Carrier multiplication (CM), where a single high-energy photon generates multiple electron–hole pairs, offers a promising route to enhance the efficiency of solar cells and photodetectors.Transition metal dichalcogenides, such as 2H-MoTe2 and 2H-WSe2, exhibit efficient CM. Given the similar electronic band structure of 2H-MoSe2, it is expected to show comparable CM efficiency. In this study, we establish the occurrence and efficiency of CM in a solution-processed thin film of bulk-like 2H-MoSe2. We characterize the dynamics of excitons and free charge carriers by using ultrafast transient optical absorption and terahertz spectroscopy. At higher photon energy the efficiency is comparable to literature results for 2H-MoTe2 grown by chemical vapor deposition (CVD) or in bulk crystalline form. At higher photon energies the experimental CM efficiency is reproduced by theoretical modeling. We also observe CM for photon energies below the energetic threshold of twice the band gap, which is most probably due to subgap defect states. Transient optical absorption spectra of 2H-MoSe2 exhibit features of trions from which we infer that photoexcitation leads to free charge carriers. We find no signatures of excitons at the indirect band gap. From analysis of the frequency dependence of the terahertz conductivity we infer that scattering of charge carriers in our sample is less than for CVD grown or bulk crystalline 2H-MoTe2. Our findings make solution-processed 2H-MoSe2 an interesting material for exploitation of CM in photovoltaic devices.

We present a theoretical model to compute the efficiency of the generation of two or more electron-hole pairs in a semiconductor by the absorption of one photon via the process of carrier multiplication (CM). The photogeneration quantum yield of electron-hole pairs is calculated from the number of possible CM decay pathways of the electron and the hole. We apply our model to investigate the underlying cause of the high efficiency of CM in bulk 2H-MoTe₂, as compared to bulk PbS and PbSe. Electronic band structures were calculated with density functional theory, from which the number of possible CM decay pathways was calculated for all initial electron and hole states that can be produced at a given photon energy. The variation of the number of CM pathways with photon energy reflects the dependence of experimental CM quantum yields on the photon energy and material composition. We quantitatively reproduce experimental CM quantum yields for MoTe₂, PbS, and PbSe from the calculated number of CM pathways and one adjustable fit parameter. This parameter is related to the ratio of Coulomb coupling matrix elements and the cooling rate of the electrons and holes. Large variations of this fit parameter result in small changes in the modeled quantum yield for MoTe₂, which confirms that its high CM efficiency can be mainly attributed to its extraordinary large number of CM pathways. The methodology of this work can be applied to analyze or predict the CM efficiency of other materials.

We studied the nature of excitons in the transition metal dichalcogenide alloy Mo0.6W0.4S2 compared to pure MoS2 and WS2 grown by atomic layer deposition (ALD). For this, optical absorption/transmission spectroscopy and time-dependent density functional theory (TDDFT) were used. The effects of temperature on A and B exciton peak energies and line widths in optical transmission spectra were compared between the alloy and pure MoS2 and WS2. On increasing the temperature from 25 to 293 K, the energy of the A and B exciton peaks decreases, while their line width increases due to exciton-phonon interactions. The exciton-phonon interactions in the alloy are closer to those for MoS2 than those for WS2. This suggests that exciton wave functions in the alloy have a larger amplitude on Mo atoms than that on W atoms. The experimental absorption spectra could be reproduced by TDDFT calculations. Interestingly, for the alloy, the Mo and W atoms had to be distributed over all layers. Conversely, we could not reproduce the experimental alloy spectrum by calculations on a structure with alternating layers, in which every other layer contains only Mo atoms and the layers in between also contain W atoms. For the latter atomic arrangement, the TDDFT calculations yielded an additional optical absorption peak that could be due to excitons with some charge transfer character. From these results, we conclude that ALD yields an alloy in which Mo and W atoms are distributed uniformly among all layers.