Lewis Acid–Base Adduct Approach for High Efficiency Perovskite Solar Cells

ConspectusSince the first report on the long-term durable 9.7% solid-state perovskite solar cell employing methylammonium lead iodide (CH₃NH₃PbI₃), mesoporous TiO₂, and 2,2′,7,7′-tetrakis­[<i>N</i>,<i>N</i>-di­(4-methoxyphenyl)­amino]-9,9′-spirobifluorene (spiro-MeOTAD) in 2012, following the seed technologies on perovskite-sensitized liquid junction solar cells in 2009 and 2011, a surge of interest has been focused on perovskite solar cells due to superb photovoltaic performance and extremely facile fabrication processes. The power conversion efficiency (PCE) of perovskite solar cells reached 21% in a very short period of time. Such an unprecedentedly high photovoltaic performance is due to the intrinsic optoelectronic property of organolead iodide perovskite material. Moreover, a high dielectric constant, sub-millimeter scale carrier diffusion length, an underlying ferroelectric property, and ion migration behavior can make organolead halide perovskites suitable for multifunctionality. Thus, besides solar cell applications, perovskite material has recently been applied to a variety fields of materials science such as photodetectors, light emitting diodes, lasing, X-ray imaging, resistive memory, and water splitting. Regardless of application areas, the growth of a well-defined perovskite layer with high crystallinity is essential for effective utilization of its excellent physicochemical properties. Therefore, an effective methodology for preparation of high quality perovskite layers is required.In this Account, an effective methodology for production of high quality perovskite layers is described, which is the Lewis acid–base adduct approach. In the solution process to form the perovskite layer, the key chemicals of CH₃NH₃I (or HC­(NH₂)₂I) and PbI₂ are used by dissolving them in polar aprotic solvents. Since polar aprotic solvents bear oxygen, sulfur, or nitrogen, they can act as a Lewis base. In addition, the main group compound PbI₂ is known to be a Lewis acid. Thus, PbI₂ has a chance to form an adduct by reacting with the Lewis base. Crystal growth and morphology of perovskite can be controlled by taking advantage of the weak chemical interaction in the adduct. We have successfully fabricated highly reproducible CH₃NH₃PbI₃ perovskite solar cells with PCE as high as 19.7% via adducts of PbI₂ with oxygen-donor <i>N</i>,<i>N</i>′-dimethyl sulfoxide. This adduct approach has been found to be generally adopted, where formamidinium lead iodide perovskite, HC­(NH₂)₂PbI₃ (FAPbI₃), with large grain, high crystallinity, and long-lived carrier lifetime was successfully fabricated via an adduct of PbI₂ with sulfur-donor thiourea as Lewis base. The adduct approach proposed in this Account is a very promising methodology to achieve high quality perovskite films with high photovoltaic performance. Furthermore, single crystal growth on the conductive substrate is expected to be possible if we kinetically control the elimination of Lewis base in the adduct

The Interplay between Trap Density and Hysteresis in Planar Heterojunction Perovskite Solar Cells

Anomalous current–voltage (<i>J</i>–<i>V</i>) hysteresis in perovskite (PSK) solar cell is open to dispute, where hysteresis is argued to be due to electrode polarization, dipolar polarization, and/or native defects. However, a correlation between those factors and <i>J</i>–<i>V</i> hysteresis is hard to be directly evaluated because they usually coexist and are significantly varied depending on morphology and crystallinity of the PSK layer, selective contacts, and device architecture. In this study, without changing morphology and crystallinity of PSK layer in a planar heterojunction structure employing FA_0.9Cs_0.1PbI₃, a correlation between <i>J</i>–<i>V</i> hysteresis and trap density is directly evaluated by means of thermally induced PbI₂ regulating trap density. Increase in thermal annealing time at a given temperature of 150 °C induces growth of PbI₂ on the PSK grain surface, which results in significant reduction of nonradiative recombination. Hysteresis index is reduced from 0.384 to 0.146 as the annealing time is increased from 5 to 100 min due to decrease in the amplitude of trap-mediated recombination. Reduction of hysteresis by minimizing trap density via controlling thermal annealing time leads to the stabilized PCE of 18.84% from the normal planar structured FA_0.9Cs_0.1PbI₃ PSK solar cell

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/cm², 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

Zn₂SnO₄‑Based Photoelectrodes for Organolead Halide Perovskite Solar Cells

We report a new ternary Zn₂SnO₄ (ZSO) electron-transporting electrode of a CH₃NH₃PbI₃ perovskite solar cell as an alternative to the conventional TiO₂ electrode. The ZSO-based perovskite solar cells have been prepared following a conventional procedure known as a sequential (or two-step) process with ZSO compact/mesoscopic layers instead of the conventional TiO₂ counterparts, and their solar cell properties have been investigated as a function of the thickness of either the ZSO compact layer or the ZSO mesoscopic layer. The presence of the ZSO compact layer has a negligible influence on the transmittance of the incident light regardless of its thickness, whereas the thickest compact layer blocks the back-electron transfer most efficiently. The open-circuit voltage and fill factor increase with the increasing thickness of the mesoscopic ZSO layer, whereas the short-circuit current density decreases with the increasing thickness except for the thinnest one (∼100 nm). As a result, the device with a 300 nm-thick mesoscopic ZSO layer shows the highest conversion efficiency of 7%. In addition, time-resolved and frequency-resolved measurements reveal that the ZSO-based perovskite solar cell exhibits faster electron transport (∼10 times) and superior charge-collection capability compared to the TiO₂-based counterpart with similar thickness and conversion efficiency

Reduced Graphene Oxide/Mesoporous TiO₂ Nanocomposite Based Perovskite Solar Cells

We report on reduced graphene oxide (rGO)/mesoporous (mp)-TiO₂ nanocomposite based mesostructured perovskite solar cells that show an improved electron transport property owing to the reduced interfacial resistance. The amount of rGO added to the TiO₂ nanoparticles electron transport layer was optimized, and their impacts on film resistivity, electron diffusion, recombination time, and photovoltaic performance were investigated. The rGO/mp-TiO₂ nanocomposite film reduces interfacial resistance when compared to the mp-TiO₂ film, and hence, it improves charge collection efficiency. This effect significantly increases the short circuit current density and open circuit voltage. The rGO/mp-TiO₂ nanocomposite film with an optimal rGO content of 0.4 vol % shows 18% higher photon conversion efficiency compared with the TiO₂ nanoparticles based perovskite solar cells

Tuning Molecular Interactions for Highly Reproducible and Efficient Formamidinium Perovskite Solar Cells via Adduct Approach

The Lewis acid–base adduct approach has been widely used to form uniform perovskite films, which has provided a methodological base for the development of high-performance perovskite solar cells. However, its incompatibility with formamidinium (FA)-based perovskites has impeded further enhancement of photovoltaic performance and stability. Here, we report an efficient and reproducible method to fabricate highly uniform FAPbI₃ films via the adduct approach. Replacement of the typical Lewis base dimethyl sulfoxide (DMSO) with <i>N</i>-methyl-2-pyrrolidone (NMP) enabled the formation of a stable intermediate adduct phase, which can be converted into a uniform and pinhole-free FAPbI₃ film. Infrared and computational analyses revealed a stronger interaction between NMP with the FA cation than DMSO, which facilitates the formation of a stable FAI·PbI₂·NMP adduct. On the basis of the molecular interactions with different Lewis bases, we proposed criteria for selecting the Lewis bases. Owed to the high film quality, perovskite solar cells with the highest PCE over 20% (stabilized PCE of 19.34%) and average PCE of 18.83 ± 0.73% were demonstrated