shuhui

Photostability is one of the most vital challenges for perovskite solar cells (PSCs). With the embedding of black phosphorus (BP), well known for its self‐healing and superior property to regulate charge recombination, into MAPbI₃ perovskites, the associated devices exhibit significant enhancement in photostability. The incorporation of BP effectively inhibits Pb⁰ defect formation and retards hot carrier recombination.

Photostability is one of the most vital challenges for organic–inorganic hybrid perovskite solar cells (PSCs). With the incorporation of black phosphorus (BP), well known for self‐healing and its superior property to regulate charge recombination, into CH₃NH₃PbI₃ perovskites (MAPbI₃/BP), the associated PSCs exhibit significant enhancement in photostability in addition to the photovoltaic (PV) performance. The MAPbI₃/BP‐based PSCs retain ≈94% of initial efficiency after 1000 h continuous white light LED illumination in a dry N₂ glovebox whereas their counterparts without the incorporation of BP decrease to ≈30%. Although BP has very small influence on the morphology and structure of the perovskite crystals, Pb⁰ defects are effectively inhibited and hot carrier recombination is found to be retarded as confirmed by femtosecond optical spectroscopy. The utilization of the material to simultaneously inhibit Pb⁰ defect formation and retard charge recombination, such as BP, is a promising strategy to enhance the photostability of organic–inorganic hybrid perovskite‐based PSCs and their siblings.

Sulfur‐incorporated bismuth‐based perovskite films are obtained by a low‐pressure vapor‐assisted solution process (LP‐VASP) method. A homogeneous and highly compact MBI film with a narrower bandgap of 1.67 eV is successfully achieved. In addition, the obtained film has a low trap‐state density of 1.9 × 10¹⁶ cm⁻³ and the optimal PCE of MA₃Bi₂I_9‐2x S _x PSCs reached 0.152%.

Methylammonium (MA) bismuth iodide ((CH₃NH₃)₃Bi₂I₉) is a promising perovskite material for solar cell application considering the air stability and the nontoxic lead‐free molecular constitution. However, the further improvement of the device performances is prohibited by the wide bandgap (≈2.1 eV) and unsatisfied crystallinity of the (CH₃NH₃)₃Bi₂I₉ films. Herein, a developed low‐pressure vapor‐assisted solution process (LP‐VASP) method is applied to obtain the sulfur‐incorporated bismuth‐based perovskites films. Due to the presence of sulfur, both the crystal quality and the energy band property are improved effectively in the as‐fabricated lead‐free perovskite films. After a systematic study of the influence of the reaction time on the device performances, the optimized reaction time is found to be 30 min, under which, the sulfur‐incorporated MA₃Bi₂I_9‐2x S _x perovskite films exhibit a reduced bandgap of 1.67 eV and a compact morphology. The corresponding optimal PCE reaches 0.152%. This study provides a new way for the incorporation of sulfur in the lead‐free bismuth‐based perovskite solar cells.

Two novel hole‐transporting materials (HTMs) based on 9‐(4‐methoxyphenyl) carbazole and benzodithiophene cores are synthesized. The impact of these cores on the physicochemical properties and performance of perovskite solar cells (PSCs) based on these HTMs are investigated. The newly developed N1,N1′‐(9‐(4‐methoxyphenyl)‐9H‐carbazole‐3,6‐diyl)bis(N1‐(4‐(bis(4‐methoxyphenyl)amino)phenyl)‐N4,N4‐bis(4‐methoxyphenyl)benzene‐1,4‐diamine) (PhCz‐4MeOTPA)‐based PSC exhibits a power conversion efficiency of 16.04% along with enhanced stability under heat and illumination.

