An Ultrathin Ferroelectric Perovskite Oxide Layer for High‐Performance Hole Transport Material Free Carbon Based Halide Perovskite Solar Cells

The hole transport material (HTM) free carbon based perovskite solar cells (C‐PSCs) are promising for its manufactural simplicity, but they currently suffer from low power conversion efficiencies (PCE) largely because of the voltage loss. Here, a new strategy to increase the PCE by incorporating an ultrathin ferroelectric oxide PbTiO₃ layer between the electron transport material and the halide perovskite is reported. The resulting C‐PSCs have achieved PCEs up to 16.37%, which is the highest record for HTM‐free C‐PSCs to date, mainly ascribable to the ferroelectric layer enhanced open circuit voltage. Detail measurements and analysis show an enhanced built‐in potential in the C‐PSCs as well as suppression of the non‐radiative recombination due to the ferroelectric PbTiO₃ layer incorporation, accounting for the boosted V_OC and photovoltaic performance.     

Multifunctional Synthesis Approach of In:CuCrO2 Nanoparticles for Hole Transport Layer in High‐Performance Perovskite Solar Cells

While there are very limited studies of doped ternary metal oxide based hole transport materials, a multifunctional synthesis approach of In doped CuCrO₂ nanoparticles (NPs) as efficient hole transport layers (HTLs) including simplifying the synthesis requirements is proposed, enabling doping and achievement of treatment‐free HTLs. Remarkably, compared with conventional methods for synthesizing CuCrO₂ NPs, the newly proposed azeotropic promoted approach dramatically reduces the reaction time by 90% and the calcination temperature by one‐third, which not only promotes high throughput production but also reduces power consumption and cost in synthesis. Equally important, indium is successfully doped into CuCrO₂, which is fundamentally difficult in low temperature processes. The In doping offers less d–d transition of Cr³⁺ and p‐type doping characteristics for improving HTL transmittance and conductivity, respectively. Interestingly, In doped CuCrO₂ HTL with these improvements can be achieved by a simple ambient‐condition process and exhibits thermal stability up to 200 °C, which allows perovskite solar cells (PSCs) to achieve a power conversion efficiency of 20.54%. Meanwhile, the devices show good repeatability and photostability. Consequently, the work contributes to establishing a simple approach to realize pristine and doped multinary oxides based HTL for the development of practical and high performing PSCs.     

P3HT为空穴传输层的碳基钙钛矿太阳电池

碳电极具有成本低、印刷方便、可有效隔离水氧等优点,因此有望利用碳电极材料实现低成本、高稳定性的钙钛矿太阳电池。无空穴传输层的传统碳基钙钛矿太阳电池面临着空穴提取率低、电子逆向传输,钙钛矿和碳电极界面的载流子复合等问题。文章引入聚(3-己基噻吩)(P3HT)作为器件的空穴传输层,使碳基钙钛矿太阳电池ITO/SnO₂/MAPbI₃/P3HT/Carbon的光伏性能得到了显著改善:器件的光电转化效率从11.16% 提高到13.37%。在氮气环境下,连续光照1000h,太阳电池的光电转化效率可保持初始值的87%,而传统器件在光照500h后,其光电转化效率已下降至初始值的60%。     

Conjugated Small Molecule for Efficient Hole Transport in High‐Performance p‐i‐n Type Perovskite Solar Cells

