Multifunctional Inverse Opal-Like TiO2 Electron Transport Layer for Efficient Hybrid Perovskite Solar Cells

Photonic Crystal Structures for Photovoltaic Applications

Photonic crystals are artificial structures with a spatial periodicity of dielectric permittivity on the wavelength scale. This feature results in a spectral region over which no light can propagate within such a material, known as the photonic band gap (PBG). It leads to a unique interaction between light and matter. A photonic crystal can redirect, concentrate, or even trap incident light. Different materials (dielectrics, semiconductors, metals, polymers, etc.) and 1D, 2D, and 3D architectures (layers, inverse opal, woodpile, etc.) of photonic crystals enable great flexibility in designing the optical response of the material. This opens an extensive range of applications, including photovoltaics. Photonic crystals can be used as anti-reflective and light-trapping surfaces, back reflectors, spectrum splitters, absorption enhancers, radiation coolers, or electron transport layers. This paper presents an overview of the developments and trends in designing photonic structures for different photovoltaic applications.

Silicon/nickel oxide core/shell nanospheres as a hole transport layer for high efficiency and light-stable perovskite solar cells

Metal halide perovskite solar cells (PSCs) possess huge potential due to their high power conversion efficiency. However, instability is still a key factor limiting their applications. Therefore, we have found a feasible strategy to improve the light stability of PSCs. Specifically, a core-shell material with a silicon nanosphere core and a nickel oxide nanosheet shell serves as the hole transport layer in our PSCs. Due to the selective absorption of ultraviolet light by the silicon nanoparticles, the ultraviolet light content of the natural light that reaches the perovskite layer is reduced. Compared with a control device (without Si), the PSCs with the silicon/nickel oxide hole transport layer possessed a higher current density of 22.09 mA cm^-2 and a higher power conversion efficiency of 18.54%, with both values increased by 2.7% and 6.1%, respectively. More importantly, the PSCs based on a silicon/nickel oxide hole transport layer maintains 85% of its initial power conversion efficiency value after 700 hours of natural light exposure. These results indicate that the silicon/nickel oxide hole transport layer is an important functional component of the PSCs, which improves the photovoltaic performance and reduces ultraviolet light-induced photodegradation, thereby improving the device stability.

Low-Temperature-Processed Monolayer Inverse Opal SnO2 Scaffold for Efficient Perovskite Solar Cells

Organic-inorganic halide perovskite solar cells (PSCs) have attracted tremendous attention in the photovoltaic field due to their excellent optical properties and simple fabrication process. However, the recombination of photogenerated electron-hole pairs at the interface severely affects the power conversion efficiency (PCE) of the PSCs. Herein, a monolayer of inverse opal SnO₂ (IO-SnO₂ ) is synthesized via a template-assisted method and used as a scaffold for perovskite layer (PSK). The porous IO-SnO₂ scaffold increases the contact area and shortens the transport distance between the electron transport layer (ETL) and PSK. Ultraviolet photoelectron spectroscopy and Kelvin probe force microscopy results indicate that the built-in electric field is enhanced with IO-SnO₂ scaffold, strengthening the driving force for charge separation. Femtosecond transient absorption spectroscopy measurements reveal that the IO-SnO₂ scaffold facilitates interfacial electron transfer from PSK to ETL. Based on the above superiorities, the IO-SnO₂ -based PSCs exhibit boosted PCE and device stability compared with the pristine PSCs. This work provides insights into the development of novel scaffold layers for high-performance PSCs.

Colorful Efficient Moiré-Perovskite Solar Cells

Light harvesting is crucial for thin-film solar cells. To substantially reduce optical loss in perovskite solar cells (PSCs), hierarchical light-trapping nano-architectures enable absorption enhancement to exceed the conventional upper limit and have great potential for achieving state-of-the art optoelectronic performances. However, it remains a great challenge to design and fabricate a superior hierarchical light-trapping nano-architecture, which exhibits extraordinary light-harvesting ability and simultaneously avoids deteriorating the electrical performance of PSCs. Herein, colorful efficient moiré-PSCs are designed and fabricated incorporating moiré interference structures by the imprinting method with the aid of a commercial DVD disc. It is experimentally and theoretically demonstrated that the light harvesting ability of the moiré interference structure can be well manipulated through changing the rotation angle (0°-90°). The boosted short-circuit current is credited to augment light diffraction channels, leading to elongated optical paths, and fold sunlight into the perovskite layer. Moreover, the imprinting process suppresses the trap sites and voids at the active-layer interfaces with eliminated hysteresis. The moiré-PSC with an optimized 30° rotation angle achieves the best enhancement of light harvesting (28.5% higher than the pristine), resulting in efficiencies over 20.17% (MAPbI₃ ) and 21.76% ((FAPbI₃ )_{1- x} (MAPbBr₃ )_x ).

