Highly Efficient and Stable Perovskite Solar Cells Using a Dopant‐Free Inexpensive Small Molecule as the Hole‐Transporting Material

The hole transporting layer (HTL) plays an important role in realizing efficient and stable perovskite solar cells (PSCs). In spite of intensive research efforts toward the development of HTL materials, low‐cost, dopant‐free hole transporting materials that lead to efficient and stable PSCs remain elusive. Herein, a simple polycyclic heteroaromatic hydrocarbon‐based small molecule, 2,5,9,12‐tetra(tert‐butyl)diacenaphtho[1,2‐b:1′,2′‐d]thiophenen, as an efficient HTL material in PSCs is presented. This molecule is easy to synthesize and inexpensive. It is hydrophobic and exhibits excellent film‐forming properties on perovskites. It has unusually high hole mobility and a desirable highest occupied molecular orbital energy level, making it an ideal HTL material. PSCs fabricated using both the n‐i‐p planar and mesoscopic architectures with this compound as the HTL show efficiencies as high as 15.59% and 18.17%, respectively, with minimal hysteresis and high long term stability under ambient conditions.     

Copper Salts Doped Spiro‐OMeTAD for High‐Performance Perovskite Solar Cells

The development of effective and stable hole transporting materials (HTMs) is very important for achieving high‐performance planar perovskite solar cells (PSCs). Herein, copper salts (cuprous thiocyanate (CuSCN) or cuprous iodide (CuI)) doped 2,2,7,7‐tetrakis(N,N‐di‐p‐methoxyphenylamine)‐9,9‐spirobifluorene (spiro‐OMeTAD) based on a solution processing as the HTM in PSCs is demonstrated. The incorporation of CuSCN (or CuI) realizes a p‐type doping with efficient charge transfer complex, which results in improved film conductivity and hole mobility in spiro‐OMeTAD:CuSCN (or CuI) composite films. As a result, the PCE is largely improved from 14.82% to 18.02% due to obvious enhancements in the cell parameters of short‐circuit current density and fill factor. Besides the HTM role, the composite film can suppress the film aggregation and crystallization of spiro‐OMeTAD films with reduced pinholes and voids, which slows down the perovskite decomposition by avoiding the moisture infiltration to some extent. The finding in this work provides a simple method to improve the efficiency and stability of planar perovskite solar cells.     

Molecular Engineering of Copper Phthalocyanines: A Strategy in Developing Dopant‐Free Hole‐Transporting Materials for Efficient and Ambient‐Stable Perovskite Solar Cells

Copper (II) phthalocyanines (CuPcs) have attracted growing interest as promising hole‐transporting materials (HTMs) in perovskite solar cells (PSCs) due to their low‐cost and excellent stability. However, the most efficient PSCs using CuPc‐based HTMs reported thus far still rely on hygroscopic p‐type dopants, which notoriously deteriorate device stability. Herein, two new CuPc derivatives are designed, namely CuPc‐Bu and CuPc‐OBu, by molecular engineering of the non‐peripheral substituents of the Pc rings, and applied as dopant‐free HTMs in PSCs. Remarkably, a small structural change from butyl groups to butoxy groups in the substituents of the Pc rings significantly influences the molecular ordering and effectively improves the hole mobility and solar cell performance. As a consequence, PSCs based on dopant‐free CuPc‐OBu as HTMs deliver an impressive power conversion efficiency (PCE) of up to 17.6% under one sun illumination, which is considerably higher than that of devices with CuPc‐Bu (14.3%). Moreover, PSCs containing dopant‐free CuPc‐OBu HTMs show a markedly improved ambient stability when stored without encapsulation under ambient conditions with a relative humidity of 85% compared to devices containing doped Spiro‐OMeTAD. This work thus provides a fundamental strategy for the future design of cost‐effective and stable HTMs for PSCs and other optoelectronic devices.     

