Predicting solar cell performance from terahertz and microwave spectroscopy

Mobilities and lifetimes of photogenerated charge carriers are core properties of photovoltaic materials and can both be characterized by contactless terahertz or microwave measurements. Here, the expertise from fifteen laboratories is combined to quantitatively model the current-voltage characteristics of a solar cell from such measurements. To this end, the impact of measurement conditions, alternate interpretations, and experimental inter-laboratory variations are discussed using a (Cs,FA,MA)Pb(I,Br)3 halide perovskite thin-film as a case study. At 1 sun equivalent excitation, neither transport nor recombination is significantly affected by exciton formation or trapping. Terahertz, microwave, and photoluminescence transients for the neat material yield consistent effective lifetimes implying a resistance-free JV-curve with a potential power conversion efficiency of 24.6 %. For grainsizes above ≈20 nm, intra-grain charge transport is characterized by terahertz sum mobilities of ≈32 cm2 V−1 s−1. Drift-diffusion simulations indicate that these intra-grain mobilities can slightly reduce the fill factor of perovskite solar cells to 0.82, in accordance with the best-realized devices in the literature. Beyond perovskites, this work can guide a highly predictive characterization of any emerging semiconductor for photovoltaic or photoelectrochemical energy conversion. A best practice for the interpretation of terahertz and microwave measurements on photovoltaic materials is presented

Time-resolved spectroscopy of charge-carrier dynamics in metal halide perovskites and other semiconductors for photovoltaics

This thesis reports optical measurements and analysis of the charge-carrier dynamics in semiconductor materials and structures that are used in photovoltaic cells. Observing the carrier dynamics in these devices is crucial for understanding the properties that are limiting efficiency, so that improvements can be achieved more quickly through design, rather than trial and error. In perovskite solar cells, the charge transport layers (CTLs) on either side of the perovskite layer provide the asymmetry required to generate a photovoltage and photocurrent. In this work, the charge-carrier transfer and recombination behaviour at each individual interface is studied by measuring the carrier dynamics when only one interface is present i.e. a bilayer of a perovskite and either an electron transport layer (ETL) or a hole transport layer (HTL). In addition to using optical techniques that are more commonly employed in the literature, optical pump terahertz probe spectroscopy (OPTP) was used to measure the carrier density accurately in the first few nanoseconds, which revealed very different behaviour for fullerene based ETLs and the commonly used HTL, Spiro-OMeTAD. These measurements were then compared to two mathematical models of differing complexity, which provided greater understanding of the measured dynamics. The different behaviours observed at different interfaces have important implications for solar cell design. A limitation of using OPTP to study semiconductors used in photovoltaics is that the carrier lifetimes are often much longer than the time range of the technique. An electronically delayed OPTP technique (E-OPTP) was developed, that has virtually unlimited time range whilst maintaining sub-nanosecond resolution. An efficient method to sample photoconductance decays longer than the laser repetition period was proposed, and was used to study the impact of surface passivation on the carrier dynamics in silicon. This technique was compared to inductively-coupled photoconductance measurements, which highlighted the superior spatial and temporal resolution of E-OPTP that allows in-plane and out-of-plane diffusion to be studied. The diffusion was modelled using analytical solutions of the ambipolar continuity equation. Overall, this work demonstrates the suitability of this technique for studying semiconductors with long carrier lifetimes

THz photoconductivity dynamics of semiconductors from sub-nanosecond to millisecond timescales

The pump-probe delay in optical pump, terahertz probe (OPTP) spectroscopy (time-resolved THz spectroscopy) is typically varied using a mechanical delay stage, which limits the delay range to a few nanoseconds. Here we demonstrate an inexpensive modification to typical OPTP setups that extends the range of pump-probe delays to beyond millisecond timescales, whilst retaining the sub-nanosecond resolution required to resolve faster processes that are often present at early times after pulsed optical excitation. We used this new method to investigate the photoconductance dynamics in a range of materials including IIIV semiconductors, metal halide perovskites, germanium and silicon, whose carrier lifetimes range from a few nanoseconds to milliseconds

Terahertz photoconductance dynamics of semiconductors from sub-nanosecond to millisecond timescales

Optical pump terahertz probe spectroscopy (OPTP) is a versatile non-contact technique that measures transient photoconductance decays with femtosecond temporal resolution. However, its maximum temporal range is limited to only a few nanoseconds by the mechanical delay lines used. We extended the temporal range of OPTP to milliseconds and longer while retaining sub-nanosecond resolution. A separate pump laser was electrically synchronized to the probe pulses, allowing the pump–probe delay to be controlled with an electronic delay generator. We demonstrated the capabilities of this technique by examining the photoconductance decays of semiconductors with lifetimes ranging over six orders of magnitude: III-Vs, metal halide perovskites, germanium, and silicon. A direct comparison of results on silicon from OPTP and inductively coupled photoconductance decay highlighted the higher spatial and temporal resolution of OPTP, which allowed in-plane and out-of-plane carrier diffusion to be studied

Ultrafast, high modulation depth terahertz modulators based on carbon nanotube thin films

The development of THz technology and communication systems is creating demand for devices that can modulate THz beams rapidly. Here we report the design and characterisation of high-performance, broadband THz modulators based on the photo-induced transparency of carbon nanotube films. Rather than operating in the standard modulation mode, where optical excitation lowers transmission, this new class of modulators exhibits an inverted modulation mode with an enhanced transmission. Under femtosecond pulsed illumination, modulation depths reaching +80% were obtained simultaneously with modulation speeds of 340 GHz. The influence of the film thickness on the insertion loss, modulation speed and modulation depth was explored over a frequency range from 400 GHz to 2.6 THz. The excellent modulation depth and high modulation speed demonstrated the significant potential of carbon nanotube thin films for ultrafast THz modulators.Peer reviewe

High-bandwidth perovskite photonic sources on silicon

Light-emitting diodes (LEDs) are ubiquitous in modern society, with applications spanning from lighting and displays to medical diagnostics and data communications. Metal-halide perovskites are promising materials for LEDs because of their excellent optoelectronic properties and solution processability. Although research has progressed substantially in optimizing their external quantum efficiency, the modulation characteristics of perovskite LEDs remain unclear. Here we report a holistic approach for realizing fast perovskite photonic sources on silicon based on tailoring alkylammonium cations in perovskite systems. We reveal the recombination behaviour of charged species at various carrier density regimes relevant for their modulation performance. By integrating a Fabry–Pérot microcavity on silicon, we demonstrate perovskite devices with efficient light outcoupling. We achieve device modulation bandwidths of up to 42.6 MHz and data rates above 50 Mbps, with further analysis suggesting that the bandwidth may exceed gigahertz levels. The principles developed here will support the development of perovskite light sources for next-generation data-communication architectures. The demonstration of solution-processed perovskite emitters on silicon substrates also opens up the possibility of integration with micro-electronics platforms