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Chen L.,Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energy | Chen L.,Southwest University | Zhang Q.,Southwest University | Lei Y.,Southwest University | And 7 more authors.
Physical Chemistry Chemical Physics | Year: 2013

In this work, we report our effort to understand the photocurrent generation that is contributed via electron-exciton interaction at the donor/acceptor interface in organic solar cells (OSCs). Donor/acceptor bi-layer heterojunction OSCs, of the indium tin oxide/copper phthalocyanine (CuPc)/fullerene (C60)/molybdenum oxide/Al type, were employed to study the mechanism of photocurrent generation due to the electron-exciton interaction, where CuPc and C60 are the donor and the acceptor, respectively. It is shown that the electron-exciton interaction and the exciton dissociation processes co-exist at the CuPc/C60 interface in OSCs. Compared to conventional donor/acceptor bi-layer OSCs, the cells with the above configuration enable holes to be extracted at the C60 side while electrons can be collected at the CuPc side, resulting in a photocurrent in the reverse direction. The photocurrent thus observed is contributed to primarily by the charge carriers that are generated by the electron-exciton interaction at the CuPc/C60 interface, while charges derived from the exciton dissociation process also exist at the same interface. The mechanism of photocurrent generation due to electron-exciton interaction in the OSCs is further investigated, and it is manifested by the transient photovoltage characteristics and the external quantum efficiency measurements. © 2013 The Owner Societies.


Liu H.,Hong Kong Baptist University | Wu Z.,Hong Kong Baptist University | Hu J.,Yunnan University | Song Q.,Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energy | And 5 more authors.
Applied Physics Letters | Year: 2013

High performance inverted bulk heterojunction organic solar cells (OSCs), based on the blend of poly[[4,8-bis[(2-ethylhexyl)oxy] benzo [1,2-b:4,5-b′] dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl] thieno[3,4-b]thiophenediyl]] (PTB7): 3′H-Cyclopropa[8,25] [5,6]fullerene-C70-D5h(6)-3′-butanoicacid, 3′-phenyl-, methyl ester (PC70BM), were achieved using an aluminum-doped zinc oxide (AZO) front transparent cathode. A structurally identical PTB7:PC70BM-based OSC having an indium tin oxide (ITO) front cathode was also made for comparison studies. The surface of AZO and ITO was modified with a 10 nm thick solution-processed ZnO interlayer to facilitate the efficient electron extraction. This work yielded AZO-based OSCs with a promising power conversion efficiency of 6.15%, slightly lower than 6.57% of a control ITO-based OSC, however, a significant enhancement in the stability of AZO-based OSCs was observed under an ultraviolet (UV)-assisted acceleration aging test. The distinctive enhancement in the lifetime of AZO-based OSCs arises from the tailored absorption of AZO electrode in wavelength <380 nm, serving as a UV filter to inhibit an inevitable degradation in ITO-based OSCs caused by the UV exposure. © 2013 AIP Publishing LLC.


Li P.,Southwest University | Li P.,Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energy | Wang G.,Southwest University | Wang G.,Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energy | And 18 more authors.
Physical Chemistry Chemical Physics | Year: 2014

In this work, we investigate the effect of the thickness of the polyethylenimine ethoxylated (PEIE) interface layer on the performance of two types of polymer solar cells based on inverted poly- (3-hexylthiophene) (P3HT):phenyl C61-butryric acid methyl ester (PCBM) and thieno[3,4-b]thiophene/ benzodithiophene (PTB7):[6,6]-phenyl C71-butyric acid methyl ester (PC71BM). Maximum power conversion efficiencies of 4.18% and 7.40% were achieved at a 5.02 nm thick PEIE interface layer, for the above-mentioned solar cell types, respectively. The optimized PEIE layer provides a strong enough dipole for the best charge collection while maintaining charge tunneling ability. Optical transmittance and atomic force microscopy measurements indicate that all PEIE films have the same high transmittance and smooth surface morphology, ruling out the influence of the PEIE layer on these two parameters. The measured external quantum efficiencies for the devices with thick PEIE layers are quite similar to those of the optimized devices, indicating the poor charge collection ability of thick PEIE layers. The relatively low performance of devices with a PEIE layer of thickness less than 5 nm is the result of a weak dipole and partial coverage of the PEIE layer on ITO. © the Owner Societies 2014.


Li P.,Southwest University | Li P.,Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energy | Cai L.,Southwest University | Cai L.,Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energy | And 12 more authors.
Synthetic Metals | Year: 2015

In this work, a cathode buffer layer (CBL) with higher charge transfer ability was fabricated by using polyethylenimine ethoxylated (PEIE) covered ZnO nanoparticles (NPs) (ZnO/PEIE). A high power conversion efficiency of 3.8% was achieved for an inverted polymer solar cell based on poly(3-hexylthiophene) (P3HT):phenyl C61-butryricacid methyl ester (PCBM) by using this ZnO/PEIE CBL, which is much higher than that of other cells with only PEIE or ZnO CBL. Transient photovoltage/photocurrent and electrochemical impedance measurements confirm that device with ZnO/PEIE bilayer has faster charge transfer ability and less interfacial charge recombination than devices with other CBLs. AFM characterization shows that ZnO NPs are uniformly covered by PEIE, which would be not only lower the work function of ZnO, but also be beneficial to reduce defects caused by oxygen adsorption on ZnO. © 2015 Elsevier B.V.All rights reserved.


Zhang Y.J.,Southwest University | Zhang Y.J.,Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energy | Li P.,Southwest University | Li P.,Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energy | And 8 more authors.
RSC Advances | Year: 2015

In solar cells, a maximum external quantum efficiency of 100% can be attained if the photocurrent originates from the dissociation of singlet excitons. However, a higher efficiency can be attained through singlet fission (SF), where multiple charge carrier pairs are generated from a single photon, thus increasing the number of excitons and hence the photocurrent generation. The verification of SF is normally difficult and costly. In this study, SF is verified in pentacene by simply measuring the external quantum efficiency (EQE) of a device with a very thick pentacene (indium tin oxide/poly(3,4-ethylenedioxythiophene):poly(4-styrenesulphonate) (PEDOT:PSS) (40 nm)/pentacene (600 nm)/fullerene (40 nm)/tris-8-hydroxy-quinolinato aluminum (8 nm)/Al). A measured EQE of 6.16% at 695 nm is achieved, which is much larger than the maximum calculated EQE (3.45%). The calculation was based on the assumption that all singlet excitons (if SF did not occur) reaching the pentacene-C60 interface contribute to the photocurrent. To account for this discrepancy, only singlet fission to double the number of excitons can be supposed, since a longer singlet diffusion length of 140 nm is not practical in pentacene. Thus, SF in pentacene and the dissociation of triplet excitons at the pentacene-C60 interface have been verified. This journal is © The Royal Society of Chemistry 2015.

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