Improving efficiency and stability of colorful perovskite solar cells with two-dimensional photonic crystals

Colorful solar cells have been much sought after because they can generate electricity and concurrently satisfy ornamentation purposes. Owing to their outstanding power conversion efficiency and flexibility in processing, perovskite solar cells (PSCs) have the great potential to become both efficient and aesthetically appealing. Here, we specially devise and fabricate two novel electron transport layers (ETLs) for PSCs with two-dimensional (2D) photonic crystal structures, namely the 2D inverse opal (IO) structured SnO₂ (IOS) and SnO₂-TiO₂ composite (IOST), using the template-assisted spin-coating method. The synergistic structure and material modifications to the ETLs lead to a number of unique features, including the remarkable electron transfer ability, vivid colors and good protection to the infiltrated perovskite films. Furthermore, the IOS and IOST ETLs are effectively incorporated into the CH₃NH₃PbI₃-based PSC devices that deliver the best efficiency of 16.8% with structural colors.

Recent progress of light manipulation strategies in organic and perovskite solar cells

Organic and perovskite solar cells are suffering from the insufficient utilization of incident light and thus low light harvesting efficiency despite their rapid progress in the past decade. In this regard, light manipulation strategies have attracted numerous attention to solve this inherent limit. Herein, the recent advances in light manipulation techniques in this area are overviewed. The light manipulation mechanisms are illustrated to classify the structures. Various light manipulation structures, fabrication techniques, and corresponding results are given and discussed, addressing the suppression of surface reflection, nano/micro-structure-induced light scattering, and the plasmonic effects with periodic metallic patterns and metallic nanoparticles. A brief perspective on future research is also proposed for pursuing broadband light harvesting.

Efficient light harvesting with a nanostructured organic electron-transporting layer in perovskite solar cells

Nanostructures have proved to be advantageous in light harvesting, improving the power conversion efficiency (PCE) of photovoltaic devices. However, the reported light-harvesting nanostructures all require extra processing beyond that for device fabrication, with multiple steps for nano-patterned structures or plasmonic nano-particles. Here we synthesized a conjugated polymer PFPDI which could simply form a nanostructured film on perovskite by spin coating. PFPDI possesses a perylene diimide-based backbone and phosphite ester pendants, which makes it a robust electron-transporting material (ETM) in perovskite solar cells. Furthermore, the perovskite solar cells with PFPDI as the electron-transporting layer (ETL) exhibited a higher light-harvesting efficiency compared to the standard phenyl-C61-butyric acid methyl ester (PCBM) devices. The JSC of the PFPDI device was enhanced from 19.71 mA cm-2 to 23.43 mA cm-2. SEM images and reflectance spectra confirmed that the PFPDI formed ditch-like nanostructures on perovskite film and induced a better light-harvesting efficiency. Further research indicated that the interaction of P[double bond, length as m-dash]O with Pb was essential to the formation of the nanostructures of PFPDI on perovskite. Therefore, our work not only offers an efficient organic ETM, but also opens up new horizons for simply constructing nanostructures with light-harvesting ability in photovoltaic devices.

Using microgels to control the morphology and optoelectronic properties of hybrid organic–inorganic perovskite films

Spin coating mixed microgel/perovskite precursor solutions gives disordered inverse opal perovskite films with morphologies and optoelectronic properties that are controlled by the microgel particles.

Tailoring the Mesoscopic TiO2 Layer: Concomitant Parameters for Enabling High-Performance Perovskite Solar Cells

Architectural control over the mesoporous TiO₂ film, a common electron-transport layer for organic-inorganic hybrid perovskite solar cells, is conducted by employing sub-micron sized polystyrene beads as sacrificial template. Such tailored TiO₂ layer is shown to induce asymmetric enhancement of light absorption notably in the long-wavelength region with red-shifted absorption onset of perovskite, leading to ~20% increase of photocurrent and ~10% increase of power conversion efficiency. This enhancement is likely to be originated from the enlarged CH₃NH₃PbI₃(Cl) grains residing in the sub-micron pores rather than from the effect of reduced perovskite-TiO₂ interfacial area, which is supported from optical bandgap change, haze transmission of incident light, and one-diode model parameters correlated with the internal surface area of microporous TiO₂ layers. With the templating strategy suggested, the necessity of proper hole-blocking method is discussed to prevent any direct contact of the large perovskite grains infiltrated into the intended pores of TiO₂ scaffold, further mitigating the interfacial recombination and leading to ~20% improvement in power conversion efficiency compared with the control device using conventional solution-processed hole blocking TiO₂. Thereby, the imperatives that originate from the structural engineering of the electron-transport layer are discussed to understand the governing elements for the improved device performance.

