雙介面修飾之組合應用於反式聚合物太陽能電池之研究

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(1)國立臺灣師範大學光電科技研究所碩士論文 Institute of Electro-Optical Science and Technology National Taiwan Normal University. 雙介面修飾之組合應用於反式聚合物太陽能電池之研究 Giant Enhancement of Inverted Polymer Solar Cells Efficiency by Manipulating Dual Interlayers with Integrated Approaches 指導教授：李亞儒、陳永芳博士研究生：謝幸樺. 中華民國. 一○三. 年六月.

(2) 摘要本研究提出使用 2-萘硫醇(2-Naphthalenethiol,2-NT)與金奈米粒子對主動層兩側之緩衝層做介面修飾，可提升含氧化鋅奈米柱(ZnO nanorod)之反式聚合物太陽能電池效率。2-NT 用於對 ZnO 奈米柱進行表面鈍化處理以減少氧缺陷，這個結果使太陽能電池之開路電壓提高，2-NT 亦給予電子一個明確的方向，使電子傳導至陰極的過程中復合的機率變小；而金奈米粒子，利用散射效果及區域性表面電漿共振效應(Localized Surface Plasmon Resonance, LSPR)提高整體元件的光子捕獲量及激子解離率，藉此提升光電流與填充因子。同時藉由兩種介面修飾可以進一步提升上述之效果，使整體元件達到更高的效率。本研究成功的整合製程與雙介面修飾法，元件經兩種方法修飾後之光轉換效率由 2.02%提升至 4.20%，其提升幅度將近 200%，這是在 ZnO 結構之有機聚合物太陽能電池上之最高紀錄，也代表著對於高效率聚合物共混結構電池上開創了一種新的修飾方法。. 關鍵字:介面修飾、表面電漿效應、導電小分子.

(3) Abstract To modify the interface on buffer layer with 2-Naphthalenethiol(2-NT) and gold nanoparticles to improve efficiency of the inverted polymer solar cell containing Znic oxide (ZnO) nanorods structure is demonstrated. Here we use the 2-Naphthalenethiol(2-NT) and gold nanoparticles to modify the interlayer between active layer and electrode on both sides. 2-NT is used for passivation treatment on the surface of ZnO nanorods, to reduce the oxygen defects in ZnO nanorods and improve the open circuit voltage of the solar cell. 2-NT also provides a clear direction for electron to transport to cathode that reduce the probability of electron recombination. Introducing gold nanoparticles improved scattering effects and surface plasmon resonance (SPR). These two phenomenon increase the amount of captured photons and the probability of exciton dissociation lead to the enhancement of photocurrent and fill factor. Modifying both two buffer layers in two ways simultaneously can further improve the overall efficiency. The results of this study also shows that the dual interface modification in the manufacturing process is indeed feasible. In addition, the enhancement of photon conversion efficiency achieved nearly 200% after dual interface modification. This is the highest record for organic polymer solar cell with ZnO nanorod structure. It also represents that we demonstrate a novel method to modifying the polymer blend structure organic solar cell.. Keyword: interface modified, surface plasmon, conductive small molecules.

(4) 謝誌在此特別感謝陳永芳老師、李亞儒老師、許芳琪老師、蕭國瑞老師，因為有這些老師的幫忙才有機會到台大學習且在一年內的時間完成這份研究論文，在週會與討論時雖然常常被糾正但也因此從各位老師身上學習到許多對於實驗的態度與精神，除此之外也很感謝宋運明學長在實驗上的各種幫忙，不管是製程上的技巧或是論文上疑惑都帶給我相當大的幫助。另外感謝師大研究室的各位同學，除了實驗上的事情外在生活上也跟大家相處得很開心，雖然沒有常常回實驗室但每次回去討論或閒聊都能有少許收穫。在此祝福各位畢業後在工作上能有所成就！在最後感謝我的二阿姨當我台北求學的六年間時常關心詢問我、照顧時有病痛的我、協助我處理生活上的瑣事，使我能堅持到最後完成碩士學位。面對家人與親友的無盡關愛與溫暖，在此致上我最大的感激，謝謝你們。.

(5) Content 1.Introduction .................................................................................................. 3 Reference .............................................................................................................................. 6. 2.Theoretical Background ..................................................................... 9 2.1The principle of solar cell .......................................................................... 9 2.1.1Solar Spectrum .................................................................................................... 9 2.1.2Photovotaic effect ............................................................................................. 10 2.1.3Short Circuit Current....................................................................................... 11 2.1.4Open Circuit Voltage ....................................................................................... 12 2.1.5Filling Factor&Efficiency ............................................................................. 13 2.1.6Device Analysis................................................................................................. 14 2.1.7 Mobility measurement by CELIV ............................................................ 14 2.1.8 Liftime measurement by OCVD ............................................................... 16 2.2Organic semiconductor ...................................................................................... 18. 2.3Organic Solar cell structure .................................................................... 19 2.3.1Bilayer heterojunction .................................................................................... 19 2.3.2Bulk heterojunction ......................................................................................... 21 Reference ............................................................................................................................ 22. 3.Equipment and material Design ............................................... 23 3.1Equipment ....................................................................................................... 23 3.1.1Scanning electron microscopy .................................................................... 23 3.1.2Incident Photo-to-Current Efficiency ....................................................... 24 3.1.3Thermal evaporation ....................................................................................... 25 3.1.4Solar simulator................................................................................................... 26. 3.2Material Design ............................................................................................ 27 3.2.1ZnO nanowires.................................................................................................. 27 3.2.2Organic materials ............................................................................................. 28 Reference ............................................................................................................................ 30 1.

(6) 4.Giant Enhancement of Inverted Polymer Solar Cells Efficiency by Manipulating Dual Interlayers with Integrated Approaches.............................................................. 31 4.1 Introduction ................................................................................................................ 31 4.2 Experiment.................................................................................................................. 32 4.3Device fabrication ..................................................................................................... 32 4.4Characterization Details ......................................................................................... 33 4.5 Results and discussion ........................................................................................... 34 Reference ............................................................................................................................ 44. 5.Conclusion .................................................................................................. 46. 2.

