Highly efficient inverted bulk-heterojunction solar cells with a gradiently-doped ZnO layer

Abstract

In this study, we demonstrate a highly efficient inverted bulk heterojunction (BHJ) polymer solar cell using a wet-chemically prepared doped ZnO precursor with a self-organized ripple nanostructure as an electron extraction layer and a blend of poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)-carbonyl]-thieno-[3,4-b]thiophenediyl]] (PTB7) and [6,6]-phenyl C71 butyric acid methyl ester (PC₇₁BM) as an active light absorbing layer. In order to enhance the electron extraction efficiency, the ZnO ripple surface was modified with various alkali metal carbonate materials including Li₂CO₃, K₂CO₃, Na₂CO₃, Cs₂CO₃, and (NH₄)₂CO₃. The inclusion of an additional metal carbonate layer led to gradient doping of the ZnO ripple layer and improved the electron extraction properties by modifying the energy levels without destroying the ZnO ripple structures. The highest performing solar cells were fabricated with Li₂CO₃ and yielded a maximum PCE of 10.08%; this value represents a ∼14% increase in the efficiency compared to solar cells without a metal carbonate treatment.

---

Broader context

Organic BHJ solar cells currently occupy an important position in the future of renewable energy strategies because they are an efficient and cost-effective solution for the utilization of solar energy. In addition to improvements in the design and synthesis of active materials, interface engineering is also considered as an important factor that should be considered to further increase the efficiency and stability of organic BHJ devices. In an early study on BHJ solar cells, which utilized a metal oxide electron transporting layer, the doping method was one possible approach used to modify the energy level of the metal oxide layer. In most cases, this doping process was done by mixing the oxide precursor and salts powder. Unfortunately, this mixing method alters the structural properties of the metal oxide layer upon introduction of particular nanostructures. In our work, we suggest a novel method that can induce gradient doping of the metal oxide layer by using an alkali metal carbonate with bilayer deposition. The inclusion of an additional metal carbonate doping layer leads to improved electron extraction properties; this improvement is due to the modification of the energy level and electron transport properties of the ZnO layer without destruction of the ZnO ripple structures. The outstanding electron extraction properties of the gradiently-doped ZnO ripple layer allow the resulting devices to exhibit significantly enhanced efficiencies compared to their pristine, ZnO ripple-based reference cells.

Introduction

Organic-based photovoltaic cells have attracted immense attention as next-generation solar cells due to their low-cost, low-temperature processing, lightweight, and flexible nature.^1–3 Since the first report on bulk heterojunction solar cells in 1995,^4,5 substantial progress in the field of organic bulk heterojunction (BHJ) solar cells has increased device efficiencies to near 10%.^6–8 Aside from improvements in the design and synthesis of active materials,^9–11 morphology control and interface engineering have emerged as key factors to further increase device efficiency and stability.^12–16 The most successful technique for morphology control is accomplished through the incorporation of processing additives to induce well-organized phase separation of the donor and acceptor materials.^12,15–17 To achieve easy extraction of photo-generated charges via interface engineering, there are two main strategies. One employs an interfacial dipole layer or a surface modifier by using conjugated polyelectrolyte (CPE) or a polar solvent to obtain suitable interfacial conditions for transporting charges.^18–20 The other strategy tunes the energy level by a doping process in order to ensure proper energy level matching between the buffer layer and the active layer.^21–24

Similar to interface engineering techniques that apply interfacial surface modification layers, we have recently demonstrated enhanced power conversion efficiency (PCE) in inverted BHJ solar cells by introducing an additional pure ZnO layer or an additional [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) layer at the interface between the active layer and the ZnO ripple layer; this was performed using a wet-chemical method.^25,26 The additional pure ZnO layer was deposited by atomic layer deposition (ALD), and the additional PCBM layer was deposited by a simple spin-coating method. Although current technology allows ALD processing even in a roll-to-roll process under near atmosphere pressure, ALD is still not a simple technique due to its complicated equipment and additional processing steps compared to spin-coating. Also, in the case of PCBM insertion,²⁶ there were some difficulties in depositing the active layer on top of the PCBM layer by spin-coating; these were caused by the fact that the PCBM layer can be washed away during active layer deposition, even though we have introduced a temporary protective layer by using an orthogonal solvent. Thus, the fabrication success yield was relatively low.

