Chen Weijie

Herein, the doping effect on semiconductor oxide‐based electron transport materials, especially for the typical TiO₂, in perovskite solar cells is reviewed and emphasized by classifying the doping ions according to the main family of elements from the critical factors of lattice optimization, carrier transporting improvement, and interface modification.

From the perspective of the device structure of perovskite solar cells (PSCs), the electron transport layer is one of the essential components and plays a significant role in suppressing carrier recombination. Furthermore, its decisiveness is related to the quality of perovskite film, the rapid interface carrier extraction, and the bandgap alignment. However, the deficiency of the semiconductor oxides based electron transport materials, especially for most studied TiO₂, is that their carrier mobility is one to three orders of magnitude lower than the most commonly used hole transport materials, leading to an imbalanced carrier flux and unpredicted hysteresis. Doping new ions are the most effective ways to improve electron mobility and tune the bandgap, while the fundamental mechanism of doping in the majority of cases are still lacking. Herein, the doping effect on semiconductor oxides is reviewed and emphasized by classifying the doping ions according to the critical factors of lattice optimization, a carrier transporting improvement, and interface modification. This review is the first systematic summary of the ion doping characteristics in oxide electron transport layers of PSCs. Finally, the implementation of doping ions in electron transport materials is briefly discussed for further enhancing the photovoltaic performance of PSCs.

A chlorinated π‐conjugated donor polymer with a siloxane‐functionalized side chain is developed and utilized in binary‐ and ternary‐based nonfullerene organic solar cells processed using a nonhalogenated solvent (toluene), which achieve high power conversion efficiencies of 10.38% and 13.25%, respectively. The thickness of the ternary blend is 300 nm. Furthermore, the binary‐ and ternary‐based devices exhibit high thermal and atmospheric stabilities.

Although several donor polymers have been synthesized for use in nonfullerene organic solar cells (NFOSCs), the number of efficient π‐conjugated donor polymers compatible with nonhalogenated solvent‐processed thick active layer NFOSCs is limited. Two wide‐bandgap π‐conjugated donor polymers functionalized with a siloxane side chain, P1 (chlorine‐free) and P2 (chlorinated), are designed and synthesized. The siloxane‐functionalized side chains and/or Cl π‐conjugated donor polymers increase the absorption coefficients, reduce the energy losses, increase the charge‐carrier mobility, and suppress the bimolecular recombination, which are beneficial to achieve high‐performance thick‐film ternary NFOSCs. Toluene‐processed devices based on P2:IT‐4F:BTP‐4Cl, and P2:IT‐4F:BTP‐4F exhibit high power conversion efficiencies (PCEs) of 13.25% and 11.02% with fill factors (FFs) of 70.03% and 71.60%, respectively. A P2:IT‐4F binary NFOSC exhibits a PCE of 10.38% with an FF of 69.78%, lower than that of the ternary NFOSC. The ternary device PCE of 13.25% is achieved using a 300 nm‐thick active layer, indicating that the siloxane‐functionalized side‐chain π‐conjugated polymer easily controls the bulk heterojunction blend film thickness of the NFOSC. The findings may potentially aid the development of nonhalogenated solvent‐processed thick‐film ternary NFOSCs that can satisfy future production requirements.

Herein, excitation wavelength‐dependent charge generation dynamics in nonfullerene solar cells is revealed by time‐dependent density functional theory (TDDFT)‐based nonadiabatic dynamics simulations.

Unraveling the charge generation dynamics at the donor–acceptor (D−A) interfaces is crucial for improving the photovoltaic performances of nonfullerene acceptor‐based organic solar cells (OSCs). Herein, time‐dependent density functional theory (TDDFT) based nonadiabatic dynamics simulations are used to explore the ultrafast photoinduced dynamics at a nonfullerene D–A PTB7@PDI interface. Based on the results, it is found that such an interface exhibits distinct charge generation processes upon excitation with different wavelengths. The excitation at ≈591 nm mainly results in the local exciton |PTB7 ^∗ >, whereas the charge transfer exciton |PTB7 ⁺ PDI ⁻ > also has minor contribution. Later on, the electron transfer from PTB7 to PDI, i.e., channel I charge generation process, occurs in 1 ps. The situations are much more complex when the excitation is conducted using ≈487 nm light. The initial populated excitons include local excitons |PDI ^∗ >, |PTB7 ^∗ >, and charge transfer exciton |PTB7 ⁺ PDI ⁻ >, after which both channel I and channel II charge generation take place ultrafast. However, in both situations, the charge generation processes occur within a few picoseconds, which is consistent with previous experimental work. Such ultrafast charge generation processes in a wide range of solar spectra are one of the reasons responsible for the excellent photovoltaic properties of such OSCs.