The π‐conjugated organic small molecule 4,4′‐cyclohexylidenebis[N,N‐bis(4‐methylphenyl) benzenamine] (TAPC) has been explored as an efficient hole transport material to replace poly(3,4‐ethylenedio‐xythiophene):poly(styrenesulfonate) (PEDOT:PSS) in the preparation of p‐i‐n type CH₃NH₃PbI₃ perovskite solar cells. Smooth, uniform, and hydrophobic TAPC hole transport layers can be facilely deposited through solution casting without the need for any dopants. The power conversion efficiency of perovskite solar cells shows very weak TAPC layer thickness dependence across the range from 5 to 90 nm. Thermal annealing enables improved hole conductivity and efficient charge transport through an increase in TAPC crystallinity. The perovskite photoactive layer cast onto thermally annealed TAPC displays large grains and low residual PbI₂, leading to a high charge recombination resistance. After optimization, a stabilized power conversion efficiency of 18.80% is achieved with marginal hysteresis, much higher than the value of 12.90% achieved using PEDOT:PSS. The TAPC‐based devices also demonstrate superior stability compared with the PEDOT:PSS‐based devices when stored in ambient circumstances, with a relatively high humidity ranging from 50 to 85%.     

Improving Performance and Stability of Flexible Planar‐Heterojunction Perovskite Solar Cells Using Polymeric Hole‐Transport Material

For realizing flexible perovskite solar cells (PSCs), it is important to develop low‐temperature processable interlayer materials with excellent charge transporting properties. Herein, a novel polymeric hole‐transport material based on 1,4‐bis(4‐sulfonatobutoxy)benzene and thiophene moieties (PhNa‐1T) and its application as a hole‐transport layer (HTL) material of high‐performance inverted‐type flexible PSCs are introduced. Compared with the conventionally used poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), the incorporation of PhNa‐1T into HTL of the PSC device is demonstrated to be more effective for improving charge extraction from the perovskite absorber to the HTL and suppressing charge recombination in the bulk perovskite and HTL/perovskite interface. As a result, the flexible PSC using PhNa‐1T achieves high photovoltaic performances with an impressive power conversion efficiency of 14.7%. This is, to the best of our knowledge, among the highest performances reported to date for inverted‐type flexible PSCs. Moreover, the PhNa‐1T‐based flexible PSC shows much improved stability under an ambient condition than PEDOT:PSS‐based PSC. It is believed that PhNa‐1T is a promising candidate as an HTL material for high‐performance flexible PSCs.     

High‐Performance Organic Solar Cells with Spray‐Coated Hole‐Transport and Active Layers

In this study, we report high performance organic solar cells with spray coated hole‐transport and active layers. With optimized ink formulations we are able to deposit films with controlled thickness and very low surface roughness (<10 nm). Specifically we deposit smooth and uniform 40 nm thick films of poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as well as films composed of a mixture of poly(3‐hexyl thiophene) (P3HT) and the C₆₀‐derivative (6,6)‐phenyl C61‐butyric acid methyl ester (PCBM) with thicknesses in the range 200–250 nm. To control film morphology, formation and thickness, the optimized inks incorporate two solvent systems in order to take advantage of surface tension gradients to create Marangoni flows that enhance the coverage of the substrate and reduce the roughness of the film. Notably, we achieve fill factors above 70% and attribute the improvement to an enhanced P3HT crystallization, which upon optimized post‐drying thermal annealing results in a favorable morphology. As a result, we could extend the thickness of the layer to several hundreds of nanometers without noticing a substantial decrease of the transport properties of the layer. By proper understanding of the spreading and drying dynamics of the inks we achieve spray coated devices with power conversion efficiency of 3.75%, with fill factor, short circuit current and open circuit voltage of 70%, 9.8 mA cm^?2 and 550 mV, respectively.     

Dithieno[3,2‐b:2′,3′‐d]pyrrol‐Cored Hole Transport Material Enabling Over 21% Efficiency Dopant‐Free Perovskite Solar Cells

