Light-intensity-dependent photoresponse time of organic photodetectors and its molecular origin.

Organic photodetectors (OPDs) exhibit superior spectral responses but slower photoresponse times compared to inorganic counterparts. Herein, we study the light-intensity-dependent OPD photoresponse time with two small-molecule donors (planar MPTA or twisted NP-SA) co-evaporated with C60 acceptors. MPTA:C60 exhibits the fastest response time at high-light intensities (>0.5 mW/cm2), attributed to its planar structure favoring strong intermolecular interactions. However, this blend exhibits the slowest response at low-light intensities, which is correlated with biphasic photocurrent transients indicative of the presence of a low density of deep trap states. Optical, structural, and energetical analyses indicate that MPTA molecular packing is strongly disrupted by C60, resulting in a larger (370 meV) HOMO level shift. This results in greater energetic inhomogeneity including possible MPTA-C60 adduct formation, leading to deep trap states which limit the low-light photoresponse time. This work provides important insights into the small molecule design rules critical for low charge-trapping and high-speed OPD applications.

Identifying the Molecular Origins of High-Performance in Organic Photodetectors Based on Highly Intermixed Bulk Heterojunction Blends.

A bulk-heterojunction (BHJ) structure of organic semiconductor blend is widely used in photon-to-electron converting devices such as organic photodetectors (OPD) and photovoltaics (OPV). However, the impact of the molecular structure on the interfacial electronic states and optoelectronic properties of the constituent organic semiconductors is still unclear, limiting further development of these devices for commercialization. Herein, the critical role of donor molecular structure on OPD performance is identified in highly intermixed BHJ blends containing a small-molecule donor and C60 acceptor. Blending introduces a twisted structure in the donor molecule and a strong coupling between donor and acceptor molecules. This results in ultrafast exciton separation (<1 ps), producing bound (binding energy ∼135 meV), localized (∼0.9 nm), and highly emissive interfacial charge transfer (CT) states. These interfacial CT states undergo efficient dissociation under an applied electric field, leading to highly efficient OPDs in reverse bias but poor OPVs. Further structural twisting and molecular-scale aggregation of the donor molecules occur in blends upon thermal annealing just above the transition temperature of 150 °C at which donor molecules start to reorganize themselves without any apparent macroscopic phase-segregation. These subtle structural changes lead to significant improvements in charge transport and OPD performance, yielding ultralow dark currents (∼10-10 A cm-2), 2-fold faster charge extraction (in µs), and nearly an order of magnitude increase in effective carrier mobility. Our results provide molecular insights into high-performance OPDs by identifying the role of subtle molecular structural changes on device performance and highlight key differences in the design of BHJ blends for OPD and OPV devices.

Origin of Open-Circuit Voltage Losses in Perovskite Solar Cells Investigated by Surface Photovoltage Measurement.

Increasing the open-circuit voltage (Voc) is one of the key strategies for further improvement of the efficiency of perovskite solar cells. It requires fundamental understanding of the complex optoelectronic processes related to charge carrier generation, transport, extraction, and their loss mechanisms inside a device upon illumination. Herein, we report the important origin of Voc losses in methylammonium lead iodide perovskite (MAPI)-based solar cells, which results from undesirable positive charge (hole) accumulation at the interface between the perovskite photoactive layer and the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hole-transport layer. We show strong correlation between the thickness-dependent surface photovoltage and device performance, unraveling that the interfacial charge accumulation leads to charge carrier recombination and results in a large decrease in Voc for the PEDOT:PSS/MAPI inverted devices (180 mV reduction in 50 nm thick device compared to 230 nm thick one). In contrast, accumulated positive charges at the TiO2/MAPI interface modify interfacial energy band bending, which leads to an increase in Voc for the TiO2/MAPI conventional devices (70 mV increase in 50 nm thick device compared to 230 nm thick one). Our results provide an important guideline for better control of interfaces in perovskite solar cells to improve device performance further.

An Efficient, "Burn in" Free Organic Solar Cell Employing a Nonfullerene Electron Acceptor.

A comparison of the efficiency, stability, and photophysics of organic solar cells employing poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3'″-di(2-octyldodecyl)-2,2';5',2″;5″,2'″-quaterthiophen-5,5'″-diyl)] (PffBT4T-2OD) as a donor polymer blended with either the nonfullerene acceptor EH-IDTBR or the fullerene derivative, [6,6]-phenyl C71 butyric acid methyl ester (PC71 BM) as electron acceptors is reported. Inverted PffBT4T-2OD:EH-IDTBR blend solar cell fabricated without any processing additive achieves power conversion efficiencies (PCEs) of 9.5 ± 0.2%. The devices exhibit a high open circuit voltage of 1.08 ± 0.01 V, attributed to the high lowest unoccupied molecular orbital (LUMO) level of EH-IDTBR. Photoluminescence quenching and transient absorption data are employed to elucidate the ultrafast kinetics and efficiencies of charge separation in both blends, with PffBT4T-2OD exciton diffusion kinetics within polymer domains, and geminate recombination losses following exciton separation being identified as key factors determining the efficiency of photocurrent generation. Remarkably, while encapsulated PffBT4T-2OD:PC71 BM solar cells show significant efficiency loss under simulated solar irradiation ("burn in" degradation) due to the trap-assisted recombination through increased photoinduced trap states, PffBT4T-2OD:EH-IDTBR solar cell shows negligible burn in efficiency loss. Furthermore, PffBT4T-2OD:EH-IDTBR solar cells are found to be substantially more stable under 85 °C thermal stress than PffBT4T-2OD:PC71 BM devices.