LiTFSI‐Free Spiro‐OMeTAD‐Based Perovskite Solar Cells with Power Conversion Efficiencies Exceeding 19%

To date, the most efficient perovskite solar cells (PSCs) employ an n–i–p device architecture that uses a 2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenyl‐amine)‐9,9′‐spirobifluorene (spiro‐OMeTAD) hole‐transporting material (HTM), which achieves optimum conductivity with the addition of lithium bis(trifluoromethane)sulfonimide (LiTFSI) and air exposure. However, this additive along with its oxidation process leads to poor reproducibility and is detrimental to stability. Herein, a dicationic salt spiro‐OMeTAD(TFSI)₂, is employed as an effective p‐dopant to achieve power conversion efficiencies of 19.3% and 18.3% (apertures of 0.16 and 1.00 cm²) with excellent reproducibility in the absence of LiTFSI and air exposure. As far as it is known, these are the highest‐performing n–i–p PSCs without LiTFSI or air exposure. Comprehensive analysis demonstrates that precise control of the proportion of [spiro‐OMeTAD]⁺ directly provides high conductivity in HTM films with low series resistance, fast hole extraction, and lower interfacial charge recombination. Moreover, the spiro‐OMeTAD(TFSI)₂‐doped devices show improved stability, benefitting from well‐retained HTM morphology without forming aggregates or voids when tested under an ambient atmosphere. A facile approach is presented to fabricate highly efficient PSCs by replacing LiTFSI with spiro‐OMeTAD(TFSI)₂. Furthermore, this study provides an insight into the relationship between device performance and the HTM doping level.     

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.     

Molecularly Engineered Phthalocyanines as Hole‐Transporting Materials in Perovskite Solar Cells Reaching Power Conversion Efficiency of 17.5%

Easily accessible tetra‐5‐hexylthiophene‐, tetra‐5‐hexyl‐2,2′‐bisthiophene‐substituted zinc phthalocyanines (ZnPcs) and tetra‐tert ‐butyl ZnPc are employed as hole‐transporting materials in mixed‐ion perovskite [HC(NH₂)₂]_0.85(CH₃NH₃)_0.15Pb(I_0.85Br_0.15)₃ solar cells, reaching the highest power conversion efficiency (PCE) so far for phthalocyanines. Results confirm that the photovoltaic performance is strongly influenced by both, the individual optoelectronic properties of ZnPcs and the aggregation of these tetrapyrrolic semiconductors in the solid thin film. The optimized devices exhibit PCE of 15.5% when using tetra‐5‐hexyl‐2,2′‐bisthiophene substituted ZnPcs, 13.3% for tetra‐tert ‐butyl ZnPc, and a record 17.5% for tetra‐5‐hexylthiophene‐based analogue under standard global 100 mW cm^?2 AM 1.5G illumination. These results boost up the potential of solution‐processed ZnPc derivatives as stable and economic hole‐transport materials for large‐scale applications, opening new frontiers toward a realistic, efficient, and inexpensive energy production.     

Fusing Nanowires into Thin Films: Fabrication of Graded‐Heterojunction Perovskite Solar Cells with Enhanced Performance

Perovskite solar cells (PSCs) have recently experienced a rapid rise in power conversion efficiency (PCE), but the prevailing PSCs with conventional mesoscopic or planar device architectures still contain nonideal perovskite/hole‐transporting‐layer (HTL) interfaces, limiting further enhancement in PCE and device stability. In this work, CsPbBr₃ perovskite nanowires are employed for modifying the surface electronic states of bulk perovskite thin films, forming compositionally‐graded heterojunction at the perovskite/HTL interface of PSCs. The nanowire morphology is found to be key to achieving lateral homogeneity in the perovskite film surface states resulting in a near‐ideal graded heterojunction. The hidden role of such lateral homogeneity on the performance of graded‐heterojunction PSCs is revealed for the first time. The resulting PSCs show high PCE up to 21.4%, as well as high operational stability, which is superior to control PSCs fabricated without CsPbBr₃‐nanocrystals modification and with CsPbBr₃‐nanocubes modification. This study demonstrates the promise of controlled hybridization of perovskite nanowires and bulk thin films for more efficient and stable PSCs.     

