A Simple Approach for Unraveling Optoelectronic Processes in Organic Solar Cells under Short‐Circuit Conditions

Abstract

efficiencies (PCEs) up to 18.2% [7][8][9] were achieved. This progress in the field of OPVs is intricately linked to the correct assessment of material and device para meters and to efforts towards deconvoluting the electronic processes and the contributions of different loss mechanisms. [10,11] Under short-circuit conditions, the interplay of exciton photogeneration in the active layer, geminate and non-geminate recombination processes, and free charge carrier extraction define the measured electron flux (photocurrent). [12][13][14][15][16] The complexity of the photoelectronic processes complicates the analysis, and advanced techniques are frequently applied to characterize a single aspect. [17][18][19][20] For example, impedance spectroscopy is commonly used to gauge the losses due to non-geminate recombination, and time-delayed collection field is employed to investigate geminate recombination losses. [17][18][19][20] In this work, we propose a comprehensive technique that allows the quantification of geminate recombination and the determination of the mobility-lifetime product (µτ). We couple optical transfer matrix method-based simulations with simple experimental methods (JV characteristics, external quantum efficiency (EQE)) that are readily available in most laboratories. The geminate recombination losses are quantified by a geminate recombination prefactor (P g ), which is a material parameter of the active layer's donor/acceptor blend. P g describes the fraction of excitons splitting into free electrons and holes; thus, P g approaching unity is desirable for efficient OPVs. [21] We determine P g from experimental current-voltage characteristics combined with optical simulations.Next, we propose a new approach to obtain the µτ based on the measurement of the EQE, the previously determined P g , and the simulation of the EQE. The effective mobility, µ, defines a charge carrier's drift velocity, v d , in a given electric field, E, and the effective lifetime, τ, describes the time between the generation of a free charge carrier and its subsequent annihilation via recombination. Thus, µτ as a single integrated parameter includes both charge transport and non-geminate recombination features. [22] Efforts to provide accessible means to determine the µτ product are reported in the OPV community only rarely, and, if so, they are based on substantial simplifications, and the results are not juxtaposed with those obtained by any independent method. [23] Since µτ determines the distance that a photogenerated charge carrier travels in the active layer before it recombines, we can quantify the extraction efficiency η under short-circuit conditions. [24][25][26] The short-circuit current (J sc ) of organic solar cells is defined by the interplay of exciton photogeneration in the active layer, geminate and non-geminate recombination losses and free charge carrier extraction. The method proposed in this work allows the quantification of geminate recombination and the determination of the mobility-lifetime product (µτ) ...

Effect of Palladium‐Tetrakis(Triphenylphosphine) Catalyst Traces on Charge Recombination and Extraction in Non‐Fullerene‐based Organic Solar Cells

“…[ 87 ] The simulated J sc is a theoretical value that would be obtained if the absorbed photon flux yields electronflux in the outer circuit without any losses. [ 80 ] The difference between the experimentally obtained J ph,sat that excludes extraction losses (and with that non‐geminate recombination losses), and the simulated J sc is therefore attributed to geminate recombination losses, and, in this system with added impurity, also to exciton quenching by Pd(PPh 3 ) 4 . The ratio of J ph,sat, and the simulated J sc , here called P e,g , is shown in Figure a.…”

“…[ 75–79 ] Not all of these coulombically bound geminate pairs split into free charge carriers, though, because of geminate recombination. [ 75–80 ] Further, since we investigate a system with an added impurity, the formed excitons likely undergo quenching even before a geminate pair can be formed. [ 81–84 ]…”

“…The optical properties of the active layer, in conjunction with those of all other device layers as well as the device architecture, determine the amount of light absorbed and the interference pattern within the device and the active layer. [ 80,85,86 ] A change in the optical properties will result in a change of the generation rate G of charge carriers that are photo‐generated in the active layer, and with that, the theoretical maximum of the J sc changes. We illuminated the devices with a solar simulator and measured the current voltage‐characteristics to obtain the saturated photocurrent J ph,sat at large reverse bias.…”

“…All non‐polymer based materials were characterized with a Woollam M‐2000DI Variable Angle Spectroscopic Ellipsometer to obtain the optical properties of all device layers as input for the transfer matrix simulation as described in our previous work. [ 80 ]…”

Organic Solar Cells Parameters Extraction and Characterization Techniques

“…A voltage ramp in the reverse direction is used to collect the charges leading to a ramp in the displacement current density . The time needed by the transient current to reach its peak is used to estimate the mobility in the device through the simple analytical formula given by ( 15 ) [ 107 , 108 , 109 ]. where is the mobility of fast carriers, d is active layer thickness, A is the ramp rate of the applied voltage, is the difference between current at and .…”