Anisotropy in Organic Single‐Crystal Photovoltaic Characteristics

Abstract

Organic single crystals have been well researched for many years.[1] Typical vapor-phase growth of organic crystals developed from vertical [2] to horizontal growth [3] 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 2 V -1 s -1 , along with anisotropic charge-transport properties. [4,5] 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, [6] and these organic crystals could be patterned on individual channels for transistors. [7] 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 (C 60 ) thin films. These organic singlecrystal devices exhibited excellent diode behavior with rectifying ratios of 10 5 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, [8] 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. [9,10] The crystal data obtained for tetracene confirm a C 18 H 6 molecular formula with a molecule weight of 222.23 g mol -1. 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...

Non‐volatile Ferroelectric Poly(vinylidene fluoride‐co‐trifluoroethylene) Memory Based on a Single‐Crystalline Tri‐isopropylsilylethynyl Pentacene Field‐Effect Transistor

“…The strong and uniform birefringence of the single crystal under a cross-polarized microscope indicates that the intrinsic single-crystalline nature is well preserved after thermal deposition of the Au electrodes, as shown in the inset of Figure 3c. [21] Due to the presence of ferroelectric P(VDF-TrFE) gate dielectric layer, a well-saturated hysteresis of source-drain current (I DS ) was obtained as a function of gate voltage, as shown in Figure 3d Figure 3d, and significantly reduced the gate-leakage current between gate and drain electrodes, leading to a very low OFF state I DS of approximately 10 À11 A. The field-effect mobility of a single-crystalline TIPS-PEN FeFET was calculated from the slope of I DS versus gate voltage (V G ) in Figure 3d, using an equation suitable for the linear regime of drain current-voltage due to the application of a low source-drain voltage (V DS ) of À5 V in our device.…”

Single-crystal tetrathiafulvalene microwire arrays formed by drop-casting method in the saturated solvent atmosphere

“…In addition, along every individual microwire the color is uniform under a fixed polarization angle. These results indicate the anisotropic crystalline nature of the oriented microwire arrays and the single-crystal structure of an individual microwire caused by the molecular packing [23][24][25]. Fig.…”

Crystals in Materials Science

“…54 Recently, Yang and coworkers assembled a solar cell based on a single crystal of tetracene (shown in Figure 9) as the light absorbing layer and a C60 electron transport layer. 55 The device showed efficiencies that were less than 1%, and efficiency depended critically on the thickness of the crystal used in addition to the crystal face that was irradiated. The authors hypothesized that the device suffered from low performance because of the poor absorption of visible light by tetracene.…”