Solar Cell Nanotechnology

Preface

Developments in human civilization have revolved around the consumption of energy. Historically, control over energy has been directly related to the centralization of power. The natural abundance of fossil fuels has helped sovereign nations in gaining better control over sociological developments. The absence of such natural resources has forced the remaining societies to greatly depend on the lucky ones that have them. In some cases, the developments have slowed to a crawl, and even basic needs have not been fulfilled. Therefore, materials scientists and engineers have taken up the challenge to reduce our dependence on fossil fuel resources. The invention of methods to harness solar energy has given new hope to the nations with limited or no fossil fuel reserves. In the last few decades the idea of technologies based on solar cells has been well established. New techniques of materials synthesis and their integration in novel engineering designs have helped the industry produce solar cells with high energy efficiency. The use of high purity silicon has dominated the solar cell industry for several decades. It appears that the efficiency of silicon-based solar cells has achieved maximum potential and no further improvements are expected. The high cost of extra pure silicon and saturation in energy efficiency has motivated scientists to look for new materials and technologies that are efficient and relatively cost effective.

Journals are flooded with research articles with claims about materials and technologies that might be able to produce highly efficient solar cells. It is therefore important to summarize the literature in order to draw meaningful conclusions based on the latest research findings. Since silicon-based solar cells are immensely popular, various books are available on the subject. It was therefore decided to look at other types of solar cells, which are either in the development phase or are likely competitors of silicon solar cells. The purpose of our efforts has been to collect articles from various experts in the field with this view in mind. This book is composed of seventeen chapters, each of which was written by an expert in their field. Topics are broadly designed to cover dye-sensitized types of solar cells and their related problems, layered types of solar cells, application of lithography in solar cells, and luminescent solar and plasmonic light trapping. Graphene being the most recent discovery, its application in organic types of solar cells is covered in this book. Finally, analytical modeling and electrical circuit design, which are another important aspect of solar cell development, are also included. The final section of the book appends to the series of articles written on putative future trends in this area. This section is specifically written for advanced practitioners to help them channel their expertise in the desired direction.

We expect that readers will gain an in-depth knowledge in new areas of solar cells, which are not commonly known and for which literature is scarcely available. This title has been edited for a broad readership that includes scholars and researchers from diverse backgrounds, and for commercial sectors looking for innovative solar cell materials and related technologies. This could be a solitary reference book for researchers in materials science, engineering and nanotechnology. We are confident that readers will gain invaluable information that can be utilized not only in research and development, but also in commercial manufacturing of solar cell panels.

Atul Tiwari, PhD

Rabah Boukherroub, PhD

Maheshwar Sharon, PhD

Part 1

CURRENT DEVELOPMENTS

Chapter 1

Design Considerations for Efficient and Stable Polymer Solar Cells

Prajwal Adhikary¹, Jing Li², Qiquan Qiao¹,*

¹Center for Advanced Photovoltaics, Department of Electrical Engineering and Computer Sciences, South Dakota State University, Brookings, South Dakota, USA

²Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing, China

Abstract

Organic photovoltaics (OPVs) are gaining more interest due to their potential for low fabrication cost, mechanical flexibility and light weight. Not long ago, lower power conversion efficiency and inferior stability compared to their inorganic counterparts were considered major issues associated with OPVs. Today, OPVs with an efficiency as high as 12% have been achieved and stability under ambient conditions has also been significantly improved, especially with the devices using inverted architecture. A major advantage of using OPVs over traditional thin film inorganic solar cells with comparable efficiency (~14–15%), is their excellent performance in real-life environments like high temperature and low light conditions. In this chapter, we discuss recent advances in realizing high performance and stable OPV devices. The chapter consists of three major parts: the role of interfacial layer for efficient Bulk heterojunction (BHJ) solar cells, the selection of interfacial layer for stable and longer life time OPVs and major interfacial materials used for conventional and inverted device architectures.

Keywords: Organic solar cells, inverted structure, normal structure, interfacial layer, cell stability

1.1 Introduction

1.1.1 Background

Organic and organic/inorganic hybrid solar cells have attracted a lot of interest due to their solution-based processing and low cost [1–15]. A completely new field of conducting polymers emerged in the 1970s when Shirakawa, MacDiarmid and Heeger found that simple doping of polyacetylene with a controlled amount of halogen (Cl, Br, I) could lead to a dramatic increase in conductivity (eleven orders of magnitude) of the film in which electrical properties could be tuned from insulator to semiconductor to metal [16]. In early days, it was fundamentally challenging to achieve high performance organic photovoltaic (OPV) cells by sandwiching a single organic layer between two metal electrodes, in which it is difficult for excitons generated in the active layer to reach the metal interface with the diffusion length of only 10–20 nm [17]. A major breakthrough in OPVs was achieved in 1986 when Tang introduced the concept of bilayer in which copper phthalocyanine as p-type and a perylene derivative as n-type, two organic materials, were sandwiched between metal electrodes [18]. This concept increased the OPVs cell efficiency to 1%, basically due to improved interface for exciton dissociation.

