Solar energy: is 50% efficiency now attainable?

Most commercial solar cells achieve 15-25% efficiencies, converting incoming solar energy into usable electricity. But a new record has been published in 2020, achieving 47.1% conversion efficiency. The paper used “a monolithic, series-connected, six-junction inverted metamorphic structure under 143 suns concentration”. Our goal in this short note is to explain the achievement and its implications.


“A monolithic, series-connected, six-junction inverted metamorphic structure under 143 suns concentration”

p-n junctions are the foundation of all solar cells. Each side of the junction is doped, so that the “p”-side will surrender electrons, while the “n”-side will accept electrons. When incoming solar energy strikes the junction, it may dislodge an electron and leave behind a hole. The liberated electron will propagate towards the “n”-side, while the holes will propagate towards the “p”-side, thus creating a direct current.

Bandgap is the energy needed to dislodge an electron from its usual orbit, so it is free to move through a p-n junction. The energy in light varies with wavelength (lower-wavelength equals lower energy). Light waves below the bandgap will not suffice to dislodge electrons: they will pass through the material and the energy will not be captured. Light waves above the bandgap will have excess energy left over after dislodging electrons: the excess energy will be lost as heat.

Single-junction solar cells are composed of p-n junctions made of a single material, most commonly crystalline silicon, in today’s commercial solar industry. Silicon atoms have a bandgap of 1.4eV and achieve optimum conversion efficiency in light with 700-1,000nm wavelengths (red and infra-red). They do not capture energy efficiently from lower energy, lower wavelength light (such as 400-700nm) or very high wavelength light (1,000nm+).

Multi-Junction solar cells aim to overcome the limitations of single-junction solar cells, combining multiple p-n junctions, made of multiple solar materials, to capture a broader range of the spectrum. For example, the six-junction solar cell discussed in this note has six separate junctions, connected in series, to capture light from c350-1700nm wavelengths, which is tantamount to c65-85% of all the energy in sunlight.

Group III-V alloys are used in different combinations in each of these junctions, to tune its bandgap, to capture a different wavelength of light. These alloys are composed of elements from Groups III and V in the periodic table. Group III includes boron, aluminium, gallium and indium. Group V includes nitrogen, phosphorus, arsenic and antimony.

The junctions are usually stacked with the highest energy absorber on top (i.e., junction 6). Photons that lack sufficient energy to dislodged electrons in junction 6 will pass through it, and have a additional chances of being absorbed in junction 5, through to junction 1.

The challenge is how to stack these six junctions on top of each other in a way that limits recombination and resistance, both of which are going to impair solar cell efficiency.

The challenge of recombination?

Recombination occurs when dislodged electron and holes re-combine in a solar cell, thereby lowering the current reaching the current collectors. If recombination re-emits photons, it is known as radiative recombination. Group III-V solar cells are particularly sensitive to recombination around dislocations.

Dislocations are abrupt changes in the crystal structures in a material. A physical effect is that dislocations allow atoms to glide or slip past one another at low stress levels. An optoelectronic effect is to impede current and encourage recombination of electrons and holes.

One type of dislocation, known as a threading dislocation because of its shape, extends beyond the surface of the strained layer and throughout the material, so it can be particularly deleterious to solar cell performance.

Multi-junction solar cells are particularly prone to dislocations because each junction is made of a different material. These materials are lattice-mismatched monoliths.

Monolithic materials are formed a single, continuous and unbroken crystal structure, all the way to its edges, with minimal defects or grain boundaries. This means it does not suffer from grain boundaries or dislocations, and in turn, efficiency losses from recombination should be minimized. But it is very difficult to manufacture monolithic materials from lattice-mismatched components.

Lattice-mismatched materials have different lattice constants. This means that they are composed of crystals of different sizes. In turn, this means they will not adhere well to one-another. Their boundaries are prone dislocations.

The solution: metamorphic epitaxy?

A technique called metamorphic epitaxy was used to create the monolithic six-junction solar cells described above, and overcome the inter-related challenges of recobination at dislocations in lattice mismatched materials.

