HTL-free Perovskite Solar Cells Research Articles

Light management in hole transport layer-free perovskite solar cell by SPP and LSPR

In recent years, light management based on localized surface plasmon resonance (LSPR) effects in perovskite solar cells (PSCs) has received significant attention. However, the use of surface plasmon polariton (SPP) excitations in PSCs has been less studied. Meanwhile, hole transport layer-free perovskite solar cells (HTL-free PSCs) have garnered interest due to their lower cost. In this study, we improve light absorption in HTL-free PSCs by simultaneously utilizing LSPR and SPP effects. Au nanotriangles are employed on the surface of the back electrode to excite SPPs. The thickness of the perovskite layer is varied from 100 nm to 400 nm. The optimal periodicity and dimensions of the triangular nanoparticles are determined for each perovskite layer thickness. In the optimal structures with perovskite layer thicknesses of 100 nm, 200 nm, 300 nm, and 400 nm, absorption enhancements of 25%, 12.4%, 13%, and 4.3% are achieved, respectively. The interaction of light with SPP and LSP modes leads to improved solar cell performance. Furthermore, the short circuit current density (JSC) in structures with layer thicknesses of 100 nm and 200 nm increased from 16.7 mA cm−2 to 20.71 mA cm−2 and from 19.8 mA cm−2 to 21.86 mA cm−2, respectively. Other photovoltaic characteristics of the solar cell were obtained through optical-electrical numerical analysis. For the improved solar cell with a perovskite thickness of 100 nm, the values of open circuit voltage, efficiency, and fill factor were 0.847 V, 0.81, and 14.24%, respectively, representing increases of 1.1%, 2.4%, and 28.7% compared to the bare device. Additionally, in the solar cell with a thickness of 200 nm, an efficiency of 17.03% was achieved, showing a 12.5% improvement compared to the bare structure. Our research results facilitate the design of high-performance, ultra-thin, semi-transparent solar cells.

Nanotextured highly efficient optical and light trapping strategies using efficient hole transport-free structure for perovskite solar cells

Device modelling of CH3NH3PbI3-based hole transport layer (HTL)-free nanotextured perovskite solar cells (PSC) was performed. In recent years, HTL-free PSCs have grown tremendously and gained widespread attention. Also, using nanostructures in PSCs significantly affected this type of solar cell's power conversion efficiency (PCE). Light trapping (LT) by nanostructures is a promising approach to optimizing the performance of PSCs in this work. We employed coupled optical and electrical modelling to optimize these nanostructures. In the current simulation, all parameters impacting the PSC were accurately examined and considered using the finite element technique (FEM). First, we obtained the photovoltaic results for two planar and free-HTL structures. The simulation results show that not using HTL reduces the PCE of PSC by up to 15 %. In this article, we emphasized nanostructures and their key features that minimize losses related to light by managing and optimizing light to achieve high PCE for the PSC. Using nano-texture at the surface of the active layer with ZnO material and optimizing the height, we were able to bring the lost efficiency in the free-HTL structure to 14.87 %, and also the short-circuit current density and open-circuit voltage for the proposed structure were 18.16 (mA/cm2) and 0.98 (V), respectively.

Improvement of performance of perovskite solar cells through BaTiO<sub>3</sub> doping regulated built-in electric field

The preparation of hybrid perovskite solar cells is expensive and environmentally demanding. Carbon-based HTL-free perovskite solar cells (C-PSCs) have attracted much attention because they replace the expensive precious metal electrode and remove the poor stability of the hole transport material. However, the improvement of efficiency is hampered by poor carrier separation and transport performance within C-PSCs, while the enhancement of the built-in electric field can improve the carrier transport performance, thus enhancing photoelectric performance. The built-in electric field can be regulated by doping. The anomalous photovoltaic effect and the built-in electric field of ferroelectric material play an important role in the field of optoelectronics. In this work, a simple and effective method is developed to improve the performance of perovskite solar cells via the combination of internal doping of ferroelectric polymer and external control of electric field. Ferroelectric material barium titanate (BaTiO<sub>3</sub>) powder is added into perovskite precursor solution as an additive to prepare C-PSCs, which can improve the perovskite film morphology, reduce the film defect density, and enhance the carrier transport performance of C-PSCs. The results show that when the addition of BaTiO<sub>3</sub> is 1.0% (mass fraction), the perovskite film is uniform and dense, and the photoelectric conversion efficiency of the cell is the highest. After the forward voltage polarization treatment, the residual polarized electric field of ferroelectric material BaTiO<sub>3</sub> increases the built-in electric field, which provides sufficient power for realizing carrier transport and extraction, thus inhibiting the occurrence of non-radiative recombination. At the same time, the depletion layer width is increased, and the reverse saturation current is reduced, so the cell performance is significantly improved. The optimal device efficiency is 9.02%. This work provides an efficient strategy for regulating the built-in electric field by doping perovskite absorption layer.

