Lawrence M. Woods

Next-Generation Thin-Film Photovoltaics

Future spacecraft and high-altitude airship (HAA) solar array technologies will require high array specific power (W/kg), which can be met using thin-film photovoltaics (PV) on lightweight and flexible substrates [1]. Thin-film array technology, with thin-film specific array support structure, begin to exceed the specific power of crystalline multi-junction arrays with thin-film device efficiencies as low as 8.5% [2]. Thin-film PV devices have other advantages in that they are more easily integrated into HAAs, and are projected to be much less costly than their crystalline PV counterparts. Furthermore, it is likely that only thin-film array technology will be able to meet device specific power requirements exceeding 1 kW/kg (photovoltaic and integrated substrate/blanket mass only).

CdTe solar cells: Electronic and morphological properties

CdTe solar cells have been produced with record efficiency and excellent stability from a low cost manufacturing line. Efficiency of 14.7% has been attained by standard manufacturing film deposition processes on soda lime glass. Modules were produced that have 10.5% active area efficiency. Long term testing of modules have shown degradation rates of less than 0.5%/year. These improvements have been linked to changes in CdTe morphology produced by recrystallization time/temperature profiles.

Ag nanowire based transparent conductor for CIGS PV

Coated silver nanowires (AgNW) have been considered as a replacement for transparent conducting oxides (TCOs) in CIGS based photovoltaic devices. The advantages of AgNW over TCOs are discussed, and optical and electrical characteristics of AgNWs on glass are presented. Similarly fabricated AgNWs with varying sheet resistance on CIGS devices were tested against ITO transparent conductor controls. The CIGS was produced using a roll-to-roll technique on a flexible polymer substrate. Variations in the ZnO layer resistivity that are adjacent to the AgNW layer in the CIGS device were also tested. Device results indicate similar Jsc, but a reduced FF for cells made with the AgNWs, and Voc dependence on the resistivity of the coated AgNW and ZnO window layers. FF and Voc losses associated with the use of AgNWs are discussed.

COMPONENTS TOWARD NEXT GENERATION TWO-TERMINAL TANDEM PHOTOVOLTAICS USING THIN-FILM CIS BASED ALLOYS

The development of monolithic tandem devices based on thin-films of high-efficiency and radiation resistant CuInSe2 (CIS)-based alloys is discussed. These next generation devices are anticipated to meet the requirements for high array specific power on future spacecraft and high-altitude airships (HAA). Widebandgap CIS alloy devices with bandgap greater than 1.65 eV are needed for optimum monolithic serial connection with high-efficiency CIS bottom cells. Early development of these wide-bandgap devices indicate that this technology will not be a simple translation from their high-efficiency low-bandgap alloy counterparts, and single junction devices have not yet exceeded 10% efficiency. The development of transparent interconnects and durable low-bandgap CIS bottom cells show some promise, with demonstrated interconnect transparency greater than 71%, and durable low-bandgap bottom cells with full AM1.5 spectrum efficiencies exceeding 8%.

Synthesis and Application of Electronic Oxides for Solar Energy

Plasma-enhanced chemical vapor deposition (PECVD) is being developed as a flexible coating technology for a variety of oxides. In this paper we discuss the synthesis of transparent conducting oxides (TCOs), insulating oxides and electrochromic oxides. Tin oxide was synthesized using mixtures of SnCl 4 and O 2 . By proper control of processing conditions the resistivity of this material may be varied from 10 -3 5 Ω-cm. Films of varying resistivity were employed as buffer layers in CdS/CdTe solar cells. Preliminary device results have demonstrated that integration of a tin oxide buffer layer was very beneficial for cell performance. In addition, we demonstrate the PECVD synthesis of WO 3 from WF 6 /O 2 /H 2 /Ar mixtures. The plasma process space that yielded adherent, transparent tungsten oxide was established. The deposited films were both amorphous and reversibly electrochromic. High temperature annealing above 400°C converted the films into a polycrystalline state.