Polymer solar cells
By Assoc.Prof.Dr. Jatuphorn Wootthikanokkhan
Nanotec-KMUTT Center of Excellence on Hybrid Nanomaterials for Alternative Energy (HyNAE)
School of Energy, Environment and Materials
King Mongkut’s University of Technology Thonburi (KMUTT), Thailand
It has been known that solar cell can be classified into three broad categories, i.e, the first generation solar cell made from bulk crystalline silicon; the second generation solar cell, also called thin film solar cells, based on amorphous silicon, CIGS, CdTe, ect; and the third generation solar cells which include dye sensitized solar cell (DSSC), polymer solar cell (Fig.1) also known as organic photovoltaic cell (OPV), quantum dot solar cell, and a recently developed perovskite solar cell.
Particularly, the polymer solar cells based on semiconducting polymers have gained immense interest over the past few years, partly stimulated by the fact that production process of the polymer based solar cell is relatively simple, inexpensive and less polluted. In addition, by tailoring made some chemical structure of the polymeric materials, flexibility and photo-electric properties of the material can be tuned. Moreover, it is also possible to be enlarges the production scale by adapting some existing industrial process such as ink-jet or doctor blade screen printing. At the present time, the maximum power conversion efficiency (PCE) of organic solar cell certified by the national renewable energy laboratory (NREL) was 11.5 % . The value is relatively low as compared to those obtained from the first and second generation solar cells. Further attempts have yet to be made in order to enlarge the cell size and increase PCE of the organic solar cells. Besides, improvements of the OPV in terms of lifetime and materials cost also deserve a consideration.
Working mechanism of the polymer solar cell
Basically, configuration of polymer solar cell comprises a layer photo-active materials being sandwiched between two electrodes (Fig.2). Additional layer capable of suppressing the electron-hole recombination may also be included. Operating mechanism of the polymer solar cells can be described as following. Upon exposure of the semiconducting polymer to sun light, the polymer was photo-doped and some tightly-bound electron-hole pairs (also known as an exciton) were created. The excitons may recombine again unless they can diffuse into an interfacial area between polymer and electrodes, where some difference in work function exist. To minimize exciton recombination and to enhance power conversion efficiency [PCE] of the polymer solar cell, it is common to blend the semiconducting polymer with some electron acceptor materials such as fullerene and its derivatives. As a result, interface between the donor material (the polymer) and the acceptor material was created. Across this donor-acceptor interface, a large HOMO-LUMO energy level offset produced a large enough internal electric field gradient, capable of splitting the exciton into free electrons and holes. This kind of photovoltaic devices containing blend of donor-acceptor materials is also known as a bulk hetero-junction [BHJ] solar cell
Figure 2. Schematic of the device structure of a polymer bulk-heterojunction solar cell. (top), Formation and splitting of exciton in an active layer of BHJ solar cell (bottom)
Research work on BHJ polymer solar cell at HyNAE
Our research work in this area has concerned the synthesis of donor-acceptor copolymers to be used as compatibilizers in BHJ solar cell. With these copolymers, morphology of the active layer had been altered [Fig. 3] and a greater power conversion efficiency of the solar cells was obtained. These include the following copolymer systems; poly(3-hexyl thiophene)-b-fullerene copolymer, poly(p-xylylene)-graft-poly(butylacrylate-g-fullerene) . Besides, attempts were also made to synthesis fuillerene functionalized polymers and explore a feasibility of using the modified polymer as a replacement of the electron acceptor material. These include the development of fullerene grafted polystyrene  and fullerene grafted dehydrochlorinated poly(vinyl chloride) . Last but not least, our team has recently synthesized a low band gap semiconducting polymer based on Poly[4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b0]dithiophene-co-quinoxaline] (PBDTQx) copolymer. By combining this polymer with a newly developped fabrication technique, namely double layer deposition (Fig.4), an improvement of power conversion efficiency of the polymer solar cell as achieved .
Figure 3. AFM images (topographic mode) of P3HT/PCBM (left) and P3HT/PCBM with P3HT-b-PSFu copolymer (right).
Figure 4. Schematic device structure of 1:0.5:0.5 of PBDTQx:PC71BM:ICBA double layer coating BHJ solar cells using convective deposition method
 Nattawoot Rattanathamwat, Jatuphorn Wootthikanokkhan, Nonsee Nimitsiriwat, Chanchana Thanachayanont,Udom Asawapirom, and Anusit Keawprajak Poly(3-hexyl thiophene)-b-Fullerene (P3HT) Functionalized Polystyrene Copolymers as compatibilizer in P3HT/ phenyl-C61-butyric acid methyl ester (PCBM) Solar Cells, International Journal of Polymeric Materials, 63(2014)448-455
 Narumon Seeponkai, Jatuphorn Wootthikanokkhan, Chanchana Thanachayanont, Sombat Thanawan, Siriwat Radabutra, and Surawut Chuangchote Synthesis of Graft Copolymers and Their Preliminary Use as a Compatibilizer in Polymer Solar Cells”, International Journal of Polymeric Materials, 63(2014)302-309
 Narumon Seeponkai, Nopparat Keaitsirisart, Jatuphorn Wootthikanokkhan,* Chanchana Thanachayanont, Surawut Chuangchote, Fullerene Functionalized Polystyrene: Synthesis, Characterizations and Application in Bulk Heterojunction Polymer Solar Cells, International Journal of Polymeric Materials and Polymeric Biomaterials, 63(2014)33-40
 Jatuphorn Wootthikanokkhan, Phapada Khunsriya, Narumon Seeponkai, Udom Asawapirom and Anusit Keawprajak Thermal Behavior and Photovoltaic Performance of Fullerene Grafted Dehydrochlorinated Poly(vinyl chloride) in Bulk Heterojunction Solar Cells, International Journal of Polymeric Materials, 64(2015)302-399
 Chanitpa Khantha, Teantong Chonsut, Anusit Keawprajak, Pisist Kumnorkaew, Jatuphorn Wootthikanokkhan, Enhanced Performance of Bulk Heterojunction Solar Cells Using Double layers Deposition of Polymer:Fullerenes Derivatives, Synthetic Metals 207(2015), pp. 72-78 DOI: 10.1016/j.synthmet.2015.06.002.