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Selenium heterostructure solar cells
Solar Cells, 1 ( 1 9 7 9 / 8 0 ) 431 - 4 3 3
431
© Elsevier S e q u o i a S.A., L a u s a n n e - - P r i n t e d in t h e N e t h e r l a n d s
Short Communication
Selenium heterostructure solar cells
R. F. S H A W * a n d A M A L K. G H O S H
Exxon Research and Engineering Company, Linden, N.J. 07036 (U.S.A.) ( Received
The fabrication o f improved selenium solar cells with an exposed area efficiency o f a b o u t 3.72% and a~ engineering efficiency of 3.04% is reported. It is projected that engineering efficiencies o f better than 10% can be achieved with these selenium devices. The present cells are easy to fabricate and stable and are therefore candidates for low cost cens. Selenium devices have played an important role in the technology of rectifiers, light meters and solar cells [1]. Loferski [2] estimates the maxim u m theoretical efficiency o f these cens to be between 11 and 19% for air mass zero (AM0). Preston's [3] work in the early fifties was the first attempt to explain the performance of the device. Sputtered CdO on selenium gave the best results. A maximum open-circuit voltage of 0.5 V and a quantum efficiency o f 70% were reported. The cell could be called a heterostructure device in t o d a y ' s terminology. The solar cell we report is fabricated in the following manner. A semipolished iron substrate previously coated with 500 £ of tenurium is heated to 250 °C. Elemental selenium is fused on the substrate and allowed to cool to 185 °C. The selenium is spread with a d o c t o r blade to form a uniform film and is cooled to 120 °C in 3 min. The temperature is then raised to 150 °C which effects complete crystallization of the selenium. The p-type selenium thus formed has a thickness o f a b o u t 8 pm. CdSe and CdO layers are then formed in one process step b y reactively sputtering cadmium metal in air at a pressure o f 10 -2 Tort (1.3 Pa) for 18 min at an r.f. p o w e r density of 0.5 W cm-2. Under these conditions a thin n-type layer contiguous with the selenium layer is deposited and thereafter, with continuous deposition, an n+-type layer is formed. The combined n and n ÷ layers are a b o u t 1500 )k thick and form the heterojunction. In another set of experiments CdSe was intentionally deposited before the CdO. No difference in performance was observed for the t w o cens. A gold current-collecting electrode is then deposited using standard photoresist masking techniques.
* P r e s e n t a d d r e s s : E x x o n E n t e r p r i s e s Inc.
432 lO0
!
z
-
7-
g
>-
5O
_~
/
400
500
600
WAVELENGTH
700 (nm)
Fig. 1. T h e s p e c t r a l r e s p o n s e c u r v e o f a s e l e n i u m s o l a r cell a n d t h e t r a n s m i s s i o n o f a 1000 A CdO thin film.
A typical photovoltaic cell produced by either of these techniques has an open-circuit voltage of 0.74, a short-circuit current of 8 mA c m - 2 and a fill factor o f 0.49 with a sunlight irradiance of 95 mW cm -2, thus giving an efficiency o f 3.05%. The dark I - V curves for the cell can be described by a regular diode equation with diode constant around 2.5. The reverse saturation current Jo is about 10 -s A cm -2. From analysis of the short-circuit photocurrent spectra we conclude t h a t the major source of the short-circuit photocurrent is the barrier within the selenium with some minor contributions from some u n k n o w n source mainly at long wavelengths. The response at short wavelengths is reduced because of the absorption edge of CdO. This is shown in Fig. 1. In ordinary solar cells such as those from crystalline silicon and GaAs, not all carriers are generated in the barrier region. Some are also generated in the bulk and then diffuse in the absence o f a field to the junction. However, in our selenium devices very few carriers generated in the bulk reach the junction because of short diffusion lengths. Although the predominant carrier generation process is due to absorption within the space charge region, the open-circuit photovoltage cannot be explained by the band bending in selenium alone. The diffusion potential estimated from C - V measurements is smaller than the open~ircuit photovoltage, in contrast with ordinary solar cells. For example, diffusion potentials of between 0.5 and 0.6 V have been estimated from C - V measurements on cells with open-circuit photovoltages of from 0.7 to 0.84 V. One possible explanation for the observed anomaly is t h a t there is a second barrier in the device which is not being probed by our
433
C - V measurements. This barrier m a y be in the same direction as the principal barrier b u t with a much larger capacitance. The fill factor cannot be accounted for b y the series resistance alone. The barrier width decreases with incident light intensity. Since the barrier is the photosensitive region, we can no longer consider the device to be a constantcurrent generator. Accordingly, this barrier shrinkage could affect the fill factor. With our present limited understanding o f device operation we estimate that an efficiency of over 10% for AM1 should be possible with such a selenium device. To achieve this level of performance, however, will probably require significant improvements in current generation from outside the barrier region.
1 G. Dietzel, P. Gorlich and A. Krohs, JanaerJahrb., 1 (1958) 203. 2 J. Loferski, Solar Cells, National Academy of Sciences, Washington, D.C., 1972, p. 48. 3 J. S. Preston, Proc. R. Soc. London, Set. A, 202 (1950) 449.
