Optimization of photovoltaic characteristics of organic solar cell by incorporating carbon nanotube

Abstract

Optimization of the photovoltaic characteristics can be a strong tool for designing organic photovoltaic (OPV) cell with high power conversion efficiency (PCE). This thesis advances the field of carbon nanotube (CNT) organic solar cells by studying the photovoltaic characteristics and parameters of the devices to understand the optimization process. In this work, two simple and efficient methods have been used for the extraction of the circuital parameters i.e. photo-current (Iph), series resistance (Rs), shunt resistance (Rsh), reverse saturation current (I0), and diode ideality factor (n) of a solar cell from current-voltage (I-V) curve. The parameters of solar devices including silicon solar cells and organic solar cells with tandem, CNT incorporated structures have been successfully extracted by using these extraction methods. For optimizing photovoltaic characteristics, origin of open circuit voltage (Voc) and short circuit current (Isc) has been thoroughly studied. Voc is interpreted with the energy offset between donor and acceptor level. Formula for estimating Voc has been modified considering the exciton levels. Quantum efficiency has been simulated considering absorption, exciton dissociation and charge collection efficiency for OPV materials. The simulated quantum efficiency summed over the AM 1.5 solar electromagnetic spectrum for each wavelength to find the Isc. Finally photovoltaic characteristics are optimized at different layers of OPV by incorporating CNTs. Semiconducting single wall nanotubes (s-SWNTs) of polychiral nature are used as donor with (6,6)-phenyl-C71- butyric acid methyl ester (PC71BM) as acceptor. By changing the chirality combinations, highest PCE of 2.96% is obtained for polychiral CNT with diameter ranges from 0.615-1.2 nm. s-SWNTs are used as the donor with (6,6)-phenyl-C61- butyric acid methyl ester (PCBM). By changing the chirality of SWCNTs, the optimized PCE of around 1% has been found for chirality (10,3) and (11,1). Then CNTs are mixed in both donor and hole transport layer of a thieno (3,4-b) thiophene/benzodithiophene (PTB7) and PC71BM solar cell. The overall efficiency for such a device is 8.18%, compared with the 7.40% of reference device without CNTs. Mixed CNTs are used in the active layer PTB7: PC71BM cell and overall PCE for sorted semiconducting SWNTs is 8.51%. Though unsorted SWNTs shows more efficiency gain than reference device but less than sorted SWNTs due to presence of metallic tubes. Boron (B) or Nitrogen (N) doped multiwall nanotubes (MWNTs) are used in the active layer and both could serve as efficient exciton dissociation centers and charge transfer channels. Overall efficiency for B doped device is 8.27% and for N doped device is 8.03%. Highest PCE of 8.98% is obtained by mixing selective chiral s-SWNTs and sorted SWNTs in the active layer of PTB7: PC71BM cell which is 21.35% more than PCE of reference device.