Improved Morphology and Efficiency of n–i–p Planar Perovskite Solar Cells by Processing with Glycol Ether Additives

Planar perovskite solar cells can be prepared without high-temperature processing steps typically associated with mesoporous device architectures; however, their efficiency has been lower, and producing high-quality perovskite films in planar devices has been challenging. Here, we report a modified two-step interdiffusion protocol suitable to preparing pinhole-free perovskite films with greatly improved morphology. This is achieved by simple addition of small amounts of glycol ethers to the preparation protocol. We unravel the impact the glycol ethers have on the perovskite film formation using in situ ultraviolet–visible absorbance and grazing incidence wide-angle X-ray scattering experiments. From these experiments we conclude that addition of glycol ethers changes the lead iodide to perovskite conversion dynamics and enhances the conversion efficiency, resulting in more compact polycrystalline films, and it creates micrometer-sized perovskite crystals vertically aligned across the photoactive layer. Consequently, the average photovoltaic performance increases from 13.5% to 15.9%, and reproduciability is enhanced, specifically when 2-methoxyethanol is used as the additive

Effects of High Temperature and Thermal Cycling on the Performance of Perovskite Solar Cells: Acceleration of Charge Recombination and Deterioration of Charge Extraction

In this work, we investigated the effects of high operating temperature and thermal cycling on the photovoltaic (PV) performance of perovskite solar cells (PSCs) with a typical mesostructured (m)-TiO₂–CH₃NH₃PbI_3–<i>x</i>Cl_<i>x</i>–spiro-OMeTAD architecture. After temperature-dependent grazing-incidence wide-angle X-ray scattering, in situ X-ray diffraction, and optical absorption experiments were carried out, the thermal durability of PSCs was tested by subjecting the devices to repetitive heating to 70 °C and cooling to room temperature (20 °C). An unexpected regenerative effect was observed after the first thermal cycle; the average power conversion efficiency (PCE) increased by approximately 10% in reference to the as-prepared device. This increase of PCE was attributed to the heating-induced improvement of the crystallinity and p doping in the hole transporter, spiro-OMeTAD, which promotes the efficient extraction of photogenerated carriers. However, further thermal cycles produced a detrimental effect on the PV performance of PSCs, with the short-circuit current and fill factor degrading faster than the open-circuit voltage. Similarly, the PV performance of PSCs degraded at high operation temperatures; both the short-circuit current and open-circuit voltage decreased with increasing temperature, but the temperature-dependent trend of the fill factor was the opposite. Our impedance spectroscopy analysis revealed a monotonous increase of the charge-transfer resistance and a concurrent decrease of the charge-recombination resistance with increasing temperature, indicating a high recombination of charge carriers. Our results revealed that both thermal cycling and high temperatures produce irreversible detrimental effects on the PSC performance because of the deteriorated interfacial photocarrier extraction. The present findings suggest that the development of robust charge transporters and proper interface engineering are critical for the deployment of perovskite PVs in harsh thermal environments

Blade-Coated Hybrid Perovskite Solar Cells with Efficiency > 17%: An In Situ Investigation

Blade-coating has recently emerged as a scalable fabrication method for hybrid perovskite solar cells, but it currently underperforms spin-coating, yielding a power conversion efficiency (PCE) of ∼15% for CH₃NH₃PbI₃ (MAPbI₃). We investigate the solidification of MAPbI₃ films in situ during spin/blade-coating using optical and X-ray scattering methods. We find that the coating method and conditions profoundly influence the crystallization process, which proceeds through intermediate crystalline solvates. The polymorphism and composition of the solvates are mediated by the solvent removal rate dictated by the process temperature in blade-coating. Low to intermediate temperatures (25–80 °C) yield solvates with differing compositions and yield poor PCEs (∼5–8%) and a large spread (±2.5%). The intermediate solvates are not observed at elevated temperatures (>100 °C), pointing to direct crystallization of the perovskite from the sol–gel ink. These conditions yield large and compact spherulitic domains of perovskite and improve the PCE to ∼13–15% with a narrower spread (< ± 0.5%), while coating at 150 °C yields 17.5% solar cells by inducing in situ decomposition of a small amount of MAPbI₃ into PbI₂. The insights into the crystallization pathway highlight the current challenges and future opportunities associated with scaling up hybrid perovskite solar cell manufacturing

Mesostructured Fullerene Electrodes for Highly Efficient n–i–p Perovskite Solar Cells

Electron-transporting layers in today’s state-of-the-art n–i–p organohalide perovskite solar cells are almost exclusively made of metal oxides. Here, we demonstrate a novel mesostructured fullerene-based electron-transporting material (ETM) that is crystalline, hydrophobic, and cross-linked, rendering it solvent- and heat-resistant for subsequent perovskite solar cell fabrication. The fullerene ETM is shown to enhance the structural and electronic properties of the CH₃NH₃PbI₃ layer grown atop, reducing its Urbach energy from ∼26 to 21 meV, while also increasing crystallite size and improving texture. The resulting mesostructured n–i–p solar cells achieve reduced recombination, improved device-to-device variation, reduced hysteresis, and a power conversion efficiency above 15%, surpassing the performance of similar devices prepared using mesoporous TiO₂ and well above the performance of planar heterojunction devices on amorphous or crystalline [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM). This work is the first demonstration of a viable, hydrophobic, and high-performance mesostructured electron-accepting contact to work effectively in n–i–p perovskite solar cells