Latest Efficiency Records: Perovskite-Silicon tandem Solar Cells

The latest efficiency record for a perovskite/silicon tandem solar cell, an impressive 33.9% set by Longi in October 2023 (Fig 1), is the current pinnacle of what has been a remarkable leap in photovoltaics. This record, surpassing the previous benchmark set by King Abdullah University of Science & Technology (KAUST) earlier in the year, is one of several set since late 2022 that surpasses the Shockley-Queisser (S-Q) limit of a single junction silicon solar cell.

So why is breaking this limit important? Due to optical, thermal and electronic losses, it is highly unlikely that a commercially viable single junction silicon solar cell will ever have an efficiency close to the S-Q limit. Continuous improvements in silicon solar cell technology (like passivation techniques, advanced light trapping, etc.) are pushing their efficiencies higher, but surpassing the S-Q limit will not be possible.

This article will explore the underpinnings of multi-junction solar cells, and explain how optical and electrical simulations can be utilized in understanding how to optimize the performance of tandem solar cells.

Fig. 1. The rise in solar cell efficiency from the 1980s to 2023, highlighting the surpassing performance of Perovskite/Silicon tandem cells over Single Crystal Silicon cells in recent years.

The Limitations of Silicon Solar Cells

Fig. 2. Optical (green), Thermal (yellow) and Electronic (Blue) losses in a silicon solar cell.

Silicon-based single Junction PV modules make up for 95% of the PV Market. The most efficient solar cell currently available has an efficiency of 26.8 %, while the theoretical limit, called the Shockley-Queisser limit, is about 32 %. The nature of the bandgap (direct - indirect) influences the number of absorbed photons. The indirect bandgap of silicon is a strong limitation to the ideal efficiency of these solar cells. Three types of losses limit the efficiency of a solar cell:

• Optical: photons with energy lower than the bandgap are not absorbed. The higher the bandgap, the higher the amount of non-absorbed photons (about 19% of the total losses for a Si solar cell).

• Thermal: photons with an energy that exceeds the Bandgap ( E>Eg) are absorbed. The generated carriers are thermalizing down to the band edge. The excess energy is released as heat to the solar cell (about 33% of the total losses for a Si solar cell).

• Electronic: loss due to radiative charge recombination, i.e. the pair recombines and, eventually, a photon is emitted. (about 15% of the total losses for a Si solar cell). Isothermal losses are an additional electronic contribution. These are losses due to the power dissipation along the band-edge which is an isothermal curve (constant temperature of the carrier).

Overall these losses result in about 68 percent of the total sunlight not being converted into electricity. The efficiency of commercial silicon solar panels is further limited to around 20 % by parasitic losses caused by non-ideality due to the large-area of the cell. the large area.

Methods for Exceeding the Shockley–Queisser Limit

This blog focuses on multi-junction solar cells. Here, we list only a few solutions the community has proposed to reduce losses in silicon solar cells. We invited the reader to search for more detailed analyses if interested in this topic:

• Hot carrier solar cells extract high-energy charges before the excess energy turns into heat.

• Intermediate bandgap solar cells absorb photons at energies smaller than a bandgap without reducing the voltage. The Intermediate band (IB) energy level is between the conduction and valence bands. The IB allows photons with energy smaller than the bandgap to excite a charge from the valence band to the conduction.

• Multiple exciton generations in a solar cell create more than one charge carrier from a single high-energy photon.

Despite the proposed solutions, theoretically, should improve the efficiency of a silicon solar cell, they are rarely employed in commercial photovoltaics.

Fig. 3. Three advanced solar cell mechanisms: hot carrier absorption, intermediate band transitions, and multiple exciton generation, each enhancing electron excitation and energy conversion efficiency.

The Rise of Multi-Junction Solar Cells

Conventional solar cells have a single PN Junction, a combination of p and n-doped semiconductor layers. The opposite type of doping creates an internal electric field that promotes the extraction of photogenerated charges. A multi-junction solar cell has two or more junctions, and each one absorbs at a different wavelength. The more junctions, the greater the portion of the solar spectrum that is absorbed, resulting in higher theoretical efficiency.

Solar cells with infinite multi-junctions have a theoretical efficiency of up to 86.8 %. -Currently, the most efficient solar cell in the world has an efficiency of 47.6 % under concentrating illumination, and it is made up of 4 junctions. The record efficiency under standard Global Spectrum instead amounts to 39.5 % for a triple junction solar cell. Both of these record solar cells have light-absorbing materials constituted by three five semiconductors. These advanced solar cells are up to a thousand times more expensive than silicon solar cells. Their use is mostly limited to special applications such as space applications where cost is less of an issue than other aspects such as weight.

