Abstract
AbstractIntegrated perovskite/organic bulk heterojunction (BHJ) solar cells have the potential to enhance the efficiency of perovskite solar cells by a simple one‐step deposition of an organic BHJ blend photoactive layer on top of the perovskite absorber. It is found that inverted structure integrated solar cells show significantly increased short‐circuit current (Jsc) gained from the complementary absorption of the organic BHJ layer compared to the reference perovskite‐only devices. However, this increase in Jsc is not directly reflected as an increase in power conversion efficiency of the devices due to a loss of fill factor. Herein, the origin of this efficiency loss is investigated. It is found that a significant energetic barrier (≈250 meV) exists at the perovskite/organic BHJ interface. This interfacial barrier prevents efficient transport of photogenerated charge carriers (holes) from the BHJ layer to the perovskite layer, leading to charge accumulation at the perovskite/BHJ interface. Such accumulation is found to cause undesirable recombination of charge carriers, lowering surface photovoltage of the photoactive layers and device efficiency via fill factor loss. The results highlight a critical role of the interfacial energetics in such integrated cells and provide useful guidelines for photoactive materials (both perovskite and organic semiconductors) required for high‐performance devices.
Highlights
The integrated cell used for this work was developed based on an inverted perovskite device structure applying indium tin oxide (ITO) as an anode, polytriarylamine (PTAA) as hole transport layer (HTL), poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) polymer layer as interfacial compatibiliser, MAPI as the photoactive layer (1.60 eV bandgap), PCBM and a thin layer of ZnO nanoparticles as electron transport layers (ETLs) and a silver cathode
The spectra show high external quantum efficiency (EQE) (70–80%) for both devices in the wavelength range of 400–800 nm, and for the integrated cell an EQE reaching up to 40% in the 800–900 nm wavelength range as evidence of short-circuit current being generated due to the absorption of TT in the bulk heterojunction (BHJ) layer
We found that a significant energetic barrier (≈250 meV) is present at the perovskite/organic BHJ interface
Summary
The rational design of device structures through optimized fabrication methods and materials selection has led to a remarkable increase in the power conversion efficiency (PCE) of perovskite solar cells, approaching 25% in the highest performing (typically mixed cation) devices.[1,2,3,4,5,6,7] there is still room for further improvement in PCE in the singlejunction perovskite solar cells,[8,9] their efficiency is fundamentally limited by the Shockley–Queisser theoretical limit,[10] for example at 30% PCE for the 1.6 eV bandgap methylammonium lead iodide (MAPI) layer.[8,11,12]. We measure a ≈250 meV energetic barrier for hole transport formed at the interface between the organic BHJ and MAPI layers that leads to interfacial charge carrier accumulation and recombination. These observations elucidate one of the important loss mechanisms in integrated perovskite/BHJ devices and suggest a potential method (by proper energy level tuning of the photoactive layers) to further improve device performance
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