Self‐Encapsulating Thermostable and Air‐Resilient Semitransparent Perovskite Solar Cells

Semitransparent perovskite solar cells (PSCs) are of interest for application in tandem solar cells and building‐integrated photovoltaics. Unfortunately, several perovskites decompose when exposed to moisture or elevated temperatures. Concomitantly, metal electrodes can be degraded by the corrosive decomposition products of the perovskite. This is even the more problematic for semitransparent PSCs, in which the semitransparent top electrode is based on ultrathin metal films. Here, we demonstrate outstandingly robust PSCs with semitransparent top electrodes, where an ultrathin Ag layer is sandwiched between SnO_x grown by low‐temperature atomic layer deposition. The SnO_x forms an electrically conductive permeation barrier, which protects both the perovskite and the ultrathin silver electrode against the detrimental impact of moisture. At the same time, the SnO_x cladding layer underneath the ultra‐thin Ag layer shields the metal against corrosive halide compounds leaking out of the perovskite. Our semitransparent PSCs show an efficiency higher than 11% along with about 70% average transmittance in the near‐infrared region (λ > 800 nm) and an average transmittance of 29% for λ = 400–900 nm. The devices reveal an astonishing stability over more than 4500 hours regardless if they are exposed to ambient atmosphere or to elevated temperatures.     

All‐Solution‐Processed Silver Nanowire Window Electrode‐Based Flexible Perovskite Solar Cells Enabled with Amorphous Metal Oxide Protection

Silver nanowire (AgNW)‐based transparent electrodes prepared via an all‐solution‐process are proposed as bottom electrodes in flexible perovskite solar cells (PVSCs). To enhance the chemical stability of AgNWs, a pinhole‐free amorphous aluminum doped zinc oxide (a‐AZO) protection layer is deposited on the AgNW network. Compared to its crystalline counterpart (c‐AZO), a‐AZO substantially improves the chemical stability of the AgNW network. For the first time, it is observed that inadequately protected AgNWs can evanesce via diffusion, whereas a‐AZO secures the integrity of AgNWs. When an optimally thick a‐AZO layer is used, the a‐AZO/AgNW/AZO composite electrode exhibits a transmittance of 88.6% at 550 nm and a sheet resistance of 11.86 Ω sq^?1, which is comparable to that of commercial fluorine doped tin oxide. The PVSCs fabricated with a configuration of Au/spiro‐OMeTAD/CH₃NH₃PbI₃/ZnO/AZO/AgNW/AZO on rigid and flexible substrates can achieve power conversion efficiencies (PCEs) of 13.93% and 11.23%, respectively. The PVSC with the a‐AZO/AgNW/AZO composite electrode retains 94% of its initial PCE after 400 bending iterations with a bending radius of 12.5 mm. The results clearly demonstrate the potential of AgNWs as bottom electrodes in flexible PVSCs, which can facilitate the commercialization and large‐scale deployment of PVSCs.     

Simultaneous Improved Performance and Thermal Stability of Planar Metal Ion Incorporated CsPbI2Br All‐Inorganic Perovskite Solar Cells Based on MgZnO Nanocrystalline Electron Transporting Layer

The high thermal stability and facile synthesis of CsPbI₂Br all‐inorganic perovskite solar cells (AI‐PSCs) have attracted tremendous attention. As far as electron‐transporting layers (ETLs) are concerned, low temperature processing and reduced interfacial recombination centers through tunable energy levels determine the feasibility of the perovskite devices. Although the TiO₂ is the most popular ETL used in PSCs, its processing temperature and moderate electron mobility hamper the performance and feasibility. Herein, the highly stable, low‐temperature processed MgZnO nanocrystal‐based ETLs for dynamic hot‐air processed Mn²⁺ incorporated CsPbI2Br AI‐PSCs are reported. By holding its regular planar “n–i–p” type device architecture, the MgZnO ETL and poly(3‐hexylthiophene‐2,5‐diyl) hole transporting layer, 15.52% power conversion efficiency (PCE) is demonstrated. The thermal‐stability analysis reveals that the conventional ZnO ETL‐based AI‐PSCs show a serious instability and poor efficiency than the Mg²⁺ modified MgZnO ETLs. The photovoltaic and stability analysis of this improved photovoltaic performance is attributed to the suitable wide‐bandgap, low ETL/perovskite interface recombination, and interface stability by Mg²⁺ doping. Interestingly, the thermal stability analysis of the unencapsulated AI‐PSCs maintains >95% of initial PCE more than 400 h at 85 °C for MgZnO ETL, revealing the suitability against thermal degradation than conventional ZnO ETL.     

