New Photovoltaic Concept: Liquid‐Crystal Solar Cells Using a Nematic Gel Template

Abstract

Liquid-crystal (LC) gels are normally used as insulating, scattering media in LC displays. Organic solar cells generate electrical power by the separation of photocreated electrons and holes at the interface between electron-donating and electron-accepting organic semiconductors. We combine these two technologies to create a new type of plastic solar cell. The efficiency of solar cells can be improved by enlarging the interface area to maximize charge separation and by vertically separating the two materials to provide different paths for electrons and holes to the electrodes. We use a new electron-donating, visible-light absorbing, LC gel to form a surface with nanometer-sized grooves with a large interface area to an overlying electron-accepting layer. This forms a photovoltaic cell, which exhibits a monochromatic power conversion efficiency of 0.6 % at an incident intensity of 45 mW cm. Improvements will provide low-cost, robust solar cells compatible with plastic electronics and roll-to-roll processing. Most organic photovoltaics use either main-chain conjugated polymers, often in conjunction with fullerenes, or low-molecular-weight glasses, achieving power-conversion efficiencies up to 4.2 %. Charge separation is achieved in solar cells by ionization of the photogenerated exciton at a heterointerface between electron-donating material with a low ionization potential (IP) and electron-accepting species with a high electron affinity (EA). The diffusion length of the exciton before recombination is of the order of 100–200 A, so that distributed, rather than planar, heterointerfaces have been used to maximize charge separation. These have been formed by phase-separated blends of electron-donating and electronaccepting organic materials. The bulk-distributed heterojunctions are formed randomly and do not guarantee a continuous pathway for charge carriers to the electrodes. The required vertical segregation of the blend materials can be obtained by surface treatment or in diffuse bilayer photovoltaics. A blend of an electron-donating columnar LC, hexabenzocoronene, and an electron-accepting perylene dye vertically segregates spontaneously to give an efficient photovoltaic effect. Unfortunately, the spatial scale of the phase separation cannot be controlled reproducibly in these devices. Another problem for phase-separated configurations is poor stability of the domain size lowering the device efficiency. To date the photovoltaic effect has only been demonstrated with calamitic LCs via an ionic mechanism, although electronic photoconductivity has been well researched. Polymerizable LCs, with two polymerizable groups attached through flexible aliphatic spacers to an aromatic core, are attracting interest as organic semiconductors for electroluminescence and organic transistors. Their advantages include spontaneous self-assembly, relatively high charge mobility (1 × 10 cm V s), easy deposition by spin-coating or ink-jet printing and lithographic photopatterning to form multilayer polymer networks. We now report the use of reactive mesogens in a novel way to demonstrate a new approach to photovoltaics. Liquid-crystalline gels are formed from a homogeneous mixture of a reactive mesogen and a non-polymerizable LC. Polymerization results in controlled phase separation of the two components and the formation of a polymer network matrix around the nematic droplets. To date, transparent LC gels have typically been used in direct-view reflective displays, switchable polarizers, directional reflectors, and so forth. Polymerization-induced phase separation of a homogeneous mixture of a LC and a polymer-forming material brings the concept of a paintable display closer. A key step in our new approach is the use of mixtures of hole-transporting reactive mesogens with a low IP and analogous LCs with the same aromatic core to form an electron-donating LC gel. An ideal LC composite photovoltaic is illustrated in Figure 1. It has a vertically separated, large-area interface between electron-donating and electron-accepting layers providing complete pathways for transport of holes and electrons. An electron-blocking polymer network of the electron-donating reactive mesogen is deposited first on an electrode surface by spin-coating from solution and then crosslinking in order to provide a high open circuit. A mixture of the same electron-donating reactive mesogen and the corresponding non-polymerizable analogue are then deposited in a similar C O M M U N IC A TI O N S