실리콘 와이어 태양전지 및 태양광/열전 융합소자 연구

Abstract

The optimal hybridization of photovoltaic (PV) and thermoelectric (TE) devices has long been considered ideal for the efficient harnessing solar energy. Our hybrid approach uses full spectrum solar energy via lossless coupling between PV and TE devices while collecting waste energy from thermalization and transmission losses from PV devices. Achieving lossless coupling makes the power output from the hybrid device equal to the sum of the maximum power outputs produced separately from individual PV and TE devices. TE devices need to have low internal resistances enough to convey photo-generated currents without sacrificing the PV fill factor. Concomitantly, a large number of p-n legs are preferred to drive a high Seebeck voltage in TE. Our simple method of attaching a TE device to a PV device has greatly improved the conversion efficiency and power output of the PV device (~30% at a 15 ºC temperature gradient across a TE device).

Photoelectrochemical (PEC) researches for hydrogen evolution have continually been strived to reduce overpotentials required for cleaving water molecules. To drive the spontaneous water splitting reaction, required energy is greater than 2.03 eV which includes Gibbs free energy for chemical reaction and loss energy in real system. We show here that the overpotentials in PEC cells are readily offset by thermovoltages generated from a temperature gradient (T) across the thermoelectric (TE) device. To convert into chemical and electrical energies, our hybrid system, consisting of a PEC cell electrically coupled in series by a TE device, is able to utilize a full solar spectrum for collecting both photon and phonon energies. By only harnessing solar energy, a photon-to-current efficiency of ~22% has been recorded, at T=18.1 °C, using a Si photocathode of a bias-free condition.

Si wire solar cells have received a great deal of attention for their potential use in next-generation photovoltaics due to their superior optical absorptance and lower silicon requirements. Although optimization of the structural parameters such as the areal wire density and the wire length have mostly focused on enhancing the light absorptance via optical impedance matching using subwavelength nanostructures,1-5 reliable electrical characterizations of the photovoltaic performances of wire solar cells have been constrained due to the difficulties in forming the top electrodes. Robust Ohmic contacts need to be integrated between the top ends of the wires and transparent conducting oxide (TCO), but some studies have reported relatively low cell efficiencies compared to conventional thin film solar cells due to serious degradation of the photovoltaic fill factor (FF).6,7 High fill factors were obtained from a single-wire solar cell (81%)8 or from very small sized cells integrated in either ITO (69.4%)4 or Indium (57%)9 without bus/finger bars. Most recent studies regarding commercially favorable wiring of top metal have commonly used TCO for Ohmic integration and metal grids as the emitter contact. These studies have reported low experimental FF values; e.g., 28% for metallization using bus bars and TCO6 and 30% for metal grids only without TCO7. We demonstrate the wrap-around top metal contacts for radial junction silicon wire solar cells. A thin Ag sheet surrounds the silicon wires close to the planar bottom contacts. A fill factor of 65.4% was obtained, which is higher than that of conventional top contacts made using transparent conductive oxides and metal grids.