The Langevin equation has been a workhorse for understanding the rate of bimolecular recombination in optoelectronic and photovoltaic materials for over a century. Countless experiments have shown that it overpredicts nongeminate radiative recombination, by as much by several orders of magnitude, prompting researchers to develop numerous correction factors that account for impacts of inhomogeneity in material composition, carrier concentration, and mobility. Here we show that the presence of free carriers leads to screening, which contributes to lowering the bimolecular recombination rate even in a homogeneous material. We develop an expression that captures carrier screening of the Coulomb attraction that leads to recombination. Our expression relates the recombination rate to the concentration of free carriers and to energetic disorder, which stems from the steepness of the density of states. We define a generalized Coulomb capture radius $r_0$, based on the energetic disorder $E_0$, as well as a screened capture radius $r_{\text{sc}}$, and then show that the recombination rate is reduced from the Langevin expression by a ratio $r_{\text{sc}}/r_0$, a significant reduction when carrier concentration is large and energetic disorder is low. We also show that the rate still factors into a term containing mobility and a term that is proportional to the $np$ product, now with a reduction due to screening, which reduces the order of this nominally quadratic bimolecular term below second order, as observed in experiments that separately quantify the contribution from mobility to the order. Our work establishes a stricter upper bound on the bimolecular recombination rate and points to materials properties that may result in higher efficiency across many future optoelectronics and photovoltaics.

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• Carrier generation & recombination
• Amorphous semiconductors
• Organic semiconductors
• Organic-inorganic hybrid perovskites
• Photovoltaic absorbers