Another simple approach that can be used to modify the metal oxide electron transport layer is doping with salts such as Cs₂CO₃ and SnCl₂.^22,24 It has been reported that the use of salts, as a source of Cs or Sn dopants for a cathodic metal oxide buffer layer such as TiO₂ and ZnO, can improve the solar cell efficiency; doping with Cs or Sn components can improve the charge extraction properties by modifying the energy level of the metal oxide buffer layer. M. H. Park et al. have demonstrated enhanced PCEs in BHJ solar cells based on poly(3-hexylthiophene) (P3HT) and PCBM by introducing nanocrystalline TiO₂ doped with Cs₂CO₃.²⁴ Additionally, M. Thambidurai and co-workers have demonstrated enhanced PCEs in inverted BHJ solar cells based on poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)-carbonyl]-thieno-[3,4-b]thiophenediyl]] (PTB7) and [6,6]-phenyl-C71-butyric acid methyl ester (PC₇₁BM) by using Sn-doped TiO₂.²²

In most cases, these doping processes have been performed by mixing the oxide precursor and salt powder. However, this mixing method is not suitable for our ZnO layer, which has a ripple structure, because the addition of salts interrupts the formation of the ripple structure. In this work, as an alternative way to dope the ZnO layer without disturbing ripple structure formation, we utilized a bilayer method using various metal carbonate materials such as lithium carbonate (Li₂CO₃), potassium carbonate (K₂CO₃), sodium carbonate (Na₂CO₃), cesium carbonate (Cs₂CO₃), and ammonium carbonate ((NH₄)₂CO₃). The inclusion of an additional metal carbonate layer leads to improved electron extraction properties via modification of the energy level and the electron transport properties of the ZnO layer (without destruction of the ZnO ripple structures). The highest performing solar cells were fabricated with Li₂CO₃ and yielded a maximum PCE of 10.08%; this value represents a ∼14% increase in the efficiency compared to solar cells without a metal carbonate treatment.

Results and discussion

Fig. 1 shows a schematic of the inverted BHJ solar cell architecture as well as the chemical structures of the photoactive layer materials (PTB7 and PC₇₁BM). After deposition of the wet-chemically prepared ZnO precursor onto the pre-patterned indium tin oxide (ITO) substrates by spin-coating under atmospheric conditions, a self-organized ripple structure was developed to enhance light absorption by scattering. Detailed procedures of the development of the ZnO ripple structures are described elsewhere.^25–28 In order to obtain good phase separation in the active layer, 3% 1,8-diiodooctane (DIO) was incorporated into the solution of PTB7 and PC₇₁BM. For the hole transport layer, thermally evaporated MoO₃ was used.

Fig. 1 Schematic illustration of the inverted solar cell structure and chemical structures of the photoactive materials.

First, in order to investigate the morphology changes initiated in the ZnO ripple layer by adding the metal carbonate layer, atomic force microscopy (AFM) measurements were performed for ZnO ripple layers covered with various alkali metal carbonate materials: Li₂CO₃, K₂CO₃, Na₂CO₃, Cs₂CO₃, and pseudo-alkali (NH₄)₂CO₃. A previous report noted that the charge extraction properties can be influenced by the use of patterned electrodes.²⁹ Since the ripple structure of the ZnO layer is essential for achieving high efficiencies from inverted BHJ solar cells, the investigation of morphological changes by doping is crucial for clearly elucidating the effect of doping.

Fig. 2a shows AFM images of the ZnO ripple structures with a root mean square (RMS) roughness value of ∼16.7 nm. Surface morphology images of ZnO ripples after spin-coating a metal carbonate layer on top are shown in Fig. 2b–f. The measured RMS values were 8.7 nm for the ZnO ripple with Li₂CO₃, 13.4 nm for the ZnO ripple with K₂CO₃, 11.4 nm for the ZnO ripple with Na₂CO₃, 11.7 nm for the ZnO ripple with Cs₂CO₃, and 10.8 nm for the ZnO ripple with (NH₄)₂CO₃. Although the RMS roughnesses of the ZnO ripple layers covered with metal carbonates were slightly lower than that of the ZnO ripple without a metal carbonate, the ripple structures themselves were not significantly changed.

