Efficient bilayer polymer solar cells possessing planar mixed-heterojunction
structures†
Jen-Hsien Huang,
aKuang-Chieh Li,
bDhananjay Kekuda,
aHari Hara Padhy,
bHong-Cheu Lin,
bKuo-Chuan Ho
cand Chih-Wei Chu*
adReceived 17th November 2009, Accepted 29th January 2010 First published as an Advance Article on the web 5th March 2010 DOI: 10.1039/b924147g
We have investigated the influence of thermal annealing on the performance of polymer bilayer solar cell devices incorporating poly{2,6-(4,4-bis[2-ethylhexyl]-4H-cyclopenta[2,1-b;3,4-b0 ]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)} (PCPDTTBT) as the donor and two kinds of fullerenes (C60, C70) as
acceptors. The higher absorption of C70increased the external quantum efficiency in the spectral range
400–600 nm. We observed morphological changes of the polymer films when the pre-annealing temperature was near the crystalline temperature (Tc, 207C). These nanostructural transformations
resulted in a modified interfacial morphology of the donor phases and, therefore, greatly influenced the device performance. Post-annealing treatment reorganized the interface between the donor and acceptor phases, leading to better contact. The highest power conversion efficiency (2.85%) was obtained when we performed device pre- and post-annealing both at 200C for 30 min; the open-circuit voltage was 0.69 V and the short-circuit current was 8.42 mA cm2.
1.
Introduction
In recent years, polymer-based solar cells have become increas-ingly attractive as a means of harnessing clean energy because of their low cost,1–4 ease of processing,5–8 and the feasibility of
fabricating them on various substrates. Since the bilayer heter-ojunction concept was proposed in 1986 by Tang et al.,9many
advances in bilayer solar cells has been achieved through the use of various material combinations.10–15Although bilayer devices
can exhibit good charge transport,16 their power conversion
efficiencies (PCEs) are limited by the small interface between the electron donor and acceptor.17 Moreover, the short exciton
diffusion lengths of the organic materials [ca. 6.8–8.5 nm for poly(3-hexylthiophene) (P3HT)18 and ca. 40 nm for fullerene
(C60)19] restrict the thickness of the active layer, leading to
inef-ficient absorbance. To overcome these problems, Heeger et al. proposed the use of bulk heterojunction (BHJ) solar cells, which mix the donor and acceptor components in a bulk volume so that a donor–acceptor interface is located within a distance less than the exciton diffusion length of each absorbing site.20Although
the exciton dissociation efficiencies for these blend structures are high relative to those of bilayer structures, several problems are typically encountered within the disordered nanostructures. For
example, a well-mixed blend of two semiconductors will lack the continuous channels required for charge transport, causing a significant fraction of the carriers to recombine at donor– acceptor interfaces before they have the chance to reach their respective electrodes.21,22In addition, phase separation between
the two semiconductors can lead to the length scale for exciton dissociation becoming too great. To overcome these problems, ordered BHJs, prepared by filling an inorganic nanostructure with conjugated polymers, have been developed for the fabrica-tion of efficient solar cells.23Although much research has been
performed in the area of polymer/TiO2 hybrid systems, their
efficiencies are limited because of incompatibility between the nanostructure dimensions of TiO2 and the exciton diffusion
length of the polymer layers.24–26
Thermotropic liquid crystalline copolymers are attracting great attention in the area of organic electronics, especially for their use in thin film transistors and solar cells.27–32 These
copolymers can undergo significant morphological changes and can form periodic nanostructures if annealed at temperatures higher than their glass transition temperatures. Such trans-formations lead to a crystalline phase and, therefore, a dramatic increase in hole mobility. In a previous study, we developed a cyclopentadithiophene (CPDT)-based copolymer, PCP-DTTBT, comprising bithiazole and thiophene units.33
PCPDTTBT exhibited a broad absorption range and good charge-transporting properties, with a hole mobility of 5.2 104 cm2 V1 s1, making it a good candidate for use in high
efficiency polymer solar cells. To increase the efficiency of the p–n junction interfacial area of bilayer solar cells, in this study we employed the thermotropic liquid crystalline PCPDTTBT and fullerenes to fabricate planar-mixed heterojunction solar cells. Furthermore, we also increased the absorbance of the solar cell devices by replacing C60, which possesses a very low absorption
coefficient in the visible spectrum, with the analogous
aResearch Center for Applied Sciences, Academia Sinica, Taipei 11529,
Taiwan. E-mail: gchu@gate.sinica.edu.tw; Fax: +886-2-2782-6680; Tel: +886-2-2789-8000 ext 70
bDepartment of Materials Science and Engineering, National Chiao Tung
University, Hsinchu 300, Taiwan
cDepartment of Chemical Engineering, National Taiwan University, Taipei
10617, Taiwan
dDepartment of Photonics, National Chiao Tung University, Hsinchu 300,
Taiwan
† This paper contains work presented at the 2009 International Conference on Carbon Nanostructured Materials held in Santorini, Greece.
