Influence of the Hole Transporting Layer on the Thermal Stability of Inverted Organic Photovoltaics Using Accelerated-Heat Lifetime Protocols

Abstract

High power conversion efficiency (PCE) inverted organic photovoltaics (OPVs) usually use thermally evaporated MoO3 as a hole transporting layer (HTL). Despite the high PCE values reported, stability investigations are still limited and the exact degradation mechanisms of inverted OPVs using thermally evaporated MoO3 HTL remain unclear under different environmental stress factors. In this study, we monitor the accelerated lifetime performance under the ISOS-D-2 protocol (heat conditions 65 °C) of nonencapsulated inverted OPVs based on the thiophene-based active layer materials poly(3-hexylthiophene) (P3HT), poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7), and thieno[3,2-b]thiophene-diketopyrrolopyrrole (DPPTTT) blended with [6,6]-phenyl C71-butyric acid methyl ester (PC[70]BM). The presented investigation of degradation mechanisms focus on optimized P3HT:PC[70]BM-based inverted OPVs. Specifically, we present a systematic study on the thermal stability of inverted P3HT:PC[70]BM OPVs using solution-processed poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) and evaporated MoO3 HTL. Using a series of measurements and reverse engineering methods, we report that the P3HT:PC[70]BM/MoO3 interface is the main origin of failure of the P3HT:PC[70]BM-based inverted OPVs under intense heat conditions, a trend that is also observed for the other two thiophene-based polymers used in this study.

Solution‐Processable Anode Double Buffer Layers for Inverted Polymer Solar Cells

“…Because of the complexity of this material, a clear understanding of the stability is lacking, and different interpretations are summoned, but it is clear that from this point of view molybdenum oxide is not an issue‐free material. For example, using three different donor:fullerene systems, Hermerschmidt et al have shown that inverted polymer solar cells with e‐MoO x undergo a photovoltaic performance decay in air and under accelerated ageing conditions ( T = 65 °C). They have attributed this behavior to a degradation of the active layer/e‐MoO x interface.…”

“…In terms of photovoltaic performances, P3HT:PCBM active blend is quite tolerant to any buffer layer, but other materials are not, as it will be shown also in the present work. As a matter of fact, direct comparisons of e‐MoO x and PEDOT:PSS are seldom reported in the literature, but in those cases, the PCEs drop dramatically (Table ) . One of the possible reasons could be the unsuitable work function of PEDOT:PSS (about 5–5.2 eV).…”

Solution‐Processed Molybdenum Oxide with Hydroxyl Radical‐Induced Oxygen Vacancy as an Efficient and Stable Interfacial Layer for Organic Solar Cells

“…The SM5‐based device maintained over 55% of its initial PCE after 1200 h of thermal treatment, whereas that of the PEDOT:PSS‐based device continuously dropped below 25% for the same measurement period. The devices were also tested under thermal treatment at 85 °C in relative humidity of 20% in average, following the environmental conditions adopted by the ISOS‐D‐2 protocol . Both the PCE of PEDOT:PSS‐ and SM‐based devices dropped rapidly within 50 h (Figure S8, Supporting Information).…”

Flexible, Large-Area Organic Solar Cells With Improved Performance Through Incorporation of CoFe2O4 Nanoparticles in the Active Layer

“…7b) is referred to as burn-in 21,22 , which is a signature of light induced degradation 24 . This burn-in has been demonstrated to be related to the photoinduced dimerization of fullerenes within the active layer 50,51 . The larger drop for CFO-OSC is attributed to the higher absolute J sc initial value.…”