I am a Doctor of Engineering candidate at National Univerisity of Singapore research in Solar Cells
Mathematical Modelling and Analysis of Perovskite Solar Cells
A perovskite solar cell is a type of solar cell which consists of perovskite structured compound as its light harvesting active layer. The most commonly used perovskite compound is a hybrid organic-inorganic lead. This type of material and its fabrication process are both at a much lower cost than the currently commercialized silicon solar cells in the market. In addition, it can absorb the complete visible solar spectrum around 500nm due to its high absorption coefficient. Therefore, it is possible to create low cost, high efficiency, thin, lightweight and flexible perovskite solar cells modules for commercialization.
Recently, the pursuit of organic-inorganic perovskite solar cells has been significantly advanced because of the promising high-efficiency potential of the perovskite materials. Based on the structure of dye sensitized solar cells, Miyasaka et al. firstly incorporated a perovskite material into a solar cell, thereby achieving 3.8% conversion efficiency. Afterward, many groups contributed to the development of perovskite solar cells and as a result, its efficiency increased quickly to 22.1% in this year. In addition to the advancement in experimental studies, the modelling and numerical simulation of perovskite solar cells also grew recently, as it can aid in elucidating the device physics, unveiling its intrinsic material properties and predicting the device performance.
Our group develops in-depth mathematical device model for perovskite solar cells. This mathematical model is able to capture the physical phenomena of all four standard types of perovskite solar cells, namely mesoporous configuration, extreme thin absorber configuration, planar n-i-p configuration and planar p-i-n configuration. After calibrating and validating the resulting model with in-house fabricated meso-structured solar cells, we perform a subsequent loss analysis for meso-structured perovskite solar cells. This can provide insights for further improving the solar cell device architecture of meso-structured perovskite solar cells and their efficiency. Our implemented device model is generic enough to compare the different physical phenomena occurring in perovskite solar cells processed in a meso-structured configuration compared to those processed in a planar configuration.
Abstract: Planar perovskite solar cells made entirely via solution-processing at low temperatures (<150°C) offer promise for simple manufacturing, compatibility with flexible substrates, and perovskite-based tandem devices; however, they require an electron-selective layer that performs well with similar processing. We report a contact passivation strategy using chlorine-capped TiO2 colloidal nanocrystal (NC) film that mitigates interfacial recombination and improves interface binding in low-temperature planar solar cells. We fabricated solar cells with certified efficiencies of 20.1% and 19.5% for active areas of 0.049 and 1.1 square centimeters, respectively, achieved via low-temperature solution processing. Solar cells with efficiency >20% retained 90% (97% after dark recovery) of their initial performance after 500 hours continuous room-temperature operation at their maximum power point under one-sun illumination.
Pub.: 06 Feb '17, Pinned: 28 Jul '17
Abstract: We demonstrate four- and two-terminal perovskite-perovskite tandem solar cells with ideally matched band gaps. We develop an infrared-absorbing 1.2-electron volt band-gap perovskite, FA0.75Cs0.25Sn0.5Pb0.5I3, that can deliver 14.8% efficiency. By combining this material with a wider-band gap FA0.83Cs0.17Pb(I0.5Br0.5)3 material, we achieve monolithic two-terminal tandem efficiencies of 17.0% with >1.65-volt open-circuit voltage. We also make mechanically stacked four-terminal tandem cells and obtain 20.3% efficiency. Notably, we find that our infrared-absorbing perovskite cells exhibit excellent thermal and atmospheric stability, not previously achieved for Sn-based perovskites. This device architecture and materials set will enable "all-perovskite" thin-film solar cells to reach the highest efficiencies in the long term at the lowest costs.
Pub.: 20 Nov '16, Pinned: 17 Aug '17
Abstract: The formation of a dense and uniform thin layer on the substrates is crucial for the fabrication of high-performance perovskite solar cells (PSCs) containing formamidinium with multiple cations and mixed halide anions. The concentration of defect states, which reduce a cell's performance by decreasing the open-circuit voltage and short-circuit current density, needs to be as low as possible. We show that the introduction of additional iodide ions into the organic cation solution, which are used to form the perovskite layers through an intramolecular exchanging process, decreases the concentration of deep-level defects. The defect-engineered thin perovskite layers enable the fabrication of PSCs with a certified power conversion efficiency of 22.1% in small cells and 19.7% in 1-square-centimeter cells.
