D. Rose

Fabrication procedures and process sensitivities for CdS/CdTe solar cells

This paper details the laboratory processes used to fabricate CdS/CdTe solar cells at the National Renewable Energy Laboratory. The basic fabrication technique includes low-pressure chemical vapor deposited SnO2 , chemical-bath deposited CdS, close-spaced sublimated CdTe, solution-CdCl2 treatment, and an acid-contact etch, followed by application of a doped-graphite paste. This paper also describes the results of a reproducibility study in which cells were produced by multiple operators with an average AM1·5 efficiency of 12·6%. And finally, this paper discusses process sensitivities and alternative cell fabrication procedures and reports the fabrication of a cell with an AM1·5 efficiency of 15·4%. Copyright

Chemical reactivity of CdCl2 wet-deposited on CdTe films studied by X-ray photoelectron spectroscopy

The authors use X-ray photoelectron spectroscopy to investigate the chemical reactivity of CdTe films exposed to a solution of CdCl2 dissolved in methanol. They show that annealing in vacuum does not instigate a chemical reaction between CdCl2 and CdTe, but that annealing in either He:O2 or pure He leads to the formation of a surface Cl residue comprising Cd, Te, Cl, and O in the form of oxides and oxychlorides. From detailed analysis of X-ray photoelectron data, they propose that CdO, TeO2, TeCl2O are building blocks for the surface Cl residue.

Intermixing at the CdS/CdTe Interface and its Effect on Device Performance

A study of the CdS/CdTe interface was performed on glass/SnO 2 /CdS/CdTe device structures. CdS layers were deposited by chemical solution growth to a thickness of 80–100 nm, and CdTe was deposited by close-spaced sublimation at substrate temperatures of 500°, 550°, and 600°C. Post-deposition CdCl 2 heat treatment was performed at 400°C. Samples were analyzed by optical spectroscopy, secondary ion mass spectrometry (SIMS), spectral response, and current-voltage measurements. SIMS analysis shows that the intermixing of CdS and CdTe is a function of substrate temperature and post-deposition CdCl 2 heat treatment. The degree of intermixing increases with increases in substrate temperature and the intensity of CdCl 2 heat treatment. Optical analysis and X-Ray diffraction data show that the phases of CdS x Te 1-x are also a function of the same parameters. Formation of a Te-rich CdS x Te 1-x alloy is favored for films deposited at higher substrate temperatures. Spectral response of the devices is affected by the degree of alloying at the interface. The degree of alloying is indicated by simultaneous changes in long wavelength response (due to the formation of lower bandgap intermixed CdS x Te 1-x ) and the short wavelength response (due to the change in CdS thickness). Device performance is heavily influenced by alloying at the interface. With optimized intermixing, improvements in V oc , and diode quality factors are observed in the resulting devices.

Influence of CdS/CdTe interface properties on the device properties

In this paper, the authors have focused on the formation and the role of the CdS/CdTe interface on CdTe solar cells. The devices were made using chemical bath deposited (CBD) CdS on SnO/sub 2//glass substrates and the CdTe was deposited by close spaced sublimation (CSS) and subsequently CdCl/sub 2/ treated and annealed. Compositional analysis showed considerable interdiffusion of Te and S as well as Cl accumulation at the interface. Micro-photoluminescence (PL) analysis reveals sulfur accumulation at the grain boundaries and a graded CdS/sub x/Te/sub 1-x/ alloy at the interface. Their analysis leads them to conclude that Cl accumulation and anion vacancies result in a one sided n/sup +/-p junction. This model could explain the collection loss in the CdS layer, seen in the spectral response of CdS/CdTe devices.

The effect of high-resistance SnO2 on CdS/CdTe device performance

In this paper, we have studied the effect of high-resistance SnO2 buffer layers, deposited by low-pressure chemical-vapor deposition, on CdS/CdTe device performance. Our results indicate that when CdS/CdTe devices have a very thin layer of CdS or no CdS at all, the i-SnO2 buffer layer helps to increase device efficiency. When the CdS layer is thicker than 600{angstrom}, the device performance is dominated by CdS thickness, not the i-SnO2 layer. If a very thin CdS layer is to be used to enhance device performance, we conclude that a better SnO2 buffer layer is needed.

