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Recent developments in amorphous silicon solar cells
Solar Energy Materials 3 (1980) 503-518 (~ North-Holland Publishing Company
RECENT DEVELOPMENTS IN AMORPHOUS SILICON SOLAR CELLS* D. E. CARLSON RCA Laboratories, Princeton, NJ 08540, USA Received 21 July 1980 This article reviews recent advances in the development of amorphous silicon solar cells. Both the glow-discharge deposition conditions and the solar-cell structures are discussed in some detail. The performance characteristics of present amorphous silicon cells are described, and the loss mechanisms that limit performance are considered. An effort has been made to point out those areas where further research is needed. Recently, amorphous silicon p-i-n cells with areas of 1.19 cm 2 have been fabricated with conversion efficiencies as high as 6.1~'o.
I. Introduction The present interest in hydrogenated amorphous silicon (a-Si :H) results mainly from its promise as a low-cost photovoltaic material foi terrestrial solar cells. The purpose of this article is to review recent developments in a-Si :H solar-cell technology and to point out those areas that require more research. In section 2, we describe the deposition conditions that have been used to make a-Si :H solar cells, and then we describe the various solar-ceU structures in section 3. Section 4 is a review of the performance history of a-Si :H solar cells while section 5 covers present performance characteristics in some detail. We discuss the loss mechanisms that appear to limit device performance in section 6 and present some thoughts about future directions in the final section. In the previous review paper in this journal, H. Fritzsche discussed the electrical, optical and structural properties of plasma-deposited a-Si :H. In a future issue, W. Paul will review the material and device properties of sputtered a-Si :H. The present article concentrates on solar cells made from the plasma decomposition of silane (Sill4).
2. Deposition conditions There are several types of plasma-decomposition or glow-discharge systems that have been used to fabricate a-Si :H solar cells. The first a-Si :H films were grown in an *Some of the research reported herein was supported by Solar Energy Research Institute, under Contract Number XJ-9-8254, and by RCA Laboratories, Princeton, NJ 08540, USA. 503
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rf inductive system that utilized an external coil surrounding a cylindrical glass discharge chamber [1]. Although this type of system has been used to produce a-Si :H solar cells with conversion efficiencies > 5 ...... .,, at RCA Laboratories, the uniformity, over large areas is poor. Similar device performance [2] has also been observed in cells made in rf systems that operate in a mode similar to diode sputtering, i.e., the rf discharge is sustained by capacitive coupling through electrodes within the discharge chamber [3]. This type of system is widely used since it is commercially available and is capable of depositing uniform films over relatively large areas ( >0.1 m 2 ). Another type of rf system employs a similar geometry but magnetic confinement is imposed on the discharge in a manner similar to that used in magnetron sputtering. Preliminary work at RCA Laboratories indicates that the a-Si:H produced by this type of discharge contains a relatively high concentration of defects, e.g., (SiH2t,, groups, and the best cells to date exhibited conversion efficiencies less than 3".~, 14]. G o o d quality a-Si:H has also been produced in dc proximity glow-discharge systems [5]. In this case, the substrate is located in close proximity of a screened cathode with an anode screen located on the other side of the cathode. In some dc systems, the substrate is the cathode, but generally the film quality is poor as a result of positive ion bombardment during growth [6]. Since the rf diode and the dc proximity systems are the most widely used, we will restrict our detailed discussion of deposition conditions to those systems. G o o d quality a-Si :H has been obtained in rf diode systems by operating at low rf power densities (<0.1 W/cm 2) and in pure Sill 4 discharge atmospheres [7]. Generally, high flow rates ( ~50-200 std. cm 3 min i) are directed at or over the substrate to minimize contaminants outgassing from walls and heaters. Relatively good material has been made over a wide range of pressures ( ~0.02 to 1.0 Torr) and substrate temperatures ( ~200-400 C). Normally, only one electrode is powered and the other is grounded. For a substrate located on the grounded electrode, the deposition rate is on the order of 5 to 20 nm/min. In the dc proximity mode, the power density is also on the order of 0.1 W/cm 2 with the discharge pressure in the range of 0.3 to 0.9 Torr. The spacing between the substrate and cathode screen is on the order of 1 cm with the anode located ~ 1 to 4 cm away from the cathode. The applied voltage is typically 650 to 800 V for pure Sill4 discharges. The flow rates and substrate temperatures are comparable to those used in rf diode systems;deposition rates are ~20-30 nm/min. While it is evident that the material quality is strongly influenced by substrate temperature and discharge power, it is not clear how important other deposition parameters are. There is some evidence that contaminants such as oxygen and nitrogen can adversely affect device quality [8-10], but much more research should be done in this area. Moreover, there is relatively little information available about the discharge kinetics or the discharge chemistry. Recent mass spectroscopy work [11-13] indicates that species such as SiH~- and SiH~- are relatively abundant in Sill4 discharges and that higher silanes (SixHy groups where x >2) are created in varying amounts depending on deposition conditions. However, there is no concensus regarding which discharge species are desirable in growing good quality a-Si :H films. Even less is
D. E. Carlson / Amorphous silicon solar cells
505
known about the chemistry that is occurring at the surface of the growing a-Si :H film, but it appears that significant reactions involving bond breaking and restructuring must be occurring since the a-Si :H films contain relatively little hydrogen compared to that in the discharge species incident on the surface.
3. Solar cell structures Most a-Si :H solar cells have been fabricated in one of three structures : Schottkybarrier, MIS or p-i-n. The Schottky-barrier cell is used mainly as a diagnostic device since it is relatively easy to fabricate. However, the open-circuit voltage of Schottkybarrier cells is limited to values less than ~600 mV, so high performance is not possible with this type of cell. One can fabricate a simple Schottky-barrier cell by depositing undoped a-Si :H on Mo and then evaporating ~5 nm of Pd onto the top of the a-Si :H film. The photovoltaic and diode characteristics can be improved by [t-- 11=t- ( ~ S n m {0)
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first depositing a thin layer ( ~20 nml of phosphorus-doped a-Si :H from a discharge containing ~1 vol°~,~,PH 3 in Sill4. To date, conversion efficiencies greater than 5% have been obtained only with the three structures shown in fig. 1. The MIS (metal-insulators-semiconductor) cell is similar to a Schottky-barrier structure, but a thin insulating layer ( ~2-3 nmi is located between the undoped a-Si :H and a high work function metal such as Pt or Ir (see fig. la). In order to assure good optical coupling into the cell, the metal film must be thin t ~5 nm) and also must be overcoated with an antireflection layer such as ~45 nm o f Z r O 2 [14]. Figs. lb and c show two p-i-n structures that have exhibited good performance. The first is fabricated by depositing a boron-doped layer ( ~20 nm) on steel from a discharge in ~ 1 vol0/o B2H 6 in Sill 4. The undoped layer is ~0.5 lzm thick and the top phosphorus-doped layer is ~8 nm thick. Finally, ~70 nm of indium-tin oxide (ITO) is deposited by electron beam evaporation on the n layer to act as both a top contact and an antireflection layer [15]. The p-i-n structure in fig lc is illuminated through the glass substrate which is coated with ~60 nm of ITO and ~10 nm of a cermet such as Pt-SiO2 [16]. The cermet helps assure a good contact to the thin p layer ( ~8 nm). The undoped layer is ~0.643.8 #m thick and the n layer is ~20 nm thick. The back contact is formed by sputter-deposition of Ti or Ti/A11 ~ 100 nmt. It is apparent from the above descriptions that a-Si :H solar cells are truly thin film devices since the thickness of the total structure (apart from the substrate) is less than 1 ltm in each case.
