High absorptivity of perovskite solar cell enhanced by metal grating

Consumption of fossil fuel has led to serious environmental pollution, and an urgent demand for solar energy. Perovskite solar cell (PSC) is a device that converts solar energy into electricity. It is cost effective and power efficient, which has attracted much attention. However, PSC shows low absorptivity due to the limited thickness of the active layer. In this paper, a bilateral L-shaped metal grating structure is introduced into the PSC to enhance the absorptivity of the active layer by the surface plasmon effect between the metaling grating and the TiO2. With the deflection angle of the inner angle connection line of the metal grating is 45°, the inner angle distance is 100 nm, and the structural period is 250 nm, the absorptivity of the active layer of the PSC is 86.5% at 715 nm, which is 28.6% higher than the conventional solar cell at the same wavelength. Such results provide an effective way to improve the absorption of PSCs.     

基于级联金属光栅结构的有机太阳能电池宽谱吸收增强

为拓宽活性材料对太阳光谱的吸收范围,实现器 件的宽谱吸收增强,本文提出一种基 于光栅结构的新型有机太阳能电池器件,该器件在一个周期内引入不同占空比的光栅来构成 级联光栅。本文采用时域有限差分法从理论上研究了级联光栅结构对器件吸收性能的影响, 结果表明该结构可激发多个表面等离激元谐振,这些不同波长下的谐振模式可以在吸收光谱 中同时存在,从而实现宽谱吸收增强。考虑太阳光谱的影响,在TM和TM/TE混合偏振模式 下,活性材料的整体吸收效率从等效平板结构的23.9%分别提高到54.2%和36.4%,分别增 强126.8%和52.3%,且基于该级联光栅结构的 太阳能电池器件可实现在0到65度入射角范 围内的广角吸收。此外,在TM偏振模式下,该器件的吸收性能对光栅参数微小误差变化的 敏感性较低,适用于实际纳米器件 的制备。这项工作的理论结果有助于纳米金属光栅在有机 太阳能电池中的应用提供新的理论指导。     

Efficient electron-blocking layer-free planar heterojunction perovskite solar cells with a high open-circuit voltage

Perovskite solar cells (PSCs) with a simple device structure are particularly attractive due to their low cost and convenient fabrication process. Herein, highly efficient, electron-blocking layer (EBL)-free planar heterojunction (PHJ) PSCs with a structure of ITO/CH₃NH₃PbI₃/PCBM/Al were fabricated via low-temperature, solution-processed method. The power conversion efficiency (PCE) of over 11% was achieved in EBL-free PHJ-PSCs, which is closed to the value of PSC devices with the PEDOT:PSS as the EBL. It is impressed that the open-circuit voltage (V_oc) up to 1.06 V, an average value of 1.0 V for 43 devices, was obtained in EBL-free PHJ-PSCs. The electrochemical impedance spectroscopy (EIS) results suggested that the high PCE and V_oc are attributed to the relatively large recombination resistance and low contact resistance in EBL-free PHJ-PSCs. The solution-processed, EBL-free PHJ structure paves a boulevard for fabricating high-efficiency and low-cost PSCs.     

Efficiency enhancement in perovskite solar cell utilizing solution-processable phthalocyanine hole transport layer with thermal annealing

We demonstrate efficiency enhancement in perovskite solar cells (PSCs) utilizing a free-dopant hole transporting material (HTM), non-peripherally substituted octapentyl phthalocyanine (C5PcH₂) with thermal annealing. Particularly, by using thermal annealing approach, the external quantum efficiency at around 480 nm increase from 78 to 84%. Hence, the fill factor and short-circuit current density are markedly improved from 0.35 ± 0.02 to 0.55 ± 0.05 and from 18 ± 1 to 18.8 ± 0.3 mA cm^-2, respectively. Finally, the best device is achieved with power conversion efficiencies of 12.2% by annealing at 130 °C for 10 min. The photoluminescence and photo-induced charge carrier extraction in linearly increasing voltages measurements indicate that the charge carrier mobility in C5PcH₂ increases, and thereby the hole extraction and transportation from the perovskite layer to the Au anode as well the photovoltaic performance of PCS is improved by using thermal annealing processing.     

