Influence of Environmentally Affected Hole-Transport Layers on Spatial Homogeneity and Charge-Transport Dynamics of Organic Solar Cells

Abstract

After organic photovoltaic (OPV) cells achieved efficiency of more than 10%, the control of stability and degradation mechanisms of solar cells became a prominent task. The increase of device efficiency due to incorporation of a hole-transport layer (HTL) in bulk-heterojunction solar cells has been extensively reported. However, the most widely used HTL material, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), is frequently suspected to be the dominating source for device instability under environmental conditions. Thereby, effects like photooxidation and electrode corrosion are often reported to shorten device lifetime. However, often in environmental device studies, the source of degradation, whether being from the HTL, the active layer, or the metal cathode is rather difficult to distinguish because the external diffusion of oxygen and water affects all components. In this study, different HTLs, namely, those prepared from traditional PEDOT:PSS and also two types of molybdenum trioxide (MoO) are exposed to different environments, such as oxygen, light, or humidity, prior to device finalization under inert conditions. This allows investigating any effects within the HTL and from reactions at its interface to the indium tin oxide electrode or the active layer. The surface and bulk chemistry of the exposed HTL has been monitored and discussed in context to the observed device physics, dynamic charge transport, and spatial performance homogeneity of the corresponding OPV device. The results show that merely humidity exposure of the HTL leads to decreased device performance for PEDOT:PSS, but also for one type of the tested MoO. The losses are related to the amount of absorbed water in the HTL, inducing loss of active area in terms of interfacial contact. The device with PEDOT:PSS HTL after humid air exposure showed seriously decreased photocurrent by microdelamination of swelling/shrinkage of the hygroscopic layer. The colloidal MoO with water-based precursor solution presents slight decay of solar cell performance, also here caused by swelling/shrinking reaction, but by a combination of in-plane particle contact and resistance scaling with particle expansion. However, the device with quasi-continuous and alcohol-based MoO showed unharmed stable electrical performance.

Solution‐Processable 2D α‐In2Se3 as an Efficient Hole Transport Layer for High‐Performance and Stable Polymer Solar Cells

“…Poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is the commonly used HTL in conventional PSCs to facilitate hole transporting, which can be attributed to its high transmittance, appropriate work function (WF), and smooth surface morphology . However, due to its strong acidity, it easily corrodes the indium tin oxide (ITO) substrate or reacts with BHJ, and its hygroscopicity is also detrimental to the stability of devices . To avoid these shortcomings, some transition metal oxides (TMOs), such as MoO 3 , V 2 O 5 , and NiO x , have been used to replace PEDOT:PSS .…”

“…[11][12][13][14][15] However, due to its strong acidity, it easily corrodes the indium tin oxide (ITO) substrate or reacts with BHJ, and its hygroscopicity is also detrimental to the stability of devices. [11,12] To avoid these shortcomings, some transition metal oxides (TMOs), such as MoO 3 , V 2 O 5 , and NiO x , have been used to replace PEDOT:PSS. [16][17][18] Nevertheless, these materials are usually deposited under high vacuum, which is not only of high cost but also not compatible with roll-to-roll technology.…”

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

“…As the solution process offers simplified device fabrication, with potentially lower processing costs than vacuum‐assisted techniques, various strategies to develop solution‐processed MoO 3 (SM) have been investigated, typically involving the use of ammonium heptamolybdate tetrahydrate as the precursor, powder‐based MoO 3 on the hydrogen peroxide solvent, or MoO 3 nanoparticles . These methods are generally sensitive to the water content, which can deteriorate the device stability . The device performance of OSCs based on SM now approaches that of PEDOT:PSS‐based devices, and recently, a doping approach for the versatile application of SM was successfully demonstrated in highly efficient OSCs using blade‐coating printing techniques .…”

“…[20][21][22][23][24][25][26][27][28][29] These methods are generally sensitive to the water content, which can deteriorate the device stability. [30] The device performance of OSCs based on SM now approaches that of PEDOT:PSS-based devices, and recently, a doping approach for the versatile application of SM was successfully demonstrated in highly efficient OSCs using blade-coating printing techniques. [29] However, research endeavors on device stability in SM-based OSCs are considerably lacking, especially for high-efficiency nonfullerene-based OSCs (NF-OSCs).…”

Role of Central Metal Ions in 8‐Hydroxyquinoline‐Doped ZnO Interfacial Layers for Improving the Performance of Polymer Solar Cells

“…Major benefits of OSCs include potentially low cost, compatibility with large‐area production methods, a lightweight structure, and a relatively short energy payback time . Previous studies have conducted the intensive research to extend the OSCs absorption range by modifying their photoactive layer, which has comprised highly functional novel donor, and the acceptor materials . New interfacial device architectures with appropriate doping materials have been explored for future commercial applications …”