Enbing Bi

Efficient and stable large-area perovskite solar cells with inorganic charge extraction layers

Perovskites go large Solar cells made of planar organic-inorganic perovskites now have reported efficiencies exceeding 20%. However, these values have been determined from small illuminated areas. Chen et al. used highly doped inorganic charge extraction layers to make solar cells on the 1 cm2 scale (see the Perspective by Sessolo and Bolink). The layers helped to protect the active layer from degradation by air. The cells achieved governmentlab–certified efficiencies of >15%. Furthermore, 90% of the efficiency was maintained after 1000 hours of operation. Science, this issue p. 944; see also p. 917 Highly doped inorganic layers both improve charge extraction and help protect the active layer from humid air. [Also see Perspective by Sessolo and Bolink] The recent dramatic rise in power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) has triggered intense research worldwide. However, high PCE values have often been reached with poor stability at an illuminated area of typically less than 0.1 square centimeter. We used heavily doped inorganic charge extraction layers in planar PSCs to achieve very rapid carrier extraction, even with 10- to 20-nanometer-thick layers, avoiding pinholes and eliminating local structural defects over large areas. The robust inorganic nature of the layers allowed for the fabrication of PSCs with an aperture area >1 square centimeter that have a PCE >15%, as certified by an accredited photovoltaic calibration laboratory. Hysteresis in the current-voltage characteristics was eliminated; the PSCs were stable, with >90% of the initial PCE remaining after 1000 hours of light soaking.

A quasi core–shell nitrogen-doped graphene/cobalt sulfide conductive catalyst for highly efficient dye-sensitized solar cells

A novel conductive catalyst was designed based on a quasi core–shell structure of N-doped graphene/cobalt sulfide. This platinum-free catalyst shows high catalytic activity and conductivity owing to close interactions between the core (cobalt sulphide) and the shell (N-doped graphene). It enables dye-sensitized solar cells (DSSCs) to obtain high energy conversion efficiency up to 10.71%, which is as far as we know the highest efficiency for DSSCs based on a platinum-free counter electrode.

Highly compact TiO2 layer for efficient hole-blocking in perovskite solar cells

A uniform and pinhole-free hole-blocking layer is necessary for high-performance perovskite-based thin-film solar cells. In this study, we investigated the effect of nanoscale pinholes in compact TiO2 layers on the device performance. Surface morphology and film resistance studies show that TiO2 compact layers fabricated using atomic layer deposition (ALD) contain a much lower density of nanoscale pinholes than layers obtained by spin coating and spray pyrolysis methods. The ALD-based TiO2 layer acts as an efficient hole-blocking layer in perovskite solar cells; it offers a large shunt resistance and enables a high power conversion efficiency of 12.56%.

A solvent- and vacuum-free route to large-area perovskite films for efficient solar modules

Recent advances in the use of organic–inorganic hybrid perovskites for optoelectronics have been rapid, with reported power conversion efficiencies of up to 22 per cent for perovskite solar cells. Improvements in stability have also enabled testing over a timescale of thousands of hours. However, large-scale deployment of such cells will also require the ability to produce large-area, uniformly high-quality perovskite films. A key challenge is to overcome the substantial reduction in power conversion efficiency when a small device is scaled up: a reduction from over 20 per cent to about 10 per cent is found when a common aperture area of about 0.1 square centimetres is increased to more than 25 square centimetres. Here we report a new deposition route for methyl ammonium lead halide perovskite films that does not rely on use of a common solvent or vacuum: rather, it relies on the rapid conversion of amine complex precursors to perovskite films, followed by a pressure application step. The deposited perovskite films were free of pin-holes and highly uniform. Importantly, the new deposition approach can be performed in air at low temperatures, facilitating fabrication of large-area perovskite devices. We reached a certified power conversion efficiency of 12.1 per cent with an aperture area of 36.1 square centimetres for a mesoporous TiO2-based perovskite solar module architecture.

Diffusion engineering of ions and charge carriers for stable efficient perovskite solar cells

Long-term stability is crucial for the future application of perovskite solar cells, a promising low-cost photovoltaic technology that has rapidly advanced in the recent years. Here, we designed a nanostructured carbon layer to suppress the diffusion of ions/molecules within perovskite solar cells, an important degradation process in the device. Furthermore, this nanocarbon layer benefited the diffusion of electron charge carriers to enable a high-energy conversion efficiency. Finally, the efficiency on a perovskite solar cell with an aperture area of 1.02 cm2, after a thermal aging test at 85 °C for over 500 h, or light soaking for 1,000 h, was stable of over 15% during the entire test. The present diffusion engineering of ions/molecules and photo generated charges paves a way to realizing long-term stable and highly efficient perovskite solar cells.

