Quantum Efficiency/Spectral Response/IPCE Measurement Technology 02
How to use EQE external quantum efficiency spectroscopy for current loss analysis of perovskite solar cells?
This article will illustrate how to use the external quantum efficiency EQE spectroscopy technique for current loss analysis of perovskite solar cells. First, it will briefly introduce that what external quantum efficiency EQE is and explain how to experimentally conduct the external quantum efficiency EQE measurement. Through five steps to understand how the characterization of each wavelength band of the external quantum efficiency EQE spectrum is related to the structure of each layer of perovskite solar cells. It is also illustrated how to calculate the loss of short-circuit current of each layer.
What is Solar Cell External Quantum Efficiency EQE?
External Quantum Efficiency (EQE) of solar cells is also known as Spectral Responsivity or IPCE (Incident Photo-Electron Conversion Efficiency). The external quantum efficiency EQE represents the ability of each incident photon to be converted into electrons that is transmitted to the external circuit, called the external quantum efficiency, EQE. The unit is expressed as a percentage.
How to perform external quantum efficiency EQE measurement?
According to IEC-60904-8 (an international standard which defines the EQE/SR testing method for PV field), the external quantum efficiency EQE/ spectral response measurement system must have the following main components.
Figure 1. The illustration of the testing configuration of external quantum efficiency EQE/ spectral response SR in IEC 60904-8 standard. The system includes a continuous wavelength white light source, monochromator, chopper, lock-in amplifier, etc.
A monochromator is first illuminated with a continuous wavelength white light source. This instrument can utilize different optical techniques such as prism, bandpass filter or grating to generate monochromatic light. Of course, the current mainstream is the single light meter using grating technology.
When the monochromator generates monochromatic light of a specific wavelength, it will be modulated into a specific frequency AC light source by an optical chopper chopper. The radiation energy of the incident monochromatic light is calibrated by a calibrated photodetector. After that, the same amount of incident photons is illuminated on the solar cell under test, and the generated photo-current signal is also demodulated and read out by the lock-in amplifier. By divided the incident photon energy and generated photon current, we can get the spectral response value. The SR spectrum of the device under test can be obtained by continuously changing the different wavelengths, and then the EQE spectrum can be obtained by unit conversion.
Why EQE spectroscopy can be used for current loss analysis of solar cells?
To reach this question, we need to know the working principle of a solar cell. The working principle of solar cells can be roughly divided into four processes:
(1) Optical Absorption,
(2) Photocarrier Generation,
(3) Charge Transport,
(4) Charge Collection.
(1) Absorption of photons
When the photon energy is greater than the band gap of the material, the semiconductor can be excited, and the photon energy can be absorbed by processes such as intrinsic absorption, extrinsic absorption, and free-carrier absorption.
(2) Photocarrier Generation
After the semiconductor material absorbs photons, electron-hole pairs are generated. This process is called the photogenerated carrier process. If the semiconductor absorbed the photons through the previous three processes (intrinsic, extrinsic, or free-carrier absorptions), but does not generate the photo-carriers. It is a kind of “energy loss” due to that the loss energy will not contribute to the electrical power in the end.
(3) Charge transfer (Transport)
If the electron-hole pair is generated in the depletion region of the PN junction, it will be dissociated into electrons and holes by the internal electric field of the PN junction. These charge carriers will be driven by the electric field (Drift) to move to the positive and negative electrodes at both ends.
If electron-hole pairs are in the intrinsic region of P-type semiconductor or N-type semiconductor, they will be transported in the form of diffusion to reach the depletion region. After reaching the depletion region, they be dissociated into electrons and holes and drifted to the metal electrodes by the electric field of the depletion region.
(4) Charge collection (Collection)
When electrons or holes reach the metal-semiconductor junction near the electrode, they are transported to the external electrodes. We call this process as charge collection. When the positive and negative electrodes are connected directly, it will make the electrical load RL become infinite small. One can this condition as “short-circuit” condition. It will generate the maximum output current called short-circuit current.
Figure 2. The “short-circuit” condition of a solar cell.
Above four processes described the known incident photons are illuminated on and absorbed by the solar cell and become photocarriers and how to transmit to the electrodes. The whole process is the external quantum efficiency EQE process, the capability/ percentage of incident photons to convert to electrons.