Progress in Tandem Solar Cells Based on Hybrid Organic–Inorganic Perovskites

Owing to their high efficiency, low‐cost solution‐processability, and tunable bandgap, perovskite solar cells (PSCs) made of hybrid organic‐inorganic perovskite (HOIP) thin films are promising top‐cell candidates for integration with bottom‐cells based on Si or other low‐bandgap solar‐cell materials to boost the power conversion efficiency (PCE) beyond the Shockley‐Quiesser (S‐Q) limit. In this review, recent progress in such tandem solar cells based on the emerging PSCs is summarized and reviewed critically. Notable achievements for different tandem solar cell configurations including mechanically‐stacked, optical coupling, and monolithically‐integrated with PSCs as top‐cells are described in detail. Highly‐efficient semitransparent PSC top‐cells with high transmittance in near‐infrared (NIR) region are critical for tandem solar cells. Different types of transparent electrodes with high transmittance and low sheet‐resistance for PSCs are reviewed, which presents a grand challenge for PSCs. The strategies to obtain wide‐bandgap PSCs with good photo‐stability are discussed. The PCE reduction due to reflection loss, parasitic absorption, electrical loss, and current mismatch are analyzed to provide better understanding of the performance of PSC‐based tandem solar cells.     

ITO‐Free and Fully Solution‐Processed Semitransparent Organic Solar Cells with High Fill Factors

Organic photovoltaic (OPV) solar cells that can be simply processed from solution are in the focus of the academic and industrial community because of their enormous potential to reduce cost. One big challenge in developing a fully solution‐processed OPV technology is the design of a well‐performing electrode system, allowing the replacement of ITO. Several solution‐processed electrode systems were already discussed, but none of them could match the performance of ITO. Here, we report efficient ITO‐free and fully solution‐processed semitransparent inverted organic solar cells based on silver nanowire (AgNW) electrodes. To demonstrate the potential of these AgNW electrodes, they were employed as both the bottom and top electrodes. Record devices achieved fill factors as high as 63.0%, which is comparable to ITO based reference devices. These results provide important progress for fully printed organic solar cells and indicate that ITO‐free, transparent as well as non‐transparent organic solar cells can indeed be fully solution‐processed without losses.     

Improving Interfacial Charge Recombination in Planar Heterojunction Perovskite Photovoltaics with Small Molecule as Electron Transport Layer

Although perovskite solar cells (PSCs) have emerged as a promising alternative to widely used fossil fuels, the involved high‐temperature preparation of metal oxides as a charge transport layer in most state‐of‐the‐art PSCs has been becoming a big stumbling block for future low‐temperature and large‐scale R2R manufacturing process. Such an issue strongly encourages scientists to find new type of materials to replace metal oxides. Except for expensive PC₆₁BM with unmanageable morphology and electrical properties, the past investigation on the development of low‐temperature‐processed and highly efficient electron transport layers (ETLs) has met some mixed success. In order to further enhance the performance of all‐solution‐processed PSCs, we propose a novel n‐type sulfur‐containing small molecule hexaazatrinaphtho[2,3‐c][1,2,5]thiadiazole (HATNT) with high electron mobility up to 1.73 × 10^?2 cm² V^?1 s^?1 as an ETL in planar heterojunction PSCs. A high power conversion efficiency of 18.1% is achieved, which is fully comparable with the efficiency from the control device fabricated with PC₆₁BM as ETL. This superior performance mainly attributes from more effective suppression of charge recombination at the perovskite/HATNT interface than that between the perovskite and PC₆₁ BM. Moreover, high electron mobility and strong interfacial interaction via S? I or S? Pb bonding should be also positive factors. Significantly, our results undoubtedly enable new guidelines in exploring n‐type organic small molecules for high‐performance PSCs.     

