Mahui

Emissive and charge-generating donor–acceptor interfaces for organic optoelectronics with low voltage losses

Emissive and charge-generating donor–acceptor interfaces for organic optoelectronics with low voltage losses, Published online: 01 April 2019; doi:10.1038/s41563-019-0324-5

High‐performance and stable perovskite solar cells with a power conversion efficiency exceeding 20% are achieved based on a low‐temperature solution‐processed ZnO electron transport layer. Dual passivation layers with TiO _x and phenyl‐C61‐butyric acid methyl ester (PCBM) enable the devices to exhibit good stability under an ambient air condition.

ZnO is considered as a potential electron transport layer (ETL) in solar cells due to its high charge carrier mobility and low‐temperature processability. However, it is challenging to obtain highly efficient and stable perovskite solar cells (PSCs) using low‐temperature ZnO ETL due to the poor chemical compatibility between ZnO and perovskite films. Hence, proper surface passivation strategies of ZnO ETL are developed to enhance the ZnO/perovskite interface stability for highly efficient PSCs. In this study, low‐temperature TiO _x post‐treatment is performed to passivate the ZnO surface, suppress the interface charge recombination, and stabilize the ZnO/perovskite interface. The fullerene treatment is further applied to enhance the electron transfer from perovskite to ZnO and reduce the hysteresis behavior. Finally, high‐performance PSCs with an efficiency of over 20% and improved stability are achieved.

A 248 nm KrF excimer laser with high photon energy and low thermal effect is employed to perform a rapid surface modification on CH₃NH₃PbI₃ films for the first time. This approach can reduce the surface trap density of CH₃NH₃PbI₃ films effectively and improve the cell performance of perovskite solar cells obviously.

For perovskite solar cells (PSCs), the surface traps of perovskite films have great influence on the charge carrier behavior at the interface of perovskite and charge transport layers. In this investigation, a 248 nm KrF excimer laser with high photon energy and shallow penetration depth is introduced to perform surface modification on the CH₃NH₃PbI₃ film through irradiation for reducing its surface trap density for the first time. A whole excimer laser surface modification (ELSM) process can be completed in few seconds, and the actual interaction time of the excimer laser and perovskite film is only several hundred nanoseconds. After ELSM, the trap density of the CH₃NH₃PbI₃ film decreases from 1.61 × 10¹⁶ cm⁻³ to 5.81 × 10¹⁵ cm⁻³, and the nonradiative recombination is suppressed effectively. As a result, the open‐circuit voltage and the power conversion efficiency (PCE) of CH₃NH₃PbI₃‐based PSCs increase from 1082 ± 27 to 1117 ± 16 mV and from 16.69% ± 0.77% to 18.50% ± 0.65%, respectively, and the PCE of the champion device reaches 19.38%. In addition, the measured larger charge recombination resistance, slower open‐circuit photovoltage decay, and longer charge recombination lifetime confirm the effective suppression effect of ELSM on the charge carrier recombination in PSCs.

Thick‐film‐induced absorption compensation at short‐wavelength band is designed based on organic donors and acceptors with comparable bandgaps of 1.61 eV and suitable energy offsets for both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), giving high J _sc and V _oc close to perovskite solar cells. As a result, a suppressed trade‐off between J _sc and V _oc among OSCs is demonstrated.

Herein, a high‐mobility polymer (Si25) pairing a nonfullerene acceptor (O‐IDTBR) is introduced to construct active layers of organic solar cells (OSCs). The OSCs based on Si25 and O‐IDTBR with comparable bandgaps of 1.61 eV show high open‐circuit voltage (V _oc) of 1.03 V. Suitable energy level offsets between the donor and acceptor as well as sufficient photon absorbance by a 400 nm thick active layer afford a notable short‐circuit current (J _sc) of 21.11 mA cm⁻², indicating a significantly suppressed trade‐off between J _sc and V _oc among OSCs. In addition, notable high power conversion efficiency (PCE) between 10.2% and 11.54% can be achieved with thick blend films from 210 to 560 nm, a thickness range beneficial to pin‐hole free printing. The maximum PCE of 11.54% corresponds to a 400 nm thick blend film, which is a rare thickness for high‐efficiency nonfullerene‐based OSCs. The corresponding fill factors (FFs) are between 51.59% and 53.33%. The inferior FF is due to a very low electron–hole mobility ratio, offering space for future FF elevation. The results highlight the high V _oc and J _sc potentials for thick‐film nonfullerene OSCs based on a high hole mobility donor as well as looking forward to a high electron mobility nonfullerene acceptor.

