Analysis on Thermodynamic and Thermoelectric Effects of Hot Carrier Solar Cells Barriers in Nanowires

Abstract

This thesis focuses on developing energy barrier models that increase photovoltaic hot carrier extraction efficiency. Hot carriers are electrons containing an excess of kinetic energy and the energy required to leave the valence band. In mitigating losses and increasing the efficiency of solar cells, hot carriers offer the opportunity to extract this kinetic energy. The study of extracting the kinetic energy has resulted in looking at nanowires with heterostructures, comprised of type III-V semiconductor materials [1]. The nanowires have an energy barrier that allows only the hot carriers to travel over or tunnel through. Studying the effects of different barriers on hot carrier flow provides insight into how to increase efficiency.

The first half of this thesis is concerned with introducing the concept of photovoltaic solar cells and the transition into hot carriers. We describe their role in band gap physics, pivoting to their role in photovoltaic devices. We introduce a base of equations and descriptions for how to model electron transport in a nanowire system. However, the focus here is to build a foundation in the theory. Therefore, the models explored through this work are all in one dimension. The Landauer-Buttiker scattering theory approach towards mapping pieces of efficiency (maximum power output, fill factor) is the main method of describing hot carrier flow in this work.

The other half of this work is concerned with introducing the models explored during this thesis work. We look at a benchmark rectangular model, testing its length limits in the infinite and finite regimes. Additionally, we explore one more infinite length model to reinforce knowledge of the system, followed by a physical rectangular and a rectangle/triangle hybrid model. We present our results, seeing which model best gives rise to a positive correlation in maximizing efforts. The hybrid model presents a possible solution towards optimizing both power and fill factor for physical experimentation. Understanding how to characterize hot carrier transport in nanowires at the fundamental level allows for further developments in the field of hot carrier physics and is an important contribution to future experiments.

Popular Abstract

Electricity is the driving force behind every single piece of technology used in today’s society. Fossil fuel-dependent energy production is still the preferred choice for generating electricity, with easy accessibility and cheap processing costs. Consequently, convenience comes in the form of adverse lasting environmental effects. Air pollution, water contamination, and critical mining of finite resources are some of the key contributors to such an adverse effect. A way to lessen the environmental blow while still producing large amounts of electricity is demonstrated in harnessing the sun’s power. Solar cells are currently the leading technology in converting sunlight into electricity through the flow of electrons.


When you excite electrons by absorbing sunlight, you produce an effect. This effect is referred to as the photovoltaic (PV) effect. Electrons can move and generate a current. Great strides in studying this effect have been made since its conception in the 1950s. Strides have been made in researching PV materials known as semiconductors to reduce costs and make the tech more accessible to all. While solar energy can show promising possibilities, the efficiency in generating electricity is an enigma that scientists continue to work on. When the sun shines light onto you, a feeling of warmth might occur on your hand, while your face stays the same. In principle, the sun’s rays radiate different energy levels. This effect is similar to how a PV solar panel works. Electrons in the panel can be very energetic, depending on how much light you shine. However, the issue is that they get very hot and don’t get turned into energy until they’ve relaxed. We lose energy that could be used to produce more electricity.

Currently, researchers are looking at how to make these extremely energetic electrons con- serve their energy over more extended periods than it takes them to relax. Currently, these electrons relax over picoseconds but could relax over larger amounts of time, such as nanoseconds. This allows us to start thinking about how best to harness the energy. Electrons expend this extra energy over tiny distances, on par with the time to relax. Even with the best efforts, they only occur over nanometer distances in objects such as nanowires made of these semiconductor materials. A sheet of paper is 100,000 nanometers thick, so these electrons only travel short distances. A way to harness the energy is to force the electrons to use their energy to overcome an obstacle. Having barriers as obstacles forces only the highly energetic electrons to move, creating a current. This current then generates electricity. These highly energetic electrons are called hot carriers.

My project focuses on understanding how changing the shape of the barriers can affect the way hot carriers travel over and through them. Additionally, we look at how changing the temperature of only one side of the barrier could further impact the way electrons flow and the overall effect on electricity generation. This allows us to gain a deep, fundamental understanding of what is happening in PV solar panels using the power of these hot carriers.

The technology of PV solar power that harnesses hot carrier energy is still unknown to a great extent. Current research in the field is focused on looking at how to fabricate these devices and the fundamental physics behind the effects produced from such a setup. Understanding how hot carriers traverse not only over the barrier, but also through it, gives a glimpse into the possibility of making readily available and accessible solar cell technology incorporating hot carriers towards more efficient PV solar panels.

• author: Toma, Alexandru
• supervisor: Peter Samuelsson; Heiner Linke
• organization: Department of Physics; Mathematical Physics
• course: FYSM64 20242
• year: 2025
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: solar-cells, hot carriers, quantum transport, Landauer-Buttiker, Scattering, fermi-distribution, solid state physics, solar energy conversion, III−V nanowire heterostructures, master thesis, theoretical physics
• language: English
• id: 9216272
• date added to LUP: 2025-12-19 08:57:33
• date last changed: 2025-12-19 08:57:33