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Carrier transport layer

Michael Corazza Dechan Angmo

ETL degradation

The electron transport layer (ETL) is a layer which has a high electron affinity and high electron mobility. These characteristics allow electrons to flow across the layer, while holes are “blocked” and cannot go through. Several different materials have been used for this task, in order to optimize both the performance and the stability.

One of the first ETL used was lithium fluoride (LiF) which has a good band alignment with adjacent layers and also a good stability with the cathode interface.DOI:10.1126/science.270.5243.1789DOI:10.1002/adma.200501825 The addition of C60 to LiF has recently proved an increase on current density and lifetime.DOI:10.1002/adma.201002738DOI:10.1063/1.3297876 TiOx and CrOx have shown to cause an increase of stability, with especially the former enhancing the stability toward both UV and oxygen.DOI:10.1063/1.2402890DOI:10.1063/1.2885724DOI:10.1016/j.solmat.2010.12.030 An analogue increase of stability was proven with the use of CuOx alone or together with LiF.DOI:10.1063/1.3580629 A very thin layer (1 nm) of lithium benzoate has also proven to increase the performance and the stability.DOI:10.1016/j.solmat.2011.01.012 Moreover phosphine oxide 2,7-bis (diphenylphosphoryl)-9,9-spirobi(fluorine) was used to increase thermostability.DOI:10.1016/j.solmat.2011.06.029

Later a study showed that Cs2CO3 was more stable than LiFDOI:10.1016/j.solmat.2010.05.043, and that an interface of 5 nm of CdSe slowed down the degradation further.DOI:10.1021/jp906277k

One of the most common materials used as ETL is ZnO due to its high electron mobility.DOI:10.1063/1.2359579 It is also compatible to R2R process since it can be coated from a precursor solution of zinc acetate or nanoparticles from water or alcohol solution. The mobility of the zinc oxide can be changed with doping and UV curing.DOI:10.1063/1.1992666DOI:10.1116/1.1714985 Oxygen radicals are in fact bounded to ZnO film, which can be removed by UV irradiation, thus increasing the mobility.DOI:10.1039/B9NR00430K In the dark oxygen slowly re-bounds. Further, it has been shown that using ZnO as a ETL, the lifetime of the cell increases compared to a cell without ZnO.

HLT degradation: PEDOT:PSS

Poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) is the most commonly used hole transport layer in both inverted and normal devices. It is a polymeric salt with PEDOT in its oxidized state carrying a positive charge and PSS having a deprotonated sulfonyl group carrying a negative charge (Figure 1). While PEDOT:PSS is a water soluble and easily printable/coatable material, it imposes major limitation to the stability of OPV devices.

Figure 1: Chemical structure of PEDOT:PSS.

PEDOT:PSS is a highly hygroscopic material which can retain a large fraction of water (10-15 wt%).DOI:10.1016/j.orgel.2008.05.006Elschner, A., Kirchmeyer, S., Lovenich, W., Merker, U., & Reuter, K. (2010).PEDOT: principles and applications of an intrinsically conductive polymer. CRC Press. The electronic properties of PEDOT:PSS display a dynamic behavior in the presence of humidity, temperature, and irradiation. To learn more about the properties of PEDOT:PSS, consult other literatureDOI:10.1016/j.orgel.2008.05.006Elschner, A., Kirchmeyer, S., Lovenich, W., Merker, U., & Reuter, K. (2010).PEDOT: principles and applications of an intrinsically conductive polymer. CRC Press.. While water uptake may impact the inherent electronic properties of PEDOT:PSS, the most pronounced effect of water uptake by PEDOT:PSS embedded in a solar cell is observed on the surrounding materials and interfaces. As written under the metal electrode section, the hygroscopic nature of PEDOT:PSS act as water reservoir channeling it throughout the device. In normal devices, water uptake by PEDOT:PSS results in increase resistivity at the PEDOT:PSS/photoactive interfaceDOI:10.1016/j.solmat.2006.06.041 while others have found that water uptake of PEDOT:PSS result in transmission of water to aluminum/active interface, thus resulting in oxidation of aluminum at this interfaceDOI:10.1016/j.orgel.2011.01.025DOI:10.1016/j.solmat.2010.09.007. As a result, the replacement of PEDOT:PSS with non-hygroscopic materials such as evaporated MoO3DOI:10.1016/j.orgel.2011.01.025DOI:10.1016/j.solmat.2010.09.007, sputtered NiODOI:10.1016/j.solmat.2010.10.014, WO3-V2O5DOI:10.1109/LED.2009.2039846 has been shown to significantly enhance stability.

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