This article introduces External Quantum Efficiency (EQE) and explains the experimental procedure for EQE spectral measurements. Through five steps, we’ll explore the characterization of EQE spectra across different wavelengths and their correlation with the layered structure of perovskite solar cells, demonstrating how to calculate short-circuit current loss for each layer.

What is External Quantum Efficiency (EQE) in Solar Cells?

The external quantum efficiency (EQE) of a solar cell is also known as spectral responsivity or IPCE (incident photon-to-electron conversion efficiency).

EQE represents the ability of a solar cell to convert each incident photon into an electron that is transported to the external circuit. It is expressed as a percentage.

How to Perform External Quantum Efficiency (EQE) Measurements?

According to the IEC-60904-8 standard, a spectral response measurement system must include the following key components:

Figure 1. IEC 60904-8 international standard, illustrating the test setup for EQE. The system includes a continuous wavelength white light source, a monochromator, a chopper, and a lock-in amplifier.

Why Can EQE Spectra Be Used to Analyze Current Loss in Solar Cells?

The operation of a solar cell can be broadly divided into four processes:

• Photon Absorption
• Photocarrier Generation
• Charge Transport
• Charge Collection

(1) Photon Absorption

When the photon energy is greater than the material’s band gap, the semiconductor can be excited. 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 photocarrier generation. If the semiconductor absorbs photons through the previous three processes (intrinsic, extrinsic, or free-carrier absorption) but does not generate photocarriers, it represents a type of “energy loss.” This is because the lost energy will not contribute to the electrical power output.

(3) Charge 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 generated in the intrinsic region of a P-type or N-type semiconductor, they will be transported by diffusion until they reach the depletion region. Upon reaching the depletion region, they will be dissociated into electrons and holes and drift to the metal electrodes due to the electric field within the depletion region.

(4) Charge Collection

When electrons or holes reach the metal-semiconductor junction near the electrode, they are transported to the external electrodes. This process is called charge collection. When the positive and negative electrodes are directly connected, the electrical load R_L becomes infinitely small. This condition is referred to as the “short-circuit” condition. It generates the maximum output current, known as the short-circuit current (I_sc).

Figure 2. “Short-circuit condition” of a solar cell.

Directly connecting the positive and negative terminals of the cell makes the external load R_L=0, resulting in a short-circuit condition. In this state, the voltage across the cell V_a = I∙R_L is zero, and only current flows through the solar cell, which is the short-circuit current I_sc.

The external quantum efficiency (EQE) of a solar cell represents the number of electrons transported to the external circuit under short-circuit conditions, after monochromatic light with a known number of photons is incident on the solar cell and undergoes the processes of photon absorption, photocarrier generation, charge transport, and charge collection. The four processes described above explain how incident photons are absorbed by the solar cell, become photocarriers, and are transported to the electrodes. This entire process constitutes the EQE process, which is the ability/percentage of incident photons converted into electrons. Therefore, the EQE spectrum reflects all the information of the four processes mentioned above.

Figure 3. Incident photons of different wavelengths penetrate to different depths in the solar cell. Therefore, the EQE spectrum contains information about the photon-to-electron conversion efficiency at various penetration depths.

For example, solar cell materials absorb photons of different energies differently. Photons with shorter wavelengths, such as UV light, have higher energy. When these photons strike the solar cell, they can immediately excite the semiconductor material, generating photocarriers. Photons with longer wavelengths, such as IR and near-infrared light, have lower energy and a longer penetration depth. These photons penetrate deeper into the materials before being absorbed and generating photocarriers. These carriers behave differently depending on their locations within the solar cells.

Photons with intermediate wavelengths are typically absorbed in the depletion region of the PN junction. Because the internal electric field in the depletion region has a strong force, it can immediately dissociate electron-hole pairs into free electrons and holes, and use the electromotive force of the electric field to conduct the charges to the metal-semiconductor junction, resulting in higher conversion efficiency. Therefore, the EQE from the UV, VIS, to IR bands reflects the quality of different structural regions such as the surface region, PN junction, and bottom layer. A higher EQE value indicates better process conditions in that region of the device.

Figure 4. Incident photons of different wavelengths penetrate to different depths in the solar cell. Taking a crystalline silicon solar cell as an example, different structural layers are reflected in the information of each band in the EQE spectrum.

How to Analyze Current Loss in Perovskite Solar Cells Using EQE Spectroscopy?

As discussed above, the different wavelength bands of the EQE spectrum reveal the quality of each structural layer’s process in the solar cell device. Therefore, we first need to 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 perovskite as the light harvester.

Since the first report of long-term durable, 9.7% efficient solid-state perovskite solar cells, organic-inorganic halide perovskites have gained widespread attention due to their excellent optoelectronic properties. As a result, power conversion efficiencies (PCEs) exceeding 25% have been 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 layer (HTL or ETL) between the electrodes and the perovskite light-absorbing layer.

When the conductive substrate, typically fluorine-doped tin oxide (FTO) or indium tin oxide (ITO), is deposited on the electron transport layer (ETL), the conductive substrate acts as the negative electrode; this structure is considered the normal structure. In cases 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.

