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Microcavity-Enhanced Light-Trapping for Highly Efficient Organic Parallel Tandem Solar Cells Lijian Zuo, Chu-Chen Chueh, Yun-Xiang Xu, Kung-Shih Chen, Yue Zang, Chang-Zhi Li, Hongzheng Chen,* and Alex K.-Y. Jen* Recently, sophisticated optimization using rational molecular design,[1–3] morphological control,[4] interlayer engineering,[5] and optical manipulation[6] have been employed as effective tools for achieving high-performance organic photovoltaics (OPVs).[7–10] Based on these efforts, OPV devices with greater than 10% power conversion efficiency (PCE) have been achieved, which greatly enhances their chances for commercialization. However, due to significant energy loss arising from low dielectric constants and charge-transporting properties of organic semiconductors,[11,12] the photoactive layer of most OPVs has been limited to thickness around 100–200 nm to avoid severe charge recombination. Such thin photoactive layers often can not completely absorb the incident light which restricts the upper limit of device external quantum efficiency (EQE) to be ∼70% despite the internal quantum efficiency (IQE) of the photoactive layer can approach over 90%.[13,14] To harvest light more efficiently, several light-trapping strategies have been exploited to enhance optical density in the photoactive layer. Among which, multi-junction stacking is considered to be a very promising approach due to its ability to extend the optical paths for harvesting light.[9,10] Till now, most of the reported tandem OPVs adopt the series connected configuration since the complementary absorption from each junction with different bandgap (Eg) allows different regions of light to be harvested in the cells.[9,15–17] In general, the two sub-cells in the series architecture are bridged by an interconnecting layer (ICL), which serves as the charge recombination channel to tunnel through the stacking junctions.[10,18] Recently, a record-high PCE (10.6%) of OPVs has been achieved in such a series connected tandem configuration.[9] According to the Kirchhoff circuit laws, the PCE of a series connected tandem cell (denoted as series tandem cell hereafter) will reach its maximum value when the short-circuit current (JSC) of each sub-cell is well matched.[19] It requires intricate control of the photoactive layer thickness in each sub-cell to L. Zuo, Dr. C.-C. Chueh, Dr. Y.-X. Xu, K.-S. Chen, Y. Zang, Dr. C.-Z. Li, Prof. A. K.-Y. Jen Department of Materials Science and Engineering University of Washington Seattle, USA E-mail: [email protected] L. Zuo, Prof. H. Z. Chen, Prof. A. K.-Y. Jen State Key Laboratory of Silicon Materials MOE Key Laboratory of Macromolecular Synthesis and Functionalization Zhejiang University Hangzhou 310027, P. R. China E-mail: [email protected]
DOI: 10.1002/adma.201402782
Adv. Mater. 2014, DOI: 10.1002/adma.201402782
accomplish optimal optical distribution in the stratified device to achieve balanced JSC of constituent sub-cells.[9,20] As for a parallel connected tandem cell (denoted as parallel tandem cell hereafter), it only needs matching of the open-circuit voltage (VOC) of each sub-cell instead.[21–24] This provides a convenient way to optimize photovoltaic performance by employing unitary materials in both sub-cells due to matched VOC. Moreover, the parallel tandem cells possess the three-terminal nature, which enables two sub-cells to work individually so it can still function even if one of the cells fails. Nevertheless, the progress of organic parallel tandem cells is lagging behind that of the series connected tandem cells because the difficulty in finding highly conductive and transparent intermediate electrodes, which limits its PCE to be around 5% at most.[22] Recently, the development of transparent electrodes has proceeded rapidly in emerging transparent electronics, including highly conductive PEDOT:PSS,[25,26] Ag nanowires,[27,28] graphene,[29] and ultra-thin metal film (UTMF).[30,31] In particular, the development of semi-transparent organic photovoltaics (STOPVs) has drawn significant attraction due to their potential for building integration. By using transparent electrodes, ST-OPVs can convert partial sunlight into electrical power and modulate the wavelengths of transmitted light for efficient energy conservation. For example, ultra-thin Ag (8–16 nm) with high conductivity and transparency has been shown to function as efficient transparent electrode for ST-OPVs to result in high PCE (∼6%) and good average visible light transmittance (AVT).[31] The decent reflectivity of UTMF enables microcavity to be formed with another reflective electrode for fabricating highly efficient and ITO-free flexible OPVs.[6,32,33] Since microcavity reinforces the optically confined incident lights that have resonant frequencies due to coherent interference, it leads to comparable or even higher PCE than those derived from the ITO-based devices.[6,34,35] Furthermore, the resonant light in an optical microcavity can be manipulated by varying the layer thickness within the stratified device, especially the thickness of the photoactive layer. Given these appealing features, it is feasible to use UTMF as an intermediate electrode to construct highly efficient parallel tandem cells. As shown in Figure 1a, the parallel tandem cell comprises a front ST-OPV sub-cell and a back top-illuminated ITO free OPV sub-cell that are bridged by an intermediate ultrathin Ag electrode. In addition to the advantage of extending light paths within the tandem structure, the microcavity formed in the back sub-cell can further enhance the absorption of the transmitted photons with specific resonant frequencies. Especially, the microcavity can also be tuned to enhance the absorption of low energy photons near the band edge of the photoactive layer.
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Figure 1. (a) Schematic diagram of the studied parallel tandem cell, constructed from a “front” semi-transparent sub-cell and a “back” top-illuminated ITO-free sub-cell. (b) Molecular structures of the studied materials.