The three main approaches for reducing lead content in perovskite solar cells while keeping high efficiency are reviewed: i) the partial replacement of Pb by another element with similar charge and size, ii) the partial replacement of lead and halide units by organic cations, and iii) the engineering of the cells to optimize the light harvesting by the perovskite layer.

The rise and commercialization of perovskite solar cells (PSCs) is hindered by the toxicity of lead present in the perovskites used as the solar light absorber. To counter this problem, lead (Pb) can be fully (lead‐free) or partially (lead‐less) replaced by diverse elements. The former compounds suffer from poor efficiency and poor stability, whereas the later appear more promising. Herein, a survey of the methods reported in the literature to reduce Pb content in PSCs to fabricate “lead‐less” (also called “lead‐deficient”) PSCs is offered. First, the comparison of Sn and Pb elements and the partial replacement of Pb by Sn are developed. Then, its substitution by either Ge, Sr, or other alkaline‐earth‐metals, transition metals, and elements from columns 12, 13, and 15 of the periodic table are detailed. The new families of perovskites based on the insertion of organic cations to replace lead and halogen units, namely the “lead‐deficient” and “hollow” halide perovskites are then presented and discussed. Finally, atypical ways to reduce the toxicity of PSCs are presented: perovskite layer thickness reduction via optimization of photon collection, integration of photonic structures, and usage of recycled lead. The current achievements and the outlook of those strategies are presented and discussed.

Semitransparent organic solar cells are excellent candidates for applications where coloration is critical; yet, little research is directed at controlling their active layer colors. Herein, a ternary system of non‐fullerene acceptors modulates coloration through a continuous range: cyan → blue → purple → reddish purple. Optimal efficiencies of up to 6.93% with average visible transmittance of 34.03% are achieved.

Semitransparent organic solar cells (STOSCs) have received increasing attention due to promising applications such as building‐integrated photovoltaics. Successful commercialization requires that STOSCs are aesthetically pleasing as well as having balanced power conversion efficiencies (PCEs) and average visible transmittances (AVTs). Non‐fullerene acceptors, which possess excellent electrical/chemical properties, have helped STOSCs to achieve high PCE and AVT; however, research related to modulating color and appearance of STOSCs has lagged behind. Herein, narrow bandgap donor and acceptor (PTB7‐Th and IEICO‐4F) and ultra‐wide bandgap acceptors (T2‐ORH and T2‐OEHRH) are used to achieve semitransparency and controllable device coloration. Blend films with controllable colors including cyan → blue → purple → reddish purple colors are successfully demonstrated, which are controlled by ratios of IEICO‐4F:T2‐ORH or IEICO‐4F:T2‐OEHRH with PTB7‐Th. By incorporating semitransparent electrodes (comprising Sb₂O₃/Ag/Sb₂O₃), STOSCs with PCEs of 6–7% are achieved for cyan, aqua, indigo, and purple and ≈4% PCEs for reddish‐purple colors, with AVTs in the range of 23–35%. Moreover, optical properties of blend films are studied via absorption and transmission measurements, whereas the range of colors achieved is quantified using commission internationale de l'éclairage (CIE) chromaticity and CIE L * a * b* color space then represented as RGB color models.

A new intermediate FABr–PbBr₂–DMSO is discovered in early stage of perovskite spin‐coating. With this crystalline compound, the crystallization dynamics can be modulated to achieve controlled morphologies of FAPbBr₃ films by simply varying the solvent composition. Mg–ZnO nanoparticles is introduced for wide bandgap perovskite solar device with an increased efficiency of over 9% based on inverted solar cell structure.

Bromide‐based organo‐metal halide perovskites have shown great potential for use in tandem solar cells (SCs), light‐emitting diodes (LEDs), and photodetectors. Herein, a new protocol using a one‐step deposition method for producing formamidinium lead bromide (FAPbBr₃) perovskites is reported, which features a solvent‐engineered intermediate phase to achieve superior films. For the first time, a FABr–PbBr₂–DMSO intermediate is identified and single crystals of the same intermediate compound are synthesized. A systematic investigation of phase evolution in the film formation process reveals that DMSO enables crystallization of the FABr–PbBr₂–DMSO intermediate and thus modulates the crystallization process of FAPbBr₃ perovskite, achieving uniform, smooth films with Volmer–Weber morphology. To prevent hole leakage arising from the larger bandgap of FAPbBr₃ than FAPbI₃, an additional layer of Mg‐doped ZnO nanoparticles is added. As a result, inverted SCs using these solvent engineered films achieve power conversion efficiencies (PCEs) of up to 9.06%, the highest reported efficiency for inverted FAPbBr₃ perovskite devices.