Later, it was found that better charge transport materials with high electron affinity are also required to have their band levels (HOMO and LUMO) aligned with respect to most donor-type polymers. In 1993, Sariciftci et al. reported C60 which possesses higher electron affinity and mobility and soon established itself as a widely used acceptor [19]. Halls et al. reported an increase in photocurrent as high as 20 times with the use of C60 as acceptors with MEH-PPV [20]. A similar increase in photocurrent was reported by Morita et al. when they used C60 with P3AT [21].

It was known that in a bilayer configuration, exciton dissociates at the donor-acceptor interface but the diffusion length of exciton is about 10 nm which limits their dissociation from the photoexcitation sites outside their diffusion length. Therefore, only a small fraction of polymer actually contributes to photocurrent generation. This limitation of bilayer structure was overcome in 1994 when Yu et al. made the first bulk heterojunction organic solar cell by dissolving MEH-PPV and C60 which showed photosensitivity of an order magnitude higher than pure polymer-based solar cells [22].

Major impedance in further improvement of PSC efficiency was due to large bandgap (~ 2 eV) of most polymers which had narrow light absorption range and poor hole mobility [23]. Synthesis of soluble polythiophenes, especially poly(3-hexylthiophene) (P3HT), was one step further in the field of OPVs. Currently, morphology optimization of P3HT/PCBM-based PSCs has increased the PCEs by higher than 4% [24]. An efficiency of over 6% was reported when PCDTBT, a low bandgap polymer that utilized cyclopentadithiophene unit as the donor block in the polymer chain, was developed by Leclerc et al. which incorporated the use of TiOx layer as optical spacer [25]. Efficiency above 7–8% was achieved when low bandgap polymers designed by Yu et al. based on thieno-thiophene (TT) and benzodithiophene (BDT) alternating units were synthesized. Higher Voc of 0.74 V was achieved owing to lowered HOMO level of polymer, and low bandgap helped to harvest more light which led to Jsc of 14.5 mA/cm² [26].

Major requirements for large area roll to roll (R2R) processing in order to make organic photovoltaic commercially successful are high efficiency and stability. Various strategies can enhance performance parameters of the cell (Voc, Jsc, FF) including choice of materials, efficient light harvesting mechanisms and morphological optimizations. On the other hand, improvement in device stability requires understanding degradation mechanisms of individual components of the OPV devices. Also, utilization of inverted devices instead of conventional architecture could help us achieve more stable OPV devices as it allows us to use materials that are more stable. For example, the acidic nature of PEDOT:PSS used as hole transport layer in conventional devices hampers stability as it etches the bottom ITO. On the other hand, low work function metals on the cathode side oxidize when exposed to ambient condition. The generally used vacuum-deposition-based technique for top electrode increases the cost.

Therefore, a technique that could overcome the above mentioned hindrances is to use inverted device structure, in which charge collecting electrodes are reversed, i.e., ITO could be used as cathode for collecting electrons and top electrodes could be used for collecting holes which are just opposite to that of regular BHJ structure. First, inverted structure does not require acidic PEDOT:PSS. Top electrodes could be any high work function metal (e.g. Ag, Au) which increase device stability, as these metals are air stable. Also, oxidation of silver into silver oxide has been reported to improve hole extraction efficiency as AgO has Fermi level closer to HOMO of polymer. Second, use of n-type metal oxides such as ZnO and TiO2 as electron transport layer saves the active layer from UV-induced degradation as most of the UV light is absorbed by this layer, and also the Fermi level of these metal oxides that lies close to PCBM LUMO provides additional interfacial area for exciton dissociation. Third, the use of solution-based metal could be employed in the form of top electrode which can avoid vacuum-based deposition technique, thus reducing cost. Fourth, inverted structure could take advantage of naturally favored vertical phase separation of active layer where PCBM rich layer lies towards ITO and polymer rich layer lies on the top, which improves overall device performance. Finally, reports suggest that in the modeling of optical field distribution in the different device structures, higher current density from inverted structures resulted due to enhanced absorption of incident light in the active layer as there is no PEDOT:PSS and Ca layer as in normal structure which is responsible for parasitic absorption of light [27]. Currently, the highest efficiency reported (9.2%) so far in single junction organic BHJ solar cells is based on inverted structure.

1.1.2 Theory

Semiconductor properties shown by polymers are due to the pi conjugation, the alternating single and double (or triple) bonds between the carbon atoms. Ground state of carbon has 1s²2s²2p² configuration in which, single bonds are associated with σ-bonds consisting of localized electrons and double bonds are associated with both σ-bonds and π-bonds where π-bonds consist of π-electrons which can delocalize along the conjugation length responsible for charge transport in carbon-based semiconductors.

