Ming Hsien Li

Low-Temperature Sputtered Nickel Oxide Compact Thin Film as Effective Electron Blocking Layer for Mesoscopic NiO/CH3NH3PbI3 Perovskite Heterojunction Solar Cells

We introduce the use of low temperature sputtered NiOx thin film, which substitutes the PEDOT-PSS and solution-processed NiOx as an effective electron blocking layer for mesoscopic NiO/CH3NH3PbI3 perovskite solar cells. The influences of film thickness and oxygen doping on the photovoltaic performances are scrutinized. The cell efficiency has been improved from 9.51 to 10.7% for devices using NiOx fabricated under pure argon atmosphere. With adequate doping under 10% oxygen flow ratio, we achieved power conversion efficiency of 11.6%. The procedure is large area scalable and has the advantage for cost-effective perovskite-based photovoltaics.

Inorganic p-type contact materials for perovskite-based solar cells

Organic–inorganic hybrid perovskite solar cells are promising low-cost emerging photovoltaic devices due to their rapid progresses in conversion efficiencies. In this perspective, we review recent published works of perovskite-based solar cells incorporated with an inorganic p-type hole transport layer. The current state-of-the-art devices of nickel oxide-based perovskite solar cells display a remarkable power of efficiency over 15%. In addition, solar cells using perovskite as light absorber and hole transporter without p-type contact material are also reviewed.

Novel spiro-based hole transporting materials for efficient perovskite solar cells

Three spiro-acridine-fluorene based hole transporting materials (HTMs), namely CW3, CW4 and CW5, are employed in the fabrication of organic-inorganic hybrid perovskite solar cells. The corresponding mesoscopic TiO2/CH3NH3PbI3/HTM devices are investigated and compared with that made with commercial spiro-OMeTAD. The best conversion efficiency of 16.56% is achieved for CW4 in the presence of tBp and Li-TFSI as additives and without a cobalt dopant. The performances of CW4 are further examined in terms of conductivity, mobility, morphology, and stability to show its potential as an alternative HTM.

Ultrafast Dynamics of Hole Injection and Recombination in Organometal Halide Perovskite Using Nickel Oxide as p-Type Contact Electrode

There is a mounting effort to use nickel oxide (NiO) as p-type selective electrode for organometal halide perovskite-based solar cells. Recently, an overall power conversion efficiency using this hole acceptor has reached 18%. However, ultrafast spectroscopic investigations on the mechanism of charge injection as well as recombination dynamics have yet to be studied and understood. Using time-resolved terahertz spectroscopy, we show that hole transfer is complete on the subpicosecond time scale, driven by the favorable band alignment between the valence bands of perovskite and NiO nanoparticles (NiO(np)). Recombination time between holes injected into NiO(np) and mobile electrons in the perovskite material is shown to be hundreds of picoseconds to a few nanoseconds. Because of the low conductivity of NiO(np), holes are pinned at the interface, and it is electrons that determine the recombination rate. This recombination competes with charge collection and therefore must be minimized. Doping NiO to promote higher mobility of holes is desirable in order to prevent back recombination.

Mixed Cation Thiocyanate-Based Pseudohalide Perovskite Solar Cells with High Efficiency and Stability

Novel organic-inorganic hybrid perovskite compounds composed of mixed A-site cation (Formamidinium and Cesium) and pseudohalides (SCN and I) ions are successfully synthesized. These new classes of hybrid perovskites photovoltaics exhibited remarkable power conversion efficiency of more than 16% with excellent stability against moisture in ambient environment and under low-light storage condition. The existence of SCN- ion inclusion is confirmed by secondary ion mass spectrometry and Fourier transform infrared spectroscopy. The SCN--doped pseudohalide is advantageous for the formation of large perovskite grains, as well as the performance and stability of the device.

Low‐Pressure Vapor‐Assisted Solution Process for Thiocyanate‐Based Pseudohalide Perovskite Solar Cells

In this report, we fabricated thiocyanate-based perovskite solar cells with low-pressure vapor-assisted solution process (LP-VASP) method. Photovoltaic performances are evaluated with detailed materials characterizations. Scanning electron microscopy images show that SCN-based perovskite films fabricated using LP-VASP have long-range uniform morphology and large grain sizes up to 1 μm. The XRD and Raman spectra were employed to observe the characteristic peaks for both SCN-based and pure CH3 NH3 PbI3 perovskite. We observed that the Pb(SCN)2 film transformed to PbI2 before the formation of perovskite film. X-ray photoemission spectra (XPS) show that only a small amount of S remained in the film. Using LP-VASP method, we fabricated SCN-based perovskite solar cells and achieved a power conversion efficiency of 12.72 %. It is worth noting that the price of Pb(SCN)2 is only 4 % of PbI2 . These results demonstrate that pseudo-halide perovskites are promising materials for fabricating low-cost perovskite solar cells.

