Recent Research Progress on Organic–Inorganic Hybrid Solar Cells
Wenjie Zhao, Na Li, Xin Jin, Shengnan Duan, Baoning Wang, Aijun Li, and Xiao-Feng Wang
Nowadays, organic-inorganic solar cells have recently gained remarkable attention from the photovoltaic research community because of their remarkable device performance and high power-conversion efficiencies (PCEs). The crystalline Si and other alternative inorganic materials are more commonly used in the manufacturing of solar cells, but their implementation in modern electronic devices is restricted as a result of the inflexibility of the solar cells and the aggravated air pollution problem. One of the possible alternatives is organic-based solar cells, where the cell can be fabricated at low temperature over a large area in an inexpensive way. Now the organic materials have been developed greatly as eco-friendly solar energy to replace non-renew'able sources.'-3 However, the low electron mobility or the inefficient charge transport due to the presence of the traps in the organic active film leads to a decrease of the efficiency of the organic photovoltaic cell structures. Later research provides the evidence that the limitation of the organic photovoltaic structures linked to the charge transport can be overcome by combining organic materials with inorganic semiconductors nanoparticles (NPs), which improve the charge transport based on the low ionization potential of the organic part and the high electron affinity of the inorganic component.4 5 The combined characteristics of the organic and inorganic semiconductors are implemented into the hybrid solar cells, which exhibit the good charge-transfer characteristics of inorganic semiconductors and the easy processing and tunability of organic semiconductors.
Among the inorganic compounds applied on hybrid photovoltaic structures, ZnO6 and TiO,7-10 with different kinds of morphologies have been proposed as promising materials in hybrid organic-inorganic photovoltaic device applications because of their optical and electrical properties. ZnO is an n-type semiconductor characterized by its unique physical and chemical properties, such as high chemical stability, high electrochemical coupling coefficient, broad range of radiation absorption, and high photostability. Ti02 as another electron acceptor for use in photovoltaic devices has the advantages of resistance to acid and base, good chemical and photochemical stability, non-toxicity, low cost, and better charge separation properties. These advantages have attracted intensive interest focusing on organic-inorganic hybrid solar cells with ZnO or TiO, as inorganic compounds and many efforts have been made to further improve their performance. Recent studies show that a better control of the organic-inorganic heterojunction morphology can develop the performance of solar cells by enhancing the generation and transfer efficiency of excitons. The grain size and morphology of inorganic compounds are proved to be critical for the performance of the solar cells, because the charge generation and transport highly rely on percolating pathways. Employing inorganic semiconductor NPs could effectively increase charge diffusion and effective carrier lifetime, thereby promoting charge transfer at the interface and bulk materials. Besides the bulk properties of organic- inorganic heterojunction, the interfacial properties of semiconductor oxide could also influence the relevant photophysical processes of it. Due to the surface-induced defect states, the doping could effectively adjust the bandgap of the inorganic semiconductor and the electrical properties, thus suppressing the change of its bulk property. The device performance of the organic-inorganic hybrid solar cells can be improved by optimizing of the donor-acceptor interface to improve the charge separation efficiency and suppress the charge recombination. An efficient way to enhance the interfacial properties is the surface modification of inorganic materials with semiconductor quantum dots and small organic molecules. Controllable nanomorphology, well-structured interfaces, and superior optoelectronic interactions are proved to be essential to develop highly efficient hybrid solar cells.11
Further efforts are made by applying both ZnO and TiO, together on hybrid bulk heterojunction to improve the efficiency solar cells. Moreover, as an abundant natural pigment with low costs and non-toxic property, a series of chlorophyll (Chi) derivatives are applied into organic-inorganic-based solar cells (OISCs) to achieve the goal of using “green" photovoltaics with high efficiency. Their outstanding photoelectron performance and strong absorption ability around near-infrared region provide new research ideas and methods for using clean and renewable energy.
ZnO organic Hybrid Solar Cells
In organic-inorganic hybrid solar cells, ZnO is widely used to replace the electron acceptor organic semiconductors of organic solar cells. And the efficiency of organic- inorganic solar cells critically not only relies on the compactness of mixing of the donor and acceptor semiconductors, but also depends on the presence of unobstructed
FIGURE 8.1 Electron tomography of РЗНТ/ZnO solar cells (a) transmission electron micrograph of a cross-section of РЗНТ/ZnO photovoltaic cell, (b) reconstructed volumes of РЗНТ/ ZnO layers obtained by electron tomography, (c) reconstructed volume of a cross-section of the active layer of a completed РЗНТ/ZnO device, (d) the green arrow indicates an isolated ZnO domain, the red arrow indicates a ZnO domain that is connected to the top, but not through a strictly rising path. Reprinted with permission from Ref. 1, Copyright 2009, Springer Nature.
transport pathways of electrons and holes to achieve efficient charge transfer and collection.