ª The Royal Society of Chemistry 2010
PAPER www.rsc.org/materials | Journal of Materials Chemistry
complementary acceptor C70. As a result, the planar-mixed
het-erojunction solar cells satisfied several requirements simulta-neously: a high-area heterojunction interface, a large absorbance, and good charge transfer. We suspect that devices based on this type of structure might one day rival the perfor-mance of BHJ solar cells.
2.
Experimental
PCPDTTBT was synthesized according to a previously pub-lished procedure.33The fabrication process of the devices was
initiated by cleaning the indium tin oxide (ITO)-coated glass. The cleaned substrates were exposed to UV ozone for 15 min. Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PED-OT:PSS) was then spun onto the ITO substrates, which were subsequently dried at 120C for 1 h. The active layer consisted of two layers. First, a copolymer layer was deposited through spin coating. A solution of PCPDTTBT polymer (10 mg mL1,
1 wt%) in 1,2,4-trichlorobenzene was spun on the substrates at 2500 rpm for 60 s. The films were then annealed at various temperatures (100 to 250C) for 1 h to modify their morphol-ogies. The samples were then patterned through a suitable mask and transferred to a vacuum chamber, where n-type fullerene C70
was deposited. The deposition rate and thickness of the fullerene (C60or C70) thin films were 0.5 A s1and 40 nm, respectively, in
each case. After deposition of the active layer, Al cathodes were deposited. The surface morphologies were visualized using atomic force microscopy (AFM). X-Ray diffraction was used to measure the crystallinity of the films. The optical absorption spectra of the PCPDTTBT films were recorded at room temperature using a UV–Vis spectrophotometer (V-650, Jasco). The external quantum efficiency (EQE) was measured using a lock-in detector and monochromatic light from a Xe lamp. The
photoluminescence (PL) spectra were obtained using a Hitachi F-4500 instrument. The illuminated current–voltage (J–V) characteristics were studied using an Agilent 4056 semiconductor parameter analyzer.
3.
Results and discussion
Fig. 1 presents the molecular structure of the low-bandgap polymer and a schematic energy level diagram of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of PCPDTTBT relative to those of the fullerenes. All photovoltaic devices were fabricated with a bilayer structure (Fig. 1c); the thicknesses of the polymer and fullerene layers were held constant at 100 and 40 nm, respec-tively.
Fig. 2 displays the absorption spectra of the pure PCPDTTBT, PCPDTTBT/C60, and PCPDTTBT/C70 films. These films were
all obtained by spin-coating PCPDTTBT onto ITO glass and then thermally depositing the fullerenes. The spectrum of the PCPDTTBT film reveals an absorption maximum at 560 nm, with its absorption onset at 660 nm. The absorption of PCPDTTBT between 350 and 550 nm was complemented by combining PCPDTTBT with C60. The absorbance of C70is much
greater than that of C60 (inset to Fig. 2) because of its much
higher absorption coefficient. The low absorption coefficient of C60 arises from its high degree of symmetry, which forbids
lowest-energy transitions. Therefore, the absorbance of PCPDTTBT/C70was much higher than that of PCPDTTBT/C60.
This phenomenon is also clearly evident in the photographs presented in Fig. 2.
Fig. 3 reveals the performances of the bilayer solar cells fabricated from PCPDTTBT/C60 and PCPDTTBT/C70. These
devices were both pre- and post-annealed at 150C. The short
Fig. 1 (a) Molecular structure of PCPDTTBT. (b) HOMO and LUMO energy levels of PCPDTTBT and the fullerenes. (c) Schematic representation of the structure of the bilayer solar cells.
