Starting at the sun, photons - the packets of energy in sunlight - travel some fantastic 150 million kilometers before they reach the earth. When such a photon falls on a photovoltaic cell, it strikes the surface of the semiconductor material, liberating free electrons from the material's atoms, giving us electricity.

But not all these packets of energy are converted. In a typical solar cell that is commercially employed today, less than 25% of the energy falling on it is converted to electricity.

What this means in practical terms is, a 1 MW solar panel can generate only about 4 MWh of electricity per day. Compare this with a 1 MW coal power plant, which can generate close to 24 MWh of electricity per day (1 MW * 24 hours).

Solar photovoltaic (PV) based power generation presents one of the most scalable - and affordable - ways to produce electricity today. Just imagine how much more attractive solar power would be if the efficiencies of solar panels were much higher. 

How much more efficient can solar panels get?

Determinants of solar panel efficiency

I first need to make a distinction between the efficiencies of solar cells and solar panels. The efficiencies of solar panels - which are cells stacked together with glass on top - are usually 1-2% lower than those for solar cells. 

That distinction made, let me quickly dive into the topic.

The efficiency of solar cells (and panels) depends on the following aspects - cell material, type of cell and theoretical efficiency limits.

Cell material

Firstly, the material with which the cells and panels are made matters. Most solar cells today are crystalline silicon solar cells. The other - minor - variant is the thin film solar cell that could be based on Cadmium Telluride (CdTe), Copper Indium Diselenide (CIS) or Gallium Arsenide (GaAs).

Within the dominant crystalline silicon solar cells, the cell material could be polycrystalline (lower efficiency), monocrystalline (higher efficiency) or monocrystalline PERC (higher efficiency than just mono).

Depending on the cell material used, the efficiency range can be between 14 - 23%.

Type of cell

Until recently, most solar cells were monofacial - they generated electricity ony from the top portion. Bifacial cells, which are becoming more common, are ones in which the cells can generate additional electricity from the sunlight reflected from the bottom surface, an additional 1-2%.

Thus, the efficiences of the best solar cells currently used in commercial applications - bifacial monocrytalline PERC cells - are inching close to 23%.

Theoretical efficiency limits

23% is a lot more than the 20% that the best in class solar cells were giving untll a few years back, but efficiencies may not have a lot more room for improvements because the theoretical upper limit for solar cell efficiency is 33.5%

So, is this the best efficiency we can expect from solar panels?

The answer is a bit more nuanced than a simple yes or no. Because, under certain circumstances, the solar cell efficiency can go beyond this theoretical limit.

Hello, how is that possible?

The theoretical efficiency mentioned above is applicable for single p-n junction cells, which is the type used in most solar power plants today - these are cells that have only one p-n junction. A p-n junction is the component of the solar cell that controls the flow of electric current.

Cells with more than one p-n junction - known as heterojunction cells - can have much higher theoretical efficiencies, and lab efficiencies of up to 45% have been reported.

Theoretical efficiencies for hetero-junction solar cells depend on the number of cells - the more junctions, the higher the upper limit of theoretical efficiency. For a 3-junction cell, this is about 56%. Increasing the number of junctions beyond three will indeed increase theoretical efficiency, but the number of junctions increases exponentially for equal efficiency improvements for every additional delta. (For those who are curious, studies indicate that the theoretical maximum for an infinite-junction solar cell is about 86% with concentrated sunlight, but this estimate is of just academic interest) 

So, that's about what is theoretically possible with crystalline silicon solar cells.

What about disruptive solar PV innovations such as Perovskite solar cells?

Most times, after I finish talking to people about solar cells they can use for their solar power plants, they complain - "But Narsi, you have not considered Pero-something solar cells."

Perovskite solar cells have been in the news for over five years, with some reporters writing about it as if their rooftop solar panels were made of these. Halide perovskites are a family of materials that have shown potential for high performance & efficiency, and low production costs in solar cells. They derive their admirably easy-to-pronounce name from the nickname for their crystal structure. This category of solar cells have indeed shown remarkable progress in recent years with rapid increases in efficiency, from reports of about 3% in 2009 to over 25% today. Their theoretical maximum for a single junction architecture is also only about 32%, but it is possible that efficiency enhancements for these cells could be much faster than they are for silicon.

But perovskite cells are still at lab & pilot stages, even though there are commercialization claims by some companies. A number of challenges remain - specifically with regard to their stability - before they can become a competitive commercial technology. While silicon based solar cells can easily last 25 years (and even up to fifty years), Perovskite cells have shown limited durability, While the durability has increased from minutes (yes, just minutes!) to months in the last few years, it still has some way to go before it can become a competitor to silicon.

Conservatiely, one can expect these types of cells to be commercialized to a certain extent in the late 2020s or early 2030s, and if lucky, hit the large scale stage five years from then.

From an efficiency perspective, we are still looking at maximum efficiencies of around 30%, for single junction perovskite cells.

Likely efficiency highs until 2030

So, there we are.

I took you around on a whirlwind tour of solar cells and their efficiencies.

For single-junction solar cells, by 2030, the best efficiencies we can look forward to are between 27% and 30%, the latter if Perovskite gets commercialized by then.

In theory, efficiencies for solar cells can touch 50% or even beyond through heterojunction cells, but these are too expensive in spite of years of research and development to bring down costs.

For at least until 2030, one can expect single junction solar cells to be the only type used commercially, on scale.

Considering all these, I do not expect solar cell efficiencies to go far beyond 25% even by 2030, perhaps 27% at best. Subtract a percentage or two for the efficiency of the solar panel.