A numerical approach to study the effect of bandgap and electron affinity in HTL-free perovskite solar cells and design of two-terminal silicon/perovskite tandem solar cell

The hole-transport layer (HTL) free perovskite solar cells with carbon electrode (c-PSCs) have garnered significant attention owing to their simple design, cost-effectiveness, and good stability. However, the power conversion efficiency (PCE) of these cells is still very low in comparison to conventional PSCs that utilize HTLs and metal electrodes. Tandem solar cells (TSCs) based on perovskite and crystalline silicon (c-Si) solar cells are considered possible contenders for surpassing the theoretical maximum limit and addressing the efficiency constraints encountered in single junction PSCs. The current research work assesses the design and optimization of a c-Si/c-PSC-based two-terminal (2-T) monolithically coupled TSC utilizing a one-dimensional solar cell capacitance simulator (SCAPS-1D) software. Initially, a detailed examination of standalone c-PSC has been conducted to evaluate bandgap and electron affinity variation. Then, the optimized c-PSC was integrated with the bottom c-Si cell by adopting a widely employed current matching technique. An investigation was undertaken to identify a current matching condition between two subcells that yielded optimal performance for the device. Under optimum conditions, a Voc of 2.138 V, Jsc of 15.60 mA/cm2, FF of 85.79%, and PCE of 28.62% were achieved in a 2-T monolithically connected tandem cell. We believe the results acquired in this study would be of significant importance in advancing highly efficient 2-T tandem cells based on HTL-free c-PSCs.

Performance improvement of HTL-free perovskite solar cells with the graded approach by numerical simulation

In the exponential growth of perovskite solar cells (PSCs), there remains certain concerns regarding the wide-range incident photons absorption. The doubly graded lead-free absorber in the PSC has been recommended for solving these problems. The absorber material in the PSC is based on the lead-free as well as tin-based perovskite i.e., CH3NH3SnI3, that is one of the precarious parameters due to the non-toxic behavior as well as the lower band gap of 1.3 eV that can be useful in broad visible absorption spectrum than the traditional CH3NH3PbI3 layer. Furthermore, the quantum efficiency (QE) as well as the power conversion efficiency (PCE) in the double-graded configuration of the PSC can be improve. The outcomes of the present numerical simulations examine the solar cells' single and double grading strategy. Further, we use a novel approach of HTL-free PSC, that can lower the manufacturing cost and the defectivity. The present work compares single and double-grading Perovskite Absorber Layer (PAL) for obtaining high PCE. The results are exciting as the highest open-circuit voltage (VOC) of 0.965V, higher short-current density (JSC) of 35.26 mA/cm2, and high fill factor (FF) of 86.40% in the doubly-graded PSC shows a much-optimized PCE of nearly 29.35% that can be convenient for fabricating much efficient PSC device.

