(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]
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.
Energy
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
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/prestaties-bij-weinig-zonlicht-cis/
[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
Chapter3
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
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.
Fig. 3.2 The IPCE instrument
Fig. 3.3 The IPCE equipment
3.1.3 Thermal evaporation
Thermal evaporation is a process wherein a solid material is heated
chopper
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.
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.
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
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
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
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
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
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.
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
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
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;
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;