Figure 1.1 shows the bonds in ethene molecule. Two σ-bonds connect to each of the hydrogen atoms and one σ-bond is formed between the carbon atoms. The fourth valence electron of each carbon atom situated in the respective pz orbital, which is oriented perpendicular to the sp2 orbitals, form a π bond, which is weaker than the σ-bond due to lesser overlap of the pz orbitals. Along with the formation of σ and π bonds, orbitals are split into bonding and antibonding (denoted with *) orbitals. In accordance with Pierls instability, formation of two delocalized band states takes place in a polymer known as highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs). Difference in energy level between HOMO and LUMO gives rise to bandgap of the polymer. Generally, energy level differences are in the order of 1–3 eV [28].

Figure 1.1 Schematic showing σ and π bond formation in an ethene molecule (left) and energy band diagram showing HOMO and LUMO energy levels (right).

In a typical inorganic solar cell, incident photon generates electron-hole pair which easily dissociates into electron and hole in the presence of ambient thermal energy. However, in organic semiconductors incident photon generates electron-hole pairs known as excitons which are rather tightly bound with energies of 0.3 to 1eV which are impossible to break in the presence of ambient thermal energy. Notably, ambient thermal energy is approximately 0.025 eV which is far lower than the energy required to break excitons into electrons and holes owing to lower dielectric permittivity (∊= 3–4) in an organic material. This energy requirement for breaking excitons makes it imperative to use a second material which could provide a built-in potential sufficient enough to dissociate excitons. Most often in an OPV cell, primary light absorber or donor material are the sites where excitons are formed. Difference in the molecular orbital energies between donor and acceptor materials helps to overcome the coulombic attraction and hence dissociation of electron and hole occurs at the donor-acceptor interface. In order to be dissociate, these excitons must find interface which is the region where donor and the acceptor meet.

1.1.2.1 Photovoltaic Processes in Donor-Acceptor (D-A) System

Buckminsterfullerene (e.g. C60) is widely used acceptor material due to its higher electron affinity. One important factor is the diffusion length that an exciton can travel to reach the D-A interface before it can recombine. Generally the D-A ratio and phase separation, carrier mobility, and bulk-morphology are the factors responsible for efficient exciton dissociation. Figure 1.2 depicts formation of excitons upon exposure to light. These excitons will not readily convert into electrons and holes after they reach the interface between donor and the acceptor within ~ 10 nm. As shown in Figure 1.2, excitons formed in the donor side get dissociated into electrons and holes when excitons find the interface. There are two substeps before charge carriers can move to their respective electrodes (electrons towards cathode, hole towards anode). First, there is the charge transfer (CT) state in which even the electron reaches the acceptor but are still bound together and might recombine as geminate recombination because they both still belong to the same exciton. A non-geminate recombination would have occurred if these charge carriers had been separated at the interface with electrons on the acceptor LUMO and holes on the donor HOMO got recombined with opposite charge originating from a different exciton present at the interface. Second, the true dissociation state, known as charge separation (CS) state, in which electrons stay at the LUMO of the acceptor and holes stay in the HOMO of the donor [29].

Figure 1.2 Energy diagram illustrating excited electrons while transferring from LUMO of donor to the LUMO of acceptor creating a charge separated (CS) state. Reproduced with permission from ref. [29].

Charge carriers once separated can now make their way to the respective electrodes through a combination of drift and diffusion mechanisms. They may still have to go through a competing recombination process [30]. Figure 1.3 shows the ultrafast charge transfer time during which carriers are dissociated with electrons transferring into acceptor and holes staying at the donor. All the absorbed photons do not necessarily lead to mobile carriers; some photogenerated carriers fall into interfacial traps or form bound interfacial charge transfer excitons which can lead to carrier recombination [31]. For efficient charge extraction, recombination time must be longer compared to the charge transport time (total time taken by the carriers from the point of dissociation to the point when they are collected at the electrode).

Figure 1.3 Ultrafast time dynamics during the operation of the BHJ solar cell. The excited charge carriers could either be swept out by the internal voltage or get trapped and recombine back into the ground state.

1.1.2.2 Equivalent Circuit Diagram of a PV Cell under Illumination

Equivalent circuit of a simplified solar cell is shown in Figure 1.4. A series resistance (Rs) represents charge carrier transport resistance and shunt resistance (Rsh) represents leakage current. The J-V characteristics of a solar cell can be described as Eq. 1.1,

Figure 1.4 Equivalent circuit diagram of a PV cell under illumination. Reproduced with permission from ref. [8].

(1.1) equation

where kB is Boltzmann’s constant, T is temperature, q is elementary charge, A is device area, n is ideality factor of the diode, J0 is reverse saturation current density, and Jph is photocurrent. The J-V curves and photovoltaic parameters including Voc and FF strongly depend on the n, J0, Rs, and Rsh [32].

