Abstracts

Stefano Amaducci

The challenge of integrating photovoltaics and plant production

Solar energy plays a major role in the energy transition, but the expansion of ground-mounted PV systems will displace other land uses. Agrivoltaic technologies are conceived and developed to enable the cohabitation of solar panels and crops and the sharing of solar radiation. In other words they are designed so as ensure an adequate conversion of solar energy to electricity while still ensuring acceptable levels of crop production. Current international policies are accelerating the process of energy transition; PV technologies are fully mature and ready to be implemented, but agrivoltaics are still in their infancy. The state of the art on research and development on agrivoltaic technologies will be presented, knowledge gaps will be highlighted and opportunities and threats will be critically reviewed.
 

Natalie Banerji

Organic semiconductors for solar cells and bioelectronics

Organic semiconductors are viable alternatives to their widely commercialized inorganic counterparts. They are favoured by tuneable optoelectronic properties, excellent mechanical characteristics and low-cost solution-processing. Applications include organic field effect transistors (OFETs), light-emitting diodes (OLEDs), photovoltaics (OPVs) and bioelectronic devices such as electrochemical transistors (OECTs). However, their implementation suffers from the low conductivity of organic semiconductors, which originates from their intrinsically low charge carrier density and mobility. Fundamentally understanding how additional conductive charges are generated and how to enhance their mobility is therefore primordial for the advancement of the field. My team uses optoelectronic expertise to investigate charge transfer (CT) reactions that add extra positive or negative charge to the conjugated backbone, a process commonly known as doping. Doping either involves CT with a dopant molecule in the ground state (chemical doping) or in the excited state (photo-induced doping), or CT with an electrode (electrochemical doping). Photo-induced CT generates charges in OPVs, while signal transduction in OECTs relies on electrochemical doping. We use ultrafast spectroscopic techniques, such as transient absorption (TA) and time-domain terahertz (TD-THz) spectroscopies, to investigate the nature and evolution of photoexcited species, as well as the charge transport properties at the nanoscale. In this talk, I will first show results about free charge dynamics in highly efficient organic solar cell materials containing non-fullerene acceptors (NFAs) with low energetic offset for CT. Then, I will describe ways to explore spectroscopy to study processes in organic bioelectronics. In operando visible, Raman and TD-THz measurements on OECTs will be presented for different polythiophenes.
 

Christoph Brabec

Accelerating the science & technology of solution processed photovoltaics 

The development of complex functional solar materials for organic or perovskite photovoltaics poses a multi-objective optimization problem in a large multidimensional parameter space. Solving it requires reproducible, user-independent laboratory work and intelligent preselection of experiments. However, experimental materials science is a field where manual routines are still predominant, although other domains like pharmacy or chemistry have introduced robotics, automation and machine learning long before. In this talk, I will present a fused knowledge-based / data-driven approach to utilize MAPs (material acceleration platforms) for autonomous materials and device discovery in the field of emerging photovoltaic technologies. The impact of this methodology for autonomous material and device discovery will be discussed in terms of contributing to overcome fundamental scientific challenges.

David Cahen

Sustainable Photovoltaics needs Sustainable Materials: Self-repair and -healing in Si, CI(G)S and Pb-Halide Perovskites

Self-healing, as distinct from self-repair, implies that a material (or device/ system) ca, without any external factor, recover from damage. Si solar cells can self-repair; Si:Li detectors self-heal. CI(G)S, for which radiation-hardness is quite unlike that of materials like InP, can self-heal. Pb-Halide Perovskites (HaPs) can self-heal, although a bit different from CIGS.  Lattice dynamics appears the origin of HaP self-healing (begging the question of what is the root-cause for those dynamics). HaP behavior is mostly mis-interpreted as defect tolerance, which can be eliminated by logic as sole basis. When we’ll have criteria for materials’ self-healing and -repair, healable or at least repairable types of relevant ETLs and HTLs, then enabling truly sustainable PV. Time permitting, I’ll discuss an example of problems with searching for new self-healing materials, or put HaP PV in perspective with other PV options, including organics and currently commercial ones.
 

Richard Friend

Coulomb interactions in organic semiconductors

Pi-conjugated organic molecules and polymers now provide a set of well-performing semiconductors that support devices, including light-emitting diodes (LEDs) as used in smart-phone displays and lighting, field-effect transistors (FETs) and photovoltaic diodes (PVs).  Semiconductor properties for the organics are however very different to traditional inorganic systems.  In particular, dielectric screening is weak, so that Coulomb interactions between charges and associated spin exchange energies are big, often much larger than kBT.  Management of transport and of excited state spin is fundamental for efficient LED and solar cells operation. I will discuss some of our recent work in Cambridge. 
 

