Rational Molecular Engineering of NiOx Interfaces for High-Performance Inverted Perovskite Solar Cells.

Reducing carrier transport barrier in anode interface enables efficient and stable inverted mesoscopic methylammonium-free perovskite solar cells

The continuing increase of the efficiency of perovskite solar cells has pushed the internal quantum efficiency approaching 100%, which means the light-to-carrier and then the following carrier transportation and extraction are no longer limiting factors in photoelectric conversion efficiency of perovskite solar cells. However, the optimal efficiency is still far lower than the Shockley-Queisser efficiency limit, especially for those inverted perovskite solar cells, indicating that a significant fraction of light does not transmit into the active perovskite layer to be absorbed there. Here, a planar inverted perovskite solar cell (ITO/PTAA/perovskite/PC61BM/bathocuproine (BCP)/Ag) is chosen as an example, and we show that its external quantum efficiency (EQE) can be significantly improved by simply texturing the poly[bis (4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) layer. By washing the film prepared from a mixed polymer solution of PTAA and polystyrene (PS), a textured PTAA/perovskite interface is introduced on the light-input side of perovskite to inhibit internal optical reflection. The reduction of optical loss by this simple texture method increases the EQE and then the photocurrent of the ITO/PTAA/perovskite/PC61BM/BCP/Ag device with the magnitude of about 10%. At the same time, this textured PTAA benefits the band edge absorption in this planar solar cell. The large increase of the short-circuit current together with the increase of fill factor pushes the efficiency of this inverted perovskite solar cell from 18.3% up to an efficiency over 20.8%. By using an antireflection coating on glass to let more light into the device, the efficiency is further improved to 21.6%, further demonstrating the importance of light management in perovskite solar cells.

Perovskite solar cells (PSCs) have recently emerged as highly efficient and cutting-edge photovoltaic technology. In inverted PSCs, challenges are focused on the insufficient interface contact and energy level misalignment between the electron transport layer (ETL) and the metal electrode. Hence, the cathode interfacial layer (CIL) plays a crucial role in regulating energy levels and enabling charge extraction in PSCs. In this study, a low-cost phenanthroline derivative, 4,7-dimethoxy-1,10-phenanthroline (Phen-OMe), is developed as an efficient CIL between the PCBM and Ag electrodes. The incorporation of Phen-OMe not only improves the interfacial contact but also effectively reduces the work function (WF) of the Ag electrode, thus promoting charge dissociation and transport at the interface. Through utilizing a wide-band-gap perovskite with the band gap of 1.77 eV as the active layer by a simple, high-throughput, and low-cost doctor-blade coating process, the power conversion efficiency (PCE) is enhanced significantly from 16.11% of the control device to 18.61% of the device with Phen-OMe as the CIL. Interestingly, Phen-OMe shows a broad application as the CIL in PSCs and tandem solar cells (TSCs), resulting in a boosted efficiency of 22.29% in intermediate-band-gap PSCs and a PCE of 22.05% with a high open-circuit voltage (VOC) of 2.12 V in the perovskite/organic TSC. This achievement shows that Phen-OMe would be a potential candidate as low-cost and efficient CILs for PSCs.

Dual-Resistance of Ion Migration and Moisture Erosion via Hydrolytic Crosslinking of Siloxane Functionalized Poly(Ionic Liquids) for Efficient and Stable Perovskite Solar Cells

Carbazole-based self-assembled monolayers (SAMs) are widely used in inverted perovskite solar cells (PSCs). However, the biased intermolecular assembly of SAM molecules, and the lack of Lewis-basic heteroatoms to efficiently tune the crystallinity of perovskites and passivate the interface defects still limited the further improvement of the efficiency and stability for PSCs involving carbazole-based SAMs. Herein, a novel 3,6-dithiophene carbazole-based SAM molecule (named CzTh) is designed via the substituent engineering strategy, which is demonstrated to effectively solve the obstacles. The theory and experiment find that the introduction of thiophene regulated SAMs with the carbazole core in terms of surface wettability for precursor solvent, energy level alignment, the crystallization of perovskite films and defects passivation, which is attributed to dipole moment changes, and the Lewis base property of S atom. Consequently, the PSCs with CzTh achieved enhanced power conversion efficiency (PCE) and excellent air stability, compared to the commercial SAMs (Me-4PACz). As an "one-stone-three-birds" design strategy for SAMs, the "thiophene-substitution" effectively tunes the perovskite crystallization, passivates the defects, and enhances the hole injection at the perovskite/SAMs interface of inverted PSCs.

