Charge Transport and Trapping in Bulk-Heterojunction Solar Cells

Abstract

Charge carrier transport and trapping was investigated in organic solar cell structures consisting of poly-3-hexylthiophene blended with the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester in 6:5 weight ratio. The analysed devices having solar efficiency of 3.7 per cent were produced in the inverted layer sequence. The fill factor of the IV characteristics was as high as 68 per cent. It was demonstrated that despite of such relatively high fill factor carrier trapping is effectively involved in the charge transport phenomena. The density of the trapping states was evaluated to be up to 10(20) division by 7 x 10(21) cm(-3) and their activation energy was about 0.18 eV. At such high densities these states may probably act as transport states, limiting carrier mobility. The results were analyzed by taking into account carrier thermal generation from traps as well their mobility variation according to the Gaussian disorder model. The mobility parameters obtained by both methods demonstrated good coincidence.

Gaussian Distribution of the Charge Traps in the Energy Gap of MDMO-PPV and Its Dependence on Synthesis Route Revealed by the Thermally Stimulated Current Spectroscopy

“…This implies that both polymers, though supposedly being identical in monomer-units, might indeed have different densities of the structural electronic defects that furthermore are characterized by the different effective activation energies. Sensitivity of other key transport parameters as, e.g., carrier mobility on the material structure was demonstrated also in, e.g., [11,30,31]. the photo-thermo-electrical methods was demonstrated.…”

“…The TSC method was proven to be a sensitive tool enabling to reveal, e.g., differences caused by polar molecular orientation in the films [10]. Moreover, it enabled to prove that trapping states, depending on their densities might act as transport states, governing in this way the whole charge transport in materials and devices [11] In this study, alongside with the integral TSC mode, we had adopted also the fractional heating TSC technique as a direct photoelectrical method to analyze distribution of trapping states in [poly-(2-methoxyl),5-(3,77dimethyloctyloxy)] paraphenylenevinylene (MDMO-PPV) synthesized in two different ways. MDMO-PPV is one of the promising conjugated polymers for organic solar cells due to ease of its processability as well as tunability of the optical and electronic properties through chemical modifications [12].…”

Non-Homogeneity of Distribution of Transport and Trapping States in Poly[2-methoxy-5-(3′,7′- dimethyloctyloxy)-1,4-phenylenevinylene]

“…[21,22], and by TSC in MEH-PPV. 18 Characteristically the greater spatial disorder parameter = 4 5 was obtained in P3HT:PCBM solar cell structures, 19 evidencing their greater spatial disorder because of the bulk heterojunction structure. On the other hand energetical disorder parameter of about 0.06-0.07 eV was higher in P3HT and P3OT, 21,22 but in their structures with PCBM it was comparable to that in our samples, i.e., 0.028-0.045, indicating more favorable transport conditions.…”

“…[18,19]. The TSCs caused by the thermal carrier generation from the traps can be described as: 20 here n t0 is the carrier density at the traps having activation energy E t , T is the temperature, is the heating rate, k is the Boltzmann constant, f t is the carrier attempt-toescape frequency, e is the elementary charge, L is the layer thickness, and A is the sample area.…”

“…[15][16][17][18] Though, for a shallower states located close to the HOMO or LUMO bands, their distribution is essential as they may start acting not only as traps but as transport states as well. 8,19 Therefore in this work we used Thermally Stimulated Current measurements as direct photoelectrical method to prove distribution of trapping and transport states in MDMO-PPV.…”