4. Performance history of a-Si:H solar cells
In fig. 2, we show the performance history of a-Si :H solar cells fabricated in the Schottky-barrier and MIS configurations. Data points that are not labeled with reference numbers refer to devices fabricated at RCA Laboratories. As shown in fig. 2, the conversion efficiency of small-area devices improved rapidly in the first few years of development [14]. However, these devices degraded slowly upon exposure to air apparently as a result of water vapor interacting with the interfacial layer between the metal and the a-Si :H [6, 12]. Although the performance of largearea Schottky-barrier and MIS cells is improving, the rate of progress is relatively slow (see fig. 2). Since the junction in these structures is formed by a thin metal contact (e.g., ~5 nm of Pt), it may be difficult to assure the uniformity needed for the commercial production of solar panels. Moreover, these devices may have to be well-encapsulated to prevent degradation. As shown in fig. 3, the performance of p-i-n cells increased only gradually in the first few years of development. However, in the last year, the efficiency of large area ( > 1 cm 2) p-i-n devices has improved significantly. This improvement resulted from a systematic optimization of fabrication parameters such as layer thicknesses, doping levels, discharge power, etc. [15]. Most p-i--n cells have exhibited good shelf life without encapsulation, and it appears to be somewhat easier to make large area
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p-i-n cells than it is to make large area Schottky-barrier or MIS cells• Fig. 4 shows a photograph of a series-connected monolithic a-Si :H solar panel with an active area of 63 cm 2. The panel consists of 17 p - i n cells connected in series, and the conversion efficiency was 3.6~o. This panel was fabricated in an rf discharge system that was making small-area (9.5 mm 2) cells with an efficiency of ~4.0~o at the time [2]. It is interesting to note that while single crystal silicon solar cells are ~ 2 to 3 times more efficient than present a-Si :H cells in sunlight, the a-Si :H cells are comparable to or better than crystalline Si cells under fluorescent light [24]. This is due to the fact that the spectral response of a-Si :H cells is very closely matched to the emission spectrum of a fluorescent light• Sanyo Corporation hopes to take advantage of this property and plans to market a line of consumer products (e.g., watches, calculators) with a-Si :H solar cells as power sources in the near future [24].
5. Present performance characteristics The I - V characteristics of a recent p-i-n cell (1.19 cm 2) under simulated AM1 illumination are shown in fig. 5. The solar simulator was an Oriel, model 6730, with a uniformity of ___3 ~ over an area of 50 cm 2. The simulator is calibrated by first measuring the performance of an a-Si :H cell in sunlight (near AM1 conditions) and
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determining the solar irradiance by means of a NASA-calibrated crystalline Si cell and a pyroheliometer. The a-Si :H cell is then placed in the solar simulator, and the light intensity is adjusted to give the same short-circuit current as measured in sunlight. A small crystalline Si cell is located on the sample holder and is used to maintain the calibration. In the last few months more than one hundred p - i n cells (each 1.19 cm 2) have been fabricated at RCA Laboratories with conversion efficiencies greater than 5.0%. Most of these cells were fabricated in the configuration shown in fig. lb, but a few were made with the structure shown in fig. lc. A record open-circuit voltage (Vo~) of 910 mV has been obtained with the latter structure. However, the best conversion efficiency (6.1%) and the best short-circuit current densities (~,12 mA/cm 2 in AMI light) have been obtained with p - i n cells illuminated through the n-layer (see fig. lb). The best fill factor observed in p-i-n cells is ~0.61 although values as high as 0.674 were observed in earlier work with Schottky-barrier cells. A spectral response curve is shown in fig. 6 for a p-i-n cell illuminated through the n layer. The curve shows a decrease in collection efficiency at short wavelengths due
D. E. Carlson / Amorphous silicon solar cells
509
Fig. 4. Photograph of monolithic a-Si:H solar panel with 17 cells connected in series (total active area =63 cm2).
mainly to absorption losses in the top doped layer and also, a decrease at long wavelengths due to the decreasing absorption coefficient of undoped a-Si :H [25]. The data in fig. 6 were obtained by illuminating the sample with chopped monochromatic light in the presence of constant AM1 illumination and calibrating the short-circuit current for the monochromatic light to give a collection efficiency (number of electron-hole pairs collected per 100 incident photons). This procedure is necessary since the spectral response of a-Si :H solar cells can change significantly with light intensity [26]; the presence of constant AM1 illumination assures that the cell is characterized under normal operating conditions. Also listed in fig. 6 is the short-circuit current density (J~) calculated from the spectral response curve and from measured solar irradiance data for direct sunlight (AMI). Short-circuit current densities measured in sunlight are typically 15-20~o larger than the calculated values due to a contribution from the indirect sunlight (scattered blue light). Therefore, to obtain good agreement between measured and calculated values of J~¢, one must either include the scattered light in the calculation or measure the performance in sunlight with a collimating tube to exclude scattered Light.
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the dark may be associated with relaxation semiconduction [31]. A semiconductor is in the relaxation regime when the dielectric relaxation time is greater than the carrier lifetime (this condition corresponds to p ;~ 108 fl cm for ~ ~ 1 #s). Another characteristic of recent p-i--n cells is that they are fully depleted s a that an electric field ( ~ 2 x 104 V cm -1) extends across the undoped layer under shortcircuit conditions. The fully depleted condition can be confirmed by comparing the spectral response of the cell under zero and reverse bias or by analyzing the wavelength dependence of the I - V characteristics [32]. An important property of a-Si :H solar cells is that they appear to be relatively stable. As mentioned earlier, some Schottky-barrier and MIS cells have exhibited degradation resulting from interactions with water vapor, but encapsulation should prevent this degradation [6]. Processing at elevated temperatures (e.g., > 1 h at 350°C) can also degrade device performance since hydrogen can evolve from a-Si :H films and leave behind dangling bonds [6, 8]. Prolonged exposure to light may change the characteristics of some a-Si :H films and devices [33]. In some cases, the lightinduced changes are caused by surface effects [34], but bulk effects may be occurring in other films [35].
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6. Performance limitations
The present performance of a-Si :H solar cells is limited by several factors. Earl). work on a-Si:H Schottky-barrier cells indicated that a major limitation was the minority carrier diffusion length [14, 36]. Recent studies have indicated that the hole diffusion length ;~n undoped a-Si:H lies in the range of ~0.03 to 0.2 ltm [3~39]. The techniques that have been used to estimate the hole diffusion length include the photoelectromagnetic (PEM) effect [37], analysis of I - V characteristics of p - i n cells under illumination [38], and the measurement of the collection efficiency as a function of the i-layer thickness [39]. In crystalline semiconductors, the minority carrier diffusion length (LI is simply related to the mobility (It) and lifetime (r) by the expression L=
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However, there is evidence that in a-Si:H there is a range of mobilities and lifetimes for the minority carriers depending on their location in the density of state distribution [40]. From PEM measurements, Moore [37] estimates that the hole lifetime in undoped a-Si :H is ~0.34 ILs while the electron lifetime is ~ 1.2 #s. Recent studies of transient diode recovery by Snell et al. [41] indicate hole lifetimes of ~10-20 ~s at current densities of ~ 10 mA/cm 2. Even longer hole lifetimes ( ~ 1 ms) have been indicated by the Xerographic discharge experiments of Mort et al. [42] in very lightly doped a-Si :H (5 vppm B2H6 in Sill4). It is not clear at the present time whether these estimates differ because the materials were different or because the experimental results were not interpreted properly. There is evidence that several types of defects may be responsible for limiting carrier lifetimes. Dangling bonds such as those created by hydrogen outdiffusion can severely reduce the photoluminescence intensity [43] and solar cell efficiency [8]. Recently, Street and Biegelsen [44] have concluded that the dangling bonds are the dominant recombination centers in undoped a-Si:H. There is also evidence that recombination centers are associated with short polymer chains or (Sill2) . groups [45]. Other recombination centers appear to be associated with impurities such as oxygen, nitrogen and phosphorus [8]. Present p-i-n cells with efficiencies of 5 - 6 ' , o typically contain ~102° oxygen atoms cm -3, ~ 5 x 10 ~9 carbon atoms cm 3 ~7 x 1018 nitrogen atoms cm -3 and ~10-14 at~o hydrogen. No (SiHz), groups are detectible in the ir absorption spectra of these films. There is some evidence that geminate recombination is responsible for limiting the transport in undoped a-Si :H [46-48]. This recombination has been observed in other amorphous semiconductors such as a-Se [49], and might be expected in any low mobility material where the photogenerated electron-hole pairs have difficulty escaping their mutual coulombic attraction. However, other recent studies of both a-Si :H [50] and a-Si :F :H [51] suggest that geminate recombination is not occurring in these materials. While some ambiquities exist regarding the recombination mechanisms, it is well-
D. E. Carlson ,; Amorphous silicon solar cells
513
accepted that the space-charge field plays an important role in the operation of a-Si :H solar cells [36]. Recent p-i-n devices are depleted over the entire thickness of the i-layer, ~0.54).7 #m [32, 52]. Crandall [38] has modeled the photovoltaic characteristics of such cells by considering the variation of the drift length within a Mott barrier (i.e., a uniform field exists throughout the undoped layer). He finds that the characteristics of a cell with a fill factor of ~0.60 can be explained with a drift length at zero bias of ~ 3.5 #m ; this corresponds to a hole diffusion length of ~0.2 #m. Thus, one must improve the lifetime (or mobility) to obtain better fill factors. Recent computer modeling of a-Si :H p - i n cells by Swartz [53] indicates that the performance is being severely limited by the quality of the doped layers. There is considerable evidence that doping increases the defect density in a-Si "H. The photoluminescence intensity at low temperatures (~50-100K) decreases [54], the electron spin density increases [55], and solar cell performance decreases [8, 20]. Doping with either BEH 6 or PH 3 creates a distribution of gap states as shown by optical absorption measurements [25], collection efficiency measurements [40] and photoemission measurements [56]. The optimum doping level in p-i-n cells is ~ 1 volvo B2H 6 in Sill 4 for the p layer
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D. E. Carlson ,, Amorphou.~ silicon ~olar cells
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and ~ 1 - 2 vol% P H 3 in Sill 4 for the n layer [14, 15, 20]. At these doping levels, the lifetime of minority carriers appears to be very short, and the spectral response can be modeled assuming that the doped layers are dead and act only as filters [25]. Fig. 7 shows that the collection efficiency at short wavelengths is strongly influenced by the thickness of the top doped layer [57]. Since the absorption coefficient of a-Si :H is ~8 x 105 cm- 1 at 2 ~0.4/tm [25], the top doped layer must be < 1 0 nm to assure a high collection efficiency at short wavelengths. Fig. 8 shows data for Vo~ and J~ as a function of the thickness of the n layer for a p - i n cell illuminated through that layer. As expected, J , decreases for large thicknesses of the n layer since the collection efficiency at short wavelengths decreases. The initial increase in J , with n-layer thickness is largely due to the increase in the built-in potential which is evident in the rapid increase in Voc.
