ZiQi Sun

Organic–inorganic hybrid perovskite solar cells are attracting the attention of researchers owing to the high level of performance they exhibit in photovoltaic device applications. However, the attainment of an even higher level of performance is hindered by their anomalous current–voltage (I–V) hysteresis behavior. Even though experimental and theoretical studies have suggested that the perovskite materials may have a ferroelectric nature, it is still far from being fully understood. In this study, the origin of the hysteresis behavior in CH₃NH₃PbI₃ perovskite thin films is investigated. The behavior of ferroelectricity using piezoresponse force microscopy is first examined. Then, by comparing the scan-rate-dependent nano/macroscopic I–V curves, it is found that ion migration assisted by the grain boundaries is a dominant origin of I–V hysteresis from a macroscopic viewpoint. Consequently, the observations suggest that, even though ferroelectricity exists in the CH₃NH₃PbI₃ perovskite materials, ion migration primarily contributes to the macroscopic I–V hysteresis. The presented results can provide fundamental guidelines to the resolution of hysteresis issues in organic–inorganic hybrid perovskite materials.

This study reports on the origin of the hysteresis in CH₃NH₃PbI₃ perovskite thin films. The presence of the ferroelectricity is demonstrated, and further, ion migration assisted by the grain boundaries is demonstrated as a dominant origin of macroscopic I–V hysteresis. Consequently, the observations suggest that although ferroelectricity exists, ion migration primarily contributes to the macroscopic I–V hysteresis.

In article number 1701048, Jang-Sik Lee and Bohee Hwang report an approach for designing high density memory devices utilizing organolead halide perovskite (OHP) materials with a cross-point array structure which is deposited by sequential vapor deposition. The OHP-based nanoscale memory devices exhibit remarkably fast switching speed, low operating voltage, and long data retention. OHPs which are a key material in solar cells have a great potential to be used in high-density information storage devices.

This study reports the development of copper(I) thiocyanate (CuSCN) hole-transport layers (HTLs) processed from aqueous ammonia as a novel alternative to conventional n-alkyl sulfide solvents. Wide bandgap (3.4–3.9 eV) and ultrathin (3–5 nm) layers of CuSCN are formed when the aqueous CuSCN–ammine complex solution is spin-cast in air and annealed at 100 °C. X-ray photoelectron spectroscopy confirms the high compositional purity of the formed CuSCN layers, while the high-resolution valence band spectra agree with first-principles calculations. Study of the hole-transport properties using field-effect transistor measurements reveals that the aqueous-processed CuSCN layers exhibit a fivefold higher hole mobility than films processed from diethyl sulfide solutions with the maximum values approaching 0.1 cm² V⁻¹ s⁻¹. A further interesting characteristic is the low surface roughness of the resulting CuSCN layers, which in the case of solar cells helps to planarize the indium tin oxide anode. Organic bulk heterojunction and planar organometal halide perovskite solar cells based on aqueous-processed CuSCN HTLs yield power conversion efficiency of 10.7% and 17.5%, respectively. Importantly, aqueous-processed CuSCN-based cells consistently outperform devices based on poly(3,4-ethylenedioxythiophene) polystyrene sulfonate HTLs. This is the first report on CuSCN films and devices processed via an aqueous-based synthetic route that is compatible with high-throughput manufacturing and paves the way for further developments.

Dissolution of copper thiocyanate (CuSCN) in aqueous ammonia enables processing of superior quality hole-transporting layers at low temperature in ambient air. Transistors based on these CuSCN layers exhibit mobilities close to 0.1 cm² V⁻¹ s⁻¹, while solar cells incorporating CuSCN interlayers yield power conversion efficiencies of 10.7% and 17.5% for organic bulk heterojunction and organometal halide cells, respectively.

Functionalized imidazolium iodide salts (ionic liquids) modified with CH₂CHCH₂, CH₂CCH, or CH₂CN groups are applied as dopants in the synthesis of CH₃NH₃PbI₃-type perovskites together with a fumigation step. Notably, a solar cell device prepared from the perovskite film doped with the salt containing the CH₂CHCH₂ side-chain has a power conversion efficiency of 19.21%, which is the highest efficiency reported for perovskite solar cells involving a fumigation step. However, doping with the imidazolium salts with the CH₂CCH and CH₂CN groups result in perovskite layers that lead to solar cell devices with similar or lower power conversion efficiencies than the dopant-free cell.

Perovskite films, grown from PbI₂:MAI in DMSO in the presence of functionalized ionic-liquid (imidazolium iodide) dopants and incorporated into perovskite solar cells, are reported. One cell has a power conversion efficiency exceeding 19%. Difference in power conversion efficiency can be traced to the physical properties of imidazolium-PbI₃ salts that form during the preparation of the film.

