His research focuses on developing high-efficiency organic solar cells and morphology manipulation, characterization in organic/perovskite semiconductor films.
The measurement of the ideality factor (nid) is a popular tool to infer the dominant recombination type in perovskite solar cells (PSC). However, the true meaning of its values is often misinterpreted in complex multilayered devices such as PSC. In this work, the effects of bulk and interface recombination on the nid are investigated experimentally and theoretically. By coupling intensity‐dependent quasi‐Fermi level splitting measurements with drift diffusion simulations of complete devices and partial cell stacks, it is shown that interfacial recombination leads to a lower nid compared to Shockley–Read–Hall (SRH) recombination in the bulk. As such, the strongest recombination channel determines the nid of the complete cell. An analytical approach is used to rationalize that nid values between 1 and 2 can originate exclusively from a single recombination process. By expanding the study over a wide range of the interfacial energy offsets and interfacial recombination velocities, it is shown that an ideality factor of nearly 1 is usually indicative of strong first‐order non‐radiative interface recombination and that it correlates with a lower device performance. It is only when interface recombination is largely suppressed and bulk SRH recombination dominates that a small nid is again desirable.
Detailed balance is a cornerstone of our understanding of artificial light-harvesting systems. For next generation organic solar cells, this involves intermolecular charge-transfer (CT) states whose energies set the maximum open circuit voltage VOC. We have directly observed sub-gap states significantly lower in energy than the CT states in the external quantum efficiency spectra of a significant number of organic semiconductor blends. Taking these states into account and using the principle of reciprocity between emission and absorption results in non-physical radiative limits for the VOC. We propose and provide compelling evidence for these states being non-equilibrium mid-gap traps which contribute to photocurrent by a non-linear process of optical release, upconverting them to the CT state. This motivates the implementation of a two-diode model which is often used in emissive inorganic semiconductors. The model accurately describes the dark current, VOC and the long-debated ideality factor in organic solar cells. Additionally, the charge-generating mid-gap traps have important consequences for our current understanding of both solar cells and photodiodes – in the latter case defining a detectivity limit several orders of magnitude lower than previously thought.
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