Organic solar cells comprising multiple-device stacked structures
exhibiting complementary absorption behavior
Wei-Ting Lin
a, Yen-Tseng Lin
b, Chu-Hsien Chou
a, Fang-Chung Chen
a,n, Chain-Shu Hsu
c aDepartment of Photonics, National Chiao Tung University, Hsinchu 30010, Taiwan
bInstitute of Lighting and Energy Photonics, National Chiao Tung University, Tainan 71150, Taiwan c
Department of Applied Chemistry, National Chiao Tung University, Hsinchu 30010, Taiwan
a r t i c l e i n f o
Available online 28 August 2013 Keywords: Organic Photovoltaic Multi-junction Absorption
a b s t r a c t
We developed organic solar cells based on multiple-device stacked structures featuring complementary absorption behavior. Thefirst, semitransparent (ST) subcell featured an inverted structure; its anode comprised a MoO3/Ag bilayer. This structure provided a transmittance of greater than 35% in the visible
region. The second subcell, featuring a low-band-gap small molecule in its photoactive layer, was stacked onto the ST device; the two subcells could be connected either in series or in parallel. Because the two subcells exhibited complementary absorption behavior, their stacked structure connected in parallel displayed a power conversion efficiency of 4.37%, greater than those of the isolated subcells.
& 2013 Elsevier B.V. All rights reserved.
1. Introduction
Organic photovoltaic devices (OPVs) are promising candidates for use in next-generation solar cells because of their attractive properties of inexpensive fabrication, light weight, and mechanical flexibility[1–6]. The presence of bulk heterojunctions between the electron donors and acceptors in photoactive thinfilms can greatly improve device efficiency [1–6]. To date, the power conversion efficiency (PCE) of OPVs has reached approximately 10% in single-junction devices[1]. Increasing photon absorption is one of the key issues that must be overcome if we are to further improve PCEs. The low mobility of organic materials, however, restricts the use of thicker photoactive layers for the harvesting of greater number of photons. Moreover, the narrow absorption ranges of single materials limit the range of the solar spectrum available for absorption. One plausible method toward solving these problems is to produce tandem cells [7–11]. Nevertheless, because multi-junction devices have relatively complicated structures, the need for sophisticated fabrication procedures generally decreases the device fabrication yield.
In 2006, Shrotriya et al. proposed an alternative approach for constructing multi-junction devices: they superimposed one semi-transparent (ST) cell onto another conventional one [12]. After connecting the two subcells, either in series or in parallel, the PCE could be doubled relative to the efficiency of the corresponding single-junction device. Nevertheless, most reported stacked cells of this type have adopted the same photoactive materials in the
two subcells[12–14]; as a result, multiple-device stacked struc-tures exhibiting complementary absorptions are rare[15]. In this present study, we used two different organic materials, with complementary absorption spectra, as the photoactive layers in the two subcells. Thefirst, ST subcell adopted an inverted struc-ture; its ST anode comprised a MoO3/Ag bilayer. The second
subcell featured a low-band-gap (LBG) small molecule (SM) in its photoactive layer; it was stacked onto the ST device such that the two subcells were connected either in series or in parallel. Because the absorption behavior of the two subcells was complementary, the stacked device exhibited improved PCE relative to that of the single-cell device.
2. Experimental
The conjugated polymer and fullerene derivative used in this study were regioregular poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)propyl-1-phenyl[6,6]methanofullerene (PCBM), respectively. The LBG SM used in the back subcell was 2,5-di(2-ethylhexyl)-3,6-bis-(5 ″-n-hexyl-[2,2′,5′,2″]terthiophen-5-yl)-pyr-rolo[3,4-c]pyrrole-1,4-dione (SMDPPEH)[16].Fig. 1(a) displays the chemical structures of these materials as well as the device structure.Fig. 1(b) presents the absorption spectra of the P3HT: PCBM and SMDPPEH:PCBM thinfilms. The two blends cover the solar spectrum over the range from 350 to 800 nm in a comple-mentary manner. To begin the fabrication of the ST device, a solution of Cs2CO3 in 2-ethoxyethanol was spin-coated onto an
indium tin oxide (ITO)-coated substrate and then the substrate was baked at 1401C for 20 min. Next, a solution of the P3HT/PCBM blend in 1,2-dicholorobenzene was coated onto the substrate. Contents lists available atScienceDirect
journal homepage:www.elsevier.com/locate/solmat
Solar Energy Materials & Solar Cells
0927-0248/$ - see front matter& 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.solmat.2013.07.043
nCorresponding author. Tel.:+886 3 5131484; fax: +886 3 5735601.