Nanostructured materials characterized by high surface–volume ratio hold the promise to constitute the active materials for next-generation sensors. Solution-processed hybrid organohalide perovskites, which have been extensively used in the last few years for optoelectronic applications, are characterized by a self-assembled nanostructured morphology, which makes them an ideal candidate for gas sensing. Hitherto, detailed studies of the dependence of their electrical characteristics on the environmental atmosphere have not been performed, and even the effect of a ubiquitous gas such as O₂ has been widely overlooked. Here, the electrical response of organohalide perovskites to oxygen is studied. Surprisingly, a colossal increase (3000-fold) in the resistance of perovskite-based lateral devices is found when measured in a full oxygen atmosphere, which is ascribed to a trap healing mechanism originating from an O₂-mediated iodine vacancies filling. A variation as small as 70 ppm in the oxygen concentration can be detected. The effect is fast (<400 ms) and fully reversible, making organohalide perovskites ideal active materials for oxygen sensing. The effect of oxygen on the electrical characteristics of organohalide perovskites must be taken into deep consideration for the design and optimization of any other perovskite-based (opto-) electronic device working in ambient conditions.

Oxygen gas is found to induce a colossal change in the electrical current flowing through organometallic hybrid perovskites, paving the way to the demonstration of fast, fully reversible, and wide-range oxygen sensors. The efficiency of the sensing element depends dramatically on the nanoscale morphology of the material, which can be controlled by optimization of the deposition process.

In this work, high-efficiency nonfullerene polymer solar cells (PSCs) are developed based on a thiazolothiazole-containing wide bandgap polymer PTZ1 as donor and a planar IDT-based narrow bandgap small molecule with four side chains (IDIC) as acceptor. Through thermal annealing treatment, a power conversion efficiency (PCE) of up to 11.5% with an open circuit voltage (V_oc) of 0.92 V, a short-circuit current density (J_sc) of 16.4 mA cm⁻², and a fill factor of 76.2% is achieved. Furthermore, the PSCs based on PTZ1:IDIC still exhibit a relatively high PCE of 9.6% with the active layer thickness of 210 nm and a superior PCE of 10.5% with the device area of up to 0.81 cm². These results indicate that PTZ1 is a promising polymer donor material for highly efficient fullerene-free PSCs and large-scale devices fabrication.

The nonfullerene polymer solar cells based on a wide-bandgap polymer PTZ1 and a narrow-bandgap acceptor IDIC exhibit weak active-layer thickness and area dependence with an optimal power conversion efficiency of 11.5%, indicating that the blend of PTZ1/IDIC has potential for the practical application of polymer solar cells.

“Nonfullerene” acceptors are proving effective in bulk heterojunction (BHJ) solar cells when paired with selected polymer donors. However, the principles that guide the selection of adequate polymer donors for high-efficiency BHJ solar cells with nonfullerene acceptors remain a matter of some debate and, while polymer main-chain substitutions may have a direct influence on the donor–acceptor interplay, those effects should be examined and correlated with BHJ device performance patterns. This report examines a set of wide-bandgap polymer donor analogues composed of benzo[1,2-b:4,5-b′]dithiophene (BDT), and thienyl ([2H]T) or 3,4-difluorothiophene ([2F]T) motifs, and their BHJ device performance pattern with the nonfullerene acceptor “ITIC”. Studies show that the fluorine- and ring-substituted derivative PBDT(T)[2F]T largely outperforms its other two polymer donor counterparts, reaching power conversion efficiencies as high as 9.8%. Combining several characterization techniques, the gradual device performance improvements observed on swapping PBDT[2H]T for PBDT[2F]T, and then for PBDT(T)[2F]T, are found to result from (i) notably improved charge generation and collection efficiencies (estimated as ≈60%, 80%, and 90%, respectively), and (ii) reduced geminate recombination (being suppressed from ≈30%, 25% to 10%) and bimolecular recombination (inferred from recombination rate constant comparisons). These examinations will have broader implications for further studies on the optimization of BHJ solar cell efficiencies with polymer donors and a wider range of nonfullerene acceptors.

Swapping main-chain substituents in a set of analogous wide-bandgap polymer donors is shown to result in gradual bulk-heterojunction (BHJ) device performance improvements when the polymers are combined with the nonfullerene acceptor “ITIC”. The gradual improvements result from better charge generation, collection, and reduced geminate and bimolecular recombination, leading to polymer-nonfullerene BHJ solar cells with power conversion efficiencies as high as 9.8%.