(7) Chapter1 Introduction Polymer based bulk heterojunction solar cells have been intensively studied in the past decades due to their potential of developing low cost and scalable renewable energy [1–3]. Tremendous progress has been made in the past few years to improve the device performance and the power conversion efficiency (PCE) of the devices has reached to 8 – 9% [4 – 6]. From the practical application perspective, there is still room for further improving their efficiency to compete with their inorganic counterparts [7 – 9]. It has been shown that an inclusion of cathode (anode) interlayer is also important to elevate cell performance because this interlayer layer can improve the charge collections at electrodes. Materials such as MoO3, PEDOT:PSS, V2O5 and so on have been widely used as the anode interlayer [10 – 12] while TiO2, ZnO, water-soluble polymers, small molecules, etc. have been demonstrated to have good electron collection ability [13 – 19]. Some of those cathode interlayers using organic molecules with designed end groups also show remarkable improvement of open-circuit voltages (Voc) [14 – 19]. Recently, several groups have tried to modify the interlayers by doping metallic nanostructures [20 – 22] or self-assembling a functional monolayer on the metal-oxide [23 – 25] to further enhance the device efficiency. It is known that metallic nanostructures, nanoparticles (NPs) for example, can scatter the incident light to extend its optical path within the photoactive layer and the plasmonic near-field from NPs can be coupled into the nearby photoactive materials to expand the cross-section of absorption [22]. This surface plasmon resonance (SPR) effect can be applied either in the front or rear interlayer of the cell, which has been shown to enhance the cell efficiency up to 20% by adopting single type metallic NPs in the front layer [20 – 22]. Very recently, Yang and co-workers [22] have tried to apply the concept of SPR effect on both sides of the cell resulting in ~13% increment in PCE.. 3.

(8) Self-assembling a monolayer on an interlayer is another approach to raise up the cell performance through the mechanisms of enhancing exciton dissociation [26–28], controlling the morphology of the photoactive layer [24,25,29], and improving Voc [30,31]. Each mechanism contributes differently to the cell efficiency ranging from 10% up to 100% of original PCE. A monolayer with additional capability of assisting exciton dissociation rate exhibits the highest potential in performance improvement and the application of this concept can be easily carried out in inverted device structure, using the noble metal as the top electrode to collect hole charges. We have previously reported the use of conjugated 2- naphthalenethiol (2-NT) molecule to modify ZnO-nanorod surface and obtained an improved performance of inverted ZnO-nanord/P3HT:PCBM/Ag solar cells [32] for ~ 100%. This conjugated 2-NT molecule has multiple effects on device improvement in the aspects of exciton dissociation rate, cathode interlayer surface passivation, and bulk heterojunction charge transport. The multiple roles of the employed 2-NT monolayer drive the device to keep the best record so far in the presented cell configuration. Since manipulating the cathode or anode interlayer improves cell performance at different extent, the result infers that it is likely to further enhance the solar cell efficiency if both interlayers are properly treated. It has been shown that the incorporation of metallic NPs into the rear interlayer of the solar cells can achieve a more effective plasmonic scattering than those at the front side because of the reduced optical loss from the destructive interference of scattered light and unscattered light [22]. Further, the technique of self-assembling is readily to be accomplished on the surface of front interlayer. Thus, considering the fabrication feasibility and performance enhancement factors enable us to drive the best way to achieve the highest efficiency, such as by the integration of SPR and surface modification effects. In this contribution, we fabricate inverted solar cells using modified cathode (front) and anode (rear) interlayers through the integrated 4.

(9) approaches as described above to sandwich the photoactive layer, poly(3-hexythiophene):(6,6)-phenyl C61 butyric acid methyl ester (P3HT:PCBM). A 2-NT monolayer is self-assembled on the ZnO-nanorod array surface, and 20 wt% gold nanoparticles (Au-NPs) embedded PEDOTPSS layer is designed as the rear (anode) interlayer. Generally, NPs of large size (> 50 nm) can cause stronger light scattering, but easily to create shorts in the thin film devices. Thus, we choose Au-NPs of 50 nm in diameter as our scatters in PEDOT:PSS. The presented work is also the first report to incorporate Au-NPs in the rear interlayer using an inverted device structure. The finished devices are composed of layers of ITO/ZnO-x/P3HT:PCBM/PEDOT:PSS:y/Ag, where x and y stand for the modified substances used. Quite interestingly, the integrated manipulation of interlayer treatment can promote the cell efficiency from 2.02% to 4.36% (best cell), approaching 120% increment in PCE. This significant enhancement of device efficiency is a result of the combination of several factors including largely improved light absorption efficiency, exciton separation efficiency, and much extended carrier lifetime arising from the joint effects of SPR and surface modification.. 5.

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(13) Chapter2 Theoretical background 2.1 The principle of solar cells 2.1.1 Solar spectrum Solar energy from the sun light, therefore the power output of solar cells depended on the intensity and spectrum of sun light. It will also affect the output voltage and current of the solar cell. However, the intensity of the sunlight spectrum can be expressed by the solar spectrum, it is related to measurement location and the angle of the incident sum light relative to ground. Generally in tetms of air mass (AM) [1] to represent. i.e. AM=1/cosθ. This is because the sunlight can be absorbed and scattered by the atmosphere to the ground. And the position and angle of these two factors, also based on air mass to represent. For example : AM 1 represents the sunlight is incident to the ground verticality. And AM 1.5 is representative of the sun to the incident angle of 42.8 degrees.[2,3] However, AM 1.5 is generally used to represent average illuminance on the surface of the earth. (Fig. 2.1,2.2). Fig.2.1 Solar Spectrum (It is excerpted from Ref. 2) 9.

(14) Fig.2.2 Solar Spectrum (It is excerpted from Ref. 2). 2.1.2 Photovoltaic effect The principle of solar cell is based on the photovoltaic effect, which arises from the p-n junction between two semiconductors under light illumination. When a diode is exposed to sun radiation, the energy of phonons will be used to generate electron-hole pairs, which are called excitons. [4,5]. (1). (2). 10.

(15) (3). (4). Fig.2.3 The process of converting light into electric current. The overall photovoltaic process of an organic/polymeric solar cell can be divided into at least four critical steps(Fig.2.3): (1) Photon absorption and exciton generation. (2)Exciton dissociation or charge carrier generation at donor/acceptor interface. (3) Carrier transport toward respective electrodes. (4) Carrier collection at the respective electrodes. For all currently reported organic/polymeric photovoltaic materials and devices, none of these four steps have been optimized.. 2.1.3 Short circuit current (Isc) The short-circuit current is due to the generation and collection of light-generated carriers. For an ideal solar cell at most moderate resistive loss mechanisms, the short-circuit current and the light-generated current are identical. Therefore, the short-circuit current is the largest current which may be drawn from the solar cell. [6] The I–V behavior of a solar cell can be described by a general single exponential diode equation: (2.1.1) 11.

(16) where I0 is the dark current, q is element charge, A is the diode ideality factor, U is the applied voltage. Under illumination, photons are absorbed to create electron-hole pairs. The exciton will be separated by the built-in electric field, and then photo-generated current flow in the opposite direction with the injected current pass through the donor/acceptor interface. So the total currents of solar cells is given by (2.1.2) where the Jph is the photo current density. When the voltage is zero in the short circuit situation, the current will pass through the external load. Equation then gives Isc=Iph where Isc is short circuit current.. 2.1.4 Open Circuit Voltage (Voc) The open-circuit voltage, VOC, is the maximum voltage available from a solar cell, and this occurs at zero current. The open-circuit voltage corresponds to the amount of forward bias on the solar cell due to the bias of the solar cell junction with the light-generated current. An equation for Voc is found by setting the net current equal to zero in the solar cell equation to give: (2.1.3) Where the Voc is open circuit voltage, Iph is the photo current, J0 is the dark current, q is element charge, A is the diode ideality factor. [6] The above equation shows that Voc depends on the saturation current of the solar cell and the light-generated current. While ISC typically has a small variation, the key effect is the saturation current, since this may vary by orders of magnitude. The saturation current, J0 depends on recombination in the solar cell. Open-circuit voltage is then a 12.

