In-Hyuk Jang

Growth of CH3NH3PbI3 cuboids with controlled size for high-efficiency perovskite solar cells

Perovskite solar cells with submicrometre-thick CH(3)NH(3)PbI(3) or CH(3)NH(3)PbI(3-x)Cl(x) active layers show a power conversion efficiency as high as 15%. However, compared to the best-performing device, the average efficiency was as low as 12%, with a large standard deviation (s.d.). Here, we report perovskite solar cells with an average efficiency exceeding 16% and best efficiency of 17%. This was enabled by the growth of CH(3)NH(3)PbI(3) cuboids with a controlled size via a two-step spin-coating procedure. Spin-coating of a solution of CH(3)NH(3)I with different concentrations follows the spin-coating of PbI(2), and the cuboid size of CH(3)NH(3)PbI(3) is found to strongly depend on the concentration of CH(3)NH(3)I. Light-harvesting efficiency and charge-carrier extraction are significantly affected by the cuboid size. Under simulated one-sun illumination, average efficiencies of 16.4% (s.d. ± 0.35), 16.3% (s.d. ± 0.44) and 13.5% (s.d. ± 0.34) are obtained from solutions of CH(3)NH(3)I with concentrations of 0.038 M, 0.050 M and 0.063 M, respectively. By controlling the size of the cuboids of CH(3)NH(3)PbI(3) during their growth, we achieved the best efficiency of 17.01% with a photocurrent density of 21.64 mA cm(-2), open-circuit photovoltage of 1.056 V and fill factor of 0.741.

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

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) cm(2)/(V s) and slow recombination rate. Average power conversion efficiency (PCE) of 18.3% was achieved from 41 cells and the best PCE of 19.7% was attained via adduct approach.

Control of I–V Hysteresis in CH3NH3PbI3 Perovskite Solar Cell

UNLABELLED Mismatch of current (I)-voltage (V) curves with respect to the scan direction, so-called I-V hysteresis, raises critical issue in MAPbI3 (MA = CH3NH3) perovskite solar cell. Although ferroelectric and ion migration have been proposed as a basis for the hysteresis, origin of hysteresis has not been apparently unraveled. We report here on the origin of I-V hysteresis of perovskite solar cell that was systematically evaluated by the interface-dependent electrode polarizations. Frequency (f)-dependent capacitance (C) revealed that the normal planar structure with the TiO2/MAPbI3/spiro-MeOTAD configuration showed most significant I-V hysteresis along with highest capacitance (10(-2) F/cm(2)) among the studied cell configurations. Substantial reduction in capacitance to 10(-3) F/cm(2) was observed upon replacing TiO2 with PCBM, indicative of the TiO2 layer being mainly responsible for the hysteresis. The capacitance was intensively reduced to 10(-5) F/cm(2) and C-f feature shifted to higher frequency for the hysteresis-free planar structures with combination of PEDOT PSS, NiO, and PCBM, which underlines the spiro-MeOTAD in part contributes to the hysteresis. This work is expected to provide a key to the solution of the problem on I-V hysteresis in perovskite solar cell.

Pseudo first-order adsorption kinetics of N719 dye on TiO2 surface.

We have investigated the adsorption kinetics of Ru-based N719 dye on TiO(2) surface in dye-sensitized solar cell using 0.5 mM and 5 mM dye solutions. The amount of adsorbed dye on TiO(2) surface of ca. 5 μm-thick film was measured as a function of immersion (adsorption) time. The amount of adsorbed dye increases with increasing the adsorption time and keeps constant after saturation. Completion of dye adsorption is found to be more than 5 times faster in 5 mM than in 0.5 mM. Since the change of dye concentration is negligible compared to that of number of TiO(2) adsorption site, reaction order and rate constant can be estimated from a pseudo reaction. Among the zeroth-, first-, and second-order simulation, the observed data follow first order reaction for both 0.5 mM and 5 mM cases. The rate constant is estimated to be 0.504 min(-1) for 5 mM and 0.094 min(-1) for 0.5 mM, which indicates that completion of dye adsorption is about 5 times shorter in 5 mM than in 0.5 mM. This is consistent with the observed adsorption time difference. Except for the difference in adsorption kinetics, best cell efficiency is similar regardless of dye solution concentration.