Fig. 2 AFM height images (10 μm × 10 μm) of (a) pristine ZnO ripple, (b) ZnO ripple covered with Li₂CO₃, (c) ZnO ripple covered with K₂CO₃, (d) ZnO ripple covered with Na₂CO₃, (e) ZnO ripple covered with Cs₂CO₃, and (f) ZnO ripple covered with (NH₄)₂CO₃.

In order to investigate the lateral morphology changes in the ZnO ripple layer due to the addition of the metal carbonate layer, fast Fourier transform (FFT) analysis and power spectral density (PSD) analysis were performed using AFM topographic images. Fig. 3a shows the 2D-FFT images of the pristine ZnO ripple and the ZnO ripple doped with various metal carbonate salts. All FFT results exhibited a ring-like pattern that indicates a patterned structure with a random orientation and a spatial period. The PSD (Fig. 3b) calculated from the FFT results showed almost identical periodicities to the ZnO ripples doped with Li₂CO₃, K₂CO₃, Na₂CO₃, or Cs₂CO₃. The periodic length scale, which is a reciprocal value of spatial frequency, of the ZnO ripples doped with Li₂CO₃, K₂CO₃, Na₂CO₃, or Cs₂CO₃ was ca. 41 nm. For the ZnO ripple with (NH₄)₂CO₃, the calculated periodic length was 19 nm, which is almost half of the value of the ZnO ripple doped with other metal carbonate salts. However, an approximately 20 nm change in the spatial period cannot significantly affect the scattering of incident light. Therefore, the scattering of incident light was not dependent on the dopant species.

Fig. 3 (a) 2D-FFT images of pristine ZnO ripple and ZnO ripple doped with various metal carbonate salts. (b) The PSD calculated from the FFT results.

In order to investigate the penetration depth of the metal carbonate dopants deposited on the ZnO ripple layers, we performed X-ray photoelectron spectroscopy (XPS). Fig. 4 shows the XPS spectra obtained from ZnO layers with various metal carbonates deposited on top and then etched by an Ar ion beam. Because of the possibility that XPS sampling of the surface might not be equivalent in the crests and valleys of the rippled ZnO, XPS measurements were also conducted on the flat ZnO samples in order to verify the quantitative accuracy. The XPS results from the rippled and flat ZnO samples were almost identical. Note that the etching thickness of each step was 10 nm. For the samples with K₂CO₃ (Fig. 4a), Na₂CO₃ (Fig. 4b), and Cs₂CO₃ (Fig. 4c), K 2p peaks, Na 1s peaks, and Cs 3d peaks, respectively, were clearly detected at an etching depth of up to 70 nm; however, the intensity of each peak gradually decreased as the depth increased. Note that the original thickness of the ZnO ripple layer was approximately 80 nm. Therefore, these XPS results indicated that the ZnO layer was formed by gradient-doping spontaneously (see Fig. 4f). For the sample with Li₂CO₃ (Fig. 4d), the Li 1s peak (shown at 55 eV) was detected at an etching depth of up to 30 nm. However, the Li 1s peak was not detected beyond 30 nm. In the case of (NH₄)₂CO₃ (Fig. 4e), the N 1s peak was not detected, even at an etching depth of 10 nm. Fig. 4f shows the normalized dopant concentration versus the etching depth. The amount of alkali metal dopants gradually decreased with increasing depth. However, in contrast to the expectation that small alkali metals would be able to penetrate more deeply, the relatively large alkali metals (Cs and Na) penetrated more deeply compared to the smaller Li atoms. This phenomenon is not yet fully understood, and additional XPS studies are currently underway to elucidate these results.