Star-like, dopant-free, corannulene-cored hole transporting materials for efficient inverted perovskite solar cells

Fluorinated triphenylamine-based dopant-free hole-transporting material for high-performance inverted perovskite solar cells

Hole-transport materials (HTMs) play an important role in perovskite solar cells (PSCs) to enhance the power conversion efficiency (PCE). The innovation of HTMs can increase the hole extraction ability and reduce interfacial recombination. Three organic small molecule HTMs with 4H-cyclopenta[2,1-b:3,4-b']dithiophene (CPDT) as the central unit was designed and synthesized, namely, CPDTE-MTP (with the 2-ethylhexyl substituent and diphenylamine derivative end-group), CPDT-MTP (with the hexyl substituent and diphenylamine derivative end-group), and CPDT-PMTP (with the hexyl substituent and triphenylamine derivative end-group), which can form bifunctional and robust hole transport layer (HTL) on ITO and is tolerable to subsequent solvent and thermal processing. The X-ray photoelectron spectroscopy (XPS) results proved that CPDT-based HTMs can both interact with ITO through the nitrogen element in them and the tin element in ITO and passivate the upper perovskite layer. It is worth noting that the champion efficiency of MAPbI3 PSCs based on CPDT-PMTP achieved 20.77%, with an open circuit voltage (VOC) of 1.10 V, a short-circuit current (JSC) of 23.39 mA cm-2, and a fill factor (FF) of 80.83%, as three new materials were introduced into p-i-n PSCs as dopant-free HTMs.

Adjusting the hole transport layer (HTL) to optimize its interface with perovskite is crucial for minimizing interface recombination, enhancing carrier extraction, and achieving efficient and stable inverted perovskite solar cells (PSCs). However, as a commonly used HTL, the self-assemble layer (SAM) of [2-(3,6-dimethoxy-9H-carbazol-9-yl)ethyl] phosphonic acid (MeO-2PACz) tends to form clusters and micelles during the deposition process, leading to inadequate coverage of the ITO substrate. Here, a Co-SAM strategy is employed by incorporating 4-mercaptobenzoic acid (SBA) and 4-trifluoromethyl benzoic acid (TBA) as additives into MeO-2PACz to fabricate a Co-SAM-based HTL. The introduced additive can interact with MeO-2PACz, facilitating cluster dispersion and thereby enabling better deposition on ITO for improved HTL coverage. Moreover, Co-SAM exhibits superior energy level alignment with perovskite to enhance interfacial contact and improve carrier extraction efficiency as well as promote growth of bottom perovskite grains. As a result, an impressive increase of the power conversion efficiency (PCE) from 21.34% to 23.31% is achieved in the inverted device based on the Co-SAM HTL of MeO-2PACz+TBA while maintaining ≈90% of its initial efficiency under continuous operation at 1-sun.

