Charge transfer state characterization and voltage losses of organic solar cells

Abstract

A correct determination of voltage losses is crucial for the development of organic solar cells with improved performance. This requires an in-depth understanding of the properties of interfacial charge transfer (CT) states, which not only set the upper limit for the open-circuit voltage of a system, but also govern radiative and non-radiative recombination processes. Over the last decade, different approaches have emerged to classify voltage losses in organic solar cells that rely on a generic detailed balance approach or additionally include CT state parameters that are specific to organic solar cells. In the latter case, a correct determination of CT state properties is paramount. In this work, we summarize the different frameworks used today to calculate voltage losses and provide an in-depth discussion of the currently most important models used to characterize CT state properties from absorption and emission data of organic thin films and solar cells. We also address practical concerns during the data recording, analysis, and fitting process. Departing from the classical two-state Marcus theory approach, we discuss the importance of quantized molecular vibrations and energetic hybridization effects in organic donor-acceptor systems with the goal to providing the reader with a detailed understanding of when each model is most appropriate.

Subgap Absorption in Organic Semiconductors

“…Deviations are seen for both midgap states and excitons, suggesting that these states are not in equilibrium with free charge carriers under open-circuit conditions. As a result, we generally expect J 0 RAD ∝ exp(− E CT / kT ) and V OC RAD ∝ E CT / q (under one sun illumination) in high-offset donor–acceptor systems, consistent with a large body of experimental evidence from fullerene-based acceptor BHJs. ,,− …”

Advanced characterization of organic–metal and organic–organic interfaces: from photoelectron spectroscopy data to energy-level diagrams

“…The functionality of virtually all organic and hybrid (opto-)electronic devices depends on interface energetics, and a thorough understanding of energy-level alignment mechanisms at organic-metal and organic-organic interfaces is indispensable for further efficiency improvements [1][2][3][4][5]. For example, the energy-level offset at the donor-acceptor interface in organic photovoltaic devices is crucial for exciton dissociation [6][7][8]; chemisorbed molecular monolayers on metals can tune the substrate work functions by an interfacial charge transfer [9,10] and allow, consequently, to lower charge injection barriers into electrodes [11,12]. The number of organic materials used for optoelectronic applications is virtually unlimited [13,14] and energy-level diagrams (ELDs) are frequently used to choose the best material for a given purpose [4,15].…”

Simplified Y6‐Based Nonfullerene Acceptors: In‐Depth Study on Molecular Structure–Property Relation, Molecular Dynamics Simulation, and Charge Dynamics

“…[47] It is determined using a reciprocity relation between Fourier transform photocurrent spectroscopy-EQE (FTPS-EQE) and electroluminescence (EL) spectra. [40,48,49] ΔE 3 is the energy loss that comes from nonradiative recombination, and it can be calculated by measuring the electroluminescent quantum efficiency (EQE EL ) according to the equation of ΔE 3 = −kT ln(EQE EL ). [48] Figure 6 shows the FTPS-EQE and EL spectra of PM6:YBO-2O and PM6:YBO-FO.…”