(17) measurement of the amount of recombination in the device.. 2.1.5 Filling Factor (FF) & Efficiency (η ) The "fill factor", more commonly known by its abbreviation "FF", is a parameter which, in conjunction with Voc and Isc, determines the maximum power from a solar cell. The FF is defined as the ratio of the maximum power from the solar cell to the product of Voc and Isc. Graphically, the FF is a measure of the "squareness" of the solar cell and is also the area of the largest rectangle which will fit in the IV curve. [6]. Fig.2.4 Current-voltage (I-V) curves of solar cell (It is excerpted from Ref. 6). A key limitation in the equations described above is that they represent a maximum possible FF, although in practice the FF will be lower due to the presence of parasitic resistive losses, which are discussed in effects of parasitic resistances. Therefore, the FF is most commonly determined from measurement of the IV curve and is defined as the maximum power divided by the product of Isc*Voc. (2.1.4) The power conversion ηpower efficiency can be defined as η. (2.1.5). The input power for efficiency calculations is 100 mW/cm2.. 13.

(18) 2.1.6 Device analysis Series resistance (RS) in a solar cell has three causes: firstly, the movement of current through the emitter and base of the solar cell; secondly, the contact resistance between the metal contact and the silicon ssemiconductor; and finally the resistance of the top and rear metal contacts. The main impact of series resistance is to reduce the fill factor, although excessively high values may also reduce the short-circuit current. [6]. Fig.2.5 Impact of RS and RSH on the Current-voltage (I-V) curves of solar cell (It is excerpted from Ref. 6). Significant power losses caused by the presence of a shunt resistance (RSH), are typically due to manufacturing defects. Low RSH causes power losses in solar cells by providing an alternate current path for the light-generated current. Such a diversion reduces the amount of current flowing through the solar cell junction and reduces the voltage from the solar cell. The loss of this current to the shunt therefore has a larger impact. [6]. 2.1.7 Mobility measurement by CELIV CELIV[7-9] is powerful method allowing the charge transport and recombination to be studied in various semiconductors. It is a complimentary technique in the sense that it allows to study materials when other techniques such as Time-of-Flight (TOF) are inapplicable.The 14.

(19) equation to calculate charge carrier mobility in case of surface photogenerated small charge was derived: (2.1.6) Organic electronic devices, such as solar cells and light emitting diodes are typically very thin, on the order of hundreds of nanometers, therefore, charge carrier are photogenerated in the volume of the films, according to Beer-Lambert law. (I)The case of low conductivity and volume carrier distribution: (2.1.7) (II) The case of high conductivity, carrier extraction from the volume: A correction factor to estimate the carrier mobility for intermediate film conductivities again in case of carrier extraction from the volume of the film was published later as determined from numerical calculations. (2.1.8) Typical CELIV setup is not different than TOF, except that when TOF can be (though usually is not) measured with large load resistances (integral mode TOF), CELIV can only be done in differential mode when the load resistance is low (RC < ttr). Triangle-shaped increasing voltage pulse is applied and the current response is measured as change in voltage on the load resistance of the oscilloscope.(Fig.2.6). 15.

(20) Fig.2.6. Pulse sequence and a schematic response of the Photo-CELIV technique. (It is excerpted from Ref. 9). Typically, Photo-CELIV is used to measure the charge carrier mobility in organic semiconductors since they are large bandgap (2 eV or so) and not much thermally generated carriers are present for extraction in the dark. The essence of this technique to measure the charge carrier mobility is very simple. The charge carrier mobility is defined as carrier drift velocity v in a given electric field E: v = μ × E. From classical mechanics, the constant speed of moving object is defined as the time required to travel the given distance d: v = d / t. In our case, the given distance is the film thickness and time is the transit time: μ = v / (ttrE) = d2 / (ttrU).. 2.1.8 Liftime measurement by OCVD Open-Circuit Voltage Decay(OCVD) [10,11] technique. This technique has certain advantages over frequency or steady-state-based methods: a. it provides a continuous reading of the lifetime as a function 16.

(21) of Voc at high-voltage resolution, b. it is experimentally much simpler, and c. the data treatment is outstandingly simple (basically, it consists of two derivatives) for obtaining the main quantities that provide information on the recombination mechanisms. Under constant illumination, the solar cell reaches a photostationary situation in which the free electron density satisfies U(n)=αabsI0 . Under these conditions, Voc corresponds to the increase of the quasi-Fermi level of the semiconductor (EFn) with respect to the dark value (EF0), which equals the electrolyte redox energy (EF0=Eredox). Therefore, it can be written as Equation(2.1.9): (2.1.9) Here, kBT is the thermal energy, e is the positive elementary charge, and n0 is the concentration in the dark. Clearly, the recombination rate has a major impact on the open-circuit voltage obtained at any light intensity. Information on the properties of the recombination process can be obtained from the correlation Voc(I0) in the steady state. A much more sensitive method is to determine the characteristic time of recovery when the system is displaced from a steady state at open circuit, that is, the electron lifetime (Շn). In the dyesolar- cell area, the dominant dynamic technique of this kind is IMVS, which measures the photovoltage in response to a small periodic modulation of the light intensity over a background steady state.[12,13] The starting point for the Voc decay measurement is the nonequilibrium steady state of a cell illuminated at constant intensity I0 . The illumination is interrupted, and Voc(t) is recorded, while the cell is kept at open circuit. During the decay, n evolves from the initial steady state value to the dark equilibrium (Voc=0) with concentration n0 . We will generally neglect the final region of decay at Voc≈50 mV or less, which is poorly resolved in the current setup, hence we can assume that n<<n0 . According to. +αabsI0, the transient is described by 17.

(22) Equation: (2.1.10) Intuitively, the electron lifetime can be defined as Equation: (2.1.11) Hence, Equation: (2.1.12) This definition is exact only for a linear system with U=krn (kr=rate constant for recombination).[6] The more general and rigorous concept of the lifetime is discussed below, and we show that Equation (2.1.12) is generally justified for the decay in nonlinear dye solar cells. Using Equations (2.1.9) and (2.1.12), we can derive the lifetime from Voc(t) by Equation (2.1.13): (2.1.13) Therefore, n(Voc) is given by the reciprocal of the derivative of the decay curve normalised by the thermal voltage.. 2.2 Organic semiconductors In organic π electron conjugated materials, the outer shell or valence π electrons are typically responsible for the electronic and optoelectronic properties. When a material rests at its lowest ground state, the highest occupied molecular orbital (HOMO) typically refers to a highest energy level and fully occupied electron bonding orbital, and the lowest unoccupied molecular orbital (LUMO) typically refers to a lowest energy level empty antibonding orbital.[14,15]. 18.

