We report on vacuum-free and metal electrode–free organic tandem solar cells that use conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the top electrode. The PEDOT:PSS top electrode was deposited via film-transfer lamination that does not need high-vacuum processing. The fabricated tandem solar cells exhibit an open-circuit voltage of 1.62 V, which is nearly the sum of the VOC of individual subcells, a high fill factor up to 0.72, and averaged power conversion efficiency of 3.6% under 100 mW cm−2 AM 1.5 illumination. The effect of the patterning of charge recombination layer and electrodes on the device performance has also been discussed.
Organic solar cells have gained much interest for their potential as a promising renewable energy source with simple and low-cost processing.1–5 Generally, organic semiconductors have relatively narrow absorption bands which do not provide an extensive overlap with the solar spectrum.6 Organic solar cells in tandem architecture where two or more photoactive layers are stacked on top of each other connected via a charge recombination layer could produce higher efficiency because the light harvesting is improved in tandem solar cells comparing to single-junction solar cells. So far, the reported champion power conversion efficiency (PCE) of organic solar cells is from tandem structure.7,8 Recently, flexible organic tandem solar cell modules have been demonstrated via roll-to-roll coating fully processed in air.9
In organic tandem solar cells, conducting polymer poly (3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT: PSS) combined with n-type metal oxides (such as PEDOT: PSS/ZnO, PEDOT:PSS/TiO2) are commonly used as the charge recombination layer.10–13 Recently, a more simple recombination layer of polymer-modified PEDOT:PSS (PEDOT: PSS/PEIE, where PEIE is polyethylenimine ethoxylated) has been reported.14–16 The modified PEDOT:PSS shows large work function contrast on its top and bottom surface and has been demonstrated as a simple and high-performance solution-processed charge recombination layer. In the polymer tandem solar cells, three layers including the top electrode, the bottom electrode, and the polymer charge recombination layer are conductive. Not like silicon-based solar cells, the photoactive layers of polymer solar cells are thin (about 100 nm) and soft. The probe is easy to penetrate the active layer, thus leading to the contact of the top (or bottom) electrode and the charge recombination layer if these conductive layers are not properly patterned. These contact will result in large leakage current (or even shortage) and poor device yield of organic tandem solar cells.17,18 Therefore, the proper patterning of the electrodes and the charge recombination layers is critical to the device performance and yield. Generally, metals (such as MoO3/Ag) are used as the top electrode deposited via thermal evaporation and patterned through a shadow mask. With the shadow mask, the metal electrodes are easily to pattern. However, the fabrication of metal electrode requires high-vacuum deposition system, which is quite expensive and the processing is highly energy-consuming. Recently, the conductivity of solution-processible conducting polymer PEDOT:PSS reaches over 1000 S/cm, which enables the PEDOT:PSS itself to serve as top electrodes replacing metal electrode. The easiest and most commonly-used method to fabricate small-area uniform PEDOT:PSS film is spin coating. However, the direct spin coating is difficult to control the pattern of the PEDOT:PSS film.
Film-Transfer lamination is that the target film layer is first deposited onto a transfer medium substrate (such as polydimethylsiloxane, plastic wrap) and then transferred onto the receiving substrate.19–21 A very important advantage of the film-transfer lamination technique is the ease in patterning the top PEDOT:PSS electrode as compared to the spin coating process. Previously, we22,23 and others24,25 reported transfer-laminated PEDOT:PSS as the top electrode for single-junction solar cells. So far, there is no report using PEDOT:PSS as the top electrode for organic tandem solar cells. The use of PEDOT:PSS top electrode will alleviates the need of high vacuum deposition for the fabrication of tandem cells.
In this work, we report on organic tandem solar cells using PEDOT:PSS as the top electrode prepared by film-transfer lamination (hereafter the transfer-laminated PEDOT: PSS top electrode referred to as PEDOT-T). The device structure is shown in Fig. 1. The tandem solar cells is in an inverted structure. Indium tin oxide (ITO) coated with PEI is used as the electron-collecting electrode of the bottom cell. PEDOT: PSS PH1000/PEI is used as the charge recombination layer. Poly(3-hexylthiophene) (P3HT):indene-C60 bis-adduct (ICBA) blend is employed as both the bottom and the top photoactive layers. With the PEDOT-T top electrode, the fully-solution processed inverted organic tandem solar cells exhibit a fill factor (FF) up to 0.72, an average open-circuit voltage (VOC) of 1.60 V which is close to the addition of the VOC of the two individual sub-cells under 100 mW cm−2 air mass (AM) 1.5 global (G) solar illumination.
