Interface Engineering to Eliminate Hysteresis of Carbon-Based Planar Heterojunction Perovskite Solar Cells via CuSCN Incorporation

Efficient and Stable CuSCN-based Perovskite Solar Cells Achieved by Interfacial Engineering with Amidinothiourea

Cuprous thiocyanate (CuSCN) emerges as a prime candidate among inorganic hole-transport materials, particularly suitable for the fabrication of perovskite solar cells. Nonetheless, there is an Ohmic contact degradation between the perovskite and CuSCN layers. This is induced by polar solvents and undesired purities, which reduce device efficiency and operational stability. In this work, we introduce amidinothiourea (ASU) as an intermediate layer between perovskites and CuSCN to overcome the above obstacles. The characterization results confirm that ASU-modified perovskites have eliminated trap-induced defects by strong chemical bonding between -NH- and C═S from ASU and under-coordinated ions in perovskites. The interfacial engineering based on the ASU also reduces the potential barrier between the perovskite and CuSCN layers. The ASU-treated perovskite solar cells (PSC) with a gold electrode obtains an improved power conversion efficiency (PCE) from 16.36 to 18.03%. Furthermore, after being stored for 1800 h in ambient air (relative humidity (RH) = 45%), the related device without encapsulation maintains over 90% of its initial efficiency. The further combination of ASU and carbon-tape electrodes demonstrates its potential to fabricate low-cost but stable carbon-based PSCs. This work finds a universal approach for the fabrication of efficient and stable PSCs with different device structures.

Uncovering the Role of Electronic Doping in Lead-free Perovskite (CH3 NH3 )2 CuCl4-x Brx and Solar Cells Fabrication

Lead halide perovskites are attractive pigments to fabricate solar cells in the laboratory, owing to their high power conversion efficiency. However, given the presence of Pb, such materials also have a high level of toxicity and are carcinogenic for humans and aquatic life. Arguably, this hampers their acceptability for immediate commercialization. This study entails the synthesis, optoelectronic properties, and photovoltaic parameters of two-dimensional copper-based perovskites as an environmentally benign alternative to lead-based perovskites. The perovskites - (CH3 NH3 )2 CuCl4-x Brx with x=0.3 and 0.66 - are derivatives of the stable (CH3 NH3 )2 CuCl4 . The single crystals and powders diffractograms suggest compositions with variations in Cl/Br ratio and dissimilar bromine localization in the inorganic framework. The copper mixed halide perovskite exhibits a narrow absorption with a bandgap of 2.54-2.63 eV related to the halide ratio disparity (crystal color variation). These findings demonstrate the impact of halides to optimize the stability of methylammonium copper perovskites and provide an effective pathway to design eco-friendly perovskites for optoelectronic applications.

Novel Materials in Perovskite Solar Cells: Efficiency, Stability, and Future Perspectives

Solar energy is regarded as the finest clean and green energy generation method to replace fossil fuel-based energy and repair environmental harm. The more expensive manufacturing processes and procedures required to extract the silicon utilized in silicon solar cells may limit their production and general use. To overcome the barriers of silicon, a new energy-harvesting solar cell called perovskite has been gaining widespread attention around the world. The perovskites are scalable, flexible, cost-efficient, environmentally benign, and easy to fabricate. Through this review, readers may obtain an idea about the different generations of solar cells and their comparative advantages and disadvantages, working mechanisms, energy alignment of the various materials, and stability achieved by applying variable temperature, passivation, and deposition methods. Furthermore, it also provides information on novel materials such as carbonaceous, polymeric, and nanomaterials that have been employed in perovskite solar in terms of the different ratios of doping and composite and their optical, electrical, plasmonic, morphological, and crystallinity properties in terms of comparative solar parameters. In addition, information on current trends and future commercialization possibilities of perovskite solar have been briefly discussed based on reported data by other researchers.