A power conversion efficiency (PCE) of 17.59% is achieved in ternary organic photovoltaic cells (OPVs) with BTP‐BO‐4F:Y6‐1O as alloyed acceptor, resulting from the simultaneously improved three photovoltaic parameters. The good compatibility of used materials is one of the prerequisites for achieving efficient ternary OPVs.

Efficient organic photovoltaic cells (OPVs) are fabricated using two structurally similar Y6 derivations (BTP‐BO‐4F and Y6‐1O) as acceptor and PM6 as donor. The two binary OPVs exhibit a high fill factor (FF) (>76%), the complementary short‐circuit‐current density (J _SC) and open‐circuit voltage (V _OC). The high FFs of binary OPVs indicate the good compatibility of corresponding materials to form efficient charge transport channels. A power conversion efficiency (PCE) of 17.59% is obtained from ternary OPVs with 15 wt% Y6‐1O in acceptors, benefiting from the simultaneously improved J _SC of 26.13 mA cm⁻², a V _OC of 0.860 V, and an FF of 78.26%. The values of V _OC of ternary OPVs can be gradually increased along with the incorporation of Y6‐1O, suggesting the preferred formation of an alloyed state between BTP‐BO‐4F and Y6‐1O due to their good compatibility. Meanwhile, the cascaded energy levels of BTP‐BO‐4F and Y6‐1O can form efficient electron transport channels in ternary active layers. The main contribution of Y6‐1O can be summarized as enhancing photon harvesting, optimizing phase separation, and adjusting molecular arrangement. The experimental results may provide new insight on developing efficient ternary OPVs by selecting two well‐compatible acceptors.

A series of nonfullerene acceptors with BTP as the central core is prepared through integrated regulation. Shortening the alkyl chain, extending the end group, and fluorination are effective strategies to improve the morphology of the active layer, increase light management and carrier management, reduce the E _loss, and enhance the performance of the device.

The photovoltaic properties and energy losses of organic solar cells (OSCs) based on nonfullerene acceptors (NFAs) are highly dependent on their molecular structures and aggregation morphologies. Charge carrier and light managements are important to optimize NFA molecules. Herein, four NFAs with different alkyl substituents and end groups, named BTP‐C11‐N2F, BTP‐C9‐N2F, BTP‐C9‐IC4F, and BTP‐C9‐N4F, are designed and synthesized by side‐chain shortening, end‐acceptor π‐extension, and fluorination. As a result, a favorable morphology is achieved in BTP‐C9‐N4F‐based OSCs by using typical high bandgap polymer PM6 as a donor, and this system obtains the highest power conversion efficiency of 17.0% with a short circuit current (J _sc) of 26.3 mA cm⁻², an open circuit current (V _oc) of 0.85 V, and a fill factor (FF) of 75.7%. In addition, its light (J _sc) and charge carrier (V _oc × FF) managements relative to the Shockley–Queisser limit are also increased. Extending the conjugation of the end groups increased the energy levels of NFAs and enabled an E _loss of 0.50 eV with a nonradiative recombination loss of as low as 0.20 eV in BTP‐C11‐N2F‐based OSCs. This work provides an efficient strategy to optimize the molecular structures of nonfullerene acceptors and further improve the properties of OSCs.

An organic photovoltaic (OPV) formulation is developed, which suggests a low synthetic complexity and a competitive figure of merit. The prepared module exhibits a maximum power conversion efficiency (PCE) of 10% when using solution‐processed material as the hole‐transporting layer and simultaneously exhibits outstanding stability, which will benefit the commercialization of highly efficient OPV products.

