Investigation on Hole Transport Layers for Enhanced Efficiency in Selenium Solar Cells

The invention of the first solid-state solar cell, based on selenium (Se), marked a pivotal moment in the history of photovoltaics (PV). Selenium is a relatively earth-abundant and nontoxic material with good chemical stability. Moreover, its wide bandgap and low processing temperature makes it very interesting as top cell for monolithic tandem solar cells and indoor photovoltaics. However, selenium solar cell efficiency stifled at 6.5% since 2017 and just recently, after more than a lustrum, it overcame the 7% threshold. This new milestone is shadowed by the fact that the record device produced by Lu et al. lacks a hole transport layer (HTL).

Historically, the improvement efforts have been focused on the electron transport layers (ETLs) of selenium cells from which we can emphasize the use of TiO_x and (Zn,Mg)O. In contrast, hole transport layer innovations have been centered around conjugated polymers like P3HT, PTAA or PEDOT:PSS. The hole transport material that currently yields the best efficiencies is molybdenum oxide (MoO_x), a HTL used in every photovoltaic technology. Despite that, many articles report devices with high efficiencies without a hole transport layer. This fact suggests that the HTL optimization and exploration for selenium solar cells is still a loose end for the selenium solar cell research community.

In this work, we investigate the use of several unexplored transition metal oxide (like VO_x, WO_x and NiO_x) HTLs for the Se PV technology, including some of the most stablished transport materials (like MoO_x and PEDOT:PSS). Several absorber thicknesses and two selenium crystallization strategies have been used to explore the influence each HTL has in the different scenarios.

Each selective layer thickness has been optimized to obtain the highest power conversion efficiency (PCE). Then, the refined HTLs have been systematically compared between each other. These devices have been extensively studied using morphological, compositional and optoelectronic characterization to investigate the source of the performance differences. SCAPS simulation has also been used to comprehend the effect of the selective layers in the device structure. Champion cells of over 4% PCE have been achieved using 20 nm MoO_x and ultrathin 1nm WO_x layers.

Indoor photovoltaic performance across many different illumination conditions will be presented. The devices presented have a resilience factor of over 90% and indoor efficiencies over 10% for several of the studied HTLs.