In this study, we demonstrate the potential of using such a parallel tandem architecture to enhance efficiency of devices (20–25%) consisting of identical junction of diverse BHJ systems compared to those obtained from their optimized single junction devices (Figure 1b). The improved EQE (∼80%) verifies that enhanced light-trapping can be achieved through this parallel tandem architecture which has better optical management. Since matched photocurrent generated from each subcell is not required, two polymers with distinctly different absorption profiles can be stacked together to explore the efficacy of the parallel tandem architecture. A very high PCE of 9.2% could be achieved without the tedious matching of JSC 2
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from each sub-cell. This demonstrates that innovative optical manipulation can be used to effectively improve the performance of OPVs. Because the performance of tandem devices is strongly dependent on the properties of the intermediate electrodes, the characteristics of UTMF electrodes were carefully evaluated through both experiments and simulations in terms of transmittance (T) and resistance (R). The transfer matrix method (TMM) was used for simulation in this study.[36] It has been shown to provide useful guidance for optimizing the optical field distribution within the device.[6] The detailed information regarding TMM simulation is described in the supporting
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information. In general, the conductivity of ultra-thin Ag decreases dramatically if its thickness is below the percolation threshold (∼10 nm) due to the formation of inhomogeneous islands. However, this problem can be alleviated by using a proper seeding layer to improve the compatibility with ultrathin Ag.[35,37] As shown in Figure S1a, an ultra-thin Ag layer with a thickness of ∼10 nm deposited on top of PEDOT:PSS has an even lower resistance (∼12 Ω/ⵧ) than that of ITO on glass (∼15 Ω/ⵧ). The result from measuring optical properties of the ultra-thin Ag (10–12 nm) depicted in Figure S1b reveals that most of the optical loss stems from the reflection while the absorption loss caused by UTMF is only accounted for ∼5% due to the intrinsically low absorption of Ag (Figure S1c). However, such reflectivity can be advantageous when it is used as the intermediate electrode in the parallel tandem structure since the reflected light can be re-coupled into the front photoactive layer. Moreover, the transmitted light can extend its pathway to the back sub-cell to enable the efficient utilization of the incident sunlight. Two highly efficient polymers, PIDTT-DFQT[38] and PIDTTDFBT,[39] were chosen to be blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the identical double-junction BHJ systems for evaluation. Their absorption spectra are shown in Figure S2. Prior to validate the parallel tandem device, two individual front/back single junction sub-cells (using PIDTT-DFQT:PC71BM BHJ as a representative system) were fabricated to investigate their optical and electrical properties. The ST-OPV was fabricated with the configuration of ITO/ZnO (20 nm)/BHJ (75 nm)/MoO3 (6 nm)/UTMF (10 nm) while the top-illuminated ITO-free device was made in the configuration of glass/opaque Ag (100 nm)/ZnO (20 nm) /BHJ (45–75 nm)/ MoO3 (6 nm)/UTMF (14 nm)/MoO3 (50 nm). As shown in Figure S3 and Table S1, the ST-OPV employing UTMF as the top electrode maintains ∼80% of the pristine single junction cell performance which exhibits a high PCE of ∼5% and a high transmittance of over 40% at 700 nm, suggesting its suitability to serve as a front sub-cell.[30,31] In the proposed parallel tandem architecture (Figure 1a), a backside top-illuminated ITO-free cell adopting the UTMF microcavity configuration was used to re-absorb the transmitted light from ST-OPV, especially for the low energy photons near the band edge of organic semiconductors. As known, the optical microcavity formed between two highly reflective surfaces could confine specific wavelengths of light by resonant recirculation.[40–43] Our recent result has demonstrated that the performance of device could be substantially improved in such UTMF microcavity architecture, due to enhanced photocurrents.[6] The oscillating characteristics of microcavity can also be manipulated by varying the thickness of photoactive layer, capping layer, and UTMF.[31] As shown from the simulation results in Figure S4, the center of the maximum oscillating light intensity of the device (based on the aforementioned configuration) red-shifts gradually from 600 to 700 nm when the thickness of the BHJ layer is increased from 45 to 75 nm to result in varied resonant length of the oscillating chamber. Benefitting from such microcavity effect, a calculated EQEMAX of over 80% (Figure S4d) can be achieved, which outperforms
that calculated from the ITO counterpart (∼70%, Figure 2c). Figure S5 shows the J−V curves and the pictures of the topilluminated ITO-free devices with different BHJ thicknesses, in which the thickness of each interlayer is identical to those used for TMM simulation. As can be seen, the colors of these devices changed apparently, indicating different optical distribution within the device chambers as suggested by the simulation (Figure S4). More importantly, the fabricated top-illuminated ITO-free device employing such a microcavity configuration showed even higher performance than that based on the regular ITO substrate (Table S1). This testifies the effectiveness of using UTMF microcavity configuration in the back sub-cell to exploit low energy photons near the band edge of polymers (Figure S4d). Based on this finding, the parallel tandem solar cell employing ultra-thin Ag intermediate anode was fabricated and the detailed layout was illustrated in Figure S6. Assisted by the TMM simulation, the JSC, optical distribution, and EQE of the corresponding devices can be pre-evaluated by modeling. Figure 2a shows the relationship of JSC on the photoactive layer thickness of each sub-cell and the resultant tandem cell based on PIDTT-DFQT:PC71BM BHJ (the thicknesses of other interlayers and electrodes are kept the same under the optimized conditions), where the green, orange, and purple surfaces represent the front, back, and the parallel tandem cell, respectively. Assuming 100% IQE, the highest achievable JSC (∼15 mA/cm2) of the parallel tandem solar cell is 25% higher than that of optimized single junction cell (∼12 mA/cm2, Figure S7a) and is very close to the upper limit value of this system (∼17 mA/cm2, Figure S7a). It is worth to note that the back cell can generate a maximum JSC at the BHJ thickness of ∼75 nm independent of the front cell BHJ thickness. It is attributed to the microcavity formed in the back sub-cell, which not only enhances the light in-coupling but also enables certain wavelengths to be trapped within the back sub-cell (Figure 2b). Thus, a broad range of BHJ thickness in sub-cells (40–200 nm for front sub-cell and 60–80 nm for back sub-cell) can be chosen without significantly sacrificing JSC (13.5–14.9 mA/cm2). On the contrary, the required JSC matching for the series connected tandem device strongly limits the thickness variation of each sub-cell.[44] Matched JSC can only be achieved along the crossed line of the green/orange surfaces (Figure 2a), indicating the intricate thickness control needed in the series tandem configuration. As shown in Figure 2c, the microcavity-enhanced light trapping from back sub-cell compensates the weak absorption of low energy photons (550–700 nm), leading to improved light harvesting across the whole absorption region of BHJ. The EQE of the parallel tandem cell is significantly enhanced (∼25%) in the region between 450 and 700 nm compared to that of the single junction devices, which correspond to a JSC of around 15 and 12 mA/cm2, respectively. For the front subcell, the broad absorption between 300 and 650 nm (comparable to the pristine single junction cell) contributes to a JSC of ∼8.0 mA/cm2. Meanwhile, the back sub-cell exhibits a lower plateau between 300 and 550 nm and an enhanced absorption band between 600 and 700 nm, which contributes to a JSC of ∼6.9 mA/cm2. By simply superimpose these two sub-cells as a parallel tandem cell, it leads to an enhanced current in the
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Figure 2. Thickness dependence of Jsc of the front sub-cell (middle surface), back sub-cell (bottom surface), and parallel tandem cell (top surface) base on (a) PIDTT-DFQT:PC71BM and (d) PIDTT-DFBT:PC71BM BHJ. Light intensity distribution in the parallel tandem solar cells based on (b) PIDTTDFQT:PC71BM and (e) PIDTT-DFBT:PC71BM; Calculated EQE of the conventional single junction solar cell, each sub-cells, and the parallel tandem solar cell for (c) PIDTT-DFQT:PC71BM and (f) PIDTT-DFBT:PC71BM based devices. All simulation is based on the assumption of 100% IQE.
