Solution Processed Vanadium Pentoxide as Charge Extraction Layer for Organic Solar Cells

Abstract

Organic solar cells (OSCs) based on polymers and small molecules have seen a tremendous increase in interest during the past few years. Signifi cant progress in this fi eld seeded the prospect for a cost-effective and easy-to-fabricate photovoltaic technology-typical advantages claimed for organic (opto-)electronic devices. Very recently, certifi ed cell effi ciencies in excess of 7% have been reported for polymer based cells. [ 1 ] For large-scale and high-throughput production of OSCs, liquid processing of the functional layers is desirable. Aside from the active organic layers, inter-layers are typically required to facilitate the extraction of the photo-generated charges. Specifi cally, on the anode side, polyethylene dioxythiophene:polystyrenesulfonate (PEDOT:PSS) is regularly used. [ 2 ] However, PEDOT:PSS is burdened with structural and electrical inhomogeneity [ 3,4 ] and has been demonstrated to be an origin of limited device lifetime. [ 5 ] Particularly, the aqueous PEDOT:PSS dispersion and the acidic nature can cause substantial degradation. [6,7 ] Very recently, transition metal-oxides (TMOs) such as molybdenum-, vanadium-, or tungsten-oxide (MoO 3 , V 2 O 5 , and WO 3 ) with high work functions (WFs) of up to 6.9 eV have been shown to be promising alternatives to PEDOT:PSS. [8][9][10][11] TMOs have also been used as constituents of the connecting architecture in stacked organic light-emitting diodes and organic tandem solar cells. [12][13][14][15] The unique energetics of these TMOs has so far been predominantly accessible for fi lms thermally evaporated in high-vacuum.The fi rst results for TMO layers obtained by solution processing from nano-particle (NP) dispersions have been reported only very recently. [ 16,17 ] Meyer et al. prepared MoO 3 layers by dispersing MoO 3 NPs using a polymer as dispersing agent. After deposition, the layers had to be treated by an oxygen plasma to remove the polymer. A high WF of the resulting layers of 5.7-6 eV was obtained. A substantial drawback of the approach, however, is the observation of larger NP aggregates with a size of 100 nm and an overall high roughness of 25 nm (rms). Owing to their roughness these NP-layers are critical sources of shorts, especially over a large device area.In contrast, TMO layers (WO 3 , V 2 O 5 and MoO 3 ) have been prepared by sol-gel deposition, predominantly for electrochromic, catalytic and sensing applications. [18][19][20][21] Post processing of the sol-gel TMO layers at high temperatures (300 ° C-600 ° C) is routinely applied in order to achieve specifi c microstructures or crystalline phases in the materials, as required by the particular application. These high processing temperatures are not compatible with the temperature-sensitive substrates (e.g. poly mer foils) envisaged for low-cost, high-throughput fabrication of organic solar cells. In spite of this limitation, Steirer et al. have very recently used NiO prepared via a sol-gel route as a replacement for PEDOT:PSS in an organic solar cell. [ 22 ] The requirement of pos...

Origin of Work Function Modification by Ionic and Amine‐Based Interface Layers

“…Deposition of V 2 O 5 was performed similarly as described in Ref. [ 27 ] . Approximately 10 nm thick organic semiconductor or insulator fi lms were deposited on top by spin coating from solution: [6,6]-phenyl-C 70 -butyric acid methyl ester (PC 70 BM, Solenne BV), poly(3-hexylthiophene) (P3HT, Aldrich, M n = 54 000-75 000 g mol −1 ) and a blend of PC 70 BM and poly[2,6-(4,4-bis-…”

“…[ 27 ] , semiconducting (intrinsic), or insulating nature was measured as shown in Figure 3 and Table 1 . The qualitative criterion for the classifi cation of these substrate materials is based on the ability to provide charges, which is proportional to the dielectric response of the substrate material to the WML.…”

Recent Advances in Nanostructured Vanadium Oxides and Composites for Energy Conversion

“…[232] Isopropanol and vanadium(V) oxitriisopropoxide solution was used to prepared 10-to 45-nm-thick V 2 O 5 layers via hydrolysis at ambient condition without any post-treatments (Figure 17a). [233] The work function (WF) of 5.6 eV was achieved, and shown even better characteristics than PEDOT:PSS (Figure 17b). Figure 17c shows that cells with 10-nm-thick V 2 O 5 layers have the optimized power conversion efficiency (PCE).…”

Metal oxide semiconducting interfacial layers for photovoltaic and photocatalytic applications

“…In comparison with MoO 3 , research on V 2 O 5 is rudimentary. For a better understanding of the charge transport mechanism at the V 2 O 5 -polymer interface, additional investigations of the interface electronic structures are indispensable [43,104,105]. The inverted OSC devices fabricated with PCDTBT in the active layer along with TiO 2 as the electron transport layer (ETL) and MoO 3 as the hole transfer layer (HTL) exhibits high Voc of about 91 % with a PCE of 7.2 % as shown in Table 1, owing to low band gap of the polymer and efficient charge transport across the interface [98].…”