Complex multilength-scale morphology in organic photovoltaics

Fundamental knowledge regarding material crystallization and phase separation in bulk heterojunction organic solar cell thin films has been summarized. The coupling and competition of the crystallization and demixing of donor and acceptor materials leads to the complex thin-film morphology in organic solar cells. The packing and phase-separated morphology of conjugated polymers and non-fullerene acceptors constitute the interpenetrating network structure. The structural details of the multilength-scale morphology and their impact on exciton kinetics, charge separation, carrier transport, and device performance are demonstrated. Control over the morphology in bulk heterojunction (BHJ) organic photovoltaics (OPVs) remains a key issue in improving the power conversion efficiency (PCE), despite the performance advances in recent years. This review summarizes the morphological features and guiding strategies of OPV blends spanning fullerene blends, non-fullerene blends, and all-polymer systems. The crystallization and liquid–liquid (L-L) demixing of materials, including donors and acceptors, are in competition to form the morphology across multiple length scales. Its character can be manipulated by controlling the kinetics and thermodynamics in film drying processes and post-treatments, affording appropriate phase separation and interfaces to promote charge generation and transport. The structure–property relationships constructed based on pivotal characterization techniques are discussed and future perspectives are provided. Control over the morphology in bulk heterojunction (BHJ) organic photovoltaics (OPVs) remains a key issue in improving the power conversion efficiency (PCE), despite the performance advances in recent years. This review summarizes the morphological features and guiding strategies of OPV blends spanning fullerene blends, non-fullerene blends, and all-polymer systems. The crystallization and liquid–liquid (L-L) demixing of materials, including donors and acceptors, are in competition to form the morphology across multiple length scales. Its character can be manipulated by controlling the kinetics and thermodynamics in film drying processes and post-treatments, affording appropriate phase separation and interfaces to promote charge generation and transport. The structure–property relationships constructed based on pivotal characterization techniques are discussed and future perspectives are provided. the diffusion coefficient of the molecules or polymer chains during the nonequilibrium processes. the ratio of the collected photogenerated charges to the number of incident photons. a solar cell parameter that is defined by FF=PmaxJSC×VOC=Pin×PCEJSC×VOC. the glass–rubber transition temperature of amorphous materials (or the amorphous phase of semicrystalline materials), above which the mobility of structural segments is significantly increased. the lowest temperature of the two-phase region in the phase diagrams of component blends, below which the homogeneous mixtures are stable. absorption spectroscopy arising by the excitation of the core-shell electrons into empty orbitals above the valence shell, obtaining structural information with elemental and bonding-environmental sensitivities. the voltage when the solar cell is at the open-circuit condition. the ratio of maximum power output (Pmax) to power input (Pin). Pmax is the product of the JSC, VOC, and FF of the solar cell. The standard solar irradiance of 100 mW cm−2 is usually used as Pin. a scattering technique combining NEXAFS and GISAXS, with the element-sensitive contrast function Δδ2 + Δβ2, where the energy-dependent indices δ and β describe the dispersion and absorption aspects of the interaction between the X-ray and the materials. the current density when the solar cell is at the short-circuit condition. the highest temperature of the two-phase region in the phase diagrams of component blends, above which the homogeneous mixtures are stable. an empirical equation describing the temperature dependence of relaxation times as a function of the temperature T and a reference temperature Ts. The form of the equation is logaT = − C1(T − Ts)/(C2 + T − Ts), where aT is the ratio of any mechanical or electrical relaxation time at T to its value at Ts; C1 and C2 are the empirical constants.