A scalable and accessible photoactive formulation with a low synthetic complexity (SC) index is utilized in organic photovoltaic (OPV) fabrication. The formulation readily dissolves in nonchlorinated solvents, and the corresponding photoactive films can be processed by various coating methods to fabricate devices with power conversion efficiencies (PCEs) of 16.1% and 15.2% when using vacuum‐based molybdenum oxide and solution‐processable conducting polymer as the hole transporting layer in the inverted structure, respectively. This prepared device shows superior stability under light exposure. The PCE is maintained 94% of the initial values after 1080 h of light soaking at 100 mW cm⁻². Furthermore, the figure of merit based on the ratio of the SC index and PCE indicates the benefit of this formulation for OPV manufacturing, showing the feasibility of commercialization. Eventually, a PCE of 10.3% is demonstrated for a mini‐module fabricated under ambient conditions, with an active area of 32.6 cm². To our knowledge, this PCE is one of the largest values reported to date for a green solvent and an all‐solution‐processed OPV module with an inverted architecture.

A universal crystal engineering method based on anion exchange is proposed and developed to grow crystalline perovskite thin films. Highly crystalline perovskite films with preferred crystal orientation and micrometer‐scale grains are presented via annealing perovskite films in saturated MAI vapor. Perovskite solar cells based on crystal engineering show improved efficiency from 21.07% to 22.26%.

Crystalline, dense, and uniform perovskite thin films are crucial for achieving high‐power conversion efficiency solar cells. Herein, a universal method of fabricating highly crystalline and large‐grain perovskite films via crystal engineering is demonstrated. Anion exchange of Cl⁻ and I⁻, and annealing perovskite films, in an ultraconfined and uniform temperature enclosed space with saturated MAI (or FAI) vapor using hot‐pressing sublimation technology are conducted. This process ensures a rapid crystal growth rate due to fast exchange between the gas phase and the crystalline film to reduce vertically oriented grain boundaries. The generation of the commonly observed PbI₂ phase is also suppressed due to the chemical equilibrium state during the thermal annealing process. Using this approach, pinhole‐free perovskite films with preferred crystal orientation and micrometer‐scale grains are obtained, leading to a high steady‐state efficiency of 22.15% based on mixed‐cation perovskite composition. In addition, devices based on different perovskite compositions all exhibit enhanced photovoltaic performance based on the crystal engineering method. The device (without encapsulation) has an efficiency loss of about only 4% after 2520 h of aging in ambient conditions and retains 87% of its initial efficiency after 1000 h of continuous 1 Sun light soaking, thus demonstrating considerably improved stability.

A record efficiency of 15.24% is achieved for organic solar cells with an area of ≥1 cm² with D18:Y6 as absorber material. The optimized cell design minimizes resistive losses due to the indium tin oxide (ITO) electrode. The cell is very homogeneous and is hardly affected by shunts, as revealed by light beam‐induced current, electroluminescence, and dark lock‐in thermography imaging.

In organic photovoltaics, high power conversion efficiencies (PCE) are mostly achieved on device areas well below 0.1 cm². Herein, organic solar cells based on a D18:Y6 absorber layer on an active area of ≥ 1 cm² with a certified PCE of 15.24% are reported. The impacts of the sheet resistance of the transparent electrode and the cell design are quantified by means of full optical device simulations and an analytical electrical model. Three imaging methods (light beam‐induced current, dark lock‐in thermography, and electroluminescence [EL]) are applied and reveal a strong homogeneity of the record cell. Nevertheless, it is found that there is substantial room for improvement mostly in current but also in fill factor and that a PCE of 18.6% on ≥1 cm² is feasible with this absorber material. Further, photoluminescence (PL) and EL spectroscopy reveal that both emissions occur at the same wavelength(s) and are very similar to the PL spectrum of a pure Y6 acceptor film. The latter points strongly toward electronic coupling between the S1 states of the acceptor and the charge transfer states at the donor/acceptor interface.

An amphiphilic dendritic block copolymer is developed to serve as an efficient surface modifier of ZnO electron‐transporting layer in an organic photovoltaic device. When using an interlayer based on its hybridization with gold nanoparticles, the device can deliver improved performance and possess a lifetime of over 1.79 years when stored at room temperature in inert conditions.

