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# Quantum Efficiency/Spectral Response/IPCE Measurement Technology 01 A Powerful Tool for Creating High-Efficiency Solar Cells

## What is Quantum Efficiency?

Before explaining what quantum efficiency (Quantum Efficiency) is, let’s first understand the spectral response.

Spectral Response (SR) is an index to evaluate the photoelectric conversion capability of optical radiation detection devices (such as photodetectors, photometers, solar cells, etc.), that is, the efficiency of incident photon-electron conversion efficiency, IPCE. For example, solar cells are also photoelectric devices that convert light into electrical energy, so the spectral response is also an important index for evaluating their conversion efficiency.

Spectral Response SR(λ) Among them, P(λ) is the incident light energy of each wavelength, in Watts (Watt); I(λ) is the current converted by the solar cell after receiving the incident light, in amperes (Amp). Its physical meaning is: the ability of a solar cell to generate ampere current when receiving one watt of light energy.

The spectral response can also be called Quantum Efficiency (QE) or IPCE (Incident Photon-Electron Conversion Efficiency. The incident light energy is converted into the number of photons, and the current generated by the solar cell and transmitted to the external circuit is converted into the number of electrons. The spectral response can represent the ability of each incident photon to be converted into electrons transported to the external circuit, called Quantum Efficiency (QE), the unit is expressed as a percentage. This can also be called the incident photon-electron conversion efficiency IPCE. ## How to calculate the quantum efficiency? (The formula of quantum efficiency)

The conversion between spectral response and quantum efficiency can be written as the following formula: q is electron quantity,  h is Plank Constant,  v is photon frequency, λ is the wavelength of incident photons (nm).

According the the above formula, the external quantum efficiency formula can be rewritten as:  ### Figure 2 The conversion between spectral response (SR) and external quantum efficiency (EQE). ## What is External Quantum Efficiency (EQE) ?

From the formula and the unit of spectral response (Amp/Watt), the ampere Amp is converted to the number of electrons per unit time (electron/sec), and the Watt is converted to the number of photons per unit time (Photons/sec). The quantum efficiency obtained by incorporating the above formula is called the external quantum efficiency.

Generally speaking, the quantum efficiency QE refers to the external quantum efficiency EQE, also known as the incident photon-electron conversion efficiency IPCE (Incident Photon-Electron Conversion Efficiency).

QE=EQE=IPCE

The external quantum efficiency EQE calculates the number of electrons produced by the total number of incident photons. Taking Figure 2-1 as an example, suppose there are a total of 10 photons incident on the solar cell, 2 photons are reflected on the surface of the solar cell, and finally 6 charges are generated. Therefore, by definition, the external quantum efficiency of this solar cell is

EQE=number of generated charges/total number of incident photons=6/10=60%

## What is Internal Quantum Efficiency (IQE)?

Internal Quantum Efficiency (IQE) is also the calculation of photon-electron conversion efficiency. Different from the external quantum efficiency EQE, it calculates the number of photons that actually enter the solar cell and the number of electrons it generates. Taking Figure 2-1 as an example, suppose there are a total of 10 photons incident on the solar cell, and 2 photons are reflected on the surface of the solar cell. Then the number of photons that actually enter the solar cell material is (10-2)=8 photons and 6 electrons are generated. Then the internal quantum efficiency of this battery

IQE=number of generated charges/number of photons incident into the internal material=6/(10-2) =75%

## What is the relation between internal quantum efficiency IQE and external quantum efficiency EQE?

The internal quantum efficiency is only calculated when incident on the inside of the material. The external quantum efficiency does not consider the interface  reflection or penetration, and calculates the total number of incident photons.

If the reflectivity of the interface is R, the relationship between the two is:  ## Why is quantum efficiency the best tool for creating high-efficiency solar cells?

Quantum efficiency/spectral response reflects the photoelectric conversion efficiency of solar cells at different wavelengths. The conversion efficiency of solar cells is affected by factors such as the material, manufacturing process, and structure of the cell itself, so that different wavelengths have different conversion efficiencies. Using spectral response/quantum efficiency measurement technology to detect and analyze the changes in the conversion efficiency of the solar cells under different conditions, we can analyze the pros/ cons of the process and find out the key factors related to improving efficiency.

Figure 3 shows the measured spectral responses A and B of the two silicon crystal cells with two different processes. From the spectral response results, it can be seen that the efficiency of cell A is higher, mainly because of the conversion in the 700~1100 nm band. The efficiency is higher than that of the B cell, and the short-circuit current contributed by it is 0.897 mA/cm^2 higher than that of the B cell. But in 300~500 nm wavelength range, the efficiency of A is slightly lower than that of B cell, and the short-circuit current density is 0.675 mA/cm^2 lower than that of B cell. Therefore, the overall short-circuit current density of cell A is still higher than cell B (0.897-0.675)=0.222 mA/cm^2.

The different wavelength ranges represent the structure and manufacturing process of different layers of the solar cells. It will be introduced in more detail in the next section. Therefore, according to the results reflected in different wavelength bands, the process of A cell in the short wavelength range can be improved to further. From the results of the spectral response, it is easy to analyze the pros and cons of solar cells in different manufacturing processes, which is a guideline for improving efficiency. ## Quantum efficiency/spectral response/IPCE application in silicon solar cell process improvement

Quantum efficiency/ spectral response/ IPCE spectrum reflects the characteristics of each layer of the solar cell. Taking silicon solar cells as an example, interface reflection will occur at the incident interface. Different wavelengths reflect different phenomena of device physics. Generally, the loss caused by reflection in the UV and the infrared wavelength band is higher, and the loss in the visible wavelength range is the lowest.

