Concept: Carrier generation and recombination
Silicon nanowire and nanopore arrays promise to reduce manufacturing costs and increase the power conversion efficiency of photovoltaic devices. So far, however, photovoltaic cells based on nanostructured silicon exhibit lower power conversion efficiencies than conventional cells due to the enhanced photocarrier recombination associated with the nanostructures. Here, we identify and separately measure surface recombination and Auger recombination in wafer-based nanostructured silicon solar cells. By identifying the regimes of junction doping concentration in which each mechanism dominates, we were able to design and fabricate an independently confirmed 18.2%-efficient nanostructured ‘black-silicon’ cell that does not need the antireflection coating layer(s) normally required to reach a comparable performance level. Our results suggest design rules for efficient high-surface-area solar cells with nano- and microstructured semiconductor absorbers.
We study the photoresponse of single-layer MoS2 field-effect transistors by scanning photocurrent microscopy. We find that, unlike in many other semiconductors, the photocurrent generation in single-layer MoS2 is dominated by the photo-thermoelectric effect and not by the separation of photoexcited electron-hole pairs across the Schottky barriers at the MoS2/electrode interfaces. We observe a large value for the Seebeck coefficient for single-layer MoS2 that, by an external electric field, can be tuned between -4x10^2 µV/K and -1x10^5 µV/K. This large and tunable Seebeck coefficient of the single-layer MoS2 paves the way to new applications of this material such as on-chip thermopower generation and waste thermal energy harvesting.
Understanding exciton behavior in organic semiconductor molecules is crucial for the development of organic semiconductor-based excitonic devices such as organic light-emitting diodes and organic solar cells, and the tightly bound electron-hole pair forming an exciton is normally assumed to be localized on an organic semiconducting molecule. We report the observation of long-range coupling of electron-hole pairs in spatially separated electron-donating and electron-accepting molecules across a 10-nanometers-thick spacer layer. We found that the exciton energy can be tuned over 100 megaelectron volts and the fraction of delayed fluorescence can be increased by adjusting the spacer-layer thickness. Furthermore, increasing the spacer-layer thickness produced an organic light-emitting diode with an electroluminescence efficiency nearly eight times higher than that of a device without a spacer layer. Our results demonstrate the first example of a long-range coupled charge-transfer state between electron-donating and electron-accepting molecules in a working device.
The nanostructuring of silicon surfaces-known as black silicon-is a promising approach to eliminate front-surface reflection in photovoltaic devices without the need for a conventional antireflection coating. This might lead to both an increase in efficiency and a reduction in the manufacturing costs of solar cells. However, all previous attempts to integrate black silicon into solar cells have resulted in cell efficiencies well below 20% due to the increased charge carrier recombination at the nanostructured surface. Here, we show that a conformal alumina film can solve the issue of surface recombination in black silicon solar cells by providing excellent chemical and electrical passivation. We demonstrate that efficiencies above 22% can be reached, even in thick interdigitated back-contacted cells, where carrier transport is very sensitive to front surface passivation. This means that the surface recombination issue has truly been solved and black silicon solar cells have real potential for industrial production. Furthermore, we show that the use of black silicon can result in a 3% increase in daily energy production when compared with a reference cell with the same efficiency, due to its better angular acceptance.
Bismuth vanadate (BiVO4) has a band structure well-suited for potential use as a photoanode in solar water splitting, but it suffers from poor electron-hole separation. Here, we demonstrate that a nanoporous morphology (specific surface area of 31.8 m(2)/g) effectively suppresses bulk carrier recombination without additional doping, manifesting an electron-hole separation yield of 0.90 at 1.23 V vs. the reversible hydrogen electrode (RHE). We enhanced the propensity for surface-reaching holes to instigate water splitting chemistry by serially applying two different oxygen evolution catalyst (OEC) layers, FeOOH and NiOOH, which reduces interface recombination at the BiVO4/OEC junction while creating a more favorable Helmholtz layer potential drop at the OEC/electrolyte junction. The resulting BiVO4/FeOOH/NiOOH photoanode achieves a photocurrent density of 2.73 mA/cm(2) at a potential as low as 0.6 V vs. RHE.
