Disorder‐Induced Open‐Circuit Voltage Losses in Organic Solar Cells During Photoinduced Burn‐In

Abstract

different effect on device performance depending on the morphology of the active layer. We investigate this infl uence of morphology in the context of two fundamental mechanisms that are able to cause a V oc loss. First, an increased trap-assisted recombination rate that lowers the charge carrier density [ 2,3 ] and second a broadening of the density of states that reduces V oc [ 4,5 ] with the same charge carrier density and unchanged recombination dynamics. Answering this question is important not only for making more stable solar cells, but also for understanding in general how traps reduce the open-circuit voltage in organic solar cells. Aging is a convenient way to add traps into an organic solar cell and see their effect on recombination and V oc without introducing additional materials into the active layer.We illuminate BHJ solar cells of various materials in inert conditions for three days. Using transient photovoltage (TPV), [ 6 ] charge extraction (CE) [ 7 ] and temperature dependent measurements of the opencircuit voltage, we compare the recombination dynamics and charge carrier densities of fresh and aged solar cells and relate them to the observed V oc losses. We find that in degraded solar cells of amorphous materials such as poly[9′hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′benzothiadiazole)] (PCDTBT), the recombination dynamics are basically unchanged. Instead, the observed open-circuit voltage losses appear to result from increased energetic disorder that causes a change in the relevant low-energy portion of the density of states such that for the same charge carrier density a lower quasi Fermi level splitting is obtained. We observe that solar cells made from crystalline materials do not show V oc losses and have a higher charge carrier density than solar cells made from amorphous materials. Apparently crystalline materials are not affected by a moderate increase of energetic disorder, most likely due to a higher charge carrier density.Besides reduced open-circuit voltages, also losses of fi ll factor [ 1,8 ] and short-circuit current [ 9,10 ] are observed upon burnin. Often a convolution of degradation mechanisms at material interfaces and within the bulk of the absorbing layer is present. Recent work suggests that a decrease in fi ll factor during degradation is related to interface defects [ 1,8 ] which are most likely related to injection or extraction barriers at the electrodes. [11][12][13] The photoinduced open-circuit voltage ( V oc ) loss commonly observed in bulk heterojunction organic solar cells made from amorphous polymers is investigated. It is observed that the total charge carrier density and, importantly, the recombination dynamics are unchanged by photoinduced burn-in. Charge extraction is used to monitor changes in the density of states (DOS) during degradation of the solar cells, and a broadening over time is observed. It is proposed that the V oc losses observed during burn-in are caused by a redistribution of charge carriers in a broader DOS. The tempe...

Indacenodithienothiophene-Based Ternary Organic Solar Cells

“…The pulse intensity was controlled to keep the height of the photovoltage transient smaller than 10 mV resulting in a voltage transient with amplitude ΔV ≪ Voc. The measured transient decays show the form of single exponentials, as expected for the pseudo-first-order kinetic (Hamilton et al, 2010;Heumueller et al, 2015).…”

“…As collected in Table 2 Understood the limitation in the transport properties, we then focus on the better ability of the ternary system in the photogeneration by employing charge extraction (CE) and photoinduced absorption (PIA) spectroscopy (Salvador et al, 2012;Löslein et al, 2013;Gasparini et al, 2015b). In CE measurements, the samples were connected to the terminal of an oscilloscope with the input impedance of 1 MΩ and illuminated with a continuous background laser to keep it in the Voc condition (Heumueller et al, 2015). In order to study the transient decay, we used a nanosecond switch that shifts the cell from open circuit to short circuit condition, allowing the calculation of charge density (n) by integrating the curves, respectively ( Figure 4A) (Heumueller et al, 2015;Gasparini et al, 2016 Table 2).…”

“…In CE measurements, the samples were connected to the terminal of an oscilloscope with the input impedance of 1 MΩ and illuminated with a continuous background laser to keep it in the Voc condition (Heumueller et al, 2015). In order to study the transient decay, we used a nanosecond switch that shifts the cell from open circuit to short circuit condition, allowing the calculation of charge density (n) by integrating the curves, respectively ( Figure 4A) (Heumueller et al, 2015;Gasparini et al, 2016 Table 2). The ability of charge generation in the BHJ solar cells is also studied with PIA spectroscopy.…”

Light‐Induced Degradation Mechanism in Poly(3‐hexylthiophene)/Fullerene Blend Solar Cells

“…Recent studies have shown severe deterioration of photovoltaic effi ciency down to 60% during the fi rst 100 h light exposure. [15][16][17][18][19] In other words, the burn-in photodegradation mechanism is still open question. Thus, it is highly required to understand the burn-in loss mechanism.…”

Side‐Chain Engineering for Enhancing the Properties of Small Molecule Solar Cells: A Trade‐off Beyond Efficiency

“…[30] In addition, McGehee and co-workers and Brabec and co-workers found that a high degree of crystallinity could significantly reduce the initial light-induced degradation, which is caused by light-induced traps. [37,44] Despite the remarkable thermal stability and photoinduced stability in these studies, the precise correlation between the molecular structure and device stability remains phenomenological, based on the reports for several thousands of photovoltaic materials. [12,18,[45][46][47] OSCs can be divided into two categories: polymer based and small molecule based devices.…”