High Electron Affinity Enables Fast Hole Extraction for Efficient Flexible Inverted Perovskite Solar Cells

Inverted perovskite solar cells (PSCs) with low‐temperature processed hole transporting materials (HTMs) suffer from poor performance due to the inferior hole‐extraction capability at the HTM/perovskite interfaces. Here, molecules with controlled electron affinity enable a HTM with conductivity improved by more than ten times and a decreased energy gap between the Fermi level and the valence band from 0.60 to 0.24 eV, leading to the enhancement of hole‐extraction capacity by five times. As a result, the 3,6‐difluoro‐2,5,7,7,8,8‐hexacyanoquinodimethane molecules are used for the first time enhancing open‐circuit voltage (V_oc) and fill factor (FF) of the PSCs, which enable rigid‐and flexible‐based inverted perovskite devices achieving highest power conversion efficiencies of 22.13% and 20.01%, respectively. This new method significantly enhances the V_oc and FF of the PSCs, which can be widely combined with HTMs based on not only NiO_x but also PTAA, PEDOTT:PSS, and CuSCN, providing a new way of realizing efficient inverted PSCs.     

Development of Dopant‐Free Organic Hole Transporting Materials for Perovskite Solar Cells

There has been considerable progress over the last decade in development of the perovskite solar cells (PSCs), with reported performances now surpassing 25.2% power conversion efficiency. Both long‐term stability and component costs of PSCs remain to be addressed by the research community, using hole transporting materials (HTMs) such as 2,2′,7,7′‐tetrakis(N,N′‐di‐pmethoxyphenylamino)‐9,9′‐spirbiuorene(Spiro‐OMeTAD) and poly[bis(4‐phenyl)(2,4,6‐trimethylphenyl)amine] (PTAA). HTMs are essential for high‐performance PSC devices. Although effective, these materials require a relatively high degree of doping with additives to improve charge mobility and interlayer/substrate compatibility, introducing doping‐induced stability issues with these HTMs, and further, additional costs and experimental complexity associated with using these doped materials. This article reviews dopant‐free organic HTMs for PSCs, outlining reports of structures with promising properties toward achieving low‐cost, effective, and scalable materials for devices with long‐term stability. It summarizes recent literature reports on non‐doped, alternative, and more stable HTMs used in PSCs as essential components for high‐efficiency cells, categorizing HTMs as reported for different PSC architectures in addition to use of dopant‐free small molecular and polymeric HTMs. Finally, an outlook and critical assessment of dopant‐free organic HTMs toward commercial application and insight into the development of stable PSC devices is provided.     

Low‐Cost N,N′‐Bicarbazole‐Based Dopant‐Free Hole‐Transporting Materials for Large‐Area Perovskite Solar Cells

Despite the recent unprecedented development of efficient dopant‐free hole transporting materials (HTMs) for high‐performance perovskite solar cells (PSCs) on small‐area devices (≤0.1 cm²), low‐cost dopant‐free HTMs for large‐area PSCs (≥1 cm²) with high power conversion efficiencies (PCEs) have rarely been reported. Herein, two novel HTMs, 3,3′,6,6′ (or 2,2′,7,7′)‐tetrakis(N,N′‐di‐p‐methoxyphenylamine)‐N,N′‐bicarbazole (3,6 BCz‐OMeTAD or 2,7 BCz‐OMeTAD), are synthesized via an extremely simple route from very cheap raw materials. Owing to their excellent film‐forming abilities and matching energy levels, 3,6 BCz‐OMeTAD and 2,7 BCz‐OMeTAD can be successfully employed as a perfect ultrathin (≈30 nm) hole transporting layer in large‐area PSCs up to 1 cm². The 3,6 BCz‐OMeTAD and 2,7 BCz‐OMeTAD based large‐area PSCs show highest PCEs up to 17.0% and 17.6%, respectively. More importantly, high performance large‐area PSCs based on 2,7 BCz‐OMeTAD retain 90% of the initial efficiency after 2000 h storage in an ambient environment without encapsulation.     