Fig. 4 XPS depth study of alkali metal-doped ZnO: (a) K 2p peaks in ZnO with K₂CO₃, (b) Na 1s peaks in ZnO with Na₂CO₃, (c) Cs 3d peaks in ZnO with Cs₂CO₃, (d) Li 1s peaks in ZnO with Li₂CO₃, (e) N 1s peak at NH₄ in ZnO with (NH₄)₂CO₃, and (f) normalized profiles of atom% versus etching depth.

Fig. 5a shows the current density (J)–voltage (V) characteristics of inverted PTB7:PC₇₁BM solar cells with and without various metal carbonate materials on the ZnO ripple layer. More than 100 devices were fabricated to optimize the device efficiency by controlling the coating parameters of the metal carbonate materials in these PTB7:PC₇₁BM-based inverted BHJ solar cells. The best reference solar cell fabricated without a metal carbonate layer yielded a PCE of 8.68%; this device had a short circuit current density (J_sc) of 16.52 mA cm⁻², an open circuit voltage (V_oc) of 0.74 V, and a fill factor (FF) of 0.71. The solar cells with an alkali metal-doped ZnO layer yielded significantly enhanced PCEs, except for the solar cell with pseudo-alkali NH₄. The solar cell with potassium-doped ZnO yielded a slightly enhanced PCE of 9.10%, while the solar cell with a sodium-doped ZnO layer showed a more enhanced PCE of 9.29%. For the device with cesium-doped ZnO, we obtained a significantly improved PCE of 9.54%, which was caused by enhancements in the J_sc and FF. However, the best performance was shown in the device with the lithium-doped ZnO layer. The champion solar cell yielded a significantly enhanced PCE of 10.08% with a J_sc of 18.93 mA cm⁻², a V_oc of 0.73 V, and a FF of 0.73. Details related to the performance of these solar cells are listed in Table 1, and the incident photon to charge carrier efficiency (IPCE) spectra and the detailed solar cell parameter distributions are enclosed in the ESI† (Fig. S4 and S8).

Fig. 5 J–V characteristics of inverted PTB7–PC₇₁BM solar cells with the doped ZnO ripple layer using various alkali metal carbonate salts.

Table 1 Summarized photovoltaic performance characteristics of inverted BHJ solar cells with various alkali metal carbonate-doped ZnO ripple layers

	V oc [V]	FF	J sc (J–V) [mA cm^−2]	J sc (IPCE) [mA cm^−2]	η [%]
a The average PCE values were calculated from more than 30 devices.
Pristine ZnO Ripple	0.74	0.71	16.52	15.91	8.68 (8.51)
w/Potassium	0.73	0.71	17.56	16.89	9.10 (8.86)
w/Sodium	0.73	0.72	17.67	16.95	9.29 (8.96)
w/Cesium	0.72	0.72	18.41	17.57	9.54 (9.27)
w/Ammonium	0.72	0.67	16.13	15.42	7.78 (6.93)
w/Lithium	0.73	0.73	18.93	18.27	10.08 (9.87)

