Hui-Seon Kim

Lead iodide perovskite sensitized all-solid-state submicron thin film mesoscopic solar cell with efficiency exceeding 9%.

We report on solid-state mesoscopic heterojunction solar cells employing nanoparticles (NPs) of methyl ammonium lead iodide (CH3NH3)PbI3 as light harvesters. The perovskite NPs were produced by reaction of methylammonium iodide with PbI2 and deposited onto a submicron-thick mesoscopic TiO2 film, whose pores were infiltrated with the hole-conductor spiro-MeOTAD. Illumination with standard AM-1.5 sunlight generated large photocurrents (JSC) exceeding 17 mA/cm2, an open circuit photovoltage (VOC) of 0.888 V and a fill factor (FF) of 0.62 yielding a power conversion efficiency (PCE) of 9.7%, the highest reported to date for such cells. Femto second laser studies combined with photo-induced absorption measurements showed charge separation to proceed via hole injection from the excited (CH3NH3)PbI3 NPs into the spiro-MeOTAD followed by electron transfer to the mesoscopic TiO2 film. The use of a solid hole conductor dramatically improved the device stability compared to (CH3NH3)PbI3 -sensitized liquid junction cells.

High Efficiency Solid-State Sensitized Solar Cell-Based on Submicrometer Rutile TiO2 Nanorod and CH3NH3PbI3 Perovskite Sensitizer

We report a highly efficient solar cell based on a submicrometer (~0.6 μm) rutile TiO2 nanorod sensitized with CH3NH3PbI3 perovskite nanodots. Rutile nanorods were grown hydrothermally and their lengths were varied through the control of the reaction time. Infiltration of spiro-MeOTAD hole transport material into the perovskite-sensitized nanorod films demonstrated photocurrent density of 15.6 mA/cm(2), voltage of 955 mV, and fill factor of 0.63, leading to a power conversion efficiency (PCE) of 9.4% under the simulated AM 1.5G one sun illumination. Photovoltaic performance was significantly dependent on the length of the nanorods, where both photocurrent and voltage decreased with increasing nanorod lengths. A continuous drop of voltage with increasing nanorod length correlated with charge generation efficiency rather than recombination kinetics with impedance spectroscopic characterization displaying similar recombination regardless of the nanorod length.

Parameters Affecting I-V Hysteresis of CH3NH3PbI3 Perovskite Solar Cells: Effects of Perovskite Crystal Size and Mesoporous TiO2 Layer.

Current-voltage (I-V) characteristics of CH3NH3PbI3 perovskite solar cells are studied using a time-dependent current response with stepwise sweeping of the bias voltage. Compared with the crystalline Si solar cell showing time-independent current at a given bias voltage, the perovskite solar cells exhibit time-dependent current response. The current increases with time and becomes steady at forward scan from short-circuit to open-circuit, whereas it is decayed and saturated with time at reverse scan from open-circuit to short-circuit. Time-dependent current response eventually leads to I-V hysteresis depending on the scan direction and the scan rate. Crystal size of CH3NH3PbI3 and the mesoporous TiO2 (mp-TiO2) film are found to influence I-V hysteresis, where the I-V hysteresis is alleviated as crystal size increases and in the presence of mp-TiO2. The capacitance observed at low frequency (0.1 to 1 Hz), associated with dipole polarization, tends to diminish as size of perovskite and mp-TiO2 layer thickness increases, which suggests that the origin of hysteresis correlates to the capacitive characteristic of CH3NH3PbI3 and the degree of hysteresis depends strongly on perovskite crystal size and mesoporous TiO2 layer.

Mechanism of carrier accumulation in perovskite thin-absorber solar cells

Photovoltaic conversion requires two successive steps: accumulation of a photogenerated charge and charge separation. Determination of how and where charge accumulation is attained and how this accumulation can be identified is mandatory for understanding the performance of a photovoltaic device and for its further optimization. Here we analyse the mechanism of carrier accumulation in lead halide perovskite, CH3NH3PbI3, thin-absorber solar cells by means of impedance spectroscopy. A fingerprint of the charge accumulation in high density of states of the perovskite absorber material has been observed at the capacitance of the samples. This is, as far as we know, the first observation of charge accumulation in light-absorbing material for nanostructured solar cells, indicating that it constitutes a new kind of photovoltaic device, differentiated from sensitized solar cells, which will require its own methods of study, characterization and optimization.