Replacing the acceptors of 3,9‐bis(2‐methylene‐(3‐(1,1‐dicyanomethylene)‐indanone)‐ 5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2, 3‐d:2’,3’‐d’]‐s‐indaceno[1,2‐b:5,6‐b’]‐dithiophene (ITIC) and another fuse ring acceptor with withdrawing units of 1,1‐dicyanomethylene‐3‐indanone and hexyl side chains (IDIC) with rhodanine‐benzothiadiazole‐coupled indacenodithiophene with branched 2‐ethylhexyl side chains (EH‐IDT) in nonfullerene organic solar cells can not only enhance power conversion efficiency but also extend device longevity under light. Good miscibility between the donor and the acceptor is found to be a key factor in stabilizing the film morphology of the active layer and contributes to an excellent photostability.

Nonfullerene organic solar cells (OSCs) have achieved an impressive power conversion efficiency (PCE) over the past few years, showing a great potential for real applications. However, the study on the photostability and degradation mechanism of nonfullerene OSCs is far behind than that of fullerene‐based solar cells, which is crucial for the commercial applications of the technology. Herein, an efficient and stable nonfullerene OSC based on PCE10:rhodanine‐benzothiadiazole‐coupled indacenodithiophene with branched 2‐ethylhexyl side chains (EH‐IDT) is fabricated from environmentally benign solvent. The PCE10:EH‐IDT solar cell shows a high PCE of 9.17% and a long operational lifetime (T ₈₀) of 2132 h, compared with other two OSCs based on 3,9‐bis(2‐methylene‐(3‐(1,1‐dicyanomethylene)‐indanone)‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2,3‐d:2’,3’‐d’]‐s‐indaceno[1,2‐b:5,6‐b’]‐dithiophene (ITIC) and another fuse ring acceptor with withdrawing units of 1,1‐dicyanomethylene‐3‐indanone and hexyl side chains (IDIC) nonfullerene acceptors, with tested lifetimes of only 221 and 558 h, respectively. As indicated by the Flory–Huggins interaction parameters, ITIC and IDIC have poor miscibility with PCE10, which leads to morphology degradation, suppressed charge generation, increased trap states, and charge recombination in the photoaging test, which accounts for the significant loss of short‐circuit current density and fill factor during operation. The improved miscibility of the donor and the acceptor results in a more stable morphology, and the PCE10:EH‐IDT solar cells thus achieve an outstanding overall performance that combines high efficiency and superior photostability and paves the way for the potential practical applications of OSCs.

All‐polymer solar cells (all‐PSCs) fabricated via slot‐die printing are obtained. In situ grazing incidence wide‐angle X‐ray scattering reveals the multiple crystallization kinetics during film drying. Printing with 1,8‐diiodooctane leads to the formation of a multi‐length‐scale phase separation and eventually improves the solar cell efficiency up to 9.10%, which is the highest efficiency for printed all‐PSCs.

Herein, high‐performance printed all‐polymer solar cells (all‐PSCs) based on a bulk‐heterojunction (BHJ) blend film are demonstrated using PTzBI as the donor and N2200 as the acceptor. A slot‐die process is used to prepare the BHJ blend, which is a cost‐effective, high‐throughput approach to achieve large‐area photovoltaic devices. The real‐time crystallization of polymers in the film drying process is investigated by in situ grazing incidence wide‐angle X‐ray scattering characterization. Printing is found to significantly improve the crystallinity of the polymer blend in comparison with spin coating. Moreover, printing with 1,8‐diiodooctane as the solvent additive enhances the polymer aggregation and crystallization during solvent evaporation, eventually leading to multi‐length‐scale phase separation, with PTzBI‐rich domains in‐between the N2200 crystalline fibers. This unique morphology achieved by printing fabrication results in an impressively high power conversion efficiency of 9.10%, which is the highest efficiency reported for printed all‐PSCs. These findings provide important guidelines for controlling film drying dynamics for processing all‐PSCs.

