ZiQi Sun

Benzene units are inserted into the backbone of a quaterthiophene-based polymer named PffBT4T, and the resulting polymer, PffBT4T-B, exhibits remarkably tight alkyl chain interdigitation, which can expel the ITIC-Th molecules from the polymer domains thus forming more pure and crystalline ITIC-Th domains. As a result, PffBT4T-B-based non-fullerene organic solar cells achieve a high power conversion efficiency of 9.4%.

Perovskite materials due to their exceptional photophysical properties are beginning to dominate the field of thin-film optoelectronic devices. However, one of the primary challenges is the processing-dependent variability in the properties, thus making it imperative to understand the origin of such variations. Here, it is discovered that the precursor solution aging time before it is cast into a thin film, is a subtle but a very important factor that dramatically affects the overall thin-film formation and crystallinity and therein factors such as grain growth, phase purity, surface uniformity, trap state density, and overall solar cell performance. It is shown that progressive aging of the precursor promotes efficient formation of larger seeds after the fast nucleation of a large density of small seeds. The hot-casting method then leads to the growth of large grains in uniform thin-films with excellent crystallinity validated using scanning microscopy images and X-ray diffraction patterns. The high-quality films cast from aged solution is ideal for thin-film photovoltaic device fabrication with reduced shunt current and good charge transport. This observation is a significant step toward achieving highly crystalline thin-films with reliability in device performance and establishes the subtle but dramatic effect of solution aging before fabricating perovskite thin-films.

A direct correlation between precursor aging time and crystalline quality of hybrid perovskite-based thin films is discovered and systematically elucidated. Progressive aging of precursor results in a pinhole-free thin film with optimal phase purity that results in high-efficiency planar solar cells.

To prevent the interfacial charge recombination between injected holes in the valence band and the redox mediator in the electrolyte in p-type dye sensitized solar cells (p-DSSC) the passivation of the recombination sites by organic insulator chenodeoxycholic acid (CDCA) layer is critically investigated in this study. Rather than classical coating of the semiconductor's surface by simultaneous co-adsorption of CDCA during the dyeing step, two other methods are investigated. The first consists in dissolving CDCA in the electrolyte, while the second consists in spin coating an ethanol solution of CDCA onto the already dyed photocathode. In this study, different sensitizers, electrolytes, and p-SCs, (NiO, CuGaO₂) are explored. Analysis of the current/voltage curves and electrochemical impedance spectroscopy provides evidence that the role of the CDCA layer is to create a physical barrier to prevent the approach of the redox mediator from the NiO surface and consequently raise the open circuit voltage (V_oc). The important finding of this study is the demonstration that the V_oc in p-DSSC is heavily limited by interfacial charge recombination and that higher V_oc values much above 100 mV and as high as 500 mV can be attained with conventional materials (NiO) if this deleterious side reaction can be suppressed or diminished.

It is demonstrated that the open circuit voltage (V_oc) in p-type dye sensitized solar cells is heavily limited by interfacial charge recombination and consequently that higher V_oc values above 100 mV and as high as 500 mV can be attained with conventional materials (NiO and electrolytes) if the deleterious interfacial charge recombination reactions can be decreased.

Deliberate halide exchange between unstable intermediate HPbI₂Br and nonstoichiometric FAI is employed to produce high-quality FAPbI_3–xBr_x (x ≈ 0.44), which eliminates the use of antisolvent dripping and other post-treatment techniques. The obtained perovskite thin film demonstrates high crystallinity and a large and compact crystal domain up to 2–3 µm. The corresponding device shows power conversion efficiency of up to around 19.0%, with reliable stability and reproducibility.

Recent advances in the efficiency of crystalline silicon (c-Si) solar cells have come through the implementation of passivated contacts that simultaneously reduce recombination and resistive losses within the contact structure. In this contribution, low resistivity passivated contacts are demonstrated based on reduced titania (TiO_x) contacted with the low work function metal, calcium (Ca). By using Ca as the overlying metal in the contact structure we are able to achieve a reduction in the contact resistivity of TiO_x passivated contacts of up to two orders of magnitude compared to previously reported data on Al/TiO_x contacts, allowing for the application of the Ca/TiO_x contact to n-type c-Si solar cells with partial rear contacts. Implementing this contact structure on the cell level results in a power conversion efficiency of 21.8% where the Ca/TiO_x contact comprises only ≈6% of the rear surface of the solar cell, an increase of 1.5% absolute compared to a similar device fabricated without the TiO_x interlayer.

