Loi Group Topics

Welcome to the website of the Photophysics and OptoElectronics Group of Prof. dr. Maria Antonietta Loi.

Here we highlight a few of the topics that we work on. In general our group researches novel materials for solar cells, photodetectors, light emitting diodes & neuromorphic microelectronics applications. The materials we work on have in common that they are solution processable. This property holds the promise of cheap production methods with a low energy demand.

The most investigated materials are:

• metal halide perovskites
• colloidal semiconducting quantum dots
• polymer-wrapped single walled carbon nanotubes
• organic semiconductors

Below you can read about our current research topics:

Metal halide perovskites

Over the past ten years, intensive research efforts have been devoted to improving the power conversion efficiency (PCE) of the organometal halide perovskite solar cells by developing strategies for perovskite film growth, the device structure, optimizing the interfacial layers, the composition in perovskite film, which have brought fruitful results with PCE increased to over 25%. In spite of the sky-rocketing increase in the performance, several key physical properties still need to be investigated.

Tin based perovskite solar cells

Despite the high performance and the impressive progress there are still obstacles hindering the commercialization of hybrid perovskite solar cells. One of the largest concerns is the toxicity of lead, which is contained in the most efficient HPSCs.

Among the alternatives to lead, tin (Sn) has great potential, as the Sn-based hybrid perovskites display equally excellent optical and electrical properties as the lead-based counterparts. Recently, extensive research efforts have been devoted to improving the photon-to-electricity conversion efficiency of tin-perovskite solar cells by developing various device structures, optimizing the film morphology of the active material, changing its composition and utilizing reducing agents. However, the efficenty stayed below 6% for many years due to the tendency of tin to change oxidation number from Sn²⁺ to Sn⁴⁺.

Very recently, our research group has demonstrated a strategy to overcome the challenges faced by the tin-based perovskite solar cells and have successfully pushed the efficiency up to 9%. The record efficiency of these Sn-based solar cells is achieved by depositing at low temperature near-single-crystalline films in out-of-plane direction. Trace amounts of layered (2D) tin perovskite initiate the homogeneous growth of the 3D material. The extended ordering and packing of the crystal planes improves the robustness and integrity of the active layer resulting not only in more efficient but also in more stable Sn-based solar cells.

[1] Highly Reproducible Sn‐Based Hybrid Perovskite Solar Cells with 9% Efficiency, Advanced Energy Materials, 8 (4), 2018, 1702019
[2] Enhancing the crystallinity and perfecting the orientation of formamidinium tin  iodide for highly efficient Sn-based perovskite solar cells, Nano Energy, 60, 2019, 810-816

(Matteo Pitaro, Qianshan Feng, Lijun Chen, Andrea Arreola Vargas)

Hot carriers

When excited with an excess of energy, charge carriers normally cool down rapidly to the band extrema of the corresponding absorber material. Their excess energy is emitted through phonons and becomes observable as heat. This process reduces the maximum attainable power conversion efficiency of single junction solar cells to around 34%, the so-called Shockley-Queisser limit.

Metal halide perovskites, especially tin-based- compounds exhibit a drastically reduced cooling time when compared to classical semiconductors. This gives rise to the potential to fabricate hot carrier solar cells, with which the excess energy could be harvested and the Shockley-Queisser limit could be overcome. Our goal is to understand the fundamental mechanism behind the prolonged cooling time and to optimise it for future exploitation in devices.

[1] Long-lived hot-carrier light emission and large blue shift in formamidinium tin triiodide perovskites, Nature Communications, 9, 2018, 243
[2] Cooling, Scattering, and Recombination—The Role of the Material Quality for the Physics of Tin Halide Perovskites, Advanced Functional Materials, 29 (35), 2019, 1902963
[3] Hot carrier solar cells and the potential of perovskites for breaking the Shockley–Queisser limit, Journal of Material Chemistry C, 7, 2019, 2471-2486

(Moti Kot)

Layered Perovskite-Like Compounds

In layered metal halide perovskites the organic cation is too large to maintain the conventional three-dimensional perovskite structure. Instead, layers of metal halide octahedra are sandwiched between long organic spacer cations. As a result of the difference in band gap and dielectric constant of these two layers, charge carriers are confined to the inorganic part of the lattice and form strongly bound electron-hole pairs, known as excitons. The thickness and composition of the inorganic layer as well as the size and type of the organic cation dictate the material’s optical properties, making layered metal halide perovskites rich platform to exploit structural versatility to tune optical properties.

Our interest in these type of materials is two-fold: through a symbiosis of device physics and optical spectroscopy, we seek to strengthen our grasp on the fundamental photophysical and optoelectronic properties of this fascinating material system, and we seek to directly apply our knowledge of these materials to improve the efficiency and stability of perovskite-based opto-electronic devices, including solar cells and LEDs.

