Chen Weijie

A series of high‐performance perylene diimide based molecular acceptors, namely, TPP‐PDI, TPO‐PDI, and TPS‐PDI, are smartly designed for efficient nonfullerene polymer solar cells. Combined with the optimization of the blend morphology through supramolecular molecular lock effect, the champion power conversion efficiency of 11.01% is realized in TPO‐PDI‐based devices.

Abstract

A series of perylene diimide (PDI) derivatives, TPP‐PDI, TPO‐PDI, and TPS‐PDI, are developed for nonfullerene polymer solar cells (NF‐PSCs) by flaking three PDI skeletons around 3D central cores with different configurations and electronic states, such as triphenylphosphine (TPP), triphenylphosphine monoxide (TPO), and triphenylphosphine sulfide (TPS). These small‐molecule acceptors have a “three‐wing propeller” structure due to their similar backbones. By changing the electron density of phosphorus atoms through oxidation and sulfuration, the “folding‐back” strength is decreased, resulting in a less twisted molecular conformation. The stronger electron‐withdrawing ability of the oxygen atom affords TPO‐PDI the least twisted conformation, which enhances the crystallinity of this complex. NF‐PSCs based on PTTEA:TPO‐PDI exhibit a high power conversion efficiency (PCE) of 8.65%. Ultimately, the joint “molecular lock” effect arising from OH⋅⋅⋅F and OH⋅⋅⋅OP supramolecular interactions is achieved by introducing 4,4′‐biphenol as an additive, which successfully promotes fibril‐like phase separation and blend morphology optimization to generate the highest PCE of 11.01%, which is currently the highest value recorded for NF‐PSCs based on PDI acceptors.

Publication date: 18 December 2019

Source: Joule, Volume 3, Issue 12

Author(s): Kui Jiang, Qingya Wei, Joshua Yuk Lin Lai, Zhengxing Peng, Ha Kyung Kim, Jun Yuan, Long Ye, Harald Ade, Yingping Zou, He Yan

Context & Scale

Summary

Graphical Abstract

Nature Communications, Published online: 09 October 2019; doi:10.1038/s41467-019-12613-8

CuGaO₂ nanoparticles modified with aminosilane are used as a hole‐transporting layer with CuSCN for perovskite solar cells. The enhanced power conversion efficiency and thermal stability compared to the cell using only CuSCN are correlated with the improved carrier extraction and reduced interfacial degradation by the inorganic nanoparticles.

Abstract

Herein, solution‐processible inorganic hole‐transport layer (HTL) of a perovskite solar cell that consists of CuGaO₂ nanoparticles and CuSCN, which leads to an improved device performance as well as long‐term stability, is reported. Uniform films of CuGaO₂ are prepared by first treating CuGaO₂ nanoparticles with aminosilane that leads to well‐dispersed CuGaO₂ solution, followed by spin‐coating of the suspension. Subsequent spin‐coating of CuSCN solution onto the CuGaO₂ forms a smooth HTL with excellent coverage and electrical conductivity. Comparing to the reference device with CuSCN HTL, the CuGaO₂/CuSCN device improves carrier extraction and reduces trap density by ≈40%, as measured by photoluminescence and capacitance analysis. Excellent thermal stability is also demonstrated: ≈80% of the initial efficiency of the perovskite solar cells with the CuGaO₂/CuSCN HTL is retained after 400 h under 85 °C/85% relative humidity environment.

A low bandgap nonfullerene acceptor (NFA) is incorporated into fullerene electron‐transporting layer (ETL) of an inverted perovskite solar cell aiming to intercept the NIR light passing through the device. However, it cannot enhance the device's NIR photoresponse. Further adding a p‐type polymer effectively enhances the device's NIR photoresponse due to better cascade energy‐level alignment and increased hole mobility.

