Furthermore, charge transport layers such as PEDOT:PSS and fullerenes used in both organic photovoltaics (OPV) and perovskite solar cells (PSCs) are also candidates for thermoelectrics. For the same dopant, the doping regime for solar cells and thermoelectrics can range from ultralow to high, respectively. In addition, similar dopants are being explored for molecular doping of organic and perovskite semiconductors. HPVK doping to some extent is inspired by doped OSCs. (16) Additive engineering mainly deals with solid or solvent additives with the goal of physical and chemical passivation, morphology control, stability improvement, enhanced miscibility, and in some cases improved transport properties. (15) In contrast to alloying, doping concentrations are generally low, while preserving the crystal structure of the host. Alloying is a solid solution between two or more materials. (14) It should be noted that doping is fundamentally different than alloying and additive engineering, which are other popular strategies being explored for perovskite and organic solar cells. (13) In solar cells, doping can influence carrier diffusion length, open circuit voltage, interfacial energy barrier, contact resistance, and charge recombination rate, all of which govern the device performance. (12) Depending on which type of carrier is found in excess, the semiconductor is referred as p-type (hole majority) or n-type (electron excess). The addition of specific dopants creates defect states in the semiconductor gap offering control over the carrier density (holes or electrons depending on doping type) and, thus, its electrical and optical properties. As in the case of traditional semiconductors, doping plays a crucial role in these emerging materials to tune their optical and electronic properties. The bandgap, chemical tunability, and large library of compositional variants as well as solution-processability accessible by OSCs and HPVKs make them suitable for next-generation energy conversion devices. (9,10) While organic semiconductors (OSCs) are being explored as potential thermoelectric materials from the past several decades, HPVKs have only started gaining attention recently due to their ultralow thermal conductivity and “phonon glass, electron crystal” (PGEC) structure. (4−8) In addition, halide perovskites (HPVK) have exhibited exceptional performance in other functional devices such as radiation detectors and LEDs. The efficacy of halide perovskites as a PV material is proved by the continuous surge in its power conversion efficiency (PCE) since its reemergence in 2009. (1−3) Another important material system that has revolutionized the PV field is halide perovskites. In the last two decades, organic semiconductors have expanded their applicability to various applications from photovoltaics (PV) and thermoelectrics to catalysis. In particular, solution-processed materials for such devices are pursued owing to their simple and cost-effective fabrication process. Energy conversion devices such as solar cells and thermoelectric generators have attracted significant interest in recent times.
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