[1] Unravelling Light‐Induced Degradation of Layered Perovskite Crystals and Design of Efficient Encapsulation for Improved Photostability, Advanced Functonal Materials, 28 (21), 2018, 1800305
[2] The Impact of Stoichiometry on the Photophysical Properties of Ruddlesden–Popper Perovskites, Advanced Functional Materials, 30 (5), 2020, 1907505
[3] Tuning the Energetic Landscape of Ruddlesden–Popper Perovskite Films for Efficient Solar Cells, ACS Energy Letters, 5 (1), 2020, 39-46
[4] Band‐Edge Exciton Fine Structure and Exciton Recombination Dynamics in Single Crystals of Layered Hybrid Perovskites, Advanced Functional Materials, 30 (6), 2020, 1907979

(Jiale Chen, Lijun Chen, Moti Kot)

Colloidal quantum dots (CQDs)

Colloidal quantum dots have been attracting attention for the last 30 years due to their tunable band-gap. However, only in the last years they became useful building blocks for the fabrication of electronic and optoelectronic devices starting from solution. The charge transport between QDs assembled in thin films is obtained by exchanging the original bulky ligands for shorter ones; this coupling is not affecting the band-gap, allowing the preparation of  thin films with fixed chemical properties but tunable band-gap. Our group has vast experience in working with Pb chalcogenides QDs. In Pb chalcogenides the optical bandgap can be varied throughout the near-infrared range (0.8-2µm) simply by changing the size of the nanocrystals. This flexibility makes them interesting as IR-photodetectors and emitters, and also makes them ideal candidates for the active layer of solar cells. Using solution-based processing techniques, we can stack organized arrays of these QDs into a thin film, the electronic properties of which we can control by changing the surface chemistry of the QDs, for example by changing the ligands, or by putting a shell around them. This gives us a large toolbox to explore the physics of these artificial solids. In our group, we focus mainly on the following topics:

• Controlling the surface chemistry via inorganic or organic ligand exchange, both in thin film and in solution.
• Fabricating transistors to explore the electronic properties and the transport mechanisms of the CQD solids.
• Fabricating solar cells and photodetectors with various structures and deposition methods.
• Controlling the self-assembly of the CQDs to form highly ordered superlattices.

[1] Temperature dependent behaviour of lead sulfide quantum dot solar cells and films, Energy Environmental Science, 9 (9), 2016, 2916-2924
[2] Stoichiometric control of the density of states in PbS colloidal quantum dot solids, Science Advances, 3 (9), 2017, eaao1558
[3] Electroluminescence Generation in PbS Quantum Dot Light-Emitting Field-Effect Transistors with Solid-State Gating, ACS Nano, 12 (12), 2018, 12805-12813
[4] Electron Mobility of 24 cm² V⁻¹ s⁻¹ in PbSe Colloidal‐Quantum‐Dot Superlattices, Advanced Materials, 30 (38), 2018, 1802665
[5] Colloidal Quantum Dot Inks for Single-Step-Fabricated Field-Effect Transistors: The Importance of Postdeposition Ligand Removal, ACS Applied Materials and Interfaces, 2018, 10 (6), 5626-5632
[6] Revealing Trap States in Lead Sulphide Colloidal Quantum Dots by Photoinduced Absorption Spectroscopy, Advanced Electronic Materials, 4 (1), 2018, 1700348

(Han Wang, Jacopo Pinna, Xin Wen, Alexandru Mednicov)

Single-walled carbon nanotubes for neuromorphic microelectronics

Single-walled carbon nanotubes (SWNTs) are among the most promising materials for future optoelectronics beyond silicon, due to its extraordinary high carrier mobility and high mechanical, thermal, and chemical stability. However, the coexistence of semiconducting and metallic species in the as-grown SWNTs remains a big challenge for its applications.

Our research group has been very active in the development of the selection mechanism for semiconducting carbon nanotubes (polymer wrapping). This has led to major advancements in the understanding of the physics of carbon nanotubes and their interaction with conjugated polymers and the fabrication of field effect transistors with outstanding performances (≥15 cm²/Vs mobility and 10⁸ on/off ratio). Recently, our group demonstrated chemical self-assembly of semiconducting single walled carbon nanotubes (s-SWNTs) on pre-patterned substrates. The network devices show superior performance (mobility up to 24 cm²/Vs), while SWNTs devices based on individual tubes show an unprecedented (100%) yield for working devices. Importantly, the SWNTs assembled by means of the thiol groups are stably anchored to the substrate and are resistant to external perturbation. Very recently we fabricated artificial synapses based SWCNT transistors. These SWCNT artificial synapses display learning capabilities when biased with simple square-shaped pulses.

Organic semiconductors

In the last years our group has been using organic semiconductors as interface materials in metal halide perovskites solar cells. These organic layers are extremely important as they act as hole and electron extraction layer but also as passivation for defects on the perovskite film surface. Furthermore, we have demonstrated that the transport properties of the organic layer determine also the behavior of the perovskite device at low temperature.

In collaboration with colleagues in many different countries, we have also contributed to enlighten the photophysics of the so-called ternary blend devices and to demonstrate the possibility of using a layer-by-layer deposition method for highly performing organic PV.

In the next months we will engage with the physics of non-fullerene acceptors in the framework of the Solar-Era project “Industrial roll-to-roll (R2R) printing of highly efficient non-fullerene acceptor (NFA)-based organic photovoltaics (OPV)”

[1] N-type polymers as electron extraction layers in hybrid perovskite solar cells with improved ambient stability, Journal of Material Chemistry A, 2016, 4, 2419-2426
[2] Elimination of the light soaking effect and performance enhancement in perovskite solar cells using a fullerene derivative, Energy & Environmental Science, 2016, 9 , 2444-2452
[3] Efficient Perovskite Solar Cells over a Broad Temperature Window: The Role of the Charge Carrier Extraction, Advanced Energy Materials, 2017, 7 (22), 1701305
[4] Favorable Mixing Thermodynamics in Ternary Polymer Blends for Realizing High Efficiency Plastic Solar Cells, Advanced Energy Materials, 2019, 9 (19), 1803394
[5] A multi-objective optimization-based layer-by-layer blade-coating approach for organic solar cells: rational control of vertical stratification for high performance, Energy & Environmental Science, 2019, 12 , 3118-3132

(David Garcia Romero)