---

Ultra-violet photoelectron spectroscopy (UPS) measurements demonstrate that the energy levels of ZnO can be effectively modified by doping with alkali metal carbonates. Fig. 6 shows the energy level diagrams extracted from the UPS data (see the ESI,† Fig. S6). The Fermi energy (E_F) was calculated from the ITO surface and all other spectra are plotted with respect to this value. The vacuum levels (VLs) of the samples were determined by linear extrapolation of the secondary electron cutoffs on the high binding energy side of the UPS spectra (15–18 eV). The valence band maximums (VBMs) for both pristine ZnO and doped ZnO were extracted from the onset in the low binding energy side of the UPS spectra. Comparing the shift in the VBM onset to the E_F of ITO provides the relative position of the VBM level. The conduction band minimum (CBM) level is estimated by using the VBM and the measured optical gaps extracted from the UV-Vis absorption spectra (see the ESI,† Fig. S3). According to the basic operational principles of organic BHJ solar cells, photo-generated electrons are initially transferred from the electron donor materials to the electron acceptor (PC₇₁BM). Then, these separated electrons are extracted to the cathode electrode through the electron transport layer (ZnO in our case). Thus, the energy level matching between PC₇₁BM and ZnO is critical to the electron extraction process. The CBM of the pristine ZnO layer prepared by a wet-chemical method was 3.51 eV. Therefore, the energy level difference between PC₇₁BM and pristine ZnO (i.e., the electron extraction barrier) was relatively large (0.69 eV). However, the energy levels of the ZnO layers were dramatically changed upon the addition of an alkali metal carbonate layer. The electron extraction barriers decreased after the introduction of an alkali metal carbonate doping layer. The CBM energy level of ZnO doped with K₂CO₃ was 3.58 eV. The CBM levels of the doped ZnO layers increased more after introducing Na₂CO₃ (3.68 eV) and Cs₂CO₃ (3.82 eV). However, the largest CBM level change was observed in Li₂CO₃-doped ZnO. The CBM level of ZnO modified by Li₂CO₃ was 4.19 eV, which is almost the same energy level as the LUMO of PC₇₁BM. Therefore, we conclude that electrons were easily extracted from the LUMO of PC₇₁BM to ITO through the modified ZnO CBM without any barrier; this led to the highest PCE performance in the solar cell device with the Li₂CO₃-doped ZnO layer. For the pseudo-alkali (NH₄)₂CO₃, the modified CBM of (NH₄)₂CO₃-doped ZnO was 3.48 eV, which is higher than that of pristine ZnO. Thus, the solar cell with the (NH₄)₂CO₃-doped ZnO layer exhibited a decreased PCE value compared to the solar cell with the pristine ZnO layer.

Fig. 6 Energy levels of ZnO ripple layers doped with various metal carbonate materials.

In general, V_oc is mainly determined by the energy level difference between the highest occupied molecular orbital (HOMO) of donor polymers and the lowest unoccupied molecular orbital (LUMO) of acceptor materials. However, it is true that the V_oc can be influenced by the work function difference of two electrodes. Although a slightly increasing trend in V_oc was clearly observed when the average values were compared (see Fig. S8 in the ESI†), the amount of V_oc change was not very significant compared to the change of the CBM level of doped ZnO. Generally, the V_oc change occurring by the influence of the electrode work function mainly originates from band bending at the interface of the metal electrode and the active semiconducting layer. Although the CBM level of the doped ZnO layer was significantly changed by the doping process, the conductivity of the doped ZnO was still significantly lower than that of metals. Thus, the band bending at the interface between the low conductivity metal oxide (ZnO) and the active organic layer was not very significant to affect V_oc.

To further study the effect of the alkali metal-doped ZnO on the inverted BHJ solar cells, impedance spectroscopy (IS) measurements were performed. IS is a useful method to analyze the interfacial properties, such as electron transport and recombination, in solar cells. Fig. 7a shows the Nyquist plots of the IS results measured under dark conditions. All solar cells showed one semicircle without transmission line (TL) behavior regardless of the type of doping. It is known that the TL pattern is observable when the transport resistance is lower than the recombination resistance. Therefore, the absence of the TL pattern indicated that all solar cells experience strong recombination, which can be interpreted using the Gerischer impedance model.^30,31 In addition, the presence of a single semicircle indicates that the interface contacts between the active layer and the ZnO layer are not likely to be rectifying contacts. The devices with doped ZnO exhibited higher recombination resistances (i.e., lower recombination rates) than the device with pristine ZnO. Because all device fabrication parameters were identical for all of the solar cells (with the exception of the doping process), the difference in the recombination rate likely originated only from the interface modification that was induced by doping. Furthermore, the order of the measured recombination resistances was consistent with solar cell performance. The solar cell with Li-doped ZnO, which showed the highest performance, exhibited the highest recombination resistance, and the solar cell with NH₄-doped ZnO, which showed the lowest efficiency, exhibited the lowest recombination resistance. The higher recombination resistance in the solar cells with alkali metal-doped ZnO can be explained by the fact that the doping process effectively modified the CBM energy level of the ZnO layer and allowed easy extraction of photo-generated charges at the ZnO interface.

Fig. 7 (a) Impedance spectra (Nyquist plot) of inverted PTB7–PC₇₁BM solar cells with and without a doped ZnO ripple layer. (b) Calculated recombination resistance versus voltage.