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D. E. Carlson / Amorphous silicon solar cells
515
The built-in potential in p-i-n cells is on the order of 1.1 V [28, 58]. However, the optical gap in undoped a-Si :H is ~ 1.65-1.70 eV, so a significant fraction of the gap is not being utilized. Conductivity and thermopower data indicate that the Fermi level in phosphorus-doped a-Si :H is ~0.2 eV below the conduction band [59] while in boron-doped a-Si :H it is ~0.5 eV above the valence band [55]. Moving either Fermi level closer to the respective band edge will result in an increase in the builtin potential. A recent development that promises to improve the situation at least for n-type films involves using glow-discharge atmospheres of SiF 4 and H2 in conjunction with I(K~--1000 vppm of AsH 3 or PH3 [60, 61]. These films are highly conductive ( ~ 10 f~- 1 cm - 1 vs. ~ 10- 2 f~- ~ cm - ~ for heavily-doped a-Si :H films), artd the Fermi level appears to be z0.05 eV below the conduction band [61]. Some recent Raman scattering data [62] suggests that these films may be somewhat polycrystalline, but they may still function as good contact layers for solar cells. Highly conductive films ( ~10 ~ - 1 cm-1) have also been made from glow discharges containing Ar, Sill4 and PH3 (with PH3/SiH4 ~0.025), and X-ray diffraction data showed that these films were at least partially polycrystalline [63]. Improving the conductivity of the doped layers should reduce the contact resistance which appears to be ~2-5 f~ cm 2 in present p--i--n cells. Reducing this resistance to less than 1 f~ cm 2 should improve the fill factor by several percent. Another complication in the case of p-i-n cells with thin top doped layers is the presence of surface states at the ITO/doped layer interface. If the doped layer is very thin ( < 10 nm) it will be fully depleted, and surface states may reduce the built-in potential. Moreover, a resistive oxide layer may develop at the ITO/doped layer interface and contribute to the series resistance. Reflection losses for a p-i-n cell averaged over a typical spectral response curve are ~7~o and absorption losses in the ITO are usually less than 3~o [25]. Thus, the single ITO layer acts as both a top contact and a relatively good antireflection coating. A small absorption loss ( ~3~o of J~) also occurs at the back contact of present p-i-n cells [25].
7. Future directions
As mentioned in the previous section, device performance would improve significantly if the quality of the doped layers were improved. The work of Ovshinsky and Madan [60] indicates that improved doped films can be made by varying deposition conditions such as the discharge atmosphere. At the present time, it is not clear what role fluorine plays in determining the properties of these films, but it appears that the improved conductivity results largely from an increase in the short range order [62, 63]. There are many other discharge atmospheres and deposition conditions that should be examined in more detail. Some preliminary studies have been conducted with discharges containing SiC14 and H 2 [64], SiC12H 2 and H2 [8], Sill4 and CH 4 [8], Sill 4 and C2H 2 [65, 66], Sill4 and GeH 4 [8, 57], Sill4 and 0 2 [67, 68] and Sill4 and
516
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N 2 [8, 67]. However~ relatively little work has been done with dopants present in these discharges. Many additives such as C2H 2, 02 and N 2 will increase the optical gap of a-Si :H so that if the modified films could also be made relatively conductive by doping, they might work well as doped layers in a p - ~ n device. Another approach to improve solar-cell performance is to develop a new type of wide bandgap, highly conductive film to form a heterojunction with a-Si :H. One can make ITO/a-Si :H or SnO//a-Si :H heterojunctions but the open-circuit voltages are relatively low (<600 mV) [14, 52]. Materials such as polycrystalline SiC or GaN might be suitable if they could be deposited in a low cost, thin film process. Another structure that might lead to higher conversion efficiencies is the stacked junction cell where p-i-n junctions are stacked on top of one another [57, 69]. However, to obtain high performance, the optical bandgap of the various junctions must be tailored so that the light is first incident on the widest bandgap material and the bandgap decreases for each subsequent junction [70]. As mentioned earlier, some wide bandgap alloys such as a-Si :C :H [65, 66] and a-Si :O :H [68] have been made, but there have been no detailed studies of their suitability for photovoltaics. Solar cells have been fabricated from narrow bandgap alloys of a-Si :Ge :H, but the device performance degraded significantly as the Ge content increased [57]. Thus, further progress with stacked junction cells will depend greatly on efforts to modify both the electrical and optical properties of a-Si :H. Another possible approach is to form stacked junctions using other semiconductor materials such as a thin film GaAs cell in conjunction with an a-Si :H p-i-n junction. If the optical gap of a-Si :H is decreased, e.g., by reducing the hydrogen content or by alloying with Ge, then the material is well-suited for photothermal energy conversion [71], and hybrid structures that convert sunlight to both heat and electricity may be fabricated. Even with a relatively large bandgap of z l.65-1.70 eV, the photothermal performance of a-Si :H cells can be improved by means of an infrared absorbing material such as a cermet as a back contact [21]. Since hydrogen evolution can degrade device performance, the operating temperature of such devices would probably be tess than ~ 150C. However, it may be possible to improve the thermal stability of a-Si :H by modifying the material; recently, Tanaka et al. [72] showed that the incorporation of Ar into the a-Si:H structure hinders hydrogen evolution. The development of an efficient solar cell based on thin amorphous semiconductor films is an enormous and exciting challenge to materials scientists. However, continued progress will require a coordinated effort between scientists of various disciplines. In particular, a better understanding of the plasma chemistry and the surface reaction chemistry is essential for further development of the glow-discharge deposition process. Also, more effort should be devoted toward both the theoretical and experimental identification of defects in a-Si :H. Research in the field of amorphous semiconductors is presently in a dynamic phase involving a succession of new discoveries and concepts. There appears to be a high probability that this research will lead to new useful products, such as low cost terrestrial solar cells, in the near future.