There has been long-standing debate on how free charges are generated in donor:acceptor blends that are used in organic solar cells, and which are generally comprised of a complex phase morphology, where intermixed and neat phases of the donor and acceptor material co-exist. Here we resolve this question, basing our conclusions on Stark effect spectroscopy data obtained in the absence and presence of externally applied electric fields. Reconciling opposing views found in literature, we unambiguously demonstrate that the fate of photogenerated electron-hole pairs-whether they will dissociate to free charges or geminately recombine-is determined at ultrafast times, despite the fact that their actual spatial separation can be much slower. Our insights are important to further develop rational approaches towards material design and processing of organic solar cells, assisting to realize their purported promise as lead-free, third-generation energy technology that can reach efficiencies over 10%.
Advancing perovskite solar cell technologies toward their theoretical power conversion efficiency (PCE) requires delicate control over the carrier dynamics throughout the entire device. By controlling the formation of the perovskite layer and careful choices of other materials, we suppressed carrier recombination in the absorber, facilitated carrier injection into the carrier transport layers, and maintained good carrier extraction at the electrodes. When measured via reverse bias scan, cell PCE is typically boosted to 16.6% on average, with the highest efficiency of ~19.3% in a planar geometry without antireflective coating. The fabrication of our perovskite solar cells was conducted in air and from solution at low temperatures, which should simplify manufacturing of large-area perovskite devices that are inexpensive and perform at high levels.
Metal halide perovskites such as methylammonium lead iodide (CH3NH3PbI3) are generating great excitement due to their outstanding optoelectronic properties, which lend them to application in high-efficiency solar cells and light-emission devices. However, there is currently debate over what drives the second-order electron-hole recombination in these materials. Here, we propose that the bandgap in CH3NH3PbI3 has a direct-indirect character. Time-resolved photo-conductance measurements show that generation of free mobile charges is maximized for excitation energies just above the indirect bandgap. Furthermore, we find that second-order electron-hole recombination of photo-excited charges is retarded at lower temperature. These observations are consistent with a slow phonon-assisted recombination pathway via the indirect bandgap. Interestingly, in the low-temperature orthorhombic phase, fast quenching of mobile charges occurs independent of the temperature and photon excitation energy. Our work provides a new framework to understand the optoelectronic properties of metal halide perovskites and analyse spectroscopic data.
Development of light-emitting diodes (LEDs) based on colloidal quantum dots is driven by attractive properties of these fluorophores such as spectrally narrow, tunable emission and facile processibility via solution-based methods. A current obstacle towards improved LED performance is an incomplete understanding of the roles of extrinsic factors, such as non-radiative recombination at surface defects, versus intrinsic processes, such as multicarrier Auger recombination or electron-hole separation due to applied electric field. Here we address this problem with studies that correlate the excited state dynamics of structurally engineered quantum dots with their emissive performance within LEDs. We find that because of significant charging of quantum dots with extra electrons, Auger recombination greatly impacts both LED efficiency and the onset of efficiency roll-off at high currents. Further, we demonstrate two specific approaches for mitigating this problem using heterostructured quantum dots, either by suppressing Auger decay through the introduction of an intermediate alloyed layer, or by using an additional shell that impedes electron transfer into the quantum dot to help balance electron and hole injection.
We use correlated confocal and widefield fluorescence microscopy to probe the interplay between local variations in charge carrier recombination and charge carrier transport in methylammonium lead triiodide perovskite thin films. We find that local photoluminescence variations present in confocal imaging are also observed in widefield imaging, while intensity-dependent confocal measurements show that the heterogeneity in non-radiative losses observed at low excitation powers becomes less pronounced at higher excitation powers. Both confocal and widefield images show that carriers undergo anisotropic diffusion due to differences in inter-grain connectivity. These data are all qualitatively consistent with trap-dominated variations in local photoluminescence intensity, and with grain boundaries that exhibit varying degrees of opacity to carrier transport. We use a two-dimensional kinetic model to simulate and compare confocal time-resolved photoluminescence decay traces with experimental data. The simulations further support the assignment of local variations in non-radiative recombination as the primary cause of photoluminescence heterogeneity in the films studied herein. These results point to surface passivation and inter-grain connectivity as areas that could yield improvements in perovskite solar cell and optoelectronic device performance.