Abstract

Herein, interfacial engineering is demonstrated to improve the thermal stability of non‐fullerene bulk‐heterojunction (BHJ) OPVs to a practical level. An amphiphilic dendritic block copolymer (DBC) is developed through a facile coupling method and employed as the surface modifier of ZnO electron‐transporting layer in inverted OPVs. Besides showing distinct self‐assembly behavior, the synthesized DBC possesses high compatibility with plasmonic gold nanoparticles (NPs) due to the constituent malonamide and ethylene oxide units. The hybrid DBC@AuNPs interlayer is shown to improve device's performance from 14.0% to 15.4% because it enables better energy‐level alignment and improves interfacial compatibility at the ZnO/BHJ interface. Moreover, the DBC@AuNPs interlayer not only improves the interfacial thermal stability at the ZnO/BHJ interface but also endows a more ideal BHJ morphology with an enhanced thermal robustness. The derived device reserves 77% of initial PCE after thermal aging at 65 °C for 3000 h and yields an extended T ₈₀ lifetime of >1100 h when stored at a constant thermal condition at 65 °C, outperforming the control device. Finally, the device is evaluated to possess a T ₈₀ lifetime of over 1.79 years at room temperature (298 K) when stored in an inert condition, showing great potential for commercialization.

Novel interface polarization induced field‐effect passivation based on amorphous transition metal oxide is developed for efficient and ambient‐air‐stable perovskite solar cells. Comprehensive insights into the interaction between the field‐effect passivation, interface polarities, and the performance of the device have been elucidated in detail.

Abstract

Organolead halide hybrid perovskite solar cells (PSCs) have become a shining star in the renewable devices field due to the sharp growth of power conversion efficiency; however, interfacial recombination and carrier‐extraction losses at heterointerfaces between the perovskite active layer and the carrier transport layers remain the two main obstacles to further improve the power conversion efficiency. Here, novel field‐effect passivation has been successfully induced to effectively suppress the interfacial recombination and improve interfacial charge transfer by incorporating interfacial polarization via inserting a high work function interlayer between perovskite and holes transport layer. The charge dynamics within the device and the mechanism of the field‐effect passivation are elucidated in detail. The unique interfacial dipoles reinforce the built‐in field and prevent the photogenerated charges from recombining, resulting in power conversion efficiency up to 21.7% with negligible hysteresis. Furthermore, the hydrophobic interlayer also suppresses the perovskite decomposition by preventing the moisture penetration, thereby improving the humidity stability of the PSCs (>91% of the initial power conversion efficiency (PCE) after 30 d in 65 ± 5% humidity). Finally, several promising research perspectives based on field‐effect passivation are also suggested for further conversion efficiency improvements and photovoltaic applications.

The excited state characteristics of organic hole transport materials in perovskite photovoltaics (PVs), such as transition dipole moment, is confirmed to be a critical factor in improving the built‐in potential of devices for efficient charge extraction along with reduced carrier recombination. Moreover, the aggregation property of the organic semiconductor can have a synergistic effect with its excited state property for high‐efficiency perovskite PVs.

Abstract

Intrinsic characteristics of organic semiconductor‐based hole transport materials (HTMs) such as facile synthesizability, energy level tunability, and charge transport capability have been highlighted as crucial factors determining the performances of perovskite photovoltaic (PV) cells. However, their properties in the excited state have not been actively studied, although PVs are operated under solar illumination. Here, the characteristics of organic HTMs in their excited state such as transition dipole moment can be a decisive factor that can improve built‐in potential of PVs, consequently enhancing their charge extraction property as well as reducing carrier recombination. Moreover, the aggregation property of organic semiconductors, which has been an essential factor for high‐performance organic HTMs to improve their carrier transport property, can induce a synergistic effect with their excited state property for the high‐efficiency perovskite PVs. Additionally, it is also confirmed that their optical bandgaps, manipulated to have their absorption in the UV region, are beneficial to block UV light that degrades the quality of perovskite, consequently improving the stability of perovskite PV in p–i–n configuration. As a proof‐of‐concept, a model system, composed of triarylamine and imidazole‐based organic HTMs, is designed, and it is believed that this strategy paves a way toward high‐performance and stable perovskite PV devices.

Perpendicular crystal orientation and orderly n‐phase distribution in quasi‐2D perovskite films are simultaneously achieved by F‐substitution in phenethylammonium (PEA⁺), leading to an impressive 18.10%‐efficiency of perovskite solar cells with n = 4. Meanwhile, the horizontal crystal orientation and random n‐phase distribution are obtained in perovskite films based on PEA and (Cl/Br)‐substituted PEA, respectively.

