JIANG ZHI XUAN

A polymerization‐assisted grain growth strategy in the sequential deposition method of perovskite thin films is demonstrated by triggering a polymerization process during PbI₂ film annealing. This strategy effectively passivates undercoordinated lead ions, reduces defect density, and boosts power conversion efficiency up to 23.0%, together with a prolonged lifetime.

Abstract

Intrinsically, detrimental defects accumulating at the surface and grain boundaries limit both the performance and stability of perovskite solar cells. Small molecules and bulkier polymers with functional groups are utilized to passivate these ionic defects but usually suffer from volatility and precipitation issues, respectively. Here, starting from the addition of small monomers in the PbI₂ precursor, a polymerization‐assisted grain growth strategy is introduced in the sequential deposition method. With a polymerization process triggered during the PbI₂ film annealing, the bulkier polymers formed will be adhered to the grain boundaries, retaining the previously established interactions with PbI₂. After perovskite formation, the polymers anchored on the boundaries can effectively passivate undercoordinated lead ions and reduce the defect density. As a result, a champion power conversion efficiency (PCE) of 23.0% is obtained, together with a prolonged lifetime where 85.7% and 91.8% of the initial PCE remain after 504 h continuous illumination and 2208 h shelf storage, respectively.

Self‐crystallized multifunctional 2D perovskite (M2P) is formed on top of a 3D perovskite light absorber. The M2P layer performs as a hole‐transfer facilitator and a surface‐trap passivator in perovskite solar cells (PSCs). PSCs using the developed 3D/2D perovskites achieve a power conversion efficiency of 20.79% with highly improved long‐term stability compared to devices without M2P.

Abstract

Recently, perovskite solar cells (PSC) with high power‐conversion efficiency (PCE) and long‐term stability have been achieved by employing 2D perovskite layers on 3D perovskite light absorbers. However, in‐depth studies on the material and the interface between the two perovskite layers are still required to understand the role of the 2D perovskite in PSCs. Self‐crystallization of 2D perovskite is successfully induced by deposition of benzyl ammonium iodide (BnAI) on top of a 3D perovskite light absorber. The self‐crystallized 2D perovskite can perform a multifunctional role in facilitating hole transfer, owing to its random crystalline orientation and passivating traps in the 3D perovskite. The use of the multifunctional 2D perovskite (M2P) leads to improvement in PCE and long‐term stability of PSCs both with and without organic hole transporting material (HTM), 2,2′,7,7′‐tetrakis‐(N,N‐di‐p‐methoxyphenyl‐amine)‐9,9′‐spirobifluorene (spiro‐OMeTAD) compared to the devices without the M2P.

A strategy is demonstrated for efficacious regulation of perovskite crystallinity using glycolic acid (GA) against nonvolatile thioglycolic acid (TGA) following dimethyl sulfoxide sublimation, resulting in enhanced device performance. A champion power conversion efficiency as high as 21.32% is achieved for the GA‐based device, which is almost 13% or 20% higher than those of the control device or TGA‐based device.

Abstract

A strategy for efficaciously regulating perovskite crystallinity is proposed by using a volatile solid glycolic acid (HOCH₂COOH, GA) in an FA_0.85MA_0.15PbI₃ (FA: HC(NH₂)₂; MA: CH₃NH₃) perovskite precursor solution that is different from the common additive approach. Accompanied with the first dimethyl sulfoxide sublimation process, the subsequent sublimation of GA before 150 °C in the FA_0.85MA_0.15PbI₃ perovskite film can artfully regulate the perovskite crystallinity without any residual after annealing. The improved film formation upon GA modification induced by the strong interaction between GA and Pb²⁺ delivers a champion power conversion efficiency (PCE) as high as 21.32%. In order to investigate the role of volatility in perovskite solar cells (PSCs), nonvolatile thioglycolic acid (HSCH₂COOH, TGA) with a similar structure to GA is utilized as an additive reference. Large perovskite grains are obtained by TGA modification but with obvious pinholes, which directly leads to an increased defect density accompanied by a decline in PCE. Encouragingly, the champion PCE achieved for GA‐based PSC device (21.32%) is almost 13% or 20% higher than those of the control device or TGA‐based device. In addition, GA‐modified PSCs exhibit the best stability in light‐, thermal‐, and humidity‐based tests due to the improved film formation.

In article https://doi.org/10.1002/smll.2019014661901466, Bo Chi, Gregory J. Wilson, and co‐workers investigate superior electronic properties of nanostructured tin (IV) oxide (SnO₂) as an ideal inorganic electron transport layer (ETL) in n–i–p perovskite solar cells. The bilayer ETL architecture attaining impressive power conversion efficiency (PCE) greater than 20% is depicted.

Perovskites with different dimensional structures attract wide attention in the field of photovoltaics. There are three main perovskite structures that can be obtained by tuning the A‐site cations, i.e., 3D perovskite, reduced‐dimensional perovskite, and 2D/3D hybrid perovskite, which exhibit different properties, characters, and intrinsic weaknesses. With the use of efficient and targeted tactics, such materials show great potential for perovskite solar cells.

