A 14% efficient nonaqueous semiconductor/liquid junction solar cell
Applied Physics Letters
We describe the most efficient semiconductor/liquid junction solar cell reported to date. Under W-halogen (ELH) illumination, the device is a 14% efficient two-electrode solar cell fabricated from ann-type silicon photoanode in contact with a nonaqueous electrolyte solution. The cell's central feature is an ultrathin electrolyte layer which simultaneously reduces losses which result from electrode polarization, electrolyte light absorption, and electrolyte resistance. The thin electrolyte layer
... also eliminates the need for forced convection of the redox couple and allows for precise control over the amount of water (and other electrolyte impurities) exposed to the semiconductor. After one month of continuous operation under ELH light at 100 mW /cm 2 , which corresponds to the passage of over 70 000 C/cm 2 , thin-layer cells retained over 90% of their efficiency. In addition, when made with Wacker Silso cast polycrystalline Si, cells yield an efficiency of9.8% under simulated AMI illumination. The thin-layer cells employ no external compensation yet surpass their corresponding experimental (three-electrode) predecessors in efficiency. Much attention has been focused on semiconductor/ liquid junction solar cells as an alternative to solid state devices. 1 ' 2 Liquid junction cells offer potential cost advantages over their solid-state counterparts. For example, the processing required to form a diffused junction in a solid-state device is replaced by the simple immersion of the semiconductor in a liquid. In 1978 Heller et a!. reported the first highly efficient ( 12%) liquid junction solar cell. 1 a Since then, Heller and others have shown that several materials can be stabilized in aqueous solvents while simultaneously achieving high efficiencies. 1 b Another approach to limiting photocorrosion has been to employ nonaqueous solvents, J-s but until recently, efficiencies in these solvents were disappointingly low. Many of the early nonaqueous results were ascribed to the presence of electronic states at the semiconductor/liquid interface. 6 However, from the current-voltage characteristics of experimental cells developed utilizing potentiostatic control, it was concluded that (I) in several systems, interface states were not limiting the cell efficiencies, and (2) that high efficiency junctions could be realized by drastically reducing the uncompensated cell resistance. Based on these ideas, studies of passivated systems have demonstrated that nonaqueous solvents can provide media where systematic design of highly efficient, nearly ideal semiconductor/liquid interfaces is possible. 7 -11 Although efficient aqueous two-electrode cells have been demonstrated by Heller and others, the only efficient nonaqueous systems to date have been three-electrode cells which utilize external potentiostatic control and do not necessarily represent practical prototypes. Nonaqueous twoelectrode cell design entails many difficulties including the following: limited stability ofboth forms of the redox couple, excessive absorption oflight by the solution, low solubility of the redox pair, insufficient conductivity of the electrolyte, and the presence of trace amounts of corrosive water. 12 • 13 The thin-layer cell design, depicted in Fig. I , simultaneously circumvents several of these difficulties. A liquid electrolyte is sandwiched between a transparent glass cover, which is coated with a conducting indium-tin-oxide (ITO) film to serve as the counterelectrode. A thin electrical insulator isolates the two electrodes and maintains the desired interelectrode spacing. Incident light passes through the glass and the solution and impinges on the semicondutor surface. The device equations in Fig. 1 indicate that decreases in electrolyte thickness will improve all of the crucial design parameters of the systems. Some of the potential advantages of a thin-layer cell design have been proposed previously, 10 • 13 • 14 but cells which have been constructed have not been efficient. 13 A key feature of our system is that the cell is sufficiently thin to effectively eliminate all of the losses which arise from the solution. Calculation of the relevant variables for typical cell parameters (Table I) reveals that, even in nonaqueous solvents, electrolyte losses can be minimized by using interelectrode spacings of 10-20 f.im. The thin-layer cell design offers advantages even when compared to conventional three-electrode cells which employ an external power supply and electronic feedback to compensate for losses. Series resistance losses are much smaller than uncompensated resistance I glass IT /coL nter electroae I electrolyte ~ semiconductor/working sealant electrode ohmic contact _/ Device equations: J _ 2FDCo L-t A=€tCo I I ~6,! insulator FIG. I. Cross section of a typical thin-layer liquid junction solar cell (not to scale) , and the three equations which govern its operation. J L is the diffusion-limited current density in an electrolyte layer of thickness t. F is Faraday's constant and D, C 0 , and E are the diffusion coefficient, concentration, and molar extinction coefficient of the redox species, respectively. Vd is the ohmic voltage drop in the electrolyte, J is the cell current density, and pis the electrolyte resistivity. 1095 Appl. Phys. Lett. 45 (1 0).