ª The Royal Society of Chemistry 2010
circuit current (JSC) and fill factor (FF) of the PCPDTTBT/C60
-based device were 1.99 mA cm2 and 42.8%, respectively;
combined with an open circuit voltage (VOC) of 0.66 V, it
delivered a PCE of 0.56%. The photocurrent underwent a significant increase after replacing C60with C70. Under a light
intensity of 100 mW cm2, the value of J
SC increased up to
4.13 mA cm2—an enhancement of greater than a 107% over that
obtained for the PCPDTTBT/C60-based device. This behavior
can be explained by considering the much larger absorbance of C70 relative to that of C60. As a result, the PCPDTTBT/C70
-based device exhibited a PCE of 1.17%. The EQE (inset to Fig. 3) also exhibited a higher photoresponse between 400 and 600 nm
for the PCPDTTBT/C70-based device. These results are in good
agreement with the UV–Vis spectra.
Fig. 4 displays the J–V characteristics of the photovoltaic devices that had undergone pre-annealing at various tempera-tures and subsequent post-annealing at 150 C after cathode deposition. The pre-annealing temperature mainly affected the value of JSC, with the value of VOCremaining largely unchanged,
except for the device annealed at 250C. As a consequence, the photocurrent increased continuously from 3.51 to 6.46 mA cm2
upon increasing the pre-annealing temperature from 100 to 200C. Because PCPDTTBT possessed a value of Tcof 207C,33
its polymer chains underwent higher ordering at higher temper-atures. This behavior can result in enhanced polymer chain stacking and a rougher morphology. Therefore, the devices
Fig. 2 Optical absorption spectra of the pure PCPDTTBT, PCPDTTBT/C60, and PCPDTTBT/C70films. Inset: Absorption
coeffi-cients of C60and C70.
Fig. 3 Performance of solar cells incorporating PCPDTTBT/C60and
PCPDTTBT/C70. The pre- and post-annealing temperatures were both
150C.
Fig. 4 J–V curves of PCPDTTBT/C70-based bilayer solar cells
fabri-cated at various pre-annealing temperatures.
Fig. 5 Tapping mode AFM images of PCPDTTBT films that had been subjected to pre-annealing at (a) 100, (b) 150, (C) 200, and (d) 250C. ª The Royal Society of Chemistry 2010
displayed superior performance after pre-annealing. The opti-mized photovoltaic device, which had been pre-annealed at 200C (Fig. 4), exhibited a value of J
SCof 6.46 mA cm2, a value
of VOCof 0.67 V, and a FF of 38.2%.
In view of the importance of the interfacial morphology of solar cell devices, we used AFM to investigate the surface morphology of the annealed PCPDTTBT surfaces (Fig. 5). After annealing at 100C, the polymer film was smooth and feature-less. As the annealing temperature increased, the surface roughness increased accordingly (Fig. 5b and c). After annealing at 200C, distorted fibrils having a characteristic width scale of ca. 100 nm were evident in the polymer film. This temperature is quite close to the value of Tc(207C) of PCPDTTBT. Therefore,
we consider the distortion arose in part from the thermally induced crystallinity of PCPDTTBT. The morphology returned to a featureless smooth surface, however, after increasing the annealing temperature to 250 C—that is, when the annealing temperature was higher than the melting temperature (Tm,
237C)33of PCPDTTBT, causing it to melt. These images reveal
that an annealing temperature between 150 and 200C resulted in a rougher morphology. Subsequent deposition of C70onto the
fibrillar structure led, therefore, to a well-mixed interface between PCPDTTBT and C70. Such planar-mixed
hetero-junction devices feature a larger interfacial area for exciton dissociation, relative to that of a conventional bilayer structure; as a result, a greater photocurrent can be generated.
We also monitored the polymer structures using XRD. Fig. 6 presents XRD spectra of the PCPDTTBT films annealed at various temperatures. Again, the XRD spectrum of the PCPDTTBT film annealed at 200 C exhibited a dramatic increase in intensity of the peak at a value of 2q of 6.2 (corres-ponding to the interchain spacing in PCPDTTBT associated with interdigitated alkyl chains). This finding suggests that the poly-mer underwent chain-reorganization and self-assembly during the annealing process. Because greater stacking of polymer chains can improve charge mobility, the polymer film annealed at 200 C not only possessed a favorable morphology but also enhanced charge mobility—two positive effects for improving solar cell efficiency.