D. E. Carlson / Amorphous silicon solar cells
517
References [1] [2] [3] [4] [5] [6]
R. C. Chittick, J. H. Alexander and H. F. Sterling, J. Electrochem. Soc. 116 (1969) 77. J. J. Hanak, RCA Laboratories, private communication. J. C. Knights, Phil. Mag. 34 (1976) 663. H. A. Weakliem, RCA Laboratories, private communication. D. E. Carlson, US Patent No. 4064521 (19771. D. E. Carlson and C. W. Magee, 2nd E. C. Photovoltaic Solar Energy Conf., Berlin, 23-26 April 1979 (Reidel, Dordrecht, 1979) p. 312. [7] R. A. Street, J. C. Knights and D. K. Biegelsen, Phys. Rev. BI8 (1978) 1880. [8] D. E. Carlson, Tech. Digest of 1977 IEEE Intern. Electron Dev. Mtg., IEEE, NY (1977) p. 214. [9] C. W. Magee and D. E. Carlson, Solar Cells, to be published. [10] R. W. Griffith, Amorphous Silicon/Materials Subcontractor's Review Meeting, Washington, DC (23-25 April 1980) [11] G. Turban, Y. Catherine and B. Grolleau, Thin Solid Films 67 (1980) 302. [12] 1. Hiller, Bull. Am. Phys. Soc. 25 (1980) 294. [13] H. A. Weakliem, AIEE 13th Fluid and Plasma Dynamics Conf., Snowmass, CO (14 July 1980). [14] D. E. Carlson, IEEE Trans. Electron Dev. ED-24 (1977) 449. [15] D.E. Carlson, R. W. Smith, G. A. Swartz and A. R. Triano, Electrochem. Soc. Meeting, Hollywood, Florida (5~10 October 1980). [16] J. J. Hanak and V. Korsun, Proc. 13th IEEE Photovoltaic Specialists Conf. Washington, DC (1978) p. 780. [17] J. 1. B. Wilson, J. McGill and S. Kinmond, Nature 272 (1978~ 153. [18] J. J. Thompson, J. Allison, M. M. A1-Kaisi and I. P. Thomas, Rev. de Phys. Appl. 13 (1978) 625. [19] Y. Uchida, H. Sakai and T. Ichimura, Fuji Electric Co., published results. [20] D. E. Carlson and C. R. Wronski, Appl. Phys. Lett. 28 (1976) 671. [21] J. J. Hanak, Proc. 14th IEEE Photovoltaic Specialists Conf., San Diego, CA (1980) p. 623. [22] H. Okamoto, Y. Nitta, T. Adachi and Y. Hamakawa, Surface Sci. 86 (1979) 486. [23] R. A. Gibson, W. E. Spear, P. G. LeComber and A. J. Snell, J. Non-Crystalline Solids 35/36 (1980) 725. [24] Y. Kuwano, T. Imai, M. Ohnishi and S. Nakano, Proc. 14th IEEE Photovoltaic Specialists Conf., San Diego, CA (1980) p. 1408. [25] D. E. Carlson, ibid, p. 291. [26] C. R. Wronski, B. Abeles, G. D. Cody, D. L. Morel and T. Tiedje, ibid, p. 1057. [27] G. A. Swartz, RCA Laboratories, private communication. [28] D. E. Carlson and C. R. Wronski, J. Electron. Mater. 6 (1977) 95. [29] S. Ashok, P. A. Lester and S. J. Fonash, to be published. [30] C. R. Wronski and D. E. Carlson, RCA Laboratories, unpublished results. [31] G. H. Dohler and H. Heyszenan, Phys. Rev. B12 (1975) 641. [32] G. A. Swartz and R. Williams, Proc. 14th IEEE Photovoltaic Specialists Conf., San Diego, CA (1980) p. 1224. [33] D. L. Staebler and C. R. Wronski, Appl. Phys. Lett. 31 (1977) 292. [34] I. Solomon, T. Dietl and D. Kaplan, J. de Phys. 39 (1978) 1241. [35] D. L. Staebler and C. R. Wronski, J. Appl. Phys. 51 (1980) 32. [36] C. R. Wronski, IEEE Trans. Electron Dev. ED-24 (1977) 355. [37] A. R. Moore, Appl. Phys. Lett. 37 (1980) 327. [38] R. S. Crandall, to be published. [39] D. L. Staebler, J. Non-Crystaline Solids 35/36 11980) 387. [40] R. S. Crandall, J. Non-Crystalline Solids 35/36 (19801 381. [41] A. J. Snell, W. E. Spear and P. G. LeComber, Phil. Mag., to be published. [42] J. Mort, S. Grammatica, J. C. Knights and R. Lujan, to be published. [43] J. 1. Pankove and D. E. Carlson, Appl. Phys. Lett. 31 (1977) 450. [44] R. A. Street and D. K. Biegelsen, Solid State Commun. 33 (1980) 1159.
518
[45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57] [58] [59] [60] [61] [62] [63] [64] [65] [66] [67] [68] [69] [70] [71] [72] [73]
D [~.'l ('(ir],]'o}J
Amorphous silicon solar (ell,~
J. C. Knights, G. Lucovsky and R. J. Nemanich, J. Non-Crystalline Solids 32 (1979) 393. R. S. Crandall, R. Williams and B. E. To~npkins, J. Appl. Phys. 50 (1979) 5506. R. Crandall, Appl. Phys. Lett. 36 (1980)607. I. Shimizu, T. Komatsu. K. Saito and E. lnoue. J. Non-Crystalline Solids 35/36 {1980) 773. D. M. Pai and S. W. lng, Phys. Rev. 173 i1968) 899. T. Tiedje, C. R. Wronski, B. Abeles and J. M. Cebulka, Solar Cells, to be published. M. Silver, A. Maden, D. Adler and W. Czubatyj, Proc. 14th IEEE Photovoltaic Specialists Conf.. San Diego, CA (1980) p. 1062. J. J. Hanak, V. Korsun and J. P. Pellicane, 2nd E. C. Photovoltaic Solar Energy Conf., Berlin, 23 26 April 1979 (Reide[, Dordrecht, 1979) p. 270. G. A. Swartz, to be published. T. S. Nashashibi, I. G. Austin and T. M. Searle, Phil. Mag. 35 (1977) 83t. Z. 1. Jan, R. H. Bube and J. C. Knights, J. Appl. Phys. 51 (1980) 3278. B. yon Roedern, L. Ley, M. Cardona and F. W. Smith, Phil. Mag. B40 (1979) 433. J. J. Hanak, B. Faughnan, V. Korsnn and J. P. Pellicane, Proc. 14th IEEE Photovoltaic Specialists Conf., San Diego, CA (1980) p. 1209. R. H. Williams, R. R. Varma, W. E. Spear and P. G. LeComber. J. Phys. C12 (1979) L209. W. E. Spear, Advan. Phys. 26 (1977) 3t2. S. R. Ovshinsky and A. Madam Nature 276 (1978) 482. A. Madam S. R. Ovshinsky and E. Benn, Phil. Mag. 40 (1979) 259. R. Tsu~ M. lzu and S. R. Ovshinsky, Bull. Am. Phys. Soc. 25 (1980) 295. S. Usui and M. Kikuchi, J. Non-Crystalline Solids 34 (1979) 1. W. W. Kruehler. R. D. Plaettner. M. Moeller, B. Rauscher and W. Stetter, J. Non-Crystalline Solids 35/36 {1980) 333 D. A. Anderson and W. E. Spear, Phil. Mag. 35 (1977~ 1. D. Engemann, R. Fischer and J. Knecht, Appl. Phys. Lett. 32 (1978) 567. R. W. Griffith, F. J. Kampas, P. E. Vanier and M. D. Hirsch, J. Non-Crystalline Solids 35/36 (1980j 391. J. C. Knights, R. A. Street and G. Lucovsky, ibid, p. 279. Y. Hamakav~a, H. Okamoto and Y. Nitta. Appl. Phys. Lctt. 35 {1979J 1871 J. J. Loferski, Proc. 12th IEEE Photovoltaic Specialists Conf., Baton Rouge, LA (1976) p. 957. M. Janai, D. D. Allred, D. C. Booth and B. O. Seraphin, Solar Energy Mater. 1 (1979) 11. K. Tanaka, S. Yamasaki, K. Nakagawa, A. Matsnda. H. Okushi, M. Matsumura and S. lizima, J. Non-Crystalline Solids 35/36 (1980) 475. A. Madan, J. McGill, W. Czubatyj, J. Yang and S. R. Ovshinsky, to be published. The structure is ZnS(35 nm )/Au :Pd(90:10) (7 nm )/Nb205(2 nm )/undoped a-Si :F :H ( ~0.5 #m )/n +a-Si :F:H (80 nm t/ Mo. The a-Si :F :H was deposited from an rf discharge with SiF4/H 2 ~ 5.
RECENT DEVELOPMENTS IN AMORPHOUS SILICON SOLAR CELLS* D. E. CARLSON RCA Laboratories, Princeton, NJ 08540, USA Received 21 July 1980 This article reviews recent advances in the development of amorphous silicon solar cells. Both the glow-discharge deposition conditions and the solar-cell structures are discussed in some detail. The performance characteristics of present amorphous silicon cells are described, and the loss mechanisms that limit performance are considered. An effort has been made to point out those areas where further research is needed. Recently, amorphous silicon p-i-n cells with areas of 1.19 cm 2 have been fabricated with conversion efficiencies as high as 6.1~'o.
