment was performed by simply adding PVP (1mg/ml) into theformulation without the NPs. The result showed that the enhanced Voc could be attributedto the incorporation of PVP，which can form into charge transfer complex (CTC) withPCBM and have a deeper HOMO level (-5.93eV) than that of P3HT (-5.21eV). Therefore,the effective bandgap of BHJ can be enlarged due to the addition of PVP, leading to ahigher build-in potential. The Voc was eventually enhanced as a result. However, the devicegot a poor FF by simply adding PVP without NPs carriers.The devices without the hole selective layer of WO3were also fabricated with astructure of ITO/TiO2/P3HT:PCBM:NaYF4/Ag. The result showed that FF can be effectively improved by the introduction of NaYF4NPs, which can tune the internalmorphology of the active layer.The non-inverted standard cell with an architecture of ITO/PEDOT: PSS/P3HT:PCBM:NPs/LiF/Al was also fabricated. The results showed that the NaYF4NPs cappedwith PVP played a positive role in both inverted and non-inverted cells.Organic/inorganic hybrid ultraviolet photodetectors:UVPDs based on individual inorganic wide-bandgap semiconductors such as GaN, SiC,or diamond would require complicated fabrication processes, such as metalorganic chemicalvapor deposition (MOCVD) and molecular beam epitaxy (MBE), leading to a high cost. Inaddition, it’s hard to modulate the bandgap of inorganic semiconductors. Organicsemiconductors or their hybrids with inorganic semiconductors by contrast have provided acomparatively simple way for UV detection to achieve low cost and large-sale application.Since the absorption of organic materials can be easily tuned by tailoring their chemicalstructure, UVPDs based on these materials seem to possess more flexibility in realizing aspectral selective response.According to the above reasons, the high spectrum selectivity hybrid ultravioletphotodetectors were fabricated with poly(N-vinylcarbazole)(PVK) and a n-type2D TiO2nanobowls (NBs) array as the electron donor and acceptor, respectively.Firstly, the highly ordered2D TiO2NBs array was prepared on the surface of ITOsubstrate via a sol–gel method and using colloidal templates of polystyrene (PS) spheres.The influences of the TiO2sol concentration on the morphology of2D TiO2NBs array wereinvestigated. The finally prepared TiO2NBs have a diameter of about375nm, and theridges are about50nm high and100nm thick. The effective area of PVK/TiO2heterojuntion can be reasonably increased due to the large surface area of TiO2NBs.Secondly, the organic/inorganic hybrid UVPDs were fabricated with a structure ofITO/TiO2NBs/PVK/WO3/Ag. The ITO substrate acted as not only an electrode but also aoptical filter for short wavelength. The spectral selectivity of the device was achieved by theinterplay of the transmission of ITO and the absorption of TiO2NBs and PVK together. The response peak of the devices centered at330nm with a full width at half maximum of38.5nm, which is very close to the ultraviolet-B band (UVB,280–320nm). The bestperformance of the device is observed at330nm by applying a bias of-5V, correspondingto a photoresponsivity of8.14A/W under the illumination of144mW/cm2330nm UVlight.The reflectance spectra of the TiO2NBs array from the top and bottom were alsostudied. An obvious reflectance peak is obtained at330nm when the NB array isilluminated from the top, which will help PVK to efficiently reuse the330nm UV lightreflected by the Ag electrode. Meanwhile, there is a significant reflectance peak at375nmwhen the NBs are illuminated from the bottom, which can also contribute to the sharplong-wave photoresponse onset at375nm.