region between 450 and 700 nm, demonstrating the effectiveness of this microcavity architecture. Similar microcavity-enhanced light-trapping phenomena can also be observed in the PIDTT-DFBT:PC71BM based BHJ. As shown in Figure 2d, the maximum JSC of the back sub-cell can be reached at a BHJ thickness of ∼80 nm independent to the BHJ thickness of the front cell. Similar enhancement (∼22%) in JSC (17.5 mA/cm2) can be achieved in this parallel tandem configuration compared to that obtained in the single junction cell (14.4 mA/cm2, Figure S7b). As shown in Figure 2f, the microcavity of the back sub-cell (Figure 2e) helps increase the EQE of parallel tandem cell in the region between 600 and 750 nm. The resulting high EQEs (over 80% for both BHJ systems) for these devices show the promise of further improving the OPV device performance. Finally, the parallel tandem devices were fabricated based on the optimal conditions generated from optical simulation (shown in Figure 2b and 2e). The JSC of such parallel tandem structure was not sensitive to the ultra-thin Ag thickness due to the trade-off between reflection and transmittance (Table S2). After thorough device engineering, an optimum intermediate anode comprising PEDOT:PSS (40 nm), ultrathin Ag (10 nm), and MoO3 (20 nm), was employed because it can: 1) provide robust solvent resistance for multilayer integration; 2) form Ohmic contact for both sub-cells; and 3) possess respectable conductivity and transparency. Figure 3 shows the J–V and EQE curves of the fabricated parallel tandem cells based on identical double-junction BHJ and their relevant parameters are summarized in Table 1. Note that the resultant
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three-terminal parallel tandem cell enables us to measure the sub-cells separately. Both parallel tandem cells show much higher performance than the pristine single junction cell as predicted from the TMM simulation. For the PIDTT-DFQT:PC71BM system, the parallel tandem cell shows a high JSC of 14.38 mA/cm2 with a VOC of 0.91 V, a FF of 0.62, and a resultant PCE of 8.12%, which is 25% higher than the PCE obtained from the single junction cell (VOC: 0.93, JSC: 11.21, FF: 0.63, and PCE: 6.52%) as shown in Figure 3a. The front sub-cell exhibits a VOC of 0.89 V, a JSC of 7.79 mA/cm2, a FF of 0.60, and a PCE of 4.16% while the back sub-cell gives a VOC of 0.92 V, a JSC of 6.64 mA/cm2, a FF of 0.66, and a PCE of 4.07%. The comparable performance between the sum of two sub-cells and the tandem cell validates the fabricated parallel tandem architecture. The enhanced JSC of parallel tandem cell is mainly due to the improved light harvesting of wavelength beyond 450 nm as the EQE curves indicated in Figure 3b. This result is the same as the optical simulation predicted using the front ST-OPV and back UTMF microcavity configuration (Figure 2c). The significantly enhanced EQE value (>80%) in the back sub-cell is among the highest values reported in the literature. For another efficient BHJ based on PIDTT-DFBT:PC71BM, a 20% PCE enhancement was also predicted by optical simulation in the parallel tandem cell (Figure 2d–f). Compared to PIDTT-DFQT, PIDTT-DFBT has a deeper HOMO and more intense intramolecular charge transfer (Figure S2b). As shown in Figure 3c, the tandem cell possesses a high JSC of 15.25 mA/cm2, a VOC of 0.96 V, a FF of 0.59, and a high PCE of
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COMMUNICATION Figure 3. (a, c) J–V curves and (b, d) EQE spectra of the single junction cell, the front/back subcells, and the parallel tandem cell based on the BHJ layer of (a, b) PIDTT-DFQT:PC71BM and (c, d) PIDTT-DFBT:PC71BM.
8.72%. The PCE is ∼ 20% higher than that obtained from single junction cell (7.32%). In which, the front sub-cell contributes a PCE of 4.33% with a VOC of 0.94 V, a JSC of 7.97 mA/cm2, and a FF of 0.57, while the back sub-cell has a PCE of 4.57%, with a VOC of 0.98 V, a JSC of 7.44 mA/cm2, and a FF of 0.63. The FF in the front sub-cell is relatively low. This may be due to mismatched energy level between the BHJ and the modified PEDOT:PSS or the poor contact created by dewetting between the hydrophilic PEDOT:PSS and the hydrophobic BHJ surface.
This can be easily resolved by inserting a thin MoO3 layer in between as the stratified structure shown in Figure 2e.[10] As a result, enhanced EQE curves could be achieved again in the region between 600 and 800 nm (Figure 3d), which is consistent with the result of simulation (Figure 2f). All these results suggest that such parallel tandem architecture is an excellent platform for exploring the versatile high-performance donor polymers. To verify this, we have further evaluated the general applicability of this parallel tandem
Table 1. Summarized device performance of the studied solar cells. Polymers PIDTT-DFQT
PIDTT-DFBT
P3HT + PIDTT-DFBT
a)Integrated
Cells
JSC (mA/cm2)
VOC (V)
FF
PCE (%)
JSCa) (mA/cm2)
JSCb) (mA/cm2)
Front cell
7.79
0.89
0.60
4.16
7.38
8.02
Back cell
6.64
0.92
0.66
4.07
6.47
6.89
Tandem cells
14.38
0.91
0.62
8.12
13.80
14.91
Single cell
11.21
0.93
0.63
6.52
Front cell
7.97
0.94
0.57
4.33
7.70
9.20
Back cell
7.44
0.98
0.63
4.57
6.90
8.30
Tandem cells
15.25
0.96
0.59
8.72
14.80
17.50
Single cell
12.50
0.97
0.61
7.32
Front cell
10.20
0.84
0.64
5.50
9.60
12.00
Back cell
6.20
0.96
0.62
3.70
6.00
5.50
Tandem cells
16.10
0.88
0.65
9.20
15.60
17.5
JSC from the EQE spectra; b)JSC calculated by optical simulation.
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Figure 4. (a) Thickness dependence of Jsc of the front P3HT:IC60BA sub-cell (middle surface), back PIDTT-DFBT:PC70BM sub-cell (bottom surface), and parallel tandem cell (top surface); (b) J−V curves and (c) EQE spectra of the front/back sub-cells and the constructed parallel tandem cell comprising two different BHJ layers.
architecture to other high performance BHJ systems, such as P3HT,[45] PTB7,[5] and PCPDT-FBT.[46] The TMM simulations of BHJ based on these polymers are summarized in Table S3 and the corresponding JSC based on the BHJ thickness of each sub-cell and the final parallel tandem cells are presented in Figure S8. As can be seen, significantly enhanced JSC can be achieved for all these BHJ systems in the parallel tandem configuration compared to that obtained from each individually optimized single junction counterparts. Due to microcavity in the back sub-cell, the maximum JSC of back sub-cell is obtained at a fixed value regardless of the BHJ thickness of the front subcell. These results demonstrate the general applicability of such device configuration to all the donor polymers studied, particularly to those with weak absorption tails. Taking advantage of the fact that parallel tandem cell does not require JSC matching, two state-of-the-art medium bandgap polymers with distinctly different absorption profiles, P3HT:IC60BA (front sub-cell) and PIDTT-DFBT:PC71BM (back sub-cell), were stacked together using the parallel tandem configuration. Each of the single junction cells exhibits high PCE (6.3% for P3HT:IC60BA (Table S1) and 7.3% for PIDTTDFBT:PC71BM) (Table 1). The EQE of these devices are shown in Figure 4d and 3d. Due to different band-gap (P3HT: 1.9 eV and PIDTT-DFBT: 1.6 eV, see Figure S2), the EQE of the P3HT:IC60BA single junction device ends at 650 nm, while that of PIDTT-DFBT:PC71BM device further extends to ∼750 nm. The JSC of the fabricated parallel tandem cell is also shown in Figure 4a. The calculated maximum JSC of the parallel tandem cell is approaching 18 mA/cm2, in which the front P3HT:IC60BA and the back PIDTT-DFBT:PC71BM sub-cell contributes 12 and 6 mA/cm2, respectively. Due to similar UTMF configuration, the microcavity feature of the back PIDTT-DFBT:PC71BM subcell resembles that shown in Figure 2d. Figure 4b-c depicts the J−V and EQE curves of the fabricated parallel tandem cell and its constituent sub-cells. The relevant photovoltaic parameters are summarized in Table 1. The front P3HT:IC60BA sub-cell shows a JSC of 10.2 mA/cm2, a VOC of 0.84 V, a FF of 0.64, and a PCE of 5.5% while the back PIDTT-DFBT:PC71BM sub-cell exhibits a JSC of 6.2 mA/cm2, a VOC of 0.96 V, a FF of 0.62, and a PCE of 3.7%. An impressive PCE of 9.2% can be obtained from the fabicated tandem device, with a JSC of 16.1 mA/cm2, a VOC 0.88 V, and a FF of 0.65 although the JSC and VOC of the subcells are mismatched. This is one of the highest PCEs obtained from organic parallel tandem cells.[22] As shown in their EQE spectra (Figure 4c), the back PIDTT-DFBT:PC71BM sub-cell 6
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contributes significantly to the enhanced light harvesting in region between 650 and 750 nm due to the microcavity effect. It helps compensate the absorption between 330 and 650 nm, leading to increased EQE and PCE. In summary, high-performance parallel tandem solar cells comprising a semi-transparent front cell and a microcavity assisted top-illuminated back cell are demonstrated to efficiently convert incident light to electricity. This device architecture takes advantage of both semi-transparent OPV and UTMF microcavity light-trapping effects by using an ultra-thin Ag film as an intermediate transparent electrode. As a result, a very high PCE of 9.2% could be achieved, which represents the best result reported for organic parallel tandem solar cells. This device configuration greatly alleviate the requirement of matching photocurrent between two sub-cells in a series connected solar cell, therefore it is easier to improve device performance by using more efficient BHJ systems.
