An-Na Cho

High‐Efficiency Perovskite Solar Cells Based on the Black Polymorph of HC(NH2)2PbI3

Perovskite solar cells with power conversion efficiencies exceeding 16% at AM 1.5 G one sun illumination are developed using the black polymorph of formamidnium lead iodide, HC(NH2)2 PbI3 . Compared with CH3 NH3 PbI3 , HC(NH2 )2 PbI3 extends its absoprtion to 840 nm and shows no phase transition between 296 and 423 K. Moreover, a solar cell based on HC(NH2 )2 PbI3 exhibits photostability and little I-V hysteresis.

High-Performance Long-Term-Stable Dopant-Free Perovskite Solar Cells and Additive-Free Organic Solar Cells by Employing Newly Designed Multirole π-Conjugated Polymers

Perovskite solar cells (PSCs) and organic solar cells (OSCs) are promising renewable light-harvesting technologies with high performance, but the utilization of hazardous dopants and high boiling additives is harmful to all forms of life and the environment. Herein, new multirole π-conjugated polymers (P1-P3) are developed via a rational design approach through theoretical hindsight, further successfully subjecting them into dopant-free PSCs as hole-transporting materials and additive-free OSCs as photoactive donors, respectively. Especially, P3-based PSCs and OSCs not only show high power conversion efficiencies of 17.28% and 8.26%, but also display an excellent ambient stability up to 30 d (for PSCs only), owing to their inherent superior optoelectronic properties in their pristine form. Overall, the rational approach promises to support the development of environmentally and economically sustainable PSCs and OSCs.

Impact of Interfacial Layers in Perovskite Solar Cells

Perovskite solar cells (PCSs) are composed of organic-inorganic lead halide perovskite as the light harvester. Since the first report on a long-term-durable, 9.7 % efficient, solid-state perovskite solar cell, organic-inorganic halide perovskites have received considerable attention because of their excellent optoelectronic properties. As a result, a power conversion efficiency (PCE) exceeding 22 % was certified. Controlling the grain size, grain boundary, morphology, and defects of the perovskite layer is important for achieving high efficiency. In addition, interfacial engineering is equally or more important to further improve the PCE through better charge collection and a reduction in charge recombination. In this Review, the type of interfacial layers and their impact on photovoltaic performance are investigated for both the normal and the inverted cell architectures. Four different interfaces of fluorine-doped tin oxide (FTO)/electron-transport layer (ETL), ETL/perovskite, perovskite/hole-transport layer (HTL), and HTL/metal are classified, and their roles are investigated. The effects of interfacial engineering with organic or inorganic materials on photovoltaic performance are described in detail. Grain-boundary engineering is also included because it is related to interfacial engineering and the grain boundary in the perovskite layer plays an important role in charge conduction, recombination, and chargecarrier life time.

Acridine-based novel hole transporting material for high efficiency perovskite solar cells

An acridine-based hole transporting material (ACR-TPA) without the spirobifluorene motif is synthesized via non complicated steps. The ACR-TPA film including Li-TFSI and 4-tert-butylpyridine (tBP) additives exhibits a hole mobility of 3.08 × 10−3 cm2 V−1 s−1, which is comparable to the mobility of the classical spiro-MeOTAD (2.63 × 10−3 cm2 V−1 s−1), and its HOMO level of −5.03 eV is slightly lower than that of spiro-MeOTAD (−4.97 eV). ACR-TPA layers with different thicknesses are applied to MAPbI3 perovskite solar cells, where power conversion efficiency (PCE) increases as the ACR-TPA layer thickness increases due to increased recombination resistance and fast charge separation. The best PCE of 16.42% is achieved from the ca. 250 nm-thick ACR-TPA, which is comparable to the PCE of 16.26% for a device with spiro-MeOTAD in the same device configuration. It is thus anticipated that ACR-TPA can be a promising alternative to spiro-MeOTAD because of its lower cost and comparable photovoltaic performance.

Role of LiTFSI in high Tg triphenylamine-based hole transporting material in perovskite solar cell

A hole transporting material based on triphenylamine with a high glass transition temperature (Tg) of 99 °C, coded as BT41, was synthesized and applied to a perovskite solar cell. The pristine BT41 showed a low power conversion efficiency (PCE) of 1.1% due to a low photocurrent density (Jsc) of ca. 6 mA cm−2 and an almost negligible fill factor of less than 0.2, which was significantly improved to 9.0%, however, owing mainly to the 3-fold improved Jsc of 17.6 mA cm−2, by adding both tert-butylpyridine (tBP) and lithium bis(trifluoromethane sulfonyl) imide (LiTFSI) as additives. The oxidation of BT41 was dominated by LiTFSI, which was responsible for the hole mobility increasing by one order of magnitude. The addition of the additive also reduced the recombination resistance, which correlates with the higher fill factor. Although both additives in BT41 contributed cooperatively to the improvement of the photovoltaic performance, LiTFSI played the major role in the enhancement.

Efficient and Reproducible CH3NH3PbI3 Perovskite Layer Prepared Using a Binary Solvent Containing a Cyclic Urea Additive

An efficient CH3NH3PbI3 perovskite solar cell whose performance is reproducible and shows reduced dependence on the processing conditions is fabricated using the cyclic urea compound 1,3-dimethyl-2-imidazolidinone (DMI) as an additive to the precursor solution of CH3NH3PbI3. X-ray diffraction analysis reveals that DMI weakly coordinates with PbI2 and forms a CH3NH3PbI3 film (film-DMI) with no intermediate phase. The surface of annealed film-DMI (film-DMI-A) was smooth, with an average crystal size of 1 μm. Photoluminescence and transient photovoltage measurements show that film-DMI-A exhibits a longer carrier lifetime than a CH3NH3PbI3 film prepared using the strongly coordinating additive dimethyl sulfoxide (DMSO) (film-DMSO-A) because of the reduced number of defect sites in film-DMI-A. A solar cell based on film-DMI-A exhibits a higher power conversion efficiency (17.6%) than that of a cell based on film-DMSO-A (15.8%). Furthermore, the performance of the film-DMI-A solar cell is less sensitive to the ratio between PbI2 and DMI, and film-DMI can be fabricated under a high relative humidity of 55%.

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.