Four‐Terminal Perovskite/Silicon Multijunction Solar Modules

Multijunction solar cells employing perovskite and crystalline‐silicon (c‐Si) light absorbers bear the exciting potential to surpass the efficiency limit of market‐leading single‐junction c‐Si solar cells. However, scaling up this technology and maintaining high efficiency over large areas are challenging as evidenced by the small‐area perovskite/c‐Si multijunction solar cells reported so far. In this work, a scalable four‐terminal multijunction solar module design employing a 4 cm² semitransparent methylammonium lead triiodide perovskite solar module stacked on top of an interdigitated back contact c‐Si solar cell of identical area is demonstrated. With a combination of optimized transparent electrodes and efficient module design, the perovskite/c‐Si multijunction solar modules exhibit power conversion efficiencies of 22.6% on 0.13 cm² and 20.2% on 4 cm² aperture area. Furthermore, a detailed optoelectronic loss analysis along with strategies to enhance the performance is discussed.     

High‐Performance Semitransparent Tandem Solar Cell of 8.02% Conversion Efficiency with Solution‐Processed Graphene Mesh and Laminated Ag Nanowire Top Electrodes

A high‐performance semitransparent tandem solar cell that uses solution‐processed graphene mesh and laminated Ag NW as a transparent anode and cathode, respectively, is demonstrated. The laminated top electrode can be deposited without causing any damage to the underneath organic solar cells. Power conversion efficiencies of 8.02% and 6.47% are obtained when the light is projected from the solution‐processed graphene mesh and laminated AgNW, respectively. The performance of the tandem cell is found to be comparable to a tandem solar cell fabricated using commercially available indium tin oxide. These findings offer a high‐performance device and open a new pathway in searching for a potential replacement to the frequently used transparent conducting electrodes.     

Revealing the Hole and Electron Transport Dynamics in the Working Devices for Efficient Semitransparent Perovskite Solar Cells

The performance and stability of perovskite solar cells (PSCs) are dominated by the electron and hole transport dynamics, which are highly desired to be characterized in the working devices. Nevertheless, for the opaque PSCs, the hole transport layer (HTL) in n–i–p devices or the electron transport layer (ETL) in p–i–n devices are typically buried beneath the metal electrode, which makes it difficult to simultaneously characterize the charge transport dynamics from both sides of the working devices. In this work, for the first time, the charge transport dynamics of the working devices from both the front and rear side of semitransparent PSCs (ST-PSCs) is characterized via femtosecond transient reflection spectroscopy (FS-TRS). A significant enhancement of the hole transport and negligible change of the electron transport is observed when the perovskite/HTL interface is treated by 2-chloro-phenethylammonium iodide (2-Cl-PEAI). A champion power conversion efficiency (PCE) of 23.3% (certified 22.3%) is achieved, which is the highest certified PCE for ST-PSCs up to date. The ST-PSCs maintained more than 90% of its initial efficiency after 2000 h of maximum power point tracking (MPPT). Additionally, the ST-PSCs are implemented in 4-terminal (4-T) perovskite/passivated emitter rear silicon cell (PERC), reaching a simulated output power of 30.8 mW cm⁻².     

Simultaneous Improvement in Efficiency and Stability of Low‐Temperature‐Processed Perovskite Solar Cells by Interfacial Control

In most current state‐of‐the‐art perovskite solar cells (PSCs), high‐temperature (≈500 °C)‐sintered metal oxides are employed as electron‐transporting layers (ETLs). To lower the device processing temperature, the development of low‐temperature‐processable ETL materials (such as solution‐processed ZnO) has received growing attention. However, thus far, the use of solution‐processed ZnO is limited because the reverse decomposition reaction that occurs at ZnO/perovskite interfaces significantly degrades the charge collection and stability of PSCs. In this work, the reverse decomposition reaction is successfully retarded by sulfur passivation of solution‐processed ZnO. The sulfur passivation of ZnO by a simple chemical means, efficiently reduces the oxygen‐deficient defects and surface oxygen‐containing groups, thus effectively preventing reverse decomposition reactions during and after formation of the perovskite active layers. Using the low‐temperature‐processed sulfur‐passivated ZnO (ZnO–S), perovskite layers with higher crystallinity and larger grain size are obtained, while the charge extraction at the ZnO/perovskite interface is significantly improved. As a result, the ZnO–S‐based PSCs achieve substantially improved power‐conversion‐efficiency (PCE) (19.65%) and long‐term air‐storage stability (90% retention after 40 d) compared with pristine ZnO‐based PSCs (16.51% and 1% retention after 40 d). Notably, the PCE achieved is the highest recorded (19.65%) for low‐temperature ZnO‐based PSCs.     