431
© Elsevier S e q u o i a S.A., L a u s a n n e - - P r i n t e d in t h e N e t h e r l a n d s
Short Communication
Selenium heterostructure solar cells
R. F. S H A W * a n d A M A L K. G H O S H
Exxon Research and Engineering Company, Linden, N.J. 07036 (U.S.A.) ( Received
The fabrication o f improved selenium solar cells with an exposed area efficiency o f a b o u t 3.72% and a~ engineering efficiency of 3.04% is reported. It is projected that engineering efficiencies o f better than 10% can be achieved with these selenium devices. The present cells are easy to fabricate and stable and are therefore candidates for low cost cens. Selenium devices have played an important role in the technology of rectifiers, light meters and solar cells [1]. Loferski [2] estimates the maxim u m theoretical efficiency o f these cens to be between 11 and 19% for air mass zero (AM0). Preston's [3] work in the early fifties was the first attempt to explain the performance of the device. Sputtered CdO on selenium gave the best results. A maximum open-circuit voltage of 0.5 V and a quantum efficiency o f 70% were reported. The cell could be called a heterostructure device in t o d a y ' s terminology. The solar cell we report is fabricated in the following manner. A semipolished iron substrate previously coated with 500 £ of tenurium is heated to 250 °C. Elemental selenium is fused on the substrate and allowed to cool to 185 °C. The selenium is spread with a d o c t o r blade to form a uniform film and is cooled to 120 °C in 3 min. The temperature is then raised to 150 °C which effects complete crystallization of the selenium. The p-type selenium thus formed has a thickness o f a b o u t 8 pm. CdSe and CdO layers are then formed in one process step b y reactively sputtering cadmium metal in air at a pressure o f 10 -2 Tort (1.3 Pa) for 18 min at an r.f. p o w e r density of 0.5 W cm-2. Under these conditions a thin n-type layer contiguous with the selenium layer is deposited and thereafter, with continuous deposition, an n+-type layer is formed. The combined n and n ÷ layers are a b o u t 1500 )k thick and form the heterojunction. In another set of experiments CdSe was intentionally deposited before the CdO. No difference in performance was observed for the t w o cens. A gold current-collecting electrode is then deposited using standard photoresist masking techniques.
* P r e s e n t a d d r e s s : E x x o n E n t e r p r i s e s Inc.
432 lO0
!
z
-
7-
g
>-
5O
_~
/
400
500
600
WAVELENGTH
700 (nm)
Fig. 1. T h e s p e c t r a l r e s p o n s e c u r v e o f a s e l e n i u m s o l a r cell a n d t h e t r a n s m i s s i o n o f a 1000 A CdO thin film.
A typical photovoltaic cell produced by either of these techniques has an open-circuit voltage of 0.74, a short-circuit current of 8 mA c m - 2 and a fill factor o f 0.49 with a sunlight irradiance of 95 mW cm -2, thus giving an efficiency o f 3.05%. The dark I - V curves for the cell can be described by a regular diode equation with diode constant around 2.5. The reverse saturation current Jo is about 10 -s A cm -2. From analysis of the short-circuit photocurrent spectra we conclude t h a t the major source of the short-circuit photocurrent is the barrier within the selenium with some minor contributions from some u n k n o w n source mainly at long wavelengths. The response at short wavelengths is reduced because of the absorption edge of CdO. This is shown in Fig. 1. In ordinary solar cells such as those from crystalline silicon and GaAs, not all carriers are generated in the barrier region. Some are also generated in the bulk and then diffuse in the absence o f a field to the junction. However, in our selenium devices very few carriers generated in the bulk reach the junction because of short diffusion lengths. Although the predominant carrier generation process is due to absorption within the space charge region, the open-circuit photovoltage cannot be explained by the band bending in selenium alone. The diffusion potential estimated from C - V measurements is smaller than the open~ircuit photovoltage, in contrast with ordinary solar cells. For example, diffusion potentials of between 0.5 and 0.6 V have been estimated from C - V measurements on cells with open-circuit photovoltages of from 0.7 to 0.84 V. One possible explanation for the observed anomaly is t h a t there is a second barrier in the device which is not being probed by our
433
C - V measurements. This barrier m a y be in the same direction as the principal barrier b u t with a much larger capacitance. The fill factor cannot be accounted for b y the series resistance alone. The barrier width decreases with incident light intensity. Since the barrier is the photosensitive region, we can no longer consider the device to be a constantcurrent generator. Accordingly, this barrier shrinkage could affect the fill factor. With our present limited understanding o f device operation we estimate that an efficiency of over 10% for AM1 should be possible with such a selenium device. To achieve this level of performance, however, will probably require significant improvements in current generation from outside the barrier region.
1 G. Dietzel, P. Gorlich and A. Krohs, JanaerJahrb., 1 (1958) 203. 2 J. Loferski, Solar Cells, National Academy of Sciences, Washington, D.C., 1972, p. 48. 3 J. S. Preston, Proc. R. Soc. London, Set. A, 202 (1950) 449.