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Ji Hoon Seo, Dong-Ho Kim, Se-Hun Kwon, Myungkwan Song, Min-Seung Choi, Seung Yoon Ryu, Hyung Woo Lee, Yun Chang Park, Jung-Dae Kwon, Kee-Seok Nam, Yongsoo Jeong, Jae-Wook Kang,* and Chang Su Kim* Tandem solar cells are multijunction photovoltaic devices in which two subcells are stacked in order to achieve higher overall solar absorption.[1–5] Tandem solar cells offer a number of advantages over single cell devices. They deliver a higher open circuit voltage (Voc), which ideally equals the sum of Voc values of subcells connected in series, and they can consist of several subcells with different band gaps that have complementary absorption characteristics and potentially cover the entire solar spectrum. Various types of tandem solar cells have been investigated thus far, including organic/organic (small molecules,[6,7] polymers[8,9]) solar cells, dye-sensitized solar cells (DSSC),[10,11] inorganic/inorganic (amorphous and microcrystalline silicon (a-Si/μc-Si)[12,13]) solar cells, and hybrid (DSSC/ Cu(In, Ga)Se2,[14,15] DSSC/Si[16,17]) solar cells. Very recently, the first reports on organic/inorganic hybrid tandem cells that utilize the entire solar spectrum have been published. These comprise a a-Si solar cell as the bottom cell and an organic solar cell as the top cell.[18] However, the efficiencies of hybrid tandem solar cells remain significantly lower than those of either of the two cells individually. The origin of such limited cell performance is the poor interfacial contact, which makes it difficult to provide an efficient recombination region for electrons and holes generated from the two cells. As this is a relatively new research field, a number of challenging issues, including interface physics and device design, remain unresolved. In this paper, we report on the development of hybrid tandem solar cells based on two well-studied organic and inorganic subcells. The effects of the interlayer combination and thickness matching on the device performance were investigated in detail. The power conversion efficiency (PCE) of the optimized hybrid tandem solar cell reached a maximum of

J. H. Seo, Dr. D. H. Kim, Dr. M. Song, M.-S. Choi, Dr. S. Y. Ryu, Dr. J.-D. Kwon, Dr. K.-S. Nam, Dr. Y. Jeong, Dr. J.-W. Kang, Dr. C. S. Kim Advanced Functional Thin Films Department Korea Institute of Materials Science. Changwon 641-831, Republic of Korea E-mail: [email protected]; [email protected] J. H. Seo, Prof. S.-H. Kwon, Prof. H. W. Lee National Core Research Center for Hybrid Materials Solution Pusan National University Busan, 609-735, Republic of Korea Dr. Y. C. Park Measurement & Analysis Team National Nanofab Center, Daejeon, 305-806, Republic of Korea

DOI: 10.1002/adma.201201419

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High Efficiency Inorganic/Organic Hybrid Tandem Solar Cells

5.72%, which is higher than that of either of the two cells individually. The Voc of the hybrid cell was 1.42 V, reaching 92% of the sum of the subcell Voc values. Figure 1(a) shows the device structure of the inorganic/ organic hybrid tandem solar cells designed and synthesized in this work. The a-Si (p-type/Intrinsic/n-type stack) thin films were deposited as the bottom inorganic cell on Al-doped ZnO (AZO)[19,20] coated glass using plasma enhanced chemical vapor deposition. The active layer of the organic top cell consisted of a blend of poly(4,8-bis-alkyloxybenzo(1,2-b:4,5-b′)dithiophene2,6-diyl-alt-(alkyl thieno(3,4-b) thiophene-2-carboxylate)-2,6diyl) (PBDTTT-C)[21,22] as a low band gap polymer donor and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as an electron acceptor. The optical band gap of PBDTTT-C in the solid state was calculated from the absorption edge and found to be ∼1.61 eV.[21] Figure 1(b) shows typical photocurrent action spectra measured separately for inorganic and organic solar cells. The external quantum efficiency (EQE) is plotted as a function of the wavelength of the light. The low band gap PBDTTTC and PCBM film showed a relatively long wavelength range extending from 600 nm to 800 nm, enabling it to compensate for the narrow absorption spectra of the inorganic a-Si thin films. Thus, the spectral response of the two systems complemented each other demonstrating the advantage of employing them in hybrid tandem solar cells. The organic and inorganic solar cells were separated by a transparent indium tin oxide (ITO) layer and a highly conductive poly(3,4-ethylenedioxylenethiophene)polystylene sulfonic acid (PEDOT:PSS) layer. The ITO, acting as an electron transport layer, was deposited using magnetron sputtering and owing to its high transparency, the light intensity reaching the top solar cell was extremely high, resulting in an increased photocurrent. PEDOT:PSS was used as the hole transport layer with electrons from the inorganic solar cells combining with holes from the organic solar cells at the ITO/PEDOT:PSS interface. The energy band diagram of the proposed hybrid tandem solar cell is shown in Figure S1. The cross-sectional image of the hybrid tandem solar cell taken with high-resolution transmission electron microscopy (TEM) shows well-defined individual layers. Figure 1(c) shows TEM energy dispersive X-ray spectrometry (EDX) element mapping of the hybrid solar cells, where uniform distributions of the component elements can clearly be observed. The interfaces appear sharp and there is no evidence of layer-by-layer mixing or physical damage that may have occurred during the ITO sputtering or PEDOT:PSS spin-coating. A major challenge in the fabrication of organic-based tandem solar cells involves the interlayer that electrically connects but physically separates the