Dopant‐free hole transport materials (HTMs) are essential for commercialization of perovskite solar cells (PSCs). However, power conversion efficiencies (PCEs) of the state‐of‐the‐art PSCs with small molecule dopant‐free HTMs are below 20%. Herein, a simple dithieno[3,2‐b:2′,3′‐d]pyrrol‐cored small molecule, DTP‐C6Th, is reported as a promising dopant‐free HTM. Compared with commonly used spiro‐OMeTAD, DTP‐C6Th exhibits a similar energy level, a better hole mobility of 4.18 × 10^?4 cm² V^?1 s^?1, and more efficient hole extraction, enabling efficient and stable PSCs with a dopant‐free HTM. With the addition of an ultrathin poly(methyl methacrylate) passivation layer and properly tuning the composition of the perovskite absorber layer, a champion PCE of 21.04% is achieved, which is the highest value for small molecule dopant‐free HTM based PSCs to date. Additionally, PSCs using the DTP‐C6Th HTM exhibit significantly improved long‐term stability compared with the conventional cells with the metal additive doped spiro‐OMeTAD HTM. Therefore, this work provides a new candidate and effective device engineering strategy for achieving high PCEs with dopant‐free HTMs.     

Dopant-Free Hole Transport Materials Based on a Large Conjugated Electron-Deficient Core for Efficient Perovskite Solar Cells

Hole transport materials (HTMs) play a significant role in device efficiencies and long-term stabilities of perovskite solar cells (PSCs). In this work, two simple dopant-free HTMs are designed with a large conjugated electron-deficient core. On the one hand, a large coplanar backbone endows enhanced π–π stacking and reduced hole hopping distance. On the other hand, the incorporation of electron-deficient unit can easily tune the energy levels as well as increase hole mobilities. Combining these two advantages together, 12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro[1,2,5]thiadiazole[3,4-e]thieno[2″,3″:4,5]thieno[2′,3′:4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5]thieno[3,2-b]indole is chosen as the large electron-deficient core to construct two novel dopant-free HTMs, Y6-T and Y-T. Both Y6-T and Y-T behave suitable highest occupied molecular orbital levels, good hole mobilities, as well as strong hydrophobicities. After careful device optimization with a passivation agent, Y-T delivers an impressive power conversion efficiency of 20.29%, which is higher than that of Y6-T (18.82%) and doped spiro-OMeTAD (19.24%). Moreover, PSCs based on Y6-T and Y-T show much better long-term stabilities than spiro-OMeTAD due to the intrinsic hydrophobicity. Therefore, this work provides a promising candidate as well as a useful design strategy for exploring dopant-free HTMs, which may pave the way for the commercialization of PSCs.     

Pyrite‐Based Bi‐Functional Layer for Long‐Term Stability and High‐Performance of Organo‐Lead Halide Perovskite Solar Cells

Organo‐lead halide perovskite solar cells (PSCs) have received great attention because of their optimized optical and electrical properties for solar cell applications. Recently, a dramatic increase in the photovoltaic performance of PSCs with organic hole transport materials (HTMs) has been reported. However, as of now, future commercialization can be hampered because the stability of PSCs with organic HTM has not been guaranteed for long periods under conventional working conditions, including moist conditions. Furthermore, conventional organic HTMs are normally expensive because material synthesis and purification are complicated. It is herein reported, for the first time, octadecylamine‐capped pyrite nanoparticles (ODA‐FeS₂ NPs) as a bi‐functional layer (charge extraction layer and moisture‐proof layer) for organo‐lead halide PSCs. FeS₂ is a promising candidate for the HTM of PSCs because of its high conductivity and suitable energy levels for hole extraction. A bi‐functional layer based on ODA‐FeS₂ NPs shows excellent hole transport ability and moisture‐proof performance. Through this approach, the best‐performing device with ODA‐FeS₂ NPs‐based bi‐functional layer shows a power conversion efficiency of 12.6% and maintains stable photovoltaic performance in 50% relative humidity for 1000 h. As a result, this study has the potential to break through the barriers for the commercialization of PSCs.     