Tailoring the SnO2 electron transport layer with hydrofluoric acid to assemble efficient and stable HTL-free perovskite solar cells

Tailoring the SnO2 electron transport layer with hydrofluoric acid to assemble efficient and stable HTL-free perovskite solar cells

Highly efficient and stable hole-transport-layer-free inverted perovskite solar cells achieved 22% efficiency through p-type molecular synergistic doping

low-cost, facile manufacturing processes are becoming more and more important for commercialization of perovskite solar cells (PSCs). In this aspect, the development of hole-transport-layer-free (HTL-free) PSC is particularly important. In this paper, we fabricated HTL-free PSC with improved photovoltaic performance via synergistic doping Cu(thiourea)Cl (Cu(Tu)Cl) and thiosemicarbazide into the perovskite precursor solution to obtain high-quality perovskite films with average grains of more than 1 µm. This synergistic doping strategy can simultaneously decrease perovskite surface and internal defects and mitigate interfacial energy barrier, which is superior to the present single passivation method which brings about defect passivation deficiency. As a result, compared to the control device with the efficiency of 13.10%, the HTL-free perovskite device prepared by synergistic doping strategy achieves a remarkable PCE of 22% with almost negligible hysteresis, which is the highest PCE values reported so far for the HTL-free PSCs. In addition, the environmental, thermal and long-term operational stabilities of the optimized devices prepared by synergistic doping strategy are significantly improved compare to the control devices. Especially, the unencapsulated optimized device retains 90% of the original efficiency under continuous illumination (100 mW cm−2) for 1000 h with the maximum power point (MPP) tracking in nitrogen-filled test box. Furthermore, this strategy has been shown to be effective in wide-bandgap perovskite. The wide-bandgap perovskite Cs0.05MA0.15FA0.8Pb(I0.75Br0.25)3 (1.65 eV) based HTL-free device achieves a PCE of 19.7% with an Voc of 1.2 V, which is the highest PCE values reported so far for the HTL-free wide-bandgap PSCs, which opens a new way to fabricate HTL-free perovskite-based tandem solar cells. This work manifests a noteworthy step towards the facile, cost-efficient fabricating of PSCs with both high efficiency and simple manufacturing processes.

Synergistic Study of Reduced Graphene Oxide as Interfacial Buffer Layer in HTL-free Perovskite Solar Cells with Carbon Electrode

The application of machine learning algorithms to the detection of fraudulent credit card transactions is a challenging problem domain due to the high imbalance in the datasets and confidentiality of financial data. This implies that legitimate transactions make up a high majority of the datasets such that a weak model with 99% accuracy and faulty predictions may still be assessed as high-performing. To build optimal models, four techniques were used in this research to sample the datasets including the baseline train test split method, the class weighted hyperparameter approach, and the undersampling and oversampling techniques. Three machine learning algorithms were implemented for the development of the models including the Random Forest, XGBoost and TensorFlow Deep Neural Network (DNN). Our observation is that the DNN is more efficient than the other 2 algorithms in modelling the under-sampled dataset while overall, the three algorithms had a better performance in the oversampling technique than in the undersampling technique. However, the Random Forest performed better than the other algorithms in the baseline approach. After comparing our results with some existing state-of-the-art works, we achieved an improved performance using real-world datasets.

Role of built-in potential over ETL/perovskite interface on the performance of HTL-free perovskite solar cells

Lead-free, Sn-based perovskite solar cells (PvSCs) have piqued the interest of researchers and their commercialization. Hole transport layer (HTL)-free PvSCs are low-cost and easy to fabricate. The built-in potential of a photovoltaic (PV) device plays an essential role in the separation of light-generated charge carriers. In this simulation work, we have investigated the built-in potential at the interface of electron transport layer (ETL) and Formamidinium tin iodide (FASnI3) with the help of Mott-Schottky plot for various types of ETLs. We have measured how the built-in potential was affected by thickness, doping density, and defect density of FASnI3 perovskite absorber layer and found that the built-in potential of ETL/FASnI3 interface had a significant impact on the device performance parameters. The built-in potential greatly affects the Shockley-Read-Hall (SRH) recombination mechanism. In this simulation, the material parameters of each layer have been optimized and obtained an optimized PCE of more than 16% for all devices. It is anticipated that the results of this study have great potential to understand the role of built-in potential in n-p junction PV devices and consequently device optimization of not only for HTL-free FASnI3-based PvSCs but also for other single n-p junction PvSCs. However, the fabrication of HTL-free FASnI3-based PvSCs is still a challenging task due to the oxidation of Sn2+ to Sn4+.