On the other hand, we can realize that the external quantum efficiency EQE spectrum can reflect the information of these four important processes.
Figure 3. The incident photons of different wavelengths penetrate to different depths in the solar cell. Therefore, the external quantum efficiency EQE spectrum carries the different penetration depths information of photon-electron conversion processes.
For example, solar cell materials have different absorption properties for photons of different energies. Photons with shorter wavelengths have higher energy, such as UV light. When they are incident on the solar cell, they can immediately excite the semiconductor material to generate photogenerated carriers.
Photons with longer wavelengths have lower energy, such as IR and near-infrared light, and longer penetration depth. When they penetrate deeper materials and are absorbed to generate photogenerated carriers. These photogenerated carriers have different behaviors when they are in different positions inside the solar cells.
The photons of intermediate wavelengths are generally absorbed in the depletion region of the PN junction. Because the electric field inside the depletion region has a strong force, the electron-hole pair can be dissociated into free electron-hole immediately, and the electric potential of the electric field can be used to conduct the charge to the metal-semiconductor junction. Therefore, photons in this wavelength range usually have a higher conversion efficiency.
The quantum efficiency EQE from UV, VIS, and IR bands reflects the quality of different structural regions such as the surface area, PN junction, and bottom layer. The higher the quantum efficiency EQE value reflects the better the processing conditions in this region of the device. Figure 4 shows the Si solar cell as an example that external quantum efficiency EQE spectrum reveals different layers information of a solar cell in different wavelength range.
Figure 4. The incident photons with different wavelengths penetrate to different depths in the solar cell. Taking a crystalline silicon solar cell as an example, different structural layers reflect information in various bands of the external quantum efficiency EQE spectrum.
Therefore, the external quantum efficiency EQE spectroscopy is often used to analyze the current loss of solar cells. In the article Quantum Efficiency/Spectral Response/IPCE Measurement Technology 01: A Powerful Tool for Creating High-Efficiency Solar Cells, a variety of different solar cells are introduced including crystallized Si, CIGS, a-Si tandem solar cells.
This article will specifically focus on perovskite solar cells, how to use the external quantum efficiency EQE spectroscopy technique to carry out the current loss analysis.
How to analyze the current loss of perovskite solar cells using external quantum efficiency EQE spectroscopy?
As discussed above that the different wavelength bands of the external quantum efficiency EQE spectrum reveal the quality of the process of each structural layer of the solar cell device. Therefore, we need to first understand the device structure of perovskite solar cells.
A. Perovskite solar cell device structure
The most common perovskite solar cell (PSC) consists of organic-inorganic lead halide perovskites that act as light absorbers. Since the first report of long-term durable solid-state perovskite solar cells with an efficiency of 9.7%, organic-inorganic halide perovskites have received extensive attention due to their excellent optoelectronic properties. As a result, a power conversion efficiency (PCE) over 25 % is certified.
Perovskite solar cells can be divided into normal- and inverted- device structures. The two structures are similar in that there is a charge transport layers (HTL or ETL) between the electrodes and the perovskite light-absorbing layer.
When a conductive substrate, usually fluorine-doped tin oxide (FTO) or indium tin oxide (ITO), is deposited on the electron transport layer (ETL), the conductive substrate is the negative electrode. This structure is considered normal. In the case where the conductive substrate has a hole transport layer (HTL), an inverted structure is formed. In this case, the polarity of the conductive substrate is positive (positive). Figure 5 shows the Normal and Inverted structure of the perovskite solar cells.
Figure 5. Normal and inverted structures of perovskite solar cells. ETL and HTL represent the electron transport layer and hole transport layer, respectively. FTO and ITO represent F-doped tin oxide and indium tin oxide, respectively. (From Chemistry Europe)
The photoelectric conversion process of perovskite solar cells can be characterized on the external quantum efficiency EQE spectrum. In the case of perovskite solar cells with normal device structure, photons penetrate the glass substrate and the electron transport layer (ETL) to reach the perovskite light absorption layer. After the photons are absorbed, excitons or free carriers are generated on the femtosecond to pico sec time scale. Free carriers are transported in the perovskite light-absorbing layer by diffusion or drift. This process usually takes a few nanoseconds. After the photocarriers are extracted by the charge transport layer (ETL and HTL), it usually takes a few microseconds before they can be collected by the electrodes. During these processes, a portion of free carriers is lost through bulk and interfacial recombination. Finally, the carriers are transported through external circuits and loads to generate electricity.