Device structure of the metal electrode–free tandem solar cells where top PEDOT-T is prepared by transfer lamination method using PDMS as the transfer medium.
Device structure of the metal electrode–free tandem solar cells where top PEDOT-T is prepared by transfer lamination method using PDMS as the transfer medium.
Starting with the deposition of the first PEI layer, all the layers are fully processed from solution. The PEI layers on both ITO and PH1000 films were spin coated from 0.1 wt. % isopropanol solution at 5000 rpm for 1 min and annealed for 100 °C for 5 min in a N2-filled glove box. The bottom P3HT:ICBA (1:1, w/w, 90 nm) active layer were spin-coated from a 24 mg/ml dichlorobenzene solution at 900 rpm for 40 s and annealed at 150 °C for 5 min in a N2-filled glove box. PH1000 film (40 nm) was spin coated at 4000 rpm for 1 min and annealed at 150 °C for 5 min. The top P3HT:ICBA (1:1, w/w, 200 nm) active layer was spin-coated from a 40 mg/ml chlorobenzene solution due to its lower boiling temperature at 1300 rpm for 40 s and annealed at 150 °C for 5 min in the N2-filled glove box. For the preparation of the top PEDOT-T electrode, 5% ethylene glycol (EG) and 0.1% nonionic surfactant polyethylene glycol 2,5,8,11-tetramethyl-6 - dodecyne-5,8-diol ether (PEG-TmDD, TOYNOL® Superwet- 340, Tianjin SurfyChem T&D Co., Ltd.) were added to the PEDOT:PSS PH1000 (Hereaus) to increase the conductivity and wettability. The film-transfer lamination process has been described in our previous report.15 In brief, a piece of PDMS was attached onto a clean glass and treated under oxygen plasma for 30 s to tune its hydrophilicity. The PEDOT:PSS was spin-coated onto the PDMS substrates with a two-step recipe of 500 rpm for 5 s and then 1000 rpm for 1 min. At the same time, the prepared samples of ITO/PEI/P3HT:ICBA/PH1000/PEI/P3HT:ICBA were exposed to oxygen plasma for about 5 s. After PEDOT:PSS film dried in air for about 10 min (Temperature: 25 °C, Moisture: 40%), the PEDOT:PSS with PDMS substrate was cut into desired shapes and transferred onto the P3HT:ICBA film with the PEDOT:PSS side facing down. Finally, the PDMS was slowly peeled off and PEDOT: PSS films left on the cells to complete the PEDOT:PSS film transfer lamination process. Ag paint (Leitsilber 200, Ted Pella Inc.) was put onto the other side of PEDOT:PSS film for electrical contact during the measurement. After device fabrication, the layers of P3HT:ICBA/PH1000/PEI/P3HT:ICBA were electrically isolated using a sharp tweezer, scratched along the perimeter of the top electrode (shown as the dash box in Fig. 1) to reduce the fringing effects and overestimation of the current density of the tandem cells. Before the solar cell performance measurement, we encapsulated the tandem solar cells in the N2-filled glove box using polyisobutylene edge seal (HelioSeal™ PVS 101, ADCO product Inc.).26 Current density-voltage of the devices were measured under ambient air using a Keithley 2400 source-measure unit. During the measurements, an aperture (5 mm2) was placed onto the tandem solar cells in order to define area for accuracy. The cells were illuminated by a 450 W Newport solar simulator (model 91192–1000) equipped with an AM 1.5 filter at a calibrated intensity of 100 mW cm−2.