Abstract

How to extend the photoresponse of perovskite solar cells (PVSCs) to the region of near‐infrared (NIR)/infrared light has become an appealing research subject in this field since it can better harness the solar irradiation. Herein, the typical fullerene electron‐transporting layer (ETL) of an inverted PVSC is systematically engineered to enhance device's NIR photoresponse. A low bandgap nonfullerene acceptor (NFA) is incorporated into the fullerene ETL aiming to intercept the NIR light passing through the device. However, despite forming type II charge transfer with fullerene, the blended NFA cannot enhance the device's NIR photoresponse, as limited by the poor dissociation of photoexciton induced by NIR light. Fortunately, it can be addressed by adding a p‐type polymer. The ternary bulk‐heterojunction (BHJ) ETL is demonstrated to effectively enhance the device's NIR photoresponse due to the better cascade‐energy‐level alignment and increased hole mobility. By further optimizing the morphology of such a BHJ ETL, the derived PVSC is finally demonstrated to possess a 40% external quantum efficiency at 800 nm with photoresponse extended to the NIR region (to 950 nm), contributing ≈9% of the overall photocurrent. This study unveils an effective and simple approach for enhancing the NIR photoresponse of inverted PVSCs.

A donor derivative is incorporated in benzodithiophene terthiophene rhodanine (BTR)‐based thick‐film all‐small‐molecule (ASM) organic solar cells (OSC), which achieves power conversion efficiency of 10.14% and fill factor of 74.2%, outperforms its binary counterparts, and stands the record value for thick‐film dual‐donor ternary ASM OSCs. The results demonstrate that the donor derivative is a promising third component to boost the performance of ASM OSCs.

Abstract

Thick‐film all‐small‐molecule (ASM) organic solar cells (OSCs) are preferred for large‐scale fabrication with printing techniques due to the distinct advantages of monodispersion, easy purification, and negligible batch‐to‐batch variation. However, ASM OSCs are typically constrained by the morphology aspect to achieve high efficiency and maintain thick film simultaneously. Specifically, synchronously manipulating crystallinity, domain size, and phase segregation to a suitable level are extremely challenging. Herein, a derivative of benzodithiophene terthiophene rhodanine (BTR) (a successful small molecule donor for thick‐film OSCs), namely, BTR‐OH, is synthesized with similar chemical structure and absorption but less crystallinity relative to BTR, and is employed as a third component to construct BTR:BTR‐OH:PC₇₁BM ternary devices. The power conversion efficiency (PCE) of 10.14% and fill factor (FF) of 74.2% are successfully obtained in ≈300 nm OSC, which outperforms BTR:PC₇₁BM (9.05% and 69.6%) and BTR‐OH:PC₇₁BM (8.00% and 65.3%) counterparts, and stands among the top values for thick‐film ASM OSCs. The performance enhancement results from the enhanced absorption, suppressed bimolecular/trap–assisted recombination, improved charge extraction, optimized domain size, and suitable crystallinity. These findings demonstrate that the donor derivative featuring similar chemical structure but different crystallinity provides a promising third component guideline for high‐performance ternary ASM OSCs.

Mesoporous boron‐doped TiO₂ (B‐TiO₂) is demonstrated as an improved electron transport layer (ETL) for perovskite solar cells for the reduction of hysteresis. The incorporation of boron dopant in TiO₂ ETL not only reduces the hysteresis but also improves device performance. Consequently, a methylammonium lead iodide photovoltaic device based on B‐TiO₂ ETL achieves a promising efficiency of 20.51% with negligible hysteresis.