1.1.2.3 Parameters Governing Performance of Solar Cells

Power conversion efficiency (PCE) of an organic solar cell depends upon how well the light is absorbed, exciton generated and diffused to the D-A interface, separated into electrons and holes, and finally collected at the respective electrode. The external quantum efficiency (EQE) is defined as the ratio between charge carriers at the electrode upon light illumination to the number of incident photons at a particular wavelength [33]. EQE can be expressed as Eq. 1.2,

(1.2)

where, λ = wavelength of the incident photon;

ηA(λ) = absorption efficiency of the organic material, a ratio of excitons generations from HOMO to LUMO upon light excitation to the total incident photons;

ηED(λ) = exciton diffusion efficiency, which is the ratio of number of excitons that diffuse and reach the D-A interface to number of total excitons generated;

ηCS(λ) = charge separation efficiency and denotes the ratio of the number of excitons that successfully undergo the CT process to the number of excitons that have reached the D–A interface;

ηCC(λ) = charge collection efficiency and is defined by the ratio of number of charge carriers that have been collected at the electrodes to the number of charge carriers that has undergone CT process.

The PCE (η) of a photovoltaic device can be expressed as Eq. 1.3,

(1.3) equation

where, Jsc, Voc, FF and Pin are short-circuit current density, open-circuit voltage, fill factor and power of incident light.

Current density is dependent upon the bandgap of the organic material, thickness and morphology of the active layer. Theoretically, Jsc is defined by Eq. 1.4,

(1.4) equation

where, h is the Planck’s constant, c the speed of light, q the elementary charge, and Psun the solar irradiance.

Open circuit voltage (Voc) is the maximum potential available from the cell which occurs at zero current. In an organic system, theoretically Voc is determined by the energy level difference between HOMO of the donor and LUMO of the acceptor.

Fill factor (FF) denotes the squareness of the IV curve is governed by several factors including carrier mobility, balanced charge transport, and planar polymer structure for molecular packing in the organic photovoltaic devices [34–36].

1.2 Role of Interfacial Layer for Efficient BHJ Solar Cells

An ideal interfacial layer is the one that provides Ohmic contact with minimum contact resistance between the active layer and respective electrode. Once the charge carriers are dissociated in the active layer, efficient charge sweep-out from active layer to respective electrode is dependent on choice and electrical property of the interfacial layer. Nature of electrical contact provided by the interfacial layer has significant impact on the key performance parameters (e.g. Jsc, Voc, FF) of the organic solar cell, hence the overall power conversion efficiency.

Choice of materials has become an important criterion to optimize the electronic/electrical properties of the interface for efficient performance of organic solar cells. Several classes of interfacial materials including metal oxides, conjugated semiconductor electrolytes, self-assembled structures, crosslinkable materials, and graphene-based materials which require efficient integration and compatibility with other layers in multilayer devices have been previousy tried [37]. The role of an interfacial layer is described below.

1.2.1 Role of Interfacial Layer on Voc

As mentioned above, maximum Voc achievable in a BHJ polymer solar cell is the difference in energy level between HOMO of the donor and LUMO of the acceptor (Voc1 in Figure 1.5). However, for this to happen there must be an Ohmic contact formed between the active layer and the electrodes (both anode and cathode). Various interfacial effects such as dipole formation, charge transfer, and formation of interfacial states can occur depending upon their interaction between the contacts. The static work function difference between the two electrodes in which active layer is sandwiched might create an internal electric field which can reduce the Voc of the cell due to the formation of Schottky contact (Voc2 in Figure 1.5). Formation of Schottky contact at the interface and its role in device performance is discussed in the last part of this section.

Figure 1.5 Energy level diagram of a BHJ OPV device where organic layer is sandwiched between transparent electrode and metal electrode. Reproduced with permission from ref. [38].

An integer charge transfer (ICT)-based model is widely used for understanding interfacial effects in OPVs. Negative ICT state (EICT-) represents Fermi-level pinning on acceptor (EF,e) defined as the energy gained by adding one electron to an organic molecule, and positive ICT state (EICT+) represents Fermi-level pinning on donor (EF,h) defined as energy required to take away one electron from organic molecule. As shown in Figure1.5, an electron can transfer from the substrate to the organic semiconductor when the Fermi level of the substrate is lower than EF,e of organic semiconductor. Also, when the Fermi level of the substrate is higher than EF,h holes will transfer from substrate to organic semiconductor [39, 40].

According to a study conducted by Mihailetchi et al. on a normal structure device, various metal cathodes such as LiF/Al, Ag, Au, and Pd were used as a top electrode in a device configuration of ITO/PEDOT:PSS/OC1C10-PPV:PCBM/metal cathode, and they found that only LiF/Al formed Ohmic contact with the active layer while other metals formed non-Ohmic contact. In the case of Ohmic contacts, Voc was dependent on HOMO of the donor and LUMO of the acceptor, while for non-Ohmic contact Voc was in agreement with work function difference between the electrodes based on MIM (metal-insulator-metal) model [41].

