Oxygen-Induced Doping as a Degradation Mechanism in Highly Efficient Organic Solar Cells

Abstract

Despite tremendous advances in improving the efficiency of organic solar cells above 14%, the environmental stability of such devices remains an essential and widely inadequately addressed challenge. Understanding the underlying principles of device degradation is a critical step toward further development and commercialization of organic photovoltaics. Herein, we report on the effect of oxygen exposure on the operation and degradation of highly efficient PffBT4T-2OD:PC 71 BM photovoltaic devices. Ultrafast pump−probe transient absorption (TA) measurements and ultrasensitive photothermal deflection spectroscopy (PDS) in combination with field-effect transistors suggest that oxygen-induced doping of the active layer is responsible for the severe degradation of the photovoltaic performance. We find that light exposure further accelerates this effect without causing photo-oxidation of the materials.

Impact of the Electron Acceptor Nature on the Durability and Nanomorphological Stability of Bulk Heterojunction Active Layers for Organic Solar Cells

“…[ 24 ] Several studies suggest that, independently of the mechanism involved in the decrease of photovoltaic parameters, simultaneous exposure to oxygen and light induces a faster drop in performances compared to individual exposure to only one of these degradation agents. [ 27,28 ] Conjugated polymers such as PBDB‐T are often thought to be prone to permanent rupture of the conjugation through chemical degradation. However, as the J sc of PBDB‐T:PC 71 BM OSCs exhibits a relatively stable value during the 45 min experimental timeframe (Figure 3a), such chemical degradation and the resulting photobleaching of PBDB‐T should be negligible during our light exposure experiments.…”

Energy Transfer to a Stable Donor Suppresses Degradation in Organic Solar Cells

“…The chemical structures of DRCN5T and PC 70 BM, along with the schematic inverted architecture structure of the investigated solar cells are illustrated in Figure a. Although reports highlighting problems with the common extraction layers MoO x /Ag exist, this structure with high work function contacts was chosen to minimize degradation effects by the electrodes and allowed us to focus on the processes affecting the active layer . In this architecture, cesium‐doped zinc‐oxide (ZnO:Cs) served as electron transport/hole blocking layer and molybdenum‐oxide (MoO x ) as hole transport/electron blocking layer; indium tin oxide (ITO) and silver acted as the contacts.…”

“…To investigate the stability of this SM system upon exposure to oxygen and light, the performances of 24 DRCN5T:PC 70 BM solar cells were monitored for 22 h under continuous illumination conditions of 1 Sun and a controlled atmosphere of 20% O 2 in a bespoke environmental setup . This setup ensures a constant flow of gas to minimize heating of the substrates with the temperature of the gas being continuously evaluated.…”

“…After a pump‐probe delay of 300 ps this trend inverts as a marginally larger exciton amplitude with increasing degradation is observed. We speculate that the initial reduction of the signal followed by an increase at later times corresponds to the formation of trap states as a consequence of degradation, efficiently quenching the excitons and creating long‐lived, trapped charges, previously observed in systems containing PC 70 BM . Accordingly, the spectrum between 800 nm and 1250 nm changes gradually during exposure to oxygen and light (see inset Figure b), indicating the formation of a second species, likely to be trapped carriers.…”

“…Under illumination, polymer solar cells typically show a strong initial decrease in device performance (“burn‐in”), caused by formation of defects in the bandgap . In addition, exposure to oxygen leads to p‐type doping in some polymer systems, which causes increased bimolecular recombination or formation of traps which in turn impede charge generation and transport . The combined influence of oxygen and light induces irreversible photo‐oxidation in many polymers through reactions involving singlet oxygen or radical superoxide, causing chemical decomposition and deactivation of the chromophores .…”