Ohmic transition at contacts key to maximizing fill factor and performance of organic solar cells

Abstract

While thermodynamic detailed balance limits the maximum power conversion efficiency of a solar cell, the quality of its contacts can further limit the actual efficiency. The criteria for good contacts to organic semiconductors, however, are not well understood. Here, by tuning the work function of poly(3,4-ethylenedioxythiophene) hole collection layers in fine steps across the Fermi-level pinning threshold of the model photoactive layer, poly(3-hexylthiophene):phenyl-C61-butyrate methyl ester, in organic solar cells, we obtain direct evidence for a non-ohmic to ohmic transition at the hole contact that lies 0.3 eV beyond its Fermi-level pinning transition. This second transition corresponds to reduction of the photocurrent extraction resistance below the bulk resistance of the cell. Current detailed balance analysis reveals that this extraction resistance is the counterpart of injection resistance, and the measured characteristics are manifestations of charge carrier hopping across the interface. Achieving ohmic transition at both contacts is key to maximizing fill factor without compromising open-circuit voltage nor short-circuit current of the solar cell.

17% Efficient Organic Solar Cells Based on Liquid Exfoliated WS2 as a Replacement for PEDOT:PSS

“…This is an interesting finding and demonstrates the ability of solution‐processed MX 2 nanosheets to alter the WF of the ITO electrode. The significant WF shift is expected to lead to the formation of a better hole extracting contact with the BHJ layer as compared to PEDOT:PSS …”

Recent Developments in the Optimization of the Bulk Heterojunction Morphology of Polymer: Fullerene Solar Cells

“…The fill factor ( FF ) [52,53,54,55], as mentioned above, depends on the series resistance, shunt resistance and diode. However, these also depend on a large number of factors that interact with each other intricately and for this reason the FF is more complex and less understood than the V oc and J sc [47,56,57,58]. Previous studies have shown that variables such as the blend composition, the blend morphology, the regioregularity of the conjugated polymer and the thickness of the BHJ influence significantly the FF affecting R s and R sh [52].…”

Insights from Device Modeling of Perovskite Solar Cells

“…In the field of emerging photovoltaic technologies based on organic materials, device modeling has been extensively used to not only develop a deeper understanding of the physical principles governing the device function, [6][7][8] but also as a tool for optimizing the device structure in order to enhance its performance. [9][10] Similarly, albeit to a lesser degree, device simulations have also been applied to colloidal quantum dot photovoltaics. 11 Since the advent of perovskite photovoltaics, significant experimental efforts have been devoted to enhancing the device power conversion efficiency (PCE), reaching remarkable improvements in just a decade.…”