Polymer inverted device structures

Inverted Device Structures The conventional device structure for PSCs is indium tin oxide (ITO)/PEDOT PSS/polymer blend/Al, where a conductive high-work-function PEDOTPSS layer is used for anode contact, and a low-work-function metal as the cathode. Both the PEDOTPSS layer and the low-work-function metal cathode can cause the degradation of PSCs [110-112]. The acidic PEDOTPSS was reported to etch the ITO and cause interface instability through indium diffusion into the polymer active layer. Low-work-fiinction metals, such as calcium and aluminum, are easily oxidized when exposed to air, increasing the series resistance at the metal/BHJ interface and degrading device performance. 

He Z, Zhong C, Su S et al (2012) Enhanced power-conversion efficiency in polymer solar cells using an inverted device structure. Nat Photon 6(9) 591-595... 

Yang T, Cai W, Qin D, Wang E, Lan L, Gong X, Peng J, Cao Y (2010) Solution-processed zinc oxide thin film as a buffer layer for polymer solar cells with an inverted device structure. JPhys Chem C 114 6849-6853. doi 10.1021/jpl003984... 

Transparent polymer solar cells (i.e., polymer solar cells with transparent electrodes) can be easily fabricated based on inverted architecture and have important application in tandem architectures as well. We can form transparent solar cells by replacing the Al top electrode with 12 nm Au in the inverted structure. The J-V curves for this transparent polymer solar cell, with light incident from ITO and Au side, are shown in Figure 11.17. The difference between the two J-V curves is due to the partial loss by the reflection and absorption at the semitransparent Au electrode. To provide sufficient electrical conductance, Au layer thickness has to be sufficient and the optical loss at Au electrode becomes significant. However, the inverted solar cell structure has the V2O5 layer which is not only transparent but also provides effective protection to the polymer layer. A transparent conductive oxides electrode, such as ITO, can therefore be deposited without compromising device performance. 

It must be considered the work function reduction was closely relative to the thickness of the polymer interlayer. For example, the work function of ZnO increased from 2.47 eV (4 nm PEI) to 3.39 eV (16 nm PEI). This was because with the increasing polymer interlayer thickness, the random molecule dipoles coimtervailed with each other and thus weakened the interfacial dipole effect between ZnO and the interlayer. When polymer interlayer was used in the inverted structure, the device performance was obviously enhanced. Fig. 7.4D demonstrates the L-Vcurves of PLEDs. The tum-on voltage (voltage at luminance of 1 cdm and... 

Studies of conjugated polymer-porous metal oxide (i.e., TiOa) structures have focused on optimization of the layer structure of the device and choice of polymer materials. By optimization of the device thickness and electrode choice, groups achieved photocurrent external quantum efficiency (EQE) values of 10,23, and 40 % [36-38]. The reduction of the polymer and TiOa layer thicknesses to the region of 50-100 nm is a key step for this optimization. In polymer-porous metal oxide structures, the porous metal oxide acceptor layer is normally deposited on top of a TCO substrate (i.e., ITO), the polymer is then fabricated on the metal oxide, and an evaporated metal is deposited on top of the active layers as a hole collecting contact. This polarity is the opposite of most commonly reported organic solar cell structures, so the hybrid cell is also called inverted organic solar cell. ... 

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