Epitaxy is the process of orientation-controlled growth of crystals on top of other crystals. The 47% efficient solar cell used a variant called organometallic vapour phase epitaxy (OMVPE).

Metamorphic epitaxy minimizes dislocations around the active site of an engineered material. This is achieved by relieving the strain around lattice-mismatched boundaries by encouraging dislocations to occur away from the active site of the material. Specifically, materials known as Compositionally Graded Buffers (CGBs) were introduced in between the fourth to sixth junctions of the six-junction solar cell, as thse were the boundaries most prone to dislocations.

Specifically, these six-junction solar cells were monolithically grown on a single 2×3 cm GaAs substrate, at 550-750C temperatures, in an atmospheric-pressure OMVPE system.  “Growth begins with the high-bandgap lattice-matched junctions [on the bottom], leaving these high-power-producing junctions without dislocations”.

Then the cell was then inverted as the high bandgap lattices need to be situated on the top of the cell. (In other words, the cell is printed upside down and then turned over). Gold was electroplated onto contact of the inverted structure (literally, “gold-plated”!), then the cell was epoxied onto a flat silicon wafer. The GaAs substrate was removed by chemical etching. A front-side grid of NiAu was deposited by photolithography. Finally an anti-reflective coating of MgF2/ZnS/MgF2/ZnS was thermally deposited on the top of the cell.

The full 6J IMM structure consisted of 140 layers, including individual compositional step-graded buffer layers. The total growth time was 7.5 h.

1 sun’s concentration?

Under 1 sun’s solar intensity, the cell described above achieved 39.2% efficiency. This is the highest 1-Sun conversion efficiency demonstrated by any technology to-date. The prior record is 38.8% for a five-junction bonded III–V solar cell.

The efficiency is very high, because the voltages of each junction add up to a high total voltage. However, the current density in each junction was low. The efficiency could have been higher with a higher current density, which in turn, is achieved by concentrating the incoming sunlight.

143 suns’ concentration?

Concentration of incident light improves solar cell efficiency. The reason is that more concentrated light dislodges more electrons. More dislodged electrons means a higher current density. In turn, a higher current density raises the bandgap for dislodging further electrons (it is harder to remove further electrons from a material that has already lost some electrons). So even more energy can be absorbed when additional light strikes the cell.

Concentrating solar light is also desirable as a way to lower costs, as multi-junction solar cells are expensive to produce. Concentrating the light from 1 square meter onto 1 square centimeter, for example, reduces the area of solar materials required by a factor of 10,000.

Joule losses set the upper limit on the solar concentration that will maximize efficiency. Joule losses are the loss of electricity as heat when electric current passes through a conductor. They are a square function of current and a linear function of resistance. So they rise quadratically as solar intensity rises linearly.

Lower resistance will help to limit joule losses. In the solar cell described above, several challenges were observed keeping resistance low.

Each junction is connected in series in the cell. The current flows between each junction through a “tunnel interconnection”. Resistance through these tunnel junctions was found to rise with current, placing a practical limit on solar concentrations.

Internal resistances within each junction were also higher than desired. They were found to have been elevated by the temperatures during epitaxy and during dopant diffusion (particularly in Zn-containing layers).

At the top junction, the 2.1eV bandgap material required a high resistance to conduct charge laterally to the metal grid fingers that serve as current collectors for the cell’s electrical circuit.

Reducing the effective series resistance to 0.015 Ω cm2 is seen to be possible, by analogy to previous four-junction solar cells, which would allow the six-junction cell described above to surpass 50% efficiency at 1,000–2,000 Suns. The maximum theoretical efficiency is 62%.

Commercial implications?

47-50% efficient solar cells are a good incremental improvement. To put the ‘breakthrough’ into context, the previous record for a multi-junction solar cell was 46% efficiency at 508 suns, using a four-junction device. There is scope for multi-junction solar cell efficiency to improve further.

The cell was also very small, at 0.1cm2. When solar ‘records’ are measured, usually the stipulation is required that a cell must be 1cm2 in area, as a testing criterion.

Its production was very complex taking 7.5-hours to assemble 140 separate layers. Complex structures are expensive and more prone to degradation, which makes commerciality challenging.