Recent progress in antimony chalcogenide‐based photovoltaic materials and devices including Sb₂S₃ solar cells, Sb₂Se₃ solar cells, and Sb₂(S _x Se_1−x)₃ solar cells is comprehensively reviewed. The fundamental properties and preparation techniques of antimony chalcogenides are discussed. The achievements and challenges in antimony chalcogenide solar cells are highlighted. In addition, the outlook for future research in this field is provided.

Antimony chalcogenides such as Sb₂S₃, Sb₂Se₃, and Sb₂(S_xSe_1−x)₃ have emerged as very promising alternative solar absorber materials due to their high stability, abundant elemental storage, nontoxicity, low‐cost, suitable tunable bandgap, and high absorption coefficient. Remarkable achievements have been made in antimony chalcogenide solar cells in the past few decades, with the power conversion efficiency (PCE) currently reaching 9.2%, which is close to the PCE level required for industrial applications. To facilitate the realization of highly efficient antimony chalcogenide solar cells in the future, a comprehensive review of antimony chalcogenide‐based materials and photovoltaic devices is presented. First, the fundamental physical properties and preparation methods of antimony chalcogenide‐based materials are outlined, and then, notable recent developments in antimony chalcogenide‐based photovoltaic devices with various architectures are highlighted. Finally, the most prominent limitations are described, and approaches to achieving remarkable advances in antimony chalcogenide solar cells in the future are provided.

Layered 2D perovskite solar cells often suffer from poor carrier transport. Herein, the authors propose a homo‐tandem structure to extract the photogenerated carriers efficiently while retaining the optical density of the absorbers. It thus improves the power conversion efficiency of resultant devices by 30% without the penalty of moisture stability.

Layered two dimensional (layered 2D) organic–inorganic metal halide perovskites have attracted tremendous interest in photovoltaics due to its acceptable materials stability, especially the moisture resistance, when compared with their three dimensional counterparts. However, the limited carrier transport capability, which originates from the insulativity of bulky organic molecules, has significantly affected the resultant device efficiency. To create a shorter carrier pathway with sufficient optical density, the homo‐tandem device structure by using layered 2D perovskite absorbers is proposed. Following this strategy, the semi‐transparent device and filter bottom cells have been investigated and optimized using the same layered 2D perovskite absorber (BA₂MA₃Pb₄I₁₃). The corresponding four‐terminal tandem device is successfully demonstrated with the champion power conversion efficiency of 14.42%, which is 30% higher than that of single BA₂MA₃Pb₄I₁₃ perovskite devices (11.02%). A stabilized efficiency of 13.57% in the optimized champion tandem device also have been achieved. These results suggest alternatives to develop layered 2D perovskite based solar cells and other optoelectronic devices.

The nonfullerene acceptor 3,9‐bis(2‐methylene‐(3‐(1,1‐dicyanomethylene)‐indanone))‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2,3‐d:2′,3′‐d′]‐s‐indaceno[1,2‐b:5,6‐b′] dithiophene (ITIC) molecular film exhibits large and negative magneto‐photocurrent in ambient conditions. The effect is attributed to the exciton‐charge reaction. By studying ITIC‐based binary and ternary organic solar cells, the interplay of spin‐dependent polaron pair dissociation and exciton‐charge reaction plays a decisive role in the photovoltaic process.

