Surface Modification of ZnO Layers via Hydrogen Plasma Treatment for Efficient Inverted Polymer Solar Cells

Abstract

Modifications of the ZnO electron extraction layer with low-pressure H plasma treatment increased the efficiency of inverted polymer solar cells (PSCs) based on four different photoactive blends, namely, poly(3-hexylthiophene):[6,6]-phenyl C71 butyric acid methyl ester (P3HT:PC71BM), P3HT:1',1″,4',4″-tetrahydro-di[1,4]methanonaphthaleno-[5,6]ullerene-C60 (P3HT:IC60BA), poly[(9-(1-octylnonyl)-9H-carbazole-2,7-diyl)-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl]:PC71BM (PCDTBT:PC71BM), and (poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-(2-ethylhexy)carbonyl]thieno[3,4-b]thiophenediyl]]):PC71BM (PTB7:PC71BM), irrespective of the donor:acceptor combination in the photoactive blend. The drastic improvement in device efficiency is dominantly attributable to the reduction in the work function of ZnO followed by a decreased energy barrier for electron extraction from fullerene acceptor. In addition, reduced recombination losses and improved nanomorphology of the photoactive blend in the devices with the H plasma treated ZnO layer were observed, whereas exciton dissociation also improved with hydrogen treatment. As a result, the inverted PSC consisting of the P3HT:PC71BM blend exhibited a high power conversion efficiency (PCE) of 4.4%, the one consisting of the P3HT:IC60BA blend exhibited a PCE of 6.6%, and our champion devices with the PCDTBT:PC71BM and PTB7:PC71BM blends reached high PCEs of 7.4 and 8.0%, respectively.

Improved Stability of Polymer Solar Cells in Ambient Air via Atomic Layer Deposition of Ultrathin Dielectric Layers

“…In particular, the estimated exciton lifetimes of PTB7 deposited Figure S15, Supporting Information) verified that charge separation occurring at the ZnO/Al 2 O 3 /P3HT interfaces represents a phenomenon predominantly governed by the suppression of the intrinsic deep defects in ZnO. [34] Finally, all findings in this study explicitly confirm the potential of ALD architecture in regulating the ZnO properties and defect states and verify the significance of ALD-controlled interface engineering in photovoltaic and other optoelectronic applications.…”

“…This has the net result of reducing the energy offset between the LUMO of P3HT and the conduction band of ALD-modified ZnO promoting fast, energetically downhill, electron transfer from P3HT to ALD dielectric-coated ZnO, therefore, reducing the exciton lifetime. [34,72] Identically, the simultaneous suppression of defect-induced hole trapping and enhancement of electron transfer in ALD-coated ZnO granted minor deviations in exciton lifetimes of 40 nm thick PTB7 films deposited on ZnO substrates; this was estimated from TRPL measurements for excitation at 468 nm and detection at 790 nm (Figure 6c; Table S3, Supporting Information) and 830 nm (Figure 6d; Table S4, Supporting information). In particular, the estimated exciton lifetimes of PTB7 deposited Figure S15, Supporting Information) verified that charge separation occurring at the ZnO/Al 2 O 3 /P3HT interfaces represents a phenomenon predominantly governed by the suppression of the intrinsic deep defects in ZnO.…”

“…[22][23][24][25][26][27][28] Development of doped ZnO-based materials with a variety of different dopants, such as aluminum, indium, cesium, nitrogen, and hydrogen, has also been shown to improve the efficiency and stability of the fabricated cells. [29][30][31][32][33][34] On the other hand, atomic layer deposition (ALD) of dielectric oxides has been established as a viable and effective approach for the surface passivation of several metal oxides (including ZnO) in order to improve their optoelectronic properties; [35,36] it has also been implemented in silicon solar cell technology. [37] Recently, ALD dielectric oxides have been considered as promising candidates for reducing undesirable surface recombination in dye-sensitized and perovskite solar cells.…”

Surface passivation effect by fluorine plasma treatment on ZnO for efficiency and lifetime improvement of inverted polymer solar cells

“…[48][49][50][51][52] Besides the metal and also nitrogen doping which has also been reported to tune the properties of the zinc oxide surface, 53 our group and others recently introduced the beneficial surface modification of ZnO by using a hydrogen (H) plasma treatment for efficient inverted PSCs. 54,55 Plasma surface treatment is a process that alters the surface energy of many materials so as to improve the bonding characteristics. In our previous work it was found that H plasma treatment altered the surface properties of ZnO films, in particular the wetting of the photoactive layer, while it also reduced surface roughness and induced significant changes in the nanomorphology/crystallinity of the photoactive blend, which was deposited on top of the ZnO surface, allowing efficient charge generation and extraction.…”

Defect Restoration of Low‐Temperature Sol‐Gel‐Derived ZnO via Sulfur Doping for Advancing Polymeric Schottky Photodiodes

“…However, ZnO thin films derived via the sol‐gel method have more unreacted organic residuals and other chemical defects than those derived via other high‐temperature methods such as magnetron sputtering, spray pyrolysis, or molecular beam epitaxy . Therefore, various surface treatment methods have been developed to decrease the surface defects of solution‐processed ZnO and realize high‐performance organic diodes . The representative examples introduce nanoparticle ZnO doped with thiophene, sulfur, or Cs in colloid phases, which result in enhanced organic photovoltaic performances as the electron transport layer.…”