Controllable Perovskite Crystallization by Water Additive for High‐Performance Solar Cells

A key issue for perovskite solar cells is the stability of perovskite materials due to moisture effects under ambient conditions, although their efficiency is improved constantly. Herein, an improved CH₃NH₃PbI_3?xCl_x perovskite quality is demonstrated with good crystallization and stability by using water as an additive during crystal perovskite growth. Incorporating suitable water additives in N,N‐dimethylformamide (DMF) leads to controllable growth of perovskites due to the lower boiling point and the higher vapor pressure of water compared with DMF. In addition, CH₃NH₃PbI_3?xCl_x · nH₂O hydrated perovskites, which can be resistant to the corrosion by water molecules to some extent, are assumed to be generated during the annealing process. Accordingly, water additive based perovskite solar cells present a high power conversion efficiency of 16.06% and improved cell stability under ambient conditions compared with the references. The findings in this work provide a route to control the growth of crystal perovskites and a clue to improve the stability of organic–inorganic halide perovskites.     

A Band‐Edge Potential Gradient Heterostructure to Enhance Electron Extraction Efficiency of the Electron Transport Layer in High‐Performance Perovskite Solar Cells

As the key component in efficient perovskite solar cells, the electron transport layer (ETL) can selectively collect photogenerated charge carriers produced in perovskite absorbers and prevent the recombination of carriers at interfaces, thus ensuring a high power conversion efficiency. Compared with the conventional single‐ or dual‐layered ETLs, a gradient heterojunction (GHJ) strategy is more attractive to facilitate charge separation because the potential gradient created at an appropriately structured heterojunction can act as a driving force to regulate the electron transport toward a desired direction. Here, a SnO₂/TiO₂ GHJ interlayer configuration inside the ETL is reported to simultaneously achieve effective extraction and efficient transport of photoelectrons. With such an interlayer configuration, the GHJs formed at the perovskite/ETL interface act collectively to extract photogenerated electrons from the perovskite layer, while GHJs formed at the boundaries of the interconnected SnO₂ and TiO₂ networks throughout the entire ETL layer can extract electron from the slow electron mobility TiO₂ network to the high electron mobility SnO₂ network. Devices based on GHJ ETL exhibit a champion power conversion efficiency of 18.08%, which is significantly higher than that obtained from the compact TiO₂ ETL constructed under the comparable conditions.     

Polyfluorene Derivatives are High‐Performance Organic Hole‐Transporting Materials for Inorganic−Organic Hybrid Perovskite Solar Cells

Photovoltaics based on organic?inorganic perovskites offer new promise to address the contemporary energy and environmental issues. These solar cells have so far largely relied on small‐molecule hole transport materials such as spiro‐OMeTAD, which commonly suffer from high cost and low mobility. In principle, polyfluorene copolymers can be an ideal alternative to spiro‐OMeTAD, given their low price, high hole mobility and good processability, but this potential has not been explored. Herein, polyfluorene derived polymers‐TFB and PFB, which contain fluorine and arylamine groups, are demonstrated and can indeed rival or even outperform spiro‐OMeTAD as efficient hole‐conducting materials for perovskite solar cells. In particular, under the one‐step perovskite deposition condition, TFB achieves a 10.92% power conversion efficiency that is considerably higher than that with spiro‐OMeTAD (9.78%), while using the two‐step perovskite deposition method, about 13% efficient solar cells with TFB (12.80%) and spiro‐OMeTAD (13.58%) are delivered. Photo­luminescence reveals the efficient hole extraction and diffusion at the interface between CH₃NH₃PbI₃ and the hole conducting polymer. Impedance spectroscopy uncovers the higher electrical conductivity and lower series resistance than spiro‐OMeTAD, accounting for the significantly higher fill factor, photocurrent and open‐circuit voltage of the TFB‐derived cells than with spiro‐MeOTAD.     