Evaporated Te on CdTe: A vacuum-compatible approach to making back contact to CdTe solar cell devices

A commonly used process for forming low-resistance contacts to thin-film p-type CdTe involves the formation of a Te layer by etching the CdTe film in a concentrated mixture of nitric and phosphoric acids. The authors compare evaporated Te back contacts with ‘control’ back contacts formed by the usual etching process, and demonstrate that evaporating Te onto a CdTe thin film is a viable process for forming a low-resistance contact. The best efficiency achieved for a CdTe solar cell made with an evaporated Te back contact is 12.1%, whereas the efficiency of the device made with the control back contact was 11.9%. The evaporation process offers numerous advantages over acid etching, most notably vacuum compatibility amenable to large-scale production of CdTe solar cell modules.

Effects of Cu from ZnTe:Cu contacts in CdS/CdTe cells

CdS/CdTe devices processed with ZnTe:Cu/Ti back contacts are studied as a function of contact deposition temperature between /spl sim/200/spl deg/C and /spl sim/400/spl deg/C. Both the open-circuit voltage (V/sub oc/) and fill factor (FF) increase with temperature. High-resolution secondary ion mass spectroscopy shows Cu concentration in the CdTe region increases due to CdCl/sub 2/ treatment, whereas Cu concentration in the CdS region increases with deposition temperature. Measurements of specific contact resistance suggest that Cu diffusing from the contact interface significantly increases the specific contact resistance at the ZnTe:Cu/Ti interface, resulting in an interface with dominant resistance loss.

The role of oxygen in CdS/CdTe solar cells deposited by close-spaced sublimation

The presence of oxygen during close-spaced sublimation (CSS) of CdTe has been previously reported to be essential for high-efficiency CdS/CdTe solar cells because it increases the acceptor density in the absorber. The authors find that the presence of oxygen during CSS increases the nucleation site density of CdTe, thus decreasing pinhole density and grain size. Photoluminescence showed that oxygen decreases material quality in the bulk of the CdTe film, but positively impacts the critical CdS/CdTe interface. Through device characterization, they were unable to verify an increase in acceptor density with increased oxygen. These results, along with the achievement of high-efficiency solar cells (13% AM1.5) without the use of oxygen, led them to conclude that the use of oxygen during CSS deposition of CdTe can be useful but is not essential.

Tin oxide stability effects—their identification, dependence on processing and impacts on CdTe/CdS solar cell performance

High efficiency polycrystalline thin film CdTe solar cells involve the growth of CdTe films on CdS/SnO2/glass substrates. The CdS layer in such a structure is commonly reported to benefit from a brief hydrogen anneal prior to the deposition of the CdTe film. In this paper, we show that the SnO2 layer can be susceptible to reduction in H2 and that the degree of susceptibility is dependent on the type of SnO2 used. Chemical vapor deposited (CVD) SnO2/glass substrates (Solarex Corp.) show the most resistance to reduction while room-temperature sputtered SnO2 films show the least resistance. When annealed under reducing conditions, Sn from the SnO2 reacts with S-containing impurities and oxygen in as-grown chemical bath deposited (CBD) CdS films to form SnS. Cd-containing impurities are more volatile resulting in a loss of Cd relative to S in films annealed in H2. These films appear dark due to the presence of SnS, a grayish-black impurity, in the CdS and possibly SnO in the SnO2. In normal CSS CdTe depositio...

Microscopic Analysis of Residuals on Polycrystalline CdTe Following Wet CdCl 2 Treatment

In this study we report on the spatial distribution and composition of residuals on the CdTe surface following a typical wet CdCl 2 treatment, and the effect that our ion-beam milling has on this residual-coated surface. Results show that residuals are spatially discrete, located primarily along grain boundaries, and are likely a cadmium oxychloride. Results also show that the residuals may penetrate deep into the CdTe surface such that typical ion-beam milling procedures do not produce complete residual removal.