The impact of CBz-PAI interlayer in various HTL-based flexible perovskite solar cells: A drift-diffusion numerical study

In perovskite solar cells (PSCs), the charge carrier recombination obstacles mainly occur at the ETL/perovskite and HTL/perovskite interfaces, which play a decisive role in the solar cell performance. Therefore, this study aims to enhance the flexible PSC (FPSC) efficiency by adding the newly designed CBz-PAI-interlayer (simply CBz-PAI-IL) at the perovskite/HTL interface. In addition, substantial work has been carried out on five different HTLs (Se/Te-Cu2O, CuGaO2, V2O5, and CuSCN, including conventional Spiro-OMeTAD as a reference HTL with and without CBz-PAI-IL), using drift-diffusion simulation to find suitable FPSC design to attain the maximum PCE. Interestingly, PET/ITO/AZO/ZnO NWs/FACsPbBrI3/CBz-PAI/Se/Te-Cu2O/Au device architecture demonstrates the highest achievable power conversion efficiency (PCE) of 27.9 %. The findings of this study confirmed that the reference device (without IL) displays a large valence band edge (VBE)/highest occupied molecular orbital (HOMO) energy level misalignment compared to the modified interface device (with CBz-PAI-IL that reduces VBE/HOMO level mismatch) that eases the hole transport, simultaneously, it reduces the charge carrier recombinations at the interface, resulting in diminished Voc losses in the device. Furthermore, the influence of perovskite absorber thickness and defect density, parasitic resistances, and working temperature are systematically examined to govern the superior FPSC efficiency and concurrently understand the device physics.

Lithium Polystyrene Sulfonate as a Hole Transport Material in Inverted Perovskite Solar Cells

Despite the exceptional efficiency of perovskite solar cells (PSCs), further improvements can be made to bring their power conversion efficiencies (PCE) closer to the Shockley-Queisser limit, while the development of cost-effective strategies to produce high-performance devices are needed for them to reach their potential as a widespread energy source. In this context, there is a need to improve existing charge transport layers (CTLs) or introduce new CTLs. In this contribution, we introduced a new polyelectrolyte (lithium poly(styrene sulfonate (PSS))) (Li:PSS) polyelectrolyte as an HTL in inverted PSCs, where Li⁺ can act as a counter ion for the PSS backbone. The negative charge on the PSS backbone can stabilize the presence of p-type carriers and p-doping at the anode. Simple Li:PSS performed poorly due to poor surface coverage and voids existence in perovskite film as well as low conductivity. PEDOT:PSS was added to increase the conductivity to the simple Li:PSS solution before its use which also resulted in lower performance. Furthermore, a bilayer of PEDOT:PSS and Li:PSS was employed, which outperformed simple PEDOT:PSS due to high quality of perovskite film with large grain size also the large electron injection barrier (ϕ_e ) impeded back diffusion of electrons towards anode. As a consequence, devices employing PEDOT:PSS / Li:PSS bilayers gave the highest PCE of 18.64%.

Combination of Metal Oxide and Polytriarylamine: A Design Principle to Improve the Stability of Perovskite Solar Cells

In the last decade, perovskite photovoltaics gained popularity as a potential rival for crystalline silicon solar cells, which provide comparable efficiency for lower fabrication costs. However, insufficient stability is still a bottleneck for technology commercialization. One of the key aspects for improving the stability of perovskite solar cells (PSCs) is encapsulating the photoactive material with the hole-transport layer (HTL) with low gas permeability. Recently, it was shown that the double HTL comprising organic and inorganic parts can perform the protective function. Herein, a systematic investigation and comparison of four double HTLs incorporating polytriarylamine and thermally evaporated transition metal oxides in the highest oxidation state are presented. In particular, it was shown that MoOx, WOx, and VOx-based double HTLs provided stable performance of PSCs for 1250 h, while devices with NbOx lost 30% of their initial efficiency after 1000 h. Additionally, the encapsulating properties of all four double HTLs were studied in trilayer stacks with HTL covering perovskite, and insignificant changes in the absorber composition were registered after 1000 h under illumination. Finally, it was demonstrated using ToF-SIMS that the double HTL prevented the migration of perovskite volatile components within the structure. Our findings pave the way towards improved PSC design that ensures their long-term operational stability.