Introduction The NiOx layer is actively used as hole extraction layer for inverted structure perovskite solar cells. However, clear guidelines on NiOx chemical composition, manufacturing methods, energy levels, and the hole-transport process remain unclear. In this conference, high-density NiOx layers with various manufacturing temperatures are produced with Nickel acetylacetonate using the SPD method in order to improve the performances of inverted perovskite solar cells. The chemical composition of the NiOx is investigated using XPS, and GIXRD. Furthermore, the influence of Na+ ions in the valence band of the NiOx layer is investigated by vacuum ultraviolet photoelectron spectroscopy using synchrotron radiation. The results showed that the conversion efficiency of the perovskite solar cell is high as the crystallite size of NiOx is large. The highest perovskite solar cell conversion efficiency was obtained at a NiOx layer production temperature of 550 °C. Inverted perovskite solar cells were fabricated with over 16.1% conversion efficiency. Conversion efficiency of solar cell The conversion efficiencies of the perovskite solar cells using NiOx films with various deposition temperatures are summarized in Table 1. The perovskite solar cells conversion efficiency increased along with the NiOx layer production temperature to 550 and 600 °C; however, it decreased slightly when the temperature increased further to 600 °C. As a result, the perovskite solar cell conversion efficiency reached its peak when the NiOx layer production temperature was 500 °C. To highlight the performance change, we herein investigated the composition, structure and the band state of the NiOx layers. Influence of valence band and valence of Ni ion of the NiOx layers To investigate the states of the regions near the edges of the valence bands of the NiOx layers, PES measurements were carried out with a BL-07B at the NewSUBARU synchrotron radiation facility, and the results are shown in Figure 1. The regions near the edges of the valence bands of the NiOx correspond to Ni 3d (label A), O 2p (label B), and the charge transfer transition from an O 2p level to Ni 3d photoionization (label C). From the PES measurement results, the region (around 4-5 eV) corresponding to O 2p was stronger at 300 and 600 °C than at 500 °C. This is because the influence of the O 2p orbital increased because the proportion of oxygen in NiOx increased owing to the loss of Ni ions. Although the ratio of Ni2+ to Ni3+ ions affects the energy levels in the vicinity of the HOMO, a change in these energy levels does not contribute to hole extraction in perovskite solar cells. From these results, it was found that the valence of the Ni ions has little influence on the performance of the solar cells. Crystal analysis of the NiOx layers Both the structures and crystallite sizes of the thin NiOx films formed by SPD were investigated via GIXRD, as shown in Figure 2. The five peaks that appeared at 2θ values of approximately 37.6° 43.6°, 63.3°, 75.6°, and 79.6° were assigned to the 111, 200, 220, 311, and 222 planes, respectively, and then attributed to the randomly oriented NiOx crystal. At 250 °C, no diffraction peaks were observed because the NiOx film did not crystallise. This implies that crystallised NiOx films formed with annealing temperatures above 300 °C. The NiOx crystallite sizes were calculated from the intensities of the 200 peaks using the Scherrer equation, and the results are shown in Figure 1, which indicates that their sizes increased with increasing SPD temperatures. The size of a NiOx crystallite and the tendency of conversion efficiency were consistent, except at 550 and 600 °C, where the FTO layer broke beyond the glass transition point. In other words, as a guideline for manufacturing high-performance inverted perovskite solar cells, it is important to enlarge the crystallites in the NiOx film. Conclusions We demonstrated that it is related to the NiOx crystallite size and the conversion efficiency of the perovskite solar cells, which can be used as a guideline to achieve high performance. The highest perovskite solar cell conversion efficiency was obtained at a NiOx layer production temperature of 500 °C. In the inverse perovskite solar cell, the conversion efficiency exceeded 16.1%, and hysteresis was not observed. Moreover, the measurements of the valence band using synchrotron radiation indicated that the ratio of Ni2+ to Ni+3 had little effect on the solar cell performance. This is an important guideline for improving the performance of perovskite solar cells. Acknowledgement This work was supported by New energy and industrial Technology Development Organization (NEDO), Japan.

The grain boundaries (GBs) instability induced by photodecomposition of residual PbI2 is long-standing challenge for further simultaneous improvement of stability and power conversion efficiency (PCE) of perovskite solar cells (PSCs). Herein, a novel GB stabilization strategy through managing unstable residual PbI2 within perovskite films is reported, which is realized by incorporating 2-iodoimidazole (2-IM) into perovskite precursor solution. The 2-IM can in situ convert unstable residual PbI2 at GBs into robust metallo-organic complex 2-IMPbI2 exhibiting an orderly hexagonal layered crystal structure. 2-IMPbI2 is uncovered to have much better defect passivation effect and stability than PbI2. The formed 2-IMPbI2 facilitates perovskite crystallization, passivates GB defect, suppresses ion migration, mitigates phase segregation, and promotes carrier transport, contributing to simultaneously enhanced PCE and stability. Owing to the ingenious GB modulation strategy, the inverted 1.66eV PSCs achieve a PCE of 24.12%, which is among the highest PCEs ever reported for 1.66eV PSCs. This strategy demonstrates good universality by accomplishing efficient 1.53eV PSCs with a PCE of 26.84%. Moreover, the inverted wide-bandgap PSCs with 2-IMPbI2 maintain 94% and 90% of their initial efficiencies after 1000h of continuous maximum power point operation and after 500h of thermal stress at 85°C, respectively.

