Electric power may, in principle, be generated in a highly efficient manner from heat created by focused solar irradiation, chemical combustion, or nuclear decay by means of thermionic energy conversion. As the conversion efficiency of the thermionic process tends to be degraded by electron space charges, the efficiencies of thermionic generators have amounted to only a fraction of those fundamentally possible. We show that this space-charge problem can be resolved by shaping the electric potential distribution of the converter such that the static electron space-charge clouds are transformed into an output current. Although the technical development of practical generators will require further substantial efforts, we conclude that a highly efficient transformation of heat to electric power may well be achieved.

In this work, we describe the formation of a reduced bandgap CeNiO phase, which, to our knowledge, has not been previously reported, and we show how it is utilized as an absorber layer in a photovoltaic cell. The CeNiO phase is prepared by a combinatorial materials science approach, where a library containing a continuous compositional spread of Ce NiO is formed by pulsed laser deposition (PLD); a method that has not been used in the past to form Ce-Ni-O materials. The library displays a reduced bandgap throughout, calculated to be 1.48-1.77 eV, compared to the starting materials, CeO and NiO, which each have a bandgap of ∼3.3 eV. The materials library is further analyzed by X-ray diffraction to determine a new crystalline phase. By searching and comparing to the Materials Project database, the reduced bandgap CeNiO phase is realized. The CeNiO reduced bandgap phase is implemented as the absorber layer in a solar cell and photovoltages up to 550 mV are achieved. The solar cells are also measured by surface photovoltage spectroscopy, which shows that the source of the photovoltaic activity is the reduced bandgap CeNiO phase, making it a viable material for solar energy.

In this Letter, we systematically explore the influence of TiO2 thickness with nanometric variations over a range of 20–600 nm on the photovoltaic parameters (open-circuit voltage, short circuit current, fill-factor, and power conversion efficiency) of CH3NH3PbI3-based solar cells. We fabricate several sample libraries of 13 × 13 solar cells on large substrates with spatial variations in the thickness of the TiO2 layers while maintaining similar properties for the other layers. We show that the optimal thickness is ∼50 nm for maximum performance; thinner layers typically resulted in short-circuited cells, whereas increasing the thickness led to a monotonic decrease in performance. Furthermore, by assuming a fixed bulk resistivity of TiO2, we were able to correlate the TiO2 thickness to the series and shunt resistances of the devices and model the variation in the photovoltaic parameters with thickness using the diode equation to gain quantitative insights.

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