Fig. 5. Various tandem solar cell configurations and their corresponding electrical circuit diagrams, demonstrating how light management and connectivity affect the overall efficiency and performance of the solar cells.

Optimizing Tandem Solar Cells

A multi-junction solar cell with only two absorbing materials is a so-called tandem solar cell, and it is the most interesting option for “standard” commercial uses. They can be fabricated with four or two electrodes, also known as four-terminal and two-terminal tandem configurations. The three-terminal tandem is the third contact variation that is less known but is catching up. In a four-terminal tandem solar cell the two sub-cells are fabricated independently and are electrically isolated from each other, the sub-cells are either mechanically stacked or coupled with a spectral splitter which is essentially a set of lenses to direct light to the appropriate sub-cell. However, this configuration has considerable parasitic absorption and reflection from the inactive layers and a higher overall material cost due to multiple substrates, complex assembly, and wiring.

Fig. 6. A solar energy system where a holographic spectrum splitter divides incident sunlight into high and low energy photons to be absorbed by corresponding high and low band-gap solar cells, enhancing the photovoltaic efficiency.

In two-terminal tandem solar cells, the sub-cells are electrically connected and must be current-matched to avoid recombination losses. This configuration is obtained by stacking the sub-cells mechanically or, most effectively, by adding a transparent conducting layer, also known as the recombination layer, which is essential to form an ohmic contact between the sub-cells. This configuration requires less wiring and is simpler to install, but the fabrication process is challenging since the top cell must be deposited without damaging the bottom cell. Additionally, with daily and seasonal variations of the solar spectrum, the current matching requirement is not always satisfied.

Three terminal tandems have a middle contact between the two sub-cells avoiding the need for the tandem junction and current matching as in two terminal tandems. However, these devices are less explored than 2-terminal and 4-terminal solar cells. We will not discuss it here.

Fig. 7. Contour plots comparing the performance of tandem solar cells, with axes representing the bandgaps of top and bottom cells and contours indicating efficiency percentages.

The highest theoretical efficiencies under standard light intensities and AM1.5 G are 46% and 45.7% for four-terminal and two-terminal tandem solar cells, respectively. Twp-terminal tandem PVs have fewer bandgap combinations to reach the highest efficiency because they require both electrical and optical matching, whereas 4-terminal tandem PVs need only optical matching, allowing a larger variation in the band gap of the top cell. Which of these two solutions could be the winner is still unclear. Both of them could have several benefits, but they have issues to be resolved before we can see them in the market.

So far, we assumed that the illumination impinges exclusively on the top subcell. It is also worth mentioning bifacial tandem solar cells. These interesting solutions collect light irradiation impinging on the rear side of the solar cells, increasing the overall amount of photons that are absorbed by the active layer.

Fig. 8. A bi-facial tandem solar cell setup where sunlight is captured by both the top and bottom cells, with the bottom cell also utilizing reflected light (albedo) for energy generation.

Maximizing Efficiency with Light Management

The most widely studied tandem solar cells are silicon/perovskite, where silicon is the bottom sub-cell. Silicon has a band gap of 1.12 eV. To maximize the efficiency, the ideal band gap of the top cells should be between 1.67 and 1.75 eV. Perovskites are great solutions for the top cell as they can be engineered to have a bandgap between 1.15 and 3.1 eV. They can be deposited at low temperatures with solution process methods like spin coating, avoiding any damage to the bottom cell during the fabrication of the two-terminal solar cell. Additionally, perovskites have a direct band gap and a strong light absorption coefficient. Therefore, thicknesses of around 500 nm are sufficient for the perovskite sub-cell. As a comparison, the silicon layer is almost a thousand times thicker than the perovskite due to its indirect band gap and low absorption coefficient.

Fig. 10. A cross-sectional diagram of a tandem solar cell, showing the layering from the superstrate to the encapsulation. The perovskite and silicon sub-cells are separated by transparent contacts and coupled optically.

Several strategies have been developed to increase the light absorption of the silicon sub-cell. The usage of a textured surface (Fig. 11) is one of the most commonly employed techniques. However, determining the optimal roughness for such surfaces is a complex process that cannot be achieved through experimental trial and error alone. That is where computer simulations come in handy. From an optical simulation, we can appreciate, for example, how much current can be recovered from a two-terminal silicon perovskite tandem solar cell by adding texture interfaces. This simulation is shown in Figure 12 and was performed with the software Setfos.

