solar cell

 The COVID-19 situation, which has hit the world from 2019 to this year, has a lot of impact on the industry beyond everyday life. Nevertheless, according to the International Energy Agency (IEA) data, Global use of renewable energy in 2020 increased by 1.5% compared to 2019.  Among them, the renewable energy sector in the form of solar power generation and wind power generation increased from an 8% share in the first quarter of 2019 to a 9% share in the first quarter of 2020. As such, it is noteworthy that the renewable energy industry maintained an upward trend, albeit slightly, despite poor conditions such as lockdown in each country and negative economic growth rate. The International Energy Agency (IEA) predicts that solar power will play a major role in the renewable energy-oriented energy market in the future. According to the report, renewables are projected to account for 80% of the growth in global electricity demand over the next decade, and solar power leads the renewable energy market growth.

what is the solar cell

 A solar cell is composed of a junction diode, albeit its construction differs somewhat from that of standard p-n junction diodes. When light reaches the p-n junction, the photons in the light energy supply the junction with enough energy to build a number of electron-hole pairs.

 The incoming light causes the junction's thermal equilibrium to be broken. The depletion region's free electrons can swiftly reach the n-type side of the junction. Similarly, depletion holes can quickly reach the p-type side of the junction. Once the freshly formed free electrons reach the n-type side of the junction, they are unable to cross it due to the junction's barrier potential.

  The p-n junction will function like a tiny battery cell when the concentration of electrons increases on one side. There will be a modest current flowing through the junction if we attach a small load across it.

Perovskite/Si tandem solar cell

what is the tandem solar cell

A solar cell that can be seen even outside is a silicon solar cell that is currently commercialized. A silicon solar cell has a world-record value of 26.7% for Si-based heterostructure solar cell. Further improvement in device PCE for Si-based single-junction solar cells becomes a bottleneck in this field since the device efficiency is approaching the Shockley-Queisser (S-Q) limit of 29% for a single-junction cell.

 

In addition, a single junction solar cell has a significant problem of solar-energy conversion. Thermalization is the loss of excess band gap energy of a photon as heat and transmission loss is the result of Photons with energies less than the band gap. In the other word, a significant portion of solar energy cannot be utilized, and the resulting light conversion efficiency is also limited.

 Journal of Energy Chemistry 58 (2021): 219-232

Tandem solar cell combines different cells (or different Eg) and Increases absorption. Tandem solar cells have multiple p–n junctions made of different semiconductor materials. In response to different wavelengths of light, the p–n junction of each material will produce electrical current. It allows for the absorption of a wider range of wavelengths, enhancing the cell’s absorption of sunlight to convert to electrical energy.

why the monolithic perovskite/silicon tandem solar cell

the monolithic perovskite/silicon tandem solar cell is considered to have the highest potential in terms of high efficiency and price competitiveness. And the perovskite/si randem solar cells can overvomes the theoretical effucuency limit of single solar cell and recorded an efficiency of 29.8%. it is attracting attention as a next generation super solar cell.

why are we studying the  monolithic perovskite/si tandem solar cells?

 

 First, monolithic configuration has the advantage of less parasitic absorption. because the two sub-cells are electrically connected, no additional substrates or electrodes are needed. and the the highest theorical PCE is 45.7%.  

 

The perovskite absorber materials have the outstanding advantages of electronic and optical properties such as wide absorption spectrum, long transport distance of carriers, fast charge separation, high charge mobility, long carrier separation lifetime, high efficiencies and can reduce processing cost.

 

 

Also, in order to maximize the efficiency of the tandem solar cell, current matching between the bottom cell and the top cell is essential. Organic–inorganic hybrid perovskite materials can be easily tuned the bandgap and thickness by simple fabricate process. so that the two sub-cells can be tandemerized.

Nature Energy 3.10 (2018): 828-838

Tandem research in ECO lab.

1. monolithic perovskite/silicon tandem solar cell

In our ECO lab, Because perovskite can be coated with a solution process at a low temperature, and it is easy to match the current with the bottom cell. So we fabricate not only perovskite/silicon (PERC, Al-BSF, HIT,,,) tandem solar cells but also perovskite/CIGS, perovskite/perovskite and so on.