E-mail address:[email protected] (F.-C. Chen).
After solvent annealing[17,18], the sample was further thermally annealed at 1101C for 15 min. Finally, MoO3(3.5 nm) and Ag (10–
100 nm) were deposited to form the top electrodes; the Ag layer was chosen because of its long skin depth and high electrical conductivity[13]. Fabrication of the SM device began with spin-coating of poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) onto an ITO-coated substrate and then baking the resultingfilm at 120 1C for 1 h. Next, a solution of the SMDPPEH: PCBM blend in chlorobenzene was coated on top of the PEDOT:PSS layer. Finally, Al (100 nm) was thermally evaporated to serve as the cathode. The photocurrent density–voltage (J–V) curves under illumination were recorded using a Keithley 2400 measurement system. The light source was a Thermal Oriel solar simulator, the illumination intensity of which was calibrated using a Si photo-diode detector equipped with a KG-5filter (Hamamatsu)[19].
3. Results and discussion
Fig. 2(a) displays the J–V curves of ST devices fabricated with various Ag thicknesses. Although the device fabricated with a 10-nm-thick Ag anode had the highest transmittance [Fig. 1(b)], it exhibited relatively poor performance, with a PCE of 2.36%. When we increased the thickness of the Ag layer to 15 nm, the transmit-tance in the visible region was greater than 35% [Fig. 1(b)]; this ST device exhibited an open-circuit voltage (Voc) of 0.57 V, a
short-circuit current (Jsc) of 8.53 mA cm2, and afill factor (FF) of 0.61,
leading to a PCE of 2.97%. In general, the device efficiency improved upon increasing the thickness of the Ag layer, suggesting
that the sheet resistance of the Ag anode limited the device performance. Although the highest device efficiency (3.29%) was obtained when the thickness of the Ag layer was 30 nm, the overall transmittance in the visible wavelength range was only approximately 10% [Fig. 1(b)], making this device unsuitable for use in stacked structures.Table 1summarizes the parameters of the devices featuring Ag anodes of various thicknesses.
Fig. 2(b) presents the J–V characteristics of the SM devices obtained under the illumination with simulated solar irradiation. For the optimized device, in which the thickness of the photo-active layer was 100 nm, the values of Voc, Jsc, and FF were 0.72 V,
10.28 mA cm2, and 0.52, respectively, yielding a PCE of 3.83%. The inset toFig. 2(b) displays the incident photon-to-electron conver-sion efficiencies (IPCEs) of both the ST and SM devices. The major wavelength response ranges of the ST and SM devices were 450– 600 and 600–750 nm, respectively, confirming the complemen-tary manner of photoabsorption of these two different devices.
We constructed multiple-device stacked structures by stacking the SM device onto the inverted ST device (Fig. 1); the back SM device absorbed solar irradiation that had passed through the ST device. We tested the performance of the two subcells connected either in parallel or in series.Fig. 3(a) displays the J–V curves of the
Fig. 1. (a) Chemical structures of P3HT, PCBM, and SMDPPEH. (b) Absorption spectra offilms of P3HT:PCBM (1:1, w/w) and SMDPPEH:PCBM (1:1, w/w).
Fig. 2. (a) J–V characteristics of ST devices prepared with Ag layers of various thicknesses. (b) J–V curves of SM devices prepared with photoactive layers of various thicknesses. Inset: EQE spectra of the two subcells in the stacked structure. Structures of the ST and SM devices: ITO/PEDOT:PSS/P3HT:PCBM (220 nm)/MoO3
(3.5 nm)/Ag and ITO/PEDOT:PSS/SMDPPEH:PCBM (100 nm)/Al, respectively.
