Hiroyuki Kanda

100 °C Thermal Stability of Printable Perovskite Solar Cells Using Porous Carbon Counter Electrodes.

Many efforts have been made towards improving perovskite (PVK) solar cell stability, but their thermal stability, particularly at 85u2009°C (IEC 61646 climate chamber tests), remains a challenge. Outdoors, the installed solar cell temperature can reach up to 85u2009°C, especially in desert regions, providing sufficient motivation to study the effect of temperature stress at or above this temperature (e.g., 100u2009°C) to confirm the commercial viability of PVK solar cells for industrial companies. In this work, a three-layer printable HTM-free CH3 NH3 PbI3 PVK solar cell with a mesoporous carbon back contact and UV-curable sealant was fabricated and tested for thermal stability over 1500u2005h at 100u2009°C. Interestingly, the position of the UV-curing glue was found to drastically affect the device stability. The side-sealed cells show high PCE stability and represent a large step toward commercialization of next generation organic-inorganic lead halide PVK solar cells.

Light stability tests of CH3NH3PbI3 perovskite solar cells using porous carbon counter electrodes

The CH3NH3PbI3 perovskite solar cells have been fabricated using three-porous-layered electrodes as, 〈glass/F-doped tin oxide (FTO)/dense TiO2/porous TiO2-perovskite/porous ZrO2-perovskite/porous carbon-perovskite〉 for light stability tests. Without encapsulation in air, the CH3NH3PbI3 perovskite solar cells maintained 80% of photoenergy conversion efficiency from the initial value up to 100 h under light irradiation (AM 1.5, 100 mW cm-2). Considering the color variation of the CH3NH3PbI3 perovskite layer, the significant improvement of light stability is due to the moisture-blocking effect of the porous carbon back electrodes. The strong interaction between carbon and CH3NH3PbI3 perovskite was proposed by the measurements of X-ray photoelectron spectroscopy and X-ray diffraction of the porous carbon-perovskite layers.

Lead-free perovskite solar cells using Sb and Bi-based A 3 B 2 X 9 and A 3 BX 6 crystals with normal and inverse cell structures

Research of CH3NH3PbI3 perovskite solar cells had significant attention as the candidate of new future energy. Due to the toxicity, however, lead (Pb) free photon harvesting layer should be discovered to replace the present CH3NH3PbI3 perovskite. In place of lead, we have tried antimony (Sb) and bismuth (Bi) with organic and metal monovalent cations (CH3NH3+, Ag+ and Cu+). Therefore, in this work, lead-free photo-absorber layers of (CH3NH3)3Bi2I9, (CH3NH3)3Sb2I9, (CH3NH3)3SbBiI9, Ag3BiI6, Ag3BiI3(SCN)3 and Cu3BiI6 were processed by solution deposition way to be solar cells. About the structure of solar cells, we have compared the normal (n-i-p: TiO2-perovskite-spiro OMeTAD) and inverted (p-i-n: NiO-perovskite-PCBM) structures. The normal (n-i-p)-structured solar cells performed better conversion efficiencies, basically. But, these environmental friendly photon absorber layers showed the uneven surface morphology with a particular grow pattern depend on the substrate (TiO2 or NiO). We have considered that the unevenness of surface morphology can deteriorate the photovoltaic performance and can hinder future prospect of these lead-free photon harvesting layers. However, we found new interesting finding about the progress of devices by the interface of NiO/Sb3+ and TiO2/Cu3BiI6, which should be addressed in the future study.

Water Soluble Aluminum Paste Using Polyvinyl Alcohol for Silicon Solar Cells

Screen-printing aluminum is still dominantly used in the solar cell fabrication process. Ethyl cellulose is one of the main contents of screen-printing pastes that require dichloromethane for its cleaning process, a substance renowned for being extremely toxic and threatening to the human body. Developing environmental friendly aluminum pastes is essential in order to provide an alternative to the commercial pastes. In this work, new, nontoxic polyvinyl alcohol-based aluminum pastes are introduced. Polyvinyl alcohol was used as a soluble polymer that can be synthesized without saponification and that is also soluble in water. Three different pastes were developed using different recipes including many aluminum particle sizes varying from 3.0 to 45u2009μm, aluminum oxide with particle sizes between 35 and 50u2009μm, and acetic acid. Evaluation of the pastes was carried out by Scanning Electron Microscope (SEM) image analysis, sheet resistance measurements, and fabricating silicon solar cells using each paste. Solar cells with 15.6% efficiency were fabricated by nonvacuum processing on CZ-Si p-type wafers using developed aluminum pastes on the back side.

All-inorganic inverse perovskite solar cells using zinc oxide nanocolloids on spin coated perovskite layer

We confirmed the influence of ZnO nanoparticle size and residual water on performance of all inorganic perovskite solar cells. By decreasing the size of the ZnO nanoparticles, the short-circuit current density (Jsc) and open circuit photovoltage (Voc) values are increased and the conversion efficiency is improved. Although the Voc value is not affected by the influence of residual water in the solution for preparing the ZnO layer, the Jsc value drops greatly. As a result, it was found that it is important to use the oxide nanoparticles with a small particle diameter and to reduce the water content in the oxide forming material in order to manufacture a highly efficient all inorganic perovskite solar cells.