I. Introduction The present interest in hydrogenated amorphous silicon (a-Si :H) results mainly from its promise as a low-cost photovoltaic material foi terrestrial solar cells. The purpose of this article is to review recent developments in a-Si :H solar-cell technology and to point out those areas that require more research. In section 2, we describe the deposition conditions that have been used to make a-Si :H solar cells, and then we describe the various solar-ceU structures in section 3. Section 4 is a review of the performance history of a-Si :H solar cells while section 5 covers present performance characteristics in some detail. We discuss the loss mechanisms that appear to limit device performance in section 6 and present some thoughts about future directions in the final section. In the previous review paper in this journal, H. Fritzsche discussed the electrical, optical and structural properties of plasma-deposited a-Si :H. In a future issue, W. Paul will review the material and device properties of sputtered a-Si :H. The present article concentrates on solar cells made from the plasma decomposition of silane (Sill4).
2. Deposition conditions There are several types of plasma-decomposition or glow-discharge systems that have been used to fabricate a-Si :H solar cells. The first a-Si :H films were grown in an *Some of the research reported herein was supported by Solar Energy Research Institute, under Contract Number XJ-9-8254, and by RCA Laboratories, Princeton, NJ 08540, USA. 503
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rf inductive system that utilized an external coil surrounding a cylindrical glass discharge chamber [1]. Although this type of system has been used to produce a-Si :H solar cells with conversion efficiencies > 5 ...... .,, at RCA Laboratories, the uniformity, over large areas is poor. Similar device performance [2] has also been observed in cells made in rf systems that operate in a mode similar to diode sputtering, i.e., the rf discharge is sustained by capacitive coupling through electrodes within the discharge chamber [3]. This type of system is widely used since it is commercially available and is capable of depositing uniform films over relatively large areas ( >0.1 m 2 ). Another type of rf system employs a similar geometry but magnetic confinement is imposed on the discharge in a manner similar to that used in magnetron sputtering. Preliminary work at RCA Laboratories indicates that the a-Si:H produced by this type of discharge contains a relatively high concentration of defects, e.g., (SiH2t,, groups, and the best cells to date exhibited conversion efficiencies less than 3".~, 14]. G o o d quality a-Si:H has also been produced in dc proximity glow-discharge systems [5]. In this case, the substrate is located in close proximity of a screened cathode with an anode screen located on the other side of the cathode. In some dc systems, the substrate is the cathode, but generally the film quality is poor as a result of positive ion bombardment during growth [6]. Since the rf diode and the dc proximity systems are the most widely used, we will restrict our detailed discussion of deposition conditions to those systems. G o o d quality a-Si :H has been obtained in rf diode systems by operating at low rf power densities (<0.1 W/cm 2) and in pure Sill 4 discharge atmospheres [7]. Generally, high flow rates ( ~50-200 std. cm 3 min i) are directed at or over the substrate to minimize contaminants outgassing from walls and heaters. Relatively good material has been made over a wide range of pressures ( ~0.02 to 1.0 Torr) and substrate temperatures ( ~200-400 C). Normally, only one electrode is powered and the other is grounded. For a substrate located on the grounded electrode, the deposition rate is on the order of 5 to 20 nm/min. In the dc proximity mode, the power density is also on the order of 0.1 W/cm 2 with the discharge pressure in the range of 0.3 to 0.9 Torr. The spacing between the substrate and cathode screen is on the order of 1 cm with the anode located ~ 1 to 4 cm away from the cathode. The applied voltage is typically 650 to 800 V for pure Sill4 discharges. The flow rates and substrate temperatures are comparable to those used in rf diode systems;deposition rates are ~20-30 nm/min. While it is evident that the material quality is strongly influenced by substrate temperature and discharge power, it is not clear how important other deposition parameters are. There is some evidence that contaminants such as oxygen and nitrogen can adversely affect device quality [8-10], but much more research should be done in this area. Moreover, there is relatively little information available about the discharge kinetics or the discharge chemistry. Recent mass spectroscopy work [11-13] indicates that species such as SiH~- and SiH~- are relatively abundant in Sill4 discharges and that higher silanes (SixHy groups where x >2) are created in varying amounts depending on deposition conditions. However, there is no concensus regarding which discharge species are desirable in growing good quality a-Si :H films. Even less is
D. E. Carlson / Amorphous silicon solar cells
505
known about the chemistry that is occurring at the surface of the growing a-Si :H film, but it appears that significant reactions involving bond breaking and restructuring must be occurring since the a-Si :H films contain relatively little hydrogen compared to that in the discharge species incident on the surface.
3. Solar cell structures Most a-Si :H solar cells have been fabricated in one of three structures : Schottkybarrier, MIS or p-i-n. The Schottky-barrier cell is used mainly as a diagnostic device since it is relatively easy to fabricate. However, the open-circuit voltage of Schottkybarrier cells is limited to values less than ~600 mV, so high performance is not possible with this type of cell. One can fabricate a simple Schottky-barrier cell by depositing undoped a-Si :H on Mo and then evaporating ~5 nm of Pd onto the top of the a-Si :H film. The photovoltaic and diode characteristics can be improved by [t-- 11=t- ( ~ S n m {0)
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first depositing a thin layer ( ~20 nml of phosphorus-doped a-Si :H from a discharge containing ~1 vol°~,~,PH 3 in Sill4. To date, conversion efficiencies greater than 5% have been obtained only with the three structures shown in fig. 1. The MIS (metal-insulators-semiconductor) cell is similar to a Schottky-barrier structure, but a thin insulating layer ( ~2-3 nmi is located between the undoped a-Si :H and a high work function metal such as Pt or Ir (see fig. la). In order to assure good optical coupling into the cell, the metal film must be thin t ~5 nm) and also must be overcoated with an antireflection layer such as ~45 nm o f Z r O 2 [14]. Figs. lb and c show two p-i-n structures that have exhibited good performance. The first is fabricated by depositing a boron-doped layer ( ~20 nm) on steel from a discharge in ~ 1 vol0/o B2H 6 in Sill 4. The undoped layer is ~0.5 lzm thick and the top phosphorus-doped layer is ~8 nm thick. Finally, ~70 nm of indium-tin oxide (ITO) is deposited by electron beam evaporation on the n layer to act as both a top contact and an antireflection layer [15]. The p-i-n structure in fig lc is illuminated through the glass substrate which is coated with ~60 nm of ITO and ~10 nm of a cermet such as Pt-SiO2 [16]. The cermet helps assure a good contact to the thin p layer ( ~8 nm). The undoped layer is ~0.643.8 #m thick and the n layer is ~20 nm thick. The back contact is formed by sputter-deposition of Ti or Ti/A11 ~ 100 nmt. It is apparent from the above descriptions that a-Si :H solar cells are truly thin film devices since the thickness of the total structure (apart from the substrate) is less than 1 ltm in each case.
4. Performance history of a-Si:H solar cells
In fig. 2, we show the performance history of a-Si :H solar cells fabricated in the Schottky-barrier and MIS configurations. Data points that are not labeled with reference numbers refer to devices fabricated at RCA Laboratories. As shown in fig. 2, the conversion efficiency of small-area devices improved rapidly in the first few years of development [14]. However, these devices degraded slowly upon exposure to air apparently as a result of water vapor interacting with the interfacial layer between the metal and the a-Si :H [6, 12]. Although the performance of largearea Schottky-barrier and MIS cells is improving, the rate of progress is relatively slow (see fig. 2). Since the junction in these structures is formed by a thin metal contact (e.g., ~5 nm of Pt), it may be difficult to assure the uniformity needed for the commercial production of solar panels. Moreover, these devices may have to be well-encapsulated to prevent degradation. As shown in fig. 3, the performance of p-i-n cells increased only gradually in the first few years of development. However, in the last year, the efficiency of large area ( > 1 cm 2) p-i-n devices has improved significantly. This improvement resulted from a systematic optimization of fabrication parameters such as layer thicknesses, doping levels, discharge power, etc. [15]. Most p-i--n cells have exhibited good shelf life without encapsulation, and it appears to be somewhat easier to make large area
D. E. Car~son ,' Amorphous silicon s'o/ar cells
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p-i-n cells than it is to make large area Schottky-barrier or MIS cells• Fig. 4 shows a photograph of a series-connected monolithic a-Si :H solar panel with an active area of 63 cm 2. The panel consists of 17 p - i n cells connected in series, and the conversion efficiency was 3.6~o. This panel was fabricated in an rf discharge system that was making small-area (9.5 mm 2) cells with an efficiency of ~4.0~o at the time [2]. It is interesting to note that while single crystal silicon solar cells are ~ 2 to 3 times more efficient than present a-Si :H cells in sunlight, the a-Si :H cells are comparable to or better than crystalline Si cells under fluorescent light [24]. This is due to the fact that the spectral response of a-Si :H cells is very closely matched to the emission spectrum of a fluorescent light• Sanyo Corporation hopes to take advantage of this property and plans to market a line of consumer products (e.g., watches, calculators) with a-Si :H solar cells as power sources in the near future [24].
