Determination of gap defect states in organic bulk heterojunction solar cells from capacitance measurements

Abstract

Energy distributions ͓density-of-states ͑DOS͔͒ of defects in the effective band gap of organic bulk heterojunctions are determined by means of capacitance methods. The technique consists of calculating the junction capacitance derivative with respect to the angular frequency of the small voltage perturbation applied to thin film poly͑3-hexylthiophene͒ ͑P3HT͒ ͓6,6͔-phenyl C 61 -butyric acid methyl ester ͑PCBM͒ solar cells. The analysis, which was performed on blends of different composition, reveals the presence of defect bands exhibiting Gaussian shape located at E Ϸ 0.38 eV above the highest occupied molecular orbital level of the P3HT. The disorder parameter , which accounts for the broadening of the Gaussian DOS, lies within the range of 49-66 meV. The total density of defects results of order 10 16 cm −3 .

Kinetics of occupancy of defect states in poly(3-hexylthiophene):fullerene solar cells

“…2(a) resulting from contributions of defect levels in the gap. We recently demonstrated that such defects belong to the polymer in analyzing P3HT diodes along with P3HT:PCBM solar cells [9]. Because a complete step is observed in ) ( f C , the inflection point serves to define a frequency m f related to the defect band kinetic response, which corresponds to the minimum of the frequency derivative Fig.…”

“…This band structure across the active layer differs from that expected using undoped materials, in which the potential drops linearly between the contacts [8]. Impedance and capacitance measurements have been used to evidence such band bending [9,10] usually exhibiting a Mott-Schottky characteristics ( V C  2 ), which can be readily interpreted as a strong indication of the presence of negatively charged defects within the P3HT energy gap.…”

“…The negative impact of oxygen-induced traps on the charge transport in poly(3-hexylthiophene) diodes has been recently addressed [16,18]. This work investigates the density, energetic, and kinetics of defect states within the gap of P3HT, following our previous results [9]. The study is performed by analyzing demonstrated stability in samples kept in standard lab conditions (no intense light and ambient temperature around 22ºC) for longer than a year.…”

“…The study is performed by analyzing demonstrated stability in samples kept in standard lab conditions (no intense light and ambient temperature around 22ºC) for longer than a year. The capacitance measuring method was described in the previous work [9]. Capacitance spectra were measured at decreased temperatures starting at 315 K. A final spectrum was again registered at room-temperature to check measurement reproducibility.…”

“…Capacitance spectra were measured at decreased temperatures starting at 315 K. A final spectrum was again registered at room-temperature to check measurement reproducibility. Photovoltaic response was observed to result in conversion efficiencies around 3.5% [9]. Current density-voltage (J-V) characteristics of devices were taken using a Keithley 4200 source unit under 1 sun AM1.5G illumination conditions from a xenon lamp based Oriel 9600 150 W Sun…”