Improved phase purity in 2D hybrid perovskite thin films with horizontal crystal orientation was achieved through slow crystallization employing lead-complexing solvent additives.

We have studied the emission and absorption properties of type II GaSb/ GaAs quantum dots embedded in a p-i-n photodiode. The excitation power evolution provides clear signatures of the spatially separated confinement of electrons and holes in these nanostructures. We have estimated the confinement potential for the holes to be ϳ500 meV, leading to an intense room temperature emission assisted by recapture processes from the wetting layer. Photocurrent measurements show strong absorption in the wetting layer and in the quantum dots at room temperature which are important for photodetection applications based in this system. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2827582͔In recent years, GaSb/ GaAs structures have aroused great interest due to their type II band alignment and intrinsically different behavior compared to the well known InAs/ GaAs system. Fundamental issues regarding their growth process, energy level structure, and optical properties in addition to their technological applications in photodetection and photovoltaics have been already investigated in different configurations such as quantum dots ͑QDs͒, 1-5 quantum wells 6 ͑QWs͒ or ternary compounds. 7 In this work, we present various results regarding the GaSb/ GaAs QDs system, which extend and complete previous works.The QDs studied here were grown by solid source molecular beam epitaxy on a n-type GaAs͑001͒ substrate after deposition of a n-type GaAs buffer layer ͑Si: 1 ϫ 10 18 cm −2 ͒. The QDs were nucleated at 480°C, using a growth rate of 0.1 ML/ s. The formation of the GaSb QDs was detected by the change of the reflection high energy electron diffraction pattern after the deposition of 1.3 ML of GaSb. The GaSb layer, with a nominal thickness of 2 ML, was then exposed to Sb flux for 20 s and then annealed for 20 s without an Sb flux to limit the amount of Sb segregated during capping. The GaAs capping was done at 0.4 ML/ s in two steps. In the first step, a 10 nm thick GaAs layer was grown at the temperature of QD nucleation to avoid their destabilization. In the second one, a 40 nm thick GaAs layer was deposited at 570°C. During growth, the As and Sb beam equivalent pressures were 1.0ϫ 10 −5 and 1.9ϫ 10 −6 mbar, respectively. This scheme was repeated six times, with a 3 min growth interruption under an As 4 flux to lower the substrate temperature before the nucleation of the next QD layer. On top, a p-type 300 nm thick GaAs layer ͑Be: 1 ϫ 10 18 cm −2 ͒ was grown at 580°C. Finally, standard optical lithography and wet etching techniques were used to define mesas and metal Ohmic contacts. Figure 1͑a͒ shows the photoluminescence ͑PL͒ spectra recorded at 20 K as a function of excitation power at 532 nm. We can clearly identify two bands centered at 1.32 and 1.05 eV. The narrow high energy band corresponds to the wetting layer ͑WL͒ recombination and dominates the spectrum at low temperatures. Its peak energy position is compatible with a GaSb WL thickness of 0.7 nm, 2 which is larger than the total amount of GaSb deposited ͑0.57 nm͒...

Bandgap engineering of strain-balanced InGaAs/GaAsP multiple quantum wells (MQWs) allows high-quality materials with an absorption edge beyond GaAs to be epitaxially grown in Ge/GaAs-based multijunction solar cells. We demonstrate MQW solar cells with effective bandgaps ranging from 1.31 eV to as low as 1.15 eV. The bandgap-voltage-offset of MQWs is found to be independent of effective bandgaps and superior to a bulk reference by approximately 0.1 V. This implies the merit of high photovoltage as compared with bulk cells with the same bandgap in addition to their widely bandgap-tunable property. Towards the realization of fully lattice-matched quad-junction devices, we demonstrate a 70-period, 1.15-eV bandgap MQW cell as a promising material in 0.66/1.15/1.51/1.99-eV quad-junction cells, whose practical efficiency has a potential to achieve over 50%. With such a large period number of MQWs, the reverse-biased external quantum efficiency reaches an average of over 60% in the spectral region corresponding to a 1.15-eV subcell; this is achieved with only a-few-percent drop at short-circuit condition. The device presented here reaches the target open-circuit voltage and over 75% of the current density required for realizing a 1.15-eV subcell in a 50%-efficient quad-junction solar cell. We believe that future devices which exploit light-trapping structures and enhanced carrier extraction will be able to reach the desired target.

Computational models can provide significant insight into the operation mechanisms and deficiencies of photovoltaic solar cells. Solcore is a modular set of computational tools, written in Python 3, for the design and simulation of photovoltaic solar cells. Calculations can be performed on ideal, thermodynamic limiting behaviour, through to fitting experimentally accessible parameters such as dark and light IV curves and luminescence. Uniquely, it combines a complete

The thermal emissivity of crystalline silicon photovoltaic (PV) solar cells plays a role in determining the operating temperature of a solar cell. To elucidate the physical origin of thermal emissivity, we have made an experimental measurement of the full radiative spectrum of the crystalline silicon (c-Si) solar cell, which includes both absorption in the ultraviolet to near-infrared range and emission in the mid-infrared. Using optical modelling, we have identified the origin of radiative emissivity in both encapsulated and unencapsulated solar cells. We find that both encapsulated and unencapsulated c-Si solar cells are good radiative emitters but achieve this through different effects. The emissivity of an unencapsulated c-Si solar cell is determined to be 75% in the MIR range, and is dominated by free-carrier emission in the highly doped emitter and back surface field layers; both effects are greatly augmented through the enhanced optical outcoupling arising from the front surface texture. An encapsulated glass-covered cell has an average emissivity around 90% on the MIR, and dips to 70% at 10 µm and is dominated by the emissivity of the cover glass. These findings serve to illustrate the opportunity for optimising the emissivity of c-Si based collectors, either in conventional c-Si PV modules where high emissivity and low-temperature operation is desirable, or in hybrid PV-thermal collectors where low emissivity enables a higher thermal output to be achieved