Experimental Section Device Fabrication: For the fabrication of inverted single cells, ITOcoated (15 Ω/sq.) glass substrates were cleaned sequentially with detergent, DI-water, acetone, and isopropanol. After the air plasma treatment of the ITO surfaces, sol-gel ZnO[46] was then spin-coated (∼20 nm thick) and annealed at 130 °C for 5 min in air. To avoid the possible contamination of oxygen or moisture, the BHJ layer (PIDTTDFQT:PC71BM or PIDTT-DFBT:PC71BM) was spin-coated from a solution of 1,2-dichlorobenzene:1-chloronaphthalene (97:3, vol%) and annealed at 110 °C for 5 min in an N2-filled glovebox. The film thickness was controlled by varying the concentration and spin-coating speed accordingly. The literature reported procedure was used to prepare the P3HT:ICBA film.[45] Afterward, these coated substrates were transferred into vacuum chamber for the deposition of 6 nm MoO3 and 100 nm Ag to complete the device fabrication. For the inverted ST-OPV, an ultrathin Ag (10 nm) was deposited (4 × 10−7 Torr, 5 Å/s) as the transparent electrode in conjunction with 6 nm MoO3. For the top-illuminated devices, the glass substrates were cleaned using the same procedure for ITO substrates. Then 100 nm Ag was thermally evaporated under high vacuum to serve as the bottom electrode. Sol-gel ZnO (20 nm) was then deposited after the further modification of Ag by 11-mercaptoundecanoic acid (MUA) self-assemble monolayer. BHJ layers with different thickness were then spin-coated and annealed at 110 °C for 5 min in an N2-filled glovebox. Finally, 6 nm MoO3, 14 nm Ag (transparent electrode, 4 × 10−7 Torr, 5 Å/s), and 50 nm MoO3 (capping layer) were sequentially evaporated to complete the devices. For the fabrication of parallel tandem cells, patterned ITO substrates spin-coated with ZnO and active layers of the front cells were prepared as the previous procedure. 40 nm modified PEDOT:PSS[46] was next
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The authors thank the support from the Air Force Office of Scientific Research (No. FA9550–09–1–0426), the Asian Office of Aerospace R&D (No. FA2386–11–1–4072), and the Office of Naval Research (No. N00014–11–1–0300). A. K.-Y. Jen thanks the Boeing Foundation for support. This work was also partly supported by the Major State Basic Research Program (973 program) (2014CB643503). L. Zuo thanks China Scholarship Council (CSC) for financial support. Received: June 23, 2014 Revised: July 27, 2014 Published online:
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deposited, followed by vacuum thermal evaporation of 10 nm Ag (4 × 10−7 Torr, 5 Å/s) and 20 nm MoO3. Subsequently, BHJ layers of the back sub-cells were spin-coated on top of MoO3 in glovebox, based on the optimized thickness predicted by optical modeling. After annealed at 110 °C for 5 min, C60-surfactant (∼8 nm) was spin-coated on top of the BHJ layer and finally the tandem cells were completed by vacuum thermal evaporation of 100 nm Ag. The device area, defined by the overlapped area of patterned intermediate anode and cathodes, is around 4.5 mm2 (re-calibrated by microscope). Device Characterization: All the J−V curves in this study were recorded using a Keithley 2400 source measure unit, under the illumination of a 450 W Thermal Oriel solar simulator (AM1.5G). The illumination intensity of the light source was accurately calibrated employing a standard Si photodiode detector equipped with a KG-5 filter, which can be traced back to the standard cell of National Renewable Energy Laboratory (NREL). The EQE spectra are obtained by the IPCE measurement, which combines a Xenon lamp (Oriel, 450 W) as the light source, a monochromator, chopper with frequency of 400 Hz, a lock-in amplifier (SR830, Stanford Research Corp), and a Si-based diode (J115711–1-Si detector) for calibration. The absorption and transmission spectra were measured by UV-visible absorption instrument (PerkinElmer Lambda-9 spectrophotometer).
[13] N.-K. Persson, X. Wang, O. Inganas, Appl. Phys. Lett. 2007, 91, 083503. [14] S. H. Park, A. Roy, S. Beaupre, S. Cho, N. Coates, J. S. Moon, D. Moses, M. Leclerc, K. Lee, A. J. Heeger, Nat. Photon. 2009, 3, 297. [15] J. Y. Kim, K. Lee, N. E. Coates, D. Moses, T.-Q. Nguyen, M. Dante, A. J. Heeger, Science 2007, 317, 222. [16] K. Li, Z. Li, K. Feng, X. Xu, L. Wang, Q. Peng, J. Am. Chem. Soc. 2013, 135, 13549. [17] W. Li, A. Furlan, K. H. Hendriks, M. M. Wienk, R. A. J. Janssen, J. Am. Chem. Soc. 2013, 135, 5529. [18] N. Li, D. Baran, K. Forberich, M. Turbiez, T. Ameri, F. C. Krebs, C. J. Brabec, Adv. Energ. Mater. 2013, 3, 1597. [19] B. E. Lassiter, C. Kyle Renshaw, S. R. Forrest, J. Appl. Phys. 2013, 113, 1597. [20] Y. Jin, J. Feng, M. Xu, X.-L. Zhang, L. Wang, Q.-D. Chen, H.-Y. Wang, H.-B. Sun, Adv. Opt. Mater. 2013, 1, 809. [21] A. Hadipour, B. de Boer, P. W. M. Blom, J. Appl. Phys. 2007, 102, 074506. [22] S. Sista, Z. Hong, M.-H. Park, Z. Xu, Y. Yang, Adv. Mater. 2010, 22, E77. [23] X. Guo, F. Liu, W. Yue, Z. Xie, Y. Geng, L. Wang, Org. Electron. 2009, 10, 1174. [24] S. Tanaka, K. Mielczarek, R. Ovalle-Robles, B. Wang, D. Hsu, A. A. Zakhidov, Appl. Phys. Lett. 2009, 94, 113506. [25] T. Ameri, G. Dennler, C. Waldauf, H. Azimi, A. Seemann, K. Forberich, J. Hauch, M. Scharber, K. Hingerl, C. J. Brabec, Adv. Funct. Mater. 2010, 20, 1592. [26] Y. Zhou, H. Cheun, S. Choi, C. Fuentes-Hernandez, B. Kippelen, Org. Electron. 2011, 12, 827. [27] C.-C. Chen, L. Dou, R. Zhu, C.-H. Chung, T.-B. Song, Y. B. Zheng, S. Hawks, G. Li, P. S. Weiss, Y. Yang, ACS Nano 2012, 6, 7185. [28] C.-C. Chen, L. Dou, J. Gao, W.-H. Chang, G. Li, Y. Yang, Energ. Environ. Sci. 2013, 6, 2714. [29] Z. Liu, J. Li, Z.-H. Sun, G. Tai, S.-P. Lau, F. Yan, ACS Nano 2011, 6, 810. [30] C.-C. Chueh, S.-C. Chien, H.-L. Yip, J. F. Salinas, C.-Z. Li, K.-S. Chen, F.-C. Chen, W.-C. Chen, A. K. Y. Jen, Adv. Energ. Mater. 2013, 3, 417. [31] K.-S. Chen, J.-F. Salinas, H.-L. Yip, L. Huo, J. Hou, A. K. Y. Jen, Energy Environ. Sci. 2012, 5, 9551. [32] H.-W. Lin, S.-W. Chiu, L.-Y. Lin, Z.-Y. Hung, Y.-H. Chen, F. Lin, K.-T. Wong, Adv. Mater. 2012, 24, 2269. [33] N. P. Sergeant, A. Hadipour, B. Niesen, D. Cheyns, P. Heremans, P. Peumans, B. P. Rand, Adv. Mater. 2012, 24, 728. [34] H. Jin, C. Tao, M. Velusamy, M. Aljada, Y. Zhang, M. Hambsch, P. L. Burn, P. Meredith, Adv. Mater. 2012, 24, 2572. [35] J.-F. Salinas, H.-L. Yip, C.-C. Chueh, C.-Z. Li, J.-L. Maldonado, A. K. Y. Jen, Adv. Mater. 2012, 24, 6362. [36] G. F. Burkhard, E. T. Hoke, M. D. McGehee, Adv. Mater. 2010, 22, 3293. [37] L. Cattin, Y. Lare, M. Makha, M. Fleury, F. Chandezon, T. Abachi, M. Morsli, K. Napo, M. Addou, J. C. Bernède, Sol. Energ. Mat. Sol. C. 2013, 117, 103. [38] Y.-X. Xu, L.-J. Zuo, C.-Z. Li, C.-C.Chueh, A. K.-Y. Jen, unpublished. [39] Y.-X. Xu, C.-C. Chueh, H.-L. Yip, F.-Z. Ding, Y.-X. Li, C.-Z. Li, X. Li, W.-C. Chen, A. K. Y. Jen, Adv. Mater. 2012, 24, 6356. [40] K. J. Vahala, Nature 2003, 424, 839. [41] H. Benisty, H. de Neve, C. Weisbuch, IEEE J. Quant. Electron. 1998, 34, 1612. [42] H. Benisty, H. de Neve, C. Weisbuch, IEEE J. Quant. Electron. 1998, 34, 1632. [43] Z.-Y. Huang, S. W. Chiu, C.-W. Chen, Y.-H. Chen, L.-Y. Lin, K.-T. Wong, H.-W. Lin, Nanoscale 2014, 6, 2316. [44] Y.-H. Chen, C.-W. Chen, Z.-Y. Huang, K.-T. Wong, L.-Y. Lin, F. Lin, H.-W. Lin, Org. Electron. 2014, 15, 1828. [45] G. Zhao, Y. He, Y. Li, Adv. Mater. 2010, 22, 4355. [46] C.-Y. Chang, L. Zuo, H.-L. Yip, Y. Li, C.-Z. Li, C.-S. Hsu, Y.-J. Cheng, H. Chen, A. K. Y. Jen, Adv. Funct. Mater. 2013, 23, 5084.