Fig. 11. An electron microscopy view of textured silicon surfaces with pyramid-like structures intended to enhance light absorption within a silicon sub-cell, illustrating the complexity of optimizing surface roughness for solar applications.

We used an optical simulation model that incorporates wave-optics to compute the reflection and transmission of coherent thin-film components, ray-optics to evaluate the angular scattering properties of the texture interfaces, and a net-radiation algorithm that uses this information to quantify the light propagation in the entire layer stack.

Fig. 13. A detailed layer stack of a perovskite/silicon tandem solar cell, specifying the materials and thicknesses of each layer from the air interface down to the substrate.

Both stand-alone optical simulation and combined optoelectronic simulation of perovskite/Si tandem solar cells are extremely important for the development of this innovative PV technology. The simulation could also help to understand the relationship between optical properties and electrical properties of the tandem PV. From the simulated absorptance of the full stack, we can calculate the photocurrent of the devices. Starting from a planar device with no texture interfaces, the addition of an anti-reflected coating improves the absorption of both the perovskite and silicon absorbers at wavelengths between 500 and 1000 nm. The texturing of both sides of the silicon sub-cell brings a clear improvement in the light absorption at wavelengths larger than 750 nm. The current increases by almost 23% compared to the planar device. It is silicon that especially benefits from light management strategies whereas perovskite does not require such treatments thanks to the high absorption coefficient.

Fig. 14. A cross-sectional scanning electron microscopy (SEM) of the front and rear side of a planar perovskites silicon tandem solar cell. Source: © D. Türkay (EPFL), C. Wolff (EPFL), F. Sahli (CSEM), Q. Jeangros (CSEM). Calculated absorptance of a full perovskite/Si tandem PV.

However, spin-coating perovskite on a textured surface leads to uneven deposition and the formation of holes. Possible solutions include reducing the texture size and depositing a thicker perovskite layer or depositing the perovskite conformally to the texture of the silicon sub-cell.

These two approaches led to the highest efficiency records for silicon perovskite tandem solar cells in 2022. The record efficiency of 32.5 % was obtained for a planarized tandem solar cell with a nano texture between the two sub-cells, improving light management and the deposition quality of the perovskite absorber. For a tandem cell with a micrometric texture and conformally deposited perovskite, the highest certified efficiency amounts to 31.3 %.

Fig. 15. The left graph plots current-voltage characteristics for different defect densities in a solar cell, and the right graph shows the increase in perovskite layer thickness with increasing defect density, highlighting the optical optimum at the lowest defect concentration.

Addressing Recombination Losses

Getting the most out of solar cells requires good light management and limiting losses by recombination at the interfaces. Non-radiative recombination at the interfaces between the perovskite and the charge of transporting layers is a typical source of performance loss.

Electrical simulations with SETFOS (Fig. 15) show that with increasing defect density at the perovskite interfaces, the open-circuit voltage (Voc) decreases considerably. To compensate for the loss of carriers, the perovskite thickness should be increased by up to 30 nm from the optimum thickness value inferred from Optical simulations (Fig. 15).

Exploring More Solar Cell Topics

In concluding this broad overview of tandem solar cells, it is evident that the field is ripe with innovation and opportunities for further exploration. Here is a quick look at some of the pivotal areas of ongoing research:

Three-Terminal Tandem Solar Cells: The introduction of the three-terminal tandem design marks a significant leap forward, overcoming the limitations of the top sub-cell bandgap and promising higher energy yields with strong economic prospects and reduced environmental impact.

Innovations to Reduce Parasitic Absorption: Addressing parasitic absorption is crucial. Recent shifts in local selective contact materials aim to minimize these losses, enhancing the overall efficiency of tandem cells.

Enhancing Light Absorption: Tuning the bandgap of perovskite materials can significantly boost light absorption and photoelectric conversion efficiency, making it a promising candidate for future low-cost solar cells.

Advances in Perovskite Absorber Material: Halide perovskite materials have been a game-changer, leading to more efficient tandem solar cells with tunable and strong light-absorbing capabilities. Research is still ongoing and we are expecting several new developments that could continue increasing the efficiency of the solar cells but, most importantly, increase their operational stability.

These developments represent just the beginning of what is possible with tandem solar cells. As research continues to advance, we can expect to see even more innovative solutions and improvements in solar cell technology, driving us toward a more sustainable and efficient energy future.