A similar study on inverted device structure were carried out by Hau et al. in which various metal cathodes including Ca/Al, Al, Ag, Cu, Au and Pd were used in a device configuration of ITO/ZnO-NPs/C60-SAM/P3HT:PCBM/metal anodes as shown in Figure 1.6 [42]. They found that Voc of the cells were reduced with a decrease in work function of the anode metals as shown in Figure 1.7. However with the insertion of a PEDOT:PSS layer between the active layer and metal anode, maximum Voc was achieved despite the choice of metals. This result indicated that PEDOT:PSS provided Fermi-level pinning to the EICT+ of the active layer for better Ohmic contact formation. The absence of Ohmic contact provided by PEDOT:PSS devices showed Voc dependent on work function difference of the electrodes based on MIM model.

Figure 1.6 J-V characteristics of inverted device with ZnO-Nps/C60-SAM as electron transport layer fabricated (a) without a PEDOT:PSS layer (top), (b) with a PEDOT:PSS layer using various top metal electrodes (bottom). Reproduced with permission from ref. [42].

Figure 1.7 Band diagram of inverted device structure ITO/ZnO–NPs/C60–SAM/P3HT: PCBM/metal electrodes (Ca/Al, Al, Ag, Cu, Au, Pd) solar cells.

Hau et al. also studied the role of SAMs in an ITO/PEDOT:PSS/P3HT:PCBM/ZnO/metal-based device and found that devices without SAMs could lead to poor performance mainly due to the formation of Schottky contact at the interface between ZnO and metal [43]. Schottky contact acts as an electron injection barrier as shown in Figure 1.8, with inclusion of SAM injection barrier reduced to zero forming an Ohmic contact. Formation of Ohmic contact and higher device efficiency with inclusion of SAM is due to the appropriate dipoles and better chemical bonding between SAM/metal interfaces.

Figure 1.8 (a) Device architecture, (b) formation of Schottky contact between ZnO and metal in absence of SAM, and (c) formation of Ohmic contact between ZnO and metal.

1.2.2 Influence on Active Layer Vertical Morphology Based on underneath Interfacial Layer

Optimum phase separation between donor and acceptor domains in a BHJ system is required for efficient exciton dissociation, charge transport and charge collection. Ideally donor-acceptor should not exceed 10 nm which is the exciton diffusion length to avoid exciton recombination. Recently, vertical phase separation has been found to play a crucial role in obtaining efficient charge transport pathways by forming interpenetrating network between the active layer and respective electrodes. Studies show that when active layer is spin coated on top of a substrate, donor and acceptor are not equally distributed throughout the film. Depending upon the choice of solvent, rate of drying, and thermal and vapor annealing conditions, morphology of an active layer could be altered. Germack et al. used surface sensitive near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and found when active layer consisting of P3HT with surface energy of 26.9 mN/m² and PCBM with surface energy of 37.8 mN/m² were coated on substrates with higher surface energy value (γ), the buried interface was selectively PCBM-rich, whereas when active layer was coated on substrates with lower γ value, they found the buried interface was selectively P3HT rich. However, the top side of the active layer was always P3HT rich owing to lower γ value of air [44].

Subiah et al. carried out XPS- and AES-based study to find out the vertical profile of a PDTS-BTD:PC71BM-based system [45]. As shown in Figure 1.9, C composition was found to be higher in the bottom than in the top. And, S composition was higher in the top and lower in the bottom.

Figure 1.9 XPS spectra of (a) C 1s region and (b) S 2p region obtained from top and bottom surfaces of the PDTS-BTD:PC71BM films. Reproduced with permission from ref. [45].

Also, in their work a comparative study was performed on normal and inverted structure BHJ cells; inverted structure yielded higher PCE owing to higher photocurrent due to better charge collection. EQE spectra in Figure 1.10 shows higher current yield in inverted device structure than in conventional geometry.

Figure 1.10 EQE spectra of conventional and inverted geometry OPV devices based on PDTS-BTD:PC71BM. Reproduced with permission from ref. [45].

1.2.3 Light Trapping Strategies and Plasmonic Effects for Efficient Light Harvesting

Typically a thicker active layer can help to absorb more sunlight. However, the low charge carrier mobility in organic materials limits the thickness of active layer to be around ~ 100 nm so as to minimize recombination. Therefore we simply need a technique where light absorption is maximized keeping thickness of the active layer constant. Various strategies have been put forward which can enhance light harvesting. Conventionally, light trapping was based on total internal reflection effects achieved by roughening of the entrance interface. Light could stay in longer interaction distance with the active material thereby increasing absorption. This technique could improve light enhancement by 4n², where n is the refractive index of the active layer. To overcome this limitation, a highly ordered and periodic nanostructure-based method are being explored which can theoretically enhance the light absorption up to 12 × 4n² [46].