5. Present performance characteristics The I - V characteristics of a recent p-i-n cell (1.19 cm 2) under simulated AM1 illumination are shown in fig. 5. The solar simulator was an Oriel, model 6730, with a uniformity of ___3 ~ over an area of 50 cm 2. The simulator is calibrated by first measuring the performance of an a-Si :H cell in sunlight (near AM1 conditions) and
D. E. Carlson/Amorphous silicon solar ('ell.~
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determining the solar irradiance by means of a NASA-calibrated crystalline Si cell and a pyroheliometer. The a-Si :H cell is then placed in the solar simulator, and the light intensity is adjusted to give the same short-circuit current as measured in sunlight. A small crystalline Si cell is located on the sample holder and is used to maintain the calibration. In the last few months more than one hundred p - i n cells (each 1.19 cm 2) have been fabricated at RCA Laboratories with conversion efficiencies greater than 5.0%. Most of these cells were fabricated in the configuration shown in fig. lb, but a few were made with the structure shown in fig. lc. A record open-circuit voltage (Vo~) of 910 mV has been obtained with the latter structure. However, the best conversion efficiency (6.1%) and the best short-circuit current densities (~,12 mA/cm 2 in AMI light) have been obtained with p - i n cells illuminated through the n-layer (see fig. lb). The best fill factor observed in p-i-n cells is ~0.61 although values as high as 0.674 were observed in earlier work with Schottky-barrier cells. A spectral response curve is shown in fig. 6 for a p-i-n cell illuminated through the n layer. The curve shows a decrease in collection efficiency at short wavelengths due
D. E. Carlson / Amorphous silicon solar cells
509
Fig. 4. Photograph of monolithic a-Si:H solar panel with 17 cells connected in series (total active area =63 cm2).
mainly to absorption losses in the top doped layer and also, a decrease at long wavelengths due to the decreasing absorption coefficient of undoped a-Si :H [25]. The data in fig. 6 were obtained by illuminating the sample with chopped monochromatic light in the presence of constant AM1 illumination and calibrating the short-circuit current for the monochromatic light to give a collection efficiency (number of electron-hole pairs collected per 100 incident photons). This procedure is necessary since the spectral response of a-Si :H solar cells can change significantly with light intensity [26]; the presence of constant AM1 illumination assures that the cell is characterized under normal operating conditions. Also listed in fig. 6 is the short-circuit current density (J~) calculated from the spectral response curve and from measured solar irradiance data for direct sunlight (AMI). Short-circuit current densities measured in sunlight are typically 15-20~o larger than the calculated values due to a contribution from the indirect sunlight (scattered blue light). Therefore, to obtain good agreement between measured and calculated values of J~¢, one must either include the scattered light in the calculation or measure the performance in sunlight with a collimating tube to exclude scattered Light.
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Another measurement that is often used to characterize a solar cell involves a determination of the diode quality factor. The diode quality factor under illumination (n') is defined by the expression
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Ill
where Jo is the reverse saturation current density. Recent p - i - n cells exhibit n' values of z 1.1 [27] indicating that relatively little recombination is occurring in the p-i-n junction. This value is similar to that obtained in earlier work with Schottky-barrier cells [28]. While the behavior under illumination is close to that of an ideal diode, the I - V characteristics of a-Si :H cells in the dark are often poor. In some cases, the data can be explained by space-charge limited currents [20, 29], while in other cells a PooleFrenkel expression provides a better fit [30]. In some cells, the poor rectification in
D. E. Carlson / Amorphous silicon solar cells T03060-3-01-04
511
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Isc(mml)= 9.27
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400
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::::::::::::::::::::::::: 800 900
NRVELENQTH--NM Fig. 6. The collection efficiency o f a p - ~ n cell as a function of wavelen~h (cell illuminated through the n layer).
the dark may be associated with relaxation semiconduction [31]. A semiconductor is in the relaxation regime when the dielectric relaxation time is greater than the carrier lifetime (this condition corresponds to p ;~ 108 fl cm for ~ ~ 1 #s). Another characteristic of recent p-i--n cells is that they are fully depleted s a that an electric field ( ~ 2 x 104 V cm -1) extends across the undoped layer under shortcircuit conditions. The fully depleted condition can be confirmed by comparing the spectral response of the cell under zero and reverse bias or by analyzing the wavelength dependence of the I - V characteristics [32]. An important property of a-Si :H solar cells is that they appear to be relatively stable. As mentioned earlier, some Schottky-barrier and MIS cells have exhibited degradation resulting from interactions with water vapor, but encapsulation should prevent this degradation [6]. Processing at elevated temperatures (e.g., > 1 h at 350°C) can also degrade device performance since hydrogen can evolve from a-Si :H films and leave behind dangling bonds [6, 8]. Prolonged exposure to light may change the characteristics of some a-Si :H films and devices [33]. In some cases, the lightinduced changes are caused by surface effects [34], but bulk effects may be occurring in other films [35].
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6. Performance limitations
The present performance of a-Si :H solar cells is limited by several factors. Earl). work on a-Si:H Schottky-barrier cells indicated that a major limitation was the minority carrier diffusion length [14, 36]. Recent studies have indicated that the hole diffusion length ;~n undoped a-Si:H lies in the range of ~0.03 to 0.2 ltm [3~39]. The techniques that have been used to estimate the hole diffusion length include the photoelectromagnetic (PEM) effect [37], analysis of I - V characteristics of p - i n cells under illumination [38], and the measurement of the collection efficiency as a function of the i-layer thickness [39]. In crystalline semiconductors, the minority carrier diffusion length (LI is simply related to the mobility (It) and lifetime (r) by the expression L=
-/tz
12t
However, there is evidence that in a-Si:H there is a range of mobilities and lifetimes for the minority carriers depending on their location in the density of state distribution [40]. From PEM measurements, Moore [37] estimates that the hole lifetime in undoped a-Si :H is ~0.34 ILs while the electron lifetime is ~ 1.2 #s. Recent studies of transient diode recovery by Snell et al. [41] indicate hole lifetimes of ~10-20 ~s at current densities of ~ 10 mA/cm 2. Even longer hole lifetimes ( ~ 1 ms) have been indicated by the Xerographic discharge experiments of Mort et al. [42] in very lightly doped a-Si :H (5 vppm B2H6 in Sill4). It is not clear at the present time whether these estimates differ because the materials were different or because the experimental results were not interpreted properly. There is evidence that several types of defects may be responsible for limiting carrier lifetimes. Dangling bonds such as those created by hydrogen outdiffusion can severely reduce the photoluminescence intensity [43] and solar cell efficiency [8]. Recently, Street and Biegelsen [44] have concluded that the dangling bonds are the dominant recombination centers in undoped a-Si:H. There is also evidence that recombination centers are associated with short polymer chains or (Sill2) . groups [45]. Other recombination centers appear to be associated with impurities such as oxygen, nitrogen and phosphorus [8]. Present p-i-n cells with efficiencies of 5 - 6 ' , o typically contain ~102° oxygen atoms cm -3, ~ 5 x 10 ~9 carbon atoms cm 3 ~7 x 1018 nitrogen atoms cm -3 and ~10-14 at~o hydrogen. No (SiHz), groups are detectible in the ir absorption spectra of these films. There is some evidence that geminate recombination is responsible for limiting the transport in undoped a-Si :H [46-48]. This recombination has been observed in other amorphous semiconductors such as a-Se [49], and might be expected in any low mobility material where the photogenerated electron-hole pairs have difficulty escaping their mutual coulombic attraction. However, other recent studies of both a-Si :H [50] and a-Si :F :H [51] suggest that geminate recombination is not occurring in these materials. While some ambiquities exist regarding the recombination mechanisms, it is well-
D. E. Carlson ,; Amorphous silicon solar cells
513
accepted that the space-charge field plays an important role in the operation of a-Si :H solar cells [36]. Recent p-i-n devices are depleted over the entire thickness of the i-layer, ~0.54).7 #m [32, 52]. Crandall [38] has modeled the photovoltaic characteristics of such cells by considering the variation of the drift length within a Mott barrier (i.e., a uniform field exists throughout the undoped layer). He finds that the characteristics of a cell with a fill factor of ~0.60 can be explained with a drift length at zero bias of ~ 3.5 #m ; this corresponds to a hole diffusion length of ~0.2 #m. Thus, one must improve the lifetime (or mobility) to obtain better fill factors. Recent computer modeling of a-Si :H p - i n cells by Swartz [53] indicates that the performance is being severely limited by the quality of the doped layers. There is considerable evidence that doping increases the defect density in a-Si "H. The photoluminescence intensity at low temperatures (~50-100K) decreases [54], the electron spin density increases [55], and solar cell performance decreases [8, 20]. Doping with either BEH 6 or PH 3 creates a distribution of gap states as shown by optical absorption measurements [25], collection efficiency measurements [40] and photoemission measurements [56]. The optimum doping level in p-i-n cells is ~ 1 volvo B2H 6 in Sill 4 for the p layer
(FrE)
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D. E. Carlson ,, Amorphou.~ silicon ~olar cells
514
and ~ 1 - 2 vol% P H 3 in Sill 4 for the n layer [14, 15, 20]. At these doping levels, the lifetime of minority carriers appears to be very short, and the spectral response can be modeled assuming that the doped layers are dead and act only as filters [25]. Fig. 7 shows that the collection efficiency at short wavelengths is strongly influenced by the thickness of the top doped layer [57]. Since the absorption coefficient of a-Si :H is ~8 x 105 cm- 1 at 2 ~0.4/tm [25], the top doped layer must be < 1 0 nm to assure a high collection efficiency at short wavelengths. Fig. 8 shows data for Vo~ and J~ as a function of the thickness of the n layer for a p - i n cell illuminated through that layer. As expected, J , decreases for large thicknesses of the n layer since the collection efficiency at short wavelengths decreases. The initial increase in J , with n-layer thickness is largely due to the increase in the built-in potential which is evident in the rapid increase in Voc.