Numerical simulation of novel designed HTL-free perovskite solar cells to realize over 25% efficiency based on CH3NH3PbI3/CH3NH3PbI2Br cascade structure

The hole transport layer-free perovskite solar cells ((HTL-free PSCs) have drawn tremendous interest because of their simple structure, superior stability and low production cost. However, the inferior charge separation at the perovskite absorber/back electrode (carbon in most cases) interface limits the solar cell performance. In the research, a novel energy band structure with CH3NH3PbI3/CH3NH3PbI2Br cascade layers is designed and extensively analyzed using the SCAPS-1D simulator to explore its potential for HTL-free PSCs. In order to obtain optimal device performance, several critical device parameters, including the thickness value, doping concentration and defect density for both the CH3NH3PbI3 and CH3NH3PbI2Br layers, are systematically investigated and selected. After optimization, a decent photoelectric conversion efficiency of 25.32% is obtained for this structure, which provides 21.5% relative improvement compared with the 20.84% efficiency for the single-layered CH3NH3PbI3 PSC. The outcome of the current study indicates that the proposed CH3NH3PbI3/CH3NH3PbI2Br cascade energy band structure will provide an innovative route to the fabrication of high performance HTL-free PSCs.

Investigation of the role of back contact work function for hole transporting layer free perovskite solar cells applications

Despite the unprecedented progress in power conversion efficiency (PCE), perovskite solar cells (PSCs) have a weak link where the organic hole transport materials are prone to degrade under stress (humidity, temperature etc.), leading to the overall device performance downfall. Hole transport layer (HTL) free perovskite solar cells could be one of the options to overcome this challenge. In HTL-free PSCs, new, suitable back contact materials are needed to further optimize the device's performance and stability by investigating the work function of back contact materials. In this study, SCAPS-1D software is used to understand the role of the back contact material work function on device performance. The optimized devices with a back contact work function of 5.4 eV or above showed a power conversion efficiency of over 22%. In addition, the effects of bulk defects, interfacial defects, and series resistance in HTL-free PSCs are also studied. It is found that defects heavily affect the solar cell characteristics, and the VOC falls rapidly with increasing defects density. On the other hand, the series resistance does not affect the VOC and JSC; however, the fill factor and PCE diminish steadily with an increase in series resistance. This study will pave the way for HTL free perovskite solar cells fabrication work to reduce the overall cost of the device.

Surface Passivation for Efficient Bifacial HTL-free Perovskite Solar Cells with SWCNT Top Electrodes

Building-integrated photovoltaics is an emerging field that demands approaches to deliver efficient and flexible photovoltaic cells at a low cost. In this work, high-quality single-walled carbon na...

Electron transport layer engineering with rubidium chloride alkali halide to boost the performance of perovskite absorber layer

The mesoscopic perovskite solar cells (PSCs) based on titanium oxide (TiO2) with a certified 25.2% efficiency are the forefront devices in the PSCs field. Hence, it can conclude the mesoporous titanium oxide (mp-TiO2) is one of the most promising candidates to use as an electron transport layer (ETL) in PSCs. Improving the conductivity of mp-TiO2 can consider as a simple route to motivate the electron extraction ability of this layer and increase the efficiency of PSCs. Herein, rubidium chloride (RbCl) was introduced as an additive source to boost the optoelectronic properties of mp-TiO2 ETL. It was observed through ETL modification with RbCl, the optical transmittance of mp-TiO2 remains constant but increases its electro-conductivity. Results showed that the morphology and crystalline properties of the perovskite layer with a modified ETL substrate is improved. It indicates a perovskite layer with enlarger grains and lower lead iodide (PbI2) surplus. Altogether, ETL modification brings a champion efficiency of 11.10% for hole transport layer (HTL)-free PSCs higher than that of 8.65% for the HTL-free PSCs based on pristine ETL. Besides, Modified PSCs compared to pristine PSCs showed higher stability response as a result of lower grain boundaries in the modified perovskite layer.

Antioxidation and Energy-Level Alignment for Improving Efficiency and Stability of Hole Transport Layer-Free and Methylammonium-Free Tin-Lead Perovskite Solar Cells.

