Perovskite solar cells (PSCs) have versatile applications, making them a promising device for daily life. In this study, researchers optimize ETL-based bifacial perovskite solar cells for flexible devices via simulation. The process is done by selecting the suitable front transparent electrode (FTE), hole transport layer (HTL), and rear transparent electrode (RTE).
ETL-Based Bifacial Perovskite Solar Cells for Flexible Devices
It was observed that the power conversion efficiency (PCE) of the perovskite cell device was improved significantly. This became possible with a well-like structure with a small conduction band offset (CBO) at the interface of FTE/perovskite. However, performance reduction was noticed with an upward shift in the valence band of HTL.
Highlights
- To optimize the ETL-free bifacial perovskite for flexible devices.
- Minimum CBO at the perovskite interface can enhance device performance.
- RTE’s bandgap and electron affinity highly affect the performance of the device.
- 1.4 eV of perovskite is the optimized one.
- For both illumination conditions, the device shows PCE >27%.
Power conversion efficiency (PCE) of perovskite solar cells has increased from 3.8% to 26.1% in a decade. Thus, organic-inorganic metal halide perovskite solar cells have gained much attention lately.
However, the development of flexible PSCs is delayed due to the high sintering temperature of the electron transport layer (ETL). In inverted PSCs, researchers mainly used [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as ETL to make them highly efficient. Since PCBM are expensive, incorporating them in the device increases the overall expenses of the device.
Researchers then tried ETL-free PSCs which are the most promising and acceptable devices. This approach has simple configuration and eliminates complex preparation, thus reducing time and energy required.
Quick fact: Liu et al. developed the first PCE having 13.5%.
The present PCE cells have 20-22% efficiency but are still lagging behind because of the unbalanced charge transfer rate.
Reason: Lack of permanent built-in field when ETL is absent.
Considering Different Approaches
Researchers considered using concentrators, PV materials with 2 or more distinct bandgaps in tandem arrangement, and a bifacial approach. All these aim to improve the device’s performance and encourage widespread adoption of the technology. Since the bifacial design is simple and inexpensive, it increases the power conversion efficiency at a slightly higher cost by adding a rear transparent electrode.
Light can enter the system from both ends by installing the transparent electrodes. With this, bifacial solar panels can potentially achieve more than 30% higher PCE in comparison to mono-facial panels. However, there are different factors determining the same, such as tilt angle, ground surface reflectivity, and height above the ground, etc. Moreover, if bifacial and flexible solar cell technology’s benefits are combined, they can result in efficient and versatile solar energy harvesting devices.
Applications of ETL-free flexible bifacial PSC:
- Folding shade on the shops
- Folding window covers
- On the sails
- Or an umbrella on the beach
It is possible to process flexible PSCs by role to role method and can be encapsulated with low-cost flexible layers. Although flexible bifacial PSCs are a new technology, still under research and development, they have made notable advances.
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With the help of simulation, it was easy for researchers to aim for the desired parameters or properties of the rear transparent electrode (RTE). This enables them to achieve optimum performance of the device. In the present simulation work, researchers have copied ETL-free bifacial as per the following diagram.
Observing flexible solar cells with different electrodes, interfacial defect layers, and hole transport layers, researchers discovered the band alignment and potential barriers to improve the overall performance. Moreover, they achieved >27% efficiency under different conditions by optimizing the perovskite absorber’s bandgap to 1.4 eV.
Structure of the Device and Simulation Parameters ETL-based Perovskite
- Researchers used one-dimensional Solar Cell Capacitance Simulator (SCAPS-1D) package for simulating the proposed device.
- Also, for designing a bifacial device from a validated one, Au was replaced with a transparent electrode Cu/Cu2O composite layer.
- Passivated-FTO (PFTO) was used to work as the FTE.
Results and Discussion
Effects of the Front Transparent Electrode (FTE)
In ETL-free PSCs, the FTE should be designed with high transparency and improved band alignment for efficient charge transportation. Researchers have exposed various FTEs like Zirconium-doped In2O3 (Zr:In2O3), ITO, Aluminum-doped ZnO (Al:ZnO), and passivated/modified-FTO (PFTO). At lower temperatures, it was easy to deposit these electrodes on a flexible substrate.