Perovskite solar cells (PSCs) possess both high‐power conversion efficiency (PCE) and good operation stability for future application. Although many different types of hole‐transporting materials (HTMs) are assessed, few dopant‐free small organic molecule HTMs‐based PSC cells exist, which exhibit excellent stability under both heat and illumination. Herein, two novel HTMs that are based on 9‐(4‐methoxyphenyl) carbazole and benzodithiophene cores are synthesized and named N1,N1′‐(9‐(4‐methoxyphenyl)‐9H‐carbazole‐3,6‐diyl)bis(N1‐(4‐(bis(4‐methoxyphenyl)amino)phenyl)‐N4,N4‐bis(4‐methoxyphenyl)benzene‐1,4‐diamine) (PhCz‐4MeOTPA) and N1,N1′‐(benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl)bis(N1‐(4‐(bis(4‐methoxyphenyl)amino)phenyl)‐N4,N4‐bis(4‐methixyphenyl)benzene‐1,4‐diamine) (BDT‐4MeOTPA). Of the two HTMs, PhCz‐4MeOTPA possesses a lower level of planarity than that of BDT‐4MeOTPA, which inhibits molecular stacking to improve film quality and increases hole‐transport mobility and charge transport. A PCE of 16.04% is achieved with the application of dopant‐free PhCz‐4MeOTPA in PSCs, which is higher than that of dopant‐free BDT‐4MeOTPA. The unencapsulated PSC devices based on PhCz‐4MeOTPA maintain 82% of their initial values under continuous sun illumination in an ambient environment at 40–45 °C after 672 h and 92% of their initial values at 80 °C in an ambient environment after 1200 h in the dark.

Publication date: August 2019

Source: Nano Energy, Volume 62

Author(s): Daniele Benetti, Efat Jokar, Che-Hsun Yu, Amir Fathi, Haiguang Zhao, Alberto Vomiero, Eric Wei-Guang Diau, Federico Rosei

Graphical abstract

An energetic cascade between mixed and pure regions assists in suppressing recombination losses in nonfullerene acceptor (NFA)‐based organic solar cells. The impact of polymer–NFA blend composition upon film morphology, energetics, charge carrier recombination kinetics, and photocurrent properties is studied.

Abstract

Here, it is investigated whether an energetic cascade between mixed and pure regions assists in suppressing recombination losses in non‐fullerene acceptor (NFA)‐based organic solar cells. The impact of polymer‐NFA blend composition upon morphology, energetics, charge carrier recombination kinetics, and photocurrent properties are studied. By changing film composition, morphological structures are varied from consisting of highly intermixed polymer‐NFA phases to consisting of both intermixed and pure phase. Cyclic voltammetry is employed to investigate the impact of blend morphology upon NFA lowest unoccupied molecular orbital (LUMO) level energetics. Transient absorption spectroscopy reveals the importance of an energetic cascade between mixed and pure phases in the electron–hole dynamics in order to well separate spatially localized electron–hole pairs. Raman spectroscopy is used to investigate the origin of energetic shift of NFA LUMO levels. It appears that the increase in NFA electron affinity in pure phases relative to mixed phases is correlated with a transition from a relatively planar backbone structure of NFA in pure, aggregated phases, to a more twisted structure in molecularly mixed phases. The studies focus on addressing whether aggregation‐dependent acceptor LUMO level energetics are a general design requirement for both fullerene and NFAs, and quantifying the magnitude, origin, and impact of such energetic shifts upon device performance.

The different aggregation forms of hole‐transporting materials (HTMs) affect intermolecular charge transfer and hole transporting in achieving highly efficient dopant‐free perovskite solar cells. The combination of twisted periphery groups with planar core units shows an efficient approach to regulate the state of molecular aggregation after a systematical investigation of 6,12‐dihydroindeno[1,2‐b]fluorine (IDF)‐HTMs with the same IDF core and the different periphery groups.

Abstract

Although several hole‐transporting materials (HTMs) have been designed to obtain perovskite solar cells (PSCs) devices with high performance, the dopant‐free HTMs for efficient and stable PSCs remain rare. Herein, a rigid planar 6,12‐dihydroindeno[1,2‐b]fluorine (IDF) core with different numbers of bulky periphery groups to construct dopant‐free HTMs of IDF‐SFXPh, IDF‐DiDPA, and IDF‐TeDPA is modified. Thanks to the contributions of the planar IDF core and the twisted SFX periphery groups, the dopant‐free IDF‐SFXPh‐based PSCs device achieves a device performance of 17.6%, comparable to the doped 2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenylamine)‐9,9′‐spirobifluorene (spiro‐OMeTAD)‐based device (17.6%), with much enhanced device stability under glovebox and ambient conditions.

