Performance Analysis of Bulk Heterojunction Organic Solar Cells

Abstract

Organic semiconductor materials are of great interest due to their

exible nature and huge potential to act as an alternative to their inorganic counterparts for certain applications. The conductivity in them is originated from delocalization of bonded electrons in organic molecules and the existence of conjugated systems. Solar cells are im- portant devices in which the organic materials can be used. There are various morphologies of organic solar cells among which bulk hetero- junction is one of the best. Performance of the bulk heterojunction organic solar cells can be improved using better materials and a suit- able device structure. ITO/PEDOT:PSS/P3HT:PCBM blend/Al de- vice is simulated using Metal-Insulator-Metal model. Various device engineering are applied to analyse the performance and hence the opti- mization of the device parameters. The active layer thickness is one of the important parameter of organic photovoltaic cells. Generally it is in the order of few tens of nanometres and hence they are also termed as thin lm solar cells in literature. Since the coe cient of optical ab- sorption of the active layer materials used in fabricating cells is nite, the layer thickness decides the percentage of photons absorbed by the cell, complex phenomenon such as exciton dissociation, exciton recom- bination, carrier recombination and hence the overall performance of the cell. The P3HT:PCBM bulk heterojunction solar cell is simulated in Silvaco TCAD using simpli ed metal-insulator-metal model. The photocurrent, open circuit voltage, ll factor and e ciency are studied for the thicknesses from 20 nm to 200 nm. As a result of the increase in percentage of photons absorbed with the higher thickness, the sig- ni cant increase in photocurrent is observed. On the other hand ll factor is decreased as the higher thickness facilitates the carrier recom- bination probability. There is gradual increase in open circuit voltage with the thickness and it is justi ed using diode analogy of organic solar cell. The devices with di erent electrode materials are simulated and the e ect of change in work functions of electrodes on the per- formance of bulk heterojunction devices is analysed. The electrode - active layer interfaces play very important role in the behaviour of the devices. The devices with di erent values of cathode-active layer and anode-active layer interface energy level o set are simulated and their current-voltage characteristics are analysed for the performance. The best case is observed in case of zero interface energy level o sets in case of the electrodes, anode and cathode. Fabrication steps for bulk heterojunction organic solar cells are discussed; devices are fabricated and characterized for dark characteristics