Photovoltaic industry - Efficiency Limits

The thermodynamic efficiency of various devices is of wide interest because of the relevance of this parameter for energy conversion. The classic limiting efficiency of a solar cell was analyzed by Shockley and Queisser (1961). They also established a model to describe the electrical behavior of the diode that constitutes the solar cell. This time, the model came from detailed balance arguments and, at first sight, does not give the same results as the standard model. According to Shockley and Queisser (1961), the thermodynamic efficiency for an ideal single homo-junction cell is 31%. The efficiency of a single-junction device is limited by transmission losses of photons with energies below the band-gap and thermal relaxation of carriers created by photons with energies above the band-gap.

In the classic case, every photon absorbed in a solar cell at most one electron–hole pair. Kodolinski et al. (1993) showed quantum efficiency higher than 1 in the short-wavelength range of a-Si solar cell. This can be explained as an optically induced Auger mechanism: the energy in excess of the band-gap that one of the carriers receives from a high-energy photon is used in a second electron–hole generation. This result has led to the revision of the Shockley–Queisser model of the ideal solar cell, widely accepted as the physical limit of PV conversion.

Several methods have been offered to increase the power conversion efficiency of solar cells, including tandem cells, impurity-band and intermediate-band devices, hot-electron extraction, and carrier multiplication, the so-called “third generation” PV. these methods will be discussed in the next posts. 

News: Scientists capture electron movements inside a solar cell

New research from the Femtosecond Spectroscopy Unit at the Okinawa Institute of Science and Technology Graduate University (OIST), published in Nature Nanotechnology, has made electron's movement measured.

"We have made a video of a very fundamental process: for the first time we are not imagining what is happening inside a solar cell, we are actually seeing it. We can now describe what we see in this time-lapse video, we no longer have to interpret data and imagine what might have happened inside a material. This is a new door to understanding the motion of electrons in semiconductors materials." Prof. Keshav Dani.

If you shine light on a material, the light energy can be absorbed by the electrons and move them from a low-energy state to a higher one. If the light pulse that you shine at the material is very, very short, a few millionths of a billionth of a second - that is a few femto seconds - it creates a very rapid change in the material. However, this change does not last long, as the material goes back to its original state on a very fast time scale. For a device to work, like in a solar cell, we have to extract energy from the material while it is still at the high energy state. Scientists want to study how materials change state and lose energy. "In reality, you cannot watch these electrons changing state on such a fast time scale. So, what you do is measure the change of reflectivity of the material," Dr. Man explained. To understand how the material changes when exposed to light, researchers expose the material to a very short, but intense, pulse of light which causes the change, and then continuing to measure the change introduced by the first pulse by probing the material with subsequent much weaker light pulses at different delay times after the first pulse.

This research provides a new insight into the movement of electrons that could potentially change the way solar cells and semiconductor devices are built. This new insight brings the technology field one step closer to building better and more efficient electronic devices.

Source
For more information:
Imaging the motion of electrons across semiconductor heterojunctions. , Nature Nanotechnology,

Photovoltaic industry - Introduction

Antoine Henri Becquerel
(1852-1908)

Presently, the world energy consumption is 10 terawatts (TW) per year, and by 2050, it is projected to be about 30 TW. The world will need about 20 TW of non-CO2 energy to stabilize CO2 in the atmosphere by mid-century.The simplest scenario to stabilize CO2 by mid-century is one in which photovoltaics (PV) and other renewables are used for electricity (10 TW), hydrogen for transportation (10 TW), and fossil fuels for residential and industrial heating (10 TW). Thus, PV will play a significant role in meeting the world future energy demand. The present is considered as the “tipping point” for PV.

The PV effect was discovered in 1839 by Becquerel while studying the effect of light on electrolytic cells. A long period was required to reach sufficiently high efficiency.

Solar cells developed rapidly in the 1950s owing to space programs and used on satellites (crystalline Si, or c-Si, solar cells with efficiency of (6–10%).

The energy crisis of the 1970s greatly stimulated research and development (R&D) for PV.

Solar cells based on compound semiconductors (III–V and II–VI) were first investigated in the 1960s. At the same time, polycrystalline Si (pc-Si) and thin-film solar cell technologies were developed to provide high production capacity at reduced material consumption and energy input in the fabrication process, and integration in the structure of modules by the deposition process and consequently cost reduction for large-scale terrestrial applications.

In the following posts, we review the current status of the PV market and recent results on several leading types of solar cells, such as c-Si, pc-Si, and amorphous-Si (a-Si), and III–V, II–VI, and I–III–VI2 semiconductors and their alloys and nano-PV.

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