Rectification ratios

The first realization of a conjugated polymer/fullerene diode [89] was achieved only recently after the detection of the ultrafasl phoioinduced electron transfer for an lTO/MEH-PPV/CW)/Au system. The device is shown in Figure 15-18. Figure 15-19 shows the current-voltage characteristics of such a bilayer in the dark at room temperature. The devices discussed in the following section typically had a thickness of 100 nm for the MEH-PPV as well as the fullerene layer. Positive bias is defined as positive voltage applied to the 1TO contact. The exponential current tum-on at 0.5 V in forward bias is clearly observable. The rectification ratio at 2 V is approximately l()4. 

The rectification ratio is obtained by multiplying JFR by the reaction resistance Rr ... 

The fundamental figure of merit for rectification, the rectification ratio, RR, is defined as the current at a positive bias V divided by the absolute value of the current at the corresponding negative bias —V RR = I(V)/ /(—V) I. Commercial doped Si, Ge, or GaAs pn junction rectifiers have RR between 10 and 100. 

All compounds were measured at room temperature in air between Au electrodes inside a Faraday cage. IVT = maximum of the optical InterValence Transfer or intramolecular charge transfer band. RR Rectification Ratio. Survives cycling means RR does not decrease with cycling of IV measurements. AR or anti-AR indicates whether electron flow is from A to D (AR) or from D to A (anti-AR)... 

Fig. 10 (a) Schematic representation of the I-V measurement of the Cgo derivative based molecular rectifier, (b) I-V characteristics of the Langmuir-Blodgett film of the fullerene penta-pod showing current rectification at bias voltage of 1.0 V measured at different junction positions, with high rectification ratios (RR) registered at each junction. (Reprinted with permission from [98])... 

The fabrication of diodes on silicon substrates was demonstrated using the supramolecular interactions between a 5,10,15,20-tetra(3-fluorophenyl)porphyrin and Ceo fullerene with a rectification ratio of 1,500 (see Fig. 11). The rectifying behavior is explained by theoretical calculations which show that the LUMO orbital is located mainly on the fullerene whereas the HOMO orbital is located on the porphyrin moiety [99]. 

SAMs of a molecular rectifier candidate - donor-jr-acceptor molecule 88 - were investigated with an alkanethiolate-covered STM probe by Ashwell et al. [25]. The molecular junction shown in Fig. 10.3 demonstrated highly asymmetric I(V) characteristics, which was taken as evidence for rectification of the monolayer. Depending on the experimental details, rectification ratios up to around 18 was observed. Reversible protonation and deprotonation of the donor moiety, C6H4N(CH3)2, in acidic or basic media, respectively, switched the rectification capability of the monolayer OFF and ON. This observation strongly indicates that the rectification behavior is an inherent property of the molecule. 

We have carried out tunneling conductance measurements on a large number of N-doped Y-junction carbon nanotubes. The junction (JA) in Fig. 5a exhibits a rectification behavior with distinct change of slope in the I-V data at —1.5 and +1 V. A high value of rectification ratio of 4.75 is obtained for bias voltages of 1.5 V. Coulomb blockade is easily identifiable in the bias range, -0.4 to +0.6 V. The arms of the tube, however, are devoid of blockade but exhibit varied behavior. The arm-1 shows distinct features at 0.4 V. In the case of arm-2, similar... 

The rectification ratio RR is defined as the current at a positive bias V divided by the absolute value of the current at the corresponding negative... 

In Fig. 2 the current-density versus voltage (IV) characteristics are presented at room temperature and at 80 K. At 295 K the IV-characteristics exhibit a current rectification ratio of 8.105 for a bias variation from -1 to +1 V. Upon cooling, the reverse dark current-density at -2 Vbias decreases from 5.10 A/cm2 at 295 K to the sub nA/cm2 region at 80 K. From an Arrhenius plot of the reverse dark current-density... 

Haak and Nolta (141, 14%) have observed rectifying phenomena when polycrystalline samples of metal-free or metal phthalocyanines are compressed between different metal electrodes. A small amount of a liquid polar impurity was found to be essential for rectification to occur. The rectification ratio (ratio of conductance in forward and reverse directions, the forward direction being movement of electrons from the least noble electrode to the sample) varies from 25 to 500. The latter value is obtained when copper phthalocyanine is sandwiched between either platinum and silver, or silver and aluminum, electrodes. Kleitman (188) has also demonstrated that metal-free phthalocyanine can act as a rectifier. 

The -V characteristics of this PV device in the dark and under 0.9 suns of simulated AM1.5 solar illumination are shown in Figure 12.7b. The device shows excellent rechfying behavior in the dark with a rectification ratio of approximately 10 at 1V, and has a very low leakage current, corresponding to a shunt resistance l sh 1 MO cm. The dark J-V characteristics can be well described using the modified diode equation (Sze and Ng, 2007) ... 

---