DEVELOPMENT AND EFFICIENCY STUDIES OF ORGANIC SOLAR CELLS

Abstract

Alhough significant progress has been made in organic solar cells based on conducting polymer-fullerene mixture as active layer, however the efficiency of current bulk heterojunction (BHJ) solar cells still does not warrant commercialization. A lack of understanding makes targeted improvement troublesome. The main theme of this thesis is to modify the morphology of active layer in inverted BHJ solar cells, relating the device performance to basic physics and material properties such as the preferential crystallization of donor polymer. The first part of the dissertation focuses on introduction to the various aspects related to the topic, literature perusal, basic principles of photovoltaic power conversion with semiconductors (both inorganic and organic). The working principle of organic bulk heterojunction solar cells and a critical review of the state of knowledge are also given in this chapter. In chapter 2, the details including principles of the various analytical tools employed for the preparation and characterization of the organic solar cells are summarized. Chapter 3 describes the effect of different cosolvents on the photovoltaic performance of an inverted organic solar cell. The effect of varying polymer crystallinity, morphology, and optical property which is produced by addition of different cosolvents in to the poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C61-butyric acid methyl ester (PCBM) solution, on the performance of an inverted polymeric solar device is considered in detail. Photovoltaic devices primed with cyclohexanone cosolvent showed the best performance with power conversion efficiency (PCE) reaching a value of 3.01±0.05%. This increase in efficiency of the inverted device is related to a combined effect of ordered P3HT crystallite growth, and precise size and phase separation of domains with the addition of cyclohexanone cosolvent. Chapter 4 focuses on optimization of processing conditions of cosolvent addition in the active blend of inverted organic solar cell. In this work, the concept of solvent induced crystallization of donor poly(3-hexylthiophene) (P3HT) polymer was extended to the photoactive blend of inverted organic solar cells. An increase in power conversion efficiency of an OSC device from 2.74±0.05% to 3.01±0.05% was realized by addition of an optimized concentration of cyclohexanone cosolvent and ageing period of 2 h. This improvement of 10% in the efficiency of inverted device with cyclohexanone addition is related to the increase in the current density and fill factor of the device. Increase in the crystallinity of P3HT for efficient photoabsorption and commensurate vertical concentration gradient observed in P3HT fractions of the blend for efficient hole transport is possibly responsible for the betterment of the photovoltaic parameters in modified device. In Chapter 5, investigation on processing pathway for cosolvent addition in active layer preparation of inverted organic solar cell is reported. The cosolvent addition pathway was changed from conventional blended mixture addition (one step addition) to two steps individually mixed addition. Such changes in the processing pathway led to an improvement of PCE from 3.01±0.05 to 3.39±0.05%. Our study indicated that improvement in the device efficacy is related to improved characteristic surface morphology as seen in AFM images, proven by improved crystallization of donor polymer supported by the UV-Visible spectroscopy, GIXRD and PL data. A fine coverage of homogenously distributed well grown P3HT crystallites was evident ~ viii ~

in the individually mixed cosolvent casted film promoting the enhanced photo absorption and charge transport properties which benefited the device in terms of increased short circuit current density (Jsc) and power conversion efficiency. GIXRD results also support more favourable upright concentration gradient in the individually mixed film which eases the charge transport. A new approach of morphology modification, i.e., Ternary solvent mixture approach to control the P3HT:PCBM blend morphology in inverted organic solar cells is been discussed in chapter 6. In this work, bulk heterojunction blend of poly(3-hexylthiophene):[6, 6]-phenyl C61-butyric acid methyl ester (P3HT:PCBM) which is prepared using a ternary solvent mixture is utilized as the active layer of an inverted organic solar cell. Ternary solvent mixture consisting of a good solvent ortho-dichlorobenzene and marginal solvents cyclohexanone and toluene offers limited solubility to the P3HT component of blended mixture. However, a decrease in power conversion efficiency from 2.74±0.05% in the unmodified OSC device to 2.64±0.07% in ternary solvent mixture modified device is observed. This decrease in the efficiency of mixed cosolvent modified active layer is due to less efficient photoabsorption by randomly crystallized P3HT domains.