Iodide–fullerene π interactions play decisive roles in n-doping and electron transport of fullerenes at the perovskite–PCBM interface in the devices of perovskite solar cells (Pero-SCs). But instability issues of perovskites due to halide anion migration greatly limit the practical application of Pero-SCs. To fully understand the properties of these interactions, we conducted systematic studies on fullerene ammonium halides containing I−, Br−, and Cl− that exist in perovskites. Our findings show that fullerene is an overwhelmingly strong electron acceptor that activates relatively inert halide anions (Br− and Cl−) and initiates the n-doping process by forming anion–π interactions, which are closely related to the performance and stability of the p–i–n Pero-SC devices based on a CH3NH3PbX3 (X = I, Br, and Cl) light absorber and a fullerene (C60 or PCBM) electron transport layer (ETL). At the perovskite–fullerene interface in the p–i–n Pero-SC, n-doping occurs by forming iodide–fullerene π interactions, which facilitates charge transport and improves the performance of the devices by suppressing hysteresis. On the basis of experimental evidence and computational results, we propose a plausible mechanism for iodide migration attributed to its n-doping at the interface and subsequent hopping of slidable iodide to a neighbouring fullerene given that space and energy are favorable. Thus, the migration of halide anions through the fullerene layer results in perovskite degradation and performance decay of the devices. This work highlights the importance of halide anion–fullerene π interactions in materials chemistry and explores the introduction of a halide anion blocking ETL in p–i–n Pero-SCs.

In this Communication, a self-organization method of [6,6]-phenyl-C61-butyric acid 2-((2-(dimethylamino)-ethyl) (methyl)amino)ethyl ester (PCBDAN) interlayer in between 6,6-phenyl C61-butyric acid methyl ester (PCBM) and indium tin oxide (ITO) has been proposed to improve the performance of N–I–P perovskite solar cells (PSCs). The introduction of self-organized PCBDAN interlayer can effectively reduce the work function of ITO and therefore eliminate the interface barrier between electron transport layer and electrode. It is beneficial for enhancing the charge extraction and decreasing the recombination loss at the interface. By employing this strategy, a highest power conversion efficiency of 18.1% has been obtained with almost free hysteresis. Furthermore, the N–I–P PSCs have excellent stability under UV-light soaking, which can maintain 85% of its original highest value after 240 h accelerated UV aging. This self-organization method for the formation of interlayer can not only simplify the fabrication process of low-cost PSCs, but also be compatible with the roll-to-roll device processing on flexible substrates.

Self-n-doped stable and highly conductive fullerene ammoniums with excellent thickness-tolerance could act as promising cathode interlayers to facilitate electron transfer and improve power conversion efficiencies (PCEs) of large-area organic solar cells (OSCs). Herein, systematic studies on electronic and spatial structure of fullerene ammonium iodide (PCBANI) have been performed to elucidate the cross self-n-doping mechanism. In PCBANI, partial electron transfer from iodide to the core fullerene could result in n-doping and high conductivity. This doping process forms strong anion-π interactions between iodides and fullerene cores accompanied by side-chain’s head-to-tail cation-π interactions that contribute to the stabilization of the n-doped fullerene. Moreover, two possible pathways of the cross self-n-doping involving intermolecular exchange and transfer of iodide have been verified by experiment combined with computational modeling. Based on all of the solid evidence, we propose an electron transfer model for PCBANI in which the iodide sandwiched in the n-doped fullerene core acts as a shuttle to transfer electrons via redox processes. This finding provides a strategy for electrically doping and assembling fullerenes to improve their performance in photovoltaic devices and endow them with new functionalities that could be applied to optoelectronics and organic electronics.

