RESUMEN
Vacuum techniques for perovskite photovoltaics (PV) are promising for their scalability but are rarely studied with techniques readily adaptable for industry. In this work, we study the use of close-space sublimation (CSS) for making perovskite solar cells, a technique that has seen widespread use in industry, including in PV, and benefits from high material-transfer and low working pressures. A pressed pellet of formamidinium iodide (FAI) can be used multiple times as an organic source, without needing replacement. Using CSS at a rough vacuum (10 mbar), efficient cesium formamidinium lead iodide perovskite based solar cells are obtained reaching a maximum photoconversion efficiency (PCE) of 18.7%. They maintain their performance for >650 h when thermally stressed at 85 °C in a nitrogen environment. To explain the initial rise in PCE upon heating, we used drift-diffusion simulations and identified a reduction in bulk trap density as the primary factor.
RESUMEN
Coevaporation of perovskite films allows for a fine control over the material stoichiometry and thickness but is typically slow, leading to several-hour processes to obtain thick films required for photovoltaic applications. In this work, we demonstrate the coevaporation of perovskite layers using faster deposition rates, obtaining 1 µm thick films in approximately 50 min. We observed distinct structural properties and obtained devices with efficiency exceeding 19%, demonstrating the relevance of this deposition process from a material perspective and also in view of potential industrialization.
RESUMEN
The Moving photocarrier Grating Technique (MGT) allows the simultaneous determination of the photocarrier drift mobilities and the small-signal recombination lifetime of photoconductive semiconductors. The technique measures the direct current (DC) induced by a monochromatic illumination consisting of a moving interference pattern superimposed on a uniform background of much higher intensity. A drawback of the technique is the low level of the signal to be measured, which can be masked by the noise at low temperatures or low light intensities. In this work, we propose implementing an alternating current (AC) version of the MGT by chopping the weak beam in the standard configuration. We call this new technique the Chopped Moving photocarrier Grating (CMG). In CMG, the AC signal can be measured with a lock-in amplifier for electrical noise removal. In this way, the signal-to-noise ratio can be increased compared to the standard DC technique. Assuming a multiple-trapping model for charge transport, we find the theoretical expression for the current density induced by CMG at fundamental frequency. By using a numerical simulation with parameters typical for hydrogenated amorphous silicon, we verify the expected equivalence between both techniques for low enough chopping frequencies. Then, we test experimentally this equivalence for an undoped hydrogenated amorphous silicon sample. For low signal levels, we demonstrate the superior performance of CMG.