Fig. 7b shows the recombination resistance versus the various forward biases deduced from equivalent circuit fitting. The solar cell with pristine ZnO showed the typical tendency of recombination resistance dependence on the forward bias.^32,33 Such decrement of the recombination resistance is attributed to an increase in the charge accumulation at the interface caused by the reduced internal potential as the forward bias is increased. However, the recombination resistance of the solar cells with a doped ZnO layer showed less dependence on the forward bias up to the ∼V_oc. Therefore, the independent recombination resistance of the solar cells with a gradiently-doped ZnO layer can be explained by the fact that the extra doping layer effectively modifies the energy level and allows less accumulation at the interface; this, in turn, enables easier extraction of photo-generated charges.

Experimental section

Preparation of ZnO sol–gel

A ZnO sol–gel solution (concentration of solution: 0.75 M) was prepared by dissolving zinc acetate [Zn(CH₃COO)₂.2H₂O] (Sigma-Aldrich Co., 99.9%) in 2-methoxyethanol (Sigma-Aldrich Co., 99.8%) containing ethanolamine as a stabilizer. The solution was stirred for 2–3 h at 60 °C using a magnetic stirrer to obtain a homogeneous solution.

OPV device fabrication

The inverted BHJ solar cells consisted of a stack of indium tin oxide (ITO)-coated glass, ZnO with a ripple structure, a heterojunction of donor and acceptor materials (as the active layer), PEDOT : PSS, and an Ag top-electrode. To prepare the ZnO ripple film, a sol–gel solution of 0.75 M ZnO was spin-coated on ITO-coated glass that had been pre-treated with UV-ozone for 60 min. After coating the ZnO precursor, the samples were heated in air to 350 °C at a constant heating rate of 20 °C min⁻¹ on a stabilized hot-plate. The details of this synthesis and the effects of the ZnO ripple film on device performance have been reported elsewhere.^25–28 Alkali metal carbonate solutions were prepared by dissolving Li₂CO₃, K₂CO₃, Na₂CO₃, Cs₂CO₃, and (NH₄)₂CO₃ (99.9%, Aldrich) in distilled water (1 wt%). An alkali metal carbonate doping layer was deposited on the ZnO ripple layer by spin-coating at 2000 rpm for 45 s in air. For the preparation of the active layer, PTB7 : PC₇₁BM (1 : 1.5 ratio) was dissolved in chlorobenzene/1,8-diiodooxtane (CB/DIO; 0.97/0.03 ml, Aldrich). This solution was then stirred at 60 °C overnight. After passing the solution through a 0.22 μm polytetrafluoroethylene (PTFE) filter, the active layer was deposited onto the ZnO-coated ITO electrode by spin-coating at 1000 rpm for 60 s in an N₂-filled glove box. The thickness of the active layer was about 80 nm. The MoO₃ hole transport layer (7 nm) was deposited by thermal evaporation onto the active layer. The Ag metal top electrode (100 nm) was also thermally evaporated onto the MoO₃ layer under 5 × 10⁻⁶ Torr. The active surface area of the device, as defined by a metal shadow mask, was 0.38 cm².

Device characterization

The current density–voltage (J–V) curves of the solar cell devices were obtained using a Keithley 2401 source measurement unit under AM1.5G simulated illumination (100 mW cm⁻²). The intensity of the simulated sunlight was measured using a standard Si-photodiode detector with a KG-3 filter (Newport Co., Oriel). The EQE spectra of each solar cell device were obtained using a solar cell spectral response/QE/IPCE measurement system (Newport Co., Oriel IQE-200). The absorption spectra were measured using a UV/Vis spectrometer (Varian, Cary5000).

AFM measurements

The surface properties and morphologies of the pristine ZnO and doped ZnO films were characterized by AFM. The AFM images (scan area: 10 μm × 10 μm and 2 μm × 2 μm) were obtained using a Seiko E-Sweep atomic force microscope in tapping mode.

