Intergrain Connection of Organometal Halide Perovskites: Formation Mechanism and Its Effects on Optoelectrical Properties.

Two-step processes are commonly used for the fabrication of organic-inorganic perovskite solar cells; they convert a PbI2 film to a perovskite film by dipping it in CH3NH3I (MAI) solution or spin-coating the MAI solution onto it. Dipping yields perovskite films with discrete and rough morphologies, whereas spinning yields films with smooth and connected morphologies. The residual MAI solution that remains after spinning is the key factor that governs the smoothness of the resulting morphology; centrifugal force has no influence. A perovskite layer forms as soon as the MAI solution is loaded onto the PbI2 film, then the MAI residues left after spinning dissolve this outermost perovskite layer. The subsequent recrystallization of the dissolved perovskites increases the connectivity and smoothness of the crystals. The final morphology is dependent on the degrees of dissolution and recrystallization, which can be controlled by varying the processing conditions. A post-thermal treatment can be applied to induce the additional dissolution of the perovskites, which results in an increase in the final grain size while maintaining good connectivity. Combining these results, we fabricated an optimal film morphology that gives rise to perovskite solar cells with improved efficiency. The optimal perovskite film has a smooth and connected morphology as well as better carrier transport than rough and discrete films. This article provides fundamental understanding of the mechanism of formation during two-step processes of connected perovskite morphologies that can guide the further development of two-step processes for the fabrication of optimal perovskite morphologies.

Enhancing the Durability and Carrier Selectivity of Perovskite Solar Cells Using a Blend Interlayer.

A mechanically and thermally stable and electron-selective ZnO/CH3NH3PbI3 interface is created via hybridization of a polar insulating polymer, poly(ethylene glycol) (PEG), into ZnO nanoparticles (NPs). PEG successfully passivates the oxygen defects on ZnO and prevents direct contact between CH3NH3PbI3 and defects on ZnO. A uniform CH3NH3PbI3 film is formed on a soft ZnO:PEG layer after dispersion of the residual stress from the volume expansion during CH3NH3PbI3 conversion. PEG also increases the work of adhesion of the CH3NH3PbI3 film on the ZnO:PEG layer and holds the CH3NH3PbI3 film with hydrogen bonding. Furthermore, PEG tailors the interfacial electronic structure of ZnO, reducing the electron affinity of ZnO. As a result, a selective electron-collection cathode is formed with a reduced electron affinity and a deep-lying valence band of ZnO, which significantly enhances the carrier lifetime (473 µs) and photovoltaic performance (15.5%). The mechanically and electrically durable ZnO:PEG/CH3NH3PbI3 interface maintains the sustainable performance of the solar cells over 1 year. A soft and durable cathodic interface via PEG hybridization in a ZnO layer is an effective strategy toward flexible electronics and commercialization of the perovskite solar cells.

Decoupling Charge Transfer and Transport at Polymeric Hole Transport Layer in Perovskite Solar Cells.

Tailoring charge extraction interfaces in perovskite solar cells (PeSCs) critically determines the photovoltaic performance of PeSCs. Here, we investigated the decoupling of two major determinants of the efficient charge extraction, the charge transport and interfacial charge transfer properties at hole transport layers (HTLs). A simple physical tuning of a representative polymeric HTL, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), provided a wide range of charge conductivities from 10(-4) to 10(3) S cm(-1) without significant modulations in their energy levels, thereby enabling the decoupling of charge transport and transfer properties at HTLs. The transient photovoltaic response measurement revealed that the facilitation of hole transport through the highly conductive HTL promoted the elongation of charge carrier lifetimes within the PeSCs up to 3 times, leading to enhanced photocurrent extraction and finally 25% higher power conversion efficiency.

Enhanced Organic Solar Cell Stability through the Effective Blocking of Oxygen Diffusion using a Self-Passivating Metal Electrode.

A new and simple strategy for enhancing the stability of organic solar cells (OSCs) was developed by using self-passivating metal top electrodes. Systematic investigations on O2 permeability of Al top electrodes revealed that the main pathways for oxidation-induced degradation could be greatly suppressed by simply controlling the nanoscale morphology of the Al electrode. The population of nanoscale pinholes among Al grains, which critically decided the diffusion of O2 molecules toward the Al-organic interfaces that are vulnerable to oxidation, was successfully regulated by rapidly depositing Al or promoting lateral growth among the Al grains, accompanied by increasing the deposition thickness. Our observations suggested that the stability of OSCs with conventional architectures might be greatly enhanced simply by controlling the fabrication conditions of the Al top electrode, without the aid of additional secondary treatments.

Naphthodithiophene-Based Conjugated Polymer with Linear, Planar Backbone Conformation and Strong Intermolecular Packing for Efficient Organic Solar Cells.

Two donor-acceptor copolymers, PBDT and PNDT, containing 4,8-bis(2-ethylhexyloxy)benzo[1,2-b:3,4-b']dithiophene (BDT) and 4,9-bis(2-ethylhexyloxy)naphtho[1,2-b:5,6-b']dithiophene (NDT), respectively, as an electron-rich unit and 5,6-difluoro-2,1,3-benzothiadiazole (2FBT) as an electron-deficient unit, were synthesized and compared. The introduction of the NDT core into the conjugated backbone was found to effectively improve both light harvesting and the charge carrier mobility by enhancing chain planarity and backbone linearity; the NDT copolymer has stronger noncovalent interactions and smaller bond angles than those of the BDT-based polymer. Moreover, the introduction of the NDT core brings about a drastic change in the molecular orientation into the face-on motif and results in polymer:PCBM blend films with well-mixed interpenetrating nanofibrillar bulk-heterojunction networks with small-scale phase separation, which produce solar cells with higher short-circuit current density and fill factor values. A conventional optimized device structure containing PNDT:PC71BM was found to exhibit a maximum solar efficiency of 6.35%, an open-circuit voltage of 0.84 V, a short-circuit current density of 11.92 mA cm(-2), and a fill factor of 63.5% with thermal annealing, which demonstrates that the NDT and DT2FBT moieties are a promising electron-donor/acceptor combination for high-performance photovoltaics.