Tin-lead (Sn-Pb) perovskites have shown great potential in applications of single-junction perovskite solar cells (PSCs) and tandem devices due to outstanding photoelectrical properties and low band gaps. Currently, Sn-Pb PSCs typically have a p-i-n structure, but choices of hole transport layer (HTL) materials are very limited and there are different concerns in each of them. Eliminating the HTL is a direct and promising strategy to address the concerns, but is rarely studied. In this work, we demonstrate HTL-free and MA-free based Sn-Pb PSCs and a synergistic integration strategy of simultaneously introducing a reducing agent and in situ surface passivation. With the integration strategy, Sn-Pb perovskite films with enhanced antioxidation, reduced trap density, prolonged carrier lifetime, and improved energy-level alignment are achieved. Consequently, final HTL-free PSCs exhibit a champion power conversion efficiency (PCE) of 17.4%, which is a new record for HTL-free and MA-free Sn-Pb PSCs. Meanwhile, the integration strategy-based HTL-free device maintains excellent stability with efficiency unchanged for the first 200 h, and finally retaining 81% of the efficiency after 480 h aging in the air. This study shows the potential of achieving desirable HTL-free and MA-free Sn-Pb PSCs and offers more opportunities for tandem solar cells and other photovoltaic devices.

Low-temperature sprayed carbon electrode in modular HTL-free perovskite solar cells: a comparative study on the choice of carbon sources

Among the three carbon sources (MWCNT, CNC, and graphene), MWCNT was identified as the best candidate for application as the back electrode in HTL-free C-PSCs. Ultimately, an improved PCE of 11.2% was achieved for modular MWCNT-based PSCs.

Engineered surface properties of MAPI using different antisolvents for hole transport layer-free perovskite solar cell (HTL-free PSC)

Perovskite solar cells (PSCs) have attained considerable success within just a few recent years. This accomplishment is critically based on compositional modifications and morphology engineering of perovskite material or dependent upon prepared mesoporous-titanium dioxide (mp-TiO2). However, no analysis of the antisolvent role used for crystallization of perovskite has been undertaken. Herein, we investigated the role of the antisolvent in the performance of hole transport layer (HTL)-free PSC (HTL-free PSC) based on the following sandwich structure: glass/fluorine-doped tin oxide (FTO)/compact-TiO2 (c-TiO2)/mp-TiO2/Perovskite (MAPI)/gold (Au). We studied perovskite layers with various porosities and layer thicknesses, and revealed that the generated pinholes had a major effect on the HTL-free PSC performance. A possible reason for this is that the pinhole in the MAPI layer does not allow the MAPI crystals to generate charge pathways to the TiO2 layer. The MAPI layer prepared by chlorobenzene demonstrated a compact and pinhole-free highly crystalline MAPI layer with enhanced optical and electrical characteristics. However, the MAPI layers prepared by toluene and diethyl ether suffered from severe recombination issues at the MAPI/TiO2 interconnect. The dark current/voltage (J–V) curve of the HTL-free PSC prepared using chlorobenzene shifted to higher voltage, suggesting a reduction of the backflow of charges at the interface. The J–V characteristics under illumination proved that the HTL-free PSC fabricated by chlorobenzene using as antisolvent in this study, had the best power conversion efficiency (PCE) of 5.65% along with open circuit voltage (Voc), short circuit current (Jsc), and fill factor (FF) values of 0.8335 V, 11.964 mA/cm2, and 0.56, respectively.

Improved Electrical and Structural Stability in HTL-Free Perovskite Solar Cells by Vacuum Curing Treatment

Device engineering with proper material integration into perovskite solar cells (PSCs) would extend their durability provided a special care is spent to retain interface integrity during use. In this paper, we propose a method to preserve the perovskite (PSK) surface from solvent-mediated modification and damage that can occur during the deposition of a top contact and furtherly during operation. Our scheme used a hole transporting layer-free top-contact made of Carbon (mostly graphite) to the side of hole extraction. We demonstrated that the PSK/graphite interface benefits from applying a vacuum-curing step after contact deposition that allowed mitigating the loss in efficiency of the solar devices, as well as a full recovery of the electrical performances after device storage in dry nitrogen and dark conditions. The device durability compared to reference devices was tested over 90 days. Conductive atomic force microscopy (CAFM) disclosed an improved surface capability to hole exchange under the graphite contact after vacuum curing treatment.