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 fluorine-doped tin oxide and indium tin oxide, respectively. (From Chemistry Europe)

The photovoltaic conversion process of perovskite solar cells can be characterized by the EQE spectrum. Taking a normal perovskite solar cell device structure as an example, photons penetrate the glass substrate and the electron transport layer (ETL) to reach the perovskite light-absorbing layer. After photons are absorbed, excitons or free carriers are generated within a timescale of femtoseconds to picoseconds. Free carriers are transported in the perovskite light-absorbing layer through diffusion or drift. This process typically takes a few nanoseconds. After the photogenerated carriers are extracted by the charge transport layers (ETL and HTL), it usually takes a few microseconds for them to be collected by the electrodes. During these processes, some free carriers are lost through bulk recombination and interface recombination. Finally, carriers are transported through the external circuit and load to generate electricity.

B. Characterization of Each Structural Layer on the EQE Spectrum Curve

This section will use the behavior of a standard perovskite solar cell in the EQE spectrum to illustrate the relevant characteristics of each device layer on the EQE curve. EQE spectroscopy is the only method that can quantitatively assess unfavorable light absorption behavior (i.e., parasitic absorption) in solar cells. This is because the EQE spectrum contains depth information due to the light penetration depth associated with each wavelength (λ). Therefore, carrier recombination near the front and rear interfaces can be further determined from the EQE spectra in the short-wavelength λ region and the long-wavelength λ region, respectively.

The EQE spectrum represents the percentage of photons that actually contribute to current generation in the solar cell. To explain how the EQE spectrum changes due to parasitic light absorption and carrier recombination in solar cells, we will break down the relationship between each wavelength band and each structural layer in 5 steps.

• • Perfect Absorber Analysis:

First, we assume the EQE spectrum of a perfect perovskite absorber with zero reflectance (R = 0), as shown in Figure (a). In this spectrum, all sunlight is completely absorbed by a semiconductor absorber with zero reflectivity (i.e., R = 0), resulting in EQE = 100%. When the incident wavelength is higher than the band gap E_g, the EQE becomes zero.

• • Metal Layer Influence:

We now incorporate the influence of the metal layer, as shown in Figure (b). In a metal/semiconductor structure, assuming interface reflected light (R>0), the EQE response from UV to infrared will be reduced. The reflectance component of this structure can be divided into front surface (R_front) and rear surface (R_rear) contributions. Typically, the reflectance R is constant in the UV/visible region, consistent with R_front. As the incident wavelength approaches the band gap wavelength λ_Eg(SC), the light absorption of the absorber layer starts to weaken, causing the reflectance R to increase significantly. When the incident wavelength exceeds the band gap wavelength λ_Eg(SC), the reflectance reaches the R_rear contribution.

• • TCO Influence:

Figure (c) illustrates the effect of adding a transparent conductive oxide (TCO) layer on the EQE. Perovskite solar cells utilize TCOs such as In₂O₃:Sn (ITO) and ZnO:Al. However, parasitic light absorption in the TCO reduces the EQE, leading to parasitic loss. There are typically two types of parasitic light absorption in TCOs: interband transition and free carrier absorption.

C. How to Calculate Short-Circuit Current Loss?

First, it is necessary to understand the integrated short-circuit current density J_sc(EQE) determined by the EQE spectrum under the AM1.5G standard spectrum. J_sc(EQE) represents the integration of the AM1.5G standard spectrum (IEC 60904-3) with the solar cell’s EQE spectrum (generally 300 nm to 1100 nm).

The EQE spectrum can be converted to spectral responsivity SR(λ), which is expressed in Amp/Watt. The AM1.5G spectrum has units of Watt/m². Thus, the integral has units of Amp/m², which is the unit of current density. Since the EQE spectrum is measured under short-circuit conditions, this is referred to as the integrated short-circuit current density J_sc(EQE) under the AM1.5G spectrum. Various losses in the EQE spectrum can be quantified as losses in short-circuit current density through the calculation of J_sc(EQE).

We will use 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 stack consisted of a spin-coated poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) hole transport layer (HTL) on a glass substrate coated with indium-doped tin oxide (ITO). Subsequently, a 470 nm thick perovskite layer was deposited via spin-coating, followed by evaporated C60 (30 nm) and Bathocuproine (BCP) (8 nm) to form the electron transport layer (ETL). A copper (Cu) electrode was used for the top contact.

By measuring the reflectance spectrum R and the EQE spectrum, and utilizing the J_sc(EQE) calculation, the major current losses in this perovskite solar cell were analyzed. The results are shown below.

Figure 6. Device absorption (1-reflectance) and EQE spectra of the FA_0.83Cs_0.17Pb_0.5Sn_0.5I₃ perovskite solar cell, along with a graphical representation of the different current losses occurring in the system. Apart from parasitic absorption and optical losses due to the thin sample not absorbing all light, significant charge collection losses are also observed.

First, the optical loss due to the total reflectance R of the device is significant, with a J_sc loss of nearly 7 mA/cm². The J_sc loss due to parasitic absorption in the TCO is about 1 mA/cm². In particular, the device has significant collection losses of nearly 2 mA/cm². This collection loss includes the combined recombination losses at the metal/perovskite interface and the HTL/perovskite interface.

Conclusion

This article explained how to use EQE spectroscopy to analyze current loss in solar cells. We illustrated the characterization of each structural layer of a perovskite solar cell on the EQE spectrum curve at different wavelength bands. Using a low-bandgap FA_0.83Cs_0.17Pb_0.5Sn_0.5I₃ perovskite solar cell as an example, we demonstrated how EQE spectral analysis provides various current loss analysis results, which can guide efficiency improvement and optimization.