Anisotropy in Organic Single-Crystal Photovoltaic Characteristics

Organic single crystals have been well researched for many years. Typical vapor-phase growth of organic crystals developed from vertical to horizontal growth in order to achieve improved crystalline quality. Recently, a field-effect study on these single-crystal semiconductors demonstrated high carrier mobilities, up to ca. 15 cm Vs, along with anisotropic charge-transport properties. These single-crystal transistors were usually fabricated with rigid and thick crystals (tens of micrometers to millimeters), which were fragile and difficult to process because of poor mechanical properties. Recently, a growth method affording thin, 150 nm thick, organic crystals demonstrated the processability of single-crystal transistors on flexible substrates, and these organic crystals could be patterned on individual channels for transistors. However, in addition to transistor studies of these organic crystals, other types of electronic applications, such as two-terminal devices, have not yet been realized, mainly because of the difficultly processing thick crystals. In this Communication, we report organic single-crystal photovoltaics fabricated from single pieces of thin tetracene crystals on bilayer heterojunctions with fullerene (C60) thin films. These organic singlecrystal devices exhibited excellent diode behavior with rectifying ratios of 10 and an external power conversion efficiency (PCE) of ca. 0.34 %. By employing these high-quality single crystals in two-terminal devices, high-performance optoelectronic devices, such as organic diodes, photovoltaics, and photodetectors, become possible alternatives for large-area, low-cost flexible electronics. The quality of the single crystals was examined by cross-polarized microscopy, X-ray powder diffraction (XRD), and single-crystal X-ray diffraction. The optical microscopy images (Fig. 1a and b) recorded at 0° and 90° from the entrance polarizer and exit analyzer, respectively, show large birefringence, confirming the anisotropic crystalline nature of the tetracene crystals. The XRD data exhibit strong and narrow first (001) and second order (002) reflections (Fig. 1c), indicating the tetracene crystal is c-oriented, with molecular-plane growth along the vertical direction. The crystal data obtained for tetracene confirm a C18H6 molecular formula with a molecule weight of 222.23 g mol. The lattice constants are a = 6.02 ± 0.025 Å, b = 7.77 ± 0.032 Å, c = 12.46 ± 0.054 Å, a =101.11 ± 0.078°, b = 99.41 ± 0.092°, and c = 94.40 ± 0.088°, and tetracene crystallizes with a triclinic crystal structure in space group P 1. The coordinates for tetracene can be obtained in the crystallographic information file (CIF) format from the Cambridge Crystallographic Data Center (CCDC), and the above crystallographic data are consistent with the data in the CCDC. Both the strong birefringence and the XRD results indicate these organic single crystals are of high quality. A schematic structure of a single-crystal solar cell is shown in Figure 2a. The device structure comprised a poly(3,4-ethylenedioxythiophene): polystyrenesulfonate (PEDOT:PSS)coated indium tin oxide (ITO) substrate, a tetracene crystal, evaporated thin films of C60 and bathocuproine (BCP), and an aluminum thin-film electrode. We note that thin crystals (ca. 200 nm) used in this study conformed better on substrates C O M M U N IC A IO N