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High-efficiency inverted tandem polymer solar cells with step-Al-doped MoO3 interconnection layer
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High-efficiency inverted tandem polymer solar cells with step-Al-doped MoO3 interconnection layer Jian Liu a,b, Shuyan Shao a, Gang Fang a,b, Jiantai Wang a,b, Bin Meng a, Zhiyuan Xie a,n, Lixiang Wang a a State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China b University of Chinese Academy of Sciences, Beijing 100039, PR China
art ic l e i nf o
a b s t r a c t
Keywords: Tandem polymer solar cells Bulk heterojunction Interconnection layer Metal oxide Work function
A highly transparent and physically robust step-Al-doped MoO3 layer was successfully utilized as the interconnection layer (ICL) to fabricate high-efficiency inverted tandem polymer solar cells (PSCs). The inverted tandem cell constructed by the same PCDTBT:PC70BM active layer showed a power conversion efficiency (PCE) of 6.88% with equivalent external quantum efficiency of nearly 80%, implying a high charge-collection efficiency in tandem structure. Incorporation of two sub-cells with complementary absorption spectra leads to further increase of PCE over 7.31%, which is the best results for tandem PSCs with PEDOT:PSS-free interconnection layer. & 2013 Elsevier B.V. All rights reserved.
1. Introduction Bulk heterojunction (BHJ) polymer solar cells (PSCs) have attracted considerable interest due to their potential to become a new energy source with low-cost, light-weight and mechanical flexibility [1–15]. The power conversion efficiency (PCE) of singlejunction polymer:fullerene PSCs has been progressively increased to over 9% with the development of advanced conjugated polymer donor materials, fine-tuning of the active layer morphology and optimization of the cell structure [16–21]. The tandem device architecture is considered to be one of the effective approaches to further boost the PCE of PSCs by extending the spectral coverage to solar light [22–29]. Very recently, a highest PCE of 10.6% was reported for PSCs by using tandem architecture [30]. Apart from the optimal combination of the low and wide band-gap donor polymers, the interconnection layers (ICLs) linking the sub-cells in a tandem structure is crucial to the final photovoltaic performance. An ideal ICL should be not only highly transparent and conductive, but also possess different work functions (WFs) at both sides, enabling energetic matching to the highest occupied molecular orbital (HOMO) level of polymer donor and the lowest unoccupied molecular orbital (LUMO) level of acceptor in sub-cells to lower the interfacial resistance and realize efficient recombination of electrons and holes generated in sub-cells. The ICL should also be robust enough to withstand organic solvents during multi-layer solution processing. Up to now, the commonly used ICL in tandem
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Corresponding author. Tel./fax: +86 431 85262819. E-mail address: [email protected] (Z. Xie).
PSCs is poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS)/n-type transition metal oxide (TiOx or ZnO) nanocrystal bi-layer structure [23,29]. The high conductivity and hydrophilic property of metallic PEDOT:PSS render the ICL very robust and efficient charge recombination layer. However, metallic PEDOT:PSS layer would lead to light attenuation loss in the infrared region [31]. In addition, the incorporation of PEDOT:PSS also raises device instability concerns due to its acidic and hygroscopic nature [32,33]. Transition metal oxides, such as MoO3 and V2O5, have been used to replace the PEDOT:PSS layer in the ICL structure [27,34]. MoO3 is an excellent hole-collecting material. With MoO3 as the hole-collecting layer, some ICL structures such as MoO3/ZnO, MoO3/TiOx, LiF/Al/MoO3 and MoO3/Ag/Al/Ca, have been explored to fabricate tandem PSCs [35–39]. Although the cross-linked n-type TiOx and ZnO prepared from the sol–gel method can prevent organic solvents from penetrating into the underlying BHJ while spincoating the upper BHJ layer, TiOx and ZnO films require high-temperature annealing for completed hydrolysis to realize sufficient conductivity and robustness, which limits the free selection of front sub-cell to optimize the performance of tandem PSCs [34,37]. A combination of MoO3 and low WF metal is also not an ideal ICL in tandem PSC due to possible charge recombination and optical loss induced by the metal layer [38,39]. Herein, we report an efficient MoO3-based ICL in inverted tandem structure by using step-Al-doping approach. Al-doping of top half MoO3 layer can change the high-WF MoO3 into low-WF MoO3–Al composite, and thus provides the large WF offset at two sides of MoO3-based ICL without sacrificing its transparency [40]. More importantly, the MoO3–Al layer was found to be physically
0927-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.solmat.2013.06.034
Please cite this article as: J. Liu, et al., High-efficiency inverted tandem polymer solar cells with step-Al-doped MoO3 interconnection layer, Solar Energy Materials and Solar Cells (2013), http://dx.doi.org/10.1016/j.solmat.2013.06.034i
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robust to withstand organic solvents during multi-layer solution processing owing to the formation of Mo–O–Al complex structure. With this step-Al-doped MoO3 ICL, inverted tandem PSC device constructed by two sub-cells based on a blend of PCDTBT:PC70BM showed a PCE of 6.88% and an equivalent external quantum efficiency (EQE) approaching 80%, implying a high chargecollection efficiency in tandem structure. Tandem PSCs with two sub-cells having complementary absorption spectra were also constructed and a PCE of 7.31% was achieved, which is the best result for tandem PSCs with PEDOT:PSS-free ICL. These results indicate that the step-Al-doped MoO3 layer is an excellent ICL for tandem PSCs. In addition, an approach was developed to measure the current–voltage (J–V) characteristics and EQE spectra of the sub-cells in a tandem structure. Different from the reported method [23,24], the corresponding J–V characteristics and EQE spectra of two sub-cells can be extracted regardless of their absorption features.
(SR830, Stanford Research System) at a chopping frequency of 280 Hz during illumination with a monochromatic light from a Xenon lamp. 2.3. Thin film characterization Transmittance and absorption spectra of the samples were measured using a Perkin-Elmer35 UV-visible spectrophotometer. The refractive index (n and k values) and the thicknesses of the various layers in the device structure were measured using spectroscopic ellipsometry (Horiba Jobin Yvon). UPS measurements were performed on Thermo ESCALAB 250 using He–I (21.2 eV) discharge lamp. A sample bias of −12 V was used in order to separate the sample and the secondary edge for the analyzer.