Michael Graetzel
Molecular Photovoltaics and the Rise of Perovskite Solar Cells

Photovoltaic cells using molecular dyes, semiconductor quantum dots or perovskite pigments as light harvesters have emerged as credible contenders to conventional devices. Dye sensitized solar cells (DSCs) use a three-dimensional nanostructured junction for photovoltaic electricity production and reach currently a power conversion efficiency (PCE) of over 15 % in full sunlight. They possess unique practical advantages in particular highly effective electricity production ambient light, ease of manufacturing, flexibility and transparency, bifacial light harvesting, and aesthetic appeal, which have fostered industrial production and commercial applications. They served as a launch pad for perovskite solar cells (PSCs) which are presently being intensively investigated as one of the most promising future PV technologies, the PCE of solution processed laboratory cells having reached 25.7%. Current research focusses on their scale up to as well as ascertaining their long-term operational stability. My lecture will cover our most recent findings in these revolutionary photovoltaic domains.

Martin Green
Evolution of silicon solar cell efficiency

An outline of the evolution of silicon solar cell efficiency and associated cell design concepts is given starting from the Al-BSF cell and the birth of “passivated contacts” in the 1970s, the rapid design and experimental progress in the 1980s with PERC and IBC cells, and subsequent progress with HJT and TOPCon cells leading to the recent 26.56% p-type and 26.81% n-type HJT3.0 outright efficiency records. The role of improved wafer quality in further increasing efficiency via fill factor “supercharging” is described, as are recent results suggesting the switch to IBC might occur more quickly than previously thought. Finally, although tandem cells represent the ultimate photovoltaic solution, barriers to their near-term commercialisation are discussed.
 

Jianhui Hou
Material Design and Device Engineering for Efficient Organic Photovoltaic Cells

The developments on organic photovoltaic (OPV) materials and device engineering have contributed greatly to the rapid advances in photovoltaic performance of OPV cells. In the past years, we focused on the conjugated polymers based on benzothiophene units and developed a few polymer donors with superior photovoltaic performance, such as PBDB-T and PBDB-TF (also known as PM6). Based on these polymers and the non-fullerene acceptors developed by other research groups, we carried out a series of studies on material design and device engineering. In this presentation, I would like to introduce some interesting and useful strategies related to molecular design and device fabrication. What is more, our recent works including the single-junction cells with over 19% efficiency, the double-junction tandem cells with over 20% efficiency and the efficient OPV cells for indoor applications will be briefly introduced.
 

Rene Janssen
Material and Device Design for Multijunction Perovskite Solar Cells

The tunable bandgap of metal halide perovskites can be used to enhance solar cell efficiency in tandem- and triple-junction solar cells. Often wide-bandgap mixed-halide perovskites and narrow-bandgap mixed-metal perovskites suffer from non-radiative recombination due to bulk traps and interfacial recombination centers that limit the open-circuit voltage in wide-bandgap sub-cells and restrict photocurrent for narrow-bandgap materials. We study the origin of these traps with photocurrent spectroscopy and absolute photoluminescence spectroscopy and employ passivation strategies to eliminate losses and increase stability. Combined with reducing parasitic absorption losses using optical simulations as guide, this allows fabricating efficient multi-junction cells in all-perovskite configurations or in combination with other semiconductors.
 

Prashant V. Kamat
Overcoming Challenges of Ion Migration in Perovskite Solar Cells

Nanostructured semiconductor materials offer new approaches to develop solar cells with relatively low carbon footprint as compared to silicon based photovoltaic devices. The solution processibility and low temperature processing of these materials significantly decreases the energy required to produce photovoltaic devices. Of particular interest are the metal halide perovskites which have delivered new record-breaking solar cell efficiencies. However, long-term stability of perovskite solar cells remains a hurdle in implementing large panel photovoltaic devices for outdoor applications. Instabilities in perovskites can occur because of both intrinsic and extrinsic changes under light irradiation. For instance, ion migration in perovskite film is one of the factors responsible for deterioration of solar cell performance.
The thermodynamic and redox properties of halide perovskites provide a strong driving force for hole trapping and oxidation of iodide species. When in contact with a non-polar solvent, the migration of iodine species is further extended to expulsion of iodine from the perovskite film. Thus, the mobility of halides and their susceptibility to hole-induced oxidation play a crucial role in determining the long-term stability of metal halide perovskite solar cells. Modification of the perovskite composition through introduction of different cations, halide ions, or introduction of low-dimensional perovskite phases suppress phase segregation.  When Ruddlesden-Popper 2D mixed-halide perovskite films with spacer cations such as butylammonium are introduced into three-dimensional (3D) perovskite films, they stabilize them against thermal- and moisture-induced degradation. This improved photostability has led to the incorporation of 2D perovskites into photovoltaic and light emitting display devices. While such passivation of 3D perovskites using 2D perovskites has been reported widely, the stability of the 2D/3D interface during long term solar cell operation is yet to be assessed fully. Thus, suppression of halide ion as well as cation mobility remains a key factor.in achieving long term stability and improving efficiency of perovskite solar cells and light emitting devices.

