Quantifying Quasi‐Fermi Level Splitting and Open‐Circuit Voltage Losses in Highly Efficient Nonfullerene Organic Solar Cells

Abstract

The power conversion efficiency (PCE) of state‐of‐the‐art organic solar cells is still limited by significant open‐circuit voltage (VOC) losses, partly due to the excitonic nature of organic materials and partly due to ill‐designed architectures. Thus, quantifying different contributions of the VOC losses is of importance to enable further improvements in the performance of organic solar cells. Herein, the spectroscopic and semiconductor device physics approaches are combined to identify and quantify losses from surface recombination and bulk recombination. Several state‐of‐the‐art systems that demonstrate different VOC losses in their performance are presented. By evaluating the quasi‐Fermi level splitting (QFLS) and the VOC as a function of the excitation fluence in nonfullerene‐based PM6:Y6, PM6:Y11, and fullerene‐based PPDT2FBT:PCBM devices with different architectures, the voltage losses due to different recombination processes occurring in the active layers, the transport layers, and at the interfaces are assessed. It is found that surface recombination at interfaces in the studied solar cells is negligible, and thus, suppressing the non‐radiative recombination in the active layers is the key factor to enhance the PCE of these devices. This study provides a universal tool to explain and further improve the performance of recently demonstrated high‐open‐circuit‐voltage organic solar cells.

A History and Perspective of Non‐Fullerene Electron Acceptors for Organic Solar Cells

“…[280,[286][287][288] The V OC loss associated with surface recombination may be seen to be non-radiative, however, this is a device-related loss as opposed to material-related mechanisms such as non-radiative decay of the CT states discussed in the previous sections. [289] To improve the contacts and reduce surface recombination, electrode interlayers are usually employed. [290][291][292] In this regard, Yao et al demonstrated that the PCE of PM6:Y6 can be increased from 15% to an impressive 17% by changing the cathode interlayer from PDINO to PDNN.…”

“…[ 280,286–288 ] The V OC loss associated with surface recombination may be seen to be non‐radiative, however, this is a device‐related loss as opposed to material‐related mechanisms such as non‐radiative decay of the CT states discussed in the previous sections. [ 289 ]…”

Cascaded energy landscape as a key driver for slow yet efficient charge separation with small energy offset in organic solar cells

“…In this respect, many studies have been conducted in order to reveal the origin of the success of the PM6/Y6 blend OSCs [11][12][13][14][15][16][17][18][19] . However, the photophysical mechanisms underlying efficient free carrier (FC) generation in this blend system remain the subject of continuing debate.…”

Direct quantification of quasi-Fermi level splitting in organic semiconductor devices

“…However, the corresponding V OC loss derived from this approach generally includes contributions from both intrinsic bulkrelated processes and surface recombination. In organic solar cells, the presence of surface recombination is usually correlated with electrode-induced photovoltage losses at the contacts [37][38][39], causing a mismatch between V OC and the associated QFLS in the bulk. As such, measurements based on electroluminscence cannot differentiate between intrinsic and electrode-induced photovoltage losses, leading to a consistent underestimation of the QFLS in organic photovoltaic devices.…”

“…As such, measurements based on electroluminscence cannot differentiate between intrinsic and electrode-induced photovoltage losses, leading to a consistent underestimation of the QFLS in organic photovoltaic devices. A method for overcoming the difficulty in quantifying the QFLS has recently been suggested using photoinduced absorption [39]. However, this method requires detailed knowledge about the absorption cross section and charge-transport parameters of the device, which must necessarily be semitransparent.…”