3. Results and discussion 2. Experimental 2.1. Materials PCDTBT (molecular weight, Mw¼24,000; polydispersity index, PDI¼1.8) was synthesized in our laboratory. PDPP3T (Mw¼ 24,000, PDI¼3.15) was purchased from Solarmer Material Inc. and PC70BM was purchased from American Dye Source Inc. MoO3 was purchased from Sigma-Aldrich (99.5%) and used as received. 2.2. Device fabrication and characterization Inverted tandem polymer solar cells were fabricated on ITOcoated glass substrates. The ITO-coated glass substrates were cleaned with detergent, ultra-sonicated in de-ionized water, acetone, and isopropyl alcohol in sequence, and subsequently dried at 120 1C in an oven overnight. The MoO3–Al composite cathode buffer layer (10 nm) with 55% Al content in weight percentage was thermally deposited on ITO substrate by co-evaporation in a vacuum chamber under a base pressure of 4 10−6 Torr. For the tandem PSCs, a solution containing a mixture of PCDTBT:PC70BM (1:4, w/w) in dichlorobenzene with a PCDTBT concentration of 3.2 or 4.3 mg mL−1 was spin-cast on top of the MoO3–Al composite layer to produce a 65 or 150-nm-thick active layer as the front BHJ. The step-Al-doped MoO3 ICL was deposited atop the first active layer with two sequential processes: firstly, 10-nm pure MoO3 without Al-doping was evaporated on top of active layer; secondly, MoO3 and Al were co-evaporated on 10-nm pure MoO3 film and the Al doped content was set as 55% in weight percentage by modulating the corresponding evaporation speeds. The thickness of Al-doped MoO3 film was 10 nm. Then, the rear BHJ layer of the PCDTBT:PC70BM (1:4, w/w; 105 nm) or PDPP3T:PC70BM (1:2, w/w; 120 nm) was deposited on top of the MoO3-based ICL via spincoating. The PDPP3T:PC70BM (1:2, w/w) blend was dissolved in a solution mixture of 1,2-dichlorobenzene/chloroform/1, 8-diiodooctane (0.76:0.19:0.05, v/v/v) with PDPP3T concentration of 8 mg mL−1. Finally, a bi-layer structure of MoO3 (6 nm)/Al (80 nm) was deposited atop the rear BHJ layer via thermal evaporation in a vacuum of 4 10−6 Torr to complete the device fabrication. The cell active area was 12 mm2, which was defined by the overlapping area of the ITO and Al electrodes. An Oriel 150 W solar simulator with AM 1.5G filter was used to provide 100 mW cm−2 simulated solar light for illumination of the photovoltaic cells. The light intensity was determined by a calibrated silicon diode with KG-5 visible color filter. Current–voltage traces were obtained with a Keithley 236 source meter. External quantum efficiency measurements were performed under short-circuit conditions with a lock-in amplifier
Fig. 1(a) shows the device configuration of the inverted tandem PSCs. The Al content in MoO3–Al composite (cathode buffer layer) is 55% in weight percentage. The two sub-cells with identical or complementary BHJ are linked by MoO3-based ICL with top half doped with 55% Al in weight percentage. In the former case, both of two sub-cells were based on PCDTBT:PC70BM BHJ; in the latter case, a PDPP3T:PC70BM BHJ was used to replace the PCDTBT: PC70BM BHJ in rear sub-cell to extend the absorption spectra coverage [41]. The molecular structures of PCDTBT, PDPP3T and PC70BM are shown in Fig. 1(b). The pure MoO3 film possesses a high work function of 5.49 eV and upon the coverage of Al-doped MoO3 film the work function was reduced to 4.07 eV (See Fig. S1 in Supporting Information). Fig. 1(c) illustrates the energy level diagram of the tandem cell structure. The MoO3–Al composite with low work function is able to form an Ohmic contact with the LUMO of PC70BM as a cathode buffer layer on the ITO electrode. The pure MoO3 layer with high work function serves as the top anode in combination with Al electrode. The step-Al-doped MoO3 ICL was used to link the two BHJ layers with the pure MoO3 side contacting with front sub-cell and the MoO3–Al side contacting with rear sub-cell. Thus, the HOMO level of polymer donor in front sub-cell is well aligned with the LUMO level of fullerene acceptor in rear sub-cell, enabling the efficient charge recombination in ICL. The effective conductivity of such ICL was measured to be 8.5 10−7 S/cm and just brought in an additional series resistance of 2.3 Ω cm2(see Fig. S2 in Supporting Information). Thus the electron can easily tunnel the ICL with negligible voltage drop. Fig. 2(a) displays the optical constants (n and k) of MoO3 and MoO3–Al layers measured with the spectroscopic ellipsometry. The extinction coefficient dispersion of MoO3–Al layer is redshifted compared to that of MoO3 layer, possibly due to reduction of effective band gap. The ICL structure of MoO3(10 nm)/MoO3–Al (10 nm) exhibits a high transparency of over 95% as shown in Fig. 2(b). Fig. 2(b) shows the absorption of the ITO/MoO3–Al/ PCDTBT:PC70BM, ICL/PCDTBT:PC70BM and ITO/MoO3–Al/PCDTBT: PC70BM ICL/PCDTBT:PC70BM structures. The superposition of the BHJ absorption in sub-cells fits well with the absorption of the tandem structure, indicating that spin-coating the upper BHJ layer did not deteriorate the underlying MoO3-based ICL and the bottom BHJ layer. Previous work demonstrated that Mo–O–Al complex structure was formed after Al doping [40]. Such complex structure makes the whole layer cross-linked and consequently physically robust to withstand organic solvents. The optical modeling based on the classic transfer matrix method was used to optimize the thickness combinations of two BHJ layers of front and rear subcells linked by the MoO3-based ICL [41]. Fig. 2(c) shows a dependence of simulated short-circuit current (JSC) of the inverted
Please cite this article as: J. Liu, et al., High-efficiency inverted tandem polymer solar cells with step-Al-doped MoO3 interconnection layer, Solar Energy Materials and Solar Cells (2013), http://dx.doi.org/10.1016/j.solmat.2013.06.034i
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Fig. 1. (a) The layout of the tandem solar cell. (b) The molecular structures of PCDTBT, PDPP3T and PC70BM materials. (c) The schematic of the energy level diagram of the inverted tandem polymer solar cell.