Abstract

Perovskite solar cells (PSCs) have witnessed astonishing improvement in power conversion efficiency (PCE), more recently, with advances in long‐term stability and scalable fabrication. However, the presence of an anomalous hysteresis behavior in the current density–voltage characteristic of these devices remains a key obstacle on the road to commercialization. Herein, sol–gel‐processed mesoporous boron‐doped TiO₂ (B‐TiO₂) is demonstrated as an improved electron transport layer (ETL) for PSCs for the reduction of hysteresis. The incorporation of boron dopant in TiO₂ ETL not only reduces the hysteresis behavior but also improves PCE of the perovskite device. The simultaneous improvements are mainly ascribed to the following two reasons. First, the substitution of under‐coordinated titanium atom by boron species effectively passivates oxygen vacancy defects in the TiO₂ ETL, leading to increased electron mobility and conductivity, thereby greatly facilitating electron transport. Second, the boron dopant upshifts the conduction band edge of TiO₂, resulting in more efficient electron extraction with suppressed charge recombination. Consequently, a methylammonium lead iodide (MAPbI₃) photovoltaic device based on B‐TiO₂ ETL achieves a higher efficiency of 20.51% than the 19.06% of the pure TiO₂ ETL based device, and the hysteresis is reduced from 0.13% to 0.01% with the B‐TiO₂ based device showing negligible hysteresis behavior.

In this work, a “perovskite” template‐assisted structure is developed to fabricate high‐quality α‐FAPbI₃. The δ‐FAPbI₃ phases are avoided. Defects are substantially reduced with an excellent light harvesting. A power conversion efficiency of 21.24% (the highest efficiency reported for pure α‐FAPbI₃) is achieved. It also realizes a great stability in 800 h thermal ageing and 500 h light soaking.

Abstract

Formamidinium (FA) lead halide (α‐FAPbI₃) perovskites are promising materials for photovoltaic applications because of their excellent light harvesting capability (absorption edge 840 nm) and long carrier diffusion length. However, it is extremely difficult to prepare a pure α‐FAPbI₃ phase because of its easy transformation into a nondesirable δ‐FAPbI₃ phase. In the present study, a “perovskite” template (MAPbI₃‐FAI‐PbI₂‐DMSO) structure is used to avoid and suppress the formation of δ‐FAPbI₃ phases. The perovskite structure is formed via postdeposition involving the treatment of colloidal MAI‐PbI₂‐DMSO film with FAI before annealing. In situ X‐ray diffraction in vacuum shows no detectable δ‐FAPbI₃ phase during the whole synthesis process when the sample is annealed from 100 to 180 °C. This method is found to reduce defects at grain boundaries and enhance the film quality as determined by means of photoluminescence mapping and Kelvin probe force microscopy. The perovskite solar cells (PSCs) fabricated by this method demonstrate a much‐enhanced short‐circuit current density (  J _sc) of 24.99 mA cm⁻² and a power conversion efficiency (PCE) of 21.24%, which is the highest efficiency reported for pure FAPbI₃, with great stability under 800 h of thermal ageing and 500 h of light soaking in nitrogen.

An all‐inorganic mixed‐halide perovskite solar cell with a power conversion efficiency of 16.42% is realized by using a Cs₂CO₃‐doped ZnO electron transport layer, which ascribes to the interfacial energy level tuning for reducing ohmic loss at the contact and enlarging the built‐in potential. A high thermostability is simultaneously obtained via surface defect passivation for improving the CsPbI₂Br film against phase transformation.

Abstract

Inorganic mixed‐halide CsPbX₃‐based perovskite solar cells (PeSCs) are emerging as one of the most promising types of PeSCs on account of their thermostability compared to organic–inorganic hybrid counterparts. However, dissatisfactory device performance and high processing temperature impede their development for viable applications. Herein, a facile route is presented for tuning the energy levels and electrical properties of sol–gel‐derived ZnO electron transport material (ETM) via the doping of a classical alkali metal carbonate Cs₂CO₃. Compared to bare ZnO, Cs₂CO₃‐doped ZnO possesses more favorable interface energetics in contact with the CsPbI₂Br perovskite layer, which can reduce the ohmic loss to a negligible level. The optimized PeSCs achieve an improved open‐circuit voltage of 1.28 V, together with an increase in fill factor and short‐circuit current. The optimized power conversion efficiencies of 16.42% and 14.82% are realized on rigid glass substrate and flexible plastic substrate, respectively. A high thermostability can be simultaneously obtained via defect passivation at the Cs₂CO₃‐doped ZnO/CsPbI₂Br interface, and 81% of the initial efficiency is retained after aging for 200 h at 85 °C.