Pub.: 01 Jul '17, Pinned: 17 Aug '17
Abstract: Of the many materials and methodologies aimed at producing low-cost, efficient photovoltaic cells, inorganic-organic lead halide perovskite materials appear particularly promising for next-generation solar devices owing to their high power conversion efficiency. The highest efficiencies reported for perovskite solar cells so far have been obtained mainly with methylammonium lead halide materials. Here we combine the promising-owing to its comparatively narrow bandgap-but relatively unstable formamidinium lead iodide (FAPbI3) with methylammonium lead bromide (MAPbBr3) as the light-harvesting unit in a bilayer solar-cell architecture. We investigated phase stability, morphology of the perovskite layer, hysteresis in current-voltage characteristics, and overall performance as a function of chemical composition. Our results show that incorporation of MAPbBr3 into FAPbI3 stabilizes the perovskite phase of FAPbI3 and improves the power conversion efficiency of the solar cell to more than 18 per cent under a standard illumination of 100 milliwatts per square centimetre. These findings further emphasize the versatility and performance potential of inorganic-organic lead halide perovskite materials for photovoltaic applications.
Pub.: 07 Jan '15, Pinned: 28 Jul '17
Abstract: Three-dimensional organic-inorganic perovskites have emerged as one of the most promising thin-film solar cell materials owing to their remarkable photophysical properties, which have led to power conversion efficiencies exceeding 20 per cent, with the prospect of further improvements towards the Shockley-Queisser limit for a single-junction solar cell (33.5 per cent). Besides efficiency, another critical factor for photovoltaics and other optoelectronic applications is environmental stability and photostability under operating conditions. In contrast to their three-dimensional counterparts, Ruddlesden-Popper phases-layered two-dimensional perovskite films-have shown promising stability, but poor efficiency at only 4.73 per cent. This relatively poor efficiency is attributed to the inhibition of out-of-plane charge transport by the organic cations, which act like insulating spacing layers between the conducting inorganic slabs. Here we overcome this issue in layered perovskites by producing thin films of near-single-crystalline quality, in which the crystallographic planes of the inorganic perovskite component have a strongly preferential out-of-plane alignment with respect to the contacts in planar solar cells to facilitate efficient charge transport. We report a photovoltaic efficiency of 12.52 per cent with no hysteresis, and the devices exhibit greatly improved stability in comparison to their three-dimensional counterparts when subjected to light, humidity and heat stress tests. Unencapsulated two-dimensional perovskite devices retain over 60 per cent of their efficiency for over 2,250 hours under constant, standard (AM1.5G) illumination, and exhibit greater tolerance to 65 per cent relative humidity than do three-dimensional equivalents. When the devices are encapsulated, the layered devices do not show any degradation under constant AM1.5G illumination or humidity. We anticipate that these results will lead to the growth of single-crystalline, solution-processed, layered, hybrid, perovskite thin films, which are essential for high-performance opto-electronic devices with technologically relevant long-term stability.
Pub.: 08 Jul '16, Pinned: 28 Jul '17
Abstract: Organic-inorganic perovskites have shown promise as high-performance absorbers in solar cells, first as a coating on a mesoporous metal oxide scaffold and more recently as a solid layer in planar heterojunction architectures. Here, we report transient absorption and photoluminescence-quenching measurements to determine the electron-hole diffusion lengths, diffusion constants, and lifetimes in mixed halide (CH3NH3PbI(3-x)Cl(x)) and triiodide (CH3NH3PbI3) perovskite absorbers. We found that the diffusion lengths are greater than 1 micrometer in the mixed halide perovskite, which is an order of magnitude greater than the absorption depth. In contrast, the triiodide absorber has electron-hole diffusion lengths of ~100 nanometers. These results justify the high efficiency of planar heterojunction perovskite solar cells and identify a critical parameter to optimize for future perovskite absorber development.
Pub.: 19 Oct '13, Pinned: 28 Jul '17
Abstract: The energy costs associated with separating tightly bound excitons (photoinduced electron-hole pairs) and extracting free charges from highly disordered low-mobility networks represent fundamental losses for many low-cost photovoltaic technologies. We report a low-cost, solution-processable solar cell, based on a highly crystalline perovskite absorber with intense visible to near-infrared absorptivity, that has a power conversion efficiency of 10.9% in a single-junction device under simulated full sunlight. This "meso-superstructured solar cell" exhibits exceptionally few fundamental energy losses; it can generate open-circuit photovoltages of more than 1.1 volts, despite the relatively narrow absorber band gap of 1.55 electron volts. The functionality arises from the use of mesoporous alumina as an inert scaffold that structures the absorber and forces electrons to reside in and be transported through the perovskite.
Pub.: 09 Oct '12, Pinned: 28 Jul '17