Designing an efficient modification molecule to mitigate non-radiative recombination at the NiOx/perovskite interface and improve perovskite quality represents a challenging yet crucial endeavor for achieving high-performance inverted perovskite solar cells (PSCs). Herein, we synthesized a novel fullerene-based hole transport molecule, designated as FHTM, by integrating C60 with 12 carbazole-based moieties, and applied it as a modification molecule at the NiOx/perovskite interface. The in situ self-doping effect, triggered by electron transfer between carbazole-based moiety and C60 within the FHTM molecule, along with the extended π conjugated moiety of carbazole groups, significantly enhances FHTM's hole mobility. Coupled with optimized energy level alignment and enhanced interface interactions, the FHTM significantly enhances hole extraction and transport in corresponding devices. Additionally, the introduced FHTM efficiently promotes homogeneous nucleation of perovskite, resulting in high-quality perovskite films. These combined improvements led to the FHTM-based PSCs yielding a champion efficiency of 25.58 % (Certified: 25.04 %), notably surpassing that of the control device (20.91 %). Furthermore, the unencapsulated device maintained 93 % of its initial efficiency after 1000 hours of maximum power point tracking under continuous one-sun illumination. This study highlights the potential of functionalized fullerenes as hole transport materials, opening up new avenues for their application in the field of PSCs.

Lead-free tin perovskite solar cells

To date, the most efficient perovskite solar cells (PSCs) require hole‐transporting materials (HTMs) that are doped with hygroscopic additives to improve their performance. Unfortunately, such dopants negatively impact the overall PSCs stability and add cost and complexity to the device fabrication. Hence, there is a need to investigate new strategies to boost the typically modest performance of dopant‐free HTMs for efficient and stable PSCs. Thionation is a simple and single‐step approach to enhance the carrier‐transport capability of organic semiconductors, yet still completely unexplored in the context of HTMs for PSCs. In this work, a novel polymeric semiconductor, P1, based on a diketopyrrolopyrrole (DPP) moiety, is proposed as a dopant‐free HTM. Its modest performance in PSCs (power conversion efficiency (PCE) = 7.1%) is significantly enhanced upon thionation of the DPP moiety. The resulting dithioketopyrrolopyrrole‐based HTM, P2, leads to PSCs with nearly 40% performance improvement (PCE = 9.7%) compared to devices based on the nonthionated HTM (P1). Furthermore, thionation also remarkably boosts the shelf‐storage and thermal stability with respect to traditional 2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenylamine)‐9,9′‐spirobifluorene‐based PSCs. This work provides useful insights to further design effective dopant‐free HTMs employing the straightforward one‐step thionation strategy for efficient and stable PSCs.

Energy loss and unstable properties of the interface and grain boundaries (GBs) in perovskite solar cells (PSCs) greatly limit the efficiency and stability of PSCs. Here, a polyvinyl pyrrolidone (PVP) treatment is proposed to overcome these challenges. The impact of PVP treatment on perovskite films and the corresponding performance of the devices are systemically investigated. The crystallinity, GBs, and PbI2 residues of the perovskite films are all improved via the interaction of PVP and perovskite crystals, which results in an increased grain size and enhanced built-in electric field in the devices. The trap density is dramatically decreased from 8.74 × 1015 to 4.37 × 1015 cm–3, and the additional interface electrical field of 1.26 × 106 V/cm is formed at the perovskite/PCBM interface, which dramatically eliminates the energy loss of the bulk and interface of inverted PSCs. Based on this strategy, a high power conversion efficiency (PCE) of 20.77% is achieved based on the MAPbI3/PCBM planar heterojunction. In addition, the stability of PSCs is also dramatically improved, and the PCE of PVP devices can retain 80% of its initial value after 14 days in air and can retain 99% of its initial value after 64 days in N2, while the control devices can only retain 44 and 85% of their initial PCE values under the same exposure conditions, respectively.