Fig. 2. (a) The refractive index (n) and extinction coefficient (k) of MoO3 and MoO3–Al layers. (b) The measured transmittance of ICL (10 nm MoO3/10 nm MoO3–Al) and absorbance of ITO/MoO3–Al(10 nm)/BHJ-1(PCDTBT:PC70BM, 65 nm), ICL/BHJ-2(PCDTBT:PC70BM, 105 nm) and their tandem structure. (c) Contour plot of simulated current density of tandem solar cells based on PCDTBT:PC70BM BHJ with different active layer thickness combination.
tandem PSCs on the thicknesses of the two PCDTBT:PC70BM layers. Details of the simulations were given in the Supporting Information (see Figs.S3 and S4). The JSC of the inverted PSCs can reach an optimal value of ca. 6.54 mA cm−2 when the thicknesses of the two PCDTBT:PC70BM BHJ layers in front and rear sub-cells are 65 and 105 nm, respectively. Fig. 3(a) shows the illuminated current density and power conversion efficiency as a function of voltage for the front PCDTBT: PC70BM and rear PCDTBT:PC70BM reference cells and the identicaljunction PCDTBT:PC70BM tandem PSC under 100 mW cm−2 AM 1.5G simulated solar illumination. The corresponding photovoltaic
parameters were summarized in Table 1. The front reference cell with 65 nm PCDTBT:PC70BM BHJ layer yielded a PCE of 6.25%, a VOC of 0.89 V, a JSC of 10.6 mA cm−2 and a FF of 66.3%. The rear reference cell with 105 nm PCDTBT:PC70BM BHJ layer exhibited a PCE of 5.10%, a VOC of 0.85 V, a JSC of 10.5 mA cm−2, and a FF of 57.1%. More than 25 inverted tandem PSCs with the step-Al-doped MoO3 based ICL were fabricated, and the average PCE was 6.7 70.2%. The best tandem cell demonstrated a PCE of 6.88%, a VOC of 1.73 V, a JSC of 6.50 mA cm−2, and a FF of 61.2%. The VOC of the tandem PSC is equal to the sum of the two sub-cells, indicating that the two sub-cells were well linked by the step-Al-doped
Please cite this article as: J. Liu, et al., High-efficiency inverted tandem polymer solar cells with step-Al-doped MoO3 interconnection layer, Solar Energy Materials and Solar Cells (2013), http://dx.doi.org/10.1016/j.solmat.2013.06.034i
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MoO3 based ICL with negligible voltage loss. The measured EQE spectra for the front and rear reference cells and the tandem cell are shown in Fig. 3b. It is noted that the measured JSCs for the front and rear reference cells agreed well with their corresponding EQE spectra. However, the calculated photocurrent of the tandem cell from EQE spectra was only 4.6 mA cm−2, much lower than the measured JSC of 6.50 mA cm−2 due to the mismatched photocurrent of the two sub-cells under monochromatic illumination. For tandem PSCs, the EQEs of the individual sub-cells are usually measured by employing monochromatic light to selectively turn on one of the sub-cells [23,29]. However, this approach is not available for tandem solar cells based on same BHJ layers. New approaches are required to measure the J–V characteristics and the EQE spectra of the sub-cells in a tandem structure. Considering that the MoO3–Al composite have similar optical property, but different WF compared to the pure MoO3 film, we constructed the following devices A–D with different structures:.
Fig. 3. (a) Current density J (filled symbols) and power conversion efficiency η (open symbols) as a function of voltage for the front reference cell (ITO/10 nm MoO3–Al/65 nm BHJ layer/10 nm MoO3/Al), rear reference cell (ITO/10 nm MoO3– Al/105 nm BHJ layer/6 nm MoO3/Al) and their tandem solar cell linked by the stepAl-doped MoO3 ICL. (b) EQE spectra of the front and rear reference cells and the tandem solar cell.
Device A: ITO/MoO3 (10 nm)/PCDTBT:PC70BM (65 nm)/MoO3 (10 nm)/MoO3–Al (10 nm)/ PCDTBT:PC70BM (105 nm)/MoO3 (6 nm)/Al Device B: ITO/MoO3 (10 nm)/PCDTBT:PC70BM (65 nm)/MoO3 (10 nm)/Al Device C: ITO/MoO3–Al (10 nm)/PCDTBT:PC70BM (65 nm)/ MoO3 (5 nm)/MoO3–Al (10 nm)/MoO3 (5 nm)/PCDTBT:PC70BM (105 nm)/MoO3 (6 nm)/Al Device D: ITO/MoO3–Al (10 nm)/MoO3 (5 nm)/PCDTBT:PC70BM (105 nm)/MoO3 (6 nm)/Al
The illuminated J–V characteristics of these devices under 100 mW cm−2 AM 1.5G simulated solar illumination are shown in Fig. 4 and the corresponding photovoltaic parameters were also summarized in Table 1. Different from the above-demonstrated tandem solar cell, the low WF MoO3–Al buffer layer in front subcell was replaced by the high WF MoO3 layer in Device A. The replacement of MoO3–Al by the MoO3 layer did not lead to any variation in optical electric field distribution profile between device A and the tandem solar cell (see Fig. S5 in Supporting Information). As shown in Fig. 4(a), the device B shows an Ohmic behavior upon light illumination. Thus, the structure of MoO3 (10 nm)/PCDTBT:PC70BM (65 nm)/MoO3 (10 nm) in device A acts as a resistance and allow us to measure the photovoltaic characteristics of the rear sub-cell in the tandem structure. Device A exhibits a VOC of 0.84 V, a JSC of 6.54 mA cm−2 and a FF of 58.2%, giving a PCE of 3.20%. The measured photovoltaic performance of device A was attributed to the rear sub-cell in device A. Based on the same principle, the photovoltaic performance of the front subcell in the tandem structure can be measured by device C. As shown in Fig. 4(b), device C exhibited a PCE of 3.60%, a VOC of 0.87 V, a JSC of 6.65 mA cm−2, and a FF of 62.2%. The EQE spectra of the front and rear sub-cells in the tandem solar cell were measured via devices C and A as shown in Fig. 4(c). Although the same PCDTBT:PC70BM BHJ layer was utilized in the two subcells, the spectral responses from the front and rear sub-cells were quite different. The front sub-cell showed a slightly higher EQE at short wavelength region, while the rear sub-cell exhibited higher EQE at long wavelength region. This phenomenon was originated from the different optical electric field distribution in the tandem structure for the different wavelength light (see Fig. S5 in Supporting Information). The calculated photocurrents of the front and rear sub-cells from their corresponding EQE were 6.60 and 6.47 mA cm−2, respectively, very close to the measured values of 6.65 and 6.54 mA cm−2 in devices C and A. As the two sub-cells were series-connected and photocurrents were well matched, an equivalent EQE spectrum of the tandem solar cell was obtained by the superposition of the EQE spectra of two sub-cells as shown in Fig. 4(c). It can be seen that the equivalent EQE spectrum peaks at nearly 80% with very flat spectral response from 400 to 600 nm. This value is higher than 70% of optimal EQE of the single-junction PCDTBT:PC70BM solar cells (the data is not shown here), implying a high charge-collection efficiency in tandem structure.