Abstract

Halide substitution in phenethylammonium spacer cations (X‐PEA⁺, X = F, Cl, Br) is a facile strategy to improve the performance of PEA based perovskite solar cells (PSCs). However, the power conversion efficiency (PCE) of X‐PEA based quasi‐2D (Q‐2D) PSCs is still unsatisfactory and the underlying mechanisms are in debate. Here, the in‐depth study on the impact of halide substitution on the crystal orientation and multi‐phase distribution in PEA based perovskite films are reported. The halide substitution eliminates n = 1 2D perovskite and thus leads to the perpendicular crystal orientation. Furthermore, nucleation competition exists between small‐n and large‐n phases in PEA and X‐PEA based perovskites. This gives rise to the orderly distribution of different n‐phases in the PEA and F‐PEA based films, and random distribution in Cl‐PEA and Br‐PEA based films. As a result, (F‐PEA)₂MA₃Pb₄I₁₂ (MA = CH₃NH₃ ⁺, n = 4) based PSCs achieve a PCE of 18.10%, significantly higher than those of PEA (12.23%), Cl‐PEA (7.93%) and Br‐PEA (6.08%) based PSCs. Moreover, the F‐PEA based devices exhibit remarkably improved stability compared to their 3D counterparts.

Highly efficient ternary all‐polymer solar cells (PSCs) based on an ultranarrow bandgap polymer acceptor are realized. The optimized ternary all‐PSCs achieve a full coverage of solar spectrum, yielding an excellent power conversion efficiency of 12.1% with a remarkable short‐circuit current density of 21.9 mA cm⁻².

Abstract

Developing organic solar cells (OSCs) based on a ternary active layer is one of the most effective approaches to maximize light harvesting and improve their photovoltaic performance. However, this strategy meets very limited success in all‐polymer solar cells (all‐PSCs) due to the scarcity of narrow bandgap polymer acceptors and the challenge of morphology optimization. In fact, the power conversion efficiencies (PCEs) of ternary all‐PSCs even lag behind binary all‐PSCs. Herein, highly efficient ternary all‐PSCs are realized based on an ultranarrow bandgap (ultra‐NBG) polymer acceptor DCNBT‐TPC, a medium bandgap polymer donor PTB7‐Th, and a wide bandgap polymer donor PBDB‐T. The optimized ternary all‐PSCs yield an excellent PCE of 12.1% with a remarkable short‐circuit current density of 21.9 mA cm⁻². In fact, this PCE is the highest value reported for ternary all‐PSCs and is much higher than those of the corresponding binary all‐PSCs. Moreover, the optimized ternary all‐PSCs show a photostability with ≈68% of the initial PCE retained after 400 h illumination, which is more stable than the binary all‐PSCs. This work demonstrates that the utilization of a ternary all‐polymer system based on ultra‐NBG polymer acceptor blended with compatible polymer donors is an effective strategy to advance the field of all‐PSCs.

Slot die coating is used to fabricate high efficiency (power conversion efficiencies (PCE) > 11.0%), stable organic solar cells based on a donor PTB7‐Th and nonfullerene acceptor IEICO‐4F. The 11% small area and 1 cm² devices with a PCE of 9.63% show the scalability of the technique. The highest light utilization efficiency of 5.26% with a PCE of 9.07% is achieved for the all solution processed semi‐transparent solar cell.

Abstract

Slot‐die (SD) coating is used to fabricate fully solution processed organic solar cells (OSCs) based on a blend of high performance donor polymer (PTB7‐Th) and a non‐fullerene acceptor (IEICO‐4F) for stable devices over extended periods of operation. The optimization of a sequential deposition process of transport and active layers, under ambient conditions, enable high efficiency slot‐die coated solar cells with remarkable power conversion efficiencies (PCE) > 11.0% to bridge the gap between lab‐to‐fab. Fully slot‐die coated inverted OSCs are demonstrated with efficiencies reaching 11% along with 1 cm² devices, proving the scalability and reproducibility of the proposed technique. Further, replacing the evaporated Ag electrode with solution processed Ag nanowire (AgNW) electrodes shows the highest light utilization efficiency of 5.26% for semi‐transparent OSC with a PCE of 9.07% and average visible transmission of 58%.

Conformation effects of Y6‐type acceptors are systematically studied based on asymmetric design strategies. Z‐shape and W‐shape conformations‐based acceptors can help reduce energy loss in devices through significantly suppressed nonradiative energy loss. Benefiting from the high open‐circuit voltage of BP5T‐4F in the devices, ternary organic solar cells based on PM6:BP5T‐4F:CH1007 achieve a 17.2% efficiency.