800 /
600
/
>
E 400
o
-4
/
200
0
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7
-
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30
d n (nm)
Fig. 8. The open-circuit voltage and short-circuit current density as a function of the thickness of the n layer (cell illuminated through the n layerJ.
D. E. Carlson / Amorphous silicon solar cells
515
The built-in potential in p-i-n cells is on the order of 1.1 V [28, 58]. However, the optical gap in undoped a-Si :H is ~ 1.65-1.70 eV, so a significant fraction of the gap is not being utilized. Conductivity and thermopower data indicate that the Fermi level in phosphorus-doped a-Si :H is ~0.2 eV below the conduction band [59] while in boron-doped a-Si :H it is ~0.5 eV above the valence band [55]. Moving either Fermi level closer to the respective band edge will result in an increase in the builtin potential. A recent development that promises to improve the situation at least for n-type films involves using glow-discharge atmospheres of SiF 4 and H2 in conjunction with I(K~--1000 vppm of AsH 3 or PH3 [60, 61]. These films are highly conductive ( ~ 10 f~- 1 cm - 1 vs. ~ 10- 2 f~- ~ cm - ~ for heavily-doped a-Si :H films), artd the Fermi level appears to be z0.05 eV below the conduction band [61]. Some recent Raman scattering data [62] suggests that these films may be somewhat polycrystalline, but they may still function as good contact layers for solar cells. Highly conductive films ( ~10 ~ - 1 cm-1) have also been made from glow discharges containing Ar, Sill4 and PH3 (with PH3/SiH4 ~0.025), and X-ray diffraction data showed that these films were at least partially polycrystalline [63]. Improving the conductivity of the doped layers should reduce the contact resistance which appears to be ~2-5 f~ cm 2 in present p--i--n cells. Reducing this resistance to less than 1 f~ cm 2 should improve the fill factor by several percent. Another complication in the case of p-i-n cells with thin top doped layers is the presence of surface states at the ITO/doped layer interface. If the doped layer is very thin ( < 10 nm) it will be fully depleted, and surface states may reduce the built-in potential. Moreover, a resistive oxide layer may develop at the ITO/doped layer interface and contribute to the series resistance. Reflection losses for a p-i-n cell averaged over a typical spectral response curve are ~7~o and absorption losses in the ITO are usually less than 3~o [25]. Thus, the single ITO layer acts as both a top contact and a relatively good antireflection coating. A small absorption loss ( ~3~o of J~) also occurs at the back contact of present p-i-n cells [25].
7. Future directions
As mentioned in the previous section, device performance would improve significantly if the quality of the doped layers were improved. The work of Ovshinsky and Madan [60] indicates that improved doped films can be made by varying deposition conditions such as the discharge atmosphere. At the present time, it is not clear what role fluorine plays in determining the properties of these films, but it appears that the improved conductivity results largely from an increase in the short range order [62, 63]. There are many other discharge atmospheres and deposition conditions that should be examined in more detail. Some preliminary studies have been conducted with discharges containing SiC14 and H 2 [64], SiC12H 2 and H2 [8], Sill4 and CH 4 [8], Sill 4 and C2H 2 [65, 66], Sill4 and GeH 4 [8, 57], Sill4 and 0 2 [67, 68] and Sill4 and
516
1). I:'. (arL~'on
,4morl~hOu.~ silicoH s~hu ~'~'l/~
N 2 [8, 67]. However~ relatively little work has been done with dopants present in these discharges. Many additives such as C2H 2, 02 and N 2 will increase the optical gap of a-Si :H so that if the modified films could also be made relatively conductive by doping, they might work well as doped layers in a p - ~ n device. Another approach to improve solar-cell performance is to develop a new type of wide bandgap, highly conductive film to form a heterojunction with a-Si :H. One can make ITO/a-Si :H or SnO//a-Si :H heterojunctions but the open-circuit voltages are relatively low (<600 mV) [14, 52]. Materials such as polycrystalline SiC or GaN might be suitable if they could be deposited in a low cost, thin film process. Another structure that might lead to higher conversion efficiencies is the stacked junction cell where p-i-n junctions are stacked on top of one another [57, 69]. However, to obtain high performance, the optical bandgap of the various junctions must be tailored so that the light is first incident on the widest bandgap material and the bandgap decreases for each subsequent junction [70]. As mentioned earlier, some wide bandgap alloys such as a-Si :C :H [65, 66] and a-Si :O :H [68] have been made, but there have been no detailed studies of their suitability for photovoltaics. Solar cells have been fabricated from narrow bandgap alloys of a-Si :Ge :H, but the device performance degraded significantly as the Ge content increased [57]. Thus, further progress with stacked junction cells will depend greatly on efforts to modify both the electrical and optical properties of a-Si :H. Another possible approach is to form stacked junctions using other semiconductor materials such as a thin film GaAs cell in conjunction with an a-Si :H p-i-n junction. If the optical gap of a-Si :H is decreased, e.g., by reducing the hydrogen content or by alloying with Ge, then the material is well-suited for photothermal energy conversion [71], and hybrid structures that convert sunlight to both heat and electricity may be fabricated. Even with a relatively large bandgap of z l.65-1.70 eV, the photothermal performance of a-Si :H cells can be improved by means of an infrared absorbing material such as a cermet as a back contact [21]. Since hydrogen evolution can degrade device performance, the operating temperature of such devices would probably be tess than ~ 150C. However, it may be possible to improve the thermal stability of a-Si :H by modifying the material; recently, Tanaka et al. [72] showed that the incorporation of Ar into the a-Si:H structure hinders hydrogen evolution. The development of an efficient solar cell based on thin amorphous semiconductor films is an enormous and exciting challenge to materials scientists. However, continued progress will require a coordinated effort between scientists of various disciplines. In particular, a better understanding of the plasma chemistry and the surface reaction chemistry is essential for further development of the glow-discharge deposition process. Also, more effort should be devoted toward both the theoretical and experimental identification of defects in a-Si :H. Research in the field of amorphous semiconductors is presently in a dynamic phase involving a succession of new discoveries and concepts. There appears to be a high probability that this research will lead to new useful products, such as low cost terrestrial solar cells, in the near future.
