Using Silicon Nanowires to Fix Solar Inefficiency Problems

SOLAR ENERGY HAS SO MUCH POTENTIAL!!!! I mean come on, the sun is a virtually limitless source of power. So why hasn’t it taken over yet? Before we get into this, we should probably first understand the basics.

Understanding Solar Cells

Solar panels convert solar energy into electrical energy

A solar panel is made up of many smaller entities called solar cells. Each solar cell follows the same formatting:

A semiconductor, typically silicon, is sandwiched 🥪 in between two layers of conductive material. Bonds keep the silicon electrons in place so that current cannot flow through.

Okay cool. But how does it actually work?

The solar cell is actually made up of two different kinds of silicon. The N-type has extra electrons and the P-type has extra holes for electrons. The P-N junction is where the two different silicon types meet.

A simplified diagram of the p-n junction

Electrons will wander around the junction, leaving a positive charge on one side and a negative on the other.

Light is the flow of tiny electrons called photons. If a photon strikes the solar cell with enough power, electrons can be moved from their bonds, leaving holes. Electrons are ‘free’ to move around from this point.

You may be wondering why ‘free’ is in quotations. There is an electric field at the p-n junction, and so, the electrons are drawn to the n-side while the holes are drawn to the p-side. Electrons are collected by thin pieces of metal at the beginning of the cell, later flowing through an external circuit, producing electricity 💡

If you don’t feel like reading, here’s a really good beginner explanation of how solar cells work 🙌

Introducing: The Shockley-Queisser Limit

Single Junction Solar Cells currently max out their energy capture at 33.7%. This phenomenon is referred to as the Shockley-Queisser Limit, which references the total amount of solar energy that can be connected in a p-n junction photovoltaic cell.

Graphing the Shockley-Queisser Limit

You see, the Shockley-Queisser limit isn’t fully concrete. By changing the semi-conductor materials we can actually improve the efficiency rate of the total solar cell.

In fact, there are solar cells that can convert 40% of solar energy 😱.

WHAT??!! WHY DON’T WE USE THEM???

Concentrated Photovoltaic Solar Cell

CVPs, or concentrated photovoltaic cells have a super high efficiency rate. The problem is that they require a solar tracker (something that moves the panel in the direction of the sun) AND a cooling system. This means that they’re super inefficient, both economically and in terms of time (it takes a while for it to reach high efficiency). They’re also multi-junction (made with multiple p-n layers), which means the Shockley-Queisser limit doesn’t directly apply.

Getting sustainable energy is crucial to our survival as a species. How can we be expected to colonize other planets, or even just live, if we are dependent on fossil fuels? We need to improve our stance on this limit NOW.

Introducing Silicon Nanowires

Silicon nanowires (SiNWs) are semiconductor nanowires formed through the use of silicon precursors.

Wait wait wait… Those are big words. What do any of those mean?

Nanowire: A thin, flexible, rod shaped piece of metal that is only 1 nanometer in diameter. (For context, a human hair is 50 000 nanometers thick 🤯. It’s hard to even imagine how small these are!!!)

Silicon: The second most abudant element on Earth. It works well in energy-related scenarios as it is a semiconductor. This essentially means it can conduct electricity, but only when certain conditions are met.

Silicon Precursor: A high purity gas or liquid material used as a base for the production of semiconductors.

Why Silicon Nanowires?

The majority of current solar cells use crystalline silicon. The problem is, these materials are dangerous to consumer health, leave lasting environmental impacts, AND are inefficient 😡

Silicosis is a lung disease brought on by exposure to crystalline silicon. Look at its impact. Do you really want your lungs looking like that???

Luckily silicon nanowires can eliminate that issue.

The cost is relatively easy to drive down and they increase power-conversion efficiency (PCE).

But how?

As you may have guessed, a silicon nanowire

Integration

It’s important to make sure that the dimensions are intact first. The most viable ratios for solar occur when:

• The diffusion length of the nanowire is smaller than the optical thickness.
  → The diffusion length is the average length a carrier (substance conveying other substances) moves between generation (producing charges) and recombination (getting rid of free-roaming charges).
• There is a higher aspect ratio while still maintaining small dimensions. The aspect ratio references the relationship between height and width, in 3D objects; a surface:volume ratio (i.e. a square having an aspect ratio of 1:1, a cube having a ratio of 24:16)

As we optimize the structure and dimensions, the efficiency increases.

SiNW solar cells work similarly to traditional solar cells. This can be explained in 3 steps:

Photon Absorption

^That’s pretty much a fancy word that describes the collection of light. The photovoltaic effect occurs when the photons in sunlight strike the semiconductor surface, giving way to atoms to roam around ‘freely’

Charge Seperation

The process of electrons travelling to another place, generally to a higher energy level (as a result of the absorption of photons). In solar cells, this is the ‘p-side’ of the p-n junction.

Charge Collection

Collecting the charges and making electricity! That’s the fun part 😎🔋

Okay… but how do these actually INCREASE the efficiency?

Quantum confinement can cause electrons to speed up or slow down

There’s this super cool concept known as the Quantum Confinement Effect. This plays a huge role in restricting the motion of random moving electrons. The Shockley-Queisser limit changes as the energy band gap (energy range in a solid where electrons can’t exist) increases or decreases.

Traditional solar cells have an indirect energy band gap (if you want to get specific, it’s Eg∼1.1). This means that both phonons () AND photons are required to prevent electron movement.

SiNWs, however, have a direct energy band gap (specifically ∼1.4 eV at a diameter of 5 nm). This means that they:

• Don’t require the use of phonons AND
• are balanced in nature

This property allows us to move away from the Shockley-Queisser limit and increase efficiency on Earth. Pretty neat, right? 🤩

Impact

There are currently 1.2 BILLION people living in energy poverty. This means they don’t have access to electricity. That’s insane. That’s 16% OF THE WHOLE GODDAMN WORLD.

There’s probably 100 people in this photo. Now imagine that times 12 MILLION 🤯. That’s the amount of people in energy poverty 😱

Imagine the doors we could open if we had clean, accessible energy available to everyone internationally? If we didn’t have to worry about polluting the Earth in exchange for innovation?

Think of all those lives we could change if we were able to revolutionize solar energy.

It starts right now. The first step in revolutionizing the world is revolutionizing energy, and that starts with collection. Using SiNWs has the potential for MAJOR impact.

🔑 Key Takeaways

→ The Shockley-Queisser limit is preventing single-junction solar cells from becoming more efficient 😔
→ Silicon nanowires are wires made at the nanoscale. These can be intergrated into the semi-conductors of solar cells. 🔍
→ Solar cells work in three main steps: photon absorption, charge seperation, and charge collection ☀️
→ As a result of their ability to restrict the motion of electrons (Quantum Confinement Effect), SiNWs can modify the Shockley-Queisser limit. 🏃‍♀️
→ If we can achieve solar efficiency, we WILL impact billions 🌎