•A highest PCE>17.2% has been achieved for the PSCs with PCBDAN.•The devices with PCBDAN show higher performance.•The improved stability of PSCs is related to the hydrophobic PCBDAN.•PCBDAN can be used in the fabrication of high efficient and stable PSCs.The recent rapid rise in power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) has attracted worldwide extensive attention. However, the PSC applications are limited by their poor stability due to perovskite degradation in moisture. We used a fullerene amine interlayer in planar PSCs to reduce the interface barrier between ETL and metal electrode and also resist the moisture. The utilization of fullerene amine interlayer allowed for the enhancement of PSCs' performance, showing a highest power conversion efficiency (PCE)>17.2% with negligible hysteresis. More importantly, the air stability of PSCs with fullerene amine was improved: the unpackaged devices stored in air can keep their high performance with no obvious PCE loss in 10% humidity and >90% of the initial PCE in 45% humidity after 20 days.Improved performance and air stability of planar perovskite solar cells via interfacial engineering using a fullerene amine interlayer.

Construction of π-conjugation network in ordered fullerenes by self-assembly remains challenging for improving their optoelectronic performance and developing advanced materials. Here, we present a layered stacking of self-n-doped fullerene ammonium iodide (PCBANI) through a delicate balance among iodide anion–C60 π, electrostatic, and C60 π–π interactions to construct an unprecedented supramolecular system. X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and computational modeling are carried out to clarify the structure. Remarkably, the formation of intermolecular iodide anion−π interactions between iodide and the surrounded fullerene cores yields an iodide-linked C60 π–π two-dimensional (2-D) network. Consequently, the ordered and tightly packed fullerenes sandwiching iodide could facilitate electron transfer along the network system. Comparative devices incorporating the disordered films show dramatically decreased current densities and manifest the importance of the π-extended network for electron transfer. This work provides a key strategy to control the packing of ordered electron-transport materials to suppress defect formation. Moreover, engineering self-assembly of self-n-doped fullerenes with novel architectures, such as nanowire, nanotube, and nanoparticle would yield new functionalities that are suitable for photovoltaic devices, nanoelectronics, etc.

An alcohol-soluble fullerene derivative functionalized with a crown-ether end group in its side chain (denoted as PCBC) was synthesized and applied as a cathode buffer layer in planar p–i–n perovskite solar cells and bulk-heterojunction polymer solar cells. It is found that the introduction of the PCBC cathode buffer layer can greatly improve the photovoltaic performance of the planar p–i–n perovskite solar cells based on CH3NH3PbI3−xClx with power conversion efficiency (PCE) reaching 15.08%. In addition, the bulk-heterojunction polymer solar cells based on PBDTTT-C-T:PC70BM with the PCBC cathode buffer layer also showed a higher PCE of 7.67%, which is improved in comparison with the traditional device with the Ca/Al cathode. This work indicates that PCBC is a promising solution-processable cathode buffer layer material for application in both planar p–i–n perovskite solar cells and bulk-heterojunction polymer solar cells.

A series of self n-doped fullerene ammonium derivatives have been synthesized and confirmed with electron paramagnetic resonance and conductivity measurements. The existence of stable C60O2˙− anion radical in these materials resulted in intrinsically high conductivities between 1.05 × 10−2 and 1.98 × 10−2 S cm−1. Among fullerenes with different numbers of ammonium and counter anions, [6,6]-phenyl-C61-butyric acid 2-((2-(trimethylammonium)ethyl)-(dimethyl)ammonium)-ethyl ester diiodides (PCBDANI) showed the best solvent resistance, which was confirmed by the measurement of film thickness and corresponding UV-vis absorption before and after rinsing with dichlorobenzene. Most importantly, the inverted polymer solar cells with the structure of ITO/PCBDANI/P3HT:PCBM/MoO3/Ag retained reasonably high power conversion efficiency even at a thickness of 82 nm of the PCBDANI film as the cathode interlayer. Thus large-area devices via printing this interlayer or printing on this interlayer could become feasible.