One-step electrodeposition of CuSCN/CuI nanocomposite and its hole transport-ability in inverted planar perovskite solar cells

The synthesis of CuSCN/CuI nanocomposite by single-step electrodeposition is developed. The surface morphology and film thickness are controlled by changing the electrochemical potential and deposition time. The mixed-phase formation of CuSCN/CuI is confirmed through x-ray diffraction and Raman spectral analysis. Nanopetal (NP) like morphology of CuSCN/CuI is observed in FESEM micrographs. Interestingly, the NPs density and thickness are increased with increasing the deposition potential and time. The device fabricated using CuSCN/CuI nanocomposite as a hole transport layer (HTL) which is grown for 2 min delivers the best photovoltaic performance. The maximum power conversion efficiency of 18.82% is observed for CuSCN/CuI NP with a density of 1153μm^-2and thickness of 142 nm. The charge transfer ability of the CuSCN/CuI NP HTL is analyzed by electrochemical impedance spectroscopy. Based on the observation, moderate charge transport resistance and optimum film thickness are required for achieving maximum photovoltaic performance in perovskite solar cells (PVSCs). Thus, the developed CuSCN/CuI NP HTL is a potential candidate for PVSCs.

Methods of Stability Control of Perovskite Solar Cells for High Efficiency

The increasing demand for renewable energy devices over the past decade has motivated researchers to develop new and improve the existing fabrication techniques. One of the promising candidates for renewable energy technology is metal halide perovskite, owning to its high power conversion efficiency and low processing cost. This work analyzes the relationship between the structure of metal halide perovskites and their properties along with the effect of alloying and other factors on device stability, as well as causes and mechanisms of material degradation. The present work discusses the existing approaches for enhancing the stability of PSC devices through modifying functional layers. The advantages and disadvantages of different methods in boosting device efficiency and reducing fabrication cost are highlighted. In addition, the paper presents recommendations for the enhancement of interfaces in PSC structures.

Recent Progress of Inverted Perovskite Solar Cells with a Modified PEDOT:PSS Hole Transport Layer

Organic-inorganic hybrid perovskite solar cells (PSCs) has achieved the power conversion efficiency (PCE) of 25.2% in the last 10 years, and the PCE of inverted PSCs has reached >22%. The rapid enhancement has partly benefited from the employment of suitable hole transport layers. Especially, poly(3,4-ethenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is one of the most widely used polymer hole transport materials in inverted PSCs, because of its high optical transparency in the visible region and low-temperature processing condition. However, the PCE and stability of PSCs based on pristine PEDOT:PSS are far from satisfactory, which are ascribed to low fitness between PEDOT:PSS and perovskite materials, in terms of work function, conductivity, film growth, and hydrophobicity. This paper summaries recent progress regarding to modifying/remedy the drawbacks of PEDOT:PSS to improve the PCE and stability. The systematically understanding of the mechanism of modified PEDOT:PSS and various characteristic methods are summarized here. This Review has the potential to guide the development of PSCs based on commercial PEDOT:PSS.

Role of Water in Suppressing Recombination Pathways in CH3NH3PbI3 Perovskite Solar Cells