XPS/UPS measurements

XPS investigations were carried out on a Thermo Scientific K-α spectrometer (Thermo Fisher) using Al Kα non-monochromatic X-ray excitation source at a power of 72 W. The analysis area was 400 μm in diameter and a pass energy of 50 eV was utilized for electron analysis.

IS measurements

Non-modulated impedance spectroscopy was performed using an impedance analyzer (IVIUM Tech., IviumStat) at various forward biases. A 30 mV voltage perturbation was applied over a constant forward applied bias between 0 and 1.5 V in the frequency range between 0.1 Hz and 1.0 MHz.

Conclusions

In summary, we have demonstrated high performance inverted BHJ solar cells with enhanced PCEs by the insertion of an additional alkali metal carbonate doping layer between the self-organized rippled ZnO layer and the photoactive layer. Bilayer formation of the ZnO layer and the alkali metal carbonate layer induced gradient doping of the ZnO layer and effectively modified the CBM of ZnO. The champion solar cell with a Li₂CO₃-doped ZnO layer exhibited a maximum PCE of 10.08%, which represents a value nearly 14% higher than those of solar cells with pristine ZnO ripple layers. ZnO doping provided substantial dynamic advantages in the operation of inverted BHJ solar cells. IS studies clearly showed enhanced recombination resistance, which was independent of the applied forward bias in the solar cells, after the insertion of an alkali metal carbonate doping layer. Therefore, we concluded that the enhanced PCE in the solar cells with a doped ZnO layer was attributable to the effective modification of the CBM level of ZnO and to the effective quenching of electron–hole recombination by reducing the accumulated charges at the surface of the ZnO ripples.

Acknowledgements

This research was supported by a National Research Foundation of Korea grant (NRF-2013R1A2A2A01067741, 2014R1A4A1071686, 2009-0093818) and KIMS. The portion of this research conducted at the KAERI was supported by a National Research Foundation of Korea grant (2012M2A2A6013183).