High performance flexible organic solar cells with efficiency above 12% for 1 cm² cells are fabricated using a Ag/Cu composite grid electrode. The excellent optical and electrical properties of the Ag/Cu electrode contribute to the high performance and good mechanical resistance of the flexible organic solar cell.

Abstract

With the rapid progress of organic solar cells (OSCs), improvement in the efficiency of large‐area flexible OSCs (>1 cm²) is crucial for real applications. However, the development of the large‐area flexible OSCs severely lags behind the growth of the small‐area OSCs, with the electrical loss due to the large sheet resistance of the electrode being a main reason. Herein, a high conductive and high transparent Ag/Cu composite grid with sheet resistance <1 Ω sq⁻¹ and an average visible light transparency of 84% is produced as the transparent conducting electrode of flexible OSCs. Based on this Ag/Cu composite grid electrode, a high efficiency of 12.26% for 1 cm² flexible OSCs is achieved. The performances of large‐area flexible OSCs also reach 7.79% (4 cm²) and 7.35% (9 cm²), respectively, which are much higher than those of the control devices with conventional flexible indium tin oxide electrodes. Surface planarization using highly conductive PEDOT:PSS and modification of the ZnO buffer layer by zirconium acetylacetonate (ZrAcac) are two necessary steps to achieve high performance. The flexible OSCs employing Ag/Cu grid have excellent mechanical bending resistance, maintaining high performance after bending at a radius of 2 mm.

Oxidants will happen: The effective oxidation of spiro‐OMeTAD is carried out by a hybrid polyoxometalate@metal–organic framework (POM@MOF) POM@Cu‐BTC which can be used as the hole transport material (HTL). This composite oxidant contributes to enhanced efficiency, as well as improved long‐time stability of perovskite solar cells (PSCs).

Abstract

The controllable oxidation of spiro‐OMeTAD and improving the stability of hole‐transport materials (HTMs) layer are crucial for good performance and stability of perovskite solar cells (PSCs). Herein, we report an efficient hybrid polyoxometalate@metal–organic framework (POM@MOF) material, [Cu₂(BTC)_4/3(H₂O)₂]₆[H₃PMo₁₂O₄₀]₂ or POM@Cu‐BTC, for the oxidation of spiro‐OMeTAD with Li‐TFSI and TBP. When POM@Cu‐BTC is introduced to the HTM layer as a dopant, the PSCs achieve a superior fill factor of 0.80 and enhanced power conversion efficiency 21.44 %, as well as improved long‐term stability in an ambient atmosphere without encapsulation. The enhanced performance is attributed to the oxidation activity of POM anions and solid‐state nanoparticles. Therefore, this research presents a facile way by using hybrid porous materials to accelerate oxidation of spiro‐OMeTAD, further improving the efficiency and stability of PSCs.

A new doping strategy is developed for poly(3‐hexylthiophene) (P3HT) as the hole transport layer (HTL) in triple‐cation/double‐halide mesoscopic perovskite solar cells (PSCs), achieving efficiencies of 19.25% and 13.3% on lab‐scale and large‐area module, respectively. Promising stability results after 1500 h air exposure (relative humidity ≈ 60%, r.t.), more than 500 h at 85 °C, and 100 h of continuous light soaking.

