Temperature and Light Modulated Open‐Circuit Voltage in Nonfullerene Organic Solar Cells with Different Effective Bandgaps

Abstract

bandgap systems for the emerging fields of organic semitransparent photovoltaics and near-infrared photodiodes. [9,10] So far, there are no systematic studies on the relationship between the effective bandgap E g,eff , defined by the HOMO of the donor and the LUMO of the acceptor, the density of states (DOS) distribution and multimechanism recombination processes in nonfullerene organic solar cells. However, all mentioned parameters both individually and synergistically are known to be strongly influential on photoelectronic processes and, thus, on the power conversion efficiency (PCE). A specific interest represents the temperature and light intensity dependence of the open-circuit voltage V oc that is selectively sensitive to these parameters and remains immune to the parasitic effects of the series resistance and large shunt. [11-13] Temperature is a macroscopic parameter, which appears in exponential functions in all major equations that describe charge carrier concentration, recombination, and transport processes. This prevalence of the temperature in most equations is mainly caused by the reliance on the Fermi-Dirac distribution or thermally activated processes. [13,14] Hence, temperature is an essential parameter to consider for the analysis of electronic processes in all types of semiconductor materials and devices. [13,15] A controllable change of temperature allows the modulation of basic electronic processes both mechanistically and quantitatively in organic bulk-heterojunction (BHJ) solar cells. In contrast to studies across different devices, uncontrollable fabrication-related deviations in morphology and defect concentrations can be excluded as a parameter of influence. However, in OPVs, the temperature effect is rarely studied and if so, only to inspect the temperature-dependent trends for the main photoelectric parameters (open circuit voltage (V oc), short circuit current density (J sc), fill factor (FF)) and PCE of organic BHJ solar cells. [16-21] Even less attention has been paid to the investigation of temperature-dependent generationrecombination processes in BHJ layers and, specifically, at their interfaces with electrical contacts. [22-24] In this contribution we analyzed temperature and light modulated V oc in a series of nonfullerene BHJ solar cells with conventional or inverted device architecture and a systematically tuned effective bandgap E g,eff in a wide range from 1.0 to 1.62 eV. The temperature-dependent V oc versus light intensity slopes The relationship of the temperature-light intensity dependence of opencircuit voltage V oc in nonfullerene-based organic solar cells with their material characteristics and multimechanism recombination parameters is described. The systematic variation of the effective bandgap E g,eff and the electrode layers allows the observation of different relative contributions of bimolecular, bulk, and surface trap-assisted recombination mechanisms. The complementary advantages of the analytical model and the established voltage-impedance spectroscopy techn...

Unraveling Device Physics of Dilute‐Donor Narrow‐Bandgap Organic Solar Cells with Highly Transparent Active Layers

“…It is commonly assumed that for slopes of 1kT/q pure or dominant bimolecular recombination occurs in the devices, whereas slopes >1kT/q indicate bulk trap-assisted recombination. [41,53,55] Thus, the data suggest an increase in trap-assisted recombination only for the 20% system. In the presence of surface trap-assisted recombination, the slopes are reduced and values <1kT/q indicate a strong relative contribution of surface recombination.…”

“…Second, to achieve higher transparency, significantly thinner films are employed than in previous works, ranging from 92 to 83 and 78 nm, for 40%, 30% and 20% donor concentration, respectively, instead of the optimized 140 nm shown in previous works. [40,41] We use a highly reproducible opaque back electrode configuration in order to elucidate the changes in the photoelectric processes that arise from the dilution of the donor. The devices studied in this work and the respective energy level diagram are shown schematically in Figure 1b,c.…”

Optical Expediency of Back Electrode Materials for Organic Near-Infrared Photodiodes

“…The parameters used are displayed in Table , with E g being the band gap, T being the temperature, N c being the effective density of states, V bi being the built-in field, χ being the Langevin prefactor for bimolecular recombination, μ eff being the effective mobility, and ε being the dielectric constant. All experimental recombination parameters were obtained from impedance spectroscopy measurements combined with JV -characteristics as described by Brus and Vollbrecht, to yield N surf , N bulk , and χ ,, …”

Non‐Fullerene Acceptor Organic Photodetector for Skin‐Conformable Photoplethysmography Applications

“…The energy levels were estimated from previous values reported in the literature. [21,22] In the proposed OPD, both PTB7-Th and COTIC-4F contribute to photogenerated current since both materials absorb in complementary regions (different optical bandgaps); therefore, it is expected that two energy cascades of charge transfer occur: electron transfer from the donor to the acceptor and hole transfer from the acceptor to the donor.…”