CH3NH3PbI3 Perovskite/Fullerene Planar‐Heterojunction Hybrid Solar Cells

We have demonstrated the performance of inverted CH3NH3PbI3 perovskite-based solar cells (SCs) with a room temperature (RT) sputtered ZnO electron transport layer by adding fullerene (C60) interlayer. ZnO exhibits a better matched conduction band level with perovskite and Al work function and around energy offset of 2.2 eV between highest occupied molecular orbital level of CH3NH3PbI3 perovskite and valance band level of ZnO. However, the CH3NH3PbI3 perovskite layer will be damaged during direct RT sputtering deposition of ZnO. Therefore, the C60 interlayer having matched conduction band level with ZnO and CH3NH3PbI3 perovskite added between the CH3NH3PbI3 perovskite and RT sputtered ZnO layers for protection prevents sputtering damages on the CH3NH3PbI3 perovskite layer. The short-circuit current density (JSC, 19.41 mA/cm2) and open circuit voltage (VOC, 0.91 V) of the SCs with glass/ITO/poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS)/perovskite/C60/RT sputtered ZnO/Al structure is higher than the JSC (16.23 mA/cm2) and VOC (0.90 V) of the reference SC with glass/ITO/PEDOT:PSS/perovskite/C60/bathocuproine (BCP)/Al structure. Although the SCs with the former structure has a lower fill factor (FF%) than the SCs with the latter structure, its conversion efficiency η% (10.93%) is higher than that (10.6%) of the latter.

Simulation of perovskite solar cell employing ZnO as electron transport layer (ETL) for improved efficiency

Plasmonic nanoparticles exhibit a great ability to dramatically improve photon harvesting in solar cells. They provide an exceptionally very innovative way of transforming the solar cell and photovoltaic cell industries. In photovoltaic cell research, nano-plasmonics especially noble metal nanoparticles have emerged as a new frontier runner plasmons to be incorporated in photovoltaic structures. These nano plasmons concentrate photons and channel them inside a perovskite layer. However, challenges on its effectiveness have emerged and need to be addressed. These challenges includes the loss in absorption fluxes, increased light trapping band, developing inexpensive fabrication techniques, scaling plasmonics into manufacturing levels and integrating them into perovskite active nanostructures. In this paper, we address the challenge posed by low total absorption fluxes and decreased total enhancement across the whole solar spectrum using silver nanoplasmonics to improve its total absorption fluxes. We therefore document a numerical analysis of total absorption enhancements of a model CH3NH3PbI3 perovskite photocell whose active absorber layer is embedded with spherical plasmonic silver nanoparticles of different diameters at different array spacing. It is still unknown to what extent the organic cation (framework) affects the optical and electronic properties of CH3NH3PbI3 perovskites. Further, there a number of questions that have been raised in relation to the inorganic framework or lattice though computational has suggested that CH3NH3PbI3 is influenced by different geometric parameters related to CH3NH3+ on its nitrogen K-edge (or the nitrogen 1s-orbital electrons). Therefore, new ways in which the organic cation can interact with the inorganic Pb–I lattice is believed to affect its XA spectrum and therefore its electronic structure. It has been suggested that hybridized CH3NH3+ levels in the (lead-and-iodide dominated) valence band appear to dominate in CH3NH3PbI3 crystals and the band gap in CH3NH3PbI3 can change due to the rotation of the CH3NH3+ cation, mostly by affecting the conduction band. The influence of particle size, array spacing and location is analyzed numerically and discussed in a realistic system. Results revealed that when silver nanoparticles are integrated into a CH3NH3PbI3 perovskite layer, total solar absorption enhancement increases by 8% in the infrared (IR) and by 48 % in the far-infrared (far-IR) spectra in layers of 135 nm thicknesses at minimum array spacing and maximum diameter. It was concluded that the total absorption enhancement can be improved by reinforcement from a plasmonic near-field influenced by its total cross sectional scattering effects. In this process absorption in the IR and far IR spectra is influenced in the CH3NH3PbI3 perovskite layer.