We present a strategy to fabricate polymer solar cells in inverted geometry by self-organization of alcohol soluble cathode interfacial materials in donor–acceptor bulk heterojunction blends. An amine-based fullerene [6,6]-phenyl-C61-butyric acid 2-((2-(dimethylamino)-ethyl)(methyl)amino)ethyl ester (PCBDAN) is used as an additive in poly(3-hexylthiophene) (P3HT) and 6,6-phenyl C61-butyric acid methyl ester (PCBM) blend to give a power conversion efficiency of 3.7% based on devices ITO/P3HT:PCBM:PCBDAN/MoO3/Ag where the ITO alone is used as the cathode. A vertical phase separation in favor of the inverted device architecture is formed: PCBDAN is rich on buried ITO surface reducing its work function, while P3HT is rich on air interface with the hole-collecting electrode. The driving force of the vertical phase separation is ascribed to the surface energy and its components of the blend compositions and the substrates. Similar results are also found with another typical alcohol soluble cathode interfacial materials, poly[(9,9-bis(3′-(N, N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN), implying that self-organization may be a general phenomenon in ternary blends. This self-organization procedure could eliminate the fabrication of printing thin film of interlayers or printing on such thin interlayers and would have potential application for roll-to-roll processing of polymer solar cells.Keywords: conjugated polymers; fullerenes; organic electronics; photovoltaic devices; self-assembly

Efficient low-band-gap polymers are one key component for constructing tandem solar cells with other higher-band-gap materials to harvest wide absorption of the solar spectrum. The N-acyldithieno[3,2-b:2′,3′-d]pyrrole (DTP) building block is used for making low-band-gap polymers. It is attractive because of its strong donating ability and relatively low highest-occupied-molecular-orbital level in comparison with the N-alkyl DTP building block. However, additional solubilizing groups on the accepting units are needed for soluble donor–acceptor polymers based on the N-alkanoyl DTP building block. Combining N-benzoyl DTP with a 4,7-dithieno-2,1,3-benzothiadiazole building block, a polymer with a low band gap of 1.44 eV, delivers a high short-circuit current of 17.1 mA/cm2 and a power conversion efficiency of 3.95%, which are the highest for the devices with DTP-containing materials. Herein, an alcohol-soluble diamine-modified fullerene cathode interfacial layer improved the device efficiency significantly more than the mono-amine analogue.Keywords: amine-modified fullerene; interfacial materials; low-band-gap polymers; N-acyldithienopyrrole; polymer solar cells;

An alcohol-soluble fullerene derivative functionalized with a crown-ether end group in its side chain (denoted as PCBC) was synthesized and applied as a cathode buffer layer in planar p–i–n perovskite solar cells and bulk-heterojunction polymer solar cells. It is found that the introduction of the PCBC cathode buffer layer can greatly improve the photovoltaic performance of the planar p–i–n perovskite solar cells based on CH3NH3PbI3−xClx with power conversion efficiency (PCE) reaching 15.08%. In addition, the bulk-heterojunction polymer solar cells based on PBDTTT-C-T:PC70BM with the PCBC cathode buffer layer also showed a higher PCE of 7.67%, which is improved in comparison with the traditional device with the Ca/Al cathode. This work indicates that PCBC is a promising solution-processable cathode buffer layer material for application in both planar p–i–n perovskite solar cells and bulk-heterojunction polymer solar cells.

A series of self n-doped fullerene ammonium derivatives have been synthesized and confirmed with electron paramagnetic resonance and conductivity measurements. The existence of stable C60O2˙− anion radical in these materials resulted in intrinsically high conductivities between 1.05 × 10−2 and 1.98 × 10−2 S cm−1. Among fullerenes with different numbers of ammonium and counter anions, [6,6]-phenyl-C61-butyric acid 2-((2-(trimethylammonium)ethyl)-(dimethyl)ammonium)-ethyl ester diiodides (PCBDANI) showed the best solvent resistance, which was confirmed by the measurement of film thickness and corresponding UV-vis absorption before and after rinsing with dichlorobenzene. Most importantly, the inverted polymer solar cells with the structure of ITO/PCBDANI/P3HT:PCBM/MoO3/Ag retained reasonably high power conversion efficiency even at a thickness of 82 nm of the PCBDANI film as the cathode interlayer. Thus large-area devices via printing this interlayer or printing on this interlayer could become feasible.