Quantifying the Nongeminate Recombination Dynamics in Nonfullerene Bulk Heterojunction Organic Solar Cells

Abstract

the continued research has led to power conversion efficiencies (PCEs) over 11% for single-junction devices. [7,8] Be that as it may, difficulties in tuning the molecular structure and electronic properties, as well as the comparatively high cost of production of fullerene-based acceptors are drawbacks that have triggered the search for nonfullerene acceptors (NFAs) as an alternative. [9][10][11][12][13][14][15][16] Noteworthy improvements have been made over the last few years, and state-of-the-art NFA-OSCs have been reported with PCEs over 16 %, thus outperforming their fullerene-based counterparts. [17,18] In tandem and ternary systems, PCEs reaching even up to 17.3% have been recently achieved. [19,20] Additionally, the reduction in the bandgap of NFAs opens up the possibility to fabricate semitransparent OSCs that could be applied in building-integrated photovoltaics or power generating greenhouses. [21][22][23][24] To further improve the performance of OSCs, loss-processes such as nongeminate recombination, the recombination of electrons and holes which do not originate from the same exciton, have to be curtailed. This includes bimolecular recombination (also known as Langevin recombination), where charge carriers recombine directly from band to band, as well as trap-assisted recombination (also known as Shockley-Read-Hall recombination), where states deep in the bandgap act as efficient recombination centers. A detailed understanding of these mechanisms responsible for the aforementioned recombination losses is required. [25] Whether or not there are considerable differences in recombination dynamics between NFA-OSCs and fullerene-based OSCs, has yet to be understood since most research in regard to recombination dynamics was performed only with OSCs employing fullerene acceptors. [26][27][28] Furthermore, the majority of studies describe recombination dynamics based on a numerical, drift-diffusion model under the assumption of an effective-medium for the BHJ active layer. [29][30][31][32][33][34][35][36][37][38] The concentration of charge carriers is the key differential parameter in the theory of recombination dynamics. However, only integral parameters (electrical conductivity, impedance, open-circuit voltage, V OC ) can be directly measured by experiments. Therefore, the primary challenge of the theoretical background of any method developed to quantify recombination processes in photovoltaic devices is based on finding the appropriate relationship between the measured In this study, a comprehensive analytical model to quantify the total nongeminate recombination losses, originating from bimolecular as well as bulk and surface trap-assisted recombination mechanisms in nonfullerene-based bulk heterojunction organic solar cells is developed. This proposed model is successfully employed to obtain the different contributions to the recombination current of the investigated solar cells under different illumination intensities. Additionally, the model quantitatively describes the experimentally measured ope...

Functionalized rGO Interlayers Improve the Fill Factor and Current Density in PbS QDs-Based Solar Cells

“…It should be noted that the influence of the middle bandgap states on the device efficiency is lower than the influence of trap states between 0.30 and 0.35 eV, since the rate of trap state emission exponentially depends on their depth [28]. Nevertheless, such states affect the V OC [23,39] via trap-assisted recombination, which correlates with the results obtained for our devices from the light J-V characteristics and slight n growth.…”

Phthalimide Polymer Donor Guests Enable over 17% Efficient Organic Solar Cells via Parallel‐Like Ternary and Quaternary Strategies

“…The term of J ph ⋅R s is added to correct the applied bias V bias for the voltage loss occurring over the series resistance. [64] Figure 6b,c shows the plots of J ph versus V eff of the optimal ternary and quaternary devices after adding PhI-Se. Enhanced photocurrent generation is seen on high V eff regime, which reflects 1) the contribution from the adding PhI-Se component that enhances the absorption of the 300-600 nm solar light and acts as additional paths for charge separation and hole transport and 2) the contribution from the increased electron mobility after adding PhI-Se.…”

On Charge Carrier Density in Organic Solar Cells Obtained via Capacitance Spectroscopy

“…In total, these six relevant methods to obtain the charge carrier density n are listed in Figure 4, where the first three (I-III) have already been used several times in past stu dies. [14,[17][18][19][20]27,31,32] The other three methods (IV-VI) are introduced to address the shortcomings mentioned earlier. The different total charge carrier densities n resulting from these methods for the investigated PTB7-Th:ITIC-2F and the PM6:Y6 solar cells are compared in Figure 5.…”

“…To probe the influence that the differently calculated total charge carrier densities n have on subsequent investigations, we can repeatedly perform the quantitative analysis of the nongeminate recombination dynamics that was introduced in ref. [18] and is summarized again in, cf., Equations (S7)-(S10) in the Supporting Information, taking into account the variations of n that we have discussed previously. This type of analysis is based on the assumption that it is possible to describe the current density lost due to nongeminate recombination, i.e., the recombination current density that we have defined above (J rec = J ph,sat -J ph ), with the help of the voltagedependent charge carrier density n. Other known factors such as the charge carrier mobility μ n/p and dielectric constant ε r are required as well, and the reduction factor ξ, the density of traps in the bulk N tb and the density of surface traps N ts , which are important to quantify the nongeminate recombination dynamics, act as fitting parameters.…”

“…[19] The datasets already published in refs. [18,19] will be used for this comparison. All relevant parameters for the analyses and calculations described below are listed in Table S1 (Supporting Information), and the J-V curves of the two studied blend systems are shown in Figure S1 in the Supporting Information.…”