Trimolecular Recombination … really?

As you might already have guessed, I am interested in loss mechanisms in organic photovoltaics. Despite considering the impact of recombination on the solar cell performance, also the physical origins are challenging… and many open questions remain.

Just a view days ago, there was another publication about recombination of free polarons (free carriers) – also called nongeminate recombination *1 – more specifically, trimolecular recombination. Abendstimmung im Vogelschutzgebiet GarstadtYou might remember that, a while ago, I already mentioned third order recombination, including a reference to private communications with Prof. Juska and another recent paper by the Durrant group [Shuttle 2008]) as well as a potential candidate for its origin. The new paper [Juska 2008] uses three different experimental methods, including photo-CELIV, to measure the temperature dependence of the trimolecular recombination rate in polymer:fullerene solar cell. The authors mention very briefly a possible mechanism responsible for the third order recombination, Auger processes. Shuttle et al. argue in their paper that a bimolecular recombination with a carrier concentration dependent prefactor could be the origin, in particular as they observe a decay law proportional to n2.5-n3.5, depending on the sample. We are also in the game, an accepted APL awaiting its publication (preprint here) Update 20.10.2008: now published online [Deibel 2008b]. We rather tend to believe the explanation by Shuttle, but that’s just an assumption at the present stage: the generally low recombination rate could also be due to a rather improbable process.

Disregarding the physical origin for a moment, it is important to note that the nongeminate polaron recombination rate is very low in solution-processed polymer:fullerene solar cells. We believe – and can also show with macroscopic solar cell simulations – that it is not a major limiting factor for the solar cell performance.

As a concluding remark, the differences between recombination in ordered inorganic semiconductors and disordered organic semiconductors also imply that the ideality factor n to describe the exponential part of the current-voltage characteristics, as applied in the Shockley equation, has a different meaning for organic solar cells. To my (limited) knowledge, this has never been openly discussed. (Update 14.10.2008: I should have known better, as I knew these two papers discussing the ideality factor, [Harada 2005, Koster 2005]. IMHO, these are not yet the complete answers, but I might be wrong;-) Some other deficiencies of applying the Shockley equation to disordered organic semiconductor devices I have scribbled down in this post.

*1 The term geminate tells us something about the history of a carrier pair: geminate pairs originate from the same (photoexcited) precursor state, such as a polaron pair generated by the dissociation of a singlet exciton. Nongeminate pairs, on the other hand, are unrelated carriers of usually opposite charge that can interact and recombine. They could either be injected in to the organic semiconductor, or originate from successfully dissociated polaron pairs.

P.S. Thanks to Martijn and Thomas for pointing me to the new Juska article:)

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