Collaborative Passivation for Dual Charge Transporting Layers Based on 4-(chloromethyl)benzonitrile Additive toward Efficient and Stable Inverted Perovskite Solar Cells

Poor carrier transport capacity and numerous surface defects of charge transporting layers (CTLs), coupled with misalignment of energy levels between perovskites and CTLs, impact photoelectric conversion efficiency (PCE) of inverted perovskite solar cells (PSCs) profoundly. Herein, a collaborative passivation strategy is proposed based on 4-(chloromethyl) benzonitrile (CBN) as a solution additive for fabrication of both [6,6]-phenyl-C61-butyric acid methylester (PCBM) and poly(triarylamine) (PTAA) CTLs. This additive can improve wettability of PTAA and reduce the agglomeration of PCBM particles, which enhance the PCE and device stability of the PSCs. As a result, a PCE exceeding 20% with a remarkable short circuit current of 23.9 mA cm^-2 , and an improved fill factor of 81% is obtained for the CBN- modified inverted PSCs. Devices maintain 80% and 70% of the initial PCE after storage under 30% and 85% humidity ambient conditions for 1000 h without encapsulation, as well as negligible light state PCE loss. This strategy demonstrates feasibility of the additive engineering to improve interfacial contact between the CTLs and perovskites for fabrication of efficient and stable inverted PSCs.

Efficient and Stable Carbon-Based Perovskite Solar Cells Enabled by Mixed CuPc:CuSCN Hole Transporting Layer for Indoor Applications

Perovskite solar cells (PSCs) are an innovative technology with great potential to offer cost-effective and high-performance devices for converting light into electricity that can be used for both outdoor and indoor applications. In this study, a novel hole-transporting layer (HTL) was created by mixing copper phthalocyanine (CuPc) molecules into a copper(I) thiocyanate (CuSCN) film and was applied to carbon-based PSCs with cesium/formamidinium (Cs_0.17FA_0.83Pb(I_0.83Br_0.17)₃) as a photoabsorber. At the optimum concentration, a high power conversion efficiency (PCE) of 15.01% was achieved under AM1.5G test conditions, and 32.1% PCE was acquired under low-light 1000 lux conditions. It was discovered that the mixed CuPc:CuSCN HTL helps reduce trap density and improve the perovskite/HTL interface as well as the HTL/carbon interface. Moreover, the PSCs based on the mixed CuPc:CuSCN HTL provided better stability over 1 year due to the hydrophobicity of CuPc material. In addition, thermal stability was tested at 85 °C and the devices achieved an average efficiency drop of approximately 50% of the initial PCE value after 1000 h. UV light stability was also examined, and the results revealed that the average efficiency drop of 40% of the initial value for 70 min of exposure was observed. The work presented here represents an important step toward the practical implementation of the PSC as it paves the way for the development of cost-effective, stable, yet high-performance PSCs for both outdoor and indoor applications.

Modifying SnO2 with Polyacrylamide to Enhance the Performance of Perovskite Solar Cells

Modification of the charge transport layers is an effective way to improve charge transport and performance of perovskite solar cells (PSCs). The ions in the ionic compounds used for the modification of SnO2 may migrate into the perovskite layer, which harms the stability of PSCs. In this work, a low-cost, water-soluble nonionic polymer polyacrylamide (PAM) is used to modify SnO2. The addition of PAM improves the uniformity, wettability, and electron mobility of the SnO2 film. Through the modification of SnO2, the defects of perovskite films are reduced and the grain size is increased. Furthermore, the energy-level alignment at the SnO2/perovskite interface is improved, which is beneficial to the transfer of electrons from perovskite to SnO2. Finally, the power conversion efficiency (PCE) of PSCs formed from modified SnO2 is enhanced to 22.59%. More importantly, the unencapsulated devices with modified SnO2 retain 90% of the initial value after storage for more than 1000 h under a relative humidity of 50%. These results indicate that modifying SnO2 using PAM is a promising strategy to improve the performance of PSCs.