Soft-cover deposition of scaling-up uniform perovskite thin films for high cost-performance solar cells

Low-cost and high energy conversion efficiency are the crucial factors for large scale application of solar cells. In recent years, a promising high cost-performance photovoltaic technology, organometal halide perovskite solar cells (PSCs), has attracted great attention. However, most of the reported high efficiencies were obtained on a small working area of about 0.1 cm2 with the material utilization ratio of only 1% during film deposition, which actually hinders the advancement in future application of PSCs. Here we present the soft-cover deposition (SCD) method where surface wettability, solution viscosity and thermal crystallization are the processing key factors for the deposition of uniform perovskite films with high material utilization ratios. Scaling-up, pinhole-free, large crystal grains and rough-border-free perovskite films were obtained over a large area of 51 cm2, which were processed continuously in ambient air with a significant enhancement in the material utilization ratio up to ∼80%. Highly reproducible power conversion efficiencies up to 17.6% were achieved in unit cells with a working area of 1 cm2, leading to a high overall cost-performance. We believe that the present SCD technology will benefit the low-cost fabrication of highly efficient perovskite solar cells and open up a route for the deposition of other solution processed thin-films.

Annealing-free perovskite films by instant crystallization for efficient solar cells

Organic–inorganic perovskite solar cells (PSCs) have attracted considerable attention around the world because they can be fabricated easily and inexpensively by solution-based processes. The key to the fabrication of high-performance PSCs is the crystallinity and morphology of the perovskite film, and thermal annealing is usually required to achieve a film with the necessary properties. Herein, we introduce a technique for instant crystallization of perovskite films without the need for thermal annealing. Specifically, a solution of methylammonium iodide and lead iodide was spin-coated onto a substrate, and ethyl acetate was dripped onto the film during spinning to induce instant crystallization of a CH3NH3PbI3 perovskite film. The resulting crystalline film exhibited large crystal grains and a high carrier lifetime. PSCs fabricated with annealing-free films prepared by means of this technique exhibited performance comparable to that of PSCs fabricated with annealed films and showed much higher efficiency than did reference cells fabricated with annealing-free films. This new instant-crystallization technique offers a way to shorten the device fabrication time, which will reduce the cost of manufacturing efficient PSCs.

Consecutive Morphology Controlling Operations for Highly Reproducible Mesostructured Perovskite Solar Cells.

Perovskite solar cells have shown high photovoltaic performance but suffer from low reproducibility, which is mainly caused by low uniformity of the active perovskite layer in the devices. The nonuniform perovskites further limit the fabrication of large size solar cells. In this work, we control the morphology of CH3NH3PbI3 on a mesoporous TiO2 substrate by employing consecutive antisolvent dripping and solvent-vapor fumigation during spin coating of the precursor solution. The solvent-vapor treatment is found to enhance the perovskite pore filling and increase the uniformity of CH3NH3PbI3 in the porous scaffold layer but slightly decrease the uniformity of the perovskite capping layer. An additional antisolvent dripping is employed to recover the uniform perovskite capping layer. Such consecutive morphology controlling operations lead to highly uniform perovskite in both porous and capping layers. By using the optimized perovskite deposition procedure, the reproducibility of mesostructured solar cells was greatly improved such that a total of 40 devices showed an average efficiency of 15.3% with a very small standard deviation of 0.32. Moreover, a high efficiency of 14.9% was achieved on a large-size cell with a working area of 1.02 cm(2).

Fullerene-Structured MoSe2 Hollow Spheres Anchored on Highly Nitrogen-Doped Graphene as a Conductive Catalyst for Photovoltaic Applications.

A conductive catalyst composed of fullerene-structured MoSe2 hollow spheres and highly nitrogen-doped graphene (HNG-MoSe2) was successfully synthesized via a wet chemical process. The small molecule diethylenetriamine, which was used during the process, served as a surfactant to stabilize the fullerene-structured MoSe2 hollow spheres and to provide a high content of nitrogen heteroatoms for graphene doping (ca. 12% N). The superior synergistic effect between the highly nitrogen-doped graphene and the high surface-to-volume ratio MoSe2 hollow spheres afforded the HNG-MoSe2 composite high conductivity and excellent catalytic activity as demonstrated by cyclic voltammetry, electrochemical impedance spectroscopy and Tafel measurements. A dye-sensitized solar cell (DSSC) prepared with HNG-MoSe2 as a counter electrode exhibited a conversion efficiency of 10.01%, which was close to that of a DSSC with a Pt counter electrode (10.55%). The synergy between the composite materials and the resulting highly efficient catalysis provide benchmarks for preparing well-defined, graphene-based conductive catalysts for clean and sustainable energy production.

A hybrid catalyst composed of reduced graphene oxide/Cu2S quantum dots as a transparent counter electrode for dye sensitized solar cells

We synthesized a hybrid catalyst of reduced graphene oxide/Cu2S quantum dots (RGO/Cu2S QDs) via a facile wet chemical approach. The synergistic effect between ultrathin-RGO and ultrasmall-QDs endowed the hybrid catalyst with a high transparent performance, excellent conductivity and catalytic activity. A dye-sensitized solar cell fabricated with the hybrid catalyst showed the overall power conversion efficiency was 7.12%, which was comparable to that of a Pt-based device.