In 2009, Oosterhout el al. first spatially resolve the morphology of 2%-efficient organic-inorganic hybrid solar cells consisting of poly(3-hexylthiophene) as the donor and ZnO as the acceptor in the nanometer range by electron tomography. And via solving the three-dimensional exciton-diffusion equation, a consistent and quantitative correlation between solar cells performance, photophysical data and the three- dimensional morphology could be obtained for solar cells with different layer thicknesses. Figure 8.1 shows the electron tomography of РЗНТ/ZnO hybrid solar cells.
The relatively poor performance of organic-inorganic solar cells is related to inefficient charge generation as a result of the low thickness of inorganic materials and the coarse phase separation, as well as the exciton losses impaired by the electrodes. However, the solar cells with thicker photoactive layers (organic-inorganic heterojunction), charge generation is much more efficient, ow'ing to a much more favorable phase separation. Therefore, as expected, a better control of the organic-inorganic heterojunction morphology could improve performance of the solar cells through enhancing the generation and transfer efficiency of excitons.
ZnO-NP Organic Hybrid Solar Cells
Because the charge generation and transport highly rely on percolating pathways to ensure that the charge carriers can arrive at their respective electrodes without recombination due to trapping in dead ends on the isolated ZnO domain, the grain size and morphology of ZnO are essentially critical for the performance of the solar cells.
In 2004. Beek et al. reported nano-crystalline ZnO as n-type semiconductor in organic-inorganic hybrid solar cells, w'hich is a cheap and environmentally-friendly material that could be synthesized in high purity and crystallinity at low temperatures.12
Figure 8.2 displays the layout of organic-inorganic hybrid solar cells employed in nano-crystalline ZnO. The nano-crystalline ZnO could be disappeared in relatively apolar solvent mixture, thus it could blend with poly (2-methoxy-5-(3',7'- dimethyloctyloxy)-l,4-phenylenevinylene](MDMO-PPV). Upon excitation, ultra-fast charge transfer could occur between the interface of nano-crystalline ZnO-MDMO- PPV, which is utilized to generate an efficient organic-inorganic hybrid solar cell with a high fill factor and open-circuit voltage.
This demonstrates that the effectiveness of using a combination of organic materials and inorganic materials by using nano-crystalline inorganic semiconductor would be benefited to the charge separation and transfer at the interface of the donor- acceptor and photovoltaic performance of the organic-inorganic hybrid solar cells.
In 2012, Wu et al. had systematically investigated poly (2-methoxy-5-(2- ethylhexyloxy)-l,4-phenylenevinylene) (MEH-PPV) and vertically aligned ZnO nanorod array (ZnO-NA).13 As shown in Figure 8.3, they had prepared the organic- inorganic solar cells with three kinds of MEH-PPV/ZnO-NA layouts and found the device layout and illuminated photoactive area impose significant effects on the steady-state and dynamic performances of the devices even though the device architecture, the materials property, and the Au electrode in the devices are not changed.13
ZnO nanorod arrays could grow from the substrate vertically and have a great potential for organic-inorganic heterojunction solar cells due to their ease of synthesis14 and high electron mobility (-102 cm2-V_1-s-1)15 with a direct transport pathway to the corresponding electrode.16 However, the inefficient charge generation and
FIGURE 8.2. (a) The schematic energy level diagram and (b) the device structure of the organic-inorganic hybrid solar cells employed in nano-crystalline ZnO. Reprinted with permission from Ref. 2, Copyright 2004. WLEY-VCH Verlaine GmbH & Co. KGaA, Weinheim.
