All-polymer bulk heterojunction solar cells with high fill factors based on blends of poly-3-hexylthiophene: poly(peryle
- PDF / 595,290 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 43 Downloads / 134 Views
All-polymer bulk heterojunction solar cells with high fill factors based on blends of poly-3hexylthiophene: poly(perylene diimide-alt-terthiophene) Dariusz Kotowski, Erika Kozma, Marinella Catellani and Silvia Luzzati Istituto per lo Studio delle Macromolecole - CNR, via Bassini 15, 20133 Milano, Italy ABSTRACT The photovoltaic characteristics of all polymer bulk heterojunction solar cells made of P3HT and a perylene diimide-based copolymer (PEK3) have been studied. Thermal annealing is needed to improve the performances. Annealing optimization induces an enhancement of the power conversion efficiency from 0.06 to 1%, Jsc from 0.24 to 2.9 mA/cm2 and FF from 0.32 to 0.59. The origin of such improvements has been investigated by studying the P3HT:PEK3 blend morphology, by means of absorption and emission spectroscopy and charge transport, from single carrier measurements on P3HT:PEK3 diodes. Upon annealing we have observed an increase in phase segregation and a 100-fold enhancement of the hole and electron mobilities, that favor the dissociation of bound electron-hole pairs and their transport to the electrodes. This explains the high FF of the annealed P3HT:PEK3 solar cell. INTRODUCTION Polymeric bulk heterojunction solar cells, PSCs, generally consist in a blend between a donor, hole-conducting, light absorbing conjugated polymer and an acceptor, electron-conducting material. The most successful acceptors to date are soluble fullerene derivatives that are characterised by high electron affinity and good electron mobilities. The replacement of fullerene by a polymer acceptor has raised the attention due to the potential advantages connected to the easier electronic properties tailoring and the good light absorption of conjugated polymer materials respect to fullerenes. Several combinations of donor and acceptor polymers have been tested as active materials in bulk heterojunction solar cells, and promising results have been obtained with efficiencies up to 2% [1-4]. Unfortunately, PSC performances are relatively poor as compared to those of fullerene-based devices. One of the main reasons limiting the efficiencies of the all-polymer solar cells is the low fill factor (FF), often non exceeding values of about 0,4. Such low FFs are commonly associated to poor electron-hole dissociation, poor or to unbalanced transport of the electrons and holes in the polymer:polymer blends [5]. A factor limiting the device performances of PSC is the difficulty in designing n-type polymers with good accepting properties and good electron mobilities to compete with fullerene. Another aspect that contributes in lowering the efficiency of all-polymer solar cells is the strong tendency to phase segregation of polymer:polymer blends [1]. The challenge to prepare efficient PSC consist in finding a pair of donor and acceptor polymers to attain a good miscibility for efficient exciton dissociation, and the right phase segregation to ensure good electron-hole separation and transport to the electrodes.
Recently, perylene diimide-based acceptor
Data Loading...
