Infrared photocurrent response of charge-transfer exciton in polymer bulk heterojunction

Abstract

We study the charge-transfer exciton absorption and photocurrent response in solution-processed bulk heterojunction based on poly(3-hexylthiophene) donor and (6,6)-phenyl-C61-butyric acid methyl ester acceptor in the near-infrared wavelength region. While the exciton absorption exists only for wavelength below 650nm, direct generation of charge-transfer exciton formed between the donor and acceptor extends the absorption wavelength to 950nm. For films with micrometer thickness, the photon-to-electron conversion efficiency is about 60% at 750nm wavelength under reverse voltage bias and the photocurrent to dark current ratio is about 8.6 at 900nm and remains 3.6 even at 1000nm. Photodetector with high sensitivity covering exclusively the 650–1000nm near infrared region can therefore be made without a low bandgap material. The charge-transfer exciton absorption coefficient and photocurrent sensitivity depend on the annealing condition which controls the donor-acceptor morphology.

Narrowband light detection via internal quantum efficiency manipulation of organic photodiodes

“…OPDs utilizing thick junctions have previously been reported 15,22,33,35 . In particular, Armin et al 35 recently described photodiodes with impressive performance metrics for broadband visible, IR-blind light detection using the archetypal bulk heterojunction organic semiconductor donor: acceptor blend of PCDTBT:PC70BM [(poly[N-9 00 -heptadecanyl-2,7-carbazole-alt-5,5-(4 0 ,7 0 -di-2-thienyl-2 0 ,1 0 ,3 0 -benzothiadiazole)]: PC70BM].…”

“…It is important to note that this is not a particularly efficient method to produce a NIR response since the CT state absorption is very weak in organic semiconductors 21 and therefore extremely thick junctions (410 mm) are required for light harvesting via CT states. As a consequence, the OPDs of Yang et al 22 could only be operated at extremely large reverse bias voltages (4100 V), and they also had a very high dark current noise. A truly narrowband (FWHMo100 nm), low noise, visible-blind NIR detector is yet to be realized.…”

“…NIR photodetectors are important in medical, industrial, communication, scientific and military applications as well as for night-vision imaging and security [14][15][16] . NIR-sensitive photodectectors have previously been demonstrated using inorganic quantum dot sensitization 17,18 , narrow optical-gap donor organic semiconductors within a conventional thin heterojunction structure 14,19,20 , and more recently by harvesting sub-gap charge transfer states (CT) 15,21,22 . Yang et al have reported on the use of CT state absorption to extend the spectral response of an OPD based on a P3HT:PC60BM blend to the NIR 22 .…”

“…NIR-sensitive photodectectors have previously been demonstrated using inorganic quantum dot sensitization 17,18 , narrow optical-gap donor organic semiconductors within a conventional thin heterojunction structure 14,19,20 , and more recently by harvesting sub-gap charge transfer states (CT) 15,21,22 . Yang et al have reported on the use of CT state absorption to extend the spectral response of an OPD based on a P3HT:PC60BM blend to the NIR 22 . It is important to note that this is not a particularly efficient method to produce a NIR response since the CT state absorption is very weak in organic semiconductors 21 and therefore extremely thick junctions (410 mm) are required for light harvesting via CT states.…”

Polymer photodetector with voltage-adjustable photocurrent spectrum

“…2,3 In general the photoresponse of photodetectors is determined by the absorption of the active layer. 11 The results showed that the photoresponse could be changed from visible to infrared by simply increasing the thickness of the active layer. Most conjugated polymers have the response in the visible range due to the fact that the band gap of most organic semiconductor is higher than 2 eV.…”