A lead‐bismuth (Pb‐Bi) binary metal based all‐inorganic perovskite film is successfully fabricated and applied as absorber layer to enhance the stability of perovskite solar cells (PSCs). High power conversion efficiency (PCE) of 11.9% is obtained for the all‐inorganic (PSC).The PCE only reduced by 10% under atmospheric humidity of 40% in 4 weeks.

Abstract

A lead‐bismuth (Pb‐Bi) binary metal based all‐inorganic perovskite film is successfully fabricated and applied as absorber layer to enhance the stability of perovskite solar cells (PSCs). Unlike the Pb‐only perovskite‐based device, the Pb‐Bi binary metal perovskite based one shows better tolerance to humidity and oxygen. High power conversion efficiency (PCE) of 11.9% is obtained for the all‐inorganic (PSC). Noticeably, the PCE only reduced by 10% under atmospheric humidity of 40% in four weeks. An electron‐only device also shows reduced trap states. The improved stability and PCE is ascribed to higher quality perovskite film with less trap states and smaller series resistance (R _s) in the device.

Publication date: August 2019

Source: Nano Energy, Volume 62

Author(s): Zhiliang Liu, Sibo Li, Xu Wang, Yuying Cui, Yuan Qin, Shifeng Leng, Yun-xiang Xu, Kai Yao, Haitao Huang

Graphical abstract

Metal oxides are used as charge transporting layers to effectively separate the photogenerated electrons and holes in perovskite solar cells (PSCs). The metal oxide layers require a wide bandgap, a good charge mobility, and a compatible band alignment with the perovskite layers. This review summarizes and correlates the preparation and performance of the various metal oxides used in PSCs.

Abstract

Currently, the efficiency of perovskite solar cells (PSCs) is ≈24%. For the fabrication of such high efficiency PSCs, it is necessary to use both electron and hole transport layers to effectively separate the charges generated by light absorption of the perovskite layer and selectively transfer the separated electrons and holes. In addition to the efficiency, the materials used for transporting charges must be resilient to light, heat, and moisture to ensure long‐term stability of PSCs; furthermore, low‐cost fabrication is required to form a charge transport layer at low temperatures by a solution process. For this purpose, metal oxides are best suited as charge transport materials for PSCs because of their advantages such as low cost, long‐term stability, and high efficiency. In this Review, the metal oxide electron and hole transport materials used in PSCs are reviewed and preparation of these materials is summarized. Finally, the challenges and future research direction for metal oxide‐based charge transport materials are described.

A thermodynamically favored crystal preferable orientation growth along the (001) crystal plane is explored in formamidinium/methylammonium mixed perovskites, and the origin is found to be the reduction of surface energy. Combined with the (001) plane lying parallel to the substrate, it affects the charge transportation and collection in the resultant perovskite solar cells, resulting in a power conversion efficiency of 21.2%.

Abstract

Crystal orientation has a great impact on the properties of perovskite films and the resultant device performance. Up to now, the exquisite control of crystal orientation (the preferred crystallographic planes and the crystal stacking mode with respect to the particular planes) in mixed‐cation perovskites has received limited success, and the underlying mechanism that governs device performance is still not clear. Here, a thermodynamically favored crystal orientation in formamidinium/methylammonium (FA/MA) mixed‐cation perovskites is finely tuned by composition engineering. Density functional theory calculations reveal that the FA/MA ratio affects the surface energy of the mixed perovskites, leading to the variation of preferential orientation consequently. The preferable growth along the (001) crystal plane, when lying parallel to the substrates, affects their charge transportation and collection properties. Under the optimized condition, the mixed‐cation perovskite (FA_{1– x} MA _x PbI_2.87Br_0.13 (Cl)) solar cells deliver a champion power conversion efficiency over 21%, with a certified efficiency of 20.50 ± 0.50%. The present work not only provides a vital step in understanding the intrinsic properties of mixed‐cation perovskites but also lays the foundation for further investigation and application in perovskite optoelectronics.