(23) Energy. Fig.2.7 Schematic energy of Organic semiconductors. In typical organic semiconductors, including most organic crystalline semiconductors, the intermolecular electronic orbital coupling are generally much poorer compared with their inorganic semiconductor counterparts. In most organic semiconductors, orbital overlap and coupling is mostly on the molecular levels, i.e. molecular shape or packing directly restricts or limits the intermolecular orbital coupling or band formation. Therefore, stable conduction bands and valence bands with substantial bandwidth (i.e. over 0.1 eV) are rare in organic semiconductors. The optical excitation energy gap Eg in organic semiconductors typically represent the smallest energy difference between discrete LUMO and HOMO orbitals, and free charge carriers will transfer or “hop” among different orbitals or sites instead of transporting in “bands”. “Band like” organic semiconductors are rare.[16] In most organic semiconductors, when a photon with energy matching the Eg excites an organic molecule, an electron first transfers from the HOMO to the LUMO.. 2.3 Organic solar cells structure 2.3.1 Bilayer heterojunction From the spatial structure point of view, the solar cells were of the donor/acceptor double layer cells.. 19.

(24) Fig.2.8 Schematic photocarrier generation processes of organic D/A junction. As shown in Fig.2.8, once a photogenerated exciton in either the donor or acceptor layer diffuses to the D/A interface, charge separation would occur where the electrons will transfer to or remain in the acceptor LUMO, and holes will transfer to or remain in the donor HOMO[17-19]. Due to both the electrode’s induced internal field and chemical potential driving forces, the electrons and holes would hop to their respective electrodes much more easily and quickly than in the single layered cells. The likelihood of charge recombination is much smaller than in the singlelayer cells because electrons and holes move in two separate domain layers. Since the successful demonstration of the Tang cell, the organic and polymeric photovoltaic field started to grow rapidly as new organic/polymeric donors and acceptors were researched extensively.. (a). (b). Fig.2.9 Energy level schemes of a photovoltaic cell in (a) open-circuit voltage mode and (b) short-circuit current mode. 20.

(25) 2.3.2 Bulk heterojunction The solar cells were categorized as “bulk heterojunction” or BHJ cells as shown in Fig.2.10 (energetic profile).. Fig.2.10 Scheme of a donor/acceptor blend type bulk heterojunction BHJ solar cell. These cells are fabricated by intimately blending a donor with an acceptor. In this way, the donor/acceptor interface are located randomly everywhere in the bulk, making it easier for an exciton to reach a nearby donor/acceptor interface and be dissociated into carriers.. References [1]http://en.wikipedia.org/wiki/Air_mass_(solar_energy) [2]http://www.szsolar.org/dictionary.php [3]http://www.twentezon.nl/kennis-platform/technische-aspecten-uitgelicht/prestatiesbij-weinig-zonlicht-cis/. 21.

(26) [4]http://en.wikipedia.org/wiki/Photovoltaic_effect [5]Myung-Su Kim,The University of Michigan, 2009 [6]http://pveducation.org/ [7]G. Juska, K. Arlauskas, and M. Vili¯unas, Applied Physics Letters, 2005, 86, 112104 [9]http://www.abo.fi/student/en/Content/Document/document/10920 [10]JOHN E. MAHAN, THOMAS W.E KSTEDT,, ROBERT I. FRANK, MEMBER, IEEAEN,D ROY KAPI,OW, 1979, 26, 733-739 [11]Arie Zaban, Miri Greenshtein, and Juan Bisquert, ChemPhysChem, 2003, 4, 859-864 [12]G. Schlichthˆrl, S. Y. Huang, J. Sprague, A. J. Frank, J. Phys. Chem. B 1997, 101, 8141. [13] A. C. Fisher, L. M. Peter, E. A. Ponomarev, A. B. Walker, K. G. U. Wijayantha, J. Phys. Chem. B 2000, 104, 949. [14]http://chemistry.umeche.maine.edu/CHY252/HOMO-LUMO.html [15]https://www.youtube.com/watch?v=tTebc4hq2sQ [16]Yongbo Yuan, Timothy J. Reece, Pankaj Sharma, Shashi Poddar, Stephen Ducharme, Alexei Gruverman, Yang Yang, Jinsong Huang, Nature Materials, 2011, 10, 296–302 [17]P.Peumans, A.Yakimov, and S.R.Forrest: Small molecular weight organic thin-filmphotodetectors and solar cells. J.Appl.Phys.2003, 93,3693 [18]L.A.A.Pettersson, L.S.Roman, and O.Inganas: Modeling photocurrent action spectra of photovoltaic devices based on organic thin films. J.Appl.Phys.1999, 86,487 [19]C.W.Tang: Two-layer organic photovoltaic cell. Appl. Phys.Lett.1986, 48,183. 22.

(27) Chapter3 Equipment and Material Design 3.1 Equipment 3.1.1 Scanning electron microscopy (SEM) The scanning electron microscopy (SEM) is a type of electron microscope that produces images of a sample by scanning it with a focused beam of electrons. The basic principle is that a beam of electrons is generated by a tungsten filament or a field emission gun[1]. The electron beam is accelerated through a high voltage (e.g.: 10^7 V/cm) and pass through a system of apertures and electromagnetic lenses to produce a thin beam of electrons, then the beam scans the surface of the specimen by means of scan coils. The electrons interact with atoms in the sample, producing various signals that can be detected and that contain information about the sample's surface topography and composition. The electron beam is generally scanned in a raster scan pattern, and the beam's position is combined with the detected signal to produce an image. The most common mode of detection is by secondary electrons emitted by atoms excited by the electron beam. The number of secondary electrons is a function of the angle between the surface and the beam. On a flat surface, the plume of secondary electrons is mostly contained by the sample, but on a tilted surface, the plume is partially exposed and more electrons are emitted. By scanning the sample and detecting the secondary electrons, an image displaying the tilt of the surface is created. The photo of SEM (JEOL, JSM-6500F) as shown in Fig.3.1 employs Schottky typefield-emission gun (T-FE, tungsten coated with ZrO2) for electron source with a probe-current range from several 10 pA to 20 nA when the No.4 objective lens aperture is used. 23.

