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To understand why organic solar cells have a non-radiative energy loss of 0.2-0.4 eV, [7,8] a significantly larger value compared to state-of-the-art inorganic counterparts, [9,10] a fundamental understanding of all non-radiative loss pathways is crucial.Sources of non-radiative energy loss in OPVs are still not well understood, [11] but recent studies suggest an intrinsic origin controlled by molecular properties. [8] However, these studies mostly consider non-radiative recombination at the donoracceptor (D-A) interface, where charge transfer (CT) excitons (either generated directly by optical excitation or via injected charge) recombine at a rate exponentially dependent on the CT state energy (E CT ), [8,11,12] prescribed by the "energy-gap law." [13] Experimentally, a linear increase in non-radiative energy loss is observed as E CT is reduced. [8,12] Theoretical modeling [12] established design rules for using stiff molecules with strongly absorbing CT states [4] to minimize such losses at the D-A interface.Despite the success of this model in capturing the general trend, for a given E CT , large variations are observed in the estimated non-radiative energy loss values. [4] These variations are attributed to recombination away from the D-A interface (i.e., in the bulk or at the contacts), surface recombination, and effects of the energetic disorder, not considered explicitly in the model. [4,11] Moreover, recently it has been shown that the "energy-gap law" fails for non-fullerene acceptor (NFA) based solar cells, [7] which has been explained through hybridization of CT and local exciton states. In addition, Gillett et al. [14] suggested back-electron transfer from CT-triplet states to NFA-triplet states as a non-radiative recombination pathway for NFA solar cells. These latest studies highlight the importance of identifying fundamental device processes beyond the "energy-gap law" that could add to and perhaps unify our understanding of non-radiative energy losses in both fullerenes-and NFA-based OPVs.One key characteristic of organic semiconductors is high binding energy excitons (0.2-0.5 eV) owing to the small dielectric constant and confinement in localized molecules. [15] This excitonic nature of organic semiconductors produces unique non-radiative loss processes like exciton-exciton annihilation

The notion of quasi-equilibrium is central to most solar cells; however, it has been questioned in organic photovoltaics (OPVs) owing to strong energetic disorder that frustrates efficient relaxation of electrons and holes within their respective density of states (DOS). Here, modulation electroluminescence (EL) spectroscopy is applied to OPVs and it is found that the frequency response of charge transfer (CT) state EL on the high energy side of the spectrum differs from that of the low energy side. This observation confirms that static disorder contributes substantially to the linewidth of the steady-state EL spectrum and is unambiguous proof that the distribution of CT states formed by electrical injection in the dark is not in quasi-equilibrium. These results emphasize the need for caution when analyzing OPV cells on the basis of reciprocity models that assume quasi-equilibrium holds, and highlight a new method to study this unusual aspect of OPV operation.

Controlling changes in the optical properties of photonic devices allows photonic integrated circuits (PICs) to perform useful functions, leading to a breadth of applications in communications, computing, and sensing. Many mechanisms to change optical properties exist, but few allow doing so in a reversible, non-volatile manner. This leads to power inefficiency in reconfigurable circuits and requires external memory elements. In this work, we propose and experimentally demonstrate reversible, non-volatile phase actuation of a silicon nitride PIC with thermally-stable photochromic organic molecules. The use of a high-core-index platform allows, for the first time, the photochemical actuation of a planar-resonator-based photonic memory unit, which enables high performance and permits integrated spectroscopic analysis. We show novel properties of this all-optical memory for a silicon photonics platform, including complete transparency in the optical C-band, as well as first-order photokinetics of the photoconversion that lead to bidirectional scalable switching rates and continuous tuning. Such features are critical for memories in analog applications, such as quantum, microwave, and neuromorphic photonics, where low loss and precision are paramount.

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