Solution‐phase van der Waals epitaxy growth of MAPbI₃ perovskite films on MoS₂ flakes is observed. The in‐plane coupling between the perovskite and the MoS₂ crystal lattices leads to perovskite films with larger grain size, lower trap density, and preferential growth orientation. Consequently, the efficiency of fabricated perovskite solar cells is substantially improved by the MoS₂ flakes as interfacial layers.

Abstract

The quality of perovskite films is critical to the performance of perovskite solar cells. However, it is challenging to control the crystallinity and orientation of solution‐processed perovskite films. Here, solution‐phase van der Waals epitaxy growth of MAPbI₃ perovskite films on MoS₂ flakes is reported. Under transmission electron microscopy, in‐plane coupling between the perovskite and the MoS₂ crystal lattices is observed, leading to perovskite films with larger grain size, lower trap density, and preferential growth orientation along (110) normal to the MoS₂ surface. In perovskite solar cells, when perovskite active layers are grown on MoS₂ flakes coated on hole‐transport layers, the power conversion efficiency is substantially enhanced for 15%, relatively, due to the increased crystallinity of the perovskite layer and the improved hole extraction and transfer rate at the interface. This work paves a way for preparing high‐performance perovskite solar cells and other optoelectronic devices by introducing 2D materials as interfacial layers.

The factors affecting the V _OC in 2D perovskite cells with different [PbI₆]⁴⁻ layer sheets (n = 2–4) are elucidated. Nonradiative recombination at the perovskite/C₆₀ interface is found to dominate except for the n = 2 system where the bulk recombination determines the properties of the cell. Substantial V _OC gains through suppression of interfacial recombination at the top interface are expected.

Abstract

2D Ruddlesden–Popper perovskite (RPP) solar cells have excellent environmental stability. However, the power conversion efficiency (PCE) of RPP cells remains inferior to 3D perovskite‐based cells. Herein, 2D (CH₃(CH₂)₃NH₃)₂(CH₃NH₃) _{n −1}Pb _n I_{3 n +1} perovskite cells with different numbers of [PbI₆]⁴⁻ sheets (n = 2–4) are analyzed. Photoluminescence quantum yield (PLQY) measurements show that nonradiative open‐circuit voltage (V _OC) losses outweigh radiative losses in materials with n > 2. The n = 3 and n = 4 films exhibit a higher PLQY than the standard 3D methylammonium lead iodide perovskite although this is accompanied by increased interfacial recombination at the top perovskite/C₆₀ interface. This tradeoff results in a similar PLQY in all devices, including the n = 2 system where the perovskite bulk dominates the recombination properties of the cell. In most cases the quasi‐Fermi level splitting matches the device V _OC within 20 meV, which indicates minimal recombination losses at the metal contacts. The results show that poor charge transport rather than exciton dissociation is the primary reason for the reduction in fill factor of the RPP devices. Optimized n = 4 RPP solar cells had PCEs of 13% with significant potential for further improvements.

This work reports a strategy that ensures the degree of nonradiative recombination can be measured accurately in low‐energetic‐offset organic photovoltaic systems and reports key observations on the relationship between the nonradiative recombination loss and properties of the donor/acceptor interface, including an observed correlation between high domain purity and high nonradiative recombination loss.