Large area perovskite solar cell module

The recent dramatic rise in power conversion efficiencies (PCE) of perovskite solar cells has triggered intense research worldwide. However, their practical development is hampered by poor stability and low PCE values with large areas devices. Here, we developed a gas-pumping method to avoid pinholes and eliminate local structural defects over large areas of perovskite film, even for 5×5 cm² modules, the PCE reached 10.6% and no significant degradation was found after 140 days of outdoor testing. Our approach enables the realization of high performance large-area PSCs for practical application.     

Broadband absorption enhancement in ultra‐thin crystalline Si solar cells by incorporating metallic and dielectric nanostructures in the back reflector

We propose a back reflecting scheme in order to enhance the maximum achievable current in one micron thick crystalline silicon solar cells. We perform 3D numerical investigations of the scattering properties of metallic nanostructures located at the back side and optimize them for enhancing absorption in the silicon layer. We validate our numerical results experimentally and also compare the absorption enhancement in the solar cell structure, both with quasi‐periodic and random metallic nanostructures. We have looked at the interplay between the metallic nanostructures and an integrated back reflector. We show that the combination of metallic nanoparticles and a metallic reflector results in significant parasitic absorption. We compared this to another implementation based on titanium dioxide nanoparticles, which act as a Lambertian reflector of light. Our simulation and experimental results show that this proposed configuration results in reduced absorption losses and in broadband enhancement of absorption for ultra‐thin solar cells, paving the way to an optimal back reflector for thin film photovoltaics. Copyright © 2014 John Wiley & Sons, Ltd.     

Performance enhancement of perovskite solar cell by controlling deposition temperature of copper phthalocyanine as a dopant-free hole transporting layer

In this study, an efficient perovskite solar cell is introduced using dopant-free and needle-like copper phthalocyanine (CuPc) in role of a hole selective layer. The needle-like structure was obtained by controlling the substrate temperature during deposition of CuPc. The photovoltaic cell power conversion efficiency was 14.89%, which is approximately similar to that of devices using a conventional doped 2,2′,7,7′-tetrakis(N,N′-di-p-methoxyphenylamine)-9, 9′-spirobifluorene (Spiro-OMeTAD) hole transport material under the same conditions. This study demonstrates thermally evaporated CuPc can be used independently with no additive materials, and it can be an efficient hole selective material to fabricate low-cost perovskite solar cells.     

Two-dimensional modeling of the MIS grating solar cell

A two-dimensional model has been developed for simulating the behavior of a metal-insulator-semiconductor (MIS) grating solar cell. In contrast with traditional methods, which solve the nonlinear semiconductor equations directly, this model solves a derived nonlinear boundary value problem, using an integral equation technique. The general characteristics of MIS grating solar cells have been calculated and are explained in terms of the underlying physical processes. In order to gain a better understanding of the collection mechanism taking place in a grating solar cell, a new concept, the "diffusion thickness of the space-charge layer" is introduced. The best-known grating solar cell is the MIS inversion layer solar cell. However, it is shown that for common values of the surface charge, an inversion layer cell with a low base doping level may not have as high an efficiency as a cell with a more heavily doped base, in which the minority-carrier flow is two-dimensional.     

Flexible,hole transporting layer-free and stable CH3NH3PbI3/PC61BM planar heterojunction perovskite solar cells

Hole transporting layer (HTL)-free CH₃NH₃PbI₃/PC₆₁BM planar heterojunction perovskite solar cells were fabricated with the configuration of ITO/CH₃NH₃PbI₃/PC₆₁BM/Al. The devices present a remarkable power conversion efficiency (PCE) of 11.7% (12.5% best) under AM 1.5G 100 mW cm⁻² illumination. Moreover, the HTL-free perovskite solar cells on flexible PET substrates are first demonstrated, achieving a power conversion efficiency of 9.7%. The element distribution in the HTL-free perovskite solar cell was further investigated. The results indicated that the PbI₂ enriched near the PC₆₁BM side for chlorobenzene treatment via the fast deposition crystallization method. Without using HTL on the ITO, the device is stable with comparison to that with poly(3,4-ethylenedioxylenethiophene): poly(styrene sulfonate) (PEDOT:PSS) as HTL. In addition, the fabricating time of the whole procedure from ITO substrate cleaning to device finishing fabrication only cost about 3 h for our mentioned devices, which is much more rapid than other structure devices containing other transporting layer. The high efficient and stable HTL-free CH₃NH₃PbI₃/PC₆₁BM planar heterojunction perovskite solar cells with the advantage of saving time and cost provide the potential for commercialization printing electronic devices.     

