RESUMO
The electrically insulating space layer takes a fundamental role in monolithic carbon-graphite based perovskite solar cells (PSCs) and it has been established to prevent the charge recombination of electrons at the mp-TiO2/carbon-graphite (CG) interface. Thick 1 µm printed layers are commonly used for this purpose in the established triple-mesoscopic structures to avoid ohmic shunts and to achieve a high open circuit voltage. In this work, we have developed a reproducible large-area procedure to replace this thick space layer with an ultra-thin dense 40 nm sputtered Al2O3 which acts as a highly electrically insulating layer preventing ohmic shunts. Herewith, transport limitations related so far to the hole diffusion path length inside the thick mesoporous space layer have been omitted by concept. This will pave the way toward the development of next generation double-mesoscopic carbon-graphite-based PSCs with highest efficiencies. Scanning electron microscope, energy dispersive X-ray analysis, and atomic force microscopy measurements show the presence of a fully oxidized sputtered Al2O3 layer forming a pseudo-porous covering of the underlying mesoporous layer. The thickness has been finely tuned to achieve both electrical isolation and optimal infiltration of the perovskite solution allowing full percolation and crystallization. Photo voltage decay, light-dependent, and time-dependent photoluminescence measurements showed that the optimal 40 nm thick Al2O3 not only prevents ohmic shunts but also efficiently reduces the charge recombination at the mp-TiO2/CG interface and, at the same time, allows efficient hole diffusion through the perovskite crystals embedded in its pseudo-pores. Thus, a stable V OC of 1 V using CH3NH3PbI3 perovskite has been achieved under full sun AM 1.5 G with a stabilized device performance of 12.1%.
RESUMO
Relating crystallization of the absorber layer in a perovskite solar cell (PSC) to the device performance is a key challenge for the process development and in-depth understanding of these types of high efficient solar cells. A novel approach that enables real-time photo-physical and electrical characterization using a graphite-based PSC is introduced in this work. In our graphite-based PSC, the device architecture of porous monolithic contact layers creates the possibility to perform photovoltaic measurements while the perovskite crystallizes within this scaffold. The kinetics of crystallization in a solution based 2-step formation process has been analyzed by real-time measurement of the external photon to electron quantum efficiency as well as the photoluminescence emission spectra of the solar cell. With this method it was in particular possible to identify a previously overlooked crystallization stage during the formation of the perovskite absorber layer. This stage has significant influence on the development of the photocurrent, which is attributed to the formation of electrical pathways between the electron and hole contact, enabling efficient charge carrier extraction. We observe that in contrast to previously suggested models, the perovskite layer formation is indeed not complete with the end of crystal growth.
