Analysis of the photocurrent action spectra of MEH-PPV polymer photodiodes

Abstract

We have measured the photocurrent action spectra of the conjugated polymers poly͓2-methoxy, 5 ethyl ͑2Ј hexyloxy͒ paraphenylenevinylene͔ ͑MEH-PPV͒ and poly͑phenylenevinylene͒ ͑PPV͒ in sandwich cells between indium tin oxide ͑ITO͒ and aluminum electrodes. Under forward bias and illumination through ITO, the photocurrent spectrum is broad and has a maximum at high energy, where the absorption coefficient is greatest ͑the symbatic response͒. Under reverse bias and illumination through ITO, the photocurrent spectrum consists of a very narrow peak ͑the full width at half maximum is 0.1 eV͒, located in the low-energy tail of the absorption profile ͑the antibatic response͒. Several established models attempt to explain this behavior and to relate the photocurrent action spectrum to the absorption coefficient, considering penetration depth of the light and diffusion of excitons or directly photogenerated charges. At a qualitative level many of these seem to provide an adequate description. In this paper, we undertake a quantitative examination of these models and we find that none of them can reproduce the very narrow antibatic response that we observe in both MEH-PPV and PPV. Upon exposure to air, we observe an enhancement of the photocurrent by a much greater factor than the dark current, from which we conclude that charge generation is mediated by exciton dissociation. As the temperature decreases we observe a progressive redshift of the absorption edge, although the photocurrent onset undergoes a much smaller redshift. We therefore conclude that the narrow antibatic peak is due to a specific enhancement of dissociation upon excitation at low energy. We propose that the particularly sharp onset of photocurrent at low energy may be due to enhanced intermolecular charge separation within crystallite grains between those neighboring conjugated segments that are more extended and more planar.

Solution-processed semiconductors for next-generation photodetectors

“…26 The potential for organic photodetectors (OPDs) to achieve filter-less narrowband colour sensing was recognised early in the first homojunctions and linear heterojunctions. 27 However, these devices were limited by the excitonic nature of organic semiconductors that typically exhibit exciton binding energies of order 100s of meV, severely impacting charge generation and hence, transport and external quantum efficiency (EQE). This problem has been largely resolved by employing the bulk heterojunction (BHJ), a blend of two organic semiconductors with different chemical potential energies, the donor and the acceptor, to facilitate charge separation.…”

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].…”

“…Harrison et al 33 have shown that a narrow response can be obtained around the absorption onset of neat MEH-PPV thick photodiodes via enhanced exciton dissociation efficiency at lower energies. In our current work, we introduce the new concept of charge collection narrowing (CCN) in thick bulk heterojunctions to achieve narrowband OPDs.…”

Doping effect of viologen on photoconductive device made of poly (p-phenylenevinylene)

“…2 reported before; 14,15 the maximum of photocurrent action spectra is located at the photon energy ͑ϳ2.4 eV͒ where the optical absorption is very weak. The spectral response shows a maximum at a photon energy that balances the competing factors of the absorbed light intensity and the position of photocarrier generation with respect to the collecting electrodes.…”