 

Laura Miranda Perez
Perovskite solar: the role of the next generation PV in addressing the climate and energy crisis.

Abstract Perovskite PV has been at the center of photovoltaic discussions during the last ten years for a variety of reasons. In one hand, it is the major breakthrough on PV in the last decades, it has achieved efficiencies unseen before, without having reached its full potential yet. On the other hand, the technology is questioned based on the intrinsic stability of the material and sustainability factors, as the presence of hazardous elements. In this presentation, I will show the evolution of this technology, its prospects for commercialization, and the role perovskite PV has in addressing climate and energy crisis.
 

Tsutomu Miyasaka
Efficiency and stability development in hybrid and inorganic perovskite photovoltaic cells

Tsutomu Miyasaka Toin University of Yokohama Superior nature of halide perovskite photovoltaic cells is their small voltage losses in photocurrent-voltage performance that have enabled high standard in power conversion efficiency (PCE). Our efforts in minimizing the voltage loss achieved high open-circuit voltage (Voc) over 1.4 V for thermally stable inorganic CsPbX3 cells, which work with PCE of 34% under indoor weak light by sustaining high Voc levels. Engineering for large grain crystallization and chemical passivation of hetero-junction interfaces are the key to recombination-free efficient carrier transfers in polycrystalline perovskite films. We have focused on compositional engineering of modifying the grain boundary and interfacial structures towards enhancement of Voc close to Shockley–Queisser (SQ) limits. For hybrid Cs-FA-MAPb(I,Br)3 perovskite cells (bandgap 1.51eV), Voc reached 1.19V (PCE>22%) that is close to the SQ limit (ca.1.21V) of the absorber. The strategy of Voc development has been also applied to plastic-film based lightweight flexible devices capable of 21% PCE.
 

Jenny Nelson
Optimising solar energy conversion in molecular electronic materials

Solar radiation will be the largest single source of electricity in a low-carbon future. To maximise the potential of solar power, new materials will be needed to harvest and convert solar energy alongside existing photovoltaic technologies. Molecular electronic materials, such as conjugated polymers and molecules, can achieve photovoltaic conversion through a process of  photon absorption, charge separation and charge collection. The materials are appealing because of the potential to tune their properties through chemical design and their compatability with high-throughput manufacture. They are also interesting model systems for photochemical energy conversion because of their parallels with natural photosynthesis. Through a remarkable series of advances in materials design, the efficiency of photovoltaic energy conversion in molecular materials has risen from 1% to around 20% within two decades, surpassing most predictions. We will discuss the factors that control the function of molecular solar cells including the nature of the charge separating heterojunction, and the impact of chemical and physical structure on phase behaviour, energy and charge transport, light harvesting, and loss pathways. Finally, we will address the limits to conversion efficiency in such systems.
 

Thuc-Quyen Nguyen
Organic Solar Cells for Energy Generation

Organic semiconductors (OSCs) are a class of carbon-based materials that can be synthesized to have band gaps from the UV to the near infrared regions of the electromagnetic spectrum. OSCs are attractive due to their unique properties: light weight, mechanical flexibility, biocompatible, low cost, low-temperature processing, and simple fabrication methods such as roll-to-roll coating, spray coating or ink-jet printing into desired size and shape. OSCs have been implement in commercial products such as displays and lightings and have potential applications in transistors, solar cells, photodetectors, thermoelectrics, ratchets, sensors, neuromorphic computing, and bioelectronics. Organic solar cells potentially can offer low cost, large area, flexible, light-weight, clean, and quiet alternative energy sources for indoor and outdoor applications. In this talk, I will give the current progress and challenges in solution-processed organic solar cells and their potential applications in energy generation.
 