Table 1 Photovoltaic parameters of the PCDTBT:PC70BM-based solar cells with different device structures under 100 mW cm−2 AM 1.5G irradiation. Device
Front BHJ thickness [nm]
Rear BHJ thickness [nm]
Jsc [mA cm−2]
Voc [V]
FF [%]
PCE [%]
Front reference cell Rear reference cell Measured tandem cell Device A Device C
65 – 65 65 65
– 105 105 105 105
10.6 10.5 6.50 6.54 6.65
0.89 0.85 1.73 0.84 0.87
66.3 57.1 61.2 58.2 62.2
6.25 5.10 6.88 3.20 3.60
Please cite this article as: J. Liu, et al., High-efficiency inverted tandem polymer solar cells with step-Al-doped MoO3 interconnection layer, Solar Energy Materials and Solar Cells (2013), http://dx.doi.org/10.1016/j.solmat.2013.06.034i
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Fig. 5. Contour plot of simulated current density of tandem solar cells using different active layer thickness combinations for PCDTBT based front sub-cell and PDPP3T based rear sub-cell.
Fig. 4. Illuminated J–V characteristics of device A and B (a), device C and D (b). The EQE spectra of the front and rear sub-cells obtained from device C and device A (c). The insets in (a, b) represent the equivalent circuits of device A and C, respectively.
Fig. 6. (a) Current density J (filled symbols) and power conversion efficiency η (open symbols) as a function of voltage for the front reference cell (ITO/10 nm MoO3–Al/150 nm PCDTBT:PC70BM layer/10 nm MoO3/Al), rear reference cell (ITO/ 10 nm MoO3–Al/120 nm PDPP3T:PC70BM layer/6 nm MoO3/Al) and their tandem solar cell linked by the step-Al-doped MoO3 ICL. (b) EQE spectra of the front and rear reference cells, and the front and rear sub-cells in tandem structure.
The tandem PSCs with complementary absorption spectra was further explored using small band-gap PDPP3T to replace PCDTBT in rear sub-cell. Low band-gap PDPP3T was first developed for photovoltaic application by Janssen and his coworkers [42]. Ye et al. have further boosted the performance of PDPP3T-based solar cell by using ternary-solvent strategy to tune the BHJ morphology, enabling this polymer an efficient low band-gap donor for tandem cell application [43]. The absorption of PDPP3T:PC70BM blend was extended to 950 nm as shown in Fig. S6. The optical simulation was also used to implement layout optimization and the corresponding results are shown in Fig. 5.
According to the simulated results, the thicknesses of the front PCDTBT:PC70BM BHJ and the rear PDPP3T:PC70BM BHJ were fixed to be 150 and 120 nm, respectively. The illuminated current density and power conversion efficiency as a function of voltage for the reference cells and tandem cell are shown in Fig. 6(a), and the corresponding photovoltaic parameters were summarized in Table 2. The VOC of tandem cell is 1.53 V, equal to the sum of two sub-cells (0.87 V and 0.67 V, respectively). The overall PCE of the tandem cell is 7.31%, much higher than those of the two reference cells (5.30% and 5.71%). EQE spectra of two reference cells and two sub-cells in tandem structure are shown in Fig. 6(b), which agree
Please cite this article as: J. Liu, et al., High-efficiency inverted tandem polymer solar cells with step-Al-doped MoO3 interconnection layer, Solar Energy Materials and Solar Cells (2013), http://dx.doi.org/10.1016/j.solmat.2013.06.034i
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Table 2 Photovoltaic parameters of the single-junction PCDTBT:PC70BM solar cell (front reference cell), the single-junction PDPP3T:PC70BM solar cell (rear reference cell) and tandem cell constructed by PCDTBT:PC70BM and PDPP3T:PC70BM combination under 100 mW cm−2 AM 1.5G irradiation. Device
Front BHJ thickness [nm]
Rear BHJ thickness [nm]
Jsc [mA cm−2]
Voc [V]
FF [%]
PCE [%]
Front reference cell Rear reference cell Measured tandem cell
150 – 150
– 120 120
10.9 13.5 8.21
0.87 0.67 1.53
55.9 63.2 58.2
5.30 5.71 7.31
with their corresponding JSC within 3% discrepancy. The EQE spectra of sub-cells were extracted using the above-demonstrated strategy. This indicates that our strategy is universal for EQE spectra extraction in two-terminal tandem solar cells regardless of their absorption features. Optical simulation results indicate that the PDPP3T-based rear sub-cell absorbed more photons than PCDTBT-based front subcell (Fig. S7 in Supporting Information). The comparable JSC of rear sub-cell to that of front sub-cell implies low internal quantum efficiency of PDPP3T-based sub-cell. The low FF of the tandem cell is attributed to the thick BHJ of the front cell, in which the holes need to travel a longer distance than electrons in rear sub-cell to reach their recombination sites (see Fig. S7 in Supporting Information) [44]. Therefore, it is expected that the PCE of tandem PSCs with step-Aldoped MoO3 based ICL can be further enhanced if a combination of two high-performance photovoltaic polymers with well-matched absorption spectra are used.
4. Conclusion A highly transparent and physically robust step-Al-doped MoO3 layer was successfully utilized as the interconnection layer to fabricate high-efficiency inverted tandem polymer solar cells. The inverted tandem cell constructed by the same PCDTBT:PC70BM active layer showed a PCE of 6.88% with equivalent EQE values of nearly 80%, implying a high charge-collection efficiency in tandem structure. Incorporation of two sub-cells with complementary absorption spectra leads to further increase of PCE up to 7.3%, which is the best result for tandem PSCs with metal oxide-based interconnection layer. This study highlights that the step-Al-doped MoO3 layer is an excellent interconnection layer for high-efficiency tandem PSCs. In addition, a new strategy is developed to measure the corresponding J–V characteristics and EQE spectra of two subcells in tandem structure regardless of their absorption features.
Acknowledgments Z.-Y. Xie acknowledges the financial support from the National Natural Science Foundation of China (Nos. 51273193) and 973 Project of Ministry of Science and Technology of China (2009CB623602, 2009CB930603). The financial support from the Science and Technology Development Project of Jilin Province is also acknowledged (201105029).