In the 350 nm ~ 500 nm band, the spectral response curve increases as the wavelength increases. Because the penetration depth of long-wavelength photons is deeper, close to the pn junction, the conversion efficiency is improved. Generally, the most efficient part is in the band of the PN junction, because the internal electric field of the pn junction can efficiently disassemble the electron-hole pairs after absorbing photons. Therefore, the highest efficiency is in the 500-800 nm band, which reflects the characteristics of the pn junction layer. The 800~1100 nm wavelength range penetrates to the lowest p-layer. The external quantum efficiency of the single crystal silicon solar cell in Figure 4 can be used to observe the reaction characteristics of each layer. ### Figure 4 Schematic diagram of the quantum efficiency spectrum of a silicon solar cell and the response of each wavelength. The illustration shows the component structure of a silicon solar cell.

The previous figure 3 is an example, and the following figure 5 can be obtained by converting the spectral response into quantum efficiency. The efficiency of A cell is lower than that of B cell at 300 nm ~ 500 nm. To further improve the efficiency of the A cell, it should focus on the process of anti-reflection layer (300 nm ~ 350 nm) and n layer (350 nm ~ 500 nm) as the directions of efficiency improvement. ## Quantum efficiency/spectral response/IPCE application in Copper Indium Gallium Senillide (CIGS) solar cells

Copper Indium Gallium Selenium CIGS (Copper Indium Gallium Selenium) is a quaternary compound semiconductor and is classified as a single-junction solar cell. Figure 6 shows its common device structure. ### Figure 6 Typical CIGS solar cell device structure. 

Copper indium gallium selenide affects the size of its energy gap with the difference of indium gallium content, so that its absorption wavelength range can be from 1.02 ev to 1.68 ev. The quantum efficiency/spectral response/IPCE can test the energy gap of different solar cells. As shown in Figure 7, when the content of gallium in the copper indium gallium selenide increases, and the quantum efficiency/spectral response/IPCE spectrum results show that the energy gap increases. Therefore, it can be used as a detection of the gallium component in the process monitoring. ### Figure 7 Under the same device structure, different gallium components changing the quantum efficiency spectra shows that as the composition of gallium increases, the energy gap of copper indium gallium selenide also increases, from 1 eV to 1.67 eV. 

The current research direction is to reduce costs and improve photoelectric conversion efficiency for this technology. As shown in Figure 8, the characteristics of each part of the device structure corresponding to the quantum efficiency/spectral response/IPCE spectrum of different bands are drawn. For example, the quantum efficiency of the Window layer (ZnO) can be observed at a wavelength of 300 nm ~ 400 nm, the quantum efficiency of the Buffer layer (CdS) can be observed at a wavelength of 400 nm ~ 540 nm, and the Absorber layer can be observed at a wavelength of 540 nm ~ 1200 nm (CIGS) quantum efficiency. ### Figure 8: Quantum efficiency spectrum of the copper indium gallium selenium solar cell and the characteristics of each layer of the reaction cell in different wavelength ranges. 

The quantum efficiency spectrum in Figure 9 changes the film thickness of CdS without changing the process conditions of CIS. The results show that the 400-500 nm band changes with the thickness of CdS (15 nm ~ 80 nm) and the efficiency changes accordingly. In the wavelength band> 500 nm, it shows that there is no significant difference in the efficiency of CIS, which means that the process conditions are stable. From the final result, it can be clearly judged that the optimal film thickness condition of CdS is 15 nm. If the CIS process conditions are the same but the spectrum > 500 nm changes, it means that there are other factors that affect the experimental results of different CdS film thicknes. Then we can analyze the impact of the related processes to achieve the effect of obtaining the most effective information in a single process experiment. Through the spectra of quantum efficiency/spectral response/IPCE, the detailed effects of process changes can be observed, and a database can be established to find problems and improve conditions when the production line yield changes. ### Figure 9 Adjusting the thickness of different CdS layers can find the difference in 400~500nm wavelength range from the quantum efficiency/spectral response/IPCE spectrum. ### Figure 10 The current and voltage curves of solar cells made by using different Buffer layer materials. ### Figure 11 Quantum Efficiency curves of CdS and ZnS(O,OH) Window layers. 

From the above description, we can realize that the quantum efficiency/spectral response/IPCE spectrum provides the information of copper indium gallium selenide solar cells (CIGS) as follows:

1. The photoelectric conversion efficiency of each layer such as Window/ Buffer/ Absorber.
2. The identification of the material energy gap by the concentration of gallium in the Absorber copper indium gallium selenide.
3. The degree of change in efficiency of each layer due to changes in process conditions.

In today’s fiercely competitive solar industry, continuously reducing costs and improving photoelectric conversion efficiency are necessary conditions for solar manufacturers to stand out! The key to the improvement of solar cell conversion efficiency lies in the improvement of manufacturing process and materials. Measuring the quantum efficiency/spectral response/IPCE of solar cells can understand the photoelectric conversion efficiency of solar cells under different light wavelengths. Users can quickly find process problems and improve them based on the results of the spectral response, which is more conducive to efficiency. promote.

Reference

 A. Pudov “IMPACT OF SECONDARY BARRIERS ON CuIn1-xGaxSe2 SOLAR‐CELL OPERATION” Dissertation, Dep. Of Physics, Colorado State University, 2005

 Markus Gloeckler “DEVICE PHYSICS OF CuIn1-xGaxSe2 SOLAR‐CELL” Dissertation, Dep. Of Physics, Colorado State University, 2005

 A.V. Shah et al./Solar Energy Materials & Solar Cells 78 （2003） 469-491

 Oerlikon Solar – Constantine, 24 Sep 08