Hye Ji Han

Bio: Hye Ji Han is an academic researcher from Kyung Hee University. The author has contributed to research in topics: Perovskite (structure) & Energy conversion efficiency. The author has an hindex of 11, co-authored 13 publications receiving 1768 citations.

Hysteresis-less inverted CH3NH3PbI3 planar perovskite hybrid solar cells with 18.1% power conversion efficiency

Abstract: Hysteresis-less inverted ITO/PEDOT:PSS/CH3NH3PbI3 (MAPbI3)/PCBM/Au planar hybrid solar cells with 18.1% average power conversion efficiency irrespective of the scan rate were fabricated by depositing dense pinhole-free MAPbI3 perovskite on a PEDOT:PSS/ITO substrate via a single-step spin-coating of solubility controlled MAPbI3 solution. The conductivities of PEDOT:PSS, PCBM, poly(triaryl amine) (PTAA):tert-butylpyrridne (tBP) + Li-bis(trifluoromethanesulfonyl)imide (Li–TFSI), MAPbI3, and TiO2 were 0.014, 0.016, 0.034, 0.015, and 0.00006 mS cm−1, respectively. The average PL lifetimes (τav) of the inverted and normal cell were 1.277 and 1.94 ns, respectively. The diffusion coefficient (Dn) and charge carrier lifetime (τn) for the inverted MAPbI3 planar hybrid solar cells were increased by 1.14-fold and 1.1-fold, respectively, compared with the conventional FTO/TiO2/MAPbI3/PTAA:tBP + Li–TFSI/Au planar hybrid cells. Hence, the inverted MAPbI3 planar hybrid solar cells exhibited better power conversion efficiency and stability than the conventional MAPbI3 cells because (i) the electron extraction from MAPbI3 to the electron conductor was improved because the electron conductivity of PCBM is higher than that of TiO2; (ii) the EQE value was increased by the better charge injection/separation efficiency between MAPbI3 and PCBM, and by the higher charge collection efficiency than the conventional cell; (iii) the fill factor is improved by the increased Dn and τn; and (iv) the air and humidity stability is improved by the absence of corrosive additives in the device architecture and the hydrophobicity of the PCBM top layer. The reduced current density–voltage (J–V) hysteresis with respect to the scan rate and scan direction in the inverted planar hybrid solar cells could be attributed to a more balanced electron flux (Je) and hole flux (Jh), and a reduced number of surface traps.

Planar CH3NH3PbI3 Perovskite Solar Cells with Constant 17.2% Average Power Conversion Efficiency Irrespective of the Scan Rate

Abstract: J. H. Heo, D. H. Song, H. J. Han, Prof. S. H. Im Functional Crystallization Center (FCC) Department of Chemical Engineering Kyung Hee University 1732 Deogyeong-daero , Giheung-gu, Yongin-si , Gyeonggi-do 446-701 , Republic of Korea E-mail: imromy@khu.ac.kr S. Y. Kim, Prof. J. H. Kim Department of Physics Incheon National University 119 Academy-ro , Yeonsu-gu , Incheon 406-772 , Republic of Korea D. Kim, Dr. H. W. Shin, Prof. T. K. Ahn Department of Energy Science Sungkyunkwan University Seobu-ro 2066 , Jangan-gu , Suwon 440-746 , Republic of Korea C. Wolf, Prof. T.-W. Lee Department of Materials Science and Engineering Pohang University of Science and Technology (POSTECH) 77 Cheongam-Ro , Nam-Gu, Pohang , Gyungbuk 790-784, Republic of Korea

Highly efficient low temperature solution processable planar type CH3NH3PbI3 perovskite flexible solar cells

