ZiQi Sun

A fullerene derivative (α-bis-PCBM) is purified from an as-produced bis-phenyl-C₆₁-butyric acid methyl ester (bis-[60]PCBM) isomer mixture by preparative peak-recycling, high-performance liquid chromatography, and is employed as a templating agent for solution processing of metal halide perovskite films via an antisolvent method. The resulting α-bis-PCBM-containing perovskite solar cells achieve better stability, efficiency, and reproducibility when compared with analogous cells containing PCBM. α-bis-PCBM fills the vacancies and grain boundaries of the perovskite film, enhancing the crystallization of perovskites and addressing the issue of slow electron extraction. In addition, α-bis-PCBM resists the ingression of moisture and passivates voids or pinholes generated in the hole-transporting layer. As a result, a power conversion efficiency (PCE) of 20.8% is obtained, compared with 19.9% by PCBM, and is accompanied by excellent stability under heat and simulated sunlight. The PCE of unsealed devices dropped by less than 10% in ambient air (40% RH) after 44 d at 65 °C, and by 4% after 600 h under continuous full-sun illumination and maximum power point tracking, respectively.

Significantly improved performance of mixed perovskite solar cells, using a facile α-bis-PCBM-containing perovskite growth method during device fabrication, is reported. The newly developed perovskite solar cell exhibits an enhanced power conversion efficiency of 20.8%, along with enhanced stability under heat and illumination.

Organometallic perovskites, solution-processable materials with outstanding optoelectronic properties and high refractive index, provide a unique platform for alldielectric metamaterials operating at visible frequencies. In article number 1604268, Cesare Soci and co-workers realize perovskite metasurfaces with structural coloring tunable across visible frequencies, which also yields a three-fold increase of luminescence emission in comparison with unstructured perovskite films.

A wide bandgap small molecular acceptor, SFBRCN, containing a 3D spirobifluorene core flaked with a 2,1,3-benzothiadiazole (BT) and end-capped with highly electron-deficient (3-ethylhexyl-4-oxothiazolidine-2-yl)dimalononitrile (RCN) units, has been successfully synthesized as a small molecular acceptor (SMA) for nonfullerene polymer solar cells (PSCs). This SMA exhibits a relatively wide optical bandgap of 2.03 eV, which provides a complementary absorption to commonly used low bandgap donor polymers, such as PTB7-Th. The strong electron-deficient BT and RCN units afford SFBRCN with a low-lying LUMO (lowest unoccupied molecular orbital) level, while the 3D structured spirobifluorene core can effectively suppress the self-aggregation tendency of the SMA, thus yielding a polymer:SMA blend with reasonably small domain size. As the results of such molecular design, SFBRCN enables nonfullerene PSCs with a high efficiency of 10.26%, which is the highest performance reported to date for a large bandgap nonfullerene SMA.

A wide bandgap small molecular acceptor, SFBRCN, containing a 3D spirobifluorene core flanked with two 2,1,3-benzothiadiazole groups and end-capped with two highly electron-deficient (3-ethylhexyl-4-oxothiazolidine-2-yl)dimalononitrile units, has been successfully synthesized as a small molecular acceptor for nonfullerene polymer solar cells with a high efficiency of 10.26%.

A ternary nonfullerene polymer solar cell with a high efficiency of 9.70% is realized by using an n-type organic semiconductor ITIC acceptor and two polymer donors of medium bandgap polymer J51 and narrow bandgap polymer PTB7-Th with the polymer weight ratio of 0.8:0.2.

Understanding charge separation and charge transport at a molecular level is crucial for improving the efficiency of organic photovoltaic (OPV) cells. Under illumination of Bulk Heterojunction (BHJ) blends of polymers and fullerenes, various paramagnetic species are formed including polymer and fullerene radicals, radical pairs, and photoexcited triplet states. Light-induced Electron Paramagnetic Resonance (EPR) spectroscopy is ideally suited to study these states in BHJ due to its selectivity in probing the paramagnetic intermediates. Advanced techniques like pulsed EPR and ENDOR spectroscopy allow the determination of hyperfine coupling tensors, while high-frequency EPR allows the EPR signals of the individual species to be resolved and their g-tensors to be determined. The magnetic resonance parameters of the various polymer donors reveal details about the delocalization of the positive polaron which is important for the efficient charge separation in BHJ systems. Time-resolved EPR can contribute to the study of the dynamics of charge separation, charge transfer and recombination in BHJ by probing the unique spectral signatures of charge transfer and triplet states. EPR also has the potential to allow characterization of intermediates and products of BHJ degradation.

Light-induced EPR spectroscopy is ideally suited to study charge separated states in BHJ since it selectively probes the paramagnetic charge carriers and excited states. This review focusses on the application of advanced EPR techniques to characterize the electronic structure of positive and negative polarons as well as dynamics of spin-dependent charge separation and charge recombination processes in organic photovoltaic materials.