(28) Fig. 3.1 Scanning electron microscopy.. 3.1.2 Incident Photon-to-Current Efficiency Our systems (as shown in Figure 3.2,3.3) include a light source, monochromator, lock-in amplifier, filters, and reflective optics to provide monochromatic light to a photovoltaic device while a broadband bias light illuminates the test device to simulate end-use conditions. A computer interfaces with the monochromator, signal conditioning equipment, and digital signal processing equipment; interprets signals; maintains calibration information; saves test data; and produces test reports. Different models include different features and can be configured with different options. IPCE value indicates the amount of current that the cell will produce when irradiated by photons of a particular wavelength. If the cell's quantum efficiency is integrated over the whole solar electromagnetic spectrum, one can evaluate the amount of current that the cell will produce when exposed to sunlight.. 24.

(29) Optica. Monoc. l Syste m1. hromat or. chopper. Lock-inAmplifi er Compu Fig. 3.2 The IPCE ter instrument. Optic. Optica l Syste Solar cell Front-En m2 d-Amplif ier. Optical System 2. al Syste m1. chopper Solar cell. & Mon ochro. Fig. 3.3 The IPCE equipment. mato 3.1.3 rThermal evaporation Thermal evaporation is a process wherein a solid material is heated 25.

(30) inside a high vacuum chamber to a temperature which generates some vapor pressure. Inside the vacuum, even a very low vapor pressure is adequate to create a vapor cloud within the chamber. This evaporated material now consists of a vapor stream, which passes through the chamber, and strikes and sticks onto the substrate as a film or coating.The process works on the vacuum chamber, usually at low pressure, about 10-6 torr, to avoid reaction between the vapor and atmosphere. The thermal evaporation technique is used as resistance heating to heat the material.. Fig. 3.4 Thermal evaporation.. 3.1.4 Solar simulator A Solar Simulation system also known as sun simulator reproduces full spectrum light equal to natural sunlight. The ground level spectrum of natural sunlight is different for various locations on earth. The constituents of the atmosphere affect both absorption and scattering. The conditions for the AM 1.5 spectra were chosen by American Society of Testing and Materials because they are representative of average conditions in the 48 contiguous states of the United States. In the USA, American Society for Test and Measurement has established such standards for Solar Simulators.. 26.

(31) Fig. 3.5 Solar simulator. 3.2 Material Design 3.2.1 ZnO nanowires[2] ZnO is a wide-band gap (3.37eV) inorganic semiconductor white powder that is insoluble in water, and it is widely used as an additive in numerous materials and products including rubbers, ceramics, glass, cement, paints. It occurs naturally as the mineral zincite, but most zinc oxide is produced synthetically. ZnO nanorods grown by hydrothermal method of the native doping of the semiconductor (due to oxygen vacancies or zinc interstitials) is n-type. For topography, ZnO nanorods or nanowires appear like hexagonal columns as shown in Fig 3.6.. 27.

(32) Fig. 3.6 The SEM image of ZnO nanorods with size bar of 100nm. This semiconductor has several favorable properties, including good transparency, high electron mobility, and wide bandgap. but the defect state of oxygen vacancies is considered as the most loss for inorganic/organic hybrid solar cells.. 3.2.2 Organic materials 1.P3HT The P3HT (poly(3-hexylthiophene)) use as a hole conducting donor. It has high mobility and a relatively low band gap which can absorb more photons in the visible region then other polymers Donor. Fig. 3.7 The chemical structure of P3HT when R=CH2(CH2)4CH3. 2.PCBM The PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) is a fullerene derivative of the C60 buckyball that is an electron acceptor 28.

(33) material and is often used in plastic solar cells. The material contains side chain that makes it soluble in common organic solvents. Acceptor. Fig3.8 The chemical structure of PCBM.. 3.PEDOT:PSS PEDOT:PSS (PEDOT = poly(3,4-ethylenedioxythiophene, and PSS = poly(styrene sulfonate)) is a polymer mixture of two ionomers. Part of the sulfonyl groups are deprotonated and carry a negative charge. The other PEDOT is a conjugated polymer and carries positive charges. This has high conductivity and easily to coating ,so usually used as a transparent, conductive polymer with high ductility in different applications.. Fig3.9 The chemical structure of PEDOT:PSS.. 4.2-Naphthalenethiol The magnetic resonance shift for a self-assembled monolayer of 2-naphthalenethiol was studied that suggested considerable promise in flexible and transparent photonic devices for biological and chemical sensing. 2-Naphthalenethiol was used in the preparation of cholesterol monolayer and multilayer Langmuir- Blodgett (LB) films[3]. The 29.

(34) electrochemical barrier properties of these films were studied using cyclic voltammetry and electrochemical impedance spectroscopy.. Fig3.10 The chemical structure of 2-Naphthalenethiol.. Reference [1]G.I. Goldstein, D.E. Newbury, P. Echlin, D.C. Joy, C. Fiori, and E. Lifshin, New York and London. 1981. [2]Jason B. Baxter, Eray S. Aydil , Journal of Crystal Growth. 2005, 274, 407–411 [3]http://en.wikipedia.org/wiki/Langmuir%E2%80%93Blodgett_film. 30.

(35) Chapter4 Giant Enhancement of Inverted Polymer Solar Cells Efficiency by Manipulating Dual Interlayers with Integrated Approaches 4.1 Introduction After several generations, the bulk heterojuction (BHJ) become today's mainstream of active layer with the development of organic solar. We usually choose metal with low work function as the cathode (Ca Al) of BHJ solar cell. However, without encapsulation, it will oxidize rapidly because of its high activity. One way to overcome this problem is using metal oxide buﬀer layers (ZnO, TiO2) onto indium tin oxide. And then we can choose a more stable metal to apply to hole collecting electrodes in inverted solar cell. The current will not be higher than the regular cell, but the stability of the element has improved significantly. Therefore, the structure can be changed to many different ways. The most common problem of hybrid solar cell is a large number of defect at inorganic-organic interface , which may reduce the current due to recombination of electrons and holds during the transfer process. From the perspective of material property, introducing a metal-oxide into organics will raise the inorganic-organic interfacial problem due to the different surface characters. The semiconducting metal-oxides tend to be hydrophilic, while polymers are hydrophobic. This caused intense charge recombination at the interface. The solution is to adsorb the conductive small molecules, 2-naphthalenethiol (2NT) , on the ZnO rods to modify the interface of buffer layer. In addition, coating the PEDOT:PSS on the other side of active layer as hole transport layer also can enhance current of inverted photovoltaic. PEDOT:PSS is a conductive plastic whose work function is 5eV ,usually be applied to electrode in the inverted photovoltaic. However, PEDOT:PSS is a hydrophilic material, this limits the vertical conductive gain properties because of the lacking of moist for 31.

