(Research Plan) : Utilization of New Polymer and Optimization Processing for Organic Solar Cell to Reach Power Conversion Efficiency Beyond 10%

Background and Statement of Research

ARTICLE IN

Organic solar cells belong to the class of photovoltaic cells known as excitonic solar cells, which are characterized by strongly bound electron–hole pairs (excitons) that are formed after excitation with light. Strongly bound excitons exist in these materials as a consequence of the low dielectric constants in the organic components, which are insufficient to affect direct electron–hole dissociation, as is found in their high dielectric inorganic counterparts. In excitonic solar cells, exciton dissociation occurs almost exclusively at the interface between two materials of differing electron affinities (and/or ionization potentials): the electron donor (or simply donor) and the electron acceptor (or simply acceptor). To generate an effective photocurrent in these organic solar cells, an appropriate donor–acceptor pair and device architecture must be selected.

In the more than 20 years since the seminal work of Tang (1986), organic solar cells have undergone a gradual evolution that has led to energy conversion efficiencies of about 5%.

 Two main approaches have been explored in the effort to develop viable devices: the donor–acceptor bi-layer, commonly achieved by vacuum deposition of molecular components, and the so-called bulk heterojunction (BHJ), which is represented in the ideal case as a bi-continuous composite of donor and acceptor phases, hereby maximizing the all-important interfacial area between the donors and acceptors. Polymer-based photovoltaic systems which can be processed in solutions, and which generally take the form of BHJ devices, most closely conform to the ultimate vision of organic solar cells as low-cost, light weight, and flexible devices. The real advantage of these BHJ devices, which can be processed in solution, over vacuum deposition is the ability to process the composite active layer from solution in a single step, by using a variety of techniques that range from inkjet printing to spin coating and roller casting.

However, regardless of the method of preparation, one feature that extends across all classes of organic solar cells is the almost ubiquitous use of fullerenes as the electron accepting component. The high electron affinity and superior ability to transport charge make fullerenes the best acceptor component currently available for these devices. The state-of-the-art in the field of organic photovoltaics is currently represented by BHJ solar cells based on poly(3-hexylthiophene) (P3HT) and the fullerene derivative [6,6]- phenyl-C61-butyric acid methyl ester (PCBM), with reproducible efficiencies approaching 5%. The widely used conjugated polymers in these bulk heterojunction solar cells are poly[2-methoxy-5-(30,70- imethyloctyloxy)-1, 4-phenylene vinylene] (MDMO-PPV) and regioregular poly(3- hexylthiophene) (P3HT) because of their relatively high hole mobility and strong light absorption. Fullerene (C60) and its derivatives have been used as effective electron acceptors, which are either blended with conjugated polymers or used as an electron acceptor layer. Ultra-fast electron transfer from conjugated polymers in their photoexcited states to C60 results in a very efficient charge separation. To attain efficiencies approaching 10% in such organic solar cells, much effort is required to understand the fundamental electronic interaction between the polymeric donors and the fullerene acceptors as well as the complex interplay of device architecture, morphology, processing, and the fundamental electronic Processes.

In the past 10 to 15 years there has been intense research focused on the optimization of polymer-based solar cells. The approach to improve device efficiency has involved varying numerous parameters such as the choice of donor and acceptor, donor : acceptor ratio, choice of casting solvent(s),and annealing treatments. In contrast to this, one of the main advantages of using block copolymers as solar cell materials is that they would not require a combinatorial screening approach to fully optimize their device efficiencies. Careful planning and judicious choice of the structure at the outset would help to target a particular donor–acceptor domain size, the type of morphology and the interfacial width between the two components. Block copolymers will proceed to the microphase-separated equilibrium structure regardless of the casting solvent (as long as the solvent is nonselective) and numerous other processing parameters.

The drive to use block copolymers in organic solar cells is mainly due to their equilibrium structure on a well-defined length scale. Ideally, the processing of the active OPV layer needs to be simple and rapid, so that it is compatible with existing roll to-roll (R2R) technology akin to the manufacture of conventional polymer films that have also recently been demonstrated for solar cell blend manufacture.

Whatever method chosen, one of the most important parameters remains the size of the domain. Work with blends has shown that domain sizes should be slightly greater than the exciton diffusion length, so as to obtain the most efficient balance between the process of charge formation from excitons, and charge transport with minimal losses by recombination. A comparable balance is required when dealing with block copolymers. The domain size needs to be exactly of the right size for the efficiency to be maximized. Too small, and charge recombination is favored, too large, and the interfacial density is too low to collect excitons and form charges. Fortunately, this can be managed by carefully adjusting the size of the polymer backbone.

Objective/motivation

To prepare a new active conjugated polymer as an electron donor layer for organic solar cell, improve donor-acceptor blend, adjust morphology and structure, all in order to provide well defined pathways for electron-hole mobility. The aimed polymer should improve Power Conversion Efficiency of organic solar cell up to more than 10% and give the solar cell 10 years life time in order to reach market interest.

