The spectral shape near the energy gap determines the radiative limit of the open-circuit voltage in organic photovoltaic devices. In this work, we employ ultrasensitive photocurrent measurements and detect sub-gap states with energies far below gap in a large number of different donor-acceptor blends. We provide evidence that these low-energy sub-gap states are associated with radiative mid-gap trap states, generating photocurrent via an optical release process. To account for the radiative mid-gap states, we implement a two-diode model which accurately describes both the dark current and the open-circuit voltage in organic solar cells. These findings provide important insights for our current understanding of organic photovoltaic devices.
Non-radiative losses to the open-circuit voltage are a primary factor in limiting the power conversion efficiency of organic photovoltaic solar cells. The dominate non-radiative loss is intrinsic to the active layer which, along with the thermodynamic limit to the open-circuit voltage, define the quasi-Fermi level splitting (QFLS). Quantification of the QFLS in organic photovoltaic devices is challenging due to the excitonic nature of photoexcitation and device-related non-radiative losses. In this presentation I will outline an experimental approach based on electro-modulated photoluminescence to quantify the QFLS in organic solar cells. Drift-diffusion simulations are used to verify the accuracy of the method, while state-of-the art PM6:Y6 solar cells are created with varying non-radiative losses. This method quantifies the QFLS in organic photovoltaics, fully characterizing the magnitude of different contributions to the non-radiative losses of the open-circuit voltage.
Probing the photovoltaic external quantum efficiency (EQE) at photon energies well below the semiconductor bandgap is an important tool for achieving a better understanding of the contribution of trap and tail states involved in charge generation processes in photovoltaic devices, notably solar cells. In this work, we present an electrical and optical noise-reduced EQE apparatus achieving 100 dB dynamic range. We carefully identify and study several device- and EQE apparatus-related factors limiting the EQE measurement sensitivity. Minimizing these factors allows us to detect photocurrents smaller than a fA, corresponding to EQE signals as small as -100 dB. We use these ultra-sensitive EQE measurements to probe weak sub-bandgap absorption features in organic, inorganic and perovskite semiconductors. In this regard, we directly observe photocurrent-contributing sub-gab trap states in organic solar cells significantly lower in energy than the corresponding charge-transfer state.
Recent advances in organic solar cell material development based around non-fullerene electron acceptors in bulk heterojunctions have propelled power conversion efficiencies to >18%, with 20% on the horizon and 25% predicted. These efficiencies are close to traditional inorganic semiconductor photovoltaics and thus focus is now turning to manufacturability and creating a viable solar cell technology. In this presentation we report the highest efficiency to date (16% with a Fill Factor >70%) in a thick junction binary organic solar cell based upon PM6:BTP-eC9. Using a very accurate approach based upon temperature dependent ultra-sensitive EQE measurements we find that this system (and a similar one based upon PM6:Y6) have near unity charge generation yields (CGY > 99%). In this regime, we observe that a small increase in CGY of only 0.5% leads to a 2.5 times more reduction in bimolecular recombination relative to the Langevin limit enabling high efficiency thick junction solar cells.
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