Rapid Directed Sintering of CdTe/CdS Thin Film Solar Cells Using Intense Pulsed Light

Abstract

Traditional printing of low cost thin film materials for solar energy applications would transform the power generation landscape. Thin film PVs have been touted as a low cost alternative to the dominant crystalline silicon (c-Si) technology. Goodrich, et al. estimated the costs of c-Si modules in 2010 to be $2.25 per watt (/W) compared to $0.76/W for Cadmium Telluride (CdTe) thin film technology. In the past two years, the costs of c-Si modules have dropped well below $1/W due to surplus production and razor thin and often negative margins from manufacturers. These cost pressures have driven a number of thin film manufacturers from the market. Although the materials costs of thin film technologies are low, the high heat, high vacuum, and specialty gas techniques are energy and capital intensive processes that drive up costs. A big challenge for thin film PV technologies is to drastically reduce the manufacturing and capital expenditures as well as installed costs to aggressively compete with the falling costs of c-Si. Therefore, printing thin films at near-atmospheric conditions has a high likelihood of driving down production costs. This will require innovative low temperature techniques that can be adapted into current production schemes.In this work we have made nanoparticle inks of CdS and CdTe that are deposited using traditional printing techniques. The inks are then exposed to extremely short pulses (~ 1 ms) of high energy light, which is absorbed by the semiconducting nanoparticles and released almost instantaneously as heat. The temperature of the films can exceed several hundred degrees celcius for very brief moments precipitating changes in the morphology of the films. This Intense Pulsed Light (IPL) process can be accomplished over an area of nearly 60 cm2 and uses a very broad spectrum of light, making this process simpler and less expensive to scale than a laser source. The deposition and processing of the thin films is accomplished at atmospheric conditions reducing manufacturing, handling and substrate costs.The IPL process has been utilized quite extensively in the sintering of metals specifically in the area of printed electronics.[1] To date very little work has been done on the processing of semiconductors such as CdS and CdTe. In this work we have studied the IPL process on thin films of CdS and CdTe nanoparticle thin films. This includes an in-depth look at the behavior of the band gap, crystal structure, grain size and defects during the process. Specific analysis included UV-Vis, SRD, SEM, TEM and PL and we also incorporated a finite element model to plot the history of the process.We observed a stark change in the band gap of CdS with increased power per pulse and determined that lower total power was required to affect a significant change in the crystallinity of the films at higher pulse powers (Fig 1).[2] We also observed the growth of CdTe grains during IPL processing that resulted in a pin-hole free film, which also exhibited fewer point defects (Fig 2).[3] Finally a CdTe/CdS solar cell was manufactured and measured. Jha, M.; Dharmadasa, R., Amos, D.; Druffel, T., Low Temperature Production of Conductive Copper Using Intense Pulsed Light Sintering, ACS Applied Materials and Interfaces, 2014, 5(24): p. 13227-13234.Dharmadasa, R.; Druffel, T., Intense Pulsed Light Sintering of CdS Thin Films, Advanced Engineering Materials, 2014, DOI: 10.1002/adem.201400008.Dharmadasa, R.; Lavery, B. W.; Dharmadasa, I. M.; Druffel, T., Light Induced Sintering of Cadmium Telluride Nanoparticle based Thin Films, ACS: Applied Materials and Interfaces, 2014, DOI:10.1021/am500124t.