Perovskite/crystalline silicon tandem solar cells fabricated by non-vacuum-process

Silicon solar cells and perovskite solar cells were fabricated by non-vacuum printed processes in our group. Screen-printing, spin coating and spray pyrolysis deposition techniques were utilized mainly. Low temperature and non-vacuum processed TiO2 layer and Al2O3 layer and Al2O3/TiO2 double layer were introduced as an alternative anti-reflection coating film. In-house developed polyvinyl alcohol based aluminum pastes were also introduced for solar cell applications. p-type crystalline silicon solar cells were fabricated with over 17% conversion efficiency with non-vacuum processes. On the other hand, perovskite solar cells using organic or inorganic hole-transporting materials can be reach the conversion efficiency over 12%. Finally, a two terminal tandem solar cells with structure of <;perovskite top cell/Au/ (w/ or w/o)Ag nano electrode/crystalline silicon bottom cell> were fabricated using these solar cells and Voc of 1.38 V was achieved.

Silica-sol-based spin-coating barrier layer against phosphorous diffusion for crystalline silicon solar cells

The phosphorus barrier layers at the doping procedure of silicon wafers were fabricated using a spin-coating method with a mixture of silica-sol and tetramethylammonium hydroxide, which can be formed at the rear surface prior to the front phosphorus spin-on-demand (SOD) diffusion and directly annealed simultaneously with the front phosphorus layer. The optimization of coating thickness was obtained by changing the applied spin-coating speed; from 2,000 to 8,000 rpm. The CZ-Si p-type silicon solar cells were fabricated with/without using the rear silica-sol layer after taking the sheet resistance measurements, SIMS analysis, and SEM measurements of the silica-sol material evaluations into consideration. For the fabrication of solar cells, a spin-coating phosphorus source was used to form the n+ emitter and was then diffused at 930°C for 35 min. The out-gas diffusion of phosphorus could be completely prevented by spin-coated silica-sol film placed on the rear side of the wafers coated prior to the diffusion process. A roughly 2% improvement in the conversion efficiency was observed when silica-sol was utilized during the phosphorus diffusion step. These results can suggest that the silica-sol material can be an attractive candidate for low-cost and easily applicable spin-coating barrier for any masking purpose involving phosphorus diffusion.

Effect of Silicon Surface for Perovskite/Silicon Tandem Solar Cells: Flat or Textured?

Perovskite and textured silicon solar cells were integrated into a tandem solar cell through a stacking method. To consider the effective structure of silicon solar cells for perovskite/silicon tandem solar cells, the optic and photovoltaic properties of textured and flat silicon surfaces were compared using mechanical-stacking-tandem of two- and four-terminal structures by perovskite layers on crystal silicon wafers. The reflectance of the texture silicon surface in the range of 750-1050 nm could be reduced more than that of the flat silicon surface (from 2.7 to 0.8%), which resulted in increases in average incident photon to current conversion efficiency values (from 83.0 to 88.0%) and current density (from 13.7 to 14.8 mA/cm2). Using the texture surface of silicon heterojunction (SHJ) solar cells, the significant conversion efficiency of 21.4% was achieved by four-terminal device, which was an increase of 2.4% from that of SHJ solar cells alone.

Novel spin coated phosphorus sources for gettering process on crystalline silicon

Three different novel liquid phosphorus dopant sources, based on a combination of phosphoric acid (H3PO4) and phosphorus pentoxide (P2O5), were spin-coated onto p-type crystalline silicon wafers. Heat treatment under an ambient atmosphere has been carried out at 925 oC for 30 min in order allowing P diffusion into the wafers. Photoconductance measurements quantified the improvement in the quality of silicon using either bulk effective minority carrier lifetime (τeff) or implied open circuit voltage (iVoc) parameter; e.g. τeff of single crystalline silicon raised up to around 760 μs where initial value was around 330 μs and iVoc of multi crystalline silicon solar cells was increased from 598 mV up to 625 mV. These primary results on newly introduced spin coated dopant sources come along with effective diffusivity by taking advantage of both H3PO4 and P2O5 and can be a promising low-cost alternative improving wafer level quality for the silicon solar cell technology.

Perovskite/p-type crystal silicon tandem solar cells

In order to combine perovskite solar cell and crystal silicon solar cell, indium tin oxide as conductive layer is utilized in terms of high transparency. However, distortion of I-V curve was observed by spattering conductive layer on organic hole transport material layer (Spiro-OMeTAD). In order to analyze and find out the reason of the distortion of I-V curve, perovskite solar cell was fabricated with conductive layer deposited by spattering on the hole transport material changing spattering time. It was turned out that distortion of I-V curve was increased with increase of spattering time and different diode factor was observed by measuring dark I-V curve, which attribute to the distortion of I-V curve.