Emergence of New Materials for Exploiting Highly Efficient Carrier Multiplication in Photovoltaics

In conventional solar cell semiconductor materials (predominantly Si) photons with energy higher than the band gap initially generate hot electrons and holes, which subsequently cool down to the band edge by phonon emission. Due to the latter process, the energy of the charge carriers in excess of the band gap is lost as heat and does not contribute to the conversion of solar to electrical power. If the excess energy is more than the band gap it can in principle be utilized through a process known as carrier multiplication (CM) in which a single absorbed photon generates two (or more) pairs of electrons and holes. Thus, through CM the photon energy above twice the band gap enhances the photocurrent of a solar cell. In this review, we discuss recent progress in CM research in terms of fundamental understanding, emergence of new materials for efficient CM, and CM based solar cell applications. Based on our current understanding, the CM threshold can get close to the minimal value of twice the band gap in materials where a photon induces an asymmetric electronic transition from a deeper valence band or to a higher conduction band. In addition, the material must have a low exciton binding energy and high charge carrier mobility, so that photoexcitation leads directly to the formation of free charges that can readily be extracted at external electrodes of a photovoltaic device. Percolative networks of coupled PbSe quantum dots, Sn/Pb based halide perovskites, and transition metal dichalcogenides such as MoTe2 fulfill these requirements to a large extent. These findings point towards promising prospects for further development of new materials for highly efficient photovoltaics

Single-particle properties of topological Wannier excitons in bismuth chalcogenide nanosheets

We analyze the topology, dispersion, and optical selection rules of bulk Wannier excitons in nanosheets of Bi2Se3, a topological insulator in the family of the bismuth chalcogenides. Our main finding is that excitons also inherit the topology of the electronic bands, quantified by the skyrmion winding numbers of the constituent electron and hole pseudospins as a function of the total exciton momentum. The excitonic bands are found to be strongly indirect due to the band inversion of the underlying single-particle model. At zero total momentum, we predict that the s-wave and d-wave states of two exciton families are selectively bright under left- or right-circularly polarized light. We furthermore show that every s-wave exciton state consists of a quartet with a degenerate and quadratically dispersing nonchiral doublet, and a chiral doublet with one linearly dispersing mode as in transition metal dichalcogenides. Finally, we discuss the potential existence of topological edge states of chiral excitons arising from the bulk-boundary correspondence

Temperature dependence of the charge carrier mobility in gated quasi-one-dimensional systems

The many-body Monte Carlo method is used to evaluate the frequency dependent conductivity and the average mobility of a system of hopping charges, electronic or ionic on a one-dimensional chain or channel of finite length. Two cases are considered: the chain is connected to electrodes and in the other case the chain is confined giving zero dc conduction. The concentration of charge is varied using a gate electrode. At low temperatures and with the presence of an injection barrier, the mobility is an oscillatory function of density. This is due to the phenomenon of charge density pinning. Mobility changes occur due to the co-operative pinning and unpinning of the distribution. At high temperatures, we find that the electron-electron interaction reduces the mobility monotonically with density, but perhaps not as much as one might intuitively expect because the path summation favour the in-phase contributions to the mobility, i.e. the sequential paths in which the carriers have to wait for the one in front to exit and so on. The carrier interactions produce a frequency dependent mobility which is of the same order as the change in the dc mobility with density, i.e. it is a comparably weak effect. However, when combined with an injection barrier or intrinsic disorder, the interactions reduce the free volume and amplify disorder by making it non-local and this can explain the too early onset of frequency dependence in the conductivity of some high mobility quasi-one-dimensional organic materials.Comment: 9 pages, 8 figures, to be published in Physical Review