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Microcavity-Enhanced Light-Trapping for Highly Efficient Organic Parallel Tandem Solar Cells Lijian Zuo, Chu-Chen Chueh, Yun-Xiang Xu, Kung-Shih Chen, Yue Zang, Chang-Zhi Li, Hongzheng Chen,* and Alex K.-Y. Jen* Recently, sophisticated optimization using rational molecular design,[1–3] morphological control,[4] interlayer engineering,[5] and optical manipulation[6] have been employed as effective tools for achieving high-performance organic photovoltaics (OPVs).[7–10] Based on these efforts, OPV devices with greater than 10% power conversion efficiency (PCE) have been achieved, which greatly enhances their chances for commercialization. However, due to significant energy loss arising from low dielectric constants and charge-transporting properties of organic semiconductors,[11,12] the photoactive layer of most OPVs has been limited to thickness around 100–200 nm to avoid severe charge recombination. Such thin photoactive layers often can not completely absorb the incident light which restricts the upper limit of device external quantum efficiency (EQE) to be ∼70% despite the internal quantum efficiency (IQE) of the photoactive layer can approach over 90%.[13,14] To harvest light more efficiently, several light-trapping strategies have been exploited to enhance optical density in the photoactive layer. Among which, multi-junction stacking is considered to be a very promising approach due to its ability to extend the optical paths for harvesting light.[9,10] Till now, most of the reported tandem OPVs adopt the series connected configuration since the complementary absorption from each junction with different bandgap (Eg) allows different regions of light to be harvested in the cells.[9,15–17] In general, the two sub-cells in the series architecture are bridged by an interconnecting layer (ICL), which serves as the charge recombination channel to tunnel through the stacking junctions.[10,18] Recently, a record-high PCE (10.6%) of OPVs has been achieved in such a series connected tandem configuration.[9] According to the Kirchhoff circuit laws, the PCE of a series connected tandem cell (denoted as series tandem cell hereafter) will reach its maximum value when the short-circuit current (JSC) of each sub-cell is well matched.[19] It requires intricate control of the photoactive layer thickness in each sub-cell to L. Zuo, Dr. C.-C. Chueh, Dr. Y.-X. Xu, K.-S. Chen, Y. Zang, Dr. C.-Z. Li, Prof. A. K.-Y. Jen Department of Materials Science and Engineering University of Washington Seattle, USA E-mail: [email protected] L. Zuo, Prof. H. Z. Chen, Prof. A. K.-Y. Jen State Key Laboratory of Silicon Materials MOE Key Laboratory of Macromolecular Synthesis and Functionalization Zhejiang University Hangzhou 310027, P. R. China E-mail: [email protected]
DOI: 10.1002/adma.201402782
Adv. Mater. 2014, DOI: 10.1002/adma.201402782
accomplish optimal optical distribution in the stratified device to achieve balanced JSC of constituent sub-cells.[9,20] As for a parallel connected tandem cell (denoted as parallel tandem cell hereafter), it only needs matching of the open-circuit voltage (VOC) of each sub-cell instead.[21–24] This provides a convenient way to optimize photovoltaic performance by employing unitary materials in both sub-cells due to matched VOC. Moreover, the parallel tandem cells possess the three-terminal nature, which enables two sub-cells to work individually so it can still function even if one of the cells fails. Nevertheless, the progress of organic parallel tandem cells is lagging behind that of the series connected tandem cells because the difficulty in finding highly conductive and transparent intermediate electrodes, which limits its PCE to be around 5% at most.[22] Recently, the development of transparent electrodes has proceeded rapidly in emerging transparent electronics, including highly conductive PEDOT:PSS,[25,26] Ag nanowires,[27,28] graphene,[29] and ultra-thin metal film (UTMF).[30,31] In particular, the development of semi-transparent organic photovoltaics (STOPVs) has drawn significant attraction due to their potential for building integration. By using transparent electrodes, ST-OPVs can convert partial sunlight into electrical power and modulate the wavelengths of transmitted light for efficient energy conservation. For example, ultra-thin Ag (8–16 nm) with high conductivity and transparency has been shown to function as efficient transparent electrode for ST-OPVs to result in high PCE (∼6%) and good average visible light transmittance (AVT).[31] The decent reflectivity of UTMF enables microcavity to be formed with another reflective electrode for fabricating highly efficient and ITO-free flexible OPVs.[6,32,33] Since microcavity reinforces the optically confined incident lights that have resonant frequencies due to coherent interference, it leads to comparable or even higher PCE than those derived from the ITO-based devices.[6,34,35] Furthermore, the resonant light in an optical microcavity can be manipulated by varying the layer thickness within the stratified device, especially the thickness of the photoactive layer. Given these appealing features, it is feasible to use UTMF as an intermediate electrode to construct highly efficient parallel tandem cells. As shown in Figure 1a, the parallel tandem cell comprises a front ST-OPV sub-cell and a back top-illuminated ITO free OPV sub-cell that are bridged by an intermediate ultrathin Ag electrode. In addition to the advantage of extending light paths within the tandem structure, the microcavity formed in the back sub-cell can further enhance the absorption of the transmitted photons with specific resonant frequencies. Especially, the microcavity can also be tuned to enhance the absorption of low energy photons near the band edge of the photoactive layer.