(36) organic polymer active layer. Nevertheless, due to the hydrophilicity, we can uniformly mixed Au NP with PEDOT:PSS which can form surface plasmon resonance then give a clear path for the carrier to overcome the limitations of vertical conduction at interface between the active layer and hole transport layer.. 4.2 Experiment Indium tin oxide (ITO)-coated glass substrates were cleaned by successive sonication in commercial wafer cleaning buffer solution, acetone and isopropanol for 15 min per step and then dried in N2 gas flow before used. Initially, a 30 nm ZnO film was sputtered onto the ITO-coated glass followed by suspending the substrate in an aqueous solution of 40 mM zinc nitrate (Acros, 98% purity) and 40 mM hexamethylenetetramine (Acros) at 90 °C for 65 min in an oven [1,2]. The well aligned ZnO-nanorod array was then grown and the process was finished by dipping the substrate into deionized water to remove the residual salts and dried in N2 gas flow. Au-NP suspension with concentration ~ 3.5×1010 particles/mL in 0.1 mM citrate buffer was purchased commercially from Aldrich. The average particle size is approximately 50 nm.. 4.3Device fabrication Inverted solar cells were fabricated on the ZnO-nanorod array substrates before and after surface treatment, respectively. A polymer blend solution composed of 25 mg P3HT (Lumin. Tech. Co.) and 15 mg PCBM (Lumin. Tech. Co.) in 1 mL 1,2-dichlorobenzene (ODCB) was spin-coated onto the ZnO-nanorod arrays to form a photoactive layer at 400 rpm for 40 sec and then dried in air. The resulting film thickness was ~ 200 nm. The PEDOT:PSS:Au solution was prepared by blending 1 32.

(37) mL Au-NP solution into 4 mL PEDOT:PSS solution. This buffer solution was then spin-coated directly on the photoactive layer. Finally, a 100 nm thick silver film was thermally deposited on the PEDOT:PSS at pressure around 2×10-6 torr to complete the device fabrication. The finished devices were composed of layers of ITO/ZnO-x/P3HT:PCBM/PEDOT:PSS:y/Ag, where x (y) can be either none (none) or 2-NT (Au-NPs (Au)). The device with no interlayer treatment; i.e., (x, y)=(none, none), was chosen as the standard cell. Devices having the rear interlayer doped with Au-NPs, the front interlayer modified with 2-NT, and dual interlayers treated with both approaches are denoted as (x, y) = (none, Au), (2-NT, none), and (2-NT, Au), respectively. The typical photo-active area defined by the overlapping of the ITO and Ag electrodes for those devices was 5 mm2. Samples for the optical characterizations were prepared on ITO-coated glass following the same preparation procedures as photovoltaic devices without metal deposition.. 4.4Characterization Details The J-V characteristics of the finished photovoltaic devices were evaluated by using a Keithley Model 2400 source meter under irradiation intensity of 100 mW/cm2 from a calibrated solar simulator (Newport Inc.) with AM 1.5G filter. The calibration was done by using a standard Si photodiode. The incident-photon-conversion-efficiency (IPCE) spectra were performed using a setup consisting of a lamp system, a chopper, a monochromator, a lock-in amplifier, and a standard silicon photodetector (ENLI Technology). The UV-visible absorption spectra were measured by using a JASCO Model V-630 UV-vis spectrophotometer. The open-circuit-voltage-decay measurements were conducted by using a Xenon lamp equipped with a copper operated at 10 Hz to produce light pulse and the voltage responses of the cells were recorded by an Agilent Model DSO 5052A oscilloscope.. 33.

(38) 4.5 Results and discussion Fig. 1(a) shows the extinction spectra of Au-NPs in adequate solution determined by UV-vis spectroscopy, exhibiting a plasmonic resonance peak located at 530 nm. The average particle size are ca. 50±7 nm, estimated from the image of scanning electron microscopy (SEM) (see inset in Fig. 1(a)). Those Au-NPs are doped into the PEDOT:PSS hole transport layer before contacting with the top Ag electrode. As an example, the finished device structure with (x, y) = (2-NT, Au) is shown in Fig. 1(b). Fig. 2(a) depicts the current density(J)–voltage(V) characteristics for those four kinds of cells and the corresponding performance parameters are summarized in Table 1. The standard cell denoted as (x, y) = (none, none) shows the short circuit current density (Jsc), open circuit voltage (Voc), fill factor (FF), and power conversion efficiency (PCE) of 9.9 mA/cm2, 0.5 V, 41.3%, and 2.02%, respectively, which agrees well with that reported in the literature [4]. Doping the rear interlayer with Au-NPs results in a moderate improvement in Jsc (~8%) and FF (~14%) at a constant V oc, and an approximately 21% enhancement in PCE. By simply modifying the ZnO-nanorod array with 2-NT molecules, there are ~ 21%, ~24%, ~22%, and ~86% improvement in Jsc, Voc, FF, and PCE, respectively. Apparently, in addition to the much higher Jsc and FF values than the Au-NP doped case, modifying the front interlayer with 2-NT molecules also enhances Voc. By manipulating both interlayers, the PCE of the device is further improved to 4.20% with a Jsc, Voc, and FF of 12.8 mA/cm2, 0.61 V, and 53.8%, respectively. For the best cell, the PCE can be up to 4.36%, an improvement factor of ~120%. The largely improved Jsc, FF and Voc suggest the joint effects of both interlayer treatments. In order to understand the underlying mechanism of the giant enhancement of the cell efficiency, Fig. 2(b) depicts the recorded IPCE responses in the wavelengths from 350 to 800 nm. It shows a clearly 34.

(39) enhancement in IPCE values over the wavelength range from 400 to 600 nm for all modified cells. Fig. 2(c) exhibits the corresponding IPCE enhancement factor with respect to the standard cell. In the wavelengths from 400 to 600 nm, the IPCE enhancement factor is about 1.2 for 2-NT modified device, while it is ~1.1 for Au-NPs doped cells. By combining both approaches, there is approximately 30% increment in the IPCE value, indicating that the collected extra charge is almost the sum of the individual case. We then measured the UV-vis absorption spectra for the BHJ film under different modification conditions. According to Fig. 3(a), the structure with neat interlayers shows typical absorption characteristics for P3HT in the wavelengths of 400 – 650 nm with three vibronic state transition peaks at 515, 550, and 600 nm [5,6]. Modifying the front interlayer with 2-NT molecules neither alters the spectra feature nor the photon absorption efficiency. However, incorporating Au NPs into the rear interlayer enhances light absorption of the photoactive film in 450 – 600 nm region. The average enhancement factor is ~1.05 (see inset in Fig. 3(a)), suggesting a harvest of additional 5% of incident photons by the photoactive layer as Au-NPs is added. Because the photoactive layer absorbs only additional 5% photons, it is unlikely to produce ~30% extra charge carriers for the collection as shown by the IPCE measurements. The largely enhanced Jsc should arise from some other factors. Based on Mihailetchi and co-workers’ analytical approach [7,8], we compared the photocurrent behavior for these cells. The photocurrent density (Jph) is defined as the current density difference between the cell under illumination and in the dark, and the effective voltage (Veff) is determined by V0 – Va, where V0 is the voltage when Jph = 0 and Va is the applied voltage. As shown in Fig. 3(b), the Jph increased linearly with Veff at low voltages and then saturated at a certain high value of Veff. The saturated photocurrent density (Jsat) is independent of the bias and temperature and can be correlated with the maximum exciton generation rate (Gmax) through Jph = qGmaxL, where q is the elementary charge and L is the thickness of the photoactive layer, by assuming that 35.