Fig. 7 displays the J–V and EQE characteristics under illu-mination for photovoltaic devices that had undergone pre-annealing at 200C and subsequent post-annealing at various temperatures after cathode deposition. All the performance parameters for the planar-mixed heterojunction device as a function of pre- and post- annealing temperatures are summarized in Table 1. We observe that the cell performance was improved further after post-treatment. The value of JSCand the
FF were both enhanced upon increasing the post-annealing temperature, with a temperature of 200 C providing the best efficiency (up to 2.85%). We find the performance of the planar-mixed heterojunction device can rival those of the soluble-pro-cessed BHJ cell.33Ayzner et al. also reported that BHJ geometry
is not necessary for high efficiency, and bilayer solar cells can be nearly as efficient as BHJ solar cells.34Consistent with the trend
in the value of JSC, the EQE magnitude initially decreased
slightly from 60.0% (post-annealed at 200C) to 54.6% (post-annealed at 150C), followed by a rapid decrease to 26.7% for the device subjected to a post-annealing temperature of 250C. The reason for the superior performance of the devices after
Fig. 6 XRD spectra of PCPDTTBT films that had been annealed at various temperatures.
Fig. 7 (a) J–V curves and (b) EQE spectra of PCPDTTBT/C70-based
bilayer solar cells that had been subjected to various post-annealing temperatures. The pre-annealing temperature was held constant at 200C. ª The Royal Society of Chemistry 2010
post-annealing can be explained by considering the elimination of voids between the PCPDTTBT and C70phases and the
crea-tion of a larger interface for exciton dissociacrea-tion.35This also can
be explained by the shape of J–V curves. For the devices annealed at a temperature lower than 200C, an inflection point (S-shape) can be seen. The S-shape is often observed when solar cell devices have a barrier to carrier transport.36–39 These
phenomena are further supported by the PL emission spectra. Fig. 8 reveals that excitons were not fully quenched in the bilayer structure, indicating a poor interface between the PCPDTTBT and C70phases. Although the pre-annealing process did roughen
the morphology of PCPDTTBT, leading to a planar-mixed het-erojunction interface, the fibrillar surface also resulted in some of the deposited C70 molecules not touching the underlying
PCPDTTBT; such defects at the interface reduced the exciton dissociation efficiency. This problem was overcome after post-annealing to reorder the polymer and C70phases. Therefore, the
PL emission decreased upon increasing the post-annealing temperature. The thermal annealing process enabled spatial
rearrangement of the polymer chains and C70, thereby improving
contact between the donor and acceptor layers.
4.
Conclusion
We have fabricated thermally induced, planar mixed-hetero-junction solar cells based on PCPDTTBT and fullerenes. Pre-annealing led to a thermally induced fibrillar morphology that increased the interfacial area between the donor and acceptor phases. Post-annealing reorganized the interface between the donor and acceptor phases to enhance their contact area. The optimal annealing temperature was 200 C for both pre- and post-annealing, yielding a PCE of 2.85%. Such processing treatment might be attractive for other light harvesting materials, such as thermotropic liquid crystal polymers, whose morphol-ogies are significantly modulated through thermal treatment.
Acknowledgements
We thank the National Science Council, Taiwan (NSC 98-2221-E-001-002), and the Academia Sinica Research Project on Nano Science and Technology for financial support.
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Condition JSC/mA cm2 VOC/V FF (%) PCE (%)
Pre-annealinga 100C 3.51 0.65 41.2 0.94 150C 4.14 0.65 43.4 1.17 200C 6.46 0.66 38.7 1.65 250C 2.18 0.39 22.3 0.19 Post-annealingb 100C 2.55 0.38 17.5 0.17 150C 6.46 0.66 38.7 1.65 200C 8.42 0.70 48.4 2.85 250C 4.01 0.51 29.8 0.61 a
The post-annealing temperature was controlled at 150C for 1 h.bThe pre-annealing temperature was controlled at 200C for 1h.
Fig. 8 PL spectra of PCPDTTBT/C70bilayer films that had been
sub-jected to various post-annealing temperatures. The pre-annealing temperature was held constant at 200C.
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