A research team from China's Soochow University and LONGi has developed a perovskite-silicon tandem solar cell with a power conversion efficiency of 34.0% by placing insulating zirconia particles at an interface. Treatments that suppress charge loss can also get in the way of extracting the charges that are needed. In this study, rather than laying an insulator continuously across the whole interface, the team scattered the particles and left pathways between them for charge to pass through. An independently certified cell recorded a steady-state efficiency of 33.5% and an open-circuit voltage of 2.014 V. The work shows that how the interfaces between materials are designed can determine power-generation performance.
The paper by Huimin Zhang, Qingshui Zheng, and colleagues was peer reviewed and published online in the journal Science Bulletin on September 3. In a press release on September 18, the publisher, Science China Press, described the interface analysis and the durability test conducted under operating conditions.
Light is shared between two absorbers, but charge is lost at the interface
A perovskite-silicon tandem solar cell stacks two different light-absorbing layers. The upper perovskite layer absorbs higher-energy light, and the silicon layer below uses the light that passes through. As the U.S. Department of Energy explains, combining materials with tuned absorption ranges can aim for higher efficiency than a single material can easily achieve.
Absorbing light alone does not produce electricity, however. The electrons generated by light and the "holes," which carry positive charge, must each be extracted to the electrodes. If electrons and holes recombine along the way, charge that could have been used for power is lost. This process, in which no light is emitted, is called non-radiative recombination.
The study targeted the buried interface beneath the perovskite layer, which is responsible for extracting holes. On textured silicon, it is also difficult to grow perovskite uniformly. If gaps form in the film or contact is uneven, spots where charge is easily lost appear.
Passivation, which treats the interface to suppress recombination, is used to address this. But if the interface is covered too heavily with an insulating layer in an attempt to reduce losses, the charge that should be extracted has a harder time passing through. The regions that suppress recombination and the pathways that extract charge must coexist.
Leaving charge pathways between insulating particles
The team introduced monoclinic zirconia (ZrO2) nanoparticles between a transparent conductive oxide and the self-assembled monolayer formed on top of it. A self-assembled monolayer is an ultrathin film formed when molecules arrange themselves regularly on a surface, and in this device it extracts holes. The key design choice was to disperse the zirconia as particles instead of forming it as a continuous film.
According to the publisher, the effect of adding the particles begins at the stage where the perovskite film is formed. The change in surface energy helps the perovskite precursor solution spread more evenly, producing a dense film with larger crystal grains and fewer gaps. Instead of only improving the electrical properties of the finished cell, the approach first prepares the foundation on which the light-absorbing layer grows.
During operation, field-effect passivation acts in the regions where zirconia particles are present, suppressing non-radiative recombination. Meanwhile, holes can be extracted through the self-assembled monolayer left between the particles. Zirconia is not used as a conductive material; instead, regions that make charge less likely to be lost and pathways that let charge flow coexist on a microscopic interface.
The team also explains that zirconia's high dielectric constant tends to suppress local fluctuations in the electric field and charge accumulation. In addition, X-ray photoelectron spectroscopy results supported the formation of Zr–O–P bonds between the zirconia and the molecules of the self-assembled monolayer. Because the monolayer also bonds to the underlying conductive oxide, the team interprets the multiple attachment points as improving the film's adhesion and leading to more uniform coverage.
Adjusting both the arrangement of the particles and the chemical bonding is what characterizes this work. Simply adding more insulator does not keep improving performance. What matters is an arrangement that suppresses charge loss while leaving regions from which current can be extracted.
What the measurements show about the interface
In time-resolved photoluminescence measurements, the average carrier lifetime of the sample with the combined interface improvements increased from 1.46 microseconds to 2.81 microseconds. This offers a clue about how the processes by which excited charge is lost have changed. However, a longer carrier lifetime alone does not allow the conclusion that charge is extracted to the electrodes faster. Making charge less likely to be lost and extracting it efficiently to the outside must each be verified separately.
The press release combines other measurement results to explain the mechanism. Conductive atomic force microscopy, which maps current distribution, and Kelvin probe measurements, which map surface potential, both reportedly showed greater uniformity after the interface modification. These results indicate reduced site-to-site variation across the interface and capture changes that average conversion efficiency alone would not reveal.