Abstract: The current density–voltage (J–V) hysteresis and power conversion efficiency (η) of planar type CH3NH3PbI3 perovskite solar cells with TiO2 and ZnO electron conductors, which are formed by high temperature spray pyrolysis deposition at 450 °C and by room temperature spin-coating and subsequent heat-treatment at 150 °C, respectively, were compared. The ZnO based perovskite solar cells exhibited better efficiency deviation (15.96 ± 1.07%) and less J–V hysteresis than the TiO2 based cells (15.20 ± 1.23%) because the ZnO based cell has 1.2 fold longer charge carrier life time (τn) than the ZnO based cell and the ZnO electron conductor has better electron conductivity (0.0031 mS cm−1) than the TiO2 electron conductor (0.00006 mS cm−1), thereby balancing the electron flux and the hole flux more. Due to the low temperature solution processability of the ZnO electron conductor, we could demonstrate a highly efficient PEN (poly-ethylenenaphthalate)/ITO/ZnO/CH3NH3PbI3 perovskite/PTAA/Au flexible planar solar cell with 1.1 V open-circuit voltage (Voc), 18.7 short-circuit current density (mA cm−2) Jsc, 75% fill factor (FF), and 15.4% η for the forward scan direction and 1.1 V Voc, 18.7 mA cm−2Jsc, 76% FF and 15.6% η for the reverse scan direction under illumination of 1 Sun.

Stable semi-transparent CH3NH3PbI3 planar sandwich solar cells

Abstract: Semi-transparent CH3NH3PbI3 (MAPbI3) planar sandwich solar cells could be fabricated by simply laminating an FTO (F doped tin oxide)/TiO2/MAPbI3/wet hole transporting material (HTM) with additives and PEDOT:PSS (poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid))/indium tin oxide (ITO). The best FTO/TiO2/MAPbI3/P3HT with additives/PEDOT:PSS/ITO planar sandwich structured solar cells exhibited a 12.8% (deviation: 11.7% ± 0.74%) average power conversion efficiency (ηavg) but poor visible transmittance due to strong absorption by P3HT. Meanwhile, the semi-transparent FTO/TiO2/MAPbI3/PTAA with additives/PEDOT:PSS/ITO planar sandwich solar cells exhibited a 15.8% (deviation: 14.45% ± 0.76%) ηavg without significant J–V hysteresis with respect to the forward and reverse scan directions. The average visible transmittance (AVT) was controlled from 17.3% to 6.3% and the corresponding ηavg changed from 12.55% to 15.8%. The unsealed sandwich planar perovskite solar cells exhibited great air and humidity stability over 20 days due to the self-passivated device architecture of the sandwich type device.

Efficiency enhancement of semi-transparent sandwich type CH3NH3PbI3 perovskite solar cells with island morphology perovskite film by introduction of polystyrene passivation layer

Abstract: Highly semi-transparent sandwich type CH3NH3PbI3 (MAPbI3) island perovskite solar cells with high efficiency were constructed by introduction of a polystyrene (PS) passivation interlayer. The PS insulator can prevent direct contact between the TiO2 electron conductor and the polytriarylamine hole conductor. The PS passivation interlayer could be selectively deposited on bare TiO2 by a consecutive spin-coating and spin-washing process. The average visible transmittance of semi-transparent sandwich type MAPbI3 island perovskite solar cells with and without PS passivation layer was ∼20.9% and ∼18.6%, respectively; its power conversion efficiency was 10.2% for forward scan condition and 10.6% for reverse scan condition (average = 6.17% ± 2.32% for 40 samples) and 5.9% for forward scan condition and 6.6% for reverse scan condition (average = 2.93% ± 1.57% for 40 samples); and its efficiency degradation for 30 days was ∼5% and ∼9%, respectively.

Highly Reproducible Perovskite Solar Cells with Average Efficiency of 18.3% and Best Efficiency of 19.7% Fabricated via Lewis Base Adduct of Lead(II) Iodide

Abstract: High efficiency perovskite solar cells were fabricated reproducibly via Lewis base adduct of lead(II) iodide. PbI2 was dissolved in N,N-dimethyformamide with equimolar N,N-dimethyl sulfoxide (DMSO) and CH3NH3I. Stretching vibration of S═O appeared at 1045 cm–1 for bare DMSO, which was shifted to 1020 and 1015 cm–1 upon reacting DMSO with PbI2 and PbI2 + CH3NH3I, respectively, indicative of forming the adduct of PbI2·DMSO and CH3NH3I·PbI2·DMSO due to interaction between Lewis base DMSO and/or iodide (I–) and Lewis acid PbI2. Spin-coating of a DMF solution containing PbI2, CH3NH3I, and DMSO (1:1:1 mol %) formed a transparent adduct film, which was converted to a dark brown film upon heating at low temperature of 65 °C for 1 min due to removal of the volatile DMSO from the adduct. The adduct-induced CH3NH3PbI3 exhibited high charge extraction characteristics with hole mobility as high as 3.9 × 10–3 cm2/(V s) and slow recombination rate. Average power conversion efficiency (PCE) of 18.3% was achieved from 41 ...