High‐Performance and Stable Perovskite Solar Cells Based on Dopant‐Free Arylamine‐Substituted Copper(II) Phthalocyanine Hole‐Transporting Materials

A power conversion efficiency (PCE) as high as 19.7% is achieved using a novel, low‐cost, dopant‐free hole transport material (HTM) in mixed‐ion solution‐processed perovskite solar cells (PSCs). Following a rational molecular design strategy, arylamine‐substituted copper(II) phthalocyanine (CuPc) derivatives are selected as HTMs, reaching the highest PCE ever reported for PSCs employing dopant‐free HTMs. The intrinsic thermal and chemical properties of dopant‐free CuPcs result in PSCs with a long‐term stability outperforming that of the benchmark doped 2,2′,7,7′‐Tetrakis‐(N,N‐di‐p‐methoxyphenylamine)‐9,9′‐Spirobifluorene (Spiro‐OMeTAD)‐based devices. The combination of molecular modeling, synthesis, and full experimental characterization sheds light on the nanostructure and molecular aggregation of arylamine‐substituted CuPc compounds, providing a link between molecular structure and device properties. These results reveal the potential of engineering CuPc derivatives as dopant‐free HTMs to fabricate cost‐effective and highly efficient PSCs with long‐term stability, and pave the way to their commercial‐scale manufacturing. More generally, this case demonstrates how an integrated approach based on rational design and computational modeling can guide and anticipate the synthesis of new classes of materials to achieve specific functions in complex device structures.     

Impact of the Doping Method on Conductivity and Thermopower in Semiconducting Polythiophenes

The development of organic semiconductors for use in thermoelectrics requires the optimization of both their thermopower and electrical conductivity. Here two fundamentally different doping mechanisms are used to investigate the thermoelectric properties of known high hole mobility polymers: poly 3‐hexylthiophene (P3HT), poly(2,5‐bis(3‐tetradecylthiophen‐2‐yl)thieno[3,2‐b]thiophene) (PBTTT‐C₁₄), and poly(2,5‐bis(thiphen‐2‐yl)‐(3,7‐diheptadecantyltetrathienoacene)) (P2TDC₁₇‐FT4). The small molecule tetrafluorotetracyanoquinodimethane (F₄TCNQ) is known to effectively dope these polymers, and the thermoelectric properties are studied as a function of the ratio of dopant to polymer repeat unit. Higher electrical conductivity and values of the thermoelectric power factor are achieved by doping with vapor‐deposited fluoroalkyl trichlorosilanes. The combination of these data reveals a striking relationship between thermopower and conductivity in thiophene‐based polymers over a large range of electrical conductivity that is independent of the means of electrical doping. This relationship is not predicted by commonly used transport models for semiconducting polymers and is demonstrated to hold for other semiconducting polymers as well.     

Enhanced Photovoltaic Performance of Amorphous Donor–Acceptor Copolymers Based on Fluorine‐Substituted Benzodioxocyclohexene‐Annelated Thiophene

