Progress in Understanding Degradation Mechanisms and Improving Stability in Organic Photovoltaics

Abstract

Understanding the degradation mechanisms of organic photovoltaics is particularly important, as they tend to degrade faster than their inorganic counterparts, such as silicon and cadmium telluride. An overview is provided here of the main degradation mechanisms that researchers have identified so far that cause extrinsic degradation from oxygen and water, intrinsic degradation in the dark, and photo-induced burn-in. In addition, it provides methods for researchers to identify these mechanisms in new materials and device structures to screen them more quickly for promising long-term performance. These general strategies will likely be helpful in other photovoltaic technologies that suffer from insufficient stability, such as perovskite solar cells. Finally, the most promising lifetime results are highlighted and recommendations to improve long-term performance are made. To prevent degradation from oxygen and water for sufficiently long time periods, OPVs will likely need to be encapsulated by barrier materials with lower permeation rates of oxygen and water than typical flexible substrate materials. To improve stability at operating temperatures, materials will likely require glass transition temperatures above 100 °C. Methods to prevent photo-induced burn-in are least understood, but recent research indicates that using pure materials with dense and ordered film morphologies can reduce the burn-in effect.

Burn‐in Free Nonfullerene‐Based Organic Solar Cells

“…[3,4] However, even in the absence of these extrinsic factors, i.e., in the case of firmly packaged devices, short-and long-term photovoltaic performance loss is still observed under operation. [8] The effect may deplete the initial power conversion efficiency (PCE) by as much as 20%-60%, [9] depending on the material system. [7] In organic polymer based solar cells, the burn-in period reflects an early, near to exponential photovoltaic performance roll-off.…”

“…[7] In organic polymer based solar cells, the burn-in period reflects an early, near to exponential photovoltaic performance roll-off. [9,10] For instance, light-induced burn-in has been experimentally and theoretically correlated, although to different extent, with photochemical reactions, [11] critical concentrations of chemical and metal impurities, [12,13] molecular weight distribution, [14] degree of crystallinity, [15] crosslinking, [16] processing additives, [17] and the formation of longlived radicals. It typically occurs so rapidly that it is generally accepted to specify the lifetime of the device post burn-in phase, i.e., neglecting the early loss in performance.…”

Reducing Burn‐In Loss of Organic Photovoltaics by a Robust Electron‐Transporting Layer

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efficiency (PCE) of OPVs has already reached 14.2%. [2][3][4][5][6] The origin of the burn-in loss is thought to be mainly related to the instability of the bulk heterojunction (BHJ) morphology and/or interface rather than the photo-oxidation of the photoactive layer.Morphological instability of photoactive layer is one of critical issues of burn-in loss in OPV. Especially, OPVs suffer from a rapid decrease in PCE during initial device operation, which is known as the "burn-in loss."

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“…[12] The interlayers minimize the energy barrier between the photoactive layer and electrodes, and thereby enhance the collection efficiencies of electrons/holes on the cathode/ anode. [5,18] Additionally, transition metal oxide ETLs provide an excellent barrier property against oxygen and metal electrode diffusion compared to organic ETL materials. [17] Among various ETL materials, transition metal oxides are commonly utilized because of their adequate highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels, stability, transparency, and excellent electron mobility.…”

“…Thus, significant efforts have been made to develop various electron-transporting layer (ETL) materials for OPVs such as transition metal oxides, [13] carbon-based materials, [14] polymers, [15] low work function metal salts, [16] and organicinorganic hybrids. [5,18] Additionally, transition metal oxide ETLs provide an excellent barrier property against oxygen and metal electrode diffusion compared to organic ETL materials. [5,18] Additionally, transition metal oxide ETLs provide an excellent barrier property against oxygen and metal electrode diffusion compared to organic ETL materials.…”