1-1 Introduction to Photovoltaic
Faced with the looming threats of global warming and limited supply associated with fossil fuels, the global community has been actively developing renewable energy sources. Photovoltaic (PV) cells gain widespread acceptance as a source of clean and renewable energy. For PV, various materials were selected with difference performance. Crystalline silicon (Si) is used as the semiconductor component in over
90% of the PV applications today. Other materials such as amorphous silicon (a-Si),
copper indium gallium arsenide (CIGS), cadmium telluride (CdTe), etc., which with
the efficiency above 10% have also been explored.
1,2,3Though inorganic photovoltaic have an advantage of high conversion efficiency, the cost per watt of solar energy is too high and hard to universalize. The reasons for
this are the high cost of raw materials and processing, and difficulty in fabrication and
installation of PV.
Polymer-based photovoltaic have introduced after the development of conjugated conducting polymer. Polymer can be a promising candidate for photovoltaic materials.
Its advantages include low-cost potential, large-area fabrication, low specific weight,
mechanical flexibility, etc.
4Specially, polymer is easily processed. Solution processes,
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including spin-coating
5, spray-casting
6, dip-coating
7, roller-coating
8and ink-jet
printing
9, can be used to coat polymers on a variety of substrates. For all their
potential advantages, polymeric solar cells are still far from commercial application,
mainly because of their low power-conversion efficiency and short lifetime. Most
current efforts on developing polymeric solar cells are focused on developments of
new materials and device structures to improve the power-conversion efficiency, but
the issue of short lifetime has received scant attention.
In this work, we aimed to develop new processes to improve the power
conversion efficiency and stability of polymer solar cells.
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1-2 Working Principle and Characteristics of Polymer Solar Cells
The active layer of polymer-based solar cells is composed of electron donor and acceptor. Solar cells transform light into electrical energy by exciting materials in active layer for carriers. The process is described through the following steps:
(1) Absorption of photos (Figure 1-1a):
In the first step, photos are absorbed and transform into energy, followed by photo-excitation of the electrons on donors. Sufficient and operative absorption from Solar Spectrum is required to generate more current. A band gap of 1.1 eV (1100nm) is capable of absorbing 77% of the solar irradiation.4 For this reason, developing new organic materials with small band gap is a way to enhance the performance of solar cells.
(2) Generation of charge carrier (excitons dissociation; Figure 1-1b):
Excitons diffuse to the interfacial area of electron donor and acceptor to dissociate. The diffusion length of excitons is around 10nm.10 Longer diffusion distance in donor increases the possibility of excitons’ recombination, which reduces the power conversion efficiency. Thus, controlling a proper morphology of active layer with nano-scale structure is quite important.
(3) Transport of the carriers to the electrodes (Figure 1-1c):
Dissociated excitons separate into holes and electrons, followed by diffusing
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through the donor and acceptor to electrodes. A bicontinuously and interpenetrating passage of morphology and the property of materials with high carrier mobility are the essential terms to assist in transporting carriers.
(4) Collection of changes at the electrodes (generation of electric current; Figure 1-1d):
At last, optional electrodes with suitable work function have great effects on device operation. Inactive metals with low work function are proper for a cathode material. For anode, high work function conductor complemented with hole transporting layer is generally used.
Through these four processes, polymer solar cells under sunlight can obtain currents. The current-voltage characteristics of a solar cell in the dark and under illumination are shown in Figure 1-2. Take the energy level schemes for example in Figure 1-3,11 when the solar cell is under illumination, incident sunlight photos are absorbed by the materials and generate excitons. Without an applied electric field (electrodes are connected and potential is zero, Figure 3a), the maximum current is so-called short current (Jsc) which is obtained at short circuited condition. Under an applied electric field, the gradient of materials’ work functions is offset, approaching no current flows. This voltage is defined as the open circuit voltage (Voc) at open circuit condition in Figure 3b.
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Fill factor (FF) is defined as the ratio of the maximum power to the external short and open circuit value:
FF V J
V J
The external yield of energy conversion efficiency η (also called power conversion efficiency, PCE) is defined as the power produced by the cell (Pm) divided by the power incident on the rep sentative area of the cell (P ). re in
PCE P
P A 100% V J FF
Where:
Pin (incident light power density) =100mW/cm2 Ac (active surface area) =1cm2
Normally, PCE is the key parameter for solar cell’s productivity, and it is also the parameter often used for comparison purposes.11
Otherwise, for calibrating the experimental condition, light with an intensity of 1000 W/cm2 and a spectral distribution AM1.5 global standard solar spectrum were used.
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a)
b)
c)
d)
Figure 1- 1 Working principle of bulk heterojunction solar cells: a) absorption of photos; b) Generation of charge carrier (excitons dissociation); c) Transport of the carriers to the electrodes; d) Collection of changes at the electrodes (generation of electric current).
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Voc Vm
Jsc Jm
Dark Current
J
V Photo Current
Pm=Vm× Jm
Figure 1- 2 J-V curve of polymeric solar cells.