Abstract

Y6, as a state‐of‐the‐art nonfullerene acceptor (NFA), is extensively optimized by modifying its side chains and terminal groups. However, the conformation effects on molecular properties and photovoltaic performance of Y6 and its derivatives have not yet been systematically studied. Herein, three Y6 analogs, namely, BP4T‐4F, BP5T‐4F, and ABP4T‐4F, are designed and synthesized. Owing to the asymmetric molecular design strategies, three representative molecular conformations for Y6‐type NFAs are obtained through regulating the lateral thiophene orientation of the fused core. It is found that conformation adjustment imposes comprehensive effects on the molecular properties in neat and blend films of these NFAs. As a result, organic solar cells (OSCs) fabricated with PM6:BP4T‐4F, PM6:BP5T‐4F, and PM6:ABP4T‐4F show high power conversion efficiency of 17.1%, 16.7%, and 15.2%, respectively. Interestingly, these NFAs with different conformations also show reduced energy loss (E _loss) in devices via gradually suppressed nonradiative E _loss. Moreover, by employing a selenium‐containing analog, CH1007, as the complementary third component, ternary OSCs based on PM6:BP5T‐4F:CH1007 (1:1.02:0.18) achieve a 17.2% efficiency. This work helps shed light on engineering the molecular conformation of NFAs to achieve high efficiency OSCs with reduced voltage loss.

4‐Dimethylaminopyridine (DMAP) is introduced to develop a facile technique for selectively passivating grain boundaries (GB) and controlling the topographical boundary of perovskite surfaces near GBs. A power conversion efficiency of 22.4% is achieved for a planar perovskite solar cell with DMAP treatment and the device stability under damp‐heat and light irradiation is improved.

Abstract

Recent progress in highly efficient perovskite solar cells (PSCs) has been made by virtue of interfacial engineering on 3D perovskite surfaces for their defect control, however, the structural stability of the modified interface against external stimuli still remains unresolved. Herein, 4‐dimethylaminopyridine (DMAP) is introduced to develop a facile technique for selectively passivating the grain boundary (GB) and controlling the topographical boundary of the perovskite surface near the GB. Through the surface treatment of DMAP, strongly bound DMAP crystals are selectively formed at the GB, which serves two functions: nonradiative recombination at GB is effectively reduced by healing the uncoordinated Pb²⁺ while adhesion strength between the perovskite and the poly(triaryl amine) (PTAA) polymer is significantly enhanced by a mechanical interlock effect. A planar PSC with DMAP treatment exhibits a champion power conversion efficiency of 22.4%, which is not only much higher than the 20.04% observed for a nontreated control device, but also the highest among the planar PSCs using PTAA polymers as a hole transport material. Furthermore, the use of DMAP leads to a substantial improvement in the device stability under damp‐heat test and light irradiation.

The Ruddlesden–Popper (RP) perovskite film quality is a governing factor determining the performance of RP perovskite solar cells (PSCs). The RP film quality for fabricating high‐performance RP perovskite‐based PSCs is discussed. Evaluation index and influential factors of RP perovskite films are reviewed. Based on this, strategies to improve RP perovskite film quality are also proposed.

Abstract

In the last decade, perovskite solar cells (PSCs) have undergone unprecedented rapid development and become a promising candidate for a new‐generation solar cell. Among various PSCs, typical 3D halide perovskite‐based PSCs deliver the highest efficiency but they suffer from severe instability, which restricts their practical applications. By contrast, the low‐dimensional Ruddlesden–Popper (RP) perovskite‐based PSCs have recently raised increasing attention due to their superior stability. Yet, the efficiency of RP perovskite‐based PSCs is still far from that of the 3D counterparts owing to the difficulty in fabricating high‐quality RP perovskite films. In pursuit of high‐efficiency RP perovskite‐based PSCs, it is critical to manipulate the film formation process to prepare high‐quality RP perovskite films. This review aims to provide comprehensive understanding of the high‐quality RP‐type perovskite film formation by investigating the influential factors. On this basis, several strategies to improve the RP perovskite film quality are proposed via summarizing the recent progress and efforts on the preparation of high‐quality RP perovskite film. This review will provide useful guidelines for a better understanding of the crystallization and phase kinetics during RP perovskite film formation process and the design and development of high‐performance RP perovskite‐based PSCs, promoting the commercialization of PSC technology.

A cation/anion co‐alloying strategy is applied to prepare Zn_0.4Cu_0.7In_1.0S _x Se_2−x (ZCISSe) quinary alloyed quantum dots (QDs). The band gap, conduction band energy level, and density of defect trap states of the QDs can be conveniently tailored through composition engineering. The quinary alloyed QDs deliver a new certified efficiency record of 14.4 % for quantum‐dot‐sensitized solar cells.