Reflection and absorption of the interfacial layer play a crucial role in the photovoltaic performance of an organic cell. Hadipour et al. have conducted a systematic study using various buffer layers on a P3HT:PCBM-based system [47]. Figure 1.11 shows simulated electric field intensity within P3HT:PCBM for incident light wavelength λ = 550 nm, where higher optical field intensity is observed in devices without use of Ca because Ca has less back reflectivity compared to the others. This indicated that one of the most widely used cathode buffer layers in conventional BHJ possesses parasitic absorption which can reduce photocurrent by 25%. A study regarding absolute optical reflection of 60 nm thin Ag, Al, Ca electrodes and its effect in Jsc by Pandey et al. showed that Ag reflects 96%, Al reflects 90% and Ca reflects 68% of light and is directly proportional to the Jsc [48].

Figure 1.11 Conventional structure (A, B, C, and D) and inverted structure (E). Various buffer layers employed in the study are shown in the figure. The active layer consists of 80 nm thick P3HT:PCBM blend for all cell structures simulated electric field intensity within the P3HT:PCBM layers for incident light of wavelength λ = 550 nm. Reproduced with permission from ref. [47].

Hadipour et al. also showed that higher refractive index material such as MoO3 can act as an optical spacer and requirement of its optimum thickness for higher device performance. An optical spacer in an organic solar cell inserted between the active layer and the reflective electrode which can redistribute the optical electric field.

Another interesting work regarding optical spacer carried out by Gilot et al. using ZnO (39 nm) between active layer and top electrode revealed the necessity of inclusion of ZnO as an optical spacer for maximum optical field to be placed in the active area [49]. Calculated optical electric field for light of different wavelengths with a 40 nm thick active layer with and without ZnO is shown in Figure 1.12. Absorption peak of P3HT:PCBM film is at about 550 nm, and we should focus on wavelengths between 500–600 nm. In the figure (bottom), the device with ZnO let the optical electrical field be maximum in the active layer while without ZnO, the maximum optical field is not placed exactly in the active area. However, for thicker active layer this effect was shown to be detrimental as the active layer itself is in the most effective position of the optical electric field.

Figure 1.12 Calculated optical electric field intensity for 400,500 and 600 nm wavelength incident light on device without (top) and with 39 nm ZnO (bottom). Active layer consisted of 40 nm P3HT:PCBM. Reproduced with permission from ref. [49]

Similarly, metallic nanoparticles have been reported that can enhance performance of an OPV cell through surface plasmonic effects. Wu et al. blended Au nanoparticles onto PEDOT:PSS based on device configuration of ITO/PEDOT:PSS-Au NPs/P3HT:PCBM/cathode metal [50]. They found incorporation of Au NPs on poly(3,4-ethylenedioxythiophene) doped by poly(styrenesulfonate) (PEDOT: PSS) enhanced light absorption induced by localized surface plasmon resonance (LSPR). LSPR occurs when frequency of the incident light matches with the resonance peak of the noble metallic materials which can then enhance electromagnetic field near the surface of the metallic NPs, thus improving light trapping inside the active material. Also in their work, dynamic PL measurements showed that the LSPR effect reduced the lifetime of photogenerated excitons in the active blend indicating reduction of geminate recombination due to interplay between metallic NPs and excitons leading to enhanced rate of exciton dissociation and hence improvement in charge transfer process. Figure 1.13a shows light trapping through forward scattering due to LSPR induced by Au NPs, while Figure 1.13(b) indicates enhancement of the electromagnetic field due to LSPR.

Figure 1.13 Schematic illustrating induction of LSPR due to forward scattering of light in presence of Au NP. (a) Increase in optical path length due to light trapping. (b) Schematic representation of the local enhancement of the electromagnetic field. Reprinted with permission from ref. [50]. Copyright © 2011 American Chemical Society.

1.2.4 Morphology Control of Active Layer and ETL by Processing

Ngo et al. reported a comparative study on active layer deposition technique that can play a significant role in morphology of film [51]. Their study was based on spin coating vs spray coating as shown in Figure 1.14. Figure 1.14(a) shows dotted polymer morphology in active layer while Fig. 1.14(b) shows self-assembled fibrillar structure arising mainly due to longer evaporation time taken by the solvents. These fibrillar structures contribute in enhancing charge carrier mobility by providing lesser resistant charge transport pathway leading to higher Jsc and FF. In the same work, they also mentioned that the morphology of widely used ZnO as ETL could be controlled into more compact and smoother film. However, deposition of both ETL and active layer via spray coating technique led to lower efficiency due to poor interface formed between these two layers. Nevertheless, utilization of simple airbrush technique to spray coat the active layer and ETL can be modified for large-scale manufacturing of polymer solar cells at a much lower cost.