Table 1
Photovoltaic parameters of OPVs featuring Ag anodes of various thicknesses. Ag thickness (nm) Voc(V) Jsc(mA cm2) FF PCE (%)
10 0.56 8.10 0.52 2.36
15 0.57 8.53 0.61 2.97
20 0.57 9.03 0.60 3.03
30 0.57 9.74 0.59 3.29
100 0.57 9.70 0.65 3.57 W.-T. Lin et al. / Solar Energy Materials & Solar Cells 120 (2014) 724–727 725
individual subcells and of the stacked device structure. Because a portion of the photons was absorbed and/or blocked by the front ST subcell, the back SM cell exhibited a lower photocurrent (i.e., 3.91 mA cm2) than that of the device measured before connec-tion with the ST device. The FF of the back SM alone was increased to 0.54, but the value of Vocdecreased slightly to 0.70 V, resulting
in a PCE of 1.49%. When we connected the subcells in series, the value of Vocof the whole stacked device was 1.26 V, very close to
the sum of the photovoltages of the two isolated subcells (0.57 V for the ST device and 0.70 V for the SM device). Because the current in the same loop should be identical according to Kirch-hoff's law, a low value of Jscof 3.78 mA cm2was obtained, limited
by the back subcell. Therefore, the PCE (only 2.70%) was even lower than that of the ST device measured individually under conditions of 1 sun. On the other hand, when we connected the subcells in parallel, the photocurrent increased significantly to 12.01 mA cm2; the values of Voc and FF were 0.60 V and 0.60,
respectively, resulting in an improved PCE of 4.37%. This value is very close to the sum of the PCEs of the individual subcells in the device (2.97% for the ST device and 1.49% for the SM device), indicating that no apparent energy loss occurred when the stacked structure was connected in parallel. Furthermore, the PCE was higher than that of either of the isolated subcells measured individually under illumination (i.e., 2.97% for the ST device and 3.83% for the SM device). Our results suggest that stacking structures is an effective approach toward assembling multiple devices containing complementary absorption behavior. Table 2
summarizes the parameters of the various stacked structures. The thickness of the photoactive layer in the back SM subcell barely affected the performance of the stacked structure. Never-theless, the effect of the thickness of the front subcell was significant. For example,Fig. 3(b) displays the device performance
of the stacked structure when we decreased the thickness of the P3HT:PCBM layer in the front subcell to 100 nm. First, the value of Jscof the front ST device decreased to 7.10 mA cm2, presumably
due to the lower degree of photon absorption; accordingly, the PCE decreased to 2.73%. Furthermore, because more photons passed through the ST cell, the value of Jscof the individual SM device in
the stacked device increased to 4.75 mA cm2, resulting in a higher PCE (1.78%). Because of the increased value of Jsc of the
current-limiting subcell (i.e., the back SM subcell) for the stacked structure connected in series, the overall PCE improved to 3.64%. Notably, the stacked structure connected in parallel exhibited even better performance, with Voc, Jsc, and FF values of 0.61 V,
11.52 mA cm2, and 0.62, respectively; its PCE was 4.18%. This value is also almost identical to the sum of the PCEs of the individual subcells (2.37% for the ST device and 1.78% for the SM device). These results reveal that, unlike the situation when the subcells were connected in series, the efficiency of the subcells connected in parallel was not limited by current matching; the subcells could still function individually when they were con-nected in parallel, leading to higher PCEs[14].Table 3summarizes the parameters of the stacked structures in which the thickness of the P3HT:PCBM layer was 100 nm.
To further support the beneficial effect of the complementary absorption behavior, we further built stacked structures contain-ing the same photoactive materials in the two subcells. Note, however, that the device performance of the SM subcell made with the inverted structure was rather poor, presumably due to the poorfilm quality on the Cs2CO3 surfaces and/or the inferior
metal/polymer contact at the cathode. We, therefore, only used P3HT:PCBM blends for device fabrication. Fig. 4 displays the device performance of the stacked structure, in which both photoactive layers were P3HT:PCBM. The back cell exhibited a typical PCE of 3.87% (Voc¼0.59; Jsc¼10.38 mA cm2; FF¼0.63)
under 100 mW cm2illumination (AM1.5G)[18]. When the device was stacked with the front cell, the PCE decreased significantly to
Fig. 3. J–V curves of the front ST subcell, the back SM subcell, and the stacked structures connected either in series or in parallel under illumination (simulated AM1.5G). Thickness of the photoactive layer of the front ST subcell: (a) 220 and (b) 100 nm.