Abstract

As the hole transport layer (HTL) for perovskite solar cells (PSCs), poly(3‐hexylthiophene) (P3HT) has been attracting great interest due to its low‐cost, thermal stability, oxygen impermeability, and strong hydrophobicity. In this work, a new doping strategy is developed for P3HT as the HTL in triple‐cation/double‐halide ((FA_1−x−yMA_xCs_y)Pb(I_1−xBr_x)₃) mesoscopic PSCs. Photovoltaic performance and stability of solar cells show remarkable enhancement using a composition of three dopants Li‐TFSI, TBP, and Co(III)‐TFSI reaching power conversion efficiencies of 19.25% on 0.1 cm² active area, 16.29% on 1 cm² active area, and 13.3% on a 43 cm² active area module without using any additional absorber layer or any interlayer at the PSK/P3HT interface. The results illustrate the positive effect of a cobalt dopant on the band structure of perovskite/P3HT interfaces leading to improved hole extraction and a decrease of trap‐assisted recombination. Non‐encapsulated large area devices show promising air stability through keeping more than 80% of initial efficiency after 1500 h in atmospheric conditions (relative humidity ≈ 60%, r.t.), whereas encapsulated devices show more than >500 h at 85 °C thermal stability (>80%) and 100 h stability against continuous light soaking (>90%). The boosted efficiency and the improved stability make P3HT a good candidate for low‐cost large‐scale PSCs.

CsPbI₂Br perovskite is doped by Eu(Ac)₃ to obtain high‐quality perovskite films with low defect density and long carrier lifetime. A high efficiency of 15.25%, an open‐circuit voltage of 1.25 V, a short‐circuit current density of 15.44 mA cm⁻², and a fill factor of 79.00% are realized for CsPbI₂Br solar cells. The devices with Eu(Ac)₃ doping demonstrate excellent air stability.

Abstract

All‐inorganic perovskite solar cells have developed rapidly in the last two years due to their excellent thermal and light stability. However, low efficiency and moisture instability limit their future commercial application. The mixed‐halide inorganic CsPbI₂Br perovskite with a suitable bandgap offers a good balance between phase stability and light harvesting. However, high defect density and low carrier lifetime in CsPbI₂Br perovskites limit the open‐circuit voltage (V _oc < 1.2 V), short‐circuit current density (J _sc < 15 mA cm⁻²), and fill factor (FF < 75%) of CsPbI₂Br perovskite solar cells, resulting in an efficiency below 14%. For the first time, a CsPbI₂Br perovskite is doped by Eu(Ac)₃ to obtain a high‐quality inorganic perovskite film with a low defect density and long carrier lifetime. A high efficiency of 15.25% (average efficiency of 14.88%), a respectable V _oc of 1.25 V, a reasonable J _sc of 15.44 mA cm⁻², and a high FF of 79.00% are realized for CsPbI₂Br solar cells. Moreover, the CsPbI₂Br solar cells with Eu(Ac)₃ doping demonstrate excellent air stability and maintain more than 80% of their initial power conversion efficiency (PCE) values after aging in air (relative humidity: 35–40%) for 30 days.

Single‐crystalline 2D Ruddlesden–Popper‐type halide perovskites (RPPs) are demonstrated to exhibit extremely large two‐photon absorption coefficients in the near‐IR due to near‐resonance with 2D excitons. Efficient polarization‐resolved sub‐bandgap photodetection is realized at room temperature by utilizing 2D‐RPP two‐photon detectors, which opens avenues for future exploration of nonlinear optics in this class of hybrid quantum materials.

Abstract

Two‐dimensional (2D) perovskites have proved to be promising semiconductors for photovoltaics, photonics, and optoelectronics. Here, a strategy is presented toward the realization of highly efficient, sub‐bandgap photodetection by employing excitonic effects in 2D Ruddlesden–Popper‐type halide perovskites (RPPs). On near resonance with 2D excitons, layered RPPs exhibit degenerate two‐photon absorption (D‐2PA) coefficients as giant as 0.2–0.64 cm MW ^{− 1}. 2D RPP‐based sub‐bandgap photodetectors show excellent detection performance in the near‐infrared (NIR): a two‐photon‐generated current responsivity up to 1.2 × 10⁴ cm² W⁻² s⁻¹, two orders of magnitude greater than InAsSbP‐pin photodiodes; and a dark current as low as 2 pA at room temperature. More intriguingly, layered‐RPP detectors are highly sensitive to the light polarization of incoming photons, showing a considerable anisotropy in their D‐2PA coefficients (β_[001]/β_[011] = 2.4, 70% larger than the ratios reported for zinc‐blende semiconductors). By controlling the thickness of the inorganic quantum well, it is found that layered RPPs of (C₄H₉NH₃)₂(CH₃NH₃)Pb₂I₇ can be utilized for three‐photon photodetection in the NIR region.

