Enhanced Charge Separation by Sieve-Layer Mediation in High-Efficiency Inorganic-Organic Solar Cells By Chien-Hung Lin, Surojit Chattopadhyay, Chia-Wen Hsu, Meng-Hsiu Wu, Wei-Chao Chen, Chien-Ting Wu, Shao-Chin Tseng, Jih-Shang Hwang, Jiun-Haw Lee, Chun-Wei Chen, Cheng-Hsuan Chen, Li-Chyong Chen, * and Kuei-Hsien Chen* Classical inorganic photovoltaic (PV) devices lose out on their cost efficiency to answer the global energy crisis with the renewable energy option. Although new-generation organic photovoltaic (OPV) devices cannot match their predecessors in terms of efficiency (h) or lifetime, they provide an ecological and economical alternative. The advent of fullerenes (C 60 ) had a major contribution in increasing h from 0.95% [1] to 3.6% [2] in an OPV device. The most-widely studied OPV device is based on mixtures of conjugated polymer-fullerenes or hybrid organic-C 60 . In low-weight molecular systems, an h of 5.0% in hybrid organic-C 60 heterojunction PV cells using Cu phthalocyanine (CuPc) was reported. [3] To date, phenyl-C 60 -butyric acid methyl ester (PCBM) with conjugated polymers has become the standard for the fabrication of plastic solar cells with a high efficiency of 4.4% to 5.0%. [4,5] The OPV devices rely on the generation of excitons and their diffusion to the donor/acceptor (DA) interface, followed by the collection of dissociated carriers. The thickness of the active region should ideally match the exciton diffusion length (L d ) which is generally <40 nm in zinc phthalocyanine (ZnPc) and <70 nm for CuPc. [6] This immediately limits the h of the OPVs in the sense that light absorption and exciton production is inefficient at such small thicknesses. A better device architecture could lead to efficient light trapping and carrier collection. Efforts in this direction resulted in double-heterostructure [7] OPVs, the use of long L d in C 60 , [2] bulk [8,9] and mixed [10,11] heterojunction materials, and multi-heterojunction or tandem cells. [12,13] Dis- ordered bulk heterojunctions based on polymers and nanocrys- tals [8,14,15] have inherent problems with phase separation over length scales exceeding L d and a slow charge transfer due to the low carrier mobility. These problems are addressed in ordered bulk heterojunction OPVs that have fabrication complexities. Progress in nanotechnology now enables us to synthesize quasiordered, inorganic, nanoporous films or nanowire arrays which can be filled with a polymer [16–18] to achieve the desired structure and performance. The introduction of the inorganic semiconductor material will ensure efficient charge transfer. This new direction of inorganic-organic (IN-OR) heterojunction PV devices holds promise, as we will show in this report. The biggest challenges facing these organic devices are the relatively poor charge transport and separation compared to their inorganic counterparts. A new component in the IN-OR PV cell design will be introduced, in the form of an electronic sieve layer that blocks hole diffusion and enhances charge separation and transport. The use of a wide bandgap and insulating electronic sieve layer at the organic and inorganic interface distinguishes the cell design to be reported here from conventional ones and hence is of central interest. Figure 1a shows a schematic of the IN-OR PV cell structure with a sieve layer incorporated and Figure 1b shows the ZnPc and Au layers with representative thicknesses. The sieve layer needs careful selection both in terms of its bandgap and its thickness. An appropriate bandgap ensures a large band discontinuity at the organic-inorganic interface to block the hole diffusion. At the same time, an appropriate thickness ensures efficient electron tunnelling through it. The electronic impor- tance of the sieve layer can be visualized from the energy-band diagram shown in Figure 1c. The use of lithium fluoride (LiF) as an electronic sieve in solar cells is inspired by the positive results it brought for organic light-emitting diodes (OLEDs) [19,20] and solar cells. [21,22] LiF is believed to lower the work function of the metal surfaces, which in turn reduces the barrier for electron injection from the metal electrode to the electron-transporting COMMUNICATION www.advmat.de [*] Dr. K. H. Chen, M. H. Wu, W. C. Chen Institute of Atomic and Molecular Sciences, Academia Sinica No. 1, Sec. 4, Roosevelt Rd., Taipei 106 (Taiwan) E-mail: chenkh@pub.iams.sinica.edu.tw Dr. L. C. Chen, Dr. C. H. Chen, Dr. K. H. Chen Center for Condensed Matter Sciences, National Taiwan University No. 1, Sec. 4, Roosevelt Rd., Taipei 106 (Taiwan) E-mail: chenlc@ntu.edu.tw C. H. Lin, Prof. J. H. Lee Graduate Institute of Photonics and Optoelectronics and Department of Electrical Engineering, National Taiwan University No. 1, Sec. 4, Roosevelt Rd., Taipei 106 (Taiwan) Prof. S. Chattopadhyay Institute of Biophotonics, National Yang Ming University 155, Section 2, Li-Nong St., Taipei 112 (Taiwan) Prof. J. S. Hwang, C. W. Hsu Institute of Optoelectronic Sciences, National Taiwan Ocean University 2 Pei-Ning Road, Keelung 20224 (Taiwan) Prof. C. W. Chen, C. T. Wu, S. C. Tseng Department of Material Science and Engineering, National Taiwan University No. 1, Sec. 4, Roosevelt Rd., Taipei 106 (Taiwan) DOI: 10.1002/adma.200701309 Adv. Mater. 2009, 21, 759–763 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 759