We conclude that a 47-50% efficient solar cell is a tremendous technical achievement. But the evidence does not yet suggest proximity to commercialising ultra-efficient multi-junction solar cells like this at mass scale.


Source: Geisz, J. F., France, R. M., Schulte, K. L., Steiner, M. A., Norman, A. G., Guthrey, H. L., Young, M. R., Song, T. & Moriarty, T. (2020). Six-junction III–V solar cells with 47.1% conversion efficiency under 143 Suns concentration. National Renewable Energy Laboratory (NREL), Golden, CO, USA.

What oil price is best for energy transition?

It is possible to decarbonize all of global energy by 2050. But $30/bbl oil prices would stall this energy transition, killing the relative economics of electric vehicles, renewables, industrial efficiency, flaring reductions, CO2 sequestration and new energy R&D. This 15-page note looks line by line through our models of oil industry decarbonization. We find stable, $60/bbl oil is ‘best’ for the transition.


Our roadmap for the energy transition is outlined on pages 2-4, obviating 45Mbpd of long-term oil demand by 2050, looking across each component of the oil market.

Vehicle fuel economy stalls when oil prices are below $30/bbl, amplifying purchases of inefficient trucks and making EV purchases deeply uneconomical (pages 5-6).

Industrial efficiency stalls when oil prices are below $30/bbl, as oil outcompetes renewables and more efficient heating technologies (page 7).

Cleaning up oil and gas is harder at low oil prices, cutting funding for flaring reduction, methane mitigation, digitization initiatives and power from shore (pages 8-9).

New energy technologies are developed more slowly when fossil fuel prices are depressed, based on R&D budgets, patent filings and venturing data (pages 10-11).

CO2 sequestration is one of the largest challenges in our energy transition models. CO2-EOR is promising, but the economics do not work below $40/bbl oil prices (pages 12-14).

Our conclusion is that policymakers should exclude high-carbon barrels from the oil market to avoid persistent, depressed oil prices (as outlined on page 15).

Will renewable growth slow down from 2020?

The growth of renewables has been revolutionary, with wind and solar costs emerging towards the bottom of the global cost curve, scaling up at a pace of 300TWH pa. However, we find unsettling evidence that the market could slow in the 2020s, as curtailment accelerates in heartland markets such as California, Germany and the UK. The rationale, and all the underlying data, are included in this Excel file.

Ramp Renewables? Portfolio Perspectives.

It is often said that Oil Majors should become Energy Majors by transitioning to renewables. But what is the best balance based on portfolio theory? Our 7-page note answers this question, by constructing a mean-variance optimisation model. We find a c0-20% weighting to renewables maximises risk-adjusted returns. The best balance is 5-13%. But beyond a c35% allocation, both returns and risk-adjusted returns decline rapidly.


Pages 2-3 outline our methodology for assessing the optimal risk-adjusted returns of a Major energy company’s portfolio, including the risk, return and correlations of traditional investment options: upstream, downstream and chemicals.

Page 4 quantifies the lower returns that are likely to be achieved on renewable investment options, such as wind, solar and CCS, based on our recent modeling.

Pages 5-6 present an “efficient frontier” of portfolio allocations, balanced between traditional investment options and renewables, with different risk and return profiles.

Pages 6-7 draw conclusions about the optimal portfolios, showing how to maximise returns, minimise risk and maximise risk-adjusted returns (Sharpe ratio).

The work suggests oil companies should primarily remain oil companies, working hard to improve the efficiency and lower the CO2-intensities of their base businesses.

Patent Leaders in Energy

Technology leadership is crucial in energy. It drives costs, returns and future resiliency. Hence, we have reviewed 3,000 recent patent filings, across the 25 largest energy companies, in order to quantify our “Top Ten” patent leaders in energy.


This 34-page note ranks the industry’s “Top 10 technology-leaders”: in upstream, offshore, deep-water, shale, LNG, gas-marketing, downstream, chemicals, digital and renewables.

For each topic, we profile the leading company, its edge and the proximity of the competition.