Organic‐free (all‐inorganic) and lead‐free halide perovskites in combination with dopant‐free hole transport materials (HTMs) are summarized in terms of potential photovoltaic performance, and progress in compositional and morphological design of solution‐processed perovskite absorbers. Strategies to enhance device efficiency are focused on preparation of high quality perovskite and HTM interface.

Abstract

Having demonstrated incredibly fast progress in power conversion efficiency, rising to a level comparable with that of crystalline silicon cells, lead‐based organic–inorganic hybrid perovskite solar cells are now facing the stability tests needed for industrialization. Poor thermal stability (<150 °C) owing to organic constituents and interlayer diffusion of materials (dopants), and environmental incompatibility due to Pb has surged the development of organic‐free, Pb‐free perovskites and dopant‐free hole transport materials (HTMs). The recent rapid increase in efficiency of cells based on inorganic perovskites, crossing 18%, demonstrates the great potential of inorganic perovskites as thermally stable and high‐efficiency cells. Although all kinds of Pb‐free perovskites lag in efficiency in comparison to the hybrid and inorganic perovskites, they also demonstrate better structural and environmental stability. The performance of dopant‐free HTMs matching/surpassing dopant‐containing HTMs makes the former a better choice for stability. Even though the efforts to enhance the stability of Pb‐based hybrid perovskites should continue by different techniques, organic‐free and lead‐free perovskites, and dopant‐free HTMs must be pursued with greater interest for the future. This review describes the present issues and possible strategies to address them, and thus will help to improve the overall performance of robust organic‐free, Pb‐free, and dopant‐free perovskite solar cells.

Nature Energy, Published online: 07 October 2019; doi:10.1038/s41560-019-0471-6

A fully automated spray‐coated technology for the commercial large‐scale solution‐based processing of colloidal CsPbI₃ quantum dot films is achieved. The solar cells based on such films show a high power conversion efficiency of 11.2%.

Abstract

A fully automated spray‐coated technology with ultrathin‐film purification is exploited for the commercial large‐scale solution‐based processing of colloidal inorganic perovskite CsPbI₃ quantum dot (QD) films toward solar cells. This process is in the air outside the glove box. To further improve the performance of QD solar cells, the short‐chain ligand of phenyltrimethylammonium bromide (PTABr) with a benzene group is introduced to partially substitute for the original long‐chain ligands of the colloidal QD surface (namely PTABr‐CsPbI₃). This process not only enhances the carrier charge mobility within the QD film due to shortening length between adjacent QDs, but also passivates the halide vacancy defects of QD by Br⁻ from PTABr. The colloidal QD solar cells show a power conversion efficiency (PCE) of 11.2% with an open voltage of 1.11 V, a short current density of 14.4 mA cm⁻², and a fill factor of 0.70. Due to the hydrophobic surface chemistry of the PTABr–CsPbI₃ film, the solar cell can maintain 80% of the initial PCE in ambient conditions for one month without any encapsulation. Such a low‐cost and efficient spray‐coating technology also offers an avenue to the film fabrication of colloidal nanocrystals for electronic devices.

The electron extracting properties of the widely used electron transporting layer (ETL) material Phen‐NaDPO are remarkably enhanced via simple addition of the wide‐bandgap inorganic material tin (II) thiocyanate (Sn(SCN)₂). Use of this hybrid ETL system in organic and perovskite solar cells results in consistent efficiency improvements due to the reduced trap‐assisted recombination and efficient electron extraction.