Table 2
Photovoltaic parameters of individual subcells and stacked structures when the thickness of the P3HT:PCBM layer was 220 nm.
Device condition Voc (V) Jsc (mA cm2) FF PCE (%) SM device under standard conditionsa
0.72 10.28 0.52 3.83 Transparent device under standard
conditionsa
0.57 8.53 0.61 2.97 SM device in stacked structure 0.70 3.91 0.54 1.49 Transparent device in stacked structure 0.57 8.53 0.61 2.97 Stacked structure connected in series 1.26 3.78 0.57 2.70 Stacked structure connected in parallel 0.60 12.01 0.60 4.37
aThe device was not connected to another subcell; it was measured individually.
Table 3
Photovoltaic parameters of individual subcells and stacked structures when the thickness of the P3HT:PCBM layer was 100 nm.
Device condition Voc (V) Jsc (mA cm2) FF PCE (%) SM device under standard conditionsa
0.72 10.28 0.52 3.83 Transparent device under standard
conditionsa
0.57 7.10 0.59 2.37 SM device in stacked structure 0.70 4.75 0.54 1.78 Transparent device in stacked structure 0.57 7.10 0.59 2.37 Stacked structure connected in series 1.27 5.12 0.56 3.64 Stacked structure connected in parallel 0.61 11.52 0.60 4.18
a
The device was not connected to another subcell; it was measured individually. W.-T. Lin et al. / Solar Energy Materials & Solar Cells 120 (2014) 724–727
0.59% (Voc¼0.53; Jsc¼1.83 mA cm2; FF¼0.61). Again, it was due
to the fact that a portion of the light was absorbed and/or blocked by the front subcell. When the subcells were connected in series, the photocurrent was only 2.03 mA cm2; the values of Vocand FF
were 1.09 V and 0.59, respectively, resulting in a low PCE of 1.31%. The PCE, however, became higher (3.29%) when the two subcells were connected in parallel. The values of Voc, Jsc and FF were
0.55 V, 9.96 mA cm2 and 0.60, respectively. Although a better performance was obtained compared with the efficiency of each subcell, the PCE was still lower than that (3.87%) of a typical standard single-junction cell. The results were consistent with our previous observation[14]. In short, the data presented inFig. 4
suggests that complementary light absorption of the active layers is indeed essential for achieving even higher device performance.
4. Conclusion
Stacking multiple solar cells is a promising approach for achieving high efficiencies by reducing absorption and thermali-zation loss [20,21]. For double-junction solar cells based on inorganic materials, such as III–V semiconductors, there are gen-erally two methods for combining the cells: one is to physically separate them and to use multiple contacts and the other is to integrate the subcells with tunnel junctions joining them in series
[21]. For OPVs, the most common approach is to connect two subcells directly in series, which is similar to the second method. Multiple OPVs connected with thefirst method using a physical separated structure are still seldom reported. In this work, we have developed organic solar cells based on multiple-device stacked structures exhibiting complementary absorption behavior. We stacked an inverted ST subcell with a second subcell featuring a LBG SM; we examined the performance of the two subcells connected either in series or in parallel. Because the two subcells exhibited complementary absorption behavior, their stacked device connected in parallel provided an improved PCE. Notably, the device efficiency remained limited by certain factors; for example, when we connected the subcells in parallel, the values of Voc were still relatively low. Further improvements, such as
tailoring the energy-level structure to increase the photovoltage, will be performed in the future.
Acknowledgments
We thank the National Science Council of Taiwan (Grant nos. NSC 102-3113-P-009-004 and NSC 101-2628-E-009-008-MY3) and the Ministry of Education of Taiwan (through the ATU program) for financial support.
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Fig. 4. J–V curves of the stacked structures connected either in series or in parallel under illumination (simulated AM1.5G). The photoactive layer of both subcells was P3HT:PCBM.