From the following band diagram, it is evident that CBO at the FTE interface which is close to 0 have drawn higher PCE. The direction of an electric field at ITO is opposite to the HTL interface, which is not suitable for efficient charge transportation.
This is considered as the potential barrier for electrons flowing towards FTEs. Device with a smaller CBO value depicts less recombination at the FTE interface as per the recombination profile. There is an increase in the electron affinity of FTE with a negative change in the CBO at the FTE interface. This was due to the difference in the electron affinity between adjacent layers.
As the thickness of the FTE layers increases, the PCE of the device decreases when illuminated from the FTO side. However, no significant changes were observed for rear illumination.
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Effect of Hole Transport Layer (HTL)
For this study, different HTLs like DM, Cul, Cu2O, and CuSCN were used on the performance reference device. The following energy band diagram shows modified band alignment at perovskite or HTL and HTL/RTE interfaces. When illuminated from the rear, the recombination in Cul and CuSCN devices shows the same SRH recombination profiles. There are higher possibilities of direct recombination with higher valence levels of the adjacent layer.
Effect of Interfacial Defect Layers
At the time of thermal annealing, defects of the interface are highlighted. These defects are promoted if the interface lacks oxygen vacancy, lattice mismatch and stoichiometry composition. The study talks about 3 types of defect interfaces mentioned below:
- HTL/back electrode: Caused due to reaction of the back electrode with HTL in the presence of oxygen.
- TCO/perovskite: A defect in this interface leads to oxygen vacancy.
- Perovskite/HTL: Any defect in this leads to lattice mismatch.
- For defect density less than 10^16 cm−3, the PCE of the device stays the same. It demonstrates a lower recombination rate in the interface layer.
- With a concentration higher than 10^16 cm−3 there is an increase in the recombination rate, which reduces the device’s efficiency.
- Similarly, with an increase in the thickness of the interface defect layer, there is a linear decrease in the PCE of the device. It leads to an increase in the recombination rate in the IDL region.
This phenomenon supports the requirements to reduce the defect in the PFTO/perovskite interface via passivation or any other suitable processing method. Mostly, surface passivation is preferred to modify the surface morphology.
Effect of the Rear Transparent Electrode (RTE)
This electrode holds an important place in determining the overall performance of the bifacial PSCs. Two principal factors affecting the performance of bifacial solar cells are electron affinity and bandgap. Bifacial PSCs have lower PCE compared to their single-faced counterpart, which impacts the RTE in important ways. A change from negative to positive in the VBO at the HTL/RTE interface is observed with an increase in the bandgap value of RTE.
For both illumination conditions, the device shows a maximum PCE at VBO of +0.29 eV (bandgap ~2.4 eV). When electron affinity is 3.3 eV for both types of illumination with VBO of +0.13 eV at HTL/RTE, the performance of the device improves.
With an increase in electron affinity of RTE, the VBO between HTL/RTE shifts towards positive. The study shows that the NAN-based device shows higher PCE for rear illumination. It indicates reduced electric field intensity in the negative direction in NAN-based devices at the HTL/RTE interface. Moreover, the PCE device increases with an increase in work function and they become saturated for large work functions.
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Optimization of the Perovskite Layer
As discussed above, we have simulated different combinations of devices using different FTE, HTL, and RTE. The charge carrier generation decreases with an increase in the perovskite absorber layer. In contrast, the VOC increases with the increase of the built-in potential of the absorber layer. The PCE device increases to an optimized bandgap of 1.4 eV and others are as follows:
- Front illumination PCE 24.65%
- Rear illumination PCE 25.48%
The absorber layer’s defect density of perovskite was reduced from 8.0 × 10^14 cm−3 to 1.0 × 10^14 cm−3. It leads to an increase in device PCE to 26.27% and 26.45% for front and rear illumination.
Moreover, after optimizing the thickness of the absorber layer to 800 nm and then decreasing the defect density to 1.0 × 1014 cm−3. This increases the device’s PCE to 26.88% (front illumination) and 27.35% (rear illumination).
Conclusions
So, with this researchers conclude that using a simulation package to optimize ETL-free bifacial PSCs. By studying the impact of different materials on the performance of the device, it was observed that certain materials improved the performance due to their specific properties. Moreover, the bandgap, defect density, and thickness are important determinants of the perovskite absorber layer. Thus, the power conversion efficiency of more than 27% was achieved with an optimized configuration for both rear and front illumination.