Abstract

Organic–inorganic hybrid lead‐halide perovskite materials (ABX₃) have attracted widespread attention in the field of photovoltaics owing to their impressive optical and electrical properties. However, obstacles still exist in the commercialization of perovskite photovoltaics, such as poor stability, hysteresis, and human toxicity. A‐site cation engineering is considered to be a powerful tool to tune perovskite structures and the resulting optoelectronic properties. Based on the selection and combination of A‐site cations, three types of perovskite structures, i.e., 3D perovskite, reduced‐dimensional (2D/quasi‐2D) perovskite, and 2D/3D hybrid perovskite can be formed. Herein, the remarkable breakthroughs resulting from these three perovskite structures are summarized, and their corresponding properties and characteristics, as well as their intrinsic disadvantages, are highlighted. By summarizing recent research progress, a new viewpoint for improving the performance and stability of perovskite photovoltaics is provided.

The undercoordinated ionic defects at heterojunction interfaces remain challenges that limit the performances and stability of perovskite photoelectric devices. A self‐phase separated doping strategy is developed to link multilayer heterojunction interfaces including both the energy level and trap states, paving a novel route for nonequilibrium distributed dopants to solve the key challenge of interface defects.

Abstract

In perovskite solar cells (PSCs), the interfaces of the halide perovskite/electron transport layer (ETL) and ETL/metal oxide electrode (MOE) always attract and trap free carriers via the surface electrostatic force, altering quasi‐Fermi level (E _Fq) splitting of contact interfaces, and significantly limit the charge extraction efficiency and intrinsic stability of devices. Herein, a graded “bridge” is first reported to link the MOE and perovskite interfaces by self vertical phase separation doping (PSD), diminishing the side effect of notorious ionic defects via both reinforced interface E _bi and the vacancies filling. Experimental and theoretical results prove that the inhomogeneous distribution of CsF in the bulk or surface of PC₆₁BM would not only form metal–oxygen (M–O) dipole on MOE, reinforcing the interface E _bi, but also create a graded energy bridge to alleviate the disadvantage of band offset raised by the enhanced interface E _bi, which significantly avoid the carrier accumulation and recombination at defective interfaces. Employing PSD, the power conversion efficiency of the devices approaches 21% with a high open‐circuit voltage (1.148 V) and delivers a high stability of 89% after aging 60 days in atmosphere without encapsulation, which is the highest efficiency of organic electron transport layers for n–i–p PSCs.

Incorporation a small portion of a novel polymer donor named PBT(E)BTz with a deeper highest occupied molecular orbital level than the host materials is proven promising to construct highly efficient ternary polymer solar cells (PSCs). In addition to the role of a “solid additive” for ternary PSCs, PBT(E)BTz shows great potential to be a thermal and light stabilizer in ternary PSCs.

Abstract

Ternary strategies have attracted extensive attention due to their potential in improving power conversion efficiencies (PCEs) of single‐junction polymer solar cells (PSCs). In this work, a novel wide bandgap polymer donor (E _g ^opt ≈ 2.0 eV) named PBT(E)BTz with a deep highest occupied molecular orbital (HOMO) level (≈−5.73 eV) is designed and synthesized. PBT(E)BTz is first incorporated as the third component into the classic PBDB‐T‐SF:IT‐4F binary PSC system to fabricate efficient ternary PSCs. A higher PCE of 13.19% is achieved in the ternary PSCs with a 5% addition of PBT(E)BTz over binary PSCs (12.14%). Similarly, addition of PBT(E)BTz improves the PCE for PBDB‐T:IT‐M binary PSCs from 10.50% to 11.06%. The study shows that the improved PCE in ternary PSCs is mainly attributed to the suppressed charge carrier recombination and more balanced charge transport. The generality of PBT(E)BTz as a third component is further evidenced in another efficient binary PSC system—PBDB‐TF:BTP‐4Cl: an optimized PCE of 16.26% is realized in the ternary devices. This work shows that PBT(E)BTz possessing a deep HOMO level as an additional component is an effective ternary PSC construction strategy toward enhancing device performance. Furthermore, the ternary device with 5% PBT(E)BTz displays better thermal and light stability over binary devices.

The undercoordinated ionic defects at heterojunction interfaces remain challenges that limit the performances and stability of perovskite photoelectric devices. A self‐phase separated doping strategy is developed to link multilayer heterojunction interfaces including both the energy level and trap states, paving a novel route of nonequilibrium distributed dopant to solve the key challenge of interface defects.