The above process includes four main processes: light absorption, photogenerated carriers, carrier transport, and charge collection. With different perovskite light absorption layer conditions, as well as the material properties of the charge transport layer, different conversion efficiencies (PCEs) will be generated. These internal carrier behaviors are all manifested in the external quantum efficiency (EQE) spectrum.
B. Characterization of Each Layer on EQE Spectral Curves
In this paragraph, the behavior of the normal structure perovskite solar cell in the external quantum efficiency (EQE) spectrum will be used to illustrate the relevant characteristics of each device layers on the external quantum efficiency (EQE) spectrum curve.
In fact, the external quantum efficiency EQE spectroscopy is the only method that can quantitatively assess unfavorable light absorption behavior (i.e. parasitic absorption) in solar cells. Since the external quantum efficiency EQE spectrum contains depth information due to the light penetration depth associated with each wavelength (λ). Therefore, the carrier recombination near the front and rear interfaces can be further determined from the quantum efficiency EQE spectra in the short-wavelength λ region and the long-wavelength λ region, respectively.
As we know, the external quantum efficiency EQE spectrum represents the percentage of photons that contributes to the generation of current in the solar cell. To explain how external quantum efficiency EQE is changed in analyzing parasitic light absorption and carrier recombination in solar cells, we will break down to 5 steps to describe the relationships between each wavelength bands and each device layers. [More details: Analysis of Optical and Recombination Losses in Solar Cells]
- First, we assume the EQE spectrum of a perovskite is a perfect absorber with zero reflectivity (R = 0), as shown in Figure (a). In this spectrum (a), all sunlight is completely absorbed by a semiconductor absorber with zero optical reflection (i.e. R = 0 and EQE = 100%). And when the incident wavelength is higher than the band gap Eg, EQE will become zero.
- Step two, we add the effect of the metal layer, as shown in Figure (b). In metal/semiconductor structures, if the interface reflects light (R>0), the EQE response from UV to IR will be reduced. The reflection components of this structure can be divided into front surface (Rfront) and rear surface (Rrear) parts. In general, the contribution of reflectance R in the UV/Vis region is constant, consistent with Rfront. While the reflectance at the incident wavelength, since the light absorption of the absorbing layer begins to weaken, the reflectance R begins to increase significantly when the incident wavelength is close to the bandgap wavelength λEg(SC). When the incident wavelength is greater than the bandgap wavelength λEg(SC), the reflectance reaches Rrear contribution.
- Step 3, adding a transparent conductive electrode TCO. The effect of TCO on the external quantum efficiency EQE is shown in Figure (c). Perovskite solar cells use transparent conductive electrodes TCO, such as In2O3:Sn (ITO) and ZnO:Al, etc. The parasitic light absorption in the TCO will reduce the external quantum efficiency EQE and become optical parasitic loss. There are usually two kinds of parasitic light absorption in TCO: interband transition and free carrier absorption.
The interband transition of TCO shows a significant decrease in the external quantum efficiency EQE spectrum in the UV band. This is because the incident light wavelength λ is smaller than the band gap of TCO, that is, λ ≤ λEg(TCO). In addition, when the wavelength λ of the incident light is larger than λEg(TCO), the absorption of TCO comes from free carrier absorption.
Due to the light absorption of the TCO layer, the internal quantum efficiency IQE of the perovskite solar cell will also decrease. The formula of IQE is as below:
IQE=EQE/(1-R).
One can refer to the definition of IQE in Quantum Efficiency/Spectral Response/IPCE Measurement Technology 01: A Powerful Tool for Creating High-Efficiency Solar Cells.
The internal quantum efficiency IQE shows the efficiency with which absorbed photons (non-incident photons) are converted to photocurrent. Therefore, the IQE spectrum is obtained by normalizing the EQE spectrum using the absorption component (i.e. 1 – R). According to many studies, the upper limit of the IQE of solar cells with TCO layers is usually 80%~95%. Therefore, the EQE current loss caused by TCO is an important loss in perovskite solar cells.