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Figure 1. (a) High-resolution TEM cross-sectional image and the corresponding device structure of the hybrid tandem solar cell. (b) EQE spectra of the organic and inorganic single cells used in this work. (c) TEM EDX element mapping of the hybrid tandem solar cell. (d) J–V characteristics of organic, inorganic single solar cells and hybrid tandem solar cells under illumination (100 mW/cm2).

two solar cells.[23,24] The use of transparent conductive oxides for the interlayer is highly promising owing to their low sheet resistance and high optical transparencies in the visible region for photon transmission. However, these materials are usually deposited by a sputtering process, which can cause plasma damage to the underlying organic cell and so is not always a viable option.[25,26] However, if the bottom solar cell is extremely robust it can remain intact during the sputtering making the process useful for interfacial engineering, such as high temperature or dry coating processes for tandem solar cells. Figure 1(d) shows the current density–voltage (J–V) characteristics of organic, inorganic, and hybrid tandem solar cells under AM1.5 simulated illumination with an intensity of 100 mW/cm2. The inorganic (a-Si) single solar cell exhibited an open circuit voltage (Voc) of 0.80 V, a short circuit current density (Jsc) of 7.86 mA/cm2, a fill factor (FF) of 0.64, and a PCE of 4.11%. The organic (PBDTTT-C:PCBM) single solar cell exhibited a Voc of 0.75 V, a Jsc of 11.54 mA/cm2, a FF of 0.53, and a PCE of 4.65%. The hybrid tandem solar cell yielded a Voc of 1.42 V, Jsc of 6.84 mA/cm2, a FF of 0.58, and a PCE of 5.72%. The Voc of the hybrid tandem solar cell is 1.42 V, reaching 92% of the sum of the subcell Voc values. This provides evidence that the two subcells have been successfully connected in series.[27,28] In addition, the PCE of the hybrid cell is calculated to be 5.72%, which is higher than

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that of either of the two cells individually (4.11% and 4.65%, respectively). Figure 2(a) shows the J–V characteristics of hybrid tandem solar cells with different interlayer combinations under illumination and Table 1 summarizes the characteristics of the device performance. The interlayer should efficiently collect electrons and from one cell and holes from another and should then act as an efficient recombination zone for them, free of potential loss.[29,30] As shown in Figure 2(a), the device performance of the tandem solar cells changes significantly with the variation of interlayers, indicating that the interlayer plays a critical role in the charge recombination process. The hybrid tandem solar cell without an interlayer exhibited a Voc of 0.87 V, a Jsc of 2.22 mA/cm2, a FF of 0.15, and a PCE of 0.30%. This poor photovoltaic performance of the device without an interlayer indicates that it is not electrically connected in series. The hybrid tandem solar cell fabricated with only a PEDOT:PSS interlayer coated on the n-type a-Si layer led to a significant increase in the Voc from 0.87 V to 1.40 V, resulting in a remarkable PCE increase from 0.30% to 2.08%. The Voc for the hybrid tandem solar cell with only a PEDOT:PSS interlayer is approximately the sum of the Voc of the individual cells. The hybrid tandem solar cell fabricated with only an ITO interlayer also showed the similar trend. However, the J–V characteristics of these devices

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Current density (mA/cm2)

Table 1. Photovoltaic parameters of hybrid tandem solar cells with different interlayer combinations.