Moisture degradation of halide perovskites is the Achilles heel of perovskite solar cells. A surprising revelation in 2014 about the beneficial effects of controlled humidity in enhancing device efficiencies overthrew established paradigms on perovskite solar cell fabrication. Despite the extensive studies on water additives in perovskite solar cell processing that followed, detailed understanding of the role of water from the photophysical perspective remains lacking; specifically, the interplay between the induced morphological effects and the intrinsic recombination pathways. Through ultrafast optical spectroscopy, we show that both the monomolecular and bimolecular recombination rate constants decrease by approximately 1 order with the addition of an optimal 1% H₂O by volume in CH₃NH₃PbI₃ as compared to the reference (without the H₂O additive). Correspondingly, the trap density reduces from 4.8 × 10¹⁷ cm^-3 (reference) to 3.2 × 10¹⁷ cm^-3 with 1% H₂O. We obtained an efficiency of 12.3% for the champion inverted CH₃NH₃PbI₃ perovskite solar cell (1% H₂O additive) as compared to the 10% efficiency for the reference cell. Increasing the H₂O content further is deleterious for the device. Trace amounts of H₂O afford the benefits of surface trap passivation and suppression of trap-mediated recombination, whereas higher concentrations result in a preferential dissolution of methylammonium iodide during fabrication that increases the trap density (MA vacancies). Importantly, our study reveals the effects of trace H₂O additives on the photophysical properties of CH₃NH₃PbI₃ films.

Understanding the Impact of Cu-In-Ga-S Nanoparticles Compactness on Holes Transfer of Perovskite Solar Cells

Although a compact holes-transport-layer (HTL) film has always been deemed mandatory for perovskite solar cells (PSCs), the impact their compactness on the device performance has rarely been studied in detail. In this work, based on a device structure of FTO/CIGS/perovskite/PCBM/ZrAcac/Ag, that effect was systematically investigated with respect to device performance along with photo-physics characterization tools. Depending on spin-coating speed, the grain size and coverage ratio of those CIGS films on FTO substrates can be tuned, and this can result in different hole transfer efficiencies at the anode interface. At a speed of 4000 r.p.m., the band level offset between the perovskite and CIGS modified FTO was reduced to a minimum of 0.02 eV, leading to the best device performance, with conversion efficiency of 15.16% and open-circuit voltage of 1.04 V, along with the suppression of hysteresis. We believe that the balance of grain size and coverage ratio of CIGS interlayers can be tuned to an optimal point in the competition between carrier transport and recombination at the interface based on the proposed mechanism. This paper definitely deepens our understanding of the hole transfer mechanism at the interface of PSC devices, and facilitates future design of high-performance devices.

Top Illuminated Hysteresis-Free Perovskite Solar Cells Incorporating Microcavity Structures on Metal Electrodes: A Combined Experimental and Theoretical Approach

Further technological development of perovskite solar cells (PSCs) will require improvements in power conversion efficiency and stability, while maintaining low material costs and simple fabrication. In this Research Article, we describe top-illuminated ITO-free, stable PSCs featuring microcavity structures, wherein metal layers on both sides on the active layers exerted light interference effects in the active layer, potentially increasing the light path length inside the active layer. The optical constants (refractive index and extinction coefficient) of each layer in the PSC devices were measured, while the optical field intensity distribution was simulated using the transfer matrix method. The photocurrent densities of perovskite layers of various thicknesses were also simulated; these results mimic our experimental values exceptionally well. To modify the cavity electrode surface, we deposited a few nanometers of ultrathin MoO₃ (2, 4, and 6 nm) in between the Ag and poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) layers provide hydrophobicity to the Ag surface and elevate the work function of Ag to match that of the hole transport layer. We achieved a power conversion efficiency (PCE) of 13.54% without hysteresis in the device containing a 4 nm-thick layer of MoO₃. In addition, we fabricated these devices on various cavity electrodes (Al, Ag, Au, Cu); those prepared using Cu and Au anodes displayed improved device stability of up to 72 days. Furthermore, we prepared flexible PSCs having a PCE of 12.81% after incorporating the microcavity structures onto poly(ethylene terephthalate) as the substrate. These flexible solar cells displayed excellent stability against bending deformation, maintaining greater than 94% stability after 1000 bending cycles and greater than 85% after 2500 bending cycles performed with a bending radius of 5 mm.

Solution processed double-decked V2Ox/PEDOT:PSS film serves as the hole transport layer of an inverted planar perovskite solar cell with high performance

Solution processed double-decked V₂O_x/PEDOT:PSS HTL film can effectively improve optoelectronic properties of PSC devices.