D. E. Carlson / Amorphous silicon solar cells
517
References [1] [2] [3] [4] [5] [6]
R. C. Chittick, J. H. Alexander and H. F. Sterling, J. Electrochem. Soc. 116 (1969) 77. J. J. Hanak, RCA Laboratories, private communication. J. C. Knights, Phil. Mag. 34 (1976) 663. H. A. Weakliem, RCA Laboratories, private communication. D. E. Carlson, US Patent No. 4064521 (19771. D. E. Carlson and C. W. Magee, 2nd E. C. Photovoltaic Solar Energy Conf., Berlin, 23-26 April 1979 (Reidel, Dordrecht, 1979) p. 312. [7] R. A. Street, J. C. Knights and D. K. Biegelsen, Phys. Rev. BI8 (1978) 1880. [8] D. E. Carlson, Tech. Digest of 1977 IEEE Intern. Electron Dev. Mtg., IEEE, NY (1977) p. 214. [9] C. W. Magee and D. E. Carlson, Solar Cells, to be published. [10] R. W. Griffith, Amorphous Silicon/Materials Subcontractor's Review Meeting, Washington, DC (23-25 April 1980) [11] G. Turban, Y. Catherine and B. Grolleau, Thin Solid Films 67 (1980) 302. [12] 1. Hiller, Bull. Am. Phys. Soc. 25 (1980) 294. [13] H. A. Weakliem, AIEE 13th Fluid and Plasma Dynamics Conf., Snowmass, CO (14 July 1980). [14] D. E. Carlson, IEEE Trans. Electron Dev. ED-24 (1977) 449. [15] D.E. Carlson, R. W. Smith, G. A. Swartz and A. R. Triano, Electrochem. Soc. Meeting, Hollywood, Florida (5~10 October 1980). [16] J. J. Hanak and V. Korsun, Proc. 13th IEEE Photovoltaic Specialists Conf. Washington, DC (1978) p. 780. [17] J. 1. B. Wilson, J. McGill and S. Kinmond, Nature 272 (1978~ 153. [18] J. J. Thompson, J. Allison, M. M. A1-Kaisi and I. P. Thomas, Rev. de Phys. Appl. 13 (1978) 625. [19] Y. Uchida, H. Sakai and T. Ichimura, Fuji Electric Co., published results. [20] D. E. Carlson and C. R. Wronski, Appl. Phys. Lett. 28 (1976) 671. [21] J. J. Hanak, Proc. 14th IEEE Photovoltaic Specialists Conf., San Diego, CA (1980) p. 623. [22] H. Okamoto, Y. Nitta, T. Adachi and Y. Hamakawa, Surface Sci. 86 (1979) 486. [23] R. A. Gibson, W. E. Spear, P. G. LeComber and A. J. Snell, J. Non-Crystalline Solids 35/36 (1980) 725. [24] Y. Kuwano, T. Imai, M. Ohnishi and S. Nakano, Proc. 14th IEEE Photovoltaic Specialists Conf., San Diego, CA (1980) p. 1408. [25] D. E. Carlson, ibid, p. 291. [26] C. R. Wronski, B. Abeles, G. D. Cody, D. L. Morel and T. Tiedje, ibid, p. 1057. [27] G. A. Swartz, RCA Laboratories, private communication. [28] D. E. Carlson and C. R. Wronski, J. Electron. Mater. 6 (1977) 95. [29] S. Ashok, P. A. Lester and S. J. Fonash, to be published. [30] C. R. Wronski and D. E. Carlson, RCA Laboratories, unpublished results. [31] G. H. Dohler and H. Heyszenan, Phys. Rev. B12 (1975) 641. [32] G. A. Swartz and R. Williams, Proc. 14th IEEE Photovoltaic Specialists Conf., San Diego, CA (1980) p. 1224. [33] D. L. Staebler and C. R. Wronski, Appl. Phys. Lett. 31 (1977) 292. [34] I. Solomon, T. Dietl and D. Kaplan, J. de Phys. 39 (1978) 1241. [35] D. L. Staebler and C. R. Wronski, J. Appl. Phys. 51 (1980) 32. [36] C. R. Wronski, IEEE Trans. Electron Dev. ED-24 (1977) 355. [37] A. R. Moore, Appl. Phys. Lett. 37 (1980) 327. [38] R. S. Crandall, to be published. [39] D. L. Staebler, J. Non-Crystaline Solids 35/36 11980) 387. [40] R. S. Crandall, J. Non-Crystalline Solids 35/36 (19801 381. [41] A. J. Snell, W. E. Spear and P. G. LeComber, Phil. Mag., to be published. [42] J. Mort, S. Grammatica, J. C. Knights and R. Lujan, to be published. [43] J. 1. Pankove and D. E. Carlson, Appl. Phys. Lett. 31 (1977) 450. [44] R. A. Street and D. K. Biegelsen, Solid State Commun. 33 (1980) 1159.
518
[45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57] [58] [59] [60] [61] [62] [63] [64] [65] [66] [67] [68] [69] [70] [71] [72] [73]
D [~.'l ('(ir],]'o}J
Amorphous silicon solar (ell,~
J. C. Knights, G. Lucovsky and R. J. Nemanich, J. Non-Crystalline Solids 32 (1979) 393. R. S. Crandall, R. Williams and B. E. To~npkins, J. Appl. Phys. 50 (1979) 5506. R. Crandall, Appl. Phys. Lett. 36 (1980)607. I. Shimizu, T. Komatsu. K. Saito and E. lnoue. J. Non-Crystalline Solids 35/36 {1980) 773. D. M. Pai and S. W. lng, Phys. Rev. 173 i1968) 899. T. Tiedje, C. R. Wronski, B. Abeles and J. M. Cebulka, Solar Cells, to be published. M. Silver, A. Maden, D. Adler and W. Czubatyj, Proc. 14th IEEE Photovoltaic Specialists Conf.. San Diego, CA (1980) p. 1062. J. J. Hanak, V. Korsun and J. P. Pellicane, 2nd E. C. Photovoltaic Solar Energy Conf., Berlin, 23 26 April 1979 (Reide[, Dordrecht, 1979) p. 270. G. A. Swartz, to be published. T. S. Nashashibi, I. G. Austin and T. M. Searle, Phil. Mag. 35 (1977) 83t. Z. 1. Jan, R. H. Bube and J. C. Knights, J. Appl. Phys. 51 (1980) 3278. B. yon Roedern, L. Ley, M. Cardona and F. W. Smith, Phil. Mag. B40 (1979) 433. J. J. Hanak, B. Faughnan, V. Korsnn and J. P. Pellicane, Proc. 14th IEEE Photovoltaic Specialists Conf., San Diego, CA (1980) p. 1209. R. H. Williams, R. R. Varma, W. E. Spear and P. G. LeComber. J. Phys. C12 (1979) L209. W. E. Spear, Advan. Phys. 26 (1977) 3t2. S. R. Ovshinsky and A. Madam Nature 276 (1978) 482. A. Madam S. R. Ovshinsky and E. Benn, Phil. Mag. 40 (1979) 259. R. Tsu~ M. lzu and S. R. Ovshinsky, Bull. Am. Phys. Soc. 25 (1980) 295. S. Usui and M. Kikuchi, J. Non-Crystalline Solids 34 (1979) 1. W. W. Kruehler. R. D. Plaettner. M. Moeller, B. Rauscher and W. Stetter, J. Non-Crystalline Solids 35/36 {1980) 333 D. A. Anderson and W. E. Spear, Phil. Mag. 35 (1977~ 1. D. Engemann, R. Fischer and J. Knecht, Appl. Phys. Lett. 32 (1978) 567. R. W. Griffith, F. J. Kampas, P. E. Vanier and M. D. Hirsch, J. Non-Crystalline Solids 35/36 (1980j 391. J. C. Knights, R. A. Street and G. Lucovsky, ibid, p. 279. Y. Hamakav~a, H. Okamoto and Y. Nitta. Appl. Phys. Lctt. 35 {1979J 1871 J. J. Loferski, Proc. 12th IEEE Photovoltaic Specialists Conf., Baton Rouge, LA (1976) p. 957. M. Janai, D. D. Allred, D. C. Booth and B. O. Seraphin, Solar Energy Mater. 1 (1979) 11. K. Tanaka, S. Yamasaki, K. Nakagawa, A. Matsnda. H. Okushi, M. Matsumura and S. lizima, J. Non-Crystalline Solids 35/36 (1980) 475. A. Madan, J. McGill, W. Czubatyj, J. Yang and S. R. Ovshinsky, to be published. The structure is ZnS(35 nm )/Au :Pd(90:10) (7 nm )/Nb205(2 nm )/undoped a-Si :F :H ( ~0.5 #m )/n +a-Si :F:H (80 nm t/ Mo. The a-Si :F :H was deposited from an rf discharge with SiF4/H 2 ~ 5.