Influence of Charge Transport Layers on Capacitance Measured in Halide Perovskite Solar Cells

Heteroleptic Tin-Antimony Sulfoiodide for Stable and Lead-free Solar Cells

In situ NMR Investigation of the Photoresponse of Perovskite Crystal

Morphology control and device optimization for efficient organic solar cells

ABX3 Perovskites for Tandem Solar Cells

Solid additives are crucial in layer‐by‐layer (LBL) polymer solar cells (PSCs). Despite its importance, the simultaneous application of solid additives into both donor and acceptor layers has been largely overlooked. In this work, two multifunctional solid additives are actively designed, and investigated the synergistic effect on both donor and acceptor layers. Incorporating the multifunctional solid additives into the donor layer could effectively enhance the aggregation and molecular stacking of the donor polymer, leading to reduced energy disorder and minimizing ΔE2. When the multifunctional solid additives are introduced into the acceptor layer, they just play a role in optimizing the morphology, thereby reducing the ΔE3. Excitedly, the simultaneous addition of the multifunctional solid additives into both donor and acceptor layers produced a synergistic effect for decreasing ΔE2 and ΔE3 simultaneously, especially adding SA2, thus enabling an excellent power conversion efficiency (PCE) of 19.95% (certified as 19.68%) with an open‐circuit voltage (Voc) of 0.921 V, a short circuit current density (Jsc) of 27.08 mA cm−2 and a fill factor (FF) of 79.98%. The work highlights the potential of multifunctional solid additives in independently regulating the properties of donor and acceptor layers, which is expected as a promising approach for further developing higher performance PSCs.

Plastic/organic /polymer photovoltaic solar cells are fourth generation cells however the efficiency, thermal stability and cost of fourth generation solar cells are still not sufficient to replace conventional solar cells. Hybrid solar cells have been one of the alternate technologies to harness solar power into electrical power to overcome the high cost of conventional solar cells. This review paper has focused on the concept of hybrid solar cells with the combination of organic/polymer materials, blended with inorganic semiconducting materials. The paper presents the importance of nanoscale materials and its shape and size, nanotubes, nanowire, nanocrystal, which can increase the efficiency of the solar cells. The study shows that nanomaterials have immense potential and application of nanomaterials (inorganic/organic/polymer) can improve the performance of photovoltaic solar cells. Tuning of nanomaterials increase the functionality, band gap, optical absorption and shape of the materials, in multiple orders compared to micro scale materials. Hybrid solar cells have unique properties of inorganic semiconductors along with the film forming properties of conjugated polymers. Hybrid materials have great potential because of their unique properties and are showing great results at the preliminary stages of research. The advantage of organic/polymer is easy processing; roll to roll production, lighter weight, flexible shape and size of the solar cells. Application of nanotechnology in hybrid solar cells has opened the door to manufacturing of a new class of high performance devices.

Enhancing the efficiency of low-temperature planar perovskite solar cells by modifying the interface between perovskite and hole transport layer with polymers