Nanowire Interconnects for Printed Large‐Area Semitransparent Organic Photovoltaic Modules

Semitransparent organic photovoltaic (OPV) cells promise applications in various transparent architectures where their opaque counterparts cannot contribute. Realizing practical applications of this technology requires the manufacturing of large‐area modules without significant performance loss compared to the lab‐scale devices. In this work, efficient semitransparent OPV modules based on ultrafast laser patterning on both glass and flexible substrates are reported. Solution‐processed metallic silver nanowires (AgNWs) are used as transparent top electrodes. The efficient low‐ohmic contact of the interconnects between the top AgNWs and the bottom electrode in combination with high‐precision laser beam positioning system allow to fabricate semitransparent modules with high electrical fill factor of ≈63% and a remarkable geometric fill factor exceeding 95%, respectively. These results represent an important progress toward upscaling of high‐performance OPV modules with reduced production costs.     

Simple,Robust, and Going More Efficient: Recent Advance on Electron Transport Layer‐Free Perovskite Solar Cells

Perovskite solar cells (PSCs) have shown great potential for photovoltaic applications with their unprecedented power conversion efficiency advancement. Such devices generally have a complex structure design with high temperature processed TiO₂ as the electron transport layer (ETL). Further careful design of device configuration to fully tap the potentials of perovskite materials is expected. Particularly, for the practical application of PSCs, it is crucial to simplify their device structures thus the associated manufacturing process and cost while maintaining their efficiency to be comparable with the conventional devices. But how simple is simple? ETL‐free PSCs promise the simplest structured, thus simple manufacturing processes and low cost large area PSCs in practical applications. They can also help the further exploration of the great potential of perovskite materials and understanding the working principle of PSCs. Within this review, the evolution of the PSC is outlined by discussing the recent advances in the simplification of device configuration and processes for cost effective, highly efficient, and robust PSCs, with a focus on ETL‐free PSCs. Their advancement, key issues, working mechanism, existing problems, and future performance enhancements. This review aims to promote the future development of low cost and robust ETL‐free PSCs toward more efficient power output.     

Donor–Acceptor Type Dopant‐Free,Polymeric Hole Transport Material for Planar Perovskite Solar Cells (19.8%)

Organic–inorganic hybrid perovskite has led to the development of new solar cells with outstanding efficiency. In perovskite solar cells (PSCs), perovskite is sandwiched between a working electrode (fluorine‐doped tin oxide) and a counter electrode (gold, Au). In order to transport charges and block opposite charges, charge transport layers are inserted between perovskite and the electrodes. In particular, a hole transport layer is important because it generally prevents perovskite from exposure to air. Therefore, it is necessary to investigate dopant‐free and hydrophobic polymeric hole transport materials (HTMs). In this study, a novel polymeric HTM (PTEG) is synthesized by controlling the solubility using a tetraethylene glycol group. The planar‐PSC employing PTEG exhibits an efficiency of 19.8% without any dopants, which corresponds to the highest value reported to date. This study offers a fundamental strategy for designing and synthesizing various polymeric HTMs.     