A nonfullerene acceptor (NFA), named 3,9‐bis(2‐methylene‐(3‐(1,1‐dicyanomethylene)‐indanone))‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2,3‐d:2′,3′‐d′]‐s‐indaceno[1,2‐b:5,6‐b′] dithiophene (ITIC) with the fused aromatic conjugated backbone structure, is an intriguing small molecular semiconductor in the application of organic bulk heterojunction (BHJ) solar cells. However, the underlying spin‐dependent photo‐physics in the photovoltaic process remains deficient. Here, ITIC‐based thin solid film, binary, and ternary organic blends are designed and fabricated with two polymeric donors, poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)‐benzo[1,2‐b:4,5‐b′]‐dithiophene))‐alt‐(5,5‐(1′,3′‐di‐2‐thienyl‐5′,7′‐bis(2‐ethylhexyl)benzo[1′,2′‐c:4′,5′‐c′]dithiophene‐4,8‐dione))] (PBDB‐T) and poly[(2,6‐(4,8‐Bis(3‐((2‐ethylhexyl)oxy)‐phenyl)‐benzo[1,2‐b:4,5‐b′]‐dithiophene))‐alt‐(5,5‐(1′,3′‐di‐2‐thienyl‐5′,7′‐bis(2‐ethylhexyl)benzo[1′,2′‐c:4′,5′‐c′]dithiophene‐4,8‐dione))] (PBPD‐Th). With the spin‐sensitive magneto‐photocurrent and magneto‐electroluminescence measurements, ITIC always exhibits large magnetic field effects with negative signs in the ambient temperature. The effect is attributed to the exciton‐charge reaction. However, both positive and negative signs are detected in the binary and ternary organic blends at small and large fields, respectively. The results elucidate that the mutual combination of the spin‐dependent polaron pair dissociation and exciton‐charge reaction plays a decisive role in the photovoltaic process. Furthermore, with photo‐induced electron paramagnetic resonance (EPR) studies, the full width at half maximum (FWHM) of the line shape for the positive magneto‐photocurrent is firmly associated to the magnitude of the effective g‐factor. The present study may shed a new light on the deep understanding of spin‐dependent photo‐physical process in NFA‐based solar cells for organic opto‐spintronic developments.

Impressive progress has been made in the last few years on producing perovskite photovoltaics using scalable printing and coating technologies. The key developments, such as the control of nucleation and crystal growth of the perovskite thin film, which have enabled this rapid progress in coated and printed perovskite photovoltaics, are highlighted.

Abstract

Hybrid organic–inorganic metal halide perovskite semiconductors provide opportunities and challenges for the fabrication of low‐cost thin‐film photovoltaic devices. The opportunities are clear: the power conversion efficiency (PCE) of small‐area perovskite photovoltaics has surpassed many established thin‐film technologies. However, the large‐scale solution‐based deposition of perovskite layers introduces challenges. To form perovskite layers, precursor solutions are coated or printed and these must then be crystallized into the perovskite structure. The nucleation and crystal growth must be controlled during film formation and subsequent treatments in order to obtain high‐quality, pin‐hole‐free films over large areas. A great deal of understanding regarding material engineering during the perovskite film formation process has been gained through spin‐coating studies. Based on this, significant progress has been made on transferring material engineering strategies to processes capable of scale‐up, such as blade coating, spray coating, inkjet printing, screen printing, relief printing, and gravure printing. Here, an overview is provided of the strategies that led to devices deposited by these scalable techniques with PCEs as high as 21%. Finally, the opportunities to fully close the shrinking gap to record spin‐coated solar cells and to scale these efficiencies to large areas are highlighted.

High‐performance perovskite resistive memory devices are made by employing a nonhalide lead source. The unipolar perovskite memory devices achieve an outstanding ON/OFF ratio with a relatively low operation voltage, a large endurance, and long retention times. The reliable fabrication of high‐yield cross‐bar array perovskite memory devices demonstrates the potential for realizing high‐density perovskite memory devices with excellent selectivity.

Abstract

Resistive random access memories can potentially open a niche area in memory technology applications by combining the advantages of the long endurance of dynamic random‐access memory and the long retention time of flash memories. Recently, resistive memory devices based on organo‐metal halide perovskite materials have demonstrated outstanding memory properties, such as a low‐voltage operation and a high ON/OFF ratio; such properties are essential requirements for low power consumption in developing practical memory devices. In this study, a nonhalide lead source is employed to deposit perovskite films via a simple single‐step spin‐coating method for fabricating unipolar resistive memory devices in a cross‐bar array architecture. These unipolar perovskite memory devices achieve a high ON/OFF ratio up to 10⁸ with a relatively low operation voltage, a large endurance, and long retention times. The high‐yield device fabrication based on the solution‐process demonstrated here will be a step toward achieving low‐cost and high‐density practical perovskite memory devices.

Organic solar cells achieve over 15% efficiency through the use of a copolymer donor, and simultaneously enhanced open‐circuit voltage and short‐circuit current density are obtained. High‐performance solar cells are adaptable for environment‐friendly solvents using a blade‐coating method, while showing better photostability than the corresponding ternary solar cells.