Inorganic-organic lead-halide perovskite solar cells have reached efficiencies above 22% within a few years of research. Achieved photovoltages of >1.2 V are outstanding for a material with a bandgap of 1.6 eV – in particular considering that it is solution processed. Such values demand for low non-radiative recombination rates and come along with high luminescence yields when the solar cell is operated as a light emitting diode. This progress report summarizes the developments on material composition and device architecture, which allowed for such high photovoltages. It critically assesses the term “lifetime”, the theories and experiments behind it, and the different recombination mechanisms present. It attempts to condense reported explanations for the extraordinary optoelectronic properties of the material. Amongst those are an outstanding defect tolerance due to antibonding valence states and the capability of bandgap tuning, which might make the dream of low-cost highly efficient solution-processed thin film solar cells come true. Beyond that, the presence of photon recycling will open new opportunities for photonic device design.

Perovskite solar cells show exceptionally high photovoltages. This progress report discusses the current understanding of the main material properties that are responsible for the high electronic quality of the metal-halide perovskites. Amongst them is a pronounced defect tolerance, which facilitates low non-radiative recombination rates and high luminescence yields.

Perovskite solar cells (PSCs) have recently demonstrated high efficiencies of over 22%, but the thermal stability is still a major challenge for commercialization. In this work, the thermal degradation process of the inverted structured PSCs induced by the silver electrode is thoroughly investigated. Elemental depth profiles indicate that iodide and methylammonium ions diffuse through the electron-trasnporting layer and accumulate at the Ag inner surface. The driving force of forming AgI then facilitates the ions extraction. Variations on the morphology and current mapping of the MAPbI₃ thin films upon thermal treatment reveal that the loss of ions occurs at the grain boundaries and leads to the reconstruction of grain domains. Consequently, the deteriorated MAPbI₃ thin film, the poor electron extraction, and the generation of AgI barrier result in the degradation of efficiencies. These direct evidences provide in-depth understanding of the effect of thermal stress on the devices, offering both experimental support and theoretical guidance for the improvement on the thermal stability of the inverted PSCs.

Silver-electrode-induced thermal degradation of the inverted perovskite solar cells is investigated with direct evidences. The diffusion of iodide and methylamine ions is directly observed in the elemental depth profile during thermal treatment only when the Ag electrode is introduced. The loss of ions leads to the reconstruction of the grain boundaries and forming thick PbI₂ gaps between crystal grains.

Mixed ion perovskite solar cells (PSC) are manufactured with a metal-free hole contact based on press-transferred single-walled carbon nanotube (SWCNT) film infiltrated with 2,2,7,-7-tetrakis(N,N-di-p-methoxyphenylamine)-9,90-spirobifluorene (Spiro-OMeTAD). By means of maximum power point tracking, their stabilities are compared with those of standard PSCs employing spin-coated Spiro-OMeTAD and a thermally evaporated Au back contact, under full 1 sun illumination, at 60 °C, and in a N₂ atmosphere. During the 140 h experiment, the solar cells with the Au electrode experience a dramatic, irreversible efficiency loss, rendering them effectively nonoperational, whereas the SWCNT-contacted devices show only a small linear efficiency loss with an extrapolated lifetime of 580 h.

A perovskite solar cell with carbon nanotube-based hole contact and drop cast 2,2,7,7-tetrakis(N,N-di-p-methoxyphenylamine)-9,90-spirobifluorene (Spiro-OMeTAD) exhibits superior stability over the standard device with spin-coated Spiro-OMeTAD and evaporated gold contact. The solar cells are subjected to a 140 h maximum power point tracking stability experiment in 1 sun illumination and 60 °C, and the carbon-based cell outperforms the gold cell clearly.

The efficiencies of the hybrid organic–inorganic perovskite solar cells have been rapidly approaching the benchmarks held by the leading thin-film photovoltaic technologies. Arguably, one of the most important factors leading to this rapid advancement is the ability to manipulate the microstructure of the perovskite layer and the adjacent functional layers within the device. Here, an analysis of the nucleation and growth models relevant to the formation of perovskite films is provided, along with the effect of the perovskite microstructure (grain sizes and voids) on device performance. In addition, the effect of a compact or mesoporous electron-transport-layer (ETL) microstructure on the perovskite film formation and the optical/photoelectric properties at the ETL/perovskite interface are overviewed. Insight into the formation of the functional layers within a perovskite solar cell is provided, and potential avenues for further development of the perovskite microstructure are identified.

The performance of perovskite solar cells is greatly affected by the microstructure of the functional layers, especially that of the perovskite film. By controlling the nucleation and crystal growth process, desirable microstructures (grains and voids size) of the perovskite films, such as dense films with large grains, can be achieved for high-efficiency solar cells.