Application of Electrospray Deposition for Efficient and Stable Organic Photovoltaic Devices''

Organic solar cells can be processed from solution at room temperature and have been considered as an alternative photovoltaic technology, thus having been receiving considerable interest in the field of optoelectronics. Most importantly, their light weight, low cost, easy fabrication over large area and flexible devices are appealing to future organic electronics market for both economic and environmental reasons, as printing and coating technologies can be incorporated during the material process. In this work, we have investigated the application of electrospray deposition technique towards achieving high efficiency and stable organic solar cells. The main advantages of electrospray deposition (ESD) over other common deposition processes are: suitability for less soluble materials, lack of solvent evaporation effects, possibility for realizing multilayered devices and efficient usage of the materials. Among them, multilayered thin film growth is most important advantage of ESD which is difficult to achieve while using conventional solution process techniques like spin coating, dip-coating, screen printing, ink jet printing or spray coating etc, as dissolution of the bottom organic layer during the growth of top layer can not be avoided. In this systematic study, a number of issues concerning ESD have been tackled, from the optimization of the deposition parameters to eventually realizing efficient and stable organic photovoltaic devices. The first part of thesis comprised the electrical characterization of the ESD deposited sandwich structure Schottky diode using poly(3-hexylthiophene)(P3HT) in order to optimize various electrospray parameters. The results provide a description of various charge carrier transport parameters along with the determination of field dependent hole mobility and electric field activation parameter from the fitting of the experimental I-V curves. The hole mobility was determined as 2.7x10−7cm2/Vs and the value of electric field parameter is 3.45x10−4/m0:5/V0:5 which are comparable to the literature values. A clear evidence of the deep localized states within the band gap was observed via forward and backward I-V measurements. Following this promising outcome, we first fabricated and characterized hybrid photovoltaic devices from dispersed nanoparticles (TiO2) and polymer blend solution, results of which will be presented in the next chapter. From the Grazing incidence X-ray diffraction (GIXRD) measurements, it was found that increasing thermal annealing improves the self organization of P3HT chain in the film. However, after optimization processes such as tuning the ratio, solvent soaking and temperature annealing, efficiency of around 0.0002% was achieved. The main reason for the lower performance of the hybrid devices is due to the aggregation of the nanoparticles in the blend films, which was observed by atomic force microscopy (AFM) and scanning electron microscopy (SEM) measurements. In the following chapter, I will demonstrate the Bulk heterojunction organic photovoltaic approach by replacing the electron accepting nanoparticles with [6,6]-phenyl C61 butyric acid methyl ester (PCBM). A number of parameters in term of thickness, donor acceptor ratio of the materials and annealing temperatures were optimized. The results show that ESD devices performed rather well at lower temperature (below 130 0C) even better than the conventional spin coated sample with the same thickness and donor-acceptor ratio. At the optimum temperature of 1250C, the maximum power conversion efficiency (PCE) was 1.4 % for the spin coated samples, while ESD samples have higher PCE of 1.6 %. Further enhancement the PCE to 2.17% was observed in case of multilayered devices in which the active layer (P3HT:PCBM blend) is sandwiched between the very thin stacking layers of P3HT and PCBM. In the last part of the thesis, I focused on realizing the inverted OPV device architectures using ESD technique, where we replaced unstable hole transport layer poly(3,4- ethylenedioxythiophene) doped with polystyrene sulfonic acid (PEDOT:PSS) and Al metal contacts with TiOx/ZnO as electron extracting layers and MoO3/Ag as hole collecting contacts respectively. Long term stability and performance were compared to the direct architecture of OPV devices. The TiOx based inverted solar cell shows the maximum power conversion efficiency (PCE) of 1.30% from trilayer devices comparing to the PCE of 0.82% from single layer of the same materials at annealing temperature of 1100C for 10 minutes. Further improvement in device efficiency of inverted OPVs was demonstrated by employing ZnO electron collecting layers instead of TiOx which has better transparency and higher mobility. The power conversion efficiency (PCE) of 1.48% from the trilayer based OPV comparing to PCE of 0.83% from the single layer device at thermal annealing of 1200C was achieved. The stability of the inverted structure devices were tested under ambient condition with continuous illumination which shows much better device performance in longer time duration as compare to direct structure devices. In summary, application of ESD deposition for organic solar cells with different device configurations was systematically studied in this thesis and promising high efficiency and stability were obtained, even comparing to the common deposition method. This places the technique in a front position as an alternative method for organic thin film growth and device fabrication.