M.K. Fung

Polymer solar cells based on very narrow-bandgap polyplatinynes with photocurrents extended into the near-infrared region

The synthesis, characterization and photophysics of some solution-processable intensely coloured polyplatinynes functionalized with the thienopyrazine-thiophene hybrid spacer and their model molecular complexes are described. These metallated polymers possess extremely low bandgaps of 1.47-1.50 eV, which extend towards the near-infrared (NIR) range of the solar spectrum, and represent the lowest optical bandgaps ever reported for metallopolyynes in the literature. Both polymers can be used to fabricate efficient solar cells with power conversion efficiencies (PCEs) of up to 0.63% under air mass (AM1.5) simulated solar illumination, and the possibility of covering the 600-900 nm solar-radiation range to harvest photocurrent has been demonstrated. The influence of the thienyl core as well as its substituent group, on the optical and photovoltaic behavior of these metallopolymers was investigated in detail. The power dependencies of the solar cell parameters (including the short-circuit current density, open-circuit voltage, fill-factor and PCE) were also studied. The present work offers an attractive avenue towards conjugated materials with broad solar absorptions and demonstrates the potential of metallopolyynes for both visible and NIR light power generation.

Influence of the polymer processing conditions on the performance of bulk heterojunction solar cells

We performed a systematic study of the effect of processing conditions on the performance of P3HT:PCBM solar cells. We have investigated the influence of the source material, solution preparation (stirring vs. sonication), additives (such as 1,8-octanedithiol), pre-formation of P3HT nanowires, and annealing on the device performance and/or morphology and phase separation in the active layer. Furthermore, the influence of spin-coating PCBM on top of P3HT and PCBM on top of the P3HT:PCBM layer has been investigated. We found that all of these factors affect the performance of the solar cells, although there are several alternative methods which can result in similar improvements of the performance. We found that the improvement trends for various procedures (additives, PCBM top layer, P3HT nanowires, etc.) are similar and also strongly dependent on the different source materials. The main factor determining in the obtainable efficiency is the solution preparation and the source materials. Obtained results and the implications on further improvement of solar cell performance are discussed in detail.

Effect of fabrication processes on bulk heterojunctions (BHJ) photovoltaic device performance

Organic photovoltaic (OPV) devices have attracted much interest in recent decades because they have a great potential for low cost solar cells. Among different kinds of organic solar cells, conjugated polymer/fullerene bulk heterojunction (BHJ) solar cells have exhibited improvements in the power conversion efficiency (PCE) in recent years. The performance of BHJ solar cells is highly dependent on different fabrication processes. To address this issue, we focus on the dependence of different photovoltaic parameters on the fabrication methods. BHJ solar cells fabricated using platinum metallopolyyne (P1) with a low band gap of 1.85 eV as an electron donor and phenyl-C61-butyric acid methyl ester (PCBM) as an electron acceptor have been studied. The fabrication parameters, such as ratios of P1 to PCBM, solvents used, thickness of the active layers and top contact materials, have been systematically investigated. Blend ratio and solvent used had most significant influence on photovoltaic performance with several times higher efficiency of the best condition compared to the worst condition. They affected all photovoltaic parameters [open circuit voltage (Voc), short circuit current density (Jsc) and the fill factor (FF)]. Top contact materials affected the Voc and the FF, while thickness of the active layer mainly affected the Jsc and FF. The influence of different fabrication conditions on photovoltaic performance has been discussed.