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.solmat.2013.06.034.
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High-efficiency inverted tandem polymer solar cells with step-Al-doped MoO3 interconnection layer Jian Liu a,b, Shuyan Shao a, Gang Fang a,b, Jiantai Wang a,b, Bin Meng a, Zhiyuan Xie a,n, Lixiang Wang a a State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China b University of Chinese Academy of Sciences, Beijing 100039, PR China
art ic l e i nf o
a b s t r a c t
Keywords: Tandem polymer solar cells Bulk heterojunction Interconnection layer Metal oxide Work function
A highly transparent and physically robust step-Al-doped MoO3 layer was successfully utilized as the interconnection layer (ICL) to fabricate high-efficiency inverted tandem polymer solar cells (PSCs). The inverted tandem cell constructed by the same PCDTBT:PC70BM active layer showed a power conversion efficiency (PCE) of 6.88% with equivalent external quantum efficiency of nearly 80%, implying a high charge-collection efficiency in tandem structure. Incorporation of two sub-cells with complementary absorption spectra leads to further increase of PCE over 7.31%, which is the best results for tandem PSCs with PEDOT:PSS-free interconnection layer. & 2013 Elsevier B.V. All rights reserved.
1. Introduction Bulk heterojunction (BHJ) polymer solar cells (PSCs) have attracted considerable interest due to their potential to become a new energy source with low-cost, light-weight and mechanical flexibility [1–15]. The power conversion efficiency (PCE) of singlejunction polymer:fullerene PSCs has been progressively increased to over 9% with the development of advanced conjugated polymer donor materials, fine-tuning of the active layer morphology and optimization of the cell structure [16–21]. The tandem device architecture is considered to be one of the effective approaches to further boost the PCE of PSCs by extending the spectral coverage to solar light [22–29]. Very recently, a highest PCE of 10.6% was reported for PSCs by using tandem architecture [30]. Apart from the optimal combination of the low and wide band-gap donor polymers, the interconnection layers (ICLs) linking the sub-cells in a tandem structure is crucial to the final photovoltaic performance. An ideal ICL should be not only highly transparent and conductive, but also possess different work functions (WFs) at both sides, enabling energetic matching to the highest occupied molecular orbital (HOMO) level of polymer donor and the lowest unoccupied molecular orbital (LUMO) level of acceptor in sub-cells to lower the interfacial resistance and realize efficient recombination of electrons and holes generated in sub-cells. The ICL should also be robust enough to withstand organic solvents during multi-layer solution processing. Up to now, the commonly used ICL in tandem
n
Corresponding author. Tel./fax: +86 431 85262819. E-mail address: [email protected] (Z. Xie).
PSCs is poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS)/n-type transition metal oxide (TiOx or ZnO) nanocrystal bi-layer structure [23,29]. The high conductivity and hydrophilic property of metallic PEDOT:PSS render the ICL very robust and efficient charge recombination layer. However, metallic PEDOT:PSS layer would lead to light attenuation loss in the infrared region [31]. In addition, the incorporation of PEDOT:PSS also raises device instability concerns due to its acidic and hygroscopic nature [32,33]. Transition metal oxides, such as MoO3 and V2O5, have been used to replace the PEDOT:PSS layer in the ICL structure [27,34]. MoO3 is an excellent hole-collecting material. With MoO3 as the hole-collecting layer, some ICL structures such as MoO3/ZnO, MoO3/TiOx, LiF/Al/MoO3 and MoO3/Ag/Al/Ca, have been explored to fabricate tandem PSCs [35–39]. Although the cross-linked n-type TiOx and ZnO prepared from the sol–gel method can prevent organic solvents from penetrating into the underlying BHJ while spincoating the upper BHJ layer, TiOx and ZnO films require high-temperature annealing for completed hydrolysis to realize sufficient conductivity and robustness, which limits the free selection of front sub-cell to optimize the performance of tandem PSCs [34,37]. A combination of MoO3 and low WF metal is also not an ideal ICL in tandem PSC due to possible charge recombination and optical loss induced by the metal layer [38,39]. Herein, we report an efficient MoO3-based ICL in inverted tandem structure by using step-Al-doping approach. Al-doping of top half MoO3 layer can change the high-WF MoO3 into low-WF MoO3–Al composite, and thus provides the large WF offset at two sides of MoO3-based ICL without sacrificing its transparency [40]. More importantly, the MoO3–Al layer was found to be physically
0927-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.solmat.2013.06.034
Please cite this article as: J. Liu, et al., High-efficiency inverted tandem polymer solar cells with step-Al-doped MoO3 interconnection layer, Solar Energy Materials and Solar Cells (2013), http://dx.doi.org/10.1016/j.solmat.2013.06.034i
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robust to withstand organic solvents during multi-layer solution processing owing to the formation of Mo–O–Al complex structure. With this step-Al-doped MoO3 ICL, inverted tandem PSC device constructed by two sub-cells based on a blend of PCDTBT:PC70BM showed a PCE of 6.88% and an equivalent external quantum efficiency (EQE) approaching 80%, implying a high chargecollection efficiency in tandem structure. Tandem PSCs with two sub-cells having complementary absorption spectra were also constructed and a PCE of 7.31% was achieved, which is the best result for tandem PSCs with PEDOT:PSS-free ICL. These results indicate that the step-Al-doped MoO3 layer is an excellent ICL for tandem PSCs. In addition, an approach was developed to measure the current–voltage (J–V) characteristics and EQE spectra of the sub-cells in a tandem structure. Different from the reported method [23,24], the corresponding J–V characteristics and EQE spectra of two sub-cells can be extracted regardless of their absorption features.
(SR830, Stanford Research System) at a chopping frequency of 280 Hz during illumination with a monochromatic light from a Xenon lamp. 2.3. Thin film characterization Transmittance and absorption spectra of the samples were measured using a Perkin-Elmer35 UV-visible spectrophotometer. The refractive index (n and k values) and the thicknesses of the various layers in the device structure were measured using spectroscopic ellipsometry (Horiba Jobin Yvon). UPS measurements were performed on Thermo ESCALAB 250 using He–I (21.2 eV) discharge lamp. A sample bias of −12 V was used in order to separate the sample and the secondary edge for the analyzer.