CuI/Spiro-OMeTAD Double-Layer Hole Transport Layer to Improve Photovoltaic Performance of Perovskite Solar Cells

The hole transport layer (HTL) is one of the main factors affecting the efficiency and stability of perovskite solar cells (PSCs). However, obtaining HTLs with the desired properties through current preparation techniques remains a challenge. In the present study, we propose a new method which can be used to achieve a double-layer HTL, by inserting a CuI layer between the perovskite layer and Spiro-OMeTAD layer via a solution spin coating process. The CuI layer deposited on the surface of the perovskite film directly covers the rough perovskite surface, covering the surface defects of the perovskite, while a layer of CuI film avoids the defects caused by Spiro-OMetad pinholes. The double-layer HTLs improve roughness and reduce charge recombination of the Spiro-OMeTAD layer, thereby resulting in superior hole extraction capabilities and faster hole mobility. The CuI/Spiro-OMeTAD double-layer HTLs-based devices were prepared in N2 gloveboxes and obtained an optimized PCE (photoelectric conversion efficiency) of 17.44%. Furthermore, their stability was improved due to the barrier effect of the inorganic CuI layer on the entry of air and moisture into the perovskite layer. The results demonstrate that another deposited CuI film is a promising method for realizing high-performance and air-stable PSCs.

Vacuum-Assisted Drying Process for Screen-Printable Carbon Electrodes of Perovskite Solar Cells with Enhanced Performance Based on Cuprous Thiocyanate as a Hole Transporting Layer

Carbon-based perovskite solar cells without a hole transport layer (HTL) are considered to be highly stable and of low cost. However, the deficient interface contact and inferior hole extraction capability restrict the further improvement of the device efficiency. Introducing a hole transporting layer, such as cuprous thiocyanate (CuSCN), can enhance the hole extraction ability and improve the interface contact. However, our further studies indicated that-at a certain temperature-for carbon-based solar cells, in the CuSCN layer, the diffusion of SCN^- into the perovskite film would produce more interfacial defects and aggravate nonradiative recombination, thus hindering the carrier transport. We further disclosed the reasons for performance attenuation during the thermal treatment of carbon electrodes, proposed a vacuum-assisted drying process for carbon electrodes to suppress the destructive effect, and finally, achieved an enhanced efficiency for perovskite solar cells with a CuSCN inorganic HTL and screen-printable carbon electrode. Also, the unencapsulated perovskite solar cell demonstrated over 80% efficiency retention after being stored in an ambient atmosphere (45-70% relative humidity (RH)) for over 1000 h and maintained over 85% efficiency retention for 309 h of 1-sun irradiation under a continuous nitrogen flow under open-circuit conditions.

Hole transport free flexible perovskite solar cells with cost-effective carbon electrodes

Low temperature derived carbon electrodes are employed to fabricate low cost hole transport layer free perovskite solar cells, in which perovskite films annealed in glovebox and ambient air are used as the absorbers, respectively. Results suggest that the air annealed sample has bigger crystal grains and higher crystallinity, and the existence of a small amount of lead iodide which passivates grain boundaries contributes to a lower trap density. As a result, a maximum power conversion efficiency (PCE) of 13.07% was obtained on the air annealed device, which is higher than those of devices annealed in glovebox (11.25%). Furthermore, the stability of unencapsulated devices stored in wet (with humidity around 90% ± 5%) air atmosphere are investigated and the results prove that our devices exhibit good stability. In addition to rigid devices, flexible perovskite solar cells are also fabricated using the same procedure. The highest PCE of 11.53% is demonstrated on the champion flexible device, and 69% of its initial PCE can be maintained even after 2000 bending cycles with a bending radius of 2 mm. Our work provides a promising and simple rout for low-cost, air-stable, high-efficiency carbon perovskite solar cells for both large area production and flexible electronic devices industry.

Organic ammonium salt-assisted pinhole-free CuSCN films for carbon-based perovskite solar cells

Organic ammonium salt, PEAI, has been utilized as additive in CuSCN film of C-PSCs. It not only eliminated the pinhole, but also passivated the perovskite film and enhanced the interfacial connection, resulting in a 11% PCE improvement.