Figure 1- 3 Energy levels and light harvesting.11
1-3 The Bulk-Heterojunction Polymer Solar Cells
The evolution of polymer solar cells was through few kinds of structures, such as bilayer structure12,13, bulk heterojunctions14,15 and ordered heterojunctions16,17.
The bulk heterojunction (BHJ) is the most successful device architecture for polymeric
photovoltaic. In BHJ solar cells, the active layer is made of mixed donor and acceptor.
The mixed donor and acceptor may phase-separate and form a large area of interface.
Excitons can easily be dissociated when they diffuse to the nearby interface of donor
and acceptor. On the other hand, these phase-separated donor and acceptor in BHJ
provide bicontinuous and undisturbed pathways for the transport of charge carriers to the electrodes. Thus, we can realize that the key points to make high efficiency BHJ solar cells are: 1) control a proper morphology of active layer with nano-scale phase separation of electron donor and acceptor; 2) bicontinuous pathways of donor and acceptor for carrier transporting.
In these years, poly(3-hexylthiophene) (P3HT, shown in Figure 1-4) and phenyl-C61-butyric acid methyl ester (PCBM , shown in Figure 1-4) mixed BHJ solar cells have been studied extensively. The fullerene (C60) derivative PCBM is the the strongest candidate of electron acceptor materials used for organic photovoltaic application. PCBM plays an important role in separating the excitons and constituting the passways for electrons’ transportation. It has a high electron mobility
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of 10-3 cm2/Vs.18 Most importantly, its good-solubility in solvent enlarges the processability in solution procedure.
As a hole conducting donor, P3HT have a relative low energy gap (1.9eV) which can absorb more photos in visible region. Highly-crystallized P3HT can approach high effect hole mobility in the range 0.05-0.1 cm2/Vs.19 It also has the advantages such as good solubility, processability, and environmental stability.20
Using region-regular P3HT (RR-P3HT) as donor and PCBM as acceptor, bulk heterojunction solar cells have been realized with external quantum efficiencies of around 75% and power conversion efficiencies up to 5%.21 It is proposed that the high efficiency of BHJ solar cells is due to the microcrystalline lamellar stacking of P3HT in the solid-state packing. It has also been improved by several techniques, such as pre-treatment of solution26, 27, 28, thermal annealing30, vapor annealing31, et al.
These processes all have a common purpose- control the morphology of active layer, which we introduce in following part.
Figure 1- 4 Chemical structure of PCBM (left) and P3HT (right).
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1-4 Morphology Control for P3HT:PCBM BHJ Solar Cells
For P3HT: PCBM mixed system of polymeric solar cells, morphology control to active layer have a great influence on the performance and stability. Proper solvent to the P3HT and PCBM and annealing process to active layers are researched for controlling the crystallinity of P3HT and distribution of PCBM. According to different processes in active layer’s manufacture, it can be divided into two parts:
1-4-1 Solution treatments
As we know, solubility (affinity) of solvent to the solute makes a great effect in solutes’ aggregation or dispersion in solution. Thus, control the solubility of P3HT and PCBM solution is one of methods for their morphology control.
Good solvents for P3HT and PCBM like dichlorobenzene (DCB), chlorobenzene (CB)22, chloroform (CF)23 and toluene24 can supply a proper environment for P3HT and PCBM’s dissolving and extending. On purpose of increasing the crystallinity of P3HT, poor solvent can play a role of the nuclear agent while adding into a good solvent.25 For instance, poor solvent - hexane adding into DCB solution had an increased UV absorption peak at 607nm by P3HT’s aggregation.26 By the same idea, 3-hexylthiophene (3HT) in CB also can reduce the difficulty of P3HT’s nucleation.27
Besides solvents mixed system, using a proper poor-solvent also can increase the aggregation of P3HT. During an aging time, P3HT in both p-xylene28 and o-xylene29
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can aggregates due to the natural poor-solubility.
P3HT fibers can be observed in solution by these processes. All processes mentioned above provide an increased crystallinity of P3HT and an improved efficiency. Besides, these solvent treatments take an annealing-free process in the device procedure.
1-4-2 Film treatments
In addition to solution treatments, making efforts to active layer during or after drying process also can effectively control the morphology of P3HT and PCBM.
Thermal-annealing is treated as a standard process for most solar cells’
manufacture. Thermal energy makes P3HT re-crystallized for approaching a high mobility. The most well-known process was developed by A. J. Heeger et al., which approached a power conversion of 5% by a 150oC post-annealing after Al cathode evaporated. Another process is solvent vapor annealing which prolong the wet time of active layer in drying process.30 During the evaporation of solvent, P3HT in good solvent such as DCB, may precipitate and self-arrange by the gradually increased concentration. This process was introduced by Y. Yang et al. and successfully obtained a solar cell performance with 4.37% efficiency.31 Thus, we understand that both re-crystallization and self-arrangement of P3HT assist in improving the device performance.
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Besides to promote the crystallinity of P3HT, the distribution of PCBM is also an important issue for device performance. PCBM is a small molecule and easily terns to mobile and aggregate by any drive force such as thermal energy and lower solubility.
