Acceptor materials for organic solar cells

Abstract

Renewable energies are considered as the energies of the future because of their small impact on the environment. Among them, solar energy is probably the most promising one because it can be directly converted into electricity using photovoltaic modules. Organic photovoltaics, that is solar cells with an organic or polymer material as photoactive layer, represent an attractive future technology as large-scale and low-cost green energy source. In organic solar cells, donor and acceptor materials are combined in the active layer to convert light in to electrical power. While new donor materials have received considerable interest and have strongly improved the efficiency of organic solar cells in the last decade, much less attention has been given to new acceptor materials. This is the main topic of the research described in this thesis. Chapters 2 and 3 aim at establishing design rules for the structure and the synthesis of acceptor polymers for organic solar cells. In a first attempt, three polymers using different combinations of electron-deficient aromatic heterocylcles (quinoxaline, benzothiadiazole and thienopyrazine) have been synthesized and characterized. The electrochemical properties of the materials reveal that it is not possible to use them as acceptor polymer. However, by correlating the frontier orbital energies to the chemical structure, a new perspective towards the design of acceptor polymers could be established. Based on the improved design, three new polymers were then synthesized by alternating one thiophene ring with one electron-deficient unit. This new design led to materials with suitable electrochemical properties. The new polymers were tested as acceptor material in solar cells with poly(3-hexylthiophene) (P3HT) as the donor, resulting in power conversion efficiencies up to 0.22% in simulated solar light. Near steady-state photoinduced absorption spectroscopy revealed that charge separation in these blends is in competition with charge recombination. In addition, the incomplete exciton dissociation prevents from achieving efficient charge generation and the low electron-mobility in the acceptor polymers hampers charge collection. A number of polymers bearing diketopyrrolopyrrole (DPP) units in the main chain display high electron mobilities and, hence, the electron-deficient DPP unit is an interesting unit for acceptor polymers. In Chapter 4, the synthesis of three new DPP-based polymers (PA, PB, and PC) were described and have been used as acceptor materials in solar cells. Among them, one shows higher electron mobility than the polymers described in Chapter 3. Photoinduced absorption spectroscopy shows that in blends of P3HT with PA or PB charge formation is limited, while for the P3HT:PC blend photogenerated charges recombine into the PC triplet state before they can separate, unless assisted by a reverse electric field. The materials show power conversion efficiencies up to 0.36%. In Chapter 3 and 4, the acceptor polymers we used had a relatively high reduction potential which caused an incomplete exciton quenching. To overcome this problem, the goal of the research described in Chapter 5 is to explore new molecular acceptor materials presenting low reduction potentials. Two molecules have been synthesized: a soluble indigo dye and an isoindigo dye, both exhibiting a reduction potential at -1.25 V vs. Fc/Fc+. Near steady-state photoinduced absorption spectroscopy revealed the formation of long-lived free charges in thin films blends of both dyes with P3HT. These observations suggest that the dyes can be very attractive acceptor material for bulk heterojunction solar cell. However, this promise is not fulfilled because both dyes fail to have appreciable electron mobility. In Chapter 6, three new DPP-based polymers with optical band gaps varying from 1.45 to 1.25 eV have been synthesized. All three polymers show excellent hole mobility, up to 0.57 cm2/Vs. The materials were applied in bulk heterojunction solar cells with [70]PCBM as acceptor. Optimization involving the use of co-solvents has led to power conversion efficiencies up to 3.3% with external quantum efficiencies up to 50% in the low energy region of the spectrum.