We prepared MoS 2 monolayers on Au nanodot (ND) and nanohole (NH) arrays. Both these sample arrays exhibited enhanced photoluminescence intensity compared with that of a bare SiO 2 /Si substrate. The reflectance spectra of MoS 2 /ND and MoS 2 /NH had clear features originating from excitation of localized surface plasmon and propagating surface plasmon polaritons. Notably, the surface photovoltages (SPV) of these hybrid plasmonic nanostructures had opposite polarities, indicating negative and positive charging at MoS 2 /ND and MoS 2 /NH, respectively. Surface potential maps, obtained by Kelvin probe force microscopy, suggested that the potential gradient led to a distinct spatial distribution of photo-generated charges in these two samples under illumination. Furthermore, the local density of photo-generated excitons, as predicted from optical simulations, explained the SPV spectra of MoS 2 /ND and MoS 2 /NH. We show that the geometric configuration of the plasmonic nanostructures modified the polarity of photo-generated excess charges in MoS 2 . These findings point to a useful means of optimizing optoelectronic characteristics and improving the performance of MoS 2 -based plasmonic devices.

Dilute donor organic solar cells (OSCs) are a promising technology to circumvent the trade‐off between open‐circuit voltage (Voc) and short‐circuit current density (Jsc). The origin of hole transport in OSCs with donor concentrations below the percolation threshold is diversely discussed in the community. Herein, both hole back transfer and long‐range hopping (tunneling) are analyzed as possible mechanisms of photocurrent in small molecule dilute donor OSCs using kinetic Monte Carlo (kMC) simulations. In contrast to previous kMC studies, the driving force for exciton dissociation is accounted for. As a study system, nitrogen‐bridged terthiophene (NBTT) molecules in a [6,6]‐phenyl‐C70‐butyric acid methyl ester (PC71BM) matrix are investigated. The simulations show that hole back transfer from the small molecule donor to the fullerene matrix explains the measured concentration dependences of the photocurrents as well as the Jsc dependence on the light intensity for donor concentrations below 5 wt%. For 5 wt%, distances between NBTT molecules decrease to reasonable ranges that long‐range hopping or tunneling cannot be discounted. Compared with polymer donors, larger hole localization is observed. The results emphasize that the barrier for hole back transfer is not only due to the highest occupied molecular orbital (HOMO) offset, but also by hole localization.

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We performed frequency-modulated (AC) and steady-state (DC) surface photovoltage spectroscopy (SPS) measurements on a bilayer structure consisting of an organic semiconductor (P3HT, P3HT:PC61BM, or PFBT2Se2Th:PC71BM) on top of a ZnO electron-transport layer. The AC spectra overlap with the absorption spectra of the organic layer, providing evidence that AC SPS corresponds to band-to-band transitions. The DC spectra are generally broader than the AC spectra, with responses extended below the absorption edge. Thus, DC SPS also probes transitions between band states and trap states within the band gap in addition to band-to-band transitions. When a hole-transport layer (HTL) is deposited on top of the organic layer, the DC spectra of P3HT and P3HT:PC61BM are narrower than those without the HTL, suggesting that the sub-band gap states exist at the surface of these organic semiconductors. In contrast, PFBT2Se2Th:PC71BM does not show signature of surface states or optically active trap states in the band gap. External quantum efficiency and capacitance measurements are employed to explain the nature of sub-band gap states that contribute to surface photovoltage signals and the differences between the two bulk heterojunction systems.

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Hybrid organic-inorganic lead halide perovskite materials show great promise in a number of optoelectronic applications, including solar cells, light emitting diodes, and photodetectors. Understanding their intrinsic material properties is critical to enhancing device performance and enabling innovative material and device designs. Here, we study lattice dynamics using far-infrared (FIR) reflectance and photogenerated carrier dynamics using surface photovoltage (SPV) measurements on high-quality methylammonium lead bromide (MAPbBr3) single crystals. FIR reflectance shows three coherent infrared-active phonon modes between 40 and 200 cm−1 that result in reststrahlen bands with much higher peak reflectance than has been previously reported. The phonon mode strength and damping are comparable to classical oxide perovskite single crystals. However, the effects of defects on photogenerated carrier recombination are still evident in SPV measurements. By performing SPV over different spectral ranges, we are able to separate the effects of surface and bulk defects on the recombination dynamics of photogenerated charge carriers. We further apply SPV measurements to obtain the minority carrier (electron) diffusion length for the MAPbBr3 crystal. This study demonstrates that both FIR reflectance and SPV measurements provide useful information on the electromagnetic response properties of halide perovskite single crystals.

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Dilute-donor (DD) organic photovoltaic (OPV) devices comprise a low concentration of donor molecules in an acceptor matrix. The open-circuit voltage (V oc) of these devices is commonly higher than their bulk heterojunction (BHJ) counterparts and has been attributed to Schottky barrier heights between the anode and the acceptor matrix or reduced bimolecular recombination due to smaller donor/acceptor interfacial areas. Here, we examine the V oc of a variety of small-molecule and polymer donors, all at 5 wt %, in both fullerene and non-fullerene acceptors by performing photovoltaic (PV) and electroluminescence (EL) measurements on the same devices. We find a substantial V oc variation for different donors in the same acceptor matrix, indicating that DD V oc cannot be adequately explained by the Schottky barrier or interfacial area. V oc values of fullerene DD devices vary linearly with the band gap value as determined from the intercept of PV external quantum efficiency and EL spectra. In contrast, the non-fullerene acceptor (NFA) DD devices show V oc variation with donors despite having the same band gap. These results show that V oc of DD OPVs is predominantly determined by non-radiative voltage loss, similar to BHJ OPVs. NFA-based DD devices show promises for low voltage loss and high charge generation.