Figure 1.14 Phase diagram indicating (a) active layer deposited via spin coating and (b) spray coating techniques. Reproduced with permission from ref. [51].

1.3 Selection of Interfacial Layer for Stable and Longer Lifetime

Stability is a major issue for organic solar cells to compete with existing inorganic solar technology. Longer lifetime is the prerequisite for commercial application. Polymer solar cells degrade both under illumination and in the dark. Therefore it is very important to understand the origins of degradation. An organic cell is susceptible to both chemical and physical degradations. Chemical degradation includes diffusion of oxygen and moisture into the active layer, oxidation of metal electrode, chemical degradation of the ITO electrode, degradation of PEDOT:PSS layer, and photo-oxidation of polymers. Major physical degradation includes change in morphology of the donor and acceptor molecules which may not remain the same as they were during the device fabrication process. Change in ambient temperature could easily change the micro/nano phase separation between donor and acceptor due to slow diffusion or recrystallization of the components inside active layer thereby reducing PCE of the cell.

1.3.1 Stability of Active Layer Materials

Degradation of active layer obviously leads to degradation of power conversion efficiency. It is important to understand the mechanisms involved in degradation of active layer and methods to improve it. Light-induced degradation is one of the major degradation processes of active layer besides oxidation of polymers in the presence of ambient oxygen and moisture. In poly-phenylenevinylene (PPV)-based polymers that were extensively used for OPV research, photochemical decomposition is from the side chain and vinylene moiety degradation [52]. Initially, highly reactive singlet oxygen was thought to be the source of degradation. However, Chambon et al., through their transient absorption spectroscopy, showed that formation of superoxide oxygen anion during electronic transfer of photoinduced carriers acts like electron acceptors and oxidize the polymer [53]. Similarly with the most popular polymer P3HT, it was previously thought that the singlet oxygen was responsible for photodegradation, but it is now known that it is the hydroperoxide formation at the benzylic position that initiates side chain oxidation as shown in Figure 1.15.

Figure 1.15 Photooxidation mechanism in P3HT. Reproduced with permission from ref. [54].

Manceu et al. studied photodegradation of polymers in which they exposed samples to light (under 1 sun and ambient air). A normalized number of absorbed photons versus ageing time was used for comparing stability for various polymers [55]. They then ranked monomers, from which polymers are made, according to their stability to light as shown in Figure 1.16. They found that donor groups with side chains are more susceptible to degradation and substitution of a carbon with silicon improves stability.

Figure 1.16 Stability order of various donor groups. Reproduced with permission from ref. [55].

Another study carried out by Xia et al. based on UV absorptions, PL spectra, and FT-IR spectra, showed that photovoltaic performance of aryl side chains is more stable than alkyl side chains [56].

Rate of degradation as studied by Kumar et al. depends upon different solvents used for desired morphology control [57]. Their work shows the choice of solvents could broaden the effective density of states which then controls the rate of degradation of active layer. Rate of degradation also depends upon regioregularity and molecular packing [58]. Ebadian et al. used 94% and 98% regioregular (RR) P3HT and found that although lower RR P3HT initially has lower PCE compared to higher RR P3HT, after a long period of time lower RR P3HT-based devices showed higher PCE, indicating electron charge transport as the major factor in determining the degradation rate [59]. In the same study, better charge transport due to higher carrier mobility was shown in higher RR P3HT as electron transport plays a critical role in polymer degradation hence leading to poor device stability.

Since side chains are responsible for degradation of the polymer, Liu et al. introduced thermo-cleavage technique in which a carboxylic ester was attached at the 3-position to every second thiophene moiety [60]. This allowed researchers to get rid of the ester group after heating to 200°C. This method paved the way for obtaining thermally stable bulk heterojunction [61]. A similar study carried out by Krebs et al. using P3MHOCT:PCBM after thermal cleavage yielded P3CT:PCBM and later showed better stability even after 4000 hours of full sun exposure [62].

1.3.2 Stability of Metal Electrodes

Aluminum and calcium are commonly used cathode metals in conventional BHJ solar cells. Although they show stable performance when stored in an inert environment, their lower work function leads to oxidation when exposed to ambient conditions converting them into insulating material, thereby lowering PCE to almost no photovoltaic response. Another mode of degradation is diffusion of oxygen and water through pinholes and aluminum grains respectively present in the thermally evaporated aluminum electrode into the organic layer. As shown in Figure 1.17, water can eventually result in the homogenous photo-oxidation of organic material, while molecular oxygen can lead to photo-oxidation of polymers at places near microscopic pinholes [54, 63]. AFM-based studies show photo-oxidation of the organic material leads to protrusion towards the outside [64].