(40) all of the photogenerated excitons are separated into free carriers and contributed to the current [7,8]. The obtained Jsat for (x, y) = (none, none), (none, Au), (2-NT, none), and (2-NT, Au) were 11.8, 12.8, 13.9, and 14.1 mA/cm2, respectively, which correspond to Gmax of 3.7 ×1027, 4.0 ×1027, 3.9 ×1027, and 4.4 ×1027 m – 3s – 1, respectively. It is known that Gmax is governed by the maximum number of photon absorbed [6,7]. The incorporation of Au-NPs in PEDOT:PSS increases Gmax, suggests that more photons are absorbed in the photoactive layer, which is consistent with UV-vis absorbance measurement in Fig. 3(a). Sample treated with only 2-NT molecules also exhibits a subtle increase in Gmax. Since the photon absorption efficiency remains the same (see Fig. 3(a)), the enhancement can be attributed to the passivation effect of 2-NT molecules on the ZnO-nanorod surface to minimize the carrier loss at the surface defect states, such that more free charges can contribute to Jph. This surface passivation effect has been resolved by the fluorescence measurement previously [3]. By taking the advantages of both treatments; i.e., SPR and surface passivation effects, Gmax displays the highest value among the cells. In fact, not all the photogenerated excitons are completely dissociated into free carriers. The excition dissociation properties P(E,T) can be related to Jph through Jph = Jsat P(E,T) [9]. The calculated P(E,T) values under Jsc condition are 83.6%, 85.3%, 86.3%, and 87.1% for (x, y) = (none, none), (none, Au), (2-NT, none), and (2-NT, Au), respectively. Therefore, modifying either the front or the rear interlayer shows an improved exciton dissociation rate at different extent, which suggests that either SPR and 2-NT modification can effectively assist exciton separation. This role for 2-NT molecule is also evidenced by the shortening of fluorescence decay lifetime [3]. According to Wu et al. [10], SPR enhanced exciton dissociation probability can be understood as the plasmon–exciton coupling participating in the charge transfer process and can also be further interpreted by the concept of “hot excitons,” which possess excess energy to overcome their initial Coulombic potential [11,12]. With the integration of both treatments, P(E,T) can be further 36.

(41) enhanced by a factor of 4.1%. The enhancement of exciton dissociation rate can have significant contribution to the largely enhanced Jsc. Open-circuit voltage decay (OCVD) technique was employed to obtain the carrier lifetime [13]. The cell was initially illuminated at a constant light intensity followed by an interruption of the illumination and the evolution of Voc with time (t) was monitored simultaneously. The Voc (t) follows the exponential decay behavior of a specific time constant τ. By exciting the cell at different initial steady states, a set of Voc (t) curves can be obtained and the corresponding τ can be calculated. Fig. 3(c) displays the recorded decay transient curves of Vocs under the application of a light pulse at various incident intensities for the standard sample. Using the same measurement procedures, the resulting decay lifetimes of Vocs for the four types of cells along with the corresponding fits are summarized in Fig. 3(d). Based on the fitting results, one can extrapolate the carrier lifetime for each type of cell to one sun condition and the obtained values for (x, y) = (none, none), (none, Au), (2-NT, none), and (2-NT, Au) cells are 25, 67, 140, and 200 μs, respectively. The carrier lifetime for the standard cell obtained in one sun using the OCVD method has similar order of magnitude as that measured by using the impedance spectroscopy [14]. The incorporation of either Au-NPs or 2-NT molecules can effectively prolong the carrier lifetime for approximately 3 or 6-fold. By combining both approaches, the carrier lifetime can be further extended to almost 8-fold. The extension of the carrier lifetime by the inclusion of Au-NPs is a result of improved carrier mobility as suggested by Lu et al. [9], which is also the same for the effect of 2-NT molecules as shown in Ref. 32. We have also measured the carrier mobility by employing charge extraction in a linearly increasing voltage (CELIV) method and the obtained mobility values for (none, none), (none, Au), (2-NT, none), and (2-NT, Au) are 4.0×10 –5, 4.6×10 –5, 7.5×10 –5, and 8.2×10 –5 cm2V – 1s – 1, respectively. There is an improvement of carrier mobility for the cells with treated interlayer and the higher mobility can reduce the probability of carrier recombination, and hence the longer carrier lifetime. Therefore, both the surface 37.

(42) modification and SPR effect can improve the charge transport through the active layer and interface to the electrode. The coupling effect of both approaches further maximizes the charge transport of the cell structure. Based upon the above results, manipulating the dual interlayers results in a significant improvement in the performance of solar cells and the underlying mechanisms can be understood as follows. Doping the rear interlayer with Au-NPs produces an approximately 5% additional harvested photon numbers, which can contribute to photocurrent. Because there is no changes in Voc, it suggests that the interlayer property is not altered in the presence of Au-NPs and still remains ohmic contact with Ag [15,16]. Though the series resistance (Rs) slightly increases from 5.6 to 6.0 Ωcm2 and the shunt resistance (Rsh) subtly decreases from 274 to 263 Ωcm2, the cell performance is improved, revealing that the cell performance does not result from a reduction in cell resistance. Instead, there is a subtle improvement in exciton dissociation rate and carrier lifetime owing to the SPR effect from Au-NPs. In general, an enhanced exciton dissociation probability reduces the carrier recombination rate, which is also supported by the improved carrier lifetime and, therefore, the FF of cells. Thus, we attribute the increased FF to the enhancement of exciton dissociation probability [10] and extended carrier lifetime [9] resulting from the locally enhanced electromagnetic field originating from the excitation of the SPR. Consequently, the application of the SPR concept can lead to an improved efficiency of the cell from 2.02% to 2.45% (~21% enhancement). Modifying the front interlayer with 2-NT molecules results in ~86% enhancement in PCE due to largely enhanced Jsc, Voc and FF. The multiple functions of 2-NT layer enhance Voc due to the surface passivation effect and Jsc as results of improved exciton dissociation rate and a longer carrier lifetime. The higher FF can be partially attributed to reduced Rs from 5.6 to 5.0 Ωcm2 and enhanced Rsh from 274 to 294 Ωcm2 and partially due to the increased exciton dissociation rate and lifetime. By integrating both approaches in a single cell, we obtain a giant enhancement in the cell performance because of taking the advantages of both treatments. Doping the rear 38.

