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Vertical Phase Separation in Poly(3-hexylthiophene):
Fullerene Derivative Blends and its Advantage for
Inverted Structure Solar Cells
By Zheng Xu, Li-Min Chen, Guanwen Yang, Chun-Hao Huang, Jianhui Hou,
Yue Wu, Gang Li, Chain-Shu Hsu, and Yang Yang*

conversion efficiency (PCE) for this system
reported so far is about 4–5%.
Morphology optimization of theactive
layer is an essential way to improve the
device efficiency. Besides the lateral phaseseparated morphology, the vertical distribution of the components in the blend film
is also critical, and vertical phase separation has been suggested in several polymer
blend systems,[9–12] as well as P3HT:PCBM
blends.[13–15] Campoy-Quiles et al. recently
used variable-angle spectroscopic ellipsometry(VASE) to model the vertical composition profile of P3HT:PCBM thin films
and reported a concentration gradient
varying from PCBM-rich near the poly (3,4-ethylenedioxythiophene)/poly (styrene-sulfonate) (PEDOT:PSS) side to P3HT-rich
adjacent to the free (air) surface. Consequently, the regular device
structure (Scheme 1a), in which the polymer blend is sandwiched
between the PEDOT:PSS-coatedindium tin oxide (ITO) anode
and low work function metal cathode, has a non-ideal
composition profile. Several approaches have been proposed
to modify the composition profile to achieve better device
performance. For example, Campoy-Quiles et al. have shown that
the compositional gradient can be switched by modifying the
surface energy of the substrate with a self-assembled monolayer
(SAM). Weiet al. also introduced a new fullerene derivative with a
fluorocarbon chain which spontaneously forms a buffer layer
near the metal cathode to improve the device performance.[16]

A method which enables the investigation of the buried interfaces without altering
the properties of the polymer films is used to study vertical phase separation of
spin-coated poly(3-hexylthiophene)(P3HT):fullerene derivative blends. X-ray
photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) analysis
reveals the P3HT enrichment at the free (air) surfaces and abundance of fullerene
derivatives at the organic/substrate interfaces. The vertical phase separation
is attributed to the surface energy difference of the components and their
interactions with the substrates. This inhomogeneousdistribution of the
donor and acceptor components significantly affects photovoltaic device
performance and makes the inverted device structure a promising choice.

1. Introduction
Polymer photovoltaic (PV) cells have the advantage of low-cost
fabrication and easy processing. The state-of-the-art device
structure is the polymer bulk heterojunction (BHJ),[1,2] blending
conjugated polymersintimately with soluble fullerene derivatives. An interpenetrating network of the donor–acceptor blend
sandwiched between the anode and cathode offers large
interfacial area for efficient charge separation and excellent
charge transport, leading to high efficiency performance.
Regioregular poly(3-hexylthiophene) (RR-P3HT) and fullerene
derivative [6,6]-phenyl C61 butyric acid methyl ester (PCBM)blend represents one of the most promising systems. Several
process conditions[3–6] and post-treatments[7,8] have been proposed to form a nano-scale phase-separated morphology with
crystalline P3HT and PCBM domains, and the highest power
[*] Prof. Y. Yang, Z. Xu, L.-M. Chen, G. Yang
Department of Materials Science and Engineering
University of California, Los Angeles
Los Angeles, CA 90095(USA)
E-mail: yangy@ucla.edu
C.-H. Huang, Prof. C.-S. Hsu
Department of Applied Chemistry
National Chiao Tung University
Hsinchu, Taiwan (Republic of China)
Dr. J. Hou, Dr. Y. Wu, Dr. G. Li
Solarmer Energy, Inc.
3445 Fletcher Ave
El Monte, CA 91731 (USA)

DOI: 10.1002/adfm.200801286

Adv. Funct. Mater. 2009, 19, 1227–1234

Scheme 1. a) Schematic depiction of the regular structure and...