Abstract

Ternary blending and copolymerization strategies have proven advantageous in boosting the photovoltaic performance of organic solar cells. Here, 15% efficiency solar cells using copolymerization donors are demonstrated, where the electron‐withdrawing unit, ester‐substituted thiophene, is incorporated into a PBDB‐TF polymer to downshift the molecular energy and broaden the absorption. Copolymer‐based solar cells suitable for large‐area devices can be fabricated by a blade‐coating method from a nonhalogen and nonaromatic solvent mixture. Although ternary solar cells can achieve comparable efficiencies, they are not suitable for environment‐friendly processing conditions and show relatively low photostability compared to copolymer‐based devices. These results not only demonstrate high‐efficiency organic photovoltaic cells via copolymerization strategies but also provide important insights into their applications in practical production.

Organic solar cells achieve over 15% efficiency through the use of a copolymer donor, and simultaneously enhanced open‐circuit voltage and short‐circuit current density are obtained. High‐performance solar cells are adaptable for environment‐friendly solvents using a blade‐coating method, while showing better photostability than the corresponding ternary solar cells.

Abstract

Ternary blending and copolymerization strategies have proven advantageous in boosting the photovoltaic performance of organic solar cells. Here, 15% efficiency solar cells using copolymerization donors are demonstrated, where the electron‐withdrawing unit, ester‐substituted thiophene, is incorporated into a PBDB‐TF polymer to downshift the molecular energy and broaden the absorption. Copolymer‐based solar cells suitable for large‐area devices can be fabricated by a blade‐coating method from a nonhalogen and nonaromatic solvent mixture. Although ternary solar cells can achieve comparable efficiencies, they are not suitable for environment‐friendly processing conditions and show relatively low photostability compared to copolymer‐based devices. These results not only demonstrate high‐efficiency organic photovoltaic cells via copolymerization strategies but also provide important insights into their applications in practical production.

An in situ back‐contact passivation strategy is adopted to optimize the photovoltaic performance of n–i–p planar perovskite solar cells. Devices with a flat‐band alignment between the perovskite and polymer passivation layer achieve a high photovoltage of 1.15 V and fill factor of 83% with 1.53 eV bandgap perovskite, leading to a stabilized power conversion efficiency of 21.6%.

Abstract

Organic–inorganic hybrid perovskite solar cells (PSCs) have seen a rapid rise in power conversion efficiencies in recent years; however, they still suffer from interfacial recombination and charge extraction losses at interfaces between the perovskite absorber and the charge–transport layers. Here, in situ back‐contact passivation (BCP) that reduces interfacial and extraction losses between the perovskite absorber and the hole transport layer (HTL) is reported. A thin layer of nondoped semiconducting polymer at the perovskite/HTL interface is introduced and it is shown that the use of the semiconductor polymer permits—in contrast with previously studied insulator‐based passivants—the use of a relatively thick passivating layer. It is shown that a flat‐band alignment between the perovskite and polymer passivation layers achieves a high photovoltage and fill factor: the resultant BCP enables a photovoltage of 1.15 V and a fill factor of 83% in 1.53 eV bandgap PSCs, leading to an efficiency of 21.6% in planar solar cells.

A new hybrid lead halide material built of deformable post‐perovskite‐type chains exhibits an enhanced intrinsic white photoemission, setting a new record of 45% for photoluminescence quantum yield. The deformable lattice of this compound enables the self‐trapping of excitons responsible for broadband emission.

Abstract

Hybrid metal halides containing perovskite layers have recently shown great potential for applications in solar cells and light‐emitting diodes. Such compounds exhibit quantum confinement effects leading to tunable optical and electronic properties. Thus, broadband white‐light emission has been observed from diverse metal halides and, owing to high color rendering index, high thermal stability, and low‐temperature solution processability, these materials have attracted interest for application in solid‐state lighting. However, the reported quantum yields for white photoluminescence (PLQY) remain low (i.e., in the range 0.5–9%) and no approach has shown to successfully increase the intensity of this emission. Here, it is demonstrated that the quantum efficiencies of hybrid metal halides can be greatly enhanced if they contain a polymorph of the [PbX₄]²⁻ perovskite‐type layers: the [PbX₄]²⁻ post‐perovskite‐type chains showing a PLQY of 45%. Different piperazines lead to a hybrid lead halide with either perovskite layers or post‐perovskite chains influencing strongly the presence of self‐trapped states for excitons. It is anticipated that this family of hybrid lead halide materials could enhance all the properties requiring the stabilization of trapped excitons.