Donor–acceptor (D‐A) type π‐conjugated copolymers with crystalline behavior have been extensively investigated as donor semiconductors in organic photovoltaics (OPVs). On the other hand, the development of high‐performance amorphous donor materials is still behind. The amorphous donor copolymer DTS‐C₀(F₂) consisting of dithieno[3,2‐b:2′,3′‐d]silole ( DTS ) donor unit and the recently developed fluorine‐substituted naphtho[2,3‐c]thiophene‐4,9‐dione ( C₀(F₂) ) acceptor unit shows moderate photovoltaic performance upon blending with PC₇₁BM. In this work, to enhance the hole‐transporting characteristics, a 3‐hexylthiophene ( HT ) spacer unit is integrated into the conjugated backbone, resulting in a new amorphous copolymer DTS‐HT‐C₀(F₂) . The strong electron‐accepting nature of C₀(F₂) allows the introduction of the HT spacer without affecting the frontier orbital energies and thus the D‐A character. Without using solvent additives and thermal annealing, OPVs based on DTS‐HT‐C₀(F₂) and [6,6]‐phenyl‐C71‐butyric acid methyl ester PC₇₁BM show an improved power conversion efficiency of 9.12%. Investigation of the device physics unambiguously reveals that the hole mobility of the copolymer in the blend is increased by an order of magnitude by the introduction of HT , while keeping an amorphous film nature, leading to higher short‐circuit current density and fill factor. These results demonstrate the realization of high‐performance OPVs based on amorphous active layers.     

Dopant-free Starlike Molecular Hole Conductor with Ordered Packing for Durable all-Inorganic Perovskite Solar Cells

All-inorganic n-i-p perovskite solar cells (PSCs) using doped Spiro-OMeTAD as hole transport material (HTM) suffer from photothermal stability due to ionic diffusion and radical-induced degradation by the dopants. In this article, dopant-free starlike molecule (N2, N2-bis(4-(bis(4-methoxyphenyl)amino)phenyl)-N5,N5-bis(4-methoxyphenyl)pyridine-2,5-diamine (BD)) is synthesized to engineer the stacking properties and delivered higher hole mobility than doped Spiro-OMeTAD (3.2 × 10⁻⁴ versus 1.76 × 10⁻⁴ cm² V⁻¹ s⁻¹) as dopant-free HTM. Starlike BD HTM has a twisted acceptor unit and strong dipole, forming crystalline and ordered packing film to ensure intramolecular charge transfer and improve mobility. The BD CsPbI₃ PSCs deliver the maximum efficiency of 19.19%, which is the highest performance for all-inorganic PSCs based on dopant-free HTMs. Meanwhile, the ordered molecules-packing blocks the migration channel of I⁻ ions to metal electrodes and improves the device stability. BD-based devices maintain more than 93% and 80% of the initial efficiency after 85 °C storage for 35 days and maximum power point (MPP) tracking at 85 °C for 1000 h, respectively.     

Solution Processable Inorganic–Organic Double‐Layered Hole Transport Layer for Highly Stable Planar Perovskite Solar Cells

Perovskite solar cells (PSCs) have reached their highest efficiency with 2,2′,7,7′‐tetrakis(N,N′‐di‐p‐methoxyphenylamine)‐9,9′‐spirobifluorene (spiro‐OMeTAD). However, this material can cause problems with respect to reproducibility and stability. Herein, a solution‐processable inorganic–organic double layer based on tungsten oxide (WO₃) and spiro‐OMeTAD is reported as a hole transport layer in PSCs. The device equipped with a WO₃/spiro‐OMeTAD layer achieves the highest efficiency (21.44%) in the tin (IV) oxide planar structure. The electronic properties of the double layer are thoroughly analyzed using photoluminescence, space‐charge–limited current, and electrochemical impedance spectroscopy. The WO₃/spiro‐OMeTAD layer exhibits better hole extraction ability and faster hole mobility. The WO₃ layer particularly improves the open‐circuit voltage (V_OC) by lowering the quasi‐Fermi energy level for holes and reducing charge recombination, resulting in high V_OC (1.17 V in the champion cell). In addition, the WO₃ layer as a scaffold layer promotes the formation of a uniform and pinhole‐free spiro‐OMeTAD overlayer in the WO₃/spiro‐OMeTAD layer. High stability under thermal and humid conditions stems from this property. The study presents a facile approach for improving the efficiency and stability of PSCs by stacking an organic layer on an inorganic layer.     

