CIGSS Thin Film Solar Cells

Year 4, Quarter 2 Report

Report no. FSEC-CR1513-05

April 18, 2005

Prepared for

National Renewable Energy Laboratory

######## 1617 Cole Boulevard

## Golden, CO 80401

## Submitted by

Neelkanth G. Dhere

Florida Solar Energy Center^®^

## 1679 Clearlake Road 

## Cocoa, FL 32922-5703

# 

![](media/image1.wmf){width="1.2in" height="1.1027777777777779in"}

**CIGSS Thin Film Solar Cells**

###### NREL contract no. NDJ-2-30630-03, UCF/FSEC Account no. 2012 6017

Year 4, Quarter 2 Report no. FSEC-CR-1513-05, April 18, 2005

**Rapid Thermal Processing**

A Rapid Thermal Processing (RTP) unit has been designed, constructed and
installed for preparation of CIGSS thin films on 10 cm x 15 cm
substrates by selenization/ sulfurization of elemental precursors using
the vacuum deposited selenium layer and N~2~:H~2~S atmosphere. For RTP,
selenium evaporation will be carried out by thermal evaporation in a
separate setup. An optimized, minute amount of NaF will be added to
improve the morphological and electrical properties of the absorber.

The configuration and schematic are shown in Fig. 1 and 2 respectively.
A quartz tube (ID = 15 cm) is mounted with a stainless steel flange
assembly. Feedthroughs are mounted on the end flange for connection to a
thermocouple which would monitor temperature. Viton rubber 'O' ring in a
groove is used to form an air-tight seal between the hard surfaces of
the flange and glass tube. The flange has two openings -- a gas inlet
(for H~2~S and Se vapors) and an exhaust. A pair of high density
infrared heaters (Research Inc., Model 5090), each with a bank of 5
T3-style quartz infrared halogen lamps directs energy onto the reaction
tube where the substrate is kept. Each lamp is rated at 1200 Watts. The
power to the lamps is controlled by a pair of single-phase (120V AC, 55
A) standalone SCR power controls (Research Inc., Model 5620). Each
T3-style quartz halogen lamp can provide an upward ramp of 100 ^0^C per
2 seconds. 90% of the radiant energy is radiated in less than 1 second.

The two heater arrays have to be controlled independently so that the
thermal gradient across the thickness of the substrate remains
symmetrical during all RTP processing phases. This would prevent the
substrate from breaking and cracking. A supporting tray prevents the
viscous substrate from sagging when the maximum process

Fig. 1: RTP System Configuration

.

Fig. 2: RTP Schematic

temperature crosses the transition point of soda lime glass substrate.
Each heater consists of a specular aluminum reflector that directs the
infrared energy supplied by the lamps on to the substrate. Connections
to supply required cooling water to the heater have been provided. The
heaters were mounted on a stand so as to face both the surfaces of the
substrate. Two aluminum sheets with 95% reflectivity and measuring
1'x1'x0.02" each were formed into arcs and installed laterally on both
sides of the quartz tube to reflect the heat back to the substrate
during RTP. Circular reflective stainless steel sheets were placed
within the quartz tube at both ends. A sliding mechanism has been
provided to quickly move the heaters away from the reaction tube after
the completion of the process for rapid cooling.

Experiments were carried out in order to determine the rate at which
substrate temperatures would rise with the current setup. A mechanical
pump was used to achieve a vacuum in the range of 10^-3^ Torr within the
quartz tube. Cool water was circulated through each of the two infrared
heaters at a rate of about 0.4 gallon per minute per heater. The two
power controllers for the infrared lamp arrays were switched to
'operate' mode. The power was raised to 100%. The temperature of the
substrate was measured by means of a K-type chromel-alumel thermocouple.
An initial temperature ramp of \>6 ^o^C per second and an average
temperature ramp of 3.96 ^o^C per second were obtained so that the
temperature increase from 25 ^o^C to 500 ^o^C could be attained in 120
seconds at 100% power (Figure 3). The control power was slowly reduced
and the power supply was switched off after reaching the temperature of
500 °C.

The next set of experiments was carried out to determine the fixed power
to the two heaters for reaching and maintaining temperature of 500 ^o^C.
Initially, the power was kept at 30% of the maximum. The temperature
rose to 430 ^o^C in 48 minutes and then remained stable. The power was
then increased to 40%. Temperature rose to 500 ^o^C in 2 minutes with a
tendency to rise further. At this stage the power was switched off
(Figure 4). In the subsequent experiment, it was found that when total
lamp power was fixed at 35%, the substrate temperature rose to 500 ^o^C
in 52 minutes and remained stable (Figure 5).

The following graphs show ramp rates for various power settings:

![](media/image2.wmf){width="5.15625in" height="2.9583333333333335in"}

Fig. 3. Plot of Temperature vs. time at 100% power

![](media/image3.wmf){width="5.385416666666667in"
height="3.7916666666666665in"}

Fig. 4. Plot of temperature vs. time for 30%, 40% and 0% power

![](media/image4.wmf){width="5.666666666666667in"
height="2.8020833333333335in"}

Fig. 5. Plot of temperature vs. time for power fixed at 35% (constant)

![](media/image5.jpeg){width="3.372916666666667in"
height="2.6590277777777778in"}

Fig. 6: RTP Setup at FSEC PV Materials Lab

**Selenization/Sulfurization Furnace:**

Three new temperature controllers from Eurotherm were procured.
Necessary enclosure was procured and all three controllers along with a
over temperature controller have been properly installed in the
enclosure while the enclosure itself has been installed securely beneath
the furnace. The controllers were connected to the
selenization/sulfurization furnace and are being used to control the
heat treatment process for the development of CIGSS thin films on glass
and stainless steel substrates. The controllers were programmed with
required parameters and initially tested for time/temperature readings.
The time/temperature control ability and ramp has been improved using
these controllers.

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