Top-Absorption Organic Photodiodes Suitable for Device Integration

We have fabricated top-absorption organic photodiodes (TA-OPDs) using sputtered indium zinc oxide (IZO) electrodes and compared their photodiode characteristics with conventional bottom-absorption organic photodiodes (BA-OPDs). In poly ([2-methoxy-5-(2'-ethylhexyloxy)]-1,4-phenylenevinylene) (MEH-PPV) films sandwiched between Al and IZO electrodes, the ratio of photoconductivity to dark conductivity, σR, was approximately 103 for both BA- and TA-OPDs. In a TA-type Al/td-PTC/α-NPD/CuPc/IZO device, the σR and the short-circuit photocurrent IP were 3.4×102 and 5.3×10-2 mA/cm2, respectively. The lower value of σR in TA-OPDs compared to BA-OPDs in this structure is ascribed to sputtering damage during IZO film preparation. We were able to prevent successfully this damage by inserting the MoO3 hole-injecting and sputter buffer layer instead of the CuPc layer. We have also investigated the frequency characteristics of several TA-OPDs and obtained relatively good response in the TA-OPDs with a MoO3 buffer layer. In terms of frequency characteristics, the cut-off frequency of the TA-OPDs with a MoO3 buffer layer was as high as 300 kHz.