Wearable Large-Scale Perovskite Solar-Power Source via Nanocellular Scaffold

Dramatic advances in perovskite solar cells (PSCs) and the blossoming of wearable electronics have triggered tremendous demands for flexible solar-power sources. However, the fracturing of functional crystalline films and transmittance wastage from flexible substrates are critical challenges to approaching the high-performance PSCs with flexural endurance. In this work, a nanocellular scaffold is introduced to architect a mechanics buffer layer and optics resonant cavity. The nanocellular scaffold releases mechanical stresses during flexural experiences and significantly improves the crystalline quality of the perovskite films. The nanocellular optics resonant cavity optimizes light harvesting and charge transportation of devices. More importantly, these flexible PSCs, which demonstrate excellent performance and mechanical stability, are practically fabricated in modules as a wearable solar-power source. A power conversion efficiency of 12.32% for a flexible large-scale device (polyethylene terephthalate substrate, indium tin oxide-free, 1.01 cm² ) is achieved. This ingenious flexible structure will enable a new approach for development of wearable electronics.

Improving the performance and reliability of inverted planar perovskite solar cells with a carbon nanotubes/PEDOT:PSS hybrid hole collector

Adopting an efficient charge transport layer is crucial to improve the photovoltaic (PV) performances of organo-lead halide perovskite (PRV) solar cells. In this study, we suggest a novel hybrid hole-transport layer (HTL) consisting of single-walled carbon nanotubes (SWNTs) and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) for inverted-planar PRV devices. The SWNTs were drop-cast on ITO/glass substrates, and they were partly grown perpendicular to the substrates. Then, we coated PEDOT:PSS to cover the SWNTs for complete electron-blocking. A PRV light-harvester was spin-cast on the hybrid HTL, and the vertical SWNTs protruded into the PRV through penetrating the PEDOT:PSS. Steady-state photoluminescence spectroscopy evidenced that the SWNTs/PEDOT:PSS hybrid HTL showed enhanced charge-carrier quenching properties. The hybrid HTL also revealed negligible parasitic absorption loss checked by UV-Vis spectroscopy. These contributed to improve the average power conversion efficiency from 9.4% to 11.0% (up to 12.5% for the best cell) based on fabricated 90 devices. Furthermore, significant suppression of current-voltage hysteresis was attained by employing the hybrid HTL. This study not only manifests unprecedented utilization of the SWNTs for the HTL in inverted planar PRV cells but also paves the way for the development of high-performance and reliable PRV solar cells compatible with flexible processing at low temperature (<150 °C).

Effect of the Microstructure of the Functional Layers on the Efficiency of Perovskite Solar Cells

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.

Thermally Induced Crystallization of High Quality CH3 NH3 PbI3 Film with Large Grains for Highly Efficient Perovskite Solar Cells

Recently, fully covered and smooth perovskite films could be fabricated by optimized coating methods; however, it is still hard to prepare perovskite films with large grain sizes and high crystallinity. Given the fact that thermal energy can promote crystallization, we combine high-temperature crystallization with the application of a solvent featuring a high boiling point, in order to produce high quality perovskite films with micrometer-sized grains. We further investigated the temperature dependence of the thermally induced synthetic strategy, whereby the grains become larger as the temperature is elevated. After solar cell device fabrication, the efficiency of the best cell can attain a high value of 15.53 % with reduced hysteresis behavior.

WB crystals with oxidized surface as counter electrode in dye-sensitized solar cells

Tungsten boride (WB) crystals, whose surface tends to be oxidized when exposed to air, were demonstrated to have a comparable activity to platinum as counter electrode material in dye-sensitized solar cells. The synergistic effect of both catalytically active surface layer WO_x and electronically conductive internal WB is considered to be responsible for the high activity of the WB crystals.

A chemical approach to perovskite solar cells: control of electron-transporting mesoporous TiO2and utilization of nanocarbon materials

This Perspective highlights recent chemical approaches to perovskite solar cells, including the control of electron-transporting mesoporous TiO₂and the utilization of nanocarbon materials.

Designing nanobowl arrays of mesoporous TiO₂ as an alternative electron transporting layer for carbon cathode-based perovskite solar cells

In this work, we have designed a mesoporous TiO2 nanobowl (NB) array with pore size, bowl size and film thickness being easily controllable by the sol-gel process and the polystyrene (PS) template diameter. Based on the TiO2 NB array, we fabricated carbon cathode based perovskite solar cells (C-PSCs) to investigate the impact of TiO2 NB nanostructures on the performance of the as-obtained C-PSCs devices. As expected, the TiO2 NB based devices show a higher power conversion efficiency (PCE) than that of the planar counterpart, mainly due to the enhanced light absorption arising from the NB-assisted light management, the improved pore-filling of high quality perovskite crystals and the increased interface contact for rapid electron extraction and fast charge transport. Leveraging these advantages of the novel TiO2 NB film, the 220 nm-PS templated TiO2 NB based devices performed the best on both light absorption capability and charge extraction, and achieved a PCE up to 12.02% with good stability, which is 37% higher than that of the planar counterpart. These results point to a viable and convenient route toward the fabrication of TiO2 ETL nanostructures for high performance PSCs.