Broadband cooling spectra of hot electrons and holes in PbSe quantum dots

Understanding cooling of hot charge carriers in semiconductor quantum dots (QDs) is of fundamental interest and useful to enhance the performance of QDs in photovoltaics. We study electron and hole cooling dynamics in PbSe QDs up to high energies where carrier multiplication occurs. We characterize distinct cooling steps of hot electrons and holes and build up a broadband cooling spectrum for both charge carriers. Cooling of electrons is slower than of holes. At energies near the band gap we find cooling times between successive electronic energy levels in the order of 0.5 ps. We argue that here the large spacing between successive electronic energy levels requires cooling to occur by energy transfer to vibrational modes of ligand molecules or phonon modes associated with the QD surface. At high excess energy the energy loss rate of electrons is 1–5 eV/ps and exceeds 8 eV/ps for holes. Here charge carrier cooling can be understood in terms of emission of LO phonons with a higher density-of-states in the valence band than the conduction band. The complete mapping of the broadband cooling spectrum for both charge carriers in PbSe QDs is a big step toward understanding and controlling the cooling of hot charge carriers in colloidal QDs

Photon recycling in CsPbBr3 All-Inorganic Perovskite Nanocrystals

Photon recycling, the iterative process of re-absorption and re-emission of photons in an absorbing medium, can play an important role in the power-conversion efficiency of photovoltaic cells. To date, several studies have proposed that this process may occur in bulk or thin films of inorganic lead-halide perovskites, but conclusive proof of the occurrence and magnitude of this effect is missing. Here, we provide clear evidence and quantitative estimation of photon recycling in CsPbBr nanocrystal suspensions by combining measurements of steady-state and time-resolved photoluminescence (PL) and PL quantum yield with simulations of photon diffusion through the suspension. The steady-state PL shows clear spectral modifications including red shifts and quantum yield decrease, while the time-resolved measurements show prolonged PL decay and rise times. These effects grow as the nanocrystal concentration and distance traveled through the suspension increase. Monte Carlo simulations of photons diffusing through the medium and exhibiting absorption and re-emission account quantitatively for the observed trends and show that up to five re-emission cycles are involved. We thus identify 4 quantifiable measures, PL red shift, PL QY, PL decay time, and PL rise time that together all point toward repeated, energy-directed radiative transfer between nanocrystals. These results highlight the importance of photon recycling for both optical properties and photovoltaic applications of inorganic perovskite nanocrystals

Change in Tetracene Polymorphism Facilitates Triplet Transfer in Singlet Fission-Sensitized Silicon Solar Cells

Singlet fission in tetracene generates two triplet excitons per absorbed photon. If these triplet excitons can be effectively transferred into silicon (Si) then additional photocurrent can be generated from photons above the bandgap of Si. This could alleviate the thermalization loss and increase the efficiency of conventional Si solar cells. Here we show that a change in the polymorphism of tetracene deposited on Si due to air exposure, facilitates triplet transfer from tetracene into Si. Magnetic field-dependent photocurrent measurements confirm that triplet excitons contribute to the photocurrent. The decay of tetracene delayed photoluminescence was used to determine a triplet transfer time of 215 ns and a maximum yield of triplet transfer into Si of ~50 %. Our study suggests that control over the morphology of tetracene during deposition will be of great importance to boost the triplet transfer yield further

Quantifying the contribution of material and junction resistances in nano-networks

Networks of nanowires and nanosheets are important for many applications in printed electronics. However, the network conductivity and mobility are usually limited by the inter-particle junction resistance, a property that is challenging to minimise because it is difficult to measure. Here, we develop a simple model for conduction in networks of 1D or 2D nanomaterials, which allows us to extract junction and nanoparticle resistances from particle-size-dependent D.C. resistivity data of conducting and semiconducting materials. We find junction resistances in porous networks to scale with nanoparticle resistivity and vary from 5 Ohm for silver nanosheets to 25 GOhm for WS2 nanosheets. Moreover, our model allows junction and nanoparticle resistances to be extracted from A.C. impedance spectra of semiconducting networks. Impedance data links the high mobility (~7 cm2/Vs) of aligned networks of electrochemically exfoliated MoS2 nanosheets to low junction resistances of ~670 kOhm. Temperature-dependent impedance measurements allow us to quantitatively differentiate intra-nanosheet phonon-limited band-like transport from inter-nanosheet hopping for the first time.Comment: 5 figure