Molecular Engineering Using an Anthanthrone Dye for Low‐Cost Hole Transport Materials: A Strategy for Dopant‐Free,High‐Efficiency,and Stable Perovskite Solar Cells

In this report, highly efficient and humidity‐resistant perovskite solar cells (PSCs) using two new small molecule hole transporting materials (HTM) made from a cost‐effective precursor anthanthrone (ANT) dye, namely, 4,10‐bis(1,2‐dihydroacenaphthylen‐5‐yl)‐6,12‐bis(octyloxy)‐6,12‐dihydronaphtho[7,8,1,2,3‐nopqr]tetraphene (ACE‐ANT‐ACE) and 4,4′‐(6,12‐bis(octyloxy)‐6,12‐dihydronaphtho[7,8,1,2,3‐nopqr]tetraphene‐4,10‐diyl)bis(N,N‐bis(4‐methoxyphenyl)aniline) (TPA‐ANT‐TPA) are presented. The newly developed HTMs are systematically compared with the conventional 2,2′,7,7′‐tetrakis(N,N′‐di‐p‐methoxyphenylamino)‐9,9′‐spirbiuorene (Spiro‐OMeTAD). ACE‐ANT‐ACE and TPA‐ANT‐TPA are used as a dopant‐free HTM in mesoscopic TiO₂/CH₃NH₃PbI₃/HTM solid‐state PSCs, and the performance as well as stability are compared with Spiro‐OMeTAD‐based PSCs. After extensive optimization of the metal oxide scaffold and device processing conditions, dopant‐free novel TPA‐ANT‐TPA HTM‐based PSC devices achieve a maximum power conversion efficiency (PCE) of 17.5% with negligible hysteresis. An impressive current of 21 mA cm^?2 is also confirmed from photocurrent density with a higher fill factor of 0.79. The obtained PCE of 17.5% utilizing TPA‐ANT‐TPA is higher performance than the devices prepared using doped Spiro‐OMeTAD (16.8%) as hole transport layer at 1 sun condition. It is found that doping of LiTFSI salt increases hygroscopic characteristics in Spiro‐OMeTAD; this leads to the fast degradation of solar cells. While, solar cells prepared using undoped TPA‐ANT‐TPA show dewetting and improved stability. Additionally, the new HTMs form a fully homogeneous and completely covering thin film on the surface of the active light absorbing perovskite layers that acts as a protective coating for underlying perovskite films. This breakthrough paves the way for development of new inexpensive, more stable, and highly efficient ANT core based lower cost HTMs for cost‐effective, conventional, and printable PSCs.     

Extrinsic Movable Ions in MAPbI3 Modulate Energy Band Alignment in Perovskite Solar Cells

Ionic movement is considered awful in perovskite solar cells (PSCs) for relating with the hysteresis and long‐term instability. However, the positive role of ions to enhance the energy band bending for high performance PSC is always overlooked, let alone reducing the hysteresis. In this work, LiI is doped in CH₃NH₃PbI₃. It is observed that the aggregation of Li⁺/I^? tunes the energy level of the perovskite and induces n/p doping in CH₃NH₃PbI₃, which makes charge extraction quite efficient from perovskite to both NiO and [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) layer. Therefore, in NiO/LiI doped perovskite/PCBM solar cells, Li⁺ and I^? modulate the interface energy band alignment to facilitate the electron/hole transport and reduce the interface energy loss. On the other hand, n/p doping enlarges Fermi energy level splitting of the PSCs to improve the photovoltage. The performance of LiI doped PSCs is much higher with reduced hysteresis compared to the undoped solar cells. This work highlights the positive effect of selective ionic doping, which is promisingly important to design the stable and efficient PSCs.     