High-performance perovskite solar cell using photonic-plasmonic nanostructure

In this paper, a coupled optical-electrical modeling method is applied to simulate perovskite solar cells (PSCs) to find ways to improve light absorption by the active layer and ensure that the generated carriers are collected effectively. Initially, a planar structure of the PSC is investigated and its optical losses are determined. To reduce the losses and enhance collection efficiency, a convex light-trapping configuration of PSC is used and the impacts of these nanostructures on all parts of the cell are investigated. In this convex nanostructured PSC, the power conversion efficiency (PCE) is found to be increased when the thickness of the absorbing layer remained unchanged. Then, a plasmonic reflector is applied to trap light inside the perovskite. In this structure, by scattering light through the surface plasmon resonance (SPR) effect of the Au back-contact, the electromagnetic field is found to concentrate in the active layer. This results in increased perovskite absorption and, consequently, a high current density of the cell. In the final structure, which is the integration of these two structures, optical losses are found to be greatly diminished and the short-circuit current density (Jsc) is increased from 18.63 mA/cm2 for the planar structure to 23.5 mA/cm2 for the proposed structure. Due to the increased Jsc and open-circuit voltage (Voc) caused by the improved carrier collection, the PCE increases from 14.62 to 19.54%.

Correlating Hysteresis and Stability with Organic Cation Composition in the Two-Step Solution-Processed Perovskite Solar Cells

The two-step solution-based process has demonstrated substantial success in fabricating high-efficiency perovskite solar cells in recent years. Despite the high performance, the underlying mechanisms that govern the formation of perovskite films and corresponding device performance are yet to be fully understood. Particularly, organic cation composition used in the two-step solution processing of mixed-cation lead halide perovskite solar cells plays a critical role in the perovskite film formation and the resultant device performance. However, little is understood about the impacts of organic cation composition on the current density-voltage (J-V) hysteretic behavior and stability of perovskite solar cells. To address this need, here, we study the effect of mixed organic cations, that is, the fraction of formamidinium (FA) and methylammonium (MA) contents, used for the two-step solution-processed perovskite thin films on solar cell performance, including efficiency, J-V hysteresis, and stability. In addition to the efficiency variations, we find that perovskite solar cells based on FA-rich and MA-rich stoichiometries show distinct characteristics in J-V hysteresis and stability. The origins of such a discrepancy are attributed to the thermodynamically driven conversion from lead iodide to perovskites, which is determined by the combination of organic cations. The perovskite solar cells based on the mixed cation FA_0.6MA_0.4PbI₃ composition show a champion power conversion efficiency of over 21% and robust stability (retaining more than 90% of initial efficiency) under maximum power-point tracking in dry nitrogen for more than 500 h. Our work provides insights on understanding the formation of perovskite films in the two-step process, which may benefit further investigation on perovskite solar cells.

Dimensionality-Controlled Surface Passivation for Enhancing Performance and Stability of Perovskite Solar Cells via Triethylenetetramine Vapor

Perovskite solar cells (PSCs) have achieved unprecedented progress in terms of enhancement of power conversion efficiency (PCE). Nevertheless, device stability is still an obstacle to the commercialization of this emerging photovoltaic technology. Though strategies such as compositional management and ligand engineering have been reported to tackle this critical issue, these methods often have drawbacks such as compromised device performance. Herein, we propose an approach combining material dimensionality control and interfacial passivation by a post-device treatment via triethylenetetramine (TETA) vapor to enhance both efficiency and stability of Cs_0.05FA_0.79MA_0.16PbI_2.5Br_0.5-based PSCs. Results of X-ray diffraction and scanning electron microscopy show the formation of low-dimensional perovskites at the interface between the perovskite film and the hole transporting layer after the TETA vapor treatment. Measurements of the energy level alignment and electrochemical properties by ultraviolet photoelectron spectroscopy and impedance spectra confirm the reduced density of trap states and improved interfacial charge transport. Consequently, TETA-based treatment significantly enhances both efficiency (from 17.07 to 18.03%) and stability (PCE retention from 73.4 to 88.9%) of the PSCs under >65% relative humidity for 1000 h compared to the controlled device without TETA treatment. Furthermore, the TETA vapor also shows an advantageous effect of dramatically improving the performance of PSC devices, which initially had poor performance (from 6.8 to 10.5%) through surface defect passivation.