Abstract

Open‐circuit voltage (V _OC) losses in organic photovoltaics (OPVs) inhibit devices from reaching V _OC values comparable to the bandgap of the donor–acceptor blend. Specifically, nonradiative recombination losses (∆V _nr) are much greater in OPVs than in silicon or perovskite solar cells, yet the origins of this are not fully understood. To understand what makes a system have high or low loss, an investigation of the nonradiative recombination losses in a total of nine blend systems is carried out. An apparent relationship is observed between the relative domain purity of six blends and the degree of nonradiative recombination loss, where films exhibiting relatively less pure domains show lower ∆V _nr than films with higher domain purity. Additionally, it is shown that when paired with a fullerene acceptor, polymer donors which have bulky backbone units to inhibit close π–π stacking exhibit lower nonradiative recombination losses than in blends where the polymer can pack more closely. This work reports a strategy that ensures ∆V _nr can be measured accurately and reports key observations on the relationship between ∆V _nr and properties of the donor/acceptor interface.

High‐throughput density functional theory (DFT) methods are used to screen 1845 halide perovskite materials in search of nontoxic, stable, optimal bandgap materials with high photovoltaic efficiencies for use in single junction, quantum dot, and tandem Si‐perovskite solar cells. A total of 15 promising halide perovskite materials, including (CH₃NH₃)_0.75Cs_0.25SnI₃, ((NH₂)₂CH)Ag_0.5Sb_0.5Br₃, CsMn_0.875Fe_0.125I₃, ((CH₃)₂NH₂)Ag_0.5Bi_0.5I₃, and ((NH₂)₂CH)_0.5Rb_0.5SnI₃, are found.

Abstract

Two critical limitations of organic–inorganic lead halide perovskite materials for solar cells are their poor stability in humid environments and inclusion of toxic lead. In this study, high‐throughput density functional theory (DFT) methods are used to computationally model and screen 1845 halide perovskites in search of new materials without these limitations that are promising for solar cell applications. This study focuses on finding materials that are comprised of nontoxic elements, stable in a humid operating environment, and have an optimal bandgap for one of single junction, tandem Si‐perovskite, or quantum dot–based solar cells. Single junction materials are also screened on predicted single junction photovoltaic (PV) efficiencies exceeding 22.7%, which is the current highest reported PV efficiency for halide perovskites. Generally, these methods qualitatively reproduce the properties of known promising nontoxic halide perovskites that are either experimentally evaluated or predicted from theory. From a set of 1845 materials, 15 materials pass all screening criteria for single junction cell applications, 13 of which are not previously investigated, such as (CH₃NH₃)_0.75Cs_0.25SnI₃, ((NH₂)₂CH)Ag_0.5Sb_0.5Br₃, CsMn_0.875Fe_0.125I₃, ((CH₃)₂NH₂)Ag_0.5Bi_0.5I₃, and ((NH₂)₂CH)_0.5Rb_0.5SnI₃. These materials, together with others predicted in this study, may be promising candidate materials for stable, highly efficient, and nontoxic perovskite‐based solar cells.

The factors affecting the V _OC in 2D perovskite cells with different [PbI₆]⁴⁻ layer sheets (n = 2–4) are elucidated. Nonradiative recombination at the perovskite/C₆₀ interface is found to dominate except for the n = 2 system where the bulk recombination determines the properties of the cell. Substantial V _OC gains through suppression of interfacial recombination at the top interface are expected.

Abstract

2D Ruddlesden–Popper perovskite (RPP) solar cells have excellent environmental stability. However, the power conversion efficiency (PCE) of RPP cells remains inferior to 3D perovskite‐based cells. Herein, 2D (CH₃(CH₂)₃NH₃)₂(CH₃NH₃) _{n −1}Pb _n I_{3 n +1} perovskite cells with different numbers of [PbI₆]⁴⁻ sheets (n = 2–4) are analyzed. Photoluminescence quantum yield (PLQY) measurements show that nonradiative open‐circuit voltage (V _OC) losses outweigh radiative losses in materials with n > 2. The n = 3 and n = 4 films exhibit a higher PLQY than the standard 3D methylammonium lead iodide perovskite although this is accompanied by increased interfacial recombination at the top perovskite/C₆₀ interface. This tradeoff results in a similar PLQY in all devices, including the n = 2 system where the perovskite bulk dominates the recombination properties of the cell. In most cases the quasi‐Fermi level splitting matches the device V _OC within 20 meV, which indicates minimal recombination losses at the metal contacts. The results show that poor charge transport rather than exciton dissociation is the primary reason for the reduction in fill factor of the RPP devices. Optimized n = 4 RPP solar cells had PCEs of 13% with significant potential for further improvements.