Surface passivation of perovskite film for efficient solar cells

Surface passivation of perovskite film for efficient solar cells, Published online: 01 April 2019; doi:10.1038/s41566-019-0398-2

Impressive progress has been made in the last few years on producing perovskite photovoltaics using scalable printing and coating technologies. The key developments, such as the control of nucleation and crystal growth of the perovskite thin film, which have enabled this rapid progress in coated and printed perovskite photovoltaics, are highlighted.

Abstract

Hybrid organic–inorganic metal halide perovskite semiconductors provide opportunities and challenges for the fabrication of low‐cost thin‐film photovoltaic devices. The opportunities are clear: the power conversion efficiency (PCE) of small‐area perovskite photovoltaics has surpassed many established thin‐film technologies. However, the large‐scale solution‐based deposition of perovskite layers introduces challenges. To form perovskite layers, precursor solutions are coated or printed and these must then be crystallized into the perovskite structure. The nucleation and crystal growth must be controlled during film formation and subsequent treatments in order to obtain high‐quality, pin‐hole‐free films over large areas. A great deal of understanding regarding material engineering during the perovskite film formation process has been gained through spin‐coating studies. Based on this, significant progress has been made on transferring material engineering strategies to processes capable of scale‐up, such as blade coating, spray coating, inkjet printing, screen printing, relief printing, and gravure printing. Here, an overview is provided of the strategies that led to devices deposited by these scalable techniques with PCEs as high as 21%. Finally, the opportunities to fully close the shrinking gap to record spin‐coated solar cells and to scale these efficiencies to large areas are highlighted.

A controllable ultraviolet/ozone (UVO) treatment is employed to prepare a high‐quality electrochemically deposited NiO_x hole transport layer (HTL). Under optimal conditions of UVO treatment, the increased hole conductivity in the HTL reduces defects at the HTL/perovskite interface, and a narrowed offset of the valence band between the HTL and perovskite film are obtained, which results in high‐performance perovskite solar cells with an efficiency of 19.67%.

Nickel oxide (NiO_x) has exhibited great potential as a hole transport layer (HTL) for fabricating efficient and stable perovskite solar cells (PSCs). However, it has been greatly limited by its fabrication and manipulation process. In this work, a simple processing method on an ultrathin electrochemical mesoporous NiO_x film manipulated by controllable ultraviolet/ozone (UVO) treatmentis employed; the duration of UVO treatment on the NiO_x film significantly affects the photovoltaic properties of the PSCs. When the exposure duration increases, the wettability, electrical conductivity, nonstoichiometry, and valence band energy of the NiO_x film are improved with varying degrees. Besides, the perovskite grain size, recombination resistance at the perovskite/NiO_x interface, and build‐in potential of the device also increase, resulting in higher short‐circuit current density (J _SC) and open‐circuit voltage (V _OC). Combining these factors together, an optimal exposure time of UVO treatment on the NiO_x film has been achieved at 5 min, which results in a significantly high performance with an efficiency of 19.67%, large V _OC (>1.1 V), and J _SC (>23 mA cm⁻²). Furthermore, the experimental results are coincide well with simulation results on the different corresponding subjects. Hopefully, this work could facilitate material manipulation toward scalable, high efficiency, and stable solar cells.

In this work, the origin of the graded phase distribution in 2D perovskite films is revealed: DMSO suppresses the graded distribution effectively. An intermediate phase involved DMSO is found to be responsible for the less graded films. Solar cells with a less graded phase distribution shows an improved power conversion efficiency exceeding 14%.

Ruddlesden‐Popper perovskites (2D perovskites) show tremendous potential in photovoltaic devices for their superior stability compared with their 3D counterparts, while their lower power conversion efficiencies severely hinder their further progress for practical application. Generally, the 2D perovskite films fabricated by spin‐coating present graded distribution of 2D phases, and the small‐n phases located at the film bottom will impede the interlayer charge transport. In this work, the origin of the graded phase distribution is explained, and the roles of DMSO in Ruddlesden‐Popper perovskites film crystallization are systematically investigated. DMSO was found to homogenize the film composition, and therefore suppress the proportion of the small‐n 2D phases. Through X‐ray diffraction and grazing‐incidence wide‐angle X‐ray scattering measurements, it has been revealed that an intermediate phase is responsible for the less graded films and rationalizes the effect of DMSO in film crystallization. More importantly, solar cells with an efficiency exceeding 14% are fabricated by this facile method, which deliver good light, thermal, and ambient stability without encapsulation. These findings significantly benefit the understanding of crystal growth in 2D perovskites film and demonstrate that 2D perovskites are promising candidates for high‐performance solar cells.