Optimizations of Pulsed Plated p and n‐type Bi2Te3‐Based Ternary Compounds by Annealing in Different Ambient Atmospheres

This work presents a comprehensive study of the fabrication and optimization of electrodeposited p‐ and n‐type thermoelectric films. The films are deposited on Au and stainless steel substrates over a wide range of deposition potentials. The influence of the preparative parameters such as the composition of the electrolyte bath and the deposition potential are investigated. Furthermore, the p‐doped (Bi_xSb_1‐x)₂Te₃ and the n‐doped Bi₂(Te_xSe_1‐x)₃ films are annealed for a period of about 1 h under helium and under tellurium atmosphere at 250 °C for 60h. Annealing in He already leads to significant improvements in the thermoelectric performance. Furthermore, due to the equilibrium conditions during the process, annealing in Te atmosphere leads to a strongly improved film composition, charge carrier density and mobility. The Seebeck coefficients increase to values up to +182 μV K^?1 for p‐doped and–130 μV K^?1 for n‐doped materials at room temperature. The power factors also exhibit improvements with 1320 μW m^?1 K^?2 and 820 μW m^?1 K^?2 for p‐doped and n‐doped films, respectively. Additionally, in‐situ XRD measurements performed during annealing of the films up to 600K under He atmosphere show stepwise improvements of the crystal structure leading to the improvements in thermoelectric parameters. The thermal conductivity is between 1.2 W m^?1 K^?1 and 1.0 W m^?1 K^?1.     

Room‐Temperature Vapor Deposition of Cobalt Nitride Nanofilms for Mesoscopic and Perovskite Solar Cells

Organic/inorganic hybrid solar cells, typically mesoscopic and perovskite solar cells, are regarded as promising candidates to replace conventional silicon or thin film photovoltaics. There have been intensive investigations on the development of advanced materials for improved power conversion efficiencies, however, economical feasibilities and reliabilities of the organic/inorganic photovoltaics are yet to reach at a sufficient level for practical utilizations. In this study, cobalt nitride (CoN) nanofilms prepared by room‐temperature vapor deposition in an inert N₂ atmosphere, which is a facile and highly reproducible procedure, are proposed as a low‐cost counter electrode in mesoscopic dye‐sensitized solar cells (DSCs) and a hole transport material in inverted planar perovskite solar cells (PSCs) for the first time. The CoN film successfully replaces conventional Pt in DSCs, resulting in a power conversion efficiency comparable to the ones based on Pt. In addition, PSCs employing the CoN manifest high efficiency even up to 15.0%, which is comparable to state‐of‐the‐art performance in the cases of PSCs employing inorganic hole transporters. Furthermore, flexible solar cell applications of the CoN are performed in both mesoscopic and perovskite solar cells, verifying the advantages of the room‐temperature deposition process and feasibilities of the CoN nanofilms in various fields.     

Outstanding Performance of Hole‐Blocking Layer‐Free Perovskite Solar Cell Using Hierarchically Porous Fluorine‐Doped Tin Oxide Substrate

Perovskite solar cells (PSCs) are of great interest in current photovoltaic research due to their extraordinary power conversion efficiency of ≈20% and boundless potentialities. The high efficiency has been mostly obtained from TiO₂‐based PSCs, where TiO₂ is utilized as a hole‐blocking, mesoporous layer. However, trapped charges and the light‐induced photocatalytic effect of TiO₂ seriously degrade the perovskite and preclude PSCs from being immediately commercialized. Herein, a simplified PSC is successfully fabricated by eliminating the problematic TiO₂ layers, using instead a fluorine‐doped tin oxide (FTO)/perovskite/hole–conductor/Au design. Simultaneously, the sluggish charge extraction at the FTO/perovskite interface is overcome by modifying the surface of the FTO to a porous structure using electrochemical etching. This surface engineering enables a substantial increase in the photocurrent density and mitigation of the hysteretic behavior of the pristine FTO‐based PSC; a remarkable 19.22% efficiency with a low level of hysteresis is obtained. This performance is closely approaching that of conventional PSCs and may facilitate their commercialization due to improved convenience, lower cost, greater stability, and potentially more efficient mass production.