Efficient and stable large-area perovskite solar cells with inorganic charge extraction layers

Abstract: The recent dramatic rise in power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) has triggered intense research worldwide. However, high PCE values have often been reached with poor stability at an illuminated area of typically less than 0.1 square centimeter. We used heavily doped inorganic charge extraction layers in planar PSCs to achieve very rapid carrier extraction, even with 10- to 20-nanometer-thick layers, avoiding pinholes and eliminating local structural defects over large areas. The robust inorganic nature of the layers allowed for the fabrication of PSCs with an aperture area >1 square centimeter that have a PCE >15%, as certified by an accredited photovoltaic calibration laboratory. Hysteresis in the current-voltage characteristics was eliminated; the PSCs were stable, with >90% of the initial PCE remaining after 1000 hours of light soaking.

Halide Perovskite Photovoltaics: Background, Status, and Future Prospects

Abstract: The photovoltaics of organic–inorganic lead halide perovskite materials have shown rapid improvements in solar cell performance, surpassing the top efficiency of semiconductor compounds such as CdTe and CIGS (copper indium gallium selenide) used in solar cells in just about a decade. Perovskite preparation via simple and inexpensive solution processes demonstrates the immense potential of this thin-film solar cell technology to become a low-cost alternative to the presently commercially available photovoltaic technologies. Significant developments in almost all aspects of perovskite solar cells and discoveries of some fascinating properties of such hybrid perovskites have been made recently. This Review describes the fundamentals, recent research progress, present status, and our views on future prospects of perovskite-based photovoltaics, with discussions focused on strategies to improve both intrinsic and extrinsic (environmental) stabilities of high-efficiency devices. Strategies and challenges regardi...

Stabilizing Perovskite Structures by Tuning Tolerance Factor: Formation of Formamidinium and Cesium Lead Iodide Solid-State Alloys

Abstract: Goldschmidt tolerance factor (t) is an empirical index for predicting stable crystal structures of perovskite materials. A t value between 0.8 and 1.0 is favorable for cubic perovskite structure, and larger (>1) or smaller (<0.8) values of tolerance factor usually result in nonperovskite structures. CH(NH2)2PbI3 (FAPbI3) can exist in the perovskite α-phase (black phase) with good photovoltaic properties. However, it has a large tolerance factor and is more stable in the hexagonal δH-phase (yellow phase), with δH-to-α phase-transition temperature higher than room temperature. On the other hand, CsPbI3 is stabilized to an orthorhombic structure (δO-phase) at room temperature due to its small tolerance factor. We find that, by alloying FAPbI3 with CsPbI3, the effective tolerance factor can be tuned, and the stability of the photoactive α-phase of the mixed solid-state perovskite alloys FA1–xCsxPbI3 is enhanced, which is in agreement with our first-principles calculations. Thin films of the FA0.85Cs0.15PbI3 p...

Inorganic caesium lead iodide perovskite solar cells

Abstract: The vast majority of perovskite solar cell research has focused on organic–inorganic lead trihalide perovskites. Herein, we present working inorganic CsPbI3 perovskite solar cells for the first time. CsPbI3 normally resides in a yellow non-perovskite phase at room temperature, but by careful processing control and development of a low-temperature phase transition route we have stabilised the material in the black perovskite phase at room temperature. As such, we have fabricated solar cell devices in a variety of architectures, with current–voltage curve measured efficiency up to 2.9% for a planar heterojunction architecture, and stabilised power conversion efficiency of 1.7%. The well-functioning planar junction devices demonstrate long-range electron and hole transport in this material. Importantly, this work identifies that the organic cation is not essential, but simply a convenience for forming lead triiodide perovskites with good photovoltaic properties. We additionally observe significant rate-dependent current–voltage hysteresis in CsPbI3 devices, despite the absence of the organic polar molecule previously thought to be a candidate for inducing hysteresis via ferroelectric polarisation. Due to its space group, CsPbI3 cannot be a ferroelectric material, and thus we can conclude that ferroelectricity is not required to explain current–voltage hysteresis in perovskite solar cells. Our report of working inorganic perovskite solar cells paves the way for further developments likely to lead to much more thermally stable perovskite solar cells and other optoelectronic devices.