Nature Communications, Published online: 07 June 2019; doi:10.1038/s41467-019-10351-5

Preventing the degradation of metal perovskite solar cells (PSCs) by humid air poses a substantial challenge for their future deployment. We introduce here a two-dimensional (2D) A₂PbI₄ perovskite layer using pentafluorophenylethylammonium (FEA) as a fluoroarene cation inserted between the 3D light-harvesting perovskite film and the hole-transporting material (HTM). The perfluorinated benzene moiety confers an ultrahydrophobic character to the spacer layer, protecting the perovskite light-harvesting material from ambient moisture while mitigating ionic diffusion in the device. Unsealed 3D/2D PSCs retain 90% of their efficiency during photovoltaic operation for 1000 hours in humid air under simulated sunlight. Remarkably, the 2D layer also enhances interfacial hole extraction, suppressing nonradiative carrier recombination and enabling a power conversion efficiency (PCE) >22%, the highest reported for 3D/2D architectures. Our new approach provides water- and heat-resistant operationally stable PSCs with a record-level PCE.

Electric field treatment during thermal annealing is used to control the vertical phase segregation of components in the organic solar cells. Residual additive solvent remaining in the active layer, which is responsible for unfavorable morphological changes, is effectively removed by this treatment. Maintaining the morphology of the active layer over time reflects long‐term stability of solar cells.

The utility of electric fields during the bulk heterojunction (BHJ) film drying process for tuning the morphology and stability of the device is demonstrated. An external electric‐field‐assisted annealing (EFTA) treatment is used to engineer the stability of amorphous donor polymer‐based BHJs without compromising device performance. Residual additive in the device post fabrication is a major source of degradation. Thermal annealing of an active layer effectively removes residual additive, which in case of amorphous polymer donor‐based BHJs, however, leads to unfavorable changes in the morphology. The detrimental effect of thermal annealing in amorphous donor polymer‐based solar cells is mitigated by the presence of an electric field during the drying stage. The complete removal of additive is ensured by this treatment procedure and leads to improved packing and a rigid morphology. The structural stability is reflected in the performance parameters monitored over the long term and electrical noise measurements. The magnitude and polarity of the applied electric field are observed to control the vertical distribution of donor and acceptor components.

Hexamethyl‐mono‐n‐butyl‐substituted zinc phthalocyanine (Me₆Bu‐ZnPc) is synthesized through a ring‐expansion method. The favored face‐on molecular alignment is observed for Me₆Bu‐ZnPc on the perovskite layer. Perovskite solar cells using Me₆Bu‐ZnPc as the dopant‐free hole‐transporting material achieve the highest power‐conversion efficiency (PCE) of 17.41% and retain over 90% of their initial PCE after 1400 h storage at 25 °C and with a relative humidity of 75%.

Efficient and stable hole‐transporting materials (HTMs) are necessary for perovskite solar cells (PSCs) with excellent efficiency and long‐term stability. Here, two A₃B‐type metal phthalocyanine (MPc) compounds are prepared as dopant‐free HTMs for conventional n‐i‐p structured PSCs. Mono‐n‐butyl‐substituted zinc phthalocyanine and hexamethyl‐mono‐n‐butyl‐substituted zinc phthalocyanine (Me₆Bu‐ZnPc) are synthesized through ring‐expansion method, and their exact structures are characterized using nuclear magnetic resonance and mass spectroscopy. The molecular orientation of the developed HTM thin films against the underlying surface is studied using X‐ray diffraction. Different substituents in MPcs can strongly affect their molecular orientation, resulting in different hole mobilities. The favored face‐on molecular alignment is only observed for Me₆Bu‐ZnPc on the perovskite layer, proving the crucial role of methyl substituents in controlling the molecular alignment through the special interactions between the MPc molecule and different sites of perovskite material on the surface. PSCs using Me₆Bu‐ZnPc as a dopant‐free HTM yields the highest reported power‐conversion efficiency (PCE) of 17.41%. With its high hydrophobicity and good coverage, Me₆Bu‐ZnPc HTM thin film acts as an encapsulation layer, which leads to significantly increased long‐term stability. The Me₆Bu‐ZnPc‐based devices retain over 90% of their initial PCE after 1400 h storage at 25 °C and with a relative humidity of 75%.