Morphology-photovoltaic property correlation in perovskite solar cells: One-step versus two-step deposition of CH3NH3PbI3

Perovskite CH3NH3PbI3 light absorber is deposited on the mesoporous TiO2 layer via one-step and two-step coating methods and their photovoltaic performances are compared. One-step coating using a solution containing CH3NH3I and PbI2 shows average power conversion efficiency (PCE) of 7.5%, while higher average PCE of 13.9% is obtained from two-step coating method, mainly due to higher voltage and fill factor. The coverage, pore-filling, and morphology of the deposited perovskite are found to be critical in photovoltaic performance of the mesoporous TiO2 based perovskite solar cells.

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.

Ferroelectric Polarization in CH3NH3PbI3 Perovskite

We report on ferroelectric polarization behavior in CH3NH3PbI3 perovskite in the dark and under illumination. Perovskite crystals with three different sizes of 700, 400, and 100 nm were prepared for piezoresponse force microscopy (PFM) measurements. PFM results confirmed the formation of spontaneous polarization in CH3NH3PbI3 in the absence of electric field, where the size dependency to polarization was not significant. Whereas the photoinduced stimulation was not significant without an external electric field, the stimulated polarization by poling was further enhanced under illumination. The retention of ferroelectric polarization was also observed after removal of the electric field, in which larger crystals showed longer retention behavior compared to the smaller sized one. Additionally, we suggest the effect of perovskite crystal size (morphology) on charge collection at the interface of the ferroelectric material even though insignificant size dependency in electric polarization was observed.

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.

High efficiency solar cells combining a perovskite and a silicon heterojunction solar cells via an optical splitting system

We have applied an optical splitting system in order to achieve very high conversion efficiency for a full spectrum multi-junction solar cell. This system consists of multiple solar cells with different band gap optically coupled via an “optical splitter.” An optical splitter is a multi-layered beam splitter with very high reflection in the shorter-wave-length range and very high transmission in the longer-wave-length range. By splitting the incident solar spectrum and distributing it to each solar cell, the solar energy can be managed more efficiently. We have fabricated optical splitters and used them with a wide-gap amorphous silicon (a-Si) solar cell or a CH3NH3PbI3 perovskite solar cell as top cells, combined with mono-crystalline silicon heterojunction (HJ) solar cells as bottom cells. We have achieved with a 550 nm cutoff splitter an active area conversion efficiency of over 25% using a-Si and HJ solar cells and 28% using perovskite and HJ solar cells.

Material and Device Stability in Perovskite Solar Cells.

Organic-inorganic halide perovskite solar cells have attracted great attention because of their superb efficiency reaching 22 % and low-cost, facile fabrication processing. Nevertheless, stability issues in perovskite solar cells seem to block further advancements toward commercialization. Thus, device stability is one of the important topics in perovskite solar cell research. In the beginning, the poor moisture resistivity of the perovskite layer was considered as a main problem that hindered further development of perovskite solar cells, which encouraged engineering of the perovskite or protection of the perovskite by a buffer layer. Soon after, other parameters affecting long-term stability were sequentially found and various attempts have been made to enhance intrinsic and extrinsic stability. Here we review the recent progresses addressing stability issues in perovskite solar cells. In this report, we investigated factors affecting stability from material and device points of view. To gain a better understanding of the stability of the bulk perovskite material, decomposition mechanisms were investigated in relation to moisture, photons, and heat. Stability of full device should also be carefully examined because its stability is dependent not only on bulk perovskite but also on the interfaces and selective contacts. In addition, ion migration and current-voltage hysteresis were found to be closely related to stability.