FIGURE 8.3. The device layouts (a-c) and architecture (d) used in the study. The black bare (b and c) identifies the ITO stripe length, which is the same to the width of the continuous ITO layer in (a). The area enclosed by dotted lines (a-c) identifies the OLA region in each device. Reprinted with permission from Ref. 3. Copyright 2012, Elsevier Ltd.
charge transfer, which are influenced by the morphology of heterojunction, affected the PCE of the solar cells effectively. In order to further improve the organic- inorganic hybrid (ZnO nanorods/polymer) solar cells, Ruankham etal. had improved the photovoltaic performances by controlling their charge dynamics via addition of ZnO NPs into poly(3-hexylthiophene) (P3HT) photoactive layer.17 The inter-rod space of ZnO nanorod substrates is completely filled with the solution-processed ZnO NPs-P3HT blends, forming homogeneous junction among the components. And the PCE of the solar cells has been achieved to 1.02% w ith 13 vol % ZnO NPs employed in ZnO nanorods/polymer. Figure 8.4 shows the device architecture and possible charge transport diagram of the solar cells studied in this work. And their research demonstrates that formation of ZnO NP domain extending across the active layer provides larger interfacial area of ZnO-РЗНТ interface and more effective percolation path for the charge carriers.17 Therefore, employing inorganic semiconductor NPs could effectively increase charge diffusion and effective carrier lifetime, thereby promoting charge transfer at the interface and bulk materials.
Modified ZnO Organic Hybrid Solar Cells
While the bulk properties of organic-inorganic heterojunction are used to described its’ interface, it is knowrn that the interfacial properties of semiconductor oxide could also influence the relevant photophysical processes of it. The doping could effectively adjust the bandgap of the inorganic semiconductor and the electrical
FIGURE 8.4. Device architecture and possible charge transport diagram of the ITO/dense ZnO/ZnO nanorods/ZnO nanoparticles/P3HT/VOx/Ag solar cells. Arrow lines show possible charge transport pathway. Reprinted with permission from Ref. 4. Copyright 2015, Springer Nature.
properties, thereby suppressing the change of its bulk property clue to the surface- induced defect states. Musselman et al. had doped zinc oxide with nitrogen (ZnO:N) to tune its electron concentration, reducing it by approximately two orders of magnitude in 2014.18 And they further studied the effect of bulk electron concentration on the surface properties of ZnO and the exciton dissociation on the double-layer ZnO- P3HT interface model. As illustrated in Figure 8.5, the formation of a space-charge region could be generated by electron trapping and oxygen chemisorption of ZnO at its surface and grain boundaries. Under illumination, photo-generated holes created in the ZnO near its surface could recombine with trapped surface electrons, potentially releasing chemisorbed oxygen molecules.18 However, more efficient light- induced de-trapping of electrons is observed form the ZnO:N surface, which enhances exciton dissociation and electron transfer from the P3HT to the ZnO.
Therefore, doping ZnO with a small amount of nitrogen could reduce its electron concentration in dramatically improved surface properties and enhance the ability of excitons dissociation.
The performance efficiency of organic-inorganic hybrid solar cells is not high yet, only about (r| = 2-3%).19 Among the limiting factors in the hybrid devices, the poor compatibility between inorganic acceptor materials and organic donor materials components and the serious charge recombination at the interface of acceptor-donor are the major interfacial difficulties for efficient devices. The incompatibility between hydrophilic NPs and hydrophobic polymers frequently causes a macroscopic phase separation and a bad interfacial contact between acceptor and donor components, resulting in a low efficiency of the charge transfer from donor to acceptor materials. Moreover, the normally low carrier mobility (ca. 10_l to 10~9 cm-V-'-s-1)20 in conjugated polymers often results in a poor hole transfer from the donor to acceptor, and
FIGURE 8.5. (a) The trapping of electrons at the surface of ZnO results in the formation of a space-charge region, (b) photogenerated holes in the ZnO can recombine with trapped electrons. releasing adsorbed species and reducing the space charge and associated band bending at the surface, (c) a thicker space-charge region is expected for the ZnO:N, which should enhance the de-trapping process and result in a greater reduction in the surface’s work function upon UV illumination (as indicated by the change in surface potential. ASP). Reprinted with permission from Ref. 5. Copyright 2014, Published by WLEY-VCH Verlaine GmbH & Co. KGaA. Weinheim.
then leading to a poor spatial separation of photo-generated charge carriers (electron and hole), yielding an easy interfacial charge recombination. Therefore, optimization of the donor-acceptor interface to improve the charge separation efficiency and suppress the charge recombination is an essential strategy to enhance the performance of the organic-inorganic hybrid solar cells.