Uwe Rau
The rise and the decay of the photovoltage and what it tells us about how solar cells work

Solar cells are devices that transform solar light into electrical energy. The generation of excess electronhole pairs in a photovoltaic absorber material is an important intermediate step and is considered rightfully as the driving force behind the photovoltaic action of a solar cell. However, the excess concentration of free carriers, representing an excess of chemical energy, does not yet imply the delivery of electrical power. Thus, an equally important step, the transformation of the chemical energy into electrical energy must be considered. The present contribution will highlight the thermodynamics of charge carrier separation and photovoltage generation in solar cells. We will see that this step has very much in common with the same transformation step in batteries. It is also shown that this transformation comes with losses that can be significant in various types of solar cells, especially in perovskite solar cells. The transformation step comes also with a time delay, illustrated by the simple fact that the photovoltage in any solar cell takes some time to develop after a short illumination pulse. The analysis of this rise/decay dynamics by transient time and/or frequency domain methods and the interpretation with the help of a generic solar cell model yields substantial information on the working principle of any type of solar cells.

Michael Saliba
Historic Overview and Design Principles for Optoelectronic Perovskite Materials

Perovskite solar cells (PSCs) have created much excitement in the past years and attract spotlight attention. This talk will provide an overview of the reasons for this development highlighting the historic development as well as the specific material properties that make perovskites so attractive for the research community.

The current challenges are exemplified using a high-performance model system for PSCs (multication Rb, Cs, methylammonium (MA), formamidinium (FA), ethylammonium (EA) perovskites). The triple cation (Cs, MA, FA) achieves high performances due to suppressed phase impurities. This results in more robust materials enabling breakthrough reproducibility and stability. The theme of multicomponent perovskites is used to achieve high open-circuit voltages towards 1.7 V.

The last part elaborates on a roadmap on how to extend the multication to multicomponent engineering providing a series of new compounds that are highly relevant candidates for the coming years, also in areas beyond photovoltaics, for example for LEDs or medical scintillation detectors.

Edward Sargent
Photophysics-by-design: Materials to devices for optical energy capture

The conversion of light – both information-bearing light (e.g. images and information signals) and energy-bearing light (e.g. solar energy) – into electricity, has seen transformational advances in recent decades. A key factor is materials-by-design: quantum dots, perovskites, and inorganic materials, whose photophysics and photochemistry are sufficiently well-understood, that they can rationally be harnessed towards societally important goals including renewable electricity at scale. I will discuss progress in these fields.

Susanne Siebentritt
Efficiency limitations in state-of-the-art chalcopyrite solar cells – and how to overcome them

Cu(InGa)(SSe)2 solar cells have large potential due to their efficiency above 23%, their proven stability, their low green house gas emissions. The open-circuit voltage of state of the art cells is limited by the recombination of and through tail states, which can be reduced by alkali treatments which may passivate the grain boundaries and increase the doping. They also employ a full-area metal back contact, which is passivated by a band gap gradient towards the side. This band gap gradient induces high non-radiative recombination and can be omitted by the use of a passivated backcontact. In addition, the diode factor is higher than 1, not because of a significant contribution of recombination in the space charge region, but because of metastable defects present in the absorber. The effect of these states can be reduced by a higher doping level. Combining these measures, the efficiency of single junctions can be improved. The ultimate efficiency improvement is by tandem solar cells, which are possible within this material class.

Richard M. Swanson
The quest to improve solar cell efficiency

One of the important threads enabling the recent emergence of photovoltaic solar energy as a low-cost, large-scale energy source is ever increasing energy conversion efficiency. There are many elements to this journey, including, a) improved understanding of the physics and operation of photovoltaic devices, including both optical and charge carrier transport and recombination properties, b) the development of highly selective contacts that effectively separate positive and negative charge carriers, c) ingot growth techniques that produce crystalline silicon with extremely low defect density, d) surface passivation methods, and e) new device structures. These efforts have increased solar cell conversion efficiency from 12 percent at the beginning of the development of terrestrial solar cells in 1975 to over 26 percent today. This talk will chronicle the breakthroughs in technology and understanding that have enabled this remarkable progress.

Eli Yablonovitch.
Counter-Intuitively: the Greater the External Luminescence, the More Efficient the Photovoltaic Cell.

Photovoltaic Cells must take account of Optical Science in 2 ways:
1.  Light enters a solar cell, restricted to a narrow refraction angle, but there is a great benefit from internal random light scattering that fills the internal optical angle space, (called ergodic evolution).  Owing to the high refractive index of typical semiconductors, the increased phase space effectively concentrates the light by the 4(n squared) factor»50.  This concentration factor significantly increases both the voltage and current, and is used in almost all commercial solar panels.
2.  More recently discovered, is the counter-intuitive role of external luminescence, which is present in all operating solar cells.  High external luminescence is a marker for high voltage, and requires very low non-radiative recombination.  This has led to the mantra “A Great Solar Cell must also be a very efficient LED”.  The 1-sun, single-junction efficiency record is 29.1%, held by a thin film GaAs solar cell, of ~50% LED efficiency.