3. Results and discussion 2. Experimental 2.1. Materials PCDTBT (molecular weight, Mw¼24,000; polydispersity index, PDI¼1.8) was synthesized in our laboratory. PDPP3T (Mw¼ 24,000, PDI¼3.15) was purchased from Solarmer Material Inc. and PC70BM was purchased from American Dye Source Inc. MoO3 was purchased from Sigma-Aldrich (99.5%) and used as received. 2.2. Device fabrication and characterization Inverted tandem polymer solar cells were fabricated on ITOcoated glass substrates. The ITO-coated glass substrates were cleaned with detergent, ultra-sonicated in de-ionized water, acetone, and isopropyl alcohol in sequence, and subsequently dried at 120 1C in an oven overnight. The MoO3–Al composite cathode buffer layer (10 nm) with 55% Al content in weight percentage was thermally deposited on ITO substrate by co-evaporation in a vacuum chamber under a base pressure of 4 10−6 Torr. For the tandem PSCs, a solution containing a mixture of PCDTBT:PC70BM (1:4, w/w) in dichlorobenzene with a PCDTBT concentration of 3.2 or 4.3 mg mL−1 was spin-cast on top of the MoO3–Al composite layer to produce a 65 or 150-nm-thick active layer as the front BHJ. The step-Al-doped MoO3 ICL was deposited atop the first active layer with two sequential processes: firstly, 10-nm pure MoO3 without Al-doping was evaporated on top of active layer; secondly, MoO3 and Al were co-evaporated on 10-nm pure MoO3 film and the Al doped content was set as 55% in weight percentage by modulating the corresponding evaporation speeds. The thickness of Al-doped MoO3 film was 10 nm. Then, the rear BHJ layer of the PCDTBT:PC70BM (1:4, w/w; 105 nm) or PDPP3T:PC70BM (1:2, w/w; 120 nm) was deposited on top of the MoO3-based ICL via spincoating. The PDPP3T:PC70BM (1:2, w/w) blend was dissolved in a solution mixture of 1,2-dichlorobenzene/chloroform/1, 8-diiodooctane (0.76:0.19:0.05, v/v/v) with PDPP3T concentration of 8 mg mL−1. Finally, a bi-layer structure of MoO3 (6 nm)/Al (80 nm) was deposited atop the rear BHJ layer via thermal evaporation in a vacuum of 4 10−6 Torr to complete the device fabrication. The cell active area was 12 mm2, which was defined by the overlapping area of the ITO and Al electrodes. An Oriel 150 W solar simulator with AM 1.5G filter was used to provide 100 mW cm−2 simulated solar light for illumination of the photovoltaic cells. The light intensity was determined by a calibrated silicon diode with KG-5 visible color filter. Current–voltage traces were obtained with a Keithley 236 source meter. External quantum efficiency measurements were performed under short-circuit conditions with a lock-in amplifier
Fig. 1(a) shows the device configuration of the inverted tandem PSCs. The Al content in MoO3–Al composite (cathode buffer layer) is 55% in weight percentage. The two sub-cells with identical or complementary BHJ are linked by MoO3-based ICL with top half doped with 55% Al in weight percentage. In the former case, both of two sub-cells were based on PCDTBT:PC70BM BHJ; in the latter case, a PDPP3T:PC70BM BHJ was used to replace the PCDTBT: PC70BM BHJ in rear sub-cell to extend the absorption spectra coverage [41]. The molecular structures of PCDTBT, PDPP3T and PC70BM are shown in Fig. 1(b). The pure MoO3 film possesses a high work function of 5.49 eV and upon the coverage of Al-doped MoO3 film the work function was reduced to 4.07 eV (See Fig. S1 in Supporting Information). Fig. 1(c) illustrates the energy level diagram of the tandem cell structure. The MoO3–Al composite with low work function is able to form an Ohmic contact with the LUMO of PC70BM as a cathode buffer layer on the ITO electrode. The pure MoO3 layer with high work function serves as the top anode in combination with Al electrode. The step-Al-doped MoO3 ICL was used to link the two BHJ layers with the pure MoO3 side contacting with front sub-cell and the MoO3–Al side contacting with rear sub-cell. Thus, the HOMO level of polymer donor in front sub-cell is well aligned with the LUMO level of fullerene acceptor in rear sub-cell, enabling the efficient charge recombination in ICL. The effective conductivity of such ICL was measured to be 8.5 10−7 S/cm and just brought in an additional series resistance of 2.3 Ω cm2(see Fig. S2 in Supporting Information). Thus the electron can easily tunnel the ICL with negligible voltage drop. Fig. 2(a) displays the optical constants (n and k) of MoO3 and MoO3–Al layers measured with the spectroscopic ellipsometry. The extinction coefficient dispersion of MoO3–Al layer is redshifted compared to that of MoO3 layer, possibly due to reduction of effective band gap. The ICL structure of MoO3(10 nm)/MoO3–Al (10 nm) exhibits a high transparency of over 95% as shown in Fig. 2(b). Fig. 2(b) shows the absorption of the ITO/MoO3–Al/ PCDTBT:PC70BM, ICL/PCDTBT:PC70BM and ITO/MoO3–Al/PCDTBT: PC70BM ICL/PCDTBT:PC70BM structures. The superposition of the BHJ absorption in sub-cells fits well with the absorption of the tandem structure, indicating that spin-coating the upper BHJ layer did not deteriorate the underlying MoO3-based ICL and the bottom BHJ layer. Previous work demonstrated that Mo–O–Al complex structure was formed after Al doping [40]. Such complex structure makes the whole layer cross-linked and consequently physically robust to withstand organic solvents. The optical modeling based on the classic transfer matrix method was used to optimize the thickness combinations of two BHJ layers of front and rear subcells linked by the MoO3-based ICL [41]. Fig. 2(c) shows a dependence of simulated short-circuit current (JSC) of the inverted
Please cite this article as: J. Liu, et al., High-efficiency inverted tandem polymer solar cells with step-Al-doped MoO3 interconnection layer, Solar Energy Materials and Solar Cells (2013), http://dx.doi.org/10.1016/j.solmat.2013.06.034i
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Fig. 1. (a) The layout of the tandem solar cell. (b) The molecular structures of PCDTBT, PDPP3T and PC70BM materials. (c) The schematic of the energy level diagram of the inverted tandem polymer solar cell.