Fig. 2 shows the current density-voltage (J-V) characteristics of the tandem solar cells and the corresponding single active layer solar cells. Their device structures are: A, B: glass/ITO/PEI/P3HT:ICBA/PEDOT-T, where P3HT:ICBA (the same as the bottom active layer of the tandem cell) in device A is processed from dichrolobenzene solution, and P3HT:ICBA (the same as the top active layer of the tandem cell) in device B is processed from chrolobenzene solution; and C: glass/ITO/PEI/P3HT:ICBA/PH1000/PEI/P3HT:ICBA/PEDOT-T. Their performance is summarized in Table I. The single-junction devices (type A) yield a VOC = 0.79 ± 0.01 V, a JSC = 5.1 ± 0.3 mAcm−2, and a FF = 0.64 ± 0.02, resulting in a PCE = 2.5 ± 0.1%. Devices (type B) exhibit a VOC = 0.83 ± 0.01 V, a JSC = 6.4 ± 0.4 mAcm−2, and a FF = 0.58 ± 0.02, resulting in a PCE = 3.0 ± 0.2%. The efficiency of the cells is relatively low which is in part because the light absorption is poorer than that in a normal device containing a metal electrode as a light reflector.27 It might also be related to the polymer batch-to-batch variation. The tandem cells (type C) exhibit a VOC = 1.60 ± 0.02 V, a JSC = 3.1 ± 0.1 mA cm−2, a FF = 0.68 ± 0.04, and a PCE = 3.4 ± 0.2%, averaged 4 devices when measured with an aperture to define the photoactive area. The VOC of tandem cells is close to the addition of that of the two individual sub-cells. The FF of the tandem reaches a very high value of 0.68 ± 0.04, higher than those of both sub-cells (0.64 ± 0.02 and 0.58 ± 0.02), which has been observed in the previous report. The low JSC of the tandem device is due to the use of both P3HT:ICBA active layers in the two subcells. The JSC of the tandem solar cells can be improved when efficient spectrally complementary active layers are employed. Overall, the results prove that transfer-laminated PEDOT-T film can work well as a top electrode for organic tandem solar cells.
J-V characteristics of devices A, B (single-junction solar cells), and device C (tandem solar cell) under AM1.5 100 mWcm−2 illumination. Inset displays the J-V characteristics of a typical tandem solar cell in the dark and under AM 1.5 100 mWcm−2 illumination in semi-logarithmic scale.
J-V characteristics of devices A, B (single-junction solar cells), and device C (tandem solar cell) under AM1.5 100 mWcm−2 illumination. Inset displays the J-V characteristics of a typical tandem solar cell in the dark and under AM 1.5 100 mWcm−2 illumination in semi-logarithmic scale.
Average photovoltaic performance of single-junction and tandem solar cells. Device A, B: glass/ITO/PEI/P3HT:ICBA/PEDOT-T; C: glass/ITO/PEI/P3HT:ICBA/PH1000/PEI/ P3HT:ICBA/PEDOT-T electrode; averages taken over 4 devices for device A, B, and C. Data in parentheses indicate performance of the tandem solar cell with the highest PCE.
| Device | VOC (V) | JSC (mAcm−2) | FF | PCE |
|---|---|---|---|---|
| A (bottom cell) | 0.79 ± 0.01 | 5.1 ± 0.2 | 0.64 ± 0.02 | 2.5 ± 0.1% |
| B (top cell) | 0.83 ± 0.01 | 6.4 ± 0.4 | 0.58 ± 0.02 | 3.0 ± 0.2% |
| C (with aperture) | 1.60 ± 0.03 (1.62) | 3.1 ± 0.1 (3.2) | 0.68 ± 0.04 (0.72) | 3.4 ± 0.2% (3.6%) |
| Device | VOC (V) | JSC (mAcm−2) | FF | PCE |
|---|---|---|---|---|
| A (bottom cell) | 0.79 ± 0.01 | 5.1 ± 0.2 | 0.64 ± 0.02 | 2.5 ± 0.1% |
| B (top cell) | 0.83 ± 0.01 | 6.4 ± 0.4 | 0.58 ± 0.02 | 3.0 ± 0.2% |
| C (with aperture) | 1.60 ± 0.03 (1.62) | 3.1 ± 0.1 (3.2) | 0.68 ± 0.04 (0.72) | 3.4 ± 0.2% (3.6%) |
It should be noted that the patterning of the conductive recombination layer is important for the tandem solar cells to work properly. The conductivity of the PH1000 film is about 1.1 S/cm. After coating with PEI from isopropanol solution, the conductivity of the PH1000 is slightly enhanced to about 2 S/cm determined by film thickness and the sheet resistance. It should also be noted that we also tried coating PEI modification from 2-methoxyethanol solution. Surprisingly, the conductivity of PH1000 films was increased from 1.1 to 550 S/cm. The high conductivity of the charge recombination layer results in poor device yield of tandem solar cells. Therefore, the selection of the solvent for processing PEI for modifying PEDOT:PSS films is critical because the solvent could strongly affect the property of the PEDOT:PSS. We use isopropanol as the processing solvent because it does not change the conductivity of the PH1000 films much. When the charge recombination layer PH1000/PEI fully covers the substrate, the probe and