Perovskite Solar Cells

Most countries have solar cells as a sustainable energy source. You can see them on houses or on wide fields, mostly in sunny areas as they convert sunlight into electricity.

But what surprised me is that they actually convert not even a third of the sunlight energy. We haven’t really figured out how to efficiently turn sunlight into electricity, although it is already used so widely. Improving the efficiency of solar cells could make solar cells one of the main energy sources worldwide.

The first practical silicon solar cell was built in 1954 at Bell Labs with an efficiency of about 6% and in the 1950s they reached even 15%. Within just a few decades solar cells had groundbreaking advancements. In the 21st century perovskite solar cells were developed and improved solar cells efficiency drastically, coming close to 30% efficiency.

Understanding why requires looking beyond the idea of “better materials” and into the physics of how these materials interact with light, charge, and energy.

How are perovskite solar cells structured?

Perovskite solar cells (PSCs) are solar cells that use perovskite materials. Those materials follow a special chemical structure: ABX3, where A and B are cations of different sizes and X is an anion. For example, CH3NH3PbI3 (CH3NH3 (A) Pb (B) I3 (X3)) or MAPbI3. This structure is extremely useful for building solar cells because perovskites are ambipolar semiconductors. A semiconductor has characteristics of an insulator and a conductor. And an ambipolar semiconductor can transport electrons and holes. Hole means the absence of an electron, so it means carrying positive charge.

In a perovskite solar cell, this perovskite material lies in between an Electron Transport Layer (ETL) and a Hole Transport Layer (HTL). The ETL consists of an n-doped material, meaning impurities are added to the material to add extra electrons. For example doping ZnO with Al increases the electron concentration and hence the ability to conduct electrons. HTL is a p-doped material, meaning impurities are added to create electron holes. Popular materials for the ETL are TiO2 doped with nitrogen and for the HTL the p-doped molecule Spiro-OMeTAD. The perovskite material in the middle is intrinsic as it is not heavily doped.

I like to remember this three-layer structure as p-i-n:  p-doped (HTL)- intrinsic (perovskite) – n-doped (ETL).

Additionally, the three layers are usually deposited on a Transparent Conducting Oxide (TCO) such as glass with fluorine-doped tin oxide (FTO) to provide a transparent, conductive path for the current.

As you might assume, there was a lot of research on choosing the materials and there still is. So, the materials change for different solar cells.

How do they work?

The underlying principle of solar cells is to convert sunlight into electricity. The incoming sunlight hits the perovskite layer and the electrons in this layer become excited. The electron moves from the perovskite to the ETL and leaves a hole which moves to the HTL. This creates a charge difference or voltage between ETL and HTL. Also, the electrons move from the ETL to the TCO and create an electrical current.

This is, in my opinion, a pretty understandable workflow and goes all back to Albert Einstein (of course). He won the noble prize for explaining the photoelectric effect: photons (sunlight) hit a material which sets free the electrons if the energies match.

This is how they generally work, however, there are many different types of perovskite solar cells, differing in structure and materials. Material based variations include lead-based cells, tin-based cells or carbon-based cells. Structure based variations include for example tandem solar cells which pair solar cells with different band-gaps (such as Perovskite/Silicon).

An example for a perovskite cell is MAPbI3 mesoporous cell. The perovskite consists of CH3NH3PbI3 (or MAPbI3), the ETL of TiO2, HTL of Spiro-OMeTAD and the FTO of TCO. It works like described above.

In addition to the normal setup, many solar cells have an anti-reflecting coating on top of them. This coating reduces reflective losses of sunlight because the efficiency depends on how much sunlight hits the perovskite.

What are differences form other solar cells and advantages?

Normal solar cells are made of silicon. Silicon acts as the middle-part semiconductor while the p- and n-doped parts as ETL and HTL is doped silicon. This is similar to perovskite cells, but perovskites use entirely different materials as doped regions.