In lately study, Yang and his group added di-thioene into DCB solution. Di-thioene has an affinity to PCBM and higher boiling point than DCB. In film forming process, DCB will evaporate first, then (di-thioene) bring P3HT aggregating but PCBM still well-disperse in active layer. In this way, annealing-free process can be approached but without a large-scale aggregation of PCBM.32
In summary, how to control the morphology with high crystallinity of P3HT and well-distribution PCBM is the key point for approaching high performance device. It can be achieved by these mechanisms: solubility control, self-arrangement and re-crystallization of P3HT.
Even though these morphology-control processes can provide an increased performance of polymer solar cells, the further observation of morphology or device’s efficiency are rare to study. Morphology of active layer is facile to change with reducing effects for solar cells’ performance. It is known as “physical degradation”
introduced in section 1-5.
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1-5 Degradation of P3HT:PCBM BHJ Solar Cells
In spite of the continuously efforts in promoting the efficiency of BHJ solar cells, the stability of the devices is also an essential issue for commercialization.
Various degradation processes occurring in BHJ solar cells. We classified this decay into physical and chemical parts.
1-5-1 Physical degradation:
In BHJ solar cells, physical degradation includes the diffusion of metal cathode33,34, interfacial contact35, and unstable morphology of active layer36,37. The issue we mostly concerned is the unstable morphology of active layer. As we mentioned above, mostly studies reported in promoting the performance of solar cells, but rare studies connected the relationship between morphology and device stability in different processes.
In first generation bulk heterojunction solar cells, study showed that “high Tg PPV” had a relatively better thermal stability of performance and bulk morphology than MDMO-PPV. This was interpreted that the free movement of the PCBM molecules was hampered due to a stiffer “high Tg PPV”: PCBM matrix.36 In recently study,
Jang Jo et al. reported that in P3HT: PCBM solar cells, solvent annealing resulted in a more stable morphology than thermal annealing. The key point was firm
and ordered P3HT can hamper PCBM’s large-scale aggregation, which was relatively
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fast mobile by thermal energy.
37These studies provide us a good aspect about the mechanism of mobile P3HT and PCBM for improvement.
1-5-2 Chemical degradation:
On the other hand, chemical degradation in BHJ solar cells also includes several parts. The main causes of chemical degradation in BHJ solar cells come from ambient oxygen and water. It may oxidize low work function cathodes such as calcium (Ca) or aluminum (Al),38 photo-oxidized P3HT with light,39 or recover the natural acidic of Poly (3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) to un-stabilize the interface between PEDOT: PSS and Indium tin oxide (ITO) conductive glass.40 All chemical reactions mentioned above decrease the performance of solar cells. In the study, we focused on the degradation by unstable interface between ITO and PEDOT: PSS, which is rare known for its effect to BHJ solar cells.
PEDOT has been one of the most successful conducting polymers in organic electrical devices for its excellent transparency and high conductivity. In most application, PEDOT with PSS (Figure 1-5) is the most widely utilized because it has a good shelf life and can be easily coated on substrates. The existence of PSS has two functions. First, it can make charge balance to stabilize the p-doped PEDOT chains.
The other function is keeping PEDOT segments dispersed in aqueous medium
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well.41,42
Though its good-stability of filmed PEDOT: PSS, it still unstable in humidity exposure by a reaction with ITO substrate. Hydrophilic PEDOT: PSS may absorb humidity to recover its acidic property.
H2O+PSS(HSO3)ÎH3O++PSS(SO3)
-Conducting ITO is constituted with In2O3 and SnO2. In device structure, the acidic property of humidified PEDOT: PSS may etch the In2O3 in ITO substrate, followed by releasing indium ions.
In2O3 +6H3O+Î9H2O+2In3+
Indium ions from dissolving In2O3 diffuse into PEDOT layer.43 It can be observation that through time indium’s concentration might existence in entire PEDOT: PSS films even reach to the interface of the active layer.44,45
The phenomenon was first observed in polymer light emitting diode, but rare studied in solar cells.
Figure 1- 5 Chemical structure of PEDOT (bottom) and PSS (top).
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1-6 Objective Statement
The goals of this research are two-fold: minimize the physical and chemical degradations of P3HT: PCBM BHJ solar cells, and maximize their power conversion efficiency. We accomplish our goals with two approaches: optimizing the active layer’s morphology by manipulating its film-forming process and eliminating anode/active layer reactions with an interface-modifying film deposited by atomic layer deposition (ALD).
In the first part (chapter 2), we focused on the physical degradation of morphology in active layer. New drying process at low temperature was adopted to approach a proper morphology. In addition to the observations of physical property, device performance and long-term stability also were examined.
In the second part (chapter 3), we make efforts in protecting the device from the unstable interface between ITO and PEDOT: PSS. To accomplish the objective, the concept of blocking layer was introduced. The effect of blocking layer inserted in the device was examined by long-term storage and depth-profile for observing the distribution of indium in thin films.
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