Companies covered by the analysis include Aramco, BP, Chevron, Conoco, Devon, Eni, EOG, Equinor, ExxonMobil, Occidental, Petrobras, Repsol, Shell, Suncor and TOTAL.

Upstream technology leaders have been discussed in greater depth in our April-2020 update, linked here.


More information? Please do not hesitate to contact us, if you would like more information about accessing this document, or taking out a TSE subscription.

Oil Companies Drive the Energy Transition?

There is only one way to decarbonise the energy system: leading companies must find economic opportunities in better technologies. No other route can source sufficient capital to re-shape such a vast industry that spends c$2trn per annum. We outline seven game-changing opportunities. Leading energy Majors are already pursuing them in their portfolios, patents and venturing. Others must follow suit.


Pages 2-3 show that today’s technologies are not sufficient to decarbonise the global energy system, which will surpass 100,000TWH pa by 2050. Better technologies are needed.

Pages 4-6 show how Oil Majors are starting to accelerate the transition, by developing these game-changing technologies. The work draws on analysis of 3,000 patents, 200 venture investments and other portfolio tilts.

Pages 7-13 profile seven game-changing themes, which can deliver both the energy transition and vast economic opportunities in the evolving energy system. These prospects cover electric mobility, gas, digital, plastics, wind, solar and CCS. In each case, we find leading Oil companies among the front-runners.

Perovskites: Lord of Light?

Perovskites are the fastest-improving solar innovation. The best test-cells hit a new record of 28% efficiency last year, with line-of-sight to the mid-30s, i.e., 2x more efficient than today’s silicon photovoltaics. A Major is at the cutting edge.


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Is the world investing enough in energy?

Global energy investment will need to rise by c$220-270bn per annum by 2025-30, according to the latest data from the IEA, which issued its ‘World Energy Investment’ report this week. We think the way to achieve this is via better energy technologies.

Specifically, the world invested $1.6bn in new energy supplies in 2018, which must be closer to $1.8-1.9bn, to meet future demand in 2025-30– whether emissions are tackled or not. The need for oil investment is most uncertain. More gas investment is needed in any scenario. And renewables investment must rise by 15-100%.

Note: data above includes $1.6trn investment in energy supplies and c$250bn in energy efficiency measures

Hence the report strikes a cautious tone: “Current market and policy signals are not incentivising the major reallocation of capital to low-carbon power and efficiency that would align with a sustainable energy future. In the absence of such a shift, there is a growing possibility that investment in fuel supply will also fall short of what is needed to satisfy growing demand”.

We do not think the conclusions are surprising. Our work surveying 50 investors last year found that fears over the energy transition are elevating capital costs for conventional energy investments (below).

Meanwhile, low returns make it challenging to invest at scale in renewables.

We argue better energy technologies are the antidote to attracting capital back into the industry. That is why Thunder Said Energy focuses on the opportunities arising from energy technologies. Please see further details in our recent note, ‘What the Thunder Said’. For all our ‘Top Technologies’ in energy, please see here.

References

IEA (2019). World Energy Investment. International Energy Agency.

Two Majors’ Secret Race for the Future of Offshore Wind?

An exciting aspiration in wind technology is to obviate large, expensive “towers”, and unleash tethered kites into the skies. They can access 2-4x more wind-power at greater altitudes, and at 50-90% lower costs. Intriguingly, we have discovered Exxon and Shell are at the forefront of pursuing this new wind opportunity offshore, based on their patents and filings.

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Under-investment risks in the energy transition?

Fears over the energy transition are now restricting investment in fossil fuels, based on our new paper, published in conjunction with the Oxford Institute for Energy Studies, linked here.    

They have elevated capital costsby 4-7% for oil and by c25% for coal, compared with the early 2010s.

  • One consequence will be to concentrate capital into renewables, gas,  and shorter-cycle oil projects (i.e.,  shale).
  • But there will also be negative consequences, risking long-run supply shortages of oil and coal.
  • Companies are also being pressured to ‘harvest’ their existing assets, rather than maximising potential value in the 2020s, which may impact valuations.  

For further details please see the full paper, linked here, or contact us.