Broadband enhancement of coaxial heterogeneous gallium arsenide single‐nanowire solar cells

Coaxial gallium arsenide single‐nanowire solar cells with multiple electrically and optically functional nanoshells are presented in this paper. Both optical absorption and light‐conversion characteristics are extensively examined by performing a comprehensive device‐oriented simulation. It is found that a window layer with a large semiconductor bandgap is necessary for the nanowire gallium arsenide solar cells, which allow internal quantum efficiency ~100% in ~75% of the absorption band of gallium arsenide. Results also reveal the role of nanofocusing effect in enhancing the performance of nanowire devices that show both absorption and external quantum efficiencies over 100% under resonances. A dielectric cladding shell is introduced and optimized, which enhances the nanofocusing effect and leads to extraordinary enhancement of both absorption and light‐conversion capabilities in a very broad band. This design contributes a short‐circuit current density increased by 2.4 times and an open‐circuit voltage over 1.1 V. Copyright © 2014 John Wiley & Sons, Ltd.     

Upper limits to absorption enhancement in thick solar cells using diffraction gratings

The application of diffraction gratings to solar cells is a promising approach to superseding the light trapping limits of conventional Lambertian structures. In this paper a mathematical formalism is derived for calculating the absorption that can be expected in a solar cell equipped with a diffraction grating, which can be applied to any lattice geometry and grating profile. Furthermore, the formalism is used to calculate the upper limit of total absorption that can theoretically be achieved using a diffraction grating. The derived formalism and limits are valid when the solar cell thickness is greater than the coherence length of the illuminating solar spectrum. Comparison is made to the upper limit achievable using an angularly selective Rugate filter, which is also calculated. Both limits are found to be considerably higher than the Lambertian limit within the range of sunlight concentration factors practically employed in photovoltaic systems (1–1000×). The upper limit of absorption using the diffraction grating is shown to be equal to the thermodynamic limit for all absorbances and concentration factors. The limit for the Rugate filter is generally lower, but tends to the thermodynamic limit for lower cell absorbances. Copyright © 2011 John Wiley & Sons, Ltd.     

基于纳米锥结构的钙钛矿材料吸收特性研究

本文对ZnO结构太阳能电池钙钛矿(CaTiO₃)薄 膜层中嵌入的Ag粒子的等离子体增强进行了理论分析。由于引入了Ag颗粒,归因于Ag颗粒的 强表面等离子体吸收和Ag与CaTiO₃之间的协同作用,太阳能电池的吸收得以增强。在计算 分析中,Ag粒子的最佳半径为80 nm。优化的纳米结构的平均吸收效 率可以达到80％,比纯CaTiO₃薄膜层结构的平均吸收效率高24％。因此认为,本文设计研究的 Ag/CaTiO₃太阳能电池结构可能以低成本和简单的制造工艺成为未来工业制造领域的焦点 。     

Self-assembled monolayers in perovskite solar cells

Perovskite solar cells (PSCs) have attracted great atten-tion due to excellent power conversion efficiency (PCE),low cost and simple solution processing.The certified PCE has reached 25.5％ from the initial efficiency of 3.8％,being com-parable to that of commercial crystalline silicon solar cells[1,2].The efficiency boosting is mainly ascribed to the excellent properties of halide perovskite materials,including suitable bandgaps,high absorption coefficient,long carrier diffusion length and high defect tolerance[3].Moreover,through the composition and interface engineering,the operational sta-bility of PSCs can exceed 1000 h under continuous illumina-tion[4].Therefore,PSCs show a great promise for commerciali-zation.     

Ion migration in perovskite solar cells

The past few years have witnessed a remarkable progress of perovskite solar cells(PSCs),which can be attributed to the high light absorption coefficient,tunable bandgap,long carrier diffusion length,solution processability at low temperature and relatively low cost of perovskite materials.     

Ionic liquids in perovskite solar cells

Metal halide perovskites have gained a lot of attention par-ticularly in recent years due to their excellent optoelectronic properties and simple scalable processability[1-3].One major application of halide perovskites is solar cells,however,des-pite the power conversion efficiencies(PCEs)have already reached around 25％[4,5],the long-term stability issue of such devices still impedes their commercialization.Ionic liquids,which contain a large asymmetric organic cation and an organ-ic or inorganic anion,have recently been applied to per-ovskite solar cells(PSCs),not only increasing the efficiency but also remarkably improving the long-term stability[6,7].     

