Organizing electroactive materials for organic solar cells

The US Government's 2008 International Energy Outlook forecasts global energy consumption of nearly 700 quadrillion British thermal units (BTUs) in 2030, a worryingly high figure compared to the 462 quadrillion BTUs consumed in 2005.¹ Therefore, the need to resort to cleaner and more abundant sources of energy is urgent, no matter one's political views. Solar energy is perhaps the cleanest and most abundant of the energy sources available. To date, silicon-based solar cells monopolize the photovoltaic market. However, their main drawback is that the associated production costs are very high, achieving cost parity with fossil-fuel energy only after 5–7 years of operation.²

Flexible and lightweight organic solar cells (OSCs)^3–5 can be fabricated using printing polymer technologies making them a cheaper alternative to inorganic photovoltaics. On the downside, they typically exhibit efficiencies below 5%. This is mostly due to charge recombination, where negatively and positively charged species recombine in the active layer before the charges manage to proceed to the electrodes. The nanometric organization of p- and n-type materials (dominated by holes and electrons, respectively) in OSCs should facilitate charge mobility and prevent recombination. We are now focusing our research on the synthesis of supramolecular systems between π-extended (including extended π conjugation) tetrathiafulvalene (TTF) derivatives and fullerenes, with an ultimate aim of more efficient OSCs (see Figure 1).

π-extended TTF (exTTF) analogs, in which the 1,3-dithiole rings are connected covalently to a conjugated core, maintain the TTF electron-donor capabilities, while also exhibiting improved light-absorption properties and large and curved surfaces. We recently succeeded in constructing receptors for fullerenes based on two different exTTF analogs, which bind fullerenes with an association constant of K_a=10³–10⁴/molar.^6–9 In collaboration with the groups of Dirk Guldi and Enrique Ortí, we showed that intracomplex photoinduced electron transfer occurs in solution.¹⁰

Figure 1. From molecular recognition to self-organized electroactive materials for organic solar cells.

Exploiting the unique combination of supramolecular and electronic complementarity of the exTTF-fullerene couple, we embarked on synthesizing different nanometric architectures through self-assembly. We constructed linear supramolecular polymers,¹¹ supramolecular dendrimers,¹² and covalent dendrimers capable of associating fullerenes.¹³ A thorough experimental approach, including variable-concentration and variable-temperature experiments using advanced spectrometric techniques unambiguously demonstrated the formation of self-assembled structures.

We plan to extend our approach to derivatives that absorb light more efficiently in the visible range.⁷ More importantly, we are currently taking the first steps to assemble OSCs based on these materials. These should help us design the next generations of self-assembled molecular structures, which we hope will constitute a small step along the path to cheaper, cleaner energy production.

The contributions of the co-authors on the original publications are gratefully acknowledged. Emilio Pérez thanks the Spanish Ministry for Science and Innovation (MICINN) for a Ramón y Cajal fellowship, which is cofinanced by European Union social funds.