Fig. 2. (a) The refractive index (n) and extinction coefficient (k) of MoO3 and MoO3–Al layers. (b) The measured transmittance of ICL (10 nm MoO3/10 nm MoO3–Al) and absorbance of ITO/MoO3–Al(10 nm)/BHJ-1(PCDTBT:PC70BM, 65 nm), ICL/BHJ-2(PCDTBT:PC70BM, 105 nm) and their tandem structure. (c) Contour plot of simulated current density of tandem solar cells based on PCDTBT:PC70BM BHJ with different active layer thickness combination.
tandem PSCs on the thicknesses of the two PCDTBT:PC70BM layers. Details of the simulations were given in the Supporting Information (see Figs.S3 and S4). The JSC of the inverted PSCs can reach an optimal value of ca. 6.54 mA cm−2 when the thicknesses of the two PCDTBT:PC70BM BHJ layers in front and rear sub-cells are 65 and 105 nm, respectively. Fig. 3(a) shows the illuminated current density and power conversion efficiency as a function of voltage for the front PCDTBT: PC70BM and rear PCDTBT:PC70BM reference cells and the identicaljunction PCDTBT:PC70BM tandem PSC under 100 mW cm−2 AM 1.5G simulated solar illumination. The corresponding photovoltaic
parameters were summarized in Table 1. The front reference cell with 65 nm PCDTBT:PC70BM BHJ layer yielded a PCE of 6.25%, a VOC of 0.89 V, a JSC of 10.6 mA cm−2 and a FF of 66.3%. The rear reference cell with 105 nm PCDTBT:PC70BM BHJ layer exhibited a PCE of 5.10%, a VOC of 0.85 V, a JSC of 10.5 mA cm−2, and a FF of 57.1%. More than 25 inverted tandem PSCs with the step-Al-doped MoO3 based ICL were fabricated, and the average PCE was 6.7 70.2%. The best tandem cell demonstrated a PCE of 6.88%, a VOC of 1.73 V, a JSC of 6.50 mA cm−2, and a FF of 61.2%. The VOC of the tandem PSC is equal to the sum of the two sub-cells, indicating that the two sub-cells were well linked by the step-Al-doped
Please cite this article as: J. Liu, et al., High-efficiency inverted tandem polymer solar cells with step-Al-doped MoO3 interconnection layer, Solar Energy Materials and Solar Cells (2013), http://dx.doi.org/10.1016/j.solmat.2013.06.034i
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MoO3 based ICL with negligible voltage loss. The measured EQE spectra for the front and rear reference cells and the tandem cell are shown in Fig. 3b. It is noted that the measured JSCs for the front and rear reference cells agreed well with their corresponding EQE spectra. However, the calculated photocurrent of the tandem cell from EQE spectra was only 4.6 mA cm−2, much lower than the measured JSC of 6.50 mA cm−2 due to the mismatched photocurrent of the two sub-cells under monochromatic illumination. For tandem PSCs, the EQEs of the individual sub-cells are usually measured by employing monochromatic light to selectively turn on one of the sub-cells [23,29]. However, this approach is not available for tandem solar cells based on same BHJ layers. New approaches are required to measure the J–V characteristics and the EQE spectra of the sub-cells in a tandem structure. Considering that the MoO3–Al composite have similar optical property, but different WF compared to the pure MoO3 film, we constructed the following devices A–D with different structures:.
Fig. 3. (a) Current density J (filled symbols) and power conversion efficiency η (open symbols) as a function of voltage for the front reference cell (ITO/10 nm MoO3–Al/65 nm BHJ layer/10 nm MoO3/Al), rear reference cell (ITO/10 nm MoO3– Al/105 nm BHJ layer/6 nm MoO3/Al) and their tandem solar cell linked by the stepAl-doped MoO3 ICL. (b) EQE spectra of the front and rear reference cells and the tandem solar cell.
Device A: ITO/MoO3 (10 nm)/PCDTBT:PC70BM (65 nm)/MoO3 (10 nm)/MoO3–Al (10 nm)/ PCDTBT:PC70BM (105 nm)/MoO3 (6 nm)/Al Device B: ITO/MoO3 (10 nm)/PCDTBT:PC70BM (65 nm)/MoO3 (10 nm)/Al Device C: ITO/MoO3–Al (10 nm)/PCDTBT:PC70BM (65 nm)/ MoO3 (5 nm)/MoO3–Al (10 nm)/MoO3 (5 nm)/PCDTBT:PC70BM (105 nm)/MoO3 (6 nm)/Al Device D: ITO/MoO3–Al (10 nm)/MoO3 (5 nm)/PCDTBT:PC70BM (105 nm)/MoO3 (6 nm)/Al
The illuminated J–V characteristics of these devices under 100 mW cm−2 AM 1.5G simulated solar illumination are shown in Fig. 4 and the corresponding photovoltaic parameters were also summarized in Table 1. Different from the above-demonstrated tandem solar cell, the low WF MoO3–Al buffer layer in front subcell was replaced by the high WF MoO3 layer in Device A. The replacement of MoO3–Al by the MoO3 layer did not lead to any variation in optical electric field distribution profile between device A and the tandem solar cell (see Fig. S5 in Supporting Information). As shown in Fig. 4(a), the device B shows an Ohmic behavior upon light illumination. Thus, the structure of MoO3 (10 nm)/PCDTBT:PC70BM (65 nm)/MoO3 (10 nm) in device A acts as a resistance and allow us to measure the photovoltaic characteristics of the rear sub-cell in the tandem structure. Device A exhibits a VOC of 0.84 V, a JSC of 6.54 mA cm−2 and a FF of 58.2%, giving a PCE of 3.20%. The measured photovoltaic performance of device A was attributed to the rear sub-cell in device A. Based on the same principle, the photovoltaic performance of the front subcell in the tandem structure can be measured by device C. As shown in Fig. 4(b), device C exhibited a PCE of 3.60%, a VOC of 0.87 V, a JSC of 6.65 mA cm−2, and a FF of 62.2%. The EQE spectra of the front and rear sub-cells in the tandem solar cell were measured via devices C and A as shown in Fig. 4(c). Although the same PCDTBT:PC70BM BHJ layer was utilized in the two subcells, the spectral responses from the front and rear sub-cells were quite different. The front sub-cell showed a slightly higher EQE at short wavelength region, while the rear sub-cell exhibited higher EQE at long wavelength region. This phenomenon was originated from the different optical electric field distribution in the tandem structure for the different wavelength light (see Fig. S5 in Supporting Information). The calculated photocurrents of the front and rear sub-cells from their corresponding EQE were 6.60 and 6.47 mA cm−2, respectively, very close to the measured values of 6.65 and 6.54 mA cm−2 in devices C and A. As the two sub-cells were series-connected and photocurrents were well matched, an equivalent EQE spectrum of the tandem solar cell was obtained by the superposition of the EQE spectra of two sub-cells as shown in Fig. 4(c). It can be seen that the equivalent EQE spectrum peaks at nearly 80% with very flat spectral response from 400 to 600 nm. This value is higher than 70% of optimal EQE of the single-junction PCDTBT:PC70BM solar cells (the data is not shown here), implying a high charge-collection efficiency in tandem structure.