A tandem organic solar cell is fabricated employing subcells with the same donor PBDB‐T and two acceptors F‐M and NNBDT with complementary absorptions. A power conversion efficiency of 14.52% is achieved with a high V _oc of 1.82 V, a notable FF of 74.7%, and a decent J _sc of 10.68 mA cm⁻².

Abstract

The tandem structure is an efficient way to simultaneously tackle absorption and thermalization losses of the single junction solar cells. In this work, a high‐performance tandem organic solar cell (OSC) using two subcells with the same donor poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)‐benzo[1,2‐b:4,5‐b′]dithiophene))‐alt‐(5,5‐(1′,3′‐di‐2‐thienyl‐5′,7′‐bis(2‐ethylhexyl)benzo[1′,2′‐c:4′,5′‐c′]dithiophene‐4,8‐dione))] (PBDB‐T) and two acceptors, F‐M and 2,9‐bis(2‐methylene‐(3(1,1‐dicyanomethylene)benz[f ]indanone))7,12‐dihydro‐(4,4,10,10‐tetrakis(4‐hexylphenyl)‐5,11‐diocthylthieno[3′,2′:4,5]cyclopenta[1,2‐b]thieno[2″,3″:3′,4′]cyclopenta[1′,2′:4,5]thieno[2,3‐f][1]benzothiophene (NNBDT), with complementary absorptions is demonstrated. The two subcells show high V _oc with value of 0.99 V for the front cell and 0.86 V for the rear cell, which is the prerequisite for obtaining high V _oc of their series‐connected tandem device. Although there is much absorption overlap for the subcells, a decent J _sc of the tandem cell is still obtained owing to the complementary absorption of the two acceptors in a wide range. With systematic device optimizations, a best power conversion efficiency of 14.52% is achieved for the tandem device, with a high V _oc of 1.82 V, a notable FF of 74.7%, and a decent J _sc of 10.68 mA cm⁻². This work demonstrates a promising strategy of fabricating high‐efficiency tandem OSCs through elaborate selection of the active layer materials in each subcell and tradeoff of the V _oc and J _sc of the tandem cells.

Two solid additives are proven to improve the molecular packing of acceptors, while devices processed with different additives exhibit different photovoltaic performances due to the different volatilities. The working mechanism and basic design rules of solid additives are revealed, and a feasible method for achieving high‐efficiency polymer solar cells is established.

Abstract

Fine‐tuning of the nanoscale morphologies of the active layers in polymer solar cells (PSCs) through various techniques plays a vital role in improving the photovoltaic performance. However, for emerging nonfullerene (NF) PSCs, the morphology optimization of the active‐layer films empirically follows the methods originally developed in fullerene‐based blends and lacks systematic studies. In this work, two solid additives with different volatilities, SA‐4 and SA‐7, are applied to investigate their influence on the morphologies and photovoltaic performances of NF‐PSCs. Although both solid additives effectively promote the molecular packing of the NF acceptors, due to the higher volatility of SA‐4, the devices processed with SA‐4 exhibit a power conversion efficiency of 13.5%, higher than that of the control devices, and the devices processed with SA‐7 exhibit poor performances. Through a series of detailed morphological analyses, it is found that the volatilization of SA‐4 after thermal annealing is beneficial for the self‐assembly packing of acceptors, while the residuals due to the incomplete volatilization of SA‐7 have a negative effect on the film morphology. The results delineate the feasibility of applying volatilizable solid additives and provide deeper insights into the working mechanism, establishing guidelines for further material design of solid additives.

Rational molecular passivation for high-performance perovskite light-emitting diodes

Rational molecular passivation for high-performance perovskite light-emitting diodes, Published online: 25 March 2019; doi:10.1038/s41566-019-0390-x