High conductive PEDOT via post-treatment by halobenzoic for high-efficiency ITO-free and transporting layer-free organic solar cells

The 4-halobenzoics (XBA) including 4-fluorobenzoic acid (FBA), 4-chlorobenzoic acid (CBA), 4-bromobenzoic acid (BBA) and 4-iodobenzoic acid (IBA) have been applied to modify poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) by simply spin-coating solvent treatment. A universal and significant improvement in the conductivity of PEDOT:PSS (from ∼1 S/cm to more than 3300 S/cm) has been achieved by XBA modification, which results from the acid XBA-induced phase segregation, depletion of PSS chains and the conformational change of the conductive PEDOT chains. Especially, the oxidation CBA treated PEDOT:PSS exhibits low sheet resistance of 43 Ω/sq and transparency of over 80% in the visible range, which are comparable to those of indium tin oxide (ITO). The modified PEDOT:PSS has been facilely applied as the transparent anode for high-efficiency ITO-free organic solar cell device without hole transport layer. The high power conversion efficiency of 7.9% has been achieved by employing CBA treated PEDOT:PSS as anode for ITO-free device based on poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] (PTB7-Th):[6,6]-phenyl-C71 butyric acid methyl ester (PC₇₁BM), which is comparable to the corresponding devices based on the traditional ITO anode.     

Absorption enhancement of silicon solar cell with Ag nanoparticles by surface plasmons resonance

The absorption enhancements of silicon layer in silicon solar cells with three kinds of Ag nanoparticles including sphere, cylinder and cuboid are studied by the finite difference time domain （FDTD） method, respectively. The results show that the light absorption of silicon is significantly improved due to the localized surface plasmon （LSP） reso- nance. The relations of the absorption enhancement with the parameters of nanoparticles are thoroughly analyzed. The optimal absorption enhancement can be achieved by adjusting the relevant parameters. Among the three types of Ag nanoparticles, i.e., sphere, cylinder and cuboid, the silicon with the cubical Ag nanopaticles shows the most efficient absorption enhancement at optimal conditions, its maximum absorption enhancement factor is 1.35, and that with the spherical Ag nanopaticles gets the lowest absorption enhancement. The work is useful for the further theoretical study and design for the plasmonic thin-film solar cell.     

N-phenylindole-diketopyrrolopyrrole-containing narrow band-gap materials for dopant-free hole transporting layer of perovskite solar cell

Novel conjugated materials, DPIO and DPIE, having same molecular configuration of both an electron donating N-phenylindole and an electron accepting diketopyrrolopyrrole derivative, exhibited different aggregation behavior because of the applied side chains. When DPIO and DPIE were applied to as hole transporting materials in perovskite solar cell, DPIO showed better device performance than ones with DPIE, mostly due to the aggregation-assisted enhanced electrical property. DPIO effectively extracted hole from the perovskite layer, providing over 10% PCE of cell efficiency without any chemical doping. Incident-photon-to-electron conversion efficiency (IPCE) measurement confirmed that DPIO’s strong absorption in the longer wavelength region partly contributed to the light harvesting of the solar cell device. In addition, time-resolved photoluminescence (TRPL) and transient photovoltage (TPV) studies proved that the DPIO-based device, compared to the conventional Spiro-MeOTAD-based device, has better charge extraction ability and reduced charge recombination.     

Design of Ag nanograting for broadband absorption enhancement in amorphous silicon thin film solar cells

Light trapping is one of the key issues to improve the light absorption and increase the efficiency of thin film solar cells. The effects of the triangular Ag nanograting on the absorption of amorphous silicon solar cells were investigated by a numerical simulation based on the finite element method. The light absorption under different angle and area of the grating has been calculated. Furthermore, the light absorption with different incident angle has been calculated. The optimization results show that the absorption of the solar cell with triangular Ag nanograting structure and anti-reflection film is enhanced up to 96% under AM1.5 illumination in the 300–800 nm wavelength range compared with the reference cell. The physical mechanisms of absorption enhancement in different wavelength range have been discussed. Furthermore, the solar cell with the Ag nanograting is much less sensitive to the angle of incident light. These results are promising for the design of amorphous silicon thin film solar cells with enhanced performance.