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Figure 1. (a) Schematic diagram of the studied parallel tandem cell, constructed from a “front” semi-transparent sub-cell and a “back” top-illuminated ITO-free sub-cell. (b) Molecular structures of the studied materials.
In this study, we demonstrate the potential of using such a parallel tandem architecture to enhance efficiency of devices (20–25%) consisting of identical junction of diverse BHJ systems compared to those obtained from their optimized single junction devices (Figure 1b). The improved EQE (∼80%) verifies that enhanced light-trapping can be achieved through this parallel tandem architecture which has better optical management. Since matched photocurrent generated from each subcell is not required, two polymers with distinctly different absorption profiles can be stacked together to explore the efficacy of the parallel tandem architecture. A very high PCE of 9.2% could be achieved without the tedious matching of JSC 2
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from each sub-cell. This demonstrates that innovative optical manipulation can be used to effectively improve the performance of OPVs. Because the performance of tandem devices is strongly dependent on the properties of the intermediate electrodes, the characteristics of UTMF electrodes were carefully evaluated through both experiments and simulations in terms of transmittance (T) and resistance (R). The transfer matrix method (TMM) was used for simulation in this study.[36] It has been shown to provide useful guidance for optimizing the optical field distribution within the device.[6] The detailed information regarding TMM simulation is described in the supporting
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information. In general, the conductivity of ultra-thin Ag decreases dramatically if its thickness is below the percolation threshold (∼10 nm) due to the formation of inhomogeneous islands. However, this problem can be alleviated by using a proper seeding layer to improve the compatibility with ultrathin Ag.[35,37] As shown in Figure S1a, an ultra-thin Ag layer with a thickness of ∼10 nm deposited on top of PEDOT:PSS has an even lower resistance (∼12 Ω/ⵧ) than that of ITO on glass (∼15 Ω/ⵧ). The result from measuring optical properties of the ultra-thin Ag (10–12 nm) depicted in Figure S1b reveals that most of the optical loss stems from the reflection while the absorption loss caused by UTMF is only accounted for ∼5% due to the intrinsically low absorption of Ag (Figure S1c). However, such reflectivity can be advantageous when it is used as the intermediate electrode in the parallel tandem structure since the reflected light can be re-coupled into the front photoactive layer. Moreover, the transmitted light can extend its pathway to the back sub-cell to enable the efficient utilization of the incident sunlight. Two highly efficient polymers, PIDTT-DFQT[38] and PIDTTDFBT,[39] were chosen to be blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the identical double-junction BHJ systems for evaluation. Their absorption spectra are shown in Figure S2. Prior to validate the parallel tandem device, two individual front/back single junction sub-cells (using PIDTT-DFQT:PC71BM BHJ as a representative system) were fabricated to investigate their optical and electrical properties. The ST-OPV was fabricated with the configuration of ITO/ZnO (20 nm)/BHJ (75 nm)/MoO3 (6 nm)/UTMF (10 nm) while the top-illuminated ITO-free device was made in the configuration of glass/opaque Ag (100 nm)/ZnO (20 nm) /BHJ (45–75 nm)/ MoO3 (6 nm)/UTMF (14 nm)/MoO3 (50 nm). As shown in Figure S3 and Table S1, the ST-OPV employing UTMF as the top electrode maintains ∼80% of the pristine single junction cell performance which exhibits a high PCE of ∼5% and a high transmittance of over 40% at 700 nm, suggesting its suitability to serve as a front sub-cell.[30,31] In the proposed parallel tandem architecture (Figure 1a), a backside top-illuminated ITO-free cell adopting the UTMF microcavity configuration was used to re-absorb the transmitted light from ST-OPV, especially for the low energy photons near the band edge of organic semiconductors. As known, the optical microcavity formed between two highly reflective surfaces could confine specific wavelengths of light by resonant recirculation.[40–43] Our recent result has demonstrated that the performance of device could be substantially improved in such UTMF microcavity architecture, due to enhanced photocurrents.[6] The oscillating characteristics of microcavity can also be manipulated by varying the thickness of photoactive layer, capping layer, and UTMF.[31] As shown from the simulation results in Figure S4, the center of the maximum oscillating light intensity of the device (based on the aforementioned configuration) red-shifts gradually from 600 to 700 nm when the thickness of the BHJ layer is increased from 45 to 75 nm to result in varied resonant length of the oscillating chamber. Benefitting from such microcavity effect, a calculated EQEMAX of over 80% (Figure S4d) can be achieved, which outperforms
that calculated from the ITO counterpart (∼70%, Figure 2c). Figure S5 shows the J−V curves and the pictures of the topilluminated ITO-free devices with different BHJ thicknesses, in which the thickness of each interlayer is identical to those used for TMM simulation. As can be seen, the colors of these devices changed apparently, indicating different optical distribution within the device chambers as suggested by the simulation (Figure S4). More importantly, the fabricated top-illuminated ITO-free device employing such a microcavity configuration showed even higher performance than that based on the regular ITO substrate (Table S1). This testifies the effectiveness of using UTMF microcavity configuration in the back sub-cell to exploit low energy photons near the band edge of polymers (Figure S4d). Based on this finding, the parallel tandem solar cell employing ultra-thin Ag intermediate anode was fabricated and the detailed layout was illustrated in Figure S6. Assisted by the TMM simulation, the JSC, optical distribution, and EQE of the corresponding devices can be pre-evaluated by modeling. Figure 2a shows the relationship of JSC on the photoactive layer thickness of each sub-cell and the resultant tandem cell based on PIDTT-DFQT:PC71BM BHJ (the thicknesses of other interlayers and electrodes are kept the same under the optimized conditions), where the green, orange, and purple surfaces represent the front, back, and the parallel tandem cell, respectively. Assuming 100% IQE, the highest achievable JSC (∼15 mA/cm2) of the parallel tandem solar cell is 25% higher than that of optimized single junction cell (∼12 mA/cm2, Figure S7a) and is very close to the upper limit value of this system (∼17 mA/cm2, Figure S7a). It is worth to note that the back cell can generate a maximum JSC at the BHJ thickness of ∼75 nm independent of the front cell BHJ thickness. It is attributed to the microcavity formed in the back sub-cell, which not only enhances the light in-coupling but also enables certain wavelengths to be trapped within the back sub-cell (Figure 2b). Thus, a broad range of BHJ thickness in sub-cells (40–200 nm for front sub-cell and 60–80 nm for back sub-cell) can be chosen without significantly sacrificing JSC (13.5–14.9 mA/cm2). On the contrary, the required JSC matching for the series connected tandem device strongly limits the thickness variation of each sub-cell.[44] Matched JSC can only be achieved along the crossed line of the green/orange surfaces (Figure 2a), indicating the intricate thickness control needed in the series tandem configuration. As shown in Figure 2c, the microcavity-enhanced light trapping from back sub-cell compensates the weak absorption of low energy photons (550–700 nm), leading to improved light harvesting across the whole absorption region of BHJ. The EQE of the parallel tandem cell is significantly enhanced (∼25%) in the region between 450 and 700 nm compared to that of the single junction devices, which correspond to a JSC of around 15 and 12 mA/cm2, respectively. For the front subcell, the broad absorption between 300 and 650 nm (comparable to the pristine single junction cell) contributes to a JSC of ∼8.0 mA/cm2. Meanwhile, the back sub-cell exhibits a lower plateau between 300 and 550 nm and an enhanced absorption band between 600 and 700 nm, which contributes to a JSC of ∼6.9 mA/cm2. By simply superimpose these two sub-cells as a parallel tandem cell, it leads to an enhanced current in the
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Figure 2. Thickness dependence of Jsc of the front sub-cell (middle surface), back sub-cell (bottom surface), and parallel tandem cell (top surface) base on (a) PIDTT-DFQT:PC71BM and (d) PIDTT-DFBT:PC71BM BHJ. Light intensity distribution in the parallel tandem solar cells based on (b) PIDTTDFQT:PC71BM and (e) PIDTT-DFBT:PC71BM; Calculated EQE of the conventional single junction solar cell, each sub-cells, and the parallel tandem solar cell for (c) PIDTT-DFQT:PC71BM and (f) PIDTT-DFBT:PC71BM based devices. All simulation is based on the assumption of 100% IQE.