Research Update: Hybrid organic-inorganic perovskite (HOIP) thin films and solar cells by vapor phase reaction

With the rapid progress in deposition techniques for hybrid organic-inorganic perovskite (HOIP) thin films, this new class of photovoltaic (PV) technology has achieved material quality and power conversion efficiency comparable to those established technologies. Among the various techniques for HOIP thin films preparation, vapor based deposition technique is considered as a promising alternative process to substitute solution spin-coating method for large-area or scale-up preparation. This technique provides some unique benefits for high-quality perovskite crystallization, which are discussed in this research update.

Highly stable perovskite solar cells with all-inorganic selective contacts from microwave-synthesized oxide nanoparticles

Although perovskite solar cells have achieved extremely high performance in just a few years, their device stability and fabrication cost are still of great concern. For inverted p–i–n perovskite solar cells, the commonly used electron-transporting layers are C60 and PCBM, which have stability issues and are very expensive. Here, we report a novel and highly stable perovskite solar cell using an inorganic electron-transporting layer made of microwave-assisted solution-processed indium-doped zinc oxide (IZO) nanoparticles. With NiO as the hole-transporting layer, the perovskite solar cells with all-inorganic selective contacts demonstrate a decent power conversion efficiency of over 16%. More importantly, the IZO-based perovskite solar cells demonstrate impressive long-term stability under air or light-soaking conditions. With encapsulation, our device retained 85% of the initial power conversion efficiency after 460 hours of light soaking. This result reveals that good device performance, low fabrication cost and impressive light-soaking stability can be realized simultaneously by employing facile microwave-synthesized oxides (IZO and NiO in this work) as inorganic selective contacts.

Robust and Recyclable Substrate Template with an Ultrathin Nanoporous Counter Electrode for Organic-Hole-Conductor-Free Monolithic Perovskite Solar Cells

A robust and recyclable monolithic substrate applying all-inorganic metal-oxide selective contact with a nanoporous (np) Au:NiOx counter electrode is successfully demonstrated for efficient perovskite solar cells, of which the perovskite active layer is deposited in the final step for device fabrication. Through annealing of the Ni/Au bilayer, the nanoporous NiO/Au electrode is formed in virtue of interconnected Au network embedded in oxidized Ni. By optimizing the annealing parameters and tuning the mesoscopic layer thickness (mp-TiO2 and mp-Al2O3), a decent power conversion efficiency (PCE) of 10.25% is delivered. With mp-TiO2/mp-Al2O3/np-Au:NiOx as a template, the original perovskite solar cell with 8.52% PCE can be rejuvenated by rinsing off the perovskite material with dimethylformamide and refilling with newly deposited perovskite. A renewed device using the recycled substrate once and twice, respectively, achieved a PCE of 8.17 and 7.72% that are comparable to original performance. This demonstrates that the novel device architecture is possible to recycle the expensive transparent conducting glass substrates together with all the electrode constituents. Deposition of stable multicomponent perovskite materials in the template also achieves an efficiency of 8.54%, which shows its versatility for various perovskite materials. The application of such a novel NiO/Au nanoporous electrode has promising potential for commercializing cost-effective, large scale, and robust perovskite solar cells.

Cu/Cu2O nanocomposite films as a p-type modified layer for efficient perovskite solar cells

Cu/Cu2O films grown by ion beam sputtering were used as p-type modified layers to improve the efficiency and stability of perovskite solar cells (PSCs) with an n-i-p heterojunction structure. The ratio of Cu to Cu2O in the films can be tuned by the oxygen flow ratio (O2/(O2 + Ar)) during the sputtering of copper. Auger electron spectroscopy was performed to determine the elemental composition and chemical state of Cu in the films. Ultraviolet photoelectron spectroscopy and photoluminescence spectroscopy revealed that the valence band maximum of the p-type Cu/Cu2O matches well with the perovskite. The Cu/Cu2O film not only acts as a p-type modified layer but also plays the role of an electron blocking buffer layer. By introducing the p-type Cu/Cu2O films between the low-mobility hole transport material, spiro-OMeTAD, and the Ag electrode in the PSCs, the device durability and power conversion efficiency (PCE) were effectively improved as compared to the reference devices without the Cu/Cu2O interlayer. The enhanced PCE is mainly attributed to the high hole mobility of the p-type Cu/Cu2O film. Additionally, the Cu/Cu2O film serves as a protective layer against the penetration of humidity and Ag into the perovskite active layer.