A novel passivated contact on crystalline silicon comprised of calcium and reduced titania is shown to result in a reduction in contact resistivity by up to two orders of magnitude compared to other titania-based contacts to crystalline silicon. This enables the fabrication of a first-of-its-kind passivated contact n-type c-Si PERC cell with an efficiency of 21.8%.

Recently, organic–inorganic hybrid perovskite materials have drawn great attention for their outstanding performance in high-efficiency solar cells. Successful synthesis has been realized either in solution-based chemical deposition or vapor deposition. However, conflicts have never ceased among quality control, growth rate, process complexity, and instrument requirement, which have limited their development toward real applications. In this work, the first electrochemical fabrication of perovskite toward high-efficiency and scalable perovskite solar cells (PSCs) is established. The morphology and crystallization of the CH₃NH₃PbI₃ film can be effectively controlled by simply modulating a few physical parameters. A detailed study on its optoelectronic properties reveals significantly improved film quality and interfacial conditions. Aided by this, the total process does not require standard annealing, which greatly reduces the total growth time from hours to minutes. Up to now, an efficiency of 15.65% has been achieved in planar PSCs under 1 sun AM 1.5 condition, with small hysteresis and efficiency loss under longtime exposure to air. Moreover, high efficiency (10.45%) can be easily attained for large cells (2 cm²). This result will hopefully facilitate research for applicable high-efficiency PSCs and other multicomponent materials as well.

The first complete electrochemical fabrication of perovskite has been achieved for perovskite solar cells, with a total time two orders of magnitude less than other presented solution-based methods. High efficiency is realized with large area with efficiency loss of 0.7% from a total of over three weeks' exposure in air, aided by modulation of just a few physical parameters without additional treatments.

A new, all room-temperature solution process is developed to fabricate efficient, low-cost, and stable perovskite solar cells (PVSCs). The PVSCs show high efficiency of 17.10% and 14.19%, with no hysteresis on rigid and flexible substrates, respectively, which are the best efficiencies reported to date for PVSCs fabricated by room-temperature solution-processed techniques. The flexible PVSCs show a remarkable power-per-weight of 23.26 W g⁻¹.

A method for resolving the diffusion length of excitons and the extraction yield of charge carriers is presented based on the performance of organic bilayer solar cells and careful modeling. The technique uses a simultaneous variation of the absorber thickness and the excitation wavelength. Rigorously differing solar cell structures as well as independent photoluminescence quenching measurements give consistent results.

Silicon/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) heterojunction solar cells with 16.2% efficiency and excellent stability are fabricated on pyramid-textured silicon substrates by applying a water-insoluble ester as capping layer. This shows that a conformal coating of PEDOT:PSS on textured silicon can greatly improve the junction quality with the main stability failure routes related to the moisture-induced poly(3,4-ethylenedioxythiophene) aggregations and the tunneling silicon oxide autothickening.

Organic–inorganic hybrid halide perovskites (e.g., MAPbI₃) have recently emerged as novel active materials for photovoltaic applications with power conversion efficiency over 22%. Conventional perovskite solar cells (PSCs); however, suffer the issue that lead is toxic to the environment and organisms for a long time and is hard to excrete from the body. Therefore, it is imperative to find environmentally-friendly metal ions to replace lead for the further development of PSCs. Previous work has demonstrated that Sn, Ge, Cu, Bi, and Sb ions could be used as alternative ions in perovskite configurations to form a new environmentally-friendly lead-free perovskite structure. Here, we review recent progress on lead-free PSCs in terms of the theoretical insight and experimental explorations of the crystal structure of lead-free perovskite, thin film deposition, and device performance. We also discuss the importance of obtaining further understanding of the fundamental properties of lead-free hybrid perovskites, especially those related to photophysics.

Recent progress on lead-free perovskite solar cells (PSCs) in terms of the theoretical insight and experimental explorations of the crystal structure of lead-free perovskites, thin-film deposition, and device performance is reviewed. The importance of understanding the fundamental properties of lead-free hybrid perovskites is discussed. Greater effort is needed to explore high-performance lead-free PSCs.