(43) interlayer with Au-NPs has the unique feature of benefiting the photon absorption quantity while manipulating the front interlayer is more crucial for diminishing the surface defect states of the metal-oxide layer to minimize surface recombination events. For instance, as shown in Fig. 3(a), the additional 5% harvested photons due to Au-NPs scattering produce ~30% increment in Jsc. The reason for this magnification effect is that, in addition to the extra absorbed photons, the carrier dynamics including both the exciton dissociation rate and the carrier lifetime are maximized through the coupling of the effects of surface modified conjugated small molecules and the SPR. In such case, more free charges can be obtained and collected at electrodes by travelling through a better charge transport pathways. Further, the surface passivation of the front metal-oxide layer can raise the Voc of the cell. Additionally, the high FF is attributed to the highly reduced recombination rate due to the much increased exciton dissociate probability from combining both approaches. Thus, the cell efficiency can be further maximized and a PCE of 4.36% for the best cell with ~ 120% increment is achieved. This sets the record of the inverted polymer solar cell using P3HT:PCBM as the photoactive layer. Acknowledgement This work is supported by the National Science Council, Taiwan (Project No. NSC 102 - 2112 - M - 239 - 001 - MY3).. 39.

(44) Figure Fig.4.1 (b). Absorbance (a.u.). (a). 300. Ag PEDOT:PSS:Au NP P3HT:PCBM ZnO-NRs ITO glass. 400. 500. 600. 700. 800. 900. Light. Wavelength (nm). Fig.4.1. (a) UV-vis absorbance for Au-NPs in adequate solution. The inset shows the SEM image of Au-NPs on a silicon wafer. (b) The schematic structure for the cell structure having the front interlayer modified with 2-NT molecules and the rear interlayer doped with Au-NPs.. Fig.4.2 (a) 5. J (mA/cm2). 0. -5. x , y none , none none , Au 2-NT , none 2-NT , Au. -10. -0.4. -0.2. 0.0. 0.2. V (V) 40. 0.4. 0.6.

(45) (b) 80. IPCE(%). 60. 40 x , y none, none none, Au 2-NT, none 2-NT, Au. 20. 0 400. 500. 600. 700. 800. Wavelength(nm). (c). Enhancement factor. 1.2. 1.0. 0.8. x , y none, Au 2-NT, none 2-NT, Au. 0.6. 400. 500. 600. 700. Wavelength (nm). Fig.4.2 Evaluation of cell performance. (a) Current density (J)- voltage (V) characteristics, (b) incident-photon-conversion-efficiency (IPCE) curves, and (c) enhancement factor of IPCE values of the standard and cells under different manipulation conditions.. 41.

(46) Fig.4.3. Absorbance (a.u.). Enhancement Factor. (a) 1.05 1.00 0.95 0.90 400. 500. 600. Wavelength(nm). x, y none, none none, Au 2-NT, none 2-NT, Au 400. 500. 600. 700. Wavelength(nm). (b). Jph(mA/cm2). 10. x,y none, none none, Au 2NT , none 2NT , Au. 1. 0.01. 0.1. Veff ( V ). 42. 1.

(47) (c) light on. light off. 0.3. Voc(V). 0.2 light intensity. 0.1. 0.0 0.00. 0.04. 0.08. 0.12. time(s). (d) -2. 10. lifetime (s). x , none , none , 2-NT , 2-NT ,. y none Au none Au. -3. 10. 0.0. 0.1. 0.2. 0.3. Voc(V). Fig.4.3 (a) UV-vis spectra for cells under different manipulation conditions. (b) Photocurrent density (Jph) vs. effective voltage (Veff) characteristics of the standard and manipulated cells. (c) The response of open circuit voltage (Voc) as a function of time to a light pulse with various light intensities. (d) The carrier lifetime as a function of Voc along with a fitting curve for each type of the cell.. 43.

(48) Reference [1] T. C. Monson, M. T. Lloyd, D. C. Olson, Y. J. Lee, and J. W. P. Hsu, Advanced Materials, 2008, 20,4755 – 4759. [2] C. Goh, S. R. Scully, and M. D. McGehee, Journal of Applied Physics.2007, 101, 114503-1 – 114503-12. [3] J.Y. Chen, F.C. Hsu, Y.M. Sung, and Y. F. Chen, Journal of Materials Chemistry.2012, 22, 15726–15731. [4] D. C. Olson, J. Piris, R. T. Collins, S. E. Shaheen, and D. S. Ginley,Thin Solid Films.2006, 496, 26–29. [5] Y. M. Sung, F. C. Hsu, D. Y. Wang, I. S. Wang, C. C. Chen, H. C. Liao, W. F. Su, and Y. F. Chen,Journal of Materials Chemistry.2011, 21, 17462–17467. [6] C. J. Barbec, N. S. Sariciftci, and J. C. Hummelen, Advanced Functional Materials.2001, 11, 15–26. [7] V. D. Mihailetchi, L. J. A. Koster, J. C. Hummelen, and P. W. M. Blom, Physical Review Letter.2004, 93, 216601-1− 216601-4. [8] V. D. Mihailetchi, H. X. Xie, B. de Boer, L. J. A. Koster, and P. W. M. Blom, Advanced Functional Materials.2006, 16, 699−708. [9] L. Lu, Z. Luo, T. Xu, and L. Yu, Nano Letters.2013, 13, 59 – 64. [10] J. L. Wu, F. C. Chen, Y. S. Hsiao, F. C. Chien, P. L. Chen, C. H. Kuo, M. H. Huang, and C. S. Hsu, ACS Nano.2011, 5, 959−967. [11] J. H. Lee, J. H. Park, J. S. Kim, D. Y. Lee, K. Cho, Organic Electronics.2009, 10, 416 – 420. [12] H. Ohkita, S. Cook, Y. Astuti, W. Duffy, S. Tiemey, W. Zhang, M. Heeney, I. McCulloch, J. Nelson, D. D. C. Bradley, and J. R. Durrent, Journal of American Chemical Society.2008, 130, 3030 – 3042.. 44.

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(50) Chapter5 Conclusion In conclusion, a giant enhancement of inverted solar cell efficiency has been demonstrated with the integration of different approaches based on the fabrication feasibility and impact on the cell performance. The front metal-oxide interlayer adopts the surface modification strategy by self-assembled a layer of 2-NT molecules on the ZnO-nanorod surface while the rear interlayer employs the surface plasmon resonance effect through doping the PEDOT:PSS with Au-NPs. Particularly, the former one can effectively passivate the metal-oxide surface and the latter one can improve the photon absorption efficiency. In addition, both approaches can also effectively enhance the exciton dissociation rate and extend the carrier lifetime. With the integration of both approaches, the fabricated cell can not only take the advantages of the individual treatment, but also have the benefits of the coupling between these two approaches. Therefore, the cell efficiency can be enhanced from 2.02 % to 4.36%, which represents the highest record reported so far in inverted solar cells using ZnO-nanorod as electron transporting layer and P3HT:PCBM as the photoactive material. The proposed method can be generalized to other polymer blend systems as well and open up a new route for designing high efficiency polymer solar cells.. 46.

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