Efficient and Stable Mesoscopic Perovskite Solar Cells Using PDTITT as a New Hole Transporting Layer

Stability is the main challenge in the field of organic–inorganic perovskite solar cells (PSCs). Finding low‐cost and stable hole transporting layer (HTL) is an effective strategy to address this issue. Here, a new donor polymer, poly(5,5‐didecyl‐5H‐1,8‐dithia‐as‐indacenone‐alt‐thieno[3,2‐b]thiophene) (PDTITT), is synthesized and employed as an HTL in PSCs, which has a suitable band alignment with respect to the double‐A cation perovskite film. Using PDTITT, the hole extraction in PSCs is greatly improved as compared to commonly used HTLs such as 2,2′,7,7′‐tetrakis[N,N‐di(4‐methoxyphenyl)amino]‐9,9′‐spirobifluorene (spiro‐OMeTAD), addressing the hysteresis issue. After careful optimization, an efficient PSC is achieved based on mesoscopic TiO₂ electron transporting layer with a maximum power conversion efficiency (PCE) of 18.42% based on PDTITT HTL, which is comparable with spiro‐OMeTAD‐based PSC (19.21%). Since spiro‐based PSCs suffer from stability issue, the operational stability in the PSC with PDTITT HTL is studied. It is found that the device with PDTITT retains 88% of its initial PCE value after 200 h under illumination, which is better than the spiro‐based PSC (54%).     

Cooperative Effect of GO and Glucose on PEDOT:PSS for High VOC and Hysteresis‐Free Solution‐Processed Perovskite Solar Cells

Hybrid organic–inorganic halide perovskites have emerged at the forefront of solution‐processable photovoltaic devices. Being the perovskite precursor mixture a complex equilibrium of species, it is very difficult to predict/control their interactions with different substrates, thus the final film properties and device performances. Here the wettability of CH₃NH₃PbI₃ (MAPbI₃) onto poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hole transporting layer is improved by exploiting the cooperative effect of graphene oxide (GO) and glucose inclusion. The glucose, in addition, triggers the reduction of GO, enhancing the conductivity of the PEDOT:PSS+GO+glucose based nanocomposite. The relevance of this approach toward photovoltaic applications is demonstrated by fabricating a hysteresis‐free MAPbI₃ solar cells displaying a ≈37% improvement in power conversion efficiency if compared to a device grown onto pristine PEDOT:PSS. Most importantly, V_OC reaches values over 1.05 V that are among the highest ever reported for PEDOT:PSS p‐i‐n device architecture, suggesting minimal recombination losses, high hole‐selectivity, and reduced trap density at the PEDOT:PSS along with optimized MAPbI₃ coverage.     

Passivated Perovskite Crystallization via g‐C3N4 for High‐Performance Solar Cells

Organometallic halide perovskite films with good surface morphology and large grain size are desirable for obtaining high‐performance photovoltaic devices. However, defects and related trap sites are generated inevitably at grain boundaries and on surfaces of solution‐processed polycrystalline perovskite films. Seeking facial and efficient methods to passivate the perovskite film for minimizing defect density is necessary for further improving the photovoltaic performance. Here, a convenient strategy is developed to improve perovskite crystallization by incorporating a 2D polymeric material of graphitic carbon nitride (g‐C₃N₄) into the perovskite layer. The addition of g‐C₃N₄ results in improved crystalline quality of perovskite film with large grain size by retarding the crystallization rate, and reduced intrinsic defect density by passivating charge recombination centers around the grain boundaries. In addition, g‐C₃N₄ doping increases the film conductivity of perovskite layer, which is beneficial for charge transport in perovskite light‐absorption layer. Consequently, a champion device with a maximum power conversion efficiency of 19.49% is approached owing to a remarkable improvement in fill factor from 0.65 to 0.74. This finding demonstrates a simple method to passivate the perovskite film by controlling the crystallization and reducing the defect density.     

High Efficiency Planar p‐i‐n Perovskite Solar Cells Using Low‐Cost Fluorene‐Based Hole Transporting Material

For commercial applications, it is a challenge to find suitable and low‐cost hole‐transporting material (HTM) in perovskite solar cells (PSCs), where high efficiency spiro‐OMeTAD and PTAA are expensive. A HTM based on 9,9‐dihexyl‐9H‐fluorene and N,N‐di‐p‐methylthiophenylamine (denoted as FMT) is designed and synthesized. High‐yield FMT with a linear structure is synthesized in two steps. The dopant‐free FMT‐based planar p‐i‐n perovskite solar cells (pp‐PSCs) exhibit a high power conversion efficiency (PCE) of 19.06%, which is among the highest PCEs reported for the pp‐PSCs based on organic HTM. For comparison, a PEDOT:PSS HTM‐based pp‐PSC is fabricated under the same conditions, and its PCE is found to be 13.9%.     