Also, perovskites have several advantages over normal solar cells. Especially the band gap, carrier diffusion length and exciton binding energy of perovskite materials are beneficial. But one of the biggest industrial advantages is that it combines all those features with relatively low costs. While silicon solar cells dominate markets with efficiencies approaching 27%, their material costs and manufacturing limits their future prospects.

In the following I will shortly describe the science of band-gaps, carrier diffusion length and exciton binding energy to provide a deeper insight into what scientists need to look for in materials.

Band-gap

Quantum mechanics states that electrons can exist in superpositions of states. Those possible states are called bands, while states that cannot be occupied by electrons are called band-gaps.

The furthest band from the atomic nucleus which an electron can occupy is called valence band. Outside the nucleus, the next band is called conducting band, where electrons are free. That means the band-gap is the energy difference between vanlence and conducting band.

This band gap is not just a name given to make more sense of the states, but it characterises elements: In metals, the valence band and conducting band overlap which allows electricity to flow through metals. In insulators the valence and conducting band have a wide gap between them, blocking electricity. Semiconductors, however, have a narrower band-gap than insulators, allowing electrons to bridge the gap through excitation. (2)

Coming to solar cells, the band-gap is crucial for the photoelectric effect. When the energy of an incoming photon is below the band-gap, it passes through without absorption. But in perovskite materials, the band-gap is „tunable “(1), meaning it can be controlled externally. They can be tuned by manipulating the chemical composition, like changing the A-cation. Perovskites have an extremely advantageous band gap for solar cells that absorb most of the incoming photons. Also, the ETL and HTL are chosen such that their energy levels create favourable band alignment with the perovskite.

Carrier diffusion length

The carrier diffusion length measures the distance an electron travels before recombining with the hole. After the photoelectric effect in the perovskite, the electron goes to the ETL and the hole to HTL, storing energy. But when electron and hole recombine, the energy is lost to light or heat. This means the longer the diffusion length, the better the efficiency because it allows more electrons to be captured. Perovskites have advantageous carrier diffusion lengths, very high in optimised single-crystal forms.

Exciton binding energy

The binding energy holds the electron to the nucleus while the exciton binding energy holds the photo-generated electron-hole pair together.

Perovskites have low exciton binding energy (only a few meV), meaning room temperature can easily separate electron-hole pairs into free carriers. The binding energy is only slightly higher than the thermal energy at room temperature due to the chemical structure of perovskites which increases their efficiency.

What has been achieved?

The most significant achievement has been the rapid increase in Power Conversion Efficency (PCE), measuring the percentage of sunlight energy converted into usable electrical energy.„Currently, the highest power conversion efficiency (PCE) of PSCs has reached >26%“ (3). Although >26% PCE might not sound that dramatic, it is a rapid increase, especially since perovskite solar cells were not too long ago discovered (about 2008).

Despite those breakthroughs, a guarantee of 20-to-25 years lifespan under high-temperatures remains a challenge. Moreover, PCSs are sensitive regarding environmental factors, especially moisture and oxygen. It is challenging to produce large-scale and cost-effective solar cells from laboratory tests.

Current research

Current research on perovskite solar cells is moving towards overcoming the three most profound challenges: high efficiency, long-term stability and scalability for mass production.

While efficiencies of 25.5% have already been achieved, scientists try now to make them commercially more available. For example, they try to replace lead from perovskite solar cells to address the environmental standards as lead is toxic.

One of the most promising research is tandem solar cells. A review about the main progress of perovskite solar cells in 2020-2021 states that „by now, the highest certified efficiency of the perovskite-based tandems has increased to over 29%“ (3)

The main idea of tandem solar cells is to stack multiple photovoltaic layers on top of each other to capture a broader range of the solar spectrum. As already discussed, perovskite have advantageous band-gaps, but they don’t use all of the solar spectrum that hits the surface. But multiple layers of different band-gaps increases the amount of light that can be used and thereby increases the efficiency.

An efficiency of 29% with tandem solar cells is really impressive and demonstrates the potential for the future.