1

Without interlayer 0 -1

Only PEDOT:PSS

-2 -3 -4

ITO & PEDOT:PSS

-5 0.0

0.3

0.6

0.9

1.2

1.5

Voltage (V)

(b)

-ZIm/Ωcm2

9000

Only PEDOT:PSS 6000

3000

ITO & PEDOT:PSS 0

Jsc (mA/cm2)

Voc (V)

Fill factor

Efficiency (%)

Without interlayer

2.22

0.87

0.15

0.30 ± 0.04

Only PEDOT:PSS

4.23

1.40

0.35

2.08 ± 0.19

Only ITO

4.38

1.35

0.37

1.90 ± 0.38

ITO & PEDOT:PSS

5.02

1.39

0.49

3.48 ± 0.11

Interlayer combination

0

5000

10000

15000

-ZRe/Ωcm

2

Figure 2. (a) J–V characteristics of hybrid tandem solar cells with different interlayer combinations under illumination. (b) Impedance spectrum of hybrid tandem solar cells with different interlayer combinations in the dark at zero bias.

have a pronounced S-shape with a high series resistance of 124 Ωcm2 and a very low FF of 0.35. The S-shape behavior indicates a significant energy barrier for charge extraction and injection. This barrier to charge carrier recombination leads to accumulation of carriers in the interlayer, increases series resistance, and decreases the FF.[31,32] The S-shape of the J–V curve can be avoided by the combination of an ITO/PEDOT:PSS interlayer, which in this case resulted in an increase in the FF to 0.49, and a decrease in the series resistance to 29 Ωcm2, with the Voc remaining unchanged. This result indicates that the ITO/PEDOT:PSS interlayer can provide an efficient recombination region for electrons and holes generated from the top and bottom cells due to the formation of better ohmic contacts. Consequently, an increase in the PCE from 2.08% to 3.48% was observed after the incorporation of an ITO/PEDOT:PSS interlayer into the hybrid tandem solar cells. To further investigate the photovoltaic parameters of solar cells with different interlayer combinations, electrical impedance spectrum measurements of the hybrid cells with only PEDOT:PSS and ITO/

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PEDOT:PSS were performed. This type of measurement is a powerful, non-destructive, characterization tool that is commonly used for the analysis of a wide variety of photovoltaic devices.[33–35] Figure 2(b) shows the Cole–Cole plot of the hybrid tandem solar cells with different interlayer combinations in the dark at zero bias. The semicircle evident for the device with only a PEDOT:PSS interlayer is larger than that for device with an ITO/PEDOT:PSS interlayer. This is attributed to the charge extraction and injection resistance associated with the electrons and holes recombination process in the interlayer. It is noteworthy that for the achievement of high-efficiency tandem solar cells, the optimized film thickness of the bottom cell is critical for balanced optical absorption and matched photocurrent.[36,37] To investigate the effect of the bottom solar cell thickness on the performance of the hybrid tandem solar cell, the J–V characteristics were measured under illumination (Figure 3(a)) with the photovoltaic parameters listed in Table 2. As shown from these results, when the a-Si thickness was decreased from 135 nm to 81 nm, the Voc remained almost the same, the FF slightly increased from 0.49 to 0.58, and the Jsc increased significantly from 5.02 mA/cm2 to 6.84 mA/cm2, for which the enhancement ratio was as much as 36%. Consequently, the PCE was improved from 3.48% to 5.72%, for which approximately 64% of the enhancement was ascribed to the contribution of the Jsc improvement. The likely explanation for these exceptional results is that the optical transmission through the bottom inorganic cell was optimized to obtain maximum optical absorption by the top organic cell. To our knowledge, this is the first time clear Jsc and PCE enhancement has been observed for hybrid tandem solar cells owing to the optimization of the thickness of the bottom solar cell. The optical cavity, and therefore the transmittance, was significantly affected by the thickness of the bottom inorganic solar cell. Given the fact that the top organic cell absorbs between 700 nm and 950 nm, the optical cavity (or layer thickness) of the bottom inorganic cell needed to be optimized in order to transmit in this wavelength range.[38,39] As shown in Figure 3(b), the optical cavity was tuned such that there was maximum overlap between the transmittance through the bottom cell and the absorption by the top cell. However, the PCE reached a maximum of 5.72% at a thickness of 81nm and decreased with further reductions in thickness. This trend is similar to that exhibited by inorganic single solar cells, where the Jsc and FF also decrease with decreasing thickness (Figure S2). This occurrence can be attributed to reduced light absorption and increased leakage current.[40,41] In summary, we have designed and fabricated highperformance inorganic/organic hybrid tandem solar cells and investigated the effects of interlayer combination and thickness

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the potential to further enhance the efficiency of future hybrid tandem solar cells. And, in order for the hybrid tandem solar cell to succeed as a technology, more effort must be directed towards large area fabrication combined with high productivity such as roll-to-roll production of inorganic solar cell[42] and organic solar cell.[43]

0

Current density (mA/cm )

a-Si thickness

2

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135nm 119nm 89nm 81nm 61nm

-2

-4

Experimental Section -6 0.0

0.3

0.6

0.9

1.2

1.5

Voltage (V) 100

1.0

80

0.8 Organic absorbance

0.6

60 40

0.4 a-Si thickness

0.2 0.0

20

81nm 135nm 400

500

600

700

800

Transmittance (%)

Normalized absorbance (a.u.)