For high efficiency of tandem solar cells, we study current matching by band gap tuning and thickness control of perovskite absorber of the top cell, energy level alignment and TCO (transparent conductive oxide) for optical and electrical connection between the top and bottom cells.

Otherwise, We can apply tandem solar cells with high open circuit voltage to various field such as Water splitting and carbon reduction.

Kim, Chan Ul, et al. "Optimization of device design for low cost and high efficiency planar monolithic perovskite/silicon tandem solar cells." Nano Energy 60 (2019): 213-221.

2. vacuum deposition perovskite solar cell

 

Scalability and long-term stability issues are essential factors for the commercialization of solar cells. In our ECO lab., We aim to study the all-vacuum deposition perovskite solar cell. All functional layers are deposited on the bottom cell by thermal evaporator, sputter, ALD and e-beam. The vacuum deposition method can form pinhole-free perovskite film. By improving the quality of the perovskite film itself, it can increase the possibility of high efficiency and long-term stability. Furthermore, since it is hardly affected by the roughness of the substrate, it is possible to fabricate tandem device with various bottom cells such as textured silicon cells.

Transparent solar cell

How to transparent solar cell work?

Transparent solar cells (TSC) absorb invisible light for human eyes. Because humans can see only a small segment of the electromagnetic spectrum, low energy photons (IR and NIR) and high energy photons (UV) remain undetected. The role of transparent solar cells is to use only undetected photons and generate electricity. Because visible light does not interact with such solar cells, so they appear invisible to our eyes.

How to make transparent solar cell?

There are three different approaches to achieve high transparency of solar cells

Luminescent solar concentrators can absorb invisible incident light and release it back. The released photon trapped in transparent polymer and redirected to solar panel. 

Solar cell materials with high band gap can absorb only highly energetic photons which are invisible for human eyes.

Making holes in naturally opaque materials can increase transparency of such materials. Some light passes through the holes while remain light is absorbed by solar cell.

Research in Ecolab.

Here, we demonstrate a flexible, color-neutral, and high-efficiency TSC based on a freestanding form of n-silicon microwires (SiMWs).

S.B. Kang, JH. Kim, M.H. Jeong et al. Stretchable and colorless freestanding microwire arrays for transparent solar cells with flexibility. Light Sci Appl  (2019).

Here, great all-around transparent solar cells (TSC) featuring high flexibility and high transparency with color-tunable solar cells are demonstrated. The TSCs exhibit an efficiency of 7.38% and 5.52% at the average visible transparencies of 45% and 60%, respectively.

S.B. Kang, B. Salimzhanov, W. J. Park, M. H. Jeong, J. Y. Kim, K. J. Choi et al. Colorful Transparent Silicon Photovoltaics with Unprecedented Flexibility.  Adv. Funct. Mater.  (2021)

stretchable solar cell

Why we need stretchable and flexible solar cell?

 Among many green energy industries, solar cells have been actively researched for decades. However, only silicon-based solar cells which is rigid and fragile have been mainly used for industrials and practical applications. Recently, stretchable and flexible solar cells have been emerging to overcome the disadvantages of silicon-based solar cells. For examples, numerous companies and researchers have started to try integrating bending solar panels on the roofs of electric cars as a supplementary source of charging and introducing the wearable solar cells in electronic watch. If those types of solar cells are commercialized it will lead to a new breakthrough in the solar cell market. So, we plan to study solar cells with high performance, high mechanical stability and high long-term stability solar cells and solar modules that can be used practically.

Stretchable and flexible solar cell application technology

1. Stretchable & flexible solar cell and module

We can fabricate stretchable solar cells with polymer-embedded nano/microstructure using dry etching,  vacuum deposition.

A stretchable solar module can be manufactured by attaching a stretchable solar cell on a stretchable substrate and connecting it with the electrodes.

2. Stretchable & flexible substrates and electrodes

For the flexible and stretchable solar module, solar cell must be stretchable and bendable itself, and the electrodes and substrates also are elastic and flexible in applications.

We realized stretchable electrodes by depositing stretchable electrode on the wrinkled elastic polymer substrate. Novel design of wrinkled polymer/organic/metal structure can increase the mechanical stability.