region between 450 and 700 nm, demonstrating the effectiveness of this microcavity architecture. Similar microcavity-enhanced light-trapping phenomena can also be observed in the PIDTT-DFBT:PC71BM based BHJ. As shown in Figure 2d, the maximum JSC of the back sub-cell can be reached at a BHJ thickness of ∼80 nm independent to the BHJ thickness of the front cell. Similar enhancement (∼22%) in JSC (17.5 mA/cm2) can be achieved in this parallel tandem configuration compared to that obtained in the single junction cell (14.4 mA/cm2, Figure S7b). As shown in Figure 2f, the microcavity of the back sub-cell (Figure 2e) helps increase the EQE of parallel tandem cell in the region between 600 and 750 nm. The resulting high EQEs (over 80% for both BHJ systems) for these devices show the promise of further improving the OPV device performance. Finally, the parallel tandem devices were fabricated based on the optimal conditions generated from optical simulation (shown in Figure 2b and 2e). The JSC of such parallel tandem structure was not sensitive to the ultra-thin Ag thickness due to the trade-off between reflection and transmittance (Table S2). After thorough device engineering, an optimum intermediate anode comprising PEDOT:PSS (40 nm), ultrathin Ag (10 nm), and MoO3 (20 nm), was employed because it can: 1) provide robust solvent resistance for multilayer integration; 2) form Ohmic contact for both sub-cells; and 3) possess respectable conductivity and transparency. Figure 3 shows the J–V and EQE curves of the fabricated parallel tandem cells based on identical double-junction BHJ and their relevant parameters are summarized in Table 1. Note that the resultant
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three-terminal parallel tandem cell enables us to measure the sub-cells separately. Both parallel tandem cells show much higher performance than the pristine single junction cell as predicted from the TMM simulation. For the PIDTT-DFQT:PC71BM system, the parallel tandem cell shows a high JSC of 14.38 mA/cm2 with a VOC of 0.91 V, a FF of 0.62, and a resultant PCE of 8.12%, which is 25% higher than the PCE obtained from the single junction cell (VOC: 0.93, JSC: 11.21, FF: 0.63, and PCE: 6.52%) as shown in Figure 3a. The front sub-cell exhibits a VOC of 0.89 V, a JSC of 7.79 mA/cm2, a FF of 0.60, and a PCE of 4.16% while the back sub-cell gives a VOC of 0.92 V, a JSC of 6.64 mA/cm2, a FF of 0.66, and a PCE of 4.07%. The comparable performance between the sum of two sub-cells and the tandem cell validates the fabricated parallel tandem architecture. The enhanced JSC of parallel tandem cell is mainly due to the improved light harvesting of wavelength beyond 450 nm as the EQE curves indicated in Figure 3b. This result is the same as the optical simulation predicted using the front ST-OPV and back UTMF microcavity configuration (Figure 2c). The significantly enhanced EQE value (>80%) in the back sub-cell is among the highest values reported in the literature. For another efficient BHJ based on PIDTT-DFBT:PC71BM, a 20% PCE enhancement was also predicted by optical simulation in the parallel tandem cell (Figure 2d–f). Compared to PIDTT-DFQT, PIDTT-DFBT has a deeper HOMO and more intense intramolecular charge transfer (Figure S2b). As shown in Figure 3c, the tandem cell possesses a high JSC of 15.25 mA/cm2, a VOC of 0.96 V, a FF of 0.59, and a high PCE of
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8.72%. The PCE is ∼ 20% higher than that obtained from single junction cell (7.32%). In which, the front sub-cell contributes a PCE of 4.33% with a VOC of 0.94 V, a JSC of 7.97 mA/cm2, and a FF of 0.57, while the back sub-cell has a PCE of 4.57%, with a VOC of 0.98 V, a JSC of 7.44 mA/cm2, and a FF of 0.63. The FF in the front sub-cell is relatively low. This may be due to mismatched energy level between the BHJ and the modified PEDOT:PSS or the poor contact created by dewetting between the hydrophilic PEDOT:PSS and the hydrophobic BHJ surface.
This can be easily resolved by inserting a thin MoO3 layer in between as the stratified structure shown in Figure 2e.[10] As a result, enhanced EQE curves could be achieved again in the region between 600 and 800 nm (Figure 3d), which is consistent with the result of simulation (Figure 2f). All these results suggest that such parallel tandem architecture is an excellent platform for exploring the versatile high-performance donor polymers. To verify this, we have further evaluated the general applicability of this parallel tandem
Table 1. Summarized device performance of the studied solar cells. Polymers PIDTT-DFQT
PIDTT-DFBT
P3HT + PIDTT-DFBT
a)Integrated
Cells
JSC (mA/cm2)
VOC (V)
FF
PCE (%)
JSCa) (mA/cm2)
JSCb) (mA/cm2)
Front cell
7.79
0.89
0.60
4.16
7.38
8.02
Back cell
6.64
0.92
0.66
4.07
6.47
6.89
Tandem cells
14.38
0.91
0.62
8.12
13.80
14.91
Single cell
11.21
0.93
0.63
6.52
Front cell
7.97
0.94
0.57
4.33
7.70
9.20
Back cell
7.44
0.98
0.63
4.57
6.90
8.30
Tandem cells
15.25
0.96
0.59
8.72
14.80
17.50
Single cell
12.50
0.97
0.61
7.32
Front cell
10.20
0.84
0.64
5.50
9.60
12.00
Back cell
6.20
0.96
0.62
3.70
6.00
5.50
Tandem cells
16.10
0.88
0.65
9.20
15.60
17.5
JSC from the EQE spectra; b)JSC calculated by optical simulation.