Abstract

The photoelectronic properties of quantum dots (QDs) have a critical impact on the performance of quantum‐dot‐sensitized solar cells (QDSCs). Currently, I‐III‐VI group QDs have become the mainstream light‐harvesting materials in high‐performance QDSCs. However, it is still a great challenge to achieve satisfactory efficiency for light‐harvesting, charge extraction, and charge collection simultaneously in QDSCs. We design and prepare Zn_0.4Cu_0.7In_1.0S _x Se_2−x (ZCISSe) quinary alloyed QDs by cation/anion co‐alloying strategy. The critical photoelectronic properties of target QDs, including band gap, conduction band energy level, and density of defect trap states, can be conveniently tailored. Experimental results demonstrate that the ZCISSe quinary alloyed QDs can achieve an ideal balance among light‐harvesting, photogenerated electron extraction, and charge‐collection efficiencies in QDSCs compared to its single anion or cation quaternary alloyed QD counterparts. Consequently, the quinary alloyed QDs boost the certified efficiency of QDSCs to 14.4 %, which is a new efficiency record for liquid‐junction QD solar cells.

A low‐cost industrial organic pigment, quinacridone (QA), was applied as surface passivation agent for perovskite solar cells (PSCs) by solution processing of a soluble QA derivative followed by thermal annealing to convert it into insoluble QA. Passivation with strong interactions between QA molecules and metal halides, together with the hydrophobicity of QA coating, enabled highly efficient PSCs with remarkable stability.

Abstract

Surface passivation of perovskite solar cells (PSCs) using a low‐cost industrial organic pigment quinacridone (QA) is presented. The procedure involves solution processing a soluble derivative of QA, N,N‐bis(tert‐butyloxycarbonyl)‐quinacridone (TBOC‐QA), followed by thermal annealing to convert TBOC‐QA into insoluble QA. With halide perovskite thin films coated by QA, PSCs based on methylammonium lead iodide (MAPbI₃) showed significantly improved performance with remarkable stability. A PCE of 21.1 % was achieved, which is much higher than 18.9 % recorded for the unmodified devices. The QA coating with exceptional insolubility and hydrophobicity also led to greatly enhanced contact angle from 35.6° for the pristine MAPbI₃ thin films to 77.2° for QA coated MAPbI₃ thin films. The stability of QA passivated MAPbI₃ perovskite thin films and PSCs were significantly enhanced, retaining about 90 % of the initial efficiencies after more than 1000 hours storage under ambient conditions.

A regio‐regular polymer acceptor PYF‐T‐o with highly conjugated polymerization sites is synthesized, exhibiting red‐shifted absorption and ordered inter‐chain packing compared to its isomeric counterparts (PYF‐T and PYF‐T‐m). When employed in all‐polymer solar cells, the PYF‐T‐o‐based devices yield a higher efficiency of 15.2 %.

Abstract

Polymerization sites of small molecule acceptors (SMAs) play vital roles in determining device performance of all‐polymer solar cells (all‐PSCs). Different from our recent work about fluoro‐ and bromo‐ co‐modified end group of IC‐FBr (a mixture of IC‐FBr1 and IC‐FBr2), in this paper, we synthesized and purified two regiospecific fluoro‐ and bromo‐ substituted end groups (IC‐FBr‐o & IC‐FBr‐m), which were then employed to construct two regio‐regular polymer acceptors named PYF‐T‐o and PYF‐T‐m, respectively. In comparison with its isomeric counterparts named PYF‐T‐m with different conjugated coupling sites, PYF‐T‐o exhibits stronger and bathochromic absorption to achieve better photon harvesting. Meanwhile, PYF‐T‐o adopts more ordered inter‐chain packing and suitable phase separation after blending with the donor polymer PM6, which resulted in suppressed charge recombination and efficient charge transport. Strikingly, we observed a dramatic performance difference between the two isomeric polymer acceptors PYF‐T‐o and PYF‐T‐m. While devices based on PM6:PYF‐T‐o can yield power conversion efficiency (PCE) of 15.2 %, devices based on PM6:PYF‐T‐m only show poor efficiencies of 1.4 %. This work demonstrates the success of configuration‐unique fluorinated end groups in designing high‐performance regular polymer acceptors, which provides guidelines towards developing all‐PSCs with better efficiencies.

Nature Materials, Published online: 28 January 2021; doi:10.1038/s41563-020-00876-2