Abstract

A simple approach that enables a consistent enhancement of the electron extracting properties of the widely used small‐molecule Phen‐NaDPO and its application in organic solar cells (OSCs) is reported. It is shown that addition of minute amounts of the inorganic molecule Sn(SCN)₂ into Phen‐NaDPO improves both the electron transport and its film‐forming properties. Use of Phen‐NaDPO:Sn(SCN)₂ blend as the electron transport layer (ETL) in binary PM6:IT‐4F OSCs leads to a remarkable increase in the cells' power conversion efficiency (PCE) from 12.6% (Phen‐NaDPO) to 13.5% (Phen‐NaDPO:Sn(SCN)₂). Combining the hybrid ETL with the best‐in‐class organic ternary PM6:Y6:PC₇₀BM systems results to a similarly remarkable PCE increase from 14.2% (Phen‐NaDPO) to 15.6% (Phen‐NaDPO:Sn(SCN)₂). The consistent PCE enhancement is attributed to reduced trap‐assisted carrier recombination at the bulk‐heterojunction/ETL interface due to the presence of new energy states formed upon chemical interaction of Phen‐NaDPO with Sn(SCN)₂. The versatility of this hybrid ETL is further demonstrated with its application in perovskite solar cells for which an increase in the PCE from 16.6% to 18.2% is also demonstrated.

The polystyrene is introduced into perovskite solar cells as the buffer layer between the SnO₂ and perovskite, which can release the stress during the perovskite annealing. A large lattice, fewer defect, and low ion‐immigration‐energy perovskite can be obtained by releasing stress. Finally, 21.89% efficiency is obtained and the cell can maintain almost 97% of the initial efficiency after 5 days.

Abstract

The mixed halide perovskites have become famous for their outstanding photoelectric conversion efficiency among new‐generation solar cells. Unfortunately, for perovskites, little effort is focused on stress engineering, which should be emphasized for highly efficient solar cells like GaAs. Herein, polystyrene (PS) is introduced into the perovskite solar cells as the buffer layer between the SnO₂ and perovskite, which can release the residual stress in the perovskite during annealing because of its low glass transition temperature. The stress‐free perovskite has less recombination, larger lattices, and a lower ion migration tendency, which significantly improves the cell's efficiency and device stability. Furthermore, the so‐called inner‐encapsulated perovskite solar cells are fabricated with another PS capping layer on the top of perovskite. As high as a 21.89% photoelectric conversion efficiency (PCE) with a steady‐state PCE of 21.5% is achieved, suggesting that the stress‐free cell can retain almost 97% of its initial efficiency after 5 days of “day cycle” stability testing.

A high‐throughput inkjet printing approach is developed, and used to fabricate 25 mixed perovskite films from the sequential inkjet printing of four pure precursors in a fast and reproducible manner. The obtained film properties database enables to efficiently screen perovskite constituents for photovoltaic applications, highlighting the benefit of this approach.

Abstract

The exploration and optimization of numerous mixed perovskite compositions are causing a strong demand for high‐throughput synthesis. Nevertheless high‐throughput fabrication of perovskite films with representative film properties, which can efficiently screen the perovskite compositions for photovoltaic applications, has rarely been explored. A high‐throughput inkjet printing approach that can automatically fabricate perovskite films with various compositions with high reproducibility and high speed is developed. The automatic sequential printing of four precursors forms 25 mixed films in a fast and reproducible manner. The obtained bandgaps, photoluminescence (PL) peak positions, and PL lifetimes allow for the efficient screening of perovskite compositions for photovoltaic applications. To exemplify this concept, among 25 tested films, two compositions CH₃NH₃PbBr_0.75I_2.25 (MA) and (HC(NH₂)₂)_0.75(CH₃NH₃)_0.25PbBr_0.75I_2.25 (FA_0.75MA_0.25) with a long (237 ns) and short (49.0 ns) PL lifetime, respectively, are screened out for device investigations. As expected, the MA‐based device exhibits a much higher efficiency (19.0%) than that (15.3%) of the FA_0.75MA_0.25 counterpart. This efficiency improvement is mainly ascribed to a smaller dark saturate current density, a lower level of energetic disorder, more efficient charge transfer and decreased charge recombination losses, which are consistent with the much longer PL lifetime in the database.