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Figure 4. (a) Thickness dependence of Jsc of the front P3HT:IC60BA sub-cell (middle surface), back PIDTT-DFBT:PC70BM sub-cell (bottom surface), and parallel tandem cell (top surface); (b) J−V curves and (c) EQE spectra of the front/back sub-cells and the constructed parallel tandem cell comprising two different BHJ layers.
architecture to other high performance BHJ systems, such as P3HT,[45] PTB7,[5] and PCPDT-FBT.[46] The TMM simulations of BHJ based on these polymers are summarized in Table S3 and the corresponding JSC based on the BHJ thickness of each sub-cell and the final parallel tandem cells are presented in Figure S8. As can be seen, significantly enhanced JSC can be achieved for all these BHJ systems in the parallel tandem configuration compared to that obtained from each individually optimized single junction counterparts. Due to microcavity in the back sub-cell, the maximum JSC of back sub-cell is obtained at a fixed value regardless of the BHJ thickness of the front subcell. These results demonstrate the general applicability of such device configuration to all the donor polymers studied, particularly to those with weak absorption tails. Taking advantage of the fact that parallel tandem cell does not require JSC matching, two state-of-the-art medium bandgap polymers with distinctly different absorption profiles, P3HT:IC60BA (front sub-cell) and PIDTT-DFBT:PC71BM (back sub-cell), were stacked together using the parallel tandem configuration. Each of the single junction cells exhibits high PCE (6.3% for P3HT:IC60BA (Table S1) and 7.3% for PIDTTDFBT:PC71BM) (Table 1). The EQE of these devices are shown in Figure 4d and 3d. Due to different band-gap (P3HT: 1.9 eV and PIDTT-DFBT: 1.6 eV, see Figure S2), the EQE of the P3HT:IC60BA single junction device ends at 650 nm, while that of PIDTT-DFBT:PC71BM device further extends to ∼750 nm. The JSC of the fabricated parallel tandem cell is also shown in Figure 4a. The calculated maximum JSC of the parallel tandem cell is approaching 18 mA/cm2, in which the front P3HT:IC60BA and the back PIDTT-DFBT:PC71BM sub-cell contributes 12 and 6 mA/cm2, respectively. Due to similar UTMF configuration, the microcavity feature of the back PIDTT-DFBT:PC71BM subcell resembles that shown in Figure 2d. Figure 4b-c depicts the J−V and EQE curves of the fabricated parallel tandem cell and its constituent sub-cells. The relevant photovoltaic parameters are summarized in Table 1. The front P3HT:IC60BA sub-cell shows a JSC of 10.2 mA/cm2, a VOC of 0.84 V, a FF of 0.64, and a PCE of 5.5% while the back PIDTT-DFBT:PC71BM sub-cell exhibits a JSC of 6.2 mA/cm2, a VOC of 0.96 V, a FF of 0.62, and a PCE of 3.7%. An impressive PCE of 9.2% can be obtained from the fabicated tandem device, with a JSC of 16.1 mA/cm2, a VOC 0.88 V, and a FF of 0.65 although the JSC and VOC of the subcells are mismatched. This is one of the highest PCEs obtained from organic parallel tandem cells.[22] As shown in their EQE spectra (Figure 4c), the back PIDTT-DFBT:PC71BM sub-cell 6
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contributes significantly to the enhanced light harvesting in region between 650 and 750 nm due to the microcavity effect. It helps compensate the absorption between 330 and 650 nm, leading to increased EQE and PCE. In summary, high-performance parallel tandem solar cells comprising a semi-transparent front cell and a microcavity assisted top-illuminated back cell are demonstrated to efficiently convert incident light to electricity. This device architecture takes advantage of both semi-transparent OPV and UTMF microcavity light-trapping effects by using an ultra-thin Ag film as an intermediate transparent electrode. As a result, a very high PCE of 9.2% could be achieved, which represents the best result reported for organic parallel tandem solar cells. This device configuration greatly alleviate the requirement of matching photocurrent between two sub-cells in a series connected solar cell, therefore it is easier to improve device performance by using more efficient BHJ systems.
Experimental Section Device Fabrication: For the fabrication of inverted single cells, ITOcoated (15 Ω/sq.) glass substrates were cleaned sequentially with detergent, DI-water, acetone, and isopropanol. After the air plasma treatment of the ITO surfaces, sol-gel ZnO[46] was then spin-coated (∼20 nm thick) and annealed at 130 °C for 5 min in air. To avoid the possible contamination of oxygen or moisture, the BHJ layer (PIDTTDFQT:PC71BM or PIDTT-DFBT:PC71BM) was spin-coated from a solution of 1,2-dichlorobenzene:1-chloronaphthalene (97:3, vol%) and annealed at 110 °C for 5 min in an N2-filled glovebox. The film thickness was controlled by varying the concentration and spin-coating speed accordingly. The literature reported procedure was used to prepare the P3HT:ICBA film.[45] Afterward, these coated substrates were transferred into vacuum chamber for the deposition of 6 nm MoO3 and 100 nm Ag to complete the device fabrication. For the inverted ST-OPV, an ultrathin Ag (10 nm) was deposited (4 × 10−7 Torr, 5 Å/s) as the transparent electrode in conjunction with 6 nm MoO3. For the top-illuminated devices, the glass substrates were cleaned using the same procedure for ITO substrates. Then 100 nm Ag was thermally evaporated under high vacuum to serve as the bottom electrode. Sol-gel ZnO (20 nm) was then deposited after the further modification of Ag by 11-mercaptoundecanoic acid (MUA) self-assemble monolayer. BHJ layers with different thickness were then spin-coated and annealed at 110 °C for 5 min in an N2-filled glovebox. Finally, 6 nm MoO3, 14 nm Ag (transparent electrode, 4 × 10−7 Torr, 5 Å/s), and 50 nm MoO3 (capping layer) were sequentially evaporated to complete the devices. For the fabrication of parallel tandem cells, patterned ITO substrates spin-coated with ZnO and active layers of the front cells were prepared as the previous procedure. 40 nm modified PEDOT:PSS[46] was next
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Adv. Mater. 2014, DOI: 10.1002/adma.201402782
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The authors thank the support from the Air Force Office of Scientific Research (No. FA9550–09–1–0426), the Asian Office of Aerospace R&D (No. FA2386–11–1–4072), and the Office of Naval Research (No. N00014–11–1–0300). A. K.-Y. Jen thanks the Boeing Foundation for support. This work was also partly supported by the Major State Basic Research Program (973 program) (2014CB643503). L. Zuo thanks China Scholarship Council (CSC) for financial support. Received: June 23, 2014 Revised: July 27, 2014 Published online:
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Adv. Mater. 2014, DOI: 10.1002/adma.201402782
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deposited, followed by vacuum thermal evaporation of 10 nm Ag (4 × 10−7 Torr, 5 Å/s) and 20 nm MoO3. Subsequently, BHJ layers of the back sub-cells were spin-coated on top of MoO3 in glovebox, based on the optimized thickness predicted by optical modeling. After annealed at 110 °C for 5 min, C60-surfactant (∼8 nm) was spin-coated on top of the BHJ layer and finally the tandem cells were completed by vacuum thermal evaporation of 100 nm Ag. The device area, defined by the overlapped area of patterned intermediate anode and cathodes, is around 4.5 mm2 (re-calibrated by microscope). Device Characterization: All the J−V curves in this study were recorded using a Keithley 2400 source measure unit, under the illumination of a 450 W Thermal Oriel solar simulator (AM1.5G). The illumination intensity of the light source was accurately calibrated employing a standard Si photodiode detector equipped with a KG-5 filter, which can be traced back to the standard cell of National Renewable Energy Laboratory (NREL). The EQE spectra are obtained by the IPCE measurement, which combines a Xenon lamp (Oriel, 450 W) as the light source, a monochromator, chopper with frequency of 400 Hz, a lock-in amplifier (SR830, Stanford Research Corp), and a Si-based diode (J115711–1-Si detector) for calibration. The absorption and transmission spectra were measured by UV-visible absorption instrument (PerkinElmer Lambda-9 spectrophotometer).
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