Semiconductor quantum dots, such as CdS, CdSe, and PbS, with a tunable band- gap in the visible region can serve as sensitizers for wide bandgap semiconductor materials.21 Furthermore, the semiconductor quantum dots could also provide new chances to utilize hot electrons or generate multiple charge carriers with a single photon.22 Thus, combination of organic-inorganic solar cells with semiconductor quantum dots could enhance the charge transfer processes between organic and inorganic materials. Hao el al. had employed a simple and low cost electro-deposition method to fabricate organic-inorganic solar cell with CdSe-modified ZnO and P3HT as active layer.23 They demonstrated that the CdSe-ZnO core-shell nanorod arrays with P3HT to form p-n heterojunctions that could largely improves the photovoltaic performance of solar cell, exhibiting a PCE of 0.88%.23
Organic sensitizers, which are available in the form of dyes, have been largely used for photovoltaic works in the form of dye-sensitized solar cells because of their high absorption coefficients that benefit the light absorption spectra of the devices. Organic sensitizers could be used to modify the inorganic semiconductor materials based on their advantages. And ZnO nanorod arrays modified with organic sensitizers display specific light-harvesting and charge-collecting properties, which are promising for enhancing the characteristic performance of organic-inorganic hybrid solar cells based on ZnO/poly(3-hexylthiophene). Ruankham et al. had used common organic sensitizers (N719 (Ru-based complex), NKX2677 (coumarin dye), and D205 (indoline dye) and synthesized derivative of square molecules)-modified ZnO nanorod arrays to improve the performance of the solar cells in 2019.24 The mechanisms of modified solar cells are summarized in Figure 8.6. For unmodified devices and D205- and N719-modified devices, the excitons in P3HT could separate and the generated free electrons could transfer through the interface (red line in Figure 8.3), while this process does not occur for NKX2677-modified devices.24 The induced space-charge layer for D205- and NKX2677-modified devices could suppress a flow of charge leakage25 (gray dotted line in Figure 8.6); however, this does not take place in unmodified devices and N719- and square-modified devices. In a work, Ruankham el al. had present a workable approach for improving the performance of organic- inorganic solar cells via organic sensitizers-modified ZnO nanorods, and square- modified devices gave the best PCE of about 0.82%, with improved structure and performance could reach as high as 1.02%.
Surface modification of inorganic materials with semiconductor quantum dots and small organic molecules has been an efficient way to enhance the interfacial properties and the device performance in the organic-inorganic hybrid solar cells. For further enhancement of the photovoltaic performance of the solar cells, Bi el al. had employed amphiphilic and carboxylated dye molecules (Z907) to modify vertically aligned ZnO nanorod arrays via grafting Z907 onto ZnO surfaces at different soaking times.26 The modification effects on the performance of the organic-inorganic hybrid solar cells with poly(2-methoxy-5-(2-ethylhexyloxy)-l,4-phenylenevinylene) (MEH-PPV) as the donor materials had been investigated systematically. Amphiphilic sensitization with Z907 at aligned ZnO nanorod arrays improves the compatibility between ZnO nanorod arrays and MEH-PPV and enhances the charge separation efficiency at the interface of MEH-PPV-ZnO as a result of the enhanced electronic coupling property at the interface for charge transfer.26 However, the presence of Z907 could reduce the surface defect concentration of ZnO nanorod arrays, but increase the defects at the heterojunction interface. This phenomenon is attributed to electron-rich carbonyl groups of Z907. Therefore, the content of Z907 on the ZnO
FIGURE 8.6. Device mechanisms along the ZnO nanorods for (a) unmodified devices, (b) D205-modified devices, (c) NKX2677-modified devices, and (d) N719- and Sq-modified devices. Ejn is the internal electric field of the ZnO nanorods. Red and blue dotted lines are the electron and hole transport pathways, and the gray dotted line is the charge leakage. Reprinted with permission from Ref. 6. Copyright 2011. American Chemical Society.
FIGURE 8.7. The device structure and photovoltaic performance of Z907 modified organic- inorganic hybrid solar cells. Reprinted with permission from Ref. 7, Copyright 2011. American Chemical Society.
nanorod arrays surface induced by modification time could generate a great effect on the performance of the solar cells. Figure 8.7 shows the device structure and photovoltaic performance of the Z907-modified organic-inorganic hybrid solar cells. Their study had demonstrated that trapping electrons induced by surface defects could promote the charge transfer at the interface of donor-acceptor for efficient charge separation in the organic-inorganic hybrid solar cells, and both open-circuit voltage and charge recombination rate in the solar cell are correlated to the occupation of injected electrons in conduction band and surface defects.