A poly(acetoxy-p-phenylene vinylene) based diode with a soft breakdown behaviour J.R. de Lima a , L.O. PeÂres b , J.R. Garcia b , J. Gruber c , I.A. HuÈmmelgen a, * a Departamento de FõÂsica, Universidade Federal do Parana Â, Caixa Postal 19081, 81531-990 Curitiba PR, Brazil b Instituto de QuõÂmica de Sa Äo Carlos, Universidade de Sa Äo Paulo, Caixa Postal 369, 13560-970 Sa Äo Carlos SP, Brazil c Instituto de QuõÂmica, Universidade de Sa Äo Paulo, Caixa Postal 26077, 05599-970 Sa Äo Paulo SP, Brazil Received 17 September 1999; accepted 13 October 1999 Abstract We present an organic device with an abrupt increase in the current when the applied voltage exceeds a critical value, behaving like a switch. The device consists in a blend of poly(acetoxy-p-phenylene vinylene) and poly(methyl methacrylate) sandwiched between two metal electrodes, one of them being Cu. The device construction is quite simple and the critical voltage can be controlled using dierent conjugated materials in spite of poly(acetoxy-p- phenylene vinylene). # 2000 Elsevier Science Ltd. All rights reserved. Several types of devices that use conjugated poly- mers as active material were described in the last years, such as light-emitting diodes [1], transistors [2], opto- couplers [3], photodetectors [4], lasers [5] and switches which operate like a thyristor [6]. In this communi- cation we report on a unipolar device made using poly(acetoxy-p-phenylene vinylene), Acet-PPV (or, in substitution, C 60 ), blended with poly(methyl methacry- late), PMMA, which presents rectifying character, but with an uncommon current dependence on applied vol- tage, I(V ), at forward bias. Acet-PPV was prepared by electrochemical synthesis following the route represented in Fig. 1 [7]. Acet- PPV:PMMA blends were prepared using CH 2 Cl 2 as solvent. Solutions with concentrations of 14 mg/ml for Acet-PPV and 27 mg/ml for PMMA (M w =120,000 g/ mol) were combined to produce a ®nal concentration of Acet-PPV in the Acet-PPV:PMMA blend. Blends with Acet-PPV concentrations of 0; 14; 25; 34; 36 and 50% w/w were prepared. Devices with 100% Acet- PPV were not prepared because the Acet-PPV ®lms were not mechanically stable. Blends of C 60 :PMMA were prepared at a concentration of 27% w/w, using CHCl 3 as a solvent. Devices were prepared in the Cr/Acet-PPV:PMMA/ Cu sandwich structure onto glass substrate: Cr was evaporated onto the glass substrate and covered with the polymer blend by spin coating the blend solution at 3000 rpm. The top electrode (Cu) was then evapor- ated, to complete the device. The sample geometry is the same as in Ref. [8]. In some devices the Cr (bot- tom) electrode was substituted by Cu and the Cu (top) electrode was substituted by Ag. The same procedure was used in case of C 60 :PMMA based devices. The polymer blend thickness was determined using a surface pro®ler and the electrical characteristics of the samples were measured in air at room temperature using a procedure described in Ref. [6]. The I(V ) characteristics of the Cr/Acet- PPV:PMMA/Cu device are presented in Fig. 2. At for- ward bias conditions (which we de®ne as Cr positively biased) there is a critical voltage V C . We de®ne V C as Solid-State Electronics 44 (2000) 565±569 0038-1101/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0038-1101(99)00290-7 * Corresponding author. E-mail address: iah@®sica.ufpr.br (I.A. HuÈmmelgen).