- Step 4, adding a doping layer between the transparent conductive electrode TCO/light-absorption-layer. The external quantum efficiency EQE is shown in Figure (d). In the solar cell structure, a doped layer is usually inserted at the TCO/semiconductor interface. In normal perovskite solar cells, the doped layer represents the electron transport layer (ETL). The electron transport layer (ETL) of the doped layer exhibits strong light absorption and reduces the response of the external quantum efficiency EQE in the short wavelength band. The loss of the EQE spectrum caused by the Doped layer as shown below.
- Step 5, considering metal/semiconductor recombination loss. The process of extracting photogenerated carriers from the perovskite light-absorbing layer is drifted or disused to the metal electrode, and then transmitted to the external circuit. The efficiency of this process between metal and absorber is an essential process. The recombination losses at the metal/semiconductor interface have proven to be significant in many solar cells. When carrier recombination occurs at the semiconductor/metal back interface, the external quantum efficiency EQE spectrum at long wavelengths will apparently reduced. Therefore, the carrier recombination in the interface region can be quantitatively characterized by detailed EQE analysis.
C. How to calculate the loss of short-circuit current?
Before explaining how to calculate the loss of short-circuit current, first, it is necessary to understand the integrated short-circuit current density Jsc(EQE) from the EQE spectrum under the AM1.5G standard spectrum. Jsc(EQE) represents the solar cell’s EQE quantum efficiency spectrum (generally 300 nm ~ 1100 nm) integrated with the AM1.5G standard spectrum (IEC 60904-3).
EQE spectrum can be converted into spectral response SR(𝜆) whose unit is Amp/Watt; while AM1.5G spectrum is in Watt/m2. In this way, the integrated unit Amp/m2 is the current density unit. The EQE quantum efficiency spectrum is measured under short-circuit conditions, therefore, it is called the integrated short-circuit current density Jsc (EQE) of the EQE spectrum under the AM1.5G spectrum.
For the various losses of the external quantum efficiency EQE spectrum, the loss of the short-circuit current density can be obtained after the calculation of Jsc(EQE).
We take the current loss study of low-bandgap perovskite solar cells published in Advanced Energy Materials by Oxford University in 2021 as an example.
The device consists of a spin-coated poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) hole transport layer on a glass substrate coated with an indium-doped tin oxide (ITO) conductive material ( HTL). Thereafter, a perovskite layer with a thickness of 470 nm was deposited by spin-coating, and the evaporated C60 (30 nm) and Bathocuproine (BCP) (8 nm) formed the electron transport layer (ETL). Copper (Cu) electrodes are used on the top.
The main current loss analysis of this perovskite solar cell is obtained through the measurement of reflectance spectrum R and external quantum efficiency EQE spectrum, and using Jsc (EQE) calculation. The result is shown in the figure below.
Figure 6. The device absorption (1-R) and external quantum efficiency spectra EQE of FA0.83Cs0.17Pb0.5Sn0.5I3 perovskite solar cells. The graphical chart of the different current losses occurring in the system is also shown. From the chart, in addition to the parasitic absorption and optical losses, the collection losses is the main bottleneck of this kind of device. Even the sample thickness is increased from 470nm to 800 nm, the collection losses (including the metal/perovskite interface and the HTL/perovskite interface) is still significant.
From the chart of Figure 6, the optical losses caused by the total reflectivity R of the device is significant. This optical loss induced the Jsc loss is nearly 7 mA/cm2. The Jsc loss due to the parasitic absorption of the TCO is about 1 mA/cm2. In particular, this device suffers from significant collection losses around 2 mA/cm2. The collection losses include the composite loss of the metal/perovskite interface and the HTL/perovskite interface.
The authors also studied the Jsc loss of the perovskite light-absorbing layer (800 nm) with different film thicknesses. The 800 nm thickness device still has a significant Jsc losses due to insufficient carrier diffusion length by the traps.
Summary
This article shows how to analyze the current losses of solar cells by using external quantum efficiency EQE spectroscopy. We illustrate the characterization of different wavelength bands on the external quantum efficiency EQE spectral curve of each layer of perovskite solar cells. At the same time, taking the low-bandgap FA0.83Cs0.17Pb0.5Sn0.5I3 perovskite solar cell as an example, it is explained that through the external quantum efficiency EQE spectral analysis, various current loss analysis results are obtained, which are used as a guide for efficiency improvement.
