Abstract:
The thesis concerns with the development of polymer solar cells under the perspective of fabrication and efficiency enhancement. The contents are separated into 3 sections. In the first section, this work introduces an alternative coating technique based on solution process named rapid convective deposition to obtain thin film in the scale of nanometer. This technique provides several advantages including, uncomplicated process, controllable film thickness as well as suitable for large area fabrication. To approve that rapid convective deposition has potential to become an alternative thin film preparation technique, the specific properties of polymer thin films prepared by conventional spin coating and rapid convective deposition were compared. The results reveal that film thickness obtained by both rapid convective deposition and spin coating are in the range of 30-150 nm commonly required for polymer solar cells applications. The same film thickness obtained from both techniques exhibit the similar optical absorption, crystalline formation as well as surface morphology. In addition, rapid convective deposition can significantly reduce waste material over 90% at the fixed thickness and area leading to the significant decrease of manufacturing cost. It is apparent that rapid convective deposition can be applied as an alternative coating technique to fabricate polymer thin film nanometer scale.
For the second section, polymer solar cells based on Poly[N-9'-heptadecanyl-2,7- carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) : [6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM) were fabricated with multilayer structure of ITO/
PEDOT:PSS/ PCDTBT:PC₇₁BM/TiOₓ/ Al via rapid connective deposition. Optimization of each layer thickness is an essential factor determining solar cell performance. To optimize the thickness of PCDTBT:PC₇₁BM layers, the efficiency of polymer solar cells with different active layer thickness were investigated. The results show that the active layer thickness are varied from 62 nm to 140 nm. The optimum thickness of 70 nm was achieved with the maximum power conversion efficiency (PCE) of 7.30%. At the highest PCE, the surface morphology of active layer provides a favorable phase separation between PCDTBT and PC₇₁BM contributed to efficient charge separation and transportation to electrode. The results obviously show that rapid convective deposition can be employed to fabricate high-performance polymer solar cells.
In the third section, adding second electron acceptor into binary blend of active layer is considered as an alternative approach to improve polymer solar cells efficiency. The polymer solar cells based on PCDTBT as electron donor and PC₇₁BM as electron acceptor is improved through adding the second electron acceptor, Indene- C60 bisadduct (ICBA), into the active layer as function of ICBA weight ratio. The ternary blend device with 25% ICBA exhibits the maximum PCE of 4.27%. The results indicate that the enhancement of solar cell efficiency is attributed to (i) the proper phase separation of electron donor and acceptor in the active layer and (ii) the balance of electron and hole mobility in the device. This research demonstrates that adding the second acceptor into the active layer has enough potential to become an alternative approach to enhance the polymer solar cell efficiency