SrTiO3/Al2O3‐Graphene Electron Transport Layer for Highly Stable and Efficient Composites‐Based Perovskite Solar Cells with 20.6% Efficiency

For practical use of perovskite solar cells (PSCs) the instability issues of devices, attributed to degradation of perovskite molecules by moisture, ions migration, and thermal‐ and light‐instability, have to be solved. Herein, highly efficient and stable PSCs based on perovskite/Ag‐reduced graphene oxide (Ag‐rGO) and mesoporous Al₂O₃/graphene (mp‐AG) composites are reported. The mp‐AG composite is conductive with one‐order of magnitude higher mobility than mp‐TiO₂ and used for electron transport layer (ETL). Compared to the mp‐TiO₂ ETL based cells, the champion device based on perovskite/Ag‐rGO and SrTiO₃/mp‐AG composites shows overall a best performance (i.e., V_OC = 1.057 V, J_SC = 25.75 mA cm^?2, fill factor (FF) = 75.63%, and power conversion efficiency (PCE) = 20.58%). More importantly, the champion device without encapsulation exhibits not only remarkable thermal‐ and photostability but also long‐term stability, retaining 97–99% of the initial values of photovoltaic parameters and sustaining ≈93% of initial PCE over 300 d under ambient conditions.     

Efficient and Highly Air Stable Planar Inverted Perovskite Solar Cells with Reduced Graphene Oxide Doped PCBM Electron Transporting Layer

Reduced graphene oxide (rGO) is added in the [6,6]‐Phenyl‐C61‐butyric acid methyl ester (PCBM) electron transport layer (ETL) of planar inverted perovskite solar cells (PSCs), resulting in a power conversion efficiency (PCE) improvement of ≈12%, with a hysteresis‐free PCE of 14.5%, compared to 12.9% for the pristine PCBM based device. The universality of the method is demonstrated in PSCs based on CH₃NH₃PbI_3?x Cl_x and CH₃NH₃PbI₃ perovskites, deposited through one step and two step spin coating process, respectively. After a comprehensive spectroscopic characterization of both devices, it is clear that the introduction of rGO in PCBM ETL results in an important increase of the ETL conductivity, together with reduced series resistance and surface roughness. As a result, a significant photoluminescence quenching of such perovskite/ETL is observed, confirming the increased measured short circuit current density. Transient absorption measurements reveal that in the rGO‐based device, the relaxation process of the excited electrons is significantly faster compared to the reference, which implies that the charge injection rate is significantly faster for the first. Furthermore, the light soaking effect is significantly reduced. Finally, aging measurements reveal that the rGO stabilizes the ELT/perovskite interface, which results in the stabilization of perovskite crystal structure after prolonged illumination.     

Solvent‐Mediated Dimension Tuning of Semiconducting Oxide Nanostructures as Efficient Charge Extraction Thin Films for Perovskite Solar Cells with Efficiency Exceeding 16%

The recent surge in efficiency and progress of organohalide perovskite solar cells (PSCs) has been significant. The PSC performance is significantly influenced by nanostructuring as this varies the intrinsic optical, electrical, and electrochemical properties. Diverse TiO₂ electron transport layers (ETLs) are solvothermally grown on the transparent conducting oxide substrate with different dimensionalities, 0D nanoparticles (TNP), 1D nanowires (TNW) to 2D nanosheets (TNS), by varying the organic solvent used. These layers feature enhanced optical transparency (≈2%–5% transmittance improvement compared to pristine fluorine doped tin oxide, FTO, glass) minimizing light absorption losses. PSCs constructed using 1D TNW or 2D TNS yield enhanced photovoltaic performance compared to the 0D TNP counterparts. This is a result of i) improved infiltration of the perovskite in the porous TNW or TNS network and ii) facilitated electron transport and charge extraction at the TNW/perovskite or TNS/perovskite interfaces, thus reduced interfacial recombination loss. Employing a bilayered ETL film consisting of a self‐assembled TiO₂ blocking layer and a subsequent TNW active layer, produces PSC devices with an efficiency exceeding 16%. This bilayered ETL film can simultaneously block the photogenerated holes and enhance electron ­extraction, therefore improving PSC performance.     

Development of Next‐Generation Organic‐Based Solar Cells: Studies on Dye‐Sensitized and Perovskite Solar Cells

Next‐generation organic solar cells such as dye‐sensitized solar cells (DSSCs) and perovskite solar cells (PSCs) are studied at the National Institute of Advanced Industrial Science and Technology (AIST), and their materials, electronic properties, and fabrication processes are investigated. To enhance the performance of DSSCs, the basic structure of an electron donor, π‐electron linker, and electron acceptor, i.e., D–π–A, is suggested. In addition, special organic dyes containing coumarin, carbazole, and triphenylamine electron donor groups are synthesized to find an effective dye structure that avoids charge recombination at electrode surfaces. Meanwhile, PSCs are manufactured using both a coating method and a laser deposition technique. The results of interfacial studies demonstrate that the level of the conduction band edge (CBE) of a compact TiO₂ layer is shifted after TiCl₄ treatment, which strongly affects the solar cell performance. Furthermore, a special laser deposition system is developed for the fabrication of the perovskite layers of PSCs, which facilitates the control over the deposition rate of methyl ammonium iodide used as their precursor.     