the silver paint penetrate the top active layer and make the contact of the top electrode PEDOT-T and the charge recombination layer PH1000/PEI. In this case, the top subcell exhibits large leakage current and hence loses the VOC and FF. Fig. 3(a) shows the J-V characteristics of tandem cells where PH1000/PEI covers the whole substrates. The cell exhibits large leakage, a low FF of 0.25 and a low VOC of 1.1 V. When the PH1000 film was coated in half of the substrate (shown in Fig. 1), the contact between the top PEDOT-T and PH1000/PEI is avoided and the cells exhibit a large VOC of about 1.60 V and a large FF of about 0.70. Besides the half coating of the charge recombination layer PH1000, the scratch along the PEDOT-T pattern (see the dash line box in Fig. 1) and the use of an aperture are also important to accurately measure the device performance because of the fringe effect induced by the high conductivity of the charge recombination layer. As showed in Fig. 3(b), we present current-voltage (I-V) of the tandem cells when measured with and without apertures. The current of the tandem cells measured without aperture is about 1.4 times as much as that measured with aperture after scratch. Sista et al.28 also reported that increase in the conductivity of the interconnecting layer would result in a significant charge collection outside of active area defined by electrodes overlap in the lateral direction. To confirm this phenomenon in our cells, we further fabricated a single-junction device using PH1000 as the interlayer under PEDOT-T electrode with the structure: glass/ITO/PEI/P3HT:ICBA/PH1000/PEDOT-T (Fig. 4). The I-V curve is shown in Fig. 4. The original fabricated cell without any pattern (scratch or aperture) shows a current of 0.47 mA and a relatively low FF of 45%. After scratch along the top electrode, the current drops down to 0.37 mA. When an aperture is used to define the photoactive area, the current further drops to 0.15 mA and the FF increases to 62%. The results show that the proper patterning of the conductive charge recombination layer is critical for device yield as well as the accurate measurement of the device performance.
J-V characteristics of tandem solar cells with different patterning of the charge recombination layer: (a) PH1000 film is not patterned (fully coverage on the whole substrate); Inset is the J-V characteristics in semi-log scale; (b) PH1000 film covers the half of the substrates (similar to the ITO pattern, Fig. 1); And, PH1000 film was further patterned by scratching along the PEDOT-T electrode (dash-line box, shown in Fig. 1), and the cells were measured with aperture.
J-V characteristics of tandem solar cells with different patterning of the charge recombination layer: (a) PH1000 film is not patterned (fully coverage on the whole substrate); Inset is the J-V characteristics in semi-log scale; (b) PH1000 film covers the half of the substrates (similar to the ITO pattern, Fig. 1); And, PH1000 film was further patterned by scratching along the PEDOT-T electrode (dash-line box, shown in Fig. 1), and the cells were measured with aperture.
Device structure and J-V characteristics of single-junction cell with PH1000 film as the interlayer (half coating) under illumination measured after different patterning: original, scratch along the PEDOT-T electrode (dash-line box), and scratch + aperture.
Device structure and J-V characteristics of single-junction cell with PH1000 film as the interlayer (half coating) under illumination measured after different patterning: original, scratch along the PEDOT-T electrode (dash-line box), and scratch + aperture.
In summary, we reported vacuum-free and metal electrode–free inverted organic tandem solar cells that use film-transfer laminated PEDOT:PSS as the top electrode. The inverted organic tandem solar cells exhibit a VOC up to1.62 V and a large FF up to 0.72. The performance of the tandem solar cells could be further enhanced when efficient spectrally-complementary active layers are employed. The results show that organic solar cells in tandem structure can be easily processed from solution and do not require high-vacuum deposition. This advances the fabrication of low-cost organic tandem solar cells.
The work is supported by the Recruitment Program of Global Youth Experts, the National Natural Science Foundation of China (Grant No. 21474035) and by the Fundamental Research Funds for the Central Universities, HUST (Grant No. 2014YQ013).