References

1. C. J. Brabec, S. Gowrisanker, J. J. M. Halls, D. Laird, S. Jia and S. P. Williams, Adv. Mater., 2010, 22, 3839.
2. S. B. Darling and F. You, RSC Adv., 2013, 3, 17633.
3. S. Cho, K.-D. Kim, J. Heo, J. Y. Lee, G. Cha, B. Y. Seo, Y. D. Kim, Y. S. Kim and D. C. Lim, Sci. Rep., 2014, 4, 4306.
4. G. Yu, J. Hummelen, F. Wudl and A. J. Heeger, Science, 1995, 270, 1789.
5. J. J. M. Halls, C. A. Walsh, N. C. Greenham, E. A. Marseglia, R. H. Friend, S. C. Moratti and A. B. Holmes, Nature, 1995, 376, 498.
6. Z. He, C. Zhong, S. Su, M. Xu, H. Wu and Y. Cao, Nat. Photonics, 2012, 6, 591.
7. L. Dou, J. You, J. Yang, C.-C. Chen, Y. He, S. Murase, T. Moriarty, K. Emery, G. Li and Y. Yang, Nat. Photonics, 2012, 6, 180.
8. L. Dou, J. You, Z. Hong, Z. Xu, G. Li, R. A. Street and Y. Yang, Adv. Mater., 2013, 25, 6642.
9. Y. J. Cheng, S. H. Yang and C. S. Hsu, Chem. Rev., 2009, 109, 5868.
10. Y. Y. Liang and L. P. Yu, Acc. Chem. Res., 2010, 43, 1227.
11. Y. F. Li, Acc. Chem. Res., 2012, 45, 723.
12. J. Peet, J. Y. Kim, N. E. Coates, W. L. Ma, D. Moses, A. J. Heeger and G. C. Bazan, Nat. Mater., 2007, 6, 497.
13. G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery and Y. Yang, Nat. Mater., 2005, 4, 864.
14. P. M. Beaujuge and J. M. J. Frechet, J. Am. Chem. Soc., 2011, 133, 20009.
15. J. K. Lee, W. L. Ma, C. J. Brabec, J. Yuen, J. S. Moon, J. Y. Kim, K. Lee, G. C. Bazan and A. J. Heeger, J. Am. Chem. Soc., 2008, 130, 3619.
16. W. Chen, M. P. Nikiforov and S. B. Darling, Energy Environ. Sci., 2012, 5, 8045.
17. H.-C. Liao, C.-C. Ho, C.-Y. Chang, M.-H. Jao, S. B. Darling and W.-F. Su, Mater. Today, 2013, 16, 326.
18. H. Choi, J. S. Park, E. Jeong, G. H. Kim, B. R. Lee, S. O. Kim, M. H. Song, H. Y. Woo and J. Y. Kim, Adv. Mater., 2011, 23, 2759.
19. J. H. Seo, A. Gutacker, Y. M. Sun, H. B. Wu, F. Huang, Y. Cao, U. Scherf, A. J. Heeger and G. C. Bazan, J. Am. Chem. Soc., 2011, 133, 8416.
20. B. R. Lee, E. D. Jung, Y. S. Nam, M. Jung, J. S. Park, S. Lee, H. Choi, S.-J. Ko, N. R. Shin, Y.-K. Kim, S. O. Kim, J. Y. Kim, H.-J. Shin, S. Cho and M. H. Song, Adv. Mater., 2014, 26, 494.
21. S. Y. Park, B. J. Kim, K. Kim, M. S. Kang, K.-H. Lim, T. I. Lee, J. M. Myoung, H. K. Baik, J. Ho Cho and Y. S. Kim, Adv. Mater., 2012, 24, 834.
22. M. Thambidurai, J. Y. Kim, H. Song, Y. Ko, N. Muthukumarasamy, D. Velauthapillai, V. W. Bergmann, S. A. L. Weberd and C. Lee, J. Mater. Chem. A, 2014, 2, 11426.
23. S. Kwon, K.-G. Lim, M. Shim, H. C. Moon, J. Park, G. Jeon, J. Shin, K. Cho, T.-W. Lee and J. K. Kim, J. Mater. Chem. A, 2013, 1, 11802.
24. M.-H. Park, J.-H. Li, A. Kumar, G. Li and Y. Yang, Adv. Funct. Mater., 2009, 19, 1241.
25. D. C. Lim, K.-D. Kim, S.-Y. Park, E. M. Hong, H. O. Seo, J. H. Lim, K. H. Lee, Y. Jeong, C. Song, E. Lee, Y. D. Kim and S. Cho, Energy Environ. Sci., 2012, 5, 9803.
26. S. Cho, K.-D. Kim, J. Heo, J. Y. Lee, G. Cha, B. Y. Seo, Y. D. Kim, Y. S. Kim, S.-Y. Choi and D. C. Lim, Sci. Rep., 2014, 4, 4306.
27. D. C. Lim, W. H. Shim, K.-D. Kim, H. O. Seo, J.-H. Lim, Y. Jeong, Y. D. Kim and K. H. Lee, Sol. Energy Mater. Sol. Cells, 2011, 95, 3036.
28. H.-Y. Park, D. Lim, K.-D. Kim and S.-Y. Jang, J. Mater. Chem. A, 2013, 1, 6327.
29. R. Meier, C. Birkenstock, C. M. Palumbiny and P. Müller-Buschbaum, Phys. Chem. Chem. Phys., 2012, 14, 15088.
30. J. Bisquert, I. Mora-Sero and F. Fabregat-Santiago, ChemElectroChem, 2014, 1, 289.
31. V. Gonzalez-Pedro, E. J. Juarez-Perez, W. S. Arsyad, E. M. Barea, F. Fabregat-Santiago, I. Mora-Sero and J. Bisquert, Nano Lett., 2014, 14, 888.
32. G. Garcia-Belmonte, A. Guerrero and J. Bisquert, J. Phys. Chem. Lett., 2013, 4, 877.
33. M.-J. Jin, J. Jo and J.-W. Yoo, Org. Electron., 2015, 19, 83.

---

Footnote

† Electronic supplementary information (ESI) available: Additional AFM images, IPCE spectra, UPS spectra, absorption spectra. See DOI: 10.1039/c5ee03045e

---