Improvement of Transparent Metal Top Electrodes for Organic Solar Cells by Introducing a High Surface Energy Seed Layer

We present highly transparent and conductive silver thin films in a thermally evaporated dielectric/metal/dielectric (DMD) multilayer architecture as top electrode for efficient small molecule organic solar cells. DMD electrodes are frequently used for optoelectronic devices and exhibit excellent optical and electrical properties. Here, we show that ultrathin seed layers such as calcium, aluminum, and gold of only 1 nm thickness strongly influence the morphology of the subsequently deposited silver layer used as electrode. The wetting of silver on the substrate is significantly improved with increasing surface energy of the seed material resulting in enhanced optical and electrical properties. Typically thermally evaporated silver on a dielectric material forms rough and granular layers which are not closed and not conductive below thicknesses of 10 nm. With gold acting as seed layer, the silver electrode forms a continuous, smooth, conductive layer down to a silver thickness of 3 nm. At 7 nm silver thickness such an electrode exhibits a sheet resistance of 19 Ω/□ and a peak transmittance of 83% at 580 nm wavelength, both superior compared to silver electrodes without seed layer and even to indium tin oxide (ITO). Top‐illuminated solar cells using gold/silver double layer electrodes achieve power conversion efficiencies of 4.7%, which is equal to 4.6% observed in bottom‐illuminated reference devices employing conventional ITO. The top electrodes investigated here exhibit promising properties for semitransparent solar cells or devices fabricated on opaque substrates.     

Targeting Ideal Dual‐Absorber Tandem Water Splitting Using Perovskite Photovoltaics and CuInxGa1‐xSe2 Photocathodes

Efficient sunlight‐driven water splitting devices can be achieved by pairing two absorbers of different optimized bandgaps in an optical tandem design. With tunable absorption ranges and cell voltages, organic–inorganic metal halide perovskite solar cells provide new opportunities for tailoring top absorbers for such devices. In this work, semitransparent perovskite solar cells are developed for use as the top cell in tandem with a smaller bandgap photocathode to enable panchromatic harvesting of the solar spectrum. A new CuIn_xGa_1‐xSe₂ multilayer photocathode is designed, exhibiting excellent performance for photoelectrochemical water reduction and representing a near‐ideal bottom absorber. When pairing it below a semitransparent CH₃NH₃PbBr₃‐based solar cell, a solar‐to‐hydrogen efficiency exceeding 6% is achieved, the highest value yet reported for a photovoltaic–photoelectrochemical device utilizing a single‐junction solar cell as the bias source under one sun illumination. The analysis shows that the efficiency can reach more than 20% through further optimization of the perovskite top absorber.     

Patterned Wettability Surface for Competition‐Driving Large‐Grained Perovskite Solar Cells

Novel photovoltaic perovskite solar cells (PSCs) with high‐efficient photovoltaic property are largely in thrall to the uncertain perovskite grain size and inevitable defects. Here, inspired by the competitive growth between tree and grass in the forest system, a competitive perovskite grain growth approach via micro‐contact print (MicroCP) method (CD disk as templates) for printing wettability‐patterned substrate is proposed, aiming to achieve large‐grained perovskite and avoid discontinuous perovskite films caused by the low wettability of substrates. A MicroCP process is employed to construct a patterned wettability surface for the perovskite competitive growth mechanism on the electrode surface. This approach modifies the substrates quickly, ensures the uniform coverage of perovskite due to the function of ‐NH₂ and Pb²⁺ bonds, and converts the perovskite films composed of small grains and pinholes into high‐quality perovskite films, free from pinholes and made up of large grains, resulting in efficiencies over 20% for the MicroCP PSCs.