Hierarchical SnO2 Nanoparticle-ZnO Nanorod Photoanode for Improving Transport and Life Time of Photoinjected Electrons in Dye-Sensitized Solar Cell

A hierarchical photoanode comprising a SnO(2) nanoparticle underlayer and a ZnO nanorod overlayer was prepared and its photovoltaic performance was compared to photoanodes consisting of SnO(2) nanoparticle only and ZnO nanorod only. The photoanode layer thickness was adjusted to about 7.6 μm to eliminate thickness effect. Ruthenium complex, coded N719, was used as a sensitizer. The photoanode composed of ZnO nanorod only showed a power conversion efficiency (PCE) as low as 0.54% with a short-circuit photocurrent density (J(SC)) of 2.04 mA/cm(2) and an open-circuit voltage (V(OC)) of 500 mV. The photoanode with SnO(2) nanoparticle only exhibited higher PCE (1.24%) because of higher J(SC) (6.64 mA/cm(2)), whereas V(OC) (340 mV) was lower than ZnO nanorod. Compared to SnO(2) nanoparticle and ZnO nanorod films, the bilayer structured film demonstrated much higher PCE (2.62%) because of both higher J(SC) (7.35 mA/cm(2)) and V(OC) (660 mV). Introduction of ZnO nanorod on the SnO(2) nanoparticle layer improved significantly electron transport and lifetime compared to the SnO(2) only film. One Order of magnitude slower charge recombination rate for the bilayer film than for the SnO(2) film was mainly responsible for the improved efficiency.

Highly efficient and recyclable nanocomplexed photocatalysts of AgBr/N-doped and amine-functionalized reduced graphene oxide.

Although silver bromide has recently drawn considerable attention because of its high photocatalytic activity, it tends to form agglomerated metallic silver under the irradiation of visible light. Therefore, photocatalytic activity decreases with time and cannot be applied for repeated uses. To overcome this limitation, in the present work, we complexed AgBr with nitrogen doped (N-doped) and amine functionalized reduced graphene oxide (GN). N-doped and/or amine functionalized graphene shows intrinsically good catalytic activity. Besides, amine groups can undergo complexation with silver ions to suppress its reduction to metallic Ag. As a result, these complexed catalysts show excellent photocatalytic activity for the degradation of methylene blue (MB) dye under the irradiation of visible light. Photocatalytic degradation of MB shows that the catalytic activity is optimized at a condition of 0.5 wt % GN, under which ∼99% of MB was degraded only after 50 min of visible light irradiation. Notably, the complexed catalyst is quite stable and retained almost all of its catalytic activity even after greater than 10 repeated cycles. Moreover, the catalyst can also efficiently decompose 2-chlorophenol, a colorless organic contaminant, under visible light exposure. Detailed experimental investigation reveals that hydroxyl (·OH) radicals play an important role for dye degradation reactions. A relevant mechanism for dye degradation has also been proposed.

Dependence of hysteresis on the perovskite film thickness: inverse behavior between TiO2 and PCBM in a normal planar structure

The effect of perovskite film thickness on the current density (J)–voltage (V) hysteresis is investigated with a normal planar perovskite solar cell (PSC) having the FTO/ETL/MAPbI3/spiro-MeOTAD/Au structure (ETL = electron transporting layer, MA = methylammonium, and spiro-MeOTAD = 2,2′,7,7′-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9′-spirobifluorene). A compact TiO2 (c-TiO2) layer is used as an ETL, which is compared with a PCBM ETL and a c-TiO2/PCBM bilayered ETL. The MAPbI3 layer thickness is varied from 100 nm to 800 nm by controlling the precursor solution concentration. The hysteresis increases with perovskite layer thickness for the c-TiO2 layer, while the hysteresis decreases with increasing the perovskite layer thickness in the presence of PCBM. Deep trap states are much more reduced upon inserting PCBM compared with those of the c-TiO2 case, indicative of fewer traps for non-radiative recombination. The ideality factor obtained from the light-dependent open-circuit voltage (Voc) increases for the c-TiO2 layer but decreases for both the c-TiO2/PCBM bilayer and the PCBM layer as the perovskite layer thickness increases, which again supports that the dependence of hysteresis on the perovskite thickness is related to trap states, that is, decreases in deep trap states can reduce hysteresis. From the capacitance–frequency studies, it is found that the low-frequency capacitance correlates with the observed hysteresis, where it increases for the c-TiO2 layer but decreases for the PCBM containing ETLs with perovskite thickness. This work provides important insight into the hysteresis behavior of PSCs, where the hysteresis depends not only on the nature of the ETL but also on the degree of recombination in the bulk perovskite.