Recent progress on additive engineering during perovskite film formation is reported according to the following common categories: Lewis acid, Lewis base, ammonium salts, low‐dimensional perovskites, and ionic liquid. Then, various additive‐assisted strategies for interface optimization are compared. Finally, an outlook on the research trends with respect to additive engineering in perovskite solar cell development is provided.

Abstract

Perovskite solar cells (PSCs) have reached a certified 25.2% efficiency in 2019 due to their high absorption coefficient, high carrier mobility, long diffusion length, and tunable direct bandgap. However, due to the nature of solution processing and rapid crystal growth of perovskite thin films, a variety of defects can form as a result of the precursor compositions and processing conditions. The use of additives can affect perovskite crystallization and film formation, defect passivation in the bulk and/or at the surface, as well as influence the interface tuning of structure and energetics. Here, recent progress in additive engineering during perovskite film formation is discussed according to the following common categories: Lewis acid (e.g., metal cations, fullerene derivatives), Lewis base based on the donor type (e.g., O‐donor, S‐donor, and N‐donor), ammonium salts, low‐dimensional perovskites, and ionic liquid. Various additive‐assisted strategies for interface optimization are then summarized; additives include modifiers to improve electron‐ and hole‐transport layers as well as those to modify perovskite surface properties. Finally, an outlook is provided on research trends with respect to additive engineering in PSC development.

Recent progress on additive engineering during perovskite film formation is reported according to the following common categories: Lewis acid, Lewis base, ammonium salts, low‐dimensional perovskites, and ionic liquid. Then, various additive‐assisted strategies for interface optimization are compared. Finally, an outlook on the research trends with respect to additive engineering in perovskite solar cell development is provided.

Abstract

Perovskite solar cells (PSCs) have reached a certified 25.2% efficiency in 2019 due to their high absorption coefficient, high carrier mobility, long diffusion length, and tunable direct bandgap. However, due to the nature of solution processing and rapid crystal growth of perovskite thin films, a variety of defects can form as a result of the precursor compositions and processing conditions. The use of additives can affect perovskite crystallization and film formation, defect passivation in the bulk and/or at the surface, as well as influence the interface tuning of structure and energetics. Here, recent progress in additive engineering during perovskite film formation is discussed according to the following common categories: Lewis acid (e.g., metal cations, fullerene derivatives), Lewis base based on the donor type (e.g., O‐donor, S‐donor, and N‐donor), ammonium salts, low‐dimensional perovskites, and ionic liquid. Various additive‐assisted strategies for interface optimization are then summarized; additives include modifiers to improve electron‐ and hole‐transport layers as well as those to modify perovskite surface properties. Finally, an outlook is provided on research trends with respect to additive engineering in PSC development.

Atomic layer deposition (ALD) is key to improving the efficiency and stability of perovskite solar cells (PSCs). Moreover, ALD unlocks novel options regarding device architecture and processing, not achievable otherwise. Herein, the state of the art in ALD‐grown functional charge transport layers for PSCs is highlighted and the most urgent scientific issues and opportunities for further research are outlined.

Within less than a decade of development, perovskite solar cells (PSCs) have reached efficiency levels that trigger the question about commercialization of the technology. However, the steadily increasing efficiency of PSCs is still accompanied by concerns of long‐term stability and open questions about upscaled manufacturing. Atomic layer deposition (ALD) is a technique that can provide unique contributions to both issues. On top of that, ALD is an enabling technology to unlock novel options regarding device architecture and processing that are not achievable by other coating techniques. As such, ALD has enjoyed some notable recent interest from the community of perovskite photovoltaics. Herein, the current state of the art in ALD‐grown functional charge transport layers for PSCs is highlighted. The most urgent scientific issues that have to be tackled are emphasized and opportunities for further research are outlined.