Dependence of mobility and charge injection on active layer thickness of bulk heterojunction organic solar cells: PCBM:P3HT

Abstract

The asymmetric behavior in the dark current of any solar cell is essential for decoupling recombination and charge extraction for efficient charge collection. Therefore current density dependence on applied voltage can be used to investigate the complex interplay between bulk charge transport, interface exchange effects and recombination mechanisms. In the present work, we investigate from dark current–density [J(V)] of bulk heterojunction solar cells, the dependence of mobility and charge injection mechanisms on different blend layer thicknesses. The active layer thickness has been established through varying the spin-coating speeds between 1000, 2000, 3000 and 4000 revolutions per minute (rpm) and confirmed by a dektak surface profilometer. The carrier mobility (μ) as a factor limiting the efficiency of organic solar cells was investigated from dark space charge limited current and trap free space charge limited current conduction mechanisms to distinguish between charge extraction and recombination. This approach allows the determination of the effects of threshold field through variation of the active layer thickness (ALT) on the potential barrier height $$(\phi_{B} )$$ at the electrode contacts. Low values of charge carrier mobilities (10–6 cm2 V−1 s−1) in the trap free space charge limited current conduction region have been correlated to the Langevin recombination constants. In the ohmic region, the highest dark carrier mobility corresponded to the 77.1 nm ALT. Further we observe a shift in the transition voltage at the inflection point of J–V curves with increasing film thickness in the forward bias.