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

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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.

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Acknowledgements

The authors would like to acknowledge the Material Physics Research Institute of the University of the Witwatersrand for research support and the incentive grant of the National Research Foundation for the financial support (UID 85675).

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Authors and Affiliations

  1. DST/Centre of Excellence in Strong Materials and Material Physics Research Institute, School of Physics, University of the Witwatersrand (Wits), Private Bag 3, Johannesburg, 2050, South Africa

    Lordwell Jhamba & Daniel Wamwangi

  2. Department of Physics, School of Mathematical and Natural Sciences, University of Venda, Private Bag X5050, Thohoyandou, 0950, South Africa

    Lordwell Jhamba

  3. Department of Physics, University of Namibia, 340 Mandume Ndemufayo Ave, Pionierspark, Private Bag 13301, Windhoek, Namibia

    Zivayi Chiguvare

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Jhamba, L., Wamwangi, D. & Chiguvare, Z. Dependence of mobility and charge injection on active layer thickness of bulk heterojunction organic solar cells: PCBM:P3HT. Opt Quant Electron 52, 245 (2020). https://doi.org/10.1007/s11082-020-02362-0

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