Table 1 Photovoltaic parameters of the PCDTBT:PC70BM-based solar cells with different device structures under 100 mW cm−2 AM 1.5G irradiation. Device
Front BHJ thickness [nm]
Rear BHJ thickness [nm]
Jsc [mA cm−2]
Voc [V]
FF [%]
PCE [%]
Front reference cell Rear reference cell Measured tandem cell Device A Device C
65 – 65 65 65
– 105 105 105 105
10.6 10.5 6.50 6.54 6.65
0.89 0.85 1.73 0.84 0.87
66.3 57.1 61.2 58.2 62.2
6.25 5.10 6.88 3.20 3.60
Please cite this article as: J. Liu, et al., High-efficiency inverted tandem polymer solar cells with step-Al-doped MoO3 interconnection layer, Solar Energy Materials and Solar Cells (2013), http://dx.doi.org/10.1016/j.solmat.2013.06.034i
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Fig. 5. Contour plot of simulated current density of tandem solar cells using different active layer thickness combinations for PCDTBT based front sub-cell and PDPP3T based rear sub-cell.
Fig. 4. Illuminated J–V characteristics of device A and B (a), device C and D (b). The EQE spectra of the front and rear sub-cells obtained from device C and device A (c). The insets in (a, b) represent the equivalent circuits of device A and C, respectively.
Fig. 6. (a) Current density J (filled symbols) and power conversion efficiency η (open symbols) as a function of voltage for the front reference cell (ITO/10 nm MoO3–Al/150 nm PCDTBT:PC70BM layer/10 nm MoO3/Al), rear reference cell (ITO/ 10 nm MoO3–Al/120 nm PDPP3T:PC70BM layer/6 nm MoO3/Al) and their tandem solar cell linked by the step-Al-doped MoO3 ICL. (b) EQE spectra of the front and rear reference cells, and the front and rear sub-cells in tandem structure.
The tandem PSCs with complementary absorption spectra was further explored using small band-gap PDPP3T to replace PCDTBT in rear sub-cell. Low band-gap PDPP3T was first developed for photovoltaic application by Janssen and his coworkers [42]. Ye et al. have further boosted the performance of PDPP3T-based solar cell by using ternary-solvent strategy to tune the BHJ morphology, enabling this polymer an efficient low band-gap donor for tandem cell application [43]. The absorption of PDPP3T:PC70BM blend was extended to 950 nm as shown in Fig. S6. The optical simulation was also used to implement layout optimization and the corresponding results are shown in Fig. 5.
According to the simulated results, the thicknesses of the front PCDTBT:PC70BM BHJ and the rear PDPP3T:PC70BM BHJ were fixed to be 150 and 120 nm, respectively. The illuminated current density and power conversion efficiency as a function of voltage for the reference cells and tandem cell are shown in Fig. 6(a), and the corresponding photovoltaic parameters were summarized in Table 2. The VOC of tandem cell is 1.53 V, equal to the sum of two sub-cells (0.87 V and 0.67 V, respectively). The overall PCE of the tandem cell is 7.31%, much higher than those of the two reference cells (5.30% and 5.71%). EQE spectra of two reference cells and two sub-cells in tandem structure are shown in Fig. 6(b), which agree
Please cite this article as: J. Liu, et al., High-efficiency inverted tandem polymer solar cells with step-Al-doped MoO3 interconnection layer, Solar Energy Materials and Solar Cells (2013), http://dx.doi.org/10.1016/j.solmat.2013.06.034i
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Table 2 Photovoltaic parameters of the single-junction PCDTBT:PC70BM solar cell (front reference cell), the single-junction PDPP3T:PC70BM solar cell (rear reference cell) and tandem cell constructed by PCDTBT:PC70BM and PDPP3T:PC70BM combination under 100 mW cm−2 AM 1.5G irradiation. Device
Front BHJ thickness [nm]
Rear BHJ thickness [nm]
Jsc [mA cm−2]
Voc [V]
FF [%]
PCE [%]
Front reference cell Rear reference cell Measured tandem cell
150 – 150
– 120 120
10.9 13.5 8.21
0.87 0.67 1.53
55.9 63.2 58.2
5.30 5.71 7.31
with their corresponding JSC within 3% discrepancy. The EQE spectra of sub-cells were extracted using the above-demonstrated strategy. This indicates that our strategy is universal for EQE spectra extraction in two-terminal tandem solar cells regardless of their absorption features. Optical simulation results indicate that the PDPP3T-based rear sub-cell absorbed more photons than PCDTBT-based front subcell (Fig. S7 in Supporting Information). The comparable JSC of rear sub-cell to that of front sub-cell implies low internal quantum efficiency of PDPP3T-based sub-cell. The low FF of the tandem cell is attributed to the thick BHJ of the front cell, in which the holes need to travel a longer distance than electrons in rear sub-cell to reach their recombination sites (see Fig. S7 in Supporting Information) [44]. Therefore, it is expected that the PCE of tandem PSCs with step-Aldoped MoO3 based ICL can be further enhanced if a combination of two high-performance photovoltaic polymers with well-matched absorption spectra are used.
4. Conclusion A highly transparent and physically robust step-Al-doped MoO3 layer was successfully utilized as the interconnection layer to fabricate high-efficiency inverted tandem polymer solar cells. The inverted tandem cell constructed by the same PCDTBT:PC70BM active layer showed a PCE of 6.88% with equivalent EQE values of nearly 80%, implying a high charge-collection efficiency in tandem structure. Incorporation of two sub-cells with complementary absorption spectra leads to further increase of PCE up to 7.3%, which is the best result for tandem PSCs with metal oxide-based interconnection layer. This study highlights that the step-Al-doped MoO3 layer is an excellent interconnection layer for high-efficiency tandem PSCs. In addition, a new strategy is developed to measure the corresponding J–V characteristics and EQE spectra of two subcells in tandem structure regardless of their absorption features.
Acknowledgments Z.-Y. Xie acknowledges the financial support from the National Natural Science Foundation of China (Nos. 51273193) and 973 Project of Ministry of Science and Technology of China (2009CB623602, 2009CB930603). The financial support from the Science and Technology Development Project of Jilin Province is also acknowledged (201105029).
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.solmat.2013.06.034.
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