Life cycle assessment of hole transport free planar–mesoscopic perovskite solar cells

Organo-metal lead halide perovskite solar cells (PSCs) attract attention due to their low cost and high power conversion efficiency. Some weak points of this technology are short lifetime, instability, and expensive metal electrode deposition. Eliminating the unstable hole transport layer (HTL) and using carbon-based materials as the counter electrode would address both. In this work, we present a cradle-to-gate life cycle assessment of two HTL-free PSC designs, which use solution phase deposition to achieve mesoscopic and planar structures. Environmental impacts of producing 1 m2 PSCs are converted to impacts per kWh electricity generation assuming 5 years of operational lifetime. We find that major impacts come from fluorine doped tin oxide (FTO) glass patterning due to the electricity consumption of FTO patterning and glass cleaning processes. Even though the electricity consumption when manufacturing both PSCs is similar, their different efficiencies make the environmental impacts per kWh of electricity higher for the mesoscopic PSC than for the planar PSC. Energy payback time values of planar PSCs and mesoscopic PSCs are 0.58 and 0.74 years, respectively, and these values are shorter than those of commercial first and second generation solar cells. However, the global warming potential (GWP) values of planar and mesoscopic PSCs are 75 and 94 g CO2-eq/kWh, respectively, and these values are still higher than those of commercial solar cells. To reach the GWP of commercial cells, the operational lifetime would have to be 8 and 10 years for planar and mesoscopic PSCs, respectively.

Influence of Charge Transport Layers on Capacitance Measured in Halide Perovskite Solar Cells

Summary Capacitance-based techniques have been used to measure the electrical properties of halide perovskite solar cells (PSCs) such as defect activation energy and density, carrier concentration, and dielectric constant, which provide key information for evaluating the device performance. Here, we show that capacitance-based techniques cannot be used to reliably analyze the properties of defects in the perovskite layer or at its interface, because the high-frequency capacitance signature is due to the response of charge carriers in the hole-transport layer (HTL). For HTL-free PSCs, high-frequency capacitance can be considered as the geometric capacitance for analyzing the dielectric constant of the perovskite layer because there is no trapping and de-trapping of charge carriers in the perovskite layer. We further find that the low-frequency capacitance signature can be used to calculate the activation energy of the ionic conductivity of the perovskite layer, but the overlapping effects with charge transport materials must be avoided.

Performance of low-cost mixed cationic carbon-based solar cells prepared through compositional engineering under ambient conditions

Perovskite solar cells (PSC) have reached a record efficiency of up to 25.2 % in a matter of few years. This has spawned a lot of interest in investigating the commercial scalability of the perovskites by reducing the cost. Carbon can act as a cheaper alternative to gold and aluminium to act as counter electrode. In the present study, we have tuned the perovskite layer by the addition of FA (Formamidinium) and Cs (Cesium) through compositional engineering. The optimized configuration of perovskite was then utilized in carbon monolithic electrode stack to form a carbon-based perovskite solar cell. Emphasis is more on the performance of these carbon-based PSC. Three batches of carbon-based PSC were prepared with two-hole transport layers (P3HT, Spiro-OMeTAD) and one with no hole transport layer. All the carbon-based PSC were then subjected to EIS studies. The carbon-based HTL-Free PSC had a power conversion efficiency (PCE) of up to 9.5 % while cells with HTLs as P3HT and Spiro-OMeTAD had 8.5 % and 13.4 %, respectively. Stability wise, HTL-free devices performed exceedingly well (up to 76 days). Under endurance testing against light and heat, P3HT- based devices managed to retain 75 % of PCE after 25 h constant illumination and performed better than HTL-free and Spiro-OMeTAD based cell under thermal stress of up to 240 ℃.