A novel photomultiplication (PM)‐type organic photodiode (OPD) that responds much faster (109 kHz bandwidth) than conventional PM‐type OPDs is demonstrated. This fast response is achieved by introducing quantum dots (QDs) as a PM‐inducing interlayer at the interface between the electrode and the photoactive layer. When the device is illuminated, the photogenerated electrons within the photoactive layer are rapidly transferred and trapped in the trap states of the QD interlayer. The electron trapping subsequently leads to charging of the QD and a consequent shift of the QD energy levels, thereby inducing hole injection from the electrode. This PM mechanism is distinct from that of conventional PM‐type OPDs, whose PM usually requires a long time to induce hole (or electron) injection because of the slow transport and accumulation of electrons (or holes) within the photoactive layer. Because of its PM mechanism, the proposed QD‐interlayer PM‐type OPD achieves high bandwidth and high specific detectivity. In addition, it is demonstrated that the response speed of the proposed device is closely related to the charge trapping/detrapping dynamics of the QDs. This work not only offers a new concept in the design of fast‐responding PM‐type OPDs but also provides comprehensive understanding of the underlying device physics.

Dimensionality engineering is an effective approach to improve the stability and power conversion efficiency (PCE) of perovskite solar cells (PSCs). A two-dimensional (2D) perovskite assembled from bulky organic cations to cover the surface of three-dimensional (3D) perovskite can repel ambient moisture and suppress ion migration across the perovskite film. This work demonstrates how the thermal stability of the bulky organic cation of a 2D perovskite affects the crystallinity of the perovskite and the optoelectrical properties of perovskite solar cells. Structural analysis of (FAPbI3)0.95(MAPbBr3)0.05 (FA = formamidinium ion, MA = methylammonium ion) mixed with a series of bulky cations shows a clear correlation between the structure of the bulky cations and the formation of surface defects in the resultant perovskite films. An organic cation with primary ammonium structure is vulnerable to a deprotonation reaction under typical perovskite-film processing conditions. Decomposition of the bulky cations results in structural defects such as iodide vacancies and metallic lead clusters at the surface of the perovskite film; these defects lead to a nonradiative recombination loss of charge carriers and to severe ion migration during operation of the device. In contrast, a bulky organic cation with a quaternary ammonium structure exhibits superior thermal stability and results in substantially fewer structural defects at the surface of the perovskite film. As a result, the corresponding PSC exhibits the PCE of 21.6% in a reverse current–voltage scan and a stabilized PCE of 20.1% with an excellent lifetime exceeding 1000 h for the encapsulated device under continuous illumination.

Photoinduced capacitances of perovskite solar cells exhibit peculiarities such as an apparently high capacitance and an inductive feature, so‐called negative capacitance, which are not easily explained with typical carrier dynamics. Consequently, the origins of the photoinduced capacitances in perovskite solar cells have been intensively debated over the past several years. Here, the photoinduced capacitances of perovskite solar cells are analyzed by impedance spectroscopy. The analysis clarifies that the photoinduced capacitances of perovskite solar cells comprise several Debye relaxation‐type capacitance components. Among these components, the photoinduced capacitance in the low‐frequency range is attributed to ionic‐to‐electronic current amplification. However, the photoinduced capacitances in the middle‐ and high‐frequency ranges originate from bipolar injection. The clear elucidation of the origins of the photoinduced capacitances in perovskite solar cells provides comprehensive insights for analyzing properties of perovskites in either the time or frequency domain.

Electrical doping is essentially required for high‐performance organic thermoelectric (TE) materials; however, the doping efficiency ηd has not been extensively investigated in highly doped organic semiconductors (OSCs). Here, it is demonstrated that the distribution of dopant molecules in a specific position in highly doped OSCs affects the ηd, which is critically related to the Seebeck coefficient S and the electrical conductivity σ. Poly(2,5‐bis(3‐hexadecylthiophen‐2‐yl)thieno[3,2‐b]thiophene) (PBTTT) films are p‐doped with 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4TCNQ) by either solution‐sequential (SSq) doping or vapor doping. SSq doping deposited F4TCNQ only in the amorphous domains of PBTTT films, whereas vapor doping deposited it in both the amorphous and crystalline domains. F4TCNQ molecules in the crystalline domains exhibited a high ηd and led to a rapid increase of the power factor with increasing σ: S2σ ∝ σ0.76. These results provide guidance for the efficient doping of highly doped OSCs and emphasize the importance of doping efficiency in obtaining high‐performance organic TE materials.