Perovskite solar cell technology is in the advent of commercial entrance. These materials offer several new value propositions that can allow short‐term monetization in emerging applications, such as Internet‐of‐things or building‐integrated photovoltaics. Prospective offerings perovskite photovoltaics could deliver for high‐value markets, such as utility‐scale photovoltaics, and the feasibility of large deployments are also discussed.

Perovskite solar cell technology is fast approaching its first commercial deployment, with the 10‐year mark since the first research having passed recently. Commercial entrance seems very tangible, but there are a number of remaining challenges related to various economic and technical factors. Conventional photovoltaic markets, such as utility scale photovoltaics, are quite rigid and very demanding for a new entrant. Perovskites offer several new value propositions, which offer monetization prospects in the near future, if properly used. In particular, functionalities such as flexibility, high specific power, and good low‐light performance enable new applications and broadening of the conventional PV usage. The specific cases of internet of things and building‐integrated photovoltaics are discussed, and market opportunities are analyzed. Technology incubation with simultaneous market presence in emerging applications can provide essential economic stability and time for the technology to develop into its full potential. Opportunities in high‐value markets and massive‐scale deployment are also addressed, with the analysis of potentially disruptive offerings being promised by perovskite photovoltaic technology.

Three enamine hole‐transporting materials (HTMs) based on Tröger's base scaffold were synthesized. These compounds are obtained in a three‐step facile synthesis from commercially available materials without the need of expensive catalysts, inert conditions or time‐consuming purification steps.

Abstract

The synthesis of three enamine hole‐transporting materials (HTMs) based on Tröger's base scaffold are reported. These compounds are obtained in a three‐step facile synthesis from commercially available materials without the need of expensive catalysts, inert conditions or time‐consuming purification steps. The best performing material, HTM3, demonstrated 18.62 % PCE in PSCs, rivaling spiro‐OMeTAD in efficiency, and showing markedly superior long‐term stability in non‐encapsulated devices. In dopant‐free PSCs, HTM3 outperformed spiro‐OMeTAD by a factror of 1.6. The high glass‐transition temperature (T _g=176 °C) of HTM3 also suggests promising perspectives in device applications.

This work reports a strategy that ensures the degree of nonradiative recombination can be measured accurately in low‐energetic‐offset organic photovoltaic systems and reports key observations on the relationship between the nonradiative recombination loss and properties of the donor/acceptor interface, including an observed correlation between high domain purity and high nonradiative recombination loss.

Abstract

Open‐circuit voltage (V _OC) losses in organic photovoltaics (OPVs) inhibit devices from reaching V _OC values comparable to the bandgap of the donor–acceptor blend. Specifically, nonradiative recombination losses (∆V _nr) are much greater in OPVs than in silicon or perovskite solar cells, yet the origins of this are not fully understood. To understand what makes a system have high or low loss, an investigation of the nonradiative recombination losses in a total of nine blend systems is carried out. An apparent relationship is observed between the relative domain purity of six blends and the degree of nonradiative recombination loss, where films exhibiting relatively less pure domains show lower ∆V _nr than films with higher domain purity. Additionally, it is shown that when paired with a fullerene acceptor, polymer donors which have bulky backbone units to inhibit close π–π stacking exhibit lower nonradiative recombination losses than in blends where the polymer can pack more closely. This work reports a strategy that ensures ∆V _nr can be measured accurately and reports key observations on the relationship between ∆V _nr and properties of the donor/acceptor interface.