Nowadays, silicon solar cells have been the most widespread for commercial use. One of the most promising structure for further improvement of photoelectric conversion efficiency is tandem structure, which connects two types of solar cell. Recently, efficiency of perovskite solar cell (PSCs) has reached over 20%1) in few years, while silicon solar cell has reported as 26.3% for long time2). From fig.1, PSC has a large band gap of 1.5 eV, which makes open circuit voltage (V oc) high. Therefore, it is desirable to apply to a tandem top cell with silicon bottom cell. Fig.2 shows the concept of this tandem solar cell. Ideally, this structure can obtain over 30% efficiency3), but it is required to fabricate thin perovskite film with large grain as an absorber layer. Since a certain range of the wavelength exists in which both perovskite and silicon absorb, we need to balance a photoabsorption between the top and bottom cell by controlling the thickness of perovskite layer for the current matching between each layer. In conventional solar cell, an absorber layer which has a small grain size with many boundary causes fast recombination. Here, the larger grain size makes less grain boundary can reduce this effect. Generally, the growth of larger grain takes a long time, therefore it is needed to prevent following two negative effects: (1) discontinuity due to crystal growth toward 3-dimentional (3D) and (2) the decomposition of the perovskite layer. In most cases, solution process was adopted to fabricate the perovskite layer, which dissolved precursor of lead iodide (PbI2) and methylammonium iodide (MAI) in an organic solvent. However, the crystals from solution process grow in 3D and large grains were achieved, resulting in the low coverage (appearing the discontinuity). Also, film morphology of the perovskite is severely affected by drying process of the solution. Interestingly, another possible approach to make thin and continuous perovskite film is a dry process. After depositing PbI2, the deposited PbI2 reacted with MAI over 150°C by using sublimation. Most of the report, this process conducted at 150°C4). This process will keep PbI2morphology at some extent and 2D crystal growth can be expected. Higher temperature annealing is expected to be more effective to obtain a larger crystal grain, but the CH3NH3PbI3 perovskite is easy to decompose when annealing at high temperature and even MAI decomposition starts at 230-250°C5). Accordingly, there are only few reports conducted at a high temperature in this process. In this study, we investigated high-temperature annealing by two kinds of dry process (1.vapor deposition 2. Solid-state reaction) to obtain large grain toward 2D crystal growth. Furthermore, we discussed the effect of high-temperature annealing at different temperature and MAI concentration. In vapor deposition process, PbI2 layer was deposited on the planar TiO2 layer, and on the other side, MAI was deposited by spin coating MAI solution on another glass. Then, both substrates put face to face sandwiching with thermostable tape as a spacer. When the temperature increased from 150 to 180°C, the grain size of the perovskite was much larger than that of PbI2. However, the maximum grain size was limited to 1μm. Photoabsorbance of perovskite increased with increase annealing temperature, but some PbI2 was still detected in this temperature, even though the reaction time extended to about 2h. It indicated that the extent of reaction is still low at this temperature. When temperature reached to 200°C, reaction of PbI2 completed, but the perovskite layer detached from the substrate. It is supposed that excess MAI was segregated after completely reacted with PbI2. In the solid state reaction, MAI powder was put on the deposited PbI2 and directly annealed at 250°C. PbI2 was completely reacted and grain size was larger, but it was still limited to 1μm. Some pores were observed in the interface between TiO2 and the perovskite. It is supposed that maximum grain size of the perovskite might be limited to the grain structure of PbI2. Thus, it is important to obtain the suitable morphology of PbI2 for further improvement of the perovskite grain size. [References] 1) Yang. W et al., science, 2015, 348, 1234-1237 2) M. Green et al., Prog. Photovolt: Res. Appl. 2017, 2016, 25, 3-13 3) P. Löper et al., Phys. Chem. Chem. Phys., 2015, 17, 1619-1629 4) Liu. M et al., Nature, 2013, 501, 395-398 5) Emilio. J et al., Energy Environ. Sci., 2016, 9, 3406-3410 Figure 1

The aim of this article is to investigate the origin of the open circuit voltage (Voc) in organic heterojunction solar cells. The studied devices consist of buckminsterfullerene C60 as acceptor material and an oligophenyl-derivative 4,4′-bis-(N,N-diphenylamino)quaterphenyl (4P-TPD) as donor material. These photoactive materials are sandwiched between indium tin oxide and p-doped hole transport layers. Using two different p-doped hole transport layers, the built-in voltage of the solar cells is independently changed from the metal contacts. The influence of the built-in voltage on the Voc is investigated in bulk and planar heterojunctions. In bulk heterojunctions, in which doped transport layers border directly on the photoactive blend layer, Voc cannot exceed the built-in voltage significantly. Though, in planar heterojunctions, Voc is identical with the splitting of quasi-Fermi levels at the donor-acceptor interface and is thus primarily determined by the difference of the lowest unoccupied molecular orbital of C60 and the highest occupied molecular orbital of 4P-TPD. In planar heterojunctions, the open circuit voltage can exceed the built-in voltage. Furthermore, the investigations show that the efficiency of organic solar cells can be improved by using p-doped charge transport layers with optimized energy level alignment to the active materials. The optimized planar heterojunction shows a fill factor of up to 65.5% and a Voc of 0.95 V. For solar cells with insufficient energy level alignment between the photoactive layer system and the hole transport layer, a reduced Voc in bulk heterojunction cells and a characteristic S shape of the I-V characteristics in planar heterojunction cells are observed.

Efficiency enhancement of Si nanostructure hybrid solar cells by optimizing non-radiative energy transfer from Si quantum dots

Using X-ray diffraction (XRD), it was confirmed that the deposition of hole-transporting materials (HTM) on a CH3NH3PbI3 perovskite layer changed the CH3NH3PbI3 perovskite crystal, which was due to the material exchanging phenomena between the CH3NH3PbI3 perovskite and HTM layers. The solvent for HTM also changed the perovskite crystal. In order to suppress the crystal change, doping by chloride ion, bromide ion and 5-aminovaleric acid was attempted. However, the doping was unable to stabilize the perovskite crystal against HTM deposition. It can be concluded that the CH3NH3PbI3 perovskite crystal is too soft and flexible to stabilize against HTM deposition.