Efficient 4,4′,4″‐tris(3‐methylphenylphenylamino)triphenylamine (m‐MTDATA) Hole Transport Layer in Perovskite Solar Cells Enabled by Using the Nonstoichiometric Precursors

To overcome the drawbacks of the acidic and hygroscopic nature of the poly(3,4‐ethylenedio‐xythiophene):poly(styrenesulfonate) hole transport layer (HTL), the p‐type polymeric hole transport materials have attracted great attention and have been applied into perovskite solar cells. Here, a starburst amine molecule 4,4′,4″‐tris(3‐methylphenylphenylamino)triphenylamine (m‐MTDATA) without any additive is demonstrated as an effective hole transport material in perovskite solar cells. Meanwhile, considering the different surface affinity of precursor's composition on m‐MTDATA, the influence of the molar ratios between lead iodine (PbI₂) and methylammonium iodide in precursor solution on the perovskite characteristics and device performance is investigated in‐depth. Ultimately, an enhanced efficiency of 17.73% and a high fill factor of 79.6% are achieved, which attribute to the strong passivation effect of traps and small resistance loss from appropriate unreacted PbI₂ left in the perovskite layer. This work not only provides a remarkable HTL, but also reveals that the adjustment of precursor ratio is necessary for the one‐step solution approach, because the affinities of precursor's composition may be different with the underlying transport layers.     

PEDOT:PSS‐Assisted Exfoliation and Functionalization of 2D Nanosheets for High‐Performance Organic Solar Cells

Here, a facial and scalable method for efficient exfoliation of bulk transition metal dichalcogenides (TMD) and graphite in aqueous solution with poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) to prepare single‐ and few‐layer nanosheets is demonstrated. Importantly, these TMD nanosheets retain the single crystalline characteristic, which is essential for application in organic solar cells (OSCs). The hybrid PEDOT:PSS/WS₂ ink prepared by a simple centrifugation is directly integrated as a hole extraction layer for high‐performance OSCs. Compared with PEDOT:PSS, the PEDOT:PSS/WS₂‐based devices provide a remarkable power conversion efficiency due to the “island” morphology and benzoid–quinoid transition. This study not only demonstrates a novel method for preparing single‐ and few‐layer TMD and graphene nanosheets but also paves a way for their applications without further complicated processing.     

Undoped Hole Transport Layer Toward Efficient and Stable Inorganic Perovskite Solar Cells

Numerous strategies have been practiced to improve the power conversion efficiency of CsPbI₂Br-based perovskite solar cells (PSCs), which definitely makes efficiency gradually approach the theoretical efficiency limit. However, sufficient device stability is still in urgent demand for commercialization, pushing to overcome some instability sources induced by hygroscopicity of spiro-OMeTAD and residual strain of perovskite layer. To address these issues, p-type semiconductor of PCPDTBT is used to replace spiro-OMeTAD, enabling dual functions of hole transport and strain regulation. On the one hand, undoped PCPDTBT performs excellent hole extraction and transport, while avoiding the perovskite degradation caused by the hygroscopicity of common additives. On the other hand, PCPDTBT assisted by a thermally spin-coating method compensates for the thermally-induced residual strain in perovskite layer owing to its high thermal expansion coefficient. Consequently, CsPbI₂Br-based PSCs with PCPDTBT layer achieve improved efficiency of 16.5% as well as enhanced stability. This study provides a simple and facile strategy to achieve efficient and stable CsPbI₂Br-based PSCs.