Figure 1.17 Schematic representing modes of water and molecular oxygen diffusion. Reproduced with permission from ref. [54].

This problem could be solved using different metal electrodes, most possibly Ag, as in inverted device structure which has higher work function and is less susceptible to oxygen and moisture.

1.3.3 Stability of Transparent Electrode

ITO is generally used as a transparent electrode in BHJ OPV devices. It is known that PEDOT:PSS layer deposited on top of ITO leads to degradation of the interface between the two due to acidic nature of PEDOT:PSS. Also, in one study led by Norrman et al. ITO/PEDOT:PSS could promote the degradation of the active layer in which PSS could diffuse to other parts of the device with a reaction that can lead to degradation of the overall PCE [65]. Another study done by Schäfer et al. showed change in work function of the ITO upon UV irradiation leading to lowering of VOC, which they conclude is a reversion of the ITO work function [66]. Kanai et al. later showed that such an effect can be overcome by introducing MoO3 between the ITO and organic layer [67].

1.3.4 Stability by Electron Transport Layers (ETLs)

Besides energy level alignment for efficient electron transport from acceptor LUMO to the cathode electrode, electron transport layer (ETL) is also reported for improving device lifetime. LiF has also been widely used as electron transport layer which can improve device stability due to moisture blocking ability [68]. TiOx is another popular material used as ETL, which could be applied both for energy alignment and better stability despite its lower conductivity in amorphous form. Li et al. showed inclusion of 20 nm TiOx in a device structure consisting of ITO/PEDOT:PSS/P3HT:PCBM/TiOx/Al improved PCE compared to those without TiOx. Also, they showed that devices without TiOx degraded significantly after continuous exposure to UV light; however the cells with TiOx layer exhibited negligible degradation. TiOx layer contains organic moieties coming from sol gel-based deposition. They conducted an IR-and ESR-based study on UV irradiated TiOx layer. Results showed that alkoxide functionalities Ti-OR (OR= alkoxide) present in the sol gel derived TiOx layer get photo-oxidized into CO2 and H2O by utilizing O2 causing oxygen gas scavenging and saving the organic layer lying underneath [69]. A study conducted by Jin et al. showed the role of thin CdSe (8 nm) layer inbetween organic layer and metal electrode could enhance the lifetime of an OPV device by blocking the moisture and oxygen when exposed to air.

ZnO is one of the most widely used ETL with high electron mobility in inverted device structure. Ferreira et al. used ZnO as ETL in conventional structure and compared stability with devices without ETL [70]. He found that devices with ZnO as ETL exhibited longer lifetime retaining almost 60% of the initial efficiency after 78 days of exposure to air. TEM-based analysis suggested that the reaction between metal electrode (Al) and active layer was minimized with the inclusion of the ZnO sol gel layer in between. Puetz et al. doped ZnO with indium (IZO) and found that it not only improved PCE but also stability [71]. They used IZO between active layer and metal electrode in a device with configuration (ITO/P3HT:PCBM/IZO/Ag) and found that PCE is enhanced compared to a device without the IZO. An energy level diagram is shown in Figure 1.18. However, although improvement in VOC was just 50mV, it demonstrated better Ohmic contact provided by inclusion of IZO layer which led to higher PCE. Also, cells with IZO layer showed longer stability even after stored under ambient conditions due to improved barrier effect exhibited by IZO layer.

Figure 1.18 Energy level diagram showing IZO acting as an electron transport layer barrier for holes and preventing further recombination losses.

1.3.5 Stability by Hole Transport Layers (HTLs)

PEDOT:PSS is the most commonly used hole transport layer in regular BHJ devices. One of the key issues associated with PEDOT:PSS is its acidic nature that can etch the underneath ITO in the long run leading to degradation in PCE. Kim et al. showed that pH of PEDOT:PSS could be adjusted by addition of NaOH [72]. They showed the optimum amount of NaOH (0.2 molar ratios) could remove sulfonic acid group (up to 23%) with device performance still remaining the same and lifetime increased by about 25%. Another disadvantage of using PEDOT:PSS is its hygroscopic nature which can ultimately oxidize the active layer. Girroto et al. compared PCE of devices made of three different hole transport layers, namely: PEDOT:PSS, thermally evaporated MoO3 and sol-gel derived MoO3. In their experiments, devices with different hole transport layer exhibited similar performance. However, stability of MoO3-based devices showed a longer lifetime compared to PEDOT:PSS [73]. Use of sol-gel derived MoO3 is beneficial in achieving a low cost technique suitable for R2R manufacture of solar devices. Another approach to replace PEDOT:PSS is to use mixed oxides as reported by Huang et al. in which they used WO3-V2O5 with remarkable stability in inverted device structure [74]. Their cells retained more than 90% of initial efficiency even after 1000 h of air exposure. Mixed oxide can suppress the leakage current as higher shunt resistance could be achieved with its inclusion. Also