Ultraviolet photoelectron spectroscopy reportedly confirmed an energy-level alignment suited to extracting holes, and impedance measurements indicated reduced recombination at the interface. Results ranging from chemical-bond analysis to electrical response each support the mechanism the team proposes. From the published explanation alone, though, it is not possible to separate how much each effect contributed to the efficiency gain.
What the 34% and 33.5% figures measure
The performance figures provided by the publisher include results for the best-performing cell in the laboratory and for a cell that received independent certification. They are summarized below.
| Evaluation | Conversion efficiency | Open-circuit voltage | Other reported electrical characteristics |
|---|---|---|---|
| Best-performing cell in the laboratory | 34.0% | 1.997 V | Short-circuit current density 20.36 mA/cm², fill factor 83.62% |
| Independently certified cell | 33.5% steady-state efficiency | 2.014 V | The publisher's press release does not name the certifying body or give the sample area |
Source: Science China Press press release. The best laboratory cell recorded 34.0% and 1.997 V, while the independently certified cell recorded a steady-state efficiency of 33.5% and 2.014 V. Figures obtained from separate cells cannot be combined and treated as a single performance value.
Open-circuit voltage is the voltage obtained when no current is flowing to the outside. Short-circuit current, by contrast, is the current that flows when the voltage between the terminals is zero. In actual power extraction, an operating point between the two is used. The fill factor indicates how much of the product of open-circuit voltage and short-circuit current is realized as actual maximum output. Even with a high open-circuit voltage, high conversion efficiency does not follow if current extraction is hindered.
It would also be wrong to take this 34.0% as the world record for perovskite-silicon tandem solar cells. In July, LONGi announced that a different perovskite-silicon cell achieved 35.5% and was certified by Europe's ESTI. The significance of this study lies in attempting to resolve the trade-off between charge loss and charge transport at the interface through both materials and structure.
Independent certification means that a third party separate from the research team measured the cell's performance. It does not mean that the third party verified the mechanism operating at the interface or reproducibility in mass production. The press release also does not give the number of samples, measurement uncertainty, or cell area. From the published materials alone, it is therefore not possible to judge performance variation or the size of the cell that achieved this performance.
What the 2,000-hour test does and does not tell us
In the durability test, the encapsulated zirconia-modified cell retained 84% of its initial efficiency after 2,000 hours. This does not mean the conversion efficiency itself was 84%; it means that 84% of the performance at the start of the test remained, taking that starting performance as 100%. The test conditions were as follows.
| Item | Conditions given by the publisher |
|---|---|
| Sample | Encapsulated cell |
| Light | Continuous illumination at sunlight-equivalent intensity (1 sun) |
| Temperature | Room temperature |
| Operation | Continuous operation at the maximum power point |
| Result | Retained 84% of initial efficiency after 2,000 hours |
Source: Science China Press press release. The result of retaining 84% after 2,000 hours shows stability for an encapsulated cell under continuous illumination at room temperature and operation at the maximum power point.
A test that keeps shining light on a cell while power is being drawn differs from one that measures performance after storage in the dark. It is meaningful in that it examines degradation that occurs during power generation. However, a result from 2,000 hours of continuous illumination at room temperature cannot be directly converted into years of outdoor use.
On evaluating the stability of perovskite solar cells, a 2020 consensus statement by researchers based on the ISOS protocols, published in Nature Energy, proposes evaluating continuous illumination, outdoor testing, thermal cycling, and other conditions separately. When temperature and humidity change, so does the stress on materials and interfaces. In environments where light and dark alternate, as between day and night, the phenomenon in which performance partly recovers when a cell is left in the dark must also be considered. The number of hours of illumination alone cannot be used to compare durability across different experiments.
On the way to practical use, challenges include not only whether the cells can withstand high heat and humidity and temperature changes, but also whether the arrangement of zirconia particles and the coverage of the self-assembled monolayer can be kept uniform at larger cell areas. If manufacturing reproducibility can be demonstrated and long-term stability under operating conditions confirmed across a range of conditions, this interface design would move closer to being a technology for extracting light-generated charge with less loss in practical tandem solar cells.