Method

To achieve the objectives, the study will be conducted as follows: (a). Literature Review, in order to understand the recent advancement in Polymer synthesis and organic solar cell processing. (b). Preparation of new polymer base on benzothiadiazole derivate. To date, one of the most efficient example of a low-bandgap polymer for use in solar cells is poly[{2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b’]dithiophene}-alt-{4,7-(2,1,3-benzothiadiazole)}. This polymer has measured optical band gap of about 1.45 eV, and in a 1:1 blend with PCBM shows a power conversion efficiency of 2.7% and a Voc value of 0.65 V, with a peak EQE value of about 30% and photocurrent production at wavelengths longer than 900 nm. The excellent performance of such polymer can be attributed to a broad absorption spectrum and high mobility of the charge carriers (2 K 10^-2 cm²V^-1 s-¹ in Field Effect Transistor (FETs)). The ability to achieve efficiencies approaching 3% in a 1:1 blend with PCBM correlates with the superior miscibility of the polymer with PCBM relative to other donor–acceptor polymers.

Preparation of new poly-benzothiadiazole derivate base solution is planned by using Suzuki coupling reaction. P3HT/bisPCBM are expected as electron acceptor layer rather than P3HT/PCBM. Such fullerene bis-adducts has recently known as brought significant efficiency increase for P3HT due to and open circuit voltage increase. Similar increase is expected for the high performance donor polymer benzothiadiazole derivate as mentioned above. 

Figure 1. bis-derivative [6,6]- phenyl-C61-butyric acid methyl ester (PCBM)

The molecular weights are measured by GPC. All the solar cells will be fabricated on indium-tin-oxide (ITO) pattern glass substrates with an active area of 2.2 mm². The ITO-pattern glass substrates are sequentially washed with deionized water, propan-2-ol, acetone, and then treated with ozone gas under UV light.

Poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS, Bayer AG ) as a buffer layer is spin coated on the top of ITO substrate. The thickness of the PEDOT: PSS film (PEDOT film) is controlled to be about 50nm. The PEDOT film is first baked at about 200 ^oC for about 10 min in air, and then baked under a vacuum at 80 ^oC for about 30 min to remove the water. The poly benzothiadiazole derivate solution is spin coated on the top of the PEDOT layer to form an active layer. A P3HT/bis-PCBM layer is then deposited under a vacuum and used as an electron acceptor layer. Finally, a silver layer of about 50 nm is deposited under vacuum. A typical heterojunction solar cell had a structure of class /ITO/PEDOT (50 nm)/polymer/P3HT/(bisPCBM) (30 nm)/Ag (50 nm).

Current–voltage (I–V) characteristics of the solar cells are measured in air with a source meter (Keithley, SMU 2400) under simulated AM1.5 irradiation (100 mW/cm²) from a solar simulator (WACOM, WXS-85-H). Photocurrent action spectra of the photocurrents are measured with a lock-in amplifier (NF Circuit Block, LI-573) under irradiation of monochromatic light chopped at a frequency of 400 Hz. UV–Vis absorption spectra of the copolymer films are recorded on a Shimadzu Multispec 1500 spectrophotometer. The differential scanning calorimetry (DSC) measurement is carried out using DSC3100SA (Bruker AXS) at a scanning rate of 5 1 C/min in nitrogen. The ionization potentials of the copolymer and P3HT/bis-PCBM are measured by a photoelectron spectrometer (Riken Keiki,AC-1) and the electron affinities are determined by combination of photoelectron spectroscopy and UV–vis absorption spectroscopy. Morphology of the active layers is characterized by using Scanning Electron Microscopy (SEM) while the layer’s cross-section is observed by using Tunneling Electron Microscopy (TEM). All the other measurements are carried out in air at room temperature.

Lifetime testing

In order for organic solar cells to fully mature from research and development into cost effective products, continuous improvement in efficiency and stability must be achieved. It is clear that the organic semiconductors and electrode materials used so far are susceptible to oxygen and moisture. To reduce the degradation of the active layer, oxygen and water vapor barrier coatings become a necessity.

Accelerated lifetime testing of the device encapsulated with super barrier films with WVTR of 0.2 g m^-2 d^-1 at 65 ° C/85% relative humidity (rh). The barrier films are characterized using electrical calcium tests and lifetime of OPV cells under accelerated conditions are correlated to the WVTR. The OPV cells are exposed to 65 ° C/85% rh (damp heat and dark), 65 ° C (high temperature, dark) and 65 ° C/1 sun (high temperature under light).

The others testing for solar cell life time are summarized in Table 1.

Table 1. Mechanism specific developmental testing. Summary of various failure modes (cause/effect) leading to reduction in efficiency in OPVs.

Stress	Response
Mechanical	De-lamination, electrode failure, packaging failure
Temperature	Acceleration, de-lamination, morphological changes, diffusion
Light: spectral response, total intensity	Photochemical oxidation, photo bleaching, yellowing, mechanical failure
Oxygen: humidity, water	Donor/acceptor oxidation, electrode oxidation, charge extraction, change in mobility, TCO etching, interface failure
Coupled effects: water and mechanical, light and mechanical	Interconnect failures (in addition to above mentioned failures)
Electrical: electric field, columbic charge	Localized heating, shorts

Expected outcome

The outcomes of the proposed research are summarized bellow; (1) A new Polymer for electron donor in organic solar cell which generate Power Conversion Efficiency (PCE) beyond 10% and 10 years life time. (2) Publication of our results in highly reputed international journals is our door to reach international achievement and industrial collaboration. (3) Contribution to this knowledge will useful especially for me and my country upon come back.

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