Breakdown mechanisms and reverse current-voltage characteristics of organic bulk heterojunction solar cells and photodetectors

Abstract

We investigate the reverse current-voltage characteristics and breakdown mechanisms of organic bulk heterojunction solar cells and photodetectors. Dark current and photo current measurements at different temperatures indicate that tunneling is the dominant mechanism at high reverse voltage. A band-to-band tunneling model that accommodates either Gaussian or exponential-parabolic density of states distributions is developed and used for simulations. At high reverse bias, the model explains the observed breakdown, which differs from the bias independent dark current behavior predicted by the Onsager-Braun model. At low reverse voltage, the incorporation of shunt resistance in the model provides good agreement between the measured reverse bias characteristics and simulations.

The impact of thermal annealing temperature on the low-frequency noise characteristics of P3HT:PCBM bulk heterojunction organic solar cells

“…Instead, in Ref. [38], it was reported that the tunneling current can be the major effect in the reverse bias. Therefore, we believe impact ionization may not play a major role in the reverse bias for P3HT:PCBM organic solar cells.…”

“…Although the reverse current of P3HT:PCBM organic solar cells could increase rapidly beyond a specific electric field value (larger than 5x10 7 V/m according to Ref. [38]), there is still no report on associating such current breakdown to the carrier avalanche. Instead, in Ref.…”

“…We attribute the excess LFN in the reverse bias from devices annealed at 110 ∘ C and 140 ∘ C to the tunneling current noise. While the total dark current in diodes may consist of diffusion, generation-recombination, shunt leakage, and tunneling, only the tunneling current, which increases with the electric field [37,38], is significantly larger in the reverse bias than forward bias. The thin active layers of the P3HT:PCBM devices favor a tunneling mechanism.…”

“…While the improved morphology as a result of higher annealing temperature should yield less pronounced trap-assisted tunneling, in fact, the tunneling current is also related to other factors such as the material band structure and the carrier effective mass [37,38]. While there have been few studies on how the carrier effective mass changes with the annealing temperature, it is observed that thermal annealing modifies the band structure of P3HT:PCBM films manifested by the red-shift in the absorption spectrum [7].…”

Characterization of Novel Optoelectronic Semiconductor Materials and Devices

“…We attribute the excess LFN in the reverse bias from devices annealed at While the improved morphology as a result of higher annealing temperature should yield less pronounced trap-assisted tunneling, in fact, the tunneling current is also related to other factors such as the material band structure and the carrier effective mass [23], [135]. While there have been few studies on how the carrier effective mass changes with the annealing temperature, it is observed that thermal annealing modifies the band structure of P3HT:PCBM films manifested by the red-shift in the absorption spectrum [126].…”