Abstract

In perovskite solar cells (PSCs), the interfaces of the halide perovskite/electron transport layer (ETL) and ETL/metal oxide electrode (MOE) always attract and trap free carriers via the surface electrostatic force, altering quasi‐Fermi level (E _Fq) splitting of contact interfaces, and significantly limit the charge extraction efficiency and intrinsic stability of devices. Herein, a graded “bridge” is first reported to link the MOE and perovskite interfaces by self vertical phase separation doping (PSD), diminishing the side effect of notorious ionic defects via both reinforced interface E _bi and the vacancies filling. Experimental and theoretical results prove that the inhomogeneous distribution of CsF in the bulk or surface of PC₆₁BM would not only form metal–oxygen (M–O) dipole on MOE, reinforcing the interface E _bi, but also create a graded energy bridge to alleviate the disadvantage of band offset raised by the enhanced interface E _bi, which significantly avoid the carrier accumulation and recombination at defective interfaces. Employing PSD, the power conversion efficiency of the devices approaches 21% with a high open‐circuit voltage (1.148 V) and delivers a high stability of 89% after aging 60 days in atmosphere without encapsulation, which is the highest efficiency of organic electron transport layers for n–i–p PSCs.

The aza[5]helicene‐based hole‐transporter is superior to its congener with the planar N‐annulated perylene π‐linker. This study has highlighted that the use of a helical π‐linker for donor−π linker−donor typed organic semiconductors can retain stronger intermolecular π⋅⋅⋅π interactions and attenuated interface charge recombination, leading to better power conversion efficiency of perovskite solar cells.

Abstract

The superior role of helical π‐linkers is demonstrated for the design of donor−π linker−donor typed molecular semiconductors in perovskite solar cells (PSCs). Flat N‐annulated perylene (NP) and contorted aza[5]helicene (A5H) are side‐functionalized with methoxyphenyl and end‐capped with dimethoxydiphenylamine electron‐donor to afford two small‐molecule hole‐transporters J3 and J4. For methoxyphenyl functionalized π‐linkers, intermolecular π⋅⋅⋅π interactions in planar NP exist more extensively than those in helical A5H. However, for the dimethoxydiphenylamine derived hole‐transporters with high highest occupied molecular orbital energy levels, a part of the π⋅⋅⋅π interaction remains for J4 with A5H, while this desirable effect for charge transport is completely deprived for J3 with NP. Thus, the theoretically predicted hole mobility of J4 single‐crystal is even over two times higher than that of J3 one. Because of the larger size of the molecular aggregate, the hole mobility of the spin‐coated J4 thin film is also over three times as high as that of the J3 analog. Due to the reduced transport resistance and enhanced recombination resistance, PSCs with J4 exhibit a power conversion efficiency of 21.0% at standard air mass 1.5 global conditions, which is higher than that of 19.4% with J3 and that of 20.3% with spiro‐OMeTAD control.

In article number https://doi.org/10.1002/aenm.2019042341904234, Feng Liu and co‐workers report a detailed structure‐performance relationship to help understand the success of Y6 non‐fullerene acceptors. Through the analysis of the single crystal structure of Y6, it is found that Y6 forms a polymer‐like conjugated backbone through its banana‐shaped structure and π‐π interactions between molecules, and forms a 2D electron transport network under the ordered arrangement of the lattice.

The doping of poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) with dopamine is reported. The doping of dopamine endows PEDOT:PSS with enhanced work function and conductivity. This work provides an efficient strategy to enhance the performances of organic solar cells.

Abstract

Poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has been is applied as hole transport material in organic electronic devices for more than 20 years. However, the redundant sulfonic acid group of PEDOT:PSS has often been overlooked. Herein, PEDOT:PSS‐DA is prepared via a facile doping of PEDOT:PSS with dopamine hydrochloride (DA·HCl) which reacts with the redundant sulfonic acid of PSS. The PEDOT:PSS‐DA film exhibits enhanced work function and conductivity compared to those of PEDOT:PSS. PEDOT:PSS‐DA‐based devices show a power conversion efficiency of 16.55% which is the highest in organic solar cells (OSCs) with (poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl)‐4‐fluorothiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]dithio‐phene))‐co‐(1,3‐di(5‐thiophene‐2‐yl)‐5,7‐bis(2‐ethylhexyl)‐benzo[1,2‐c:4,5‐c′]dithiophene‐4,8‐dione))] (PM6):(2,2′‐((2Z,2′Z)‐((12,13‐bis(2‐ethylhexyl)‐3,9‐diundecyl‐12,13‐dihydro‐[1,2,5]thiadiazolo[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‐2,10‐diyl)bis(methanylylidene))bis(5,6‐difluoro‐3‐oxo‐2,3‐dihydro‐1H‐indene‐2,1‐diylidene))dimalononitrile) (Y6) as the active layer. Furthermore, PEDOT:PSS‐DA also exhibits enhanced performance in three other donor/acceptor systems, exhibiting high compatibility in OSCs. This work demonstrates that doping PEDOT:PSS with various amino derivatives is a potentially efficient strategy to enhance the performance of PEDOT:PSS in organic electronic devices.