Environmental Engineering Reference
In-Depth Information
LWF metals as cathode are easily oxidized, causing dark spots and degrading
devices very fast. One way is to use inverted structure as mentioned before.
The other way is to develop advanced encapsulation technology to protect the
devices from moisture and oxygen.
6.5.2.2 Photoactive Materials
Organic semiconductors are easily oxidized by oxygen, especially under
illumination. However, OPVs must be exposed to solar light during operation.
Therefore, it is challenging to synthesize and develop air-stable organic materials
with deep HOMO levels and low band gaps.
6.5.2.3 Device Design
BHJ structure offers an interpenetrating network for charge transport, which is
controlled by annealing, solvents, additives, etc. The morphology is changeable by
varying the annealing temperature and processing time, i.e., not at the thermo-
dynamic equilibrium. Thus, it is necessary to exploit an effective and stable
morphology beneficial to both efficiency and stability, providing bi-continuous
pathways for charge transport in their individual phase. More importantly, it
should be temperature and light stable, especially under illumination when the
device is heated up.
On the other hand, inverted structure facilitates the stability improvement due
to the reverse of anode and cathode, where HWF metal as anode at top could
sustain the oxidization.
6.5.2.4 Encapsulation
Low-cost encapsulation technology leads to limited protection against oxygen and
water. Therefore, a proper and powerful encapsulation is necessary to further
improve the device stability. Encapsulation of alternating inorganic-organic
multilayer actually serves as a high barrier, fulfilling the requirements for flexible
OPVs. The encapsulating layer should be possessed with easy process, light
weight, no defects, and flexibility.
In summary, all the factors influencing device stability should be emphasized in
order to prolong device lifetime and achieve commercial products. Taking various
interface parameters and factors into account, a much more comprehensive model
should be established by improving previous models mentioned before. Deep
insight and knowledge into degradation mechanism, which could disclose the
relationship between degradation mechanisms and material design, preparation
technique, heat effect during operation, annealing treatment, and encapsulation, are
significantly beneficial to optimization of organic material design and device
stability.
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