Buffer layers in inverted organic solar cells and their impact on the interface and device characteristics: An experimental and modeling analysis

• Role of ZnO and MoO3 buffer layers on the performance of inverted organic solar cells has been studied theoretically and experimentally.
• Both ZnO and MoO3 layers aid to exciton dissociation at the interface between the electrode and the active layer.
• Absence of ZnO gives rise to S-shaped J-V characteristics due to hindered charge extraction and non-selectivity of carrier flow towards cathode.
• Presence of MoO3 between the active layer and ITO anode assists in making an energetically favourable contact and improves carrier extraction.

Buffer layers play crucial role in increasing the power conversion efficiencies (η) in organic solar cells (OSCs) and hence it is important to understand the underlying microscopic mechanisms behind the improvements aiding the existing qualitative understanding. In this manuscript, we have investigated the role of zinc oxide (ZnO) and molybdenum oxide (MoO3) buffer layers on the current density - voltage (J-V) characteristics of inverted organic solar cell (IOSC) devices combining experimental results with drift-diffusion based transport modeling, on the active layer (ActL) blend of P3HT:PCBM (Poly 3-hexylthiophene (P3HT): (6, 6) phenyl C61 butyric acid methyl ester (PC61BM)), a workhorse system with well-established device data. The results show that while ZnO alone improves the open circuit voltage (VOC) significantly, use of both the ZnO and MoO3 layers help in improving the short-circuit current density (JSC), aided by contributions to exciton dissociation by both layers at the electrode ActL interface. Absence of ZnO, in particular, causes S-shaped J-V curve which is attributed to reduced surface recombination velocity of majority carriers due to hindered charge extraction and non-selectivity of carrier flow towards cathode. On the other hand, presence of MoO3film between ActL and the anode assists in making an energetically favourable contact with ActL and improves the extraction of photo generated charge carriers. Further in conjunction with dark J-V characteristics, impedance spectroscopy (IS) carried out under dark and illuminated conditions establishes the role of buffer layers in modifying the barrier heights at the contacts and the interfacial structure.

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