Terahertz photonics

Abstract:

Organic semiconductors are commonly used as charge-extraction layers in metal-halide perovskite solar cells. However, parasitic light absorption in the sun-facing front molecular layer, through which sun light must propagate before reaching the perovskite layer, may lower the power conversion efficiency of such devices. Here, we show that such losses may be eliminated through efficient excitation energy transfer from a photoexcited polymer layer to the underlying perovskite. Experimentally observed energy transfer between a range of different polymer films and a methylammonium lead iodide perovskite layer was used as basis for modelling the efficacy of the mechanism as a function of layer thickness, photoluminescence quantum efficiency and absorption coefficient of the organic polymer film. Our findings reveal that efficient energy transfer can be achieved for thin (≤10 nm) organic charge-extraction layers exhibiting high photoluminescence quantum efficiency. We further explore how the morphology of such thin polymer layers may be affected by interface formation with the perovskite.

Abstract:

Hybrid organic-inorganic perovskites are exciting materials for solar-energy applications whose microscopic properties are still not well understood. Atomic-resolution (scanning) transmission electron microscopy, (S)TEM, has provided invaluable insights for many crystalline solar-cell materials, and is used here to successfully image CH(NH2)2PbI3 thin films with low electron-radiation dose. Such images reveal a highly ordered atomic arrangement of sharp grain boundaries and coherent perovskite/PbI2 interfaces, with a striking absence of long-range disorder in the crystal. We demonstrate that beaminduced degradation of the perovskite leads to an initial loss of CH(NH2)2 + ions, leaving behind a partially unoccupied perovskite lattice, which explains the unusual regenerative properties of these materials. We further observe aligned point defects and climbdissociated dislocations. Our findings thus provide an atomic-level understanding of technologically important lead-halide perovskites.

Abstract:

Although metal halide perovskite photovoltaics have shown an unprecedented rise in power conversion efficiency (PCE), they remain far from their theoretical PCE limit. Among the highest efficiencies to date are delivered when polycrystalline films are enhanced via “molecular passivation”, but this can introduce new instabilities, in particular under severe accelerated aging conditions (e.g., at 85 °C in the dark or under full spectrum simulated sunlight). Here, we utilize a benzylammonium bromide passivation treatment to improve device performance, achieving the champion stabilized power output (SPO) of 19.5 % in a p-i-n device architecture. We correlate the improved device performance with a significant increase in charge carrier diffusion lengths, mobilities, and lifetimes. Furthermore, treated devices maintain an increased performance during 120 h combined stressing under simulated full spectrum sunlight at 85 °C, indicating that enhancement from this passivation treatment is sustained under harsh accelerated aging conditions. This is a crucial step toward real-world operation-relevant passivation treatments.

Abstract:

Mixed tin‐lead halide perovskites are promising low‐bandgap absorbers for all‐perovskite tandem solar cells that offer higher efficiencies than single‐junction devices. A significant barrier to higher performance and stability is the ready oxidation of tin, commonly mitigated by various additives whose impact is still poorly understood for mixed tin‐lead perovskites. Here, the effects of the commonly used SnF2 additive are revealed for FA0.83Cs0.17SnxPb1−xI3 perovskites across the full compositional lead‐tin range and SnF2 percentages of 0.1–20% of precursor tin content. SnF2 addition causes a significant reduction in the background hole density associated with tin vacancies, yielding longer photoluminescence lifetimes, decreased energetic disorder, reduced Burstein–Moss shifts, and higher charge‐carrier mobilities. Such effects are optimized for SnF2 addition of 1%, while for 5% SnF2 and above, additional nonradiative recombination pathways begin to appear. It is further found that the addition of SnF2 reduces a tetragonal distortion in the perovskite structure deriving from the presence of tin vacancies that cause strain, particularly for high tin content. The optical phonon response associated with inorganic lattice vibrations is further explored, exhibiting a shift to higher frequency and significant broadening with increasing tin fraction, in accordance with lower effective atomic metal masses and shorter phonon lifetimes.

Abstract:

The analysis of terahertz transmission through semiconducting thin films has proven to be an excellent tool for investigating optoelectronic properties of novel materials. Terahertz time-domain spectroscopy (THz-TDS) can provide information about phonon modes of the crystal, as well as the electrical conductivity of the sample. When paired with photoexcitation, optical-pump-THz-probe (OPTP) technique can be used to gain an insight into the transient photoconductivity of the semiconductor, revealing the dynamics and the mobility of photoexcited charge carriers. As the relation between the conductivity of the material and the THz transmission function is generally complicated, simple analytical expressions have been developed to enable straightforward calculations of frequency-dependent conductivity from THz-TDS data in the regime of optically thin samples. Here, we assess the accuracy of these approximated analytical formulas in thin films of highly doped semiconductors, finding significant deviations of the calculated photoconductivity from its actual value in materials with background conductivity comparable to 102Ω− 1cm− 1. We propose an alternative analytical expression, which greatly improves the accuracy of the estimated value of the real photoconductivity, while remaining simple to implement experimentally. Our approximation remains valid in thin films with high dark conductivity of up to 104Ω− 1cm− 1 and provides a very high precision for calculating photoconductivity up to 104Ω− 1cm− 1, and therefore is highly relevant for studies of photoexcited charge-carrier dynamics in electrically doped semiconductors. Using the example of heavily doped thin films of tin-iodide perovskites, we show a simple experimental method of implementing our correction and find that the commonly used expression for photoconductivity could result in an underestimate of charge-carrier mobility by over 50%.