(b)

0 900

Wavelength (nm) Figure 3. (a) J–V characteristics of hybrid tandem solar cells with different amorphous silicon thicknesses under illumination. (b) Normalized absorption spectra of organic thin film and transmittance of the amorphous silicon thin film as function of thickness.

matching on the device performance. The obtained results indicate that the ITO/PEDOT:PSS interlayer can provide an efficient recombination region for electrons and holes generated from the top and bottom cells, owing to the formation of superior ohmic contacts. Furthermore, the optical transmission through the bottom inorganic solar cell was optimized to maximize the optical absorption of the top organic solar cell. The power conversion efficiency of the optimized hybrid tandem solar cells reached a maximum PCE of 5.72% and the measured Voc was 1.42V, reaching 92% of the sum of the subcell Voc values. This study opens up a new direction towards the achievement of a universal optimal device layout and provides

Device Fabrication: The architecture of the hybrid tandem solar cell device is shown in Figure 1(a). AZO thin films were deposited on glass substrates by employing the RF magnetron sputtering technique. The 2 wt% of Al-doped ZnO target was the optimum composition to achieve the minimum resistivity. The a-Si (p-type/Intrinsic/n-type stack) thin films, as the bottom inorganic solar cells, were deposited on AZO-coated glass using plasma enhanced chemical vapor deposition. A mixture of SiH4 and H2 was used to obtain the intrinsic layer, whereas further addition of BH4 and PH3 was used to obtain p- and n-type layers, respectively. The layers were optimized individually by tuning the deposition parameters to obtain properties suitable for solar cells. To prepare interlayers on the n-type a-Si layer, ITO and PEDOT:PSS were applied. The ITO was deposited using magnetron sputtering and the PEDOT:PSS layer was spin coated onto ITO and dried at 150 °C for 1 min. An optimized interlayer thickness is critical. Because too thin a layer leads to a lower Voc and leakage current due to incomplete coverage, while too thick a layer increases the series resistance and reduces the Jsc and FF.[44] The highest efficiency of the hybrid tandem solar cell is obtained when the thickness of the ITO and PEDOT:PSS is about 50nm and 70 nm, respectively. The 2 wt% PBDTTT-C:PCBM solution (weight ratio of 1:2) in dichlorobenzene was spin coated on top of the PEDOT:PSS and dried for 10 min under ambient conditions. The device fabrication was completed by thermal evaporation of Al as the top electrode. Electrical, Optical, and Microscopic Characterization of Photovoltaic Cells and Thin Films: The J–V characteristics of the solar cell devices were measured under AM1.5 simulated illumination with an intensity of 100 mW/cm2. For each set of experimental conditions, over 10 devices were fabricated and the average PCE parameters together with their standard deviations are reported. External quantum efficiency (EQE) was measured using an incident photo-to-current efficiency measurement system. Absorption and transmittance spectra were obtained using an ultraviolet-visible spectrophotometer. The crosssectional microstructure of the devices was observed by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX) was employed for elemental analysis in spherical aberration corrected scanning transmission electron microscope (Cs-corrected STEM) mode.

Supporting Information Supporting Information is available from the Wiley Online library or from the author.

Table 2. Photovoltaic parameters of hybrid tandem solar cells with different amorphous silicon thicknesses.

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a-Si thickness

p-type

Intrinsic

n-type

Jsc (mA/cm2)

Voc (V)

Fill factor

135 nm

10 nm

100 nm

25 nm

5.02

1.39

0.49

3.48 ± 0.11

119 nm

6 nm

100 nm

13 nm

5.57

1.38

0.55

4.27 ± 0.08

89 nm

6 nm

70 nm

13 nm

6.28

1.36

0.59

5.10 ± 0.20

81 nm

3 nm

70 nm

8 nm

6.84

1.42

0.58

5.72 ± 0.29

61 nm

3 nm

50 nm

8 nm

6.54

1.35

0.51

4.60 ± 0.17

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Efficiency (%)

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The present research was supported by the research fund (2012PNK2860) of the Korea Institute of Materials Science. This research was also supported in part by the New and Renewable Energy (20103020010050) of the Korea Institute of Energy Technology Evaluation and Planning. Prof. H. W. Lee gratefully acknowledges support form the Basic Science Research Program (2011-0014709) through the National Research Foundation of Korea. Received: April 7, 2012 Revised: June 12, 2012 Published online: July 16, 2012

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